Fluorinated High-Voltage Electrolytes To Stabilize Nickel-Rich Lithium Batteries.

As state-of-the-art (SOA) lithium-ion (Li-ion) batteries approach their specific energy limit (∼250 Wh kg-1), layer-structured, nickel-rich (Ni-rich) lithium transition metal oxide-based cathode materials, e.g., LiNi0.8Mn0.1Co0.1O2 (NMC811), have attracted great interest owing to their practical high specific capacities (>200 mAhg-1). Coupled with their high average discharge voltages (∼4 V vs Li/Li+), Ni-rich cathode-based lithium batteries possess a great potential to achieve much higher specific energies (>350 Wh kg-1 at the cell level) than the SOA Li-ion counterparts. In addition, Ni-rich oxides are low-cost battery cathode materials due to their low cobalt contents. However, Ni-rich cathode-based lithium batteries suffer quick capacity degradations upon cycling, particularly at high upper cutoff voltages (e.g., ≥4.5 V vs Li/Li+), due to crystal structure changes of the active cathode materials and parasitic side reactions at the electrolyte/electrode interfaces. In this study, a fluorinated-solvent-based, high-voltage stable electrolyte (HVE), i.e., 1 M Li bis(trifluoromethanesulfonyl)imide (LiTFSI) in fluoroethylene carbonate (FEC), bis(2,2,2-trifluoroethyl) carbonate (FDEC), and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) with Li difluoro(oxalate)borate (LiDFOB) additive, was formulated and evaluated in Li/NMC811 battery cells. To the best of our knowledge, this class of electrolyte has not been investigated for Ni-rich cathode-based lithium batteries. Li/NMC811 cells with HVE exhibited a superior long-term cycle performance stability, maintaining ∼80% capacity after ∼500 cycles at a high cutoff voltage of 4.5 V (vs Li/Li+) than a baseline carbonate-solvent-based electrolyte (BE). The superior cycle stability of the Li/NMC811 cells is attributed to the inherently high-voltage stability of HVE, supported by the physical and electrochemical analyses. This conclusion is supported by our density functional theory (DFT) modeling where HVE shows a less tendency of deprotonation/oxidation than BE, leading to the observed cycle stability. The findings in this study are important to help tackle the technical challenges facing Ni-rich cathode-based lithium batteries to realize their high energy density potentials with a long cycle life.

Ab initio study of alloying of MnBi to enhance the energy product.

High energy density magnets are preferred over induction magnets for many applications, including electric motors used in flying rovers, electric vehicles, and wind turbines. However, several issues related to cost and supply with state-of-the-art rare-earth-based magnets necessitate development of high-flux magnets containing low-cost earth-abundant materials. Here, by using first-principles density functional theory, we demonstrate the possibility of tuning magnetization and magnetocrystalline anisotropy of one of the candidate materials, MnBi, by alloying it with foreign elements. By using density functional theory in the high-throughput fashion, we consider the possibility of various metal and non-metal elements in the periodic table occupying empty sites of MnBi and found that MnBi-based alloys with Rh, Pd, Li, and O are stable against decomposition to constituent elements and have larger magnetization energy product compared to MnBi. Combined with other favorable properties of MnBi, such as high Curie temperature and earth abundancy of constituent elements, we envision the possibility of MnBi-based high-energy-density magnets.