Alginate Based Binder Materials for High Capacity Silicon Anodes
Authors: Mokkelbost Tommy , Wagner Nils P. , Tøndervik Anne , Tolchard Julian Richard , Sheridan Edel , Mæhlen Jan Petter , Voje Jorunn , Helgerud Trond , Svensson Ann Mari 
Published: February 2020

Lithium-ion secondary cells are one of the most advanced energy storage systems currently available, and the battery concept with the highest specific energy and energy density. Due to rapidly falling cost following mass production, Li-ion batteries are making a significant impact on markets previously dominated by other technologies, including stationary storage. However, new materials are urgently required in order to address the need for higher energy density, longer cycle/calender life, and improved safety, while satisfying very demanding cost and sustainability targets. Silicon as an anode material is among the most promising materials having a very high theoretical specific charge capacity of about 3600 mAh/g. In addition, it is a low cost, non-toxic and readily available material.
For both electrodes, binders are necessary to maintain electrode structure. The state of the art binder material is PVDF, which requires toxic solvents such as NMP. For materials, such as silicon, which have large volume changes during (de)alloying there is a need for new binders which can accommodate such volume changes. Recently, there have been several reports on alginates [1-3] as binder material for anodes in Li ion batteries; however the alginate materials itself used in the studies reported were poorly characterized.
The overall content and distribution of the repetition units, mannuronate and guluronate (M/G) in the alginate have an impact on the 3D structure and properties. Na alginates with varying levels of G are evaluated. 1) Na alginate with approx. 70% G obtained from brown seaweed containing long G-blocks. 2) Na alginate with approx. 47% G obtained from brown seaweed containing shorter G-blocks 3) Na alginate produced from Pseudomonas fluorescens with approx. 25% G, and where the G residues are only present as single G's i.e. there are no G-blocks present. Also, the molecular weight (MW) of the alginate is an important determinant for the chemical and physical properties (e.g. viscosity in solution), hence the effect of MW on binder properties of the above mentioned alginates is examined.
Here we present a systematic study where different alginate samples are characterized in order to identify the best possible combinations of active anode material and the binder component. Both in-house prepared and commercial Na alginate based binder materials are used to understand how the properties of the different alginate binders effect the overall electrode structure. Seaweed alginates are obtained from FMC Biopolymer, whereas bacterial alginates are prepared at SINTEF using well established methods, including fermentation and extraction. The materials are evaluated as binders for anodes using commercial available silicon materials from Elkem. In addition, the electrochemical performance of Na alginate based electrodes are compared to other proposed binders such as CMC, PVA and PAA.
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