Single Lithium-Ion Conducting Solid Polymer Electrolyte with Superior Electrochemical Stability and Interfacial Compatibility for Solid-State Lithium Metal Batteries

Tackling the Challenges of Aqueous Zn-Ion Batteries via Polymer-Derived Strategies

Zn-ion batteries (ZIBs) have gathered unprecedented interest recently benefiting from their intrinsic safety, affordability, and environmental benignity. Nevertheless, their practical implementation is hampered by low rate performance, inferior Zn2+ diffusion kinetics, and undesired parasitic reactions. Innovative solutions are put forth to address these issues by optimizing the electrodes, separators, electrolytes, and interfaces. Remarkably, polymers with inherent properties of low-density, high processability, structural flexibility, and superior stability show great promising in tackling the challenges. Herein, the recent progress in the synthesis and customization of functional polymers in aqueous ZIBs is outlined. The recent implementations of polymers into each component are summarized, with a focus on the inherent mechanisms underlying their unique functions. The challenges of incorporating polymers into practical ZIBs are also discussed and possible solutions to circumvent them are proposed. It is hoped that such a deep analysis could accelerate the design of polymer-derived approaches to boost the performance of ZIBs and other aqueous battery systems as they share similarities in many aspects.

Accelerated Single Li-Ion Transport in Solid Electrolytes for Lithium-Sulfur Batteries: Poly(Arylene Ether Sulfone) Grafted with Pyrrolidinium-Terminated Poly(Ethylene Glycol)

Polymeric solid electrolytes have attracted tremendous interest in high-safety and high-energy capacity lithium-sulfur (Li─S) batteries. There is, however, still a dilemma to concurrently attain high Li-ion conductivity and high mechanical strength that effectively suppress the Li-dendrite growth. Accordingly, a rapidly Li-ion conducting solid electrolyte is prepared by grafting pyrrolidinium cation (PYR+)-functionalized poly(ethylene glycol) onto the poly(arylene ether sulfone) backbone (PAES-g-2PEGPYR). The PYR+ groups effectively immobilize anions of Li-salts in Li-conductive PEGPYR domains phase-separated from PAES matrix to enhance the single-ion conduction. The tailored PAES-g-2PEGPYR membrane shows a high Li-ion transference number of 0.601 and superior ionic conductivity of 1.38 mS cm-1 in the flexible solid state with the tensile strength of 1.0 MPa and Young's modulus of 1.5 MPa. Moreover, this PAES-g-2PEGPYR membrane exhibits a high oxidation potential (5.5 V) and high thermal stability up to 200 (C. The Li/PAES-g-2PEGPYR/Li cell stably operates for 1000 h without any short circuit, and the rechargeable Li/PAES-g-2PEGPYR/S cell discharges a capacity of 1004.7 mAh g-1 at C/5 with the excellent rate capability and the prominent cycling performance of 95.3% retention after 200 cycles.

Borate-containing triblock copolymer electrolytes for improved lithium-ion transference number and interface stability

The electrolytes with high lithium-ion transference number (tLi+) can reduce the formation of concentration polarization during charge/discharge process and improve the electrochemical performance of lithium-ion batteries (LIBs). Herein, we report triblock copolymer electrolytes (PBOEE) containing borate. The sp2 hybridized boron atoms acting as Lewis acids can anchor the anions of lithium salts, enabling PBOEE to achieve high tLi+ of up to 0.53. Also, the borate groups can promote the formation of stable organic-rich solid electrolyte interphase (SEI) film, which enables the Li symmetric cell to cycle stably at 0.1 mA cm-2/0.1 mAh cm-2 for more than 3100 h with a low overpotential of 0.08 V under 50 °C. The optimized PBOEE_24 has an ionic conductivity of 1.41 × 10-4 S cm-1 and electrochemical stability window of 4.8 V vs. Li+/Li at 50 °C. Combining these advantages, the LiFePO4/PBOEE_24/Li cell exhibits an initial discharge specific capacity of 157.3 mA h g-1 at 0.5C with a capacity retention of 85 % after 600 cycles under 50 °C. At a higher current density of 1C, the discharge capacity maintains at 128.0 mA h g-1 after 400 cycles with a capacity retention of 84.88 %. These results suggest that block copolymer containing sp2 hybridized boron atoms is a promising all-solid-state polymer electrolyte.

Polyphosphazene-Based Anion-Anchored Polymer Electrolytes For All-Solid-State Lithium Metal Batteries

Safety concerns of traditional liquid electrolytes, especially when paired with lithium (Li) metal anodes, have stimulated research of solid polymer electrolytes (SPEs) to exploit the superior thermal and mechanical properties of polymers. Polyphosphazenes are primarily known for their use as flame retardant materials and have demonstrated high Li-ion conductivity owing to their highly flexible P = N backbone which promotes Li-ion conduction via inter- and intrachain hopping along the polymer backbone. While polyphosphazenes are largely unexplored as SPEs in the literature, a few existing examples showed promising ionic conductivity. By anchoring the anion to the polymer backbone, one may primarily allow the movement of Li ions, alleviating the detrimental effects of polarization that are common in conventional dual-ion conducting SPEs. Anion-anchored SPEs, known as single Li-ion conducting solid polymer electrolytes (SLiC-SPEs), exhibit high Li-ion transference numbers (tLi+), which limits Li dendrite growth, thus further increasing the safety of SPEs. However, previously reported SLiC-SPEs suffer from inadequate ionic conductivity, small electrochemical stability windows (ESWs), and limited cycling stability. Herein, we report three polyphosphazene-based SLiC-SPEs comprising lithiated polyphosphazenes. The SLiC polyphosphazenes were prepared through a facile synthesis route, opening the door for enhanced tunability of polymer properties via facile macromolecular nucleophilic substitution and subsequent lithiation. State-of-the-art characterization techniques, such as differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and solid-state nuclear magnetic resonance spectroscopy (ssNMR) were employed to probe the effect of the polymer structure on Li-ion dynamics and other electrochemical properties. Produced SPEs showed thermal stability up to ∼208 °C with ionic conductivities comparable to that of the best-reported SLiC-SPEs that definitively comprise no solvents or plasticizers. Among the three lithiated polyphosphazenes, the SPE containing dilithium poly[bis(trifluoroethylamino)phosphazene] (pTFAP2Li) exhibited the most promising electrochemical characteristics with tLi+ of 0.76 and compatibility with both Li metal anodes and LiFePO4 (LFP) cathodes; through 40 cycles at 100 °C, the PEO-pTFAP2Li blend showed 81.2% capacity utilization and 86.8% capacity retention. This work constitutes one of the first successful demonstrations of the cycling performance of a true all-solid-state Li-metal battery using SLiC polyphosphazene SPEs.

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.

Toward a molecular understanding of the conductivity of lithium-ion conducting polyanion polymer electrolytes by molecular dynamics simulation

With the improved lithium-ion transference number near unity, the low conductivity of single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) still hinders their application in high-rate batteries. Though some empirical conclusions on the conducting mechanism of SLIC-SPEs have been obtained, a more comprehensive study on the quantitative relationship between the molecular structure factors and ionic conduction performance is expected. In this study, a model structure that contains adjustable main chain and anion groups in the polyethylene oxide (PEO) matrix was used to clarify the influence of molecular structural factors on ionic conductivity and electrochemical stability of SLIC-SPEs. The anionic group was further disassembled into the intermediate group and end group while the main chain structure was distinguished into different degrees of polymerization and various lengths of the spacers between anions. Therefore, a well-defined molecular structure was employed to describe its relationship with ionic conductivity. In addition, the dissociation degree of salts and mobility of ions changing with the molecular structure were also discussed to explore the fundamental causes of conductivity. It can be concluded that the anion group affects the conductivity mainly via the dissociation degree, while the main chain structure impacts the conductivity by both dissociation degree and mobility.

Advanced Composite Solid Electrolytes for Lithium Batteries: Filler Dimensional Design and Ion Path Optimization

Composite solid electrolytes are considered to be the crucial components of all-solid-state lithium batteries, which are viewed as the next-generation energy storage devices for high energy density and long working life. Numerous studies have shown that fillers in composite solid electrolytes can effectively improve the ion-transport behavior, the essence of which lies in the optimization of the ion-transport path in the electrolyte. The performance is closely related to the structure of the fillers and the interaction between fillers and other electrolyte components including polymer matrices and lithium salts. In this review, the dimensional design of fillers in advanced composite solid electrolytes involving 0D-2D nanofillers, and 3D continuous frameworks are focused on. The ion-transport mechanism and the interaction between fillers and other electrolyte components are highlighted. In addition, sandwich-structured composite solid electrolytes with fillers are also discussed. Strategies for the design of composite solid electrolytes with high room temperature ionic conductivity are summarized, aiming to assist target-oriented research for high-performance composite solid electrolytes.

Chemistry-Informed Machine Learning for Polymer Electrolyte Discovery

Solid polymer electrolytes (SPEs) have the potential to improve lithium-ion batteries by enhancing safety and enabling higher energy densities. However, SPEs suffer from significantly lower ionic conductivity than liquid and solid ceramic electrolytes, limiting their adoption in functional batteries. To facilitate more rapid discovery of high ionic conductivity SPEs, we developed a chemistry-informed machine learning model that accurately predicts ionic conductivity of SPEs. The model was trained on SPE ionic conductivity data from hundreds of experimental publications. Our chemistry-informed model encodes the Arrhenius equation, which describes temperature activated processes, into the readout layer of a state-of-the-art message passing neural network and has significantly improved accuracy over models that do not encode temperature dependence. Chemically informed readout layers are compatible with deep learning for other property prediction tasks and are especially useful where limited training data are available. Using the trained model, ionic conductivity values were predicted for several thousand candidate SPE formulations, allowing us to identify promising candidate SPEs. We also generated predictions for several different anions in poly(ethylene oxide) and poly(trimethylene carbonate), demonstrating the utility of our model in identifying descriptors for SPE ionic conductivity.

Enabling Stable Interphases via In Situ Two-Step Synthetic Bilayer Polymer Electrolyte for Solid-State Lithium Metal Batteries

Poly(ethylene oxide) (PEO)-based electrolyte is considered to be one of the most promising polymer electrolytes for lithium metal batteries. However, a narrow electrochemical stability window and poor compatibility at electrode-electrolyte interfaces restrict the applications of PEO-based electrolyte. An in situ synthetic double-layer polymer electrolyte (DLPE) with polyacrylonitrile (PAN) layer and PEO layer was designed to achieve a stable interface and application in high-energy-density batteries. In this special design, the hydroxy group of PEO-SPE can form an O-H---N hydrogen bond with the cyano group in PAN-SPE, which connects the two layers of DLPE at a microscopic chemical level. A special Li+ conducting mechanism in DLPE provides a uniform Li+ flux and fast Li+ conduction, which achieves a stable electrolyte/electrode interface.LiFePO4/DLPE/Li battery shows superior cycling stability, and the coulombic efficiency remains 99.5% at 0.2 C. Meanwhile, LiNi0.6Co0.2Mn0.2O2/DLPE/Li battery shows high specific discharge capacity of 176.0 mAh g−1 at 0.1 C between 2.8 V to 4.3 V, and the coulombic efficiency remains 95% after 100 cycles. This in situ synthetic strategy represents a big step forward in addressing the interface issues and boosting the development of high-energy-density lithium-metal batteries.

Strategies and characterizations enabling high performance PEO-based solid-state lithium-ion batteries

Polyethylene oxide (PEO) based polymer electrolytes have been widely used in solid-state lithium batteries (SSBs) owing to their high solubility of lithium salt and favourable ionic conductivity, flexibility for improved...

Single-Ion Lithium Conducting Polymers with High Ionic Conductivity Based on Borate Pendant Groups

A family of single‐ion lithium conducting polymer electrolytes based on highly delocalized borate groups is reported. The effect of the nature of the substituents on the boron atom on the ionic conductivity of the resultant methacrylic polymers was analyzed. To the best of our knowledge the lithium borate polymers endowed with flexible and electron‐withdrawing substituents presents the highest ionic conductivity reported for a lithium single‐ion conducting homopolymer (1.65×10⁻⁴ S cm⁻¹ at 60 °C). This together with its high lithium transference number tLi+ =0.93 and electrochemical stability window of 4.2 V vs Li⁰/Li⁺ show promise for application in lithium batteries. To illustrate this, a lithium borate monomer was integrated into a single‐ion gel polymer electrolyte which showed good performance on lithium symmetrical cells (<0.85 V at ±0.2 mA cm⁻² for 175 h).

Single-ion conducting polymer electrolytes as a key jigsaw piece for next-generation battery applications

As lithium-ion batteries have been the state-of-the-art electrochemical energy storage technology, the overwhelming demand for energy storage on a larger scale has triggered the development of next-generation battery technologies possessing high energy density, longer cycle lives, and enhanced safety. However, commercial liquid electrolytes have been plagued by safety issues due to their flammability and instability in contact with electrodes. Efforts have focused on developing such electrolytes by covalently immobilizing anionic groups onto a polymer backbone, which only allows Li+ cations to be mobile through the polymer matrix. Such ion-selective polymers provide many advantages over binary ionic conductors in battery operation, such as minimization of cell polarization and dendrite growth. In this review, the design, synthesis, fabrication, and class are reviewed to give insight into the physicochemical properties of single-ion conducting polymer electrolytes. The standard characterization method and remarkable electrochemical properties are further highlighted, and perspectives on current challenges and future directions are also discussed.

A Critical Review for an Accurate Electrochemical Stability Window Measurement of Solid Polymer and Composite Electrolytes

All-solid-state lithium batteries (ASSLB) are very promising for the future development of next generation lithium battery systems due to their increased energy density and improved safety. ASSLB employing Solid Polymer Electrolytes (SPE) and Solid Composite Electrolytes (SCE) in particular have attracted significant attention. Among the several expected requirements for a battery system (high ionic conductivity, safety, mechanical stability), increasing the energy density and the cycle life relies on the electrochemical stability window of the SPE or SCE. Most published works target the importance of ionic conductivity (undoubtedly a crucial parameter) and often identify the Electrochemical Stability Window (ESW) of the electrolyte as a secondary parameter. In this review, we first present a summary of recent publications on SPE and SCE with a particular focus on the analysis of their electrochemical stability. The goal of the second part is to propose a review of optimized and improved electrochemical methods, leading to a better understanding and a better evaluation of the ESW of the SPE and the SCE which is, once again, a critical parameter for high stability and high performance ASSLB applications.

Recent Advances on Materials for Lithium-Ion Batteries

Environmental issues related to energy consumption are mainly associated with the strong dependence on fossil fuels. To solve these issues, renewable energy sources systems have been developed as well as advanced energy storage systems. Batteries are the main storage system related to mobility, and they are applied in devices such as laptops, cell phones, and electric vehicles. Lithium-ion batteries (LIBs) are the most used battery system based on their high specific capacity, long cycle life, and no memory effects. This rapidly evolving field urges for a systematic comparative compilation of the most recent developments on battery technology in order to keep up with the growing number of materials, strategies, and battery performance data, allowing the design of future developments in the field. Thus, this review focuses on the different materials recently developed for the different battery components—anode, cathode, and separator/electrolyte—in order to further improve LIB systems. Moreover, solid polymer electrolytes (SPE) for LIBs are also highlighted. Together with the study of new advanced materials, materials modification by doping or synthesis, the combination of different materials, fillers addition, size manipulation, or the use of high ionic conductor materials are also presented as effective methods to enhance the electrochemical properties of LIBs. Finally, it is also shown that the development of advanced materials is not only focused on improving efficiency but also on the application of more environmentally friendly materials.

Double-Layered Multifunctional Composite Electrolytes for High-Voltage Solid-State Lithium-Metal Batteries

The need for safe storage systems with a high energy density has increased the interest in high-voltage solid-state Li-metal batteries (LMBs). Solid-state electrolytes, as a key material for LMBs, must be stable against both high-voltage cathodes and Li anodes. However, the weak interfacial contact between the electrolytes and electrodes poses challenges in the practical applications of LMBs. In this study, a double-layered solid composite electrolyte (DLSCE) was synthesized by introducing an antioxidative poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP)-10 wt % Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) to the cathode interface, whereas a lithium-friendly poly(oxyethylene) (PEO)-5 wt % LATP was made to come into contact with Li metal. Owing to the heterogeneous double-layered structure of the DLSCE, a high ionic transfer number (0.43), high ionic conductivity (1.49 × 10^-4 S/cm), and a wide redox window (4.82 V) were obtained at ambient temperature. Moreover, the DLSCE showed excellent Li-metal stability, thereby enabling the Li-Li symmetric cells to stably run for over 600 h at 0.2 mA/cm² with effective lithium dendrite inhibition. When paired with a high-voltage LiNi_1/3Co_1/3Mn_1/3O₂ cathode, the Li/DLSCE/NCM111 cell exhibited excellent electrochemical performance: long-term cyclability with 85% capacity retention could be conducted at 0.2C after 100 cycles corresponding to 100% Coulombic efficiencies.

Ultraviolet-cured polyethylene oxide-based composite electrolyte enabling stable cycling of lithium battery at low temperature

The room and low-temperature performances of solid-state lithium batteries are crucial to expand their practical application. Polyethylene oxide (PEO) has received great attention as the most representative polymer electrolyte matrix. However, most PEO-based solid-state batteries need to operate at high temperature due to low room temperature ionic conductivity. Improving the ionic conductivity by adding plasticizers or reducing the crystallinity of PEO often compromises its mechanical strength. Here, an amorphous PEO-based composite solid-state electrolyte is obtained by ultraviolet (UV) polymerizing PEO and methacryloyloxypropyltrimethoxy silane (KH570)-modified SiO₂ which demonstrates both satisfactory mechanical performance and high ionic conductivity at room (3.37 × 10^-4 S cm^-1) and low temperatures (1.73 × 10^-4 S cm^-1 at 0 °C). In this electrolyte, the crystallinity of PEO is reduced through cross-linking, and therefore provides a fast Li⁺ ions transfer area. Moreover, the KH570-modified SiO₂ inorganic particles promote the dissociation of lithium salts by Lewis acid centers to increase the ionic conductivity. Importantly, this kind of cross-linking networks endows the final electrolyte much higher mechanical strength than the pure PEO polymer electrolyte or PEO-inorganic filler blended systems. The solid-state LiFePO₄/Li cell assembled with this electrolyte exhibits excellent cycling performance and high capacity at room and low temperatures.

100th Anniversary of Macromolecular Science Viewpoint: Solid Polymer Electrolytes in Cathode Electrodes for Lithium Batteries. Current Challenges and Future Opportunities

Solid polymer electrolytes (SPEs) are an important class of ion-transporting materials for enabling safe and high-energy-density all-solid lithium batteries. Within the composite cathode electrode (CCE), an SPE plays a critical role as both binder material for mechanical integrity and electrolyte to facilitate ion transport. The inclusion of an SPE within the CCE leads to the formation of distinctive heterogeneous SPE/solid interfaces that are not present in traditional liquid electrolyte-containing CCE. Here, the viewpoint emphasizes the importance of understanding the interfacial behavior of SPEs in all-solid CCEs. Challenges and opportunities are highlighted in achieving and maintaining good interfacial contact, and the role of interfacial dynamics and nanoconfinement on ion transport. Additionally, routes to achieving high-voltage electrochemical stability through stabilization of interfaces and the development of SPEs with inherently higher oxidative stability are discussed. SPEs with high-voltage stability will provide a pathway to using cathode active materials operating at 4.5 V versus Li/Li⁺ and beyond, which are essential to attaining next-generation higher-energy batteries. Overall, the viewpoint clarifies the importance of targeted research and development of SPEs for enabling all-solid CCEs for lithium batteries.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.

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.