Commercial Sodium‐Ion Batteries: Understanding Mechanism and Function Using In Operando Neutron Diffraction and X‐Ray Tomography Studies

Abstract Sodium‐ion batteries (SIBs) are emerging as a promising complement to lithium‐ion batteries due to the abundance and low cost of sodium‐based materials. However, their widespread adoption may be limited by electrochemical and structural considerations. In this study, X‐ray diffraction (XRD) and energy‐dispersive X‐ray spectroscopy confirm the presence of O3‐structured Na x Ni 1 /3 Mn 1/ 3 Fe 1 /3 O 2 as the active cathode material and hard carbon templated with CaCO 3 as the anode material in commercial SIBs. The battery demonstrates a gradual capacity decline when operated continuously at 100 mA (89% capacity retention over 100 cycles), with accelerated capacity degradation observed at 1 A, particularly after ≈70 cycles (73% capacity retention over 100 cycles). Continuous cycling at 1 A leads to battery death after about 225 cycles. In operando neutron diffraction is used to reveal reversible phase transitions between O3‐ and P3‐type structures within the 1.5–4.0 V voltage window and solid solution reactions within the O3 and P3 regions. Additionally, computed tomography is employed to assess the internal structural integrity. These findings provide a comprehensive understanding of the structural evolution during function and stability of commercially available SIBs and thus provide essential structural and mechanistic information for further improving their commercial viability.