Multifunctional MXene inks for printed electrochemical energy storage devices
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Abstract
Since the discovery of MXenes, the family has expanded rapidly in the past decade. With their fascinating properties, including high electrical conductivity, solution processability, tunable surface functionality, and excellent mechanical properties, MXenes have garnered significant enthusiasm from the academic community and industrial relevance. The most extensively studied of the many applications for MXene-based devices is electrochemical energy storage (EES). Importantly, MXene inks allow quick yet efficient production of personal EES devices through additive manufacturing. However, there are relatively few comprehensive summaries of reports on the processing of MXene inks for EES devices. This paper provides a comprehensive review of MXene synthesis, additive manufacturing strategies and the latest advancements in the printing of MXene-based high-performance EES devices including micro-supercapacitors and batteries. Besides, the current challenges for high precision and high-performance printing technology are also discussed. This review is expected to provide valuable insights for solution processing of MXene inks and may shed light on the large-scale application of MXenes toward the next generation of wearable electronics.
Keywords
INTRODUCTION
With the rapid development of wearable and portable smart electronic devices, there is a growing demand for high-performance electrochemical energy storage (EES) devices[1-7], especially for rechargeable batteries[8,9] and supercapacitors (SCs)[10,11]. Rechargeable batteries are able to store more charge due to the redox reactions occurring in the electrode-electrolyte interface, thus possessing a much higher energy density compared to that of SCs, but their lower power densities restrict their suitability for high-power applications and fast charging. In contrast, SCs can be charged in seconds and provide higher power densities. To achieve high-performance EES devices (EESDs), numerous efforts have been devoted to investigating electrode materials with large specific surface area (SSA), high electrochemical activity, and fast charge transport[12-14].
Owing to the exceptional physical and chemical properties, along with the unique structural characteristics including atomic-level thickness, which provide more exposed surface area and better ion transport, two-dimensional (2D) materials have recently gained attention of researchers as prospective options for energy storage[15-18]. After the discovery of graphene, different classes of 2D materials, including transition metal dichalcogenides (TMDs)[19,20], silicene[21,22], layered double hydroxides (LDHs)[23,24], phosphorene[25,26], and MXenes[27,28] have been successively developed. As a novel 2D material, MXenes are a novel class of transition metal carbides, nitrides, and carbonitrides[29]. Discovered in 2011[30], MXenes have the general formula Mn+1XnTx (n = 1,2,3 or 4), where M represents an early transition metal, X stands for C and/or N, and Tx refers to various functional groups such as -OH, -O, -F. The surface functional groups endow a good hydrophilicity while the metallic backbone provides excellent electrical conductivity. MXenes have excellent electrochemical, electronic, optical, and mechanical properties and show promising applications in various fields, such as energy storage[31], sensors[32], optoelectronics[33], electronics[34], biomedicine[35], and catalysis[36]. The predominant strategy for large-scale preparation of 2D material nanosheets is intercalation exfoliation due to low cost, solution processability and high yield of monolayers[37].
When the exfoliation flakes are redispersed in solvents and assisted by additives, printable 2D materials inks can be formulated, allowing EESDs to be fabricated using simple, efficient and scalable printing technologies. Compared to traditional manufacturing techniques, the simplicity, low cost and high efficiency of printing technology, as well as its scalability, demonstrate significant potential for manufacturing energy storage devices. The first thing to consider when printing EESDs is the appropriate printing technology. Different printing technologies vary in terms of resolution, printing speed, film thickness, and so on. The appropriate printing technology should be selected for different application requirements. Inks with specific rheological requirements should be formulated for different printing technologies to ensure smooth printing. In order to meet the rheological requirements, the ink additives, such as binders, surfactants, and rheology modifiers, are typically employed to promote the deposition and molding of materials. However, the subsequent high-temperature heat treatment to remove additives leads to the degradation of material properties and the complexity of the manufacturing process. In addition, the thermal annealing treatment also limits the choice of soft substrates, including plastics, polymers, plant leaves, or even human skin. The 2D materials inks, such as graphene[38], molybdenum disulfide[39], black phosphorus[40], and MXene inks[41,42], can be printed on different substrates using various printing methods and show good performance.
Printable MXene inks for EESDs (mainly MSCs and batteries) have received a lot of attention due to the excellent rheological, conductive and mechanical properties of MXenes[43,44]. The unique surface chemistry of MXenes allows them to be stably dispersed in both aqueous and polar solvents and to be formulated directly into inks without additives, which is the prerequisite of additive manufacturing of various functional devices using diverse printing technologies, such as inkjet printing[45,46], screen printing[47,48], three-dimensional (3D) printing[49,50], etc. [Figure 1]. However, printable MXene inks still present certain challenges that researchers are striving to address. The research group is currently focusing on improving the stability of MXene inks and achieving scale production. MXene oxidizes easily in humid environments, which compromises the structural integrity of the nanosheets and thus reduces the performance of MXene inks. Consequently, improving the oxidation stability of MXene inks is an approach for realizing high-performance MXene-based printed devices. To accomplish the large-scale production of MXene inks, a few areas of research need to be explored, involving the synthesis methods of MXenes, the rheology of inks, and printing processes. The performance of the fabricated EESDs depends mainly on the quality of the print. Thus, ink formulation is crucial to improving print quality and involves scalable production of high quality MXene, ink formulation and printing techniques. Besides, the research and application of non-titanium-based MXene materials with outstanding properties is still relatively limited and needs further exploration[28].
Figure 1. Various printing and coating technologies and applications of MXene for printed electronics. The technologies including inkjet printing, screen printing, transfer printing, 3D printing, patterned coating, etc. Reproduced with permission[51]. Copyright 2018, John Wiley and Sons. Reproduced under terms of the CC-BY license[52]. Copyright 2019, Springer Nature. Reproduced with permission[53]. Copyright 2020, John Wiley and Sons. Reproduced with permission[54]. Copyright 2019, John Wiley and Sons. The applications including sensors, optoelectronics, energy storage and EMI shielding. Sensors: Reproduced with permission[55]. Copyright 2018, Elsevier. Reproduced with permission[56]. Copyright 2021, John Wiley and Sons. Optoelectronics: Reproduced with permission[57]. Copyright 2021, John Wiley and Sons. Reproduced with permission[58]. Copyright 2021, John Wiley and Sons. Energy storage: Reproduced with permission[59]. Copyright 2020, Elsevier. Reproduced with permission[60]. Copyright 2019, John Wiley and Sons. EMI shielding: Reproduced with permission[61]. Copyright 2021, Elsevier.
This review summarizes recent advancements in MXene-based printed energy storage devices and flexible electronics. First, various strategies for MXene ink formulation are briefly discussed. Next, the advantages and limitations of different printing technologies are presented. Besides, multiple applications of MXene-based printing technologies, such as batteries, SCs, and flexible electronics, are exemplified. Finally, the paper concludes the main challenges and lists the opportunities for MXene-based printing technologies.
SYNTHESIS METHODS OF MXENE AND INK FORMULATION
Bottom-up strategy to synthesize MXenes
Distinct synthesis methods affect the quality and properties of subsequent MXene. Bottom-up synthesis methods have proven to be a versatile strategy for the synthesis of high-quality 2D materials with large areas. This strategy makes it possible to directly grow ultrathin 2D crystals while bypassing the precursor, extending the possibilities for more intriguing applications. Currently, the bottom-up strategy is mainly based on chemical vapor deposition (CVD), which has the advantage of efficiently synthesizing MXene with unique surface chemistry and high yield[62,63] [Figure 2A]. For example, Wang et al. developed the direct synthesis of Ti2CCl2 flowers supported on Ti foil by choosing TiCl4 and graphite as the Ti and C sources, respectively, and heated up at 950 C[64]. They also demonstrated the synthesis of Ti2N using this strategy [Figure 2B]. Besides, Xiang et al. chose gaseous TiCl3 and CH4 as the Ti and C sources, respectively[65]. The reaction temperature is lower (850 C); the product Ti2CCl2 is highly crystalline with a much higher yield, which can be produced in kg in a single batch. Moreover, when dispersing Ti2CCl2 in water, a homogeneous solution is thus formed without the necessity of any surfactant, similar to conventional MXene flakes. The gas-phase assembled Ti2CCl2 film also possesses a higher electrical conductivity and much longer shelf life than conventional Ti2CTx films and solutions, demonstrating the unique advantage of their methodology. Over the past decade, the top-down strategy has been studied extensively by researchers. Since the first reported MXene in 2011, many methods of selective etching have been developed, including hydrofluoric acid (HF) etching[66], in-situ formed HF etching[67], alkali etching[68], and molten salt etching[69], electrochemical etching[70], and so on. We therefore briefly review several different classical methods of selective etching.
Figure 2. Synthesis of MXenes by bottom-up and top-down strategies. Schematic of 2D Mo2C (A) and Ti2CCl2 (B) produced by CVD method. (C) Schematic illustration of the synthesis process by top-down strategy. Scanning electron microscope (SEM) image of
Top-down strategy to synthesize MXenes
Since the successful etching of the Al layer from Ti3AlC2 to obtain MXene in 2011, the top-down strategy has been the main preparation strategy for MXene, consisting of two stages: selective etching and delamination [Figure 2C][71].
Selective etching
The MAX phase serves as the MXene precursor and typically refers to a class of ternary layered compounds with a formula of Mn+1AXn, where M is an early transition metal (e.g., Sc, Ti, V, Cr, Zr), A represents a group III A or IV A element (Al, Ga), and X is C and/or N[72,73] [Figure 2D]. Selectively etching the A element in the MAX phase yields MXene with alternating M and X elements. Currently, there are four known main structures of MXene: M2X, M3X2, M4X3 and M5X4. Similar to the MAX phase, MXene has a hexagonal close packing crystal structure with P63/mmc space group symmetry, with M atoms in a close packing arrangement and X atoms in the octahedral gap[74].
Selective etching of the MAX phase is the first step to obtain monolayered or few-layered MXenes. In the MAX phase, the M-X bonds are covalent and chemically strong, while the M-A bonds are metallic, with weaker bonding energy and stronger chemical activity[75]. Therefore, it is difficult to obtain MXene directly by the mechanical exfoliation method used for graphene preparation. Instead, chemical etching should be used to prepare MXene by preferentially breaking the M-A bond while retaining the M-X bond, taking advantage of the weaker chemical bonding energy of M-A compared to M-X. The etching conditions are associated with the MAX phase and rely on factors such as M-A atomic binding strength, particle size, and crystal structure[31].
Since Naguib et al. first employed HF selective etching route at room temperature (RT) to prepare Ti3C2Tx, etching the MAX phase with HF has become the most widely used method to produce MXene with an accordion-like morphology[30]. However, this method relies heavily on hazardous HF solutions.
Intercalation and delamination strategies
Therefore, weakening the interaction force between MXene layers is crucial to effectively delaminate multilayer MXene[80]. After selective etching, the multilayered (m-) MXene surface is terminated by hydrophilic, electronegative functional groups, mainly -F, -OH, -O, etc. [Figure 2E][81]. Strong forces between the multilayer MXene layers and the incomplete etching process lead to delamination difficulties. Through intercalating molecules and ions into stacked MXene layers to enlarge the interlayer spacing and weaken the interlayer interaction, the ultrasonication or manual handshaking is able to delaminate the swollen solids into predominantly single-layer MXene flakes [Figure 2F][82]. Organic intercalating agents, such as polar organic molecules (e.g., dimethyl sulfoxide (DMSO)[83], urea[84], hydrazine[85]), and other large organic alkali molecules[86] [e.g., tetrabutylammonium hydroxide (TBAOH) or tetramethylammonium hydroxide (TMAOH)], have been effectively employed. In the early stages of the study, researchers had already reported that chemical intercalation using DMSO successfully enlarged the layer spacing
Following the swelling the m-MXene, delamination is the crucial step to realize single-or-few-layered MXene dispersions, which is the prerequisite of forming aqueous inks. The easiest way to delaminate the swollen MXene solids is through ultrasonication, whose condition determines the quality of MXene flakes (concentration, flake size, pin-holes, defects quantity and density, etc.[88]). For instance, longer time and higher power ultrasonication produces MXene flakes with many defects and reduces the lateral size. Therefore, it is essential to develop delamination methods that do not require ultrasonic treatment. For instance, Shahzad et al. developed the minimally intensive layer delamination (MILD) method, which produces large clean flakes with fewer defects by manual shaking instead of ultrasonic treatment[89]. This method used mixed lithium fluoride and hydrochloric acid to achieve etching and intercalation at the same time in which hydrated lithium ions intercalate into the interlayer, significantly increasing the interlayer spacing. Currently, this method has become the main route to prepare high quality MXene with large flakes. Top-down methods are more suitable for scale production than bottom-up strategies. However, their etching process and subsequent delamination can cause the MXene to become smaller and introduce defects. In addition, the choice of MAX precursors limits the types and structures of the prepared MXene.
MXene inks formulation and rheological properties
In general, inks are composed of three parts: active materials, solvents and additives. The active material is the main component of the ink, such as the 2D materials graphene, molybdenum disulfide, MXene, and so on. Solvents are divided into two categories: aqueous and organic. The selection of solvents depends on the solute-solvent interactions; only those possessing surface tension similar to that of the solute result in good dispersions. In addition, the solvents should showcase a certain degree of volatility such that the ink drying kinetics is fast enough without interrupting the printing process. Additives include binders, surfactants and rheology modifiers. Although the use of additives greatly improves the rheological properties of the ink, thereby improving processability, the additive removal treatments require high temperatures, which not only reduce the performance of the ink, but also complicate the whole process.
MXene inks are suspensions or dispersions containing MXene flakes with different rheological properties that can be deposited using printing or coating techniques. Different printing technologies have distinct rheological requirements for MXene inks. Figure 3A illustrates the relationship between viscosity, throughput, resolution, film thickness and transparency with multiple printing technologies. The rheological behavior of MXene inks is mainly described by their viscosity. Viscosity can be measured as the ratio of shear stress to shear rate. MXene inks are pseudoplastic fluids with a shear thinning behavior; the viscosity decreases with the increase in the shear stress. The shear thinning behavior ensures the smooth flow of the ink through the nozzle without clogging, enabling the deposition on the substrate. The viscosity of MXene inks is related to a number of factors[90], including MXene ink concentration, MXene flake size distribution, solvents, etc., with the concentration of the dispersion being the main parameter.
Figure 3. Rheological properties of MXene inks. (A) Relationship between viscosity, yield, resolution, film thickness and transparency with different printing technologies. Reproduced with permission[54]. Copyright 2021, Elsevier. (B) Plots of relative viscosity vs. volume fraction and measured viscosity vs. concentration for single and multilayer Ti3C2Tx dispersions. Reproduced with permission[91]. Copyright 2018, American Chemical Society.
Various concentrations of MXene inks exhibit distinct rheological behaviors [Figure 3B]. For example, for aqueous monolayer Ti3C2Tx dispersions, as each order of magnitude increase in MXene concentration, the viscosity increased with it by three orders of magnitude (at low shear rate)[91]. The viscosity of MXene inks can be controlled by modulating the size and the thickness of the MXene laminates. The study of two aqueous inks with two different MXene flake sizes, denoted as L-Ti3C2Tx ink (average size 2 µm), and
In addition, MXene inks dispersed in varying solvents showcase differing rheological behaviors. Water is the most common dispersing medium because MXene has surface hydrophilic functional groups that allow stable dispersion in water or hydrophilic media[93]. For organic solvents, solvents with a surface tension that better matches that of the MXene are more stable in dispersion. Common good solvents are DMSO,
MXene-based MSCs
Micro-SCs (MSCs) have received great attention owing to their high-power density, long cycle life, fast charging speed and small area[96-98]. At present, the common MSC structure is a planar interdigital structure with two adjacent electrodes placed separately. The narrow gap between the interdigital electrodes shortens the ion diffusion distance, thus accelerating the ion transport kinetics which correspondes to higher charge/discharge rates in the MSC devices. The areal capacitance is commonly used as an indicator to evaluate the performance of MSCs. Currently, efficient fabrication strategies for fabricating MSCs include but are not limited to photolithography[99], laser scribing[100] and printing technology[101]. The complex and time-cost fabrication process of photolithography and the selection of active materials for laser scribing have limited its development. Printing technology is an additive manufacturing technology that has gained increased attention due to its simplicity, low cost, and scalability, allowing for the manufacturing of devices on a large scale[102]. Printing technologies can be classified as digital and non-digital categories based on whether a template is utilized during the printing process. Inkjet and 3D printing are examples of digital printing, while screen and transfer printing fall under non-digital printing. Each printing technique requires specific fluidic properties and the ink rheology. The advantages and limitations in the application of different printing technologies are summarized in Table 1. It is noteworthy that the printed MSCs are typically operated at a voltage below 0.6 V owing to the unique properties of MXene. Its narrow voltage window greatly limits the energy density of the MSCs. This limitation can be mitigated by optimizing the electrolyte and designing asymmetric MSCs. In the following paragraphs, we focus on the MXene MSCs achieved via different printing techniques. Table 2 is a summary of the reported printed/coated MSCs.
Summary of the advantages and disadvantages of different printing technologies
| Printing methods | Advantages | Disadvantages |
| Inkjet printing | Less material required Environmentally friendly Low-cost | Nozzle clogging |
| Screen printing | High-speed production Low waste | Low resolution High roughness |
| Transfer printing | Small wastage | High volume inks Extra step |
| Direct ink writing | Scalable | Low resolution |
Summary of printed MXene-based MSCs
| Ref | Materials | Printing/coating methods | Electrolyte | Electrochemical performance |
| [103] | SA-Ti3C2Tx | Inkjet printing | PVA/H2SO4 | 108.1 mF cm-2 |
| [104] | Ti3C2Tx/graphene | Inkjet printing | PVA/Na2SO4 | 3.84 mF cm-2 |
| [105] | Ti3C2Tx-N | Screen printing extrusion printing | H2SO4 | 70.1 mF cm-2 8.2 F cm-2 |
| [106] | Ti3C2Tx/CNF | 3D printing | H2SO4 | 2.02 F cm-2 |
| [107] | Ti3C2Tx | 3D printing | PVA/H2SO4 | 2.337 F cm-2 |
| [108] | Ti3C2Tx/MnO2 | Inkjet printing | PVA/Na2SO4 | 0.51 μWh cm-2 |
| [109] | NiS/N-Ti3C2Tx | Inkjet printing | PVA/KOH | 33.5 mWh cm-3 |
| [52] | Ti3C2Tx | Inkjet printing extrusion printing | PVA/H2SO4 | 562 F cm-3 |
| [110] | Ti3C2Tx@C | Inkjet printing | PVA/H2SO4 | 40 mF cm-2 |
| [111] | Ti3C2Tx | 3D printing | PVA/H2SO4 | 1,035 mF cm-2 |
| [112] | V2CTx | Direct ink writing | PVA/Zn (CF3SO3)2 | 3.82 F cm-2 |
| [113] | Ti3C2Tx | Inkjet printing | PVA/H3PO4 | 192 μF cm-2 |
| [114] | Ag@Ti3C2 MnO2@Ti3C2 | Direct ink writing | KOH | 38.16 Wh kg-1 |
| [51] | Ti3C2Tx | Stamping | PVA/H2SO4 | 61 mF cm-2 |
| [115] | Ti3C2Tx | Blade coating | PVA/H2SO4 | 241 mF cm-2 |
| [116] | MX/CCNS | Vacuum filtration Screen printing | PVA/H2SO4 | 114.9 mF cm-2 |
| [117] | Ti3C2Tx/PH1000 | Vacuum filtration | H2SO4 | 23 mWh cm-3 |
Inkjet printing
Inkjet printing is a widely used digital, high-resolution, non-contact printing technology that is employed in the manufacturing of various devices, including energy storage[118] and optoelectronic devices[119]. This method is able to produce miniatured devices with much reduced dimensions while demonstrating a high integrity substrate/geometry compatibility and scalability. The 2D materials such as graphene[120] and MoS2 nanosheets[121] have been formulated into functional inks for inkjet printing. The quality of the inkjet-printed patterns depends on three factors: the substrate, ink rheology, and printing parameters. In addition, ink parameters such as Reynolds number (Re), Ohnesorge number (Oh), and Weber number (We) are frequently used to evaluate the ink printability. By calculating the inverse Ohnesorge number based on the following equation, Z of the ink, researchers can better understand the ink's properties[122]. The Z value depends on surface tension (γ), density (ρ), diameter of the jetting nozzle (a), and viscosity (η), respectively.
To form stable droplets, the Z value should be between 1 and 14[123]. For water-based inks, the high surface tension of water results in a Z value higher than 14. According to the equation, it is necessary to increase the viscosity appropriately to reduce the Z value [Figure 4A]. The maximum viscosity value depends on the diameter of the jetting nozzle that allows the ink to pass through the printer without clogging.
Figure 4. Inkjet printing of MXenes. (A) Z-value range for stable inkjet printing. Reproduced with permission[123]. Copyright 2010, Annual Reviews. (B) Key factors for additive-free Ti3C2Tx ink formulation: ink concentration, flake size and viscosity, and SEM image of printing ink onto fabric surface. Reproduced with permission[92]. Copyright 2020, Wiley-VCH. (C) Diagram of inkjet printing using MXene organic ink and the corresponding high printing resolution patterns. The process consists of three steps: jetting, wetting, and drying. Reproduced under terms of the CC-BY license[52]. Copyright 2019, Springer Nature (D) Schematic of the inkjet printing MP-MSCs and SEM and Cross-sectional image of MP-MSC. Reproduced with permission[124]. Copyright 2021, Wiley-VCH.
However, the stacking of layered MXene makes ion diffusion difficult and reduces electrochemical performance. To address this issue, Ma et al. reported a hybrid ink of MXene/PH1000 to fabricate MSCs[124]. The highly conductive and water-soluble PH1000 was attached to the surface of the MXene sheets, effectively relieving the stacking of the MXene and facilitating ion transfer. In addition to inkjet-printed MSCs, a temperature sensor was also printed, based on which flexible, self-powered integrated systems were realized by connecting MSCs and the temperature sensor in series with a silicon film solar cell [Figure 4D].
Indeed, the practical application of MXene inks is hindered by the MXene stability issue. When MXene inks are exposed to humid environments, the edge oxidation of MXene begins, generating TiO2 and reducing device performance[125]. To enhance the MXene ink stability, a feasible strategy is to protect the edges by capping. For instance, formulating sodium ascorbate (SA)-capped MXene inks (with Triton X-100 and propylene glycol as a modifier) significantly enhances the stability of printed electrodes[103]. In a related study, Sangili et al. prepared MXene@C using a hydrothermal method to inhibit the oxidation of Ti by H2O and O2, and the resulting ink exhibited long-term stability[110].
Screen printing
Screen printing involves transferring a template design onto a flat substrate using a mesh screen made of silk or nylon[126]. The process is simple, low-cost, highly productive, and highly scalable. This requires that the rheological properties of inks meet the high requirements of the printing process. In particular, the ink must remain in a liquid state on the screen for a long time, and dry quickly on the substrate to ensure that the printed pattern is clear and well-defined.
The rheological behavior of ink under high shear stress (> 500 Pa) is mainly influenced by the loss modulus G″ (G″ > G′), which governs the ink in the form of liquid. This ensures that the ink flows easily through the screen mesh onto the substrate during printing, resulting in a well-defined printed pattern. At lower shear stresses (< 100 Pa), the storage modulus G (G′ > G″) becomes the main influencing factor, causing the ink to behave like a solid and facilitating the solidification of the ink during printing. The viscosity of inks used for screen printing generally ranges between 2-50 Pa·s[127]. Although screen printing has the advantage of low cost and scalable printing capability, its resolution is relatively low (hundreds of µm). Therefore, screen printing is unsuitable for applications that require precise control of the film at nanometer size.
The main challenge in screen-printed MXene MSCs is the low areal capacitance. This is due to the low solid loading of MXene inks that meet the rheological requirements for screen printing, as well as the severe nanosheet restacking issue which greatly suppresses the ion diffusion process. There are three main strategies to improve the electrochemical properties: (1) increasing the ink concentration; (2) surface functionalization; and (3) incorporating spacers to prevent sheet stacking.
For instance, Abdolhosseinzadeh et al. reported the formulation of highly concentrated MXene sediments inks for screen printing of MSCs[128]. Delaminated-MXene (d-MXene) can be used as a conductive binder, by adding a little to the sediments, those unetched MAX and unexfoliated m-MXene solids through the formation of hydrogen bonds and van der Waals force between MXenes, resulting in a mass loading of
Figure 5. Screen printing of MXene inks. (A) The MXene sediment inks with shear thinning behavior for screen printed patterns. Reproduced with permission[128]. Copyright 2020, Wiley-VCH. (B) Schematic of synthesis of MXene-N and the HRTEM images of crumpled MXene-N nanosheets and comparison of the printed MSC areal capacitances under different scan rates from recently published studies. Reproduced with permission[105]. Copyright 2019, Wiley-VCH. (C) The synthesis of RuO2·xH2O@MXene nanocomposite. Reproduced with permission[130]. Copyright 2019, Wiley-VCH.
It is also believed that incorporating spacers prevents the sheet stacking and improves the electrochemical performance of the MSCs. Adjusting the material composition to prepare printed MXene nanocomposite inks can effectively enhance the electrochemical performance of the devices[129]. For instance, Li et al. developed a simple in situ method to produce RuO2-xH2O@MXene nanocomposites that can be combined with silver nanowires (AgNWs) as composite inks for high-performance, fully printed MSCs[130]. The RuO2 nanoparticles in the nanocomposite possess a high pseudocapacitance while acting as spacers to prevent the self-restacking of MXene nanosheets, providing an effective network for ion diffusion. Meanwhile, the AgNWs form a highly conductive network structure and effectively transfer electrons while maintaining a flexible structure that can withstand repeated bending strain. As a result, screen-printed MSCs using RuO2-xH2O@MXene-AgNWs nanocomposite inks exhibit a volumetric capacity of 864.2 F cm-3 at a scan rate of
Transfer printing
Transfer printing is a method of transferring the ink from one substrate to another. In direct transfer printing, the material is directly transferred from the original substrate to the target substrate, similar to the process of gravure printing. Therefore, this method does not require expensive/complex printing unit, and is suitable for many types of substrates. However, the ink printing process needs to be optimized to reduce the diffusion effect. In contrast, indirect transfer printing requires an intermediate transfer medium. This process involves four steps: first, the ink is deposited on the original substrate; second, an intermediate transfer medium, typically a flexible polymer, is chosen to transfer the ink from the original substrate; third, the intermediate transfer medium carrying the ink is transferred to the target substrate, and the ink is attached to the target substrate by certain methods; finally, the intermediate transfer medium and ink are stripped. Flexographic printing is the typical example of indirect transfer printing strategy. Indirect transfer printing typically results in a higher resolution than direct transfer printing. However, more complicated procedures suggest a higher cost for the indirect transfer printing.
The performance of transfer printing is influenced by a range of factors, including the characteristics of the substrate material and intermediate transfer medium, as well as environmental parameters such as temperature, atmosphere and pressure. By adjusting these parameters, it is possible to alter the binding forces between the ink and different substrates. To produce high-quality printed patterns, it is crucial that the bonding force between the ink and the intermediate transfer medium is stronger than the bonding force with the original substrate, but weaker than the bonding force with the target substrate[131]. MXene aqueous inks can be printed on a variety of substrates using stamps with various pattern designs[93]. Ultrathin MXene micropatterns can be obtained by applying aqueous MXene ink to a polydimethylsiloxane (PDMS) stamp and then printed onto a coverslip, as shown in Figure 6A[132]. Zhang et al. have developed a method that uses aqueous inks (Ti3C2Tx and Ti3CNTx) and transfer printing technology (with the help of a 3D-printed stamp) to fabricate flexible all-MXene MSCs[133]. Ti3C2Tx MSCs with interdigitated fingers are produced by coating 3D-printed stamps with ink and then pressing them firmly onto flexible substrates, resulting in an areal capacitance of 61 mF cm-2 at 25 μA cm-2 and 50 mF cm-2 at 800 μA cm-2. The stamping strategy is cost-effective and is capable of producing high-performance MXene-based MSCs on a large scale [Figure 6B].
Figure 6. (A) Illustration of using microcontact printing (μCP) to print the ultrathin MXene micropatterns. Reproduced with permission[132]. Copyright 2016, Advanced Materials. (B) A 3D printing technology enables the creation of various stamp patterns and the use of stamps to fabricate MSCs by pressing them on the substrate. Reproduced with permission[51]. Copyright 2018, Wiley-VCH.
3D printing
A 3D printing technology, also known as additive manufacturing technology, allows the creation of complex 3D objects without wasting materials by using designed structures. Fused deposition, photopolymerization, and direct ink writing (DIW) are typical 3D printing strategies. Currently, fused deposition modeling (FDM) is the most common 3D printing method. By repeatedly extruding functionalized ink from a nozzle, depositing it on a substrate, cooling and solidifying, 3D structures can be obtained. For production of miniature devices, the DIW process is an excellent method to transfer conductive inks to substrates, offering a low-cost and convenient solution for manufacturing electronic and energy storage devices[133]. This process can be categorized into two types: solid- and liquid-state writing. Solid-state writing involves the transfer of ink without solvent volatilization, whereas liquid writing uses the solvent volatilization to solidify the ink, which is dependent on the ink's wettability on the substrate. One of the primary advantages of the DIW process is that conductive inks can be directly deposited onto the substrate upon demand, thus no materials are wasted. The DIW technique is a simple and cost-effective printing method that allows the direct deposition of materials onto a variety of substrates. In addition, the emerging electrohydrodynamic printing technology, with its high resolution, wide range of materials and scalability, has shown great potential for development in the field of printed electronics.
Additive-free aqueous MXene inks with desirable viscoelastic properties were formulated for 3D printing of MSCs. Figure 7A illustrated the manufacturing process of aqueous MXene ink for 3D printing solid MSCs. The 3D structures were successfully fabricated by continuously extruding the additive-free aqueous MXene inks layer by layer [Figure 7B]. Well-defined shaped freestanding 3D structures were obtained after freeze-drying. The assembled solid-state symmetric MXene MSCs exhibited good capacitance, with an areal capacitance of the electrodes of 2.1 F cm-2 at an active material loading of about 8.5 mg cm-2[134]. To enhance the electrochemical properties, the concentration of MXene inks can be increased by utilizing superabsorbent polymer (SAP) beads to realize the high concentration of Ti3C2Tx inks (up to 290 mg/mL) [Figure 7C]. The highly conductive ink can be directly utilized for 3D printing of MSCs as both the current collector and active material for charge storage. Furthermore, the inks can be 3D-printed on a variety of substrates, including flexible polymers and paper. The printed MSCs exhibited excellent electrochemical properties, with an areal capacitance of up to 1,035 mF cm-2[111].
Figure 7. (A) Schematic of 3D-printed MSCs fabrication process. (B) Photographs of 3D-printed microlattice and rectangular hollow prism structures. Reproduced with permission[134]. Copyright 2019, John Wiley and Sons. (C) Schematic representation of the increase in MXene dispersion concentration due to the addition of SAP beads to absorb water. The printed MSC retains the complete pattern even after bending and twisting. Reproduced with permission[111]. Copyright 2019, American Chemical Society (D) Rheological properties of 3D printable gel inks and the electrochemical properties of the symmetric SC device. Reproduced with permission[106]. Copyright 2022, John Wiley and Sons. (E) The fabrication process of MSCs using 3D printing with unidirectional freezing. Reproduced with permission[135]. Copyright 2020, John Wiley and Sons.
Furthermore, by compositing MXene with other nanomaterials, composite inks can be formulated. For instance, Zhou et al. developed MXene/cellulose nanofibers (CNFs) composite inks and successfully printed various 3D structures at low MXene loadings[106]. CNFs were used as rheology modifiers to improve the rheological properties of MXene inks at low loading for more printability. Furthermore, the 3D-printed solid-state interdigitated symmetrical SCs were fabricated, exhibiting a high areal capacity (2.02 F cm-2 at
A nanocomposite ink composed of MXene, manganese dioxide nanowires (MnONWs), AgNWs, and fullerene (C60) was reported for high-resolution 3D printing[135]. By mixing high capacitance MnONWs and highly conductive AgNWs, boosted charge transfer kinetics and improved electrochemical performance were achieved. In this composite ink, C60 acts as the lubricant, reducing friction between neighboring layers and allowing the MXene layer to stretch. The 3D-printed MSC ultimately experienced less than a 20% degradation in areal capacitance at up to 50% tensile strain and maintained approximately 75% of its initial capacitance after 1,000 stretch/release cycles [Figure 7E].
Coating methods
In addition to the printing methods discussed above, solution-based coating of MXene inks can also be employed to fabricate MXene conductive patterns. Unlike printing techniques, coating methods require less stringent ink rheological properties and equipment. Conventional coating methods include spray coating, blade coating, spin coating, slot die coating, etc.[33,136-138].
Spray coating works based on the ejection of MXene ink using pressurized carrier gas from a spray gun through a nozzle, resulting in an aerosol that rapidly dried upon depositing on the substrate, forming large-area conductive films. Factors such as solvent type, velocity, distance between the substrate and the nozzle and temperature all affect the uniformity of the obtained film. Xie et al. developed high-voltage asymmetric, on-chip MXene MSCs using Ti3C2Tx as the negative electrode and activated carbon as the positive electrode with a simple cutting-spraying method[139]. Results showed that the prepared MSCs can operate at a wide voltage window of 1.6 V and deliver 3.5 mWh cm-3 (with a power density of 100 mW cm-3), which is superior to reported on-chip energy storage devices. Besides, the MSCs exhibit good capacitance retention of ~91.4% after 10,000 cycles [Figure 8A].
Figure 8. Coating methods. (A) SEM image of interdigital gap for on-chip MSCs manufactured by cutting-spraying method and digital photograph of integrating MXene-based on-chip MSCs. Reproduced with permission[139]. Copyright 2020, Elsevier. (B) By blade coating with large flake of MXene aqueous ink, a high conductivity film can be produced. Reproduced with permission[141]. Copyright 2023, American Chemical Society. (C)Slot die coating of transparent and non-transparent films on different substrates. Reproduced with permission[142]. Copyright 2023, John Wiley and Sons. (D) The preparation of the delaminated-Ti3C2 via the freeze-and-thaw (FAT) method and SEM images of delaminated-Ti3C2 with obvious wrinkles on the surface. Reproduced with permission[143]. Copyright 2020, Wiley-VCH.
Blade coating is a versatile and scalable method that has been extensively studied due to its simplicity. In this method, the ink is deposited onto the substrate in front of the blade and then moved across the substrate to form uniform, large-area thin films. The thickness of the film obtained by blade coating varies based on the speed of the blade, the distance between the blade and substrate, the substrate wettability, and the viscosity of the ink[140]. Using the blade coating technique, Ti3C2Tx films have been reported as transparent conductive electrodes (TCEs) with excellent optoelectronic properties (19,325 S cm-1). Based on Ti3C2Tx TCEs, the electrochemical properties of MSCs were investigated, with a maximum energy density of 0.004 μWh cm-2 (at a power density of 0.23 μW cm-2) and maintaining 90% of their initial capacity after 10,000 cycles[141] [Figure 8B].
Similar to blade coating, slot-die coating is a very versatile coating technique in which a solution is delivered to the substrate and deposits to form films through a narrow slot with thicknesses ranging from a few nanometres to a few micrometers. The advantages of the technique lie mainly in the fact that the films can be produced on a large scale and are suitable for different solvent types and viscosities. By carefully regulating the speed of coating and the height of the gaps, it is possible to form highly aligned, compact films with a high degree of homogeneity. For instance, Guo et al. optimized the rheological properties of MXene aqueous inks to produce large-area, low-roughness films at RT, and then used direct laser scribing to fabricate MSCs[142]. It is noted that the rate performance of the thick film is poor due to its highly compact and aligned flake, which impedes the ion transport kinetics [Figure 8C]. On the other hand, vacuum-assisted filtration is the easiest way to produce MXene freestanding films with a desired thickness. For instance, MXene MSCs can also be fabricated by mask-assisted vacuum filtration. However, the limited filtration speed and filter size, as well as the subsequent transfer process, renders this approach difficult to scale up[143] [Figure 8D].
PRINTED ENERGY STORAGE DEVICES BEYOND MSCs
The unique structure and high conductivity of MXene allow it to be employed as a conductive host for anodes in rechargeable batteries[144]. Lithium metal is considered as an ideal anode for rechargeable lithium-based batteries due to its low redox potential (-3.04 V vs. the standard hydrogen electrode) and high theoretical capacity (3,860 mAh g-1)[145]. Nevertheless, the low Coulombic efficiency, the irregular solid electrolyte interphase (SEI) deposition and uncontrollable growth of lithium dendrites in lithium-metal batteries pose great safety risks[146]. The safety of lithium-based batteries is determined by whether the lithium dendrites formation is under control. Indeed, these dendrites are formed due to the uncontrolled electrochemical deposition of lithium ions on the negative side of the lithium metal, and their sharp tips can not only pierce the battery diaphragm, but also short circuits the positive and negative battery electrodes, causing explosion and combustion. Therefore, finding ways to prevent the formation of dendrites and improve the safety of lithium-ion batteries (LIBs) is crucial for their successful development and widespread application.
By constructing 3D MXene scaffolds, the volume expansion can be suppressed and homogeneous local current distribution in MXene-Li composite is facilitated, which is of critical significance in prolonging the lithium-metal batteries life and improving the safety.
For instance, Shen et al. used 3D-printed MXene arrays with wide gaps to prepare lithium anodes
Figure 9. (A) Schematic of 3D-printed MXene arrays and lattices. Reproduced with permission[147]. Copyright 2020, Elsevier. (B) Schematic and coulombic efficiency plots of the lithium plating/stripping behavior of vertical MXene electrodes at 1.0 mA cm-2 and SEM image of v-Ti3C2Tx electrodes. Reproduced with permission[148]. Copyright 2022, Wiley-VCH. (C) Diagram of 3D-printed LMB. Working principle of printing MXene scaffold and super-thick LFP lattice for LMB (top), freeze-drying prepare of porous MXene lattices (bottom). Reproduced with permission[149]. Copyright 2023, Elsevier. (D) Preparation process and rheological properties of Si/MXene inks and SEM images of nSi/MX-C electrode and Gr-Si/MX-C electrode. Reproduced with permission[152]. Copyright 2019, Springer Nature.
To enhance the performance of lithium-based batteries, a multi-dimensional LFP was developed as an ultra-thick cathode by 3D printing[149]. The cathode was assembled with a dendrite-free MXene@Li anode, achieving unprecedented areal capacity (25.3 mAh cm-2) and energy density (81.6 mWh cm-2) at an ultra-high mass loading of 171 mg cm-2, exceeding all reported 3D-printed batteries [Figure 9C].
In addition to lithium anodes, silicon-based anodes have received a lot of attention due to their superior theoretical capacity and abundant reserves, but the volumetric expansion caused by the lithiation process can crack the surface of the material and reduce the capacity[150,151]. Viscous Ti3C2Tx (MX-C) inks have been reported as conductive binders for the manufacture of Si/MXene anodes with high mass loading and hence high areal capacitance[152]. The MXene nanosheets in the prepared MX-C inks formed a continuous conductive network that enwrapped the Si nanoparticles, resulting in electrodes with excellent mechanical properties and the ability to produce thick electrodes [Figure 9D]. Compared to conventional additives, the thickness of the MX-C ink could reach ~350 μm before cracking occurs, while the critical cracking thickness (CCT) of conventional additives was less than 100 μm. This is due to the much higher apparent viscosity of MX-C than that of conventional two-component additives such as polyacrylic acid/carbon black (PAA/CB)-water system. Besides, the electrochemical properties of the composite electrodes were investigated. The results indicated that nanoscale Si (nSi)/MX-C exhibited good stability after 280 cycles of charging and discharging. The electrode could adapt well to changes in volume and maintain a good electron transport network, thereby addressing the stability problem of conventional polymeric binders.
The challenges of lithium-sulfur batteries include the insulating nature of sulfur and the shuttling effect of the soluble polysulfides[153]. To address these challenges, Tang et al. reported a viscous aqueous ink with nanoscale S uniformly decorated on titanium carbide MXene nanosheets (S@Ti3C2Tx)[154]. The conductive Ti3C2Tx effectively chemisorbed soluble lithium polysulfide, forming a thick sulfate layer that significantly suppresses the polysulfide shuttling during cycling and improves sulfur utilization. Eventually, S@Ti3C2Tx exhibited high capacity at all C-rates, excellent rate handling, and ultra-low capacity decay rate [Figure 10A]. Similarly, the preparation of nitrogen-doped porous Ti3C2 MXene (N-p Ti3C2Tx) frameworks that regulate both S cathodes and Li anodes using 3D printing techniques has also been reported[155]. This framework possesses a hierarchical porous structure, high electrical conductivity, and structural robustness, endowing it with the lithiophilic-sulfiphilic property and the ability to provide fast charge transport while inhibiting polysulfide shuttle and dendrite growth. As a result, the printed Li-S full cells (3DP N-p Ti3C2Tx/S||3DP N-p Ti3C2Tx @Li) can operate continuously for 60 cycles at 12.02 mg cm-2 with a cycling capacity of 8.47 mAh cm-2 [Figure 10B]. Table 3 summarized the reviewed printed/coated Mxene-based batteries.
Figure 10. (A) Schematic demonstration of Ti3C2Tx entrapping the polysulfides by forming a sulfate complex protective barrier. (B) The preparation process of LSB dual electrodes by 3D printing framework with N-p Ti3C2Tx. Reproduced with permission[154]. Copyright 2018, John Wiley and Sons. Reproduced with permission[155]. Copyright 2021, Elsevier.
The performance of printed/coated MXene-based batteries for EESDs
| Refs. | Printing/coating methods | Type | Electrodes | Electrolyte | Performance |
| [156] | Inkjet printing | Li-ion batteries | Lithium//V2O5/Ti3C2Tx | LiPF6 | 112 mAh g-1, 680 cycles (91.8%) |
| [147] | 3D printing | Li-ion batteries | Ti3C2Tx arrays-Li//LFP | LiPF6 | 149.4 mAh g-1, 300 cycles (99.4%) |
| [157] | Blade coating | Li-ion batteries | LTO/Ti3C2Tx//NMC/Ti3C2Tx | LiPF6 | 154 mAh g-1, 200 cycles (101.4%) |
| [152] | Slurry casting | Li-ion batteries | Si/Ti3C2Tx//Lithium | LiPF6 | 23.3 mAh cm-2 |
| [155] | 3D printing | Li-S batteries | N-pTi3C2Tx@Li//N-pTi3C2Tx/S | LiTFSI/LiNO3 | 8.47 mAh cm-2, 250 cycles (decay of 0.06% per cycle) |
| [154] | Vacuum filtration | Li-S batteries | Lithium//S@Ti3C2Tx | LiTFSI/LiNO3 | 1,350 mAh g-1, 175 cycles (decay of 0.035% per cycle) |
| [148] | Blade coating | Lithium metal batteries | Li@v-Ti3C2Tx//LFP | LiTFSI/LiNO3 | 135 mAh g-1, 300 cycles (85.4%) |
| [158] | Blade coating | Zinc ion batteries | Zn-Ti3C2Tx//NVO | ZnSO4/Na2SO4 | 4 mAh cm-2, 100 cycles (90%) |
| [159] | Patterning | Zn metal batteries | Ti3C2Tx@Zn//MnO2 | ZnSO4/MnSO4 | 281.9 mA h g-1, 200 cycles (79.7%) |
| [160] | 3D printing | Na metal batteries | Na@V2CTx/rGO-CNT//NVP@C-rGO | NaPF6 in diglyme | 400 cycles (86.27%) |
| [161] | 3D printing | Sodium metal batteries | Na@Ti3C2Tx/rGO//NVP@C-rGO | NaPF6 in diglyme | 500 cycles (85.3%) |
Alternatively, MXene inks can be coated for conductive substrates. For instance, the researchers developed a new gelation-assisted method for the formulation of versatile MXene gel ink, which exhibited improved rheological properties. In addition, the MXene anode prepared by blade coating showed[158] uniform zinc deposition and high plating/stripping reversibility with coulombic efficiency values of up to 99.7% at a high rate of 20 mA cm-2. The assembled aqueous zinc ion batteries exhibited a high capacitance retention of 90% after 100 cycles at a rate of 2 A g-1.
Besides, separator modification is an effective strategy for dendrite-free zinc anodes. For instance, an MXene-decorated separator by spray-printing MXene inks on one side of a commercial glass fiber (GF) has been reported[162]. The abundant surface polar groups, good electrolyte wettability, and high ionic conductivity of the MXene-GF separator facilitate uniform local current distribution and promote zinc nucleation kinetics. The full cell of the aqueous zinc ion battery assembled with Janus MXene-GF separator maintained 77.9% capacity after 1,000 cycles at 5.0 A g-1. In addition, there is an urgent need to develop micro-batteries with high power and energy density. Micro-batteries based on MXene ink printing have been reported to exhibit excellent electrochemical properties. For example, VS2/MXene cathodes have been successfully designed for aqueous micro zinc ion batteries, exhibiting high capacitance and excellent cycle life.
SUMMARY AND PERSPECTIVES
In summary, MXenes have been intensively investigated in the printed energy storage field owing to their excellent electrochemical charge storage performances, mechanical/electrical properties, and, in particular, the excellent solution processability of MXene inks. So far, a variety of methods have been investigated to fabricate MXene MSCs by employing the charge storage or electrical properties, respectively. However, the development of large-scale, reproducible and cost-effective printing/coating processes for MXene inks remains a major challenge for the commercial production of high-performance EESDs. Most critically, scaling up the production of MXene inks and improving their stability to meet industrial demand are major challenges for commercial applications. To meet industrial requirements, novel methods, such as supercritical carbon dioxide-assisted etching or gaseous assembly of MXene flakes, should be further developed with kg yields. In addition, improving the oxidative stability of inks should also be the priority when considering the scale-up of the application of MXene inks toward printed energy storage devices and beyond.
It should be highlighted that synthesizing MXene with controllable and uniform surface terminations is of significance but quite challenging. Currently, researchers have found that the surface terminations affect the lithium-ion flux and diffusion kinetics, resulting in the controllable deposition of SEI layers and suppressing the lithium dendrites formation if proper MXene surface terminations are achieved. Moreover, these surface terminations have important implications for the rheological properties, film formation, and device performance of MXene inks. The precise tuning of MXene surface chemistries should also benefit the film adhesion.
Finally, the EESD integration based on MXene with other sensors or energy conversion allows the development of more functions, opening vast opportunities of MXenes. For example, by integrating self-powered device nanogenerators with MXene EESDs, continuous energy supply can be achieved from the wind to meet the electricity demand in specific areas. Integration with sensors is also a promising direction. The integrated device does not require an external power source for charging, improving its flexibility. We are quite optimistic that through continuous investigation of MXenes, more killer applications can be unleashed very soon.
DECLARATIONS
Authors’ contributions
Selecting the topic of review, funding acquisition and supervision: Zhang, C.
Investigation and analysis: He, Q.; Chen H.; Chen, Y.; Zeng, C.
Writing and editing the review: Li, Z.; Xu, T.
Revising the review: Deng, S.
Availability of data and materials
Not applicable.
Financial support and sponsorship
Zhang, C. thanks the generous support from the Fundamental Research Funds for the Central Universities (1082204112A26), the National Natural Science Foundation of China (22209118), and the Open Research Fund of the State Key Laboratory of Mesoscience and Engineering (MESO-23-D06).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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Li, Z.; He, Q.; Chen, H.; Chen, Y.; Zeng, C.; Xu, T.; Deng, S.; Zhang, C. Multifunctional MXene inks for printed electrochemical energy storage devices. Energy Mater. 2025, 5, 500005. http://dx.doi.org/10.20517/energymater.2024.31
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