A 3D printed freestanding ZnSe/NC anode for Li-ion microbatteries

LIU Huai-zhi, LI Xiao-jing, LI Qiang, LIU Xiu-xue, CHEN Feng-jun, ZHANG Guan-hua

（State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, College of Mechanical and Vehicle Engineering, Hunan University, Changsha 410082, China）

Abstract： The rapid development of micro/nanomanufactured integrated microsystems in recent years requires high performance micro energy storage devices (MESDs). Li-ion microbatteries (LIMBs) are the most studied MESDs, but the low mass loading of active materials and the less-than-perfect energy density hinder their further application. A 3D printed ZnSe/N-doped carbon (ZnSe/NC)composite electrode was designed and fabricated by extrusion-based 3D printing and a post-treatment strategy for use as the anode of LIMBs. The high capacity ZnSe nanoparticles are confined in the NC, where the NC not only improves the conductivity but also acts as a buffer layer to reduce the volume expansion and provide additional active sites for electrochemical reactions. The interconnected design of the 3D printed electrode is good for fast mass transfer and ion transport. A freestanding 3D printed ZnSe/NC electrode with a high mass loading of 3.15 mg cm-2 was achieved by direct ink printing, which had a superior energy density and decent reversibility in high-power LIMBs. This strategy can be used for other high-performance electrodes to achieve a high-mass-loading of active materials for microbatteries, opening up a new way to construct advanced MESDs.

Key words: Microsystems；Microbatteries；ZnSe/NC microelectrode；High mass loading；3D printing.

1 Introduction

Micro/nano technology is undergoing rapid development in recent years. Various micro/nano systems have been developed and applied in the emerging fields such as micro-electromechanical systems(MEMSs), micro/nano robots, smart portable/wearable microsystems and implantable micro medical devices[1-6]. As the energy supply for micro/nano systems, micro energy storage devices (MESDs) attract great attention due to the multiple merits like high energy/power output, customized design and feasible integration[7-13]. MESDs can not only be used as energy supply for micro/nano systems, but also can be directly integrated with micro/nano devices, thus meeting the requirements of integration, intelligence and lightness[14-17]. Normally, the microelectrode size of MESDs is in the micrometer range, and the whole footprint area of the MESDs is at millimeter level or centimeter level[18-20]. Among the commonly studied MESDs, metal-ion microbatteries especially Li-ion microbatteries (LIMBs) and Na-ion microbatteries(NIMBs) have gained abundant attention due to their high energy density, high open-circuit voltage, and the massive research foundations for Li-ion batteries(LIBs) and Na-ion batteries (NIBs)[21-26]. For example,Zheng et al. reported the integrated NIMBs and realized the excellent flexibility, tunable voltage and capacity output, as well as excellent areal energy density (145 μWh cm-2)[27]. Lee et al. reported the bendable LIMBs with a sandwich structure of lithium metal anode, lithium phosphorus oxynitride electrolyte and lithium cobalt oxide cathode[28]. For the microelectrode preparation, David Pech et al. reported a porous Prussian blue-based cathode through electrodeposition and Chuang Yue et al. fabricated the surface-engraved Si/TiN/Ge anode through femtosecond-laser direct writing and thin film deposition techniques for the LIMBs[29,30]. However, the excellent electrochemical performance of electrode materials was generally obtained at relatively low mass loading by conventional blade-coating technology, which needs large amounts of binders and easily causes the uneven distribution of active materials. On the one hand, increasing the electrode thickness by bladecoating will cause the proportional increase in electron and ion transport distance and the electrical resistance. On the other hand, thick electrodes with high mass loading tend to have poor charge transport kinetics, which will eventually lead to worse rate performance and hinder the increase of energy density.Moreover, the fracture and delamination of thick electrodes during drying also pose great challenges to fabricate robust electrodes and to obtain stable battery performance. Therefore, it is important to explore novel microelectrodes with high mass loading for the advanced micro energy storage systems.

3D printing technology is an emerging fabrication technology that can achieve high mass loading for microelectrodes and has been widely used in micro/nano devices[31-33]. 3D printed microelectrodes have great potential in the field of energy storage system due to their optimal structural design, rationally planned porosity and controllable loading amount[34-37]. As a widely used extrusion technology,direct ink writing (DIW) based 3D printing has been widely used in metal ion batteries, supercapacitors and micro energy devices benefitting from its advantages of rapid prototyping, simple operation, low cost and multiple printable materials[38-40]. As for the material selection, transition metal selenides (TMSs) such as ZnSe, CoSex, NiSe2, MoSe2and Cu2Se have been widely studied as anode materials for metal ion batteries on account of their high theoretical capacity, high safety, and high practicability[41,42]. Among them,ZnSe manifests the promising candidate as an anode by the virtue of low cost, semiconductivity and high electrochemical activity[43,44]. However, ZnSe-based materials suffer from low electrical conductivity and unavoidable volume expansion during the lithiation and delithiation process, possessing the unsatisfactory cycling stability. Moreover, the typical ZnSe-based electrodes are blade-coated on a planar current collector like the metal foil, showing a relatively low mass loading and inferior energy output. Metal-Organic Frameworks (MOFs) derived carbon-based materials have been proved as a class of components to effectively enhance the conductivity and stability of electrodes, which is essential for improving the comprehensive performance of high-mass-loading ZnSe microelectrodes in the microbatteries[45,46]. Thus, it is quite necessary to utilize 3D printing to construct high-mass-loading microelectrode with novel material design for high-performance microbatteries.

In this work, the ZnSe nanoparticles embedded in N-doped carbon (ZnSe/NC) composite electrode is first prepared by one-step selenization and carbonization. Notably, the NC derived from MOFs effectively alleviates the nanoparticle pulverization and electrical contact loss caused by the volume expansion of ZnSe.Besides, the introduce of N-dopant provides additional active sites and defects for fast chemical reaction,contributing to the enhanced specific capacity and rate performance. Therefore, the ZnSe/NC electrode demonstrates an outstanding discharge capacity of 1 170 mAh g-1at a current density of 100 mA g-1and excellent stability over 1 000 cycles. The superior performance is found to be determined by the combination of surface pseudocapacitive reaction and ion diffusion control. Moreover, a freestanding ZnSe/NC microelectrode with high mass loading is further realized by DIW-based 3D printing technology and post treatment. Increasing the mass loading amount to 3.15 mg cm-2, the 3D printed ZnSe/NC microelectrode still shows great reversibility and maintains a stable discharge capacity over 100 cycles. This work provides a novel approach for the development of high-performance microelectrodes with high mass loading, showing the prospect in developing advanced 3D printed microelectrodes in microbatteries.

2 Experimental

2.1 Synthesis of ZnSe/NC composite

Zeolitic Imidazolate Framework (ZIF-8) precursor was prepared by an improved room temperature coprecipitation method. Firstly, 6.16 g 2-methylimidazole and 5.95 g Zn (NO3)2·6H2O were dissolved in 150 mL methanol to form solution A and B, respectively. Then solution A and B were mixed and left for 24 h to collect a white precipitate, which was washed with methanol until clear. After drying at 80 °C for 6 h, the ZIF-8 precursor was successfully obtained. Finally, ZIF-8 precursor and Se powder were annealed at 650 °C for 2 h at a mass ratio of 1∶2.5, under the atmosphere of argon gas with a heating rate of 3 °C min-1. After that, the final active material of ZnSe/NC composite was collected.

2.2 3D printing of ZnSe/NC microelectrode

At the beginning, ZnSe/NC active material,polyvinylidene fluoride (PVDF), and carbon nanotubes (CNTs) were dispersed in N-Methyl pyrrolidone (NMP) at a mass ratio of 8∶1∶1 and stirred for 12 h until the bottle was inverted and the slurry did not flow down. Meanwhile, the stirred slurry was transferred into the syringe, and the cellular structure was printed on the glass substrate, where the printing speed was 5 mm s-1and the extrusion pressure was 60 pounds per square inch (psi). The 3D printed ZnSe/NC electrode was separated from the glass plate by soaking in deionized water and freeze dried for 8 h ultimately.

2.3 Characterization and electrochemical measurement

Morphologies of the samples were obtained by the field emission scanning electron microscope(SEM, from Carl-Zeiss SIGMA HD) at an accelerating voltage of 10 kV. The crystal structure of samples was detected through X-ray diffraction (XRD,from Rigaku TTR III, CuKαλ= 0.154 056 nm). The chemical binding states, types of elements, and the atom concentrations in the sample were comprehensively studied by X-ray photoelectron spectroscopy(XPS, from Thermo Fisher Scientific K-Alpha 1063 with an AlKα source). Raman spectra for identifying substances and analyzing was obtained by the confocal microscopy system (WITec Alpha-300R, the 532 nm laser wavelength). The 1 mol L-1LiPF6, Celgard 2500 membrane, Li foil of 0.6 mm were utilized as the electrolyte, separator and counter electrode, respectively. Finally, the electrochemical data were measured at the Land CT2001 test system and electrochemical workstation (CHI660E, Shanghai Chenhua instrument Co., Ltd).

3 Results and discussion

The 3D printed ZnSe/NC microelectrode based on DIW technology is illustrated in Fig. 1 (a). Briefly,the ZnSe/NC active material, CNTs conductive additive and PVDF binding agent were well-mixed and then were dispersed in NMP to form uniform, highviscous, thixotropic composite ink for printing. Specifically, the ZnSe/NC composite means the composite material of ZnSe nanoparticles embedded in the Ndoped carbon, where the ZnSe comes from the selenization of ZIF-8 precursor (Zn resource) with Se powder (Se resource), and the N-doped carbon was obtained by the carbonization of ZIF-8 (N resource and C resource). The ZIF-8 precursor was collected by the co-precipitation of Zn (NO3)2·6H2O and C4H6N2, which happened ahead of one-step selenization and carbonization[47]. The interconnected 3D structure of microelectrode was obtained on the hard substrate by the computer aided predesign. Finally,the post treatments including soaking and freeze drying were employed and the freestanding 3D microelectrode could be released from the substrate. It is worth noting that the microelectrode configuration could be easily adjusted as various structures such as cellular structure through the DIW-based printing.The microstructure of ZIF-8 precursor is shown in Fig. S1 (a), where the ZIF-8 demonstrates a typical polyhedron structure and a uniform size distribution.After one-step selenization and carbonization, the ZnSe/NC composite was obtained, with detailed SEM images shown in Fig. 1 (b) and Fig. S1 (b). It can be seen that the ZnSe nanoparticles less than 100 nm are tightly packed and encapsulated by the N-doped carbon (derived from ZIF-8). More importantly, the 3D printed electrodes can not only be formed into various geometries and letter patterns as shown in Fig. 1(c), but also can be printed onto different substrates, such as glass, polyimide (PI), paper, polyethylene terephthalate (PET), stainless steel mesh(SSM) and Cu foil, presenting the versatility of the DIW-based 3D printing technique for preparing 3D arbitrary microelectrodes[48].

One of the most challenging aspects for constructing 3D printed microelectrodes is to develop the printable ink with good rheological behavior and desirable formulation[49]. As shown in Fig. 2 (a), the prepared ink inverting at the bottom of the bottle did not flow down after turning around the bottle for 48 h, reflecting a good printing behavior of the obtained ink and the easily self-supporting characteristic. The ink concentration for 3D printing is around 400 mg mL-1and the diameter of printing needle is 400 μm, which can be used to fabricate the microelectrode with a precise width of about 500 μm, shown in Fig. 2 (b). Additionally, the 3D printed microelectrodes with different layers can be easily fabricated without destroying the cellular configuration. As can be seen from Fig. 2(a), a three-layer microelectrode with the area of 1×1 cm2and the thickness of 1.2 mm (each layer is around 0.4 mm) is controllably obtained after 3D printing, whose microstructure is then studied by SEM images. Fig. 2 (b) and 2 (c) show that the 3D printed ZnSe/NC microelectrode after post treatment displays no internal collapse and still maintains the microstructure as designed. It is worth noting that the optimal designed cross-grid structure possesses highly continuous conductive framework and abundant channels for the electrolyte penetration, which is beneficial for the fast electron transfer and ion transport, contributing to the enhanced electrochemical performance[50].Besides, each layer with a thickness of about 400 μm is continuously and tightly connected, offering a continuous channel for the penetration of electrolyte and for Li ion transport[51]. The CNTs in the electrode can be clearly seen in Fig. 2 (d), and the ZnSe/NC nanoparticles are uniformly distributed around the CNTs,which is beneficial for the enhanced electrical conductivity and facilitated electron/ion transport[52].Fig. 2 (e) shows the XRD patterns of ZIF-8 precursor and ZnSe/NC composite. All of the characteristic peaks of ZnSe/NC are well indexed to the crystalline phase (JCPDS card No. 37-1463) and no other impure characteristic peaks are detected. Besides, the thermogravimetric (TG) curve of ZnSe/NC composite is then displayed in Fig. 2 (f), showing that the percentage of ZnSe is 74.1%. The slight loss of mass was caused by the evaporation of the adsorbed water in the pores of ZnSe/NC before 200 °C. From 200 to 800 °C, oxygen reacted with ZnSe and carbon, and then the final obtained solid product was ZnO powder,further evincing the successful fabrication of the 3D printed ZnSe/NC microelectrode[53].

The detailed morphology investigation of ZnSe/NC composite is shown in Fig. 3. Two strong peaks at 1 336 (Dband) and 1 574 cm-1(Gband) are observed through the Raman spectra in Fig. 3 (a), representing the disordered carbon and ordered carbon, respectively[54]. Notably, there is a small bulge at 1 437 cm-1,which is related to the stretching vibration of the N＝N bond in the N-doped carbon matrix[55]. The high-intensity-ratio ofD-band toG-band (ID/IG=1.1)indicates that the doping of heteroatom N successfully introduces abundant defect sites, which is beneficial for the fast chemical reactions[56]. Moreover, XPS spectra for the entire ZnSe/NC composite and each element are further displayed in Fig. 3 (b) to Fig. 3 (f).As shown in Fig. 3 (b), the Zn, Se, C, N and O elements are all detected in the full XPS spectra. Besides,the specific element contents with Zn (33.7%), Se(30.7%), C (18.7%), N (7.2%) and O (9.7%) are displayed as Table S1 in the supporting information,where the element contents are influenced by the content of ZIF-8 precursor and Se powder, and the onestep selenization and carbonization process. There are two strong characteristic peaks appearing at 1 019 and 1 043 eV in Fig. 3 (c), which correspond to the Zn2p3/2and Zn2p1/2states, respectively[57]. For the Se element, the two characteristic peaks at 51.5 and 52.4 eV in Fig. 3 (d) are related to Se3d5/2and Se3d3/2[58]. Besides, the peak at 53.4 eV can be ascribed to Se3d3/2, and the broad peak at 56.8 eV may be related to the Se-O bond due to the annealing and localized surface oxidation of ZnSe/NC[59]. Combined with the high-resolution XPS spectra of the Zn2p and Se3d, it can be concluded that the ZnSe was successfully synthesized after selenization and carbonization.It is worth noting that the N element comes from the N-containing organic ligands in the ZIF-8 precursor,which proves that the N element is in-situ doped into the ZnSe/NC composite during the annealing process.The N1s in Fig. 3 (e) can be divided into pyridine N(396 eV) with lithophilic advantage and graphitic N(398 eV) with increased conductivity, which are helpful to improve the lithium storage performance of ZnSe/NC composites[60,61]. In addition, Fig. 3 (f)shows that ZnSe/NC composite contains chemical bonds such as C-N (283 eV), C＝O (285 eV) and C-C (282 eV), reflecting the good conductivity of the composite and fast electron and ion transfer during the reaction process.

The electrochemical performance of ZnSe/NC anode is firstly studied by cyclic voltammetry (CV) at 0.5 mV s-1ranging from 0.01-3.0 V in the initial 3cycles, as shown in Fig. 4 (a). Specifically, the redox peak at 0.46 V in the first cycle is related to the transition of ZnSe and it switches to 0.57 V in the next 2 cycles, which might be attributed to the gradual activation process of multiple lithiation/delithiation[62,63].The nearly overlapping curves of the second and the third cycle indicate the excellent reversibility of the ZnSe/NC anode. In addition, the rate capability and cycling stability of the ZnSe/NC anode are further investigated for the practical long-term application.Fig. 4 (b) demonstrates that the ZnSe/NC anode displays a superior discharge capacity of 1 130 mAh g-1at 100 mA g-1, and it still maintains 433 mAh g-1even when the current increases 20 times to 2 000 mA g-1.Surprisingly, the discharge capacity reaches a high value of 1 028 mAh g-1when the current density returns to 100 mA g-1, showing a good reversibility and a high coulombic efficiency of the ZnSe/NC anode under both small and large current densities. Besides,the long-term stability of ZnSe/NC anode is investigated at the high current density of 1 000 mA g-1,demonstrating a stable discharge capacity over 300 mAh g-1and maintaining nearly 100% coulombic efficiency even after 1 000 cycles, which is comparable to the recently reported microbatteries[64-70], as shown in Table S2. It can be noticed that the specific capacity of the ZnSe/NC anode increases in the following cycles, which may be the conversion reactions in the initial lithiation/delithiation states. Besides, the gradual infiltration of electrolyte and the activation process can also contribute to the increased specific capacity, which can also be observed in the rate performance in Fig. 4 (b). The decent rate capability and high reversibility might be attributed to the fast ion transport and mass transfer of the well-designed ZnSe/NC composite, where the ZnSe acts as highly active material and the NC as a buffer layer to maintain the structural stability and enhance the overall conductivity of electrode.

In order to explore the superior lithium-storage performance of the ZnSe/NC electrode, CV curves recorded at growing scan rates from 0.1 to 10 mV s-1are displayed in Fig. 4 (d). Each CV curve presents a similar shape as the scan rate gets larger, indicating the stable electrochemical behavior at different situations. The corresponding currents (i) of the reduction peaks and oxidation peaks are observed regularly increasing, implying that the ZnSe/NC electrode has fast reaction kinetics and high uniformity. According to the previous reports, the current (i) response to the scan rate (v) will be different when it is mainly influenced by the surface pseudocapacitive reaction(k1v) or the diffusion-controlled reaction (k2v1/2)[18,71],which can be denoted as

The individual contribution of surface pseudocapacitive reaction (k1v) and the diffusion-controlled reaction (k2v1/2) could be quantitatively computed based on the equation (1) and equation (2), where thebvalue approaching to 0.5 indicates a diffusion-controlled behavior and b value close to 1.0 implies a surface pseudocapacitive process[24]. The fitting b value for the anodic peaks and cathodic peaks are calculated as 0.728 2 and 0.727 9 respectively, as shown in Fig. S2. It reveals that the reaction kinetics is influenced by the main surface pseudocapacitive reaction and minor ion diffusion control[71-73]. Besides,Fig. 4 (e) further demonstrates the contribution of pseudocapacitive capacitance and diffusion-controlled capacitance at different scan rates, followed by the individual pseudocapacitive contribution at the scan rate of 0.1, 0.2, 0.5, 2, 5 and 10 mV s-1in Fig. S3. The surface pseudocapacitive contribution accounts for 31% at 0.1 mV s-1and it keeps increasing with increasing the scan rate. Especially under the high scan rate of 10 mV s-1, the contribution of surface pseudocapacitive process reaches to 98%, manifesting the superior reaction kinetics in the process of charging and discharging. The electrochemical impedance spectra (EIS) is further investigated in Fig. S4 and the homologous linear fitting line is displayed in Fig. S5, where the small radius and steep slope reveal an excellent electrical conductivity, in favor of the superior reaction kinetics and rate capability. The excellent electrochemical properties of the ZnSe/NC electrode are attributed to the optimal design of advanced ZnSe nanoparticles and stable carbon with N-doping,which could also be applied in other highly reactive electrode materials.

Besides the investigation for routine planar electrode prepared by the traditional blade coating method, 3D printing is a highly efficient and highly controllable micromanufacturing technology and possesses the ability to achieve high-mass-loading active electrodes for high-power microsystems[74]. Thus, the lithium storage electrochemical performance of a 3D printed ZnSe/NC microelectrode with a high mass loading (3.15 mg cm-2) for LIMBs is further explored.Compared with the blade-coating method, the optimal design of the 3D printed microelectrode is beneficial for the full penetration of electrolyte and the freestanding interconnected framework can facilitate the fast electron transfer and ion transport, thus enabling the superior performance under the high-mass-loading condition. As shown in Fig. 5 (a), the high-massloading ZnSe/NC microelectrode exhibits similar redox peaks as the planar electrode in Fig. 4 (a), while the actual peak current is much larger, which implies the greater capacity and energy density in the given size. The EIS curve of the ZnSe/NC microelectrode is also displayed in Fig. 5 (b). It can be seen that it still maintains great electrochemical reactivity and electrical conductivity even under high mass loading conditions. Fig. 5 (c) demonstrates the long-term cycling test at a current density of 1 000 mA g-1, where the 3D printed ZnSe/NC microelectrode delivers a decent discharge specific capacity of 224 mAh g-1in the beginning, and remains over 122 mAh g-1in the following 100 cycles, with a stable coulombic efficiency during the whole process. The successful construction of the 3D printed ZnSe/NC microelectrode with a high mass loading shows good prospect for achieving high-performance microelectrodes and high-power microbatteries in the next-generation energy storage devices.

4 Conclusion

In conclusion, we successfully designed and fabricated the 3D printed freestanding ZnSe/NC microelectrode for constructing high-performance Li-ion batteries and Li-ion microbatteries. The ZnSe/NC composite was prepared by one-step selenization and carbonization using the MOF-based precursor and Se powder. Importantly, the introduced carbon with Ndopant can effectively improve the conductivity,provide more active sites and alleviate volume expansion of ZnSe nanoparticles, favoring the superior electrochemical performance during the chemical reaction. As a result, the ZnSe/NC anode demonstrated excellent discharge capacity, rate performance and longterm stability. The fast kinetic reaction and pseudocapacitive contribution were also systematically investigated. Moreover, the 3D printed ZnSe/NC microelectrode with a high mass loading of 3.15 mg cm-2was further achieved for Li-ion microbatteries by DIWbased 3D printing technology, which also exhibited decent electrochemical performance and long-term stability. This work shows promising prospect in the fabrication of high-performance microelectrodes for use in high-power micro energy storage devices.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (52175534, 51975204), the Science and Technology Innovation Program of Hunan Province(2021RC3052), Natural Science Foundation of Hunan Province (2021JJ30103), and the Fundamental Research Funds for the Central Universities(531118010016).