For batteries to be a viable solution for large-scale energy storage, higher capacity or higher voltage (effectively higher energy) electrode materials are required. Present-day technology employs transition metal oxides where lithium ions undergo reversible intercalation, which fall short of capacity and voltage requirements for the next-generation lithium batteries. Lithium-rich layered oxides (LRLO, a composite of classical layered oxide LiTMO₂ and Li₂TMO₃ where TM stands for a transition metal) do meet these requirements; however, their lifespan is severely limited given the performance degradation over electrochemical charge-discharge. Specifically, the origins of voltage fade, the decrease in electrode voltage over time, remain poorly understood.

Voltage fade manifests as a curious phenomenon as it is revealed only during operation at large voltages (≥4.3 V). Shirley Meng and her co-workers from the University of California-San Diego, the Chinese Academy of Sciences, Argonne National Laboratory, and Deutsches Elektronen-Synchrotron suspected structural changes to be the root cause of voltage fade in LRLO. They investigated the physicochemical transformations at different electrochemical states in LRLO using in situ Bragg coherent diffraction imaging setup. Similar experiments were also carried out on traditional nickel cobalt aluminum oxide (NCA) to delineate the fundamental difference between the intercalation dynamics of LRLO and NCA.

The researchers identified the presence of dislocations at higher voltages in the LRLO particles, as seen in parts (b) and (c) in the Figure. Specialized, in situ cells were used to simultaneously carry out electrochemical cycling and x-ray diffraction studies. The successive diffraction profiles detail displacement (i.e., strain) field. The dislocations appear as a singularity in the displacement field and indirectly identified. These dislocations nucleate as voltage is increased and gradually evolve into a clustered network. They give rise to edge, and plane defects, which in turn locally alter the microstructure in the bulk material. Such disorder leads to structural changes, and even when the dislocations disappear, the ordering of the microstructure is unlikely to recover (disordered materials are by nature metastable with an energetically unfavorable local lithium environment). On the other hand, similar experiments on NCA exhibit much fewer dislocations. The voltage fade in each was recorded and a direct correlation between the increased disorder and voltage fade is identified.

The researchers hypothesized the association between dislocation-induced disorder and voltage fade. If this mechanistic interpretation of the complex evolution were true, reordering should recover the voltage. The researchers extracted LRLO electrodes and annealed them above 150°C. Such heat-treated electrodes exhibited fairly good relapse toward the pristine response, thus affirming the connection between structural evolution and electrochemical degradation. The conventional materials have a higher lithium diffusivity in the bulk and lower ductility as compared to the LRLO and, in turn, would lead to the much fewer dislocations formation and structural changes.

Jesse Ko from the US Naval Research Laboratory in Washington DC, not affiliated with the work, says that the use of operando three-dimensional imaging by in situ Bragg coherent diffractive imaging technique elucidates the complexity of charging dynamics and the associated structural changes caused by defects formed at high potentials. He further says that the restoration of the voltage in the lithium-rich oxide material by subsequent heat-treatment presents a new direction for studying the anionic activity of these materials.

Meng, from UC-San Diego, says that in the past five years, the research community has made great progress on understanding key features and dynamic phenomena that govern the performance limitations of LRLO. However, voltage decay during the cycling process is still unsolved, even believed to be an intrinsic and inevitable consequence of anionic activity that results in the extra capacity. With this publication, for the first time known, Meng says her team demonstrated that the dislocation defects in the cycled LRLO cathode are responsible for the voltage decay during the cycling process. “And more importantly, we design a path—heat treatment—to eliminate the defects in the bulk structure of the cycled material to achieve voltage recovery,” she says.

Although the heat treatment approach to reversing the defects is not a scalable solution yet, the fundamental understanding of the structure reversibility will provide an opportunity for identifying a more practical pathway to fully address the voltage decay issue of high-capacity LRLO cathodes. Moving forward, high-voltage, high-capacity battery technology requires advancements in various interrelated aspects such as stable electrolytes and suitable anodes to pair with these high-voltage cathodes.

Read the abstract in Nature Energy.