Ionic Conductivity, Polymer Electrolyte, Membranes, Electrochemical Stability, Separators

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The most commonly used separators in Li-ion batteries are the polyolefin membranes due to their satisfactory electrochemical stability, useful thickness and advantageous mechanical strength.

3.1 Introduction

The most commonly used separators in Li-ion batteries are the polyolefin membranes due to their satisfactory electrochemical stability, useful thickness and advantageous mechanical strength [1]. The most commonly used separators in LIBs’ low porosity, inferior thermal dimensional turbulence and inadequate electrolyte wettability might give rise to high cell resistance or an internal short circuit; this circuit seriously hinders the electrochemical and safety performance of Li-ion batteries (Lee and others [20]; Yanilmaz and others [21]), [1]. Some attempts have been devoted to the development of new separators with excellent thermal stability and satisfactory electrolyte wettability [1]. Cellulose-based materials have been examined as separators in Li-ion batteries due to their outstanding properties, such as excellent thermal stability (Chun and others [22]; Jabbour and others [23]; Kim and others [24]; Xu and others [25]; Zhang and others [26, 27]; Jiang and others [28]; Weng and others [29]; Liao and others [30]) and the desired electrochemical stability in recent decades [1]. Via an electrospinning method, co-workers (Zhang and others [26]) and Cui have prepared a superior and renewable thermal-resistant cellulose/poly(vinylidene fluoride-co-hexafluoropropylene) composite non-woven separator for high-performance Li-ion batteries [1]. Through a force-spinning method, Alcoutlabi and others (Weng and others [29]) have mass-produced fibrous cellulose membranes as a fruitful alternative to commercial polyolefin separators [1]. The BC nanofibrous membrane has been devised as a separator for Li-ion batteries through a facile hot-pressing fabrication approach by co-workers (Jiang and others [28]) and Zhang [1]. The tensile strength of the devised BC separator is meager as compared to the commercial Celgard membrane [1]. Hydrophilic inorganic Al2O3 powders are normally utilized as supports to alter the surface of the separators due to their chemical inertness, excellent thermal stability (Deng and others [31]; Jeon and others [32]) and satisfactory wettability [1]. A BC/Al2O3 nanofibrous composite membrane as an LIB separator had been appropriately prepared by coating Al2O3 on BC nanofibres via a straightforward in situ thermal decomposition technique [1]. The porosity, thermal shrinkage, tensile strength, ionic electrical conductivity and electrolyte wettability of the devised BC/Al2O3 membrane were typified [1].

The separator does not entail directly in any cell reactions, though properties and its structure play important roles in determining the battery performance, which comprises cycle life, safety, power density [33], and energy density, in Li-ion batteries [2]. Some modification methodologies, including polymer coatings [34], electrospinning technique [35], and polydopamine treatment [36], have been examined to circumvent the shortcomings of polyolefin-based separators [2]. A ZrO2-added composite separator had been devised by Scrosati and others [2, 37]. A new silica tube-coated PE composite separator had been devised by Zhang and others [2, 38]. It could be concluded that multiple kinds of ceramic-coated composite separators have been demonstrated, resulting in enhanced separator, which wets thermal shrinkage and ability, at high temperature [2]. That incorporation of pottery particles into the separators is an appealing technique to derive high-performance separators is demonstrated by some reports [2]. The nanosized pottery particles including TiO2, and Al2O3, SiO2, can substantially enhance thermal stability the mechanical strength, including their thermal resistance [33] and ionic electrical conductivity of separators because of their high hydrophilicity and high surface area [2]. It could be surmised that the new composite separator retains superior performances compared with traditional separators if the traditional pottery oxides were substituted by zeolites or molecular sieves [2]. In terms of morphology, structure, thermal stability, the electrochemical performances, and electrolyte wettability, the attributes of the separator are assessed [2].

Porous polyolefin separators including polyethylene (PE), PP/PE/PP, and polypropylene (PP), are the most popular separators for Li-ion batteries due to advantages including high mechanical strength, low cost [39, 40, 41], and satisfactory electrochemical stability [3]. In high-performance Li-ion batteries [42], low porosity and their meager thermal stability restrict their application [3]. Developing an optimum separator to ameliorate the shortcomings of traditional polyolefin separators is required to satisfy the increased performance and safety demands of Li-ion batteries to this effect [3]. Some recent research initiatives have been carried out in effort to circumvent the drawbacks of the polyolefin separator to satisfy the requirements of high-performance Li-ion batteries [3, 43]. Zhu and others devised a new, ceramic-grafted PE separator, which is fabricated with electron beam irradiation; the ceramic-grafted PE separator showed highly enhanced dimensional thermal stability without any deviation from its original primitive thickness and pore structure [3, 44]. That a PE-coated PI nonwoven composite membrane not just indicates excellent thermal shutdown function though displays considerably greater thermal stability, lower internal resistance than the PP/PE/PP separator [45], and better wettability with the polar electrolyte, had been shown by Shi and others, [3]. The incorporation of pottery particles and nonwoven fabrics has been shown that nanosized inorganic fillers can substantially enhance the mechanical strength, ionic electrical conductivity of composite separators [46], and thermal stability [3]. Various basic properties of the composite separator, which comprises its surface morphology, porosity, thermal stability, ionic electrical conductivity, and electrolyte wettability, were assessed and compared to those of the commercial Celgard 2500 separator [3].

All of the above exploit of the special properties of Li-ion batteries: no memory effect, high energy density, high operation voltages, and long cycle life (Fang and others [47]; Liang and others [48]; Kumar and others [49]), which self-discharges low [4]. A critical element of a LIB is the separator membrane [4]. Architecture and the separator membrane’s property play a crucial role in affecting the cell performance, which comprises energy density, power density, service life, and safety, even though the separator does not take part directly in electrochemical reaction of a LIB cell [4]. Lots of contextual factors should to be regarded whilst choosing useful separators for Li-ion batteries [4].

The pore structure and thickness of the separator should be carefully controlled, as a satisfactory balance between mechanical strength and ionic electrical conductivity should be kept (Arora and Zhang [40]; Lee and others [33]; Zhang [50]) in order to satisfy these two functions [5]. The pore structure and porosity of the material are clearly quite crucial to the performance of the separator in a battery in addition to the separator material [5]. To enhance systematic researches of the impact of these properties on the performance of Li-ion batteries there is hence a need for clear-cut techniques by which the pore structure and porosity of a specific material could be varied conveniently [5]. We are, however, not cognizant of any researches on cellulose premised LIB separators revolved around the thickness reliance of the pore structure, porosity and i.e. the pore size distribution [5]. A clear-cut paper-making filtration process by which CC separators with pore structures and various thicknesses could be manufactured simply by differing the quantity of cellulose utilized is explained by us [5]. CC separators with pore structures and various thicknesses are permitted by this strategy to be manufactured without various drying methods or utilizing external pressure [5]. It is demonstrated that thinner separators display a less compact structure with greater porosities and bigger pores and that this leads to Li-ion batteries with lower cell resistances [5].

Lithium-ion battery with light weight, long cycle life, and huge capacity, has demonstrated to be an optimal choice for electric vehicles [51] under the nervous circumstance in the energy resources [6]. The safety problem of high-power lithium-ion battery has aroused more and more attention [52] with the rapid development of lithium ion battery industry [6]. High temperature running of battery systems normally triggers the shrinkage of conversional battery separator, leading to quite fast internal shorting of the battery [6]. Due to the high-power charge and discharge, violent oscillation, long time, and collision, which might give rise to the contraction of the conventional separators [53, 54], which works, overcharge, the normal working temperature can be surpassed by the local temperature inside battery, as reasonably well [6]. The amelioration of thermodynamic stability for the separators of lithium batteries is problematic [55, 56, 57, 58] and crucial [6]. It is urgently required to build heatproof and anti-shrink separators for a safer lithium-ion battery [6]. Through porous polyethylene (PE), polypropylene (PP), and/or PP/PE/PP membrane because of their satisfactory electrochemical performance [46], the separators for commercial lithium-ion batteries are normally comprised [6]. Some researches focus on the amelioration of the safety and stability of lithium-ion battery [6]. It is pointed out that electrospun is a convenient technique to prepare lithium-ion battery separator with some excellent performances, including small holes, high porosity, distribution homogeneity [57, 59], and huge specific surface area [6].

Safety problem is still a largest obstacle for the large-scale applications [60] in spite of the widespread application of lithium-ion batteries [7]. The safety concern of lithium-ion batteries is a serious obstacle in the technology development [7]. Since the state-of-art electrolytes of lithium-ion batteries use highly flammable carbonate-based organic electrolytes; these electrolytes may be ignited and then presumably cause serious hazards of firing and upsurge under abused conditions (heat, overcharge, short circuit, etc.) [7]. A novel sort of multi-component (MC) additive is summarized by us for lithium-ion battery electrolyte (1.0 mol/L LiPF6/ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1 wt.%)) [7]. The impacts of MC additive in enhancing the cell performance of the lithium-ion cell and the thermal stability of the electrolyte were investigated [7]. How the MC additive might behave on the surface of electrodes needs additional research, and its possibility as the flame retardant for commercial lithium-ion battery needs to assess [7].

Electrolyte cum separator Beside electrodes, the separator cum electrolyte is an indispensable element of the battery, as it plays a significant role in the transportation of ions during charging and discharging mechanisms, so it should be in stable form during battery operation [8]. Lithium batteries is superior to all existing systems because of its safety, high energy density, light weight, cost effective, and shape flexibility, among these [8]. Mostly fast ionic conductors are based upon inorganic materials with excellent transport properties, though inferior electrolyte stability limits their use as electrolyte cum electrode separator [8]. Through Michael Faraday, the phenomena of ionic conduction had been identified in the 1800s on solid electrolytes PbF2; later in 1964, first-time polymer electrolytes came in existence which had been based upon CdCl2 and multivalent salts HgCl2 [8]. The presence of both crystalline stage and amorphous stage makes attempting to investigation the properties of polymer electrolytes correctly [61, 62] as multivalent cations are easier to deal with and cheap than widely used alkali metal salts [8]. The electrolyte along with a separator serves as a medium to keep electrodes individually when liquid electrolytes are utilized and for transportation of ions in any battery system [8]. The polymer electrolyte is utilized as a thin-film membrane which can function for both ion conduction and separation of electrodes for the solid-state Li-ion batteries [8]. Polymer electrolytes formed by dissolving Li salt in a high molecular weight polymer host including PEO are dual ionic conductors [8]. In the electrolyte, polymers with low T g are preferred for excellent flexibility and fast ion transport [8].

In a matrix of organic solvent display, lithium batteries based upon liquid electrolytes consisting of a lithium salt dissolved a low flash point and are susceptible to leakage (Tasaki and others [63]), [9]. Suitable electrochemical stability, satisfactory mechanical strength, tremendous lithium ion transference number, high ionic electrical conductivity, advantageous compatibility with electrode materials (He and others [64]; Li and others [65]; Isken and others [66]) and thermal stability must be possessed by polymer electrolytes as separator in lithium batteries [9]. The most often investigated polymer electrolytes for Lithium-ion batteries (Sil and others [67]) is the complexes of Li salts with high molecular weight PEO [9]. To combine the benefits of SPEs and liquid electrolytes gel polymer electrolytes have been introduced (Deka and Kumar [68]) that display high ionic electrical conductivity at ambient temperature by retaining huge quantity of liquid electrolyte in the polymer host (Sil and others [67]), [9]. While the electrolyte introduces high mechanical stability for application as separator in lithium batteries (Ramesh and others [69]), high ionic conductivities could be accomplished [9]. LiClO4 retains huge size anions, low dissociation energy and is exceedingly soluble in most organic solvents, offering so high ionic electrical conductivity (Park [70]) and a high concentration of free ions [9]. EC and PC solvents are normally utilized as high dielectric constant of 66 and 89 [71], respectively and high-permittivity element in fabrication of lithium batteries because of their low-viscosity [9]. Applying low concentrations of PMMA caused in formation of GPEs, which constitutes high ionic conductivities comparable to liquid electrolytes [9]. At high concentration of PMMA the ionic electrical conductivity of GPEs had been declined which had been attributed to greater relationships between the electrolyte and the polymer matrix [9]. That the kind of utilized polymer, Li salt and plasticizer play crucial role in determining electrochemical performances of the prepared GPEs [70] and the ionic electrical conductivity is revealed by these studies [9].

An polymer system with a polymer as separator/electrolyte, which conducts ionically, is of extreme interest because of their various applications, such as super capacitors, fuel cells, solar cells, Li ion batteries, and electro chromic windows [10]. The advantageous electrolyte for any application in energy storage/conversion tools should have (a) high ionic electrical conductivity, (b) an electrochemical stability window (>4 V), (c) a low melting point, (d) a high boiling point, (e) high chemical stability, (f) low cost and satisfactory compatibility with electrodes [40, 72, 73, 74], and (g) non-toxicity [10]. The majority of of the tools are based upon liquid/gel polymer electrolytes because of compatibility with electrodes and their high ionic electrical conductivity (10−3 to 10−2 S cm−1) [10]. That elicits the scholars toward replacement of the traditional liquid polymer electrolyte with a solvent-free polymer electrolyte, which haves high ionic electrical conductivity, leak proofing, better flexibility, a wide electrochemical window, satisfactory mechanical strength, ease of preparation [75, 76, 77], and light weight [10]. Polyethylene oxide (PEO) is undoubtedly the optimal host polymer utilized as SPEs with strong, unstrained C–O, C, and C–C, –H ties and it has a SPEs dielectric constant, easy availability, high ionic electrical conductivity in the amorphous stage, low glass transition temperature, satisfactory dimensional and chemical stability, and high flexibility, out of the aforesaid host polymers [10]. Incorporation of the nanofiller enables in impeding the recrystallization tendency of the polymer chain and decreases the glass transition temperature of the composite polymer electrolyte; this electrolyte suggests enhancement in ionic electrical conductivity [10, 78] The nanofiller supports ion dissociation and strengthens the ion migration by offering further conducting paths for the cation within the host polymer matrix [10, 79, 80].

Given the practical applications, the SPEs must owns the properties of high ionic conductivities (exceeded 10−4 S cm−1 at operator temperature), huge Li+ transference number (close to unity), wide electrochemical stability window (4–5 V vs. Li/Li+), excellent mechanical strength, low interfacial resistance between the electrolyte and the electrode [81, 82, 83, 84, 85], and satisfactory thermal stability [11]. A block copolymer, which is comprised which of soft and hard segment, is polyurethane [11]. A polyurethane-based gel polymer electrolyte for Li-ion batteries had been synthesized by Liu and others [11, 86]. A single-ion electrolyte, which is based upon polyurethane and the LiFePO4/Li cell, which employs, this electrolyte that revealed outstanding rate performance, had been indicated by Porcarelli and others [11, 87]. At least a handful works have focus on the SPEs based upon polyurethane, Liu and others [88] indicated series of cationic PU for SPEs; this PU revealed high ionic electrical conductivity at room temperature [11]. The performance of all-solid-state Li-ion batteries employing these PU-based SPEs have not been evaluated even though these polyurethane-based SPEs have been confirmed to owns excellent mechanical properties and high ionic conductivities [11]. Through changing the composition of the hard and soft segment [89], the properties of polyurethane could be readily tailored [11]. The polyether-based soft segment in polyurethane with (−CH2–CH2–O) n unit can offer the transport path way for the Li+ ions [90] and dissolve the cations [11]. Isophorone diisocyanate (IPDI) [91] and 4, 4′-methylenediphenyl diisocyanate (MDI) [88, 92, 93] had been utilized by the earlier literatures for the hard segment, and the rigidity alicyclic and aromatic structure with strong hydrogen bonding between the hard and soft segment confined the segmental movements of give rise to low ionic electrical conductivity and the polyurethane [11]. Given that the high ionic electrical conductivity depends upon the rapid segmental movements of the polymer matrix [94], both hard segment and soft segment are flexible will useful [11].

Such difficulties have been resolved by replacing liquid electrolyte with solid polymer electrolyte in the batteries [12, 73]. Due to their distinctive properties, including leakage proof, flexibility and satisfactory toughness, and easy fabrication into shapes [95, 96] and advantageous sizes, polymer electrolyte films attract an increased interest [12]. Some shortcomings of the polymer electrolyte films, including electrochemical stability and comparatively low ionic electrical conductivity, still deter their practical application in batteries [12, 97]. Confronted with these limitations, gel polymer electrolyte films have attracted considerable attention as they owns the enhanced ionic electrical conductivity and improve the interfacial property substantially [12, 98, 99]. In organic solvents, this sort of salt could be dissolved totally, so that the Li-ions could be transferred in polymer electrolyte films [12]. A 85PVdF–HFP: 15LiBF4 complexed polymer electrolyte had been devised, displaying ionic electrical conductivity [100] and comparatively high amorphicity [12]. The 85PVdF–HFP: 15LiBF4 polymer electrolyte had been selected to be the host system for preparing gel polymer electrolyte films in the present study [12]. Ion pairing could be lowered by the plasticizer, improve the flexibility of polymer chains, increase ionic electrical conductivity and lithium-ion battery property, and enhance the stability of electrode/electrolyte interface [12]. Firstly, the gel polymer electrolyte films were prepared based upon the 85PVdF–HFP: 15LiBF4 system along with plasticizers PC and EC in a weight ratio of 1:1 [12]. The 2032 coin cells were assembled employing their charge-discharge cycling performance had been assessed employing a computerized battery cycling unit and the self-made gel polymer electrolyte films [12].

Polymer electrolytes are the crucial class of materials for the applications in clean energy tools including batteries, sensors [101, 102, 103] because of its flexibility and safety, and fuel cells, and it is non-corrosive [13]. Ion transport mechanism, mechanical stability in polymer electrolyte and improving electrical conductivity, is critically crucial in the past century [13]. Through the incorporation of inert fillers including TiO2, SiO3, Al2O3, etc. [104, 105], nanocomposite polymer electrolyte is prepared in order to attain the greater ionic electrical conductivity at ambient temperature [13]. To attain the hybrid polymer electrolyte for the fabrication of Li-ion polymer battery, electrochemical and electrical performance has been examined in the present work [13]. The principal aim of the current study is to prepare lithium-ion: PVdF, which is incorporated with TiO2 nanofiller by solution casting method, which conducts hybrid polymer electrolyte based upon PVA [13]. Structural, vibrational, electrical conductivity, mechanical, thermal, electrochemical behaviour of the prepared samples, and morphological, have been examined employing XRD, FTIR, AC impedance spectroscopic method, stress-strain measurement, DSC, TGA, SEM, cyclic voltammetry (CV), respectively, and linear sweep voltammetry (LSV) [13].

There are many fatal shortcomings such as leakage, blast [106] in the liquid Li-ion batteries, and flame, even though lower cost of organic liquid electrolyte and the high ion electrical conductivity can preferably derive economical battery and the high energy density [14]. Upon Fenton and others observed the polymer electrolyte with ionic electrical conductivity; this electrical conductivity is composed of poly(ethylene oxide) (PEO) and alkali metal salt in 1973 [107], polymer electrolyte has been extensively researched due to ease of processing, satisfactory plasticity, and efficient prevention of electrolyte leakage [14, 108]. Solid macromolecule, which limits the migration of lithium ions to a certain extent and additional gives rise to a decline in the ionic electrical conductivity of the electrolyte is the matrix of polymer electrolyte [14]. That issue has been demonstrated to be solved by gel polymer electrolyte (GPE) because it comprises of polymer matrix, plasticizer, and lithium [14]. There are handful applications for commercial Li-ion batteries with cellulose as the GPE matrix due to transfer number [109, 110, 111] and low ionic electrical conductivity even though cellulose is the most plentiful biopolymer resources on the earth [14]. The second plentiful biopolymer on the earth [112], a novel kind of biopolymer of lignin and extensively exists in the cell wall of plants, as the GPE matrix, which not just can dramatically lessen the cost, though can derive high ionic electrical conductivity, lithium-ion transference number, wide electrochemical stability window, and stable cycling performance had been founded by us [14].

The LiFePO4 (LFP) is one of the most useful cathode materials for LIB because of its environmental compatibility, low cost, high theoretical capacity (170 mAh g−1), non-toxicity [113, 114], and excellent thermal stability [15]. That cathode material usually suffers from many shortcomings including deterioration of capacity, inferior stability, rate performance, and Li+ ion diffusivity (~10−14 cm2 s−1), at high discharge/charge current density [15, 115, 116]. That inferior performance, which is correlated with defects in side reaction between cathode material and electrolyte decreases or olivine structure of LFP, Li+ electronic electrical conductivity and ion diffusion rate [15]. That graphitic and carbon carbon coating is an effective strategy for enhancing electronic electrical conductivity and electrochemical performance of the cathode material in a LIB [117] is revealed by the most recent report [15]. Since a blocking layer is formed between electrolyte and active material particles, carbon coating method has concentrated on enhancing the electrochemical performance of LFP [15]. The carbon-coated active material particle is not an effective way to enhance discharge and charge performance at high current rate because most of the carbon is amorphous [118] and the contact area between LFP particles is quite less [15]. Salt LiTFSI has been selected because it could be readily disassociated into anions and cations [119] and it indicates low lattice energy, and IL EMIMFSI is utilized because it indicates low viscosity, satisfactory electrochemical/thermal stability, high ionic electrical conductivity, and satisfactory plasticizer effect including a supplier of free charge carriers [15, 120]. We have examined thermal stability, electrochemical performance of the prepared ILGPE, and ionic electrical conductivity, and assembled coin cell by employing 80 wt% IL containing GPE with lithium metal foil as anode with a standard GO@LFP and LFP (i.e., without coating) cathode [15].

Ionic liquid electrolyte has devised into entire solid polymer electrolyte [121, 122] and gel polymer electrolyte [16]. Gel polymer electrolyte with better security and greater electrical conductivity has aroused more attention [16, 123, 124, 125]. Ionic liquids recognize environment protection, the cleaner generate, and cycle economics because of its advantages such as non-flammable and non-toxic property, low vapour pressure, high electrical conductivity, high ion transference number, wide electrochemistry window, and satisfactory stability performance [16, 126, 127, 128]. Ionic liquids are one of main elements in the ionic liquid polymer electrolyte (IL-PE) membrane since it is one of principal carry out mediums [16]. The gel polymer electrolyte is prepared by Jae-Kwang Kim with N-methyl-N-butyl pyrrolidinium bis (trifluoromethanesulfonyl) imide (Py14TFSI), which the electrostatic spinning technique this technique indicates the electrical conductivity of 1.1 × 10−4 S cm−1 at 0 °C [129] based upon poly(vinylidene fluoride-co-hexafluoropropylene) (P (VdF–HFP)); this technique indicates the electrical conductivity of 1.1 × 10−4 S cm−1 at 0 °C [16, 129]. Polymer matrix is one of the essential materials as supporting conceptual framework of gel polymer electrolytes [16, 130]. There are primarily four systems as polymer matrix for polymer lithium-ion battery, such as polyvinylidene fluoride (PVDF) system [131], polyether (PEO) system [132, 133, 134], polyacrylonictrile (PAN) system [135], and polymethyl methacrylate (PMMA) [16, 136]. BF4/PVDF–HFP/PMMA gel polymer electrolyte by the stage inversion technique which has the electrochemistry stability window of 4.5 V and the electrical conductivity of 1.4 × 10−3 S cm−1 is prepared by Wei Zhai [16]. Light-sensitive urethane acrylate (PUA) and the polymethyl methacrylate (PMMA) is contained by the IL-PE membrane as the electrolyte matrix, the lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) as the lithium salt and the N-methyl-N-propyl pyrrolidinium bis (trifluoromethanesulfonyl) imide (Py13TFSI) ionic liquid as enhancing actor of the electrical conductivity [16].

Lithium ion batteries have been enhanced employing polymer nanofibrous electrolyte membrane with its highly porous structure, ionic electrical conductivity and high electrolyte uptake to transport as considerably as lithium ions via it [17]. Scholars in this field are more involved to prepare polymer electrolyte membranes by blending or forming composites employing metal oxides or various polymers to increase ionic electrical conductivity, electrochemical stability than that of pure polymer electrolytes [137, 138, 139, 140, 141], and electrolyte uptake [17]. Some various synthesis techniques are utilized to prepare polymer electrolyte membrane by stage inversion [142], solution casting [139, 143] and electrospinning [17, 144]. A straightforward technique for preparation of nanofibrous membranes with high porosity because of tunable fiber diameter, which is controlled by differing utilized electric field, distance between grounded collector and syringe needle, flow rate of viscous polymer solution, and polymer solution concentration, is Electrospinning [17]. Porosity is the size-dependent property; hence, electrospun nanofibres of blended polymers synthesized by electrospinning owns high porosity which is responsible for high ionic electrical conductivity at room temperature [145, 146] and increase in electrolyte uptake [17]. Attempts have been made to maximize the composition of PMMA and PVdF to fabricate their composites nanofibrous membrane by electrospinning so as to increase the ionic electrical conductivity [17]. The fabrication of polymer nanofibrous electrolyte membranes of PVdF–PMMA composites in various percentage (PVdF: PMMA = 100:0, 80:20 and 50:50) by electrospinning is indicated to examine the impact of PMMA on Li-ion battery performance [17].

Ionic liquids (ILs) are comprised of completely charged species and owns high ionic electrical conductivity, wide electrochemical window, excellent thermal stability, non-volatility, nonflammability [147, 148, 149] and ecologically benignity [18]. Solubility of Li salt is high in BF4− and TFSI ions, and in particular, TFSI anion-based ionic liquids indicate greater electrochemical and thermal stability than the other counter anions of the IL [18, 150]. It is reluctant to generate effective solid and stable electrolyte interface (SEI) [151, 152] even if the ILs owns wide electrochemical window and high ionic electrical conductivity [18]. Menne and others has researched the combination of LiTFSI salt with an organic solvent, which is dissolved in N-butyl, N-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide (Pyr14-TFSI) (50:50 wt%) as an electrolyte for Li battery, and the results show better thermal stability including greater performance than the pristine ILs [18, 153]. Of the various kinds of the ILs examined till date, quaternary ammonium-based acyclic and cyclic cations including pyrrolidinium, tetraalkylammonium and piperidinium high cathodic stability towards lithium metal [154] and display wide electrochemical window [18]. LiTFSI mixed with alkyl carbonate, which includes Pyr24TFSI indicates nonflammability, enhanced electrical conductivity and electrochemical stability window, which is compared when to traditional alkylcarbonate-based electrolyte, as pointed out by Lombardo and others [18, 155]. The symmetric salts owns high thermal including wide electrochemical window and cathodic stability [18]. Olivine structured transition metal phosphates, including LiFePO4 (LFP), have gained considerably attention because of its cost efficacy this IL’s high theoretical capacity, nontoxicity, thermal, chemical including high cycling stability, electrochemical stability and flat voltage plateau [156, 157] among the various cathode materials [18].

Binder, as a required functional material of electrode in Li-ion battery, has a crucial impact on the electrochemical performance [19]. Polyvinylidene fluoride (PVDF), which is costly, difficult to recycle [158] is the most frequent binder, which is utilized in the Li-ion battery [19]. He and others [159] have utilized the cyanoethylated carboxymethyl chitoan (CN–CCTS) as the binder for cathode LiFePO4 and it displayed better resistance to the organic electrolyte solvents than that with sodium carboxymethylcellulose (CMC) and PVDF [19]. Qiu and others [160] have indicated the carboxymethylcellulose derivative (CMC–Li) as the binder for LiFePO4 electrode [19]. In contrast with the PVDF, these water-soluble binders are environment-friendly and carry out excellent electrochemical performance [19]. Very few studies have been carried out on humics as the binder for LiFePO4 cathode till now [19]. The electrochemical performance could be considerably enhanced with many reasonable regulation with the humics as the principal binder [19].

3.2 Separators, Porosity, Shrinkage, Uptake, Ionic Conductivity, Thermal Stability, Membranes

3.2.1 A Bacterial Cellulose/Al2O3 Nanofibrous Composite Membrane for a Lithium-Ion Battery Separator [1]

The bacterial cellulose (BC)/Al2O3 nanofibrous composite membrane as a lithium-ion battery separator has been efficiently prepared by coating Al2O3 on the BC nanofibres via a straightforward in situ thermal decomposition of Al(NO3)3·9H2O [1]. The half lithium-ion battery, which is assembled with the BC/Al2O3 separator, reveals satisfactory cycling performance and huge discharge capacity, implying that the BC/Al2O3 membrane could be utilized as a lithium-ion battery separator [1]. That the tensile strength of the BC membrane is considerably lower than that of the PP-PE-PP membrane, whilst the value of the BC–Al2O3 membrane substantially increasing, similar to that of the PP-PE-PP membrane is revealed by the results [1].

3.2.2 Hollow Mesoporous Silica Sphere-Embedded Composite Separator for High-Performance Lithium-Ion Battery [2]

A high-performance composite separator based upon hollow mesoporous silica spheres, poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF–HFP), and poly(ethylene terephthalate) non-woven had been examined as lithium-ion battery separator through a dip coating method, which a stage separation moist process followed [2]. Due to comparatively polar components and the preferable microstructure, the composite separator displays greater porosity, outstanding thermal stability, and superior electrolyte wettability [2]. A straightforward strategy to prepare high-performance separator, which is shown to be a satisfactory candidate for lithium-ion batteries is summarized by this research [2].

3.2.3 Al2O3/Poly(Ethylene Terephthalate) Composite Separator for High-Safety Lithium-Ion Batteries [3]

Separators have garnered considerable attention from developers and scholars in respect to their important role in the safety of lithium-ion batteries [3]. Through scanning electron microscopy and other specific measurements in regard to a composite separator’s morphology, electrolyte wettability, porosity, and a composite separator’s application in lithium-ion batteries including thermal shrinkage, the basic properties of the Al2O3-coated PET nonwoven composite separator were typified [3]. The lithium-ion battery assembled with this composite separator indicates better electrochemical performance (e.g., cycling and discharge C-rate capability) compared to that with the Celgard 2500 separator [3]. The results of the present study constitute a straightforward strategy to preparing high-performance separators; these separators could be utilized to improve the safety of lithium-ion batteries [3].

3.2.4 Recent Developments of Cellulose Materials for Lithium-Ion Battery Separators [4]

The recent advancements of cellulose materials for lithium-ion battery separators are studied [4].

3.2.5 Thickness Difference Induced Pore Structure Variations in Cellulosic Separators for Lithium-Ion Batteries [5]

The pore structure of the separator is important to the performance of a lithium-battery as it affects the cell resistance [5]. It is shown that the pore size and porosity of the CC separator could be increasing simply by declining the thickness of the CC separator by employing less CC in the manufacturing of the separator [5]. It is clear that the various pore structure of the separators had been an crucial factor, which affects the battery performance in addition to the separator thickness, as the results revealed that a greater ionic electrical conductivity had been obtained for the 10 µm thick CC separator than for the 20 and 40 µm thick CC separators [5]. The present clear-cut, yet effective, approach for modifying the pore structure consequently holds significant promise for the manufacturing of separators with performance, which is enhanced, including for fundamental researches of the impact of the properties of the separator on the performance of lithium-ion cells [5]. Contemporary LIB separators usually are made from polyolefin-based polymer materials and usually suffer from low thermal stabilities and electrolyte wettabilities (Chun and others [22]; Prasanna and others [161]; Ryou and others [36]; Weng and others [29]; Xu and others [25, 162]; Zhang and others [46, 163]; Zhou and others [164]); these wettabilities has caused in a search for alternative separator materials [5]. That CC is a fruitful separator material that is worth additional researches especially as such separators most most likely could be manufactured employing up-scalable paper-making mechanisms is demonstrated by these results [5]. A LiFePO4/Li battery, which includes a 10 µm CC separator, displaying the largest pores and the highest porosity, is demonstrated to characteristic a specific capacity of about 100 mAh g−1 at a rate of 2 C. Ionic electrical conductivity data, clearly indicate that the cell resistance for a thinner separator had been substantially declined as a consequence of its more open pore structure [5]. The SEM images indicate that the CC-25 separator featured bigger pores and a less compact structure than the CC-50 and CC-100 separators, despite the fact that the fiber morphology and overall structure (entangled CC fibres with a thickness of around 30 nm) were the identical in all three instances [5].

3.2.6 A Heatproof Separator for Lithium-Ion Battery Based on Nylon66 Nanofibers [6]

Membrane is utilized as lithium-ion battery separator, demonstrating satisfactory thermodynamics properties through thermo-gravimetric analysis (TG), tension test, and thermal shrinkage experiment, had been nanofiber-based by Electrospun nylon66 (PA66) [6]. Electrospun nylon66 separator premised battery displays better safety than the battery, which applys Celgard commerce separator under the condition of serious vibration and high temperature [6]. Such facts confirm that the electrospun nylon66 separator is an optimal separator candidate for power lithium-ion battery of electric vehicles [6].

3.2.7 The Effect of Multicomponent Electrolyte Additive on LiFePO4-Based Lithium-Ion Batteries [7]

In a LiPF6 baseline electrolyte, which is called flame-retarding electrolyte, one multi-component (MC) additive is utilized to enhance the safety of lithium-ion battery [7]. The electrochemical performances of LiFePO4/Li half cells with flame-retarding electrolytes and baseline were assessed, respectively [7]. The MC additive enhances the thermal stability and does not deteriorate the battery electrochemical performance with LiFePO4 cathode, hence, the combination is a fruitful additive for the safer lithium-ion battery with the MC additive, the cycling performances of LiFePO4/Li half cells were enhanced efficiently at the rate of 0.1 C. Therefore, and the resistance of flame-retarding electrolyte didn’t increase [7]. It could be observed that the rest mass of both electrolytes are equal (both are 0.316 g), though compared to the extinguishing time (60 s) of baseline electrolyte, flame-retarding electrolyte employs more time (66.8 s) [7]. It could be observed that the electrical conductivity of flame-retarding electrolyte is 9.42 mS/cm, which is greater than that of baseline electrolyte (7.80 mS/cm), suggesting better electrical conductivity [7]. It could be observed that there are two principal exothermic peaks at 244.2 °C and 220.3 °C, respectively, with the total heat generation of −158.5 J/g [7].

3.3 Ionic Conductivity, Electrochemical Stability, Polymer Electrolytes, Salt

3.3.1 Polymer Electrolytes for Lithium-Ion Batteries: A Critical Study [8]

The present review essay on a brief history, polymer electrolytes (PEs)’s brief application of polymer electrolyte systems, and advantage [8]. The essay began with a brief introduction of polymer electrolytes followed by extreme employs and their varieties [8]. The role of host polymer matrix by taking several examples of polymer electrolyte, which the various renowned group of the preoccupied field published, has been examined [8]. The criteria for selection of suitable host polymer, salt, aprotic solvents to be utilized in polymer electrolyte, and inorganic filler/clay, have been outlined in detail [8]. That essay includes various methods for the preparation of polymer electrolyte films [8]. The various self-proposed processes (like VTF, WLF, free volume theory, dispersed/intercalated processes, etc.) have been outlined in order to elucidate the Li-ion conduction in polymer electrolyte systems [8].

3.3.2 Electrochemical Investigation of Gel Polymer Electrolytes Based on Poly(Methyl Methacrylate) and Dimethylacetamide for Application in Li-Ion Batteries [9]

Solution-casting method had been utilized to fabricate GPEs containing various weight proportion of PMMA [9]. Spectroscopy had been utilized to investigation the level of relationships between PMMA and lithium salt in the prepared GPEs had been infra-red by fourier reshape [9]. Through estimating the bulk resistance of polymer electrolytes from Nyquist plot, Li-ion electrical conductivity of GPEs had been dictated [9]. Increased PMMA content of GPEs caused in an amelioration in the electrochemical potential window from 4.2 to 4.5 V [9]. Moreover the optimal electrochemical properties and the highest lithium transference number (0.42) were obtained for GPE containing 10 wt% PMMA and 0.75 M LiClO4 [9]. Optimized electrochemical properties and the highest lithium transfer number (0.42) were obtained for GPEs containing 10 wt% of PMMA [9].

3.3.3 Effect of Variation of Different Nanofillers on Structural, Electrical, Dielectric, and Transport Properties of Blend Polymer Nanocomposites [10]

The effect of multiple nano-fillers with various particle sizes and dielectric constants (BaTiO3, Er2O3, CeO2, or TiO2) on blend solid polymer electrolyte, which includes PVC and PEO, complexed with bulky LiPF6 has been examined [10]. Evidence of interaction among the functional groups of the nanofiller in terms of shifting and change of the peak profile and the polymer with the ions is offered by FTIR [10]. The particle size and the dielectric constant indicate an abnormal trend with various nano-fillers [10]. “The particle size of the pristine nanofiller follows the trend Er2O3 ≅ BaTiO3 > CeO2 > TiO2” [10]. The particle size of CeO2 is greater than that of TiO2, though the dielectric constant of the polymer nanocomposite, which is dispersed with TiO2, is greater than that of the CeO2 [10]. The AC electrical conductivity follows the universal Jonscher power law, and an efficient mechanism has been devised to comprehend the nanofiller interaction with cation co-ordinated polymer [10]. The high-frequency dispersion region corresponding to bulk relaxation phenomena falls outside the assessed frequency variety and can not be detected for the high-conductivity system [10]. That dielectric constant and the particle size have an crucial effect on systemic, microstructural, dielectric, and electric, properties is shown by the results [10]. The ion transference number (~0.99) results suggest the SPE films to be predominately ionic with a broad voltage stability window (~3.5 V) [10].

3.3.4 Effect of the Soft and Hard Segment Composition on the Properties of Waterborne Polyurethane-Based Solid Polymer Electrolyte for Lithium-Ion Batteries [11]

An increase in hard segment content declined the crystallinity and thermal stability of WPU [11]. The ionic electrical conductivity increasing first with the increased of the hard segment content and declined [11]. The ionic electrical conductivity against the temperature of WPU10–25% Li is linear, suggesting that the ionic electrical conductivity of this electrolyte showed Arrhenius-like behaviour [11, 92]. Through the hard segment content, the compatibility of WPU-based SPEs with lithium electrode had been shaped [11]. The compatibility of WPU-based SPEs with lithium electrode were examined conducting the AC impedance spectra of Li/SPE/Li cell [11]. All-solid-state LiFePO4/SPE/Li battery, which is based upon WPU12–20% Li electrolyte, delivered discharge specific capacities of 159 and 162 mAh g−1 under 60 and 80 °C at 0.1 C, respectively [11]. All-solid-state LiFePO4/SPE/Li battery, which is based upon WPU12–20% Li electrolyte delivered the discharge capacities of 159 mAh g−1 at 60 °C and 162 mAh g−1 at 80 °C at 0.1 C [11]. Tuning the suitable soft and hard segment composition of WPU might eventually give rise to the successful use of WPU-based SPEs for all-solid-state Li-ion batteries [11]. The LSV results indicate that the WPU-based SPEs showed an electrochemical stability up to 5.0 V and offer feasibility for the application in Li-ion batteries [11]. The consequence suggests that the battery can deliver a comparatively high specific capacity at high temperature and low rates [11]. The consequence suggests that the WPU-based battery can deliver similar capacities at low rates to that of PEO one [11]. The uncontrolled passivation, which gives rise to the continuous impedance growth [165], is revealed by this consequence [11].

3.3.5 Preparation, Properties, and Li-Ion Battery Application of EC + PC-Modified PVdF–HFP Gel Polymer Electrolyte Films [12]

Following poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF–HFP) and lithium tetrafluoroborate (LiBF4) salt along with blending plasticizers, propylene carbonate (PC) and ethylene carbonate (EC), high Li-ion-conducting gel polymer electrolyte films are devised [12]. Lithium-ion batteries based upon the gel polymer electrolyte film display striking charge-cycling and discharge performances [12]. The initial discharge capacity of this battery is as high as 165.1 mAh g−1 at 0.1 C and indicates a small capacity fading of 4.8% after 120 cycles, suggesting that the 85PVdF–HFP: 15LiBF4 + 150 (EC + PC) system is an excellent electrolyte candidate for lithium-ion battery applications [12]. The charge-discharge performance of the lithium ion cell, which is fabricated with this gel polymer electrolyte film, is apparently better than that of the previously indicated lithium ion cells fabricated with other PVdF–HFP-based gel polymer electrolyte films [12]. Performances of the button cells fabricated with the present gel polymer electrolyte films were assessed at ambient temperature are-discharged by the charge [12].

3.3.6 Influences of LiCF3SO3 and TiO2 Nanofiller on Ionic Conductivity and Mechanical Properties of PVA: PVdF Blend Polymer Electrolyte [13]

Solid polymer electrolytes have been intensively studied because of its flexibility, safety, electrochemical stability, and long life for its applications in multiple electrochemical tools in recent decades [13]. Interaction of LiCF3SO3 and TiO2 nanofiller in the optimized composition of PVA:PVdF (80:20-system-A possessing ~2.8 × 10−7 S cm−1 at 303 K) blend polymer electrolyte have been examined in the current study [13]. The effect of various concentration of TiO2 in system-B has been examined and the optimized system is regarded as system-C (~3.7 × 10−3 S cm−1 at 303 K) [13]. Vibrational, systemic, mechanical, electrical conductivity, electrochemical properties, and thermal, have been investigated employing FTIR, DSC, XRD, stress-strain, AC impedance spectroscopic method and TGA, LSV, and CV respectively to ascertain the optimized system [13].

3.3.7 A High-Performance and Environment-Friendly Gel Polymer Electrolyte for Lithium-Ion Battery Based on Composited Lignin Membrane [14]

Mechanical property, the morphology, and thermal stability of the composite lignin-PVP membrane and the electrochemical properties of LP-GPE are examined [14]. A high ionic electrical conductivity of 2.52 × 10−3 S cm−1 at room temperature, outstanding electrochemical stability of LP-GPE, and excellent lithium-ion transference number of 0.56, are revealed for electrochemical properties [14]. That LP-GPE could be utilized as a new electrolyte for Li-ion battery with high-performance, environmentally friendly properties, and cheap, is shown by all these results [14].

3.3.8 Electrochemical Characterization of Ionic Liquid Based Gel Polymer Electrolyte for Lithium Battery Application [15]

The 80 wt% IL containing GPE indicates satisfactory thermal stability (~200 °C), ionic electrical conductivity (6.42 × 10−4 S cm−1), wide electrochemical stability window (~4.10 V versus Li/Li+ at 30 °C), and Li-ion electrical conductivity (1.40 × 10−4 S cm−1 at 30 °C) [15]. LiFePO4 cathode indicates enhanced electrochemical performance with cyclic stability up to 50 cycles and a satisfactory discharge/charge capacity at 1 C rate, as compared with the without coated LiFePO4 had been oxide-coated by the graphene [15]. The discharge capacity reaches a maximal value of 104.50 and 95.0 mAh g−1 for graphene oxide-coated LiFePO4 and without coated LiFePO4 at 1 C rate respectively at 30 °C [15]. LiFePO4 cathode after coating with graphene oxide, these results revealed enhanced electrochemical performance of pristine [15]. That the GO@LFP cathode indicates electronic electrical conductivity and satisfactory electrochemical reactivity is revealed by this consequence [15]. Complex impedance spectroscopic researches indicate that the 80 wt% IL containing GPE has ionic electrical conductivity of 6.42 × 10−4 S cm−1 at 30 °C [15].

3.3.9 A Novel and Shortcut Method to Prepare Ionic Liquid Gel Polymer Electrolyte Membranes for Lithium-Ion Battery [16]

The ionic liquid polymer electrolyte (IL-PE) membrane is prepared by ultraviolet (UV) cross-linking technology with polyurethane acrylate (PUA), methylmethacrylate (MMA), ionic liquid (Py13TFSI), lithium salt (LiTFSI), benzoyl peroxide (BPO), and ethylene glycol dimethacrylate (EGDMA) [16]. The room temperature the lithium ions transference number of 0.22 and ionic electrical conductivity of 1.37 × 10−3 S cm−1 is displayed by the resultant electrolyte membranes [16]. The interfacial resistances between the electrodes and the IL-PE have the less change after 10 cycles than before 10 cycles [16]. Upon 10 cycles, IL-PE has better compatibility with the Li electrode and the LiFePO4 electrode [16]. 131.9 mAh g−1 with 95.5% columbic efficiency after 80 cycles is the discharge capacity [16]. The battery, which employs the IL-PE, displays a satisfactory rate and cycle performance [16].

3.3.10 Poly(Methyl Methacrylate) Reinforced Poly(Vinylidene Fluoride) Composites Electrospun Nanofibrous Polymer Electrolytes as Potential Separator for Lithium-Ion Batteries [17]

Fabrication of nanofibrous polymer electrolyte membranes in various percentage (PVdF: PMMA = 100:0, 80:20 and 50:50) by electrospinning and poly(methylmethacrylate) (PMMA) of poly(vinylidene fluoride) (PVdF) is indicated to examine the impact of PMMA on Li-ion battery performance of PVdF membrane as separator [17]. At room temperature, PVdF–PMMA (50:50) polymer electrolyte membrane revealed ionic electrical conductivity 0.15 S/cm and electrolyte uptake 290% [17]. Nanofibrous PVdF–PMMA (50:50) polymer electrolyte membrane had been observed to be a potential separator for Li-ion batteries [17]. That the PVdF–PMMA membrane has a satisfactory thermal stability with minimum weight% deterioration is revealed by these results [17].

3.3.11 Asymmetric Tetraalkyl Ammonium Cation-Based Ionic Liquid as an Electrolyte for Lithium-Ion Battery Applications [18]

Performance of N-butyl N,N,N-triethylammonium bis (trifluoromethanesulfonyl)-imide (N2224TFSI) as a room temperature ionic liquid (RTIL), which includes ethylene carbonate (EC)/diethylcarbonate (DEC) and lithium salt has been examined as an electrolyte for lithium-ion battery [18]. The electrolyte is highly susceptible to fire during direct exposure to the flame, suggesting an electrolyte for lithium-ion battery’s non-flammable character [18]. The performance of the IL electrolyte has been evaluated with lithium-ion half cells employing mesocarbon and LiFePO4 microbead (MCMB) electrodes, demonstrating satisfactory galvanostatic cycling with high capacity retention of about 84 and 90%, respectively [18]. The impedance plots show that when the lithium metal is in contact with IL electrolyte, the impedance notably increases with time [18].

3.3.12 The Investigation of Humics as a Binder for LiFePO4 Cathode in Lithium-Ion Battery [19]

Binder, as a required functional material of electrode in Li-ion battery, has a crucial impact on the electrochemical performance [19]. The methods of galvanostatic discharge/charge and cyclic voltammetry (CV) were carried out to assess the performance of humics binder in LFP electrode [19].

3.4 Conclusion

Through coating Al2O3 on the BC nanofibres via a straightforward in situ thermal decomposition technique, we have efficiently prepared a BC–Al2O3 nanofibrous composite membrane as an LIB separator [1]. Smaller interfacial resistance, superior thermal dimensional stability and better electrochemical stability as compared to the PP-PE-PP and BC separators is shown by the BC–Al2O3 membrane [1]. The BC–Al2O3 composite membrane must be a quite fruitful candidate separator for high-power Li-ion batteries [1].

Hollow mesoporous SiO2 spheres/PVdF–HFP-coated PET non-woven composite separators had been devised by us as an advanced separator for lithium-ion battery [2]. With greater porosity, greater ionic electrical conductivity which have significant affects on the cell performances, and superior electrolyte wettability, the incorporation of the distinctive SiO2 spheres allows the composite separator [2].

Higher porosity, enhanced electrolyte wettability, greater ionic electrical conductivity, which is compared to the commercial Celgard 2500 separator, and greater electrolyte uptake, had been shown by the Al2O3/PET separator [3]. The Al2O3/PET separator displayed superior dimensional thermal stability, implying that it can be utilized to noticeably improve the safety of Li-ion batteries [3]. The cell, which includes the Al2O3/PET separator, showed superior electrochemical performance (e.g., cycling and C-rate capability) compared to the cell containing the Celgard 2500 [3]. Electrochemical performance and nonwoven PET’s excellent thermal makes the Al2O3/PET composite separator a fruitful candidate as the next-generation separator for high-safety Li-ion batteries [3].

In the application of LIB separators, Cellulose has received considerably attention due to thermal stabilities and its satisfactory chemical [4]. Cellulose can not play a role as a supporter, a gelator in LIB separators, and an enhancer, though also be endowed new functions by modification [4]. The preparation techniques for cellulose LIB separators include traditional coating, casting, electrospinning, papermaking, including ISISA and stage inversion and forcespinning [4]. The use of cellulose materials for LIB separators is supposed to be more extensive with diversified applications of Li-ion batteries and the increasingly restrictive environmental requirements [4]. The preparation technique for cellulose LIB separators needs to be enhanced to lessen the environmental influence and decline the production cost [4]. Structural design of cellulose-based LIB separators and functional modification are satisfactory directions for the next generation of high-performance/high-safety batteries [4].

A surprisingly facile paper-making strategy has been explained for the manufacturing of stratified Cladophora cellulose (CC) separators with pore structures and various thicknesses [5]. Through differing the quantity of CC, which is utilized in the manufacturing process, as an increasing thickness leads to a declined peak and porosity pore size in the paper-making process, the pore size distribution and porosity of the separators could be controlled simply [5]. The technique enables the manufacturing of CC separators with thicknesses down to 10 µm and fosters the manufacturing of separators premised with various pore structures and consequently ionic conductivities though on the identical material [5]. It has been demonstrated that a decline in the CC separator thickness from 40 to 10 µm results in an increase in the peak pore size from 12 to 21 nm including an increase in the porosity from 33 to 44%, resulting in an increase in the ionic electrical conductivity of the electrolyte soaked separators from 0.69 to 0.82 mS cm−1 [5].

Electrospun nylon66 nanofiber separator displays excellent mechanical and thermal properties [6].

The MC additive had been added into the baseline electrolyte as the flame retardant, and the impacts of the flame-retarding electrolyte were examined in this work [7]. The flame-retarding electrolyte, which includes MC, additive indicates the better nonflammability and electrical conductivity than that of baseline electrolyte [7]. The specific capacity of flame-retarding electrolyte indicates more excellent cycling performance than that of baseline one at 0.1 in the electrochemical cycling tests C [7]. Following nonflammability, electrochemical performance, and thermal stability, it could be concluded that the MC additive enhances the thermal stability substantially and indicates better electrochemical performance in the LiFePO4/Li half cells; hence, the MC additive is a possible excellent choice of electrolyte additive [7].

A thorough critical review of the PEs (published over the last three decades) utilized in electrochemical tools has been outlined and examined appropriately [8]. A thorough ion transport processes for Li-ion transport like VTF, WLF, free volume theory, dispersed/intercalated processes, etc., in polymer electrolysis are examined and studied [8]. The fillers/clays have been utilized for improving the properties like ionic electrical conductivity, surface structure/microstructure, electrochemical, stability, etc. [8]. A thin/safe/flexible/cheap electrolyte cum separator is asserted for electrochemical tools, and it could be achieved by employing solid polymer electrolytes cum separators [8].

The PMMA-based gel polymer electrolytes were efficiently prepared employing solution-casting method with differing polymer content from 2 to 10 wt% [9]. The PMMA-based gel polymer electrolytes has been demonstrated that the ionic electrical conductivity of the GPEs increases with decline in polymer content displays a maximal value of 2.3 × 10−3 S cm−1 at ambient condition from AC impedance spectroscopy [9]. Electron transfer and lithium transfer number number enhanced substantially which is requirement for Li ion batteries whilst in increasing polymer content [9]. Through estimating the bulk resistance of polymer electrolytes from Nyquist plot, Li-ion electrical conductivity of GPEs had been dictated [9]. Based on 4.2–4.5 V, the electrochemical potential window had been enhanced by increased PMMA content in GPEs [9]. Optimized electrochemical properties and the highest lithium transfer number (0.42) were obtained for GPEs containing 10 wt% of PMMA [9].

Free-standing polymer nanocomposite films consisting of (PEO–PVC) + LiPF6 with 10 wt% nanofiller, which haves dielectric constant (BaTiO3 and a various particle size, CeO2, TiO2), and Er2O3, have been prepared via the solution cast method [10]. That dielectric constant and the particle size have an crucial effect on systemic, microstructural, dielectric, and electric, properties is shown by the results [10]. X-ray diffraction results confirmed the polymer nanocomposite formation [10]. The FTIR investigation revealed clear empirical evidence for polymer-ion, ion-ion, and polymer-ion-nanofiller interaction [10]. The dielectric spectroscopy offers important information of the increase in three to four orders of the dielectric constant as compared to the nanofiller free polymer salt matrix [10]. The supremacy of the erbium oxide nanofiller as compared to other nano-fillers for enhancing the energy-storage performance of nano-composites is revealed by the present study [10].

Solid polymer electrolytes (SPEs) based upon LiTFSI and WPU were fabricated through an organic solvent free process [11]. The crystallinity of WPU declined and the surface of WPU membrane became homogenous and smooth with the increase of hard segment content [11]. The ionic electrical conductivity increasing first with the increased hard segment content and declined [11]. The WPU12–20% Li electrolyte (55 wt% soft segment content) revealed an ion electrical conductivity of 5.14 × 10−5 S cm−1 at 25 °C and 1.26 × 10−3 S cm−1at 60 °C with the electrochemical stability window reached around (vs. Li+/Li) 5.0 V [11]. All-solid-state LiFePO4/SPE/Li battery, which is based upon WPU12–20% Li electrolyte delivered the discharge capacities of 159 mAh g−1 at 60 °C and 162 mAh g−1 at 80 °C at 0.1 C [11].

“The 85PVdF–HFP: 15LiBF4 + x (EC + PC) (x = 0, 50, 100, 150, and 200 wt%) gel polymer electrolyte films are prepared” [12]. The 85PVdF–HFP: 15LiBF4 + 150 (EC + PC) film displays the optimal properties, whose crystallinity, ionic electrical conductivity, melting temperature, and electrochemical stability window are 9.5%, 115 °C, 4.6 V, respectively, and 8.1 × 10−4 S cm−1 [12]. The performance of the present cell is in the optimal position in comparison to the indicated lithium ion cells fabricated with PVdF–HFP-based gel polymer electrolyte films [12]. The 85PVdF–HFP: 15LiBF4 + 150 (EC + PC) gel polymer electrolyte film can be an excellent electrolyte candidate for lithium-ion batteries [12].

Influence of nanoTiO2 and LiCF3SO3 with the polymer blend PVA: PVdF (system-A) have been investigated [13]. XRD pattern demonstrates that the interaction of nanoTiO2 and LiCF3SO3 with the host polymer by the inference of greater width of peak and change in peak intensity [13]. The incorporation of TiO2 improves the properties of polymer electrolyte and it has been assessed its electrochemical performance and configured as the cell [13].

The prepared lignin-PVP composite membrane displays satisfactory mechanical property and advantageous thermal stability [14]. A high liquid electrolyte uptake of lignin-PVP composite membrane gives rise to good electrochemical performances of the corresponding LP-GPE, such as excellent ionic electrical conductivity, high Li-ion transference number, better compatibility with active electrode, and wide electrochemical stability window [14]. The exploration of the LP-GPE will be a brand-new candidate to satisfy zero environmental influence Li-ion batteries and high-performance [14].

Free-standing 80 wt% IL, which includes GPE membrane based upon polymer PVdF–HFP, salt LiTFSI, and the electrochemical properties of membrane, which is prepared, were examined and IL EMIMFSI had been prepared for lithium battery application [15]. Complex impedance spectroscopic researches indicate that the 80 wt% IL containing GPE has ionic electrical conductivity of 6.42 × 10−4 S cm−1 at 30 °C [15]. We have computed electrochemical stability window (~4.10 V vs. Li/Li+) for 80 wt% IL, which includes GPE membrane which is useful for lithium battery application, and the Li-ion electrical conductivity (Li+ = 1.40 × 10−4 S cm−1) [15]. The surface of LiFePO4 cathode particle had been modified with coating of graphene oxide [15]. The graphene oxide coating not just offers as a protective layer though also enhanced the electronic electrical conductivity of cathode [15]. Upon surface coating with graphene oxide, electrochemical performance of pristine LFP cathode material enhances [15].

A series of tests on the Py13TFSI/LiTFSI/PUA/PMMA ionic liquid polymer the Li/IL-PE/LiFePO4 half-cell and electrolyte membrane were undertaken and examined [16]. An electrochemical stabilization window of about 4.8 V had been obtained, and the electrolyte membrane had sufficient electrochemical stability to act as an electrolyte material in the Li/IL-PE/LiFePO4 half-cell [16]. The ionic liquid polymer electrolyte membrane had a satisfactory compatibility with Li electrode and LiFePO4 electrode; the discharge and charge performance of lithium ion battery had been investigated [16]. In lithium-ion battery, the Py13TFSI/LiTFSI/PUA/PMMA kind ionic liquid polymer electrolyte membranes were fruitful [16].

PVdF–PMMA composite fibres with diameter in nano-scale membranes were efficiently prepared by electrospinning [17]. The increase in proportion of PMMA improves electrolyte uptake and ionic electrical conductivity of PVdF–PMMA composites membranes [17]. That the preparation approach for PVdF/PMMA composites membranes by electrospinning with PVdF/PMMA (50:50) nanofibrous polymer electrolyte membrane had been observed to be fruitful and potential separator for Li-ion batteries than that of PVdF and pure PVdF–PMMA (80:20) had been demonstrated by these results [17].

The addition of EC/DEC solvent mixture to the IL improves the efficient SEI layer formation and prevents the graphitic disorder of MCMB during discharge/charge cycling [18]. An initial high irreversible capacity in the variety of 557 mAh g−1 at 0.1 C rate is demonstrated by the MCMB half cell [18]. High reversible capacity and satisfactory rate capability are obtained, where the capacity retention (94%) is kept up to 75 cycles due to the presence of high content of graphitic behaviour and microspores [18]. Preliminary capacity of 132 mAh g−1 at 0.1 C rate after the formation cycle with more than 85% capacity retention for 50 cycles is demonstrated by the LFP [18].

The way raw materials are mixed is an crucial factor for the quality of electrode and can substantially affect the electrochemical performance [19].

3.5 Related Work

Lee H, Yanilmaz M, Toprakci O, Fu K, Zhang XW (2014) A review of recent developments in membrane separators for rechargeable lithium-ion batteries. Energy Environ Sci 7:3857–3886 []

The separator does not entail directly in any cell reactions, though properties and its structure play important roles in determining the battery performance, which comprises cycle life, safety, power density [33], and energy density, in LIBs [2]. The nanosized pottery particles including TiO2, and Al2O3, SiO2, can substantially enhance thermal stability the mechanical strength, including their thermal resistance [33] and ionic electrical conductivity of separators because of their high hydrophilicity and high surface area [2]. The pore structure and thickness of the separator should be carefully controlled, as a satisfactory balance between mechanical strength and ionic electrical conductivity should be kept (Arora and Zhang [40]; Lee and others [33]; Zhang [50]) in order to satisfy these two functions [5].


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  1. 1.HeidelbergGermany

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