Understanding the Effect of Cation Disorder on the Voltage Profile of Lithium Transition-Metal Oxides

Abstract

Cation disorder is a phenomenon that is becoming increasingly important for the design of high-energy lithium transition metal oxide cathodes (LiMO2) for Li-ion batteries. Disordered Li-excess rocksalts have recently been shown to achieve high reversible capacity, while in operando cation disorder has been observed in a large class of ordered compounds. The voltage slope is a critical quantity for the design of cation-disordered rocksalts, as it controls the Li capacity accessible at voltages below the stability limit of the electrolyte (∼4.5–4.7 V). In this study, we develop a lattice model based on first principles to understand and quantify the voltage slope of cation-disordered LiMO2. We show that cation disorder increases the voltage slope of Li transition metal oxides by creating a statistical distribution of transition metal environments around Li sites, as well as by allowing Li occupation of high-voltage tetrahedral sites. We further demonstrate that the voltage slope increase upon disorder is generally smaller for high-voltage transition metals than for low-voltage transition metals due to a more effective screening of Li–M interactions by oxygen electrons. Short-range order in practical disordered compounds is found to further mitigate the voltage slope increase upon disorder. Finally, our analysis shows that the additional high-voltage tetrahedral capacity induced by disorder is smaller in Li-excess compounds than in stoichiometric LiMO2 compounds.

Reversible Delithiation of Disordered Rock Salt LiVO2

“…The corresponding voltage profiles (Figure b) reveal a sloping behavior, supposing a single‐phase insertion process (see ex‐situ XRD refinements, Figure ). The steepness of the discharge voltage profile slope corresponds to the Li + insertion into a fully disordered structure as proposed by Ceder et al . Nevertheless, the first charging step distinguishes from the further charges.…”

“…Intermixing of the cations, due to Li diffusion within these layers, is regarded as ageing process, which lowers the battery performance ,. Therefore, materials with Li and TM sharing the same sub‐lattice in a cubic close packed array have been rather out of scope of the battery community in the past decades, until the paradigm change induced by various works of theoretical and experimental studies on disordered rock salt structures (DRS) …”

“…[3,4] Therefore, materials with Li and TM sharing the same sub-lattice in a cubic close packed array have been rather out of scope of the battery community in the past decades, until the paradigm change induced by various works of theoretical and experimental studies on disordered rock salt structures (DRS). [5][6][7][8][9][10][11] Only few reports related to the electrochemical behavior of DRS-type LiTMO 2 compounds have been published so far. Above all, following elaborated investigations of Obrovac et al with TM= Ti, Mn, Fe, Co and Ni, the DRS oxides showed poor electrochemical performance, compared to their layered analogous compounds (space group R " 3m).…”

Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

“…Thus the distribution of Li site energies describes the impact of structure on voltage, with a wide spread of local configurations and energies corresponding to an increase in voltage slope characteristic of disordered rocksalts. 60 Here, we identify two mechanisms influencing this distribution in the high-voltage region -the presence of strong Li-F bonds, and the formation of high-voltage tetrahedral Li sites upon delithiation.…”

“…Thus, any Li-conductive ''0-TM'' channel may create a single tetrahedral Li upon charge. 60 Based on the distribution shown in Fig. 5d, 16% of the Li in ST-LMVO could be expected to move to a tetrahedral site.…”