aThe electrochemical kinetics of battery electrodes at the single-particle scale are measured as a function of state-of-charge, and interpreted with the aid of concurrent transmission X-ray microscopy (TXM) of the evolving particle microstructure. An electrochemical cell operating with near-picoampere current resolution is used to characterize single secondary particles of two widely-used cathode compounds, NMC333 and NCA. Interfacial charge transfer kinetics are found to vary by two orders of magnitude with state-of-charge (SOC) in both materials, but the origin of the SOC dependence differs greatly. NCA behavior is dominated by electrochemically-induced microfracture, although thin binder coatings significantly ameliorate mechanical degradation, while NMC333 demonstrates strongly increasing interfacial reaction rates with SOC for chemical reasons. Micro-PITT is used to separate interfacial and bulk transport rates, and show that for commercially relevant particle sizes, interfacial transport is rate-limiting at low SOC, while mixed-control dominates at higher SOC. These results provide mechanistic insight into the mesoscale kinetics of ion intercalation compounds, which can guide the development of high performance rechargeable batteries. Broader contextThe performance of high performance Li-ion batteries, central to both electric transportation and grid scale storage, is ultimately reliant on the performance of critical components such as their cathode and anode compounds. For ease of manufacturing, the prevailing technological forms of these materials are secondary particles of nearly spherical morphology containing many nanocrystallites. The electrochemical kinetics at this critical length scale have been difficult to assess; particle-level behavior has primarily been deduced from macroscale cell measurements. Thus the microelectrode technique developed in this work, combined with state-of-the-art TXM imaging, allows for the first time the direct measurement of electrochemical kinetics of particles as they are charged and discharged. Surprising behavior is revealed -interfacial charge transfer kinetics are found to vary greatly with state-of-charge and cycling history, both for intrinsic chemical reasons (in NMC333) and because of massively damaging ''electrochemical shock'' (in NCA). Moreover, a thin coating of polymer binder is found to ameliorate fracture damage. These results, and the techniques demonstrated, provide a bridge between macroscopic battery function and microscale electrode kinetics as influenced by electrochemomechanical stress and charging history.

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In this work, we demonstrate the stable cycling of more than one Li in solid-state-synthesized ε-LiVOPO4 over more than 20 cycles for the first time. Using a combination of density functional theory (DFT) calculations, X-ray pair distribution function (PDF) analysis and X-ray absorption near edge structure (XANES) measurements, we present a comprehensive analysis of the thermodynamics, kinetics, and structural evolution of ε-Li x VOPO4 over the entire lithiation range. We identify two intermediate phases at x = 1.5 and 1.75 in the low-voltage regime using DFT calculations, and the computed and electrochemical voltage profiles are in excellent agreement. Operando PDF and EXAFS techniques show a reversible hysteretic change in the short (<2 Å) VO bond lengths coupled with an irreversible extension of the long VO bond (>2.4 Å) during low-voltage cycling. Hydrogen intercalation from electrolyte decomposition is a possible explanation for the ∼2.4 Å VO bond and its irreversible extension. Finally, we show that ε-Li x VOPO4 is likely a pseudo-1D ionic diffuser with low electronic conductivity using DFT calculations, which suggests that nanosizing and carbon coating is necessary to achieve good electrochemical performance in this material.

The path toward Li-ion batteries with higher energy densities will likely involve use of thin lithium (Li)-metal anode (<50 µm thickness), whose cyclability today remains limited by dendrite formation and low coulombic efficiency (CE). Previous studies have shown that the solid–electrolyte interface (SEI) of the Li metal plays a crucial role in Li-electrodeposition and -stripping behavior. However, design rules for optimal SEIs are not well established. Here, using integrated experimental and modeling studies on a series of structurally similar SEI-modifying model compounds, we reveal the relationship between SEI compositions, Li deposition morphology, and CE and identify two key descriptors for the fraction of ionic compounds and compactness, leading to high-performance SEIs. We further demonstrate one of the longest cycle lives to date (350 cycles for 80% capacity retention) for a high specific-energy Li||LiCoO2 full cell (projected >350 watt hours [Wh]/kg) at practical current densities. Our results provide guidance for rational design of the SEI to further improve Li-metal anodes.