(616i) Crosslinked Hairy Nanoparticle Membrane for Enabling High Reversibility in Lithium Metal Batteries

Lithium metal batteries have been regarded as the ‘holy-grail’ of energy storage devices due to their promise of providing platform for large-scale energy storage devices with high rechargeability. However, safety concerns related to their operation have limited their practical usage. The instabilities in such batteries occur due to the uneven electrodeposition of lithium ions that take the shape of needle-like dendrites on repeated cycling, which are capable of migrating across to the counter electrode to cause short-circuits that often results in fire or explosion. In this regard, there have been several studies in the literature dedicated to the prevention of dendrite growth by means of a high modulus physical barrier. However, electrolytes/separators with high mechanical strength tend to have low room temperature ionic conductivity, thus limiting their practical use. A recent theoretical study based on linear stability analysis of dendrite growth by Tikekar et al. (JECS 2014, Science Advances 2016) pointed out the importance of separator-pore size in addition to its modulus. In a nutshell, they concluded that dendrites can be prevented from crossing over to the counter electrode using battery separator with pore-diameter lower than critical (smallest) size of the dendritic nucleate. In this study, we designed a crosslinked nanoparticle-polymer composite electrolyte with tunable pore size and carried out stability test for dendrite growth. The pore size of this membrane (or the inter-particle distance) can be effectively tuned by controlling the particle volume fraction in the membrane. Transmission electron microscopy and small-angle X-ray scattering measurements were performed for measuring the inter-particle distances. In contrast to most previously reported polymer electrolytes, the crosslinked membrane simultaneously showed good mechanical strength (G’~1MPa) and high ionic conductivity at room temperature (s~5mS/cm), which is a consequence of the high crosslinking density. Since each nanoparticle has multiple node points (~55) for linkage with other nanoparticles, high modulus is achieved even with a relatively low volume fraction of nanoparticles. Direct visualization experiments were conducted using optical microscopy for understanding the nucleation morphology and dendrite-growth rate with different pore sizes. The crosslinked membrane with a pore size of ~25nm was further used as composite electrolyte in a symmetric lithium cell and the dendrite induced short circuit time was quantified using polarization (continuous charging) test at various current densities. The crosslinked membrane indeed showed very high short circuit time compared to similar electrolytes reported in the literature, owing to the pore-size cut-off effect.