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Pressure to reduce carbon emissions， global warming and over reliance on fossil fuels are a few of the factors that make electric and hybrid electric vehicles （EV／ HEV） more attractive alternatives to the burning of fossil fuels．Lithium-ion （Li-ion） batteries represent a much more advanced alternative to lead-acid batteries since they have a much higher power to weight ratio （2. 59 MJ／ Kg） ［2］［3］． Moreover， Li-ion batteries possess favorable versatility in many other applications， e． g． ， in computers， cell phones， cameras，camcorders， and medical devices． Nevertheless， current Li-ion batteries have limitations that must be overcome before they can be used on a mass scale． Three of the most important challenges are power density， cost， and safety of operation． Electrolytes are the second most expensive （ after the cathode） component of Li-ion batteries． Battery companies are desperately seeking low cost， safer， and high performance electrolytes．
Current state-of-the-art Li-ion batteries employing liquid electrolytes consists of two components： a Li-ion source and an organic solvent． The Li-ion source is generally a single or mixed fluorinated Li-salt ［e． g． ， LiPF 6 ， CF 3 SO 3 Li， and （CF 3 SO 2 ）2 NLi］． A commonly used organic solvent is ethylene carbonate （EC） because of its low cost， good electrochemical stability， and high dielectric constant， which facilitates dissociation of the Li-ion source，leading to high ionic conductivities． Other carbonates， such as dimethyl carbonate （DMC） and propylene carbonate （PC） are often used in conjunction with EC to reduce viscosity， as well as to increase the wettability of the electrolytic solution with battery components， e． g． ， separator and electrodes．Despite these significant attributes of carbonate solvents， they are not recommended for EV／ HEV batteries because of their highly flammable characteristics in the absence of thermal ／ flame protection devices． A small Li-ion battery requires ～ 5 mL of liquid electrolyte， while an EV battery， which contains many small batteries connected in series， requires anywhere from 500 －1000 mL of electrolyte． Thus， liquid electrolytes based full size batteries can potentially be explosive under abusive conditions （ e． g． ， shorting， crushing，or excessive overcharging） and occasionally， under normal conditions （ e．g． ， over discharge， resistive and ／ or forced over discharge）， because the batteries may undergo thermal runaway that generates a sharp rise in temperature and results in serious hazards of fire and explosion ［21］． Accordingly，there is a compelling need for strategic design and development of an advanced solvent free electrolyte system， which is free of leakage and possesses high ionic conductivity and desired electrochemical and mechanical properties．
The availability of lithium rechargeable batteries featuring solvent-free highly conductive solid polymer electrolyte （SPE） systems will have a major impact on the EV／ HEV industry， leading to a significant reduction in environmental pollution and improved performance， compared with current carbonate based liquid electrolytes． The driving force for this advance is the strong potential for achieving high energy densities， high cell voltage， and superior self-discharge characteristics， while largely mitigating deficiencies，such as leakage， instability， and difficulty in the manufacture of the major flat types are currently prevalent in Li-ion batteries containing liquid electrolytes．
The electrochemical properties of SPEs required for EV／ HEV applications include： ionic conductivity greater than 10 －3 S ／ cm at room temperature，electrical conductivity less than 10 －7 S ／ cm， electrical breakdown at greater than 5 V／ m， and stability of the electrolyte adjacent to cathode material up to at least 4. 5 V versus lithium． Other desired features include high mechanical strength， glass transition temperature （ T g ） much lower than room temperature， thin film processability， good interface properties （ compatibility and adhesion）， and smooth operation at ambient temperature． Moreover， the system must be amenable to mass production at reasonable cost．
At this time， a serious drawback of all SPEs is their inadequate conductivities （much lower than 10 －3 S ／ cm） at ambient temperatures． Good ionic conductivity is essential to ensure that a battery system is capable of delivering usable amounts of power at a high rate， a critical requirement for EV／ HEV batteries． It has been predicted that an SPE possessing room temperature conductivity near 10 －3 S ／ cm would lead to the mass scale production of long awaited and significantly safer， high energy density batteries ［46］． Among the solvent free polymer electrolyte systems that have been the most investigated in past decades is polyethylene oxide （PEO）． The main advantages of PEO as a host are its chemical， mechanical and electrochemical stabilities since it contains only strong unstrained C-O， C-C， and C-H bonds． PEO is very flexible（T g ＝ －61 o C） because of the presence of swivel ether linkages and the repeat unit （－ CH 2 CH 2 O －） providing just the right spacing for maximum dissolution of lithium salts． Owing to the presence of sufficient interchain entanglement， PEO electrolyte behaves like a rubbery material but contains both crystalline and amorphous regions． It should be noted that lithium ion conduction takes place in the amorphous phase via diffusion， which occurs through a complex mechanism involving the PEO segmental mobility ［28 ］（ Figure 1. 2 ）． PEO electrolytes also exhibit excellent melt processing capability which is very desirable for the scale mass production of batteries．
Despite these positive attributes， all PEO based electrolytes exhibit poor room temperature ionic conductivity （＜ 10 －5 S ／ cm）， because the degree of crystallinity in a PEO system increases with an increase in concentrations of lithium salts， leading to a marked decrease in ionic conductivity before an acceptable value is attained． As a result， the potential of the PEO system will remain impractical unless the room temperature conductivity is increased from＜ 10 －5 S ／ cm to the “magic number” of ＞ 10 －3 S ／ cm．
Recently， there have been many attempts to improve ionic conductivity of PEO electrolytes with regard to （i） the minimization of the crystallinity of PEO to make a fully amorphous polymer ［30］，（ii） the investigation of the effect of mixing nano sized inorganic filler on the ionic conductivity of PEO composite ［32］， and （iii） the addition of liquid plasticizer （both low and high molecular weight） to prepare PEO electrolyte ［31 ］． Unfortunately，these approaches have yet to achieve anywhere near the magic number． Even though extensive work has been done with other low T g based SPE systems（viz． polyphosphazene and polysiloxanes）， they are far behind the current PEO systems， because of the low dissolution of lithium salts in those polymer matrices ［33］． Consequently， finding the ideal SPEs having good transport properties still gives the impression to be a challenging task．
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 ［14］．Along with decreasing glass transition temperature （T g ） of the polymer electrolyte system and improving the capacity of lithium ion dissociation ［21］，the methods consist of blocking， grafting， crosslinking， compositing and blending various polymers materials ［23］．
Accordingly， investigation of new polymeric salts is essential， although several factors are still needed to be considered while making a choice for the best lithium salt， and the key components include performance， price， and safety． The performance of salt is related to its conductivity at various temperatures， thermal and electrochemical stability．
Based on these， this paper examines the performance of three crosslinked polystyrene lithium salts （PSLSs）： PSTFSILi， PSPhSILi， and PSDTTOLi， which belong to the category of low lattice energy lithium salts． To obtain a balance for the compatibility of ionic conductivity and mechanical properties， this series of polymer salts are blended with high molecular weight PEO （Mw ＝ 4x10 6 g／ mol）， PEG （Mw ＝ 1000 g／ mol） with high swelling capability， complexed with LiTFSI ［34］．
The most important aspect which has prompted us to study this class of SPEs is that the current state-of-the-art liquid electrolytes exhibits very low cationic transference numbers （ t ＋ ＝ 0. 2 －0. 3）， which causes polarization of the electrolyte， leading to an increase in resistivity， especially near or below subzero temperatures ［6］． Recharging the cell then requires more time，energy， and electrochemical potential． In order to circumvent this problem，it is essential that， in addition to high conductivity， an electrolyte exhibits a high cationic transference number （＞ 0. 6） for smooth and efficient charging and discharging characteristics． Since the anionic part of the lithium salt is immobile （covalently linked to crosslinked polystyrene microparticles） in the proposed SPEs， we anticipate that these SPEs will behave similarly to a single-ion conducting polymer， which exhibits a higher cationic transference number， leading to the efficient charging and discharging． Addressing the issue of cationic transference number is significantly important for EV／ HEV application since these vehicles are expected to survive extreme weather conditions． In order to accomplish the aforementioned goals， we present herein a new synthetic strategy to develop a commercially viable product．
In choosing this system， PEG has been used as a plasticizer to improve the electrical properties of solid polymer electrolytes since 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 ［30］． PEG is also considerably cheaper compared to commercial high molecular weight PEO ［33 ］． Among that， incorporating cross-linked nano-sized particles with good swelling property ［36］ into the electrolytes is the better way to improve the membrane strength， rigidity as well as the conductivity ［13］． Mechanically stable cross-linked polymer electrolyte membranes are also able to safety separation of anodes and cathodes， preventing short circuits and dissolution of electrode components ［31］．
The primary goal of our study is to identify new， stable， and environmentally friendly PSLSs which exhibit excellent electrochemical and thermal properties for potential use in Li-ion batteries．