Multifunctional composite designs for structural energy storage

Structural batteries have emerged as a promising alternative to address the limitations inherent in conventional battery technologies. They offer the potential to integrate energy storage functionalities into stationary constructions as well as mobile vehicles/planes. The development of multifunctional composites presents an effective avenue to realize the structural plus concept, thereby mitigating inert weight while enhancing energy storage performance beyond the material level, extending to cell‐ and system‐level attributes. Specifically, multifunctional composites within structural batteries can serve the dual roles of functional composite electrodes for charge storage and structural composites for mechanical load‐bearing. However, the implementation of these multifunctional composites faces a notable challenge in simultaneously realizing mechanical properties and energy storage performance due to the unstable interfaces. In this review, we first introduce recent research developments pertaining to electrodes, electrolytes, separators, and interface engineering, all tailored to structure plus composites for structure batteries. Then, we summarize the mechanical and electrochemical characterizations in this context. We also discuss the reinforced multifunctional composites for different structures and battery configurations and conclude with a perspective on future opportunities. The knowledge synthesized in this review contributes to the realization of efficient and durable energy storage systems seamlessly integrated into structural components.

The rapid development of mobile electronics and electric vehicles has created increasing demands for highperformance energy storage technologies.5][6] An emerging battery technology known as structural batteries, composed of multifunctional components, presents a solution to address the limitations of conventional batteries.This innovative approach involves integrating energy storage directly into the structural framework of devices, mobile vehicles, or aircraft.][9][10][11][12] Initially introduced by US military labs, the concept of structural batteries aimed to enhance performance and reduce weight in systems like ground vehicles and unmanned aerial vehicles (UAVs). 13,14Since lithium-ion batteries consist of current collectors, electrodes, separators, and electrolyte, the multifunctionality of structural battery is allocated to the multifunctional design of these battery components.6][17] Figure 1A provides an illustration of the overall design concept of structural batteries.The development of multifunctional composites for structural lithium-ion batteries is the essential component.6][27][28][29][30] CFs, in particular, are widely used due to their high stiffness, favorable strength-to-weight ratios, and excellent electrical conductivity. 18,31This review paper extensively discusses the structural-plus concept in electrodes, electrolytes, separators, as well as interface designs for structural batteries.
Utilizing structural batteries in an electric vehicle offers a significant advantage of enhancing energy storage performance at cell-or system-level.If the structural battery serves as the vehicle's structure, the overall weight of the system decreases, resulting in improved energy storage performance (Figure 1B).For instance, replacing traditional components like roofs with structural batteries in electric cars can reduce mass by 20%, allowing for additional batteries and increased F I G U R E 1 Overview of structural battery concept; (A) multifunctional composite designs for structural batteries [18][19][20][21] ; (B) mass saving results of structural batteries 22 ; (C) various applications of structural batteries 23 ; (D) manufacturing strategy of multifunctional composite materials for structural batteries. 24riving range. 22,32Structural batteries can also be extended to other applications, such as aircraft, drones, and even future humanoid robots (Figure 1C).Ongoing research focuses on developing safe, high energy-density, and lightweight structural energy storage for the use in hybrid-electric aircraft. 33Notably, cylindrical structural batteries have been developed, exhibiting substantially higher stiffness and yield strength compared to conventional structures. 15This advancement has demonstrated an extended hover time for drones, showcasing the potential of structural batteries in enhancing performance and enabling new capabilities in aerial systems.
With the increasing demand for structural batteries, there has been a surge in research on manufacturing strategies.This research encompasses the development of novel manufacturing processes to seamlessly integrate batteries into building structures without compromising their performance and durability.Thereby, efforts are directed towards enhancing multicomponent compatibility, manufacturing efficiency, and cost-effectiveness, aiming to make structural batteries more accessible and affordable. 34,35or instance, a 3D printing technique co-extruded continuous CFs and doped functional photopolymer resin, cured by a UV laser, enabling the fabrication of multifunctional composite materials.Notably, this approach enables the printing of a functional lithium-ion structural battery in a single step (Figure 1D). 24These advancements in manufacturing strategies contribute to the progress and widespread implementation of structural batteries in diverse applications.
Therefore, multifunctional composite designs for structural batteries are rapidly evolving, offering numerous opportunities for future research and practical applications.The incorporation of composite materials and multifunctional capabilities has demonstrated the potential to realize structure-plus concept for structural batteries.This review aims to provide a comprehensive overview of recent advances and developments in multifunctional composites for structural batteries.It covers a wide range of topics, including composite designs, performance characterization, fabrication strategies, and various composite structures and battery configurations.Specifically, Sections 2 and 3 present the latest research progresses on electrodes, electrolytes, separators, and interface designs, as well as associated characterizations of mechanical and electrochemical properties for structural energy storage.Section 4 focuses on the elaboration of reinforced multifunctional composites for structural batteries.Section 5 further explores the multifunctionalities for structure-plus concept on different composite structures and battery configurations.The last section discusses a perspective on future opportunities.

| MULTIFUNCTIONALITIES OF STRUCTURAL BATTERIES
Structural batteries exhibit the unique ability to serve as both electrochemical energy storage and structural components capable of bearing mechanical loads with the frameworks or devices they are integrated into.These structural batteries, functioning as rechargeable batteries, adhere to the same electrochemical behavior seen in commonly used lithium-ion batteries.Their energy storage relies on the reversible oxidation-reduction reactions of lithium and the lithium-ion couple (Li/Li + ) to store energy.Typically, metal oxide (LiMO 2 , M = Co, Ni, Mn) or metal phosphate (LiFePO 4 ) are used as active material in the cathode, while the anode is composed of electrode material like graphite, silicon, or other metal oxides with low Li/Li + reaction potential (SnO 2 , NiO, etc.).A liquid electrolyte-incorporated separator or solid-state electrolyte separates the cathode and anode, exclusively allowing the passage of lithium ions.7][38] The structural battery, in its role as the structural component of the device, necessitates substantial mechanical strength and resilience to forestall severe deformation or damage.0][41] This amalgamation of energy storage principles and mechanical fortification has positioned structural batteries as a transformative solution for reshaping electrified devices or vehicles.
Owing to distinct material subsystems present in electrodes, electrolytes, and separators, the advancements in multifunctionality within structural batteries are explored separately.Striving to concurrently enhance mechanical properties and energy storage performance, several approaches have been reported.These include embedding CFs or polymer additives as reinforced inclusions to augment the mechanical properties and interface engineering within multifunctional composites to reduce the interfacial resistance for improved charge transport kinetics.

| Interface engineering of multifunctional structural electrodes and devices
The successful implementation of structural batteries in diverse applications, including automobiles and aircraft, necessitates the development of lightweight composite materials with efficient energy storage capabilities.Previous researchers have made strides in developing structural electrodes using CFs not only as active electrode materials but also as support structures.0][41] Given these properties, CFs are often incorporated with other active materials to form composite electrodes, bolstering mechanical attributes and expediting electron transport within the electrode, such as LiFePO 4 / GO@CFs, 18 Li 4 Ti 5 O 12 @CF, 42 Sb 2 S 3 @CF, 43 CuO@CF, 44 CuNPs@CF, 45 ZnCo 2 O 4 -urchins-CF, 46 and so forth.][49] However, a notable challenge lies in the balance between mechanical properties and electrochemical performance.1][52] While CFs play the role of a scaffold to accommodate other highcapacity active materials, ensuring strong adhesion between these materials and the CFs is crucial for maintaining stable battery performance. 53,54[57] To improve the affinity between CFs and active materials, Huang et al. conducted a heat treatment on pristine CFs to derive oxidized carbon fibers (OCFs). 35This oxidation process increased the surface area, facilitating better electrochemical deposition of active materials onto the OCFs.During the cycling, sulfur and Li 2 S 2 /Li 2 S were uniformly deposited onto the OCFs, establishing strong contacts between the active materials and the OCFs.On the anode side, molten lithium was infiltrated into the OCF matrix to enhance the strength of the anode.The top and middle panels in Figure 2A illustrate the propensity of particle-based electrodes to delaminate from planar current collectors due to the weak solid-solid interactions among particles.However, conformallycoating structural electrodes illustrate the stable mechanical robustness.The fabricated battery cell integrated with these structural components delivered charge-discharge processes under compression up to 20 MPa for 20 cycles at 0.2 C. Sanchez et al. employed electrophoretic deposition (EPD) to deposit durable coating of electrode active materials on CFs, as demonstrated in Figure 2B. 18The cathodic composite material composed of LiFePO 4 and electrochemically exfoliated graphene oxide (EGO), which demonstrated enhanced electrical conductivity and good adhesion on CFs.Consequently, it allowed for loading active F I G U R E 2 (A) Conformally coated electrodes 58 ; (B) electrophoretic coating on carbon fibers as cathode 18 ; (C) a multifunctional structural battery composite with carbon fiber as the negative electrode 59 ; (D) tree-root inspired electrode/separator interface adhesion 60 ; (E) copper-coordinated cellulose ion conductors 61 ; (F) aramid nanofiber separators 62 ; (G) structural vertically aligned nanofiber separator. 63aterial exceeding 90%, while simultaneously preventing degradation and reducing charge transfer resistance during cycling in the structural battery.
In the realm of lithiation and delithiation processes within graphitized phase, CFs have been ingeniously harnessed as anodes to enhance lithium storage capacity.Leveraging the one-dimensional structure of CFs, it is feasible to create both 2D films and 3D networks, offering the advantage of constructing binder-free anodes.Remarkably, this approach yields a notable rate performance, even under high loading.When incorporating CF anode with LiFePO 4 cathode, the full cell performance outperforms that of conventional graphite anode-LiFePO4 cathode batteries.This outcome underscores how CFs not only curtail the need for electrode additives but also enhance both power density and energy density. 64,65Given CFs' demonstrable prowess as an anode material, recent designs of structural batteries have capitalized on their multifunctionality, wherein mechanical and electrochemical performance seamlessly converge.For example, a novel approach was introduced to construct structural batteries using multifunctional constituents, as depicted in Figure 2C. 59The cross-section SEM image showcased a CF negative electrode and a LiFePO 4 positive electrode, separated by a separator comprised of glass fiber embedded polymer matrix.The CFs not only acted as active anode materials for ion storage and electron transports but also contributed to the overall mechanical strength of the structural battery.The glass fiber reinforced separator facilitated lithium-ion transport and transferred mechanical stress between different battery components.The resulting structural battery composite exhibited impressive electrochemical and mechanical properties, boasting an energy density of 24 Wh kg −1 and an elastic modulus of 25 GPa.The study emphasized the influence of component properties, such as separator thickness and structure, as well as negative electrode thickness and fiber volume fraction, on the performance of the structural batteries.This understanding of the interplay between electrochemical and mechanical functions enables the future design of structural batteries with desired energy density, elastic stiffness, and mechanical strength.These findings highlight multifunctional battery electrodes, providing valuable insight into structural batteries with stable cycling and high energy density.

| Mechanical enhancement by interface and solid-state electrolyte designs
Considering the structural battery is composed of multiple components, the interfaces among them assume a pivotal role in shaping its mechanical properties and electrochemical performance.Drawing inspiration from how trees withstand strong winds by anchoring themselves and the soil, Jin et al. developed a tree-root-like interfacial adhesion strategy between electrodes and separators, as depicted in Figure 2D.By infiltrating polymeric binding materials into the electrodes and laminating them to the separator, they created a continuous network that firmly bonded the electrodes to the separator.This approach led to a significant enhancement in the flexural properties of the batteries, with a flexural modulus 11 times higher than that of conventional batteries.The method involved the formation of a tree-root-like structure of P(VdF-HFP) binder in the subsurface region of the electrode using a phase inversion technique.As a result, a trilayer configuration comprising an anode, separator, and cathode, with strong interfacial bonding, exhibited remarkable rigidity. 19Moreover, the realization of dependable structural batteries hinges on the development of solid electrolytes with robust mechanical properties and favorable processability, as opposed to traditional liquid electrolytes.Various strategies have been explored to enhance the mechanical integrity of solid polymer electrolytes.These approaches encompass the incorporation of ceramic nanoparticles, 66 ceramic/polymer fibers, 67,68 and the integration of 3D networks. 69For example, a noteworthy method involves a fiber-reinforced structural electrolyte that utilizes poly(ethylene oxide)-lithium bis(trifluoromethane) sulfonimide (PEO-LiTFSI) as the polymer electrolyte matrix, while employing lithium aluminum titanium phosphate (LATP) and glass fiber (GF) as reinforcing fillers.This configuration results in a remarkable tensile strength of 33.1 MPa, a large Li + transfer number of 0.37, and moderate ionic conductivity of 6.3 × 10 −5 S cm −1 at 25°C. 70Nevertheless, the inclusion of inert reinforcement materials like GF introduces a trade-off between ionic conduction and mechanical strength.To circumvent this dilemma, a general strategy involves the utilization of the second network with inherent high ionic conductivity.For example, through the coordination of copper ions with one-dimensional cellulose nanofibrils, a cellulose nanofiber-based electrolyte was developed with a high ionic conductivity of 1.5 × 10 −3 S cm −1 , a high transference number of 0.78, a wide electrochemical stability window of 0-4.5 V, and a high mechanical strength of 29.2 MPa (Figure 2E). 20This design concept can be extended to other polymers and cations, enabling the creation of high-performance solid-state ion conductors with potential applications for high-energy-density and safe solid-state batteries.The advanced design of structural battery electrolytes ensures mechanical integrity under flexural loads or impact, thereby influencing the electrochemical and mechanical performance of structural battery devices. 21By considering these factors and leveraging the strategies presented, the development of robust solid-electrolyte and interfaces between electrodes and electrolytes pave the way for structural batteries.

| Mechanically robust separators
Separators hold immense importance in structural batteries as they act as crucial elements between anodes and cathodes, preventing failures under challenging conditions such as high loads and extreme working temperatures.Polymer-based separators with strong secondary bonds between the polymer chains have been extensively explored and tailored for application in structural batteries.Examples of such separators include poly(phthalazinone ether sulfone ketone) (PPESK) 71 and aramid nanofiber (ANF). 62,72In one previous research study, the ANF separators were demonstrated with high modulus and self-extinguishing properties, as depicted in Figure 2F. 62These separators are produced through the dissolution of Kevlar fibers and subsequent vacuum-assisted self-assembly.Notably, the ANF separators possess a high decomposition temperature of 447°C, and a Young's modulus of 8.8 GPa which is 10 times higher than commercial separators.Furthermore, the ANF separators are self-extinguishing and demonstrate resistance to melting or dripping when exposed to flames.The results highlight the favorable combination of mechanical and thermal stability, flame resistance, and electrochemical stability offered by ANF separators, unlocking the applications under extreme working conditions.Inspired by the damage-tolerant architecture, separators with intentional porous pattern/alignment and robust microstructure provide another direction for developing structural batteries. 73or example, vertically aligned insulating nanostructures (VANS) were developed as the separator between electrodes (Figure 2G). 36The VANS separator effectively isolates rough electrode interfaces, ensuring unimpeded ion transport while preventing electrical contact.This separator not only facilitates charge transfer but also imparts mechanical strength to the system.Compared to traditional polymeric separators, the structural separator demonstrated enhanced ionic conductivity in the supercapacitor cell and a 50% increase in strength.These findings highlight the potential of functional composite separators to improve the electrochemical and mechanical performance of energy storage devices.

| CHARACTERIZATIONS OF MULTIFUNCTIONAL COMPOSITES FOR STRUCTURAL BATTERIES
The development of advanced structural battery cells requires the discovery and utilization of the composite structure as well as the performance optimization of its critical components: electrode materials, separator, electrolyte, current collector, and assembly materials.This comprehensive understanding of the relationship between the composite structure and the corresponding mechanical and electrochemical performance of the battery is highly desirable.Various characterization techniques can provide valuable insights into the mechanical behavior and the coupling effect of mechano-electrochemical properties.These techniques reveal the structural evolution, strengthening mechanism, and charge transport properties during the battery operation conditions under various mechanical loads.To measure the mechanical properties of the composite or the assembled structural battery, the tensile test, compression test, and three-point bending test are powerful methods for investigating the mechanical behaviors of critical materials/components in structural batteries, which are commonly used in the field of mechanics.As shown in Figure 3A-C, the stress-strain curves can be derived from these testing profiles to obtain critical mechanical values such as modulus, yield strength, ultimate tensile stress, fracture strength, and toughness.Finite element methods can be utilized to map strain and stress distributions within the composite or assembled cells.This modeling approach enables the prediction of flexural performance for different configurations of structural batteries. 19,37Moreover, the mechanical properties of structural batteries can be evaluated using puncture tests, which subject the cells to harsh risk assessments involving tensile and compressive stresses (Figure 3D).The resulting load-displacement curves and fracture images distinguish the mechanical properties of different structural designs under severe deformation. 35hese mechanical testing methods are specifically developed to analyze stress changes in various deformation scenarios, providing a comprehensive evaluation of the mechanical properties of both composite materials and the final assembled cells.Combining these techniques allows for a thorough characterization of mechanical behavior and performance optimization in advanced structural batteries.
Given that structural batteries are designed to simultaneously store electric energy and bear mechanical loads, it is crucial to evaluate their electrochemical performance under external mechanical loads. 38Consequently, operando characterization methods, which combine mechanical and electrochemical tests, are employed to investigate the coupling effect of these properties. 77For instance, the tensile test of a structural battery cell, conducted using the Deben miniature test rig, enables the study of reversible charge-discharge performance under mechanical loading, and vice versa (Figure 3E).By comparing preconditioning chargedischarge cycling with cycling at specific strains under designed mechanical loads, the strain-dependent discharge capacity and cycling retention can be assessed, thereby evaluating the multifunctional capabilities of the structural batteries. 78Similarly, the combination of a three-point bending test and electrochemical test allows for the examination of the impact of flexural stress on charge-discharge behavior and cycling stability (Figure 3F).Furthermore, a scaled-up structural battery utilizing high-strength carbon-fiber composites and interlocking polymer rivets to stabilize the electrode layer stack is demonstrated in an electric scooter configuration under quasi-static three-point bending, as shown in Figure 3G. 79By employing these operando characterization methods, researchers can comprehensively assess the mechanical-electrochemical coupling in structural batteries, enabling a better understanding of their performance and advancing their multifunctional capabilities.
Over the past decade, significant strides have been made in the development and application of advanced mechanical and operando characterizations in structural battery research.These techniques offer innovative approaches for studying structural characteristics, strengthening mechanisms of composites and assembled cells, and their correlation with electrochemical battery performance.By integrating these characterization methods with simulations or numerical models, it becomes possible to optimize the design of advanced structural batteries for diverse application scenarios. 39This integration enables optimized designs for the functionalities of structural batteries.

| MULTIFUNCTIONAL PERFORMANCE OF STRUCTURAL BATTERY USING CFS AND OTHER STRENGTHENING ADDITIVES
In the realm of structural batteries, CFs have gained widespread usage due to their ability to enhance mechanical strength.The incorporation of CFs into structural battery composites can be achieved through various strategies, leading to distinct mechanical and electrochemical performances.For instance, the conformal coating of CFs with catholyte as the cathode and lithium metal as the anode demonstrates improved ultimate tensile strength and Young's modulus compared to non-modified lithium-sulfur electrodes (Figure 4A,B).Electrode preparation involving electrodeposition-like reactions on a carbon fabric skeleton enhances the mechanical robustness of the structural battery. 35Additionally, the alignment of CFs in different directions relative to the applied force results in varied mechanical properties.Aligning fibers parallel to the tensile force direction increases fracture forces by a factor of 10 compared to fibers aligned perpendicular to the loading direction 24 (Figure 4C,D).In the electrode composite, the interfaces between active material particles and embedded CFs play a crucial role in achieving good electrochemical performance and mechanical strength.Interface engineering, such as coating PAN on CFs' surface, "locks-in" the interface between active materials The multifunctional performance by introducing carbon fiber and other reinforcement components; (A, B) the mechanical strength comparison before and after embedding carbon fibers in the lithium-sulfur structural battery 58 ; (C, D) The tensile behavior of the glass fiber reinforced separator with the fiber orientation relative to the loading directions 59 ; (E, F) The mechanical property of the PAN coated carbon fibers in the graphite and LiFePO4 composite electrodes 80 ; (G, H) the layer up stacking structural full cell battery with carbon fiber reinforced electrodes and the fiber-based current collector 81 ; (I) the carbon nanofiber reinforced cathode with or without carboxylic function group modification 82 ; (J, K) the MnO 2 nanosheet reinforced PEO/LiTFSI based solid-state electrolyte. 83All with permission of reproduction.
and the structural CF backbone (Figure 4E,F), leading to improved ultimate tensile strength for both the graphite composite anode (>40%) and lithium iron phosphate (LFP) composite cathode (>80%). 40Furthermore, incorporating CFs as the current collector with graphite/ carbon fiber anodes and LFP/carbon fiber cathodes using a traditional composite lamination process, enables the development of a pouch-free battery configuration as a full cell for CubeSat structural walls (Figure 4G,H). 41his design increases the utility of interior CubeSat volume while maintaining a discharge capacity of 16 and 8 mAh g −1 (normalized to the overall battery weight) under tensile stresses of 100 and 200 MPa, respectively.Notably, the absence of additional packaging materials showcases the advantages of structural composites for practical structural energy storage systems.
Furthermore, the essential reinforcement component has also been explored for multifunctional capabilities.Surface modification of CFs with carboxylic functional groups further improves both the mechanical strength (Figure 4I) and electronic conductivity of the composite electrodes, resulting in enhanced electrochemical performance. 42Beyond fibers, two-dimensional reinforcements have shown potential for improving mechanical properties and electrochemical stability.For example, MnO 2 nanosheets were introduced into a solid-state polymer-based electrolyte, poly(ethylene oxide) (PEO)/LiTFSI in Figure 4J. 43These nanosheets combined with PEO polymer chains, not only exhibit a higher tensile strength (2.3 times) compared to a PEO/ LiTFSI solid electrolyte but also enable long-range migration of Li ions on the surface of the MnO 2 nanosheets. 43Significant progress has been made in integrating CFs and other reinforcement components into multifunctional composites as well as structural batteries, enabling them to withstand mechanical stress while maintaining stable electrochemical behavior.However, there is still ample room for improvement in terms of both mechanical properties and electrochemical performance, considering the current energy density of lithium-ion batteries and the practical requirements for structural frame bodies.

| NOVEL STRUCTURE DESIGNS FOR STRUCTURAL BATTERIES
The dimensionality of reinforcement components plays a crucial role in enhancing the multifunctional performance of structural batteries.Advanced manufacturing processes have enabled the successful preparation and investigation of fabrics, bio-inspired structures, and programmable frameworks, all of which have shown promising effects in enhancing the multifunctional performance of structural batteries.For instance, a fiber-based composite cathode was developed by conformally depositing iron (III) phosphate (FP) over a mechanically strong and electrically conductive carbon nanotube (CNT) backbone, infiltrated with a PEO-LiTFSI electrolyte. 44The presence of the polymer electrolyte within the CNT@FP fabric resulted in a fracture toughness twice that of pristine CNT fabric (Figure 5A).Additionally, incorporating Al 2 O 3 fiber-reinforced PEO/LiTFSI electrolyte enabled stable charge-discharge cycles in the all-solid-state nanofiberreinforced electrodes.This demonstrated that nanofibers significantly improved the electrochemical and mechanical performance of the polymer-ceramic composite electrodes (Figure 5B). 44At the cell-level, a layered-up lamination design was employed, using polymer rivets inserted into perforations created through local heating.These rivets fused with the barrier layer, encapsulating the carbon-fiber-reinforced-polymer (CFRP) face sheets and the electrode-separator-electrode stacking (Figure 5C).The interlocking rivets allowed the effective transfer of shear stress through the battery stack to the CFRP face sheets.The resulting multifunctional energy storage composite structure exhibited enhanced mechanical robustness and stabilized electrochemical performance.It retained 97%-98% of its capacity after 1000 three-point bending fatigue cycles, making it suitable for applications such as energy-storing systems in electric vehicles. 79io-inspired structures offer valuable insights for the development of effective structural batteries.For instance, the bio-inspired tree-root structure enhances adhesion at the electrode/separator interfaces, enabling higher bending forces in three-point bending tests (Figures 2D and 5E,F).These results underscore the critical role of adhesion in strengthening the battery structure. 19Biostructures also serve as inspiration for solid-state electrolyte design.A Nacre-inspired multilayer ceramic/polymer composite electrolyte, featuring a "brick-and-mortar" arrangement of brittle platelets and thin polymer layers, demonstrates superior mechanical properties. 45The composite electrolyte, comprising NASICON-type electrolyte Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) and polymers such as PEO or poly(ether-acrylate) (PEA), exhibits high ultimate flexural strength (30.2 MPa) and strain (1.1%), indicative of its toughness.The assembled LiFePO 4 /LAGP-PEA/Li cell can be charge-discharged after 100 cycles with a capacity retention of 95.6% under a 10 N bending force.These results highlight the improved mechanical properties of the nacre-like composite electrolyte, ensuring enhanced stability against external impacts in solid-state batteries.By drawing inspiration from natural structures, researchers can design and fabricate structural batteries with improved adhesion, mechanical strength, and stability.These bioinspired approaches have the potential to revolutionize the development of advanced materials for multifunctional energy storage systems.
The utilization of 3D printing technology enables the creation of 3D programmable structures capable of withstanding mechanical forces, similar to struts in architectural constructions.Through precise control of structural factors and architectural dimensions during the digital light processing and subsequent pyrolysis process (Figure 5I), 3D architecturally designed carbon electrodes were developed as a mechanically robust anode for lithium-ion batteries.These micro-architected pyrolytic carbon electrodes are monolithic, additive-free, and capable of withstanding uniaxial compression up to a maximum stress of 27 MPa, enabling easy electrode recycling.Notably, the specific strength of these electrodes is 101 kN m kg −1 , comparable to that of 6061 aluminum alloy (Figure 5J).Moreover, the 3D-architected pyrolytic carbon electrodes demonstrate a high areal capacity of 4 mAh cm −2 when subjected to The multifunctional performance of novel structure design for structural energy storage; (A, B) the mechanical and electrochemical performance of the fabric-reinforced batteries 84 ; (C, D) the schematic of the interlayer locking of the layered-up batteries and the corresponding mechano-electrochemical behaviors 76 ; (E, F) the tree-root like adhesion enhanced mechanical property for lithiumsulfur batteries 60 ; (G, H) the schematic for the nacre-inspired composite electrolyte and electrochemical performance under mechanical loading 85 ; (I-L) the fabrication and multifunctional performance of 3D architectural carbon electrodes. 86All with permission for reproduction.cycling at 0.38 mA cm −2 over 100 cycles (Figure 5K). 86This emphasizes their potential for enhancing the performance of multifunctional systems, with electrode engineering factors being independently controllable.
The multiscale structures derived from fabrics, interlayer locking configurations, bio-inspired composites, and programmable architectures exhibit potential for advancing multifunctional energy storage systems.The interplay between mechanical and electrochemical performance, along with the associated factors and coupling effects, expands the boundaries of optimization in this rapidly evolving field.

| DISCUSSIONS AND OUTLOOK
Structural batteries, capable of storing energy while simultaneously bearing mechanical loads, offer a means to extend the usage of conventional battery devices for broader applications.The utilization of multifunctional composites that enhance both mechanical properties and electrochemical performance has emerged as an effective approach for designing structural batteries.In contrast to merely adding mechanically robust materials as protection shields, employing multifunctional materials facilitates the creation of structural composites that can serve as cathodes, anodes, separators, or solid-state electrolytes within the battery system.Capitalizing on the inherent advantage of simultaneously fortifying mechanical properties and enhancing electrochemical performance, we have summarized the performance of the current state-of-the-art structural batteries in Table 1.
Drawing from conventional toughening micromechanisms, the fiber toughening involves both the introduction of fiber fillers and the modification of matrix-filler interfaces.This combination encourages shear yielding within the matrix material and prevents the development of crazes into catastrophic cracks. 77,91he endeavors to incorporate CFs into electrodes, glass/ polymer fibers into solid-state electrolytes, and to engineer interfaces among battery components mirror the fiber toughening mechanism observed in ceramic and polymer matrices.By harnessing fiber-reinforced composites, structural batteries achieve a design characterized by high strength, high toughness, and lightweight attributes while maintaining the electrochemical capability of lithium storage and charge transports.
The characteristics of mechanical properties in these works vary due to the multifunctional design nuances, leading to diverse measurement technologies.Collating Young's modulus information from these works reveals that reinforced electrodes can attain values exceeding 10 GPa.The introduction of reinforcement not only enhances the mechanical attributes of polymer-based solid-state electrolytes/separators but also bolsters the properties of ceramic-based electrolytes, endowing them with respectable ionic conductivities and stability.
At the full-cell level, these works, incorporating mechanical strength values from distinct measurement methods, encompass diverse aspects of energy storage evaluation, spanning material-level assessments, celllevel examinations, and capacity measurements under mechanical loads.While direct comparisons might be challenging, the improved mechanical properties and augmented energy densities validate the efficacy of the introduced multifunctional design in structural batteries.For example, the coextruded LiFePO 4 and CF with a solid polymer electrolyte in a micro-battery showcases an elevated modulus of 124 GPa and a tensile strength of 1.1 GPa.This configuration yields an energy density of 77 Wh kg −1 at a current density of 0.5 C, holding promise for electric devices reliant on structural battery designs. 90otably, its tensile strength rivals that of commercial fiber-reinforced polymer composites (CFRP: axial modulus: 155-400 GPa, axial tensile strength: 1.29-2.8GPa) used for structural applications. 92However, a notable discrepancy arises in terms of elastic modulus.
The introduction of ductile-phase and the enhancement of the ceramic-ductile phase interface bonding strength have proven efficient in toughening brittle materials. 93,94Within this toughening mechanism, the incorporation of ductile polymer phases into electrode or inorganic electrolyte materials serves to bridge microcracks, thus elevating toughness.Noteworthy examples encompass the integration of bio-inspired ductile polymer phases into electrode material LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532), 60 LAGP-based electrolytes, 85 and the use of multilayer cathode-separator-anode stacks designed as rivets. 76These reinforced composites enable structural batteries to endure charge-discharge cycles under mechanical loads without evident damage.However, the utilization of inert or less electrochemically active ductile phases results in a relatively lower energy density when compared to commercial lithium-ion batteries (exceeding 200 Wh kg −1 ). 95Moreover, the incorporation of excessive ductile phases might compromise modulus, potentially stressing the battery's active materials and overall performance.
Alternative reinforcements, including lattice structural design and innovative networks, also bolster the mechanical strength of the composite.For example, 3D printed architectures featuring mechanically robust lattice structures can be introduced to electrode or electrolyte composites, resulting in acceptable electrochemical performance. 86While these architectures fall  short of matching the overall mechanical strength of commercial structural composites, they open up possibilities through the application of 3D printing technology.Significant strides have been made in recent years within the realm of multifunctional composite design for structural batteries.These encompass the use of CFs, interface modifications between active materials and CFs, incorporation of other polymer/ceramic fibers into separators or solid-state electrolytes, nature-inspired microstructural designs, and additive manufacturing-enabled programming, all of which have demonstrated improved mechanical robustness as discussed in Sections 2 and 4.However, the balance between multifunctional and electrochemically active materials ultimately shapes the extent of performance enhancement.Specifically, CF-reinforced electrodes exhibit limited capacity retention, prompting the need for further improvements to enhance cycling stability. 21,60Thus, designing effective reinforcements for practical scenarios remains crucial.
A potential approach involves the reinforcement of hierarchical fiber networks across multiple scales, ensuring efficient charge transport and contact adhesion between the multifunctional network and active materials.This concept mirrors a hybrid transportation network with vertically and horizontally distributed highways, alongside locally scattered county routes.In the case of ductile phases and other reinforcements, efforts to strengthen bonds between multiple phases and optimize material content hold promise in achieving the balance between mechanical and electrochemical performances.These strategies present avenues for enhancing overall structural battery performance to facilitate effective integration into practical applications.
Distinct from batteries using liquid electrolytes, reinforcements employed in solid-state batteries offer broader possibilities for multifunctional implementation in real-world scenarios.In multifunctional composites, where only solid-solid contacts exist, the bonds between multiple phases exhibit heightened strength, rendering them capable of supporting larger mechanical loads. 86By integrating reinforcements with solid polymer electrolytes, substantial improvements in mechanical strength can be achieved. 66,67,83Through the careful selection of compatible fillers and polymer matrices, this approach holds the potential to rival commercial CFRP in terms of mechanical strength.However, challenges arise with respect to electrochemical performance, as interface considerations become paramount to attain moderate rate performance and cycling stability, which are essential for high-power density solid-state batteries. 96,97Beyond variations in battery configurations, numerous additional challenges must be addressed, including the absence of standardized evaluation methods for structural battery devices, which is a crucial aspect for incorporating batteries as structural components in vehicles and aircraft.Implementing the structural concept into industrial settings necessitates considerations of manufacturing techniques and safety standards, presenting practical concerns within structural battery research.
Manufacturing techniques represent a critical area for advancement in the field of structural lithium-ion batteries.Researchers are exploring novel manufacturing approaches for integrating batteries into building structures, while also seeking ways to optimize their performance and durability, as summarized in previous research. 24,60,63However, the fabrication of structural batteries currently relies on specific techniques and materials, lacking a standardized method.This leads to increased production costs and hinders their widespread implementation in electric vehicles and other devices.The manufacturing process for structural batteries is still in its nascent stages and has yet to reach cost-effectiveness.
Lastly, safety standards are also of paramount importance for structural batteries.These batteries are designed to be integrated into the structural framework of systems, such as electric vehicles, exposing them to harsher mechanical conditions compared to conventional batteries.They are subjected to vibrations, stress, and other physical forces that can affect their performance and long-term stability.Ensuring the mechanical and electrochemical stability of structural batteries in such demands is thus crucial.Therefore, it is imperative to conduct rigorous safety tests as the field of structural batteries progresses toward practical applications. 98To achieve greater multifunctional features, researchers could also explore the integration of safety measures in the design, such as the use of non-flammable electrolytes and thermally stable separators. 99,100

F I G U R E 3
Mechanical properties and operando characterizations for structural batteries; (A, B) tensile/compression test and stressstrain curve for the battery composites74 ; (C, D) three-point bending test with the finite element simulation for the structural batteries60 ; (E, F) the puncture test for a structural battery in a pouch cell configuration 58 ; (G) in operando tensile tests under battery operation conditions75 ; (H) in operando three-point bending test under battery operation conditions; (I) in operando demonstration of a structural battery integrated with an electric scooter under the three-point bending configuration.76All with permission of reproduction.

T A B L E 1
The performance of state-of-the-art structural batteries.