Dynamic Structure Evolution of Extensively Delithiated High Voltage Spinel Li₁₊Ni_0.5Mn_1.5O₄ x < 1.5

High voltage spinel is one of the most promising next-generation cobalt-free cathode materials for lithium ion battery applications. Besides the typically utilized compositional range of LiNi_0.5Mn_1.5O4 0 < x < 1 in the voltage window of 4.90-3.00 V, additional 1.5 mol of Li per formula unit can be introduced into the structure, in an extended voltage range to 1.50 V. Theoretically, this leads to significant increase of the specific energy from 690 to 1190 Wh/kg. However, utilization of the extended potential window leads to rapid capacity fading and voltage polarization that lack a comprehensive explanation. In this work, we conducted potentiostatic entropymetry, operando XRD and neutron diffraction on the ordered stoichiometric spinel LiNi_0.5Mn_1.5O₄ within 0 < x < 2.5 in order to understand the dynamic structure evolution and correlate it with the voltage profile. During the two-phase reaction from cubic (x < 1) to tetragonal (x > 1) phase at ∼2.8 V, we identified the evolution of a second tetragonal phase with x > 2. The structural evaluation during the delithiation indicates the formation of an intermediate phase with cubic symmetry at a lithium content of x = 1.5. Evaluation of neutron diffraction data, with maximum entropy method, of the highly lithiated phase LiNi_0.5Mn_1.5O₄ with 2 < x < 2.5 strongly suggests that lithium ions are located on octahedral 8a and tetrahedral 4a positions of the distorted tetragonal phase I4₁amd. Consequently, we were able to provide a conclusive explanation for the additional voltage step at 2.10 V, the sloping voltage profile below 1.80 V, and the additional voltage step at ∼3.80 V.