JP2008300244A - Positive electrode for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode for a nonaqueous electrolyte secondary battery in which cycle characteristics of charge and discharge are improved. <P>SOLUTION: A positive electrode for the nonaqueous electrolyte secondary battery having an active material layer containing an active material, contains in the active material layer a chemical compound expressed by LiNi<SB>1-x</SB>Ti<SB>x</SB>O<SB>2</SB>(in the formula, 0<x<0.7) apart from the active material. In the chemical compound, I (003)/I (104) which is a peak ratio of a face (003) and a face (104) in its X-ray diffraction pattern is preferably 0.4 to 0.9. Proportion of an amount of the chemical compound against the amount of the active material in the active material layer is preferably 5 to 30 wt%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a positive electrode used for a nonaqueous electrolyte secondary battery such as a lithium secondary battery. The present invention also relates to a non-aqueous electrolyte secondary battery including the positive electrode.

As an active material of a positive electrode used for a non-aqueous electrolyte secondary battery such as a lithium secondary battery, a compound having a layered rock salt structure such as LiCoO ₂ and LiNiO ₂ or a compound having a spinel structure such as LiMn ₂ O ₄ Lithium transition metal composite oxides such as are known. In addition, compounds in which part of the transition metal in these composite oxides is substituted with other metals have been proposed.

For example, it has been studied to use a compound in which a part of Ni in LiNiO ₂ is substituted with Ti as a positive electrode active material of a lithium secondary battery (see, for example, Non-Patent Document 1). In this document, the characteristics of a compound represented by Li _x Ni 1− _y Ti _y O ₂ (0.1 ≦ y ≦ 0.5) as a positive electrode active material of a lithium secondary battery are being studied. It cannot be said that sufficient capacity is obtained.

By the way, apart from the positive electrode active material, various Si-based and Sn-based materials having higher theoretical capacities than graphite-based materials generally used at present have been studied as negative electrode active materials for lithium secondary batteries. ing. Negative electrode active materials made of these Si-based and Sn-based materials generally have a large irreversible capacity during the initial charge. Therefore, in order to improve the charge / discharge cycle characteristics with high capacity by utilizing the high capacity characteristics of these negative electrode active materials, these negative electrode active materials have a high capacity and a positive electrode having an appropriate irreversible capacity. It is necessary to use it in combination with an active material. However, it cannot be said that LiCoO ₂ or the like that is currently generally used as a positive electrode active material has sufficient irreversible capacity to be used in combination with Si-based or Sn-based materials.

S. H. Chang et al., Solid State Ionics, 86-88 (1996) p.171-175

  Accordingly, an object of the present invention is to provide a positive electrode for a non-aqueous electrolyte secondary battery that can eliminate the various drawbacks of the above-described conventional technology.

The present invention provides a positive electrode for a non-aqueous electrolyte secondary battery having an active material layer containing an active material, wherein LiNi1− _x Ti _x O ₂ (where 0 <x <0.7) is provided, and a positive electrode for a non-aqueous electrolyte secondary battery is provided.

The present invention, (wherein, 0 <x <0.7 and is) LiNi1 over _x Ti _x O ₂ positive active material for a nonaqueous electrolyte secondary battery characterized by comprising a compound represented by An additive is provided.

  Furthermore, the present invention provides a non-aqueous electrolyte secondary battery provided with the positive electrode.

  Since the positive electrode of the present invention can supply a large amount of lithium to the negative electrode active material during the initial charge, the irreversible capacity can be reduced and the charge / discharge cycle characteristics are improved.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The positive electrode of the present invention has a positive electrode active material layer formed on at least one surface of a current collector, for example. The positive electrode active material layer contains an active material. As an active material, what is conventionally known in the said technical field can be especially used without a restriction | limiting. For example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ , Li (Li _{x Examples} thereof include lithium transition metal composite oxides such as Mn _2x Co _1-3x ) O ₂ (where 0 <x <1/3). In particular, it is preferable to use a substance in which the crystal structure is not easily destroyed even when the cutoff voltage at the first charge is set high. Moreover, it is also preferable to use a thing with a large irreversible capacity at the time of first charge. Examples of such a positive electrode active material include LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and Li (Li _x Mn _2x Co _1-3x ) O ₂ (where 0 <x <1/3. And the like.

In addition to the positive electrode active material, the active material layer contains a compound represented by the following formula (1). This compound is a kind of the above-mentioned lithium transition metal composite oxide in a broad sense, and can theoretically be used as a positive electrode active material for a non-aqueous electrolyte secondary battery, but it is sufficient to use this compound alone. A secondary battery having excellent performance cannot be obtained. Therefore, in this invention, the compound represented by Formula (1) is not included in a positive electrode active material. As a result of various studies by the present inventors, it has been found that the compound represented by the formula (1) has a large amount of lithium released as an irreversible capacity during the first charge. In particular, it was found that the amount of lithium released as irreversible capacity was large when the cut-off voltage at the first charge was set high. Therefore, by including this compound in the active material layer together with the positive electrode active material as an additive for the positive electrode active material having a high capacity, it becomes possible to supply a sufficient amount of lithium to the negative electrode during the initial charge, The characteristics of the positive electrode active material can be sufficiently extracted.
LiNi1- _x Ti _x O ₂ (1)
(Where 0 <x <0.7)

  In the compound represented by the formula (1), x is a number greater than 0 and less than 0.7 as described above. Since it becomes difficult to obtain a desired irreversible capacity even if the value of x, that is, the amount of Ti is large or small, the value of x is set within this range. Considering the charge / discharge capacity and irreversible capacity of the positive electrode, x is preferably 0.01 to 0.5, particularly 0.05 to 0.3.

The compound represented by formula (1) has a peak ratio of (003) plane to (104) plane in its X-ray diffraction pattern, that is, I (003) / I (104) is 0.4 to 0.9, In particular, it is preferably 0.5 to 0.8. The compound represented by the formula (1) is an α-NaFeO ₂ type, that is, a compound having a layered rock salt structure. This peak ratio is a measure of the layered structure, and the larger the peak ratio, the more the formula (1 The layer structure of the compound represented by) is further enhanced. And by making this peak ratio of the compound represented by Formula (1) into the said range, elimination | release of lithium at the time of first charge progresses favorably.

The compound represented by the formula (1) is generally used in the form of a solid powder. The various positive electrode active materials described above are also generally used in the form of a solid powder. The particle size of these powders is not particularly critical in the present invention, but the positive electrode active material particles and the compound particles represented by formula (1) are filled with high density by filling the gaps between them. In view of the preferable, the preferable particle diameter of these particles is 5 to 30 μm in terms of D ₅₀ . In this case, these in two types of particles, the value of [D ₅₀ is larger particles towards the particle / D ₅₀ small] it is preferably 1.5 to 5.

The compound represented by (1) is obtained, for example, by using Li ₂ CO ₃ , Ni (OH) ₂ , and TiO ₂ as raw materials, mixing these powders, and firing them at a predetermined temperature in the atmosphere. . By appropriately adjusting various firing conditions such as the temperature and time during firing, the above-mentioned peak ratio of I (003) / I (104) can be set within a desired preferable range. A major factor is the firing temperature. By setting the firing temperature to, for example, 600 to 800 ° C., a compound having a peak ratio within the above range can be easily obtained.

  In the active material layer of the positive electrode, the compound represented by the formula (1) is used in combination with the positive electrode active material to supplement the supply of lithium to the negative electrode active material. The ratio of the amount of the compound represented by the formula (1) to the amount of the substance is preferably 5 to 30% by weight, particularly 5 to 20% by weight. By setting this ratio to 5% by weight or more, the charge / discharge cycle characteristics can be improved. From this viewpoint, the ratio of the amount of the compound represented by the formula (1) to the amount of the positive electrode active material is preferably as high as possible. However, the initial capacity tends to decrease with this, so this ratio is 30% by weight. By making it below, it is possible to balance the reduction of the initial capacity and the improvement of the cycle characteristics.

  In the positive electrode of the present invention, for example, a positive electrode active material and a compound represented by the formula (1) are suspended in a suitable solvent together with a conductive agent such as acetylene black and a binder such as polyvinylidene fluoride to produce a positive electrode mixture. Then, this is applied to at least one surface of a current collector such as an aluminum foil, dried, and then rolled and pressed.

In order to improve the cycle characteristics of the secondary battery provided with the positive electrode of the present invention, it is preferable to adjust the precharging and the first and subsequent charging conditions. Regarding the precharge, it is preferable that the cutoff potential is set high, the amount of lithium released from the compound represented by formula (1) is increased, and the negative electrode active material is accumulated as an irreversible capacity. From this viewpoint, the precharge cutoff potential is preferably set to 4.2 to 5.0 V based on Li / Li ^+, and more preferably set to 4.4 to 5.0. Preliminary charging is charging performed for the first time after the battery is assembled, and is generally performed before the battery manufacturer ships the product to the market. In other words, lithium secondary batteries sold in the market are usually precharged already. Therefore, the first charge / discharge after the preliminary charge and the subsequent discharge after the preliminary charge corresponds to the first charge / discharge. In that sense, in the following description, “charge / discharge after discharge after preliminary charge” is referred to as “charge / discharge after first time”.

On the other hand, it is preferable to set the cut-off voltage in the first and subsequent charging to be the same as or lower than the cut-off voltage in the preliminary charge. However, if the cut-off voltage is too low, the advantage of using the compound represented by formula (1) cannot be fully utilized. On the other hand, if the cut-off voltage is too high, the non-aqueous electrolyte tends to be damaged. Therefore, the cut-off potential in the first and subsequent charging is preferably 4.2 to 5.0 V, particularly 4.25 to 4.5 V, based on Li / Li ⁺ .

  The positive electrode of the present invention is used together with a negative electrode, a separator, a non-aqueous electrolyte and the like to constitute a non-aqueous electrolyte secondary battery. The negative electrode is formed, for example, by forming a negative electrode active material layer on at least one surface of a current collector. The negative electrode active material layer contains an active material. The negative electrode active material is made of a material capable of occluding and releasing lithium. As the negative electrode active material, those conventionally known in the art can be used without particular limitation. From the viewpoint of taking advantage of the positive electrode of the present invention, the irreversible capacity at the first charge is large, and the theoretical capacity It is preferable to use a material having a high value. Examples of such a material include a material containing Si or Sn.

  The negative electrode active material containing Si is capable of occluding and releasing lithium ions. For example, silicon alone, an alloy of silicon and metal, silicon oxide, or the like can be used. These materials can be used alone or in combination. Examples of the metal include one or more elements selected from the group consisting of Cu, Ni, Co, Cr, Fe, Ti, Pt, W, Mo, and Au. Among these metals, Cu, Ni, and Co are preferable, and Cu and Ni are preferably used from the viewpoint of excellent electronic conductivity and a low ability to form a lithium compound. Further, lithium may be occluded in the active material made of a material containing Si before or after the negative electrode is incorporated in the battery. A particularly preferable Si-containing material is silicon or silicon oxide in view of the high occlusion amount of lithium.

  On the other hand, as an example of the negative electrode active material containing Sn, tin alone, an alloy of tin and metal, or the like can be used. These materials can be used alone or in combination. Examples of the metal that forms an alloy with tin include one or more elements selected from the group consisting of Cu, Ni, Co, Cr, Fe, Ti, Pt, W, Mo, and Au. Of these metals, Cu, Ni, and Co are preferable, and Cu and Ni are particularly preferable.

  The negative electrode active material layer can be, for example, a continuous thin film layer made of the negative electrode active material, a coating layer containing particles of the negative electrode active material, a sintered body layer containing particles of the negative electrode active material, and the like. Further, it may be a layer having the structure shown in FIG. 1 described below. When the negative electrode active material is in the form of particles, the negative electrode active material layer may include a mixture of the negative electrode active material composed of particles and metal particles.

  FIG. 1 shows a schematic diagram of a cross-sectional structure of a preferred embodiment of a negative electrode used in the present invention. The negative electrode 10 of this embodiment includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. 1 shows a state in which the active material layer 12 is formed only on one side of the current collector 11 for convenience, the active material layer may be formed on both sides of the current collector. .

  In the active material layer 12, at least a part of the surface of the active material particles 12 a containing Si is covered with a metal material having a low lithium compound forming ability. The metal material 13 is a material different from the constituent material of the particles 12a. Gaps are formed between the particles 12a coated with the metal material. That is, the metal material covers the surface of the particle 12a in a state in which a gap is ensured so that the non-aqueous electrolyte containing lithium ions can reach the particle 12a. In FIG. 1, the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. This figure is a schematic view of the active material layer 12 viewed two-dimensionally. In reality, each particle is in direct contact with other particles or through a metal material 13. “Low lithium compound forming ability” means that lithium does not form an intermetallic compound or solid solution, or even if formed, lithium is in a very small amount or very unstable.

  The metal material 13 has conductivity, and examples thereof include copper, nickel, iron, cobalt, and alloys of these metals. In particular, the metal material 13 is preferably a highly ductile material in which even if the active material particles 12a expand and contract, the coating on the surface of the particles 12a is not easily broken. It is preferable to use copper as such a material.

  The metal material 13 is preferably present on the surface of the active material particles 12 a over the entire thickness direction of the active material layer 12. The active material particles 12 a are preferably present in the matrix of the metal material 13. Thus, even if the particles 12a are pulverized due to expansion and contraction due to charge / discharge, the particles are less likely to fall off. In addition, since the electronic conductivity of the entire active material layer 12 is ensured through the metal material 13, the electrically isolated active material particles 12 a are generated, and in particular, the electrically isolated active material is deep in the active material layer 12. Generation of the particles 12a of the substance is effectively prevented. The presence of the metal material 13 on the surface of the active material particles 12a over the entire thickness direction of the active material layer 12 can be confirmed by electron microscope mapping using the material 13 as a measurement target.

  The metal material 13 coats the surface of the particle 12a continuously or discontinuously. When the metal material 13 continuously covers the surfaces of the particles 12a, it is preferable to form fine voids in the coating of the metal material 13 that allow the non-aqueous electrolyte to flow. When the metal material 13 discontinuously coats the surface of the particle 12a, the non-aqueous electrolyte is supplied to the particle 12a through a portion of the surface of the particle 12a that is not covered with the metal material 13. In order to form the coating of the metal material 13 having such a structure, the metal material 13 may be deposited on the surfaces of the particles 12a by, for example, electrolytic plating in accordance with conditions described later.

  The average thickness of the metal material 13 covering the surface of the active material particles 12a is preferably 0.05 to 2 μm, more preferably 0.1 to 0.25 μm. That is, the metal material 13 covers the surface of the active material particles 12a with a minimum thickness. As a result, while the energy density is increased, the particles 12a are prevented from falling off due to expansion / contraction and pulverization due to charge / discharge. Here, the “average thickness” is a value calculated based on a portion of the surface of the active material particle 12 a that is actually covered with the metal material 13. Accordingly, the portion of the surface of the active material particle 12a that is not covered with the metal material 13 is not used as a basis for calculating the average value.

  The voids formed between the particles 12a coated with the metal material 13 have a function as a flow path of the non-aqueous electrolyte containing lithium ions. Since the non-aqueous electrolyte smoothly flows in the thickness direction of the active material layer 12 due to the presence of the voids, cycle characteristics can be improved. Further, the voids formed between the particles 12a also have a function as a space for relieving stress caused by the volume change of the active material particles 12a due to charge and discharge. The increase in the volume of the active material particles 12a whose volume has been increased by charging is absorbed by the voids. As a result, pulverization of the particles 12a is difficult to occur, and significant deformation of the negative electrode 10 is effectively prevented.

  The active material layer 12 is preferably formed by subjecting a coating obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying it, and performing electrolytic plating using a predetermined plating bath. It is formed by depositing a metal material 13 between 12a.

  In order to form necessary and sufficient voids in the active material layer 12 in which the non-aqueous electrolyte can be circulated, it is preferable to sufficiently infiltrate the plating solution into the coating film. In addition to this, it is preferable to make conditions suitable for depositing the metal material 13 by electrolytic plating using the plating solution. The plating conditions include the composition of the plating bath, the pH of the plating bath, and the current density of electrolysis. Regarding the pH of the plating bath, it is preferable to adjust this to 7.1-11. By controlling the pH within this range, the dissolution of the active material particles 12a is suppressed, the surface of the particles 12a is cleaned, and plating on the particle surface is promoted. Is formed. The value of pH is measured at the temperature at the time of plating.

When using copper as the metal material 13 for plating, it is preferable to use a copper pyrophosphate bath. When nickel is used as the metal material, for example, an alkaline nickel bath is preferably used. In particular, it is preferable to use a copper pyrophosphate bath, even if the active material layer 12 is thick, because the voids can be easily formed over the entire thickness direction of the layer. Further, since the metal material 13 is deposited on the surface of the active material particles 12a and the metal material 13 is less likely to be deposited between the particles 12a, the voids between the particles 12a are also successfully formed. preferable. When using a copper pyrophosphate bath, the bath composition, electrolysis conditions and pH are preferably as follows.
Copper pyrophosphate trihydrate: 85-120 g / l
-Potassium pyrophosphate: 300-600 g / l
-Potassium nitrate: 15-65 g / l
-Bath temperature: 45-60 ° C
・ Current density: 1 to 7 A / dm ²
PH: Ammonia water and polyphosphoric acid are added to adjust the pH to 7.1 to 9.5.

In particular, when a copper pyrophosphate bath is used, it is preferable to use one having a P ratio defined by a ratio of P ₂ O ₇ weight to Cu weight (P ₂ O ₇ / Cu) of 5 to 12. . When the P ratio is less than 5, the metal material 13 covering the active material particles 12a tends to be thick, and it may be difficult to form desired voids between the particles 12a. Further, when a P ratio exceeding 12 is used, current efficiency is deteriorated, and gas generation is likely to occur, so that production stability may be lowered. When a copper pyrophosphate bath having a P ratio of 6.5 to 10.5 is used as a more preferable copper pyrophosphate bath, the size and number of voids formed between the active material particles 12a may be reduced. This is very advantageous for the flow of the water electrolyte.

When an alkaline nickel bath is used, the bath composition, electrolysis conditions and pH are preferably as follows.
Nickel sulfate: 100 to 250 g / l
Ammonium chloride: 15-30 g / l
・ Boric acid: 15-45 g / l
-Bath temperature: 45-60 ° C
・ Current density: 1 to 7 A / dm ²
-PH: 25% by weight ammonia water: Adjust to pH 8-11 within the range of 100-300 g / l.
When this alkaline nickel bath and the above-described copper pyrophosphate bath are compared, there is a tendency that moderate voids are formed in the active material layer 12 when the copper pyrophosphate bath is used, and the life of the negative electrode is increased. It is preferable because it is easy to achieve.

  The ratio of the voids in the entire active material layer formed by the various methods described above, that is, the void ratio is preferably about 15 to 45% by volume, particularly about 20 to 40% by volume. By setting the porosity within this range, it is possible to form necessary and sufficient voids in the active material layer 12 that allow the nonaqueous electrolyte to flow. The void amount of the active material layer 12 is measured by a mercury intrusion method (JIS R 1655). The mercury intrusion method is a method for obtaining information on the physical shape of a solid by measuring the size and volume of pores in the solid. The principle of the mercury intrusion method is to apply a pressure to mercury to inject it into the pores of the object to be measured, and to measure the relationship between the pressure applied at that time and the volume of mercury that has been pushed in (intruded). In this case, mercury enters sequentially from the large voids present in the active material layer 12. In the present invention, the void amount measured at a pressure of 90 MPa is regarded as the entire void amount. The porosity (%) of the active material layer 12 is obtained by dividing the void amount per unit area measured by the above method by the apparent volume of the active material layer 12 per unit area and multiplying it by 100. .

  In the negative electrode, in addition to the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method being within the above range, the active material layer 12 measured by the mercury intrusion method at 10 MPa. It is preferable that the porosity calculated from the void volume is 10 to 40%. Moreover, it is preferable that the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method at 1 MPa is 0.5 to 15%. Furthermore, it is preferable that the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method at 5 MPa is 1 to 35%. As described above, in the mercury intrusion measurement, mercury intrusion conditions are gradually increased. Under low pressure conditions, mercury is pressed into large gaps, and under high pressure conditions, mercury is pressed into small gaps. Therefore, the porosity measured at a pressure of 1 MPa is mainly derived from large voids. On the other hand, the porosity measured at a pressure of 10 MPa reflects the presence of small voids.

  The large voids described above are mainly derived from the spaces between the active material particles 12a. On the other hand, the above-mentioned small voids are considered to originate mainly from the space between the crystal grains of the metal material 13 deposited on the surfaces of the active material particles 12a. The large void mainly functions as a space for relieving stress caused by expansion and contraction of the active material particles 12a. On the other hand, the small gap mainly serves as a path for supplying the non-aqueous electrolyte to the active material particles 12a. By balancing the abundance of these large voids and small voids, the cycle characteristics are further improved.

The porosity can also be controlled by appropriately selecting the particle size of the active material particles 12a. From this viewpoint, the particle 12a has a maximum particle size of preferably 30 μm or less, more preferably 10 μm or less. Moreover, when the particle diameter of the particle is expressed by a D ₅₀ value, it is preferably 0.1 to 8 μm, particularly preferably 0.3 to 4 μm. The particle size of the particles is measured by laser diffraction / scattering particle size distribution measurement and electron microscope observation (SEM observation).

  If the amount of the active material relative to the whole negative electrode is too small, it is difficult to sufficiently improve the energy density of the battery. Conversely, if the amount is too large, the strength decreases and the active material tends to fall off. Considering these, the thickness of the active material layer is 10 to 40 μm, preferably 15 to 30 μm, and more preferably 18 to 25 μm.

  In the negative electrode 10 of the present embodiment, a thin surface layer (not shown) may be formed on the surface of the active material layer 12. Further, the negative electrode 10 may not have such a surface layer. The thickness of the surface layer is 0.25 μm or less, preferably 0.1 μm or less. There is no restriction | limiting in the lower limit of the thickness of a surface layer. By forming the surface layer, the pulverized active material particles 12a can be further prevented from falling off. However, in the present embodiment, by setting the porosity of the active material layer 12 within the above-described range, it is possible to sufficiently prevent the pulverized active material particles 12a from dropping without using a surface layer. Is possible.

  When the negative electrode 10 has the thin surface layer or does not have the surface layer, a secondary battery is assembled using the negative electrode 10 to reduce the overvoltage when the battery is initially charged. Can do. This means that lithium can be prevented from being reduced on the surface of the negative electrode 10 when the secondary battery is charged. The reduction of lithium leads to the generation of dendrites that cause a short circuit between the two electrodes.

  When the negative electrode 10 has a surface layer, the surface layer covers the surface of the active material layer 12 continuously or discontinuously. When the surface layer continuously covers the surface of the active material layer 12, the surface layer has a large number of microscopic voids (not shown) that are open at the surface and communicate with the active material layer 12. It is preferable. The fine voids are preferably present in the surface layer so as to extend in the thickness direction of the surface layer. The fine voids allow the non-aqueous electrolyte to flow. The role of the fine voids is to supply a non-aqueous electrolyte into the active material layer 12. When the surface of the negative electrode 10 is viewed in plan by electron microscope observation, the fine voids are such that the ratio of the area covered with the metal material 13, that is, the coverage is 95% or less, particularly 80% or less, especially 60% or less. It is preferable that the size is large. When the coverage exceeds 95%, it is difficult for the non-aqueous electrolyte having high viscosity to enter, and the range of selection of the non-aqueous electrolyte may be narrowed.

  The surface layer is made of a metal material having a low lithium compound forming ability. This metal material may be the same as or different from the metal material 13 present in the active material layer 12. The surface layer may have a structure of two or more layers made of two or more different metal materials. Considering the ease of manufacture of the negative electrode 10, the metal material 13 present in the active material layer 12 and the metal material constituting the surface layer are preferably the same type.

  The negative electrode 10 having the above structure can be manufactured, for example, by the method described in Japanese Patent Application Laid-Open No. 2006-253095 related to the previous application of the present applicant.

  As the separator used together with the positive electrode and the negative electrode described above, a synthetic resin nonwoven fabric, a polyolefin such as polyethylene or polypropylene, a polytetrafluoroethylene porous film, or the like is preferably used. From the viewpoint of suppressing the heat generation of the electrode that occurs when the battery is overcharged, it is preferable to use a separator in which a thin film of a ferrocene derivative is formed on one side or both sides of a polyolefin microporous membrane. The separator preferably has a puncture strength of 0.2 N / μm thickness or more and 0.49 N / μm thickness or less, and a tensile strength in the winding axis direction of 40 MPa or more and 150 MPa or less. This is because even when a negative electrode active material that greatly expands and contracts with charge and discharge is used, damage to the separator can be suppressed, and occurrence of internal short circuits can be suppressed.

The nonaqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent. The lithium _{salt, CF 3 SO 3 Li, (} CF 3 SO 2) NLi, (C 2 F 5 SO 2) 2 NLi, LiClO 4, LiA1Cl 4, LiPF 6, LiAsF 6, LiSbF 6, LiCl, LiBr, LiI And LiC ₄ F ₉ SO ₃ . These can be used alone or in combination of two or more. Of these lithium salts, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) NLi, and (C ₂ F ₅ SO ₂ ) ₂ NLi are preferably used because of their excellent water decomposition resistance. Examples of the organic solvent include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and the like. In particular, 0.5 to 5% by weight of vinylene carbonate and 0.1 to 1% by weight of divinyl sulfone and 0.1 to 1.5% by weight of 1,4-butanediol dimethanesulfonate with respect to the whole non-aqueous electrolyte It is preferable from the viewpoint of further improving the charge / discharge cycle characteristics. The details are not clear, but 1,4-butanediol dimethanesulfonate and divinylsulfone are decomposed stepwise to form a film on the positive electrode, so that the film containing sulfur becomes denser. It is thought that it is to become.

  The non-aqueous electrolyte secondary battery composed of the above members and the like can be in the form of a cylinder, a square, a coin, or the like. However, it is not limited to these forms.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples.

[Example 1]
(1) Production of positive electrode LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ (D ₅₀ = 13 μm) as a positive electrode active material, LiNi _0.9 Ti _0.1 O ₂ (D ₅₀ = 21 μm) as an additive, conductivity Acetylene black as a material and polyvinylidene fluoride as a binder were suspended in polyvinylpyrrolidone as a solvent to obtain a positive electrode mixture. This positive electrode mixture was applied to a current collector made of an aluminum foil and dried, followed by roll rolling and pressing to obtain a positive electrode. The ratio of the amount of LiNi _0.9 Ti _0.1 O _{2 to} the amount of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ was 13% by weight. Further, the peak ratio of I (003) / I (104) in the X-ray diffraction pattern of LiNi _0.9 Ti _0.1 O ₂ was 0.6. LiNi _0.9 Ti _0.1 O ₂ was prepared by the following procedure.

Li ₂ CO ₃ , Ni (OH) ₂ , and TiO ₂ were weighed so as to have a molar ratio of Li: Ni: Ti = 1: 0.9: 0.1. These were mixed and slurried with a wet pulverizer, then dried and granulated with a spray dryer. The obtained granulated powder was fired at 800 ° C. for 20 hours to obtain the target LiNi _0.9 Ti _0.1 O ₂ .

(2) Production of negative electrode A current collector made of an electrolytic copper foil having a thickness of 18 μm was acid-washed at room temperature for 30 seconds. After the treatment, it was washed with pure water for 15 seconds. A slurry containing silicon particles was applied on both sides of the current collector to a thickness of 15 μm to form a coating film. The composition of the slurry was particles: styrene butadiene rubber (binder): acetylene black = 100: 1.7: 2 (weight ratio). The average particle diameter D ₅₀ of the particles was 2 μm. The average particle diameter D ₅₀ was measured using a Microtrac particle size distribution measuring device (No. 9320-X100) manufactured by Nikkiso Co., Ltd.

The current collector on which the coating film was formed was immersed in a copper pyrophosphate bath having the following bath composition, and copper was electroplated on the coating film by electrolysis to form an active material layer. The electrolysis conditions were as follows. DSE was used for the anode. A DC power source was used as the power source.
Copper pyrophosphate trihydrate: 105 g / l
-Potassium pyrophosphate: 450 g / l
・ Potassium nitrate: 30 g / l
-P ratio: 7.7
・ Bath temperature: 50 ° C
・ Current density: 3 A / dm ²
-PH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

  The electrolytic plating was terminated when copper was deposited over the entire thickness direction of the coating film. In this way, a target negative electrode was obtained. SEM observation of the longitudinal section of the active material layer confirmed that the active material particles were covered with a copper film having an average thickness of 240 nm in the active material layer. The porosity of the active material layer was 30%.

The positive electrode and the negative electrode thus obtained were opposed to each other with a polypropylene separator having a thickness of 20 μm interposed therebetween. As an electrolyte, a lithium secondary battery was used by adding 2% by volume of vinylene carbonate to a solution of 1 mol / l LiPF ₆ dissolved in a 1: 1 volume% mixed solvent of ethylene carbonate and diethyl carbonate. Manufactured. In the secondary battery, the amounts of the positive electrode active material and the negative electrode active material were adjusted so that the capacity ratio between the positive electrode and the negative electrode was 1: 2 at a charge cut-off voltage of 4.3 V.

[Example 2]
A positive electrode was produced in the same manner as in Example 1 except that the ratio of the amount of LiNi _0.9 Ti _0.1 O _{2 to} the amount of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ was 20% by weight. Thereafter, a lithium secondary battery was obtained in the same manner as in Example 1.

[Comparative Example 1]
A positive electrode was produced in the same manner as in Example 1 except that LiNi _0.9 Ti _0.1 O ₂ was not used. Thereafter, a lithium secondary battery was obtained in the same manner as in Example 1.

[Evaluation]
The lithium secondary batteries obtained in the examples and comparative examples were repeatedly charged and discharged, and the capacity retention rate at that time was measured. The result is shown in FIG. Charging / discharging was performed under the following conditions. Preliminary charge was 0.05 C, cut-off voltage was set to 4.5 V, and constant current / constant voltage was set (the cut-off current value of constant voltage charge was 1/5 of the constant current value). The discharge following the precharge was 0.05 C, the cut-off voltage was set to 2.7 V, and a constant current was set. The charge after the first time was 0.5 C, the cut-off voltage was set to 4.2 V, and constant current / constant voltage was set. The discharge after the first time was 0.5 C, the cut-off voltage was set to 2.7 V, and a constant current was set. The capacity maintenance rate was a ratio to the capacity of the discharge following the precharge.

  As is clear from the results shown in FIG. 2, the batteries obtained in the respective examples are maintained at a higher capacity maintenance ratio when the charge / discharge cycles are repeated as compared with the batteries obtained in the comparative examples. You can see that From these results, it can be seen that the batteries obtained in each Example have good charge / discharge cycle characteristics.

It is a schematic diagram which shows the cross-sectional structure of one Embodiment of the negative electrode used suitably by this invention. It is a graph which shows a capacity | capacitance maintenance factor when charging / discharging the lithium secondary battery obtained by the Example and the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Negative electrode for non-aqueous electrolyte secondary batteries 11 Current collector 12 Active material layer 12a Active material particles 13 Metal material with low lithium compound forming ability

Claims (8)

In the positive electrode for a non-aqueous electrolyte secondary battery having an active material layer containing an active material, LiNi ₁ - _x Ti _x O ₂ (where 0 <x <0. 7), and a positive electrode for a non-aqueous electrolyte secondary battery. _2. The non-aqueous electrolyte 2 according to claim 1, wherein the active material is a lithium transition metal composite oxide (excluding LiNi 1− _x Ti _x O ₂ (where 0 <x <0.7)). Positive electrode for secondary battery.   3. The non-specificity according to claim 1, wherein I (003) / I (104), which is a peak ratio between the (003) plane and the (104) plane in the X-ray diffraction pattern of the compound, is 0.4 to 0.9. Positive electrode for water electrolyte secondary battery.   The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein a ratio of the amount of the compound to the amount of the active material is 5 to 30% by weight. LiNi1 (wherein, 0 <x <0.7) over _x Ti _x O ₂ additive of the positive electrode active material for a nonaqueous electrolyte secondary battery characterized by comprising a compound represented by.   A non-aqueous electrolyte secondary battery comprising the positive electrode according to claim 1.   The nonaqueous electrolyte secondary battery according to claim 6, comprising a negative electrode having an active material layer containing a negative electrode active material made of a material containing Si or Sn.   The active material layer of the negative electrode contains particles of a negative electrode active material containing Si or Sn, and at least a part of the surface of the particles is coated with a metal material having a low lithium compound forming ability, and the metal material The nonaqueous electrolyte secondary battery according to claim 7, wherein voids are formed between the particles coated with.

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