Discovery of lithium in copper current collectors used in batteries

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Scripta Materialia 67 (2012) 669–672 www.elsevier.com/locate/scriptamat

Discovery of lithium in copper current collectors used in batteries Shrikant C. Nagpure,a,b R. Gregory Downing,c Bharat Bhushana,⇑ and S.S. Babud a

Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics (NLBB), Ohio State University, Columbus, OH 43210, USA b Center for Automotive Research, Ohio State University, Columbus, OH 43212, USA c National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, USA d Department of Material Science and Engineering, Ohio State University, Columbus, OH 43210, USA Received 11 June 2012; accepted 8 July 2012 Available online 16 July 2012

Aging studies of Li-ion batteries have been concentrated on degradation of cathode, anode and electrolyte materials with very limited attention to degradation in current collectors. Our data shows the presence of lithium beyond the active material and in the copper current collector (CCC). The lithium impurity in the CCC will lead to degradation in its thermal and electrical behavior and thus cannot be ignored for overall efforts in understanding the aging mechanisms predicting the life and performance of the batteries. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Li-ion batteries; Aging; Current collector; Degradation mechanism

The world energy consumption is expected to double in next 50 years. Environmental concerns have stressed the use of clean renewable sources to satisfy this increasing demand. Batteries play an important role in storage of electrical energy obtained from these renewable sources. The automotive application of batteries has led the development of high energy and power density Li-ion battery technology. Aging characteristics of Li-ion battery systems for hybrid electric, plug-in hybrid electric and electric vehicles must be understood and quantified for the commercial success of these vehicles. Several studies on the degradation mechanisms for cathode, anode and electrolyte materials have been published [1]. For a better understanding of the aging mechanisms, all the components in a battery, including the separator and current collector, have to be tested under real-life driving conditions. Even as new anode and cathode materials are developed, certain components, such as the current collectors, are made of the same material in the new generation of Li-ion batteries. In

⇑ Corresponding

author. Tel.: +1 614 292 0651; fax: +1 614 292 0325; e-mail: [email protected]

the case of Li-ion batteries, copper is used as the current collector for the anode and aluminum is used as the current collector for the cathode. The corrosion of the current collectors has already been studied to demonstrate their electrochemical stability; however, these studies were not performed on the current collectors extracted from real-life Li-ion batteries. Degradation in the performance of current collectors is never anticipated when predicting the cycle life of the battery. In a Li-ion battery Li is the single most important element as it directly participates in the electrochemical reactions during the cycling of the battery. Measuring the Li concentration in the electrodes is crucial for understanding the migration of Li across the battery. Electron-based techniques are unable to measure the Li due to the presence of a thin beryllium window on the detectors that blocks the low-energy X-rays from any element with an atomic number less than five. We have successfully applied neutron depth profiling (NDP) to measure the Li concentration profiles in the anode and cathode of unaged and aged batteries [2]. We adopted a similar approach to measure the effect of aging and Li migration on a copper current collector (CCC). The data presented here suggests that the degra-

1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.07.009

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dation of current collectors cannot be ignored in the overall effort to understand the aging of Li-ion batteries. NDP is a non-destructive analytical technique based on the nuclear fission reaction between a beam of neutrons with certain elements, such as lithium, throughout the sample. The cold neutrons are delivered through a neutron guide to the NDP facility. In this work, the “cold” neutrons refer to neutrons with energy of less than 5 meV. Since cold neutrons have extremely low energy and momentum, there is no center-of-mass motion in the neutron–lithium reaction. Furthermore, the neutron event rate is not sufficient to cause a significant temperature rise in the sample, nor is there significant radiation damage to the sample during the measurement period. All NDP experiments were conducted at the NIST Center for Neutron Research (NCNR). A schematic of the NDP chamber at the NISTNCNR facility is shown in Figure 1. The sample is attached to an aluminum disk (Fig. 1) and held vertically in the center of a vacuum chamber by the alignment grooves provided on the sample mount. The sample mount is oriented facing a surface barrier-type charged particle detector. The chamber is maintained at a vacuum of less than 1.33 mPa (106 torr). An 0.8 cm2 area of the sample is exposed to the cold neutron beam long enough for the statistical error in counting the energy particles to be no greater than 3%. Upon absorption of the neutron by the Li in the sample, monoenergetically charged 4He and 3H particles are emitted, as per the reaction below. These particles travel diametrically opposite from the site of the reaction. 6

Li þ n ! 4 Heð2055 keVÞ þ 3 Hð2727 keVÞ 4

ð1Þ

3

The energies of the He and H particles at the reaction site are known to be 2055 and 2727 keV, respectively [3]. These heavy charged particles lose energy via a stochastic collision with electrons along their outward path towards the surface. The count rate and the residual energy are simultaneously measured from all sample depths for the particle species emerging in the direction of the detector (Fig. 1). The charged particles do not lose any energy after leaving the surface of the sample as they travel through the vacuum towards the detectors.

Figure 1. Schematic layout of the cold neutron depth profiling chamber at NIST.

The reaction center of mass is coincident with the site of the lithium atom. Thus the 4He and 3H particles originate from the same location as the original lithium atom, and their respective energies are directly related to the location of the lithium atom in the sample. The energy loss of the charged particle per unit length traveled through the sample is given by the stopping power function of the sample. Mathematically, to the first-order approximation, the depth is related to the stopping power by Bragg’s law, which is given as: Z E0 dE ð2Þ x¼ EðxÞ SðEÞ Here x is the path length traveled by the particle through the material, E0 is the initial energy of the particle, EðxÞ is the energy of the particle emerging from the surface and SðEÞ is the stopping power of the sample material [4]. The Stopping and Range of Ions in Matter (SRIM) code developed by Ziegler et al. [5] is then used to obtain the stopping power of copper, and to assign the residual energies of the charged particle to the corresponding depth in the sample. The concentration of 6Li within the sample is determined by comparing the count rate observed from the sample with that of a well-characterized boron concentration standard, labeled as N6 [6]. Since the natural abundance of 6Li in the sample is only 7.5%, the total Li elemental concentration is obtained by dividing the determined 6Li concentration by 0.075. The full-width-half maximum resolution of the NDP system had geometric and energy contributions corresponding to 0.27 lm in Cu. Figure 2a shows the dissembled commercial Li-ion battery used in this experiment. This commercial Liion battery has a graphite anode, a cathode comprising LiFePO4 nanoparticles (40–50 nm) and lithium hexafluorophosphate salt in 1:1 ethylene carbonate and dimethyl carbonate as the electrolyte. Each electrode has an active material bonded onto the current collector using a polyvinylidene difluoride binder. In the case of the anode, graphite is bonded onto a CCC, as shown in Figure 2b. For cathode layers of LiFePO4, nanoparticles are bonded onto an aluminum current collector (ACC) (Fig. 2c). The anode and cathode strips, with a separator in between, are rolled and then packed into a can to form a cylindrical cell. The battery has an operating voltage of 3.3 V and a nominal discharge capacity of 2.3 Ah. It was cycled from 65% to 75% state of charge with an average of 6 C-rate at a temperature of 45 °C until it reached 80% of its rated capacity. At this residual capacity the battery was considered to be dead for automotive applications, consistent with the definition of a dead battery used by the automotive industry [7]. The cell was completely discharged after it reached 80% of their rated capacity. The cylindrical cell was then opened in a glove box filled with an argon atmosphere. The oxygen level was maintained at 88 ppm and the dew point was 34 °C. The cell was unrolled, and the long anode and cathode strips were separated. The total surface area of the CCC was 0.193 m2 (length = 1520 mm, width = 63.5 mm). As such, the current density in CCC with 6 C-rate cycling is 71.5 A m2. A small area of the anode where the CCC

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Figure 2. (a) Disassembled commercial Li-ion battery showing the cathode, anode and separator; high-resolution optical cross-section image of the (b) anode (graphite–Cu–graphite) and (c) cathode (LiFePO4–Al–LiFePO4), showing the electrode structure; and (d) measured lithium concentration profile from the two surfaces of the copper current collector.

was exposed due to the delamination of the active graphite material was chosen as the target area for the experiment. No surface deposits were visible on this area. The energy spectrum of the 2727 keV 3H particle is used here because it has two advantages over the corresponding energy spectrum of the 2055 keV 4He particle. First, since the 3H particle has higher energy and less mass, concentration profiles can be obtained to a greater depth in the sample. Second, the 3H energy spectrum is not overlapped by the 4He energy spectrum, but the 4He energy spectrum is interfered with by the 3H energy spectrum at low energies, i.e. by the charged particles generated from deeper below the sample surface. Figure 2b shows a high-resolution optical microscopy image of a cross-section of the anode. Similarly, Figure 2c shows a high-resolution optical microscopy image of a cross-section of the cathode. A CCC of 6 lm thickness is visible between the graphite coatings on either side. The lithium concentration is measured for this thin CCC from both faces. Figure 2d shows the measured Li concentration profile from the two surfaces of the CCC. The profiles from each surface resemble a classic diffusion profile from surfaces similar to metalliding or carburization. The profile was reproducible for

different CCC samples. The profile from the front face has a maximum of 0.025% atomic Li and the profile from the back face has a Li concentration of 0.08% atomic Li. The measured profiles do not have the same maximum at the surface, but show similar exponential decay along the thickness of the CCC. The data should not be confused with a plating of Li on the CCC surface, as the NDP measurements indicate that the Li has penetrated into the copper subsurface. The first-principles calculations by Van de Walle et al. [8] have shown enhanced Li solubility (30 at.%) in face-centered cubic copper at 400 K. Also, the calculated phase diagram using differential thermal analysis and electromotive force measurements at high temperature have suggested 18 at.% solubility of Li in copper [9]. Therefore, a tendency for Li to diffuse into the CCC should be expected. During operation of the battery there exists a high concentration of Li atoms at the CCC surface, especially when the battery is fully charged. However, at the operating temperature of the batteries, diffusion of Li to form copper alloys is not expected due to the high activation energies [10]. Suzuki et al. [11] have concluded that it is possible for mass transfer across a copper film coated on a carbon fiber using electrochemical insertion and extraction mecha-

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nisms. However, they have not reported measurements showing the presence of Li within CCC films. The presence of Li in the CCC indicates the loss of cycleable Li. This loss of active Li into the CCC is responsible for the loss of electrical capacity of the battery. The diffusion of Li into the CCC can alter its both, the electrical and thermal behavior. The increased levels of Li in Cu would change the battery’s impedance and eventually degrade its performance through ohmic losses. The residual resistivity of the CCC will be affected due to the Li impurity, as given by Nordheim’s rule [12]: qr ðxÞ ffi Ax

ð3Þ

where qr ðxÞ is the residual resistivity of Cu due to Li impurity, A is a constant and x is the impurity concentration. Also, changes in the thermal properties and local heating due to increased impedance can cause delamination between the CCC and the active graphite coating, leading to a further increase in the overall battery impedance. These effects, now evident from our measurements, should be given consideration while building the battery performance and aging models. Only then can accurate estimates of the performance and the aging of the Li-ion battery will be established and used to guide the successful deployment in electric vehicles. Even though a specific mechanism for the above phenomenon is still elusive, the discovery is significant for aging studies of Li-ion batteries. In summary, neutron depth profiling has been used to characterize the CCC used in a real life Li-ion battery. The data shows the presence of lithium in the CCC, with a profile similar to the metalliding or carburization process. The presence of lithium in CCC will affect its thermal and electrical behavior during the operation of the battery. The presence of lithium beyond the active material and in the CCC suggests that the degradation and loss of active Li in current collectors cannot be ignored in the attempt to understand the aging mechanisms predicting the life and performance of batteries.

The authors sincerely thank the Institute for Materials Research (IMR) at The Ohio State University for providing the financial support for this research. The support of the National Institute of Standards and Technology, U.S. Department of Commerce is acknowledged in providing the neutron research facilities used in this work. [1] J. Vetter, P. Nova´k, M.R. Wagner, C. Veit, K.-C. Mo¨ller, J.O. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, J. Power Sources 147 (2005) 269–281. [2] S.C. Nagpure, R.G. Downing, B. Bhushan, S.S. Babu, L. Cao, Electrochim. Acta 56 (2011) 4735–4743. [3] R.G. Downing, G.P. Lamaze, J.K. Langland, S.T. Hwang, J. Res. Natl. Inst. Stand. Technol. 98 (1993) 109–126. [4] J.F. Ziegler, G.W. Cole, J.E.E. Baglin, J. Appl. Phys. 43 (1972) 3809–3815. [5] J.F. Ziegler, J.P. Biersack, M.D. Ziegler, SRIM – The Stopping and Range of Ions in Matter, Lulu Press Co., Morrisville, NC, 2008. [6] D.M. Gilliam, G.P. Lamaze, M.S. Dewey, G.L. Greene, Nucl. Instrum. Methods Phys. Res. Sect. A: Accel. Spectrom. Dect. Assoc. Equip. 334 (1993) 149–153. [7] Anonymous, USABC Requirements of End of Life Energy Storage Systems for PHEVs. Southfield, MI: USABC, 2006, available at . [8] A. Van de Walle, Z. Moser, W. Gasior, Arch. Metall. Mater. 49 (2004) 535–544. [9] W. Gasior, B. Onderka, Z. Moser, A. Debski, T. Gancarz, CALPHAD 33 (2009) 215–220. [10] Z. Xiong, S. Shi, C. Ouyang, M. Lei, L. Hu, Y. Ji, Z. Wang, L. Chen, Phys. Lett. A 337 (2005) 247–255. [11] J. Suzuki, M. Yoshida, C. Nakahara, K. Sekine, M. Kikuchi, T. Takamura, Electrochem. Solid State 4 (2001) A1–A4. [12] R.M. Rose, J. Wulff, L.A. Shepard, Structure and Properties of Materials, Vol. 4: Electronic Properties, John Wiley & Sons, New York, 1966.