HIGH MODULI POLYMER GELS AND NON-AQUEOUS ELECTROLYTES WITH MUTLI-IONIC LITHIUM SALTS HAVING HIGH THERMAL STABILITY AND LITHIUM ION TRANSFERENCE NUMBERS

Abstract

Rechargeable Li-ion batteries play pivotal role in the growth of the market for portable electronic devices like mobile phones, laptops etc. Several key drawbacks with the Lithium-ion Batteries (LIBs) put a limitation on their use in more advanced applications especially, those that require more power output and rapid charging times such as electric vehicles. One issue is the safety concerns that arise due to the growth of lithium dendrites especially at rapid charging conditions, leading to fire hazards at extreme thermal runaway scenarios. Another issue is that the current state of the art LIBs are fast approaching their maximum theoretical energy density limits. The energy density of the present-day LIBs is insufficient to deliver satisfactory performance especially when used in advanced applications such as electric vehicles which require long driving range per charge. One possible approach suggested to increase the energy density of the battery is by replacing intercalated graphite anode with lithium metal which has greater theoretical capacity. Utilizing lithium metal would open the possibility to use cathodes with conversion chemistry that would store more charge per cycle and there by resulting in the overall increase in the energy density of the battery. One of the key issues impeding the commercialization of Li-metal batteries is safety. Safety issues arises due to the uneven deposition of Li on the surface of the Li metal which would give rise to needle like protrusions (dendrites) that are capable of piercing the separator and reaching all the way to cathode. The result is an internal short circuit of the cell , which ultimately results in fire or explosion. There are many solutions that have been proposed over the years, courtesy of some extensive theoretical and experimental research to tackle the issue of dendrite growth. This thesis describes about the research work along the lines of a couple of possible approaches to prevent the dendrite growth. The first approach to be discussed is the development of solid electrolytes/separators to address this safety issue. The major drawback with most of the solid electrolytes/separators is that they have very low conductivity and lithium ion transference number (tLi+)values. Transference number of an ion by definition is the fraction of current carried by that particular ion, here in the case of LIBs is Li+, out of the total current carried by all the ions in the solution. Ideally a tLi+ of unity is aspired. The goal was to develop solid electrolytes having good room temperature conductivity and high transference numbers and at the same time having good mechanical strength. To that extent high moduli polymer gel electrolytes with high thermal stability were developed by in-situ encapsulation of ionic liquids and solvate ionic liquids into nanofibrillar methyl cellulose networks. The separators/iongels prepared possessed good room temperature conductivity and lithium ion transference numbers. The other approach described is developing non-aqueous liquid electrolytes having high ionic conductivity and high Li+ transference numbers (tLi+). Electrolytes with high tLi+ are efficient in minimizing the concentration polarization and ultimately mitigating the dendrite growth. As a part of this approach a functionalized symmetric, multi-ionic polyhedral oligomeric silsesquioxane (POSS) with dissociative lithium salt (POSS-(LiNSO2CF3)8 ) salt was dissolved in tetraglyme (G4), CH3–O–(CH2CH2O)4–CH3 in a specific O/Li ratio and the solution mixture was used as a electrolyte. Good ionic conductivities (σ10-4 S cm-1) and lithium ion transference numbers of tLi+= 0.65 are achieved in these electrolyte systems.