Rechargeable Mg/S batteries have the potential to provide a compelling battery for a range of applications owing to their high capacity and gravimetric energy density, safety, and low-cost construction. However, the Mg/S energy storage is not widely developed and deployed due to technical challenges, which include short cycle lifespan and lack of suitable electrolyte. To study the microstructure degradation of Mg/S batteries, multiscale X-ray tomography, an inherently nondestructive method, is performed on dismantled Swagelok Mg/S cells comprising a graphene–sulfur cathode and a super-P separator. For the first time, 3D microstructure visualization and quantification reveal the dissolution (volume fraction decreases from 13.5% to 0.7%, surface area reduces from 2.91 to 1.74 µm µm ) and agglomeration of sulfur particles, and the carbon binder densification after 10 cycles. Using tomography data, the image-based simulations are then performed. The results show that the insoluble polysulfides can inevitably block the Mg transportation via shuttle effect. The representative volume should exceed 8200 µm to represent bulk cathode. This work elucidates that the Mg/S cell performance is significantly affected by microstructural degradation, and moreover demonstrates how multiscale and multimodal characterization can play an indispensable role in developing and optimizing the Mg/S electrode design. 2 −3 2+ 3