Effects of gradient concentration on the microstructure and electrochemical performance of LiNi 0.6 Co 0.2 Mn 0.2 O 2 ca

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RESEARCH ARTICLE

Effects of gradient concentration on the microstructure and electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode materials Wenming Li, Weijian Tang, Maoqin Qiu, Qiuge Zhang, Muhammad Irfan, Zeheng Yang (✉), Weixin Zhang (✉) School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China

© Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract Nickel(Ni)-rich layered materials have attracted considerable interests as promising cathode materials for lithium ion batteries (LIBs) owing to their higher capacities and lower cost. Nevertheless, Mn-rich cathode materials usually suffer from poor cyclability caused by the unavoidable side-reactions between Ni4+ ions on the surface and electrolytes. The design of gradient concentration (GC) particles with Ni-rich inside and Mn-rich outside is proved to be an efficient way to address the issue. Herein, a series of LiNi0.6Co0.2Mn0.2O2 (LNCM622) materials with different GCs (the atomic ratio of Ni/Mn decreasing from the core to the outer layer) have been successfully synthesized via rationally designed co-precipitation process. Experimental results demonstrate that the GC of LNCM622 materials plays an important role in their microstructure and electrochemical properties. The as-prepared GC3.5 cathode material with optimal GC can provide a shorter pathway for lithium-ion diffusion and stabilize the near-surface region, and finally achieve excellent electrochemical performances, delivering a discharge capacity over 176 mAh$g–1 at 0.2 C rate and exhibiting capacity retention up to 94% after 100 cycles at 1 C. The rationally-designed co-precipitation process for fabricating the Ni-rich layered cathode materials with gradient composition lays a solid foundation for the preparation of high-performance cathode materials for LIBs. Keywords gradient concentration, Ni-rich, LiNi0.6Co0.2-Mn0.2O2, electrochemical performance, lithium-ion battery

Received September 18, 2019; accepted December 25, 2019 E-mails: [email protected] (Zhang W), [email protected] (Yang Z)

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Introduction

With the rapid development of electric vehicles and largescale energy storage areas, the higher requirements for energy and power density of lithium ion batteries (LIBs) are attractive features [1]. The important way to increase the energy density and power density of LIBs is to develop promising cathode materials [2]. One kind of promising cathode materials for LIBs are based on lithiated layered oxides of NCM-family of materials with the general formula LiNixCoyMnzO2 (x + y + z = 1). Among them, Nirich oxides LiNixCoyMnzO2 (x ≥ 0.5, x + y + z = 1) with high energy density and low-cost are anticipated as technologically important cathode materials for future LIBs [3]. However, Ni-rich layered materials have a series of problems pertinent to the structural instability, causing serious capacity decay. Electrolyte decomposition, Li+/Ni2+ ions replacement and phase transition of the electrode materials will gradua

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