Engineering electrode microstructures for advanced lithium-ion batteries
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Abstract
The architecture of anode materials is an essential factor in improving the performance of energy storage devices, which meets the increasing demand for energy storage and helps achieve environmental sustainability targets. Atomic manufacturing allows the makeup of electrodes to be changed precisely at the atomic level. This facilitates the creation of electrode materials with specific physical properties and enhanced performance. This Perspective reviews the details of how the microstructure design influences key electrode material characteristics. Finally, we anticipate the potential of materials and manufacturing techniques for materials microstructure in the future. A thorough grasp of the materials microstructure in electrode materials is offered by this article.
Keywords
INTRODUCTION
For the optimal function of modern human society, energy storage technology must be reliable, cost-effective, and secure. Rechargeable lithium-ion batteries (LIBs) are the industry standard for energy storage in the rapidly growing sector of electric devices, grid storage, and portable gadgets. The current cost of LIBs is around US$150 per kilowatt-hour (kWh-1), with a downward trend. According to the US Department of Energy, LIBs will be more economical than internal combustion engine vehicles once the cost falls below approximately US$100 kWh-1[1,2]. Because of this, it is anticipated that sales of electric vehicles, which already surpass one million units annually, will expand dramatically to over thirty million units annually by 2030. As the world increasingly relies on intermittent yet alternative energy, such as solar and wind, the grid energy storage business is expected to expand rapidly during the next decade. Consequently, it is predicted that the global demand for LIBs would increase to 1,200~1,600 GWh by 2030, exceeding ten times the demand of 2020.
While LIBs stand out as the leading rechargeable battery technology, there is still a significant difference between LIBs and gasoline in terms of energy and power density[3]. Therefore, there is a compelling need for further advancements in LIB performance. The active electrode substance composition and microstructure significantly affect energy and power density of LIBs due to their effect on the electrode production process[4-8]. Consequently, battery engineers must consider selecting and deploying active electrode materials with the desired microstructure during the battery design.
For batteries, a microstructured electrode contains active material particles within a well-structured domain with a minimum dimension of 1 μm. Microtechnology advancements have significantly enhanced the efficacy of microstructured electrodes over the last two decades. These electrodes provide advantages in power density, high-rate capability, gravimetric capacity, memory effect reduction, fracture toughness, and durability against fatigue. Furthermore, functionalizing a battery material concerning its superior rate capability, extended cycling performance and high specific capacity for LIBs has been the focus of morphology engineering[9-16]. Using synergistic benefits of each structure design, incorporating active materials into functionalized microstructural morphology can help develop novel electrochemical performance-governing materials[17-19]. While substantial research in recent decades has evaluated various nanostructured anode materials for LIBs employing distinct lithium storage, there remains a need for a comprehensive review and discussion on the intricate microengineering methodologies[20-22].
This Perspective concentrates on the discussion of developing specific morphological features, including (a) low-dimensional microstructures; (b) 3D hierarchical structures; and (c) distinctive hollow structures. These themes have gained prominence due to their unique characteristics and the significant advantages they offer in the realm of battery technology. What makes these themes so prevalent? Low-dimensional microstructures, in general, retain larger, more electrochemically active surface areas with smaller diameters. This characteristic facilitates improved contact between the electrode and the electrolyte and shorter diffusion distances. The lowest dimensional patterns that may effectively transport Li+ and electrons are represented by 3D hierarchical porous microstructures. The advantages of micron-sized assemblies - often distinguished by a high tap density and thermodynamic stability - combine with those of nano-sized blocks in unique hollow structures.
LOW-DIMENSIONAL MICROSTRUCTURES
Extensive research has been devoted to nanomaterials as electrode materials for many years due to their numerous advantageous properties, including a substantial contact region between the electrode's surface and electrolyte, a short distance for electron and ion transport, and in some instances, the capability to conduct electrochemical reactions that are impracticable with bulk materials. Furthermore, the remarkable characteristics of the microstructure of materials can be modified by manipulating their morphologies. This enables the distinction between different microstructures based on their dimensional characteristic. According to their intrinsic dimensionality, micromaterials may be grouped according to their morphologies or intrinsic holistic form. To illustrate, (i) nanospheres are classified as zero-dimensional (0D) materials since their dimensions are exclusively at the nanoscale; (ii) anisotropic nanowires, nanorods, or nanotubes are one-dimensional (1D) materials that possess at least one dimension beyond the nanometer range; and (iii) single or few layers of atoms or molecules make up two-dimensional (2D) materials. The layers are joined by strong covalent or ionic bonds and bound by weak van der Waals interactions.
0D nanomaterials
In contrast to bulk materials, 0D nanoparticles offer several benefits for lithium battery applications, including enhanced interfacial interaction with electrolytes, mitigation of inherent alterations in volume resulting from cyclic processes, and accelerated Li+ transport[23]. Zhu et al. recently reported on applying 0D nanomaterials to enable innovative lithium battery chemistries[24]. The synthesis of spherical 5-nm lithium oxide (Li2O) incorporated in a sub-10-nm tricobalt tetraoxide (Co3O4) matrix [Figure 1A] used as a cathode material in a lithium-ion battery was accomplished via a straightforward sintering procedure. By forming a condensed phase of metastable lithium peroxide (LiO2) and lithium re-insertion, this nanocomposite enabled lithium extraction and reformation of Li2O. For Li2O to be stabilized by interfacial precipitation over the Co3O4 substrate surface, the nanoconfinement of Li2O within the Co3O4 nano porous skeleton is crucial. By utilizing a confined, entirely condensed-phase configuration, this system circumvented many challenges in lithium-air batteries. The device exhibited excellent performance, as shown in Table 1.
Figure 1. Illustrative examples of low-dimensional microstructure samples. (A) Transmission electron microscopy (TEM) of the nanocomposite Li2O and Co3O4 powder, the circled area is a representative structure of amorphous Li2O confined within the Co3O4 skeleton. Reproduced with the permission of Ref.[24] Copyright 2016, Springer Nature. (B1) SEM image, (B2) TEM image, (B3) high-resolution HRTEM image, (B4) TEM image and the elemental mapping images taken from the selected area of NbC nanospheres. Reproduced with the permission of Ref.[25] Copyright 2017, John Wiley & Sons. (C1) TEM and (C2) magnified TEM images of the boxed area in (C1) of NiS nanoparticles. Reproduced with the permission of Ref.[28] Copyright 2016, Elsevier. (D) TEM images of CuP2 nanowires. Reproduced with the permission of Ref.[34] Copyright 2016, American Chemical Society. (E) Schematic illustrating the synthetic method used for NW growth and schematic showing the cumulative effect of cycling on the NW architecture. The pristine Ge NW array is transformed into a porous, interconnected network of active material due to the charge/discharge process. Reproduced with the permission of Ref.[35] Copyright 2014, American Chemical Society. (F) Aligned ceramic Li0.33La0.557TiO3 nanowires for hybrid solid electrolyte. Li+ conduction paths in solid polymer electrolyte containing random and aligned nanowires. Express Li+ conduction along the ceramic/polymer interface. Reproduced with the permission of Ref.[37] Copyright 2017, Springer Nature. (G1) Crystal structure and surface energy of LiFePO4 crystals and the schematic illustration of shortening the lithium-ion diffusion distance along the [010]; structural and phase purity characterization of the LiFePO4 nanosheets; (G2) representative SEM image of the as-synthesized LiFePO4 particles; (G3) SEM image showing the latitudinally accumulated nanosheets. Inset: an individual nanosheet obtained after exfoliation. Reproduced with the permission of Ref.[44] Copyright 2014, American Chemical Society. (H1) Schematic illustration for the synthesis of the Si@G network. (H2 and H3) TEM images of the 3D interconnected graphene network at different magnifications. Reproduced with the permission of Ref.[50] Copyright 2014, American Chemical Society.
Various morphologies of nanomaterials in energy storage, their electrochemical performance, and the importance of controlling morphology
| Type | Electrode material | Function | Application | Performance | Advantages | Ref. |
| 0D | nanocomposite Li2O and | Cathode | LTO/NC | • Initial discharge capacity: 502 Ah kg-1, then increased to 587 Ah kg-1 in a few cycles • Discharge capacity loss: 4.9% after | • Zero-dimensional morphology stabilized the synthesis of LiO2, suppressing the growth of O2 • Hermetically sealed condensed-phase apparatus | [24] |
| 0D | nanocrystalline NbC | Membrane | Li-S | • Discharge capacity loss: 0.037% cycle-1 over 1,500 cycles • C-rate: 840 and 730 mAh g-1 at 2 and 5 C | • The conductive NbC interlayer functions as a protective barrier, confining polysulfides. This confinement serves the dual purpose of preventing passivation of the lithium anode and mitigating self-discharge tendencies in the battery • Furthermore, the interlayer assumes the role of a “second current collector”, facilitating the reutilization of trapped active materials and substantially augmenting sulfur utilization in the battery system | [25] |
| 0D | Graphene-wrapped nickel sulfide nano prisms | Anode | Li-ion | • C-rate: 834, 656, 542, 278 and 141 mAh g-1 at 0.07, 0.5, 1, 3 and 5 A g-1 | • Increased specific area of surface and pore size facilitated enhanced electrolyte penetration within the electrochemically active region • A conductive network was created by encircling graphene | [28] |
| 1D | Phosphorus-rich copper phosphide nanowires | Anode | Li-ion | • Initial discharge capacity and coulombic efficiency: 865 mAh g-1 and 57% (0.1 C) • Discharge capacity loss: 0.12% per cycle • C-rate: 1,000 and 600 mAh g-1 at 0.2 and 6 C (87% retention) | • The CuP2 nanowires were designed to withstand volume expansion, resulting in increased capacity retention and decreased electrode resistance • Creating easy lithium diffusion pathways results in reduced diffusion resistance, likely linked to the development of the Li-ion conductive structure Li3P | [34] |
| 1D | Germanium nanowire | Anode | Li-ion | • Initial discharge capacity and coulombic efficiency: 1,103 mAh g-1 and 97% (0.5 C) • Discharge capacity loss: 888 mAh g-1 after 1,100 cycles • C-rate: 1,250, 1,174, 1,050, 821, 722, and back to 1,188 mAh g-1 at 0.01, 0.2, 0.5, 1, 2, and 0.01 C | • A porous network of interconnected Ge nanowires is mechanically robust to the capacity retention of the electrode. | [35] |
| 1D | Well-aligned Li0.33La0.557TiO3 nanowires | electrolyte | Solid battery | • Total ionic conductivity: 6.05 × 10-5 S cm-1 at 30 °C • Surface ionic conductivity: | • Nanowire surfaces provide a fast ion conduction pathway without crossing junctions. | [37] |
| 2D | Single-crystalline LiFePO4 nanosheets | cathode | Li-ion | • C-rate (2D type): 151, 140, 120, 85, 72 mAh g-1 at 0.5, 1 5, 10, and 20 C • C-rate (bulk type): 147, 135, 85, 58, 40 mAh g-1 at 0.5, 1 5, 10, and 20 C • Discharge capacity loss: no obvious fading after 1,000 cycles at 10 C | • 2D nanosheets considerably enhance lithium-ion insertion/extraction efficiency, thus leading to an enhanced rate capability. | [44] |
| 2D | Graphene-wrapped porous silicon | Anode | Li-ion | • Discharge capacity loss: 1,299.6 mAh g-1 after 25 cycles at 0.05 C • C-rate: 1,467.5, 1,210.6, 970.7, 697.8, and back to 1,096.8 mAh g-1 at 0.05, 0.1, 0.2, 0.5, and 0.05 C | • 2D morphology stabilized structure • Graphene maintained stability and conductivity | [50] |
| 3D | Mesoporous crystalline MoSe2 | Anode | Li-ion | • Discharge capacity loss: 630 mAh g-1 after 35 cycles • C-rate: 646, 604, 593, 557, 499, and 372 mAh g-1 at 0.05, 0.1, 0.2, 0.5, 1, and 2 C | • The structural stability of the 3D mesoporous MoSe2 electrode leads to stable cycling performance • The element “Se” plays a vital role in achieving enhanced rate performance and better kinetics | [53] |
| 3D | Melamine-based porous organic frameworks | Cathode | Li-S | • Discharge capacity loss: 0.0508% cycle-1 over 500 cycles • C-rate: 1,382, 1,255, 1,134, 996, 875, and 737 mAh g-1 at 0.1, 0.2, 0.3, 0.5, 1, and 2 C | • Melamine-based porous organic frameworks have a porous structure with very high nitrogen content and customizable porosity | [55] |
| 3D | Urchin-like MnO/C microspheres | Anode | Li-ion | • Initial discharge capacity: 845 mAh g-1 • Discharge capacity loss: 723mAh g-1 after 80 cycles • C-rate (u-MnO): 1,157.1, 478.4, 270.5, and 81.7 mAh g-1 at 0.1, 0.2, 0.5, and 1 mA cm-2 • C-rate (u-MnO/c): from 1,197.1 to | • A particular carbon film on u-MnO may buffer volume expansion during cycling • A needle-like carbon layer on u-MnO improves its electronic conductivity | [54] |
| 3D | Sea urchin-like Si@MnO2@rGO | Anode | Li-ion | • C-rate: 1,323.87, 971.85, 701.12, 491.85 and 272.47 mAh g-1 at 0.1, 0.2, 0.5, 1, and 2 A g-1 • Initial discharge capacity: 1,378.14 mAh g-1 • Discharge capacity loss: 960.21 mAh g-1 after 150 cycles | • The construction that looks similar to a sea urchin keeps the anodes from getting excessively huge during cycling | [55] |
| 3D | Flower-like Si@NiO/rGO composites | Anode | Li-ion | • C-rate: from 2,500 to 500 mAh g-1 at different current densities (0.1~2.0 A g-1) • Discharge capacity loss: 1,081.34 mAh g-1 after 700 cycles at 1 A g-1 | • The highly percolated nanoflowers facilitated fast Li+ diffusion and efficiently mitigated the volume expansion of Si NPs during cycling | [68] |
| Hollow Spheres | Hierarchical Fe3O4 hollow spheres | Anode | Li-ion | • Initial discharge capacity and coulombic efficiency (hollow sphere): 1,063 mAh g-1 and 66% (0.5 A g-1) • Initial discharge capacity and coulombic efficiency (solid sphere): 800 mAh g-1 and 57% (0.5 A g-1) • C-rate (hollow sphere): 992, 853, 716, and 548 mAh g-1 at 1, 2, 4, and 8 A g-1 • Discharge capacity loss (hollow sphere): 700 mAh g-1 without notable fading after 200 cycles at 3 A g-1 • Discharge capacity loss (solid sphere): | • A multilayer hierarchical structure made up of nanosheet sections and a hollow structure provides a strong and spongy framework with an extensive electrode/electrolyte interface and a shorter Li+ diffusion path • Architecture might more effectively handle the enormous changes in volume generated with frequent lithiation and delithiation | [71] |
| Hollow Spheres | Yolk-shell MoO2 microspheres | Anode | Li-ion | • Initial discharge capacity and coulombic efficiency: 955.4 mAh g-1 and 66% • Discharge capacity loss: 662 mAh g-1 after 50 cycles at 0.5 A g-1 (retention rate: 94.7%) • C-rate: 850, 714, and 450 mAh g-1 at 0.05, 0.5, and 2 A g-1 | • The space between individual nanoparticles (porous shell) acts as an effective buffer for the material's volume fluctuations • The yolk-shell structure aids the diffusion of Li+ during intercalation and deintercalation by the electrolyte | [77] |
| Hollow Spheres | NiCo2O4 hollow spheres with complex interior structures | Anode | Li-ion | • Initial discharge capacity and coulombic efficiency: 928 mAh g-1 and 64% (0.15 A g-1) • C-rate: 834, 745, 662, and 533 mAh g-1 at 0.3, 0.6, 1, and 2 A g-1 • Discharge capacity loss: 706 mAh g-1 after 100 cycles at 0.2 A g-1 (retention rate: 78%) | • Small primary nanoparticles and holes may enhance the movement of Li+, yielding a high capacity and outstanding rate capabilities • A three-layer core-in-double-shell interior construction may mitigate the significant volume changes that occur during repeated Li+ insertion/extraction cycles | [87] |
In addition to being effective at storing lithium, 0D nanomaterials can also enhance battery performance via additional mechanisms[25]. By utilizing their polar characteristics and electronic conductivity, nanospheres of niobium carbide (NbC) were fabricated [Figure 1B] and coated on a separator to trap polysulfide (PS) intermediates. The utilization of 0D NbC for effective PS trapping, coupled with the intrinsic electronic conductivity of NbC, led to a substantial enhancement compared to the conventional configuration. Specifically, the capacity loss rate was reduced from 0.497% to 0.24%.
Despite possessing numerous electrochemical benefits compared to bulk materials, some 0D materials, including ZnO, remain intrinsically non-conductive and have restricted electronic conductivity. Carbon coating is a common way to mitigate this concern[26]. Kim et al. investigated carbon-coated 0D nanomaterials early[27]. Carbon was deposited onto these nanoparticles via chemical vapor deposition. The structure exhibited the most favorable characteristics: carbon-coated 10-nm Si particles demonstrated superior performance, retaining initial coulombic efficiency of 89% and 96% of their first cycle capacity after 40 cycles at 0.2 C.
Nanoparticles remain susceptible to aggregation and readily detach in intense fragmentation. A commonly utilized approach to tackle this challenge involves incorporating 0D nanomaterials onto or within carbon structures. The extended 2D structure of graphene, which provides electronic conductivity, stability, and support against pulverization during cycling, renders it an especially appealing material for loading nanoparticles. Graphene-encapsulated nickel sulfide (NiS) nanoparticles with a consistent tetrahedral structure were synthesized by AbdelHamid et al. in an organic solvent via a colloidal method
1D nanomaterials
The merits of this category of nanostructures render them appealing for various LIB applications. Charge movement along the lengthwise direction is facilitated by their elongated dimension along one axis. Their advantageous stress distribution in the high aspect ratio shape helps avoid pulverization during riding. Furthermore, they are particularly advantageous for forming flexible membranes and are simple to cultivate on current collectors[29-33].
For instance, Li et al. developed a distinctive supercritical fluid-liquid-solid growth technique to fabricate copper phosphide (CuP2) nanowires[34]. The synthesized CuP2 nanowires exhibit a 1D characteristic, as evident from morphological observations in Figure 1D. Consistent with expectations, the CuP2 nanowire sample that underwent calcination exhibits remarkable cycling performance and capacity. After undergoing 100 cycles at 100 mA g-1, it maintained a capacity of 945 mAh g-1, indicating its promise as an anode material for LIBs. Kennedy et al. documented the direct growth of Ge nanowires on the copper foil (see Figure 1E)[35]. The material exhibited a notable capacity of 1,103 mAh g-1 at 0.5 C, which was sustained for 1,100 cycles at 888 mAh g-1. The exceptional durability of the battery can be attributed to the direct development of 1D nanowires, which formed a durable electrode that retained its exceptional stability despite material restructuring caused by protracted cycling.
Enhancing electronic conductivity is one of the benefits of the 1D morphology. An analogous pattern is observed with regard to ionic conductivity, whereby ions can swiftly traverse the longitudinal axis. Cui et al. reported Li0.33La0.557TiO3 nanowires in the polymer[36,37]. By combining electrospinning and calcination, polycrystalline Li0.33La0.557TiO3 nanowires (d < 140 nm) were produced. As shown in Figure 1F, an analysis was conducted to examine how the 1D structure affects the Li+ conductivity compared to its 0D form. An investigation was conducted on how nanowire alignment affects the transit of Li+. The research found that 0D nanoparticles increased the Li+ conductivity of the polymer tenfold. This improvement was primarily attributed to the increased concentration and mobility of Li+ ions on the surface of the nanoparticles, where a very ionically conductive contact was formed with the polymer. Nevertheless, their potential was limited by the distinct features of 0D nanoparticles included in the composite and their tendency to cluster together. Replacing 0D morphology with 1D nanowires resulted in a fivefold improvement in ionic conductivity. The increase was only caused by the 1D morphology, which created a continuous channel for Li+. Through nanowire alignment, interwire cross-junctions were effectively eliminated, resulting in a smooth flow of Li+ across the interface between the nanowire and polymer.
Another intriguing characteristic that distinguishes 1D nanoparticle morphologies from others is that They are very beneficial for creating adaptable, freestanding membranes. The distinctive characteristic arises from the fibrous nature of 1D nanostructures, allowing them to be easily manipulated or integrated into flexible, durable structures that resemble fabric or foil. Lei et al. illustrated this phenomenon when they fabricated a flexible separator without binders using Al2O3 nanowires[38]. The resulting separator exhibited improved charge transfer, thermal stability, and wettability, all contributing to enhanced battery performance. Substituting Al2O3 nanowires for cellulose fiber separators resulted in a doubling of the capacity of a graphite/lithium-iron-phosphate (LFP) battery.
2D nanomaterials
The preferential exposure of specific facets in the 2D structures plays a pivotal role in facilitating rapid ion and electron transfer, thereby enhancing the efficiency of lithium storage. In addition, the mechanical flexibility of 2D nanomaterials is enhanced by their ultrathin thickness, making them ideal for creating elastic, thin, and flexible LIBs[39,40]. The substantial area-to-volume ratio of the nano-thin materials enables excellent contact with the electrolyte and promotes in-plane electron and ion transport. Additionally, the prolonged lateral dimensions establish uninterrupted ion transport pathways. Furthermore, the impact of stress-strain cycles during battery operation is mitigated by the 2D nanostructures, thereby improving overall battery stability[41,42]. Electrode materials exhibiting anisotropic Li+ diffusion could benefit significantly from implementing 2D nanomaterials. The 2D morphology of these materials was intentionally structured to restrict the diffusion of Li+ along the 2D plane, substantially improving both the rate capability and diffusion velocity of Li+. Concurrently, in contrast to the 0D morphology, sizable 2D sheets exhibit greater volumetric energy density and stability due to their reduced side reactions with the electrolyte.
An investigation comparing the LFP with different carbon materials revealed that 2D graphene nanosheets disperse more evenly in the electrode matrix than carbon nanotubes (CNT) and acetylene black (AB)[43]. The LFP-graphene composite in the study demonstrated a diffusion coefficient of 5.9 × 10-9 cm2/s and an interface resistance value of around 130 Ω cm-2. The figures were around 150%-300% greater than those of the CNT and AB. This pertains to the development of extended electron routes and the elevated electron conductivity shown by this category of 2D materials. Zhao et al. produced LFP nanosheets that are single-crystalline and have a thickness ranging from 30 to 60 nm in the [010] direction [Figure 1G][44]. The nanosheets also have micrometric lateral dimensions. The synthesis process controlled the morphology and crystal structure, with diethylene glycol stabilizing the planes throughout the solvothermal reaction. This stabilization was achieved due to the interaction energy, effectively inhibiting growth in the [010] orientation. Consequently, the surface coverage of the [010] plane exceeded 93%, significantly improving Li+ by tenfold compared to traditional LFP powder. This improvement further resulted in a 50% reduction in polarization and an increased capability to operate at high rates. Specifically, a capacity of 90 mAh g-1 was reached at a rate of 10 C, and the stability over 1,000 cycles was found to be extremely excellent. This method limited the nanoscale size in the best direction for Li+ diffusion while maintaining a micrometer size in the other directions. Consequently, it improved the transit of Li+, decreased surface area, and minimized negative interactions with the electrolyte.
The remarkable mechanical properties of a network created by 2D materials around the electrode particles can prevent fracturing and improve the stability of the electrochemical reaction. Typically, an amount of weight between 10% and 60% of the mass of active materials is utilized to improve the mechanical strength and electrochemical efficiency of electrodes[45-49]. Wu et al. investigated the pulverization of silicon nanoparticles that occurs throughout the cycling process[50]. They achieved this by creating a hybrid nanostructure consisting of graphene-wrapped silicon/carbon nanofibers (Si/CNF-G). Figure 1H shows the conceptual design, transmission electron microscopy (TEM) image of the Si/CNF-G nanosheet composite and the corresponding cycle performance at different silicon ratios. The electrode without a binder showed a reversible capacity of 878 mA h g-1 over 100 cycles for the optimized sample. However, the silicon electrode could only sustain a reversible discharge capacity of 200 mA h g-1 after 100 cycles.
3D HIERARCHICAL STRUCTURES
Hierarchical structures offer significant benefits in the field of LIBs. Firstly, using nanosized building blocks ensures short distances for both electronic and Li+ transport, enhancing electrochemical activity. Secondly, the porous nature of these nanostructures facilitates effective impregnation of electrolytes and ensures a suitable region of contact, thereby improving rate performance. Lastly, the ordered structure of these nanostructures allows them to endure the stress from lithium insertion or extraction, therefore prolonging the battery cycle life[51].
Mesoporous structures
Typically, hierarchical nanostructures formed by nanoparticles consist of numerous fundamental building blocks with diameters falling within the range of tens of nanometers. This results in the creation of a porous framework with a fairly large surface area[52]. Shi et al. developed a nanocoating method using mesoporous silica SBA-15 as a stiff template to manufacture well-organized mesoporous crystallized MoSe2[53]. The MoSe2 particles created have a rod-like shape and structured mesostructure formed from the SBA-15 precursors [Figure 2A]. The mesoporous MoSe2 sample exhibits distinctive structural characteristics, resulting in a discharge capacity of 630 mAh g-1 after 35 cycles. This capacity significantly surpasses its predicted capacity achieved by the four-electron conversion process.
Figure 2. Illustration of the preparation, morphology and performance of 3D hierarchical nanostructures. (A1 and A2) SEM images of the synthesized mesoporous MoSe2 materials using SBA-15 as the template via nanocoating method. Reproduced with the permission of Ref.[53] Copyright 2012, John Wiley & Sons. (B) Synthetic schematic of SNW/S and working mechanism of LiPSs trapping. Reproduced with the permission of Ref.[55] Copyright 2021, IOPScience. (C) Schematic illustration for the preparation process of u-MnO and
Porous organic polymers (POPs) may provide an alternative porous structure. Given the extensive range of different types of persistent organic pollutants, the capabilities and applications of POPs are likewise very diverse and include several sectors, particularly in batteries based on lithium chemistry. The use of POPs for immobilizing active substances has mostly been limited to the domain of Li-S batteries[54,55]. As shown in Figure 2B, the performance of Li-S batteries is influenced by the well-documented PS shuttle effect, which arises from the solubility of intermediate reaction products known as PSs. POPs have primarily been utilized in two manners: (1) by chemisorbing intermediate PS species to hinder their dissolution and diffusion out of the cathode; and (2) by forming covalent bonds between sulfur and insoluble POPs, thereby preventing PS dissolution during discharge (the covalent bonds between sulfur and the POP host are ideally maintained during lithiation/delithiation).
POPs produced by porous carbon exhibit a high number of active sites and improved conductivity, enabling quick and reversible lithium-ion intercalation/de-intercalation. Additionally, it has enough void spaces to tolerate volume variations, facilitating rapid redox reactions. The physical characteristics such as shape, size, surface area, and the presence of different elements in the structure of porous carbon materials may significantly influence their ability to conduct electricity. Porous carbon materials have been synthesized using POPs in order to attain a substantial specific surface area, which facilitates efficient electrolyte penetration and redox processes. The presence of enriched micro and mesopores allows for rapid and effective ion transport and enhances the accessibility to the active surface. Furthermore, the simple modification of POPs allows for the inclusion of diverse heteroatoms inside their molecular framework. Lithium ions in the electrolyte are propelled towards lithium-affinitive sites on the negative electrode surface by combining electric potential gradients, lithium-ion concentration gradients, and electrostatic interactions with electronegative sites within the electrode framework. Doping atoms should establish electronegative sites (doping atoms or adjacent carbon atoms) as Lewis bases to interact with lithium ions acting as Lewis acids. Therefore, sites carrying a greater negative charge are believed to induce stronger interactions with lithium ions. Moreover, lithium affinity is not solely determined by acid-base interactions; it is also influenced by other factors such as the local geometric and electronic structure of the dopant. Thus, introducing the concept of local dipoles becomes necessary: doping atoms form dipoles with adjacent carbon atoms, thereby further enhancing the binding energy. These factors collectively contribute to the enhancement of lithium storage capacity. Subsequently, these heteroatoms may be transformed into heteroatom-doped carbon upon exposure to high temperatures, hence enhancing the capacity for lithium-ion storage[56-58].
Urchin-like nanospheres
The urchin-like hierarchical structures, formed by assembling 1D building blocks, possess structural benefits derived from both the overarching hierarchical structures and the fundamental low-dimensional components. Consequently, these complex formations exhibit improved lifespan and quicker response kinetics[39,59].
The Urchin-like materials have a spherical shape composed of nanorods, with a hollow macroporous core. To achieve this category, the gradual decomposition of functional ligands plays a pivotal role in the genesis of intricate hierarchical nanostructures[60]. This core may function as an electrolyte reservoir, and the nanorods facilitate rapid diffusion of the electrolyte and effectively accommodate the strain generated during cycling[61]. Zhang et al. manufactured urchin-like MnO/C microspheres by a hydrothermal technique and thermal degradation of polyvinyl alcohol (PVA) on the MnO surface [Figure 2C][62]. The electrochemical performance for lithium storage is enhanced in u-MnO/C due to its distinctive hierarchical nanostructures and carbon coatings. The u-MnO/C anode has a notable initial discharge capacity of
Liu et al. recommended a straightforward hydrothermal technique to synthesize sea urchin-like
Flower-like nanostructures
Flower-shaped structures have gotten numerous attention among various forms inspired by nature. This is attributed to their notably larger surface area than circular particles, leading to more effective interactions at the surface[64]. The morphology and dimensions of artificially fabricated flower-like structures may be manipulated to regulate both the stability of the structure and the performance of surface activities[65].
The flower-shaped configuration enhances the contact area of electrodes, increases the wetting properties of electrolytes, and expedites the movement of lithium ions, improving rate performance[66,67]. Furthermore, these self-supported electrodes can eliminate the need for additional binders or conductive additives, enhancing the specific capacity even further[30]. Pan et al. produced a Si@NiO/rGO anode with a distinctive flower-like structure[68]. This was achieved using a straightforward and effective one-step synthesis process to encapsulate Si inside the flower-like NiO structure [Figure 2E1 and E2] while evenly embedding rGO nanosheets in the "petals" of NiO. Figure 2E3 displays the rate performance of these anodes at different current densities ranging from 0.1 to 2.0 A g-1. The performance of all three anodes decreases with increasing current density from 0.1 to 2 A g-1. Si@NiO/rGO composite had superior capacity maintenance than Si@NiO and Si/rGO, despite all recovering and remaining stable when current density returns to 0.1 A g-1. The Si@NiO/rGO hybrid maintains a capacity maintenance rate of about 77.08% with a reversible capacity of 1,794.22 mAh g-1. The unique layout of silicon nanoparticles encased in nickel oxide nanoflowers and rGO nanosheets within the nickel oxide “petal” structure makes for excellent performance. This construction allows quick electrolyte penetration and lithium-ion diffusion and improves silicon nanoparticle swelling buffer zones.
DISTINCTIVE HOLLOW STRUCTURES
Hollow structures are particularly valuable as hosts for active electrode materials, offering a functionality exclusive to morphologies with dimensions of 0D and 2D. Additionally, 1D nanostructures can include active substances within their empty core. However, the small volume of nanotubes makes it challenging to load a significant quantity of active substances. Therefore, 3D hollow nanostructures are well-documented and very effective for this particular purpose. Various hollow 3D structures, including single-shell and multi-level architectures, have been created to meet the growth in volume of anode materials in LIBs, offering both physical and chemical confinement that enables greater storage of Li+.
Single-shell architecture
Hollow structures are built with a single layer of active material around an interior void. The synthesis techniques for these structures typically employ either template-engaged or template-free procedures[69,70]. Ma et al. created hollow balls using extremely thin Fe3O4 nanosheets[71]. The self-engaged template consists of solid spheres containing a precursor of iron, which is composed of Fe-ions and isopropyl alcohol, referred to as Fe-IPA. During the solvothermal process, Fe-IPA spheres gradually change to hierarchical
Figure 3. Structural information of distinctive hollow structures. TEM images of the Fe-IPA with hollow structure obtained after reaction for (A1) 1 h, (A2) 2 h, (A3) 4 h, and (A4) 12 h. The scale bars are 300 nm; (A5) Schematic illustration of the formation process of Fe-glycerate hollow spheres: (I) formation of Fe-glycerate nanosheets on the surface of the initially formed Fe-IPA solid spheres; (II) further growth of Fe-glycerate nanosheets and consumption of Fe-IPA spheres; (III) formation of hierarchical Fe-glycerate hollow spheres. Reproduced with the permission of Ref.[71] Copyright 2015, John Wiley & Sons. (B) Schematic illustration of the formation process of yolk-shell MoO2 microspheres. Reproduced with the permission of Ref.[77] Copyright 2013, Royal Society of Chemistry. (C) Schematic illustration of the formation process of NiCo2O4 core-in-double-shell hollow spheres. Reproduced with the permission of Ref.[87] Copyright 2014, John Wiley & Sons.
Multi-level architecture
The advantages of single-shelled hollow constructions are offset by the excess vacant space, which greatly reduces the tapping density of the active materials, resulting in low volumetric power and energy density[52,69]. Hollow nanostructures with multi-level architecture, such as those with yolk-shells and multi-shells, have been suggested to avoid these problems[72-79]. By optimizing the use of the interior cavity of hollow structures, these intricate designs successfully raise the weight fraction of the electrochemically active components[52]. Furthermore, the structural stability might be further improved using the multi-level design as physical supports[80]. In addition, the electrolyte might penetrate the inner area via the void spaces of these hollow structures, which could also handle the volume fluctuations that occur during repeated charging and discharging cycles.
The yolk-shell architecture is a distinct kind of core-shell structure defined by a certain core-void-shell arrangement[78,81,82]. Metal oxides with a yolk-shell structure are being closely studied as potential electrode materials in LIBs because they can prevent the clumping of active nanoparticles inside them and handle the substantial volume changes occurring throughout cycling. Yolk-shell structured SnO2, Co3O4, V2O5, and
Multi-shelled micro-/nano hollow structures with more than two shells are expected to have superior properties than single-shell structures because of their larger area of surface, lower density, and improved loading capacity. Hollow constructions with many shells may be readily created using hollow shells as templates. Due to the permeable nature of the template's shell, the chosen shell material will coat both the inner and outer surfaces of the templates, forming uninterrupted layers of coating. Shen et al. proposed a novel and effective approach to produce homogenous NiCo2O4 hollow spheres with intricate internal architecture[87]. This method first entails synthesizing NiCo-glycerate solid spheres using a straightforward solvothermal technique. Subsequently, the solid spheres of NiCo-glycerate may be transformed into complicated hollow spheres of NiCo2O4 by a straightforward process of heat annealing in the presence of air [Figure 3C]. The complicated internal structure that forms during the thermal disintegration of metal glycerate is a consequence of the combined influence of contraction and adhesion activities resulting from non-equilibrium heat treatment. This technology has been shown to have the potential to be used widely for synthesizing metal oxide complex hollow spheres. The NiCo2O4 hollow spheres have a distinctive inner structure consisting of a core surrounded by two concentric shells. The porous shells comprise tiny nanocrystalline particles. After 100 cycles of continuous cycling at a current density of 200 mAh g-1, the battery retains a reversible discharge capacity of 706 mAh g-1, equivalent to 78% of the discharge capacity seen in the second cycle. More precisely, the existence of minute primary nanoparticles and holes might enhance the movement of Li+ ions, leading to a greater storage capacity and exceptional ability to charge and discharge quickly. Significantly, the distinctive three-layer core-in-double-shell interior structure can absorb the substantial volume change that occurs during the repeated insertion and extraction of Li+ ions during cycling. This structure also provides structural integrity, thereby reducing the problem of pulverization and improving the stability of the cycling process.
One essential factor for electrode materials is the mechanical property, particularly concerning the calendering process employed during optimized electrode film production. This approach focuses on tightly packing electrode materials to improve the electrical connection between the active material and conductive ingredients and enhance adhesion between the active material and foil. The calendering process involves substantial pressure, reaching up to 80 MPa. Under such conditions, numerous nanostructures could fall apart, potentially limiting the practical application of these structures. Fortunately, composition, crystallinity, nano subunit aggregation, shell thickness, and shell number may be controlled to make hollow structures resistant to high pressure. For instance, an amorphous carbon shell is brittle and prone to cracking with minimal deformation, while a graphene cage exhibits mechanical strength and flexibility, displaying exceptional resistance to pressure from the exterior[88]. Moreover, metal oxides typically demonstrate better mechanical robustness than carbon materials. Experimental results from in situ TEM indentation show that a TiO2 hollow sphere containing a 16 nm shell thickness remains structurally intact at a pressure of 160 MPa, while an amorphous carbon hollow sphere disintegrates at pressures below
Lastly, it is imperative to underscore that the hollow structure may not universally benefit electrode materials[90]. Specifically, when employed as electrode materials, hollow structured materials often exhibit comparatively lower Coulombic efficiency and tap density. The critical consideration lies in judiciously applying the appropriate structure in a context-specific manner, thereby maximizing the advantages of hollow materials in mitigating volumetric deformation and ion dissolution. While the hollow structure proves advantageous, it is not omnipotent.
CONCLUSION
In summary, each of these discussed morphologies has distinct advantages that make them particularly well-suited for use in LIBs. Various nanoparticle shapes are used to store electricity according to unique material needs (refer to Table 1). Nanostructures have similar characteristics such as large surface areas and fast Li+ transport dynamics, but each morphological category has distinct qualities ideal for certain applications. The basic 0D structure was first used in energy storage, demonstrating the promise of nanotechnology in progressing this subject. However, a recent trend involves transitioning to higher-order morphologies to mitigate drawbacks associated with 0D nanoparticles, such as electrolyte side reactions, interparticle contact resistance, and aggregation tendencies. Nevertheless, 0D nanoparticles remain optimal for certain applications, particularly in designs involving nano-confined active materials and situations where the maximization of catalytic and adsorption properties is crucial. Moreover, the effectiveness of these nanoparticles is notable when incorporated into higher-order morphologies, resulting in hybrid structures that exhibit synergistic performance while mitigating the drawbacks associated with 0D configurations. La Mer’s mechanism primarily considers the size homogeneity of nanoparticles by temporally separating the nucleation stage from the growth process. However, a universal theory to elucidate the uniformity in nanoparticle morphology is currently lacking, which requires future studies.
The 1D materials present many appealing characteristics, including high aspect ratios, reduced lattice boundaries, which is the interface between two grains, or crystallites, in a polycrystalline material but seldom found in 1D materials, and a restricted presence of harmful flaws, making them appealing as electrode materials. Notably, the easy convertibility of 1D shape into cloth-like membranes holds great promise for applications in separators and flexible electronics. The 1D structure may have a hollow core, which improves mass dispersion and exposure of catalytic sites. This makes it useful as an active material host in Li-O2 and Li-S battery systems.
The 2D topology allows for unlimited charge transport in two planar directions while constraining just the in-plane dimension. This enhances structural stability, minimizes side reactions, and preserves favorable nanostructured attributes, notably rapid kinetics. To achieve an ideal design, it is important to restrict the flow direction perpendicular to Li+ diffusion pathways in order to enhance the Li+ transit efficiency. The flat 2D shape is beneficial for catalytic purposes in Li-O2 batteries and may be used as a base for attaching 0D nanoparticles to form advanced composite nanomaterials. However, the dearth of scientific factors elucidating the heightened mechanical flexibility of 2D nanomaterials remains conspicuous. Diverse methodologies have been deployed to investigate the mechanical behaviors of 2D materials across varied conditions. Predominantly, mechanical assessments rely on the nanoscale indentation technique of atomic force microscopy, characterized by a markedly uneven stress distribution concentrated at the contact point of the indenter. Interestingly, this method is infrequently applied in battery research. Therefore, the exigency for quantitative scientific metrics is pronounced to systematically analyze the mechanical flexibility of 2D nanomaterials in the context of battery research, providing valuable insights into the influence of material thickness on their performance.
The 3D nanostructures provide several intrinsic and crucial benefits for energy storage compared to electrodes composed solely of 0D nanoparticles or 1D nanowires of electroactive materials. Firstly, they have a significantly large active surface area crucial for enhancing the accessibility of lithium ions and improving electrochemical reactions between metal oxide materials and the electrolyte. Secondly, these motifs feature an adjustable overall free volume that accommodates the expansion of metal oxides during lithium insertion. Lastly, they contribute to reduced Li+ diffusion distances, resulting in shorter transport lengths for the movement of Li+ in both intercalation materials and conversion or alloying reactions. In order to enhance the utilization of nanostructures with varying shapes in lithium rechargeable batteries, it is crucial to focus specifically on the following aspects.
DISCUSSION AND PERSPECTIVES
Future investigations on hollow micro-/nanostructures should prioritize examining both fundamental and practical elements. It is crucial to comprehend current synthetic processes to create more dependable and scalable approaches for generating void micro-/nanostructures in an economical and environmentally friendly way. Improved insight into the underlying mechanisms may facilitate the generalization of these approaches, paving the way for their applications in industrial settings. Additionally, it can enable the design and production of materials with intricate structures and desired qualities that were previously unattainable. The extensive development of synthetic techniques will allow the use of such hollow frameworks as adaptable basic units that can be readily included in developing and manufacturing novel practical substances and technologies.
Defective electrode materials, encompassing heteroatom doping, inherent defects, vacancy defects, and other structural modifications, are highly desirable in advanced batteries. As a result, these defects can enhance ion diffusion and electron transfer. Introducing defects into nanomaterials has two main benefits. Firstly, it expands the storage capacity for foreign ions, improving battery capacity. Secondly, these defects act as effective adsorption sites, particularly for intermediates, preventing undesirable shuttle effects.
Anode-free lithium batteries are an advanced technology that has the potential to outperform the current gradual improvements in lithium battery lifespan. They offer a remarkable increase of up to 60% in energy density. In an anode-free lithium battery, a naked copper foil is used as the anode, allowing lithium to migrate from the cathode and deposit onto the copper current collector during the first formation cycle. However, challenges arise from high consumption of Li+ in an unstable solid electrolyte interphase (SEI) and dendrite formation, resulting in significant instability. Nanomaterials with precisely regulated shapes at both the nano and macroscales have significant promise for addressing this issue. Various strategies with regulated structure and permeability have been shown to effectively control and redistribute the flow of Li+, hence reducing the production of dendrites and stabilizing the SEI. This research path involves using creative structural design and suitable material composition to achieve significant breakthroughs in anode-free technology.
The extensive library of nanomaterial production technologies facilitates the creation of hybrid nanomaterials, composed of multiple components, each uniquely tailored for particular purposes. This method allows for the development of versatile nanostructures, particularly useful for intricate cells such as Li-S[91-93] and solid batteries[94-96], in which various components can conduct distinct tasks within the same system. Examples include sulfur storing, PS capture, transformation, and conductivity improvement. To improve battery performance, integrating emerging technologies, such as artificial intelligence, will undoubtedly aid in designing novel nanomaterials and nanocomposites and their combination with organic substances such as binders and additives. The synergistic benefits of these advancements make further developments of this technology highly promising for lithium rechargeable batteries.
The design of electrode structure is a pivotal determinant of battery performance, necessitating considering multiple factors for optimization. Key considerations in this area include: (1) Surface Area and Porosity: Enhancing electrode surface area through micro- or nano-architectures with well-defined porosity facilitates efficient electrolyte interaction, augmenting ion transport and thereby improving battery capacity; (2) Particle Dimensions and Morphology: Optimal sizing and shaping of electrode particles influence the rate of electron and ion transport, with nanostructures particularly effective in reducing electron pathways and enhancing battery performance; and (3) Conductivity and Pathway Optimization: Ensuring high conductivity by including conductive materials or additives is imperative for efficient electron transport within the electrode structure. These design principles, contingent on battery type and application, form the basis for achieving enhanced battery performance and reliability.
Two main issues hinder the effective deployment of nanotechnology for LIBs. One primary factor is the intricate and expensive techniques used in producing nanomaterials, particularly those with intricate structures. The second reason is the use of non-commercial criteria for evaluating novel nanostructures, including minimal bulk loading, surplus electrolyte, and low electrode density. Despite these challenges, alloy-based anode nanomaterials have shown significant advancements in commercial applications after a decade of extensive research and refinement. Initiatives in expansion and automobile production are expected to integrate industrial perspectives into developing lithium rechargeable batteries, considering factors such as practicality, cost, and adherence to industry testing requirements.
DECLARATIONS
Authors’ contributions
Drafted the manuscript: Chen Z
Participated in the writing, revision and literature discussion: Chen Z, Zhang C
Finalized the manuscript: Chen Z, Zhang C
Conflict of interest
Both authors declared that there are no conflicts of interest.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work is supported by the Australian Research Council (LP220100036) and the University of Queensland Knowledge Exchange & Translation Grants for funding. Zhang C acknowledges the Australian Research Council for his Discovery Early Career Researcher Award fellowship (DE230101105).
Ethical approval and consent to participate
Not applicable.
Consent for publication
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Copyright
© The Author(s) 2024.
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Chen Z, Zhang C. Engineering electrode microstructures for advanced lithium-ion batteries. Microstructures 2024;4:2024033. http://dx.doi.org/10.20517/microstructures.2023.89
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