Specific Surface Area and Bulk Strain: Important Material Metrics Determining the Electrochemical Performance of Li- and Mn-Rich Layered Oxides

Abstract

Li- and Mn-rich layered oxides are a promising next-generation cathode active material (CAM) for automotive applications. Beyond well-known challenges such as voltage fading and oxygen release, their commercialization also depends on practical considerations including cost and energy density. While the cost requirement for these materials could be satisfied by eliminating cobalt, the volumetric energy density requirement might imply the transition from the most widely used porous structure to a more densely packed structure. Here, we investigated five Li- and Mn-rich layered oxides which were synthesized by various routes to obtain CAMs with different morphologies (porous vs dense), transition-metal compositions (Co-containing vs Co-free), and agglomerates sizes (≈6−12 μm). The as-received materials were characterized, e.g., by gas physisorption, Hg intrusion porosimetry, as well as X-ray powder diffraction, and were electrochemically tested by a discharge rate test. Thus, we identified two important material metrics which determine the initial electrochemical performance of Li- and Mn-rich CAMs, and which might be used as performance predictors: (i) the surface area in contact with the electrolyte that defines the effective current density which is applied to the surface of the CAMs, and (ii) the microstrain in the bulk that affects distinct redox features during cycling.