Electrode materials for rechargeable electrochemical cells and method of making same

A method for preparing a lithiated transition metal oxide electrochemical charge storage material for use in an electrochemical cell. The cell (10) includes a cathode (20), an anode (30) and an electrolyte (40) disposed therebetween. The method involves the preparation of the lithiated, transition metal oxide material in an inert environment. The materials are characterized by improved electrochemical performance, and an identifiable x-ray diffraction matter.

TECHNICAL FIELD 
This invention relates in general to secondary lithium electrochemical 
cells, and more particularly to secondary lithium batteries having high 
capacity positive electrodes. 
BACKGROUND OF THE INVENTION 
Secondary lithium electrochemical cells, and particularly lithium 
batteries, using an intercalation compound as the positive electrode, or 
cathode of the battery have been studied intensely during the past decade. 
Heretofore, the cathode material used in these batteries was typically a 
lithiated cobalt oxide, nickel oxide, or manganese oxide. Lithiated 
transition metal oxide batteries are being studied as an alternative to 
current nickel-cadmium and nickel-metal hydride cells because they possess 
several attractive characteristics, e.g., high cell voltage, long shelf 
life, a wide operating temperature range, and use of non-toxic materials. 
The earliest reports of LiNiO.sub.2 and LiCoO.sub.2 as the positive 
electrode materials in rechargeable lithium batteries occurred more than a 
decade ago and are shown in, for example, U.S. Pat. Nos. 4,302,518 and 
4,357,215 to Goodenough, et al. 
These materials have been intensively investigated, and one of them, 
LiCoO.sub.2 is currently used in commercial lithium ion batteries. 
Numerous patents have been issued for different improvements in these 
materials as the positive electrode for lithium cells. An example of a 
recent improvement is illustrated in U.S. Pat. No. 5,180,547 to Von Sacken 
for "HYDRIDES OF LITHIATED NICKEL DIOXIDE AND SECONDARY CELLS PREED 
THEREFROM". The Von Sacken reference teaches fabricating the hydroxides of 
lithium nickel dioxide fabricated in an atmosphere including a partial 
pressure of water vapor greater than about 2 torr. 
Regardless of the particular material used in such cells, each material is 
synthesized in an oxidizing environment such as O.sub.2 or air at 
temperatures higher than about 700.degree. C. using nickel or cobalt and 
lithium containing salts. For example, a publication to Ohzuku, et al 
published in the Journal of the Electrochemical Society, Vol. 140, No. 7, 
Jul. 19, 1993, illustrates at Table 1 thereof, the typical processing 
methods for preparing LiNiO.sub.2. Each of the methods illustrated in the 
Ohzuku, et al reference show preparing the material in an oxidizing 
environment of either oxygen or air. 
Charge and discharge of the materials fabricated according to these 
processes proceeds by a charge mechanism of de-intercalation and 
intercalation of lithium ions from and into these materials. The materials 
synthesized by the prior art methods have a reversible capacity of about 
135 mAh/g. In other words, about 0.5 lithium ions can be reversibly 
deintercalated and intercalated from and into each mole of LiNiO.sub.2 or 
LiCoO.sub.2. 
A significant amount of the capacity of these materials resides at 
potentials higher than about 4.2 volts versus lithium. If more than 0.5 
lithium ions is removed from each of either a LiNiO.sub.2 or LiCoO.sub.2 
electrode, potentials higher than 4.2 volts versus lithium are required 
causing decomposition of most electrolytes. Further, removal of more than 
0.5 lithium ions will result in irreversible changes in the structure of 
these materials, causing a decrease in their capacity during charge and 
discharge cycles. This result was reported in a publication by Xie, et al 
prepared at the Electrochemical Society Fall Meeting, 1994, Extended 
Abstract No. 102, Miami, October 1994. 
The reversible capacities of the most commonly used materials synthesized 
in O.sub.2 and air atmospheres are very sensitive to residual inactive 
lithium salts such as Li.sub.2 O, LiOH, and LiCoO.sub.3, each of which 
result from the synthesis process. However, to make stoichiometric 
LiNiO.sub.2, which is perceived to have the best performance of any of the 
prior art materials, excess lithium salt is normally used in precursor 
materials. As a result, the presence of residual lithium salt is 
inevitable in the final product fabricated according to prior art methods. 
In addition to causing a decrease in the capacity of LiNiO.sub.2, the 
presence of residual lithium salts often causes gas evolution such as 
CO.sub.2, H.sub.2 and O.sub.2 at the positive electrode during charging. 
Further, it is normally observed that the initial charge efficiency is 
much lower for LiNiO.sub.2 (i.e., less than about 80%) than that for 
LiCoO.sub.2 when the two materials are made in a similar fashion. In order 
to reduce these problems, manufacturers typically try to minimize or 
eliminate residual lithium salts from the product. 
Accordingly, there exists a need to develop a new cathode material for 
rechargeable electrochemical systems, which is fabricated of materials 
which are relatively environmentally friendly, may be fabricated at 
relatively low temperatures and which demonstrate performance 
characteristics superior to those of the prior art. Specifically, such 
materials should have: (1) high capacity greater than 170 mAh/g at 
potentials between 3.5 and 4.2 volts; (2) an easy synthesis process which 
can be highly controlled; (3) insensitivity to residual lithium salts; (4) 
high initial charge efficiency; and (5) high reversible charge/discharge 
reactions so as to provide a material having good cycle life.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the specification concludes with claims defining the features of the 
invention that are regarded as novel, it is believed that the invention 
will be better understood from a consideration of the following 
description in conjunction with the drawing figures, in which like 
reference numerals are carried forward. 
Referring now to FIG. 1, there is illustrated therein a schematic 
representation of an electrochemical cell 10 including a lithiated 
transition metal oxide electrode in accordance with the instant invention. 
The electrochemical cell includes a positive electrode 20 and a negative 
electrode 30 and has an electrolyte 40 disposed between said electrodes. 
The cell 10 further includes a positive electrode 20 fabricated of a 
transition metal oxide such as a nickel oxide or a cobalt oxide 
electrochemical charge storage material which is described in greater 
detail hereinbelow. The negative electrode 30 or anode of the cell 10 may 
be fabricated of materials selected from the group of materials consisting 
of, but not limited to, Li metal, Li alloying metals such as Al, Sn, Bi, 
carbon (including graphite and petroleum coke), low voltage Li 
intercalation compounds, such as TiS.sub.2, V.sub.6 O.sub.13, MoS.sub.2, 
and combinations thereof. The electrolyte 40 disposed between the 
electrodes can be any of the electrolytes known in the art, including, for 
example, LiClO.sub.4 in propylene carbonate or polyethylene oxide, 
impregnated with a lithiated salt. The electrolyte may be either a solid, 
gel, or aqueous electrolyte. The electrolyte 40 may also act as a 
separator between the positive and negative electrodes. 
In accordance with the instant invention, there is provided a method for 
fabricating a lithiated transition metal oxide material which is capable 
of storing and discharging electrical charge. The material disclosed 
herein is therefore useful as, for example, the cathode in lithium 
rechargeable batteries. The stabilized material has the formula 
LiTMyO.sub.2.Li.sub.2 O where TM is a transition metal selected from the 
group of Mn, Ni, Co, and combinations thereof; 0.05.ltoreq.x.ltoreq.1.0; 
y.gtoreq.1.0; and where Li.sub.2 O may exist as a separate phase. The 
valence state of the TM may be less than the 3+ state. It is to be noted 
that Li.sub.2 O may be tolerated in the instant material, and does not 
cause the deleterious effects observed in the prior art lithiated 
transition metal oxide cathode materials. The material may further include 
one or more modifiers selected from the group of Ti, Bi, Fe, Zn, Cr, and 
combinations thereof. 
Referring now to FIG. 2, there is illustrated therein a flowchart 50 
describing the steps for preparing a lithiated transition metal oxide 
material in accordance with the instant invention. The first step in 
preparing the lithiated transition metal oxide material is illustrated in 
Box 52 of Flowchart 50. Box 52 recites the step of providing a transition 
metal precursor material. Precursor materials which may be used include, 
for example, first transition metal compounds such as TM(OH).sub.2, TMO, 
TM(NO.sub.3).sub.2, and TM(CO.sub.2) where TM is a first transition metal, 
such as Co, Ni, or Mn. Specific examples of materials include, 
Ni(OH).sub.2, Ni(NO.sub.3).sub.2.6H.sub.2 O, NiO, 
Co(OH).sub.2.Co(NO.sub.3).sub.2.6H.sub.2 O, CoO, MnO, Mn(OH).sub.2, 
Mn(NO.sub.3).sub.2.6H.sub.2 O, Mn.sub.2 O.sub.3, and combinations thereof. 
In one preferred embodiment, the transition metal precursor material is 
Ni(OH).sub.2. In a second preferred embodiment, the transition metal 
precursor material is Co(OH).sub.2. 
The second step illustrated in Flowchart 50 is shown in Box 54 and 
comprises the step of providing a lithium containing compound. Examples of 
lithium-containing compounds include, for example, LiNO.sub.3, LiOH, 
Li.sub.2 O, Li hydrocarbonate salts and combinations thereof. It is to be 
understood that in selecting the first transition metal precursor material 
and the lithium containing material, at least one of the them must include 
an oxidizing group, such as NO.sub.3- to provide an oxidizing agent for 
the reaction. In one preferred embodiment, the transition metal precursor 
material is TM(OH).sub.2, such as Ni(OH).sub.2, and the lithium containing 
material is LiNO.sub.3, providing the required NO.sub.3- oxidizing agent. 
Thus, the reaction for this preferred combination is as follows: 
##STR1## 
This combination is preferred because the transition metal hydroxide has a 
layered structure, and both Ni(OH).sub.2 +LiNO.sub.3 can mix 
homogeneously, as LiNO.sub.3 becomes liquids at temperatures above 
260.degree. C. Further, Ni(OH).sub.2 has a crystalline structure similar 
to that of LiNiO.sub.2 (a layered structure) and does not go through a NiO 
phase before forming LiNiO.sub.2. The transition metal precursor material 
and the lithium-containing compound are mixed together via conventional 
mixing techniques such as, for example, ball milling. This step is 
illustrated in Box 56 of Flowchart 50. 
Thereafter, the materials are reacted, as by heating as illustrated in Box 
58 of Flowchart 50. The conditions and environment in which the heating 
takes place is critical to forming a material having high capacity as 
illustrated herein. More particularly, the mixed materials are heated in 
an inert environment. By an inert environment, it is meant that the 
principle components of the atmosphere in which the heating takes place 
are not reactive with the materials therein. Accordingly, the heating 
illustrated in Step 58 of Flowchart 50 is carried out in a helium, 
nitrogen or argon environment. In one preferred embodiment, the heating 
generates reaction conditions, and takes place in a N.sub.2 atmosphere, at 
temperatures between about 500.degree. C.-800.degree. C., and preferably 
between 600.degree. C.-700.degree. C. Heating continues for at least four 
and preferably at least ten hours. This is a substantial departure from 
the prior art which uniformly teaches the use of an oxidizing element to 
facilitate the activity of the oxidizing agent. Indeed, the prior art 
teaches away from any nonoxidizing environment. 
There is an optimal reaction time for each temperature and for different 
ratios of Ni.sup.2+ to Li.sup.+ in the starting materials. The optimal 
reaction time can be determined by examining x-ray diffraction patterns of 
resulting materials. Specifically, the novel material resulting from the 
process described in FIG. 2 can be identified by its unique powder x-ray 
diffraction ("XRD") pattern. Specifically, the XRD pattern for a high 
capacity LiNiO.sub.2.0.7Li.sub.2 O material in accordance with the instant 
invention is shown in FIG. 3. The XRD pattern has several peaks 
illustrated therein, though only two, identified as 70 and 72, are 
examined herein. 
Peak 70 corresponds to an x-ray diffraction intensity at the degrees 
2.theta. angle of approximately 18.7.degree., using CuK.alpha..sub.1 as 
the x-ray source. Peak 72 corresponds to the x-ray diffraction intensity 
at the degrees 2.theta. of approximately 44.2.degree., again using a 
CuK.alpha..sub.1 x-ray source. XRD patterns of prior art materials 
demonstrate a ratio between these peaks of no more than 1.40:1.00, and 
typically about 1.1:1.0. Conversely, the signature ratio of the instant 
high capacity material is at least 1.60:1.0 and may be considerably 
higher. This ratio is demonstrated in FIG. 3. 
Accordingly, and contrary to the state of the art methods disclosed in the 
prior art, the synthesis of LiNiO.sub.2 or LiCoO.sub.2 can be accomplished 
through melt-solid reaction using an NO.sub.-3 containing salt as the 
oxidizing agent in an inert environment such as helium or nitrogen at 
temperatures below 700.degree. C. Materials made in an inert environment 
have higher reversible capacity and charge efficiency than those made by 
the conventional method, i.e., in air or oxygen. Further, the 
reversibility of the intercalation/deintercalation of these materials is 
better, as will be demonstrated hereinbelow. 
The materials fabricated in accordance with the method described herein, 
demonstrates distinct differences in defined structures of XRD patterns, 
as described above. In addition to the differences illustrated in FIG. 3, 
materials fabricated according to the instant invention have a 
significantly different physical appearance as compared with conventional 
materials. Materials fabricated according to the instant invention have a 
deep black color, such as carbon black, and have a "slippery" consistency 
similar to that of graphite powder. Conversely, materials according to the 
prior art are gray in color and do not possess the "slippery" 
graphite-like feeling. 
The invention may be better understood from the examples presented below: 
EXAMPLES 
Examples I 
A lithiated transition metal oxide material was prepared in accordance with 
the instant invention. Ni(OH).sub.2 and LiNO.sub.3 were provided in the 
molar ratio of 1.0:2.5, and mixed thoroughly in a ball mixer and pressed 
into a pellet. Thereafter the pellet was heated to 300.degree. C. in 
helium for four hours, heated to 600.degree. C. for 20 hours in helium, 
with two intermittent grinding and heating steps. The weight of the 
resulting product was consistent with LiNiO.sub.2.0.75Li.sub.2 O. An XRD 
analysis of the material was conducted on the material and is illustrated 
in FIG. 3 described hereinabove. The XRD pattern of the material indicates 
that it contained LiNiO.sub.2 and Li.sub.2 O only. 
The electrochemical behavior of the material fabricated according to this 
example was evaluated in a test cell with 1M LiPF.sub.6 in a solution of 
50% ethylene carbonate and 50% dimethylethylene as the electrolyte and a 
lithium metal foil as the negative electrode (anode). The charge and 
discharge profiles of the cell voltage of the cell fabricated accordingly 
to this Example I is illustrated in FIG. 4 hereof. More specifically, it 
may be appreciated from FIG. 4, that nearly one lithium ion may be removed 
from each LiNiO.sub.2 on charging at potentials below 4.2 volts and that 
approximately 0.9 lithium ion can be intercalated into the material for 
each nickel atom on discharge at a potential higher than about 3.0 volts. 
It should be pointed out here that this material has the following 
characteristics that are different from those synthesized by a prior art 
method: 1. The peak ratio of the XRD intensity at the 2.theta. angle of 
about 18.7.degree. to that at 44.3.degree. is greater than 1.6 as 
illustrated in FIG. 3, compared to less than 1.4 for those by a prior art 
method; 2. the existence of Li.sub.2 O does not affect the charge and 
discharge capacity; and 3. there is a flat plateau near 4.2 on charge and 
a corresponding one on discharge for this material as illustrated at 
points 80 and 82 respectively in FIG. 4. No plateau is observed on the 
charge curve at this potential for materials synthesized by a prior art 
method. 
Referring now to FIG. 5, there is illustrated therein the discharge 
capacity (line 84) and the charge efficiency (line 86) as a function of 
cycle life for a coin type cell using a lithiated nickel oxide material 
fabricated in accordance with the instant invention, as described in this 
Example I. The lithiated nickel oxide served as the positive electrode 
material and commercially available graphite was used as the negative 
material. The separator used in the cell was porous polypropylene and is 
commercially available under the name Celgard 2500.RTM.. The electrolyte 
used in the cell was 1M LiPF.sub.6 in a mixture of ethylene carbonate, 
diethylene carbonate, and propylene carbonate. The cell was charged and 
discharged at a rate of about C/3. The mass ratio of the positive 
electrode material to the negative electrode material was approximately 
2:1. As shown in FIG. 5, the capacity of the cell does not fade with 
increasing cycle number. 
Example II 
Ni(OH).sub.2 and LiNO.sub.3 were provided in the molar ratio of 1:1.05 and 
ground and mixed in a ball mill. The mixture was heated to 300.degree. C. 
in air for 8 hours and then at 600.degree. C. in air for 40 hours. The 
resulting product was ground and examined by x-ray diffraction. The XRD 
patterns indicated that the material consisted of Li.sub.2 Ni.sub.8 
O.sub.10 and LiNO.sub.3, and demonstrated poor electrochemical properties. 
However, a similar mixture was converted completely into high capacity 
LiNiO.sub.2.Li.sub.2 O having a capacity greater than .about.170 mAh/g 
within 18 hours when calcined in a helium environment at temperatures of 
600.degree. C. This example indicates that partial oxidation of Ni.sup.2+, 
by O.sub.2 in air slows decomposition of LiNO.sub.3 as in the prior art 
method. A high capacity LiNiO.sub.2 can be synthesized at a low 
temperature such as 600.degree. C. in an inert environment, but cannot be 
made in air or O.sub.2 at the same temperature. 
Example III 
Co(OH).sub.2 and LiNO.sub.3 were mixed in the molar ratio of 1:2.5 and 
ground and mixed in a ball mill. The mixture was heated to 300.degree. C. 
in helium for 8 hours and then at 600.degree. C. in helium for 20 hours. 
The resulting product was ground and examined using x-ray diffraction. The 
XRD patterns indicate that the material contained LiCoO.sub.2 and Li.sub.2 
O. The material also showed a capacity in excess of 140 mAh/g. 
While the preferred embodiments of the invention have been illustrated and 
described, it will be clear that the invention is not so limited. Numerous 
modifications, changes, variations, substitutions and equivalents will 
occur to those skilled in the art without departing from the spirit and 
scope of the present invention as defined by the appended claims.