Ultrahigh‐Capacity Rocksalt Cathodes Enabled by Cycling‐Activated Structural Changes

Abstract

Mn-redox-based oxides and oxyfluorides are considered the most promising earth-abundant high-energy cathode materials for next-generation lithium-ion batteries. While high capacities are obtained in high-Mn content cathodes such as Li-and Mn-rich layered and spinel-type materials, local structure changes and structural distortions ( often lead to voltage fade, capacity decay, and impedance rise, resulting in unacceptable electrochemical performance upon cycling. In the present study, structural transformations that exploit the high capacity of Mn-rich oxyfluorides while enabling stable cycling, in stark contrast to commonly observed structural changes that result in rapid performance degradation, are reported. It is shown that upon cycling of a cation-disordered rocksalt (DRX) cathode (Li 1.1 Mn 0.8 Ti 0.1 O 1.9 F 0.1 , an ultrahigh capacity of ≈320 mAh g −1 (energy density of ≈900 Wh kg −1 ) can be obtained through dynamic structural rearrangements upon cycling , along with a unique voltage profile evolution and capacity rise. At high voltage, the presence of Mn 4+ and Li + vacancies promotes local cation ordering, leading to the formation of domains of a "𝜹 phase" within the disordered framework. On deep discharge, Mn 4+ reduction, along with Li + insertion transform the structure to a partially ordered DRX phase with a 𝜷′-LiFeO 2 -type arrangement. At the nanoscale, domains of the in situ formed phases are randomly oriented, allowing highly reversible structural changes and stable electrochemical cycling. These new insights not only help explain the superior electrochemical performance of high-Mn DRXbut also provide guidance for the future development of Mn-based, high-energy density oxide, and oxyfluoride cathode materials.

Li–Mn–O Li-rich cation disordered rock-salt cathode materials do not undergo reversible oxygen redox during cycling

“…S15†). Similar voltage profile evolutions were reported for other cation-disordered rocksalt cathodes like Li 1.1 Mn 0.8 Ti 0.1 O 1.9 F 0.1 , 43 and have been attributed to the formation of a partially ordered disordered rocksalt phase with a β′-LiFeO 2 -type arrangement.…”

The Limited Incorporation and Role of Fluorine in Mn-rich Disordered Rocksalt Cathodes

“…A total of six samples were studied: ss-LMT53, ss-LMT63, ss-LMT81, mw-LMT53, mw-LMT62, and mw-LMT81, corresponding to samples prepared via standard solid-state (ss) and microwave (mw) synthesis methods. While the solid-state synthesis of all three compositions has been reported, ,, the lack of a comprehensive analytical framework able to determine the purity and composition of the resulting product powders has so far prevented the optimization of the synthesis conditions. Here, we employed the hybrid experimental–computational methodology described earlier to identify the optimal reaction parameters to (1) maximize the F content in the DRX phase and (2) reduce the amount of impurity phases in the as-prepared LMT53, LMT62, and LMT81 samples.…”

“…Lithium excess disordered rocksalt oxide (DRX) cathodes have received increasing attention over the past few years due to their high energy densities, compositional flexibility, and promise for more sustainable electrochemical energy storage involving Fe- and Mn-based redox processes. − In contrast to their layered oxide counterparts, significant fluorination has been achieved in DRX compounds, enabled by disorder-induced Li-rich local environments amenable to fluorination. ,,, Many studies have examined the influence of partial F substitution for O on the electrochemical performance of DRX, showing that higher fluorination levels result in a higher and more reversible capacity, and a slower capacity fade. − The benefits of F substitution have largely been attributed to the lower valence of F – compared to O 2– , allowing for a greater fraction of low valent redox-active species, such as Mn 2+ or Mn 3+ , to occupy the cation sites. , In turn, this increased TM-based redox reservoir reduces the dependence on O-based redox processes that lead to greater irreversibilities, oxygen loss, voltage hysteresis, , and particle cracking . Additional benefits of fluorination include more interconnected Li transport pathways through the DRX structure, and less severe Jahn–Teller distortions due to modulation of the crystal field stabilization energy by the F – anions …”

Direct Mapping of Fluorine in Cation Disordered Rocksalt Cathodes

“…With the advent of large-scale energy storage systems, such as electric vehicle batteries and smart power grids, the demand for high energy density, low-cost, and stable cathode materials is continually increasing. , A class of lithium excess disordered rocksalt (DRX) oxides has emerged as a promising cathode candidate for Li ion batteries (LIBs) due to a high specific energy density (∼1000 W-h/kg) and the capability to incorporate earth-abundant materials. − However, the high level of oxygen redox required to deliver a high capacity generally triggers oxygen loss from the Li-excess cathode materials, making them unusable in the long run. − To overcome this problem, fluorination of DRX oxides was proposed . Theoretically, the partial substitution of oxygen by fluorine can effectively decrease the average anion valance and increase the number of redox-active transition metals per formula unit .…”