Design of Battery Electrodes with Dual‐Scale Porosity to Minimize Tortuosity and Maximize Performance

Advances in materials and electrode architecture have facilitated remarkable improvements in the power performance of rechargeable batteries over the past decade, as represented by laboratory demonstrations of fast discharge [ 1–3 ] and commercial realizations such as packaged Li-ion cells with > 20 kW/kg at ∼ 65 Wh/kg. [ 4 ] However, even the most advanced of today’s batteries continue to have poor materials utilization, with only ∼ 50% of cell volume devoted to active materials in cells designed for high-energy density. [ 5 ] Thus, emphasis has now shifted towards maximizing energy density while retaining suffi cient rate capability to power critical applications such as portable devices and electric vehicles. In batteries using porous electrodes, cell-level energy density increases with electrode thickness and density, until limitations on accessible capacity are imposed by inadequate ion transport through a diminishing volume of tortuous, electrolyte-fi lled porosity. [ 6,7 ] For electrodes in which electronic conductivity and charge-transfer are not rate-limiting, ion transport in the percolating electrolyte network becomes limiting as current density increases beyond a critical value. [ 6 , 8 ] Since tortuosity τ , increases nonlinearly with density (e.g., following the Bruggeman relationship τ ∝ ε − 1/2 , where ε is pore fraction), attempts to increase cell energy density by densifying the electrodes rapidly reach diminishing returns. (Throughout, we defi ne τ = ε ( σ 0 / σ ), with ε the volume fraction porosity, σ a measured transport coeffi cient, and σ 0 the value of the transport coeffi cient for the porefi lling medium). Efforts to increase cell-level energy density by increasing electrode thickness (i.e., reducing inactive materials such as separators and current collectors) likewise incur electrolyte-transport limitations. Measured electrode tortuosities vary widely with electrode porosity and preparation, but commonly range between 2.5–30, [ 8–10 ] implying signifi cant room