Nanostructured birnessite exhibits high specific capacitance and nearly ideal capacitive behaviour in aqueous electrolytes, rendering it an important electrode material for low-cost, high-power energy storage devices. The mechanism of electrochemical capacitance in birnessite has been described as both Faradaic (involving redox) and non-Faradaic (involving only electrostatic interactions). To clarify the capacitive mechanism, we characterized birnessite’s response to applied potential using ex situ X-ray diffraction, electrochemical quartz crystal microbalance, in situ Raman spectroscopy and operando atomic force microscope dilatometry to provide a holistic understanding of its structural, gravimetric and mechanical responses. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite, ReaxFF reactive force field-based molecular dynamics and ReaxFF-based grand canonical Monte Carlo simulations on the dynamics at the birnessite–water–electrolyte interface. We show that capacitive charge storage in birnessite is governed by interlayer cation intercalation. We conclude that the intercalation appears capacitive due to the presence of nanoconfined interlayer structural water, which mediates the interaction between the intercalated cation and the birnessite host and leads to minimal structural changes.

纳米结构的水钠锰矿在水性电解质中表现出高比电容和近乎理想的电容行为，使其成为低成本、高功率储能装置的重要电极材料。水钠锰矿中电化学电容的机制被描述为法拉第（涉及氧化还原）和非法拉第（仅涉及静电相互作用）。为了阐明电容机制，我们使用异位 X 射线衍射、电化学石英晶体微天平、原位拉曼光谱和操作原子力显微镜膨胀法表征了水钠锰矿对施加电位的响应，以提供对其结构、重量和机械响应的整体理解。这些观察得到了原子尺度模拟的支持，该模拟使用密度泛函理论对水钠锰矿的阳离子插层结构、基于 ReaxFF 反应力场的分子动力学和基于 ReaxFF 的对水钠锰矿-水-电解质界面动力学进行的大规范蒙特卡罗模拟. 我们表明水钠锰矿中的电容性电荷存储受层间阳离子插层控制。我们得出结论，由于存在纳米限制的夹层结构水，插层呈现电容性，这介导了插层阳离子与水钠锰矿主体之间的相互作用并导致最小的结构变化。基于 ReaxFF 反应力场的分子动力学和基于 ReaxFF 的大规范蒙特卡罗模拟水钠锰矿-水-电解质界面的动力学。我们表明水钠锰矿中的电容性电荷存储受层间阳离子插层控制。我们得出结论，由于存在纳米限制的夹层结构水，插层呈现电容性，这介导了插层阳离子与水钠锰矿主体之间的相互作用并导致最小的结构变化。基于 ReaxFF 反应力场的分子动力学和基于 ReaxFF 的大规范蒙特卡罗模拟水钠锰矿-水-电解质界面的动力学。我们表明水钠锰矿中的电容性电荷存储受层间阳离子插层控制。我们得出结论，由于存在纳米限制的夹层结构水，插层呈现电容性，这介导了插层阳离子与水钠锰矿主体之间的相互作用并导致最小的结构变化。