Lithium-sulfur batteries have for some time promised to be the successor to lithium-ion batteries, as they offer a fantastic capacity—the amount of electric charge a battery can deliver at a given voltage—at least in principle. But so far in practice, they have not at all lived up to their promise. Two opposing approaches, both aiming at reducing the volume of electrolyte required, potentially offer a pathway to solving the problem. A new review paper compares the two options and considers the applications for which they are most appropriate.
Lithium-ion batteries have revolutionized portable electronics, assisted the rollout of electric vehicles, and permitted the development of smart grids in the past few decades.
But for many applications, not least those supporting decarbonization of our economy, further advance in battery technology will be necessary. A major barrier to the electrification of heavy transport (such as long-haul trucking, shipping and aviation), for example, is the size and mass of the battery: beyond a certain point, the vehicle is simply moving around a very large, very heavy battery.
Any future advancements will thus depend upon achieving higher energy density (amount of energy per unit of volume)—to get that weight and volume down, as well as longer service life to improve sustainability.
Unfortunately, the performance of lithium-ion batteries along these lines is rapidly approaching their theoretical limit.
Lithium-sulfur batteries have in recent years been investigated as a promising successor to lithium-ion because their theoretical capacity—the amount of electric charge a battery can deliver at a given voltage—is substantially higher. Lithium-sulfur (Li-S) batteries would also benefit from the low price of sulfur, as well as from the element’s great abundance in the earth and its relative environmental friendliness.
But there has been a gap between theory and the real world. In practice, it has proven to be very difficult to fully take advantage of the large theoretical capacity of sulfur. The actual energy density of Li-S batteries developed to date has clocked in at far below its expected values.
One of the major factors that contributes to the gap between theory and practice is the requirement of a large volume of electrolytes (what allows the conduction of ions in a battery). The electrochemical reaction involved requires a large amount of electrolyte to fully solubilize polysulfide intermediates (chains of sulfur atoms that play an intermediary role in the reaction) in order to accelerate the reaction kinetics.
Even at some lower electrolyte-to-sulfur ratios that have been achieved in labs, electrolytes still account for almost half of the weight of the whole battery cell, while in conventional lithium-ion batteries, electrolytes make up only about a fifth of the weight.
“If the successor has worse energy density than its predecessor, then it’s not much of a successor,” said Yanguang Li, an electrocatalysis specialist at the Soochow University and co-author of the review paper.
While it remains a challenge for Li–S batteries to perform well under a “lean electrolyte” condition when using conventional ether-based electrolytes, there are two emerging electrolyte systems that offer promise. The review paper considers the two competing options—highly solvating electrolytes (HSEs) and sparingly solvating electrolytes (SSEs).
Solvation describes the interaction of a solute (the substance being dissolved) and the solvent (the substance into which the solute is being dissolved), which produces a stabilization of the solute in the solution. The solvent and solute molecules are reorganized into new structures—solvation complexes. Thus the HSEs are those that have strong solvation capability towards polysulfide intermediates, while the SSE approach solvates a little of them.
The HSE approach involves the use of electrolyte solvents with high donicity and permittivity, such as dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA), and 1-methylimidazole (MeIm). They have the feature of promoting the dissolution of polysulfide intermediates.
SSEs take the opposite approach of inhibiting the dissolution of polysulfide intermediates by using electrolytes that sparingly solvate (hence the name), such as ionic liquids, hydrofluoric ethers, and highly concentrated electrolytes.
The researchers conclude that it remains unclear at this moment which electrolyte system, HSEs or SSEs, is better for practical lean-electrolyte Li–S batteries, since each system has its benefits and drawbacks.
They believe that instead, battery researchers should recognize that the two approaches are optimal for different applications. They think that HSEs have greater potential to be used in Li–S batteries for applications requiring high power and high energy demands, such as unmanned drones and heavy transport, while SSEs would be more promising for Li–S battery applications requiring long life-cycles such as microelectronics.
At the same time, the researchers hope in their own work to optimize the practical performance of both HSEs and SSEs in lean-electrolyte Li–S batteries. Greater effort can be devoted to integrating HSEs or SSEs with more advanced sulfur cathodes and lithium anodes to do this. Ultimately, the researchers hope to produce Li–S batteries with large energy density of over 500 watt-hours per kilogram and a long cycle life of over 1000 cycles for a cost of under $100 per kilowatt-hour.