Towards High-Performance Li-rich NCM∥Graphite Cells by Germanium-Polymer Coating of the Positive Electrode Material

The high specific capacity of Li-rich layered oxides of up to 300 mAh g(-1) renders them a promising class of positive electrode materials for high energy lithium ion batteries (LIBs). However, this material class suffers from poor capacity retention, voltage fade and structural degradation. The degradation phenomena include phase transformation and particle cracking during operation and take place in the bulk as well as the active material's surface. In this work, we demonstrate a straightforward approach for the surface modification of Li1.15Ni0.15Co0.15Mn0.55O2 for LIB cells. This strategy is based on a bis(carboxyethylgermanium)-based (Ge-132) coating, which is prepared via a simple sol-gel process. The coating layer is composed of a Ge-132 polymer network, which can coordinate on the oxygen atoms at the material's surface. As a result, LIB full-cells with coated Li-rich NCM vs a graphite negative electrode provide reversible capacities of approximate to 180 mAh g(-1) within a voltage range of 2.5-4.65 V for more than 200 cycles. The coated positive electrode exhibits a capacity retention improved by more than 20% compared to the uncoated reference material, which can be attributed to the suppression of parasitic side reactions at the cathode/electrolyte interface, less particle cracking upon electrochemical load and reduced manganese dissolution. (C) 2020 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.