Ionically conducting Li- and Na-phosphonates as organic electrode materials for rechargeable batteries

Facilitating rapid charge transfer in electrode materials necessitates the optimization of their ionic transport properties. Currently, only a limited number of Li/Na-ion organic cathode materials have been identified, and those exhibiting intrinsic solid-phase ionic conductivity are even rarer. In this study, we present tetra-lithium and sodium salts with the generic formulae: A₄-Ph-CH₃P and A₄-Ph-PhP, wherein A = Li, Na; Ph-CH₃P = 2,5-dioxido-1,4-phenylene bis(methylphosphinate); Ph-PhP = 2,5-dioxido-1,4-phenylene bis(phenylphosphinate), as novel alkali-ion reservoir cathode materials. Notably, A₄-Ph-PhP exhibits impressive Li-ion and Na-ion conductivities, measured at 2.6 × 10^-7 and 1.4 × 10^-7 S cm^-1, respectively, in a dry state at 30 °C. To the best of our knowledge, these represent the first example of small-molecule organic cathode materials with intrinsic Li⁺ and Na⁺ conductivity. Theoretical calculations provide further insight into the electrochemical activity of the Li/Na-phenolate groups, as well as the enhanced electron affinity resulting from -phenyl and -Na substitutions. Additionally, Na₄-Ph-PhP displays two distinct charge-discharge plateaus at approximately 2.2 V and 2.7 V, and 2.0 V and 2.5 V <i>vs.</i> Na⁺/Na, respectively, and demonstrates stable cycling performance, with 100 cycles at a rate of 0.1C and an impressive 1000 cycles at 1C. This study not only expands the portfolio of phenolate-based organic salts for use in metal-ion batteries but also underscores the potential of phosphonate-based organic materials in advancing energy storage technologies.