Enhanced Electrochemical Properties of LiFePO4 Electrodes with

The electrode area was 1.77 cm2, thickness was about 120 μm before pressing ... The ASI measurements were performed using a 30 s current interruption...
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Enhanced Electrochemical Properties of LiFePO4 Electrodes with Carboxylated Poly(vinyl difluoride) in Lithium-Ion Batteries: Experimental and Theoretical Analysis Ki Chun Kil,† Maeng Eun Lee,‡ Gu Yeon Kim,§ Chae-Woong Cho,*,z Kijun Kim,z Geunbae Kim,|| and Ungyu Paik*,† †

WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea CRD) Core Technology Lab, and zCell) Precedence Material Development Group, Samsung SDI Co. Ltd., Yongin, Kyeonggi-Do 449-092, Korea § Cell) Advanced Product Development Group, and Cell) Material Development Team, Samsung SDI Co. Ltd., Cheonan, Chungcheongnam-Do 330-300, Korea )

‡

ABSTRACT: The adhesion strengths between LiFePO4 cathodes and aluminum (Al) current collectors as well as the corresponding electrochemical properties of electrodes with carboxylated poly(vinyl difluoride) (C-PVdF) were experimentally and theoretically investigated. The adhesion strength of LiFePO4 cathodes with C-PVdF on Al current collectors were increased, compared with that of as-received PVdF, resulting in the reduction of the internal resistance of the electrode. These results were supported by theoretical simulations based on the Metropolis Monte Carlo method. Electrochemical experiments indicated that the cyclability and rate capabilities of the cathode were improved as the weight fraction of C-PVdF in the electrode increased up to 70% of the polymer binders. In addition, 18 650-sized full cells employing the electrodes with C-PVdF showed promising performance in terms of rate capability as well as long-term cyclability, which suggests that this strategy can be beneficial for the realization of high power sources such as electric vehicles, electric bikes, and various power tools.

1. INTRODUCTION Large-scale lithium-ion batteries (LIBs) have gained attention as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and stationary energy storage for solar and wind electricity generation. Many studies focusing on cathodes, anodes, electrolytes, and separators have been conducted for the improvement of battery performance. In the case of cathodes, numerous studies1 5 have explored the demand characteristics of cathode materials, such as uniform flat discharge potential, high capacity, high rate capability (high ionic and electronic conductivity), long-term cyclability, and structural stability during the discharge and charge process. Lithium transition metal phosphates with olivine structure, such as LiFePO4, are expected to be a promising cathode material for large-scale LIBs since it has excellent cyclability, high safety, and good thermal stability in the fully charged state.6 10 Nonetheless, the poor electronic conductivity and low lithium-ion diffusivity of LiFePO4 adversely have a negative influence on the capacity and rate capabilities of devices, which impedes its expansion to the commercial field.11 13 In order to improve the intrinsic properties of LiFePO4, particularly the low electrical conductivity, a variety of strategies such as conductive carbon coating,14,15 metal doping,11,16 and particle size control17,18 have been reported. r 2011 American Chemical Society

Moreover, electrochemical properties of a cathode are closely related to the physical parameters of the electrode including film thickness, active loading, packing density, and adhesion strength.19,20 In particular, the adhesion strength between the electrode and current collector can be a key factor for enhancing the electrochemical properties of the electrode.21 23 Chen et al.21 noted that the enhanced adhesion strength of the Si/C composite anode on the Cu substrate by the addition of acrylic adhesive leads to better cycling performance of the electrode. Huang et al.22 showed that the adhesion strength of the active material on the current collector and cycle efficiency of the cathode are increased by the addition of carbon nanotubes (CNTs) into a LiFePO4 composite electrode. Generally, the adhesion strength of the electrode on the current collector is correlated with the physicochemical properties of the polymeric binder.24,25 Therefore, adequate control of the adhesion strength by changing the physicochemical properties of the binder may lead to the improvement of the electrochemical properties of LIBs. Received: June 3, 2011 Revised: July 11, 2011 Published: July 13, 2011 16242

dx.doi.org/10.1021/jp205233h | J. Phys. Chem. C 2011, 115, 16242–16246

The Journal of Physical Chemistry C In this study, we experimentally and theoretically investigated improvements in the adhesion strengths of LiFePO4 cathodes on the Al current collector and the corresponding electrochemical behavior of the LIBs through the carboxylation of poly(vinyl difluoride) (hereinafter denoted C-PVdF) as a binder in the electrode. Thus, the effects of carboxylic groups in PVdF on the adhesion strengths between electrodes and current collectors were investigated by measuring the adhesion strengths of LiFePO4 cathodes on Al current collectors. The influence of C-PVdF on the internal resistance of the electrode was evaluated by area specific impedance (ASI). These results are supported by calculations of binding energy conducted using a Metropolis Monte Carlo method.26 Cycle-life and rate capabilities for the LiFePO4 cathode of LIBs were examined in order to confirm the correlation between the adhesion strength of the electrode with C-PVdF and electrochemical properties. Cylindrical 18 650sized full cells were fabricated, and the corresponding electrochemical behaviors (rate capabilities and cycle performance) were investigated to confirm the realization.

2. EXPERIMENTAL SECTION 2.1. LiFePO4 Electrode Preparation. Partially carbon-coated LiFePO4 powder was obtained from a commercial source (P2, Phostech, Lithium Inc.). Partially carbon-coated LiFePO4 powder has an average particle size of 0.5 1 μm, surface area (BET) of 12 18 m2/g, and carbon content of 2 wt %, respectively. Carbon black (Denka Black) with a primary particle size of 25 50 nm was used as a conducting agent. Conventional PVdF (6020, Solvay Solexis, Belgium) and/or 3 wt % carboxylic group substituted PVdF (5320, Solvay Solexis, Belgium) were used as binders. For the preparation of cathode slurry, 90% LiFePO4 powder, 5% carbon black, 5% PVdF binder (the weight fraction of carboxylated PVdF in the binder system was controlled from 10% to 100%), and a suitable amount of polyurethane dispersant were mixed in 1-methyl-2-pyrrolidinone anhydrous (NMP, Sigma-Aldrich) solvent. The slurries were coated onto aluminum foil using a doctor blade. Cast slurries were dried at about 120 °C for 30 min to vaporize the solvent and then dried further in a vacuum oven at 120 °C for 2 h to remove the residual solvent. The electrode area was 1.77 cm2, thickness was about 120 μm before pressing, and active loading of LiFePO4 was 12.4 mg/cm2. 2.2. Characterization. Adhesion strength tests were based on ASTM D903-98 and were measured by using an Instron tensiletest apparatus of Model 5500 fitted with Load Cell Model 2511104 with a constant cross-head speed of 1 cm/min at ambient room temperature. The samples were tested mostly with a 180° pull on a custom-modified Parker-Daedal Model 14615 Linear Motion Slide. The ASI measurements were performed using a 30 s current interruption method during charge/discharge cycling. The details of this procedure were described previously.27 29 Adsorption behaviors (binding energies) of PVdF and C-PVdF on an alumina (Al2O3) surface were investigated using computational methods. In this model, R-alumina (001)30 is used for the surface, which is covered with oxygen atoms. The surface crystal structure of alumina consists of 54 aluminum and 81 oxygen atoms with lengths A, B, and C of 14.28, 14.28, and 5.66 Å and angles R, β, γ of 90°, 90°, and 60° respectively. CH3CH2CF2CH3 for PVdF and CH3CH(COOH)CF2CH3 for C-PVdF are considered analogues in this model. The C-PVdF

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Figure 1. Adhesion strength of LiFePO4 cathodes on Al current collectors and the area specific impedance (ASI) at 80% depth of discharge (DOD) for LiFePO4 electrodes as a function of weight fraction of C-PVdF in the PVdF binder system.

was modeled by substituting one COOH group for one hydrogen in the PVdF monomer unit. Methyl groups are added to the elongation axes to represent the eliminated parts. In order to find the minimum energy interactions between PVdF, C-PVdF analogues, and the alumina surface, 10 000 iterations of a Metropolis Monte Carlo simulation with the Sorption Locate31,32 module were performed in Material Studio 5.0 (Accelrys Inc.). The periodic boundary condition with a 15 Å vacuum slab has been applied to represent bulkiness of the alumina surface. The Universal force field33 35 and the Dreiding force field36 were used with a QEq charge37 for the simulation. The cutoff radius was chosen as 12.5 Å for long-range interactions. The electrochemical performance of the LiFePO4 electrodes was evaluated in coin-type half cells (2032R type). Pure lithium metal foil (Cyprus-Foote Mineral Co., Kings Mountain, NC) was used as an anode. A separator (Celgard 2320) was placed between the electrodes. The electrolyte solution was 1.0 M LiFP6 in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC). All the cells were fabricated in a dry room. The cycle performance and rate capabilities were performed at a rate of 1 C and at various rates from 0.1 to 5 C, respectively, between 3.0 and 4.2 V at 25 °C using a battery cycle tester (TOSCAT 3000, Toyo Systems, Japan). Each 18 650-sized full cell was fabricated with a LiFePO4 cathode, a graphite anode, a separator (Celgard 2325), and 1.0 M LiPF6 in a 1:1 volume ratio of EC and DMC. The cycle performance and rate capabilities were investigated at a rate of 1 C and at various currents between 3.0 and 4.2 V at 25 °C using an Arbin cycler (ABTS 4.0).

3. RESULTS AND DISCUSSION Figure 1 shows the adhesion strength of the LiFePO4 cathode on the Al current collector as a function of the C-PVdF weight fraction in the PVdF binder system of the electrode. The value of adhesion strength for the LiFePO4 cathode with the as-received PVdF is ∼0.92 N/cm2. As expected, it was shown that the adhesion strength is linearly increased in proportion to the weight fraction of C-PVdF in the electrode. In particular, the value of adhesion strength for the LiFePO4 cathode with 70% C-PVdF binder is ∼12.4 N/cm2, which is a factor of approximately 15 higher than that with as-received PVdF. This result agrees with those of previous studies.38,39 Khongtong et al.38 reported that adhesion strength at the interface between oxidized-1,4-polybutadiene (1,4-PBD) with a carboxylic group and 16243

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Table 1. Calculated Binding Energies between PVdF, C-PVdF Analogues, and Alumina Surfaces with Different Force Fields adsorbents

universal FF (kJ/mol) Dreiding FF (kJ/mol)

CH3CH2CF2CH3

378.65

366.18

CH3CH(COOH)CF2CH3

635.80

626.89

Figure 2. Snapshot of minimum energy interaction positions between (a) PVdF and (b) C-PVdF analogues and an alumina surface.

Al/Al2O3 surfaces is higher than that for unoxidized 1,4-PBD. Zhai et al.39 reported that the generation of a carboxylic group at the interface between the epoxy adhesive modified by nanoAl2O3 and steel leads to an increase in adhesion strength. However, beyond 70% C-PVdF binder, the electrode cannot be prepared on the current collector because the slurries instantly form into a gel after the mixing process, which is probably due to a strong chemical reaction between the carboxylic group in PVdF and functional groups of the carbon layer formed on the surface of LiFePO4 particles. Such an improvement in adhesion strength between the electrode and the current collector is directly related to the internal resistance of the electrode. Figure 1 shows that ASI of the electrodes at 80% depth of discharge (DOD) decreased as a function of weight fraction of C-PVdF in the binder system of the electrode, which indicates that the adhesion strength is inversely proportional to the internal resistance of the electrode. It is expected that the amount of carboxyl functional groups in the binder system is increased with C-PVdF fraction in the binder system of the electrode. Therefore, the electrode with high C-PVdF fraction in the binder system shows enhanced adhesion strength between the electrode and the current collector. Also, high contents of C-PVdF in the binder system could reduce the distance between the electrode and current collector through short binding distance between them, which leads to enhancement in electronic conductivity in the electrode by reduced electron transport distance. In order to verify these results, the binding energy was theoretically calculated using the Metropolis Monte Carlo method. The binding energies between PVdF, C-PVdF analogues, and alumina surfaces are presented in Table 1. Even though two different force fields are used, the results show the same tendency. It was confirmed that CH3CH(COOH)CF2CH3 binds more strongly than CH3CH2CF2CH3. The positions presented in Figure 2a show that fluorine atoms are located opposite to the alumina surface to avoid electrostatic repulsion

Figure 3. (a) Discharge cycle performance curves, (b) initial discharge capacity and discharge capacity after 50 cycles, and (c) discharge rate capabilities for LiFePO4 electrodes as a function of weight fraction of C-PVdF in the PVdF binder system.

with oxygen atoms. In Figure 2b, the hydrogen of the carboxyl group in C-PVdF directs to an oxygen atom, resulting in hydrogen bonding. Even though the fluorine in C-PVdF is closer than that of PVdF, the repulsion is compensated with extra hydrogen bonding of C-PVdF. Due to the terminal methyl groups at each end point, minimum structures can be easily extended to represent long chains without large distortion of current conformation. From these results, a simplified monomer model can reasonably be used to explain how C-PVdF is adsorbed on the surface more strongly than PVdF, as shown in the results given in the Experimental Section. Based on simulations, it is expected that the stronger interaction between C-PVdF and the current collector inevitably leads to a decrease in the distance between the electrode and the current collector, which results in a reduction in the electron conducting pathway through the carbon black network toward the current collector and the corresponding internal resistance of the electrode. Therefore, it is suggested that the enhanced adhesion strength of the electrode on the current collector based on the carboxylation 16244

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increase in weight fraction of C-PVdF is correlated with the enhancement of the rate capabilities. Thus, the enhanced electrical conductivity (i.e., reduced internal resistance of the electrode) through the improved adhesion strength of the electrode on the current collector can lead to enhancement of electrochemical properties for the electrode. In order to verify the application and realization of the cathode system utilizing C-PVdF for the full cells, we fabricated two different types of cylindrical 18 650-sized full cells with and without 70% C-PVdF. Figure 4a shows the rate capabilities of the full cells having LiFePO4 cathodes with as-received PVdF and 70% C-PVdF, respectively, at 25 °C under various currents. Normalized discharge energies for the electrodes with C-PVdF present higher rate capabilities at various currents, compared to that for the electrodes with as-received PVdF. Moreover, after 500 cycles, the discharge capacity of the full cells having a LiFePO4 cathode with C-PVdF is higher than that with the asreceived PVdF at 25 °C under a 1 C rate, as shown in Figure 4b. In addition, the discharge capacity retention of the full cells having electrodes with C-PVdF is about 87% of the initial discharge capacity, whereas that of electrodes with as-received PVdF is only about 78% of the initial capacity.

Figure 4. (a) Discharge rate capabilities at various currents and (b) discharge cycle performance curves for the 18 650-sized full cells with and without C-PVdF.

of PVdF results in the improved electron conductivity via shorter electron conducting pathways toward the current collector. These results may contribute to improved electrochemical properties of the electrode, as shown in Figure 3. Figure 3a shows the cycle-life performance curves of the LiFePO4 cathode with PVdF binder system having different weight fractions of C-PVdF, at 25 °C under a 1 C discharge rate. In Figure 3b, the initial discharge capacity of the electrodes with as-received PVdF is 149.7 mAh/g, which is almost identical regardless of the weight fraction of C-PVdF. However, after 50 cycles, remarkable differences in discharge capacity for electrodes with different weight fractions of C-PVdF were observed. The discharge capacity for the electrodes with as-received PVdF is 127.4 mAh/g, which implies that it retains only ∼85% of the initial discharge capacity. On the other hand, the discharge capacities and retentions for the electrodes increased in proportion to the weight fraction of C-PVdF. In particular, the discharge capacity for electrodes with 70% C-PVdF is 138.3 mAh/g and retains ∼92% of the initial discharge capacity, indicating that the discharge capacity for electrodes with 70% C-PVdF has the highest cyclability compared to that for electrodes with other weight fractions of C-PVdFs. Moreover, discharge rate capabilities for electrodes with different weight fractions of C-PVdF also showed similar behavior in cyclability. At lower rates below 0.2 C, the discharge rate capabilities of all electrodes with or without C-PVdF are almost identical, as shown in Figure 3c. On the other hand, as the C rate is increased, a correlation between the discharge rate capabilities of the electrodes and C-PVdF having different weight fractions in the electrode was apparent. For example, at 5 C, the rate capability of the electrode with as-received PVdF is 107.5 mAh/g, whereas that of the electrodes with 70% C-PVdF is 121.2 mAh/g, exhibiting the highest value. In other words, the

4. CONCLUSIONS We experimentally and theoretically investigated the adhesion strengths of LiFePO4 cathodes on Al current collectors and examined the electrochemical properties of electrodes fabricated with the C-PVdFs. It was found that the carboxylation of PVdF (70%) provides the highest adhesion strength and the corresponding lowest internal resistance of the electrode. After performing theoretical simulations using the Metropolis Monte Carlo method, it was confirmed that the carboxylic groups in PVdF impart significantly higher binding energies with the alumina surface on Al current collectors, compared to PVdF. Thus, enhanced adhesion strength and the corresponding lowered internal resistance of the electrode led to improved discharge capacity and rate capabilities (i.e., 92% of retention after 50 cycles and a 121.2 mAh/g rate capability at 5 C). Cylindrical 18 650-sized full cells were fabricated to evaluate performance. Cells fabricated with electrodes with 70% C-PVdF showed higher rate capabilities and discharge capacity retention compared to that of as-received PVdF. Therefore, it can be expected that this study can impact the realization and commercialization of the high power sources such as electric vehicles, electric bikes, and various power tools. ’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected], tel. +82-31-288-4677, fax +8231-288-4667 (C.-W.C); e-mail [email protected], tel. +82-22220-0502, fax +82-2-2281-0502 (U.P).

’ ACKNOWLEDGMENT This work was financially supported by National Research Foundation of Korea (NRF) through Grant No. K20704000003TA050000310, Global Research Laboratory (GRL) Program provided by the Korean Ministry of Education, Science and Technology (MEST) in 2011, WCU (World Class University) program through the National Research Foundation 16245

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The Journal of Physical Chemistry C of Korea funded by the Ministry of Education, Science and Technology (R31-10092), and Samsung SDI Co. Ltd.

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