The Challenge of New Compositions for Layered Oxides Rich in Lithium and in Manganese As Positive Electrode Materials for Lithium-Ion Batteries

Abstract

The family of layered materials currently attracting most interest for practical applications is that of Li and Mn-rich Li1+xM1-xO2 layered oxides, with the overall Li/M ratio >1, M being Mn, Ni and Co 3d transition metals. Indeed, they offer very high reversible capacities (> 230 mAh/g) and the composition rich in manganese fulfills sustainability, availability and cost issues. The exceptional capacity delivered by these layered oxides is in fact explained by the reversible participation of oxygen anions to the redox processes. [1-4] This reaction is reversible within the bulk, occurring without any major structural modification, whereas oxidized oxygen ions are destabilized at the surface, leading to oxygen loss and structural reorganization at the outer part of the particle. [5-6] This structural reorganization is at the origin of a voltage profile evolution upon cycling and of a continuous decrease in energy, and its kinetics is obviously highly dependent on the composition of the pristine material.[7] The challenge in the field is now to develop alternative compounds with low cost and environmentally friendly metals being able to promote the participation of oxygen anions in the redox processes, with optimized and stabilized electrochemical performance over long range cycling. A large number of compositions are currently under study in our group at ICMCB, screening the composition in transition metal ions in a series of materials Li(NiIIMnIV x+3yCoIII 1-2(x+2y))O2 recently reported to stabilize cationic vacancies on the transition metal sites (i.e. in the slabs). [8-10] We focused our efforts on compositions showing a Li/M ratio ranging between 1 and 1.5 and a Mn content being at least 45 at.% of M, as they deliver very attractive reversible capacities with a limited first cycle irreversible capacity. The phase diagram was established as function of the M composition and of the Li/M ratio using the combination of Synchrotron X-ray and neutron powder diffraction analyses. During this presentation, we will discuss in details the relationship between the synthesis conditions, the composition, the structure and the electrochemical performance of these materials, but also the subtle differences identified in the structural modifications observed in different cycling conditions to determine the optimized formation of the electrode material for its cyclability upon long range cycling. Acknowledgments The authors thank Cathy Denage, Laëtitia Etienne and Eric Lebraud (ICMCB) for SEM, ICP-OES and routine XRD analyses respectively, as well as the ANR and DGA for the funding of the project SILMARILION ANR-16-CE05-0015-02.