Thermodynamics and Na kinetics in P2-type oxygen redox Mn-Ni binary layered oxides manipulated via Li substitution

Abstract Sodium-ion batteries (SIBs) have attracted significant attention as promising replacements for lithium-ion batteries (LIBs) due to the wide availability of sodium. However, compared with LIBs, the relatively low energy density, accompanied by fast cycling and rate-performance degradation of SIBs, impedes their practical use. Exploiting oxygen redox reactions increases energy density but induces severe thermodynamic instability with concomitant irreversible capacity decay and large hysteresis. Herein, a combined experimental and theoretical understanding of the P2-type oxygen redox Na2/3[Li1/8Mn5/8Ni2/8]O2 is elucidated from three perspectives: i) electrochemical and structural characterization coupled with thermodynamic phase stability, ii) Na kinetics in terms of various physicochemical factors, and iii) charge compensation mechanism based on Ni and oxygen redox, which is compared with a binary oxide without Li substitution. The experimental results indicate that a quasi-single-phase reaction of Na2/3[Li1/8Mn5/8Ni2/8]O2 occurs during (de)intercalation. It is supported by the thermodynamic formation energies using first-principles calculations. Experimental and computational results consistently reveal improved phase stability compared with the pristine Mn-Ni binary oxide. The kinetic favorability of Na2/3[Li1/8Mn5/8Ni2/8]O2 is theoretically explained by elastic softness, lowered Na migration barrier, and phase stability. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy identifies oxygen redox (above 4.0 V) occurring after the dominant two-electron Ni2+/Ni4+ redox reactions, which is also confirmed by qualitative and quantitative electronic structure analyses. Based on these concrete considerations, the superior cycle and rate performance features of Na2/3[Li1/8Mn5/8Ni2/8]O2 compared with Na2/3[Mn6/8Ni2/8]O2 are demonstrated. This study can serve as a guide to exploit the full potential of oxygen redox properties enabling further advances in SIBs.