Unveiling Conversion Reaction on Intercalation‐Based Transition Metal Oxides for High Power, High Energy Aqueous Lithium Battery

Aqueous lithium batteries are reaching their energy and power limits partly due to limited capacity from intercalation chemistry. Alternatively, conversion reactions bring added capacity that can significantly increase the capacity ceiling of the cell. However, such reactions can only be realized in organic electrolytes, and similar redox chemistry in aqueous lithium batteries is not reported yet. In this work, it is discovered that a large work function difference (up to 1.78 eV) between crystalline LiMn 2 O 4 (cLMO) and semicrystalline MnO 2 (scMO) makes energy bands of MnO 2 bend downward toward their interface, resulting in the accumulation of free electrons on scMO and inducing a reversible conversion reaction between scMO and Li + in aqueous media. Based on this mechanism, a rechargeable hybrid aqueous battery employing hierarchical porous cLMO–scMO composite as a cathode delivers an extremely high cell‐level energy (171 Wh kg −1 ) and power density (5118 W kg −1 ) with excellent cycling stability (up to 3000 cycles) in a wide working temperature range (25 to 60 °C). More significantly, similar conversion reactions can also be achieved on LiCoO 2 , which provides a general design principle to obtain unprecedented energy and power density in commonly used transition metal oxides.