** Contributed from Nature Communications Open Source Article
The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result.
Herein, we show that two-dimensional titanium carbide or carbonitride nano sheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives.
“The nano sheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450μm). Consequently, very high areal capacity anodes (up to 23.3 mAh cm-2) have been demonstrated.” Utilization of Li-ion chemistry to store the energy electro-chemically can address the ever-increasing demands from both portable electronics and hybrid electrical vehicles.
Such stringent challenges on the battery safety and lifetime issues require high-performance battery components, with most of the focus being on electrodes or electrolytes with novel nano-structures and chemistries.
However, equally important is the development of electrode additives, which are required to main-tain the electrode’s conductive network and mechanical integrity. Traditionally, electrode additives are made of dual components based on a conductive agent (i.e. carbon black, CB) and a poly-meric binder.
While the former ensures the charge transport throughout the electrode, the latter mechanically holds the active materials and CB together during cycling. Although these traditional electrode additives have been widely applied in Li-ion battery technologies, they fail to perform well in high-capacity electrodes, especially those displaying large volume changes.
This is because the polymeric binder is not mechanically robust enough to withstand the stress induced during lithiation/deli-thiation, leading to severe disruption of the conducting networks. This results in rapid capacity fade and poor lifetime.
In summary, the efficient utilization of 2D MXene nanosheets as a new class of conductive binder for high volume-change Si electrodes is of fundamental importance to the electrochemical energy storage field.
The continuous network of MXene nanosheets not only provides sufficient electrical conductivity and free space for accommodating the volume change issue but also well resolves the mechanical instability of Si. Therefore, the combination of viscous MXene ink and high-capacity Si demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance.
Of equal importance is that the formation of these high-mass-loading Si/MXene electrodes can be achieved by means of a commercially compatible, slurry-casting technique, which is highly scalable and low cost, allowing for large-area production of high-performance, Si-based electrodes for advanced batteries.
Considering that more than 30 MXenes are already reported, with more predicted to exist, there is certainly much room for further improving the electrochemical performance of such electrodes by tuning the electrical, mechanical and physicochemical properties of this exciting 2D MXene family.
Professor Valeria Nicolosi
Professor of Nanomaterials and Advanced Microscopy at Trinity College Dublin