Grid scale storage requires a low cost, safe, and environmentally benign battery system. Na is an earth abundant material and Na-ion batteries fulfill these requirements better than Li-ion batteries. Widespread implementation of Na-ion batteries is limited by several factors: (1) slow Na ion diffusion kinetics, (2) large volume changes and structural pulverization during charging/discharging, and (3) difficulty in maintaining a stable solid electrolyte interphase (SEI). These challenges are related to the large size of the Na ion (372% larger in volume than Li ion for a coordination number of four...), which makes it impossible to simply adopt the recent knowledge and strategies developed for high-performance Li-ion batteries.

...Sn is a promising anode material because it alloys with Na at a high specific capacity of 847 mAh/g when Na₁₅Sn₄ is formed. Studies of Sn film and nanostructured anodes were reported; the cycle life, however, is limited to 20 cycles due to pulverization. The pulverization is primarily due to a 420% volume expansion associated with the formation of Na₁₅Sn₄.

...In this study, we develop a nature-inspired low cost electrode consisting of an electro-deposited Sn film on conductive wood fiber. Conductivity is achieved by a solution-based coating of carbon nanotubes (CNT) on the fiber surface. We find that the wood fiber increases the cyclability of Sn for Na-ion batteries by alleviating: 1) the capacity loss due to electrode pulverization, and 2) the poor rate performance as a result of slow ion diffusion kinetics...The Sn anode described is ideal for grid scale storage. The materials used are earth abundant and environmentally friendly, and electrodeposition and conductive fiber substrates are scalable for large throughput manufacturing.

—Zhu et al.

Wood fibers, the authors note, are tracheids—hollow elongated cells that transport water and mineral salts. Pores in the fiber wall allow for intercellular fluid transportation. Natural wood fibers with diameters on the order of 25 µm serve as the substrate for the Sn film.

The team initially coats the fibers with a thin layer (10 nm) of single-walled carbon nanotubes (SWCNTs) to provide electrical conductivity. Various other conductive materials, including graphene, metal nanowires, and conductive polymers could be deposited on wood fibers with similar solution-based processes, they suggested.

During the sodiation/desodiation process, the substrate deforms together with the Sn film to release high stresses and prevent the delamination and pulverization characteristic of Sn anodes. Additionally, the researchers noted, the wood fiber has a high capacity for electrolyte absorption. Liquid electrolytes penetrate the porous structure of the fiber, allowing for Na ion diffusion through the fiber cell walls in addition to diffusion at the Sn film surface. This creates a dual ion transport path that effectively addresses the slow kinetics of Sn anodes for Na-ion batteries.

The key metrics for Na-ion batteries are low cost and material abundance, as opposed to high-energy density for Li-ion batteries. The target application for Na-ion batteries, therefore, is grid-scale energy storage. This removes some design constrains for materials and structures.

...The abundance and large scale roll-to-roll processability of wood fibers make them an excellent candidate for energy storage applications where low costs are desired.

—Zhu et al.

Resources

• Hongli Zhu, Zheng Jia, Yuchen Chen, Nicholas Weadock, Jiayu Wan, Oeyvind Vaaland, Xiaogang Han, Teng Li, and Liangbing Hu (2013) Tin Anode for Sodium-Ion Batteries Using Natural Wood Fiber as a Mechanical Buffer and Electrolyte Reservoir. Nano Letters doi: 10.1021/nl400998t