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Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are potential alternatives to lithium-ion batteries. However, knowledge about the solid electrolyte interphase (SEI) in SIBs and PIBs is still limited. Here, the formation and stability of SEI in SIBs and PIBs are compared to understand ageing related to SEI characteristics in electrolyte solutions based on 1 M KPF₆ or 1 M NaPF₆ in ethylene carbonate:diethyl carbonate (EC:DEC). Galvanostatic cycling coupled with pause testing was used to quantify the amount of charge consumed for electrolyte reduction for initial SEI formation and for SEI reformation required due to the dissolution of SEI. Proton nuclear magnetic resonance (1H-NMR) spectroscopy was used to reveal changes in the composition of electrolyte solutions due to SEI formation and dissolution. ¹H-NMR findings were supported by X-ray photoelectron spectroscopy (XPS) analysis showing the evolution of SEI composition during a 50 h pause.

Knowledge about capacity losses related to the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs) is still limited. One major challenge in SIBs is that the solubility of SEI species in liquid electrolytes is comparatively higher than the corresponding species formed in Li-ion batteries. This study sheds new light on the associated capacity losses due to initial SEI formation, SEI dissolution and subsequent SEI reformation, charge leakage via SEI and subsequent SEI growth, and diffusion-controlled sodium trapping in electrode particles. By using a variety of electrochemical cycling protocols, synchrotron-based X-ray photoelectron spectroscopy (XPS), gas chromatography coupled with mass spectrometry (GC-MS), and proton nuclear magnetic resonance (¹H-NMR) spectroscopy, capacity losses due to changes in the SEI layer during different open circuit pause times are investigated in nine different electrolyte solutions. It is shown that the amount of capacity lost depends on the interplay between the electrolyte chemistry and the thickness and stability of the SEI layer. The highest capacity loss is measured in NaPF₆ in ethylene carboante mixed with diethylene carbonate electrolyte (i.e., 5 µAh h^−1/2_pause or 2.78 mAh g·h^−1/2_pause) while the lowest value is found in NaTFSI in ethylene carbonate mixed with dimethoxyethance electrolyte (i.e., 1.3 µAh h^−1/2_pause or 0.72 mAh g·h^−1/2_pause).

Manganese-based layered oxides have gained wide attention as cathode materials for sodium-ion batteries due to their cost-effectiveness and nontoxicity. Among them, Na2Mn3O7, which shows promising electrochemical properties as a host material for sodium ions, has been extensively investigated recently. However, the charge compensation mechanisms during battery operation are still ambiguous. Herein, we investigate the electronic structure of Na2Mn3O7 using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering techniques. Mn L-II,L-III-edge XAS spectra show that manganese ions do not undergo any oxidation reaction during the first charge process, suggesting that sodium removal is instead charge compensated by oxygen-ion redox reactions. This, in turn, has an impact on the cycling performances delivered by the material, especially the capacity retention over cycles and also the electrochemical kinetics of sodium ions in Na2Mn3O7.

Oxygen anion redox offers an effective strategy to enhance the energy density of layered oxide positive electrodes for sodium- and lithium-ion batteries. However, lattice oxygen loss and irreversible structural transformations over the first cycle may result in large voltage hysteresis, thereby impeding practical application. Herein, ribbon superstructure ordering of Li/transition-metal-ions was applied to suppress the voltage hysteresis combined with Ti-substitution to improve the cycling stability for P3-Na0.67Li0.2Ti0.15Mn0.65O2. When both cation and anion redox reactions are utilized, Na0.67Li0.2Ti0.15Mn0.65O2 delivers a reversible capacity of 172 mA h g(-1) after 25 cycles at 10 mA g(-1) between 1.6-4.4 V vs. Na+/Na. Ex-situ X-ray diffraction data reveal that the ribbon superstructure is retained with negligible unit cell volume expansion/contraction upon sodiation/desodiation. The performance as a positive electrode for Li-ion batteries was also evaluated and P3-Na0.67Li0.2Ti0.15Mn0.65O2 delivers a reversible capacity of 180 mA h g(-1) after 25 cycles at 10 mA g(-1) when cycled vs. Li+/Li between 2.0-4.8 V.

Activation of oxygen redox represents a promising strategy to enhance the energy density of positive electrode materials in both lithium and sodium-ion batteries. However, the large voltage hysteresis associated with oxidation of oxygen anions during the first charge represents a significant challenge. Here, P3-type Na0.67Li0.2Mn0.8O2 is reinvestigated and a ribbon superlattice is identified for the first time in P3-type materials. The ribbon superstructure is maintained over cycling with very minor unit cell volume changes in the bulk while Li ions migrate reversibly between the transition metal and Na layers at the atomic scale. In addition, a range of spectroscopic techniques reveal that a strongly hybridized Mn 3d-O 2p favors ligand-to-metal charge transfer, also described as a reductive coupling mechanism, to stabilize reversible oxygen redox. By preparing materials under three different synthetic conditions, the degree of ordering between Li and Mn is varied. The sample with the maximum cation ordering delivers the largest capacity regardless of the voltage windows applied. These findings highlight the importance of cationic ordering in the transition metal layers, which can be tuned by synthetic control to enhance anionic redox and hence energy density in rechargeable batteries.

Sodium transition metal oxides with a layered structure are one of the most widely studied cathode materials for Na+-ion batteries. Since the mobility of Na+ in such cathode materials is a key factor that governs the performance of material, electrochemical and muon spin rotation and relaxation techniques are here used to reveal the Na+-ion mobility in a P2-type Na0.5MgxNi0.17-xMn0.83O2 (x = 0, 0.02, 0.05 and 0.07) cathode material. Combining electrochemical techniques such as galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique with mu+SR, we have successfully extracted both self-diffusion and chemical-diffusion under a potential gradient, which are essential to understand the electrode material from an atomic-scale viewpoint. The results indicate that a small amount of Mg substitution has strong effects on the cycling performance and the Na+ mobility. Amongst the tested cathode systems, it was found that the composition with a Mg content of x = 0.02 resulted in the best cycling stability and highest Na+ mobility based on electrochemical and mu+SR results. The current study clearly shows that for developing a new generation of sustainable energy-storage devices, it is crucial to study and understand both the structure as well as dynamics of ions in the material on an atomic level.

The interfacial reactions in sodium-ion batteries (SIBs) are not well understood yet. The formation of a stable solid electrolyte interphase (SEI) in SIBs is still challenging due to the higher solubility of the SEI components compared to lithium analogues. This study therefore aims to shed light on the dissolution of SEI influenced by the electrolyte chemistry. By conducting electrochemical tests with extended open circuit pauses, and using surface spectroscopy, we determine the extent of self-discharge due to SEI dissolution. Instead of using a conventional separator, beta-alumina was used as sodium-conductive membrane to avoid crosstalk between the working and sodium-metal counter electrode. The relative capacity loss after a pause of 50 hours in the tested electrolyte systems ranges up to 30 %. The solubility of typical inorganic SEI species like NaF and Na2CO3 was determined. The electrolytes were then saturated by those SEI species in order to oppose ageing due to the dissolution of the SEI.

Increasing dependence on rechargeable batteries for energy storage calls for the improvement of energy density of batteries. Toward this goal, introduction of positive electrode materials with high voltage and/or high capacity is in high demand. The use of oxygen chemistry in lithium and sodium layered oxides has been of interest to achieve high capacity. Nevertheless, a complete understanding of oxygen-based redox processes remains elusive especially in sodium ion batteries. Herein, a novel P3-type Na0.67Ni0.2Mn0.8O2, synthesized at low temperature, exhibits oxygen redox activity in high potentials. Characterization using a range of spectroscopic techniques reveals the anionic redox activity is stabilized by the reduction of Ni, because of the strong Ni 3d-O 2p hybridization states created during charge. This observation suggests that different route of oxygen redox processes occur in P3 structure materials, which can lead to the exploration of oxygen redox chemistry for further development in rechargeable batteries.