Crystallographic and morphological data and the galvanostatic cycling and rate performance of the cobalt-substituted Li
2Fe
0.5Mn
0.5SiO
4/C compounds Li
2Fe
xMn
xCo
1−2xSiO
4/C (
x = 0.33, 0.40, 0.45) were evaluated and compared with those of unsubstituted Li
2Fe
0.5Mn
0.5SiO
4/C. The hydrothermally synthesized Li
2Fe
xMn
xCo
1−2xSiO
4, consisting of uniform nanosized primary particles and no impurities, were indexed on the basis of the orthorhombic unit cell in space group Pmn2
1 and exhibited a solid solution at least beyond
x = 0.33. The primary particle sizes of Li
2Fe
xMn
xCo
1−2xSiO
4 decreased because of the substitution of cobalt for iron and manganese. In addition, although the capacity fades of Li
2Fe
xMn
xCo
1−2xSiO
4/C were similar to those of Li
2Fe
0.5Mn
0.5SiO
4/C, the discharge capacity and rate capability of Li
2Fe
0.5Mn
0.5SiO
4/C were improved by the substitution of cobalt for iron and manganese. Li
2Fe
xMn
xCo
1−2xSiO
4/C (
x = 0.45) exhibited the best electrochemical performance with first discharge capacities of 242.5 and 163.6 mAh g
−1 at current rates of 3.3 and 165 mA g
−1, respectively. The good electrochemical performance of Li
2Fe
xMn
xCo
1−2xSiO
4/C cathode materials is attributed to their smaller primary particle size compared with that of Li
2Fe
0.5Mn
0.5SiO
4/C.
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