Recent Advances in Innovative Polymer Electrolytes based on Poly(ionic liquid)s

Phase Behavior of Melts of Diblock-Copolymers with One Charged Block

In this work, we investigated the phase behavior of melts of block-copolymers with one charged block by means of dissipative particle dynamics with explicit electrostatic interactions. We assumed that all the Flory-Huggins χ parameters were equal to 0. We showed that the charge- correlation attraction solely can cause microphase separation with a long-range order; a phase diagram was constructed by varying the volume fraction of the uncharged block and the electrostatic interaction parameter λ (dimensionless Bjerrum length). The obtained phase diagram was compared to the phase diagram of "equivalent" neutral diblock-copolymers with the non-zero χ-parameter between the beads of different blocks. The neutral copolymers were constructed by grafting the counterions to the corresponding co-ions of the charged block with further switching off the electrostatic interactions. Surprisingly, the differences between these phase diagrams are rather subtle; the same phases in the same order are observed, and the positions of the order-disorder transition ODT points are similar if the λ-parameter is considered as an "effective" χ-parameter. Next, we studied the position of the ODT for lamellar structure depending on the chain length N. It turned out that while for the uncharged diblock copolymer the product χcrN was almost independent of N, for the diblock copolymers with one charged block we observed a significant increase in λcrN upon increasing N. This can be attributed to the fact that the counterion entropy prevents the formation of ordered structures, and its influence is more pronounced for longer chains since they undergo the transition to ordered structures at smaller values of λ, when the electrostatic energy becomes comparable to kbT. This was supported by studying the ODT in diblock-copolymers with charged blocks and counterions cross-linked to the charged monomer units. The ODT for such systems was observed at significantly lower values of λ, with the difference being more pronounced at longer chain lengths N. The fact that the microphase separation is observed even at zero Flory-Huggins parameter can be used for the creation of "high-χ" copolymers: The incorporation of charged groups (for example, ionic liquids) can significantly increase the segregation strength. The diffusion of counterions in the obtained ordered structures was studied and compared to the case of a system with the same number of charged groups but a homogeneous structure; the diffusion coefficient along the lamellar plane was found to be higher than in any direction in the homogeneous structure.

Influence of Counterion Structure on Conductivity of Polymerized Ionic Liquids

We performed long-time all-atom molecular dynamics simulations of cationic polymerized ionic liquids with eight mobile counterions, systematically varying size and shape to probe their influence on the decoupling of conductivity from polymer segmental dynamics. We demonstrated rigorous identification of the dilatometric glass-transition temperature (T_g) for polymerized ionic liquids using an all-atom force field. Polymer segmental relaxation rates are presumed to be consistent for different materials at the same glass-transition-normalized temperature (T_g/T), allowing us to extract a relative order of decoupling by examining conductivity at the same T_g/T. Size, or ionic volume, cannot fully explain decoupling trends, but within certain geometric and chemical-specific classes, small ions generally show a higher degree of decoupling. This size effect is not universal and appears to be overcome when structural results reveal substantial coordination delocalization. We also reveal a universal inverse correlation between ion-association structural relaxation time and absolute conductivity for these polymerized ionic liquids, supporting the ion-hopping interpretation of ion mobility in polymerized ionic liquids.

Innovative Electrolytes Based on Ionic Liquids and Polymers for Next-Generation Solid-State Batteries

Electrolytes based on organic solvents used in current Li-ion batteries are not compatible with the next-generation energy storage technologies including those based on Li metal. Thus, there has been an increase in research activities investigating solid-state electrolytes, ionic liquids (ILs), polymers, and combinations of these. This Account will discuss some of the work from our teams in these areas. Similarly, other metal-based technologies including Na, Mg, Zn, and Al, for example, are being considered as alternatives to Li-based energy storage. However, the materials research required to effectively enable these alkali metal based energy storage applications is still in its relative infancy. Once again, electrolytes play a significant role in enabling these devices, and research has for the most part progressed along similar lines to that in advanced lithium technologies. Some of our recent contributions in these areas will also be discussed, along with our perspective on future directions in this field. For example, one approach has been to develop single-ion conductors, where the anion is tethered to the polymer backbone, and the dominant charge conductor is the lithium or sodium countercation. Typically, these present with low conductivity, whereas by using a copolymer approach or incorporating bulky quaternary ammonium co-cations, the effective charge separation is increased thus leading to higher conductivities and greater mobility of the alkali metal cation. This has been demonstrated both experimentally and via computer simulations. Further enhancements in ion transport may be possible in the future by designing and tethering more weakly associating anions to the polymer backbone. The second approach considers ion gels or composite polymer electrolytes where a polymerized ionic liquid is the matrix that provides both mechanical robustness and ion conducting pathways. The block copolymer approach is also demonstrated, in this case, to simultaneously provide mechanical properties and high ionic conductivity when used in combination with ionic-liquid electrolytes. The ultimate electrolyte material that will enable all high-performance solid-state batteries will have ion transport decoupled from the mechanical properties. While inorganic conductors can achieve this, their rigid, brittle nature creates difficulties. On the other hand, ionic polymers and their composites provide a rich area of chemistry to design and tune high ionic conductivity together with ideal mechanical properties.

Chemically crosslinked liquid crystalline poly(ionic liquid)s/halloysite nanotubes nanocomposite ionogels with superior ionic conductivity, high anisotropic conductivity and a high modulus

A novel type of chemically crosslinked liquid crystalline nanocomposite ionogel electrolyte based on poly(ionic liquid) (PIL) with superior ionic conductivity and high anisotropic conductivity was designed and synthesized using the in situ photopolymerization of sheared soft ionogels containing charged halloysite nanotubes (HNTs) and ionic liquid monomers. The oriented structure was investigated using scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS). The chemically crosslinked backbone of the PIL and the addition of HNTs endowed the flexible ionogels with a combined very high modulus (up to 26.7 MPa) and mechanical strength (up to 4.4 MPa). Crucially, the obtained ionogels exhibited high mechanical stability even at temperatures up to 200 °C. Remarkably, in terms of the conductivities, the resulting pre-sheared ionogels displayed superior room temperature ionic conductivity (up to 6 mS cm-1) and a very high conductivity anisotropy ratio (up to 1600), owing to the alignment of the HNTs with oppositely charged surfaces and the high ionic conductivity of the polyelectrolyte PILs. Furthermore, flexible solid-state supercapacitor devices based on the high ion-conductive nanocomposite ionogels were fabricated, which demonstrated high and temperature-dependent specific capacitance, and remarkable cycling stability and flexible performance.

Free ion diffusivity and charge concentration on cross-linked polymeric ionic liquid iongel films based on sulfonated zwitterionic salts and lithium ions

The properties of various mixtures of a zwitterionic ionic liquid (ZIs-1) and LiNTf2, including their conductivity, have been studied showing how they can be adjusted through their molar composition. Conductivity tends to increase with the LiNTf2 content although it presents a minimum at the region close to the eutectic point. These mixtures also provide excellent features as liquid phases for the preparation of composite materials based on crosslinked PILs. The prepared films display excellent and tuneable properties as conducting materials, with conductivities that can be higher than 10-2 S cm-1 above 100 °C. The selected polymeric compositions show very good mechanical properties and thermal stability, even for low crosslinking degrees, along with a suitable flexibility and good transparency. The final properties of the films correlate with the composition of the monomeric mixture used and with that of the ZIs-1:LiNTf2 mixture.

Impact of building block structure on ion transport in cyclopropenium-based polymerized ionic liquids

Cation geometry, size, and polarity all contribute to conductivity in PS-TAC PILs, with highest conductivity from the isopropyl cation geometry.

Co-Poly(ionic liquid) Films via Anion Exchange for the Continuous Tunability of Ion Transport and Wettability

This manuscript details a novel and simple approach to achieve surface-tethered co-poly(ionic liquid) (coPIL) films through the exchange of the resident anion of a poly(ionic liquid) (PIL) film with two or more anions. Initially, surface-tethered PIL films were prepared by the surface-initiated ring-opening metathesis polymerization of the ionic liquid monomer 3-[(bicyclo[2.2.1]hept-5-en-2-yl)methyl]-1,2-dimethylimidazol-3-ium hexafluorophosphate ([N1-dMIm][PF6]) whose PF6 - anion was easily interchanged with aqueous solutions containing a binary mixture of the PF6 - anion, along with perchlorate (ClO4 -) or bis(fluorosulfonyl)imide (FSI-) anions. The binary mole fraction of each anion in the film was determined from the infrared spectra of the coPIL films. The thermodynamically driven anion selectivity for exchange from the liquid phase into the coPIL films was determined to follow the order ClO4 - < PF6 - < FSI-. The aqueous wettability of p[N1-dMIm] coPIL films containing both the PF6 - and ClO4 - anions (p[N1-dMIm][PF6][ClO4]) was quantified by contact angle goniometry with the observation that the surface showed an enrichment in the ClO4 - anion compared to the average binary anion mole fraction of ClO4 - in the film (y ClO4 - ). The rate of ion transport through the p[N1-dMIm][PF6][ClO4] coPIL films, quantified by electrochemical impedance spectroscopy, linearly depends on the binary anion mole fraction of ClO4 - in solution (x ClO4 - ), enabling continuous tunability by over three orders of magnitude for ion conductivity in the coPIL films.

Covalently Crosslinked 1,2,3-Triazolium-Containing Polyester Networks: Thermal, Mechanical, and Conductive Properties

Azide-alkyne "click" cyclization was used to prepare a series of polymerizable acetoacetate monomers containing a 1,2,3-trizolium ionic liquid group. The monomers were subsequently polymerized using base-catalyzed Michael addition chemistry, producing a series of covalently crosslinked 1,2,3-triazolium poly(ionic liquid) (TPIL) networks. Structure-activity relationships were conducted to gauge how synthetic variables, such as counteranion ([Br], [NO3], [BF4], [OTf], and [NTf2]), and crosslink density (acrylate/acetoacetate ratio) effected thermal, mechanical, and conductive properties. TPIL networks were found to exhibit ionic conductivities in the range of 10-6-10-9 S/cm (30 °C, 30% relative humidity), as determined from dielectric relaxation spectroscopy, despite their highly crosslinked nature. Temperature-dependent conductivities demonstrate a dependence on polymer glass transition, with free-ion concentrations impacted by various ions' Lewis acidity/basicity and ion mobilities impacted by freely mobile anion size.

Fast Switching Properties and Ion Diffusion Behavior of Polytriphenylamine Derivative with Pendent Ionic Liquid Unit

A novel triphenylamine derivative-linked ionic liquid unit, 1-(6-((4-(bis(4-(thiophen-2-yl)phenyl)amino)benzoyl)oxy)hexyl)-3-methyl-imidazolium tetrafluoroborate (TTPAC₆IL-BF₄), was designed and synthesized successfully, and its corresponding polymer PTTPAC₆IL-BF₄ was obtained by the electropolymerization method. The highest occupied molecular orbital energy band of TTPAC₆IL-BF₄ is higher and the onset oxidative potential lower compared with that of 6-bromohexyl 4-(bis(4-(thiophen-2-yl)phenyl)amino) benzoate (TTPAC₆Br) without modifying the ionic liquid unit. Both PTTPAC₆IL-BF₄ and PTTPAC₆Br show similar color change and optical contrast under different redox states. However, PTTPAC₆IL-BF₄ presents a faster electrochromic switching time than PTTPAC₆Br owing to the improved ionic conductivity and ion diffusion coefficient with the introduction of a pendent ionic liquid unit. It is more intriguing that PTTPAC₆IL-BF₄ could show electrochromism under different potentials even without supplying any additional electrolyte. The particular behavior further proves that BF₄^- ions around imidazole cations at the side chain may participate in balancing the charge of the polymer backbone when redox reaction happens, resulting in faster movement of ions during the doping process. The results imply that introducing an ionic liquid unit to the side chain is an efficient method to improve the switching time of conjugated polymers and would be inspirational for the design and preparation of novel bifunctional electrochromic polymeric electrolytes.

High Performance Polymer/Ionic Liquid Thermoplastic Solid Electrolyte Prepared by Solvent Free Processing for Solid State Lithium Metal Batteries

A polymer/ionic liquid thermoplastic solid electrolyte based on poly(ethylene oxide) (PEO), modified sepiolite (TPGS-S), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) ionic liquid is prepared using solvent free extrusion method. Its physical-chemical, electrical, and electrochemical properties are comprehensively studied. The investigated solid electrolyte demonstrates high ionic conductivity together with excellent compatibility with lithium metal electrode. Finally, truly Li-LiFePO₄ solid state coin cell with the developed thermoplastic solid electrolyte demonstrates promising electrochemical performance during cycling under 0.2 C/0.5 C protocol at 60 °C.

Single lithium-ion conducting solid polymer electrolytes: advances and perspectives

Electrochemical energy storage is one of the main societal challenges to humankind in this century. The performances of classical Li-ion batteries (LIBs) with non-aqueous liquid electrolytes have made great advances in the past two decades, but the intrinsic instability of liquid electrolytes results in safety issues, and the energy density of the state-of-the-art LIBs cannot satisfy the practical requirement. Therefore, rechargeable lithium metal batteries (LMBs) have been intensively investigated considering the high theoretical capacity of lithium metal and its low negative potential. However, the progress in the field of non-aqueous liquid electrolytes for LMBs has been sluggish, with several seemingly insurmountable barriers, including dendritic Li growth and rapid capacity fading. Solid polymer electrolytes (SPEs) offer a perfect solution to these safety concerns and to the enhancement of energy density. Traditional SPEs are dual-ion conductors, in which both cations and anions are mobile and will cause a concentration polarization thus leading to poor performances of both LIBs and LMBs. Single lithium-ion (Li-ion) conducting solid polymer electrolytes (SLIC-SPEs), which have anions covalently bonded to the polymer, inorganic backbone, or immobilized by anion acceptors, are generally accepted to have advantages over conventional dual-ion conducting SPEs for application in LMBs. A high Li-ion transference number (LTN), the absence of the detrimental effect of anion polarization, and the low rate of Li dendrite growth are examples of benefits of SLIC-SPEs. To date, many types of SLIC-SPEs have been reported, including those based on organic polymers, organic-inorganic hybrid polymers and anion acceptors. In this review, a brief overview of synthetic strategies on how to realize SLIC-SPEs is given. The fundamental physical and electrochemical properties of SLIC-SPEs prepared by different methods are discussed in detail. In particular, special attention is paid to the SLIC-SPEs with high ionic conductivity and high LTN. Finally, perspectives on the main challenges and focus on the future research are also presented.

Pyrrolidinium FSI and TFSI-Based Polymerized Ionic Liquids as Electrolytes for High-Temperature Lithium-Ion Batteries

Promising electrochemical and dynamical properties, as well as high thermal stability, have been the driving forces behind application of ionic liquids (ILs) and polymerized ionic liquids (PILs) as electrolytes for high-temperature lithium-ion batteries (HT-LIBs). Here, several ternary lithium-salt/IL/PIL electrolytes (PILel) have been investigated for synergies of having both FSI and TFSI anions present, primarily in terms of physico-chemical properties, for unique application in HT-LIBs operating at 80 °C. All of the electrolytes tested have low Tg and are thermally stable ≥100 °C, and with TFSI as the exclusive anion the electrolytes (set A) have higher thermal stabilities ≥125 °C. Ionic conductivities are in the range of 1 mS/cm at 100 °C and slightly higher for set A PILel, which, however, have lower oxidation stabilities than set B PILel with both FSI and TFSI anions present: 3.4–3.7 V vs. 4.2 V. The evolution of the interfacial resistance increases for all PILel during the first 40 h, but are much lower for set B PILel and generally decrease with increasing Li-salt content. The higher interfacial resistances only influence the cycling performance at high C-rates (1 C), where set B PILel with high Li-salt content performs better, while the discharge capacities at the 0.1 C rate are comparable. Long-term cycling at 0.5 C, however, shows stable discharge capacities for 100 cycles, with the exception of the set B PILel with high Li-salt content. Altogether, the presence of both FSI and TFSI anions in the PILel results in lower ionic conductivities and decreased thermal stabilities, but also higher oxidation stabilities and reduced interfacial resistances and, in total, result in an improved rate capability, but compromised long-term capacity retention. Overall, these electrolytes open for novel designs of HT-LIBs.

Database and new models based on a group contribution method to predict the refractive index of ionic liquids

Refractive index is one of the important physical properties, which is widely used in separation and purification. In this study, the refractive index data of ILs were collected to establish a comprehensive database, which included about 2138 pieces of data from 1996 to 2014. The Group Contribution-Artificial Neural Network (GC-ANN) model and Group Contribution (GC) method were employed to predict the refractive index of ILs at different temperatures from 283.15 K to 368.15 K. Average absolute relative deviations (AARD) of the GC-ANN model and the GC method were 0.179% and 0.628%, respectively. The results showed that a GC-ANN model provided an effective way to estimate the refractive index of ILs, whereas the GC method was simple and extensive. In summary, both of the models were accurate and efficient approaches for estimating refractive indices of ILs.

Counteranion-Mediated Intrinsic Healing of Poly(ionic liquid) Copolymers

Fabrication of self-healing/healable materials using reversible interactions that are governed by their inherent chemical features is highly desirable because it avoids the introduction of extra groups that may present negative effects on their functions. The present study exploits the inherently featured electrostatic interactions of the ion pairs in polymeric ionic liquids (PILs) as the driving force to fabricate healable PIL copolymers. The healable PIL copolymers are fabricated through the copolymerization of the IL monomers with ethyl acrylate followed by the replacement of Br^- counteranions with bulkier ones such as bis(trifluoromethanesulfonyl)imide (TFSI^-). Without modifying the chemical structures of the PIL moieties, the healing performance of the as-prepared PIL copolymers can be effectively mediated by their counteranions. The PIL copolymers that do not possess healability when paired with Br^- counteranions become healable after exchanging the Br^- counteranions with larger-sized ones (e.g., TFSI^-). The PIL copolymers paired with bulky counteranions exhibit enhanced chain mobility and highly reversible ion-pair interactions, which facilitate the healing process. The PIL copolymers paired with TFSI^- anions can completely heal the damage/cut upon heating at 55 °C for 7.5 h. Meanwhile, the counteranions with larger sizes not only benefit the healing performance of the PIL copolymers but also enhance their ion conductivity. The ion conductivity of the PIL copolymers paired with TFSI^- is an order of magnitude higher than that of the PIL copolymers paired with Br^-. Therefore, the as-prepared healable PIL copolymers are potentially useful as solid electrolytes in PIL-based energy devices to improve their safety and reliability.

Ionic liquids and derived materials for lithium and sodium batteries

A comprehensive review of various applications of ionic liquids and derived materials in lithium and sodium batteries with an emphasis on recent advances.

Mesostructure and physical properties of aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl sulfate doped with divalent sulfate salts in the liquid and the mesomorphic states

Mesomorphic liquid crystal character and effect on physical properties.