Synthesis and Comparison of Chitosan and Methocel Based Flexible Gels for Potential Use As Organic Electrolytes in Batteries
Authors: Poosapati Aswani , Madan Deepa 
Published: February 2020

Reversibility, power output, and energy storage capability are considered to be the primary requirements when fabricating a rechargeable battery. These attributes are majorly based on the composition along with the interactions (redox reactions) occurring between them. An electrolyte is majorly responsible for the redox-reactions to occur at interfaces, and thus its preparation is a vital step in constructing a battery. Liquid electrolytes were mostly used during the advent of batteries, but due to hindrances like toxicity, corrosive nature, safety and leakage issues, and dendrite formation leading to battery failure; polymeric electrolytes (PEO, PVS, PEG, etc.) were found to be the best alternative. This was due to their similar ionic conductivity (σi), electrochemical stability and transport properties to their liquid counterparts but with better dynamic properties suitable for varied applications. Although relatively high values of σi were already reported by various groups for various polymer systems, they are made with synthetic polymers along with acids and other hazardous components. This leaves the possibility of leaching into the environment due to the corrosion of the casing unless safely disposed. Also, most of their preparation processes are complex, time and energy intensive.

Our study focuses on the possibility of the use of the naturally available class of polymers called polysaccharides to form free-standing, flexible films that act as gel electrolytes. Chitosan and Methocel are chosen among various polysaccharides for testing due to the ease of preparation from the shells of crabs, shrimps and prawns and cellulose respectively. Also, they are naturally abundant, renewable, biodegradable, cost-effective, eco-friendly, environmentally benign and have a high degree of functionality which is not available in most synthetic polymers. During initial testing, the ionic conductivity values obtained for pristine chitosan and methocel were approximately 10-6S/cm, which is at least two orders of magnitude higher than the conventionally used synthetic polymers (PEO, PVP, etc.). This inherent advantage of these biopolymers encouraged us to explore the possibility of their use as an electrolyte layer in batteries. It was noted that a semi-stable gel is formed when these biopolymers were vigorously mixed with minute amounts of acetic acid and dissolved in deionized water and allowed to sit for a while. It was also noted from the literature that chitosan films have an enormous surface area per unit volume (high aspect ratio), which makes us anticipate higher efficiencies. The possibility of forming a semi-stable gel eliminates the need to base the electrolyte primarily on synthetic materials such as PEO, which are non-biodegradable and also possess a crystallization tendency which results in lowering of ionic conductivity and flexibility of the films.

In our effort to make stable yet flexible organic films, various amounts of PVA (x=0.2, 0.4, 0.6, 0.8, 1) were added individually to Chitosan (CPx, CP=Chitosan: PVA) and methocel (MPx, MP=Methocel: PVA), vortex mixed, cast into films, dried and then tested against SS blocking electrodes. Unlike most other polymer preparation processes that are complex and energy intensive, we prepare organic electrolytes in much simpler stages and dry at temperatures close to room temperature. Ionic conductivity studies were performed on the various films obtained by AC impedance spectra analysis. The best average ionic conductivity values recorded were 6.804mS/cm and 0.0442mS/cm for CP0.2 and MP1 respectively. These samples CP0.2 and MP1 were then chosen and used for future experimentations due to enhanced conductivity and flexibility properties in comparison to pristine chitosan and methocel respectively. To further enhance the ionic conductivities, varying amounts of KOH (y=0.1, 0.2, 0.3, 0.4, 0.5) were added individually to CP0.2 (CPKy, CPK= CP0.2: KOH) and MP1 (MPKy, MPK= MP1: KOH) respectively, and cast into films and tested in a similar process as earlier. An average highest ionic conductivity of 105mS/cm and 7mS/cm was obtained for CPK0.3 (Chitosan: PVA: KOH= 1:0.2:0.3) sample among the CP0.2 samples, and for MPK0.4 (Methocel: PVA: KOH= 1:1:0.4) sample among the MP1 samples respectively. When comparing the results obtained, though the stability and flexibility of chitosan and methocel films were relatively similar their ionic conductivity values were extremely different. Chitosan films with PVA and KOH had much higher ionic conductivity than methocel films and we attribute this to the inherent high aspect ratio nature of chitosan. The best average conductivity value of 105mS/cm obtained for CPK0.3 is comparable to the values of other existing synthetic polymer electrolytes. For future experimentation, we plan to test the CPK0.3 electrolyte in a full cell against suitable cathode and anode and characterize it.