Synthesis Study Of Cathode Material LiMnPO₄for Lithium Ion Batteries

The olivine LiMPO4(M=Fe, Mn, Co and Ni) phosphates have been widely investigated aspotential cathodes for lithium-ion batteries due to their large theoretical capacities, excellentcycle life and fewer safety issues. Among these LiMPO4materials, LiFePO4has been widelyinvestigated and commercialized. However, its relatively low potential (Fe3+/Fe2+:3.4V vs.Li/Li+) causes a low overall energy density, which limits its application for power batteries.LiMnPO4exhibits a redox potential of4.1V vs. Li/Li+, corresponding to the theoretical energydensity increase of20%compared with LiFePO4. This can better meet the requirement ofhigh-power secondary batteries. However, the commercial application of LiMnPO4still faces agreat challenge due to the low ionic and electronic conductivities. In this paper, by means ofexploring the synthesis of nano-LiMnPO4materials and then for the purpose of improving theelectrochemical performance, LiMnPO4materials were prepared via Polyol and liquid phasemethods, respectively. The main contents and conclusions were summarized as follows:1. Nano-LiMnPO4samples are synthesized via two-step heating Polyol method. We focuson the role of the first thermal plateau temperature T1(T1=100,110,120,130,140,150°C) ontheir morphologies and electrochemical performances. Results show that samples at T1=100~120°C contain some impurities and their specific surface areas are less than15m2.g-1. Purenano-LiMnPO4sample can be achieved at T1=130°C, and shows the largest specific surfacearea (46.3m2.g-1). With the further increase of T1, the specific surface areas of samples are keptat35~37m2.g-1. The electrochemical performance of nano-LiMnPO4samples exhibits the sametendency as the specific surface area. The nano-LiMnPO4at T1=130°C presents the bestelectrochemical performance. It delivers a discharge capacity of129mAh.g-1at C/10and81mAh.g-1at5C. This indicates that the specific surface area is one of the key factors indetermining the electrochemical performance of LiMnPO4.2. LiMnPO4samples are prepared via the liquid phase method assisted by DMSO. We studythe effect of the polyvinylpyrrolidone (PVP) as a surfactant on the particle size andelectrochemical properties. Results show that LiMnPO4/C prepared with PVP has a uniform,smaller size than that without PVP, exhibiting the superior performance: it delivers a dischargecapacity of157.3mAh.g-1at1/20C,119mAh.g-1at1C, and92mAh.g-1at5C. The decreasingof particle size reduces the solid-state diffusion path, thus improving the lithium-ion kineticbehavior. Also, the relatively high surface area for the smaller particles promotes fast charge transfer.3. In order to further improve the electrochemical performance of LiMnPO4,LiMn0.8Fe0.2PO4was prepared via liquid-phase method assisted by dimethyl sulfoxide (DMSO).Results show that Fe-doping LiMnPO4materials exhibit excellent electrochemical performance.The LiMn0.8Fe0.2PO4with uniform, smaller nanoparticles and thus more excellentelectrochemical properties can be obtained with PVP. At room temperature, the PVP-assistedLiMn0.8Fe0.2PO4material delivers a high discharge capacity of160.3mAh.g-1at1/20C, anexcellent rate capability (102mAh.g-1at20C and83mAh.g-1at50C), a better capacityretention during cycling (above97%capacity retention after50cycles when discharged at C/2rate), and superior low-temperature performance (97mAh.g-1at C/2when discharge at-15°C).This makes it one of preferred cathode materials for power lithium ion batteries.