Reversible Li Intercalation in Layered Cathodes Enabled by Dopant-Induced Medium-Range Orders

Doping could effectively tune the electrochemical performance of layered oxide cathodes in Li-ion batteries, whereas the working mechanism is usually oversimplified (i.e., a "pillar" effect). Although the Jahn–Teller effect is generally regarded as the fundamental origin of structural instability in some oxides, more polyhedral distortions are associated with pseudo-JTE (PJTE), which involves vibronic couplings. In this work, the atomic structures of doped LiCoO2 by Mg cations, F anions, and both were investigated thoroughly to reveal the atomic environments of these dopants and their influence on electrochemical performance. The function of these dopants as pillars is well discussed from the view of PJTE manipulation. Briefly, the MgO4 tetrahedra in Mg-doped LiCoO2 could suppress the charge transfer from the ligand to Co in neighboring octahedra, thus depressing PJTE. Although F doping does increase the ligand-field strength, the induced octahedral distortion reduces the structural stability dramatically. Comparatively, Mg/F co-doping generates the CoO5F–MgO4F2–CoO5F medium-range orders (MROs), which could depress both structural distortion and charge transfer in Co-centered octahedra for reduced PJTE. The reduced PJTE accounts for the improved electrochemical performance, making the co-doped LiCoO2 offer the best performance: a 70% capacity retention after 200 cycles within the potential range of 2.8–4.6 V, followed by Mg-doped LiCoO2. In contrast, although F-doping could induce an extra rock salt-like surface layer for higher capacity, its cycling improvement is rather limited. These results highlight the importance of structural modulation in enhancing the material performance and propose that the manipulation of PJTE would be an effective strategy in developing novel high-performance oxide cathodes.