Identification of the Key Structure Factor to Achieve High Energy Density in Spinel LiNi0.5Mn1.5O4

Extending the lower cutoff potential of spinel LiNi0.5Mn1.5O4 to 2 V holds the potential to significantly increase the energy density from approximately 600 to around 900 W h/kg by utilizing an extra ∼2.7 V plateau. However, precisely adjusting the length of the ∼2.7 V plateau has proven to be a challenging task, and the decisive structural factor has remained elusive. In this study, a thorough and comparative investigation employing time-of-flight neutron powder diffraction, aberration-corrected scanning transmission electron microscopy, combined with theoretical calculations, has unveiled, for the first time, the key structural feature influencing the length of the ∼2.7 V plateau, a specific point defect, that is, Ni occupation at the 16c octahedral site. It drives Li ions at the neighboring 8a tetrahedral site to move to the 16c site and accelerates the T1 → T2 phase transition, resulting in a shorter ∼2.7 V plateau. Ultimately, the cathode material with the vacant 16c site exhibits a prolonged ∼2.7 V plateau, achieving a higher energy density of approximately 900 W h/kg than that of the cathode material with ∼5.4% Ni at the 16c site (around 800 W h/kg). These findings offer valuable insights for achieving high-energy-density and cost-effective spinel cathodes through meticulous microstructure regulation.