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Improved swelling behavior of Li ion batteries by microstructural engineering of anode
Journal of Industrial and Engineering Chemistry 71 (2019) 270–276
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Improved swelling behavior of Li ion batteries by microstructural engineering of anode Keemin Parka , Seungcheol Myeonga , Donghyeok Shina , Chae-Woong Chob , Soo Chan Kimb , Taeseup Songa,* a b
Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea Lab) Electrode Development Group, Samsung SDI, Suwon, Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 August 2018 Received in revised form 15 November 2018 Accepted 18 November 2018 Available online 26 November 2018
Swelling behavior of the graphite electrode hinders the increase in volumetric density of Li ion batteries as the free space in the cell is necessary to ensure battery safety by accommodating the volume change of the anode associated with Li ions. Here, we report a simple electrode microstructural engineering strategy to control swelling behavior of the anode by employing a two-step pressing process. Two-step pressing enables the uniform distribution of the pores throughout the electrode without cracking or pulverization of the active material. The anode prepared by a two-step pressing process under an optimized condition exhibits superior swelling behavior (4.47% swelling after 25 cycles) compared to that of the anode prepared with a one-step pressing process (5.00% swelling after 25 cycles). The electrochemical properties could be also further improved due to a uniform pore distribution and enhanced adhesion strength. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Swelling Lithium Ion Batteries Two-step Pressing Microstructure
Introduction The development of lithium ion batteries (LIBs) with high volumetric energy and power densities is crucial to their use in electric vehicles and to meet the demand of prolonged operation in portable electronic devices [1–5]. Although various electrode materials with high gravimetric and volumetric capacities, such as Si anode and polyanion-type cathode materials, have been intensively studied to replace the active materials adopted in commercial LIBs, their practical use has been limited due to unsatisfactory electrochemical properties [6–11]. The high volumetric energy density of commercial LIBs has been achieved by employing electrodes with high density and high active material loading. However, the decrease in pore size and volume with increasing density and loading level in the electrode results in the degradation of its electrochemical properties due to poor electrolyte permeability [12–14]. More importantly, the problems resulting from the swelling of the graphite electrodes due to the expansion of the crystalline lattice during lithiation as well as the formation of a solid electrolyte interface (SEI) layer become more severe with increasing density and loading level [15–18]. The
* Corresponding author. E-mail address: [email protected] (T. Song).
swelling behavior of the electrode is also directly related to its safety, especially battery swelling and its potential for explosion. Therefore, a spatial margin in the battery cell is necessary to accommodate the volume change of the anode associated with Li ions, which induces a decrease in the volumetric capacity of the cell. For this reason, it is crucial to suppress anode swelling to achieve high volumetric capacity in the cell and improve its safety. To realize high volumetric capacity of the cell and improve its safety, many researchers have investigated swelling behavior during battery cycling [17,19,20]. Lee et al. developed an in situ thickness measuring system and quantitatively analyzed thickness changes in the cell during cycling processes that occur in various types of cells [17]. Zhang et al. designed an asymmetric electrode configuration with different loading levels of active material on the front and back sides of the current collector to suppress electrode swelling by inducing compressive stress in the jelly-roll [19]. Although various approaches have been suggested to address the electrode swelling issue, further improvement by a simple and facile method is necessary to achieve LIBs with high volumetric energy and power densities. Here, we report a novel electrode processing strategy to achieve LIBs with high volumetric energy density by controlling the swelling behavior of the anode. Generally, a one-step roll pressing process on the electrode has been employed to adjust the target density of the electrode in the conventional LIBs manufacturing
https://doi.org/10.1016/j.jiec.2018.11.035 1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
K. Park et al. / Journal of Industrial and Engineering Chemistry 71 (2019) 270–276
process. Higher roll pressing pressure is necessary to achieve higher volumetric capacity. However, harsh pressing conditions induce high stress and/or damage, such as cracking or pulverization, in the active material as well as nonhomogeneous pore distribution in the electrode (Scheme 1), which leads to poor electrochemical properties and severe electrode swelling behavior. To address these issues, we employed a two-step pressing process on the electrode. The first soft roll pressing enables the smooth reorientation of the graphite perpendicular to the pressing force, which reduces stress and mechanical damage within the graphite and promotes uniform pore distribution throughout the electrode. The target density of the electrode is adjusted by the following second roll pressing. The electrode prepared by the two-step roll pressing process exhibits significantly reduced spring back and swelling behaviors. The anodes prepared by a two-step pressing process and one-step pressing process exhibit the 4.47% swelling and 5.00% swelling after 25 cycles, respectively as the uniform pore distribution in the electrode prepared by a two-step pressing process plays an important role on the efficient relaxation of the stress associated with Li ion. Furthermore, uniform pore distribution in the electrode improves the electrolyte permeability, Li ion kinetics, and adhesion strength, which improves the electrochemical properties. The electrode with two-step pressing process exhibits enhanced cycle and rate retention compared to those of the electrode with one-step pressing process (Cycle retention: 85.83/82.77% @0.5C, Rate retention: 89.22/87.72% @2C/0.2C ratio, respectively). Our simple and novel strategy for electrode configuration engineering could be utilized directly for battery manufacturing at the commercial level. Experimental To prepare the slurry, graphite, 1.0 wt% aqueous carboxymethyl cellulose (CMC) solution, and 40% styrene butadiene rubber (SBR) emulsion were put into a vial at a weight ratio of 97.8: 1.0: 1.2 based on solid state. Then, the slurry was mixed at 2000 rpm for 5 min,
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coated onto a copper current collector (thickness 6 mm) using a doctor blade, and dried at 110 C for 10 min. After drying, the thickness of the unpressed electrode was approximately 110 mm. To achieve a target density of 1.6 g cc1 at the graphite electrode, the thickness of the electrode must be approximately 70 mm, which was obtained by controlling the roll pressing process under different conditions. Table 1 shows information on the samples obtained through this pressing process. In this table, first pressing refers to the ratio of compression in the first pressing at a thickness of 40 mm that should be reduced to achieve a target density of 1.6 g cc1 in the graphite electrode. The remaining thickness was compressed through the second pressing. For example, in 100%–0% electrode, the value of the first pressing is 100%. This means that the thickness of 40 mm was reduced by the first pressing without a second pressing. In the case of the 40%–100% electrode, the value of the first pressing is 40%, which means that 16 mm corresponding to 40% of 40 mm was reduced through the first pressing, and the remaining 24 mm were reduced through the second pressing. The as-prepared graphite electrodes were characterized using various instruments as follows. The cross-sectional images of the electrodes were observed by a field-emission scanning electron microscope (FE-SEM, JSM-7600F, JEOL, Tokyo, Japan). An X-ray microscope (Xradia 810 Ultra, ZEISS, Germany) was used to assess the pore distribution of the graphite electrodes. For the electrolyte permeability, 20 mg of the electrolyte (1.15 M LiPF6 in EC/DEC/DMC (3:5:2 v/v) + 5% FEC) was dropped onto the surface of the graphite electrode. At the same time, the time required for the electrolyte to completely penetrate into the graphite electrode was measured. A universal testing machine (AGS-J, Shimadzu, Kyoto, Japan) was used to measure the adhesion strength by the peeling test. Defect level of the graphite electrodes was measured using Raman spectroscopy (LabRam HR, Horiba, France). For the full cell test, the cathode was produced with LiCoO2 (LCO), carbon black, and polyvinylidene fluoride at a weight ratio of 97.7: 1.3: 1.0. The mass loading level and the density of LCO and graphite electrodes were 21/11.2 mg cm2 and 4.1/1.6 g cc1,
Scheme 1. Schematic illustration of the graphite electrode configuration depending on pressing condition.
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Table 1 Thickness and density of three graphite electrodes based on first and second pressing conditions. First pressing
100%–0% electrode 40%–100% electrode 75%–100% electrode
Second pressing
Pressing (%)
Thickness (mm)
Density (g cc
100 40 75
70.9 93.8 81.9
1.58 1.19 1.37
1
)
Pressing (%)
Thickness (mm)
Density (g cc1)
N/A 100 100
N/A 70.0 70.3
N/A 1.60 1.59
Fig. 1. SEM cross-sectional images of the non, first, and second pressing graphite electrodes after drying with three different conditions: 100%–0%, 40%–100%, and 75%–100%.
K. Park et al. / Journal of Industrial and Engineering Chemistry 71 (2019) 270–276
respectively. The prepared electrode was placed in an aluminum pouch, and an electrolyte (1.15 M LiPF6 in EC/DEC/DMC (3:5:2 v/v) + FEC 5%) was inserted. The thickness of the pouch full cell was measured in situ using a linear gauge (LGK-0110, Mitutoyo, Japan) with constant loading during cycling. All the electrochemical properties were tested using a battery cycle tester (TOSCAT 3000, Toyo System, Tokyo, Japan). Electrochemical measuring equipment (ZIVE BP2A, Won-A tech, Korea) was used to measure the electrochemical impedance spectroscopy (EIS) spectra. Results and discussion To explore the pressing condition effects on the electrochemical properties and swelling behavior, three types of graphite electrodes were prepared as a function of the roll pressing process condition; (1) the electrode prepared by the one-step roll pressing condition (subsequently referred to as the “100%–0% electrode”); (2) the electrode prepared by the two-step roll pressing conditions — 40% pressing of the target density at the first pressing and the second pressing to achieve an electrode density of 1.6 g cc1 (subsequently referred to as the “40%–100% electrode”); and (3) 75% pressing of the target density at the first pressing and the second pressing to achieve an electrode density of 1.6 g cc1 (subsequently referred to as the “75%–100% electrode”). Scheme 1 shows a schematic illustration of the graphite electrode configuration depending on pressing condition. Even though the electrodes have the same apparent density, the pore distribution and the arrangement of the graphite particles in the electrode could be different depending on the pressing process conditions. If the pressing is performed with a one-step process, the stress applied to the graphite could be relatively high. The induced stress caused by applied high pressure is relaxed in such a way that the graphite active materials are broken before the arrangement of the graphite is sufficiently reoriented, resulting in a graphite electrode with uneven pore distribution. On the other hand, when the first pressing condition is soft, such as the 40%– 100% condition, the graphite particles could be reoriented rather than cracking to relieve the stress applied to the graphite, which leads to the uniform distribution of the pores. These results were confirmed by the intensity ratio of D and G peaks (ID/IG) in Raman spectroscopy on the graphite electrode surface after one-step pressing and two-step pressing (Fig. S1). In Raman spectroscopy, ID/IG implies the defect level in the graphite particles, and the larger is the ID/IG value, the more defects are present [21–23]. When graphite particles become more disordered due to stress, the defect ratio of the graphite particles increases, and the value of ID/ IG increases [24]. It is clearly shown that the graphite electrode
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prepared with two-step pressing has lower ID/IG value compared to that of the graphite electrode prepared with one-step pressing. Fig. 1 shows the cross-sectional SEM images of the graphite electrodes as a function of the pressing condition. The thickness and apparent density of the electrodes, corresponding to the pressing condition, was summarized in Table 1. The as casted graphite electrode (without pressing) has the thickness of 109 mm. After the pressing process, the 100%–0%, 40%–100%, and 75%–100% graphite electrodes have the thickness of around 70 mm and apparent density of around 1.6 g cc1. Based on the SEM images, noticeable difference in the microstructure was not observed. However, it is clear that two-step pressing is helpful to reduce the defect generation caused by high pressure pressing (Fig. S1). The three-dimensional X-ray microscopy (3D-XRM) reconstruction method was employed to understand the pore distribution in the graphite electrodes depending on the pressing condition. This method have been widely used on graphite-based anodes to analyze the pore distribution of the electrode [25,26]. Fig. 2(a) shows the 3D-XRM reconstruction images for the 100%– 0%, 40%–100%, and 75%–100% graphite electrodes. It is observed that the electrodes prepared with two-step pressing have lager pore size compared to that of the electrode prepared with one-step pressing. These large pore plays an important role on the electrolyte permeation and Li ion kinetics [27]. The reconstructed image of the graphite electrode was divided into top/middle/ bottom layers, and the porosity of each layer was analyzed (Fig. 2(b)). The 40%–100% electrode exhibits the most uniform pore distribution at top/middle/bottom layers with 25.61/26.79/25.50%. The 100%–0% and 75%–100% graphite electrodes have high porosity at the top layer and lower porosity at the bottom layer. This nonuniformity of the pore distribution could induce significant swelling behavior and poor Li ion kinetics. Based on the reconstruction images, the pore size distribution in the overall electrode and the top/middle/bottom layers for the 100%–0%, 40%– 100%, and 75%–100% graphite electrodes are displayed in Fig. S2. For the 40%–100% electrode, the volume fraction of pore size with 1.4 mm accounted for 55% of the total pore volume and showed the most uniform pore size and volume distribution. However, the 100%–0% and 75%–100% graphite electrodes have broad pore size distribution in overall electrode. Based on these results, it is expected that the different rearrangement behavior of graphite particles during the first pressing could induce different pore size and volume distribution even under the identical apparent electrode density. The electrolyte permeability is closely related to the pore distribution of the electrode [27], which plays an important role on the electrode swelling behavior. Fig. 3(a) shows the electrolyte
Fig. 2. (a) 3D-XRM reconstruction images with overall electrode and (b) porosity of the top, middle, and bottom layers of the 100%–0%, 40%–100%, and 75%–100% electrodes.
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Fig. 3. (a) Electrolyte permeability in terms of the time it takes for the electrolyte to penetrate completely through the electrode. (b) Adhesion strength measured by peeling test.
permeation characteristics of the 100%–0%, 40%–100%, and 75%– 100% graphite electrodes. The 40%–100% electrode exhibits the best electrolyte permeation behavior compared to those of other two electrodes, which is attributed to the uniform pore distribution in top/middle/bottom layers in the electrode. The adhesion strength between the Cu substrate and the graphite-binder composite film for the three electrodes was measured by the peeling test (Fig. 3(b)) [28,29]. The adhesion strength related to the pore and binder distribution of the electrode could affect the electrochemical properties as well as the swelling behavior. The highest adhesion strength could be obtained in the 40%–100% electrode. This result indicates that the adhesion strength could be also controlled by adjusting pressing conditions. However, further study is necessary to understand the relationship between the pressing condition and adhesion strength. The swelling behavior of the three electrodes were monitored during cycling (Fig. 4). The pouch full cell consisting of LCO and graphite electrodes was prepared and cycled at 0.5C with the twostep formation (0.1 and 0.2C) at the initial two cycles. As the swelling behavior of the pouch full cell is mainly ascribed to a volume change in the graphite electrode associated with Li ion intercalation/deintercalation [30], we assumed that the thickness change of the pouch full cell results from the thickness change of the graphite electrode. To quantify the degree of the electrode swelling, a swelling ratio is calculated as thickness change during
Fig. 4. Swelling ratio of the graphite electrodes measured by the pouch full cell consisting of an LCO and graphite electrodes during two-step formation (0.1 and 0.2C) and cycling (0.5C).
cycling divided by the initial thickness. swelling ratio ¼
thickness change initial thickness
To better understand the swelling behavior depending on the pressing condition, we introduced following two concepts; initial and cycling swelling ratio. The initial swelling ratio refers to the ratio of thickness swelling occurring during the charging process (lithiation to the graphite electrode) of the first formation step (0.1C). This initial swelling ratio, as reported in a previous report, is largely related to the formation of the SEI layer by electrolyte decomposition on the graphite surface [17], the lattice expansion of the graphite particles resulting from Li ion intercalation, and stacking variation of the graphite particles by the lattice expansion [20]. The cycling swelling ratio refers to the difference in swelling ratio between 0 and 25 cycles at the discharged state after initial two formation cycling. This behavior is attributed to the accumulation of microscopic stress applied to graphite during Li ion intercalation/deintercalation in the cycling process, which causes macroscopic stress on the graphite electrode and consequently swells it [20]. The 40%–100% graphite electrode exhibits much reduced swelling behavior compared to 100%–0% and 75%– 100% graphite electrodes, with an initial/cycling swelling ratio of 19.23/4.47%. The uniform pore size and distribution in the 40%– 100% electrode effectively relieves the stress from the formation of the SEI layer and accommodates the volume change associated with Li ion during cycling, which significantly reduces battery swelling. The thickness change of the electrodes after 25 cycles was also observed by SEM (Fig. S3). The 100%–0%, 40%–100%, and 75%– 100% graphite electrodes have the thickness of 83.8, 81.9 and 83.4 mm, respectively. The 40%–100% graphite electrode exhibits the least thickness expansion. This result is well matched with swelling behavior. The electrochemical properties of the pouch full cells with 100%–0%, 40%–100%, and 75%–100% graphite electrodes were also evaluated. Fig. 5(a) shows the voltage profiles at the current density of 0.5C after the formation step. The cells with 100%–0%, 40%–100%, and 75%–100% graphite electrodes deliver the charge/ discharge capacities of 157.61/152.07, 156.56/152.13, and 157.33/ 152.58 mAh g1, respectively. Especially, the cell with 40%–100% graphite electrode exhibits the low polarization during charging and discharging, which is related to the improved Li ion kinetics caused by the electrode microstructure with uniform pore distribution. The cycle performances of the cells were also carefully monitored for 200 cycles (Fig. 5(b)). The cell with 40%– 100% graphite electrode also exhibits improved cycling performance (85.83%) compared to those of the cells with 100%–0% and
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0% and 75%–100% graphite electrodes. EIS spectra in Fig. S4 revealed the lowest charge transfer resistance (Rct) in the 40%– 100% electrode, which shows good correlation with electrolyte permeability and rate capability results. The electrochemical property results including the swelling behavior, polarization during cycling, cycle retention and rate capability clearly indicate that the electrode configuration engineering plays an important role on the improvement in the performance and reliability of LIBs. Conclusions The microstructural engineering of the graphite electrode was achieved by simple two-step pressing condition control. The first softened pressing enables the rearrangement of the graphite particles, which induces the uniform pore size and pore distribution in overall electrode without introducing defects in the graphite particle, resulting in the improvement in the swelling behavior and electrochemical properties. Our approach could be expanded to the cathode for LIBs as well as the electrodes for fuel cell devices that need uniform pore size and pore distribution characteristics. Acknowledgment This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20168510050080) and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy, Republic of Korea (No. 20174010201240). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.11.035. References
Fig. 5. Electrochemical properties of the pouch full cell consisting of an LCO and graphite electrodes with different pressing processes: (a) voltage profiles at 0.5C after two-step formation at 0.1 and 0.2C, (b) cycle stability over 200 cycles at 0.5C, and (c) rate capability from 0.2 to 2C.
75%–100% graphite electrodes (82.77% and 83.10%). The enhanced adhesion strength and reduced swelling degree in the 40%–100% graphite electrode enables the improvement in the cycle retention. Fig. 5(c) shows the rate capability of the cells. Even though the cells show the similar rate capability under low current density (0.2C and 0.5C), the cell with 40%–100% graphite electrode exhibits improved rate capability at high C rates (1C and 2C) due to the enhanced Li ion kinetics compared to those of the cells with 100%–
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Improved swelling behavior of Li ion batteries by microstructural engineering of anode Keemin Parka , Seungcheol Myeonga , Donghyeok Shina , Chae-Woong Chob , Soo Chan Kimb , Taeseup Songa,* a b
Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea Lab) Electrode Development Group, Samsung SDI, Suwon, Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 August 2018 Received in revised form 15 November 2018 Accepted 18 November 2018 Available online 26 November 2018
Swelling behavior of the graphite electrode hinders the increase in volumetric density of Li ion batteries as the free space in the cell is necessary to ensure battery safety by accommodating the volume change of the anode associated with Li ions. Here, we report a simple electrode microstructural engineering strategy to control swelling behavior of the anode by employing a two-step pressing process. Two-step pressing enables the uniform distribution of the pores throughout the electrode without cracking or pulverization of the active material. The anode prepared by a two-step pressing process under an optimized condition exhibits superior swelling behavior (4.47% swelling after 25 cycles) compared to that of the anode prepared with a one-step pressing process (5.00% swelling after 25 cycles). The electrochemical properties could be also further improved due to a uniform pore distribution and enhanced adhesion strength. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Swelling Lithium Ion Batteries Two-step Pressing Microstructure
Introduction The development of lithium ion batteries (LIBs) with high volumetric energy and power densities is crucial to their use in electric vehicles and to meet the demand of prolonged operation in portable electronic devices [1–5]. Although various electrode materials with high gravimetric and volumetric capacities, such as Si anode and polyanion-type cathode materials, have been intensively studied to replace the active materials adopted in commercial LIBs, their practical use has been limited due to unsatisfactory electrochemical properties [6–11]. The high volumetric energy density of commercial LIBs has been achieved by employing electrodes with high density and high active material loading. However, the decrease in pore size and volume with increasing density and loading level in the electrode results in the degradation of its electrochemical properties due to poor electrolyte permeability [12–14]. More importantly, the problems resulting from the swelling of the graphite electrodes due to the expansion of the crystalline lattice during lithiation as well as the formation of a solid electrolyte interface (SEI) layer become more severe with increasing density and loading level [15–18]. The
* Corresponding author. E-mail address: [email protected] (T. Song).
swelling behavior of the electrode is also directly related to its safety, especially battery swelling and its potential for explosion. Therefore, a spatial margin in the battery cell is necessary to accommodate the volume change of the anode associated with Li ions, which induces a decrease in the volumetric capacity of the cell. For this reason, it is crucial to suppress anode swelling to achieve high volumetric capacity in the cell and improve its safety. To realize high volumetric capacity of the cell and improve its safety, many researchers have investigated swelling behavior during battery cycling [17,19,20]. Lee et al. developed an in situ thickness measuring system and quantitatively analyzed thickness changes in the cell during cycling processes that occur in various types of cells [17]. Zhang et al. designed an asymmetric electrode configuration with different loading levels of active material on the front and back sides of the current collector to suppress electrode swelling by inducing compressive stress in the jelly-roll [19]. Although various approaches have been suggested to address the electrode swelling issue, further improvement by a simple and facile method is necessary to achieve LIBs with high volumetric energy and power densities. Here, we report a novel electrode processing strategy to achieve LIBs with high volumetric energy density by controlling the swelling behavior of the anode. Generally, a one-step roll pressing process on the electrode has been employed to adjust the target density of the electrode in the conventional LIBs manufacturing
https://doi.org/10.1016/j.jiec.2018.11.035 1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
K. Park et al. / Journal of Industrial and Engineering Chemistry 71 (2019) 270–276
process. Higher roll pressing pressure is necessary to achieve higher volumetric capacity. However, harsh pressing conditions induce high stress and/or damage, such as cracking or pulverization, in the active material as well as nonhomogeneous pore distribution in the electrode (Scheme 1), which leads to poor electrochemical properties and severe electrode swelling behavior. To address these issues, we employed a two-step pressing process on the electrode. The first soft roll pressing enables the smooth reorientation of the graphite perpendicular to the pressing force, which reduces stress and mechanical damage within the graphite and promotes uniform pore distribution throughout the electrode. The target density of the electrode is adjusted by the following second roll pressing. The electrode prepared by the two-step roll pressing process exhibits significantly reduced spring back and swelling behaviors. The anodes prepared by a two-step pressing process and one-step pressing process exhibit the 4.47% swelling and 5.00% swelling after 25 cycles, respectively as the uniform pore distribution in the electrode prepared by a two-step pressing process plays an important role on the efficient relaxation of the stress associated with Li ion. Furthermore, uniform pore distribution in the electrode improves the electrolyte permeability, Li ion kinetics, and adhesion strength, which improves the electrochemical properties. The electrode with two-step pressing process exhibits enhanced cycle and rate retention compared to those of the electrode with one-step pressing process (Cycle retention: 85.83/82.77% @0.5C, Rate retention: 89.22/87.72% @2C/0.2C ratio, respectively). Our simple and novel strategy for electrode configuration engineering could be utilized directly for battery manufacturing at the commercial level. Experimental To prepare the slurry, graphite, 1.0 wt% aqueous carboxymethyl cellulose (CMC) solution, and 40% styrene butadiene rubber (SBR) emulsion were put into a vial at a weight ratio of 97.8: 1.0: 1.2 based on solid state. Then, the slurry was mixed at 2000 rpm for 5 min,
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coated onto a copper current collector (thickness 6 mm) using a doctor blade, and dried at 110 C for 10 min. After drying, the thickness of the unpressed electrode was approximately 110 mm. To achieve a target density of 1.6 g cc1 at the graphite electrode, the thickness of the electrode must be approximately 70 mm, which was obtained by controlling the roll pressing process under different conditions. Table 1 shows information on the samples obtained through this pressing process. In this table, first pressing refers to the ratio of compression in the first pressing at a thickness of 40 mm that should be reduced to achieve a target density of 1.6 g cc1 in the graphite electrode. The remaining thickness was compressed through the second pressing. For example, in 100%–0% electrode, the value of the first pressing is 100%. This means that the thickness of 40 mm was reduced by the first pressing without a second pressing. In the case of the 40%–100% electrode, the value of the first pressing is 40%, which means that 16 mm corresponding to 40% of 40 mm was reduced through the first pressing, and the remaining 24 mm were reduced through the second pressing. The as-prepared graphite electrodes were characterized using various instruments as follows. The cross-sectional images of the electrodes were observed by a field-emission scanning electron microscope (FE-SEM, JSM-7600F, JEOL, Tokyo, Japan). An X-ray microscope (Xradia 810 Ultra, ZEISS, Germany) was used to assess the pore distribution of the graphite electrodes. For the electrolyte permeability, 20 mg of the electrolyte (1.15 M LiPF6 in EC/DEC/DMC (3:5:2 v/v) + 5% FEC) was dropped onto the surface of the graphite electrode. At the same time, the time required for the electrolyte to completely penetrate into the graphite electrode was measured. A universal testing machine (AGS-J, Shimadzu, Kyoto, Japan) was used to measure the adhesion strength by the peeling test. Defect level of the graphite electrodes was measured using Raman spectroscopy (LabRam HR, Horiba, France). For the full cell test, the cathode was produced with LiCoO2 (LCO), carbon black, and polyvinylidene fluoride at a weight ratio of 97.7: 1.3: 1.0. The mass loading level and the density of LCO and graphite electrodes were 21/11.2 mg cm2 and 4.1/1.6 g cc1,
Scheme 1. Schematic illustration of the graphite electrode configuration depending on pressing condition.
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Table 1 Thickness and density of three graphite electrodes based on first and second pressing conditions. First pressing
100%–0% electrode 40%–100% electrode 75%–100% electrode
Second pressing
Pressing (%)
Thickness (mm)
Density (g cc
100 40 75
70.9 93.8 81.9
1.58 1.19 1.37
1
)
Pressing (%)
Thickness (mm)
Density (g cc1)
N/A 100 100
N/A 70.0 70.3
N/A 1.60 1.59
Fig. 1. SEM cross-sectional images of the non, first, and second pressing graphite electrodes after drying with three different conditions: 100%–0%, 40%–100%, and 75%–100%.
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respectively. The prepared electrode was placed in an aluminum pouch, and an electrolyte (1.15 M LiPF6 in EC/DEC/DMC (3:5:2 v/v) + FEC 5%) was inserted. The thickness of the pouch full cell was measured in situ using a linear gauge (LGK-0110, Mitutoyo, Japan) with constant loading during cycling. All the electrochemical properties were tested using a battery cycle tester (TOSCAT 3000, Toyo System, Tokyo, Japan). Electrochemical measuring equipment (ZIVE BP2A, Won-A tech, Korea) was used to measure the electrochemical impedance spectroscopy (EIS) spectra. Results and discussion To explore the pressing condition effects on the electrochemical properties and swelling behavior, three types of graphite electrodes were prepared as a function of the roll pressing process condition; (1) the electrode prepared by the one-step roll pressing condition (subsequently referred to as the “100%–0% electrode”); (2) the electrode prepared by the two-step roll pressing conditions — 40% pressing of the target density at the first pressing and the second pressing to achieve an electrode density of 1.6 g cc1 (subsequently referred to as the “40%–100% electrode”); and (3) 75% pressing of the target density at the first pressing and the second pressing to achieve an electrode density of 1.6 g cc1 (subsequently referred to as the “75%–100% electrode”). Scheme 1 shows a schematic illustration of the graphite electrode configuration depending on pressing condition. Even though the electrodes have the same apparent density, the pore distribution and the arrangement of the graphite particles in the electrode could be different depending on the pressing process conditions. If the pressing is performed with a one-step process, the stress applied to the graphite could be relatively high. The induced stress caused by applied high pressure is relaxed in such a way that the graphite active materials are broken before the arrangement of the graphite is sufficiently reoriented, resulting in a graphite electrode with uneven pore distribution. On the other hand, when the first pressing condition is soft, such as the 40%– 100% condition, the graphite particles could be reoriented rather than cracking to relieve the stress applied to the graphite, which leads to the uniform distribution of the pores. These results were confirmed by the intensity ratio of D and G peaks (ID/IG) in Raman spectroscopy on the graphite electrode surface after one-step pressing and two-step pressing (Fig. S1). In Raman spectroscopy, ID/IG implies the defect level in the graphite particles, and the larger is the ID/IG value, the more defects are present [21–23]. When graphite particles become more disordered due to stress, the defect ratio of the graphite particles increases, and the value of ID/ IG increases [24]. It is clearly shown that the graphite electrode
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prepared with two-step pressing has lower ID/IG value compared to that of the graphite electrode prepared with one-step pressing. Fig. 1 shows the cross-sectional SEM images of the graphite electrodes as a function of the pressing condition. The thickness and apparent density of the electrodes, corresponding to the pressing condition, was summarized in Table 1. The as casted graphite electrode (without pressing) has the thickness of 109 mm. After the pressing process, the 100%–0%, 40%–100%, and 75%–100% graphite electrodes have the thickness of around 70 mm and apparent density of around 1.6 g cc1. Based on the SEM images, noticeable difference in the microstructure was not observed. However, it is clear that two-step pressing is helpful to reduce the defect generation caused by high pressure pressing (Fig. S1). The three-dimensional X-ray microscopy (3D-XRM) reconstruction method was employed to understand the pore distribution in the graphite electrodes depending on the pressing condition. This method have been widely used on graphite-based anodes to analyze the pore distribution of the electrode [25,26]. Fig. 2(a) shows the 3D-XRM reconstruction images for the 100%– 0%, 40%–100%, and 75%–100% graphite electrodes. It is observed that the electrodes prepared with two-step pressing have lager pore size compared to that of the electrode prepared with one-step pressing. These large pore plays an important role on the electrolyte permeation and Li ion kinetics [27]. The reconstructed image of the graphite electrode was divided into top/middle/ bottom layers, and the porosity of each layer was analyzed (Fig. 2(b)). The 40%–100% electrode exhibits the most uniform pore distribution at top/middle/bottom layers with 25.61/26.79/25.50%. The 100%–0% and 75%–100% graphite electrodes have high porosity at the top layer and lower porosity at the bottom layer. This nonuniformity of the pore distribution could induce significant swelling behavior and poor Li ion kinetics. Based on the reconstruction images, the pore size distribution in the overall electrode and the top/middle/bottom layers for the 100%–0%, 40%– 100%, and 75%–100% graphite electrodes are displayed in Fig. S2. For the 40%–100% electrode, the volume fraction of pore size with 1.4 mm accounted for 55% of the total pore volume and showed the most uniform pore size and volume distribution. However, the 100%–0% and 75%–100% graphite electrodes have broad pore size distribution in overall electrode. Based on these results, it is expected that the different rearrangement behavior of graphite particles during the first pressing could induce different pore size and volume distribution even under the identical apparent electrode density. The electrolyte permeability is closely related to the pore distribution of the electrode [27], which plays an important role on the electrode swelling behavior. Fig. 3(a) shows the electrolyte
Fig. 2. (a) 3D-XRM reconstruction images with overall electrode and (b) porosity of the top, middle, and bottom layers of the 100%–0%, 40%–100%, and 75%–100% electrodes.
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Fig. 3. (a) Electrolyte permeability in terms of the time it takes for the electrolyte to penetrate completely through the electrode. (b) Adhesion strength measured by peeling test.
permeation characteristics of the 100%–0%, 40%–100%, and 75%– 100% graphite electrodes. The 40%–100% electrode exhibits the best electrolyte permeation behavior compared to those of other two electrodes, which is attributed to the uniform pore distribution in top/middle/bottom layers in the electrode. The adhesion strength between the Cu substrate and the graphite-binder composite film for the three electrodes was measured by the peeling test (Fig. 3(b)) [28,29]. The adhesion strength related to the pore and binder distribution of the electrode could affect the electrochemical properties as well as the swelling behavior. The highest adhesion strength could be obtained in the 40%–100% electrode. This result indicates that the adhesion strength could be also controlled by adjusting pressing conditions. However, further study is necessary to understand the relationship between the pressing condition and adhesion strength. The swelling behavior of the three electrodes were monitored during cycling (Fig. 4). The pouch full cell consisting of LCO and graphite electrodes was prepared and cycled at 0.5C with the twostep formation (0.1 and 0.2C) at the initial two cycles. As the swelling behavior of the pouch full cell is mainly ascribed to a volume change in the graphite electrode associated with Li ion intercalation/deintercalation [30], we assumed that the thickness change of the pouch full cell results from the thickness change of the graphite electrode. To quantify the degree of the electrode swelling, a swelling ratio is calculated as thickness change during
Fig. 4. Swelling ratio of the graphite electrodes measured by the pouch full cell consisting of an LCO and graphite electrodes during two-step formation (0.1 and 0.2C) and cycling (0.5C).
cycling divided by the initial thickness. swelling ratio ¼
thickness change initial thickness
To better understand the swelling behavior depending on the pressing condition, we introduced following two concepts; initial and cycling swelling ratio. The initial swelling ratio refers to the ratio of thickness swelling occurring during the charging process (lithiation to the graphite electrode) of the first formation step (0.1C). This initial swelling ratio, as reported in a previous report, is largely related to the formation of the SEI layer by electrolyte decomposition on the graphite surface [17], the lattice expansion of the graphite particles resulting from Li ion intercalation, and stacking variation of the graphite particles by the lattice expansion [20]. The cycling swelling ratio refers to the difference in swelling ratio between 0 and 25 cycles at the discharged state after initial two formation cycling. This behavior is attributed to the accumulation of microscopic stress applied to graphite during Li ion intercalation/deintercalation in the cycling process, which causes macroscopic stress on the graphite electrode and consequently swells it [20]. The 40%–100% graphite electrode exhibits much reduced swelling behavior compared to 100%–0% and 75%– 100% graphite electrodes, with an initial/cycling swelling ratio of 19.23/4.47%. The uniform pore size and distribution in the 40%– 100% electrode effectively relieves the stress from the formation of the SEI layer and accommodates the volume change associated with Li ion during cycling, which significantly reduces battery swelling. The thickness change of the electrodes after 25 cycles was also observed by SEM (Fig. S3). The 100%–0%, 40%–100%, and 75%– 100% graphite electrodes have the thickness of 83.8, 81.9 and 83.4 mm, respectively. The 40%–100% graphite electrode exhibits the least thickness expansion. This result is well matched with swelling behavior. The electrochemical properties of the pouch full cells with 100%–0%, 40%–100%, and 75%–100% graphite electrodes were also evaluated. Fig. 5(a) shows the voltage profiles at the current density of 0.5C after the formation step. The cells with 100%–0%, 40%–100%, and 75%–100% graphite electrodes deliver the charge/ discharge capacities of 157.61/152.07, 156.56/152.13, and 157.33/ 152.58 mAh g1, respectively. Especially, the cell with 40%–100% graphite electrode exhibits the low polarization during charging and discharging, which is related to the improved Li ion kinetics caused by the electrode microstructure with uniform pore distribution. The cycle performances of the cells were also carefully monitored for 200 cycles (Fig. 5(b)). The cell with 40%– 100% graphite electrode also exhibits improved cycling performance (85.83%) compared to those of the cells with 100%–0% and
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0% and 75%–100% graphite electrodes. EIS spectra in Fig. S4 revealed the lowest charge transfer resistance (Rct) in the 40%– 100% electrode, which shows good correlation with electrolyte permeability and rate capability results. The electrochemical property results including the swelling behavior, polarization during cycling, cycle retention and rate capability clearly indicate that the electrode configuration engineering plays an important role on the improvement in the performance and reliability of LIBs. Conclusions The microstructural engineering of the graphite electrode was achieved by simple two-step pressing condition control. The first softened pressing enables the rearrangement of the graphite particles, which induces the uniform pore size and pore distribution in overall electrode without introducing defects in the graphite particle, resulting in the improvement in the swelling behavior and electrochemical properties. Our approach could be expanded to the cathode for LIBs as well as the electrodes for fuel cell devices that need uniform pore size and pore distribution characteristics. Acknowledgment This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20168510050080) and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy, Republic of Korea (No. 20174010201240). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.11.035. References
Fig. 5. Electrochemical properties of the pouch full cell consisting of an LCO and graphite electrodes with different pressing processes: (a) voltage profiles at 0.5C after two-step formation at 0.1 and 0.2C, (b) cycle stability over 200 cycles at 0.5C, and (c) rate capability from 0.2 to 2C.
75%–100% graphite electrodes (82.77% and 83.10%). The enhanced adhesion strength and reduced swelling degree in the 40%–100% graphite electrode enables the improvement in the cycle retention. Fig. 5(c) shows the rate capability of the cells. Even though the cells show the similar rate capability under low current density (0.2C and 0.5C), the cell with 40%–100% graphite electrode exhibits improved rate capability at high C rates (1C and 2C) due to the enhanced Li ion kinetics compared to those of the cells with 100%–
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