Highly Conductive, Flexible, and Nonflammable Double-Network Poly(ionic liquid)-Based Ionogel Electrolyte for Flexible Lithium-Ion Batteries

High-Performance Ionogels from Dynamic Polyrotaxane-Based Networks

The swelling of a polymer matrix by ionic liquids and additional lithium salts may lead to the formation of ionogel electrolytes. However, the introduction of additional ions usually results in a decreased lithium-ion transference number, because of the trapping of the lithium ions in clusters and polymer-ion complexes. Achieving highly efficient lithium-ion migration and increasing lithium-ion transference number (tLi +) are however crucial for the successful application of ionogel electrolytes. Herein, we design a crosslinked polyrotaxane network and then introduce ionic liquid and a lithium salt to obtain an ionogel electrolyte based on the principle of competitive coordination with the lowest binding energy for lithium ions coordinated with both the polymer network and ionic liquid clusters. This facilitates their migration within the ionogel and their release from the coordination environment, thereby improving lithium-ion transport efficiency (ionic conductivity of 2.2×10-3 S cm-1 and tLi +=0.45 at 20 °C). As proof of concept, the lithium-lithium symmetrical cells achieve stable cycling for 2000 hours, while the NMC622||Li battery demonstrates good rate performance and excellent cycling stability at 20 °C (theoretical initial capacity, 300 cycles with a single cycle capacity loss of 0.03 %). This work provides new insights for the design and synthesis of ionogel electrolytes, facilitating lithium ion migration within the electrolytes.

Regulated solvation structure and solid electrolyte interfaces via imidazolium ionic gel electrolytes with high Li-ion transference number for Li-metal batteries

Solid lithium metal batteries (LMBs) are faced with problems such as the solvation structure of lithium ion and the instability of solid electrolyte interface (SEI), which lead to poor cycling stability and anode interface damage. Here, the introduced 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMIM][FSI]) ionic liquid (IL) interacts strongly with Lithium salt to form a new ionic gel electrolyte (IGE) based on the poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), which facilitates the excellent Li-ion transference number up to 0.506 and improves the mechanical properties. The reconstruction of Li-ion solvation environment by [EMIM][FSI] and PVDF-HFP functional groups, as well as the formation of SEI protective layer rich in LiF and Li3N, make the IGE with excellent electrochemical properties and interfacial stability. As a result, the Li||Li symmetric batteries demonstrate outstanding cycle stability, while the LiFePO4||Li batteries exhibit a superior capacity of 154.04 mAh/g at 0.2 C, maintaining a capacity retention rate as high as 94.5 % even after 200 cycles. The results not only demonstrate a new approach to improve Li-ion transference number in IGEs, but also open a potential avenue to achieve LMBs with high performance.

Hypercrosslinked Metal-Organic Polyhedra Electrolyte with High Transference Number and Fast Conduction of Li Ions

Solid-state electrolytes (SSEs) with high Li-ion transference numbers and fast ionic conductivity are urgently needed for technological innovations in lithium-metal batteries. To promote the dissociation of ion pairs and overcome the mechanical brittleness and interface defects caused by traditional fillers in polymeric electrolytes, we designed and fabricated a cationic hypercrosslinking metal-organic polyhedra (HCMOPs) polymer as SSE. Benefiting a three-component synergistic effect: cationic MOPs, branched polyethyleneimine macromonomer and polyelectrolyte units, the Li-HCMOP electrolyte possesses a high Li-ion conductivity, a high Li-ion transference number and a low activation energy. The LiFePO4/Li battery exhibits high capacity with superior rate performance and cycling stability. Moreover, soluble MOPs serve as high crosslinking nodes to provide excellent mechanical strength for electrolytes and good compatibility with polymers. This work highlights an effective idea of high-performance MOP-based solid-state electrolytes applied in LMBs.

Recent Progress on Advanced Flexible Lithium Battery Materials and Fabrication Process

Flexible energy storage devices have attracted wide attention as a key technology restricting the vigorous development of wearable electronic products. However, the practical application of flexible batteries faces great challenges, including the lack of good mechanical toughness of battery component materials and excellent adhesion between components, resulting in battery performance degradation or failure when subjected to different types of deformation. It is imperative to develop flexible batteries that can withstand deformation under different conditions and maintain stable battery performance. This paper reviews the latest research progress of flexible lithium batteries, from the research and development of new flexible battery materials, advanced preparation processes, and typical flexible structure design. First, the types of key component materials and corresponding modification technologies for flexible batteries are emphasized, mainly including carbon-based materials with flexibility, lithium anode materials, and solid-state electrolyte materials. In addition, the application of typical flexible structural designs (buckling, spiral, and origami) in flexible batteries is clarified, such as 3D printing and electrospinning, as well as advanced fabrication techniques commonly used in flexible materials and battery components. Finally, the limitations and coping strategies in the practical application of flexible lithium batteries are discussed, which provides new ideas for future research.

Composite Ionogel Electrodes for Polymeric Solid-State Li-Ion Batteries

Realizing rechargeable cells with practical energy and power density requires electrodes with high active material loading, a remaining challenge for solid-state batteries. Here, we present a new strategy based on ionogel-derived solid-state electrolytes (SSEs) to form composite electrodes that enable high active material loading (>10 mg/cm2, ~9 mA/cm2 at 1C) in a scalable approach for fabricating Li-ion cells. By tuning the precursor and active materials composition incorporated into the composite lithium titanate electrodes, we achieve near-theoretical capacity utilization at C/5 rates and cells capable of stable cycling at 5.85 mA/cm2 (11.70 A/g) with over 99% average Coulombic efficiency at room temperature. Finally, we demonstrate a complete polymeric solid-state cell with a composite anode and a composite lithium iron phosphate cathode with ionogel SSEs, which is capable of stable cycling at a 1C rate.

Buoyancy-Driven Micro/-Nanomotors: From Fundamentals to Applications

The progression of self-powered micro/-nanomotors (MNMs) has rapidly evolved over the past few decades, showing applications in various fields such as nanotechnology, biomedical engineering, microfluidics, environmental science, and energy harvesting. Miniaturized MNMs transduce chemical/biochemical energies into mechanical motion for navigating through complex fluidic environments with directional control via external forces fields such as magnetic, photonic, and electric stimuli. Among various propulsion mechanisms, buoyancy-driven MNMs have received noteworthy recognition due to their simplicity, efficiency, and versatility. Buoyancy force-driven motors harness the principles of density variation-mediated force to overcome fluidic resistance to navigate through complex environments. Restricting the propulsion in one direction helps to control directional movement, making it more efficient in isotropic solutions. The changes in pH, ionic strength, chemical concentration, solute gradients, or the presence of specific molecules can influence the motion of buoyancy-driven MNMs as evidenced by earlier reports. This review aims to provide a fundamental and detailed analysis of the current state-of-the-art in buoyancy-driven MNMs, aiming to inspire further research and innovation in this promising field.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Spatial Effect on the Performance of Carboxylate Anode Materials in Na-Ion Batteries

Developing low-voltage carboxylate anode materials is critical for achieving low-cost, high-performance, and sustainable Na-ion batteries (NIBs). However, the structure design rationale and structure-performance correlation for organic carboxylates in NIBs remains elusive. Herein, the spatial effect on the performance of carboxylate anode materials is studied by introducing heteroatoms in the conjugation structure and manipulating the positions of carboxylate groups in the aromatic rings. Planar and twisted organic carboxylates are designed and synthesized to gain insight into the impact of geometric structures to the electrochemical performance of carboxylate anodes in NIBs. Among the carboxylates, disodium 2,2'-bipyridine-5,5'-dicarboxylate (2255-Na) with a planar structure outperforms the others in terms of highest specific capacity (210 mAh g-1), longest cycle life (2000 cycles), and best rate capability (up to 5 A g-1). The cyclic stability and redox mechanism of 2255-Na in NIBs are exploited by various characterization techniques. Moreover, high-temperature (up to 100 °C) and all-organic batteries based on a 2255-Na anode, a polyaniline (PANI) cathode, and an ether-based electrolyte are achieved and exhibited exceptional electrochemical performance. Therefore, this work demonstrates that designing organic carboxylates with extended planar conjugation structures is an effective strategy to achieve high-performance and sustainable NIBs.

Studies on Composite Solid Electrolytes with a Dual Selective Confinement Interface Structure of Anions for High-Performance Lithium Metal Batteries

Solid-state lithium batteries (SSLBs) have attracted much attention due to their good thermal stability and high energy density. However, solid-state electrolytes with low conductivity and prominent interfacial issues have hindered the further development of SSLBs. In this research, inspired from a selective confinement structure of anions, a novel HMOF-DNSE composite solid electrolyte with a dual selective confinement interface structure is proposed based on the semi-interpenetrating structure generated by poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), poly(di-n-butylmethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMATFSI), and a metal-organic frameworks MOF derivative (HMOF) as a filler. The dual-network structure of PVDF-HFP/PDADMATFSI combined with HMOF formed a dual selective confinement interface structure to confine out the movement of large anions TFSI-, thereby enhancing the transfer ability of Li+. Subsequently, the addition of HMOF further improves the transfer of Li+ by binding up TFSI- through its crystal structure. The results show that HMOF-DNSE possesses a high room-temperature ionic conductivity (0.7 mS cm-1), a wide electrochemical window (up to 4.5 V), and a high Li+ transfer number (tLi+) (0.56). LiFePO4/HMOF-DNSE/Li cell shows an excellent capacity of 141.5 mAh g-1 at 1C rate under room temperature, with a high retention of 80.1% after 500 cycles. The material design strategy, which is based on selective confinement interface structures of anions, offers valuable insights into enhancing the electrochemical performance of solid-state lithium batteries.

A self-healing polymerized-ionic-liquid-based polymer electrolyte enables a long lifespan and dendrite-free solid-state Li metal batteries at room temperature

The implementation of high-safety Li metal batteries (LMBs) needs more stable and safer electrolytes. The solid-state electrolytes (SSEs) with their advantageous properties stand out for this purpose. However, low Li/electrolyte interfacial instability and uncontrolled Li dendrites growth trigger unceasing breakage of the solid electrolyte interphase (SEI), leading to fast capacity degradation. In response to these shortcomings, a new type of polymer electrolyte with self-healing capacity is introduced by grafting ionic liquid chain units into the backbones of polymers, which inherits the chemical inertness against the Li anode, allowing high Li⁺ transport, wide electrochemical window, and self-healing traits. Benefiting from the strong external H-bonding interactions, the obtained polymer electrolyte can spontaneously reconstruct dendrite-induced defects and fatigue crack growth at the Li/electrolyte interface, and, in turn, help tailor Li deposition. Owing to the resilient Li/electrolyte interface and dendrite-free Li plating, the equipped Li|LFP batteries display a high initial specific capacity of 134.7 mA h g^-1, rendering a capacity retention of 91.2% after 206 cycles at room temperature. The new polymer electrolyte will undoubtedly bring inspiration for developing practical LMBs with highly improved safety and interfacial stability.

Highly conductive, Transparent, Adhesive and Self-healable Ionogel Based on a Deep Eutectic Solvent with Widely Adjustable Mechanical Strength

Ionogels have attracted intensive attentions as promising flexible conductive materials. However, simultaneous integration of excellent mechanical properties, high conductivity, outstanding self-healing ability and strong adhesiveness is still challenging. Here, an ingenious composition design is proposed to address this long-standing challenge of ionogels. High-performance PEI/PAA/CMC ionogels, consisted of a loosely cross-linked poly(acrylic acid) (PAA) network, dynamically cross-linked network based on polycationic polyethyleneimine (PEI) and polyanionic PAA, and carboxymethyl cellulose (CMC) reinforcing filler, are formed in a deep eutectic solvent (DES) composed of choline chloride and urea. Benefiting from the loose PAA network and dynamic noncovalent interactions, ionogels with both highly enhanced mechanical robustness and excellent conductivity are obtained at high loading of DES, overcoming the strength-ductility/conductivity trade-off dilemma. By adjusting PEI/PAA mass ratio, the tensile strength and strain of PEI/PAA/CMC ionogels are effectively controlled in a wide range of 0.15-7.9 MPa and 232-1161%, respectively, while maintaining the desirable conductivity of ∼10^-4 S cm^-1 . Besides, healed tensile strength over 2.1 MPa and adhesion strength up to 0.2 MPa are achieved for the PEI_0.06 /PAA_0.25 /CMC_0.01 ionogel. The delicate design strategy provides a feasible approach to prepare ionogels with outstanding comprehensive performance, which have potential for applications in flexible electronics. This article is protected by copyright. All rights reserved.

Rheological scaling of ionic-liquid-based polyelectrolytes in ionic liquid solutions: the effect of the ion diameter of ionic liquids

We investigate the effect of the ion diameter a of ionic liquids (ILs) on the shear viscosity of polymerized ionic liquids (PILs) in IL solutions. When both the PIL and IL contain large PFSI anions (a ≈ 0.57 nm), the specific viscosity η_sp first decreases with increasing IL concentration c_IL in the low c_IL regime, reaches a minimum and then increases with increasing c_IL in the high c_IL regime. By comparing the measured η_sp with the modified charge screening model proposed in our previous study [Matsumoto et al., Macromolecules, 2021, 54, 5648-5661], we attribute the observed non-monotonic trend of η_sp against c_IL to the charge underscreening phenomenon, i.e., an increase of the screening length at high c_IL leads to the upturn of η_sp. On the other hand, when the PIL and IL contain small BF₄ anions (a ≈ 0.34 nm), the η_sp decreases asymptotically with increasing c_IL, because the charge on the PIL chain is likely screened fully in the entire c_IL regime. Our results demonstrate that the ion diameter of ILs plays an important role in governing the charge screening mechanism of PILs in IL solutions, and thus influencing the viscoelastic properties of PIL solutions.

Novel poly(epichlorohydrin)-based matrix for monolithic ionogel electrolyte membrane with high lithium storage performances

Based on gelling matrices and ionic liquids (ILs), monolithic ionogel electrolyte membranes (MIEMs) have become a research focus. However, further application is limited by lack of functional matrices. Herein, we proposed the introduction of an ionized polymer, i.e., polyether polymer with side-chain ionic groups obtained via the reaction of quaternary ammonium with uncrystallizable poly (epichlorohydrin) (PECH), as the matrix into the gels to balance the mechanical properties and the ionic conductivity. In combination with lithium bis-(fluorosulfonyl) imide (LiFSI) and 1-ethyl-3-methylimidazolium bis-(fluorosulfonyl)-imide (EMImFSI) via a solvent casting technique, a flexible MIEM was successfully prepared. The as-obtained MIEM exhibited good thermal stability (up to about 250 °C) and a high ionic conductivity of 1.21 mS cm-1 at 20 °C. Moreover, Li|LiFePO4 coin cells using this MIEM delivered high capacity (150.0 mA h g-1 at 0.2C) with good cycling stability, and an excellent C-rate response. This work discloses a novel and paramount route to exploit PECH-based MIEMs for Li storage, as well as energy storage systems beyond Li.

Ternary Deep Eutectic Solvent (DES) with a Regulated Rate-Determining Step for Efficient Recycling of Lithium Cobalt Oxide

Deep eutectic solvents (DESs) have attracted extensive research for their potential applications as leaching solvent to recycle valuable metal elements from spent lithium ion batteries (LIBs). Despite various advantages like being economical and green, the full potential of conventional binary DES has not yet been harnessed because of the kinetics during leaching. Herein, we consider the fundamental rate-determining-step (RDS) in conventional binary DES and attempt to design ternary DES, within which the chemical reaction kinetics and diffusion kinetics can be regulated to maximize the overall leaching rate. As a proof of concept, we show that the ternary choline chloride/succinic acid/ethylene glycol (ChCl/SA/EG) type ternary DES can completely dissolve LCO powder at 140 °C in 16 h. By systematically studying the leaching process at various conditions, the energy barrier during leaching can be calculated to be 11.77 kJ/mol. Furthermore, we demonstrate that the extraction of the cobalt ions from the leaching solution can be directly achieved by adding oxalic ions without neutralizing the solution. The precipitate can be used to regenerate LCO with high purity. The recycled materials show comparable electrochemical performance with commercial LCO. Our design strategy of ternary DES with regulated RDS is expected to have both scientific and technological significance in the field of hydrometallurgical recycling of LIBs.

Solid electrolytes for solid-state Li/Na–metal batteries: inorganic, composite and polymeric materials

This feature article presents the electrolyte synthetic approaches, design strategies, and merging materials that may address the critical issues of solid electrolytes for solid-state Li/Na–metal batteries.

Highly Stretchable, Fast Self-Healing, and Waterproof Fluorinated Copolymer Ionogels with Selectively Enriched Ionic Liquids for Human-Motion Detection

The development of waterproof ionogels with high stretchability and fast self-healing performance is essential for stretchable ionic conductors in sophisticated skin-inspired wearable sensors but can be rarely met in one material. Herein, a semicrystalline fluorinated copolymer ionogel (SFCI) with extremely high stretchability, underwater stability, and fast self-healability was fabricated, among which hydrophobic ionic liquids ([BMIM][TFSI]) were selectively enriched in fluoroacrylate segment domains of the fluorinated copolymer matrix through unique ion-dipole interactions. Benefiting from the reversible ion-dipole interactions between the [BMIM][TFSI] and fluoroacrylate segment domains as well as the physical cross-linking effects of semicrystalline oligoethylene glycol domains, the SFCI exhibited ultrastretchability (>6000%), fast room-temperature self-healability (>96% healing efficiency after cutting and self-healing for 30 min), and outstanding elasticity. In addition, the representative SFCI also exhibited high-temperature tolerance up to 300 °C, antifreezing performance as low as -35 °C, and high transparency (>93% visible-light transmittance). As a result, the as-obtained SFCI can readily be used as a highly stretchable ionic conductor in skin-inspired wearable sensors with waterproof performance for real-time detecting physiological human activities. These attractive features illustrate that the developed ultrastretchable and rapidly self-healable ionogels with unique waterproofness are promising candidates especially for sophisticated wearable strain sensing applications in complex and extreme environments.