Page 398 - Handbook of Battery Materials

12.16 Silicate Li 2 MSiO 4 369
realized in practical cells so far, and the cycle life of LiCoPO 4 is also poor because
of the instability of the commonly used LiPF 6 in ethylene carbonate (EC):diethyl
carbonate (DEC) electrolyte at these high operating voltages [111]. The redox po-
tential of LiNiPO 4 is even higher at 5.1 V, which makes it difficult to analyze with
the currently available electrolytes.
Recently, the mixed transition-metal ion systems have attracted considerable
interest. Among these, the LiFe 1−y Mn y PO 4 solid solution system has attracted
particular attention as it exhibits higher energy density and improved redox kinetics
due to improved electronic conductivity compared to LiMnPO 4 [116,121–124].
Alternatively, the LiMn 1−y Co y PO 4 and LiFe 1−y Co y PO 4 solid solutions are appealing
because of their high operating voltages, which arise from the Co 2+/3+ redox couple.
Even though LiMnPO 4 ,LiCoPO 4 , and their solid solutions have higher theoretical
energy densities than LiFePO 4 , their experimental energy densities are lower than
or equal to that of LiFePO 4 owing to their lower capacity values and high polarization
losses. Further improvements in high-voltage electrolytes could allow efficient use
of the high-voltage Co 2+/3+ redox couples and enhance the energy density.

12.16
Silicate Li 2 MSiO 4

Even though LiFePO 4 has become successful because of its low cost and good safety
characteristics, its energy density is limited because of the lower operating voltage
−1
(3.4 V), lower theoretical capacity (∼170 mAh g ), and lower crystallographic
density. In this regard, polyanion-containing frameworks that can accept two
lithium ions per transition metal atom would help to realize high capacities and
energy densities. Recently, a new class of silicates, Li 2 MSiO 4 (M = Fe, Mn, and Co),
has been introduced, which offers the possibility of reversibly extracting/inserting
two lithium ions per formula unit with a theoretical capacity around ∼333 mAh g −1
[125, 126]. Li 2 MSiO 4 crystallizes in an orthorhombic β-Li 3 PO 4 structure, with all
the cations occupying tetrahedral sites.
Li 2 FeSiO 4 has been shown to undergo an initial charge at around 3.1 V with
a capacity of around 160 mAh g −1 and stable cycle life. The redox voltage of the
Fe 2+/3+ couple is considerably lower than the 3.45 V in LiFePO 4 because of the
lower electronegativity of Si compared to P. The stronger P–O covalent bonding
in LiFePO 4 leads to a weaker Fe–O covalence and a consequent lowering of the
Fe 2+/3+ redox energy in LiFePO 4 compared to the weaker Si–O covalent bonding
in Li 2 FeSiO 4 , resulting in an increase in the cell voltage by ∼0.3 V in LiFePO 4 .
It is important to note that the capacity corresponding to the Fe 3+/4+ couple
has not been realized in Li 2 FeSiO 4 at room temperatures [125]. Nevertheless,
close to two lithium atoms per formula could be extracted in Li 2 MnSiO 4 at ∼4V
4+
corresponding to the oxidation of Mn 2+ to Mn 3+ and then Mn 3+ to Mn .In
contrast to Li 2 FeSiO 4 , the extraction of both the electrons occurs around the
same potential range in Li 2 MnSiO 4 because of the extraction of the electron
from the same e g redox level for both the Mn 2+/3+ and Mn 3+/4+ redox couples.