Long Cycle‐Life Ca Batteries with Poly(anthraquinonylsulfide) Cathodes and Ca−Sn Alloy Anodes

Calcium (Ca) batteries are attractive post-lithium battery technologies, due to their potential to provide high-voltage and high-energy systems in a sustainable manner. We investigated herein 1,5-poly(anthraquinonylsulfide) (PAQS) for Ca-ion storage with calcium tetrakis(hexafluoroisopropyloxy)borate Ca[B(hfip)$_4$]$_2$ [hfip=OCH(CF$_3$)$_2$] electrolytes. It is demonstrated that PAQS could be synthesized in a cost-effective approach and be processed environmentally friendly into the electrodes. The PAQS cathodes could provide 94 mAh g$^{−1}$ capacity at 2.2 V vs. Ca at 0.5C (1C=225 mAh g$^{−1}$). However, cycling of the cells was severely hindered due to the fast degradation of the metal anode. Replacing the Ca metal anode with a calcium-tin (Ca−Sn) alloy anode, the PAQS cathodes exhibited long cycling performance (45 mAh g$^{−1}$ at 0.5C after 1000 cycles) and superior rate capability (52 mAh g$^{−1}$ at 5C). This is mainly ascribed to the flexible structure of PAQS and good compatibility of the alloy anodes with the electrolyte solutions, which allow reversible quinone carbonyl redox chemistry in the Ca battery systems. The promising properties of PAQS indicate that further exploration of the organic cathode materials could be a feasible direction towards green Ca batteries.