Preparation And Doping Modification Of LiMnPO₄ Via Solvothermal Synthesis In Polyol/Water System

Olivine framed LiMnPO₄ is one of the most promising cathode material for high-energy lithium ion batteries due to its high thermal stability, high energy density?170 mAh·g-1? and high voltage?4.1 vs. Li/Li⁺?. However, the intrinsically low electronic and ionic conductivity limit its application. For the purpose of improving the electrochemical performances of LiMnPO₄, the preparation conditions of LiMnPO₄ material were explored, and doping modification were done. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy and charge/discharge test of the synthetic material have been investigated. The work is as follows:In this paper, LiMnPO₄ has been synthesized via an efficient solvothermal synthesis in polyol/water system. On the basis of the original synthesis conditions, different manganese sources?manganese acetate, manganese nitrate and manganese sulfate? and different polyol?ethylene glycol, 1, 4-butanediol and diethylene glycol? for synthesis of LiMnPO₄ have been investigated. The results show that the optimum manganese source and polyol are MnSO₄ and diethylene glycol. The discharge capacity of LiMnPO₄/C is 115.1 mAh·g-1 with capacity retention 85.5 % after 50 cycles at 0.1 C.The effects of different doping content of Sr²⁺ on the structure and performance of LiMnPO₄ cathode material have been studied. The optimal doping content of Sr²⁺ is 1 at.%. The Sr²⁺ doping can improve cycle performance of LiMnPO₄. After 50 cycles, the LiMn_0.99Sr_0.01PO₄/C?1 at.% Sr? can deliver the discharge capacity of 128.3 mAh g-1 with 95.2 % cycle retention at 0.1 C. EIS test results show that the Sr²⁺ doping is benefit to reduce the charge transfer resistance and promote the diffusion of lithium ion.In addition, the study about Ce³⁺ doping in LiMnPO₄ has been reported in this paper. The optimal doping content of Ce³⁺ is 3 at.%?The LiMn_0.955Ce_0.03PO₄/C?LCe₃? shows the best cycling stability, which can deliver the highest discharge capacity of 132.3 mAh g-1 with 95.4 % capacity retention after 50 cycles at 0.1 C. Meanwhile, LCe₃ also exhibits excellent rate capability among all the samples. The discharge capacity of LCe₃ is 78.2 mAh·g-1 at 10 C,which is 55.9 % of that at 0.1 C. The above results are closely related with the effect of Ce³⁺ doping, which can stabilize the olivine structure of LiMnPO₄ and improve the Li⁺ diffusion. This can be proved by the results of XRD, CV and EIS.