PROGRESS REPORT Energy Storage

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Smart Electrochemical Energy Storage Devices with Self-Protection and Self-Adaptation Abilities Yun Yang, Dandan Yu, Hua Wang,* and Lin Guo* and longevity of the devices. Development of smart energy storage devices which can take actions and self-adapt in response to these stimuli is a promising strategy to expand the service life of future electronics, as well as to improve their operational stability and safety. In recent years, a number of smart electrochemical energy storage devices have been rationally designed and fabricated to address these challenges.[24] Different from the multifunctional devices fabricated by integrating several independent functional modules,[25–27] delicate designs have been developed to confer innate intelligence onto these advanced devices, including in situ safety monitoring, realtime display of energy storage level, and self-protection capabilities.[28–32] In order to cope with external mechanical damage, self-healing supercapacitors and batteries have been successfully fabricated, and could improve the reliability and lifetime of these devices.[33,34] Moreover, to avoid the irreversible recovery of flexible energy storage devices caused by deformation over time, shape-memory supercapacitors have also been developed.[34–36] In this progress report, we summarize the main characteristics of current smart electrochemical energy storage devices with enriched functionalities, ranging from self-protection to intelligent responses to external stimuli. In particular, smart energy storage devices with different types of abilities, such as self-monitoring of internal shorting caused by dendrite formation, self-guard against thermal runaway, as well as selfadjustment in response to mechanical damage and deformation are discussed in detail. Finally, the challenges and future perspectives of smart rechargeable energy storage devices are discussed.

Currently, with booming development and worldwide usage of rechargeable electrochemical energy storage devices, their safety issues, operation stability, service life, and user experience are garnering special attention. Smart and intelligent energy storage devices with self-protection and selfadaptation abilities aiming to address these challenges are being developed with great urgency. In this Progress Report, we highlight recent achievements in the field of smart energy storage systems that could early-detect incoming internal short circuits and self-protect against thermal runaway. Moreover, intelligent devices that are able to take actions and self-adapt in response to external mechanical disruption or deformation, i.e., exhibiting self-healing or shape-memory behaviors, are discussed. Finally, insights into the future development of smart rechargeable energy storage devices are provided.

1. Introduction Electrochemical energy storage devices play significant roles in our daily and industrial activities.[1–13] Due to the ubiquitous usage of these devices, their safe operation, performance stability and service life are fundamental to achieve a smooth user experience, and thus have drawn increasing attention.[14–17] The internal shorting caused by dendrite formation, thermal runaway as a result of overcharging or short-circuiting of these devices can cause explosions, fires and even casualties if these risks are left uncontrolled.[18,19] Although battery management system with sensing and feedback modules can regulate stacked cells on a macroscopic scale, the potential hazards arising from the inner reactions of batteries are still disconcerting. The design of smart energy storage devices with intrinsic in situ monitoring and self-protection abilities without the aid of external control systems is urgently needed.[20–23] In addition, energy storage devices may suffer from external mechanical stimuli, such as long-time shape deformation and accidental damage, which can seriously impair the reliability

2. Smart Designs for Self-Protection Dr. Y. Yang, D. Yu, Prof. H. Wang, Prof. L. Guo School of Chemistry Beijing Advanced Innovation Center for Biomedical Engineering Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education Beihang University Beijing 100191, P. R. China E-mail: [email protected]; [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201703040.

DOI: 10.1002/adma.201703040

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Internal shorting and overheating are two severe safety issues facing the state-of-the-art electrochemical energy storage devices. Nevertheless, smart electronics with multiple functionalities in addition to the energy storage ability, are emerging to overcome these problems, mainly by means of incorporating additional signal-sensing electrodes and temperature-responsive smart materials. By utilizing sophisticated architectures and engineered materials, these introduced smart modules are compatible with the original electrochemically active components in the devices, generating a series of

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smart rechargeable energy storage systems that are capable of self-protection with only minimal compromise regarding their energy storage performance.

Yun Yang received her Ph.D. degree in Chemical and Biomedical Engineering from Nanyang Technological University in 2015. She is currently a research fellow at Beihang University. Her research interests focus on nature-inspired energystorage systems and microbial fuel cells.

2.1. Smart Design to Avoid Internal Shorting With the development of high energy/power density energy storage systems, concerns regarding their safe operation are increasing, and are being extensively studied.[28,37] Lithium-ion batteries are renowned for their outstanding energy densities, while they are challenged by safety hazards due to the inevitable and uncontrollable growth of lithium dendrites on the negative electrodes during electrochemical cycling.[38–44] With the gradual growth of lithium dendrites during charge/discharge cycles, these dendrites will ultimately reach the separator and penetrate through the porous polymer, thereby forming a short circuit between the anode and the cathode, which in turn can cause fires or even explosions.[45,46] In the past decades, several approaches have been developed in order to inhibit the growth of Li dendrites, such as improving the electrode materials,[47–49] using electrolyte additives,[50–56] employing solid or gel electrolytes,[57–59] or modifying the separator.[60–65] While these studies have made some progress, it is still not possible to completely suppress the growth of dendrites using present technologies. In view of these problems, smart strategies have been designed to detect the early growth of dendrites, before the emergence of internal short circuits. For example, internal battery health was in situ monitored with the aid of a novel bifunctional separator reported by Wu et al.[29] In comparison to traditional battery separators, which are insulating polymer layers with porous structures, the bifunctional separator was in a polymer-metal-polymer triple layer configuration (Figure 1a,b). The incorporated conducting metal intermediary layer offers an additional voltage sensing terminal, so that the extent of dendrite growth can be indicated by the voltage value between the intermediary layer and the negative electrode. Due to the electrochemical potential difference between the anode and the intermediary metal, there is a considerable initial voltage signal at the safe operating stage. Once the lithium dendrites grow long enough to reach the intermediate conducting layer, and before further puncturing through the entire separator, an immediate and sharp drop in the monitored voltage signal could be observed, indicating the upcoming safety hazard. The bifunctional separator was fabricated by stacking two commercially available 12-µm-thick polyethylene (PE) separators, one of which was pre-coated with a 50-nm-thick copper layer via sputtering. Such sandwiched triple layer separator possessed several advantageous features fundamental to their contribution in the smart batteries with both a traditional separator function and a voltage sensing function. The copper coating implemented by magneto sputtering was too thin to influence the porous morphology of the separator. Consequently, the mechanical strength and ionic conductivity of the separator were not affected by the new conductive metal layer. Although the metal coating offered conductivity in plane, the electrical resistance was still very high across the separator, ensuring the insulating property of the stacked separator. When assembling the bifunctional separator into a lithium metal battery with a

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Hua Wang is an associate professor in the School of Chemistry at Beihang University. He received his Ph.D. degree in Materials Science and Engineering from Beihang University in 2012. He then worked as a research fellow at Nanyang Technological University for 2 years. His research interest is the development of advanced nanomaterials for energy conversion and storage, including lithium/sodium-ion batteries, photocatalytic water splitting, and water purification. Lin Guo is a professor and vice dean of the School of Chemistry in Beihang University. He received his Ph.D. in Materials Science and Engineering from Beijing University of Institute of Technology in 1997. He worked as a visiting scholar in Hong Kong University of Science and Technology in 1999, and Dresden Technology University for 2 years. His research interests include synthesis and characterization of sophisticated nanomaterials, high-strength nanomaterials with light weight, and functional nanomaterials for energy storage.

third recording terminal connected to the conducting copper layer, the VCu-Li was observed to suddenly drop at 3.5 h with a charging current density of 4.0 mA cm−2, indicating that the lithium dendrites had penetrated through half of the entire separator, in the case where the intermediate layer was put in the middle. By varying the relative position of the conducting

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Figure 1.  Schematic illustration of the smart battery capable of early-detecting dendrite growth and predicting dendrite pinhole puncture position. a) In a traditional lithium battery, dendrite formation cannot be detected until they completely penetrate through the entire separator, causing internal shorting and safety hazards. b) By comparison, in a safe lithium battery with a bifunctional triple-layer separator (a conducting copper layer sandwiched between two conventional separators), a safety alarm could be activated once the growing lithium dendrites contact the conducting copper layer, giving a sharply dropping signal in VCu-Li as a warning of the upcoming safety hazard. c) By comparing the measured resistance between the left side of the metal layer and the negative electrode (R1 + Rjunction), the resistance between the right side of the conducting layer and the negative electrode (R2 + Rjunction), and the resistance between the two ends of the sandwiched separator (R1 + R2), the value of R1 and R2 can be calculated. The relative position of the pinhole could be deduced by the ratio of R1 to R2. The accuracy of the prediction method (d) and a comparison of actual and calculated pinhole positions (e). Reproduced with permission.[29] Copyright 2014, Nature Publishing Group.

layer within the sandwiched triple layer separator, it is possible to adjust the critical penetrating ratio to raise a safety alarm at an appropriate threat level, e.g., at 25%, 60% or 80% of the separator thickness being penetrated by dendrites. What’s more, the presence of this novel separator in the smart battery also makes it feasible to determine the position of dendrite penetration. As a demonstration, a pinhole in the separator was created in advance to induce dendrite puncturing through it. Once a safety alarm was activated, the location of the pinhole could be

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predicted by measuring the electronic resistances as shown in Figure 1c. This method showed an accuracy as high as 98% in predicting the position of the dendrite (Figure 1d,e). Thus, the bifunctional separator with two polyolefin separator layers and a sandwiched metallic voltage sensing layer is effective in early detection of dendrite growth with high sensitivity and accuracy. However, the triple layer configuration of such separators raises the risk of higher internal resistance in the corresponding battery, as well as causing difficulties

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in cell assembly. Lin et al. further designed a fully integrated bifunctional separator for dendrite detection to overcome these problems.[66] Incorporating the Cu interlayer into the polyolefin separators is technically difficult, since a horizontal stretching process for final pore formation will destroy the Cu layer. Consequently, a solution synthesis method without a stretching step was developed to fabricate a polyimide (PI) separator, and a nanoporous PI membrane was created using LiBr salt as the template at the polyamic acid (PAA) precursor stage. The as-prepared PI separator displayed superior thermal stability, high porosity, good mechanical strength and excellent electrolyte wettability. With the subsequent Cu sputtering and doctor-blading processes, an all-integrated bifunctional PI/Cu/ PI separator was generated with strong interface adhesion.[66] When tested in a pouch-cell with Li foils as both the anode and cathode, the fully integrated separator enabled efficient Li dendrite detection, as indicated by a severe voltage drop from ≈3 V to nearly 0 V between the Cu layer and the negative electrode.

2.2. Smart Design to Avoid Overheating Thermal runaway is another challenging issue plaguing electrochemical energy storage systems, especially during ultrafast charge/discharge processes, which results in the hazard of fire or explosions.[67–70] Two types of techniques have been developed to address the thermal runaway problem. One method is to inhibit heat generation by adopting alternative electrolytes, e.g., polymer gel electrolytes and solid-state electrolytes with low ionic conductivities.[71–74] The other route is to release or absorb the heat before overheating, e.g., employing safety vents, extinguishing agents, or a thermal fuse.[70,75–78] Although these related studies showed some effects in thermal protection, they are limited by the passive strategies with significant sacrifices of energy storage performance and irreversible self-protection reactions. Currently, smart strategies to enable stimulus-responsive and active protection against thermal runaway have been reported.[79] A temperature-inducible self-guard approach against overheating without significantly compromising the electrochemical energy storage performance was reported by Yim et al.[80] In most cases, direct addition of extinguishing additives into the electrolyte would greatly impair the ionic conductivity of the electrolyte and the overall electrochemical performance. What’s more, some extinguishing agents, i.e., 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane) (DMTP), have poor miscibility with commercial electrolytes, which would significantly decrease their protecting effect. To address these problems, DMTP was rationally designed to be encased in microcapsules with a thermo-responsive surface-capping layer, in order to realize stimulus-actuated and reliable self-protection, while leaving the electrochemical energy storage capability intact (Figure 2a). These microcapsules were prepared via an oil-in-water emulsion-based poly­ merization process, with methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) serving as the mono­mer and cross-linking agent, respectively. The polymeriza­ tion and cross-linking reactions generated a polymeric layer, i.e., a poly(methyl methacrylate) (PMMA) shell, with prompt responsiveness to temperature changes, good chemical and

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electrochemical stability, and superior ability to hold the highly volatile DMTP content. Consequently, such microcapsules have excellent compatibility with commercial electrolytes and separators without significant deterioration of ionic conductivity. Nevertheless, when the temperature exceeded 70 °C, the encapsulated DMTP agent was released likely due to increased internal pressure as a result of rapid DMTP vaporization, since the boiling point of the DMTP content (95 °C) was lower than the glass transition temperature of the PMMA shell (127 °C) (Figure 2b). When tested in a full cell with a graphite anode and a LiNi0.5Co0.2Mn0.3O2 cathode, the microcapsules were coated onto the PE separator (0.636 g g−1 PE) and mixed with the electrolyte (10 wt%) (Figure 2c). Remarkably, the specific capacity and cycling performance of the as-prepared lithium-ion battery was not influenced by the incorporation of these microcapsules. In a nail penetration test, a normal cell with pristine separator and electrolyte suffered from a dramatic temperature rise from 25 °C to 72.3 °C. By contrast, the internal temperature of the microcapsule-embedded cell only increased to 37.2 °C, i.e., 74% of the overheating was effectively suppressed (Figure 2d). These thermo-responsive self-extinguishing microcapsules thus offer an effective method to provide self-protection against thermal runaway without any performance fading of the host battery. In order to achieve repeated protection, Yang et al. developed an electrolyte with reversible sol–gel transitions in response to temperature changes, and fabricated a smart supercapacitor which could actively and reversibly protect itself against overheating (Figure 3a).[81] The thermal copolymer poly(N-isopropylacrylamide-co-acrylamide) (PNIPAM/AM) was synthesized, and the PNIPAM/AM solution was employed as the smart electrolyte. When the temperature exceeded the transition temperature of the electrolyte, PNIPAM changed from the hydrophilic state to the hydrophobic state as a result of hydrogen bonds breaking between N-isopropyl groups and water molecules, and the PNIPAM solution turned into a hydrogel in which ion migration was greatly inhibited. Importantly, when the temperature was decreased, all the transition reactions of the electrolyte were reversed. The sol–gel transition temperature could be varied from 40 to 47 °C by adjusting the ratio of NIPAM to AM (Figure 3b). A symmetric supercapacitor was fabricated with carbon nanotube films as the electrodes, and an aqueous solution with 0.5 M LiOH and 0.2 g mL−1 PNIPAM/AM was applied as the electrolyte. The resulting supercapacitor showed a 35.2% decreased of capacitance, from 52.5 to 34.0 F g−1 upon heating from 20 to 70 °C, turning to a relatively muted “off” state to prevent further thermal runaway and safety hazard caused by operation under overheated conditions. Conversely, its energy storage capacity was completely restored, and it resumed an “on” state after cooling down (Figure 3c). The control over this smart supercapacitor could be reproduced over multiple overheating-cooling cycles. This work represents a successful demonstration of a novel strategy in active self-protection against thermal runaway with reversibly regulated energy storage performance. However, the relatively narrow range of transition temperature and solvent of the PNIPAM/AM based smart electrolyte limits its wider applicability. Another kind of thermally protective electrolyte with similar thermo-responsive sol–gel transition reactions and a broader transition temperature range was presented by Shi et al., on

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Figure 2.  a) Synthesis route of the fire-suppressing microcapsules composed of a DMTP (extinguishing agent) core and a temperature-responsive PMMA shell via an oil-in-water emulsion-based polymerization reaction, using MMA (monomer), EGDMA (cross-linking agent), and ADVN (initiator). b) DSC curves of the self-extinguishing microcapsules. The inset SEM images show the morphology of the microcapsules at two different temperatures. The initial spherical shapes of these microcapsules can not be preserved once the internal DMTP is released. c) IIustration of the temperature-responsive microcapsules-coated PE separator. When the surrounding temperature increases, the internal DMTP can be released from the microcapsules, so that the rising temperature can be suppressed. d) Temperature curves during the a nail penetration test of the fully charged cells (graphite/LiNi0.5Co0.2Mn0.3O2 (NCM523)) with or without self-extinguishing microcapsules. Reproduced with permission.[80] Copyright 2015, American Chemical Society.

the basis of the thermoplastic elastomer, Pluronic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO).[82] The transition temperature of the Pluronic solution could be adjusted from 20 °C to over 90 °C by varying its concentration and molecular weight (MW). Moreover, the extent of shut-down of the corresponding supercapacitors at high temperatures was dependent on the MW of the Pluronic solution, ranging from 50% to around 100%. This Pluronic-based aqueous electrolyte showed good compatibility with a series of electrode materials and conductive ions. Moreover, a Pluronic polymer that is soluble in organic solvents could be obtained by modifying its block segments. These features make the Pluronic-based electrolytes to be promising smart electrolyte candidates applicable in versatile thermally self-protecting energy storage systems. In addition to these sol–gel transiting electrolytes, repeated overheating protection could also be realized by taking advantages of a thermo-responsive polymer switching (TRPS) material which was incorporated onto the current collector (Figure 3d).[83] This material was composed of a polymer matrix

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with a high thermal expansion coefficient and graphene-coated spiky nanostructured nickel (GrNi) particles embedded within the matrix. Polyethylene (PE) was chosen as the matrix polymer, due to its electrochemical stability and high thermal expansion coefficient (≈10−4 K−1). The GrNi particles had diameters of 1 to 3 µm, and a graphene layer with a thickness of 5–10 nm was coated onto each GrNi particle in order to stabilize the Ni-electrolyte interface. Benefitted from the field-assisted tunneling effect of the nano-spikes, the TRPS material possessed an electrical conductivity (σ) as high as 50 S cm−1 at normal operating temperatures. However, once the temperature exceeded the switching temperature (Ts), the polymer matrix expanded and separated the embedded conductive particles. Consequently, the σ value decreased sharply, dropping seven to eight orders of magnitude within one second (Figure 3e). When the temperature returned below Ts, the polymer matrix contracted and restored the conducting pathways along the nano-spiky particles. The Ts value of TRPS could be tuned over a wide range by employing different polymer matrix materials

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Figure 3.  a) Illustration of a thermally responsive sol–gel transiting electrolyte capable of suppressing the conductive ion migration upon overheating. When increasing the temperature, the electrolyte solution turns into a hydrogel via hydrophobic association. b) The transmittance changes (UV–Vis transmittance at 800 nm) with increasing temperature from 33 to 58 °C. The concentration of the three kinds of copolymer was 0.2 g mL−1, and the molar ratios of NIPAM to AM were 8:1, 8:2 and 8:3 for copolymer 1, 2 and 3, respectively. c) The reversible specific capacitance changes of the supercapacitor accompanying the sol–gel electrolyte transitions. Reproduced with permission.[81] Copyright 2015, Wiley. d) Schematic view of a safe battery design. In normal LIBs, the separator will melt upon overheating, resulting in internal circuit shorting between the anode and the cathode. In the safe battery, the current collector of the positive electrode is coated with a thin TRPS layer. The TRPS layer has high conductivity at room temperature and ensures normal operation of the battery. Once the temperature exceeds the transition temperature, the TRPS becomes insulating as a result of expanded polymer matrix and contact loss of the embedded conductive fillers, completely shutting down the battery. e) Change of resistance of PE-based matrices with different conductive fillers in response to overheating. CB is short for carbon black. f) Cycling performance of the safe battery under repeated temperature switching from 25 °C to 70 °C. Reproduced with permission.[83] Copyright 2016, Nature Publishing Group.

and adjusting the ratio between the polymer and GrNi. The sensitivity of this TRPS material was superior, and the changes in σ levels were reversible with varied temperatures. Furthermore, a safe battery was constructed by coating a TRPS layer

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onto the cathodic current collector, which responded quickly to overheating. After cycling at 25 °C, the capacity of the smart battery immediately dropped to nearly zero when the temperature reached 70 °C. When the battery was cooled down to 25 °C,

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the capacity was restored to its initial level with no significant sacrifice of energy storage performance (Figure 3f). Since the changes in σ of the TRPS film are reversible, such smart batteries are capable of repeatable self-protection and performance recovery with a fast reaction speed.

3. Smart Function for Self-Recovery Recently, intelligent energy storage systems which have robust performance against mechanical damage and deformation are emerging. Along with the extensive progress in the development of dynamic materials, e.g., healable and deformable shape-recovery materials, some multifunctional smart energy storage devices have been endowed with self-healing and shapememory abilities.

3.1. Self-healing Abilities in Response to External Mechanical Damage Portable and wearable energy storage devices are in high demand due to the increasing popularization of personalized

electronics.[84–87] However, these devices are inevitably susceptible to structural fracture and mechanical damage during practical operation, resulting in malfunction, inconvenience brought by usage interruption, electronic waste and safety hazards.[34,88] Motivated by living organisms whose injuries can be healed spontaneously by innate regenerative systems, smart energy storage devices with self-healing ability in response to external mechanical damage are very appealing, and are being studied intensely,[34,89,90] facilitated by the rapid development of healable materials.[91–97] As a consequence, the service time, reliability and durability of these devices can be significantly enhanced. The first prototype of a self-healing supercapacitor was developed by Wang et al. (Figure 4a).[98] An artificial healable material based on a supramolecular network was introduced, and the dynamic and reversible hydrogen bonding within the network realized the healable functionality of the material in response to mechanical damage. Since the material displayed gel-like properties due to its low glass transition temperature below room temperature, 47 wt% of TiO2 nanoflowers was incorporated to elevate the overall mechanical strength in order to obtain a free-standing substrate. The as-prepared composite still maintained the healable capability, and could be

Figure 4.  a) Schematic for the design and the fabrication process of a smart supercapacitor with self-healing capability. Numerous hydrogen-bond acceptors (blue rods) and donors (green rods) exist in the supermolecular network (red wires) of the self-healable material. In the composite, hierarchical flower-like TiO2 nanostructures (black spheres) were mixed with the healable material, and the composite was thermally compressed to form a free-standing substrate. By spreading CNT films as active materials onto the self-healing substrates, the composite electrodes were formed and assembled into a symmetric self-healing supercapacitor. b) Optical microscope images of the composite comprising 47 wt% TiO2 after cutting (b1) and scar healing (b2) after 5 min at 50 °C. c) Photographs of a flexible self-repairing electrode on a PET sheet modified with CNT films (c1) and an integrated self-healing supercapacitor (c2). d) Specific capacitances of the original supercapacitor, and of the same device after self-healing for increasing number of times. The inset illustrates a schematic of the structure and self-healing principle of the device. Reproduced with permission.[98] Copyright 2014, Wiley. e) Schematic illustration of a self-healable fiber-shaped electrode. Reproduced with permission.[33] Copyright 2014, Wiley.

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completely healed in 5 min at 50 °C after cutting (Figure 4b). A film composed of single-walled carbon nanotube (SWCNT) was spread onto the self-recovery substrate to fabricate the electrodes, and a poly(vinyl alcohol) (PVA)-H2SO4 gel with a certain capacity for self-recovery was employed as both the electrolyte and the separator (Figure 4c). The resulting symmetric supercapacitor demonstrated a good specific capacitance of 35 F g−1 as well as a self-healing capability upon complete cutting. Lateral movements of the substrates during the selfrecovery process brought the separated areas of the SWCNT layer into contact to efficiently restore the device’s mechanical integrity and electrode conductivity. The capacitance of the selfrepairing supercapacitor can be restored to nearly 85.7% of the original value after 5 cutting/healing cycles (Figure 4d). With a similar architectural design containing self-recovery substrates and coatings of electrochemical active materials, self-healable energy storage devices in other shapes have also been reported (Figure 4e).[33] Compared to large planar or fibrous electrodes, the delicate electrodes with multiple tiny components are more difficult to precisely reconnect in order to restore electric conductivity after damage. To address this challenge, Huang et al. further designed a magnetism-assisted self-healing supercapacitor (Figure 5a).[99] A stainless steel yarn composed of numerous tiny fibers was uniformly and firmly covered with Fe3O4 nano­ particles which possess permanent magnetism. After that, a 2-µm-thick layer of polypyrrole (PPy) was electrodeposited onto the steel yarn with adhering Fe3O4 nanoparticles. The PPy layer not only served as the pseudocapacitive energy storage material, but also protected the magnetic Fe3O4 particles from falling off the composite yarn electrode. Two such composite electrodes were covered with a PVA-H2SO4 gel electrolyte and assembled together to form a symmetric supercapacitor. Finally, the supercapacitor was wrapped in a self-healable carboxylated polyurethane (PU) shell. After complete cutting, the outermost PU shell is responsible for the integrity restoration of the asprepared supercapacitor via the dynamic hydrogen bonds in the self-healable supramolecular PU network, while the magnetic force helped to accurately align each of the tiny bisected fibers within the electrodes. With the synergistic effect of both the PU shell and the magnetic electrodes, this smart supercapacitor was able to self-recover its electrochemical performance after mechanical disruption. After four cutting/healing cycles, the magnetism-assisted self-healable supercapacitor retained a specific capacitance of 44.1 mF cm−2, i.e., 71.8% of its initial specific capacitance (61.4 mF cm−2). Such design strategies offer the promise of efficient self-repair after breakage, even for energy storage devices with complex cross-sections. Although some prototypes of self-mending supercapacitors have been successfully demonstrated, their self-healing efficiencies are not satisfying, since their specific capacitances significantly decreased by 14.3 to 28.2% within 5 breaking/ healing cycles.[100–102] Among these studies, efforts were mainly made concerning the recovery of electrode conductivities and integrity of the overall the configuration. Furthermore, the conventional PVA-based electrolytes with relatively low self-repair capability were frequently employed, which likely limited the overall self-healing performance of the corresponding supercapacitors. Consequently, novel polymeric electrolytes which

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are intrinsically self-healable have been studied further.[100–102] Recently, Huang et al. has reported a new self-healing electrolyte composed of polyacrylic acid and hybrid vinyl-silica nanoparticles (VSNPs-PAA) (Figure 5b).[103] The PAA polymer chains provided abundant reversible intermolecular hydrogen bonding, while the VSNPs reinforced the mechanical properties and covalently crosslinked the PAA polymer chains. By varying the water and proton ion content in the VSNPsPAA electrolyte, its ionic conductivity could be tuned over a wide range. Benefitted from the dynamic hydrogen bonds crosslinking the VANPs-PAA network, the polyelectrolyte was not only self-healable but also stretchable to >3700%. A symmetric supercapacitor was assembled using a VSNPs-PAA film as the electrolyte, and CNT papers electrodeposited with PPy served as the electrodes (Figure 5c). This supercapacitor displayed a comparable energy storage performance to those using conventional electrolytes, indicating that the VSNPsPAA film is a promising multifunctional electrolyte that can be used to replace conventional PVA-based electrolytes without compromising the electrochemical performance. Confronted with complete cutting, the as-prepared supercapacitor demonstrated a healing efficiency of ≈100%, assisted by a gentle pressure as well as some small patches of CNT paper placed on the “wounds” to restore the electrical conductivity of the electrodes (Figure 5c). Such a patch-assisted non-autonomic supercapacitor could restore ≈100% of its initial energy-storage performance even after the 20th cutting (Figure 5d). In comparison to self-mending supercapacitors, the development of self-healing lithium-ion batteries is more challenging, due to the difficulties in constructing appropriate self-healable battery electrodes and the safety hazard posed by the exposure of the organic electrolytes to air. While silicon anodes with healing capabilities have already been employed to extend the cycle life of lithium ion batteries,[97] development of selfrecovery electrolyte that can be safely used in rechargeable batteries remains to be a technical hurdle. The first demonstration of a self-healable aqueous lithium-ion battery was reported by Zhao et al.[104] LiMn2O4 and LiTi2(PO4)3 nanoparticles with diameters of several hundred nanometers were uniformly attached onto an aligned CNT sheet, respectively. A number of such aligned CNT sheets were then stacked onto the former composite sheets to form the CNT/LiMn2O4/CNT and CNT/ LiTi2(PO4)3/CNT films. The sandwiched configuration of these films with embedded LiMn2O4 or LiTi2(PO4)3 nanoparticles could prevent the leakage of the active materials upon fracture. Further, these CNT/LiMn2O4/CNT and CNT/LiTi2(PO4)3/CNT films adhered to a self-healing polymer substrate, and formed the cathode and anode, respectively. Once the electrodes were cut, their electrical conductivities could self-repair with the aid of the dynamic hydrogen bonds in the self-healing polymers and the Van der Waals forces between the broken aligned CNT films, whereby their electrical resistances only increased by 2.3% after a single cutting/healing cycle. Aqueous Li2SO4/ sodium carboxymethylcellulose (CMC), which has a considerable ionic conductivity of 0.12 S cm−1 at room temperature, was adopted as the gel electrolyte. In contrast to the conventional organic electrolytes which are normally toxic and combustible, the Li2SO4/CMC gel electrolyte is relatively safe, nontoxic and stable in air. A self-healing aqueous lithium-ion battery was

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Figure 5.  a) Schematic illustration of the self-healing process of the yarn-based supercapacitor with the assistance of magnetic attraction. The inset image illustrates the reconnection of the tiny fibers in the broken yarn electrodes when the seperated parts are brought together. Reproduced with permission.[99] Copyright 2015, American Chemical Society. b) Preparation of the VSNPs-PAA electrolyte via polymerization using acrylic acid (AA) monomer and VSNPs crosslinker in the presence of ammonium persulfate (APS) as the initiator and phosphoric acid as a regulator of pH and water content. c) Schematics for the configuration of a solid-state supercapacitor composed of a VSNPs-PAA polyelectrolyte and PPy@CNT paper composite electrodes (left), as well as the non-autonomic self-healing process with the assistance of small patches of conductive CNT paper (right). d) Healing efficiency during breaking/healing cycles calculated from CV (red circles) and GCD (blue stars) tests. Reproduced with permission.[103] Copyright 2015, Nature Publishing Group.

assembled using the self-repairing anode and cathode, together with the Li2SO4/CMC gel electrolyte, which displayed a considerable specific capacity of 28.2 mAh g−1 and a high energy density of 32.04 Wh kg−1. Its specific capacitance was maintained at 69.3% after five repeated cutting-healing cycles, and the healed lithium battery after the fifth fracture still kept a good rate capability and cycling performance. Even more importantly, no safety events happened when the battery was damaged.

3.2. Shape-Memory Ability in Response to External Mechanical Deformation When energy storage devices are subjected to practical applications, they are possibly confronted with deformation over time and then performance decay. Recently, in order to facilitate durable and reliable power supply especially for wearable electronics, shape memory energy storage devices are emerging, which have the appealing ability to memorize a pre-designed shape and recover it after deformation in response to external stimuli, such as heat, magnetic force, pressure, etc.[24,34,35,105] Shape memory materials are the fundamental components of such devices, and mainly contain three types of materials, i.e., shape-memory polymers, alloys and some ceramic materials.[106–108]

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In the past two years, shape memory materials have been applied as substrates or cores to support charge-storage active components, and have been assembled into plane- or fibershaped memory supercapacitors. Yan et al. reported a smart design for a watchband-like supercapacitor based on a TiNi alloy, with a shape memory ability inducible by body temperature (Figure 6a).[109] The shape-memory TiNi alloy is renowned to its outstanding flexibility, stability and biocompatibility, and its shape restoration is driven by the reversible crystalline phase transformation between martensite and austenite triggered by heat. For device assembly, reduced graphene oxide (rGO) coated on TiNi alloy (TNA) flake was used as the negative electrode, while MnO2 deposited on ultrathin Ni foil served as the positive electrode. And aqueous- or ion-liquid-based gel electrolyte also functioned as the separator. The as-prepared devices not only exhibited stable electrochemical properties under different bending conditions, but also demonstrated a distinctive shapememory ability. Such supercapacitor was used to constitute the smart watchband of an electronic watch, which could realize the integrated functions of powering the electronic watch and automatically wrapping around the human wrist (Figure 6b). The phase transition temperature of the TiNi alloy used in this work was 15 °C. When this watchband touched the human wrist, the skin temperature of about 35 °C could directly actuate

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Figure 6.  a) Schematic configuration of a shape-memory supercapacitor. b) The shape memory process of a watchband-like supercapacitor induced by contact with a warm human wrist. Reproduced with permission.[109] Copyright 2016, Wiley. c) CV curves of the SMSCs before and after shape restoration. d) Illustration of the SMSCs interwoven with traditional yarns, and the fabrication process of the smart energy storage textiles (17 × 12 cm) with a shape-recovery capability. Reproduced with permission.[36] Copyright 2016, Royal Society of Chemistry.

its shape restoration to the preprogrammed ring shape, and thus the capacitor could spontaneously wrap around the wrist. Similarly, smart energy storage textiles have been demonstrated by Zhi’s group on the basis of supercapacitors based on a fibrous TiNi alloy.[36] The TiNi alloy served as the current collector and substrate for the deposition of active materials. This flexible wire-shaped device showed excellent electrochemical stability before and after shape recovery, conserving 96% of the initial capacitance after shape restoration (Figure 6c). This fibrous supercapacitor can be interwoven into traditional fabrics to construct a shape-memory textile with energy storage capability (Figure 6d). Furthermore, the shape memory elements can endow the smart textile with additional functionalities, such as temperature alarm or automatic cooling when the temperature is high, demonstrating the dramatic potential of shape-memory materials in smart wearable electronics. Despite the superior mechanical and electrical properties of shape-recovery alloys, the corresponding electronic systems may suffer from an increased weight burden, posing problems for the design of lightweight devices. By contrast, shape-memory polymers are frequently lighter, and can achieve reversible deformation driven by the elastic spring energy stored in the elastic domain of the polymer. By winding aligned CNT sheets on a shape-memory polyurethane substrate to construct deformable electrodes and employing PVA gel electrolyte, Peng et al. fabricated a shape-memory, fiber-shaped supercapacitor (SFSCs) (Figure 7a).[35] Such a supercapacitor could be deformed and fixed into various shapes by bending

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or stretching when the temperature exceeds the transition temperature. After cooling down to room temperature, the polymer network of polyurethane will be physically cross-linked to maintain the deformed shape. Once the temperature exceeds the thermal transition temperature again, the physical crosslinks in the middle phase will be cleaved, and the supercapacitor can restore to its original linear shape from bending or elongating states. At the same time, the electrochemical performances of the supercapacitor were well maintained during the deformation and shape recovery processes, even after 500 cycles of deformation and shape restoration at an elongation strain of 50% (Figure 7b). Due to its fibrous shape and flexibility, this shape-memory supercapacitor was successfully woven into electronic textiles via in-series or parallel connection (Figure 7c,d). Consequently, the resulting textiles possessed not only an augmented electrochemical performance for self-powering but also a shape-recovery capability to realize smart functionalities.

4. Conclusion The development of intelligent energy storage devices is well on its way towards a smart era of electronics. The latest research focusing on electrochemical energy storage devices capable of self-protecting or self-adapting in response to external or internal stimuli is summarized. In particular, active strategies to establish innate intelligent devices, which facilitate coping with internal shorting, overheating, mechanical damages and

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Figure 7.  a) Schematic of fiber-shaped supercapacitors comprising a shape-memory polyurethane substrate and the resulting textile, which can be automatically recovered from flexural or elongated states to the original shape, once the temperature exceeds Ttran. b) Capacitance ratio of SFSCs before and after different numbers of deformation cycles. C0 and C represent the specific capacitances before and after stretching restoration, respectively. c) GCD curves of SFSCs connected in series (left) and parallel (right), respectively. The current density of the GCD tests was 0.5 A g−1. d) The photograph of the resulting textiles woven from SFSCs. Reproduced with permission.[35] Copyright 2015, Wiley.

deformation, are reviewed. These exciting developments and novel ideas aim to significantly improve user experience by addressing the fundamental challenges of present energy storage systems, i.e., safe, reliable and durable operation. Despite the success of many proof-of-concept demonstrations, further improvements need to be made before the practical application of these lab-scale smart devices. In particular, the reaction times of available sol–gel transiting electrolytes, healable materials and shape-memory materials are relatively long, which necessitates the development of novel materials with faster reaction speeds. In addition, system optimization or other smart designs are needed to improve the compatibility between the incorporated smart materials and the resting energy storage components in the host device, in order to develop multifunctional intelligent electronics without significantly sacrificed electrochemical energy storage performance or volumetric energy density. Moreover, by utilizing other smart materials and dynamic materials as well as taking inspirations from natural sophisticated systems, the prototypes of smart rechargeable energy storage systems could be greatly enriched.

Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (51502009, 51532001), National Key

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Basic Research Program of China (2014CB31802) and the 111 Project (B14009).

Conflict of Interest The authors declare no conflict of interest.

Keywords batteries, electrochemistry, self-protection

energy

storage,

self-adaptation,

Received: May 31, 2017 Revised: June 28, 2017 Published online:

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