WO2024003202A2

WO2024003202A2 - Method and apparatus for characterising an electrochemical device during operation

Info

Publication number: WO2024003202A2
Application number: PCT/EP2023/067749
Authority: WO
Inventors: Fabian Alexander KRETH; Andrea BALDUCCI; Marcus OSTERMANN; Lars Henning HESS
Original assignee: Friedrich-Schiller-Universität Jena
Priority date: 2022-06-28
Filing date: 2023-06-28
Publication date: 2024-01-04
Also published as: WO2024003202A3

Abstract

A method for characterising an electrochemical device during operation, comprising connecting the electrochemical device to a power source, followed by placing the electrochemical device within a sampler. The method further comprises probing a liquid electrolyte within the electrochemical device during operation by the sampler, detecting liquid decomposition products in the electrolyte, and analysing the decomposition products, thereby characterising the electrochemical device during operation.

Description

Title: Method and Apparatus for characterising an electrochemical device during operation

Cross-Reference to Related Applications

[0001] This application claims priority to German Patent Application 10 2022 002 412.8, filed on 28 June 2022. The entire disclosure of German Patent Application 10 2022 002 412.8 is hereby incorporated herein by reference.

Field of the Invention

[0002] The invention comprises a method and an apparatus for characterising an electrochemical device during operation.

Background of the Invention

[0003] A variety of in-situ/in-operando characterisation tools and techniques have been and are still in development for investigating and understanding physicochemical limits and aging mechanisms of electrochemical energy storage (EES) devices. These techniques enable the development of improved electrolytes and electrode materials for the electrochemical energy storage devices [Refs. 1-7],

[0004] Such EES devices, such as electrochemical capacitors and batteries, are widespread in today's society. The electrochemical energy storage devices enable, for example, implementing electric transportation, i.e., transportation vehicles which are powered by an electric engine, such as cars, trains, or buses. The electrochemical energy storage devices further enable energy self-sufficiency.

[0005] The EES devices have, however, their limits regarding specific energy, i.e., how much charge is stored in the electrochemical energy storage devices. The specific energy is the mean available energy from the EES device nomalised to the mass of the EES device. A further limit is specific power, i.e., how fast the energy is delivered by the electrochemical energy storage devices. The specific energy is the available energy normalized to the mass of the EES device. Attention must be paid concerning which definition of mass is chosen when calculating these values as there are two possible values. The mass is defined in research as the mass of an active material. In other words, the mass is the mass of the material actively storing the charge. The mass is defined in industry as the mass of the EES device, i.e., cell, or of a whole battery pack.

[0006] Commercial lithium ion batteries substantially show values of specific energy in the range of 100-300 Wh/kg and values of specific power in the range of 1000 - 3000 W/kg. Commercial electric double layer capacitors, also referred to as super- or ultracapacitors, can achieve values of specific energy of substantially 5-10 Wh/kg and values of specific power up to 25 kW/kg.

[0007] Lifetime of the electrochemical energy storage devices is a further limit, e.g. cyclability or stability in the charged state. An increase of the specific energy, the specific power and the lifetime is required to meet society’s increasing demand for renewable energy generation and storage [Refs. 8, 9],

[0008] Aging processes occur within an electrolyte, an electrode, and an electrolyteelectrode interphase in the EES devices during operation above their rated parameters.

[0009] Primary aging processes include electrolyte-depleting and electrode pore-blocking side-reactions in case the EES devices are electrochemical capacitors. The primary aging processes further include mechanical detachment of the active material from a current collector, such as delamination, and an increase of internal cell pressure. The increase of the internal cell pressure is due to evolution of volatile side-products in the electrochemical capacitors. Expressions of the primary aging processes are, for example, a capacitance fading and an increase of an equivalent series resistance (ESR) [Refs. 10-12],

[0010] The primary aging processes include parasitic side reactions, poor solid electrolyte interphase formation, dendrite formation, active material dissolution, electrolyte decomposition, delamination, gas evolution and an increase of pressure in the EES devices in the case that the EES devices are batteries. The primary aging process results in a capacity loss, a power fading and an increase of the ESR [Ref. 13],

[0011] Monitoring and understanding of side reactions that lead to the cell aging in the EES devices will enable the possibility of optimizing cell performance in the EES device. The optimizing of the cell performance can be carried out by adapting the electrolyte and potential developing of side reaction blocking additives.

[0012] Various in-situ/in-operando techniques have been developed to investigate the aging processes. For example, US Patent application US 2020/0256921 Al discloses a chamber and a system for real-time analysis of gas generated inside a secondary battery. The chamber comprises a first housing which is insulative, a second housing which is thermally conductive and surrounding the first housing, an inlet configured to connect a pump module for generating a flow of an induction medium into the chamber. The chamber further comprises an outlet for connecting an analysis module for analysing the generated gas in the secondary battery by the flow of the induction medium.

[0013] The in-situ/in-operando techniques are further intended to identify possible causes of the aging processes [Refs. 1-7], It is known that the terms “in-situ” and “in-operando” are often freely exchanged with each other. In the present document, a distinct differentiation is made between the terms “in-situ” and “in-operando”. The term “in-situ” is defined as ‘in its (original) place; in position/at the place or locality in question’ based on a definition from Latin origin. The term “in-operando” is defined as ‘working or operating’ . In other words, the term “in-operando” encompass an additional notion of time- resolved measurements compared to the term “in-situ” [Refs. 14, 15],

[0014] The time-resolved measurements refer to a type of experimental technique, i.e., methodology, that involves capturing and analyzing data at different points in time. The experimental technique of the time-resolved measurements is useful in studying dynamic processes or phenomena that occur over a specific time scale. The time-resolved measurements in the context of electrochemistry refer to investigate reactions that take place at an electrode interface or within an electrochemical cell. These time-resolved measurements enable researchers tracking changes in various parameters of the electrochemical cell, such as potential, current, concentration, or spectroscopic properties, as a function of time.

[0015] Applications of in-situ/in-operando techniques for the electrochemical capacitors and for the batteries are known and are, for example, calorimetry, simultaneous thermal analysis (STA), differential or online electrochemical mass spectroscopy (DEMS/OEMS), electrochemical quartz microbalance (EQCM), (surface-enhanced) Raman spectroscopy (SERS, Raman), transmission electron microscopy (TEM), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), nuclear magnetic resonance spectroscopy (NMR), electrochemical atomic force microscopy (EC-AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-Ray emission spectroscopy (XES), resonant inelastic X-Ray scattering (RIXS), and small-angle X-ray scattering (SAXS). It is noted that not every in-situ/in-operando technique can be applied for the above-mentioned applications due to the nature of the different charge-storage mechanisms of the electrochemical capacitors and the batteries, i.e., faradic, capacitive, or pseudocapacitive [Refs. 1-7],

[0016] Kurzweil and Chwistek [Ref. 10] reported investigations regarding aging mechanisms and decomposition products of an electrolyte of electric double-layer capacitors (EDLCs). The electrolyte is a one molar solution of tetraethyl ammonium tetrafluoroborate in acetonitrile. The investigations were conducted via gas chromatography -mass spectrometry (GC-MS). Kbps et al. [Ref. 17] showed in 2021 that a choice of conducting salts for EDLCs influences an amount and a type of decomposition products. Gachot et al. [Ref. 18] investigated composition of a liquid phase and a gas phase of carbonate based Li-ion batteries using similar techniques.

[0017] The investigations of Kurzweil and Chwistek, Kbps et al. and Gachot et al. have been conducted in a post-mortem approach. In the present document, the term “postmortem” means a disassembly of electrochemical cells and an analysis of parts of the disassembled parts of the electrochemical cells by various means. It is still unknown in the post-mortem approach what happens within the electrochemical cell within a time-resolved scale, as it is in the case of in-operando techniques.

[0018] The prior art describes the investigations regarding the aging mechanisms of the electrochemical devices by analysing decomposition products within a liquid electrolyte via gas chromatography-mass spectrometry (GC-MS). The prior art has only reported investigations conducted in the post-mortem approach.

[0019] Existing automated liquid samplers and/or different autosampler systems are known to investigate, i.e., characterise, trace amounts of dissolved analytes within an appropriate solvent, i.e., a solvent wherein the analytes are soluble in the solvent. The existing automated liquid samplers are suited for investigating decomposition products within the electrolyte of the electrochemical storage device. The existing automated liquid samplers are further quite abundant in analytical, chemistry, electrochemistry, and material development laboratories. The electrochemical cell can be used for electrochemical applications, such as batteries, electrochemical capacitors, i.e., electric double-layer capacitors, or pseudo capacitors, hybrid devices, fuel cells or electrocatalysts. [0020] Float measurements or “floating” are known methods for measuring the operation of the electrochemical device. The float measurements comprise testing a stability of the electrochemical cell by holding a maximum rated cell voltage for a fixed time. The maximum rated cell voltage is dependent on the choice of the electrode and the electrolyte. The maximum rated cell voltage is in the range of 2.7 to 3.0 V cell voltage for electric double layer capacitors, comprising, for example, carbon based electrodes combined with an organic solvent, for example, acetonitrile or propylene carbonate. The fixed time is dependent on the applied voltage on the electrochemical device and dependent on the stability of the electrochemical cell. Typical values of the fixed time reported in literature are in the range of 50 to 500 h.

[0021] A remaining energy density, a remaining capacitance, and a remaining capacity of the electrochemical cell are measured after holding of the electrochemical cell at the maximum rated cell voltage at the fixed time. The float measurements enable an understanding a degradation and the stability of the electrochemical cell.

[0022] No methods combining the electrochemical measurements with the in-operando analysis of the liquid electrolyte via GC-MS, either for the electrochemical capacitors, nor for the batteries, has been reported so far to the knowledge of the inventors.

[0023] There is a need for measuring electrochemical processes, for example, energy and power storage, energy and power consumption, aging, an increase of resistance, corrosion, self-discharge, and plating of the electrochemical devices during operation. The decomposition reactions occurring within the electrode-electrolyte interphase or within the electrolyte are of interest. An investigation of the decomposition reactions occurring at the positive electrode or the negative electrode of the electrochemical devices is also carried out.

Summary of the Invention

[0024] A method for characterising an electrochemical device during operation is taught in this disclosure. The electrochemical device is an energy storage and/or a conversion device, such as an electrochemical capacitor, a battery, a hybrid device, a fuel cell, an electrochemical cell or an electrocatalyst. The electrochemical device may be a single electrochemical cell or may refer to a device comprising at least two electrochemical cells in series or in parallel An electrochemical cell comprises exactly one positive electrode and one negative electrode with optionally comprising an additional reference electrode, resulting in a maximum of three electrodes. The term “operation” refers to a period of time in which actions are performed on the electrochemical cell. In other words, the term “operation” refers to a period of time during voltage or current is applied to the electrochemical device, i.e., the electrochemical cell. The method comprises connecting the electrochemical device to a power source. The power source is a device that provide energy or power to the electrochemical device in means of current or electric potential. The method further comprises placing the electrochemical device within a sampler and probing, during operation, a liquid electrolyte within the one or more electrochemical cells in the sampler, i.e., the probing is an in-operando technique. The method further comprises detecting liquid decomposition products in the electrolyte and analysing the liquid decomposition products, thereby characterising the electrochemical device during operation. The detecting and the analysing of the liquid decomposition products enable determining the chemical composition of the electrolyte.

[0025] In one aspect, the characterising is conducted by at least one of an impedance spectroscopy, charging and/or discharging, cyclic voltammetry, floating, pulsed charging, pulsed discharging, amperometry, voltammetry, or a combination thereof.

[0026] In one further aspect, the sampler is chosen from an automated liquid sampler, or an autosampler system, preferably the autosampler system is at least one of a gas chromatograph, a gas chromatograph coupled with a mass spectrometer, a gas chromatograph coupled with another detector, or a combination thereof.

[0027] In one further aspect, the probing is followed by injecting the electrolyte into a gas chromatograph. The injecting is further followed by separating the liquid decomposition products via gas chromatography prior to the detecting. The detecting and the analysing are conducted via mass spectrometry.

[0028] In one aspect, a manual and/or an automated sampling of the electrochemical device is triggered by the power source.

[0029] The automated sampling enables a time-resolved measuring and detecting of the liquid decomposition products. The automated sampling makes possible the investigation of specific aging processes within the electrochemical devices.

[0030] In one aspect, the power source is a potentiometer, a galvanostat, or a combination thereof. [0031] In one further aspect, the method further comprises disassembling the electrochemical device for extracting components, such as electrodes and the electrolyte, from the electrochemical device for a post-mortem analysis.

[0032] An apparatus is taught in this disclosure. The apparatus for performing the method comprises a mean for extracting a sample of the electrolyte from the electrochemical device during operation.

[0033] In one aspect, the apparatus comprises a spacer for separating the electrodes of the electrochemical device.

[0034] In one further aspect, the apparatus further comprises current collectors made of stainless steel, copper (Cu), aluminium (Al), titanium (Ti), platinum (Pt), or a combination thereof.

[0035] In one aspect, the electrochemical device further comprises two electrodes or three electrodes located in the interior of the electrochemical device. The electrodes are chosen from at least one of a positive electrode and a negative electrode. The electrochemical device further comprises at least two current collectors, an active material, a spacer, an electrolyte, and a septum. The septum is gastight and pierceable.

[0036] In one aspect, the electrochemical device comprises three electrodes, wherein one of the electrodes is a reference electrode. The electrochemical device enables to give detailed time-resolved measurements, in other words insight into the formation and possible causes of degradation products in the electrochemical device. Further, the electrochemical device with the three electrodes makes possible the differentiation between aging processes that occur at the positive electrode or at the negative electrode.

[0037] In one aspect, the detailed time-resolved measurements are carried out from 5 to 10 min to up to 5 hours.

[0038] In one aspect, the positive electrode and the negative electrode are separated by the spacer. The inventors empirically found out that a spacer with a shape of an O-ring with a cut on top of the spacer enables sufficient diffusion of the liquid decomposition products from an electrode-electrode interphase towards the electrolyte. The sufficient diffusion of the liquid decomposition products enable the detection of the liquid decomposition products via mass spectrometry (MS).

[0039] In one aspect, the current collectors, the electrodes, the electrolyte, the spacer, and the septum are removable and replaceable. [0040] In another aspect, the automated liquid sampler (ALS) and/or the autosampler system directly probe the electrolyte within the electrochemical device. In other words, the ALS probes a sample of the electrolyte, followed by injecting the sample into a gas chromatograph-mass spectrometry (GC-MS).

[0041] In another aspect, the interior of the electrochemical device is assembled using gaskets. In other words, the electrolyte has no direct contact with used glue.

[0042] In one aspect, the electrochemical cell is connected with several cables outside the ALS to a current source, preferably the current source is a potentiometer and/or a galvanostat.

[0043] In another aspect, the electrochemical device is assembled within an inert gas atmosphere.

[0044] In a further aspect, electrochemical device is assembled within an argon filled glovebox.

[0045] In another aspect, the electrochemical device is assembled and disassembled in air, in an oxygen rich atmosphere, and in an inert gas atmosphere.

[0046] An electrochemical device is described that is easily, i.e., freely implemented into existing, i.e. commercially available, automated liquid samplers and/or in different autosampler systems, i.e. exchanged with an original turret placed inside the ALS, in a matter of a few minute. Certain requirements should be addressed when implementing the electrochemical cell into the existing automated liquid samplers and/or in the different autosampler systems. A requirement is related to the tightness. In other words, the electrochemical cell has to be impermeable to liquids and gases. A further requirement is related to piercability. In other words, a probe has to have access to the interior of the electrochemical cell, i.e. to the electrolyte chamber. Impermeability for liquids and gases has to be maintained. A further requirement is related to diffusion. The diffusion of the liquid decomposition products from the electrode-electrolyte interphase to and within the electrolyte has to be achieved even though GC-MS can detect trace amounts of analytes. Another requirement is related to mounting. The electrochemical cell has to be freely exchangeable with an original turret of the automated liquid sampler and/or be installed in other autosampler systems in such a way that direct probing is possible. Another requirement is related to durability. The electrochemical cell needs to endure elevated currents and potentials as well as an aggressive chemical environment. A further requirement is related to performance. The electrochemical cell has to show a similar electrochemical performance as commonly used lab-scale cells, i.e. Swagelok®-type or coin-cells. The apparatus of the present invention fulfils all of the above mentioned requirements.

Description of the figures

[0047] Figs. 1 (a) to (e) are technical drawings of an in-operando gas chromatography - mass spectrometry (GC-MS) cell.

[0048] Fig. 2 shows a capacitance and an equivalent circuit resistance (ESR) of a negative electrode during aging tests of a positive electrode of an electrochemical cell.

[0049] Fig. 3a-c shows charge-discharge profiles of the positive electrode and (oversized) negative electrode at three different points during operation at 0 hour, 15 hours, and 30 hours.

[0050] Fig. 4 shows electrochemical impedance spectra of the positive electrode obtained each 5 hours during aging tests of the positive electrode.

[0051] Fig. 5 shows a time-resolved total ion chromatogram obtained during aging tests of the positive electrode.

[0052] Fig. 6 shows an outline of a method for characterising an electrochemical device during operation.

[0053] Fig. 7 shows an apparatus for performing the method.

Detailed description of the invention

[0054] The invention will now be described on the basis of the drawings. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with the feature of a different aspect or aspects and/or embodiments of the invention.

[0055] Electrochemical testing was performed, i.e., conducted with a SP-150 from Biologic. A gas chromatography-mass spectrometry as a sampler 20 was performed in step S50 with an Agilent 5977A EI-MSD - 7890 B GC System. [0056] An electrochemical cell as part of an electrochemical device 10 was equipped with an Agilent 7683 Series automatic liquid sampler 20 and a HP-5ms column from Agilent (30 m, ID 0.25, DF 1, Temp range -60 to 325/350 °C). Applied temperature ramp was from 40°C to 280°C at 15 K min'¹. An injection volume of liquid samples from a liquid electrolyte 8 has been set to 1 pL by a 10 pL syringe.

[0057] Standard carbon electrodes 4 were produced by mixing 3 g with 90% Kuraray YP-50F, 5% IMERYS Super C65 and 5% Dow Chemical Walocell CMC in 8 ml water, thereby obtaining a slurry. The slurry was stirred in a dissolver for 30 min until the slurry yielded a homogenous suspension. The homogeneous suspension was cast on aluminium foil with a doctor blade set to 150 pm. Circular cut-outs were made from the aluminium foil using an in-house made punch press.

[0058] Oversized ones of the carbon electrodes 4 were produced by mixing Kuraray YP- 50F (85%) with IMERYS Super C65 (10%) and PTFE (10%) in 50 mL ethanol (EtOH), thereby obtaining a suspension after stirring for 2 hours and heating to 80°C. The suspension was transferred onto a glass plate and rolled flat until a thickness of 1-2 mm is reached. 5 mm electrodes 4 were punched and dried. An in-house build spacer 3 was used as a separator the two electrodes 4.

[0059] The in-house build spacer 3 was made from polyether ketone (PEEK) and had the shape of an O-ring with a cut on top of the spacer 3. The spacer 3 enabled sufficient diffusion of liquid decomposition products from an electrode-electrode interphase towards the electrolyte 3 of the electrochemical cell 10. Use of a commonly used separator, i.e., spacer 3, for example made of paper, polymer or glass fiber membrane, pressed between the two electrodes 4 was not made, as this commonly used separator did not allow sufficient diffusion of the liquid decomposition products. This lack of sufficient diffusion results in a much higher limit of detection, which results in an inferior method. It will be appreciated that sufficient amounts of liquid decomposition products are required to generate a responding signal during analysis. The amount of liquid decomposition products required is dependent on the analytical devices and the analytes.

[0060] A I M solution of tetraethylammonium-tetrafluoroborate (Et4NBF4) (Sigma Aldrich) in acetonitrile (ACN) (Sigma Aldrich) was chosen as the electrolyte 8. The electrolyte 8 was prepared in an Argon-filled dry box (Labmaster Pro, MBraun). All used solid materials were dried in a vacuum glass oven. The solvent was dried using molar sieves 3 A (Kostrolith). Water content of the electrolyte 8 was measured to be below 20 ppm by Karl Fischer Titration.

[0061] An electrochemical three-electrode in-operando GC-MS cell 10 as the electrochemical device 10 was developed and is illustrated on Figs. la-e. The electrochemical three-electrode in-operando GC-MS cell 10 comprises poly ether ketone (PEEK).

[0062] No glue was used during the fabrication and the assembly of the electrochemical three-electrode in-operando GC-MS cell. All screw-type parts are sealed either using gaskets and/or septa 9 of fluoroelastomers (Viton®) or polytetrafluorethylene (PTFE).

[0063] Two current collectors 1, 2 were used. The two current collectors 1, 2 were made from stainless steel (1.4301, X5CrNil8-10). As the reference electrode 5, a silver wire (Alfa Aesar, 99.9985%) was used.

[0064] The GC-MS cell 10 was assembled in an argon-filled dry box and filled with 1000-1200 pL of the electrolyte 8.

[0065] In one example, the GC-MS cell 10 was assembled symmetrically, in other words with two standard electrodes 4. The symmetrical assembly is performed if processes occurring simultaneously on the two standard electrodes 4 are to be characterised in step S60. In the example, processes occurring one of the standard electrode 4 can’t be characterised.

[0066] In another example, the GC-MS cell 10 was assembled asymmetrically, in other words with one standard electrode 4 and one oversized electrode 4. The asymmetrical assembly is performed if processes occurring on one singular electrode are to be characterised in step S60.

[0067] Applied potentials are either positive or negative depending on the characterised electrode 4. A standard electrode 4 has been used as the positive electrode and an oversized electrode 4 has been used as the negative electrode for the investigating of the positive electrode.

[0068] Fig. 6 illustrates a method for characterising the electrochemical device 10 during operation. The method comprises connecting in a connection step S 10 the electrochemical device 10 to a power source 15, followed by placing in a placing step S20 the electrochemical device 10 within the sampler 20. The method further comprises probing, during operation, in a probing step S30 the liquid electrolyte 8 within one or more electrochemical cells in the electrochemical device 10 by the sampler 20. The probing step S30 is followed by detecting in a detection step 40 liquid decomposition products in the electrolyte 8 and analysing in an analysis step 50 the liquid decomposition products. The detecting step 40 and the analysis step 50 of the liquid decomposition products are conducted, for example, via mass spectrometry.

[0069] In one example, the sampler is an automated liquid sampler (ALS). The probing step S30, i.e., the sampling step, comprises, for example, piercing a septum 9 of the electrochemical device 10 with a needle of the automated liquid sampler. A liquid sample is taken from the liquid electrolyte 8 and injected in an injection step S33 into a gas chromatograph 20 -mass spectrometer. An electrochemical response is measured and/or applied by the power source 15, for example using a potentiometer and/or a galvanostat.

[0070] In one further example, the method comprises a separating step S35 for separating the liquid decomposition products via gas chromatography after the injection step S33.

[0071] Fig. 7 illustrates a diagrammatic view of an apparatus 100 for performing the method set out in this description. The apparatus 100 comprises the electrochemical device 10 connected to a power source 15. The electrochemical device 10 comprises one or more electrochemical cells containing the liquid electrolyte 8 within the electrochemical device 10. The apparatus 100 further comprises the sampler 20 within which is placed in the electrochemical device 10. The sampler 20 is adapted to probe in the probing step S30 the liquid electrolyte 8 during operation. The apparatus 100 comprises a detector 25 for detecting, in the detection step 40, and analysing, in the analysis step 50, the liquid decomposition products in the electrolyte 8.

[0072] The apparatus 100 comprises, in one example, an extractor 30 for extracting a liquid sample of the electrolyte 8 from the electrochemical device 10 during operation.

[0073] The electrochemical device 10 further comprises current collectors 1, 2 made of stainless steel, copper (Cu), aluminium (Al), titanium (Ti), platinum (Pt), or a combination thereof.

[0074] In one example, the electrochemical device 10 further comprises two or three electrodes 4 located in the interior of the electrochemical device 10 as illustrated in Fig. 1c. The electrodes 4 are chosen from at least one of a positive electrode and a negative electrode. The electrochemical device 10 further comprises an active material 12 and the septum 9. The active material 12 is the material which is actively engaging in the energy storage process. The electrodes 4 comprise the active material, a binder and further additives for enhancing electric conductivity, for example. The septum 9 is gastight and pierceable. The GC-MS cell 10 was electrochemically cycled within the following sequence. Electrochemical impedance spectroscopy (EIS) for the analysis step S50 was performed within the range of 500 kHz to 10 mHz. The EIS was followed by three cycles of galvanostatic charging and discharging up to a set potential of 1 V vs. Ag/Ag+ were performed by a power source 15, i.e. a galvanostat followed the EIS.

[0075] The three cycles of galvanostatic charging and discharging were followed by a floating as a further analysing in step 50. The floating lasted 1 hour at the previously set potential of 1 V vs. Ag/Ag+ and was repeated five times. The set voltage is increased by 0.25 V, after the five repetitions at which point the sequence is repeated. The voltage was further increased by additional 0.25 V to 1.5 V vs Ag+/Ag.

[0076] The increase of the set voltage was repeated until a voltage of 2.25 V vs. Ag/Ag+ is reached.

[0077] The measurement ended with a final EIS measurement as a further analysing in step 50. A liquid sample was automatically taken, i.e. extracted, by an extractor 30, via the automated liquid sampler 20, injected in the injection step S33, and analysed in the analysis step S50 via GC-MS for each 5 hours of floating, in other words five times for one hour at one potential were floated, followed by an increase of the voltage resulting in a floating time of 5 hours at each potential.

[0078] Fig. 2 shows a calculated capacitance (F g-1) and an equivalent circuit resistance (ESR, cm2) as a function of an accumulated floating time. The applied potential is increased by 0.25 V as illustrated in Fig. 2 for each 5 hours of floating . The capacitance and the ESR show stable values until a potential of 1.75 V vs. Ag/Ag+ is reached. A sharp decrease of the capacitance and an increase of the ESR is observed at potentials higher than 2 V vs. Ag/Ag+. That indicates strong (significant) aging of the GC-MS cell and thus a generation of liquid decomposition products.

[0079] Fig. 3a-c show a selection of galvanostatic charge-discharge profiles of the positive standard electrode 4 (thick grey) and the negative oversized electrode 4 (thin black) at three different points, i.e. at 0 hour, 15 hours, and 30 hours during operation, i.e. within operation. [0080] At 0 hour (set potential of 1 vs. Ag/Ag+) and at 15 hours (set potential of 1.50 vs. Ag/Ag+) the two electrodes show an ideal EDLC-like linear behavior. At 30 hours (set potential of 2.25 vs. Ag/Ag+) the profiles show a curved line. This example shows strong degradation of the electrode 4 -electrolyte 8 interphase. The negative electrode 4 shifts further towards negative potential when aging processes occur on the positive electrode 4. The shifting was expected and attributed to an irreversible oxidation on the positive electrode 4. The irreversible oxidation results in an accumulation of electrons on the negative electrode. The shifting can be neglected as long as the negative electrode 4 is within the stability window of the characterised electrochemical device 10. The stability window is the potential range in which the investigated system, i.e. the characterised electrochemical device 10 is stable, meaning no degradation of the electrode-electrolyte interphase is occurring. The shift does not influence the measurement of voltage as long as the shift of the electrodes 4 does not exceed the respective stability limits, i.e., the stability window.

[0081] Fig. 4 shows Nyquist diagrams of the obtained impedance spectra. An EDLC-like impedance spectrum is obtained, illustrated by a small half circle followed by a steep increase of the imaginary part, at the start the operation. This shifts slightly towards higher resistances with increasing floating time and increasing potential. The resistance increases at 20 hours of floating (set potential of 1.75 vs. Ag/Ag+). The resistance shows the characteristics of a lab-scale EDLC. The resistance only increases after reaching a potential of 2.00 V vs. Ag+/Ag, that is equal to a floating time of 20 hours.Non-existent half-circle indicates a strong degradation of the electrode-electrolyte interphase and a strong increase of the internal resistances.

[0082] Fig. 5 shows the three-dimensional time-resolved total ion chromatograms (TICs) of in-operando GC-MS measurements. The measurements were obtained during aging tests of the positive electrode.

[0083] Samples of the electrolyte 8 were taken by the sampler 20 in-operando each 5 hours and were analysed in the analysis step S50 via GC-MS. Mass spectra were continuously obtained after injecting in the injection step S33 the sample into the gas chromatography. Different analytes were separated in the separation step S35 via gas chromatography at which point the analytes reach the mass spectrometer (MS). The obtained mass spectra were compared with NIST databases when the analytes reached the MS. Possible liquid decomposition products were detected in the detection step S40 and identified.

[0084] TICs display an accumulated intensity, rel. abundance, across a detected range of masses at every point during analysis as function of the time, i.e., retention time (tR) of the analytes. Each peak of the spectra correlates to a specific mass spectra and indicates one specific degradation product. The characteristic retention time of each compound of the liquid decomposition products does not change as long as parameters of the analysis do not change.

[0085] A first peaks arise at 1.75 vs. Ag/Ag+ by comparing the obtained TICs. The height and the area increase with increasing potential. Further, the total amount of detected signals increases with increasing potential. This shows the stronger degradation at higher potentials.

[0086] During the aging tests, four degradation products were identified and were assigned to aging processes occurring at the positive electrode: acetamide (tR of 8 min), N- ethyl -acetamide (tR of 12 min), 2,4,6-Trimethyl-l,3,5-triazine (tR of 12.5 min) and diacetamide (tR of 14 min). Acetamide, N-ethyl -acetamide and diacetamide are oxidation and hydrolysis products of the solvent acetonitrile. 2,4,6-Trimethyl-l,3,5-triazine is a trimerization and cyclization product of acetonitrile. This shows no change of oxidation states of the liquid decomposition products. In other words, no electrons are consumed in the electrochemical device 10. The inventors assumed that the cyclization is a surface- confined reaction that needs partial oxidation during the surface-confined reaction.

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01948U_0007.GIF. Reference numerals

1, 2 current collectors

3 spacer

4 electrodes (positive and negative electrode) 5 reference electrode

6, 7 gaskets

8 electrolyte

9 septum/gasket

10 electrochemical device 12 active material

15 power source

20 sampler

25 detector

30 extractor 100 apparatus

Claims

Claims A method for characterising an electrochemical device (10) during operation, comprising: connecting (S10) the electrochemical device (10) to a power source (15), followed by placing (S20) the electrochemical device (10) within a sampler (20); probing (S30), during operation, a liquid electrolyte (8) within one or more electrochemical cells in the electrochemical device (10) by the sampler (20); detecting (S40) liquid decomposition products in the electrolyte (8); and analysing (S50) the liquid decomposition products, thereby characterising (S60) the electrochemical device (10) during operation. The method of claim 1, wherein the characterising (S60) is conducted by at least one of an impedance spectroscopy, charging and/or discharging, cyclic voltammetry, floating, pulsed charging, pulsed discharging, amperometry, voltammetry, or a combination thereof. The method of claim 1 or 2, wherein the sampler (20) is chosen from an automated liquid sampler, or an autosampler system, preferably the autosampler system is at least one of a gas chromatograph, a gas chromatograph coupled with a mass spectrometer, a gas chromatograph coupled with a detector, such as a flame ionization detector (FID), a thermal conductivity detector (TCD), or an electron capture detector (ECD). The method of any of the above claims, wherein: the probing (S30) is followed by injecting (S33) the liquid electrolyte (8) into a gas chromatograph ; the injecting (S33) is followed by separating (S35) the liquid decomposition products via gas chromatography prior to the detecting (S40); and wherein the detecting (S40) and the analysing (S50) are conducted via mass spectrometry. The method of any of the above claims, wherein the power source (15) is a potentiometer, a galvanostat, or a combination thereof. The method of any of the above claims, wherein the probing (S30) and the injecting (S33) are automatically and/or manually triggered by one of the power source (15) or by a device external to the electrochemical device (10), preferably a potentiometer, during operation. The method of any of the above claims, further comprising disassembling (S70) the electrochemical device (10) for extracting components, such as electrodes (4) and the electrolyte (8), from the electrochemical device (10) for a post-mortem analysis. An apparatus (100) for performing a method for characterising an electrochemical device (10) during operation, comprising: the electrochemical device (10) connected to a power source (15), wherein the electrochemical device (10) comprises a liquid electrolyte (8) in one or more electrochemical cells; a sampler (20) within which is placed the electrochemical device (10), wherein the sampler (20) is adapted to probe the liquid electrolyte (8) during operation; and a detector (25) for detecting and analysing liquid decomposition products in the electrolyte (8). The apparatus of claim 8, further comprising an extractor (30) for extracting a liquid sample of the electrolyte (8) from the electrochemical device (10) during operation. The apparatus of claim 8 or 9, further comprising a spacer (3) for separating electrodes (4) of the electrochemical device (10). The apparatus of claims 8 to 10, further comprising current collectors (1, 2) made of stainless steel, copper (Cu), aluminium (Al), titanium (Ti), platinum (Pt), or a combination thereof.