24 March 2020
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Batteries are indispensable in modern life. Not only is our reliance on batteries increasing with ever more power-hungry portable devices, but our need for a way to capture and retain renewable energy necessitates more reliable and higher capacity energy storage solutions. We cannot control when the wind will blow or the sun will shine, so we need to be able to store this energy when available and discharge it when needed. As well as this, there is a growing interest in electrically powered transportation which has fuelled the need for large capacity, fast charging energy storage systems.

Lithium ion batteries are perhaps the type of battery most used in everyday life. In fact the contribution of lithium ion batteries was recognised in 2019 with a Nobel Prize in Chemistry. Typically lithium ion batteries are composed of a lithium based cathode, such as LiCoO2 or LiMn2O4, a lithium based electrolyte, and a graphite based anode.

Our recent blog post examined emerging battery technologies including solid state lithium metal batteries and metal-air batteries. After reading a recent review of MXenes in battery anodes we decided to explore recent innovations in the use of MXenes in batteries.

MXenes

MXenes are two-dimensional materials, like the more well-known material graphene, and at their heart are layers of transition metal carbides, nitrides and/or carbonitrides. Much like graphene, which when layered sufficiently becomes graphite, MXenes can be layered into multi-layer MXenes and are incredible electrical conductors. They therefore present a possible alternative to graphite anodes in batteries. MXenes also possess characteristics suitable for use in supercapacitors, another energy storage mechanism.

MXenes have some distinct advantages over current graphite anodes. A wide interlayer distance enables MXenes to incorporate a greater number of lithium ions between their 2D layers. Due to the larger interlayer gap present in MXenes, sodium and potassium ions can more easily intercalate into MXenes than graphite.

A further advantage of MXene anodes is their ability to retain their high capacity over many charge/discharge cycles, leading to a battery that can satisfy our energy needs over and over again for longer.

Researchers from Drexel University, often seen as the birth place of MXenes, have found that thin MXene electrodes can be charged in milliseconds. These electrodes are synthesised with a hydrogel electrode design and have a microporous structure which enables ions to easily flow in and out of the structures during charging and discharging.

Recent work from a team at Manchester University has resulted in the 3D printing of MXene electrodes for use in supercapacitors. The electrodes, which are formed of over 20 printed layers, demonstrated superb capacity retention and high energy capacity. This technology could be used to create a vast range of MXene structure and shapes for use in different media.

Furthermore, scientists have found that the choice of electrolyte can have a huge impact on MXene supercapacitor electrode capacity. By altering the electrolyte, the energy storage of titanium carbide could be doubled. With around 30 MXenes successfully synthesised so far, this work shows how much research and optimisation has yet to be done.

Combining MXenes with other materials

The unique properties of MXenes can be combined with those of other materials to make composites which are greater than the sum of their parts.

For example, MXene combined with carbon nanotubes provides a scaffold for potassium anodes which prevents detrimental dendritic growths in potassium metal batteries. Dendrites can lead to breakdown of the battery and can catch fire, which has prevented the widespread use of alkali metal anodes. However dendrite prevention could enable these types of anodes, which have a huge energy storage capacity, to become more mainstream.

Recently MXenes have also been used to enhance the properties of other known anode materials. Silicon anodes have demonstrated potential for a huge lithium ion capacity, but the intercalation of lithium leads to expansion of the anode by up to 300 percent, which soon causes cracking and ultimately breakdown. One way around this is to add carbon and polymer binders which detract from the capacity and conductivity of the anodes. However, scientists at Trinity College Dublin and Drexel University have discovered that addition of MXenes to silicon prevents expansion and the synergistic combination enhances the conductive properties of the silicon whilst increasing the lithium ion capacity of the MXene.

MXenes can even lend their conductive properties to materials that would otherwise be unsuitable in batteries. Researchers from KTH Royal Institute of Technology have developed an electrode made using a wood-MXene composite. The wood is in the form of tiny cellulose nanofibers which strengthen the electrode. This composite results in a flexible electrode that may prove valuable in wearable technology or foldable devices.

Wearable technology could also receive a boost from newly developed MXene yarn. The yarn integrates small flakes of MXene into cotton, linen or bamboo fibres before covering the fibres in larger flakes. The yarn can then be knitted into fabric which retains the flexibility required of fabric and can be washed multiple times without losing any conductive properties. This could result in easy integration of batteries into a garment itself.

Summary

MXenes are a promising addition to available battery materials. A lot of this research is in its early stages and the cost of the transition metals necessary for MXenes may prevent their ubiquitous use. However, MXenes have a lot to offer the world of energy storage. In addition to opening up more possibilities for multivalent ion batteries, they are a useful ingredient for composite materials with enhanced properties for energy storage. As demonstrated by graphene, the different structures and capabilities of 2D materials such as MXenes are incredibly varied. This means there are many more forms and functions to explore before the versatility of MXenes reaches its maximum potential in energy storage technology. It is clear that MXenes present an exciting area of research to achieve our energy storage needs.

Alison is a trainee patent attorney working in our chemistry team. Alison has an MChem from the University of St Andrews with an industrial placement year at GSK, Stevenage. Her DPhil is from the University of Oxford, focussing on developing new methods of synthesising asymmetric molecules with axial chirality.
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