Formulation and Characterization of Biopolymer for Solid-State Electrolytes

Abstract

Flammable organic electrolytes and narrow oxidative stability still prevent next-generation solid state lithium-ion batteries from using high-voltage cathodes with sustainable polymers. This dissertation addresses this problem by developing a bio-sourced, chitosan-based solid polymer electrolyte reinforced with a tannic-acid–lithium metal–organic framework (TALi). The TALi is novel because it combines phenolic anchoring sites from tannic acid with lithium coordination, creating both Li⁺ binding sites and mesoscale porosity in a single additive. Casting 4:1 chitosan:TALi films followed by controlled protonation produces dense 120 µm membranes with room-temperature ionic conductivity of 4.38 × 10⁻⁴ S cm⁻¹ about 12 times higher than pure chitosan. Conductivity increases to 1.90 × 10⁻³ S cm⁻¹ at 80 °C with an activation energy of 0.252 eV. Linear-sweep voltammetry shows practical stability up to ≈ 4.9 V vs Li/Li⁺ at 0.10 mA cm⁻². Li ‖ 4:1 film ‖ NMC 532 coin cells deliver 127 mAh g⁻¹ at C/20 (85% of liquid-electrolyte capacity) and retain 109 mAh g⁻¹ after 34 cycles with 93–95% coulombic efficiency, giving a fade rate of 0.06% per cycle. Rate tests show 56.8 mAh g⁻¹ at C/5 and 98% capacity recovery when returned to C/20. Spectroscopic and dielectric analyses show the high conductivity comes from dual Li⁺ transport pathways. These include segmental motion along protonated chitosan chains and vacancy-assisted hopping across catecholate sites in the MOF. Maxwell–Wagner polarization inside 50–200 nm pores also increases free-ion density. A percolation optimum near 20 wt% TALi balances conductivity, mechanical properties, and electrochemical window. This work demonstrates a practical approach toward safer, high-voltage, bio-derived electrolytes using scalable solution processing methods