The continuous porous structure of 3D NMS scaffolds allows them a large specific surface area, which is important for EES devices. Inherently, the large specific surface area can provide more electrochemical reaction interfaces and reaction active sites, which means that it can accommodate more charges and reach a higher energy density [48]. Intuitively, the large specific surface area can increase the contact area between the electrodes and the active material, thus accelerating the transfer speed between electrons or ions and the active material, which helps to improve the multiplicity performance of the energy storage device [49]. Moreover, the large specific surface area helps to spread out the distribution of electrons and ions in the electrodes, preventing the occurrence of side reactions caused by uneven local current density and improving the stability of the energy storage device.

Transport kinetics in EES refers to the transport behavior of electrons and ions. In electrodes based on 3D NMS scaffolds, the transport path of electrons is based on a 3D conductive collector network, whereas the transport path of ions relies on the negative space that can be penetrated by the electrolyte left by the electrode. In electrodes based on interconnected porous 3D NMS scaffolds, the presence of collectors is divided into two cases, including the conductive scaffolds acting directly as collectors and the scaffolds themselves being unconducive, integrating a conformal conductive thin film on their surfaces acting as collectors. Both cases are complete with a 3D interconnected porous structure, which also ensures a convenient electron transport pathway. Ion transport, on the other hand, drives the attention of researchers. Over the past decade, 3D nano- and micro-structuring have been recognized as an effective strategy for tuning ion transport mobility and selectivity. On the one hand, nanostructures can modulate the local electric field, regulate the ion concentration in the electrolyte, and alleviate the problem of slow local kinetics. On the other hand, nano- and micro-structures can shorten the ion diffusion distance. The diffusion time of ions in nanostructures can be described by Einstein's formula T =  λ²/Di (where λ denotes the lateral size of the nanostructure and Di is assumed to be independent of nanosize) [49,50]. That is, under the same conditions, the size reduction will greatly shorten the ion transport time. Because of this, in electrodes based on 3D NMS scaffolds, the thin film-shaped active material is able to complete the electrochemical reaction with the electrolyte quickly. Another point to note is the tortuosity of the 3D NMS scaffolds. According to the expression of effective ionic conductivity, D_eff =  D(ε/τ), where ε represents porosity, τ is tortuosity, and D means intrinsic ionic conductivity; it can be concluded that the effective ionic conductivity is inversely proportional to the tortuosity of the 3D scaffold [9,51]. In other words, for 3D NMS scaffolds, utilizing scaffolds with low tortuosity in the vertical direction to ensure a low-tortuosity transport path for the electrolyte can effectively enhance the electrochemical performance of the electrode.

As previously described in Sect. 2.2, simply increasing the thickness of the active material brings about a serious loss of mechanical properties, which in turn shortens the cycling life of the device. The preparation of electrodes using 3D NMS scaffolds can effectively solve this problem. For example, 3D NMS scaffolds made of one-dimensional materials such as carbon cloth, carbon fiber, and other woven or stacked materials have excellent mechanical toughness and can meet the electrochemical and mechanical requirements of flexible wearable devices [52]. Of course, the material and structure of 3D NMS scaffolds are the decisive factors for their mechanical properties. The influence of structure on mechanical properties is more fascinating than that of material. The recently emerging honeycomb structure, with its in-plane cells in ordered or disordered two-dimensional arrays and parallel stacking in the out-of-plane direction, is characterized by a periodic topology assignment and thus has high specific stiffness and strength and specific energy absorption [18,53]. Specifically, for in-plane stresses, the honeycomb absorbs them through bending deformation of the cell walls and plastic hinges at the cell-wall junctions; out-of-plane stresses are absorbed through cell wall buckling and membrane deformation. Cells are Voronoi honeycombs when they are disordered in both size and shape, and foam structures when they are also disordered in the out-of-plane direction. Thus, the introduction of suitable NMS scaffolds is certainly going to enhance the mechanical properties of electrodes. For the mechanical properties of NMS scaffold, both toughness and strength properties are influenced by several factors: (1) the basic mechanical properties of the scaffold material; (2) the relative density of the scaffold, which is the ratio of the density of the porous scaffold to the density of the corresponding bulk material [47]; (3) the geometry of the scaffold and holes; and (4) the dimensions of the scaffold in terms of porosity, pore size, and wall thickness. For example, thicker scaffolds are generally more likely to resist some degree of damage, but may also compromise flexibility. Moderate porosity improves the bending capacity of the scaffold, but excessive porosity will undoubtedly reduce its strength. Therefore, application requirements should be considered from various angles when designing the scaffold construction. When selecting the appropriate 3D NMS scaffolds for specific applications, it is necessary to consider not only the high stresses inside the electrodes due to the volume expansion of the active materials, side reactions, and inhomogeneous current distributions during the charging and discharging process, but also the mechanical stresses that need to be withstood during the device integration and encapsulation process [47,54].

The structure of 3D NMS scaffolds determines the structure of electrodes to some extent, especially for electrodes with conformal thin-film active materials. Therefore, it is a very effective strategy to design the electrode structure and performance by designing and tuning the 3D NMS scaffolds. Due to the rapid development of processing technologies and integration processes in recent years, we have progressed from simply utilizing NMS scaffolds in nature to directly designing and preparing desired NMS scaffolds according to different application conditions. The main preparation processes, including the "top-down" template method and the "bottom-up" 3D printing technology, have been at the center of research and practical applications. The ability to prepare precisely defined 3D NMS scaffolds, and thus precisely defined electrodes and devices, is of great significance, both in terms of performance enhancement and the understanding of the relationship between the relevant structure and electrochemical properties in conjunction with simulation techniques [55,56,57,58]. This, in turn, will help researchers design and prepare more rational EES devices with better performance. It is important to note that high energy and power densities, good multiplicity performance, and cycle life are always the relentless pursuits of EES devices. Therefore, the design principle of 3D NMS scaffolds, which are complementary and useful to the main application mechanism, is to maintain or increase the EES capacity per unit of active material while minimizing the ratio of inactive components.

Based on the above unique advantages, NMS scaffolds are increasingly employed in the next generation of EES devices. In the next chapter, we summarize the main preparation methods and characteristics of NMS scaffolds.

During the past decades, the emerging of nano- and micro-fabrication technics combined with novel assembly modes makes it possible to tailor scaffolds in a wide range from the nanoscale to microscale, which brings up more opportunities for NMS scaffolds. Simultaneously, in contrast, the demand for advanced NMS scaffolds with unrivalled structures at nano- and micro-scale is continually facilitating the rapid development of nano- and micro-fabrication technics [82]. It is imperative to emphasize the fact that despite that NMS scaffolds act as complementary and facilitating roles during the charge-discharge energy storage process, the versatile scaffolds endowed by advanced fabrication technics significantly affect the chemical and physical properties both of the EDLCs and pseusocapacitors [47,53,54]. It is imperative to keep in mind that optimal NMS scaffold design is a key aspect of many of these electrodes, as it endows them with some good properties such as filtration, robustness, and kinetics. Inspired by evolved wood structure with a wealth of NMS pores at multiscale in nature, Chen et al. reported a typical all-wood-structured SCs, in which the obtained activated wood carbon directly acts as anode while MnO₂ was electrochemical deposited onto both of the surface and its inside channels of the carbonized carbon with original thin wood membrane as separator without further treatment [33]. In this manner, natural wood directly transforms into wood carbon NMS scaffolds designed by nature via carbonization technology. Moreover, after the synergie treatment between delignification steps via chemical (e.g., a mixed solution of H₃PO₄, adenine, and H₂O₂) or biological (e.g., enzymolysis) and carbonization processes, hierarchically porous carbon-based NMS scaffolds, which feature vertical open microchannels and hierarchical nanopore properties, are successfully obtained simultaneously [83,84,85]. Recently, different from most parts, Chen’s group outlines an interesting strategy that straightforwardly uses natural wood as an NMS scaffold without any treatment, resulting in a hierarchically reconstructed multiscale porous structure consisting of Ti₃C₂ and wood vessels (as shown in Fig. 5a) [13]. The ultimate 3D continuous network structure endowed by porous wood NMS scaffold and Ti₃C₂ aerogels exhibits advantages such as abundant active sites with boosted capacitance and hierarchically arranged channels with accelerated electronic transportation. With these salience features, the electrode exhibits an aerial capacitance of 930 mF cm⁻² at 0.5 mA cm⁻² while the corresponding peak energy density of symmetric supercapacitors is 23 μWh cm⁻². Although great progress has been made regarding wood-based NMS scaffolds for SCs, many limitations still exist, especially for flexible applications. To this end, constructing soft scaffolds based on carbon network with interconnected and porous structure is an appealing trade-off strategy to realize high energy density via mass loading enhancement of pseudomaterials without the expense of wearable attributes (for example, lightweight, mechanical flexibility and flame-retardant safety) for practical wearable SCs applications, including portable communications, wearable backups, and implantable medical devices [38,86,87,88,89]. Case in point, Shang et al. devised a soft hybrid scaffold strategy in terms of directly vacuum filtration of carbon nanotubes (CNTs) and reduced holey graphene oxides (rHGOs), in which the properties of CNTs endow the electrode with excellent electric conductivity and mechanical robustness (Fig. 5b). Benefiting for the hierarchically porous structure, the hybrid scaffold comprising of nanotube and nanosheet, not only offers rapid ion and electron transport pathway but also exhibits good accommodate ability to load more pseudomaterials for better electrochemical performance [38]. Given the outstanding softness of all materials, the 3D network-based device holds high flexibility, enlisting fully fulfil the practical application demands for wearable SCs. Eventually, the electrode based on soft hybrid scaffold has areal capacitance of 6000 mF cm⁻². The areal energy density and power density of the asymmetric supercapacitors are performed as 1050 μWh cm⁻² and 17,200 mW cm⁻², respectively, along with 90.9% capacitance retention after 3000 cycles at 50 mA cm⁻². Other than retaining and utilizing the intrinsic nano- and micro-structure of NMS materials, as mentioned above, there is now a growing interest in emphasizing the control to precisely tailor and fabricate the NMS scaffolds in order to further uncover the structure-function relationship and make full use of active materials for performance optimization of advanced EES devices [4,90]. In the realm of this, the 3D printing technique provides reliable bottom-up manufacturing processing that is capable of devising predictable 3D geometrically architected NMS scaffolds. Over the past decade, the current ink formulation and 3D printing technics enable the design and construction of scaffolds with nano- or micro-structures, almost any desired stereoscopic geometry at the multiscale, by being synergized with other processing technologies [91]. Specifically, Lin et al. reported a gradient porous design of graphene aerogel-graded NMS scaffold, the center-to-center ligament spacing of which is decreasing from the outer to the inner layers, facilitating sufficient diffusion of ion/electrolyte for the electrode [92]. Along with the same design concept that NMS scaffolds possess and are capable of realizing complementary and facilitating roles for the main mechanism of application, like Chen (Fig. 3a) and Lei (Fig. 4e), Xue et al. directly utilize polymer lattices as NMS scaffolds for electrodes rather than select conductive materials that act both as scaffolds and current collectors [16,18]. As indicated in Fig. 5c, based on digital light processing (DLP) and digital mirror device (DMD) techniques, an octet-truss lattice polymer scaffold was synthesized via acrylate-based UV photosensitive resin. Furthermore, by virtue of the dynamic hydrogen bubble template (DHBT) (discussed in Fig. 6a) method, the resulted electrodes exhibit impressing structural features provided by hollow and hierarchically porous 3D NMS scaffolds, such as the fine intrinsic conductivity endowed by the rGO and NiP metal layers, fast transport for charge kinetics, ultra-large active sites, unique electrical conductivity, facilitated electron transport and ionic diffusion channels as pathways, and sufficient voids both for accommodating enough pseudomaterials and electrolyte penetration, and so forth. Ultimately, the areal capacitance performances of the rGO electrode is 293.4 mF cm⁻² at 0.5 mA cm⁻², together with a peak energy density of 8 μWh cm⁻² and a power density of 12.56 mW cm⁻² with a long lifespan (96% after 5000 cycles). As what we can concluded from the aforementioned, even as the same material (carbon or carbon derivatives), processing techniques play powerful roles in enhancing the properties of a material by virtue of optimizing their shape and size from a structural point of view.

To satisfy the increasing energy storage capability requirements from implementation of the Internet of Things (IoT), despite a surge of new technologies, unmet challenges for energy density still remain for MSCs, for which they only cover a limited footprint area close to several square millimeters. Different from 2D thick electrodes with NMS materials, utilizing the capability of designable as advantage of 3D NMS scaffolds is crucial to foster strengths and circumvent the weakness of MSCs, enlisting better utilization of the space from a fundamental goal point of view [81,93]. As an additional part within limited space, the exist of scaffold comes at the expense of some volume ratio of the device to some extent while the designable of scaffold is another key advantageous point to significantly affect the properties both of SCs and MSCs. Following this concept, it is imperative to tailor structures of NMS scaffolds precisely, even featuring the synergy among new technologies, which unleashes the potential of scaffolds in terms of boosting the energy storage performance of MSCs. When compared with more mature and precise silicon-based architectural techniques such as photolithography, the template-assisted method is another attractive, full-fledged approach in terms of tailoring the architecture and functions of NMS scaffolds. Typically, DHBT is a promising method for preparing porous single metals (e.g., Au, Ag, Pt, Cu, Sn, etc.) and binary alloy (e.g., CuAu, CuAg, PtAu, CuZn, etc.) metallic NMS scaffolds due to the advantageous both in terms of its cleanliness and ease of preparation (as shown in Fig. 6a-c) [39,62,94]. In this case, with numerous diverse nano- and micro-pores, such metallic NMS scaffold reported by Pech’s group suited for fully electrolyte penetration, higher active surface area value (150 times of flat Au film), ion transport facilitation, and sufficient mechanical robustness to overcome the huge volume changes during cycles [39]. Based on these advantages, accompanying with RuO_xN_yS_z as pseudomaterials, the high areal electrode capacitance of 14,300 mF cm⁻² is obtained. Simultaneously, the areal energy density as well as power density of 432 mJ cm⁻² and 421 mW cm⁻², are yielded, respectively, coupling with the excellent cycling life (100% of initial capacitance even after 5000 cycles). In many cases, fabrication of porous scaffolds followed by in situ deposition of pseudomaterials in a conformal manner is an effective strategy. However, for some MSCs obtained by freeze casting (ice-templating), simultaneous preparation of the scaffolds and pseudomaterials in a hierarchical porous structure is possible. This ice-template assisted processing technic endowing low dimensional materials (such as nanowires, nanoplates, CNT, and etc.) with 3D structural properties and broadening their applicability. Particularly, when freeze casting is integrated with other processing routes (e.g., additive manufacturing, electrospinning, and laser etching), it is possible to obtain diverse scaffolds with nano- and micro-scale geometries. More recently, a pseudoplastic nanocomposite fine gel for 3D printed interdigitated MSCs was reported by Liang’s group. After printing step, a honeycomb-like porous scaffold was obtained by virtue of the following unidirectional freezing and freeze-drying techniques (Fig. 6d). Freeze-drying is a particularly versatile technique that enables the use of diverse assembly units for NMS scaffolds with a wide range of porosity and pore morphology (lamellar, honeycomb, etc.) tailored by chemical or physical methods [13,14,95,96,97]. Eventually, the MSCs based on honeycomb-like scaffold delivers areal capacitance of 216.2 mF cm⁻² (at 10 mV s⁻¹) and the areal energy density and power density are performed as 19.2 μWh cm⁻² and 58.3 mW cm⁻², respectively [14]. Given the distinct physical/mechanical attributes of honeycomb structure, anodic aluminum oxide (AAO) template method is another promising candidate strategy toward controlling the design of MSC electrodes with clear and periodic finer structure, as depicted in Fig. 6e [18,98,99]. Impressively, as reported by Lei’s group, this honeycomb alumina nano-scaffold (HAN) possesses surprisingly high mechanical stability despite the thickness of the cell wall is only 16 ± 2 nm, which features the synergy between the alumina physical property and honeycomb structural mechanism [18]. Meanwhile, the electrodes based on aligned HAN (conformal SnO₂ as current collector) enabling better effective ionic transport pathways relative to random porous structures mentioned before. Finally, this MSC based on robustly stable HAN yields a high capacitance of 128 mF cm⁻² at 0.5 mA cm⁻², coupling with the peak areal energy densities and power density of 160 μW cm⁻² and 40 mW cm⁻², respectively.

Although the 3D NMS holder is not directly involved in the reaction, application-driven selection of the right material can greatly affect the overall device performance. Generally, the materials of 3D NMS scaffolds can be carbon, metal, semiconductor, ceramic, polymer, and so forth. The 3D NMS scaffolds can be used in SCs either directly as a collector or simply as a mechanical support for a thin film-shaped current collector [14,18]. The use of metallic 3D collectors should pay attention to the voltage window range that the metal is suitable for and the stable electrolyte environment. For example, metallic collectors are typically inert in alkaline electrolyte environments, which limits the use of active materials suitable for acidic electrolyte environments. However, this problem can be perfectly solved by directly depositing active materials onto 3D MNS scaffolds that are inert or even non-conducting but have a layer of thin film-shaped collectors adapted to the environment. This provides new opportunities to enrich the choice of materials for SC collectors. At the same time, since the 3D scaffold is an additional extra part, the design of its structure should follow the lowest occupy ratio in the active volume, which is especially an important point in MSCs.

In the past few years, designing NMS scaffolds for AIBs has emerged not only as an attractive strategy to realize enhancement both of power density and rate performance without at the expense of a much lower energy density, but it is also used to ensure sufficient charge-discharge life cycles. Conductive network scaffolds based on one-dimensional nanostructures (such as CNTs, carbon nanofiber, and cellulose nanofiber (CNF)) show significant potential in the field of flexible wearable energy storage devices due to numerous advantages. Case in point, Kuang and coworkers reported a conductive nanofiber network-based CNF scaffold that possesses mechanical robustness and good electrolyte retention, offering decoupled ion and electron transfer pathways to ensure fast charge transfer kinetics of the electrode [38,42,86,87]. As shown in Fig. 7a, after pretreated process of CNF, neutral carbon black particles are assembled onto the surface of negatively charged CNF via spontaneous electrostatic self-assembly technique, forming the versatile scaffold for thick electrodes with compact structure [120]. Attributing to the unique interconnected 3D network with tightly wrapped lithium ion phosphate (LFP) endowed by freeze-drying process, the close-packed nano-paper electrode obtained due to the densification treatment performs high active material mass loading up to 60 mg cm⁻². Eventually, the Li-LFP batteries deliver high areal capacity and volumetric energy density of 8.8 mAh cm⁻² and 538 Wh L⁻¹, along with better cycling stability and capacity retention (90% after 150 cycles). As for potassium-ion batteries (PIBs), considering the large radius of K⁺ (1.38 vs 1.02 Å of Na⁺ and 0.76 Å of Li⁺), a robust scaffold with a larger space is needed to buffer the huge volume change so as to facilitate the fast K ions insertion-extraction processes during the cycles, preventing the subsequent pulverization of active materials. Compared to graphitic carbon with confined interspace, owing to its larger interlayer spacing, hard carbon is a more suitable candidate as anode material for PIBs to form stage-1 potassium intercalation compounds. Based on this concept, Li and coworkers illustrated a hierarchically nitrogen-doped porous carbon (NPC) synthesized via a self-template technique as both an electrode and a scaffold for K ion insertion-extraction processes (Fig. 7b). It is noteworthy that urea is employed as a nitrogen source in the synthesis process to graft various nitrogen doping groups into the obtained porous carbon matrix, both for enlisting more electronegative to generate a stronger attraction toward ions and for expanding carbon layer space via tuning electronic structure [15]. Finally, the obtained NPC electrode results in a rate capability of 185 mAh g⁻¹ at 10.0 A g⁻¹ and a high reversible capacity of 342.8 mAh g⁻¹ after 500 cycles, which exhibits good enough electrochemical performance as PIB carbonaceous anode.

In the era of the Internet of Things (IoT), the requirement for autonomous power supplies from numerous miniaturized electronic devices is becoming more urgent. In order to be compatible with other miniaturized devices, the footprint area of AIMBs is limited within 1 cm² as well (similar to MSCs mentioned in Sect. 4.1.2). Therefore, utilizing NMS scaffolds to construct 3D electrodes for AIMBs has been recognized as a potent way to achieve optimized power density and cycling stability without sacrificing energy density. In detail, despite the enlarged amount of active material loaded, the usage of 3D space in the NMS scaffold ensures a shorter ion transport diffusion length than that of 2D thick electrodes, which is crucial for AIMBs to realize simultaneous enhancement of the energy and power. Over the past few years, there has been a growing realization that tailoring the structures of NMS scaffolds precisely is a key aspect of many of these AIMBs, as it endows the devices with some predictable physical or chemical properties and functions in the context of fast processing technology development. Among various processing techniques, chemically de-alloying is a desirable approach to obtaining interconnected 3D NMS metallic scaffolds [119,121]. As reported by Li and coworkers, endowed by HNO₃ de-alloying treatment of interdigital-patterned Ag₇₅Au₂₅ (at%), the porous Au simultaneously acts both as micro-current collectors and NMS scaffolds, with each microelectrode structure consisting of periodic Au ligaments and nanopore channels (Fig. 8a). When integrating the c-K_xMnO₂ cathode and ac-K_xV₂O₅ anode on Au porous NMS current collectors as full cell, this really realizes Li-ion micro-battery-like capacity as well as supercapacitor-level rate performance and cycle life span [121]. The maximum energy density of the device exhibits ~ 103 mWh cm⁻³ which is 14-fold higher than the Li-film battery (4 V/500 μAh), along with ~ 600 W cm⁻³ power density.

To precisely tailoring the structures of NMS scaffolds to comprehensively optimize the electronic and mechanical performance of AIMBs, a well-known processing technique is photolithography. Over the past two decades, to some extent, the photolithography technique has been the central aspect of intensive investigations and practical applications for manufacturing miniaturized electronic devices. A surge in the development of photolithography has allowed the rapid upscaling of the AIMBs both in design and innovation in the past 5 years. Ning and coworkers demonstrated a strategy that features the synergy between 3D holographic lithography and conventional photolithography, enlisting simultaneous control of both the scaffold structure and spatial arrangement of AIMBs. From an application point of view, this is a compatible strategy for commercial manufacturing. Thanks to recent advances, which have enabled a technological upgrade that enables the complex construction of well-defined periodic NMS porous electrodes using a single incident beam and standard photoresist treatments by varying the beam patterns and exposure parameters. Specifically, by varying the parameters of the holographic lithography beam (for instance, intensity, polarization, and angle), various structures can be realized via photoresist templates for confining the electrodeposition growth behavior of scaffold materials, resulting in customized 3D NMS scaffolds after removing the photoresist templates. As shown in Fig. 8b, following the concept of effective electron and ion transportation pathways, both of interdigitated LiMnO₂ cathodes and Ni-Sn anodes are based on 3D NMS Ni scaffolds, resulting in supercapacitor-like power for Li-ion micro-batteries (3600 μW cm⁻² μm⁻¹ peak) [29]. In another recent work, Sun and coworkers reported a promising strategy for merging imprint lithography, self-assembly, and electrochemical deposition to obtain an interconnected porous NMS scaffold for high-performance interdigitated AIMBs (Fig. 8c) [71]. Distinguished from the conventional photolithography used above (Fig. 8b), they impress silicon template into melted a poly (methyl methacrylate) (PMMA) and then separated the silicon after cooling to obtain the interdigitated patterns. Moreover, essential steps in the formation of an interconnected porous NMS Ni scaffold are the self-assembly in the trenches of polystyrene (600 nm colloids), Ni electrodeposition from the bottom to fully fill the trenches, and removal the polystyrene and PMMA by toluene. This interconnected NMS Ni current collector possesses inverse opal structure as shown in SEM image (Fig. 8c), which ensures high active volume fraction simultaneously at high-enough mass loading of active materials, sufficient electrolyte penetration, and fast kinetics. After electrodeposition of V₂O₅ (cathode) and Li metal (anode), capillary force guided gel electrolyte infilling, and packaging processes, the assembled AIMBs based on NMS Ni scaffold performs high areal energy and power densities of 1.24 J cm⁻² and 75.5 mW cm⁻², respectively.

The use of 3D scaffolds is a promising approach for high-performance ion battery electrodes, enlisting high mass loading of active materials, fast electron and ion kinetic transport pathways, and robust mechanical properties. Active materials can be simply formed as thin films on the surface of NMS scaffolds in a conformal manner by infiltration, spraying, or deposition. For network-based 3D scaffolds based on 1D and 2D materials, the active material is simply wrapped into the scaffolds, and then the interconnected network structure becomes a stable support for the active material with good interfacial connectivity without the need for additional binders. For 3D scaffolds, the conformal attachment of the active material becomes extra important. On the one hand, the 3D porous interconnected structures provided by NMS scaffolds offer abundant active sites and excellent transfer kinetics; on the other hand, despite being thin films, the use of 3D NMS scaffolds endows active materials with sufficient mass loading for energy storage due to the utilization of third-dimensional space. Therefore, especially for AIMBs, thin film deposition techniques are more suitable for loading active materials onto NMS scaffolds, including atomic layer deposition (ALD), electrochemical deposition, chemical vapor deposition (CVD), and so forth [18,46,86,103]. In fact, although significant progress has been made in recent years, designing and constructing 3D NMS scaffolds for ideal batteries and AIMBs are still in its infancy. The main technological barrier restraining the practical application of NMS scaffolds for batteries and AIMBs is focusing on rational design and precious manufacturing techniques. Moreover, a deeper understanding of the relationship between electrochemical reactions and electrode structure also plays a crucial role in further pursuit in the realm of batteries, where theoretical simulation and analysis are practically necessary.

Due to the uneven electric field and ion flux on the surface of conventional planar electrodes, the instability of the metal anode during the plating-stripping process usually results in dendrites [148]. The construction of porous conductive substrates based on NMS scaffolds can dissipate the inhomogeneous local current density and ion flux, effectively avoiding the generation of dendrites [149]. At the same time, the interconnection network effectively mitigates the volume change of the metal material during repeated deposition/stripping processes and stabilizes the electrode electrochemical performance. Moreover, the sufficient internal space of the porous structure can fully accommodate the metal deposition while ensuring smooth ion transport channels, which assists in managing the metal deposition effect. Lei's group has synthesized composite NMS scaffolds that can serve as Na metal anode hosts by electrochemically dispersing RuO₂ nanoparticles homogeneously on the fiber surface of carbon paper (RuCP), as shown in Fig. 9a [150]. The interwoven carbon nanofibers in the RuCP scaffolds provide an electron-conducting network. While the RuO₂ nanoparticles exhibit a strong affinity for Na ions, which promotes the ability of the Na metal to uniformly wrap around the surface of the scaffold and fill the voids, significantly reducing the surface energy of carbon paper. SEM images of the scaffolds before and after Na metal deposition indicated that Na metal evenly coated the RuCP scaffolds and produced a dense and smooth surface following a Na deposition procedure of 20 mAh cm⁻². The electrode overpotential was just 13 mV at 1 mA cm⁻², allowing for lengthy periods of steady dendrite-free operation (1500 cycles). Alexander et al. created and employed a single-phase mixed ion- and electron-conducting garnet (MIEC) NMS scaffold in Li-metal batteries [63]. As shown in Fig. 9b, the MIEC has approximately conductivity for both lithium ions and electrons. Combined with the continuous and porous structure, it is capable of uniformly distributing potential across the whole surface throughout the charging and discharging process. It is an effective strategy to alleviate the tensions on the solid electrolyte surface, hence decreasing localized hot spots that can contribute to dendrite formation. The authors demonstrate that the critical current in a sandwich symmetric structure comprised of a MIEC support and a garnet solid electrolyte can achieve an unprecedented 100 mA cm⁻² without dendritic short-circuiting. In addition, the hybrid solid-state battery also exhibits a long cycle life (350 and 500 cycles at 1.15 and 2.3 mA cm⁻² current densities, with a cathode area capacity of 2.3 mAh cm⁻²).

In addition to the impacts of electric field and ion flux on dendrites, internal stresses during metal deposition are recognized as an essential aspect in dendrite existence, as demonstrated by theoretical model of Jiang's group [35]. The stress-driven metal diffusion flux transfers and deposits metal ions at the interface of the solid electrolyte interface (SEI) bottom layer during Li deposition. When the SEI layer is defective at this point, Li dendrites form here. The mechanical property of the continuous porous NMS scaffold serves to counteract the internal stresses in the material, thus mitigating the generation of dendrites at this level. Based on this concept, the group reports a 3D porous Cu collector supported by a soft PDMS substrate (shown in Fig. 10). It is shown that the internal stress during Li metal deposition leads to folds in the soft collector and gradually transforms from a 1D to a 2D wrinkling pattern. The wrinkling releases the internal stresses during Li plating and suppresses the emergence of dendrites. The half-cells fitted with soft Cu collectors have a Coulombic efficiency of more than 98% at a current density of 1 mA cm⁻² and more than 200 cycles.

Another important aspect that affects the stability of the metal cell anode is SEI layer. As known, the SEI layer refers to a layer of film formed between electrolyte and electrode material that can prevent side reactions between electrode material and electrolyte, so as to improve the performance and safety of the battery. In practice, however, the metal battery anode will cause the SEI layer to break/reorganize continuously under the huge volume change during the reaction process, which will lead to an increase in lithium ion diffusion resistance at the interface and an elevation of voltage polarization. Therefore, utilizing NMS scaffolds as artificial SEI layers is another appealing strategy to enhance electrode stabilization. Generally speaking, the artificial SEI layer should fulfill the following requirements: (1) excellent chemical and electrochemical stability, with electronic insulation to prevent the successive depletion of the electrolyte and the metal; (2) good mechanical strength and flexibility, which can inhibit the huge volume change and dendrite growth of the anode in the process of charging and discharging; and (3) the ability for fast and uniform cross-layer transport of metal ions. The properties of NMS scaffolds can fully satisfy the demands of an artificial SEI layer and realize the effective protection of metal electrodes. As shown in Fig. 11a, Zhai et al. designed and prepared insulator/metal/insulator 3D sandwich structure composite scaffold consisting of g-C₃N₄/graphene/g-C₃N₄ for Li-metal battery anodes by a combination of 3D self-assembly and in situ calcination [40]. Among them, g-C₃N₄ serves as an artificial SEI layer with insulating properties that inhibit Li metal deposition on its surface, while its unique structured nanopores allow Li⁺ penetration to reach the interior and insulate the electrolyte. Since g-C₃N₄ is connected to graphene by van der Waals forces, there is enough internal gap to allow Li deposition on the graphene surface. The interconnected structure and mechanical properties provided by the graphene network greatly lower the local current density and volume change during lithium deposition and exfoliation. And the amorphous morphology of g-C₃N₄ results in more homogenous and protective characteristics due to the lack of brittle grain boundaries. The synergistic effect of g-C₃N₄ with graphene enables the 3D scaffold anode to maintain high Coulombic efficiency (99.89%) and reliable long-term cycling (180 cycles) under high cathode capacity (3.5 mAh cm⁻²) and lack of electrolyte (25 μL per cell). Otherwise, the NMS scaffold can also assist in the formation of the SEI layer and stabilize its function. Besides, in comparison, the ordered 3D structure not only has better mechanical properties and stability but also can better reveal the structure-property relationship through theoretical simulation, so as to investigate the influence mechanism and provide guidance for structural design. Ni and colleagues assembled ordered micropore-structured graphene oxide scaffolds doped with S and N atoms (SNGO) (Fig. 11b) [20]. Following a molten lithium infusion process, stable Li₃N and Li₂S can be formed on the film at the same time as the anode SEI layer to realize the rapid transmission of Li ions. The lithiophilic properties of N and S functional groups can promote the uniform deposition of Li ions. Moreover, the graphene oxide scaffolds provide sufficient accommodation space while alleviating the volume expansion caused by Li deposition, thus improving the strength and toughness of the SEI layer and avoiding the occurrence of fracture. COMSOL simulations also confirmed that the 3D-printed ordered microchannels further shorten the Li ion transport pathways and uniformly disperse the internal electric field, heat, and stress distribution, preventing dendrite formation. The measured results of the assembled Li-S full battery show that the Li anode composed of SNGO has a stable cycling performance with a reversible discharge capacity of up to 861.7 mAh g⁻¹ after 250 charge/discharge cycles at 2 C rate. This presents a new concept for the design of artificial SEI layer. In addition, some other typical examples of NMS used for metal battery anodes are summarized in Table 3.

As mentioned before, for metal battery anodes, NMS scaffold not only improves electron/ion conductivity but also modulates the electric field density and ionic flux on the anode surface, preventing dendrite formation and significantly improving electrode stability. This 3D open structure provides an ideal strategy for the development of metal battery anodes with high energy density and extended cycle life. It is worth mentioning, however, that when utilized as a collector structure, NMS scaffold undoubtedly adds weight to the metal anode while occupying a certain load volume. Constructing NMS scaffolds with hollow structures and high porosity can lessen this influence, but it also affects mechanical characteristics; therefore, the two must be balanced. Furthermore, the relationship between scaffold size and batteries performance when used as SEI layer is not clear. Hole size, porosity, arrangement, and scaffold thickness are all important influencing factors. In addition to experimental studies, in-depth analyses and predictions need to be combined with theoretical calculations, simulations, and machine learning to assist in accurate design.