JP4547587B2 - Lithium secondary battery - Google Patents

Description

【００５９】
（３）耐久性の評価
上記表１から明らかなように、易黒鉛化性炭素材料であるコークスを混合してない、つまり黒鉛質材料のみからなる炭素物質を負極活物質に用いた比較例３のリチウム二次電池は、極めて放電容量維持率が低く、サイクル特性が悪いものとなってる。また、易黒鉛化性炭素材料を混合した炭素物質混合物を負極活物質に用いた比較例１のリチウム二次電池であっても、主たる炭素物質として粒子が球形の黒鉛質材料を混合していることから、放電容量維持率はやはり低く、サイクル特性は良好ではない。
【００６０】
これに対し、ホウ素を添加した非球晶黒鉛質材料と易黒鉛化性炭素材料とを混合した炭素物質混合物を負極活物質とする実施例のそれぞれのリチウム二次電池は、いずれも高い放電容量容量維持率を示し、サイクル特性の良好なリチウム二次電池となっている。図１は、サイクル劣化の違いを巧く示しており、この図からも実施例のリチウム二次電池が良好なサイクル特性を有することが判る。これらのことから、本発明のリチウム二次電池が耐久性に優れたリチウム二次電池となることが確認できる。なお、充放電サイクル試験が６０℃という高温環境下で行われていることから、高温使用時の耐久性にも優れたリチウム二次電池であるといえる。
【００６１】
炭素材料混合物における非球晶黒鉛質材料と易黒鉛化性炭素材料との混合比の異なる実施例１および実施例２のリチウム二次電池の両者を比較すれば、易黒鉛化性炭素材料であるコークスの混合比が大きい実施例１のリチウム二次電池のほうが、初期放電容量が小さくなるものの放電容量維持率において高い値を示すことが判る。したがって、この結果から、非球晶黒鉛質材料と易黒鉛化性炭素材料との混合比は、目的とするリチウム二次電池の特性に応じ、好適な範囲で任意に選択すればよいことが確認できる。
【００６２】
正極活物質となるリチウムニッケル複合酸化物の組成の異なる実施例１および実施例３のリチウム二次電池の両者を比較すれば、Ｎｉサイトの一部をＣｏおよびＡｌで置換したものを用いた実施例１のリチウム二次電池のほうが、Ｍｎで置換したものを用いた実施例２のリチウム二次電池より、初期放電容量および容量維持率の両者に優れていることが判る。このことから、より高い耐久性を有するリチウム二次電池とするためには、正極活物質となるリチウム遷移金属複合酸化物の組成についても考慮するのが望ましいことが確認できる。
【００６３】
なお、微細な１次粒子が凝集して２次粒子を形成するリチウムニッケル複合酸化物を正極活物質用いた参考例のリチウム二次電池では、放電容量維持率が低いことが判る。このことから、サイクル劣化等の耐久性悪化の主要因の一つが、上述したようなリチウム遷移金属複合酸化物の２次粒子の崩壊にあることが確認できる。したがって、実施例のそれぞれのリチウム二次電池との比較からすれば、非球晶黒鉛質材料と易黒鉛化性炭素材料とを混合した炭素物質混合物を負極活物質とする本発明のリチウム二次電池は、通常充放電範囲におけるリチウムイオンの正負極間の移動を効果的に制限できることでこの２次粒子の崩壊が抑制され、耐久性の高いリチウム二次電池となることが確認できる。
【００６４】
【発明の効果】
本発明は、リチウム遷移金属複合酸化物を正極活物質として用いた正極を有するリチウム二次電池を、対向する負極の負極活物質をホウ素を添加した非球晶黒鉛質材料と易黒鉛化性炭素材料とを混合した炭素物質混合物からなるように構成するものである。このような構成をもつ本発明のリチウム二次電池では、電池容量を大きく保ちつつ、通常充放電範囲におけるリチウムイオンの正負極間の移動を効果的に制限して、大量のリチウムの吸蔵・離脱に起因する正極活物質であるリチウム遷移金属複合酸化物の２次粒子粒子の崩壊を抑制することができる。したがって、本発明のリチウム二次電池は、サイクル特性等の耐久性の良好な二次電池となる。
【図面の簡単な説明】
【図１】 充放電サイクル試験の結果として、実施例１および比較例１のリチウム二次電池の各サイクルにおける放電容量維持率をグラフにして示す。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a lithium secondary battery using a lithium occlusion / desorption phenomenon, and more particularly to a lithium secondary battery excellent in durability.
[0002]
[Prior art]
With the miniaturization of personal computers, video cameras, mobile phones, etc., in the fields of information-related equipment and communication equipment, lithium secondary batteries have been put into practical use because of their high energy density as the power source used for these equipment. It has become widespread. On the other hand, in the field of automobiles, the development of electric vehicles has been urgently caused by environmental problems and resource problems, and lithium secondary batteries have been studied as power sources for the electric vehicles.
[0003]
Lithium secondary batteries are expensive compared to other secondary batteries, and therefore require excellent durability. In other words, the cycle characteristics such that the capacity does not extremely decrease even when the charge / discharge cycle is repeated, and the storage characteristics such that the capacity does not decrease even when left for a long time in the charged state are good. Is required. Furthermore, the requirement is particularly severe when the vehicle is placed in a harsh environment such as the above-described automobile power source.
[0004]
Lithium secondary batteries that are currently mainstream are LiCoO ₂ The main components are a positive electrode using a lithium transition metal composite oxide as a positive electrode active material, a negative electrode using a carbon material as a negative electrode active material, and a non-aqueous electrolyte in which a lithium salt is dissolved in an organic solvent. Yes. Various technologies have been developed for each component aiming at improving durability.
[0005]
[Problems to be solved by the invention]
As a result of repeated experiments and researches on the durability of lithium secondary batteries, the present inventor is one of the main causes of deterioration of the durability of lithium secondary batteries. I arrived. In general, a lithium transition metal composite oxide serving as a positive electrode active material is a powdery body in which fine primary particles close to a single crystal are aggregated to form secondary particles, and the secondary particles become powder particles. The positive electrode is formed by binding such a lithium transition metal composite oxide together with a conductive material with a binder. Since the primary particles of the lithium transition metal composite oxide expand and contract with the insertion and extraction of lithium, the bonds are released at the grain boundaries of the primary particles aggregated by the stress generated in the secondary particles, Secondary particles collapse. The collapse of the secondary particles inhibits the electron conduction in the positive electrode, and the ratio of the lithium transition metal composite oxide used as the active material is reduced, so that the capacity of the lithium secondary battery is reduced. The above is one cause for deteriorating the durability found by the present inventors.
[0006]
In a lithium secondary battery, a carbon material is generally used as a negative electrode active material used for a negative electrode opposed to the positive electrode. Among various carbon materials, a graphite material having a high degree of crystallinity is preferably used because of its large theoretical capacity. Further, among graphite materials, mesophase small spheres having optical anisotropy obtained in the process of heating pitches at around 400 ° C. are sintered at a temperature of about 2800 ° C. (graphitized mesophase small). It is considered more suitable to use a sphere (MCMB). The reason for this is spherical particles, which contributes to an improvement in packing density, and in addition, the crystallites are arranged in a lamellar shape and the crystallite end faces are exposed on the entire surface of the particles, so lithium is occluded / desorbed. This is because the process is performed smoothly and the large current characteristics are also excellent.
[0007]
However, the present inventors have confirmed through experiments that when the lithium transition metal composite oxide is used as the positive electrode active material and the carbon material consisting only of the graphite material is used as the negative electrode active material, the durability is inferior. This is because the potential of the negative electrode becomes too low, the amount of lithium occluded / desorbed in the lithium transition metal composite oxide, which is the positive electrode active material, cannot be controlled, and this leads to the collapse of the secondary particles. It is done. And the present inventor has found that it is effective for improving the durability to increase the potential of the negative electrode by some means and to control the amount of lithium occluded / desorbed in the lithium transition metal composite oxide. Got.
[0008]
Further, according to experiments conducted by the present inventor, it has been clarified that durability, particularly storage characteristics, is deteriorated even when the spherulite artificial graphite is used as the negative electrode active material. The negative electrode is formed by binding a carbon material with a binder, and electrical conduction in the negative electrode is maintained by contact of the carbon material particles. When spherulite artificial graphite is used, the contact between the active material particles in the negative electrode is close to point contact due to its shape characteristics. Therefore, the volume change of the negative electrode active material accompanying the swelling of the negative electrode and the insertion / extraction of lithium cannot be followed, the contact between the active material particles is easily cut off, and the electrical conductivity in the negative electrode is lowered. This is considered to be a cause of a decrease in durability when spherulite artificial graphite is used as the negative electrode active material. And this inventor acquired the knowledge that optimizing the particle shape of the carbon substance used as a negative electrode active material is also effective in the improvement of durability.
[0009]
The present invention has been made based on the above knowledge, and in a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material and a carbon material as a negative electrode active material, the cycle characteristics are improved by improving the negative electrode active material. An object of the present invention is to provide a lithium secondary battery excellent in durability such as storage characteristics.
[0010]
[Means for Solving the Problems]
(1) The lithium secondary battery of the present invention is a carbon obtained by mixing a positive electrode using a lithium transition metal composite oxide as a positive electrode active material, a non-spherulitic graphite material added with boron, and a graphitizable carbon material. And a negative electrode using the material mixture as a negative electrode active material. In other words, the graphitic material that is the main carbon material is a graphitic material that is not carbonized as a carbon material that is supplementarily mixed with the graphitized material, while the particle shape is optimized and other elements are added separately. It is a lithium secondary battery using a mixture with a carbon material as a negative electrode active material.
[0011]
Graphite material has high crystallinity and has a clear stage in which lithium ions are occluded, so that lithium ions are occluded / desorbed smoothly, and has a low reaction potential and a large capacity for reversible charge / discharge. In contrast, graphitizable carbon materials are carbon materials typified by coke, etc., which are substances with low graphitization degree and low crystallinity, relatively high reaction potential and large irreversible capacity, but reversible. The capacity to store and desorb lithium ions is small. When used as a negative electrode active material, this carbon material mixture obtained by mixing two different types of carbon materials keeps the battery capacity large and effectively restricts the movement of lithium ions between the positive and negative electrodes in the normal charge / discharge range. Thus, the collapse of the secondary particle particles of the lithium transition metal composite oxide, which is the positive electrode active material, caused by the large amount of lithium insertion / extraction can be suppressed. Therefore, the lithium secondary battery of the present invention using such a carbon material mixture as the negative electrode active material is a secondary battery having good durability such as cycle characteristics.
[0012]
In addition, due to the irreversible capacity of the graphitizable carbon material, even when the secondary battery is once overcharged, it is possible to effectively prevent dendrite from being deposited on the negative electrode surface. The lithium secondary battery is excellent in safety.
[0013]
The graphite material has non-spherical particles. In other words, without using a graphitic material composed of spherical particles such as graphitized mesophase spherules, which is suitable in that occlusion / desorption of lithium ions can be performed smoothly, it is plate-shaped, square-shaped, imperfect. A graphite material composed of non-spherical particles such as regular shapes is used as the main carbon substance. When the negative electrode is formed of such a carbon material, the number of contact points between particles in the negative electrode increases, so that the electron conductivity in the negative electrode can be improved, that is, the negative electrode internal resistance can be reduced. Therefore, the lithium secondary battery of the present invention using a carbon material mixture containing a non-spherulitic graphite material as a constituent carbon material as a negative electrode active material is a lithium secondary battery having a large discharge capacity.
[0014]
In addition, the large number of contact points between particles in the negative electrode keeps the electric conductivity in the negative electrode high due to the swelling of the negative electrode and the volume change of the carbon material, which is the negative electrode active material accompanying the insertion and extraction of lithium. Therefore, it is possible to configure a lithium secondary battery that has good cycle characteristics and little capacity deterioration due to storage in a charged state, that is, good storage characteristics. That is, the lithium secondary battery of the present invention is a lithium secondary battery excellent in durability. In particular, the lithium secondary battery is excellent in durability at high temperatures because the function is sufficiently exhibited even in a high temperature environment where the battery reaction is activated.
[0015]
Furthermore, the non-spherulitic graphite material is one to which boron is added. Boron contributes to increasing the crystallinity of the graphite material. By adding boron and improving crystallinity, insertion / extraction of lithium to / from the graphite material becomes smooth. Therefore, the lithium secondary battery of the present invention using the carbon material mixed with the graphite material added with boron as the additive element as the negative electrode active material is a lithium secondary battery having a large capacity and excellent input / output characteristics.
(2) The lithium secondary battery of the present invention comprises a lithium transition metal composite oxide serving as a positive electrode active material and a composition formula LiNi _x M1 _y M2 _z O ₂ (M1 is at least one selected from Co and Mn; M2 is at least one selected from Al, B, Fe, Cr and Mg; x + y + z = 1; 0.5 <x <0.95; 0.01 <y <0.4; 0.001 <z <0.2) It can implement in the aspect made into the lithium nickel complex oxide of the regular arrangement layered rock salt structure represented.
[0016]
In the lithium secondary battery, various lithium transition metal composite oxides can be used as the positive electrode active material. At present, the basic composition is LiCoO. ₂ It is an ordered array layered rock salt structure lithium cobalt complex oxide. However, in this lithium cobalt composite oxide, Co, which is the main constituent element, is extremely expensive, so a lithium secondary battery using this as a positive electrode active material must be expensive. The lithium secondary battery of this embodiment is an inexpensive lithium secondary battery by using a regularly arranged layered rock salt structure lithium nickel composite oxide whose main constituent element is relatively inexpensive Ni as the positive electrode active material.
[0017]
However, the lithium-nickel composite oxide has a drawback in that the volume change accompanying the insertion and extraction of lithium is large compared to the lithium-cobalt composite oxide, and the secondary particles are easily collapsed. Considering this point, it is possible to effectively limit the movement of lithium ions between the positive and negative electrodes in the normal charge / discharge range, and the lithium transition metal composite oxide, which is a positive electrode active material resulting from the insertion and extraction of a large amount of lithium. The negative electrode function of suppressing the collapse of the secondary particle particles is sufficiently exhibited. Therefore, combining the carbon material mixture with a negative electrode using a negative electrode active material provides a lithium secondary battery that can fully utilize the merit of improved durability.
[0018]
An ordered layered rock salt structure lithium nickel composite oxide has a basic composition of LiNiO. ₂ However, in the lithium secondary battery of this embodiment, a part of the Ni site in the crystal structure is substituted with another element. The substitution by other elements is aimed at stabilizing the crystal structure of the lithium nickel composite oxide, and the lithium nickel composite oxide in which the Ni site is substituted is used as the positive electrode active material. The secondary battery is a lithium secondary battery with higher durability.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, with respect to an embodiment of the lithium secondary battery of the present invention, a carbon material mixture serving as a negative electrode active material, a lithium transition metal composite oxide serving as a positive electrode active material, and a lithium secondary battery using these as a positive electrode active material and a negative electrode active material. Detailed description will be given in the order of the configuration of the next battery.
(1) Carbon material mixture
(A) Graphite material
The main carbon substance which comprises the carbon substance mixture used as the negative electrode active material of the lithium secondary battery of the present invention is a non-spherulitic graphite material to which boron is added. The artificial graphite material is a graphitizable carbon material (for example, coke, mesophase pitch-based carbon fiber, pyrolytic vapor-grown carbon fiber, etc.) made from pitches obtained from coal or petroleum. It can be manufactured by firing at a high temperature.
[0020]
The particles only need to be non-spherulitic, that is, they should not be graphite materials composed of spherical particles such as graphitized mesophase microspheres, and various artificial graphite materials can be used. Actually, according to experiments conducted by the present inventors, in a lithium secondary battery using graphitized mesophase spherules as a negative electrode active material, a charge / discharge cycle test at an environmental temperature of 60 ° C. was performed at a temperature of 20 ° C. As a result of being left for a week and then conducting a charge / discharge cycle test, the capacity drop was remarkable.
[0021]
Considering the boron addition described below, it is desirable that the graphite material has irregular particles obtained by heat-treating the pitch. Further, the graphite material is preferably crystallized in advance, and the interplanar spacing d in the crystal structure is a parameter indicating crystallization. ₀₀₂ If d is adopted, d ₀₀₂ A graphite material that satisfies ≦ 0.34 nm is desirable. Compared to this preferred range, d ₀₀₂ This is because the irreversible capacity is increased and the utilization rate of lithium is decreased when the thickness is> 0.34 nm.
[0022]
Boron is added to the pitch during the production of artificial graphite. ₂ O _Three ). More specifically, for example, a predetermined amount of boron oxide is added to the pitch, and after heat treatment at a temperature of 300 to 700 ° C. for about 3 to 24 hours while bubbling with nitrogen gas, the inertness is obtained. A non-spherulitic graphite material to which boron is added can be produced by firing at a temperature of 2800 to 3000 ° C. for about 30 minutes to 24 hours in a gas atmosphere.
[0023]
The addition ratio of boron is desirably 0.05 to 10 wt% when the entire non-spherical graphitized material is 100 wt%. Compared to this preferred range, if it is less than 0.05 wt%, the graphitization is insufficient, that is, the crystallinity is poor, so the irreversible capacity increases, and if it exceeds 10 wt%, the crystal of the added boron This is because the strain is large and the number of stages in which lithium is occluded decreases, resulting in a decrease in capacity. In the case of the above manufacturing method, the predetermined amount of boron oxide added to the pitch may be 0.05 to 10 wt% in terms of boron. More desirably, the content is 0.1 to 5 wt%.
[0024]
The graphite material is desirably a powdery material having an average particle size of 5 μm or more and 50 μm or less. If it is less than 5 μm compared to this preferred range, the irreversible capacity of Li will increase when the battery is constructed, and if it exceeds 50 μm, the foil will be used when applying the negative electrode mixture in electrode preparation. This is because there is a high possibility that the current collector will be cut.
[0025]
(B) Easily graphitizable carbon material
The carbon material supplementarily mixed with the carbon material mixture that becomes the negative electrode active material of the lithium secondary battery of the present invention is an easily graphitizable carbon material. An easily graphitizable carbon material is a carbon material whose structure can be easily replaced by heat treatment, and means a carbon material that is easily graphitized by firing at a high temperature. Various materials such as coke, mesophase pitch carbon fiber, and pyrolytic vapor grown carbon fiber can be used. Desirably, the material is not graphitized, and the above-mentioned parameter is the surface spacing d. ₀₀₂ If d is adopted, d ₀₀₂ It is desirable that the graphitizable carbon material be ≧ 0.34 nm.
[0026]
Among various graphitizable carbon materials, it is desirable to use coke in the case of the lithium secondary battery of the present invention. Coke has the advantage of being extremely inexpensive. In addition, while graphite stores lithium in a stage between carbon nets, coke does not have a clear stage, so it is considered that there are more sites where lithium is stored than graphite. Although the amount of lithium that can be reversibly occluded / desorbed is small, it is a material with a high potential (irreversible) lithium occlusion capacity. Therefore, by limiting the movement of lithium ions between the positive and negative electrodes in the normal charge / discharge range, the collapse of the crystal structure of the lithium transition metal composite oxide, which is the positive electrode active material, is effectively suppressed, and the cycle characteristics of the lithium secondary battery The durability when the secondary battery is overcharged can also be improved.
[0027]
The graphitizable carbon material is desirably a powdery material having an average particle size of 1 μm or more and 50 μm or less. This is because, when the thickness is less than 1 μm, the irreversible capacity of Li increases when it is less than 1 μm, and when it exceeds 50 μm, the electron conductivity of the electrode decreases and the internal resistance of the battery increases.
[0028]
(C) Mixing of carbon materials
The mixing of the non-spherulitic graphite material to which boron is added and the graphitizable carbon material is not particularly limited, and for example, it may be sufficiently uniform using a device such as a ball mill or ultrasonic dispersion. You can go to
[0029]
The mixing ratio of the two is preferably such that the ratio of non-spherical graphite material: graphitizable carbon material = 9: 1 to 6: 4 in the weight ratio. This is because when the proportion of the non-spherical graphite material is too large as compared with this preferred range, the initial capacity is large but the cycle deterioration is large, and the effect of improving the durability cannot be sufficiently obtained. This is because when the ratio of the graphitizable carbon material is too large, the capacity of the lithium secondary battery itself is too low.
(2) Lithium transition metal composite oxide
(A) Composition and crystal structure
In the lithium secondary battery of the present invention, a lithium transition metal composite oxide is used as the positive electrode active material. Various lithium transition metal composite oxides exist depending on the type of transition metal and the crystal structure, and a known lithium transition metal oxide may be used as the positive electrode active material.
[0030]
From the viewpoint that a lithium secondary battery having a high battery voltage of 4V class can be constructed, the basic composition is LiCoO. ₂ The ordered composition layered rock salt structure lithium cobalt composite oxide, the basic composition is LiNiO ₂ A regularly arranged layered rock salt structure lithium nickel composite oxide with a basic composition of LiMnO ₂ Layered rock salt structure lithium manganese composite oxide with basic composition LiMn ₂ O _Four It is desirable to use a spinel structure lithium manganese composite oxide. Among these, considering the advantages of large theoretical capacity and relatively low cost, the basic composition is LiNiO. ₂ It is more desirable to use a lithium-nickel composite oxide with an ordered layered rock salt structure.
[0031]
Here, “the basic composition is assumed to be” means not only the composition represented by the composition formula but also a part of sites such as Li, Co, Ni, Mn, etc. in the crystal structure to other elements. It is meant to include those substituted with. Furthermore, it means that not only a stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient or the like is included.
[0032]
The basic composition is LiNiO ₂ When using a regularly arranged layered rock salt structure lithium nickel composite oxide, the composition is LiNi _a M _b O ₂ (M is preferably at least one selected from Co, Mn, Al, B, Fe, Cr, and Mg; 0.5 <a <0.95; a + b = 1). And further, the composition formula LiNi _x M1 _y M2 _z O ₂ (M1 is at least one selected from Co and Mn; M2 is at least one selected from Al, B, Fe, Cr and Mg; x + y + z = 1; 0.5 <x <0.95; 0.01 <y <0.4; 0.001 <z <0.2) is more preferable.
[0033]
This LiNi _x M1 _y M2 _z O ₂ Is a part of the Ni site substituted by two or more elements of M1 and M2 having different roles. The ratio of Ni remaining without replacement, that is, the value of x in the composition formula is preferably 0.5 <x <0.95. Compared with this preferred range, when x ≦ 0.5, not only the layered rock salt structure but also the second phase such as the spinel structure is formed, and the capacity is too low. In the case of ≧ 0.95, the replacement effect is too small, and the intended battery with good durability cannot be constructed. In addition, it is more preferable to set it as the range of 0.7 <x <0.9.
[0034]
The element M1 selected from Co and Mn mainly serves to stabilize the crystal structure of the lithium nickel composite oxide. By stabilizing the crystal structure at M1, the cycle characteristics of the non-aqueous electrolyte secondary battery are kept good, and battery capacity deterioration due to charging / discharging at high temperature and storage at high temperature is suppressed. In order to sufficiently exhibit the effect of improving the cycle characteristics, it is desirable that the substitution ratio of M1, that is, the value of y in the composition formula is 0.01 <y <0.4. When y ≦ 0.01, the durability of the secondary battery is not sufficiently stabilized when y ≦ 0.01, and when y ≧ 0.4, lithium nickel composite The crystallinity of the oxide is undesirably lowered. It is more preferable that 0.05 <y <0.3. Further, in Co, capacity reduction due to element substitution is suppressed, and Li (Co, Ni) O ₂ Is an all solid solution type and has the advantage of minimizing the decrease in crystallinity, so considering this, it is more desirable to use Co for M1.
[0035]
The element M2 selected from Al, B, Fe, Cr, and Mg plays a role of mainly suppressing the decomposition reaction of the active material accompanying oxygen release and improving the thermal stability. Because of this role, the substitution ratio of M2, that is, the value of z in the composition formula is preferably 0.001 <z <0.2. Compared to this preferred range, when z ≦ 0.001, a sufficient effect on safety cannot be obtained, and when z ≧ 0.2, the capacity of the positive electrode is reduced, which is not preferable. Note that 0.004 <z <0.1 is more preferable. Furthermore, since Al has the advantage of minimizing capacity reduction while improving thermal stability, it is more desirable to use Al for M2 in consideration of this.
[0036]
(B) Particle structure
Generally, a lithium transition metal composite oxide used as a positive electrode active material is in a powder form. In this case, in the powder particles, primary particles close to single crystals are aggregated to form secondary particles. The primary particle size and the secondary particle size also affect the durability of the lithium secondary battery. As described above, as one of the main causes for deteriorating the durability of the lithium secondary battery, the bond is broken at the grain boundary of the primary particles with the occlusion / desorption of lithium, and the secondary particles are collapsed. This phenomenon can be considered. It is considered that this phenomenon cannot be completely eliminated by the above means as long as the lithium transition metal composite oxide is used. Therefore, even when the secondary particles collapse, it is desirable to form secondary particles by aggregating a relatively small number of relatively large primary particles in order to suppress a decrease in electrical conductivity in the positive electrode.
[0037]
More specifically, the lithium transition metal composite oxide has a particle structure in which the average particle size of the primary particles is 0.5 to 10 μm and the average particle size of the secondary particles is 5 to 50 μm. A powdery body is desirable. If the average particle size of the secondary particles is less than 5 μm, compared to this preferred range, this paste is likely to gel when adjusting the paste-like positive electrode mixture, making it difficult to apply to the current collector, Further, if it exceeds 50 μm, it is difficult to produce a uniform electrode. When the average particle size of the primary particles is less than 0.5 μm, many small primary particles are aggregated, resulting in a large decrease in electron conductivity when the secondary particles are collapsed. This is because the secondary particles also become large and it is difficult to produce a uniform electrode.
[0038]
(C) Synthesis method
The method of synthesizing the lithium transition metal composite oxide is not particularly limited, and may be synthesized by various methods such as a solid phase reaction method, a method of firing it after precipitation from a solution, a spray combustion method, a molten salt method, and the like. Can do.
[0039]
For example, the composition formula LiNi _x Co _y Al _z O ₂ LiOH · H used as a lithium source in the case of producing a lithium nickel composite oxide with a regularly arranged layered rock salt structure represented by the solid phase reaction method ₂ O, Li ₂ CO _Three And Ni (OH) as a nickel source ₂ Etc., Co to be a cobalt source and an aluminum source, respectively. _Three O _Four , Al (OH) _Three Are mixed in proportions according to the composition of the target lithium nickel composite oxide and fired at a temperature of about 750 to 900 ° C. for about 12 to 120 hours in an oxygen stream or in air. Can be synthesized.
(3) Configuration of lithium secondary battery
The lithium secondary battery of the present invention includes a positive electrode using the lithium transition metal composite oxide as a positive electrode active material, a negative electrode using the carbon material mixture as a negative electrode active material, and a separator sandwiched between the positive electrode and the negative electrode In addition, a nonaqueous electrolytic solution or the like can be configured as a main component.
[0040]
(A) Positive electrode
A positive electrode is a mixture of a conductive material and a binder mixed with the above lithium transition metal composite oxide powder that becomes a positive electrode active material, and an appropriate solvent is added to form a paste-like positive electrode mixture such as aluminum. It can be formed by applying and drying on the surface of a current collector made of metal foil and, if necessary, compressing to increase the electrode density. The conductive material is for ensuring the electrical conductivity of the positive electrode, and a mixture of one or two or more carbon material powders such as carbon black, acetylene black, and graphite can be used. The binder plays a role of connecting the active material particles and the conductive material particles, and a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene can be used. . An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active material, conductive material, and binder.
[0041]
(B) Negative electrode
The negative electrode is prepared by mixing a binder into a powdery body of the above carbon material mixture that becomes a negative electrode active material, and adding a paste as a solvent to the surface of a metal foil current collector such as copper. It can be formed by applying and drying, and compressing to increase the electrode density as necessary. As with the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as the binder, and an organic solvent such as N-methyl-2-pyrrolidone is used as a solvent for dispersing the active material and the binder. Can do.
[0042]
(C) Separator, non-aqueous electrolyte
The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.
[0043]
The nonaqueous electrolytic solution is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent, and becomes lithium ions and exists in the electrolytic solution. LiBF that can be used is LiBF _Four , LiPF ₆ LiClO _Four , LiCF _Three SO _Three , LiAsF ₆ , LiN (CF _Three SO ₂ ) ₂ , LiN (C ₂ F _Five SO ₂ ) ₂ Etc. These lithium salts may be used alone, or two or more of these may be used in combination.
[0044]
As the organic solvent for dissolving the lithium salt, an aprotic organic solvent is used. For example, a mixed solvent composed of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether and the like can be used. Examples of cyclic carbonates are ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, etc., examples of chain carbonates are dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc., examples of cyclic esters are gamma butyrolactone, gamma valero. Examples of lactones include cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, and examples of chain ethers include dimethoxyethane and ethylene glycol dimethyl ether. Any one of these may be used alone, or two or more may be mixed and used.
[0045]
Instead of the separator and the non-aqueous electrolyte, a high molecular weight polymer such as polyethylene oxide and LiClO are used. _Four And LiN (CF _Three SO ₂ ) ₂ It is also possible to use a solid polymer electrolyte using a lithium salt such as a gel electrolyte, or a gel electrolyte obtained by trapping the non-aqueous electrolyte in a solid polymer matrix such as polyacrylonitrile.
[0046]
(D) Other
The lithium secondary battery of the present invention constituted by the above-described constituent elements can have various shapes such as a cylindrical type, a laminated type, and a coin type. Regardless of the shape, the above components are housed in a battery case, and a current collecting lead or the like is used between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal. The battery case is sealed and the battery system is separated from the outside to complete the lithium secondary battery.
(4) As mentioned above, about embodiment of the lithium secondary battery of this invention, the carbon material mixture used as a negative electrode active material, the lithium transition metal complex oxide used as a positive electrode active material, and these were used for the positive electrode active material and the negative electrode active material. Although the description is divided into the configuration items of the lithium secondary battery, the embodiment described above is only one embodiment, and the lithium secondary battery of the present invention is based on the knowledge of those skilled in the art including the above embodiment. It can be implemented in various forms with various changes and improvements.
[0047]
【Example】
The lithium secondary battery of the present invention was actually produced as an example based on the above embodiment. In addition, in order to compare with this, a lithium secondary battery using a carbon material having a configuration deviating from the above embodiment as a negative electrode active material was produced as a comparative example. And the lithium secondary battery of the Example and the comparative example was used for the charging / discharging cycle test, and it confirmed that the lithium secondary battery of this invention was excellent in durability. Hereinafter, these will be described.
(1) Produced lithium secondary battery
(A) Example 1
The carbon material used as the negative electrode active material was a carbon material mixture of a non-spherulitic graphite material to which boron was added and a graphitizable carbon material. The graphitized material was prepared by adding a predetermined amount of boron oxide to pitch, heat-treating this pitch at 400 ° C. for 5 hours under nitrogen bubbling, and then firing it at 3000 ° C. for 30 minutes in an argon atmosphere. The addition ratio of boron was 1 wt% when the entire graphite material was 100 wt%. This artificial graphite material had an irregular particle shape and an average particle diameter of about 20 μm. Further, coke having an average particle diameter of about 10 μm was used as the easily graphitizable carbon material to be mixed. The mixing ratio of the non-spherical graphite material and the easily graphitizable carbon material was 7: 3.
[0048]
The lithium transition metal composite oxide used as the positive electrode active material includes a composition formula LiNi _0.85 Co _0.1 Al _0.05 O ₂ A lithium nickel composite oxide with an ordered layered rock salt structure represented by This lithium nickel composite oxide is synthesized by a solid phase reaction method, and the primary particles have an average particle diameter of about 20 μm, most of which are in the range of 0.8 to 3 μm. It has a particle structure that aggregates to form secondary particles having an average particle size of about 13 μm.
[0049]
First, the positive electrode active material is mixed with 85 parts by weight of the lithium nickel composite oxide, 10 parts by weight of acetylene black as a conductive material, and 5 parts by weight of polyvinylidene fluoride as a binder, and an appropriate amount as a solvent. N-methyl-2-pyrrolidone was added to prepare a paste-like positive electrode mixture, and then this paste-like positive electrode mixture was applied to both sides of an aluminum foil current collector having a thickness of 20 μm and dried. Thereafter, the sheet was compressed by a roll press to produce a sheet-like material having a positive electrode mixture thickness of 40 μm per side. This sheet-like positive electrode was cut into a size of 54 mm × 450 mm and used.
[0050]
For the negative electrode to be opposed, first, 90 parts by weight of the above carbon material mixture serving as the negative electrode active material is mixed with 10 parts by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone is added as a solvent. The paste-like negative electrode mixture is then applied to both sides of a 10 μm thick copper foil current collector, dried, and then compressed by a roll press. A sheet having a thickness of 50 μm per side was prepared. This sheet-like negative electrode was cut into a size of 56 mm × 500 mm and used.
[0051]
Each of the positive electrode and the negative electrode was wound with a polyethylene separator having a thickness of 25 μm and a width of 58 mm interposed therebetween to form a roll-shaped electrode body. Then, the electrode body was inserted into a 18650 type cylindrical battery case (outer diameter 18 mmφ, length 65 mm), a non-aqueous electrolyte was injected, and the battery case was sealed to produce a cylindrical lithium secondary battery. . The non-aqueous electrolyte is LiPF in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 7: 3. ₆ Was dissolved at a concentration of 1M. This lithium secondary battery was referred to as the lithium secondary battery of Example 1.
[0052]
(B) Example 2
The lithium secondary battery of Example 1 is a lithium secondary battery in which the mixing ratio of the carbon material mixture serving as the negative electrode active material is changed. In the carbon substance mixture, the mixing ratio of the non-spherical graphite material and the graphitizable carbon material was 9: 1. Other configurations were the same as those of the lithium secondary battery of Example 1. This lithium secondary battery was referred to as the lithium secondary battery of Example 2.
[0053]
(C) Example 3
The lithium secondary battery of Example 1 is a lithium secondary battery in which a lithium transition metal composite oxide serving as a positive electrode active material is changed. The lithium transition metal composite oxide has a composition formula LiNi _0.85 Mn _0.15 O ₂ A lithium nickel composite oxide with an ordered layered rock salt structure represented by This lithium nickel composite oxide is synthesized by a solid phase reaction method, and has a particle structure in which primary particles having an average particle diameter of about 1 μm aggregate to form secondary particles having an average particle diameter of about 15 μm. Is. Other configurations were the same as those of the lithium secondary battery of Example 1. This lithium secondary battery was referred to as the lithium secondary battery of Example 3.
[0054]
(D) Comparative Example 1
In the lithium secondary battery of Example 1, a lithium secondary battery using graphitized mesophase microspheres as the main carbon material instead of the non-spherulitic graphitized material constituting the carbon material mixture as the negative electrode active material. is there. The particle shape was substantially spherical, and the average particle size was about 25 μm. Other configurations were the same as those of the lithium secondary battery of Example 1. This lithium secondary battery was used as the lithium secondary battery of Comparative Example 1.
[0055]
(E) Comparative Example 2
In the lithium secondary battery of Example 1 above, the lithium secondary battery in which the carbon material that is the negative electrode active material is not mixed with the graphitizable carbon material, and the carbon material that is made of only the non-spherulitic graphitized material is the negative electrode active material. Next battery. Other configurations were the same as those of the lithium secondary battery of Example 1. This lithium secondary battery was used as the lithium secondary battery of Comparative Example 2.
[0056]
(F) Reference example
This lithium secondary battery is a lithium secondary battery manufactured in order to demonstrate that the main cause of deterioration in durability is the collapse of secondary particles of a lithium transition metal composite oxide that becomes a positive electrode active material. The lithium transition metal composite oxide serving as the positive electrode active material has the same composition formula LiNi as in the lithium secondary battery of Example 1. _0.85 Co _0.1 Al _0.05 O ₂ A lithium nickel composite oxide with a regular array layered rock salt structure represented by: This lithium nickel composite oxide is synthesized by a solid phase reaction method, and the primary particles have an average particle size of about 0.15 μm, most of which are in the range of 0.1 to 0.2 μm. It has a particle structure in which primary particles aggregate to form secondary particles having an average particle size of about 25 μm. Other configurations were the same as those of the lithium secondary battery of Example 1. This lithium secondary battery was used as a lithium secondary battery of a reference example.
(2) Charge / discharge cycle test
Each of the lithium secondary batteries of the above Examples, Comparative Examples, and Reference Examples was subjected to a charge / discharge cycle test. The charge / discharge cycle test was performed in a high temperature environment of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the lithium secondary battery. The condition is that the current density is 2 mA / cm. ² The battery is charged to a charging upper limit voltage of 4.1 V at a constant current of 2 mA / cm ² Charging / discharging for discharging to a discharge lower limit voltage of 3.0 V at a constant current of 1 cycle was defined as 1 cycle, and this cycle was repeated 3000 cycles or more. The discharge capacity in each cycle was measured.
[0057]
As a result of this charge / discharge cycle test, the following Table 1 shows the initial discharge capacity per unit weight of the positive electrode active material (discharge capacity at the first cycle) and the discharge capacity maintenance rate after 3000 cycles (3000). Cycle discharge capacity / initial discharge capacity × 100%). FIG. 1 is a graph showing the discharge capacity maintenance ratio in each cycle of the lithium secondary batteries of Example 1 and Comparative Example 1.
[0058]
[Table 1]

[0059]
(3) Durability evaluation
As is clear from Table 1 above, the lithium secondary battery of Comparative Example 3 in which coke, which is an easily graphitizable carbon material, is not mixed, that is, a carbon material made of only a graphite material is used as the negative electrode active material. The discharge capacity retention rate is extremely low, and the cycle characteristics are poor. Further, even in the lithium secondary battery of Comparative Example 1 using a carbon material mixture in which an easily graphitizable carbon material is mixed as a negative electrode active material, a graphite material having spherical particles as a main carbon material is mixed. Therefore, the discharge capacity maintenance rate is still low, and the cycle characteristics are not good.
[0060]
On the other hand, each of the lithium secondary batteries of the examples in which the carbon material mixture obtained by mixing the non-spherulitic graphite material added with boron and the easily graphitizable carbon material is used as the negative electrode active material has a high discharge capacity. The lithium secondary battery has a capacity retention rate and good cycle characteristics. FIG. 1 skillfully shows the difference in cycle deterioration, and it can be seen from this figure that the lithium secondary battery of the example has good cycle characteristics. From these things, it can confirm that the lithium secondary battery of this invention turns into a lithium secondary battery excellent in durability. In addition, since the charge / discharge cycle test is performed in a high temperature environment of 60 ° C., it can be said that the lithium secondary battery is excellent in durability at high temperature use.
[0061]
Comparing the lithium secondary batteries of Example 1 and Example 2 having different mixing ratios of the non-spherulitic graphite material and the easily graphitizable carbon material in the carbon material mixture, it is an easily graphitizable carbon material. It can be seen that the lithium secondary battery of Example 1 having a larger coke mixing ratio shows a higher value in the discharge capacity retention ratio although the initial discharge capacity is smaller. Therefore, from this result, it is confirmed that the mixing ratio of the non-spherulitic graphite material and the graphitizable carbon material may be arbitrarily selected within a suitable range according to the characteristics of the target lithium secondary battery. it can.
[0062]
Comparing both the lithium secondary batteries of Example 1 and Example 3 having different compositions of the lithium nickel composite oxide serving as the positive electrode active material, an example in which a part of the Ni site was replaced with Co and Al was used. It can be seen that the lithium secondary battery of Example 1 is superior in both the initial discharge capacity and the capacity retention rate than the lithium secondary battery of Example 2 using the one substituted with Mn. From this, it can be confirmed that in order to obtain a lithium secondary battery having higher durability, it is desirable to consider the composition of the lithium transition metal composite oxide serving as the positive electrode active material.
[0063]
In addition, it can be seen that the lithium secondary battery of the reference example using the lithium nickel composite oxide in which fine primary particles aggregate to form secondary particles uses a positive electrode active material has a low discharge capacity retention rate. From this, it can be confirmed that one of the main causes of durability deterioration such as cycle deterioration is the collapse of the secondary particles of the lithium transition metal composite oxide as described above. Therefore, according to the comparison with each lithium secondary battery of the example, the lithium secondary battery of the present invention using a carbon substance mixture obtained by mixing a non-spherulitic graphite material and a graphitizable carbon material as the negative electrode active material. It can be confirmed that the battery can effectively limit the movement of lithium ions between the positive and negative electrodes in the normal charge / discharge range, so that the secondary particles are prevented from collapsing and become a highly durable lithium secondary battery.
[0064]
【The invention's effect】
The present invention relates to a lithium secondary battery having a positive electrode using a lithium transition metal composite oxide as a positive electrode active material, a non-spherulitic graphitic material to which boron is added as a negative electrode active material of an opposing negative electrode, and graphitizable carbon. It comprises so that it may consist of a carbon substance mixture which mixed material. In the lithium secondary battery of the present invention having such a configuration, a large amount of lithium is occluded / released by effectively restricting the movement of the lithium ion between the positive and negative electrodes in the normal charge / discharge range while keeping the battery capacity large. The secondary particle particles of the lithium transition metal composite oxide that is the positive electrode active material resulting from the above can be prevented from collapsing. Therefore, the lithium secondary battery of the present invention is a secondary battery having good durability such as cycle characteristics.
[Brief description of the drawings]
FIG. 1 shows, as a graph, discharge capacity retention rates in respective cycles of lithium secondary batteries of Example 1 and Comparative Example 1 as a result of a charge / discharge cycle test.

Claims (2)

A positive electrode using a lithium transition metal composite oxide as a positive electrode active material;
A negative electrode using, as a negative electrode active material, a carbon material mixture obtained by mixing a non-spherulitic graphite material added with boron and an easily graphitizable carbon material;
A lithium secondary battery provided. The lithium transition metal composite oxide, the composition formula _{LiNi x M1 y M2 z O 2} (M1 is Co, at least one selected from Mn; at least one M2 is selected Al, B, Fe, Cr, from Mg; x + y + z = 1; 0.5 <x <0.95; 0.01 <y <0.4; 0.001 <z <0.2) The lithium secondary battery according to claim 1.

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