Batteries will be key in our efforts to reduce CO2 emissions but require major progress in sustainability, cost, and energy density. Liquid-electrolyte metal-sulfur batteries would be game-changers in many respects: a theoretical capacity amongst the highest of all batteries paired with the low cost and sustainability of sulfur. However, intrinsic obstacles are imposed by the electronically and ionically insulating nature of sulfur. Converting sulfur during discharge/charge is fundamentally different from mixed-conducting storage materials. While Li-ion battery materials transform in the solid-state, sulfur converts to metal sulfides in a solid-liquid-solid process. This causes poor cycle life and insufficient energy densities. In this project, we approach the fundamental challenge of sulfur phase transformation in a novel way: high-rate conversion in the bulk solid-state. We will pioneer advanced metrologies such as cryo-electron microscopy and in situ grazing incidence scattering with stochastic modeling to quantify the phase evolution during electrochemical sulfur conversion at atomic and mesoscopic (1-1000 nm) length scales. Based on systematic experiments on 2D transition metal carbide (MXene) substrates, we will establish the scientific foundations of solid-liquid-solid and solid-state sulfur phase transformation. Finally, we will form cathodes as artificial solid mixed conductors by structuring sulfides and MXenes to enable high-rate bulk solid-state sulfur conversion. This will solve the cycle life issue of Me-S batteries and boost the stored energy by maximizing the sulfur packing density. Foundation of SOLIDCON is a systems materials engineering approach, identifying how mutual structuring of storage materials, electron conductors, and ion conductors defines the physicochemical processes across length scales: electron, ion and mass transport, and electrochemical conversion.