The lithium ion battery chemistries are reaching their theoretical limits. The transition to sustainable energy sources such as solar- and wind power requires large energy storage systems to counter their intermittency. Furthermore, the electrification of the transportation sector increases the demand for higher specific capacities i.e., the capacity to weight ratio. The needs are right now being met by lithium ion batteries but as they approach their limit it is necessary to either improve the existing battery chemistries or develop new ones [i]. The former has been presented here. The latter is the subject of this article.
The lithium sulfur (Li-S) battery chemistry where sulfur, and lithium metal, are used as the cathode and anode, respectively, is very promising due to its high theoretical energy density of 2600 Wh kg-1. The sulfur is very attractive as cathode material due to its high theoretical specific capacity of 1675 mAh g-1 which is six times higher than that of the common LiCoO2 cathode. Likewise lithium metal is attractive as anode material because of its specific capacity of 3860 mAh g-1 which is about ten times that of graphite which is used in most commercial lithium ion batteries. [i, ii, iii]
Some of the challenges that keep Li-S batteries from commercialization are trying to be solved by employing nanostructured graphene and sulfur particles. The results are promising and have led to significant improvements in the last ten years.
– The working principle of the Li-S battery
– Challenges with Li-S batteries and possible solutions
– Concluding remarks
The Inner Workings of the Li-S Chemistry
The lithium sulfur battery cells are commonly composed of a sulfur cathode, a lithium metal anode, a separator, and an organic liquid electrolyte. When the cell is in its charged state, the sulfur will mainly be on the form: Octasulfur since it is the most stable configuration at room temperature [ii]. As the cell discharges, lithium ions and electrons move internally and externally, respectively, from the anode to the cathode. The electrons are inhibited from flowing through the electrolyte because of the separator as it is electrically insulating. When the lithium ions and electrons approach the sulfur cathode, the cyclic octasulfur molecules undergo reduction reactions i.e., the octasulfur molecules accept the electrons which reduces their oxidation level. These reductions result in structural changes in the sulfur molecules which facilitate the formation of lithium polysulfides. At first, these polysulfides will in the early stages of the discharge be on the form Li2S8 and Li2S6, but as the cell discharges further, more lithium ions and electrons arrive at the cathode which reduces the lithium polysulfides to Li2S4 and Li2S3. When the cell is completely discharged the cathode will consist of Li2S molecules [i, ii, iv].
The sulfur is a conversion cathode material since it converts from cyclic octasulfur to the linear Li2S molecules. This is different from the intercalation mechanism being utilized in the common commercial lithium ion batteries.
The Intrinsic Challenges of the Li-S Chemistry
The lithium sulfur chemistry is subject to several issues that must be overcome or at least mitigated before commercialization is possible. One of the issues is that both sulfur and Li2S are electrical insulating which lowers the amount of sulfur/Li2S that partakes in the charging/discharging of the battery. The result of this issue is a lower capacity. Another issue is that the intermediate compounds, Li2S8, Li2S6, Li2S4, and Li2S3 are soluble in the organic electrolyte and will therefore, alongside lithium, shuttle between the anode and cathode where they participate in side reactions. These side reactions deteriorate the anode and the cathode. A third issue is the change in volume when the cathode converts between sulfur and Li2S which may result in structural degradation of the cathode. [i]
These issues can be solved by encapsulating the nanosized sulfur particles with a material that can enhance the electrical conductivity, trap the soluble lithium polysulfides, and buffer the volume changes. Graphene and graphene oxide (GO) have both been shown to meet these requirements.[i]
Graphene is a sheet of carbon atoms arranged in a honeycomb lattice. Graphene oxide is obtained by treating graphene with oxidizers which introduces oxygen and hydrogen groups, and structural defects into the lattice. The oxygen and hydrogen form functional groups which increase the trapping capability of lithium polysulfides. However, the electrical conductivity of graphene and GO is reduced when defects and functional groups are introduced. Another approach is to enhance the physical trapping by constructing porous graphene or GO structures. The pores can be divided into micro- (<2 nm), meso- (2-50 nm), and macropores (>50 nm). Micropores inhibit the diffusion of the lithium polysulfides, while the mesopores trap the polysulfides and increase the amount of sulfur/Li2S that partakes in the charging/discharging. The macropores do not trap the lithium polysulfides but act as a buffer for the volume change instead. [i]
Sulfur and graphene can be combined into different nanostructures. Three of the possible structures are shown in the figure below. The first structure is the in plane configuration where sulfur is deposited onto a sheet of graphene. This structure does not facilitate a strong trapping of the soluble polysulfides. The entrapment can be enhanced by using a sandwich structure where the sulfur is deposited between layers of graphene. This also ensures great electrical conductivity. The polysulfides can however escape through the edges of the sandwich structure. This leakage can be overcome by wrapping the sulfur with a graphene sheet as seen in the core-shell structure. The last structure is very promising as it traps the polysulfides to a great extent, can buffer the volume changes, and provides high sulfur loadings. The electrical conductivity is however lower compared to the in plane- and sandwich structure. This is due to the larger sulfide particles.
Schematic of three configurations of Li2S and graphene. Lithium and sulfur are depicted as orange and yellow, respectively.
Porous 3D foams of graphene oxide have also been utilized. They show great sulfur loading but due to the increased weight of the cathode, the specific capacity is reduced. The structure can also be combined to achieve e.g., high electrical conductivity and great entrapment of the polysulfides. This complicates the production process which in turn increases the cost. [i]
Besides the challenges with the cathode, the lithium metal anode also poses a serious issue. Lithium is highly reactive and will therefore participate in undesired side reactions some of which consume electrolyte. When the Li-S battery charges, lithium ions are shuttled from the cathode to the anode where they form dendrites that can grow through the separator and cause an internal short circuit.
These issues can be solved in multiple ways. One technique is to coat the surface of the lithium metal with a carbonaceous material such as carbon nanotubes, carbon nanofibers, or graphite particles. Other techniques involve changing from a liquid electrolyte to a solid or gel polymer-based one, or modifying the separator such that it blocks the soluble lithium polysulfides. [i, v]
The Li-S battery chemistry is promising due to the high theoretical energy density of 2600 Wh kg-1, their low cost, and environmental friendliness, but some major issues must be overcome before they can be commercialized [i]. Graphene and graphene oxide are promising candidates to solve the issues.
Before the way for commercialization is paved the issues with the lithium polysulfides, the sulfur loading, and the stability of the lithium anode must be solved. However, since 2010 the lifespan and the capacity fade per cycle has increased and decreased, respectively, for Li-S batteries significantly showing that progress is being made. [i]
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[i] Yunya Zhang et al., “Graphene and its derivatives in lithium-sulfur batteries”, Materials Today Energi, 9, 2018, pp. 319-335
[ii] Arumugam Manthiram, Yongzhu Fu, and Yu-Sheng Su, “Challenges and Prospects of Lithium-Sulfur Batteries”, Accounts of Chemical Research, 46, 2013, pp. 1125-1134
[iii] Wei Zhao, Woosung Choi, and Won-Sub Yoon, “Nanostructured Electrode Materials for Rechargeable Lithium-Ion Batteries”, Journal of Electrochemical Science and Technology, 11, 2020, pp. 195-219
[iv] Seung-Ho Yu et al., “Understanding Conversion-Type Electrodes for Lithium Rechargeable Batteries”, 51, Accounts of Chemical Research, 2018, pp. 273-281
[v] Xiaosong Xiong et al., “Methods to Improve Lithium Metal Anode for Li-S Batteries”, Frontiers in Chemistry, 7, 2019, 827