Stable Cyclability of Novel Triblock Polyelectrolyte Separators for Lithium Metal Battery

Abstract

Lithium metal is a promising anode for next generation lithium batteries due to its high theoretical energy density (3860mAh g-1). However, despite the attractive feature of lithium metal, its dendrite could form short circuit inside the cells and cause serious safety problems during lithium plating and stripping process. Polymer electrolyte with high lithium transference number and high modulus would substantially mitigate dendrite growth in lithium metal battery. According to previous models, structuring the electrolyte to immobilized the anions with high shear modulus is of the various options to suppress dendrite growth. In this work, we designed a single-ion conductor (SIC) based on a block copolymer using sulfonic anion, whose structure covalently bonded with hydoxylated polyisoprene (named as sPI) to reach high transference number. Polystyrene can be used to support the mechanical properties of matrix and the adjacent polyisoprene with double bond main chain plays a decoupling role (PI(1,4)) which separate the stiff PS domain, and the lithium conducting sPI domain (referred to SII block copolymer). Using small angle X-ray scattering and transmission electron microscopy, nanostructures of membranes were identified as a bi-continuous network structure. Remarkably, the maximum conductivity was 1.4×10-4 S cm-1 at ambient temperature with merely 30 wt% solvent uptake. The lithium transference number (TLi +) was nearly unity in liquid electrolyte. The Young’s modulus of SII was 53 MPa and storage modulus was nearly 3 GPa at room temperature. This bi-continuous morphology with high TLi +, high modulus, and moderate conductivity is believed to suppress dendrite grow effectively. From galvanostatic cycling experiments, we found that the presence of PS and the sPI in SII membrane in liquid electrolyte with lithium symmetric cell provides over 245 hr in cell life time. Batteries containing SII membranes in liquid electrolyte are able to cycle at least 20 times with high coulombic efficiency (98%). This results warrant that the sPI can effectively delay lithium dendrite formation due to reduced uneven lithium deposition while maintaining the mechanical property with moderate values by adjusting PS and PI(1,4) length, and this architecture design also made the application of SIC in lithium metal polymer battery possible in the future.