Direct ink writing of metal‐based electrocatalysts for Li–S batteries with efficient polysulfide conversion

Thanks to the significantly higher energy density compared with universal commercialized Li‐ion batteries, lithium–sulfur (Li–S) batteries are being investigated for use in prospective energy storage devices. However, the inadequate electrochemical kinetics of reactants and intermediates hinder commercial utilization. This limitation results in substantial capacity degradation and short battery lifespans, thereby impeding the battery's power export. Meanwhile, the capacity attenuation induced by the undesirable shuttle effect further hinders their industrialization. Considerable effort has been invested in developing electrocatalysts to fix lithium polysulfides and boost their conversion effectively. In the conventional process, the planar electrodes are prepared by slurry‐casting, which limits the electron and ion transfer paths, especially when the thickness of the electrodes is relatively large. Compared with traditional manufacturing methods, direct ink writing (DIW) technology offers unique advantages in both geometry shaping and rapid prototyping, and even complex three‐dimensional structures with high sulfur loading. Hence, this review presents a detailed description of the current developments in terms of Li–S batteries in DIW of metal‐based electrocatalysts. A thorough exploration of the behavior chemistry of electrocatalysis is provided, and the adhibition of metal‐based catalysts used for Li–S batteries is summarized from the aspect of material usage and performance enhancement. Then, the working principle of DIW technology and the requirements of used inks are presented, with a detailed focus on the latest advancements in DIW of metal‐based catalysts in Li–S battery systems. Their challenges and prospects are discussed to guide their future development.

export. Meanwhile, the capacity attenuation induced by the undesirable shuttle effect further hinders their industrialization. Considerable effort has been invested in developing electrocatalysts to fix lithium polysulfides and boost their conversion effectively. In the conventional process, the planar electrodes are prepared by slurry-casting, which limits the electron and ion transfer paths, especially when the thickness of the electrodes is relatively large. Compared with traditional manufacturing methods, direct ink writing (DIW) technology offers unique advantages in both geometry shaping and rapid prototyping, and even complex three-dimensional structures with high sulfur loading. Hence, this review presents a detailed description of the current developments in terms of Li-S batteries in DIW of metal-based electrocatalysts. A thorough exploration of the behavior chemistry of electrocatalysis is provided, and the adhibition of metal-based catalysts used for Li-S batteries is summarized from the aspect of material usage and performance enhancement. Then, the working principle of DIW technology and the requirements of used inks are presented, with a detailed focus on the latest advancements in DIW of metal-based catalysts in Li-S battery systems.
Their challenges and prospects are discussed to guide their future development.

K E Y W O R D S
direct ink writing, efficient polysulfides conversion, Li-S batteries, metal-based electrocatalysts 1 | INTRODUCTION As the demand for secondary batteries continues to grow, the theoretical energy density of commercially available lithium-ion batteries has reached its peak. As a result, there is a greater requirement for growing energy storage. [1,2] Against this background, the remarkable development of lithium-sulfur (Li-S) batteries has provided promise in terms of their potential usefulness. They can offer a remarkable theoretical gravimetric energy density (2600 Wh kg −1 ). Moreover, the cost is low, as sulfur, the active component, is abundantly available and easily accessible. [3][4][5][6][7][8][9][10] Nevertheless, the current utilization of Li-S batteries does indeed have a few inherent challenges regarding the sulfur cathode: (1) the insulation properties of the reactant and the final discharge product; (2) the drastic volume change (up to 80%) between S and Li 2 S during chemical reactions; and (3) the undesirable shuttle effect of soluble intermediate (lithium polysulfides [LiPSs]). [11][12][13] These problems often together lead to significant depletion of active sulfur and a marked decrease in battery capacity.
Introduction of electrocatalysts is considered a more promising strategy for addressing the severe polysulfide shuttle effect. [14,15] Electrocatalysis, by increasing the standard rate constant of internal electrode reactions, enhances the Faraday current. [16,17] Catalytic materials play a major role in this, as they not only have the ability to capture LiPSs but also improve the redox kinetics by facilitating electron/ion transfer. [18] Moreover, they also reduce the contact between long-chain LiPSs and electrolytes during charge/discharge cycles. [19] To date, various catalytic materials, such as metal (Ni, Mo, and Co), [20] metal oxides (TiO 2 , Co 3 O 4 , and Fe 3 O 4 ), [21] metal sulfides (Co 9 S 8 , MoS 2 , and TiS 2 ), [22,23] metal nitrides (TiN and VN), [24] metal phosphides (CoP, Ni 2 P, and Cu 3 P), [25] and metal carbides (TiC, Mo 2 C, and Fe 3 C) have been studied as cathode host materials. [2] The published literature on Li-S battery electrocatalysts has also grown exponentially since 2012, with the trend shown in Figure 1A. Previous reports have indeed provided comprehensive reviews of the classification of advanced electrocatalysts for systems, with a focus on their intrinsic physicochemical properties. These evaluations help in understanding the key characteristics of different types of electrocatalysts and their potential suitability for Li-S battery applications. [1] The catalytic activity of electrocatalysts depends mainly on their electronic structures, and therefore, a rational manipulation of the electronic structure is a key factor to achieve efficient electrocatalysis. [1] Specifically, the traditional cathodes usually adopt the method of slurry casting with an aluminum foil as the current collector. Slurries usually contain 10-15 wt% of binders to confer strong support to the current collector. [26] However, the introduction of the binder weakens the electron transport path and adversely affects the total energy density of the batteries. [9] Moreover, a significant challenge is the construction of cathodes capable of accommodating high levels of sulfur loading. This is primary for accomplishing high areal capacities while maintaining excellent cycling stability throughout the battery's operational life. [27] This is because the cathode is prone to cracking/delamination from the current collector after drying at high loadings, consequently disrupting the electron transmission route and confining the electron flow to the active material, highareal sulfur. [28,29] The ion transport distance increases in thicker planar electrodes, leading to an increase in interfacial resistance and consequent reduction in rate capability and energy density. [30] Besides, the traditional battery fabrication process based on slurry-casting electrodes has difficulty meeting the specific requirements of miniaturization and customization. [31,32] F I G U R E 1 (A) Statistical data of articles on the topic of "lithium-sulfur (Li-S)" and "catalysis" published from 2010 to 2023. (B) Direct ink writing (DIW) technology of metal-based electrocatalysts for Li-S batteries. Therefore, to shorten the diffusion pathways of ions and lower resistance, it is imperative to incorporate a threedimensional (3D) architecture into the electrodes with larger surface areas. [31] As an advanced manufacturing technology, 3D printing or the additive manufacturing technique is receiving increasing attention for its possible applications in aerospace, automotive, medical, and energy storage fields. [33][34][35][36] With well-developed inks, sophisticated electrode geometry and structure can be easily designed by a combination of computer-aided design and rapid manufacturing processes. [37][38][39][40][41] Generally, the printed structure is a grid with parallelly arranged rods in alternating layers. Therefore, a multichannel structure with the largest surface area and rich porosity can boost the permeability of the electrolyte and promote rapid ion transport to the active material. Meanwhile, 3D printing technology affords a potential solution to address the challenges related to electrode thickness and active material loading in energy storage systems. [29,30,42,43] Despite many guiding reviews on 3D printed electrochemical systems, few reviews cover the recent progress and future potential of use of direct ink writing (DIW) for metal-based electrocatalysts targeting commercially feasible Li-S batteries. Herein, an extensive and thorough presentation of the latest advancements in DIW of metal-based electrocatalysts with efficient LiPSs conversion for Li-S batteries is provided, see Figure 1B, including insightful descriptions of design principles, mechanistic electrocatalysis functions, and practical prospects.

| ELECTROCATALYSIS IN Li-S BATTERIES
The typical Li-S battery principle is controlled by several highly complex and unbalanced multistep transformation reactions. [44] Taking the discharge process as an example, several intermediate LiPSs (Li 2 S n , n ≤ 8) and final products are generated from sulfur (S 8 ). Specifically, during the initial discharge phase, long-chain soluble LiPSs (Li 2 S 8 and Li 2 S 6 ) are generated, which are further reduced in the middle discharge stage, producing the middle-stage LiPSs. The middle-chain LiPSs undergo transformation into solid-phase end products, namely, Li 2 S 2 /Li 2 S, and deposit on the cathode. [16] Notably, the first step is considerably easy because of its low energy barrier, while in sulfur redox kinetics, the last step of the reaction is slow thanks to the high energy barrier. [45] Meanwhile, the discharge products tend to deposit on the surface of active sites, which gradually diminishes their catalytic activity. [46] This phenomenon can cause battery performance to degrade over time. It is worth noting that approximately 75% of the total discharge capacity originates from the last step of sulfur redox kinetics, where Li 2 S is formed. To promote the decomposition of Li 2 S 2 /Li 2 S, which show insulating properties, an additional driving force is necessary during the subsequent charging process. [1] Therefore, suitable electrocatalysts are needed to promote Li 2 S 2 /Li 2 S precipitation and decomposition, which will determine battery capacity. [1,2] Electrocatalysis is a type of catalytic behavior that occurs at the interface between two phases and involves the diversion of charges and electrons. In the electrocatalytic system, this nonhomogeneous process necessitates the presence of two distinct phases and one interface. [16,47] Consequently, it becomes imperative to gain a comprehensive understanding of the underlying mechanism of electrocatalysis. This will enable efficient introduction of metal-based catalysts to effectively improve the cycle stability and rate performance of sulfur electrodes.

| Electrocatalysis mechanism
Catalysis is a fundamental concept in chemistry, playing a crucial role in expediting reactions by effectively lowering the activation energy needed for the process to occur. [48] Efficient introduction of electrocatalysts to speed up the reaction rate and promote complete transformation of LiPSs is an efficient path to improve the electrochemical performance. The introduction of an electrocatalyst in the sulfur electrode can cause changes during the surface transformation steps and electron conversion processes. This alteration results in the solid sulfur being reduced to soluble LiPSs, which are subsequently adsorbed by the electrocatalyst. Afterward, the electrocatalyst facilitates the gradual conversion of the soluble LiPSs into solid Li 2 S, enabling efficient conversion while minimizing any loss of the active material in the process. [49] Notably, both practical experiments and theoretical calculations have played a major role in unraveling the significance of adsorption and electron transfer in the electrocatalytic process. The adsorption of reactants is a key aspect of the pre-surface transformation reaction, serving as a prerequisite for their catalytic conversion. Carbon-based materials of distinct forms, including 0D quantum dots, 1D nanofibers and nanotubes, two-dimensional (2D) nanoflakes and nanowalls, 3D porous hollow spheres, and so forth, are extensively used for fabricating sulfur hosts. [2,50] However, the ability to immobilize LiPSs on the surface of nonpolar carbon materials is typically limited to weak physisorption interactions driven by van der Waals forces. [51] Insufficient adsorption capacity results in ineffective immobilization of reactants on the surface of the electrocatalyst, consequently leading to a decrease in conversion efficiency. In this respect, metal compounds with chemical entrapments in Li-S chemistry were investigated by relatively strong interactions with LiPSs. [52] However, more than physical and chemical adsorption is required to improve the electrochemical performance of LiPSs. Under extreme situations (e.g., high sulfur loading, a lean electrolyte/sulfur [E/S] ratio, or a low-temperature test environment), the physicochemical process of anchoring/conversion of LiPSs in batteries often shows sluggish behavior. This slow kinetics can result in uncontrolled deposition of Li 2 S, which can adversely impact the battery's performance.
Therefore, the research focus extends beyond solely controlling the decomposition and diffusion of LiPSs. It also involves improving the kinetics of LiPSs' transformation into the final Li 2 S 2 /Li 2 S products. This approach represents a potentially useful strategy to attain highenergy battery systems. Catalytic materials have the capability to capture LiPSs and facilitate their oxidation-reduction conversion through rapid electron/ ion transfer. This process leads to an unsaturated state, effectively mitigating the undesired effects of LiPSs dissolution and diffusion in Li-S batteries. [2,53] Consequently, it is feasible to capture LiPSs and further achieve stable adsorption conversion. Moreover, electrocatalysts can also increase the rate of Li 2 S dissolution by weakening the energy barrier associated with Li 2 S dissolution. This lowered energy barrier facilitates the efficient dissolution of Li 2 S and contributes to improved performance in Li-S batteries. [16,47] However, both adsorption and conversion processes are intricately linked to the characteristic of different LiPSs. Consequently, exploring the electrocatalytic performance of each step plays a critical role in assessing the electrocatalysts' efficiency in Li-S battery systems. This comprehensive analysis enables a better understanding of the overall performance and effectiveness of electrocatalysts. [54] The immobilization of LiPSs by electrocatalysts is influenced by the sulfur atoms present in the LiPSs species. The number of sulfur atoms involved in the process affects the adsorption behavior and highlights the importance of considering the specific interactions between electrocatalysts and LiPSs, based on their sulfur content, for effective performance in Li-S battery systems. [55] Hence, determining the rate-limiting step is crucial in increasing the overall reaction rate of the process. By identifying and addressing the step that poses the most significant hindrance to the reaction kinetics, it becomes possible to enhance the overall speed of the process. [54] When multiple rate-limiting steps are present, the addition of multiple catalysts becomes an option.
In cases where both the adsorption procedure and the electron diversion step act as rate-limiting processes within a single reaction, a combination of two differentacting electrocatalysts (one with remarkable adsorption capability and the other with exceptional electrocatalytic ability) can be used to improve efficiency and accelerate the overall reaction rate. By leveraging the complementary strengths of these catalysts, improved performance can be achieved in the system. [16] 2.2 | Metal-based catalysts for Li-S batteries Currently, an increasing number of electrocatalysts specifically designed for sulfur cathodes are available. These electrocatalysts play a vital role in facilitating LiPSs' adsorption, enhancing electronic conductivity, and promoting efficient electron transfer. [16] Among these electrocatalytic materials, metal-based catalysts hold a prominent position as they have intriguing structural and electronic properties. As a result, they have found widespread use in Li-S batteries, contributing to improved performance and overall battery efficiency. [56,57] Among them, some typical metals, metallic compounds, and carbide-based materials have been studied long and intensively, and groundbreaking advances have been achieved. [2,[58][59][60][61][62][63] Besides, the emergence of novel catalytic materials (e.g., metal boride, metal phosphides, metal selenide, and monatomic metals, as well as the adoption of defect engineering) significantly advanced the LiPSs chemical kinetics. These developments have efficiently prevented the dissolution of LiPSs in the electrolyte and reduced the loss of active sulfur. [64] In this section, we will summarize some of the metal-based catalysts that are being extensively used for Li-S battery systems, highlighting their contributions to the field.
Metals have excellent electrical conductivity and can boost interfacial electrochemical processes. [60,65] As an example, a porous carbon flower (Figure 2A,B) decorated with nickel nanoparticles as sulfur hosts was reported by Bao and co-workers. [66] The Ni-CF/S electrodes enable short ionic transport lengths and can provide stable chemical effects and rapid chemical kinetics of LiPSs. Consequently, the batteries can maintain 87% of the initial discharge capacity after 50 cycles ( Figure 2C) at 0.1C. Indeed, the utilization of diverse electrocatalysts and the optimization of their properties represent a simple yet effective means to achieve excellent performance of battery systems. By increasing the electronic transmission rate, suppressing undesired effects, and improving sulfur utilization, these approaches contribute to the overall performance and feasibility of Li-S batteries for more field applications. Because of the considerable cost, convenient synthesis methods, and the strong chemical binding ability of LiPSs with strong O 2− anion polarity, metal oxides (VO 2 [58,63,[67][68][69] Yushin and co-workers [70] first reported mesoporous Magnéli-phase Ti 4 O 7 microspheres with uniform size to develop an atom-economic comproportionation reaction. The prepared Ti 4 O 7 microspheres have relatively narrow pores (about 2-5 μm, Figure 2D,E), which are conducive to the physical restriction of LiPSs, and the rough surface is conducive to reducing electrode polarization. Therefore, the obtained Ti 4 O 7 /S cathode shows high-capacity utilization and shows a capacity decay rate of 0.09% per cycle, and promising rate properties ( Figure 2F). In metal-based sulfides, the presence of different electronegativities among the elements leads to the formation of polar covalent bonds. This polarization of bonds creates a favorable environment for the sharing of electron pairs between atoms. [71] Moreover, the excellent electrical conductivity of metal sulfides can shorten the energy barrier of Li 2 S formation and boost the reversibility of redox processes between Li 2 S and LiPSs. By facilitating these reactions, metal sulfides contribute to improved performance and increased efficiency in Li-S batteries. Metal-based sulfides (Co, V, Ni, Mn, Ti, and Mobased sulfides and their compounds), as functional sulfur hosts with dual adsorption and catalytic functions, are applied to manufacture high-load Li-S batteries. [23,59,62,72] Co-based sulfides are widely applied to obtain high-sulfurloading cathode hosts. Meng et al. [73] designed distinct highly puffed Co 9 S 8 @CNFs as sulfur hosts ( Figure 2G,H). The obtained S⊆Co 9 S 8 @CNFs cathodes deliver a large specific capacity of 1080 mAh g −1 and a low-capacity  [70] Copyright 2021, Elsevier. (G) Schematic for the synthetic procedure of S⊆Co 9 S 8 @CNFs. (H) SEM images of Co 9 S 8 @CNF and S⊆Co 9 S 8 @CNFs. Reproduced with permission. [73] Copyright 2019, American Chemical Society. decay rate of 0.03% per cycle. The interconnected structures within the host play a critical role in accommodating a large amount of sulfur, facilitating efficient Li + and electron transfer, and contributing to additional reversible capacity. Moreover, the physical robustness and polarized nature of each Co 9 S 8 subunit collectively enable space constraints and chemical bonding, effectively limiting the pulverization of electrodes and anchoring soluble LiPSs in the cycling process. This synergistic effect of structural and chemical characteristics can boost LiPSs' adsorption and catalytic performance.
Metal nitrides with excellent electrochemical activity and good LiPSs chemisorption can be designed to store sulfur in specific structures. However, their nanostructures are not easy to control. Aiming to tackle this problem, Lim et al. [74] synthesized a well-developed macro-and mesoporous TiN (h-TiN) using a one-pot method ( Figure 3A,B). DFT calculations confirmed the strong mutual effect between LiPSs and h-TiN (200) with lower adsorption energies, as shown in Figure 3C-E. As a result, the prepared h-TiN host with 72 wt% sulfur content has a capacity decline of only 0.016% per cycle. MXene is a 2D inorganic compound consisting of transition-metal carbides, carbonitrides, or nitrides. [75] It consists of multiple atomic layers and has received significant attention in material science research since its discovery in 2011. One of the key reasons for its importance is the metallic conductivity present at the hydroxyl or terminal oxygen surfaces. This unique property makes MXene a highly promising material for various fields. [71,76] The exfoliated MXene nanosheets are usually hydrophilic under the action of valuable surface functional groups (e.g., -OH, -O, or -F). [77] In addition, MXenes also have high electrical conductivity (ranging from 6000 to 80 000 S cm −1 ). Various MXenes are used to store active sulfur and extensively explored for Li-S batteries. Gu et al. [78] synthesized a hierarchically porous membrane decorated with a completely dispersed monolayer of MXene using a simple phase inversion strategy. The 3D layered porous frameworks can enhance the transport of ions and electrons, adapt to volume expansion, achieve high-level sulfur loading, and afford sufficient active sites for anchoring LiPSs, as shown in Figure 3F. Thus, the Ti 3 C 2 T x -CNT@C batteries have remarkable rate performance and outstanding cycling stability, even when operating under the conditions of 4.5 mg cm −2 ( Figure 3G).
Metal composites have received widespread attention in Li-S battery electrocatalysis because there are various types and sources of metal composites. However, an individual metal composite often has limitations and cannot achieve the best performance simultaneously. [48] The concept of bifunctional heterostructures has been proposed to address this issue. [79][80][81] The combination of highly adsorbent metal oxides and highly conductive metal nitrides can effectively enable preparation of heterogeneous structured electrocatalysts. Zhao and co-workers designed a Ti 4 O 7 /TiN/C heterojunction with a porous microdisk structure to store sulfur. [82] The presence of an abundant polar bond (N-Ti-O) at the boundary between Ti 4 O 7 and TiN, as depicted in Figure 4A-C, is noteworthy. These covalent bonds provide the host with stable adsorption capability for LiPSs while also facilitating their efficient electron transfer during the conversion. The TiN/Ti 4 O 7 /C heterojunction shows excellent electrochemical performance (a high discharge capacity of 615.5 mAh g −1 at 4C). Catalysis acceleration of the conversion reactions can alleviate the LiPSs' shuttle effect to a large extent. However, most synthesized catalysts tend to function in only one direction, either for the reduction or oxidation reaction in the redox process. The ability to improve both reduction and oxidation reactions at the electrodes is then necessary. Therefore, an oxide-sulfide heterostructure (TiO 2 -Ni 3 S 2 , Figure 4D) is designed to improve reductions of soluble LiPSs and oxidation of the final Li 2 S by designing TiO 2 nanoparticles on the Ni 3 S 2 surface. [83] As shown in Figure 4E, the stable adsorption ability of TiO 2 and the catalytic effect of Ni 3 S 2 efficiently boost the reduction process of LiPSs and promote the deposition of Li 2 S in the discharge step. Moreover, both TiO 2 and Ni 3 S 2 have demonstrated high catalytic oxidation for the dissolution of Li 2 S during the charging process. These catalysts effectively promote the oxidation reaction, facilitating the conversion of Li 2 S back into soluble LiPSs. This oxidation process helps renew the catalyst surface, ensuring its continuous functionality in subsequent cycles. Figure 4F compares the cyclic stability with a low sulfur content at 0.5C, revealing a low-capacity decay rate of 0.04% per cycle for the TiO 2 -Ni 3 S 2 /rGO cathode. In addition, the improved catalytic ability ensures good sulfur utilization, thereby achieving a Coulombic efficiency of about 100% after 900 cycles. This study provides updated methods to realize the high energy density of battery systems by improving the redox process simultaneously.

| 3D PRINTING-DIW TECHNIQUE
Despite their significance, use of metal-based cathode hosts still involves a major challenge. Traditional fabricating methods, such as roll-to-roll processing, are not able to achieve high geometrical complexity. Moreover, as the sulfur content increases and the E/S ratio decreases, the accumulation of LiPSs on the substrate surface becomes more pronounced during the cycling process. This phenomenon poses a challenge in meeting the demands for high specific capacity in various applications. 3D printing technology is a disruptive method of processing. It uses digital design models to manufacture 3D objects by depositing active material inks layer by layer. [42,84] Compared with commercial manufacturing, it enables cost-effective fast prototyping and free-form programming. [41,85,86] Among various 3D printing technologies, DIW has become a commonly adopted technology for fabricating Li-S batteries owing to its unique characteristics and advantages. [87] However, DIW technology has certain limitations and challenges. Some of these include high ink requirements, the relatively weak mechanical properties of printed structures, and the need for additional post-processing steps. [40] These factors highlight the importance of having a comprehensive understanding of the principles and requirements of printing. In the following section, we will present a detailed discussion of these principles and requirements, aiming to provide a deeper insight into the printing process and address the associated challenges and considerations.

| Printing principles and requirements
DIW is the most common extrusion-based 3D printing system, offering advantages of ease of use, low cost, and the availability of a wide range of material sources.
Moreover, it can be used to build complicated 3D structures of any shape and high precision, as shown in Figure 5. [87] Unlike other printing methods, DIW has high geometric orientation and design adaptability that will facilitate both in-plane and out-of-plane structure design. Additionally, the thickness of the electrodes can be controlled to remain between a few hundred nanometers and millimeters. One way to enhance the energy density of printed structures is by improving the load of active material within a limited footprint. By optimizing the printing parameters such as nozzle diameter, ink extrusion features, and applied pressure, it is possible to control the resolution of the printed structures. These parameters collectively determine the precision and fidelity of the printed features. [88] The nozzle is more customizable and can be downsized to tens of micrometers or even nanometers. [89] In the DIW process, the material ink is extruded directly from the masterbatch without melting or solidifying. Therefore, the ink must have a low viscosity when extruded from the nozzle to maintain its fluidity. After extrusion, it is crucial for the material to have high viscosity to retain its F I G U R E 4 (A) Schematic illustration of the merits of a Ti 4 O 7 /TiN/C microdisk for a high-performance lithium-sulfur battery. High resolution of (B) Ti 2p and (C) N 1s XPS spectra. Reproduced with permission. [82] Copyright 2022, Elsevier. (D) SEM image of TiO 2 -Ni 3 S 2 / rGO. (E) Li 2 S deposition and dissolution measurements. (F) Long-term cycling performance at 0.5C. Reproduced with permission. [83] Copyright 2020, John Wiley and Sons.
shape on the printing bed. [90] Generally, the printing ink has a viscosity in the range of 10 2 -10 6 mPa·s as well as a shear rate of about 0.1 s −1 to insure the printability of the ink. [91] Ensuring smooth ink flow is essential to prevent clogging of the deposition nozzle during the printing process. The loss modulus (G″) to storage modulus (G′) ratio is a significant indicator of the ink's ability to maintain shape during the printing process. Typically, it is desirable for the value of G′ to be larger than that of G″ of the DIW ink. [91] Furthermore, it is important that the value of G″ be maintained relatively constant at low shear stress. The rheological properties of inks are usually evaluated by shear rheological experiments using a rotary rheometer. Based on this, DIW enables the printing of materials using a wide range of raw feedstock. [92] In the context of Li-S systems, DIW technology offers two approaches for preparing S-based cathodes: (1) the as-prepared 3D printing host, typically a porous structure or scaffold, is subjected to vapor infiltration of active sulfur and (2) the ink used for DIW contains a suspension of sulfur particles. Among these, the latter ink usually consists of sulfur-containing active materials, conductive additives such as activated carbon, and polymers as binders dissolved in organic solvents. The DIW technology offers flexibility in terms of materials class for ink formulation. By regulating the composition of the precursor ink, including the addition of plasticizers and rheology modifiers, it is possible to tune the rheological properties to meet the specific requirements of the printing process.

| DIW of metal-based electrocatalysts for Li-S batteries
Three-dimensional printing's hierarchical structure offers several advantages in Li-S batteries. Carefully designed large pores and a hierarchical architecture can improve the overall performance of the sulfur electrode. [93] Gao et al. [29] prepared a freestanding 3D-printed sulfur/carbon composite that had a high active material areal loading ( Figure 6A). Figure 6B shows the electrochemical performance of the cathode due to its excellent ion and electron transport capabilities at both the micro-and nanoscale. Importantly, the precise control of the stacked layers in the 3D printing operation allows for accurate manipulation of the loading of the active material. In the second year, the researchers introduced an electrode that was independent of thickness for the first time ( Figure 6C). By combining DIW technology with the icetemplate process, the electrode can be transformed from a thick electrode into a "thin electrode." It is vertically aligned and has a constant thickness of about 20 μm. [94] The "thin electrode" is the effective thickness of the layer of active material within the printed electrode, rather than the entire thickness of the printed electrode itself. Therefore, although the total thickness may be reduced through 3D printing, the emphasis is on how to achieve printed electrodes with a thin and well-optimized active material, which is critical for improving the electrochemical performance.
While 3D-printed carbon-based hosts can offer high conductivity and a porous structure, there may be F I G U R E 5 Schematic diagrams and advantages based on direct ink writing. Reproduced with permission. [87] Copyright 2023, Royal Society of Chemistry. challenges in mitigating the shuttle effect. This is particularly true for highly mass-loaded cathodes. [40] Therefore, it is necessary and relevant to introduce metal-based electrocatalytic mediators in 3D-printed hosts to improve reaction kinetics and electrochemical performances. Along this line, Ouyang et al. [27] reported Co nanoparticles and nitrogen-doped porous carbon fiber composites and applied them as an advanced sulfur host ( Figure 7A). The non-Newtonian behavior of the printed ink enables continuous flow during extrusion, facilitating the printing of complex patterns and structures, and allowing for tailoring of the performances of printed materials, as shown in Figure 7B. In addition, the relationship of G′ and G″ with shear stress plays a major role in maintaining the stability of the extruded structure during the printing process ( Figure 7C). Consequently, the as-assembled Li-S batteries can provide remarkable areal capacity and long cycle life with sulfur mass loadings of 4.5 and 7.1 mg cm −2 (see Figure 7D,E). The weak metal-carbon interaction can hinder the full utilization of catalytic properties in certain systems. Through synergistic metal-metal interactions, bimetal alloying modulates the electronic structure of single metals and reduces the energy barrier of the ratedetermining step. For example, a CoFe alloy embedded with porous carbon spheres (CoFe-MCS) is a notable example of a bimetallic catalyst developed by Sun and coworkers for the dual-directional conversion of LiPSs. [95] Through detailed electrical analysis characterization, theoretical calculations, and in situ instrument detection, it has been successfully revealed that CoFe-MCS has excellent electrocatalytic activity involving bidirectional redox processes ( Figure 7F,G). Encouragingly, this printed cathode offers a low-capacity decay rate of 0.062% per F I G U R E 6 (A) Schematic illustration of the 3DP sulfur/carbon (S/BP-2000) cathode. (B) Unique advantage of the application of threedimensional (3D) printing fabrication technology. Reproduced with permission. [29] Copyright 2019, Elsevier. (C) Mechanism of lamellar growth of a 3D-printed vertical aligned electrode. Reproduced with permission. [94] Copyright 2020, Elsevier.
cycle and a high areal capacity of 6.0 mAh cm −2 at 7.7 mg cm −2 , thanks to optimized redox kinetics and a refined grid structure ( Figure 7H-J).
Cobalt-based sulfides (e.g., CoS 2 , Co 3 S 4 , and Co 9 S 8 ) are being designed carefully to store sulfur hosts for the improvement of redox reaction kinetics at the electrode. One of the notable features is their excellent roomtemperature conductivity. [7] Zheng et al. [96] designed 3D printed C/S/CoS x cathodes with extensive microchannels and high sulfur loading to achieve ideal deposition of LiPSs with enhanced Li + and e − transport ( Figure 8A). Consequently, such a designed electrode shows a high initial discharge capacity (1118.8 mA h g −1 at 3 mA cm −2 ) and a low-capacity attenuation of 0.2% per cycle after 150 cycles with a sulfur loading of 8 mg cm −2 . Metal phosphides are regarded as prominent examples in the F I G U R E 7 (A) Schematic illustration of a three-dimensional (3D)-printed Co/Co-N@NPCF-S cathode. (B) Apparent viscosity and (C) storage modulus and loss modulus of the Co/Co-N@NPCF-S ink. Cycling performance of 3D-printed Co/Co-N@NPCF-S electrodes with sulfur loadings of (D) 4.5 mg cm −2 and (E) 7.1 mg cm −2 , respectively. Reproduced with permission. [27] Copyright 2022, John Wiley and Sons. (F) Operando Raman spectra of the electrolyte with respect to the S@CoFe-MCS cathode. (G) Schematic illustration of CoFe-MCS to accelerate the dual-directional polysulfide electrocatalysis in comparison with a bare Fe cluster for lithium-sulfur chemistry. (H) Rate performance and cycling performance of a 3DP cathode with sulfur loadings of (I) 5.3 and (J) 7.7 mg cm −2 , respectively. Reproduced with permission. [95] Copyright 2020, John Wiley and Sons. emerging areas of nonprecious metal electrocatalysts. [97] The electronegative nature of phosphorus (P) atoms in metal phosphides allows them to efficiently absorb electrons from metal atoms, making them effective electron acceptors. [98] By providing electron-rich sites, metal phosphides facilitate the redox processes and improve catalytic activity. The study of the reaction kinetics of LiPSs catalyzed by metal phosphides is still in its infancy. [57] By understanding the importance of surface-oxidized compounds and optimizing the exposure of active sites, researchers aim to boost the catalytic performance of metal phosphides and unlock their full potential as electrocatalysts in Li-S batteries. [98] In one work, Zhang et al. [99] reported polar Ni 2 P hollow microspheres as effective sulfur hosts. As shown in Figure 8B, the multilayer nested morphology of Ni 2 P provides not only multilayer barriers for LiPSs but also large cavity spaces to accept high-sulfur loading and volume variations. Notably, the redox peak of the 3D printed Ni 2 P@S (3DP-Ni 2 P@S) cathode is significantly narrower ( Figure 8C), indicating enhanced reaction kinetics for polysulfide conversion. Meanwhile, it can afford an outstanding areal capacity of 9.8 mAh cm −2 at 0.05C when the sulfur loading is 7.3 mg cm −2 . The 3DP-Ni 2 P@S cathode still maintains excellent capacity retention even with a higher sulfur loading of 10.4 mg cm −2 ( Figure 8D). Recently, metal borides have garnered considerable attention in the production of advanced Li-S battery systems. Therefore, lanthanum hexaboride (LaB 6 ) is considered a viable candidate due to its similar characteristics and potential advantages. Sun and coworkers designed a free-standing hybrid cathode with a metallic LaB 6 electrocatalyst. [38] Figure 8E [38] Copyright 2020, Elsevier. decolorization of the Li 2 S 6 solution by LaB 6 highlights its potential to improve the catalytic activity and selectivity of Li-S battery systems ( Figure 8F). Consequently, the printed LaB 6 /SP@S electrodes can achieve an areal capacity (7.75 mAh cm −2 ) over 4.0 mAh cm −2 at 0.05 C with a sulfur loading of 9.3 mg cm −2 , as shown in Figure 8G. This work provides more choices of electrocatalyst materials for high energy density and long-term cycling performance in Li-S batteries.
Combining the strong adsorption capacity of oxides on LiPSs and the catalytic activity of sulfides on LiPSs reduction and Li 2 S decomposition offers significant scientific insights and practical benefits. [51] Wei et al. [100] proposed a versatile way to synthesize MO x -MXene (M: Ti, V, and Nb) heterostructures as multifunctional hosts with an excellent capability for immobilizing and transforming LiPSs. As shown in Figure 9A, these heterostructures all show an accordion-like morphology. More encouragingly, the 3D-printed representative VO x −V 2 C/S electrode with increased sulfur loading of 10.78 mg cm −2 yields an areal capacity (9.74 mAh cm −2 ) at 0.05C. This is attributed to smooth mass transport and rapid carrier diffusion, as well as the layered porous matrix from 3D printing, which provides ample room for sulfur redox/volume changes ( Figure 9B,C). Beyond the cathode, the uneven Li deposition/dissolution at the anode leads to uncontrolled dendrite growth, resulting in low Coulombic efficiency and short life in practical Li-S batteries. To address issues with the use of Li anodes, proposed solutions include a 3D framework with lithiophilic surfaces, optimized electrolyte composition for tailoring the chemical/physical properties of the solid electrolyte interphase, an artificial interlayer for Li anode protection, and modified separators for even Li + deposition. By using the "two-in-one" strategy, researchers aim to tackle the dual challenges of the LiPSs shuttle effect and Li dendrite growth, ultimately improving the electrochemical performance of Li-S batteries. [101] For instance, Cai et al. [102] prepared a V 8 C 7 -VO 2 heterostructure as a promising host for both S and Li electrodes ( Figure 9D). The as-designed V 8 C 7 -VO 2 heterostructure with polarity and conductivity can offer significant advantages in terms of strong immobilization and rapid conversion of LiPSs. At the same time, this heterostructure with lithiophilic properties can promote uniform growth of Li, efficiently reducing the nucleation overpotential and inhibiting the dendritic formation, as shown in Figure 9E. As a result, integrated 3D printing full batteries delivered a high areal capacity of 7.36 mAh cm −2 with a Coulombic efficiency of over 99.7% ( Figure 9F). When directly made into a bracelet battery, it can successfully power an electronic watch (see Figure 9G), providing excellent potential for use for wearable energy applications. Similarly, a multifunctional 3DP framework consisting of nitrogen-doped Ti 3 C 2 MXene (N-pTi 3 C 2 T x ) was prepared for conditioning difunctional electrodes by Wei et al. [103] This framework has stratified porosity, abundant nitrogen-active sites, and high electrical conductivity for synergistic lithiophilic-sulfiphilic characteristics. Impressively, the printed Li-S full battery shows a remarkable areal capacity of 8.47 mAh cm −2 at 0.1C with an elevated loading of 12.02 mg cm −2 . This work reveals the reasonable design of the 3D printed frameworks, providing potential avenues for the growing number of applications for energy storage systems in the real world.

| SUMMARY AND OUTLOOK
The efficiency and electrochemical properties of Li-S batteries are severely affected by slow chemical reactions and undesired intermediate dissolution. The development of suitable electrocatalysts with stable immobilization for LiPSs and a powerful catalytic ability for their transformation is indeed crucial in the advancement of battery systems. The DIW 3D-printed Li-S batteries have comparable or even higher volumetric power density/energy densities than conventional 2D planar electrodes. They also demonstrate the advantages of areal capacity enhancement, mainly due to high sulfur mass loading, thick electrodes, and hierarchical porous structures for improved electrolyte-active material contact and rapid ion transport. Besides, the printed electrode does not usually have a current collector. In this rereview, the mechanism of metal-based electrocatalysts for Li-S battery systems was discussed first. Varieties of catalytic substrates, including metals and metal compounds, have been applied to store active sulfur. Also, the specific electrochemical performance with different electrocatalysts was compared ( Figure 10A and Table 1). Then, we described the promising DIW technology and discussed the working principle of the techniques as well as the requirements of printing inks. In detail, ink rheology is crucial to the printable characteristics of fluid ink. Finally, the research progress of DIW metal-based catalysts for Li-S batteries was summarized ( Figure 10B and Table 2). Printed metalbased catalysts as the sulfur host can improve LiPSs' trapping ability and facilitate the intermedium transformation. This, in turn, effectively inhibits the shuttle effect. While existing strategies have effectively guided researchers in selecting suitable sulfur hosts, the practical Li-S batteries obtained by DIW-printed metal-based electrocatalysts have some challenges. The following factors need to be considered to improve the electrochemical properties: (1) it is critical to enhance the trapping capability of LiPSs and increase the transition rate. Therefore, it is very important to choose a catalyst that has chemisorption on LiPSs and has conductivity, mass transfer, and catalysis properties. This provides electron transfer pathways and facilitates fast electrocatalysis. To meet higher energy requirements, Li-S battery systems must have high sulfur content (>70%), high sulfur loading (>7 mg cm −2 ), and a low E/S ratio (6 μL mg −1 ). Nevertheless, the higher sulfur content, the higher polysulfide yield, and the small amount of electrolyte also lead to greater challenges in terms of wettability and ion migration, which in turn lead to higher requirements in terms of the electrocatalysts. Printed metal-based electrocatalysts with high catalytic activity could increase the redox conversion of LiPSs even under the above-mentioned harsh conditions. (2) The optimal design of catalysts needs to be further investigated. Using DIW technology, it is possible to regulate the pore size and structures, and even the assignment of active components of catalytic materials. Therefore, optimizing the structure of catalysts to maximize their catalytic performance becomes increasingly important. Meanwhile, such designed 3D structures show outstanding structural stability, allowing them to withstand the stresses induced by volume expansion in charge and discharge processes. (3) Although DIW seems to be a feasible technique to overcome the difficult problem of high loading in conventional planar electrodes, there are still some drawbacks to overcome to make the technique more widely available in actual applications. To tackle the above problems, a framework with satisfactory printing accuracy is needed to better design the micro/mesoscopic structures of the printed catalytic material. Synthesis methods for inks are essential for better control of nanoscale structures and improved productivity for industrial applications. On the other hand, other ways of 3D printing techniques might be extended to the catalytic aspect to obtain higher-precision electrode structures for Li-S batteries with more exposed active sites. The selection of a catalyst with the aforementioned properties would ensure achievement of high load capacities and fast charge and discharge rates in Li-S batteries.