A Novel Single-Ion-Conducting Polymer Electrolyte Derived from CO2

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A Novel Single-Ion-Conducting Polymer Electrolyte Derived from CO2-Based Multifunctional Polycarbonate Kuirong Deng, ShuanJin Wang, Shan Ren, Dongmei Han, Min Xiao, and Yuezhong Meng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11384 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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ACS Applied Materials & Interfaces

A Novel Single-Ion-Conducting Polymer Electrolyte Derived from CO2-Based Multifunctional Polycarbonate Kuirong Deng, Shuanjin Wang, Shan Ren, Dongmei Han, Min Xiao*, Yuezhong Meng* The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province / State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected];

KEYWORDS: electrolyte;

CO2-based multifunctional polycarbonate; single-ion-conducting polymer

all-solid-state

electrolyte;

ionic

conductivity;

thiol-ene

click

chemistry;

environmentally friendly.

ABSTRACT

This work demonstrates the facile and efficient synthesis of a novel environmentally friendly CO2-based multifunctional polycarbonate single-ion-conducting polymer electrolyte with good electrochemistry performance. The terpolymerizations of CO2, propylene epoxide (PO) and allyl glycidyl ether (AGE) catalyzed by zinc glutarate (ZnGA) were performed to generate

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poly(propylene carbonate allyl glycidyl ether) (PPCAGE) with various alkene groups contents which can undergo clickable reaction. The obtained terpolymers exhibits an alternating polycarbonate structure confirmed by 1H NMR spectra and amorphous microstructure with glass transition temperature (Tg) lower than 11.0 °C evidenced by differential scanning calorimetry (DSC) analysis. The terpolymers were further functionalized with 3-mercaptopropionic acid (MPA) via efficient thiol-ene click reaction followed by reacting with lithium hydroxide to afford single-ion-conducting polymer electrolytes with different lithium contents. The all-solidstate polymer electrolyte with 41.0 mol.% lithium containing moiety shows high ionic conductivity of 1.61×10−4 S/cm at 80 °C and high lithium ion transference number of 0.86. It also exhibits electrochemical stability up to 4.3 V vs. Li+/Li. This work provides an interesting design way to synthesize an all-solid-electrolyte used for different lithium batteries.

INTRODUCTION Lithium-ion batteries are promising and clean power sources for a wide variety of applications such as consumer portable electronics and electric vehicles because they are superior to most of batteries in terms of energy density.1 Great progress has been made on classical lithium-ion batteries technology based on liquid electrolytes in the past two decades.2 However, liquid electrolytes suffer from safety issues due to highly flammable organic liquid and formation of dendrite responsible for explosion hazards.3,4 Polymer electrolytes have attracted ever-increasing interest. Some breakthroughs in gel polymer electrolytes such as composite of PVDF with glass fiber, nonwoven fabrics and cellulose have been achieved.5−8 Solid polymer electrolytes (SPEs) provide an ideal solution to those safety issues, because of their nonflammable property and potential application in all-solid-state lithium-ion batteries.9−10 The development of SPEs,


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however, has been hampered by issues including low ionic conductivity at room temperature and low lithium ion transference number.11 Efforts have been devoted to modify polymer structure and properties of polymeric electrolyte to improve performance.12 Many new structured polymers have been used for the preparation of polymer electrolytes. Notably, polycarbonates synthesis from CO2 and epoxides have received considerable attention.13−26 CO2 is well-known to be mainly responsible for global warming.27 Conversion of CO2 to desirable, economically competitive polycarbonates has received considerable interest since the pioneering work of Inoue and co-workers in 1969.28 Over the 40 years, numerous heterogeneous and homogeneous catalytic systems have been developed for the copolymerization CO2 and epoxides.29−32 CO2-based polycarbonates have been applied to polymer electrolytes as these polymers contain a large fraction of oxygen to promote salt dissociation and possess low glass transition temperatures (indicating increased segment mobility and ion transport). Tominaga et al.13−20, 33−35 and Brandell et al.36−42 have developed various electrolytes based on poly(ethylene carbonate) and alternating copolymers of carbon dioxide with glycidyl ethers. What’s more, poly(propylene carbonate) (PPC), an alternating copolymer of CO2 and propylene oxide, one of the most extensively studied CO2-based polycarbonates43 has been studied as solid polymer electrolytes (SPEs)22−24 and gel polymer electrolytes (GPEs)21, 26 due to its amorphous nature, low glass transition temperature, biodegradable properties and good interfacial contact with commonly used electrodes.28 The local relaxation and segmental motion of PPC chain is favorable for the transport of lithium ions. In our previous work, poly(propylene carbonate)/poly(ethylene oxide) (PPC/PEO) polymer electrolytes were developed. Owing to the addition of amorphous PPC, the glass transition temperature and crystallinity of PEO/PPC decrease. Consequently, the ionic conductivity increases to 6.83 × 10-5 S/cm.23 Dukhanin


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prepared SPEs based on the poly(propylene carbonate)–lithium perchlorate system24 and the poly(ethylene carbonate)–lithium perchlorate system44, and declared that SPEs based on poly(propylene carbonate) exhibited better conducting properties compared to those based on poly(ethylene carbonate). Recently, Konieczynska synthesized poly(ether 1,2-glycerol carbonate)s as a thermally stable SPE, which exhibits temperature-dependent conductivity with values comparable to those of optimized PEO-based polymer electrolytes.25 On the whole, the SPEs composite of CO2-based polycarbonates and lithium salts could solve most of the safety issues encountered with liquid electrolytes and achieve relatively high ionic conductivity. On the other hand, lithium ion transference number is also an important factor for practical application of lithium ion batteries.45 Generally speaking, the SPEs composed of polymer matrix and lithium salt suffer from the fact that only a small fraction (1/5th) of the all ionic current is carried by the motion of lithium ion and lithium ion transference number is far below the value of 0.5, which gives rise to the formation of a large concentration gradient with harmful effects such as limited power delivery and dendritic growth.2, 46 In order to minimize the polarization, approaches have been adopted to reduce the mobility of anions. One method is to anchor anions to the polymer backbone, which is a common method to achieve single-ion conducting polymer electrolytes.47 Here we describe a novel single-ion-conducting polymer electrolyte based on CO2-based multifunctional polycarbonate to tune lithium transport number and lithium ionic conductivity at the same time. This polymer electrolyte was synthesized by the terpolymerization of propylene oxide (PO), allyl glycidyl ether (AGE) and CO2 followed by grafting 3-mercaptopropionic acid (MPA) to side chains via thiol-ene click chemistry and subsequently lithiation. The structure of the polymer electrolyte is similar to poly(propylene carbonate), therefore, it possesses


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amorphous nature. Compared to poly(propylene carbonate), the longer flexible pendant group of AGE units is in favor of segmental motion. So a lower glass transition temperature than that of poly(propylene carbonate) is expected, which benefits the transport of lithium ions. The introduction of a flexible spacer between the main polymer chain and the attached anion increases mobility of ions.48 What's more, carboxylic acid lithium salt is anchored to the polymer side chain to suppress the migration of anions toward the anode, thus a high lithium ion transference number is expected. EXPERIMENTAL Materials. Allyl glycidyl ether and propylene oxide were freshly distilled after reflux over CaH2 for 20 h under dry nitrogen gas flow prior to use. Carbon dioxide of 99.99% purity was supplied in a high pressure cylinder equipped with a relief valve and copper pipe. Zinc glutarate (ZnGA) was prepared according to previous work.49 3-Mercaptopropionic acid (MPA) (99%) was purchased from J&K. 2,6-di-tert-butyl-4-methylphenol (99.5%), Lithium hydroxide monohydrate (99.0%) and 2,2-dimethoxy-2-phenylacetophenone (DMPA) (99%) was obtained from Aladdin. Solvents such as methanol, tetrahydrofuran, dichloromethane and diethyl ether were of analytical reagent grade and used as received without further treatment. Terpolymerization of CO2, PO and AGE. Terpolymerization of CO2, PO and AGE was carried out in a 500 mL autoclave equipped with a mechanical stirrer and a programmable temperature controller. Typically, 1.0 g ZnGA was introduced into the autoclave, then the autoclave with the catalyst inside was further dried for 6 h under vacuum at 80 °C. Upon cooling down, PO and AGE was immediately injected into the autoclave, and then the reactor was pressurized to 5.0 MPa with CO2 and maintained at 60 °C for 40 h. The autoclave was cooled


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down followed by releasing the pressure. The bulk polymer was dissolved in dichloromethane and ca. 0.5 g of 2,6-di-tert-butyl-4-methylphenol was added to the solution to forestall selfcross-linking. Ethanol solution of hydrochloric acid (5%) was added to the polymer solution under continuously stirring until it became clear. The viscous solution was precipitated by being poured into high-speed stirred cold ethanol. This procedure was repeated twice to completely purify the polymer. The final polymer precipitates (PPCAGE) were dried under vacuum at 45 °C for at least 12 h. Representative Procedure for the Thiol-ene Click Reaction between PPCAGE and MPA. PPCAGE (2.00 g, 8.2 mmol alkenes) and MPA (49.2 mmol) were dissolved in 35.0 mL THF under dry nitrogen gas. After the addition of DMPA (1.0 mmol), the solution was exposed to UV light (365 nm) under stirring for 2 h. The reaction mixture was precipitated by being poured into vigorously stirred cold diethyl ether. This procedure was repeated twice to completely purify the polymer. The graft polymer (PPCAGE-g-COOH) was dried at 60 °C under high vacuum for at least 12 h thereafter. Lithiation of PPCAGE-g-COOH. PPCAGE-g-COOH (2.00 g, 4.88 mmol carboxylic acid) was dissolved in 40.0 mL DMAc. 0.5 M lithium hydroxide aqueous solution was added into PPCAGE-g-COOH solution dropwise under continuously stirring until PH reach 8.0. Then the solution was purified by being dialyzed against DMAc/H2O (8:2 vol.) for 24h and dried in vacuum at 60 °C for at least 24 h to obtain PPCAGE-g-COOLi. Preparation of Polymer Electrolytes. Solution-casting method was used to prepared polymer electrolytes. Given amounts of PPCAGE-g-COOLi was dissolved in DMAc/H2O (8:2 vol.). The resulting solutions were poured on PTFE dishes and dried under high vacuum at 60 °C for 48 h. The obtained films were punched into circular pieces (d = 17mm) and further dried in a vacuum


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oven at 80 °C for at least 24 h to remove the trace amount of solvent. Later, the pieces were transferred into a dry-argon filled glove box (H2O and O2 content

A Novel Single-Ion-Conducting Polymer Electrolyte Derived from CO2

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