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

Abstract

We have designed and tested several synthesis routes targeting a highly fluorinated disordered rocksalt (DRX) cathode, Li 1.2 Mn 0.4 Ti 0.4 O 1.6 F 0.4 , with each route rationalized by thermochemical analysis. Precursor combinations were screened to raise the F chemical potential and avoid the formation of LiF, which inhibits fluorination of the targeted DRX phase. MnF 2 was used as a reactive source of F, and Li 6 MnO 4 , LiMnO 2 , and Li 2 Mn 0.33 Ti 0.66 O 3 were tested as alternative Li sources. Each synthesis procedure was monitored using a multi-modal suite of characterization techniques including X-ray diffraction, nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry. From the resulting data, we advance the understanding of oxyfluoride synthesis by outlining the key factors limiting F solubility. At low temperatures, MnF 2 consistently reacts with the Li source to form LiF as an intermediate phase, thereby trapping F in strong Li−F bonds. LiF can react with Li 2 TiO 3 to form a highly lithiated and fluorinated DRX (Li 3 TiO 3 F); however, MnO is not easily incorporated into this DRX phase. Although higher temperatures typically increase solubility, the volatility of LiF above its melting point (848 °C) inhibits fluorination of the DRX phase. Based on these findings, metastable synthesis techniques are suggested for future work on DRX fluorination.

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

“…Recent work has shown that LiF is highly volatile above its melting point (848 °C) when placed under flowing Ar. [ 27 ] This conclusion is further supported by the lower‐than‐stoichiometric amounts of Li detected in all LMTOF samples reported here (see ICP results in Table S2, Supporting Information). Notably, fluorination levels reported here are much lower than those previously reported for comparable DRX materials synthesized using a solid‐state approach (generally 2.5‐5 at%).…”

“…[12,[16][17][18][19][20][21][22][23][24][25][26] However, it remains challenging to synthesize DRX compositions with high fluorine content (>10%) using synthesis techniques that are scalable and can provide control over the particle morphology, warranting the exploration of alternate synthesis strategies. [27] DRX cathodes are typically synthesized via high temperature solid-state routes. [17,18,20,28,29] Although such routes often result in the desired DRX phase, they require that precursor powders are well mixed through mechanical mixing/milling to ensure a fast and homogenous reaction upon heating, and they provide poor control over particle morphology.…”

“…[ 12,16–26 ] However, it remains challenging to synthesize DRX compositions with high fluorine content (>10%) using synthesis techniques that are scalable and can provide control over the particle morphology, warranting the exploration of alternate synthesis strategies. [ 27 ]…”

“…However, energy dispersive X‐ray spectroscopy (EDX) and solid‐state nuclear magnetic resonance (ssNMR) spectroscopy measurements reveal that the fluorinated samples have lower‐than‐expected F contents (0.1 – 0.6 at%), likely due to LiF volatilization during heating. [ 27 ] Despite their low F contents, they show promising electrochemical performance with capacities up to ≈275 mAh g −1 and improved capacity retention after 50 cycles compared to the baseline oxide. Notably, some Mn‐rich compositions exhibit an increase in discharge capacity with cycling, which warrants further investigations into the redox mechanisms of these materials.…”

Phase Transition Induced Enhanced Performance of Sodium-Rich Na1.2Mn0.8O2–yFy (y = 0–0.5) Cathodes

“…22,23 In addition, F substitution at the oxygen site further improves the capacity retention and cycle stability by cation valency reduction, suggesting that fluorination is the key to maximizing performance. 10,24 Typically, these cathodes are synthesized by solid-state synthesis and Na−F as the only source of F. However, the dissociation energy of NaF is very high, such that its miscibility into metal oxides is very low when concentration is increased. In the case of cationdisordered rock salt (DRX) structures, fluorine preferentially incorporates into the cation-rich site (such as a Li-rich site), whereas this is severely hindered in ordered rock salt structures.…”