Coordination Chemistry Guides the Design of Mn-Rich Phosphate Cathodes with Fewer Structure Defects

Abstract

Mn-rich phosphate of Na3.3Mn1.15Ti0.85(PO4)3 (NMTP), only containing high-abundance elements, was considered a promising cathode for Na-ion batteries (NIBs) owing to its 2.3-electron reactions and high working voltage based on Mn2+/Mn3+/Mn4+ redox couples between 2.5 and 4.2 V. However, excessive Mn content provides more chances for Mn2+ delocalization to sodium vacancies (Mn/Na□) to generate intrinsic antisite defects (IASDs), which leads to more serious voltage hysteresis in charge/discharge profiles, limiting the energy density of the NMTP system. Herein, we propose to introduce dopants with low valence and electronegativity into the titanium sites of NMTP to regulate Mn–O coordination chemistry. Following this rule, monovalent and low-electronegativity Li+ dopants are selected to build an enhanced Mn–O framework with a Na-rich environment, effectively reducing Mn/Na□ IASDs during the synthesis of materials. Benefited from facilitated Na+ diffusion kinetics because of the less Mn/Na□ IASD-affected pathway in the structure, Li+-doped NMTP (NMTLiP) shows suppressed voltage hysteresis behavior, achieving an energy density increase from 331 to 426 W h kg–1. This current work provides directional guidance from a coordination chemistry regulation perspective to suppress structure defects in Mn-rich phosphate cathodes toward high-energy NIBs.