Correlation between Ionic Conductivity and Mechanical Properties of Solid-like PEO-based Polymer Electrolyte

Poly(ethylene glycol) methyl ether methacrylate polymer networks (PEO-based networks), with or without anionic bis(trifluoromethanesulfonyl)imide (TFSI)-grafted groups, are promising electrolytes for Li-metal all solid-state batteries. Nevertheless, there is a need to enhance our current understanding of the physicochemical characteristics of these polymer networks to meet the mechanical and ionic conductivity property requirements for Li battery electrolyte materials. To address this challenge, our goal is to investigate the impact of the cross-linking density of the PEO-based network and the ethylene oxide/lithium ratio on mechanical properties (such as glass transition temperature and storage modulus) and ionic conductivity. We have synthesized a series of cross-linked PEO-based polymers (si-SPE for single ion solid polymer electrolyte) via solvent-free radical copolymerization. These polymers are synthesized by using commercially available lithium 3-[(trifluoromethane)sulfonamidosulfonyl]propyl methacrylate (LiMTFSI), poly(ethylene glycol)methyl ether methacrylate (PEGM), and [poly(ethylene glycol) dimethacrylate] (PEGDM). In addition, we have synthesized a series of cross-linked PEO-based polymers (SPE for solid polymer electrolyte) using LiTFSI as the ionic species. Most of the resulting polymer films are amorphous, self-standing, flexible, homogeneous, and thermally stable. Interestingly, our research has revealed a correlation between ionic conductivity and mechanical properties in both the SPE and si-SPE series. Ionic conductivity increases as glass transition temperature, α relaxation temperature, and storage modulus decrease, suggesting that Li+ transport is influenced by polymer chain flexibility and Li+/EO interaction.

Contributing to the Revolution of Electrolyte Systems via In Situ Polymerization at Different Scales: A Review

Solid-state batteries have become the most anticipated option for compatibility with high-energy density and safety. In situ polymerization, a novel strategy for the construction of solid-state systems, has extended its application from solid polymer electrolyte systems to other solid-state systems. This review summarizes the application of in situ polymerization strategies in solid-state batteries, which covers the construction of polymer, the formation of the electrolyte system, and the design of the full cell. For the polymer skeleton, multiple components and structures are being chosen. In the construction of solid polymer electrolyte systems, the choice of initiator for in situ polymerization is the focus of this review. New initiators, represented by lithium salts and additives, are the preferred choice because of their ability to play more diverse roles, while the coordination with other components can also improve the electrical properties of the system and introduce functionalities. In the construction of entire solid-state battery systems, the application of in situ polymerization to structure construction, interface construction, and the use of separators with multiplex functions has brought more possibilities for the development of various solid-state systems and even the perpetuation of liquid electrolytes.

Enabling high rate capability and stability all-solid-state batteries via cationic surfactant modification of composite electrolyte

The garnet-type solid electrolyte Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was modified with a cationic surfactant Cetyltrimethylammonium Bromide (CTAB) to improve the dispersion of LLZTO inorganic particles in Poly (ethylene oxide) (PEO) electrolyte (PEO-LLZTO@CTAB) by a liquid phase casting method. During fabrication, the cationic modifier CTAB is uniformly adsorbed on the surface of LLZTO particles, which can effectively reduce their surface energy and lead to a thin CTAB surface coating layer. This fabricated PEO-LLZTO@CTAB can avoid the aggregation of LLZTO particles in the composite solid-state electrolyte (CSSE) and improve the interfacial contact at the PEO/LLZTO interface, thus reducing the overall resistance of PEO-LLZTO@CTAB/Li half-cell and inhibiting the dendrite growth during cycling. The all-solid-state batteries (ASSBs) with LiFePO4 (LFP) as the cathode, PEO-LLZTO@CTAB as the electrolyte and Li as the anode exhibit a high initial discharge capacity of 146.6 mAh-g-1, excellent rate performance and high-capacity retention of 91.0 % after 300 cycles at 0.2 C multiplier and 60 °C within the voltage range of 2.7-4.0 V.

Binding of Li+ to Negatively Charged and Neutral Ligands in Polymer Electrolytes

Conceptually, single-ion polymer electrolytes (SIPE) with the anion bound to the polymer could solve major issues in Li-ion batteries, but their conductivity is too low. Experimentally, weakly interacting anionic groups have the best conductivity. To provide a theoretical basis for this result, density functional theory calculations of the optimized geometries and energies are performed for charged ligands used in SIPE. Comparison is made to neutral ligands found in dual-ion conductors, which demonstrate higher conductivity. The free energy differences between adding and subtracting a ligand are small enough for the neutral ligands to have the conductivity seen experimentally. However, charged ligands have large barriers, implying that lithium transport will coincide with the slow polymer diffusion, as observed in experiments. Overall, SIPE will require additional solvent to achieve a sufficiently high conductivity. Additionally, the binding of mono- and bidentate geometries varies, providing a simple and clear reason that polarizable force fields are required for detailed interactions.

Toward a molecular understanding of the conductivity of lithium-ion conducting polyanion polymer electrolytes by molecular dynamics simulation

With the improved lithium-ion transference number near unity, the low conductivity of single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) still hinders their application in high-rate batteries. Though some empirical conclusions on the conducting mechanism of SLIC-SPEs have been obtained, a more comprehensive study on the quantitative relationship between the molecular structure factors and ionic conduction performance is expected. In this study, a model structure that contains adjustable main chain and anion groups in the polyethylene oxide (PEO) matrix was used to clarify the influence of molecular structural factors on ionic conductivity and electrochemical stability of SLIC-SPEs. The anionic group was further disassembled into the intermediate group and end group while the main chain structure was distinguished into different degrees of polymerization and various lengths of the spacers between anions. Therefore, a well-defined molecular structure was employed to describe its relationship with ionic conductivity. In addition, the dissociation degree of salts and mobility of ions changing with the molecular structure were also discussed to explore the fundamental causes of conductivity. It can be concluded that the anion group affects the conductivity mainly via the dissociation degree, while the main chain structure impacts the conductivity by both dissociation degree and mobility.

Single-Ion Conducting Polymer Electrolyte for Superior Sodium-Metal Batteries

Sodium-metal batteries (SMBs) are considered a potential alternative to high-energy lithium-metal batteries (LMBs). However, the high reactivity of metallic sodium towards common liquid organic electrolytes renders such battery technology particularly challenging. Herein, we propose a multi-block single-ion conducting polymer electrolyte (SIPE) doped with ethylene carbonate as suitable electrolyte system for SMBs. This novel SIPE provides a very high ionic conductivity (2.6 mS cm-1 ) and an electrochemical stability window of about 4.1 V at 40 °C, enabling stable sodium stripping and plating and excellent rate capability of Na||Na3 V2 (PO4 )3 cells up to 2 C. Remarkably, such cells provide a capacity retention of about 85 % after 1,000 cycles at 0.2 C thanks to the very high Coulombic efficiency (99.9 %), resulting from an excellent interfacial stability towards sodium metal and the Na3 V2 (PO4 )3 cathode.

Sequencing polymers to enable solid-state lithium batteries

Rational designs of solid polymer electrolytes with high ion conduction are critical in enabling the creation of advanced lithium batteries. However, known polymer electrolytes have much lower ionic conductivity than liquid/ceramics at room temperature, which limits their practical use in batteries. Here we show that precise positioning of designed repeating units in alternating polymer sequences lays the foundation for homogenized Li+ distribution, non-aggregated Li+-anion solvation and sequence-assisted site-to-site ion migration, facilitating the tuning of Li+ conductivity by up to three orders of magnitude. The assembled all-solid-state batteries facilitate reversible and dendrite-mitigated cycling against Li metal from ambient to elevated temperatures. This work demonstrates a powerful molecular engineering means to access highly ion-conductive solid-state materials for next-generation energy devices.

Asymmetric Trihalogenated Aromatic Lithium Salt Induced Lithium Halide Rich Interface for Stable Cycling of All-Solid-State Lithium Batteries

Solid polymer electrolytes (SPEs) are the key components for all-solid-state lithium metal batteries with high energy density and intrinsic safety. However, the low lithium ion transference number (t+) of a conventional SPE and its unstable electrolyte/electrode interface cannot guarantee long-term stable operation. Herein, asymmetric trihalogenated aromatic lithium salts, i.e., lithium (3,4,5-trifluorobenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiFFF) and lithium (4-bromo-3,5-difluorobenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiFBF), are synthesized for polymer electrolytes. They exhibit higher t+ values and better compatibility with Li metal than conventional lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). Due to the trihalogenated aromatic anions, LiFFF- and LiFBF-based electrolytes are prone to generate an LiF- and LiBr-rich solid electrolyte interphase (SEI), therefore increasing the stability of the solid electrolyte/anode interface. Particularly, LiFBF could induce a LiF/LiBr hybrid SEI, where LiF shows a high Young's modulus and high surface energy for homogenizing Li ion flux and LiBr exhibits an extremely low Li ion diffusion barrier in the SEI layer. As a result, the Li/Li symmetric cells could remain stable for more than 1200 h without a short circuit and the LiFePO4/Li batteries showed superb electrochemical performance over 1200 cycles at 1 C. This work provides valuable insights from the perspective of lithium salt molecular structures for high-performance all-solid-state lithium metal batteries.

Anion-tethered Single Lithium-ion Conducting Polyelectrolytes through UV-induced Free Radical Polymerization for Improved Morphological Stability of Lithium Metal Anodes

Single Li+ ion conducting polyelectrolytes (SICs), which feature covalently tethered counter-anions along their backbone, have the potential to mitigate dendrite formation by reducing concentration polarization and preventing salt depletion. However, due to their low ionic conductivity and complicated synthetic procedure, the successful validation of these claimed advantages in lithium metal (Li0 ) anode batteries remains limited. In this study, we fabricated a SIC electrolyte using a single-step UV polymerization approach. The resulting electrolyte exhibited a high Li+ transference number (t+ ) of 0.85 and demonstrated good Li+ conductivity (6.3×10-5  S/cm at room temperature), which is comparable to that of a benchmark dual ion conductor (DIC, 9.1×10-5  S/cm). Benefitting from the high transference number of SIC, it displayed a three-fold higher critical current density (2.4 mA/cm2 ) compared to DIC (0.8 mA/cm2 ) by successfully suppressing concentration polarization-induced short-circuiting. Additionally, the t+ significantly influenced the deposition behavior of Li0 , with SIC yielding a uniform, compact, and mosaic-like morphology, while the low t+ DIC resulted in a porous morphology with Li0 whiskers. Using the SIC electrolyte, Li0 ||LiFePO4 cells exhibited stable operation for 4500 cycles with 70.5 % capacity retention at 22 °C.

Engineering the Structural Uniformity of Gel Polymer Electrolytes via Pattern-Guided Alignment for Durable, Safe Solid-State Lithium Metal Batteries

Ultrathin and super-toughness gel polymer electrolytes (GPEs) are the key enabling technology for durable, safe, and high-energy density solid-state lithium metal batteries (SSLMBs) but extremely challenging. However, GPEs with limited uniformity and continuity exhibit an uneven Li+ flux distribution, leading to nonuniform deposition. Herein, a fiber patterning strategy for developing and engineering ultrathin (16 µm) fibrous GPEs with high ionic conductivity (≈0.4 mS cm-1 ) and superior mechanical toughness (≈613%) for durable and safe SSLMBs is proposed. The special patterned structure provides fast Li+ transport channels and tailoring solvation structure of traditional LiPF6 -based carbonate electrolyte, enabling rapid ionic transfer kinetics and uniform Li+ flux, and boosting stability against Li anodes, thus realizing ultralong Li plating/stripping in the symmetrical cell over 3000 h at 1.0 mA cm-2 , 1.0 mAh cm-2 . Moreover, the SSLMBs with high LiFePO4 loading of 10.58 mg cm-2 deliver ultralong stable cycling life over 1570 cycles at 1.0 C with 92.5% capacity retention and excellent rate capacity of 129.8 mAh g-1 at 5.0 C with a cut-off voltage of 4.2 V (100% depth-of-discharge). Patterned GPEs systems are powerful strategies for producing durable and safe SSLMBs.

Tuning electron delocalization of hydrogen-bonded organic framework cathode for high-performance zinc-organic batteries

Stable cathodes with multiple redox-active centres affording a high energy density, fast redox kinetics and a long life are continuous pursuits for aqueous zinc-organic batteries. Here, we achieve a high-performance zinc-organic battery by tuning the electron delocalization within a designed fully conjugated two-dimensional hydrogen-bonded organic framework as a cathode material. Notably, the intermolecular hydrogen bonds endow this framework with a transverse two-dimensional extended stacking network and structural stability, whereas the multiple C = O and C = N electroactive centres cooperatively trigger multielectron redox chemistry with super delocalization, thereby sharply boosting the redox potential, intrinsic electronic conductivity and redox kinetics. Further mechanistic investigations reveal that the fully conjugated molecular configuration enables reversible Zn2+/H+ synergistic storage accompanied by 10-electron transfer. Benefitting from the above synergistic effects, the elaborately tailored organic cathode delivers a reversible capacity of 498.6 mAh g-1 at 0.2 A g-1, good cyclability and a high energy density (355 Wh kg-1).

A reflection on polymer electrolytes for solid-state lithium metal batteries

Before the debut of lithium-ion batteries (LIBs) in the commodity market, solid-state lithium metal batteries (SSLMBs) were considered promising high-energy electrochemical energy storage systems before being almost abandoned in the late 1980s because of safety concerns. However, after three decades of development, LIB technologies are now approaching their energy content and safety limits imposed by the rocking chair chemistry. These aspects are prompting the revival of research activities in SSLMB technologies at both academic and industrial levels. In this perspective article, we present a personal reflection on solid polymer electrolytes (SPEs), spanning from early development to their implementation in SSLMBs, highlighting key milestones. In particular, we discuss the SPEs' characteristics taking into account the concept of coupled and decoupled SPEs proposed by C. Austen Angell in the early 1990s. Possible remedies to improve the physicochemical and electrochemical properties of SPEs are also examined. With this article, we also aim to highlight the missing blocks in building ideal SSLMBs and stimulate research towards innovative electrolyte materials for future rechargeable high-energy batteries.

Recent Development in Topological Polymer Electrolytes for Rechargeable Lithium Batteries

Solid polymer electrolytes (SPEs) are still being considered as a candidate to replace liquid electrolytes for high-safety and flexible lithium batteries due to their superiorities including light-weight, good flexibility, and shape versatility. However, inefficient ion transportation of linear polymer electrolytes is still the biggest challenge. To improve ion transport capacity, developing novel polymer electrolytes are supposed to be an effective strategy. Nonlinear topological structures such as hyperbranched, star-shaped, comb-like, and brush-like types have highly branched features. Compared with linear polymer electrolytes, topological polymer electrolytes possess more functional groups, lower crystallization, glass transition temperature, and better solubility. Especially, a large number of functional groups are beneficial to dissociation of lithium salt for improving the ion conductivity. Furthermore, topological polymers have strong design ability to meet the requirements of comprehensive performances of SPEs. In this review, the recent development in topological polymer electrolytes is summarized and their design thought is analyzed. Outlooks are also provided for the development of future SPEs. It is expected that this review can raise a strong interest in the structural design of advanced polymer electrolyte, which can give inspirations for future research on novel SPEs and promote the development of next-generation high-safety flexible energy storage devices.

Zwitterionic Separator Featured with Superdesolvating Properties for High Performance Lithium-Sulfur Batteries

Lithium-sulfur chemistry has greatly expanded the boundaries of lithium batteries, but the persistent parasitic reaction of soluble sulfur intermediates with lithium anode remains a primary challenge. Understanding and regulating the solvation structures of lithium ions (Li+) and polysulfides (LiPSs) are critical to addressing the above issues. Herein, inspired by the natural superhydrophilic resistance to contamination, we developed a zwitterionic nanoparticles (ZWP) separator capable of modulating the solvated of Li+ and LiPSs. The dense solvated layer induced by ZWP effectively prevents the movement of LiPSs without compromising Li+ transport. Moreover, the high electrolyte affinity of the ZWP effectively results in minimizing the deposition of LiPSs on the separator. Furthermore, the structure of the solvated Li+ and LiPSs is also unveiled by molecular simulation and nuclear magnetic resonance (NMR). In addition, in situ UV setup proved the ZWP separator can effectively suppress the shuttle of LiPSs. The restricted space formed by the tightly packed ZWP stabilizes the lithium deposition and regulates dendrite growth. Consequently, the performance of lithium-sulfur batteries is significantly improved and good cycle stability is maintained even at high sulfur loadings (5 mg cm-2). This contribution provides a new insight into the rational design of lithium-sulfur battery separators.

Application of Biomass Materials in Zinc-Ion Batteries

Currently, aqueous zinc-ion batteries, with large reserves of zinc metal and maturity of production, are a promising alternative to sustainable energy storage. Nevertheless, aqueous solution has poor frost resistance and is prone to side reactions. In addition, zinc dendrites also limit the performance of zinc-ion batteries. Biomass, with complex molecular structure and abundant functional groups, makes it have great application prospects. In this review, the research progress of biomass and its derived materials used in zinc-ion batteries are reviewed. The different regulation strategies and characteristics of biomass used in zinc-ion battery electrodes, electrolyte separators and binders are demonstrated. The regulation mechanism is analyzed. At the end, the development prospect and challenges of biomass in energy materials application are proposed.

Enhanced Electrochemical Performance of PEO-Based Composite Polymer Electrolyte with Single-Ion Conducting Polymer Grafted SiO2 Nanoparticles

In order to enhance the electrochemical performance and mechanical properties of poly(ethylene oxide) (PEO)-based solid polymer electrolytes, composite solid electrolytes (CSE) composed of single-ion conducting polymer-modified SiO₂, PEO and lithium salt were prepared and used in lithium-ion batteries in this work. The pyridyl disulfide terminated polymer (py-ss-PLiSSPSI) is synthesized through RAFT polymerization, then grafted onto SiO₂ via thiol-disulfide exchange reaction between SiO₂-SH and py-ss-PLiSSPSI. The chemical structure, surface morphology and elemental distribution of the as-prepared polymer and the PLiSSPSI-g-SiO₂ nanoparticles have been investigated. Moreover, CSEs containing 2, 6, and 10 wt% PLiSSPSI-g-SiO₂ nanoparticles (PLi-g-SiCSEs) are fabricated and characterized. The compatibility of the PLiSSPSI-g-SiO₂ nanoparticles and the PEO can be effectively improved owing to the excellent dispersibility of the functionalized nanoparticles in the polymer matrix, which promotes the comprehensive performances of PLi-g-SiCSEs. The PLi-g-SiCSE-6 exhibits the highest ionic conductivity (0.22 mS·cm^-1) at 60 °C, a large t_Li+ of 0.77, a wider electrochemical window of 5.6 V and a rather good lithium plating/stripping performance at 60 °C, as well as superior mechanical properties. Hence, the CSEs containing single-ion conducting polymer modified nanoparticles are promising candidates for all-solid-state lithium-ion batteries.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Polystyrene-Based Single-Ion Conducting Polymer Electrolyte for Lithium Metal Batteries

Lithium metal batteries are one of the more promising replacements for lithium-ion batteries owing to their ability to reach high energy densities. The main problem limiting their commercial application is the formation of dendrites, which significantly reduces their durability and renders the batteries unsafe. In the present work, we used a single-ion conducting gel polymer electrolyte based on a poly(ethylene-ran-butylene)-block-polystyrene (SEBS) block copolymer, which was functionalized with benzenesulfonylimide anions and plasticized by a mixture of ethylene carbonate and dimethylacetamide (SSEBS-Ph-EC-DMA), with a solvent uptake of 160% (~12 solvent molecules per one functional group of the membrane). The SSEBS-Ph-EC-DMA electrolyte exhibits an ionic conductivity of 0.6 mSm∙cm−1 at 25 °C and appears to be a cationic conductor (TLi+ = 0.72). SSEBS-Ph-EC-DMA is electrochemically stable up to 4.1 V. Symmetrical Li|Li cells; further, with regard to SSEBS-Ph-EC-DMA membrane electrolytes, it showed a good performance (~0.10 V at first cycles and <0.23 V after 700 h of cycling at ±0.1 mA∙cm−2 and ±0.05 mAh∙cm−2). The LiFePO4|SSEBS-Ph-EC-DMA|Li battery showed discharge capacity values of 100 mAh∙g−1 and a 100% Coulomb efficiency, at a cycling rate of 0.1C.

Ion slippage through Li+-centered G-quadruplex

Single-ion conductors have garnered attention in energy storage systems as a promising alternative to currently widespread electrolytes that allow migration of cations and anions. However, ion transport phenomena of most single-ion conductors are affected by strong ion (e.g., Li⁺)-ion (immobilized anionic domains) interactions and tortuous paths, which pose an obstacle to achieving performance breakthroughs. Here, we present a Li⁺-centered G-quadruplex (LiGQ) as a class of single-ion conductor based on directional Li⁺ slippage at the microscopic level. A guanine derivative with liquid crystalline moieties is self-assembled to form a hexagonal ordered columnar structure in the LiGQ, thereby yielding one-dimensional central channels that provide weak ion-dipole interaction and straightforward ionic pathways. The LiGQ exhibits weak Li⁺ binding energy and low activation energy for ion conduction, verifying its viability as a new electrolyte design.

Improving Li-ion interfacial transport in hybrid solid electrolytes

The development of commercial solid-state batteries has to date been hindered by the individual limitations of inorganic and organic solid electrolytes, motivating hybrid concepts. However, the room-temperature conductivity of hybrid solid electrolytes is still insufficient to support the required battery performance. A key challenge is to assess the Li-ion transport over the inorganic and organic interfaces and relate this to surface chemistry. Here we study the interphase structure and the Li-ion transport across the interface of hybrid solid electrolytes using solid-state nuclear magnetic resonance spectroscopy. In a hybrid solid polyethylene oxide polymer-inorganic electrolyte, we introduce two representative types of ionic liquid that have different miscibilities with the polymer. The poorly miscible ionic liquid wets the polymer-inorganic interface and increases the local polarizability. This lowers the diffusional barrier, resulting in an overall room-temperature conductivity of 2.47 × 10^-4 S cm^-1. A critical current density of 0.25 mA cm^-2 versus a Li-metal anode shows improved stability, allowing cycling of a LiFePO₄-Li-metal solid-state cell at room temperature with a Coulombic efficiency of 99.9%. Tailoring the local interface environment between the inorganic and organic solid electrolyte components in hybrid solid electrolytes seems to be a viable route towards designing highly conducting hybrid solid electrolytes.

Ion-Dipole Interaction Regulation Enables High-Performance Single-Ion Polymer Conductors for Solid-State Batteries

Solid polymer electrolytes with large ionic conductivity, high ionic transference number, and good interfacial compatibility with electrodes are highly desired for solid-state batteries. However, unwanted polarizations and side reactions occurring in traditional dual-ion polymer conductors hinder their practical applications. Here, single-ion polymer conductors (SIPCs) with exceptional selectivity for Li-ion conduction (Li-ion transference number up to 0.93), high room-temperature ionic conductivity of about 10^-4 S cm^-1 , and a wide electrochemical stability window (>4.5 V, vs Li/Li⁺ ) are prepared by precisely regulating the ion-dipole interactions between Li⁺ and carbonyl/cyano groups. The resulting SIPCs show an excellent electrochemical stability with Li metal during long-term cycling at room temperature and 60 °C. LiFePO₄ -based solid-state cells containing the SIPCs exhibit good rate and cycling performance in a wide temperature range from -20 to 90 °C. By the same way of ion-dipole interaction regulation, sodium- and potassium-based SIPCs with both high ionic conductivity and high cationic transference numbers are also prepared. The findings in this work provide guidance for the development of high-performance SIPCs and other metal-ion systems beyond Li⁺ .

Solid Polymer Electrolytes for Lithium Batteries: A Tribute to Michel Armand

In a previous publication, a tribute to Michel Armand was provided, which highlighted his outstanding contribution to all aspects of research and development of lithium-metal and lithium-ion batteries. This area is in constant progress and rather than an overview of the work of Armand et al. since the seventies, we mainly restrict this review to his contribution to advances in solid polymer electrolytes (SPEs) and their performance in all-solid-state lithium-metal batteries in recent years.

Phase behavior of binary and ternary fluoropolymer (PVDF-HFP) solutions for single-ion conductors

A fluoropolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) has a dielectric constant of ∼11, providing charge screening effects. Hence, this highly polar PVDF-HFP material has been employed as a matrix for solid polymer electrolytes (SPEs). In this study, the phase behavior of binary PVDF-HFP solutions was analyzed using the Flory-Huggins theory, in which ethylene carbonate, propylene carbonate, dimethyl carbonate, γ-butyrolactone, and acetone were employed as model solvents. In particular, for the binary PVDF-HFP/acetone system, the solid-liquid and liquid-liquid phase transitions were qualitatively described. Then, the phase diagram for ternary acetone/PVDF-HFP/polyphenolate systems was constructed, in which the binodal, spinodal, tie-line, and critical point were included. Finally, when a polyelectrolyte lithium polyphenolate was mixed with the PVDF-HFP matrix, it formed a single-ion conductor with a Li⁺ transference number of 0.8 at 23 °C. In the case of ionic conductivity, it was ∼10^-5 S cm^-1 in solid state and ∼10^-4 S cm^-1 in gel state, respectively.

Soluble and Perfluorinated Polyelectrolyte for Safe and High-Performance Li-O2 Batteries

The severe performance degradation of high-capacity Li-O₂ batteries induced by Li dendrite growth and concentration polarization from the low Li⁺ transfer number of conventional electrolytes hinder their practical applications. Herein, lithiated Nafion (LN) with the sulfonic group immobilized on the perfluorinated backbone has been designed as a soluble lithium salt for preparing a less flammable polyelectrolyte solution, which not only simultaneously achieves a high Li⁺ transfer number (0.84) and conductivity (2.5 mS cm^-1 ), but also the perfluorinated anion of LN produces a LiF-rich SEI for protecting the Li anode from dendrite growth. Thus, the Li-O₂ battery with a LN-based electrolyte achieves an all-round performance improvement, like low charge overpotential (0.18 V), large discharge capacity (9508 mAh g^-1 ), and excellent cycling performance (225 cycles). Besides, the fabricated pouch-type Li-air cells exhibit promising applications to power electronic equipment with satisfactory safety.

Li-Ion Transport and Solvation of a Li Salt of Weakly Coordinating Polyanions in Ethylene Carbonate/Dimethyl Carbonate Mixtures

Electrolytes with a high Li-ion transference number (t_Li) have attracted significant attention for the improvement of the rapid charge-discharge performance of Li-ion batteries (LIBs). Nonaqueous polyelectrolyte solutions exhibit high t_Li upon immobilization of the anion on a polymer backbone. However, the transport properties and Li-ion solvation in these media are not fully understood. Here, we investigated the Li salt of a weakly coordinating polyanion, poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)amide] (poly(LiSTFSA)), in various ethylene carbonate and dimethyl carbonate mixtures. The highest ionic conductivity was unexpectedly observed for the lowest polar mixture at the highest salt concentration despite the low dissociation degree of poly(LiSTFSA). This was attributed to a unique conduction phenomenon resulting from the faster diffusion of transiently solvated Li ions along the interconnected aggregates of polyanion chains. A Li/LiFePO₄ cell using such an electrolyte demonstrated improved rate capability. These results provide insights into a design strategy of nonaqueous liquid electrolytes for LIBs.

Does Cell Polarization Matter in Single-Ion Conducting Electrolytes?

Single-ion conducting polymer electrolytes (SIPE) are particularly promising electrolyte materials in lithium metal-based batteries since theoretical considerations suggest that the immobilization of anions avoids polarization phenomena at electrode|electrolyte interfaces. SIPE in principle could allow for fast charging while preventing cell failure induced by short circuits arising from the growth of inhomogeneous Li depositions provided that SIPE membranes possess sufficient mechanical stability. To date, different chemical structures are developed for SIPE, where new compounds are often reported through electrochemical characterization at low current rates. Experimental counterparts to model-based assumptions and determination of system limitations by correlating both models and experiments are rare in the literature. Herein, Chazalviel's model, which is derived from ion concentration gradients, is applied to theoretically determine the limiting current density (J_Lim) of a SIPE. Comparison with the experimentally obtained J_Lim reveals a large deviation between the theoretical and practical values. Beyond that, charge-discharge profiles show a distinct arcing behavior at moderate current densities (0.5 to 1 mA cm^-2), indicating polarization of the cell, which is not so far reported for SIPE. In this context, by application of various electrochemical and physiochemical methods, the details of cell polarization and the role of the solid electrolyte interphase in producing arcing behavior in the voltage profiles in stripping/plating experiments are revealed, which eventually also elucidate the inconsistency of J_Lim.

Photo-Crosslinked Single-Ion Conducting Polymer Electrolyte for Lithium-Metal Batteries

Polymer electrolytes are considered potential key enablers for lithium-metal batteries due to their compatibility with the lithium-metal negative electrode. Herein, crosslinked selfstanding single-ion conducting polymer electrolytes were obtained via a facile UVinitiated radical polymerization using pentaerythritol tetraacrylate as the crosslinker and lithium (3methacryloyloxypropylsulfonyl)-(trifluoromethylsulfonyl)imide as ionic functional group. Incorporating propylene carbonate as charge-transport supporting additive allowed for achieving single-ion conductivities of 0.21 mS cm^-1 at 20°C and 0.40 mS cm^-1 at 40°C, while maintaining a suitable electrochemical stability window for 4V-class positive electrodes (cathodes). As a result, this single-ion polymer electrolyte featured good cycling stability and rate capability in Li||LiFePO₄ and Li||LiNi_0.6 Mn_0.2 Co_0.2 O₂ cells. These results render this polymer electrolyte as potential alternative to liquid electrolytes for high-energy lithium-metal batteries. This article is protected by copyright. All rights reserved.

Designing Versatile Polymers for Lithium-Ion Battery Applications: A Review

Solid-state electrolytes are a promising family of materials for the next generation of high-energy rechargeable lithium batteries. Polymer electrolytes (PEs) have been widely investigated due to their main advantages, which include easy processability, high safety, good mechanical flexibility, and low weight. This review presents recent scientific advances in the design of versatile polymer-based electrolytes and composite electrolytes, underlining the current limitations and remaining challenges while highlighting their technical accomplishments. The recent advances in PEs as a promising application in structural batteries are also emphasized.

Investigating miscibility and lithium ion transport in blends of poly(ethylene oxide) with a polyanion containing precisely-spaced delocalized charges

Synthesis of a precision single ion conductor with a phenylsulfonyl (TFSI) lithium salt pendant at every 5th carbon is reported and miscibility, conductivity, and transference studies are performed upon blending with PEO at varying compositions.

Single-Ion Lithium Conducting Polymers with High Ionic Conductivity Based on Borate Pendant Groups

A family of single‐ion lithium conducting polymer electrolytes based on highly delocalized borate groups is reported. The effect of the nature of the substituents on the boron atom on the ionic conductivity of the resultant methacrylic polymers was analyzed. To the best of our knowledge the lithium borate polymers endowed with flexible and electron‐withdrawing substituents presents the highest ionic conductivity reported for a lithium single‐ion conducting homopolymer (1.65×10⁻⁴ S cm⁻¹ at 60 °C). This together with its high lithium transference number tLi+ =0.93 and electrochemical stability window of 4.2 V vs Li⁰/Li⁺ show promise for application in lithium batteries. To illustrate this, a lithium borate monomer was integrated into a single‐ion gel polymer electrolyte which showed good performance on lithium symmetrical cells (<0.85 V at ±0.2 mA cm⁻² for 175 h).

An integrated cathode and solid electrolyte via in situ polymerization with significantly reduced interface resistance

Reducing the interfacial resistance between solid electrolytes and electrodes is critical for developing high-energy density solid-state batteries. In the present study, a simple strategy of designing an integrated cathode and solid electrolyte (ICSE) to avoid a contact interface is proposed and successfully fulfilled with the help of UV curving. Firstly, a porous polymer film (PVDF-HFP/PVDF) was formed on the surface of the porous LiFePO₄ electrode via PVP dissolution. Secondly, curable monomers, including PEGDA/PETMP/TFEMA, were filled into the porous membrane via infiltration and concentration. Lastly, the ICSE was obtained via curing with ultraviolet light. The as-prepared LiFePO₄//ICSE//Li solid battery displays excellent electrochemical performance with a high reversible capacity of 153 mA h g^-1 and a capacity of over 140 mA h g^-1 was retained after 150 cycles at 0.1C and 25 °C. This ICSE strategy may effectively contribute to the practical application of all-solid-state batteries.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Unraveling the Role of Neutral Units for Single-Ion Conducting Polymer Electrolytes

With the cationic transference number close to unity, single-ion conducting polymer electrolytes (SICPEs) are recognized as an advanced electrolyte system with improved energy efficiency for battery application. The relatively low ionic conductivity for most of the SICPEs in comparison with liquid electrolytes remains the major "bottleneck" for their practical applications. Polyethylene oxide (PEO) has been recognized as a benchmark for solid polymer electrolytes due to its high salt solubility and reasonable ionic conductivity. PEO has two advantages: (i) the polar ether groups coordinate well with lithium ions (Li⁺) providing good dissociation from anions, and (ii) the low T_g provides fast segmental dynamics at ambient temperature and assists rapid charge transport. These properties lead to active use of PEO as neutral plasticizing units in SICPEs. Herein, we present a detailed comparison of new SICPEs copolymerized with PEO units vs SICPEs copolymerized with other types of neutral units possessing either flexible or polar structures. The presented analysis revealed that the polarity of side chains has a limited influence on ion dissociation for copolymer-type SICPEs. The Li⁺-ion dissociation seems to be controlled by the charge delocalization on the polymerized anion. With good miscibility between plasticizing neutral units and ionic conductive units, the ambient ionic conductivity of synthesized SICPEs is still mainly controlled by the T_g of the copolymer. This work sheds light on the dominating role of PEO in SICPE systems and provides helpful guidance for designing polymer electrolytes with new functionalities and structures. Furthermore, based on the presented results, we propose that designing polyanions with a highly delocalized charge may be another promising route for achieving sufficient lithium ionic conductivity in solvent-free SICPEs.

Copper-coordinated cellulose ion conductors for solid-state batteries

Although solid-state lithium (Li)-metal batteries promise both high energy density and safety, existing solid ion conductors fail to satisfy the rigorous requirements of battery operations. Inorganic ion conductors allow fast ion transport, but their rigid and brittle nature prevents good interfacial contact with electrodes. Conversely, polymer ion conductors that are Li-metal-stable usually provide better interfacial compatibility and mechanical tolerance, but typically suffer from inferior ionic conductivity owing to the coupling of the ion transport with the motion of the polymer chains^1-3. Here we report a general strategy for achieving high-performance solid polymer ion conductors by engineering of molecular channels. Through the coordination of copper ions (Cu²⁺) with one-dimensional cellulose nanofibrils, we show that the opening of molecular channels within the normally ion-insulating cellulose enables rapid transport of Li⁺ ions along the polymer chains. In addition to high Li⁺ conductivity (1.5 × 10^-3 siemens per centimetre at room temperature along the molecular chain direction), the Cu²⁺-coordinated cellulose ion conductor also exhibits a high transference number (0.78, compared with 0.2-0.5 in other polymers²) and a wide window of electrochemical stability (0-4.5 volts) that can accommodate both the Li-metal anode and high-voltage cathodes. This one-dimensional ion conductor also allows ion percolation in thick LiFePO₄ solid-state cathodes for application in batteries with a high energy density. Furthermore, we have verified the universality of this molecular-channel engineering approach with other polymers and cations, achieving similarly high conductivities, with implications that could go beyond safe, high-performance solid-state batteries.

Single-ion conducting polymer electrolytes as a key jigsaw piece for next-generation battery applications

As lithium-ion batteries have been the state-of-the-art electrochemical energy storage technology, the overwhelming demand for energy storage on a larger scale has triggered the development of next-generation battery technologies possessing high energy density, longer cycle lives, and enhanced safety. However, commercial liquid electrolytes have been plagued by safety issues due to their flammability and instability in contact with electrodes. Efforts have focused on developing such electrolytes by covalently immobilizing anionic groups onto a polymer backbone, which only allows Li+ cations to be mobile through the polymer matrix. Such ion-selective polymers provide many advantages over binary ionic conductors in battery operation, such as minimization of cell polarization and dendrite growth. In this review, the design, synthesis, fabrication, and class are reviewed to give insight into the physicochemical properties of single-ion conducting polymer electrolytes. The standard characterization method and remarkable electrochemical properties are further highlighted, and perspectives on current challenges and future directions are also discussed.