Operando Time-Slicing Neutron Reflectometry Measurements of Solid Electrolyte Interphase Formation on Amorphous Carbon Surfaces of a Li-Ion Battery

Abstract

A solid-electrolyte interphase (SEI) is widely recognized to improve the safety and durability of lithium ion batteries. In this work, we investigate the structure and chemistry of the carbon electrode and SEI in operando during two-cycle battery operation for further understanding of the electrochemical reactions, and the effect of the hysteresis using in situ neutron reflectivity (NR) and ex situ hard X-ray photoelectron spectroscopy (HAXPES). The results revealed the structural evolution of the electrode and SEI layer, such as the change in the thickness and scattering length density (SLD) in connection with the chemical composition during the lithiation/delithiation processes. Next, the HAXPES results at the point before and after the charging/discharging process revealed the change in the chemical composition of the SEI layer due to the chemical reactions on the formation/degradation. Based on the combination of these analyses, the results showed that the SLDs of the SEI layer evaluated by NR analysis were consistent with those determined by HAXPES. Concerning the difference in the first and second cycles, the structure of the amorphous carbon electrode exhibited hysteresis due to lithiation/delithiation, whereas the chemical composition of the SEI layer after charge/discharge was almost independent of the number of cycles.

Electrochemical investigation of ion-beam sputter-deposited carbon thin films for Li-ion batteries

“…Although graphite is vastly used in LIBs and was studied thoroughly , investigations on amorphous sp 2 -bonded, sputter-deposited thin carbon films as active material are relatively rare [17,18,21,[23][24][25][26][27][28]. It is stated that thin amorphous carbon films do not show reversible Li intercalation, not even at the lowest possible potential of 0 V vs Li reference material [21].…”

“…The potential for SEI formation is not a fixed value, but it is generally thought to range from 0.3 to 2 V [15,19,22]. Operando experiments on the investigation of SEI formation on a 70 nmthin amorphous carbon film produced by magnetron sputtering was done by neutron reflectometry (NR) [27,28]. NR is very sensitive on layer thickness changes and was applied for the first two cycles [28].…”

“…Operando experiments on the investigation of SEI formation on a 70 nmthin amorphous carbon film produced by magnetron sputtering was done by neutron reflectometry (NR) [27,28]. NR is very sensitive on layer thickness changes and was applied for the first two cycles [28]. The analysis of the NR pattern allowed to gain results on the SEI layer growth and, separately, on changes of carbon film [27,28].…”

Application of precise neutron focusing mirrors for neutron reflectometry: latest results and future prospects

“…Taking these reductions into consideration, the exposure time is estimated be less than 2 min by reference to the latest mirror for lower angles with a Q z range from 0.08 to 0.56 nm À1 , and less than 2 h is required for higher angles up to 2.6 nm À1 without taking the background into account. Furthermore, operando measurement with a time resolution of 10 min up to 2.6 nm À1 is possible by applying a carbon anode with an area of 35 Â 35 mm, which is three times larger than that used in previous work (Kawaura et al, , 2020 but is a commercially available size.…”

“…For example, the electrodes of lithium ion batteries (LIBs) are a key target for NR because they are composed of a complex of Li compounds and organic binders containing light elements that are sensitive to neutrons, and the interface with a liquid electrolyte where electrochemical reactions occur is at the frontline of research. More specifically, interfacial layers formed at electrode-electrolyte interfaces and the distributions of Li ions not only under constant voltages (Hirayama, Yonemura et al, 2010;Owejan et al, 2012;Browning et al, 2014;Minato et al, 2016;Browning et al, 2019) but also in operando studies during charging/discharging processes (Kawaura et al, 2016;Kawaura et al, 2020) have been investigated using NR to date.…”