Organic electrode materials are very attractive for electrochemical energy storage devices because they can be flexible, lightweight, low cost, benign to the environment, and used in a variety of device architectures. They are not mere alternatives to more traditional energy storage materials, rather, they have the potential to lead to disruptive technologies. Although organic electrode materials for energy storage have progressed in recent years, there are still significant challenges to overcome before reaching large-scale commercialization. This review provides an overview of energy storage systems as a whole, the metrics that are used to quantify the performance of electrodes, recent strategies that have been investigated to overcome the challenges associated with organic electrode materials, and the use of computational chemistry to design and study new materials and their properties. Design strategies are examined to overcome issues with capacity/capacitance, device voltage, rate capability, and cycling stability in order to guide future work in the area. The use of low cost materials is highlighted as a direction towards commercial realization.
Bryony McAllister earned her BSc in Chemistry from the University of Victoria in 2015. She is currently an NSERC CGS-D doctoral student in the Seferos Lab in the Chemistry Department at the University of Toronto. Her current research focuses on the design of organic conjugated and pendant polymers for high-voltage supercapacitors and batteries.Luke Kyne obtained his HBSc in Chemistry and Psychology as a C. David Naylor Scholar at the University of Toronto in 2018. As an NSERC USRA and SOUSCC Award recipient, Luke has been investigating the applications of novel organic cathode materials in lithium-and magnesium-ion batteries as a member of the Seferos Lab.Tyler Schon received his BSc in chemistry from the University of Western Ontario in 2012 and completed his PhD at the University of Toronto in 2017, under the supervision of Prof. Seferos, where he developed novel organic polymers for energy storage devices. Tyler is currently CEO of Pliant Power Devices, a company he co-founded in 2017 commercializing sustainable battery technology.
Lithium-ion batteries have achieved commercial success; however, work remains to increase the capacity and safety of both the anode and cathode electrodes. Organic anodes have the potential to replace conventional graphite anodes because they are abundant, safe, and high-capacity materials. Superlithiated organic anodes achieve capacities in excess of 1500 mA h g–1; however, the mechanism of superlithiation and how it relates to different materials is an open question. Here, we disclose a pyrene-fused azaacene polymer that undergoes superlithiation and exhibits a continuous activation process, whereby the capacity increases with the number of cycles, reaching values up to 1775 mA h g–1 (1535 mA h g–1, subtracting the carbon additive contribution). This high performance is attributed to the stability and extended conjugation afforded by the polymer design. Ex situ studies suggest cycling results in deformation of the electrode structure, from an amorphous electrode material to one with increased crystallinity and sp2 character. Importantly, this superlithiated electrode maintains the same capacity across a 10-fold increase in rate during the activation process, showing that the kinetic limitations of superlithiation can be overcome and suggesting that commercial practical superlithiation anodes are within reach.
Organic emitters exhibiting delayed fluorescence (DF) are promising luminescent materials for next-generation organic light-emitting diodes (OLEDs). Faster intersystem crossing rates and shorter emission lifetimes can be achieved in luminescent molecules through the incorporation of heavy atoms, which enhance spin−orbit coupling and promote intersystem crossing between singlet and triplet states. DF molecules often contain a sulfur atom, and reports of selenium-containing DF OLEDs also exist. However, the literature lacks a direct exploration of the effect of spin−orbit coupling on reverse intersystem crossing in a delayed fluorescence emitter by the substitution of selenium for sulfur. Here we show that substitution of selenium for sulfur in a modified thioxanthenone-triphenylamine analogue increases the rate of forward intersystem crossing by a factor of over 250 and the rate of reverse intersystem by a factor of 22. We attribute the increased rates to enhanced spin−orbit coupling from heavy atom substitution, and computational and electron spin resonance studies support this. This work provides an insight into future molecular design strategies for heavy-atomcontaining, DF emitters.
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