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Nowadays， people are facing severe environmental problems， such as running out of fossil fuels， fluctuation of oil prices， global warming， and pollution impact． With growing demanding of energy， it becomes more and more necessary to find alternative energy storage systems （ESS） ［2］．
To utilize other sustainable energy sources， for instance， solar and the wind， it is necessary to develop effective ESS to ensure the energy delivery and mediate power fluctuations ［3 ］． Rechargeable lithium ion batteries（LIBs） have been considered as one of the most promising ESS， which are widely used in mobile electronic equipment， such as laptop computers， cellular phones， and digital cameras and so on ［4］， because of their high specific energy density （up to 150 Wh ／ kg）， long cycle life and， highefficiency advantages．
Lithium-ion batteries have been manufactured yearly， based mainly on the use of LiPF 6 dissolved in liquid organic aprotic solvents， a major drawback for commercial production problem of leakage which still compromises safety and forces a non-flexible design． Rechargeable solid lithium batteries（RSLB） are particularly attractive because they combine high power density from the Li ／ Li ＋ electrochemical couple with the safety and durability characteristics of plastics． The currently best available salt ［5］， Lithium bis （trifluoromethyl sulfonyl） imide （LiTFSI） is used with high molecular weight polyethylene oxide （PEO； MW： 4，000，000）， but the SPE functions poorly below 60 ℃， so the salt requires the added plasticizers to maintain the amorphous character of the polymer host． As for LiTFSI， which is more expensive than LiPF 6 ， the main drawback to its use in lithium-ion batteries is related to corrosion of the aluminum current collector of the positive electrode． Another drawback in using lithium polymer batteries is their low cationic transference numbers in poly （ oxyethylene） based host polymers ［6］． The high anion mobility induces a concentration gradient for both species through the electrolyte， which results in increased internal resistance and voltage losses ［7］．LiTFSI does not provide adequate electrolyte characteristics， viz． ，（ i） cationic conductivity of at least 10 －2 mS ／ cm at room temperature， （ ii） wide electrochemical stability （0 －5 V），（ iii） safety through solvent-free design and lower reactivity with the lithium anode ［8］．
Figure 1. 1. A schematic of the structure of the lithium ion battery ［9］．
In the lithium ion battery， there are positive and negative electrodes，called cathode and the anode respectively． The cathode is the positive electrode， is made of pure lithium metal oxide． The more uniform the composition， the better the performance and the longer battery life． The anode， a negative electrode which is located on the other side， is made of graphite，full of carbon with layer structure． The battery is filled with transport medium： the electrolyte． So the lithium ion carrying lithium charge can flow freely． The electrolyte may extremely pure， as free of water as possible for the purpose of assuring efficient charging and discharging． Between the two electrodes， there is a layer， the separator， which could prevent the short circuit． To the tiny lithium ions， the separator is permittable． The positively charged lithium ions pass from the cathode through the separator into the layered graphite structure of the anode， where they stored now the battery is charged． When the battery discharges that is the energy removed from the cell， the lithium ions travel via the electrolyte to the anode through the separator back to the cathode， so the light is lighted up．
Until now continually researches are on improving battery performance，cycle life， safety， lightness， shape design variety， no leakage system， and low volatility． Also to meet the requirement for applications in electric vehicles （EV） and hybrid electric vehicles （HEV）， it is necessary to develop high energy density and low-cost materials．
For decades， many polymers， for instance， poly （ acrylonitrile）， poly（methyl methacrylate）， poly （ vinylidene fluoride-co-hexafluoropropylene），and poly （vinylidene fluoride） （PVdF） have been used as mediums for polymer electrolytes by soaking or swelling polymeric matrix with a liquid electrolyte． Nevertheless， the limit of this process lies in the poor stability and low electrolyte retention capability ［10］． Since liquid organic solvents in the electrolytes significantly decrease lithium battery lifetime and safety． Alternatively， solid polymer electrolytes （ SPEs ） with promising electrochemical properties （ e． g． ， conductivity， interfacial stability， and ionic transport properties）［11］ and mechanical properties （ e． g． ， viscous and elastic moduli， yield stress） provide an ideal way to solve the safety issue in the secondary lithium battery application ［12］．
The batteries based on metallic lithium anode and solid polymer electrolytes have encountered several problems： （ i） A low ambient conductivity．（ ii） Side reaction between electrolyte and lithium metal， giving rise to capacity loss during cycling． （iii） Safety hazards caused by dendrite formation during cycling． The conventional poly （ ethylene oxide） （ PEO） -based solid polymer electrolytes have been studied comprehensively due to the ability to form complexes with a wide variety of lithium salts ［13］．
Electrolytes are a major concern related to lithiumion batteries for the reason that their properties， such as viscosity， ionic conductivity， and thermal stability， are capable of determining the performance of a battery． Polymer electrolytes now are expected to be the alternative electrolytes to satisfy the needs for power storage device as well as electric vehicles with high efficiency， high energy density， and long life． Polymer electrolytes can be divided into two types： liquid ／ gel electrolytes and solid polymer electrolytes．Gel electrolytes formed by immersing a significant amount of organic liquid electrolytes into polymer framework have very high ambient temperature ionic conductivities， but they still suffer from several disadvantages， such as worsening of mechanical properties， increased reactivity towards the metal electrode and release of volatiles．
On the other hand， the solid polymer electrolytes have commonly been prepared by dissolving the salt into the polymer framework as the case in the conversant polyethylene oxide （ PEO） membrane． Finding the ideal SPEs maintaining good transport properties give the impression to be a challenging task， most solid electrolyte systems could not perform as well as liquids at room temperature， owing to their lower segmental chain mobility in the crystalline regions compared to the amorphous regions， even though various modification methods could be adopted to decrease the crystallinity of the polymer，e． g． ， by adding inorganic fillers， forming a copolymer ［14］．
Currently， organic solvents based electrolytes are most commonly used in LIB， because of their excellent properties such as high conductivity， low viscosity， and high ionic mobility． Aurbach et al． reported that when selecting electrolytes and their composition， the conductivity， electrochemical stability， operating temperature range， and safety concerns should be well considered． Satisfying all of these demands by using a single solvent is nearly impossible． Therefore， solvents exhibiting different physicochemical properties are frequently mixed for use in a variety of lithiumion battery applications［15］．
At present， most commercial electrolytes for LIBs are composed of lithium salts， such as LiPF 6 ， and mixed solvents ethylene carbonate （EC） and linear carbonates， such as dimethyl， diethyl and ethyl-methyl carbonates（DMC， DEC， EMC， respectively） ［16］． Cyclic EC with a high dielectric constant is a key component for the lithium salt dissociation and sufficient negative electrode passivation． However， EC offers a poor low-temperature performance． To extend the usable range of the electrolyte， linear carbonates essential to be added are， however， highly flammable because of their very low flash point and high vapor pressure ［17］．
Propylene carbonate （PC） is also a promising alternative to the state of theart electrolyte solvents attributable to its excellent properties like low melting point （55 ℃）， high boiling point （240 ℃）， and high flash point （132℃）． Additionally， it enables high conductivities and complete salt dissociation due to its high permittivity （64. 92 at 25 ℃）， which is close to that of EC （89. 78 at 25 ℃） as well as its moderate viscosity （2. 53 mPa s at 25 ℃）［18］． Unfortunately， PC is not capable of forming a good SEI on graphite electrodes ［19］．
The resulting binary or mixed ternary solutions can achieve more rapid ion transport as a result of lower viscosity and subsequently higher ionic conductivity． The choice of co-solvent is a critical issue that significantly affects not only the conductivity but also the electrochemical performance， including the physicochemical properties of solid electrolyte interface （ SEI） formation［20］．
Liquid ／ gel electrolyte batteries show high ionic conductivity （10 －2 ～10 －3 S ／ cm）． Nonetheless， it limits temperature range， low energy density，lack of mechanical stability， corrosion of the electrodes and growth of lithium dendrites from the anode to the cathode． Using liquid electrolytes also may perhaps lead to safety hazards such as fire and explosion， which can be caused by leakage of a fluid electrolyte comprising highly flammable linear carbonates as a solvent ［21］．
Over the past 30 years， solid polymer electrolytes （SPEs） have been intensively studied for the applications in solid-state lithium rechargeable batteries， which are predominantly motivated by their superior advantages over liquid electrolytes， such as avoiding high volatility， fluid leakage， and flammability of traditional liquid electrolytes or gel polymer． They also possess advantageous features of light weight， shape versatility， and easy processability． Therefore， they are promising components for high energy density power sources ［13］．
Conventional SPEs are typically prepared by dissolving lithium salt in a polymer matrix， in some cases， additionally containing plasticizers． The polymer matrix need contain a unit of Lewis base， usually ethylene oxide unit（ － OCH 2 CH 2 －）， to solvate lithium salt． It is generally acknowledged that the motion of Li ions is coupled to the motion of the polymer backbone and ／ or segment through the coordinating interactions between mobile Li ＋ cations and Lewis base （e． g． ， ether oxygen atoms） ［22］．
Several polymer materials such as poly （ vinylidene fluoride） （ PVdF），polyacrylonitrile （PAN）， poly （ methyl methacrylate） （ PMMA）， polyurethane （ PU）， and polyvinyl chloride （ PVC）， have been used as the host polymers for the preparation of polymer electrolytes． However， one major limitation to the SPEs applications is their electrochemical and mechanical properties． To obtain a balance for the compatibility of ionic conductivity and mechanical properties， numerous investigations have been carried out such as the synthesis of the new polymer matrix， the preparation of polymer single-ion conductor， and the doping with nanoparticles． Then through blocking， grafting， crosslinking， compositing and blending methods ［23 ］， researchers can improve the performance of the polymer electrolyte by reducing the crystallinity of the polymer， increasing the concentration of ions and the proportion of the amorphous region contained in the system， along with decreasing glass transition temperature （ T g ） of the polymer electrolyte system and improving the capacity of lithium ion dissociation． SPEs， which consist of polyethylene oxide （PEO） in which a lithium salt is dissolved， would be an ideal candidate ［21］．
The development of solid polymer electrolytes began in the early 1970's with the discovery of complex ionic formation of PEO with alkali metal salts by P． V． Wright ［24］［25］． Since then， the numbers of researches to PEO based SPEs as promising candidates to prepare thinner， lighter and safer LIBs have grown enormously ［26］． Owing to the excellent chain flexibility and polar groups； therefore， the polarity of PEO causes lithium salts to be easily dissolved and promotes the smooth movement of dissociated ions．
Figure 1. 2 Schematic representation of lithium ion migration associ-ated with the segmental mobility of the PEO chains．
As is known， a Li-ion is coordinated with about four ether oxygen atoms of the same or different PEO chains． Lithium cation mobility in the solid phase is regulated by lithium cation-polymer interaction involving lithium cation with oxygen， nitrogen or sulfur coordination bonding， and can transport the dissociated ions by the segmental motion of the main chain． However，the lithium ion transportation can only happen in the amorphous phase of polymer hosts． Usually， the crystalline phase has much lower ionic conductivity than the amorphous phase ［27］， due to the segmental mobility of the polymer matrix that is frozen at the crystallinity phase． PEO based electrolytes exhibit destitute ambient temperature ionic conductivity due to the very high degree of crystallinity present in PEO at room temperature and， therefore， exhibit relatively low ionic conductivity （10 －8 ～ 10 －7 S ／ cm） ［28］．
As the ion moves along the polymer chain， old coordination links break and the new connections form． This cation motion is facilitated by the flexing of the polymer chain segments producing a strong coupling between the segmental motion of the polymer and the ionic transport． As the polymer segments are flexible in the amorphous phase， the polymer electrolyte regularly has a higher conductivity in the amorphous phase． Along with that， the anion is relatively weakly bound to the polymer chain， but the flexibility of the polymer chain is the rate also determining for the anion transport ［29］．
In these electrolytes， the ether oxygen atoms interact with the cations and cause salt solvation． The cation transport is assisted by the segmental motion of the polymer chains． Recognizing the fact that ion conduction takes place in the amorphous phase of polyethylene oxide， considerable researches have been focused on tailoring a flexible host polymer chemical structure with a larger proportion of amorphous phases such as PEO-based block copolymers，star-branched copolymers， and network cross-linked polymers ［13］．
Various approaches， for instance， the synthesis of branched polymers，copolymers with low molecular weight PEO or the incorporation of plasticizing agents， have been undertaken to decrease the crystallinity and T g of the electrolyte to enhance the motion of polymer chains that is responsible for improving the ambient temperature conductivity ［30］．
The most widely used technique to lower the operational temperature of PEO based electrolytes is to add liquid plasticizers， but this gives rise to electrolyte films with poor mechanical properties and higher reactivity towards lithium anode ［31］．
Few polymer blending methods such as PEO／ Polyvinylidene fluoride，PEO／ Polyacrylonitrile， PEO／ Poly （methyl methacrylate） and PEO／ Polyurethanes have been reported， and the addition of various inorganic fillers has been proposed ［32 ］． However， there is obvious phase separation in the PEO-based blend electrolytes because the compatibility of PEO and other polymer is not sufficient． Moreover， the mechanical properties and thermal stability of aliphatic polymers are poor， and they cannot be used as electrolytes without additional separator or reinforcing additives． When the third component， such as solid non-ionic plastic crystal material or inorganic fillers of silica， titanium， zeolites， aluminum oxides and others， is introduced into the PEO-based electrolytes to form the composite polymer electrolytes （CPEs），all of the above performances could be， more or less， improved ［13］．
Therefore， the development of amorphous PEO-based solid polymer electrolytes capable of combining high ionic conductivity with excellent mechanical property is the main goal of the present research．
Methyl group capped polyethylene glycol （PEG） with a melting point below room temperature， readily dissolves lithium salts similar to high molecular weight PEO but has better ionic conductivity due to the lower viscosity and higher ionic mobility of lithium ions． As a plasticizer， PEG has been used to improves the electrical properties of solid and gel polymer electrolytes because it provides the ability to decrease the T g resulting in polymer matrices with increased flexibility． The addition of plasticizers is to effectively improve the ionic conductivity of PEO based SPEs． PEG is also considerably cheaper when compared to commercial high molecular weight PEO ［33］．
In the research， PEO-based polymer membranes were blended with PEG of a high swelling capability to combine better with PS nanoparticles ［34］．High plasticity is beneficial to electrolyte processability， and the composites can be easily manufactured for batteries in various sizes and shapes．
A way to decrease the segment of crystalline phase in the electrolyte is a selection of an appropriate type of anion， which could disturb the regular alignment of polymer chains during crystallization and act like a plasticizer of the polymer matrix． So far， numerous lithium salts have been explored as conducting salts for the PEO-based polymer electrolytes， which primarily include those with weakly coordinating nature， such as hexafluorophosphate（PF 6 － ）， tetrafluoroborate （ BF 4 － ）， perchlorate （ ClO 4 － ）， triflate （ CF 3 SO 3 － ）， bis （ trifluoromethane sulfonyl） imide （［N （ SO 2 CF 3 ） 2 ］－， TFSI－）， bis （pentafluoroethane sulfonyl） imide （［N （ SO 2 C 2 F 5 ） 2 ］－， BETI－）， and so on．
Among them， LiPF 6 is widely used in the electrolytes of LIBs， however，which has relatively poor thermal stability （decomposes at 150℃ ） and easily forms corrosive HF on exposure to moisture．
On the other hand， LiTFSI has been most extensively studied as a highly dissociative conducting salt for SPEs． This is generally attributed to several intrinsic features of the large anion TFSI－， including （1） the high flexibility of － SO 2 － N － SO 2 － of TFSI－， being favorable for reducing the crystallinity of PEO matrix （plasticizing effect）；（2） the highly delocalized charge distribution of TFSI－， being pivotal for effectively reducing the interactions between Li ＋ and TFSI－， thus increasing the dissociation and solubility of LiTFSI in PEO matrix， then and there allows for achieving high values of ionic conductivity ［35］； and （3） excellent thermal， chemical and electrochemical stability， being required for stable electrolytes． Attributable to the flexible S-N-S bonds in LiTFSI， the Tg of electrolytes with LiTFSI salt is ordinarily lower than that of electrolytes with other sorts of salts like LiClO 4 or LiCF 3 SO 3 ． These significant properties of TFSI－are helpful for developing reliable conductive SPEs for LIBs and other electrochemical devices ［27］．
Even though LiTFSI-PEO electrolytes show suitable conductivity only at a temperature higher than the melting point （Tm） of PEO， due to the crystallization of PEO， conductivity as low as about 10 －6 S cm － 1 ， LiTFSI-PEO electrolytes still have other good characteristics， such as light weight， high flexibility and easy for molding．
Also， the concentration of lithium salt in the polymer electrolytes is determined by the molar ratio of － CH 2 CH 2 O －（EO） ／ Li ＋ （i． e． ，［EO unit］ to ［Li ＋ ］）． In the meantime， a molar ratio of ethylene oxide （－ CH 2 CH 2 O －， EO） unit ／ Li ＋ （i． e． ， a molar ratio of ［EO］ to ［lithium salt］） of 20 is thus chosen for LiTFSI-PEO， since the concentration of lithium salt for LiTFSI-PEO electrolyte around this ratio region has been found to afford relatively high ionic conductivities at medium-high temperaturess ［22］．
Practically， an ionic conductivity of about 10 －3 S cm － 1 is required to apply solid polymer electrolytes to LIBs． Accordingly， plasticized polymer electrolytes have been recently prepared and they exhibit the ionic conductivities of 10 －3 S cm － 1 ． Unfortunately， their mechanical properties are weak． In the aim at overcome these problems， the application of cross-linked polymer with good swelling property ［36］ provides another alternative strategy for the improvement of electrolyte properties that can provide mechanical support while ionic salts and plasticizers can also be contained in the polymer networks［13］．
Among them， incorporating cross-linked nano-sized particles into the electrolytes is the better way to improve the membrane strength， rigidity as well as the conductivity， which is attributable to the cross-linked structure and chain extension increases the bonding quality and thus enhances the composite membrane strength ［31］． Mechanically stable cross-linked polymer electrolytes countenance safety separation of anodes and cathodes， preventing shortcircuits and dissolution of electrode components．
Accordingly， we have prepared three cross-linked polymer electrolytes and reported on the synthesis and characterization， regarding their structural，thermal and morphological features．