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.

Content

–          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]

Conclusion

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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References

[i] Yunya Zhang et al., “Graphene and its derivatives in lithium-sulfur batteries”, Materials Today Energi, 9, 2018, pp. 319-335
https://www.sciencedirect.com/science/article/abs/pii/S2468606918300728?via%3Dihub 

[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
https://pubs.acs.org/doi/10.1021/ar300179v 

[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
https://www.jecst.org/journal/view.php?number=335

[iv] Seung-Ho Yu et al., “Understanding Conversion-Type Electrodes for Lithium Rechargeable Batteries”, 51, Accounts of Chemical Research, 2018, pp. 273-281
https://pubs.acs.org/doi/10.1021/acs.accounts.7b00487

[v] Xiaosong Xiong et al., “Methods to Improve Lithium Metal Anode for Li-S Batteries”, Frontiers in Chemistry, 7, 2019, 827
https://www.frontiersin.org/articles/10.3389/fchem.2019.00827/full

The post Lithium Sulfur the Next-Gen Lithium Ion Battery? appeared first on The Nano Future.

Today it is hard to imagine a world where smartphones, laptops, smartwatches, and electric vehicles are not rechargeable. This indispensable attribute is thanks to the lithium-ion battery (LIB) which was first commercialized by Sony and Asahi Kasei in 1991 [i]. Our demand for LIBs does not seem to decrease or stagnate as more personal devices and especially electric vehicles are introduced to the market. The global electric vehicle stock increased exponentially from 1.18 million in 2016 to 4.79 million in 2019 [ii] and the trend is expected to continue. Therefore, continuous development and improvement of LIBs are of great importance.

Content

–          The working principle of lithium-ion batteries

–          Nanostructured anode and cathode materials

–          Outlook at commercial products and the successor to lithium-ion batteries

Working Principle of Lithium-Ion Batteries

The common present-day LIBs are similar to the ones commercialized in 1991 and are composed of two electrodes, a separator, and a liquid electrolyte, as seen in the figure below.

The positive electrode (cathode) is commonly a lithium metal oxide such as LiCoO₂ while the negative electrode (anode) is made from graphite. When the LIB is charged the lithium ions shuttle from the cathode to the anode while electrons are moved away from the cathode and supplied at the anode.
    When the LIB is discharged the process is reversed and the lithium ions shuttle internally from the anode to the cathode where they intercalate. The electrons move externally from the anode to the cathode and as they move, they can be used to power a device such as a smartphone. The capacity of the LIB is hence limited by the amount of lithium that can be stored in the cathode and the anode structure. It can be increased by enlarging the batteries but since this is often not desired, much attention is given to increase the capacity per kilogram of battery i.e. the specific capacity.
    This can be done by creating new cathode and anode compounds which can accommodate more lithium per kilogram. However, this often comes with great challenges e.g. silicon as an anode material has about ten times higher specific capacity compared to graphite but it suffers from drastic volume changes during charge/discharge [iii]. Another way to increase the specific capacity is to optimize the structure of the anode and cathode material such that a larger portion of it participates in the intercalation process.

Nanostructured Anode and Cathode

The anode in commercial LIBs is based on graphite which is the same material used in pencils. It is used due to its price, availability, and relatively good theoretical specific capacity of 372 mAh g^-1 [iv]. Despite the success graphite has had, it is still relevant to improve the anode in terms of e.g. the specific capacity. Some of the promising nanostructured carbon allotropes that could be used instead are: Carbon nanotubes (CNTs), graphene nanosheets, and carbon with nanosized pores. [iv]
    It has been demonstrated that the CNTs, which are rolled sheets of graphene, can accommodate lithium ions both on the inside and outside, and that they have excellent electronic conductivity. It was also reported that by introducing defects into the CNTs the capacity improved. [iv]
    Another option is graphene nanosheets, which can be prepared by exfoliation from bulk graphite since it is composed of stacked layers of graphene. Similar to the CNTs the surface area is greater compared to graphite and since both the plane and the edge of the nanosheets can accommodate lithium ions the theoretical specific capacity can reach 744 mAh g^-1. Furthermore, it can be increased to over 1000 mAh g^-1 by introducing defects. One of the difficulties with graphene is its low density and hence it requires a larger anode volume which ultimately leads to a larger battery. Another difficulty is that it participates in undesired reactions leading to irreversible loss of capacity. [iv]
    A third alternative is to create nanosized pores in carbonaceous materials. One approach is to make hollow spheres with a double-shelled nanostructure which increases the surface area and thus the specific capacity. The specific capacity for such a structure has been reported to reach 920 mAh g^-1. However, the increased surface area inevitably results in obstacles and defects which may trap lithium ions which in turn causes an irreversible capacity loss. [iv]

While the anode material is based on carbon, the cathode material in common commercial LIBs is in general either a lithium metal oxide such as LiCoO₂ (which was introduced by J. B. Goodenough and was the first commercial cathode) and LiNiMnCoO₂,  or a phosphate-based compound such as LiFePO4. These cathode materials are used today along with multiple other types depending on the desired voltage and working environment. LiCoO₂ and LiNiMnCoO₂  are some of the most common cathode materials due to their great specific capacity of about 150 mAh g^-1, and good cyclability. They, however, rely on cobalt which is expensive, toxic, and poses both environmental and health concerns due to the conditions in which it is mined and extracted.
    In order to lower the amount of cobalt and increase the specific capacity, it has been proposed to substitute some of the cobalt with manganese (Mn) and Nickel (Ni). It has been reported that a nanosheet structure of LiNi_1/3Co_1/3Mn_1/3O₂ with width and thickness of 0.7-1.5 µm and 10-100 nm, respectively, delivered an increased specific capacity of 193 mAh g^-1. This is due to the high surface area of the nanosheets. [iv]

Outlook

Many of the nanostructured cathode and anode materials are still in the research phase but some have already been commercialized. An example is a LiFePO₄/Graphite based LIB from LithiumWerks which utilizes their trademarked Lithium Nanophosphate technology [v]. Another example is the porous carbon material usable in both the anode and cathode developed by Graphene Batteries [vi].

Lithium-Ion batteries have and will be important for our transition to green energy but as our demand for inter alia higher capacities and higher power output keeps increasing we may have to look towards other materials for both the anode, cathode, and the electrolyte. This will give us new problems which we have to overcome, perhaps through nanostructuring?

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References    

[i] George Crabtree, Elizabeth Kócs, and Lynn Trahey, ”The energy-storage frontier: Lithium-ion batteries and beyond”, MRS Bulletin, 40, 2015, pp. 1067-1078
https://www.cambridge.org/core/journals/mrs-bulletin/article/energystorage-frontier-lithiumion-batteries-and-beyond/A0CE1F1D2F344EB6B362DA3C29DC2BD1/core-reader

[ii] International Energy Agency, “Global EV Outlook 2020 – Entering the decade of electric drive?”, Technology report, 2020
https://www.iea.org/reports/global-ev-outlook-2020

[iii] Xuefeng Song, Xiaobing Wang, Zhuang Sun, Peng Zhang, and Lian Gao, “Recent Developments in Silicon Anode Materials for High Performance Lithium-Ion Batteries”, Merck, [Accessed on: 08/11-2020]
https://www.sigmaaldrich.com/technical-documents/articles/materials-science/recent-developments-in-silicon-anode-materials.html

[iv] 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
https://www.jecst.org/journal/view.php?number=335

[v] LithiumWerks, [Accessed on: 08/11-2020]
https://lithiumwerks.com/

[vi] Graphene Batteries, [Accessed on: 08/11-2020]
https://www.graphenebatteries.no/

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