Journal of The Electrochemical Society, 163 (6) A1095-A1100 (2016) A1095 Decomposition of LiPF 6 in High Energy Lithium-Ion Batteries Studied with Online Electrochemical Mass Spectrometry Aur´ elie Gu´ eguen, a, z Daniel Streich, a Minglong He, a Manuel Mendez, b Frederick F. Chesneau, b Petr Nov´ ak, a and Erik J. Berg a a Electrochemistry Laboratory, Paul Scherrer Institute, 5232 Villigen, Switzerland b BASF SE, GCN/EE - M311, 67056 Ludwigshafen, Germany The chemical and electrochemical instabilities of LiPF 6 in carbonate electrolytes for Li-ion batteries were studied with online electrochemical mass spectrometry (OEMS). Decomposition of carbonate electrolytes based on LiPF 6 eventually results in the formation of POF 3 , which is readily detected and followed in situ during operation of Li-rich HE-NCM-based Li-ion cells. Electrode potentials above ∼4.2 V leads to carbonate solvent oxidation and presumably the formation of ROH species, which subsequently hydrolyze the LiPF 6 salt and initiate a thermally activated autocatalytic electrolyte decomposition cycle involving POF 3 as a reactive intermediate. Activation of the Li 2 MnO 3 domains of the Li-rich cathode contributes along with electrolyte and separator impurities to further POF 3 generation. Electrode potentials below ∼2.5 V vs. Li + /Li impede POF 3 formation and presumably also further electrolyte decomposition by scavenging reactive intermediate species. As a result, much less POF 3 gas was detected upon the 2 nd charge when using Li metal counter electrode, contrary to delithiated LiFePO 4 . In situ OEMS confirm that the parasitic reactions involving LiPF 6 constitute an intricate reaction scheme, but more importantly, provide further evidence about what the components of this scheme are and how these may interact with each other. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0981606jes] All rights reserved. Manuscript submitted December 8, 2015; revised manuscript received March 3, 2016. Published March 29, 2016. Rechargeable Li-ion batteries are nowadays extensively used to power electronics and are entering the transportation sector by pow- ering electric vehicles (EV). A wide range of both negative (e.g. graphite) and positive electrode materials (e.g. layered cobalt ox- ides, spinel-type manganese oxides, and olivine-type iron phosphates) have been thoroughly investigated and are now in widespread use in commercial batteries. The specific energy of Li-ion batteries is limited mainly by the positive electrode materials, having typical practical specific charges of ∼150 mAh/g and average operating po- tentials of ∼3.8 V vs. Li + /Li, which significantly inhibits the in- troduction of Li-ion batteries as power source in new applications. In recent years, the layered Li-rich cobalt-nickel-manganese oxides xLi 2 MnO 3 (1−x)(LiMO 2 )(x ∼ 0.5, M = Co, Ni, Mn), hereafter called HE-NCM, have been shown to exhibit a high and reversible specific charge (∼250 mAh/g) and a competitive average operating potential (∼3.75 V vs. Li + /Li). 1–4 The origin of such a high specific charge is not yet completely understood, as the exact structure of the HE-NCM materials is highly dependent on the synthesis conditions and models coming from structural characterization are still under debate. Several reports have shown the presence of so-called Li 2 MnO 3 domains in the compound 5,6 whereas other groups demonstrated the monophasic character of their materials. 7 However, during the first charge, a long potential plateau at ∼4.5 V vs. Li + /Li, not observed for conventional layered oxides, results from the delithiation process of the Li 2 MnO 3 domains accompanied by oxygen extraction. The extracted oxygen species are believed to be very reactive and partly evolve as O 2 gas, but can also further react with the electrolyte and the products present at the electrode/electrolyte interface. 8 For example, Freunberger et al. proposed a mechanism for reactions between oxygen radicals and the carbonate solvents resulting in formation of lithium dicarbonate, formate, acetate, Li 2 CO 3 , CO 2 and H 2 O. 9 Several electrochemically initiated side reactions implicating the solvents (notably oxidation of carbonates HRCO 3 ) and the LiPF 6 salt have been proposed: 10–19 HRCO 3 → RO · + CO 2 + H + + e − [1] LiPF 6 ↔ LiF + PF 5 [2] PF 5 + ROH → POF 3 + HF + RF [3] z E-mail: aurelie.gueguen@psi.ch POF 3 + ROH → POF 2 (OR) + HF [4] POF 3 + RCO 3 R → POF 2 OR + CO 2 + RF [5] For instance, when the cyclic ethylene carbonate (R = C 2 H 3 in Equa- tion 1) is oxidized, 10 it may (via Equation 1) lead to the formation of RO . radicals, which subsequently may (via Equation 3) hydrolyze the salt and cause the formation of various organofluorine compounds, such as flouroethylene C 2 H 3 F. LiPF 6 is known to form an equilib- rium (Equation 2) with LiF and PF 5 . 11,12 POF 3 can further react with ROH and/or carbonate solvents to form phosphate products, such as alkylfluorophoshate and alkyldifluorophosphate, respectively (Equa- tions 4 and 5). 13–19 Thermally activated decomposition of carbonate and LiPF 6 based electrolytes has however been extensively studied (c.f. Nowak et al. 20 and citations therein). For instance, the presence of several phosphate species was evidenced by ex situ GC-MS and NMR when studying the stability and chemical decomposition of carbon- ate electrolytes at elevated temperatures (60–100 ◦ C), which strongly accelerated such decomposition reactions. 16,21 In situ spectroscopic evidence of electrochemically initiated electrolyte decomposition in- volving the LiPF 6 salt and high voltage cathodes is however more scarce. In previous reports, we used online electrochemical mass spec- trometry (OEMS) to follow the evolution of gases, such as O 2 and CO 2 , from the interface of the HE-NCM positive electrodes in Li-ion cells containing typical carbonate electrolytes. 22 The results, com- bined with X-ray photoelectron spectroscopy (XPS), allowed us to propose a mechanism for reactions taking place in different potential windows of the two first charge and discharge cycles. Further improve- ments on the OEMS experimental setup allow us now to monitor the evolution of other gases, such as H 2 and POF 3 , originating from the decomposition of the carbonate electrolyte. The aim of the present work is not only to investigate the formation of O 2 and CO 2 , but also the H 2 and POF 3 gas evolution from HE-NCM during cycling to further disclose the underlying electrolyte decomposition processes. Experimental Electrode preparation.—The positive electrodes were prepared by coating thin glass fiber sheets (Whatman, GF/C) with a slurry of 93 wt% HE-NCM or stoichiometric NCM111 (LiNi 1/3 Co 1/3 Mn 1/3 O 2 ) (BASF SE), 3 wt% polyvinylidene fluoride (PVDF Kynar HSV ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.82 Downloaded on 2018-07-20 to IP