PEO-based Solid Polymer Electrolytes Research Articles

A novel approach for the atomic scale characterization of Li-ion battery components probed by positron annihilation lifetime spectroscopy

Recent research has focused on solid polymer electrolytes (SPEs) as the best liquid electrolyte replacements. Poly(ethylene oxide) (PEO) is the most frequent one because of its fast segmental movements in the free volume which controls lithium-ion transport between electrodes. Therefore, PEO-based copolymer electrolytes that can be tailored for mechanical strength and ionic conductivity are often studied. The most popular material is PEO chains containing PMMA because PEO transports ions, and PMMA has good mechanical properties. Further investigations on PEO-based solid polymer electrolytes (SPEs) have shown that chain designs with more nonlinear branches prevent crystallinity and maintain fast segmental dynamics. For this purpose, we worked on PEO-grafted-PMMA copolymers in which the free volume probed by positron annihilation lifetime spectroscopy considerably affects the structure-ionic conductivity relationship. Finally, we investigated the free volume theory of ionic condcutivity using Yahsi-Ulutas-Tav (YUT) theory.

Novel PEO-based Solid Polymer Electrolyte with High Elasticity and High Ionic Conductivity for Room Temperature Lithium Metal Batteries

The development and application of polyethylene oxide (PEO) based solid polymer electrolyte (SPE) is severely constrained by its low ionic conductivity and poor tensile resistance. To improve the electrochemical performance of SPE without losing its mechanical properties, a benzene sulfonate-based additive is adopted into crosslinking system composed of -CH2-CH2-O- segments to obtain a membrane with a high ionic conductivity of 1.47 × 10−4 S cm−1 and an ionic transference number of 0.70 at room temperature. The activation energy value of 0.128 eV gives evidence for a favorable migration mechanism of PTH-SPE. Anti-dendrite growth and contact optimization can be realized by molecular structure design with a tensile elongation of 490%. The reversible overpotential of Li||Li symmetric cell within 1000 h demonstrates that the compact PTH-SPE can inhibit the growth of lithium dendrite. This work provides a new strategy for designing high-performance solid electrolytes for room temperature via a green solvent-free method.

Flexible High Lithium-Ion Conducting PEO-Based Solid Polymer Electrolyte with Liquid Plasticizers for High Performance Solid-State Lithium Batteries.

Lithium-ion secondary batteries (LIB) with high energy density have attracted much attention for electric vehicle (EV) applications. However, LIBs have a safety problem because these batteries contain a flammable organic electrolyte. As such, all-solid secondary batteries that are not flammable have been extensively reported recently. In this study, we have focused on polymer electrolytes, which is flexible and is expected to address the safety problem. However, the conventional polymer electrolytes have low electrial conductivity at room temperature. Various attempts have been made to solve this problem, such as the addition of inorganic fillers and ionic liquids; however, these composite polymer electrolytes have not yet reached a practical level of lithium-ion conductivity. In this study, high electrical conductivity and lithium dendrite formation-free PEO based composite electrolytes are developed with both a filler of Li6,4La3Zr1.4Ta0.6O12 and liquid plasticizers of tetraethylene glycol dimethyl ether and 1,2 dimethoxyethane. The proposed flexible polymer electrolyte shows a high electrical conduciviy of 6.01×10-4 S cm-1 at 25 °C.

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 Na3 Zr2 Si2 PO12 (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 (4.5V vs Na/Na+ ). Furthermore, the SPE enables intimate contact with the electrode. The Na||Na3 V2 (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 20µm 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.

Different-grain-sized boehmite nanoparticles for stable all-solid-state lithium metal batteries.

PEO is one of the common composite polymer electrolyte vehicles; however, the presence of crystalline phase at room temperature, high interface impedance, and low oxidation resistance (<4.0 V) limit its application in stable all-solid-state lithium metal batteries. Herein, we designed a PEO-based solid polymer electrolyte (SPE) by adding boehmite nanoparticles to address the above-mentioned issues. Different-grain-sized boehmite nanoparticles were synthesized by adjusting the hydrothermal temperature. Moreover, the impacts of these distinct grain-sized boehmite nanoparticles used to fabricate boehmite/PEO polymer electrolytes (BPEs) on the performance of all-solid-state lithium metal batteries were investigated. It was found that with the increase in boehmite's grain size, BPEs show better performance. The best BPE exhibited an improved Li+ transference number (0.59), high ionic conductivity (1.25 × 10-4 S m-1), and wide electrochemical window (∼4.5 V) at 60 °C. The assembled lithium symmetric battery can stably undergo 500 hours of lithium plating/stripping at 0.1 mA cm-2. At the same time, the LiFePO4/BPE/Li battery exhibits excellent cycling stability after 100 cycles at 0.5C. This reasonable design strategy with a superior capacity retention rate (86%) demonstrates great potential in achieving high ionic conductivity and good interface stability for all-solid-state lithium metal batteries simultaneously.

(Best Poster Award - 1st Place) Characterisation of the Li Metal|Electrolyte Interface of PEO-Based SPEs with Borate Salts, Using in Situ Li Deposition PES Measurements Aided by AIMD Simulations

Lithium metal and anode-free batteries has the potential to increase the energy density and specific energy of the batteries drastically. However, when regular electrolyte materials are tried for these concepts they are plagued with problems due to degradation of the electrolyte at the interface and processes like dendrite formation which leads to poor capacity retention.[1] A strategy to mitigate these problems is to use Solid Polymer Electrolytes (SPEs), as these have better stability towards lithium metal. In order to find appropriate chemistries to use in SPE-based cells it is important to investigate and understand their anode interface, as this is where the Solid Electrolyte Interphase (SEI) is created from the degradation products during cycling.This work investigates three polyethylene oxide (PEO)-based SPEs, using lithium tetrafluoroborate (LiBF4), lithium bis(oxalate)borate (LiBOB), and lithium difluoro(oxalate)borate (LiDFOB) as the lithium conducting salts. The interfaces of these SPEs (PEO:LiBF4, PEO:LiBOB, and PEO:LiDFOB) was studied using Photo-Electron Spectroscopy (PES) with in situ lithium deposition, where lithium atoms are evaporated on top of the SPE film. The analysis of the core-level spectra before and after lithium deposition is compared to Ab Initio Molecular Dynamics (AIMD) simulations of the interface, where lithium atoms are gradually added during the simulation. These experimental and theoretical methods are meant to mimic the electrochemical plating of lithium at the lithium|SPE interface during charging of a lithium metal or anode-less cell.The interfaces of PEO:LiBOB and PEO:LiBF4 show, among other compounds, polyethylene as a breakdown product at the interface. PEO:LiDFOB shows a much lesser degree of degradation compared to the other two systems. The degradation layer is however still effective at protecting the SPE from further breakdown, as PEO:LiDFOB has a relatively high capacity retention compared to PEO:LiBF4 and PEO:LiBOB. Suggested is that the LiF provides a better electrochemical stability, while the boron and oxalate breakdown products provides a higher mechanical stability at the interface.[1] Huang WZ, Zhao CZ, Wu P, Yuan H, Feng WE, Liu ZY, Lu Y, Sun S, Fu ZH, Hu JK, Yang SJ. Anode‐Free Solid‐State Lithium Batteries: A Review. Advanced Energy Materials. 2022 Jul;12(26):2201044. Figure 1

Multifunctional single-ion conductor-integrated PEO-based solid polymer electrolytes endow highly stable and dendrite-free lithium metal batteries

Poly(ethylene oxide) (PEO) based solid polymer electrolytes (SPEs) hold great promise for circumventing the safety issues caused by liquid electrolytes, which are widely used as one of the leading technical strategies for constructing solid-state lithium metal batteries (LMBs). Unfortunately, developing PEO-based SPEs applicable to high-performance LMBs remains challenging due to the uncontrolled formation of lithium dendrites and sluggish ion migration. In this study, we present the fabrication of highly conductive and mechanically robust composite SPEs (CSPEs) for use in dendrite-proof LMBs by adding a new bis(sulphonyl)imide-based single-ion conductor (LiPEBI) as functional polymer fillers into PEO SPEs network. As expected, the as-prepared CSPEs can simultaneously achieve an excellent mechanical strength of 2.1 MPa, a considerable elongation at a break of 1491.0%, and an optimized degree of crystallization of 37.1%. In addition, introducing the LiPEBI additive promotes the homogeneous distribution of Li-ions and accelerates the ionic transport, resulting in an enhanced ionic conductivity of 4.17 × 10−4 S cm−1 and lithium ion transference number (tLi+) of 0.45 at 60 °C for CSPEs. Such comprehensive physical and electrochemical performances are favorable to suppressing lithium dendrite growth and prolonging the lifespan of LMBs. As a result, the Li||Li symmetrical cells assembled by the CSPEs display a remarkably stable cyclic performance with an extremely low voltage polarization of 60.3 mV over 500 h under 50 μA cm−2 at 60 °C. Also, the CSPEs-based Li||LiFePO4 batteries illustrate superior rate performance up to 1 C and deliver long-term cycling stability with a high discharge capacity of 128.8 mAh g−1 after 100 cycles under 0.4 C at 60 °C. We believe the novel CSPEs have great potential for actual application in the construction of high-safety and dendrite-free solid-state LMBs.

Enhancing interfacial Li+ transport and dielectric properties in poly(ethylene oxide)-based all-solid electrolytes via inactive g-C3N4 nanosheets filler incorporation

The advancement of all-solid-state Li metal batteries (ASSLMBs) faces a major challenge in the growth of lithium dendrites on the anode-electrolyte interface. In this study, we propose a dual-filler approach using poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) that combine Li1.4Al0.4Ti1.6(PO4)3 (LATP) ion-conductive particles with graphitic carbon nitride (g-C3N4) nanosheets. Analysis through second harmonic resonance enhanced electrostatic force microscopy and critical current density (CCD) tests reveal that the g-C3N4 additives form nano-capacitors at the SPE-lithium interface, effectively reducing sudden changes in current densities. The distribution of relaxation time constant (DRT) measurements confirms that the g-C3N4 filler suppresses uncontrolled Li dendrite growth, effectively mitigating battery aging caused by anode interfacial degradation. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis indicates that the nitrogen-containing organic groups in g-C3N4 are reduced to form a stable interfacial layer with lithium metal. As a result of these enhancements, the electrolyte demonstrates remarkable interfacial stability in Li/Li symmetrical cells at 0.65 mA/cm2 and delivers promising performance in assembled Li-LiFePO4 batteries, achieving a reversible capacity of 121.6 mAh/g at 1 C after 200 cycles. These findings highlight the potential of dual-filler PEO-based SPEs for promoting interfacial lithium-ion transport in all-solid-state Li metal batteries.

Effect of anion size and salt concentration on ion transport in PEO-based solid polymer electrolytes at high temperatures

Effect of anion size and salt concentration on ion transport in PEO-based solid polymer electrolytes at high temperatures

3D core-shell nanofibers framework and functional ceramic nanoparticles synergistically reinforced composite polymer electrolytes for high-performance all-solid-state lithium metal battery

Satisfactory ionic conductivity, excellent mechanical stability, and high-temperature resistance are the prerequisites for the safe application of solid polymer electrolytes (SPEs) in all-solid-state lithium metal batteries (ASSLMBs). In this study, a novel poly(m-phenylene isophthalamide) (PMIA)-core/poly(ethylene oxide) (PEO)-shell nanofiber membrane and the functional Li6.4La3Zr1.4Ta0.6O12 (LLZTO) ceramic nanoparticle are simultaneously introduced into the PEO-based SPEs to prepare composite polymer electrolytes (CPEs). The core PMIA layer of composite nanofibers can greatly improve the mechanical strength and thermal stability of the CPEs, while the shell PEO layer can provide the 3D continuous transport channels for lithium ions. In addition, the introduction of functional LLZTO nanoparticle not only reduces the crystallinity of PEO, but also promotes the dissociation of lithium salts and releases more Li+ ions through its interaction with the Lewis acid-base of anions, thereby overall improving the transport of lithium ions. Consequently, the optimized CPEs present high ionic conductivity of 1.38×10−4 S/cm at 30 °C, significantly improved mechanical strength (8.5 MPa), remarkable thermal stability (without obvious shrinkage at 150 °C), and conspicuous Li dendrites blocking ability (>1800 h). The CPEs also both have good compatibility and cyclic stability with LiFePO4 (>2000 cycles) and high-voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) (>500 cycles) cathodes. In addition, even at low temperature (40 °C), the assembled LiFePO4/CPEs/Li battery still can cycle stably. The novel design can provide an effective way to exploit high-performance solid-state electrolytes.

PEO-Based Solid Composite Polymer Electrolyte for High Capacity Retention All-Solid-State Lithium Metal Battery.

The limited ionic conductivity at room temperature and the constrained electrochemical window of poly(ethylene oxide) (PEO) pose significant obstacles that hinder its broader utilization in high-energy-density lithium metal batteries. The garnet-type material Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) is recognized as a highly promising active filler for enhancing the performance of PEO-based solid polymer electrolytes (SPEs). However, its performance is still limited by its high interfacial resistance. In this study, a novel hybrid filler-designed SPE is employed to achieve excellent electrochemical performance for both the lithium metal anode and the LiFePO4 cathode. The solid composite membrane containing hybrid fillers achieves a maximum ionic conductivity of 1.9 × 10-4 S cm-1 and a Li+ transference number of 0.67 at 40°C, respectively. Additionally, the Li/Li symmetric cells demonstrate a smooth and stable process for 2000 h at a current density of 0.1mA cm-2 . Furthermore, the LiFePO4 /Li battery delivers a high-rate capacity of 159.2 mAh g-1 at 1 C, along with a capacity retention of 95.2% after 400 cycles. These results validate that employing a composite of both active and inactive fillers is an effective strategy for achieving superior performance in all-solid-state lithium metal batteries (ASSLMBs).

Semi-interpenetrating-network PEO-based polymer electrolytes with fast Li-ion conduction for solid Li-metal batteries

Semi-interpenetrating-network (semi-IPN) PEO-based solid polymer electrolytes (SPEs) are synthesized for the application of Li-metal batteries. Owing to the special semi-IPN structure and synergistic effects of EO segments and PVDF-HFP, the SIN solid electrolytes demonstrate great flexibility, high ionic conductivity of 2.45 × 10−4 S cm−1 at 25 °C, outstanding thermal stability and broad electrochemical window (4.9 V vs. Li+/Li). In addition, SIN owns excellent electrochemical stability with Li-metal with a low over-potential <70 mV after 1400 h Li plating/stripping. Moreover, benefiting from the stable interfacial contact, LiFePO4|SIN|Li cells provide high discharge capacity of 146 mAh g−1 with superior capacity retention of 96.6 % after 500 cycles at 0.1C under room temperature. Hence, the semi-IPN PEO-based SPEs appear to be promising for use in high-performance solid-state Li-metal batteries.

Finite elastic-viscoplastic deformation behaviour of PEO-based solid polymer electrolyte: Experimental investigation and constitutive modelling

Polyethene oxide (PEO) has emerged as a preferred candidate for solid electrolytes due to its excellent solubility towards lithium salts, stable chemical properties, high flexibility, and low cost. However, the poor knowledge of the mechanical behaviours of PEO electrolytes leads to the road facing vast challenges in developing all-solid-state lithium batteries. In this study, we conducted the uniaxial tensile tests on PEO-LiTFSI films at 20–40 °C, and the strain rate ranges from 0.001 s−1 to 0.01 s−1 while investigating the potential effect mechanisms. Based on a rheological scheme of semi-crystalline polymers, we developed a new constitutive model for simulating the stress-strain curves of PEO-based electrolytes. The results indicate that PEO-LiTFSI exhibits the finite viscoelastic-plastic deformation characteristic with a minimal elastic strain range, low elastic modulus and yield strength. Increasing LiTFSI content and temperature can reduce the crystallinity of the PEO, resulting in significant softening of the polymer electrolyte; the opposite is true with raising the strain rate. Notably, the present model can capture the mechanical response of PEO electrolytes across a range of temperatures and strain rates, and the predicted tensile curves align closely with the experimental results. In addition, the simulation reveals that subpar mechanical performance would occur for PEO-LiTFSI under the recommended electrochemical operation conditions (0.05–5 C and 50 °C).

AlTiMgLiO medium entropy oxide additive for PEO-based solid polymer electrolytes in lithium ion batteries

Solid polymer electrolytes (SPE) have attracted considerable attention as electrolytes for solid-state batteries due to their toughness, high safety, and ionic conductivities that can be comparable with liquid electrolytes, especially at higher temperatures. However, polymers have low elastic moduli, which decrease at higher temperatures, limiting their ability to reduce dendrite formation. Mechanical blocking is one method of improving the interfacial layer and reducing dendritic growth but requires the elastic modulus of the polymer to be high enough to suppress lithium dendrites growth. Previous studies have focused on using unary metal oxides, which are limited by the percent of additives that can be included in the polymer before causing negative effects on electrochemical properties. In this study, we demonstrate a new strategy for improving the performance of polymers by synthesizing a multielement oxide (MEO) filler, AlTiMgLiO, to create a composite SPE with enhanced electrochemical performance. The synthesized AlTiMgLiO-containing SPE resulted in a voltage window of 0–6.18 V and a lithium transference number of 0.42. The overpotential voltage during galvanostatic cycling was reduced due to the improvements made to the morphology. The improvement of the interfacial layer reduced Li dendritic growth, resulting in a capacity of 99.68 mAh g−1 after 500 cycles, and a capacity retention of 78.69 %. The possible reasons for the improvement are discussed, providing a direction for future studies on the use of multielement materials as fillers in solid polymers electrolytes.

Robust ion-rectifying polymer electrolyte membrane for high-rate solid-state lithium metal batteries

The uncontrolled dendrite growth and poor rate performance originating from crystallization-induced anisotropic ion transport and insufficient mechanical strength of solid polymer electrolytes (SPE) have significantly impeded the practical application of high-rate solid-state lithium metal batteries. Herein, we present a robust ion-rectifying PEO-based SPE membrane by incorporating titanium-oxo clusters (TOC) composed of a diameter of 2.69 nm and a height of 1.04 nm rigid backbone and hairy PEG chains into PEO-LiN(CF3SO2)2 (LiTFSI) electrolyte, denoted as PEO-TOC membrane. This membrane realizes dendrite-free lithium metal deposition where TOC serves as a rhecology-tuning (by reducing crystallinity) and anion-trapping (through Lewis acid-base interactions) agent, thus achieving an ion-rectifying effect in the electrolyte membrane. Consequently, the rapid and homogeneous Li+ flux is achieved, resulting in a high ionic conductivity of 5.5 × 10−5 S cm−1 at 30 °C and Li+ transference number of 0.51. Furthermore, the rigid backbone enables a robust compressive strength of 940 MPa for dendrite suppression. As a result, the robust ion-rectifying PEO-TOC membrane endows the Li-LFP cells with high-rate performance, achieving 111.9 mAh g−1 at 3.5 C, and good cycling performance of 92.9% capacity retention over 250 cycles at 1 C, which routine SPE hardly fulfills with anisotropic Li+ flux and mechanical properties.

Enhanced ionic conductivity of PEO-based solid electrolyte through synergistic dissociation effect

Low ionic conductivity is an obstacle to realize the commercial application of PEO-based solid polymer electrolyte due to high crystallinity and poor segment motion of PEO. Herein, a kind of Co-based metal-organic framework (Co-MOF) is synthesized as the filler to inhibit PEO crystallization and produce a synergistic dissociation effect between LiTFSI and PEO so as to improve the ionic conductivity. The Lewis acid-base pairs formed between metal Co2+ and TFSI− facilitates the dissociation of LiTFSI. While the nitro group of the organic framework drags the ether unit of PEO, releases Li+ and improves Li+ hopping. The ionic conductivity of PEO/Co-MOF-5% reaches 1.26×10−4 S cm−1 and a reversible capacity of 111 mAh g−1 is obtained for Li|(PEO/Co-MOF-5%)|LiFePO4 cell after 120 cycles at room temperature.

Single-ion conductors functionalized graphene oxide enabling solid polymer electrolytes with uniform Li-ion transport toward stable and dendrite-free lithium metal batteries

The utilization of poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) has long been deemed promising for the application of state-of-the-art lithium metal metals (LMBs), but still severely impeded by the insufficient Li-ion transporting channels and inhomogeneous Li deposition. Herein, the design of single-ion conductors (PSLi) modified graphene oxide nanosheets (GO-PSLi)-reinforced PEO-based SPEs to address these long-standing issues is proposed. The electronegative sulfonate-rich groups with lithiophilic features on GO-PSLi surface not only effectively enhance the dispersion of nanoparticles within the PEO matrix, but also provide fast Li-ion diffusion pathways. Meanwhile, introducing PSLi chains is also favorable to highly reducing the crystallization of composite SPEs through hydrogen interactions derived from the desired amide functional groups. This, in turn, facilitates segmental relaxation of the PEO backbone, leading to accelerates the Li-ion migration. As a proof of concept, the as-prepared composite SPEs demonstrate a good ionic conductivity of 2.2 × 10−4 S cm−1 and a high lithium transference number of 0.53, significantly outperforming the pure PEO SPEs (1.0 × 10−4 S cm−1 and 0.28, respectively) at 60 °C. In addition, the symmetrical Li||Li cells based on the composite SPEs show excellent reversible lithium plating/stripping performances with dendrite-free uniform lithium deposition over 1100 h under a current of 0.1 mA cm−2 at 60 °C. The Li||LiFePO4 batteries also achieve an impressive capacity of 120.2 mAh g−1 at a high current density of 4C and deliver remarkable cycling abilities with 84.6% retention after 250 cycles under 1C at 60 °C. This study offers a facile and practical strategy to develop advanced composite PEO-based SPEs for safe and dendrite-free solid-state LMBs.

A copolyether with pendant cyclic carbonate segment for PEO-based solid polymer electrolyte

The application of PEO-based solid polymer electrolyte (SPE) is limited by the low ionic conductivity and lithium-ion transference number near room temperature. In this paper, a pendant polyether poly (EO-co-PCGE) (PECP) with low molecular weight is prepared by copolymerization of ethylene oxide (EO) monomer with typical lithium-ion conduction group and propylene carbonate glycidyl ether (PCGE) monomer with high dielectric group. By blended with PECP, the PEO-based SPE is obtained and its ionic conductivity can be increased to 3.54 × 10−5 S cm−1 at room temperature and 1.32 × 10−4 S cm−1 at 40 °C. In one aspect, the pendant structure and low molecular weight are beneficial to the reduction of the crystallinity of PEO matrix; in a further aspect, the cyclic carbonate pendant offers a competitive effect on segmental motion and the lithium-ion complexation of PEO, thus affecting the conduction mechanism of lithium-ions in the conventional PEO-based SPE and significantly increasing the lithium-ion transference number. The synergistic effect of these two aspects makes the prepared PEO-based SPE have excellent properties of ionic conduction especially at room temperature, and the LiFePO4/Li batteries with the SPE present excellent cycling performance at the temperature of 40 °C with a discharge capacity of about 100 mA h g−1 after 150 battery cycles.

Oxygen Vacancy-Rich Ultrathin Co3O4 Nanosheets as Nanofillers in Solid-Polymer Electrolyte for High-Performance Lithium Metal Batteries

The development of high-performance solid-polymer electrolytes (SPEs) is a key to the practical application of lithium metal batteries (LMBs). The use of two-dimensional (2D) inorganic nanofiller is an efficient way to build poly(ethylene oxide) (PEO)-based SPEs with high ionic conductivity and stability. Herein, a series of 2D oxygen vacancy-rich Co3O4-y−x (x = 1, 2 and 3) with well-defined 2D nanostructures, a high surface area and controllable oxygen vacancy contents (Co3O4-y) was synthesized via a facile self-assembly method and NaBH4 reduction. When the 2D Co3O4-y−x (x = 1, 2 and 3) nanosheets are introduced as nanofillers in PEO-based SPEs, they can interact with the PEO to form a three-dimensional (3D) PEO/Co3O4-y film with uniform Li+ distribution and vertical diffusion channels, as well as strong adsorption of NO3− from LiNO3 electrolyte salt at the defective sites. As a result, the PEO/Co3O4-y−2 film reached a high ionic conductivity of 4.9 × 10−5 S cm−1, high Li+ a transference number of 0.51 and a wide electrochemical window over 4.6 V at 80 °C. The PEO/Co3O4-y−2 film enables the Li||PEO/Co3O4-y−2||LiFePO4 cell to deliver a high reversible capacity of 117.7 mAh g−1 at 2 C and to maintain 126.7 mAh g−1 at 1 C after 250 cycles with an initial capacity retention of 87.9%.

Effects of Molecular Structure of Cross-Linked Solid Polymer Electrolytes on Ionic Conduction Behavior

To improve the safety and energy density of Li batteries, all-solid-state batteries (ASSBs) are desired. Solid polymer electrolytes (SPEs) are expected to be promising candidates, owing to features such as their high flexibility and formability. To achieve high-performance ASSBs, we investigated molecular structure design by fabricating two types of SPEs. First, chemically cross-linked polyethylene oxide/polypropylene oxide (P(EO/PO))-based SPEs were formed with low-molecular-weight PEO as side chains. Second, physically cross-linked SPEs were fabricated with short PEO chains (G2) as soft-units and norbornene (NB) as hard-units with intramolecular phase separation. Evaluation of the physicochemical and electrochemical properties of these SPEs was carried out, and the degree of dissociation of the Li salt was investigated by Raman spectroscopy. The phenomenon of decoupling between the segmental motion of the SPEs and ionic conduction was confirmed by Vogel-Fulcher-Tamman equation analysis. P(NB/G2)-based SPEs also exhibited high thermal stability, which originated from the NB unit, according to TG-DTA and DSC measurements. Furthermore, ionic conductivity increased monotonically with the Li salt concentration, in contrast to conventional PEO-based SPEs. Physically cross-linked SPEs achieved the decoupling of ionic conduction from their segmental motion owing to their differences between the ionic and structural components of the SPEs.