Dual Roles of Li-Excess for Disordered-Rocksalt Li-Ion Battery Cathodes

Abstract

Disordered-rocksalt (DRX)-type lithium transition-metal (TM) oxides/oxyfluorides are a promising class of cathode materials for Li-ion batteries [1]. These materials deliver high capacity (>250 mAh/g) and energy density (>800 Wh/kg). They can also be made with inexpensive cations (e.g., Mn, Fe, Ti) and anions (e.g., O, F, S), leading to new opportunities towards sustainable high-energy Li-ion batteries with a low cost [1].The interest to DRX-cathodes grew from observations that DRX Li-TM oxides with Li-excess compositions (e.g., x=1.2 in Li x TM2-x O2) could deliver high capacities (>250 mAh/g), while those without Li-excess (e.g., x=1.0 in Li x TM2-x O2) typically exhibited 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) showed a large improvement in reversible capacity with the increasing Li-excess level [3,4]. Also, 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 a de facto design principle of the DRX cathodes.While Li-excess facilitates Li diffusion, it increases the initial average TM valence and decreases the TM content and thus may trade the theoretical TM-redox capacity for O-redox capacity. O-redox can provide alternative electron-capacities, but it also can trigger structural damage (e.g., O loss), resulting in poor cycling stability [1,3,6]. Therefore, recent efforts concentrated on minimizing the O-redox-triggered structural damage in the Li-excess DRX structure. 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 talk, we demonstrate in a case study of Li1.05Mn0.90Nb0.05O2 and Li1.2Mn0.60Nb0.20O2 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 [9]. Our finding that Li-excess is not critical for Mn-based DRX-cathodes with small particles motivates addressing why other DRX chemistries, such as Li-Ni-Ti-Mo oxides, do require Li-excess to deliver high capacity even with nano-sized active particles. By contextualizing our results with other reports from the DRX literature and confirming with Density-Functional-Theory calculations, we elucidate that the 0-TM percolation is not critical at the nanoparticle scale. Instead, Li-excess is necessary to decrease the operating voltage of certain DRX cathodes (Ni/Co/Fe-based DRX), which otherwise would face difficulties in charging due to their very high redox potential. Overall, introducing Li-excess in DRX cathodes modifies the cathode voltage in addition to facilitating Li diffusion through 0-TM percolation, and these dual roles must be simultaneously considered to design for high capacity. References R. J. Clement et al., Energy Environ. Sci. 13, 345–373 (2020).J. Lee et al., Science 343, 519–522 (2014)J. Lee et al., Energy Environ. Sci. 8, 3255 (2015).M. Yang et al., ACS Appl. Mater. Interfaces 11, 44144–44152 (2019).R. Chen et al., Adv. Energy Mater. 5, 1401814 (2015).J. Lee et al., Nature 556, 185–190 (2018).J. Lee et al., Nat. Commun. 8, 981 (2017).Z. Lun et al., Adv. Energy. Mater. 1802959 (2019).J. Lee et al., Adv. Energy. Mater. (in print).