Sodium-ion batteries have recently aroused the interest of industries as possible replacements for lithium-ion batteries in some areas. With their high theoretical capacities and competitive prices, P2-type layered oxides (Na_xTMO₂) are among the obvious choices in terms of cathode materials. On the other hand, many of these materials are unstable in air due to their reactivity toward water and carbon dioxide. Here, Na_0.67Mn_0.9Ni_0.1O₂ (NMNO), one of such materials, has been synthesized by a classic sol-gel method and then exposed to air for several weeks as a way to allow a simple and reproducible transition toward a Na-rich birnessite phase. The transition between the anhydrous P2 to the hydrated birnessite structure has been followed via periodic XRD analyses, as well as neutron diffraction ones. Extensive electrochemical characterizations of both pristine NMNO and the air-exposed one vs sodium in organic medium showed comparable performances, with capacities fading from 140 to 60 mAh g^-1 in around 100 cycles. Structural evolution of the air-exposed NMNO has been investigated both with ex situ synchrotron XRD and Raman. Finally, DFT analyses showed similar charge compensation mechanisms between P2 and birnessite phases, providing a reason for the similarities between the electrochemical properties of both materials.

Batteries based on organic electrolytes have been raising safety concerns due to some associated fire/explosion accidents caused by the unusual combination of highly flammable organic electrolytes and high energy electrodes. Nonflammable aqueous batteries are a good alternative to the current energy storage systems. However, what makes aqueous batteries safe and viable turns out to be their main weakness, since water molecules are prone to decomposition because of a narrow electrochemical stability window (ESW). In this perspective we introduce aqueous batteries and then discuss the state-of-the-art of water-in-salt (WIS) electrolytes for aqueous energy storage systems. The main strategies to improve ESW are reviewed, including: (i) the use of fluorinated salts to make a solid electrolyte interphase (SEI); (ii) the use of cost-effective and highly soluble salts to reduce water activity through super concentration; and (iii) the use of hybrid electrolytes combining the advantages of both aqueous and non-aqueous phases. Then, we discuss different battery chemistries operated with different WIS electrolytes. Finally, we highlight the challenges and future technological perspectives for practical aqueous energy storage systems, including applications in stationary storage/grid, power backup, portable electronics, and automotive sectors.

Since their appearance on the scene, MXenes have been recognized as promising anode materials for rechargeable batteries, thanks to the combination of structural and electronic features. The layered structure with a suitable interlayer distance, good electronic conductivity, and moldability in composition makes MXenes exploitable both as active and support materials for the fabrication of nanocomposites providing both capacitive and Faradaic contributions to the final capacity. Although a variety of possibilities has been explored, the fundamental mechanism of the electrode reaction is still hazy. We herein report the investigation of Ti₃C₂T _x MXenes, the benchmark composition for application in energy storage, through the combined operando X-ray absorption spectroscopy (XAS) and Raman analysis supported by density functional theory (DFT) calculations with the aim of clarifying the origin and nature of capacity when the material was cycled vs Na. The electrode reaction determined was Ti₃C₂X₂ + 1Na → Na₁Ti₃C₂X₂, defining the theoretical capacity.