UC San Diego

The rechargeable Li ion batteries are approaching their energy density limitation, while the prosperous growth of electric vehicle market is demanding cheaper and more sustainable batteries with higher energy density.To meet this goal, new battery material is needed to replace the current battery cathode, namely the LiCoO2 and LiNixMnyCo1-x-yO2 (NMC), which both contains the increasingly expensive transition metal, cobalt. One way to limit the cobalt usage is to increase the nickel substitution, as Ni is cheaper and more abundant compared to Co. Additionally, high Ni NMC delivers more capacity than their low Ni counterparts. However, transition metal substituent introduced an unexpected problem, i.e., the 1st cycle capacity loss. With electrochemical characterization and synchrotron X-ray diffraction, we have identified the sluggish Li intercalation at the end of discharge is the root-cause of this problem, which provided guidance for future improvement on these materials. In addition to optimizing the NMC cathode material, designing new cathode chemistry is another promising approach. Sulfur is a good cathode candidate for next generation energy storage system, due to its high capacity (~1675 mAh cm-2, 8 times as high as NMC), low price, and abundance in earth’s crust. However, elemental sulfur cathode suffers from its insulating nature and polysulfide dissolution problem. Sulfurized polyacrylonitrile (SPAN) is a sulfur based conductive polymer, which prevents sulfur dissolution by forming covalent bonding with sulfur and provides electron pathway by the chemical backbone. Although SPAN typically shows extraordinary stable cycling performance due to its unique structure and high specific capacity (~700 mAh cm-2), the Li-SPAN batteries reported in literature are yet to satisfy the industry demand due to its low areal capacity and incompatibility with ether electrolyte, which is commonly used in Li metal batteries. We discovered that LiNO3 as an electrolyte additive, enables SPAN to stably cycle in ether electrolyte, by forming a LiF-rich CEI layer. Its reaction mechanism in different electrolytes was investigated by X-ray absorption spectroscopy, where Li2Sx dissolution was observed in ether electrolyte without additive. Besides the electrolyte optimization, we replaced the traditionally used PVdF binder with mechanically robust CMC binder, which prevents the mechanical disintegration of the high areal loading cathode (> 6 mAh cm-2) and enables its stable cycling with reduced porosity (30%). When it comes to the anode, Li metal is the ultimate choice of rechargeable battery anode material due to its highest gravimetric capacity (3862 mAh cm-2) and lowest electrochemical potential (-3.04 V vs SHE.). However, the irregular morphology of electrochemically deposited Li leads to lots of problems, such as parasitic reactions, electrochemically isolated “dead” Li formation, and dendrite shorting. Many approaches have been developed to suppress the dendritic lithium formation and increase the lithium metal stripping/plating efficiency to > 99.0%. However, the porosity of lithium anode increases upon long cycling is a real challenge, which causes electrolyte depletion, increases cell impedance, and ultimately dictates the end of cell life. We demonstrated a bottom-up approach that an Fe/LiF nanocomposite substrate promotes the nucleation and growth of hexagonal single crystal Li at the initial stage of Li deposition, inducing dense Li deposition on top of the nuclei. Leveraging the low porosity Li, we have shown >1000 (Coulombic efficiency (CE) = 99.17%) and >600 (CE=99.06%) cycle in half cells under exceptionally high current density, 3 and 5 mA cm-2. Further, the full cell tests using NMC811 cathode with practical areal capacity of > 3 mAh cm-2, 1-fold excess of Li, lean electrolyte (3 g Ah-1), and cycled at high current density of 3 mA cm-2 retains > 80% cell capacity for more than 130 cycles, which is a 550% improvement over the baseline cells. We believe that through proper design and optimization of cathode and anode materials, the commercialization route for rechargeable Li metal battery with high energy density will be realized in the coming years.