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High-capacity thick cathode with a porous aluminum current collector for lithium secondary batteries
Journal of Power Sources 334 (2016) 78e85
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High-capacity thick cathode with a porous aluminum current collector for lithium secondary batteries Hidetoshi Abe a, b, Masaaki Kubota b, Miyu Nemoto b, Yosuke Masuda b, Yuichi Tanaka c, Hirokazu Munakata a, Kiyoshi Kanamura a, * a
Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan Research Department, R&D Managing Division, R&D Institute, Furukawa Battery, 23-6 Kuidesaku Shimofunao, Jyoban, Iwaki, Fukushima 972-8501, Japan c Foil and Foil Stock Development Section, No.3 Department, Nagoya Center, Research and Development Division, UACJ, 3-1-12 Chitose, Minato, Nagoya 455-8670, Japan b
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A thick LiFePO4 cathode with a porous current collector has been developed. 8.4 m Ah cm2 has been realized for the LiFePO4 cathode with 400 mm thickness. Good rate capability (discharge capacity ratio of 1.0C/0.2C ¼ 0.980) was obtained. 80% of the initial capacity was retained at 2000th cycle.
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Article history: Received 9 August 2016 Received in revised form 16 September 2016 Accepted 4 October 2016
A high-capacity thick cathode has been studied as one of ways to improve the energy density of lithium secondary batteries. In this study, the LiFePO4 cathode with a capacity per unit area of 8.4 m Ah cm2 corresponding to four times the capacity of conventional cathodes has been developed using a three-dimensional porous aluminum current collector called “FUSPOROUS”. This unique current collector enables the smooth transfer of electrons and Liþ-ions through the thick cathode, resulting in a good rate capability (discharge capacity ratio of 1.0 C/0.2 C ¼ 0.980) and a high charge-discharge cycle performance (80% of the initial capacity at 2000th cycle) even though the electrode thickness is 400 mm. To take practical advantage of the developed thick cathode, conceptual designs for a 10-Ah class cell were also carried out using graphite and lithium-metal anodes. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium secondary battery Thick cathode Porous current collector High energy density
1. Introduction Recently, the lithium secondary battery has a higher energy density compared to other secondary batteries. Therefore, the
* Corresponding author. E-mail address: [email protected] (K. Kanamura). http://dx.doi.org/10.1016/j.jpowsour.2016.10.016 0378-7753/© 2016 Elsevier B.V. All rights reserved.
lithium secondary battery, especially the lithium ion battery, has been used for various applications, such as portable devices, electric vehicles (xEV), smart grids, and energy storage system (ESS). However, the electric power consumption of portable devices is increasing due to the multifunction applications of such devices. In addition, electric vehicles do not have an adequate cruising range. Therefore, a higher energy density lithium secondary battery has been strongly demanded.
H. Abe et al. / Journal of Power Sources 334 (2016) 78e85
In order to improve the energy density of the lithium secondary battery, high capacity anode and cathode materials are necessary. Lithium metal and its alloys have been investigated as promising candidates for high-capacity electrodes [1e15]. On the other hand, candidates for a high-capacity cathode are currently limited [16e18]. In order to use a high-capacity anode, such as the lithium metal anode, the cathode capacity per unit area should be improved by using new materials or a thick electrode. Conventionally, an aluminum foil, which is electrochemically stable in a high potential environment, is used as the cathode current collector for lithium secondary batteries [19e23]. The thickness of the aluminum foil is generally from 10 to 30 mm. The cathode is prepared by coating a slurry containing the active material, conductive material, binder, and dispersing medium on the aluminum foil followed by drying and pressing processes. The thickness of the coated cathode is usually at most 150 mm. In order to increase the energy density of the lithium secondary battery, the combination of thick cathodes and thin high capacity anodes is a practical solution. The thicker cathode provides a higher capacity per unit area [24,25]. A schematic illustration of the thick electrode is shown in Fig. 1. However, the thick cathode has many problems. Many cracks, dropouts, and exfoliations are caused by the volume change in the coated layer during charge and discharge. In addition, the ohmic resistance increases with the increasing thickness of the electrode due to the low electronic conductivity of the composite matrix. FUSPOROUS®, which has been developed by UACJ Co., Ltd., is a porous aluminum. It has a three-dimensional open porous structure. When a cathode material mixture is filled into the pores of the FUSPOROUS®, the cathode material is tightly held in the threedimensional current collector, resulting in an increase in the cathode material amount per unit area [26e29]. In this study, an improvement in the energy density has been investigated by increasing the amount of cathode material. This thick electrode may decrease the cost due to reducing the number of components in the cell. The possibility of a high-capacity thick cathode with a porous aluminum current collector for lithium secondary batteries is described. 2. Experimental The amount of active material per unit area was controlled by changing the amount of the filled slurry and thickness of the porous aluminum current collector. The thickness was adjusted by the pressing process. The slurry consisted of carbon-coated lithium iron phosphate (LFP) powder as the cathode material, acetylene black, sodium carboxymethyl cellulose, and polymethylacrylate binder at the weight ratio of 100: 6.8: 2: 3. Sodium carboxymethyl cellulose was used as a dispersant and a binder. The viscosity of the slurry was adjusted by adding the
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proper amount of water. The prepared slurry was filled into the pores of the porous aluminum current collector under vacuum. The electrode was then dried and pressed to obtain the proper density. The electrode was next formed with a 20-mm diameter. An aluminum foil was welded to the current tab. On the other hand, the conventional composite electrode was prepared using the same slurry. The specifications of the prepared cathodes are summarized in Table 1. Cross sections of porous aluminum current collector and prepared electrodes were observed by a scanning electron microscope (SEM) (JEOL JSM-5310LV). From the SEM images, their microstructures were confirmed, and the thicknesses of the electrodes were estimated. The basic electrochemical properties of the thick cathodes with a porous aluminum current collector were evaluated by a half-cell test. In order to compare this thick electrode with a conventional cathode, a half-cell test was also conducted using the standard composite electrode. The cathode with a 20-mm diameter was used as the working electrode, lithium metal foil as the counter electrode and the reference electrode, and 1.3 mol dm3 lithium hexafluorophosphate dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate with the volume ratio of 2:5:3 as the electrolyte. Each electrochemical property was measured by a battery test system (Toyo system TOSCAT-3100). The working electrode was charged to 4.2 V vs. Li/ Liþ and then kept at 4.2 V vs. Li/Liþ until the charging current decreased to less than 0.05 C, then discharged to 2.0 V vs. Li/Liþ at various currents at 25 C. A 0.1 C charge and a 0.1 C discharge were carried out for the first cycle. As the activation cycles, a 0.2 C charge-discharge cycle was then carried out for four cycles. The fifth and the sixth discharge tests were conducted at 0.2 C and 1.0 C with a 0.5 C charge, respectively, as an evaluation of the discharge rate performance. In addition, the 0.5 C charge - 0.5 C discharge cycle performance was also evaluated. Electrochemical impedance spectroscopy (EIS) of the cells with LFP1 and LFP3 was then measured by a Solartron SI 1280 at the 0% state of charge (SOC), 50% SOC, and 100%SOC, and the measuring potential was fixed at each SOC. The scanning range was from 20 kHz to 0.01 Hz. In order to evaluate the battery performance with these cathodes, full cells were constructed using these cathodes and graphite anodes. The specifications of the test cells are shown in Table 2. A cell with the thick cathode (LFP2) could not be assembled due to peeling off of the coated layer. The evaluation of the test cells was carried out using the conventional thin cathode (LFP1) and the thick cathode with porous aluminum as the current collector. The slurry of the anode material mixture was coated on a copper foil with a 10-mm thickness. The anode slurry was prepared by the mixing of graphite as the anode material, sodium carboxymethyl cellulose and polystyrene-butadiene as the binder in the ratio of 98:1:1 and adding water for viscosity adjustment. They were then
Fig. 1. Schematic structure of two kinds of lithium secondary batteries, (a) Conventional battery which consists of thin cathodes and thin anodes, (b) High energy density battery which consists of thick cathodes and thin anodes with high capacity.
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Table 1 Specifications of prepared cathodes. Cathode
Dimensions (mm)
Current collector
Cathode material
Thickness (mm)
Density (g cm3)
Capacity per unit area (mAh cm2)
LFP1 LFP2 LFP3
40 40 40 40 40 40
A1 foil A1 foil Porous A1
LFP LFP LFP
100 210 400
1.8 1.8 1.8
1.9 4.0 8.4
Table 2 Specifications of test cells. Test cell
Cathode designed capacity (mAh)
Anode Anode material
Total thickness (mm)
LFP1 LFP2 LFP3
25 e 100
Graphite e Graphite
50 e 200
dried and pressed. Finally, the anodes were obtained with the dimensions of 46 mm 50 mm, and they were welded with nickel tape as the current lead. The full cell was designed with the capacity ratio of the anode versus cathode of about 1.10, and it consisted of two pieces of anode and one piece of cathode surrounded by a membrane separator made of polyethylene. The assembled electrode components were packed in an aluminum laminate film, then filled with electrolyte. 1.3 mol dm3 lithium hexafluorophosphate dissolved in the solvent, which contained ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate at the volume ratio of 2:5:3 with 1 wt % vinylene carbonate added was used as the electrolyte. For evaluation of the cycleability of the thick cathode with the porous Al current collector, the following conditions were adopted. The cell was charged at 3.6 V until the current was reduced to 0.05 C, then discharged to 2.0 V at various currents at 25 C. The discharge and charge cycle at 0.1 C for the charge and 0.1 C for the discharge were carried out at the first cycle. For activation, the following discharge and charge cycle at 0.2 C were then carried out for four cycles. Another cycle test was conducted at 0.2 C and 1.0 C discharge after a 0.5 C charge at the fifth and sixth cycles, respectively. The 0.5 C charge-0.2 C discharge cycle performance was then also evaluated.
3. Results and discussion An SEM image of the cross section of the porous aluminum current collector “FUSPOROUS®” is shown in Fig. 2 (a). The porous
aluminum has an open three-dimensional structure that is prepared by the sintering of flake-like aluminum particles. An SEM image of the cross section of the thick cathode with the porous aluminum current collector is shown in Fig. 2 (b). This cathode was approximately four times thicker than the conventional coated electrode, and it was confirmed that the porous aluminum was uniformly distributed over the entire cross section. The discharge curves of the half-cells are shown in Fig. 3 (a) and (b) at 0.2 C and 1.0 C, respectively. The conventional coated cathode (LFP1) and the thick cathode with the porous aluminum (LFP3) exhibited the same discharge performance. In contrast, the conventional thick coated cathode (LFP2) had a higher polarization and lower capacity than those of LFP1 and LFP3, especially at the 1.0 C discharge. In the case of LFP2, it is considered that the discharge performance at the 0.2 C discharge is already influenced by the lithium ion diffusion in the electrolyte contained in the coating layer. In addition, the discharge performance at 1.0 C may indicate the high ohmic resistance of the entire coating layer. As shown in Fig. 3 (a) and (b), the three kinds of cathodes exhibited interesting and different performances. Fig. 4 shows the schematic cross sections of each cathode. Although LFP3 is four times thicker than LFP1, lithium ions were adequately supplied into the porous electrode and electron transfer through the thick electrode smoothly occurred. In the case of the porous Al current collector, the electrochemical reaction was smoothly carried out due to the high diffusivity of lithium ions from both sides of the cathode. In contrast, in LFP1 and LFP2, the penetration of the electrolyte is limited by the aluminum foil current collector at the center of the
Fig. 2. The scanning electron microscope image of cross section for (a) porous aluminum current collector “FUSPOROUS®”, (b) thick cathode with porous aluminum current collector.
H. Abe et al. / Journal of Power Sources 334 (2016) 78e85
Fig. 3. The result of half-cell test for (a) 0.2 C discharge curves, (b) 1.0 C discharge curves, and (c) change in discharge capacities of half cells during cycling.
Fig. 4. Schematic cross sections of each cathode.
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electrodes. This is one of the advantages of the porous Al current collector. Fig. 3 (c) shows the cycle performance. LFP1 and LFP3 showed a stable discharge capacity during the course of 20 cycles, but LFP2 showed a degradation in the discharge capacity after the 10th cycle. The exfoliation of the coated cathode layer from the aluminum foil current collector was caused by a volume change in the coated layer during the charge and discharge cycles. Nyquist plots were obtained by EIS measuring of the half-cells using LFP1 and LFP3 (Supplemental Fig. 1). Two kinds of equivalent circuits are suggested. The first has an arc as a charge transfer resistance (Rct) [30e32], and the second has two arcs as the Rct and solid electrolyte interphase resistance (RSEI) [33e37]. Since the spectrum obtained by this experiment exhibited two arcs and a diffusion part, the equivalent circuit shown in Fig. 5 (a) was adopted. Each resistance was fitted by the equivalent circuit. The calculated values are shown in Table 3 and Fig. 5 (b) and (c). Also, R1 þ R2 was defined as the interfacial resistance (Rir) in this study. The resistances that were calculated at the various SOCs showed that Rir of the LFP3 cell was lower than that of LFP1, but not very much different from each other. However, the capacity of LFP1 is lower than that of LFP3 due to the different thickness of the electrode. In order to normalize the electrode capacity, the parallel connection of the LFP1 electrodes is assumed in the estimation of the resistances. The calculation results are shown in Table 4, Fig. 5 (d) and (e). In this case, the resistances of LFP1 were lower than those of LFP3. The resistances at the various SOCs were approximately 2 times greater than those of LFP1. It is well known that the electrical resistance is proportional to l, and the diffusive resistance is proportional to l2, where l is the thickness of the porous composite electrode. If the resistance is calculated only based on the electrode thicknesses of LFP1 and LFP3, the diffusive resistance of LFP3 should become 18.2 times higher and the electrical resistance should become 4.27 times that of LFP1. However, the resistance differences between LFP1 and LFP3 obtained from the experiment were lower than these values. Based on these results, it can be said that the thick cathode (LFP3) using the porous aluminum current collector provides a lower resistance compared to the coated electrode on Al foil with the same thickness, according to the model shown in Fig. 4. In addition, LFP3 has a larger amount of cathode material and higher surface area per one piece of electrode than those of LFP1, so that the practical resistance became lower than the expected resistance from the thickness of the electrode. On the other hand, the resistance of LFP1 was higher than that of LFP3 with regard to normalization of the electrode capacity, indicating that the charge-discharge performance of LFP3 becomes lower at the higher rate condition. In the future, a thick cathode will be applied to high energy and low rate battery applications. In order to apply the thick electrode with porous Al to the middle rate battery applications with a high energy density, it is necessary to optimize the length of the conductive path and diffusion of ions in the bulk of the porous electrode. Fig. 6 (a) shows the 0.2 C discharge and 0.5 C charge curves, and Fig. 6 (b) shows the 1.0 C discharge and 0.5 C charge curves of the full cells. Though the cell using the thick cathode with the porous aluminum current collector (LFP3) had the same cathode area as that of the cell using the conventional normal cathode (LFP1), LFP3 obtained a four times higher cell capacity than that of LFP1. The ratio of the discharge capacities at 1.0 C and 0.2 C was 0.979 and 0.980 for LFP1 and LFP3, respectively. LFP1 and LFP3 showed the same discharge performance during the discharge rate test. Fig. 6 (c) and Fig. 6 (d) show the discharge capacity change and the change in the discharge capacity retention
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Fig. 5. Result of electrochemical impedance spectroscopy measurement for (a) equivalent circuit, relationship between SOC and each calculated resistance; (b) R1, R2, and R3, (c) interfacial resistance (Rir ¼ R2 þ R3), (d) normalized R1, R2, and R3, and (e) normalized interfacial resistance (Rir ¼ R2 þ R3).
Table 3 Calculated each resistance. Electrode
Thickness (mm)
LFP1
97
LFP3
414
SOC (%)
0 50 100 0 50 100
Interfacial resistance Rir ¼ R2 þ R3 (Ohms)
Resistance R1 (Ohms)
R2 (Ohms)
R3 (Ohms)
3.88 7.81 2.18 2.44 3.10 2.00
48.6 2.03 8.18 16.8 6.39 3.31
20.5 8.42 2.99 20.0 1.00 1.25
69.1 10.5 11.2 36.8 7.39 4.56
Table 4 Each calculated resistance and resistances of LFP1 were normalized in thickness, such as the same thickness of LFP3. Electrode
Thickness (mm)
SOC (%)
Resistance R1 (Ohms)
R2 (Ohms)
R3 (Ohms)
LFP1
(414)
LFP3
414
0 50 100 0 50 100
0.91 1.83 0.51 2.44 3.10 2.00
11.38 0.48 1.92 16.8 6.39 3.31
4.80 1.97 0.70 20.0 1.00 1.25
corresponding to the discharge capacity at the first cycle, respectively. Both LFP1 and LFP3 retained 80% of the initial
Interfacial resistance Rir ¼ R2 þ R3 (Ohms)
16.2 2.45 2.62 36.8 7.39 4.56
capacity at the 2000th cycle, indicating an excellent chargedischarge cycle performance. LFP3 had a slightly higher
H. Abe et al. / Journal of Power Sources 334 (2016) 78e85
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Fig. 6. The result of full cell test for (a) 0.2 C discharge and 0.5 C charge curves, (b) 1.0 C discharge and 0.5 C charge curves, (c) change of discharge capacities during cycling, and (d) change of discharge capacity retentions.
Fig. 7. Scanning electron micrograph of cross sectional view for LFP3 (a) before and (b) after the charge-discharge cycle test.
performance than that of LFP1. Thus, the thick cathode with the porous aluminum current collector suppressed the volume change in the electrode during cycling. LFP3 exhibited an adequate cycle performance, indicating the possibility of application of a thick electrode as the cathode for higher energy density batteries. The test cell with LFP3 was disassembled at the 2000th cycle. Gas generation and drying of the electrolyte associated with decomposition of the electrolyte were not observed, and also lithium dendrite growth in the anode did not occur. Fig. 7 shows SEM images of the cross sectional view for LFP3 before and after the charge-discharge cycle test. No significant difference in the electrode thickness and no exfoliation of the active material in LFP3 were observed. Under the experiment conditions, the discharge rate performance and cycle performance of the conventional thin coated cathode was similar to those of the thick cathode with the porous
aluminum current collector. When the charge-discharge current is higher than 1.0 C or when the capacity per unit area is increased, a larger amount of lithium ions and anions should transfer between the cathode and anode due to the higher current density or longer diffusion pathway, respectively. In fact, the amount of lithium ions, which transfer between the anode and
Fig. 8. Schematic illustration of cells using LFP1 and LFP3.
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H. Abe et al. / Journal of Power Sources 334 (2016) 78e85
Table 5 Estimated specifications for three cases. Cell Volume (dm3) Case 1 0.16 Case 2 0.16 Case 3 0.16
Cathode
Anode
Weight (Kg)
Capacity (Ah)
Voltage Energy density (V) (Wh dm3)
Energy density (Wh kg1)
Pieces of electrodes (Cat./Ano.)
Pieces of separators
Type Cathode material weight (g)
Anode material
0.31 0.31 0.30
11.83 12.59 15.94
3.2 3.2 3.3
123 (100%) 130 (106%) 177 (144%)
28/29 13/14 15/16
58 28 32
LFP1 81.11 (100%) LFP3 86.32 (106%) LFP3 99.60 (123%)
Graphite Graphite Li metal
242 (100%) 257 (106%) 336 (139%)
*Case 3 cell was designed the Li metal anode capacity as 2000 mAh/g (utilization ¼ 51.8%).
cathode in the cell using LFP3, is four times greater than that in the cell using LFP1. Therefore, it can be expected that the charge transfer process slightly influences the discharge and charge behavior of the electrodes, and lithium ion transfer, which mainly occurred by a diffusion process, mostly limits the electrochemical reaction in the cell using LFP3. Fig. 8 shows a schematic illustration of cells using LFP1 and LFP3. According to this model, the rate performance may be improved by using a high porosity separator, such as a three-dimensional ordered macroporous separator (3DOM separator) [38e40] or using a high concentration of electrolyte in the future. Finally, a conceptual design with three cases of a 10-Ah class cell with the same volume (dimensions) has been completed. The conventional cell consisted of LFP1 and a thin coated anode (Case1). The second cell consisted of LFP3 and a thick-coated anode (Case2). The third cell consisted of LFP3 and a lithium metal anode (Case3). Their specifications are summarized in Table 5. The cells using the thick cathode (LFP3) can significantly reduce the number of electrodes, and separators. Therefore, it is expected that the electrode processing costs and the separator material cost decrease. The energy density does not improve in Case2 with a graphite anode. In addition, its rate capability may be very low due to the diffusion of Liþ ions in the thick graphite anode. On the other hand, a large improvement is expected for Case3 with a high capacity lithium metal anode. Though, lithium iron phosphate was used as the cathode material in this experiment, further improvement in the energy density is expected using a high potential and high density cathode material, which may provide more than a 400 W h kg1 energy density.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
4. Conclusion [21]
The new cathode with porous Al as the current collector has been developed, and evaluated. This cathode holds a larger amount of active material than the conventional coated cathode, so that the capacity per unit area can be increased. Generally, when the increasing thickness of an electrode, the rate capability of the conventional coated cathode decreases. In contrast, the new cathode containing several times a higher amount of active material than the conventional coated cathode exhibited an excellent charge-discharge characteristic including its rate capability and cycle performance. This is due to three factors. (1) Highly efficient current collection by the porous aluminum current collector, (2) High capability of lithium ion supply from the electrolyte due to the three-dimensional porous structure. and (3) High mechanical strength of electrode and strong holding of active materials due to three-dimensional porous structure. In the future, a high energy density and low cost lithium battery will be expected by combining this cathode and a high capacity anode, such as Si, Sn, and Li metal anodes.
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High-capacity thick cathode with a porous aluminum current collector for lithium secondary batteries Hidetoshi Abe a, b, Masaaki Kubota b, Miyu Nemoto b, Yosuke Masuda b, Yuichi Tanaka c, Hirokazu Munakata a, Kiyoshi Kanamura a, * a
Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan Research Department, R&D Managing Division, R&D Institute, Furukawa Battery, 23-6 Kuidesaku Shimofunao, Jyoban, Iwaki, Fukushima 972-8501, Japan c Foil and Foil Stock Development Section, No.3 Department, Nagoya Center, Research and Development Division, UACJ, 3-1-12 Chitose, Minato, Nagoya 455-8670, Japan b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A thick LiFePO4 cathode with a porous current collector has been developed. 8.4 m Ah cm2 has been realized for the LiFePO4 cathode with 400 mm thickness. Good rate capability (discharge capacity ratio of 1.0C/0.2C ¼ 0.980) was obtained. 80% of the initial capacity was retained at 2000th cycle.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2016 Received in revised form 16 September 2016 Accepted 4 October 2016
A high-capacity thick cathode has been studied as one of ways to improve the energy density of lithium secondary batteries. In this study, the LiFePO4 cathode with a capacity per unit area of 8.4 m Ah cm2 corresponding to four times the capacity of conventional cathodes has been developed using a three-dimensional porous aluminum current collector called “FUSPOROUS”. This unique current collector enables the smooth transfer of electrons and Liþ-ions through the thick cathode, resulting in a good rate capability (discharge capacity ratio of 1.0 C/0.2 C ¼ 0.980) and a high charge-discharge cycle performance (80% of the initial capacity at 2000th cycle) even though the electrode thickness is 400 mm. To take practical advantage of the developed thick cathode, conceptual designs for a 10-Ah class cell were also carried out using graphite and lithium-metal anodes. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium secondary battery Thick cathode Porous current collector High energy density
1. Introduction Recently, the lithium secondary battery has a higher energy density compared to other secondary batteries. Therefore, the
* Corresponding author. E-mail address: [email protected] (K. Kanamura). http://dx.doi.org/10.1016/j.jpowsour.2016.10.016 0378-7753/© 2016 Elsevier B.V. All rights reserved.
lithium secondary battery, especially the lithium ion battery, has been used for various applications, such as portable devices, electric vehicles (xEV), smart grids, and energy storage system (ESS). However, the electric power consumption of portable devices is increasing due to the multifunction applications of such devices. In addition, electric vehicles do not have an adequate cruising range. Therefore, a higher energy density lithium secondary battery has been strongly demanded.
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In order to improve the energy density of the lithium secondary battery, high capacity anode and cathode materials are necessary. Lithium metal and its alloys have been investigated as promising candidates for high-capacity electrodes [1e15]. On the other hand, candidates for a high-capacity cathode are currently limited [16e18]. In order to use a high-capacity anode, such as the lithium metal anode, the cathode capacity per unit area should be improved by using new materials or a thick electrode. Conventionally, an aluminum foil, which is electrochemically stable in a high potential environment, is used as the cathode current collector for lithium secondary batteries [19e23]. The thickness of the aluminum foil is generally from 10 to 30 mm. The cathode is prepared by coating a slurry containing the active material, conductive material, binder, and dispersing medium on the aluminum foil followed by drying and pressing processes. The thickness of the coated cathode is usually at most 150 mm. In order to increase the energy density of the lithium secondary battery, the combination of thick cathodes and thin high capacity anodes is a practical solution. The thicker cathode provides a higher capacity per unit area [24,25]. A schematic illustration of the thick electrode is shown in Fig. 1. However, the thick cathode has many problems. Many cracks, dropouts, and exfoliations are caused by the volume change in the coated layer during charge and discharge. In addition, the ohmic resistance increases with the increasing thickness of the electrode due to the low electronic conductivity of the composite matrix. FUSPOROUS®, which has been developed by UACJ Co., Ltd., is a porous aluminum. It has a three-dimensional open porous structure. When a cathode material mixture is filled into the pores of the FUSPOROUS®, the cathode material is tightly held in the threedimensional current collector, resulting in an increase in the cathode material amount per unit area [26e29]. In this study, an improvement in the energy density has been investigated by increasing the amount of cathode material. This thick electrode may decrease the cost due to reducing the number of components in the cell. The possibility of a high-capacity thick cathode with a porous aluminum current collector for lithium secondary batteries is described. 2. Experimental The amount of active material per unit area was controlled by changing the amount of the filled slurry and thickness of the porous aluminum current collector. The thickness was adjusted by the pressing process. The slurry consisted of carbon-coated lithium iron phosphate (LFP) powder as the cathode material, acetylene black, sodium carboxymethyl cellulose, and polymethylacrylate binder at the weight ratio of 100: 6.8: 2: 3. Sodium carboxymethyl cellulose was used as a dispersant and a binder. The viscosity of the slurry was adjusted by adding the
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proper amount of water. The prepared slurry was filled into the pores of the porous aluminum current collector under vacuum. The electrode was then dried and pressed to obtain the proper density. The electrode was next formed with a 20-mm diameter. An aluminum foil was welded to the current tab. On the other hand, the conventional composite electrode was prepared using the same slurry. The specifications of the prepared cathodes are summarized in Table 1. Cross sections of porous aluminum current collector and prepared electrodes were observed by a scanning electron microscope (SEM) (JEOL JSM-5310LV). From the SEM images, their microstructures were confirmed, and the thicknesses of the electrodes were estimated. The basic electrochemical properties of the thick cathodes with a porous aluminum current collector were evaluated by a half-cell test. In order to compare this thick electrode with a conventional cathode, a half-cell test was also conducted using the standard composite electrode. The cathode with a 20-mm diameter was used as the working electrode, lithium metal foil as the counter electrode and the reference electrode, and 1.3 mol dm3 lithium hexafluorophosphate dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate with the volume ratio of 2:5:3 as the electrolyte. Each electrochemical property was measured by a battery test system (Toyo system TOSCAT-3100). The working electrode was charged to 4.2 V vs. Li/ Liþ and then kept at 4.2 V vs. Li/Liþ until the charging current decreased to less than 0.05 C, then discharged to 2.0 V vs. Li/Liþ at various currents at 25 C. A 0.1 C charge and a 0.1 C discharge were carried out for the first cycle. As the activation cycles, a 0.2 C charge-discharge cycle was then carried out for four cycles. The fifth and the sixth discharge tests were conducted at 0.2 C and 1.0 C with a 0.5 C charge, respectively, as an evaluation of the discharge rate performance. In addition, the 0.5 C charge - 0.5 C discharge cycle performance was also evaluated. Electrochemical impedance spectroscopy (EIS) of the cells with LFP1 and LFP3 was then measured by a Solartron SI 1280 at the 0% state of charge (SOC), 50% SOC, and 100%SOC, and the measuring potential was fixed at each SOC. The scanning range was from 20 kHz to 0.01 Hz. In order to evaluate the battery performance with these cathodes, full cells were constructed using these cathodes and graphite anodes. The specifications of the test cells are shown in Table 2. A cell with the thick cathode (LFP2) could not be assembled due to peeling off of the coated layer. The evaluation of the test cells was carried out using the conventional thin cathode (LFP1) and the thick cathode with porous aluminum as the current collector. The slurry of the anode material mixture was coated on a copper foil with a 10-mm thickness. The anode slurry was prepared by the mixing of graphite as the anode material, sodium carboxymethyl cellulose and polystyrene-butadiene as the binder in the ratio of 98:1:1 and adding water for viscosity adjustment. They were then
Fig. 1. Schematic structure of two kinds of lithium secondary batteries, (a) Conventional battery which consists of thin cathodes and thin anodes, (b) High energy density battery which consists of thick cathodes and thin anodes with high capacity.
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Table 1 Specifications of prepared cathodes. Cathode
Dimensions (mm)
Current collector
Cathode material
Thickness (mm)
Density (g cm3)
Capacity per unit area (mAh cm2)
LFP1 LFP2 LFP3
40 40 40 40 40 40
A1 foil A1 foil Porous A1
LFP LFP LFP
100 210 400
1.8 1.8 1.8
1.9 4.0 8.4
Table 2 Specifications of test cells. Test cell
Cathode designed capacity (mAh)
Anode Anode material
Total thickness (mm)
LFP1 LFP2 LFP3
25 e 100
Graphite e Graphite
50 e 200
dried and pressed. Finally, the anodes were obtained with the dimensions of 46 mm 50 mm, and they were welded with nickel tape as the current lead. The full cell was designed with the capacity ratio of the anode versus cathode of about 1.10, and it consisted of two pieces of anode and one piece of cathode surrounded by a membrane separator made of polyethylene. The assembled electrode components were packed in an aluminum laminate film, then filled with electrolyte. 1.3 mol dm3 lithium hexafluorophosphate dissolved in the solvent, which contained ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate at the volume ratio of 2:5:3 with 1 wt % vinylene carbonate added was used as the electrolyte. For evaluation of the cycleability of the thick cathode with the porous Al current collector, the following conditions were adopted. The cell was charged at 3.6 V until the current was reduced to 0.05 C, then discharged to 2.0 V at various currents at 25 C. The discharge and charge cycle at 0.1 C for the charge and 0.1 C for the discharge were carried out at the first cycle. For activation, the following discharge and charge cycle at 0.2 C were then carried out for four cycles. Another cycle test was conducted at 0.2 C and 1.0 C discharge after a 0.5 C charge at the fifth and sixth cycles, respectively. The 0.5 C charge-0.2 C discharge cycle performance was then also evaluated.
3. Results and discussion An SEM image of the cross section of the porous aluminum current collector “FUSPOROUS®” is shown in Fig. 2 (a). The porous
aluminum has an open three-dimensional structure that is prepared by the sintering of flake-like aluminum particles. An SEM image of the cross section of the thick cathode with the porous aluminum current collector is shown in Fig. 2 (b). This cathode was approximately four times thicker than the conventional coated electrode, and it was confirmed that the porous aluminum was uniformly distributed over the entire cross section. The discharge curves of the half-cells are shown in Fig. 3 (a) and (b) at 0.2 C and 1.0 C, respectively. The conventional coated cathode (LFP1) and the thick cathode with the porous aluminum (LFP3) exhibited the same discharge performance. In contrast, the conventional thick coated cathode (LFP2) had a higher polarization and lower capacity than those of LFP1 and LFP3, especially at the 1.0 C discharge. In the case of LFP2, it is considered that the discharge performance at the 0.2 C discharge is already influenced by the lithium ion diffusion in the electrolyte contained in the coating layer. In addition, the discharge performance at 1.0 C may indicate the high ohmic resistance of the entire coating layer. As shown in Fig. 3 (a) and (b), the three kinds of cathodes exhibited interesting and different performances. Fig. 4 shows the schematic cross sections of each cathode. Although LFP3 is four times thicker than LFP1, lithium ions were adequately supplied into the porous electrode and electron transfer through the thick electrode smoothly occurred. In the case of the porous Al current collector, the electrochemical reaction was smoothly carried out due to the high diffusivity of lithium ions from both sides of the cathode. In contrast, in LFP1 and LFP2, the penetration of the electrolyte is limited by the aluminum foil current collector at the center of the
Fig. 2. The scanning electron microscope image of cross section for (a) porous aluminum current collector “FUSPOROUS®”, (b) thick cathode with porous aluminum current collector.
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Fig. 3. The result of half-cell test for (a) 0.2 C discharge curves, (b) 1.0 C discharge curves, and (c) change in discharge capacities of half cells during cycling.
Fig. 4. Schematic cross sections of each cathode.
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electrodes. This is one of the advantages of the porous Al current collector. Fig. 3 (c) shows the cycle performance. LFP1 and LFP3 showed a stable discharge capacity during the course of 20 cycles, but LFP2 showed a degradation in the discharge capacity after the 10th cycle. The exfoliation of the coated cathode layer from the aluminum foil current collector was caused by a volume change in the coated layer during the charge and discharge cycles. Nyquist plots were obtained by EIS measuring of the half-cells using LFP1 and LFP3 (Supplemental Fig. 1). Two kinds of equivalent circuits are suggested. The first has an arc as a charge transfer resistance (Rct) [30e32], and the second has two arcs as the Rct and solid electrolyte interphase resistance (RSEI) [33e37]. Since the spectrum obtained by this experiment exhibited two arcs and a diffusion part, the equivalent circuit shown in Fig. 5 (a) was adopted. Each resistance was fitted by the equivalent circuit. The calculated values are shown in Table 3 and Fig. 5 (b) and (c). Also, R1 þ R2 was defined as the interfacial resistance (Rir) in this study. The resistances that were calculated at the various SOCs showed that Rir of the LFP3 cell was lower than that of LFP1, but not very much different from each other. However, the capacity of LFP1 is lower than that of LFP3 due to the different thickness of the electrode. In order to normalize the electrode capacity, the parallel connection of the LFP1 electrodes is assumed in the estimation of the resistances. The calculation results are shown in Table 4, Fig. 5 (d) and (e). In this case, the resistances of LFP1 were lower than those of LFP3. The resistances at the various SOCs were approximately 2 times greater than those of LFP1. It is well known that the electrical resistance is proportional to l, and the diffusive resistance is proportional to l2, where l is the thickness of the porous composite electrode. If the resistance is calculated only based on the electrode thicknesses of LFP1 and LFP3, the diffusive resistance of LFP3 should become 18.2 times higher and the electrical resistance should become 4.27 times that of LFP1. However, the resistance differences between LFP1 and LFP3 obtained from the experiment were lower than these values. Based on these results, it can be said that the thick cathode (LFP3) using the porous aluminum current collector provides a lower resistance compared to the coated electrode on Al foil with the same thickness, according to the model shown in Fig. 4. In addition, LFP3 has a larger amount of cathode material and higher surface area per one piece of electrode than those of LFP1, so that the practical resistance became lower than the expected resistance from the thickness of the electrode. On the other hand, the resistance of LFP1 was higher than that of LFP3 with regard to normalization of the electrode capacity, indicating that the charge-discharge performance of LFP3 becomes lower at the higher rate condition. In the future, a thick cathode will be applied to high energy and low rate battery applications. In order to apply the thick electrode with porous Al to the middle rate battery applications with a high energy density, it is necessary to optimize the length of the conductive path and diffusion of ions in the bulk of the porous electrode. Fig. 6 (a) shows the 0.2 C discharge and 0.5 C charge curves, and Fig. 6 (b) shows the 1.0 C discharge and 0.5 C charge curves of the full cells. Though the cell using the thick cathode with the porous aluminum current collector (LFP3) had the same cathode area as that of the cell using the conventional normal cathode (LFP1), LFP3 obtained a four times higher cell capacity than that of LFP1. The ratio of the discharge capacities at 1.0 C and 0.2 C was 0.979 and 0.980 for LFP1 and LFP3, respectively. LFP1 and LFP3 showed the same discharge performance during the discharge rate test. Fig. 6 (c) and Fig. 6 (d) show the discharge capacity change and the change in the discharge capacity retention
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Fig. 5. Result of electrochemical impedance spectroscopy measurement for (a) equivalent circuit, relationship between SOC and each calculated resistance; (b) R1, R2, and R3, (c) interfacial resistance (Rir ¼ R2 þ R3), (d) normalized R1, R2, and R3, and (e) normalized interfacial resistance (Rir ¼ R2 þ R3).
Table 3 Calculated each resistance. Electrode
Thickness (mm)
LFP1
97
LFP3
414
SOC (%)
0 50 100 0 50 100
Interfacial resistance Rir ¼ R2 þ R3 (Ohms)
Resistance R1 (Ohms)
R2 (Ohms)
R3 (Ohms)
3.88 7.81 2.18 2.44 3.10 2.00
48.6 2.03 8.18 16.8 6.39 3.31
20.5 8.42 2.99 20.0 1.00 1.25
69.1 10.5 11.2 36.8 7.39 4.56
Table 4 Each calculated resistance and resistances of LFP1 were normalized in thickness, such as the same thickness of LFP3. Electrode
Thickness (mm)
SOC (%)
Resistance R1 (Ohms)
R2 (Ohms)
R3 (Ohms)
LFP1
(414)
LFP3
414
0 50 100 0 50 100
0.91 1.83 0.51 2.44 3.10 2.00
11.38 0.48 1.92 16.8 6.39 3.31
4.80 1.97 0.70 20.0 1.00 1.25
corresponding to the discharge capacity at the first cycle, respectively. Both LFP1 and LFP3 retained 80% of the initial
Interfacial resistance Rir ¼ R2 þ R3 (Ohms)
16.2 2.45 2.62 36.8 7.39 4.56
capacity at the 2000th cycle, indicating an excellent chargedischarge cycle performance. LFP3 had a slightly higher
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Fig. 6. The result of full cell test for (a) 0.2 C discharge and 0.5 C charge curves, (b) 1.0 C discharge and 0.5 C charge curves, (c) change of discharge capacities during cycling, and (d) change of discharge capacity retentions.
Fig. 7. Scanning electron micrograph of cross sectional view for LFP3 (a) before and (b) after the charge-discharge cycle test.
performance than that of LFP1. Thus, the thick cathode with the porous aluminum current collector suppressed the volume change in the electrode during cycling. LFP3 exhibited an adequate cycle performance, indicating the possibility of application of a thick electrode as the cathode for higher energy density batteries. The test cell with LFP3 was disassembled at the 2000th cycle. Gas generation and drying of the electrolyte associated with decomposition of the electrolyte were not observed, and also lithium dendrite growth in the anode did not occur. Fig. 7 shows SEM images of the cross sectional view for LFP3 before and after the charge-discharge cycle test. No significant difference in the electrode thickness and no exfoliation of the active material in LFP3 were observed. Under the experiment conditions, the discharge rate performance and cycle performance of the conventional thin coated cathode was similar to those of the thick cathode with the porous
aluminum current collector. When the charge-discharge current is higher than 1.0 C or when the capacity per unit area is increased, a larger amount of lithium ions and anions should transfer between the cathode and anode due to the higher current density or longer diffusion pathway, respectively. In fact, the amount of lithium ions, which transfer between the anode and
Fig. 8. Schematic illustration of cells using LFP1 and LFP3.
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Table 5 Estimated specifications for three cases. Cell Volume (dm3) Case 1 0.16 Case 2 0.16 Case 3 0.16
Cathode
Anode
Weight (Kg)
Capacity (Ah)
Voltage Energy density (V) (Wh dm3)
Energy density (Wh kg1)
Pieces of electrodes (Cat./Ano.)
Pieces of separators
Type Cathode material weight (g)
Anode material
0.31 0.31 0.30
11.83 12.59 15.94
3.2 3.2 3.3
123 (100%) 130 (106%) 177 (144%)
28/29 13/14 15/16
58 28 32
LFP1 81.11 (100%) LFP3 86.32 (106%) LFP3 99.60 (123%)
Graphite Graphite Li metal
242 (100%) 257 (106%) 336 (139%)
*Case 3 cell was designed the Li metal anode capacity as 2000 mAh/g (utilization ¼ 51.8%).
cathode in the cell using LFP3, is four times greater than that in the cell using LFP1. Therefore, it can be expected that the charge transfer process slightly influences the discharge and charge behavior of the electrodes, and lithium ion transfer, which mainly occurred by a diffusion process, mostly limits the electrochemical reaction in the cell using LFP3. Fig. 8 shows a schematic illustration of cells using LFP1 and LFP3. According to this model, the rate performance may be improved by using a high porosity separator, such as a three-dimensional ordered macroporous separator (3DOM separator) [38e40] or using a high concentration of electrolyte in the future. Finally, a conceptual design with three cases of a 10-Ah class cell with the same volume (dimensions) has been completed. The conventional cell consisted of LFP1 and a thin coated anode (Case1). The second cell consisted of LFP3 and a thick-coated anode (Case2). The third cell consisted of LFP3 and a lithium metal anode (Case3). Their specifications are summarized in Table 5. The cells using the thick cathode (LFP3) can significantly reduce the number of electrodes, and separators. Therefore, it is expected that the electrode processing costs and the separator material cost decrease. The energy density does not improve in Case2 with a graphite anode. In addition, its rate capability may be very low due to the diffusion of Liþ ions in the thick graphite anode. On the other hand, a large improvement is expected for Case3 with a high capacity lithium metal anode. Though, lithium iron phosphate was used as the cathode material in this experiment, further improvement in the energy density is expected using a high potential and high density cathode material, which may provide more than a 400 W h kg1 energy density.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
4. Conclusion [21]
The new cathode with porous Al as the current collector has been developed, and evaluated. This cathode holds a larger amount of active material than the conventional coated cathode, so that the capacity per unit area can be increased. Generally, when the increasing thickness of an electrode, the rate capability of the conventional coated cathode decreases. In contrast, the new cathode containing several times a higher amount of active material than the conventional coated cathode exhibited an excellent charge-discharge characteristic including its rate capability and cycle performance. This is due to three factors. (1) Highly efficient current collection by the porous aluminum current collector, (2) High capability of lithium ion supply from the electrolyte due to the three-dimensional porous structure. and (3) High mechanical strength of electrode and strong holding of active materials due to three-dimensional porous structure. In the future, a high energy density and low cost lithium battery will be expected by combining this cathode and a high capacity anode, such as Si, Sn, and Li metal anodes.
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