3D printed electrodes and separators make flexible battery

Novel stretchable or wearable electronics require a rechargeable flexible energy supply or means to store energy. Now researchers from the University of Maryland, College Park have designed 3D-printed, deformable electrodes and separators based on nanocellulose that are promising for stretchable Li-ion batteries [Qian et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.02.015].

Battery components are inherently brittle and fracture easily when flexed, so the researchers, led by Teng Li, turned to nanofibrillated cellulose (NFC), which is made up of long fibers. When combined with carbon nanotubes (CNTs) functionalized with carboxylic groups, strong hydrogen bonding forms a robust 3D network with the cellulose. The CNT/NFC network wraps around the active materials, in the form of powdered graphite (Gr) and lithium iron phosphate (LiFePO₄ or LFP) nanoparticles, creating a stable and viscous ink suitable for 3D printing. The NFC acts as a surfactant to disperse the CNTs and active materials throughout the solution, without the need for an additional surfactant. The hydrogen bonding between the CNTs and NFC also improves the viscosity of the ink and eliminates the need for an additional binder. Moreover, water can be used as an eco-friendly, cheap and readily available solvent.

Using the new inks in extrusion-based 3D printing, the team fabricated serpentine-shaped networks. The tessellating open pattern of active material enables the electrode structure to be stretched by up to 50% without breaking or losing performance. Thanks to the NFC and CNT network, which holds the active Gr and LFP microparticles in place, electrical conductivity is maintained throughout stretching. Resistance increases by around 1% during the process but returns to its original value when the strain is released. Repeatedly stretching the electrode structure leads to only a 3% increase in resistance over 50 cycles.

To create a complete battery, the researchers fabricated a separator in the same way from the same NFC embedded with Al₂O₃ nanoparticles to improve the uptake of electrolyte and ionic conductivity. Using layer-by-layer 3D printing, the researchers fabricated a sandwich-type structured stretchable battery from the electrode and separator inks.

The team believe that their approach is promising for the low-cost manufacturing of high-performance stretchable Li-ion batteries for energy storage in wearable, flexible electronics.