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Physical Methods

Electrospinning

Electrospinning is a simple and popular technique to synthesize 1D nanostructures with fiber diameters ranged from tens of nanometers up to micrometers [109]. This preparation is difficult to achieve by the approaches mentioned above. This technique can produce polymers, organic, and inorganic composites with dense, hollow, or porous structures [110], from polymer solutions based on electrostatic forces [111]. An electrospinning unit generally consists of a syringe and a needle, a grounded collector, and a high-voltage supply, as shown in Fig. 7a, b [117]. During the electrospinning process, polymer solutions are loaded in the syringe and move into the needle to form a droplet. When a high voltage is applied between the needle and the collector, the electrostatic force at the surface of droplet would drive it to elongate to form a fiber. Finally, the solid polymer fibers would deposit onto the collector.

The schemes of a single axial and b coaxial electrospinning [111, 112]. ㄷ Inorganic fibers [113]. d Inorganic particles encapsulated carbon fibers [114]. 이 The modification of carbon fibers [115]. 에 Carbon fiber membrane with nanoparticles [38]. 지 Highly flexible carbon fiber membrane [116]

The polymer solutions and needle are the key points for the success of fiber fabrication. Polymer solution should reach the minimum viscosity for the formation of homogeneous fiber structure. The solvent of polymer should have a lower evaporation rate, which allows the polymer solidification after leaving the needle. The needle should be designed with coaxial structure to achieve hollow or core-shell fiber structure (Fig. 7b). For the coaxial electrospinning, the core and shell solutions should be adjusted to be immiscible or non-precipitable. Besides, during the electrospinning process, solution flow rates, voltage, temperature, distance from needle to the collector, and diameter of the needle have a huge influence on the fiber structure.

The obtained electrospun membrane needs further treatment to be a binder-free electrode. Carbon, ceramic, or metal nanofibers can be synthesized from the carbonization of electrospun fibers that contain polymer, metal salts, or metal atoms, respectively. Their composites such as metal/C and ceramic/C can be also obtained from their corresponding mixed precursors followed by a one-step or multi-step heat treatment. A wide range of electrospun materials have been investigated for LIBs including metal oxides (e.g., TiO₂ , Fe₂ O₃ , ZnO, NiO, CuO, LiCoO₃ , Li₄ Ti₅ O₁₂ , and LiMn₂ O₄ ) [118, 119], hybrids [120] (e.g., SnO_x /C, SiO_x /C, Co₃ O₄ /C, SnO_x /C, TiO₂ /C) [113, 121,122,123,124,125,126,127,128,129,130], and polymers (e.g., polyvinyl alcohol (PVA), PAN and PVP, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and polyethylene oxide (PEO)) [131].

Conventional electrospinning generally disperses metal salts and nanoparticles inside the fibers. However, the nanoparticles can adhere to the outside of the fibers as well (Fig. 7c). Lan, Yang, and coworker prepared 3D free-standing spider-web-like membranes with high mass loading of bismuth (Bi) nanoparticle clusters followed by carbonization in nitrogen gas [132]. The 3D Bi/C membrane provides good mechanical properties and stabilizes the Bi nanoparticles up to 200 cycles.

The architecture of fibers can be optimized to accommodate large volume changes and instability of the electrode materials during cycling process. The adjustment of the fiber structure can be started from either inside or outside of the fiber. The internal fiber can be regulated by the polymer solution and post-treatment, while the external fiber structure is controlled by post-treatment. When the polymer solution contains etchable materials, a porous fiber structure can be prepared after carbonization and template etching (Fig. 7d). This porous materials is capable of accommodating higher sulfur and suppressing the polysulfides shuttle effects [114]. The polymer can individually form an active material at the expense of flexibility self-standing property. This disadvantage can be addressed by additives. Liu et al. showed the PAN fibers with an appropriate amount of CNTs can still be self-standing after sulfurization [115]. The sulfur only exists in the form of Li₂ S₂ and Li₂ S₃ rather than polysulfides in the sulfurized PAN. Therefore, it shows ultra-stable cycling performance up to 1000 cycles (Fig. 7e).

Alternatively, the post-treatment of the surface of electrospun fibers is another way to prepare the high-performance binder-free electrode (Fig. 7f) . After carbonization, the three-dimensional conductive network is formed to provide good electronic conductivity. The fiber surface also provides a large number of sites for the growth of active materials with easy access to electrolyte [38]. Another post-treatment is to coat the nanofibers with a protective surface layer. Generally, the nanoparticles spinning out with the polymer solution is inevitably exposed at the surface of the fiber. This part of the material may fall off from fibers during the cycling process, so the surface coating is equivalent to the protection of the fiber [133].

In addition to polymer solution, the needle is also of importance to the fibers design. The core-shell composite nanofiber can be prepared by a dual nozzle coaxial electrospinning setup (Fig. 7g) [116]. This needle can achieve a great core-shell fiber structure. Besides, hollow fibers can be prepared by designing the inner and outer solutions. When the hollow fiber is filled with the active material, there is sufficient space to allow the volume to expand [112].

Vacuum Filtration

The vacuum filtration method is a rapid manufacturing process to assemble different kinds of nanoscale materials into the macroscopic film for various applications. This process is low-cost, rapid, and efficient, which demonstrates a promising strategy for various functional films. 2D materials can be easily assembled into flexible self-standing paper-like materials, which can be directly used as flexible binder-free electrodes in energy storage devices [134, 135]. In general, the active materials are randomly dispersed between the supporting materials. Therefore, high mechanical strength and flexibility are preserved for the papers (Fig. 8) [136, 137].

The scheme of vacuum filtration process [136]

The vacuum filtration features as the following strengths. Firstly, active materials can adhere on the conductive substrate, leading to the improvement of electron conductivity. For example, the electron conductivity of MoS₂ can be largely improved; therefore, better rate performance can be obtained [138, 139]. Secondly, the large surface area is in favor of the contact between active materials and lithium ions, which facilitates the transportation of Li-ion. When the active material is added into the 2D material, the interlayer spacing becomes large; thus, the electrolyte can be immersed. The lithium ions are more accessible to the material; thereby, the interface impedance of material is reduced [140]. Thirdly, the effective material utilization is also facilitated by hindering the aggregation of 2D materials [141,142,143]. Lastly, the material agglomerations and electrode instabilities result from the huge volume change of active materials during Li insertion/extraction [144, 145]. Supporting sheets can absorb stress induced by volume expansion, similar to the role of elastic buffer [146, 147].

Different types of nanostructures can be assembled into 2D materials. For example, the nanoparticles, nanotubes, nanosheets, nanorods, etc. can fabricate into the graphene sheets [148]. When CNTs as additive are assembled into the nanosheets, the restacking of the nanosheets can be prevented, and the conductivity of ion and electron can be greatly increased [149]. The electrode chemical properties can be enhanced by coating or mixing active materials on other conductive materials and then assembling into 3D functional materials [150,151,152]. It is mainly attributed to the synergistic effects that 3D structure not only serves as a flexible scaffold for strains/stresses release and volume expansion, but also offers a three-dimensional conductive architecture with open channels for electron transfer and Li-ion diffusion. Besides, pre-protection of active materials is a way to improve material stability. The surface modified anode materials in graphene exhibit high capacities, long cycle-life, and excellent rate performance [153]. The Mn₂ P₂ O₇ -carbon in graphene electrode delivers a capacity of 585 mA h g ⁻¹ at a current density of 1000 mA g ⁻¹ . When increasing the current density to 5000 mA g ⁻¹ , a high capacity of 400 mA h g ⁻¹ can be remained even after 2000 cycles [153].

Physical Vapor Deposition

At certain temperature and airflow rate, the elemental vapor can be easily deposited onto the porous supporting materials [154,155,156]. Solid sulfur and red P nanoparticles are the typical materials, which can be deposited into porous carbon materials. The commercialization of sulfur as cathode materials is blocked by several intrinsic problems, including low electronic/ionic conductivity, large volumetric expansion, and shuttle effect of intermediate polysulfides (Li₂ S_x (4 ≤ x ≤ 8)). Particularly, the shuttle effect of polysulfides results in transport of sulfur from cathode to anode and the reaction with Li metal, which leads to significant capacity loss and safety issues. So far, the design of porous structure is the basic strategy to suppress the polysulfides shuttle effect, and sulfur vapor deposition is an effective way for the fabrication of S/C composite. It is an environmentally friendly, solvent-free method in which the sulfur powder undergoes a physical deposition process with no changes of chemical properties [157]. With proper absorbent in the structure, the shuttle effect of polysulfides can also be fixed. Recently, Yang, Zhang, and coworkers reported Ti₃ C₂ T_x paper is a good host for sulfur deposition (Fig. 9a). This Ti₃ C₂ T_x paper shows no cracks after 25 convexly and concavely bending cycles (Fig. 9b, c) [158]. Yu and coworkers [159] demonstrated porous carbon fibers encapsulated with red P shows high capacity of 2030 mAh g ⁻¹ at 0.1 C rate after 100 cycles. It is worth noting that physical vapor deposition (PVD) is only one of the procedures of immobilizing S or P onto carbon materials. Therefore, the most important research direction is how to design a porous conductive matrix.

아 The scheme of fabrication of robust, freestanding, and conductive Ti₃ C₂ T_x /S paper. Photographs of freestanding Ti₃ C₂ T_x /S paper when bending b convexly and c concavely, showing good mechanical flexibility similar to that of the pure Ti₃ C₂ T_x paper [158]

Application in Flexible Batteries

Flexible devices, such as wearable displays, sensors, sportswear, mobile communication devices, rollup displays, and so on, are one of the important directions for intelligent and smart world [160]. The development of these new devices requires the power of a flexible battery system [161,162,163]. However, current advanced pouch and 18,650 cells cannot be used on flexible devices due to the rigid material properties. Each component of the flexible battery, such as electrodes, separator, and solid electrolyte, must be flexible (Fig. 10a) [164]. The conventional electrode is generally adhered to the metal foil by a coating method to physically bond the active material and the conductive agent. During repeated bending and folding, the active material separates from the current collector, ending up with deactivation. For example, the Li₄ Ti₅ O₁₂ (LTO)-based electrode folded about 100 cycles would present the detachment of LTO from Al foil. The impedance of the electrode increases from the first fold, and the higher the active material loading, the faster the impedance increases (Fig. 10b). At the same time, the pouch cell bending 30° results in serious capacity fade (Fig. 10c).

아 Assembly and bending tests of flexible batteries with flexible electrodes [164]. ㄴ Electrical resistance change with folding cycles [165]. ㄷ Capacity retention of folded cells at different angles at 1 C [165]

There are many strategies to fabricate flexible electrodes. Song et al. reported that coating LTO particles and Ag nano wires onto the polyethylene terephthalate (PET) web can greatly improve the electrode flexibility and stability. The electrical resistance of Ag@LTO@PET electrode does not change during 1000 folding cycles (Fig. 10b). Pouch-type Ag@LTO@PET-based half cells showed great cycling performance with little capacity decay when the electrode was bent at any angle (Fig. 11c) [165]. The most mature method is to fix the active material on a flexible substrate. As described in the “Introduction” section, the direct growth of the active material on the conductive substrate can improve battery energy density and rate performance. Herein, we take the carbon cloth and carbon materials as the example to show the application of binder-free electrodes in flexible devices.

아 Schematic illustration for the structural features of the flexible SnO₂ nanosheets on flexible carbon cloth electrode during the folding (I), the rolling (II), and twisting (III) tests. ㄴ Current-time curves of the composite samples at various bending angles of the 1st and 200th cycles, and the inset images show the corresponding bending angles for measurement and photographs [166]

Most carbon materials cannot be used in flexible electronics. For example, a binder-free electrode based on graphite paper can only maintain 25 cycles in a bent state [167]. Comparing with other carbon materials, carbon cloth with excellent flexibility and electrical conductivity is one of the most promising materials for the flexible battery application. Even after the surface modification of inorganic materials, carbon cloth still shows excellent flexibility. As shown in Fig. 11a, there are no apparent changes of the electrode after bending, rolling, twisting, folding, and crumpling tests. After the mechanical test, the active materials on the carbon cloth can maintain structural integrity. Also, after 200 bending cycles, the current value slightly decreases from 17.3 to 16.8 mA, which demonstrates great stability (Fig. 11b) [166].

It is particularly difficult to synthesize flexible carbon materials. For example, the PAN film becomes much more brittle and fracture after carbonization, which is difficult to use in flexible batteries. The ideal carbon material, like the clothes we wear, bending and folding many times can still remain intact. The flexibility of the material can be greatly improved through reasonable design such as the addition of functional additives. Wang et al. reported that the carbonized PAN film with SiO₂ filler can fully recover to its original state after repeated rolling or folding process [114]. When assembled into the pouch cell, it can withstand at different bending angles up to 180°. Yu et al. demonstrated that Zn(CH₃ COO)₂ assists the uniform carbonization of PAN, which relieved the stress concentration [130]. The film obtained by this method can return to the initial state after folding four times (Fig. 12a). When assembled into the pouch cell, it can light the LED at any folding angle. When the pouch cell is disassembled, the binder-free electrode remains intact while the slurry-based electrode is completely destroyed (Fig. 12b–e).

아 Digital photographs of Zn(CH₃ COO)₂ -PAN film, which can be folded four times. LED lighting tests of a full battery when b flat, c folded once, and d folded twice; 및 e digital photographs of the electrode after the LED lighting test [130]

결론

In conclusion, recent research progress on the preparation of binder-free electrodes for LIBs has been summarized. The fabrication methods focus on the chemical, physical, and electrical treatment, such as thermal treatment, hydrothermal treatment, CBD, ALD, CVD; vacuum filtration, electrospinning; and electrophoretic deposition, anodization, electrodeposition. Thermal treatment is the most commonly used chemical method to carbonize polymer for free-standing structure or decompose of the precursor of metallic oxide. The hydrothermal and CBD methods are very attractive due to accurate control of the size and morphology of nanomaterials. CBD and hydrothermal methods present in situ growth of active materials on the substrate through a chemical reaction. CVD is defined as the deposition of a gas carrier on a heated surface by a chemical reaction, while the ALD technique is a vapor phase chemical deposition process that is capable of producing high-quality nanoscale thin films in an atomic layer-by-layer manner. The vacuum filtration and electrospinning are the representative physical methods. The former is a physical manufacturing process to assemble different materials like nanoplatelets and nanoparticles into the macroscopic film. The latter can produce 1D nanoscale materials with fiber diameters ranged from tens of nanometers up to micrometers. The electrical method is a widely used technique to make coatings and thin films. However, it is not often used to prepare binder-free electrode. Among these methods, CVD and CBD are excellent ways to prepare silicon-based and sulfur-based materials, respectively.

The binder-free electrode shows better electrochemical performances than the traditional slurry system. The binder-free electrode can improve ionic and electronic transportation, cycling performance, and energy density of the electrodes. In addition, nanoscale materials are uniformly anchored on the supporting materials, which can effectively prevent the agglomeration of nanoparticles and mitigate the volumetric expansion during the repeated cycling process.

The conductive matrix plays a crucial role in the electrochemical properties and performances of the binder-free electrode. The ultra-flexible film has great potential to make a big breakthrough in the field of wearable and flexible devices. However, existing substrates are still unable to meet the requirements. The flexible device requires the binder-free electrode to bend and fold for numerous times with no damage and no separation from the substrate. According to current research process, ultra-flexible and ultra-stable carbon materials become the most promising candidate for next-generation flexible binder-free electrode.

Despite the difficulties, the future is expected. The uniform and large-scale growth of the active material on the conductive substrate is one of the necessary conditions for practical application. Fortunately, it is now possible to achieve. Practical applications need to consider the basic properties of the electrode in the battery, such as the initial Coulombic efficiency and voltage profiles. Therefore, the active materials for both anodes and cathodes should be carefully selected. For example, Si, Sn, or carbon materials serve as promising candidates for anode materials while the cathode materials may be selected from S matching with Li metal, or the existing Li metal oxides. In addition, flexible batteries can be achieved with all of flexible components, such as electrodes, separators, and electrolytes. Although these aspects have been studied for a long time, breakthrough is needed to facilitate the research progress.

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약어

LIB:

리튬 이온 배터리

PVDF:

폴리불화비닐리덴

ACF:

Active carbon fiber

PAN:

폴리아크릴로니트릴

rGO:

환원그래핀옥사이드

PVP:

폴리비닐피롤리돈

LBL:

Layer-by-layer

이동:

산화 그래핀

PDDA:

Poly (diallyldimethylammonium chloride)

CNT:

탄소나노튜브

CBD:

화학조 증착

CVD:

화학 기상 증착

SEI:

Solid electrolyte interface

시 NW:

실리콘 나노와이어

ALD:

원자층 증착

EPD:

Electrophoretic deposition

PVA:

폴리비닐알코올

PVDF-HFP:

Poly(vinylidene fluoride-co-hexafluoropropylene)

PEO:

Polyethylene oxide

Bi:

Bismuth

PVD:

Physical vapor deposition

LTO:

리₄ Ti₅ O₁₂

PET:

폴리에틸렌 테레프탈레이트

악성 흑색종에 대한 상승적 항종양 효능을 위한 지질 나노제형을 사용한 다카르바진 및 전-트랜스 레티노산(ATRA)의 공동 전달 고성능 광검출을 위한 인터페이스 유도 WSe2 면내 동종접합