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

Doping could effectively tune the electrochemical performanceoflayered oxide cathodes in Li-ion batteries, whereas the working mechanismis usually oversimplified (i.e., a "pillar" effect).Although the Jahn-Teller effect is generally regarded as thefundamental origin of structural instability in some oxides, morepolyhedral distortions are associated with pseudo-JTE (PJTE), whichinvolves vibronic couplings. In this work, the atomic structures ofdoped LiCoO2 by Mg cations, F anions, and both were investigatedthoroughly to reveal the atomic environments of these dopants andtheir influence on electrochemical performance. The function of thesedopants 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 neighboringoctahedra, thus depressing PJTE. Although F doping does increase theligand-field strength, the induced octahedral distortion reduces thestructural stability dramatically. Comparatively, Mg/F co-doping generatesthe CoO5F-MgO4F2-CoO5F medium-range orders (MROs), which could depress both structuraldistortion and charge transfer in Co-centered octahedra for reducedPJTE. 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 rangeof 2.8-4.6 V, followed by Mg-doped LiCoO2. In contrast,although F-doping could induce an extra rock salt-like surface layerfor higher capacity, its cycling improvement is rather limited. Theseresults highlight the importance of structural modulation in enhancingthe material performance and propose that the manipulation of PJTEwould be an effective strategy in developing novel high-performanceoxide cathodes.