Engineered networking in a family of solvent-free single-ion conducting borate network polymer electrolytes for Li-metal battery applications

As the state-of-the-art energy storage technology, lithium-ion batteries have been attracting lots of attention, but their finite energy densities cannot satisfy the overwhelming demand for large energy storage and commercial flammable liquid electrolytes are also plagued by safety concerns. Solvent-free single-ion polymer electrolytes with excellent electrochemical properties are expected to solve these issues and enhance the energy density of the next-generation batteries technology. Here, we engineered the networking of a series of solvent-free anionic network polymer electrolytes to improve ionic transport for Li-metal battery applications. The anionic network polymers formed as a diamondoid structure consisting of borate anions bridged by branched ethylene glycol linkers of differing stoichiometric ratios, enabling the controlled segmental mobility of network polymers. The increasing segmental mobility offered an elevated ionic conductivity, revealing ionic transport was mostly controlled by engineering the segmental mobilities of polymers, especially at the given interanionic distance. However, there was a restricted ionic transport in fast segmental dynamics of the network polymers featuring free branches, implying that the branching would less contribute to ionic transport compared to the interanionic distance likely due to the frustration in changing the coordination site. Standout network polymer exhibited notable ion selectivity in Li+ cation transport and high oxidative stability. Galvanostatic cycling reveals outstanding resistance to dendrite growth, suggesting that the solvent-free network polymer can serve as a powerful electrolyte for Li-metal batteries.