2011 saw the discovery of a new class of 2D materials, "MXenes" that has piqued the interest of materials scientists around the globe due to their unique properties, making them appealing for applications ranging from biomedical to transparent conducting electrodes, water purification, and electronics. This article will discuss the most recent advances in MXenes for energy storage devices, particularly lithium sulfur batteries.
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MXenes - A New Class of 2D Wonder Materials
The general formula for ternary carbides is Mn+1AXn, where n = 1,2,3, M is a transition metal such as Cr, V, Ti, Sc, etc., A is an element such as aluminum or silicon, and X is either carbon or nitrogen. Researchers refer to these phases as MAX phases.
M-A bonds in MAX phases are weaker than M-X bonds, so A layers are easily susceptible to etching and produce the Mn+1XnTx phase (a common formula for MXenes), where Tx represents surface functional groups such as oxygen, hydroxyl, or fluorine.
The first MXenes nanomaterial reported in 2011 was Ti3C2, which was synthesized by removing Al atoms selectively from layered hexagonal ternary carbide Ti3AlC2. Since then, over 30 MXenes materials have been synthesized to date, such as Ti2C, Nb2C, Mo2C, Ti3CN, Ti4N3, and Mo2TiC2.
Properties of MXenes
MXenes have sparked substantial interest due to their superior metallic conductivity, hydrophilicity, and mechanical properties, which are much better than graphene.
It has already been demonstrated that the electron mobilities of various MXenes can outperform graphene (2.5x 105cm2/Vs) with values up to 106 cm2/Vs.
Ti3C2Tx sheets have a metallic conductance of around 6500 S/cm, higher than that of other 2D materials like carbon nanotubes and graphene. MXenes thin films are transparent, and the Ti3C2Tx monolayer transmits more than 97% of visible light.
The elastic constant of Ti4C3-MXene, for example, was calculated to be 512 GPa. Wear resistance, high strength, and hardness are just a few of the properties that accompany a material's exceptional bending stiffness.
Role of MXenes in Lithium Sulphur Batteries
Existing lithium-ion battery (LIBs) technology continues to be ineffective due to its low energy density, which comes at a high price. Lithium-sulfur (Li-S) batteries are viable replacements for traditional LIBs due to their low cost and high energy density.
Nonetheless, Li–S batteries suffer from poor cycling stability, low sulfur utilization, and low coulombic efficiency, all of which limit their future applications.
To achieve desirable Li-S cells, it is necessary to have high electronic conductivity, the potential to accelerate sulfur redox, and a capability to prevent the shuttling effect to enhance cycling stability. MXenes can fulfill the aforementioned requirements and have thus piqued the interest of researchers in the field of Li–S batteries.
In general, MXenes act as a conductive framework in MXenes-based sulfur hosts, which can significantly increase the capacities after long-term cycling of the assembled Li-S cells.
Meanwhile, the insulating nature of sulfur reduces the redox reaction kinetics, lowering rate capabilities and sulfur utilization. MXenes' highly conductive nature potentially enhances charge transfer kinetics, allowing for significant improvements in sulfur utilization.
During the discharge cycle, the intermediate lithium polysulfide species dissociate in the electrolyte and diffuse between the cathode and anode, typically known as the shuttling effect, resulting in poor coulombic efficiency and rapid capacity fading of the Li-S cells. MXenes prevent the shuttling effect and reduce active material loss through a strong Ti-S interaction.
Development of MXenes Based Cathode and Anode Materials for Li-S Battery
The Li-S battery consists of four major components: a ring-structured octasulfur (S8) cathode with a theoretical specific capacity of 1675 mAh/g, a Li metal anode with a theoretical specific capacity of 3840 mA/g, a separator, and an electrolyte. When these are combined into a cell, the theoretical energy density of the battery can reach 2600 Wh/kg.
Titanium carbide MXenes have shown strong polysulfide adsorption capabilities and high electronic conductivity, attracting significant research attention for Li-S applications. Scientists from Nanjing Tech University in China, for example, conﬁrmed that when MXenes were used as a cathode material in a Li-S battery, the capacity was 1169 mAh/g with a coulombic efficiency of 90.5%.
Aside from Ti-based MXenes, CNTs doped Mo2CTx have been synthesized to inhibit the shuttling effect of lithium polysulfide. In addition, the functionalization with CNTs will prevent the restacking of MXenes nanosheets. A capacity of 954 mAh/g has been reported in the Mo2CTx-CNT composites when used as cathode electrode in Li-S batteries.
Metallic lithium is regarded as one of the most promising Li-S battery anode materials. However, the formation of dendrites continues to obstruct its practical application. Scientists discovered that when MXenes were used as a lithium anode, they acted as "artificial solid electrolyte interface (SEI) films" for metallic lithium and remained intact during repeated charge-discharge cycles.
Furthermore, the conductive Ti3C2 MXenes hinder the vertical growth of lithium dendrites, preventing them from piercing the separators, thereby preventing the overheating and explosion in batteries.
MXenes Based Separator
Besides modifying the sulfur cathode and anode, another effective strategy for preventing lithium polysulfide migration between the cathode and anode is modifying a commercial polymeric separator.
When MXenes are coated on a glass fiber separator, it improves porosity and electrolyte solution uptake, inhibiting the lithium polysulfide shuttle effect when compared to a commercial GF separator.
The Future of MXenes in Li-S Battery
MXene research is still in its early stages. However, there is certainly much room for improvement in the electrochemical performances of Li-S batteries by tuning the chemical and physical properties of existing MXenes and exploring other MXenes members.
Despite the fact that there are still many obstacles to overcome, MXenes research is constantly expanding. MXenes in LSBs have excellent prospects for future applications, thanks to the ongoing discovery of new family members.
Just wait a little longer, and MXenes-based Li-S batteries will soon be a reality in our daily life.
References and Further Reading
Zhang, C., et al. (2020). Two‐dimensional MXenes for lithium‐sulfur batteries. InfoMat, 2(4), 613-638. https://onlinelibrary.wiley.com/doi/10.1002/inf2.12080
Zhang, Y., et al. (2021). MXene and MXene-based materials for lithium-sulfur batteries. Progress in Natural Science: Materials International, 31(4), 501-513. https://www.sciencedirect.com/science/article/pii/S1002007121000897
Zhang, T., et al. (2022). MXenes: Synthesis strategies and lithium-sulfur battery applications. eScience.https://www.sciencedirect.com/science/article/pii/S2667141722000222