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Materials Science, Composites
The inner constituents of lithium-ion batteries (LIBs) are easy to deform during charging and discharging processes, and the accumulation of these deformations would result in physical fractures, poor safety performances, and short lifespan of LIBs. The introduction of self-healing (SH) materials into electrodes or electrolytes can bring about great enhancements in their mechanical strength, thus optimizing the cycle stability of the batteries. Due to the self-healing property of these special functional materials, the fractures/cracks generated during repeated cycles could be spontaneously cured.
- self-healing
- lithium-ion batteries
- electrode
- electrolyte
1. Introduction
With the widespread use of renewable and clean energy, the technology of energy storage has become one of the most practically significant research hotspots nowadays [1]. Among multiple kinds of energy storage devices, lithium-ion batteries (LIBs) have played an important role and dominated the portable electronics and electric vehicles (EVs) markets due to their massive advantages such as high energy density, reusability, and long service life [2]. Although LIBs have greatly improved our lives, many limitations still remain. Since the most widely used electrode materials are currently quite brittle and poorly scalable, they are susceptible to drastic structural changes and mechanical fractures in cycle processes, bringing about irreversible capacity loss and poor cycle stability [3]. To solve these problems, many efforts have been made to eliminate the cracks and pulverization in electrodes by optimizing the structural design and developing functional composite materials. Some recent research works have shown that introducing the concept of self-healing (SH) into battery materials can effectively enhance the stability and durability [4]. The LIBs containing self-healing materials and possessing self-healing features are defined as self-healable LIBs.
Self-healable LIBs consist of materials that can spontaneously heal the internal or external damages [5]. This concept takes inspiration from nature, since many animals such as starfish, gecko and even human skin can reconstruct injured parts and retain their original functions. The capability of the damaged part to repair itself is significant for the survival rates of these natural creatures. It is efficient to lengthen the cycle life and improve the stability and reliability of LIBs with the help of SH materials. Early research works concerning SH materials focused on encapsulation or hollow fibers that can release healing agents and polymerize to achieve a limited number of healings [6]. With further research, it was found that the multiple reversible healing can be obtained by physico-chemical approaches such as liquid metal and alloys, which can heal the cracks in electrodes via reversible solid–liquid conversion reaction [7]. In addition, the SH polymer binders have been carefully designed via dynamic bonds to stimulate spontaneous repair of the physical damages in electrodes. Furthermore, the highly stretchable solid-state SH electrolytes have been proven to effectively prolong the cycle life of solid-state LIBs.
2. The Mechanisms of Self-Healing Materials
“SH” can be defined as the behavior of repairing itself and restoring functionality by using available resources in the event of damage [15]. The strategies associated with self-healing usually involve either physical or chemical events at the molecular level [5]. Depending on the differences in repair driving forces, SH mechanisms can be divided into physical approaches (i.e., phase-separated morphologies [21] and shape-memory recovery [22]), chemical approaches and physico-chemical approaches (i.e., encapsulation [23] and microvascular networks [24]).
Chemical approaches are characterized by the reformation of reversible chemical bonds or supramolecular interactions. These approaches depend on inherent dynamic chemistry rather than introducing external repairing agents, so they are also known as the “intrinsic self-healing”. Due to the special repair mechanisms, there are two advantages to apply these approaches to LIBs: (1) Benefiting from the reversible bonding, the healing process is theoretically infinite; (2) Since no redundant electrochemical inactive components would be introduced to the battery system, the energy density of the battery could be optimized at the beginning. However, since the inherent dynamic chemical responses of the SH materials are usually slow, this self-healing mechanism dictates that the healing process may take longer.
2.1. Supramolecular Interactions
Supramolecular chemistry can reestablish 3D networks through dynamic formation of supramolecular bonds. The new “colloidal” fracture interfaces composed of several unbonded supramolecular bonds will get reformed once the network structure damaged, leading to the recombination of the interfaces and self-repair of damages [25]. Supramolecular bonds include various chemical interactions, such as hydrogen bonds [26]], metal-ligand coordination [27], host–guest interactions [28], ionic interactions [29] and π–π stacking [30].
Supramolecular rebonding processes are usually bottom-up and involve non-equilibrium states, the low bonding energy of these bonds (≈10 kcal mol−1) makes them strongly susceptible to external environmental influences and become easy to spontaneously reform due to their reversibility and sensitivity, which contributes to the reversibility of the process and their application in various room-temperature SH materials [31]. Another attractive feature of supramolecular chemistry is that the matrix can be repaired rapidly and autonomously, but the mechanical strength of the repaired sites is usually much lower compared to covalent and free-radical rebonding.
Supramolecular chemistry is favored in SH materials and complex battery systems due to the ease of dissociation–association and high energy efficiency. Among them, hydrogen bonds are widely used owing to their directionality and affinity. Hydrogen bonding refers to the dipole–dipole attraction between a hydrogen atom and an electro-negative atom with a lone pair of electrons (i.e., N, O, or F). In particular, hydrogen bonds based on urea-pyrimidine (UPy) groups are most widely utilized in SH materials because of their ease of application, good reversibility and strong temperature-mechanical property correlation [26].
2.2. Covalent and Free-Radical Rebonding
Traditionally, the molecular synthesis of organic compounds is kinetically controlled, which will irreversibly form strong covalent bonds and single products [35]. During the last couple of years, researchers have found that, under equilibrium control by heating or irradiation, some covalent bonds can be reversibly formed, broken, or even reformed [36]. Since then, diverse methods have been exploited to design self-healing polymers with free-radical or covalent rebonding, which can be divided into three categories: condensation reactions, addition reactions and exchange reactions [11].
Condensation reaction is one of the most common chemical bonding methods in SH polymers, where two functional molecules react to construct a new bond and form a small molecule simultaneously. In most cases, this new molecule (usually water) is formed during bond formation and consumed in turn.
Another typical reaction type is addition reaction, including the most widely investigated Diels–Alder (DA) reaction. This [4 + 2] cycloaddition reaction of a dienophile and a diene is controlled by dynamics and thermodynamics, making it particularly suitable for self-healing because temperature can be used as a convenient trigger.
The third category is the exchange reaction, where functional groups exchange at equilibrium state continuously and rapidly, making it easy to repair damages. Such exchange reactions include imines, oximes, siloxane, disulfide bonds and so on. Unlike reversible addition reactions, exchange reactions require lower stabilities of the corresponding reversible linker to achieve self-repair [37].
3. Self-Healing Electrodes
The electrochemical performance decay of LIBs is usually caused by volume expansion/contraction during repeated charging/discharging processes due to the electrochemical reactions. Such a volume change will bring the formation of cracks, separate the active material from the collector and disrupt the electronically conductive network within the electrode. To overcome these problems, several research works have been done to improve the durability of electrodes, such as adding buffer materials, alloying the Li-active materials with inactive elements and building nanostructured electrodes. However, the addition of inactive or buffering components can reduce the gravimetric and volumetric energy densities of the battery. Nanostructured electrodes possessed low packing density and faced many other problems such as excessive consumption of electrolyte and repeated formation of the solid electrolyte interphase (SEI).
Recent studies have shown that SH materials can limit the formation of cracks/fractures on electrodes, increase the durability of electrode materials and thus enhance the cycle life of LIBs. One approach highlights the self-healing electrode material itself. Another approach is to coat the active material with a self-healing binder.
3.1. Liquid Metal and Alloys
Benefiting from fluidity and surface tension, liquid materials show great potential for self-healing. However, the batteries in previous studies usually need to operate at extremely high temperatures, which limits its application [38]. Therefore, gallium (Ga) draws much attention among potential candidates due to the fact that it is the only nontoxic metal with a low melting point at near room temperature (29.8 °C). This unique feature makes it a highly promising self-healing anode material, which possibly enables recoverable morphologies during repeated charging/discharging processes. Cracks caused by lithium insertion will disappear after deintercalation as a result of the ideal fluidity and surface tension of Ga. Such SH properties can potentially lead to an ultralong cycle life.
The self-healing behavior and electrochemical property of pure Ga was initially investigated. By forming the Li2Ga alloy, one Ga atom hosts two Li atoms when fully intercalated with lithium and provides a theoretical specific capacity of 769 mAh g−1 [39]. Saint et al. prepared single line phases LixGay by ball-milling and emulated the electrochemical reactivity of Ga towards Li. By in situ X-ray diffraction (XRD) measurements, they demonstrated that electrochemical-driven transformation from LiGa to Li2Ga was initially fully reversible, but adversely affects battery capacity retention during subsequent cycles. Eventually they stabilized the cell capacity at about 300 mAh g−1 beyond 20 cycles bypassing the structural transition [40]. Deshpande et al. performed electrochemical tests at 40 °C to ensure the liquid state of pure Ga [41]. Three intermetallic phases (Li2Ga7, LiGa, and Li2Ga) were formed. More importantly, they certificated the reversible solid–liquid transition upon lithiation and delithiation of Ga. The Ga electrode underwent crystallization upon lithium and transformed into solid state. Conversely, the solid-state Ga electrode would transform back to a liquid phase upon delithiation. Cracks formed in the solid state could be healed once the electrode returned to the liquid state. Therefore, it was easy to realize self-healing through the solid–liquid phase transition and improve electrode durability. In addition, the SH behavior of gallium nanodroplets (GaNDs) was demonstrated by Liang and co-workers via in-situ transmission electron microscopy (in-situ TEM) [42]. Similarly, those GaNDs encountered a liquid–solid phase transition and delivered SH properties.
3.1.1. Working Temperature
High temperature is needed to ensure the liquid state of Ga for reasonable diffusivity. To decrease the melting point of Ga and improve capacity, Sn has been introduced to form Ga–Sn liquid metal (LM) alloy [43]. The melting point of the alloy was 20 °C, which is below the typical RT (25 °C). It was further stabilized in a skeleton formed by reduced graphene oxide together with a carbon nanotube. In situ microscopy was used to examine the self-healing ability of the LM alloy. As a result, the novel Ga-Sn alloy anode delivered an ultra-long cycle life (>4000 cycles with a capacity of about 400 mAh g−1 at 4 A g−1).
3.1.2. Capacity
Among diverse anode materials, Si owns the highest theoretical capacity of 4200 mAh g−1 (lithiated to Li4.4Si). However, Si-based electrodes typically suffer from poor capacity retention due to the extreme volume change, which leads to material pulverization, loss of electrical contact with electrodes, continuous solid electrolyte interface (SEI) growth and consumption of electrolytes [49]. Based on this situation, Si can be the ideal reinforcement to make up for the lack of capacity of liquid metal and improve the energy density of the battery, At the same time, liquid metal can act as a liquid buffer and heal the fractures caused by the volume expansion/contraction of Si.
3.1.3. Aggregation of Nanoparticles
Ga NPs are easy to get aggregated during charge/discharge processes owing to their low surface energy, so the cycling stability of electrodes based on monodispersed GaNPs was not satisfactory. To address this issue, the utilization of solid Ga-based compounds (e.g., GaSx, CuGa2, Ga2Se3, Ga2O3) seems to be the wise choice [51,52,53,54]. On the one hand, Ga-basd compounds could retain the SH properties of Ga metal upon the conversion reaction. On the other hand, it is easy to prepare various nanostructured anode materials, which can shorten the transport way and provide more active sites.
3.1.4. Volume Changes
In addition, the most critical factor affecting the performances of LIBs is the volume expansion of the bulk materials. Carbon encapsulation is a well-known method of buffering volume changes. For example, Lee et al. prepared a liquid Ga electrode within a carbon substrate to minimize the detachment among solid-state LixGa particles [57]. In addition, the porous carbon matrix can suppress the volume changes of the embedded Ga. A recovery to initial liquid phase after multiple cycles was observed in the experiment sample, and the self-healing was not impressive without the confining matrix, which demonstrated the importance of a confining porous matrix. Moreover, core–shell fibers with encapsulated nanosized SH LM particles with well-designed void space were prepared by coaxial electrospinning and a carbonization process [48]. The free-standing anode offered a capacity of 552 mAh g−1 after 1500 cycles at 1 A g−1. The impressive electrochemical performance can be attributed to the unique core–shell structure and well-designed void spaces which can effectively alleviate the volume changes of the LMNPs during the lithiation/delithiation processes.
3.2. Self-Healing Binders
Incorporating self-healing binders into electrodes, especially Si anodes, is another feasible way to enhance performance of LIBs. Most of the binders used in LIBs are polymers. Different from conventional polymer binders, SH polymer binders are stretchable and can spontaneously restore the cracks that arise from the volumetric changes of Si anodes. In this way, the mechanical and electrical connections among Si particles could be more stable, and better cycle stability could be achieved. According to self-healing mechanism, different types of SH binders based on supramolecular interactions as well as covalent and free-radical rebonding have been extensively researched.
3.2.1. Supramolecular Interactions
Supramolecular interactions include hydrogen bonding, metal–ligand coordination, host–guest interaction and dynamic ionic bonding. The vast majority of the SH polymer binders developed so far are based on supramolecular interactions. Although dynamic covalent bonds can be used to synthesis self-healing materials, supramolecular interactions are more favored in complex battery systems. This may be attributed to its energy-efficient feature (easy to dissociate-associate) and repeatable healing capability. This interaction can be formed between the binder as well as the active material and/or between the polymer chains.
Among different supramolecular interactions, hydrogen bonds are the most common. Hydrogen bonds can induce self-repair autonomously and repeatedly at room temperature. Polar functional groups on SH polymers could act as both hydrogen donors and acceptors [58]. Bridel et al. indicated that the strong hydrogen bonds were proposed to be critical for the electrochemical properties of Si-based anodes because of inducing the SH property [59]. This unique adaptable interaction can accommodate the dramatic volume changes of active materials and maintain the electronic wiring and integrity within the electrode.
3.2.2. Covalent and Free-Radical Rebonding
Although self-healing polymers based on supramolecular interactions have been demonstrated as potential binders for Si anodes, there are still some issues that need to be solved. For example, the supramolecular network solely based on hydrogen bonds deforms easily, and the diffusivity of lithium ion is insufficient. To solve the above problems, boronic crosslinker (BC) was incorporated into the guar gum by Ryu et al. to generate boronic esters spontaneously art room temperature [76]. Guar gum were rich in hydroxyl groups and ensured good contact between the current collector and the silicon particles. BC reacted with vicinal alcohol without additional driving forces, while the other component, polyethylene oxide (PEO), contributed to the enhanced Li-ion conductivity. As a result, the boronic crosslinked guar (BC-g) binder extended the cycle life of either SiNPs anodes or SiMPs anodes. The electrode exhibited a good capacity retentivity and rate performance even under rigorous conditions (i.e., with high mass loading).
3.2.3. Dual Crosslinking
In addition to the physical mechanism based on supramolecular interactions and chemical mechanism based on dynamic covalent bonds mentioned above, dual crosslinking, through the complex structural crosslinking of the polymer itself and the abundance of hydrogen bonds, has become another mechanism that can also realize the polymer binder to achieve self-healing ability.
4. Self-Healing Electrolytes
Organic liquid electrolytes cause the major safety issues of commercial LIBs due to its flammability, danger of leakage as well as high sensitivity to water vapor. Replacing the liquid electrolytes with solid-state electrolytes is considered to be an effective solution to the above problems. Solid-state electrolytes could be grouped into two categories: gel polymer electrolytes (GPEs) and all-solid-state electrolytes (ASSEs). GPEs have both good safety and high ionic conductivity similar to organic liquid electrolytes. In addition, they are light, viscoelastic, easy to processs, and have low reactivity with the electrode material. However, due to the swelling of the polymer in the electrolyte, GPEs usually have poor mechanical properties and flammable nature, which still presents hidden danger. Moreover, the development of ASSEs, including polymers, ceramics and their hybrids, was given high expectations. Solid polymer electrolytes (SPEs) possess good flexibility as well as process ability, but the low ion conductivity (<10−4 S cm−1) and easy-to-fragment features limit their utilization. Based on these considerations, the strategy of modification of solid electrolytes containing polymers to give them self-healing capabilities has become a quite promising method to solve the unstable cycle problems of LIBs and to extend the life of batteries through spontaneous healing damages.
4.1. Self-Healing Gel Polymer Electrolytes
4.1.1. Liquid Electrolytes
Flammable organic electrolytes are subject to leakage problems in extreme cases. In addition, as a result of the poor adhesion between the separator and the electrode, undesirable slips or gaps with high contact resistance may form at the separator/lithium metal interface during prolonged cycles, resulting in significantly non-uniform Li+ flux distribution. Subsequent growth of lithium dendrites and pulverization of lithium metal can be expected. Gel polymer electrolytes (GPEs), which combine traditional liquid electrolytes and solid polymer electrolytes, could largely improve the safety of LIBs and enhance the electrolyte/electrode interface contact.
4.1.2. Ionic Liquids
Compared with conventional liquid electrolytes, ionic liquids (ILs) exhibit many favorable features, including high ionic conductivity, strong thermal stability, and flame retardancy [87]. ILs can also be incorporated into polymer networks to fabricate GPEs for LIBs, and the polymeric ionic liquids (PILs) also exhibit flame retardancy and superior processability [88]. For example, Watanabe and co-workers synthesized a micellar ion gel composed of IL and a diblock copolymer through multiple hydrogen bonds [89]. The formed ionic gels showed self-healing properties after 3 h in the absence of external stimuli at room temperature.
4.2. Self-Healing All-Solid-State Electrolytes
4.2.1. Solid Polymer Electrolytes
Emerging technologies are driving the continuous development of all-solid-state electrolytes (ASSEs) in LIBs to enhance safety and room-temperature performance. A successful ASSE must possess high conductivity, keep good contact with electrodes and encompass a cost-effective, scalable processability. Different from inorganic solid electrolytes which suffer from the unstable and high resistance electrolyte/electrode interface, solid polymer electrolytes (SPEs) possess superior processability.
Given that some breakable SPEs may lead to catastrophic battery failure caused by a short circuit, Xue et al. have designed a series of self-healing SPEs via quadruple hydrogen bonding [91,92,93]. One of the formed SPEs was able to cure the cutting damage within 2 h at 30 °C with no external stimulus and provide high stretchability. However, the healing efficiency was not satisfactory, and it usually needed some stimulus such as high temperature or a period of time. To enhance the healing efficiency at mild temperature, they prepared a novel SPE carrying urea groups and disulfide bonds (Figure 7a,b) [94].
4.2.2. Solid Composite Electrolytes
Whiteley et al. developed a solid electrolyte-in-polymer matrix to form the electrolyte layer in LIB (Figure 7c,d) [95]. The membranes were synthesized by the hot pressing of powders of Li2S–P2S5 inorganic electrolytes and polyimine. Taking advantage of the void space among solid-state electrolyte pellets, they constructed an in-situ derived polymer matrix through reversible cross-links without sacrificing good contact. In this way, the weight proportion of solid electrolyte reached 80% and the thickness of the separator was reduced to about 4 µm. The battery based on this membrane with FeS2 as the cathode cycled stably for more than 200 cycles.
5. Self-Healing Current Collectors
In addition to the electrodes and electrolytes, current collectors are indispensable elements in LIBs, but are easily overlooked. Although current collectors cannot provide energy support, their quality greatly affects the cycling stability of LIBs, since the cracks or fractures will lead to the failure of the battery directly. Conventional current collectors generally rely on strain-adaptive structures requiring complex processing, and thus have poor stretchability, low device packaging density, and robustness, which is unacceptable especially for flexible LIBs. Therefore, the development of self-healing current collectors is of great importance to enhance the safety and stability of LIBs. However, current research on SH materials in LIBs is mainly focused on self-healing electrodes, self-healing binders, and self-healing electrolytes, while relatively little research has been conducted on self-healing current collectors.
Lee and co-workers demonstrated a self-healing current collector consisting of nickel sheets, eutectic gallium indium particles (eGaInPs) as well as carboxylated polyurethane (CPU) [18]. It delivered an initial electronic conductivity of 2479 S cm−1 and high stretchability with 700% strain. Benefiting from the flow of liquid metal to the damaged sites together with mechanical healing through interfacial hydrogen bonding of the CPU matrix, the impaired areas can be electrically restored. When used in graphene nanosheet supercapacitors, the self-healing electrodes exhibited a long lifespan of 1000 stretching/releasing cycles and 600 charging/discharging cycles.
6. Self-Healing Interfaces
6.1. Self-Healing Electrolyte–Electrode Interfaces
Among inorganic solid-state electrolytes, Li1.5Al0.5Ge1.5P3O12 (LAGP) electrolytes draw a lot of attention thanks to the high ionic conductivity at room temperature as well as good air stability. However, there has been little progress in the successful application of LAGP in Li metal batteries due to the interfacial incompatibility between LAGP and lithium metal. It is assumed that the use of a self-healing polymer electrolyte (SHPE) as an interface in LAGP-based Li metal cells can maintain a stable and integrated electrode/electrolyte contact during cycling, thus eliminating Li|LAGP interfacial side reactions [19].
6.2. Self-Healing Artificial Solid–Electrolyte Interfaces
The direct utilization of Li metal as anodes in LIBs is hampered by the instability of the Li/electrolyte interface, continuous dendrite growth, and drastic volume changes during cycling, resulting in rapid capacity degradation, internal short circuits, and even safety hazards [100]. Therefore, it is reasonable to prepare artificial solid–electrolyte interfaces (ASEIs) layers on lithium metal via in-situ or ex-situ methods, and these ASEIs usually have higher strength and thickness than natural SEIs. Even so, they still cannot meet the requirements because the mechanical stability of the in-situ formed ASEIs is usually poor, and the fabrication process of ex-situ ASEIs is tedious and stringent [101].
6.2.1. Liquid Metal and Alloys
Inspired by Wu’s work [43], Zhang et al. considered that coating LM on Li metal was a promising way to achieve stable ASEI layers and dendrite-free Li metal [102]. LM could not only repair the cracks of SEI layers caused by drastic volume changes during long-term cycling, but also regulate uniform deposition of Li+, resulting in excellent full-cell cycling performance. When paired with LTO cathodes, the LMNP-Li|LTO full cell showed smaller polarization and better electrochemical performance. Moreover, gallium-indium (GaIn) alloys have been used as anodes to improve the capacity utilization and cycle durability of LIBs at room temperature due to their low melting point (<25 °C) [103].
6.2.2. Supramolecular Interactions
Wang et al. prepared SHPs based on poly (ethylene oxide) (PEO) segments and UPy groups as robust and strongly adherent ASEIs for Li metal to achieve long-term Li plating/peeling cycles [99]. This supramolecular polymer (PEO-UPy) can be further stabilized after reacting with Li and in-situ forming an LiPEO–UPy coating layer. Similarly, self-healing capabilities were guaranteed by the UPy quadruple hydrogen bonds that triggered the intrinsic restoration of this ASEI. The strong adhesion between Li and the coating together with the homogenization of the fast Li+ flux prevented uncontrolled nucleation and inhibited dendritic growth. As a result, the electrochemical performances of symmetric cells and full-cells based on this LiPEO–UPy-coated Li (LiPEO–UPy@Li) were both improved.
7. Conclusions
Depending on the differences of healing mechanisms, self-healing behaviors can be categorized into physical approaches, chemical and physico-chemical approaches. The SH materials used in LIBs currently are mainly based on chemical approaches which can be grouped into supramolecular reactions and covalent/free radical rebonding. These two strategies achieve self-healing by means of either supramolecular interactions or reversible chemical bonds, which can achieve theoretically permanently healing but have relatively lower efficiency. Due to the deformation and fractures of the materials in LIBs, the capacity of the active materials would decrease in repeated charging/discharging processes and the battery lifespan become shortened. Fortunately, self-healing strategies provide new ideas to solve the above problems by healing the damaged parts to enhance the performance.
A large number of LIBs with self-healing properties have been constructed. Liquid metals and alloys as well as self-healing binders are instrumental in reducing mechanical cracks/fractures on electrodes, especially for Si based anodes, which can help increase the durability of the electrode materials. In addition, SH materials have also been used to improve the mechanical properties of solid polymer electrolytes through spontaneous repair of the damages. Therefore, the interfacial properties, dendrite suppression as well as mechanical flexibility can be improved. In addition to the internal components, self-healing materials can also be applied to the current collectors. Furthermore, self-healing strategy can also be used to solve the problem of interfacial stability due to the generated by-products during cycling processes, such as self-healing artificial SEI layers. Self-healable LIBs exhibit longer durability, higher safety, and better performance, and the developing of LIBs based on self-healing materials is one of the most promising directions for their practical application. Although numerous efforts have been made in this area, many challenges still exist, such as further improvements of healing efficiency.
This entry is adapted from the peer-reviewed paper 10.3390/nano12203656
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