Mechanical and Thermal Failure Induced by Contact between a Li1

Oct 3, 2017 - Chemical reactions at the solid electrolyte (SE) and Li metal interface form an interphase before electrochemical reactions occur. This ...
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Mechanical and Thermal Failure Induced by Contact between a Li1.5Al0.5Ge1.5(PO4)3 Solid Electrolyte and Li Metal in an All Solid-State Li Cell Habin Chung and Byoungwoo Kang* Department of Materials Science and Engineering (MSE), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea S Supporting Information *

ABSTRACT: Chemical reactions at the solid electrolyte (SE) and Li metal interface form an interphase before electrochemical reactions occur. This study investigates the effects of the chemically formed interphase between Li metal and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) on cell failures under various experimental conditions. LAGP forms a black interphase by chemically reacting with Li metal. The interphase comprises a stoichiometrically changed LAGP and Li-related oxides and behaves as a mixed ionic and electronic conductor with the electronic conductivity dominating. Thus, upon application of an electrical current to Li metal anode, most of the Li ions can be reduced at the SE side surface of the interphase rather than the Li metal side, causing a local volumetric increase that triggers cracks in the SE. This crack formation process continues the pulverization of SE, leading to a gradual increase in cell resistance. Under cell operating conditions, electrochemical reactions with the chemically formed interphase can lead to the mechanical deterioration of the SE, leading to cell failure. Furthermore, the chemically formed interphase between melted Li and LAGP above 200 °C induces a rigorous chemical reaction with Li that leads to a thermal runaway. The chemical stability of the SE against Li metal can strongly affect the solid-state cell’s electrical properties, mechanical integrity, and thermal stability.

1. INTRODUCTION The demand for safer Li secondary batteries with higher energy densities has increased.1,2 In this respect, Li-ion batteries containing typical liquid electrolytes have fundamental limitations because liquid electrolytes can act as fuels in thermal runaway behavior, leading to a fire or the explosion of a battery, and can decompose at high potentials (>4.5 V), which leads to the restricted use of high-potential cathodes.3,4 Moreover, liquid electrolytes cause severe safety problems with Li metal, which is the best anode material with respect to a high energy density with low potential and high capacity because Li metal in liquid electrolytes easily leads to the formation of dendrites, which can produce a short circuit between the cathode and anode.5,6 Applying proper solid electrolytes (SEs) can solve these problems, and therefore, much effort has been focused on developing these.7−12 Li1.5Al0.5Ge1.5(PO4)3 (LAGP), a wellknown oxide-based SE, which contains no transition metals, was believed to be stable with Li metal.13 However, increased resistance was observed in a Li/LAGP/Li symmetric cell over time and in an all-solid-state cell after several cycles.14,15 When LAGP is in direct contact with Li metal, an interphase is chemically formed from a Ge reduction reaction.15 However, the relation between the increased resistance and the chemically formed interphase is still not fully understood given that the interphase can have a mixed conducting property. The effects of the chemically formed interphase on © XXXX American Chemical Society

the mechanical and thermal stabilities of SEs are also not yet fully understood. In this study, we tried to understand the characteristics of the chemically formed interphase between Li metal and the SE, LAGP, and its effects on cell stability. We found that the chemically formed interphase shows a mixed ionic and electronic conductivity with a relatively high electronic conductivity. The electrical properties of the interphase strongly affect the mechanical integrity of the SE; fracture of LAGP can easily occur because of the mixed conducting property of the interphase, leading to severely increased resistance and failure of the cell, and this can be accelerated with an applied dc or ac current. Furthermore, we first observe rigorous thermal runaway of the interface between the melted Li metal and LAGP at high temperatures (>200 °C), especially in the chemically formed interphase region. This combusting phenomenon informs that the chemically formed interphase can be susceptible to the easy release of O2 gas at high temperatures even though the SE is very stable at this temperature. We clearly demonstrate that the chemical stability of the SE against Li metal can be very important for operating solid-state cells because it strongly affects not only the Received: June 5, 2017 Revised: September 18, 2017

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DOI: 10.1021/acs.chemmater.7b02301 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials mechanical integrity and the electrical resistance of an SE but also its thermal stability.

2. EXPERIMENTAL SECTION 2.1. LAGP Synthesis. Li1.5Al0.5Ge1.5(PO4)3 was synthesized via a solid-state reaction. Li2CO3 (Junsei, 99.0%), Al2O3 (Sigma-Aldrich, 98%), GeO2 (Alfa Aesar, 99.999%), and (NH4)2HPO4 (Sigma-Aldrich, 98%) were properly weighed. The precursors were mixed overnight by ball milling with 3, 5, and 10 mm zirconia balls and acetone solvent at 200 rpm. The mixture was dried on a hot plate and then ground using an agate mortar with a pestle. The mixture was then calcined at 900 °C for 6 h in a box furnace in air to evaporate volatile material and form the LAGP phase. The LAGP powder was ground again in an agate mortar with a pestle and then pulverized to a size of approximately half a micrometer using a planetary ball mill with 1 mm zirconia balls and acetone solvent in a zirconia container at 500 rpm for 2 h. After being dried and ground, the pulverized particles were sieved through a 50 μm mesh, placed in a mold, and pressed at ∼500 MPa for 1 min to form a 12.9 mm diameter pellet. The pellet was sintered in a box furnace again at 800 °C for 6 h in air. The surfaces of the sintered pellets were polished. 2.2. Li/SE Chemical Reaction. Li metal foil 10 mm in diameter and 150 μm in thickness was attached to the surface of a sintered LAGP pellet in an Ar-filled glovebox, with both O2 and moisture pressures of 1 MHz, which is not shown in these Nyquist plots.8 Moreover, the semicircle in the high-frequency range, which is located at the low-resistance side in the plot, can represent Li ionic grain boundary conduction. Another semicircle that is overlapped with the right side of the grain boundary semicircle may represent interfacial conduction. The semicircular tail at a low frequency can be expressed by element O (Cothyperbol), which may be related to the existence of a stagnant layer for Liion diffusion. This could be from the chemically formed interphase.17 Furthermore, this low capacitive tail indicates that a redox reaction of Li ions and electrons can occur in the interphase or Li metal. The tendencies of the semicircles in the Nyquist plots could be divided into three stages during the measurement for 50 days. During stage 1, the semicircle for the

interfacial conduction continuously decreased over 18 h, as shown in Figure 4b. To understand the physical process, another cell in the same stage 1 was disassembled and the black interphase was observed with no cracks (inset of Figure 4b). The chemical properties of this observed interphase were similar to those of the interphase formed by contact only. At stage 1, imperfect physical contact of Li/LAGP could be improved by the interfacial chemical reaction that leads to the formation or growth of the interfacial product. Given that the interfacial resistance decreased with the formation or growth of the interfacial product, the chemically formed interphase is an electrical conducting phase. Further reaction for 2 days is represented by stage 2 in Figure 4c; here, the size of the semicircle remains constant, indicating that the electrical resistance is almost saturated. This indicates stable formation of the interfacial product. At stage 3 in Figure 4d, the size of the semicircle gradually and continuously increased for ≤50 days D

DOI: 10.1021/acs.chemmater.7b02301 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 6. Sequential images as a function of time for contact of sintered LAGP pellet and melted Li metal at 200 °C in the glovebox (H2O and O2 levels of 200 °C) with melted Li metal, and the decomposition of the interphase may provide the available O2 for the thermal runaway. This indicates that the thermal stability of the cell at high temperatures strongly depends on the electrical properties of the chemically formed interphase between the Li metal and SE. The chemical stability and electrical properties of the chemically formed products at the interphase should be considered with respect to operating reversible and long-term stable solid-state cells with Li metal anodes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02301. Table of EDS line scanning intensities, XRD data for LAGP and the interphase, Bode plots for the Li/LAGP/ Li symmetric cell, Nyquist plots for the Li/LAGP/Li symmetric cells, XRD data for the product of the melted Li/LAGP reaction, and DSC data for Li and the interphase mixture (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Habin Chung: 0000-0001-5220-5516 Byoungwoo Kang: 0000-0002-8081-1908 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Fundamental R&D Program for Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy (Grant 10037918). This work was also supported by the Future Semiconductor Device Technology Development Program (10045226) funded by the Ministry of Trade, Industry & Energy (MOTIE) and the Brain Korea 21 PLUS project for the Center for Creative Industrial Materials (F14SN02D1707).

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ABBREVIATIONS SE, solid electrolyte; LAGP, Li1.5Al0.5Ge1.5(PO4)3; SEM, scanning electron microscopy; EDS, energy dispersive spectroscopy; EDX, energy dispersive X-ray; XPS, X-ray photoelectron spectroscopy; EIS, electrochemical impedance spectroscopy; XRD, X-ray diffraction; NaSICON, Na super ionic conductor; DSC, differential scanning calorimetry; SEI, solid electrolyte interface; ALD, atomic layer deposition. H

DOI: 10.1021/acs.chemmater.7b02301 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials 0 ≤ x ≤ 0·7 for use as Li+ sensitive membranes. J. Eur. Ceram. Soc. 1999, 19, 2931−2940. (23) Lide, D. R. CRC handbook of chemistry and physics; CRC Press: Boca Raton, FL, 2004; Vol. 85, pp 12−130. (24) Han, X.; Gong, Y.; Fu, K.; He, X.; Hitz, G. T.; Dai, J.; Pearse, A.; Liu, B.; Wang, H.; Rubloff, G.; Mo, Y.; Thangadurai, V.; Wachsman, E. D.; Hu, L. Negating interfacial impedance in garnet-based solid-state Li metal batteries. Nat. Mater. 2017, 16, 572−579.

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DOI: 10.1021/acs.chemmater.7b02301 Chem. Mater. XXXX, XXX, XXX−XXX