Technical Note pubs.acs.org/ac
Online Continuous Flow Differential Electrochemical Mass Spectrometry with a Realistic Battery Setup for High-Precision, LongTerm Cycling Tests Balázs B. Berkes,*,† Anna Jozwiuk,† Miloš Vračar,† Heino Sommer,†,‡ Torsten Brezesinski,*,† and Jürgen Janek†,§ †
Battery and Electrochemistry Laboratory, Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ BASF SE, 67056 Ludwigshafen, Germany § Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany S Supporting Information *
ABSTRACT: We describe the benefits of an online continuous flow differential electrochemical mass spectrometry (DEMS) method that allows for realistic battery cycling conditions. We provide a detailed description on the buildup and the role of the different components in the system. Special emphasis is given on the cell design. The retention time and response characteristics of the system are tested with the electrolysis of Li2O2. Finally, we show a practical application in which a Li-ion battery is examined. The value of long-term DEMS measurements for the proper evaluation of electrolyte decomposition is demonstrated by an experiment where a Li1+xNi0.5Mn0.3Co0.2O2 (NMC 532)/graphite cell is cycled over 20 charge/discharge cycles. n the field of battery research and development, electrochemical investigations play necessarily a dominant role. However, apart from well-established measurement techniques and cycling protocols, it is inevitable to extend the range of characterization methods to obtain new and/or different information about the processes occurring in different types of battery cells. Processes that cannot be perceived unambiguously from purely electrochemical data include dissolution/loss of active material components,1−4 fading of certain electrode regions,5,6 and reactions that occur in the electrolyte phase, such as decomposition1,7−9 and gas evolution.10−12 To track these often deleterious side reactions and their consequences on the functioning of the battery, sophisticated measurement techniques are needed and used, such as in situ atomic force microscopy, X-ray (fluorescence) microscopy, scanning electron microscopy, X-ray diffraction, and Raman spectroscopy, to mention only a few.13−18 Nevertheless, behind the general ideas and the demonstration of some impressive measurement results, there is usually a lack of information about the construction details of the analytical system which, however, have a tremendous effect on the reliability and reproducibility of research data. The design (and implementation) of gas analysis systems for the study of the reaction products during the operation of lithium-ion batteries (or more generally, electrochemical systems) is not a novel problem.
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© 2015 American Chemical Society
Differential electrochemical mass spectrometry (DEMS) was first introduced to the literature by Gadde and Bruckenstein.19 They demonstrated how to interface a mass spectrometer to the site of a reaction. Then, eventually, over the years, more advanced versions of their rudimentary setup were gradually established and are now being used actively in electrochemical research.20−22 Later, DEMS was also applied to the study of reactions in Li-ion batteries.11,23−27 There are systems in which, similarly, a mass spectrometer is connected to an electrochemical cell; yet, they are not unequivocally “differential”. This means that the condition of monitoring potential or timeresolved ion current and electrode current simultaneously is not fulfilledmore of an integral signal is detected.28 In this study, we give a detailed description and analysis of an improved measurement technique, namely, the combination of online mass spectrometry with electrochemical battery testing. In comparison to the common and well-established systems, where a porous membrane is integrated between the electrochemical cell (or half-cell) and the vacuum inlet,29 our device shows substantial differences. We intend to examine batteries such that their construction and operating conditions are as close as possible to practical (realistic) systems. Therefore, full Received: April 1, 2015 Accepted: May 12, 2015 Published: May 12, 2015 5878
DOI: 10.1021/acs.analchem.5b01237 Anal. Chem. 2015, 87, 5878−5883
Technical Note
Analytical Chemistry
diameter), the separator was 42-mm-diameter GF/A; and 600 μL of LP57 served as an electrolyte. Instrumentation. Two commercially available quadrupole mass spectrometers with a scan range of 1−200 Da and detection limits down to 10 ppb and with secondary electron multiplier (SEM) detectors were used for gas analysis (GSD 320 O2, OmniStar Gasanalysesystem, Pfeiffer Vacuum GmbH, Germany). One of the mass spectrometers was used to monitor the entire mass scale range (depending on the battery system and electrolyte solution), while the other was set up to track specific m/z values and monitor the detector currents of the expected fragments over time. The batteries were cycled using a BioLogic VSP300 potentiostat. The synchronization of the ion current signal with the electrochemical measurement was accomplished with the help of the computer clock. The duration of the measurements was usually several days, i.e., the reconcilement of the time scales does not require high precision. The mass flow controller is a product of Bronkhorst High-Tech B.V. (EL-FLOW Select), and the helium flow was typically set to 2 mL/min. Stainless steel metal tubing with nominal diameters of 1/4 in. and 1/16 in., tube fittings, quick connects, and membrane valves were purchased from Swagelok, with special cleaning suited for clean room conditions. KF metal flanges served as connections between the glass gas bubbler and metal tubing. Swagelok fittings containing Teflon ferrules were used to link the glass cold trap to the metal tubing. The other components and their properties, such as the cell design and the construction of the interface between the cell and the spectrometers, are discussed in much more detail in the following sections.
cells with two typical battery electrodes are needed, with the conventional anode−separator−cathode arrangement. The evolved gases are transferred into the mass spectrometer through an inert carrier gas with the consequence of having higher retention time (∼30 s), compared to vacuum-based systems. This might be a drawback in electrochemical systems requiring fast scanning. However, the method is highly suited for most battery studies in which cycling measurements take hours, or even days. The electrode materials are coated on common current collectors, such as aluminum or copper foil, close to real-world battery applications and are not sputter-deposited, doctorbladed, or painted directly onto the cell components.12 Over the last 30 years, several cell types have been presented, although the characterization of the “interfacing” between the cell and the vacuum system is often missing. Here, we demonstrate a new interface which enables the continuous transfer of gaseous reaction products from the battery cell into the spectrometer. In this paper, we focus on the details of the experimental instrumentation rather than the systematic study of a specific cell chemistry in the hope of accelerating research progress in the field and helping a more widespread use of online mass spectrometry in batteries.
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EXPERIMENTAL SECTION Materials. LiPF6 (1 M) in ethylene carbonate/ethyl methyl carbonate (EC/EMC, 3:7 by weight), SelectiLyte LP57, was provided by BASF SE. The water content was determined to be
