Carboxyphenolate Coordination Frameworks for High‐Voltage Calcium Storage

ABSTRACT 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 Ca ²⁺ limits diffusion. Accommodating Ca ²⁺ requires flexible or disordered frameworks that facilitate calcium‐ion mobility. Herein, we investigate conjugated carboxyphenolate coordination frameworks (Ca ₂ ‐ M ‐ THBPD; M = Mg ²⁺ , Ca ²⁺ , Ba ²⁺ ; wherein THBPD = 2,2′,5,5′‐tetraoxido‐[1,1′‐biphenyl]‐4,4′‐dicarboxylate) as amorphous organic positive electrode materials. Ca ₂ ‐ M ‐THBPD operates above 3.5 V vs. Ca ²⁺ /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.