Synthesis of charged Li.sub.x CoO.sub.2 (0<.times.<1) for primary and secondary batteries

A method for producing stable pre-charged Li.sub.x CoO.sub.2 as the cathode active metal in primary or secondary active metal non-aqueous cells and cells using such material are disclosed.

1. Field of the Invention 
The present invention is directed generally to the field of high energy, 
non-aqueous electrochemical cells and, more particularly, to improvements 
in such cells employing Li.sub.x CoO.sub.2 cathode material which enable 
the cathode material to be synthesized in a pre-charged state prior to 
incorporation in the cell. 
2. Related Art 
Non-aqueous, active metal cells have become well known for achieving very 
high energy densities or energy to weight ratios, i.e., higher than was 
previously known with other types of electrochemical cells. Active metal 
cells typically consist of a light, strongly reducing anode, normally of 
an alkali metal such as lithium (an aprotic, non-aqueous solvent into 
which an appropriate quantity of the salt of the anode metal has been 
dissolved to form a conductive solution, and an oxidizing agent as the 
cathode material. Such cells can be in the form of primary or secondary 
(rechargeable) cells. 
It is further known to employ the material Li.sub.x CoO.sub.2 (0&lt;x&lt;1) as 
the active cathode material of such cells. For example, its use is 
disclosed in U.S. Pat. No. 4,497,726 and further discussed in Mizushima, 
K. et al, "Li.sub.x CoO.sub.2 (0&lt;x&lt;1): A New Cathode Material for 
Batteries of High Energy Density," Mat. Res. Bull., Vol. 15, 783 (1980). A 
lithium non-aqueous secondary electrochemical cell having an ester-based 
organic electrolyte solution and a cathode active material comprising 
Li.sub.x CoO.sub.2 (0&lt;X&lt;1) is illustrated and described in U.S. Pat. No. 
4,804,596 to Walter B. Ebner and Hsiu-Ping W. Lin (an inventor in the 
present application) which is also assigned to the same assignee as the 
present application. That reference describes the use of Li.sub.x 
CoO.sub.2 as the active cathode material in a cell in combination with an 
ester-based electrolyte solution that can withstand the high operating and 
charging potentials characteristic of that system. The Li.sub.x CoO.sub.2 
cathode material in that system, however, must be incorporated in the 
discharged state and thereafter charged. Furthermore, because of corrosion 
problems in stainless steel, an aluminum grid is required to withstand the 
initial charging voltage. 
Lithium-cobalt oxide (LiCoO.sub.2) and lithium-cobalt-nickel oxides 
(LiCo.sub.1-y Ni.sub.y O.sub.2) (0.ltoreq.y.ltoreq.1) are described for 
use as electrodes for rechargeable lithium cells by R. J. Gummow and M. M. 
Thackeray in "Characterization of LT-Li.sub.x Co.sub.1-y Ni.sub.y O.sub.2 
Electrodes for Rechargeable Lithium Cells", J. Electrochem. Soc., Vol. 
140, No. 12, December (1993). They describe the use of acid leaching to 
improve the recycling properties of certain materials. Data supplied in 
the reference for the charge/discharge profiles of acid leached LT (Low 
Temperature) LiCoO.sub.2, however, shows achievement of only about 63 
mAh/g for the first discharge and this degrades quickly to &lt;20 mAh/g in 
only four cycles. Certain Ni doped Li/LT-LiCoNiO.sub.2 cells assembled in 
a charged state were found to be significantly more cycle tolerant. 
However, success was limited to Ni doped materials. 
Thus, Li.sub.x CoO.sub.2 heretofore has been available for incorporation as 
a successful cathode material only in a fully discharged state because 
Li.sub.x CoO.sub.2 as it is known to exist in the charged state is not 
stable with respect to elevated temperatures normally required in the 
manufacturing environment. The batteries have, therefore, been assembled 
in the discharged state and charged prior to first use. The charging 
process has certain drawbacks. It results in the plating of an amount of 
lithium from the cathode onto the anode, and batteries have had to be 
designed to accommodate the extra lithium plated out of the cathodes 
during the initial charging after assembly; otherwise, internal shorting 
of the battery could be a problem. Also, the high voltage required to the 
initial charging of the battery has required the cathode collector to be 
made from aluminum rather than the preferred material, stainless steel. 
Other approaches have been tried to improve the cycle efficiency of 
Li.sub.x CoO.sub.2. Electrochemical titration has been used on 
pre-fabricated cathodes to obtain charged Li.sub.x CoO.sub.2 material. 
However, this process has been used with limited success as it produces 
only limited quantities of charged material and the final products have to 
be determined by the pre-fabricated shapes and compositions. 
Accordingly, it is a primary object of the present invention to provide a 
synthesis for charged Li.sub.x CoO.sub.2 (0&lt;X&lt;1) suitable for use as the 
cathode active material in primary and secondary battery applications. 
It is a further object of the present invention to provide a synthesis for 
charged Li.sub.x CoO.sub.2 (0&lt;X&lt;1) for primary and secondary cell 
applications in which the charged material is in a stable powdered form 
which can than be shaped and incorporated in any composition of cathode 
desired. 
Other objects and advantages with respect to the present invention will 
occur to those skilled in the art through familiarity with the 
specification and claims herein. 
SUMMARY OF THE INVENTION 
The present invention provides a new cathode process in which charged 
Li.sub.x CoO.sub.2 cathodes can be manufactured in an efficient and 
cost-effective manner. The product is superior to acid-treated 
embodiments, delivering twice the capacity of those materials with 
improved cycling efficiency. The positive current collector for the 
cathode is not limited to aluminum. It may be stainless steel. The present 
invention provides a process to manufacture charged Li.sub.x CoO.sub.2, 
preferably where 0.ltoreq.x.ltoreq.0.5, in a powdered form for use as a 
raw material in the subsequent manufacture of cathodes. The powdered form 
can be combined in any desired cathode composition and worked into any 
configuration or shape. 
In the preferred process, pure LiCoO.sub.2 commercially obtainable from FMC 
Corporation, for example, is used as the starting material. A small amount 
of solvent, for example, methyl formate (MF) is added to the LiCoO.sub.2 
powder to wet the powder and to form a wet slurry or paste. The material 
is then formed as a layer on a pre-cut metal grid, preferably of aluminum, 
and suitably provided with an electrical lead, and the surface thereafter 
smoothed. The pasted material is then sealed inside microporous separators 
which may be a polyethylene envelope which is itself thereafter sandwiched 
between two sections of lithium anode of approximately the same dimensions 
also provided with electrical connections. The three-plate stack which 
itself forms a large lithium cell is then confined in an alluminated 
trilaminated envelope with anode and cathode leads protruding out from the 
envelope. Next, electrolyte is injected into the bag and the large cell 
charged. After charging the desired amount, the cell is opened in a dry 
room and the cathode envelope opened and the material rinsed with solvent 
and vacuum dried. 
The charged Li.sub.x CoO.sub.2 powder is now ready to be removed from the 
original aluminum charging grid and utilized in a cathode mixture in any 
manner desired. The value of x can be controlled by the amount of 
coulombic titration, and is preferably less than about 0.5. 
The charged material is still in powdered form and can be used as raw 
cathode material for any batteries. The material is normally mixed with a 
conductive diluent such as carbon or graphite in a binder such as 
polytetrafluoroethylene (PTFE) and the cells can be used for primary or 
secondary applications without initial charging.

DETAILED DESCRIPTION 
The present invention enables the advantages associated with the unusually 
high energy density of Li.sub.x CoO.sub.2 cathode material to be 
incorporated in a pre-charged state by subjecting it to a pre-charging 
process prior to incorporation in the cathode mixture. The process makes 
use of Li.sub.x CoO.sub.2 in the uncharged state and transforms it into a 
pre-charged Li.sub.x CoO.sub.2 (0&lt;x&lt;1) powdered raw cathode material for 
incorporation in a cathode mix suitable for any battery, primary, 
secondary, etc. in which such cathode material is desired. As a powder, 
the material can be worked into any shape or mixture combination required. 
A system for pre-charging the LiCoO.sub.2 powder for use as the cathode 
active material according to the invention is depicted in FIG. 5. The 
charging system generally takes the form of a rather large lithium cell 
shown generally at 10 and includes a metallic retaining shell having a 
retaining rim as shown at 12 which may be stainless steel and which 
further supports a metallized plastic bag 14, preferably a trilaminated 
envelope having an aluminized inner surface (not shown). Protruding anode 
and cathode leads shown respectively at 16 and 18 are designed for 
external connection to a source of charging voltage. The LiCoO.sub.2 20 is 
pressed onto both sides of the metallic grid, preferably aluminum, 22. 
Grid 22 which with a pair of semipermeable microporous polymer separators 
24 is sandwiched between a pair of lithium anodes 26 with metallic, 
preferably nickel, current collectors 28. This forms what is known as a 
three-plate stack cell with the cathode material sandwiched between a pair 
of large area anodes. This may be covered by a layer of material such as 
Tefzel 30 within the trilaminated metallized envelope 14. 
In the preferred embodiment of the process, finely divided LiCoO.sub.2 
powder, approximately -325 mesh, obtained in a substantially pure form 
from FMC Corporation, is combined with a small amount of solvent, such as 
methyl formate (MF), to form a heavy slurry or paste. The paste is then 
spread onto both sides of the pre-cut metallic grid 22 which is of a metal 
which can withstand the required charging voltage without corroding, such 
as aluminum. The thickness of the paste is typically 0.25" and that of the 
grid is 0.01". The surface of the paste may be made generally smooth using 
a stainless steel plate, or the like. The pasted material is then sealed 
inside separators 24 which may each be a layer of microporous polymer 
material, normally a polyethylene envelope, represented by separators 
which itself is thereafter sandwiched between the pair of lithium anodes 
26 with nickel grids 28. This sandwich or three-plate stack is thereafter 
confined inside the metallized (aluminized) trilaminated envelope (which 
may be polyethylene terephthalate). An electrolyte material is then 
injected into the bag to activate the cell. The electrolyte is preferably 
a 2 molar double salt methyl formate (LiAsF.sub.6 +LiBF.sub.y) system but 
any suitable material including methyl acetate may be employed. The cell 
is typically charged at a potential of 4.3 volts. After charging, one 
trilaminated envelope was opened in a dry room and the internal cathode 
envelope cut open and the material rinsed with solvent and vacuum dried. 
The result was a charged Li.sub.x CoO.sub.2 powder in ready-to-use form in 
which the value of x can be controlled by the amount of coulombic 
titration and is preferably less than 0.5. 
The typical grid size used experimentally has been about 9 cm by 19 cm 
which can process about 100 grams of LiCoO.sub.2. This is enough material 
for about 25 size "AA" rechargeable Li.sub.0.5 CoO.sub.2 cells. The system 
works well for fairly high production rates. The normal charging voltage 
is about 4.3V and about 5 mA of current. The metallized trilaminate 
envelope may be any compatible gas-tight system which is easy to apply and 
remove in the process. 
As can be seen from the above, the construction of the cell charging system 
is simple and inexpensive and the charged material requires no special 
handling. Desired quantities of conductive diluent, such as carbon or 
graphite, and binder, such as polytetrafluoroethylene (PTFE), can be added 
and the material processed into the finished cathode. Such cathodes, of 
course, can be used for primary or secondary applications without the 
initial charging LiCoO.sub.2 requires. 
The discharge performance of pre-charged Li.sub.0.5 CoO.sub.2 cathodes is 
depicted in FIG. 1 for the discharge rates of 1 and 5 mA/cm.sup.2. In 
addition, the delivered capacity, which was 113 mAh/g based on total 
cathode weight, is quite comparable to the typical delivered capacity of 
approximately 120 mAh/g LiCoO.sub.2 after charging a cell manufactured in 
the discharge state. FIG. 2 confirms that the material is cyclable and so 
suitable for use in secondary cells. 
The materials as processed in accordance with the present invention 
represent a drastic improvement over the acid leached materials previously 
known and are comparable to materials utilized in the cells previously 
assembled in the uncharged state. In addition, the positive current 
collector material for the cathode need not be limited to aluminum in the 
case of the pre-charged material and can be made from stainless steel or 
other materials. It had previously been found that stainless steel 
corroded because of the high required charging voltages in cells built 
with the discharged LiCoO.sub.2 cathodes. FIGS. 3 and 4 compare cells 
utilizing aluminum and stainless steel positive current collectors. FIG. 3 
depicts discharge performance of pre-charged Li.sub.0.5 CoO.sub.2 at 1 
mA/cm.sup.2 during four discharge cycles. The Figure shows that results 
are comparable using either material as the positive current collector. 
FIG. 4 makes a similar comparison for secondary cell cycling and indicates 
that either material would also be satisfactory in this respect. 
This invention has been described herein in considerable detail in order to 
comply with the Patent Statutes and to provide those skilled in the art 
with the information needed to apply the novel principles and to construct 
and use embodiments of the example as required. However, it is to be 
understood that the invention can be carried out by specifically different 
devices and that various modifications can be accomplished without 
departing from the scope of the invention itself.