Carboxyphenolate Coordination Frameworks for High‐Voltage Calcium Storage

Calcium batteries are attractive candidates for next-generation energy storage offering both their high theoretical energy density and the advantage of abundant, naturally available calcium. However, the design of high-voltage positive electrodes for Ca-ion-containing systems remains challenging, with only few examples reported to date. The main difficulties arise from the synthesis and ion-storage characteristics of Ca-based materials, as the high polarizing power of Ca2+ limits diffusion. Accommodating Ca2+ requires flexible or disordered frameworks that facilitate calcium-ion mobility. Herein, we investigate conjugated carboxyphenolate coordination frameworks (Ca2-M-THBPD; M = Mg2+, Ca2+, Ba2+; wherein THBPD = 2,2′,5,5′-tetraoxido-[1,1′-biphenyl]-4,4′-dicarboxylate) as amorphous organic positive electrode materials. Ca2-M-THBPD operates above 3.5 V vs. Ca2+/Ca (median discharge voltage 3.55 V) with low hysteresis and polarization, enabled by the synergy of amorphous disorder, enolate-quinone redox activity, and inductive spectator-cation effects. The electrode delivers a discharge capacity of 120 mAh g−1 with a Coulombic efficiency of 99.8%, retaining 75% of its initial capacity after 200 cycles at a C/20 rate. This study demonstrates, the use of reduced-state conjugated carboxyphenolate frameworks as active materials for high voltage divalent cation storage, highlighting how spectator cations and framework flexibility influence redox potential and long-term stability in Ca-ion batteries.

Affiliations

UCLouvainSST/IMCN/MOST - Molecular Chemistry, Materials and Catalysis

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Bakuru, V. R., Darsi Rambabu, Lin, X., Markowski, R., Apostol, P., Frano, V., Guo, X., Purkait, T., Pal, S., Goossens, T., Tie, D., Ramackers, A., & Vlad, A. (2026). Carboxyphenolate Coordination Frameworks for High‐Voltage Calcium Storage. Advanced Science. Published. https://doi.org/10.1002/advs.75481 (Original work published 2026)