As one of the important components within lithium batteries, the electrolyte plays a vital role in aspects of safety and achieving long-term stable electrochemical performance. Next-generation lithium metal batteries (LMBs) are currently limited by the occurrence of inhomogeneous lithium deposits, eventually dendrite formation, or disadvantageous flammability arising from organic liquid electrolytes. In contrast, solid-state polymer electrolytes (SPE) are regarded as promising candidates for high-energy density LMBs affording enhanced safety, mechanical stability and stability against dendrite formation. Polycaprolactone (PCL) is considered as one of the most promising biocompatible polymers due to its degradation in contact with microorganisms, yet little work has been conducted on pure PCL SPEs especially against high-voltage cathode materials.
In the present work, polycaprolactone-based star polymers (Bt-PCL & xBt-PCL) are synthesized and applied as solid polymer electrolytes in Li||NMC622 cells. Neither in synthesis nor in membrane processing was any solvent present, enabling a fully dry polymer electrolyte. While Bt-PCL exhibits a wide electro-chemical stability window (up to 5.3 V, against stainless steel) and decent lithium ion transference number (0.5) at 60 ℃, insufficient mechanical strength due to low melting point leads to unstable cell performance. To further improve its thermal and mechanical stability, cross-linked structure xBt-PCL is developed by simply introducing hydrogen-abstraction photoinitiator and then UV-radiating, thus facilitating a more durable membrane in suppressing dendritic/mossy lithium growth at high temperature (60 ℃). Anions mobility limited by cross-linked structure is seen in enhanced transference number from 0.5 to 0.74, achieving more homogeneous lithium deposition where better interfacial properties and capacity retention (88% after 60 cycles) in Li|SPE|NMC622 cells are demonstrated. In summary, xBt-PCL indeed reflects a sustainable solid electrolyte being attractive candidate for high-voltage lithium metal batteries.