School of Advanced Materials, Peking University, Shenzhen Graduate School, Shenzhen 518055, People's Republic of China.
Phys Chem Chem Phys. 2018 Jun 27;20(25):17497-17503. doi: 10.1039/c8cp01949e.
A recent discovery of anionic redox activity in Li-rich layered compounds opens a new direction for the design of high-capacity cathode materials for lithium-ion batteries. Here using extensive ab initio calculations, the thermodynamic existence of the Li-rich phase in LiFePO4 to form Li1+xFe1-xPO4 with x not exceeding 12.5% has been proved. Anionic redox activity and structural stability during delithiation are further investigated. Interestingly, it is found that Li1+xFe1-xPO4 cannot be delithiated completely and thus cannot achieve extra capacity by anionic redox activity, because the local oxygen-ion redox will cause the fracture of the rigid framework formed by phosphate tetrahedral polyanions. Although an extra capacity cannot be realized, the excess Li-ions at Fe sites can enhance the Li-ion diffusivity along the adjacent [010] channel and contribute to the shift from 1D to 2D/3D diffusion. This study provides a fresh perspective on olivine-type LiFePO4 and offers some important clues on designing Li-rich cathode materials with high energy density.
最近在富锂层状化合物中发现的阴离子氧化还原活性为锂离子电池高容量正极材料的设计开辟了新的方向。在此，我们使用广泛的从头算计算，证明了在 LiFePO4 中存在富锂相，可形成 Li1+xFe1-xPO4，其中 x 不超过 12.5%。进一步研究了脱锂过程中的阴离子氧化还原活性和结构稳定性。有趣的是，我们发现 Li1+xFe1-xPO4 不能完全脱锂，因此不能通过阴离子氧化还原活性获得额外的容量，因为局部氧离子的氧化还原会导致由磷酸四面体多阴离子形成的刚性骨架断裂。虽然不能实现额外的容量，但 Fe 位的过量 Li 离子可以增强锂离子沿相邻 [010] 通道的扩散能力，并有助于从一维扩散向二维/三维扩散转变。本研究为橄榄石型 LiFePO4 提供了新的视角，并为设计具有高能量密度的富锂正极材料提供了一些重要线索。
