Rechargeable metal batteries, such as Li metal batteries are considered the “holy grail” of energy storage systems.  However, dendritic metal growth and limited Coulombic efficiency (CE) during metal deposition/stripping have prevented their practical applications in rechargeable batteries.^1-3 During the last a few years, we have developed several approaches to suppress metal dendrite growth and enhance the Coulombic efficiency (CE) of metal deposition/stripping processes.^2-7 Several electrolyte additives, including CsPF₆, RbPF₆, and trace-amount of H₂O (25-50 ppm) have been found to be effective for achieving dendrite-free Li metal deposition in LiPF₆-based electrolytes.  Furthermore, we have developed a highly concentrated electrolytes composed of the lithium bis(fluorosulfonyl)imide (LiFSI) salt and 1,2-dimethoxyethane (DME) solvent which enables high rate cycling of Li metal anode at high CE (up to 99.1 %) without dendrite growth. It is demonstrated that a Li|Li cell can be cycled at high rates (10 mA cm^-2) for more than 6,000 cycles with no increase in the cell impedance and no dendritic Li growth.  A Li|Cu cell can be cycled at 4 mA cm^-2 for more than 1,000 cycles with an average CE of 98.4%.  We also demonstrate that the dendrite growth and CE of Li deposition is strongly depends on several other factors, such as substrate treatment, charge and discharge protocols, and cell pressure.  By optimizing the electrolyte compositions and various operating parameters, CE of Li deposition/stripping has been further improved without dendrite growth.  In addition to the high CE cycling of Li metal electrode, novel electrolyte was also developed which enables cycling of Na metal anode with an excellent CE (> 99.1%) as compared to a very low CE of less than 30% in conventional electrolytes. Further development of these approaches may lead to long term stable operation of Li and Na metal batteries.

Acknowledgements

This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U. S. Department of Energy, Office of Science, Basic Energy Sciences (BES) and by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE). PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830.

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