(Invited) Mechanistic Studies on Electrode/Electrolyte Interfaces in Li-Ion Batteries Using in-Situ Neutron Reflectometry
Authors: Hirayama Masaaki 
Published: December 2024

A thorough understanding of electrochemical interfaces is critical for the development of next-generation energy storage/conversion devices with high energy densities and long-life operations. The direct observation of structural changes provides invaluable information about charge transfer, ionic diffusion, and side reactions in electrochemical reaction processes. Neutron reflectometry is a powerful technique for gaining a nanoscopic understanding of solid/liquid interfaces as a function of depth. Neutrons are capable of providing electrochemical interfaces owing to their strong penetrating power through the substrate. In addition, lithium can be sensitively detected by neutrons because the scattering length of lithium is significant in comparison with that of other elements in the electrode for neutrons, whereas it is small and almost invisible to X-rays. The depth profile of the neutron scattering length density (SLD) obtained by analyzing the reflectivity profile can reveal the distribution of diffusing ions at the interface. Our previous studies on electrode/electrolyte interfaces using neutron reflectometry have mainly focused on the electrode-side interface (surface structural changes and/or surface electrolyte interphases) (1-3). There are no reports of neutron reflectometry used to discuss the depth distribution on the liquid-electrolyte side. In this study, we elucidate lithium transfer by directly observing structural changes within the cathode, through the interface, and into the electrolyte using in situ neutron reflectometry conducted on a time-of-flight reflectometer (J-PARC, BL16 SOFIA) (4, 5). We studied two different films—a Li2ZrO3-modified film and an unmodified LiCoO2 film—and found that the modified film exhibited a superior rate capability. In situ NR studies indicated that surface modification facilitates the formation of a dense cathode electrolyte interphase (CEI), primarily composed of inorganic species. In contrast, the unmodified surface was covered by a relatively sparse electrolyte-impregnated CEI. These structural observations suggest that lithium desolvation during intercalation occurs primarily on the CEI and LiCoO2 surfaces of the modified and unmodified films, respectively. The fast desolvation of Li on the CEI may contribute to the superior rate capability of the surface-modified cathodes. This suggests a mechanism of fast intercalation achieved by surface modification of low-ionically conductive oxides. The simultaneous detection of chemical composition and morphological information is a powerful way to elucidate the dynamics at cathode/liquid electrolyte interfaces suitable for high-power operation. Furthermore, neutron reflectometry is useful for understanding interfacial phenomena in all-solid-state batteries (6).
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