A Rechargeable Chromium Battery
Multivalent ions exchange multiple electrons during redox reactions, leading to the possibility of improved energy storage performance. A variety of multivalent ions, including zinc (Zn$^{2+}$), magnesium (Mg$^{2+}$), calcium (Ca$^{2+}$), aluminum (Al$^{3+}$), and indium (In$^{3+}$), have been deployed in rechargeable batteries with varying degrees of success \cite{1-9}. While chromium (Cr$^{3+}$) offers a superior volumetric capacity (approximately $11117\ \mathrm{mAh\ cm^{-3}}$) compared to the aforementioned cations, there is no report of a rechargeable chromium battery. This is because chromium metal spontaneously oxidizes to form a passivating oxide layer \cite{10} that blocks Cr$^{3+}$ ingress and egress. Here, we show that this fundamental limitation can be overcome by developing a chromium-rich high-entropy alloy. The alloy consists of five elements (Cr, bismuth (Bi), copper (Cu), tin (Sn), and nickel (Ni)), producing a multi-element native oxide rich in heterointerfaces. Some of these interfaces (such as Cr$_2$O$_3$/Bi$_2$O$_3$) exhibit a very low barrier for Cr$^{3+}$ diffusion, offering multiple pathways for efficient Cr$^{3+}$ insertion and extraction, while others (such as Cr$_2$O$_3$/CuO) block oxygen transport, thereby suppressing further oxidation. In a symmetric cell configuration, the chromium alloy supports approximately $10000$ hours (about $5000$ cycles) of reversible chromium insertion and extraction at an overpotential of only $20$ mV. The chromium-rich alloy anode was also successfully paired with a sulfur cathode to cycle reversibly in a full-cell configuration. These findings could stimulate fundamental studies on chromium-ion batteries and high-entropy alloy electrodes, opening new pathways for multivalent energy storage.