Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2022 / Paper 1 / Section B / 11 / i / b / Solution

A solid polymer electrolyte is flexible, lightweight and potentially easier to make into a thin continuous separator. It can reduce free-liquid leakage and give compliant contact with a rough electrode. Ether-containing polymers, for example, coordinate lithium ions; ionic motion can be coupled to segmental motion of the polymer chains.

Many such polymers have inadequate conductivity at room temperature because crystallinity and restricted chain motion impede ion transport. Heating, adding plasticizer or reducing crystallinity can improve conductivity, but may sacrifice mechanical stiffness or reintroduce liquid-like hazards. Oxidative stability and compatibility with lithium-rich negative electrodes also constrain the usable voltage range. A soft polymer film is not automatically a complete barrier to lithium filament penetration; defects and local current concentration remain important.