Molecular redox species for next-generation batteries

This Tutorial Review describes how the development of dissolved redox-active molecules is beginning to unlock the potential of three of the most promising ‘next-generation’ battery technologies – lithium–air, lithium–sulfur and redox-flow batteries. Redox-active molecules act as mediators in lithium–air and lithium–sulfur batteries, shuttling charge between electrodes and substrate systems and improving cell performance. In contrast, they act as the charge-storing components in flow batteries. However, in each case the performance of the molecular species is strongly linked to their solubility, electrochemical and chemical stability, and redox potentials. Herein we describe key examples of the use of redox-active molecules in each of these battery technologies and discuss the challenges and opportunities presented by the development and use of redox-active molecules in these applications. We conclude by issuing a “call to arms” to our colleagues within the wider chemical community, whose synthetic, computational, and analytical skills can potentially make invaluable contributions to the development of next-generation batteries and help to unlock of world of potential energy-storage applications

Structure and chemical composition of the Mg electrode during cycling in a simple glyme electrolyte

The volumetric energy density of magnesium exceeds that of lithium, making magnesium batteries particularly promising for next-generation energy storage. However, electrochemical cycling of magnesium electrodes in common battery electrolytes is coulombically inefficient and significant charging and discharging overpotentials are observed. Several additives and electrolyte formulations based on Mg(TFSI)2-glyme electrolytes have been proposed as solutions to these problems. However, the impact and value of these advances is often hard to discern due to a lack of knowledge of the composition and performance of the Mg electrode in the underlying Mg(TFSI)2-glyme electrolyte. In this paper, the chemical and structural changes that occur during electrochemical cycling of Mg in Mg(TFSI)2-glyme electrolyte solutions are described for the first time. Using focused ion beam-scanning electron microscopy, we show that the Mg deposited during cycling consists of a shell of degradation products, which in turn surrounds an active Mg core. These structures undergo expansion and contraction during cycling due to incorporation of Mg into the core, resulting in structural deformation and degradation of the deposits. Using this structural model, we discuss the complexities observed during electrochemical cycling of Mg electrodes and elucidate the origins of the overpotentials observed during charging. The new understanding and methodology presented here will allow the impact of electrolyte additives on the performance of the Mg electrode to be resolved

Competitive Oxygen Reduction Pathways to Superoxide and Peroxide during Sodium-Oxygen Battery Discharge

The sodium-air battery offers a sustainable, high-energy alternative to lithium-ion batteries. Discharge in the cell containing glyme-based electrolytes can lead to formation of large cubic NaO2 particles via a solution-precipitation mechanism. While promising, high rates result in sudden death. The exact nature of the discharge product has been a matter of contention, and Na2O2 has never been directly detected in a dry glyme Na−O2 cell. If Na2O2 were to form during discharge in the Na−O2 cell it would have a detrimental impact on cell performance. Here we show that Na2O2 forms during discharge at high overpotential in the glyme-based Na−O2 batteries. Na2O2 formation is confirmed by spectroscopic and electrochemical analysis and electron microscopy demonstrates that it results in thin insulating films at the electrode surface. The formation of these thin films results in rapid cell death during discharge, introducing an inherent chemical limitation to the Na−O2 battery

2021 roadmap on lithium sulfur batteries

Abstract: Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK’s independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space

The effect of H₂O on O₂ reduction in Li-O₂ batteries: The effect of H2O on O2 reduction in Li-O2 batteries

There is significant interest in aprotic lithium-air batteries due to their high theoretical specific energy. During discharge, O2 is reduced at the positive electrode and forms Li2O2. Cells are typically discharged in O2, not air, because CO2 and H2O can interfere with the discharge reaction. However, lithium-air batteries will need to use atmospheric O2 if they are to supplant state-of-the-art lithium-ion cells. Therefore, it is important to establish how detrimental CO2 and H2O are to the battery. The former is well-known to induce the formation of Li2CO3 in Li-O2 batteries, but the recent work has suggested that H2O appears to be beneficial at low concentrations in some solvents (e.g. glyme ethers), increasing discharge capacities and still forming Li2O2, while in other solvents, such as acetonitrile (CH3CN), LiOH is the discharge product. Several mechanisms have been proposed to rationalise these findings, but as yet, there is no consensus on the role of H2O on O2 reduction. The purpose of this work was to understand how H2O affects O2 reduction in electrolytes using acetonitrile (CH3CN), dimethyl sulfoxide (DMSO) and tetraethylene glycol dimethyl ether (TEGDME) solvents, and, why the 4e- reduction appears to be unfavourable at low H2O concentrations. Electrochemical and spectroscopic analysis found that, in CH3CN, 4e- reduction occurred at lower H2O concentrations than in DMSO and TEGDME. A mechanism based on the ability of the H2O/solvent mixture to stabilise OH- was proposed, with mixtures that stabilise OH- promoting 4e- O2 reduction. The mechanism was confirmed by using pressure cells to identify the electrochemical reaction occurring during discharge. Cells using DMSO and TEGDME solvents underwent 2e- O2 reduction, even with 1 M H2O concentrations. Finally, TEGDME cells were discharged in a 13% RH at 25 °C O2 atmosphere, corresponding to 1 M H2O in solution, and Li2O2 was confirmed as the discharge product, demonstrating that it is possible for electrolytes to withstand a near-atmospheric humidity

Critical Role of the Interphase at Magnesium Electrodes in Chloride-Free, Simple Salt Electrolytes

Magnesium (Mg) batteries are a potential beyond lithium-ion technology but currently suffer from poor cycling performance, partly due to the interphase formed when magnesium electrodes react with electrolytes. The use of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) electrolytes would enable high-voltage intercalation cathodes, but many reports identify poor Mg plating/stripping in the electrolyte solution due to a passivating interphase. Here, we have assessed the Mg plating/stripping mechanism at bulk Mg electrodes in a Mg(TFSI)2-based electrolyte by cyclic voltammetry, ex situ Fourier-transform infrared spectroscopy, and electron microscopy and compared this to the cycling of a Grignard-based electrolyte. Our studies indicate a nontypical cycling mechanism at Mg surfaces in Mg(TFSI)2-based electrolytes that occurs through Mg deposits rather than the bulk electrode. Fourier-transform infrared spectroscopy demonstrates an evolution in the interphase chemistry during conditioning (repeated cycling) and that this is a critical step for stable cycling in the Mg(TFSI)2-tetraglyme (4G) electrolyte. The fully conditioned electrode in Mg(TFSI)2-4G is able to cycle with an overpotential o

Understanding of the electrogenerated bulk electrolyte species in sodium-containing ionic liquid electrolytes during the oxygen reduction reaction

An understanding of the species generated in the bulk ionic liquid electrolyte in the presence of superoxide anion, O2•–, is of interest due to its close relationship to the nature of the electrode reduction products. Unlike conventional organic solvents, ionic liquids are composed entirely of ions, thereby requiring an understanding of the intermediates generated in the bulk electrolyte. The generation of a complex species, [O2•–][C4mpyr+]n [Na+]m, is envisioned in the bulk sodium cation pyrrolidinium-based ionic liquid with a composition depending on the Na+ concentration. In this work, the superoxide anion, O2•–, has been considered in theoretical calculations regarding the oxygen reduction reaction in order to determine its average coordination number and also its dynamics in these mixtures. Most interestingly, the final reduction product can be tuned depending on the Na+ concentration, whereby a limited supply of Na+ favors the superoxide product while a sufficient excess of Na+ leads to the formation of the peroxide product. These findings have been identified using a pressure cell and corroborated by rotating ring–disk electrode measurements. Thus, the preferential generation of Na2O2 over NaO2 could drastically improve the specific energy of the Na–air battery due to a higher number of electrons exchanged

Hydroperoxide-mediated degradation of acetonitrile in the lithium–air battery

Understanding and eliminating degradation of the electrolyte solution is arguably the major challenge in the development of high energy density lithium–air batteries. The use of acetonitrile provides cycle stability comparable to current state-of-the-art glyme ethers and, while solvent degradation has been extensively studied, no mechanism for acetonitrile degradation has been proposed. Through the application of in situ pressure measurements and ex situ characterization to monitor the degradation of acetonitrile in the lithium–air battery, a correlation between H2O concentration within the cell and deviation from the idealized electron/oxygen ratio is revealed. Characterization of the cycled electrolyte solution identifies acetamide as the major degradation product under both cell and model conditions. A new degradation pathway is proposed that rationalizes the formation of acetamide, identifies the role of H2O in the degradation process, and confirms lithium hydroperoxide as a critical antagonistic species in lithium–air cells for the first time. These studies highlight the importance of considering the impact of atmospheric gases when exploring lithium–air cell chemistry and suggest that further exploration of the impact of hydroperoxide species on the degradation in lithium–air cells may lead to identification of more effective electrolyte solvents

2021 roadmap on lithium sulfur batteries

Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK's independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space