Prof. Dr. Steven Renault

For n-type organic electrode materials, a particularly critical point is the very limited number of positive organic electrode materials that are prepared directly in their reduced state (eg. lithiated) for a Li-ion cell configuration.

In the last fifteen years, our lab has developed lithiated p-dihydroxyterephthalate-based (p-DHT) positive electrode material for Li-organic batteries, starting with dilithium (2,5-dilithium-oxy)-terephthalate salt (Li₄-p-DHT) using an original, solvent-free synthetic route.¹ Galvanostatic cycling delivered approximately one Li⁺ per formula unit corresponding to a reversible capacity of ∼118 mAh g⁻¹ at an average potential of 2.55 V vs Li⁺/Li. The full two-electron theoretical capacity (241 mAh g⁻¹) was not achieved, since oxidation stalled before reaching the fully quinoid state. Instead, the material stabilized in a semiquinone radical intermediate, limiting practical capacity but defining a clear one-electron redox plateau.

Spectator-cation substitution offers an excellent, structure-preserving route to tune the electrochemistry of the p-DHT⁴⁻ redox center in the solid state by acting on coordination electrostatics rather than on the redox-active framework itself. In these salts, Li⁺ remains bound to the two phenolate oxygen atoms that host the reversible hydoquinonate ⇄ semiquinone ⇄ quinone chemistry, while non-redox active “spectator” cations are deliberately chosen in the s-block.

Especially, Mg(Li₂)-p-DHT provides a reference framework for rationalizing the spectator-cation effect in this series of material.² Because of the large ionic potential of Mg²⁺, the Mg–O interaction shows an enhanced covalent character, which significantly reduces the donor inductive effect of the phenolate group conjugated with the aromatic ring, resulting in the increased operating potential observed during reversible Li⁺ extraction (∼3.4 V), despite still being limited to a one-electron reaction.

Our recent extension of this concept from s-block spectator cations to a 3d-block (with Zn²⁺ and Cu²⁺)^3,4 derivative demonstrates its extended validity. While bulk copper (2,5-dilithium-oxy)-terephthalate exhibits limited electrochemical activity when processed as a conventional powder electrode, direct synthesis of this material onto a high-surface-area conductive carbon scaffold enabled access to the second electrochemical process associated with oxidation of the phenolate units to the quinone form. This carbon-supported architecture enabled specific capacities approaching the theoretical capacity for the complete two-electron redox process of the p-dihydroxyterephthalate framework (∼190 mAh g⁻¹), corresponding to the extraction of nearly two lithium ions per formula unit at an average operating potential close to 3.5 V vs Li⁺/Li.

Moreover, the integration of magnesium- and zinc-based (2,5-dilithium-oxy)-terephthalate positive electrodes with polyester-based solid polymer electrolytes was investigated, as well as a recycling route for Mg(Li₂)-p-DHT electrodes. A reprotonation step enabled regeneration of the parent organic precursor 2,5-dihydroxyterephthalic acid, together with the recovery of lithium and magnesium as corresponding salts.

References:

1. Renault, S.; Gottis, S.; Barrès, A.-L.; Courty, M.; Chauvet, O.; Dolhem, F.; Poizot, P. A Green Li-Organic Battery Working as a Fuel Cell in Case of Emergency. Energy and Environmental Science 2013, 6, 2124–2133. https://doi.org/10.1039/C3EE40878G.
2. Jouhara, A.; Dupré, N.; Gaillot, A.-C.; Guyomard, D.; Dolhem, F.; Poizot, P. Raising the Redox Potential in Carboxyphenolate-Based Positive Organic Materials via Cation Substitution. Nat Commun 2018, 9 (1), 4401. https://doi.org/10.1038/s41467-018-06708-x.
3. Shyma Sajeevan, A.; Bernard, L.; Tran-Van, P.; Brandell, D.; Renault, S.; Poizot, P. Combining Polyester-Based Solid Polymer Electrolytes with Lithiated Organic Cathodes for 3.5 V-Class Li-Organic Rechargeable Batteries. ACS Appl. Polym. Mater. 2024. https://doi.org/10.1021/acsapm.4c00511.
4. Shyma Sajeevan, A.; Brandell, D.; Moreau P.; Renault, S.; Poizot, P. Unlocking the Second Electron in the Electrochemical Delithiation/Lithiation of Copper (2,5-Dilithium-oxy)-Terephthalate via a Carbon-Supported Electrode Architecture. ChemSusChem. 2025. https://doi.org/10.1002/cssc.202501002.