Graduate Thesis Or Dissertation
 

Pushing the limits of graphite : the potential beyond conventional applications in aqueous dual-ion batteries

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  • The increase in the demand for energy is estimated to be 28 TW by a year of 2050, which is twice of the energy produced in 2010, in response to the unprecedented growth of the global population. Current energy market is heavily relying on the fossil fuel, which brings the concerns on its scarcity and the emission of CO2. Consequently, the direction of energy generation is transitioning to increasing the portion of renewable sources, such as wind, solar and hydropower, etc. These energy sources are inherently intermittent, thereby craving for the grid-scale energy storage systems to make the best use of it. The application purposes diversify the metrics of focus in the development of rechargeable batteries. For the mobile phase energy storage, e.g., electric vehicle, portable electronic devices, etc., the key parameters include high-energy density, high-power output, and fast-charging ability. On the other hand, long-term safety and cost effectiveness are more prioritized in the stationary storage. The state-of-the-art Li-ion batteries (LIBs) feature high energy contents, which make LIBs suitable for power batteries. Nonetheless, LIBs are not a viable choice for the grid-scale energy storage due to anticipated depletion and uneven global distribution of lithium. With respect to the development of alternative energy storage systems, dual-ion batteries (DIBs) have engendered increasing attention. The operation mechanism of DIBs differs from the conventional “rocking-chair” charge storage; where simultaneous storage of the cations and the anions occurs on the anode and cathode, respectively. The ability of graphite to electrochemically hosting ions by forming graphite-intercalation compounds (GICs) have been actively investigated and constitutes the cost competitiveness of DIBs by serving as both anode and cathode. Reports on acceptor-type GICs for the storage of anions in graphite remain limited in employing organic solvents or ionic liquids (ILs) electrolytes as anion intercalation into graphite takes place at a potential above 4.5 V vs. Li/Li+; which readily decomposes most organic solutions. Moreover, flammability of organic electrolytes and high cost of ILs raise concerns on their applicability to DIBs for grid-scale energy storage. In this regard, aqueous DIBs represent the promising alternative. The major challenge for aqueous electrolytes in DIBs roots from its narrow electrochemical stability window. In particular, high anion intercalation potential of graphite cathode in DIBs triggers the oxygen evolution reaction (OER) prior to the formation of acceptor-type GICs. A strategy to resolve such a problem is deliberate manipulation of the solvation sheath in the electrolyte. Recent studies uncovered a disparity between the solvation structure of highly concentrated water-in-salt electrolytes (WiSE) or aqueous deep eutectic solvents (DES) and salt-in-water type dilute electrolyte. According to the Nernstian relationship, anodic stability of water is enhanced by reduced activity of water molecules; whereas increase in its activity lowers the insertion potential of anions in WiSE or aqueous DES electrolytes. The primary focus of this dissertation is to extend the application of graphite as cathode in aqueous DIBs by superchlorides as the active charge carriers formed in aqueous DES electrolytes. Superhalides, the complex anions containing metal-halide bonding, are the promising oxidative intercalate for graphite host as they hold the advantage of structural compactness. We present our recent studies on the reversible storage of Mg-Cl and Li-Cl superhalides in graphite from aqueous DES electrolytes. In addition, we report the construction of graphite || 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) aqueous DIB, where Mg-Cl superhalides and tributylmethylammonium (TBMA)-ion serve as the charge carriers for graphite cathode and PTCDI anode, respectively. We have discovered the formation of Mg-Cl superhalides, [MgCl3]-, in aqueous DES electrolyte comprising 9 m MgCl2 + 30 m choline chloride (ChCl) and its reversible storage in graphite. Graphite showed the initial anion-hosting capacity of 151 mAh g-1 with the stable cycling over 300 cycles. For storing Mg-Cl superhalides, rigid graphite structure turned into turbostratic with partial defects created. This study provides the potential application of new charge carriers, superhalides, in the development of aqueous DIBs. We have explored the possibility of forming Li-Cl superhalides, the lightest supercholride available, in aqueous DES that comprises 20 m LiCl and 20 m ChCl. Systematic analysis of the electrolytes revealed [Li(H2O)2Cl2]- as a dominant anion, which serves as the major anionic intercalates in graphite. Spectroscopic characterizations suggested bi-layered graphene energetically favors the insertion of hydrated [Li(H2O)2Cl2]- over dehydrated Li-Cl superhalide. Lastly, we report a study on aqueous graphite || PTCDI full cell. Ch+ in aqueous DES electrolyte comprises of 9 m MgCl2 + 30 m ChCl is unable to serve as cationic charge carrier due to highly reactive hydroxyl functional group. We examined the viability of quaternary ammonium chloride salts, i.e., NH4Cl, tetramethylammonium chloride (TMACl), and TBMACl, as a co-salt for MgCl2 in formation of [MgCl3]-. With the least Lewis acidic cation, 5 m TBMACl as an additive to saturated 5 m MgCl2 enabled reversible storage of Mg-Cl superhalide in graphite cathode. Unlike Ch+, TBMA+ does not decompose with inert alkyl groups and can be reversibly inserted / de-inserted in the PTCDI anode via conversion of carbonyl groups. A full cell was assembled based on the initial charge capacity of the graphite cathode and the initial discharge capacity of the PTCDI anode, which delivered reasonable reversible capacity of 41 mAh g-1 and exhibited stable cycling over 400 cycles.
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