A Superior Polymer Electrolyte with Rigid Cyclic Carbonate Backbone

May 10, 2017 - Qingdao Industrial Energy Storage Technology Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sc...
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A Superior Polymer Electrolyte with Rigid Cyclic Carbonate Backbone for Rechargeable Lithium Ion Batteries Jingchao Chai,†,§ Zhihong Liu,*,† Jianjun Zhang,†,§ Jinran Sun,‡ Zeyi Tian,‡ Yanying Ji,† Kun Tang,‡ Xinhong Zhou,‡ and Guanglei Cui*,† †

Qingdao Industrial Energy Storage Technology Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China ‡ College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, 266042 Qingdao, China § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The fabricating process of well-known Bellcore poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP)-based polymer electrolytes is very complicated, tedious, and expensive owing to containing a large amount of fluorine substituents. Herein, a novel kind of poly(vinylene carbonate) (PVCA)based polymer electrolyte is developed via a facile in situ polymerization method, which possesses the merits of good interfacial compatibility with electrodes. In addition, this polymer electrolyte presents a high ionic conductivity of 5.59 × 10−4 S cm−1 and a wide electrochemical stability window exceeding 4.8 V vs Li+/Li at ambient temperature. In addition, the rigid cyclic carbonate backbone of poly(vinylene carbonate) endows polymer electrolyte a superior mechanical property. The LiFe0.2Mn0.8PO4/graphite lithium ion batteries using this polymer electrolyte deliver good rate capability and excellent cyclability at room temperature. The superior performance demonstrates that the PVCA-based electrolyte via in situ polymerization is a potential alternative polymer electrolyte for high-performance rechargeable lithium ion batteries. KEYWORDS: lithium ion battery, polymer electrolyte, poly(vinylene carbonate), poly(vinylidene fluoride-hexafluoropropylene), in situ polymerization

1. INTRODUCTION Lithium ion batteries with high energy density and stable cyclability have attracted intensive attention in energy storage fields, such as portable devices, electric vehicles, and smart grids.1−3 Unfortunately, commercially available liquid electrolytes used in power batteries would lead to potential safety problems due to their low flash point. The safety issues are emerging prominently when researchers are pursuing higher energy power batteries.4,5 Currently, all-solid-state lithium batteries using solid electrolytes instead of the volatile and flammable carbonate-based liquid electrolytes are in great progress, which are considered to be promising in solving the safety problems.6−8 However, the major challenge to realize allsolid-state lithium batteries is to develop considerable ionic conductivity of solid electrolytes and simultaneously mitigate the interfacial impedance between the solid electrolytes and the electrodes. The insufficient ionic conductivity of all solid electrolytes at room temperature and poor interfacial contact between these solid materials hinders their rapid growth of allsolid-state batteries. Gel polymer electrolytes (GPE), possessing both important merits of organic liquid electrolytes and allsolid-state polymer electrolytes, deliver decent ionic conductivity at ambient temperature, considerable electrochemical © XXXX American Chemical Society

performance, and no leakage of liquid. Thus, as a compromise, developing GPE may be an alternative solution to balance the safety and the high-energy-density of lithium batteries at present.9−11 The well-known Bellcore-processed poly(vinylidene fluoridehexafluoropropylene) (PVdF-HFP) polymer electrolytes were extensively used in lithium ion batteries, which was fabricated by a solvent-induced phase inversion method.12,13 This process consumed a large amount of solvents, which was also very complicated and tedious. It was also reported that the C−F bonds in PVdF-HFP were not stable to lithium metal anode, and PVDF-HFP underwent α-hydrogen elimination under oxidizing condition or at high voltage.14 Besides, PVdF-HFP contains a large quantity of fluorine substituents, which makes it very expensive (above $30/kg) for future boosting the developments of power batteries. Therefore, it is highly desirable to develop alternative polymer matrixes at a low cost, environmental benignity, wide electrochemical window, and easy fabrication as well. Received: February 27, 2017 Accepted: May 10, 2017 Published: May 10, 2017 A

DOI: 10.1021/acsami.7b02844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

Some other polymer matrices have also been widely studied until now such as polyacrylonitrile (PAN), poly(ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), etc. However, in terms of mechanical strength, ionic conductivity, and electrochemical stability, these polymer electrolytes can hardly meet the comprehensive requirement of commercial lithium ion batteries, i.e., poly(methyl methacrylate)-based electrolyte suffers from poor mechanical properties,15 polyacrylonitrile-based electrolyte exhibits poor compatibility with lithium anode,16−18 and poly(ethylene oxide)-based electrolyte has a narrow electrochemical window below 4.0 V vs Li+/Li. More recently, great efforts have been made to develop novel polymer electrolyte matrices, such as poly(propylene carbonate),8,19 poly(ethylene carbonate),20−22 poly-α-cyanoacrylate,11,23 and so on. Most of those novel polymer electrolytes have higher ionic conductivity or a wider electrochemical window than the traditional polymer electrolytes. Zhang et. al reported that poly(propylene carbonate)-based all-solid polymer electrolyte had a considerable ionic conductivity of 10−4 S cm−1 at ambient temperature.8 Poly-α-cyanoacrylate-based gel polymer electrolyte could keep its electrochemical stability even at a high voltage of 5.0 V (vs Li+/Li).24 It was noted that all of the above polymer electrolytes were made from ex situ polymerization technology. Gel polymer electrolytes from ex situ polymerization were always tedious and environmentally unfriendly, which consumed a large amount of organic solvents to prepare the porous structure. Very recently, Kang et al. developed a kind of cyanoethyl poly(vinyl alcohol)-based polymer electrolyte with less interfacial resistance by in situ polymerization.6,25 Kim et al. and Cui et al. also reported an in situ poly(ethylene glycol) divinyl ether matrix for polymer electrolytes in lithium ion battery and sodium ion battery, respectively.26,27 In situ polymerization was demonstrated to be a simple but powerful technology for high-performance polymer electrolyte lithium batteries. Vinylene carbonate (VC) is a well-known additive in liquid electrolytes,28−30 which can be polymerized into poly(vinylene carbonate) (PVCA) as a main ingredient in the solid electrolyte interface (SEI) on the surface of anodes at low potentials or on the surface of cathode at high potentials.28,31,32 Recently, we reported a wide-electrochemical window PVCA-based solid polymer electrolyte with excellent interfacial compatibility for LiCoO2 cathode.33 The high-voltage LiCoO2/Li batteries using this solid polymer electrolyte displayed stable charge/discharge profiles, considerable rate capability, and excellent cycling performance at the elevated temperature of 50 °C. In addition, poly(vinylene carbonate) possesses a rigid cyclic carbonate backbone, endowing PVCA-based polymer electrolyte superior mechanical property, which is beneficial for the safety of lithium batteries. However, the insufficient ion conductivity of PVCAbased solid polymer electrolyte cannot satisfy the requirement of operating at ambient temperature and better rate input. Herein, a kind of PVCA-based gel polymer electrolyte was presented via an in situ polymerization method. The high energy density 2032-type LiFe0.2Mn0.8PO4/graphite battery and pouch-type LiNi0.6Co0.2Mn0.2O2/graphite battery using PVCAbased polymer electrolyte were explored in this paper. The superior performance of these lithium batteries demonstrated that PVCA-based polymer electrolyte possessed promising prospect toward future electric vehicle application.

Lithium difluoro(oxalate) borate (LiDFOB) is a common lithium salt with the combined chemical property of lithium bis(oxalate) borate (LiBOB) and lithium tetrafluoroborate (LiBF4).34−36 At the same time, LiDFOB could participate in the solid electrolyte film on graphite, which improved the cycle performance of lithium batteries.37 Thus, LiDFOB was chosen as lithium salt in this paper. The solutions of 1 M LiDFOB in VC and 1 M LiDFOB in ethylene carbonate/ dimethyl carbonate (EC/DEC, 1/1, v/v) were prepared under argon atmosphere. Varied amounts of the above two solutions were mixed, and the sample’s compositions are shown in Table S1. Azodiisobutyronitrile (0.2 wt %) (AIBN, vs VC) was added into the mixed solution. The electrolyte solutions were stored at 60 °C for 24 h and then 80 °C for another 2 h to obtain PVCA-based gel polymer electrolytes (hereafter referred to as PVCA-GPE). The static state contact angle was tested by a contact angle meter (JC2000, Shanghai zhongchen,). The infrared spectra were collected with a VERTEX 70 (Bruker) in the attenuated total reflectance method. X-ray diffraction data were collected at room temperature on a Bruker D8 diffractometer using Cu Kα radiation over the 5− 60°range. The thermal properties of polymer electrolyte were evaluated by a simultaneous thermal analyzer (TGA/DSC2 1600LF, METTLER TOLEDO). Sample was scanned from room temperature to 600 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Stress−strain curves were obtained by a tensile machine (TST-604BS) at a stretching speed of 10 cm min−1 with a sample about 1 cm wide and 8 cm long. The morphology of the samples was observed by field emission scanning electron microscopy (Hitachi S-4800). The ionic conductivity of PVCA-GPE between two stainless steels was tested via the electrochemical impedance spectroscopy (EIS) measurement in the frequency range from 1 to 106 Hz. Cellulose nonwoven (with a thickness of 25 μm) was used to separate electrodes in all cells in this paper. The ionic conductivity was calculated from σ = L/(R·S). Cyclic voltammetry curves of Li/PVCA-GPE/stainless-steel asymmetric cell were performed using an electrochemical workstation (VSP 300, Bio-Logic) in the potential range from −0.5 to 6.0 V at a scanning rate of 1.0 mV s−1. The electrochemical stability window of PVCA-GPE was determined by a linear sweep voltammetry experiment performed on a working electrode of stainless steel and a counter electrode of lithium metal at a scan rate of 1.0 mV s−1. The lithium ion transfer number of electrolyte was usually calculated from Bruce− Vincent−Evans equation:24,38 tLi+ = Iss/Io·(V − Io·Ro)/(V − Iss·Rss), where Io is the initial current and Iss is the steady-state current of the Li/PVCA-GPE/Li symmetrical cell after polarization for 10 000 s at an applied polarization voltage of 10 mV. Ro and Rss are the initial interfacial resistance and steady-state interfacial resistance after the polarization process, respectively. Interface impedance between PVCA-GPE and lithium metal was obtained by EIS measurements in the frequency range from 1 to 106 Hz. Those 2032-type coin cells are assembled by sandwiching the electrolyte between LiFe0.2Mn0.8PO4 cathode and graphite anode. Aluminum pouch cell was assembled by sandwiching the electrolyte between LiNi0.6Co0.2Mn0.2O2 cathode and graphite anode. LiFe0.2Mn0.8PO4 cathode, LiNi0.6Co0.2Mn0.2O2 cathode, and graphite anode were prepared in a conventional casting process by mixing 80 wt % active material, 10 wt % conductive carbon black, and 10 wt % PVCA dissolved in dimethylformamide (DMF). There are two reasons why PVCA was chosen as the binder instead of the conventional PVDF. On one hand, there are no fluorine elements in PVCA, which is of benefit to lower the cost of lithium ion batteries. On the other hand, the polymer electrolyte matrix in this paper is PVCA. PVCA-based binder would reduce the interfacial impendence between electrolyte and electrode. The loading density of LiFe0.2Mn0.8PO4 cathode and LiNi0.6Co0.2Mn0.2O2 cathode was about 2.6 mg cm−2, and the loading density of graphite anode was about 1.2 mg cm−2.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Physical Characterization of PVCAGPE. In situ polymerization would simplify the preparation B

DOI: 10.1021/acsami.7b02844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic diagram of the preparation process of PVCA-GPE. Photos of liquid electrolyte with varied amounts of VC before (b) and after (c) the polymerization process. (d) FT-IR adsorption spectra of lithium difluoro(oxalate) borate (LiDFOB), VC, LiDFOB-VC/EC/DMC, and PVCA-GPE.

Figure 2. (a) XRD pattern of PVCA-GPE. (b) TGA and DSC of PVCA-GPE. (c) Strain−stress curves of PVCA-GPE and PVdF-HFP-GPE. Contact angle measurement on PVCA (d) and PVdF-HFP (e) slice with LiDFOB-EC/DMC.

process of polymer electrolyte and generate a good interface compatibility between polymer electrolyte and electrode. The in situ polymerization process of PVCA-GPE is vividly illustrated in Figure 1a. In EC/DEC-based liquid electrolyte, VC can be polymerized into PVCA by a thermal radical initiator at elevated temperature. The photos of liquid electrolyte with varied amounts of VC before and after the polymerization process are shown in Figure 1b and 1c. It could be seen that the gel polymer electrolyte was kept immobile and seemed like solid state when the VC concentration was over 35%. As a compromise between the mechanical property and the ionic conductivity (Figure S1), the 40% of the VC volume fraction was considered to be the ideal value. Figure 1d displays the FT-IR adsorption spectra of LiDFOB, VC, LiDFOB-VC/

EC/DEC, and PVCA-GPE. The disappearance of the unsaturated C−H vibration band of VC at 3166 cm−1 is evidence of the polymerization reaction. In addition, in comparison with liquid LiDFOB-VC/EC/DEC, PVCA-GPE did not show obvious changes of the CO vibration band at 1823 cm−1 and C−O−C vibration band at 1021 cm−1, which implied that the polymerization of VC would not affect the chemical structure of OC−O−C. As we know, in polymer electrolyte cation transport is related to segment motion of polymer chains, which means that a low degree of crystallinity is beneficial for improved ionic conductivity. The X-ray diffraction pattern of PVCA-GPE (Figure 2a) showed that there was a broad diffraction peak at about 2θ = 23°, which meant that PVCA-GPE was completely C

DOI: 10.1021/acsami.7b02844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Temperature-dependent ionic conductivity of PVCA-GPE. (b) Polarization curves obtained by chronoamperometry for the Li/PVCAGPE/Li symmetrical cell at 25 °C. (Inset) Nyquist plots of the symmetrical cell. (c) Cyclic voltammetry curves and (d) linear sweeping voltammetry curve of Li/PVCA-GPE/ss asymmetrical cell at 25 °C.

flame (Figure S4), in favor of the superior safety performance of PVCA-GPE. 3.2. Electrochemical Properties. A major challenge to the development of polymer electrolyte is to improve its ionic conductivity at room temperature. The polymer electrolyte with a high ionic conductivity would endow the batteries with a better power performance at a high charging/discharging current density. The temperature-dependent ionic conductivity of PVCA-GPE and liquid electrolyte (with PP separator) is shown in Figure 3a. At 25 °C, the ionic conductivity of PVCAGPE was up to 5.59 × 10−4 S cm−1, which was on the same order of magnitude of the LiDFOB-EC/DMC liquid electrolyte (9.56 × 10−4 S cm−1). In addition, the ionic conductivity of PVCA-GPE from in situ generation was higher than that of PVCA-GPE from ex situ generation, which testified that in situ polymerization technology possessed advantages of both facile preparation and higher ionic conductivity. The temperature dependence of ionic conductivity of PVCA-GPE can be fitted by the Vogel−Tamman−Fulcher (VTF) empirical equation:6 σ = AT1/2 exp(−Ea/(R(T − To))) (seen Figure S5), where A is the conductivity pre-exponential factor, corresponding to the number of carrier ions, and Ea represents the activation energy for conduction which is related to polymer segmental mobility. The fitting activation energy for PVCA-GPE was 7.05 kJ mol−1, which was slightly lower than that for PVdF-HPF-based polymer electrolyte.42,43 Lithium ion transfer number is also an important factor for both liquid electrolyte and polymer electrolyte, because a large lithium ion transfer number would reduce electrode polarization and suppress undesirable side reactions on the electrodes. From the Bruce−Vincent−Evans equation, the lithium ion transfer number of PVCA-GPE was obtained to be 0.34 at room temperature (Figure 3b and Table S2), which was slightly higher than that of liquid electrolyte (0.33).44 The electrochemical stability of PVCA-GPE was examined using cyclic voltammetry on a stainless-steel working electrode

amorphous, resulting in an increase of ionic conductivity. The thermal behavior of the polymer electrolyte was investigated by TGA and DSC (Figure 2b). Mass loss above 120 °C could be ascribed to the evaporation of EC/DEC. The DSC curve of PVCA-based gel polymer electrolyte showed two small endothermic peaks centered at about 120 and 148 °C, which might correspond to evaporation of DEC and EC, respectively. It can be found that the mass retention of PVCA-GPE at 200 °C was 83.8%, which was much better than traditional liquid electrolyte.6,39 A sharp mass loss around 350 °C was due to the thermal decomposition of PVCA. Figure 2c shows the mechanical property of PVCA-GPE. PVdF-HFP-based gel polymer electrolyte (PVdF-HFP-GPE) was compared as the reference. The tensile strength of PVCA-GPE was 41.1 MPa, which was higher than that of PVdF-HFP-GPE (28.6 MPa). The Young’s modulus of PVCA-GPE was obtained by an atomic force microscope, which has been reported by Guo et.al.40,41 As can be seen from Figure S2, PVCA-GPE displayed a considerable Young’s modulus of 2 GPa due to its rigid cyclic carbonate backbone. The high Young’s modulus of polymer electrolyte is beneficial for suppressing the growth of lithium dendrite, which indicates that PVCA-GPE would provide higher safety characteristics for lithium batteries than PVdFHFP-based polymer electrolyte. The contact angle on PVCA slice with LiDFOB-EC/DMC (18 o, Figure 2d) was smaller than that on PVdF-HFP (22°, Figure 2e), suggesting that there was a better wetting between PVCA and liquid electrolyte. In addition, the efficiency of the GPE to hold liquid electrolyte can be traced by the relative absorption ratio. The change in the relative absorption ratio of the electrolyte with time is depicted in Figure S3. It could be found that after being stored at 80 °C for 24 h, the relative absorption ratio of PVCA-GPE remained 80%, much higher than that of PVdF-HFP-GPE (52%), suggesting that PVCA-GPE had a better uptake capacity for liquid electrolyte. Further, the combustion experiment verified that PVCA-GPE would extinguish after removing from a fire D

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Figure 4. Time-dependent interface impedance of lithium metal asymmetric cells assembled with PVCA-GPE obtained from (a) in situ polymerization and (b) ex situ polymerization. (c) Lithium plating−stripping galvanostatic cycling of the Li/PVCA-GPE/Li symmetric cell cycled for up to 1000 h with a constant current density of 0.1 mA cm−2.

transport capability across PVCA-GPE and the lithium metal interface. Figure 4c shows 1000 h of cycling at a current density of 0.1 mA cm−2 periodically charging and discharging for 2 h with an area capacity of 0.2 mAh cm−2. The symmetric cell presented a stable voltage at 40 mV, which confirmed the relatively stable lithium/electrolyte interphase in the cell. The stable interphase on lithium anode would be beneficial for longer cycling performance of lithium ion batteries. 3.3. Performance of the LiFe0.2Mn0.8PO4/PVCA-GPE/ Graphite Battery. A full battery composed of LiFe0.2Mn0.8PO4 (cathode), graphite (anode), and PVCA-GPE is illustrated in Figure 5a. Typical cross-section SEM images of the LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery are shown in Figure 5b. As shown in the cross-section SEM image, it could be easily found that the sandwiched battery consisted of three layers: the top layer was LiFe0.2Mn0.8PO4 cathode with a thickness of 45 μm; the inner one was PVCA-GPE with a thickness of 23 μm; the bottom layer was graphite anode with a thickness of 19 μm. In addition, cross-section SEM images clearly demonstrated the close interfacial contact between the electrode and the electrolyte. The EDS mapping of P, Mn, Fe, F, O, and C is shown in Figure 5c.The EDS demonstrated the homogeneous distribution of elements in corresponding materials, i.e., P, Mn, Fe, F, O, and C, in the LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery. The rate capability and cycling performance of LiFe0.2 Mn0.8 PO 4 /PVCA-GPE/Li batteries and graphite/ PVCA-GPE/Li batteries are depicted in Figure S6. The capacity retention of LiFe0.2Mn0.8PO4/PVCA-GPE/Li batteries was found to be 95.4% after 100 cycles. Graphite/PVCA-GPE/ Li batteries presented a discharge capacity of 292 mAh g−1 after 100 cycles at the current density of 0.5 C, demonstrating a good capacity retention. Those charging/discharging curves indicated that LiFe0.2Mn0.8PO4-based/PVCA-GPE/Li batteries and graphite/PVCA-GPE/Li batteries displayed an excellent rate capability and cycling performance, which was attributed to

and lithium metal foil as a reference electrode (Figure 3c). For the first cycle, an obvious reduction current was observed from −0.10 V in the negative scan, which was due to a lithium plating process on the stainless-steel electrode. In the following positive scan, a peak at 0.45 V was found, corresponding to the lithium stripping process.6 It was noted that the reversibility of lithium plating−striping was quite consistent with high Coulombic efficiency. The anodic stability of PVCA-GPE at room temperature was further studied by linear sweep voltammetry at room temperature at a scan rate of 1 mV s−1. Figure 3d displays the current as a function of voltage for PVCA-GPE and liquid electrolyte. PVCA-GPE possesses an oxidation decomposition onset potential on 4.8 V, which was higher than that of pristine liquid electrolyte (4.5 V). The improved electrochemical stability of the developed PVCAGPE verified the better performance for lithium batteries. The interface between electrolyte and electrode plays an important role for cycling performance in lithium ion batteries.45−47 The interfacial compatibility could be analyzed by detecting the time evolution of the impedance spectra of a symmetric lithium coin cell under open circuit. Figure 4a and 4b shows the interface impedance variation of Li/PVCA-GPE/ Li cells from in situ polymerization and ex situ polymerization, respectively. The preparation of PVCA-GPE obtained from ex situ polymerization was a solution casting process.48,49 As can be seen from Figure 4, the interface impedance between PVCAGPE and lithium metal from the in situ polymerization process increased slightly after 30 days, from 190 to 320 Ω (Figure 4a), while in the case of ex situ polymerization (Figure 4b), the interface impedance increased significantly after 30 days: from 1400 to 5800 Ω, indicating that the in situ polymerization process had a distinct advantage over the ex situ polymerization process. The large increases of interface impedance were likely due to the poor contact between lithium metal and polymer electrolyte.50,51 The lithium plating and stripping experiment was used to evaluate the interfacial impedance and lithium ion E

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decreased with increasing current density, which was due to the ohmic polarization of lithium ion batteries.52,53 The discharge capacity of LiFe0.2Mn0.8PO4/PVCA-GPE/graphite lithium ion batteries at a current density of 0.1 C was 137 mAh g−1, which was slightly lower than that of the traditional liquid electrolyte. However, LiFe0.2Mn0.8PO4/PVCA-GPE/ graphite lithium ion batteries exhibited superior cycling stability with a 88.7% capacity retention and nearly 100% columbic efficiency over 1000 cycles at a current density of 1.0 C (Figure 6c). It was noted that LiFe0.2Mn0.8PO4/PVCA-GPE/graphite lithium ion batteries showed a superior rate performance (90 mAh g−1) even at a high charging/discharging current density (8.0 C). To gain a further understanding of the excellent cycling performance of LiFe0.2Mn0.8PO4/PVCA-GPE/graphite lithium ion batteries, the ac impedance spectra of fully charged cells after the 2nd, 10th, 50th, and 100th cycles were analyzed. These Nyquist plots were fitted using the equivalent circuit shown in Figure 6d. According to the literature,54,55 Rb represents the bulk resistance. The semicircle at the highfrequency range is assigned to diffusion of Li+ ions through the solid electrolyte interface on active materials (Rf) and its capacitance in parallel. Since the semicircle is depressed, a constant phase element (CPE) was used in the equivalent circuit. The semicircle observed at the medium-to-lowfrequency region was related to the charge transfer resistance between the active materials and polymer electrolyte (Rct) and double-layer capacitance. The low-frequency region reflected the diffusion of Li+ ions in the solid phase (Zdiff) and the occupation of Li+ ions into the inserted sites (CPEdiff). In the case of LiFe0.2Mn0.8PO4/PVCA-GPE/graphite batteries, there was almost no change for Rb, which meant that the inner structure of PVCA-GPE had not changed during the charge/ discharge process for a long cycle life. In addition, Rf and Rct increased indistinctively even after 100 cycles, which suggested

Figure 5. (a) Schematic illustration of the LiFe0.2Mn0.8PO4/PVCAGPE/graphite battery. (b) Typical cross-section SEM images of LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery. (c) Energy-dispersive X-ray spectroscopy (EDS) of cross-section SEM of the LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery.

the high ionic conductivity and electrochemical interfacial compatibility of PVCA-GPE. The electrochemical performance of LiFe0.2Mn0.8PO4/graphite-based lithium ion battery was evaluated in terms of the charging/discharging capacity and rate capability. The rate capacity of LiFe0.2Mn0.8PO4/graphite-based lithium ion batteries that were charged and discharged at varied current densities ranged from 0.1 to 8.0 C (1.0 C = 0.15 mA cm−2) under a voltage range of 2.5−4.35 V. Figure 6a shows the charge/discharge profiles of lithium ion batteries at varied current densities, and the discharge capacities are given in Figure 6b. The discharge capacity of the cells gradually

Figure 6. Battery performance of the LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery. (a) Charge/discharge profiles and (b) rate capability at varied rates (1.0 C = 0.15 mA cm−2) with a voltage range of 2.5−4.35 V. (c) Cycling performance of LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery at a current density of 1.0 C. (Inset) Corresponding charge/discharge profiles. (d) Nyquist plots of the LiFe0.2Mn0.8PO4/PVCA-GPE/graphite battery at varied cycles. F

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that there was a stable interface between polymer electrolyte and electrodes. These superior performance demonstrated that the PVCA-based electrolyte is an alternative polymer electrolyte for high-performance rechargeable lithium ion batteries. In order to further validate the universal reliability of PVCAGPE, LiNi0.6Co0.2Mn0.2O2/PVCA-GPE/graphite pouch-type batteries with a capacity of 30 mAh were assembled. The charging/discharging profiles at a voltage range of 3.0−4.2 V are depicted in Figure S7a. The battery displayed stable charging/discharging profiles, which indicated that there was no phase transformation during the charging/discharging process.56 The pouch-type battery could light the purple LED lamp in a normal state (Figure S7b) and in a bended state (Figure S7c). It is noteworthy that the aluminum pouch cells continued to power the lamp after being cut a half away (Figure S7d). The whole cutoff process is displayed in Figure S8. Interestingly, the detached corner of the LiNi0.6Co0.2Mn0.2O2/PVCA-GPE/ graphite cell was successfully able to power the LED lamp. To find the internal configuration of LiNi0.6Co0.2Mn0.2O2/ PVCA-GPE/graphite aluminum pouch batteries, a cell was disassembled, and the photograph is shown in Figure S7f. It is clear that the sandwich-like battery had a superior interfacial compatibility in which electrodes and PVCA-GPE were kept closely together. Thus, they could have a superior interfacial compatibility.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +8653280662745. *E-mail: [email protected]. Tel.: +8653280662746. ORCID

Guanglei Cui: 0000-0002-8008-7673 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (51625204), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010105), and the “135” Projects Fund of the CAS-QIBEBT Director Innovation Foundation. We thank Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, for fruitful help.

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REFERENCES

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4. CONCLUSION In this article, PVCA was presented as an alternative polymer matrix in gel polymer electrolyte, which showed an enhanced tensile strength and Young’s modulus (2 GPa) than known Bellcore-processed PVdF-HPF-based polymer electrolyte. In addition, PVCA-GPE from in situ polymerization technology endows the lithium ion batteries a lower interfacial resistance. The as-prepared PVCA-GPE displayed a decent ionic conductivity of 5.59 × 10−4 S cm−1 at ambient temperature and a wide electrochemical window over 4.8 V vs Li+/Li. Furthermore, LiFe0.2Mn0.8PO4/graphite full batteries employing PVCA-GPE delivered excellent capacity retention (88.7%) after 1000 cycles at 1.0 C and outstanding rate capability (90 mAh g−1 at 8.0 C). Nyquist plots of the lithium ion battery at varied cycles indicated that there was a stable interface between polymer electrolyte and electrodes. In addition, the LiNi0.6Co0.2Mn0.2O2/PVCA-GPE/graphite aluminum pouch cell testified the safety of PVCA-GPE in practical application. These results demonstrated that the PVCA as polymer electrolyte provided an effective solution for next-generation highperformance lithium ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02844. VC volume fraction-dependent ionic conductivity, Young’s modulus mapping, relative absorption radio, combustion experiment, VTF fitting results, charge/ discharge performance, safety performance, varied volume fraction of VC in liquid electrolytes, lithium ion transference number, and fitting results of ac impedance spectra (PDF) G

DOI: 10.1021/acsami.7b02844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b02844 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX