Lithium-ion (Li-ion) batteries are a ubiquitous energy source in modern society, powering our portable devices, power tools and even some new electric vehicles (EVs). The development of EVs in particular is a crucial aspect of the fight against greenhouse gas emissions, climate change and localised pollution in urban areas.
Despite this, Li-ion batteries are still limited by their relatively low energy density, a drawback which is most keenly felt in the EV sector. For an EV to have a sufficiently long range, or for aerial vehicles (whether manned or unmanned) to be able to remain aloft efficiently, the battery delivering power to the vehicle should be as light as possible. Batteries of high energy density (the energy deliverable per unit weight of the battery) will therefore be crucial to future EV applications.
Enter the lithium-sulfur (Li-S) battery. Li-S batteries in theory are cheap, with a large specific capacity (up to 1675 mAhg-1, compared with only 155 mAhg-1 for Li-ion), high specific energy (up to 2567 Wh kg-1, compared with only 387 Wh kg-1 for Li-ion) and lower environmental impact than existing Li-ion batteries. Sulfur is a naturally abundant element, is very safe and non-toxic.
All rechargeable batteries work by converting chemical energy to electrical energy and vice versa depending on whether the battery is charging or discharging. A Li-S battery reacts lithium (Li) with sulfur (S8) to form lithium sulfide (Li2S), generating electrical energy along the way. This reaction is a source of very high levels of electrical energy, five times as much as the reactions in a Li-ion battery, hence the very high specific energy for a Li-S battery. But before we see Li-S technology replace fossil fuels or other battery technologies in our phones, cars and helicopters there are some important problems which researchers are currently working hard to solve. Some exciting breakthroughs have already been made.
The reaction to form Li2S in the battery first forms long-chain lithium polysulfides such as Li2S8 and Li2S6 as intermediates. These polysulfides can dissolve in the electrolyte within the battery and diffuse between electrodes. This “polysulfide shuttle” effect can ultimately cause a layer of Li2S to form on the anode which blocks the passage of lithium, causing aging of the battery and loss of capacity over time.
There is also a large change in volume of the electrode during the reaction which forms Li2S. This volume change can cause physical degradation of the electrodes which again leads to capacity fade and ultimately failure of the battery.
Recent research has focussed on various types of carbon-sulfur conductive matrix materials for use in the cathode of the battery to address some of these problems. The types of carbon used include fullerene, carbon nanotubes (CNTs) and graphene. Some of these rely on embedding the sulfur within carbon nanopores, which prevents the diffusion of polysulfides and reduces the shuttle effect. Graphene in particular shows promise, with recent research into a graphene aerogel “catholyte” being only one of many developments attempting to merge the two exciting technologies of graphene and Li-S batteries.
A related area of research is the binder which is used to hold the carbon and sulfur together in the matrix within the cathode. It was recently reported that a research group at Monash University have developed, and applied for patent protection for, a Li-S battery billed as the “world’s most efficient”, based on the use of a binder which allows for greater volumetric expansion during charging. The result, the team think, could provide smart phones that only need charging every five days and electric cars that can drive over 1000km without charging.
There are still hurdles to the complete commercial availability of viable Li-S batteries, but at the current pace of research a sulfur-powered world could be a real possibility before the decade is out.
