Li2−x MnSiO4 compounds with mesoporous structure are first proposed in the present work. It is interesting to note that the lithium deficient compounds exhibit much higher electrochemical performance in comparison with the stoichiometric one. Among these compounds, Li1.8MnSiO4 shows the best electrochemical performance. It is found that mesoporous Li1.8MnSiO4 without carbon coating delivers a maximum discharge capacity of 110.9 mAh g−1 at 15 mA g−1, maintaining 90.8 mAh g−1 after 25 cycles, while that of the stoichiometric one is only 48.0 mAh g−1, with 12.5 mAh g−1 remaining. The superior properties are mainly due to the great improvement of electronic conductivity and structure stability, as well as suppressed charge-transfer resistance.