Ion Transport in Semicrystalline Solid Polymer Electrolytes

Shan Cheng

doi:10.17918/etd-7050

Solid polymer electrolytes (SPEs) with both high room temperature ionic conductivity and mechanical integrity are highly desirable for all-solid-state lithium batteries. Linear polyethylene oxide (PEO) represents the simplest yet most attractive solvating polymer due to its capability to form complex with a selected number of alkali metal salt, especially lithium salt. The ether oxygen on PEO backbone coordinates with Li⁺ and the transport of the latter is facilitated through segmental motion of the polymer chain. However, the highly crystalline nature due to stereoregularity and flexibility of the polymer chain complicate the ion transport mechanism. PEO crystallization has been long considered to be detrimental to ion transport as it results in a decrease of the effective fraction of amorphous conducting phase, slower polymer chain dynamics and more tortuous pathways for ion transport. However, a quantitative analysis of crystallization effect on the ionic conductivity is challenging since these factors are always coupled. 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. We further used graphene oxide to enhance the crystal orientation, hence the conductivity anisotropy. To quantitatively characterize the crystallization effect, a model electrolyte system consists of PEO single crystals with well controlled crystal structure, size, crystallinity and orientation were fabricated. 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. Contradictory to the general view, the dynamic effect was negligible at moderate ion contents and the overall conductivity was mainly controlled by crystal orientation, strong Li-PEO interaction and Li⁺ aggregation. Our results demonstrated that semicrystalline polymer can be viewed as a two phase model which morphologically mimicking the popular systems such as block copolymers and polyolefin porous membranes. By controlling crystallization behavior, mechanically robust semi-crystalline SPE with high room temperature conductivity is feasible.

Ion Transport in Semicrystalline Solid Polymer Electrolytes

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Ion Transport in Semicrystalline Solid Polymer Electrolytes

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