Effect of Fluorination on Lithium Transport and Short‐Range Order in Disordered‐Rocksalt‐Type Lithium‐Ion Battery Cathodes

Abstract

Fluorine substitution is a critical enabler for improving the cycle life and energy density of disordered rocksalt (DRX) Li‐ion battery cathode materials which offer prospects for high energy density cathodes, without the reliance on limited mineral resources. Due to the strong Li–F interaction, fluorine also is expected to modify the short‐range cation order in these materials which is critical for Li‐ion transport. In this work, density functional theory and Monte Carlo simulations are combined to investigate the impact of Li–F short‐range ordering on the formation of Li percolation and diffusion in DRX materials. The modeling reveals that F substitution is always beneficial at sufficiently high concentrations and can, surprisingly, even facilitate percolation in compounds without Li excess, giving them the ability to incorporate more transition metal redox capacity and thereby higher energy density. It is found that for F levels below 15%, its effect can be beneficial or disadvantageous depending on the intrinsic short‐range order in the unfluorinated oxide, while for high fluorination levels the effects are always beneficial. Using extensive simulations, a map is also presented showing the trade‐off between transition‐metal capacity, Li‐transport, and synthetic accessibility, and two of the more extreme predictions are experimentally confirmed.

Tailoring the Redox Reactions for High‐Capacity Cycling of Cation‐Disordered Rocksalt Cathodes

“…Note that the greater discharge capacity at less fluorination (less Mn) suggests oxygen redox is more prone to participate in less fluorinated samples, meanwhile, the fluorination seems to affect the Li + extraction due to the possibility in modifying the cation short-range ordering. [11] Upon cycling, the capacity decay in less fluorinated samples is consistent with the loss of the highvoltage charge plateau and polarization buildup (Figure 2c and Figure S3, Supporting Information), which is also revealed in the dQ/dV profiles ( Figure S4, Supporting Information). The hysteresis and kinetics are examined by galvanostatic intermittent titration technique (GITT).…”

A Fluorination Method for Improving Cation‐Disordered Rocksalt Cathode Performance

“…[ 1–3 ] In recent years, lithium‐rich cation‐disordered rocksalt oxide cathodes have received significant attention because of their Co‐free chemistry, high energy density (>1000 W h kg −1 ) and high operating voltages, [ 4,5 ] making them attractive alternatives to the conventional layered cathode materials such as lithium nickel manganese cobalt oxides (LiNi x Mn y Co 1‐ x ‐ y O 2 , NMCs). [ 6,7 ] The large charge storage capacity in the DRX materials is attributed to the collective redox activities from a single‐ or multi‐electron pairs of TM cations, such as Ni 2+ /Ni 3+ , [ 8 ] Ni 2+ /Ni 4+ , [ 9–11 ] Mn 2+ /Mn 4+ , [ 12–14 ] Mn 3+ /Mn 4+ , [ 9,13,15–17 ] V 3+ /V 4+ , [ 18,19 ] V 4+ /V 5+ , [ 14 ] Cr 3+ /Cr 6+ , [ 19 ] Mo 3+ /Mo 5+ , [ 20 ] and Mo 3+ /Mo 6+ , [ 21 ] and oxygen anions (O 2− /O n − , 0 ≤ n < 2). [ 22–25 ] On the other hand, the d 0 TM elements ( e.g , 4 d Ti 4+ and Cr 6+ as well as 5 d Nb 5+ and Mo 6+ ) are essential for structural stability but they remain electrochemically inactive.…”

“…[ 27 ] In developing mitigation approaches to the drawback of irreversible oxygen redox behavior, computational modeling and experimental studies on partial substitution of oxygen by fluorine were attempted. [ 8–21,28 ] Table S1, Supporting Information, lists the fluorinated‐DRX compounds reported in the literature so far. In general, the F‐DRX cathodes were found to have better performance compared to their oxide counterparts, with less oxygen release, less impedance rise and better cycling stability.…”

“…), synthesis temperature and duration were found to influence F solubility. [ 8–21 ] For example, higher levels of F substitution were recently achieved on DRX compounds such as Li(1.333)‐Mn 3+ ‐Mn 4+ ‐O‐F, [ 13 ] Li 2 (Mn 2+ ) 2 Nb 1/3 O 2 F and Li 2 (Mn 2+ ) 1/2 Ti 1/2 O 2 F. [ 12 ] These samples were synthesized through a prolonged high‐energy ball‐milling route rather than the typical solid‐state process, making them difficult for commercial scale‐up. For a given composition space such as the Li(1.2)‐Mn 3+ ‐Nb 5+ ‐O‐F (LMNOF) system using the solid‐state synthesis method, it was found that significant LiF phase segregation occurs upon increasing the F content to beyond ≈7.5 at%.…”

Increasing Capacity in Disordered Rocksalt Cathodes by Mg Doping

“…Lun et al 18 argued that the degree of fluorination has a significant impact on cathode material design, by improving the Li percolating network and thus achieving faster ionic diffusion. In addition, Ouyang et al 19 recently showed that fluorination can substantially affect SRO and, at sufficiently high concentrations, is beneficial to Li-ion transport.…”