Solid-state NMR Study Of High-voltage LiCoO₂ And Its Doped Product

LiCoO₂ is an important category of cathode materials in lithium-ion batteries due to its high compact density,high specific capacity,and good thermal stability.In the last decade,many studies on LiCoO₂ have focused on the realizing of higher charging voltage by element doping to obtain greater specific capacity.In order to explore the mechanism of phase transition and capacity decay during charging and discharging,atomic-level local probe is essential for the understanding of structure-function correlation.Here we employ high-resolution solid-state NMR spectroscopy,together with other characterization methods,to study the microstructure and local atomic environments in LiCoO₂ synthesized with three common sintering methods.The local atomic environments and high-voltage charge/discharge reaction mechanism of Mg/F doped LiCoO₂ were further discussed.This work provides a technical route for further study of high-voltage LiCoO₂ by using magnetic resonance technology.The main contents of this work include the following two parts:（a）We obtained a series of LiCoO₂ samples with controlled Li/Co stoichiometric ratio by using different solid-state sintering methods.It is found that the linewidth and signal of one-dimensional 7Li NMR are very sensitive to defects of lithium overstoichiometry,and the T1 relaxation of 7Li and 59Co is highly dependent on sintering method and non-stoichiometric defects.Combined with electron paramagnetic resonance（EPR）and Raman characterizations,we proved that the two-step sintering method favors the elimination of unreacted and surface Co3O4,leading to the higher discharge capacity and capacity retention in all-solid-state lithium batteries,which is in accordance with its narrowest 7Li linewidth and the longest 7Li/59Co T1.（b）On the basis of two-step sintering method,we doped LiCoO₂ with Mg/F dual ions.Compared with the original LiCoO₂,the cycle stability of the doped cathode material at high voltage of 4.6 V was significantly improved.Firstly,we employed solid-state nuclear magnetic resonance technology to explore the defect structure introduced by doping.Through one-dimensional 7Li,19F,59Co and two-dimensional HECTOR experiments,we conclude that a pair of Co3+ and O2-in the bulk phase tend to be replaced by a pair of Mg2+ and F-to keep the charge balance,while LiF species are formed on the surface.We further studied the high-voltage charge/discharge mechanism by solid-state nuclear magnetic resonance and synchrotron X-ray diffraction.The results show that Mg/F doped LiCoO₂ can restrain the high-voltage adverse phase transition and maintain the redox stability of Co3+/Co4+ in the bulk structure.The surface LiF probably inhibits the irreversible degradation of the layer structure to spinel structure.