Shaping the future of hybrid ion capacitors

Vanchiappan Aravindan

Vanchiappan Aravindan is an assistant professor at the Department of Chemistry, IISER, Tirupati, India. His research interests include the development of high-performance electrodes and electrolytes for Li-ion batteries, including recycling spent Li-ion batteries and discarded solar panels. He has authored and co-authored over 270 articles with an h-index of 70. He is a Fellow of both the Royal Society of Chemistry (FRSC) and the Institute of Physics (FInstP), UK. Also, he is the recipient of the prestigious Swarnajayanti Fellowship (2020) from DST, Govt. of India. He is the recipient of notable recognition: the MRSI Medal (2020) and the MRSI Materials Science (2021) Annual Research Prize by the Materials Research Society of India. He is also an editor of the Journal of Industrial and Engineering Chemistry.

Martin Oschatz

Martin Oschatz is a full professor (W3) in the Chemistry of Materials for Energy Applications group at the Institute of Technical and Environmental Chemistry at the Friedrich Schiller University Jena. Martin studied chemistry at the Technische Universität Dresden. He carried out his PhD in the group of Professor Kaskel and graduated in 2015 (summa cum laude). After a postdoctoral stay at Utrecht University with Professor de Jong, Martin worked as group leader led by Professor Antonietti at the MPI of Colloids and Interfaces from 2016–2020, supported by a Liebig Fellowship from the German Chemical Industry fund. In 2019 and 2020 he, in parallel, became Professor in replacement of Inorganic Chemistry at Potsdam University. His research interests are on the molecular design of porous carbon materials with defined chemical architecture and finding new phenomena in electrochemical energy storage, catalysis, and separation applications.

Konstantin Schutjajew

Konstantin Schutjajew is a postdoctoral researcher in the group of Professor Martin Oschatz at the Institute of Technical and Environmental Chemistry at the Friedrich Schiller University Jena. He studied chemistry at the Freie Universität Berlin and performed his PhD work at the Max Planck Institute of Colloids and Interfaces in Potsdam on the topic of negative electrode materials for sodium ion batteries, for which he was awarded the PhD prize from the Division of Chemistry and Energy of the GDCh. His research interest revolves around the mechanistic investigation of processes occurring in electrochemical energy storage and conversion and synthetic methods for the preparation of novel materials and structures.

Marta Sevilla

Marta Sevilla is a Sustainable Energy & Fuels associate editor. She is a scientific researcher in the Institute of Carbon Science and Technology (INCAR-CSIC). Her research focuses on the development of sustainable synthesis procedures for the preparation of functional carbon materials with controlled textural, structural and chemical properties. Such sustainable schemes are based on the use of biomass or biomass derivatives/residues as carbon precursors and environmentally friendly transformation processes. The targeted applications are mainly energy-related applications (supercapacitors, rechargeable batteries, fuel cells or hydrogen storage).

Energy storage is a key pillar in the transition to the use of more sustainable and cleaner energy sources that allow us to work against climate change and environmental pollution. Electrochemical energy storage (EES) technologies such as batteries and supercapacitors have enabled the portabilization of electronic devices, making our daily life easier. They allow the electrification of transportation, and are starting to play a leading role in integrating renewable energy. However, EES technologies cannot keep up with the pace of technological innovations and the increasing demand of these applications, falling short respectively in power density/cyclability and energy density/self-discharge. Even though significant improvements in EES technology have been achieved in recent years, it remains difficult to attain all the requirements of such systems using one storage technology system.

As a breakthrough in this field, hybrid ion capacitors have emerged as promising next-generation EES devices that combine the features of both rechargeable batteries and supercapacitors, i.e., high energy and high power capability with long cycling stability. The present themed collection brings together some of the most recent developments towards hybrid ion capacitors with original and review-type articles, addressing advances being made in all of the key components, from the electrode materials to the electrolyte. The contributions cover both the already commercialized Li-ion capacitors (LICs) and other emerging systems like Na-ion (NICs), K-ion (KICs), and Zn-ion capacitors. Other important issues in hybrid capacitors, like electrode pre-metallation, the role of the dielectric constant, and the solid electrolyte interphase (SEI) layer formation process are covered as well.

On account of the increasing demand for EES, the sustainability and environmental impact of such materials, devices, and their manufacturing processes are becoming significant concerns, along with cost aspects. This is pushing the research from LICs, whose fundaments, development, and commercialization have been thoroughly reviewed in the article by Bhattacharjee et al. (https://doi.org/10.1039/D3SE00269A), to metal-ion capacitors based on more abundant metals such as sodium, potassium, and zinc. This latest technology has been comprehensively addressed by Devi et al. (https://doi.org/10.1039/D3SE00565H). These authors cover all the aspects involved in the technology, from electrode materials (including conventional carbon, polymer, and metal-based materials, but also novel 2D materials such as MXenes, phosphorene, or metal carbides/nitrides) to the modification of electrolytes and separators to suppress dendrite growth in the zinc metal electrode.

Carbon materials are ubiquitous in EES electrodes, and their sustainable and green manufacturing from biomass-based substances has become a hot topic. In this regard, Payá et al. (https://doi.org/10.1039/D3SE00273J) show in their study the sustainable synthesis of high-performance positive and negative electrode materials for the so-called dual carbon NICs using benign chemicals such as sodium/potassium carbonates/chlorides, sulfur, and biomass-derived substances. Besides biomass, harnessing wastes to synthesize energy materials is the most promising approach to support the circular economy. In their report, Bhattacharjee et al. (https://doi.org/10.1039/D3SE00170A) report that they are able to upcycle graphite anodes from spent Li-ion batteries (LIBs) into graphene-based positive and negative electrode materials for LICs. This work lays the groundwork for recycling the graphite anode present in all commercial LIBs into high-added-value materials and effective re-utilization in charge storage devices. Schenk et al. (https://doi.org/10.1039/D3SE00642E) explore conductive additives (Vulcan® XC72R and Printex® 140V) used in the conventional Li-ion battery industry as potential negative electrodes for LIC and NIC applications after post-treatment. Two orders of magnitude higher diffusion are observed for the Li⁺ over Na⁺ ions, which is clearly evident from the diffusion studies conducted through the galvanostatic intermittent titration technique. In other words, Li-ions prefer a more ordered graphitic structure, whereas a disordered structure is beneficial for the Na-ions. In addition, sustainable organic electrodes are proposed for charge storage applications (https://doi.org/10.1039/D3SE00406F).

Pre-metallation is a key step in hybrid ion capacitors to optimize their performance in terms of energy storage and cycle life. Divya et al. (https://doi.org/10.1039/D2SE01081J) show the importance of the different levels of pre-lithiation of Li_4+xTi₅O₁₂, 0 ≤ x ≤ 3 anodes from a LIC point of view along with commercial activated carbon as the counter electrode. Granados-Moreno et al. (https://doi.org/10.1039/D2SE01459A) attempt to incorporate LiFePO₄ as a battery-type additive with a graphene-AC matrix paired with an alloy-type SnP₂O₇ anode. This assembly delivered a remarkable improvement in the power capability of the LICs.

Electrolytes are the other key component governing the performance of EES. The works by Naskar et al. (https://doi.org/10.1039/D3SE00117B) and Bao et al. (https://doi.org/10.1039/D3SE00163F) are representative examples of current research efforts to improve the safety of conventional devices and enable flexible counterparts through the development of next-generation electrolytes such as gel electrolytes for Zn-ion capacitors. On the other hand, Eleri et al. (https://doi.org/10.1039/D3SE00122A) study the role of the dielectric constant on the electrochemical properties of the symmetric AC/AC assembly by altering the electrolyte solutions. Apparently, the higher dielectric constant solution yields better results with less electrolyte degradation upon prolonged cycling.

Yvenat et al. (https://doi.org/10.1039/D3SE00594A) attempt to address the fundamental process involved in the formation of the SEI layer on the graphite anode used in the KICs, especially in relation to the KF concentration.

The articles compiled in this themed collection represent the efforts the research community is undertaking to understand and develop a wide range of systems. Further, elucidating the working principles of these devices, as well as advancing the development of high-performance materials and electrolytes, are crucial aspects for commercialization. Except for LICs, the rest of the metal-ion capacitors are still at the starting stage or on a laboratory R&D scale. Hence, we believe that this collection is beneficial to understand and address the challenges posed by hybrid ion capacitors and will help eventually bring such systems toward commercial reality.

Acknowledgements

V. Aravindan acknowledges financial support from the Science and Engineering Research Board, a statutory body of the Department of Science & Technology (DST), Govt. of India, through Swarnajayanti Fellowship (SB/SJF/2020-21/12). M.Sevilla thanks funding by projects IDI/2018/000148 (FICYT/FEDER) and PID2021-123648OB-I00 CIN/AEI/10.13039/501100011033/and ERDF a way of making Europe).

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