Capacity-weighted figures-of-merit for battery transport metrics

Abstract

New battery materials with improved transport are needed. Typical measurements yield widely varying voltage-dependent diffusivities and reporting practices are diverse. Some materials (e.g. first-order phase change) have most redox occur at a specific voltage and may be sufficiently represented by singular transport metrics. Many rapid intercalation materials, however, exhibit second-order phase transitions with redox over a broad voltage range. How should such cases be compared? The use of capacity-weighted average values is suggested where voltage-dependent metrics are consolidated into representative descriptors as figures-of-merit. Examples are elaborated where differential capacity (dQ/dV) is used to derive a weighting function to calculate a diffusivity figure-of-merit (D_Qav). Furthermore, it is shown that galvanostatic techniques can provide transport values with even capacity-weighting such that their mean value (D_Tav) is capacity-weighted. Though equivalent conceptually, the latter approach avoids derivative noise and subjective smoothing. Computational diffusion values can similarly include capacity-weighted figures-of-merit. Lastly, diffusivity uncertainty is addressed which is dominated by surface area error due to the second-power dependence. Best-practices can reduce the diffusivity error from ∼40% to ∼2% using appropriate BET sorbents or SAXS with thickness measurements. These perspectives improve the comparison of battery materials with a diffusivity figure-of-merit that supports performance-ranking with attention to uncertainty.