Preparation and Properties of a High-Performance EOEOEA-Based Gel-Polymer-Electrolyte Lithium Battery

Article Preparation and Properties of a High-Performance EOEOEA-Based Gel-Polymer-Electrolyte Lithium Battery

Wenwen Ding 1,2, Chun Wei 1,2,* , Shiqi Wang 1,2, Linmin Zou 1,2 , Yongyang Gong 1,2,* , Yuanli Liu 1,2 and Limin Zang 1,2 1 College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China 2 Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin 541004, China * Correspondence: [email protected] (C.W.); [email protected] (Y.G.)

Received: 18 June 2019; Accepted: 29 July 2019; Published: 2 August 2019

Abstract: Gel polymer electrolyte (GPE) is a promising candidate for lithium-ion batteries due to its adhesion property (like a solid), diffusion property (like a liquid), and inhibition of the growth of lithium dendrite. In this paper, 2-(2-ethoxyethoxy)ethyl acrylate (EOEOEA) and LiBF4 electrolyte were mixed as precursors of gel polymer electrolytes. Through thermal curing, a thermally stable GPE with high ionic conductivity (5.60 10 4 s/cm at 30 C) and wide room temperature electrochemical window × − ◦ (4.65 V) was prepared, and the properties of the GPE were measured by linear sweep voltammetry (LSV), AC impedance spectroscopy, Thermogravimetric analysis (TG), and X-ray diffraction (XRD) techniques. On the basis of the in-situ deep polymerization on a LiFePO4 electrode and cellulose membrane in a battery case, EOEOEA-based GPE could be derived on both LiFePO4 electrode and cellulose membrane. Meanwhile, the contact between GPE, LiFePO4 electrode, and lithium electrode was promoted. The capacity retention rate of the as-prepared LiBF4-EOEOEA 30% gel lithium battery reached 100% under the condition of 0.1 ◦C after 50 cycles, and the Coulombic efficiency was over 99%. Meanwhile, the growth of lithium dendrite could be effectively inhibited. GPE can be applied in high-performance lithium batteries.

Keywords: lithium battery; GPE; EOEOEA; deep in-situ polymerization

1. Introduction Due to the popularization of portable mobile electronic devices and electric vehicles [1,2], high-performance lithium batteries and their safety have drawn increasing attention. The organic solvent in traditional liquid lithium batteries evaporates and leaks. The lithium dendrite growing in charge-discharge cycles can puncture the membrane, causing short circuits in batteries. At the same time, the heat released during the cycles can cause safety threats such as thermal runaway, or even spontaneous combustion [3]. In view of these issues, polymer lithium batteries, using polymer electrolytes instead of traditional organic liquid electrolytes, are free of leakage of organic liquid and growth of lithium dendrite owing to the isolation function of polymer electrolytes, making polymer lithium batteries promising candidates in the future [4,5]. According to the presence or absence of plasticizer, polymer electrolytes are divided into solid polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs), respectively. Polymer electrolytes should possess the following properties: (1) High room-temperature lithium-ion conductivity to ensure fast charge-discharge processes; (2) high chemical, electrochemical, and thermal stability—the premise for the safety and stability of the batteries; and (3) strong mechanical strength to support the assembly of lithium batteries [6].

Polymers 2019, 11, 1296; doi:10.3390/polym11081296 www.mdpi.com/journal/polymers Polymers 2019, 11, 1296 2 of 17

At present, the ionic conductivity of SPEs is too low to meet the requirements of lithium batteries [7]. Moreover, an SPE contacts with cathodes and Li electrodes in a solid-solid mode, where the absence of wettability on the contact surface readily leads to voids, which greatly increase the internal resistance of lithium batteries and further leads to the deterioration of cycling performances of batteries [8]. Huang S et al. applied in-situ polymerization to effectively alleviate this problem in SPE lithium batteries, and 91% of the capacity was retained after 100 cycles [9]. Tong Y et al. prepared an interpenetrating network polymer electrolyte with ionic conductivity more than 1 10 5 s/cm at room temperature and × − an electrochemical window wider than 4.5 V. The assembled all-solid-state lithium battery possessed a Coulomb efficiency of 99% after 200 cycles, showing excellent performance [10]. Zeng X et al. also prepared a polymer electrolyte with an interpenetrating network structure, which greatly inhibited the crystallization of Polyethylene oxide (PEO), and the ionic conductivity was improved; after 200 cycles, 85% of the capacity of the all-solid-state lithium battery was retained [11]. GPE is a special material, which has the properties of both adhesion (like a solid) and diffusion (like a liquid). Lithium dendrite is difficult to grow in GPE lithium batteries, therefore the safety and cycling life of lithium batteries can be greatly improved. Currently, GPE has been extensively studied [12]. Dong X et al. obtained refractory GPE materials by using Li6.4Ga0.2La3Zr2O12 as ion-conductive in GPE [7]. Jimin S et al. combined ionic liquid and GPE for the first time, and the obtained GPE showed resistance to fire. The battery had a specific discharge capacity of about 120 mAh/g after 300 cycles, showing excellent recycling performance and safety [13,14]. Karuppasamy. K et al. proposed the concept of a novel ternary gel polymer electrolyte. After 100 cycles, the discharge specific capacity was about 120 mAh/g, and 80% of the capacity was retained, showing better charge/discharge performance [15]. Modern polymer lithium batteries are produced with a simple casting method [16,17]. Typically, an electrolyte precursor is poured into a Polytetrafluoroethylene or glass mold in a glove box, and the excess organic solvent is evaporated at a high temperature to form a thin film for the assembly of a lithium battery. However, because of the surface tension between the precursor electrolyte and Polytetrafluoroethylene mold, the resulting membrane is subject to air bubbles, shrinkage, and uncontrollable thickness. When a battery is assembled, voids are easy to generate between the film and solid electrodes, greatly increasing the internal resistance and shortening the cycling lives of the batteries. We cannot ignore the impact of volatile organic solvents on the glove box environment at the same time. Considering the poor contact between polymer electrolytes and solid electrodes, Chen X and co-workers [18] proposed an in-situ polymerization technique on LiFePO4 electrodes. In detail, a polymer precursor electrolyte was infiltrated into LiFePO4 electrodes in air. The mixture was heated to initiate polymerization. Then, the as-treated electrodes were sliced for assembly. In this technique, the polymer electrolyte is able to fully contact the active material in the electrodes, but the electrode slices have burrs on the edge, still leading to poor contact and increase of resistance. Meanwhile, the lithium salts in the precursor electrolyte can absorb water in air, probably affecting the occurrence of polymerization. Cui Y proposed another in-situ polymerization technique on LiFePO4 electrodes in a battery container. However, because only one layer of polymer electrolyte was left on the surface of the electrode, the active materials could not fully contact the polymer electrolyte, resulting in incomplete charging and discharging of the lithium battery [19]. Guo et al. directly coated a polymer electrolyte on the surface of lithium electrodes via UV-curing. This method is limited by the surface tension between the precursor electrolyte and the Li electrode. If the Li electrode is not completely covered by the polymer electrolyte membrane, short circuits take place [11]. Compared with the method of immersing polypropylene film (PP) in an electrolyte to prepare a composite electrolyte membrane [20], we propose to inject the precursor electrolyte into the LiFePO4 electrode and the cellulose membrane for deep in-situ polymerization, to derive GPE simultaneously on LiFePO4 electrodes and cellulose membranes where the contact between them could be greatly promoted. This method is simple and efficient. Furthermore, the physical, chemical, and electrochemical Polymers 2019, 11, 1296 3 of 17 properties of EOEOEA-based GPE and relative EOEOEA-based GPE lithium batteries are studied for the application of novel polymer lithium batteries.

2. Materials and Methods

2.1. Materials

Main experimental materials: LiFePO4 (Kejing Material Technology Co., Ltd., Hefei, China), N-methyl pyrrolidone (NMP, Xilong Chemical Co., Ltd., Shantou, China), conductive carbon black (Kejing Co., Ltd., Shenzhen, China), EOEOEOEA (Aladdin Biochemical Technology Co., Ltd., Shanghai, China; average molecular weight = 600, dried with molecular sieves), benzoyl peroxide (BPO, Aladdin Biochemical Technology Co., Ltd., Shanghai, China), lithium bisoxalate (LiBOB), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium bistrifluoromethyl sulfimide (LiTFSI), lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC, Dadao New Material Technology Co., Ltd., Huizhou, China), Polythylene terephthalate film (90 µm thick, Lvcheng Electrical Machinery Co., Ltd., Shanghai, China), and cellulose membranes (NKK Company, Kochi, Japan).

2.2. Preparation of LiFePO4 Electrode

To start, 13.18 g of NMP solution was placed in a 20 mL beaker. Then, 9.2 g of LiFePO4 powder, 0.4 g of PVDF, and 0.4 g of conductive carbon black were gradually and successively added to the beaker, along with stirring with a high-speed mixer. The slurry was stirred for 2 h for uniformity. During the stirring, the beaker was sealed with a ﬁlm to prevent PVDF from deliquescence. Afterwards, the uniform slurry was coated on tin foil to prepare a 100 µm-thick electrode. For drying, the LiFePO4 electrode was ﬁrst placed in an oven at 60 ◦C for 20 min to dry the surface, and was then placed in a vacuum oven at 90 ◦C for a night to dry completely. The dried LiFePO4 electrode was cut into 16 mm-diameter wafers. The mass of each wafer was about 0.0175–0.0185 g, and that of the active species was about 0.0161–0.0170 g. The LiFePO4 wafers absorb water when exposed to air, so they need to be dried at 90 ◦C for 3 h before being placed in the glove box.

2.3. Preparation of EOEOEA-Based GPE In this experiment, EOEOEA-based GPE was prepared with a thermal curing method. All the materials required were stored in the glove box and were protected with argon gas. The entire experiment was carried out in the glove box as follows:

2.3.1. Preparation of 1 mol/L LiBF4 Electrolyte

LiBF4 was dissolved in a mixture solution (containing 2 vol% VC) of EC/DEC/DMC at a volumetric ratio of 1:1:1. The mixture was thus a traditional 1 mol/L LiBF4 electrolyte. Similarly, 1 mol/L LiBOB, LiDFOB, LiTFSI, and LiPF6 electrolytes were prepared.

2.3.2. Screening of Lithium Salts The anions of lithium salts influence the chemical, electrochemical, and physical properties of GPE, and ultimately affect the performance of the GPE lithium battery [21,22]. In this paper, EOEOEA was selected as the gel polymer substrate. LiBOB, LiDFOB, LiTFSI, LiBF4, and LiPF6 were selected as the lithium salts to prepare different lithium-salt-EOEOEA GPEs. In detail, 0.60 g of LiBOB electrolyte was mixed with 0.30 g of an EOEOEA solution (50% of the LiBOB electrolyte in mass) and 0.0045 g of BPO (0.5% of the LiBOB electrolyte and EOEOEA in mass). Polymerization was initiated after heating and LiBOB-EOEOEA 50% GPE was obtained. Similarly, LiDFOB-EOEOEA 50% GPE, LiTFSI-EOEOEA 50% GPE, LiBF4-EOEOEA 50% GPE, and LiPF6-EOEOEA 50% GPE were prepared. Based on the ionic conductivity and electrochemical window of the five EOEOEA GPEs, the most suitable lithium salt for EOEOEA was LiBF4. Polymers 2019, 11, 1296 4 of 17

2.3.3. Preparation of LiBF4-EOEOEA GPEs

To start, 0.6 g of the LiBF4 electrolyte was mixed with 0.18 g of the EOEOEA solution (30% of the mass of LiBF4 electrolyte) and 0.004 g of BPO. The mixture was the precursor electrolyte, which was then heated at 75 ◦C for 12 h to derive a gel polymer electrolyte, denoted by LiBF4-EOEOEA 30%. Similarly, GPEs of LiBF4-EOEOEA 40%, 50%, and 60% were prepared.

2.4. Characterizations of GPE Fourier infrared spectroscopy (FTIR) analysis of the gel polymer electrolytes was performed with an NICOLETNEXUS470 spectrometer (Perkin-Elmer, The States, Waltham, MA, USA). The test 1 wavelength range was 4000–400 cm− . Field emission scanning electron microscopy (FESEM) analysis of the original LiFePO4 electrodes, surfaces and cross-sections of LiFePO4 electrodes after battery cycling, original cellulose membrane, cellulose composite membranes after battery cycling, original lithium electrodes, and lithium electrodes after battery cycling was performed with an S-4800 field emission scanning electron microscope (HITACHI Company, Tokyo, Japan). The acceleration voltage was 5 kV and samples were coated with gold for 30 s. Thermogravimetric analysis (TG) of gel polymer electrolytes was performed with a Q-500 integrated thermal analyzer (TA Company, The States, Newcastle, DE USA). The samples were first dried overnight in a vacuum oven at 80 ◦C. Then, the samples were heated in a N2 atmosphere furnace with a temperature range of 20–800 ◦C at a heating rate of 10 ◦C/min. X-ray diffraction (XRD): The powder of original LiFePO4 electrodes and the powder of LiFePO4 electrodes after battery cycling were performed with an X’Pert PRO X-ray diffractometer (PANalytical, Almelo, The Netherlands). 2θ of 5◦–80◦ was scanned.

2.5. Characterizations of Electrochemical Properties of GPE

2.5.1. Electrochemical Stability The voltage that the electrolyte can withstand is limited, otherwise the electrolyte will decompose due to chemical reactions. The electrochemical window represents the voltage range in which the electrolyte can be stably present. Electrochemical window is an important index to the electrochemical stability of an electrolyte, and can be measured by linear sweep voltammetry (LSV). In the present work, a 90 µm-thick PET film was shredded, with a microtome, into a ring film with an inner diameter of 10 mm and an outer diameter of 16 mm. The inner circle is the contact area between the GPE and electrodes. The ring was sonicated for 30 min, and was then dried in a vacuum oven at 70 ◦C overnight. After drying, the ring was fast transferred to a glove box for reservation. In the glove box filled with argon gas, a 1 mm-thick stainless steel sheet was placed into a container on the anode side. The PET ring film was placed on a stainless steel sheet, and then a precursor electrolyte was dropped into the inner circle of the PET ring until the hole was filled up. Then, a new lithium sheet and anode container were inserted. All these components were sealed at 6 MPa to compose a stainless steel sheet/GPE/Li button battery, as shown in Figure1. Then, the quasi-battery was heated at 75◦ C for 12 h until the precursor electrolyte was polymerized. The stainless steel functioned as the working electrode, and the lithium sheet served as the reference and counter electrode. The as-treated battery was tested by LSV with a CHI660 electrochemical workstation, Chenhua Company, Shanghai, China The scanning rate was set to be 1 mV/s and the scanning voltage was in the range of 0.5–6.5 V. Polymers 2019, 11, 1296 5 of 17 Polymers 2019, 11, x FOR PEER REVIEW 5 of 17

FigureFigure 1. 1. SStructuretructure of of the the s stainlesstainless steel steel sheet/ sheet/ggelel polymer electrolyte ( (GPE)GPE)/Li/Li button battery.battery.

2.5.2. Conductivity 2.5.2. Conductivity Similarly, following the method stated in Section 2.5.1, a stainless steel sheet/GPE/stainless steel Similarly, following the method stated in Section 2.5.1, a stainless steel sheet/GPE/stainless steel sheet button battery was assembled, and the AC impedance measurements of this battery at different sheet button battery was assembled, and the AC impedance measurements of this battery at temperatures was performed with the electrochemical workstation. The test frequency was in the range different temperatures was performed with the electrochemical workstation. The test frequency was of 100 mHz–1 MHz and the amplitude was 5 mV. Zview software was used for the fitting of the AC in the range of 100 mHz–1 MHz and the amplitude was 5 mV. Zview software was used for the impedance spectra to derive the values of bulk resistance, which were used to calculate conductivity fitting of the AC impedance spectra to derive the values of bulk resistance, which were used to according to Equation (1). calculate conductivity according to Equation (1). L σ = (1) R LA 휎 = ∗ (1) where the thickness of GPE is L in units of cm; theR bulk∗ A resistance of GPE is R in units of Ω; and A is contactwhere the area, thickness which is of the GPE area is of Lthe in units inner of circle cm; of the the bulk PET resistance membrane of in GPE this i experiment,s R in units inof unitsΩ; and of cmA is2. Thus,contact the area, conductivity which is theσ atarea diff oferent the inner temperatures circle of the would PET be membrane calculated. in Accordingly,this experiment the, in activation units of energycm2. Thus, required the forconductivity the movement σ at of different lithium ions temperatures could be alsowould calculated be calculate accordingd. Accordingly, to the Arrhenius the equation,activation Equationenergy required (2). for the movement of lithium ions could be also calculated according to Ea the Arrhenius equation, Equation (2). σ = Aexp( ) (2) −RT Ea where A represents the pre-exponential factor;휎 = Aexp Ea stands(− ) for activation energy, kJ/mol; R is the ideal(2) RT 1 1 gas constant 8.314 J mol− K− ; and T is the temperature (K) in the test. where A represents· the pre· -exponential factor; Ea stands for activation energy, kJ/mol; R is the ideal 2.5.3.gas constant Assembly 8.314 of J·mol GPE− Lithium1·K −1; and Batteries T is the temperature (K) in the test.

2.5.3.Using Assembly the LiFePO of GPE4 Lithiumsheet as Batteries the cathode, Li sheet as the anode, and the GPE cellulose composite membrane as the electrolyte and isolation membrane, we assembled EOEOEA-based GPE batteries. Using the LiFePO4 sheet as the cathode, Li sheet as the anode, and the GPE cellulose composite In detail, a LiFePO4 sheet was placed in the middle of a container on the cathode side. Three drops of membrane as the electrolyte and isolation membrane, we assembled EOEOEA-based GPE batteries. EOEOEA 30% precursor electrolyte were added to the LiFePO4 sheet and then it was left to maintain In detail, a LiFePO4 sheet was placed in the middle of a container on the cathode side. Three drops of for 2 h to make the precursor electrolyte fully penetrate into the LiFePO4 sheet. An original cellulose isolationEOEOEA membrane30% precursor was electrolyte inserted and were was added fully to inﬁltrated the LiFePO with4 she precursoret and then electrolyte. it was left Then, to maintain a new lithiumfor 2 h to sheet make and the nickel precursor foam electrolyte were inserted. fully Thepenetrate anode into container the LiFePO was assembled.4 sheet. An Theoriginal quasi cellulose battery wasisolation sealed membrane at 5.5 MPa was with inserted a button and battery was sealer.fully infiltrate Consideringd with the precursor fact that theelectrolyte. polymerization Then, a of new the precursorlithium sheet electrolyte and nickel on thefoam cellulose were inserted. membrane The isanode favorable container to eﬀ wasective assembled control. of The the quasi thickness battery of was sealed at 5.5 MPa with a button battery sealer. Considering the fact that the polymerization of the gel polymer electrolyte membrane [23], the as-assembled quasi battery was heated at 75 ◦C for polymerization.the precursor electrolyte The as-prepared on the cellulose battery wasmembrane denoted is asfavorable an EOEOEA to effective 30% lithium control battery. of the thickness Similarly, of the gel polymer electrolyte membrane [23], the as-assembled quasi battery was heated at 75 °C for LiBF4-EOEOEA 40%, LiBF4-EOEOEA 50%, and LiBF4-EOEOEA 60% lithium batteries were assembled. polymerization. The as-prepared battery was denoted as an EOEOEA 30% lithium battery. Similarly, 2.5.4.LiBF4- AssemblyEOEOEA of40%, Traditional LiBF4-EOEOEA LiBF4 Electrolyte 50%, and Liquid LiBF Lithium4-EOEOEA Batteries 60% lithium batteries were assembled. Using the LiFePO4 sheet as the cathode, Li sheet as the anode, and cellulose membrane as the battery membrane, LiBF4 electrolyte liquid lithium batteries were assembled. In detail, a LiFePO4 sheet 2.5.4. Assembly of Traditional LiBF4 Electrolyte Liquid Lithium Batteries was placed in the middle of a container on the cathode side and then was covered with a cellulose membrane.Using the Three LiFePO drops4 she of LiBFet as4 theelectrolyte cathode were, Li she addedet as onto the anode the cellulose, and cellulose separator. membrane Then, a lithium as the battery membrane, LiBF4 electrolyte liquid lithium batteries were assembled. In detail, a LiFePO4 sheet was placed in the middle of a container on the cathode side and then was covered with a

Polymers 2019, 11, 1296 6 of 17 Polymers 2019, 11, x FOR PEER REVIEW 6 of 17 sheetcellulose and a nickel membrane foam. wereThree inserted.drops of LiBF The4 electrolyte anode container were add wased on assembled.to the cellulose The separator quasi battery. Then, was sealeda at lithium 5.5 MPa sheet with and aa buttonnickel foam battery were sealer. inserted. The The assembled anode container liquid lithium was assembled battery. wasThe quasi removed from thebattery glove was box sealed and at was 5.5 MPa maintained with a button overnight, battery so sealer. that The the LiBFassembled4 electrolyte liquid lithium could battery fully penetrate was removed from the glove box and was maintained overnight, so that the LiBF4 electrolyte could fully the LiFePO4 electrode and cellulose membrane. penetrate the LiFePO4 electrode and cellulose membrane. 2.5.5. Electrochemical Performances of Lithium Batteries 2.5.5. Electrochemical Performances of Lithium Batteries The charge-discharge cycling stability is a common index in the performance of a battery. In this The charge-discharge cycling stability is a common index in the performance of a battery. In this study, a Neware high-performance battery detection system was used to measure the cycling and rate study, a Neware high-performance battery detection system was used to measure the cycling and performancesrate performance of thoses EOEOEA-basedof those EOEOEA lithium-based lithium batteries. batter Theies. ACTheimpedance AC impedance of those of those batteries batteries before and afterbefore the and charge-discharge after the charge- cyclingdischarge was cycl measureding was measure to studyd to the study interface the interface stability stabi oflity those of those batteries. batteries. 3. Results and Discussions 3. Results and Discussions 3.1. Screening of Lithium Salts 3.1. Screening of Lithium Salts Figure2a indicates the ionic conductivity of di fferent lithium salt-EOEOEA 50% GPEs at 30 ◦C. AmongFigure these 2a indicate five GPEs,s the ionic the conductivity LiBF4-EOEOEA of different 50% GPElithium showed salt-EOEOEA the highest 50% GPE ions conductivity, at 30 °C. Among these4 five GPEs, the LiBF4-EOEOEA 50% GPE showed the highest ion conductivity, about about 1.23 10− S/cm, resulting from the combination of its excellent ionic mobility and dissociation 1.23 ×× 10−4 S/cm, resulting from the combination of its excellent ionic mobility and dissociation constant and the fact that LiBF4 has the lowest charge transfer resistance among electrolytes [24,25]. constant and the fact that LiBF4 has the lowest charge transfer4 resistance among electrolytes [24,25]. LiTFSI exhibited an ionic conductivity of about 1.17 10− S/cm because LiTFSI could reduce the LiTFSI exhibited an ionic conductivity of about 1.17 ×× 10−4 S/cm because LiTFSI could reduce the crystal region in the EOEOEA polymer. At the same time, LiTFSI-GPEs could corrode aluminum foil, crystal region in the EOEOEA polymer. At the same time, LiTFSI-GPEs could corrode aluminum foil, thus limiting its applications. LiPF yielded non-conductive LiF during heating, so LiPF -EOEOEA thus limiting its applications. LiPF6 6 yielded non-conductive LiF during heating, so LiPF6-EOEOEA6 50% GPE50% showedGPE showed the lowest the lowest ionconductivity. ion conductivity. It is It well is well known known that that LiDFOB LiDFOB generates generates LiPF LiPF6 and6 and LiBF 4 duringLiBF heating,4 during so heating, the ionic so the conductivity ionic conductivity of LiDFOB-EOEOEA of LiDFOB-EOEOEA 50% 50% GPE GPE was was between between those those of of LiPF 6 and LiBFLiPF46 [and19]. LiBF4 [19]. FigureFigure2b shows 2b the shows room-temperature the room-temperature electrochemical electrochemical windows window of di fferents of lithiumdifferent salt-EOEOEA lithium 50%-basedsalt-EOEOEA GPEs. Among50%-based them, GPEs. LiBOB-EOEOEA Among them, LiBOB 50%-EOEOEA GPE and 50% LiPF6-EOEOE GPE and LiPF6 50%-EOEOE GPE exhibited 50% relativelyGPE narrowexhibited windows. relatively LiBF4-EOEOEA narrow window 50%s. LiBF4 GPE- possessedEOEOEA 50% the widest GPE possessed electrochemical the widest window electrochemical window of about 5.05 V, much wider than the values of traditional liquid lithium of about 5.05 V, much wider than the values of traditional liquid lithium batteries and PEO-based solid batteries and PEO-based solid polymer electrolytes (~3.9 V). Moreover, the electrochemical window polymer electrolytes (~3.9 V). Moreover, the electrochemical window profiles of LiDFOB-EOEOEA profiles of LiDFOB-EOEOEA 50%, LiTFSI-EOEOEA 50%, LiBF4-EOEOEA 50%, and LiPF6-EOEOEA 50%, LiTFSI-EOEOEA50% GPE exhibited 50%, a spike LiBF4-EOEOEA at around 0.2550%, V, indicating and LiPF6-EOEOEA the occurrence 50% of oxidation GPE exhibited reaction as spike, in at aroundwhich 0.25 lithium V, indicating ions immigrated the occurrence from lithium of oxidation sheetsreactions, to stainless in steel which sheets lithium [19]. However, ions immigrated the high from lithiumhardness sheets of to LiBOB stainless-EOEOEA steel sheets 50% GPE [19]. ma However,de it difficult the highfor lithium hardness ions of to LiBOB-EOEOEA migrate. Therefore, 50% no GPE madespike it di ffi wascult observed for lithium at around ions to 0.25migrate. V for Therefore, LiBOB-EOEOEA no spike 50% was GPE observed, as shown at around in Figure 0.25 2b V. for LiBOB-EOEOEAConsidering 50%the high GPE, ionic as shownconductivity in Figure and 2wideb. Considering electrochemical the highwindow, ionic LiBF4 conductivity was selected and as wide electrochemicalthe lithium salt window, for batteries LiBF4 in was the selectedfollowing as sections the lithium. salt for batteries in the following sections.

Figure 2. (a) Ionic conductivity at 30 ◦C and (b) room-temperature electrochemical windows of diﬀerent Figure 2. (a) Ionic conductivity at 30 °C and (b) room-temperature electrochemical windows of lithium salt-EOEOEA 50% GPEs. different lithium salt-EOEOEA 50% GPEs.

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3.2. Mechanism of Polymerization of EOEOEA and LiBF4 Electrolyte 3.2. Mechanism of Polymerization of EOEOEA and LiBF4 Electrolyte Figure 3a shows the polymerization of EOEOEA initiated by BPO. Figure 3b shows the Figure3a shows the polymerization of EOEOEA initiated by BPO. Figure3b shows the precursor precursor electrolytes with different contents of EOEOEA. Figure 3c shows the gel polymer electrolytes with different contents of EOEOEA. Figure3c shows the gel polymer electrolytes with electrolytes with different contents of EOEOEA. As shown in Figure 3a, under the heating condition, different contents of EOEOEA. As shown in Figure3a, under the heating condition, BPO decomposed BPO decomposed to generate free radicals, causing polymerization of C=C in EOEOEA, and finally to generate free radicals, causing polymerization of C=C in EOEOEA, and finally giving a GPE [26]. giving a GPE [26]. C=O had a higher affinity for BF4−, resulting in the large conductivity of C=O had a higher affinity for BF4−, resulting in the large conductivity of LIBF4-EOEOEA GPE. Figure3c LIBF4-EOEOEA GPE. Figure 3c shows that LiBF4-EOEOEA 10% and LiBF4-EOEOEA 20% precursor shows that LiBF4-EOEOEA 10% and LiBF4-EOEOEA 20% precursor electrolytes were still liquids after electrolytes were still liquids after the heating. This is because the content of the EOEOEA monomer the heating. This is because the content of the EOEOEA monomer was too small, and the EOEOEA was too small, and the EOEOEA gel polymer electrolytes that formed could not contain all the gel polymer electrolytes that formed could not contain all the organic electrolytes. However, along organic electrolytes. However, along with the increase of EOEOEA content, the degree of gelation with the increase of EOEOEA content, the degree of gelation increased. Transparent GPEs were increased. Transparent GPEs were successfully obtained after the heating of LiBF4-EOEOEA 30%, successfully obtained after the heating of LiBF4-EOEOEA 30%, LiBF4-EOEOEA 40%, LiBF4-EOEOEA LiBF4-EOEOEA 40%, LiBF4-EOEOEA 50%, and LiBF4-EOEOEA 60% precursor electrolytes, and the 50%, and LiBF4-EOEOEA 60% precursor electrolytes, and the hardness of the GPEs obtained were hardness of the GPEs obtained were gradually increased in the same order. gradually increased in the same order.

Figure 3. (a) The polymerization of EOEOEA initiated by BPO; precursor electrolytes with diﬀerent contents of EOEOEA before (b) and after (c) heating. Figure 3. (a) The polymerization of EOEOEA initiated by BPO; precursor electrolytes with different contents of EOEOEA before (b) and after (c) heating. Figure4a shows the FTIR spectra of LiBF 4-EOEOEA 30% precursor electrolyte before heating and 1 LiBF4-EOEOEA 30% GPE after heating, and Figure4b is an enlarged view of 2000–500 cm − in Figure4a. Figure 4a shows the FTIR spectra of LiBF4-EOEOEA 30% precursor1 electrolyte before heating In the spectrum of the precursor electrolyte, the band at 1730 cm− corresponds to the coupling of and LiBF4-EOEOEA 30% GPE after heating, and Figure 4b is an enlarged view of1 2000–500 cm−1 in vibration of two carbonyl groups in BPO anhydride. The bands at 780 and 720 cm− correspond to the −1 Figuredeformation 4a. In vibration the spectr ofum C= ofC-H the groups precursor in EOEOEA electrolyte monomer, the band [27]. at After 1730 heating cm correspond and polymerization,s to the coupling of vibration of two carbonyl groups in BPO anhydride. The bands at 780 and 720 cm−1 LiBF4-EOEOEA 30% GPE was yielded. In the spectrum of the LiBF4-EOEOEA 30% GPE, the bands at correspond to the1 deformation vibration of C=C1 -H groups in EOEOEA monomer [27]. After heating 780 and 720 cm− (C=C-H) and band 1730 cm− completely disappeared, proving that the BPO-initiated andpolymerization polymerization, of EOEOEA30% LiBF4-EOEOEA precursor 30% GPE electrolyte was yielded. could takeIn the place. spectr Theum bands of the at LiBF 2971,4- 1806,EOEOEA 1625, 30% GPE, the bands at 1780 and 720 cm−1 (C=C-H) and band 1730 cm−1 completely disappeared, 1382, 1110, and 854 cm− correspond to the stretching vibration of CH2, symmetric stretching of C=O provanding asymmetric that the stretching BPO-initiated of C= pO,olymerization asymmetric stretching of EOEOEA30% of C-O-C, precursor vibration of electrolyte C=O-O vibration could take and −1 placebending. The vibration bands at of2971, B–F, 1806, respectively. 1625, 1382, All 1110, these and bands 854 were cm observed correspond in the to precursorthe stretching electrolyte vibration and of CH2, symmetric stretching of C=O and asymmetric stretching of C=O, asymmetric stretching of LiBF4-EOEOEA 30% GPE. C-O-C, vibration of C=O-O vibration and bending vibration of B–F, respectively. All these bands were observed in the precursor electrolyte and LiBF4-EOEOEA 30% GPE.

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Figure 4. FTIR spectra of LiBF4-EOEOEA 30% precursor electrolyte and LiBF4-EOEOEA 30% GPE.

Figure 4. FTIR spectra of LiBF4-EOEOEA 30% precursor electrolyte and LiBF4-EOEOEA 30% GPE. 3.3. Electrochemical Properties of LiBF4-EOEOEA-Based GPEs

3.3. EThelectrochemical conductivities Properties of GPEs of LiBF with4-EOEOEA diﬀerent- EOEOEABased GPE contentss at diﬀerent temperatures are shown in Figure5a. The conductivity of EOEOEA 30%, EOEOEA 40%, EOEOEA 50%, and EOEOEA 60% The conductivities of GPEs with different EOEOEA contents at different temperatures are gel electrolytes at 30 C was 5.60 10 4, 4.81 10 4, 4.00 10 4, and 3.42 10 4 S/cm, much higher shown in Figure 5a. T◦he conductivity× of− EOEOEA× −30%, EOEOEA× − 40%, EOEOEA× − 50%, and EOEOEA than that of PEO-based solid polymer electrolytes (1.0 10 8 s/cm) and lower than that of a traditional 60% gel electrolytes at 30 °C was 5.60 × 10−4, 4.81 × 10−×4, 4.00− × 10−4, and 3.42 × 10−4 S/cm, much higher liquidPolymers lithium 2019, 11 battery, x FOR PEER (1.0 REVIEW10 3 S/cm) by only one order of magnitude. These GPEs can meet9 the of 17 than that of PEO-based solid× − polymer electrolytes (1.0 × 10−8 s/cm) and lower than that of a requirement for ionic conductivity of lithium batteries in practical applications [28]. traditional liquid lithium battery (1.0 × 10−3 S/cm) by only one order of magnitude. These GPEs can meet the requirement for ionic conductivity of lithium batteries in practical applications [28]. The high conductivity of those EOEOEA GPEs should be attributed to the ester and ether groups in EOEOEA, resulting in a high solubility for lithium salts [29,30]. On the other hand, as shown in Figure 5a, at a certain temperature, the conductivity of gel electrolytes decreased with the increase of EOEOEA content, because a higher EOEOEA content led to a lower lithium salt content and lower conductivity [31,32]. For a certain GPE, the conductivity increases with the elevation of temperature, because a higher temperature prompts the movement of lithium salts; and, the changes in ionic conductivity against temperature agreed with the Arrhenius equation. It was calculated that the Ea of LiBF4-EOEOEA 30%, 40%, 50%, and 60% GPEs was 7.515, 9.994, 11.532, and 12.099 kJ/mol, respectively. The activation energy increased with the increase content of EOEOEA, because the hardness of gel electrolytes increased with the increase of EOEOEA content, which required more energy for the transmission of lithium ions in electrolytes.

FigureFigure 5.5(ba )shows Ionic conductivity the room at-temperature 30 ◦C and (b) room-temperature electrochemical electrochemical windows of windows GPEs with of diﬀ erent different EOEOEALiBFFigure4 -EOEOEA content 5. (as.) Ion GPEs. Theic windowconductivitys of atEOEOEA 30 °C and 30%, (b) room EOEOEA-temperature 40%, EOEOEA electrochemical 50%, and window EOEOEAs of 60% GPEsdifferent were LiBF wider4-EOEOEA than GPE 4.65s. V, meeting the requirement for electrochemical stability of The high conductivity of those EOEOEA GPEs should be attributed to the ester and ether groups high-voltage lithium batteries. All the widths were far greater than that (~3.9 V) of traditional liquid in EOEOEA, resulting in a high solubility for lithium salts [29,30]. On the other hand, as shown in lithium3.4. XRD batteries Results and of LiBF PEO4--EOEOEAbased solid-Based electrolyte GPEs s [33]. Noticeably, the electrochemical window of Figure5a, at a certain temperature, the conductivity of gel electrolytes decreased with the increase of LiBF4-EOEOEA 60% GPE was the widest (~5.05 V) among these GPEs, proving that the increase of EOEOEAFigure content, 6 shows because the XRD a higher patterns EOEOEA of LiBF content4-EOEOEA led to 30% a lower and lithium60% GPEs salt. LiBF content4-EOEOEA and lower 30% EOEOEA content is favorable to the electrochemical stability of gel electrolytes. To sum up, conductivityGPE only [exhibited31,32]. For one a certain diffraction GPE, thespike conductivity at 2θ of increases23°, indicating with the that elevation the crystalline of temperature, zone of EOEOEA is a gel electrolyte polymer substrate with great electrochemical stability. becauseLiBF4-EOEOEA a higher 30% temperature was veryprompts limited. With the movement the increase of of lithium EOEOEA salts; content, and, thethe changescrystalline in zone ionic of conductivityLiBF4-EOEOEA against 60% temperature GPE was extended agreed with, averse the Arrheniusto the transmission equation. of It lithium was calculated ions and that resulting the Ea ofin a decrease in conductivity. The XRD results are consistent with the results shown in Figure 5a, LiBF4-EOEOEA 30%, 40%, 50%, and 60% GPEs was 7.515, 9.994, 11.532, and 12.099 kJ/mol, respectively. Theverif activationying that energy the ionic increased conductivity with thedecrease increases when content the EOEOEA of EOEOEA, content because increase thes hardness. of gel electrolytes increased with the increase of EOEOEA content, which required more energy for the transmission of lithium ions in electrolytes.

Figure5b shows the room-temperature1000 electrochemical windows of GPEs with di ﬀerent EOEOEA EOEOEA30% contents. The windows of900 EOEOEA 30%, EOEOEA 40%, EOEOEA EOEOEA60% 50%, and EOEOEA 60% GPEs were wider than 4.65 V, meeting the requirement for electrochemical stability of high-voltage lithium 800 batteries. All the widths were far greater than that (~3.9 V) of traditional liquid lithium batteries and 700

600 500

Intensity 400

300 200

100 10 20 30 40 50 60 2θ degree （）

Figure 6. XRD patterns of different gel electrolytes.

3.5. Thermal Analysis of LiBF4-EOEOEA-Based GPEs During charging and discharging cycles, lithium batteries should release a certain amount of heat, so GPE should possess good thermal stability. Figure 7a,b show TG and Derivative thermogravimetry ( DTG) curves for EOEOEA 30% GPE and EOEOEA 60% GPE, respectively. As illustrated in Figure 7a,b, LiBF4-EOEOEA 30% GPE lost weight at 312 – 506 °C,

corresponding to the thermal decomposition of EOEOEA polymer. For LiBF4-EOEOEA 60% GPE, many weak signals were observed at 30–340 °C, which should be due to the decomposition of a larger amount of unreacted monomers in LiBF4-EOEOEA 60% GPE. Furthermore, a pronounced signal was observed at 340–530 °C for LiBF4-EOEOEA 60% GPE, similar to LiBF4-EOEOEA 30%. The

Polymers 2019, 11, x FOR PEER REVIEW 9 of 17

Polymers 2019, 11, 1296 9 of 17

PEO-basedFigure solid 5. (a electrolytes) Ionic conductivity [33]. Noticeably, at 30 °C theand electrochemical (b) room-temperature window electrochemical of LiBF4-EOEOEA window 60%s of GPE was thedifferent widest LiBF (~5.054-EOEOEA V) among GPE theses. GPEs, proving that the increase of EOEOEA content is favorable to the electrochemical stability of gel electrolytes. To sum up, EOEOEA is a gel electrolyte polymer substrate3.4. XRD with Results great of LiBF electrochemical4-EOEOEA-Based stability. GPEs

Figure 6 shows the XRD patterns of LiBF4-EOEOEA 30% and 60% GPEs. LiBF4-EOEOEA 30% 3.4. XRD Results of LiBF4-EOEOEA-Based GPEs GPE only exhibited one diffraction spike at 2θ of 23°, indicating that the crystalline zone of Figure6 shows the XRD patterns of LiBF -EOEOEA 30% and 60% GPEs. LiBF -EOEOEA 30% GPE LiBF4-EOEOEA 30% was very limited. With4 the increase of EOEOEA content, the4 crystalline zone of only exhibited one diﬀraction spike at 2θ of 23 , indicating that the crystalline zone of LiBF -EOEOEA LiBF4-EOEOEA 60% GPE was extended, averse◦ to the transmission of lithium ions and resulting4 in a 30%decrease was very in conductivity.limited. With theThe increase XRD results of EOEOEA are consistent content, the with crystalline the results zone shown of LiBF in4 -EOEOEAFigure 5a, 60%verif GPEyingwas that extended,the ionic conductivity averse to the decrease transmissions when ofthe lithium EOEOEA ions content and resulting increase ins. a decrease in conductivity. The XRD results are consistent with the results shown in Figure5a, verifying that the ionic conductivity decreases when the EOEOEA content increases.

1000 EOEOEA30% 900 EOEOEA60% 800

700

600

500

Intensity 400

300 200

100 10 20 30 40 50 60 2θ degree （）

Figure 6. XRD patterns of diﬀerent gel electrolytes. Figure 6. XRD patterns of different gel electrolytes. 3.5. Thermal Analysis of LiBF4-EOEOEA-Based GPEs

3.5. DuringThermal charging Analysis andof LiBF discharging4-EOEOEA cycles,-Based lithium GPEs batteries should release a certain amount of heat, so GPE should possess good thermal stability. Figure7a,b show TG and Derivative thermogravimetry During charging and discharging cycles, lithium batteries should release a certain amount of ( DTG) curves for EOEOEA 30% GPE and EOEOEA 60% GPE, respectively. heat, so GPE should possess good thermal stability. Figure 7a,b show TG and Derivative As illustrated in Figure7a,b, LiBF 4-EOEOEA 30% GPE lost weight at 312–506 ◦C, corresponding thermogravimetry ( DTG) curves for EOEOEA 30% GPE and EOEOEA 60% GPE, respectively. to the thermal decomposition of EOEOEA polymer. For LiBF4-EOEOEA 60% GPE, many weak signals As illustrated in Figure 7a,b, LiBF4-EOEOEA 30% GPE lost weight at 312 – 506 °C, were observed at 30–340 ◦C, which should be due to the decomposition of a larger amount of unreacted corresponding to the thermal decomposition of EOEOEA polymer. For LiBF4-EOEOEA 60% GPE, monomers in LiBF4-EOEOEA 60% GPE. Furthermore, a pronounced signal was observed at 340–530 ◦C many weak signals were observed at 30–340 °C, which should be due to the decomposition of a for LiBF4-EOEOEA 60% GPE, similar to LiBF4-EOEOEA 30%. The decomposition temperature of larger amount of unreacted monomers in LiBF4-EOEOEA 60% GPE. Furthermore, a pronounced LiBF4-EOEOEA 60% GPE was slightly higher than that of LiBF4-EOEOEA 30% GPE, indicating that signal was observed at 340–530 °C for LiBF4-EOEOEA 60% GPE, similar to LiBF4-EOEOEA 30%. The the increase of EOEOEA content can increase the decomposition temperature of GPE.

Polymers 2019, 11, x FOR PEER REVIEW 10 of 17 decomposition temperature of LiBF4-EOEOEA 60% GPE was slightly higher than that of LiBF4-EOEOEA 30% GPE, indicating that the increase of EOEOEA content can increase the Polymersdecomposition2019, 11, 1296 temperature of GPE. 10 of 17

Figure 7. (a) TG and (b) DTG curves of diﬀerent gel electrolytes. Figure 7. (a) TG and (b) DTG curves of different gel electrolytes.

3.6. Electrochemical Performances of LiBF4-EOEOEA-Based GPEs Lithium Batteries 3.6. Electrochemical Performances of LiBF4-EOEOEA-Based GPEs Lithium Batteries Cycling performance is an important index in the quality of a battery. Figure8a shows the specific dischargeCycling capacities performance of diff iserent an EOEOEA-basedimportant index GPEin the lithium quality batteries of a battery. during Fig 50ure cycles 8a shows under the specific discharge capacities of different EOEOEA-based GPE lithium batteries during 50 cycles condition of 0.1 ◦C, with a voltage range of 2.6–4.2 V. As illustrated in Figure8a, the specific discharge under the condition of 0.1 °C, with a voltage range of 2.6–4.2 V. As illustrated in Figure 8a, the capacities of LiBF4-EOEOEA 30%, 40%, 50%, and 60% lithium batteries in the first cycle were 116.45, 131.35,specific 110.47, discharge and capacities 86.00 mAh of/g, LiBF respectively.4-EOEOEA The 30%, corresponding 40%, 50%, and specific 60% discharge lithium batteries capacities in were the first still casycl highe we asre 117.52, 116.45, 129.36, 131.35, 113.79, 110.47 and, 77.67and mAh 86.00/g, mAh/g, respectively, respectively. in the fiftieth The cycle.corresponding The corresponding specific dischargecapacity retention capacities rates were after still 50 as cycles high as were 117.52, 100%, 129.36, 98%, 100%,113.79 and, and 90%, 77.67 respectively. mAh/g, respectively The 50 cycles, in the of fiftiethEOEOEA cycl 30%e. The and corresponding EOEOEA 40% lithiumcapacity batteries retention seemed rates after to be 50 very cycles stable, we andre 100%, the specific 98%, 10 discharge0%, and 90%,capacities respectively. of both batteries The 50 cycles were presentedof EOEOEA as straight30% and horizontal EOEOEA lines. 40% lithium The specific batteries discharge seemed capacity to be veryof the stable EOEOEA, and 40% the lithium specific battery discharge was higher capacit thanies that of ofboth the EOEOEAbatteries 30%were lithium presented battery, as showing straight horizontalthat in a certain lines. range, The specific ionic conductivity discharge capacity cannot of be the the EOEOEA only factor 40% affecting lithium the batter specificy wa discharges higher than that of the EOEOEA 30% lithium battery, showing that in a certain range, ionic conductivity capacities of lithium batteries. Because the hardness of LiBF4-EOEOEA 40% GPE was slightly larger cannot be the only factor affecting the specific discharge capacities of lithium batteries. Because the than that of LiBF4-EOEOEA 30% GPE, the former was able to realize a better contact of GPE with hardness of LiBF4-EOEOEA 40% GPE was slightly larger than that of LiBF4-EOEOEA 30% GPE, the the positive and negative ions in the LiBF4-EOEOEA 40% GPE battery, so the ion migration ability formerat the interface was able was to larger,realize which a better ultimately contact led of to GPE a better with discharge the positive performance and negative compared ions in to the LiBF4-EOEOEA 40% GPE battery, so the ion migration ability at the interface was larger, which LiBF4-EOEOEA 30% lithium battery [34]. The discharge specific capacity of EOEOEA 60% lithium ultimatelybattery is the led smallest to a better and the discharge fluctuation performance is the largest, compared because the to minimum the LiBF4 conductivity-EOEOEA 30% of EOEOEA lithium battery60% gel [ electrolyte34]. The discharge leads to incompletespecific capacity charging of EOEOEA and discharging 60% lithium of the battery battery, is the the discharge smallest a specificnd the fluctuation is the largest, because the minimum conductivity of EOEOEA 60% gel electrolyte leads to capacity is the smallest. At the same time, the hardness of LiBF4-EOEOEA 60% GPE is the largest; incompletelithium ion charging transport and activation discharging energy of becomes the battery, larger, the and discharge lithium specific ion transport capacity becomes is the smallest. difficult, At the same time, the hardness of LiBF4-EOEOEA 60% GPE is the largest; lithium ion transport resulting in a relatively unstable interface chemical reaction, so LiBF4-EOEOEA 60% lithium battery activationdischarge is energy unstable. becomes larger, and lithium ion transport becomes difficult, resulting in a relativelyFigure unstable8b shows interface the Coulomb chemical e ffi reaction,ciencies so of diLiBFfferent4-EOEOEA EOEOEA-based 60% lithium GPEs battery lithium discharge batteries is unstable. during 50 charge-discharge cycles under the condition of 0.1 ◦C (the internal image is an enlarged versionFigure of the 8b EOEOEA shows the 30% Coulomb GPE battery efficienc and theies EOEOEAof different 40% EOEOEA GPE battery).-based The GPE Coulombs lithium effi batteriesciencies during 50 charge-discharge cycles under the condition of 0.1 °C (the internal image is an enlarged of LiBF4-EOEOEA 30%, 40%, 50%, and 60% GPE lithium batteries in the first cycle were 75%, 80%, version of the EOEOEA 30% GPE battery and the EOEOEA 40% GPE battery). The Coulomb 73%, and 81%, respectively. The Coulomb efficiencies of LiBF4-EOEOEA 30% and LiBF4-EOEOEA efficiencies40% batteries of LiBF were4- presentedEOEOEA 30%, as straight 40%, 50% horizontal, and 60% lines GPE during lithium the batteries 50 cycles, in the indicating first cycl thate w theere 75%,charge-discharge 80%, 73%, and cycling 81%, processes respectively. of both The batteries Coulomb were efficienc very stable.ies of InLiBF contrast,4-EOEOEA the Coulomb30% and LiBF4-EOEOEA 40% batteries were presented as straight horizontal lines during the 50 cycles, efficiencies of LiBF4-EOEOEA 50% and LiBF4-EOEOEA 60% lithium batteries fluctuated to a small indicating that the charge-discharge cycling processes of both batteries were very stable. In contrast, extent, so not as stable as those of LiBF4-EOEOEA 30% and LiBF4-EOEOEA 40% batteries. the CoulombFigure8c shows efficiencies the specific of LiBF charge4-EOEOEA and discharge 50% and capacities LiBF4 versus-EOEOEA voltage 60% relationships lithium batteries of the fluctuateEOEOEAd 30% to a lithiumsmall extent, battery so in not the as first, stable 25th, as andthose 50th of LiBF cycles.4-EOEOEA The charging 30% platformsand LiBF4- wereEOEOEA located 40% at batteries.about 3.4 and 3.6 V, which correspond to the insertion and removal processes of lithium ions in lithium iron phosphate electrodes [19]. The specific capacities of first discharge were 153.27 and 116.45 mAh/g, respectively. The Coulomb efficiency in the first cycle was 75%. In the 50th cycle, the specific Polymers 2019, 11, 1296 11 of 17 discharge capacity was 117.52 mAh/g, comparable to the value in the first cycle, which proves that the charge-discharge cycling process of this battery was very stable and the capacity retention rate was as high as 100%. All the profiles in Figure8c are very smooth without spikes, indicating that this type of GPE was very stable in the charge-discharge reactions and no side reactions occurred. Figure8d shows the specific discharge capacities of the LiBF 4-EOEOEA 30% lithium battery at different rates. The values were 113.95, 105.39, 79.92, and 24.28 mAh/g under different C-rates of 0.1, 0.2, 0.5, and 1.0 ◦C, respectively. After charging and discharging cycles under the condition of high current density, the specific discharge capacity still reached 118.32 mAh/g under the condition of 0.1 ◦C, even slightly higher than that in the first discharging cycle, proving that this battery can bear high-current-density charging and discharging processes. Figure8e shows the specific discharge capacities, and Figure8f shows Coulomb e fficiencies of the LiBF4-EOEOEA 30% GPE lithium battery and the liquid LiBF4 lithium battery. For comparison of charge-discharge stability, both batteries were subjected to 50 charge-discharge cycles under the condition of 0.1 ◦C. As shown in Figure8e, the specific discharge capacity and stability of the LiBF4-EOEOEA 30% lithium battery were better than those of the traditional liquid lithium battery. In Figure8f, the Coulomb e fficiency of the LiBF4-EOEOEA 30% lithium battery was presented as a straight horizontal line, indicating the Coulomb efficiency was very stable. In contrast, the Coulomb efficiency of the traditional liquid LiBF4 battery fluctuated greatly, and the overall Coulomb efficiency was much lower. Thus, gel polymer lithium batteries will probably replace traditional liquid lithium batteries in the future. Figure9 shows the AC impedance plots of LiBF 4-EOEOEA 30% and LiBF4-EOEOEA 60% lithium batteries before and after cycling. The AC impedance plots were treated with Zview software through fitting, and the bulk resistance values of LiBF4-EOEOEA 30% and LiBF4-EOEOEA 60% lithium batteries were 19.12 and 45.36 Ω, respectively, in great match with the ionic conductivity shown in Figure5a. The diameters of the semicircles in the AC impedance plots represent the resistance values of lithium batteries. The resistance values of LiBF4-EOEOEA 30% and LiBF4-EOEOEA 60% lithium batteries before the cycling were 618.5 and 1137 Ω, respectively. Due to the passivation of lithium electrodes and gel electrolytes during the charging and discharging processes, the resistance values increased after 50 cycles. The values of LiBF4-EOEOEA 30% and LiBF4-EOEOEA 60% batteries were increased to 881.3 and 1663 Ω, respectively. The resistance of EOEOEA 30% was increased to a small extent, implying that the interface was relatively stable. In contrast, LiBF4-EOEOEA 60% contained a larger amount of unreacted monomers, and the internal passivation was more serious, increasing the contact resistance between gel polymer electrolyte and Li anode [6,35]. Polymers 2019, 11, x FOR PEER REVIEW 12 of 17

PolymersPolymers 20192019,, 1111,, 1296x FOR PEER REVIEW 1212 ofof 1717

Figure 8. (a) Specific discharge capacities and (b) Coulomb efficiencies of different EOEOEA lithium Figure 8. (a) Specific discharge capacities and (b) Coulomb efficiencies of different EOEOEA lithium batteries;Figure 8. ((ca)) specificSpecific discharge discharge capacit capacitiesies of and the ( bLiBF) Coulomb4-EOEOEA efficienc 30% ieslithium of different battery EOEOEA in the first lithium, 25th, batteries; (c) specific discharge capacities of the LiBF4-EOEOEA 30% lithium battery in the first, 25th, and 50th cycles; (d) discharging cycles of the LiBF4-EOEOEA 30% lithium battery at different rates; batteries; (c) specific discharge capacities of the LiBF4-EOEOEA 30% lithium battery in the first, 25th, and 50th cycles; (d) discharging cycles of the LiBF4-EOEOEA 30% lithium battery at different rates; (e) (e) specific discharge capacities and (f) Coulomb efficiencies of the LiBF4-EOEOEA 30% lithium and 50th cycles; (d) discharging cycles of the LiBF4-EOEOEA 30% lithium battery at different rates; specific discharge capacities and (f) Coulomb efficiencies of the LiBF4-EOEOEA 30% lithium battery battery and the liquid LiBF4 lithium battery. 4 and(e) s thepecific liquid discharge LiBF4 lithium capaci battery.ties and (f) Coulomb efficiencies of the LiBF -EOEOEA 30% lithium battery and the liquid LiBF4 lithium battery.

Figure 9. AC impedance plots of LiBF -EOEOEA 30% and LiBF -EOEOEA 60% gel lithium batteries Figure 9. AC impedance plots of LiBF4-EOEOEA 30% and LiBF4-EOEOEA 60% gel lithium batteries before and after 50 cycles.cycles. Figure 9. AC impedance plots of LiBF4-EOEOEA 30% and LiBF4-EOEOEA 60% gel lithium batteries before and after 50 cycles.

Polymers 2019, 11, 1296 13 of 17

For learning the changes in internal components of the LiBF4-EOEOEA 30% battery after 50 cycles, Polymers 2019, 11, x FOR PEER REVIEW 13 of 17 the battery was disassembled in the glove box at 5.5 MPa with the sealer, and then the LiFePO4 electrode, celluloseFor learning composite the changes membrane, in internal and components lithium electrode of the LiBF after4-EOEOEA the cycling 30% battery were after characterized 50 by SEM.cycles, the battery was disassembled in the glove box at 5.5 MPa with the sealer, and then the FigureLiFePO 10a4 exhibitselectrode, the cellulose original composite LiFePO 4membraneelectrode, and with lithium an uneven electrode surface after and the acycling maximum were particle characterized by SEM. diameter of about 1 µm. Figure 10b shows the microscopic morphology of LiFePO4 electrode before the batteryFigure cycling. 10a Clearly, exhibits the original surface LiFePO was coated4 electrode with with a layeran uneven of polymer surface and electrolyte a maximum due to the particle diameter of about 1 μm. Figure 10b shows the microscopic morphology of LiFePO4 electrode occurrencebefore of the EOEOEA battery cycl polymerization,ing. Clearly, the andsurface the was surface coated was with much a layer smoother of polymer than electrolyte the surface due to shown in Figurethe 10 occurrencea. Figure of10 EOEOEAc shows the polymerization LiFePO4 electrode, and the surface after batterywas much cycling. smoother After than 50 the cycles, surface the GPE membraneshown still in existed, Figure 1 and0a. Figure no small 10c shows particles the LiFePO were found4 electrode on the after surface, battery indicatingcycling. After that 50 sidecycles, reactions did nott occurhe GPE on membr this GPEane still membrane, existed, and which no small possessed particles stablewere found electrochemical on the surface performances., indicating that side reactions did not occur on this GPE membrane, which possessed stable electrochemical Figure 10d–f show the elemental mapping results of the surface of LiFePO4 electrode after performances. 50 cycles. The surface of the LiFePO4 electrode contains the characteristic elements F (Figure 10e) Figure 10d–f show the elemental mapping results of the surface of LiFePO4 electrode after 50 and B (Figure 10f) of LiBF -EOEOEA 30% GPE, which proves that the LiBF -EOEOEA 30% precursor cycles. The surface of4 the LiFePO4 electrode contains the characteristic elements4 F (Figure 10e) and B electrolyte(Figure can 10 bef) polymerized of LiBF4-EOEOEA on its30% surface, GPE, which and the proves LiBF that4-EOEOEA the LiBF4- 30%EOEOEA GPE 30% still precursor remained after 50 cycles.electrolyte can be polymerized on its surface, and the LiBF4-EOEOEA 30% GPE still remained after 50 cycles. Figure 10g,h show the EDX results of the LiFePO4 electrode cross section after 50 cycles. At the interface betweenFigure 10 activeg,h show species the EDX and results Al of foil, the theLiFePO characteristic4 electrode cross elements section after F and 50 cycl B (Figurees. At the 10h) in interface between active species and Al foil, the characteristic elements F and B (Figure 10h) in LiBF4-EOEOEA 30% GPE were detected, showing that the LiBF4-EOEOEA 30% precursor electrolyte LiBF4-EOEOEA 30% GPE were detected, showing that the LiBF4-EOEOEA 30% precursor electrolyte had completelyhad completely penetrated penetrated into into the the LiFePO LiFePO44 electrode and and had had polymerized polymerized to form to gel form electrolyte. gel electrolyte. It It is conjecturedis conjecture thatd that the the gel gel electrolyte electrolyte precursor precursor ha hadd a agood good wetting wetting performance performance on the on LiFePO the LiFePO4 4 electrode.electrode LiFePO. LiFePO4 can4 can be completely be completely wrapped wrapped by GPE GPE by by deep deep in-situ in-situ polymerization polymerization based on based on LiFePO4 electrode [18]. LiFePO4 electrode [18].

Figure 10. SEM images of the surfaces of (a) the original LiFePO electrode, (b) the LiFePO electrode Figure 10. SEM images of the surfaces of (a) the original LiFePO4 electrode4 , (b) the LiFePO4 electrode4 before thebefore cycling, the cycling and, (andc) the (c) LiFePOthe LiFePO4 electrode4 electrode afterafter 50 50 cycles cycles.. Elemental Elemental mapping mapping results results of the of the surface (surfaced–f) and (d–f EDX) and EDX results results of the of the cross cross section section ((gg,,h) of LiFePO LiFePO4 electrode4 electrode after after 50 cycles 50 cycles..

Figure 11 shows the XRD patterns of LiFePO4 electrode powder of the original, before and after 50 cycles. The XRD spikes of the original LiFePO4 electrode were observed at 2θ of 20.54◦, 25.39◦, 29.52◦, 35.39◦, and 36.36◦, while these spikes were weakened or even disappeared for the spent LiFePO4 Polymers 2019, 11, x FOR PEER REVIEW 14 of 17

Polymers 2019, 11, 1296 14 of 17 Figure 11 shows the XRD patterns of LiFePO4 electrode powder of the original, before and after 50 cycles. The XRD spikes of the original LiFePO4 electrode were observed at 2θ of 20.54°, 25.39°, 29.52°, 35.39°, and 36.36°, while these spikes were weakened or even disappeared for the spent electrode.LiFePO This is4 electrode because. This the is spentbecause LiFePO the spent4 LiFePOelectrode4 electrode after after cycling cycling was was tightly tightly wrapped wrapped by by the gel the gel electrolyte, proving the occurrence of in-situ polymerization on the LiFePO4 electrode. electrolyte, proving the occurrence of in-situ polymerization on the LiFePO4 electrode.

Figure 11.FigureXRD 11 patterns. XRD patterns of LiFePO of LiFePO4 electrode4 electrode powder powder of original, of original, before before and after and 50 cycles. after 50 cycles.

Because theBecause solid the content solid content in a gel in electrolytea gel electrolyte is lower is lower than than that that ina in solid a solid electrolyte, electrolyte, the the mechanical strength ofmechanical the former strength is not of as the high former as thatis not of as the high latter. as that Cellulose of the latter has. a Cellulose large length has a /diameterlarge ratio, length/diameter ratio, and in-situ polymerization on a cellulose membrane is beneficial to the and in-situmechanical polymerization strength of ongel electrolyte a cellulose composite membrane membrane is beneﬁcial [36,37]. to the mechanical strength of gel electrolyte compositeFigure 12amembrane shows the original [36,37 cellulose]. isolation membrane. The membrane was uneven on the Figuresurface, 12a shows and contained the original many voids cellulose that were isolation heterogeneously membrane. distributed The membrane. Figure 12b wasshows uneven the on the surface, andcomposite contained isolation many membrane voids of cellulose that were and heterogeneouslygel electrolyte after the distributed. cycling. The voids Figure have 12 beenb shows the completely filled, and the surface is smoother. After 50 cycles, no small particles were observed on compositethe isolation surface, which membrane indicates of that cellulose side reaction ands did gel not electrolyte occur during after the 50 the cycles. cycling. The voids have been completely ﬁlled,Figure and 12c the shows surface the SEM is smoother. images of After the original 50 cycles, lithium no electrode. small particles The original were lithium observed on the surface, whichelectrode indicates is uneven that on side the surface. reactions During did the not 50 occur cycles during of traditional the 50 liquid cycles. lithium batteries, Figurelithium 12c shows dendrite the grow SEMs. Some images types of ofthe polymer original electrolytes lithium electrode. can inhibit Thethe growth original of lithium lithium electrode dendrite [38]. Figure 12d shows the SEM image of the lithium electrode after 50 cycles. The surface is is unevensmooth, on the and surface. lithium During nanoparticles the 50 are cycles observed of traditional on the surface liquid, indicat lithiuming that batteries,this gel electrolyte lithium dendrite grows. Somecan inhibit types the of growth polymer of lithium electrolytes dendrite can [39]. inhibit the growth of lithium dendrite [38]. Figure 12d shows the SEM image of the lithium electrode after 50 cycles. The surface is smooth, and lithium nanoparticles are observed on the surface, indicating that this gel electrolyte can inhibit the growth of lithium dendritePolymers 2019 [39, 11]., x FOR PEER REVIEW 15 of 17

Figure 12.FigureSEM 12. images SEM images of ( a of) the(a) the original original cellulosecellulose membrane membrane and and(b) the (b )cellulose the cellulose composite composite isolation membraneisolation membrane after 50 after cycles, 50 cycles and (, c and) the (c original) the original lithium lithium electrode electrode and and ( d (d)) the the lithiumlithium electrode after 50 cycles.electrode after 50 cycles.

4.Conclusions

By heating EOEOEA and LiBF4 electrolyte precursors via a thermal curing method, we prepared a family of gel polymer electrolytes with high ionic conductivity (5.6 × 10−4 S/cm) at 30 °C , and wide electrochemical windows (4.65 V) at room-temperature and good thermal stability. The ionic conductivity decreased as the EOEOEA content increased, whereas the electrochemical window and thermal stability increased as the EOEOEA content increased. On the basis of the simple in-situ deep polymerization on the LiFePO4 electrode and cellulose membrane in the battery container on the cathode side, GPE generated on both the LiFePO4 electrode and cellulose membrane. Meanwhile, the contact between the gel electrolyte with LiFePO4 electrode and Li electrode, respectively, was enhanced, and the stability of gel electrolyte lithium was also enhanced. In addition, the growth of lithium dendrite was effectively inhibited. The capacity retention rate of the LiBF4-EOEOEA 30% lithium battery was 100% after 50 cycles under the condition of 0.1 °C, and the Coulomb efficiency surpassed 99%. This preparation approach provides scaffolds for the application of high-performance gel polymer electrolyte lithium batteries.

Funding: The authors would like to acknowledge the National Natural Science Foundation of China (Grant No.:51863006), and the Guangxi Natural Science Foundation of China (Grant No.:2018GXNSFAA138131).

Acknowledgments: The authors would like to acknowledge the National Natural Science Foundation of China (Grant No.:51863006), and the Guangxi Natural Science Foundation of China (Grant No.:2018GXNSFAA138131).

Conflicts of Interest: The authors declare no conflict of interest.

Polymers 2019, 11, 1296 15 of 17

4. Conclusions

By heating EOEOEA and LiBF4 electrolyte precursors via a thermal curing method, we prepared a family of gel polymer electrolytes with high ionic conductivity (5.6 10 4 S/cm) at 30 C, and wide × − ◦ electrochemical windows (4.65 V) at room-temperature and good thermal stability. The ionic conductivity decreased as the EOEOEA content increased, whereas the electrochemical window and thermal stability increased as the EOEOEA content increased. On the basis of the simple in-situ deep polymerization on the LiFePO4 electrode and cellulose membrane in the battery container on the cathode side, GPE generated on both the LiFePO4 electrode and cellulose membrane. Meanwhile, the contact between the gel electrolyte with LiFePO4 electrode and Li electrode, respectively, was enhanced, and the stability of gel electrolyte lithium was also enhanced. In addition, the growth of lithium dendrite was effectively inhibited. The capacity retention rate of the LiBF4-EOEOEA 30% lithium battery was 100% after 50 cycles under the condition of 0.1 ◦C, and the Coulomb efficiency surpassed 99%. This preparation approach provides scaffolds for the application of high-performance gel polymer electrolyte lithium batteries.

Author Contributions: W.D. proposed the project and prepared the manuscript; W.D. performed the syntheses and measurements of the samples; C.W. and Y.G. analyzed the data; S.W., L.Z. (Linmin Zou), Y.L. and L.Z. (Limin Zang) had valuable discussions and edited the manuscript. Funding: The authors would like to acknowledge the National Natural Science Foundation of China (Grant No.:51863006), and the Guangxi Natural Science Foundation of China (Grant No.:2018GXNSFAA138131). Acknowledgments: The authors would like to acknowledge the National Natural Science Foundation of China (Grant No.:51863006), and the Guangxi Natural Science Foundation of China (Grant No.:2018GXNSFAA138131). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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