Defect-Engineered β-MnO2−δ Precursors Control the Structure–Property Relationships in High-Voltage Spinel LiMn1.5Ni0.5O4−δ

This study examines the role of defects in structure-property relationships in spinel LiMn1.5Ni0.5O4 (LMNO) cathode materials, especially in terms of Mn3+ content, degree of disorder, and impurity phase, without the use of the traditional high-temperature annealing (≥700 °C used for making disordered LMNO). Two different phases of LMNO (i.e., highly P4332-ordered and highly Fd3m-disordered) have been prepared from two different β-MnO2-δ precursors obtained from an argon-rich atmosphere (β-MnO2-δ (Ar)) and a hydrogen-rich atmosphere [β-MnO2-δ (H2)]. The LMNO samples and their corresponding β-MnO2-δ precursors are thoroughly characterized using different techniques including high-resolution transmission electron microscopy, field-emission scanning electron microscopy, Raman spectroscopy, powder neutron diffraction, X-ray photoelectron spectroscopy, synchrotron X-ray diffraction, X-ray absorption near-edge spectroscopy, and electrochemistry. LMNO from β-MnO2-δ (H2) exhibits higher defects (oxygen vacancy content) than the one from the β-MnO2-δ (Ar). For the first time, defective β-MnO2-δ has been adopted as precursors for LMNO cathode materials with controlled oxygen vacancy, disordered phase, Mn3+ content, and impurity contents without the need for conventional methods of doping with metal ions, high synthetic temperature, use of organic compounds, postannealing, microwave, or modification of the temperature-cooling profiles. The results show that the oxygen vacancy changes concurrently with the degree of disorder and Mn3+ content, and the best electrochemical performance is only obtained at 850 °C for LMNO-(Ar). The findings in this work present unique opportunities that allow the use of β-MnO2-δ as viable precursors for manipulating the structure-property relationships in LMNO spinel materials for potential development of high-performance high-voltage lithium-ion batteries.