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Energy, Environmental, and Catalysis Applications 2
Quasi-Solid-State Rechargeable Li–O Batteries with High Safety and Long Cycle Life at Room Temperature Sung Man Cho, Jimin Shim, Sung Ho Cho, Jiwoong Kim, Byung Dae Son, Jong-Chan Lee, and Woo Young Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00529 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018
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ACS Applied Materials & Interfaces
Quasi-Solid-State Rechargeable Li–O2 Batteries with High Safety and Long Cycle Life at Room Temperature
Sung Man Cho,† Jimin Shim,‡ Sung Ho Cho,† Jiwoong Kim,† Byung Dae Son, † Jong-Chan Lee,*,‡ and Woo Young Yoon*,†
†
Department of Materials Science and Engineering, Korea University, 1, 5-Ga, Anam-dong,
Sungbuk-gu, Seoul 136-701, Republic of Korea ‡
School of Chemical and Biological Engineering and Institute of Chemical Process, Seoul
National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea
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ABSTRACT
As interest in electric vehicles and mass energy storage systems continues to grow, Li–O2 batteries are attracting much attention as a candidate for next-generation energy storage systems owing to their high energy density. However, safety problems related to the use of lithium metal anodes have hampered the commercialization of Li–O2 batteries. Herein, we introduced a quasisolid polymer electrolyte with excellent electrochemical, chemical, and thermal stabilities into Li–O2 batteries. The ion-conducting QSPE was prepared by gelling a polymer network matrix consisting of poly (ethylene glycol) methyl ether methacrylate, methacrylated tannic acid, lithium trifluoromethanesulfonate, and nano-fumed silica with a small amount of liquid electrolyte. The quasi-solid-state Li–O2 cell consisted of a lithium powder anode, a quasi-solid polymer electrolyte, and a Pd3Co/multiwalled carbon nanotube cathode, which enhanced the electrochemical performance of the cell. This cell, which exhibited improved safety owing to the suppression of lithium dendrite growth, achieved a lifetime of 125 cycles at room temperature. These results show that the introduction of a quasi-solid electrolyte is a potential new alternative for the commercialization of solid-state Li–O2 batteries.
Keywords: lithium–oxygen battery, solid polymer electrolyte, quasi-solid-state, palladium cobalt, lithium powder
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ACS Applied Materials & Interfaces
INTRODUCTION Li–O2 batteries have recently attracted intensive attention as a candidate for next-generation energy storage systems. Owing to their high specific energy (3505 Wh kg-1) and energy density (3436 Wh l-1), such batteries have the potential to meet the expected market expansion of electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and mass energy storage systems (MESS).1–4 However, as Li–O2 batteries are still in an early stage of development, to realize their commercialization, many obstacles need to be overcome, such as poor energy efficiency, low cycle life, and safety issues.5–8 In particular, safety issues arising from the direct use of lithium metal as an anode material are the biggest challenge for commercialization.9–12 Localization of the current density resulting from surface defects (pits, cracks) or a nonuniform solid electrolyte interphase (SEI) causes the formation of lithium dendrites. Lithium dendrite growth not only leads to safety problems in cells by penetrating conventional separators, but also induces low columbic efficiency by generating “dead lithium” during the dissolution process. In addition, when a liquid electrolyte is used, as the exterior cathode surface in Li–O2 batteries is exposed to air, lithium may be contaminated by moisture or CO2 gas, which may deteriorate the cycle stability of the cell.13–16 To solve these problems, various solid electrolytes, such as perovskite-type (Li3xLa(2/3)-xTiO3 (x = 0.11))17 and glass ceramic-type (Li1.3Al0.3Ti1.7(PO4)3, Li1.6Al0.6Ge1.4(PO4)3),18–20 have been introduced into Li–O2 cells. Although these types of solid electrolytes have high ionic conductivities and excellent mechanical strength, poor wettabilities and interfacial instability with lithium have been reported.21–24 In contrast, solid polymer electrolytes (SPEs) have somewhat lower elastic moduli than the above-mentioned inorganic solid electrolytes, but SPEs are flexible and exhibit excellent interfacial stability with lithium.25–28
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In this study, electrochemical tests and analyses were carried out by applying a quasi-solid polymer electrolyte (QSPE) with excellent electrochemical, thermal, and chemical stabilities, as well as an appropriate elastic modulus, to Li–O2 cells. The QSPE was prepared simply by using poly(ethylene glycol) methyl ether methacrylate (PEGMA) as an ion-conducting agent, methacrylated tannic acid (MTA) as a cross-linker, nano-fumed silica (SiO2) as a filler, and liquid electrolyte as a plasticizer, which has a honeycomb-branched structure that can improve the conduction of lithium ions. This QSPE has an ionic conductivity of 0.14 × 10-3 S cm-1 at room temperature (RT) and high thermal stability at high temperatures. It also exhibits superior electrochemical stability, with a wide voltage window of up to 5.1 V. To investigate the effect of the QSPE on Li–O2 cells, lithium dendrite growth behavior and electrochemical performance were analyzed systematically. Surprisingly, when combined with a lithium powder electrode (LPE), the QSPE effectively inhibited the growth of lithium dendrites. In addition, electrochemical tests were performed on Li–O2 cells using the Pd3Co/multiwalled carbon nanotube (MWCNT) cathode with high catalytic activity reported in our previous study.29 These cells showed good energy efficiency at RT and achieved a stable and long cycle life of 125 cycles. These significant results demonstrate the applicability of QSPEs for future commercialization of Li–O2 batteries.
RESULTS AND DISCUSSION Synthesis and Characterization of QSPEs. Figure 1 shows a schematic of the procedure used to synthesize the polymer composite electrolyte reinforced with a nano-fumed SiO2 filler. As shown in Figure 1a, the cross-linking agent (MTA) was synthesized by promoting a ringopening reaction between tannic acid (TA) and glycidyl methacrylate (GMA) with a catalyst
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(triphenylphosphine, TPP). The QSPE was prepared by simple one-pot polymerization using PEGMA as an ion-conducting monomeric unit, MTA as a cross-linking agent, SiO2 particles as a nanofiller, and 2,2-dimethoxy-2-phenylacetophenone (DMPA) as an initiator (Figure 1b). To confirm that polymerization was successful, Fourier transform infrared (FTIR) analyses were performed before and after polymerization (Figure S1). The C=C peak (1637 cm-1) of the methacryl group in MTA disappeared after UV irradiation, indicating that a cross-linked polymer network was formed.30
Figure 1. Schematic of the synthesis procedure for (a) methacrylated tannic acid (MTA) and (b) the quasi-solid polymer electrolyte (QSPE) with 10 wt% SiO2. AC impedance, linear sweep voltammetry (LSV), thermogravimetric analysis (TGA), and high-temperature shrinkage tests were performed to characterize the ionic conductivity, electrochemical stability, interfacial stability with Li, and thermal stability of the prepared QSPE. Figure 2a shows the effect of temperature on the ionic conductivities of the QSPE. The ionic conductivity of the QSPE is 0.14 × 10-3 S cm-1 at RT, which is similar to the ionic conductivities of liquid electrolytes (~10-3 S cm-1) at temperatures above 80 °C. Compared with the ionic
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conductivities of conventional poly(ethylene oxide) (PEO)-based SPEs (
