Natural Rubber-Based Ionogels

Natural rubber(NR),besides being an abundant renewable resource for the elastomer industry,can be a potential resource for the design of innovative biobased polymer networks.The present work is based on“telechelic”liquid natural rubber oligomers obtained by controlled chemical degradation of NR.The chain ends of such oligomers can then be functionalized(with acrylate functions in the present case)and reacted with multifunctional crosslinkers in order to form networks.What’s more,the initial solubility of such thermosetting system in an ionic liquid(IL)can be used for the formulation of ionogels.Such solid networks typically containing 80%of IL were produced,resulting in high ionic conductivity performances.The oligomer chain length was shown to affect IL fragility due to confinement and specific interactions of ions with the host polymer network.

Polymer ionogels and their application in flexible ionic devices

Polymer ionogel(PIG)is a new type of flexible,stretchable,and ion-conductive material,which generally consists of two components(polymer matrix materials and ionic liquids/deep eutectic solvents).More and more attention has been received owing to its excellent properties,such as nonvolatility,good ionic conductivity,excellent thermal stability,high electrochemical stability,and transparency.In this review,the latest research and developments of PIGs are comprehensively reviewed according to different polymer matrices.Particularly,the development of novel structural designs,preparation methods,basic properties,and their advantages are respectively summarized.Furthermore,the typical applications of PIGs in flexible ionic skin,flexible electrochromic devices,flexible actuators,and flexible power supplies are reviewed.The novel working mechanism,device structure design strategies,and the unique functions of the PIG-based flexible ionic devices are briefly introduced.Finally,the perspectives on the current challenges and future directions of PIGs and their application are discussed.

Ultrastable Viologen Ionic Liquids-Based Ionogels for Visible Strain Sensor Integrated with Electrochromism,Electrofluorochromism,and Strain Sensing

Flexible electronics play a key role in the development of human society and our daily activities.Currently they are expected to revolutionize personal health management.However,it remains challenging to fabricate smart sensors with high robustness,reliability,and visible readout.Herein,high-performance electrochromic(EC),electro-fluorochromic(EFC),and double-network ionogels with excellent transmissivity,high mechanical robustness,and ultrastable reversibility are prepared by combination of thienoviologen-containing ionic liquids with poly(ethyl acrylate)elastomer.The ionogels exhibit good mechanical properties(1000%stretchability and 3.2 kJ m^(−2) fracture energy).The ionogel-based EC devices have a significantly simplified device fabrication process as well as superior cycling stability in which 88%of the contract ratio is maintained at 88%at 500 cycles,even after being stored for 2 years under ambient atmosphere(relative humidity:30%∼40%,25°C).The conductivity of ionogels showed a fast and reproducible response to strain,and the conductivity decreased with increased strain.By virtue of the EC and EFC properties of the thienoviologen component,the EC and EFC efficiency decreased with the increased strain loaded on the ionogels,and almost no EC or EFC phenomena were observed when the strain was above 300%.This feasible strategy provides an opportunity for the development of visible strain sensors to monitor the body’s movements through color and fluorescence emission.

Durable and stretchable nanocomposite ionogels with high thermoelectric property for low-grade heat harvesting

Stretchable ionic thermoelectric(i-TE) materials have attracted growing interest in converting low-grade thermal energy into electricity. However, substantial improvement on i-TE performance of quasi-solid ionogels remains a significant challenge.Here, a nanocomposite ionogel with skin-like stretchability, high i-TE performance, thermostability and durability is prepared by hybridizing ionic liquid(IL) and Laponite nanosheets into waterborne polyurethane(WPU). With multiple H-bond, WPU can accommodate a higher content of IL, thereby improving its ionic conductivity. After cation exchange between IL and Laponite,the negatively charged Laponite sheets and released Na+can enhance the ionic Seebeck coefficient by enlarging thermophoretic mobility difference between the cations and anions in ionogel. Besides, incorporation of Laponite causes the decrease of thermal conductivity. Thus, the WPU-IL-Laponite ionogel exhibits a high ionic thermopower of 44.1 m V K-1, high ionic conductivity of 14.1 m S cm-1and low thermal conductivity of 0.43 W m-1K-1at a relative humidity of 90%. The corresponding ionic figure of merit of the ionogel is 1.90±0.27. Moreover, the ionogel demonstrates excellent durability during repeated stretching process.The stretchable ionogel can be fabricated into ionic thermoelectric capacitor to convert thermal energy from solar radiation into electricity.

Finely Tuning the Lower Critical Solution Temperature of Ionogels by Regulating the Polarity of Polymer Networks and Ionic Liquids

Nonvolatile ionogels have recently emerged as promising soft electrolyte materials due to their high ionic conductivity and good durability.However,the compatibility between polymer networks and ionic liquids(ILs),which show significant influence on the physicochemical properties of the ionogels,has been rarely studied.Herein,we elucidate a lower critical solution temperature(LCST)-type phase behavior of ionogels composed of polyacrylates and hydrophobic 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ILs.We systematically study the structural effects of ILs and monomers on the LCST of ionogels.Our work illustrates that the LCST of ionogels is primarily determined by the polarity of polymer side chains and the alkyl chain on cations of ILs.The oriented solvation between polymers and ILs caused by hydrogen-bonding effects and van der Waals interactions may serve as the driving force for the LCST phase behavior in our system.Furthermore,by varying the mixing ratio of two structurally similar ILs in their blends,the LCST of ionogels can be tuned to exhibit a linear variation within a wide temperature range(from subzero to over 200℃).Finally,thermoresponsive ionogels with desired patterns are prepared using photomasks.These nonvolatile ionogels with tunable LCST enriched the functionality of state-of-the-art ionogels,which provides insight into the design and fabrication of smart and flexible electronic/optical devices.

Boosting lithium batteries under harsh operating conditions by a resilient ionogel with liquid-like ionic conductivity

New chemistries are being developed to increase the capacity and power of rechargeable batteries. However, the risk of safety issues increases when high-energy batteries using highly active materials encounter harsh operating conditions. Here we report on the synthesis of a unique ionogel electrolyte for abuse-tolerant lithium batteries. A hierarchically architected silica/polymer scaffold is designed and fabricated through a facile soft chemistry route, which is competent to confine ionic liquids with superior uptake ability (92.4 wt%). The monolithic ionogel exhibits high conductivity and thermal/mechanical stability, featuring high-temperature elastic modulus and dendrite-free lithium cycling. The Li/LiFePO_(4) pouch cells achieve outstanding cyclability at different temperatures up to 150 ℃, and can sustain cutting, crumpling, and even coupled thermal–mechanical abuses. Moreover, the solid-state lithium batteries with LiNi_(0.60)Co_(0.20)Mn_(0.20)O_(2), LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2), and Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_(2) cathodes demonstrate excellent cycle performances at 60 ℃. These results indicate that the resilient and high-conductivity ionogel electrolyte is promising to realize high-performance lithium batteries with high energy density and safety.

High-Performance Ionic Thermoelectric Supercapacitor for Integrated Energy Conversion-Storage

Converting low-grade waste heat into usable electricity and storing it simultaneously requires a new technology that realize the directional migration of electrons or ions under temperature difference and enrichment on the electrodes.Although the urgent demand of energy conversion-storage(ECS)has emerged in the field of wearable electronic,achieving the integrated bi-functional device remains challenge due to the different mechanisms of electrical transportation and storage.Here,we report an ionic thermoelectric supercapacitor that relies on the synergistic functions of thermoelectricity and supercapacitor in the thermoelectric ionogel electrolyte and high-performance hydrogel electrodes to enhance the ECS performance under a thermal gradient.The thermoelectric electrolyte is composed of polyacrylamide hydrogel and sodium carboxymethyl cellulose(PMSC),possessing cross-linked network with excellent cation selectivity,while the ionic thermoelectric properties are further improved in the presence of NaCl.The corresponding Seebeck coefficient and ionic conductivity of the NaCl–PMSC electrolyte reach 17.1 mV K^(-1)and 26.8 mS cm^(-1),respectively.Owing to good stretchability of both gel-based electrolyte and electrode,the fullstretchable integrated ECS device,termed ionic thermoelectric supercapacitor,presents promising thermal-charge storage capability(~1.3 mC,ΔT≈10 K),thus holds promise for wearable energy harvesting.

An interfacial robust and entire self-healing ionogel-elastomer hybrid for elastic electronics enables discretionary assembly and reconfiguration

Inspired by the multi-layer architecture of mammal skins,interfacial robust,stretchable,and entirely healable gel-elastomer hybrids hold great potential in diverse fields including biomedical devices,wearable electrical devices,and soft robotics.However,existing gel-elastomer hybrids have numerous limitations including low interfacial bonding toughness,complex and time-consuming preparation process,unhealable,and non-reconfiguration.Herein,we propose a simple and general chemical strategy through the interfacial dynamic bonding between gel and elastomer to simultaneously address the abovementioned obstacles.Dynamic covalent bonds readily and repeatably covalent bonding ionogel and elastomer(interfacial toughness:390 J m^(-2)),endowed the hybrids with entire self-healing features like skin and enabled discretionary assembly and reconfiguration.Moreover,this strategy resolved the troublesome contradiction between interfacial stability and reconfiguration.Taking advantage of the aforementioned features,we readily constructed a multi-module,self-healing,self-powered,and realtime monitoring of personal status integrated elastic electronics,which could simply reconfigure the output signal of elastic electronics into an input signal of the devices-braille keyboard.

Highly stretchable,conductive,and wide-operating temperature ionogel based wearable triboelectric nanogenerator

The rapid development of wearable electronic products brings challenges to corresponding power supplies.In this work,a thermally stable and stretchable ionogel-based triboelectric nanogenerator(SI-TENG)for biomechanical energy collection is proposed.The ionic conductivity of the ionogel increased to 0.53 S·m^(−1) through optimal regulation of the amount of aminoterminated hyperbranched polyamide(NH2-HBP),which also has high strain of 812%,excellent stretch recovery,and wide operating temperature range of−80 to 250°C.The SI-TENG with this ionogel as electrode and silicone rubber both as the triboelectric layer and encapsulation layer exhibits high temperature stability,stretchability,and washability.By adding appropriate amount of nano SiO2 to triboelectric layer,the output performance is further improved by 93%.Operating in singleelectrode mode at 1.5 Hz,the outputs of a SI-TENG with an area of 3 cm×3 cm are 247 V,11.7μA,78 nC,and 3.2 W·m^(−2),respectively.It was used as a self-charging power supply to charge a 22μF capacitor to 1.6 V in 167 s with the palm patting and then to power the electronic calculator.Furthermore,the SI-TENG can also be used as a self-powered motion sensor to detect the amplitude and frequency of finger bending,human swallowing,nodding,and shaking of the head motion changes through the analysis of the output voltage.

Hierarchically Crosslinked Gels Containing Hydrophobic Ionic Liquids towards Reliable Sensing Applications

Human skin can function steadily regardless of surrounding circumstarices(dry or wet),while it is still a challenge for artificial ionic skins,which tend to release solvents in dry air and leach electrolytes in wetted state.Herein,a series of hierarchically crosslinked ionogels containing hydrophobic ionic liquids(ILs)is fabricated by combining a crystalline fluorinated copolymer with hydrophobic ILs.With a reasonable combination of nonvolatility,transparency,stretchablility,and sensitivity,such ionogels can work as reliable sensors for real-time monitoring huma n motions and operate steadily in complex environments as human skin does,which can contribute to the developme nt of durable sen sing devices with a simple design.

Highly Stretchable,Elastic,Healable,and Ultra-Durable Polyvinyl Alcohol-Based Ionic Conductors Capable of Safe Disposal

Highly durable and stretchable ionic conductors are indispensable components of flexible electronics.However,fabricating such ionic conductors that are also non-toxic and biodegradable remains a challenge.In this study,highly stretchable,elastic,healable,and ultra-durable ionic conductors capable of non-hazardous disposal are conveniently fabricated by complexation of vanillin-grafted polyvinyl alcohol(VPVA)and ionic liquids(ILs)(denoted as VPVA-IL).

Transparent,stretchable,temperature-stable and self-healing ionogel-based triboelectric nanogenerator for biomechanical energy collection

A flexible and stable power supply is essential to the rapid development of wearable electronic devices.In this work,a transparent,flexible,temperature-stable and ionogel electrode-based self-healing triboelectric nanogenerator(IS-TENG)was developed.The ionogel with excellent stretchability(1,012%),high ionic conductivity(0.3 S·m^(−1))and high-temperature stability(temperature range of−77 to 250℃)was used as the electrode of the IS-TENG.The IS-TENG exhibited excellent transparency(92.1%)and stability.The output performance did not decrease when placed in a 60℃oven for 48 h.In addition,the IS-TENG behaved like a stable output in the range of−20 to 60℃.More importantly,the IS-TENG could also achieve self-healing of electrical performance at temperatures between−20 and 60℃and its output can be restored to its original state after healing.When the single-electrode IS-TENG with an area of 3 cm×3 cm was conducted under the working frequency of 1.5 Hz,the output values for open-circuit voltage,short-circuit current,short-circuit transferred charge,and maximum peak power density were 189 V,6.2μA,57 nC,and 2.17 W·m^(−2),respectively.The IS-TENG enables to harvest biomechanical energy,and drive electronic devices.Furthermore,the application of IS-TENGs as self-driven sensors for detecting human behavior was also demonstrated,showing good application prospects in the field of wearable power technology and self-driven sensing.

Highly bonded T-Nb2O5/rGO nanohybrids for 4 V quasisolid state asymmetric supercapacitors with improved electrochemical performance

Orthorhombic niobium pentoxide （T-Nb2O5）/reduced graphene oxide nanohybrids were fabricated via the hydrothermal attachment of Nb2Os nanowires to dispersed graphene oxide nanosheets followed by a high-temperature phase transformation. Electrochemical measurements showed that the nanohybrid anodes possessed enhanced reversible capacity and superior cycling stability compared to those of a pristine T-Nb205 nanowire electrode. Owing to the strong bonds between graphene nanosheets and T-Nb2O5 nanowires, the nanohybrids achieved an initial capacity of 227 mAh·g^-1. Additionally, non-aqueous asymmetric supercapacitors （ASCs） were fabricated with the synthesized nanohybrids as the anode and activated carbon as the cathode. The 3 V Li-ion ASC with a LiPF6-based organic electrolyte achieved an energy density of 45.1 Wh·kg^-1 at 715.2 W·kg^-1. The working potential could be further enhanced to 4 V when a polymer ionogel separator （PVDF-HFP/LiTFSI/EMIMBF4） and formulated ionic liquid electrolyte were employed. Such a quasi-solid state ASC could operate at 60℃ and delivered a maximum energy density of 70 Wh·kg^-1 at 1 kW·kg^-1.

Highly Transparent,Stretchable,and Self‑Healable Ionogel for Multifunctional Sensors,Triboelectric Nanogenerator,and Wearable Fibrous Electronics

Ionogels with high transparency,stretchability and self-healing capability show great potential for wearable electronics.Here,a kind of highly transparent,stretchable and self-healable ionogels are designed using double physical cross-linking including hydrogen bonding and dipole–dipole interaction.Owing to the dynamic and reversible nature of the ion–dipole interaction and hydrogen bonds of polymeric chains,the ionogel possesses good self-healing capability.The multifunctional sensors for strain and temperature are fabricated based on ionogel.The ionogel can serve as strain sensor that exhibited high sensitivity[gauge factor(GF)=3.06]and durability(1000 cycles)to a wide range of strains(0–300%).Meanwhile,the ionogel shows rapid response to temperature,due to the temperature dependence of its ionic conductivity.Furthermore,the ionogel fbers with excellent antifreezing(−20°C)capability are fabricated,and the fbers show the good sensing performance to human motions and temperature.Importantly,the antifreezing ionogel-based triboelectric nanogenerator(ITENG)is assembled for efcient energy harvesting.The ITENG shows a short circuit current(ISC)of 6.1μA,open circuit voltage(VOC)of 115 V,and instantaneous peak power density of 334 mW m−2.This work provides a new strategy to design ionogels for the advancement of wearable electronics.

Organosilicon-group-derived silica-ionogel electrolyte for lithium ion batteries

In order to avoid leakage problem caused by liquid electrolyte, a new ionogel electrolyte was developed by in situ immobilizing organosilicon-functionalized ionic liquid within a nanoporous silica matrix. The ionic liquid evenly coats on the surface of porous silica and fills in the silica framework pores with no strong chemical interaction. The ionogel electrolyte has the dual advantages of a silica solid support and a wide electrochemical stability window of ionic liquid （4.87 V vs. Li^＋/Li）. The half-cells assembled with this electrolyte and LiFePO4 electrode have excellent performance at room temperature and 60 ℃. The Li/SiO2-IGE/LiFePO4 cell displays a discharge capacity of 129.1 mAh·g^-1 after 200 charge/discharge cycles at room temperature.

Ionogel-based flexible stress and strain sensors

Ionogels is a kind of hybrid materials composed of ionic liquids(ILs)and solid polymer network matrix,has been extensively investigated in the most recent decade.Due to the excellent mechanical properties and ionic conductivity,their promising applications in flexible stress and strain sensors have been proposed and explosively developed.In this review,we briefly summarize research progresses on ionogel based flexible stress and strain sensors(IFSSs)from five aspects,including material synthesis,device fabrication,working principles,characteristics and performances,and potential applications.Some outlooks and perspectives are also proposed at the end of review.The review is expected to provide reference and new insights into the research of IFSS.