Styrene-Based Poly(ethylene oxide) Side-Chain Block Copolymers as Solid Polymer Electrolytes for High-Voltage Lithium-Metal Batteries

A Large-Scale Fabrication of Flexible, Ultrathin, and Robust Solid Electrolyte for Solid-State Lithium-Sulfur Batteries

All-solid-state lithium metal batteries (ASSLMBs) are considered as the most promising candidates for the next-generation high-safety batteries. To achieve high energy density in ASSLMBs, it is essential that the solid-state electrolytes (SSEs) are lightweight, thin, and possess superior electrochemical stability. In this study, a feasible and scalable fabrication approach to construct 3D supporting skeleton using an electro-blown spinning technique is proposed. This skeleton not only enhances the mechanical strength but also hinders the migration of Li-salt anions, improving the lithium-ion transference number of the SSE. This provides a homogeneous distribution of Li-ion flux and local current density, promoting uniform Li deposition. As a result, based on the mechanically robust and thin SSEs, the Li symmetric cells show outstanding Li plating/stripping reversibility. Besides, a stable interface contact between SSE and Li anode has been established with the formation of an F-enriched solid electrolyte interface layer. The solid-state Li|sulfurized polyacrylonitrile (Li|SPAN) cell achieves a capacity retention ratio of 94.0% after 350 cycles at 0.5 C. Also, the high-voltage Li|LCO cell shows a capacity retention of 92.4% at 0.5 C after 500 cycles. This fabrication approach for SSEs is applicable for commercially large-scale production and application in high-energy-density and high-safety ASSLMBs.

Design of Fluorinated Elastomeric Electrolyte for Solid-State Lithium Metal Batteries Operating at Low Temperature and High Voltage

This work demonstrates the low-temperature operation of solid-state lithium metal batteries (LMBs) through the development of a fluorinated and plastic-crystal-embedded elastomeric electrolyte (F-PCEE). The F-PCEE is formed via polymerization-induced phase separation between the polymer matrix and plastic crystal phase, offering a high mechanical strain (≈300%) and ionic conductivity (≈0.23 mS cm-1) at -10 °C. Notably, strong phase separation between two phases leads to the selective distribution of lithium (Li) salts within the plastic crystal phase, enabling superior elasticity and high ionic conductivity at low temperatures. The F-PCEE in a Li/LiNi0.8Co0.1Mn0.1O2 full cell maintains 74.4% and 42.5% of discharge capacity at -10 °C and -20 °C, respectively, compared to that at 25 °C. Furthermore, the full cell exhibits 85.3% capacity retention after 150 cycles at -10 °C and a high cut-off voltage of 4.5 V, representing one of the highest cycling performances among the reported solid polymer electrolytes for low-temperature LMBs. This work attributes the prolonged cycling lifetime of F-PCEE at -10 °C to the great mechanical robustness to suppress the Li-dendrite growth and ability to form superior LiF-rich interphases. This study establishes the design strategies of elastomeric electrolytes for developing solid-state LMBs operating at low temperatures and high voltages.

External Pressure in Polymer-Based Lithium Metal Batteries: An Often-Neglected Criterion When Evaluating Cycling Performance?

Solid-state batteries based on lithium metal anodes, solid electrolytes, and composite cathodes constitute a promising battery concept for achieving high energy density. Charge carrier transport within the cells is governed by solid-solid contacts, emphasizing the importance of well-designed interfaces. A key parameter for enhancing the interfacial contacts among electrode active materials and electrolytes comprises externally applied pressure onto the cell stack, particularly in the case of ceramic electrolytes. Reports exploring the impact of external pressure on polymer-based cells are, however, scarce due to overall better wetting behavior. In this work, the consequences of externally applied pressure in view of key performance indicators, including cell longevity, rate capability, and limiting current density in single-layer pouch-type NMC622||Li cells, are evaluated employing cross-linked poly(ethylene oxide), xPEO, and cross-linked cyclodextrin grafted poly(caprolactone), xGCD-PCL. Notably, externally applied pressure substantially changes the cell's electrochemical cycling performance, strongly depending on the mechanical properties of the considered polymers. Higher external pressure potentially enhances electrode-electrolyte interfaces, thereby boosting the rate capability of pouch-type cells, despite the fact that the cell longevity may be reduced upon plastic deformation of the polymer electrolytes when passing beyond intrinsic thresholds of compressive stress. For the softer xGCD-PCL membrane, cycling of cells is only feasible in the absence of external pressure, whereas in the case of xPEO, a trade-off between enhanced rate capability and minimal membrane deformation is achieved at cell pressures of ≤0.43 MPa, which is considerably lower and more practical compared to cells employing ceramic electrolytes with ≥5 MPa external pressure.

Phase Behavior and Conformational Asymmetry near the Comb-to-Bottlebrush Transition in Linear-Brush Block Copolymers

This study explores how conformational asymmetry influences the bulk phase behavior of linear-brush block copolymers. We synthesized 60 diblock copolymers composed of poly(trifluoroethyl methacrylate) as the linear block and poly[oligo(ethylene glycol) methyl ether methacrylate] as the brush block, varying the molecular weight, composition, and side-chain length to introduce different degrees of conformational asymmetry. Using small-angle X-ray scattering, we determined the morphology and phase diagrams for three different side-chain length systems, mainly observing lamellar and cylindrical phases. Increasing the side-chain length of the brush block from three to nine ethylene oxide units introduces sufficient asymmetry between the blocks to alter the phase behavior, shifting the lamellar-to-cylindrical transitions toward lower brush block compositions and transitioning the brush block from the dense comb-like regime to the bottlebrush regime. Coarse-grained simulations support our experimental observations and provide a mapping between the composition and conformational asymmetry. A comparison of our findings to strong stretching theory across multiple phase boundary predictions confirms the transition between the dense comb-like regime and the bottlebrush regime.

Understanding Polymerized Ionic Liquids as Solid Polymer Electrolytes for Sodium Batteries

Solid polymer electrolytes (SPEs) have emerged as promising candidates for sodium-based batteries due to their cost-effectiveness and excellent flexibility. However, achieving high ionic conductivity and desirable mechanical properties in SPEs remains a challenge. In this study, we investigated an AB diblock copolymer, PS-PEA(BuImTFSI), as a potential SPE for sodium batteries. We explored binary and ternary electrolyte systems by combining the polymer with salt and [C3mpyr][FSI] ionic liquid (IL) and analyzed their thermal and electrochemical properties. Differential scanning calorimetry revealed phase separation in the polymer systems. The addition of salt exhibited a plasticizing effect localized to the polyionic liquid (PIL) phase, resulting in an increased ionic conductivity in the binary electrolytes. Introducing the IL further enhanced the plasticizing effect, elevating the ionic conductivity in the ternary system. Spectroscopic analysis, for the first time, revealed that the incorporation of NaFSI and IL influences the conformation of TFSI- and weakens the interaction between TFSI- and the polymer. This establishes correlations between anions and Na+, ultimately enhancing the diffusivity of Na ions. The electrochemical properties of an optimized SPE in Na/Na symmetrical cells were investigated, showcasing stable Na plating/stripping at high current densities up to 0.7 mA cm-2, maintaining its integrity at 70 °C. Furthermore, we evaluated the performance of a Na|NaFePO4 cell cycled at different rates (C/10 and C/5) and temperatures (50 and 70 °C), revealing remarkable high-capacity retention and Coulombic efficiency. This study highlights the potential of solvent-free diblock copolymer electrolytes for high-performance sodium-based energy storage systems, contributing to advanced electrolyte materials.

Advancements in Performance Optimization of Electrospun Polyethylene Oxide-Based Solid-State Electrolytes for Lithium-Ion Batteries

Polyethylene oxide (PEO)-based solid-state electrolytes for lithium-ion batteries have garnered significant interest due to their enhanced potential window, high energy density, and improved safety features. However, the issues such as low ionic conductivity at ambient temperature, substantial ionic conductivity fluctuations with temperature changes, and inadequate electrolyte interfacial compatibility hinder their widespread applications. Electrospinning is a popular approach for fabricating solid-state electrolytes owing to its superior advantages of adjustable component constitution and the unique internal fiber structure of the resultant electrolytes. Thus, this technique has been extensively adopted in related studies. This review provides an overview of recent advancements in optimizing the performance of PEO solid-state electrolytes via electrospinning technology. Initially, the impacts of different lithium salts and their concentrations on the performance of electrospun PEO-based solid-state electrolytes were compared. Subsequently, research pertaining to the effects of various additives on these electrolytes was reviewed. Furthermore, investigations concerning the enhancement of electrospun solid-state electrolytes via modifications of PEO molecular chains are herein detailed, and lastly, the prevalent challenges and future directions of PEO-based solid-state electrolytes for lithium-ion batteries are summarized.

Magnesium Polymer Electrolytes Based on the Polycarbonate Poly(2-butyl-2-ethyltrimethylene-carbonate)

Magnesium electrolytes based on a polycarbonate with either magnesium tetrakis(hexafluoroisopropyloxy) borate (Mg(B(HFIP)₄)₂) or magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) for magnesium batteries were prepared and characterized. The side-chain-containing polycarbonate, poly(2-butyl-2-ethyltrimethylene carbonate) (P(BEC)), was synthesized by ring opening polymerization (ROP) of 5-ethyl-5-butylpropane oxirane ether carbonate (BEC) and mixed with Mg(B(HFIP)₄)₂ or Mg(TFSI)₂ to form low- and high-salt-concentration polymer electrolytes (PEs). The PEs were characterized by impedance spectroscopy, differential scanning calorimetry (DSC), rheology, linear sweep voltammetry, cyclic voltammetry, and Raman spectroscopy. A transition from classical salt-in-polymer electrolytes to polymer-in-salt electrolytes was indicated by a significant change in glass transition temperature as well as storage and loss moduli. Ionic conductivity measurements indicated the formation of polymer-in-salt electrolytes for the PEs with 40 mol % Mg(B(HFIP)₄)₂ (HFIP40). In contrast, the 40 mol % Mg(TFSI)₂ PEs showed mainly the classical behavior. HFIP40 was further found to have an oxidative stability window greater than 6 V vs Mg/Mg²⁺, but showed no reversible stripping-plating behavior in an Mg||SS cell.

Polyaspartate Polyurea-Based Solid Polymer Electrolyte with High Ionic Conductivity for the All-Solid-State Lithium-Ion Battery

The existing in situ preparation methods of solid polymer electrolytes (SPEs) often require the use of a solvent, which would lead to a complicated process and potential safety hazards. Therefore, it is urgent to develop a solvent-free in situ method to produce SPEs with good processability and excellent compatibility. Herein, a series of polyaspartate polyurea-based SPEs (PAEPU-based SPEs) with abundant (PO)_x(EO)_y(PO)_z segments and cross-linked structures were developed by systematically regulating the molar ratios of isophorone diisocyanate (IPDI) and isophorone diisocyanate trimer (tri-IPDI) in the polymer backbone and LiTFSI concentrations via an in situ polymerization method, which gave rise to good interfacial compatibility. Furthermore, the in situ-prepared PAEPU-SPE@D₁₅ based on the IPDI/tri-IPDI molar ratio of 2:1 and 15 wt % LiTFSI exhibits an improved ionic conductivity of 6.80 × 10^-5 S/cm at 30 °C and could reach 10^-4 orders of magnitude when the temperature was above 40 °C. The Li|LiFePO₄ battery based on PAEPU-SPE@D₁₅ had a wide electrochemical stability window of 5.18 V, demonstrating a superior interface compatibility toward LiFePO₄ and the lithium metal anode, exhibited a high discharge capacity of 145.7 mAh g^-1 at the 100th cycle and a capacity retention of 96.8%, and retained a coulombic efficiency of above 98.0%. These results showed that the PAEPU-SPE@D₁₅ system displayed a stable cycle performance, excellent rate performance, and high safety compared with PEO systems, indicating that the PAEPU-based SPE system may play a crucial role in the future.

Improved Route to Linear Triblock Copolymers by Coupling with Glycidyl Ether-Activated Poly(ethylene oxide) Chains

Poly(ethylene oxide) block copolymers (PEO_z BCP) have been demonstrated to exhibit remarkably high lithium ion (Li⁺) conductivity for Li⁺ batteries applications. For linear poly(isoprene)-b-poly(styrene)-b-poly(ethylene oxide) triblock copolymers (PI_xPS_yPEO_z), a pronounced maximum ion conductivity was reported for short PEO_z molecular weights around 2 kg mol^-1. To later enable a systematic exploration of the influence of the PI_x and PS_y block lengths and related morphologies on the ion conductivity, a synthetic method is needed where the short PEO_z block length can be kept constant, while the PI_x and PS_y block lengths could be systematically and independently varied. Here, we introduce a glycidyl ether route that allows covalent attachment of pre-synthesized glycidyl-end functionalized PEO_z chains to terminate PI_xPS_y BCPs. The attachment proceeds to full conversion in a simplified and reproducible one-pot polymerization such that PI_xPS_yPEO_z with narrow chain length distribution and a fixed PEO_z block length of z = 1.9 kg mol^-1 and a Đ = 1.03 are obtained. The successful quantitative end group modification of the PEO_z block was verified by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). We demonstrate further that with a controlled casting process, ordered microphases with macroscopic long-range directional order can be fabricated, as demonstrated by small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has already been shown in a patent, published by us, that BCPs from the synthesis method presented here exhibit comparable or even higher ionic conductivities than those previously published. Therefore, this PEO_z BCP system is ideally suitable to relate BCP morphology, order and orientation to macroscopic Li⁺ conductivity in Li⁺ batteries.

Quasi-Solid Polymer Electrolyte with Multiple Lithium-Ion Transport Pathways by In Situ Thermal-Initiating Polymerization

Solid polymer electrolytes (SPEs) are considered to be attractive candidates for rechargeable batteries on account of their high safety and flexible processability. However, the restricted polymer segmental dynamics limit the Li⁺ conduction of SPEs. Herein, a composite electrolyte membrane was prepared via in situ thermal-initiating polymerization of diethylene glycol diacrylate (DEGDA) in a poly(vinylidene fluoride) frameworks (PVDF FMs) electrospun in advance. As a quasi-solid polymer electrolyte (QSPE), it provides multiple transport highways for Li⁺ built by the C═O or C-O or C═O/C-O groups in poly(diethylene glycol) diacrylate (PDEGDA), respectively, proved by density functional theory calculations together with the high-resolution ⁷Li solid-state nuclear magnetic resonance spectra. Since the interaction between Li⁺ and C═O is weaker than that between Li⁺ and C-O, Li⁺ tends to move along C═O dominating paths in PDEGDA/PVDF FMs QSPEs, even skipping back to C═O nodes from the original C-O dominating way. Multiple transport patterns facilitate Li⁺ migration within PDEGDA/PVDF FMs QSPEs, contributing to the ionic conductivity of 1.41 × 10^-4 S cm^-1 at 25 °C and the Li⁺ transference number of 0.454. Ascribing to the wetting capability of the monomer to the electrodes in use, compatible electrolyte/electrode interfaces with low interface resistance and compact cells were acquired by the in situ polymerization. Protective lithiated oligomers (RCOOLi) and LiF are enriched at the Li anode surface, promoting a lasting stable Li plating/stripping over 2000 h. By applying the QSPEs in LiFePO₄ cell, a capacity of 157.7 mAh g^-1 with almost 100% coulombic efficiency during 200 cycles is achieved at 25 °C.

Molecular Self-Assembled Ether-Based Polyrotaxane Solid Electrolyte for Lithium Metal Batteries

Poly(ethylene oxide) has been widely investigated as a potential separator for solid-state lithium metal batteries. However, its applications were significantly restricted by low ionic conductivity and a narrow electrochemical stability window (<4.0 V vs Li/Li⁺) at room temperature. Herein, a novel molecular self-assembled ether-based polyrotaxane electrolyte was designed using different functional units and prepared by threading cyclic 18-crown ether-6 (18C6) to linear poly(ethylene glycol) (PEG) via intermolecular hydrogen bond and terminating with hexamethylene diisocyanate trimer (HDIt), which was strongly confirmed by local structure-sensitive solid/liquid-state nuclear magnetic resonance (NMR) techniques. The designed electrolyte has shown an obviously increased room-temperature ionic conductivity of 3.48 × 10^-4 S cm^-1 compared to 1.12 × 10^-5 S cm^-1 without assembling polyrotaxane functional units, contributing to the enhanced cycling stability of batteries with both LiFePO₄ and LiNi_0.8Co_0.15Al_0.05O₂ cathode materials. This advanced molecular self-assembled strategy provides a new paradigm in designing solid polymer electrolytes with demanded performance for lithium metal batteries.

Enhanced Electrochemical Performance of PEO-Based Composite Polymer Electrolyte with Single-Ion Conducting Polymer Grafted SiO2 Nanoparticles

In order to enhance the electrochemical performance and mechanical properties of poly(ethylene oxide) (PEO)-based solid polymer electrolytes, composite solid electrolytes (CSE) composed of single-ion conducting polymer-modified SiO₂, PEO and lithium salt were prepared and used in lithium-ion batteries in this work. The pyridyl disulfide terminated polymer (py-ss-PLiSSPSI) is synthesized through RAFT polymerization, then grafted onto SiO₂ via thiol-disulfide exchange reaction between SiO₂-SH and py-ss-PLiSSPSI. The chemical structure, surface morphology and elemental distribution of the as-prepared polymer and the PLiSSPSI-g-SiO₂ nanoparticles have been investigated. Moreover, CSEs containing 2, 6, and 10 wt% PLiSSPSI-g-SiO₂ nanoparticles (PLi-g-SiCSEs) are fabricated and characterized. The compatibility of the PLiSSPSI-g-SiO₂ nanoparticles and the PEO can be effectively improved owing to the excellent dispersibility of the functionalized nanoparticles in the polymer matrix, which promotes the comprehensive performances of PLi-g-SiCSEs. The PLi-g-SiCSE-6 exhibits the highest ionic conductivity (0.22 mS·cm^-1) at 60 °C, a large t_Li+ of 0.77, a wider electrochemical window of 5.6 V and a rather good lithium plating/stripping performance at 60 °C, as well as superior mechanical properties. Hence, the CSEs containing single-ion conducting polymer modified nanoparticles are promising candidates for all-solid-state lithium-ion batteries.

PEO-Based Block Copolymer Electrolytes Containing Double Conductive Phases with Improved Mechanical and Electrochemical Properties

In this work, the advanced all solid-state block copolymer electrolytes (SBCPEs) for lithium-ion batteries with double conductive phases, poly(ethylene oxide)-b-poly(trimethyl-N-((2-(dimethylamino)ethyl methacrylate)-7-propyl)-ammonium bis(trifluoromethanesulfonyl) imide) (PEO-b-PDM-dTFSI)/LiTFSI, were fabricated, in which the charged PDM-dTFSI block contained double quaternary ammonium cations and the PEO block was doped with LiTFSI. The disordered (DIS) and ordered lamellae (LAM) phase structures were achieved by adjusting the composition of the block copolymer and the doping ratio r. In addition, the presence of the hard PDM-dTFSI block and the formation of the LAM phase structure resulted in a good mechanical strength of the solid PEO-b-PDM-dTFSI/LiTFSI electrolyte, and it could maintain a high level of 10⁴ Pa at 100 °C, which was around 10,000 times stronger than that of the PEO/LiTFSI electrolyte. Based on the good mechanical and electrochemical properties, the PEO-b-PDM-dTFSI/LiTFSI SBCPE exhibited excellent long-term galvanostatic cycle performance, indicating the strong ability to suppress lithium dendrites.