Letter www.acsami.org
Stabilization of Garnet/Liquid Electrolyte Interface Using Superbase Additives for Hybrid Li Batteries Biyi Xu,† Huanan Duan,*,† Hezhou Liu,† Chang−An Wang,‡ and Shengwen Zhong*,§ †
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China ‡ State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China § School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, P.R. China S Supporting Information *
ABSTRACT: To improve the solid-electrolyte/electrode interface compatibility, we have proposed the concept of hybrid electrolyte by including a small amount of liquid electrolyte in between. In this work, n-BuLi, a superbase, has been found to significantly improve the cycling performance of LiFePO 4 /Li hybrid cells containing Li7La3Zr1.5Ta0.5O12 (LLZT) and conventional carbonate-based liquid electrolyte. The modified cells have been cycled for 400 cycles at 100 and 200 μA cm−2 at room temperature, indicating excellent solid/liquid electrolyte interface stability. The role of n-BuLi may be 3-fold: to retard the decomposition reaction of LE, to suppress the Li+/ H+ exchange, and to lithiate the garnet/LE interface, inhibiting side reactions and enhancing interfacial lithium-ion transport.
KEYWORDS: interfacial resistance, lithium garnets, hybrid electrolyte, additives, cyclibility 2LiF/Li interfacial resistance of 345 Ω cm2 in symmetric cells (v.s. 1260 Ω cm2 without LiF doping).16 Another strategy has been suggested using hybrid electrolyte composed of solid electrolyte and liquid electrolyte,17,18 where a thin, stable, and compact disk of solid electrolyte acts as a transport medium for lithium ions and a separator for blocking lithium dendrites; small amount of liquid electrolyte acts as wetting agent to guarantee intimate electrolyte/electrode contact. The hybrid electrolyte may also find applications in Li−air, Li-redox-flow batteries and Li−S cells to suppress the unwanted shuttling effect.19,20 Indeed, the hybrid electrolyte consisting of lithium garnets and conventional carbonate-based liquid electrolyte has been used to build hybrid LIB cells,17 but the cyclibility of the cells was poor with the best results to be merely 100 cycles. The poor cyclibility may partially be attributed to the unstable interface between lithium anode and ceramic electrolyte,18 and the LLZO/liquid electrolyte interface may be another reason. To date, few reports have been dedicated to studying the compatibility of garnets with liquid electrolyte (LE), which is composed of lithium salt LiPF6 and organic carbonates such as ethylene carbonate (EC) and dimethyl carbonate (DMC). Very recently, M. R. Busche et al. reported an elegant work about the chemical stability of NASICON-type Li1+xAlxGe2−x(PO4)3
I
n the past few years, lithium batteries containing solid electrolytes (SE) have become of great interest because of their potential remarkable advantages compared to conventional Li-ion batteries, such as good safety, wide operation temperature range, more compact configuration, and enabling the use of new high capacity cathode materials (i.e., sulfur,1−3 Mn-based cathodes4) and lithium metal.5,6 Among the inorganic solid Li-ion electrolytes, Li7La3Zr2O12 (LLZO) garnets and their doped variants have been highlighted due to their chemical stability with Li metal, decent Li ion conductivity (>10−4 S cm−1 at room temperature), wide electrochemical windows, and reasonable air stability.7−9 Despite these merits, when assembling LLZO into a cell, the high interfacial resistance between the solid electrolyte and electrodes leads to interior cycling performance.10,11 To improve the interface property, people have come up with different strategies that can be classified into three main types. The first one is to add an intermediate coating to construct an artificial solid electrolyte interphase (SEI) layer.12,13 For example, C.L. Tsai et al. reported that the total surface area resistance of LLZO/Li interface can be decreased from ∼3000 Ω cm2 to ∼380 Ω cm2 after coating the LLZO with a ∼ 20 nm thick Au layer.12 The second strategy is to focus on the bulk material of LLZO by microstructure modification and doping.14−16 Recently, Y. Li, et al. reported that doping LiF was able to effectively suppress the formation of resistive Li2CO3 phase on the garnet surface, resulting a low LLZO© 2017 American Chemical Society
Received: April 21, 2017 Accepted: June 14, 2017 Published: June 14, 2017 21077
DOI: 10.1021/acsami.7b05599 ACS Appl. Mater. Interfaces 2017, 9, 21077−21082
Letter
ACS Applied Materials & Interfaces (LAGP) and conventional liquid electrolyte and shed light on the chemistry and the kinetics of the resistive solid−liquid electrolyte interphase (SLEI).21 On the other hand, it has been proven experimentally and computationally that the LE may decompose on electrode material surfaces and the decomposition products (e.g., LiF, Li2CO3, CH3OLi) exhibit low Li+ conductivity and subsequently result in increasing cell resistance and capacity loss during cycling.22 Besides LE decomposition, Li2CO3 formation on the surface of garnet in ambient air has been demonstrated to contribute to high interface resistance with lithium metal.23,24 In the present work, we investigate the compatibility of garnet with LE and provide a strategy to stabilize the garnet/LE interface. We first prepare the fast garnet of Ta doped Li7La3Zr1.5Ta0.5O12 (LLZT) by a solid-stare reaction route,24 with high relative density (>92%) and room-temperature Li+ ion conductivity of 6 × 10−4 S cm−1. Detail experimental process were reported in our previous work16,23,24 and the Supporting Information. Hybrid electrolyte consists of a 0.5 mm thick garnet disk (as electrolyte and separator) wetted with small amount of commercial liquid electrolyte (1 M LiPF6 in EC/DEC = 50:50 v/v). The hybrid cells were assembled by placing lithium foil (Aladdin) and LiFePO4 cathode deposited on Al foil onto both sides of hybrid electrolyte in a Swagelok cell, so had a configuration of Li/LE/LLZO/LE/LiFePO4. The kinetics, chemistry, and stability of the LLZT/LE interface is then investigated. The interface is optimized based on the assumption that by suppressing Li+/H+ exchange on garnet surface, the decomposition of electrolyte and the formation of resistive SLEI can be inhibited. A superbase, n-BuLi, is introduced as an example to suppress the Li+/H+ exchange and lithiate the garnet/LE interface; a similar strategy was used to lithiate Li4Ti5O12.25 The optimized hybrid cells exhibit excellent electrochemical stability and much improved cyclability. This work aims to contribute to understanding the stability of the garnet/LE interface and provide a simple yet effective approach to improve the stability, rendering hybrid cells with a long cycling life and good capacity retention. The as-synthesized LLZT solid electrolyte exhibits roomtemperature Li-ion conductivity of 6 × 10−4 S cm−1 (Figure S1). To investigate the chemical compatibility between garnet and LE, the LLZT pellet was soaked into the LE (a solution of 1 M LiPF6 in a 50:50 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) by volume) for a week. Figure 1a, b show the cross-section images of LLZT before and after LE treatment. Both images show rather dense ceramic electrolyte composed of grains in size of 10−20 μm with thin grain boundaries; not much morphology change is observed after being soaked except that some grains near the interface get rough by scrutiny. On the other hand, Figure S1 shows a considerable impedance increase after soaking. Moreover, as shown in Figure 1c, the XRD patterns of the LLZT pellets before and after LE soaking match well with the standard pattern of cubic garnet phase (PDF #45−0109). But the relative intensity of peaks varies slightly and the peaks become broader after being soaked. Similar observation was reported for lithium garnet reacting with water and getting protonated,22 suggesting that Li+/H+ exchange may also occur in the present work. We believe that the Li+/H+ exchange may lead to Li+ deficiency along the LE/LLZO interface and poor interfacial Li+ conduction. The LLZT pellets before and after LE soaking for 1 week were examined with Thermogravimetric analysis (TGA) (Figure S2). The weight loss of LLZT before soaking is
Figure 1. Typical SEM images of (a) as-synthesized LLZT and (b) LLZT after being soaked in LE. (c) XRD patterns of LLZT before and after being soaked in LE. (d) Galvanostatic charge/discharge curves of the hybrid cell for cycle 1, 2, and 10.
obviously smaller than that of the LLZT after soaking, which confirms the possible Li+/H+ exchange between garnet and LE. Figure 1d depicts the galvanostatic charge/discharge curves of the lithium ion battery consisting of hybrid electrolyte with a current density of 100 μA cm−2 at room temperature. The voltage plateaus of charge and discharge are around 3.55 and 3.35 V, respectively. For the first discharge, a specific capacity of approximately 160 mAh g−1 is delivered, which is 94% of the theoretical capacity of LiFePO4. This behavior indicates that hybrid electrolyte of LE/SE/LE wets the electrodes well and constructs a good conduction networks for both electrons and lithium ions. However, after 10 cycles, the specific capacity decreases to 127 mAh g−1 with increasing polarization between charge and discharge, which implies side reactions in the hybrid cell. Indeed, the increasing interfacial impedance (Figure S3) was observed by measuring the impedance of Swagelok cells consisting of stainless steel/LE/SE/LE/stainless steel over time, which confirms the reactivity between garnet and liquid electrolyte. Though the exact reaction mechanism between garnet and liquid electrolyte is unclear, it has been suggested that the decomposition of liquid electrolyte, LiPF6 in particular, may lead to capacity fading based on the following equation: LiPF6 ↔ LiF + PF5.26 The decomposition reaction can be autocatalytic if the product of PF5 reacts with trace protic impurities in the electrolyte, such as water and alcohol.26 Lewis basic additives can retard the decomposition reaction and increase the electrolyte stability dramatically.26 To improve the interface property, n-BuLi, a superbase (pKa ≈ 50), is used to deprotonate the liquid electrolyte and provides Li+ for the Li+ deficiency interface regions. Figure 2a shows clearly that after being soaked in the LE with n-BuLi, the grains near the interface become smooth, same as the as-synthesized LLZO as shown in Figure 1a. The XRD patterns of the LLZT pellets after being soaked in the LE with n-BuLi (Figure 2b) match 21078
DOI: 10.1021/acsami.7b05599 ACS Appl. Mater. Interfaces 2017, 9, 21077−21082
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ACS Applied Materials & Interfaces
Figure 2. (a) SEM image and (b) XRD patterns of LLZT after being soaked in LE with n-BuLi. Raman spectra of (c) as-synthesized LLZT, LLZT after soaking in LE (d) without and (e) with n-BuLi.
the LE and LLZT. When the current density increases to 200 μA cm−2 in cycle 201, the discharge capacity drops to 82.4 mAh g−1, but good interface stability is maintained thereafter until cycle 400. As shown in Figure 3b, the reversible discharge capacity after 400 cycles at 200 μA cm−2 was approximately 99% that at the 201th cycle. Figure 3c and d depicts the impedance evolution of the hybrid cell without and with n-BuLi, before and after cycling. Experimental data are compared with simulated spectra using equivalent circuits comprising an ohmic serial resistor (RLE) and three (RQ) circuits associated with contributions by bulk, grain boundary, and SE/LE interface.21 As shown in Figure 3c, the cell impedance grows significantly after 1 week when no nBuLi is added. Specifically, the SE/LE interface areal resistance increases from ∼1056 to ∼2419 Ω cm2 after cycling, implying that the garnet undergoes degradation reactions at the interface. In contrast, Figure 3d shows that the impedance exhibits little change even after 400 cycles, emphasizing the interface stability achieved by the addition of n-BuLi. In fact, the interface areal resistance is as low as 478 Ω cm2 by fitting. The interface stability was also confirmed by measuring the impedance (Figure S4) of Swagelok cells consisting of stainless steel/LE +n-BuLi/LLZT/LE+n-BuLi/stainless steel over time. It is generally accepted that the solvents and lithium salts in conventional liquid electrolyte decompose and form SEI layer at the surface of electrodes. As for solid electrolyte in contact with liquid electrolyte, M.R. Busche et al. recently demonstrated that a SLEI forms between LAGP and an ether-based liquid electrolyte and this SLEI growth is promoted by the presence of water.21 As for lithium garnets in particular, our previous research shows that Li+/H+ exchange may occur when lithium garnets are in contact with water or stored in ambient atmosphere, which is in accord with the literature.29,30 So we speculate that when lithium garnets contact with LE, a SLEI may form as a result of electrolyte decomposition and/or Li+/
well with the cubic garnet phase LLZT with not much intensity change and no peak broadening comparing with pristine LLZT in Figure 1c, which suggests possible depression of the Li+/H+ exchange by using n-BuLi. Raman spectroscopy is employed to further analyze the role of n-BuLi. Figure 2c−e shows the Raman spectra of LLZT pellets before and after contacting with LE, with and without nBuLi. The Raman bands of the pristine LLZT can be readily assigned to the cubic phase of LLZ with the bands in 100−150, 200−300, 300−600, and about 640 cm−1 corresponding to the vibration of the heavy La cations, the oxygen bending, the Zr− O bond stretching, and the lithium vibrations, respectively.23 The occurrence of the peaks from 773 to 743 cm−1 can be associated with the vibrational stretching mode of TaO6 octahedral units.27 Figure 3d clearly reveals that besides the LLZT peaks, the peaks at 156, 192, and 1090 cm−1 that can be assigned to Li2CO3 appear, implying the formation of Li2CO3 after LE treatment. Li2CO3 has been well-proven to be a product of electrolyte decomposition on electrode surfaces.28 On contrast, the Li2CO3 peaks disappear for the LLZT contacting with LE adding n-BuLi (Figure 2e). Typical electrochemical performance of the hybrid lithium cells with n-BuLi additive is displayed in Figure 3. As shown in Figure 3a, the modified cell delivers two distinguishable smooth charge and discharge voltage plateaus at around 3.55 and 3.35 V, respectively. The initial specific charge capacity at 100 μA cm−2 is about 176.7 mAh g−1, which is more than the theoretical capacity of LiFePO4 (i.e., 170 mAh g−1). This is probably due to the extra Li+ introduced by n-BuLi that participates in the charge process. The hybrid cell can run 200 cycles at 100 μA cm−2 with capacity retention of 86.58%, with the first Coulombic efficiency of 80.13% and above 95% in the following cycles, as shown in Figure 3b. After 10 cycles, the charge and discharge plateaus almost coincide with those of the second cycle, implying great stability and compatibility between 21079
DOI: 10.1021/acsami.7b05599 ACS Appl. Mater. Interfaces 2017, 9, 21077−21082
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ACS Applied Materials & Interfaces
Figure 3. (a) Charge/discharge profiles and (b) capacity and efficiency of the LiFePO4/Li cell using hybrid electrolyte with n-BuLi at different current density at room temperature. EIS results of the hybrid LiFePO4/Li cell (c) without and (d) with n-BuLi before and after cycling tests.
H+ exchange. This SLEI layer may be Li+ deficient or consist of poor lithium ion conductors such as Li2CO3 (Figure 2c, d) that leads to rising interfacial impedance over time. Water or existence of proton will facilitate this decomposition and/or Li+/H+ exchange process. Unfortunately, trace amounts of water can inevitably be introduced by garnet sample handling and from commercial LE solvents, even though the water concentration in the atmosphere in glovebox has been strictly controlled under 0.1 ppm. On the other hand, when n-BuLi is added in the LE (Figure 4), this superbase may have 3-fold functions: (1) this Lewis basic additive may retard the decomposition reaction of liquid electrolyte; (2) it is highly proton withdrawing and suppresses the Li+/H+ exchange and related side reactions; (3) it lithiates the garnet/LE interface and consequently improve the interfacial conductivity. As a result, stable and Li+ conductive SLEI is formed and maintained throughout the cycling process, yielding good capacity retention of the hybrid cell.
Figure 4. Schematics illustrating the effect of n-BuLi on stabilizing the SE/LE interface.
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In this work, we focused on the compatibility of lithium garnets and liquid electrolyte. We constructed the hybrid electrolyte consisting of fast lithium garnet, Ta doped Li7La3Zr1.5Ta0.5O12, with room-temperature Li+ ion conductivity of 6 × 10−4 S cm−1 and conventional carbonate-based liquid electrolyte and assembled LiFePO4/Li batteries using the hybrid electrolyte. The as-assembled batteries show poor cyclibility with the SE/LE interface areal resistance increasing from ∼1056 to ∼2419 Ω cm2 after 10 cycles. By adding a small amount of n-BuLi, the interface stability is greatly enhanced. The hybrid LiFePO4/Li cell with n-BuLi is successfully cycled for 400 cycles at current densities of 100 and 200 μA cm−2 at room temperature, with the SE/LE interface areal resistance almost constant during cycling. Microstructure and surface chemistry results show that n-BuLi may play an important role in suppressing the Li+/H+ exchange and limiting the formation of resistive and Li+ deficient SLEI layer near the garnet/LE interface. This work contributes to understanding the stability of the garnet/LE interface and provide a simple yet effective approach to improve the stability, which may inspire further research in hybrid lithium batteries and shuttle-effect-free lithium batteries such as Li-redox-flow batteries and Li−S cells.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05599. Experimental Section, impedance spectra for the LLZT and LLZT/LE samples (Figure S1), TGA profiles for LLZT and LLZT/LE samples (Figure S2), impedance spectra of stainless steel/LE/SE/LE/stainless steel cells against time (Figure S3), impedance spectra of the LLZT, LLZT/LE+n-BuLi samples, and stainless steel/LE +n-BuLi/LLZT/LE+n-BuLi/stainless steel cells against time (Figure S4) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Huanan Duan: 0000-0003-3052-3905 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS B.X. and H.D. thank Dr. Han Chen and Prof. Peng Zhang at Shanghai Jiao Tong University for their help in electrochemical measurements. This work is supported by the National Natural Science Foundation of China (11304198), the SAST-SJTU fund (USCAST-2015-40), the SJTU Materials Genome Initiative Center grant (15X190030002), and SMC-Chen Xing Young Scholar Award of SJTU. Instrumental Analysis Center of Shanghai Jiao Tong University and National Engineering Research Center for Nanotechnology are gratefully acknowledged for assisting with relevant analyses.
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