Printable 2D Materials for Thin Film Batteries and Supercapacitors
Authors: ten Elshof Johan E. , Wang Yang , Mehrali Mohammad , Huijben Mark 
Published: February 2024

Two-dimensional transition metal oxide (TMO) nanosheets are the oxide equivalents of the well-known 2D graphene phase. Compared with graphene, a much wider range of compositions, structures and properties can be realized from TMOs. 2D metal oxides typically have thicknesses of 0.45-2.5 nm, and lateral dimensions between 50 nm and tens of micrometers. Interestingly, although they are oxides, they are very flexible and bendable owing to their thinness. So far, several dozen nanosheet phases have been reported in literature, but many more compositions are possible.
Dispersions of TMO nanosheets can be made following one of several solution-phase strategies. Often, they are prepared by chemical delamination of isomorphous layered metal oxide parent phases. In a few cases, the 2D TMO phase can be grown directly bottom-up from an aqueous chemical solution. Either way, the resulting colloidal solutions of 2D single crystal nanosheets can serve in subsequent process step as the basis of a chemical ink suitable for ink jet printing on arbitrary substrates like paper, silicon or flexible polymers. Redox-active nanosheets may also be used as active electrode elements for micro-supercapacitors and flexible thin film batteries.
For example, 2D MnO2 nanosheets that are isomorphous to the layered δ-MnO2 (birnessite) phase exhibit high pseudocapacance owing to their beneficial redox properties, relatively high in-plane conductivity and very large specific surface area. Ink-jet printed 2D MnO2 based thin film microsupercapacitors were realized that showed comparable performance with other state of the art systems. However, undoped MnO2 nanosheets exhibit relatively low sheet conductivity. Substitutional doping of 3d metal ions (Co, Fe and Ni) into MnO2 nanosheets improved the volumetric energy density to 1.13 mWh cm−3 at a power density of 0.11 W cm−3. The dopants introduced new electronic states near the Fermi level which enhanced the electronic conductivity within the nanosheets and contributed to the further formation of redox-active 3d surface states. Among these, Fe-doped MnO2 exhibited a significant increase in the carrier density and conductivity, while doping with Ni or Co had a much more limited effect on conductivity. These Fe-doped MnO2 nanosheet-based microsupercapacitors show long cycle life and bendability without performance loss on flexible plastic substrates [1].
Following a similar inkjet printing approach, thin film cathodes for Li ion batteries were made using V2O5 nanosheets as charge-storage medium. In this case the conductivity of the electrode was improved by addition of highly conductive 2D MXene (Ti3C2) platelets derived from the parent MAX phase Ti3AlC2. Using LiPF6 in EC/DMC as electrolyte, the hybrid V2O5-MXene electrodes showed a high capacity of 321 mAh g-1 at 1C, 112 mAh g-1 at 10.5C and 92% capacity retention after ~700 cycles [2].
As third example, an all inkjet-printed MXene/graphene oxide (GO)/MXene thin film supercapacitor with GO as solid electrolyte phase will be discussed. Here, free water molecules trapped between GO layers enabled proton transport through the electrolyte. The as-made capacitor showed high areal capacitance, good cyclability and power densities comparable with state of the art printed supercapacitors. The specific capacitance could be increased further by addition of liquid electrolyte [3].
[1] Y. Wang, Y.-Z. Zhang, Y.-Q. Gao, G. Sheng, J.E. ten Elshof, Nano Energy 68 (2020) 104306.
[2] Y. Wang, T. Lubbers, R. Xia, Y.-Z. Zhang, M. Mehrali, M. Huijben, J.E. ten Elshof, J. Electrochem. Soc. 168 (2021) 020507
[3] Y. Wang, M. Mehrali, Y.-Z. Zhang, M.A. Timmerman, B.A. Boukamp, P.-Y. Xu, J.E. ten Elshof, Energy Storage Mater. 36 (2021) 318-325.