Electrochemical behaviour of Na-ion and Na-S batteries under external magnetic fields.

External magnetic fields (MF) represent a promising, non-invasive method for enhancing battery performance, yet their impact on sodium-based battery chemistry remains poorly understood. In this novel study, we systematically investigate the electrochemical behaviour of sodium symmetric (Na|Na) and sodium-sulphur (Na|S) half-cells under static magnetic fields ranging from 50 to 450 mT, in the absence of magnetic additives, in order to isolate the effects of the external field itself. Through galvanostatic cycling and impedance spectroscopy, we observe clear MF-dependent changes in interfacial polarisation and charge-transfer resistance, with the most pronounced effects at 250 mT. In Na|Na cells, magnetic field exposure led to more stable potential profiles and reduced polarisation compared to controls, which exhibited signatures of dendrite formation and soft short-circuiting. In Na|S cells, magnetic field application influenced redox behaviour and capacity retention, particularly under moderate-rate cycling (0.5 C), where MF-treated cells delivered higher capacities (~ 100 mAh g−1 more) and improved cycling stability over 100 cycles. Post-mortem analysis revealed differences in separator staining and cathode morphology consistent with moderated migration of sulphur-containing intermediates. These findings demonstrate that magnetic fields, even in the absence of magnetic additives, can noticeably influence interfacial kinetics and ion transport in sodium-based batteries, contributing to a broader understanding of field-assisted electrochemical systems.