Deposition of MnO2 on KOH-activated laser-produced graphene for a flexible planar micro-supercapacitor

XI Shuang*, GAO Xing-wei CHENG Xi-ming LIU Hui-long*

（1. College of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, China;2. State Key Laboratory of Precision Electronic Manufacturing Technology and Equipment & School of Electromechanical Engineering,Guangdong University of Technology, Guangzhou 510006, China）

Abstract： The rapid development of flexible supercapacitors has been impeded by the difficulty of preparing flexible electrodes.We report the fabrication of a highly flexible and conductive microporous graphene-based substrate obtained by direct laser writing combined with KOH activation, which we call activated laser-produced graphene (a-LPG), which is then decorated with electrochemically deposited MnO2 to form a flexible a-LIG/MnO2 thin-film electrode. This hybrid electrode has a high areal capacitance of 304.61 mF/cm2 at a current density of 1 mA/cm2 in a 1 mol/L Na2SO4 aqueous electrolyte. A flexible asymmetric supercapacitor with a-LIG/MnO2 as the anode, a-LIG as the cathode and PVA/ H3PO4 as a gel electrolyte was assembled, giving an areal energy density of 2.61 μWh/cm2 at a power density of 260.28 μW/cm2 and an ultra-high areal capacitance of 18.82 mF/cm2 at 0.2 mA/cm2, with 90.28% capacitance retained after 5 000 cycles. It also has an excellent electrochemical performance even in the bent state. This work provides an easy and scalable method to design high-performance flexible supercapacitor electrodes and may open a new way for their large-scale fabrication.

Key words: Micro-supercapacitor；Laser processing；Flexible electrode；Porous graphene

1 Introduction

The booming development of wearable electronics has triggered the need for miniature, lightweight and highly flexible electrochemical energy storage devices with great energy and power densities[1-4].Among these devices, micro-supercapacitor (MSC)has been increasingly considered for its advantages of ultrahigh power density[5-6], superior high-frequency response[7], good cycling stability[8]and long life span[9]. To facilitate the application of flexible MSCs,highly flexible and ultrathin current-collectors as the substrates of electrode materials are typically crucial[10-12].

Nowadays, much effort has been paid to fabricating flexible substrates based on metal oxide nanowires, carbon nanotubes and graphene, etc[13].However, among these synthesized substrates, either the as-prepared film is brittle and fragile even with adhesive added, or the synthesis process is too complicated with high cost thus to hinder their scalable applications[14-15]. Thanks to the advent of laser direct writing technology, which can convert polymers into large-area porous graphene films in one-step process,the production of flexible graphene-based electrodes with low cost and high efficiency comes to reality[16-17]. Laser-induced graphene (LIG) has attracted widespread interest in the supercapacitor field since Tour’s group discovered that porous graphene films can be obtained from polyimide (PI) by CO2laser irradiation[18-19]. For the mechanism, the laser irradiation can induce high temperature (＞2 500 °C),which would break the C＝O, C—O and N—C bonds of PI and then facilitate atomic recombination to form graphene[20]. In addition to PI, other materials such as polybenzoxazine resin[21], phenolic resin[22]and polysulfone-class polymers[23]are also employed as carbon precursors for LIG, providing a simple approach to fabricate flexible graphene-based electrodes[24].

However, LIG generally demonstrates hydrophobic nature and macropore-dominated porous structure, which would influence the loading of active materials on its surface and further limit the electrochemical performance of the assembled MSCs[25]. Therefore, it is necessary to introduce nanopores in graphene and also improve the wettability of LIG for high performance MSCs[26]. KOH activation can be regarded as a good choice to make nanopores on carbon-based materials[27].

Herein, laser induction and KOH activation were combined synchronously to fabricate porous graphene on PI in one step, named activated LIG (a-LIG). Followed with electrochemically depositing MnO2nanoparticles on its surface, the obtained a-LIG/MnO2electrode would act as the anode of MSCs. The influence of KOH concentration and deposition duration on the performance of a-LIG/MnO2electrode was investigated. The outcomes show that higher concentration of KOH contributes to the formation of carbon defects and mesoporous structures, effectively improving the hydrophilicity and wettability of a-LIG based electrodes, and thus to improve the ion transferring between electrode and electrolyte. Particularly,with superior wettability, MnO2nanoparticles can be uniformly coated on a-LIG, which further enhances the electrochemical performance of a-LIG/MnO2electrode. Then, planar a-LIG/MnO2@a-LIG MSCs were assembled with a-LIG/MnO2as the anode, a-LIG as the cathode and PVA/H3PO4gel as the electrolyte.Compared with the reported LIG/MnO2electrode[28],the a-LIG/MnO2electrode in this work combined the micro-porous structure of a-LIG and the excellent pseudocapacitive property of MnO2, which would facilitate ion transport during charging/discharging process and thus improve the electrochemical performance. Electrochemical tests indicate that the prepared MSC has favorable cycling stability at 0.2 mA/cm2(90.28% capacitance after 5 000 cycles), improved capacitance (18.82 mF/cm2compared with LIG/MnO2MSC of 15.04 mF/cm2[28]), excellent mechanical flexibility and remarkable modular integration in series and/or parallel.

2 Materials and methods

2.1 Structural design

The choice of the shape and geometry of the forked-finger structure electrode will directly affect the interaction between the electrode and the ion as well as the electron transport. The fork-finger structure electrode and its geometrical parameters are optimally designed as shown in Fig. S1, where the key geometrical parameters mainly include the number of fork-finger electrode pairs, fork-finger electrode width, fork-finger electrode gap distance, effective length and width of the electrode. The larger the forkfinger electrode aspect ratio and the smaller the effective width per unit area, the better it is for electron transfer, thus enabling the planar micro supercapacitor to achieve faster high frequency response[6]. Forked finger electrodes with more pairs of electrodes and larger aspect ratio are suitable for good conductivity,excellent cycling stability and double layer capacitance. The different spacing of the electrodes can affect the uneven distribution of ions when storing/releasing charge at the electrode-electrolyte interface[27].The electrode thicknesses also have effect on the electrochemical properties, while the electrode thickness here mainly depends on the mass loading of MnO2deposited on a-LIG. Although thicker MnO2film could provide sufficient active materials, too thick film may limit its electrochemical performance due to that the inner part of MnO2couldn’t participate in the electrochemical charge storage process, which would increase the contact resistance and decrease the effective surface area for the reversible redox reaction and specific capacitance[29]. Nanoscopically thin MnO2film could offer higher gravimetric capacitance and better utilization of the MnO2film, and the diffusion distances for the solid-state transport of insertion cations can be reduced.

2.2 Experimental materials

Potassium hydroxide (KOH), crystalline sodium sulfate (Na2SO4·10H2O), manganese sulfate monohydrate (MnSO4·H2O), and polyvinyl alcohol (PVA)were received from Sinopharm Chemical Reagent(Shanghai, China). The PI film was supplied by Xuchang Erik Insulation Products Co. Conductive silver paste was supplied by Shenzhen Xinwei New Material Co. Absolute ethanol and concentrated H3PO4was used as received.

2.3 Preparation of a-LIG

The PI films were flattened against the glass surface and then rinsed with absolute ethanol and deionized water. Then, the PI films were coated with different concentrations of KOH solutions (280, 400, 560,and 1 120 g/L-supersaturated) followed by baking at 70 °C for 3 min. Next, the samples were placed in the processing area of the CO2laser engraver(CMA4030B, China) and scanned by overlapping light spots to synthesize a-LIG. The laser scanning speed, laser power, and processing resolution were optimized and fixed to 100 mm/s, ～7.12 W and 1 000 dpi, respectively, taking into account the processing efficiency and activation effect. With the pre-designed pattern, interdigital a-LIG can be obtained(Fig. 1a), in which one part will act as cathode of MSC and the other will be used as the substrate of the anode for the following deposition. Finally, the processed samples were soaked in deionized water for several times to remove the residual KOH and dried in a 40 °C oven (DZF-6020A, Shanghai, China). For comparison, laser-induced graphene (LIG) without KOH activation was fabricated via PI film directly irradiated by CO2laser engraver.

2.4 Preparation of a-LIG/ MnO2 anode

To electrodeposit MnO2nanoparticles on a-LIG,MnSO4·H2O (0.6 g) and Na2SO4·10H2O (0.71 g) were firstly mixed in deionized water until they were completely dissolved to act as depositing electrolyte.Secondly, 3-electrode system electrochemical deposition was conducted using an electrochemical workstation (CHI660E, Shanghai, China), with the as-prepared a-LIG as the working electrode, platinum sheet as the counter electrode and silver chloride as the reference electrode. By adjusting the depositing parameters, i.e., galvanostatic mode with current density of 30 mA/cm2, MnO2nanoparticles can be uniformly deposited on the a-LIG substrate.

2.5 MSC device assembly

Fig. 1 Schematic diagram of the fabrication process of flexible planar a-LIG/MnO2@a-LIG MSC on PI precursor

Flexible copper foil and viscous conductive silver paste were applied on the common area of the interdigital electrode for electrical conduction. The conductive silver paste was cured by heating at ～80 °C for 0.5 h. The PI tape was then covered on the copper foil to protect the copper from being corroded by acid electrolyte. The procedure for preparing the PVA/H3PO4gel electrolyte was to first put PVA (1.5 g) into ionized water (25 mL) and then heat the mixed solution to 90 °C and stir for 2-3 h. Subsequently, 2 mL H3PO4was slowly dropped into the PVA solution and stirred for 10 min to form a transparent gel solution. The translucent gel electrolyte of PVA/H3PO4was obtained by standing at room temperature environment. Then, the prepared gel electrolyte was uniformly covered on the inserted finger electrode and stewed for 6 h to completely encapsulate and penetrate into the patterned electrode. Finally, a thinner PI film (～50 μm) was used to encapsulate the copper foil and electrolyte to prevent moisture volatilization.The planar a-LIG/MnO2@a-LIG MSC was successfully fabricated using a-LIG/MnO2as the anode, a-LIG as the cathode and PVA/H3PO4as the gel electrolyte. The areal size of MSC device is 25 mm×25 mm.

2.6 Characterizations

Raman spectroscopy (DXR532, United States),X-ray diffraction (XRD, Ultima IV, Japan) pattern and X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD, United Kingdom) were utilized to analyze the chemical structure and composition of LIG, a-LIG and a-LIG/MnO2. The morphology and microstructures were explored by scanning electron microscopy(SEM, Quanta 200, FEI, USA), transmission electron microscopy (TEM, JEM-1400, Japan), and high-resolution transmission electron microscopy (HRTEM,JEM-2100 UHR, Japan). Electrochemical properties of the as-prepared electrode was explored by cyclic voltammetry (CV) curve and galvanostatic charge-discharge (GCD) profile with a 3-electrode system in 1 mol/L Na2SO4electrolyte. The CV, GCD and electrochemical impedance spectroscopy (EIS) tests of the assembled a-LIG/MnO2@a-LIG symmetric MSC devices were conducted with a standard two-electrode configuration. All the electrochemical tests were performed using an electrochemical workstation(CHI660E, Chenhua Instruments, Inc., Shanghai,China).

3 Results and discussion

3.1 Structure and morphology analysis

Raman spectra were used to characterize the chemical structure of LIG and a-LIG (Fig. 2a), in which 3 distinctive peaks can be observed located at～1 311, ～1 602 and ～2 658 cm-1, which corresponds toD,GandG' peak of graphene,respectively[30]. TheDpeak is used to measure the disorder degree of the material, which is caused by defects or distortion of the sp2carbon network in the aromatic ring that makes the carbon atoms undergo symmetric stretching vibrations[31]. TheGpeak is caused by the stretching vibrations between the sp2carbon atoms[32]. TheG' peak is induced by second order zone-boundary phonons correlated to graphene layers[33]. As the number of graphene layers increases,the intensity of theG' peak decreases, with its shape turning broad, short, and asymmetric. The intensity of theG' band relative to theGband (ID/IG) can reflect the defect density in obtained graphene[34]. TheID/IGvalue of a-LIG is calculated to be 1.56, which is much lower than that of the LIG (1.82), indicating the higher crystalline quality of graphene after KOH activation. Compared with a-LIG, the Raman spectrum of a-LIG/MnO2(Fig. 2a) demonstrates a new strong peak at 638 cm-1, which corresponds to the symmetric stretching vibration of Mn-O, verifying the formation MnO2composite has been synthesized.

In Fig. 3a, the XRD pattern of a-LIG shows 2 characteristic diffraction peaks at around 25.8° and 44.7°, corresponding to the (002) and (100) crystallographic planes of graphitic carbon, respectively. The layer spacinglof LIG can be obtained by Bragg formulanλ= 2lsinθ(λ= 1.54 Å is the wavelength of Xray,θ= 12.9° is the incident contact angle,n=1 for the first-order diffraction), and the value was evaluated to be 0.34 nm, indicating the high graphitization degree of LIG. The a-LIG/MnO2hybrid presents weaker graphene diffraction peaks compared to a-LIG, which may be ascribed to the overlaying MnO2on a-LIG.Furthermore, the characteristic peaks at ～29.7°, 36.7°and 66.1° of a-LIG/MnO2can be attributed to the(310), (211) and (002) planes of MnO2(JCPDS: 44-0141), comfirming the existence of MnO2, which is consistent with the results of Raman spectra[35-36].

Fig. 2 (a) Raman spectra of LIG, a-LIG and a-LIG/MnO2. (b) XRD patterns of a-LIG and a-LIG/MnO2. (c) XPS survey, (d) C 1s spectra, (e) N 1s spectra and(f) Mn 2p spectra of a-LIG/MnO2

The XPS survey of a-LIG/MnO2in Fig. 2c demonstrates the typical spectra with C 1s peak at 284.42 eV, O 1s peak at 529.85 eV and Mn 2p peak at 642.18 eV. The C 1s spectra (Fig. 2d) can be devided into 3 obvious peaks located at 284.7, 286.3 and 288.6 eV, referring to C—C, C—O—C, O—C＝O, respectively[27,37-38]. It’s obvious that chemical structure of a-LIG/MnO2are dominated by sp2C—C, in accordance with the Raman and XRD results. The O 1s spectra can be deconvoluted into 3 subpeaks: Mn—O—Mn(530.1 eV), Mn—O—H (531.7 eV), H—O—H (533.0 eV), verifying the successful incorporation of manganese oxide. The obtained Mn 2p spectrum (Fig. 2f)consists of 2 main peaks, i.e., the Mn 2p3/2at a 641.8 eV and the Mn 2p1/2at 653.4 eV, which indicate the sole existence of Mn4+, and thus confirm the oxide nanocrystals are MnO2.

The typical SEM image of LIG and a-LIG(Fig. 3a-e) exhibits the 3D flake foam-like structure of laser-induced graphene. The LIG sample presents macropore-dominated structure. While after activated by KOH, massive meso-, macropores produced on its wall, which allows a-LIG to have a sizeable ion-accessible surface area and abundant ion transport paths[39-40], and thus to effectively improve the capacitance and multiplicity performance of graphene-based electrodes[41]. SEM images of a-LIG prepared at different KOH concentrations (Fig. S2) reveal that as KOH concentration increase, the quantity of pores on a-LIG gradually increases along with the reduced sizes. After MnO2deposition, the a-LIG skeleton is fully and evenly wrapped by MnO2nanoparticles, as shown in Fig. 3c and f. The MnO2nanoparticles loosely aggregate on the surface of activated graphene, which is beneficial for the ions to penetrate into the active materials from the electrolyte and then fully participate in the reactions at their interfaces.Moreover, contributed from the 3D network structure of a-LIG combined with the abundant spherical nanoparticles, the a-LIG/MnO2composite possesses increased accessible surface area, which would further facilitate the electrochemical performance. With excellent chemical and mechanical properties, superior ion/electron transport ability[42-43], graphene-based electrode materials are considered ideal for applications in energy storage. Here, MnO2nanoparticles deposited on graphene-based materials can be directly used as flexible self-supporting electrode avoiding the use of adhesives and conductive agents, thus to simplify the fabricating process and improve the participation of active materials.

The interior and detailed structure of LIG, a-LIG and a-LIG/MnO2was further investigated using representative TEM images (Fig. 4). The typical TEM image of LIG in Fig. 4a shows that LIG is lamellar and similar to the thin silk towel. After activated by KOH, the porosity of graphene is obviously enhanced in Fig. 4b, in which plenty of pores appear on the surface of a-LIG, which is consistent with the SEM results. The TEM image of a-LIG/MnO2in Fig. 4c present evenly distributed MnO2nanoparticles embedded in a-LIG flake with a normal distribution size,while the size was estimated to be ～40 nm in average based on TEM results in Fig. 4d. This hierarchical porous network with porous characteristic facilitates the penetration and diffusion of electrolyte as well as transportation of electrons. From the HRTEM images of LIG, we can observe a few layers of graphene with a ripple-like polycrystalline structure at the edges (Fig. 4e) due to laser-induced thermal expansion. The interlayer spacing of 0.34 nm corresponds to the (002) plane of graphene, which is consistent with the interlayer spacing (i.e., 0.34 nm) obtained by the Bragg formula from the XRD pattern.Fig. 4f shows the image of MnO2at high magnification. MnO2is tightly anchored on the LIG substrate and the lattice is not as obvious as that of LIG because of the low crystallinity of MnO2.

Fig. 4 TEM images of (a) LIG, (b) a-LIG and (c) a-LIG/MnO2. (d-f) HRTEM images of a-LIG/MnO2

3.2 The influence of KOH concentrations on the performance of a-LIG/MnO2

To investigate the effect of KOH concentrations on the performance of a-LIG/MnO2, several samples of a-LIG activated from different KOH concentrations (denoted asX) were synthesized, whereXis identified to be 0, 280, 400, 560 and 1 120 g/L (X=0 correspondes to LIG without KOH activation). With the following deposition of MnO2, a-LIG-X/MnO2electrodes were obtained and their electrochemical properties are tested as illustrated in Fig. 5. Fig. 5a depictes the comparative CV plots of a-LIG-X/MnO2electrodes at a scan rate of 20 mV/s. There is no significant redox peak, indicating the ideal capacitive behavior[43]. Specifically, the enclosed space of CV curves increases with the concentration of KOH concentration, revealing that the higher concentration KOH can induce more nanopores on the surface of graphene and thus to increase the surface area to load active materials. However, with the increment of KOH activation concentration, the CV curve tends to deform from rectangular shape, which might be due to the gradual decrease of electrode conductivity. CV curves of a-LIG-X/MnO2at different scanning rates are shown in Fig. S3. It can be found that at low scanning rate, a-LIG-X/MnO2exhibits a deformation of its shape as the scan rate increases, which is arisen from the kinetically limited pseudocapacitive reactions of MnO2films.

Fig. 5b shows that all the GCD curves of a-LIGX/MnO2exhibit a nearly triangular-shape, verifying the good capacitive characteristics at low current density[40]. The small voltage drop (IR drop) at the top of the discharge curve is caused by the internal resistance of composite electrodes with a high MnO2mass loading. Apparently, the a-LIG-1120/MnO2electrode has the highest capacitance, which is consistent with the CV results. Fig. S4 further provides the GCD curves of diverse a-LIG-X/MnO2electrodes at different current densities. These GCD curves always keep symmetrical shapes, indicating the satisfying rate capability of a-LIG-X/MnO2electrodes. The specific areal capacitances of a-LIG-X/MnO2varied with current density were calculated (according to Eqn. S1)and plotted in Fig. 5c. The highest capacitance of 556.2 mF/cm2occurs at the KOH concentration of 1 120 g/L (supersaturated solution), claiming the excellent charge storage performance of a-LIG/MnO2-1120.

3.3 The effect of MnO2 depositing durations on the performance of a-LIG/MnO2

The effect of MnO2depositing durations (denoted as Y) on the performance of composite electrodes has been further explored. Fig. 6a demonstrates the CV curves of a-LIG/MnO2-Y at different deposition durations (150, 300, 600 and 1 000 s) at a scanning rate of 20 mV/s. In Fig. S5, the CV exhibits semi-rectangular shape for short deposition time and low sweep rate, while the CV shapes deform with longer deposition time, indicating the incomplete pseudocapacitive reaction with high MnO2mass loading[44]. The a-LIG/MnO2-300 electrode shows the largest curve area compared with others, revealing that the deposition time of 300 s could be the optimized depositing time for high-performance MSC.

Fig. 5 Electrochemical performance of a-LIG-X/MnO2 electrode treated with different KOH concentrations. (a) CV curves at scan rate of 20 mV/s. (b) GCD curves at current density of 1 mA/cm2. (c) Specific areal capacitance in the current density range of 1 to 5 mA/cm2

Fig. 6 Electrochemical performance of a-LIG/MnO2-Y electrode treated with different KOH concentrations. (a) CV curves at scan rate of 20 mV/s. (b) GCD curves at current density of 1 mA/cm2. (c) Specific areal capacitance in the current density range of 1 to 5 mA/cm2

The GCD curves of a-LIG/MnO2-Y electrodes(Fig. 6b and S6) demonstrate near triangular shapes for different deposition times at current densities from 1 to 5 mA/cm2, indicating their good rate capability.As the deposition time continues to increase, IR drop also increases, illustrating the declined conductivity along with MnO2accumulating. The dependence of specific capacitance of a-LIG/MnO2-Y electrode on depositing durations is shown in Fig. 6c. The a-LIG/MnO2-300 shows the best performance of 371.11 mF/cm2among all samples at the current density of 1 mA/ cm2. As the deposition time further extended, the mass loading of MnO2increases, and then the capacitance of the electrode decreases, which could be due to the agglomeration of excess MnO2.

3.4 Electrochemical performance of a-LIG/MnO2@a-LIG MSC

As mentioned above, a-LIG/MnO2electrode-activated 1 120 g/L KOH solution with the depositing duration of 300 s has the best electrochemical performance. Thus, it was used as anode, along with LIG as the cathode and PVA/H3PO4as the gel electrolyte to assemble a-LIG/MnO2@a-LIG MSC. The CV curves of the prepared MSC were tested at scan rates of 10, 20, 50 and 100 mV/s as shown in Fig. 7a, and their rectangular shapes indicate that the capacitance is mainly attributed to the electric double-layer capacitance (EDLC) behaviour produced by graphenebased electrode. Whereas, small humps and sluggish tails occur at the corners of CV enclosed curves, implying the pseudocapacitive reaction exists besides EDLC behaviour. Moreover, there is no significant distortion in the curve when the sweep frequency is varied from 10 to 100 mV/s, indicating that the assembled device has fast charging/discharging capability and good cycling stability.

Meanwhile, the GCD curves of the MSC devices have been tested (Fig. 7b), which show an approximately triangular shape at current densities of 0.1-0.5 mA/cm2, impelling the good reversibility. The specific areal capacitances of MSC devices varied with current density (0.1, 0.2, 0.3, 0.4, 0.5 mA/cm2)are calculated according to Equation S1 to be 24.83,18.82, 9.77, 8.16 and 5.89 mF/cm2, respectively. Also,the GCD curves demonstrate curving upward and relatively straight downward, verifying the conjoint result of EDLC and pseudocapacitance.

To further evaluate the influence of KOH activation on the a-LIG/MnO2@a-LIG MSC device, EIS measurements were performed as shown in Fig. 7c. In the high frequency region, the equivalent series resistance of the KOH-treated MSC (34.15 Ω) was relatively lower than that of the KOH-free MSC (46.61 Ω),indicating a lower interfacial resistance between the electrode and the electrolyte. In addition, KOH-treated MSC exhibits a lower charge transfer resistance of 5.3 Ω which corresponds to the semicircle diameter, indicating the higher rate capability. The cycle life of the a-LIG/MnO2@a-LIG MSC was also evaluated by extended constant GCD cycles as shown in Fig. 7d. It can be observed that after 5 000 charge/discharge cycles at 0.2 mA/cm2, the a-LIG/MnO2@a-LIG MSC showed 90.28% capacitance retention and 86.38%Coulombic efficiency. The capacitance fading upon cycling might be due to the different charge storage mechanism between graphene-based material (electrostatic charge accumulated at the interface of electrolyte and the charged electrodes) and MnO2film (the surface and bulk faradic reaction). The difference will lead to a certain degree of volume change in the asfabricated hybrid electrode during cycling, which was reported to cause mechanical degradation and thus resulted in capacitance fading[45-46]. The cycling stability could be further improved through optimizing the fabricating process of the hybrid electrode, which would be our future work.

Fig. 7 Electrochemical properties of a-LIG/MnO2@a-LIG MSC. (a) CV curves at a scan rate of 10-100 mV/s. (b) GCD curves at current density range of 0.1-0.5 mA/cm2. (c) Nyquist plots of a-LIG/MnO2@a-LIG MSC with/without KOH treatment, the inset shows the enlarged area.(d) Cycling stability at 0.2 mA/cm2 current density

Multiple MSCs of a-LIG/MnO2@a-LIG have been expanded and assembled in series or parallel configurations (Fig. S7) to validate the MSCs in realworld applications. At the same current density, 3 MSCs connected in series exhibit a nearly 3-fold voltage window with similar discharge times compared to a single MSC. The discharge time of 3 MSCs connected in parallel also increases to 3 times that of a single MSC.

The flexibility and stability of the a-LIG/MnO2@a-LIG MSCs is verified by testing their electrochemical performance in the bending state. Fig.8b and c show the CV and GCD of MSC at different bending degrees from 0° to 180° at 20 mV/s and 0.2 mA/cm2, respectively. It can be demonstrated that the CV and GCD curves of the proposed a-LIG/MnO2@a-LIG MSC are almost identical in shape with negligible change significantly at different bending states. Moreover, the specific capacity retention of the prefabricated a-LIG/MnO2@a-LIG MSC is close to 100% at different bending angles from 0°-180°.These remarkable results confirm the excellent flexibility stability of our proposed a-LIG/MnO2@a-LIG MSC. Fig. 8d shows the Ragone plot, which compares the energy/power performance of a-LIG/MnO2@a-LIG MSC with other reported LIGbased supercapacitors. As can be seen, a-LIG/MnO2@a-LIG MSC exhibits satisfying energy density of 2.61 μWh/cm2at a power density of 260.28 μW/cm2, which is higher than the nitrogen-phosphorus co-doped LIG[44], MoS2decorated LIG[4], multimodality treated LIG[16,45], boron-doped LIG[46]and untreated LIG[28].

Fig. 8 Flexibility test of a-LIG/MnO2@a-LIG MSC. (a) Bending photograph of the device. The angle marked as θ in the image is defined as the bending angle, (b) CV curves for different degrees of bending from 0°-180°, (c) GCD curves for different degrees of bending from 0°-180°, (d) Ragone plots of a-LIG/MnO2@a-LIG MSC and MSC devices with LIG-based electrodes of various electrolytes. Data are reproduced from ref. (N-LIG-SC), ref. (MoS2-LIG),(S-LIG-SC), ref. (LIG-O2), ref.(B-LIG), and ref. (LIG)

4 Conclusions

In summary, a simple method for fabricating all solid-state flexible MSCs with interdigital finger electrodes using hybrid a-LIG composites has been demonstrated, which is realized by the activated laser direct writing technique combined with electrochemical deposition. Optimized parameters were obtained for the activation and deposition process to further enhance the electrochemical performance of assembled MSCs, showing high specific capacitance (18.82 mF/cm2), high energy density (2.61 μWh/cm2) and high power density (260.28 μW/cm2), as well as good cycling stability and mechanical flexibility. The LIG technique involved here can not only simplify the device fabrication process, but also easily control the size and scalability of the devices, providing a lowcost and effective method to fabricate flexible energy storage devices with high precision and repeatability.

Data availability statement

The data that support the findings of this study are openly available in Science Data Bank at https://www.doi.org/10.57760/sciencedb.j00125.00053 or https://resolve.pid21.cn/31253.11.sciencedb.j00125.00 053.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Guangdong Province,China (2022A1515011334) and National Natural Science Foundation of China (52205457).