Wrinkled graphene cages as hosts for high capacity Li metal anodes shown by cryogenic electron microscopy.

Lithium (Li) metal has long been considered the "holy grail" of battery anode chemistry, but is plagued by its low efficiency and poor safety due to its high chemical reactivity and large volume fluctuation, respectively. Here we introduce a new host of wrinkled graphene cage (WGC) for Li metal. Different from recently reported amorphous carbon spheres, WGC shows highly improved mechanical stability, better Li ion conductivity and excellent solid electrolyte interphase (SEI) for continuous robust Li metal protection. At low areal capacities, Li metal is preferentially deposited inside the graphene cage. Cryogenic electron microscopy (Cryo-EM) characterization shows that a uniform and stable SEI forms onto the WGC surface that can shield the Li metal from direct exposure to electrolyte. With increased areal capacities, Li metal is plated densely and homogeneously into the outer pore spaces between graphene cages with no dendrite growth or volume change. As a result, a high Coulombic Efficiency (CE) of ~98.0% was achieved under 0.5 mA/cm2 and 1-10 mAh/cm2 in commercial carbonate electrolytes, and a CE of 99.1% was realized with high concentration electrolytes under 0.5 mA/cm2 and 3 mAh/cm2 . Full cells using WGC electrodes with pre-stored Li paired with Li iron phosphate showed better specific capacity as well as greatly improved cycle lifetime. With 10 mAh/cm2 Li metal loading, the WGC/Li composite electrode was able to provide a high specific capacity of ~2785 mAh/g. With its roll-to-roll compatible fabrication procedure, WGC serves as a highly promising material for the practical realization of Li metal anode in next generation high energy density secondary batteries.