Kirigami Patterning of MXene/Bacterial Cellulose Composite Paper for All‐Solid‐State Stretchable Micro‐Supercapacitor Arrays

Abstract Stretchable micropower sources with high energy density and stability under repeated tensile deformation are key components of flexible/wearable microelectronics. Herein, through the combination of strain engineering and modulation of the interlayer spacing, freestanding and lightweight MXene/bacterial cellulose (BC) composite papers with excellent mechanical stability and a high electrochemical performance are first designed and prepared via a facile all‐solution‐based paper‐making process. Following a simple laser‐cutting kirigami patterning process, bendable, twistable, and stretchable all‐solid‐state micro‐supercapacitor arrays (MSCAs) are further fabricated. As expected, benefiting from the high‐performance MXene/BC composite electrodes and rational sectional structural design, the resulting kirigami MSCAs exhibit a high areal capacitance of 111.5 mF cm−2, and are stable upon stretching of up to 100% elongation, and in bent or twisted states. The demonstrated combination of an all‐solution‐based MXene/BC composite paper‐making method and an easily manipulated laser‐cutting kirigami patterning technique enables the fabrication of MXene‐based deformable all‐solid‐state planar MSCAs in a simple and efficient manner while achieving excellent areal performance metrics and high stretchability, making them promising micropower sources that are compatible with flexible/wearable microelectronics.


Preparation of multi-layered Ti 3 C 2 Tx powders:
First, an etching solution consisting of 1.0 g LiF and 20 ml 6 M HCl was employed to extract the contained Al in the Ti 3 AlC 2 MAX raw powders (1.0 g) under 35 °C for 24 h. Then, after centrifugation, removed the etching solution and washed the sediment with deionized (DI) water several times until the pH of the discarded upper liquid achieved 6. Finally, the multi-layered Ti 3 C 2 Tx powders were obtained after drying under normal atmospheric temperature.

Preparation of fully delaminated few-layered Ti 3 C 2 Tx flakes:
First, 0.2 g as-obtained multi-layered Ti 3 C 2 Tx powders were dispersed in 100 ml deionized water, followed by ultrasound exposure for 60 minutes in ice water bath.
Then the unexploited multi-layered Ti 3 C 2 Tx powders were removed by centrifugation at 4000 rpm for 5 minutes. Finally, a colloidal solution containing fully delaminated few-layered Ti 3 C 2 Tx flakes (0.5 mg ml -1 ) can be obtained.

Preparation of MXene/BC composite papers with different mass fraction of 1D BC fibers:
Typically, a variety of commercial BC (0.5 mg ml -1 ) dispersions were controllably added to as-prepared MXene colloid solution (0.5 mg ml -1 , 50 ml) to prepare the composite inks with BC content from 0-57.1 wt% under stirring. Then, the MXene/BC composite papers were prepared by rapid filtration of the obtained ink in batches through a mixed cellulose ester membrane (47 mm in diameter, 0.45 um pore size, Whatman, Germany), which were finally peeled off from the filter paper and dried for 24 h. Pure MXene papers were also prepared with the same procedure.

Fabrication of standardized MSCs units based on MXene/BC composite papers:
First, interdigital-fingers-like circuit patterns were designed with AutoCAD software on a personal computer. Then, input the software into a laser cutting machine (TR-5030, professional CO 2 Universal Laser System, Shenzhen Triumph Industrial Co.,LTD). The laser power was set to 20% (10 W) and the speed was set to 200 mm s -1 , respectively. The Z-distance between the laser and the sample was 1.0 cm, while the laser beam size was about 100 μm. Before the laser-cutting process, a thin Au layer with thickness of about 80 nm was magnetron sputtered on the surface of the MXene/BC composite papers, served as the current collector and for better conductivity. Finally, with the aid of the laser cutting, the designed coplanar interdigital electrodes based on the MXene/BC composite papers can be fabricated.
The polyvinyl alcohol (PVA)/sulfuric acid (H 2 SO 4 ) gel electrolyte was used as the solid electrolytes, which was prepared through dissolving PVA powder (5 g

Fabrication of MSCAs based on MXene/BC composite papers:
MSCAs were also fabricated according to the above procedure. First, honeycomb patterns were designed with AutoCAD software on a personal computer. After the laser-cutting process, an identical ~0.03-mm-thick double faced adhesive tape is used to bond the patterned MXene/BC composite electrodes to the bottom flexible PET film supporter with the similar honeycomb structure, for the purpose of supporting the pattered electrodes and further enhancing the robustness of the as-fabricated MSCAs to prevent being torn when the devices suffer from rough stretching, twisting and bending, resulted in improved deformational stability. Finally, the PVA-H 2 SO 4 gel electrolyte was drop-casting onto the MSC islands for ionic transport. A schematic diagram of the fabrication procedure of the MSCAs is shown in Figure 1.
The areal capacitance (Cs, mF cm -2 ) and areal energy density (Ws, mWh cm -2 ) of the MSCs and MSCAs were calculated from the charge-discharge curves according to the following equations: where C is the total capacitance, Q is the total charge, I is the discharge current, t is the discharge time, ΔE is the potential window during the discharge process after IR drop, and S is the total area of the positive and the negative electrodes.

Tensile test and electrochemical measurements:
Tensile property of the MXene/BC composite paper samples of 3.0*1.0 cm (length*width) was characterized using a dynamic mechanical analysis (DMA) Q800 apparatus (TA Instruments Inc., U.S.) at room temperature. At least five specimens were used for each sample in the tensile test. Electrochemical properties of all the fabricated devices were investigated in a two-electrode configuration. Two Cu wires were connected to the pad of each microelectrode using Ag paste to make a connection to the electrochemical instruments. CV, EIS, and GCD measurements of MSCs and MSCAs were carried out on an electrochemical workstation (CHI 660E, Chenhua, Shanghai). Impedance spectroscopy measurements were performed at open circuit voltage with ±10 mV amplitude.

Material Characterization:
The micromorphology and phase composition of all samples were characterized by Field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan), Transmission electron microscopy (TEM, JEM-2100, JEOL, Japan), and X-ray powder diffraction (XRD Bruker D8-ADVANCE) with an 18 kW advanced X-ray diffrac-tometer with Cu K α radiation (λ=1.54056 Å). Sheet resistance of vacuum-assisted deposited MXene-based film on paper was measured by a standard four-point probe method (RST-9, Four-Probe Tech.). A Nikon D3100 digital single lens reflex camera was employed to take all the optical pictures. Figure S1. Optical photographs of a mixed solution of colloidal 2D Ti 3 C 2 Tx sheets and 1D BC fibers during continually standing for 24 h. Figure S2. XPS survey spectrum of the pure BC fibers paper, pure MXene (Ti 3 C 2 Tx) paper, and MXene/BC-1.5:1 composite paper. Figure S3. Typical photo graphs and corresponding cross-sectional SEM images of as-prepared MXene/BC composite papers with different mass fraction of the added 1D BC wires, a) and b) for pure MXene paper; c) and d) for MXene/BC-5:1 composite paper; e) and f) for MXene/BC-2.5:1 composite paper; g) and h) for MXene/BC-0.75:1 composite paper. Figure S4. Galvanostatic charge-discharge curves of the as-prepared MXene/BC composite papers with different mass fraction of the added 1D BC wires, a) for pure MXene paper; b) for MXene/BC-5:1 composite paper; c) for MXene/BC-2.5:1 composite paper; d) for MXene/BC-1.5:1 composite paper; e) for MXene/BC-0.75:1 composite paper.