Surface-stabilization of LMR-NCM by Washing with Aqueous Buffers to Reduce Gassing and Improve Cycle-Life

Abstract

Li- and Mn-rich battery active materials like Li1.14(Ni0.26Co0.14Mn0.60)0.86O2 (LMR-NCM) are promising cathode active materials (CAMs) for next-generation Li-Ion batteries, as they combine a higher energy density than state-of-the-art materials at lower materials cost. However, they suffer from high gassing, particularly during formation, and from voltage and capacity fading over their cycle life. While electrolyte additives are known to alleviate these issues, it will be shown that a surface modification of LMR-NCM CAMs can also reduce their gassing during formation and improve their long-term stability. Here, we present a detailed analysis of a water-based post-treatment for LMR-NCMs, which not only lowers their initial gassing by more than 90%, but also avoids the issue of transition metal dissolution observed with conventionally used washing procedures at low ph. We thus utilized a buffered solution to increase the pH into the stability window of LMR-NCM materials (near/above pH 7), while also providing a controlled Li+/H+-exchange that is not possible with unbuffered aqueous solutions like pure H2O, H2O/alcohol mixtures, or acids. TGA-MS experiment showed that heating of Li+/H+-exchanged LMR-NCM leads to the formation of a protective (near-)surface spinel layer, and full-cell cycling displayed a drastic improvement of the capacity retention.