Carbon Nanohorn and Ionic Liquid Mixture: Towards High Performance Organic Catholyte for Rechargeable Lithium Metal Battery
Authors: Hu Shan , Tang Xiaohui , Merhi Abdul Rahman 
Published: February 2020

Recenlty, several novel prototypes of lithium metal battery that use lithium metal as anode and soluble halogen redox couple (such as Br2/Br- and I2/I-) as catholyte have been developed.1-3 These emerging prototypes have several advantages compared with existing lithium-oxygen (Li-O2) and lithium-sulfur (Li-S) batteries. The high solubility of redox couple circumvents the deposition of insoluble and electrical-insulating product on the carbon anode that has caused large overpotential for Li-O2 batteries. And the cell potentials of these prototypes are much larger than those of Li-S batteries, because of the high redox potential (vs Li+/Li) of the halogen redox couples: 3.6 V for I2/I- and 4.1 V for Br2/Br-, compared with 2-2.4 V for polysulfide. Probably the most distinct feature of these new lithium metal battery is the use of an aqueous solution as cathode that provides much higher ion conductivity and thus better power performance than solid-state cathodes. However, the aqueous electrolyte also poses several challenges to other components of the battery. First, a strictly water-tight ion exchange membrane is needed to protect the lithium metal anode from the catholyte and any water leakage will cause safety issues considering the violent reaction between lithium and water. Secondly, corrosive I2 or Br2 has to be added to the electrolyte to ensure reversible redox reactions. Finally, the small electrochemical window of water limit the further improvement on cell potential. To address these issues with existing aqueous catholyte, this paper proposes an organic catholyte with acetonitrile as solvent, I2-free ionic liquid as redox active species, and carbon nanohorn as additives. Compared with aqueous electrolyte, organic electrolyte has much higher voltage window and poses less safety threat to the lithium metal anode, but unfortunately, it also has lower ion conductivity. The carbon nanohorn additives mitigate the low conductivity problems by forming a percolating nanoscale conductive network inside the organic electrolyte.4 This network extend the reaction site from the surface of the carbon current collector into the bulk of the organic catholyte, hence it reduces the I- ion diffusion length to compensate for the lower ion conductivity of organic electrolyte. The good electrical conductivity of the carbon nanohorn also reduce the internal resistance of the battery to improve power formances. Last but not the least, the carbon nanohorn has catalytic effect towards the reduction of I2 to I-. As a result, even without adding corrosive I2 to the electrolyte, the I- to I2 reduction reaction can still proceed with low overpotential. Based on the aforementioned analysis, we developed a novel organic catholyte by mixing carbon nanohorn (CNH) into 37 mg/ml acetonitrile solution of 1-ethyl-3-methylimidazolium iodide ([EMIM]I) ionic liquid. The weight ratio between CNH and [EMIM]I is 1:1. The CNH is from Graphene Laboratories Inc. Calverton, NY, USA and the [EMIM]I (97%) and acetonitrile (anhydrous, 99.8%) are from Sigma Aldrich, Milwaukee, WI, USA. All materials and chemicals are used without further processing or purification. To elucidate the effect of CNH additives, 37 mg/ml [EMIM]I in acetonitrile solution are used as a reference catholyte. These two catholyte systems are studied inside a glovebox using a three-electrode beaker cell with glassy carbon of 7.07 mm2 area as working electrode, silver wire as non-aqueous reference electrode, and platinum mesh as counter electrode. In cyclic voltammetry study, the cell using CNH-added electrolyte has higher cathodic current (corresponding to reduction of I2 to I-) than that of CNH-free cell, demonstrating that this reduction reaction is more facile in the CNH-added cell. Electrochemical impedance spectroscopy of CNH-added electrolyte at 0.1Hz-100kHz demonstrates lower impedance at high frequency region and lower charge transfer resistance in mid-frequency region, which proves that the CNH can reduce the internal resistance of the cell and facilitate the I-/I2 redox reaction as expected. Ongoing work is to assemble the CNH-added electrolyte with lithium metal anode and evaluate the lithium metal battery performance.

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