Bulk Structure and Surface Properties of Lithium- and Manganese-Rich Layered Oxides and Their Impact on Electrochemical Performance

Abstract

Lithium- and manganese-rich layered oxides, Li1+x M1-x O2 (M=Mn, Ni, Co), are promising cathode materials for lithium-ion batteries due to their high capacities, typically more than 250 mAh/g. Despite the intense research and development in the past decade, their commercial adoption remains hindered by severe shortcomings including a large first-cycle irreversible capacity loss, [1] voltage and capacity fade, [2] DC resistance rise at low state of charge (SOC) and transition metal dissolution. [3] Studies have attributed these issues to the structural changes occurring during first-cycle activation and prolonged cycling, yet the crystal structure of the pristine oxides is still a matter of debate. Fundamental studies on the conventionally synthesized, aggregated secondary particles often lead to ambiguous results due to the structural complexity in this class of materials. In this presentation, we report the synthesis of well-formed, discrete layered Li1+x M1-x O2 crystals with various morphologies and surface properties using a molten salt method. Complementary microscopy and spectroscopy techniques at multi-length scale, including aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy and X-ray energy dispersive spectroscopy, were used to reveal the structural make-up of the bulk as well as the surface of the primary particles. Surface sensitive soft X-ray absorption spectroscopy was further used to reveal the roles of key surface properties, particularly surface composition, surface area and surface crystalline orientation, in the material challenges facing Li1+x M1-x O2 cathodes. Future directions and research to enable this class of materials will also be discussed.