Self-Healing Systems on Anodes for Next Generation Energy Storage Devices

Self-healing is the capability of materials to repair themselves after damage has occurred, usually by interaction between molecules or chains. Physical and chemical processes are applied for the preparation of self-healing systems. There are different approaches for these systems such as heterogeneous systems, shape memory effects, hydrogen bonding or covalent-bond interaction, diffusion and flow dynamics. Self-healing mechanisms can occur in particular by heat and light exposure or by reconnection without direct effect. The applications of these systems display an increasing trend in both R&D and industry sectors. Moreover, self-healing systems and their energy storage applications are currently getting great importance. This review aims to provide general information on recent developments in self-healing materials and their energy applications in view of the critical importance of self-healing systems for lithium-ion batteries (LIBs). In the first part of the review, an introduction about self-healing mechanisms and design strategies of self-healing materials is given. Then, selected important healing materials in the literature for the anodes of LIBs are mentioned in the second part. The results and future perspectives are stated in the conclusion section.


Introduction
The use of composite materials is increasing day-by-day. The composite materials have gradually increased in studies carried out in the aviation industry, where technological processes are rapidly adapted and which is important in the technologies development. With the development of technology, the ability of composite materials to adapt to the conditions of their environments and to respond appropriately to these conditions is also important. These materials, called smart materials, develop stimuli in a way that changes their mechanical, electrical, optical or magnetic properties in response to external stimuli. The production of such smart materials leads to the emergence of research topics such as increasing the durability of use, prolonging their life and/ or reducing the cost of healing, and engineers do many studies on these issues. In fact, this can be achieved by a perfect mechanism called self-healing in biological systems. In order to apply this mechanism to materials, biological systems are studied and tried to be imitated. In this context, studies on new generation smart materials have created a new research area called self-healing materials, and research in this area continues rapidly. Thanks to this behavior, it is argued that the life and reliability of materials that are defective due to production or damaged as a result of an external effect can be increased, and thus healing costs can be reduced [1][2][3][4][5][6][7].
Self-healing can be defined as the ability of a material to heal (recover/repair) damages automatically and autonomously without any outside intervention. Many general terms are used to describe such a property in materials, such as self-healing, autonomic healing, and autonomic heal.
When self-healing properties are added to man-made materials, often the self-healing action cannot be performed without an external trigger. There are several systems used to impart self-healing ability to materials ( Figure 1). These systems can be grouped into two main groups as capsulebased healing (bead, fiber and/or vascular type and mechanochemical), which is basically autonomous, and healing by the action of non-autonomous external stimuli [1,[8][9][10][11][12][13].  [7] Capsule-based healing systems involve microencapsulations and are the process of protecting micron-sized solid particles, liquid droplets or gas by isolating them from the external environment with an inert shell. The capsule ensures that the healing agent is retained within the system until a break or crack occurs in the self-healing materials ( Figure 2). In capsule-based heal systems, interfacial, in-situ, co-acidification, soluble solution encapsulation techniques are among the most basic techniques. Self-healing occurs using microencapsulated healing agents and catalytic chemical triggers in the epoxy matrix. In this system, with the progression of the crack, the embedded microcapsules are disintegrated and the healing agent advances to the crack plane by capillary action. The polymerization of the healing agent is triggered by contact with the catalysts embedded in the epoxy, causing the cracked surfaces to bond together. Thus, the injury-induced trigger mechanism provides region-specific autonomic healing control. The biggest disadvantage of microcapsule-based self-healing systems is the limited number of healing agents, and it is not known when the healing agent will be completely depleted, especially locally, in cases of multiple  [10,[18][19][20][21][22][23].
In vascular network systems, on the other hand, since there are multiple connection points at any point, a higher amount of healing agent reaches the damaged area and causes an increase in mechanical healing reliability. When there is damage to the human skin, blood flow is triggered from the capillaries in the reticulated structure to the damaged part and coagulation occurs rapidly.
Due to the vascular nature of this feeding system, minor injuries in the same area can be healed repeatedly. On the other hand, the production process of such systems is complex and it is very difficult to obtain synthetic materials with such meshes for practical applications. The other method used to gain self-healing ability is non-autonomous mechanisms and occurs through latent effects that allow self-healing of damages.

π-π Stacking Interactions Based Self-Healing Polymers
Although π-π stacking interactions are weaker than hydrogen bonds and ionic interactions, they have an important place in supramolecular systems due to their low probability of degradation by environmental factors such as humidity. The interactions between aromatic rings of different sizes, shapes, and displacement patterns are called π-π stacking or π-π interactions. Aromatic π interactions first emerged in the early 1980s and have been applied in many fields, especially biological systems, molecular recognition, self-assembly, asymmetric reduction catalyst and organic transistors. The interaction usually occurs between the π-deficient electron unit and the πelectron rich unit, and the interaction can occur mainly in two ways; face-to-face stacking and face-to-side stacking. π-π stacking interactions in self-healing supramolecular polymer materials were first obtained by Burattini et al. by combining polyimide containing multiple π-electron deficient acceptor sites and siloxane polymer containing π-electron-rich pyrenyl functional groups.
In such interactions, the nature of the electron-poor components is critical in terms of interactions, and it also affects the bond strength of the material to be obtained by playing a role in determining the bond strength of the stack [33,34].

Metal Ligand-Based Self-Healing Polymers
In the field of supramolecular chemistry, special attention has recently been paid to metallosupramolecular polymers. Metallo-supramolecular polymers can also work well in a self-healing system. Unlike polymers formed by hydrogen bonds and π-π stacking interactions, the reversibility and stimulus-response of metallopolymers are directly affected by metal ligand binding resistance.
Therefore, the choice of polyvalent metal-ligand interaction is effective in obtaining a stable, dynamic and reversible cross-linked network. In a given field, the specific properties of materials can be altered by changing the ligand and metal [35].

Ionic Interactions Based Self-Healing Polymers
Ionic interactions in polymers are mainly manifested by the formation of ionomers. Ionomers can be defined as polymers in which the volumetric properties are governed by ionic interactions in discrete regions of the material. Ionomers; Since they contain ionic, dipole-dipole and/or iondipole bonds, they also occupy an important place among supramolecular self-healing polymer materials. These ionic groups can aggregate together to form a complex, and when ionomers appear in a crack, they can self-heal through these strong intermolecular interactions between the ionic groups. In self-healing polymers, the polymer matrix must provide sufficient mobility to the polymer chains so that ionic interactions can take place at the damaged sites, thus allowing the chains to be intertwined and rearranged. In addition, many factors such as the nature of ionic groups and counter-ions, degree of neutralization, temperature, content of ionic groups and dielectric constant also play an important role in the properties of materials that self-heal by ionic interactions [14,36].

Hydrogen Bond Based Self-Healing Polymers
Among the various self-healing mechanisms in supramolecular polymers, healing by hydrogen bonding has attracted the attention of many research groups because the hydrogen bonds can be easily separated and reconnected at room temperature, and the recovery properties can be easily adjusted by manipulating the number of hydrogen bonds. Self-healing supramolecular polymers contain both covalent and non-covalent bonds in their structure. The basis of damage to materials is the breaking of chemical bonds. In self-healing materials containing hydrogen bonds, hydrogen bonds are easier to break than covalent bonds. When cracks occur as a result of applying external force to a supramolecular polymer, multiple free, unbonded hydrogen bonds are formed at the new interfaces. These free hydrogen bonding parts come together and form new hydrogen bonds, allowing the cracks to close and the damaged areas to heal. However, the activity of free hydrogens can continue for a period of time; The self-healing abilities of the new surfaces will decrease due to the recombination of free hydrogens in the same regions. On the other hand, the reduced selfhealing property can be significantly increased by the heat treatment applied to the fracture surfaces [18,37,38]. is brought together by contacting the damaged surfaces. Thus, hydrogen bonds are allowed to form the reticulated structure. Hydrogen bond formation in this material was provided by amidoethyl imidazolidone and diaminoethyl urea groups, and it was observed that no crystalline region was formed during self-healing. It has been reported that the material produced by this method elongates up to the breaking point with 500% strain. In addition, it has been explained that less than 5% residual stress is seen with the removal of the applied force and it has the capacity to recover after 300% strain. The result in tests for damage and healing in rubber is that the specimens self-heal over time at room temperature when cut into two pieces and then re-contacted. It has been emphasized that the healeded sample can be deformed up to 200% without breaking with a contact time of 15 minutes. It was also stated that the amount of recovery in the materials decreased as the time elapsed before reassembling the damaged surfaces. With this mechanical intervention, it has been proven by the tests that the healing cycle can be successfully performed many times by contacting the broken or broken parts without using any chemicals [13,[38][39][40][41][42][43][44].

Effect of Nanoparticle Additive on Self-Healing Properties
In the literature, nanoparticle doping has been done to increase the healing properties of selfhealing systems. The healing process in nanoparticle-doped polymers does not consist of steps such as breaking or recombining polymer chains. As cracks and defects occur, nanoparticles dispersed in the polymer phase fill the cracked or damaged part. Firstly, Lee et al. combined computer simulation with micromechanics to demonstrate the self-healing effect of nanoparticles in polymers, and conducted research on multilayer composites produced [45]. It has been observed that such polymer-nanoparticle composites actively respond to damage and potentially multiple self-healing of the polymer system as long as the nanoparticles continue to exist in the system. In another publication, they modeled the functionality of applied nanocomposite coatings to heal nanoscale defects on the surface with molecular dynamics and lattice spring simulations. The modeling results showed that nanoparticles tend to migrate to the damaged areas with a polymerinduced attraction force, that small particles are more effective in healing the damaged area than large particles, and that small particles are transported to the damaged area in a shorter time interval. Gupta et al. experimentally proved the transport and aggregation of nanoparticles around cracks in multilayer composite structures in the simulation studies in the literature. In the study, 3.8 nm CdSe/ZnS nanoparticles were embedded in the SiO2 layer (50 nm) deposited on the PMMA film (300 nm), and it was observed that the nanoparticles in the fragile SiO2 layer were transported to the polymer phase along the crack. It is stated that the transport of nanoparticles depends on the enthalpic and entropic interactions between the PMMA matrix and the nanoparticles. As a result of the TEM analysis applied to the cross-sectional area of the composite material, it was observed that nanoparticles whose surface was modified with fluorescent PEO ligands were deposited on the interface of PMMA and SiO2 layers. The place of nanoparticles in the self-healing phenomenon is explained by the stretching and stretching movements of the polymer chains close to the damaged area, and the tendency to decrease the nanoparticle-polymer interaction with the accumulation of nanoparticles in the crack and pre-crack regions is stated to be the driving force [46].

Physical Interaction based Self-Healing Materials
For self-healing materials that can exhibit reversible properties, there were originally two noncovalent approaches, hydrogen bonding and π-π stacking [2,47]. and auto-repair at low temperature [24]. For the first time, a self-assembled supramolecular gel of metal-ligand and polypyrrole hydrogel with high conductivity and a hybrid gel based on nanostructured polypyrrole [35]. Li et al reported a self-healing network crosslinked by coordination complexes that it consists of ligands via both nitrogen and oxygen atoms of the carboxamide groups. in room conditions [28]. Firstly, Yan and colleagues demonstrated that such a synthetic hydrogel material is prepared from polyethylene glycol and polyethyleneimine that exhibit self-healing abilities. [56]. Nishimura at all demonstrated incorporated networks of silyl ether linkages into covalently cross-linked polymer reprocessability [57]. Urban  and their derivatives, can self-heal upon mechanical damage with the key and lock commodity self-healing behavior [6]. Zn 2+ -imidazole crosslinks are distributed in a hydrogen bonded/Diels-Alder dynamic covalent double crosslinked network, ideal sea cucumber inspired materials that can transform into tough but tough materials after exposure to external stimuli and better resist external influences In contrast to moisture-affected non-covalent crosslinks SCIMs with reversible self-healing system have been reported [26].

Hydrogen bonded supramolecular self-healing
Phase separation effects at polymeric interfaces are also determinants of self-healing. Kovalenko and coated it with polymer that self-healing functionalization with covalent and hydrogen bonds.
The thickness of the self-healing polymer coating on the electrode was affecting the percent of strain and electrochemical capacity of the cell. Each time the polymer coating on the carbon/Si electrode was increased by 2X, the strain increased by a factor of 2, while the capacitance resulted in 722 mAhg −1 and 584 mAhg −1 at the 100 th cycle, respectively. It was determined that more coating caused a rapid capacity fading [63]. The dual crosslinking polymer that shown in figure 4f heals visible cracks on the electrodes and no obvious delamination between electrode surface and copper foil is proposed by Gendensuren and Oh [64].

Ionically Bonded Interaction
Polymeric materials with macromolecules consisting of ionic and/or ionizable groups can be respectively. Coulombic efficiency of electrodes that use SBR and PVDF is lower than coulombic efficiency of self-healing systems [11].

Multiple Functional Interaction Self-healing Mechanism
Self-healing properties can be intra-molecular and intermolecular, as well as a self-healing material with physical interaction combined with chain movements and multi-level chemical interactions obtained by repairing more than one type of chemical entity in a single material. Lim et al. reported PAA-PBI binding using supremolecular interaction with ionic bond and hydrogen bond. This highlights that the structure using only PAA binder with 0.45 peeling and the structure using 2% by weight PBI relative to PAA show close mechanical properties. Thus, it shows that a tight conducting network (Figure 10a) is obtained using PAA-PBI-2. This mechanical property is related to the proportional reversibility of hydrogen bonding and ionic interactions. The electrode with the PAA-PBI-2 connector showed a high initial capacity of 1376.7 mAh/g and improved capacity retention of 54.6% after 100 cycles, which was much better than the other two connectors.
The bond strength of the bonding network with Si will decrease with increasing PBI ratio because the ionic interaction between PBI and PAA provides a lower amount of carboxylic acid to adhere to the Si surface [85].

Chemical Interaction based Self-Healing Materials
Reversibility of covalent bonds can use condensation, exchange, and addition reactions. nanolayers that serve as both chemical and physical crosslinkers [88]. Li and coworkers reported a supramolecular polymer type using a host-guest complex of visible light-labile picolinium βcyclodextrin nanogels (β-CD) ultrastability against electrolytes, and photodegradation properties [27]. Most self-healing artificial materials are polymer-based [17]. Self-healing mechanisms can be classified in various ways according to the way of breaking and joining of bonds, intramolecular and intermolecular interactions, external excitations and polymer network structures. In its simplest form can be considered two main types as covalent and non-covalent. self-healing mechanisms with dynamic non-covalent bonds are hydrogen bonding, ionic interactions, metal coordination, π-π stacking or hydrophobic interactions, while self-healing mechanisms with dynamic covalent bonds include diels-alder reaction, disulfide, acylhydrazone, ester, imine. The Diels-Alder (DA) reaction for crosslinking linear polymers has been pioneered by Kennedy and Wagener over the last four decades [89,90]. The crosslinkable and reversible groups of the thermoreversible polymers were attached to linear polymer backbones, but the links of crosslinkers to polymer backbones were not reversible, using a completely reversible crosslinking covalently formed macromolecular macromolecular network reported by Chen et al [91]. A mechanically self-healing electrode was successfully developed by Lee et al by placing Ag nanowires and polydimethylsiloxane-based polyurethane (PDMS-CPU) crosslinked with Diels-Alder (DA) adducts. A combination of DA reaction coated AgNWs on the surface of the polymer, smoothing the polymer surface, greatly improving the surface mechanical sustanability of the electrode [92].
As a resemblance to this work, a transparent electrode, a thermally replaceable electrode was developed by Pyo et al, again containing polyurethane Ag nanowires as crosslinkers [93].
As an alternative to the self-healing chemistry of covalently bonded rubber materials, the Disulfide mechanism is used [94].

Imine bond based self-healing systems
According to Cao

Ester bond based self-healing systems
Ryu et al investigated the natural guar gum component BC-g on Si anode. This binder, which will maintain the electrode integrity over long cycles, adheres strongly to the surface of the Si particles with its hydroxyl content. In the polymer, the bonding between the boronic acid side groups on the polystyrene backbone and the hydroxyl groups on the guar gum increases the mechanical strength.
Hydroxy H-bonds and borate ester bonds form the self-healing mechanism. By putting a drop of electrolyte solvent on the broken surfaces, the polymer was able to reconnect the new surfaces.
The prepared Si electrode containing the developed polymer binders retained 70% capacity after 300 cycles at 1C [106].
Jung et al improved that Si anode, that have properties stabilizing the SEI layer, and preventing the volumetric expansion of Si aggregation with used binder (Figure 12), produced with polymeric binder via covalent bond formation between −OH groups on produced Si's surface and an esterification with polyacrylic acid. This combination exhibited a capacity of 1500 mA h g −1 after 500 cycles at 1000 mA g −1 [107].

Disulfide bond based self-healing systems
It affects the molecular behavior of the types and steric hindrance of self-healing fragments between different polymer chains and can self-heal and mechanical properties. The urea groups are self-healing by differences between the thiourea and urea hydrogen bond moieties.
A double-wrapped binder polyacrylic acid (PAA) and binder using outer polyurethane (BFPU) polymers ( Figure 13) to address the large internal stress of low Young's modulus bifunctional silicone was developed by Jiao. BFPU acts as a buffer layer to disperse the internal tension and stress during lithiation. This prevents structural damage to the hard PAA. Thus, large volume changes are prevented during the charge-discharge process. Si anodes that developed with PAA-BFPU binder have capacity of 3.5 mAh cm −1 and over 88% capacity retention for 200 cycles [108].   cycles. The Diels-Alder-PAA binder that shown in figure 14, outperformed commercially available silicone binders such as PAA, CMC, SA and PVdF with its 3D network structure and self-healing [22].

Conclusion
Self-healing materials have been investigated from electronics to the building industry and the biomedical fields. In addition to these areas, the application of self-healing materials to electrochemical-based devices such as batteries and supercapacitors is rapidly increasing.
Concerning self-healing systems, many researches are also being performed in the manufacture of other electronic and electrochemical devices such as dielectric actuators and electrochemical sensors. Conducting polymers are very important for actuators, solar cells, sensors and energy storage devices. However, damage to these materials causes serious problems in device performance. The main requirement in the development of self-healing and conductive materials is to maintain a high conductivity level after damage. The most basic strategy for producing selfhealing conductors is to add dynamic reversible bonds to conductive polymers. There are many researches in the literature.
Sensors are widely used in our daily life to detect external signals, which are chemical or physical signals. Most mobile devices (such as cell phones, tablets, and laptops) have capacitive electric touchscreens. However, they are quite fragile in cases where accidental dropping or scratching causes the touch sensor to malfunction. The idea that if self-healing sensors are used, it could potentially increase the lifespan of such devices, especially for functional and aesthetic purposes, has increased research in this area.
Supercapacitors are promising energy storage devices that attract attention due to their fast chargedischarge rates, long life cycles, and high power densities. There are important studies in the literature to develop supercapacitors with high flexibility and lightness. However, deformation or breakage from stress or mechanical damage limits reliability and shortens the life of supercapacitors. The use of self-healing materials in the manufacture of supercapacitors is considered a good alternative for restoring electrical properties after mechanical damage.
In conclusion, although promising developments have been achieved so far, innovative materials strategies are still needed at the application level of self-healing materials and tools for practical use and eventual commercialization.