Increasing Capacity in Disordered Rocksalt Cathodes by Mg Doping

Zhong, Peichen; Cai, Zijian; Zhang, Yaqian; Giovine, Raynald; Ouyang, Bin; Zeng, Guobo; Chen, Yu; Clément, Raphaële J.; Lun, Zhengyan; Ceder, Gerbrand

doi:10.1021/acs.chemmater.0c04109

Cited by 28 publications

(27 citation statements)

References 46 publications

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“…Indeed, no LiF peak is found in the NMR spectrum (δ iso = – 204 ppm) from the product synthesized from conventional precursors. Furthermore, the signal-to-noise ratio in the 19 F NMR of our synthesis product is much lower than typically observed for DRX materials with a nominally high F content, ,, synthesized by ball milling rather than by a traditional solid-state method. These findings support our conclusion that LiF is absent from the sample and that the synthesized DRX composition likely contains very little F because most of the LiF precursor evaporated during synthesis.…”

Section: Resultsmentioning

confidence: 66%

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

et al. 2022

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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.

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Section: Resultsmentioning

confidence: 66%

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

et al. 2022

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“…We suggest that these features will tend to reduce the magnitude of off-lattice anion displacements at high states of charge and inhibit TM migration. Achieving a lower level of Li-excess in Li 2 MnO 2 F, if the level of fluorination is kept constant, opens the possibility of using low-valent dopants such as Mg 2+ and Zn 2+ 58 , in contrast to high-valent d 0 dopants such as Ti 4+ and Nb 5+ that are often used in disordered rocksalt cathodes 39 .…”

Section: Resultsmentioning

confidence: 99%

Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

et al. 2022

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Lithium-rich disordered rocksalt cathodes display high capacities arising from redox chemistry on both transition-metal ions (TM-redox) and oxygen ions (O-redox), making them promising candidates for next-generation lithium-ion batteries. However, the atomic-scale mechanisms governing O-redox behaviour in disordered structures are not fully understood. Here we show that, at high states of charge in the disordered rocksalt Li2MnO2F, transition metal migration is necessary for the formation of molecular O2 trapped in the bulk. Density functional theory calculations reveal that O2 is thermodynamically favoured over other oxidised O species, which is confirmed by resonant inelastic X-ray scattering data showing only O2 forms. When O-redox involves irreversible Mn migration, this mechanism results in a path-dependent voltage hysteresis between charge and discharge, commensurate with the hysteresis observed electrochemically. The implications are that irreversible transition metal migration should be suppressed to reduce the voltage hysteresis that afflicts O-redox disordered rocksalt cathodes.

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“…We suggest that these features will tend to reduce the magnitude of off-lattice anion displacements, and inhibit TM migration. Achieving a lower level of Li-excess in Li 2 MnO 2 F, if the level of fluorination is kept constant, opens the possibility of using low-valent dopants such as Mg 2+ and Zn 2+ , [67] in contrast to high-valent d 0 dopants such as Ti 4+ and Nb 5+ that are often used in disordered rocksalt cathodes. [41] An alternative approach to the design of Li-rich disordered rocksalt cathodes will be to allow O 2 formation and then aim to achieve O-O dimerisation with reversible TM migration while preventing irreversible TM migration.…”

Section: Discussion: Strategies To Harness Reversible O-redoxmentioning

confidence: 99%

Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

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et al. 2021

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Lithium-rich disordered rocksalt cathodes display high capacities arising from redox chemistry on both transition-metal and oxygen ions and are potential candidates for next-generation lithium-ion batteries. The atomic-scale mechanisms governing this O-redox behaviour, however, are not fully understood. In particular, it is not clear to what extent transition metal migration is required for O-redox and what role this may play in explaining voltage hysteresis in these materials. Here, we reveal an O-redox mechanism linking transition metal migration and O2 formation in the disordered rocksalt Li2MnO2F. At high states of charge, O-ions dimerise to form molecular O2 trapped in the bulk structure, leaving vacant O sites surrounding neighbouring Mn ions. This undercoordination drives Mn movement into new fully-coordinated octahedral sites. Mn displacement can occur irreversibly, which results in voltage hysteresis, with a lower voltage upon discharge as observed experimentally. Alternatively, Mn displacement may take place into interstitial octahedral sites, which permits a reversible return of the Mn ion to its original site upon discharge, recovering the original Li2MnO2F structure and resulting in reversible O-redox without voltage loss. These new findings suggest that reversible transition metal ion migration provides a possible design route to retain the high energy density of O-redox disordered rocksalt cathodes on cycling.

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Increasing Capacity in Disordered Rocksalt Cathodes by Mg Doping

Cited by 28 publications

References 46 publications

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

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

Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

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