Printed for Performance: Harnessing the Electrochemical Potential of Prussian White Thick Electrodes v
ia 3D Printing
Authors: Khanom Halima , Reynolds Carl , Kendrick Emma 
Published: December 2024

The development of Prussian White (PW, Na1.92Fe[Fe(CN)6]) for water-based inks for thick, structured electrodes will be presented. PW is a derivative of Prussian blue analogues (PBA) and has the structural formula of AxM2[M1(CN)6]1-y, where A an alkaline metal such as Na or K, and M is usually a transitional metal such as Fe or Mn. Here, our PW is a strong Na-ion contender to replace Li-ion usage due to their low cost, efficient synthesis methods and comparable capacity to that Lithium iron phosphate of around 160 mAh/g. However, to match the performance of LFP, thick PW electrodes will need to be fabricated and here the mass transport of the electrode can be limiting. Thus, it is imperative to develop a Na-ion battery (NIB), that can exhibit both high energy and high power density, whilst maintaining cost and sustainability. To overcome this limitation of PW, the manufacturing of electrodes through careful battery design can be studied further to achieve a high performing cell.
Thicker electrodes (>100 µm) have the potential to have both high power and higher energy, however there is a trade-off between both. When increasing the thickness of a standard two-dimensional (2D) electrode this lengthens the ionic pathways, limiting ionic transport across these thick electrodes. To overcome these restraints, we can introduce a 3D architecture within the electrode microstructure to maintain favourable, short ionic pathways hence, reducing the tortuosity within the electrode microstructure. This overcomes the mass transport limiting effects and now only the electron kinetic mechanism will be restrictive. One electrode deposition method that is used to create structured electrodes is by using a 3D printer (i.e., direct ink writing). Where electrode ink is loaded into a syringe and pressure is applied to extrude the ink through a nozzle.
Prussian White is investigated here to see whether it has potential to be 3D printed via extrusion to create our desired thick and structured electrodes. 3D structured electrodes are achieved through tuning the ink rheology, these are compared against our standard 2D electrodes and with hopes to overcome the ionic and mass transport limits of 2D thick electrodes.
Here, the overpotentials, impedance measurements and operando XRD from a synchrotron source will be evaluated, to understand how PW will transform under various electrochemical techniques. Further, there will be a direct electrochemical performance comparison between our 3D printed and 2D electrodes, through rate testing and Galvanostatic Intermittent Titration Technique (GITT). Here, we have seen the mass transport improve in our thick 2.8 mAh/cm2 3D printed electrodes, compared against our standard 2D electrode due to differences in the overpotentials. Thus, demonstrating our 3D printed electrodes can surpass the constraints linked to mass transport in thick electrodes, as we have now changed the kinetics at the electrode surface. Overall, we have achieved a multi-disciplinary project as we are discovering new electrode manufacturing processes, whilst using a Na-ion material to create the next-generation of batteries.

Figure 1