Organic/Inorganic Hybrid Block Copolymer Electrolytes with Nanoscale Ion-Conducting Channels for Lithium Ion Batteries

Recent Development in Topological Polymer Electrolytes for Rechargeable Lithium Batteries

Solid polymer electrolytes (SPEs) are still being considered as a candidate to replace liquid electrolytes for high-safety and flexible lithium batteries due to their superiorities including light-weight, good flexibility, and shape versatility. However, inefficient ion transportation of linear polymer electrolytes is still the biggest challenge. To improve ion transport capacity, developing novel polymer electrolytes are supposed to be an effective strategy. Nonlinear topological structures such as hyperbranched, star-shaped, comb-like, and brush-like types have highly branched features. Compared with linear polymer electrolytes, topological polymer electrolytes possess more functional groups, lower crystallization, glass transition temperature, and better solubility. Especially, a large number of functional groups are beneficial to dissociation of lithium salt for improving the ion conductivity. Furthermore, topological polymers have strong design ability to meet the requirements of comprehensive performances of SPEs. In this review, the recent development in topological polymer electrolytes is summarized and their design thought is analyzed. Outlooks are also provided for the development of future SPEs. It is expected that this review can raise a strong interest in the structural design of advanced polymer electrolyte, which can give inspirations for future research on novel SPEs and promote the development of next-generation high-safety flexible energy storage devices.

Graphene Oxide-Grafted Hybrid Diblock Copolymer Brush (GO-graft-PEG6k-block-P(MA-POSS)) as Nanofillers for Enhanced Lithium Ion Conductivity of PEO-Based Nanocomposite Solid Polymer Electrolytes

Nanocomposite solid polymer electrolytes (NSPEs) with PEO as the matrix and (i) GO or (ii) GO-graft-PEG_6k or (iii) GO-graft-PEG_6k-block-P(MA-POSS) as nanofillers have been fabricated to elucidate the impact of the filler morphology on the lithium ion conductivity. GO-graft-PEG_6k was obtained by grafting PEG_6k onto GO via esterification. GO-graft-PEG_6k-block-P(MA-POSS) was prepared via surface-initiated atom transfer radical polymerization. Fourier-transform infrared spectroscopy revealed enhanced salt dissociation and complexation between the filler and PEO host that could be attributed to Lewis acid-base interactions. Electrochemical impedance spectroscopy revealed the improved ion conductivity of the fabricated NSPEs as compared with the pristine PEO-LiClO₄. As an example, at 50 °C, the ion conductivity increased to 4.01 × 10^-5 and 6.31 × 10^-5 S cm^-1 with 0.3% GO and 0.3% GO-graft-PEG_6k, respectively, from 2.36 × 10^-5 S cm^-1 of PEO-LiClO₄, suggesting that the filler with brush-like architecture (GO-graft-PEG_6k) is more efficient in enhancing the ion conductivity. Further increase in filler content resulted in lowering of the ion conductivity that could be ascribed to aggregation of the filler. The most dramatic impact on conductivity was observed with the incorporation of brush-like GO-graft-PEG_6k-block-P(MA-POSS) as a nanofiller (3.0 × 10^-4 S cm^-1 at 50 °C with 1.0 wt % filler content). The increase in ion conductivity in the current systems, as opposed to the conventional view, could not be correlated with the content of the amorphous phase of the matrix. The conduction mechanism is still unclear; nevertheless, it could be assumed that in addition to the ion conduction through the PEO matrix, the filler forms additional low-energy ion conducting channels at its interface with the matrix. The pendent POSS nanocages of GO-graft-PEG_6k-block-P(MAPOSS) might probably increase the free volume at the interface with the matrix that is associated with higher chain and ion mobility, thus further enhancing the ion conductivity as compared with GO and GO-graft-PEG_6k. The faster ion dynamics in 1.0 wt % GO-graft-PEG_6k-block-P(MAPOSS) NSPEs has also been verified by the dielectric relaxation studies. Thus, integration of both the PEG and POSS nanocages into GO-grafted brush-like architecture offers a new tool for tuning the lithium ion conductivity for potential Li ion battery applications.

Block copolymers as (single-ion conducting) lithium battery electrolytes

Solid-state batteries are considered the next big step towards the realization of intrinsically safer high-energy lithium batteries for the steadily increasing implementation of this technology in electronic devices and particularly, electric vehicles. However, so far only electrolytes based on poly(ethylene oxide) have been successfully commercialized despite their limited stability towards oxidation and low ionic conductivity at room temperature. Block copolymer (BCP) electrolytes are believed to provide significant advantages thanks to their tailorable properties. Thus, research activities in this field have been continuously expanding in recent years with great progress to enhance their performance and deepen the understanding towards the interplay between their chemistry, structure, electrochemical properties, and charge transport mechanism. Herein, we review this progress with a specific focus on the block-copolymer nanostructure and ionic conductivity, the latest works, as well as the early studies that are fr"equently overlooked by researchers newly entering this field. Moreover, we discuss the impact of adding a lithium salt in comparison to single-ion conducting BCP electrolytes along with the encouraging features of these materials and the remaining challenges that are yet to be solved.

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.

Enhanced Conductivity via Extraction of Hydrocarbon Templates from Nanophase-Separated PEO-LiOTf Polymer Electrolyte Films

A series of poly(ethylene oxide)-LiOTf electrolyte films were prepared using a variety of hydrocarbon templates as nanofillers, resulting in observable nanophase separation in the polymer electrolyte. Upon partial extraction of the nanofiller template, an enhanced conductivity over 2 orders of magnitude was measured using ac impedance. Scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis were employed to characterize the porosity, composition, and mass loss of template-extracted and nonextracted film samples.

Sustainable Lignin-Derived Cross-Linked Graft Polymers as Electrolyte and Binder Materials for Lithium Metal Batteries

This study concerns the development of a well-defined synthetic route to obtain lignin-derived multifunctional graft polymers by simple chemical modification and atom-transfer radical polymerization. By grafting ion-conducting and cross-linkable moieties onto the lignin, star-shaped functional polymers are prepared. Upon cross-linking under ultraviolet light irradiation, the resulting polymer network exhibits mechanical stability even at high temperature, whereas the chain mobility is maintained despite the cross-linked structure. Their use as solid polymer electrolytes (SPEs) and binders for all-solid-state lithium metal batteries (LMBs) is also evaluated. The lignin-derived graft polymers provide a facile ion conduction pathway and also efficiently suppress lithium dendrite growth during cycling, thereby attaining excellent cycling performance for the LMB cell compared to that with a conventional liquid electrolyte-Celgard system.

Regulating electrodeposition morphology of lithium: towards commercially relevant secondary Li metal batteries

Rational approaches for achieving fine control of the electrodeposition morphology of Li are required to create commercially-relevant rechargeable Li metal batteries.

Synthesis, self-assembly and self-healing properties of organic–inorganic ABA triblock copolymers with poly(POSS acrylate) endblocks

A novel organic–inorganic ABA triblock copolymer with a poly(acrylate amide) (PAA) midblock and poly(POSS acrylate) [P(POSS)] endblocks was synthesized via sequential reversible addition–fragmentation chain transfer (RAFT) polymerization.

Anomalous Self-Assembly and Ion Transport in Nanostructured Organic-Inorganic Solid Electrolytes

Nanostructured solid electrolytes containing ion-conducting domains and rigid nonconducting domains are obtained by block copolymer self-assembly. Here, we report on the synthesis and characteristics of mixtures of a hybrid diblock copolymer with an organic and inorganic block: poly(ethylene oxide)-b-poly(acryloisobutyl polyhedral oligomeric silsesquioxane) (PEO-POSS) and a lithium salt. In the neat state, PEO-POSS exhibits a classical order-to-disorder transition upon heating. Dilute electrolytes exhibit a dramatic reversal; a disorder-to-order transition upon heating is obtained, indicating that the addition of salt fundamentally changes interactions between the organic and inorganic chains. At higher salt concentrations, the electrolytes primarily form a lamellar phase. Coexisting lamellae and cylinders are found at intermediate salt concentrations and high temperatures. The conductivity and shear modulus of PEO-POSS are significantly higher than that of an all-organic block copolymer electrolyte with similar molecular weight and morphology, demonstrating that organic-inorganic block copolymers provide a promising route for developing the next generation of solid electrolytes for lithium batteries.

Electrochemical Characterization of Single Lithium-Ion Conducting Polymer Electrolytes Based on sp3 Boron and Poly(ethylene glycol) Bridges

A novel series of single lithium-ion conducting polymer electrolytes (SLICPE) based on sp³ boron and poly(ethylene glycol) (PEG) bridges is presented, in the context of the development of a new generation of batteries, with the aim to overcome the problems related to concentration overpotential and low ion transport numbers in conventional solid polymer electrolytes (SPE). The phase separation generated by the physical mixture of SPE with plasticizers such as poly(ethylene oxide) is still a serious problem. In this work, the use of PEG with different chain lengths, for the polycondensation reaction with LiB(OCH₃)₄, to synthesize SLICPE allows preventing phase separation while tuning the predominant conduction mechanism, and thus the electrical properties, especially the lithium-ion transference number. The ionic transport is promoted by chain mobility as the chain length is increased. SLICPE with the best ionic conductivity values (4.95 ± 0.05) × 10^-6 S cm^-1 was the one synthesized from poly(ethylene glycol) with an average M_N of 400 (BEG8), having an O/Li⁺ ratio of 20. The lithium transference number ( t_Li⁺) and electrochemical stability window of SLICPE membranes at 25 °C decreased as the PEG bridge length between sp³ boron atoms increased from 0.97 to 0.88 and 5.4 to 4.2 V vs Li⁰/Li⁺, respectively, for SLICPE synthesized from PEG with an average M_N of 50-400 (BEG1 to BEG8).

Cross-Linked Nanohybrid Polymer Electrolytes With POSS Cross-Linker for Solid-State Lithium Ion Batteries

A new class of freestanding cross-linked hybrid polymer electrolytes (HPEs) with POSS as the cross-linker was prepared by a one-step free radical polymerization reaction. Octavinyl octasilsesquioxane (OV-POSS) with eight functional corner groups was used to provide cross-linking sites for the connection of polymer segments and the required mechanical strength to separate the cathode and anode. The unique cross-linked structure offers additional free volume for the motion of EO chains and provides fast and continuously interconnected ion-conducting channels along the nanoparticles/polymer matrix interface. The HPE exhibits the highest ionic conductivity of 1.39 × 10⁻³ S cm⁻¹, as well as excellent interfacial compatibility with the Li electrode at 80°C. In particular, LiFePO₄/Li cells based on the HPE deliver good rate capability and long-term cycling performance with an initial discharge capacity of 152.1 mAh g⁻¹ and a capacity retention ratio of 88% after 150 cycles with a current density of 0.5 C at 80°C, demonstrating great potential application in high-performance LIBs at elevated temperatures.

High Ion-Conducting Solid-State Composite Electrolytes with Carbon Quantum Dot Nanofillers

Solid-state polymer electrolytes (SPEs) with high ionic conductivity are desirable for next generation lithium- and sodium-ion batteries with enhanced safety and energy density. Nanoscale fillers such as alumina, silica, and titania nanoparticles are known to improve the ionic conduction of SPEs and the conductivity enhancement is more favorable for nanofillers with a smaller size. However, aggregation of nanoscale fillers in SPEs limits particle size reduction and, in turn, hinders ionic conductivity improvement. Here, a novel poly(ethylene oxide) (PEO)-based nanocomposite polymer electrolyte (NPE) is exploited with carbon quantum dots (CQDs) that are enriched with oxygen-containing functional groups. Well-dispersed, 2.0-3.0 nm diameter CQDs offer numerous Lewis acid sites that effectively increase the dissociation degree of lithium and sodium salts, adsorption of anions, and the amorphicity of the PEO matrix. Thus, the PEO/CQDs-Li electrolyte exhibits an exceptionally high ionic conductivity of 1.39 × 10-4 S cm-1 and a high lithium transference number of 0.48. In addition, the PEO/CQDs-Na electrolyte has ionic conductivity and sodium ion transference number values of 7.17 × 10-5 S cm-1 and 0.42, respectively. It is further showed that all solid-state lithium/sodium rechargeable batteries assembled with PEO/CQDs NPEs display excellent rate performance and cycling stability.

Dendrite Suppression by Synergistic Combination of Solid Polymer Electrolyte Crosslinked with Natural Terpenes and Lithium-Powder Anode for Lithium-Metal Batteries

Lithium-metal anode has fundamental problems concerning formation and growth of lithium dendrites, which prevents practical applications of next generation of high-capacity lithium-metal batteries. The synergistic combination of solid polymer electrolyte (SPE) crosslinked with naturally occurring terpenes and lithium-powder anode is promising solution to resolve the dendrite issues by substituting conventional liquid electrolyte/separator and lithium-foil anode system. A series of SPEs based on polysiloxane crosslinked with natural terpenes are prepared by facile thiol-ene click reaction under mild condition and the structural effect of terpene crosslinkers on electrochemical properties is studied. Lithium powder with large surface area is prepared by droplet emulsion technique (DET) and used as anode material. The effect of the physical state of electrolyte (solid/liquid) and morphology of lithium-metal anode (powder/foil) on dendrite growth behavior is systematically studied. The synergistic combination of SPE and lithium-powder anode suggests an effective solution to suppress the dendrite growth owing to the formation of a stable solid-electrolyte interface (SEI) layer and delocalized current density.

Solid Polymer Electrolytes with Excellent High-Temperature Properties Based on Brush Block Copolymers Having Rigid Side Chains

A series of brush block copolymers (BBCPs) with polynorbornene backbones containing poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS, which is a rigid chain) and poly(ethylene oxide) (PEO) side chains were synthesized by tandem ring-opening metathesis polymerizations. The weight fractions of PEO in BBCPs are similar, and the degrees of polymerization (DPs) of PEO side chains are the same while the DPs of PMPCS are different. The bulk self-assembling behaviors were studied by small-angle X-ray scattering (SAXS). The neat BBCPs cannot form ordered nanostructures. However, after the doping of lithium salt, the BBCPs self-assemble into lamellar (LAM) structures. When the DPs of the PEO and PMPCS side chains are similar, the LAM structure is more ordered, which is attributed to the more flat interface between PMPCS and PEO phases. The ionic conductivity (σ) values of the BBCP/lithium salt complex with the most ordered LAM structure at different temperatures were measured. The σ value increases with increasing temperature in the range of 40-200 °C, and the relationship between σ and T fits the Vogel-Tamman-Fulcher (VTF) equation. The σ value at 200 °C is 1.58 × 10^-3 S/cm, which is one of the highest values for PEO-based polymer electrolytes. These materials with high σ values at high temperatures may be used in high-temperature lithium ion batteries.

Multi-POSS cluster-wrapped polymers and their block copolymers with a PEO bottlebrush polymer: synthesis and aggregation

In this paper, we report an approach for synthesizing multicluster-wrapped polymers and their block copolymers with a bottlebrush polymer with controlled chain length and composition via living ring-opening metathesis polymerization.

Gel Polymer Electrolytes Containing Anion-Trapping Boron Moieties for Lithium-Ion Battery Applications

Gel polymer electrolytes (GPEs) based on semi-interpenetrating polymer network (IPN) structure for lithium-ion batteries were prepared by mixing boron-containing cross-linker (BC) composed of ethylene oxide (EO) chains, cross-linkable methacrylate group, and anion-trapping boron moiety with poly(vinylidene fluoride) (PVDF) followed by ultraviolet light-induced curing process. Various physical and electrochemical properties of the GPEs were systematically investigated by varying the EO chain length and boron content. Dimensional stability at high temperature without thermal shrinkage, if any, was observed due to the presence of thermally stable PVDF in the GPEs. GPE having 80 wt % of BC and 20 wt % of PVDF exhibited an ionic conductivity of 4.2 mS cm^-1 at 30 °C which is 1 order of magnitude larger than that of the liquid electrolyte system containing the commercial Celgard separator (0.4 mS cm^-1) owing to the facile electrolyte uptake ability of EO chain and anion-trapping ability of the boron moiety. As a result, the lithium-ion battery cell prepared using the GPE with BC showed an excellent cycle performance at 1.0 C maintaining 87% of capacity during 100 cycles.

Recent advances in organic–inorganic well-defined hybrid polymers using controlled living radical polymerization techniques

Controlled living radical polymerizations, such as ATRP and RAFT polymerization, could be utilized for the preparation of well-defined organic–inorganic hybrid polymers based on POSS, PDMS, silica nanoparticles, graphene, CNTs and fullerene.

Nanocomposite polymer electrolytes based on poly(poly(ethylene glycol)methacrylate), MMT or ZSM-5 formulated with LiTFSI and PYR11TFSI for Li-ion batteries

Novel poly(poly(ethylenglycol)methacrylate) nanocomposite electrolytes based on montmorillonite and zeolite; and functionalized with LiTFSI and PYR₁₁TFSI are synthetized for Li-ion batteries.

Solid Polymer Electrolytes Based on Functionalized Tannic Acids from Natural Resources for All-Solid-State Lithium-Ion Batteries

Solid polymer electrolytes (SPEs) for all-solid-state lithium-ion batteries are prepared by simple one-pot polymerization induced by ultraviolet (UV) light using poly(ethylene glycol) methyl ether methacrylate (PEGMA) as an ion-conducting monomeric unit and tannic acid (TA)-based crosslinking agent and plasticizer. The crosslinking agent and plasticizer based on natural resources are obtained from the reaction of TA with glycidyl methacrylate and glycidyl poly(ethylene glycol), respectively. Dimensionally stable free-standing SPE having a large ionic conductivity of 5.6×10(-4)  Scm(-1) at room temperature can be obtained by the polymerization of PEGMA into P(PEGMA) with a very small amount (0.1 wt %) of the crosslinking agent and 2.0 wt % of the plasticizer. The ionic conductivity value of SPE with a crosslinked structure is one order of magnitude larger than that of linear P(PEGMA) in the waxy state.

Polymer composite electrolytes having core-shell silica fillers with anion-trapping boron moiety in the shell layer for all-solid-state lithium-ion batteries

Core-shell silica particles with ion-conducting poly(ethylene glycol) and anion-trapping boron moiety in the shell layer were prepared to be used as fillers for polymer composite electrolytes based on organic/inorganic hybrid branched copolymer as polymer matrix for all-solid-state lithium-ion battery applications. The core-shell silica particles were found to improve mechanical strength and thermal stability of the polymer matrix and poly(ethylene glycol) and boron moiety in the shell layer increase compatibility between filler and polymer matrix. Furthermore, boron moiety in the shell layer increases both ionic conductivity and lithium transference number of the polymer matrix because lithium salt can be more easily dissociated by the anion-trapping boron. Interfacial compatibility with lithium metal anode is also improved because well-dispersed silica particles serve as protective layer against interfacial side reactions. As a result, all-solid-state battery performance was found to be enhanced when the copolymer having core-shell silica particles with the boron moiety was used as solid polymer electrolyte.