(40c) In situ Generation of Stable Interphases in Lithium Oxygen Battery

Recent safety issues related to overcharging or fast-charging lithium-ion battery in addition to its limitations in terms of capacity and energy density have compelled researchers to look beyond lithium ion batteries. Replacing the graphite anode in Li-ion battery with a thin metallic lithium can result in significant improvements in multiple aspects like cost, weight and size. Perhaps the greatest advantage of this anodic replacement lies in the possibility of using of a Li-free high-capacity cathode like oxygen that can enhance the gravimetric energy density of battery from ~0.3kWh/kg to ~12kWh/kg (comparable to gasoline). However, these metal batteries, particularly the lithium-oxygen cell, have several fundamental roadblocks relating to both anode and cathode. The major anode specific problems are dendrite induced short circuits caused by uneven electrodeposition and rapid capacity fade due to internal side reactions. On the other hand, the oxygen cathode has several issues like low round trip efficiency, low specific capacity and poor rechargeability due to the insulating nature of the discharge product, lithium peroxide. In this study, we design ionomer based additives for forming in-situ solid-electrolyte interphases (SEIs) comprising of bromide salts and fixed anions that simultaneously addresses the issues of anode and cathode in a lithium-oxygen battery. The formation of these interphases was confirmed using scanning electron microscopy after focused ion milling at cryo temperatures as well as by chemical bond mapping using X-ray Photoelectron Spectroscopy. The ionomer SEIs protect the lithium anode against parasitic reactions and also stabilize the electrodeposition forming more compact and smooth lithium nucleates compared to that in usual batteries. The working mechanism of the ionomer additive is related to the role of bromide salts in reducing the energy barrier of ion transport at anode surface, as well as the effect of tethered anions in preventing the formation of a space charge region by depletion of free salts. Further, the bromine species, liberated in the ionomer anchoring reaction, act as a redox mediator in the oxygen evolution reaction (OER) at the cathode, thus reducing the charge overpotential. An added advantage of designing ionomer based SEI is the remarkable chemical stability it imparts to the lithium metal, enabling the usage of reactive electrolytes (like dimethyl sulfoxide and N,N dimethylacetamide), which also have very high-Gutmann donor number. These high donor number electrolytes have been reported to exhibit rare stability against nucleophilic attack by superoxides and at the same time they solvate intermediate cathode reaction products to impart high discharge capacity. We believe, these ionomer-based SEIs can rationally regulate the ion and mass transport in the lithium oxygen electrochemical cell to address the major barriers in lithium oxygen technologies.