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

Abstract: 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₋<italic>_x</italic>Mg<italic>_y</italic>][Mn₁₋<italic>_z</italic>M<italic>_z</italic>]O₂ (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₋<italic>_x</italic>[Mn₁₋<italic>_z</italic>M<italic>_z</italic>]O₂, which is attributed to the fact that the Mn‐to‐O electron transfer enhances the attractive interaction between Mg²⁺ dopants and electronegative O<italic>^δ</italic>⁻ 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. [ABSTRACT FROM AUTHOR]