Differential Analysis of Galvanostatic Cycle Data from Li-Ion Batteries: Interpretative Insights and Graphical Heuristics

Differentiation of a Li-ion battery cycling profile (galvanostatic voltage vs charge) yields a pair of complementary measures: differential capacity (dQ/dV vs voltage, also called incremental capacity) and differential voltage (dV/dQ vs charge). These metrics, especially when obtained under experimental conditions approximating cell equilibrium, are widely used to diagnose cell state and mechanisms related to cell aging. Central to the resulting diagnosis is the understanding of how the chemistry of individual electrode materials manifests in the full-cell differential plots. In this article, a fundamental theory of differential analysis is advanced from the starting point of idealized single electrode (half-cell) measurements, building up to the nontrivial translation to full cell data, with consideration of cell balancing and degradation. Various complementary approaches from the existing literature are harmonized with the aid of a novel graphical heuristic in which cell voltage and differential capacity data are superimposed on a 2D representation (“square plot”) of the cell thermodynamic state. Interpretative analysis based on the new approach is demonstrated using data from a commercial pouch cell with the well-characterized LiCoO2|graphite chemistry. A brief review of current literature on practical uses of differential analysis is also provided.