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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 38892−38899
Origin of Carbon Dioxide Evolved during Cycling of Nickel-Rich Layered NCM Cathodes Toru Hatsukade,†,∥ Alexander Schiele,†,∥ Pascal Hartmann,*,†,‡ Torsten Brezesinski,*,† and Jürgen Janek†,§
ACS Appl. Mater. Interfaces 2018.10:38892-38899. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/14/18. For personal use only.
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Battery and Electrochemistry Laboratory, Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ BASF SE, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany § Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany S Supporting Information *
ABSTRACT: Gas formation caused by parasitic side reactions is one of the fundamental concerns in state-of-theart lithium-ion batteries because gas bubbles might block local parts of the electrode surface, hindering lithium transport and leading to inhomogeneous current distributions. Here, we elucidate on the origin of CO2, which is the dominant gaseous species associated with the layered lithium nickel cobalt manganese oxide (NCM) cathode, by implementing isotope labeling and electrolyte substitution in differential electrochemical mass spectrometry−differential electrochemical infrared spectroscopy measurements. Li2CO3 on the NCM surface was successfully labeled with 13C via a process that involves its removal followed by intentional growth. In situ gas analytics on such NCM samples with 13C-labeled Li2CO3 clearly indicate that Li2CO3 decomposition contributes to CO2 evolution, especially during the first charge. At the same time, the greater contribution of electrolyte decomposition was indicated by the large amount of 12CO2 observed. Employment of butyronitrile as the electrolyte solvent in further measurements helped determine that the majority of electrolyte decomposition occurs via a reaction that involves the lattice oxygen of NCM. KEYWORDS: differential electrochemical mass spectrometry, lithium nickel cobalt manganese oxide, lithium carbonate, gas evolution, electrolyte decomposition
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INTRODUCTION The ever-increasing energy consumption of the modern day society and its heightened concerns for environmental wellness have led to soaring demands for energy storage in various sectors, ranging from portable electronics to large-scale storage applications. Lithium-ion batteries (LIBs) have been heavily relied on to meet such demands, dominating in the area of portable electronics and rapidly making its way into electric vehicles. Such reliance on LIBs can be attributed to their favorable combination of reversible cycling efficiency, lifetime, energy density, and safety. However, further research is required to fulfill the demands of the future years and those of larger scale applications.1 Although research progress has led to the development of state-of-the-art LIBs based on Ni-rich layered oxides, there remain many issues that must be addressed for their widespread applications.2,3 One key issue is gas formation caused by parasitic side reactions, which can adversely affect the performance of a battery cell and cause safety issues. Among the numerous gaseous species known to be produced, the generation of carbon dioxide (CO2) is arguably the most © 2018 American Chemical Society
notable in cells with Ni-rich layered oxides because of its overall contribution. As previously shown by Berkes et al. in their work on Li1+x Ni 0.5 Co0.2Mn0.3 O 2 (NCM523), CO 2 production was observed in every cycle during charging, resulting in CO2 making the largest impact on the overall gas accumulation when considering multiple cycles.4 Thus, its suppression is important, making the determination of its origin highly valuable. There have been many studies, both old and recent, offering insights into the origin of CO2 evolution.4−13 However, a consensus is yet to be reached, with several differing mechanisms proposed. This includes electrochemical oxidation of an organic carbonate electrolyte, decomposition of residual lithium carbonate (Li2CO3) on the NCM surface, and chemical oxidation of the organic carbonate electrolyte by lattice oxygen released from NCM at high potentials.4,7−12 Commonalities among these are their apparent potential Received: August 2, 2018 Accepted: October 18, 2018 Published: October 18, 2018 38892
DOI: 10.1021/acsami.8b13158 ACS Appl. Mater. Interfaces 2018, 10, 38892−38899
Research Article
ACS Applied Materials & Interfaces
procedure of carbonate titration can be found in the Supporting Information (Figure S1b). In Situ Gas Analytics. The electrochemical performance and the corresponding gas generation behavior were investigated by DEMS− DEIRS using He (6.0 purity, 2.5 mLn/min) as the carrier gas. The extracted gas was analyzed using a mass spectrometer (GSD 320, OmniStar Gas Analysis System, Pfeiffer Vacuum GmbH) and Fourier transform infrared (FTIR) spectrophotometer (TENSOR II, Bruker Optik GmbH). The measurements were performed at 25 °C. Additional information is provided in earlier works.16,17 Electrodes were prepared using the diversely treated NCM622 powders, with a composition of 94% active material, 3% binder (Solef5130), 1% carbon black (Timcal C65), and 2% graphite (Timcal SFG6L) and an areal loading of 11.4−12.4 mgNCM622/cm2. The NCM powders were dried under vacuum for 12 h at 300 °C prior to their implementation. Lithium iron phosphate (LiFePO4, LFP) electrodes with 90% active material (20.2−20.8 mgLFP/cm2) were electrochemically delithiated prior to their usage as the counter electrode. The custom cells were assembled inside an Ar-filled glovebox by stacking the LFP electrode (32 mm diameter), separator (GF/A, GE Healthcare Life Sciences, Whatman, 36 mm diameter), and NCM622 electrode (30 mm diameter) above each other. Either 400 μL of LP57 (1 M LiPF6 in ethylene carbonate/ethyl methyl carbonate, 3:7 by wt, BASF SE) or 1 M LiPF6 in butyronitrile (99.9% purity, Sigma-Aldrich) was used as the electrolyte. For measurements using LP57, a lithium reference electrode was applied.
dependence and their onset of evolution at high potentials. Because of this, differentiation between the mechanisms is challenging and requires a targeted approach. In this work, we aim to determine the mechanism that chiefly contributes to the production of CO2 at the Ni-rich NCM cathode at high potentials by implementing isotope labeling and electrolyte substitution in differential electrochemical mass spectrometry−differential electrochemical infrared spectroscopy (DEMS−DEIRS) measurements. Specifically, successful isotope labeling of the residual Li2CO3 layer on the surface of NCM622 (Li1+xNi0.6Co0.2Mn0.2O2) via its removal and regrowth helped in determining the contribution of Li2CO3 to CO2 evolution. Additionally, the most likely pathway for CO2 evolution from electrolyte degradation was determined by replacing the carbonate solvents of the electrolyte with butyronitrile, which is a solvent without any oxygen atoms.
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EXPERIMENTAL SECTION
Sample Preparation. NCM622 powders provided by BASF SE were used in this work. Li2CO3 removal from NCM622 involved a high-temperature treatment in a tube furnace (Nabertherm GmbH) at 740 °C for 2 h under pure O2 flow, where the flow rate was controlled by a mass flow controller (MFC, EL-FLOW Select, Bronkhorst HighTech B.V.). A temperature ramp rate of 4 °C/min and an O2 flow rate of 0.5 Ln/min were used. The samples remained under O2 flow during cooldown and were immediately transferred to an Ar-filled glovebox (MBraun) in order to minimize air exposure. The samples prepared above (622-R) were subsequently stored in a custom cell under a pressurized atmosphere (∼2.5 bar) consisting solely of 13C-labeled CO2 (99 at. % 13C,
