Lithium‐ion batteries are ubiquitous in modern society, and their importance is rapidly increasing with the popularization of electric vehicles (EVs). Consumer electronics and EVs greatly benefit from lithium‐ion batteries (LIBs) despite their high cost and limited materials abundance. However, large‐scale applications, such as grid‐scale energy storage require an alternative solution. Sodium‐ion batteries (SIBs) have gained increasing attention within the last decade as an alternative solution to grid‐scale storage as they utilize several cheaper and more abundant materials compared with LIBs. The most likely candidate for SIB industrialization will use a layered‐oxide cathode, allowing comparisons to be drawn to the industrialization of lithium layered oxide cathodes. A notable difference between sodium and lithium layered oxides is the broader range of viable metals that reversibly insert sodium, and an even larger set of possible metal combinations. To predict the optimal compositions for SIB commercialization, herein, the fundamental crystal‐chemical and electrochemical properties of each 3d transition‐metal ion is examined and the history of LIB industrialization is discussed for further insight.

Sodium-ion batteries can be a practical alternative to lithium-ion batteries due to the relatively high abundance of sodium and the projected scarcity of lithium. Both of these factors are critical considerations for grid-scale energy storage, but the central challenge to implementing sodium layered oxides in sodium-ion batteries is their relatively poor cycle life. Single-crystal particles with micrometer size can mitigate several failure mechanisms related to sodium layered oxides and can improve performance when compared to the commonly used polycrystalline particles. This work demonstrates a novel two-step moltensalt synthesis method using sodium chloride and metal oxides to form "single crystals" of a mixed-phase, spinel/rock-salt intermediate that crystallizes as micron-sized truncated octahedra. The mixed-phase spinel/rock-salt material is effectively used as a precursor to form O3-type NaNi 0.5 Mn 0.5 O 2 with large primary particles and substantially improved cycle life. This synthesis route offers the added benefit of using simple metal oxides instead of hydroxide precursors, eliminating the need for coprecipitation. Particle morphology is found to be a critical factor in mitigating the structural damages incurred during phase transformations and maintaining the electrochemical performance.

In this work, we report a method for producing a thin (<50 μm), mechanically robust, sodium-ion conducting composite solid electrolyte (CSE) by infiltrating the monomers of polyethylene glycol diacrylate (PEGDA) and polyethylene glycol (PEG) and either NaClO4 or NaFSI salt into a silica-based glass-fiber matrix, followed by an UV-initiated in situ polymerization. The glass fiber matrix provided mechanical strength to the CSE and enabled a robust, self-supporting separator. This strategy enabled the development of CSEs with high loadings of PEG as a plasticizer to enhance the ionic conductivity. The fabrication of these CSEs was done under ambient conditions, which was highly scalable and can be easily implemented in roll-to-roll processing. While NaClO4 was found to be unstable with the sodium–metal anode, the use of a NaFSI salt was found to promote stable stripping and plating in a symmetric cell, reaching current densities of as high as 0.67 mA cm–2 at 60 °C. The PEGDA + PEG + NaFSI separators were then used to form solid-state full cells with a cobalt-free, low-nickel layered Na2/3Ni1/3Mn2/3O2 cathode and a sodium–metal anode, achieving a full capacity utilization exhibiting 70% capacity retention after 50 cycles at a cycling rate of C/5 at 60 °C.