The growth of sodium dendrites and the associated solid electrolyte interface (SEI) layer is a critical and fundamental issue influencing the safety and cycling lifespan of sodium batteries. In this work, we use in-situ ²³Na magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) techniques, along with an innovative analytical approach, to provide space-resolved and quantitative insights into the formation and evolution of sodium metal microstructures (SMSs; that is, dendritic and mossy Na metal) during the deposition and stripping processes. Our results reveal that the growing SMSs give rise to a linear increase in the overpotential until a transition voltage of 0.15 V is reached, at which point violent electrochemical decomposition of the electrolyte is triggered, leading to the formation of mossy-type SMSs and rapid battery failure. In addition, we determined the existence of NaH in the SEI on sodium metal with ex-situ NMR results. The poor electronic conductivity of NaH is beneficial for the growth of a stable SEI on sodium metal.

钠树枝状晶体和相关的固体电解质界面（SEI）层的生长是影响钠电池安全性和循环寿命的关键和根本性问题。在这项工作中，我们使用原位²³Na磁共振成像（MRI）和核磁共振（NMR）技术，以及创新的分析方法，可为钠金属微结构（SMS）的形成和演化提供空间分辨和定量的见解；即树突状和生苔的Na金属）在沉积和剥离过程中。我们的结果表明，不断增长的SMS会导致过电势线性增加，直到达到0.15 V的过渡电压为止，此时触发电解质的剧烈电化学分解，从而导致生成苔藓型SMS和快速电池失败。此外，我们用异位NMR结果确定了钠金属中SEI中NaH的存在。NaH的不良电导率有利于在钠金属上生长稳定的SEI。