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

Abstract

Cation-disordered rocksalts (DRXs) have emerged as a new class of highcapacity Li-ion cathode materials. One unique advantage of the DRX chemistry is the structural flexibility that substantially lessens the elemental constraints in the crystal lattice, such as Li content, choice of transition metal redox center paired with appropriate d 0 metal, and incorporation of F anion, which allows optimization of the key redox reactions. Herein, a series of the DRX oxyfluorides based on the Mn redox have been designed and synthesized. By tailoring the stoichiometry of the DRX compositions, high-capacity cycling by promoting the cationic Mn 2+ /Mn 4+ redox reactions while suppressing those from anionic O is successfully demonstrated. A highly fluorinated DRX compound, Li 1.2 Mn 0.625 Nb 0.175 O 1.325 F 0.675 (M 0.625 F 0.675), delivers a capacity of ≈170 mAh g −1 at C/3 for 100 cycles. This work showcases the concept of balancing the cationic and anionic redox reactions in the DRX cathodes for improved electrochemical performance through the rational composition design.

Understanding the Fluorination of Disordered Rocksalt Cathodes through Rational Exploration of Synthesis Pathways

“…7−11 Recent work on Mn/Nbbased DRX oxyfluorides demonstrated that capacity retention improves upon increasing substitution of O with F, suggesting that maximal fluorination is key to optimizing performance. 12,13 DRX oxyfluorides are conventionally made by solid-state synthesis with LiF as the single source of F. Because LiF contains highly ionic Li−F bonds with a large dissociation energy, the solubility limit between LiF and most lithium transition metal oxides is low. For DRX oxyfluorides with a composition of Li 1.2 (Mn/Ti) 0.8 O 2−x F x , first-principles calculations indicate that only 5% O/F substitution (x ≤ 0.1) can be achieved at a synthesis temperature of 1000 °C.…”

Fluorination‐Enhanced Surface Stability of Disordered Rocksalt Cathodes

“…[12a] Correspondingly, the increased TM redox capacity accommodated upon fluorination reduces the capacity reliability on O redox and thus mitigates irreversible O loss. [2b,5,9,12a,13] Due to the presence of local TM‐poor, Li‐rich environments in the DRX lattice that are highly favorable for F – , [ 13,14 ] substantial levels of fluorine (F) substitution have been achieved in various DRX materials, such as Li 2 VO 2 F, [ 15 ] Li 1.2 Mn 0.7 Nb 0.1 O 1.8 F 0.2 , [ 5 ] Li 1.2 Mn 0.75 Nb 0.05 O 1.7 F 0.3 , [ 16 ] Li 1.2 Mn 0.625 Nb 0.175 O 1.325 F 0.675 , [ 17 ] Li 2 Mn 2/3 Nb 1/3 O 2 F, [2b] Li 1.2 Ti 0.2 Mn 0.6 O 1.8 F 0.2 , [ 18 ] and Li 2 MoO 2 F. [ 19 ] In general, fluorination has been demonstrated as a very effective route for improving the cycling stability in all these fluorinated DRX (F‐DRX) cathodes.…”

Exposure History and its Effect Towards Stabilizing Li Exchange Across Disordered Rock Salt Interfaces

“…Although this has been proven true in some cases, experimental and theoretical studies [11] conducted by Ceder et al [12,13] have indicated that Li and TM crystallized in a disordered rock salt (DRX) lattice have the potential to reversibly store Li by exploiting the Li percolation network. [13] The discovery of the percolation network has opened up the material space for cathodes, allowing the use of various transition metal redox chemistries for charge compensation during (de)lithiation, for example, Mn 3þ =Mn 4þ , [14,15] Mn 2þ =Mn 4þ , [13,16] Cr 3þ =Cr 5þ , [17] Mo 3þ =Mo 6þ , [18] and V 3þ =V 5þ . [17] DRX cathodes unlike their cation-ordered counterparts permit only minimal isotropic volume changes during cycling, remedying issues of structural degradation.…”

“…In this work we focus on optimizing the electrolyte exposure conditions for Li 1:2 Mn 0:625 Nb 0:175 O 1:95 F 0:05 (LMNOF), [13,16,[25][26][27] a promising high-capacity DRX cathode material, by exposing it to electrolyte at elevated temperatures (60 °C) to understand the effect this procedure has on the cathode's performance. Previous work on NMC and Li rich rock salt oxides indicate surface reconstruction [28] under conditions much less severe than this (60 °C vs. room temperature and instantaneous instead of 24-240 hours).…”