Figure 1. A brief illustration of past, today and future battery technologies. The ultimate goal has always been to fabricate batteries that are lighter and smaller for the same amount of energy storage.
Rechargeable batteries: non-stop evolution
Figure 1 compares a few different battery technologies; the history of the rechargeable battery technology can go back to 1860 when the first lead-acid battery was developed. They were cheap and powerful, and still in use today for many applications. Though it was an innovation breakthrough at the time, they are considered an old technology today and are rather heavy and bulky compared to the newer technologies that makes them inappropriate for many of today’s high-end applications. Later on, Ni-Cd and Ni-Metal Hydride batteries were invented with higher energy density and longer lifetime. The first commercial Li-ion battery appeared in the market in 1991. Li metal facilitated the design of high energy density rechargeable batteries, due to its light weight and highest electropositivity among other metals. Not very long after their first introduction, Li-ion batteries quickly found popularity in applications for watches, calculators and implantable medical devices. The cost of LIB production has immensely decreased from its early days. Only during the last decade, the price for production of 1 kWh-battery has reduced from approximately 1000 USD to 200 USD. Prices below 100 USD/kWh, however, seems essential to trigger a widespread adoption of lithium-ion batteries in electric vehicles at a global scale.
Li-Sulfur: a promising candidate for future
There are some drawbacks and environmental concerns associated with LIBs which should be addressed, such as mining of the metallic constituents of LIBs, that includes mostly Li, Co, Ni, Mn, Cu and Al. The biggest concern among these metals is related to cobalt. Later we will explain that cobalt appears as a part of the positive electrode in the LIB. Particles emitted during cobalt mining have radioactive emissions and cancer-causing effects which end up in the lungs and heart and can cause vision problems. There are also serious socioeconomic issues with mining of these elements, specifically Li and Co. The largest reserves of Li are located in Chile, Argentina and Australia, while more than half of the world reserves of cobalt are located in Congo. Hence, these resources are not evenly distributed and their production could raise social or political tension.
One promising alternative chemistry is Li-sulfur battery (LSB). The LSB holds the potential to surpass current battery technologies at two essential aspects: chemistry and sustainability. “chemistry” refers to the nature of the materials and their charge storage mechanisms and capacity. “sustainability” refers to the human implications, so on a political, socio-economic and environmental impacts. Both are linked as the cheapest and easiest-to-make battery would not be worth pursuing if its chemistry was of poor quality. Thankfully, this is not the case with LSB.
Figure 2. a) comparison of LIB and LSB technology. LSB cell employs elemental sulfur as the cathode active mass, and lithium as the anode. The design result in significantly thinner cell stack compare to LIB. b) electrodes are basically made of a mixture of active material, a conductive agent like carbon black and a binder. This mixture is applied as a form of thin coating on top of a metallic foil like copper or aluminum known as the current collectors c-d) a comparison of cell thickness and weight for the two battery technologies. LSBs are significantly lighter and smaller than LIB rivals.
In Figure 2a, we are comparing the structure of a LSB with that of LIB. The chemistry of LSB presents a couple of particularities compared to the classic LIB but the general structure and main concept remain the same: electrons and lithium ions travel from one pole (electrode) to the other. The cathode and anode electrodes in both cases, are made of a tiny particles mix. Among those small particles, the so-called “active material” is the heart of the battery because that is where the electrochemical reaction to store or release electrical energy takes place. Most of the commercial and state-of-the art LIBs employ cathode active materials that are made of oxides of transition metals such as the Co, Mn and Ni, like LiCoO2 (LCO), LiMn2O4 (LMO), LiNixMnyCozO2 (NMC); the anode active materials are usually graphite, or recently, a mixture of graphite and silicon. In case of LSB the cathode’s active material is the elemental sulfur, and lithium metal replaces the graphite anode. To make the electrodes, we usually mix the active material with a conductive agent like a carbon derivate (graphene, carbon black, carbon nanotubes, etc.) to facilitate the transport of electrons when the reaction occurs and add a suitable binder like PVDF in order to give the cathode some robustness and cohesion. Although conceptually very easy, in practice there are some factors, like proportions, choice of specific material, etc., that complicate things. A typical recipe to fabricate a LSB cathode is depicted in Figure 2b.
The theoretical capacity of sulfur is 2500 Wh/kg, compared to about 740 Wh/kg for nickel-rich LiNixMnyCozO2, which is among the highest for Li-ion batteries. Bearing these numbers in mind, we want to show how sulfur can help us to make smaller and lighter batteries. An approximate total cell thickness for both LIB and LSB cells as a function of cell nominal energy is sketched in Figure 2c-d. The combination of high-capacity elemental sulfur and lithium metal helps to reduce the total thickness of the battery cell by approximately 30% and achieve the same levels of energy as of today’s high-energy density LIBs. That means we can potentially store the same amount of energy in considerably much smaller volume. The LSB cell can ideally be 35% lighter than LIB cells for storing the same amount of energy. At the same time the price of sulfur is about 100 times cheaper than that of the materials of a LIB cathode, and therefore it will be more cost-effective.
There are a few other differences between the two technologies: the electrodes of LIB act more like a container that absorb and release both electrons and lithium. On the other hand, LSB cathodes actually react chemically to create brand new chemical species. When charged, the cathode contains elemental sulfur, the yellow powder that most of us associate with the element. When it discharges, it gradually reacts with lithium, the solid sulfur breaks down in soluble sulfur-chains that each accommodate with Li atoms. The further the discharge process goes, the shorter the chains become. Those are known by the general term of “(lithium) polysulfides” regardless of their size. The evolution of polysulfides follows a path of chemical chain-reactions. The process is represented in Figure 3. The reaction chain is complex, dependent on the surroundings (temperature, solvent nature, structure of the electrodes) and is still not well understood but, against all odds, it is always reversible, meaning the batteries can be recharged and used again.
Polysulfides have different properties depending on their length, for example they have different colours and are soluble in the battery electrolyte like table-salt in water. The end product of this chain-reaction appears at the very end of discharge. It is the shortest “chain” possible: a single-atom of sulfur linked to two Li atoms. At this point, the designation of “chain” or “polysulfide” is senseless and we call it simply “lithium sulphide”. Those small molecules are insoluble; therefore, the cathode reverts back to a solid state. The intermediary state of the cathode, as dissolved polysulfides in the electrolyte, is called a “catholyte”, from the contraction of “cathode” and “electrolyte”. 1–3
Figure 3. The highlighted areas correspond to the energy contained in one gram of either LIB cathode (grey) or LSB cathode (orange) if it were made exclusively of sulfur. The polysulfides are represented as a chain of sulfur atoms attached to one or two atoms of lithium. As the LSB discharges, the chemical reactions evolve to produce smaller and smaller polysulfides. The LSB operates at lower voltage than the LIB, but delivers a lot more capacity, making it more energy dense in theory. In practise, the energy that can actually be extracted from LSB is way smaller than this limit due to inherent draw-backs and limitation of sulfur.
Li-Sulfur: Technical challenges
There exist some technical challenges that have complicated and delayed the maturity of LSB technology. Some issues originate from the reactions at the lithium anode, others arise from the sulfur cathode. Because the lithium problems are common to a lot of lithium-based batteries, we will mention here solely the issues surrounding the sulfur cathode. There are three major issues with the sulfur electrode. The first deals with the poor conductivity of sulfur and its products. The second two arise from the dissolved state of the polysulfides.3
Sulfur, in its every form, is poorly conductive and the flow of electrons in the cathode is thus hindered. The conductivity limitation of sulfur holds the reaction rate down, resulting in less efficient and slower charges and discharges.
Because it goes through a dissolved state during its cycling, the electrode partially deconstructs and reconstructs itself after every charge/discharge cycle. In addition, the cathode will expand and shrink as it depletes and recharge. Consequently, the initial structure erodes, cracks, and gets torn apart. That leads to an accelerated decay of the battery performance, meaning it has a shorter lifetime.
The polysulfides chains can wander off everywhere, including to battery parts with which they should not be involved: the lithium anode. There, the polysulfides will participate in many sorts of undesirable side-reactions. Most notable phenomenon is the back-and-forth conversion of polysulfides that travel from one compartment to the other, creating a sort of chemical short-circuit. This phenomenon is called the “shuttle effect”4 ( Figure 4) and is unique to LSB systems and limits their ability to maintain stable capacity over time.
Figure 4. The polysulfide shuttle is the phenomena where polysulfides, produced by the cathode reactions, go back and forth between the two electrodes, effectively creating a path for electrons to move without having to go through the wires; chemical short circuit (see Figure 2a). The normal course of action would have reaction (e.) occurring only at the cathode side, hence creating a neat and contained reaction chain with no leak to the anode side. The cathode expands when the end-product of discharge, the lithium sulphide, precipitate; note how the bottom part of the cathode is larger than the top part.
Li-Sulfur: Recent progress
Luckily, LSB systems benefit from a substantial and long research history with some proven strategies, like new designs or additives, to help the technology circumvent the difficulties and become competitive. Outside a couple of precursor works in the 20-30’s, the first concrete works over lithium-sulfur electrochemistry date back to the late 60’s - early 70’s. The first design of the LSB system was the elaboration of a liquid sulfur cathode, then called catholyte, because pure solid sulfur was not able to produce electricity. The catholyte design was however limited to a limited voltage range because deepening the charge and discharge would solidify sulfur and render it useless.2
The whole LSB idea did not became widespread for decades due to the limitations that previous generation could not overcome. However, with the progress of nanotechnologies in the 2000’s and the growing demand for next generation lithium battery, interest was slowly renewed, and more investigations were undertaken. With nanotechnologies, we could create and control electrode structures on a much finer scale. This leads to the establishment of a new standard for LSB design which cemented the LSB status as one of the most promising alternatives for the future generation of batteries and sparked a huge boost of interest in the technology.5
In the recent designs, the conductive additive, most often a carbon-derivate, takes on the additional role of trapping the polysulfides. Now, the polysulfide shuttle can be drastically reduced, and the challenge resides more in making the new design viable on an economic standpoint. Already, the industry produces LSB able to answer some niche demands of the market competitively.6
Li-S batteries: Potential market and future perspective
As explained, there are many challenges ahead of LSBs before penetration into market. Being a potentially very light-weight battery, LSB is a suitable choice for applications where the battery mass is a concern. For instance in heavy duty electric transport to waste less energy in moving the battery mass itself. Similarly in drones and electric bikes the duration of flight and biking is very sensitive to the weight of battery. Weight is specifically very important in aviation industry. These applications usually do not require a long lifetime like that of a smartphone and as the end of life of the battery approaches, it is rather feasible to replace them with a new one. Instead, here the higher energy density is of greater importance as it allows to complete a mission with just a single charge. The other potential application is for satellites, where there is an increasing demand for them in the telecommunication industry. It is much easier and cost-saving for the manufacturers to launch lighter and smaller satellite into the space and therefore the weight and size of the battery matter. Li-S batteries have also been demonstrated in luxury day boats where a single charge is sufficient for a significantly long-distance trip. A widespread commercialization of LSBs, however, requires more effort and investigation. With high energy density, low cost and more importantly high safety, LSBs are unique candidates for electric buses and trucks. Unfortunately, the current short cycle life of LSBs, reportedly about 100 cycles, has been a big obstacle for their maturity.
Today, researchers are still going strong, fuelled by parallel fundamental research which unveils the complexities of the reactions inside LSBs and lead to discovery of new forms of materials. The next major step would be to consider all designs and materials that were only proven in the lab and submit them to practical conditions. Specifically, this means bigger electrodes, higher currents, extreme temperatures, take costs into account and optimise the ratio of active mass versus supporting mass. An electrode without support will not be able to perform, but too little of active mass would result in very heavy batteries for little capacity.7,8
LSB is hence at a very interesting point, where new routes and new designs are being proposed every day, and the process of making them viable is equally intense. The EnergyVille researchers from battery groups of UHasselt, VITO, and imec, have recently started a project in collaboration with University of Antwerp and University of Ghent to develop better lithium-sulfur batteries. This cSBO project is called FUGELS and is funded by the SIM/Vlaio in the context of the new battery program SIMBA (https://www.sim-flanders.be/project/fugels).
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