Today, most electrical devices and electric vehicles use lithium-ion batteries for power supply. These batteries, however, suffer from certain drawbacks concerning energy density and performance. One type of these batteries, called lithium-sulphur batteries, is able to offer more energy density and lower cost than the traditional graphite/metal oxide lithium-ion battery. The only problem is that their performance is often impaired by a parasitic reaction inside the battery that prevents it from cycling efficiently.
Now (2019), scientists at the U.S. Department of Energy’s Argonne National Laboratory have discovered that certain electrolyte materials can reduce the frequency of this reaction, which may lead to more effective lithium-sulphur batteries. While charging a lithium-sulphur battery, an unavoidable side reaction called lithium polysulfide shuttling often occurs. Lithium sulphide is converted to sulphur on the cathode, but some lithium-sulphur compounds that are not completely oxidized can dissolve from the cathode into the electrolyte. At this point, the lithium-sulphur compounds can diffuse and become reduced on the anode and oxidize back on the cathode. This process can repeat itself several times so that the battery’s charge is lost without being able to use it. The reason for polysulfide shuttling is due largely to the fact that they can dissolve readily in an electrolyte containing a solvent mixture of two compounds called dioxolane (DOL) and dimethoxyethane (DME). Therefore, scientists have put a lot of effort into developing a new type of electrolyte material that could address both of these issues. This material, called a hydrofluoroether, or HFE, has a much lower solvating ability while still maintaining generally good conductivity.
Research on how to avoid polysulfide shuttling has been carried out for more than a decade. In 2018, scientists created a lithium-sulphur battery by directly coating a thin layer of reduced graphene oxide (rGO)/sodium lignosulfonate (SL) composite on the standard polypropylene (PP) separator and producing a [email protected]/PP separator with abundant negatively charged sulfonic groups in the porous lignin network, which effectively counteracted the translocation of the negatively charged polysulfide (PS) ions without compromising the transport of positively charged Li+ ions. Using the [email protected]/PP separator, they achieved a robust Li-S battery with a capacity retention of 74% over 1,000 cycles.
In 2017, scientists demonstrated a novel approach to the problem of polysulfide shuttle by using a “mixed conduction membrane” (MCM). The MCM was a thin, non-porous lithium-ion conducting barrier which restricted the soluble polysulphides to the positive electrode. Lithium-ion conduction occurred through the MCM by electrochemical intercalation or insertion reactions and concomitant solid-state diffusion. Since the lithium ions were transported rapidly in the MCM, the internal resistance of the battery was not higher than that of a conventional lithium-sulphur battery. The MCM was as effective as the lithium nitrate additive in suppressing the polysulfide shuttle reactions. However, unlike the lithium nitrate, the MCM was not used up during cycling and thus provided extended durability and cycle life.
The main problem with lithium-sulphur batteries is that the reduction of sulphur is achieved through a multi-step mechanism and a variety of intermediate polysulphides of the general formula Li2Sn are formed. The polarity of those sulphur compounds varies widely with sulphur being non-polar and Li2S, the terminal species, being polar. The polysulphides however are of intermediate polarity and often well soluble in electrolytes. The dissolution of polysulphides depletes electrodes from active materials. Furthermore, when the polysulphides diffuse into the electrolyte, they can easily travel between cathode and anode and instead of useful oxidation and reduction cycles, parasitic reduction and oxidation of intermediate polysulphides occurs on electrodes, resulting in the capacity fading mentioned above. Lastly, if Li2S forms at the anode, it tends to form an insoluble layer and therefore blocks lithium transport. The new type of electrolyte material could solve all the aforementioned problems. It has a much lower solvating ability while still maintaining generally good conductivity. This is because dimethoxyethane is not a very suitable solvent for hydrofluoroether. “Like water is a really good solvent for table salt, dimethoxyethane is a very good solvent for lithium. But with HFEs it’s like trying to dissolve salt in gasoline,” the scientists claim.
Although the new electrolyte material is very promising, there is no magic bullet for lithium-sulphur batteries yet. So, searching for ways to improve the chemistries will still need to continue.