Sulfonated Polyoxadiazole as a Novel Single-Ion Polymer Electrolyte in Lithium-Ion Batteries

A single-ion conductor polymer electrolyte was prepared and the effects of the ratio of isophthalic acid (IPA) to 4, 4 ’ -diphenyl ether dicarboxylic acid (DPE) on the structure and electrochemical properties have been investigated. Results show that all the lithium sulfonated polyoxadiazoles (Li-SPODs) have excellent thermal stability (510 °C). The ionic conductivity of the Li-SPOD is 7.2 × 10 − 4 S cm − 1 at room temperature when the IPA/DPE ratio is 6/4. A high Coulombic ef ﬁ ciency of 99.1% for the LiFePO 4 /6/4 li-SPOD SPEs/Li half-cell could be achieved with a capacity retention of 90.63% at a current density of 0.1 C after 25 cycles.

Lithium-ion batteries (LIBs) are a kind of excellent recyclable energy storage devices, with relatively high energy density, long cycle life, good rate capability, and low self-discharge, which have been widely used in portable consumer electronic devices and promoted the development of electric vehicle (EV) industry. [1][2][3][4] However, the safety of LIBs still needs to be improved to meet with the requirements of EVs and other applications. 5,6 The main reason for the explosion and combustion of LIBs after impact or puncture is the electrolyte leakage of the conventional flammable liquid electrolytes. 7 Therefore, the solid or quasi-solid electrolytes have been considered as alternatives to solve this severe problem of LIBs. 8 Gel polymer electrolytes (GPEs) are a kind of quasi-solid electrolytes which have not only relatively high ionic conductivity compared with solid electrolytes, but also better safety compared with liquid electrolytes. Moreover, they are flexible and easy to construct all types of devices. Typically, GPEs are prepared from polymers, lithium salts, and plasticizers. Thus, the lithium ions are still transferred by the small molecular lithium salts, which leads to GPEs low lithium-ion transference numbers (t Li+ ) (⩽0.5). 9 Various lithium single-ion polymer electrolytes (SPEs) have been explored to address this problem. The anions in SPEs can be fixed on the main polymer chain, and only Li + can move in the electrolyte, which leads to their high t Li+ (close to 1). 10 Meanwhile, the high lithiumion transference number reduces the concentration polarization of near the electrodes during charge and discharge, contributing to the better battery performance. 11,12 Aromatic poly (1, 3, 4-oxadiazole) (POD) is a kind of polymer with excellent thermal stability. After the sulfonic acid group is grafted on the POD main chain to obtain the sulfonated polyoxadiazole (SPOD), which can compete with Nafion membrane for fuel-cells. [13][14][15] From the properties of the single-ion conductor and the Nafion membrane, we were inspired to neutralize SPOD with lithium hydroxide to fabricate lithium sulfonated polyoxadiazole (Li-SPOD). Moreover, the electron absorption effect of the oxadiazole ring can reduce the electron cloud density of the benzene ring and the sulfonate which is connected with it, making lithium ions more easily dissociated and contributing to the improvement of electrical conductivity.
In this communication, we are committed to the preparation of low-cost and high-performance single-ion polymer electrolytes using simple synthetic methods, so we synthesized a series of Li-SPOD polymers by adjusting the ratios of two kinds of diacid monomers, and the effect of different diacid monomer ratios on the structure and electrochemical properties of the Li-SPOD SPEs are studied.
Synthesis of Li-SPOD SPEs.-The HS with different mole ratios of IPA and DPE were dissolved in fuming sulfuric acid and poured into a three-mouth flask under mechanical stirring. The reaction condition is programmed as pre-polymerization at 85°C for 2 h, polymerization at 120°C for 3 h, and finally dehydration and cyclization at 140°C for 2 h. The obtained SPOD polymer solution was cast on a glass plate and immersed in a water coagulation bath to form a SPOD film. Then the film was neutralized with 0.1 mol l −1 LiOH solution for 48 h and washed to form Li-SPOD SPEs. Finally, the films were dried in a vacuum oven at a temperature of 100°C for 48 h, cut into a disk with a diameter of 16 mm, and swollen by 500 wt% DMSO for later use. The Li-SPOD prepared from various mole ratios of IPA, and DPE (6:4, 7:3, and 8:2) were recorded as 6/4 Li-SPOD, 7/3 Li-SPOD, and 8/2 Li-SPOD, respectively. The synthesis diagram of the polymer is shown in Scheme 1.

Results and Discussion
The ATR-FTIR spectra of Li-SPOD polymers are shown in Fig. 1a, in which the bands at 1240 and 1198 cm −1 correspond to the symmetric stretching vibration and asymmetric stretching vibration z E-mail: memoggy@126.com of O=S=O, respectively. The peak at 1032 cm −1 ascribes to the stretching vibration of S-O. The peak of 1602 cm −1 is the stretching vibration of C=N on the oxadiazole ring. 15 The presence of the characteristic peaks above indicates that the structure of sulfonated polyoxadiazole with sulfonate groups can be obtained by the reaction conditions used in this study. 16 Figure 1b shows the XRD patterns of Li-SPOD polymers. Two broad diffraction peaks at around 8°and 25°appear in the range of 5°∼ 40°for all three samples. With the increase of DPE monomer content, the 25°diffraction peak slightly offset to the direction of low angles. According to the Bragg formula, 17 the crystalline interplanar spacing of the polymer increases, but the crystal structure has not changed. At the same time, the higher the content of the DPE monomer is, the greater intensity of the diffraction peak is, because the DPE monomer can provide more ether bonds, which can improve the flexibility of the polymer chain and make it easier to crystallize.
TGA tests were carried out to characterize the thermal stability of Li-SPOD polymers. Figures 1c and 1d show the TG curves and DTG curves of Li-SPOD polymers, respectively. The mass of Li-SPOD polymers has a small loss before 200°C, which indicates the combined water loss in the polymer. With the increase of DPE content and the sulfonation degree, the mass loss becomes greater. When the temperature increases to 477 ∼ 484°C, the Li-SPOD polymers are decomposed rapidly due to the removal of -SO 3 Li and the degradation of the polymer chain. The maximum thermal  decomposition temperature of the Li-SPOD polymers is 510°C, and the carbon residue is above 50 wt% at 800°C. The above results demonstrate that Li-SPOD polymers have excellent thermal stability. The results of the thermal stability of Li-SPOD polymers are summarized in Table SI (see Supporting Information is available online at stacks.iop.org/JES/167/070518/mmedia). The T 5% of the polymers decreases gradually with the increase of DPE content, which may be influenced by the steric hindrance effect of -SO 3 Li. 18 The Nyquist plots of the electrochemical impedance for Li-SPOD SPEs in different temperatures are displayed in Fig. 2. The AC impedance spectra of the Li-SPOD SPEs are straight lines, which indicate that the samples have good contact and do not react with the stainless steel electrode. 19 Meantime, the bulk resistance of the Li-SPOD SPEs decreases gradually with the increase of temperature.
The detailed values of ion conductivity at different temperatures were calculated, as shown in Table I. The ion-conductivities of all the Li-SPOD SPEs at room temperature reaches 10 −4 ∼ 10 −3 and increase with the increment of sulfonation degree at the same temperature, which are higher than those of most reported SPEs 20-26 (see Table SII in Supporting Information). Moreover, with the increase of DPE content, the lithium-ion transference number of the Li-SPOD SPEs increases (as shown in Fig. S1 and Table SIII Table I. The values of E a are all about 17 kJ mol −1 , which is comparable to that of most sulfonatebased SPEs. The electrochemical window is also an important parameter of electrolyte performance, which determines the using range of the electrolytes. The CV curves of Li-SPOD SPEs are shown in Fig. 3. There is no significant difference in CV curves of Li-SPOD SPEs, showing a similar redox reaction, which indicates that the copolymerization ratio has little effects on the electrochemical window of the SPEs. A reduction reaction occurs from about −0.3 V at the working electrode, indicating that Li + in the SPEs obtain electrons and deposits them on the surface of the working electrode. At about 0.5 V, an oxidation reaction occurs on the working electrode, corresponding to the lithium depositing on the working electrode loses electrons and returning to the Li-SPOD SPEs. The above two processes prove that after being swollen by DMSO, Li-SPOD SPEs can achieve the Li + reversible redox reaction process and be used as an SPE membrane. When the voltage reaches 4 V, the Li-SPOD SPEs begins to decompose. In other words, the electrochemical window of Li-SPOD SPEs is 4 V (vs Li/Li + ). Besides, as shown in the supporting information in Fig. S3, when the half cell was run in the range of −1.5 ∼ 2 V, only the redox peak of lithium deposition and removal on the working electrode could be observed. However, when the half-cell was run in the range of 1 ∼ 4.5 V, an obvious reduction peak appears near 1.3 V is observed, and a relatively gentle oxidation peak appeared in the range of 1.5 ∼ 2.5 V. These reversible REDOX peaks ascribe to the doping and un-doping process of SPOD because POD is a kind of conjunctive polymer, which could be n-doped in a reductive environment. The SPOD was prepared by copolymerized POD with diphenyl ether structure, so the n-doped SPOD is still electronic insulating and can work as an SPE.
The specific capacity of discharge and Coulombic efficiency of Li-SPOD SPEs half cells under 0.1 C are shown in Fig. 4. The inserted pictures are the first charge-discharge curves of the half cells, which indicates that the Coulombic efficiency of the cell with 8/2 Li-SPOD, 7/3 Li-SPOD, and 6/4 Li-SPOD are 98.1%, 98.0%, and 99.1%, respectively. With the increase of cycle numbers, the specific discharge capacity of the cell with Li-SPOD SPEs decreases gradually, in which the 8/2 Li-SPOD SPEs decreases most significantly, from 141.4 to 119.6 mAh g −1 after 25 cycles. The Coulombic efficiency changes little, indicating that the active electrode material may be consumed due to the reaction between the electrolyte during the charging and discharging cycle, or the integrity of the electrode may be gradually destroyed during the cycle leading some active materials cannot participate in the reaction. 27 After 25 cycles of the cell with 7/3 Li-SPOD SPEs, the capacity decreases from the highest 145 to 128.2 mAh g −1 , and the Coulombic efficiency also decreases slightly in the cycle process, which indicates that the electrolyte might decompose slightly in the charge-discharge process. The capacity of 6/4 Li-SPOD SPEs falls to 132.6 from 146.3 mAh g −1 after 25 cycles and Coulombic efficiency slightly lower. When the current density increases to 0.5 C or 1 C, the conductivity is too small to transfer Li + in time, as well as the side reactions of electrolyte increase. Hence the capacity of the battery decreases significantly, and the rate performance is poor (see Fig. S4 in Supporting Information). As shown in results, with the increment of DPE content, the ionic conductivity increases gradually, contributing to the improvement of SPEs cycle stability.

Summary
Lithium sulfonated polyoxadiazoles with different degrees of sulfonation were synthesized and used as gel polymer electrolytes in LIBs. The ATR spectrum proved that the sulfonic acid group was successfully grafted into SPOD. The TG test showed that the polymers have good thermal stability, and the initial thermal decomposition temperature decreases slightly with the increase of DPE monomer content. The conductivity of SPOD SPEs is ∼10 −4 S cm −1 at room temperature, which is at a high level in polyanionic solid electrolytes. However, the electrochemical window of Li-SPOD is narrow. The 6/4 Li-SPOD SPEs have a good cycle performance, and the Coulombic efficiency is always maintained at a high level. Although its basic research has made great progress, the application is still in the initial stage, and further research is needed.