Contemporary Micro-Battery Technologies: Advances in Microfabrication, Nanostructuring, and Material Optimisation for Lithium-Ion Batteries

Abstract

The miniaturisation of electronic devices has intensified the demand for compact, high-performance lithium-ion batteries. This review synthesises recent progress in microscale battery development, focusing on microfabrication techniques, nanostructured materials, porosity-engineered architectures, and strategies for reducing non-active components. It explores both top–down and bottom–up fabrication methods, the integration of nanomaterials, the role of gradient electrode architectures in enhancing ion transport and energy density, along with strategies to reduce non-active components, such as separators and current collectors, to maximise volumetric efficiency. Advances in top–down and bottom–up fabrication methods, including photolithography, laser structuring, screen printing, spray coating, mechanical structuring, and 3D printing, enable precise control over electrode geometry and enhance ion transport and material utilisation. Nanostructured anodes, cathodes, electrolytes, and separators further improve conductivity, mechanical stability, and cycling performance. Gradient porosity designs optimise ion distribution in thick electrodes, while innovations in ultra-thin separators and lightweight current collectors support higher energy density. Remaining challenges relate to scalability, mechanical robustness, and long-term stability, especially in fully integrated micro-battery architectures. Future development will rely on hybrid fabrication methods, advanced material compatibility, and data-driven optimisation to bridge laboratory innovations with practical applications. By integrating microfabrication and nanoscale engineering, next-generation LIBs can deliver high energy density and long operational lifetimes for miniaturised and flexible electronic systems.

1. Introduction

The rapid advancement of portable electronics, medical devices, and Internet of Things (IoT) technologies has intensified the demand for compact, high-performance energy storage systems [1]. Lithium-ion batteries (LIBs), known for their high energy density, long cycle life, and efficiency, have become the cornerstone of modern energy storage, as they offer a high energy density of 90–300 Wh/kg, significantly exceeding that of lead-acid batteries, which typically range from 35 to 40 Wh/kg [2]. In addition, the cost of LIBs has steadily decreased over the years from around USD 800/kWh to less than USD 140/kWh, according to the International Energy Agency’s special report [3]. However, conventional LIB designs face limitations when scaled down for microscale applications, particularly in terms of volumetric energy density, integration flexibility, and manufacturing scalability [4].

To address these challenges, researchers have explored a range of innovative strategies aimed at enhancing LIB performance at the microscale. Microfabrication techniques, such as photolithography, laser structuring, screen printing, and 3D printing, enable precise control over electrode architecture, allowing for optimised ion transport and material utilisation. Simultaneously, the incorporation of nanostructured materials, including silicon (Si) nanowires, graphene, and metal oxides, has significantly enhanced electrochemical properties, offering higher capacity, improved conductivity, and increased mechanical stability. Recent micro-batteries (MBs) have been shown to provide power densities ranging from 4.4 µWh/kg to 3.0 mWh/kg and energy densities between 17 µW/kg and 1.5 mW/kg [5].

This review evaluates the physical design strategies that have recently emerged as the most promising routes for improving the performance and manufacturability of microscale LIBs. By integrating multidisciplinary approaches, the next generation of LIBs can achieve superior performance, scalability, and adaptability for emerging technological applications [6].

2. Microfabrication

Miniaturised electromechanical systems require compact battery solutions, and microfabrication is an intricate process that involves component compactness and alignment within the battery cell [7]. MBs integrate compact electrode designs with high energy density and are typically fabricated using either top–down (e.g., photolithography or laser-based) techniques or bottom–up (e.g., screen or 3D printing) techniques [8]. These approaches determine the geometry and spatial arrangement of cell elements, directly influencing electrochemical performance, mechanical stability, and integration with other microsystems [9]. Each method offers distinct advantages and trade-offs in terms of resolution, scalability, material compatibility, and cost, which are examined in detail across the following subsections.

2.1. Photolithography

Photolithography, a top–down microfabrication method, patterns MB electrodes by exposing a photoresist-coated substrate through a mask under UV light [10,11,12]. The procedure transfers geometries with nanometre precision (Figure 1). Photolithography employs two types of photoresists: positive resists, which become soluble after exposure, and negative resists, which become insoluble when exposed to UV. Positive resists are favoured for their superior resolution and edge definition [13].

The choice of photoresist is tied to the subsequent patterning steps. In the lift-off processes, positive resists enable clean removal of unwanted metal (Figure 2a), whereas negative resists act as durable etch masks for material removal (Figure 2b). Precise control of exposure and development is essential for fine MB patterns, but despite enabling high-resolution, computer-aided design (CAD)-based designs, this approach may reduce active material content and impact performance [14].

Photolithography has proven versatile across various electrode types (carbon-based materials, polymers, metal oxides, and MXenes-2D transition-metal carbides and nitrides). It is compatible with emerging data-driven optimisation strategies, further highlighting its promise for large-scale, high-performance MB manufacturing [15].

Refino et al. [16] fabricated Si nanowire anodes via photolithography and dry etching. A carbon layer was uniformly deposited using thermal evaporation, confirming that conventional chip-fabrication techniques are compatible with advanced energy storage materials, although capacity retention was only ca. 16% after 100 cycles. The authors attributed the sharp decline in capacity retention to several interconnected factors. First, the amorphous carbon coating, while initially boosting capacity by providing additional lithiation sites, suffers from irreversible capacity loss over repeated cycles. Second, the presence of carbon promotes the formation of a bulky and non-uniform solid electrolyte interphase (SEI), which consumes electrolyte and degrades ionic transport, leading to increased impedance. Third, non-uniform carbon deposition causes instability in lithiation/delithiation processes, reflected in a fluctuating Coulombic efficiency (CE) and spiky differential capacity profiles. Collectively, these effects drive the rapid deterioration of battery health despite the initial performance gains.

While photolithography remains a cornerstone of MB fabrication due to its resolution and material compatibility, its complexity, reliance on planar substrates, and possible loss of active material limit its use in high-capacity designs. To stay relevant for 3D and flexible MBs, it will need to be adapted or combined with complementary techniques.

2.2. Laser Structuring

Laser structuring enhances LIB electrode performance by using short-pulsed lasers to create micro-patterns that reduce tortuosity and improve ion transport, especially in thick electrodes. This boosts fast-charging and cycling stability, although active material loss (typically 5–10%) and process-induced defects from laser ablation remain barriers for scalability [17,18]. An example of the process of laser structuring is shown in Figure 3. Park et al. [19] showed that microgrooves in ultra-thick LCO (700 µm) cathodes reduce internal resistance without thermal damage. Despite 5–10% mass loss, the structured electrodes delivered five-fold higher areal capacity at 0.1 C and markedly improved rate capability by increasing diffusion. The laser structuring was performed using a femtosecond Yb: KGW laser (1030 nm, 190 fs, 6 W) operating at 30 kHz, with grooves spaced at 300 μm and scanned at 18 mm/s to create high-aspect-ratio channels in ultra-thick electrodes for improved ionic diffusion.

Bryntesen et al. [20] used ultrashort-pulsed lasers to pattern NMC111 cathodes with polyvinylidene fluoride (PVDF) binder, producing spaced-electrode designs, enhancing ion transport, and improving retention at 2 C (~75% vs. ~60% for unstructured) after 35–45 cycles, although up to 30% material loss and surface damage were reported. Laser structuring was performed using an ultrashort-pulsed mid-IR laser (~2.1 μm, few-ps pulses) operating at 20 kHz, with a scan speed of 0.4 m/s to create the parallel lines of ~20 μm width spaced 200 μm apart, ensuring minimal heat accumulation and precise removal of cathode material for improved ionic pathways.

Nasreldin et al. [21] used a two-step laser structuring (ablation) method to create 3D micropillar electrodes and serpentine current collectors (CCs) for stretchable MBs. These engineered structures improved mechanical stretchability and CE, delivering 2.5 mAh/cm² with stable operation for up to 150 cycles (92% retention at 100 cycles, C/2; 81% at 150 cycles, 1 C) under 30% strain. Laser structuring was carried out using an LPKF ProtoLaser S (LPKF Laser, Naklo, Slovenia) operating at 1064 nm and 75 kHz, with a 25 μm beam diameter; micropillars (100 μm sides, 25 μm spacing) were patterned at 3 W, while serpentine lines were formed at 10 W, enabling precise microstructuring for stretchable Li-ion MB electrodes.

Liu et al. [5] applied laser-direct writing to fabricate interdigitated microelectrodes with high-dimensional accuracy. The resulting MB delivered a capacity of 56.5 mAh/cm³ and retained over 90% capacity after 300 cycles, highlighting the compatibility of laser-patterned architectures with advanced MB designs. Laser-direct writing was employed to fabricate interdigitated graphite electrodes with dimensions of 3 mm length, 0.5 mm width, and 0.3 mm spacing, achieving high-resolution patterning for flexible dual-ion MB integration.

Figure 3. Laser structuring technique for micro-battery production. Adapted from Dai et al. [22], licensed under CC BY-NC 4.0.

Figure 3. Laser structuring technique for micro-battery production. Adapted from Dai et al. [22], licensed under CC BY-NC 4.0.

Pope et al. [23] applied femtosecond laser scribing (via ultrafast laser ablation) to high-loading Si/pitch anodes, creating hexagonal stress-relief patterns that mitigated cracking and delamination during lithiation. The structured electrodes enabled pre-lithiation and achieved over 500 stable cycles C/3 with ~70–80% capacity retention, showing an effective strategy for stabilising mechanically fragile anodes. Laser ablation was performed using a femtosecond ultrafast laser (1030 nm, ~600 fs) at 100 mm/s scan speed to inscribe hexagonal patterns (250–500 μm) into Si anodes, with two-pass ablation at 28 μJ and 14 μJ pulse energies to achieve full-depth channels while preserving the copper (Cu) CC.

Overall, laser structuring enhances ion transport, rate capability, and cycling stability, but drawbacks such as debris generation, surface damage, and capacity loss at low C-rates remain. Optimisation of parameters and integration with complementary fabrication techniques are needed for a scalable application.

2.3. Mask-Assisted Filtration

Mask-assisted filtration fabricates binder-free, patterned thin electrodes by vacuum-filtering an active-material suspension through a shadow mask, producing films with well-defined architectures [24]. The resulting films are often self-supporting, as conductive additives such as carbon nanotubes (CNTs), graphene, or polymers provide both mechanical stability and efficient electron transport [25]. Mask-assisted filtration offers a scalable, low-temperature approach for fabricating planar MBs with precise architecture (Figure 4) [26]. Zhao et al. [27] produced interdigitated electrodes composed of manganese dioxide on reduced graphene oxide (MnO₂@rGO), achieving a flexible structure with a thickness of 13.6 μm, retaining 89.5% capacitance after 10,000 cycles at 10 A/g. The assembled device delivered an areal energy density of 1.01 μWh/cm² with almost no capacitance fade over 8700 cycles and nearly 100% retention after 2000 bending cycles. Zheng et al. [26] developed all-solid-state planar MBs using mask-assisted filtration to form interdigitated electrodes on a flexible substrate. Graphene ink served as CCs, with lithium titanate oxide (LTO) and lithium iron phosphate (LFP) dispersions as anode and cathode layers. This binder- and metal-free design achieved a volumetric energy density of 146 mWh/cm³, stable operation up to 100 °C, and long-term cycling stability (3300 cycles at room temperature and ~0.0069 mAh/cm³ per cycle loss over 1000 cycles at 100 °C).

While mask-assisted filtration is scalable, it is primarily limited to planar configurations and requires precise mask design for each pattern. Further development is needed to expand its applicability to more complex geometries and improve compatibility with high-throughput production environments.

2.4. Screen Printing

Screen printing is an up-and-coming method for fabricating battery electrodes with a secondary porous network, offering advantages in resolution, deposition rate, mass loading, and cost. The process involves attaching a stencil to the printer, applying an electrode ink (a slurry composed of active material, conductive additives, binder, and solvent), and using a squeegee to transfer the ink through the mesh onto the substrate (Figure 5) [28,29]. Key parameters like mesh density, material type, and ink viscosity affect the resulting film’s resolution, adhesion, and thickness [29].

Wang et al. [30] printed an NMC622 cathode featuring vertically aligned porous channels (~10 mg/cm² loading), delivering 182 mAh/g at 0.1 C and 96 mAh/g at 4 C, and a charge capacity about seven times higher (72 mAh/g at 6 C) than bar-coated electrodes, while keeping the CE above 98.5% over 100 cycles. Zhu et al. [31] introduced scalable, screen-printed, solid-state MBs, achieving 1431 μAh/cm² areal capacity cycling stability (ca. 6500 cycles in the solid-state and ca. 8000 cycles with liquid electrolyte), with monolithically integrated cells reaching 12.5 V. Tunca et al. [32] demonstrated a water-based NMC811/rGO ink achieving up to 235 mAh/g with 75% capacity retention after 50 cycles in half-cells; in full cells with graphite, the fully water-processed system started at ~163 mAh/g and maintained ~88% of this capacity after 50 cycles. Sevinç et al. [33] developed a screen-printable Si/rGO nanocomposite ink that achieved 2671 mAh/g with 89% capacity retention after 50 cycles due to improved charge-transfer kinetics using a carboxymethyl cellulose (CMC)/polyethylene oxide (PEO) binder. Wang et al. [34] screen printed graphite anodes with a structured porous network capable of fast charging up to 6 C; in Li/graphite half-cells, they delivered ~250 mAh/g, with ~91% capacity retention after 50 cycles at 2 C, and in NMC622/graphite full cells, they provided ~140 mAh/g (~80% SOC) with essentially no capacity fade over ~170 cycles at 2 C and stable CE.

Figure 5. Visual overview of the screen-printing method for electrode production. Inspired by Sousa et al. [35].

Figure 5. Visual overview of the screen-printing method for electrode production. Inspired by Sousa et al. [35].

Screen printing offers a low-cost, scalable method for fabricating flexible battery electrodes, but resolution is limited, and achieving uniformity and high performance across large areas remains challenging. Continued optimisation of printable inks and process parameters is essential to fully realise their potential in advanced MB applications.

2.5. Spray Coating

Spray coating has gained significant attention as a scalable and cost-effective method for fabricating high-performance flexible electrodes. Unlike conventional slurry casting, it enables uniform, layer-by-layer deposition of active materials over large areas and can produce additive-free or binder-free architectures when droplet impact and solvent evaporation ensure particle adhesion (Figure 6). The absence of binder reduces non-active mass, combined with controlled sequential, enabling denser or gradient architectures that support improvements in gravimetric and volumetric capacity [36,37].

Azeemi et al. [37] fabricated a layered rGO-Si-rGO anode whose sandwich structure mitigated Si volume expansion, achieving 1089 mAh/g at 1 C after 50 cycles with ~97% CE and stable performance at current rates up to 5 C. Lee et al. [36] introduced a sequential spray coating method for additive-free LTO/LFP electrodes on 20 × 20 cm² substrates, achieving ~310 Wh/kg and ~1500 W/kg; the full cell delivered ~165 mAh/g at 0.1 C (~100 mAh/g at 10 C) and showed excellent durability (ca. 230 Wh/kg after 500 cycles at 1 C) with CE near 100% after the initial cycles.

Recently, Kwon et al. [38] created a self-supporting electrode/separator design by applying LTO and LFP layers on opposite sides of a separator via spray coating. This collector-free setup enhanced the volumetric performance (~85 Wh/L, 2400 W/L), with 95% capacity retention after 300 cycles, and retained ~87% of its initial volumetric energy density after 400 cycles at 1 C.

Kim et al. [39] applied a supersonic kinetic spray to fabricate binder-free rGO electrodes directly onto Cu CCs. This technique yielded high interfacial adhesion (~210 kPa) and a self-healing characteristic, maintaining ~295 mAh/g over 100 cycles (at 1 C), with the CE stabilising near ~99% after the initial cycles, in contrast to slurry-based rGO electrodes, which showed rapid degradation. Spray coating is a promising route for scalable, binder-free electrode fabrication, offering uniform deposition and compatibility with flexible substrates. Remaining challenges include controlling thickness, dispersion stability, interfacial adhesion, and solvent management. Refining ink formulations and process parameters is essential for performance consistency and reproducibility. A deeper understanding of impact dynamics and physical adhesion mechanisms that replace chemical binders will be crucial for industrial implementation.

2.6. Mechanical Structuring

Mechanical structuring modifies the electrode surface by applying a perpendicular compressive force to create predefined patterns (Figure 7a), without removing material or altering its chemistry [20,40]. This scalable technique can be integrated into calendaring lines and enhances Li-ion transport by shortening diffusion pathways while preserving active mass [40]. Bryntesen et al. [20] demonstrated precise line and pit patterns using compressive blades (Figure 7b), achieving optimal performance at around 40% porosity. Compared with laser processing, mechanical methods avoid debris and heat effects but offer lower spatial precision.

While promising as a low-cost, scalable alternative to laser-based methods, research remains limited regarding the impact of compression geometry, mechanical fatigue, and long-term cycling stability. Comprehensive studies are needed to determine the limits and adapt techniques for high-energy-density electrodes in MB manufacturing.

2.7. Three-Dimensional Printing

Three-dimensional printing has emerged as a transformative bottom–up approach for MB manufacturing, offering layer-by-layer fabrication of complex, material-efficient architectures with minimal waste and rapid prototyping [41,42]. Extrusion-based printing remains the most widely used owing to its simplicity, low cost, and compatibility with viscoelastic inks [43,44]. A schematic overview of the process is shown in Figure 8.

Sun et al. [45] achieved 9.7 J/cm² areal energy and 2.7 mW/cm² power using LFP/LTO electrodes. Their cell cycled stably for about 30 cycles with little capacity fade, though durability was limited by packaging. Sun et al. [45] 3D-printed interdigitated LFP/LTO microbatteries, delivering ~1.5 mAh/cm² at ~1.8 V (9.7 J/cm² areal energy, 2.7 mW/cm² power) and showing almost unchanged areal capacity over ~30 cycles, with longer-term stability mainly limited by imperfect packaging.

Zhou et al. [46] combined direct ink writing with freeze-drying to fabricate hierarchically porous electrodes (>5 mAh/cm², ca. 97% retention after 100 cycles). Zhou et al. [46] combined direct ink writing with freeze-drying to fabricate hierarchically porous LFP microelectrodes with areal loadings up to 32 mg/cm², delivering 5.05 mAh/cm² at 0.2 C and retaining 4.88 mAh/cm² (96.6%) after 100 cycles.

Assa et al. [47] demonstrated drop-on-demand (DOD) printing of distinct anode, separator, and cathode layers. The resulting device demonstrated high CE (~99.99%) and stable cycling across C-rates, with the separator lasting 160 cycles, the anode ca. 50 cycles, and the full cell to 25 cycles, supporting the viability of the DOD technique for scalable MB production.

Three-dimensional printing offers high design flexibility and material efficiency, enabling complex MB architectures. Its adoption is hindered by slow printing speeds, limited resolution, and the need for specialised inks. While promising results have been demonstrated, the technology remains largely confined to laboratory-scale studies, with challenges in reproducibility, scalability, and integration into established manufacturing workflows [48]. Improving throughput, material compatibility, and scalable manufacturing will be essential in commercial MB production.

2.8. Challenges for MB Fabrication

The microfabrication techniques reviewed in this section each offer unique advantages for tailoring Li-ion MB architectures, yet none independently fulfil all design and scalability requirements.

Table 1. Summary of microfabrication techniques.

Technique	Description	Advantages	Disadvantages
Photolithography	UV lithography using photomasks	High resolution, CAD, versatile	Complex, critical control lowers the active ratio
Laser structuring	Ultrafast lasers ablate electrodes	Precise, minimal heat boosts ion transport	30% material loss, residuals, scale-up issues
Mask filtration	Filters ink through custom masks	Scalable, binder-free, modular, low-temp use	Planar only, needs mask design, sequential
Screen printing	Stencils deposit inks	Simple, cost-effective, flexible-friendly	Lower resolution, ink/mesh limitations
Spray coating	Sprays inks layer-by-layer	Uniform coating, flexible-compatible	Ink control, overspray risk
Mechanical structuring	Compressive force or mechanical removal	No heat, mass preserved, scalable	Less precise, limited patterns
3D printing	Additive layer-by-layer manufacturing	Precise, low-waste, multi-material capable	Ink development needed, slow, resolution varies

summarises their key advantages and limitations, highlighting the trade-offs between precision, throughput, and material efficiency. Top–down approaches provide excellent resolution and alignment but involve complex, multi-step processes that reduce active-material utilisation and limit scalability. Bottom–up techniques are low-cost fabrication and are compatible with flexible substrates, yet their lower resolution and strong dependence on ink rheology hinder reproducibility and high-rate capability. Mechanical structuring, while simple and scalable, remains underexplored with limited data on long-term performance and integration potential.

Future progress will likely depend on hybrid approaches that combine the precision of top–down methods with the scalability of bottom–up processes (e.g., combining photolithography with screen printing to achieve both fine resolution and scalable deposition, or integrating laser structuring with 3D printing for precise yet flexible architectures). Wei et al. [49] demonstrated this concept by merging coating and screen printing with 3D printing to produce multilayer, spatially complex architectures with enhanced functionality and precision. Advancing material compatibility, process control, and digital approaches will be key to unlocking the full potential of microscale energy storage systems.

3. Nanostructures

Nanomaterials enhance LIB performance by increasing energy density, extending cycle life, and enhancing safety [50]. While microfabrication defines device geometry, electrochemical behaviour is increasingly governed by nanoscale material properties [8]. Through controlled design of surface area, porosity, conductivity, and mechanical resilience, nanostructured materials optimise charge transport, capacity retention, and cycling stability [51]. Si and metal-oxide enhance the SEI uniformity and reduce degradation [52], while nanocoating and electrolyte additives can improve interfaces and thermal stability [53].

3.1. Three-Dimensional Architectures

Three-dimensional electrode architectures have emerged as a powerful strategy to overcome transport limitations in MBs, enhancing ion diffusion and electron conductivity, and active material utilisation [54]. For instance, interdigitated 3D LFP/LTO electrodes have demonstrated areal energy up to 9.7 J/cm² and power densities of 2.7 mW/cm² [42]. Examples include interdigitated electrode structures (plate-type and rod-type) and sandwiched architectures, as illustrated in Figure 9.

To improve electrode material loading, the electrode support can be arranged into 3D forms (nanopillars, vertical tubes, or microchannels), enhancing the effective surface area and enabling a sandwiched or other 3D MB design. However, such architectures face limitations in electrode thickness due to stress at the layer interfaces, motivating the transition to interdigitated designs, where the anode and cathode are arranged in an interlocking, comb-like pattern to shorten ion diffusion paths and reduce interface stress [56].

Early 3D interdigitated designs mimicked sandwiched structures, using arrays of cylindrical microelectrodes [57]. Later, this evolved into arrays of interdigital plates, which offer a larger active surface area and more efficient ion transport pathways. This geometry enables simultaneous improvements in both power and energy density while maintaining mechanical stability in compact microscale configurations [58,59]. Placing the anode and cathode on a coplanar interdigitated layout permits the use of thicker electrodes, while a patterned or conformal separator layer prevents electronic contact between adjacent fingers. Tailored porosity further improves electrolyte infiltration and overall energy storage efficiency [56].

McKelvey et al. [58] explained that mechanical stress at the electrode–electrolyte interface can significantly influence performance when coupled with ionic transport. Simulations of solid polymer electrolytes in 3D trench architectures showed that stress levels could exceed the elastic limit, indicating a risk of structural damage during cycling. Interestingly, incorporating electro-chemo-mechanical coupling improved cell conductivity by about 15%, but also revealed a non-uniform pressure distribution that compromises long-term stability. These findings highlight the need to balance geometry optimisation with mechanical integrity in 3D battery designs. In addition, Rafino et al. [59] explained that Si anodes experience up to 300% volume expansion during lithiation, causing stress and structural failure. Quantitative data show that microstructured Si can withstand a shear stress of 4 GPa and nanowire tensile stress over 12 GPa, while 3D designs buffer expansion and reduce localised stress. For example, Si towers expanded 31–37% yet delivered 1093 mAh/g with 98.9% efficiency, and mesoporous Si retained 97% capacity over 70 cycles at 1910 mAh/g. These results confirm that 3D architectures, such as pillars and porous structures, are essential for mitigating stress and improving cycling stability in Si-based MBs.

While 3D electrode architectures offer significant advantages in enhancing ion transport, increasing areal capacity, and improving energy density, their practical implementation remains challenging due to complex fabrication, mechanical stress at interfaces, and layer thickness constraints. Trade-offs between structural complexity and manufacturability must be addressed for commercial adoption.

3.2. Microtubular Structures

Microtubular MB architectures employ hollow, cylindrical electrodes that offer large surface areas and short ion-diffusion paths. These structures are typically fabricated by self-rolling strained multilayers released from a sacrificial layer or template-assisted microfabrication [60]. In the self-rolling approach, internal stresses drive the film to curl concentrically, causing the material to curl once released from a sacrificial layer, forming tube-in-tube geometries (Figure 10) [61].

The tube-in-tube design by Weng et al. [62] demonstrated an area energy density of 313 µWh/cm² (power density 52 mW/cm²), maintaining 63% capacity after 45 cycles with CE up to 98.8%, significantly exceeding earlier planar MBs. The curved tubular shape spreads mechanical stress evenly and helps prevent cracks, allowing stable cycling without short circuits.

Figure 10. Schematic microtubular structure for battery fabrication. Inspired by Yan et al. [63].

Figure 10. Schematic microtubular structure for battery fabrication. Inspired by Yan et al. [63].

Microtubular architectures maximise surface area and volumetric energy density, through compact, self-rolled designs ideal for space-constrained MB applications. However, reliance on sacrificial layers and stress-engineered rolling limits scalability. Mechanical stability and packaging remain underexplored, requiring simpler fabrication and long-term performance evaluation.

3.3. Thin-Film Structures

Thin-film micro-batteries (TFMBs) deliver compact and durable energy storage systems with high energy output. Planar designs are typically either side-by-side, using liquid or solid-state electrolytes (Figure 11) [61,64,65]. In contrast to conventional LIBs, TFMBs incorporate a ceramic base layer and a metallic CC that supports layer deposition and enables sufficient charge transport [66]. They integrate easily with microfabrication techniques but face capacity limits from small active volume and poor ion diffusion in thicker electrodes, which reduces conductivity and performance [46,67].

Le Cras et al. [68] made an all-solid-state Li-ion TFMB with lithiated titanium oxysulfide (Ti–O–S) cathode and a Si nanofilm anode. The cathode exhibited ~100% CE and <0.01% capacity fade per cycle at current densities up to 130 µA/cm², while full cells enabled ~80% charge in 1 min and stable cycling for >1200 cycles with ~0.006% loss per cycle. Hallot et al. [69] applied atomic layer deposition (ALD)-coated lithium manganese oxide (LiMn₂O₄, LMO) on 30 µm Si microtube scaffolds (area enhancement factor ~50–60), achieving ~180 µAh/cm² at C/20 and >40 µAh/cm² at C/2, and maintaining 95% CE over 50 cycles (C/2).

Figure 11. Illustration of an all-solid-state thin-film micro-battery architecture (stacked design). Reproduced from Wu et al. [70].

Figure 11. Illustration of an all-solid-state thin-film micro-battery architecture (stacked design). Reproduced from Wu et al. [70].

TFMB architectures offer excellent integration potential and high energy output in compact formats, making them ideal for microscale applications. However, their capacity is limited by electrode thickness and ion-transport challenges, while solid-state electrolytes and semiconductor-based fabrication add complexity and cost. Further research depends on enhancing ion transport, mechanical flexibility, and manufacturing processes.

3.4. Nanomaterials and Nanostructures

Nanoparticles, nanowires, nanopillars, and nanotubes are nanomaterials that provide increased surface area, allowing for higher energy density, faster charge–discharge rates, and improved LIB performance [71]. These materials exist in various forms, including 0D structures such as nanoparticles, 1D structures like nanotubes and nanowires, 2D materials such as graphene and MXenes, and 3D architectures like porous foams and aerogels (Figure 12) [72].

3.4.1. Nanostructured Anode Materials

Nanostructured anodes increase surface area and conductivity, while buffering volume changes during cycling. They are classified by their Li storage mechanism: insertion, conversion, or alloying [73].

Insertion materials like graphene, CNTs, and titanium-based oxides (TiO₂, Li₄Ti₅O₁₂) [74] store Li via intercalation. Graphene’s structure offers high conductivity and flexibility, with capacities up to 1264 mAh/g, exceeding graphite [75,76]. CNTs offer stability, particularly when aligned or integrated via binder-free methods [77]. LTO and TiO₂ provide stable cycling and safety, with nanowires and composites enhancing their limited capacity (~175–330 mAh/g), sustaining ca. 100 cycles in pure TiO₂ and extending to thousands of cycles in TiO₂/graphene composites [78]. Chen et al. [79] achieved 215 mAh/g at 1 C and 168 mAh/g at 10 C after 1000 cycles using mesoporous LTO/rGO composites.

Conversion materials like iron oxide (Fe₃O₄) and cobalt (II) oxide (CoO) react with Li to form metal nanoparticles and lithium oxide (Li₂O), reaching capacities around 800–1000 mAh/g [80]. Fang et al. [81] demonstrated that 1D hollow nanostructures improve conductivity and durability: Si-silicon oxide (SiOx) nanotubes operated reliably for 6000 cycles, while tin (Sn) nanoparticles encapsulated in amorphous CNTs (Sn@aCNT) showed 749 mAh/g over 100 cycles at 0.2 A/g.

Wu et al. [82] developed a silica gel-zinc (SG@Zn) nanocomposite to address the low conductivity and volume expansion in SiO₂ anodes, retaining 72% after 300 cycles due to improved Li-ion diffusion and structural stability. Alloying materials like Si and germanium (Ge) form Li-rich alloys with high capacities. Si (~4211 mAh/g) benefits from nanostructuring (e.g., nanowires, porous Si) that reduces mechanical degradation [83]. Song et al. [84] reported a graphene-coated Si composite delivering 2589–1351 mAh/g, as current density increased from 0.3 to 2 A/g, and maintaining 1606 mAh/g after 500 cycles with 91.1% retention, attributed to graphene-enabled electronic conductivity and buffering of the Si volume change. Ge has a lower theoretical capacity (1384 mAh/g) than Si and offers strong power capability due to higher conductivity and faster Li-ion diffusion. Ge nanowire anodes delivered ~1248 mAh/g after 100 cycles and ~900 mAh/g at 10 C (with 1 C charge), with CE > 95% after the initial formation cycles [85]. To counter its ca. 300% volume change, Ge nanowires and nanoparticles are used [86]; Yuan et al. [87] achieved ~1130 mAh/g (0.1 C) and ~550 mAh/g (11 C) with thiol-passivated Ge nanowires, with CE approaching ~99% after early cycles. Wang et al. [88] synthesised a porous Si/MWCNT@C composite (multi-walled carbon nanotubes coated with carbon), retaining 80.5% capacity after 300 cycles, with CE stabilising at approximately 99% during extended cycling. A performance comparison of representative nanostructured anode materials is summarised in Table 2.

Nanostructured anodes suffer from low initial CE due to unstable SEI. High surface area and large volume change, especially in Si and Ge, repeatedly break and reform the SEI, consuming Li and electrolyte, and causing rapid capacity loss [89,90]. For practical deployment, progress must focus on scalable synthesis and stable interphases that maintain performance over long cycling.

In summary, nanostructured anodes improve LIB performance by enhancing surface area and conductivity, but each Li storage mechanism (insertion, conversion, and alloying) presents distinct trade-offs between capacity and cycle stability. Insertion materials offer excellent cycling stability and safety, though their capacities remain modest. Conversion materials provide higher capacities and can achieve exceptional durability when engineered as hollow nanostructures. However, some conversion materials often suffer from structural degradation and limited cycle life. Alloying materials such as Si and Ge deliver the highest theoretical capacities, but their large volume changes during lithiation cause severe mechanical stress and unstable SEI, leading to rapid capacity fade. Advanced nanostructuring and composite designs partially mitigate these issues, achieving improved capacity retention, yet stability remains a challenge. Overall, insertion mechanisms prioritise long-term reliability at the expense of energy density, conversion mechanisms offer a balance of capacity and durability with careful engineering, while alloying mechanisms promise exceptional capacity but demand complex strategies to overcome poor cycle stability.

Table 2. Performance comparison of nanostructured anode materials.

Material/Class	Strategy	Key Performance	Ref.
LTO, TiO2 (Insertion)	Nanowires, composites	~175–330 mAh/g, ~100 cycles (pure TiO2)	[78]
LTO/rGO (Insertion)	Mesoporous composite	168 mAh/g at 10 C after 1000 cycles	[79]
Si-SiOx nanotubes (Conversion)	1D hollow structures	Stable for 6000 cycles	[81]
SnO2 nanotubes (Conversion)	Carbon-coated	749 mAh/g (100 cycles)	[81]
SiO2 (Conversion)	SG@Zn nanocomposite	830 mAh/g; 72% (300 cycles)	[82]
Si/Graphene (Alloying)	3D layered composite	1606 mAh/g (500 cycles)	[84]
Ge (Alloying)	Nanowires,	1248 mAh/g (100 cycles); 900 mAh/g (10 C; 1 C charge); CE > 95%	[85]
Ge (Alloying)	Thiol-passivated nanowires	1130 mAh/g (0.1 C); 550 mAh/g (11 C); CE ~99%	[87]
Si/MWCNT@C (Alloying)	Porous 3D network	80.5% retention (300 cycles)	[88]

Table 2. Performance comparison of nanostructured anode materials.

Material/Class	Strategy	Key Performance	Ref.
LTO, TiO2 (Insertion)	Nanowires, composites	~175–330 mAh/g, ~100 cycles (pure TiO2)	[78]
LTO/rGO (Insertion)	Mesoporous composite	168 mAh/g at 10 C after 1000 cycles	[79]
Si-SiOx nanotubes (Conversion)	1D hollow structures	Stable for 6000 cycles	[81]
SnO2 nanotubes (Conversion)	Carbon-coated	749 mAh/g (100 cycles)	[81]
SiO2 (Conversion)	SG@Zn nanocomposite	830 mAh/g; 72% (300 cycles)	[82]
Si/Graphene (Alloying)	3D layered composite	1606 mAh/g (500 cycles)	[84]
Ge (Alloying)	Nanowires,	1248 mAh/g (100 cycles); 900 mAh/g (10 C; 1 C charge); CE > 95%	[85]
Ge (Alloying)	Thiol-passivated nanowires	1130 mAh/g (0.1 C); 550 mAh/g (11 C); CE ~99%	[87]
Si/MWCNT@C (Alloying)	Porous 3D network	80.5% retention (300 cycles)	[88]

3.4.2. Nanostructured Cathode Materials

Nanoscale engineering of cathode materials can enhance reaction kinetics and rate capability, and may improve cycling stability by enabling faster Li insertion/extraction [91]. Layered oxides like lithium cobalt oxide (LiCoO₂) and LMO benefit significantly from nanostructuring. Zheng et al. [92] developed a nonplanar LiCoO₂ (LCO) cathode with an areal loading of 71 mg/cm² that delivered ~10 mAh/cm² (~138 mAh/g) at 0.8 mA/cm², retaining 84% of its initial capacity after 20 cycles at 3 mA/cm². LCO synthesised below 30 nm improves capacity (~274 mAh/g) and shows better rate capability than conventional micron-sized LCO, but calorimetric studies indicate that the onset of exothermic decomposition and oxygen release remains largely unchanged, so the intrinsic thermal instability of high-voltage LCO persists [93]. Li et al. [94] enhanced high-voltage cycling using an aluminium phosphate/lithium phosphate (AlPO₄/Li₃PO₄) coating, achieving 88% capacity retention at 4.6 V over 200 cycles at 45 °C. Xia et al. [95] demonstrated self-supported LMO nanowall array films, delivering 131.8 mAh/g at 1 C and 97.1 mAh/g at 20 C, with 96% retention after 200 cycles, while preserving a clear capacity advantage over conventional LMO at 50 °C, supporting the finding that LMO nanowire/3D architectures enhance elevated-temperature performance and Li⁺ transport. Mao et al. [96] showed that hollow LMO microspheres deliver 132.2 mAh/g at 0.2 C and 86.3 mAh/g at 10 C, retaining 83.2% of the initial capacity after 100 cycles at 0.5 C, consistent with faster Li⁺ transport and reduced polarisation compared with aggregated LMO. Labyedh et al. [97] fabricated spinel LMO thin films by solid-state conversion at 350 °C, delivering ~1.18 Ah/cm³ at 0.1 C and still ~0.4 Ah/cm³ at 100 C, demonstrating very high volumetric capacity and rate capability. Olivine phosphates like LFP (LiFePO₄) are widely used because their phosphate structure is very stable, which improves safety (including thermal stability) and supports long-term cycling stability [98]. Wu et al. [99] reported nanostructured LFP by charging and discharging it between 2.5 and 4.2 V. They reported 154.5 mAh/g at 0.1 C and 118.4 mAh/g at 10 C; under high-rate cycling, the electrode sustained operation at 10 C for 500 cycles, with retention dependent on synthesis conditions (e.g., 71.8% for one representative sample).

Peng et al. [100] demonstrated that carbon-coated LFP nanowires deliver ~150 mAh/g at 1 C and ~110 mAh/g at 30 C, with CE approaching ~100% and retaining 86% capacity after 1000 cycles at 10 C, demonstrating durable high-rate cathode performance enabled by nanoscale Li⁺ transport pathways. Tuo et al. [101] demonstrated that boron (B)/phosphorus (P) dual-doped carbon coatings to olivine-structured lithium metal phosphate (LiFeMnPO) enhanced its capacity to 159.6 mAh/g after 1000 cycles at 0.2 C, with 99.95% retention, while retaining 97.1 mAh/g at 20 C and sustaining high CE throughout cycling. A performance comparison of representative nanostructured cathode materials is summarised in

Table 3. Performance comparison of nanostructured cathode materials.

Material	Modification/Strategy	Key Performance	Ref.
LCO	Carbon cloth nonplanar architecture	71 mg/cm2 loading; ~9 mAh/cm2 areal capacity: 84% retention (20 cycles); 69–91% (35 cycles)	[92]
LCO	AlPO4/Li3PO4 coating	88% retention at 4.6 V (200 cycles, 45 °C)	[94]
LMO	Hollow microspheres	132.2 mAh/g (0.2 C); 86.3 mAh/g (10 C); 83.2% retention (100 cycles at 0.5 C)	[95]
LMO	Self-supported nanowall array/3D films	131.8 mAh/g (1 C); 97.1 mAh/g (20 C); 96% retention (200 cycles)	[96]
LMO	Spinel thin films (solid-state conversion, 350 °C)	~1.18 Ah/cm3 at 0.1 C; ~0.4 Ah/cm3 at 100 C	[97]
LFP	Nanostructured LFP	154.5 mAh/g (0.1 C); 118.4 mAh/g (10 C); 10 C for 500 cycles; ~71.8% retention	[99]
LFP	Nanowires + C coating	110 mAh/g (30 C); 86% retention (1000 cycles, 10 C)	[100]
LFP	B, P dual-doped C coating	159.6 mAh/g; 99.95% retention (100 cycles)	[101]

.

Nanostructuring enhances cathode kinetics and stability but often reduces electrode density and increases synthesis complexity. Scalability and structure robustness remain key barriers, requiring future work to balance nanoscale improvements with cost, processability, and long-term reliability.

3.4.3. Electrolyte and Separator Nanomaterials

Nanoparticles can improve electrolyte conductivity and safety in liquid electrolytes [102]. Ceramic fillers, such as Al₂O₃ [103], SiO₂ [104], and zirconium dioxide (ZrO₂) [105], can significantly raise ionic conductivity [106]. Zhang et al. [107] showed that incorporating lithium lanthanum zirconium tantalum oxide (LLZTO) nanoparticles and fluoroethylene carbonate (FEC) into polyethylene oxide (PEO)-based electrolytes enhances ionic conductivity, mechanical strength, and interfacial stability via formation of a lithium fluoride (LiF)-rich SEI.

For separators, traditional polyethylene/polypropylene (PE/PP) films suffer from shrinkage and poor wettability. Nanoceramic coatings (Al₂O₃, SiO₂, TiO₂) improve thermal stability and electrolyte retention [108].

Despite these gains, challenges remain. For example, achieving uniform, durable coatings, maintaining mechanical and chemical stability during cycling, and managing the effect of filler loading on cell impedance. Continued research is needed to optimise formulations and scalable deposition techniques that balance performance with manufacturability and safety.

3.5. Challenges for Nanostructured Batteries

Nanostructuring enhances microscale LIB performance by increasing surface area, shortening Li-ion diffusion paths, and improving mechanical stability. These effects raise capacity, rate capability, and thermal resilience across anodes, cathodes, electrolytes, and separators, which is especially valuable for compact MB applications.

However, most nanomaterials require complex, costly, and hard-to-scale fabrication, and achieving uniform, reproducible structure remains a significant barrier. High surface area can increase side reactions and interface instability, reducing initial CE and long-term stability. In addition, inconsistent testing protocols and challenges in nanostructured component integration into complete cells complicate performance comparison and practical implementation. Future research requires scalable fabrication techniques, stable interfaces, and integration strategies, enabling nanoscale advantages to translate into manufacturable MB technologies.

4. Porosity Gradient Architectures in LIB Electrodes

Porosity gradient architectures introduce controlled variations in pore size and distribution through the electrode thickness. Unlike conventional LIB electrodes with uniform porosity, gradient structures can accelerate ion transport, reduce internal resistance, and improve active material utilisation, especially in thick or high-loading electrodes. By tailoring the internal architecture of electrodes, gradient designs offer a promising pathway to enhance both energy and power density without altering the underlying chemistry [109].

4.1. Experimental Studies Demonstrating Benefits of Gradient Porosity

Zhang et al. [110] designed an electrode with vertically aligned pores (20–40 μm on one side, 60–120 μm on the other). Orienting the larger-pore side toward the separator led to improved transport and lithiation uniformity, yielding 122.8 mAh/g. The study reported no volumetric data, leaving uncertainty in energy density trade-offs.

Ezeigwe et al. [111] experimentally validated porosity, tortuosity, and binder gradients in thick electrodes. Reducing tortuosity from 12.67 to 2.20 via multilayer architectures improved ionic conductivity, achieving up to 74% higher capacity at 2 C without sacrificing energy density, highlighting tortuosity reduction as a key transport-efficiency factor.

While experimental studies demonstrate the performance advantages of porosity gradients, most results remain limited to lab-scale prototypes. The lack of standardised fabrication methods and long-term cycling data presents a barrier to broader adoption. Validation of these operating conditions and scalable manufacturing processes is still required.

4.2. Physics-Based Modelling of Porosity Gradients

Modelling studies reinforce experimental observations. Ramadesigan et al. [112] employed a simplified pseudo-two-dimensional (P2D) model and demonstrated that porosity gradients in positive electrodes can decrease internal resistance by 15–33%. Golmon et al. [113] discovered that combining gradients with reduced particle sizes improves capacity utilisation and reduces mechanical stress. Hosseinzadeh et al. [114] demonstrated that multi-layered porosity profiles can enhance energy and power densities by 8.37% and 2.6%, while mitigating heat generation. Zhang et al. [115] also predicted that positioning higher porosity near the separator increases discharge capacity by up to 13% at 0.5 mA/cm², with stable cycling for 120 cycles.

Modelling provides valuable insights into the benefits of porosity gradients, but they often rely on idealised assumptions and simplified geometries. Bridging the gap between simulation and experimental validation is critical for practical designs.

4.3. Advanced Multi-Physics and Data-Driven Optimisation

More advanced approaches combine multi-physics simulation with machine learning optimisation. Yu et al. [116] confirmed that optimised porosity distributions support more uniform lithiation and reduce electrode heating. Amiri et al. [117] combined a P2D model with a neural network surrogate to investigate how different gradient porosity profiles influence energy density in NMC-graphite cells (Figure 13). Configurations with higher porosity near the separator show an over fourfold increase at high discharge rates.

Despite promising results, data-driven optimisation of porosity gradients is still in its early stages. Most approaches rely on simulated data, and their effectiveness in real-world battery systems remains largely untested. Experimental validation and integration into practical design workflows are the essential next steps.

4.4. Challenges for Porosity Gradients

Gradient architectures have shown strong potential for improving LIB performance by enhancing ion transport, reducing internal resistance, and optimising active material utilisation. However, the practical implementation of gradient porosity electrodes faces significant challenges despite their demonstrated benefits in improving ion transport and reducing internal resistance. Current fabrication methods, such as multilayer coating, templating, and advanced printing, remain largely confined to laboratory-scale prototypes due to their complexity and limited throughput. Achieving precise control over pore size distribution across electrode thickness requires stringent process parameters and often involves sequential steps that are difficult to scale for industrial production. Furthermore, maintaining mechanical integrity and uniformity in high-loading electrodes while introducing porosity gradients adds additional constraints on slurry rheology and drying behaviour. Although modelling and machine-learning approaches have optimised theoretical designs, translating these into reproducible, cost-effective manufacturing workflows is still an unresolved barrier. Future progress will depend on integrating gradient architectures into scalable techniques like roll-to-roll processing or hybrid printing methods, coupled with robust quality control to ensure consistency under real-world operating conditions.

5. Non-Active Material Reduction

Reducing non-active components (separators, CCs, and packaging) directly improves gravimetric and volumetric energy density in LIBs. Because these elements make up a large fraction of cell mass and volume, redesigning or minimising them can yield substantial energy gains without changing cell chemistry [118].

5.1. Separator Optimisation

In LIBs, separator selection depends on properties like insulation, stability, and wettability; since it does not store energy, minimising its volume is key to maximising energy density [119]. Thin separators can enhance battery energy density by allowing more room for electrodes, but if too thin, they risk punctures and short circuits [120].

Horvath et al. [121] showed that increasing separator thickness in NMC/CNT half-cells prolongs charge/discharge time due to higher ionic resistance, which increases approximately linearly with total separator thickness up to ca. 65 μm, while the low-rate specific capacity (~170 mAh/g at 16 mA/g) remains essentially unchanged and stable over 25 cycles.

Future work must prioritise the development of robust, ultra-thin separators with mechanical strength and thermal stability to ensure safe operation under real-world operating conditions.

5.2. Increasing Active Material Loading

Non-active components account for a significant amount of the total mass, so increasing the ratio of active to non-active materials will improve the energy density [122].

Chen et al. [123] showed that high-tap-density TiO₂/carbon microclusters achieved 4.9 mAh/cm², while vertically aligned, and folded electrodes exceeded 9 mAh/cm², with stable performance over hundreds of cycles. These strategies reduce the relative contribution of non-active materials.

However, high-loading designs remain difficult to scale or integrate into commercial formats. Future efforts must focus on balancing loading with structural optimisation to ensure both performance and manufacturability.

5.3. Electrode–Separator Integration

Directly coating electrodes onto separators offers clear benefits in reducing weight and boosting energy density, but it also introduces challenges in mechanical durability, uniform coating, and long-term reliability. Kim et al. [124] coated electrodes directly onto polyvinyl alcohol (PVA)-modified separators, with CNT conductivity pathways. This increased volumetric energy density by more than 20%. 30 mAh/cm² and up to 445 Wh/kg in half- and full-cell tests (20–50+ cycles).

Most demonstrations remain proof-of-concept, requiring further research under real-world cycling and manufacturing conditions.

5.4. Lightweight and Thin Current Collectors

CCs represent 10–15% of the total cell mass [125]. Further improvements require lightweight multifunctional designs that maintain electrical performance [126].

Zhang et al. [127] reviewed metallised plastic CCs (metal–polymer–metal layers), reducing Al foil thickness from 12 µm to 6 µm and Cu from 6 µm to 4.5 µm. The resulting mass reduction raised the specific energy by 3.8–6.5%, but challenges like low conductivity and weak adhesion remain. Furthermore, the authors emphasise that the thin Cu CCs exhibit weak mechanical properties and are prone to wrinkles and breakage during electrode coating and calendaring.

Xie et al. [128] electroplated a 4 μm, ultra-thin Cu foil using leveller ADT-DMAP. Compared to conventional 8–10 μm foils, the resulting surfaces were significantly smoother (Ra, a measure of average surface deviation, reduced from 466 nm to 109 nm), with enhanced tensile strength (from 576 to 829 MPa), enabling >40% mass reduction.

Wu et al. [129] developed a bromine-assisted electrodeposition method to produce 6 μm CCs with a nano-twinned grain structure, doubling the tensile strength up to 521.5 N/mm² compared to 247.9 N/mm² for conventional foils deposited using only chloride ions.

While ultra-thin CCs can significantly enhance energy density, they often compromise mechanical strength, conductivity, adhesion, and scalability, which must be addressed before industrial adoption.

5.5. Ultra-Thin Electrode Engineering

Engineering electrodes at the nanoscale offers a direct route to reduce non-active mass in thin-film LIBs. Soltani et al. [130] used ALD to fabricate 100 nm-thick Cu cathodes, achieving a high-rate capacity of 596 mAh/g and enabling a Cu-antimony (Sb) full cell with a capacity of 117 mAh/g and an energy density of 316 Wh/kg at an output voltage of 2.7 V, with stable performance over 500 cycles.

5.6. Wire-Based Current Collectors

The potential direction for improving LIB design involves replacing traditional metal foils with metallic wire-based structures as CCs [125]. Wire-based CCs could reduce weight, improve mechanical flexibility, and enable better electrolyte infiltration and ion transport. Their higher surface area may also enhance heat dissipation, supporting safer and more stable cycling. However, challenges such as ensuring uniform current distribution, maintaining structural integrity under repeated cycling, and scaling up production methods must be addressed. Continued research into advanced materials (e.g., carbon-coated or alloyed wires) and fabrication techniques will be essential for evaluating the feasibility of such architectures in commercial battery systems. Nanowire array CCs have shown potential to enhance electrode cycle life and rate performance, but template-based fabrication methods used for electroplating remain costly and difficult to scale beyond laboratory settings [131].

Cao et al. [132] developed Cu-Si nanocable arrays with a conductive Cu core and an Al₂O₃ coating, achieving a high specific capacity of ~1800 mAh/g at 0.3 A/g and maintaining ~1560 mA/g after 100 cycles at 1.4 A/g; the first-cycle CE was ~56% and exceeded 90% thereafter.

Wang et al. [133] developed 3D Cu-Ge core–shell nanowire anodes showing an initial discharge capacity of ~1987 mAh/g with CE ~80.1%, sustaining over 1400 mAh/g after 40 cycles (0.5 C) and retaining ~734 mAh/g after 80 cycles at 60 C, far exceeding the ≤5 C rates typical for conventional LIB.

Taberna et al. [134] designed an Fe₃O₄ electrode on Cu nanorod CCs (200 nm diameter, 1.8 µm height), delivering six-times-higher power density increase over planar electrodes and retaining 80% capacity at 8 C for 100 cycles.

5.7. Challenges for Reducing Non-Active Materials

Despite the clear potential of reducing non-active materials in LIBs, each strategy presents trade-offs that limit practical implementation. Thinner separators can increase energy density but often compromise mechanical integrity and thermal stability. High-loading electrodes enhance capacity but face ion-transport bottlenecks and thermal limitations. Integrated electrode–separator systems reduce component redundancy but remain at the proof-of-concept stage due to issues with coating uniformity, durability, and manufacturing compatibility. Ultra-thin CCs reduce mass yet often sacrifice conductivity, adhesion, and structural robustness. Wire-based structures show strong performance potential but remain difficult to scale.

Many of these innovations are demonstrated under idealised laboratory conditions, with limited validation in full-cell configurations or real-world environments. Future research requires holistic design approaches that balance performance gains with manufacturability, safety, and long-term reliability. Cross-disciplinary collaboration between materials science, mechanical engineering, and manufacturing technology will be essential to translate these promising concepts into commercially viable solutions.

6. Conclusions

The development of high-performance microscale LIBs is increasingly shaped by advances in microfabrication, nanostructuring, porosity-engineered architectures, and reduction in non-active materials. These physical innovations define the practical pathways for improving energy density, cycling stability, mechanical robustness, and integration into compact or flexible devices.

Nanostructured electrodes, advanced separator and CC designs, and porosity-engineered architectures continue to enhance electrochemical kinetics and mechanical robustness, while improving transport efficiency in high-loading systems. Complex processing routes, limited scalability, interfacial instability, and reproducibility issues hinder the transition of many promising concepts, such as gradient electrodes, ultra-thin collectors, and 3D architectures, from laboratory demonstrations to practical devices.

Balancing cost and performance is also a critical consideration for the adoption of advanced MB technologies. Top–down approaches such as photolithography and laser structuring deliver exceptional precision and enable high-performance architectures, but their reliance on cleanroom environments, expensive equipment, and multi-step processes significantly increases manufacturing costs, limiting scalability. Conversely, bottom–up techniques like screen printing, spray coating, and mask-assisted filtration offer low-cost, high-throughput fabrication compatible with flexible substrates, yet they often sacrifice resolution and uniformity, which can constrain energy density and rate capability. Emerging methods such as 3D printing provide design flexibility and material efficiency but remain cost-intensive due to slow printing speeds and specialised ink requirements. Similarly, nanostructuring and thin-film designs enhance capacity and cycle life but involve complex synthesis routes and costly precursors, challenging large-scale implementation. Ultimately, achieving an optimal cost-to-performance ratio will depend on hybrid strategies that combine the precision of high-cost methods with the scalability of low-cost processes, alongside innovations in material formulations and digital optimisation to reduce processing complexity.

Future progress will depend on scalable manufacturing, robust interface engineering, and reliable characterisation methods that connect laboratory results with practical device performance. With the sustained development of physically engineered architectures, microscale LIBs are moving toward commercially viable technologies capable of delivering high energy density and long operational lifetimes for next-generation electronic and sensing applications.