Reevaluating the Criticality of Li-Excess for Disordered-Rocksalt Li-Battery Cathodes
Authors: Lee Jinhyuk , Li Ju 
Published: July 2021

Disordered-rocksalt (DRX)-type lithium transition-metal (TM) oxides/oxyfluorides are a promising class of cathode materials for rechargeable Li-batteries [1]. These materials deliver high capacity (>250 mAh/g) and energy density (>800 Wh/kg), and they can be made with all kinds of inexpensive cations (e.g., V, Cr, Mn, Fe, Nb, Zr, Mo, W, Ta) and anions (e.g., O, F, S), leading to new opportunities to develop sustainable high-energy Li-batteries with a low cost [1].

The attention to DRX-cathodes grew from observations that DRX Li-TM oxides with Li-excess compositions (e.g., x=1.25 in Li
x
TM2-x
O2) could deliver superior capacities (>250 mAh/g), while those without Li-excess (e.g., x=1.0 in Li
x
TM2-x
O2) typically showed limited performance (<100 mA/g)[1,2]. Theoretical studies attribute the difference in performance to percolation of the so-called “0-TM channels”, promoting Li diffusion in the DRX-cathodes, only possible if the Li-excess level exceeds a critical threshold (e.g., x>1.1 in Li
x
TM2-x
O2) [1,2]. Indeed, several new DRX materials (e.g., Li-Ni-Ti-Mo-oxides, Li-Fe-Ti-oxides) showed a large improvement in reversible capacity with the increasing Li-excess level [3,4]. Also, various Li-excess DRX oxyfluorides and sulfides were developed (e.g., Li2VO2F), achieving a very high capacity above 300 mAh/g [1,5,6]. Based on these results, the Li-excess strategy has become the primary consideration for designing the DRX cathodes.

Meanwhile, introducing excess Li for 0-TM percolation sacrifices the theoretical TM-redox capacity for O-redox capacity, as demonstrated in many Li-rich cathodes shown to operate on oxygen redox as an additional electron reservoir. O-redox can provide alternative electron-capacities, but it also can trigger structural damage (e.g., O loss at the particle surface), resulting in poor cycling stability [1,3,6]. As a result, recent efforts reported in the DRX literature have concentrated on minimizing the O-redox-triggered structural damage, which is coupled with improving the Li-transport through a high-level of Li-excess. For example, F-substitution for O could improve the capacity retention of the Li-excess DRX oxides by increasing the TM-redox capacity at a given Li-excess level [7,8].

In this presentation, we will demonstrate that once particle size is sufficiently reduced, Mn-based DRX cathodes can deliver high capacity (>250 mAh/g) regardless of the Li-excess level, effectively removing the Li-excess “constraint” without sacrificing capacity and also reducing the O-redox-related side reactions. Our finding that Li-excess is not critical for cycling Mn-rich DRX-cathodes with small particles motivates addressing why other DRX cathodes with different chemistries, such as Li-Ni-Ti-Mo oxides, do require a high level of Li-excess to deliver high capacity, even with nano-sized active particles. By contextualizing our results with other reports from the DRX literature and investigating through Density-Functional-Theory (DFT) calculations, we find a strong correlation between the TM-redox potential and the measured capacity, revealing that Li-excess is more critical to cycling DRX-cathodes with high TM-redox potential than the ones with lower redox potential. Our work provides new insight into the multifaceted role of Li-excess to the performance of the DRX-cathode beyond 0-TM percolation.

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