Synthesis of Single Crystalline Ni-Rich NMC811 Cathode Materials and Improved Performance Enabled By Ultrathin Surface Coating Layer

Abstract

Ni-rich LiNi x Mn y Co1-x-y O2 (NMC, x ≥ 0.5) cathode materials have attracted intensive attention in developing high energy-density lithium ion batteries, owing to their high capacity and relatively low cost. Among them, LiNi0.8Mn0.1Co0.1O2 (NMC811) is one of the most promising candidates to realize higher energy density, particularly operating at high cut-off voltage (e.g. 4.5 V vs. Li/Li+). However, the limited cycle life is becoming the plague for its practical application due to the structural instability, microcracks formation, and electrolyte decomposition at the electrode/electrolyte interface.[1,2] As well known, single crystal NMC materials have more stable surface activity because of the low surface area and better mechanical stability than conventional polycrystalline materials. Moreover, single crystalline NMC materials can provide not only high gravimetric energy density, but also high volumetric energy density due to the increased tap density compared to the conventional polycrystalline NMC materials.[2-4] In this study, a single crystalline NMC811 sample with a large particle size of around 5 μm and polyhedral shape was synthesized by sol-gel method. Furthermore, a homogeneous ultrathin surface coating layer (~2 nm) was built on the surface of the single crystalline NMC811 particles to act as protective layer to prevent the direct contact between electrolyte and active particles. As a result, the coating layer effectively enhances an overall electrochemical performance of single crystalline NMC811 materials and especially leads to a higher reversible capacity as well as a remarkable improvement of the capacity retention (90% after 200 cycles between 3 and 4.5 V vs. Li/Li+). The influences of the coating layer on the structure and interfacial stability and electrochemical properties will be presented in details later.