Nanoporous Adsorption Effect on Alteration of the Li+ Diffusion

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Nanoporous adsorption effect on altering Li diffusion pathway by a highly ordered porous electrolyte additive for high rate all-solid-state lithium metal batteries Wenwen Li, Sanpei Zhang, Bangrun Wang, Sui Gu, Dong Xu, Jianing Wang, Chunhua Chen, and Zhaoyin Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06574 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Materials & Interfaces

Nanoporous adsorption effect on altering Li+ diffusion pathway by a highly ordered porous electrolyte additive for high rate all-solid-state lithium metal batteries Wenwen Li,a,b Sanpei Zhang,a Bangrun Wang,a,b Sui Gu,a,b Dong Xu, a,b Jianing Wang,a,b Chunhua Chenc and Zhaoyin Wena,* a.

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, P. R. China. b.

c.

University of Chinese Academy of Science, Beijing 100049, P. R. China. University of Science and Technology of China, Hefei 230026, Anhui, China.

KEYWORDS: polymer electrolytes, nanoporous adsorption, lithium metal, all-solid-state batteries, ionic conductivity, SSZ-CPE

ACS Paragon Plus Environment

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ABSTRACT

Solid polymer electrolytes (SPEs) have shown extraordinary promise for all-solid-state lithium metal batteries with high energy density and flexibility but are mainly limited by the low ionic conductivity and their poor stability with lithium metal anode. In this work, we propose a highly ordered porous electrolyte additive derived from SSZ-13 for high-rate all-solid-state lithium metal batteries. The nanoporous adsorption effect provided by the highly ordered porous nanoparticles in the poly (ethylene oxide) (PEO) electrolyte are found to significantly improve the Li+ conductivity (1.91×10-3 S cm-1 at 60°C, 4.43×10-5 S cm-1 at 20°C) and widen the electrochemical stability window to 4.7 V vs Li+/Li. Meanwhile, the designed PEO-based electrolyte demonstrates enhanced stability with the lithium metal anode. Through systematically increasing Li+ diffusion, widening the electrochemical stability window and enhancing the interfacial stability of the SSZ-CPE electrolyte, the LiFePO4/SSZ-CPE/Li cell is optimized to deliver high-rate capability and stable cycling performance, which demonstrates great potential for all-solid-state energy storage application.

TOC

We propose that nanoporous absorption effect can alter Li+ diffusion pathway, further improve the conductivity and rate performance of electrolytes.


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INTRODUCTION

The solid-state electrolytes, with high safety and possibility of increased energy density have attracted much research interest and are considered to be the critical materials of all-solid-state lithium metal batteries.1-3 But the challenges of low Li+ conductivity and poor stability to lithium anode still limit the application of advanced electrolytes.4 Altering the diffusion pathway of Li+ to easy conduction area could be efficient on obtaining a high ionic conductivity. And at the same time, controlling the side reaction of solid electrolyte during the battery cycle is also important. Therefore, great attentions have been paid on improving Li+ conduction and interfacial stability of modified all-solid-state electrolytes.5-7 PEO based solid electrolytes not only have better safety characteristic than liquid organic electrolytes, but also demonstrate flexible and easy-processing properties among all solid state electrolytes.8-13 However, low room temperature ionic conductivity and poor stability to lithium anode are the two main important factors that hinder the electrochemical performances of PEO based electrolytes for secondary batteries. In order to solve these problems, recent researches in PEO-based solid-state electrolytes (SSEs) have centred on the design of additive for SPEs, including ionic liquid, oxide particles and etc. The addition of ionic liquid and organic solvent could serve as the plasticizers, which were found to be efficient on improving the ionic conductivities of SPEs.14-16 On the other hand, various modifications of SPEs by introducing oxide particles have been reported.17-20 Experimental researches have demonstrated that ionic conductivity can be increased by the addition of oxide nanoparticles to the SPEs, such as SiO221, TiO222-23, Al2O324, LAGP25, YSZ26. For example, after adding fillers, ionic conductivity at the room temperature can be significantly improved to ~10-6 S cm-1.25 Researchers attributed the enhanced ionic conductivity to that the highly dispersed nanoparticles be able to inhibit the


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recrystallization of polymer segment and further accelerate Li+ transport.9, 11, 27 The solid-state lithium ion batteries based on the nano-sized mesoporous SiO2 modified electrolyte exhibited discharge capacity of 120 mA h g-1 after 25 cycles at 70°C.17 One MOF material containing porous channel was used as an additive of high ionic conductivity CPE(1.62×10-4 S cm-1 at 80°C). The Li-S cells based on the CPE showed discharge capacity of 1520 mA h g-1 at first cycle.28 However, there is 325 mA h g-1 are remained after numbers of cycles. These results indicated the porous nanoparticles may be a relatively effective additive to increase the ionic conductivity than the non-porous particles, which is mainly attributed to the pores creating more space-charge regions to facilitate Li+ transport. In addition to the aforementioned advantages of the porous electrolyte additive, the nanoporous adsorption effect of the porous additive has been rarely illustrated. It has been intensively investigated that the porous materials can be applied in the catalyst and mass or gas adsorption fields because of the porous adsorption effect.29-31 During the catalysis or adsorption process, the surface pore structure provides amounts of active sites on the mass adsorption to fast the kinetic process. Inspired by this, we tried to design and introduce the porous materials into the PEO-based electrolyte. The implanted materials with rich nanoporous structure can effectively promote the formation of Li+ enrichment area through adsorption effect. In the newly formed percolated interface between additives and polymer, the continuous area could act as a high-speed pathway for the fast lithium ion diffusion. In addition, the highly porous material could not only serve as plasticizer to facilitate the transport of lithium ions by enlarging amorphous polymer regions, but also maintain good dispersion in the polymer matrix to offer more interaction sites. Thus, designing and searching for a suitable inorganic filler with the


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favorable properties, including abundant surface charge, strong absorbability and high specific surface area, which is desirable for the PEO-based electrolyte.32 In this work, we utilize and design the nanoparticles derived from SSZ-13 with highly ordered porous structure as the inorganic additive in the PEO electrolyte to prepare a high ionic conductivity SSZ-13@PEO composite electrolyte (SSZ-CPE). SSZ-13 is one aluminosilicate with three dimensional open structure consisting of Si and Al connected by O atoms.33 The abundant silicon hydroxyl and aluminium hydroxyl at the surface introduced Lewis acid-base sites. By addition of the nanoporous SSZ-13, the ionic conductivity of the CPE and the interfacial compatibility with electrodes both have been improved. The battery applicability and cycle performance based the SSZ-CPE are demonstrated. The ionic conductivity of the asobtained SSZ-CPE-10% electrolyte is 1.91×10-3 S cm-1 at 60°C; 4.43×10-5 S cm-1 at 20°C, which is several times higher than pure SPE. As expected, the LiFePO4/Li battery based on the SSZCPE exhibited outstanding cycling stability (high discharge capacity of 155 mAh g-1 after 160 cycles at 0.1C and 58°C) and superior rate capability (130 mAh g-1 after 100 cycles at 1C and 58°C). Furthermore, all-solid-state lithium-sulfur battery with the SSZ-CPE exhibit an excellent cycle performance (a reversible capacity of 980 mA h g-1 has been obtained after 40 cycles at 0.1C).

RESULTS AND DISCUSSION

Characterization of nanoporous SSZ-13 and CPEs

The pristine SSZ-13 shows a uniform particle size of 500 nm, as shown in Figure S1a. The pristine micro-scale powders were processed with high-energy milling methods to produce nanoparticles of ~150 nm in size (Figure 1a). The granularity distribution of SSZ-13


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demonstrates the uniform particle size. The diameters of nanoparticles in terms of D10, D50 and D90 are found to be 90 nm, 150 nm and 230 nm, respectively. As can be seen from the corresponding enlarged high-resolution transmission electron microscope (HRTEM) image (Figure 1b), the highly ordered nanopores of 0.38 nm of the SSZ-13 nanoparticles are clearly demonstrated. As shown in Figure 1c, the SSZ-13 based composite polymer electrolyte (CPE) membrane shows smooth top and cross-section surface with homogeneous dispersion of the additive nanoparticles, and the thickness of the CPE membrane is evaluated to be ~70 µm. The nitrogen adsorption-desorption isotherms results (Figure 1d) reveal that the SSZ-13 nanoparticles have a high specific surface area of 534.56 m2 g-1 with the pore diameters less than 2 nm. The Xray

diffraction

(XRD)

patterns

of

SSZ-13,

SPE

(PEO+LiTFSI)

and

CPE-10%

(PEO+LiTFSI+10% SSZ-13) are presented in Figure 1e, in which the characteristic peaks of the SSZ-13 can be well fit to the chabazite (JCPDS No. 52-0784). There are two wide peaks in SPE corresponding to the characteristic peaks of PEO.34 It is worth noted that the two main diffraction peaks become weaker and broader, indicating the lower crystallinity of CPE-10% due to the introduction of nanoporous SSZ-13. As the previous researches reported, low crystallinity of polymers would be beneficial to the motion of coordinated cation to achieve enhanced ionic conductivity.35 Meanwhile, as shown in Figure 1f, the addition of nanoporous SSZ-13 has negligible impact on the semitransparency and flexibility of the PEO-based membrane, which facilitate us to construct flexible and wearable electronic devices.


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Figure 1. (a) TEM image of SSZ-13 nanoparticles; Inset: The granularity distribution of SSZ-13 nanoparticles; (b) Corresponding HRTEM image of SSZ-13; (c) Cross-sectional SEM image of electrode-electrolyte interface; (d) Nitrogen absorption/desorption isotherms of SSZ-13; Inset: Pore diameter distribution of SSZ-13; (e) XRD pattern of SSZ-13, SPE and CPE-10%; (f) Digital photo of the SSZ-CPE film.

Surface and mechanical properties of CPE

To investigate the impact of SSZ-13 on the physical properties of CPE, mechanical strength analysis and thermal properties were carried out. Figure 2a shows zeta potential measurements of suspensions, which is consist of SSZ-13 nanoparticles dispersing in acetonitrile. The zeta potential (-25 mV) indicates that the surface of nanoparticles exhibits a Lewis basic property and forms a stable dispersion system in suspension without aggregation.36 Thermal property is one important parameter for assessing the crystallinity of polymer and the disorder degree of segment. Different scanning calorimeter (DSC) curves of PEO powder, SPE and CPE-10% from


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-80 to 80 °C are presented in Figure 2b. The data of Tg (temperature of glass transition) and Tm (temperature of melting) and ∆Hf (melting enthalpy) were obtained and listed in the Table. S1. It is observed that sharp endothermic peaks appeared at 65 °C, 52 °C, and 48 °C, respectively, corresponding to the Tm of polymer. In addition, Tg of SPE and CPE-10% were -38 °C and -43 °C, respectively. The plasticizing effect of LiTFSI and SSZ-13 reduces the Tm and Tg, leading to the low crystalline state of PEO. The result is also consistent with that of XRD in Figure 1e. The mechanical strength is a vital criterion of all-solid-state electrolyte, which would determine the mechanical integrity of the electrolytes in the collision working conditions. The stress-strain curves of CPE-10% and SPE are shown in Figure 2c, in which the calculated Yang’s Modulus of CPE (7.52 MPa) is 1.7 times larger than that of pristine SPE. The improved strain strength and Modulus of electrolytes were due to the strong cross-link interaction between PEO and SSZ-13. In addition, the special nanoporous structure and high surface area can provide more physical cross-link centres.37 Thermal stability of electrolytes also effect on the practical application for lithium metal batteries significantly, which is evaluated by TGA analysis. According to the TGA curves in Figure 2d, the SPE exhibits two weight loss platforms. The first degradation starts from 180 °C and ends near 250 °C, which can be ascribed to the decomposition of small molecular weight PEG and loss of trace water, residual solvent. The second degradation located at 400 °C accords with the decomposition of PEO. In contrast, only one stage of CPE-10% sample can be observed because of the nanoporous capture effect. Difference of the residuals for the electrolytes is derived from SSZ-13 thermal property. Thus, after introducing the porous SSZ-13 nanoparticles, the polymer electrolyte still possesses a highly dispersed system and displays excellent thermal stability.


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Figure 2. (a) Zeta potential of SSZ-13 nanoparticles; Inset: surface schematic of SSZ-13 (b) DSC curves of PEO powder, SPE and CPE-10%; (c) Stress-strain curves of the SPE and CPE10%; (d) TGA curves of SPE and CPE-10%.

Ionic conductivity and electrochemistry of CPE

In order to confirm our hypothesis of the increasing conductivity by the nanoporous SSZ-13, the ionic conductivity and electrochemistry of CPE were further investigated. Figure 3a presents the ionic conductivities temperature curves for the CPE membranes composed of different SSZ13 content. There is a high slope linear appearing below the melting temperature (Tm), while a low slope is presented above the Tm. The results clearly show the enhancement of ionic conductivity after the addition of nanoporous SSZ-13. Comparing the different concentration of additive, the optimum SSZ-13 content for CPE is found to be ~10 wt % at different temperatures. Moreover, the CPE-10% sample shows the highest ionic conductivity in the various electrolytes (4.43×10-5 S cm-1 at 20 °C, 2.0×10-3 S cm-1 at 70 °C), while the ionic conductivity of pure SPE is


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only 1×10-6 S cm-1 at 20 °C. The conductivity as function of the SSZ-13 content under different temperatures is depicted in Figure 3b. Owing to the Lewis acidic-basic interactions between the lithium salt and SSZ-13, the lithium ions are absorbed to the surface of nanopores and form a high Li+ concentration layer cover in SSZ-13 nanoparticles. When the filler content is 10 wt %, there is an opportune enrichment area of Li+, which create continuous percolating pathways for lithium ion diffusion. Nonetheless, the increased content of redundant inactive fillers would hinder lithium ionic conduction according to effective medium theory (EMT).38-39 Additionally, the conducting properties of CPE-10% at selected temperatures are investigated by electrochemical impedance spectrum and the Nyquist plots are presented in Figure 3c. It clearly shows that the impedance value gradually decreases with the increase of temperature.

Figure 3. (a) Temperature dependence of ionic conductivities at different contents of SSZ-13 from 20 °C to 70 °C. (b) The conductivities of SSZ-CPE with different contents of SSZ-13 at different temperatures. (c) The EIS spectrum of CPE-10% at different temperatures. (d) LSV


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curves of SS/electrolyte/Li cells based on CPE (red) and SPE (black) at 60 °C. (e) Chronoamperometry curve of the symmetric lithium metal cells based on CPE-10% at 10 mV and a duration time of 3600 s at 60 °C. (f) The AC impedance spectrums of the cells before and after the polarization. Then we further study the detail electrochemistry of the CPE by linear sweep voltammetry curves(LSV). As shown in Figure 3d, the CPE-10% possesses a steady platform at about 4.6 V at 60 °C, higher than that of SPE. As a comparison, the oxidation voltage of commercial organic liquid electrolytes is found to be ~3.8 V at room temperature (Figure S2). The wider electrochemical stability window brought by the strong interaction and capture of small molecules indicates that the CPE could be applicable for high-voltage positive cathode in highenergy lithium battery. A high t+ of CPE was obtained by an impedance potential polarization analysis (Figure 3e) combined with AC spectroscopy (Figure 3f). In the early stage, the motion of Li+ and TFSI- are both increasing the current. While at the end of polarization, the current reaches a constant value due to the transfer of only Li+ from one lithium electrode to another. The calculated t+ of the CPE is 0.5, which is 2 times higher than that of SPE(0.25)40. The high Li+ ionic transference number is merited attributed to the nanoporous SSZ-13.

The interfacial compatibility and stability of CPE to lithium metal.

The interfacial stability between the lithium anode and solid electrolyte interface is the major factor for lithium metal batteries, which was evaluated by circularly galvanostatic plating and striping Li metal in symmetric sandwiched structure cells (Li/CPE/Li). Figure 4a shows a voltage window with current of 0.1 mA (0.09 mA cm-2) for 1h each cycle at 60 °C. The charge-discharge voltage of the Li/CPE/Li cell exhibits a voltage range between −0.07 to 0.07 V in the beginning


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and then the voltage slightly decreases to −0.05~0.05 V after 1000 h, indicating the high stability of the interfaces between Li metal and polymer. In contrast, symmetric lithium battery based on pure SPE shows relatively unstable interface with short circuit after long cycles. As shown in Figure 4b, the AC impedance spectra of symmetric cell after different cycles confirm that the CPE have a great compatibility with Li anode. SSZ-13 nanoparticles with high specific area and abundant nanopores could capture small molecular gas and solvent (H2O and O2) to restrict side reactions between lithium anode and impurities.41 The slight reduction of bulk and interfacial impedance reveals that the PEO is completely wetted with electrodes by the interface fusion effect. It is noteworthy that the distributed nanoporous SSZ-13 can form filling layer and prevent the short circuit. According to previous studies, the well-organized porous structure could reduce lithium dendrite effectively.42 Figure 4e, 4f and Figure S3 show SEM images of lithium metal faced to CPE membrane. We cannot observe the obvious formation of lithium dendrite in the CPE-10% cell, while mossy dendrite appears in the SPE cell (Figure 4c, 4d). The comparative experiments suggest that the SSZ-13 modified PEO electrolyte can effectively improve the interfacial stability and reduce the lithium dendrite during long cycles.

Figure 4. (a) Li plating/striping cycling and curves of symmetric cells with SPE and CPE-10% at 0.1 mA (0.09 mA cm-2). (b) EIS impedance spectra of symmetric lithium cells before cycling, after 360 h cycles and after 1000 h cycles. Inset: the equivalent circuit used for fitting. SEM


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images of lithium metal with SPE (c-d) in Li symmetric cells after 400 h and CPE-10% (e-f) after 1000 h, at same current: 0.1 mA.

Mechanism of lithium diffusion in the SSZ-CPE

The results of aforementioned suggest that the nanoporous structure of the additives introduce continuous lithium ion diffusion pathway, leading to a significant increase of ionic conductivity. Because of the good dispersion, the additives maintain stable in polymer and are less likely to deposit. Among previous studies, oxide nanoparticles with low surface area and single surface acidic could result in the disconnection of fast lithium-ion transport pathways.43-44 In addition, the particles with high density and low surface charge would deposit and aggregate into the interface between electrode and electrolyte, causing rapid cycling capacity decay.45 Due to the highly coupling with the EO units, the Li+ cations are less mobile than their anionic counterpart, which accounts for the low lithium-ion transference number (t+99.5%) have been achieved in LFP/Li ASSB. Moreover, soft-package cells with CPE demonstrate highly reversible electrochemical performance even under deformation and truncation states. These results demonstrate that the novel CPE holds great promise for the potential application as the solid electrolyte for the all-solid-state lithium metal battery and even flexible and safe thin film batteries.

EXPERIMENTAL

Preparation of the SSZ-CPE.


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The polyethylene oxide (PEO, Mw 6×106, 99.9%, from Acros) was dried at 65°C for 48 h in vacuum. The lithium bis (trifluoromethanesulfonyl) imide (LiN(SO2CF3)2, LiTFSI, 99 %, from Sigma-Aldrich) was dried at 100 °C for 24 h in vacuum. Zeolite SSZ-13 was provided by Tianjin Haisai nanomaterials Co., Ltd., the size of nanoparticles was reduced to ~100 nm with the highenergy ball milling at a rotation speed of 400 rpm for 5 h on a Planetary Mill P-5 (Fritsch, Germany). SSZ-13 nanoparticles, PEO and LiTFSI were dispersed in the acetonitrile (CH3CN, AR grade, H2O≤0.001%, from aladdin) at a certain proportion to form a homogeneous colloidal solution (EO: Li=16:1). The solution was cast to a PTFE plate and acquired electrolyte membrane at 65 °C for 24 h in vacuum of the glovebox. The as-obtained electrolyte without SSZ-13 is denoted as SPE and containing X% content of SSZ-13 is denoted as CPE-X% respectively.

Electrode preparation and battery assembly.

LiFePO4 cathode slurry was fabricated by ball-milling mixing. LiFePO4 (from Tianjin STL Energy Technology Co., Ltd.) as the active material, Super P as the conductive carbon black, and poly (vinylidene fluoride) (PVDF) as the binder dissolved in 1-methyl-2-pyrrolidinone(NMP) at 8:1:1. The slurry was casted on Al foil, and dried at 70 °C for 24 h in vacuum. The active material area mass is about 4 mg cm-2. The S/C (sulfur, 99.999%, from aladdin) cathode was prepared by similar synthetic process with the average active material loading of 1.0 mg cm-2. The battery assembled with lithium foil (12 mm, from China Energy Lithium Co., Ltd) or stainless steel as the anode was sealed by a battery sealing machine.

Characterization methods of the composite solid electrolyte.


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The scanning electron microscope (SEM, S-3400, Hitachi) are used to characterize the morphology of the samples. X-ray diffraction (XRD) analysis was performed on Rigaku Ultima from 10° to 80° with Cu Kα radiation. differential scanning calorimetry (DSC) thermogram was characterized with PerkinElmer DSC 8000 in the range of -80 °C to 80 °C, 10 °C min-1 as the heating rate. The stress-strain behavior results of materials were given by Instron 3366 Testing System. The Yang’s Modulus can be calculated by the equation: E=

σ F/A = ∆/

Where F is the tension force applied on the electrolyte membrane; A is area of actual crosssection; L0 is pristine length of the material and the ΔL is the amount of length change. The thermal stability of the electrolyte was characterized with thermogravimetric analysis (TGA, Netzsch STA 409PC) in N2 atmosphere, 10 °C min-1 as the heating rate. The N2 sorption measurements were carried out on Micromeritics Tristar 3020 at 77.3K. The pore size distribution and specific surface area were measured by the Brunauer-Emmett-Teller (BET) methods and Barrett-Joyner-Halenda (BJH). Zeta potential and surface charge characteristics of the samples were tested by Brookhaven ZetaPULS. And the size distribution of the nanoparticles was measured by a laser particle size analyzer (Malvern Mastersizer 2000).

Electrochemical tests.

Ionic conductivities of electrolytes were measured by electrochemical impedance spectroscopy (EIS) (Autolab Metrohn AUT85167 system) ranging from 0.1 Hz to 106 Hz. The electrolytes membranes were clamped between the two stainless steel (SS) disks (diameter=15.5 mm) and then sealed in CR2025 coin cells.


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The behaviour of ionic conductivity can be calculated by the equation:

=

Where L is the thickness of electrolyte membrane, Rb is the bulk electrolyte resistance and S is the area of the SS electrodes. The lithium-ion transference number (t+) of the electrolyte was calculated by AC impedance and DC polarization for symmetric cell (Li/electrolyte/Li). The t+ can be calculated by the following equation: =

∆ − ∆ −

Where ∆V is applied polarization voltage of 10 mV, I0 and IS are initial and 3600 seconds current measured by DC polarization, respectively. R0 and RS represent the resistance before and after the polarization obtained by AC impedance spectra, relatively. The liner sweep voltammograms (LSV) curves of the electrolyte were determined from 2.5 to 6 V at a scanning rate of 5 mV s-1 for SS/electrolyte/Li cell at 60 °C. The galvanostatic chargedischarge curves of batteries were measured on a Land CT2001A battery test system. The LiFePO4/electrolyte/Li and S/electrolyte/Li batteries were measured using CR2025 coin cells and 5 cm × 7 cm soft package cells. All batteries were assembled in an argon filled glove box (O2

Nanoporous Adsorption Effect on Alteration of the Li+ Diffusion

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