Li-rich layered oxides, e.g. Li [Li 0.20 Ni 0.13 Mn 0.54 Co 0.13 ]O 2 (LR-NMC), lead high energy density Li-ion battery cathodes, thanks to the reversible redox of oxygen anions that boost charge storage capacity. Unfortunately, their commercialization has been stalled by practical issues (i.e. voltage hysteresis, poor rate capability, and voltage fade) and hence it is necessary to investigate whether these problems are intrinsically inherent to anionic redox and its structural consequences. To this end, the 'model' Li-rich layered oxide Li 2 Ru 0.75 Sn 0.25 O 3 (LRSO) is here used as a fertile test-bed for scrutinizing the effects of cationic and anionic redox independently since they are neatly isolated at low and high potentials, respectively. Through an arsenal of electrochemical techniques, we demonstrate that voltage hysteresis is triggered by anionic redox and grows progressively with deeper oxidation of oxygen in conjunction with the deterioration of both interfacial charge-transfer kinetics and bulk diffusion coefficient. We equally show that this anionic-driven poor kinetics keeps deteriorating further with cycling and we also find that voltage fades faster if oxygen is kept oxidized for longer. Our findings, which are in fact harsher for LR-NMC, convey caution that anionic redox risks practical problems; hence, when chasing larger capacities with this class of materials, we encourage considering real-world applications.

Monitoring the dynamic chemical and thermal state of a cell during operation is crucial to making meaningful advancements in battery technology as safety and reliability cannot be compromised. Here we demonstrate the feasibility of incorporating optical fiber Bragg grating sensors inside commercial 18650 cells. By adjusting fiber morphologies, wavelength changes associated with both temperature and pressure are decoupled with high accuracy, and this allows for tracking of chemical events such as solid electrolyte interphase formation and structural evolution. Additionally, we demonstrate how multiple sensors can function as a microcalorimeter to monitor the heat generated by the cell. Resolving this heat in detail is not possible with conventional isothermal calorimetry and the importance of assessing the cell's heat capacity contribution is presented. Collectively, these findings offer a scalable solution for screening electrolyte additives, rapidly identifying the best formation processes of commercial batteries, and designing thermal battery management systems with enhanced safety.