An Integrated Interface Strategy towards Room Temperature Solid

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An Integrated Interface Strategy towards Room Temperature Solid-state Lithium Batteries Jiangwei Ju, Yantao Wang, Bingbing Chen, Jun Ma, Shanmu Dong, Jingchao Chai, Hongtao Qu, Longfei Cui, Xiuxiu Wu, and Guanglei Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

An Integrated Interface Strategy towards Room Temperature Solid-state Lithium Batteries ‡

Jiangwei Ju†, Yantao Wang†, , Bingbing Chen†, Jun Ma†, Shanmu Dong†, Jingchao Chai†, Hongtao Qu†, Longfei Cui¶, Xiuxiu Wu¶, and Guanglei Cui*,† †

Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and

Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, P. R. China ‡

School of Future Technology, University of Chinese Academy of Sciences, Beijing, 100049, P. R.

China ¶

Qingdao University of Science & Technology, Qingdao, 266042, P. R. China

ABSTRACT Solid-state lithium batteries have drawn wide attention to address the safety issues of power batteries. However, the development of solid-state lithium batteries is substantially limited by the poor electrochemical performances originating from the rigid interface between solid electrodes and solid-state electrolytes. In this work, a composite of poly(vinyl carbonate) and Li10SnP2S12 solid-state electrolyte is fabricated successfully via in-situ polymerization to improve the rigid interface issues. The composite electrolyte presents a considerable room temperature conductivity of 0.2 mS cm-1, electrochemical window exceeding 4.5 V, and Li+ transport number of 0.6. It is demonstrated that solid-state lithium metal battery of LiFe0.2Mn0.8PO4 (LFMP)/composite electrolyte/Li can deliver a high capacity of 130 mA h g-1 with considerable capacity retention of 88 % and Coulombic efficiency of exceeding 99 % after 140 cycles at the rate of 0.5 C at room temperature. The superior electrochemical performance can be ascribed to the good compatibility of the composite electrolyte with Li metal and the integrated compatible interface between solid electrodes and the composite electrolyte engineered by in-situ polymerization, which leads to a significant interfacial impedance decrease from 1292 Ω cm2 to 213 Ω cm2 in solid-state Li-Li 1

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symmetrical cells. This work provides vital reference for improving the interface compatibility for room temperature solid-state lithium batteries. KEYWORDS: solid-state lithium batteries, interface compatibility, sulfide solid electrolyte, poly(vinyl carbonate), in-situ polymerization 1.

INTRODUCTION Lithium-ion batteries (LIBs) are changing our life all over the world in many aspects, such as

automobile, grids, and electric vehicles. However, the flammable and volatile liquid electrolytes (LEs) raise the critical safety issues of LIBs.1-3 In addition, the LEs usually cannot be well compatible from high voltage cathode or low voltage lithium metal anode due to the limited high voltage stability or lithium dendrite growth, respectively. The practical energy densities of LIBs hence have seen their limits. Solid-state lithium batteries (SLBs) without LEs are considered to be the ultimate solution. Consequently, their key materials, solid-state electrolytes (SEs) have drawn increasing attentions due to their high Young's modulus, wide electrochemical window, and superior thermal stability.4-6 These properties enable the SEs to compatibly match high voltage cathode and low voltage lithium metal anode simultaneously, endowing SLBs promising high energy density and security. In addition, as another very important advantage, compared to liquid electrolytes, solid state electrolytes can avoid the dissolution of active materials during cycles.7,8 Development of SEs have progressed rapidly in recent years and favorable room temperature ionic conductivity has been presented in lithium garnets, NASICON-type oxides, perovskite, and sulfides based on Li2S:P2S5 system.9-12 Frequently-used SEs are listed in Table 1. It is noted that, although the conductivity of all kinds of SEs is higher than 10-4 S cm-1, SLBs deliver relatively poor electrochemical performance. When the Ohmic resistance of the SLBs has been alleviated dramatically by the high ionic conductivity of SEs, the interfacial resistance originating from the solid-solid interface 2


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between solid electrodes and SE becomes significantly pronounced, resulting in the inferior electrochemical performance.14,15 Table 1. Li+ conductivity for different SEs and the corresponding SLBs performance. Solid-state electrolytes Composition Li+ Conductivity (mS cm-1) Garnet Li6.4La3Zr1.4Ta0.6O12 1.6@RTa NASICON Li1.5Al0.5Ge1.5(PO4)3 0.18@RT Thio-LISICON Li10GeP2S12 12@RT Anti-perovskite Li3OCl 0.20@RT Perovskite Li0.5La0.5TiO3 1.0@RT Polymer-Garnet PEO:12vol.% 0.21@30 °C Li6.4La3Zr1.4Ta0.6O12 PolymerPVCA:1wt.% 0.20@30 °C Thio-LISICON Li10SnP2S12 Type

a

Solid-state lithium batteries Cathode/ Performance Anode

Reference

LFPb/Li Sulfur/Li LNMOc/Li LCOd/graphite LFP or LFMP/Li LFMP/Li

0.05 C@60 °C 0.1 C@RT 7.3 mA g-1@RT 10 mA g-1@RT 0.1 C@60 °C

Du4 Wang5 Oh9 Lü13 Inaguma11 Zhang10

0.5 C@30 °C

This work

RT, room temperature; b LFP, LiFePO4; c LNMO, LiNi0.5Mn1.5O4; d LCO, LiCoO2 .

The exact mechanisms by which the interfacial resistance generates are difficult to confirm. Takada et al. proposed a space-charge layer model to explain the origin of the large interfacial resistance,16,17 while Sakuda et al. suggested that the large interfacial resistance originates from the mutual diffusions of the elements between the cathode and SEs.18 Coating active materials of the electrode with a buffer layer has been proven to be an effective strategy,16 but most of these batteries still see obvious performance degradation only after several cycles, which is mainly caused by the contact loss between the SE and electrode active materials during lithiation/delithiation, highlighting the significance of engineering integrated interface between solid electrodes and SE.19,20 Usually, some amount of liquid electrolytes are added into between the solid electrode and SE to ensure good contact.21,22 However, the battery performance still undergoes severe degradation. In addition, to engineer an efficient Li+ transport pathway from the electrodes inside to electrolyte, SE phase is indispensible in the solid electrodes. Nevertheless, the addition of large amounts of SEs 3



will decrease the active materials proportion substantially and thus the energy density.19,20 To minimize the SEs proportion, Sakuda et al. employed laser vapor deposition method,23 and Kim et al. used wet chemical method20 to encapsulate the active materials with a thin SE layer. These tedious processes amplify the fabrication difficulties, increase the cost of SLBs and bring about side reactions. The above scenario motivates us to present a simple strategy of in-situ polymerization to engineer the integrated solid electrodes/SE interface with efficient Li+ transport pathway to significantly decrease the interfacial resistance and improve the interface compatibility in SLBs.24,25 During the in-situ polymerization processes, the liquid-state monomers with lithium salt are injected into and penetrated the cell, establishing a continuous 3D Li+ conducting pathway. After polymerization, the characteristics of the continuous 3D Li+ conducting pathway still maintain. Meanwhile, via in-situ polymerization, the preparation of SEs and fabrication of SLBs is integrated in one step. This strategy simplifies the preparation process of SEs and alleviates the cost of fabrication of SLBs. Vinylene carbonate (VC) is one typical solid electrolyte interface (SEI) additive to enhance the interfacial compatibility toward lithium anode and high voltage cathode, which can be polymerized into Poly(vinyl carbonate) (PVCA).26 In our previous work, PVCA was fabricated successfully via in-situ polymerization. It has been proven to be stable exceeding 4.5 V vs Li+/Li, which is verified by matching high voltage cathode LiCoO2.27 These advantages suggest PVCA a promising SE candidate for high energy density SLBs. However, the inferior ionic conductivity of PVCA (< 10-5 S cm-1 at room temperature) limits its application in SLBs. It is worth noting that one of the efficient solutions reported by the literatures to improve the polymers' conductivity is introducing inorganic fillers into organic matrix.28,29 For example, by adding 20 wt.% Li1.5Al0.5Ge1.5(PO4)3 powders into 4


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PEO matrix, the conductivity of PEO is significantly improved from 5.6 × 10-6 to 3.2 × 10-5 S cm-1 at room temperature.29 Inspired by the positive effects of the organic-inorganic hybrids, in this work, highly conductive inorganic thio-LISICON type Li10SnP2S12 (LSnPS) powders are dispersed into the VC liquid-state monomers. After polymerization, the PVCA-LSnPS composite electrolyte with satisfactory room temperature conductivity and wide electrochemical window is obtained. In addition, the PVCA-LSnPS composite electrolyte is compatible with lithium metal anode. Most importantly, via in-situ polymerization, compatibly integrated solid-solid interface between solid electrodes and PVCA-LSnPS are engineered successfully, which would account for the optimized interfacial resistance. This work can serve as a valuable reference for engineering compatible interface between solid electrodes and SE and improving the interface compatibility in solid-state lithium batteries. 2.

EXPERIMENTAL SECTION The thio-LISICON Li10SnP2S12 (LSnPS) were synthesized based on solid state reaction

method. For this purpose, Li2S (Alfa Aesar, 99.9 %), P2S5 (MACKLIN, ≥ 99 %), and SnS2 (MACKLIN, ≥ 99 %) were mixed in an Ar-filled glove box according to the chemistry composition. These powders were then sealed into a stainless-steel pot and ball milled for 4 hrs using a milling instrument (FRITSCH, pulverisette 7). Subsequently, the powders were poured out in an agate mortar and grinded again. The grinded powders were then pressed into pellets, sealed in a tube (Ar filled) and heated at 550 °C for 8 hrs. After that, the pellets were taken out from the tube and ball milled again to obtain the fine LSnPS powders. The LSnPS pellets for ionic conductivity test were fabricated by die pressing the obtained LSnPS powders and sintered at 550 °C for 5 hrs under Ar atmosphere. The pellets were 0.6 mm in thickness and 10 mm in diameter. The VC solution was obtained via dissolving 1.43 g Lithium Difluoro(oxalate)borate (LiDFOB, 5



Innochem(Beijing) Technology Co., Ltd., 99.9 %) and 30 mg AIBN (Sinopharm Chemical Reagent Co., Ltd.), which was used as the initiator, into 10 mL VC (Energy Chemica., 99.9%). The VC-LSnPS solution was obtained via dispersing 3 wt.% LSnPS into the above VC solution. All the processes were conducted in an Ar-filled glove box. The symmetrical CR2032 cells fabricated via in-situ processes were prepared by injecting 40 µL VC or VC-LSnPS solutions into cellulose separator sandwiched between Li metal discs. The half cells fabricated via in-situ processes were prepared by injecting 40 µL VC or VC-LSnPS solutions into cellulose separator sandwiched between LiFe0.2Mn0.8PO4 (LFMP) cathode and Li metal anode. The cells to test the linear sweep voltammetry (LSV) were prepared by injecting 40 µL VC-LSnPS solutions into cellulose separator sandwiched between steel disc and Li metal disc. All the cells were then moved to an oven and thermally treated at 60 °C for 120 hrs to ensure successful polymerization. The LSnPS electrolyte based cells were fabricated via Ex-situ processes. To fabricate the 'Li/LSnPS/Li' symmetrical cell, the LSnPS pellet was firstly obtained by die pressing 0.1 g LSnPS powders in one 10 mm diameter mold. The LSnPS pellet was then sandwiched between Li metal electrodes in CR2032 cell. To fabricate the 'LFMP/LSnPS/Li' half cell, 0.02 g LSnPS powders were spread on the LFMP cathode composite (diameter, 10 mm) and pelletized by pressing at 370 MPa, forming LFMP/LSnPS bi-layer. The bi-layer was then packaged into CR2032 coin cell using Li metal as anode. The LFMP cathode composite was obtained as follows: The cathode slurry was prepared by grinding 80 wt. % LFMP powders, 10 wt. % Super P, and 10 wt. % LA133 binder in deionized water. Then the slurry was casted on an Al foil. After drying in vacuum oven for 24 hrs at 60 °C, the cathode@Al foil was punched into disks in diameter of 10 mm and used as the LFMP cathode composite. The active material loading of the LFMP cathode is 1.34 ± 0.11 mg. 6


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X-ray diffraction (XRD, Bruker D8 ADVANCE) analysis was used to confirm the LSnPS crystal phase. The simulated XRD patterns are obtained through Mercury software based on the relaxation structure. The microstructure morphologies of sintered LSnPS pellet, cellulose separator, PVCA-cellulose, PVCA-LSnPS-cellulose were observed by scanning electron microscopy (SEM, Hitachi S-4800). The Nuclear Magnetic Resonance (NMR, Bruker AVANCE III 600 MHz) and Fourier transform infrared spectrometer (FTIR, Bruker VERTEX 70) were used to analyze the molecular structures of VC, PVCA, and PVCA-LSnPS. Gel-permeation chromatograph was used to estimate the average molecular weight of PVCA and PVCA-LSnPS. Thermogravimetric (TGA) analysis of PVCA and PVCA-LSnPS was conducted on TG 209F1 Iris (NETZSCH). The ionic conductivity of PVCA-LSnPS (σcom) is determined according to =

(1),

where Lcom is the thickness of PVCA-LSnPS composite, 50 µm, Rohm is the Ohmic resistance of PVCA-LSnPS composite estimated via electrochemical impedance spectroscopy (EIS), and Acom is the area of cellulose separator, 2.1 cm2. of PVCA-LSnPS composite is estimated according to ) (∆

=

(∆ )

(2),

where ∆V is the applied amplitude voltage, 10 mV, the superscripts of o and ss represent the initial and steady state, respectively. I is the current, Rohm and Rp are the Ohmic and interfacial resistance of Li/PVCA-LSnPS/Li symmetrical cells, respectively. The LSV test was conducted at a scanning rate of 1 mV s−1. The D.C. polarization tests of Li-Li symmetrical cells based on PVCA, PVCA-LSnPS, and LSnPS were conducted at current densities varied from 5 µA cm-2 to 0.5 mA cm-2. The galvanostatic charge-discharge tests of LFMP-Li half cells based on PVCA, PVCA-LSnPS, and LSnPS were conducted. Both the tests were carried out via LAND testing system (Wuhan LAND electronics Co., 7



Ltd.). The theoretical calculations were performed using the first principles with the projected augmented wave (PAW) plane-wave basis based on the density function theory (DFT), implemented in the Vienna ab initio simulation package.30 For this model, the kinetic energy cutoff was employed at 450 eV, and the optimized structure was completed with the conjugate gradient method31 when a force convergence criterion was less than 0.05 eV Å-1 without any symmetric restrictions. The single molecule was modeled with 4 × 4 × 1 grid for the k-point sampling. In this work, three optimized repeating units molecules of PVCA were chosen to simplify calculations. They were localized in a cell of 20 × 20 × 20 Å3 and modeled with only gamma point. In addition, the LSnPS (0 0 1) slab was employed in the model. The dipole corrections for the LSnPS slab were adopted to avoid the spurious interactions. The F atom absorption energy Ed could be defined as the Ed = Etotal - EF - Eb, where Etotal was the total energy of the adsorption model, EF and Eb were the energy of F atom, and LSnPS slab or PVCA, respectively. 3.

RESULTS AND DISCUSSION

3.1. 3.1.1

Characterization of PVCA-LSnPS Composite Structure Characterization of PVCA-LSnPS Composite The characterization of the synthesized LSnPS is shown in Figure S1 in the supporting

information. The as prepared LSnPS is well crystallized and its Li+ conductivity is comparable to that of the previous reports.32 Figure 1a shows that the VC liquids can be absolutely solidified after thermally treated at 60 °C for 24 hrs no matter with or without LSnPS addition. The color of PVCA is light brown and is darkened by the LSnPS powders. The average molecular weight (weight-average, Mw and number-average, Mn) is determined via gel-permeation chromatograph. The mean values of Mw and Mn are 1.38 × 107 and 9.28 × 106, respectively. 1H-NMR spectra of VC, 8


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PVCA, and PVCA-LSnPS are shown in Figure 1b. The peak at about 2.5 ppm represents the hydrogen in dimethyl sulphoxide (DMSO) solvent. For VC monomers, the peak position at 7.8 ppm is caused by the hydrogen in HC=CH bonds. After polymerization for both PVCA and PVCA-LSnPS, a new peak at about 5.3 ppm appears, which is a response of the hydrogen in

.

The FTIR results in Figure 1c show that the characteristic peaks of C=C-H and C=C groups in VC monomers at about 3166 and 1560 cm-1 disappear when the VC monomers are polymerized. And the PVCA-LSnPS composite has the same FTIR spectra as PVCA. The NMR and FTIR results suggest that the polymerization of VC is carried out by opening the C=C bonds, but remaining other functional groups. The FTIR and 1H NMR spectra of PVCA and PVCA-LSnPS are nearly the same, implying the LSnPS do not react with VC or PVCA. The thermal stability of PVCA and PVCA-LSnPS is determined by Thermo-gravimetric analysis (TGA) under N2 atmosphere from room temperature to 500 °C at a heating rate of 10 °C min-1 (Figure 1d). TGA analysis shows that the onset temperature for PVCA-LSnPS composite is 243 °C, higher than the blank PVCA, 226 °C, suggesting the LSnPS can improve the thermal stability of PVCA.

9



Figure 1. (a) The digital image of polymerization of VC into PVCA after thermally treated at 60 °C for 24 hrs with LSnPS or without LSnPS. (b) 1H NMR spectra of VC, PVCA, and PVCA-LSnPS using dimethyl sulphoxide as solvent. (c) FTIR spectra of VC, PVCA, and PVCA-LSnPS. (d) Thermo-gravimetric analysis curves of PVCA-LSnPS and PVCA.

3.1.2.

Morphology and Electrochemical Characterization of PVCA-LSnPS Composite

The morphologies of cellulose, PVCA, and PVCA-LSnPS composite are examined by SEM imaging. Figure 2a shows the blank cellulose separator, which are comprised of countless fibers. After PVCA injected, the cellulose fibers are covered by PVCA blocks (Figure 2b). Further, with the addition of LSnPS powders, as shown in Figure 2c, the LSnPS particles distribute randomly on the surface of PVCA-cellulose. The morphology of the LSnPS particles is spherical. It can be speculated that the LSnPS particles are capsulated by PVCA, which is certified by elements mapping analysis. As presented in Figure 2d, the elements mapping analysis displays that only C, O, 10


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F, and B can be detected from the particles surface, where C comes from PVCA, F and B from LiDFOB, while O from both. However, the representative of LSnPS, i.e., elements Sn, P, and S are not detected. Sn and P are shown in Figure S2. From SEM images in Figure 2a-d, it can be seen that via in-situ polymerization, the cellulose separator, PVCA, and LSnPS particles are integrated together.

Figure 2. Typical SEM images of (a) Surface morphology of blank cellulose separator. (b) Cross-sectional view of PVCA-cellulose. (c) Surface morphology of PVCA-LSnPS composite. (d) Element mapping analysis (B, C, F, O, and S) of PVCA-LSnPS composite before cycling. (e) Thermal evolution of Ohmic and interfacial resistance for 11



Li/PVCA-LSnPS/Li symmetrical cell. Inset is the typical EIS plots of pristine Li/VC-LSnPS/Li symmetrical cell and Li/PVCA-LSnPS/Li symmetrical cell after thermally treated at 60 °C for 120 hrs. (f) Li+ conductivity of different SEs: PEO-12 vol% LLZTO,10 PEO-LiTFSI-40 wt.% LLZTO,33 PEO-LiTFSI-LGePS,34 and PVCA-LSnPS (this work). (g) Current variation with time during polarization of Li/PVCA-LSnPS/Li symmetrical cell at room temperature.

The electrochemical properties of PVCA-LSnPS composite is investigated based on CR2032 cells. The inset in Figure 2e displays typical impedance spectra of Li/PVCA-LSnPS/Li symmetrical cells. The Ohmic resistance of PVCA-LSnPS composite, Rohm, and the interfacial resistance, Rp, are marked, respectively. The detailed discussion about the impedance spectra are shown below. According to the marked values, the thermal evolution of Rohm and Rp for Li/PVCA-LSnPS/Li symmetrical cell is plotted in Figure 2e. Obviously, both Rohm and Rp increase substantially in the first 72 hrs, which means the increasing difficulties in Li+ transportation, implying the polymerization of VC under thermal treatment. After 96 hrs, Rohm and Rp reach about 20 and 200 Ω cm2, respectively, and remain unchanged then. Consequently, to ensure successful polymerization, the pristine VC-LSnPS are thermally treated at 60 °C for 120 hrs first. A high ionic conductivity of SE is the key to improve the charge-discharge performance and the power density of SLBs. Based on Equation 1, the conductivity of PVCA-LSnPS composite, σcom is estimated and plotted against temperature (Figure 2f). At room temperature, σcom is 2 × 10-4 S cm-1, which is 3 orders of magnitude higher than that of PVCA (10-7 S cm-1 as discussed below), but one order of magnitude lower than LSnPS (2.0 mS cm-1, Figure S1c). Compared with the popular PEO-based SEs, PVCA-LSnPS delivers a relatively high value. As presented in Figure 2f, σcom is comparable to the SE of PEO with 12 vol. % LLZTO added, 2.1 × 10-4 S cm-1 reported by Zhang et al.10 and several times higher than PEO : LiTFSI : 40 wt. % LLZTO, 10-4 S cm-1,33 PEO : LiTFSI : 12


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1 wt. % LGePS, 8 × 10-5 S cm-1.34 The electrochemical window is another key to one kind of outstanding electrolytes. By compositing PVCA and LSnPS, the electrochemical window of PVCA-LSnPS can be broadened to higher than 5V (Figure S3). High decomposition voltage means the PVCA-LSnPS can match high voltage cathode, thus promote the energy density. In Figure 2g, the Li+ transport number, is estimated to be about 0.60 according to Equation 2. This value is larger than that of the LE (1 M LiPF6 in EC/EMC/DMC), which is only 0.38.25 The high for PVCA-LSnPS would be ascribed to two points: first, the movement of

large anion group, [DFOB]-,

, which is dissociated from Li salt, LiDFOB, can be greatly

hindered by PVCA solid-state matrix and, second, the contribution from the LSnPS inorganic particles have a high of nearly one unit. This high of PVCA-LSnPS is expected to decrease the polarization and suppress the formation of lithium dendrite.25,35 The high ionic transference number, wide electrochemical window, and high room temperature ionic conductivity suggest strongly the PVCA-LSnPS a much suitable SE candidate for SLBs. 3.2.

Charge-discharge Properties of LFMP-Li Half Cells PVCA-LSnPS is matched LFMP cathode and Li metal anode to fabricate LFMP-Li half cells.

After thermally treated at 60 °C for 120 hrs, one cell is unpacked. As shown in Figure S4, the black LFMP cathode composite, which was adhered to the Al foil, is now peeled off and adhered to PVCA-LSnPS composite. In addition, the PVCA-LSnPS composite is no longer a whole piece, but torn up by the steel disc or Li metal. The Li metal becomes dark grey for the exposure to air. This inner state confirms the successful fabrication processes and the integrated microstructures of LFMP/PVCA-LSnPS/Li cell fabricated via in-situ polymerization. The galvanostatic charge and discharge tests are conducted from 2.5 to 4.35 V, followed by 13



potentiostatic charge-discharge for 30 mins at 4.35 V at room temperature. Figure 3a displays the charge and discharge curves at different rates. For PVCA-LSnPS based cell at the rate of from 0.1 to 0.5 C, the two discharge plateaus are nearly the same, about 4.0 and 3.6 V, corresponding to the charge plateaus of 4.1 and 3.5 V, respectively. The small plateau differences between charge and discharge indicate reversible processes. When the rate increases to 1 C, the discharge and charge plateaus extend to 3.9 and 3.4 V, 4.2 and 3.7 V due to the polarization effect, respectively. The specific discharge capacity of LFMP/PVCA-LSnPS/Li at the rate of 0.1 C is about 139 mA h g-1, 82 % of the theoretical value, where 1 C is defined as 170 mA h g-1. As the current density increases to 1 C, an acceptable discharge capacity of 89 mA h g-1 is achieved, still 52 % of the theoretical value. The cell based on PVCA can only deliver a discharge capacity of 18 mA h g-1 at the rate of 0.05 C, 11 % of the theoretical capacity. Considering the same fabrication processes of LFMP/PVCA-LSnPS/Li and LFMP/PVCA/Li, the poor electrochemical performance of LFMP/PVCA/Li mainly originates from the inferior ionic conductivity of PVCA. Moreover, the cell based on LSnPS can barely work even at the rate of 0.05 C at room temperature. To elucidate the big difference, cross-sectional SEM images are firstly investigated. As illustrated in Figure 3c, for the LFMP/PVCA-LSnPS/Li cell fabricated via in-situ processes, the PVCA-LSnPS contacts to cathode layers closely, forming the integrated interface. However, for the LFMP/LSnPS/Li cell fabricated via ex-situ processes, large cracks are formed between the LSnPS and cathode layer as shown in Figure 3d. Generally, the electrochemical reactions in SLBs initiate at the points of contact at the interface between the SE and electrode phase.7 Due to the cracks, the contact area between SE and active materials will lose. As a result, in the LFMP/LSnPS/Li cell, the Li+ will transport inefficiently, thus much poor electrochemical performance is delivered. While for the 14


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LFMP/PVCA-LSnPS/Li cell, the integrated interface can ensure intimate contact between SE and active materials, thus the efficient Li+ transportation pathway and outstanding electrochemical performance. Considering the higher ionic conductivity of LSnPS than that of the PVCA and PVCA-LSnPS, i.e., 2.0 mS cm-1 vs. 10-7 S cm-1 and 0.2 mS cm-1, respectively, it can be concluded that: (1) the ionic conductivity of higher than 10-4 S cm-1 is enough to achieve good SLB performance with the indispensable (2) integrated interface between the SE and the solid electrode, and (3) the integrated interface plays a more important role than the ionic conductivity of SE in the SLBs system.

Figure 3. (a) Galvanostatic charge-discharge curves at the rate of 0.1, 0.3, 0.5, and 1.0 C from 2.5 to 4.35 V,

followed by potentiostatic tests for 30 mins at 4.35 V at room temperature. The LFMP/PVCA/Li cell is charged and discharged at the rate of 0.05C at room temperature. (b) Galvanostatic charge-discharge curves of the LFMP/LSnPS/Li cell at the rate of 0.05 C at room temperature. The SEM cross-sectional view of (c) LFMP/PVCA-LSnPS/Li cell and (d) LFMP/LSnPS/Li cell. 15



3.3.

EIS and D.C. Galvanostatic Cycle Analysis of the Li-Li Symmetrical Cells To further illuminate the huge electrochemical performance difference between the LFMP-Li

cells based on different SEs, EIS and d.c. galvanostatic cycle tests on Li-Li symmetrical cells are conducted. The typical impedance spectra of Li-Li symmetrical cells are shown in Figure 4a-c. The fitting models and the refined parameters are shown in Figure S5 and Table S1. The EIS plots of the pristine Li-Li symmetrical cells are nearly the same, which all show a semicircle with a tail connected. According to the previous reports, the interfacial resistance are determined by the span of the semicircle and the Ohmic resistance are determined by the high frequency intercept with the real axis.5 After treated, all the EIS plots change a lot. For Li/PVCA/Li and Li/PVCA-LSnPS/Li symmetrical cells, another semicircle emerges at the high frequency, which represents the response to the Ohmic resistance in the SEs,12 while the semicircle at the middle frequency represents the interfacial resistance. However, for treated Li/LSnPS/Li, the EIS spectrum is still comprised of one enlarged semicircle with a tail connected. According to the fitting results (Table S1), the Ohmic and interfacial resistance of the symmetrical cells are estimated and plotted in Figure 4d,e. In Figure 4d, the Ohmic resistance of pristine Li/VC/Li and Li/VC-LSnPS/Li is nearly the same, i.e., 2.0 and 1.8 Ω cm2, respectively, which are smaller than the Ohmic resistance of the pristine Li/LSnPS/Li, 30 Ω cm2. After treated, the Ohmic resistance of Li/LSnPS/Li remains unchanged, 24 Ω cm2. However, after thermally treated at 60 °C for 120 hrs, the Ohmic resistance of Li/PVCA-LSnPS/Li increases to 21 Ω cm2, 10 times larger than that of the pristine Li/VC-LSnPS/Li, implying the successful polymerization. For Li/PVCA/Li, the value increases sharply to 73751 Ω cm2, corresponding to the ionic conductivity of about 10-7 S cm-1. In Figure 4e, the interfacial resistance of pristine Li/VC-LSnPS/Li is 17 Ω cm2, smaller than Li/VC/Li, 28 Ω cm2 and Li/LSnPS/Li, 50 Ω cm2. After thermally treated at 60 °C for 16


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120 hrs, the interfacial resistance of Li/PVCA-LSnPS/Li increases to 213 Ω cm2, much smaller than that of the Li/PVCA/Li, 26642 Ω cm2. For Li/LSnPS/Li symmetrical cell, the interfacial resistance increases substantially to 1292 Ω cm2 only after stored at room temperature for 48 hrs. Further thermal treatment at 60 °C for 120 hrs increases the interfacial resistance to ~ 6000 Ω cm2 (Figure S6). The above EIS analysis suggests that the Ohmic and interfacial resistance are reduced effectively through the in-situ polymerizing PVCA-LSnPS.

Figure 4. EIS plots of pristine and after treated Li-Li symmetrical cells based on (a) VC, VC-LSnPS, (b) LSnPS,

(c) VC-LSnPS, PVCA-LSnPS before cycling. The characteristic frequencies are marked and the treated conditions are given. (d) Ohmic and (e) interfacial resistance comparison between the pristine and after treated Li-Li symmetrical cells. (f) d.c. galvanostatic cycle of Li-Li symmetrical cells based on PVCA, LSnPS, and PVCA-LSnPS at current densities of 0.5 mA cm-2, 0.5 mA cm-2, and 5 µA cm-2, respectively. The area capacities of Li-Li symmetrical cells based on PVCA, LSnPS, and PVCA-LSnPS are 0.5 mA h cm-2, 0.5 mA h cm-2, and 5 µA h cm-2, respectively.

It has been reported that the cell performance depends strongly on the compatibility between 17



electrolyte and electrodes.9 Consequently, the compatibility of PVCA, LSnPS, PVCA-LSnPS with Li metal is examined via d.c. polarization. As shown in Figure 4f, the voltage of Li/PVCA-LSnPS/Li symmetrical cell keeps stable (17 mV) for over 900 hrs at a current density of 0.5 mA cm-2. For Li/LSnPS/Li, the voltage increases substantially from 0.7 to 5 V in only 19 hrs at a current density of 0.5 mA cm-2. For Li/PVCA/Li, the voltage also increases obviously even at a much lower current density of 5 µA cm-2. These results suggest that PVCA-LSnPS is very compatible with Li metal while the individual LSnPS or PVCA are not. Consequently, based on the above analysis, the favorable LFMP-Li cell performance based on PVCA-LSnPS than that of the cells based on PVCA and LSnPS are mainly due to, (1) the relatively high room temperature ionic conductivity of PVCA-LSnPS, (2) the integrated interface between PVCA-LSnPS and electrodes, (3) the reduced Ohimic and interfacial resistance, and (4) the good compatibility of PVCA-LSnPS with Li metal. 3.4.

Cycle Performance of LFMP/PVCA-LSnPS/Li Cell The rate and cycle performance of LFMP/PVCA-LSnPS/Li half cell are presented in Figure 5a.

The specific discharge capacities at the rate of 0.1, 0.3, 0.5, and 1 C are consistent with the results shown in Figure 5a. At the rate of 0.5 C, the LFMP/PVCA-LSnPS/Li cell can cycle more than 140 cycles with the Coulombic efficiency of over 99 %. After 140 cycles, the cell still has a specific capacity of 116 mA h g-1 with the capacity retention of 88 %. The electrochemical performance of SLB based on PVCA-LSnPS is superior to that of the SLBs based on other types of SEs listed in Table 1. To show visual comparison, the electrochemical performance of LiFe0.15Mn0.85PO4/Li SLB based on PEO-LLZTO10 is plotted in Figure 5a. Though the ionic conductivity of PEO-LLZTO is 5.6 × 10-4 S cm-1 at 60 °C, which is higher than that of PVCA-LSnPS, 3.4 × 10-4 S cm-1 at 60 °C, the LFMP/PEO-LLZTO/Li cell can only operate at the rate of 0.1 C at 60 °C and deliver lower 18


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specific capacity than LFMP/PVCA-LSnPS/Li cell operate at the rate of 0.5 C at room temperature. The SLB based on PVCA-LSnPS is also more stable than the LIB based on LE (1 M LiPF6 in EC : DMC : DEC of volume ration, 1 : 1 : 1, Figure 5a).36 The good stability suggests the solid-state PVCA-LSnPS can make good restriction to Mn dissolution, which usually occurs in the LIBs when using LFMP cathode.36,37 Element mapping analysis in Figure 5b displays that Mn element cannot be detected from the PVCA-LSnPS composite after cycling. However, compared with the mapping analysis results of PVCA-LSnPS before cycling in Figure 2d, it is interesting that the distribution of F is no longer uniform but concentrated on the surface of LSnPS particles while the distribution of B, C, and O is still uniform. Density function theory (DFT) analysis calculates the adsorption energy of PVCA and LSnPS to F-, which are 3.15 eV and -0.89 eV, respectively. Positive value means that PVCA dose not attract F- while negative value means the LSnPS particles have attraction to F-. It could be speculated that along with cycling, the F- will be trapped by the LSnPS particles, Figure 5c. The aggregated F- would affect the Li+ transport behavior in the PVCA-LSnPS composite, which is an explanation for the performance degradation of LFMP/PVCA-LSnPS/Li cell.

19



Figure 5. (a) Rate performance of LFMP/PVCA-LSnPS/Li cell at the rate of 0.1, 0.3, 0.5, 1 C, and cycle

performance at the rate of 0.5 C at room temperature. The discharge cycle performance at the rate of 0.5 C of LiFe0.3Mn0.7PO4-Li cell based on (1 M LiPF6 in EC : DMC : DEC) at room temperature36 and the discharge cycle performance at the rate of 0.1 C of LiFe0.15Mn0.85PO4-Li cell based on PEO:LLZTO electrolyte at 60 °C.10 (b) Element mapping analysis of PVCA-LSnPS composite after cycling. (c) Possible complex structures in PVCA-LSnPS composite after cycling based on the signatures detected by element mapping analysis and the DFT calculation results.

4.

CONCLUSIONS 20


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In summary, solid-state poly(vinyl carbonate) - Li10SnP2S12 (PVCA-LSnPS) composite is fabricated successfully via in-situ polymerization. The PVCA-LSnPS composite satisfies the indispensable combination of high ionic conductivity, wide electrochemical window, large Li+ transport number, and good compatibility with Li metal for applications in solid-state lithium batteries. By in-situ polymerization, the integrated interface between solid electrodes and solid-state electrolyte is engineered and the impedance is decreased substantially. As a result, the electrochemical performance of solid-state lithium batteries are promoted substantially. Specifically, the LFMP-Li solid-state lithium battery based on PVCA-LSnPS composite can deliver a high capacity of 130 mA h g-1 and cycle more than 140 cycles at the rate of 0.5 C at room temperature. In addition, LFMP-Li solid-state lithium battery based on PVCA-LSnPS composite is more stable than the lithium-ion battery based on liquid electrolyte. Element mapping analysis demonstrates that the solid-state PVCA-LSnPS can make good restriction to Mn dissolution, which usually occurs in the lithium-ion batteries when using LFMP cathode. According to the element mapping analysis and Density Function Theory calculation results, the possible complex structures in PVCA-LSnPS composite after cycling are also proposed. This work provides much valuable reference for improving the rigid interface between solid electrodes and solid-state electrolytes in solid-state lithium batteries and suggests PVCA-LSnPS composite a much suitable solid-state electrolyte candidate for solid-state lithium batteries. ASSOCIATED CONTENT Supporting Information. The structure, morphology, and conductivity of as prepared Li10SnP2S12; element Sn and P mapping analysis for PVCA-LSnPS composite before cycling; linear sweep voltammetry measurement for PVCA-LSnPS composite; photo graph of one unpacked LFMP/PVCA-LSnPS/Li cell after thermally treated at 60 °C for 120 hrs; equivalent circuit models 21



and fitting impedance results for pristine Li/VC/Li, Li/LSnPS/Li, Li/VC-LSnPS/Li, and after treated Li/PVCA/Li, Li/LSnPS/Li, Li/PVCA-LSnPS/Li symmetric cells; EIS plot of Li/LSnPS/Li symmetrical cell after thermally treated at 60 °C for 120 hrs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Guanglei Cui: 0000-0001-5987-7569 Author Contributions Jiangwei Ju and Yantao Wang contribute equally to this work. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. ACKONWLEDGEMENTS This work was supported by the China Postdoctoral Science Foundation - Chinese Academy of Sciences (CPSF-CAS) Joint Foundation for Excellent Postdoctoral Fellows, the National Natural Science Foundation for Distinguished Young Scholars of China (No. 51625204), and the China Postdoctoral Science Foundation (No. 2017M612366). REFERENCES (1) Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H. H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.; Shao-Horn, Y. Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chem. Rev. 2016, 116, 140-162. (2) Sun, C.; Liu, J.; Gong, Y.; Wilkinson, D. P.; Zhang, J. Recent Advances in All-Solid-State Rechargeable Lithium Batteries. Nano Energy 2017, 33, 363-386. 22


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Table of Contents. Via an in-situ polymerization integrated strategy, the tri-layers, i.e., LFMP cathode layer, PVCA-LSnPS solid-state electrolyte layer, and Li metal anode layer are integrated together via an analogous mortise and tenon joint. As a result, the rigid interface between solid-state electrolyte and solid electrodes, i.e., LFMP cathode/PVCA-LSnPS/Li metal anode, is modified effectively and favorable charge-discharge performance is obtained at room temperature.

28


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Figure 1. (a) The digital image of polymerization of VC into PVCA after thermally treated at 60 °C for 24 hrs with LSnPS or without LSnPS. (b) 1H NMR spectra of VC, PVCA, and PVCA-LSnPS using dimethyl sulphoxide as solvent. (c) FTIR spectra of VC, PVCA, and PVCA-LSnPS. (d) Thermo-gravimetric analysis curves of PVCALSnPS and PVCA. 135x107mm (300 x 300 DPI)



Figure 2. Typical SEM images of (a) Surface morphology of blank cellulose separator. (b) Cross-sectional view of PVCA-cellulose. (c) Surface morphology of PVCA-LSnPS composite. (d) Element mapping analysis (B, C, F, O, and S) of PVCA-LSnPS composite before cycling. (e) Thermal evolution of Ohmic and interfacial resistance for Li/PVCA-LSnPS/Li symmetrical cell. Inset is the typical EIS plots of pristine Li/VC-LSnPS/Li symmetrical cell and Li/PVCA-LSnPS/Li symmetrical cell after thermally treated at 60 °C for 120 hrs. (f) Li+ conductivity of different SEs: PEO-12 vol% LLZTO,10 PEO-LiTFSI-40 wt.% LLZTO,33 PEO-LiTFSI-LGePS,34 and PVCA-LSnPS (this work). (g) Current variation with time during polarization of Li/PVCA-LSnPS/Li symmetrical cell at room temperature. 204x191mm (300 x 300 DPI)


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Figure 3. (a) Galvanostatic charge-discharge curves at the rate of 0.1, 0.3, 0.5, and 1.0 C from 2.5 to 4.35 V, followed by potentiostatic tests for 30 mins at 4.35 V at room temperature. The LFMP/PVCA/Li cell is charged and discharged at the rate of 0.05C at room temperature. (b) Galvanostatic charge-discharge curves of the LFMP/LSnPS/Li cell at the rate of 0.05 C at room temperature. The SEM cross-sectional view of (c) LFMP/PVCA-LSnPS/Li cell and (d) LFMP/LSnPS/Li cell. 146x104mm (300 x 300 DPI)



Figure 4. EIS plots of pristine and after treated Li-Li symmetrical cells based on (a) VC, VC-LSnPS, (b) LSnPS, (c) VC-LSnPS, PVCA-LSnPS before cycling. The characteristic frequencies are marked and the treated conditions are given. (d) Ohmic and (e) interfacial resistance comparison between the pristine and after treated Li-Li symmetrical cells. (f) d.c. galvanostatic cycle of Li-Li symmetrical cells based on PVCA, LSnPS, and PVCA-LSnPS at current densities of 0.5 mA cm-2, 0.5 mA cm-2, and 5 µA cm-2, respectively. The area capacities of Li-Li symmetrical cells based on PVCA, LSnPS, and PVCA-LSnPS are 0.5 mA h cm-2, 0.5 mA h cm-2, and 5 µA h cm-2, respectively. 205x112mm (300 x 300 DPI)


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Figure 5. (a) Rate performance of LFMP/PVCA-LSnPS/Li cell at the rate of 0.1, 0.3, 0.5, 1 C, and cycle performance at the rate of 0.5 C at room temperature. The discharge cycle performance at the rate of 0.5 C of LiFe0.3Mn0.7PO4-Li cell based on (1 M LiPF6 in EC : DMC : DEC) at room temperature36 and the discharge cycle performance at the rate of 0.1 C of LiFe0.15Mn0.85PO4-Li cell based on PEO:LLZTO electrolyte at 60 °C.10 (b) Element mapping analysis of PVCA-LSnPS composite after cycling. (c) Possible complex structures in PVCA-LSnPS composite after cycling based on the signatures detected by element mapping analysis and the DFT calculation results. 207x194mm (300 x 300 DPI)



Table of Contents. Via an in-situ polymerization integrated strategy, the tri-layers, i.e., LFMP cathode layer, PVCA-LSnPS solid-state electrolyte layer, and Li metal anode layer are integrated together via an analogous mortise and tenon joint. As a result, the rigid interface between solid-state electrolyte and solid electrodes, i.e., LFMP cathode/PVCA-LSnPS/Li metal anode, is modified effectively and favorable charge-discharge performance is obtained at room temperature. 240x90mm (300 x 300 DPI)


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An Integrated Interface Strategy towards Room Temperature Solid

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