Physical insights into the potential-dependent Na+ transport in SnO2−x alloying-type anodes for Na-ion micro-batteries

The modern world of microelectronics necessitates cost-effective, durable, high-performance micro-batteries. Na-ion micro-batteries with conversion-alloy-based anodes undergo phase transitions that modify their structural properties, affecting Na+ transport and making them difficult to probe. Here, we investigate Na+ transport in defect-engineered SnO2−x thin films deposited via radio frequency sputtering. Using cyclic voltammetry, the galvanostatic intermittent titration technique, and the distribution of relaxation time (DRT) derived from electrochemical impedance spectroscopy, along with ex situ scanning electron microscopy (SEM), we assess the interfacial and bulk Na+ kinetics. DRT-derived resistance reveals the breathing effect of the solid electrolyte interface layer and the evolution of electronic properties, resulting in lower total interfacial resistance at 0 V during cycling. Ex situ SEM images at the 1st, 100th, 650th, and 5000th cycles demonstrate that the activation process proceeds with pulverization induced by chemo-mechanical stress. These findings will help improve the performance of Na-ion micro-batteries undergoing complex conversion-alloying process.