Alginate Nanofibers as a Binder in Lithium-Ion Cells

It is well known that the volume expansion upon Li-alloying, followed by pulverization, deleterious side reaction, and electronic isolation limits the commercial use of high-capacity anode materials such as silicon and tin. Despite these challenges, researchers aspire to discover means to integrate high-capacity materials into electrodes. The electrode binder, one of the least investigated components in a lithium-ion battery, is posited as a potential remedy to many of the issues that lead to cell failure. Manipulating the composition and morphology of the binder impacts the longevity of the cell as well as the capacity retention. Here we show that alginate nanofibers as an anode binder can be used to enhance the performance of Si anodes for lithium-ion batteries.

Alginate is an anionic polysaccharide derived from brown algae [1].  Although it has many industrial uses such as a textile processing and a thickening agent in foods, alginate has recently been reported [1] as an eco-friendly alternative to the conventional PVDF binder. Sodium alginate has been shown to protect the active material by forming an inert passivating layer with common organic carbonate electrolyte solvents while providing ample Li conductivity [1]. Recently, our group has shown that alginate nanofibers can be formed via electrospinning alginate-PEO blends [2]. PEO, albeit electrochemically stable, can be leached away to form porous alginate nanofibers (Figure 1). Electrospinning provides a desirable fiber extrusion technique because it allows for a continuous and facile method to produce multi-component nanofibers. The small size scale of these fibers can further enhance the electronic and ionic transport through the cell.

Here we report physical and electrochemical characteristics of alginate fibers as the binding component in both graphite and silicon anodes. X-ray diffractograms and energy-dispersive x-ray spectroscopy are used to determine the phase and location of the electrochemically active materials, respectively. Cyclic voltammetry test are employed to observe reversibility of the cell. Lithium diffusion rates into the anode materials are also determined. Charge and discharge curves demonstrate alginate fibers may be used in lieu of PVDF as a binding material in Li-ion cells.

Figure 1: SEM image showing alginate fibers. The fibers were not sputter-coated and slight charging effects are evident.

[1] I. Kovalenko et al., Science 334, 75 (2011)

[2] C. A. Bonino et al., Carbohydrate Polymers 85, 111(2011)