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Selecting inactive materials with low electrolyte reactivity for lithium-ion cells
Journal of Power Sources 397 (2018) 374–381
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Selecting inactive materials with low electrolyte reactivity for lithium-ion cells
T
Zilai Yana, M.N. Obrovaca,b,∗ a b
Department of Chemistry, Dalhousie University, Halifax, N. S. B3H 4R2, Canada Department of Physics and Atmospheric Science, Dalhousie University, Halifax, N. S. B3H 4R2, Canada
H I GH L IG H T S
reactivity was measured on inactive materials at the negative electrode. • Electrolyte used inactive materials, Cu and Ni, have high electrolyte reactivities. • Commonly reactivity at inactive materials can contribute to cell fade and impedance. • Electrolyte • Ti and TiN were found to be the least reactive inactive materials of those tested.
A R T I C LE I N FO
A B S T R A C T
Keywords: Inactive materials Electrolyte reactivity Cell fade SEI formation
Electrolyte reactivity on Cu, Ni, 304 stainless steel, Ti, TiN, Mo, and Fe, was investigated at the negative electrode in VC and non-VC containing electrolytes. TiN and Ti were found to have the lowest cathodic electrolyte reactivity in both VC and non-VC containing electrolytes, with a reactivity less than graphite. In contrast, inactive materials commonly used in Li-ion batteries at the negative electrodes, Cu (current collector) and Ni (tabs), have high irreversible capacities, high electrolyte reaction rates (2–3.5 times graphite) and high impedance growth. These studies show that the current materials widely in use today at the negative electrode could be contributing to fade and impedance growth in Li-ion batteries. The use of more inert materials, such as Ti or TiN, may result in an increase of Li-ion battery cycle life.
1. Introduction
still lacking. The types of inactive materials for LIBs are numerous and their electrochemical properties, such as electrocatalytic behavior and corrosion resistance, are significantly different [8]. Due to large differences in electrochemistry on inactive metal surfaces, tremendous efforts have been made in other fields for the development of novel materials for water splitting [9,10], oxygen reduction [11,12], and other electrocatalytic processes [13,14]. It is surprizing that few studies of electrolyte reactivity on inactive materials in LIBs have been undertaken, especially since electrolyte reactivity is a key parameter influencing cycle life. An excellent inactive material for LIB negative electrodes should have the following key properties: low initial irreversible capacity to increase the overall cell capacity; low long-term electrolyte reactivity to lengthen battery service life; and low electrochemical impedance to prevent Li plating, which can cause safety issues. In a previous study, we found that copper can have significantly higher electrolyte reactivity at the negative electrode compared to graphite in vinylene
To lengthen service life and to improve safety of lithium ion batteries (LIBs), tremendous efforts have been made to deepen our understanding of parasitic reactions between electrolytes and active materials, such as graphite and silicon [1–3]. At the same time, previous research has shown that inactive materials, e.g. copper and nickel, are also not fully inert to electrolytes at low potentials [4,5]. Inactive materials play important roles in Li-ion cells, such as in cell hardware (current collectors, tabs, the cell can, etc.), where they can be exposed to electrolyte at very high or very low potentials. Inactive materials are also used as components of active materials to restrict the volumetric expansion of electrode materials as a matrix phase, and even participate in conversion reactions with the solid-electrolyte interphase (SEI) layer [6,7]. Although inactive materials have been widely used in LIBs over decades, a comprehensive study of their interactions with electrolyte is
∗
Corresponding author. Department of Chemistry, Dalhousie University, Halifax, N. S. B3H 4R2, Canada. E-mail address: [email protected] (M.N. Obrovac).
https://doi.org/10.1016/j.jpowsour.2018.07.042 Received 13 February 2018; Received in revised form 24 May 2018; Accepted 9 July 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
Journal of Power Sources 397 (2018) 374–381
Z. Yan, M.N. Obrovac
carbonate (VC)-containing electrolytes [5]. This has serious implications for Li-ion cells, which almost universally use Cu-current collectors at the negative electrode and often use VC containing electrolytes. Therefore we decided to see if other materials might have less reactivity than Cu and therefore be more suitable for use as a current collector. In this paper, the initial irreversible capacity, electrolyte reactivity, electrochemical impedance, and the SEI morphology of a variety inactive metals at the negative electrode in Li-ion cells are discussed. Some materials were chosen for their practicality of use, such as copper, iron, nickel, and stainless steel, which are widely used in current LIBs. Other inactive metals, such as molybdenum, titanium, and titanium nitride were chosen because of their known electrochemical stability in other systems [15–17]. These inactive metals were comprehensively investigated for their stability at the negative electrode.
Table 2 Electrode compositions in weight percent.
Electrodes consisted of powdered inactive metal, graphite (MAG-E, Hitachi, 20 μm average size), carbon black (CB) (Super C65, Imery's Graphite and Carbon), and polyvinylidene fluoride (PVDF) (HSV 900, KYNAR). The inactive metal powders studied were Ti (99.9%, 60–80 nm, Skyspring Nanomaterials, Inc.), Fe (99%, 40–60 nm, SigmaAldrich), Mo (≥99.9%, 1–5 μm, Sigma-Aldrich), TiN (< 3 μm, SigmaAldrich), stainless steel (SS) (316L, 60–80 nm, Skyspring Nanomaterials, Inc.), Ni (99.8%, APS 0.08–0.15 μm, Alfa Aesar), and copper (99%, −625 mesh, Alfa Aesar). The specific surface areas (SSA) of these inactive powders are listed in Table 1. Inactive metal, graphite and carbon black components of the electrodes were formulated based on a (SA) ratio of inactive metal powder:graphite:CB = 47:16:37. All electrodes contained 8 wt% PVDF binder. The corresponding formulations of all electrodes based on weight percent are listed in Table 2. Electrodes were cast from 1-methyl-2-pyrrolidinone (NMP) (99.5%, Sigma-Aldrich) slurries onto copper foil with a 0.006-inch coating bar. This resulted in an electrode loading of about 4 mg of coating per square centimeter or about 1.5 mAh cm−2. Electrode loadings were purposely kept high to minimize the impact of parasitic currents from current collectors and cell hardware [17]. Graphite and TiN/G electrodes were coated in air and then air dried at 120 °C for 1 h. To prevent oxidation of elemental metal powders, the other electrodes were coated in an argon atmosphere and then vacuum dried at 120 °C overnight. These air-sensitive electrodes were stored in an argon-filled glovebox to prevent oxidation. Electrode disks were punched from electrode coatings using a 1.35 cm2 area circular punch. Coin cell construction was conducted in an Ar-filled glovebox using standard 2325 coin cell hardware. 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (1:2 v/v, all battery grade from BASF) was used as control electrolyte. Vinylene carbonate (VC) (BASF), was added to the control electrolyte at specific weight ratios. Two layers of Celgard-2300 separator and one interleaving layer of BMF separator (3 M Company) were used as cell separators. Three formats of cells were used in this study: half-cells, double half cells (DHCs), and conventional symmetric cells. A brief description of these three cell formats is presented here. More details can be found in Reference [5]. In half-cells, each 1.35 cm2 working electrode was paired with a 2.57 cm2 circular lithium metal foil counter electrode (thickness of 0.38 mm, 99.9%, Sigma-Aldrich). DHCs were constructed simply by connecting the Li terminals of two nominally identical half-cells. Construction pathways for making conventional symmetric cells and DHCs
Cu
TiN
Mo
Ni
Fe
SS
Ti
2.35(1)
62
1.54(4)
1.9(2)
2.1(1)
2.5(1)
11.4(2)
5.09(1)
12.8(2)
Inactive Materials
CB
PVDF
Graphite Cu/G TiN/G Mo/G Ni/G Fe/G SS/G Ti/G
90.0% 16.3% 15.0% 20.5% 23.6% 53.9% 37.2% 56.2%
N/A 74.3% 75.3% 69.7% 66.3% 33.3% 51.5% 30.8%
2.0% 1.4% 1.7% 1.8% 2.1% 4.8% 3.3% 5.0%
8.0% 8.0% 8.0% 8.0% 8.0% 8.0% 8.0% 8.0%
3. Results and discussion Fig. 1 shows the initial cycles of half-cells containing inactive metal/ graphite electrodes and a graphite electrode, for comparison, in control and +2 wt% VC electrolytes. All blended electrodes have a larger initial irreversible capacity than graphite. This increase of initial irreversible capacity is from the reduction reactions on the surface of inactive materials, which consume extra capacity. Blended electrodes were formulated based on the same surface area ratio of carbon and inactive materials. Thus, their irreversible capacity is a direct measure of the inactive component reactivity. Therefore, according to Fig. 1, TiN and Ti have the lower initial irreversible capacity per unit area than other inactive materials in control electrolyte and in +2 wt% VC electrolyte.
Table 1 Measured specific surface areas (in m2 g−1) of the inactive powders investigated. *As specified by the manufacturer. CB*
MAG-E
are illustrated in Fig. S1. One half cell discharged to 5 mV and then charged to 0.9 V at C/10 rate. The other half cell was discharged to 5 mV, charged to 0.9 V once, discharged to 5 mV at a C/10 rate, and then trickle discharged until the current was less than C/200. All cells were evaluated with a Maccor Series 4000 Automated Test System while the cells were thermostatically controlled at 30.0 ± 0.1 °C. DHCs cells were cycled at a C/10 rate. All specific cell capacities were calculated with respect to the mass of graphite in the electrodes. Electrochemical impedance spectroscopy (EIS) spectra were measured using a Bio-Logic VMP3 potentiostat. EIS measurements were performed at 30 °C, with a 10 mV amplitude excitation and a frequency range from 100 kHz to 10 mHz. Electrodes cycled over 240 h in DHCs were studied. Before electrodes were recovered from DHCs, DHCs were discharged or charged to 0 V at C/10 rate, and then trickled until the current dropped below C/200. The electrodes were then harvested from the cycled DHCs in an Ar-filled glovebox and then incorporated into conventional symmetric cells. The EIS experimental setup did not allow accurate solution resistance measurement due to cable and connection resistance. For this reason, all impedance spectra were shifted along with the real axis so that the highest frequency data point is located at 0 Ω cm2. The total electrode interfacial resistance, R, which includes the charge transfer resistance and the resistance due to ion diffusion in the SEI layer, was obtained from measuring the diameter of the mid-frequency semicircle(s) in the Nyquist plot. Unless otherwise specified, error bars were calculated as the range of two samples. The morphology of fresh and cycled electrodes was studied using a scanning electron microscope (SEM) (TESCAN MIRA3) with an accelerating voltage of 20 kV. After 240 h cycling and then over 30 days resting in DHCs, the cycled electrodes were recovered in an Ar-filled glove box, rinsed in dimethyl carbonate (DMC) (battery grade from BASF) to remove residual electrolyte salt and then dried under vacuum. Pop-off specimen holders (polytetrafluoroethylene, DPM solutions Inc.) were used to transfer air-sensitive samples from glovebox to the SEM without air exposure. Specific surface areas (SSAs) were measured using a Micromeritics Flowsorb II 2300 surface area analyzer by the single-point Brunauer-Emmett-Teller (BET) method. Prior to measurement, powders were placed in a glass holder and degassed at 160 °C for over half an hour. Elemental metal powders were loaded into the holder in an argon-filled glove box to prevent oxidation.
2. Experimental
MAG-E
Coating
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Fig. 1. Voltage curves of graphite and inactive metal/graphite electrodes in control electrolyte (solid lines) and in +2 wt% VC electrolyte (dashed lines). The initial CE is the average of four samples.
electrode CE based on their capacity fade, as described in Reference [21]. All electrodes suffer continual capacity fade in DHCs, which is indicative of irreversible electrolyte reactions. The reversible capacity of blended electrodes is due to the graphite component, which has an excellent capacity retention. Thus, the increased capacity fade of blended electrodes is caused by electrolyte decomposition on carbon and inactive metals. Among these cells, the amount of capacity loss and CE is related to the type of inactive metal used and the electrolyte composition. For example, Cu/G and Ni/G electrodes have a relatively high CE with no-VC electrolyte but low CE with VC-containing electrolyte, while some electrodes, such as TiN/G, have high CE when cycled in both VC and no-VC electrolytes. Fig. 4 shows the total irreversible capacity versus cycle number (left column) and irreversible capacity per hour per unit area versus time−1/2 (right column) of the blended electrodes. Both working electrodes in each DHC were included in the calculation of irreversible capacity. The irreversible capacity grows with each cycle for all materials, but at different rates. In all materials there is a larger initial growth of
Generally, (within measurement standard deviation) the irreversible capacity is generally higher when VC is present in the electrolyte. This is expected, considering VC's tendency to reduce at the negative electrode. Fig. 2 shows initial differential capacity (dQ/dV) curves of half-cells containing inactive metal/graphite electrodes with control and VCcontaining electrolyte. The cathodic peaks are strongly related to the electrode compositions, but are not significantly by the presence of VC. For example, in both electrolytes Cu/G electrodes have one weak cathodic peak at 2.27 V and two strong cathodic peaks at 0.9 V and 1.3 V, but TiN/G electrodes only have one peak at ∼0.8 V. One explanation for these differences in dQ/dV curves is the electrochemical reduction of impurities, e.g. oxides, on the surface of inactive metals. Since TiN has a higher oxidation resistance than the elemental metal substrates [18–20], it has a relatively low initial irreversible capacity even when stored in air. Fig. 3 shows the cycling capacity and the coulombic efficiency (CE) of blended electrodes in DHCs. DHCs can accurately determine
Fig. 2. Initial discharge differential capacity of graphite and inactive metal/graphite electrodes in a potential range of 0.2–3 V. 376
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Fig. 3. Capacity (left column) and CE (right column) versus cycle number of graphite and inactive metal/graphite electrodes cycled in DHCs.
well. However, in the case of Cu and Ni in VC containing electrolyte, parabolic growth behavior is only followed for the first 5–10 cycles, after which a sharp drop in reactivity is seen. However, for these electrodes, the capacity is so far reduced at this point (Fig. 3), and the electrode impedance has grown so large (see below) that this is not surprizing, since these electrodes are essentially not functioning. The slope of the irreversible capacity per hour per unit area versus time−1/2 plot in the right column of Fig. 4 corresponds to the electrolyte reaction rate coefficient (k). To quantify the electrolyte reactivity on the inactive metals tested, the areal parasitic reaction rates of each constituent was deconvoluted from the other electrode components using the following equation [5]:
irreversible capacity, which then slows to a steady state growth after a number of cycles. For Ti, Fe, Mo, TiN containing electrodes, the irreversible capacity growth rate stabilizes after a few cycles and the relative growth is small in both VC and non-VC containing electrolyte. For Cu and Ni, the irreversible capacity growth rate is large, especially in VC containing electrolyte. In Fig. 4 (right column), the irreversible capacity per hour per unit inactive metal area of the blended electrodes is plotted to show the areal parasitic reaction rates on these metal surfaces. This plot is predicted to be linear according to a parabolic SEI growth law [5,22]:
r = kt −1/2
(1)
where r, k, and t represent the parasitic reaction rate per unit area, the electrolyte reaction rate coefficient, and time, respectively. This law has been shown to describe SEI growth on graphite well [22], which is also verified here. All of the electrodes shown in Fig. 4 also obey this law
kblend Sblend = kinactive Sinactive + kC SC
(2)
where k and S represent the reaction rate per unit area and the surface area of the whole electrode (blend), inactive component, and carbon/ 377
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Fig. 4. Cumulative irreversible capacity versus cycle number (left column) and irreversible capacity per hour per area versus time−1/2 (left column) of graphite and inactive metal/graphite electrodes cycled in DHCs.
materials at low potentials in Li-ion cells. However, these are the very materials that are used for the current collector and tabs for the negative electrode in essentially all commercial Li-ion cells. It should be noted that while cycling amongst duplicate cells was nominally identical, resulting in small error bars in k, large error bars still occurred when the reactivity was large (i.e. for SS in control electrolyte and Cu and Ni in VC containing electrolytes). This large reactivity resulted in deviations from the parabolic rate law. These deviations were responsible for the larger error bars in these samples, despite multiple replicates. In any case, it is clear that these samples have reactivity in VC containing electrolytes than the other metals that is much larger than can be explained by statistical deviation. Fig. 6 shows the impedance spectra of cycled graphite and inactive metal/graphite blended electrodes. The addition of VC additives increases the impedance of graphite electrodes. This impedance growth
graphite component, respectively, where kC was determined, from the electrolyte reactivity of the graphite electrodes. Fig. 5 shows k values of graphite and inactive metals in control electrolyte and VC-containing electrolyte. A high value of k indicates that faster parasitic reactions occurred. In both VC and non-VC containing electrolytes, Ti, Fe, Mo, and TiN have lower electrolyte reactivity than graphite. The electrolyte reactivity of Ti, Fe, Mo, and TiN in non-VC containing electrolyte is too low to be measured. Such materials are excellent candidates for use on the surfaces of current collectors and other cell components that are exposed to low potentials. Cu and Ni have low reactivity with electrolyte only if VC is not present. SS only has low reactivity only if VC is present and about the same reactivity as graphite if VC is not present. In contrast, Cu and Ni have high electrolyte reactivities that are 2–3.5 times higher than graphite in VC containing electrolyte. These are not good candidates for use as inactive 378
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Fig. 5. The k values of different materials, as determined from inactive metal/ graphite electrodes. Error bars were calculated from measurements of 2–3 sample replicates.
Fig. 7. Total electrode interfacial resistances, R, of graphite and blended electrodes in control and VC-containing electrolytes.
Fig. 8 shows the morphology of fresh electrodes and electrodes cycled in VC-containing electrolyte. Prior to cycling all electrode materials have smooth surfaces. After cycling, a thin conformal coating (the SEI layer) can be easily observed near particle edges on some materials, i.e. Ti, SS, Fe, Cu, Mo, and Ni. The SEI layer would be formed from a combination of the irreversible capacity during the first cycle and from electrolyte decomposition reactions during cycling. The SEI layer on TiN is relatively difficult to observe, which is consistent with the relatively low initial irreversible capacity and electrolyte reactivity on TiN. The same trend is observed in these electrodes when cycled in control electrolyte (as shown in Fig. S2). These electrochemical and SEM results confirm that inactive materials can participate in the parasitic reactions with electrolyte in negative electrodes, resulting in the formation of an SEI layer on their surfaces. It is important to consider the possible impact of these results on full cells. In VC containing electrolyte, the value of k measured here for graphite is 0.149 mA h0.5 m−2. Therefore, for a low surface area
has also been found previously [23]. The total electrode interfacial resistance (R) was obtained through the measurement of the diameter of mid-frequency semicircle(s) (as shown in Fig. 7). The R values of cycled electrodes are summarized in Fig. 7. All electrodes in non-VC containing electrolyte have relatively low resistance. The addition of VC increases the R value of all electrodes, especially for Cu, SS, Ni, (values large, but indeterminate, since semicircle(s) in EIS spectra are indistinct) and Ti containing electrodes. While SEI growth on the Ti and SS containing electrodes in VC containing electrolyte was relatively low, the SEI formed apparently caused high impedance growth. The high impedance growth of Cu and Ni containing electrodes in VC containing electrolyte is not surprizing, considering the large amount of electrolyte decomposition in these electrodes, and suggests substantial SEI growth. Electrodes containing TiN and Fe were found to have the lowest impedance growth in VC containing electrolytes and were comparable to the pure control graphite electrode. These materials are good candidates for inactive surfaces in Li-ion cells.
Fig. 6. Nyquist plots for graphite and blended electrodes in control and VC-containing electrolyte. 379
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Fig. 8. SEM images of fresh and cycled electrodes in VC-containing electrolyte. Scale bars represent a length of 500 nm. The arrows point out the edges of inactive metals where the SEI layer is easy to observe.
graphite of about 1.5 m2 g−1, this would correspond to 20% capacity fade in 3 years. The reactivity of Cu in VC containing electrolyte was found here to be k = 0.63 mA h0.5 m−2, much higher than that of graphite. However the surface area of Cu exposed to electrolyte will be much less than the plurality of graphite particles in a composite coating. Assuming a roughness factor of 5 for the Cu foil and a loading of 3.5 mAh cm−2, then a Cu current collector could account for 3% of cell fade in 3 years (or about 13% of the total cell fade). This is not insignificant and will depend highly on the actual surface roughness of the Cu foil. As our results show, capacity loss from the current collector might be significantly reduced by selecting better inactive materials.
addition, Cu and Ni had among the highest first cycle irreversible capacity. Such materials were deemed poor candidates for Li-ion cell components exposed to low potentials. However, these are the very materials that are used for the current collector and tabs for the negative electrode in essentially all commercial Li-ion cells! In contrast, all of the other inactive metals tested had lower reactivity than graphite in VC-containing electrolyte. Ti and TiN had the lowest reactivity with electrolyte during cycling, the lowest irreversible capacity, low impedance growth and showed no sign of electrolyte decomposition products on its surfaces. From these electrochemical results, Ti and TiN are very good candidate for inactive surfaces in LIBs in VC or non-VC containing electrolyte. These results show that the current inactive materials used in Li-ion cells may be contributing to cycling fade and that other much more stable materials exist that could improve cell lifetime and rate performance.
4. Conclusions The initial irreversible capacity, electrolyte reactivity, impedance growth, and morphology changes of Cu, Ni, 304 stainless steel, Ti, TiN, Mo, and Fe, during cycling at low potentials have been investigated in VC and non-VC containing electrolytes using recently developed double half-cells. The electrochemical behavior of inactive materials is strongly related to their composition and the composition of electrolytes. These inactive metals were found have reaction rates with electrolytes that are similar to graphite in non-VC containing electrolytes. However, in VC containing electrolytes, Cu and Ni reacted with electrolyte at 2–3.5 times the rate as graphite, with high concurrent impedance growth. In
Acknowledgments The authors acknowledge financial support from NSERC (IRCPJ 407487-09), 3 M Canada, the Canada Foundation for Innovation, and the Atlantic Innovation Fund for this work. Zilai Yan thanks the funding support from the Nova Scotia Graduate Scholarship. Zilai Yan would like to thank Dr. T.D. Hatchard, Patricia Scallion, Simon Trussler, and Dr. Xiang Yang for their kind assistance in the use of pop-off holders for 380
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SEM. Zilai Yan would also like to thank Dr. Xiang Yang for his assistance in operating SEM and to Dr. Jeff Dahn for use of his BET instrument.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Selecting inactive materials with low electrolyte reactivity for lithium-ion cells
T
Zilai Yana, M.N. Obrovaca,b,∗ a b
Department of Chemistry, Dalhousie University, Halifax, N. S. B3H 4R2, Canada Department of Physics and Atmospheric Science, Dalhousie University, Halifax, N. S. B3H 4R2, Canada
H I GH L IG H T S
reactivity was measured on inactive materials at the negative electrode. • Electrolyte used inactive materials, Cu and Ni, have high electrolyte reactivities. • Commonly reactivity at inactive materials can contribute to cell fade and impedance. • Electrolyte • Ti and TiN were found to be the least reactive inactive materials of those tested.
A R T I C LE I N FO
A B S T R A C T
Keywords: Inactive materials Electrolyte reactivity Cell fade SEI formation
Electrolyte reactivity on Cu, Ni, 304 stainless steel, Ti, TiN, Mo, and Fe, was investigated at the negative electrode in VC and non-VC containing electrolytes. TiN and Ti were found to have the lowest cathodic electrolyte reactivity in both VC and non-VC containing electrolytes, with a reactivity less than graphite. In contrast, inactive materials commonly used in Li-ion batteries at the negative electrodes, Cu (current collector) and Ni (tabs), have high irreversible capacities, high electrolyte reaction rates (2–3.5 times graphite) and high impedance growth. These studies show that the current materials widely in use today at the negative electrode could be contributing to fade and impedance growth in Li-ion batteries. The use of more inert materials, such as Ti or TiN, may result in an increase of Li-ion battery cycle life.
1. Introduction
still lacking. The types of inactive materials for LIBs are numerous and their electrochemical properties, such as electrocatalytic behavior and corrosion resistance, are significantly different [8]. Due to large differences in electrochemistry on inactive metal surfaces, tremendous efforts have been made in other fields for the development of novel materials for water splitting [9,10], oxygen reduction [11,12], and other electrocatalytic processes [13,14]. It is surprizing that few studies of electrolyte reactivity on inactive materials in LIBs have been undertaken, especially since electrolyte reactivity is a key parameter influencing cycle life. An excellent inactive material for LIB negative electrodes should have the following key properties: low initial irreversible capacity to increase the overall cell capacity; low long-term electrolyte reactivity to lengthen battery service life; and low electrochemical impedance to prevent Li plating, which can cause safety issues. In a previous study, we found that copper can have significantly higher electrolyte reactivity at the negative electrode compared to graphite in vinylene
To lengthen service life and to improve safety of lithium ion batteries (LIBs), tremendous efforts have been made to deepen our understanding of parasitic reactions between electrolytes and active materials, such as graphite and silicon [1–3]. At the same time, previous research has shown that inactive materials, e.g. copper and nickel, are also not fully inert to electrolytes at low potentials [4,5]. Inactive materials play important roles in Li-ion cells, such as in cell hardware (current collectors, tabs, the cell can, etc.), where they can be exposed to electrolyte at very high or very low potentials. Inactive materials are also used as components of active materials to restrict the volumetric expansion of electrode materials as a matrix phase, and even participate in conversion reactions with the solid-electrolyte interphase (SEI) layer [6,7]. Although inactive materials have been widely used in LIBs over decades, a comprehensive study of their interactions with electrolyte is
∗
Corresponding author. Department of Chemistry, Dalhousie University, Halifax, N. S. B3H 4R2, Canada. E-mail address: [email protected] (M.N. Obrovac).
https://doi.org/10.1016/j.jpowsour.2018.07.042 Received 13 February 2018; Received in revised form 24 May 2018; Accepted 9 July 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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carbonate (VC)-containing electrolytes [5]. This has serious implications for Li-ion cells, which almost universally use Cu-current collectors at the negative electrode and often use VC containing electrolytes. Therefore we decided to see if other materials might have less reactivity than Cu and therefore be more suitable for use as a current collector. In this paper, the initial irreversible capacity, electrolyte reactivity, electrochemical impedance, and the SEI morphology of a variety inactive metals at the negative electrode in Li-ion cells are discussed. Some materials were chosen for their practicality of use, such as copper, iron, nickel, and stainless steel, which are widely used in current LIBs. Other inactive metals, such as molybdenum, titanium, and titanium nitride were chosen because of their known electrochemical stability in other systems [15–17]. These inactive metals were comprehensively investigated for their stability at the negative electrode.
Table 2 Electrode compositions in weight percent.
Electrodes consisted of powdered inactive metal, graphite (MAG-E, Hitachi, 20 μm average size), carbon black (CB) (Super C65, Imery's Graphite and Carbon), and polyvinylidene fluoride (PVDF) (HSV 900, KYNAR). The inactive metal powders studied were Ti (99.9%, 60–80 nm, Skyspring Nanomaterials, Inc.), Fe (99%, 40–60 nm, SigmaAldrich), Mo (≥99.9%, 1–5 μm, Sigma-Aldrich), TiN (< 3 μm, SigmaAldrich), stainless steel (SS) (316L, 60–80 nm, Skyspring Nanomaterials, Inc.), Ni (99.8%, APS 0.08–0.15 μm, Alfa Aesar), and copper (99%, −625 mesh, Alfa Aesar). The specific surface areas (SSA) of these inactive powders are listed in Table 1. Inactive metal, graphite and carbon black components of the electrodes were formulated based on a (SA) ratio of inactive metal powder:graphite:CB = 47:16:37. All electrodes contained 8 wt% PVDF binder. The corresponding formulations of all electrodes based on weight percent are listed in Table 2. Electrodes were cast from 1-methyl-2-pyrrolidinone (NMP) (99.5%, Sigma-Aldrich) slurries onto copper foil with a 0.006-inch coating bar. This resulted in an electrode loading of about 4 mg of coating per square centimeter or about 1.5 mAh cm−2. Electrode loadings were purposely kept high to minimize the impact of parasitic currents from current collectors and cell hardware [17]. Graphite and TiN/G electrodes were coated in air and then air dried at 120 °C for 1 h. To prevent oxidation of elemental metal powders, the other electrodes were coated in an argon atmosphere and then vacuum dried at 120 °C overnight. These air-sensitive electrodes were stored in an argon-filled glovebox to prevent oxidation. Electrode disks were punched from electrode coatings using a 1.35 cm2 area circular punch. Coin cell construction was conducted in an Ar-filled glovebox using standard 2325 coin cell hardware. 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (1:2 v/v, all battery grade from BASF) was used as control electrolyte. Vinylene carbonate (VC) (BASF), was added to the control electrolyte at specific weight ratios. Two layers of Celgard-2300 separator and one interleaving layer of BMF separator (3 M Company) were used as cell separators. Three formats of cells were used in this study: half-cells, double half cells (DHCs), and conventional symmetric cells. A brief description of these three cell formats is presented here. More details can be found in Reference [5]. In half-cells, each 1.35 cm2 working electrode was paired with a 2.57 cm2 circular lithium metal foil counter electrode (thickness of 0.38 mm, 99.9%, Sigma-Aldrich). DHCs were constructed simply by connecting the Li terminals of two nominally identical half-cells. Construction pathways for making conventional symmetric cells and DHCs
Cu
TiN
Mo
Ni
Fe
SS
Ti
2.35(1)
62
1.54(4)
1.9(2)
2.1(1)
2.5(1)
11.4(2)
5.09(1)
12.8(2)
Inactive Materials
CB
PVDF
Graphite Cu/G TiN/G Mo/G Ni/G Fe/G SS/G Ti/G
90.0% 16.3% 15.0% 20.5% 23.6% 53.9% 37.2% 56.2%
N/A 74.3% 75.3% 69.7% 66.3% 33.3% 51.5% 30.8%
2.0% 1.4% 1.7% 1.8% 2.1% 4.8% 3.3% 5.0%
8.0% 8.0% 8.0% 8.0% 8.0% 8.0% 8.0% 8.0%
3. Results and discussion Fig. 1 shows the initial cycles of half-cells containing inactive metal/ graphite electrodes and a graphite electrode, for comparison, in control and +2 wt% VC electrolytes. All blended electrodes have a larger initial irreversible capacity than graphite. This increase of initial irreversible capacity is from the reduction reactions on the surface of inactive materials, which consume extra capacity. Blended electrodes were formulated based on the same surface area ratio of carbon and inactive materials. Thus, their irreversible capacity is a direct measure of the inactive component reactivity. Therefore, according to Fig. 1, TiN and Ti have the lower initial irreversible capacity per unit area than other inactive materials in control electrolyte and in +2 wt% VC electrolyte.
Table 1 Measured specific surface areas (in m2 g−1) of the inactive powders investigated. *As specified by the manufacturer. CB*
MAG-E
are illustrated in Fig. S1. One half cell discharged to 5 mV and then charged to 0.9 V at C/10 rate. The other half cell was discharged to 5 mV, charged to 0.9 V once, discharged to 5 mV at a C/10 rate, and then trickle discharged until the current was less than C/200. All cells were evaluated with a Maccor Series 4000 Automated Test System while the cells were thermostatically controlled at 30.0 ± 0.1 °C. DHCs cells were cycled at a C/10 rate. All specific cell capacities were calculated with respect to the mass of graphite in the electrodes. Electrochemical impedance spectroscopy (EIS) spectra were measured using a Bio-Logic VMP3 potentiostat. EIS measurements were performed at 30 °C, with a 10 mV amplitude excitation and a frequency range from 100 kHz to 10 mHz. Electrodes cycled over 240 h in DHCs were studied. Before electrodes were recovered from DHCs, DHCs were discharged or charged to 0 V at C/10 rate, and then trickled until the current dropped below C/200. The electrodes were then harvested from the cycled DHCs in an Ar-filled glovebox and then incorporated into conventional symmetric cells. The EIS experimental setup did not allow accurate solution resistance measurement due to cable and connection resistance. For this reason, all impedance spectra were shifted along with the real axis so that the highest frequency data point is located at 0 Ω cm2. The total electrode interfacial resistance, R, which includes the charge transfer resistance and the resistance due to ion diffusion in the SEI layer, was obtained from measuring the diameter of the mid-frequency semicircle(s) in the Nyquist plot. Unless otherwise specified, error bars were calculated as the range of two samples. The morphology of fresh and cycled electrodes was studied using a scanning electron microscope (SEM) (TESCAN MIRA3) with an accelerating voltage of 20 kV. After 240 h cycling and then over 30 days resting in DHCs, the cycled electrodes were recovered in an Ar-filled glove box, rinsed in dimethyl carbonate (DMC) (battery grade from BASF) to remove residual electrolyte salt and then dried under vacuum. Pop-off specimen holders (polytetrafluoroethylene, DPM solutions Inc.) were used to transfer air-sensitive samples from glovebox to the SEM without air exposure. Specific surface areas (SSAs) were measured using a Micromeritics Flowsorb II 2300 surface area analyzer by the single-point Brunauer-Emmett-Teller (BET) method. Prior to measurement, powders were placed in a glass holder and degassed at 160 °C for over half an hour. Elemental metal powders were loaded into the holder in an argon-filled glove box to prevent oxidation.
2. Experimental
MAG-E
Coating
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Fig. 1. Voltage curves of graphite and inactive metal/graphite electrodes in control electrolyte (solid lines) and in +2 wt% VC electrolyte (dashed lines). The initial CE is the average of four samples.
electrode CE based on their capacity fade, as described in Reference [21]. All electrodes suffer continual capacity fade in DHCs, which is indicative of irreversible electrolyte reactions. The reversible capacity of blended electrodes is due to the graphite component, which has an excellent capacity retention. Thus, the increased capacity fade of blended electrodes is caused by electrolyte decomposition on carbon and inactive metals. Among these cells, the amount of capacity loss and CE is related to the type of inactive metal used and the electrolyte composition. For example, Cu/G and Ni/G electrodes have a relatively high CE with no-VC electrolyte but low CE with VC-containing electrolyte, while some electrodes, such as TiN/G, have high CE when cycled in both VC and no-VC electrolytes. Fig. 4 shows the total irreversible capacity versus cycle number (left column) and irreversible capacity per hour per unit area versus time−1/2 (right column) of the blended electrodes. Both working electrodes in each DHC were included in the calculation of irreversible capacity. The irreversible capacity grows with each cycle for all materials, but at different rates. In all materials there is a larger initial growth of
Generally, (within measurement standard deviation) the irreversible capacity is generally higher when VC is present in the electrolyte. This is expected, considering VC's tendency to reduce at the negative electrode. Fig. 2 shows initial differential capacity (dQ/dV) curves of half-cells containing inactive metal/graphite electrodes with control and VCcontaining electrolyte. The cathodic peaks are strongly related to the electrode compositions, but are not significantly by the presence of VC. For example, in both electrolytes Cu/G electrodes have one weak cathodic peak at 2.27 V and two strong cathodic peaks at 0.9 V and 1.3 V, but TiN/G electrodes only have one peak at ∼0.8 V. One explanation for these differences in dQ/dV curves is the electrochemical reduction of impurities, e.g. oxides, on the surface of inactive metals. Since TiN has a higher oxidation resistance than the elemental metal substrates [18–20], it has a relatively low initial irreversible capacity even when stored in air. Fig. 3 shows the cycling capacity and the coulombic efficiency (CE) of blended electrodes in DHCs. DHCs can accurately determine
Fig. 2. Initial discharge differential capacity of graphite and inactive metal/graphite electrodes in a potential range of 0.2–3 V. 376
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Fig. 3. Capacity (left column) and CE (right column) versus cycle number of graphite and inactive metal/graphite electrodes cycled in DHCs.
well. However, in the case of Cu and Ni in VC containing electrolyte, parabolic growth behavior is only followed for the first 5–10 cycles, after which a sharp drop in reactivity is seen. However, for these electrodes, the capacity is so far reduced at this point (Fig. 3), and the electrode impedance has grown so large (see below) that this is not surprizing, since these electrodes are essentially not functioning. The slope of the irreversible capacity per hour per unit area versus time−1/2 plot in the right column of Fig. 4 corresponds to the electrolyte reaction rate coefficient (k). To quantify the electrolyte reactivity on the inactive metals tested, the areal parasitic reaction rates of each constituent was deconvoluted from the other electrode components using the following equation [5]:
irreversible capacity, which then slows to a steady state growth after a number of cycles. For Ti, Fe, Mo, TiN containing electrodes, the irreversible capacity growth rate stabilizes after a few cycles and the relative growth is small in both VC and non-VC containing electrolyte. For Cu and Ni, the irreversible capacity growth rate is large, especially in VC containing electrolyte. In Fig. 4 (right column), the irreversible capacity per hour per unit inactive metal area of the blended electrodes is plotted to show the areal parasitic reaction rates on these metal surfaces. This plot is predicted to be linear according to a parabolic SEI growth law [5,22]:
r = kt −1/2
(1)
where r, k, and t represent the parasitic reaction rate per unit area, the electrolyte reaction rate coefficient, and time, respectively. This law has been shown to describe SEI growth on graphite well [22], which is also verified here. All of the electrodes shown in Fig. 4 also obey this law
kblend Sblend = kinactive Sinactive + kC SC
(2)
where k and S represent the reaction rate per unit area and the surface area of the whole electrode (blend), inactive component, and carbon/ 377
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Fig. 4. Cumulative irreversible capacity versus cycle number (left column) and irreversible capacity per hour per area versus time−1/2 (left column) of graphite and inactive metal/graphite electrodes cycled in DHCs.
materials at low potentials in Li-ion cells. However, these are the very materials that are used for the current collector and tabs for the negative electrode in essentially all commercial Li-ion cells. It should be noted that while cycling amongst duplicate cells was nominally identical, resulting in small error bars in k, large error bars still occurred when the reactivity was large (i.e. for SS in control electrolyte and Cu and Ni in VC containing electrolytes). This large reactivity resulted in deviations from the parabolic rate law. These deviations were responsible for the larger error bars in these samples, despite multiple replicates. In any case, it is clear that these samples have reactivity in VC containing electrolytes than the other metals that is much larger than can be explained by statistical deviation. Fig. 6 shows the impedance spectra of cycled graphite and inactive metal/graphite blended electrodes. The addition of VC additives increases the impedance of graphite electrodes. This impedance growth
graphite component, respectively, where kC was determined, from the electrolyte reactivity of the graphite electrodes. Fig. 5 shows k values of graphite and inactive metals in control electrolyte and VC-containing electrolyte. A high value of k indicates that faster parasitic reactions occurred. In both VC and non-VC containing electrolytes, Ti, Fe, Mo, and TiN have lower electrolyte reactivity than graphite. The electrolyte reactivity of Ti, Fe, Mo, and TiN in non-VC containing electrolyte is too low to be measured. Such materials are excellent candidates for use on the surfaces of current collectors and other cell components that are exposed to low potentials. Cu and Ni have low reactivity with electrolyte only if VC is not present. SS only has low reactivity only if VC is present and about the same reactivity as graphite if VC is not present. In contrast, Cu and Ni have high electrolyte reactivities that are 2–3.5 times higher than graphite in VC containing electrolyte. These are not good candidates for use as inactive 378
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Fig. 5. The k values of different materials, as determined from inactive metal/ graphite electrodes. Error bars were calculated from measurements of 2–3 sample replicates.
Fig. 7. Total electrode interfacial resistances, R, of graphite and blended electrodes in control and VC-containing electrolytes.
Fig. 8 shows the morphology of fresh electrodes and electrodes cycled in VC-containing electrolyte. Prior to cycling all electrode materials have smooth surfaces. After cycling, a thin conformal coating (the SEI layer) can be easily observed near particle edges on some materials, i.e. Ti, SS, Fe, Cu, Mo, and Ni. The SEI layer would be formed from a combination of the irreversible capacity during the first cycle and from electrolyte decomposition reactions during cycling. The SEI layer on TiN is relatively difficult to observe, which is consistent with the relatively low initial irreversible capacity and electrolyte reactivity on TiN. The same trend is observed in these electrodes when cycled in control electrolyte (as shown in Fig. S2). These electrochemical and SEM results confirm that inactive materials can participate in the parasitic reactions with electrolyte in negative electrodes, resulting in the formation of an SEI layer on their surfaces. It is important to consider the possible impact of these results on full cells. In VC containing electrolyte, the value of k measured here for graphite is 0.149 mA h0.5 m−2. Therefore, for a low surface area
has also been found previously [23]. The total electrode interfacial resistance (R) was obtained through the measurement of the diameter of mid-frequency semicircle(s) (as shown in Fig. 7). The R values of cycled electrodes are summarized in Fig. 7. All electrodes in non-VC containing electrolyte have relatively low resistance. The addition of VC increases the R value of all electrodes, especially for Cu, SS, Ni, (values large, but indeterminate, since semicircle(s) in EIS spectra are indistinct) and Ti containing electrodes. While SEI growth on the Ti and SS containing electrodes in VC containing electrolyte was relatively low, the SEI formed apparently caused high impedance growth. The high impedance growth of Cu and Ni containing electrodes in VC containing electrolyte is not surprizing, considering the large amount of electrolyte decomposition in these electrodes, and suggests substantial SEI growth. Electrodes containing TiN and Fe were found to have the lowest impedance growth in VC containing electrolytes and were comparable to the pure control graphite electrode. These materials are good candidates for inactive surfaces in Li-ion cells.
Fig. 6. Nyquist plots for graphite and blended electrodes in control and VC-containing electrolyte. 379
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Fig. 8. SEM images of fresh and cycled electrodes in VC-containing electrolyte. Scale bars represent a length of 500 nm. The arrows point out the edges of inactive metals where the SEI layer is easy to observe.
graphite of about 1.5 m2 g−1, this would correspond to 20% capacity fade in 3 years. The reactivity of Cu in VC containing electrolyte was found here to be k = 0.63 mA h0.5 m−2, much higher than that of graphite. However the surface area of Cu exposed to electrolyte will be much less than the plurality of graphite particles in a composite coating. Assuming a roughness factor of 5 for the Cu foil and a loading of 3.5 mAh cm−2, then a Cu current collector could account for 3% of cell fade in 3 years (or about 13% of the total cell fade). This is not insignificant and will depend highly on the actual surface roughness of the Cu foil. As our results show, capacity loss from the current collector might be significantly reduced by selecting better inactive materials.
addition, Cu and Ni had among the highest first cycle irreversible capacity. Such materials were deemed poor candidates for Li-ion cell components exposed to low potentials. However, these are the very materials that are used for the current collector and tabs for the negative electrode in essentially all commercial Li-ion cells! In contrast, all of the other inactive metals tested had lower reactivity than graphite in VC-containing electrolyte. Ti and TiN had the lowest reactivity with electrolyte during cycling, the lowest irreversible capacity, low impedance growth and showed no sign of electrolyte decomposition products on its surfaces. From these electrochemical results, Ti and TiN are very good candidate for inactive surfaces in LIBs in VC or non-VC containing electrolyte. These results show that the current inactive materials used in Li-ion cells may be contributing to cycling fade and that other much more stable materials exist that could improve cell lifetime and rate performance.
4. Conclusions The initial irreversible capacity, electrolyte reactivity, impedance growth, and morphology changes of Cu, Ni, 304 stainless steel, Ti, TiN, Mo, and Fe, during cycling at low potentials have been investigated in VC and non-VC containing electrolytes using recently developed double half-cells. The electrochemical behavior of inactive materials is strongly related to their composition and the composition of electrolytes. These inactive metals were found have reaction rates with electrolytes that are similar to graphite in non-VC containing electrolytes. However, in VC containing electrolytes, Cu and Ni reacted with electrolyte at 2–3.5 times the rate as graphite, with high concurrent impedance growth. In
Acknowledgments The authors acknowledge financial support from NSERC (IRCPJ 407487-09), 3 M Canada, the Canada Foundation for Innovation, and the Atlantic Innovation Fund for this work. Zilai Yan thanks the funding support from the Nova Scotia Graduate Scholarship. Zilai Yan would like to thank Dr. T.D. Hatchard, Patricia Scallion, Simon Trussler, and Dr. Xiang Yang for their kind assistance in the use of pop-off holders for 380
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SEM. Zilai Yan would also like to thank Dr. Xiang Yang for his assistance in operating SEM and to Dr. Jeff Dahn for use of his BET instrument.
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