Recent development of LiMnPO4 as cathode materials of lithium-ion batteries

Abstract

Similar to LiFePO4, LiMnPO4 possesses the following advantages: eco-friendliness, low cost and excellent safety performance. Moreover, high energy density (~700 Wh•kg-1) of LiMnPO4 is 20% larger than LiFePO4, which is due to LiMnPO4 with high operating voltage ~4.1 V vs. Li falling within the electrochemical stability window of conventional electrolyte solutions. Therefore, LiMnPO4 is considered as a next generation cathode material for lithium-ion batteries. However, the lithium ion conductivity of LiMnPO4 is lower than that of LiFePO4. The main difference between the kinetics in the initial stage of charging of two olivine materials may originate from the formation energy of vacancy-polaron complex in LiMnPO4 than in LiFePO4. In addition, the anisotropic lattice distortion of MnO6 octahedron during repeated charge/discharge process hinders lithium removal/uptake reactions in LiMnPO4 and thus leads to the large volume change. This distortion is not a strict Jahn-Teller effect but is a preferential elongation of two of the equatorial Mn-O bonds (edge-sharing with the PO4). These intrinsic defects result in rapid capacity fading upon extended cycling and poor rate capability during cycles. To overcome these problems, it is an effective approach to prepare nanometer-sized and rod/sheet-like materials through proper synthesis method. These special morphologies in LiMnPO4 can stimulate the rate of Li extraction/insertion. However, the surface structural instability of particles may occur at size d<35 nm. The awkward situation may be solved by effective surface coating. Surface coating can decrease the disorder toward amorphous state and the poison of impurities on the surface of particles; especially, coating can promote the ionic and electronic conductivity. Therefore, a coating leads to a remarkable improvement of the electrochemical performance of LiMnPO4. In addition, doping is also a method for improving the electrochemical property of LiMnPO4. As a Fe-doped material, LiMnyFe1-yPO4 has been widely investigated. In this paper, the recent advances in LiMnPO4 as cathode materials of lithium-ion batteries are reviewed. The characteristics, morphologies, and possible reaction mechanisms of the material were summarized systematically. Some open questions (e.g. the facticity of Jahn-Teller effect in LiMnPO4, the thermal stability of LiMnPO4, etc.) are discussed in detail. Moreover, some improved methods (controlling the particle morphology, surface coating, and doping) for the electrochemical property of LiMnPO4 are expounded.