Method for producing lithium transition metallates

Details Method for producing lithium transition metallates Method for producing lithium transition metallates

1999-08-19

2002-09-10

Griffin, Steven P. (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Alkali metal

C423S594120, C423S593100, C423S596000, C423S599000, C252S182100

Reexamination Certificate

active

06447739

ABSTRACT:

The present invention relates to a process for preparing lithium transition metallates of the general formula
Li
x
(M
1
y
M
2
1−y
)
n
O
nz
wherein
M
1
represents nickel, cobalt or manganese,
M
2
represents a transition metal which is different from M
1
and is chromium, cobalt, iron, manganese, molybdenum and/or aluminium,
n is 2 if M
1
is manganese, and n is 1 if M
1
is nickel or cobalt, wherein
x has a value from 0.9 to 1.2,
y has a value between 0.5 and 1 and
z has a value between 1.9 and 2.1.
These types of lithium transition metallates are used as electrode materials, in particular as cathode materials for non-aqueous lithium storage battery systems, so-called lithium ion batteries.
A number of proposals has already been made relating to methods of preparation of these types of lithium transition metallates, but these are mostly unsuitable for large-scale production or lead to products which have imperfect electrochemical properties.
The use of LiCoO
2
has recently gained acceptance, but this is extremely expensive due to the limited availability, and thus high price, of cobalt and is therefore not suitable for mass production (e.g. to provide the power for electrically operated vehicles). Therefore intensive efforts have already been made to replace some or all of the LiCoO
2
with, for instance, LiNiO
2
and/or LiMn
2
O
4
as a cathode material.
Synthesis of the corresponding cobalt compound LiCoO
2
is generally regarded as a non-critical procedure. Due to the thermal stability of LiCoO
2
, it is even possible, with this system, to react cobalt carbonate and lithium carbonate, as reaction components, directly at relatively high temperatures without troublesome concentrations of carbonate being left in the final product.
The transfer of this method to LiNiO
2
has been possible only at temperatures of 800 to 900° C. These high calcination temperatures, however, lead to partly decomposed lithium nickelates with relatively low storage capacities and/or unsatisfactory resistance to cyclic operation.
For this reason, carbonate-free mixtures are proposed for preparing LiNiO
2
, in which, in most cases, &bgr;-nickel hydroxide is favoured as the nickel component, such as is described, for instance in U.S. Pat. No. 5,591,548, EP 0 701 293, J. Power Sources 54 (95) 209-213, 54 (95) 329-333 and 54 (95) 522-524. Moreover, the use of nickel oxide was also recommended in JP-A 7 105 950 and that of oxynickel hydroxide NiOOH in DE-A 196 16 861.
According to U.S. Pat. No. 4,567,031, the intimate mixture is prepared by co-precipitation of soluble lithium and transition metal salts from solution, drying the solution and calcining. Relatively finely divided crystals of the lithium transition metallate are obtained in this way at comparatively low calcining temperatures and within comparatively short times. The allocation of lithium and transition metal ions to particular layers in the crystal lattice, however, is greatly distorted so that, to a large extent, lithium ions occupy nickel layer lattice positions and vice versa. These types of crystals have unsatisfactory properties with regard to their use as electrodes in rechargeable batteries. Other processes (EP-A 205 856, EP-A 243 926, EP-A 345 707) start with solid, finely divided carbonates, oxides, peroxides or hydroxides of the initial metals. The intimate mixture is prepared by joint milling of the starting metals. The formation of lithium transition metallates takes place by solid diffusion during calcination. Solid diffusion requires comparatively high temperatures and comparatively long calcining times and does not generally lead to phase-pure lithium metallates with outstanding electronic properties. Extensive observations appear to prove that, in the case of the nickel system, decomposition of LiNiO
2
with the production of Li
2
O and NiO is initiated during prolonged thermal treatment at temperatures above about 700° C.
Therefore, in order to intensify the intimate mixing procedure, it has already been proposed, according to EP-A 468 942, to start the preparation of lithium nickelate with powdered nickel oxide or hydroxide, suspending the powder in a saturated lithium hydroxide solution and extracting the water from the suspension by spray drying. This should lead to a reduction in the calcining time and calcining temperature. Due to the relatively low solubility of lithium hydroxide in water, however, the homogeneity of this mixture is limited.
U.S. Pat. No. 5,591,548 proposes milling a powdered oxygen-containing transition metal compound with lithium nitrate and then calcining under an inert gas. The advantage of this process is the low melting point of lithium nitrate, 264° C., which means that intimate mixing takes place after heating to, for example, 300° C. in the form of a suspension of transition metal particles in molten lithium nitrate, which favours reaction with the solid.
The disadvantage of this process is that, during calcination, the gases released (H
2
O, NO
x
, O
2
) do not escape, or escape only very slowly, from the viscous molten suspension so that the intimate contact required for the solid reaction and diffusion is hindered and on the other hand only a few suspended particles are present due to concentration inhomogeneities in the geometric spacing. Therefore, interruptions in the calcining process and intermediate milling to homogenise the reaction material are required.
According to the invention, it is now proposed that calcination be performed for at least some of the time under an at least partial vacuum. This produces a significant reduction in reaction times and temperatures required.
The invention therefore provides a process for preparing lithium transition metallates of the general formula
Li
x
(M
1
y
M
2
1−y
)
n
O
nz
wherein
M
1
represents nickel, cobalt or manganese,
M
2
represents a transition metal which is different from M
1
and is chromium, cobalt, iron, manganese, molybdenum and/or aluminium,
n is 2 if M
1
is manganese, and n is 1 if M
1
is nickel or cobalt, wherein
x has a value from 0.9 to 1.2,
y has a value between 0.5 and 10 and
z has a value between 1.9 and 2.1,
by preparing an intimate mixture of finely divided, oxygen-containing compounds of the transition metals and one or more oxygen-containing lithium compounds and calcining the mixture in a reactor, which is characterised in that calcination takes place for at least some of the time under an absolute pressure of less than 0.5 bar absolute.
At least one of the lithium compounds preferably has a melting point of less than 600° C., in particular at least 90% of the lithium compounds used.
Calcination is preferably performed for at least some of the time under a partial vacuum corresponding to a pressure of 0.01 to 0.4 bar absolute, in particular at a pressure of 0.0 1 to 0.2 bar absolute.
Furthermore, it is also preferred that calcination be initially started at atmospheric pressure so that the molten lithium compound becomes supersaturated with dissolved gases resulting from the evolution of gases during reaction and still sub-stable bubble nuclei with diameters in the range of a few micrometers under atmospheric pressure are produced. This may take place, in industrial-scale batches, over a period of 2 to 12 hours, in the event that oxides are used as the transition metal compounds, or also over a longer period of time. The reactor is preferably evacuated, optionally also stepwise, only after this initial calcination stage under atmospheric pressure, so that, on the one hand, the volume of the bubble nuclei already present increases due to pressure reduction and, on the other hand, supersaturation of the molten material with dissolved gases and thus the diffusion pressure of the dissolved molecules in the direction of the gas bubbles is increased. Gas bubbles enlarged in this way, which have several times the volume of the suspended solid particles, come into contact with each other, coagulate and rise in the molten suspension until they are emitted into the reactor atmosphere at the surface of the susp

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