All‐Solid‐State Sodium Batteries with a Polyethylene Glycol Diacrylate–Na 3 Zr 2 Si 2 PO 12 Composite Electrolyte

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.

Incorporating Sodium-Conductive Polymeric Interfacial Adhesive with Inorganic Solid-State Electrolytes for Quasi-Solid-State Sodium Metal Batteries

Inorganic solid-state electrolytes have attracted enormous attention due to their potential safety, increased energy density, and long cycle-life benefits. However, their application in solid-state batteries is limited by unstable electrode-electrolyte interface, poor point-to-point physical contact, and low utilization of metallic anodes. Herein, interfacial engineering based on sodium (Na)-conductive polymeric solid-state interfacial adhesive is studied to improve interface stability and optimize physical contacts, constructing a robust organic-rich solid electrolyte interphase layer to prevent dendrite-induced crack propagation and security issues. The interfacial adhesive strategy significantly increases the room-temperature critical current density of inorganic Na-ion conductors from 0.8 to 3.2 mA cm-2 and markedly enhances the cycling performance of solid-state batteries up to 500 cycles, respectively. Particularly, the Na3V2(PO4)3-based full solid-state batteries with high cathode loading of 10.16 mg cm-2 also deliver an excellent cycling performance, further realizing the stable operation of solid-state laminated pouch cells. The research provides fundamental perspectives into the role of interfacial chemistry and takes the field a step closer to realizing practical solid-state batteries.

Virtues of Cold Isostatic Pressing for Preparation of All-Solid-State-Batteries with Poly(Ethylene Oxide)

All-solid-state-batteries (ASSBs) necessitate the preparation of a solid electrolyte and an electrode couple with individually dense and compact structures with superior interfacial contact to minimize overall cell resistance. A conventional preparation method of solid polymer electrolyte (SPE) with polyethylene-oxide (PEO) generally consists in employing uni-axial hot press (HP) to densify SPE. However, while uni-axial press with moderate pressure effectively densifies PEO with Li salts, excessive pressure also unavoidably results in perpendicular elongation and deformation for polymer matrix. In this research, to overcome this limitation for the uni-axial press technique, a cold isostatic press (CIP) is applied to the fabrication of ASSB with PEO and LiFePO4 . CIP effectively and uniformly applies pressure as high as 500 MPa without deformation. Characterizations confirm that CIP treated SPE has enhanced mechanical puncture strength, increasing from 499.3±22.6 to 539.3±22.6 g, and ionic conductivity, increasing from 1.04×10-4 to 1.91×10-4  S cm-1 at 50 °C. ASSB treated by CIP demonstrates remarkably enhanced rate capability and cyclability compared with the cell processed by HP, which is further evidenced by improvement of the apparent Li ion diffusion constant based on Sand equation analysis. The improvement enabled by CIP treatment originates from the superior interface uniformity between electrodes and SPE and from the densification of the LiFePO4 and SPE composite electrode.

Efficient Surface Passivation of Ti-Based Layered Materials by a Nonfluorine Branched Copolymer for Durable and High-Power Sodium-Ion Batteries

There is a crucial need for low-cost energy storage technology based on abundant sodium ions to realize sustainable development with renewable energy resources. Poly(vinylidene fluoride) (PVDF) is applied as a binder in sodium-ion batteries (SIBs). Nevertheless, PVDF is also known to suffer from a larger irreversible capacity, especially when PVDF is used as the binder of negative electrode materials. In this research, a poly(acrylonitrile)-grafted poly(vinyl alcohol) copolymer (PVA-g-PAN) is tested as a binder with Ti-based layered oxides as potential negative electrode materials for SIBs. The chemical stability tests of PVDF and PVA-g-PAN contacted with metallic sodium have been conducted, which reveals that PVDF experiences a defluorination process, while PVA-g-PAN demonstrates excellent chemical stability. Composite electrodes with PVA-g-PAN demonstrate superior electrochemical performances when compared with the PVDF binder, allowing improvement for initial CE, higher rate capability, and long cyclability over 1500 cycles. Detailed characterization of electrodes via soft X-ray photoelectron spectroscopy and field emission scanning electron microscopy demonstrates that the PVA-g-PAN branched structure allows a more uniform distribution of acetylene black with higher coatability, unlocking enhanced rate performances and efficient passivation of Ti-based oxides without the excessive electrolyte decomposition. These findings open a new way to design practical and durable sodium-ion batteries with a high-power density.

Composite Membrane for Sodium Polysulfide Hybrid Redox Flow Batteries

Non-aqueous redox flow batteries (NARFBs) using earth-abundant materials, such as sodium and sulfur, are promising long-duration energy storage technologies. NARFBs utilize organic solvents, which enable higher operating voltages and potentially higher energy densities compared with their aqueous counterparts. Despite exciting progress throughout the past decade, the lack of low-cost membranes with adequate ionic conductivity and selectivity remains as one of the major bottlenecks of NARFBs. Here, we developed a composite membrane composed of a thin (<25 µm) Na+-Nafion coating on a porous polypropylene scaffold. The composite membrane significantly improves the electrochemical stability of Na+-Nafion against sodium metal, exhibiting stable Na symmetric cell performance for over 2300 h, while Na+-Nafion shorted by 445 h. Additionally, the composite membrane demonstrates a higher room temperature storage modulus than the porous polypropylene scaffold and Na+-Nafion separately while maintaining high Na+ conductivity (0.24 mS/cm at 20 °C). Our method shows that a composite membrane utilizing Na+-Nafion is a promising approach for sodium-based hybrid redox flow batteries.

Scalable Glass-Fiber-Polymer Composite Solid Electrolytes for Solid-State Sodium-Metal Batteries

In this work, we report a method for producing a thin (<50 μm), mechanically robust, sodium-ion conducting composite solid electrolyte (CSE) by infiltrating the monomers of polyethylene glycol diacrylate (PEGDA) and polyethylene glycol (PEG) and either NaClO₄ or NaFSI salt into a silica-based glass-fiber matrix, followed by an UV-initiated in situ polymerization. The glass fiber matrix provided mechanical strength to the CSE and enabled a robust, self-supporting separator. This strategy enabled the development of CSEs with high loadings of PEG as a plasticizer to enhance the ionic conductivity. The fabrication of these CSEs was done under ambient conditions, which was highly scalable and can be easily implemented in roll-to-roll processing. While NaClO₄ was found to be unstable with the sodium-metal anode, the use of a NaFSI salt was found to promote stable stripping and plating in a symmetric cell, reaching current densities of as high as 0.67 mA cm^-2 at 60 °C. The PEGDA + PEG + NaFSI separators were then used to form solid-state full cells with a cobalt-free, low-nickel layered Na_2/3Ni_1/3Mn_2/3O₂ cathode and a sodium-metal anode, achieving a full capacity utilization exhibiting 70% capacity retention after 50 cycles at a cycling rate of C/5 at 60 °C.

Integrated Design for Regulating the Interface of a Solid-State Lithium-Oxygen Battery with an Improved Electrochemical Performance

A composite solid-state electrolyte (SSE) with acceptable safety and durability is considered as a potential candidate for high-performance lithium-oxygen (Li-O₂) batteries. Herein, to address the safety issues and improve the electrochemical performance of Li-O₂ batteries, a solvent-free composite SSE is prepared based on the thermal initiation of poly(ethylene glycol) diacrylate radical polymerization, and an integrated battery is achieved by injecting an electrolyte precursor between electrodes during the assembly process through a simple heat treatment. The Li-metal symmetric cells based on this composite SSE achieve a critical current density of 0.8 mA cm^-2 and a stable cycle life of over 900 h at a current density of 0.2 mA cm^-2. This composite SSE effectively inhibits the erosion of O₂ on the Li metal anode, optimizes the interface between the electrolyte and cathode, and provides abundant reaction sites for the electrochemical reactions during cycling. The integrated solid-state Li-O₂ battery prepared in this work achieves stable long cycling (118 cycles) at a current density of 500 mA g^-1 at room temperature, showing the promising future application prospects.

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.

Hybrid Polyethylene Glycol/Sodium Metaphosphate Composites Prepared via Coacervation

We report on the fabrication and characterization of homogeneous, monophasic sodium metaphosphate and polyethylene glycol hybrid composites achieved via coacervation in aqueous solution. After separation and drying, an amorphous plastic solid is formed, composed mostly of hydrated sodium phosphate moieties amalgamated with polyethylene glycol chains. These composites are largely X-ray amorphous and can contain up to 8 weight percent of polymer. Impedance spectroscopic measurements reveal DC conductivity values of 12 μS/m at room temperature, an enhancement of three orders of magnitude when compared to glassy sodium metaphosphate, and the presence of the polyethylene glycol is reflected in the equivalent circuit and ionic hopping analyses.

Rationally Designed PEGDA-LLZTO Composite Electrolyte for Solid-State Lithium Batteries

A novel composite electrolyte is rationally designed with a polyethylene glycol diacrylate (PEGDA) polymer and a garnet-type fast lithium-ion conductor (Li_6.4La₃Zr_1.4Ta_0.6O₁₂, LLZTO) for solid-state lithium batteries. The LLZTO ceramic phase is incorporated into the PEGDA polymeric matrix as nanoparticles. The ionic conductivity of the composite is further optimized with a succinonitrile plasticizer. The solid composite membranes are synthesized via a tape casting process followed by a UV curing procedure. The resulting solid-state composite electrolyte delivers a room-temperature Li⁺-ion conductivity of 3.1 × 10^-4 S cm^-1 and can sustain an electrochemical polarization potential up to 4.6-4.7 V (vs Li⁺/Li). The compositing approach harnesses the advantages of both polymeric PEGDA and ceramic LLZTO. In addition to enhancing the ionic conductivity, the LLZTO ceramic filler can suppress Li dendrites. The polymeric phase of PEGDA facilitates good interfacial contact between the solid electrolyte and the electrodes. The solid-state cells fabricated with the composite solid electrolyte, lithium-metal anode, and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC 811) cathode show long cyclability.