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Title:Enabling safe and stable cycling of lithium metal batteries using advanced electrolytes and interfaces
Author(s):Philip, Maria Antoin
Director of Research:Gewirth, Andrew A
Doctoral Committee Chair(s):Gewirth, Andrew A
Doctoral Committee Member(s):Nuzzo, Ralph G; Rodriguez-Lopez, Joaquin; Sottos, Nancy R
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):lithium metal batteries
solid electrolytes
highly concentrated electrolytes
oxide conversion cathode
Abstract:Lithium-ion batteries (LIBs) are ubiquitous in daily life, with applications in portable electronics, electric vehicles, and grid energy storage. LIBs have a limited capacity output determined by the anode, graphite. Replacing the graphite anode with lithium metal yields lithium metal batteries (LMBs), which exhibit higher capacity and energy density than LIBs. However, LMBs suffer from dendrite growth issues and instability of many electrolytes against the anode. This dissertation describes research on LMBs comprising advanced electrolytes, including solvates, solid electrolytes, and highly concentrated electrolytes, to develop stable electrode|electrolyte interfaces, enabling safe and stable cycling of these batteries with oxide-based cathodes. Improving Cell Resistance and Cycle Life with Solvate-Coated Thiophosphate Solid Electrolytes in Lithium Batteries. Solid electrolytes (SEs) have become a practical option for lithium-ion and lithium metal batteries because of their improved safety over commercially available liquid electrolytes. The most promising of the SEs are the thiophosphates, whose excellent ionic conductivities at room temperature are comparable to those of commercially utilized liquid electrolytes. Hybrid solid–liquid electrolytes exhibit higher ionic conductivities than their bare SE counterparts because of decreased grain boundary resistance, enhanced interfacial contact with electrodes, and decreased degradation at the interface. In this study, we add lithium bis(trifluoromethane sulfonyl)imide and a hydrofluoroether solvate electrolyte to the surface of Li7P3S11 (LPS) and Li10GeP2S12 (LGPS) pellets and evaluate their overall cell resistance in Li–Li symmetric cells relative to their bare Li/SE/Li counterparts. Time-resolved electrochemical impedance spectroscopy shows an order of magnitude lower cell resistance for the LGPS-solvate system than for its bare LGPS counterpart. In contrast, the LPS-solvate system exhibits a higher cell resistance than bare LPS. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show that LGPS allows for the total permeation of the solvate into the bulk SE. Although LPS has smaller grain sizes and higher porosity, it has a higher solubility in 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), which results in an LPS-TTE interlayer on the surface of the pellet, thereby increasing overall cell resistance. Cyclic voltammetry of the bare and hybrid SE cells shows an order of magnitude higher current density for the LGPS-solvate cell over the bare LGPS. Bare LPS shorts after two cycles, whereas the LPS-solvate cell does not short within the timeframe of the experiment (100 cycles). This study suggests that solvates can be used to improve the cell resistance and current density of SEs by altering the grain boundary structures and the interphase between electrode and electrolyte. Enabling High Capacity and Coulombic Efficiency for Li-NCM811 Cells Using a Highly Concentrated Electrolyte. Lithium metal batteries suffer from dendrite formation and the associated safety hazards of thermal runaway reactions. In this study, we report the performances of a highly concentrated electrolyte (HCE) and a dilute LiPF6 electrolyte in lithium metal cells using LiNi0.8Co0.1Mn0.1O2. While the HCE exhibits lower bulk ionic conductivity than the dilute LiPF6 electrolyte, the overall cell conductivity is higher for the HCE system, indicating higher thermodynamic stability of the electrolyte against the electrodes. Full cell cycling demonstrates a higher capacity for the HCE system, which declines as a function of cycle number due to the formation of decomposition products, similar to the dilute LiPF6 system. The origin of the enhanced performance is the higher stability of the HCE against Li metal anode as compared to the dilute LiPF6 electrolyte. Cycling at higher temperatures further enhances the performance of the HCE, which is more thermally stable than the dilute LiPF6 electrolyte. Effect of Glyme Solvent on Solvate Performance in Li-Fe0.9Co0.1OF Cells. To develop practical lithium metal batteries, high-capacity cathodes, such as conversion cathodes, will be required to match the high capacity of the lithium metal anode. Furthermore, practical batteries will require the use of highly concentrated solvate electrolytes which are stable against lithium metal anode. In this study, we report the performance of lithium metal cells comprising Fe0.9Co0.1OF conversion cathode with equimolar solvate electrolytes containing lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in diglyme (G2), triglyme (G3), or tetraglyme (G4) with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) hydrofluoroether additive to form [Li(Gx)TFSI](TTE)1.5 (x = 2, 3, 4) solvates. We show that cells containing [Li(G4)TFSI](TTE)1.5 (the G4 solvate) exhibit the highest capacities at all cycling rates due to the G4 solvate’s higher ionic conductivity and higher thermodynamic stability against the Fe0.9Co0.1OF cathode. Furthermore, despite the G4 solvate’s higher instability against Li metal anode, the lower amount of LiF formed on the surface of Li during cycling results in higher capacity for the full cell. Cell performance is therefore an interplay between bulk electrolyte conductivity and stabilities against both anode and cathode. In this study, the cycling capacity of the Li|[Li(G4)TFSI](TTE)1.5|Fe0.9Co0.1OF cell is higher than those for cells using [Li(G3)TFSI](TTE)1.5 and [Li(G2)TFSI](TTE)1.5 due to the G4 solvate’s higher bulk conductivity and higher stability against the Fe0.9Co0.1OF cathode.
Issue Date:2021-04-19
Rights Information:Copyright 2021 Maria Antoin Philip
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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