Highly Conductive Polymeric Ionic Liquid Electrolytes for Ambient-Temperature Solid-State Lithium Batteries

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.

Lithium salt of a pro-mesogenic [closo-CB11H12]- derivative: anisotropic Li+ ion transport in liquid crystalline electrolytes

Li+ ion conduction in two aligned liquid crystalline electrolytes consisting of 10 mol% Li+ salt of a pro-mesogenic anion derived from [closo-1-CB11H12]- in non-ionic hosts was investigated. Using electrochemical impedance spectroscopy (EIS), the ionic conductivity in the parallel (σ‖) and perpendicular (σ⊥) directions of the electrolyte samples was determined using two types of cells: an interdigitated gold electrode and a nylon 6-coated ITO cell. The ratio of ionic conductivities σ⊥/σ‖ in the electrolyte with a nona(ethylene oxide) spacer was about 3 in the entire SmA phase, while in the shorter homologue, the ratio monotonically increases from about 0.4 to 2.9. The liquid crystalline behavior of the hosts and the electrolytes was investigated by optical, thermal, and powder XRD methods.

Rigid-flexible coupling network solid polymer electrolytes for all-solid-state lithium metal batteries

Poor mechanical strength at working temperature and low ionic conductivity seriously hinder the application of poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) in high performance all-solid-state lithium metal batteries (LMBs). Here, we design and prepare a series of rigid-flexible coupling network SPEs (RFN-SPEs) with soft poly(ethylene glycol) (PEG) chains and rigid crosslinkers containing the benzene structure. Compared with soft crosslinkers, rigid crosslinkers provide the same amount of active crosslinking points with smaller molecular weight, and meanwhile enhance the mechanical strength of the network. Therefore, based on the rigid crosslinkers, RFN-SPEs exhibit synchronously improved ionic conductivity and mechanical strength. With these RFN-SPEs, symmetrical cells can be cycled for over 2100 h at 0.5 mA cm-2. Meanwhile, stable cycling and high-rate capability could be achieved for LMBs, revealing that SPEs with the rigid-flexible coupling network are promising electrolyte systems for all-solid-state LMBs.

Metal-Organic Framework Glass as a Functional Filler Enables Enhanced Performance of Solid-State Polymer Electrolytes for Lithium Metal Batteries

Polymers are promising candidates as solid-state electrolytes due to their performance and processability, but fillers play a critical role in adjusting the polymer network structure and electrochemical, thermal, and mechanical properties. Most fillers studied so far are anisotropic, limiting the possibility of homogeneous ion transport. Here, applying metal-organic framework (MOF) glass as an isotropic functional filler, solid-state polyethylene oxide (PEO) electrolytes are prepared. Calorimetric and diffusion kinetics tests show that the MOF glass addition reduces the glass transition temperature of the polymer phase, improving the mobility of the polymer chains, and thereby facilitating lithium (Li) ion transport. By also incorporating the lithium salt and ionic liquid (IL), Li-Li symmetric cell tests of the PEO-lithium salt-MOF glass-IL electrolyte reveal low overpotential, indicating low interfacial impedance. Simulations show that the isotropic structure of the MOF glass facilitates the wettability of the IL by enhancing interfacial interactions, leading to a less confined IL structure that promotes Li-ion mobility. Finally, the obtained electrolyte is used to construct Li-lithium iron phosphate full batteries that feature high cycle stability and rate capability. This work therefore demonstrates how an isotropic functional filler can be used to enhance the electrochemical performance of solid-state polymer electrolytes.

A comparative study of ammonia solubility in imidazolium-based ionic liquids with different structural compositions

Four imidazolium-based ionic liquids (ILs) with two cations 1-pentyl-3-butylimidazolium [PBIM]+ and 1-benzyl-3-butylimidazolium tetrafluoroborate [BzBIM]+, and two anions tetrafluoroborate (BF4-) and trifluoromethanesulfonate (OTf-) were synthesized for NH3 solubility enhancement. The structural, thermal, and electrochemical stabilities, ionic conductivity, and viscosity of the four ILs, namely, [PBIM]BF4, [BzBIM]BF4, [PBIM]OTf, and [BzBIM]OTf, were investigated. Due to the intermolecular interaction of the benzyl group attached to the imidazolium ring, [BzBIM]+-based ILs exhibited higher thermal stability but lower ionic conductivity compared to [PBIM]+-based ILs. Further, the NH3 solubility in all ILs was measured using a custom-made setup at temperatures ranging from 293.15 to 323.15 K and pressures ranging from 1 to 5 bar. The effects of the cation and anion structures of ILs, as well as pressure and temperature, on the NH3 solubility in the ILs were also investigated. [PBIM]BF4 showed the best solubility because of its high free volume and low viscosity. Density functional calculations validated the superior NH3 solubility in [PBIM]BF4, attributable to the minimal reorganization of the [cation]anion complex geometry during the solvation process, yielding a low solvation free energy. The findings of this study suggest that ILs exhibit a high NH3 solubility capacity and cation and anion structures considerably affect the NH3 solubility in ILs.

A Thin Composite Polymer Electrolyte Functionalized by a Novel Antihydrolysis Additive to Enable All-Solid-State Lithium Battery with Excellent Rate and Cycle Performance

Composite solid-state electrolyte (CSE) incorporated with fluorine-containing functional additives usually endows the assembled cell with improved electrochemical performance by forming stable electrode/electrolyte interfaces. However, most of fluorine-containing additives are prone to hydrolysis, which is not suitable for the large-scale preparation of CSEs. In this work, an antihydrolysis and fluorine-containing additive of magnesium 2,3,4,5,6-pentafluorophenylacetate (MgPFPAA) is successfully synthesized and then used to regulate the properties of the electrode/electrolyte interfaces of the all-solid-state lithium batteries (ASSLBs). The antihydrolysis property of MgPFPAA facilitates the large-scale preparation of the ultrathin CSEs in atmospheric environment. Both theoretical calculations and experimental results indicate that MgPFPAA can effectively improve the composition and structure of the generated solid electrolyte interface film by providing rich F sources and Mg2+ , thus leading to a stable CSE/Li interface. Furthermore, an ultrathin PEO/PVDF-based CSE (≈30 µm) functionalized by this novel MgPFPAA additive enables the assembled LiFePO4 -based ASSLB with greatly enhanced electrochemical performances, with high discharge specific capacity of 93.7 mAh g-1 at 10 C and a high capacity retention of 74.9% after 1500 cycles at 5.0 C. Also, this MgPFPAA functionalized CSE can be compatible with the high-areal-capacity LiFePO4 and the high-voltage LiNi0.8 Co0.1 Mn0.1 O2 cathodes.

Modulating the Coordination Environment of Lithium Bonds for High Performance Polymer Electrolyte Batteries

The new-generation lithium metal batteries require polymer electrolytes with high ionic conductivity and mechanical properties. However, the performance of the polymer electrolytes is severely influenced by the lithium bond formation between the functional groups and lithium ions (Li⁺), which has barely been considered in the past. Herein, a lithium bond enriched polymer gel (PAEV) is elaborately designed by copolymerizing 4-acryloylmorpholine (ACMO) and 1-vinyl-3-ethyl imidazolium bis(trifluoromethylsulfonyl)imide ([VEIM][TFSI]) in 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) with the presence of LiFSI. The lithium bonds formed between LiFSI and carbonyl groups in PACMO can be regulated by the Li⁺ coordination number, and further weakened by the hydrogen bonds with [EMIM][TFSI] and poly[VEIM][TFSI], to effectively render the polymer electrolyte with adjustable ionic conductivity and tunable mechanical property. In addition, with the regulated coordination environment of Li⁺, the LiF and Li₃N layer can be uniformly formed on the Li surface to facilitate Li⁺ nucleation and deposition. As a consequence, the PAEV electrolyte confers the Li/LiFePO₄ (LFP) battery with high capacity of 124 mA h g^-1 at 1 C under 25 °C, and 152 mA h g^-1 under 50 °C. This work can promote the development of high performance polymer electrolyte via lithium bond manipulation.

Bifunctional Solid-State Copolymer Electrolyte with Stabilized Interphase for High-Performance Lithium Metal Battery in a Wide Temperature Range

Solid-state polymer electrolytes (SPEs) are expected to guarantee safe and durable operations of lithium metal batteries (LMBs). Herein, inspired by the salutary poly(vinyl ethylene carbonate) (PVEC) component in the solid electrolyte interphase, cross-linking vinyl ethylene carbonate and ionic liquid copolymers were synthesized by in-situ polymerization to serve as polymer electrolyte for LMBs. On one hand, due to rich ester bonds of PVEC, Li⁺ could transfer by coupling/decoupling with oxygen atoms. On the other hand, the imidazole ring of ionic liquid could facilitate the dissociation of lithium salt to promote the free movement of Li⁺ . The bifunctional component synergistically increased the ionic conductivity of the SPE to 1.97×10^-4  S cm^-1 at 25 °C. Meanwhile, it also showed a wide electrochemical window, superior mechanical properties, outstanding non-combustibility, and excellent interfacial compatibility. The bifunctional copolymer-based LiFePO₄ batteries could normally operate at 0 to 60 °C, making them a promising candidate for wide-temperature-rang LMBs.

Room temperature all-solid-state lithium batteries based on a soluble organic cage ionic conductor

All solid-state lithium batteries (SSLBs) are poised to have higher energy density and better safety than current liquid-based Li-ion batteries, but a central requirement is effective ionic conduction pathways throughout the entire cell. Here we develop a catholyte based on an emerging class of porous materials, porous organic cages (POCs). A key feature of these Li⁺ conducting POCs is their solution-processibility. They can be dissolved in a cathode slurry, which allows the fabrication of solid-state cathodes using the conventional slurry coating method. These Li⁺ conducting cages recrystallize and grow on the surface of the cathode particles during the coating process and are therefore dispersed uniformly in the slurry-coated cathodes to form a highly effective ion-conducting network. This catholyte is shown to be compatible with cathode active materials such as LiFePO₄, LiCoO₂ and LiNi_0.5Co_0.2Mn_0.3O₂, and results in SSLBs with decent electrochemical performance at room temperature.

While solid-state batteries offer higher energy densities than liquid-based batteries, such devices require effective ion conduction pathways. Here, authors prepare porous organic cages as solution-processable catholytes that are enable excellent performances from various cathode active materials.

Recent advances in poly(ionic liquid)s for biomedical application

Poly(ionic liquid)s (PILs) are polymers containing ions in their side-chain or backbone, and the designability and outstanding physicochemical properties of PILs have attracted widespread attention from researchers. PILs have specific characteristics, including negligible vapor pressure, high thermal and chemical stability, non-flammability, and self-assembly capabilities. PILs can be well combined with advanced analytical instruments and technology and have made outstanding contributions to the development of biomedicine aiding in the continuous advancement of science and technology. Here we reviewed the advances of PILs in the biomedical field in the past five years with a focus on applications in proteomics, drug delivery, and development. This paper aims to engage pharmaceutical and biomedical scientists to full understand PILs and accelerate the progress from laboratory research to industrialization.

Poly(ionic liquid)s Based on Copolymers of Poly(ethylene oxide) and Cationic Glycidyl Triazolyl Polymers with Tribranched Side Chains

Copolymers comprising poly(ethylene oxide) and cationic glycidyl triazolyl polymer with tribranched side chains (PEO-co-GTP·3X) were synthesized from glycidyl azide copolymer (PEO-co-GAP) and the tricationic alkyne. A synthetic route for the tricationic alkyne was also improved. Bis(trifluoromethanesulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI) were used as counteranions. Copolymers PEO-co-GTP·3TFSI and PEO-co-GTP·3FSI were characterized by NMR, IR, size exclusion chromatography, DSC, TGA, rheological, and impedance measurements. The NMR results suggested that the main chain of the copolymer was more flexible than that of the homopolymer. However, no major changes were detected in the glass transition temperature and ionic conductivity of the homopolymer and copolymer with TFSI counteranions. The counterion exchange from TFSI to FSI resulted in an increase in the storage modulus and complex viscosity because of the ionic association. Despite its unfavorable viscoelastic properties, PEO-co-GTP·3FSI exhibited higher ionic conductivity than PEO-co-GTP·3TFSI (3.9 × 10^-5 S cm^-1 at 25 °C under anhydrous conditions).

Poly(ionic liquid)s with branched side chains: polymer design for breaking the conventional record of ionic conductivity

Poly(ionic liquid)s with branched side chains can break the conventional record of ionic conductivity of single-ion conductors.