In Situ Electrochemical Zn2+-Doping for Mn-Rich Layered Oxides in Li-Ion Batteries

Abstract

Mn-rich layered oxide materials have been considered as promising cathode materials for large scale Li-ion batteries because Mn is more inexpensive than Co and Ni. In this connection, a variety of doped-materials have been examined to improve the electrochemical performance of Mn-rich cathode materials. Doped-materials are conventionally synthesized using solid state synthesis at high temperatures, where most dopants are located at transition metal sites. The amount of redox-active transition metals decreases with increasing the amount of dopants in transition metals sites, resulting in the reduced reversible capacity of doped-materials. This paper demonstrates an in situ electrochemical doping of Zn2+ that is site-selective. Li+ at Li sites in Mn-rich layered oxides is selectively replaced by Zn2+ during cycling. Zn2+ ions in electrolytes are irreversibly inserted to Li sites in delithiated Mn-rich cathode materials during discharge, leading to the formation of Zn2+-doped Mn-rich layered oxides, [Li1-xZny] [Mn1-xMz]O-2 (M = Ni and Co). In contrast to conventional doped-materials, Zn2+ dopants at Li sites do not reduce the reversible capacity of Mn-rich oxide materials. Zn2+ at the Li sites diminishes both cation disorder and electrolyte decomposition during cycling, leading to the improved capacity retention over 100 cycles. In addition, the Zn2+ intercalation is dependent on the amount of Mn in layered oxides and, thereby, only available for Mn-rich cathode materials. Moreover, the in situ electrochemical Zn2+-doping is facile and practical in the aspect of processability, because this only requires an electrolyte additive, such as Zn(TFSI)(2).