PNNL

Abstract

Mn-redox based oxides and oxyfluorides are considered most promising as earth-abundant high-energy cathode materials for next-generation lithium-ion batteries. While high capacities are often obtained in high-Mn content cathodes such as Li- and Mn-rich layered and spinel-type oxides and oxyfluorides, local structure changes and structural distortions (particularly those related to layered-to-spinel phase transformations) often lead to voltage fade, capacity decay and impedance rise, resulting in unacceptable electrochemical performance upon cycling. In the present study, we report structural transformations that exploit the high capacity of Mn-rich oxyfluorides while enabling stable cycling, in stark contrast to commonly observed structural changes that result in rapid performance degradation. We show that upon cycling of a cation-disordered rocksalt cathode (Li1.1Mn0.8Ti0.1O1.9F0.1, M80), an ultrahigh capacity of about 320 mAh/g (energy density of about 900 Wh/kg) can be obtained through dynamic structural rearrangements on charge and discharge, along with a unique voltage profile evolution and capacity rise. At high voltage, the presence of Mn4+ and Li+ vacancies promotes local cation ordering, leading to the formation of domains of a so-called “d phase” within the disordered framework. On deep discharge, Mn4+ reduction, along with Li insertion, transform the structure to a partially disordered rocksalt phase with a ß’ LiFeO2-type arrangement. At the nanoscale, domains of the in situ formed phases are randomly oriented, allowing highly reversible structural changes and stable electrochemical cycling. These new insights not only help explain the superior electrochemical performance of high-Mn DRX, such as M80, but also provide guidance for the future development of Mn-based, high energy density oxide and oxyfluoride cathode materials.