On Ionic and Covalent Cross-Linking of Alginic Acid Derived Binders for High Capacity Silicon Anodes
Authors: Wagner Nils P. , Hanetho Sidsel Meli , Tøndervik Anne , Sletta Hårvard , Juel Mari , Mæhlen Jan Petter , Svensson Ann Mari , Voje Jorunn , Helgerud Trond 
Published: February 2020

The anode market for automotive Li-ion batteries is still governed by carbonaceous materials with a market share of 96%.[1] The graphite electrodes used today can only deliver a moderate capacity of 300-350 mAh/g, and improvements are highly sought. Silicon, often in the form of a silicon/carbon composite, is a promising alternative material with its high theoretical capacity of about 3600 mAh/g. As an alloy former this capacity is accompanied with a huge volume change of over 300%, deeming PVDF unsuitable as binder as it cannot withstand the volume changes upon cycling. A variety of technical and biopolymers were shown to drastically increase the cyclability of silicon based anodes. Prominent examples are s poly (acrylic acid) (PAA), carboxymethyl cellulose (CMC) and alginic acid (algin) derivatives.
Algin, often purified as water soluble Na alginate, is a polysaccharide found in the cell walls of brown algae. It consists of the two epimers D-mannuronic acid (M) and L-guluronic acid (G), which's content and distribution have a strong impact on the 3D structure and properties. Superior cycling of Na alginate bound Si anodes was first described by Kovalenko et al..[2] The incorporation of divalent ions such as calcium, strontium, barium or transition metals causes strong chelation of the ions resulting in a hydrogel. Several studies have described that this ionic cross-linking has a profound effect on the cyclability of Si based anodes. [3],[4],[5],[6] Divalent ions are usually chelated by two adjacent G units on two polymer chains forming a so called egg-box and high degrees of saturation cause gelling, which obstructs processing.
Since chelation including G units only is much stronger than the interaction with M groups, the M to G ratio should have a strong effect on the stability of the network. Covalent cross-linking should have an even stronger effect on the stability of the binder network as it is constructed of chemical bonds rather than ionic interactions. This study concerns the effect of different saturation degree of calcium and barium cross-linking on alginates with different M to G ratio as binder for Si based anodes. Furthermore, we discuss routes to form a covalently cross-linked algin derived network that is grafted to the Si surface.
[1] Schmuch R., Wagner R., Hoerpel G., Placke T., Winter M., Performance and cost of materials for lithium-based rechargeable automotive batteries, Nature Energy, 3 (2018), 267-278.
[2] Kovalenko, I., et al., A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries. Science, 2011. 334(6052): p. 75-79.
[3] Wua Z-Y., Deng L., Li J-T., et al. Multiple hydrogel alginate binders for Si anodes of lithium-ion battery. Electrochimica Acta, 245 (2017), 371–378.
[4] Gu Y., Yang S., Zhu G., et al. The effects of cross-linking cations on the electrochemical behavior of
silicon anodes with alginate binder, Electrochimica Acta, 269 (2018), 405-414.
[5] Zhang L., Zhang L., Peng L. C., et al. A coordinatively cross-linked polymeric network as a functional binder for high-performance silicon submicro-particle anodes in lithium-ion batteries, J. Mater. Chem. A,2 (2014), 19036–19045
[6] Yoon J., Oh D. X., Jo C. et al., Improvement of desolvation and resilience of alginate binders for Si-based anodes in a lithium ion battery by calcium-mediated cross-linking, PCCP, 16 (2014), 25628-25635.

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