相较于磷酸铁锂,磷酸铁锰锂LiMn1-xFexPO4(LMFP,0 < x < 1)可提供更高的放电电压,然而其低电子、离子电导率和Jahn-Teller晶格畸变导致了低可逆容量和短循环寿命。本文使用溶剂热合成工艺,将物质的量比例为1%的Ni2+掺入到LiMn0.75Fe0.25PO4正极材料。结果表明Ni2+的掺入可导致Li-O键的伸长,拓宽Li+传输通道,从而有利于Li+的脱出和嵌入,提高了材料的放电比容量和倍率性能。同时掺入的Ni2+缩短了Mn/Fe-O键的平均键长,增强了其结合能,进而抑制了Jahn-Teller晶格畸变,改善了材料的结构和循环稳定性。与未掺杂的LiMn0.75Fe0.25PO4相比,Ni2+掺杂后的LiMn0.75Fe0.24Ni0.01PO4具有较低的电荷转移电阻和较高Li+扩散系数。在0.1 C倍率下,其放电比容量从未掺杂的136 mAh•g-1上升到147 mAh•g-1,且在0.5 C下循环150圈后,容量保持率仍有88.2%。因此,Ni2+掺杂策略可被视为一种有潜力且有效的改性手段,用于实现LMFP的快速充放电和长循环。
Compared with lithium iron phosphate, lithium iron manganese phosphate LiMn1-xFexPO4 (LMFP, 0 < x < 1) has a higher discharge voltage. However, its low electronic and ionic conductivities, along with Jahn-Teller lattice distortion, result in low reversible capacity and short cycle life of LMFP. In this study, the solvothermal synthesis process is used to incorporate Ni2+ with a molar ratio of 1% into the LiMn0.75Fe0.25PO4 cathode material. The effects of Ni2+ doping on the structure and electrochemical performance of LiMn0.75Fe0.25PO4 were systematically investigated. Results of X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy confirm the successful incorporation of Ni2+ ions into the crystal lattice of LiMn0.75Fe0.25PO4. In addition, Rietveld refinement results of XRD data indicate that the incorporation of Ni2+ leads to the elongation of the Li-O bonds and widening of the Li⁺ transport channels. Consequently, this is conducive to the extraction and insertion of Li+, improving the discharge specific capacity and rate performance of the material. Moreover, the incorporated Ni2+ shortens the average bond lengths of the Mn-O and Fe-O bonds, suppresses the Jahn-Teller lattice distortion, and improves the structure and cycling stability of the material. Furthermore, cyclic voltammetry tests reveal that Ni2+ doping could reduce the degree of polarization of the material and enhance the reversibility of the reaction. Analysis of electrochemical impedance spectroscopy reveals that, compared with undoped LiMn0.75Fe0.25PO4, LiMn0.75Fe0.24Ni0.01PO4 exhibits a lower charge-transfer resistance and a higher lithium-ion diffusion coefficient. At a rate of 0.1 C, its discharge specific capacity has been increased from 136 mAh•g-1 of the undoped material to 147 mAh•g-1. The capacity retention rate is still 88.2% after 150 cycles at a rate of 0.5 C. Therefore, the Ni2+ doping strategy can be regarded as a promising and effective modification method for achieving rapid charge and discharge and long cycle of LMFP.
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