DFT-Based Molecular Analysis of Imidazole Derivatives as Additives to Enhance Ionic Conductivity in Polymer Electrolyte Membranes

Solid-state electrolytes provide substantial improvements to safety and electrochemical stability in lithium-ion batteries when compared with conventional liquid electrolytes, which makes them a promising alternative technology for next-generation high-energy batteries. Currently, the low mobility of lithium ions in solid electrolytes limits their practical application. The ongoing research over the past few decades on dispersing of ceramic nanoparticles into polymer matrix has been proved effective to enhance ionic conductivity although it is challenging to form the efficiency networks of ionic conduction with nanoparticles. In this work, we first report that ceramic nanowire fillers can facilitate formation of such ionic conduction networks in polymer-based solid electrolyte to enhance its ionic conductivity by three orders of magnitude. Polyacrylonitrile-LiClO4 incorporated with 15 wt % Li0.33La0.557TiO3 nanowire composite electrolyte exhibits an unprecedented ionic conductivity of 2.4 × 10(-4) S cm(-1) at room temperature, which is attributed to the fast ion transport on the surfaces of ceramic nanowires acting as conductive network in the polymer matrix. In addition, the ceramic-nanowire filled composite polymer electrolyte shows an enlarged electrochemical stability window in comparison to the one without fillers. The discovery in the present work paves the way for the design of solid ion electrolytes with superior performance.

Nanoscale Mapping of Extrinsic Interfaces in Hybrid Solid Electrolytes

Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

Solid Polymer electrolytes are versatile, highly processible and electrochemically compatible with solid electrode materials. The versatility of these materials is a result of the existence of many possible conductive polymer‐salt, polymer‐polymer and salt‐salt combinations. Despite the wide array of available lithium salts, most polymer electrolyte materials are made using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) due to its long history of achieving relatively high ionic conductivities in polymer electrolyte systems with the most famous being poly(ethylene oxide) (PEO). It is however possible that better ionic conductivities can be achieved with different salts and/or in polymer matrices containing different functional groups. This is because ionic conductivity in polymer electrolytes is partially based on the ability of the polymer matrix to dissolve and bond to the salt. These interactions impact local‐scale ion mobility which can be measured via NMR spectroscopy using pulsed field gradient experiments. In this work, polymer electrolytes are prepared using PEO, hydrogenated nitrile butadiene rubber and poly(propylene) carbonate. Ion mobility, lithium conductivity and salt‐polymer interactions are investigated to compare interactions between LiTFSI and lithium cyano(trifluorosulfonyl)imide in polymers with common salt‐dissociating functional groups such as ethyl, nitrile and carbonate to determine the impact of these interactions on ionic conductivity.

Polymer electrolytes, which eliminate the hazards of using liquid electrolytes, including electrolyte leakage, thereby showing high safety and stability, exhibit high potential for application in lithium‐ion batteries (LIBs). However, for applicability in high‐performance LIBs, improving the ionic conductivity of polymer electrolytes is vital. This study proposes a new scheme for fabricating polyvinylidene fluoride (PVDF)/polyvinyl butyral (PVB) composite polymer electrolyte membranes with high conductivity. As expected, the PVDF/PVB composite membranes synthesized in this study show a high porosity (48.8%) and liquid electrolyte uptake (110%) owing to low crystallinity; PVDF/PVB (9:1) gel polymer electrolytes (GPEs) exhibit an excellent ionic conductivity of 3.90 × 10−3 S/cm at 20 °C and a good Li+ transference number (0.62). Consequently, Li||LiFePO4 cells with PVDF/PVB (9:1) GPEs show excellent cycling performance, a high discharge capacity, which changes only slightly from 153.98 to 132.09 mAh/g over 100 cycles at 0.2 °C, thereby showing a capacity retention of 93.5%, and a high coulombic efficiency of 98.35%. Finally, a mechanism for Li‐ion transport in PVDF/PVB GPEs is proposed. Strong non‐bonded interactions between the anhydride polar groups in PVB and Li+ could facilitate Li+ migration via cationic Li+ conduction by fixing anions (PF6−), thereby improving the electrochemical performance of the system. Thus, this study confirms that PVDF/PVB GPEs, with excellent ionic conductivity and cycling performance, exhibit promising application prospects in the field of LIBs. Therefore, the results of this study could open new frontiers in the fabrication of high‐performance and stable LIBs.

Single‐ion conducting polymer electrolytes (SIPE) have attracted a lot of interest for application in high energy density lithium metal batteries. SIPEs possess lithium transport numbers close to unity, which does not provoke concentration gradients and holds the promise of limiting lithium dendrite formation. In this article, we have optimized a single‐ion polymer incorporating the most successful chemical units in polymer electrolytes, such as ethylene oxide, carbonate, and a lithium sulfonimide. This single‐ion poly(ethylene oxide carbonate) copolymer was synthesized by polycondensation between polyethylene glycol, dimethyl carbonate, and a functional diol including the pendant sulfonamide anionic group and the lithium counter‐cation. By playing with the monomer stoichiometry, the crystallinity and ionic conductivity were optimized. The best copolymer showed high ionic conductivity values of 1.2×10−4 S cm−1 at 70 °C. Lithium interactions and mobility were studied by lithium‐pulsed field gradient, lithium diffusion, NMR relaxation time measurements, and FTIR‐ATR analysis. High lithium mobility is observed, which is due to the weakly coordinating chemical environment in the polymer and also that the sulfonamide in the SIPE adopts to a greater extent the cis conformation, which is known to promote lithium mobility. Finally, the performance of the singe‐ion conducting poly(ethylene oxide carbonate) was compared in lithium symmetric cells versus an analogous conventional salt in polymer electrolyte, showing improved performance in lithium plating and stripping.

Various polymers such as PVA, PVAc, PVP and etc., doped with ammonium salts have been studied for proton conduction. But the study of proton conduction in polymer PAN is scarce. The proton conducting polymer electrolytes composed of polyacrylonitrile (PAN) with ammonium nitrate (NH4NO3) in different molar ratios, have been prepared by solution casting method, using DMF as solvent. The increase in amorphous nature of the polymer electrolytes has been confirmed by X-ray diffraction analysis (XRD). The complex formation between polymer and dissociated salt has been confirmed by Fourier transform infrared spectroscopy (FTIR). The ionic conductivity, dielectric permittivity (e*) and electric modulus (m*) have been calculated from the ac impedance spectroscopy in the frequency range 42HZ-5MHZ. The activation energy of doped PAN polymer electrolyte is calculated using Arrhenius plot. Based on the study of relaxation spectra, it is found that the relaxation time decreases with increase in temperature. INTRODUCTION In recent years, proton conducting polymer electrolytes have attracted considerable attention owing to their application in fuel cells, humidity & gas senors and electrochromic displays.1,2 The main advantages of polymer electrolytes are their good mechanical properties, the ease of fabrication of films of desirable sizes, and their ability to form good electrode – electrolyte contact. Solid polymer electrolytes (SPE), consisting of an ionic conductive polymer matrix and a supporting electrolyte salt, were firstly introduced by Fenton et al.3 The development of polymer system with high ionic conductivity is one of the main objectives in polymer research. A variety of polymers such as Poly (acrylonitrile) (PAN), Poly (vinyl chloride) (PVC) and Poly (methyl methacrylate) (PMMA) has been used as polymer matrices.4-5 Proton conducting polymer electrolytes can be obtained by doping the polymer either with alcohol, amine, amide or amide groups6 or with strong acids or ammonium salts. Ammonium salts have been reported as good donors of proton to the polymer matrix. Literature survey indicates that the synthesis and characterization of ammonium salts doped proton conducting polymer electrolytes based on Poly (acrylonitrile) (PAN) is rare. PAN is one of the most important fibers – forming polymers and has been widely used because of its high strength, abrasion resistance and good insect resistance.7 PAN is used to produce large variety of products including ultra filtration membranes, hollow fibers for reverse osmosis, fibers for textiles, oxidized frame retardant fibers like PANOX and carbon fiber. However the conductivity of PAN is < 10-14 S cm-1 and the static problem restricts its further applications. PAN is usually synthesized using free radical polymerization. Usually they are copolymers of acrylonitrile and methyl acrylate or acrylonitrile and methyl methacrylate. PAN has a melting point of about 3190C, and it also decomposes at this temperature. So PAN membrane is prepared by solution casting technique. In the present work, NH4NO3 doped PAN polymer electrolytes have been prepared and subjected to various characterizations such as XRD, FTIR and Ac impedance spectroscopy. EXPERIMENTAL DETAILS The polymer PAN (Aldrich) of average molecular weight 1, 50, 000 and the salt NH4NO3 were used as the raw materials in this study. Dimethyl formamide (DMF) was used as the solvent. The polymer electrolytes PAN doped with NH4NO3 in different compositions such as 100:0; 95:05; 90:10; 85:15; 80:20 and 75:25 were prepared by solution casting technique. The mixture of PAN and NH4NO3 was stirred continuously with a magnetic stirrer for several hours at temperature 600 C to obtain a homogenous solution. The solution was then poured into propylene petri dishes and allowed to evaporate in vacuum oven at 650C. After 48 hours, free standing transparent films were obtained. X-ray diffraction patterns of the prepared samples were recorded at room temperature on a Philips X’Pert PRO diffractometer using Cukα radiation. FTIR spectra were recorded for the polymer electrolyte films using a SHIMADZU IR Affinity -1 Spectrometer in the range of 400 4000 cm-1 at room temperature. Conductivity measurements were carried out by using a HIOKI 3532 LCZ meter in the frequency range of 42 Hz – 1MHz over the temperature range of 303 – 343 K. Possible interaction between PAN and NH4NO3. The possible interaction between PAN and NH4NO3 has been pictorially represented in the scheme 1 along with the structure. The proton from the salt interacts with the polar groups of the host polymer matrix.

The application of polymer electrolytes in Lithium Ion batteries (LIB), has been proposed as an effective method to alleviate the problems associated to the limited voltage window and safety issues associated to the traditional liquid organic electrolytes. However, polymer electrolytes studied so far based on poly (ethylene glycol) and its derivatives have been unable to reach the required Li+ ionic conductivity needed for a broader application for commercial devices. Common polymer electrolytes contain a polymer with specific chain length at a defined C:O ratio, the conducting salt is added either during the polymerization process or afterwards by a dissolution process; however, the salt is often not completely dissociated while the conduction mechanism has been described as hoping of Li+ (often Li+ and counter ions) between oxygen functional groups and between chains induced by its intrinsic mobility. In order to increase the polymer ionic conductivity, it has been recently proposed the modification of the polymer for the introduction of negatively charge moieties which allows the direct interaction of Li+ with the polymer having no free counter ions; these structures present two big advantages, the Li+ transport number is close to unity and the mobility of Li is not lowered by the presence of the counter Ion. In order to identify the possible modifications to the polymer which allows higher Li conductivity, a detailed study of sp3-Boron Poly (ethylene-glycol) is reported. In this work, by means of density functional theory simulations, different electron withdrawing/donating ligands have been included as modifiers of the Boron site electron density. It was found that the effect of the ligand on the electron density of the Boron site has a great influence on the Bader charge of the boron site and also in the interaction energy with Lithium Ions was. A further study by ab-initio molecular dynamics (AIMD) shows that the mobility of Lithium ion can be modified up to two-fold by the inclusion of withdrawing electron density ligands. This methodology might lead to the discovery of new polymer electrolytes with enhanced Li+ conductivity.

Lithium ion batteries are a crucial technologies for electric vehicles and portable electronics due to their high energy density. However, flammable liquid electrolytes create a safety concern and transitioning to all solid state batteries allows for the potential of higher energy density by utilizing lithium metal anodes. Two classes of solid materials have been explored to replace liquid electrolytes: ceramic and polymer electrolytes. Composite electrolytes, typically in the form of dispersed ceramic particles or nanowires in polymer matrix, combine these two materials to have good ionic conductivity while maintaining the mechanical properties of the polymer. While they have achieved fairly high conductivity (typically 10-4 S/cm), the mechanism for such ion conduction is not well understood.The polymer likely plays a crucial role as the ceramic volume fractions are typically under 20%.1,2 Four possible ways that polymers could increase in conductivity in the composite have been identified: decreased crystallinity, increased free volume, disruption of polymer alignment, and increased lithium ion dissolution due to acidic surface groups on the ceramic.1,3,4 We have developed thin film model systems to probe the interface of the polymer with and ceramic surfaces. We found that when confined to thin films, the conductivity of the polymer electrolyte increased compared to the bulk. We utilized differential scanning calorimetry to identify the crystallinity and found that the small crystallinity variation could not account for the variation in conductivity. Subsequently, we utilized surface conductivity and transference number measurements to understand the effects of acidic surface groups. X-ray Absorption measurements will be similarly utilized to probe the variation in lithium ion environments within these materials to understand any variation in the lithium salt dissolution. It was found that the variation in conductivity based on this effect is small.In order to probe the possible variation in free volume, Small Angle Neutron Scattering will be utilized to quantify the free volume within thin films, bulk polymer, and composite electrolytes. This can quantify the volume associated with each monomer and therefore how much space is between polymer chains for lithium ion movement.5 Lastly, while the polymer chain alignment has not been extensively discussed composite literature, it has been identified that when polymer electrolytes are cast on a surface, polymer electrolytes typically show anisotropy with measurements parallel to the surface having higher conductivity due to the alignment of the chain.6 Therefore, ceramic fillers in the composites may disrupt this packing and allow for higher conductivity perpendicular to the cast surface, the direction in which they are most commonly measured. In order to probe this effect, we measure the anisotropy of both our thin film electrolytes as well as composite electrolytes to compare the significance of the anisotropy and utilize small angle x-ray scattering to measure any anisotropy in the structure.

A stretchable all-solid-state polymer electrolyte with decoupled ion transport and mechanical supporting networks to achieve high and stable ion-conductivity

Fluoride shuttle battery (FSB)1 is expected as one of the candidates for the post lithium-ion battery. In FSBs, fluoride ions are charge carriers in the electrolyte, and fluorination and defluorination reactions of transition metal compounds are the charging and discharging reactions. Since many transition metals can be multivalent, FSBs are expected to have high theoretical capacities. All-solid-state FSBs have the advantage of high safety due to the non-flammability of the inorganic solid electrolytes. However, inorganic solid electrolytes have problems in that the effective ionic conductivity is lowered because of the poor contact between solid electrolyte particles. Although high-temperature sintering increases the effective ionic conductivity, it is energy-consuming and might transelement the interface between electrode and electrolyte in the case of integral sintering. This becomes an obstacle to constructing all-solid-state batteries.From the view of the practical process, it may be effective to fill the voids between electrolyte particles with a flexible electrolyte, such as a polymer electrolyte, to construct an electrolyte layer without sintering. Research on electrolytes for FSB is limited, not only on composite electrolytes but also on polymer electrolytes.2 In this study, we synthesized a new fluoride-ion conductive polymer electrolyte and prepared composite electrolytes by adding the polymer electrolyte to the lanthanum fluoride (LaF3) as the sintering-free electrolytes.Acetonitrile solutions of a polymer electrolyte using polyethylene oxide (PEO, M w 600,000) and potassium fluoride (KF) were prepared with/without the addition of an oligomer having boroxine rings as a nonvolatile anion acceptor (AA). The polymer solutions were vacuum-dried to obtain a polymer electrolyte membrane. Differential scanning calorimetry (DSC) and ionic conductivity measurements were performed for the polymer electrolytes. Composite electrolyte powder was obtained by mixing the LaF3 powder and polymer solution containing AA, followed by vacuum drying. The composite electrolyte powder was pelletized using a uniaxial press to obtain a composite electrolyte pellet. Ionic conductivity measurements were performed for the composite electrolyte pellet, pellet of LaF3 powder, and dense LaF3 disk.In the case of the polymer solution without adding AA, KF remained undissolved, but by adding AA, it completely dissolved. DSC of the polymer electrolyte showed that the softening point of the polymer electrolyte decreased from 69°C to 57°C with the addition of AA (Fig. 1a). The ionic conductivity of the polymer electrolyte is shown in Fig. 1b. The slope of ionic conductivity with respect to the temperature changed at 50–70°C, corresponding to the softening point. The polymer electrolyte with AA had higher ionic conductivity by three orders of magnitude and lower activation energy in the low-temperature range than without AA. The AA works to increase carrier density by increasing the solubility of KF to the polymer and to activate carrier conduction by plasticizing the polymer electrolyte.Ionic conductivities of the composite electrolyte and LaF3 pellet are shown in Fig. 1c. The behavior of the ionic conductivity of the composite electrolyte changed at 50–60°C. This was influenced by the softening point of the PEO-based polymer electrolyte, indicating that the polymer electrolyte is involved in ion conduction in the composite electrolytes. The main ion conduction path in the composite electrolyte is probably the polymer electrolyte/LaF3 interfacial layer. Although the composite electrolyte increased the ionic conductivity by more than an order of magnitude compared with the LaF3 pellet, it was still an order of magnitude lower than the dense LaF3 disk. Further investigation to form an optimal ionic conduction path is necessary to construct a sintering-free solid electrolyte for all-solid-state FSBs. M. A. Reddy and M. Fichtner, J. Mater. Chem., 21, 17059 (2011). K. Takahashi, A. Yokoo, Y. Kaneko, T. Abe, and S. Seki, Electrochemistry, 88, 310 (2020). This research is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Polymer electrolytes are important enablers for safe, thin, future flexible electronics. Alkaline electrolytes such as KOH have been extensively used in energy storage devices such as batteries and electrochemical capacitors (ECs). Although aqueous KOH is useful, it has limitations when serving in a polymer electrolyte due to crystallization that compromises hydroxide (OH-) ion conductivity. An alternative alkaline electrolyte, tetraethylammonium hydroxide (TEAOH) was shown to be more stable with polymers compared to KOH.[1,2] Polyacrylamide (PAM) is a promising polymer host material due to its hygroscopic nature, amorphous structure, and useful functional groups that support ion conduction. When combined with TEAOH, a good ionic conductivity (> 10 mS cm-1) was achieved under ambient conditions (room temperature and 45% relative humidity (RH)). Thin, lightweight, electrochemical double layer capacitors (EDLC) were also demonstrated with TEAOH-PAM that outperformed their liquid analogues.[3] One of the important properties of TEAOH-PAM is its OH- ion-conduction dependence on hydration. As its degree of hydration changes with environmental factors such as RH and temperature, so does the conductivity of TEAOH-PAM as shown in Fig. 1. An additional consequence of this environmental vulnerability is poor dimensional stability of the polymer electrolyte which can lead to premature failure. To address these issues, the use of inert inorganic fillers as additives is explored. Inorganic fillers such as SiO2, TiO2, and Al2O3 have been commonly used to enhance the ionic conductivity, strengthen the mechanical properties, and improve the thermal stability of polymer electrolytes.[4–6] Here, we leverage different inorganic fillers to improve the environmental stability, expand the working temperature range, and provide better structural support for TEAOH-PAM polymer electrolytes. In this talk, the effects of different inorganic fillers such as SiO2 and TiO2 on OH- ion-conduction will be presented. For example, SiO2 showed both advantageous and detrimental effects on OH- ion conductivity depending on the RH condition and the size of SiO2. The possibility to enhance the OH- ion conductivity of TEAOH-PAM in low and high temperatures using SiO2 and TiO2 will be discussed. A combination of electrochemical and spectroscopy experiments is used to study the interaction of these inorganic fillers with the TEAOH-PAM alkaline polymer electrolyte. These results will help further clarify OH- ion-conduction in polymer electrolytes. [1] H. Gao, J. Li, K. Lian, Alkaline quaternary ammonium hydroxides and their polymer electrolytes for electrochemical capacitors, RSC Adv. 4 (2014) 21332–21339. doi:10.1039/C4RA01014K. [2] J. Li, K. Lian, A comparative study of tetraethylammonium hydroxide polymer electrolytes for solid electrochemical capacitors, Polymer. 99 (2016) 140–146. doi:10.1016/j.polymer.2016.07.001. [3] J. Li, J. Qiao, K. Lian, Investigation of polyacrylamide based hydroxide ion-conducting electrolyte and its application in all-solid electrochemical capacitors, Sustain. Energy Fuels. 1 (2017) 1580–1587. doi:10.1039/C7SE00266A. [4] S. Ketabi, K. Lian, Effect of SiO2 on conductivity and structural properties of PEO-EMIHSO4 polymer electrolyte and enabled solid electrochemical capacitors, Electrochim. Acta. 103 (2013) 174–178. doi:10.1016/j.electacta.2013.04.053. [5] J.E. Weston, B.C.H. Steele, Effects of inert fillers on the mechanical and electrochemical properties of lithium salt-poly(ethylene oxide) polymer electrolytes, Solid State Ionics. 7 (1982) 75–79. doi:10.1016/0167-2738(82)90072-8. [6] J. Przyluski, M. Siekierski, W. Wieczorek, Effective medium theory in studies of conductivity of composite polymeric electrolytes, Electrochim. Acta. 40 (1995) 2101–2108. doi:10.1016/0013-4686(95)00147-7. Figure 1

A solid polymer blend electrolyte is prepared using poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) polymers with different molecular weight percentage (wt%) of ammonium thiocyanate (NH4SCN) by solution casting technique with tetrahydrofuran (THF) as a solvent. The structural, morphological, vibrational, thermal and electrical properties of the prepared polymer blend electrolytes have been studied. The incorporation of NH4SCN into the polymeric matrix causes decrease in the degree of crystallinity of the samples. The complex formation between the polymer and salt has been confirmed by FTIR technique. The increase in T g with increase in salt concentration has been investigated. The maximum conductivity of 3.684 × 10−3 S cm−1 has been observed for the composition of 70PVAc/30PMMA/30 wt% of NH4SCN at 303 K. This value of ionic conductivity is five orders of magnitude greater than that of 70PVAc/30PMMA polymer membrane. Dielectric and transport studies have been done. The highest conducting polymer electrolyte is used to fabricate proton battery with the configuration Zn/ZnSO4·7H2O (anode) ||polymer electrolyte||PbO2/V2O5 (cathode). The open circuit voltage of the fabricated battery is 1.83 V, and its performance has been studied.

Lithium salt, LiX (where X = BF4−, I−, CF3SO3−, COOCF3− or ClO4−), was incorporated into epoxidized natural rubber (ENR). Thin films of LiX-ENR polymer electrolytes (PEs) were obtained via solvent casting method. These electrolytes were characterized using SEM/X-mapping, FTIR, differential scanning calorimeter, thermogravimetry analysis, and impedance spectroscopy. The trend in thermal stability and ionic conductivity of LiX-ENR PEs follow LiBF4 > > LiCF3SO3 ~ LiCOOCF3 > LiI > > LiClO4. The LiClO4 hardly dissociates and formed LiClO4 aggregates within the polymer matrix that resulted in a PE with low thermal stability and low ionic conductivity. The LiCF3SO3, LiCOOCF3, and LiI, however, exert moderate interactions with the ENR, and their respective PEs exhibit moderate ionic conductivity and thermal property. The occurrence of epoxide ring opening and complexation or cross-linking reactions in and between the ENR chains that involve BF4− ions have produced a LiBF4-ENR PE with superior thermal property and ionic conductivity as compared to other PEs studied in this work.

Adsorption of alkali and alkaline earth ions on nanocages using density functional theory