Lithium-ion batteries are considered one of the most attractive technologies for rechargeable energy storage for electrical vehicles due to their high energy density and long service life. However, there is still great concern about the cost of lithium-ion batteries and the potential limit of lithium supplies available in terrestrial reserves, especially for large-scale energy storage applications for renewable energy and grid. Therefore, alternative energy storage mechanisms and devices using abundant and environmentally friendly materials are highly desirable.

A battery that uses sodium ions instead of lithium ions is attractive because it could be potentially much cheaper and safer, and it is more environmentally benign. However, a sodium ion storage mechanism is scientifically challenging because sodium ions are about 70% larger in radius than that of lithium ions. This makes it difficult to find a suitable host material to accommodate the sodium ions and allow reversible and rapid ion insertion/extraction.

To address this issue, PNNL researchers have developed crystalline nanowires that can be synthesized using a simple polymer-pyrolysis method from polyacrylates of sodium and manganese precursor compounds. The optimized Na₄Mn₉O₁₈ materials display high crystallinity and a homogeneous nanowire structure, which provides a mechanically stable structure as well as a short diffusion path for sodium-ion intercalation and extraction. The Na₄Mn₉O₁₈ nanowires have shown a high reversible capacity (128 mA h g-1 at 0.1C), excellent cyclability (77% capacity retention for 1000 cycles at 0.5C), and promising rate capability for sodium-ion battery applications. The outstanding performance of the Na₄Mn₉O₁₈ nanowires makes them a promising candidate to construct a viable and low-cost sodium-ion battery system for upcoming power and energy storage systems.