Rational Design of a Cross-Linked Composite Solid Electrolyte for Li-Metal Batteries

The interfacial problem caused by solid-solid contact is an important issue faced by a solid-state electrolyte (SSE). Herein, a cross-linked composite solid electrolyte (CSE) poly(vinylene carbonate) (PVCA)─ethoxylated trimethylolpropane triacrylate (ETPTA)─Li1.5Al0.5Ge1.5(PO4)3 (LAGP) (PEL) is prepared by in situ thermal polymerization. The ionic conductivity and Li+ transference number (tLi+) of PEL increase significantly due to the addition of LAGP, which can reach 1.011 × 10-4 S cm-1 and 0.451 respectively. The electrochemical stable window is also widened to 4.68 V. Benefiting from the integrated interfacial structure, the assembled coin cell shows low interfacial resistance. The all-solid-state NCM622|PEL|Li coin cell exhibits an initial discharge capacity of 169.7 mA h g-1 and 70% capacity retention over 100 cycles at 0.2 C, demonstrating excellent cycling stability.

All-solid-state lithium-sulfur batteries enabled by single-ion conducting binary nanoparticle electrolytes

We designed solid-state hybrid electrolytes with single-ion conducting properties by co-assembling binary core-shell polymer nanoparticles. By controlling the nanoparticle size and number, we created superlattices that optimized the Li+ concentration and transport. The electrolytes exhibited a remarkable ionic conductivity (10-4 S cm-1), lithium transference number (0.94), electrochemical stability (up to 6 V), and modulus (0.12 GPa) at 25 °C. The mechanical strength of these electrolytes depended minimally on temperature at 25-150 °C because of the robustness of the cores. When implemented in Li-S batteries with no liquids, they demonstrated an initial discharge capacity of 1090 mA h g-1 at 0.05C, a cycle life of over 200 cycles, and a rate capability with a discharge capacity of 627 mA h g-1 at 3C.

A mini review of current studies on metal-organic frameworks-incorporated composite solid polymer electrolytes in all-solid-state lithium batteries

All-solid-state lithium batteries (ASSLBs) using solid polymer electrolytes (SPEs) are believed to be future next-generation batteries aiming to replace high-risk traditional batteries using liquid electrolytes, which have a wide application range in portable electronic devices, portable power supplies, and especially in electric vehicles. Moreover, the appearance of SPEs can overcome the electrolyte leakage and flammability problems in conventional lithium-ion batteries. Nevertheless, ASSLBs still face some limitations due to the low ionic conductivity of solid-state electrolytes (SSEs) at room temperature and the poor contact electrode/electrolyte interface, which can be solved by suitable strategies. Currently, the research strategies of metal-organic frameworks that can be incorporated into solid polymer electrolytes offer a remarkable method for producing uniform solid polymer electrolytes that have good electrode/electrolyte contact interfaces and high ionic conductivity. Herein, the updates of current studies about metal-organic framework-incorporated composite solid polymer electrolytes are discussed in this mini-review.

Tailoring Practically Accessible Polymer/Inorganic Composite Electrolytes for All-Solid-State Lithium Metal Batteries: A Review

Highlights

The current issues and recent advances in polymer/inorganic composite electrolytes are reviewed. The molecular interaction between different components in the composite environment is highlighted for designing high-performance polymer/inorganic composite electrolytes. Inorganic filler properties that affect polymer/inorganic composite electrolyte performance are pointed out. Future research directions for polymer/inorganic composite electrolytes compatible with high-voltage lithium metal batteries are outlined.

Abstract

Solid-state electrolytes (SSEs) are widely considered the essential components for upcoming rechargeable lithium-ion batteries owing to the potential for great safety and energy density. Among them, polymer solid-state electrolytes (PSEs) are competitive candidates for replacing commercial liquid electrolytes due to their flexibility, shape versatility and easy machinability. Despite the rapid development of PSEs, their practical application still faces obstacles including poor ionic conductivity, narrow electrochemical stable window and inferior mechanical strength. Polymer/inorganic composite electrolytes (PIEs) formed by adding ceramic fillers in PSEs merge the benefits of PSEs and inorganic solid-state electrolytes (ISEs), exhibiting appreciable comprehensive properties due to the abundant interfaces with unique characteristics. Some PIEs are highly compatible with high-voltage cathode and lithium metal anode, which offer desirable access to obtaining lithium metal batteries with high energy density. This review elucidates the current issues and recent advances in PIEs. The performance of PIEs was remarkably influenced by the characteristics of the fillers including type, content, morphology, arrangement and surface groups. We focus on the molecular interaction between different components in the composite environment for designing high-performance PIEs. Finally, the obstacles and opportunities for creating high-performance PIEs are outlined. This review aims to provide some theoretical guidance and direction for the development of PIEs.

Effect of Ultrasonication on the Morphology, Mechanical Property, Ionic Conductivity, and Flame Retardancy of PEO-LiCF3SO3-Halloysite Nanotube Composites for Use as Solid Polymer Electrolyte

PEO-LiCF₃SO₃-halloysite nanotube (HNT) composites were fabricated by solution casting together with hot compression to form a solid polymer electrolyte (SPE) membrane. Different ultrasonic exposure times were used to disperse HNT nanoparticles in the PEO-20%LiCF₃SO₃-HNT composite solutions prior to casting. An exposure time of 15 min gave the highest ionic conductivity in the SPE membrane, the ionic conductivity significantly increased by two orders of magnitude from 6.6 × 10⁻⁶ to 1.1 × 10⁻⁴ S/cm. TEM, FE-SEM, and EDS-mapping were used to study the dispersion of HNTs in the SPE membrane. ATR-FTIR revealed that the bonding of PEO-LiCF₃SO₃ and PEO-HNT was created. XRD and DSC showed a reduction in the crystallinity of PEO due to HNT addition. The ultrasonication for an optimal period gave uniform dispersion of HNT, reduced the polymer crystallinity and strengthened the tensile property of SPE membrane. Moreover, the electrochemical stability, flame retardance and dimensional stability were improved by the addition of HNT and by ultrasonication.

Electrochemical Impedance Spectroscopy of PEO-LATP Model Multilayers: Ionic Charge Transport and Transfer

Solid-state batteries are seen as a possible revolutionary technology, with increased safety and energy density compared to their liquid-electrolyte-based counterparts. Composite polymer/ceramic electrolytes are candidates of interest to develop a reliable solid-state battery due to the potential synergy between the organic (softness ensuring good interfaces) and inorganic (high ionic transport) material properties. Multilayers made of a polymer/ceramic/polymer assembly are model composite electrolytes to investigate ionic charge transport and transfer. Here, multilayer systems are thoroughly studied by electrochemical impedance spectroscopy (EIS) using poly(ethylene oxide) (PEO)-based polymer electrolytes and a NaSICON-based ceramic electrolyte. The EIS methodology allows the decomposition of the total polarization resistance (R_p) of the multilayer cell as being the sum of bulk electrolyte (migration, R_el), interfacial charge transfer (R_ct), and diffusion resistance (R_dif), i.e., R_p = R_el + R_ct + R_dif. The phenomena associated with R_el, R_ct, and R_dif are well decoupled in frequencies, and none of the contributions is blocking for ionic transport. In addition, straightforward models to deduce R_el, R_dif, and t⁺ (cationic transference number) of the multilayer based on the transport properties of the polymer and ceramic electrolytes are proposed. A kinetic model based on the Butler-Volmer framework is also presented to model R_ct and its dependency with the polymer electrolyte salt concentration (C_Li⁺). Interestingly, the polymer/ceramic interfacial capacitance is found to be independent of C_Li⁺.

A Three-Dimensional Electrospun Li6.4La3Zr1.4Ta0.6O12-Poly (Vinylidene Fluoride-Hexafluoropropylene) Gel Polymer Electrolyte for Rechargeable Solid-State Lithium Ion Batteries

Developing high-quality solid-state electrolytes is important for producing next-generation safe and stable solid-state lithium-ion batteries. Herein, a three-dimensional highly porous polymer electrolyte based on poly (vinylidenefluoride-hexafluoropropylene) (PVDF-HFP) with Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) nanoparticle fillers (PVDF-HFP-LLZTO) is prepared using the electrospinning technique. The PVDF-HFP-LLZTO gel polymer electrolyte possesses a high ionic conductivity of 9.44 × 10^–4 S cm⁻¹ and a Li-ion transference number of 0.66, which can be ascribed that the 3D hierarchical nanostructure with abundant porosity promotes the liquid electrolyte uptake and wetting, and LLZTO nanoparticles fillers decrease the crystallinity of PVDF-HFP. Thus, the solid-state lithium battery with LiFePO₄ cathode, PVDF-HFP-LLZTO electrolyte, and Li metal anode exhibits enhanced electrochemical performance with improved cycling stability.

Organophosphorus decorated lithium borate and phosphate salts with extended π-conjugated backbone

Several new bifunctional salts, Li[B(DPN)₂], Li[B(DPN)(ox)] and Li[P(DPN)₃], have been prepared from the phosphorus(v)-containing chelating ligand 2,3-dihydroxynaphthalene-1,4-(tetraethyl)bis(phosphonate) (H₂-DPN). The new lithium salts were characterized by a variety of spectroscopic techniques. Both H₂-DPN and Li[B(DPN)₂] have been structurally characterized by X-ray crystallographic methods. These salts are related to materials being examined for use in electrolyte solutions for lithium-ion battery (LIB) applications.

Ion Liquid Modified GO Filler to Improve the Performance of Polymer Electrolytes for Li Metal Batteries

Polymer electrolytes for Li metal batteries (LMBs) should be modified to improve their ionic conductivity and stability against the lithium electrode. In this study, graphene oxide (GO) was modified by ion liquid (IL), and the IL modified GO (GO-IL) had been used as a filler for polyethylene oxide (PEO). The obtained solid polymer electrolyte (SPE) is of high ionic conductivity, low crystallinity and excellent stability against the lithium electrode. The PEO/GO-IL was characterized by various techniques, and its structure and performance were analyzed in detail. By addition of 1% GO-IL, the ionic conductivity of the PEO/GO-IL SPE reaches 1.8 × 10-5 S cm-1 at 25°C, which is 10 times higher than PEO (1.7 × 10-6 S cm-1), and the current density for stable Li plating/stripping in PEO/GO-IL can be increased to 100 μA cm-2 at 60°C. LiFePO4/Li cell (using PEO/GO-IL SPE) tests indicated that the initial discharge capacity can reach ~145 mA h g-1 and capacity retention can maintain 88% even after 100 cycles at a rate of 0.1C and at 60°C. Our creative work could provide a useful method to develop SPEs with excellent performance, thus accelerating the commercial application of LMBs.

Progress and Perspective of Ceramic/Polymer Composite Solid Electrolytes for Lithium Batteries

Solid composite electrolytes (SCEs) that combine the advantages of solid polymer electrolytes (SPEs) and inorganic ceramic electrolytes (ICEs) present acceptable ionic conductivity, high mechanical strength, and favorable interfacial contact with electrodes, which greatly improve the electrochemical performance of all-solid-state batteries compared to single SPEs and ICEs. However, there are many challenges to overcome before the practical application of SCEs, including the low ionic conductivity less than 10-3 S cm-1 at ambient temperature, poor interfacial stability, and high interfacial resistance, which greatly restrict the room temperature performance. Herein, the advances of SCEs applied in all-solid-state lithium batteries are presented, including the Li ion migration mechanism of SCEs, the strategies to enhance the ionic conductivity of SCEs by various morphologies of ICEs, and construction methods of the low resistance and stable interfaces of SCEs with both cathode and anode. Finally, some typical applications of SCEs in lithium batteries are summarized and future development directions are prospected. This work presents how it is quite significant to further enhance the ionic conductivity of SCEs by developing the novel SPEs with the special morphology of ICEs for advanced all-solid-state lithium batteries.

New Concepts in Electrolytes

Over the past decades, Li-ion battery (LIB) has turned into one of the most important advances in the history of technology due to its extensive and in-depth impact on our life. Its omnipresence in all electric vehicles, consumer electronics and electric grids relies on the precisely tuned electrochemical dynamics and interactions among the electrolytes and the diversified anode and cathode chemistries therein. With consumers' demand for battery performance ever increasing, more and more stringent requirements are being imposed upon the established equilibria among these LIB components, and it became clear that the state-of-the-art electrolyte systems could no longer sustain the desired technological trajectory. Driven by such gap, researchers started to explore more unconventional electrolyte systems. From superconcentrated solvent-in-salt electrolytes to solid-state electrolytes, the current research realm of novel electrolyte systems has grown to unprecedented levels. In this review, we will avoid discussions on current state-of-the-art electrolytes but instead focus exclusively on unconventional electrolyte systems that represent new concepts.

A review of composite solid-state electrolytes for lithium batteries: fundamentals, key materials and advanced structures

All-solid-state lithium ion batteries (ASSLBs) are considered next-generation devices for energy storage due to their advantages in safety and potentially high energy density.

Nanohybrid electrolytes for high-energy lithium-ion batteries: recent advances and future challenges

Next-generation lithium-ion batteries (LIBs) will have a two to three times increase in energy density compared to today's technology due to the adoption of new cathode and anode materials. In addition, their safety properties need to be further enhanced to allow large-scale applications. In this context, new electrolytes with high lithium-ion (Li⁺) conductivity as well as good stability should be developed. Recently, there has been a growing interest in developing nanohybrid electrolytes. By combining organic (polymers, ionic liquids) and/or inorganic (Li⁺-conductive ceramics and glasses) functional constituents, a broad range of nanohybrid electrolytes with interesting chemical, mechanical and electrochemical properties have been designed and evaluated in different cell chemistry. This article aims to conduct a comprehensive review on the development of nanohybrid electrolytes in recent years (2012 to present). Specifically, we summarize and analyze the recent progress of gel-, inorganic- and polymer-based nanohybrid electrolytes with enhanced physicochemical properties and specified functionalities for their application in LIBs. Challenges and perspectives for future development of better nanohybrid LIB electrolytes are also discussed.

Fast Ion Transport Pathway Provided by Polyethylene Glycol Confined in Covalent Organic Frameworks

Covalent organic frameworks (COFs) with well-tailored channels are able to accommodate ions and offer their conduction pathway. However, due to strong Coulombic interaction and the lack of transport medium, directly including lithium salts into the channels of COFs results in limited ion transport capability. Herein, we propose a strategy of incorporating low-molecular-weight polyethylene glycol (PEG) into COFs with anionic, neutral, or cationic skeletons to accelerate Li⁺ conduction. The PEG confined in the well-aligned channels retains high flexibility and Li⁺ solvating ability. The ion conductivity of PEG included in a cationic COF can reach 1.78 × 10^-3 S cm^-1 at 120 °C. The simplicity of this strategy as well as the diversity of crystalline porous materials holds great promise to design high-performance all-solid-state ion conductors.

Effect of morphological change of copper-oxide fillers on the performance of solid polymer electrolytes for lithium-metal polymer batteries

Solid polymer electrolytes (SPEs) for Li-metal polymer batteries are prepared, in which poly(ethylene oxide) (PEO), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and copper-oxide fillers are formulated. Their structural and electrochemical properties are analyzed when the morphology of the copper-oxide fillers has been modulated to spherical or dendritic structure. The ionic conductivity obtained by electrochemical impedance spectroscopy (EIS) has been increased to 1.007 × 10⁻⁴ S cm⁻¹ at 30 °C and 1.368 × 10⁻³ S cm⁻¹ at 60 °C, as the 5 wt% dendritic fillers have been added to the SPEs. This ionic conductivity value is 1.3 times higher than that of 5 wt% spherical filler-contained SPEs. The analyses of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) indicate that the increase of ionic conductivity is due to the remarkable decrease of crystallinity upon the addition of copper-oxide filler into PEO matrix of SPEs. The fabricated SPEs with the dendritic copper-oxide fillers present a total ionic transference number of 0.99 and a lithium-ion transference number of 0.38. More importantly, it presents a stable potential window of 2.0–4.8 V at 25 °C and high thermal stability up to 300 °C. The specific discharge capacity of the prepared cell with the dendritic filler-contained SPEs is measured to be 51 mA h g⁻¹ and 125 mA h g⁻¹ under 0.1 current-rate (C-rate) at 25 °C and 60 °C, respectively. In this study, the ionic conductivity and the electrochemical performance of the PEO-based polymer electrolyte have been evaluated when morphologically different copper-oxide fillers have been incorporated into the PEO matrix. We have also confirmed the safety and the flexibility of the prepared solid polymer electrolytes when they are used in flexible lithium-metal polymer batteries (LMPBs).

Solid polymer electrolytes (SPEs) for Li-metal polymer batteries are prepared, in which poly(ethylene oxide) (PEO), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and copper-oxide fillers are formulated.

Nanoporous Adsorption Effect on Alteration of the Li+ Diffusion Pathway by a Highly Ordered Porous Electrolyte Additive for High-Rate All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) have shown extraordinary promise for all-solid-state lithium metal batteries with high energy density and flexibility but are mainly limited by low ionic conductivity and their poor stability with lithium metal anodes. In this work, we propose a highly ordered porous electrolyte additive derived from SSZ-13 for high-rate all-solid-state lithium metal batteries. The nanoporous adsorption effect provided by the highly ordered porous nanoparticles in the poly(ethylene oxide) (PEO) electrolyte is found to significantly improve the Li⁺ conductivity (1.91 × 10^-3 S cm^-1 at 60 °C, 4.43 × 10^-5 S cm^-1 at 20 °C) and widen the electrochemical stability window to 4.7 V vs Li⁺/Li. Meanwhile, the designed PEO-based electrolyte demonstrates enhanced stability with the lithium metal anode. Through systematically increasing Li⁺ diffusion, widening the electrochemical stability window, and enhancing the interfacial stability of the SSZ-composite electrolyte (CPE) electrolyte, the LiFePO₄/SSZ-CPE/Li cell is optimized to deliver high rate capability and stable cycling performance, which demonstrates great potential for all-solid-state energy storage application.

Solid-Liquid Electrolyte as a Nanoion Modulator for Dendrite-Free Lithium Anodes

Rechargeable lithium (Li) metal batteries are considered the most promising of Li-based energy storage technologies. However, tree-like dendrite produced by irregular Li⁺ electrodeposition restricts it wide applications. Herein, based on a cation-microphase-regulation strategy, we create solid-liquid electrolytes (SLEs) by absorbing commercial liquid electrolytes into polyethylene glycol (PEG) engineered nanoporous Al₂O₃ ceramic membranes. By means of molecular dynamics simulations and comprehensive experiments, we show that Li ions are regulated and promoted in the two microphases, the channel phase and nonchannel phase, respectively. The channel phase can achieve homogeneous Li⁺ flux distribution by multiple mechanisms, including its uniform array of nanochannels and ability to suppress lateral dendrite growth by its high modulus. In the nonchannel phase, PEG chains swollen by electrolyte facilitate desolvation and fast conduction of Li⁺. As a result, the studied SLEs exhibit high ionic conductivity, low interfacial resistance, and the unique ability to stabilize deposition at the Li anode. By means of galvanostatic cycling studies in symmetric Li cells and Li/Li₄Ti₅O₁₂ cells, we further show that the materials open a path to Li metal batteries with excellent cycling performance.

High-Strength Internal Cross-Linking Bacterial Cellulose-Network-Based Gel Polymer Electrolyte for Dendrite-Suppressing and High-Rate Lithium Batteries

Lithium is a promising anode material for high energy density batteries. However, the growth of lithium dendrite causes serious safety issues, which inhibits the application of lithium anode. Herein, a novel gel polymer electrolyte based on high-strength internal cross-linking bacterial cellulose network was prepared via an environmentally friendly and simple fast freeze-drying method. The as-obtained gel polymer electrolyte demonstrates an excellent lithium ion conductivity of 4.04 × 10^-3 S cm^-1 with an exceptional lithium ion transference number of 0.514 at 25 °C. The lithium metal battery with this gel polymer electrolyte shows an initial reversible capacity of 141.2 mA h g^-1 with a capacity retention of 104.2% (compared with the initial reversible capacity) after 150 cycles at 0.5 C. An average reversible capacity of 75.2 mA h g^-1 is maintained at high rate of 9 C. Moreover, this gel polymer electrolyte possesses superior mechanical strength of 49.9 MPa with a maximum strain of 56.33%. Therefore, the vertical growth of lithium dendrite is effectively suppressed. This research indicates the potential of applying low cost bacterial cellulose into high performance energy storage devices.

Emerging crystalline porous materials as a multifunctional platform for electrochemical energy storage

Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are two emerging and explosively growing families of crystalline porous materials (CPMs). These robust frameworks are characterized by their extraordinary porosity, tremendous structural diversity, and versatile functional tunability with precision at the molecular level. In this review, we present recent milestones of MOFs and COFs in the fields of batteries and supercapacitors, two important technologies in electrochemical energy storage (EES), and highlight the functions that a CPM can offer in EES devices, including the storage of electrochemical energy, stabilization of electrode materials, pathways for charge transport, manipulation on mass transport, and promotion of electrochemical reactions. Key requirements for each function are discussed, with future directions provided for further development.

A 3D Nanostructured Hydrogel-Framework-Derived High-Performance Composite Polymer Lithium-Ion Electrolyte

Solid-state electrolytes have emerged as a promising alternative to existing liquid electrolytes for next generation Li-ion batteries for better safety and stability. Of various types of solid electrolytes, composite polymer electrolytes exhibit acceptable Li-ion conductivity due to the interaction between nanofillers and polymer. Nevertheless, the agglomeration of nanofillers at high concentration has been a major obstacle for improving Li-ion conductivity. In this study, we designed a three-dimensional (3D) nanostructured hydrogel-derived Li_0.35 La_0.55 TiO₃ (LLTO) framework, which was used as a 3D nanofiller for high-performance composite polymer Li-ion electrolyte. The systematic percolation study revealed that the pre-percolating structure of LLTO framework improved Li-ion conductivity to 8.8×10^-5  S cm^-1 at room temperature.