DEVELOPMENT OF CARBOXYMETHYL CHITOSAN AND EMIMTFSI IONIC LIQUID-BASED SOLID POLYMER ELECTROLYTES FOR SOLID-STATE LITHIUM BATTERIES

• Main Author: Deliana Dewi Sundari, Citra
• Format: Dissertations
• Language: Indonesia
• Subjects: Kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/81302
• Institution: Institut Teknologi Bandung

Lithium-ion batteries have achieved success in terms of commercialization and have been widely used due to their high energy density. However, despite their advantages, lithium-ion batteries still have several problems, including safety issues, environmental issues, limited energy capacity, and design limitations. Commercial lithium-ion batteries use liquid electrolytes based on organic solvents which have a risk of flammability and explosion if there is leakage of electrolyte material and improper use of the battery. To overcome this, the liquid electrolyte needs to be replaced with a non-flammable solid electrolyte. Solid polymer electrolyte (SPE) has several advantages compared to liquid electrolyte, i.e., enabling high voltage lithium-ion batteries, simple manufacturing process, high safety, flexible, and enable the use of lithium metal anodes for higher energy density of the battery. One of the materials that has the potential to be developed as SPE is carboxymethyl chitosan (CMCS), due to its higher ionic conductivity than polyethylene oxide (PEO), the commonly used polymer as SPE, and has an excellent ability to dissolve lithium salts. The addition of lithium salts with large anions such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was reported to increase ionic conductivity because it dissociates readily in the polymer matrix. In addition, 1-ethyl-3 methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) ionic liquid can potentially increase the ionic conductivity and electrochemical stability of SPE. This research aims to develop SPE based on CMCS and EMImTFSI to be applied as SPE in lithium-ion batteries. LiTFSI salt is used as a source of lithium ions. The addition of LiTFSI to CMCS SPE has increased its ionic conductivity from 2.09 x 10-7 S cm-1 to its highest value, i.e., 7.81 x 10-5 S cm-1 at room temperature with the addition of 25% LiTFSI. Further addition of LiTFSI reduces the ionic conductivity and mechanical properties of SPE due to the formation of salt crystallites which cause a structural inhomogeneity. SPE CMCS/LiTFSI-25% has a high tensile strength of 35.6 MPa, thermal stability up to ~244 °C, high electrochemical stability up to 5.4 V (vs. Li/Li+), and galvanostatic lithium plating/stripping cycle stability with low overpotential (~30 mV). Furthermore, the addition of EMImTFSI by 10-20% causes a decrease in SPE crystallinity, thereby increasing the ionic conductivity. The highest ionic conductivity was obtained with additions of 20% EMImTFSI, leading to increased ionic conductivity up to 1.38 x 10-4 S cm-1 at room temperature. The SPE tensile stress increased with the addition of up to 30% EMImTFSI but the increase was limited (28.1 – 31.2 MPa). Meanwhile, the tensile strain value of SPE decreases quite significantly with the addition of EMImTFSI, resulting in a stiffer SPE. SPE CMCS/LiTFSI/EMImTFSI-20% is electrochemically stable up to 5.5 V vs Li/Li+. This SPE also shows stable cycling up to more than 1000 hours on galvanostatic lithium plating/stripping cycles, with small overpotential values (~4 mV). DFT computational methods and molecular dynamics are employed to investigate the ion conduction mechanisms, interparticle interactions, and to support experimental findings. Initial computational studies on the EMImTFSI/LiTFSI binary electrolyte system show that the intermolecular interactions that occur are non-covalent and weak electrostatic interactions. The strength of the intermolecular interactions in the EMImTFSI/LiTFSI system is stronger than in the pure EMImTFSI system. Li+ ions navigate through cooperative cation-anion movements, with the Li+ ions conductivity reaching a maximum value at intermediate LiTFSI concentrations (0.1<xLi<0.2). DFT calculation on the CMCS/LiTFSI system shows that the interaction energies between Li+ - TFSI- and Li+ - CMCS (13.73 and 12.22-14.52 kcal/mol) are much lower than the Li+-TFSI- interaction energy in pure LiTFSI system (38.15 kcal/mol), which supports the experimental findings of an increase in SPE ionic conductivity upon the addition of LiTFSI. However, at high LiTFSI concentrations, the strong interaction between Li+ and TFSI- becomes dominant, thereby triggering a decrease in ionic conductivity. The findings are confirmed by molecular dynamics simulations, in which the coordination number between Li+ and oxygen in TFSI- increases as the LiTFSI concentration increased. The lithium ion conduction mechanism in the CMCS/LiTFSI SPE is a combination of ion hopping and polymer segmental motion. Furthermore, DFT calculations on the CMCS/LiTFSI/EMImTFSI system show that the presence of EMIm+ cations can limit the coordination of Li+ with TFSI ions, even though the interaction energy between Li+ and TFSI- ions increases. Molecular dynamics simulations on the CMCS/LiTFSI/EMImTFSI system indicate an increase in Li+ diffusion coefficient by 3.42% and a decrease in TFSI- diffusion coefficient by 11.62% due to the addition of EMImTFSI. The increase in Li+ ionic mobility and the decrease in TFSI- ionic mobility may lead to a positive impact on the ionic conductivity of CMCS/LiTFSI/EMImTFSI SPEs for battery applications.