Zwitterionic Matrix with Highly Delocalized Anionic Structure as an Efficient Lithium Ion Conductor

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES16 Aug 2022Zwitterionic Matrix with Highly Delocalized Anionic Structure as an Efficient Lithium Ion Conductor Shuaishuai Yan†, Yang Lu†, Fengxiang Liu, Yingchun Xia, Qiao Li and Kai Liu Shuaishuai Yan† State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084 , Yang Lu† State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084 , Fengxiang Liu State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084 , Yingchun Xia State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084 , Qiao Li State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084 and Kai Liu *Corresponding author: E-mail Address: [email protected].edu.cn State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.022.202202198 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The leakage and volatilization of liquid electrolytes raise potential safety risks in the development of electrochemical energy storage devices with high energy density. Herein, novel solid-state zwitterionic materials containing sulfonyl imide as a highly delocalized anionic structure were synthesized for highly targeted lithium ion conducting matrices. The influences of the molecular structure characteristics on thermal behavior and electrochemical property were investigated comprehensively. Due to the weak Coulomb interaction between the sulfonyl imide moiety and cationic species, the rationally designed zwitterionic electrolytes showed a high conductivity of 0.44 mS cm−1. And the obtained high lithium ion transference number of 0.43 is four times higher than that of the widely employed sulfonate analogues. Additionally, excellent cycling stability of the lithium plating/stripping process and super resistance to electrochemical oxidation (up to 5.5 V) were observed. This molecular engineering strategy for lithium ion conductor advances new possibilities for developing solvent-free and non-migrating electrolyte matrix materials for lithium metal batteries. Download figure Download PowerPoint Introduction To pursue high energy density and high safety of electrochemical energy storage devices, the development of light-weight, low vapor pressure, non-leaking, and nonflammable solid-state ion conductors is crucial.1,2 Much attention has been devoted in recent years to exploit organic polymer and inorganic ceramics solid-state electrolytes.3 Traditional polymer electrolytes, such as poly(ethylene oxide) (PEO), where the dynamics of ion motion is strongly coupled to the chain segment of the backbone, exemplify fluctuation-driven migration. Lithium ions are well solvated and trudge along the polar ether oxygen, thus the cation's transference number (t+) is <0.2, which means a low effective conductivity of lithium ions.4,5 For inorganic solid electrolyte, in which the anion framework is fairly rigid, the ions are hopping through crystal lattice site vacancies and show high ion selectivity (t+ ∼ 1). However, the brittleness of ceramic slices and poor interface contact greatly hinder their practical applications.6,7 Zwitterions are a unique class of locally charged but overall neutral molecules in which the cationic and anionic groups are covalently attached.8 The molecular structure is similar to that of ionic liquids, but the covalent bond increases the intramolecular interactions and decreases the motional degrees of freedom, which results in their higher melting point compared to ionic liquid.9 The melting temperature tends to be higher than 100 °C, and zwitterions are in the solid-state at room temperature. Impressively, zwitterions are not only stagnant under an electric field gradient due to their charge neutrality, but also have great lithium salt solubility and a very high dipole moment, practically twice that of their analogous ionic liquids.10 Specifically, with the equimolar addition of lithium salt, especially bis(trifluoromethane)sulfonimide lithium (LiTFSI), a homogenous eutectic solution can form.11 The mixture has a glass transition temperature (Tg) below 0 °C, but it cannot be regarded as a liquid solution due to its poor fluidity at room temperature. This conductor composed of ion pairs shows an intermediate state between solid and liquid, similar to organic ionic plastic crystal, where the glass phase facilitates the motion of Li+ via a paddle wheel type mechanism.12,13 Thanks to these unique properties, the zwitterion matrix has been explored substantially as a solvent-free, nonvolatile and nonflammable solid lithium-ion conductor to support targeted ion transport for batteries.14–18 Zwitterionic materials can serve as one kind of novel electrolyte for the lithium batteries and fuel cells.8,19,20 The zwitterions can be used as "ion dissociators" to promote Li+ conduct in single-ion polymer electrolytes18 and additives to increase the targeted ions transport in ionic liquid electrolytes.21–23 However, up to now, the current molecular structures of zwitterions are still very limited. The cationic groups are mainly heterocyclic imidazolium, and the anionic groups are mainly sulfonate,20 carboxylate,9 and dicyanoethenolate.15 Unfortunately, these negatively charged groups have too strong Coulomb interaction with cationic species, such as lithium ions in the electrolyte, which severely hinders the migration of lithium ions under potential and leads to a quite low transference number.24,25 In this study, a range of novel zwitterions containing sulfonyl imide as a highly delocalized anionic structure were synthesized for highly conducting lithium ion matrix. By consistently using the cationic imidazolium group, the effect of the structural characteristics of the anion moiety on the thermal property, ionic conductivity, lithium ion selectivity, and electrochemical stability was investigated carefully. When mixed with lithium salts, that is, LiTFSI or bis(fluorosulfonyl)imide lithium (LiFSI), the resulting eutectic glass-like electrolytes show relatively high mobility (0.44 mS cm−1). And the lithium ion transference number reaches 0.43, which is four times higher than that of the analogues with poorly delocalized sulfonate. Thus, the sulfonyl imide-based zwitterionic electrolytes enable reversible plating/stripping of lithium metal and super resistance to electrochemical oxidation (up to 5.5 V). This molecular design strategy demonstrates that the charged anionic structures of zwitterions have a significant impact on the electrochemical performance of batteries and provides more choices to exploit new functional materials as lithium ion conductors. Experimental Methods Materials synthesis and electrolyte preparation The three kinds of zwitterionic materials used as electrolytes of lithium metal batteries were synthesized according to the modified methods reported in the previous study.9,26 More details about the synthetic route and specific process can be found in the Supporting Information. The zwitterions with different molecular ratio of LiTFSI or LiFSI (DoDoChem Co., Suzhou, China) were dissolved in anhydrous acetonitrile and stirred for 30 min to obtain a homogeneous solution. The mass solvent was evaporated at 80 °C and then transferred to vacuum oven to dry at 80 °C for an additional 24 h. For the following various electrochemical measurements, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), Li/Li symmetric cell, and Li/NMC811 full cell, the electrolytes were first dissolved in acetonitrile and cast on a 5 μm polyethylene separator (Newmi Tech., Chongqing, China) as the battery intermediate layer. Then the solvent was removed by drying the separator under high vacuum at 80 °C for 24 h. The electrolytes were stored in an argon-filled glove box with an oxygen and water content <0.01 ppm. Characterizations Nuclear magnetic resonance (NMR) experiments were conducted on a Bruker AVANCE III 400 MHz spectrometer (Bruker, Billerica, USA) for 1H, 19F, 13C, and 7Li. Inductive coupled plasma emission spectrometry was performed on an iCAP 6000 (Thermo Fisher Scientific, Waltham, USA) to detect the content of alkali metal. Thermogravimetry (TG) was tested on a Q5000IR (TA Instruments, New Castle, USA) from room temperature to 800 °C at a heating rate of 10 °C min−1 under N2 atmosphere to measure the thermal stability. Differential scanning calorimetry (DSC) was performed on a Q200 (TA Instruments, New Castle, USA) in a temperature range from −90 to 200 °C at a ramp rate of 10 °C min−1 under N2 atmosphere. X-ray photoelectron spectroscopy (XPS) was carried out to detect the surface components by an ESCALAB Xi+ X-ray photoelectron spectrometer system (Thermo Fisher Scientific, Waltham, USA). Electrochemical measurement CR2032 type coin cells were assembled in an argon-filled glove box to evaluate the electrochemical performance. The ionic conductivity was measured by sandwiching the electrolytes between two stainless steel electrodes (SS) with a fixed 75 μm thick polyimide ring on a Princeton Applied Research Station Multiple Channels (PARSTAT MC) electrochemical workstation (AMETEK, Philadelphia, USA). The test frequency range of the impedance was from 0.1 to 100 kHz with an Alternating Current (AC) oscillation voltage of 10 mV. Each temperature was allowed to equilibrate at least 40 min and the conductivity (σ) was calculated by the following equation: σ = L / R b S (1)where L is the thickness of electrolyte, R b is the bulk resistance, and S is the contact area between the SS electrode and the electrolyte. The activation energy (Ea) data were obtained from the following Vogel–Tammann–Fulcher equation: σ = σ 0 T − 1 / 2 exp ( − E a / ( R ( T T 0 ) ) ) (2)where σ0 is the pre-exponential factor, R is the molar gas constant, and T0 is an empirically determined temperature value below Tg. The lithium transference number (t+) was measured by assembling a Li/Li symmetric cell with a thin 5 μm polyethylene separator soaked with the electrolytes according to the Bruce–Vincent method. The AC impedance spectroscopy measurements were conducted before and after the potentiostatic polarization. The polarized voltage ( Δ V ) was fixed at 10 mV. The t+ was calculated by the following equation: t + = I s ( Δ V I 0 R 0 ) / ( I 0 ( Δ V I s R s ) ) (3)where Is and I0 are the steady-state and initial currents, respectively. Rs and R0 are the steady-state and initial interfacial resistance, respectively; and I Ω is defined in the following equation: I Ω = Δ V / ( R 0 , bulk + R 0 , interface )(4)where R 0 , bulk and R 0 , interface are the bulk resistance and interface resistance at initial state, respectively. The CV tests of Li/Li cell, Li/Cu cell, and Li/NMC811 full cells were performed on a CHI760E electrochemical workstation with a scan rate of 0.1 mV s−1. The LSV measurements of Li/Al cell were performed on a PARSTAT MC electrochemical workstation with a scan rate of 0.5 mV s−1. The NMC811 cathode was prepared in lab by the slurry coating method with NMC811 powder (1 C = 200 mAh g−1), Super P, and poly(vinylidene fluoride) binder with a mass ratio of 9∶0.5∶0.5 on carbon-coated aluminum foil, followed by drying under vacuum at 80 °C overnight. The active material loading of the cathode was about 3.8 mg cm−2 and the thickness of Li foil anode was 450 μm. The constant-voltage floating test was conducted for Li/NMC811 cells, which were first charged to 4.2 V and then gradually increased to higher voltages over 10 h. The galvanostatic charge/discharge tests of the Li/Li symmetric cell and NMC811/Li full cell were conducted on a LAND CT2001A battery testing system (Lanhe, Wuhan, China). Results and Discussion Three kinds of zwitterionic molecules were synthesized through a modified method9,26 and characterized by 1H, 19F, and 13C NMR spectroscopy and metal elemental analysis ( Supporting Information Figures S1–S7). The heterocyclic imidazolium was selected as the cationic structure of these zwitterions; the planar ring with delocalized charge is partly responsible for lowering the melting points of plastic crystals,27 ionic liquids,28 and zwitterions.9 The imidazolium was grafted with sulfonate and two sulfonyl imides, which were labeled as Imi-SO3, Imi-CF3, and Imi-C4F9, respectively (Figure 1a). The former was obtained as pure white solid, and the latter two were light yellowish solids at room temperature. TG testing shows that the decomposition temperature calculated using 5 wt % weight loss of all these zwitterions is above 310 °C (Figure 1b). Imi-CF3 has the best thermal stability of 322.7 °C, which is high enough for application in electrochemical devices. The three different anionic structures have a great influence on their own thermal transition behaviors of zwitterions. Figure 1c shows the diagram of DSC on their first heating trace (10 °C min−1). The two distinct peaks of Imi-SO3 at 64 and 153 °C correspond to crystallization temperature and melting point, respectively, which are consistent with values reported in the literature.8 Imi-C4F9 shows a solid–solid phase transition at 113 °C, followed by a melting point at 141 °C. The solid-phase transition may be a result of the crystallographic change of the rigid-plastic crystal transition, which is one of the indicators of structure disorder.27 The zwitterionic Imi-CF3 has the lowest melting point among the three at 135 °C and no obvious visual solid-phase change from room temperature to melting temperature. In summary, the two sulfonyl imide-based zwitterions have lower melting temperature than the sulfonate-based one, implying that the highly delocalized anionic structure weakens the intermolecular forces between the positive and negative charged moieties of one zwitterion. The reorientation process of the crystal lattice may occur with little hindrance, especially after adding lithium salts.29 More details will be discussed later. Figure 1 | (a) The chemical structures, (b) TG, and (c) DSC traces of the three zwitterionic materials. Download figure Download PowerPoint Ionic conductivity is one of the most crucial electrochemical parameters for practical electrolytes, which determines the impedance, overpotential, and rate capability of batteries.30 The ionic conductivity of pure zwitterionic species is quite low (<10−9 S cm−1). Because the positive and negative groups are tethered covalently on one molecule, it cannot migrate under an electric field gradient due to its charge neutrality. This transport property for the electrolyte matrix is highly desirable and far surpasses that of their analogues, ionic liquids, especially in an electrochemical system where only targeted ions are expected to move, such as the lithium ions of lithium battery. Next, we prepared the lithium ion conductors by adding lithium salt, LiTFSI into the zwitterionic matrixes. The temperature dependence of ionic conductivity for the mixture of Imi-SO3, Imi-CF3, and Imi-C4F9 with different amounts of LiTFSI is shown in Figure 2a–c. The molar ratio of zwitterion to LiTFSI is mainly concentrated in the range of 1–4. The ionic conductivity of all three waxy solids increases with increasing LiTFSI content, which is attributed to the boost in number of charge carriers. In the system of Imi-CF3, on one hand, the bulk conductivity substantially changes when the lithium salt content is in the range of 0.25 to 1.2 equiv. On the other hand, the conductivity at the ratio of 1∶0.25 exceeds 0.05 mS cm−1 at 100 °C, which is about 6–11 times higher than Imi-SO3 and Imi-C4F9. Considering the same lithium salt used, this evident difference indicates that the anionic structure seems to have a significant effect on the mobility of lithium ions. The negative charge of the sulfonyl imide group on Imi-CF3 is more delocalized than the sulfonate group on Imi-SO3 and has weaker Coulomb interaction with the cationic species, which reduces the migration barrier of lithium ions to enhance conductivity. However, the ion conductivity of Imi-C4F9, with the stronger electron-withdrawing effect of perfluoroalkyl, decreases instead. Perhaps the low polarity of the bulky perfluoroalkyl group31,32 reduces the local charge density thus affecting the transport of lithium ions and lowering the ion conductivity. Figure 2 | The temperature dependence of conductivity for (a) Imi-SO3, (b) Imi-CF3, and (c) Imi-C4F9 mixed with different amounts of LiTFSI. (d) The DSC traces, (e) the ion conductivity, and (f) the activation energy of the mixture of three zwitterions and 50 mol % LiTFSI. Download figure Download PowerPoint This trend resulting from charge delocalization is also reflected in the DSC curves of the mixture of zwitterions and LiTFSI on their second heating trace (Figure 2d). Consistent with previous zwitterions with various molecular structure, a eutectic glass solution with only a Tg below 0 °C was obtained after adding an equimolar amount of LiTFSI. The homogeneous mixture is an intermediate state between solid and liquid, which is like plastic crystal. The fluidity of this compound is very poor at room temperature and cannot be regarded as a liquid solution ( Supporting Information Figure S8). This huge change in thermal behavior is due to the strong coupling interaction between the cathodic and anionic moieties of the zwitterion being disrupted by the electrostatic shielding effect9 after the addition of lithium salts. Imi-CF3-based mixture shows the lowest Tg at −41 °C, much lower than Imi-SO3 (−21 °C) and Imi-C4F9 (−12 °C), demonstrating the high plasticity of sulfonyl imide on Imi-CF3 and relaxed chemical environment resulting from the highly delocalized anionic structure. What is more, the nonflammability of the zwitterionic electrolytes was checked ( Supporting Information Figure S9 and Videos S1–S3). The mixture of zwitterions and LiTFSI-immersed glass fiber cannot be ignited by an external heating source for several seconds, demonstrating their excellent nonflammable property. The Ea of the three zwitterionic electrolytes at the molecular molar ratio of 1∶1 was calculated by the Vogel–Tammann–Fulcher model33 (Figure 2e,f). This parameter reflects the influence of two aspects, that is, solvation/dissociation in bulk electrolyte and the subsequent migration under electric field. In other words, the Ea reflects the migration barrier of ionic species in the electrolyte. The conductivity of all the tested electrolytes increases with temperature, and the mixture of Imi-SO3 and LiTFSI shows the highest value. The Imi-CF3 and Imi-SO3-based electrolytes exhibit close migration barrier (0.165 eV, 0.154 eV) among the three, which is lower than Imi-C4F9 (0.203 eV). The Imi-C4F9 with 50 mol % LiTFSI has a higher migration barrier probably due to its large volume and the low polarity of the bulky perfluoroalkyl group on the zwitterion, which also attenuate its ionic conductivity. Therefore, it can be concluded that the anionic structures on zwitterions with high delocalization, high plasticity,34 and relatively small volume make the zwitterionic electrolytes better ion conductors. However, it should be noted that the ion conductivity and Ea data obtained through electrochemical impedance spectroscopy are the result of overall migration of mobile ionic species, while only the portion of the current that is carried by lithium ions matters and determines the operating rate for lithium batteries. Given that the internal current of lithium batteries depends on lithium-ion flux rather than the total conductivity of electrolyte, the limiting current fraction or cation transference number t+ in the mixture of zwitterion and lithium salt is a crucial factor to evaluate the ionic mobility. An excessively low lithium transference number manifests an overwhelming anion movement and causes especially when is high as ionic liquid and polymer To measure the lithium ion transference the potentiostatic was performed on a Li/Li symmetric cell with AC impedance spectroscopy measurements at 100 °C. The current was by I Ω due to the different and with testing (Figure The current at first and reaches a state Imi-SO3 with 50 mol % LiTFSI has the most significant trend and the current value is below The steady-state current of Imi-CF3 is with a value of the higher the steady-state the higher the transference of the impedance The impedance before and after Current are shown in Figure and Supporting Information Figure the on the Li/Li symmetric cell mainly from bulk resistance as well as interfacial An was used to the For in the 1∶1 ratio of the in the range the bulk resistance and of the bulk electrolyte. The in the range to the resistance and the electric when electrodes lithium are In the the of about degrees is to the three zwitterionic electrolytes the same The in the three impedance curves may be a result of the different contact and the bulk electrolyte with various small charged the Bruce–Vincent the t+ of Imi-CF3 and 50 mol % LiTFSI is calculated to be 0.43 at 100 °C (Figure which is higher than the Imi-C4F9 and polymer electrolyte Imi-SO3 has the lowest value the negatively charged Coulomb interaction with cationic species is too which hinders the movement of lithium ions under Figure | The current as a of testing is shown in (b) The initial and steady-state impedance of three kinds of (c) The calculated lithium ion conductivity and transference number of electrolyte at 100 °C. Download figure Download PowerPoint the huge difference in transference number of Imi-CF3 and Imi-SO3 demonstrates that the highly delocalized anionic structure of a zwitterionic matrix is for the migration of lithium ions. The conductivity of lithium ion can be obtained by the total conductivity by the transference The system of Imi-CF3 and 50 mol % LiTFSI shows the highest conductivity up to mS far the other The excellent lithium selectivity of Imi-CF3 that the design of the anionic structure of zwitterion is an effective NMR in was to the local environment of lithium ion in these zwitterions with 50 mol % LiTFSI (Figure NMR is to the species in the The of pure LiTFSI salt is at ppm. When adding equimolar zwitterion, the anionic group with the density and chemical The sulfonate group on Imi-SO3, which at has the with lithium ion among the three and shows the The chemical change of Imi-CF3 is quite small at about demonstrating the highly delocalized sulfonyl imide group has relatively weak which will not hinder the movement of lithium ions in The of Imi-C4F9 may be due to the strong on the The NMR spectroscopy of these three zwitterions with 50 mol % LiFSI the same trend ( Supporting Information Figure the fixed cationic structure of heterocyclic imidazolium or zwitterionic electrolytes from the with various anion groups including and are in Figure The transference number of Imi-CF3 with 50 mol % LiTFSI shows much value to the reported Therefore, the of highly delocalized sulfonyl imide on zwitterions electrolytes with high lithium ion conductivity and high transference Figure | (a) NMR of three electrolytes with (b) of lithium transference number among reported zwitterionic electrolytes and Imi-CF3 with 50 mol % LiTFSI. Download figure Download PowerPoint with the sulfonate group, the more delocalized sulfonyl imide group has unique property of a strong which can promote the and local homogeneous of lithium and zwitterions. To the effect of the group of sulfonyl imide on Imi-CF3, kind of lithium salt used in recent was to be mixed with the Imi-SO3 and Imi-CF3 molecules in different LiFSI also but it not have an plasticity like LiTFSI. The phase state of of Imi-SO3 and 50 mol % LiFSI is a solid powder ( Supporting Information Figure where no obvious Tg was but only a crystallization temperature and melting point of pure Imi-SO3 ( Supporting Information Figure The conductivity is much lower than the one of LiTFSI and can only S cm−1 at °C when increasing the salt content up to (Figure and Supporting Information Figure This indicates that the positive and negative charge group on zwitterions not be too and should be as delocalized as the crystal lattice not when adding poor lithium salt, resulting in poor and low ionic On the the Imi-CF3 with 50 mol % LiFSI shows a homogeneous eutectic glass solution like before with fluidity at room temperature ( Supporting Information Figure and relatively high conductivity (Figure the of the sulfonyl imide group of Imi-CF3 still provides a strong effect for the Figure 5 | The temperature dependence of conductivity for the mixture of (a) Imi-SO3 and (b) Imi-CF3 with different amounts of (c) The conductivity of Imi-CF3 electrolyte with equimolar LiTFSI and LiFSI at and 100 °C. cyclic voltammetry curves of Li/Li symmetric cell at °C. (e) The cycling performance at 0.05 at °C. the cells were first at cm−2 with 1 for 5 (f) The voltage of lithium plating/stripping cycling with different current density at 80 °C. LSV curves of the Imi-CF3 electrolyte with a scan rate of 0.5 mV The cyclic voltammetry curves of Li/NMC811 cells performed at °C. at the first two for Li/NMC811 cell cycling at °C. Download figure Download PowerPoint should be noted that the Tg of Imi-CF3 with 50 mol % LiFSI obtained from DSC is °C, which is °C higher than Imi-CF3 with 50 mol % LiTFSI ( Supporting Information Figure the conductivity of the former is higher over a temperature range from 10 to 100 °C (Figure The highest ionic conductivity data can 0.44 mS cm−1 at 100 °C, three times

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