Exceptional Cycling Performance Enabled by Local Structural Rearrangements in Disordered Rocksalt Cathodes

Abstract

cost and thermal stability of Mn. [4] However, both families of cathodes face significant challenges in maintaining their structural integrity and rate capability upon cycling, with rapid capacity fade, significant voltage hysteresis, and impedance rise often accompanying the gradual transformation in local and/or long-range structure upon Li (de)intercalation. [5][6][7][8] At the material level, side reactions with the electrolyte, TM reduction and dissolution, TM migration and structural transformation as well as irreversible O redox chemistry, have all been suggested to contribute to these issues. [9][10][11][12][13][14][15] To minimize capacity loss in DRX, increasing Mn redox contribution at the expense of O-based redox processes is an effective strategy, as the former chemistry is inherently more reversible than the latter. [15][16][17][18] Fluorine substitution into the O anion sublattice not only stabilizes O redox, as shown by a number of recent studies, [10,[18][19][20][21][22] but also lowers the overall anionic valence and allows for an increased amount of Mn to be incorporated into the material. In contrast to the layered structure, F substitution into the cubic DRX structure can easily be achieved during synthesis. Substitution levels up to 10 at.% and 33 at.% have previously been reported on fluorinated Mn-based DRX samples prepared by solid-state and high-energy ball milling synthesis methods, respectively. [10,18,19,20] Although electrochemical performance improvements have been observed even at low levels of FThe capacity of lithium transition-metal (TM) oxide cathodes is directly linked to the magnitude and accessibility of the redox reservoir associated with TM cations and/or oxygen anions, which traditionally decreases with cycling as a result of chemical, structural, or mechanical fatigue. Here, it is shown that a capacity increase over 125% can be achieved upon cycling of high-energy Mn-and F-rich cation-disordered rocksalt oxyfluoride cathodes. This study reveals that in Li 1.2 Mn 0.7 Nb 0.1 O 1.8 F 0.2 , repeated Li extraction/reinsertion utilizing Mn 3+ /Mn 4+ redox along with some degree of O-redox participation leads to local structural rearrangements and formation of domains with off-stoichiometry spinel-like features. The effective integration of these local "structure-domains" within the cubic disordered rocksalt framework promotes better Li diffusion and improves material utilization, consequently increased capacity upon cycling. This study provides important new insights into materials design strategies to further exploit the rich compositional and structural space of Mn chemistry for developing sustainable, high-energy cathode materials.

Alternate Synthesis Method for High‐Performance Manganese Rich Cation Disordered Rocksalt Cathodes

“…One possible explanation is the formation of a spinel‐like phase upon extended cycling as was recently reported for other Mn‐based DRX cathodes. [ 22,47,48 ] Electron diffraction patterns collected on pristine and cycled LMTOF07 cathode (15 cycles) support this conclusion as shown in Figure S10 (Supporting Information). Notably, these changes are not observed in the LMTO04 and LMTOF05 compositions which maintain sloping voltage profiles after extended cycling.…”

“…The superior cycling stability of LMTOF05, LMTOF06, and LMTOF07 is attributed to an increased contribution from Mn 3+ /Mn 4+ redox and therefore reduced dependence on oxygen redox. In LMTOF06 and LMTOF07, the increasing capacity with cycling may be due to the formation of spinel‐like phases with improved Li + diffusion and hence higher active material utilization, [ 47,48 ] although we note that additional characterization studies are needed to support this conclusion. In LMTOF07, although the discharge capacity increases over the first 50 cycles, it reaches a peak value of 275 mAh g −1 after 20 cycles before decreasing to 250 mAh g −1 after 50 cycles, with more rapid capacity fade upon extended cycling (Figure S12, Supporting Information).…”

Unraveling the Nature and Role of Layered Cation Ordering in Cation-Disordered Rock-Salt Cathodes

“…It is well known that lithium transition metal oxides can form a rich variety of structures, depending on the physicochemical properties of the transition metal element, composition, and synthesis condition. Here, we briefly discuss the possibility of other types of Li–TM ordering in DRXs, considering that the Raman activity of rock salts can be activated by various mechanisms. − In recent years, it has been reported that other types of short-range structural features can form in DRXs due to various reasons. ,,, A few of these works are based on Raman spectroscopic analyses. ,− For example, electrochemical cycling of Mn-rich DRXs has been shown to generate spinel-like cation arrangements. , To examine the role of cation arrangement in Raman spectroscopic features, we compared the Raman spectra of DRXs with other lithium transition metal oxide compounds. All of these compounds are based on an fcc or near-fcc oxygen sublattice.…”

“…37,41−43 For example, electrochemical cycling of Mnrich DRXs has been shown to generate spinel-like cation arrangements. 41,43 To examine the role of cation arrangement in Raman S15a. These compounds are also selected for comparison because cations in these compounds fully occupy the octahedral sites of the oxygen sublattice (1:1 cation/anion ratio), similar to LiCoO 2like layer-structured cathodes.…”

Towards commercialization of fluorinated cation-disordered rock-salt Li-ion cathodes

“…Then, the percolation of Li-ion is enabled by the continuous connection of the 0-TM local structures. The discovery of the Li-ion percolation mechanism has led to significant attention to the DRXs (Yabuuchi et al, 2016;Cai et al, 2021;Ahn et al, 2022;Pei et al, 2022;Zhou et al, 2022). However, many thresholds must be overcome for the commercialization of DRXs, such as low cyclic retention, wide operation voltage, and unsuitable synthetic procedures.…”