In lithium-ion batteries (LIBs), many promising electrodes that are based on transition metal oxides exhibit anomalously high storage capacities beyond their theoretical values. Although this phenomenon has been widely reported, the underlying physicochemical mechanism in such materials remains elusive and is still a matter of debate. In this work, we use in situ magnetometry to demonstrate the existence of strong surface capacitance on metal nanoparticles, and to show that a large number of spin-polarized electrons can be stored in the already-reduced metallic nanoparticles (that are formed during discharge at low potentials in transition metal oxide LIBs), which is consistent with a space charge mechanism. Through quantification of the surface capacitance by the variation in magnetism, we further show that this charge capacity of the surface is the dominant source of the extra capacity in the Fe₃O₄/Li model system, and that it also exists in CoO, NiO, FeF₂ and Fe₂N systems. The space charge mechanism revealed by in situ magnetometry can therefore be generalized to a broad range of transition metal compounds for which a large electron density of states is accessible, and provides pivotal guidance for creating advanced energy storage systems.

在锂离子电池（LIB）中，许多基于过渡金属氧化物的有前途的电极都显示出超出其理论值的异常高的存储容量。尽管已经广泛报道了这种现象，但是这种材料中潜在的物理化学机制仍然难以捉摸，仍然是一个有争议的问题。在这项工作中，我们使用原位磁力分析法来证明金属纳米颗粒上存在强表面电容，并表明大量自旋极化电子可以存储在已经还原的金属纳米颗粒中（在放电过程中形成过渡金属氧化物（LIBs）中的低电势，这与空间电荷机制一致。通过磁性变化来量化表面电容，₃ O ₄ / Li模型系统，并且它也存在于CoO，NiO，FeF ₂和Fe ₂ N系统中。因此，通过原位磁力测定法揭示的空间电荷机理可以推广到广泛的过渡金属化合物范围内，对于这些过渡金属化合物而言，可以获得大的电子态密度，并为创建先进的能量存储系统提供了关键指导。