Lithium-metal batteries — which can hold up to ten times more charge than lithium-ion batteries — haven't been commercialized because of a fatal flaw: as the batteries charge and discharge, lithium is deposited unevenly on the electrodes. This buildup cuts the lives of these batteries too short to make them viable, and more importantly, can cause the batteries to short-circuit and catch fire.
A graphene-oxide-coated nanosheet was developed that, when placed in between the two electrodes of a lithium-metal battery, prevents uneven plating of lithium and allows the battery to safely function for hundreds of charge/discharge cycles. Two-dimensional materials — in this case, graphene oxide — can help regulate lithium deposition in such a way that extends the life of lithium-metal batteries.
Lithium-metal batteries are useful because of their high energy density and relatively light weight compared with conventional batteries. Over the course of many charge-discharge cycles, however, lithium builds up unevenly on the battery's lithium metal electrode in a branching, or dendritic pattern, and ultimately causes the battery to go dead. If the dendrites grow through the electrolyte solution and make contact with the other electrode, the battery may experience a catastrophic event — an explosion or fire.
In lithium-ion batteries, a separator is placed in the electrolyte. Usually made of a porous polymer or glass ceramic fibers, the separator allows lithium ions to flow through while keeping the other components blocked to prevent electrical shorts that can lead to fires.
A modified separator was used in a lithium-metal battery to modulate the flow of lithium ions to control the rate of lithium deposition and prevent dendrites from forming. A fiberglass separator was sprayed with graphene oxide, producing a nanosheet. Using scanning electron microscopy and other imaging techniques, it was shown that when the nanosheet was used in a lithium-metal battery, a uniform film of lithium formed on the lithium electrode's surface, which actually improves battery function and makes the battery much safer.
Molecular simulations suggested that the lithium ions become temporarily bonded to the graphene oxide, and then diffuse through areas of nanoscopic defects in the sheet. This delays the passage of lithium ions enough to prevent the formation of dendritic deposition of lithium on the electrode. Results of phase-field modeling computations indicated that graphene oxide can also mechanically suppress the growth of lithium dendrites.