Superior Blends Solid Polymer Electrolyte with Integrated Hierarchical

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36886-36896

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Superior Blends Solid Polymer Electrolyte with Integrated Hierarchical Architectures for All-Solid-State Lithium-Ion Batteries Dechao Zhang,† Long Zhang,*,† Kun Yang,‡ Hongqiang Wang,§ Chuang Yu,⊥ Di Xu,† Bo Xu,† and Li-Min Wang† †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, China The Development and Reform Commission of Zhangjiakou City, Zhangjiakou, Hebei 075000, China § College of Chemistry & Environmental Science, Hebei University, Baoding, Hebei 071000, China ⊥ Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, Delft 2629 JB, The Netherlands ‡

S Supporting Information *

ABSTRACT: Exploration of advanced solid electrolytes with good interfacial stability toward electrodes is a highly relevant research topic for all-solid-state batteries. Here, we report PCL/SN blends integrating with PAN-skeleton as solid polymer electrolyte prepared by a facile method. This polymer electrolyte with hierarchical architectures exhibits high ionic conductivity, large electrochemical windows, high degree flexibility, good flameretardance ability, and thermal stability (workable at 80 °C). Additionally, it demonstrates superior compatibility and electrochemical stability toward metallic Li as well as LiFePO4 cathode. The electrolyte/electrode interfaces are very stable even subjected to 4.5 V at charging state for long time. The LiFePO4/Li allsolid-state cells based on this electrolyte deliver high capacity, outstanding cycling stability, and superior rate capability better than those based on liquid electrolyte. This solid polymer electrolyte is eligible for next generation high energy density all-solidstate batteries. KEYWORDS: solid polymer electrolytes, all-solid-state lithium-ion batteries, succinonitrile, poly(ε-caprolactone), poly(acrylonitrile), interfacial stability

1. INTRODUCTION

Poly(ε-caprolactone) (PCL) as an ester-based polymer is processable, cost-effective, and biocompatible.20 The biodegradable merit is beneficial to reduction in environmental problems. More important, the low glass transition temperature (Tg = −60 °C) and large electrochemical stability window up to 5 V make PCL a promising polymer electrolyte for LIBs.21−25 From the structural view, the ester oxygen can complex with Li+ in lithium salts and thereby promote ion transport driven by segmental motion of the polymer chain. The amorphous phase with activated polymer chain segments above the glass transition temperature is generally considered to be favor for ion transportation in contrast to the crystalline regions.26 However, similar to poly(ethylene oxide), PCL is semicrystalline at room temperature and its ester oxygen is Lewis base, thus limiting its ionic conductivity. Succinonitrile (SN) exhibits a single plastic phase in the temperature range from −35 °C to its melting point (Tm), which allows a trans−gauche isomerism structure involving molecule rotation for the central C−C bonds of SN. SN-based electrolytes are thus achieve the room temperature ionic

Lithium-ion batteries (LIBs) have been widely used in portable devices because of the advantages of large specific capacity, high voltage, long cycle life, and low self-discharge. However, increasing demands for higher energy density and large-scale batteries boosted by smart devices have raised safety concerns due to the instability of organic liquid electrolytes and the hazards of electrolyte leakage.1−6 In addition, post LIBs require metal anodes instead of graphite anodes currently used, which may enlarge the safety issues regarding the formation of Li dendrites.7,8 All-solid-state (ASS) battery with substitute of liquid electrolyte by solid electrolyte (SE) may overcome this issue and in parallel accompany with the advantages of wide electrochemical windows, broad operating temperature range, and direct-stacking technique.9−11 Polymer electrolytes (polymer-Li salt complexes) are one series of the most promising solid electrolytes because of relative high ionic conductivity, flexibility, processability, and compatibility with current manufacturing technique.6,7,12−18 Typically, ether-, nitrile-, and ester-based polymer electrolytes have been intensively studied.14,19 Although many efforts have been made, their room temperature ionic conductivity and electrochemical performance still need to be improved. © 2017 American Chemical Society

Received: August 15, 2017 Accepted: October 6, 2017 Published: October 6, 2017 36886

DOI: 10.1021/acsami.7b12186 ACS Appl. Mater. Interfaces 2017, 9, 36886−36896

Research Article

ACS Applied Materials & Interfaces

Ar-filled glovebox (H2O, O2 ≤ 1 ppm). For characterization purposes, PCL, SN, and PCL/SN films were prepared by same solutionevaporation method above-mentioned. PCL/SN-SPE polymer electrolyte similar to PPS-SPE but without PAN-skeleton was also prepared for comparison. 2.3. Batteries Assembling of LiFePO4/PPS-SPE/Li Cell. The cathodes were prepared by mixing 70 wt % LiFePO4 active materials, 10 wt % carbon black, 15 wt % PCL/SN/LiTFSI (20/10/9 wt. ratio), and 5 wt % polyvinylidene fluoride (PVDF) binder dissolved in NMethyl pyrrolidone (NMP). The resulting slurry was pasted onto carbon-coated aluminum foil and dried at 100 °C for 12 h under vacuum. The mass loading of the active material (LiFePO4) on the electrodes is ∼1.9 mg/cm2. The electrode was well prepared with components homogeneously distributed (Figure S1). The electrochemical properties were investigated using CR2032 coin-type cells assembled in an argon-filled glovebox using lithium−metal foil as the counter/reference electrode, and PPS-SPE as solid electrolyte. For comparison, liquid electrolyte based cells were also assembled with LiPF6 dissolved carbonate-based electrolyte (code No.: LBC305−1, Kejingstar LTD) and Celgard 2400 as a separator. The preparation processes for PPS-SPE hierarchical architectures and assembling cell are illustrated in Figure 1.

conductivity over 1 mS/cm.14 SN has been broadly used as electrolyte matrix or addictive because of the additional advantages including: large electrochemical window, high thermal stability, or copper corrosion inhibition.14,27−43 However, SN-based electrolytes exist the issues of poor mechanical strength, poor catholic stability, and incompatibility with Li metal.14,27,29,44 These shortcomings may be mitigated by blending, cross-linking, and copolymerization with polymers and forming cathode/solid electrolyte interface.26 SN as an additive to polymer matrix is an effective way to maintain good mechanical properties and meanwhile to get fast ion transport.14,26 Polymer blending is a facile and cost-effective method to develop new polymeric materials with superior performance.45 The strategy of this work is to incorporate SN as a plastic additive blending with PCL (PCL/SN) to prepare a biocompatible/biodegradable solid polymer electrolyte (SPE) with high ionic conductivity and particularly good compatibility and stability against metallic lithium electrode. Lithium bistrifluoromethanesulfonimide (LiTFSI) is chosen as Li salt because of its large anionic radius and low dissociation energy, leading to a high conductivity.46 Increase of SN or LiTFSI concentration is favorable to better ion transport but may also decrease the strength of the SPE. Frameworks composed of electrospun poly(acrylonitrile) (PAN) or nonwovens are thus adopted to further improve the mechanical performance.47 Additionally, the ratio between PCL and SN/LiTFSI is optimized to reach a compromise between the ion transport and the mechanical performance. Herein, the fabricated PANsupported PCL-SN-based SPE with LiTFSI salt (PPS-SPE) is mechanical robust and high degree flexible. Fourier transform infrared (FTIR) spectra and the X-ray diffraction (XRD) profiles reveal the coordination/reaction inside PPS-SPE, which have modified the −CN, O, and −O− bonds. The ion transport is thus significantly enhanced. Moreover, the interfacial behavior between electrolyte/electrode is well characterized. PPS-SPE exhibits superior compatibility and electrochemical stability toward both positive and Li negative electrodes. The assembled ASS cells employing PPS-SPE as solid polymer electrolyte illustrate outstanding electrochemical performance.

Figure 1. Schematic illustration for the synthesis route of PPS-SPE.

2.4. Materials Characterization. XRD was performed using a Rigaku D/MAX-2500/PC (Cu Kα, 40 kV 200 mA). FTIR spectra was recorded with a E55+FRA106 instrument at ambient temperature from 600−4000 cm−1. Differential scanning calorimetry (DSC) was measured using a PerkinElmer DSC8000 with a scan rate of 5 °C/min. Thermogravimetric analysis (TGA) measurements were carried out on a Netzsch STA thermoanalyzer in argon atmosphere. Morphological characterizations were taken with a scanning electron microscope (SEM, Hitachi S-4800 II FESEM) equipped with an energy dispersive spectrometry (EDS) instrument. 2.5. Electrochemical Characterization. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 2 MHz−0.1 Hz at room temperature using a Princeton P4000 impedance analyzer. For ionic conductivity measurements, PPS-SPE film was sandwiched between two stainless steel (SS) as blocking electrodes. To evaluate the compatibility of PPS-SPE toward lithium metal, a Li/PPS-SPE/Li symmetric cell was examined by measuring time evolution of EIS at ambient temperature. To evaluate the electrochemical stability of PPS-SPE toward lithium metal, lithium plating-stripping galvanostatic cycling test was performed on a Li/PPS-SPE/Li symmetric cell for 600 h with a constant current density of 0.1 mA/cm2. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) of PPS-SPE were tested on Li/PPS-SPE/SS cells using Princeton P4000 at a scanning rate of 1 mV/s, respectively, where SS is a working electrode and Li counter/reference electrodes. Constant voltage impedance measurement was performed (Princeton P4000) to evaluate the stability of PPS-SPE against the electrodes at high voltage with a LiFePO4/PPS-SPE/Li cell.48 The cell was first charged to a scheduled voltage (4.2 and 4.5 V, respectively) by a constant current and then maintained the voltage for potentiostatic charging; meanwhile, AC impedance measurements were performed

2. EXPERIMENTAL SECTION 2.1. Fabrication of Electrospun Fiber Skeleton. PAN (Mw = 150 000, Aldrich) was dissolved in N,N-dimethylformamide (DMF) with the weight ratio of 7% under continuous stir at 50 °C for 12 h. The resultant solution was spun into fiber membrane via a positively charged syringe nozzle at 15 kV with a flow rate of 0.015 mL/min and a distance of 12 cm from the nozzle to the aluminum foil collector. The obtained PAN electrospun fiber skeleton (PAN-skeleton) was dried at 80 °C for 12 h under vacuum before further use. 2.2. Synthesis of PPS-SPE. PCL (Mn = 80 000, Aldrich), SN (99%, J&K Chemical), and LiTFSI (99%, J&K Chemical) were used without further purification. The detailed component optimizations for SN and LiTFSI were described in the following main text. In a typical procedure, the optimized component was weighted according to a mass ratio of PCL/SN/LiTFSI = 20:10:9. PCL and SN were first dissolved into anhydrous acetonitrile with a magnetic stirring at 60 °C for 2 h to form a homogeneous solution. Then LiTFSI was added into the solution and continuously stirred for 12 h. The resultant solution precursor was injected into the electrospun PAN fiber membrane and casted evenly on a Teflon plate. PPS-SPE was obtained by evaporating the solvent at 60 °C for 6 h in Ar atmosphere and followed by another evaporation at 60 °C for 24 h under vacuum. The thickness of the resultant PPS-SPE was ∼45 μm. All procedures were carried out in an 36887


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ACS Applied Materials & Interfaces for selected time from 4 MHz to 0.01 Hz by applying a potential of 10 mV. Lithium ion transference number (tLi+) of PPS-SPE was evaluated by a steady-state technique involving a combination of AC impedance and DC-bias measurements on a Li/PPS-SPE/Li cell. A small DC potential with ΔV = 10 mV was applied. The ac impedance measurements were carried out, in the frequency range of 100 kHz−0.1 Hz, at initial state before potentiostatic polarization as well as at steady state after the polarization current reaching a constant. tLi+ was calculated with the following equation:49

increases with increasing LiTFSI (or SN) concentrations, reaches a maximum, and then decreases. The highest value is achieved at 30 wt % for LiTFSI and 33 wt % for SN, respectively. The conductivity depends on the carrier concentration and carrier mobility. When LiTFSI content is below 30 wt %, the salt is fully dissociated and the number of Li+ increases with increasing LiTFSI concentration. Further increase of LiTFSI results in redundant dissociated Li+ and TFSI−, which can form neutral contact ion pairs to reduce the number of mobile ions.50 The ionic conductivity is therefore reduced. The decreased ionic conductivity with SN content over 33% can be explained by the presence of excess SN that is inactive as an insulator.40 Figure 2b illustrates the ionic conductivities of the polymer electrolytes based on different frameworks. PAN-based SPE achieves the highest value, though the difference is not significant. Additionally, the polymer frameworks including the cellulose filter paper and the polyester nonwoven fabric are ion insulators. Therefore, ion transport of these solid polymer electrolytes mainly depends on the PCL/SN-Li salt system. Overall, the optimized component for PPS-SPE is PCL/SN/LiTFSI = 20:10:9 (wt. ratio) along with PAN-skeleton, which is designated as PPS-SPE in the following discussion. Different batches of PPS-SPE show good repeatable ionic conductivity (Figure S2). 3.2. Structural and Physical Properties. The morphology images obtained from SEM or digital camera are displayed in Figure 3. PAN-skeleton (Figure 3a) is composed of randomly oriented fibers with a diameter size of 150 nm. The distribution of the fibers is loose and the abundant spare space in the skeleton is favorable for immersion of polymer solution. Figure 3b displays the surface image of PPS-SPE. PAN-skeleton is completely covered by PCL-SN-based solid polymer. A flat and homogeneous surface free of pore is observed from SEM, though it is somewhat rough. The compact structure is further verified by the cross-section observation cutting from detached LiFePO4/PPS-SPE/Li cell after peeling off Li sheet. As can be seen from Figure 3c, PCLSN-based polymer completely fills in PAN-skeleton and is absence of pore. It can hardly differentiate the skeleton from the filler. The thickness of PPS-SPE is about 45 μm. The crosssection image also shows a tight contact between PPS-SPE and the positive electrode, indicating a small interface resistance and a favorable lithium ion transportation that are beneficial to a good electrochemical performance of ASS LIBs. Figure 3d is the photographs of PPS-SPE presenting a dry, uniform, and flexible film under a folded and rolled and unrolled states. The films are fully recovered after folding and rolling without trace of impress, indicating an excellent flexibility and mechanical capability. Although the modulus of PPS-SPE is not evaluated, the film is pretty strong stretched manually by hand. This dry, dense, and strong SPE is believed to be beneficial to suppress lithium dendrite growth and solve the safety risk of lithium batteries. The structural evolution of PPS-SPE is characterized by FTIR and XRD to investigate the changes of the various functional groups during synthesis. Figure 4a shows the FTIR spectra of SN, PCL, PCL/SN, and PPS-SPE. The magnification of the selected region is displayed in Figure 4b. In both the functional group region and the fingerprint region, the peak positions of SN and PCL are in line with the results previously reported.31,51 The major characteristic peaks at 1429, 2256, and 2990 cm−1 for SN are assigned to CH2, CN, and C−H, respectively, and peaks at 1180, 1730, 2865, and 2945 cm−1 for

t Li + = I ssR bss(ΔV − I oR i o)/I oR b0(ΔV − I ssR i ss) where I, Rb, and Ri represent the direct current, bulk resistance, and resistance of the passivation layers, respectively. The superscripts of “0” and “SS” represent the initial and steady state, respectively. The galvanostatic charge and discharge of the LiFePO4/PPS-SPE/ Li cells between 2.5 and 4.2 V were performed on a LAND cell test instrument at ambient temperature and 45 °C. The EIS of the cells were carried out on a Princeton P4000 electrochemical workstation from 100 kHz to 0.01 Hz by applying a potential of 10 mV.

3. RESULTS AND DISCUSSION 3.1. Componential Optimization of PPS-SPE. The component of PPS-SPE is optimized through three ways including optimization of the concentration for LiTFSI and SN, respectively, and skeleton selection. The ionic conductivities (σ) of the PPS-SPE specimens with various LiTFSI and SN concentrations are shown in Figure 2a. The ionic conductivity

Figure 2. (a) Ionic conductivity as a function of SN and LiTFSI concentration, respectively. (b) Ionic conductivity of SPEs using different skeletons. 36888


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Figure 3. Typical SEM images for (a) electrospun PAN-skeleton, (b) surface of the PPS-SPE film, and (c) cross-section of PPS-SPE together with the LiFePO4 electrode. (d) Optical images of the PPS-SPE film.

Figure 4. (a) FTIR spectra of SN, PCL, PCL/SN, and PPS-SPE, and (b) their magnification. (c) XRD patterns of SN, PCL, PCL/SN, PPS-SPE, and PPS-SPE after remelting at 70 °C and cooling. (d) DSC profiles of SN, PCL, and PPS-SPE. 36889


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active. Table 1 lists Tm, Tg, and the melting enthalpy (ΔHf) obtained from the DSC curves. PPS-SPE shows lower ΔHf than

PCL are assigned to C−O−C, CO, CH3, and CH2, respectively. Although no new peak or shift of peak position is observed after blending PCL and SN, the peaks at 1730 and 2256 cm−1 are broadened and a shoulder (1689 cm−1) next to 1730 cm−1 appears. This result indicates that reaction happens, and the CO and CN bonds varied after blending. The modification of the bonds after blending is also seen in other polymer blends.45 Several observations can be made with adding LiTFSI to PCL/SN. First, new peaks further appear in the functional group region induced by combination of C−S, S−N, and S−O,52 and at 1666 cm−1 in the fingerprint region originated from the interaction between Li+ and CO of PCL.53 Second, the intensity ratio of I1180/I1730 is 0.59, 0.75, and 0.86 for PCL, PCL/SN, and PPS-SPE, respectively. The higher relative intensity at 1180 cm−1 indicates an strong disturbance of −O− bond in PCL. Third, the peaks of CN at ∼2256 cm−1 nearly vanished, indicating an intensive interaction between Li+ and CN. No characteristic peak of PAN is observed from PPS-SPE, indicating a fully enclosed PANskeleton by PCL/SN polymer electrolyte. With PVDF as a skeleton, PCL/SN-PVDF (Figure S3) exhibits the same FTIR spectrum as PPS-SPE. The XRD patterns of SN, PCL, PCL/SN, PPS-SPE, and PPS-SPE after remelting at 70 °C and cooling are shown in Figure 4c. No obvious amorphous background is observed for all measured specimens. SN demonstrates two typical plastic crystal peaks at 2θ = 19.8° and 28°; PCL demonstrates two highest characteristic peaks at 2θ = 21.5° and 23.8° and several other weak peaks, indicating long-range ordered structure in accordance with the result previously reported.40,54 However, it is notable that the sharp peak of SN at 2θ = 28° becomes very weak (inset of Figure 4c) and the peak intensities of PCL decrease for PCL/SN blends. This is in agreement with the FTIR result and again confirms the strong interaction of PCL with SN to decrease crystallinity. Upon the addition of LiTFSI, the characteristic peaks of SN completely disappear. These results are different to those of PEO/SN,40 PAN/SN,55 and PVA-CN/SN,31 where the relative intensities of SN peaks still keep high. PCL/SN blends and coordination of lithium ions to −CO and −CN groups lead to a predominant amorphous nature of PPS-SPE. In view of the results combining FTIR and XRD, blending PCL with SN results in the modification of C− O−C, CO, and CN bonds and decreasing the degree of crystallinity. This would not only enhance the mobility of the polymer chains, but also increase the concentration of trans isomers in SN to enhance the lattice defects.43 After remelting at 70 °C and cooling, PPS-SPE keeps the same XRD profile as its origin, indicating no structural change during the heat treatment. Therefore, PCL/SN blends are favorable to enhance the interaction between Li+ and the functional groups and thus improve ion transport inside the polymer segment. Fast ion transport is anticipated in such plastic−polymer−salt electrolyte. Figure 4d shows the DSC curves of SN, PCL, and PPS-SPE measured from −100 to 80 °C. Two endothermic peaks at −37.11 and 57.82 °C for SN are, respectively, attributed to the transition from the normal crystal to the plastic-crystalline phase (Tpc) and the melting process. The melting process of PCL starts at 59.77 °C, while the peak for Tg is too weak to be seen. With incorporation of LiTFSI, Tm of PPS-SPE is down to 37.11 °C with peak broadening because of the reduced crystallinity by ion complexation. In addition, the characteristic peak of Tpc of SN disappears, indicating SN in PPS-SPE is fully

Table 1. Thermodynamic Data Including: Tm (onset), ΔHf, and Tg for SN, PCL, and PPS-SPE sample

Tm (°C)

ΔHf (J g−1)

Tg (°C)

SN PCL PPS-SPE

57.82 59.77 37.11

45.62 56.38 37.98

−60 −17.02

SN and PCL, and thus a lower degree of crystallinity, which is in agreement with XRD result. Although the PPS-SPE film would be partially melted when the operating temperature over 37.1 °C, the geometric shape of PPS-SPE is stable, and the battery can properly work at least up to 80 °C (Figure S4). No volatilization of PPS-SPE is observed below 90 °C from the TGA thermograms (Figure S5). Moreover, PPS-SPE is incombustible during the combustion test (Figure S6), while the commercial liquid electrolyte is flammable. The outstanding thermal stability of PPS-SPE would avoid the short-circuit risk induced by temperature overshoot, which is of great significance for practical use of LIBs. 3.3. Electrochemical Characterizations and Interfacial Stability of PPS-SPE. The temperature dependence of the ionic conductivity of PPS-SPE is shown in Figure 5a. The room temperature ionic conductivity is 4 × 10−4 S/cm. With increasing temperature, the ionic conductivity monotonously increases to 4 × 10−3 S/cm at 85 °C. A flexion point at 55 °C represents phase transition caused by partial melting of the composite. The difference between the transition temperature and the melting temperature can be explained by two reasons. First, temperature at 37 °C (Figure 4d) is just the onset point of melting process. PPS-SPE is only partially melted in this temperature section. The transition temperature for Li+ conductivity is also impacted by amorphous PCL or SN. Second, to ensure a good contact between PPS-SPE and stainless steel, a pressure load was added by a screw on the testing mold. This pressure may affect transition temperature. Similar results are also seen from other polymer electrolytes.19 The successful measurement of the ionic conductivity up to 85 °C confirms an undamaged PPS-SPE film at such high temperature, otherwise a short circuit should occur. Good high temperature stability is attributed to the strong PAN skeleton. As shown in the inset of Figure 5a, the activation energy Ea for the Li+ conduction can be obtained by fitting the σ ≈ 1/T relation using the Vogel−Tamman−Fulcher (VTF) empirical equation with respect to the coupling between ions and polymer chains dynamics:56 σ(T ) = AT −1/2 exp(−Ea /R(T − T0))

where A represents the pre-exponential parameter, R the ideal gas constant, and T0 a parameter correlated to Tg (usually about 30 K lower than Tg). The calculated activation energy of PPSSPE is Ea = 0.034 eV. This is a relatively low activation energy among polymer electrolytes and is comparable to those for many other polymer electrolytes.31,35,38,40 Low Ea can be attributed to the presence of SN with high polarity and thus resulting in a highly dissociable Li salt and decreasing the degree of crystallinity. The lithium ion transference number (tLi+) was determined by a dc polarization combined with impedance spectroscopy using a symmetrical Li/PPS-SPE/Li cell. As shown in Figure 36890


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A wide electrochemical stability window of polymer electrolytes is crucial for practical application in high voltage batteries. Figure 5c shows a linear sweep voltammetry of PPSSPE in comparison with PCL/SN-SPE, which are sandwiched between stainless steel and Li metal electrodes and swept from open-circuit voltage to 5.5 V at a scanning rate of 1 mV/s. There is no obvious current increase until 4.8 V for PPS-SPE while 4.5 V for PCL/SN-SPE. This means no obvious oxidative decomposition of the electrolyte at least at an operating voltage up to 4.8 V versus Li+ /Li. Additionally, the CV measurements of PPS-SPE were scanned from −0.5 to 4.8 V (inset of Figure 5c). Two observations can be made from the CV curves. First, no reaction appears up to 4.8 V other than lithium plating on SS electrode and lithium stripping next to 0 V. Second, the first and the second cycles of the CV measurements are entirely repeatable. Therefore, PPS-SPE possesses a wide electrochemical window. Nitrile-based polymer electrolytes have been reported with bad compatibility toward lithium metal and poor catholic stability.14 The electrochemical stability of PPS-SPE against metallic lithium was evaluated with Li/PPS-SPE/Li symmetric cells. Figure 6a displays the time-dependent voltage profile of the symmetric Li/PPS-SPE/Li cell cycled at room temperature for over 600 cycles at a current density of 0.1 mA/cm2, periodically charging and discharging for 30 min (totally 600 h). The positive and the negative voltage is respectively relevant to the Li stripping and the Li plating process. No short circuit feature is observed and the voltage almost keeps constant at 0.1 V during long-time cycling, confirming a stable PPS-SPE/Li interphase and excellent dielectric properties. The theoretical study shows that dendrite growth can only be prevented when the shear modulus of polymer electrolyte is significantly higher than that of Li metal without a significant decrease in ionic conductivity.59 However, PPS-SPE in this work demonstrates stable Li plating/stripping processes and good cycling stability toward metallic Li over hundreds of circles, despite that it would not reach so high shear modulus. Similarly, many of the other polymer electrolytes also demonstrate a great capability to suppress Li dendrites.15,19,60 These polymers as well as PPSSPE have the common feature in using framework or block/ graft copolymers, which potentially enhances the shear modulus and plays an important role to suppress dendrite growth.8 On the other hand, it is reported that mechanical modulus is not an essential prerequisite to inhibit Li dendrite growth, and instead, good interfacial contact of PPS-SPE/Li with enhanced diffusivity may suppress the growth and proliferation of Li dendrite.61 The interfacial compatibility of PPS-SPE toward lithium metal was evaluated by measuring the EIS of the Li/PPS-SPE/ Li symmetrical cell after selected aging time at room temperature, as shown in Figure 6b. The two semicircles in the impedance spectra can be attributed to the bulk resistance at high frequency and the interfacial resistance between PPSSPE and Li metal at low frequency. The former keeps unchanged during long-term contact, indicating good stability of the PPS-SPE material. A slight fluctuation of the interfacial resistance (inset of Figure 6b) with increasing time indicates formation of a stable resistive layer on the surface of Li metal. Therefore, PPS-SPE possesses superior interfacial compatibility with Li metal. A stable interphase at high voltage is also critical for cycling stability. The viability of PPS-SPE toward the electrodes under high voltage is evaluated by using constant voltage impedance

Figure 5. (a) Temperature dependent ionic conductivity of PPS-SPE. The inset displays the VTF fitting. (b) Current variation profile of a symmetric Li/PPS-SPE/Li cell at a polarization voltage of 10 mv. The inset shows the AC impedance spectra of symmetrical cell before and after polarization. (c) LSV curves of Li/PPS-SPE/SS and Li/(PCL/ SN-SPE)/SS cells. The inset shows the CV curves of Li/PPS-SPE/SS at a scan rate of 1 mV s−1.

5b, the polarization changes the interfacial resistance from 470 Ω to 550 Ω. Meanwhile, the current varies from an initial value of 11.9 μA to a stabilized value of 5.14 μA. With I, Rb, and Rel at the initial and steady state, tLi+ is calculated to be 0.32. This value is comparable to that of commercial liquid electrolyte (0.31) and higher than those of PEO-based polymer electrolytes (∼0.2).32 tLi+ is significant as it relates to the concentration polarization of electrolyte during the charging discharging processes. A large tLi+ indicates a high rate capability. tLi+ could be further improved by adding Lewis-acidic polymers or inorganic fillers.57,58 36891


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Figure 6. (a) Cyclability of PPS-SPE in a symmetric Li/PPS-SPE/Li cell with a current density of 0.1 mA/cm2 at room temperature. Magnification of selected cycles is shown in the inset. (b) AC impedance spectra of Li/PPS-SPE/Li symmetrical cell for different aging time. The inset shows the time dependent bulk and interfacial resistance. (c) Time dependent interface-resistances between PPS-SPE and LiFePO4 (Rcathode) as well as between PPS-SPE and Li (Rlithium) at charging state by applying the constant voltages at 4.2 and 4.5 V, respectively.

confirmed by FTIR and XRD, avoid the unstable factor of “excess” SN, which is incompatible with metallic lithium.27 Similar to the behavior of ionic liquid P13 in polymer matrix.66 Finally, PAN or PAN-based framework can inhibit dendrite growth and strengthen the mechanical properties.8 Overall, the rigid framework and flexible matrix coupled solid polymer electrolyte enables the excellent dendrite suppression ability. The former provides fast ion transport and good interfacial contact and the latter maintains mechanical robust and good ability for suppressing dendrite growth.67 3.4. Electrochemical Performance of LiFePO4/PPSSPE/Li All-Solid-State Batteries. Figure 7a shows the room temperature charge−discharge curves of the LiFePO4/PPSSPE/Li ASS cell in comparison with that of the corresponding liquid cell at 0.1 C in the potential range of 2.5−4.2 V. The charge and discharge voltage plateaus at around 3.4 V represent the characteristic redox processes of LiFePO4. The flat and reversible plateaus for ASS cell indicate a small polarization. Remarkably, the ASS cell shows only slightly higher overpotential than the liquid cell. Although the ASS cell delivers a low Coulombic efficiency of 81% at the first cycle (initial charge and discharge capacity of 187 mAh/g and 151 mAh/g, respectively), the Coulombic efficiency is over 99% at the second cycle and maintains over 99.6% after fifth cycle. This LiFePO4/PPS-SPE/Li ASS cell is able to light on a LED lamp (3.2 V) at room temperature for a long time (Figure S8). Figure 7b displays long-term cycling performance at 0.2 and 1 C as well as Coulombic efficiency at 1 C for the LiFePO4/

measurement method at both 4.2 and 4.5 V with a LiFePO4/ SPE/Li cell (Figure S7). Figure 6c displays time dependent interface-resistance between PPS-SPE and LiFePO4 (Rcathode) as well as between PPS-SPE and Li (Rlithium). Neither of the resistance shows the tendency of increase at either 4.2 or 4.5 V charging stage, indicating a negligible degradation/decomposition. The superior electrochemical stability and compatibility of the PPS-SPE/Li and PPS-SPE/LiFePO4 interfaces would be greatly beneficial for a long cycling stability. We proposed that the mechanism for the long-term interfacial stability of PPS-SPE/Li-electrode lies on several advantages of PPS-SPE. First, it is reported that soft polymer electrolyte can adhere to Li electrode when Li electrode recedes during cell discharging process.62 Therefore, the soft matrix of PPS-SPE is beneficial to keep good contract between Li electrode and electrolyte during cell cycling. This is difficult if the electrolyte is a simple solid with a high shear modulus due to loss of the adhesive ability.63 Second, according to the relation between dendrite formation and ion diffusivity,62 the dendrite formation onset time is closely related to Li ion diffusivity and the resistance of the SEI of electrolyte/Lielectrode. Therefore, the high ionic conductivity along with high ion concentration of PPS-SPE is beneficial for suppressing lithium dendrite formation. Third, an optimized preparation procedure and the use of controlled environment in this work to prepare dry and densified polymer are useful to improve the interfacial compatibility of electrolyte/Li-electrode.64,65 Fourthly, complete interaction of PCL with SN in PPS-SPE, 36892


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Figure 7. (a) Charge and discharge curves of LiFePO4/Li cells using PPS-SPE and commercial liquid electrolyte at 0.1 C. (b) Cycling stability of LiFePO4/PPS-SPE/Li ASS cell at 0.2 and 1 C, respectively. (c) Nyquist plots of LiFePO4/PPS-SPE/Li ASS cell for selected cycles after cycling at 0.2 C. The inset shows Rhf (sum of the bulk and the SEI resistance, high frequency region) and Rmf (the charge-transfer resistance at the interface, middle frequency region) as a function of cycle number. Rate performance of the LiFePO4/PPS-SPE/Li ASS cell (d) at room temperature and (e) at 45 °C.

frequency ascribed to the charge-transfer resistance at the interface (Rmf).68 Both of the resistances (inset of Figure 7c) increase with cycling, reach maximum values, and after that decrease to be close to the initial values. Such superior electrochemical behavior benefits from high conductance and flexibility of PPS-SPE as well as superior interfacial stability of PPS-SPE toward both positive and negative electrodes. The surface morphology (Figure 8) of the lithium negative electrode was observed by using SEM to further understand the cycling behavior. Compared with the rough surface of initial lithium metal electrode, the surface forms smooth and homogeneous SEI layer without lithium dendrite after 200th cycling. This can be attributed to the stable interfaces and good dendritesuppression capability of PPS-SPE. Besides the cycling stability, a good rate capability is also a critical performance for practical application of ASS batteries. The rate performances of the LiFePO4/PPS-SPE/Li ASS

PPS-SPE/Li ASS cells during charge/discharge processes between 2.5 and 4.2 V. The ASS cell maintains a reversible discharge capacity of 139.5 mAh/g at 0.2 C over 200 cycles and 101.6 mAh/g at 1 C over 400 cycles with almost 100% retention rate. The Coulombic efficiency is higher than 99.6% at a current density of 1 C. The reproducible evaluation for different batches of cell (Figure S9) tested in different instrument and different environment (with and without aircondition) demonstrates good repeatable results. This outstanding cycling stability with a Coulombic efficiency approaching 100% indicates that PPS-SPE is robust enough to sustain the ASS cells. EIS measurements after selected cycles were performed to evaluate the change of the resistance after cycling. As shown in Figure 7c, each Nyquist impedance plot contains a partial semicircle in the high frequency intercepting at the real axis ascribed to the sum of the bulk and the SEI resistance (Rhf), and irregular semicircles in the middle 36893


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of SN and LiTFSI and skeleton selection, PPS-SPE achieves the high ionic conductivity of 4 × 10−4 S/cm and low activation energy of 0.034 eV. Notably, FTIR measurements reveal the modification of C−O−C, CO, and CN bonds after blending PCL with SN. Enhanced modification of these bonds is further observed with adding LiTFSI due to the coordination of Li ions. The ion transport is thereby reinforced. Additionally, PPS-SPE possesses wide electrochemical window up to 4.8 V. Another remarkable feature is that, through systematic interfacial investigations, PPS-SPE demonstrates superior compatibility and electrochemical stability toward metallic Li as well as LiFePO4 cathode even subjected to a charging voltage of 4.5 V for long time. The LiFePO4/Li ASS cells using this solid polymer electrolyte achieve high capacity, outstanding cycling stability, and superior rate capability that is better than the corresponding cell based on liquid electrolyte. PPS-SPE is thus very competitive with other polymer electrolytes and promising for next-generation all-solid-state LIBs.

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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.7b12186. SEM/EDS mapping, FTIR, TG-DSC, and combustion tests; LiFePO4/PPS-SPE/Li ASS cell cycling; photos of PPS-SPE; EIS spectra; reproducible evaluations; photo of lighting on the LED lamp; rate capability of LiFePO4/Li battery (PDF)

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Figure 8. SEM surface morphology of Li metal electrode (a) at the initial state and (b) after 200th cycles at 0.2 C.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 335 8057047. Fax: +86 335 8074545.

batteries were evaluated in the current range of 0.1−4 C at both room temperature and 45 °C, as shown in Figure 7d and e. The specific discharge/charge capacity of the ASS cell with good retention varies from 155 mAh/g at 0.1 C, 140 mAh/g at 0.2 C, 116 mAh/g at 0.5 C, and 101 mAh/g at 1 C. The recoverable capacity is over 99% when the current density turns back to 0.1 C. By increasing the temperature over the melting point at 45 °C, the rate capability significantly enhanced. As can be seen from Figure 7e, the ASS cell delivers the specific capacity of 166, 154, 143, 131, 118, 107, and 98 mAh/g at 0.1, 0.2, 0.5, 1, 2, 3, and 4 C, respectively. Moreover, the recoverable capacity is 98% when the current density turns back to 0.1 C. Rate performance of the ASS cell at high temperature is better than that of the corresponding cell based on liquid electrolyte at room temperature (Figure S10). Enhancement of the rate capability at higher temperature originates from (1) the improved ionic conductivity because of increasing temperature and (2) denser contact inside the positive electrode as well as on the electrode/electrolyte interface due to partial melt of PPS-SPE. The enhanced rate capability at the temperature over melting point again confirms the good rigidity of PPS-SPE at high temperature. It is thus believed that PPS-SPE possesses the practicability and reliability for ASS LIBs.

ORCID

Long Zhang: 0000-0002-4861-1471 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51525205) and the Foundation of Higher Education of Hebei (ZD2016033).

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REFERENCES

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Superior Blends Solid Polymer Electrolyte with Integrated Hierarchical

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