- Widespread installation of renewable energy sources, such as wind and solar, has been delayed by the lack of suitable stationary energy storage solutions. Current large-scale technologies, such as pumped hydro and compressed air, are geographically restricted. This is where electrochemical energy storage - batteries or hybrid capacitors can play a crucial role. The metrics for large-scale storage batteries are vastly different from power batteries, where top priorities in storage batteries should be low acquisition cost and minimum maintenance. As sustainable solutions, these systems cannot afford the usage of rare elements, and must exhibit extremely long cycling life. High energy and power densities are desirable; however, they are not the top priority. Lithium-ion batteries (LIBs) currently lead the market of power batteries; nevertheless, LIBs face the challenge of long-term sustainability due to the foreseen shortage of lithium (0.1% of lithium in Earth’s crust), and geopolitical issues related to its uneven global distribution. Therefore, LIBs would not be a viable option for large-scale storage batteries.
There are alternatives, however, and among different alternatives, DIBs can be very competitive in cost for large-scale stationary storage because both the cathode and anode of DIBs can be made of low-cost redox-amphoteric carbonaceous materials. DIBs do not operate by the rocking-chair principle, where its cathode and anode incorporate anions and cations, respectively, during charging. The enabler for DIBs is the cathode, as there are a plethora of anode candidates, in which Li-ions, Na-ions, or K-ions can be stored. To date, the primary cathode in DIBs is graphite, which was proposed more than three decades ago. This battery was initially known as a dual-graphite battery, where both electrodes are graphitic carbon. The primary challenge of dual-graphite batteries is the very high operation potential of its cathode, often requiring an upper cutoff potential above 5 V vs. Li+/Li. Such a potential readily oxidizes alkyl and alkylene carbonate based electrolytes. The anode side, in fact, can employ any anode of most metal-ion batteries although, to date, the focus has still been the Li-graphite anode. Recent progress has significantly advanced the technology readiness level for this battery.
Recently, some attention has been shifted to organic-based electrodes. Reversible anion storage in a redox active metal-organic framework (MOF) and the anion storage in crystalline aromatic amines, for example. Despite being known as crystals of uniform hydrogenated graphene nanosheets, polycyclic aromatic hydrocarbons (PAH) cathodes were not even known for its anion storage properties in DIBs although they have been considered as anodes for storing cations. This discovery uncovers the great potential in PAHs for DIBs and have revealed that optimization in DIBs is crucial, where several technical issues have prevented the market penetration of this technology. This inspired a deviation from non-aqueous electrolytes. Currently, DIBs are studied quite extensively using mostly all organic electrolytes or expensive ionic liquid. However, aqueous electrolytes have not been studied quite as extensively in the literature due to HER/OER issues, consequently, there is a research gap when it comes to using aqueous electrolytes. Aqueous electrolytes would lower the levelized cost for DIBs and not to mention the safety of the batteries would also improve.
The primary focus of this dissertation is to elucidate that PAH molecular solids can serve as high-performing cathodes in dual-ion batteries for reversible storage of bulky anions in non-aqueous and aqueous electrolytes, where PAHs exhibit better structural flexibility than graphite because they are assembled by van der Waals forces from all directions. Therefore, the structural flexibility and potential tunability of the PAH molecules by organic syntheses could bring high capacity values and long-term cycling stability. We report our recent results on PF6- storage in coronene, perylene, and triphenylene solids in non-aqueous electrolytes, the TFSI- storage in coronene in an aqueous electrolyte, and the insertion of large Mg-ions in a PAH perylene anhydride derivative—3,4,9,10-perylenetetracarboxilicdianhydride (PTCDA). We have explored pure PAHs as electrode materials for dual-ion batteries; a family of crystalline PAHs, namely, coronene, perylene, and triphenylene. All showing flat plateaus during charge and discharge at lower voltages (~4.1 V, ~3.9 V, and ~2.6 V, respectively) than graphite electrodes in a standard alkyl carbonate electrolyte. A voltage of <4.5 V avoids the use of expensive ionic liquids used with the conventional graphite anion insertion electrodes. This discovery will open new doors in the battery field with an expanded library of available cathode materials. These coronene, perylene, and triphenylene electrodes deliver reversible discharge capacities of ~ 40 mA h g–1, ~80 mA h g–1 and ~100 mA h g–1, respectively. The coronene and perylene electrodes display extreme cycling stability with well over 4,000 cycles for coronene (>100% capacity retention) and 99.0% coulombic efficiency. Perylene exhibits stability for over 1,700 cycles with 94.5% capacity retention (after structural deformations) and 98.6% coulombic efficiency. We also show that the PAH electrode delivers a reversible capacity of 64.5 mA h g-1, giving a CE of 73.1% in the first cycle in an aqueous electrolyte. Lastly, we reveal that a PTCDA Mg-ion and Ca-ion electrode delivers a reversible capacity of 125 mA h g-1, of over 80 mA h g-1, respectively. PTCDA also offers a good rate capability of retaining 75 mA h g-1 at 500 mA g-1 (or 3.7 C), for Mg-ions.