In recent years, as portable electronic devices such as portable telephones, notebook type personal computers and the like have become widespread, there is a need for developing a compact and lightweight secondary battery that has a high energy density. Moreover, there is also a need for development of a high-output secondary battery as the battery for electric automobiles such as hybrid automobiles. As a nonaqueous-electrolyte secondary battery that satisfies such needs, there is a lithium-ion secondary battery. A lithium-ion secondary battery includes a negative electrode, a positive electrode, an electrolyte and the like, and a material from which lithium ions can be desorbed and to which lithium ions can be inserted is used as the active material for the negative electrode and positive electrode.
A lithium-ion secondary battery that uses a lithium transition metal composite oxide, and particularly a lithium cobalt composite oxide that is comparatively easy to form as the positive-electrode material, is able to obtain a 4V high voltage, so is expected to be a battery having high-energy density, and practical use of such a battery is progressing. For a battery that uses a lithium cobalt composite oxide, much development is being performed in order to obtain excellent initial capacity characteristics and cycling characteristics, and various results have already been obtained.
However, lithium cobalt composite oxide uses expensive cobalt compounds for the raw material, so the unit cost per capacity of a battery that uses this kind of lithium cobalt composite oxide becomes much higher than that of a nickel hydride battery, and thus the applicable uses are limited. Therefore, lowering the cost of the positive-electrode material and making possible the production of a less expensive lithium ion secondary battery for not only a compact secondary battery for portable electronic devices, but also for a large secondary battery for electric-power storage for electric automobiles is highly anticipated, and realization of such would have large industrial significance.
As an example of new material for the active material for a lithium-ion secondary battery, there is lithium cobalt oxide that uses nickel that is less expensive than cobalt. This lithium nickel composite oxide shows a lower electrochemical potential than lithium cobalt composite oxide, so it is difficult for decomposition due to oxidation of the electrolyte to become a problem, and high capacity can be expected, and because a high battery voltage similar to that of a lithium cobalt composite oxide is possible, much development is being performed. However, a lithium-ion secondary battery that uses a lithium nickel composite oxide that is formed with purely nickel only has disadvantages in that the cycling characteristics are inferior when compared with a lithium cobalt composite oxide, and the loss of battery performance due to usage and storage in high-temperature environments occurs comparatively easily.
In order to solve such disadvantages, a lithium nickel-containing composite oxide in which part of the nickel has been replaced with other added elements has been proposed. For example, for the purpose of improving discharge characteristics and cycling characteristics of a lithium-ion secondary battery, JPH08-213015 (A) proposes a lithium nickel-containing composite oxide that is expressed by the general formula: LixNiaCobMcO2 (where 0.8≤x≤1.2, 0.01≤a≤0.99, 0.01≤b≤0.99, 0.01≤c≤0.3, 0.8≤a+b+c≤1.2, and M is at least one kind of element selected from among Al, V, Mn, Fe, Cu and Zn). Moreover, for the purpose of improving capacity characteristics and cycling characteristics of a lithium-ion secondary battery that can be used or stored in a high-temperature environment, JPH08-045509 (A) proposes a lithium nickel-containing composite oxide that is expressed by the general formula: LiwNixCoyBzO2 (where 0.05≤w≤1.10, 0.5≤x≤0.995, 0.005≤z≤0.20, x+y+z=1).
According to the technology described in the above literature, it is possible to obtain a lithium nickel-containing composite oxide having higher charge/discharge capacity and more excellent cycling characteristics than a lithium cobalt composite oxide. However, with the technology described in literature above, the lithium nickel-containing composite oxide described above is obtained by mixing and firing of the metal salt raw material, so the irreversible capacity that is defined by the difference between the initial charge capacity and the initial discharge capacity is large, and when forming a battery, an extra amount of negative-electrode material must be used that is equal to an amount that corresponds to the irreversible capacity of the positive-electrode material. As a result, not only does the unit mass and the battery capacity per unit volume of the overall battery decrease, but there is also a problem in the terms of safety since there is an increase in the lithium amount used for the negative-electrode material.
In regard to this, JP 3,614, 670 (B2) describes technology of obtaining a lithium nickel-containing composite oxide having a layered hexagonal crystal structure by first obtaining nickel-containing composite hydroxide by adding alkali to a mixed aqueous solution of metal salt as raw material and performing co-precipitation, then mixing this with a lithium compound and firing; the lithium nickel-containing composite oxide being expressed by the general formula: [Li]3a[Ni1−x−yCoxAly]3b[O2]6c (where subscripts of [ ] express sites, and the conditions 0<x≤0.20 and 0<y≤0.15 are satisfied). This lithium nickel-containing composite oxide is such that the site occupancy of metal ions other than lithium at site 3a that is obtained from Rietveld analysis of X-ray diffraction (hereafter, this is referred to as the “non-lithium occupancy”) is 1.7% or less, and includes secondary particles that are formed by an aggregation of plural primary particles having an average particles size of no less than 0.1 μm and no greater than 1 μm, and the crystal particle size that is calculated from the X-ray diffraction 003 peak is 73 nm or greater.
In a secondary battery that uses this lithium nickel-containing composite oxide as a positive-electrode active material, an initial discharge capacity of 160 mAh/g or greater, an irreversible capacity of 46 mAh/g or less and a Coulomb efficiency of 78% or greater are achieved. In other words, when this lithium nickel-containing composite oxide is used as positive-electrode active material, it is possible to sufficiently maintain the contact surface area with the electrolyte, and improve the diffusion rate of Li ions in the liquid phase, and it is possible to maintain the Li diffusion path in the solid phase, so it becomes possible to simultaneously improve the initial discharge capacity and the irreversible capacity.
On the other hand, in order to be able to further increase the energy density of a lithium-ion secondary battery, it is necessary to improve the filling ability of the positive-electrode active material. In order for that, increasing the particle size of the secondary particles of the lithium nickel-containing oxide that forms the positive-electrode active material is effective. However, lithium nickel-containing composite oxide has a low firing temperature of about 850° C., so it is difficult to increase the size of secondary particles by growing the primary particles by firing lithium nickel-containing composite oxide at a high temperature as in the case of lithium cobalt composite oxide. Therefore, in order to improve the filling ability of lithium nickel-containing composite oxide, it is necessary to increase the particle size of secondary particles in the precursor stage. However, the average particle size of nickel-containing composite hydroxide that is obtained by the co-precipitation described in JP 3,614,670 (B2) is at a maximum about 12 μm, and in this method, producing nickel-containing composite hydroxide that includes secondary particles having a large particle size is difficult.
In regard to this, JP2011-201764 (A) describes technology for obtaining a nickel-containing composite hydroxide having an average particle size of 15 μm to 50 μm by making the ratio of the supply of mixed aqueous solution that includes metal salt as raw material with respect to the amount of reaction solution at the supply port to 0.04% by volume/min or less while stirring the reaction solution using stirring blades having an angle of 45° or less with respect to a horizontal plane when coprecipitating the nickel-containing composite hydroxide.
With this technology, it is considered possible to improve the filling ability of the positive-electrode active material by increasing the particle size of the nickel-containing composite hydroxide, and to increase the energy density of the secondary battery that is obtained. However, in a positive-electrode active material that is obtained by taking the nickel-containing composite hydroxide described in JP2011-201764 (A) as a precursor, when the average particle size of the secondary particles becomes 20 μm or greater, it becomes difficult to suppress an increase in the non-lithium occupancy, and thus it is not possible to obtain a secondary battery having high energy density and sufficiently small irreversible capacity.