JP7640331B2 - Electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery.

Lithium-ion secondary batteries are lighter and have a higher energy density than nickel-cadmium batteries, nickel-metal hydride batteries, etc., and are therefore widely used as power sources for portable electronic devices. They are also a promising candidate as power sources for hybrid and electric vehicles. As portable electronic devices have become smaller and more functional in recent years, there are hopes for even higher energy densities for the lithium-ion secondary batteries that power these devices.

Current lithium-ion secondary batteries have a high level of safety, but due to their high capacity and high output, further improvements in safety are required. For example, if a lithium-ion secondary battery is overcharged, it may generate heat. It may also generate heat due to the occurrence of an internal short circuit. Furthermore, because lithium-ion secondary batteries contain a non-aqueous electrolyte that contains an organic solvent, the organic solvent may chemically decompose as a result of heat generation, generating gas, which may cause problems such as an increase in the internal pressure of the battery.

To address this issue, Patent Document 1 proposes a technology in which a conductive layer with PTC function is provided on the surface of the current collector.

International Publication No. 2014/050653

However, the conventional collector with a conductive layer having a PTC function described in Patent Document 1 had the problem that it was insufficient as a countermeasure against heat generation caused by an internal short circuit due to an impact. After extensive research, the inventors discovered that this was because the shell layer that forms the conductive layer was difficult to detach from the core particle when an impact was applied.

The invention was made in consideration of these problems, and aims to provide an electrode that suppresses heat generation in lithium-ion secondary batteries due to external impact.

In order to achieve the above object, the electrode for a lithium ion secondary battery according to the present invention comprises a metal foil, a conductive layer formed on at least a portion of the metal foil, and an active material layer formed on at least a portion of the surface of the conductive layer opposite the surface facing the metal foil, the conductive layer having composite particles made of inorganic particles and conductive particles, and the composite particles having the conductive particles on at least a portion of the surface of the inorganic particles.

In the lithium ion secondary battery electrode according to the present invention, when an impact is applied to the lithium ion secondary battery, the conductive particles are detached from the composite particles that make up the conductive layer, exposing the inorganic particles. At this time, the conductive layer functions as a resistance layer due to the electrical resistance of the inorganic particles. This makes it possible to suppress internal short circuits in the lithium ion secondary battery and the resulting heat generation.

The inorganic particles are preferably a lithium compound.

In addition to being stable in the electrolyte, it is important that the conductive layer allows the conductive particles to easily detach when the lithium ion secondary battery is subjected to an impact. If the hardness is too low, the impact on the conductive particles is absorbed, making it difficult for the conductive particles to be detached from the inorganic particles, and if the hardness is too high, the conductive particles are difficult to stretch, making it difficult for the conductive particles to be detached from the inorganic particles. With this configuration, the lithium compound has an appropriate hardness, and when an impact is applied, the conductive particles easily detach from the inorganic particles. This makes it possible to further improve the effect of suppressing heat generation in the lithium ion secondary battery.

Moreover, the inorganic particles are preferably _LiVOPO4 .

This configuration improves the thermal stability of the inorganic particles in the conductive layer. Therefore, even if heat is generated due to the electrical potential applied to the resistive layer while internal short circuits are suppressed, the function of the resistive layer is maintained, and the expansion of the internal short circuit and the associated temperature rise are suppressed. In addition, it is also preferable that the conductive particles have a hardness that makes them easy to remove. Therefore, a lithium ion secondary battery using the above-mentioned lithium ion secondary battery electrode can further improve the heat generation suppression effect.

According to the present invention, it is possible to obtain an electrode for a lithium ion secondary battery that can suppress heat generation even when the lithium ion secondary battery is subjected to an external impact and an internal short circuit occurs, and a lithium ion secondary battery using the electrode.

FIG. 1 is a schematic cross-sectional view of a laminate of a lithium ion secondary battery according to one embodiment of the present invention.

The following describes a preferred embodiment of the present invention. Note that the present invention is not limited to the following embodiment.

<Lithium-ion secondary battery>
FIG. 1 shows a schematic cross-sectional view of a laminate of a lithium ion secondary battery according to this embodiment.

A lithium ion secondary battery laminate 10 can be produced by preparing a positive electrode composed of 1, 2, and 3, a negative electrode composed of 5, 6, and 7, and a separator 4 impregnated with an electrolyte as shown in FIG. 1. Here, the positive electrode can be produced by forming a positive electrode active material layer 1 on a positive electrode collector 3 or on a conductive layer 2 provided on the positive electrode, and the negative electrode can be produced by forming a negative electrode active material layer 5 on a negative electrode collector 7 or on a conductive layer 6 provided on the negative electrode. However, in order to achieve the effects of the present invention, it is necessary to form a conductive layer 2 between the positive electrode collector 3 and the positive electrode active material layer 1, or to form a conductive layer 6 between the negative electrode collector 7 and the negative electrode active material layer 5. In addition, 8 and 9 in the drawing indicate the lead electrodes of the positive electrode and the negative electrode, respectively.

<Metal foil having conductive layer>
The metal foil having a conductive layer according to this embodiment is an electrode for a lithium ion secondary battery having a conductive layer formed on at least a part of one side or the entire surface thereof and an active material layer formed on the conductive layer. The conductive layer contained in this electrode is characterized by including a core particle which is an inorganic particle, and a composite particle having conductive particles on at least a part of the surface of the core particle.

During normal use of the lithium ion secondary battery, the conductive particles on the surface of the core particle ensure a good electronic path and function as a conductive layer. On the other hand, when the lithium ion secondary battery is subjected to an external impact and an internal short circuit occurs, the conductive particles come off the surface of the core particle, and the electronic path is lost, so that the battery functions as a high resistance layer. Below, the embodiments necessary to provide this function are described.

The metal foil of this embodiment may be any conductive plate material. For example, for the negative electrode, a thin metal plate (metal foil) of copper, nickel or an alloy thereof, stainless steel, etc. may be used, and for the positive electrode, a thin metal plate (metal foil) of aluminum or an alloy thereof, stainless steel, etc. may be used.

The inorganic particles of this embodiment are preferably inorganic particles having a high resistance value sufficient to suppress the generation of a large current when an internal short circuit is caused by an external impact. Specifically, the resistance value is preferably 1.0×10 ⁶ [Ωcm] or more. For example, ceramics such as alumina and zirconia, and oxides having high electrical resistance such as lithium ion compounds such as LiFePO ₄ and LiVOPO ₄ can be used.

Considering the ease with which the conductive particles can be removed from the surface, it is desirable for the inorganic particles to not only have high resistance but also a moderate hardness. In other words, the ease with which the conductive particles can be removed from the surface of the inorganic particles is affected by the hardness of the inorganic particles. If the hardness of the inorganic particles is too low, external impacts tend to be absorbed inside the conductive particles, making it difficult for the conductive particles to be removed. On the other hand, if the hardness is too high, the conductive particles tend to be difficult to deform, making it difficult to form a base point that eliminates the adhesion between the inorganic particles and the conductive particles, and as a result, it tends to be difficult for the conductive particles to be removed.

Of the lithium ion compounds, the inorganic particles are preferably _LiVOPO4 . _LiVOPO4 has a suitable strength among inorganic particle materials, making it easy to form a high resistance layer, and has a secondary safety advantage in that it has high thermal stability and can maintain electrical resistance even if an internal temperature rise in the battery occurs due to an internal short circuit.

The conductive particles of this embodiment are not particularly limited as long as they are made of a material having good conductivity, and examples of such materials include carbon-based materials, fine metal powders such as copper, nickel, stainless steel, and iron, carbon materials, conductive oxides such as ITO, and compounds and mixtures thereof. Considering the ease of detachment from the inorganic particle surface, carbon-based materials that are more flexible than inorganic particles are preferred. Examples of such carbon-based materials include carbon black, graphene, carbon nanofibers, carbon nanotubes, carbon nanowalls, and graphite.

The conductive particles may be present on at least a portion of the surface of the inorganic particles, but it is preferable that they cover 50% or more of the inorganic particle surface. At the same time, it is preferable that the resistance value of the composite particle in which the conductive particles are provided on a portion of the surface of the inorganic particle is 50 Ωcm or less. This tends to suppress a decrease in output during normal use of the lithium ion secondary battery.

<Measurement of Coverage Ratio of Inorganic Particles Covered with Conductive Particles>
To measure the coverage of the composite particles by the conductive particles, X-ray photoelectron spectroscopy (hereinafter referred to as "XPS") can be used. That is, a spectrum derived from the inorganic particles and a spectrum derived from the conductive particles are obtained by XPS, and each component is quantified, and the quantitative value of the spectrum derived from the conductive particles can be regarded as the coverage of the inorganic particles by the conductive particles.

<Measurement of Resistance Value of Inorganic Particles Coated with Conductive Particles>
The resistance value of the composite particles is measured using a resistance meter. To measure the resistance value, the composite particles are molded into pellets, and the produced pellets are measured with the resistance meter to obtain the resistance value of the composite particles.

<Production of composite particles>
The composite particles of this embodiment are manufactured by filling inorganic particles and conductive particles in a planetary ball mill, grinding and mixing, and recovering. Here, the filling amount of the inorganic particles is preferably 5 to 30 parts by mass, and more preferably 5 to 15 parts by mass, when the filling amount of the conductive particles is 1 part by mass. The filling amount of the inorganic particles and the conductive particles is such that the composite particles have a rich amount of conductive particles so as not to impair the output when normally used as a lithium ion secondary battery, and have a small amount of conductive particles so that the inorganic particles are exposed due to peeling of the conductive particles and become a high resistance layer when an impact is applied from the outside to the lithium ion secondary battery. The above range is a preferable filling amount range that allows both functions to be exerted.

<Formation of conductive layer on current collector>
The composite particles, the conductive assistant, and the binder are mixed and dispersed in a solvent such as water or N-methyl-2-pyrrolidone to prepare a paste-like slurry. Next, the slurry is applied to one or both sides of a current collector such as aluminum foil or copper foil in a predetermined thickness using, for example, a comma roll coater, and the solvent is evaporated by introducing the slurry into a drying furnace. When the slurry is applied to both sides of the current collector, it is desirable that the thickness of the coating film that becomes the conductive layer is the same on both sides. After the solvent is evaporated, pressure molding may be performed by a roller press. The thickness of the coating film that becomes the conductive layer is preferably 1 [μm] or more and less than 10 [μm]. This makes it possible to fully function as a high resistance layer when an impact is applied to the lithium ion secondary battery from the outside and an internal short circuit may occur, and does not reduce the output during normal use.

<Positive electrode>
As described below, the positive electrode can be produced by forming a positive electrode active material layer 1 on a positive electrode current collector 3 or on a conductive layer 2 provided on the positive electrode.

(Positive electrode current collector)
The positive electrode current collector 3 may be any conductive plate material, and may be, for example, a thin metal plate or foil of aluminum or an alloy thereof, stainless steel, or the like.

(Positive Electrode Active Material Layer)
The positive electrode active material layer 1 is mainly composed of a positive electrode active material, a positive electrode binder, and a necessary amount of a positive electrode conductive assistant.

(Positive Electrode Active Material)
The positive electrode active material is not particularly limited as long as it is capable of reversibly absorbing and releasing lithium ions, desorbing and inserting lithium ions (intercalating them), or doping and dedoping lithium ions with their counter anions (e.g., PF ₆ ⁻ ), and any known electrode active material can be used. For example, lithium cobalt oxide ( _LiCoO2 ), lithium nickel oxide ( _LiNiO2 ), lithium manganese spinel ( _{LiMn2O4 )} , composite metal oxides represented by the general formula: LiNi _x Co _y Mn _{z MaO2} (x+y+z+a=1, 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and _Cr ), lithium vanadium compound ( _LiV2O5 ), olivine-type _{LiMPO4 (} wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), lithium titanate ( _{Li4Ti5O12 )} , LiNi _x Co _Examples of the composite metal oxide include composite metal oxides such as _yAlzO2 ( _0.9 <x+y+z<1.1).

(Positive electrode binder)
The positive electrode binder binds the positive electrode active materials together and also binds the positive electrode active materials to the current collector. The binder may be any material capable of the above-mentioned bonding, such as fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition to the above, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, etc. may be used as the binder. In addition, an electronically conductive conductive polymer or an ionically conductive conductive polymer may be used as the binder. For example, polyacetylene may be used as the electronically conductive conductive polymer. In this case, the binder also functions as a conductive additive particle, so that a conductive additive does not need to be added. As the ionically conductive conductive polymer, for example, one having ion conductivity such as lithium ion can be used, and examples thereof include a composite of a monomer of a polymer compound (polyether polymer compounds such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.) and a lithium salt or an alkali metal salt mainly composed of lithium, such as LiClO ₄ , LiBF ₄ , and LiPF _6. Examples of the polymerization initiator used for the composite include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-mentioned monomer.

(Conductive assistant for positive electrode)
The conductive assistant for the positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 1, and a known conductive assistant can be used. Examples of the conductive assistant include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO.

<Negative Electrode>
As described below, the negative electrode can be produced by forming a negative electrode active material layer 5 on a negative electrode current collector 7 or on a conductive layer 6 provided on the negative electrode.

(Negative electrode current collector)
The negative electrode current collector 7 may be any conductive plate material, and may be, for example, a thin metal plate (metal foil) of copper, nickel, or an alloy thereof, stainless steel, or the like.

(Negative Electrode Active Material Layer)
The negative electrode active material layer 5 is mainly composed of a negative electrode active material, a negative electrode binder, and a necessary amount of a negative electrode conductive assistant.

(Negative Electrode Active Material)
Examples of the negative electrode active material include graphite, silicon oxide (SiO _x ), and metal silicon (Si).

(Negative electrode binder)
The negative electrode binder is not particularly limited, and the same binder as the positive electrode binder described above can be used.

The amount of binder contained in the negative electrode active material layer 5 is not particularly limited, but is preferably 1 to 20 parts by mass of the entire negative electrode active material layer.

(Conductive assistant for negative electrode)
The conductive assistant for the negative electrode is not particularly limited, and the same conductive assistant for the positive electrode as described above can be used.

<Electrolytes>
As the electrolyte, _salts such as _LiPF6 , LiClO4, _LiBF4 , _LiAsF6 , _{LiCF3SO3 , LiCF3} , CF2SO3, LiC( _{CF3SO2 ) 3} , LiN _{( CF3SO2 ) 2} , LiN( _{CF3CF2SO2 ) 2} , LiN( _{CF3SO2 )} ( _C4F9SO2 ) _{, LiN} ( _{CF3CF2CO ) 2 ,} LiBOB, etc. can be used. These salts may be used alone or in combination of two or _more .

Although the above describes a preferred embodiment of the present invention, the present invention is not limited to the above embodiment.

The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

Example 1
(Preparation of composite particles having inorganic core particles and conductive particles on at least a part of the surface of the core particles)
50 g _of LiVOPO4 and 5 g of acetylene black were placed in a planetary ball mill container, and zirconia beads were further added. The mixture was pulverized and mixed at a rotation speed of 500 rpm for 10 minutes, and the resulting powder was recovered to obtain composite particles having conductive particles on at least a portion of the surface of inorganic particles.

(Formation of conductive layer on current collector)
90 parts by mass of the composite particles obtained by the above method, 5 parts by mass of acetylene black, 5 parts by mass of PVdF, and N-methylpyrrolidone as a solvent were mixed to prepare a slurry for forming a conductive layer. This slurry was applied to both sides of an aluminum foil having a thickness of 12 μm, and dried at 100° C. to obtain a positive electrode current collector having a conductive layer having a thickness of 5 μm.

(Preparation of Positive Electrode)
A slurry for forming an active material layer was prepared by mixing 96 parts by mass of _LiCoO2 as a positive electrode active material, 2 parts by mass of acetylene black as a conductive assistant, 2 parts by mass of PVdF as a binder, and N-methylpyrrolidone as a solvent. This slurry was applied to both sides of the positive electrode current collector on which the conductive layer obtained above was formed, dried at 100 [°C], and then pressure-molded by a roller press to obtain a positive electrode having a positive electrode active material layer.

(Preparation of negative electrode)
A slurry for forming an active material layer was prepared by mixing 83 parts by mass of Si as a negative electrode active material, 2 parts by mass of acetylene black as a conductive assistant, 15 parts by mass of polyamideimide as a binder, and N-methylpyrrolidone as a solvent. This slurry was applied to both sides of a copper foil having a thickness of 10 μm, dried at 100° C., pressed by a roller press, and heat-treated in a vacuum at 350° C. for 3 hours to obtain a negative electrode having a negative electrode active material layer.

(Preparation of Lithium-Ion Secondary Battery for Evaluation)
The positive electrode and the negative electrode prepared above were placed in an aluminum laminate pack with a separator made of a polyethylene microporous film sandwiched between them, and a 1 M _LiPF6 solution (solvent: ethylene carbonate/diethyl carbonate=3/7 (volume ratio)) was injected into this aluminum laminate pack as an electrolyte, followed by vacuum sealing to prepare a lithium ion secondary battery for evaluation.

(Measurement of rate characteristics)
The lithium ion secondary battery for evaluation prepared in Example 1 was subjected to one cycle of charging and discharging at a current value of 0.05 [C] in a thermostatic chamber at a temperature of 25 [°C] using a secondary battery charge and discharge tester (manufactured by Hokuto Denko Corporation) with a voltage range of 2.8 [V] to 4.2 [V], and confirmed to have a normal capacity. Similarly, the battery was charged at a current value of 0.05 [C], and then discharged at a current value of 0.2 [C] or 2 [C], and the discharge capacity at each rate was obtained, and the rate characteristic (100 x 2 [C] discharge capacity / 0.2 [C] discharge capacity) was obtained. When the resistance value of the conductive layer formed on the positive electrode current collector is low, the movement of electrons at a high rate is not inhibited, and a high retention rate is shown.

(Measurement of battery surface temperature)
The evaluation lithium ion secondary battery prepared in Example 1 was charged to 4.2 [V] in a thermostatic chamber at a temperature of 25 [°C] using a secondary battery charge/discharge tester (manufactured by Hokuto Denko Corporation), and then a nail penetration test was performed. In the nail penetration test, the evaluation lithium ion secondary battery was fixed on a phenol resin plate with a hole of 10 [mm] in a thermostatic chamber at a temperature of 25 [°C], and an iron nail of 3 [mm] in diameter and 65 [mm] in length was pierced vertically into the evaluation lithium ion secondary battery at a speed of 10 [mm/s], penetrating 10 [mm] from the battery, and the nail was held for 3 minutes and then pulled out. The battery surface temperature was measured 30 seconds after the nail was pierced into the battery.

<Examples 2 to 18>
Lithium ion secondary batteries of Examples 2 to 18 were obtained in the same manner as in Example 1, except that the material of the inorganic particles, the coverage by the conductive particles, and the thickness of the conductive layer were changed as shown in Table 1. Furthermore, using the obtained lithium ion secondary batteries, the rate characteristics and the battery surface temperature of Examples 2 to 18 were measured in the same manner as in Example 1.

The evaluation results of Examples 1 to 18 are shown in Table 1. By providing a conductive layer containing inorganic particles covered with conductive particles on a positive electrode current collector as in Examples 1 to 18, a heat generation suppression effect was obtained while maintaining the rate characteristics. Furthermore, among the inorganic particles, when a lithium compound was used, a higher heat generation suppression effect was observed, and when _LiVOPO4 was used, even more favorable results were obtained. Furthermore, regarding the rate characteristics, good results were obtained by setting the coverage rate of the conductive particles and the conductive layer in the optimal range.

<Comparative Examples 1 and 2>
Lithium ion secondary batteries of Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the material of the inorganic particles, the coverage by the conductive particles, and the thickness of the conductive layer were changed as shown in Table 1. In addition, using the obtained lithium ion secondary batteries, the rate characteristics and battery surface temperature of Comparative Examples 1 and 2 were measured under the same conditions as in Example 1.

The evaluation results for Comparative Examples 1 and 2 are shown in Table 1. Comparative Example 1 had no inorganic particles and the conductive layer was composed only of conductive particles, and showed a relatively high temperature rise. Comparative Example 2 had no conductive particles and did not function as a conductive layer, so although the battery surface temperature was good, it showed extremely low rate characteristics.

A conductive layer is provided on a current collector, and the conductive layer contains inorganic core particles and composite particles having conductive particles on at least a portion of the surface of the core particles, thereby providing a lithium-ion secondary battery that suppresses heat generation while maintaining rate characteristics against external impact.

1... positive electrode active material, 2... conductive layer provided on positive electrode, 3... positive electrode current collector, 4... separator, 5... negative electrode active material, 6... conductive layer provided on negative electrode, 7... current collector, 8, 9... leads, 10... lithium ion secondary battery laminate.

Claims (5)

Metal foil;
A conductive layer formed on at least a portion of the metal foil;
an active material layer formed on at least a part of the surface of the conductive layer opposite to the surface facing the metal foil;
the conductive layer has composite particles including inorganic particles and conductive particles,
The composite particle has the conductive particle on at least a part of the surface of the inorganic particle,
The electrode for a lithium ion secondary battery, wherein the inorganic particles are _LiVOPO4 . 2. The electrode for a lithium ion secondary battery according to claim 1, wherein the conductive particles cover 50% or more of the surface of the inorganic particles. The composite particles have a resistance of 50 Ωcm or less,
3. The electrode for a lithium ion secondary battery according to claim 1, wherein the resistance value of the composite particle is measured by preparing a pellet of the composite particle and using a resistance meter. The electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the conductive particles of the composite particles are detached from the surfaces of the inorganic particles when an impact sufficient to cause an internal short circuit is applied from the outside. A lithium ion secondary battery using the electrode for lithium ion secondary batteries according to any one of claims 1 to 4 .

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