Page 399 - Handbook of Battery Materials

370 12 Lithium Intercalation Cathode Materials for Lithium-Ion Batteries

However, Li 2 MnSiO 4 suffers from poor cycle life, which is most likely caused
by Jahn–Teller distortion and loss of crystallinity during cycling. In addition,
these materials also suffer from poor electronic conductivity and the consequent
slow reaction kinetics. Therefore, various synthetic routes such as sol-gel and
microwave-solvothermal methods have been employed to prepare nanostructured
materials, and coating with electronically conductive agents has been carried out to
improve their electrochemical performance [127–129].

12.17
Other Polyanion-containing Cathodes

Polyanion-containing compounds other than LiMPO 4 ,such asLi 3 M 2 (PO 4 ) 3 (M =
V, Fe, or Ti), LiVPO 4 F, Li 5 V(PO 4 ) 3 , and LiVOPO 4 , have also attracted a great deal
of interest in recent years because of their high thermal stability and attractive
electrochemical properties [130–134]. For example, LiVPO 4 F is isostructural with
the mineral tavorite (LiFePO 4 OH) and has been shown to exhibit a capacity of
∼156 mAh g −1 with a flat voltage profile (two-phase reaction) at 4.2 V corresponding
to the V 3+/4+ redox couple. The main drawbacks of these materials are their poor
electronic conductivity and consequently slow reaction kinetics.
Recently, LiFeSO 4 F synthesized by a nonaqueous solvothermal process employ-
ing ionic liquids (iono-thermal) has been shown to deliver capacities of around
140 mAh g −1 at 3.6 V [135]. Even though the theoretical specific capacity of LiFeSO 4 F
−1
−1
(151 mAh g ) is slightly lower than that of LiFePO 4 (170 mAh g ), the mate-
rial is proposed to provide better ionic and electronic conductivities, which could
eliminate the need for carbon coating or nanoparticles.

12.18
Summary
This chapter presents an overview of the structural characteristics, chemical stabil-
ity, and electrochemical properties of various lithium-insertion cathode materials.
The materials systems that are currently used are layered, spinel oxides, and
polyanion-containing cathodes. Although only 50% of the lithium can be extracted
−1
from LiCoO 2 , limiting its practical capacity to ∼140 mAh g , a recently discovered
class of layered oxide solid solution cathodes belonging to the series xLi 2 MnO 3 –
−1
(1 – x)LiMO 2 deliver capacities of ∼250 mAh g . However, these cathodes release
oxygen during first charge, and their adoption needs more robust electrolytes that
can operate up to 4.8 V. Although the conventional LiMn 2 O 4 spinel is plagued by
severe capacity fade, spinel cathodes have been optimized by doping the cation
sites, as in the case of LiMn 1.8 Ni 0.1 O 4 , which provides more stable cycle life and
higher rate performance. These characteristics make spinels an attractive option
for electric vehicle applications, but they commonly have low energy densities.
This can be mitigated by using the 5 V region in LiMn 1.5 Ni 0.5 O 4 to realize higher