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Enhanced cycle stability of LiCoPO4 by using three-dimensionally ordered macroporous polyimide separator
Journal of Power Sources 350 (2017) 103e108
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced cycle stability of LiCoPO4 by using three-dimensionally ordered macroporous polyimide separator Yuta Maeyoshi, Shohei Miyamoto, Hirokazu Munakata, Kiyoshi Kanamura* Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo, 192-0397, Japan
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Enhanced cycle stability of LiCoPO4 is achieved by using 3DOM PI separator. 3DOM PI separator with high anodic stability reduces irreversible reactions. The ordered porous structure of the separator provides uniform current distribution. The uniform current distribution prevents increase in cell impedance.
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Article history: Received 11 January 2017 Received in revised form 17 February 2017 Accepted 14 March 2017
To enhance the low cycle stability of LiCoPO4, the development of stable separators in batteries is required, since they are oxidized in the high-voltage system at around 5 V, affecting the cycle performance of high-voltage lithium-ion batteries. The performance of the batteries also depends on the porous structure of separators. Here we report improved coulombic efficiency and capacity retention of LiCoPO4 by using a three-dimensionally ordered macroporous polyimide (3DOM PI) separator, compared with a conventional polypropylene separator with heterogeneous pore structure. The enhanced cycle stability of the cell using 3DOM PI separator is attributed to its ordered macroporous structure and high anodic stability. The uniform current distribution created by the ordered macroporous structure results in the low overpotential during charge process, preventing the oxidation of electrolyte and the growth of the resistive film on the cathode surface during cycling. Furthermore, the high anodic stability of 3DOM PI separator maintains its chemical and macroporous structures after cycling at high potentials, leading to the superior stability of the cell. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion battery High voltage Separator Polyimide Three-dimensionally ordered macroporous structure Lithium cobalt phosphate
1. Introduction The energy density and safety of lithium-ion batteries (LIBs) demand to be improved for requirements of plug-in hybrid electric vehicles, electric vehicles, and smart grids [1]. The performance of
* Corresponding author. E-mail address: [email protected] (K. Kanamura). http://dx.doi.org/10.1016/j.jpowsour.2017.03.053 0378-7753/© 2017 Elsevier B.V. All rights reserved.
LIBs highly depends on the characteristics of cathode materials. Lithium cobalt phosphate (LiCoPO4) is considered a promising cathode material, since it possesses a high operating potential (4.8 V vs. Li/Liþ), a flat voltage profile and a good theoretical capacity (167 mA h g1), improving the energy density of LIBs [2]. Moreover, LiCoPO4 with olivine structure has high thermal and structural stability derived from the strong PeO covalent band, realizing high safety in LIBs [3]. However, the high operating potential of LiCoPO4 triggers irreversible reactions assigned to
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decomposition of electrolyte, which generates undesired byproducts and forms the resistive film on the surface of the cathode, resulting in pronounced capacity fading upon cycling [4e8]. The capacity fading has also resulted from the degradation of LiCoPO4 by HF included in electrolyte solution as an impurity [9]. Several strategies have been employed to enhance the cycle stability, such as doping with metal ions [10e12], surface modification of the cathode material [13e15], substitution of anodic stable electrolyte for conventional one [5,6], and inclusion of additives in electrolyte [7,8,16]. Most of the strategies have focused on the cathode material and electrolyte. To overcome the low cycle stability of LiCoPO4, the development of stable separators in batteries is also required. The separators are considered to be inert in conventional LIBs at 4 V, while they are oxidized in the high-voltage system at around 5 V, affecting the cycle performance of high-voltage LIBs. Chen et al. have reported that polyethylene (PE) separator was more stable than polypropylene (PP) separator under high potentials due to lower HOMO energy of PE. The high anodic stability of PE separator has been found to improve the cycle stability of high-voltage cathode material LiCr0.05Ni0.45Mn1.5O4 [17]. Therefore, the separators with high anodic stability are essential to enhance the cycle performance of LiCoPO4. Generally, porous membranes are used as separators in LIBs, whose role is to prevent electrical short circuits between an anode and a cathode but easy permeation of ions. Thus, the safety and performance of LIBs strongly depend on the porous separator, and many advanced porous membranes have been developed for the batteries [18e24]. We have previously developed an ultrafine porous polyimide (PI) membrane with three-dimensionally ordered macroporous (3DOM) structure illustrated in Fig. 1, and investigated its performance as a battery separator [25e28]. 3DOM PI membranes has a high porosity of ca. 70% and uniform distribution of pores in a hexagonal close-packed arrangement, providing uniform current distribution during chargeedischarge processes in batteries. Actually, the uniform current distribution of 3DOM PI separator has prevented the increase in surface area of lithium-metal electrodes and dendrite-formed lithium deposition during charge process in LIBs [25e27]. Taking account of the result, the uniform current distribution of 3DOM PI separator is expected to lower overpotential during charge process, reducing the decomposition of electrolyte and the consequent accumulation of the resistive film on the cathode surface. Furthermore, PI are known for high thermal and chemical stability, and has been reported to be electrochemical stable at around 5.0 vs. Li/Liþ [29]. Therefore, the cycle performance of LiCoPO4 should be enhanced by using 3DOM PI separator. In this work, we investigated the effect of 3DOM PI separator on the electrochemical performance of LiCoPO4jLi cell compared with
Fig. 1. Schematic of three-dimensionally ordered macroporous structure.
conventional PP separator. The cycle stability of LiCoPO4 was discussed in terms of the anodic stability and the pore structure of the separators, and charge-transfer resistance of the cells related to the formation of the resistive film on the cathode surface. 2. Experimental 2.1. Synthesis and characterization of LiCoPO4 Carbon-coated LiCoPO4 was synthesized by a one-pot hydrothermal process according to our previous report [30]. Briefly, 0.09 mol of CoSO4$7H2O (Wako Pure Chemical Industries, Ltd.), 0.09 mol of Li3PO4 (Kojundo Chemical Lab. Co., Ltd.), and 2.0 g of carboxymethylcellulose sodium salt (Mw ¼ 9.0 104 g mol1, Sigma-Aldrich Co.) were dissolved into degassed water (30 ml) and heated at 200 C for 24 h under N2 atmosphere. After the hydrothermal treatment, the resulting precipitation was separated centrifugally, washed, and subsequently freeze-dried. The dried powder was heat-treated at 700 C for 1 h under 97% Ar/3% H2 atmosphere and carbon-coated LiCoPO4 was obtained. The X-ray diffraction (XRD, RINT 2000/PC, Rigaku Co.) pattern of the product was assigned to phospho-olivine LiCoPO4 with orthorhombic Pnma space group. The size and morphology of the prepared LiCoPO4 were observed by a scanning electron microscope (SEM, JSM7500F, JEOL Ltd.). The specific surface area of the sample was calculated using the BrunauereEmmeteTeller (BET) equation from the adsorption isotherm of N2 gas at 77 K determined by a BELSORP-mini II apparatus (MicrotracBEL Corp.). The carbon content of the sample was determined by a thermogravimetric analysis (DTG-60H, Shimadzu Co.). 2.2. Preparation of 3DOM PI separator 3DOM PI separator was prepared by using a colloidal crystal template composed of mono-disperse spherical particles [31]. Mono-dispersed silica particles in aqueous solution were accumulated by filtration process, and opal-silica template was obtained. The vacant space of silica template was filled with a precursor solution of PI, and then heated at 320 C to convert the precursor to PI. After that, the sample was immersed in 10 wt% HF aqueous solution at room temperature for 24 h to eliminate the opal-silica template. After this elimination, PI membrane remained and had inverse-opal structure. 2.3. Characterization and electrochemical measurement of separators The cathode was fabricated by coating the slurry of mixture consisting of 80 wt% LiCoPO4, 10 wt% acetylene black (Li-100, Denka Co., Ltd.), and 10 wt% polyvinylidene difluoride (Kureha Co.) in Nmethyl pyrrolidone on Al foil current collector (20 mm in thickness) using a doctor blade. The electrode was dried at 120 C under vacuum for 12 h and punched into a circle shape (14 mm in diameter). The loading amount of the cathode material was 2.2e2.7 mg cm2 with a thickness of 16e20 mm. LiCoPO4jLi halfcells were assembled in the 2032 coin type cell in an Ar filled glove box. 1 mol dm3 LiPF6/ethylene carbonate:diethyl carbonate ¼ 1:2 (v/v) (Kishida chemical Co., Ltd.) was used as an electrolyte solution. Two types of separators were used for the preparation of the cells, namely, PP and 3DOM PI separators. Galvanostatic chargeedischarge tests of the cells were carried out with a chargeedischarge unit (HJ1001SD8, Hokuto Denko Co.) in the potential range of 3.0e5.1 V at 0.1 C (1 C ¼ 167 mA g1). Current densities and specific capacities of LiCoPO4 cathodes were calculated on the basis of the weight of LiCoPO4. Electrochemical
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impedance spectroscopy was conducted using a Solartron SI 1287 electrochemical interface with an AC modulation amplitude of ±5 mV in the frequency range from 1 MHz to 0.01 Hz. The spectra were measured at a fresh state and after 100 cycles of the galvanostatic chargeedischarge test. The cycled cells were disassembled in the glove box, and the separators were washed with dimethyl carbonate and dried under vacuum. The morphology of the separators before and after cycling was observed by the SEM. Reflection spectra of the separators before and after cycling were collected using a Fourier transform infrared (FT-IR) spectrometer (FT/IR6100, Jasco Co.) by an attenuated total reflectance method. Liquid electrolyte uptakes of the separators were determined by using following equation:
Electrolyte uptake ð%Þ ¼ ðWf WiÞ=Wi 100 where Wi and Wf are the weights of the separators before and after soaking in the electrolyte for 2 h. 3. Results and discussion
Fig. 3. Initial chargeedischarge curves of LiCoPO4jLi half-cells with PP and 3DOM PI separators. The cells were tested in the potential range of 3.0e5.1 V at 0.1 C.
The SEM images of conventional PP and 3DOM PI separators are displayed in Fig. 2. 3DOM PI separator has ordered uniform macropores with pore size of 280 nm, while PP separator possesses heterogeneous pore structure. The porosity of 3DOM PI separator is ca. 70% [31], which is about twice porosity of conventional PP separator (ca. 40%) [18]. In addition, the 3DOM PI separator has high electrolyte wettability due to the relatively polar constituent of imide structure [29]. As a result, 3DOM PI separator affords a larger electrolyte uptake of 450% relative to that of PP separator (120%). To investigate the effect of 3DOM PI separator on the electrochemical performance of LiCoPO4, galvanostatic chargeedischarge test of LiCoPO4jLi half-cells with PP and 3DOM PI separators were carried out at 0.1 C (16.7 mA g1) in the potential range of 3.0e5.1 V. LiCoPO4 employed in this work has the average particle diameter of approximately 650 nm estimated by the measurement of one hundred particles on the basis of their SEM images. The specific surface area and carbon content of the LiCoPO4 are determined to be 9.0 m2 g1 and 2.0% by the BET method and the thermogravimetric analysis, respectively. Fig. 3 presents the initial chargeedischarge curves of the cells with PP and 3DOM PI separators. Both cells exhibit typical chargeedischarge features of LiCoPO4 as reported in the literature [2,4,30]. The cells using PP and 3DOM PI separators show almost the same initial discharge capacity of 122.8 and 125.3 mA h g1, respectively. However, the cycle stability of LiCoPO4 strongly depends on the separators. The coulombic efficiency and discharge capacity versus cycle number of the cells with PP and 3DOM PI separators are presented in Fig. 4(a) and (b), respectively. Both cells show low coulombic efficiency of below 100% during cycling, indicating the parallel
occurrence of irreversible anodic reactions. The irreversible reactions are probably assigned to oxidation of electrolyte and separator at high potentials [4,17]. The cell with 3DOM PI separator exhibits much higher coulombic efficiency than that with PP separator during cycling. At the 100th cycle, coulombic efficiency of the cells with PP and 3DOM PI separators are 89.8 and 96.7%, respectively. This result demonstrates that 3DOM PI separator reduces the irreversible reactions during cycling. The high coulombic efficiency of the cells using 3DOM PI separator prevents deposition of electrolyte decomposition products on the surface of LiCoPO4 cathode and the separator [4e8,17], enhancing its cycle stability relative to that with PP separator (Fig. 4(b)). The cell using 3DOM PI separator delivers the discharge capacity of 48.4 mA h g1 at the 100th cycle, and the capacity retention is 38.7%, much higher than those of the cell with PP separator (7.6 mA h g1 and 6.2%). To confirm the change of functional groups in the separators after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells, the separators before and after cycling were analyzed by the FT-IR measurement. 3DOM PI separator before cycling shows typical spectrum of polyimide with the sharp absorption peaks at 1775, 1720, 1380, and 725 cm1 related to C]O symmetric stretching, C]O asymmetric stretching, CeN stretching, and C]O bending, respectively [20,29]. The spectrum of 3DOM PI is maintained after cycling (Fig. 5), demonstrating that PI has high anodic stability and its chemical structures are not changed at the high operating potential of LiCoPO4 (4.8 V vs. Li/Liþ). The high anodic stability enhances the coulombic efficiency of the cell using
Fig. 2. SEM images of PP and 3DOM PI separators.
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Fig. 4. (a) Coulombic efficiency and (b) discharge capacity versus cycle number of LiCoPO4jLi half-cells with PP and 3DOM PI separators. The cells were tested in the potential range of 3.0e5.1 V at 0.1 C.
Fig. 5. FT-IR spectra of PP and 3DOM PI separators before and after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells.
3DOM PI separator (Fig. 4). On the other hand, the spectrum of PP separator after cycling has new peaks at 1250 and 1740 cm1 related to CeO stretching [32] and C]O stretching [33,34] (arrowed in Fig. 5). This result indicates that PP is anodically unstable and oxidized at high potentials, lowering the coulombic efficiency of the cell using PP separator. Fig. 6 displays SEM images of PP and 3DOM PI separators after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells. The number of pores in PP separator decreases after cycling compared with its fresh state (Figs. 2 and 6). It has been reported that polyolefin separators are oxidized at around 5 V vs. Li/ Liþ [35]. Additionally, the side reactions occurring among the highvoltage cathode material, PP separator, and carbonate-based electrolyte at high potentials have been reported to produce deposits on the surface of the separator [17]. Thus, the clogged pores observed for PP separator is considered to be due to the gradual decomposition of its surface during charge and discharge cycles in
addition to the deposition of decomposition products of electrolyte. In contrast to PP separator, the macroporous structure in 3DOM PI separator is maintained after cycling (Fig. 6), which is ascribed to its high anodic stability. To further investigate the interfacial reaction resistance of the cathodes, electrochemical impedance spectroscopy was conducted. The Nyquist plots of the LiCoPO4jLi half-cells using PP and 3DOM PI separators at the fresh state and after 100 cycles of the galvanostatic chargeedischarge test are shown in Fig. 7(a) and (b), respectively. All spectra are composed of a semi-circle in the high frequency range and a sloping line in the low frequency range. The semi-circle and the sloping line correspond to the charge-transfer resistance and the Warburg impedance associated with diffusion of the lithium-ion, respectively [10]. The spectra were fitted using the equivalent circuit model (inset of Fig. 7(a)) with ohmic resistances of electrolyte (Re), charge-transfer resistance (Rct), constant phase elements associated with the double-layer capacitance
Fig. 6. SEM images of PP and 3DOM PI separators after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells.
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Fig. 7. Electrochemical impedance spectra of LiCoPO4 with PP and 3DOM PI separators. The LiCoPO4 electrodes in Li half-cells were measured (a) at the fresh state and (b) after 100 cycles of the galvanostatic chargeedischarge test. Inset of (a) is the equivalent circuit established for simulation of impedance spectra.
between cathode particles and electrolyte (CPE), and the Warburg impedance (W) [10,36,37]. At the fresh state, the Re of the cells using PP and 3DOM PI separators are 3.4 and 1.7 U, respectively. The smaller Re of the cell using 3DOM PI separator is due to its high electrolyte wettability. The cell using PP separator shows the increased Re of 4.6 U after 100 cycles owing to the clogged pores as displayed in Fig. 6. As reported in many literature, the decomposed products of electrolyte, the so-called solid-electrolyte interphase have lithium-ion conductivity [38]. For this reason, the Re of the cell using PP separator did not increase so much even when the pores of PP separator were clogged with decomposed materials. The cell using 3DOM PI separator with high anodic stability shows the Re of 1.4 U after 100 cycles, which is equivalent to the value at the fresh state. The Rct of the cells using PP and 3DOM PI separators are almost the same values of 202.1 and 199.0 U, respectively, at the fresh state. Meanwhile, after 100 cycles, the cell with 3DOM PI separator presents smaller Rct of 113.3 U than that for the cell with PP separator (209.4 U). Furthermore, it has proposed that the circularity of semi-circle in impedance spectrum is directly related to the uniformity of electrode surface [39,40]. The circularity of semi-circle for the cell using 3DOM PI separator is higher than that of the cell using PP separator, which indicates that the surface of the cathode using 3DOM PI separator is more uniform after cycling. The smaller Rct and the uniform cathode surface should be due to the ordered macroporous structure of 3DOM PI separator which provides uniform current distribution. The uniform current distribution lowers overpotential during charge process, reducing the decomposition of electrolyte and growth of the resistive film on the cathode surface. Furthermore, the high anodic stability of 3DOM PI separator also contributes to the superior stability of the cell, since the chemical and macroporous structures in the separator are not changed after cycling at high potentials (Figs. 5 and 6). The small Rct and the uniform cathode surface of the cell using 3DOM PI separator after cycling accord well with its high cycle stability as discussed above (Fig. 4). On the other hand, the larger Rct and low uniform cathode surface for the cell using PP separator is attributed to its low anodic stability which causes the oxidation of separator itself and the side reactions among the active material, the separator, and electrolyte at high potentials [17]. These undesirable reactions block the pores in PP separator, which leads to heterogeneous current distribution. The heterogeneous current distribution results in the high overpotential during charge process, accelerating the decomposition of electrolyte and the consequent accumulation of the resistive film on the cathode surface. It is also noteworthy that the Rct of the cell with 3DOM PI separator decreases after cycling, which possibly results from gradual
percolation of electrolyte or other activating effects [37]. This work demonstrates that the cycle stability of LiCoPO4 is enhanced by applying 3DOM PI separator in spite of using carbonate-based electrolytes which are unstable at around 5 V vs. Li/Liþ [41e43]. The cycle performance will be further improved by combining 3DOM-PI separator with promising electrolytes and additives such as fluoroethylene carbonate [5,6] and lithium difluoro(oxalato) borate [8], reported by other research groups. 4. Conclusions The improved coulombic efficiency and capacity retention of LiCoPO4 were achieved by using 3DOM PI separator, compared with conventional PP separator. The ordered macroporous structure in 3DOM PI separator provides uniform current distribution, which results in the low overpotential during charge process, preventing the decomposition of electrolyte and the increase in Rct during cycling. The high anodic stability of 3DOM PI separator maintains its chemical and macroporous structures after cycling at high potentials, leading to the superior stability of the cell. The ordered structure and high anodic stability of 3DOM PI separator enhance the cycle stability of LiCoPO4. On the other hand, the cell using PP separator with poor anodic stability shows low coulombic efficiency related to a large amount of irreversible reactions such as oxidation of electrolyte and separators. The irreversible reactions block the pores in PP separator, which leads to heterogeneous current distribution. The heterogeneous current distribution increases overpotential during charge process, accelerating the decomposition of electrolyte and the growth of the resistive film on the cathode surface. This work demonstrates that the pore structure and anodic stability of separators are important factors to improve the cycle performance of high-voltage cathode materials. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] M. Hu, X. Pang, Z. Zhou, J. Power Sources 237 (2013) 229e242. [2] K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid-State Lett. 3 (2000) 178e179. [3] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188e1194. [4] N.N. Bramnik, K. Nikolowski, C. Baehtz, K.G. Bramnik, H. Ehrenberg, Chem. Mater. 19 (2007) 908e915. [5] R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach,
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced cycle stability of LiCoPO4 by using three-dimensionally ordered macroporous polyimide separator Yuta Maeyoshi, Shohei Miyamoto, Hirokazu Munakata, Kiyoshi Kanamura* Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo, 192-0397, Japan
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
Enhanced cycle stability of LiCoPO4 is achieved by using 3DOM PI separator. 3DOM PI separator with high anodic stability reduces irreversible reactions. The ordered porous structure of the separator provides uniform current distribution. The uniform current distribution prevents increase in cell impedance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2017 Received in revised form 17 February 2017 Accepted 14 March 2017
To enhance the low cycle stability of LiCoPO4, the development of stable separators in batteries is required, since they are oxidized in the high-voltage system at around 5 V, affecting the cycle performance of high-voltage lithium-ion batteries. The performance of the batteries also depends on the porous structure of separators. Here we report improved coulombic efficiency and capacity retention of LiCoPO4 by using a three-dimensionally ordered macroporous polyimide (3DOM PI) separator, compared with a conventional polypropylene separator with heterogeneous pore structure. The enhanced cycle stability of the cell using 3DOM PI separator is attributed to its ordered macroporous structure and high anodic stability. The uniform current distribution created by the ordered macroporous structure results in the low overpotential during charge process, preventing the oxidation of electrolyte and the growth of the resistive film on the cathode surface during cycling. Furthermore, the high anodic stability of 3DOM PI separator maintains its chemical and macroporous structures after cycling at high potentials, leading to the superior stability of the cell. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion battery High voltage Separator Polyimide Three-dimensionally ordered macroporous structure Lithium cobalt phosphate
1. Introduction The energy density and safety of lithium-ion batteries (LIBs) demand to be improved for requirements of plug-in hybrid electric vehicles, electric vehicles, and smart grids [1]. The performance of
* Corresponding author. E-mail address: [email protected] (K. Kanamura). http://dx.doi.org/10.1016/j.jpowsour.2017.03.053 0378-7753/© 2017 Elsevier B.V. All rights reserved.
LIBs highly depends on the characteristics of cathode materials. Lithium cobalt phosphate (LiCoPO4) is considered a promising cathode material, since it possesses a high operating potential (4.8 V vs. Li/Liþ), a flat voltage profile and a good theoretical capacity (167 mA h g1), improving the energy density of LIBs [2]. Moreover, LiCoPO4 with olivine structure has high thermal and structural stability derived from the strong PeO covalent band, realizing high safety in LIBs [3]. However, the high operating potential of LiCoPO4 triggers irreversible reactions assigned to
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decomposition of electrolyte, which generates undesired byproducts and forms the resistive film on the surface of the cathode, resulting in pronounced capacity fading upon cycling [4e8]. The capacity fading has also resulted from the degradation of LiCoPO4 by HF included in electrolyte solution as an impurity [9]. Several strategies have been employed to enhance the cycle stability, such as doping with metal ions [10e12], surface modification of the cathode material [13e15], substitution of anodic stable electrolyte for conventional one [5,6], and inclusion of additives in electrolyte [7,8,16]. Most of the strategies have focused on the cathode material and electrolyte. To overcome the low cycle stability of LiCoPO4, the development of stable separators in batteries is also required. The separators are considered to be inert in conventional LIBs at 4 V, while they are oxidized in the high-voltage system at around 5 V, affecting the cycle performance of high-voltage LIBs. Chen et al. have reported that polyethylene (PE) separator was more stable than polypropylene (PP) separator under high potentials due to lower HOMO energy of PE. The high anodic stability of PE separator has been found to improve the cycle stability of high-voltage cathode material LiCr0.05Ni0.45Mn1.5O4 [17]. Therefore, the separators with high anodic stability are essential to enhance the cycle performance of LiCoPO4. Generally, porous membranes are used as separators in LIBs, whose role is to prevent electrical short circuits between an anode and a cathode but easy permeation of ions. Thus, the safety and performance of LIBs strongly depend on the porous separator, and many advanced porous membranes have been developed for the batteries [18e24]. We have previously developed an ultrafine porous polyimide (PI) membrane with three-dimensionally ordered macroporous (3DOM) structure illustrated in Fig. 1, and investigated its performance as a battery separator [25e28]. 3DOM PI membranes has a high porosity of ca. 70% and uniform distribution of pores in a hexagonal close-packed arrangement, providing uniform current distribution during chargeedischarge processes in batteries. Actually, the uniform current distribution of 3DOM PI separator has prevented the increase in surface area of lithium-metal electrodes and dendrite-formed lithium deposition during charge process in LIBs [25e27]. Taking account of the result, the uniform current distribution of 3DOM PI separator is expected to lower overpotential during charge process, reducing the decomposition of electrolyte and the consequent accumulation of the resistive film on the cathode surface. Furthermore, PI are known for high thermal and chemical stability, and has been reported to be electrochemical stable at around 5.0 vs. Li/Liþ [29]. Therefore, the cycle performance of LiCoPO4 should be enhanced by using 3DOM PI separator. In this work, we investigated the effect of 3DOM PI separator on the electrochemical performance of LiCoPO4jLi cell compared with
Fig. 1. Schematic of three-dimensionally ordered macroporous structure.
conventional PP separator. The cycle stability of LiCoPO4 was discussed in terms of the anodic stability and the pore structure of the separators, and charge-transfer resistance of the cells related to the formation of the resistive film on the cathode surface. 2. Experimental 2.1. Synthesis and characterization of LiCoPO4 Carbon-coated LiCoPO4 was synthesized by a one-pot hydrothermal process according to our previous report [30]. Briefly, 0.09 mol of CoSO4$7H2O (Wako Pure Chemical Industries, Ltd.), 0.09 mol of Li3PO4 (Kojundo Chemical Lab. Co., Ltd.), and 2.0 g of carboxymethylcellulose sodium salt (Mw ¼ 9.0 104 g mol1, Sigma-Aldrich Co.) were dissolved into degassed water (30 ml) and heated at 200 C for 24 h under N2 atmosphere. After the hydrothermal treatment, the resulting precipitation was separated centrifugally, washed, and subsequently freeze-dried. The dried powder was heat-treated at 700 C for 1 h under 97% Ar/3% H2 atmosphere and carbon-coated LiCoPO4 was obtained. The X-ray diffraction (XRD, RINT 2000/PC, Rigaku Co.) pattern of the product was assigned to phospho-olivine LiCoPO4 with orthorhombic Pnma space group. The size and morphology of the prepared LiCoPO4 were observed by a scanning electron microscope (SEM, JSM7500F, JEOL Ltd.). The specific surface area of the sample was calculated using the BrunauereEmmeteTeller (BET) equation from the adsorption isotherm of N2 gas at 77 K determined by a BELSORP-mini II apparatus (MicrotracBEL Corp.). The carbon content of the sample was determined by a thermogravimetric analysis (DTG-60H, Shimadzu Co.). 2.2. Preparation of 3DOM PI separator 3DOM PI separator was prepared by using a colloidal crystal template composed of mono-disperse spherical particles [31]. Mono-dispersed silica particles in aqueous solution were accumulated by filtration process, and opal-silica template was obtained. The vacant space of silica template was filled with a precursor solution of PI, and then heated at 320 C to convert the precursor to PI. After that, the sample was immersed in 10 wt% HF aqueous solution at room temperature for 24 h to eliminate the opal-silica template. After this elimination, PI membrane remained and had inverse-opal structure. 2.3. Characterization and electrochemical measurement of separators The cathode was fabricated by coating the slurry of mixture consisting of 80 wt% LiCoPO4, 10 wt% acetylene black (Li-100, Denka Co., Ltd.), and 10 wt% polyvinylidene difluoride (Kureha Co.) in Nmethyl pyrrolidone on Al foil current collector (20 mm in thickness) using a doctor blade. The electrode was dried at 120 C under vacuum for 12 h and punched into a circle shape (14 mm in diameter). The loading amount of the cathode material was 2.2e2.7 mg cm2 with a thickness of 16e20 mm. LiCoPO4jLi halfcells were assembled in the 2032 coin type cell in an Ar filled glove box. 1 mol dm3 LiPF6/ethylene carbonate:diethyl carbonate ¼ 1:2 (v/v) (Kishida chemical Co., Ltd.) was used as an electrolyte solution. Two types of separators were used for the preparation of the cells, namely, PP and 3DOM PI separators. Galvanostatic chargeedischarge tests of the cells were carried out with a chargeedischarge unit (HJ1001SD8, Hokuto Denko Co.) in the potential range of 3.0e5.1 V at 0.1 C (1 C ¼ 167 mA g1). Current densities and specific capacities of LiCoPO4 cathodes were calculated on the basis of the weight of LiCoPO4. Electrochemical
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impedance spectroscopy was conducted using a Solartron SI 1287 electrochemical interface with an AC modulation amplitude of ±5 mV in the frequency range from 1 MHz to 0.01 Hz. The spectra were measured at a fresh state and after 100 cycles of the galvanostatic chargeedischarge test. The cycled cells were disassembled in the glove box, and the separators were washed with dimethyl carbonate and dried under vacuum. The morphology of the separators before and after cycling was observed by the SEM. Reflection spectra of the separators before and after cycling were collected using a Fourier transform infrared (FT-IR) spectrometer (FT/IR6100, Jasco Co.) by an attenuated total reflectance method. Liquid electrolyte uptakes of the separators were determined by using following equation:
Electrolyte uptake ð%Þ ¼ ðWf WiÞ=Wi 100 where Wi and Wf are the weights of the separators before and after soaking in the electrolyte for 2 h. 3. Results and discussion
Fig. 3. Initial chargeedischarge curves of LiCoPO4jLi half-cells with PP and 3DOM PI separators. The cells were tested in the potential range of 3.0e5.1 V at 0.1 C.
The SEM images of conventional PP and 3DOM PI separators are displayed in Fig. 2. 3DOM PI separator has ordered uniform macropores with pore size of 280 nm, while PP separator possesses heterogeneous pore structure. The porosity of 3DOM PI separator is ca. 70% [31], which is about twice porosity of conventional PP separator (ca. 40%) [18]. In addition, the 3DOM PI separator has high electrolyte wettability due to the relatively polar constituent of imide structure [29]. As a result, 3DOM PI separator affords a larger electrolyte uptake of 450% relative to that of PP separator (120%). To investigate the effect of 3DOM PI separator on the electrochemical performance of LiCoPO4, galvanostatic chargeedischarge test of LiCoPO4jLi half-cells with PP and 3DOM PI separators were carried out at 0.1 C (16.7 mA g1) in the potential range of 3.0e5.1 V. LiCoPO4 employed in this work has the average particle diameter of approximately 650 nm estimated by the measurement of one hundred particles on the basis of their SEM images. The specific surface area and carbon content of the LiCoPO4 are determined to be 9.0 m2 g1 and 2.0% by the BET method and the thermogravimetric analysis, respectively. Fig. 3 presents the initial chargeedischarge curves of the cells with PP and 3DOM PI separators. Both cells exhibit typical chargeedischarge features of LiCoPO4 as reported in the literature [2,4,30]. The cells using PP and 3DOM PI separators show almost the same initial discharge capacity of 122.8 and 125.3 mA h g1, respectively. However, the cycle stability of LiCoPO4 strongly depends on the separators. The coulombic efficiency and discharge capacity versus cycle number of the cells with PP and 3DOM PI separators are presented in Fig. 4(a) and (b), respectively. Both cells show low coulombic efficiency of below 100% during cycling, indicating the parallel
occurrence of irreversible anodic reactions. The irreversible reactions are probably assigned to oxidation of electrolyte and separator at high potentials [4,17]. The cell with 3DOM PI separator exhibits much higher coulombic efficiency than that with PP separator during cycling. At the 100th cycle, coulombic efficiency of the cells with PP and 3DOM PI separators are 89.8 and 96.7%, respectively. This result demonstrates that 3DOM PI separator reduces the irreversible reactions during cycling. The high coulombic efficiency of the cells using 3DOM PI separator prevents deposition of electrolyte decomposition products on the surface of LiCoPO4 cathode and the separator [4e8,17], enhancing its cycle stability relative to that with PP separator (Fig. 4(b)). The cell using 3DOM PI separator delivers the discharge capacity of 48.4 mA h g1 at the 100th cycle, and the capacity retention is 38.7%, much higher than those of the cell with PP separator (7.6 mA h g1 and 6.2%). To confirm the change of functional groups in the separators after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells, the separators before and after cycling were analyzed by the FT-IR measurement. 3DOM PI separator before cycling shows typical spectrum of polyimide with the sharp absorption peaks at 1775, 1720, 1380, and 725 cm1 related to C]O symmetric stretching, C]O asymmetric stretching, CeN stretching, and C]O bending, respectively [20,29]. The spectrum of 3DOM PI is maintained after cycling (Fig. 5), demonstrating that PI has high anodic stability and its chemical structures are not changed at the high operating potential of LiCoPO4 (4.8 V vs. Li/Liþ). The high anodic stability enhances the coulombic efficiency of the cell using
Fig. 2. SEM images of PP and 3DOM PI separators.
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Fig. 4. (a) Coulombic efficiency and (b) discharge capacity versus cycle number of LiCoPO4jLi half-cells with PP and 3DOM PI separators. The cells were tested in the potential range of 3.0e5.1 V at 0.1 C.
Fig. 5. FT-IR spectra of PP and 3DOM PI separators before and after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells.
3DOM PI separator (Fig. 4). On the other hand, the spectrum of PP separator after cycling has new peaks at 1250 and 1740 cm1 related to CeO stretching [32] and C]O stretching [33,34] (arrowed in Fig. 5). This result indicates that PP is anodically unstable and oxidized at high potentials, lowering the coulombic efficiency of the cell using PP separator. Fig. 6 displays SEM images of PP and 3DOM PI separators after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells. The number of pores in PP separator decreases after cycling compared with its fresh state (Figs. 2 and 6). It has been reported that polyolefin separators are oxidized at around 5 V vs. Li/ Liþ [35]. Additionally, the side reactions occurring among the highvoltage cathode material, PP separator, and carbonate-based electrolyte at high potentials have been reported to produce deposits on the surface of the separator [17]. Thus, the clogged pores observed for PP separator is considered to be due to the gradual decomposition of its surface during charge and discharge cycles in
addition to the deposition of decomposition products of electrolyte. In contrast to PP separator, the macroporous structure in 3DOM PI separator is maintained after cycling (Fig. 6), which is ascribed to its high anodic stability. To further investigate the interfacial reaction resistance of the cathodes, electrochemical impedance spectroscopy was conducted. The Nyquist plots of the LiCoPO4jLi half-cells using PP and 3DOM PI separators at the fresh state and after 100 cycles of the galvanostatic chargeedischarge test are shown in Fig. 7(a) and (b), respectively. All spectra are composed of a semi-circle in the high frequency range and a sloping line in the low frequency range. The semi-circle and the sloping line correspond to the charge-transfer resistance and the Warburg impedance associated with diffusion of the lithium-ion, respectively [10]. The spectra were fitted using the equivalent circuit model (inset of Fig. 7(a)) with ohmic resistances of electrolyte (Re), charge-transfer resistance (Rct), constant phase elements associated with the double-layer capacitance
Fig. 6. SEM images of PP and 3DOM PI separators after 100 cycles of the galvanostatic chargeedischarge test in LiCoPO4jLi half-cells.
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Fig. 7. Electrochemical impedance spectra of LiCoPO4 with PP and 3DOM PI separators. The LiCoPO4 electrodes in Li half-cells were measured (a) at the fresh state and (b) after 100 cycles of the galvanostatic chargeedischarge test. Inset of (a) is the equivalent circuit established for simulation of impedance spectra.
between cathode particles and electrolyte (CPE), and the Warburg impedance (W) [10,36,37]. At the fresh state, the Re of the cells using PP and 3DOM PI separators are 3.4 and 1.7 U, respectively. The smaller Re of the cell using 3DOM PI separator is due to its high electrolyte wettability. The cell using PP separator shows the increased Re of 4.6 U after 100 cycles owing to the clogged pores as displayed in Fig. 6. As reported in many literature, the decomposed products of electrolyte, the so-called solid-electrolyte interphase have lithium-ion conductivity [38]. For this reason, the Re of the cell using PP separator did not increase so much even when the pores of PP separator were clogged with decomposed materials. The cell using 3DOM PI separator with high anodic stability shows the Re of 1.4 U after 100 cycles, which is equivalent to the value at the fresh state. The Rct of the cells using PP and 3DOM PI separators are almost the same values of 202.1 and 199.0 U, respectively, at the fresh state. Meanwhile, after 100 cycles, the cell with 3DOM PI separator presents smaller Rct of 113.3 U than that for the cell with PP separator (209.4 U). Furthermore, it has proposed that the circularity of semi-circle in impedance spectrum is directly related to the uniformity of electrode surface [39,40]. The circularity of semi-circle for the cell using 3DOM PI separator is higher than that of the cell using PP separator, which indicates that the surface of the cathode using 3DOM PI separator is more uniform after cycling. The smaller Rct and the uniform cathode surface should be due to the ordered macroporous structure of 3DOM PI separator which provides uniform current distribution. The uniform current distribution lowers overpotential during charge process, reducing the decomposition of electrolyte and growth of the resistive film on the cathode surface. Furthermore, the high anodic stability of 3DOM PI separator also contributes to the superior stability of the cell, since the chemical and macroporous structures in the separator are not changed after cycling at high potentials (Figs. 5 and 6). The small Rct and the uniform cathode surface of the cell using 3DOM PI separator after cycling accord well with its high cycle stability as discussed above (Fig. 4). On the other hand, the larger Rct and low uniform cathode surface for the cell using PP separator is attributed to its low anodic stability which causes the oxidation of separator itself and the side reactions among the active material, the separator, and electrolyte at high potentials [17]. These undesirable reactions block the pores in PP separator, which leads to heterogeneous current distribution. The heterogeneous current distribution results in the high overpotential during charge process, accelerating the decomposition of electrolyte and the consequent accumulation of the resistive film on the cathode surface. It is also noteworthy that the Rct of the cell with 3DOM PI separator decreases after cycling, which possibly results from gradual
percolation of electrolyte or other activating effects [37]. This work demonstrates that the cycle stability of LiCoPO4 is enhanced by applying 3DOM PI separator in spite of using carbonate-based electrolytes which are unstable at around 5 V vs. Li/Liþ [41e43]. The cycle performance will be further improved by combining 3DOM-PI separator with promising electrolytes and additives such as fluoroethylene carbonate [5,6] and lithium difluoro(oxalato) borate [8], reported by other research groups. 4. Conclusions The improved coulombic efficiency and capacity retention of LiCoPO4 were achieved by using 3DOM PI separator, compared with conventional PP separator. The ordered macroporous structure in 3DOM PI separator provides uniform current distribution, which results in the low overpotential during charge process, preventing the decomposition of electrolyte and the increase in Rct during cycling. The high anodic stability of 3DOM PI separator maintains its chemical and macroporous structures after cycling at high potentials, leading to the superior stability of the cell. The ordered structure and high anodic stability of 3DOM PI separator enhance the cycle stability of LiCoPO4. On the other hand, the cell using PP separator with poor anodic stability shows low coulombic efficiency related to a large amount of irreversible reactions such as oxidation of electrolyte and separators. The irreversible reactions block the pores in PP separator, which leads to heterogeneous current distribution. The heterogeneous current distribution increases overpotential during charge process, accelerating the decomposition of electrolyte and the growth of the resistive film on the cathode surface. This work demonstrates that the pore structure and anodic stability of separators are important factors to improve the cycle performance of high-voltage cathode materials. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] M. Hu, X. Pang, Z. Zhou, J. Power Sources 237 (2013) 229e242. [2] K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid-State Lett. 3 (2000) 178e179. [3] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188e1194. [4] N.N. Bramnik, K. Nikolowski, C. Baehtz, K.G. Bramnik, H. Ehrenberg, Chem. Mater. 19 (2007) 908e915. [5] R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach,
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