A Fluorination Method for Improving Cation‐Disordered Rocksalt Cathode Performance

Abstract

In Li‐rich cation‐disordered rocksalt oxide cathodes (DRX), partial fluorine substitution in the oxygen anion sublattice can increase the capacity contribution from transition‐metal (TM) redox while reducing that from the less reversible oxygen redox. To date, limited fluorination substitution has been achieved by introducing LiF precursor during the solid‐state synthesis. To take full advantage of the fluorination effect, however, a higher F content is desired. In the present study, the successful use of a fluorinated polymeric precursor is reported to increase the F solubility in DRX and the incorporation of F content up to 10–12.5 at% into the rocksalt lattice of a model Li‐Mn‐Nb‐O (LMNO) system, largely exceeding the 7.5 at% limit achieved with LiF synthesis. Higher F content in the fluorinated‐DRX (F‐DRX) significantly improves electrochemical performance, with a reversible discharge capacity of ≈255 mAh g−1 achieved at 10 at% of F substitution. After 30 cycles, up to a 40% increase in capacity retention is achieved through the fluorination. The study demonstrates the feasibility of using a new and effective fluorination process to synthesize advanced DRX cathode materials.

Fluorination‐Enhanced Surface Stability of Cation‐Disordered Rocksalt Cathodes for Li‐Ion Batteries

“…[39] This indicates that the cyclinginduced spinel-like structure in LTMOF is partially disordered (rather than a perfect spinel structure), which is consistent with our microscopic observation (Figure 4). In fact, similar electrochemical signatures suggesting the formation of spinellike structures have also been observed during the cycling of Li 1.2 Mn 0.625 Nb 0.175 O 1.95 F 0.05 cathodes, [40,41] indicating that the rocksalt to spinel-like structural transformation may be a common phenomenon existing in Mn-based DRX cathodes.…”

An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes

“…Mn-redox-based oxides and oxyfluorides are by far the most studied so far. ,,,,,,,− Due to the rich chemistry, earth abundance, and low cost of Mn, they are currently viewed as the most promising DRX cathodes. Figure a shows the voltage profiles of a Li 1.3 Nb 0.3 Mn 0.4 O 2 cathode based on the Mn 3+ /Mn 4+ redox couple, one of the most studied DRX compounds. ,,, The initial charge profile consists of two regions that are associated with Mn oxidation at lower voltages and O oxidation at higher voltages, respectively.…”

“…The local chemical inhomogeneity, driven by metal-poor and lithium-rich environments in the disordered rocksalt structure, enables the incorporation of fluorine anions into the oxygen sublattice at a level well above that of surface doping achieved on the ordered layered cathode materials. , Fluorination is typically achieved by using LiF as the F precursor in either SSS or mechanochemical synthesis. While the former limits F content to ∼10 at.%, , much higher F solubility can be achieved in the latter. ,,, Several other factors were also shown to play a role in F solubility, including the chemical nature, content, and valence of both M and M′ cations. − ,,,,,,, …”

“…Average voltages are shown in the labeled lines. Sources of DRX performance data: Mn­(III)-based, refs , , , , , , and ; Mn­(II)-based, ref ; Ni­(II)-based, refs − , , and ; V­(III)-based, refs , , and ; Mo­(III)-based, refs , , and ; and Fe­(III)-based, refs , , and . (b, c) Schematics comparing the crystal structure of (b) a “classic” rocksalt and (c) a cation-disordered rocksalt.…”

Multi-dimensional correlation of layered Li-rich Mn-based cathode materials

“…Therefore, bulk doping of the material is expected to improve the electrochemical performance and structural stability of the material radically. Generally, bulk doping can be carried out at three positions: Li sites [178] ; TM sites [107] ; oxygen active sites [96,179,180] , which can produce different effects and results.…”