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A polyacrylonitrile (PAN)-based double-layer multifunctional gel polymer electrolyte for lithium-sulfur batteries
Journal of Membrane Science 582 (2019) 37–47
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A polyacrylonitrile (PAN)-based double-layer multifunctional gel polymer electrolyte for lithium-sulfur batteries
T
Xiuli Wang∗,1, Xiaojing Hao1, Yan Xia, Yanfei Liang, Xinhui Xia, Jiangping Tu State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, And School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Quasi-solid-state lithium-sulfur battery Polyacrylonitrile Polyethylene oxide Gel polymer electrolyte
Owing to high theoretical capacity, lithium-sulfur batteries (LSBs) are receiving extensive researches. However, cyclic instability and safety issues hugely confine the commercial applications of traditional liquid LSBs. In this work, for the sake of fully leveraging the high ionic conductivity of polyacrylonitrile while avoiding Li anode “passivation effect” caused by CN group, we prepare double-layer gel polymer electrolytes for quasi-solid-state LSBs. The transition layer composed of polyacrylonitrile, polyethylene oxide and Li1.3Al0.3Ti1.7(PO4)3 (LATP) is located on Li anode side to reduce “passivation effect” triggered by pure polyacrylonitrile. Meanwhile, the high ionic conductivity layer composed of polyacrylonitrile and LATP in contact with cathode can utilize the high intrinsic ionic conductivity of polyacrylonitrile to enhance the rate performance. Furthermore, LATP with higher ionic conductivity embedded in the membrane serves as Li+ transport channels to further increase ionic conductivity. Prominently, the designed double-layer electrolytes exhibit a high Li+ transference number of 0.55 and superior mechanical property. Moreover, stable coulombic efficiency of 99.6–100.0% over 100 cycles and good capacity retention of 79.0% after 100 cycles at 0.1C can be achieved. Our newly designed double-layer electrolytes with multiple functions exhibit potential applications in safer LSBs.
1. Introduction Nowadays, the cathodes of the commercial lithium-ion batteries (LIBs) are mainly LiFePO4, LiCoO2 and LiMn2O4 and so on, and the anodes are mostly modified carbon materials, leading to extensive applications of LIBs for efficient energy storage equipment as well as new energy vehicles [1,2]. However, they are far from satisfaction owing to the low energy density and safety issues. Nowadays, using high specific capacity sulfur as the cathode active materials can fulfill the dream of high energy density and efficiency of LIBs, which makes the sulfur stand out from many traditional insertion-host materials [3]. What's more, sulfur is cheap, nontoxic and environmentally friendly [4,5]. Nonetheless, prior to the commercialization of LSBs, some technical challenges must be addressed, such as Li anode passivation, the intrinsic low conductivity of sulfur, harmful “shuttle effect” caused by intermediate polysulfides and electrolyte decomposition [6]. Typically, some strategies can eliminate part of the above problems, such as protecting the Li metal anode [7,8], modifying sulfur with conductive
materials [9–11] or metal oxides [12] and leveraging solid-state electrolytes (SSEs) [13,14]. And one of the most promising strategies is fabricating SSEs as modified separators since they can not only reduce the capacity decay, but also drastically reduce the probability of inflammation [15]. According to this strategy aforementioned, diverse Li+ conducting inorganic and polymer electrolytes have been developed. But none of them are commercialized owing to various shortcomings. Li+ conductive inorganic substances possess high ionic conductivities and wide electrochemical windows. LATP is a typical oxide solid electrolyte, which is about 3 × 10−3 S cm−1 at 25 °C [16]. However, poor interfacial compatibility with electrolyte hugely limits the extensive applications of inorganic electrolytes. Furthermore, the Li dendrites form and grow even faster in the grain boundaries of inorganic electrolyte membranes than they do in the traditional liquid electrolyte (LE) [17,18]. So it is of little significance to directly assemble the inorganic electrolytes into solid-state batteries (SSBs). Attractively, the gel polymer electrolytes (GPEs) become outstanding due to the excellent
Abbreviations: PPL, monolayer electrolyte membrane composed of PAN, PEO and LATP; PL, monolayer electrolyte membrane composed of PAN and LATP; PPL-PL, double-layer electrolyte membrane ∗ Corresponding author. E-mail address: [email protected] (X. Wang). 1 These authors contributed equality to this work. https://doi.org/10.1016/j.memsci.2019.03.048 Received 19 January 2019; Received in revised form 14 March 2019; Accepted 16 March 2019 Available online 20 March 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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interfacial wetting ability, flexibility and good electrochemical performance. The batteries assembled with GPEs can be referred as quasisolid-state batteries (QSSBs). Various polymers like polyacrylonitrile (PAN) [19], poly(methyl methacrylate) (PMMA) [20], polyethylene oxide (PEO) [21,22], poly(vinylidene fluoride) (PVDF) [23], poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) [24], poly(propylene carbonate) (PPC) [25] and many other newly synthesized polymers were used due to their unique advantages. Among them, PAN is known for excellent mechanical properties and ultra-high intrinsic ionic conductivity of about 10−3 S cm−1 at 298 K. But CN groups in pure PAN have reaction with Li anode, which will form a “passivation layer” between Li anode and PAN electrolyte, increasing impedance, blocking ion transport and deteriorating electrochemical performance [26,27]. Fortunately, PAN mixing with other polymeric or inorganic materials can attenuate this issue to some extent. Li et al. demonstrated that the hydrogen bond between CN groups in PAN and hydroxyl groups in graphene oxide can effectively weaken polarization and improve electrochemical stability [28]. As another traditional polymer material, PEO exhibits low lattice energy and high capacity of dissolving alkali metal salts, but the high degree of crystallinity, poor thermo-stability and low mechanical strength hinder its independent usage in SSBs. Therefore, in recent years, PEO often serve as an additive polymer to improve the performance of the matrix polymer [29]. Similarly, electron-withdrawing group hydroxyl in PEO can form intermolecular hydrogen bond with CN group in PAN and reduce the “passivation effect” caused by the pure PAN, which is first demonstrated in our work. On the other hand, another effective way to improve the efficiency of LSBs is to design the electrolyte configuration. Goodenough et al. designed a double-layer electrolyte with a high-voltage stable layer contacting cathode and a low-voltage stable layer contacting anode, realizing the high-voltage SSBs [30]. Gao et al. created a surface modified PVDFbased GPE with polydopamine [31]. And the created polydopamine layer can effectively increase the interfacial stability with the Li anode, demonstrating the reasonable electrolyte configuration. Herein, we propose a model of double-layer configuration with both high mechanical moduli and ionic conductivity. The designed electrolyte is composed of two layers and has different functions. The transition layer in contact with Li anode is created by rational combination of PAN, PEO and LATP. It can be referred as PPL, which is used as the protect layer to overcome the “passivation effect” of pure PAN by forming hydrogen bond with PEO. The high ionic conductivity layer PL is fabricated by rational combination of PAN and LATP, but without PEO. It can exert the high intrinsic ionic conductivity of PAN when contacting with the cathode, enhancing the total ionic conductivity of the PPL-PL electrolyte membranes. LATP with high ionic conductivity embedded in the membrane can serve as the Li+ transport channels to further increase ionic conductivity of the composite membrane. Accordingly, QSSBs assembled with this double-layer electrolytes display desirable electrochemical properties.
10 min was to prepare a semi-dry layer so that the high ionic conductivity PL layer could tightly adhere to the transition layer with no seams. And the PL layer was prepared via casting the second slurry on the first PPL electrolyte membrane. Then the PPL-PL double-layer electrolyte membrane was volatilized in vacuum at 40 °C for 16 h. After that, the membranes were collected into glove box after cutting into disks (19 mm in diameter). Finally, the above round PPL-PL electrolytes were immersed in the commercial LE consisted of 1.0 m lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio of 1:1) with 1.0 wt % LiNO3 solution. Fig. 1 also illustrates the working mechanism and stacking model of PPL-PL electrolyte in a solid-state cell, in which the two layers exert different electrochemical functions. In order to demonstrate the super performance of our PPL-PL electrolyte membrane, another monolayer electrolyte was also fabricated as the control group. All the process was similar except the original composition. The control electrolyte consisted of PAN (1.5 g), PEO (1.5 g) and LATP (0.3 g), named PPL electrolyte. 2.2. Structural characterization The morphology of all these membranes were obtained by a Hitachi S4800 Field emission scanning electron microscopy (FESEM, Japan). The element mappings of cross section were investigated by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) patterns were characterized on X'Pert PRO X-ray diffractometer with CuKα radiation from 10° to 80° in 2θ. Differential scanning calorimetry (DSC) testing was operated under N2 atmosphere via DSC Q100 equipment from −40 °C to 150 °C at the heating rate of 10 °C min−1. Thermogravimetric analysis (TGA) was tested with SDT Q600 instrument under N2 flow at a heating rate of 10 °C min−1 over the temperature range of 25–900 °C. Stress-train tests were characterized on Zwick/Roell Z020 material testing machine with the crosshead speed at 20 mm min−1. 2.3. Electrochemical evaluation Firstly, we prepared the S@C composite (Ws:Wsuper P = 3:2) by melt-diffusion strategy. After grinding together, the sublimed sulfur and super P were heated in the autoclave with a Teflon liner at 155 °C for 12 h. Then, the cathode sludge was obtained by dissolving synthesized S@C composite (90 wt %), PVDF binder (10 wt %) into the N-methy-12-pyrrolidone (NMP). After stirring homogeneously, the sludge was evenly spread onto Al foils and volatilized at 60 °C for 10 h in vacuum. The mass loading of the active materials were about 1.0–1.3 mg cm−2. Subsequently, we assembled CR2025 coin-type S@C/PPL-PL/Li cells in the Ar-filled glove box, using S@C composite as cathode, PPL-PL as electrolyte membrane and Li foil as anode. For comparison, the S@C/ LE/Li and S@C/PPL/Li cells were also assembled with the same way but using commercial LE separator or PPL as the electrolyte membrane respectively. By soaking weight-measured membrane in the commercial Li-S liquid electrolyte, we can figure out the electrolyte uptake (EU). The EU value was calculated by Eq. (1):
2. Experiment 2.1. Preparation of PPL-PL electrolyte First, we synthesized the fine LATP particles through a sol-gel method by a previous report [32]. The preparation of PPL-PL electrolyte membrane is presented schematically in Fig. 1. 1.5 g PAN and 1.5 g PEO were added into the N,N-Dimethylformamide (DMF) to form the first solution, then LATP particles (0.3 g) with a ratio of 10% PAN and PEO by weight were introduced into the above solution. After ultrasonic treatment for 30 min and magnetic stirring for 8 h, we acquired the first homogeneous solution. Similarly, we prepared the second solution without any PEO but 3.0 g PAN and 0.3 g LATP particles only. The first slurry was scraped on the tempered glass by a square blade, afterwards it was volatilized at 40 °C for 10 min to obtain the transition layer PPL electrolyte membrane. The drying time controlled within
EU% = [(ωwet − ωdry )/ ωdry] × 100%
(1)
Where ωdry represents the weight of dry membrane, and ωwet is the weight of LE-absorbing membrane. The volume value (Vm) of absorbed LE corresponding to unit active substance in an assembled LSB was calculated by Eq. (2):
Vm = (ωwet − ωdry )/(ρ × msulfur )
(2)
Where the density of liquid electrolyte (ρ) is 1.174 g/mL, msulfur is the weight of active material sulfur on a cathode. The ionic conductivity measurements were performed on a Princeton multi-channel electrochemical workstation by AC 38
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Fig. 1. Schematic of the preparation of PPL-PL electrolyte membrane and the working mechanism and stacking model of PPL-PL electrolyte in a QSSB.
3. Results and discussion
electrochemical impedance spectroscopy (EIS). The electrolyte membrane was sandwiched by two stainless steel electrodes on two sides. The ionic conductivities (σ, in S cm−1) can be calculated via Eq. (3):
σ = d/(Rb × S )
The cross section of PPL-PL electrolyte membrane is presented in Fig. 2a with the thickness of roughly 20 μm, which is much thinner than many present works and can compete with the commercial LE separators of 16–20 μm (Fig. 2b). The intimate binding of two layers is demonstrated since there is no stratification between them, which is in favor of the stability of the electrolyte membranes during the working process. The photograph of PPL-PL electrolyte membrane before soaking the LE is shown in Fig. 2c, which shows a semi-transparent physical properties. Fig. 2d displays the surface of the side that will contact with Li anode, demonstrating a lot of small pits (d < 400 nm) evenly (the SEM morphology of the side that will contact cathode is identical, with a lot of small pits, Fig. S1). Herein it will impart a high surface area and accommodate more active Li+ in the membrane, enhancing the EU and improving the ionic conductivity [33]. Surface SEM images of layer in contact with Li anode in PPL-PL electrolyte membrane with various amounts of LATP are presented in Fig. S2. It displays different surface morphologies. Compared with the membrane with 10 wt % LATP, the membrane with 5 wt % LATP is smooth without any pits to store more Li+, while the membranes with 20 wt % or 30 wt % LATP are so tough that will exhibit high surface resistance. We can ascertain from the morphology that system with 10 wt % LATP is the optimal one to exhibit good electrochemical ability, which can be further confirmed by EU and ionic conductivity of LE-absorbing PPL-PL electrolytes with various amounts of LATP at 25 °C (Fig. 1e–f). The EU of PPL-PL electrolyte membranes with 5 wt %, 10 wt %, 20 wt %, 30 wt % LATP are 145.3%, 166.5%, 136.4%, 125.7%, and the ionic conductivities of those are 5.11 × 10−4, 8.61 × 10−4, 5.34 × 10−4,
(3)
Where d and Rb represent the thickness and bulk resistance of the electrolyte, respectively. S represents the contact area between electrolyte and stainless steel electrode. The lithium ion transference number (tLi+) was calculated by combining the AC impedance and DC polarization of the Li/electrolyte/ Li cells. The tLi+ was computed via Bruce Vincent Eq. (4):
tLi + =
Is (ΔV − I0 R 0) I0 (ΔV − Is Rs )
(4)
Where the initial polarization current: I0 and steady state polarization current: Is can be acquired via DC polarization test simultaneously, the initial interfacial resistance: R0 and steady state interfacial resistance: Rs can be measured by EIS before and after the polarization, in which the voltage amplitude is 0.01 V. The linear sweep voltammetry (LSV) was used to test the electrochemical stability window of the electrolyte membranes. The voltage range is 2.0–6.0 V, and scanning rate is 0.1 mV s−1. The cyclic voltammetry (CV) curves were obtained under the voltage of 1.7–3.0 V at 0.1 mV s−1. The galvanostatic charge-discharge (GCD) were carried out on LAND battery-testing instrument from 1.7 to 2.8 V at various current densities at room temperature. EIS tests of LSBs before and after cycling were performed under the voltage amplitude of 10 mV with the frequency range 106–10−1 Hz. 39
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Fig. 2. Cross section SEM images of (a) PPL-PL electrolyte membrane and (b) LE separator; (c) Optical photograph of PPL-PL electrolyte membrane; (d) Surface SEM image of layer that contact with Li anode; (e) electrolyte uptake of PPL-PL electrolyte membrane with different LATP content; (f) ionic conductivity of PPL-PL electrolyte membrane with different LATP content.
3.52 × 10−4 S cm−1 respectively, demonstrating the optimal amount of 10 wt % LATP. Therefore, in subsequent works, the PPL-PL electrolyte membrane with 10 wt % LATP was selected in order to exert its best electrochemical potentials. At the same time, the PPL-PL electrolyte membrane shows good liquid retention ability compared with PPL electrolyte membrane and LE separator. Fig. S3 shows the thermogravimetric results of the PPL-PL, PPL electrolyte membranes and LE separator with 20 wt % liquid electrolyte. The liquid electrolyte only conserves within the micropores of the LE separator temporarily instead of forming incorporation, so it evaporates rapidly in LE separator. The polymer membranes of PPL, PPL-PL form gel via integrating with the LE, locking LE and Li+ in the membrane and obtaining excellent liquid retaining property. More importantly, the addition of PEO in PL layer of the PPL-PL membrane decreases the crystallinity and increases the disorder of polymers, which further increases LE locking ability of the PPL-PL electrolyte membrane. The elemental mapping can shed more lights on to further confirm the double-layer structure. In Fig. 3, C, P, Ti and Al are evenly presented in the cross section, while N is distributed differently due to the difference in the amount of original PAN in the two layers. The EDS results clearly demonstrate the hierarchical structure of our electrolyte membrane (shown by dotted red line in Fig. 3). However, there is no obvious stratification in SEM (Fig. 2a), suggesting the intimate contact between two layers. From the SEM of PPL-PL electrolyte membrane after 100 cycles (shown in Fig. S4), the thickness of membrane exhibits a little increase to 25 μm due to the intake of liquid electrolyte. However, there
is no obvious layered interface between the two layers, demonstrating the excellent stability of PPL-PL electrolyte membrane. XRD was conducted to identify the interaction between different materials. In Fig. 4a, the diffraction peaks of PAN+PEO membrane suggest just a blend of PAN and PEO. The diffraction peaks of PAN +PEO+LATP membrane is a blend of the three compositions, and LATP in the PAN+PEO+LATP membrane are relatively weak, indicating the amount of LATP exposed on the membrane surface is very small. In addition, the diffraction peaks from 13° to 20° of the membrane are broadened and weakened, which is attributed to the introduce of LATP. Because LATP can interact with the long chain in the polymer, then expand the amorphous region and reduce the polymeric crystallinity. The increase of amorphous region will increase the ionic conductivity and then benefit the comprehensive electrochemical properties of QSSBs, matching well with the results of ionic conductivity (Fig. 2f). Simultaneously, LATP with higher ionic conductivity embedded in the membrane can serve as the Li+ transport channels to further raise the ionic conductivity of the PPL-PL composite membrane. Tensile strength measurement was carried out to investigate the mechanical properties of LE separator, PPL and PPL-PL electrolyte membranes. Currently, one of the most important features for separators to be commercially applied is the excellent mechanical properties [34]. As shown in Fig. 4b and Table S1, the commercial LE separator's maximal mechanical tensile and strain are 94.3 MPa and 35.5% respectively. The corresponding features of PPL electrolyte membrane are 35.1 MPa and 135.2%. However, the mechanical tensile and strain of 40
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Fig. 3. EDS spectrum of the PPL-PL electrolyte membrane.
Fig. 4. (a) XRD patterns of the compositions of PPL-PL electrolyte membrane; (b) Stress-strain curves of LE, PPL, PPL-PL electrolyte membranes; (c) DSC curves of PAN, PAN+PEO, PAN+PEO+LATP; (d) TGA curves of LATP, PAN, PAN+PEO, PAN+PEO+LATP; (e) Schematic diagram of the intermolecular “hydrogen bonding effect” between PAN and PEO and the passivation weakening mechanism of CN groups by hydrogen bond. 41
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effect”. We assembled LSBs and conducted different electrochemical tests. Fig. 5e shows the ionic conductivities of the LE separator, PPL and PPL-PL electrolyte membranes within 20–90 °C. Apparently, ionic conductivities raise with the raise of temperature because of the increasing movements of polymer chains and free Li+ [36]. At 25 °C, the ionic conductivities of LE separator, PPL and PPL-PL electrolyte membranes are calculated to be 9.37 × 10−4, 2.39 × 10−4, 8.61 × 10−4 S cm−1, respectively. Notably, the PPL-PL electrolyte membrane exhibits the high ionic conductivity due to the hierarchical structure, while the monolayer of PPL electrolyte membrane possesses the lowest ionic conductivity because of the low intrinsic conductivity of PEO. Therefore, putting the high ionic conductivity PL layer on the other hand can significantly raise the total ionic conductivity of the membrane compared with monolayer PPL electrolyte membrane. Lithium ion transference number exhibits a great influence on the electrode polarization during the lithium plating and stripping process. So a high tLi+ is essential for the SSEs [37]. As we can see from Fig. 5f, after polarization, the interfacial resistance of PPL-PL electrolyte membrane increases from 103.7 to 130.1 Ω, the current changes from initial value of 82.6 mA to the steady state of 61.6 mA. Following the Bruce-Vincent formula, tLi+ is computed to be 0.55. However, the value of the commercial LE separator and PPL electrolyte membrane are 0.35 [38] and 0.39 (Fig. S5), respectively. Although the transition PPL layer can protect the Li anode from passivation, actually this protection comes at the cost of reducing electrochemical performance compared with PL layer because of the comparatively lower ionic conductivity of PPL layer. Therefore, our tests demonstrate that high ionic conductivity PL layer in PPL-PL electrolyte indeed increases ionic conductivity and lithium ion transference number compared with monolayer of PPL electrolyte membrane, which powerfully proves the effectiveness of the PL layer in our PPL-PL electrolyte membrane. The LSV curves (Fig. 6a) of the stainless steel (SS)/PPL-PL/Li, SS/ PPL/Li, and SS/LE/Li cells were measured to test the electrochemical stability of different electrolytes. For the SS/PPL-PL/Li, the LSV curve maintains stable until 5.0 V, so PPL-PL electrolyte membrane would not decompose under 5.0 V. Nevertheless, there is a sharp rise of current at 4.53 V for SS/LE/Li cell, which can be explained by the drastic decomposition of LE during the oxidation process. The current of the SS/ PPL/Li cell rises at 4.70 V, which is still lower than the SS/PPL-PL/Li cell. Fig. 6b–c is the EIS tests of SS/PPL/Li and SS/PPL-PL/Li cells at bias voltage of 4.8 V for different time at room temperature to analyse electrochemical stability under high voltage. The EIS of the SS/PPL-PL/ Li cell has little change within 48 h, because it has an oxidation voltage at 5 V and stable under 4.8 V. By contrast, the impedance of SS/PPL/Li cell is doubled, because the membrane of PPL can be oxidized and degraded under 4.8 V. However, when the SS/LE/Li cell is measured under 4.8 V, it is electrically broken down because of the massive decomposition of liquid electrolyte. Fig. 6d characterizes the third cycle of CV curves concerning the S@C/LE/Li, S@C/PPL/Li and S@C/PPL-PL/Li cells. There are three characteristic peaks, reduction peaks: 2.09 V and 2.29 V, oxidation peak: 2.51 V, which are all in accordance with typical charge-discharge characteristic peaks of LSBs. Furthermore, in terms of the S@C/PPL-PL/ Li cell, the higher reduction peak and lower oxidation peak indicate the lowest polarization [39]. Simultaneously, the highest capacity further demonstrates its elevated electrochemical performance. To further highlight the desired structure and excellent electrochemical performance of our hierarchical design of PPL-PL electrolyte membrane, we assembled S@C/LE/Li, S@C/PPL/Li and S@C/PPL-PL/ Li LSBs. As shown in Table S3, the volume value (Vm) of absorbed LE corresponding to unit active substance in an assembled S@C/PPL/Li and S@C/PPL-PL/Li LSBs was calculated to be 4 and 5 μL/mg respectively, which is quite fewer than the traditional liquid LSBs of about 60 μL/mg [40]. It seems that the PPL-PL electrolyte membrane has a high value of EU, but the volume of absorbed LE is really small. Actually, the high EU but low Vm in an assembled LSB contributes to the
the PPL-PL electrolyte membrane are 22.6 MPa and 180.4%, demonstrating an improved fracture toughness. It is worth to mention that with the increase amount of the LATP, modulus and stress of the membrane have a gradual increase at the cost of a sharp decrease of the strain (Table S2). Because the inorganic fillers embedded in the polymer matrix can hinder the sliding of grain boundaries and absorb a lot of deformation work under the action of external force, increasing the modulus and strain of the membrane. However, with the addition of inorganic fillers, the inhomogeneity in the membrane may occur, which may result in stress concentration and decrease the strain at break. Based on the above results, the PPL-PL electrolyte membrane with 10% LATP content exhibits excellent mechanical properties and shows commercial potential in rechargeable SSBs. DSC and TGA were used to analyse the thermal properties. Tg and Tm of pure PAN are about 90 °C and 317 °C. Tg and Tm of pure PEO are around −45 °C and 64 °C [35]. Fig. 4c characterizes the DSC curves within −40 °C-150 °C. The pure PAN electrolyte membrane has a Tg at around 90.1 °C. After adding PEO, the Tg of the PAN decreases to the 86.0 °C. At the same time, the new endothermic peak at around 50.4 °C refers to the Tm of PEO. The slightly decrease of Tg of the PAN and Tm of the PEO is because the lone pair electrons on the N in CN group can form HeN hydrogen bond with PEO. Fig. 4e is the schematic diagram of the intermolecular “hydrogen bonding effect” between PAN and PEO. The “hydrogen bonding effect” enable polymer chains to be more chaotic, which increases amorphous regions and reduces the crystallinity. The endothermic peak of PAN+PEO+LATP electrolyte membrane further reduces to low temperature after adding LATP (the Tg of the PAN becomes indistinct), demonstrating the addition of LATP can further reduce the crystallinity. Fig. 4d illustrates the TGA curves of LATP particles, pure PAN, PAN +PEO, and PAN+PEO+LATP electrolyte membranes. During the heating process, the pure LATP keeps weight stable, exhibiting negligible mass loss over the entire temperature range 25 °C–900 °C. At 900 °C, the weight retention rate of pure PAN, PAN+PEO, and PAN +PEO+LATP three electrolyte membranes are 36.9%, 29.2% and 42.4%, respectively. The degradation temperatures at a 5% weight loss values (Td,5%) of the three membranes are 321 °C, 298 °C, 282 °C, respectively, implying all the electrolyte membranes could live up to the working temperature requirements of SSBs (60–80 °C). The slightly decrease of the Td,5% values ascribes to the reduction of the crystalline segments and the increase of the amorphous regions, which are coincide with the DSC consequences. Fig. 5a–b are the combustion test of commercial LE separator and PPL-PL electrolyte membrane. LE separator is rapidly ignited and scorched quickly due to its poor thermal stability. By contrast, PPL-PL electrolyte membrane displays much better thermal stability. Furthermore, the two membranes are heated at 150 °C for 2 h. LE separator shrinks obviously (Fig. 5c). However, the shape of the PPL-PL membrane maintains well except little shrink on the edge (Fig. 5d). Thus, the PPL-PL electrolyte membrane possesses much better thermal stability compared with commercial LE separator. For the sake of appraising the electrochemical properties of our PPLPL electrolyte membrane, in which PPL layer serves as the transition layer to protect Li anode from passivation caused by pure PAN, and PL layer is designed to take advantage of the high intrinsic ionic conductivity of PAN. The addition of PEO can form HeN hydrogen bond with nitrogen atom in CN group (Fig. 4e), and this kind of intermolecular interaction can increase the amorphous regions, more importantly, it plays an effective way to weaken the “passivation effect” of strong polar CN group. Without the hydrogen bond to restrict the strong polar CN group, more free CN groups will react with active Li anode, forming a passivation layer on the Li anode and impeding the exertion of its electrochemical potential. This so called “passivation effect” can deteriorate the electrochemical performances. However, via the restriction of the hydrogen bond, reactivity between CN group and Li anode is hugely reduced, which can efficiently weaken the “passivation 42
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Fig. 5. The flammability test of the (a) LE separator and (b) PPL-PL electrolyte membrane; Heating experiments operated at 150 °C for 2 h on the (c) LE separator and (d) PPL-PL electrolyte membrane; (e) Ionic conductivity of LE separator, PPL and PPLPL electrolyte membranes in the temperature range of 20–90 °C; (f) Current-time curve obtained from chronoamperometry at a DC polarization of 0.01 V, inset: EIS plot of the Li/PPL-PL/Li cell before and after polarization.
Fig. 6. (a) LSV curves of LE separator and PPL-PL, PPL electrolyte membranes; EIS of (b) SS/PPL-PL/Li cell, (c) SS/PPL/Li cell at bias voltage 4.8 V for different time at 25 °C, (d) The third cycle of CV curves concerning the three cells in the potential range of 1.7–3 V (Li/Li+) at a scan rate of 0.1 mV s−1. 43
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Fig. 7. The initial charge/discharge-voltage curves of (a) S@C/LE/Li, (b) S@C/PPL/Li and (c) S@C/PPL-PL/Li cells; (d) Rate performance of three cells from 0.05 to 0.5C; (e) Cycling performance of three cells at 0.1C.
The rate performance (Fig. 7d) displays the same results, the specific capacities of S@C/LE/Li cells decrease quickly at different current density. The dissolution of polysulfides into the LE and spread to the Li anode through commercial separator (shuttle effect) drastically harm the performance of LSBs. The S@C/PPL-PL/Li cell displaces discharge capacities up to 1020.0, 883.5, 721.4, 409.6 mAh g−1 at rate of 0.05, 0.1, 0.2 and 0.5 C at room temperature respectively (shown in Fig. 7c–d), which is more stable than other contrast groups. The excellent electrochemical properties can be also demonstrated by cycling performance in Fig. 7e, S@C/PPL-PL/Li cell shows a much higher discharge capacity retention about 79.0% (from 903.5 to 714.1 mAh g−1) at 0.1C compared with S@C/LE/Li cell (32.1%), and S@C/PPL/Li cell (69.8%). Meanwhile, its superior coulombic efficiency about 99.6–100.0% exhibits its good cyclic stability. The long-time discharge ability was tested at 0.05C for commercial S@C/LE/Li, S@C/PPL/Li and S@C/PPL-PL/Li cells in Fig. S7. The S@C/PPL-PL/Li cell exhibits more stable capacity retention about 80.0% after 300 cycles compared with that of 61.9% after 300 cycles in terms of S@C/PPL/Li cell and 63.5% only after 120 cycles in terms of S@C/LE/Li cell. And 300 cycles later, discharge capacity of S@C/PPL-PL/Li still maintains at 803.1 mAh g−1. However, 120 cycles later, S@C/LE/Li cell is damaged,
superior electrochemical performance as well as the reliable security. Fig. 7a–c are the initial GCD curves of different LSBs that we picked out from the charge-discharge test at different current density. Obviously, S@C/LE/Li cells have the inferior performance. Their specific discharge capacities at 0.05, 0.1, 0.2, 0.5C are 1041.5, 712.2, 509.7 and 203.5 mAh g−1, while those of S@C/PPL/Li cells are 943.4, 801.1, 541.4 and 312.6 mAh g−1, and S@C/PPL-PL/Li cells are 1018.3, 903.5, 726.1 and 411.6 mAh g−1. It is known that PAN is a promising electrolyte for its high intrinsic ionic conductivity. However, it will react with Li anode, causing anode passivation due to the CN group. And the detrimental effect of PAN is demonstrated when we assemble S@C/ PAN/Li cells (Fig. S6). Without the protection of PPL layer, the pure PAN electrolyte (with 10 wt % LATP) dies only after 40 cycles. Besides, its coulombic efficiency hugely deviates from 100%, because the strong polarity of PAN causes the passivation of the Li anode during charging and discharging, which leads to the loss of capacity and low coulombic efficiency for the assembled full cells. Furthermore, its initial impedance is large, and the impedance increases hugely after 40 cycles, which is caused by the anode passivation resulted from PAN. Therefore, the effective function of PPL layer in our PPL-PL electrolyte membrane is also strongly demonstrated. 44
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Fig. 8. EIS plots for (a) S@C/LE/Li cell, (b) S@C/PPL/Li cell, (c) S@C/PPL-PL/Li cell before cycles and after 100 cycles; (d) Equivalent circuit.
Fig. 9. Schematic illustrations of (a) Li dendrite growth behaviors in S@C/LE/Li cell; (b) no Li dendrite growth in S@C/PPL-PL/Li cell during plating and stripping process; The SEM of Li anode (c, e) and the corresponding EDS mapping (d, f) of S in (c, d) S@C/LE/Li cells and (e, f) S@C/PPL-PL/Li cells after 100 cycles.
designed PPL-PL electrolyte membrane. As mentioned above, the LSBs assembled with our double-layer electrolyte PPL-PL possess the superior comprehensive performances, which can be a potential for commercialization of the LSBs. Fig. 9 is the schematic illustrations of Li dendrite growth behaviors in different cells. Lithium ions have uneven nucleation on the Li anode of S@C/LE/ Li cell, causing the serious Li dendrites after 100 cycles. However, the uniform SEI rather than detrimental Li dendrites on the Li anode in S@C/PPL-PL/Li cell contributes to higher safety of LSBs [42]. Simultaneously, the uniform SEI can further impede the reaction between polysufides and Li anode, protecting the Li anode from degradation [43,44]. The morphological change process of Li anode in S@C/LE/Li and S@C/PPL-PL/Li cells after charging/discharging cycles are observed in Fig. S8. The initial state of Li anode of S@C/LE/Li and S@C/ PPL-PL/Li cells are identically smooth. However, after 10 cycles their surface morphology exhibits a great difference. For the S@C/LE/Li cell, there are many particles on the Li anode, because the commercial
because the commercial LE separator cannot suppress the Li dendrite, triggering the short circuit of battery. At the same time, adopting the LE separator cannot weaken the shuttle effect, resulting in the low coulombic efficiency, the loss of capacity and reduction of cycle lives. Furthermore, we also investigate the electrochemical process of batteries with three types of electrolyte membranes. Fig. 8a–c are the EIS plots of the three LSBs recorded before and after charge-discharge 100 cycles. Fig. 8d is the equivalent circuit. The Rw, Rb, Rf and Rct represent the Warburg resistance, the bulk resistance of electrolyte, the interfacial resistance and the charge transfer resistance, respectively [41]. Compared with S@C/PPL/Li cell, the S@C/PPL-PL/Li cell displays a low Rf, Rct. And it also demonstrates a smaller increase of Rf and Rct after cycling for 100 cycles. It is well known that the continuous increase of Rf and Rct is triggered by the passivation of both cathode and anode during the cycling process. Therefore, our refined battery has low resistance and passivation amplitude. The EIS results can shed more lights on the good design and electrochemical performance of our 45
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separator cannot suppress “shuttle effect” of Li2Sx (4 ≤ X ≤ 8) and Li dendrite formation [45,46]. And after 100 cycles, the coarse dendrite covers the whole surface of the Li anode, which is really detrimental to the electrochemical performances. However, the surface of the Li anode of S@C/PPL-PL/Li cell has no obvious change after 10 cycles. Although 100 cycles later, there are a few blocks and cracks on the Li surface due to the volume expansion, it is still insignificant compared with that in S@C/LE/Li cell. Simultaneously, lower S content can be found on the Li surface of the S@C/LE/Li cell due to the lower degree of “shuttle effect” (Fig. 7d, f). So the PPL-PL electrolyte membranes with excellent mechanical property have the power to significantly suppress the polysulfides diffusion and lithium dendrite growth [47].
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4. Conclusions In summary, LSBs with superior performance have been assembled with our novel designed double-layer electrolyte membranes. The excellent mechanical property and high ionic conductivity of PPL-PL electrolytes hugely suppress the growth of Li dendrite as well as promoting the cyclic stability and rate capability. Meanwhile, the LSBs assembled show a more stable coulombic efficiency of 99.6–100.0% over 100 cycles compared with the single PPL electrolyte membrane or LE. Herein our cost-effective and facile approach to design a novel electrolyte provides a potential to commercialize LSBs in the field of new energy. Acknowledgements We sincerely appreciate the financial and technological supports of National Natural Science Foundation of China Grant No. 51502263, the Qianjiang Talents Plan D Grant No. QJD1602029 and Startup Foundation for Hundred-Talent Program of Zhejiang University. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.03.048. References [1] J. Chen, W.A. Henderson, H. Pan, B.R. Perdue, R. Cao, J.Z. Hu, C. Wan, K.S. Han, K.T. Mueller, J.-G. Zhang, Y. Shao, J. Liu, Improving lithium-sulfur battery performance under lean electrolyte through nanoscale confinement in soft swellable gels, Nano Lett. 17 (2017) 3061–3067. [2] Z. Gao, H. Sun, L. Fu, F. Ye, Y. Zhang, W. Luo, Y. Huang, Promises, challenges, and recent progress of inorganic solid-state electrolytes for all-solid-state lithium batteries, Adv. Mater. 30 (2018) 1705702. [3] S.S. Jeong, Y. Lim, Y.J. Choi, G.B. Cho, K.W. Kim, H.J. Ahn, K.K. Cho, Electrochemical properties of lithium sulfur cells using PEO polymer electrolytes prepared under three different mixing conditions, J. Power Sources 174 (2007) 745–750. [4] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928–935. [5] Z. Lin, C.D. Liang, Lithium-sulfur batteries: from liquid to solid cells, J. Mater. Chem. A 3 (2015) 936–958. [6] A. Song, Y. Huang, X. Zhong, H. Cao, B. Liu, Y. Lin, M. Wang, X. Li, Novel lignocellulose based gel polymer electrolyte with higher comprehensive performances for rechargeable lithium-sulfur battery, J. Membr. Sci. 556 (2018) 203–213. [7] S. Xiong, K. Xie, Y. Diao, X. Hong, Characterization of the solid electrolyte interphase on lithium anode for preventing the shuttle mechanism in lithium-sulfur batteries, J. Power Sources 246 (2014) 840–845. [8] Y. Ye, L. Wang, L. Guan, F. Wu, J. Qian, T. Zhao, X. Zhang, Y. Xing, J. Shi, L. Li, R. Chen, A modularly-assembled interlayer to entrap polysulfides and protect lithium metal anode for high areal capacity lithium-sulfur batteries, Energy Storage Mater. 9 (2017) 126–133. [9] J.-j. Chen, Q. Zhang, Y.-n. Shi, L.-l. Qin, Y. Cao, M.-s. Zheng, Q.-f. Dong, A hierarchical architecture S/MWCNT nanomicrosphere with large pores for lithium sulfur batteries, Phys. Chem. 14 (2012) 5376–5382. [10] X.L. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat. Mater. 8 (2009) 500–506. [11] Y. You, Y. Ye, M. Wei, W. Sun, Q. Tang, J. Zhang, X. Chen, H. Li, J. Xu, Threedimensional MoS2/rGO foams as efficient sulfur hosts for high-performance lithium-sulfur batteries, Chem. Eng. J. 355 (2019) 671–678.
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15542–15549. [44] H.-K. Jing, L.-L. Kong, S. Liu, G.-R. Li, X.-P. Gao, Protected lithium anode with porous Al2O3 layer for lithium–sulfur battery, J. Mater. Chem. A 3 (2015) 12213–12219. [45] J. Steiger, D. Kramer, R. Moenig, Microscopic observations of the formation, growth and shrinkage of lithium moss during electrodeposition and dissolution, Electrochim. Acta 136 (2014) 529–536. [46] V. Yurkiv, T. Foroozan, A. Ramasubramanian, R. Shahbazian-Yassar, F. Mashayek, Phase-field modeling of solid electrolyte interface (SEI) influence on Li dendritic behavior, Electrochim. Acta 265 (2018) 609–619. [47] Y.S. Su, A. Manthiram, Lithium-sulphur batteries with a microporous carbon paper as a bifunctional interlayer, Nat. Commun. 3 (2012) 6.
electrolyte to stabilize the lithium anode for a quasi-solid-state lithium–sulfur battery, J. Mater. Chem. A 6 (2018) 18627–18634. [41] L. Suo, D. Oh, Y. Lin, Z. Zhuo, O. Borodin, T. Gao, F. Wang, A. Kushima, Z. Wang, H.-C. Kim, Y. Qi, W. Yang, F. Pan, J. Li, K. Xu, C. Wang, How solid-electrolyte interphase forms in aqueous electrolytes, J. Am. Chem. Soc. 139 (2017) 18670–18680. [42] M.F. Wu, Z.Y. Wen, J. Jin, Bobba V.R. Chowdari, Trimethylsilyl chloride-modified Li anode for enhanced performance of Li−S cells, ACS Appl. Mater. Interfaces 8 (2016) 16386–16395. [43] F. Wu, J. Qian, R.J. Chen, J. Lu, L. Li, H.M. Wu, J.Z. Chen, T. Zhao, Y.S. Ye, Khalil Amine, An effective approach to protect lithium anode and improve cycle performance for Li−S batteries, ACS Appl. Mater. Interfaces 6 (2014)
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A polyacrylonitrile (PAN)-based double-layer multifunctional gel polymer electrolyte for lithium-sulfur batteries
T
Xiuli Wang∗,1, Xiaojing Hao1, Yan Xia, Yanfei Liang, Xinhui Xia, Jiangping Tu State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, And School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Quasi-solid-state lithium-sulfur battery Polyacrylonitrile Polyethylene oxide Gel polymer electrolyte
Owing to high theoretical capacity, lithium-sulfur batteries (LSBs) are receiving extensive researches. However, cyclic instability and safety issues hugely confine the commercial applications of traditional liquid LSBs. In this work, for the sake of fully leveraging the high ionic conductivity of polyacrylonitrile while avoiding Li anode “passivation effect” caused by CN group, we prepare double-layer gel polymer electrolytes for quasi-solid-state LSBs. The transition layer composed of polyacrylonitrile, polyethylene oxide and Li1.3Al0.3Ti1.7(PO4)3 (LATP) is located on Li anode side to reduce “passivation effect” triggered by pure polyacrylonitrile. Meanwhile, the high ionic conductivity layer composed of polyacrylonitrile and LATP in contact with cathode can utilize the high intrinsic ionic conductivity of polyacrylonitrile to enhance the rate performance. Furthermore, LATP with higher ionic conductivity embedded in the membrane serves as Li+ transport channels to further increase ionic conductivity. Prominently, the designed double-layer electrolytes exhibit a high Li+ transference number of 0.55 and superior mechanical property. Moreover, stable coulombic efficiency of 99.6–100.0% over 100 cycles and good capacity retention of 79.0% after 100 cycles at 0.1C can be achieved. Our newly designed double-layer electrolytes with multiple functions exhibit potential applications in safer LSBs.
1. Introduction Nowadays, the cathodes of the commercial lithium-ion batteries (LIBs) are mainly LiFePO4, LiCoO2 and LiMn2O4 and so on, and the anodes are mostly modified carbon materials, leading to extensive applications of LIBs for efficient energy storage equipment as well as new energy vehicles [1,2]. However, they are far from satisfaction owing to the low energy density and safety issues. Nowadays, using high specific capacity sulfur as the cathode active materials can fulfill the dream of high energy density and efficiency of LIBs, which makes the sulfur stand out from many traditional insertion-host materials [3]. What's more, sulfur is cheap, nontoxic and environmentally friendly [4,5]. Nonetheless, prior to the commercialization of LSBs, some technical challenges must be addressed, such as Li anode passivation, the intrinsic low conductivity of sulfur, harmful “shuttle effect” caused by intermediate polysulfides and electrolyte decomposition [6]. Typically, some strategies can eliminate part of the above problems, such as protecting the Li metal anode [7,8], modifying sulfur with conductive
materials [9–11] or metal oxides [12] and leveraging solid-state electrolytes (SSEs) [13,14]. And one of the most promising strategies is fabricating SSEs as modified separators since they can not only reduce the capacity decay, but also drastically reduce the probability of inflammation [15]. According to this strategy aforementioned, diverse Li+ conducting inorganic and polymer electrolytes have been developed. But none of them are commercialized owing to various shortcomings. Li+ conductive inorganic substances possess high ionic conductivities and wide electrochemical windows. LATP is a typical oxide solid electrolyte, which is about 3 × 10−3 S cm−1 at 25 °C [16]. However, poor interfacial compatibility with electrolyte hugely limits the extensive applications of inorganic electrolytes. Furthermore, the Li dendrites form and grow even faster in the grain boundaries of inorganic electrolyte membranes than they do in the traditional liquid electrolyte (LE) [17,18]. So it is of little significance to directly assemble the inorganic electrolytes into solid-state batteries (SSBs). Attractively, the gel polymer electrolytes (GPEs) become outstanding due to the excellent
Abbreviations: PPL, monolayer electrolyte membrane composed of PAN, PEO and LATP; PL, monolayer electrolyte membrane composed of PAN and LATP; PPL-PL, double-layer electrolyte membrane ∗ Corresponding author. E-mail address: [email protected] (X. Wang). 1 These authors contributed equality to this work. https://doi.org/10.1016/j.memsci.2019.03.048 Received 19 January 2019; Received in revised form 14 March 2019; Accepted 16 March 2019 Available online 20 March 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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interfacial wetting ability, flexibility and good electrochemical performance. The batteries assembled with GPEs can be referred as quasisolid-state batteries (QSSBs). Various polymers like polyacrylonitrile (PAN) [19], poly(methyl methacrylate) (PMMA) [20], polyethylene oxide (PEO) [21,22], poly(vinylidene fluoride) (PVDF) [23], poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) [24], poly(propylene carbonate) (PPC) [25] and many other newly synthesized polymers were used due to their unique advantages. Among them, PAN is known for excellent mechanical properties and ultra-high intrinsic ionic conductivity of about 10−3 S cm−1 at 298 K. But CN groups in pure PAN have reaction with Li anode, which will form a “passivation layer” between Li anode and PAN electrolyte, increasing impedance, blocking ion transport and deteriorating electrochemical performance [26,27]. Fortunately, PAN mixing with other polymeric or inorganic materials can attenuate this issue to some extent. Li et al. demonstrated that the hydrogen bond between CN groups in PAN and hydroxyl groups in graphene oxide can effectively weaken polarization and improve electrochemical stability [28]. As another traditional polymer material, PEO exhibits low lattice energy and high capacity of dissolving alkali metal salts, but the high degree of crystallinity, poor thermo-stability and low mechanical strength hinder its independent usage in SSBs. Therefore, in recent years, PEO often serve as an additive polymer to improve the performance of the matrix polymer [29]. Similarly, electron-withdrawing group hydroxyl in PEO can form intermolecular hydrogen bond with CN group in PAN and reduce the “passivation effect” caused by the pure PAN, which is first demonstrated in our work. On the other hand, another effective way to improve the efficiency of LSBs is to design the electrolyte configuration. Goodenough et al. designed a double-layer electrolyte with a high-voltage stable layer contacting cathode and a low-voltage stable layer contacting anode, realizing the high-voltage SSBs [30]. Gao et al. created a surface modified PVDFbased GPE with polydopamine [31]. And the created polydopamine layer can effectively increase the interfacial stability with the Li anode, demonstrating the reasonable electrolyte configuration. Herein, we propose a model of double-layer configuration with both high mechanical moduli and ionic conductivity. The designed electrolyte is composed of two layers and has different functions. The transition layer in contact with Li anode is created by rational combination of PAN, PEO and LATP. It can be referred as PPL, which is used as the protect layer to overcome the “passivation effect” of pure PAN by forming hydrogen bond with PEO. The high ionic conductivity layer PL is fabricated by rational combination of PAN and LATP, but without PEO. It can exert the high intrinsic ionic conductivity of PAN when contacting with the cathode, enhancing the total ionic conductivity of the PPL-PL electrolyte membranes. LATP with high ionic conductivity embedded in the membrane can serve as the Li+ transport channels to further increase ionic conductivity of the composite membrane. Accordingly, QSSBs assembled with this double-layer electrolytes display desirable electrochemical properties.
10 min was to prepare a semi-dry layer so that the high ionic conductivity PL layer could tightly adhere to the transition layer with no seams. And the PL layer was prepared via casting the second slurry on the first PPL electrolyte membrane. Then the PPL-PL double-layer electrolyte membrane was volatilized in vacuum at 40 °C for 16 h. After that, the membranes were collected into glove box after cutting into disks (19 mm in diameter). Finally, the above round PPL-PL electrolytes were immersed in the commercial LE consisted of 1.0 m lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio of 1:1) with 1.0 wt % LiNO3 solution. Fig. 1 also illustrates the working mechanism and stacking model of PPL-PL electrolyte in a solid-state cell, in which the two layers exert different electrochemical functions. In order to demonstrate the super performance of our PPL-PL electrolyte membrane, another monolayer electrolyte was also fabricated as the control group. All the process was similar except the original composition. The control electrolyte consisted of PAN (1.5 g), PEO (1.5 g) and LATP (0.3 g), named PPL electrolyte. 2.2. Structural characterization The morphology of all these membranes were obtained by a Hitachi S4800 Field emission scanning electron microscopy (FESEM, Japan). The element mappings of cross section were investigated by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) patterns were characterized on X'Pert PRO X-ray diffractometer with CuKα radiation from 10° to 80° in 2θ. Differential scanning calorimetry (DSC) testing was operated under N2 atmosphere via DSC Q100 equipment from −40 °C to 150 °C at the heating rate of 10 °C min−1. Thermogravimetric analysis (TGA) was tested with SDT Q600 instrument under N2 flow at a heating rate of 10 °C min−1 over the temperature range of 25–900 °C. Stress-train tests were characterized on Zwick/Roell Z020 material testing machine with the crosshead speed at 20 mm min−1. 2.3. Electrochemical evaluation Firstly, we prepared the S@C composite (Ws:Wsuper P = 3:2) by melt-diffusion strategy. After grinding together, the sublimed sulfur and super P were heated in the autoclave with a Teflon liner at 155 °C for 12 h. Then, the cathode sludge was obtained by dissolving synthesized S@C composite (90 wt %), PVDF binder (10 wt %) into the N-methy-12-pyrrolidone (NMP). After stirring homogeneously, the sludge was evenly spread onto Al foils and volatilized at 60 °C for 10 h in vacuum. The mass loading of the active materials were about 1.0–1.3 mg cm−2. Subsequently, we assembled CR2025 coin-type S@C/PPL-PL/Li cells in the Ar-filled glove box, using S@C composite as cathode, PPL-PL as electrolyte membrane and Li foil as anode. For comparison, the S@C/ LE/Li and S@C/PPL/Li cells were also assembled with the same way but using commercial LE separator or PPL as the electrolyte membrane respectively. By soaking weight-measured membrane in the commercial Li-S liquid electrolyte, we can figure out the electrolyte uptake (EU). The EU value was calculated by Eq. (1):
2. Experiment 2.1. Preparation of PPL-PL electrolyte First, we synthesized the fine LATP particles through a sol-gel method by a previous report [32]. The preparation of PPL-PL electrolyte membrane is presented schematically in Fig. 1. 1.5 g PAN and 1.5 g PEO were added into the N,N-Dimethylformamide (DMF) to form the first solution, then LATP particles (0.3 g) with a ratio of 10% PAN and PEO by weight were introduced into the above solution. After ultrasonic treatment for 30 min and magnetic stirring for 8 h, we acquired the first homogeneous solution. Similarly, we prepared the second solution without any PEO but 3.0 g PAN and 0.3 g LATP particles only. The first slurry was scraped on the tempered glass by a square blade, afterwards it was volatilized at 40 °C for 10 min to obtain the transition layer PPL electrolyte membrane. The drying time controlled within
EU% = [(ωwet − ωdry )/ ωdry] × 100%
(1)
Where ωdry represents the weight of dry membrane, and ωwet is the weight of LE-absorbing membrane. The volume value (Vm) of absorbed LE corresponding to unit active substance in an assembled LSB was calculated by Eq. (2):
Vm = (ωwet − ωdry )/(ρ × msulfur )
(2)
Where the density of liquid electrolyte (ρ) is 1.174 g/mL, msulfur is the weight of active material sulfur on a cathode. The ionic conductivity measurements were performed on a Princeton multi-channel electrochemical workstation by AC 38
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Fig. 1. Schematic of the preparation of PPL-PL electrolyte membrane and the working mechanism and stacking model of PPL-PL electrolyte in a QSSB.
3. Results and discussion
electrochemical impedance spectroscopy (EIS). The electrolyte membrane was sandwiched by two stainless steel electrodes on two sides. The ionic conductivities (σ, in S cm−1) can be calculated via Eq. (3):
σ = d/(Rb × S )
The cross section of PPL-PL electrolyte membrane is presented in Fig. 2a with the thickness of roughly 20 μm, which is much thinner than many present works and can compete with the commercial LE separators of 16–20 μm (Fig. 2b). The intimate binding of two layers is demonstrated since there is no stratification between them, which is in favor of the stability of the electrolyte membranes during the working process. The photograph of PPL-PL electrolyte membrane before soaking the LE is shown in Fig. 2c, which shows a semi-transparent physical properties. Fig. 2d displays the surface of the side that will contact with Li anode, demonstrating a lot of small pits (d < 400 nm) evenly (the SEM morphology of the side that will contact cathode is identical, with a lot of small pits, Fig. S1). Herein it will impart a high surface area and accommodate more active Li+ in the membrane, enhancing the EU and improving the ionic conductivity [33]. Surface SEM images of layer in contact with Li anode in PPL-PL electrolyte membrane with various amounts of LATP are presented in Fig. S2. It displays different surface morphologies. Compared with the membrane with 10 wt % LATP, the membrane with 5 wt % LATP is smooth without any pits to store more Li+, while the membranes with 20 wt % or 30 wt % LATP are so tough that will exhibit high surface resistance. We can ascertain from the morphology that system with 10 wt % LATP is the optimal one to exhibit good electrochemical ability, which can be further confirmed by EU and ionic conductivity of LE-absorbing PPL-PL electrolytes with various amounts of LATP at 25 °C (Fig. 1e–f). The EU of PPL-PL electrolyte membranes with 5 wt %, 10 wt %, 20 wt %, 30 wt % LATP are 145.3%, 166.5%, 136.4%, 125.7%, and the ionic conductivities of those are 5.11 × 10−4, 8.61 × 10−4, 5.34 × 10−4,
(3)
Where d and Rb represent the thickness and bulk resistance of the electrolyte, respectively. S represents the contact area between electrolyte and stainless steel electrode. The lithium ion transference number (tLi+) was calculated by combining the AC impedance and DC polarization of the Li/electrolyte/ Li cells. The tLi+ was computed via Bruce Vincent Eq. (4):
tLi + =
Is (ΔV − I0 R 0) I0 (ΔV − Is Rs )
(4)
Where the initial polarization current: I0 and steady state polarization current: Is can be acquired via DC polarization test simultaneously, the initial interfacial resistance: R0 and steady state interfacial resistance: Rs can be measured by EIS before and after the polarization, in which the voltage amplitude is 0.01 V. The linear sweep voltammetry (LSV) was used to test the electrochemical stability window of the electrolyte membranes. The voltage range is 2.0–6.0 V, and scanning rate is 0.1 mV s−1. The cyclic voltammetry (CV) curves were obtained under the voltage of 1.7–3.0 V at 0.1 mV s−1. The galvanostatic charge-discharge (GCD) were carried out on LAND battery-testing instrument from 1.7 to 2.8 V at various current densities at room temperature. EIS tests of LSBs before and after cycling were performed under the voltage amplitude of 10 mV with the frequency range 106–10−1 Hz. 39
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Fig. 2. Cross section SEM images of (a) PPL-PL electrolyte membrane and (b) LE separator; (c) Optical photograph of PPL-PL electrolyte membrane; (d) Surface SEM image of layer that contact with Li anode; (e) electrolyte uptake of PPL-PL electrolyte membrane with different LATP content; (f) ionic conductivity of PPL-PL electrolyte membrane with different LATP content.
3.52 × 10−4 S cm−1 respectively, demonstrating the optimal amount of 10 wt % LATP. Therefore, in subsequent works, the PPL-PL electrolyte membrane with 10 wt % LATP was selected in order to exert its best electrochemical potentials. At the same time, the PPL-PL electrolyte membrane shows good liquid retention ability compared with PPL electrolyte membrane and LE separator. Fig. S3 shows the thermogravimetric results of the PPL-PL, PPL electrolyte membranes and LE separator with 20 wt % liquid electrolyte. The liquid electrolyte only conserves within the micropores of the LE separator temporarily instead of forming incorporation, so it evaporates rapidly in LE separator. The polymer membranes of PPL, PPL-PL form gel via integrating with the LE, locking LE and Li+ in the membrane and obtaining excellent liquid retaining property. More importantly, the addition of PEO in PL layer of the PPL-PL membrane decreases the crystallinity and increases the disorder of polymers, which further increases LE locking ability of the PPL-PL electrolyte membrane. The elemental mapping can shed more lights on to further confirm the double-layer structure. In Fig. 3, C, P, Ti and Al are evenly presented in the cross section, while N is distributed differently due to the difference in the amount of original PAN in the two layers. The EDS results clearly demonstrate the hierarchical structure of our electrolyte membrane (shown by dotted red line in Fig. 3). However, there is no obvious stratification in SEM (Fig. 2a), suggesting the intimate contact between two layers. From the SEM of PPL-PL electrolyte membrane after 100 cycles (shown in Fig. S4), the thickness of membrane exhibits a little increase to 25 μm due to the intake of liquid electrolyte. However, there
is no obvious layered interface between the two layers, demonstrating the excellent stability of PPL-PL electrolyte membrane. XRD was conducted to identify the interaction between different materials. In Fig. 4a, the diffraction peaks of PAN+PEO membrane suggest just a blend of PAN and PEO. The diffraction peaks of PAN +PEO+LATP membrane is a blend of the three compositions, and LATP in the PAN+PEO+LATP membrane are relatively weak, indicating the amount of LATP exposed on the membrane surface is very small. In addition, the diffraction peaks from 13° to 20° of the membrane are broadened and weakened, which is attributed to the introduce of LATP. Because LATP can interact with the long chain in the polymer, then expand the amorphous region and reduce the polymeric crystallinity. The increase of amorphous region will increase the ionic conductivity and then benefit the comprehensive electrochemical properties of QSSBs, matching well with the results of ionic conductivity (Fig. 2f). Simultaneously, LATP with higher ionic conductivity embedded in the membrane can serve as the Li+ transport channels to further raise the ionic conductivity of the PPL-PL composite membrane. Tensile strength measurement was carried out to investigate the mechanical properties of LE separator, PPL and PPL-PL electrolyte membranes. Currently, one of the most important features for separators to be commercially applied is the excellent mechanical properties [34]. As shown in Fig. 4b and Table S1, the commercial LE separator's maximal mechanical tensile and strain are 94.3 MPa and 35.5% respectively. The corresponding features of PPL electrolyte membrane are 35.1 MPa and 135.2%. However, the mechanical tensile and strain of 40
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Fig. 3. EDS spectrum of the PPL-PL electrolyte membrane.
Fig. 4. (a) XRD patterns of the compositions of PPL-PL electrolyte membrane; (b) Stress-strain curves of LE, PPL, PPL-PL electrolyte membranes; (c) DSC curves of PAN, PAN+PEO, PAN+PEO+LATP; (d) TGA curves of LATP, PAN, PAN+PEO, PAN+PEO+LATP; (e) Schematic diagram of the intermolecular “hydrogen bonding effect” between PAN and PEO and the passivation weakening mechanism of CN groups by hydrogen bond. 41
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effect”. We assembled LSBs and conducted different electrochemical tests. Fig. 5e shows the ionic conductivities of the LE separator, PPL and PPL-PL electrolyte membranes within 20–90 °C. Apparently, ionic conductivities raise with the raise of temperature because of the increasing movements of polymer chains and free Li+ [36]. At 25 °C, the ionic conductivities of LE separator, PPL and PPL-PL electrolyte membranes are calculated to be 9.37 × 10−4, 2.39 × 10−4, 8.61 × 10−4 S cm−1, respectively. Notably, the PPL-PL electrolyte membrane exhibits the high ionic conductivity due to the hierarchical structure, while the monolayer of PPL electrolyte membrane possesses the lowest ionic conductivity because of the low intrinsic conductivity of PEO. Therefore, putting the high ionic conductivity PL layer on the other hand can significantly raise the total ionic conductivity of the membrane compared with monolayer PPL electrolyte membrane. Lithium ion transference number exhibits a great influence on the electrode polarization during the lithium plating and stripping process. So a high tLi+ is essential for the SSEs [37]. As we can see from Fig. 5f, after polarization, the interfacial resistance of PPL-PL electrolyte membrane increases from 103.7 to 130.1 Ω, the current changes from initial value of 82.6 mA to the steady state of 61.6 mA. Following the Bruce-Vincent formula, tLi+ is computed to be 0.55. However, the value of the commercial LE separator and PPL electrolyte membrane are 0.35 [38] and 0.39 (Fig. S5), respectively. Although the transition PPL layer can protect the Li anode from passivation, actually this protection comes at the cost of reducing electrochemical performance compared with PL layer because of the comparatively lower ionic conductivity of PPL layer. Therefore, our tests demonstrate that high ionic conductivity PL layer in PPL-PL electrolyte indeed increases ionic conductivity and lithium ion transference number compared with monolayer of PPL electrolyte membrane, which powerfully proves the effectiveness of the PL layer in our PPL-PL electrolyte membrane. The LSV curves (Fig. 6a) of the stainless steel (SS)/PPL-PL/Li, SS/ PPL/Li, and SS/LE/Li cells were measured to test the electrochemical stability of different electrolytes. For the SS/PPL-PL/Li, the LSV curve maintains stable until 5.0 V, so PPL-PL electrolyte membrane would not decompose under 5.0 V. Nevertheless, there is a sharp rise of current at 4.53 V for SS/LE/Li cell, which can be explained by the drastic decomposition of LE during the oxidation process. The current of the SS/ PPL/Li cell rises at 4.70 V, which is still lower than the SS/PPL-PL/Li cell. Fig. 6b–c is the EIS tests of SS/PPL/Li and SS/PPL-PL/Li cells at bias voltage of 4.8 V for different time at room temperature to analyse electrochemical stability under high voltage. The EIS of the SS/PPL-PL/ Li cell has little change within 48 h, because it has an oxidation voltage at 5 V and stable under 4.8 V. By contrast, the impedance of SS/PPL/Li cell is doubled, because the membrane of PPL can be oxidized and degraded under 4.8 V. However, when the SS/LE/Li cell is measured under 4.8 V, it is electrically broken down because of the massive decomposition of liquid electrolyte. Fig. 6d characterizes the third cycle of CV curves concerning the S@C/LE/Li, S@C/PPL/Li and S@C/PPL-PL/Li cells. There are three characteristic peaks, reduction peaks: 2.09 V and 2.29 V, oxidation peak: 2.51 V, which are all in accordance with typical charge-discharge characteristic peaks of LSBs. Furthermore, in terms of the S@C/PPL-PL/ Li cell, the higher reduction peak and lower oxidation peak indicate the lowest polarization [39]. Simultaneously, the highest capacity further demonstrates its elevated electrochemical performance. To further highlight the desired structure and excellent electrochemical performance of our hierarchical design of PPL-PL electrolyte membrane, we assembled S@C/LE/Li, S@C/PPL/Li and S@C/PPL-PL/ Li LSBs. As shown in Table S3, the volume value (Vm) of absorbed LE corresponding to unit active substance in an assembled S@C/PPL/Li and S@C/PPL-PL/Li LSBs was calculated to be 4 and 5 μL/mg respectively, which is quite fewer than the traditional liquid LSBs of about 60 μL/mg [40]. It seems that the PPL-PL electrolyte membrane has a high value of EU, but the volume of absorbed LE is really small. Actually, the high EU but low Vm in an assembled LSB contributes to the
the PPL-PL electrolyte membrane are 22.6 MPa and 180.4%, demonstrating an improved fracture toughness. It is worth to mention that with the increase amount of the LATP, modulus and stress of the membrane have a gradual increase at the cost of a sharp decrease of the strain (Table S2). Because the inorganic fillers embedded in the polymer matrix can hinder the sliding of grain boundaries and absorb a lot of deformation work under the action of external force, increasing the modulus and strain of the membrane. However, with the addition of inorganic fillers, the inhomogeneity in the membrane may occur, which may result in stress concentration and decrease the strain at break. Based on the above results, the PPL-PL electrolyte membrane with 10% LATP content exhibits excellent mechanical properties and shows commercial potential in rechargeable SSBs. DSC and TGA were used to analyse the thermal properties. Tg and Tm of pure PAN are about 90 °C and 317 °C. Tg and Tm of pure PEO are around −45 °C and 64 °C [35]. Fig. 4c characterizes the DSC curves within −40 °C-150 °C. The pure PAN electrolyte membrane has a Tg at around 90.1 °C. After adding PEO, the Tg of the PAN decreases to the 86.0 °C. At the same time, the new endothermic peak at around 50.4 °C refers to the Tm of PEO. The slightly decrease of Tg of the PAN and Tm of the PEO is because the lone pair electrons on the N in CN group can form HeN hydrogen bond with PEO. Fig. 4e is the schematic diagram of the intermolecular “hydrogen bonding effect” between PAN and PEO. The “hydrogen bonding effect” enable polymer chains to be more chaotic, which increases amorphous regions and reduces the crystallinity. The endothermic peak of PAN+PEO+LATP electrolyte membrane further reduces to low temperature after adding LATP (the Tg of the PAN becomes indistinct), demonstrating the addition of LATP can further reduce the crystallinity. Fig. 4d illustrates the TGA curves of LATP particles, pure PAN, PAN +PEO, and PAN+PEO+LATP electrolyte membranes. During the heating process, the pure LATP keeps weight stable, exhibiting negligible mass loss over the entire temperature range 25 °C–900 °C. At 900 °C, the weight retention rate of pure PAN, PAN+PEO, and PAN +PEO+LATP three electrolyte membranes are 36.9%, 29.2% and 42.4%, respectively. The degradation temperatures at a 5% weight loss values (Td,5%) of the three membranes are 321 °C, 298 °C, 282 °C, respectively, implying all the electrolyte membranes could live up to the working temperature requirements of SSBs (60–80 °C). The slightly decrease of the Td,5% values ascribes to the reduction of the crystalline segments and the increase of the amorphous regions, which are coincide with the DSC consequences. Fig. 5a–b are the combustion test of commercial LE separator and PPL-PL electrolyte membrane. LE separator is rapidly ignited and scorched quickly due to its poor thermal stability. By contrast, PPL-PL electrolyte membrane displays much better thermal stability. Furthermore, the two membranes are heated at 150 °C for 2 h. LE separator shrinks obviously (Fig. 5c). However, the shape of the PPL-PL membrane maintains well except little shrink on the edge (Fig. 5d). Thus, the PPL-PL electrolyte membrane possesses much better thermal stability compared with commercial LE separator. For the sake of appraising the electrochemical properties of our PPLPL electrolyte membrane, in which PPL layer serves as the transition layer to protect Li anode from passivation caused by pure PAN, and PL layer is designed to take advantage of the high intrinsic ionic conductivity of PAN. The addition of PEO can form HeN hydrogen bond with nitrogen atom in CN group (Fig. 4e), and this kind of intermolecular interaction can increase the amorphous regions, more importantly, it plays an effective way to weaken the “passivation effect” of strong polar CN group. Without the hydrogen bond to restrict the strong polar CN group, more free CN groups will react with active Li anode, forming a passivation layer on the Li anode and impeding the exertion of its electrochemical potential. This so called “passivation effect” can deteriorate the electrochemical performances. However, via the restriction of the hydrogen bond, reactivity between CN group and Li anode is hugely reduced, which can efficiently weaken the “passivation 42
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Fig. 5. The flammability test of the (a) LE separator and (b) PPL-PL electrolyte membrane; Heating experiments operated at 150 °C for 2 h on the (c) LE separator and (d) PPL-PL electrolyte membrane; (e) Ionic conductivity of LE separator, PPL and PPLPL electrolyte membranes in the temperature range of 20–90 °C; (f) Current-time curve obtained from chronoamperometry at a DC polarization of 0.01 V, inset: EIS plot of the Li/PPL-PL/Li cell before and after polarization.
Fig. 6. (a) LSV curves of LE separator and PPL-PL, PPL electrolyte membranes; EIS of (b) SS/PPL-PL/Li cell, (c) SS/PPL/Li cell at bias voltage 4.8 V for different time at 25 °C, (d) The third cycle of CV curves concerning the three cells in the potential range of 1.7–3 V (Li/Li+) at a scan rate of 0.1 mV s−1. 43
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Fig. 7. The initial charge/discharge-voltage curves of (a) S@C/LE/Li, (b) S@C/PPL/Li and (c) S@C/PPL-PL/Li cells; (d) Rate performance of three cells from 0.05 to 0.5C; (e) Cycling performance of three cells at 0.1C.
The rate performance (Fig. 7d) displays the same results, the specific capacities of S@C/LE/Li cells decrease quickly at different current density. The dissolution of polysulfides into the LE and spread to the Li anode through commercial separator (shuttle effect) drastically harm the performance of LSBs. The S@C/PPL-PL/Li cell displaces discharge capacities up to 1020.0, 883.5, 721.4, 409.6 mAh g−1 at rate of 0.05, 0.1, 0.2 and 0.5 C at room temperature respectively (shown in Fig. 7c–d), which is more stable than other contrast groups. The excellent electrochemical properties can be also demonstrated by cycling performance in Fig. 7e, S@C/PPL-PL/Li cell shows a much higher discharge capacity retention about 79.0% (from 903.5 to 714.1 mAh g−1) at 0.1C compared with S@C/LE/Li cell (32.1%), and S@C/PPL/Li cell (69.8%). Meanwhile, its superior coulombic efficiency about 99.6–100.0% exhibits its good cyclic stability. The long-time discharge ability was tested at 0.05C for commercial S@C/LE/Li, S@C/PPL/Li and S@C/PPL-PL/Li cells in Fig. S7. The S@C/PPL-PL/Li cell exhibits more stable capacity retention about 80.0% after 300 cycles compared with that of 61.9% after 300 cycles in terms of S@C/PPL/Li cell and 63.5% only after 120 cycles in terms of S@C/LE/Li cell. And 300 cycles later, discharge capacity of S@C/PPL-PL/Li still maintains at 803.1 mAh g−1. However, 120 cycles later, S@C/LE/Li cell is damaged,
superior electrochemical performance as well as the reliable security. Fig. 7a–c are the initial GCD curves of different LSBs that we picked out from the charge-discharge test at different current density. Obviously, S@C/LE/Li cells have the inferior performance. Their specific discharge capacities at 0.05, 0.1, 0.2, 0.5C are 1041.5, 712.2, 509.7 and 203.5 mAh g−1, while those of S@C/PPL/Li cells are 943.4, 801.1, 541.4 and 312.6 mAh g−1, and S@C/PPL-PL/Li cells are 1018.3, 903.5, 726.1 and 411.6 mAh g−1. It is known that PAN is a promising electrolyte for its high intrinsic ionic conductivity. However, it will react with Li anode, causing anode passivation due to the CN group. And the detrimental effect of PAN is demonstrated when we assemble S@C/ PAN/Li cells (Fig. S6). Without the protection of PPL layer, the pure PAN electrolyte (with 10 wt % LATP) dies only after 40 cycles. Besides, its coulombic efficiency hugely deviates from 100%, because the strong polarity of PAN causes the passivation of the Li anode during charging and discharging, which leads to the loss of capacity and low coulombic efficiency for the assembled full cells. Furthermore, its initial impedance is large, and the impedance increases hugely after 40 cycles, which is caused by the anode passivation resulted from PAN. Therefore, the effective function of PPL layer in our PPL-PL electrolyte membrane is also strongly demonstrated. 44
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Fig. 8. EIS plots for (a) S@C/LE/Li cell, (b) S@C/PPL/Li cell, (c) S@C/PPL-PL/Li cell before cycles and after 100 cycles; (d) Equivalent circuit.
Fig. 9. Schematic illustrations of (a) Li dendrite growth behaviors in S@C/LE/Li cell; (b) no Li dendrite growth in S@C/PPL-PL/Li cell during plating and stripping process; The SEM of Li anode (c, e) and the corresponding EDS mapping (d, f) of S in (c, d) S@C/LE/Li cells and (e, f) S@C/PPL-PL/Li cells after 100 cycles.
designed PPL-PL electrolyte membrane. As mentioned above, the LSBs assembled with our double-layer electrolyte PPL-PL possess the superior comprehensive performances, which can be a potential for commercialization of the LSBs. Fig. 9 is the schematic illustrations of Li dendrite growth behaviors in different cells. Lithium ions have uneven nucleation on the Li anode of S@C/LE/ Li cell, causing the serious Li dendrites after 100 cycles. However, the uniform SEI rather than detrimental Li dendrites on the Li anode in S@C/PPL-PL/Li cell contributes to higher safety of LSBs [42]. Simultaneously, the uniform SEI can further impede the reaction between polysufides and Li anode, protecting the Li anode from degradation [43,44]. The morphological change process of Li anode in S@C/LE/Li and S@C/PPL-PL/Li cells after charging/discharging cycles are observed in Fig. S8. The initial state of Li anode of S@C/LE/Li and S@C/ PPL-PL/Li cells are identically smooth. However, after 10 cycles their surface morphology exhibits a great difference. For the S@C/LE/Li cell, there are many particles on the Li anode, because the commercial
because the commercial LE separator cannot suppress the Li dendrite, triggering the short circuit of battery. At the same time, adopting the LE separator cannot weaken the shuttle effect, resulting in the low coulombic efficiency, the loss of capacity and reduction of cycle lives. Furthermore, we also investigate the electrochemical process of batteries with three types of electrolyte membranes. Fig. 8a–c are the EIS plots of the three LSBs recorded before and after charge-discharge 100 cycles. Fig. 8d is the equivalent circuit. The Rw, Rb, Rf and Rct represent the Warburg resistance, the bulk resistance of electrolyte, the interfacial resistance and the charge transfer resistance, respectively [41]. Compared with S@C/PPL/Li cell, the S@C/PPL-PL/Li cell displays a low Rf, Rct. And it also demonstrates a smaller increase of Rf and Rct after cycling for 100 cycles. It is well known that the continuous increase of Rf and Rct is triggered by the passivation of both cathode and anode during the cycling process. Therefore, our refined battery has low resistance and passivation amplitude. The EIS results can shed more lights on the good design and electrochemical performance of our 45
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separator cannot suppress “shuttle effect” of Li2Sx (4 ≤ X ≤ 8) and Li dendrite formation [45,46]. And after 100 cycles, the coarse dendrite covers the whole surface of the Li anode, which is really detrimental to the electrochemical performances. However, the surface of the Li anode of S@C/PPL-PL/Li cell has no obvious change after 10 cycles. Although 100 cycles later, there are a few blocks and cracks on the Li surface due to the volume expansion, it is still insignificant compared with that in S@C/LE/Li cell. Simultaneously, lower S content can be found on the Li surface of the S@C/LE/Li cell due to the lower degree of “shuttle effect” (Fig. 7d, f). So the PPL-PL electrolyte membranes with excellent mechanical property have the power to significantly suppress the polysulfides diffusion and lithium dendrite growth [47].
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