Optimization of Full Concentration Gradient Li[NixCo0.16Mn0.84-X]O2 (x=0.64, 0.59, 0.51) Cathode Material for Lithium-Ion Batteries

Abstract

Lithium batteries are being developed as power sources for plug-in hybrid electric and electric vehicles, and for these applications, high energy cathodes are necessary to reduce battery size because the mounting space is significantly limited (1-2). However, these cathodes exhibit some serious problems such as capacity fade and poor thermal stability, which hinders their use in vehicle applications. In particular, oxygen evolution from the delithiated cathode can cause serious safety concerns. These limitations necessitate the development of new materials to resolve the above-mentioned difficulties. Recently, we introduced unique materials, which have a core-shell (core with gradient shell) or full concentration gradient (FCG) structure in a particle level (3). The structure is basically composed of a Ni-rich core that delivers high capacity and a Mn-rich shell that provides outstanding thermal stability. However, the Mn concentration could not be varied because of the difficulty of the rod-shaped particle synthesis.We report the design of FCG Li[NixCo0.16Mn0.84-x]O2 (x = 0.64, 0.59, and 0.51) cathode materials with fixed Mn contents of 20, 25, and 33% in the transition metal layer. These cathode materials with concentration gradients followed the general performance trend of conventional layered materials; an increase in Ni content improved the capacity, whereas a higher amount of Mn delivered better capacity retention and thermal properties at the expense of capacity. We also report the effects of Mn concentration in terms of structural, electrochemical, and thermal characteristics of the FCG materials. As a result, we determined an optimal level of Mn concentration among the tested FCG cathodes, which maximized the discharge capacity of 188 mAh g-1 and had an excellent capacity retention of 96 % over 100 cycles operated up to 4.3 V at 25 oC, with a composition of FCG Li[Ni0.59Co0.16Mn0.25]O2.Reference1. J. B. Goodenough, Y. Kim, Chem. Mater.22 (2010) 5872. V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci.4 (2011) 32433. Y.-K. Sun, S.-T. Myung, M.-H. Kim, J. Prakash, K. Amine, J. Amer. Chem. Soc. 127 (2005) 13411