The cathode spinel Li1-X[Mn2]o4 provides another
type of electrode-electrolyte reaction; an electrode sur- face disproportionation reaction 2Mn3þ=Mn2þ þ Mn4þ results in dissolution of the Mn2þ from the electrode into the electrolyte.51 This reaction, unless suppressed, gives an irreversible capacity loss of the cathode and migration of the Mn2þ across the electrolyte to the anode during charge to block Liþ-ion insertion into the anode. The result is an intolerable limitation of the service life of
the cell.
In addition to chemical stability vis a vis the electrodes and higher temperatures, the electrolyte should not be decomposed by an anode μA at a higher energy than the electrolyte LUMO or a cathode μC at a lower energy than the HOMO. However, if μA or μC lie outside the window of the electrolyte, kinetic stability may be achieved by formation of a passivating SEI layer on the surface of the electrode, but at the expense of the loss in capacity to form the layer. Moreover, during a fast charge, the concentra-
tion of Liþ ions may build up on the surface of the SEI
layer, and where a change in volume of the electrode
Figure
2.
(a)
Voltage
profiles
versus
Liþ/Li0
of
the
discharge
curves
of
LixC6,
LixTiS2
and
Lix[Ti2]S4,
LixCoO2,
and
LixCoPO4.
(b)
Schematic
of
their
corresponding
energy
vs
density
of
states
showing
the
relative
positions
of
the
Fermi
energy
in
an
itinerant
electron
band
for
LixC6,
the
Ti4þ/Ti3þ
redox
couple
for
LixTiS2
and
Lix[Ti2]S4,
the
Co4þ/Co3þ
redox
couple
for
LixCoO2,
and
the
Co3þ/Co2þ
redox
couple
for
LixCoPO4.
breaks the SEI layer, Li0 may be plated out before the break is healed. Li plating can result in dendrites that grow across the electrolyte. This problem creates a safety issue that has haunted the use of a carbon anode in large- scale power batteries. These problems need to be mana- ged if safety standards are to be met with any anode, including carbon, that has its μA above the LUMO of the electrolyte.
A cathode μC at a lower energy than the electrolyte HOMO must be distinguished from an intrinsic voltage limit of the cathode, as is discussed below. As with anodes, passivating layers on cathodes are best formed in situ so that electronic contact with the cathode current collector is not broken. Preliminary work52-54 on pas- sivating SEI layers on oxide cathodes has found them to be unstable. This field has yet to be adequately researched.