Method for preparation of LiMn.sub.2 O.sub.4 intercalation compounds and use thereof in secondary lithium batteries

Method is described for the low temperature preparation of the spinel LiMn.sub.2 O.sub.4 phase which is an intercalable compound of interest for use in lithium secondary batteries. This phase can be prepared in bulk or thick film form at temperatures less than 400.degree. C. using acetate precursors.

FIELD OF THE INVENTION 
This invention relates to a method for the preparation of fine powders 
and/or thick films of lithium containing ternary oxides. More 
specifically, the present invention relates to the low temperature 
synthesis of LiMn.sub.2 O.sub.4 and LiCoO.sub.2 which are intercalable 
compounds of interest for secondary batteries. 
Heretofore, the lithium containing ternary oxides have been prepared by 
mixing the carbonates and oxides of the constituent compounds and heating 
the mixture at temperatures within the range of 700.degree.-800.degree. C. 
Although the resultant compositions have proven satisfactory for most 
purposes, studies have revealed that the high temperatures employed in the 
synthesis thereof ofttimes adversely affect the electrochemical properties 
of the compositions. In light of the fact that the lithium-based 
intercalation compounds of LiMn.sub.2 O.sub.4 and LiCoO.sub.2 have sparked 
widespread interest for use in the next generation of rocking chair 
batteries, workers in the art have focused their attention upon the 
development of alternate techniques for obtaining these compositions. 
Specifically, new routes have been sought to attain a method yielding 
materials of controlled morphology and grain size to improve battery 
behavior. 
BACKGROUND OF THE INVENTION 
In accordance with the present invention, this end has been attained by a 
novel processing sequence wherein a weak acetate ligand in combination 
with a hydroxide solution with a balanced pH permits the formation of fine 
particles of a mixed hydroxide-acetate composition. More specifically, 
there is described herein a method for the synthesis of LiMn.sub.2 O.sub.4 
and LiCoO.sub.2 phases by a novel sol-gel process involving the 
condensation of oxide networks from solution precursors. Briefly, this 
involves hydrolyzing manganese or cobalt acetates or other carboxylates in 
an aqueous solution, the hydrolysis being promoted by the addition of the 
hydroxides of lithium and ammonium which control the pH of the solution. 
Hydrolysis is initiated by the addition of lithium hydroxide and completed 
by the use of a base that can be removed thermally. This base may be 
selected from among any organic base or ammonium hydroxide which is 
preferred for use herein. This low temperature process yields a gel-like 
product which may be used to prepare either bulk or thick films of 
LiMn.sub.2 O.sub.4 or LiCoO.sub.2 which evidence electrochemical 
properties suitable for use in rocking chain batteries.

DETAILED DESCRIPTION 
The initial step in the practice of the present invention involves 
preparing an acetate precursor for the ternary oxide. This end is effected 
by admixing, with rapid stirring, an acetate of manganese or cobalt and 
the hydroxides of lithium and ammonium in an amount sufficient to yield 
the stoichiometric phase of the ternary oxide. In this process, the 
ammonium hydroxide is employed primarily for the purpose of adjusting the 
pH to a value of approximately 7, the point at which a controlled 
precipitation occurs. The required amount of ammonium is defined by the 
sum of hydroxyl groups from lithium hydroxide and from ammonium hydroxide 
equal to the normality of the transition metal cation, Mn, or for Co in 
LiCoO.sub.2. Upon such mixing, a gelatinous precipitate is formed 
instantaneously. Studies of the resultant precipitates reveal that the 
manganese composition is stable for only a few hours in contrast with the 
cobalt composition which is stable for several weaks. Accordingly, the 
manganese must be protected from oxygen to avoid the formation of 
Mn.sup.3+ leading to the precipitation of Mn.sub.2 O.sub.3. 
Following, the gelatinous precipitate so formed is dried thermally at a 
temperature ranging from 60.degree. C.-150.degree. C., the specific 
temperature chosen being dependent upon the composition and desired use 
thereof. Thus, for example, heating the manganese composition at a 
temperature of 90.degree. C. permits formation of a viscous solution which 
can be deposited upon a suitable substrate by spin coating which permits 
subsequent synthesis of LiMn.sub.2 O.sub.4 thick films. Heating of the 
gelatinous precipitate at the higher temperature (150.degree. C.) results 
in the formation of a xerogel of small grain size. 
Then, the resultant xerogels of manganese and cobalt are heated to a 
temperature within the range of 200.degree.-500.degree. C. to remove the 
acetate. At 400.degree. C. the manganese composition becomes a pure 
LiMn.sub.2 O.sub.4 phase. In order to attain the cobalt composition of 
corresponding purity, heating should be continued to 500.degree. C. 
An exemplary embodiment of the practice of the present invention is set 
forth below. It will be appreciated by those skilled in the art that this 
embodiment is presented solely for purposes of exposition and is not to be 
construed as limiting. 
EXAMPLE 
Compositions selected for use included a 0.8 M/l solution of manganese 
acetate, lithium hydroxide (1 M/l) and ammonium hydroxide (3 M/l). The 
manganese and lithium solutions were employed in stoichiometric amounts to 
yield the required phase of LiMn.sub.2 O.sub.4. The ammonium hydroxide was 
employed in an amount sufficient to furnish 2 hydroxyl ions per metal ion. 
The hydroxides were quickly added to the manganese acetate solution with 
violent stirring, so resulting in the instantaneous formation of a 
gelatinous precipitate, the manganese solution being protected against 
oxygen to avoid formation of Mn.sup.+3. The precipitate was then dried by 
heating up to 150.degree. C. to yield a homogeneous xerogel in which the 
lithium and manganese ions were well mixed. Finally, the dried precipitate 
was annealed at a temperature within the range of 200.degree.-400.degree. 
C. to yield the acetate free LiMn.sub.2 O.sub.4 phase which comprised 
grains or crystallites ranging in size between 0.3 .mu.m and 1 .mu.m. The 
resulting LiMn.sub.2 O.sub.3 powders prepared at 300.degree. C. and 
400.degree. C. were then compared with similar powders prepared at 
temperatures of 500.degree.,600.degree., and 800.degree. C. and their 
intercalation properties assessed. This end was attained using swagelock 
test cells that were assembled in a helium dry-box. Approximately 20 mg of 
LiMn.sub.2 O.sub.4 powder was mixed with 10% carbon black, pressed into a 
pellet and used as the positive electrode with lithium as the negative 
electrode. Both electrodes were separated by a porous glass filter soaked 
in an electrolyte prepared by dissolving 1 M/l LiClO.sub.4 and 1M and 
12-crown-4 ether in propylene carbonate. Cycling data was then obtained 
and plotted in graphical form. 
With reference now to FIG. 1, there is shown a graphical representation on 
coordinates of Li.sub.x Mn.sub.2 O.sub.4 against voltage in volts showing 
the cycling data over a range of potential from 4.5-3.5 volts for the 
foregoing compositions annealed at temperatures from 
300.degree.-800.degree. C. at a current density of 600 .mu.A/cm.sup.2. The 
assembled cell is first charged to remove the Li ions within Li.sub.x 
Mn.sub.2 O.sub.4, so that the cathode then becomes the open structure 
spinel .lambda.-Mn.sub.2 O.sub.4. 
The assembled cells containing Li.sub.x Mn.sub.2 O.sub.4 powders were 
automatically tested, equivalently charged and discharged up to four 
cycles at a constant current while potential was monitored as a function 
of time. A review of FIG. 1 reveals that the cycling data was in the range 
of potential of 4.5-3.5 volts which corresponds to the first lithium 
intercalation plateau for intercalation of 1 Li into .lambda.-Mn.sub.2 
O.sub.4, and over the range of potential 3.5-2.2 volts (shown on the same 
coordinates in FIG. 2) which corresponds to the second lithium 
intercalation plateau into LiMn.sub.2 O.sub.4 to give Li.sub.2 Mn.sub.2 
O.sub.4. In both FIGURES, it will be noted that the capacity of the cells 
and their cycling behavior are comparable to or better than similar 
properties for the samples prepared at the higher temperatures. 
Accordingly, the data reveals that the low temperature process, which 
yields finer size particles of LiMn.sub.2 O.sub.4, does not affect the 
capacity of the cells and enhances their cycling behavior. 
With reference now to FIG. 3, there is shown a graphical representation on 
coordinates of Li.sub.x Mn.sub.2 O.sub.4 content against voltage in volts 
showing the cycling characteristics between 4.5 and 2 volts, covering both 
plateaus. Once again, it will be noted that the charge/discharge curves 
are similar to those previously reported for cells using the LiMn.sub.2 
O.sub.4 phase prepared at 400.degree. C. as the positive electrode. 
A still further advantage of the described solution technique over the 
prior art solid state reactions is that thick films are attainable. With 
reference now to FIG. 4, there is shown a graphical representation on 
coordinates of Li.sub.x Mn.sub.2 O.sub.4 content (thick film) against 
voltage in volts showing cycling behavior over the range of 4.5-3.0 volts 
at 400 .mu.A/cm.sup.2. The electrode was prepared by forming a 10 .mu.m 
thick film of Li.sub.x Mn.sub.2 O.sub.4 by dipping a stainless steel 
substrate into a viscous acetate aqueous solution prepared as described 
above and then fired for 16 hours at 600.degree. C. The cycling data are 
similar to that shown for the bulk material. 
It will be understood by those skilled in-the-art that the described 
technique can be used with equivalent efficacy in the preparation of 
LiCoO.sub.2. However, the initial Co-acetate solution will be of a 
different concentration to attain the required composition which may be 
prepared in bulk or thick film form. It has also been found that an 
annealing temperature of 500.degree. C. is generally required to obtain 
the LiCoO.sub.2 phase. Lastly, it has also been found that it is feasible, 
using acetate precursors, to prepare Na.sub.x MnO.sub.2 or Na.sub.x 
CoO.sub.2 in accordance with the described process with NaOH being 
substituted for LiOH.