Proton Intercalation into an Open-Tunnel Bronze Phase with Near-Zero Volume Change

Managing safety and supply-chain risks associated with lithium-ion batteries (LIBs) is an urgent task for sustainable development. Aqueous proton batteries are attractive alternatives to LIBs because using water and protons addresses these two risks. However, most host materials undergo large volume changes upon H⁺ intercalation, which induces intraparticle cracking to accelerates parasitic reactions. Herein, we report that Mo₃Nb₂O₁₄ bronze exhibits reversible H⁺ intercalation (200 mAh g^-1) with a Coulombic efficiency of 99.7 % owing to near-zero volume change and solid-solution-type phase transition. Combination of experimental and theoretical analyses clarifies that rotation and shrinkage of open tunnels, which consist of flexible corner-sharing Mo/NbO_n polyhedra, relieve local structural distortions upon H⁺ intercalation to suppress intraparticle cracking. The prototype full cell of an aqueous proton battery with a Mo₃Nb₂O₁₄ anode operates stably over 1000 charge/discharge cycles. This study reveals the importance of implementing distortion-relieving voids in host materials to reduce volume change upon charge/discharge.