JP2013246989A - Negative electrode active material for lithium ion secondary battery, negative electrode for lithium ion secondary battery including the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a negative electrode active material for a lithium ion secondary battery, suppressing deterioration due to the expansion and contraction of a negative electrode active material caused by absorbing and desorbing Li and capable of achieving a lithium ion secondary battery having excellent cycle characteristics; a negative electrode for a lithium ion secondary battery including the negative electrode active material; and a lithium ion secondary battery.SOLUTION: A negative electrode active material for a lithium ion secondary battery includes a core particle capable of repeatedly absorbing and desorbing lithium and a conductive coating film on the surface of the core particle. The compressive strength of the conductive coating film is relatively high on a side closer to the surface of the core particle, and is lower at least at a part on a side far from the surface of the core particle than that on the side closer to the surface of the core particle.

Description

  The present invention relates to a negative electrode active material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries are widely used in portable electronic devices such as personal computers, mobile phones, audio, and video cameras. In recent years, the power of vehicles such as hybrid vehicles, electric vehicles, electric motorcycles, electric assist bicycles, and construction machinery is being used. Applications are expanding to sources and even power storage. Accordingly, it is required to increase the capacity per volume or weight, and the filling rate of the electrode into the battery case and the coating amount and density of the active material of the electrode itself are increasing. In addition, for large-capacity applications, battery cells are also becoming larger.

  In addition, in order to further increase the energy density, development of an active material capable of inserting and extracting more Li is being promoted. In the negative electrode, an active material containing an element that forms an alloy with Li, such as Si or Sn, is known to have a large amount of Li storage. For example, Si can occlude and release up to 4.4 Li atoms per atom by forming an alloy with Li.

  The active material forming the alloy greatly increases in volume when Li is occluded. In Si, the volume expands about 3 times by the occlusion of Li. Due to this large volume change, repeated charge / discharge causes the active material particles themselves to break, the active material to fall off due to deterioration of the binder that fixes the particles to the current collector foil, and the current collector foil to be pulled by the electrode mixture. Deformation may occur.

  Various techniques for suppressing the influence of the volume change of the active material on the electrode have been proposed.

  Patent Document 1 (Japanese Patent Application Laid-Open No. 2011-70892) discloses a method of coating a current collector foil with a mixture obtained by mixing silicone rubber powder with an active material, a conductive additive, and a binder. In this method, when the active material expands, the silicone rubber is compressed by the force, and it is expected that the expansion of the composite material layer as a whole is suppressed. However, there are concerns about the reduction in charge / discharge capacity, the increase in electrode resistance, and restrictions during electrode production, such as the type of binder solvent and the drying process temperature, due to the mixing of silicone rubber particles.

  Patent Document 2 (Japanese Patent Laid-Open No. 2008-277232) discloses the use of particles obtained by combining finely divided Si particles and carbon particles with amorphous carbon. Patent Document 3 (Japanese Patent Laid-Open No. 2010-287505) discloses a technique for surrounding Si particles and carbon particles with a conductive polymer. By using minute Si, the absolute amount of expansion associated with the occlusion of Li is reduced, and by further surrounding the Si particles with carbon particles and polymers, the expansion of Si is increased by the surrounding carbon, amorphous carbon, and high conductivity. It has been proposed to relax with molecules. However, it is not easy to design and produce composite particles of fine Si particles, carbon, and a conductive polymer.

  The method of absorbing the volume change of the active material accompanying the occlusion of Li with a coating film provided on the surface of the active material is disclosed in Patent Document 4 (Patent No. 4629027) and Patent Document 5 (Special Table 2009-514165). ). In this technique, a coating film composed of an amorphous carbon layer and a crystalline carbon layer is formed in order from the surface of the core particle forming an alloy with Li such as Si or Sn. At this time, the crystalline carbon layer is formed by collecting carbon layer units having a plate-like structure. Since this coating is formed by stacking a plurality of carbon layer units, there is a gap between the carbon layer units and the carbon layer units. And when a core particle expand | swells in a charge process, expansion | swelling of an active material is absorbed because this clearance gap deform | transforms. However, since the gaps between the carbon layer units are naturally formed, it is difficult to control the size and number so that the expansion of the active material can be sufficiently relaxed.

  Further, it is not easy to form a coating film on the core particle surface in the order of amorphous carbon and crystalline carbon. In order to form crystalline carbon by the vapor phase method, it is necessary to increase the film formation temperature. However, alteration of the core particle or crystallization of the amorphous film already formed on the surface of the core particle is not possible. Concerned. In Patent Document 4 (Japanese Patent No. 4629027) and Patent Document 5 (Japanese Translation of PCT International Publication No. 2009-514165), the core particle surface is a mechanical alloying method in which core particles and crystalline carbon particles are mixed and integrated at high speed. An active material having an amorphous carbon layer and a crystalline carbon layer in this order is prepared. In this method, the carbon particles are reacted with the core particles and fixed by kinetic energy, so that it is not necessary to apply heat energy from the outside. It is presumed that the carbon particles having the kinetic energy collide with the core particles and the crystals of the carbon particles break and become amorphous, and at the same time, the carbon particles adhere to the core particles. However, since the core particles are surrounded and surrounded by carbon particles, the carbon particles are required to be fine particles that are sufficiently smaller than the core particles, and because the fine particles are mechanically mixed at a high speed, the apparatus is required for mass production. It is difficult to increase the size.

On the other hand, a technique using SiO _x as the main component of the negative electrode active material instead of single Si is disclosed in, for example, Patent Document 6 (Japanese Patent No. 4531762). When SiO _x is the main component, the ratio of Si in the active material is lowered, so that the chargeable / dischargeable capacity is reduced, but the expansion of the active material is reduced as compared with single Si. However, since the expansion of the active material itself exists, if the thickness of the negative electrode mixture is large, the negative electrode may be deformed due to the expansion of the mixture, or the adhesion of the mixture to the current collector foil may be reduced. .

Among the techniques using SiO _x as an active material, Patent Document 7 (Japanese Patent No. 4081676) discloses an active material that is a particle having a structure in which Si microcrystals are dispersed in SiO ₂ . An active material having this structure has an advantage that it can be manufactured by a relatively simple process. However, although the SiO ₂ volume increase due to charging is mitigated by SiO ₂ , since the SiO ₂ is hard, when the Si clusters expand, the volume of the active material particles increases. For this reason, there is a concern that the negative electrode including the current collector foil may be deformed due to falling off of the active material due to the destruction of the binder or increase in the volume of the negative electrode mixture.

JP 2011-70892 A JP 2008-277232 A JP 2010-287505 A Japanese Patent No. 4629027 Special table 2009-514165 gazette Japanese Patent No. 4531762 Japanese Patent No. 4081676

  In view of the situation as described above, the present invention can suppress deterioration associated with expansion and contraction of a negative electrode active material caused by insertion and extraction of Li, and can realize a lithium ion secondary battery having excellent cycle characteristics. It aims at providing the negative electrode active material for lithium ion secondary batteries, the negative electrode for lithium ion secondary batteries containing this, and a lithium ion secondary battery.

  The first embodiment of the present invention includes a core particle capable of repeatedly inserting and extracting lithium, and a conductive coating film on the surface of the core particle, and the compressive strength of the conductive coating film is relatively high. In particular, the present invention relates to a negative electrode active material for a lithium ion secondary battery, characterized in that it is higher on the side closer to the surface of the core particle and lower on at least part of the side far from the surface of the core particle.

The second embodiment of the present invention is a method for producing a negative electrode active material for a lithium ion secondary battery comprising core particles capable of repeatedly inserting and extracting lithium, and a conductive coating film on the surface of the core particles. And
Forming a high-strength layer having relatively high compressive strength on the core particles by a thermal CVD method in which the temperature of the core particles is 800 ° C. or higher using hydrocarbon as a raw material;
By mixing the core particles with the high-strength layer and the organic compound and heating it above the carbonization temperature of the organic compound, the low-strength layer whose compressive strength is lower than that of the high-strength layer is changed to the high-strength layer. Forming on top;
It is related with the method characterized by including.

  3rd embodiment of this invention is related with the negative electrode for lithium ion secondary batteries containing the negative electrode active material of 1st embodiment of this invention, and a binder.

  4th embodiment of this invention is related with the lithium ion secondary battery characterized by having the negative electrode of 3rd embodiment of this invention.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery which can suppress the deterioration accompanying the expansion | swelling and shrinkage | contraction of the negative electrode active material produced by insertion and extraction of Li, and can implement | achieve the lithium ion secondary battery which has the outstanding cycling characteristics A negative electrode active material can be provided. Moreover, the negative electrode for lithium ion secondary batteries containing this negative electrode active material and the lithium ion secondary battery which has the outstanding cycling characteristics can be provided.

It is a figure which shows typically the cross-sectional structure of the negative electrode active material for lithium ion secondary batteries of one Embodiment of this invention. It is a figure which shows the weight change when the negative electrode active material of Example 1 is heated in a pure oxygen atmosphere. It is a figure which shows typically the battery and holding plate in the charging / discharging test of an Example, a comparative example, and a reference example. In the charging / discharging test of the comparative example 1 and the reference example 4, it is a figure which shows typically the area | region where the wrinkle generate | occur | produced in the negative electrode by the first charging / discharging.

  The negative electrode active material for a lithium ion secondary battery of the present invention includes core particles capable of repeatedly inserting and extracting lithium, and a conductive coating film on the surface of the core particles. The conductive coating film is characterized in that the compressive strength is relatively high on the side closer to the surface of the core particle and lower on at least part of the side far from the surface of the core particle. The region or layer whose compressive strength is lower than the side close to the surface of the core particle may be on the surface of the negative electrode active material, or may be inside (intermediate portion) of the conductive coating film.

  That is, in the present invention, the conductive coating film has a relatively high compressive strength on the side close to the surface of the core particle, in particular, the area in contact with the surface of the core particle, and is far from the outside, that is, the surface of the core particle. And a region close to the surface of the negative electrode active material (but not necessarily the surface of the negative electrode active material) having a relatively low compressive strength.

  The negative electrode active material according to the present invention has an increase in volume when the core particles have expanded and occluded Li in the first charging process. It is absorbed by being compressed by being pressed against a region having a relatively high compressive strength on the side close to the surface of the active material or a binder around the negative electrode active material. As a result, the elongation of the negative electrode mixture accompanying the expansion of the core particles is suppressed, and deformation of the negative electrode is prevented.

  Next, when the core particle releases Li during the discharge process and contracts, the region where the compressive strength is low in the conductive coating film that has been compressed once remains substantially as it is, that is, in a compressed state. Voids are generated in the coating film. In the subsequent charge / discharge process, the core particles are expanded and contracted in the voids. Therefore, the stress accompanying charging / discharging is not repeatedly applied to the binder around the active material. Therefore, the deterioration of the binder can be suppressed and the active material can be prevented from degranulating.

  The electronic conductivity between the core particle and the surrounding particles and current collector foil is determined by the compression existing between the binder and the core particle and the region where the compressive strength of the conductive coating film on the core particle surface is relatively high. The compressive strength of the conductive coating film made is maintained in a low region.

  As described above, by using the negative electrode active material according to the present invention, a lithium ion secondary battery having excellent cycle characteristics and having both a high capacity and a long life can be realized.

  Contrary to the present invention, the compressive strength is low on the side close to the surface of the core particle, that is, the hardness is low, and the side far from the surface of the core particle (the side close to the surface of the active material) has high compressive strength, That is, when the hardness is high, when the core particle once expanded contracts, the conductive coating film (the region or layer having a low compressive strength on the core particle surface) is largely separated from the core particle surface. The conductivity of the core particle surface may be lost.

  The conductive coating film may be a laminate in which two or more layers having different compressive strengths are laminated, or may have a continuously changing compressive strength. Moreover, the combination may be sufficient.

  The conductive coating film has a relatively high compressive strength on the surface of the core particle, and a region having a lower compressive strength on the region or on the layer (the side far from the surface of the core particle), Alternatively, there may be a layer, and for example, a region or a layer where the compressive strength is increased may be further provided thereon. The region having a low compressive strength or the layer may not be formed immediately above the region having a relatively high compressive strength on the surface of the core particle or the layer, and the compressive strength is higher than that immediately above the core particle. It may be formed via a region or a layer.

  In one embodiment of the present invention, the conductive coating film has a high-strength layer having a relatively high compressive strength on the surface of the core particle, and a low-strength layer having a lower compressive strength than the high-strength layer. It is on the strength layer.

  Another aspect of the present invention is that the conductive coating film has a high-strength layer having a relatively high compressive strength on the core particle surface, and the low-strength layer having a lower compressive strength than the high-strength layer. On the high-strength layer, and at least one layer on the low-strength layer (side closer to the surface of the negative electrode active material). The layer formed on the low-strength layer can be, for example, a layer having a higher compressive strength than the low-strength layer, and can also be a layer having a lower compressive strength than the low-strength layer. In some embodiments, at least one of the layers on the low strength layer is a layer having a higher compressive strength than the low strength layer.

  In the above-described embodiment, each layer may have a continuously changing compressive strength, and may include a portion in which the compressive strength continuously changes. Also good.

  In another aspect of the present invention, the conductive coating film has a compressive strength continuously changing from a high strength to a low strength from the side close to the surface of the core particle toward the surface side of the negative electrode active material. It has the part which is. In an embodiment, the conductive coating film has a compressive strength continuously changing from a high strength to a low strength from the surface of the core particle to the surface of the active material. In another embodiment, the conductive coating film includes a portion in which the compressive strength continuously changes from a high strength to a low strength from the side close to the surface of the core particle toward the surface side of the active material. , And a portion where the compressive strength continuously changes from low strength to high strength. There may be two or more portions where the compressive strength continuously changes from high strength to low strength, and two portions where the compressive strength continuously changes from low strength to high strength. It may be the above. Moreover, it can also have a part with a substantially constant compressive strength. The conductive coating film only needs to have a region having a lower compressive strength than a region having a relatively high compressive strength on the surface of the core particle, and the pattern of change in the compressive strength is not particularly limited.

  FIG. 1 schematically shows a cross-sectional structure of a negative electrode active material for a lithium ion secondary battery of one embodiment of the present invention.

  The negative electrode active material particle 1 includes a core particle 2 that can repeatedly occlude and release lithium, and a conductive coating film composed of a layer 3 having a high compressive strength and a layer 4 having a low compressive strength on the core particle surface. Prepare. The layer 3 having a high compressive strength is relatively thin and formed on the side close to the surface of the core particle, and the layer 4 having a low compressive strength is formed thereon, that is, on the side far from the surface of the core particle (of the negative electrode active material). Surface).

  As will be described later, the negative electrode active material is usually fixed by a binder to become a negative electrode. When the first charge starts, the core particle 2 expands, and a pressure is generated between the binder fixing the active material particle 1 and the surface coating film of the active material. With this pressure, the layer 4 having a low compressive strength is deformed and the expansion of the core particles is absorbed. On the other hand, the layer 3 having a high compressive strength in the region close to the surface of the core particle has a high strength and thus has a small deformation.

  Next, in the discharge process, the core particle 2 releases Li and contracts. At this time, the deformed layer 4 having a low compressive strength is not completely restored from the compressed state. A part of the coating on the core particle 2 is separated in the thickness direction inside the low compressive strength layer 4 or in the vicinity of the boundary between the low compressive strength layer 4 and the high compressive strength layer 3. A space is generated at least partly between the surface of the core particle 2 that expands and contracts with the occlusion / release and the binder. In the second and subsequent charge / discharge processes, the volume change of the core particle 2 is absorbed in this space.

  In this embodiment, a negative electrode binder having a higher compressive strength than the layer 4 having a lower compressive strength in the conductive coating film is used. Thereby, before the whole negative electrode mixture expand | swells, the layer 4 with low compression strength in an electroconductive coating film is compressed.

  Note that the conductive coating film is on a region having a low compressive strength or on a layer having a low compressive strength, that is, on a side closer to the surface of the negative electrode active material, or a region having a higher compressive strength, or a compressive strength. In the case of having a high strength layer, the compressive strength of the binder is not particularly limited. In this case, a binder having a lower compressive strength than a region having a low compressive strength in the conductive coating film or a layer having a low compressive strength may be used.

  The electrical conductivity of the surface of the active material particle 1 is maintained by the layer 3 having a high compressive strength on the core particle 2. The electronic conductivity between the surface of the core particle 2 and the surrounding particles and the current collector foil is such that the layer 3 having a high compressive strength on the surface of the core particle 2 and the compressive strength existing between the binder and the core particle 2. Of the lower layer 4.

  The material constituting the core particle of the negative electrode active material is not particularly limited as long as it is a substance capable of inserting and extracting lithium. Specific examples include metals such as silicon (Si), tin (Sn), aluminum, zinc, gallium, germanium, silver, cadmium, indium, antimony, lead, and bismuth, or compounds containing these elements. Among these, a compound containing Si, Sn, Si or Sn has a large capacity per weight and is preferable. In particular, Si or a compound containing Si is preferable because of high heat resistance. Further, when silicon oxide is used, Si clusters may be contained inside.

  The size of the core particles is preferably an average particle size of 30 μm or less. By making the average particle size 30 μm or less, it is possible to suppress the breakage of the core particles due to the volume change.

  The conductive coating film of the negative electrode active material preferably contains carbon as a main component and contains amorphous carbon. Since carbon has electrical conductivity and is lighter than metal, it is suitable for improving the capacity per weight of the battery. Carbon is not oxidized under normal temperature and normal pressure conditions, is stable, and is suitable as a coating film.

As described above, the strength of the conductive coating film has a distribution in the thickness direction. For example, a high-strength layer and a low-strength layer are laminated from the core particle side (in addition, the low-strength layer on the active material surface side). It may have a high-strength outermost layer on top. Alternatively, it continuously changes from high strength to low strength from the core particle side toward the active material surface. The strength of the carbon film can be changed by differences in crystallinity, composition, for example, a large proportion of sp ² bonds, containing hydrogen or nitrogen, and a film having a low number density of atoms. Needs to contain not only the crystalline phase but also the amorphous phase. A method of forming a film with carbon having different strengths will be described later.

  When the compressive strength on the surface of the core particle is relatively high or the layer is too thick, the conductive coating film tends to be peeled off from the core particle due to the volume change of the core particle. For this reason, it is preferable that the thickness be as small as possible to ensure the electron conductivity.

  Specifically, when a high-strength layer is formed on the core particle surface and a low-strength layer is formed on the high-strength layer, the ratio of the high-strength layer of the conductive coating film is It is preferably 3 parts by weight or more and 10 parts by weight or less based on 100 parts by weight of the total weight of the core particles.

  In the case of a conductive coating film having a portion where the compressive strength continuously changes from high strength to low strength from the side close to the surface of the core particle toward the surface side of the negative electrode active material, The area in which the compressive strength on the surface of the core particle of the conductive coating film is 250 or more in terms of Vickers hardness is defined as a high compressive area, and the ratio is 100 parts by weight of the total weight of the area having the high compressive strength and the core particle. The amount is preferably 3 parts by weight or more and 10 parts by weight or less.

  When a high-strength layer is formed on the core particle surface and a low-strength layer is formed on the high-strength layer, the compressive strength of the high-strength layer is preferably 250 or more in terms of Vickers hardness.

  On the other hand, the region or layer having a relatively low compressive strength preferably has a certain thickness in order to sufficiently relax the expansion of the core particles. On the other hand, when the compressive strength is relatively low, or the layer becomes excessively thick, the ratio of the core particles in the electrode decreases, and the charge / discharge capacity density of the battery decreases.

  Specifically, when a high-strength layer is formed on the surface of the core particle and a low-strength layer is formed on the high-strength layer, the ratio of the low-strength layer of the conductive coating film is the low-strength layer. The amount is preferably 5 parts by weight or more and 15 parts by weight or less with respect to the high-strength layer (high-strength layer on the surface of the core particles) and the total weight of the core particles of 100 parts by weight.

  In the case of a conductive coating film having a portion where the compressive strength continuously changes from high strength to low strength from the side close to the surface of the core particle toward the surface side of the negative electrode active material, The area where the compressive strength of the conductive coating film is 100 or less as the Vickers hardness is defined as a low compressive strength area, and the ratio is a low compressive strength area and a high compressive strength area on the core particle surface. The amount of the core particles is preferably 5 to 15 parts by weight with respect to 100 parts by weight as a whole.

  When a high-strength layer is formed on the surface of the core particles and a low-strength layer is formed on the high-strength layer, the compressive strength of the low-strength layer is preferably 100 or less as Vickers hardness.

  The amount of the carbon coating film is such that the core particles having the surface coating film, that is, the negative electrode active material of the present invention is heated in an oxidizing atmosphere, for example, in a pure oxygen atmosphere, and the carbon film is oxidized to become a gas. It can be determined from the weight change accompanying Here, the change in weight on the low temperature side corresponds to the disappearance of oxidation of carbon having a low compressive strength, and the change in weight on the high temperature side corresponds to the disappearance of oxidation of carbon having a high compressive strength. That is, the temperature disappearing in the oxidizing atmosphere of the carbon film having low compressive strength is low, and the temperature disappearing in the oxidizing atmosphere of the carbon film having high compressive strength is relatively high.

  Next, a method of forming a conductive coating film, particularly a carbon film containing carbon as a main component and containing amorphous will be described.

It is known that the strength of an amorphous carbon film varies greatly depending on the ratio of sp ² bonds and sp ³ bonds in the film, and the strength increases as the ratio of sp ³ bonds increases. In addition, the film density varies depending on the ratio of hydrogen and nitrogen contained, and the film strength is also affected. For example, in the thermal CVD method, the strength of the film varies depending on the film forming temperature, the concentration of the source gas, and the type of the source gas. This is because the ratio of sp ³ bonds and sp ² bonds and the type and amount of components other than carbon incorporated into the film change. Therefore, in the thermal CVD method, a carbon film having a high compressive strength and a carbon film having a low compressive strength can be formed separately by controlling the source gas and the film forming temperature.

  A carbon film having high compressive strength can be formed by a thermal CVD method using a hydrocarbon gas as a raw material. For example, if vapor such as methane, ethane, or propane, or vapor generated by bubbling volatile liquid such as toluene with an inert gas is supplied to the surface of particles heated in a reactor, thermal decomposition of the gas occurs. Carbon is deposited on the particle surface to form a film. If necessary, an inert gas may be used as the carrier gas. At this time, if the temperature of the particles is preferably about 800 ° C. or more, the amount of hydrogen taken into the formed film is reduced, the carbon-carbon recombination proceeds to form a dense film, and the compressive strength is high. A conductive carbon film is formed.

  On the other hand, lowering the strength of the carbon film is achieved by lowering the atomic density of the film. Low density membranes have low strength and are easily compressed. Reduction of the atom number density of the film can be achieved by forming voids in the film or making the atom bonds sparse.

A carbon film having a low compressive strength can be formed, for example, as follows. When a carbon film is formed using a hydrocarbon gas as a raw material by a CVD method, if the film formation temperature is low, the hydrocarbon is not completely decomposed and a film containing hydrogen is formed. A film containing a lot of hydrogen is non-conductive, but if it is dehydrogenated by heating in a vacuum atmosphere or in an inert atmosphere after film formation, the state of carbon atom bonds changes and becomes soft and conductive at the same time. Have sex. In this way, a carbon film having a low compressive strength can be formed. In addition, the temperature of the heat treatment after the film formation is usually set to a temperature at which the core particles change or less. For example, when SiO _x is used as the core particle, the upper limit temperature of the heat treatment after film formation is about 900 ° C. to 1000 ° C. immediately before Si clustering occurs. Conversely, in order to intentionally promote Si clustering, the temperature can be raised to 900 ° C. or higher, and further to 1000 ° C. or higher.

  A carbon film having a low compressive strength can also be formed by carbonization of a solid organic material. More specifically, core particles formed with a carbon film having a high compressive strength and, for example, polyethylene terephthalate (PET) powder are mixed, and preferably heated in an inert gas atmosphere such as nitrogen or argon. The heating temperature should just be more than the carbonization temperature of an organic compound, and can be determined suitably. When using PET powder, it heats, for example to 700 degreeC or more. During the temperature rise, the PET melts to cover the core particles, and furthermore, the PET is thermally decomposed to cause degassing, and the residue is carbonized to form a carbon film covering the core particles. Since the carbon film thus formed becomes a low-density film having fine voids, a carbon film having a low compressive strength can be formed on the surface of the core particle on which the carbon film having a high compressive strength is formed. .

  As the solid organic material used as a raw material for the carbon film in this method, any carbon-containing material that melts by heating can be used in addition to PET. For example, polyvinyl alcohol, polystyrene, and other thermoplastic resins can be used. Citric acid, sucrose, and the like can also be used as a raw material for the carbon film. The heat treatment is usually performed at a temperature lower than the temperature at which the core particles change, such as the generation of Si clusters in SiOx, but may be intentionally increased to modify the core particles.

  The negative electrode active material particles according to the present invention are applied to a current collector foil as a composite layer together with a binder to form a negative electrode.

  In general, the binder preferably has a compressive strength higher than the compressive strength of the conductive coating film of the negative electrode active material where the compressive strength is low or the low strength layer.

  Among these, it is preferable to use polyimide or polyamideimide. Polyimide and polyamideimide are both high in strength, and even when the mixing ratio of the binder in the mixture is increased, the increase in the internal resistance of the battery is small. On the other hand, for example, in the case of a polyvinylidene fluoride (PVdF) binder, when the ratio of the binder in the mixture is increased, the resistance of the battery may increase.

  The amount of the binder is preferably 8 parts by weight or more and 22 parts by weight or less with respect to 100 parts by weight of the total amount of the mixture constituent materials (that is, the active material and the binder). If the amount of the binder is not sufficient to cover most of the surface of the active material, there is a concern that the active material may fall off due to expansion and contraction of the active material. On the contrary, when the amount of the binder becomes excessive, it becomes a resistance when Li enters and exits the active material, and the electrode resistance increases. Further, when an excessive amount of the binder is mixed, the ratio of the active material in the mixture is lowered, so that the capacity of the battery is lowered.

  The negative electrode current collector foil is preferably copper or an alloy containing copper as a main component. Examples of the shape of the current collector foil include foil, flat plate, and mesh.

  The lithium ion secondary battery of the present invention has a negative electrode containing the negative electrode active material of the present invention and a binder.

  The lithium ion secondary battery according to the present invention is characterized by a negative electrode active material and a negative electrode using the negative electrode active material, and other elements constituting the battery, such as a positive electrode active material, a conductive additive, a binder, a separator, and an electrolytic solution The shape and material of the exterior body are not particularly limited. Hereinafter, although an example of the positive electrode, electrolyte solution, separator, and exterior body which comprise a secondary battery is demonstrated, it is not limited to these.

The positive electrode is formed, for example, by binding a positive electrode active material so as to cover a positive electrode current collector with a positive electrode binder. As the positive electrode active material, lithium manganate having a layered structure such as LiMnO ₂ , Li _x Mn ₂ O ₄ (0 <x <2) or lithium manganate having a spinel structure; LiCoO ₂ , LiNiO ₂ or a transition metal thereof Lithium transition metal oxides in which a specific transition metal such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ does not exceed half the lithium transition metal oxides; In which Li is made excessive in comparison with the stoichiometric composition. Examples of the Li excess composition include Li _α Ni _β Co _γ Al _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0.2) or Li _α Ni _β Co _γ Mn. _{There is δ} O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6, γ ≦ 0.2). A positive electrode active material can be used individually by 1 type or in combination of 2 or more types.

  As the positive electrode binder, the same negative electrode binder can be used. As the positive electrode binder, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost. The amount of the positive electrode binder to be used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship.

  As the current collector foil of the positive electrode, aluminum is preferable.

  A conductive auxiliary material may be added to the positive electrode active material layer containing the positive electrode active material for the purpose of reducing impedance. Examples of the conductive auxiliary material include carbonaceous fine particles such as graphite, carbon black, and acetylene black.

  The electrolytic solution includes a nonaqueous electrolytic solution that is stable at the operating potential of the battery and a supporting salt. Specific examples of the non-aqueous electrolyte include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC); dimethyl carbonate (DMC), diethyl carbonate (DEC) Chain carbonates such as ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC); propylene carbonate derivatives; aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; diethyl ether and ethyl propyl ether And aprotic organic solvents such as phosphate esters. Moreover, the fluorinated aprotic organic solvent which substituted some of them with the fluorine is mentioned.

  In particular, non-aqueous electrolytes include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (MEC). Cyclic or chain carbonates such as dipropyl carbonate (DPC) are preferred. A non-aqueous electrolyte can be used individually by 1 type or in combination of 2 or more types.

Specific examples of the supporting salt to be used include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , Li (CF ₃ SO ₂ ) ₂ , LiN ( And lithium salts such as CF ₃ SO ₂ ) ₂ . The supporting salt can be used alone or in combination of two or more. LiPF ₆ is preferable from the viewpoint of low cost.

  As the separator, a porous film such as polypropylene or polyethylene or a nonwoven fabric can be used. Moreover, what laminated | stacked them can also be used as a separator.

  As an exterior body, as long as it is stable to an electrolytic solution and has a sufficient water vapor barrier property, it can be appropriately selected and used. For example, in the case of a laminated laminate type secondary battery, a laminate film made of aluminum, silica-coated polypropylene, polyethylene, or the like can be used as the outer package. In particular, it is preferable to use an aluminum laminate film from the viewpoint of suppressing volume expansion.

  Next, the present invention will be described in detail using examples, comparative examples, and reference examples.

  In Examples, Comparative Examples, and Reference Examples, a negative electrode active material and a negative electrode were prepared in the following manner, a battery was assembled, and characteristics were evaluated by a charge / discharge test.

(Preparation of negative electrode)
As the core particles of the negative electrode, silicon monoxide (SiO) particles manufactured by Kojundo Chemical Laboratory Co., Ltd. were used after passing through a sieve having a mesh diameter of 20 μm.

  A carbon film having a high compressive strength was formed on the core particles by a thermal CVD method. Ethylene was used as the carbon source gas, and Ar gas was mixed at 1: 1. This gas was supplied to the core particles heated to 900 ° C. to form a film on the surface of the core particles. The hardness of the film formed on the Si substrate was Vickers hardness of 800 to 1000 as measured with a minute indentation hardness meter.

  The carbon film having a low compressive strength was formed using PET powder as a carbon source. Mix core particles with high compressive strength carbon film and PET powder, put in crucible and hold at 800 ° C for 1 hour in nitrogen flow atmosphere to form carbon coated membrane with low compressive strength did. Citric acid was also used as a carbon source. Citric acid was made into an aqueous solution, and this was mixed with core particles on which a carbon film having high compressive strength was formed, and the mixture was dried at 100 ° C. Thereafter, as in the case of using PET, the carbon coating film having a low compressive strength was formed by heating to 800 ° C. in a nitrogen atmosphere. In either case, after the carbon coating film was formed, the particles were adhered via the carbon, and thus pulverized with a ball mill. In the method of forming a carbon film having a low compressive strength using a solid carbon source, a continuous film could not be formed on a 1 cm square Si substrate due to peeling. Therefore, when the hardness was measured with the peeled film piece, the Vickers hardness was less than 10.

  The amount of the carbon coating film (a carbon film having a high compressive strength and a carbon film having a low compressive strength) is obtained by heating the core particles on which the surface coating is formed to 700 ° C. in a pure oxygen atmosphere. It was determined from the change in weight associated with gas. During the measurement, the temperature was increased at 10 ° C. per minute. FIG. 2 shows the change in weight of the active material used in Example 1 described later. In FIG. 2, the active material weight at 100 ° C. is assumed to be 100%, and the weight at each temperature is displayed as a percentage. Two weight reductions were observed from around 450 ° C and from around 600 ° C. The decrease in weight from the low temperature side, that is, around 450 ° C. corresponds to the disappearance of oxidation of carbon having a low compressive strength, and the decrease in weight from around 600 ° C. corresponds to the disappearance of oxidation of carbon having a high compressive strength. The weight increased again at the high temperature part after the disappearance of the carbon because the SiO of the core particles was exposed and oxidation started.

  As the binder for the negative electrode, polyimide (PI), polyamideimide (PAI), or polyvinylidene fluoride (PVdF) was used. Polyimide was manufactured by Ube Industries Co., Ltd., trade name “U Varnish A”, polyamide-imide was manufactured by Toyobo Co., Ltd., trade name “Viromax”, PVdF was manufactured by Kureha Co., Ltd. . The current collector foil was copper.

  The negative electrode was prepared by mixing an active material, a binder, and a solvent, applying the mixture to a copper current collector foil, and drying by heating. Furthermore, the electrode using the binder of polyimide and polyamideimide was heat-cured.

(Production of battery)
Lithium manganate LiMn ₂ O ₄ was used as a main component for the positive electrode active material, and a conductive additive was mixed. Aluminum was used for the current collector foil. Then, in the same manner as the negative electrode, an active material, a conductive additive, a binder and a solvent were mixed and applied to an aluminum current collector foil, followed by drying by heating to produce a positive electrode.

  A negative electrode was laminated on each of both sides of the positive electrode cut into a rectangular shape via a polypropylene separator to form an electrode body. And the electrode tab was ultrasonically welded to the non-application part of the active material provided in each edge part of a positive electrode and a negative electrode. The approximate dimension of the plane of the electrode is 9 cm long and 18 cm wide. The electrode body welded with the electrode tab was housed in an exterior body made of a laminate film in which an aluminum film and a resin were laminated.

The electrolyte used was a mixture of propylene carbonate (PC), ethylene carbonate (EC), diethylene carbonate (DEC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) as a solvent, and LiPF ₆ as a supporting salt. Was dissolved at a concentration of 1 mol / L. After injecting the electrolytic solution into the laminate film outer package containing the electrode body, the opening of the outer package was sealed under reduced pressure.

(Charge / discharge test)
As shown in FIG. 3, the charge / discharge test was performed by pressing a flat portion housing the battery electrode body with a flat plate. In FIG. 3, the pressing plate is opened so that the cell can be seen.

  Batteries were prepared as examples, comparative examples, and reference examples, and observation of the surface irregularities (cell flatness evaluation) after the first charge / discharge and a charge / discharge cycle test at room temperature were performed.

  In the evaluation of the cell flatness, those in which no wrinkle was generated on the cell surface after the first charge / discharge were determined to be acceptable (◯). On the other hand, those in which wrinkles occurred on the cell surface after the first charge / discharge were regarded as rejected (x) because the negative electrode was deformed. When the battery cell was disassembled and observed, wrinkles were generated near the outer edge of the electrode as schematically shown in FIG. This is because the absolute amount of the negative electrode mixture increases as the distance from the center of the electrode increases.

  The charge / discharge cycle test at room temperature is a lower limit voltage of 2.5 V and an upper limit voltage of 4.2 V. Charging is performed in a constant current mode at 1 C (1 C is a 1 hour rate current), and discharging is performed in a constant current mode of 1 C. It was. The ratio of the discharge capacity at the 150th cycle to the initial discharge capacity was taken as the discharge capacity retention rate.

  In any of the examples, comparative examples, and reference examples, the ratio of the carbon film having a high compressive strength is shown as a value with respect to 100 parts by weight of the core particles and the film having a high compressive strength. The ratio is shown as a value relative to a total of 100 parts by weight of the core particle, the film having a high compressive strength, and the carbon film having a low compressive strength, and the amount of the binder is shown as a value relative to a total of 100 parts by weight of the active material and the binder.

Example 1
A carbon film having a high compressive strength (hereinafter referred to as a high strength carbon film) is 3 parts by weight, a carbon film having a low compressive strength (hereinafter referred to as a low strength carbon film) is 10 parts by weight, and the binder is polyimide. A battery was prepared and evaluated with an amount of 7 parts by weight. PET was used as a raw material for the low-strength carbon film. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 86%.

(Example 2)
In Example 1, a battery was prepared and evaluated with a binder amount of 10 parts by weight. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 88%.

(Example 3)
In Example 1, a battery was prepared and evaluated with a binder amount of 15 parts by weight. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 87%.

Example 4
In Example 1, a battery was prepared and evaluated with a binder amount of 22 parts by weight. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 84%.

(Example 5)
A battery was prepared and evaluated with 3 parts by weight of the high-strength carbon film, 5 parts by weight of the low-strength carbon film, polyimide as the binder, and 15 parts by weight of the binder. PET was used as a raw material for the low-strength carbon film. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 82%.

(Example 6)
In Example 5, a battery was prepared and evaluated using 15 parts by weight of a low-strength carbon film. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 89%.

(Example 7)
A battery was prepared and evaluated with 3 parts by weight of the high-strength carbon film, 10 parts by weight of the low-strength carbon film, polyamideimide as the binder, and 10 parts by weight of the binder. PET was used as a raw material for the low-strength carbon film. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 87%.

(Example 8)
In Example 7, a battery was fabricated and evaluated with 10 parts by weight of the high-strength carbon film and 15 parts by weight of the binder. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 90%.

Example 9
In Example 7, a battery was fabricated and evaluated with 10 parts by weight of the high-strength carbon film, 10 parts by weight of the low-strength carbon film, and 22 parts by weight of the binder. The cell surface after the first charge / discharge was flat, and the discharge capacity retention after 150 cycles was 85%.

(Example 10)
A battery was prepared and evaluated with 3 parts by weight of the high-strength carbon film, 10 parts by weight of the low-strength carbon film, polyimide as the binder, and 15 parts by weight of the binder. The low strength carbon film was made from citric acid. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 88%.

(Reference Example 1)
A battery was prepared and evaluated with 3 parts by weight of high-strength carbon, 10 parts by weight of the low-strength carbon film, polyimide as the binder, and 3 parts by weight of the binder. PET was used as a raw material for the low-strength carbon film. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 55%.

(Reference Example 2)
In Reference Example 1, a battery was prepared and evaluated with a binder amount of 5 parts by weight. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 72%.

(Reference Example 3)
In Reference Example 1, a battery was prepared and evaluated with a binder amount of 25 parts by weight. The cell surface after the first charge / discharge was flat, and the discharge capacity retention after 150 cycles was 71%.

(Comparative Example 1)
A battery was fabricated and evaluated with 10 parts by weight of a high-strength carbon film and no low-strength carbon film, but with a binder of polyimide and a binder amount of 15 parts by weight. In this battery, wrinkles occurred on the cell surface after the first charge / discharge.

(Reference Example 4)
A battery was prepared and evaluated with 3 parts by weight of the high-strength carbon film, 2.5 parts by weight of the low-strength carbon film, polyimide as the binder, and 15 parts by weight of the binder. In this battery, wrinkles occurred on the cell surface after the first charge / discharge.

(Comparative Example 2)
A battery was fabricated and evaluated without forming a high-strength carbon film, using 10 parts by weight of the low-strength carbon film, polyimide as the binder, and 15 parts by weight of the binder. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 56%.

(Reference Example 5)
A battery was fabricated and evaluated with 15 parts by weight of the high-strength carbon film, 10 parts by weight of the low-strength carbon film, polyamideimide as the binder, and 15 parts by weight of the binder. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 49%.

(Reference Example 6)
A battery was prepared and evaluated with 3 parts by weight of the high-strength carbon film, 10 parts by weight of the low-strength carbon film, polyamideimide as the binder, and 25 parts by weight of the binder. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 77%.

(Reference Example 7)
A battery was prepared and evaluated with 3 parts by weight of the high-strength carbon film, 10 parts by weight of the low-strength carbon film, polyvinylidene fluoride as the binder, and 10 parts by weight of the binder. The cell surface after the first charge / discharge was flat, and the discharge capacity retention rate after 150 cycles was 44%.

  Examples are summarized in Table 1, and Comparative Examples and Reference Examples are summarized in Table 2. In the table, PI represents polyimide, PAI represents polyamideimide, and PVdF represents polyvinylidene fluoride.

実施例５、実施例６と、比較例１から、低強度炭素膜が必要なことが分かる。また、参考例４から、この実施形態では、低強度炭素膜の厚さとして５重量部以上が必要なことが分かる。低強度炭素膜が薄い比較例１、参考例４はセル表面にしわが発生していた。これは、負極活物質の膨張が緩和されずに負極の集電箔が伸びてしまったためである。

From Examples 5 and 6, and Comparative Example 1, it can be seen that a low-strength carbon film is necessary. Further, it can be seen from Reference Example 4 that in this embodiment, the thickness of the low-strength carbon film needs to be 5 parts by weight or more. In Comparative Example 1 and Reference Example 4 in which the low-strength carbon film is thin, wrinkles were generated on the cell surface. This is because the negative electrode current collector foil was stretched without the expansion of the negative electrode active material being relaxed.

  From Comparative Example 2, it can be seen that a high-strength carbon film is necessary. Reference Example 5 shows that a high-strength carbon film is not suitable even if it is too thick. In Reference Example 5, since the high-strength carbon film is thick, it can be considered that it cracked and peeled off during the charge / discharge cycle test.

  From the results of Examples 1 to 4 and Reference Examples 1 to 3 and Reference Example 6, it can be seen that the cycle characteristics are inferior when the amount of the binder is too small or too large.

  Examples 7 to 9 show that the effect of the present invention can be obtained even when the binder is polyamideimide, and Example 10 shows that citric acid other than the thermoplastic resin can also be used as a raw material for the low-strength carbon film.

  In Reference Example 7 using PVdF as the binder, the cycle retention rate was low. This seems to be because when PVdF covers the active material, it prevents Li from entering and exiting.

  As described above, according to the present invention, it is possible to provide a lithium ion secondary battery that suppresses deformation of the negative electrode by relaxing volume change of the negative electrode material due to insertion and extraction of Li and has excellent cycle characteristics. it can.

  This embodiment can be used in all industrial fields that require a power source, and in industrial fields related to the transport, storage, and supply of electrical energy. Specifically, power supplies for mobile devices such as mobile phones and laptop computers; power supplies for transportation and transportation media such as trains, satellites, and submarines, including electric vehicles such as electric cars, hybrid cars, electric bikes, and electric assist bicycles A backup power source such as a UPS; a power storage facility for storing power generated by solar power generation, wind power generation, etc .;

DESCRIPTION OF SYMBOLS 1 Active material particle 2 Core particle 3 High compression strength layer 4 Low compression strength layer

Claims (15)

Core particles capable of repeatedly inserting and extracting lithium, and a conductive coating film on the core particle surface,
The lithium ion secondary characterized in that the compressive strength of the conductive coating film is relatively high on the side closer to the surface of the core particle and lower on at least part of the side far from the surface of the core particle. Negative electrode active material for batteries. The conductive coating film is at least
Having a high strength layer with relatively high compressive strength on the core particle surface;
The negative electrode active material according to claim 1, further comprising a low-strength layer having a lower compressive strength than the high-strength layer on the high-strength layer. The conductive coating film further comprises:
3. The negative electrode active material according to claim 2, comprising a layer having a higher compressive strength than the low strength layer on a side farther from the surface of the core particle and closer to the surface of the negative electrode active material than the low strength layer. .   The ratio of the high strength layer of the conductive coating film is 3 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the total weight of the high strength layer of the conductive coating film and the core particles. The negative electrode active material according to claim 2 or 3.   The ratio of the low-strength layer of the conductive coating film is 5 parts by weight or more and 15 parts by weight or less with respect to 100 parts by weight of the total weight of the low-strength layer, the high-strength layer, and the core particles of the conductive coating film. The negative electrode active material according to claim 2, wherein the negative electrode active material is a negative electrode active material.   The conductive coating film has a portion in which the compressive strength continuously changes from a high strength to a low strength from the side close to the surface of the core particles toward the surface side of the negative electrode active material. The negative electrode active material according to claim 1.   On the surface of the core particle of the conductive coating film, the ratio of the high compressive strength region where the compressive strength is 250 or more in terms of Vickers hardness is equal to the high compressive strength region of the conductive coating film and the core particle. The negative electrode active material according to claim 6, wherein the negative electrode active material is 3 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight as a whole.   The ratio of the low compressive strength region in which the compressive strength of the conductive coating film is 100 or less in terms of Vickers hardness is equal to the low compressive strength region of the conductive coating film and the compression on the core particle surface. 8. The negative electrode active material according to claim 6, wherein the negative electrode active material is 5 parts by weight or more and 15 parts by weight or less with respect to the high strength region and 100 parts by weight of the total weight of the core particles.   The negative electrode active material according to claim 1, wherein the conductive coating film contains amorphous carbon. A core particle capable of repeatedly inserting and extracting lithium, and a method for producing a negative electrode active material for a lithium ion secondary battery comprising a conductive coating film on the surface of the core particle,
Forming a high-strength layer having relatively high compressive strength on the core particles by a thermal CVD method in which the temperature of the core particles is 800 ° C. or higher using hydrocarbon as a raw material;
By mixing the core particles with the high-strength layer and the organic compound and heating it above the carbonization temperature of the organic compound, the low-strength layer whose compressive strength is lower than that of the high-strength layer is changed to the high-strength layer. Forming on top;
A method comprising the steps of:   The negative electrode for lithium ion secondary batteries containing the negative electrode active material of any one of Claims 1-9, and a binder.   The negative electrode according to claim 11, wherein a ratio of the binder is 8 parts by weight or more and 22 parts by weight or less with respect to 100 parts by weight of the total amount of the active material and the binder.   The compressive strength of the binder is higher than the compressive strength of a relatively low compressive strength region or a low strength layer of the conductive coating film of the negative electrode active material. The negative electrode described.   The negative electrode according to claim 11, wherein the binder contains at least one of polyimide and polyamideimide.   A lithium ion secondary battery comprising the negative electrode according to claim 11.

Images