Chemical extraction of lithium from LiCoO2 has been investigated with
various oxidizing agents - Cl-2, Br-2, and I-2 - and with dilute sulfu
ric acid. A considerable amount of lithium could be extracted with bot
h chlorine and acid to give a final lithium content(1 - x) approximate
to 0.3 in Li1-xCoO2. The stronger oxidizing power of Cl-2, and the re
lative instability associated with the Li-extracted samples lead to th
e dissolution of a considerable amount of the sample during chlorine o
xidation. A deeper lithium extraction with chlorine also leads to the
occurrence of oxygen vacancies in Li1-xCoO2-delta. Lithium extraction
with acid proceeds predominantly by a disproportionation of Co3+ to Co
2+ and Co4+ analogous to that in the spinel LiMn2O4 with a small degre
e of ion exchange of Li+ by H+. However, the results of both chlorine
oxidation and acid treatment are strongly influenced by the nature of
the initial material. An Li/Co ratio < 1 and/or a disorder between Li
and Co in the initial Li1-xCo1-zO2 result in a competition of Co extra
ction from Li planes with Li extraction as evidenced by the Li/Co rati
o in the filtrate as well as the changes in the relative intensities o
f the (003) and (104) reflections. Extraction of Co from Li planes by
this process might prove to be useful to obtain improved electrode mat
erials for lithium batteries. The degree of lithium extraction that ca
n be achieved with different oxidizing agents follows the trend in the
ir oxidation potential. In addition, the literature data that Na can b
e extracted more easily from NaCoO2 than Li from LiCoO2 is explained o
n the basis of the relative energies of the Co-3+/4+ redox couple in t
he two compounds. (C) 1996 Academic Press, Inc.