A1544 Journal of The Electrochemical Society, 160 (9) A1544-A1550 (2013) 0013-4651/2013/160(9)/A1544/7/$31.00 © The Electrochemical Society Synergistic Effect of Oxygen and LiNO 3 on the Interfacial Stability of Lithium Metal in a Li/O 2 Battery Vincent Giordani, *, z Wesley Walker, * Vyacheslav S. Bryantsev, Jasim Uddin, Gregory V. Chase, and Dan Addison *, z Liox Power, Inc., Pasadena, California 91106, USA Identification of electrolytes compatible with both electrodes of a nonaqueous Li/O 2 battery is a considerable challenge. One solvent of interest, N,N-dimethylacetamide (DMA), has been shown to be stable toward the reactions of the air electrode, but decomposes rapidly when exposed to Li metal. Herein we investigate the electrochemical behavior of lithium metal electrodes in an electrolyte consisting of DMA and lithium nitrate, a combination that has been shown to have a dramatic effect on the cycle life of Li/O 2 cells. Electrochemical impedance spectroscopy, in situ pressure analysis, mass spectrometry, scanning electron microscopy and theoretical calculations are used to study the electrode/solution interface. Remarkably efficient, long-term Li metal cycling in DMA is reported through the cooperative effect of dissolved oxygen and lithium nitrate. © 2013 The Electrochemical Society. [DOI: 10.1149/2.097309jes] All rights reserved. Manuscript submitted May 31, 2013; revised manuscript received July 4, 2013. Published July 13, 2013. This was Paper 300 presented at the Toronto, ON, Canada, Meeting of the Society, May 12–16, 2013. Rechargeable organic electrolyte Li/O 2 batteries are a focus of current research interest due to very high theoretical specific energy that may enable electric vehicles with driving ranges approaching gasoline-powered vehicles. Efforts to build practical Li/O 2 batteries with suitable power density and long cycle life are hindered by a num- ber of formidable technical challenges. 1 Among these challenges, the unsolved problem of electrolyte instability is perhaps most critical. With increasing cycle number, parasitic electrolyte decomposition re- actions begin to dominate O 2 reduction and evolution reactions in the positive electrode. The oxidative stability of many solvents is low- ered in the presence of the discharge product, Li 2 O 2 , 2 and electrolyte decomposition products, such as Li alkyl carbonates, are often elec- trochemically oxidized at voltages that overlap with Li 2 O 2 oxidation. Consequently, ambiguity exists in determining whether battery cy- cling is driven by parasitic processes vs. actual O 2 electrode processes without the use of in situ electrochemical mass spectrometry or other specialized analytical methods. Carbonates, ethers, lactones, phos- phates, certain ionic liquids and siloxanes all exhibit clear evidence of decomposition in Li/O 2 batteries, 2–7 while the stability of DMSO is a matter of debate. 8,9 Electrolytes based on nitriles or amides show im- proved (though still imperfect) stability in the O 2 electrode compared to other examined solvent classes, yet these solvents are interfacially unstable against Li metal electrodes. 5,6,10,11 A frequently mentioned approach for solving the problem of Li metal reactivity in Li/O 2 batteries is the use of a protective solid Li-ion conducting membrane to prevent crossover of materials from the O 2 electrode to the Li electrode. 1 While this architecture in principle al- lows the use of Li-unstable solvents in the O 2 electrode, long cycling Li/O 2 batteries having an organic electrolyte in the O 2 electrode and a protected Li electrode architecture have not yet been experimentally demonstrated. Thus the discovery of liquid electrolyte formulations that are bistable toward O 2 and Li metal electrodes remains an impor- tant goal. Recently, Li/O 2 cells employing an electrolyte consisting of 1 M LiNO 3 in N,N-dimethylacetamide (DMA) was shown to cycle with comparatively high efficiency and stability for >2000 hours. 12 Forma- tion of Li 2 O 2 during discharge and O 2 evolution during charge were confirmed via XRD and in situ electrochemical mass spectrometry, respectively. An unexpected feature of this system is the remarkably stable cycling of the Li electrode in an electrolyte using DMA as solvent. Li metal reacts vigorously with neat DMA leading to the formation of gaseous and soluble products. 6,12 The present study ex- amines the role of LiNO 3 in suppressing the reaction between Li metal and DMA. We report a cooperative effect of O 2 and LiNO 3 on the formation of a stable solid-electrolyte interphase (SEI) that supports ∗ Electrochemical Society Active Member. z E-mail: vincent@liox.com; dan@liox.com surprisingly low overpotential cycling in symmetric Li/Li cells for >4 months. The cycling behavior of Li metal in liquid electrolytes has been a subject of extensive investigation for several decades and issues with safety and efficiency have ultimately prevented the adoption of Li metal electrodes with liquid electrolytes for most commercial rechargeable battery applications. 13 In particular, the problems of den- drite formation and irreversible capacity loss due to SEI growth con- strain practical cycle life of Li cells. Despite these issues, improved cycling of Li metal in liquid electrolytes remains an important tech- nological goal, especially for battery systems in which a high capacity negative electrode is desired for coupling to a high capacity positive electrode (e.g. S or O 2 electrodes). Regarding the Li/S battery sys- tem, LiNO 3 was previously shown to improve battery performance by forming a stable SEI that blocks reduction of soluble polysulfides released into the electrolyte during operation of the S electrode. 14 The effect of dissolved O 2 on the cycling behavior of Li metal in rechargeable Li/O 2 batteries has received little attention, perhaps due to the difficulty of achieving long-term, stable O 2 electrode cycling. Published results are contradictory. Aurbach et al. 15 reported a stabi- lizing effect of dissolved O 2 on the operation of Li metal electrodes. Contrarily, Assary et al. 16 and Younesi et al. 17 argued that crossover of dissolved O 2 to the Li metal electrode negatively impacts Li/O 2 cell performance. These conclusions were all based on observations in ether- and carbonate-based electrolytes, which are unstable in the O 2 electrode. 2–7 In this work, we study the effect of O 2 on Li electrodes in LiNO 3 - DMA electrolyte utilizing both a variety of electrochemical measure- ments and a number of analytical techniques, including electrochemi- cal impedance spectroscopy (EIS), mass spectroscopy, in situ pressure analysis and scanning electron microscopy (SEM) in order to gain a greater understanding of material degradation. Our results support the conclusion that dissolved O 2 can exert a stabilizing effect on Li metal electrodes. Interestingly, we note that while O 2 and LiNO 3 both improve Li metal cycling in DMA, the presence of either species alone is not sufficient to stabilize the Li electrode for many cycles. Rather, only both species present together allows long-term Li metal cycling in DMA with low polarization and coulombic efficiency that is comparable to Li metal cycling in other relatively effective liquid electrolytes previously disclosed. 13,18–21 Experimental Materials.— N,N-Dimethylacetamide (Sigma-Aldrich, over molecular sieve, ≥99.5%) was used as received. LiNO 3 (Sigma- Aldrich, ≥99.0%) and battery grade LiTFSI (Novolyte) salts were dried at 200 ◦ C for 24 h under vacuum. All electrolytes had a final ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 131.215.225.9 Downloaded on 2013-07-14 to IP