X-ray photoelectron spectroscopy (XPS) is a key method for studying (electro-)chemical changes in metal-ion battery electrode materials. In a recent publication, we pointed out a conflict in binding energy (BE) scale referencing at alkali metal samples, which is manifested in systematic deviations of the BEs up to several eV due to a specific interaction between the highly reactive alkali metal in contact with non-conducting surrounding species. The consequences of this phenomenon for XPS data interpretation are discussed in the present manuscript. Investigations of phenomena at surface-electrolyte interphase regions for a wide range of materials for both lithium and sodium-based applications are explained, ranging from oxide-based cathode materials via alloys and carbon-based anodes including appropriate reference chemicals. Depending on material class and alkaline content, specific solutions are proposed for choosing the correct reference BE to accurately define the BE scale. In conclusion, the different approaches for the use of reference elements, such as aliphatic carbon, implanted noble gas or surface metals, partially lack practicability and can lead to misinterpretation for application in battery materials. Thus, this manuscript provides exemplary alternative solutions.

P2-type layered oxides with the general Na-deficient composition Na _x TMO₂ (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na⁺ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na_0.67(Mn_0.55Ni_0.21Co_0.24)O₂, Na_0.67(Mn_0.45Ni_0.18Co_0.24Ti_0.1Mg_0.03)O₂ and Na_0.67(Mn_0.45Ni_0.18Co_0.18Ti_0.1Mg_0.03Al_0.04Fe_0.02)O₂ with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications.

Sodium metal is the ultimate anode for next generation high-energy-density sodium metal batteries due to its superior theoretical specific capacity, low redox potential, and natural abundance. However, sodium metal suffers from extreme and uncontrollable dendrite growth and gas evolution problems. These incidents result in a low coulombic efficiency and safety issues such as dangerous short circuits. Herein, an effective protective layer is fabricated on the Na metal anode via an extremely facile pretreatment method with 1,3-dioxolane. The protective layer exhibits fast interfacial transport and a lower resistance. Direct optical visualization shows that dendrite growth and gas evolution are suppressed due to the introduction of the protective layer. As a result, an outstanding cycling stability for 2800 h (1400 cycles) at 1 mA cm⁻² in a symmetric cell is obtained. Moreover, the full cell using the protected Na metal anode shows superior electrochemical performance in comparison to the untreated Na metal anode. Furthermore, large format protected Na metal anodes fabricated by spraying 1,3-dioxolane were demonstrated and successfully assembled in pouch cells, showing a stable specific capacity of around 95 mA h g⁻¹. Thus, our work presents a facile, efficient and scalable protection strategy to stabilize Na metal anodes towards high-energy-density sodium batteries.