Conversion-type transition metal oxides (TMOs) have emerged as promising anode materials for lithium-ion batteries (LIBs) owing to their high theoretical capacities and low cost. Intriguingly, many TMOs exhibit extra capacity that surpasses the limits predicted by conversion reaction mechanisms, challenging traditional electrochemical models and offering opportunities for next-generation high-energy storage. This review highlights the phenomenon of extra capacity in TMO anodes, emphasizing its mechanistic origins and practical implications. While these materials face well-known challenges such as low initial coulombic efficiency, solid electrolyte interphase (SEI) instability, and severe structural degradation due to large volume changes, they offer promising opportunities for achieving high energy density. Special emphasis is placed on understanding the underlying mechanisms that contribute to this anomalous capacity, including the role of reversible SEI formation, lithium-rich phases, reversible formation of LiOH, and interfacial storage phenomena. By clarifying these mechanisms and performance-enhancement approaches, this paper aims to guide future research toward the practical application of high-capacity conversion-type TMO anodes in next-generation LIBs.

Li-ion batteries (LIBs) are seeing increasingly widespread adoption across consumer electronics, electric vehicles, and grid-scale energy storage systems, yet their susceptibility to thermal runaway remains a concern. This study evaluates ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as an electrolyte additive to improve electrolyte flammability and thermal stability without compromising electrochemical performance. Electrolyte flammability was quantified using Self-Extinguishing Time (SET) measurements, which revealed that PFPN significantly suppresses combustion. At 4 wt% PFPN, 67% of electrolyte samples failed to ignite despite extended ignition exposure, and the average SET decreased by 43% (from 51 s g⁻¹ to 29 s g⁻¹). Differential Scanning Calorimetry (DSC) further demonstrated improved thermal stability, with the onset of solvent decomposition delayed by ~30 °C at 4 wt% PFPN. Ionic conductivity modestly decreases (11%, from 10.26 to 9.12 mS cm⁻¹ at 4 wt% PFPN). Electrochemical testing showed negligible impact on battery performance. Graphite||Li and NMC811||Li half-cells containing PFPN exhibited comparable capacity retention to baseline cells. NMC811||Graphite pouch cells were used to further evaluate extended cycling and rate capability, PFPN containing cells demonstrated similar capacities even after prolonged cycling and high-rate operation. Overall, PFPN provides effective flame retardance at concentrations as low as 4 wt% while maintaining electrochemical compatibility, making it a promising additive for enhancing thermal stability of LIB electrolytes.

Li-ion batteries (LIBs) are seeing increasingly widespread adoption across consumer electronics, electric vehicles, and grid-scale energy storage systems, yet their susceptibility to thermal runaway remains a concern. This study evaluates ethoxy (pentafluoro) cyclotriphosphazene (PFPN) as an electrolyte additive to reduce electrolyte flammability and thermal stability without significantly compromising electrochemical performance. Electrolyte flammability was quantified using self-extinguishing time (SET) measurements, which revealed that PFPN significantly suppresses combustion. At 4 wt% PFPN, 60% of electrolyte samples failed to ignite despite extended ignition exposure, and the average SET decreased from 51.15 s g⁻¹ to 34.60 s g⁻¹. Differential scanning calorimetry (DSC) further demonstrated improved thermal stability, with the onset of solvent decomposition delayed by ~30 °C at 4 wt% PFPN. Ionic conductivity modestly decreases (14%, from 8.13 to 6.97 mS cm⁻¹ at 4 wt% PFPN). Electrochemical testing showed negligible impact on battery performance. Graphite||Li and NMC811||Li half-cells containing PFPN exhibited comparable capacity retention to baseline cells. NMC811||graphite pouch cells were used to further evaluate extended cycling and rate capability; PFPN-containing cells demonstrated similar capacities even after prolonged cycling and high-rate operation. Overall, PFPN provides effective flame retardance at 4 wt% while maintaining electrochemical compatibility, making it a promising additive for enhancing thermal stability of LIB electrolytes.