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In this dissertation, we demonstrated that the two factors, namely tethered chain/dynamic and tortuosity/structural effects can be decoupled by preparing polymer membranes with controlled crystal orientation and measuring the in-plane and through-plane conductivity of the orientated membrane.Moderate conductivity anisotropy as a result of PEO lamellar orientation was first observed in a solution cast PEO SPE.
Lithium bis(trifluoromethanesulfonyl)imide (Li TFSI) was incorporated as a source of lithium ions, while a solid plasticizer succinonitrile (SCN) and a liquid plasticizer tetraethylene glycol dimethyl ether (TEGDME) were incorporated in the respective systems.
The important role of plasticizer on the enhancement of ionic conductivity (σ) to the superionic conductive level (10 S/cm) was revealed in both systems.
Ion conduction was confined within the chain fold region, and guided by the crystalline lamellae.
We demonstrated that at low ion content, the in-plane conductivity was 1000-2000 times greater than through-plane one due to the tortuosity effect, which was described using Nielsen's permeability model.
Interestingly, improved ionic conductivity was achieved simultaneously.
The dual improvement may be attributed to the less chemical crosslinked points and the more flexible chain motion in the looser network of PEGDA6000-PEM relative to its PEGDA700 counterpart.
Detailed characterization on Li Fe PO4 electrodes and PEMs retrieved from these cells implied that the observed battery failure might be triggered by an amide-forming side reaction that took place at the interface of a SCN plasticized PEM and a lithium electrode at high temperature. "Studies on Ionic Conductivity and Electrochemical Stability of Plasticized Photopolymerized Polymer Electrolyte Membranes for Solid State Lithium Ion Batteries." Electronic Thesis or Dissertation. He, Ruixuan "Studies on Ionic Conductivity and Electrochemical Stability of Plasticized Photopolymerized Polymer Electrolyte Membranes for Solid State Lithium Ion Batteries." Electronic Thesis or Dissertation.
Of particular importance is the fact that this detrimental side reaction was effectively suppressed upon Li BOB electrolyte modifier addition.
Finally, the investigation of the charge/discharge cycling performance of solid-state Li Fe PO half-cells at an elevated temperature of 60 °C is discussed in Chapter VII.
In the half-cells, particularly, SCN plasticized PEMs with and without electrolyte modifier lithium bis(oxalato)borate (Li BOB) were respectively employed.