A thermochromic polyoxovanadate with a 200 000-fold conductivity gain for boosting zinc-ion battery performance

The performance of zinc-ion batteries is hampered by the cathode's limited Zn²⁺ intercalation capacity and sluggish kinetics. Herein, we propose, for the first time, a thermochromic charge-separation strategy that simultaneously enhances capacity, rate performance, and cycling stability. We synthesized a new organic-inorganic hybrid thermochromic polyoxovanadate (POV), MV₂[H₂V₁₀O₂₈] (MV₂V₁₀, MV = methyl viologen cation) as a cathode, which undergoes thermo-induced electron transfer from O to V, accompanied by a color change from yellow to dark green, and the formation of an ultra-stable charge-separated state lasting over one year, as well as a transition from an insulator to a semiconductor with a 205 000-fold increase in electrical conductivity. After coloration, the capacity increased significantly by 57.3% from 172.8 to 271.8 mAh g^-1 at 0.1 A g^-1, while also exhibiting remarkable rate performance with 61.1% retention at a 100-fold higher current density, and cycling stability with 97.6% retention over 6000 cycles at a high current density of 10 A g^-1. This work presents the first successful application of electron-transfer thermochromism to enhance ZIB performance, offering a promising strategy for the development of advanced cathode materials.