Incombustible solid polymer electrolytes:A critical review and perspective

Since the advent of the solid-state batteries,employing solid polymer electrolytes(SPEs)to replace routine flammable liquid electrolytes is regarded to be one of the most promising solutions in pursing highenergy-density battery systems.SPEs with superior thermal stability,good processability,and high mechanical modulus obtain increasing attentions.However,SPE-based batteries are not impenetrable due to their decomposition and combustibility under extreme conditions.Researchers believe incorporating appropriate flame-retardant additives/solvents/fragments into SPEs can intrinsically reduce their flammability to solve the battery safety issues.In this review,the recent research progress of incombustible SPEs,with special emphasis on flame-retardant structural design,is summarized.Specifically,a brief introduction of flame-retardant mechanism,evaluation index for safety of SPEs,and a detailed overview of the latest advances on diverse-types SPEs in various battery systems are highlighted.The deep insight into thermal ru naway process,the free-standing incombustible GPEs,and the ratio nal design of pouch cell structures may be the main directions to motivate revolutionary next-generation for safety batteries.

Solid polymer electrolytes in all-solid-state lithium metal batteries:From microstructures to properties

All-solid-state lithium(Li)metal batteries(ASSLMBs)are considered one of the most promising secondary batteries due to their high theoretical capacity and high safety performance.However,low room-temperature ionic conductivity and poor interfacial stability are two key factors affecting the practical application of ASSLMBs,and our understanding of the mechanisms behind these key problems from microscopic perspective is still limited.In this review,the mechanisms and advanced characterization techniques of ASSLMBs are summarized to correlate the microstructures and properties.Firstly,we summarize the challenges faced by solid polymer electrolytes(SPEs)in ASSLMBs,such as the low roomtemperature ionic conductivity and the poor interfacial stability.Secondly,several typical improvement methods of polymer ASSLMBs are discussed,including composite SPEs,ultra-thin SPEs,SPEs surface modification and Li anode surface modification.Finally,we conclude the characterizations for correlating the microstructures and the properties of SPEs,with emphasis on the use of emerging advanced techniques(e.g.,cryo-transmission electron microscopy)for in-depth analyzing ASSLMBs.The influence of the microstructures on the properties is very important.Until now,it has been difficult for us to understand the microstructures of batteries.However,some recent studies have demonstrated that we have a better understanding of the microstructures of batteries.Then we suggest that in situ characterization,nondestructive characterization and sub-angstrom resolution are the key technologies to help us further understand the batteries'microstructures and promote the development of batteries.And potential investigations to understand the microstructures evolution and the batteries behaviors are also prospected to expect further reasonable theoretical guidance for the design of ASSLMBs with ideal performance.

Thermal, Mechanical and Electrical Properties of the PEO-based Solid Polymer Electrolytes Filled by Yttrium Oxide Nanoparticles

The novel composite lithium solid polymer electrolytes （SPEs） composed of polyethylene oxide （PEO） matrix and yttrium oxide （Y2O3） nanofillers were prepared by a solution casting method. The crystal morphology of the SPEs was characterized by polarized optical microscope （POM） and wide-angle X-ray diffraction （WAXD）. The induced nucleation and steric hindrance effects of Y2O3 nanofillers result in the increased amount as well as decreased size of PEO spherulites which are closely related to the crystallinity of the SPEs. As the Y2O3 contents increase from 0 wt% to 15 wt%, the crystallinity of the SPEs decreases proportionally. The thermal, mechanical and electrical properties of the SPEs were investigated by thermal gravimetric analysis （TGA）, dynamic mechanical analysis （DMA） and AC impedance method, respectively. The physical properties including thermal, mechanical and electrical performances, depending remarkably on the polymer-filler interactions between PEO and Y2O3 nanoparticles, are improved by different degrees with the increase of Y2O3 contents. The （PEO）21LiI/10 wt%Y2O3 composite SPE exhibits the optimal room-temperature ionic conductivity of 5.95×10-5 Scm-1, which satisfies the requirements of the conventional electrochromic devices.

Lithium bis(trifluoromethanesulfonyl)imide blended in polyurethane acrylate photocurable solid polymer electrolytes for lithium-ion batteries

The increased demand of electronic devices promotes the development of advanced and more efficient energy storage devices, such as batteries. Lithium-ion batteries (LIBs) are the most studied battery systems due to their high performance. Among the different battery components, the separator allows the control of lithium ion diffusion between the electrodes. To overcome some drawbacks of liquid electrolytes, including safety and environmental issues, solid polymer electrolytes (SPEs) are being developed. In this work, a UV photocurable polyurethane acrylate (PUA) resin has been blended with lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) up to 30 wt% LiTFSI content to reach a maximum ionic conductivity of 0.0032 mS/cm at room temperature and 0.09 mS/cm at 100 ℃. Those values allowed applying the developed materials as photocurable SPE in Swagelok type Li/C-LiFePO_(4) half-cells, reaching a battery discharge capacity value of 139 mAh.g^(−1) at C/30 rate. Those results, together with the theoretical studies of the discharge capacity at different C-rates and temperatures for batteries with LiTFSI/PUA SPE demonstrate the suitability of the developed photocurable SPE for LIB applications.

Functional additives for solid polymer electrolytes in flexible and high-energy-density solid-state lithium-ion batteries

Solid polymer electrolytes(SPEs)have become increasingly attractive in solid-state lithium-ion batteries(SSLIBs)in recent years because of their inherent properties of flexibility,processability,and interfacial compatibility.However,the commercialization of SPEs remains challenging for flexible and high-energy-density LIBs.The incorporation of functional additives into SPEs could significantly improve the electrochemical and mechanical properties of SPEs and has created some historical milestones in boosting the development of SPEs.In this study,we review the roles of additives in SPEs,highlighting the working mechanisms and functionalities of the additives.The additives could afford significant advantages in boosting ionic conductivity,increasing ion transference number,improving high-voltage stability,enhancing mechanical strength,inhibiting lithium dendrite,and reducing flammability.Moreover,the application of functional additives in high-voltage cathodes,lithium-sulfur batteries,and flexible lithiumion batteries is summarized.Finally,future research perspectives are proposed to overcome the unresolved technical hurdles and critical issues in additives of SPEs,such as facile fabrication process,interfacial compatibility,investigation of the working mechanism,and special functionalities.

In-situ polymerized PEO-based solid electrolytes contribute better Li metal batteries:Challenges,strategies,and perspectives

Polyethylene oxide(PEO)-based solid polymer electrolytes(SPEs)with good electrochemical stability and excellent Li salt solubility are considered as one of the most promising SPEs for solid-state lithium metal batteries(SSLMBs).However,PEO-based SPEs suffer from low ionic conductivity at room temperature and high interfacial resistance with the electrodes due to poor interfacial contact,seriously hindering their practical applications.As an emerging technology,in-situ polymerization process has been widely used in PEO-based SPEs because it can effectively increase Li-ion transport at the interface and improve the interfacial contact between the electrolyte and electrodes.Herein,we review recent advances in design and fabrication of in-situ polymerized PEO-based SPEs to realize enhanced performance in LMBs.The merits and current challenges of various SPEs,as well as their stabilizing strategies are presented.Furthermore,various in-situ polymerization methods(such as free radical polymerization,cationic polymerization,anionic polymerization)for the preparation of PEO-based SPEs are summarized.In addition,the application of in-situ polymerization technology in PEO-based SPEs for adjustment of the functional units and addition of different functional filler materials was systematically discussed to explore the design concepts,methods and working mechanisms.Finally,the challenges and future prospects of in-situ polymerized PEO-based SPEs for SSLMBs are also proposed.

Porous garnet as filler of solid polymer electrolytes to enhance the performance of solid-state lithium batteries

In order to enhance the ionic conductivity of solid polymer electrolytes(SPEs)and their structural rigidity against lithium dendrite during lithium-ion battery(LIB)cycling,we propose porous garnet Li6.4La3Zr2Al0.2O12(LLZO),as the filler to SPEs.The porous LLZO with interlinked grains was synthesized via a resol-assisted cationic coordinative co-assembly approach.The porous structure of LLZO with high specific surface area facilitates the interaction between polymer and filler and provides sufficient entrance for Li^(+)migration into the LLZO phase.Furthermore,the interconnection of LLZO grains forms continuous inorganic pathways for fast Li^(+)migration,which avoid the multiple diffusion for Li^(+)in interface.As a result,the SPEs with porous LLZO(SPE-PL)show a high ionic conductive of 0.73 mS·cm^(-1) at 30℃ and lithium-ion transference number of 0.40.The porous LLZO with uniformly dispersed pores also acts as an ion distributor to regulate ionic flux.The lithium-symmetrical batteries assembled with SPE-PL show a highly stable Li plating/stripping cycling for nearly 3000 h at 0.1 mA·cm^(-2).The corresponding Li/LiFePO_(4) batteries also exhibit excellent cyclic performance with capacity retention of 75%after nearly 500 cycles.This work brings new insights into the design of conductive fillers and the optimization of SPEs.

Polymer dispersed ionic liquid electrolytes with high ionic conductivity for ultrastable solid-state lithium batteries

Solid polymer electrolytes(SPEs)have emerged as one of the most promising candidates for building solid-state lithium batteries due to their excellent flexibility,scalability,and interfacial compatibility with electrodes.However,the low ionic conductivity and poor cyclic stability of SPEs do not meet the requirements for practical applications of lithium batteries.Here,a novel polymer dispersed ionic liquid-based solid polymer electrolyte(PDIL-SPE)is fabricated using the in situ polymerization-induced phase separation(PIPS)method.The as-prepared PDIL-SPE possesses both outstanding ionic conductivity(0.74 mS cm^(-1) at 25℃)and a wide electrochemical window(up to 4.86 V),and the formed unique three-dimensional(3D)co-continuous structure of polymer matrix and ionic liquid in PDIL-SPE can promote the transport of lithium ions.Also,the 3D co-continuous structure of PDIL-SPE effectively accommodates the severe volume expansion for prolonged lithium plating and stripping processes over 1000 h at 0.5 mA cm^(-2) under 25℃.Moreover,the LiFePO_(4)//Li coin cell can work stably over 150 cycles at a 1 C rate under room temperature with a capacity retention of 90.6%from 111.1 to 100.7 mAh g^(-1).The PDIL-SPE composite is a promising material system for enabling the ultrastable operation of solid-state lithium-metal batteries.

Solid polymer electrolytes:Ion conduction mechanisms and enhancement strategies

Solid polymer electrolytes(SPEs)possess comprehensive advantages such as high flexibility,low interfacial resistance with the electrodes,excellent film-forming ability,and low price,however,their applications in solid-state batteries are mainly hindered by the insufficient ionic conductivity especially below the melting temperatures,etc.To improve the ion conduction capability and other properties,a variety of modification strategies have been exploited.In this review article,we scrutinize the structure characteristics and the ion transfer behaviors of the SPEs(and their composites)and then disclose the ion conduction mechanisms.The ion transport involves the ion hopping and the polymer segmental motion,and the improvement in the ionic conductivity is mainly attributed to the increase of the concentration and mobility of the charge carriers and the construction of fast-ion pathways.Furthermore,the recent advances on the modification strategies of the SPEs to enhance the ion conduction from copolymer structure design to lithium salt exploitation,additive engineering,and electrolyte micromorphology adjustion are summarized.This article intends to give a comprehensive,systemic,and profound understanding of the ion conduction and enhancement mechanisms of the SPEs for their viable applications in solid-state batteries with high safety and energy density.

Exploring porous zeolitic imidazolate frame work-8(ZIF-8)as an efficient filler for high-performance poly(ethyleneoxide)-based solid polymer electrolytes

Nano Research volume 13,pages2259–2267(2020)Cite this article 277 Accesses 1 Altmetric Metrics details Abstract The incorporation of inorganic fillers into poly(ethyleneoxide)(PEO)-based solid polymer electrolytes(SPEs)is well known as a low-cost and effective method to improve their mechanical and electrochemical properties.Porous zeolitic imidazolate framework-8(ZIF-8)is firstly used as the filler for PEO-based SPEs in this work.Due to the introduction of ZIF-8,an ionic conductivity of 2.2×10^−5 S/cm(30℃)is achieved for the composite SPE,which is one order of magnitude higher than that of the pure PEO.ZIF-8 also accounts for the broader electrochemical stability window and lithium ion transference number(0.36 at 60℃)of the composite SPE.Moreover,the improved mechanism of ZIF-8 to the composite SPE is investigated by zeta potential and Fourier transform infrared spectrograph characterizations.The stability at the composite SPE/lithium interface is greatly enhanced.The LiFePO4||Li cells using the composite SPE exhibit high capacity and excellent cycling performance at 60℃,i.e.,85%capacity retention with 111 mA·h/g capacity retained after 350 cycles at 0.5 C.In comparison,the cells using the pure PEO show fast capacity decay to 74 mA·h/g maintaining only 68 capacity.These results indicate that the PEO-based SPEs with ZIF-8 are of great promise for the application in solid-state lithium metal batteries.

Ion transport and structural design of lithium-ion conductive solid polymer electrolytes:a perspective

Solid polymer electrolytes(SPEs)possess several merits including no leakage,ease in process,and suppressing lithium dendrites growth.These features are beneficial for improving the cycle life and safety performance of rechargeable lithium metal batteries(LMBs),as compared to conventional non-aqueous liquid electrolytes.Particularly,the superior elasticity of polymeric material enables the employment of SPEs in building ultra-thin and flexible batteries,which could further expand the application scenarios of high-energy rechargeable LMBs.In this perspective,recent progresses on ion transport mechanism of SPEs and structural designs of electrolyte components(e.g.conductive lithium salts,polymer matrices)are scrutinized.In addition,key achievements in the field of single lithium-ion conductive SPEs are also outlined,aiming to provide the status quo in those SPEs with high selectivity in cationic transport.Finally,possible strategies for improving the performance of SPEs and their rechargeable LMBs are also discussed.

Status and prospect of garnet/polymer solid composite electrolytes for all-solid-state lithium batteries

Solid polymer electrolytes(SPEs), such as polyethylene oxide(PEO), are characteristic of good flexibility and excellent processability, but they suffer from low ionic conductivity and small Li+transference number at ambient temperature. Inorganic solid electrolytes(ISEs), garnet-type Li7La3Zr2O12 and its derivatives(LLZO-based) in particular, possess high ionic conductivity at room temperature, wide electrochemical stability window, large Li+transference number as well as good stability against Li metal anode.Nevertheless, lithium dendrites growth, interfacial contact issue and brittle nature of LLZO-based ceramic electrolytes prevent their practical applications. In response to these shortcomings, LLZO-based/polymer solid composite electrolytes(SCEs), taking complementary advantages of two kinds of electrolytes, and thus simultaneously improving the electrode wettability, ionic conductivity and mechanical strength, have been made to develop high-performance SCEs in recent years. Herein, the intrinsic properties and research progress of LLZO-based/polymer SCEs, including LLZO-based/PEO SCEs(LLZO-based/PEO SCEs with uniform dispersion of LLZO-based fillers and LLZO-based/PEO layered SCEs) and LLZO-based/novel polymers SCEs, are summarized. Besides, comprehensive updates on their applications in solid-state batteries are also presented. Finally, challenges and perspectives of LLZO-based/polymer SCEs for advanced allsolid-state lithium batteries(ASSLBs) are suggested. This review paper aims to provide systematic research progress of LLZO-based/polymer SCEs, to allow for more efficient and target-oriented research on improving LLZO-based/polymer SCEs.

Recent progress of composite solid polymer electrolytes for all-solid-state lithium metal batteries

In comparison with lithium-ion batteries(LIBs)with liquid electrolytes,all-solid-state lithium batteries(ASSLBs)have been considered as promising systems for future energy storage due to their safety and high energy density.As the pivotal component used in ASSLBs,composite solid polymer electrolytes(CSPEs),derived from the incorporation of inorganic fillers into solid polymer electrolytes(SPEs),exhibit higher ionic conductivity,better mechanical strength,and superior thermal/electrochemical stability compared to the single-component SPEs,which can significantly promote the electrochemical performance of ASSLBs.Herein,the recent advances of CSPEs applied in ASSLBs are presented.The effects of the category,morphology and concentration of inorganic fillers on the ionic conductivity,mechanical strength,electrochemical window,interfacial stability and possible Li+transfer mechanism of CSPEs will be systematically discussed.Finally,the challenges and perspectives are proposed for the future development of high-performance CSPEs and ASSLBs.

SYNTHESIS, CHARACTERIZATION AND ION TRANSPORT PROPERTIES OF HOT-PRESSED SOLID POLYMER ELECTROLYTES (1-x) PEO:x KI

Synthesis and ion transport properties of hot-pressed solid polymer electrolytes （SPEs）, （l-x） PEO： x KI, where x is the content of KI in wt%, are reported. A hot-press technique has been used for the formation of the polymeric membranes in place of the usual solution cast method. The composition （80 PEO：20 KI） was identified as the highest conducting polymer electrolyte on the basis of compositional dependent conductivity studies of PEO：KI films. A conductivity enhancement of more than two orders of magnitude from that of the pure PEO was achieved. Materials characterization and ion transport mechanism were explained by using various experimental techniques.

A critical review on composite solid electrolytes for lithium batteries:Design strategies and interface engineering

The rapid development of new energy vehicles and 5G communication technologies has led to higher demands for the safety,energy density,and cycle performance of lithium-ion batteries as power sources.However,the currently used liquid carbonate compounds in commercial lithium-ion battery electrolytes pose potential safety hazards such as leakage,swelling,corrosion,and flammability.Solid electrolytes can be used to mitigate these risks and create a safer lithium battery.Furthermore,high-energy density can be achieved by using solid electrolytes along with high-voltage cathode and metal lithium anode.Two types of solid electrolytes are generally used:inorganic solid electrolytes and polymer solid electrolytes.Inorganic solid electrolytes have high ionic conductivity,electrochemical stability window,and mechanical strength,but suffer from large solid/solid contact resistance between the electrode and electrolyte.Polymer solid electrolytes have good flexibility,processability,and contact interface properties,but low room temperature ionic conductivity,necessitating operation at elevated temperatures.Composite solid electrolytes(CSEs) are a promising alternative because they offer light weight and flexibility,like polymers,as well as the strength and stability of inorganic electrolytes.This paper presents a comprehensive review of recent advances in CSEs to help researchers optimize CSE composition and interactions for practical applications.It covers the development history of solid-state electrolytes,CSE properties with respect to nanofillers,morphology,and polymer types,and also discusses the lithium-ion transport mechanism of the composite electrolyte,and the methods of engineering interfaces with the positive and negative electrodes.Overall,the paper aims to provide an outlook on the potential applications of CSEs in solid-state lithium batteries,and to inspire further research aimed at the development of more systematic optimization strategies for CSEs.

Improving ionic conductivity of polymer-based solid electrolytes for lithium metal batteries

Because of its superior safety and excellent processability,solid polymer electrolytes(SPEs)have attracted widespread attention.In lithium based batteries,SPEs have great prospects in replacing leaky and flammable liquid electrolytes.However,the low ionic conductivity of SPEs cannot meet the requirements of high energy density systems,which is also an important obstacle to its practical application.In this respect,escalating charge carriers(i.e.Li^(+))and Li^(+)transport paths are two major aspects of improving the ionic conductivity of SPEs.This article reviews recent advances from the two perspectives,and the underlying mechanism of these proposed strategies is discussed,including increasing the Li^(+)number and optimizing the Li^(+)transport paths through increasing the types and shortening the distance of Li^(+)transport path.It is hoped that this article can enlighten profound thinking and open up new ways to improve the ionic conductivity of SPEs.

Recent advances in solid polymer electrolytes for lithium batteries

Solid polymer electrolytes are light-weight, flexible, and non-flammable and provide a feasible solution to the safety issues facing lithium-ion batteries through the replacement of organic liquid electrolytes. Substantial research efforts have been devoted to achieving the next generation of solid-state polymer lithium batteries. Herein, we provide a review of the development of solid polymer electrolytes and provide comprehensive insights into emerging developments. In particular, we discuss the different molecular structures of the solid polymer matrices, including polyether, polyester, polyacrylonitrile, and polysiloxane, and their interfacial compatibility with lithium, as well as the factors that govern the properties of the polymer electrolytes. The discussion aims to give perspective to allow the strategic design of state-of-the-art solid polymer electrolytes, and we hope it will provide clear guidance for the exploration of high-performance lithium batteries.

Mechanism of high Li-ion conductivity in poly(vinylene carbonate)-poly(ethylene oxide)cross-linked network based electrolyte revealed by solid-state NMR

Solid polymer electrolytes(SPEs)have become increasingly important in advanced lithium-ion batteries(LIBs)due to their improved safety and mechanical properties compared to organic liquid electrolytes.Cross-linked polymers have the potential to further improve the mechanical property without trading off Li-ion conductivity.In this study,focusing on a recently developed cross-linked SPE,i.e.,the one based on poly(vinylene carbonate)-poly(ethylene oxide)cross-linked network(PVCN),we used solid-state nuclear magnetic resonance(NMR)techniques to investigate the fundamental interaction between the chain segments and Li ions,as well as the lithium-ion motion.By utilizing homonuclear/heteronuclear correlation,CP(cross-polarization)kinetics,and spin-lattice relaxation experiments,etc.,we revealed the structural characteristics and their relations to lithium-ion mobilities.It is found that the network formation prevents poly(ethylene oxide)chains from crystallization,which could create sufficient space for segmental tumbling and Li-ion co nductio n.As such,the mechanical property is greatly improved with even higher Li-ion mobilities compared to the poly(vinylene carbonate)or poly(ethylene oxide)based SPE analogues.

3D flame-retardant skeleton reinforced polymer electrolyte for solid-state dendrite-free lithium metal batteries

For solid polymer electrolytes(SPEs),improving their mechanical and electrochemical properties is the key to obtaining batteries with higher safety and higher energy density.Herein,a novel synergistic strategy proposed is preparing a 3D flame-retardant skeleton(3DPA)and adding nano-multifunctional fillers(Li-ILs@ZIF-8).In addition to providing mechanical support for the polyethylene oxide(PEO)matrix,3DPA also has further contributed to the system’s flame retardancy and further improved the safety.Simultaneously,the electrochemical performance is fully guaranteed by rigid Li-ILs@ZIF-8,which provides fast migration channels forLi^(+),reduces the crystallinity of PEO and effectively inhibits lithium dendrites.The limiting oxygen index of the optimal sample(PL3Z/PA)is as high as 20.5%,and the ionic conductivity reaches 2.89×10^(-4) and 0.91×10^(-3) S cm^(-1) at 25 and 55°C,respectively.The assembled Li|PL3Z/PA|Li battery can be cycled stably for more than 1000 h at a current density of 0.1 m A cm^(-2) without short circuit being pierced by lithium dendrites.The specific capacity of the LFP|PL3Z/PA|Li battery was 160.5 m Ah g^(-1) under a current density of 0.5 C,and the capacity retention rate was 90.0%after 300 cycles.

Bifunctional flame retardant solid-state electrolyte toward safe Li metal batteries

Solid polymer electrolytes(SPEs)are one of the most promising alternatives to flammable liquid electrolytes for building safe Li metal batteries.Nevertheless,the poor ionic conductivity at room temperature(RT)and low resistance to Li dendrites seriously hinder the commercialization of SPEs.Herein,we design a bifunctional flame retardant SPE by combining hydroxyapatite(HAP)nanomaterials with Nmethyl pyrrolidone(NMP)in the PVDF-HFP matrix.The addition of HAP generates a hydrogen bond network with the PVDF-HFP matrix and cooperates with NMP to facilitate the dissociation of Li TFSI in the PVDF-HFP matrix.Consequently,the prepared SPE demonstrates superior ionic conductivity at RT,excellent fireproof properties,and strong resistance to Li dendrites.The assembled Li symmetric cell with prepared SPE exhibits a stable cycling performance of over 1200 h at 0.2 m A cm^(-2),and the solid-state LiFePO_4||Li cell shows excellent capacity retention of 85.3%over 600 cycles at 0.5 C.