SYNTHESIS AND CHARACTERIZATION OF HYBRID ELECTROLYTES WITH TETHERED IONIC LIQUID FOR LITHIUM ION BATTERIES

Restricted (Penn State Only)
Author:
Yang, Guang
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 08, 2018
Committee Members:
  • Qing Wang, Dissertation Advisor
  • Qing Wang, Committee Chair
  • Robert Hickey, Committee Member
  • Enrique Gomez , Committee Member
  • Jian Yang, Outside Member
Keywords:
  • lithium metal battery
  • octasilsesquioxane
  • hybrid electrolyte
  • polymers
  • ionic conductivity
Abstract:
Rechargeable lithium ion batteries are revolutionary energy storage systems widely used in portable electronic devices (e.g., mobile phones, laptops) and more recently electrical vehicles. The conventional liquid electrolytes in the lithium ion battery brought about safety problems such as fire and explosion. Related safety accidents (e.g., cell phone explosion, laptop fire, plane smoldering, etc.) have been reported many times. This also eliminates the possibility of using lithium metal as anode material which has much higher theoretical specific capacity in comparison with commercial graphite electrode because of the growth of uncontrolled lithium dendrites can lead to short circuit and other serious accidents. Solid polymer electrolytes have many advantages over conventional liquid electrolytes. They are light-weighted, non-volatile and have much better safety features than liquid electrolyte. Meanwhile, they are also better than the ceramic electrolyte in terms of their excellent flexibility and processability. Currently, low ionic conductivity of solid polymer electrolytes (e.g., polyethylene oxide (PEO)) at ambient temperature still hinders their practical application. Ionic liquids (ILs) are non-flammable and have negligible volatility. Its ionic conductive nature, excellent chemical stability, and good electrochemical stability enable them to be regarded as useful components for next generation battery electrolytes. In this thesis work, focus will be placed on synthesis and characterization of ionic liquid tethered organic/inorganic hybrid polymer electrolyte with high room temperature ionic conductivity. Moreover, their electrochemical properties and prototype battery performances were also looked into. The use of highly conductive solid-state electrolytes to replace conventional liquid organic electrolytes enables radical improvements in reliability, safety and performance of lithium batteries. Here in chapter 2, we report the synthesis and characterization of a new class of nonflammable solid electrolytes based on the grafting of ionic liquids onto octa-silsesquioxane. The electrolyte exhibits outstanding room-temperature ionic conductivity (~4.8 × 10-4 S/cm), excellent electrochemical stability (up to 5 V relative to Li+/Li) and high thermal stability. All-solid-state Li metal batteries using the prepared electrolyte membrane are successfully cycled with high coulombic efficiencies at ambient temperature. Good cycling stability of the electrolyte against lithium has been demonstrated. This work provides a new platform of solid polymer electrolyte for the application of room-temperature lithium batteries. In chapter 3, an organic-inorganic hybrid solid electrolyte with ionic liquid moieties tethered onto dumbbell-shaped octasilsesquioxanes through oligo(ethylene glycol) spacers was synthesized. The hybrid electrolyte is featured by its high room-temperature ionic conductivity (1.2×10-4 S/cm at 20 oC with LiTFSI salt), excellent electrochemical stability (4.6 V vs Li+/Li), and great thermal stability. Excellent capability of the hybrid electrolyte to mediate electrochemical deposition and dissolution of lithium has been demonstrated in the symmetrical lithium cells. No short circuit has been observed after more than 500 hrs in the polarization tests. Decent charge/discharge performance has been obtained in the prepared electrolyte based all-solid-state lithium battery cells at ambient temperature. In chapter 4, hybrid polymer electrolyte network (XPOSS-IL) synthesized by crosslinking the individual dendritic POSS-IL was investigated. To be specific, after grafting mono-broninated hexaethylene glycol to the POSS cage, 1-vinyl imidazole was adopted for the subsequent quarternization reaction. Then the chain end double bonds underwent free radical crosslinking process to produce XPOSS-IL. The ionic conductivity of LiTFSI dissolved XPOSS-IL is 5.4 ×10-5 S/cm at 30 ℃. By adding a small fraction of ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), the ionic conductivity increases to 1.4 ×10-4 S/cm at room temperature. It is also found that EMITFSI will enhance the anodic stability of XPOSS-IL. The Li/LTO and Li/LFP cell assembled with X-POSS-IL-LiTFSI/EMITFSI demonstrates capability of delivering high specific capacities at room temperature and elevated temperature.