Safety-Enhanced Polymer Electrolytes for Sodium Batteries: Recent Progress and Perspectives

Gel polymer electrolytes based on sulfonamide functional polymer nanoparticles for sodium metal batteries

In this work, we investigate the development of polymer electrolytes for sodium batteries based on sulfonamide functional polymer nanoparticles (NaNPs). The synthesis of the polymer NaNPs is carried out by emulsion copolymerization of methyl methacrylate and sodium sulfonamide methacrylate in the presence of a crosslinker, resulting in particle sizes of 50 nm, as shown by electron microscopy. Then, gel polymer electrolytes are prepared by mixing polymer NPs and different organic plasticizers including carbonates, glymes, sulfolanes and ionic liquids. The chemical nature of the plasticizer resulted in different effects on the sodium coordination shell, which in turn impacted the properties of each membrane as investigated by FTIR. The transport properties were investigated by EIS and solid-state NMR. Among the organic gel polymer electrolytes (GPEs), the system comprising NaNPs and sulfolanes achieved the best ionic conductivity (1.1 × 10-4 S cm-1 at 50 °C) and sodium single-ion properties while for the ionogels, the best ionic conductivity was obtained by NaNPs mixed with pyrrolidinium-FSI IL (4.7 × 10-4 S cm-1 at 50 °C). From sodium metal symmetrical cell cycling, the use of ILs as plasticizers proved to be more beneficial for SEI formation and its evolution during cell cycling compared to the systems based on NPs and organic solvents. However, NPs + PC led to lower cell overvoltage than NPs + ILs (<0.4 V vs. >0.5 V). This study shows the potential of using Na-sulfonamide functional polymer nanoparticles to immobilize different plasticizers and thereby obtain soft-solid electrolytes for Na metal batteries.

Composite Polymer Solid Electrolytes for All-Solid-State Sodium Batteries

Sodium-ion batteries (SIBs) are emerging as a promising alternative to lithium-ion batteries, primarily due to their plentiful raw materials and cost-effectiveness. However, the use of traditional organic liquid electrolytes in sodium battery applications presents significant safety risks, prompting the investigation of solid electrolytes as a more viable solution. Despite their advantages, single solid electrolytes encounter challenges, including low conductivity of sodium ions at room temperature and incompatibility with electrode materials. To overcome these limitations, the researchers develop composite polymer solid electrolytes (CPSEs), which merge the strengths of high ionic conductivity of inorganic solid electrolytes and the flexibility of polymer solid electrolytes. CPSEs are usually composed of inorganic materials dispersed in the polymer matrix. The final performance of CPSEs can be further improved by optimizing the particle size, relative content, and form of inorganic fillers. CPSEs show great advantages in improving ionic conductivity and interface compatibility, making them an important direction for future solid-state sodium battery research. Therefore, this paper summarizes recent advancements in composite solid electrolytes, discusses the impact of their preparation processes on performance, and outlines potential future developments in sodium-ion solid-state batteries.

Hot-Pressing Enhances Mechanical Strength of PEO Solid Polymer Electrolyte for All-Solid-State Sodium Metal Batteries

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are widely utilized in all-solid-state sodium metal batteries (ASSSMBs) due to their excellent flexibility and safety. However, poor ionic conductivity and mechanical strength limit its development. In this work, an emerging solvent-free hot-pressing method is used to prepare mechanically robust PEO-based SPE, while sodium superionic conductors Na3Zr2Si2PO12 (NZSP) and NaClO4 are introduced to improve ionic conductivity. The as-prepared electrolyte exhibits a high ionic conductivity of 4.42 × 10-4 S cm-1 and a suitable electrochemical stability window (4.5 V vs Na/Na+). Furthermore, the SPE enables intimate contact with the electrode. The Na||Na3V2(PO4)3@C ASSSMB delivers a high-capacity retention of 97.1% after 100 cycles at 0.5 C and 60 °C, and exhibits excellent Coulombic efficiency (CE) (close to 100%). The ASSSMB with the 20 µm thick electrolyte also demonstrates excellent cyclic stability. This study provides a promising strategy for designing stable polymer-ceramic composite electrolyte membranes through hot-pressing to realize high-energy-density sodium metal batteries.

Investigation of Solid Polymer Electrolytes for NASICON-Type Solid-State Symmetric Sodium-Ion Battery

The inevitable shift toward renewable energy and electrification necessitates earth-abundant sodium reserves for next-generation Na-based energy storage technologies. By coupling the benefits of solid electrolytes over traditional nonaqueous electrolytes due to their safety hazards, solid-state sodium-ion batteries hold huge prospects in the future. This work presents a comprehensively developed solid-state sodium-ion symmetric full cell operating at room temperature enabled through a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based polymer electrolyte and modified NASICON-structured positive and negative electrodes. Among the investigated polymer electrolytes, PVDF-HFP-NaTFSI was found to outperform other counterparts by achieving a higher ionic conductivity and delivered an appreciable electrochemical stability window. By further delving into the properties of PVDF-HFP-NaTFSI, it was found to possess the least crystallinity, minimal porous structure, lowest melting point, and fusion enthalpy, indicating better ion transport than other investigated polymer electrolytes. The as-assembled solid-state battery revealed a storage capacity of 74 mAh g-1 at 0.1 C with a specific energy density of 130 Wh kgcathode_active_material-1 and demonstrated an impressive capacity retention of 84% of the initial capacity after 200 cycles. The structure and morphology retention of the cycled electrode and electrolyte through postmortem analysis bolster the electrochemo-mechanical stability of the developed solid cell. The findings reported here on polymer electrolytes persuade expedient solutions for developing ambient temperature solid-state sodium-ion batteries with promising electrochemical performance for commercialization in the near future.

Multiple Na+ transport pathways and interfacial compatibility enable high-capacity, room-temperature quasi-solid sodium batteries

Sodium-metal batteries (SMBs) are ideal for large-scale energy storage due to their stable operation and high capacity. However, they have safety issues caused by severe dendrite growth and side reactions, particularly when using liquid electrolytes. Therefore, it is critically important to develop electrolytes with high ionic conductivity and improved safety that are non-flammable and resistant to dendrites. Here, we developed polymerized polyethylene glycol diacrylate (PEGDA)-modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes (PPEs) with highly conductive sodium bis(trifluoromethanesulfonyl)imide and corrosion-inhibitive sodium bis(oxalato)borate salts for SMBs. Well-complexed PEGDA not only increases the amorphicity of the PVDF matrix, but also offers numerous Lewis basic sites through the polar groups of carbonyl and ether groups (i.e., electron donors). The presence of the Lewis basic sites facilitates the dissociation of sodium salt and transportation of Na+ within the PVDF matrix. This results in the generation of additional Na+ transport pathways, which can enhance the performance of the battery. Among PPEs, the optimized PPE-50 exhibits a high ionic conductivity of 3.42 × 10-4 S cm-1 and a mechanical strength of 14.0 MPa. A Na||Na symmetric cell with PPE-50 displays high stability at 0.2 mA cm-2 for 800 h. PPE-50 further displays high capacity, e.g., a Na3V2(PO4)3|PPE-50|Na battery delivers a decent discharge capacity of 101.5 mAh g-1 at 1.0C after 650 cycles. Our work demonstrates the development of high-performance quasi-solid polymer electrolytes with multiple transport pathways suitable for room-temperature SMBs.

Cellulose-Encapsulated Composite Electrolyte Design: Toward Chemically and Mechanically Enhanced Solid-Sodium Batteries

Sulfide- and halide-based ceramic ionic conductors exhibit comparable ionic conductivity with liquid electrolytes and are candidates for high-energy- and high-power-density all-solid-state batteries. These materials, however, are inherently brittle, making them unfavorable for applications. Here, we report a mechanically enhanced composite Na+ conductor that contains 92.5 wt % of sodium thioantimonate (Na3SbS4, NSS) and 7.5 wt % of sodium carboxymethyl cellulose (CMC); the latter serves as the binder and an electrochemically inert encapsulation layer. The ceramic and binder constituents were integrated at the particle level, providing ceramic NSS-level Na+ conductivity in the NSS-CMC composite. The more than 5-fold decrease of electrolyte thickness obtained in NSS-CMC composite provided a 5-fold increase in Na+ conductance compared to NSS ceramic pellets. As a result of the CMC encapsulation, this NSS-CMC composite shows increased moisture resistivity and electrochemical stability, which significantly promotes the cycling performance of NSS-based solid-state batteries. This work demonstrates a well-controlled, orthogonal process of ceramic-rich, composite electrolyte processing: independent streams for ceramic particle formation along with binder encapsulation in a solvent-assisted environment. This work also provides insights into the interplay among the solvent, the polymeric binder, and the ceramic particles in composite electrolyte synthesis and implies the critical importance of identifying the appropriate solvent/binder system for precise control of this complicated process.

High-Strength, Thin, and Lightweight Solid Polymer Electrolyte for Superior All-Solid-State Sodium Metal Batteries

The utilization of solid polymer electrolytes (SPEs) in all-solid-state sodium metal batteries has been extensively explored due to their excellent flexibility, processability adaptability to match roll-to-roll manufacturing processes, and good interfacial contact with a high-capacity Na anode; however, SPEs are still impeded by their inadequate mechanical strength, excessive thickness, and poor stability with Na anodes. Herein, a robust, thin, and cost-effective polyethylene (PE) film is employed as a skeleton for infiltrating poly(ethylene oxide)-sodium bis(trifluoromethanesulfonyl)imide (PEO/NaTFSI) to fabricate PE-PEO/NaTFSI SPE. The resulting SPE features a remarkable thickness of 25 μm, lightweight property (2.1 mg cm-2), superior mechanical strength (tensile strength = 100.3 MPa), and good flexibility. The SPE also shows an ionic conductivity of 9.4 × 10-5 S cm-1 at 60 °C and enhanced interfacial stability with a sodium metal anode. Benefiting from these advantages, the assembled Na-Na symmetric cells with PE-PEO/NaTFSI show a high critical current density (1 mA cm-2) and excellent long-term cycling stability (3000 h at 0.3 mA cm-2). The all-solid-state Na||PE-PEO/NaTFSI||Na3V2(PO4)3 coin cells exhibit a superior cycling performance, retaining 93% of the initial capacity for 190 cycles when matched with a 6 mg cm-2 cathode loading. Meanwhile, the pouch cell can work stably after abuse testing, proving its flexibility and safety. This work offers a promising strategy to simultaneously achieve thin, high-strength, and safe solid-state electrolytes for all-solid-state sodium metal batteries.

Silica-Poly(Vinyl Alcohol) Composite Aerogel: A Promising Electrolyte for Solid-State Sodium Batteries

The transition from fossil fuels is in part limited by our inability to store energy at different scales. Batteries are therefore in high demand, and we need them to store more energy, be more reliable, durable and have less social and environmental impact. Silica-poly(vinyl alcohol) (PVA) composite aerogels doped with sodium perchlorate were synthesized as novel electrolytes for potential application in solid-state sodium batteries. The aerogels, synthesized by one-pot synthesis, are light (up to 214 kg m-3), porous (~85%), exhibit reduced shrinkage on drying (up to 12%) and a typical silica aerogel microstructure. The formation of a silica network and the presence of PVA and sodium perchlorate in the composite were confirmed by FTIR and TGA. The XRD analysis also shows that a predominantly amorphous structure is obtained, as crystalline phases of polymer and salt are present in a very reduced amount. The effects of increasing polymer and sodium salt concentrations on the ionic conductivity, assessed via electrochemical impedance spectroscopy, were studied. At a PVA concentration of 15% (w/w silica precursors), the sodium conduction improved significantly up to (1.1 ± 0.3) × 10-5 S cm-1. Thus, this novel material has promising properties for the envisaged application.

Self-reporting and Biodegradable Thermosetting Solid Polymer Electrolyte

To date, significant efforts have been dedicated to improve their ionic conductivity, thermal stability, and mechanical strength of solid polymer electrolytes (SPEs). However, direct monitoring of physical and chemical changes in SPEs is still lacking. Moreover, existing thermosetting SPEs are hardly degradable. Herein, by overcoming the limitation predicted by Flory theory, self-reporting and biodegradable thermosetting hyperbranched poly(β-amino ester)-based SPEs (HPAE-SPEs) are reported. HPAE is successfully synthesized through a well-controlled "A2+B4" Michael addition strategy and then crosslinked it in situ to produce HPAE-SPEs. The multiple tertiary aliphatic amines at the branching sites confer multicolour luminescence to HPAE-SPEs, enabling direct observation of its physical and chemical damage. After use, HPAE-SPEs can be rapidly hydrolysed into non-hazardous β-amino acids and polyols via self-catalysis. Optimized HPAE-SPE exhibits an ionic conductivity of 1.3×10-4  S/cm at 60 °C, a Na+ transference number (t N a + ${{t}_{Na}^{+}}$ ) of 0.67, a highly stable sodium plating-stripping behaviour and a low overpotential of ≈190 mV. This study establishes a new paradigm for developing SPEs by engineering multifunctional polymers. The self-reporting and biodegradable properties would greatly expand the scope of applications for SPEs.

Solid-State Electrolytes and Electrode/Electrolyte Interfaces in Rechargeable Batteries

Solid-state batteries (SSBs) are considered to be one of the most promising candidates for next-generation energy storage systems due to the high safety, high energy density and wide operating temperature range of solid-state electrolytes (SSEs) they use. Unfortunately, the practical application of SSEs has rarely been successful, which is largely attributed to the low chemical stability and ionic conductivity, ineluctable solid-solid interface issues including limited ion transport channels, high energy barriers, and poor interface contact. A comprehensive understanding of ion transport mechanisms of various SSEs, interactions between fillers and polymer matrixes and the role of the interface in SSBs are indispensable for rational design and performance optimization of novel electrolytes. The categories, research advances and ion transport mechanism of inorganic glass/ceramic electrolytes, polymer-based electrolytes and corresponding composite electrolytes are detailly summarized and discussed. Moreover, interface contact and compatibility between electrolyte and cathode/anode are also briefly discussed. Furthermore, the electrochemical characterization methods of SSEs used in different types of SSBs are also introduced. On this basis, the principles and prospects of novel SSEs and interface design are curtly proposed according to the development requirements of SSBs. Moreover, the advanced characterizations for real-time monitoring of interface changes are also brought forward to promote the development of SSBs.

Anion-Immobilized Gel Polymer Electrolyte with a High Ion Transference Number for High-Performance Lithium/Sodium Metal Batteries

Due to their high energy density, lithium/sodium metal batteries (LMBs/SMBs) are expected to be the next generation of energy storage systems. However, the further application of alkali metal batteries based on liquid electrolytes is limited due to increasing safety concerns. Gel polymer electrolytes (GPEs), which combine the advantages of the high ionic conductivity of liquid electrolytes and excellent mechanical properties of solid polymer electrolytes, are considered to play an irreplaceable role in the realization of high-performance alkali metal batteries. In this work, a flexible boron-containing GPE (B-GPE) with a cross-linked polymer network structure is prepared by a UV-induced process. The as-prepared B-GPE exhibits good ionic conductivity and has an extremely high ion transference number due to the electron-withdrawing effect of the boron moiety and the facile electrolyte uptake ability of the ethylene oxide chain. Furthermore, a "gentle" electrode/electrolyte contact is designed by a one-step in situ polymerization method, which can enhance ion transport within the electrode and at the electrode/electrolyte interface due to the presence of a continuous polymer phase for ion conduction. Therefore, LMBs and SMBs containing B-GPE are able to effectively inhibit the growth of dendrites while exhibiting excellent cycling stability. These comprehensive results indicate that this novel B-GPE possesses potential applications for high-performance alkali metal batteries.

Deciphering Electrolyte Dominated Na+ Storage Mechanisms in Hard Carbon Anodes for Sodium-Ion Batteries

Although hard carbon (HC) demonstrates superior initial Coulombic efficiency, cycling durability, and rate capability in ether-based electrolytes compared to ester-based electrolytes for sodium-ion batteries (SIBs), the underlying mechanisms responsible for these disparities remain largely unexplored. Herein, ex situ electron paramagnetic resonance (EPR) spectra and in situ Raman spectroscopy are combined to investigate the Na storage mechanism of HC under different electrolytes. Through deconvolving the EPR signals of Na in HC, quasi-metallic-Na is successfully differentiated from adsorbed-Na. By monitoring the evolution of different Na species during the charging/discharging process, it is found that the initial adsorbed-Na in HC with ether-based electrolytes can be effectively transformed into intercalated-Na in the plateau region. However, this transformation is obstructed in ester-based electrolytes, leading to the predominant storage of Na in HC as adsorbed-Na and pore-filled-Na. Furthermore, the intercalated-Na in HC within the ether-based electrolytes contributes to the formation of a uniform, dense, and stable solid-electrolyte interphase (SEI) film and eventually enhances the electrochemical performance of HC. This work successfully deciphers the electrolyte-dominated Na+ storage mechanisms in HC and provides fundamental insights into the industrialization of HC in SIBs.

Research on Wide-Temperature Rechargeable Sodium-Sulfur Batteries: Features, Challenges and Solutions

Sodium-sulfur (Na-S) batteries hold great promise for cutting-edge fields due to their high specific capacity, high energy density and high efficiency of charge and discharge. However, Na-S batteries operating at different temperatures possess a particular reaction mechanism; scrutinizing the optimized working conditions toward enhanced intrinsic activity is highly desirable while facing daunting challenges. This review will conduct a dialectical comparative analysis of Na-S batteries. Due to its performance, there are challenges in the aspects of expenditure, potential safety hazards, environmental issues, service life and shuttle effect; thus, we seek solutions in the electrolyte system, catalysts, anode and cathode materials at intermediate and low temperatures (T < 300 °C) as well as high temperatures (300 °C < T < 350 °C). Nevertheless, we also analyze the latest research progress of these two situations in connection with the concept of sustainable development. Finally, the development prospects of this field are summarized and discussed to look forward to the future of Na-S batteries.

A soft co-crystalline solid electrolyte for lithium-ion batteries

Alternative solid electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft solid electrolyte, (Adpn)₂LiPF₆ (Adpn, adiponitrile), is synthesized and characterized that exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A substantially high ion conductivity (~10^-4 S cm^-1) and lithium-ion transference number (0.54) are obtained due to weak interactions between 'hard' (charge dense) Li⁺ ions and the 'soft' (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li⁺ ions migrate at the co-crystal grain boundaries with a (preferentially) lower activation energy E_a and within the interstitial regions between the co-crystals with higher E_a values, where the bulk conductivity is a smaller but extant contribution. These co-crystals establish a special concept of crystal design to increase the thermal stability of LiPF₆ by separating ions in the Adpn solvent matrix, and also exhibit a unique mechanism of ion conduction via low-resistance grain boundaries, which contrasts with ceramics or gel electrolytes.

Boosting Cycling Stability of Polymer Sodium Battery by "Rigid-Flexible" Coupled Interfacial Stress Modulation

The discontinuous interfacial contact of solid-state polymer metal batteries is due to the stress changes in the electrode structure during cycling, resulting in poor ion transport. Herein, a rigid-flexible coupled interface stress modulation strategy is developed to solve the above issues, which is to design a rigid cathode with enhanced solid-solution behavior to guide the uniform distribution of ions and electric field. Meanwhile, the polymer components are optimized to build an organic-inorganic blended flexible interfacial film to relieve the change of interfacial stress and ensure rapid ion transmission. The fabricated battery comprising a Co-modulated P2-type layered cathode (Na_0.67Mn_2/3Co_1/3O₂) and a high ion conductive polymer could deliver good cycling stability without distinct capacity fading (72.8 mAh g^-1 over 350 cycles at 1 C), outperforming those without Co modulation or interfacial film construction. This work demonstrates a promising rigid-flexible coupled interfacial stress modulation strategy for polymer-metal batteries with excellent cycling stability.

High-Ionic-Conductivity Sodium-Based Ionic Gel Polymer Electrolyte for High-Performance and Ultrastable Microsupercapacitors

Due to the lower cost and greater natural abundance of the sodium element on the earth than those of the lithium element, sodium-based ionic gel polymer electrolytes (IGPEs) are becoming a more cost-effective and popular material choice for portable and stationary energy solutions. The sodium-based IGPEs, however, appeared relatively inferior to their lithium-based counterparts for use in high-performance microsupercapacitors in terms of ionic conductivity and electrochemical stability. To tackle these issues, poly(ethylene glycol) diacrylate (PEGDA) with fast polymerization to build a polymer matrix and sodium perchlorate (NaClO₄) with high chemical stability and high thermal stability are employed to generate free ions for an ionic conducting phase with the support of tetramethylene glycol ether (G4) and 1-ethyl-3-methylimidazolium bis(triflouromethylsulfonyl)imide (EMIM-TFSI). It was found that the ionic conductivity (σ_dc) of this sodium-based IGPE reaches up to 0.54 mS/cm at room temperature. To manifest a high-conductivity sodium-based IGPE (SIGPE), a microsupercapacitor (MSC) with an area of 5 mm² is designed and fabricated on an interdigital reduced graphene oxide electrode. This MSC demonstrates prominent performance with a high power density of ∼2500 W/kg and a maximum energy density of ∼0.7 Wh/kg. Furthermore, after 20,000 cycles at an operating potential window from 0.0 to 1.0 V, it retains approximately 98.9% capacitance. An MSC array in 3 series × 3 parallels (3S × 3P) was successfully designed as a power source for a basic circuit with an LED. Therefore, we believe that our sodium-based IGPE microsupercapacitor holds its promising role as a solid-state energy source for high-performance and high-stability energy solutions.

High-Entropy Microdomain Interlocking Polymer Electrolytes for Advanced All-Solid-State Battery Chemistries

All-solid-state polymer electrolytes (ASPEs) with excellent processivity are considered one of the most forward-looking materials for large-scale industrialization. However, the contradiction between improving the mechanical strength and accelerating the ionic migration of ASPEs has always been difficult to reconcile. Herein, a rational concept is raised of high-entropy microdomain interlocking ASPEs (HEMI-ASPEs), inspired by entropic elasticity well-known in polymer and biochemical sciences, by introducing newly designed multifunctional ABC miktoarm star terpolymers into polyethylene oxide for the first time. The tailor-made HEMI-ASPEs possess multifunctional polymer chains, which induce themselves to assemble into micro- and nanoscale dynamic interlocking networks with high topological structure entropy. HEMI-ASPEs achieve excellent toughness, considerable ionic conductivity, an appreciable lithium transference number (0.63), and desirable thermal stability (T_d  > 400 °C) for all-solid-state lithium metal batteries. The Li|HEMI-ASPE-Li|Li symmetrical cell shows a stable Li plating/stripping performance over 4000 h, and a LiFePO₄ |HEMI-ASPE-Li|Li full cell exhibits a high capacity retention (≈96%) after 300 cycles. This work contributes an innovative design concept introducing high-entropy supramolecular dynamic networks for ASPEs.

Recent Advances in Polymers for Potassium Ion Batteries

Potassium-ion batteries (KIBs) are considered to be an effective alternative to lithium-ion batteries (LIBs) due to their abundant resources, low cost, and similar electrochemical properties of K+ to Li+, and they have a good application prospect in the field of large-scale energy storage batteries. Polymer materials play a very important role in the battery field, such as polymer electrode materials, polymer binders, and polymer electrolytes. Here in this review, we focus on the research progress of polymers in KIBs and systematically summarize the research status and achievements of polymer electrode materials, electrolytes, and binders in potassium ion batteries in recent years. Finally, based on the latest representative research of polymers in KIBs, some suggestions and prospects are put forward, which provide possible directions for future research.

Task-Specific Phosphonium Iongels by Fast UV-Photopolymerization for Solid-State Sodium Metal Batteries

Sodium metal batteries are an emerging technology that shows promise in terms of materials availability with respect to lithium batteries. Solid electrolytes are needed to tackle the safety issues related to sodium metal. In this work, a simple method to prepare a mechanically robust and efficient soft solid electrolyte for sodium batteries is demonstrated. A task-specific iongel electrolyte was prepared by combining in a simple process the excellent performance of sodium metal electrodes of an ionic liquid electrolyte and the mechanical properties of polymers. The iongel was synthesized by fast (<1 min) UV photopolymerization of poly(ethylene glycol) diacrylate (PEGDA) in the presence of a saturated 42%mol solution of sodium bis(fluorosulfonyl)imide (NaFSI) in trimethyl iso-butyl phosphonium bis(fluorosulfonyl)imide (P111i4FSI). The resulting soft solid electrolytes showed high ionic conductivity at room temperature (≥10−3 S cm−1) and tunable storage modulus (104−107 Pa). Iongel with the best ionic conductivity and good mechanical properties (Iongel10) showed excellent battery performance: Na/iongel/NaFePO4 full cells delivered a high specific capacity of 140 mAh g−1 at 0.1 C and 120 mAh g−1 at 1 C with good capacity retention after 30 cycles.

Enhancement of Electrochemical Stability Window and Electrical Properties of CNT-Based PVA-PEG Polymer Blend Composites

New polymer blend composite electrolytes (PBCEs) were prepared by the solution casting technique using poly(vinyl alcohol) (PVA)-polyethylene glycol (PEG), sodium nitrate (NaNO₃) as a doping salt and multiwalled carbon nanotubes (MWCNTs) as fillers. The X-ray diffraction pattern confirms the structural properties of the polymer blend composite films. FTIR investigations were carried out to understand the chemical properties and their band assignments. The ionic conductivity of the 10 wt % MWCNTs incorporated PVA-PEG polymer blend was measured as 4.32 × 10^-6 S cm^-1 at 20 °C and increased to 2.253 × 10^-4 S/cm at 100 °C. The dependence of its conductivity on temperature suggests Arrhenius behavior. The equivalent circuit models that represent the R _s(Q₁(R₁(Q₂(R₂(CR₃))))) were used to interpret EIS data. The dielectric behavior of the samples was investigated by utilizing their AC conductance spectra, dielectric permittivity, dielectric constant (εⁱ and ε^r), electric modulus (Mⁱ and M^r), and loss tangent tan δ. The dielectric permittivity of the samples increases due to electrode polarization effects in low frequency region. The loss tangent's maxima shift with increasing temperature; hence, the peak height rises in the high frequency region. MWCNTs-based polymer blend composite electrolytes show an enhanced electrochemical stability window (4.0 V), better transference number (0.968), and improved ionic conductivity for use in energy storage device applications.

Gel Polymer Electrolytes Design for Na-Ion Batteries

Na-ion battery has the potential to be one of the best types of next-generation energy storage devices by virtue of their cost and sustainability advantages. With the demand for high safety, the replacement of traditional organic electrolytes with polymer electrolytes can avoid electrolyte leakage and thermal instability. Polymer electrolytes, however, suffer from low ionic conductivity and large interfacial impedance. Gel polymer electrolytes (GPEs) represent an excellent balance that combines the advantages of high ionic conductivity, low interfacial impedance, high thermal stability, and flexibility. This short review summarizes the recent progress on gel polymer Na-ion batteries, focusing on different preparation approaches and the resultant physical and electrochemical properties. Reasons for the differences in ionic conductivity, mechanical properties, interfacial properties, and thermal stability are discussed at the molecular level. This Review may offer a deep understanding of sodium-ion GPEs and may guide the design of intermolecular interactions for high-performance gel polymer Na-ion batteries.

Regulating Na Electrodeposition by Sodiophilic Grafting onto Porosity-Gradient Gel Polymer Electrolytes for Dendrite-Free Sodium Metal Batteries

Sodium metal batteries have been emerging as promising candidates for post-Li battery systems owing to the natural abundance, low costs, and high energy density of Na metal. However, exploiting an Na metal anode is accompanied by uncontrolled Na electrodeposition, particularly concerning dendrite growth, hampering practical Na metal battery applications. Herein, we propose sodiophilic gel polymer electrolytes with a porosity-gradient Janus structure to alleviate Na dendrite growth. Tethering only 1.1 mol % sodiophilic poly(ethylene glycol) to poly(vinylidene fluoride-co-hexafluoropropylene) suppresses Na dendrites by regulating homogeneous Na⁺ distribution, which relies on molecular-level coordination between Na⁺ and the sodiophilic functional groups. By exploiting the porosity-gradient Janus structure, we have demonstrated that regular porosity and well-defined morphology of polymer electrolytes, particularly at the Na/electrolyte interface, significantly impact dendrite growth. This study provides new insights into the rational design of Na dendrite-suppressing polymer electrolytes, primarily focusing on the ion-regulating ability achieved by surface engineering.

Recent Advances in the Synthesis of Complex Macromolecular Architectures Based on Poly(N-vinyl pyrrolidone) and the RAFT Polymerization Technique

Recent advances in the controlled RAFT polymerization of complex macromolecular architectures based on poly(N-vinyl pyrrolidone), PNVP, are summarized in this review article. Special interest is given to the synthesis of statistical copolymers, block copolymers, and star polymers and copolymers, along with graft copolymers and more complex architectures. In all cases, PNVP is produced via RAFT techniques, whereas other polymerization methods can be employed in combination with RAFT to provide the desired final products. The advantages and limitations of the synthetic methodologies are discussed in detail.

New Diglyme-based Gel Polymer Electrolytes for Na-based Energy Storage Devices

This work presents for the first time a new diglyme-based gel polymer (DOBn-GPE) suitable for Na-based energy storage devices. The DOBn-GPE, which contains a methacrylate-based polymer, exhibited an excellent high ionic conductivity (2.3 mS cm-1 at 20 °C), broad electrochemical stability (>5.0 V), and high mechanical stability. DOBn-GPE could be successfully used for the realization of Na-ion capacitors, sodium-metal batteries, and sodium-ion batteries, displaying performance comparable with those of systems containing liquid electrolytes at room temperature and at 60 °C. The results of these investigation indicated that the development of diglyme-based gel polymer electrolytes represents a promising strategy for the realization of advanced Na-based energy storage devices.

Root Reason for the Failure of a Practical Zn-Ni Battery: Shape Changing Caused by Uneven Current Distribution and Zn Dissolution

In recent years, with the increasing application of lithium-ion batteries in energy storage devices, fire accidents caused by lithium-ion batteries have become more frequent and have arisen wide concern. Due to the safety of aqueous electrolyte, aqueous Zn-based batteries have attracted vast attention, among which Zn-Ni batteries stand out by virtue of their excellent rate performance and environmental friendliness. However, poor cycling life limits the application of Zn-Ni batteries. To figure out the main cause, a failure analysis of a practical Zn-Ni battery has been carried out. During the cycling of the Zn-Ni battery, the evolution of gas, the shape changing, and the aggregation of additive and binder of Zn anode can be observed. Combined with the finite element analysis, we finally reveal that the key factor of battery failure is the shape changing of the Zn anode caused by uneven current distribution and the dissolution of Zn. The shape changing of the Zn anode reduces the effective surface area of anode and increases the possibility of dead Zn, which makes the battery unable to discharge even in the presence of a large amount of Zn. These findings are helpful to deepen the understanding of the working and failure mechanisms of the Zn anode and provide effective guidance for subsequent research.

BaF(p-BDC)0.5 as the Catalyst Precursor for the Catalytic Dehydrochlorination of 1-Chloro-1,1-Difluoroethane to Vinylidene Fluoride

A BaF(p-BDC)0.5 catalyst was prepared by solid state reaction at room temperature with Ba(OH)2 as precursor, NH4F as F source, and H2(p-BDC) as organic ligand. The calcined samples were used as catalysts for dehydrochlorination of 1-chloro-1,1-difluoroethane to generate vinylidene fluoride (VDF) at 350 °C. Commercial production of VDF is carried out at 600–700 °C. Clearly, pyrolysis of the BaF(p-BDC)0.5 catalyst provided a promising way to prepare VDF at low temperatures. Prior to calcination, the activity of the BaF(p-BDC)0.5 catalyst was low. Following calcination at high temperatures, BaF(p-BDC)0.5 decomposed to BaF2 and BaCO3, and then the catalyst was chlorinated and fluorinated to BaClF, which showed high activity and stable VDF selectivity for dehydrochlorination of 1-Chloro-1,1-Difluoroethane to VDF.

A molecular dynamics study of a fully zwitterionic copolymer/ionic liquid-based electrolyte: Li+ transport mechanisms and ionic interactions

The development of polymer electrolytes (PEs) is crucial for advancing safe, high-energy density batteries, such as lithium-metal and other beyond lithium-ion chemistries. However, reaching the optimum balance between mechanical stiffness and ionic conductivity is not a straightforward task. Zwitterionic (ZI) gel electrolytes comprising lithium salt and ionic liquid (IL) solutions within a fully ZI polymer network can, in this context, provide useful properties. Although such materials have shown compatibility with lithium metal in batteries, several fundamental structure-dynamic relationships regarding ionic transport and the Li⁺ coordination environment remain unclear. To better resolve such issues, molecular dynamics simulations were carried out for two IL-based electrolyte systems, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) with 1 M LiTFSI salt and a ZI gel electrolyte containing the IL and a ZI copolymer: poly(2-methacryloyloxyethyl phosphorylcholine-co-sulfobetaine vinylimidazole), poly(MPC-co-SBVI). The addition of ZI polymer decreases the [TFSI]^- -[Li]⁺ interactions and increases the IL ion diffusivities, and consequently, the overall ZI gel ionic conductivity. The structural analyses showed a large preference for lithium-ion interactions with the polymer phosphonate groups, while the [TFSI]^- anions interact directly with the sulfonate group and the [BMP]⁺ cations only display secondary interactions with the polymer. In contrast to previous experimental data on the same system, the simulated transference numbers showed smaller [Li]⁺ contributions to the overall ionic conductivities, mainly due to negatively charged lithium aggregates and the strong lithium-ion interactions in the systems.

Recent Advances and Perspectives on the Polymer Electrolytes for Sodium/Potassium-Ion Batteries

Owing to the low cost of sodium/potassium resources and similar electrochemical properties of Na⁺ /K⁺ to Li⁺ , sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) are regarded as promising alternatives to lithium-ion batteries (LIBs) in large-scale energy storage field. However, traditional organic liquid electrolytes bestow SIBs/KIBs with serious safety concerns. In contrast, quasi-/solid-phase electrolytes including polymer electrolytes (PEs) and inorganic solid electrolytes (ISEs) show great superiority of high safety. However, the poor processibility and relatively low ionic conductivity of Na⁺ and K⁺ ions limit the further practical applications of ISEs. PEs combine some merits of both liquid-phase electrolytes and ISEs, and present great potentials in next-generation energy storage systems. Considerable efforts have been devoted to improving their overall properties. Nevertheless, there is still a lack of an in-depth and comprehensive review to get insights into mechanisms and corresponding design strategies of PEs. Herein, the advantages of different electrolytes, particularly PEs are first minutely reviewed, and the mechanism of PEs for Na⁺ /K⁺ ion transfer is summarized. Then, representative researches and recent progresses of SIBs/KIBs based on PEs are presented. Finally, some suggestions and perspectives are put forward to provide some possible directions for the follow-up researches.

Ionic Liquid (IL) Laden Metal-Organic Framework (IL-MOF) Electrolyte for Quasi-Solid-State Sodium Batteries

An ionic liquid (IL) laden metal-organic framework (MOF) sodium-ion electrolyte has been developed for ambient-temperature quasi-solid-state sodium batteries. The MOF skeleton is designed according to a UIO-66 (Universitetet i Oslo) structure. A sodium sulfonic (-SO₃Na) group grafted to the UIO-based MOF ligand improves the Na⁺-ion conductivity. Upon lading with a sodium-based ionic liquid (Na-IL), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) in 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Bmpyr-TFSI), the Na-IL laden sulfonated UIO-66 (UIOSNa) quasi-solid electrolyte exhibits a Na⁺-ion conductivity of 3.6 × 10^-4 S cm^-1 at ambient temperature. Quasi-solid-state sodium batteries with the Na-IL/UIOSNa electrolyte are demonstrated with a layered Na₃Ni_1.5TeO₆ cathode and sodium-metal anode. The quasi-solid-state Na∥Na-IL/UIOSNa∥Na₃Ni_1.5TeO₆ cells show remarkable cycling performance.

Effects of Comonomers on the Performance of Stable Phosphonate-Based Gel Terpolymer Electrolytes for Sodium-Ion Batteries with Ultralong Cycling Stability

Gel polymer electrolyte (GPE) is one of the most promising alternatives to solve the bottlenecks of nonaqueous liquid electrolytes such as decomposition, safety hazards, and growth of dendrites. In this work, three novel methyl phosphonate-based crosslinking gel terpolymer electrolytes with different comonomers are designed and prepared by in situ radical polymerization. The gel polymer electrolytes have excellent thermal stability, wide electrochemical windows (≥4.9 V), and high ionic conductivities (±3 mS cm^-1), and may be used as less-flammable electrolytes for sodium-ion batteries. ³¹P NMR spectra, Arrhenius plot, and density functional theory (DFT) calculations confirm that multifunctional phosphonate structural units promote the dissociation of NaClO₄ and help to transport the sodium ions freely in the polymer framework. X-ray photoelectron spectroscopy (XPS) results show that the gel polymer electrolytes have the capability of inhibiting liquid electrolyte decomposition and the formation of the stable solid electrolyte interphase (SEI) film. The Na₃V₂(PO₄)₃/GPE/Na cells exhibit better ultralong cycling stability and enhanced temperature performance than those of liquid cells. Strikingly, GPE1 has the best comprehensive electrochemical performance, especially the rate performance and long-term cycling stability with a capacity retention ratio of 82.6% after 3500 cycles, which indicates that different comonomers have obvious effects on the performance. Therefore, the full cell of SnS₂/GPE1/Na₃V₂(PO₄)₃ is evaluated and delivers good cycling stability of 500 cycles, holding a great prospect for the design and production of phosphorus-containing electrolytes for safer sodium-ion batteries.

Prevention of Na Corrosion and Dendrite Growth for Long-Life Flexible Na-Air Batteries

Rechargeable Na-air batteries (NABs) based on abundant Na resources are generating great interest due to their high energy density and low cost. However, Na anode corrosion in ambient air and the growth of abnormal dendrites lead to insufficient cycle performance and safety hazards. Effectively protecting the Na anode from corrosion and inducing the uniform Na plating and stripping are therefore of vital importance for practical application. We herein report a NAB with in situ formed gel electrolyte and Na anode with trace residual Li. The gel electrolyte is obtained within cells through cross-linking Li ethylenediamine at the anode surface with tetraethylene glycol dimethyl ether (G4) from the liquid electrolyte. The gel can effectively prevent H₂O and O₂ crossover, thus delaying Na anode corrosion and electrolyte decomposition. Na dendrite growth was suppressed by the electrostatic shield effect of Li⁺ from the modified Li layer. Benefiting from these improvements, the NAB achieves a robust cycle performance over 2000 h in opened ambient air, which is superior to previous results. Gelation of the electrolyte prevents liquid leakage during battery bending, facilitating greater cell flexibility, which could lead to the development of NABs suitable for wearable electronic devices in ambient air.

Electrolytes for Lithium- and Sodium-Metal Batteries

High-energy-density batteries have attracted significant attention due to the huge demand in electric transportation in future. Metal-based batteries, especially lithium metal batteries (LMBs) and sodium metal batteries (SMBs), have been hot research topics nowadays. The uncontrolled growth of metal dendrites has retarded the development of LMBs and SMBs. Various electrolytes have been explored to meet the demand of high-performance metal-based batteries, such as additives-contained electrolytes, polymer electrolytes, and solid-state electrolytes. To guide the development of electrolytes in LMBs and SMBs, we organize this roadmap to give out the status of present research and future challenges in this field. We also hope that the readers can get the knowledge and ideas from this roadmap.

Electrolyte Technologies for High Performance Sodium-Ion Capacitors

Bridging the energy gap between batteries and capacitors, while in principle delivering a supercapacitor-like high power density and long lifespan, sodium-ion capacitors (SIC) have been considered promising energy storage devices that could be commercialized in the near future due to the natural abundance of sodium sources and the performance superiority of SIC devices. Therefore, in the past decade, substantial research efforts have been devoted to their structure and property improvements. With regard to the electrochemical performance of an ion capacitor, except for the electrode, the composition and structure of the electrolytes are also of great importance. Thus, in this mini review, we present a brief summary of the electrolytes developed recently for high performance SIC, including aqueous, organic, and ionic liquid based electrolytes. The influence factors such as ionic conductivities, electrolyte concentrations, electrochemical stable windows, as well as the cost and safety issues are discussed. Furthermore, the future perspectives and challenges in the science and engineering of new electrolytes are also considered. We hope that this review may be helpful to the energy storage community regarding the electrolytes of advanced SIC systems.

PY13FSI-Infiltrated SBA-15 as Nonflammable and High Ion-Conductive Ionogel Electrolytes for Quasi-Solid-State Sodium-Ion Batteries

Exploring electrolytes of high safety is essential to pave the practical route for sodium-ion batteries (SIBs) toward their important applications in large-scale energy storage and power supplies. In this regard, ionogel electrolytes (IEs) have been highlighted owing to their high ionic conductivity, prominent electrochemical and thermal stability, and, more crucially, high interfacial wettability. However, present studies lack an understanding of the interaction of IEs, which determines the ion desolvation and migration. In this article, IEs comprising an SBA-15 host, an ionic liquid, sodium salt, and poly(vinylidene fluoride)-hexafluoro propylene (PVDF-HFP) have been proposed by mechanical ball milling and roller pressing. The component ratio has been optimized based on the balance between ionic conductivity and self-supporting capability of IEs. The optimal IEs showed sufficiently high ionic conductivity (2.48 × 10^-3 S cm^-1 at 30 °C), wide electrochemical window (up to 4.8 V vs Na⁺/Na), and high Na⁺ transference number (0.37). Due to the presence of SBA-15 and an ionic liquid, the IEs exhibited much improved thermal resistance than that of the conventional organic liquid electrolytes (OLEs). Furthermore, Fourier transform infrared (FT-IR) spectroscopy revealed the hydrogen bonding interaction between silanols and the dissolved salts, not only anchoring anions for immobilization but also promoting the dissociation of sodium salts. After being matched with the Na₃V₂(PO₄)₃ (NVP) cathode and metallic Na anode, the SIBs presented a specific discharge capacity of up to 110.7 mA h g^-1 initially at room temperature with 92% capacity retention after 300 cycles. The improved safety and electrochemical performance provided insights into rationally regulating IEs and their interactions with the prospect of strengthening their practical applications in SIBs.

Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering

An up-to-date review about MXenes based on their distinguishing properties, namely, large interlayer spacing and rich surface chemistry.

Ultrastable Sodium Storage in MoO3 Nanotube Arrays Enabled by Surface Phosphorylation

Molybdenum trioxide (MoO₃) has been considered as an appealing choice of anode for sodium-ion batteries because of its high theoretical capacity (1117 mA h g^-1). However, the large volume change upon Na⁺ storage results in poor cycling stability and capacity fade of MoO₃. Here, we demonstrate a surface phosphorylation strategy to mitigate the degradation of three-dimensional MoO₃ array electrodes. Such a phosphorylation strategy allows MoO₃ arrays to sustain a capacity of 265 mA h g^-1, or ∼90% of the initial value, at a rate of 2 A g^-1 over 1500 cycles, outperforming most reported MoO₃ electrodes. Moreover, kinetic analysis unveils a capacitance-dominated Na⁺ storage feature of MoO₃ arrays, owing to the enhanced electron mobility imparted by oxygen vacancies that are simultaneously introduced by phosphorylation. Hence, surface phosphorylation might offer new possibilities to bypass multiple materials challenges facing current sodium electrodes.

An Ordered Ni6 -Ring Superstructure Enables a Highly Stable Sodium Oxide Cathode

Sodium-based layered oxides are among the leading cathode candidates for sodium-ion batteries, toward potential grid energy storage, having large specific capacity, good ionic conductivity, and feasible synthesis. Despite their excellent prospects, the performance of layered intercalation materials is affected by both a phase transition induced by the gliding of the transition metal slabs and air-exposure degradation within the Na layers. Here, this problem is significantly mitigated by selecting two ions with very different MO bond energies to construct a highly ordered Ni₆ -ring superstructure within the transition metal layers in a model compound (NaNi_2/3 Sb_1/3 O₂ ). By virtue of substitution of 1/3 nickel with antimony in NaNiO₂ , the existence of these ordered Ni₆ -rings with super-exchange interaction to form a symmetric atomic configuration and degenerate electronic orbital in layered oxides can not only largely enhance their air stability and thermal stability, but also increase the redox potential and simplify the phase-transition process during battery cycling. The findings reveal that the ordered Ni₆ -ring superstructure is beneficial for constructing highly stable layered cathodes and calls for new paradigms for better design of layered materials.