Influence of Transition-Metal Order of LNMO Spinel on the Li Ion Diffusion

Abstract

Cathode materials contribute a substantial portion to the weight, cost, and environmentally sustainability of LIBs. A key strategy to enhance energy density at reduced cost and improved sustainability is to develop more cost-effective, cobalt-free, higher-energy-density alternatives. Among the Co-free electrode cathode chemistries, the LiNi0.5Mn1.5O4 (LNMO) spinel stands out as most promising. It exhibits a high specific energy density of 650 Wh/kg, attributed to its elevated operating voltage of approximately 4.7 V vs. Li+/Li. Furthermore, LNMO is more thermally stable than NMC and has high ionic conductivity. However, LNMO suffers from rapid capacity decay leading to short battery life, and high-voltage electrolytes that are resistant to decomposition at the higher operating voltages are required.LNMO forms two phases depending on the ordering of transition metals (TM). In the disordered phase, Ni and Mn ions are randomly distributed on the 16d sites of the Fd3̅m cubic unit cell, whereas in the ordered phase, the which Ni and Mn atoms occupy 4b sites and 12d sites of the P4332 cubic cell, respectively. In this work, the impact of TM ordering on fundamental parameters such as the unit cell size, electrode expansion, and Li-ion diffusion coefficients at different states of charge were explored using techniques such as in-operando x-ray diffraction, in-situ dilatometry, galvanostatic intermittent titration technique, and electrochemical impedance spectroscopy. The electrodes for these studies were prepared using both PVDF and water-based binders such as CMC in order to gain key insights into the effect of aqueous electrode processing on the Li+-ion transport parameters, interface resistances, and electrode expansion during cycling.