Mechanistic Insights into Sodium Storage in N-Doped Graphene/MoS2 Heterostructures Revealed by Operando Raman Spectroscopy

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Two-dimensional (2D) heterostructures offer tunable interlayer spacing and accessible surfaces that can overcome key limitations of conventional sodium-ion battery (SIBs) anodes. Here, we investigate Na+ storage mechanisms in three-layer 2D heterostructures composed of horizontally stacked graphene (G), nitrogen-doped graphene (NG), and MoS2 (M) by combining cyclic voltammetry with operando spectroelectrochemical Raman spectroscopy. Restricting the active thickness to three atomic layers enables direct spectroscopic access to interfacial charge transfer and strain during cycling. Electrochemical analysis reveals that Na+ storage is governed by two coupled factors: the permeability of the electrolyte-facing outer layer, and the electronic–mechanical interactions within the interlayer space. NG outer layers facilitate ion access and deliver up to 40-fold higher capacities than pristine graphene stacks, whereas MoS2 top layers suppress ion infiltration. Operando Raman correlates faradaic features with reversible structural changes, allowing separation of charge-doping and strain effects during cycling. The results demonstrate that Na+ storage proceeds predominantly via reversible accumulation in confined interlayer and blister-like reservoirs formed during solvent cointercalation. Incorporating MoS2 as a middle layer redistributes charge and buffers mechanical strain, enhancing cycling stability. These findings provide insights into charge storage mechanisms in 2D materials.