JP6642710B2 - Positive electrode material for lithium secondary battery, positive electrode for lithium secondary battery using the same, and lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode material used for a positive electrode of a lithium secondary battery and a lithium secondary battery using the same.

  Lithium rechargeable batteries are power supplies for mobile communications such as mobile phones and notebook computers, power supplies for home electric appliances, power storage devices, stationary power supplies such as uninterruptible power supplies, and drive power supplies for ships, railways, automobiles, etc. Practical application is being promoted widely in the field of. In a lithium secondary battery, for example, lithium metal, a lithium alloy, a metal oxide, carbon, or the like is used as a negative electrode active material, and a lithium composite compound containing lithium and a transition metal is used as a positive electrode active material.

  For a positive electrode for a lithium secondary battery, a paste-like positive electrode mixture is prepared by kneading a positive electrode active material and a binder in a dispersion medium, and the positive electrode mixture is applied to a base material such as a current collector. It is generally manufactured by drying, molding and forming. When a positive electrode is manufactured using a raw material containing lithium, when a binder such as polyvinylidene fluoride (PVDF) is exposed to an alkali component, the molecular chains of the binder react with the alkali component to form a three-dimensional structure. It is known that a positive electrode mixture in a slurry state is gelled by forming crosslinks.

The lithium composite compound used in the positive electrode for lithium secondary battery, LiMO ₂ with alpha-NaFeO ₂ type crystal structure (M is, Ni, Co, indicates a metal element such as Mn.) Is known. For example, Patent Literature 1 discloses a general formula Li _x Ni _1-y A _y O ₂ (0.98 ≦ x ≦ 1.10, 0.05 ≦ y ≦ 0.30, where A is Co, Al, Mg, Ca , Ti, V, Cr, Mn, Zr, Nb, Mo and W) at the surface of the positive electrode active material particles having a high proportion of Ni represented by the following formula: A technique for providing a coating layer is disclosed.

  According to Patent Document 1, after a specific organic zirconium compound and / or an organic titanium compound is chelated with acetylacetone, a specific solvent is added to prepare a coating liquid, and this coating liquid is mixed with nickel composite oxide particles. It is described that a positive electrode active material having a coating layer is obtained by heat treatment, and it is possible to suppress gelation of a paste composition used for manufacturing a positive electrode.

Meanwhile, a lithium secondary battery is configured to include an electrolytic solution in which a lithium salt such as LiPF ₆ or LiBF ₄ is dissolved. In order to improve the battery performance, the interface resistance between the electrolytic solution and the active material, the electrolytic solution It is important to reduce the decomposition of. For example, Patent Literature 2 discloses a technique of adding a cyclic boroxine compound to LiBF ₄ and at least one solvent selected from a carbonate ester and a borate ester. According to Patent Literature 2, a positive electrode active material subjected to surface treatment by applying a voltage in an electrolytic solution to which a cyclic boroxine compound is added has a low interface resistance. Patent Document 3 discloses a positive electrode in which a lithium transition metal oxide and a boron-containing compound are dry-mixed and heat-treated, and the surface of the lithium transition metal oxide is coated with boron lithium oxide. It is said that heat treatment of the boron-containing compound can convert lithium impurities present on the lithium transition metal oxide into a structurally stable boron lithium oxide.

JP 2016-24968 A International Publication No. WO2012 / 133556 Japanese Patent Publication No. 2015-536558

  In producing a positive electrode for a lithium secondary battery, if the binder reacts with an alkali component, the viscosity increases due to gelation, and the coatability of the positive electrode mixture deteriorates. In addition, since the binding property of a binder such as PVDF decreases due to defluorination, the internal resistance of the battery may increase. In particular, when the content of nickel in the positive electrode active material is high, the oxidation of nickel tends to be insufficient during the firing of the positive electrode active material, so that unreacted lithium remains after the firing, and the reaction between the binder and the alkali component. Tends to occur easily.

  According to the technique disclosed in Patent Literature 1, gelling of a positive electrode mixture used for manufacturing a positive electrode can be suppressed by a zirconium and / or titanium inorganic coating layer formed on the surface of a positive electrode active material. . However, in the technique disclosed in Patent Document 1, since a coating layer of an inorganic material having high insulating properties is formed, a charge / discharge reaction in a lithium secondary battery is hindered, and it is inevitable to increase the resistance of the battery. is there.

On the other hand, according to the technique disclosed in Patent Literature 2, when the positive electrode active material is subjected to surface treatment in an electrolyte solution containing LiBF ₄ as an electrolyte and cyclic boroxine added, the interface resistance is reduced. However, in the technique disclosed in Patent Literature 2, since a high potential of about 4.8 V (vs Li + / Li) is applied, insertion of the electrolyte into the electrolyte itself, the binder, and the carbon component constituting the positive electrode is performed. Decomposition reaction or the like may occur, and it may be unavoidable to increase the resistance of the battery. Further, according to the technique disclosed in Patent Literature 3, since a coating layer is formed on the surface of the lithium transition metal oxide, electrochemical characteristics may be deteriorated due to an increase in initial resistance.

  Therefore, the present invention provides a positive electrode material for a lithium secondary battery capable of manufacturing a positive electrode of a lithium secondary battery by suppressing the modification of the positive electrode mixture, a positive electrode for a lithium secondary battery using the same, and a lithium secondary battery using the same. It is intended to provide a battery.

The positive electrode material for a lithium secondary battery according to the present invention in order to solve the above problems, a cathode active material represented by the following formula (1), and the boric acid ester represented by the following formula (2), binder agent, only contains the binder, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, copolymer of vinylidene fluoride, a copolymer of tetrafluoroethylene and chlorotrifluoroethylene And any one or more of the above copolymers .
_{Li 1 + a Ni b Mn c} Co d M e O 2 + α ··· (1)
[However, in the formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo and Nb, and a, b, c, d, e and α are , −0.1 ≦ a ≦ 0.2, 0.7 <b ≦ 0.9, 0 ≦ c <0.3, 0 ≦ d <0.3, 0 ≦ e ≦ 0.25, b + c + d + e = 1, And -0.2 ≦ α ≦ 0.2. ]
(BO) ₃ (OR) ₃ ... (2)
[However, in the formula (2), R is an organic group having 1 or more carbon atoms. ]

  Further, a positive electrode for a lithium secondary battery according to the present invention includes the above-described positive electrode material for a lithium secondary battery.

  The lithium secondary battery according to the present invention is a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes the positive electrode material for a lithium secondary battery. I do.

  According to the present invention, a positive electrode material for a lithium secondary battery capable of producing a positive electrode of a lithium secondary battery by suppressing the modification of a positive electrode mixture, a positive electrode for a lithium secondary battery using the same, and a lithium secondary battery A battery can be provided.

1 is a cross-sectional view schematically illustrating a lithium secondary battery according to one embodiment of the present invention.

  Hereinafter, a positive electrode material for a lithium secondary battery according to an embodiment of the present invention (hereinafter, may be simply referred to as a positive electrode material), a positive electrode for a lithium secondary battery using the same, and a lithium secondary battery using the same will be described in detail. I do.

  The positive electrode material according to the present embodiment is a material used for manufacturing a positive electrode of a lithium secondary battery, and includes a powdery positive electrode active material. The positive electrode material according to the present embodiment is specifically a lithium composite compound (positive electrode active material) having a layered rock salt type crystal structure (hereinafter, sometimes referred to as a layered structure) belonging to the space group R-3m. And a borate having a boroxine ring structure. This positive electrode material becomes a material of a positive electrode mixture used in the production of a positive electrode of a lithium secondary battery, together with a binder for binding particles such as a positive electrode active material and a conductive material added to supplement conductivity. Are mixed in a dispersion medium to form a positive electrode mixture.

The lithium composite compound as the positive electrode active material has the following formula (1):
_{Li 1 + a Ni b Mn c} Co d M e O 2 + α ··· (1)
[However, in the formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo and Nb, and a, b, c, d, e and α are , −0.1 ≦ a ≦ 0.2, 0.7 <b ≦ 0.9, 0 ≦ c <0.3, 0 ≦ d <0.3, 0 ≦ e ≦ 0.25, b + c + d + e = 1, And -0.2 ≦ α ≦ 0.2. ].

  Hereinafter, the specified ranges of a, b, c, d, e, and α in the formula (1) will be described.

“A” in the above formula is −0.1 or more and 0.2 or less. a have the general formula: the stoichiometric ratio of the cathode active material represented by LiM'O _2, namely Li: M': O = 1: 1: represents the excess or deficiency of Li from 2. Here, M ′ represents a metal element other than Li in the formula (1). The smaller the amount of lithium, the higher the valence of the transition metal before charging, the lower the rate of change in the valence of the transition metal when lithium is desorbed, and the better the charge / discharge cycle characteristics of the positive electrode active material. On the other hand, if the amount of lithium is excessive, the charge / discharge capacity of the positive electrode active material decreases. Therefore, by defining a in the above range, the charge / discharge cycle characteristics of the positive electrode active material can be improved and the charge / discharge capacity can be increased. The more preferable range of a is -0.05 or more and 0.1 or less. When a is -0.05 or more, a sufficient amount of lithium for contributing to charge and discharge is secured, so that the capacity of the positive electrode active material can be increased. Further, when a is 0.1 or less, the charge compensation due to the change in the valence of the transition metal is sufficiently performed, so that both a high charge / discharge capacity and good charge / discharge cycle characteristics can be achieved.

  In the above formula, b is set to be more than 0.7 and 0.9 or less. The more nickel, the more advantageous in increasing the charge / discharge capacity. On the other hand, if nickel is excessive, the thermal stability of the positive electrode active material may be reduced. Therefore, by defining b in the above range, it is possible to stably increase the capacity of the positive electrode active material. A more preferable range of b is 0.75 or more and 0.85 or less. When b is 0.75 or more, the charge / discharge capacity becomes higher.

  In the above formula, c is equal to or more than 0 and less than 0.3. When manganese is added, the layered structure is stably maintained even if lithium is desorbed by charging. On the other hand, when the amount of manganese is excessive, the ratio of other transition metals such as nickel decreases, and the charge / discharge capacity of the positive electrode active material decreases. Therefore, by defining c in the above range, it becomes possible to stably maintain the crystal structure of the positive electrode active material even when insertion and desorption of lithium are repeated by charging and discharging. Therefore, good charge / discharge cycle characteristics, thermal stability, and the like can be obtained together with high charge / discharge capacity. A more preferable range of c is 0.10 or more and 0.25 or less. When c is 0.10 or more, the crystal structure of the positive electrode active material is further stabilized. When c is 0.25 or less, the ratio of other transition metals such as nickel increases, so that the charge / discharge capacity of the positive electrode active material is less likely to be impaired.

  In the above formula, d is 0 or more and less than 0.3. When cobalt is added, the charge / discharge cycle characteristics are improved without significantly impairing the charge / discharge capacity. On the other hand, when the amount of cobalt is excessive, the cost of the raw material becomes high, and thus there is a possibility that it is disadvantageous in industrial production of the positive electrode active material. Therefore, by setting d in the above range, it is possible to achieve both high charge / discharge capacity and good charge / discharge cycle characteristics with good productivity. A more preferable range of d is 0.10 or more and 0.25 or less. When d is 0.10 or more, the charge / discharge capacity and the charge / discharge cycle characteristics are further improved. If d is 0.25 or less, the raw material cost becomes lower, and the productivity of the positive electrode active material is improved.

  In the above formula, e is equal to or more than 0 and equal to or less than 0.25. When at least one element (M) selected from the group consisting of Mg, Al, Ti, Zr, Mo and Nb is added, the crystal structure of the positive electrode active material can be maintained while maintaining the electrochemical activity. Electrode performance such as stability and charge / discharge cycle characteristics can be improved. On the other hand, when M is excessive, the ratio of other transition metals such as nickel decreases, and the charge / discharge capacity of the positive electrode active material decreases. Therefore, by defining e in the above range, it is possible to achieve both high charge / discharge capacity and good electrochemical characteristics.

In the above equation, α is equal to or more than −0.2 and equal to or less than 0.2. α represents the excess or deficiency of oxygen (O) from the stoichiometric ratio of the cathode active material of Formula LiM'O _2. When α is within the above range, the crystal structure of the positive electrode active material has few defects, and good electrochemical characteristics can be obtained. However, α is preferably −0.1 or more and 0.1 or less from the viewpoint of more stably maintaining the layered structure depending on the performance required for the positive electrode active material.

  The positive electrode active material according to the present embodiment may include primary particles of a lithium composite compound in which individual particles are separated, and a plurality of primary particles include secondary particles combined by granulation and sintering. Is also good. The secondary particles may be granulated by either dry granulation or wet granulation. As the granulation means, for example, a granulator such as a spray dryer or a tumbling fluidized bed apparatus can be used.

  The average particle size of the primary particles of the positive electrode active material is preferably 0.1 μm or more and 2 μm or less. When the average particle size is in this range, the filling property of the positive electrode active material in the positive electrode is improved, so that a positive electrode having a high energy density can be manufactured. In addition, since scattering and aggregation of the powdery positive electrode active material are reduced, handleability is improved. The average particle size of the secondary particles of the positive electrode active material can be, for example, 3 μm or more and 50 μm or less.

The BET specific surface area of the positive electrode active material is preferably from 0.2 m ² / g to 2.0 m ² / g. When the BET specific surface area of the powdery positive electrode active material composed of an aggregate of primary particles and secondary particles is within this range, the filling property of the positive electrode active material in the positive electrode is improved, and a positive electrode having a higher energy density can be produced. Will be possible. The BET specific surface area can be measured using, for example, an automatic specific surface area measuring device.

  The crystal structure of the positive electrode active material can be confirmed by, for example, X-ray diffraction (XRD). The composition of the positive electrode active material can be confirmed by high frequency inductively coupled plasma (ICP) emission spectroscopy, atomic absorption spectroscopy (AAS), or the like.

  The particle breaking strength of the positive electrode active material is preferably 50 MPa or more and 100 MPa or less. When the particle breaking strength per particle of the positive electrode active material is within this range, the particles of the positive electrode active material are not easily broken in the process of manufacturing the electrode, and the positive electrode mixture slurry containing the positive electrode active material in the positive electrode current collector is used. When forming the positive electrode mixture layer by coating, coating defects such as peeling are less likely to occur. The particle breaking strength of the positive electrode active material can be measured using, for example, a micro compression tester.

  The cathode active material according to the present embodiment is synthesized using a compound containing lithium and a compound containing a metal element other than Li constituting the cathode active material as raw materials, mainly through a mixing step and a firing step. Can be.

  In the mixing step, a compound containing lithium and a compound containing a metal element other than Li constituting the positive electrode active material are mixed. Examples of the compound containing lithium include lithium carbonate, lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium sulfate, and the like. Among these, lithium carbonate is particularly preferred. Lithium carbonate has a stable supply, good procurement, and is inexpensive. Further, since the melting point is high, the damage to the manufacturing apparatus is small, and the industrial use and the practicality are excellent.

  As the compound containing a metal element other than Li constituting the positive electrode active material, a compound containing nickel, a compound containing manganese, a compound containing cobalt, or a compound containing an element represented by M is mixed.

  As the compound containing nickel, the compound containing manganese, and the compound containing cobalt, for example, oxides, hydroxides, carbonates, acetates, and the like can be used. Among these, it is particularly preferable to use an oxide, a hydroxide, or a carbonate. Further, as the compound containing the element represented by M, for example, carbonate, oxide, hydroxide, acetate, nitrate and the like can be used. Among these, it is particularly preferable to use a carbonate, an oxide, or a hydroxide.

  In the mixing step, specifically, each compound as a raw material is weighed at a predetermined elemental composition ratio corresponding to the above formula, and each compound is pulverized and mixed to prepare a powdery mixture in which each compound is mixed. As a pulverizer for pulverizing the compound, for example, a general precision pulverizer such as a ball mill, a jet mill, and a sand mill can be used.

  The pulverization of the raw material compound is preferably wet pulverization, and from an industrial viewpoint, wet pulverization using water as a dispersion medium is particularly preferable. The solid-liquid mixture obtained by wet pulverization may be dried using, for example, a dryer. As the dryer, for example, a spray dryer, a fluidized bed dryer, an evaporator and the like can be used.

  In the firing step, the mixture obtained in the mixing step is fired to obtain a lithium composite compound having a layered structure. In the firing step, heat treatment can be performed using an appropriate heat treatment apparatus. Specifically, for example, a roller hearth kiln, a tunnel furnace, a pusher furnace, a rotary kiln, a batch furnace, and the like can be used.

The firing step preferably includes a first heat treatment step of forming a first precursor, a second heat treatment step of forming a second precursor, and a third heat treatment step of finishing heat treatment. The positive electrode active material represented by the general formula: LiNiO ₂ has a higher capacity than the positive electrode active material represented by the general formula: LiCoO ₂ , but has a greater alkali component after firing than the positive electrode active material represented by LiCoO _2. Are likely to remain. According to the method of sequentially performing each of these heat treatment steps, the residual of the alkali component can be reduced.

  In the first heat treatment step, the mixture obtained in the mixing step is preferably heat-treated at a heat treatment temperature of 200 ° C. or more and 400 ° C. or less for 0.5 hour or more and 5 hours or less. By this heat treatment, a first precursor is obtained. The first heat treatment step is performed mainly for the purpose of removing a highly vaporizable component such as moisture which hinders the synthesis reaction of the positive electrode active material from the mixture obtained in the mixing step. In this step, carbon dioxide and the like generated due to thermal decomposition of raw materials such as lithium carbonate and combustion of impurities are removed from the mixture together with moisture. In the first heat treatment step, if the heat treatment temperature is lower than 200 ° C., the combustion reaction of impurities and the thermal decomposition reaction of the raw material may be insufficient. On the other hand, when the heat treatment temperature exceeds 400 ° C., crystallization of the lithium composite compound proceeds in this step, and a crystal structure with many defects may be formed in the presence of a gas containing moisture, impurities, and the like. On the other hand, at the heat treatment temperature described above, moisture, impurities, and the like are sufficiently removed, and a first precursor suitable for subsequent firing can be obtained.

  The heat treatment temperature in the first heat treatment step is preferably from 250 ° C. to 400 ° C., more preferably from 250 ° C. to 380 ° C. When the heat treatment temperature is within this range, the progress of crystallization in this step can be suppressed while efficiently removing moisture, impurities, and the like. Note that the heat treatment time in the first heat treatment step can be set to an appropriate time according to, for example, the heat treatment temperature, the amount of moisture and impurities contained in the mixture, the target for removing moisture and impurities, and the like.

  The first heat treatment step may be performed in an oxidizing gas atmosphere, may be performed in a non-oxidizing gas atmosphere, or may be performed in a reduced-pressure atmosphere. The oxidizing gas atmosphere may be either an oxygen gas atmosphere or an air atmosphere. In the case of the air atmosphere, the configuration of the heat treatment apparatus can be simplified, and the manufacturing cost of the positive electrode active material can be reduced. The reduced pressure atmosphere may be, for example, a reduced pressure condition of an appropriate degree of vacuum such as an atmospheric pressure or less.

  The first heat treatment step is preferably performed under an atmosphere gas flow or under exhaust by a pump. By performing the heat treatment in such an atmosphere, gas generated from the mixture can be efficiently removed. It is preferable that the flow rate of the atmosphere gas and the flow rate of the exhaust gas by the pump be larger than the volume of the gas generated from the mixture. The volume of the gas generated from the mixture is calculated, for example, by estimating the substance amount of the generated gas based on the mass of the raw material contained in the mixture and the ratio of components expected to be desorbed from the raw material, and setting the temperature. What is necessary is just to calculate about conditions.

  In the second heat treatment step, it is preferable to heat-treat the first precursor obtained in the first heat treatment step at a heat treatment temperature of 450 ° C. or more and 900 ° C. or less for 0.1 hour or more and 50 hours or less. By this heat treatment, a second precursor is obtained. The second heat treatment step is performed mainly for oxidizing nickel in the first precursor from divalent to trivalent to crystallize a lithium composite compound having a layered structure. In the second heat treatment step, if the heat treatment temperature is lower than 450 ° C., the reaction speed of the solid phase reaction is slowed, and there is a possibility that a raw material such as lithium carbonate may remain excessively. On the other hand, when the heat treatment temperature exceeds 900 ° C., the grain growth of the lithium composite compound proceeds excessively in this step, and there is a high possibility that a high-capacity positive electrode active material cannot be obtained. On the other hand, at the heat treatment temperature described above, the second precursor having few coarse crystal grains can be obtained while the solid-phase reaction proceeds as a whole.

  The heat treatment temperature in the second heat treatment step is more preferably set to 600 ° C. or higher. When the temperature is 600 ° C. or higher, the reaction efficiency of the solid phase reaction is further improved. Further, the heat treatment temperature in the second heat treatment step is more preferably set to 800 ° C. or less. If the temperature is 800 ° C. or lower, the crystal grains are less likely to become coarse.

  It is more preferable that the heat treatment time in the second heat treatment step is 0.1 hours or more and 5 hours or less. When the heat treatment time is 5 hours or less, the time required for manufacturing the positive electrode active material is reduced, and the productivity can be improved.

In order for a positive electrode active material having a nickel content of more than 70 atomic% to exhibit a high capacity, it is particularly important to sufficiently oxidize the valence of nickel from divalent to trivalent. Bivalent nickel, in LiM'O ₂ having a layered structure will be readily substituted for the lithium site, because the cause of reducing the capacity of the positive electrode active material. Therefore, in the second heat treatment step, it is preferable that the first precursor be heat-treated in an oxidizing atmosphere in which oxygen is sufficiently supplied to surely change the valence of nickel from divalent to trivalent.

  Specifically, the second heat treatment step is preferably performed in an oxidizing atmosphere having an oxygen concentration of 90% or more, more preferably in an oxidizing atmosphere having an oxygen concentration of 95% or more, and more preferably in an oxidizing atmosphere having an oxygen concentration of 95% or more. More preferably, the atmosphere is an oxidizing atmosphere. Further, it is preferable that the second heat treatment step is performed under a stream of oxidizing gas. When the heat treatment is performed in a stream of an oxidizing gas having a high oxygen concentration, nickel can be oxidized reliably, and a gas generated by thermal decomposition of a raw material can be reliably removed.

  In the third heat treatment step, it is preferable to heat-treat the second precursor obtained in the second heat treatment step at a heat treatment temperature of 700 ° C. or more and 900 ° C. or less. By this heat treatment, a lithium composite compound having a layered structure is obtained. The third heat treatment step is performed mainly for the purpose of sufficiently oxidizing nickel in the second precursor from divalent to trivalent and growing crystal grains of the lithium composite compound having a layered structure. That is, this step is a heat treatment step for performing an oxidation reaction of nickel in the second precursor and growing crystal grains of the lithium composite compound. In the third heat treatment step, if the heat treatment temperature is lower than 700 ° C., there is a possibility that the grain growth of the lithium composite compound does not proceed quickly. On the other hand, when the heat treatment temperature exceeds 900 ° C., the grain growth of the lithium composite compound proceeds excessively, or the layer structure is decomposed to generate divalent nickel, so that a high-capacity positive electrode active material is obtained. There is a high possibility that it will disappear. On the other hand, at the above-described heat treatment temperature, a high-capacity lithium composite compound can be efficiently obtained.

  In the third heat treatment step, the heat treatment time is preferably from 0.1 hour to 50 hours, more preferably from 0.5 hour to 5 hours. In the third heat treatment step, when the oxygen partial pressure is low, heat is required to promote the oxidation reaction of nickel. Therefore, when the supply of oxygen to the second precursor is insufficient in the third heat treatment step, it is necessary to increase the heat treatment temperature. However, when the heat treatment temperature is increased, the decomposition of the layered structure becomes unavoidable, so that a high capacity lithium composite compound cannot be obtained. On the other hand, if the heat treatment time is 0.1 hour or more, the second precursor can be sufficiently reacted with oxygen.

  Specifically, the third heat treatment step is preferably performed in an oxidizing atmosphere having an oxygen concentration of 90% or more, more preferably in an oxidizing atmosphere having an oxygen concentration of 95% or more, and more preferably performed in an oxidizing atmosphere having an oxygen concentration of 95% or more. More preferably, the atmosphere is an oxidizing atmosphere. Further, it is preferable that the third heat treatment step is performed under an air current of an oxidizing gas. When the heat treatment is performed in an air stream of an oxidizing gas having a high oxygen concentration, the partial pressure of oxygen in the atmosphere does not easily decrease, and nickel can be oxidized reliably without increasing the heat treatment temperature.

  In the positive electrode active material synthesized through the above mixing step and firing step, the coefficient b of nickel (Ni) in the above formula (1) is more than 0.7 and not more than 0.9, and the content of nickel is Due to the high rate, unreacted alkali components may remain after firing. The remaining alkali component reacts with an organic binder such as PVDF used as a binder when preparing the positive electrode mixture, and gelling of the positive electrode mixture is caused by a polymerization reaction of the organic binder, resulting in a paste-like state. The positive electrode mixture may be thickened. In addition, the reaction between the alkali component and the organic binder such as PVDF denatures the molecules of the organic binder, and causes a defluorination reaction such as PVDF. As a result of such modification of the binder, the binding strength of the positive electrode active material, the conductive material, the current collector, and the like due to the binder is reduced, and it is difficult to stably and substantially uniformly prepare the electrode. . In order to solve such a problem, in the positive electrode material according to the present embodiment, the modification of the positive electrode mixture is suppressed by coexisting the borate having a boroxine ring structure and the positive electrode active material.

The borate having a boroxine ring structure is specifically represented by the following formula (2):
(BO) ₃ (OR) ₃ ... (2)
[However, in the formula (2), R is an organic group having 1 or more carbon atoms. ].

  The borate represented by the formula (2) is a boroxine derivative having a molecular structure in which hydrogen of boroxine is substituted. The organic group (R) in the formula (2) may be linear or branched, and may have a ring structure or an arbitrary substituent. The substituent may be a halogen atom such as a fluorine atom, a chlorine atom and a bromine atom, or may be a group having a nitrogen atom, a sulfur atom and the like. The upper limit of the number of carbon atoms of the organic group (R) is not particularly limited, but is preferably 6 or less from the viewpoint of facilitating synthesis.

  As the organic group (R) in the formula (2), an alkyl group or a cycloalkyl group is preferable. The alkyl group may be any of a chain alkyl group and a branched alkyl group. Specific examples of the chain alkyl group include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, and n-nonyl groups. , N-decyl group and the like. Specific examples of the branched alkyl group include an isopropyl group, a 1-methyl-propyl group, a 1-ethyl-propyl group, a 2-methyl-propyl group, a 1-methyl-butyl group, a 1-ethyl-butyl group, -Methyl-butyl group, 2-ethyl-butyl group, 3-methyl-butyl group, 1-methyl-pentyl group, 1-ethyl-pentyl group, 1-propyl-pentyl group, 2-methyl-pentyl group, 2- Ethyl-pentyl group, 2-propyl-pentyl group, 3-methyl-pentyl group, 3-ethyl-pentyl group, 4-methyl-pentyl group, 1-methyl-hexyl group, 1-ethyl-hexyl group, 1-propyl -Hexyl group, 1-butyl-hexyl group, 1-pentyl-hexyl group, 2-methyl-hexyl group, 2-ethyl-hexyl group, 2-propyl-hexyl group, 2-butyl-hexyl Group, 3-methyl - hexyl group, 3-ethyl - hexyl group, 3-propyl - hexyl group, 4-methyl - hexyl group, 4-ethyl - hexyl, 5-methyl - hexyl.

The borate represented by the formula (2) is particularly suitable for triisopropoxyboroxine ((BO) ₃ (O (CH) (CH ₃ ) ₂ ) ₃ ), trimethoxyboroxine ((BO) ₃ ( OCH ₃ ) ₃ ) and tricyclohexoxyboroxine ((BO) ₃ (O (C ₆ H ₁₁ )) ₃ ) are preferred. According to these borate esters corresponding to the alkoxy-substituted boroxine, an effect of suppressing the denaturation of the binder can be effectively obtained as compared with the alkyl-substituted product such as trimethylboroxine. Note that among the borate esters represented by the formula (2), there are substances having high hydrolyzability. A borate having high hydrolyzability may be hydrolyzed by exposure to air. Therefore, when a borate having high hydrolyzability is used, it is preferable to handle in an inert atmosphere. Particularly, the positive electrode material mixture slurry is preferably prepared in an inert atmosphere.

  The ratio of the borate represented by the formula (2) to the total amount of the positive electrode active material and the borate is preferably less than 5.0% by mass, and is 0.5% by mass or more and 2.0% by mass or more. It is more preferred that: When the ratio of the borate is less than 5.0% by mass, it is expected that the lithium secondary battery having a low internal resistance can be produced by suppressing the modification of the positive electrode mixture. When the content is 0.5% by mass or more, the effect of suppressing the modification of the positive electrode mixture becomes more effective. On the other hand, if the content is 2.0% by mass or less, side effects such as deterioration of the adhesion of the positive electrode mixture and increase in internal resistance caused by the borate ester added to suppress the denaturation of the positive electrode mixture are surely prevented. Can be reduced.

  The borate represented by the formula (2) may coexist with the positive electrode active material in a powder state, or may coexist with the positive electrode active material in a liquid state. However, from the viewpoint of quality control of the positive electrode active material, it is preferable that the positive electrode material is mixed with the positive electrode active material in a powder state to obtain a positive electrode material.

  In preparing the positive electrode mixture, the borate represented by the formula (2) is preferably allowed to coexist with the positive electrode active material before the positive electrode active material and the binder are put in contact with each other. . When the positive electrode active material comes into contact with the binder, the alkali component contained in the positive electrode active material reacts with the binder, and fluorinated polyolefin such as PVDF is defluorinated, and fluorine generated by defluorination is generated. However, this is because the electrolyte solution is deteriorated in the lithium secondary battery. Specifically, for example, the positive electrode active material represented by the formula (1) or a solution thereof (dispersion) is mixed with the borate represented by the formula (2) or a solution thereof (dispersion). Then, a binder or a solution (dispersion liquid) thereof is added to the obtained mixture and kneaded to prepare a positive electrode mixture, and the positive electrode mixture is applied to a base material such as a positive electrode current collector, and dried and pressed. It is preferable to manufacture the positive electrode for a lithium secondary battery by molding. In such a production method, it is preferable to use a non-aqueous solvent such as N-methyl-2-pyrrolidone (NMP) as a solvent (dispersion medium) for preparing a solution (dispersion). Note that the conductive material may be added to the positive electrode active material at any stage.

  As described above, according to the positive electrode material in which the positive electrode active material represented by the above formula (1) and the borate ester represented by the above formula (2) are used in combination, the boric acid represented by the above formula (2) The ester reacts with the alkali component remaining after the firing of the positive electrode active material, neutralizes the alkali component, and suppresses the modification of the organic binder such as PVDF. For this reason, the gelation of the positive electrode mixture and the defluorination reaction of the binder are suppressed, and the denaturation of the positive electrode mixture is suppressed, so that the internal resistance is low without difficulty due to gelation in the manufacturing process. A positive electrode for a lithium secondary battery and a lithium secondary battery can be provided. Furthermore, by maintaining the function of the binder appropriately, the bonding strength between the particles and between the particles and the base material is increased, and the uniformity of the distribution of the particles in the positive electrode mixture layer is also increased. can get. That is, unlike the case where the boroxine derivative is directly added to the electrolytic solution, the positive electrode material can reliably eliminate the factor of increasing the resistance that occurs before assembling the battery, and can recharge the lithium secondary battery. Discharge cycle characteristics can also be improved.

  Next, the lithium secondary battery and the positive electrode for a lithium secondary battery according to the present embodiment will be described.

FIG. 1 is a sectional view schematically showing a lithium secondary battery according to one embodiment of the present invention.
As shown in FIG. 1, the lithium secondary battery 1 according to the present embodiment includes a positive electrode 10, a separator 11, a negative electrode 12, a battery container 13, a positive current collecting tab 14, a negative current collecting tab 15, an inner lid 16, and an internal pressure release. A valve 17, a gasket 18, a positive temperature coefficient (PTC) resistance element 19, a battery lid 20, and a shaft center 21 are provided. The battery cover 20 is an integrated component including the inner cover 16, the internal pressure release valve 17, the gasket 18, and the resistance element 19.

  The positive electrode 10 and the negative electrode 12 are provided in a sheet shape, and overlap each other with the separator 11 interposed therebetween. The positive electrode 10, the separator 11, and the negative electrode 12 are wound around the axis 21 to form a cylindrical electrode group.

  The shaft center 21 can be provided in any cross-sectional shape suitable for supporting the positive electrode 10, the separator 11, and the negative electrode 12. Examples of the cross-sectional shape include a cylindrical shape, a cylindrical shape, a rectangular tube shape, and a square shape. Further, the shaft center 21 can be provided by any material having good insulation properties. Examples of the material of the shaft center 21 include polypropylene and polyphenylene sulfide. Glass fiber or the like may be added to improve the mechanical strength of the shaft core.

  The battery container 13 can be formed of an active material having corrosion resistance to an electrolytic solution, for example, aluminum, stainless steel, nickel-plated steel, or the like. When the battery container 13 is electrically connected to the positive electrode 10 or the negative electrode 12, the active material should be protected from corrosion of the battery container 13 and deterioration of the active material due to alloying with lithium in a portion in contact with the electrolytic solution. Is selected. The inner surface of the battery container 13 may be subjected to a surface treatment for improving corrosion resistance and adhesion.

  A positive electrode current collecting tab 14 and a negative electrode current collecting tab 15 for extracting current are connected to the positive electrode 10 and the negative electrode 12 by spot welding, ultrasonic welding, or the like. An electrode group provided with a positive electrode current collecting tab 14 and a negative electrode current collecting tab 15 is housed in a battery container 13. The positive electrode current collecting tab 14 is electrically connected to the bottom surface of the battery lid 20. Further, the negative electrode current collecting tab 15 is electrically connected to the inner wall of the battery container 13. A plurality of positive electrode current collecting tabs 14 and negative electrode current collecting tabs 15 may be provided for the electrode group as shown in FIG. Provision of a plurality of components makes it possible to cope with a large current.

  The positive electrode 10 includes, as a positive electrode active material capable of reversibly occluding and releasing lithium ions, lithium containing a positive electrode active material represented by the above formula (1) and a borate represented by the following formula (2). It is configured to include a positive electrode material for a secondary battery. For example, the positive electrode 10 includes, for example, a positive electrode mixture layer including a positive electrode active material, a conductive material, and a binder; And an electric body. That is, the positive electrode 10 is manufactured using a positive electrode material for a lithium secondary battery as a material.

  As the conductive material, for example, carbon particles such as graphite, carbon black, acetylene black, Ketjen black, and channel black, and carbon fibers can be used. These conductive materials may be used alone or in combination of two or more. The amount of the conductive material is preferably 5% by mass or more and 20% by mass or less based on the positive electrode active material. When the amount of the conductive material is in such a range, good conductivity can be obtained, and high capacity can be secured.

  As the binder in the positive electrode, for example, an organic binder such as polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, or a copolymer of vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, or the like is used. be able to. By using only the organic binder as the binder in the positive electrode, hydrolysis of the borate represented by the formula (2) can be avoided. One of these organic binders may be used alone, or a plurality of them may be used in combination. Further, a thickening material such as carboxymethyl cellulose may be used in combination. As the organic binder, a halogenated polyolefin, more preferably a fluorinated polyolefin, and particularly preferably a polyvinylidene fluoride is used from the viewpoint of making the effect of the borate represented by the formula (2) effective.

  The amount of the binder in the positive electrode is preferably 1% by mass or more and 7% by mass or less based on the total amount of the positive electrode active material, the conductive material, and the binder. When the amount of the binder is in such a range, the capacity is less likely to be reduced and the internal resistance is not excessively increased. Further, the applicability and moldability of the positive electrode mixture layer and the strength of the positive electrode mixture layer are not easily impaired.

  As the positive electrode current collector, for example, an appropriate material such as a metal foil made of aluminum, stainless steel, titanium, or the like, a metal plate, a foamed metal plate, an expanded metal, a punching metal, or the like can be used. The metal foil may be, for example, a perforated foil having a hole diameter of about 0.1 mm to 10 mm. It is preferable that the thickness of the metal foil is substantially 10 μm or more and 100 μm or less.

  The positive electrode 10 is formed, for example, by mixing a positive electrode active material, a conductive material, and a binder with an appropriate solvent to form a positive electrode mixture, applying the positive electrode mixture to a positive electrode current collector, drying, and compression molding. By doing so, it can be produced. As a method of applying the positive electrode mixture, for example, a doctor blade method, a dipping method, a spray method, or the like can be used. Further, as a method of compression-molding the positive electrode mixture, for example, a roll press or the like can be used.

  The thickness of the positive electrode mixture layer can be set to an appropriate thickness in consideration of the specifications of the lithium secondary battery to be manufactured and the balance with the negative electrode, but when applied to both surfaces of the positive electrode current collector. , 50 μm or more and 200 μm or less. The thickness of the positive electrode mixture layer can be set according to the specifications of the lithium secondary battery, such as the capacity and the resistance value.However, if the coating amount is such a degree, the distance between the electrodes becomes excessive, It is unlikely that the distribution of occlusion and release of lithium ions occurs.

  The particle size of the positive electrode active material is generally set to be equal to or less than the thickness of the positive electrode mixture layer. When coarse particles are present in the powder of the synthesized positive electrode active material, it is preferable to perform sieve classification, airflow classification, or the like in advance to make the average particle size of the positive electrode active material smaller than the thickness of the positive electrode mixture layer.

  The density of the positive electrode mixture layer can be set to an appropriate density in consideration of the specifications of the lithium secondary battery to be manufactured and the balance with the negative electrode, but from the viewpoint of securing the capacity of the lithium secondary battery. For this reason, the density is preferably set to 60% or more of the true density.

  The separator 11 is provided to prevent a short circuit from occurring due to the direct contact between the positive electrode 10 and the negative electrode 12. As the separator 11, a microporous film of polyethylene, polypropylene, aramid resin, or the like, a film in which the surface of such a microporous film is coated with a heat-resistant substance such as alumina particles, or the like can be used. The function of the separator 11 may be provided in the positive electrode 10 and the negative electrode 12 as long as the battery performance is not impaired.

  The negative electrode 12 includes a negative electrode active material capable of reversibly occluding and releasing lithium ions. The negative electrode 12 includes, for example, a negative electrode active material, a binder, and a negative electrode current collector.

  As the negative electrode active material, an appropriate type used in a general lithium secondary battery can be used. Specific examples of the negative electrode active material include those obtained by treating a readily graphitizable active material obtained from natural graphite, petroleum coke, pitch coke, or the like at a temperature of 2500 ° C. or more, mesophase carbon, amorphous carbon, and graphite. A material coated with amorphous carbon, a carbon material whose surface crystallinity is reduced by mechanically treating the surface of natural graphite or artificial graphite, an active material in which an organic substance such as a polymer is coated and adsorbed on the carbon surface, Carbon fiber, lithium metal, alloy of lithium and aluminum, tin, silicon, indium, gallium, magnesium, etc., active material having metal supported on the surface of silicon particles or carbon particles, metal such as tin, silicon, iron, titanium Oxides and the like. Examples of the metal to be supported include lithium, aluminum, tin, silicon, indium, gallium, magnesium, and alloys thereof.

  As the binder in the negative electrode, any of an aqueous binder that dissolves, swells, or disperses in water, and an organic binder that does not dissolve, swell, or disperse in water, can be used. Specific examples of the aqueous binder include a styrene-butadiene copolymer, an acrylic polymer, a polymer having a cyano group, and a copolymer thereof. Specific examples of the organic binder include polyvinylidene fluoride, polytetrafluoroethylene, and a copolymer thereof. One of these binders may be used alone, or two or more thereof may be used in combination. Further, a thickening material such as carboxymethyl cellulose may be used in combination.

  The amount of the binder in the negative electrode is preferably 0.8% by mass or more and 1.5% by mass or less based on the total amount of the negative electrode active material and the binder for the aqueous binder. On the other hand, the content of the organic binder is preferably 3% by mass or more and 6% by mass or less based on the total amount of the negative electrode active material and the binder. When the amount of the binder is within such a range, the battery capacity and the internal resistance are rarely reduced. Further, the applicability and moldability of the negative electrode mixture layer and the strength of the negative electrode mixture layer are not easily impaired.

  As the negative electrode current collector, for example, a suitable active material such as copper, a metal foil made of a copper alloy containing copper as a main component, a metal plate, a foamed metal plate, an expanded metal, a punching metal, or the like can be used. . The metal foil may be, for example, a perforated foil having a hole diameter of about 0.1 mm to 10 mm. It is preferable that the thickness of the metal foil be 7 μm or more and 25 μm or less.

  The negative electrode 12 is produced, for example, by mixing a negative electrode active material and a binder together with an appropriate solvent to form a negative electrode mixture, applying the negative electrode mixture to a negative electrode current collector, drying, and compression molding. can do. As a method of applying the negative electrode mixture, for example, a doctor blade method, a dipping method, a spray method, or the like can be used. In addition, as a method of compression-molding the negative electrode mixture, for example, a roll press or the like can be used.

  The thickness of the negative electrode mixture layer can be set to an appropriate thickness in consideration of the specifications of the lithium secondary battery to be manufactured and the balance with the positive electrode, but when applied to both surfaces of the negative electrode current collector. , 50 μm or more and 200 μm or less. The thickness of the negative electrode mixture layer can be set according to the specifications of the lithium secondary battery, such as the capacity and the resistance value. It is unlikely that the distribution of occlusion and release of lithium ions occurs.

  The electrolytic solution enclosed in the battery container 13 is generally composed of an electrolyte and an electrolytic solution solvent, and has a low content of free acids such as water and hydrofluoric acid. .

At least lithium hexafluorophosphate (LiPF ₆ ) is used as the electrolyte. LiPF ₆ has a high solubility in carbonate used as a non-aqueous solvent described later, and has a high degree of dissociation of Li ⁺ due to a bulky PF ₆ ⁻ group. High degree. LiPF ₆ reacts with the borate represented by the formula (2), and the reaction product suppresses an increase in battery resistance. As the electrolyte, only LiPF ₆ may be used alone, or another lithium salt may be used in combination. Other lithium salts used in combination with LiPF ₆ include, for example, LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₂ , Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, Li (F ₂ SO ₂ ) ₂ N, LiF, Li ₂ CO ₃ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₄ (CF ₃ SO ₂ ) ₂ , LiBF ₃ (CF ₃ ), LiBF ₂ (CF ₃ SO ₂ ) _{2 and the} like.

  The concentration of the electrolyte is preferably in the range of 0.6 mol / L to 1.5 mol / L. When the concentration is 0.6 mol / L or more, good ion conductivity can be realized. Further, when the concentration is 1.5 mol / L or less, the resistance of ion conduction is suppressed to a small value, and the reaction speed of lithium ions is increased, so that a decrease in battery characteristics can be suppressed.

  Examples of the non-aqueous solvent used for the electrolytic solution include chain carbonate, cyclic carbonate, chain carboxylate, cyclic carboxylate, chain ether, cyclic ether, organic phosphorus compound, and organic sulfur compound. One of these compounds may be used alone, or two or more of them may be used in combination.

  Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. In addition, examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, vinylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate.

  Examples of the chain carboxylate include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Examples of the cyclic carboxylate include γ-butyrolactone, γ-valerolactone, δ-valerolactone, and the like.

  Examples of the chain ether include dimethoxymethane, diethoxymethane, 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, and 1,3-dimethoxypropane. Examples of the cyclic ether include tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, and the like.

  As the organic phosphorus compound, for example, phosphate esters such as trimethyl phosphate, triethyl phosphate and triphenyl phosphate, and phosphite esters such as trimethyl phosphite, triethyl phosphite and triphenyl phosphite, Trimethylphosphine oxide and the like. Examples of the organic sulfur compound include 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, sulfolane, sulfolene, dimethyl sulfone, ethyl methyl sulfone, methyl phenyl sulfone, and ethyl phenyl sulfone. .

  These compounds used as the non-aqueous solvent may have a substituent, or may be a compound in which an oxygen atom is substituted with a sulfur atom. Examples of the substituent include a halogen atom such as a fluorine atom, a chlorine atom, and a bromine atom. When two or more compounds are used in combination as a nonaqueous solvent, a compound having a relatively high dielectric constant and a relatively high viscosity such as a cyclic carbonate or a cyclic lactone, and a relatively high viscosity such as a chain carbonate or the like. It is preferred to combine with low compounds. From the viewpoint of the ionic conductivity of the mixed electrolytic solution, a composition containing ethylene carbonate and either ethyl methyl carbonate or diethyl carbonate is preferable.

  The electrolyte preferably contains a carbonate such as vinylene carbonate or monofluorinated ethylene carbonate as an additive. On the surface of the negative electrode active material, functional groups such as C ＝ O, CH, and COO exist, and these functional groups react irreversibly with a non-aqueous solvent along with the battery reaction to form SEI ( Solid Electrolyte Interphase) A surface film called a film is formed. The SEI coating has an action of suppressing the decomposition of the non-aqueous solvent, but is generated by consuming electric charges in the battery reaction, and thus contributes to reducing the capacity of the battery. On the other hand, if vinylene carbonate or the like is added, the SEI film can be formed while suppressing a decrease in capacity. The amount of additives such as vinylene carbonate is preferably 2% by mass or less based on the total weight of the electrolytic solution. When the amount of addition is in such a range, it is advantageous in that a decrease in capacity and output can be small when excessive vinylene carbonate or the like is oxidatively decomposed.

  The electrolyte may contain a sulfur compound such as carboxylic anhydride, 1,3-propane sultone, or a boron compound such as lithium bisoxalate borate (LiBOB) or trimethyl borate (TMB) as an additive. Good. The electrolyte is an overcharge inhibitor that suppresses overcharging of the battery, a flame retardant that improves the flame retardancy (self-extinguishing properties) of the electrolyte, and a wettability improver that improves the wettability of the electrodes and separators. And an additive for suppressing elution of Mn from the positive electrode active material, an additive for improving the ionic conductivity of the electrolytic solution, and the like. The total amount of these additives is preferably less than 10% by mass based on the electrolyte.

  Examples of the overcharge inhibitor include biphenyl, biphenyl ether, terphenyl, methyl terphenyl, dimethyl terphenyl, cyclohexylbenzene, dicyclohexylbenzene, triphenylbenzene, hexaphenylbenzene, adiponitrile, dioxane, and the like. As the flame retardant, for example, an organic phosphorus compound such as trimethyl phosphate and triethyl phosphate, a fluoride of the above-mentioned non-aqueous solvent such as a borate ester, and the like can be used. As the wetting agent, for example, chain ethers such as 1,2-dimethoxyethane and the like can be used.

  With the lithium secondary battery having the above configuration, the battery lid is used as a positive electrode external terminal, and the bottom of the battery can is used as a negative electrode external terminal, and the externally supplied power is stored in the wound electrode group and stored in the wound electrode group. This power can be supplied to an external device or the like. The lithium secondary battery of the present embodiment is, for example, a small power supply such as a portable electronic device or a household electric device, a stationary power supply such as an uninterruptible power supply or a power leveling device, a ship, a railway, a hybrid vehicle, an electric vehicle, or the like. It can be used as a drive power supply.

  In the above embodiment, the electrode group and the battery case 13 are formed in a cylindrical shape. However, the form of the electrode group is variously exemplified, for example, a form wound in a flat circular shape, a form in which strip-shaped electrodes are laminated, and a form in which a bag-shaped separator in which electrodes are stored is laminated to form a multilayer structure. It is also possible to take the form of In addition, the battery container 13 can have an appropriate shape such as a cylindrical shape, a flat elliptical shape, an oblong elliptical shape, a square shape, a coin shape, a button shape, or the like, depending on the shape of the electrode group. Further, it is also possible to adopt a form without the shaft center 21.

  As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

  Hereinafter, the present invention will be described specifically with reference to Examples, but the technical scope of the present invention is not limited thereto.

(Example 1)
Using a positive electrode material containing the positive electrode active material represented by the above formula (1) and triisopropoxyboroxine as the borate ester represented by the above formula (2), lithium secondary was prepared according to the following procedure. A battery was manufactured.

  First, lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate were prepared as starting materials for the positive electrode active material. Next, a mixing step of mixing these starting materials was performed. Specifically, the starting materials are weighed and pulverized and mixed so that the atomic ratio of Li: Ni: Co: Mn is 1.04: 0.80: 0.15: 0.05, and the starting material is powdered. Was obtained.

  Next, a firing step of firing the obtained mixture was performed. Specifically, the mixed powder obtained in the mixing step was subjected to a first heat treatment step to obtain a first precursor. Thereafter, the first precursor was subjected to a second heat treatment step to obtain a second precursor, and then the second precursor was subjected to a third heat treatment step to fire the lithium composite compound.

  In the first heat treatment step, 1 kg of the mixed powder obtained in the mixing step is filled in an alumina container having a length of 300 mm, a width of 300 mm, and a height of 100 mm, and is heated at a heat treatment temperature of 350 ° C. in an air atmosphere by a continuous transfer furnace for one hour. To obtain a powder of the first precursor. In this step, water vapor accompanying the thermal decomposition of nickel hydroxide and carbon dioxide accompanying the thermal decomposition of cobalt carbonate and manganese carbonate are generated.

  In the second heat treatment step, the first precursor powder obtained in the first heat treatment step is replaced with an atmosphere having an oxygen concentration of 90% or more in a continuous transfer furnace in an oxygen stream at a heat treatment temperature of 600 ° C. for 10 hours. Heat treatment was performed to obtain a second precursor powder. In this step, cobalt carbonate and manganese carbonate that did not react in the first heat treatment step are thermally decomposed to generate carbon dioxide. Furthermore, carbon dioxide is generated as the reaction between lithium carbonate and Ni, Co, and Mn proceeds.

The positive electrode active material obtained by the above steps was subjected to elemental analysis by ICP emission spectroscopy, and was confirmed to be Li _1.02 Ni _0.8 Mn _0.05 Co _0.15 O ₂ .

Next, massive graphite and acetylene black were mixed with the obtained positive electrode active material. Further, the ratio of triisopropoxyboroxine ((BO) ₃ (O (CH) (CH ₃ ) ₂ ) ₃ ) to the total amount of the positive electrode active material and the borate is 1% by mass as the borate. Was mixed as follows. Furthermore, a solution in which polyvinylidene fluoride (PVDF) was dissolved in N-methyl-2-pyrrolidone (NMP) was mixed to prepare a positive electrode mixture slurry. In addition, the positive electrode active material, massive graphite, acetylene black, and PVDF were mixed in such amounts that the mass ratio became 85: 8: 2: 5. Then, this positive electrode mixture slurry was uniformly applied on one surface of an aluminum foil (positive electrode current collector) having a thickness of 20 μm, and dried at a temperature of 80 ° C. Further, the positive electrode mixture slurry was similarly applied to the surface on the opposite side of the aluminum foil and dried. Then, it compression-molded with the roll press machine, and cut | disconnected so that it might become application width 5.4cm and application length 50cm, and it was set as the positive electrode. Observation of the produced positive electrode confirmed that no coating layer was formed on the surface of the positive electrode active material.

The negative electrode was made of natural graphite having a plane spacing of 0.368 nm, an average particle size of 20 μm, and a specific surface area of 5 m ² / g as a negative electrode active material. First, natural graphite as a negative electrode active material and an aqueous dispersion of carboxymethyl cellulose were sufficiently mixed, and an aqueous dispersion of a styrene-butadiene copolymer was added to prepare a negative electrode mixture slurry. Natural graphite, carboxymethylcellulose, and styrene-butadiene copolymer were mixed in an amount such that the mass ratio became 98: 1: 1. Then, the negative electrode mixture slurry was uniformly applied to a rolled copper foil (negative electrode current collector) having a thickness of 10 μm, and then dried and compression-molded in the same manner as the positive electrode to produce a negative electrode.

Next, a cylindrical lithium secondary battery shown in FIG. 1 was produced using the produced positive electrode and negative electrode. A positive electrode lead and a negative electrode lead for current extraction were formed on the positive electrode and the negative electrode, respectively, by ultrasonic welding. The positive electrode lead and the negative electrode lead are made of the same material as the current collector, respectively. A polyethylene separator is sandwiched between the tabbed positive electrode and negative electrode, and spirally wound to form an electrode group, which is housed in a cylindrical battery container. did. Then, an electrolytic solution was injected into the battery container and sealed to obtain a lithium secondary battery. As an electrolytic solution, a mixed solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC: EMC = 1: 2 was mixed with 1.0 mol / L of lithium hexafluorophosphate (LiPF ₆ ). L was used so that the concentration became L. The design capacity of this lithium secondary battery was 1.8 Ah.

(Example 2)
Lithium was prepared in the same manner as in Example 1 except that the amount of triisopropoxyboroxine ((BO) ₃ (O (CH) (CH ₃ ) ₂ ) ₃ ) mixed with the positive electrode active material was 0.5% by mass. A secondary battery was manufactured.

(Example 3)
Lithium was prepared in the same manner as in Example 1 except that the amount of triisopropoxyboroxine ((BO) ₃ (O (CH) (CH ₃ ) ₂ ) ₃ ) mixed with the positive electrode active material was changed to 2.0% by mass. A secondary battery was manufactured.

(Example 4)
Lithium was prepared in the same manner as in Example 1 except that the amount of triisopropoxyboroxine ((BO) ₃ (O (CH) (CH ₃ ) ₂ ) ₃ ) mixed with the positive electrode active material was changed to 5.0% by mass. A secondary battery was manufactured.

(Example 5)
A lithium secondary battery was produced in the same manner as in Example 1 except that the borate ester mixed with the positive electrode active material was trimethoxyboroxine ((BO) ₃ (OCH ₃ ) ₃ ).

(Example 6)
Lithium secondary was prepared in the same manner as in Example 1 except that the borate ester mixed with the positive electrode active material was trimethoxyboroxine ((BO) ₃ (OCH ₃ ) ₃ ) and the amount was 0.5% by mass. A battery was manufactured.

(Example 7)
The same procedure as in Example 1 was repeated except that the borate ester mixed with the positive electrode active material was trimethoxyboroxine ((BO) ₃ (OCH ₃ ) ₃ ) and the amount was 2.0% by mass. A battery was manufactured.

(Example 8)
The same procedure as in Example 1 was repeated except that the borate mixed with the positive electrode active material was trimethoxyboroxine ((BO) ₃ (OCH ₃ ) ₃ ) and the amount was 5.0% by mass. A battery was manufactured.

(Example 9)
A lithium secondary battery was produced in the same manner as in Example 1, except that the borate mixed with the positive electrode active material was tricyclohexoxyboroxine ((BO) ₃ (O (C ₆ H ₁₁ )) ₃ ). .

(Example 10)
Example 1 was repeated except that the borate mixed with the positive electrode active material was tricyclohexoxyboroxine ((BO) ₃ (O (C ₆ H ₁₁ )) ₃ ) and the amount was 0.5% by mass. Similarly, a lithium secondary battery was manufactured.

(Example 11)
Example 1 was repeated except that the borate mixed with the positive electrode active material was tricyclohexoxyboroxine ((BO) ₃ (O (C ₆ H ₁₁ )) ₃ ) and the amount was 2.0% by mass. Similarly, a lithium secondary battery was manufactured.

(Example 12)
Example 1 was repeated except that the borate ester mixed with the positive electrode active material was tricyclohexoxyboroxine ((BO) ₃ (O (C ₆ H ₁₁ )) ₃ ) and the amount was 5.0 mass%. Similarly, a lithium secondary battery was manufactured.

(Comparative Example 1)
A lithium secondary battery was produced in the same manner as in Example 1, except that borate esters were not added to the positive electrode mixture slurry.

(Comparative Example 2)
A lithium secondary battery was produced in the same manner as in Example 1 except that 1.0% by mass of lithium bisoxalate borate was mixed in the positive electrode mixture slurry instead of the borate esters.

(Comparative Example 3)
A lithium secondary battery was produced in the same manner as in Example 1, except that 1.0% by mass of trimethylboroxine ((BO) ₃ (CH ₃ ) ₃ ) was mixed in the positive electrode mixture slurry instead of borate esters. did.

  The initial DC internal resistance of the manufactured lithium secondary batteries according to Examples 1 to 12 and Comparative Examples 1 to 3 was measured by the following procedure. First, the battery was charged at a constant current and constant voltage for 5 hours with an upper limit voltage of 4.2 V and a current value of 1.8 A. Next, the battery was discharged at a constant current with a final voltage of 3.0 V and a current value of 1.8 A. Thereafter, this charge / discharge operation was repeated up to a total of three cycles. Subsequently, the battery was charged at a constant current and constant voltage for 5 hours with an upper limit voltage of 4.2 V and a current value of 1.8 A. After a pause of 15 minutes, the battery was discharged for 10 seconds at a current value of 1.8 A, and a voltage drop due to the discharge at this time was measured. Then, the initial DC internal resistance value was obtained by dividing the displacement of the voltage value by the current value.

  DC internal resistance values (internal resistance) obtained for the lithium secondary batteries according to Examples 1 to 12 and the lithium secondary batteries according to Comparative Examples 1 to 3, and the results of the positive electrode mixture slurry properties visually confirmed Are shown in Table 1. In the slurry properties in the table, “○” indicates that no gelation of the positive electrode mixture slurry was observed, and “△” indicates that no gelation of the positive electrode mixture slurry was observed but the adhesion of the positive electrode mixture was deteriorated. ", And the case where gelation of the positive electrode mixture slurry is observed is indicated by" x ". The internal resistance is a relative value when the result of Comparative Example 1 is set to 100.

  As shown in Table 1, in Examples 1 to 12, no gelation of the positive electrode mixture slurry was observed, and when the positive electrode mixture slurry was applied to the positive electrode current collector, the viscosity did not become too high. . When the positive electrode active material represented by the formula (1) containing nickel at a high ratio as in Examples 1 to 12 is used, the alkalinity tends to increase when the positive electrode mixture slurry is prepared. It is considered that the coexistence of the positive electrode active material represented by the formula (1) and the borate ester represented by the formula (2) suppresses the modification of the binder by an alkali component. In Examples 1 to 3, 5 to 7, and 9 to 11, the internal resistance of the lithium secondary battery was lower than that in Comparative Example 1. The reason for this is that, by suppressing the modification of the binder, the particles in the positive electrode mixture layer and the bonding strength between the particles and the positive electrode current collector are improved, and the uneven distribution of the particles is reduced. It is considered to be.

  In Examples 4, 8, and 12, although no gelation of the positive electrode mixture slurry was observed, the adhesion of the positive electrode mixture was deteriorated, and the internal resistance was higher than that of Comparative Example 1. Therefore, it can be said that the amount of the borate represented by the formula (2) is preferably less than 5.0% by mass. In Comparative Examples 2 and 3, gelation of the positive electrode mixture slurry was observed, and the internal resistance was higher than in Examples 1, 5, and 9, respectively. Therefore, it is considered that the borate represented by the formula (2) is effective in suppressing the denaturation of the binder, and a boroxine derivative in which hydrogen has been alkyl-substituted and other boric acids are not suitable.

In Examples and Comparative Examples, the effect of suppressing the gelation of the positive electrode mixture slurry for the positive electrode active material represented by Li _1.02 Ni _0.8 Mn _0.05 Co _0.15 O ₂ was examined. The other positive electrode active materials represented by (1) also tend to have a high Ni content and a large amount of residual alkali. Therefore, the addition of the borate represented by the above formula (2) was carried out. As in Examples 1 to 12, gelation of the positive electrode mixture slurry can be suppressed.

DESCRIPTION OF SYMBOLS 1 Lithium secondary battery 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery container 14 Positive current collecting tab 15 Negative current collecting tab 16 Inner lid 17 Internal pressure release valve 18 Gasket 19 Positive temperature coefficient resistance element 20 Battery lid 21 Axis

Claims (8)

It is seen containing a cathode active material represented by the following formula (1), and the boric acid ester represented by the following formula (2), a binder, a,
The binder is polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, a copolymer of vinylidene fluoride, a copolymer of tetrafluoroethylene, and a copolymer of chlorotrifluoroethylene, A positive electrode material for a lithium secondary battery, wherein the material is any one or a plurality of types .
_{Li 1 + a Ni b Mn c} Co d M e O 2 + α ··· (1)
[However, in the formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo and Nb, and a, b, c, d, e and α are , −0.1 ≦ a ≦ 0.2, 0.7 <b ≦ 0.9, 0 ≦ c <0.3, 0 ≦ d <0.3, 0 ≦ e ≦ 0.25, b + c + d + e = 1, And -0.2 ≦ α ≦ 0.2. ]
(BO) ₃ (OR) ₃ ... (2)
[However, in the formula (2), R is an organic group having 1 or more carbon atoms. ] A positive electrode material for a lithium secondary battery according to claim 1,
The positive electrode material for a lithium secondary battery, wherein the borate is any one of triisopropoxyboroxine, trimethoxyboroxine and tricyclohexoxyboroxine. It is a positive electrode material for a lithium secondary battery according to claim 1 or 2,
A positive electrode material for a lithium secondary battery, wherein a ratio of the borate to the total amount of the positive electrode active material and the borate is less than 5.0% by mass. A positive electrode material for a lithium secondary battery according to claim 3, wherein
A positive electrode material for a lithium secondary battery, wherein a ratio of the borate to the total amount of the positive electrode active material and the borate is 0.5% by mass or more and 2.0% by mass or less. It is a positive electrode material for a lithium secondary battery according to any one of claims 1 to 4,
The positive electrode material for a lithium secondary battery, wherein the amount of the binder is 1% by mass or more and 7% by mass or less. A positive electrode for a lithium secondary battery comprising a positive electrode material for a lithium secondary battery according to any one of claims 1 to 5. A lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte,
The positive electrode, a lithium secondary battery which comprises a positive electrode material for a lithium secondary battery according to any one of claims 1 to 5. The lithium secondary battery according to claim 7,
The lithium secondary battery, wherein the electrolyte includes lithium hexafluorophosphate.

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