Materials for use as cathodes in lithium electrochemical cells

DETAILED DESCRIPTION This invention relates to a novel compound of molecular formula Li.
sub.
2 Cr.
sub.
x Mn.
sub.
2-x O.
sub.
4, wherein 0<x<2, and to the use of this compound as a cathode material in secondary lithium ion cells.
The impetus for this invention was the recent, great increase in demand for batteries having high energy density and low weight, such as had already been achieved with metallic lithium systems, but which are also rechargeable.
However, lithium electrodes do not meet this requirement over extended periods of time because even in an aprotic organic solvent the cycling stability is severely limited, and problems with cell safety arise.
DESCRIPTION OF THE PRIOR ART One approach to improving the reversibility of metallic lithium electrodes involves the use of lithium intercalation compounds.
The intercalation compound serves as a host structure for lithium ions which are either stored or released depending on the polarity of an externally applied potential.
During discharge, the electromotive force reverses the forced intercalation or deintercalation thereby producing current.
Previous reports and patents on cathodes for lithium ion cells have proposed using various mixed oxides of lithium, such as LiCoO.
sub.
2, LiNiO.
sub.
2 and LiMn.
sub.
2 O.
sub.
4 as the active material.
Specifically, LiCrO.
sub.
2 is too poor an electronic conductor to be useful in an electrochemical cell.
Doping with a small amount of Mn (10% or perhaps less) dramatically improves the material's electronic conductivity as evidenced by its color.
LiCrO.
sub.
2 has an olive green color while Li.
sub.
2 Cr.
sub.
x Mn.
sub.
2-x O.
sub.
4 is dark brown to black in color for 0.
1.
ltoreq.
x<2.
0.
Both LiMn.
sub.
2 O.
sub.
4 and .
lambda.
-Li.
sub.
2 Mn.
sub.
2 O.
sub.
4 have been used as cathodes in lithium ion cells.
.
lambda.
-Li.
sub.
2 Mn.
sub.
2 O.
sub.
4 has twice the nominal capacity of LiMn.
sub.
2 O.
sub.
4 but it is reported to be hydroscopic and metastable [A.
Mosbah, A.
Verbaere and M.
Tournoux, Mat.
Res.
Bull.
, 18 1375 (1983)].