Li-rich disordered rock salt transition metal oxyfluorides as novel cathode materials in lithium-ion batteries

Abstract

Li-rich disordered rock salt compounds are a promising material class for next-generation cobalt-free cathodes in Li-ion batteries. Their cubic close-packed lattice, in which cations and anions are randomly occupying the same lattice site within their sub-lattices, offers a stable host for Li-extraction and reinsertion of more than 1 Li-ion per formula unit. The Li-excess in the structure enables not only macroscopic diffusion of Li in the bulk, but also yields theoretical capacities considerably higher compared to commonly used cathode materials. One of the early representatives of Li-rich disordered rock salt materials is the oxyfluoride Li2VO2F. Widening the knowledge of the intriguing structural and electrochemical properties of Li2VO2F was the main focus of my thesis. Up to now, the synthesis of Li2VO2F in involves high-energy ball milling, a versatile and straightforward approach, which however leads to certain disadvantages, e.g. the nanoscale particle and crystallite dimensions of the material. Alternative synthesis methods with the capability of yielding a more crystalline material are unknown so far. In this work, we investigated the synthesis and structural properties of disordered rock salt LiVO2, a novel compound and intermediate product in the synthesis towards Li2VO2F. We studied the phase transition between layered and disordered rock salt LiVO2 in a combined experimental and theoretical study and gave an explanation for the lack of synthesis reports of disordered rock salt V-containing phases. We further demonstrated reversible electrochemical delithiation of LiVO2, despite the disordered rock salt structure. In electrochemical cycling as cathode material, Li2VO2F exhibits an extraordinary high initial capacity but suffers from severe capacity fading in the subsequent cycles. The reasons for the poor stability were unclear. We investigated Li2VO2F electrodes before and after cycling and revealed several degradation processes affecting the cycling stability of the material. Furthermore, we developed a strategy to reduce the capacity fading by improving the chemical and structural stability of Li2VO2F by partially substituting V with disordered rock salt promoting elements. We synthesized two novel Li-rich disordered rock salt oxyfluoride materials, Li2V0.5Ti0.5O2F and Li2V0.5Fe0.5O2F, which show significantly better cycling stability. We further studied the mechanism of Li-extraction and reinsertion in Li2VO2F and the changes in the disordered rock salt structure by advanced X-ray powder diffraction and computational techniques. When the material is charged (Li-extraction), the disordered rock salt phase transforms partially into an amorphous phase with V being tetrahedrally coordinated. In addition, the oxygen atoms in the lattice get oxidized to superoxides and contribute to the charge compensation during Li-extraction, indicating a so-called anionic redox activity. In summary, this dissertation comprises an extensive study of the structural and electrochemical properties of Li-rich disordered rock salt Li2VO2F as cathode material in Li-ion batteries. Our results widened the knowledge of this material in particular but also act as a guideline to overcome the challenges for future application of other Li-rich disordered rock salt related materials.