Metal-Organic Framework (MOF) Assisted Ion Conduction In Solid Polymer Electrolytes For Application in Lithium-ion Batteries
Open Access
- Author:
- Zerin, Nagma
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 22, 2021
- Committee Members:
- Xueyi Zhang, Major Field Member
Janna Maranas, Chair & Dissertation Advisor
Michael Janik, Major Field Member
Harry Allcock, Outside Unit & Field Member
Phillip Savage, Program Head/Chair - Keywords:
- Solid polymer electrolyte
Metal-Organic Framework (MOF)
Lithium-ion battery
PEO6/LiClO4
PEO3/LiClO4
Ion conduction - Abstract:
- Solid polymer electrolytes (SPEs) are safer and cleaner alternatives to the flammable organic liquid electrolytes used in rechargeable lithium-ion batteries. Due to having the potential to tune stiffness, SPEs can prevent dendrite formation while being used with the lithium-metal anode of high energy density. The most common SPE is polyethylene oxide (PEO) dissolving a lithium salt (e.g., LiClO4). PEO is popular due to its ability to dissolve lithium salts easily and commercial availability at a reasonable cost. However, these electrolytes have very low ionic conductivity at room temperature (less than 10^-5 S/cm), which does not meet the performance demands for practical battery applications. It is commonly believed that lithium-ion conduction in PEO-based electrolytes only depends on polymer segmental motion in amorphous regions. Thus, to increase polymer mobility associated conductivity, a considerable amount of research is focused on lowering the glass transition temperature (Tg) of SPEs. However, this approach compromises the SPE stiffness, which makes it incompatible with the lithium-metal anode. Crystalline solid polymer electrolytes, commonly known as polymer-salt co-crystals, are promising alternatives to dissociate conductivity from polymer mobility and increase stiffness. Unfortunately, these crystals are poor ion conductors on their own. There are not many studies that attempt to increase conduction through polymer-salt co-crystals. This research aims to improve conduction through these crystals and understand their mechanism through structural analysis. To achieve our goal, we incorporate Metal-Organic framework (MOF), with a high aspect ratio, as the nanofiller. We choose MOF due to its flexibility to tune surface chemistry and dimension as well as filler shape. We analyze the effect of two shapes of MOF, one nanosheet (Cu MOF/C2n2d) and the other nanowhisker (Ni MOF/Nnd), on 5 different SPE compositions: EO:Li=14:1, 10:1, 8:1, 6:1, and 3:1 [EO= Ether oxygen, Li= Lithium]. These compositions form 3 types of crystals depending on the lithium-ion content: PEO, PEO6, and PEO3. While PEO6 and PEO3 are conductive polymer-salt co-crystals, PEO is an insulator. In PEO6, two PEO chains wrap around each other to form a cylindrical tunnel-like structure. The lithium ions conduct through the tunnels, while the anions stay outside. In PEO3, the lithium ions and anions form an ion chain, which wraps around PEO. The lithium ions slide along PEO while interacting with the anions. We have achieved significant improvement in crystalline conduction with only 2 wt% Cu MOF loading. We obtain our best conductivity at EO:Li=8:1, where 2 wt% Cu MOF produces greater than 10^-5 S/cm conductivity at room temperature. At EO:Li=6:1, the % conductivity increase is between 200-900% in the entire temperature range with 2 wt% Cu MOF, which is a significant achievement for an electrolyte containing high molecular weight PEO6. We have discovered that the shape of the MOF filler and thermal annealing time play very important roles in crystalline conductivity. 2D Cu MOF has higher conductivity than 1D Ni MOF for all the compositions, and 10 days of thermal annealing produces more effective conductive crystals than those produced after a longer annealing time. Conductivity drops significantly after one month as a result of excessive bulk crystallization. For practical battery applications, it would be imperative to control the extent of the crystallinity of the crystalline polymer electrolytes. We recommend crosslinking the composites during initial thermal annealing to hold the effective crystal structures in place. Although we are still quite far away from reaching the target room-temperature conductivity (10^-3 S/cm) for commercial battery applications, this study is the first demonstration of MOF-assisted crystalline conduction. In the field of MOF nanofillers, this opens up the opportunity to explore a novel mechanism for improving conductivity, where ion conduction is decoupled from polymer segmental motion. The combined use of crystalline polymer electrolyte and MOF has the potential to limit anion movement and dendrite formation, which paves the way to design a safe and efficient battery.