Electrochemical Characterization of Single Lithium-Ion Conducting Polymer Electrolytes Based on sp3 Boron and Poly(ethylene glycol) Bridges

Alkali metal alkoxyborate ester salts; a contemporary look at old compounds

Research into the use of sodium tetraalkoxyborate salts for different chemical applications including synthetic catalysis, hydrogen storage, or battery applications has been investigated, however, understanding of the structural, thermal and electrochemical properties of these salts has been lacking since the 1950s and 1960s. A review of the synthesis, as well as a thorough characterization using 1H NMR, 11B NMR, 13C{1H} NMR, FTIR, XRD, in situ XRD, DSC-TGA, RGA-MS, TPPA, and EIS has newly identified polymorphic phase changes for Na[B(OMe)4], K[B(OMe)4], Li[B(OMe)4], Na[B(OEt)4], Na[B(OBu)4], and Na[B(OiBu)4]. The crystal structure of K[B(OMe)4] was also solved in I41/a (a = 22.337(2) Å, c = 7.648(3) Å, V = 3815.6(4) Å3, ρ = 1.128(1) g cm-3). Ionic conductivity of the different salts was analyzed, however it was found that the compounds with longer alkyl chains had no measurable ionic conductivity compared to the shorter chained samples, Na[B(OMe)4] and K[B(OMe)4] with 9.6 × 10-8 S cm-1 and 1.6 × 10-7 S cm-1, at 114 °C respectively.

Single-Ion Conductive Polymer-Based Composite Electrolytes for High-Performance Solid-State Lithium Metal Batteries

Flexible composite polymer electrolytes (CPEs) with inorganic electrolyte fillers dispersed in polymer electrolytes integrate the merits of the polymer and inorganic electrolytes and have attracted much attention in recent years. In order to increase the electrochemical performance, especially the low lithium (Li)-ion transference number in traditional dual-ion Li salt-containing CPEs, single-ion conductive CPEs are synthesized with a single-ion polymer conductor (SIPC) as the matrix and Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) particles as the active fillers. The single-ion conductive CPEs show a high Li-ion transference number (up to 0.96), high room-temperature (RT) ionic conductivity (>1.0 × 10-4 S cm-1 ), wide electrochemical stability window (>5.0 V, vs Li/Li+ ), and excellent long-term cycling stability with Li metal at RT (3200 h). Based on the SIPC-LLZTO CPE, the solid-state lithium metal batteries with LiFePO4 - and LiCoO2 -based cathodes deliver average discharge capacities of 159 mAh g-1 for 600 cycles and 119 mAh g-1 for 200 cycles at RT, respectively. This study sheds light on the design of high-performance CPEs for next-generation solid-state lithium metal batteries.

High Performance Ternary Solid Polymer Electrolytes Based on High Dielectric Poly(vinylidene fluoride) Copolymers for Solid State Lithium-Ion Batteries

Renewable energy sources require efficient energy storage systems. Lithium-ion batteries stand out among those systems, but safety and cycling stability problems still need to be improved. This can be achieved by the implementation of solid polymer electrolytes (SPE) instead of the typically used separator/electrolyte system. Thus, ternary SPEs have been developed based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) as host polymers, clinoptilolite (CPT) zeolite added to stabilize the battery cycling performance, and ionic liquids (ILs) (1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN])), 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([PMPyr][TFSI]) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), incorporated to increase the ionic conductivity. The samples were processed by doctor blade with solvent evaporation at 160 °C. The nature of the polymer matrix and fillers affect the morphology and mechanical properties of the samples and play an important role in electrochemical parameters such as ionic conductivity value, electrochemical window stability, and lithium-transference number. The best ionic conductivity (4.2 × 10^-5 S cm^-1) and lithium transference number (0.59) were obtained for the PVDF-HFP-CPT-[PMPyr][TFSI] sample. Charge-discharge battery tests at C/10 showed excellent battery performance with values of 150 mAh g^-1 after 50 cycles, regardless of the polymer matrix and IL used. In the rate performance tests, the best SPE was the one based on the P(VDF-TrFE-CFE) host polymer, with a discharge value at C-rate of 98.7 mAh g^-1, as it promoted ionic dissociation. This study proves for the first time the suitability of P(VDF-TrFE-CFE) as SPE in lithium-ion batteries, showing the relevance of the proper selection of the polymer matrix, IL type, and lithium salt in the formulation of the ternary SPE, in order to optimize solid-state battery performance. In particular, the enhancement of the ionic conductivity provided by the IL and the effect of the high dielectric constant polymer P(VDF-TrFE-CFE) in improving battery cyclability in a wide range of discharge rates must be highlighted.

Influence of Solvent Evaporation Temperature on the Performance of Ternary Solid Polymer Electrolytes Based on Poly(vinylidene fluoride-co-hexafluoropropylene) Combining an Ionic Liquid and a Zeolite

Solid polymer electrolytes (SPEs) will allow improving safety and durability in next-generation solid-state lithium-ion batteries (LIBs). Within the SPE class, ternary composites are a suitable approach as they provide high room-temperature ionic conductivity and excellent cycling and electrochemical stability. In this work, ternary SPEs based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as a polymer host, clinoptilolite (CPT) zeolite, and 1-butyl-3-methylimidazolium thiocyanate ([Bmim][SCN])) ionic liquid (IL) as fillers were produced by solvent evaporation at different temperatures (room temperature, 80, 120, and 160 °C). Solvent evaporation temperature affects the morphology, degree of crystallinity, and mechanical properties of the samples as well as the ionic conductivity and lithium transference number. The highest ionic conductivity (1.2 × 10^-4 S·cm^-1) and lithium transference number (0.66) have been obtained for the SPE prepared at room temperature and 160 °C, respectively. Charge-discharge battery tests show the highest value of discharge capacity of 149 and 136 mAh·g^-1 at C/10 and C/2 rates, respectively, for the SPE prepared at 160 °C. We conclude that the fine control of the solvent evaporation temperature during the preparation of the SPE allows us to optimize solid-state battery performance.

Cross-Linking of Sugar-Derived Polyethers and Boronic Acids for Renewable, Self-Healing, and Single-Ion Conducting Organogel Polymer Electrolytes

This report describes the synthesis and characterization of organogels by reaction of a diol-containing polyether, derived from the sugar d-xylose, with 1,4-phenylenediboronic acid (PDBA). The cross-linked materials were analyzed by infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (FE-SEM), and rheology. The rheological material properties could be tuned: gel or viscoelastic behavior depended on the concentration of polymer, and mechanical stiffness increased with the amount of PDBA cross-linker. Organogels demonstrated self-healing capabilities and recovered their storage and loss moduli instantaneously after application and subsequent strain release. Lithiated organogels were synthesized through incorporation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into the cross-linked matrix. These lithium-borate polymer gels showed a high ionic conductivity value of up to 3.71 × 10^-3 S cm^-1 at 25 °C, high lithium transference numbers (t ₊ = 0.88-0.92), and electrochemical stability (4.51 V). The gels were compatible with lithium-metal electrodes, showing stable polarization profiles in plating/stripping tests. This system provides a promising platform for the production of self-healing gel polymer electrolytes (GPEs) derived from renewable feedstocks for battery applications.

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⁺ .

Toward Sustainable Solid Polymer Electrolytes for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most widely used energy storage system because of their high energy density and power, robustness, and reversibility, but they typically include an electrolyte solution composed of flammable organic solvents, leading to safety risks and reliability concerns for high-energy-density batteries. A step forward in Li-ion technology is the development of solid-state batteries suitable in terms of energy density and safety for the next generation of smart, safe, and high-performance batteries. Solid-state batteries can be developed on the basis of a solid polymer electrolyte (SPE) that may rely on natural polymers in order to replace synthetic ones, thereby taking into account environmental concerns. This work provides a perspective on current state-of-the-art sustainable SPEs for lithium-ion batteries. The recent developments are presented with a focus on natural polymers and their relevant properties in the context of battery applications. In addition, the ionic conductivity values and battery performance of natural polymer-based SPEs are reported, and it is shown that sustainable SPEs can become essential components of a next generation of high-performance solid-state batteries synergistically focused on performance, sustainability, and circular economy considerations.

High-Performance Room Temperature Lithium-Ion Battery Solid Polymer Electrolytes Based on Poly(vinylidene fluoride-co-hexafluoropropylene) Combining Ionic Liquid and Zeolite

The demand for more efficient energy storage devices has led to the exponential growth of lithium-ion batteries. To overcome the limitations of these systems in terms of safety and to reduce environmental impact, solid-state technology emerges as a suitable approach. This work reports on a three-component solid polymer electrolyte system based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]), and clinoptilolite zeolite (CPT). The influences of the preparation method and of the dopants on the electrolyte stability, ionic conductivity, and battery performance were studied. The developed electrolytes show an improved room temperature ionic conductivity (1.9 × 10^-4 S cm^-1), thermal stability (up to 300 °C), and mechanical stability. The corresponding batteries exhibit an outstanding room temperature performance of 160.3 mAh g^-1 at a C/15-rate, with a capacity retention of 76% after 50 cycles. These results represent a step forward in a promising technology aiming the widespread implementation of solid-state batteries.

High-Performance Room Temperature Lithium-Ion Battery Solid Polymer Electrolytes Based on Poly(vinylidene fluoride-co-hexafluoropropylene) Combining Ionic Liquid and Zeolite

The demand for more efficient energy storage devices has led to the exponential growth of lithium-ion batteries. To overcome the limitations of these systems in terms of safety and to reduce environmental impact, solid-state technology emerges as a suitable approach. This work reports on a three-component solid polymer electrolyte system based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]), and clinoptilolite zeolite (CPT). The influences of the preparation method and of the dopants on the electrolyte stability, ionic conductivity, and battery performance were studied. The developed electrolytes show an improved room temperature ionic conductivity (1.9 × 10^-4 S cm^-1), thermal stability (up to 300 °C), and mechanical stability. The corresponding batteries exhibit an outstanding room temperature performance of 160.3 mAh g^-1 at a C/15-rate, with a capacity retention of 76% after 50 cycles. These results represent a step forward in a promising technology aiming the widespread implementation of solid-state batteries.

Transference Number Determination in Poor-Dissociated Low Dielectric Constant Lithium and Protonic Electrolytes

Whereas the major potential of the development of lithium-based cells is commonly attributed to the use of solid polymer electrolytes (SPE) to replace liquid ones, the possibilities of the improvement of the applicability of the fuel cell is often attributed to the novel electrolytic materials belonging to various structural families. In both cases, the transport properties of the electrolytes significantly affect the operational parameters of the galvanic and fuel cells incorporating them. Amongst them, the transference number (TN) of the electrochemically active species (usually cations) is, on the one hand, one of the most significant descriptors of the resulting cell operational efficiency while on the other, despite many years of investigation, it remains the worst definable and determinable material parameter. The paper delivers not only an extensive review of the development of the TN determination methodology but as well tries to show the physicochemical nature of the discrepancies observed between the values determined using various approaches for the same systems of interest. The provided critical review is supported by some original experimental data gathered for composite polymeric systems incorporating both inorganic and organic dispersed phases. It as well explains the physical sense of the negative transference number values resulting from some more elaborated approaches for highly associated systems.

Silica-assisted cross-linked polymer electrolyte membrane with high electrochemical stability for lithium-ion batteries

This study aims to prepare an organic-inorganic reticular polymer electrolyte. Isocyanate acts as a bridge that connects fumed silica and PEO molecular chains. The PEO-TDI-SiO₂ solid polymer electrolytes developed can significantly have improved ionic conductivity of 0.12 mS cm^-1 at ambient temperature. This is because the TDI-SiO₂ nanoparticles inhibits polymer crystallization which provides more continuous Li-ion transport pathways. Tests at 60 °C indicate that the cross-linked structure of covalent TSI bonded to PEO effectively enlarges the electrochemical window of the polymer electrolyte to 5.6 V. Also, the PTSI electrolyte membrane has a higher Li⁺ transference number of 0.33 compared to the PEO-LiTFSI electrolytes. It is worth noting that the assembled LiFePO₄|PTSI8%|Li cells deliver outstanding rate performance and stable cycling performance. All these considerable merits of PTSI membrane demonstrate that PTSI is a promising candidate that can be used as solid polymer electrolytes for the next-generation Li-ion batteries.

Molecular Design of a Highly Stable Single-Ion Conducting Polymer Gel Electrolyte

Single-ion conducting (SIC) polymer electrolytes with a high Li transference number (t_Li⁺) have shown the capability to enable enhanced battery performance and safety by avoiding liquid-electrolyte leakage and suppressing Li dendrite formation. However, issues of insufficient ionic conductivity, low electrochemical stability, and poor polymer/electrode interfacial contact have greatly hindered their commercial use. Here, a Li-containing boron-centered fluorinated SIC polymer gel electrolyte (LiBFSIE) was rationally designed to achieve a high t_Li⁺ and high electrochemical stability. Owing to the low dissociation energy of the boron-centered anion and Li⁺, the as-prepared LiBFSIE exhibited an ionic conductivity of 2 × 10^-4 S/cm at 35 °C, which is exclusively contributed by Li ions owing to a high t_Li⁺ of 0.93. Both simulation and experimental approaches were applied to investigate the ion diffusion and concentration gradient in the LiBFSIE and non-cross-linked dual-ion systems. Typical rectangular Li stripping/plating voltage profiles demonstrated the uniform Li deposition assisted by LiBFSIE. The interfacial contact and electrolyte infiltration were further optimized with an in situ UV-vis-initiated polymerization method together with the electrode materials. By virtue of the high electrochemical stability of LiBFSIE, the cells achieved a promising average Coulombic efficiency of 99.95% over 200 cycles, which is higher than that of liquid-electrolyte-based cells. No obvious capacity fading was observed, indicating the long-term stability of LiBFSIE for lithium metal batteries.

Toward safer solid-state lithium metal batteries: a review

The solid-state lithium metal battery (SSLMB) is one of the most optimal solutions to pursue next-generation energy storage devices with superior energy density, in which solid-state electrolytes (SSEs) are expected to completely solve the safety problems caused by direct use of a lithium metal anode. Most previous work has mainly focused on improving the electrochemical performance of SSLMBs, but the safety issues have been largely ignored due to the influence of the stereotype that batteries with SSEs are always safe. In the actual research process, however, some potential dangers of SSLMBs have been gradually revealed, so extra attention should be paid to this issue. This minireview summarizes several aspects that could raise safety concerns and provides a brief overview of the corresponding solutions to each aspect. Finally, general conclusions and perspectives on the research of SSLMBs with ultra-high safety are presented.