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Enhanced cycle stability of rechargeable Li-O2 batteries by synergy effect of LiF protective layer on the Li and DMTFA additive Eunjoo Yoo, and Haoshen Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017
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Enhanced cycle stability of rechargeable Li-O2 6
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batteries by synergy effect of LiF protective layer on 1
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the Li and DMTFA additive 15
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Eunjoo Yoo* and Haoshen Zhou* 27 28 29 30 31 32 34
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Energy Technology Research Institute, 35 37
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National Institute of Advanced Industrial Science and Technology, 39
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Umezono 1-1-1, Central 2, Tsukuba, Ibaraki 305-8568, Japan 40 41 42 43 4 45 46 47 48 49 50 52
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KEYWORDS: Li-O2 battery, MWCNT, LiF layer. Cycle stability, the symmetric Li|Li cell. 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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ABSTRACT 4 5 7
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Li metal is an ideal anode for rechargeable Li-O2 batteries because of its large theoretical capacity 9
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(3860 mAhg‒1). However, problems with the growth of dendrites and reaction with electrolytes 10 1
and moisture during cycling have prevented its practical application. Herein, we report that the use 12 14
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of a 2 wt% N,N-dimethyltrifluoroacetamide (DMTFA) additive in a dimethyl sulfoxide (DMSO) 16
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electrolyte with a LiF layer on the Li anode allows for good cycling performance in Li-O2 batteries. 17 18
Indeed, a Li-O2 cell with a multi-walled carbon nanotube (MWCNT) cathode, 1.0 M 21
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LiNO3/DMTFA+DMSO (2:98 v/v) electrolyte, and a LiF layer on the Li anode could be cycled 92 23
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times at a current density of 1000 mAg‒1 with a 1000 mAhg‒1. 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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INTRODUCTION 5
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Li-O2 batteries have attracted considerable attention because of their high energy density and 6 7
gravimetric energy, which are three‒five times greater than those of Li-ion batteries. 1-4 However, 10
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Li-O2 batteries require further development to improve their cycle life. In general, their cycle life 12
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is limited owing to several fading mechanisms such as the reaction of Li with electrolytes, dendrite 13 14
formation on the Li anode, corrosion of the carbon cathode, and electrolyte decomposition during 17
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the charge process.5-7 Many research groups have focused on improving the reversibility of the 19
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cathode by developing advanced catalysts to overcome this issue.8-12 As expected, these effects 20 2
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have improved the cycling performance. However, the stability of the metallic Li anode during 24
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cycling remains unclear. In fact, studying the stabilization of the metallic Li anode is challenging 25 26
because metallic Li reacts spontaneously with most organic electrolytes and moisture. Recently 27 29
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reported approaches for improving the stability of the Li anode in Li-O2 batteries include the use 31
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of a protective polymer layer, modified separators, and electrolyte additives.13-17 Bryantsev. V.S. et 32 3
al. also reported that rechargeable Li-O2 batteries with a Li bis(trifluoromethanesulfonyl)imide 36
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(LITFSI)/N,N-dimethylacetamide (DMA)+N,N-dimethyltrifluoroacetamide (DMTFA) (98:2 v/v) 38
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electrolyte and a DMTFA-pretreated Li anode exhibited good cycling performance because the 39 41
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immersion of the Li in DMTFA led to the formation of LiF, which is believed to play a crucial role 43
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in stabilizing the Li/electrolyte interface.18 DMTFA is one of the amide solvent with dimethyl and 45
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trifluoromethyl groups. The chemical structure of DMTFA is shown in Figure S1. Although the 46 48
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LiF layer on a Li anode can prevent its corrosion during cycling when this strategy is used, its 50
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effects have been rarely studied. Thus, investigating the effects of the LiF layer on Li anodes would 51 52
be very interesting for the further development of rechargeable Li-O2 batteries. Furthermore, 53 5
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Lithium nitrate (LiNO3) has been proven to be an effective Li salt to decrease the charge potential 56 57 58 59 60 ACS Paragon Plus Environment
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and increase the efficiency of Li-O2 batteries by positively influencing the interfacial properties of 5
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Li.13,19 Thus, LiNO3 was selected as a Lithium salt in this study. 6 7
In this work, we examine the components and morphologies of a LiF layer generated by 10
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immersing Li in a DMTFA solution. We show that the presence of a LiF layer on a Li anode with 12
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a dimethyl sulfoxide (DMSO) electrolyte containing DMTFA additives results in good cycling 13 14
performance. The effect on the electrochemical performance of Li-O2 batteries of the LiF layer on 17
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the Li anode is also discussed. 18 19 20 2
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EXPERIMENTAL SECTION 24
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Protection of the Li anode: Li metal was washed with tetrahydrofuran (THF) for 5 mins to remove 25 26
contamination from the Li and then dried for 10 mins in a glove box. The dried Li was immersed 27 29
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in 100% DMTFA for 20 mins. Subsequently, the protected Li was dried in a glove box. Li-O2 31
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battery assembly and electrochemical test: To prepare the air cathode, multi-walled carbon 32 3
nanotubes (MWCNTs) and the poly(tetrafluoroethylene) (PTFE) binder at a weight ratio of 90:10 36
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were thoroughly mixed and pressed onto a carbon paper, which served as a current collector. After 38
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pressing, the air cathode was dried at 100 °C under vacuum for 1 day to remove the residual 39 40
moisture. The mass loading of carbon materials was roughly ~0.5 mg cm‒2. The LiF layer on a Li 43
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was employed as the anode, and a 1.0 M LiNO3/DMTFA+DMSO (98:2 v/v) was used as the 45
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electrolyte. The amount of electrolyte in each coin cell was approximately 60‒70 μL. A glass fiber 46 48
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filter paper was employed as the separator. The battery assembly was performed in an Ar-filled 50
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glove box (
