Reaction Mechanism and Surface Film Formation of Conversion Materials for Lithium- and Sodium-Ion Batteries: An XPS Case Study on Sputtered Copper Oxide (CuO) Thin Film Model Electrodes

Abstract

Charge storage based on conversion reactions is a promising concept to store electrical energy. Many studies have been devoted to conversion reactions with lithium; however, still many scientific questions remain due to the complexity of the reaction mechanism combined with surface film formation. Replacing lithium by sodium is an attractive approach to widen the scope of conversion reactions and to study whether the increase in ion size changes the reaction mechanisms and whether the cell performance benefits or worsens. In this study, we use thin film electrodes as a additive-free model system to study the conversion reaction of CuO with sodium (CuO/Na) by means of electrochemical methods, microscopy, and X-ray photoelectron spectroscopy. The reaction mechanism and film formation are being discussed. Some important differences to the analogue lithium-based system (CuO/Li) are found. Whereas CuO has been reported as charge product in CuO/Li cells, charging is incomplete in the case of CuO/Na and only Cu2O is formed. As an important finding, oxygen appears to be redox active and Na2O2 forms during charging from Na2O. Moreover, surface film formation due to electrolyte decomposition is much more severe as compared to CuO/Li. Depth profiling is used to probe the inner composition of the surface film, revealing a much thicker surface film with more inorganic components as compared to the lithium system. It is also found that the surface film disappears to a large extent during charging.

Self‐Templated Synthesis of Cuprous Oxide Nanofiber‐Assembled Hollow Spheres for High‐Performance Electrochemical Energy Storage

“…The XPS result in Figure (f) could well demonstrate that the valence state of copper of the product is +1. Because the XPS curve in Cu 2p region displays two main peaks located at 951.2 eV and 931.5 eV with a splitting energy of 19.7 eV, originating from Cu 2p 1/2 and Cu 2p 3/2 , respectively, which is typical XPS spectra of Cu +1 ,…”

Interfacial Phenomena/Capacities Beyond Conversion Reaction Occurring in Nano‐sized Transition‐Metal‐Oxide‐Based Negative Electrodes in Lithium‐Ion Batteries: A Review

“…Whereas the CuO/Na system showed a homogeneous distribution of inorganic compounds such as carbonates. In addition, a pronounced decrease of the surface film to a huge extent (remaining film ∼1 μm) was observed during charging (Figure ) …”

“…In newer literature, authors have started to clarify the notation of surface films (e. g. SEI or gel‐like polymeric film) for their articles . In general, latest research results regarding film formation processes in nano‐sized TMO/Li systems revealed a layered structure with an inorganic‐rich inner part, and an organic‐rich outer part, as for instance investigated for CuO and Fe 2 O 3 (and solely true for measurements against Li/Li + , since a different situation is found when these materials are applied in Na batteries) ,. For this review article, we use the notations given by the respective authors for detailed presentations of research results about film formation processes.…”

“…In contrast to other stable SEI compounds, the unstable gel‐like one is assumed to reversibly form and decompose open discharge and charge processes along with electron transfer and energy storage at potentials below 0.8 V . In newer literature, authors have started to clarify the notation of surface films (e. g. SEI or gel‐like polymeric film) for their articles . In general, latest research results regarding film formation processes in nano‐sized TMO/Li systems revealed a layered structure with an inorganic‐rich inner part, and an organic‐rich outer part, as for instance investigated for CuO and Fe 2 O 3 (and solely true for measurements against Li/Li + , since a different situation is found when these materials are applied in Na batteries) ,.…”

“…The exact interaction of nano‐sized TMOs and further battery compounds with Li + (and between them) is still under debate. Also the reversibility of conversion reaction is a point of discussion and determined by several thermodynamic and kinetic factors . For instance, the classical conversion reaction for Co 3 O 4 is stated as Co 3 O 4 +8Li + +8e − →4Li 2 O+3Co, but also subsequent reactions according to Co+Li 2 O→CoO+2Li, or a coexistence of Co 3 O 4 and CoO for the following cycles were reported .…”