STRUCTURE AND DYNAMICS OF SOFT MATERIALS FOR FLEXIBLE ELECTRONICS AND LITHIUM ION BATTERY

Restricted (Penn State Only)
Author:
Zhan, Pengfei
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 10, 2017
Committee Members:
  • Janna Kay Maranas, Dissertation Advisor
  • Janna Kay Maranas, Committee Chair
  • Michael John Janik, Committee Member
  • Scott Thomas Milner, Committee Member
  • Ralph H Colby, Outside Member
Keywords:
  • polymer
  • neutron scattering
  • organic semiconductor
  • li ion battery
  • solid polymer electrolyte
Abstract:
Organic semiconductors and solid polymer electrolytes are promising soft materials for the realization of future electronics and better batteries. Both series of materials demonstrate considerable advantages over the current materials used in application. For example, organic semiconductors such as poly(3-alkylthiophene)s (P3ATs) and 2,7-dioctyl-benzothenobenzothiophene (dC8-BTBT) demonstrate significantly higher mechanical flexibility over inorganic semiconducting materials. These organic materials dissolve in various organic solvents and can be printed on plastic substrates, thus decreasing the processing cost. Replacing silicon with organic semiconductors not only increases the flexibility and lowers the cost, but also makes the device smaller and lighter. In Lithium ion battery, polyethylene oxide (PEO) based solid polymer electrolytes are an attractive alternative to the flammable and toxic liquid/gel electrolytes currently used in rechargable lithium ion batteries. The higher mechanical modulus of solid PEO can slow the growth of dendrites (Li metal that grows through the electrolyte and causes battery failure) and prolong battery life. Because the membrane is mechanically stable, a hard casing is not required and thus the battery is lighter and flexible. In polymer semiconductors, side-chains are added to increase the solubility of the conjugated polymer in common solvents. However, our study of amorphous materials suggested adding side-chains can harm the charge transport. It is well established that structure is a critical factor for charge transport. In amorphous materials, side-chains length has minimal effect on structure. We suspect the polymer dynamic is influenced by side-chain length. Although the role of molecular motion on charge mobilities is still not well understood. We experimentally measured the dynamic in amorphous conjugated polymers poly(3-alkylthiophene)s (P3ATs) with quasi-elastic neutron scattering (QENS). The analysis of the QENS data shows that longer side-chains relax faster compared with shorter side-chains and our further analysis of the elastic incoherent structure factor (EISF) suggests that the amplitude of proton motion on the thiophene rings increases by a factor of 3 as the side-chain length increases from 6 to 12, demonstrating that longer side chains lead to enhanced motion of conjugated rings. To fully understand the effect of side-chain on dynamic of charge transport unit, we expand our investigation to highly ordered crystalline material. The lattice vibration of benzothinobenzothiophene (BTBT), 2-octyl-benzothionbenzothiophene (mC8-BTBT), and 2,7-dioctyl-benzothionbenzothiophene (dC8-BTBT) is measured with inelastic neutron scattering (INS). The charge mobility of BTBT and its alkylated derivatives has shown an increase upon the addition of alkyl side-chains and INS experiment shows a suppressed vibration in the sample with two alkyl side chains. We hypothesize the lattice vibration can influence the charge transport via electron-phonon interaction and lattice dynamics should be another factor to be taken into consideration in future material design. In solid polymer electrolyte, crystalline PEO6LiX complex has shown to be more conductive than amorphous counterparts. Since its discovery in 1999, it has attracted tremendous amount of attention in the field of solid polymer electrolyte. However, as of now, none of commercialized electrolyte is based on this complex structure. This is because the original study used low molecular weight PEO that is liquid at room temperature. Increasing the molecular weight of PEO introduces higher degree of disorder to the complex which sacrifices the PEO6LiX crystallinity and decreases the conductivity. In this study, the PEO molecular weight we use is 600K Da. We use acidic cellulose nanowhisker to assist the nucleation of PEO6LiX crystalline complex at room temperature. These polymer-cellulose composite electrolytes have demonstrated room temperature conductivity higher than 10-5 S/cm and low Li+ transport activation energy. X-ray diffraction (XRD) shows high cellulose surface acidity increases the PEO6LiX crystallinity and promotes the conduction around the room temperature. The energy barrier for Li+ hopping through PEO6 channel decreases significantly as the cellulose nanowhisker surface acidity increases. With quasi-elastic neutron scattering (QENS) we demonstrate ion transport is decoupled with polymer relaxation time, suggesting PEO6 is likely the conduction media. To fully take advantage of the conduction mechanism of PEO6, the channel should be aligned. In this work, we investigate different options for alignment and proposed a device design for this purpose. Another important salt concentration in PEO-salt mixture is the eutectic concentration. From many polymer electrolyte studies, the conductivity maximized at eutectic concentration. To understand the influence of eutectic concentration on conduction, we select a series of PEOxLiX electrolytes with eutectic concentrations (ester oxygen to Li+ ratio) that vary from 7 to 100. We demonstrate that conductivity directly correlates to the distance from the eutectic concentration. We demonstrate the maximum gain when these electrolytes are filled with α-Al2O3 nanoparticles is at the eutectic concentration. Both findings are important for effective design of polymer electrolytes for Li ion batteries.