JP2016058334A - Positive electrode material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery small in irreversible capacity and high in volume energy density.SOLUTION: A positive electrode material for a lithium ion secondary battery comprises: a positive electrode active material expressed by the compositional formula, Li(NiMnCoM)O(where x+y=2; 1.19<x/y≤1.63; 0.06<a≤0.5; 0.5≤b<0.88; 0≤c<0.25; 0≤d≤0.03; a+b+c+d=1; M represents at least one element of V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu and the like); and MoOlarger than the positive electrode active material in average particle diameter. Of the total mass of the positive electrode active material and MoO, MoOaccounts for 0-30 mass%, preferably 10-25 mass%. The particle diameter of MoOis 500-2000 nm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode material for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery including the positive electrode material, and a lithium ion secondary battery.

  The problem of the electric vehicle is that the energy density of the driving battery is low and the travel distance in one charge is short. Therefore, there is a need for a secondary battery that is inexpensive and has a high energy density.

  Lithium ion secondary batteries have a higher energy density per weight than secondary batteries such as nickel metal hydride batteries and lead batteries. Therefore, application to electric vehicles and power storage systems is expected. However, in order to meet the demands of electric vehicles, it is necessary to further increase the energy density. In order to realize high energy density, it is necessary to increase the energy density of the positive electrode and the negative electrode.

As a positive energy active material having a high energy density, a layered solid solution compound represented by a composition formula Li ₂ MO ₃ —LiM′O ₂ is expected. The layered solid solution compound utilizes a highly active property while extracting high capacity by electrochemically inactive Li ₂ MO ₃ and electrochemically active LiM′O ₂ . The layered solid solution compound can also be represented by the composition formula Li _{1 + x} M _1-x O ₂ .

Patent Document 1, the composition formula _{xLi 2 MnO 3 - (1-} x) LiMn a Co b Ni c O 2 ( wherein, 0.2 ≦ x ≦ 0.5,0.3 ≦ a ≦ 0.5, At least part of the surface of the solid solution represented by 0.1 ≦ b ≦ 0.3, 0.3 ≦ c ≦ 0.5, a + b + c = 1), where Me is Mn, Co and The present invention provides a lithium ion secondary battery in which gas is not easily generated from a positive electrode active material during charging by using a positive electrode containing a positive electrode active material coated with an oxide or a lithium oxide oxide. ing.

JP 2012-129166 A

  A lithium ion secondary battery using a layered solid solution compound as a positive electrode active material has a disadvantage that its irreversible capacity is large. The reason is that a part of lithium extracted from the positive electrode by the first charge / discharge does not return to the positive electrode active material at the time of discharge. Therefore, an extra negative electrode is required to accept these lithium. As a result, the amount of the active material not involved in the reaction in the battery is increased, and the capacity and energy density per unit volume are reduced.

  Accordingly, an object of the present invention is to provide a lithium ion secondary battery having a high volumetric energy density and a small irreversible capacity.

The positive electrode material for a lithium ion secondary battery according to the present invention has a composition formula Li _x (Ni _a Mn _b Co _c M _d ) _y O ₂ (x + y = _2, 1.19 <x / y ≦ 1.63, 0. 06 <a ≦ 0.5, 0.5 ≦ b <0.88, 0 ≦ c <0.25, 0 ≦ d ≦ 0.03, a + b + c + d = 1, M is V, Mo, W, Zr, Nb, Positive electrode active material particles represented by: at least one element of Ti, Al, Fe, Mg, Cu, and the like; and MoO ₃ particles having an average particle diameter larger than that of the positive electrode active material particles. The ratio of the MoO ₃ particles to the total amount of the MoO ₃ particles is larger than 0% by mass and smaller than 30% by mass. In particular, the content is preferably 5% by mass or more and 25% by mass or less.

  ADVANTAGE OF THE INVENTION According to this invention, a high volume energy density is obtained and the lithium ion secondary battery with a small irreversible capacity | capacitance can be provided.

It is sectional drawing which shows the structure of a lithium ion secondary battery typically. It is a figure which shows the irreversible capacity | capacitance and energy density of an Example and a comparative example.

<Positive electrode material>
When a lithium ion secondary battery is employed in an electric vehicle, a long travel distance per charge is required. A positive electrode active material using a layered solid solution compound has a large theoretical capacity and an advantage that a high capacity can be obtained with an active material of the same weight, but the electrode density is low. Therefore, the energy density per unit volume is low. Furthermore, there is a drawback that the irreversible capacity is large. In order to provide a battery having a long travel distance per charge, a positive electrode material having a high energy density and a small irreversible capacity is required.

  As a result of intensive studies by the inventors, it is not possible to charge at the first charge, but lithium can be inserted in the discharge process, and after the second cycle, a compound that can be stably charged and discharged should be used together with the layered solid solution compound. Thus, it was found that the irreversible capacity of the positive electrode using the layered solid solution compound can be relaxed.

MoO ₃ can be charged and discharged as an active material having a capacity of about 290 Ah / kg at 2.0 to 3.5 V by occluding lithium. Therefore, MoO ₃ does not release lithium in the initial charge, but can occlude lithium released from the negative electrode active material in the subsequent discharge process. Therefore, among the lithium extracted from the layered solid solution compound by the first charge / discharge, the lithium that causes irreversible capacity without returning to the layered compound at the time of discharge can be occluded, and active material performance can be exhibited.

Furthermore, the reaction potential of MoO ₃ matches the region where the resistance of the layered solid solution compound is high. In the low potential region, the resistance of the layered solid solution compound is high, and mixing with MoO ₃ contributes to the reduction in resistance. Therefore, these are preferred combinations for reducing the resistance of the battery.

MoO ₃ does not exhibit active material performance in a state where lithium is not absorbed, and since the discharge potential is low, if it is excessively contained, the energy density is lowered. Since the irreversible capacity of the layered solid solution compound is generally about 10% to 30%, the ratio of MoO _{3 in} the positive electrode material needs to be 18 mol% or less with respect to the layered solid solution compound. The mass ratio needs to be less than 30% by mass with respect to the total amount of the positive electrode active material and the MoO ₃ . The mixing amount is preferably an amount capable of absorbing irreversible capacity of lithium, and is preferably 5% by mass or more. A particularly desirable ratio is 10% by mass or more and 20% by mass or less.

The layered solid solution compound according to the present invention has a composition formula Li _x (Ni _a Mn _b Co _c M _d ) _y O ₂ (x + y = _2, 1.19 <x / y ≦ 1.63, 0.06 <a ≦ 0). 0.5, 0.5 ≦ b <0.88, 0 ≦ c <0.25, 0 ≦ d ≦ 0.03, a + b + c + d = 1, M is V, Mo, W, Zr, Nb, Ti, Al, Fe , At least one of elements such as Mg and Cu).

  x / y indicates the ratio of lithium to the transition metal in the layered solid solution compound. When x / y is 1.19 or less, the amount of lithium contributing to the reaction is reduced, and a high capacity cannot be obtained. On the other hand, if x / y is larger than 1.63, the crystal lattice becomes unstable, and the discharge capacity decreases.

  a represents the proportion of Ni in the entire transition metal element. When a is 0.06 or less, the number of elements involved in the reaction decreases, and the capacity decreases. On the other hand, when a is larger than 0.5, the activation reaction of oxygen in the initial charging process hardly occurs, and thus the capacity is reduced.

  b shows the ratio of Mn in the whole transition metal element. When b is less than 0.5, the activation reaction of oxygen hardly occurs, and thus the capacity decreases. On the other hand, when b is 0.88 or more, the amount of Ni and Co contributing to the reaction is lowered, and thus the capacity is lowered.

  c represents the proportion of Co in the entire transition metal element. If c is 0.25 or more, the oxygen activation reaction in the initial charging process is unlikely to occur, so the capacity is reduced.

  d represents the ratio of other transition metal elements M (M is at least one element of V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc.) in the entire transition metal element. If it is contained appropriately as an additive, an impurity, etc. for improving safety, and d is larger than 0.03, the proportion of elements contributing to the reaction is lowered, so that the discharge capacity is lowered.

The positive electrode material according to the present invention includes positive electrode active material particles made of a layered solid solution compound and MoO ₃ particles having an average particle size larger than that of the positive electrode active material particles. By mixing MoO ₃ having a particle diameter larger than that of the positive electrode active material, the content ratio of MoO ₃ capable of relaxing the irreversible capacity can be secured, the electrode density can be increased, and the volume energy density can be improved.

Compared with the case where the average particle diameter of the MoO ₃ particles is made smaller than that of the positive electrode active material particles, or when the MoO ₃ particles are coated and provided on the positive electrode active material particles, the packing density can be improved and the volume energy density is improved. Further, when the average particle diameter of the MoO ₃ particles is reduced, the ratio of MoO ₃ tends to be reduced and the irreversible capacity is increased.

By using the positive electrode material having this feature, a high volume energy density can be obtained, and a positive electrode having a small irreversible capacity can be provided.
Further, by mixing MoO ₃ particles, the ratio of MoO ₃ can be increased as compared with the case where the surface coating of the MoO ₃ compound is provided on the positive electrode active material particles. If the coating of the MoO ₃ compound is too thick, the output tends to decrease. If the coating is too thin, Li cannot be occluded during discharge, and the improvement of the irreversible capacity becomes insufficient. Also, mixing of MoO ₃ particles and positive electrode active material particles is preferable because there is a difference in the size of the two types of particles, and an improvement in electrode density can be expected.

Further, in order to reduce the irreversible capacity, it is necessary to mix MoO ₃ that can relax the irreversible capacity. MoO ₃ can be charged and discharged at 3.5 to 2.0 V by occluding lithium, and can exhibit a capacity of about 290 Ah / kg. The reaction potential of MoO ₃ coincides with the region where the resistance of the layered solid solution compound is high, and is a preferable combination for reducing the resistance of the battery. However, since MoO ₃ has a low discharge potential, if it is excessively contained, the energy density is lowered. Since the irreversible capacity of the layered solid solution compound is generally about 70% to 90%, MoO ₃ in the positive electrode material needs to be less than 30% by mass. A particularly desirable ratio is 10% by mass or more and 20% by mass or less.

<Lithium ion secondary battery>
A lithium ion secondary battery according to the present invention includes the above positive electrode material. By using the above positive electrode material for the positive electrode, a high volume energy density is obtained and the irreversible capacity is small. The lithium ion secondary battery according to the present invention can be preferably used for an electric vehicle.

  A lithium ion secondary battery includes a positive electrode including a positive electrode material, a negative electrode including a negative electrode material, a separator, an electrolytic solution, an electrolyte, and the like.

  The negative electrode material is not particularly limited as long as it is a substance that can occlude and release lithium ions. Substances generally used in lithium ion secondary batteries can be used as the negative electrode material. For example, graphite, a lithium alloy, etc. can be illustrated.

  As a separator, what is generally used in a lithium ion secondary battery can be used. Examples thereof include polyolefin microporous films and nonwoven fabrics such as polypropylene, polyethylene, and a copolymer of propylene and ethylene.

As the electrolytic solution and the electrolyte, those generally used in lithium ion secondary batteries can be used. For example, diethyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, methyl acetate, ethyl methyl carbonate, methyl propyl carbonate, dimethoxyethane and the like can be exemplified as the electrolytic solution. In addition, as the electrolyte, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN Examples thereof include (CF ₃ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₃ .

  One embodiment of the structure of a lithium ion secondary battery according to the present invention will be described with reference to FIG. The lithium ion secondary battery 10 includes an electrode group having a positive electrode 1 in which a positive electrode material is applied on both sides of a current collector, a negative electrode 2 in which a negative electrode material is applied on both sides of the current collector, and a separator 3. The positive electrode 1 and the negative electrode 2 are wound through a separator 3 to form a wound electrode group. This wound body is inserted into the battery can 4.

  The negative electrode 2 is electrically connected to the battery can 4 via the negative electrode lead piece 6. A sealing lid 7 is attached to the battery can 4 via a packing 5. The positive electrode 1 is electrically connected to the sealing lid 7 via the positive electrode lead piece 8. The wound body is insulated by the insulating plate 9.

  The electrode group may not be the wound body shown in FIG. 1, but may be a laminated body in which the positive electrode 1 and the negative electrode 2 are laminated via the separator 3.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example and a comparative example, the technical scope of this invention is not limited to this.

<Preparation of positive electrode active material>
The positive electrode active material used in each example was produced by the following method. Lithium carbonate, nickel carbonate, manganese carbonate, and cobalt carbonate were mixed at a predetermined ratio by a ball mill to obtain a precursor. The obtained precursor was calcined in the atmosphere at 500 ° C. for 12 hours to obtain a lithium transition metal oxide. The obtained lithium transition metal oxide was pelletized and then fired at 850 to 1050 ° C. for 12 hours in the air. The fired pellets were pulverized in an agate mortar. Table 1 shows the composition and average particle diameter of the produced positive electrode active material.

<Production of prototype battery>
In Examples 1 to 6 and Comparative Examples 1 to 4, trial batteries were produced using the mixture of the positive electrode active material produced as described above and MoO ₃ having an average particle diameter of 1050 nm as the positive electrode material.

A total of 85% by mass of the synthesized positive electrode active material and MoO ₃ , a conductive agent (mass ratio 10%) and a binder (mass ratio 5%) were uniformly mixed to prepare a positive electrode slurry. The positive electrode slurry was applied onto an aluminum current collector foil having a thickness of 20 μm, dried at 120 ° C., and pressed at 40 MPa to obtain an electrode plate. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

The negative electrode was produced using metallic lithium. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate having a volume ratio of 1: 2 was used.

Similarly, in Comparative Example 5, a prototype battery was fabricated using the same positive electrode active material as in Example 1 and a mixture of MoO ₃ having an average particle diameter of 200 nm as the positive electrode material. In Comparative Example 6, a prototype battery was fabricated using the same positive electrode active material as in Example 1 and a mixture of MoO ₂ having an average particle diameter of 900 nm as the positive electrode material.

<Charge / discharge test>
In each of the examples and comparative examples, a charge / discharge test was performed on the prototype battery manufactured as described above.

  A charge / discharge test was performed on the prototype battery. Charging was constant current constant voltage charging (CC-CV mode), and the upper limit voltage was 4.6V. The discharge was constant current discharge (CC mode), and the lower limit voltage was 2.0V. The charge / discharge current was 0.05 C, and the charge cutoff current was 0.005 C. In each example and comparative example, the discharge capacity at the second cycle was defined as the rated capacity. A value obtained by dividing the product of the rated capacity and the average discharge potential by the electrode density was defined as the volume energy density. The value obtained by subtracting the initial discharge capacity from the initial charge capacity was defined as the irreversible capacity. However, in the case of minus, it was set to zero.

Table 1 shows the composition of the positive electrode material, the ratio of MoO ₃ and MoO _{2 to} the positive electrode material, the volume energy density, and the irreversible capacity in each Example and Comparative Example.

The irreversible capacity | capacitance and volume energy density of each Example and a comparative example are shown in FIG. MoO ₃ and Examples 1 to 4 containing 5 to 30 mass%, Comparative Example 2, in each of the examples, the irreversible capacity compared with Comparative Example 1 containing no MoO ₃ is reduced. Furthermore, in Examples 1-4, the volume energy density improved. This is because it contains MoO ₃ that relieves irreversible capacity and has a large particle diameter and can improve the electrode density. On the other hand, in Comparative Example 2, the volume energy density is low. On the other hand, in Comparative Example 2 including the MoO ₃ 30 wt%, the volumetric energy density compared to Comparative Example 1 not containing MoO ₃ was reduced. This is thought to be because the proportion of MoO _{3 having} a low average discharge potential was too large. Therefore, the addition amount of MoO ₃ is preferably more than 0% by mass and less than 30% by mass, particularly 25% by mass or less. Further, since the half or less irreversible capacity by the addition of 10 wt% of MoO _3, and more preferably 10 to 25 mass%.

Compared with the batteries not containing MoO _{3 in} Comparative Examples 1, _3, and 4, the batteries containing 15% by mass of MoO ₃ in Examples 1, 5, and 6 have greatly reduced irreversible capacity. Further, the volume energy density increased in any of the examples. This is because the irreversible capacity is large regardless of the composition, and in a positive electrode using a layered solid solution compound with a low electrode density, the volume energy density is improved by reducing the irreversible capacity and improving the electrode density by partially mixing MoO _3. It shows that it is effective.

Although the irreversible capacity could be suppressed even in Comparative Example 5 in which the particle diameter of MoO ₃ was 200 nm, the volume energy density was smaller than that in Example 1 in which the particle diameter of MoO ₃ was 1050 nm. This is presumably because the electrode density could not be improved because the particle size of the mixed MoO ₃ was smaller than the particle size of the positive electrode active material. Therefore, the electrode density can be improved by making the particle diameter of the mixed MoO ₃ larger than the particle diameter of the positive electrode active material. In particular, since the difference from the particle diameter of the solid solution appears clearly by setting it to 500 nm or more, the effect of improving the electrode density is large and preferable. On the other hand, the particle diameter of MoO ₃ is preferably 2 μm (2000 nm) or less. If it is too large, it may be difficult to occlude lithium, and charge and discharge efficiency will be reduced.

The battery mixed with MoO _{2 of} Comparative Example 6 had no effect of reducing the irreversible capacity, and the volume energy density was decreased. This is because the molybdenum oxide added to the positive electrode active material was MoO ₂ that hardly reacted in the range of 2.0 to 4.6 V in this electrochemical test. Therefore, the molybdenum oxide added to the positive electrode active material needs to be MoO ₃ .

As described above, the volume energy density is high by adding MoO ₃ particles having a larger average particle diameter than the layered solid solution compound particles in a mixing ratio of more than 0% by mass and less than 30% by mass to the positive electrode material containing the layered solid solution compound. A positive electrode having a small irreversible capacity can be provided.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Battery can 5 Packing 6 Negative electrode lead piece 7 Sealing lid 8 Positive electrode lead piece 9 Insulation board 10 Lithium ion secondary battery

Claims (5)

Composition formula Li _x (Ni _a Mn _b Co _c M _d ) _y O ₂ (x + y = _2, 1.19 <x / y ≦ 1.63, 0.06 <a ≦ 0.5, 0.5 ≦ b < 0.88, 0 ≦ c <0.25, 0 ≦ d ≦ 0.03, a + b + c + d = 1, M is at least one of V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc. A positive electrode material for a lithium ion secondary battery comprising positive electrode active material particles represented by the element) and MoO ₃ particles,
The MoO ₃ particles have a larger average particle size than the positive electrode active material particles,
The positive active material and the proportion of the MoO ₃ in the total amount of MoO ₃ is a positive electrode material for a lithium ion secondary battery, characterized in that 0 less than weight percent greater than 30 wt%. The positive electrode material for a lithium ion secondary battery according to claim 1,
The positive active material and the proportion of the MoO ₃ in the total amount of MoO ₃ is a positive electrode material for a lithium ion secondary battery, characterized in that at most 10 mass% to 25 mass%. The positive electrode material for a lithium ion secondary battery according to claim 1,
The positive electrode material for a lithium ion secondary battery, wherein the MoO ₃ particles are 500 nm to 2000 nm.   A positive electrode for a lithium ion secondary battery comprising the positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 3.   A lithium ion secondary battery comprising the positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 3.