Implementation of highly crystalline polyketones as solid polymer electrolytes in high-temperature lithium metal batteries

Highly crystalline polymers have superior mechanical properties to amorphous polymers due to the stabilizing effect of the crystalline regions. In the context of solid polymer electrolytes, where the ionic transport is confined to the amorphous phases and segmental motion is critical for the conduction mechanism, crystallinity in general is detrimental. This work explores the properties and implementation in lithium metal cells of the highly crystalline polyketone poly(1-oxoheptamethylene) (POHM) and a modified, partially unsaturated and more amorphous analog (POHM-70). Interestingly, both polyketones demonstrate comparable ionic conductivity to PEO at high salt concentrations simultaneously as they are mechanically stable at 80 °C. Furthermore, due to the cross-linking of POHM-70 upon heating, it showed mechanical stability even at 120 °C. In contact with lithium metal, these polyketones are initially prone to degradation. Yet, over time, while POHM continues to degrade, the cross-linked POHM-70 passivates the interface with the electrodes and prevents it from continued decomposition. In a lithium iron phosphate (LFP)││Li cell, POHM initially shows a specific capacity close to the theoretical capacity of LFP, that becomes inaccessible due to increased internal resistance as a result of its reactivity with Li. The same capacity fade is not observed for POHM-70, where instead a stable and consistent – albeit low – capacity is observed. Despite the observed challenges with both POHM and POHM-70, they reveal promising opportunities for utilizing both semi-crystalline and amorphous polyketones as solid polymer electrolytes, and in particular finding ways to fine-tune the material for high conductivity in the semi-crystalline state.