The Effect of Molecular Weight, M/G Ratio and pH on the Cyclability of Alginate Bound High Capacity Silicon Anodes

Classical rocking chair Li-ion batteries are dominating the market as they are currently the battery concept with the highest energy density, both per weight and volume. As the technology is rather mature, only small increases in energy density are expected without changing the electrode chemistry. Though the positive electrode currently limits the capacity of a Li-ion battery most, the state of the art graphite negative electrodes only deliver a moderate capacity of 300-350 mAh/g and improvements to this are also highly desirable. Silicon is a promising alternative negative electrode material. Under cathodic polarisation it forms a Li rich alloy, delivering a very high specific capacity of about 3600 mAh/g. Alloying, in contrast to an intercalation reaction, is associated with immense volume changes of the material. The currently used binder (PVDF) has been shown to be unsuitable for Si based electrodes as it cannot withstand the volume changes associated with cycling. Lately, a variety of biopolymers have been described as potential binders for Si based electrodes. These usually come with the advantage of being water soluble and hence makes the electrode production more environmentally benign. One class of biopolymers which have shown encouraging results are derivates of alginic acid. Most commonly applied as water soluble Na salts, alginates have shown encouraging results in accommodating the volume changes and hence can improve the cyclability of Si based electrodes. [1-3] Most published studies describe the alginates they are using rather incompletely. Alginic acid consists of the two epimers D-mannuronic acid (M) and L-guluronic acid (G). The content and distribution of the repetition units have a strong impact on the 3D structure and hence the properties. Furthermore, the chain length influences the properties of the corresponding alginates. Another crucial factor is the pH of the binder solution, which influences the degree of protonation of the carboxyl groups of the alginate. Protonated carboxyl groups ease esterification of the alginate with the native oxide layer on the Si surface which has a strong influence on the binding strength. [4] This systematic study addresses the effect of pH, Mw and M/G ratio of Na alginates as a binding agent for high capacity Si anodes using commercially available materials. The examined electrodes consisted of Si as main active material, Graphite and carbon black as conductivity enhancers and different Na alginates as binding agents. Na alginates with different Mw and M/G ratio were supplied by FMC BioPolymer AS (DuPont) and a commercially available battery grade Si (Silgrain ®, e-Si 400) was provided by ELKEM AS Technology. 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. Erk, C., et al., Toward Silicon Anodes for Next-Generation Lithium Ion Batteries: A Comparative Performance Study of Various Polymer Binders and Silicon Nanopowders. Acs Applied Materials & Interfaces, 2013. 5 (15): p. 7299-7307. Zhang, L., et al., A coordinatively cross-linked polymeric network as a functional binder for high-performance silicon submicro-particle anodes in lithium-ion batteries. Journal of Materials Chemistry A, 2014. 2 (44): p. 19036-19045. Mazouzi, D., et al., New insights into the silicon-based electrode's irreversibility along cycle life through simple gravimetric method. Journal of Power Sources, 2012. 220 : p. 180-184.