The rechargeable batteries of the future

Trinity team investigates the use of carbon nanotubes-bridged MoO3 nanosheets as high performance anodes in lithium ion batteries.

It’s difficult for us to imagine a world without portable electronic devices. Whether we like it or not, they have become an integral part of our world, providing information, entertainment, and communication. The amazing power behind these handheld devices lies in the rechargeable batteries that power them.

Innovation in energy storage has revolutionised the way we live in the 21st century. A particularly relevant example of a modern rechargeable battery is a lithium-ion battery (LIB). In the early 1970s, the first non-rechargeable lithium batteries became available to consumers. Nowadays, lithium-ion is the fastest growing and most promising battery chemistry there is. Lithium has a large electrochemical potential and the largest energy density for weight, both of which are essential for good battery design.

As the planet warms due to the combustion of fossil fuels, need for the development of high performance rechargeable batteries to accompany renewable energy intensifies. Put simply, an electric battery is a device, made up of an electrochemical cell, which powers electrical devices. In power supply, the positive terminal of the battery is called the cathode, and the negative terminal the anode.

The aspect of a rechargeable battery which determines its electrochemical performance is the specific capacity of that battery. This is the number of ampere-hours per unit weight – in other words, the electrical charge transferred by a steady current flow over the duration of an hour, when the battery is discharged.

LIBs are limited by the specific capacity of commercially used materials, such as the graphite anode. Due to this, a comprehensive research effort is being made to investigate the use of alternative anode materials. Recently, a paper investigating a method of improving the efficiency of LIBs has been compiled and accepted for publication by The Institute of Physics in Ireland.

The research team, which included Trinity College’s Damien Hanlon, John B. Boland and Jonathan N. Coleman, demonstrated the application of multi-layer molybdenum trioxide (MoO3) nanosheets combined with single wall carbon nanotubes (SWNTs) as an active binder-free material for LIBs.

This technique built upon a previous strategy of replacing the standard graphitic-based anode in a LIB with other layered anode materials. Despite graphene-based materials theoretically yielding a high specific capacity, in practise they are severely limited, which enhances the need for an alternative.

MoO3 is a good example of a metal oxide which is an alternative layered material for use in LIBs. In bulk, electrochemical performance of MoO3 is limited by poor conductivity. However, If the lateral size of the compound is reduced, and the resulting nanostructures are mixed with carbon-based nanomaterials, this problem may be overcome.

Resultant composite materials show improvement in terms of specific capacity and electrochemical stability. Thus far, a costly production process has led to challenges in increasing scale of production – a common issue when moving from research and development to industrial production.

To remedy this high cost, Hanlon, Boland, Coleman and colleagues have used a technique called liquid phase exfoliation (LPE). Forces between nanosheets were broken by way of either shear energy – in which forces push different parts of a body in different directions – or ultrasonic energy – which is mechanical energy characterised by vibrating particles within a medium.

The MoO3 exfoliation process and SWNT de-bundling were both carried out in isopropanol which acted as a stabilising liquid. Using this method, the researchers produced a combination of multilayer MoO3 nanosheets, with solution processed SWNTs, to act as a high performing hybrid anode in LIBs.

Addition of the nanotubes brought a network structure to the MoO3 which improved the mechanical and electrochemical performance of the anode, by effectively providing long channels for electronic charge transport, and an active anode material. The solution processed hybrid MoO3/SWNT anode demonstrated a specific capacity with a Coulombic efficiency of 99.7% and a capacity fading of 0.02% per cycle – an impressive result.

Their research concluded that this low-cost hybrid anode could boost the development of high-performance anodes for LIBs and have very useful and powerful implications in the future. Interestingly, the most notable aspect of the group’s research is the scalability of the method of preparation of the anode used. The low cost of the method makes it a very feasible option for industrial scale production in the future.

The research, combined with other developments in the field of LIBs, moves us ever closer towards a transition from dependence on fossil fuels into a future where renewables and electric vehicles dominate the market. The only question remaining, is whether this transition will happen fast enough, which is a story for another day.