(Invited) Rational Design of Nanoporous Polymer Electrolytes and Solid-Liquid Interphases for Lithium Metal Batteries

Advances in the basic science and engineering principles of electrochemical energy storage is imperative for significant progress in portable electronic devices. In this regard, metal based batteries comprising of a reactive metal (like Li, Na, Al) as anode have attracted remarkable attention due to their promise of improving the anode-specific capacity by as much as 10-fold, compared to the current state-of-art Li-ion battery that uses a graphitic anode. Perhaps their greatest advantage lies in the possibility of using of a Li-free high-capacity cathode like Oxygen that can improve the gravimetric energy density of batteries from ~0.3kWh/kg to ~12kWh/kg (i.e. comparable to the useful energy available from combustion of hydrocarbons). A persistent challenge with batteries based on metallic anodes, concerns their propensity to fail by short-circuits produced by dendrite growth during battery recharge, as well as by runaway of the cell resistance due to internal side reactions with liquid electrolytes. In this talk, I will discuss my research that utilizes multiscale transport modeling and experiments to fundamentally understand and to thereby develop rational designs for polymer electrolytes and electrode-electrolyte interphases that overcome these challenges (Tikekar, Choudhury et al. Nat. Energy 2016). There have been several studies in 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 ionic conductivity, thus limiting their practical use. A recent theoretical study from our group reported a linear stability analysis of dendrite growth (Tikekar et al., Sci Adv. 2016) of metal electrodeposition and showed that the length-scale on which transport occurs near the electrodes could be as important as electrolyte modulus in stabilizing metals against dendrite formation. In a nutshell, this study concluded that dendrites can be prevented from crossing over to the counter electrode using battery separators with pore-diameter lower than critical (smallest) size of the dendritic nucleate. To evaluate this proposal, we designed cross-linked nanoparticle-polymer composite electrolytes with tunable pore size and quantified the stability of metal electrodeposition in these systems. In contrast to most previously reported polymer electrolytes, the crosslinked membrane simultaneously showed good mechanical strength (~1MPa) and high ionic conductivity at room temperature (~5mS/cm), which is a consequence of the high crosslinking node points in these membranes (Choudhury et al. Nat. Commn. 2015). Direct visualization experiments were performed to understand the effect of pore-size on dendrite growth, which showed remarkable agreement with the theoretical predictions. Furthermore, when operated in a battery, the crosslinked membrane showed the highest short circuit time compared to similar electrolytes reported in the literature. Importantly, these studies showed that while the tendency for battery failure by dendrite-induced short-circuits can be reduced in polymer electrolytes, the issue of capacity-fading as a result of continuous reactions of the metal with liquid electrolyte persists. An additional striking fact in the electrodeposition literature not addressed by the linear stability analysis is that certain metals, including Magnesium, do not form dendrites. In the second part of my talk, I will show how multiscale analysis of transport at electrochemical interfaces enables design of stable solid-liquid interphases for reactive metal batteries. Recently, we used Density Functional Theory (DFT) calculations to quantify the diffusion energy barrier of ions on Mg, Li, Na surfaces and interestingly it seen that the diffusion barrier of Mg (0.02eV/atom) is several fold lower than Li (0.14eV/atom) or Na (0.16eV/atom) metals. In fact, the diffusion barrier of Li2CO3, Li2O (the commonly found compounds in lithium interface) is even higher, which is consistent with the dendritic electrodeposition in such batteries. However, in quest for finding stable interfaces, we observed that most metal halides (LiF, LiBr, NaF etc.) have much lower diffusion barrier. In other words, halide-rich interfaces on lithium or sodium can lead to stable electrodeposition similar to Mg deposition (Choudhury et al. Nat. Commn. 2017). We further utilize Classical Nucleation Theory (CNT) to understand implication of the ab-initio model in macroscopic metal deposition. The predictions from the coupled DFT-CNT model were validated using ex-situ scanning electron microscopy as well as in-situ optical microscopy. The nucleation pattern, indeed, showed a strike difference between usual (carbonate-rich) and halide-rich lithium interfaces. Based on these fundamental understanding, the solid electrolyte interphase in lithium metal batteries were artificially modified using organo-metallic reactions to enable enhanced reversibility in high energy density Lithium-Oxygen battery that demonstrated extended capacity retention and longer cycle life (Choudhury et al. Sci. Adv. 2017). Figure 1