Abstract
Li-rich layered oxides, e.g. Li[Li0.20Ni0.13Mn0.54Co0.13]O2 (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 Li2Ru0.75Sn0.25O3 (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.
Highlights
Results‘Staircase-like’ first-charge - first step at 3.6 V.—The typical voltage vs. composition trace of a Li half-cell, having either LR-NMC or LRSO as the positive electrode, shows a stair-case voltage profile on oxidation that is modified into an ‘S-shaped’ one on reduction (Figure 1a)
A2975 phenomena that take place together in LRSO and quantifying their individual contributions to performance decay calls for the development of electrochemical and phenomenological models, similar to those recently implemented for LR-NMC.[59,64]
The staggering increase in capacity brought by anionic redox in Li-rich layered materials comes at a price, i.e. it simultaneously triggers hysteresis, promotes voltage fade, and builds up impedance upon cycling
Summary
‘Staircase-like’ first-charge - first step at 3.6 V.—The typical voltage vs. composition trace of a Li half-cell, having either LR-NMC or LRSO as the positive electrode, shows a stair-case voltage profile on oxidation that is modified into an ‘S-shaped’ one on reduction (Figure 1a). Such a correlation between D/R2 and dQ/dV again characterizes a typical intercalation mechanism, known for graphite[40,41] and spinel-LiMn2O4,42 that has theoretical origins in non-idealities expressed by a thermodynamic factor for activity correction.[43,44]. Post-activation – hysteresis and path dependence.—Fundamental exploration of the first cycle is essential, but when considering realworld applications, it is important to know how the cell behaves after reaching its transformed state It takes around five cycles with a CCCV protocol (at C/5 with C/100 cutoff) or ten cycles with a CC protocol (at C/5) within 2–4.6 V for LRSO to achieve a fairly stable voltage profile on charge and discharge. Post-activation, GITT findings in Figure 4a show a significant voltage gap (∼100 mV) between OCVs
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