Site-Selective In Situ Electrochemical Doping for Mn-Rich Layered Oxide Cathode Materials in Lithium-Ion Batteries

Various doped materials have been investigated to improve the structural stability of layered transition metal oxides for lithium-ion batteries. Most doped materials are obtained through solid state methods, in which the doping of cations is not strictly site selective. This paper demonstrates, for the first time, an in situ electrochemical site-selective doping process that selectively substitutes Li+ at Li sites in Mn-rich layered oxides with Mg2+. Mg2+ cations are electrochemically intercalated into Li sites in delithiated Mn-rich layered oxides, resulting in the formation of [Li1-xMgy][Mn1-zMz]O-2 (M = Co and Ni). This Mg2+ intercalation is irreversible, leading to the favorable doping of Mg2+ at the Li sites. More interestingly, the amount of intercalated Mg2+ dopants increases with the increasing amount of Mn in Li1-x[Mn1-zMz]O-2, which is attributed to the fact that the Mn-to-O electron transfer enhances the attractive interaction between Mg2+ dopants and electronegative O(delta-)atoms. Moreover, Mg2+ at the Li sites in layered oxides suppresses cation mixing during cycling, resulting in markedly improved capacity retention over 200 cycles. The first-principle calculations further clarify the role of Mg2+ in reduced cation mixing during cycling. The new concept of in situ electrochemical doping provides a new avenue for the development of various selectively doped materials.