An advanced organic cathode for non-aqueous and aqueous calcium-based dual ion batteries

The development of calcium batteries remains a grand challenge because of the lack of appropriate cathodes and electrolytes as well as their compatibility with promising anodes. Herein, we probe the electrolyte chemistry for realizing an advanced organic polymer cathode, polytriphenylamine (PTPAn), and demonstrate the potential of building reliable Ca-based dual ion batteries in both organic and aqueous electrolytes. Complementary experimental and theoretical studies reveal that the cathode reaction mechanism lies in the reversible combination/release of anions with C–N bond in PTPAn, leading to a capacity of 88 mAh g−1 with an average voltage of 3.8 V (vs. Ca/Ca2+) at 0.1 A g−1. Coupling with a graphite anode in Ca(TFSI)2/tetraglyme electrolytes, the graphite|PTPAn full cell shows a decent voltage of 2.45 V. It exhibits superior stability of over 2000 cycles with extremely fast kinetics up to 50C rate (1C = 0.1 A g−1). Interestingly, PTPAn is also highly compatible with 6.25 M Ca(TFSI)2/H2O electrolytes, allowing the construction of an all-organic aqueous calcium-based dual ion battery by coupling with a 3,4,9,10-perylene-tetracarboxylic-diimide anode. This study demonstrates the potential of building ultra-stable Ca batteries through anion-hosting cathodes coupled with customized electrolyte chemistry.