Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P. R. China.
Phys Chem Chem Phys. 2018 Jun 6;20(22):15192-15202. doi: 10.1039/c8cp00354h.
F-Doping effects on polyaniline-derived carbon coated Li3V2(PO4)3 (Li3V2(PO4)3-xFx@C) as a cathode for high performance Li rechargeable batteries are systematically investigated with a combined experimental and DFT theoretical calculation approach. The results clearly indicate that the doping amount has a significant impact on the rate capability and long cycle life. The optimal material (Li3V2(PO4)2.88F0.12@C) delivers 123.16 mA h g-1@2C, which is close to the theoretical value (133 mA h g-1), while showing a greatly improved cycle stability. Rietveld refinements show that the F- doping does not obey Vegard's Law, which may be attributed to the generated lower valence of V ions. AC impedance spectroscopy shows that the F-doping can achieve faster interfacial charge transfer for higher reaction reversibility. DFT calculations confirm that the lower V2+ (t2g↑)3 does exist in Li3V2(PO4)2.88F0.12, and the mean nearest neighbor Li-O bond length also increases for faster electrochemical kinetics, and further reveal that there is a tendency for a transition from the insulator to the n-type semiconductor due to the F dopant. The combined experimental and calculated results suggest that F-doping indeed greatly facilitates the charge transfer rate of the Li+ insertion/de-insertion process for better reversibility and enhances the Li+ diffusion rate to access the reaction sites, thus resulting in high rate capacity and cycling stability. This work not only offers a facile and effective approach to synthesize high performance Li-ion battery material for very promising practical applications, but also discloses scientific insights on element coating and doping to guide the electrode material design for fast electrode kinetics in energy storage devices.
采用实验与密度泛函理论（DFT）计算相结合的方法，系统研究了 F 掺杂对聚邻苯二胺衍生碳包覆的 Li3V2(PO4)3（Li3V2(PO4)3-xFx@C）作为高性能可再充电锂电池正极材料的影响。结果表明，掺杂量对倍率性能和长循环寿命有显著影响。最佳材料（Li3V2(PO4)2.88F0.12@C）在 2C 时的比容量为 123.16 mA h g-1，接近理论值（133 mA h g-1），同时表现出优异的循环稳定性。Rietveld 精修表明，F 掺杂并不遵循维加定律，这可能是由于 V 离子价态降低所致。交流阻抗谱表明，F 掺杂可以实现更快的界面电荷转移，从而提高反应的可逆性。DFT 计算证实，Li3V2(PO4)2.88F0.12 中确实存在较低的 V2+（t2g↑）3，且平均最近邻 Li-O 键长也增加，从而加快了电化学动力学过程。进一步表明，由于 F 掺杂剂的存在，材料有从绝缘体向 n 型半导体转变的趋势。实验与计算结果表明，F 掺杂确实极大地促进了 Li+插入/脱插过程中的电荷转移速率，从而提高了可逆性，并增强了 Li+的扩散速率，使其能够到达反应位点，从而获得高倍率容量和循环稳定性。这项工作不仅为合成高性能锂离子电池材料提供了一种简便有效的方法，为实际应用提供了很有前途的选择，还揭示了元素涂层和掺杂在指导储能器件中快速电极动力学的电极材料设计方面的科学见解。
