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.