Passivation Layer and Cathodic Redox Reactions in Sodium‐Ion Batteries Probed by HAXPES

Abstract

The cathode material P2-Nax Co2/3 Mn2/9 Ni1/9 O2, which could be used in Na-ion batteries, was investigated through synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Nondestructive analysis was made through the electrode/electrolyte interface of the first electrochemical cycle to ensure access to information not only on the active material, but also on the passivation layer formed at the electrode surface and referred to as the solid permeable interface (SPI). This investigation clearly shows the role of the SPI and the complexity of the redox reactions. Cobalt, nickel, and manganese are all electrochemically active upon cycling between 4.5 and 2.0 V; all are in the 4+ state at the end of charging. Reduction to Co(3+), Ni(3+), and Mn(3+) occurs upon discharging and, at low potential, there is partial reversible reduction to Co(2+) and Ni(2+). A thin layer of Na2 CO3 and NaF covers the pristine electrode and reversible dissolution/reformation of these compounds is observed during the first cycle. The salt degradation products in the SPI show a dependence on potential. Phosphates mainly form at the end of the charging cycle (4.5 V), whereas fluorophosphates are produced at the end of discharging (2.0 V).

Influence of Using Metallic Na on the Interfacial and Transport Properties of Na-Ion Batteries

“…However, the -CF 2 signal is still present, suggesting that the SEI layer is thin, in agreement with the previous results obtained from C 1s and O 1s peaks. Moreover, the intensity of the NaF component slightly increases during Na + extraction in Na 4 Ti 3 O 7 , probably due to the fact that more NaF is formed upon Na + extraction than upon insertion, which would agree with the behavior that is observed in other Na-and Li-based cathode materials [35,36].…”

Surface analytical approaches to reliably characterize lithium ion battery electrodes

“…To achieve a reliable assignment, it is inevitable to take additionally the Mn 3s splitting energy as well as the Mn 3p binding energy into account. In the case of the Mn 3s, splitting energies of 4.5, 5.5, and 6.5 eV are, respectively, correlated to Mn 4+ , Mn 3+ , and Mn 2+ ions, and therefore, the splitting of about 5.5 eV shown in Figure b for the Li‐Mn‐O system clearly corroborates the Mn 3+ state . According to the literature, the Mn 3p spectrum was deconvoluted into peaks originating from 5 P and a broad peak from 7 P .…”

“…Obviously, the major reason for this fact is mainly due to the complex multiplet splitting, peak overlaps, and additional shake‐up and plasmon features in the respective 2p XP spectra, although fundamental studies are available mainly by the work of Biesinger et al, who considered a semiempirical approach combining the analysis of high purity oxide/hydroxide reference samples and theoretically calculated free‐ion multiplet structures of core 2p vacancy levels by Gupta and Sen . Aside presenting only raw data sets and solely assigning expected oxidation states, simplifying approaches such as reducing the complex multiplet splitting to single Voigt peak shapes are often used, which, in consequence, could lead at least to uncertainties in the quantitative chemical information. To the best of our knowledge, only a few groups apply complex multiplet fitting procedures during XPS characterization of LIB active materials …”

Evidence of a Pseudo‐Capacitive Behavior Combined with an Insertion/Extraction Reaction Upon Cycling of the Positive Electrode Material P2‐NaxCo0.9Ti0.1O2 for Sodium‐ion Batteries

“…We are confronting two cases, when the charge process is limited to 4.3 V a small and non‐reversible plateau is recognized, but when the voltage is increased to 4.5 V the plateau became partially reversible. Indeed, the electrolyte may decompose at higher voltages and the products, resulting from electrolyte degradation, can react with the electrode surface and can be the main origin of the plateau …”