where 𝜑 is the volume fraction occupied by the SE and 𝐷𝑒𝑓𝑓 is the effective diffusion coefficient for Li ions in the SE medium. 𝐷𝑒𝑓𝑓 is in turn calculated by solving Fick's first law in the SE domain, with a concentration difference 𝛥𝑐 between the outer xy planes. 𝐷𝑒𝑓𝑓 is obtained from the overall diffusive flux 𝑗 as: