Direct Operando Quantification of Sodium Transport and Speciation in Hard Carbon Anodes

An operando, chemistry-resolved approach is presented for the quantitative analysis of sodium transport and speciation in hard carbon anodes during electrochemical cycling. By combining Na K-edge X-ray absorption spectroscopy with reference-free X-ray fluorescence analysis, absolute, SI-traceable sodium inventories and species-resolved sodium concentrations are determined directly under operando conditions without calibration standards. Reference spectra of relevant sodium compounds were acquired and used for spectral deconvolution, enabling quantitative determination of Na-containing interphase and conversion products. Focusing on the first galvanostatic cycle, the methodology resolves distinct sodiation and desodiation regimes characterized by different sodium uptake and release rates. The operando measurements reveal the evolution of individual sodium species and distinguish reversible sodium storage from irreversible sodium immobilization within the electrode. By directly linking electrochemical performance to localized sodium inventories and chemical speciation, the approach establishes a quantitative operando framework for mechanistic studies, material optimization, and future sodium-ion battery development.