Improving Solid State Batteries Using Bio-derived Alginates

Abstract

Advancements in energy storage technologies need to keep pace with the accelerating transition to renewable energy sources. Li metal anodes and high voltage cathode materials are widely considered to be the path into next generation Li-ion batteries, while the scare amounts of lithium in the earth's crust has prompted research into other battery materials. Of these, Sodium-ion batteries are extremely interesting due to the abundance of Na, similar insertion chemistry to Li as well as the possibility of using aqueous electrolytes due to higher redox potential. Next generation Li-ion batteries are currently held back due to issues like - dendrite formation at the metal anode which may lead to internal short circuiting and thermal runaway of the organic electrolyte. Similarly, Na-ion systems have issues with organic electrolytes due to the limited solubility of Na electrolyte salts. Solid state electrolytes (SSEs) have rapidly garnered interest as a potential alternative. SSEs vastly improve battery safety due to their superior thermal stability and mechanical strength. However, there are still limitations for SSEs in both these systems that need to be resolved prior to implementing these electrolytes in commercial batteries.In this study, alginate salts, which are natural polysaccarides extracted from brown algae, have been examined as a solution to the problems in these systems. Initially, sodium and lithium salts (NaAlg and LiAlg) of alginic acid were synthesized followed by their physical characterization. Conductivity for sodium and lithium alginate (with 0% water) were both promising at 0.2 mS/cm. Additionally, both LiAlg and NaAlg exhibit excellent performance as binders in anode systems compared to PVdF (polyvinylidene difluoride) binders.For utilizing the LiAlg in Li solid-state batteries, NaSICON-based LAGP electrolyte was synthesized and characterized for this study. The degradation of LAGP on Li contact has already been well researched. To combat the issue, we try to utilize a layer of LiAlg as protective layer at the LAGP surface. The coated LAGP remains stable against Li without undergoing degradation. The Li cyclability and electrochemical performance of LiAlg was further analysed by coating it on a various electrodes and studying their rate capabilities against Li. LiAlg displayed excellent performance as a secondary polymer electrolyte on the surface and in the bulk of the electrodes. These tests show exceptional cycling performances and establish a proof of concept for LiAlg as a polymer electrolyte.NaAlg which was chosen as the electrolyte in Na-ion batteries, displays better gelation properties. The effects of water content and operating temperature on Na conductivity were investigated. For a fixed water content, increasing the temperature results in improved conductivity - though this effect is more prominent at higher water concentrations and within the margin of error at lower water concentrations. At a fixed temperature, the conductivity shows linear improvement in at low (<25%) and high (>90%) water concentrations, while remaining constant between 25% and 90%. Additionally, the performance of NaAlg electrolytes (2w% NaAlg aq. solution) and GPE (20w% NaAlg Gel Polymer Electrolyte) was compared with the conventional Na2SO4 electrolyte. During these tests, the electrolyte underwent degradation possibly caused due to cross-linking induced by the dissolution of Mn2+ from the cathode on de-sodiation. Nevertheless, as a proof of concept, the NaAlg showed promise as an electrolyte (aq. and GPE) but further system optimization needs to be done to fully establish its scope as electrolytes for Na batteries.