Li Storage in Olivine Structure LiFexMn1-XPO4

Abstract

LiMnPO4 shows higher specific energy than that of LiFePO4, but suffers from poor kinetics. The mixed-cation olivine material LiFexMn(1-x)PO4 combined the good rate capability of LiFePO4 and high potential of LiMnPO4. Commonly, LiFe0.5Mn0.5PO4 is considered to exhibit two pairs of plateau in charge-discharge curves associated with MnPO4/LiMnPO4 and FePO4/LiFePO4 phase transformation. We prepared carbon coated LiFe0.5Mn0.5PO4 solid solution material by solvothermal method. This material delivers typical two plateaus in charge/discharge curves at low current rate, but a third plateau appears only during discharge at relative high current rate. The discharge capacity in the addition medium plateau increases with the current rate increases. LiFe0.5Mn0.5PO4 discharge mainly proceeds via the medium plateau at high current rate. Electrochemical measurements demonstrate that the additional discharge plateau can still be attributed to Mn3+ reduction, but with a different mechanism than in pure LiMnPO4 due to the presence of Fe in LiMnPO4 framework that gives the conditions for better kinetics. Also the battery performance of a full battery consisting of LiFe0.2Mn0.8PO4 and layered transition-metal oxides (NMC) composite cathode and graphite anode was extensively studied. The addition of LMP in NMC greatly improved the battery safety.