Surface Modification of Lithium Rich NMC for High Rate Lithium Ion Batteries

Abstract

Recently, lithium manganese-rich layered oxides like Li1+xM1-xO2 (M = Ni, Mn, Co) have been extensively scrutinized as one of the most promising cathode materials for the next-generation lithium rechargeable batteries primarily due to their high reversible capacity over 250 mAh/g. These materials are targeted to be potentially utilized in such mid- and/or large- scale applications as electric vehicles (EV) and commercial electricity storage systems (ESS). However, recent studies show that they exhibit a poor electrochemical performance at high power rate and suffer from structural instability in long-term cycling. During the repeated charge and discharge, severe cation mixing and segregations are reported to occur, which causes a series of phase transitions from layered to a defect spinel, and eventually to a rock-salt structure. This phenomenon deteriorates the high-rate performance significantly because it tends to block the diffusion channels for Li+ ions. Herein, we developed a novel surface-modified lithium manganese-rich layered oxide, which shows a substantial improvement in the high-rate performance and long-term cycle life stability. A thin nano-scaled zirconium-doped surface layer was formed on Li1.2Ni0.13Co0.13Mn0.54O2 particle by using potassium nitrate as an additive to guide the crystal habit during the synthetic process. The addition of potassium nitrate is critical in promoting the preferential development of crystal facets open for the diffusion channel. By the low surface barrier driven by the facets growth, zirconium ions could easily enter the lithium octahedral sites within the structure. The Zr-doped surface layer is formed possibly having a cation-disordered rock-salt structure of a few nano-meter scales. This thin surface layer is believed to prevent the adverse phase transitions and cation segregations as well as to shorten the diffusion path for Li+ion within the particle. As a result, the high rate performances are significantly enhanced; a specific capacity at 30 C-rate is observed to be as high as 110 mAh/g and stable performance was obtained for 300 cycles. Figure. Zr-doped Li1.2Ni0.13Co0.13Mn0.54O2 nanoparticles (a) SEM image, (b) EDS mapping image of Zr-doped surface, (c) Z-contrast STEM-HADDF image taken along the [100] zone axis. This work is supported by the National Research Foundation of Korea (NRF-2011-0030539). Figure 1