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Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state lithium batteries
Journal Pre-proof Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries Jiangkui Hu, Pingge He, Bochen Zhang, Bingyao Wang, Li–Zhen Fan PII:
S2405-8297(20)30012-X
DOI:
https://doi.org/10.1016/j.ensm.2020.01.006
Reference:
ENSM 1056
To appear in:
Energy Storage Materials
Received Date: 2 December 2019 Revised Date:
31 December 2019
Accepted Date: 8 January 2020
Please cite this article as: J. Hu, P. He, B. Zhang, B. Wang, L.–Z. Fan, Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.01.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.
Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries Jiangkui Hu, Pingge He, Bochen Zhang, Bingyao Wang, Li–Zhen Fan*
Beijing Advanced Innovation Center for Materials Genome Engineering, Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
E-mail: [email protected]
Graphical Abstract for
Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries
3D composite solid electrolytes with mechanically robust, porous polyimide film as a host are prepared by solution casting technique. As-prepared electrolytes exhibit outstanding mechanical properties and could effectively impede the growth of lithium dendrites. The assembled high-voltage NCM/Li pouch cells exhibit high safety and excellent cycling performance, demonstrating a promising strategy to achieve high-safety, high-energy density lithium batteries.
Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries
Abstract: Conventional lithium-ion batteries with liquid organic electrolytes generally suffer potential security risks concerning volatilization, flammability and explosion. Nonflammable and thin solid-state electrolytes particularly composite solid electrolytes (CSEs) that integrate the merits of different electrolyte systems have attracted increasing attention for advanced lithium batteries with improved
energy
density
and
high
safety.
In
this
work,
a
three-dimensional
(3D)
fiber-network-reinforced CSE, which consists of a mechanically robust, porous polyimide (PI) film as a host, Li6.75La3Zr1.75Ta0.25O12 (LLZTO) nanoparticles and poly (vinylidene fluoride) (PVDF) polymer matrix with bis-trifluoromethanesulfonimide lithium salt as electrolyte filler, is designed and fabricated. Such unique 3D CSE films with PI fiber network holding for uniform dispersion of LLZTO in PVDF show continuous lithium ion transfer pathways and effective prevention for lithium dendrite growth, consequently exhibiting improved mechanical property (high tensile strength of 11.5 MPa) and high cyclic stability (more than 1000 h of cycling for Li symmetric batteries). Furthermore, solid-state LiNi0.5Co0.2Mn0.3O2/Li pouch cells with the PI-PVDF/LLZTO CSE exhibit excellent cyclic stability (152.6 mA h g-1 with capacity retention of 94.9% at 0.1 C after 80 cycles) at room temperature, and high functionality and safety (withstand harsh environments such as folding, cutting and nail penetration) in practical applications.
Keywords: Porous film; composite solid electrolyte; solid-state lithium batteries; ionic conductivity; ambient temperature. 1
1. Introduction To satisfy the ever-increasing demands for clean and efficient energy storage devices, rechargeable lithium ion batteries (LIBs) are highly developed due to their high volumetric and gravimetric energy densities [1-3]. Lithium metal has been considered as the most promising anode with the advantages of ultrahigh theoretical specific capacity (3860 mA h g-1) and extremely low potential (-3.040 V vs standard hydrogen electrode) [4-6]. However, lithium metal is thermodynamically unstable in traditional liquid electrolytes and lithium dendrite formed during the Li plating/stripping process would penetrate the separator, resulting in short-circuit of batteries and eventually causing fire and tragedy [7-10]. Thus, the design of high-energy density and high-safety LIBs remains an unmet challenge. To address the aforementioned concerns, solid-state electrolytes (SSEs) are proposed as an effective alternative for traditional liquid electrolytes which generally suffer leakage, flammability, and poor chemical stability [11-14]. SSEs could be classified as three categories: inorganic electrolytes, polymer electrolytes and composite solid electrolyte (CSE) [15-18]. Among them, inorganic electrolytes generally exhibit relatively high ionic conductivity and wide electrochemical stability window. However, they are brittle and usually suffer from poor contact with electrodes [19]. Compared to inorganic electrolytes, polymer electrolytes are light weight and easy to form films with good viscoelasticity. However, the low ionic conductivities, poor mechanical and electrochemical stability limit their practical application [20]. CSE type with combining of inorganic and polymer electrolytes has been considered as the most promising candidate electrolyte for high-performance lithium batteries since it conquers the defects of inorganic as well as polymer electrolyte [21-24]. Meanwhile, great efforts have been made to design and fabricate high-performance CSEs to meet
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increasing demands for high-safety, high-energy lithium batteries. Among numerous attempts, dispersing inorganic fillers such as Li-ion conductive garnet oxides in a polymer matrix is an effective strategy to furthest integrate the merits of different electrolyte systems [25-29]. However, the uniformity of inorganic fillers in polymer matrix and high mechanical strength of self-supporting CSE films still come out a daunt challenge. To solve these issues, introducing a mechanically robust and porous host to prepare 3D CSEs is an effective strategy to ameliorate the performance of solid electrolytes. Cui’s group exploited a cellulose film as the backbone with PPC as ionic transport material to synthesis a rigid-flexible solid-state polymer electrolyte, suggesting that such a cellulose-supporting PPC solid-state polymer electrolyte exhibited high mechanical strength and high electrochemical stability [7]. Moreover, Guo’s group reported the fabrication of a 3D gel-polymer electrolyte using cellulose nonwoven as a mechanical support. The 3D gel-polymer electrolyte exhibits a high ionic conductivity and is demonstrated to effectively hinder the Li dendrite growth [29]. Therefore, exploring suitable host materials to prepare 3D composite solid-electrolyte structures with outstanding electrochemical performance is urgently demanded for achieving high-safety, high-energy density lithium batteries. In this work, we report a three-dimensional fiber-network-reinforced CSE with a robust, porous polyimide (PI) film host, Li6.75La3Zr1.75Ta0.25O12 (LLZTO) fillers and poly (vinylidene fluoride) (PVDF) matrix with bistrifluoromethanesulfonimide lithium salt (LiTFSI), which is denoted as the PI-PVDF/LLZTO CSE. Compared with the PVDF/LLZTO electrolytes in previous studies, we firstly combine the mechanically robust PI film with PVDF/LLZTO electrolytes [27,31]. In such a composite electrolyte system, PI has been demonstrated to possess high mechanical strength, thermal and chemical stability. The high porosity (80%) and large average-pore diameter (2.8 µm) of the
3
used PI film could allow the full permeation of nano-sized LLZTO filler and PVDF matrix during the casting process and then ensure the smooth bottom-surface of the electrolyte. Moreover, the mechanically robust PI host with fiber network homogenously dispersed in composite electrolyte remarkably improves the mechanical property of the CSE and prevents potential dendrite penetration. On the other hand, PVDF could be the most promising candidate as matrix with higher electrochemical stability at room temperature and much better thermal and mechanical performance compared to their counterpart PEO [30] due to its high polarization beneficial to dissociating lithium salts and enhancing the ionic conductivity. As a result, the PI-PVDF/LLZTO CSE exhibits significantly improved mechanical property (high tensile strength of 11.5 MPa) and thermal stability. Furthermore, the practical applicability and functionality of such CSEs have been demonstrated through the as-assembled Li symmetric and high-voltage NCM/Li batteries with excellent electrochemical performance, showing great promise in applications of high-safety, high-energy density energy storage systems. 2. Experimental Section 2.1 Materials The nano-sized powders of cubic LLZTO were synthesized through conventional solid-state reaction by high-energy ball-milling with a protection atmosphere of Ar according to previous work [32-33]. LiTFSI (99.99%, Sigma-Aldrich) and PVDF (Arkema, HSV 900) were dried at 80 °C under vacuum for 24 h to remove trapped water. The porous PI film was purchased from Jiangxi Advanced Nanofiber S&T Co., Ltd. 2.2 Preparation of composite solid-state electrolyte membrane The CSE was prepared by a simple solution-casting method. Firstly, PVDF and LiTFSI (with a
4
weight ratio of 1:2) were dissolved in N-methyl pyrrolidone (NMP) with a polymer concentration of 10 wt.% and the mixed suspension was magnetically stirred at 25 °C for 6 h to obtain homogeneous solution. After that LLZTO was added into the above solution with different weight percentages of LLZTO (0, 10, 20, …, 80, and 90 wt.%) in the total amount of PVDF and LLZTO. Subsequently, the homogenized mixture was spread on a clean glass plate and PI film by a doctor blade to achieve the LLZTO/PVDF and PI-LLZTO/PVDF CSEs, respectively. Finally, the LLZTO/PVDF (~50 µm in thickness) and PI-LLZTO/PVDF (~20 µm in thickness) CSEs were obtained by drying in vacuum oven at 80 °C for extra 24 h to remove the NMP solvent. 2.3 Structural characterization The crystal structure of the samples was investigated by XRD equipped with Cu Kα (λ=0.154 nm) radiation in the 2θ range of 10~70°. A field-emission scanning electron microscopy (FE-SEM, JSM 6330) was used for characterizing the morphologies of the CSEs. The mechanical property of the CSEs membranes were investigated by a Zwick testing machine at a stretching speed of 10 mm min-1 at room temperature. Thermo-gravimetric analysis (TGA) was tested with a Discovery TGA instrument under dry N2 flow at a heating rate of 10 °C min-1 over a temperature range of 50~800 °C. 2.4 Electrochemical characterization of the composite solid-state electrolyte The ionic conductivities of CSEs were measured by the electrochemical impedance spectroscopy (EIS) method with a Solartron electrochemical station 1260+1287. The impedance spectra were recorded from 25 to 90 °C using stainless steel (SS) as a blocking electrode in a frequency range from 1 MHz to 0.01 Hz with an alternating current (AC) amplitude of 10 mV. The electrochemical window of CSEs was examined by linear sweep voltammetry (LSV) at a sweep rate of 1 mV s-1 between 2 and 6 V at 25 °C with a stainless steel as working electrode and lithium metal as the
5
reference and counter electrode. The ambient temperature transference numbers of lithium ion (tLi+) were measured by AC impedance and direct-current (DC) polarization (with a DC voltage of 10 mV) using a symmetric Li/electrolyte/Li cell. Periodic stripping/plating experiments and interface stability measurements were carried out on lithium symmetrical batteries at 25 °C. 2.5 Pouch cell performance To evaluate the practical applicability of CSEs, a pouch cell with a LiNi0.5Co0.2Mn0.3O2 (NCM)-based composite cathode (length: 5 cm, width: 4 cm), a Li foil (length: 5.3 cm, width: 4.3 cm) and the PI-LLZTO/PVDF CSE (length: 5.5 cm, width: 4.5 cm) was assembled in the glovebox. The composite cathode with an optimal weight ratio of NCM: PVDF: carbon black (Super-P): SN: LiTFSI=81.1: 4: 6.75: 6.75: 1.4 was coated on Al foils with NMP as solvent to form the composite cathode. The mass loading of NCM in the composite cathode was controlled to be 9~10 mg cm-2. The charge/discharge tests of batteries were performed between 2.5 and 4.3 V on a LAND-CT2001A testing system (Wuhan Jinnuo Electronics, Ltd.). 3. Results and Discussion The functional mechanism of PI film as a host for high-performance NCM/Li batteries is schematically illustrated in Figure 1. As to single-component PVDF polymer electrolyte, the lithium dendrites form and propagate rapidly during the charge/discharge process, finally penetrating the electrolyte and leading to short circuit of batteries. Through the introduction of ceramic fillers in the polymer electrolyte, though the lithium dendrite growth has been suppressed to some extent, the ununiform lithium deposition still deteriorates the battery performance. Herein, dendrite-free Li anode with uniform lithium deposition is achieved with the application of PI-LLZTO/PVDF CSE in a full battery, where a synergistic effect between mechanically robust PI film and LLZTO/PVDF
6
CSE is beneficial to regulate uniform Li deposition and prevent dendrite growth, which significantly improves the electrochemical performance of batteries. Moreover, the PVDF matrix serves as a soft component to accommodate the interfacial volume fluctuation as well as to guarantee a closely contacted interface and sufficient channels for cross-boundary ion transportation. As-prepared LLZTO/PVDF CSE membranes without and with PI fiber network reinforcement are presented in Figure 2a and 2b, respectively, both of which exhibit good flexibility. Notably, the pristine LLZTO/PVDF CSE membranes without PI film are brown, while after the PI host is introduced, the color of the composite electrolyte membrane turns to dark yellow which can be attributed to the permeation of LLZTO/PVDF CSE into the light-yellow PI host (Figure S1). SEM images of the LLZTO powders show that the particle size of LLZTO is approximately 350 nm (Figure S2a and S2b). Comparative XRD patterns in Figure 2c indicate no obvious changes of the crystallinity of LLZTO have been observed after compositing with PVDF polymer. As shown in Figure 2d, the PI film consists of interconnected fibers forming large pores, which could allow the full permeation of LLZTO/PVDF CSE to form continuous Li-ion diffusion paths. After permeation of LLZTO/PVDF, the pores are filled up and the final CSE with a 3D fiber network holding for uniform dispersion of LLZTO in PVDF exhibits a relatively dense and smooth surface (Figure 2e). Moreover, the EDS mapping images shows that S, F, Ta and La are homogeneously dispersed in the electrolyte, which confirm the uniform distribution of the LLZTO, PVDF and LiTFSI among the CSE. The cross-section SEM images of PVDF/LLZTO and PI-PVDF/LLZTO CSE membranes are shown in Figure 2f and 2g, which indicate the thicknesses of composite electrolytes are approx. 50 µm and 20 µm, respectively, coinciding with the thickness measured via the vernier caliper in Figure S3. Also, the SEM image (Figure 2g) of the PI-PVDF/LLZTO CSE indicates the electrolyte has
7
successfully penetrated the PI film. As shown in figure 2h, the tensile strength of LLZTO/PVDF CSE was merely 3.9 MPa, while after PI fiber-network reinforcement, the composite electrolyte showed significantly enhanced mechanical properties with high tensile strength of 11.5 MPa. The improved mechanical property of the PI fiber-network-reinforced CSE could endow Li metal batteries with high safety against short circuit during charge/discharge process. In order to optimize the proportion of LLZTO, PVDF and LiTFSI in the CSE membrane, the effects of different contents of ceramic, polymer and lithium salt on the film-formation ability and ionic conductivity of electrolytes are systematically investigated. As seen from Figure S4, the self-supporting ability of the CSE membranes enhances with the raise of LLZTO content but when the LLZTO content increases up to 90 wt%, the CSE failed to form a film. Notably, PVDF polymer electrolyte is transparent, while introduction of LLZTO particles turns the color into brown. In such electrolyte system, La atoms in LLZTO can complex with the N atoms and C=O groups of typical solvent molecules, such as DMF, with the N atoms in high-electron-density state. Analogous to Lewis bases, this complex leads to partial dehydrofluorination in the CPEs and thus enhances the interactions between the PVDF matrix, lithium salt, and LLZTO particles [27]. Moreover, the temperature dependence of ionic conductivities of the CSEs is shown in Figure S5, indicating that the CSE with 50% LLZTO exhibited the highest ionic conductivity of 1.85×10-4 S cm-1 at 25 °C (Figure S6). Additionally, from Figure S7, with the decrease of lithium salt content, there is few changes on surface morphologies and flexibility of CSEs, but the ambient ion-conductivity of electrolyte decreases with lower lithium salt content (Figure S8). When the lithium salt content reaches at 40%, the ion conductivity of CSE decreases to less than 10-4 S cm-1. Therefore, taking both the film-forming state and electrochemical properties into consideration, the CSE membranes
8
prepared according to a mass ratio of LLZTO : PVDF : LiTFSI of 1:1:2 are chosen in this work. Figure 3a shows the comparative Li ionic conductivities of CSEs, and the results demonstrated that the introduction of PI film has few effects on ionic conductivity of electrolyte at room temperature. Meanwhile, for PI-LLZTO/PVDF electrolyte, the electronic conductivity is 3.73×10-10 S cm-1, which is negligible compared to the ionic conductivity (1.23×10-4 S cm-1) (figure S9). Moreover, LSV was applied to investigate the electrochemical stabilities of the LLZTO/PVDF CSE and PI fiber-reinforced CSE membranes in a potential range from 2 to 6.0 V at a scanning rate of 1 mV s-1 with stainless steel (SS) as a working electrode and Li metal as both reference and counter electrodes. As shown in Figure 3c, the LLZTO/PVDF CSE begins to decompose at~4.5 V while the PI-LLZTO/PVDF CSE decomposes at~4.8 V, which demonstrates the high suitability of PI-LLZTO/PVDF CSE for application in higher-voltage batteries. Lithium transference number (tLi+) is an essential factor to evaluate the mobility of Li ions, since the moving of anions exerts a great effect on the battery performance. The tLi+ of the CSE membrane was determined by the Bruce-Vincent equation with a symmetric Li/CSEs/Li cell. As shown in Figure 3d, the calculated tLi+ value for the PI-LLZTO/PVDF CSE is 0.51, even higher than the values for the liquid electrolyte mainly ranging from approximately 0.2 to 0.4. The high tLi+ value is attributed to the continuous lithium ion transfer pathways in CSE, which contributes to the fast mobility of Li ions. Furthermore, PI-LLZTO/PVDF CSE assembled in symmetrical Li batteries were examined by AC impedance method to investigate the interfacial stability between electrodes and electrolyte. As shown in Figure 3d, the interfacial resistance shows a few changes during the initial period and then keeps steady with the evolution of time. The result demonstrates that a thin and stable Li+ conductive passivation layer forms on the surface of the Li metal electrode and further inhibiting side reactions. Figure 3e
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shows the comparative thermal-gravimetric analysis (TGA) traces of CSE membranes. Both CSEs with and without PI show negligible weight loss until temperature reaches up to about 380°C, which is highly stable compared with commercial PP separator. The LLZTO/PVDF and PI-LLZTO/PVDF CSEs are tested in air and the solid electrolytes absorb little moisture. Moisture loss after 100 ℃, so there is a slight drop peak in the test curve. Moreover, digital photographs of the PP separator and LLZTO/PVDF and PI-LLZTO/PVDF CSE before and after storage at 150 °C for 30 min have been shown in Figure 3f, displaying that the commercial PP separator suffers a severe shape shrinkage, while LLZTO/PVDF and PI-LLZTO/PVDF CSE remains the same shape under the same condition. The phenomenon reveals that such composite electrolytes can effectively eliminate the safety threat caused by internal short-circuit and improve the battery performance under high-temperature circumstances. Symmetric lithium batteries with different CSEs are assembled to investigate their practical applicability. As shown in Figure 4a, the battery with LLZTO/PVDF CSE was short-circuited only after 426 h of cycling at 0.1 mA cm-2 (0.1 mAh cm-2). In a sharp contrast, the mechanically robust PI-LLZTO/PVDF CSE based battery presented a stable cycling for 1000 h under the same condition. Moreover, the battery with PI-LLZTO/PVDF CSE enabled stable cycling for 400 h in higher current density of 0.5 mA cm-2 (Figure S10). Figure 4b shows the voltage profiles of two symmetric cells during Li plating and stripping at a fixed capacity of 0.1 mAh cm-2 with a step-increased current density. Initially, both cells showed a similar increase in Li plating/stripping overpotentials with the increment of current density. However, when the current density increased to 0.5 mA cm-2, the Li/ LLZTO/PVDF CSE /Li symmetric cell showed an dramatical increase in the overpotential during the charging process, possibly leading to a short circuit in the battery, which indicates a critical value of
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current density has been reached. On the contrary, no voltage drop could be observed for the Li/ PI-LLZTO/PVDF CSE /Li cell even as the current density increased to over 0.8 mA cm-2. These results clearly demonstrate that the PI fiber-network can significantly increase the critical value of current density and suppress the growth of Li dendrites. The lithium plating/stripping processes of symmetrical cells are further investigated and the related results are shown in Figure 4c-4f. Before cycling, the pristine lithium anode without any reaction is smooth and dense (Figure S11). During the initial cycles, the PVDF/LLZTO CSE could inhibit the growth of lithium dendrites to a certain extent (Figure 4c). With the continuous plating/stripping, the propagation and growth of dendrites occur at the Li surface (Figure 4d) owing to the formation of an inhomogeneous SEI layer. In a sharp contrast, the PI-PVDF/LLZTO CSE can effectively inhibit the formation of lithium dendrites and regulate uniform Li deposition (Figure 4e) that few lithium dendrites emerged on the lithium anode surface after cycling (Figure 4f), which could be attributed to a synergistic effect between the uniform SEI formation and enhanced mechanical strength of CSE. To demonstrate the feasibility of the PI-LLZTO/PVDF composite electrolyte in high-energy LMBs, solid-state NCM/Li pouch cell based on LLZTO/PVDF and PI-LLZTO/PVDF CSEs were respectively assembled. The SEM image and corresponding element mappings of the Al2O3@NCM cathodes are shown in Figure S12. In order to ameliorate the poor solid-solid contact between CSEs and electrodes, little liquid electrolyte (~4 µL/cm2) is added to the interface of CSE/cathode to reduce the interface resistance. The electrochemical performance of pouch cells was evaluated via cycling in a voltage range of 2.5~4.3 V at a rate of 0.1 C and a temperature of 25 °C. Typical charge/discharge curves over cycles are presented in Figure 5a, showing that the cell delivers an initial discharge capacity of 160.7 mAh g-1. Over 80 cycles, high capacity retention of 152.6 mAh g-1
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could be obtained, implying outstanding stability of electrode/electrolyte interfaces during the cycling process. Figure 5b represented the rate performance of the cell with PI-LLZTO/PVDF CSE at various rates from 0.1 to 1 C at 25 °C. The cell delivered discharge capacities of 161, 155.6, 140, 135 mAh g-1 at rates of 0.1, 0.2, 0.5 and 1 C, respectively. When returning to 0.1 C, the discharge capacity recovered to the similar value as that before high-rate cycling. The NCM/ PI-LLZTO/PVDF CSE /Li pouch cell shows highly stable cycling for more than 80 cycles at 0.1C (with a capacity retention of 94.9%), whereas the NCM/ LLZTO/PVDF CSE /Li cell experiences a markable decay in capacitance within 60 cycles (Figure 5c). Furthermore, the NCM/ PI-LLZTO/PVDF CSE /Li pouch cells were subjected to a series of flexibility and safety test. As shown in Figure 5d, the solid-state pouch cell operates well (light a red LED) even under harsh conditions such as being folded, cut into pieces and nail penetrated, demonstrating high safety and functionality of such PI-LLZTO/PVDF CSEs applied in LIBs. 4. Conclusion In summary, a 3D LLZTO-PVDF composite solid electrolyte with a robust, porous PI film as a host was fabricated for high-voltage lithium batteries, exhibiting high safety and excellent electrochemical performance at ambient temperature. Benefiting from the mechanically robust PI host, as-prepared PI-LLZTO/PVDF CSE effectively prevents Li dendrite propagation through constructing physical obstacles in the composite electrolyte. As a result, the cyclic stability of PI-LLZTO/PVDF CSE is superior to that of LLZTO/PVDF CSE in symmetrical Li batteries under the same current density. Furthermore, the NCM/ PI-LLZTO/PVDF CSE /Li full batteries exhibit good rate performance, outstanding cyclic stability, high functional flexibility and high safety that the NCM/ PI-LLZTO/PVDF CSE /Li solid-state pouch cells can work well under folding, cutting
12
and nail penetrating situations. All present results indicate that the PI-LLZTO/PVDF CSE is suitable for application in safe, durable, high-voltage and high-energy density LIBs. Acknowledgements Li-Zhen Fan thanks Dr. Guannan Zhu and Dr. Yongfei Liu for helpful discussion. Financial supports from National Natural Science Foundation of China (51532002 and 51872027), Beijing Natural Science Foundation (L172023) are gratefully acknowledged.
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Fig. 1. Schematic illustration of lithium plating/stripping processes in NCM/Li batteries based on different electrolyte systems.
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Fig. 2. Digital photographs of (a) LLZTO/PVDF CSE and (b) PI-LLZTO/PVDF CSE at extending and bent states, (c) XRD patterns of PI-LLZTO/PVDF CSE, LLZTO and pure PVDF powders, SEM images of (d) the PI film and (e) PI-LLZTO/PVDF CSE membrane and EDS maps of S, F, Ta and La in the sample marked in (e), Cross-section SEM images of (f) LLZTO/PVDF CSE and (g) PI -LLZTO/PVDF CSE, (h) Stress-strain curves of LLZTO/PVDF CSE and PI-LLZTO/PVDF CSE.
17
Fig. 3. (a) Temperature-dependence ion conductivities of the LLZTO/PVDF and PI-LLZTO/PVDF CSEs, (b) Comparison of liner sweep voltammograms of LLZTO/PVDF and PI-LLZTO/PVDF CSEs, (c) DC polarization result for Li/ PI-LLZTO/PVDF CSE /Li symmetrical battery at 10 mV s-1 (the inset shows EIS variation before and after polarization), (d) Time evolution of interfacial resistance of Li/ PI-LLZTO/PVDF CSE after various storage times at 25 °C, (e) Thermogravimetric analysis of PP separator, LLZTO/PVDF CSE and PI-LLZTO/PVDF CSE, (f) Photographs of the PP separator (left) and PI-LLZTO/PVDF CSE (right) before and after storage at 150 °C for 30 min.
18
Fig. 4. (a) The voltage profiles were obtained from galvanostatic cycles at a current density of 0.1 mA cm-2 with 1 h stripping and 1 h plating alternating steps at 25 °C, (b) Galvanostatic cycling of Li/ LLZTO/PVDF CSE /Li and Li/ PI-LLZTO/PVDF CSE /Li symmetric cells, stepping the current density from 0.02 to 0.8 mA cm-2, Schematic illustrations of Li plating/stripping behaviors in Li/ LLZTO/PVDF CSE /Li (c) and Li/ PI-LLZTO/PVDF CSE /Li (e), SEM images of the Li surfaces obtained from lithium symmetrical cells assembled with (d) PVDF/LLZTO CSE after 426h and (f) PI-PVDF/LLZTO CSE after 1000 h cycling at 0.1 mA cm-2 and 25 °C, respectively.
19
Fig. 5. (a) Typical charge-discharge curves of PI-LLZTO/PVDF CSE in NCM/Li pouch cell at 0.1C, 25 °C, (b) rate capability (0.1-1 C) of a pouch type cell with a structure of NCM/ PI-LLZTO/PVDF CSE /Li operated at 25 °C, (c) cyclic stability with Coulombic efficiency under 0.1 C of NCM/ LLZTO/PVDF CSE /Li and NCM/ PI-LLZTO/PVDF CSE /Li solid-state battery at 25 °C, (d) illustration of solid-state pouch Li metal cell showing well-running under bending conditions and work normally after being cut into many pieces and after nail test.
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Highlights: Three-dimensional composite solid electrolytes with mechanically robust, porous polyimide film as a host were designed and prepared. As-prepared electrolytes exhibit outstanding mechanical properties and could effectively impede the growth of lithium dendrites. The assembled high-voltage NCM/Li pouch cells exhibit high safety and excellent cycling performance at ambient temperature.
CRediT author statement
Jiangkui Hu: Formal analysis; Investigation; Writing - Original Draft; Writing Review & Editing Pingge He: Writing - Review & Editing Bochen Zhang: Formal analysis Bingyao Wang: Validation Li–Zhen Fan*: Formal analysis; Investigation; Conceptualization; Writing - Review & Editing
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2405-8297(20)30012-X
DOI:
https://doi.org/10.1016/j.ensm.2020.01.006
Reference:
ENSM 1056
To appear in:
Energy Storage Materials
Received Date: 2 December 2019 Revised Date:
31 December 2019
Accepted Date: 8 January 2020
Please cite this article as: J. Hu, P. He, B. Zhang, B. Wang, L.–Z. Fan, Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.01.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.
Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries Jiangkui Hu, Pingge He, Bochen Zhang, Bingyao Wang, Li–Zhen Fan*
Beijing Advanced Innovation Center for Materials Genome Engineering, Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
E-mail: [email protected]
Graphical Abstract for
Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries
3D composite solid electrolytes with mechanically robust, porous polyimide film as a host are prepared by solution casting technique. As-prepared electrolytes exhibit outstanding mechanical properties and could effectively impede the growth of lithium dendrites. The assembled high-voltage NCM/Li pouch cells exhibit high safety and excellent cycling performance, demonstrating a promising strategy to achieve high-safety, high-energy density lithium batteries.
Porous film host-derived 3D composite polymer electrolyte for high-voltage solid state Lithium batteries
Abstract: Conventional lithium-ion batteries with liquid organic electrolytes generally suffer potential security risks concerning volatilization, flammability and explosion. Nonflammable and thin solid-state electrolytes particularly composite solid electrolytes (CSEs) that integrate the merits of different electrolyte systems have attracted increasing attention for advanced lithium batteries with improved
energy
density
and
high
safety.
In
this
work,
a
three-dimensional
(3D)
fiber-network-reinforced CSE, which consists of a mechanically robust, porous polyimide (PI) film as a host, Li6.75La3Zr1.75Ta0.25O12 (LLZTO) nanoparticles and poly (vinylidene fluoride) (PVDF) polymer matrix with bis-trifluoromethanesulfonimide lithium salt as electrolyte filler, is designed and fabricated. Such unique 3D CSE films with PI fiber network holding for uniform dispersion of LLZTO in PVDF show continuous lithium ion transfer pathways and effective prevention for lithium dendrite growth, consequently exhibiting improved mechanical property (high tensile strength of 11.5 MPa) and high cyclic stability (more than 1000 h of cycling for Li symmetric batteries). Furthermore, solid-state LiNi0.5Co0.2Mn0.3O2/Li pouch cells with the PI-PVDF/LLZTO CSE exhibit excellent cyclic stability (152.6 mA h g-1 with capacity retention of 94.9% at 0.1 C after 80 cycles) at room temperature, and high functionality and safety (withstand harsh environments such as folding, cutting and nail penetration) in practical applications.
Keywords: Porous film; composite solid electrolyte; solid-state lithium batteries; ionic conductivity; ambient temperature. 1
1. Introduction To satisfy the ever-increasing demands for clean and efficient energy storage devices, rechargeable lithium ion batteries (LIBs) are highly developed due to their high volumetric and gravimetric energy densities [1-3]. Lithium metal has been considered as the most promising anode with the advantages of ultrahigh theoretical specific capacity (3860 mA h g-1) and extremely low potential (-3.040 V vs standard hydrogen electrode) [4-6]. However, lithium metal is thermodynamically unstable in traditional liquid electrolytes and lithium dendrite formed during the Li plating/stripping process would penetrate the separator, resulting in short-circuit of batteries and eventually causing fire and tragedy [7-10]. Thus, the design of high-energy density and high-safety LIBs remains an unmet challenge. To address the aforementioned concerns, solid-state electrolytes (SSEs) are proposed as an effective alternative for traditional liquid electrolytes which generally suffer leakage, flammability, and poor chemical stability [11-14]. SSEs could be classified as three categories: inorganic electrolytes, polymer electrolytes and composite solid electrolyte (CSE) [15-18]. Among them, inorganic electrolytes generally exhibit relatively high ionic conductivity and wide electrochemical stability window. However, they are brittle and usually suffer from poor contact with electrodes [19]. Compared to inorganic electrolytes, polymer electrolytes are light weight and easy to form films with good viscoelasticity. However, the low ionic conductivities, poor mechanical and electrochemical stability limit their practical application [20]. CSE type with combining of inorganic and polymer electrolytes has been considered as the most promising candidate electrolyte for high-performance lithium batteries since it conquers the defects of inorganic as well as polymer electrolyte [21-24]. Meanwhile, great efforts have been made to design and fabricate high-performance CSEs to meet
2
increasing demands for high-safety, high-energy lithium batteries. Among numerous attempts, dispersing inorganic fillers such as Li-ion conductive garnet oxides in a polymer matrix is an effective strategy to furthest integrate the merits of different electrolyte systems [25-29]. However, the uniformity of inorganic fillers in polymer matrix and high mechanical strength of self-supporting CSE films still come out a daunt challenge. To solve these issues, introducing a mechanically robust and porous host to prepare 3D CSEs is an effective strategy to ameliorate the performance of solid electrolytes. Cui’s group exploited a cellulose film as the backbone with PPC as ionic transport material to synthesis a rigid-flexible solid-state polymer electrolyte, suggesting that such a cellulose-supporting PPC solid-state polymer electrolyte exhibited high mechanical strength and high electrochemical stability [7]. Moreover, Guo’s group reported the fabrication of a 3D gel-polymer electrolyte using cellulose nonwoven as a mechanical support. The 3D gel-polymer electrolyte exhibits a high ionic conductivity and is demonstrated to effectively hinder the Li dendrite growth [29]. Therefore, exploring suitable host materials to prepare 3D composite solid-electrolyte structures with outstanding electrochemical performance is urgently demanded for achieving high-safety, high-energy density lithium batteries. In this work, we report a three-dimensional fiber-network-reinforced CSE with a robust, porous polyimide (PI) film host, Li6.75La3Zr1.75Ta0.25O12 (LLZTO) fillers and poly (vinylidene fluoride) (PVDF) matrix with bistrifluoromethanesulfonimide lithium salt (LiTFSI), which is denoted as the PI-PVDF/LLZTO CSE. Compared with the PVDF/LLZTO electrolytes in previous studies, we firstly combine the mechanically robust PI film with PVDF/LLZTO electrolytes [27,31]. In such a composite electrolyte system, PI has been demonstrated to possess high mechanical strength, thermal and chemical stability. The high porosity (80%) and large average-pore diameter (2.8 µm) of the
3
used PI film could allow the full permeation of nano-sized LLZTO filler and PVDF matrix during the casting process and then ensure the smooth bottom-surface of the electrolyte. Moreover, the mechanically robust PI host with fiber network homogenously dispersed in composite electrolyte remarkably improves the mechanical property of the CSE and prevents potential dendrite penetration. On the other hand, PVDF could be the most promising candidate as matrix with higher electrochemical stability at room temperature and much better thermal and mechanical performance compared to their counterpart PEO [30] due to its high polarization beneficial to dissociating lithium salts and enhancing the ionic conductivity. As a result, the PI-PVDF/LLZTO CSE exhibits significantly improved mechanical property (high tensile strength of 11.5 MPa) and thermal stability. Furthermore, the practical applicability and functionality of such CSEs have been demonstrated through the as-assembled Li symmetric and high-voltage NCM/Li batteries with excellent electrochemical performance, showing great promise in applications of high-safety, high-energy density energy storage systems. 2. Experimental Section 2.1 Materials The nano-sized powders of cubic LLZTO were synthesized through conventional solid-state reaction by high-energy ball-milling with a protection atmosphere of Ar according to previous work [32-33]. LiTFSI (99.99%, Sigma-Aldrich) and PVDF (Arkema, HSV 900) were dried at 80 °C under vacuum for 24 h to remove trapped water. The porous PI film was purchased from Jiangxi Advanced Nanofiber S&T Co., Ltd. 2.2 Preparation of composite solid-state electrolyte membrane The CSE was prepared by a simple solution-casting method. Firstly, PVDF and LiTFSI (with a
4
weight ratio of 1:2) were dissolved in N-methyl pyrrolidone (NMP) with a polymer concentration of 10 wt.% and the mixed suspension was magnetically stirred at 25 °C for 6 h to obtain homogeneous solution. After that LLZTO was added into the above solution with different weight percentages of LLZTO (0, 10, 20, …, 80, and 90 wt.%) in the total amount of PVDF and LLZTO. Subsequently, the homogenized mixture was spread on a clean glass plate and PI film by a doctor blade to achieve the LLZTO/PVDF and PI-LLZTO/PVDF CSEs, respectively. Finally, the LLZTO/PVDF (~50 µm in thickness) and PI-LLZTO/PVDF (~20 µm in thickness) CSEs were obtained by drying in vacuum oven at 80 °C for extra 24 h to remove the NMP solvent. 2.3 Structural characterization The crystal structure of the samples was investigated by XRD equipped with Cu Kα (λ=0.154 nm) radiation in the 2θ range of 10~70°. A field-emission scanning electron microscopy (FE-SEM, JSM 6330) was used for characterizing the morphologies of the CSEs. The mechanical property of the CSEs membranes were investigated by a Zwick testing machine at a stretching speed of 10 mm min-1 at room temperature. Thermo-gravimetric analysis (TGA) was tested with a Discovery TGA instrument under dry N2 flow at a heating rate of 10 °C min-1 over a temperature range of 50~800 °C. 2.4 Electrochemical characterization of the composite solid-state electrolyte The ionic conductivities of CSEs were measured by the electrochemical impedance spectroscopy (EIS) method with a Solartron electrochemical station 1260+1287. The impedance spectra were recorded from 25 to 90 °C using stainless steel (SS) as a blocking electrode in a frequency range from 1 MHz to 0.01 Hz with an alternating current (AC) amplitude of 10 mV. The electrochemical window of CSEs was examined by linear sweep voltammetry (LSV) at a sweep rate of 1 mV s-1 between 2 and 6 V at 25 °C with a stainless steel as working electrode and lithium metal as the
5
reference and counter electrode. The ambient temperature transference numbers of lithium ion (tLi+) were measured by AC impedance and direct-current (DC) polarization (with a DC voltage of 10 mV) using a symmetric Li/electrolyte/Li cell. Periodic stripping/plating experiments and interface stability measurements were carried out on lithium symmetrical batteries at 25 °C. 2.5 Pouch cell performance To evaluate the practical applicability of CSEs, a pouch cell with a LiNi0.5Co0.2Mn0.3O2 (NCM)-based composite cathode (length: 5 cm, width: 4 cm), a Li foil (length: 5.3 cm, width: 4.3 cm) and the PI-LLZTO/PVDF CSE (length: 5.5 cm, width: 4.5 cm) was assembled in the glovebox. The composite cathode with an optimal weight ratio of NCM: PVDF: carbon black (Super-P): SN: LiTFSI=81.1: 4: 6.75: 6.75: 1.4 was coated on Al foils with NMP as solvent to form the composite cathode. The mass loading of NCM in the composite cathode was controlled to be 9~10 mg cm-2. The charge/discharge tests of batteries were performed between 2.5 and 4.3 V on a LAND-CT2001A testing system (Wuhan Jinnuo Electronics, Ltd.). 3. Results and Discussion The functional mechanism of PI film as a host for high-performance NCM/Li batteries is schematically illustrated in Figure 1. As to single-component PVDF polymer electrolyte, the lithium dendrites form and propagate rapidly during the charge/discharge process, finally penetrating the electrolyte and leading to short circuit of batteries. Through the introduction of ceramic fillers in the polymer electrolyte, though the lithium dendrite growth has been suppressed to some extent, the ununiform lithium deposition still deteriorates the battery performance. Herein, dendrite-free Li anode with uniform lithium deposition is achieved with the application of PI-LLZTO/PVDF CSE in a full battery, where a synergistic effect between mechanically robust PI film and LLZTO/PVDF
6
CSE is beneficial to regulate uniform Li deposition and prevent dendrite growth, which significantly improves the electrochemical performance of batteries. Moreover, the PVDF matrix serves as a soft component to accommodate the interfacial volume fluctuation as well as to guarantee a closely contacted interface and sufficient channels for cross-boundary ion transportation. As-prepared LLZTO/PVDF CSE membranes without and with PI fiber network reinforcement are presented in Figure 2a and 2b, respectively, both of which exhibit good flexibility. Notably, the pristine LLZTO/PVDF CSE membranes without PI film are brown, while after the PI host is introduced, the color of the composite electrolyte membrane turns to dark yellow which can be attributed to the permeation of LLZTO/PVDF CSE into the light-yellow PI host (Figure S1). SEM images of the LLZTO powders show that the particle size of LLZTO is approximately 350 nm (Figure S2a and S2b). Comparative XRD patterns in Figure 2c indicate no obvious changes of the crystallinity of LLZTO have been observed after compositing with PVDF polymer. As shown in Figure 2d, the PI film consists of interconnected fibers forming large pores, which could allow the full permeation of LLZTO/PVDF CSE to form continuous Li-ion diffusion paths. After permeation of LLZTO/PVDF, the pores are filled up and the final CSE with a 3D fiber network holding for uniform dispersion of LLZTO in PVDF exhibits a relatively dense and smooth surface (Figure 2e). Moreover, the EDS mapping images shows that S, F, Ta and La are homogeneously dispersed in the electrolyte, which confirm the uniform distribution of the LLZTO, PVDF and LiTFSI among the CSE. The cross-section SEM images of PVDF/LLZTO and PI-PVDF/LLZTO CSE membranes are shown in Figure 2f and 2g, which indicate the thicknesses of composite electrolytes are approx. 50 µm and 20 µm, respectively, coinciding with the thickness measured via the vernier caliper in Figure S3. Also, the SEM image (Figure 2g) of the PI-PVDF/LLZTO CSE indicates the electrolyte has
7
successfully penetrated the PI film. As shown in figure 2h, the tensile strength of LLZTO/PVDF CSE was merely 3.9 MPa, while after PI fiber-network reinforcement, the composite electrolyte showed significantly enhanced mechanical properties with high tensile strength of 11.5 MPa. The improved mechanical property of the PI fiber-network-reinforced CSE could endow Li metal batteries with high safety against short circuit during charge/discharge process. In order to optimize the proportion of LLZTO, PVDF and LiTFSI in the CSE membrane, the effects of different contents of ceramic, polymer and lithium salt on the film-formation ability and ionic conductivity of electrolytes are systematically investigated. As seen from Figure S4, the self-supporting ability of the CSE membranes enhances with the raise of LLZTO content but when the LLZTO content increases up to 90 wt%, the CSE failed to form a film. Notably, PVDF polymer electrolyte is transparent, while introduction of LLZTO particles turns the color into brown. In such electrolyte system, La atoms in LLZTO can complex with the N atoms and C=O groups of typical solvent molecules, such as DMF, with the N atoms in high-electron-density state. Analogous to Lewis bases, this complex leads to partial dehydrofluorination in the CPEs and thus enhances the interactions between the PVDF matrix, lithium salt, and LLZTO particles [27]. Moreover, the temperature dependence of ionic conductivities of the CSEs is shown in Figure S5, indicating that the CSE with 50% LLZTO exhibited the highest ionic conductivity of 1.85×10-4 S cm-1 at 25 °C (Figure S6). Additionally, from Figure S7, with the decrease of lithium salt content, there is few changes on surface morphologies and flexibility of CSEs, but the ambient ion-conductivity of electrolyte decreases with lower lithium salt content (Figure S8). When the lithium salt content reaches at 40%, the ion conductivity of CSE decreases to less than 10-4 S cm-1. Therefore, taking both the film-forming state and electrochemical properties into consideration, the CSE membranes
8
prepared according to a mass ratio of LLZTO : PVDF : LiTFSI of 1:1:2 are chosen in this work. Figure 3a shows the comparative Li ionic conductivities of CSEs, and the results demonstrated that the introduction of PI film has few effects on ionic conductivity of electrolyte at room temperature. Meanwhile, for PI-LLZTO/PVDF electrolyte, the electronic conductivity is 3.73×10-10 S cm-1, which is negligible compared to the ionic conductivity (1.23×10-4 S cm-1) (figure S9). Moreover, LSV was applied to investigate the electrochemical stabilities of the LLZTO/PVDF CSE and PI fiber-reinforced CSE membranes in a potential range from 2 to 6.0 V at a scanning rate of 1 mV s-1 with stainless steel (SS) as a working electrode and Li metal as both reference and counter electrodes. As shown in Figure 3c, the LLZTO/PVDF CSE begins to decompose at~4.5 V while the PI-LLZTO/PVDF CSE decomposes at~4.8 V, which demonstrates the high suitability of PI-LLZTO/PVDF CSE for application in higher-voltage batteries. Lithium transference number (tLi+) is an essential factor to evaluate the mobility of Li ions, since the moving of anions exerts a great effect on the battery performance. The tLi+ of the CSE membrane was determined by the Bruce-Vincent equation with a symmetric Li/CSEs/Li cell. As shown in Figure 3d, the calculated tLi+ value for the PI-LLZTO/PVDF CSE is 0.51, even higher than the values for the liquid electrolyte mainly ranging from approximately 0.2 to 0.4. The high tLi+ value is attributed to the continuous lithium ion transfer pathways in CSE, which contributes to the fast mobility of Li ions. Furthermore, PI-LLZTO/PVDF CSE assembled in symmetrical Li batteries were examined by AC impedance method to investigate the interfacial stability between electrodes and electrolyte. As shown in Figure 3d, the interfacial resistance shows a few changes during the initial period and then keeps steady with the evolution of time. The result demonstrates that a thin and stable Li+ conductive passivation layer forms on the surface of the Li metal electrode and further inhibiting side reactions. Figure 3e
9
shows the comparative thermal-gravimetric analysis (TGA) traces of CSE membranes. Both CSEs with and without PI show negligible weight loss until temperature reaches up to about 380°C, which is highly stable compared with commercial PP separator. The LLZTO/PVDF and PI-LLZTO/PVDF CSEs are tested in air and the solid electrolytes absorb little moisture. Moisture loss after 100 ℃, so there is a slight drop peak in the test curve. Moreover, digital photographs of the PP separator and LLZTO/PVDF and PI-LLZTO/PVDF CSE before and after storage at 150 °C for 30 min have been shown in Figure 3f, displaying that the commercial PP separator suffers a severe shape shrinkage, while LLZTO/PVDF and PI-LLZTO/PVDF CSE remains the same shape under the same condition. The phenomenon reveals that such composite electrolytes can effectively eliminate the safety threat caused by internal short-circuit and improve the battery performance under high-temperature circumstances. Symmetric lithium batteries with different CSEs are assembled to investigate their practical applicability. As shown in Figure 4a, the battery with LLZTO/PVDF CSE was short-circuited only after 426 h of cycling at 0.1 mA cm-2 (0.1 mAh cm-2). In a sharp contrast, the mechanically robust PI-LLZTO/PVDF CSE based battery presented a stable cycling for 1000 h under the same condition. Moreover, the battery with PI-LLZTO/PVDF CSE enabled stable cycling for 400 h in higher current density of 0.5 mA cm-2 (Figure S10). Figure 4b shows the voltage profiles of two symmetric cells during Li plating and stripping at a fixed capacity of 0.1 mAh cm-2 with a step-increased current density. Initially, both cells showed a similar increase in Li plating/stripping overpotentials with the increment of current density. However, when the current density increased to 0.5 mA cm-2, the Li/ LLZTO/PVDF CSE /Li symmetric cell showed an dramatical increase in the overpotential during the charging process, possibly leading to a short circuit in the battery, which indicates a critical value of
10
current density has been reached. On the contrary, no voltage drop could be observed for the Li/ PI-LLZTO/PVDF CSE /Li cell even as the current density increased to over 0.8 mA cm-2. These results clearly demonstrate that the PI fiber-network can significantly increase the critical value of current density and suppress the growth of Li dendrites. The lithium plating/stripping processes of symmetrical cells are further investigated and the related results are shown in Figure 4c-4f. Before cycling, the pristine lithium anode without any reaction is smooth and dense (Figure S11). During the initial cycles, the PVDF/LLZTO CSE could inhibit the growth of lithium dendrites to a certain extent (Figure 4c). With the continuous plating/stripping, the propagation and growth of dendrites occur at the Li surface (Figure 4d) owing to the formation of an inhomogeneous SEI layer. In a sharp contrast, the PI-PVDF/LLZTO CSE can effectively inhibit the formation of lithium dendrites and regulate uniform Li deposition (Figure 4e) that few lithium dendrites emerged on the lithium anode surface after cycling (Figure 4f), which could be attributed to a synergistic effect between the uniform SEI formation and enhanced mechanical strength of CSE. To demonstrate the feasibility of the PI-LLZTO/PVDF composite electrolyte in high-energy LMBs, solid-state NCM/Li pouch cell based on LLZTO/PVDF and PI-LLZTO/PVDF CSEs were respectively assembled. The SEM image and corresponding element mappings of the Al2O3@NCM cathodes are shown in Figure S12. In order to ameliorate the poor solid-solid contact between CSEs and electrodes, little liquid electrolyte (~4 µL/cm2) is added to the interface of CSE/cathode to reduce the interface resistance. The electrochemical performance of pouch cells was evaluated via cycling in a voltage range of 2.5~4.3 V at a rate of 0.1 C and a temperature of 25 °C. Typical charge/discharge curves over cycles are presented in Figure 5a, showing that the cell delivers an initial discharge capacity of 160.7 mAh g-1. Over 80 cycles, high capacity retention of 152.6 mAh g-1
11
could be obtained, implying outstanding stability of electrode/electrolyte interfaces during the cycling process. Figure 5b represented the rate performance of the cell with PI-LLZTO/PVDF CSE at various rates from 0.1 to 1 C at 25 °C. The cell delivered discharge capacities of 161, 155.6, 140, 135 mAh g-1 at rates of 0.1, 0.2, 0.5 and 1 C, respectively. When returning to 0.1 C, the discharge capacity recovered to the similar value as that before high-rate cycling. The NCM/ PI-LLZTO/PVDF CSE /Li pouch cell shows highly stable cycling for more than 80 cycles at 0.1C (with a capacity retention of 94.9%), whereas the NCM/ LLZTO/PVDF CSE /Li cell experiences a markable decay in capacitance within 60 cycles (Figure 5c). Furthermore, the NCM/ PI-LLZTO/PVDF CSE /Li pouch cells were subjected to a series of flexibility and safety test. As shown in Figure 5d, the solid-state pouch cell operates well (light a red LED) even under harsh conditions such as being folded, cut into pieces and nail penetrated, demonstrating high safety and functionality of such PI-LLZTO/PVDF CSEs applied in LIBs. 4. Conclusion In summary, a 3D LLZTO-PVDF composite solid electrolyte with a robust, porous PI film as a host was fabricated for high-voltage lithium batteries, exhibiting high safety and excellent electrochemical performance at ambient temperature. Benefiting from the mechanically robust PI host, as-prepared PI-LLZTO/PVDF CSE effectively prevents Li dendrite propagation through constructing physical obstacles in the composite electrolyte. As a result, the cyclic stability of PI-LLZTO/PVDF CSE is superior to that of LLZTO/PVDF CSE in symmetrical Li batteries under the same current density. Furthermore, the NCM/ PI-LLZTO/PVDF CSE /Li full batteries exhibit good rate performance, outstanding cyclic stability, high functional flexibility and high safety that the NCM/ PI-LLZTO/PVDF CSE /Li solid-state pouch cells can work well under folding, cutting
12
and nail penetrating situations. All present results indicate that the PI-LLZTO/PVDF CSE is suitable for application in safe, durable, high-voltage and high-energy density LIBs. Acknowledgements Li-Zhen Fan thanks Dr. Guannan Zhu and Dr. Yongfei Liu for helpful discussion. Financial supports from National Natural Science Foundation of China (51532002 and 51872027), Beijing Natural Science Foundation (L172023) are gratefully acknowledged.
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Fig. 1. Schematic illustration of lithium plating/stripping processes in NCM/Li batteries based on different electrolyte systems.
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Fig. 2. Digital photographs of (a) LLZTO/PVDF CSE and (b) PI-LLZTO/PVDF CSE at extending and bent states, (c) XRD patterns of PI-LLZTO/PVDF CSE, LLZTO and pure PVDF powders, SEM images of (d) the PI film and (e) PI-LLZTO/PVDF CSE membrane and EDS maps of S, F, Ta and La in the sample marked in (e), Cross-section SEM images of (f) LLZTO/PVDF CSE and (g) PI -LLZTO/PVDF CSE, (h) Stress-strain curves of LLZTO/PVDF CSE and PI-LLZTO/PVDF CSE.
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Fig. 3. (a) Temperature-dependence ion conductivities of the LLZTO/PVDF and PI-LLZTO/PVDF CSEs, (b) Comparison of liner sweep voltammograms of LLZTO/PVDF and PI-LLZTO/PVDF CSEs, (c) DC polarization result for Li/ PI-LLZTO/PVDF CSE /Li symmetrical battery at 10 mV s-1 (the inset shows EIS variation before and after polarization), (d) Time evolution of interfacial resistance of Li/ PI-LLZTO/PVDF CSE after various storage times at 25 °C, (e) Thermogravimetric analysis of PP separator, LLZTO/PVDF CSE and PI-LLZTO/PVDF CSE, (f) Photographs of the PP separator (left) and PI-LLZTO/PVDF CSE (right) before and after storage at 150 °C for 30 min.
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Fig. 4. (a) The voltage profiles were obtained from galvanostatic cycles at a current density of 0.1 mA cm-2 with 1 h stripping and 1 h plating alternating steps at 25 °C, (b) Galvanostatic cycling of Li/ LLZTO/PVDF CSE /Li and Li/ PI-LLZTO/PVDF CSE /Li symmetric cells, stepping the current density from 0.02 to 0.8 mA cm-2, Schematic illustrations of Li plating/stripping behaviors in Li/ LLZTO/PVDF CSE /Li (c) and Li/ PI-LLZTO/PVDF CSE /Li (e), SEM images of the Li surfaces obtained from lithium symmetrical cells assembled with (d) PVDF/LLZTO CSE after 426h and (f) PI-PVDF/LLZTO CSE after 1000 h cycling at 0.1 mA cm-2 and 25 °C, respectively.
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Fig. 5. (a) Typical charge-discharge curves of PI-LLZTO/PVDF CSE in NCM/Li pouch cell at 0.1C, 25 °C, (b) rate capability (0.1-1 C) of a pouch type cell with a structure of NCM/ PI-LLZTO/PVDF CSE /Li operated at 25 °C, (c) cyclic stability with Coulombic efficiency under 0.1 C of NCM/ LLZTO/PVDF CSE /Li and NCM/ PI-LLZTO/PVDF CSE /Li solid-state battery at 25 °C, (d) illustration of solid-state pouch Li metal cell showing well-running under bending conditions and work normally after being cut into many pieces and after nail test.
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Highlights: Three-dimensional composite solid electrolytes with mechanically robust, porous polyimide film as a host were designed and prepared. As-prepared electrolytes exhibit outstanding mechanical properties and could effectively impede the growth of lithium dendrites. The assembled high-voltage NCM/Li pouch cells exhibit high safety and excellent cycling performance at ambient temperature.
CRediT author statement
Jiangkui Hu: Formal analysis; Investigation; Writing - Original Draft; Writing Review & Editing Pingge He: Writing - Review & Editing Bochen Zhang: Formal analysis Bingyao Wang: Validation Li–Zhen Fan*: Formal analysis; Investigation; Conceptualization; Writing - Review & Editing
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: