Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case

Jan 21, 2016 - (9, 10) Optimization of the morphology of active materials is also reported as an effective approach to improve the packing efficiency ...
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Kinetic Study of Parasitic Reactions in LithiumIon Batteries: A Case Study on LiNi0.6Mn0.2Co0.2O2 Xiaoqiao Zeng, Gui-Liang Xu, Yan Li, Xiangyi Luo, Filippo Maglia, Christoph Bauer, Simon F. Lux, Odysseas Paschos, Sung-Jin Kim, Peter Lamp, Jun Lu, Khalil Amine, and Zonghai Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11800 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case Study on LiNi0.6Mn0.2Co0.2O2 Xiaoqiao Zeng1, Gui-Liang Xu1, Yan Li1, Xiangyi Luo2, Filippo Maglia3, Christoph Bauer3, Simon Franz Lux4, Odysseas Paschos3, Sung-Jin Kim3, Peter Lamp3, Jun Lu1, Khalil Amine1*, and Zonghai Chen1* 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA

2

Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA 3

4

BMW Group, Petuelring 130, 80788 Munich, Germany

BMW Group Technology Office, 2606 Bayshore Parkway, Mountain View, Ca 94043, USA

* Corresponding author: [email protected], [email protected]

Abstract The side reactions between the electrode materials and the non-aqueous electrolytes have been the major contributor to the degradation of electrochemical performance of lithium-ion batteries. A home-built high-precision leakage current measuring system was deployed to investigate the reaction kinetics between the delithiated LiNi0.6Mn0.2Co0.2O2 and a conventional non-aqueous electrolyte. It was found that the rate of parasitic reaction had strong dependence on the upper cutoff potential of the cathode material. The kinetic data also indicated a change of reaction mode at about 4.5 V vs. Li+/Li.

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Keywords: leakage current, parasitic reaction, lithium-ion battery, cathode, interfacial reaction

Introduction High energy-density lithium-ion batteries have been long pursued worldwide to significantly improve the mobility of modern portable electronics.1-3 This global R&D effort was also driven by the emerging application of high energy-density lithium-ion batteries to electrify the transportation system so that a significant reduction on both the consumption of non-renewable fossil fuels and the emission of greenhouse gas could be achieved.4-8 From the perspective of engineering optimization, the energy-density of lithium-ion batteries can be improved by proper engineering designs to reasonably reduce the volumetric and gravimetric contribution from the supporting components, like current collectors and cell packaging materials.9-10 Optimization of the morphology of active materials is also reported as an effective approach to improve the packing efficiency of the active material for an improved volumetric energy-density.11-15 On the chemistry side, advanced nonaqueous electrolytes with an improved electrochemical/chemical compatibility with the electrode materials have been developed to extend the capacity/energy retention of lithium-ion cells so that small cells/packs can be designed to meet the end-of-life electrochemical requirements for targeted applications.16-18 On the top of above approaches, developing high capacity/energy-density electrode materials is the most straightforward approach to substantially increase the energy density of battery packs.19-21 For instance, Si-based metallic alloys

22-25

have been pursued as an extremely

high capacity anode material for lithium-ion batteries, aiming at saving portion of volume/weight in lithium-ion cells to accommodate more active cathode material for an increase of practical battery

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energy density. The same approach has also been utilized for the development of advanced cathode materials with a higher working potential window and/or a higher reversible specific capacity.26-28 Using above approaches, a higher initial energy-density can generally be obtained with the price of a reduction on safety characteristics and the life of lithium-ion batteries, both of which are fundamentally connected to the parasitic reactions between the active electrode materials and the non-aqueous electrolyte at different temperatures and potentials.1,19,29,30 The safety issue is chemically related to violent reactions occurring at elevated temperatures and “extreme” potentials while the battery lifetime is more directly influenced by the extremely slow reactions continuously running under “normal” operating conditions, leading to continuous consumption of lithium reservoir and the buildup of side products inside the lithium-ion cell.

3,31,32

Hence, a practical high

energy-density lithium-ion chemistry needs to be carefully balanced between energy-density and safety/lifetime of the chosen chemistry. The electrochemical validation of the effectiveness of those approaches for improving8 battery life is practically trivial, but extremely time-consuming, especially for those chemistries that have already been demonstrated capable of being charged/discharged for 1000+ cycles.

29-35

A fundamental understanding/measurement of parasitic

reactions not only helps to select a proper life improving strategy, 8 but also substantially shortens the lengthy electrochemical validating process. In this work, a home-built high precision leakage current measurement system was implemented. The kinetic study revealed a strong dependence of parasitic reactions on the practical upper cutoff potential in terms of both the reaction mechanism and the reaction rate.

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Experimental Preparation of electrode - LiNi0.6Mn0.2Co0.2O2 (NMC 622, secured from an industrial partner) working electrodes were prepared by laminating slurry using a doctor blade onto aluminum foil substrates. The slurry was composed of 91.5 wt% active material (NMC622), ~4.5 wt% conductivity agent (Timcal C45 carbon black), and ~4 wt% binder (Solvay 5130 poly (vinylidene difluoride), PVDF). After laminating, the electrodes were first dried at 70 °C in air and subsequently punched and pinch-rolled to a fixed thickness. All punched electrodes were vacuum-dried at 85oC for 8h prior to assembling cells. The final electrode has a loading of ~13 mg/cm2 of active material. The physical characterization of the materials was carried out by high-resolution field emission scanning electron microscope (FESEM) (Hitachi S-4700II) with energy-dispersive x-ray spectroscopy (EDX) in the Center of Nano-scale Materials (CNM), Argonne National Laboratory, and Powder X-ray diffraction (XRD) from 17-BM-B (λ≈0.7291 Å) at Advanced Photon Source (APS), Argonne National Laboratory. All diffraction images were recorded on a PerkinElmer amorphous Si-based area detector. Electrochemical characterization - Charge-discharge tests were conducted on CR2032-type coin cells (area=1.6cm2) with a lithium metal reference electrode and a LiNi0.6Mn0.2Co0.2O2 working electrode at 30oC. The electrolyte used for this study was 1.0 M LiPF6 in a mixture solvent of EC/EMC (3:7 by weight, BASF LP-57). Porous polypropylene separator (Celgard 2325) was used to

electrically

separate

the

cathode

electrode

and

the

anode

electrode.

All

cells

(LiNi0.6Mn0.2Co0.2O2/Li half cell) were assembled in an Ar-filled air and moisture free glovebox, and tested at various temperature and specific rates on a MACCOR Series 4000 battery tester. The formation cycles was carried out by charge/discharge the cell between 2.8V and a specific upper cutoff voltage with a constant current of C/10 (0.32-0.42 mA) for 4 cycles. 4 ACS Paragon Plus Environment

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Kinetic study of parasitic reactions - After formation, the coin cells were conditioned in an environmental chamber at a specific temperature, and a high precision source meter (Keithley 2401) was used to charge/discharge the cell to a specific potential before measuring the leakage current (see Figure 1a). Figure 1b schematically shows the principle of the leakage current measurement. The working electrode was held at a specific potential using the source meter, presuming that the state of the charge (SOC) or the lithium concentration in the working electrode will reach an equilibrium state after the constant-voltage charge/discharge. During this process, the electron obtained from the environment by oxidizing the solvent, will be electrochemically monitored by the external circuit (Keithley 2401). The measured leakage current is practically proportional to the reaction rate of parasitic (side) reactions between the working electrode and the electrolyte. Hence, the static leakage current can be used as a quantitative indicator for the reaction rate of the side reactions. Figure 1c shows a typical current relaxation curve collected; an exponential decay function was used to extract the static current (y0 in Fig. 1C), and to (1) minimize the impact of the high frequency noise, and (2) to get rid of the potential impact of slow intercalation/deintercalation reaction.

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Figure 1 (a) Picture of a 16-channel high precision leakage current measuring system; (b) schematics showing the connection between the leakage current and the rate of parasitic reactions; (c) a typical current relaxation curve collected to extract the static leakage current.

Results and discussion In this work, LiNi0.6Mn0.2Co0.2O2 was used as the model material to investigate the parasitic reactions between the delithiated cathode material and the non-aqueous electrolyte at different degrees of delithiation. Figures 2a and 2b show typical scanning electron microscopy images of the 6 ACS Paragon Plus Environment

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cathode material investigated here. Figure 2a shows that the material has a roughly spherical morphology with a wide distribution of particle size for maximizing the packing efficiency; the larger particles show a diameter of about 10 µm. Figure 2b shows that secondary particles are composed of small pallet-shape primary particles, about 0.5 µm in thickness and 1.5 µm in diameter. The primary particles are closely aggregated together to minimize the active electrochemical surface area that can have a direct exposure to the electrolyte. The EDX analysis confirmed the coexistence of the three different transition metals (see Figure 2c), with a measured ratio close, although not identical, to the nominal value (Ni=6, Co=2, Mn=2). The high-energy Xray diffraction pattern confirms that the material is a single-phase material with a layered structure (space group R-3m) (see Figure 2d).

Figure 2. Characterizations of the NMC622 particles: (a and b) SEM images, (c) EDX spectra, and (d) HEXRD pattern. 7 ACS Paragon Plus Environment

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Figure 3a shows the voltage profiles of a LiNi0.6Mn0.2Co0.2O2/Li half cell during the initial cycling between 2.8 V and 4.6 V with a constant current of about C/10 (0.42 mA).

During the

initial cycling up to 4.6 V vs. Li+/Li, the cell delivered an initial charge capacity of 230.65 mAh·g-1, and an initial discharge capacity of 208.43 mAh·g-1, leading to an initial irreversible capacity loss of 22.22 mAh·g-1 (or 9.63 %). At the 4th cycle, the irreversible capacity loss is barely visible at the plotting scale of Figure 3a. As commonly observed in nickel-rich cathode materials, there is no clear plateau in the potential range above 4.2 V vs. Li+/Li (see Figure 3a). When charged to 4.2 V vs. Li+/Li, the cell delivered a reversible capacity of about 162 mAh·g-1 while the reversible capacity increased to about 208 mAh g-1 by raising the upper cutoff potential to 4.6 V vs. Li+/Li. This significant amount of capacity increase can also be confirmed with the long flat tail in the differential

capacity

profile

(dQ/dV

vs.

voltage)

as

shown

in

Figure

3b.

The

chemical/electrochemical stability of delithiated LiNi0.6Mn0.2Co0.2O2 at a potential higher than 4.6 V vs. Li+/Li was not explored in this work primarily because the direct electrochemical oxidation of solvent can dominate the side reactions at such a high potential.36,37 Figure 4a shows the dependence of the initial charge capacity and initial discharge capacity on the upper cutoff potential of LiNi0.6Mn0.2Co0.2O2, showing that both the charge capacity and the discharge capacity of LiNi0.6Mn0.2Co0.2O2 increased almost linearly with the upper cutoff potential. It was also observed that the initial irreversible capacity remains roughly constant at about 20 mAh g-1 up to 4.45 V vs. Li+/Li. Above this potential the initial irreversible capacity loss increased quickly. This indicates a potential change of reaction mode at elevated potentials; this issue will be further discussed later. Figure 4b shows the specific discharge capacity and specific discharge power of Li/NMC 622 cells after they were stabilized at the 4th cycle. It is clearly visible that the percentage increase on the discharge energy was slightly higher than the increase of the specific discharge capacity due to the 8 ACS Paragon Plus Environment

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increase of average working potential with the increase of the upper cutoff potential. Figure 4b shows that about 23% gain on capacity/energy-density can be obtained by simply pushing the upper cutoff potential from 4.2 V to 4.6 V vs. Li+/Li.

Figure 3. Voltage profiles (a) and differential capacity profile (b) of a LiNi0.6Mn0.2Co0.2O2/Li half cell. 9 ACS Paragon Plus Environment

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Figures 3 and 4 clearly depict the possibility of substantial gain on both the specific capacity and the energy density by pushing the upper cutoff potential of LiNi0.6Mn0.2Co0.2O2 to a higher value. Nevertheless, for real world applications, it is equally important to ensure that the long-term cycling performance of the material is not significantly compromised by the potential gain in energy density. Considering the instability of lithium metal, testing full cells using graphitic material as the counter electrode is more practical for long-term validation, but the whole process is timeconsuming. In addition, the control on the upper cutoff potential of LiNi0.6Mn0.2Co0.2O2 can only be roughly achieved by controlling the P/N ratio of the cell, the ratio between the reversible capacity of positive electrode and that of the negative electrode. Moreover, the parasitic reactions on the graphitic anode also significantly contribute to the overall performance degradation of the full cell and are difficult to be separated in such a setup. Additionally, the continuous consumption of the lithium reservoir at the anode side also constantly pushes the upper cutoff potential of the positive electrode to higher values, leading to more difficulty in quantifying the performance degradation of the cathode component. One possible alternative is to use our home-built high precision leakage current measurement system, which can measure the static leakage current as a quantitative indicator of the rate of parasitic reactions in half-cells with controlled potentials.45 The major advantage of the leakage current measurement is that the system has no capability to pick up the contribution from the chemical reaction between lithium metal and the non-aqueous electrolyte. The static leakage current measured on half cells are solely contributed from electrochemical/chemical reactions occurring on the working electrode. Figure 5a shows the static leakage current measured for a set of half cells that were characterized at a constant temperature of 30oC; each data point in Figure 5a represents an average number over 4 individual cells. When the upper cutoff potential was lower than 4.5 V vs. 10 ACS Paragon Plus Environment

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Li+/Li, the static leakage current increased roughly exponentially with the upper cutoff potential, which can be described using the Tafel equation (equation 1).    exp  /

(1)

Figure 4 (a) Dependence of first cycle capacity on the upper cutoff potential, and (b) the dependence of the available capacity and energy density on the upper cutoff potential. 11 ACS Paragon Plus Environment

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Figure 5 (a) Variation of the static current at 30 oC as a function of the upper cutoff potential, and (b) temperature-dependent static leakage current as a function of the upper cutoff potential. 12 ACS Paragon Plus Environment

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Table 1 values of i0 and ε obtained from the linear fitting Upper cutoff potential (V)

Ea (kJ mol-1)

i0 (mA)

ε (eV)

4.2

26.3±0.1

11.9±0.3

4.473±0.001

4.3

25.3±0.2

9.7±0.6

4.562±0.002

4.4

23.5±0.3

5.9±0.9

4.644±0.003

4.5

25.6±0.1

16.3±0.4

4.765±0.001

4.6

40.7±0.5

10221±2

5.022±0.005

In above equation, i represents the static leakage current measured at a constant potential E, i0 is the exchange current density that is kinetic constant for a specific electrochemical reaction and that is proportional to the active electrochemical surface area of the working electrode, and ε is the characteristic redox potential of the underline electrochemical reaction on the active electrochemical surface. Apparently, the reduced static leakage current at 4.5 V and 4.6 V (see Figure 5a) cannot be simply explained using the Tafel equation. A reasonable explanation is a change on the reaction mode that leads to the reduction of active electrochemical surface area of the composite electrode and/or a change on the characteristic redox potential of the undergoing reaction. To gain more insight of reaction mechanisms, a series of temperature-dependent experiments were carried out to decouple the interference between the exchange current density (i0) and the characteristic redox potential (ε). Figure 5b shows the experimental static leakage current measured at different cutoff potentials and different temperatures, as well as their nonlinear fit using an exponential decay function. When assuming the undergoing reactions are simple one-electron transfer electrochemical reactions, then the simple fitting gives the kinetics parameter of i0 and ε that are summarized in Table 1. Table 1 clearly show that the fitted redox potential increased consistently with the holding potential (E), indicating that the parasitic reaction cannot be considered as a well-defined simple 13 ACS Paragon Plus Environment

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electrochemical reaction.38,39 Alternatively, we can also assume that the investigated reactions are chemical reactions between the delithiated cathode (LiNi0.6Mn0.2Co0.2O2) and the non-aqueous electrolyte, whose kinetic data can be fitted using an Arrhenius equation (also in the form of exponential decay function) to extract the kinetic prefactor (i0) and the characteristic activation energy (Ea) (see Table 1). It can be seen that roughly constant activation energies were obtained at a potential below 4.5V vs Li+/Li. The slight fluctuation of obtained Ea at low potentials implies that the parasitic reaction can be a combination of electrochemical reaction and chemical reaction, but dominated by the chemical reaction. When the cutoff potential was higher than 4.5 V, a substantial change on both the redox potential and the activation energy were observed (see Table 1). This evidences a change of the reaction mode that was triggered at a higher potential (an increase in ε) and a substantially higher exchange current density. It is speculated here that the new reaction mode could be related to: 1) formation of high valence-state transition metal oxide on the particle surface and generation of new chemical species on the surface of the electrode,40-42 2) more decomposition products and change of interface at higher potentials,43, 44 and 3) internal resistance change of the decomposition layer on the electrode,11,44 that take place on the surface of the delithiated LiNi0.6Mn0.2Co0.2O2. Conclusion A home-build high precision leakage current measurement system was implemented to investigate the parasitic reactions in lithium-ion batteries using LiNi0.6Mn0.2Co0.2O2 as the cathode material. It was found, for the first time, that the parasitic reactions on the cathode side had a strong dependence on the upper cutoff potential. A significant change on the reaction mode was found at a potential higher than 4.5 V vs. Li+/Li. The study implies that a different strategy might be needed

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for effective mitigation of the parasitic reactions for materials targeted for operation at a potential higher than 4.5 V vs. Li+/Li. Acknowledgement This research was supported by BMW Corporation. Argonne National Laboratory is operated for the US Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. The authors also acknowledge the use of the Advanced Photon Source (APS) and the Center for Nanoscale Materials (CNM) that are supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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