No abstract

Two ionic liquids, EMI-AlCl 4 and N 1114 -TFSI, that support both lithium and sodium deposition/dissolution were studied as potential electrolytes for lithium metal batteries. In both cases, lithium's dendritic growth was suppressed by adding a small amount of sodium to a lithium electrolyte. This results in a co-deposition or alloying process that hinders dendrite growth. SEM images show a significant difference in morphology obtained by the addition of sodium. A smooth deposit was not enough for stable cycling of the lithium anode because of lithium's reactivity with the electrolyte. Vinylene carbonate (VC) was added to the N 1114 -TFSI to form a stable SEI layer. Cyclic voltammetry and chronopotentiometry was carried out on tungsten and stainless steel electrodes to obtain efficiency measurements. The combination of a small amount of sodium in the electrolyte, along with VC as an SEI former, lead to significant improvements in cycling performance and efficiency.The deposition and re-oxidation of lithium metal is of interest for its potential use as the anode in lithium metal batteries. The lithium-metal anode has the highest possible theoretical capacity since only a current collector is needed to support the deposition of the metal. The density of lithium metal is 534 kg/m 3 giving it a capacity of 3862 mAh/g or 2047 mAh/cm 3 . 1 The use of lithium metal reduction/oxidation as the anode half reaction would eliminate the need for an anode structure, such as carbon or silicon, thus lowering cost, size and weight of the battery as well as the assembly complexity.However, the dendritic growth of lithium during cycling lowers the coulombic efficiency and poses safety concerns. Dendrites are needle-like structures that are formed during lithium electrodeposition. They have been found to grow on different substrates, especially at scratches or other defects on the surface. The deposition and stripping of lithium causes defects so that simply changing the substrate preparation is not an effective route to preventing dendrite growth. 2 Dendrites grow in organic as well as ionic liquid electrolytes showing that their growth is related to lithium itself rather than an electrolyte effect. The needle-like growth adds to lithium's inefficient cycling because the dendrite can become isolated from the anode if the dendrite breaks or if the base of the dendrite is oxidized before the tip. Lithium dendrites are a severe safety concern because dendrites can short circuit the anode and cathode. Anode-cathode short circuits are especially dangerous when a flammable organic solvent is used as the electrolyte.Dendritic growth has been studied and various mechanisms have been proposed, 2-5 however, only modest progress has been made in their elimination. Previous reports have shown that the lithium-anode cycle life can be extended by assembling a pressurized coin cell; however, this does not eliminate dendrite growth or address the fundamentals of their formation. 6 Restricting the anode volume and applying pressure forces denser de...

Higher energy density batteries are desired, especially for mobile electronic devices. Lithium metal anodes are a possible route to achieving high energy and power density due to their light weight compared to current graphite anodes. However, whisker growth during lithium electrodeposition (i.e. charging) represents a serious safety and efficiency concern for both lithium metal batteries and overcharging of graphite anodes in lithium-ion batteries. The initial morphology of deposited lithium nuclei can have a significant impact on the bulk material deposited. The nucleation of lithium metal from an organic ethylene carbonate: dimethyl carbonate (EC:DMC) and an ionic liquid (trimethylbutylammonium bis(triflouromethanesulfonyl)imide) electrolyte has been studied. Whisker extrusion and tip-based dendrite growth was observed ex-situ, and confirmed by in-situ optical microscopy experiments. The nucleation of a non-dendritic sodium co-deposit is also discussed. A model based on nuclei geometry is provided which gives insight into the deposition rate at constant overpotential.