LI Xi-juan, LIU Guo, WU Qing-feng, WANG Xu-kun, SUI Xin-yi,WANG Xin-ge, FAN Zi-ye, XIE Er-qing, ZHANG Zhen-xing

（Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, School of Physical Science and Technology,Lanzhou University, Lanzhou 730000, China）

Abstract： Following the fast growth of micro-energy storage devices, there is an urgent need to develop miniaturized electronic devices with excellent performance that are both green and safe. Planar interdigitated rechargeable Zn microbatteries (MBs) have gained widespread attention in recent years due to their ease of series-parallel integration, mechanical flexibility and no need for traditional separators. We prepared a patterned cathode of NiCo layered double hydroxide (LDH)@indium tin oxide (ITO) nanowires(NWs) @carbon cloth (CC) by the chemical vapor deposition of ITO NWs on the carbon fibers in a CC, laser patterning, and finally the electrodeposition of NiCo-LDH to coat the ITO NW@carbon fibers. The cathode was combined with a patterned Zn foil anode to form a planar MB. Because of the highly conductive ITO NWs@CC current collector, the interdigitated MB had a satisfactory performance. The planar MB has a high specific capacity of 453.5 mAh g-1 (corresponding to 0.56 mAh cm-2) in an alkaline water-based electrolyte at 1 mA cm-2. After 4 000 cycles the capacity increased to 216% of the initial value due to gradual penetration of electrolyte into the three-dimensional NiCo-LDH@ITO NW@CC network. It also had excellent energy (798.4 μWh cm-2, corresponding to 649.9 Wh kg-1) and power densities (4.1 mW cm-2, corresponding to 3 282.7 mW kg-1). Furthermore, MBs connected in series-parallel in lighting tests illustrate the excellent performance of the device. Therefore, these fast and simple Zn MBs with an in-plane interdigital structure provide a reference for next-generation high-performance, environmentally-friendly, and scalable planar micro-energy storage systems.

Key words: Micro-energy storage devices；Micro batteries；Laser etching；ITO NWs；Carbon cloth

1 Introduction

Micro-energy storage devices mainly include micro-supercapacitors (MSCs)[1-4]and micro-batteries(MBs)[5-7], which have become the focus of research due to their great application potential in smart electronics[8-10], wireless sensor nodes[11], medical implantation and drug delivery devices[12], health detection and diagnosis[13], environmental monitoring[14,15],etc.However, compared with MSCs[16]and micro lithium ion batteries (LIBs)[17,18], micro zinc ion batteries (ZIBs)are seen as the best candidates for next generation micro-energy storage devices due to their outstanding theoretical specific capacity (820 mAh g-1), high safety,-inexpensive, aqueous electrolytes, environmentally friendliness[10,19-21]. In addition, planar interdigitated rechargeable Zn MBs (ZMBs) have received extensive attention due to their scalability, high safety and no separators of conventional batteries[5,20,21]. Unlike conventional sandwich-shaped cells, planar interdigitated MBs rarely show cell short-circuiting due to less zinc dendrite growth. Very recently, Wang et al. reported a screen printed aqueous-based Zn//MnO2planar interdigitated MBs, which delivered a high specific capacity of 393 mAh g-1[22]. Li et al. reported that the flexible planar Zn//PANI MBs delivered a high energy density of 0.25 mWh cm-2[23]. Moreover, vanadium-based[24,25]and manganese-based materials[5],due to their poor electronic conductivity, structural deformation and poor stability, are often combined with graphene, carbon nanotubes, etc. as cathode materials for batteries.

Herein, three-dimensional indium tin oxide nanowires (ITO NWs) were first grown on carbon cloth (CC) by chemical vapor deposition (CVD), and the obtained ITO NWs@CC served as the substrate for growing NiCo-LDH (NiCo-LDH@ITO NWs@CC). With the help of laser cutting technology, NiCo-LDH@ITO NWs@CC and Zn foils were patterned into interdigitated cathode and anode and assembled into ZMBs. The three-dimensional ITO NWs@CC conductive network can increase the mass loading of active materials, facilitate fast charge transfer, and tolerate structure changes in cycling, leading to improved the energy storage performance of ZMBs. The prototype aqueous Zn//NiCo-LDH@ITO NWs@CC MB delivers an areal capacity of 0.56 mAh cm-2(correspond to 453.5 mAh g-1) at 1 mA cm-2, satisfactory rate performance (63.8% capacity retention at 3 mA cm-2), and excellent cycling performance(216% capacity retention after 4 000 cycles at 5 mA cm-2). The maximum energy density of 798.4 μWh cm-2(～ 649.9 Wh kg-1) and the maximum power density of 4.1 mW cm-2(～ 3 282.7mW kg-1) can be obtained. Furthermore, the seriesparallel lighting tests of ZMBs demonstrate the good device consistency and integrability.

2 Experimental

2.1 Preparation of three-dimensional ITO NWs

ITO NWs were grown on CC by CVD. Firstly,CC was ultrasonically cleaned with acetone, hydrochloric acid and deionized water each for 10 min, and dried at 65 °C for 10 min. Secondly, Au nanoparticles were deposited on CC as catalyst by ion sputtering(SBC-12) for ITO NWs growth. Typically, the sputtering current was 10 mA for 20 s. Thirdly, Au-coated CC was placed 5 cm downstream of the heating point in the quartz tube, In and Sn metal sources were mixed with a mass ration of 24∶5 and placed at the tube furnace center. Fourthly, the furnace was ramped up to 835 °C at 20 °C min-1with Ar gas (4 mL min-1)atmosphere, and O2gas was introduced for 30 min. Finally, the tube furnace was turned off and cooled to room temperature naturally, and the three-dimensional ITO NWs on CC were obtained (ITO NWs@CC).

2.2 Preparation of NiCo-LDH@ITO NWs@CC cathode

The obtained ITO NWs@CC was laser patterned into interdigitated electrodes. Then the interdigitated ITO NWs@CC electrode with an area of about 1.75 cm2, Pt sheet, and Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The NiCo-LDH electrodeposition on ITO NWs@CC was performed at a constant voltage of -1 V for 200 s in the mixed solution consisting of 0.05 mol L-1Ni(NO3)2·6H2O,0.05 mol L-1Co(NO3)2·6H2O, 0.05 mol L-1C4H6NiO4·4H2O, 0.05 mol L-1C4H6CoO4·4H2O in 40 mL deionized water at room temperature. Subsequently, the obtained NiCo-LDH@ITO NWs@CC electrodes were kept at 65 °C for 2 h after being rinsed for several times (NiCo-LDH mass loading: ～1.23 mg cm-2).The NiCo-LDH on CC (NiCo-LDH@CC) was prepared by the same conditions for control experimental(NiCo-LDH mass loading: ～1.11 mg cm-2).

2.3 Preparation of Zn anode

Commercial zinc foil was laser patterned into interdigitated electrodes with a single electrode area of 1.75 cm2. The parameters of laser etching in N2environment are as follows: a cutting speed of 200 mM s-1,a power ratio of 80%, a frequency of 40 kHz for the first cut, and a cooling for 1 min, then a cutting speed of 50 mM s-1, a power ratio of 70%, and a frequency of 40 kHz for the second cut.

2.4 Preparation of micro Zn batteries

The NiCo-LDH@ITO NWs@CC interdigitated electrode as cathode, the interdigitated zinc foil as anode, the glass slide as the support, and AB glue as the binder, were assembled into ZMBs. Similarly, the Zn//NiCo-LDH@CC ZMBs were also as-sembled.Meanwhile, the AB glue is alkaline resistant at room temperature. Therefore, the long-term cycling tests of the ZMBs in an alkaline environment do not cause the cathode and anode to fall.

2.5 Characterization and electrochemical tests

The surface morphologies were characterized by field emission electron microscopy (FESEM, Hitachi,S4800, Japan) and high-resolution electron microscopy (HR-TEM, FEI, Techni G2 F30, operated at 300 kV). The elemental compositions were determined by X-ray photoelectron spectroscopy (XPS,PHI5702, MgKα X-ray, 1 253.6 eV) and EDS (HRTEM, FEI, Techni G2 F30, operated at 300 kV). The crystal structure and phases were analyzed by X-ray diffraction (XRD, Philips X'Pert pro, CuKα, 0.154 06 nm) and Raman micro spectrometer (JY-HR800,532 nm YAG laser). The cyclic voltammetry (CV)was performed using an electrochemical workstation(CHI 660E), the galvanostatic charge-discharge(GCD) using a blue cell test system in 2 mol L-1KOH mixed 0.2 mol L-1C4H6O4Zn·2H2O aqueous electrolyte at 25 °C. The discharge curve was used to calculate the specific capacity (mass specific capacity and areal capacity) of the battery, according to the equation ofC=(I×△t) /X(X=morA), whereCis the specific or areal capacity (mAh g-1or mAh cm-2),Iis the discharge current (mA), △t is the discharge time (h),Xrepresents the mass or area,mandAare the mass(g) and area (cm2) of the active material in the cathode of the battery, respectively.

3 Results and discussion

The brief synthesis process and device assembly steps of the planar interdigitated Zn//NiCo-LDH@ITO NWs@CC MBs are shown in Fig. 1.Firstly, indium tin oxide nanowires (ITO NWs) are grown on carbon cloth (CC) by CVD. Secondly, two independent interdigitated finger ITO NWs@CC electrodes and Zn finger electrodes from Zn foil are laser patterned. Thirdly, NiCo-LDH nanosheets are electrodeposited on ITO NWs@CC electrode. Finally,planar interdigitated Zn and NiCo-LDH@ITO NWs@CC electrodes, with the help of a binder, conductive gel and CC wire, are assembled into ZMBs on glass slides. The dimensional diagram of ZMBs is shown in Fig. S1.

The XRD pattern of the ITO NWs@CC is shown in Fig. 2a. ITO NWs@CC exhibits five main characteristic peaks at 21.5°, 30.6°, 35.5°, 51.1° and 57.6°,corresponding to the (211), (222), (400), (440) and(620) crystal planes of cubic In2O3(JCPDS#06-0416)[26], respectively. Thus, ITO NWs with a high crystallinity are successfully grown on CC. Comparatively, the XRD pattern of the NiCo-LDH@ITO NWs@CC in Fig. 2b exhibits the (001) plane of Co(OH)2at 9.5° (JCPDS#51-1731)[27]. The peaks at 33.5° and 38.8° belong to (101) and (015) planes of α-Ni(OH)2·0.75H2O (JCPDS#38-0715)[28], respectively.Similarly, typical peaks of Co(OH)2and α-Ni(OH)2·0.75H2O also appear in the XRD pattern of NiCo-LDH@CC (Fig. S2). The weak and broad peaks indicate a lower crystallinity of NiCo-LDH. As shown in the XPS survey spectra of Fig. 2c, the ITO NWs@CC presents In, Sn and O elemental peaks besides C1s peak from carbon cloth, and the corresponding fine spectra are shown in Fig. S3a-d. NiCo-LDH@ITO NWs@CC presents obvious Ni and Co elements peaks in Fig. 2d. The Ni2p XPS fine spectrum in Fig. 2e can be well fitted into 4 peaks at 855.85, 857.8, 873.15 and 875.10 eV with satellite peaks, consistent well with Ni 2p3/2and Ni2p1/2levels,respectively[29]. The Co2p XPS fine spectrum in Fig. 2f can be fitted to Co3+at 781.13 and 796.73 eV,and Co2+at 783.7 and 798.69 eV[30]. The O1s XPS fine spectrum shows 2 oxygen contributions (Fig. S4).The main peak at 531.22 eV is the oxygen in the OHgroup, and the tiny peak at 532.22 eV is related to the oxygen in the absorbed water or the surface absorbed oxygen[31]. These XRD and XPS results indicate that a low crystallinity NiCo-LDH is successfully synthesized on ITO NWs@CC.

Surface morphologies and microstructures are shown in Fig. 3. CC has a very uniform surface(Fig. 3a), and ITO nanowires are densely and uniformly grown on the surface of CC (Fig. 3b) with a diameter of about 350±10 nm (Fig. S5). The growth of ITO NWs on the Au nanoparticle-loaded CC belongs to the “bottom-up” process. The vaporized metal source dissolves into the Au particles with the gas flow, and forms Au/In-Sn alloy droplets as the temperature increase[32]. When the droplets reach saturation, nucleation occurs, and then the nanowires begin to grow. Therefore, the final product deposited on the carbon cloth is Au-tipped ITO NWs[33]. Overall, the Au catalyst determines the lateral size and density of the nanowires[34]. The as-prepared NiCo-LDH@ITO NWs@CC composite has an interconnected nanosheet structure (Fig. 3c). NiCo-LDH nanosheets are uniformly riveted on the ITO NWs surface with a mean thickness of approximately 300±10 nm (Fig. S5). The three-dimensional conductive network of ITO NWs@CC facilitates the overall uniform growth of NiCo-LDH active material (Fig. S6). TEM images further confirm the entanglement of NiCo-LDH nanosheets (Fig. 3d-e). The corresponding HRTEM image in Fig. 3f has a clear lattice pitch of 0.22 nm,fitting well with the (015) plane of Ni(OH)2·0.75H2O.However, a small amount of amorphous structure appears, verifying the poor crystallinity of NiCo-LDH with XRD results. The elemental mapping image clearly shows that Ni, Co and O elements are uniformly distributed on the NiCo-LDH@ITO NWs@CC composite (Fig. 3g-j). Similarly, the overall microscopic morphologies of NiCo-LDH@CC exhibit interconnected thin nanosheets, and flower-like growth occurs due to the uneven distribution of the local electric field. As shown from the i-t curves of Fig. S7, NiCo-LDH tends to grow more steadily on ITO NWs@CC than CC after the nucleation process.Consequently, the NiCo-LDH@CC has obvious cracks (Fig. S8a-d) compared with NiCo-LDH@ITO NWs@CC, which may be ascribed to the affected growth of NiCo-LDH by the non-uniform electric force. Therefore, ITO NWs on CC can harmonize the electric field and create more space for the uniform growth of NiCo-LDH nanosheets.

The electrochemical behavior of ZMBs explored by cyclic voltammetry (CV) indicates that the CV curve is not significantly distorted as the scan rate increases from 1 to 10 mV s-1, as shown in Fig. 4a. The observed redox peaks can be attributed to the following reversible reactions:

According to the equation logi= loga+blogυ,υis the scan rate,iis the highest current at a certain scan speed,aandbbelong to adaptable factors[35]. The current is entirely contributed by diffusion control only when the value ofbis 0.5. Whenbis 1, the current is contributed by capacitive behavior. Whenbtakes a value between 0.5 and 1, the current contribution consists of the two above parts. The calculatedbof the oxidation peak is 0.54, indicating that the current contribution is dominated by diffusion control, as shown in Fig. 4b. In addition, in the light of the equationi(at fixed voltage) =k1υ + k2υ1/2,k1υrepresents the capacitive current andk2υ1/2is the diffusion current[36]. The calculated proportions of capacitive behavior and diffusion control are shown in Fig. 4c.Clearly, diffusion control dominates at low scan rates,while capacitive behavior dominates as the scan rate increases. The phenomenon indicate that the electrolyte ions have enough time to enter the spaces of the active material to cause redox reactions at low scan rates, while at high scan rates, rapid adsorption and desorption occur on the electrode surface.

The GCD curves of ZMBs at various current densitiesare shown in Fig. 4d, and a maximum capacity of 453.5 mAh g-1can be obtained (～0.56 mAh cm-2,Fig. S9a). The device exhibits outstanding rate performance, as shown in Fig. 4e, and the capacity is 508.6, 418.0, 367.6, 302.5 and 194.0 mAh g-1(corresponding to areal capacity of 0.63, 0.51, 0.45, 0.37 and 0.24 mAh cm-2, respectively (Fig. S9b)) at 1, 1.5, 2,2.5 and 3 mA cm-2, respectively. Surprisingly, when the current density returns to the minimum value of 1mA cm-2, the capacity surpasses its initial value, implying perfect electrochemical stability and charge/discharge reversibility of ZMBs. What's more, the energy storage performance of ZMBs is compared with other recently reported micro-energy storage devices in the Ragon plot of Fig. 4f including a quasi-solidstate HOP Ni-Zn MB (260 μWh cm-2)[21], aqueous Zn//MnO2in-plane MB (168 μWh cm-2)[20], NiCo-LDH@CC//Zn MB (135.6 μWh cm-2)[19], a flexible planar Zn-PANI MB (250 μWh cm-2)[23], a flexible Zn//Ti3C2Tx(20 μWh cm-2)[37], Zn//AC MSC (115.4 μWh cm-2)[38]. Satisfactorily, our ZMBs have a maximum energy density output of 798.5 μWh cm-2(～649.9 Wh kg-1) at power density of 1.4 mW cm-2(～1 167.2 mW kg-1), and even a relatively high energy density of 478.2 μWh cm-2(～389.2 Wh kg-1) at the maximum power density of 4.1 mW cm-2(～3 282.7 mW kg-1) (Fig. S9c).

The devices in series and parallel were analyzed by CV and GCD tests to explore the integrability of ZMBs. As shown in Fig. 4g, the highest current of two MBs in parallel is twice that of a single device,while the operating voltage of 2 devices in series extends from 1.1 to 2.2 V. Moreover, the shapes of the CV curves are not distorted from each other at 5 mV s-1. At the same time, the two devices in series can achieve a potential window of 1.6-3.8 V and the charge/discharge time of two MBs in parallel is extended to twice that of a single device (Fig. 4h), which reflects the excellent integratability and consistency of ZMBs. Remarkably, two ZMBs in series can light up 36 different parallel LEDs after charged at 3 mA cm-2(Fig. 4i) and power 20 red LEDs for more than 5 min(Fig. S9d). As a comparison, the performance of Zn//NiCo-LDH@CC MBs was evaluated under the same conditions. The CV curves are similar to Zn//NiCo-LDH@ITO NWs@CC MBs but with significantly small peak currents (Fig. S10a). The rate performance is shown in Fig. S10b, and the specific capacity is 316.6, 199.6, 107.7, 54.2 and 35.2 mAh g-1at the current density of 1, 1.5, 2, 2.5 and 3 mA cm-2,respectively. Unfortunately, the specific capacity does not return to its original level when the current density returns back. Specifically, the capacity is only 269.0 mAh g-1when the current density decreases to 1 mA cm-2, showing poor stability and charge/discharge reversibility. GCD curves in Fig. S10c indicate a maximum capacity of 315.3 mAh g-1(～0.35mAh cm-2, Fig. S10d). Benefiting from the uniform electric field environment and more growth sites of NiCo-LDH nanosheets by the three-dimensional ITO NWs@CC conductive network, Zn//NiCo-LDH@ITO NWs@CC MBs exhibit superior energy storage performance.

The cycling stability of ZMBs was further investigated. It exhibits 101% capacity retention and satisfactory coulombic efficiency (about 111%) for 1 000 cycles at 1 mA cm-2, as shown in Fig. 5a. Moreover, a long time cycling tests of 4 000 times at 5 mA cm-2proves that ZMBs has excellent cycling performance,which is 119 mAh g-1at the beginning, as the cycle progresses, the capacity shows a gradual upward trend, and it increases to 258 mAh g-1after the cycle process (Fig. 5b). This is mainly due to the large specific surface area of the three-dimensional structured ITO NWs, which leads to a large mass loading of active material and thus a longer activation time of the cathode material. Fig. 5c reveals the first, the first thousandth, the second thousandth, the third thousandth, and the fourth thousandth charge/discharge curves within the open circuit voltage of 0.8-1.9 V at 5 mA cm-2. Obviously, the discharge plateau becomes longer, and no side reactions are generated during cycling. The increased discharge plateaus implies the increased capacity at the same discharge current,which is ascribed to the slow electrolyte penetration into the three-dimensional NiCo-LDH@ITO NWs@CC networks during the cycling tests. Furthermore, the charge/discharge time increases during cycling (Ⅰ: 210-220 h and Ⅱ: 390-400 h) (Fig. 5d). Surprisingly, SEM images after long cycles only show slight agglomeration at the interconnected nanosheets,almost the same morphologies as the pristine NiCo-LDH@ITO NWs@CC (Fig. 5e-h). The possible reason is that the three-dimensional structure of ITO NWs as the carrier of active material, can buffer the volume effect of the cathode nanosheet structure during the cycling process. However, NiCo-LDH nanosheets of Zn//NiCo-LDH@CC MBs have been completely agglomerated after long-term cycling, and there are still exfoliated blocks in a large area (Fig. S11). Therefore,Zn//NiCo-LDH@ITO NWs@CC MBs perform better cycling stability than Zn//NiCo-LDH@CC MBs.

4 Conclusion

Planar interdigitated Zn//NiCo-LDH@ITO NWs@CC MBs have been fabricated by a combined method of CVD, laser patterning and electrodeposition. Benefiting from the fast ion transport provided by the highly conductive three-dimensional ITO NWs@CC network and the interconnected nanosheet structure of NiCo-LDH, the aqueous ZMB exhibits excellent specific capacity (453.5 mAh g-1, ～0.56 mAh cm-2at 1 mA cm-2), satisfactory rate performance (capacity retention rate of 63.8%at 3 mA cm-2), outstanding cycle life (216% of the capacity is maintained). In addition, ZMB delivers an impressive energy density (798.4 μWh cm-2, ～649.9 Wh kg-1) and power density (4.1 mW cm-2, ～3 282.7 mW kg-1). Furthermore, ZMBs in series and parallel tests and lighting tests prove the considerable consistency and integrability, and impressive energy storage performance. Therefore, high-performance, safe, and scalable planar interdigitated Zn//NiCo-LDH@ITO NWs@CC MBs have huge potential in developing next-generation micro-energy storage devices.

Acknowledgements

This work was supported by National Natural Science Foundation of China (51972154), and Natural Science Foundation of Gansu Province(20JR5RA244)..