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Electrochemical performance of LiFePO4@C composites with biomorphic porous carbon loading and nano-core–shell structure
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Electrochemical performance of LiFePO4@C composites with biomorphic porous carbon loading and Nano-core-shell structure Peng-Zhao Gao, Ling Wang, Dong-Yun Li, Bing Yan, Wei-Wei Gong
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Cite this article as: Peng-Zhao Gao, Ling Wang, Dong-Yun Li, Bing Yan, Wei-Wei Gong, Electrochemical performance of LiFePO4@C composites with biomorphic porous carbon loading and Nano-core-shell structure, Ceramics International, http://dx.doi.org/ 10.1016/j.ceramint.2014.04.164 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical Performance of LiFePO4@C Composites with Biomorphic Porous Carbon Loading and Nano-Core-Shell Structure Peng-Zhao Gaoa*, Ling Wanga, Dong-Yun Lib, Bing Yana, Wei-Wei Gonga a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China
b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
*
Corresponding author. Tel.: +86 731 88822269; fax: +86 731 88823554.
Email: [email protected]
Abstract Effect of biomorphic porous carbon (BPC) addition on the composition, microstructure, and electrochemical performance of LiFePO4@C/C composites was investigated. Results indicated that network pores of BPC were almost completely filled by LiFePO4@C nanoparticles, which were formed by an olivine structure LiFePO4 core with size that ranged from 58.6 nm to 80.1 nm and an amorphous carbon shell with a thickness of approximately 2 nm. Double electrical conductive networks formed in the composites improved the electrical properties of samples from 2.59 × 10-6 S·cm-1 (sample A-0) to 5.76 × 10-2 S·cm-1 (samples A-20). Synergy effect of electric double layer energy storage produced by BPC and lithium-ion extraction/insertion energy storage by LiFePO4 clearly reduced the capacity reduction rate of composites, and obtained a charge/discharge capacity of 114.2/110.5 mA·h·g-1 (samples A-5) at 10C. Moreover, addition of BPC showed a significant advantage in reducing the interfacial resistance of the electrode reaction in composites from 86.72 Ω (samples A-0) to 37.58 Ω (samples A-20). The electrical conductive mechanism of LiFePO4@C/C composites is discussed.
1
Keywords LiFePO4@C/C composites; nanostructures; electrochemical properties; double electrical conductive network; synergy effect
1. Introduction Lithium-ion battery has gained popularity in the portable electronics market and is under development for electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs)[1] because of its high energy density and low self-discharge rate. However, the low power density and defective charge/discharge rate performance of lithium-ion battery limits its use in high rate charge/discharge applications [2]. An electric double layer capacitor can form an electric double layer capacitance interface for energy storage depending on polarized electrode and electrolyte. The formation of an electric double layer capacitance interface leads to high power density and excellent rate performance, but the energy density is relatively low. Numerous recent studies have attempted to overcome the drawbacks of single storage component and develop a new kind of high-energy storage device by combining two energy storage mechanisms
[3]
. At present, two ways of combining two energy storage devices are available,
namely, the “outside” and “inside” combinations. In the outside combination, two monomers are combined by a power management system into an energy storage component to achieve larger capacity and power[4]. However, this method has many shortcomings such as having too-large bulk, low specific power, and complex system management and control[5]. In the inside combination, lithium-ion extraction/insertion energy storage and electric double layer capacitance energy storage 2
mechanism are realized in the same electrode, thus ensuring monomer consistency and reduced system management complexity. The presence of lithium-ion extraction/insertion energy storage and electric double layer capacitance energy storage mechanism in the same electrode is an example of inside combination. Current studies on lithium-ion extraction/insertion energy storage and electric double layer capacitance energy storage mechanism in the same electrode is primarily oriented towards the combination of lithium-inserted compounds and activated carbon (AC). In charge and discharge processes, lithium-inserted compounds can function as lithium-ion extraction/insertion energy storage, whereas AC can be the electric double layer capacitance energy storage because of its high specific surface area and electrochemical stability. When the battery is charging, lithium ion can be extracted from the lattice of the cathode made up of lithium-inserted compounds that are spontaneously inserted into the lattice of the anode material through electrolyte. AC on the cathode surface forms an electric double layer as a result of charge adsorption and solvation ion reconstruction. When the battery is discharging, the lithium ion can extract from the anode material spontaneously and insert into the lattice of the cathode of lithium-inserted compounds through the electrolyte, and the charge gathered around the bipolar surface will produce displacement current because of the reconstruction[6]. Hu
[7]
prepared LiFePO4 with AC (LAC) and direct-mixed LAC (DMLAC) composite cathode
materials with different LiFePO4 contents. Hybrid battery-capacitors LAC/Li4Ti5O12 and DMLAC/Li4Ti5O12 with a Li4Ti5O12 anode were subsequently assembled. The overall electrochemical performance of the hybrid battery capacitors was best when the content of LiFePO4 in the composite cathode materials ranged from 11.8 wt.% to 28.5 wt.%. In addition, the hybrid
3
battery-capacitor devices showed good life cycle performance at high rates. The capacity loss of the DMLAC/Li4Ti5O12 hybrid battery-capacitor device at 4C was no more than 4.8% after 1000 cycles, and did not exceed 9.6% after 2000 cycles at 8C. Omar[8] investigated the performance of various lithium-ion chemistries used in PHEVs and compared them with several other rechargeable energy storage system technology, such as lead-acid, nickel-metal hydride, and electrical double layer capacitors. The nickel manganese cobalt oxide cathode stands out with high energy density up to 160 Wh/kg, compared with 70 Wh/kg to 10 Wh/kg for lithium iron phosphate cathode, as well as 90 and 71 Wh/kg for lithium nickel cobalt aluminum cathode and lithium titanate oxide anode battery cells, respectively. The values are considerably higher than those of lead-acid (23 Wh/kg to 28 Wh/kg) and nickel-metal hydride (44 Wh/kg to 53 Wh/kg) battery technologies. Hu[9,10] used LAC as cathode material, and Li4Ti5O12 as the anode material to form the LAC/Li4Ti5O12 system. The hybrid battery-capacitor LAC/Li4Ti5O12 has the advantage of having the high-rate capability of a hybrid capacitor AC/Li4Ti5O12 and the high battery capacity of LiFePO4/Li4Ti5O12. The hybrid battery-capacitor LAC/Li4Ti5O12 is an energy storage device in which the capacitor and secondary battery coexist in one cell system. Few studies have reported on the LiFePO4@C compound with biomorphic porous carbon (BPC), which has highly ordered macroporous structures, controlled specific area, and excellent electric double layer energy storage properties [11,12]. In the present paper, the inside combination compound of LiFePO4@C/C composites was prepared through a modified in-situ restriction polymerization method, using BPC as matrix[13]. The effect of BPC content on the composition, microstructure, and electrochemical performance of LiFePO4@C/C composites was studied by controlling the BPC content. Interesting results obtained are discussed in the succeeding sections. 4
2. Experimental process 2.1. Raw materials The BPC was self-made and smashed into small particles with a size of 50 ± 10 μm and a specific area of 771.6 m2·g-1[13]. Subsequently, BPC particles were dried at 80 °C until a constant weight was reached. Lithium acetate (CH3COOLi⋅2H2O), ferric chloride (FeCl3⋅6H2O), ammonium dihydrogen phosphate (NH4H2PO4), aniline (ANI, C6H5NH2), ascorbic acid (C6H8O6), and ethanol (CH3CH2OH) were used in the experiment. All reagents are of analytical grade. Deionized water was used as main solvent. Ferric chloride, ANI, and ascorbic acid were obtained from the Chinese Medicine Group Chemical Reagent Co., Ltd. Lithium acetate was from Tianjin Kermel Chemical Reagent Co., Ltd. Ammonium dihydrogen phosphate was from Guangdong Taishan Chemical Plant Co., Ltd., and ethanol was from Changsha Antaeus Fine Chemical Industrial Co., Ltd.
2.2. Synthesis of FePO4@polyaniline (PANI)/C Ferric chloride aqueous solution was dropped into ammonium dihydrogen phosphate and ANI mixture aqueous solution at a rate of 10 ml·h-1 under rapid stirring to produce FePO4@PANI. BPC particles with a mass fraction of 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% (the fraction was relative to the weight of the obtained LiFePO4) were subsequently added into the mixture under rapid stirring. The FePO4@PANI/C material was obtained when the mixture was stirred for 10 h at room temperature[13]. The obtained powder was centrifuged and washed thrice with deionized water and dried at 80 °C until a constant weight was obtained.
5
2.3. Synthesis of LiFePO4@C/C composites An appropriate content of FePO4@PANI/C powder was added into the lithium ethanol solution. The solution was stirred for 2 h, and then ascorbic acid was added into the suspension. The mixture was stirred for 8 h at 60 °C. The final precipitates were centrifuged and washed thrice with deionized water and ethanol until a neutral pH was obtained. The precipitate was then dried at 80 °C until a constant weight was obtained. Finally, the precursor was sintered at 700 °C for 3 h to obtain LiFePO4@C/C composites[13]. The samples were labeled as A-0, A-5, A-10, A-15, and A-20, according to the BPC addition.
2.4. Characterization of materials The interfacial nanostructures of LiFePO4@C material was identified by high resolution transmission electron microscopy (HRTEM, JEM-3010, Jeol). The surface morphology of LiFePO4@C/C composites was characterized by scanning electron microscopy (SEM, JSM-6490LV, Jeol). The phase identification of LiFePO4@C/C composites was performed by X-ray diffraction (XRD, Rigaku D/max2200 VPC) using monochromic Cu Kα radiation of 0.15405 nm operated at 40 kV and 40 mA. The conductivity of LiFePO4@C/C composites was measured by 61/2 digital multimeter (34401A, Agilent) and electrical resistivity instrument (GM-II, Shanxi Coal and Chemical Institute). The electrochemical measurement was performed with the CR2025 button cell. The cathode electrode was created using the following procedures. LiFePO4@C/C active material was mixed 6
with acetylene black as conducting agent, and polyvinylidene fluoride as binder in a weight ratio of 80:10:10. After blending in N-methyl pyrrolidone, the mixed slurry was spread uniformly on an aluminum foil and dried in vacuum for 12 h at 120 °C. With the lithium metal anode as anode, a Celgard2400
microporous
membrane
as
separator,
and
1 mol/L
LiPF6
in
ethylene
carbonate–dimethyl carbonate (1:1 in volume) as electrolyte, the cell was assembled in a nitrogen-filled (RH<3%) glove box. The charge/discharge characteristics and cycle voltammetry test were examined using LAND battery program control cell tester (LAND CT2001) between 2.5 and 4.0 V, with a theoretical current of 170 mAh/g and various rates (vs. Li/Li+) at room temperature. The electrochemical impedance spectroscopy (EIS) was performed on CHI660C electrochemical station (CH Instruments, Inc., Shanghai, China) with a voltage between 2.5 and 4.1 V at a scanning rate of 0.1mV/s. The EISs were potentiostatically measured at the open circuit voltage (OCV) of the cell with an alternating current oscillation of 5 mV amplitude over the frequencies in the range 105 Hz to 10-2 Hz. Stable OCV was obtained by cycling the cell at a constant charge/discharge rate to the desired value and subsequently leaving the cell at open circuit for 10 min. The collected EISs were fitted using Zviw 2.0 software (Scribner Associates Company).
3. Results and discussion 3.1. Structure and morphology of LiFePO4@C/C composites TEM images of LiFePO4@C nanoparticles (sample A-0) are shown in Figure 1. Each primary crystallite is completely coated by a carbon layer to form a core-shell structure, where the carbon shell is derived from the carbonization of Polyaniline(PANI)[13]. The particle sizes range from 58.6 nm to 80.1 nm (Fig. 1a). The particles are connected to each other by the carbon shell to form a 7
local network that helps improve the electrical conductive properties of LiFePO4 [14]. The HRTEM image shown in Fig. 1b reveals that the inner LiFePO4 material (black) is uniformly coated with a carbon layer (gray) with a thickness of approximately 2 nm.
Insert Figure 1 here
SEM images of LiFePO4@C particles existing in the BPC are shown in Figure 2. The network pores of BPC are almost completely filled by LiFePO4@C particles, which helps improve specific capacity and electrical properties of the material (Fig. 2a)[14]. Two types of LiFePO4@C particles exist on the wall surface of BPC (Fig. 2b). One type is grown from the active point of BPC surface and labeled as A with a particle size of approximately 60 nm [15], whereas the other type is deposited from the solution and labeled as B with a particle size of approximately 200 nm to 500 nm[14]. According to the previous researches [14, 15], in a simple precipitation process, particle formation begins with the nucleation, growth, and then coalescence of micro-particles in solution. In this experiment, two different nucleation sites exist, one appears in solution, and the other exists on the active point of BPC surface. For the nucleus formed in solution, ions can deposit and then grow on the surface of it; subsequently, micro-particles in solution can also coalesce with each other. It means that two processes work and produce the big particles of approximately 200 nm to 500 nm (type B). As for the nucleus formed on the active point of BPC surface, only half surface of the nucleus expose to the solution, and therefore the opportunity of ions deposited on the surface of it is noticeable reduced, also the micro-particles formed on the active point of BPC surface are stabled and cannot coalesce with each other. As a result, tiny particles of approximately 60 nm are obtained on the surface of BPC (type A). 8
The EDS spectra of particles A and B are shown in Fig. 2C. The molar ratio of Fe:P:O for both types of particles are nearly 1:1:4 and is very close to the composition of LiFePO4 (Fig. 2b). Particles A and B were obviously LiFePO4@C[13].
Insert Figure 2 here
The XRD patterns of LiFePO4@C/C composites are shown in Figure 3. An olivine orthorhombic structure for both samples(LiFePO4@C and LiFePO4@C/C composites) is presented, as opposed to the standard diffraction peak pattern (space group Pmnb (62), JCPDS #40–1499, Figure 3). In addition, unexpected Li3PO4, Fe2P2O7 phases (labeled as “A” and “B”) are observed in LiFePO4@C/C composites, these are derived from the decomposition of LiFePO4, similar to the results of Soon–Chur Ur
[16]
. The peaks of Li3PO4 and Fe2P2O7 phases are too weak to change the
main crystal structure of LiFePO4. No carbon peak is detected in LiFePO4@C, suggesting that the carbon shell from PANI was weak. Combined with the TEM images, the possible reason may be that the carbon shell is amorphous [15]. In addition, a weak diffraction peak of BPC is tested in LiFePO4@C/C (labeled as “C”). The weak diffraction peak indicates that the carbon from BPC was amorphous [17]. In addition, lattice constants of LiFePO4 in LiFePO4 @C and LiFePO4@C/C are calculated (listed in Table 1) and the negligible change is observed for the two samples. In addition, all data obtained are extremely closed to Zhao’s result [18], which also indicate that the olivine orthorhombic structure for both samples is obtained.
Insert Figure 3 here 9
Insert Table 1 here
3.2. Electrical conductivity of LiFePO4@C/C composites The electrical conductivity values of LiFePO4@C/C composites are listed in Table 2. The values indicates that the addition of BPC has a significant effect on the improvement of electrical conductivity of LiFePO4@C/C composites, and the value of sample A-15 and sample A-20 are close to each other.
Insert Table 2 here
Carbon material mainly acts as a conductor in LiFePO4/C composites and improves electrical conductivity[14]. A local conductive network produced by carbon shell in core-shell structure LiFePO4@C in composites can better improve the electrical properties of LiFePO4 (Fig. 1a), compared with that in Masquelie’s result
[19]
. With the increase of BPC content, a thoroughly
electrical conductive network produced by BPC in composites is formed slowly. When the BPC content is sufficient to form the thoroughly conductive network (content of BPC equals to 15% in this study), further increase in the BPC content will have little effect on the improvement of electrical conductivity of the composites, which can explain why the electrical conductivity of samples A-15 and A-20 are close to each other. In this study, a series of schematic models (Figure 4) is used to describe the form process of the two kinds of electrical conductive networks in the composites (local network formed by carbon shell in LiFePO4@C and thoroughly network formed by BPC in LiFePO4@C/C).
10
Insert Figure 4 here Current study on electrical conductivity and the mechanism of composites are focused on
multi-phase composites, in which one phase works as the matrix, and the remaining phase distribute evenly among it. If only one phase involves an electrical conductive material, then the conductivity is mainly affected by the content of the conductive phase, the key is the formation of percolation network [20]. If the electrical conductive phase exceeds one, improved mixing rules can be applied to investigate the effect of electrical properties and volume fraction of every conductive phase on the electrical properties of the composites
[21]
. However, reports on the double electrical network
structure cathode composites are few. In this study, the improved mixing rules are used to explore the influence of BPC addition on double electrical networks and the combination of LiFePO4@C nanoparticles and BPC. All calculations assume that LiFePO4@C nanoparticles work as a matrix, and that BPC are uniformly distributed within the matrix. Given that the low content of impurity Li3PO4 and Fe2P2O7, the author ignores the effect of impurity content on the electrical properties of the composites [16]. Equation (1) is used in the following form[21]: ln σC = I (VClnσC +VLFP@ClnσLFP@C)
(1)
In the equation, σC represents the electrical conductivity of composites, VC represents the volume fraction of BPC, σC represents the electrical conductivity of BPC, VLFP@C represents the volume fraction of LiFPO4@C, σLFP@C represents the electrical conductivity of LiFPO4@C, and I represents the in-factor of stack density of composites during electrical property test and the combination of LiFPO4@C and BPC. The results are shown in Table 2. Table 2 shows that the value of I, which initially increases and then decreases with the increase in 11
BPC content in composites. BPC has low density and large size (compared with that of LiFePO4). BPC addition can reduce the stack density and increase the porosity of composites. This characteristic has a negative influence on the electrical properties of composites, but the excellent electrical property of BPC has a positive influence on the electrical properties of composites. Furthermore, an increase in BPC content in the composites results in more LiFePO4@C particles held in the pores of BPC, which improves the combination of LiFePO4@C particles with BPC and results in an increase in electrical properties. The effects that increase electrical properties and the quantitative relationship between the compositions and electrical properties of composites will be studied in future studies.
3.3. Galvanostatic charge/discharge behavior of LiFePO4@C/C composites The initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate are shown in Figure 5. The capacity of the composites decreased gradually with increasing BPC content (Fig 5). According to the literature[22], porous carbon can play the role of electric double layer energy storage during charge/discharge cycles, thus increasing the charge/discharge capacity of the composites to some extent. However, this phenomenon is not observed in the current experiment probably because the specific surface area of the BPC we used is relatively small[12], and the storage capacitance of electric double layer may not be sufficient to offset the capacity loss caused by the reduction of LiFePO4@C content. In addition, at the end of the charging/discharging platform, the increasing and decreasing trends gradually change with increasing BPC content, which indicates that the proportion of electric double layer energy storage increases with increasing BPC content[23].
Insert Figure 5 here 12
The initial charge/discharge platform curves of LiFePO4@C/C composites at 0.1C rate are shown in Figure 6. The platform voltage difference decreases with increasing BPC content, which shows that the increase in BPC content in composites can decrease polarization rate [13].
Insert Figure 6 here
The charge/discharge curves of LiFePO4@C/C composites at various charge rates are shown in Figure 7. The charge/discharge capacity decreases with increasing charge current rate for all samples, and this trend is more readily observed in sample A-0 compared with other samples. The
charge/discharge
capacity
of LiFePO4
closely
corresponds to the lithium-ion
extraction/insertion energy storage itself and depends on the migrated number of lithium-ion per mole during the charge/discharge process. For sample A-0 (LiFePO4@C), larger charge/discharge capacity results in a faster rate of capacity reduction with increasing current rate, which indicates that the migrated lithium-ion number of LiFePO4@C nanoparticles decreases quickly during high current charging/discharge processes and results in a rapid decrease in lithium-ion extraction/insertion energy storage, as indicated by the low electrical conductivity of LiFePO4@C nanoparticles and the low lithium-ion migration rate of LiFePO4 [19]. Given that the BPC content increases, the charge/discharge capacity reduction rate of composites slowly decrease. When charging/discharge current rate equals 10C, the charge/discharge capacity are 114.2/110.5 mA·h·g-1 for sample A-5 and 103.7/95.4 mA·h·g-1 for sample A-0, these data appears a slight higher compared to that of Chen’s results [24]. The results may have been determined by the electric double layer energy storage created by 13
BPC addition during the charge/discharge process[22]. The electric double layer energy storage helps to release the current loaded rate and result in a slow decrease of the migrated lithium-ion number of LiFePO4@C nanoparticles during charging/discharge process, which in turn slows down the capacity decrease rate. The synergy effect of electric double layer energy storage produced by BPC and lithium-ion extraction/insertion energy storage by LiFePO4 reduces the charge/discharge capacity reduction rate of composites.
Insert Figure 7 here
Figure 8 shows the electrochemical cycling behavior of LiFePO4@C/C composites at 1C. All samples have good cycling capabilities and sample A-5 owns the best performance with a discharge capacity of 124 mAh g−1 after 50 cycles and the capacity retention rate of it is higher than 95%.
Insert Figure 8 here
3.4. Cyclic performance of LiFePO4@C/C composites The cyclic voltammetry curves of LiFePO4@C/C composites and BPC matrix with a scan rate of 0.1 mV s−1 are illustrated in Figure 9. Compared with the curves in Figs. 9a and b, LiFePO4@C (sample A-0) exhibits a sharp redox peak during cycling, which indicates that a redox reaction happened, and that the lithium extraction/insertion is a single reversible energy storage mechanism. The ratio between cathodic (ipc) and anodic (ipa) peak currents is approximately equal to one, which indicates that the sample has a good reversibility during the redox reaction process. The cathodic (Epc) and anodic (Epa) peak voltages are approximately 3.35 and 3.49 V, respectively. The potential 14
difference between Epc and Epa is approximately 0.14 V, which is consistent with the initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate, as shown in Figure 5. As shown in Fig. 9a, the obvious redox peak is observed with the increase in BPC content, but the peak broadens and the height of the peak is gradually reduced. The curve of sample A-20 is similar to that of BPC in Fig. 9b. The larger proportion of electric double layer capacitance effect results in an approximately rectangular shape for the cyclic voltammetry curve of BPC[25]. The broadening peak of the samples A-5, A-10, A-15, and A-20 in Fig. 9a indicates that the samples produce an electric double layer capacitance effect in charging and discharging process, and that the effect of the electric double layer capacitance increases with increased amount of BPC[11].
Insert Figure 9 here
3.5. Electrochemical impedance spectra (EIS) of LiFePO4@C/C composites The EIS of LiFePO4@C/C composites is shown in Figure 10. A simplified equivalent circuit model (Figure 11) is constructed to analyze the impedance spectra in Fig. 10. The EIS of the composites consists of a semicircle at high frequency and an inclined line of nearly 45 °C at low frequency (Fig. 10). The semicircle shows the charge transfer resistance (Rct in Fig. 11), double-layer capacitance, and the intercept in the high frequency region of the Z’ real axis corresponds to ohmic resistance (Rs in Fig.11), which represents the resistance of the electrolyte and electrode material. The inclined line of 45 °C to the real axis in low frequency region represents the Warburg impedance (Zw in Fig.10) of long-range lithium-ion diffusion in the LiFePO4 particles[26–28]. The constant phase element (CPE in Fig. 11) represents the double layer capacitance used in Fig. 9. Zviw2.0 software is used to fit the curves in Fig. 10. 15
The parameters in the equivalent circuit are calculated and listed in Table 3. From the table, we can see that value of Rs does not change, but that of Rct decreases dramatically as the BPC content increases. The charge transfer resistance of LiFePO4@C is 86.72 Ω, whereas that of sample A-20 is only 37.58 Ω. A small Rct is beneficial to the kinetic behavior in the course of charge/discharge process and result in better electrochemical performance of the active materials.
Insert Figure 10 here Insert Figure 11 here
The exchange current density is a parameter that indicates the reversibility of the electrode. A high exchange current density value indicates a small resistance of the electrode reaction, thus making the charge/discharge process easier. The exchange current density (i0) of the composites is calculated from the following equation [29–31]
: i0 = RT / n FRct
(2)
where n represents the number of electron transferred per molecule during the intercalation, and is 1 for LiFePO4@C, R represents the gas constant (8.314J K−1·mol−1), T represents the absolute degree (K), and F represents the Faraday Constant (9.64 × 104 C/mol). All calculated results are listed in Table 3, which shows that the increase in BPC content, the value of i0 increases from 0.30 mA (sample A-0) to 0.70 mA (sample A-20). Therefore, BPC addition can reduce the resistance of the electrode reaction and facilitate the charge/discharge process.
16
Insert Table 3 here
4. Conclusions In the present study, the effect of BPC content on the composition, microstructure, and electrochemical performance of LiFePO4@C/C composites was examined by controlling the BPC content. The following conclusions could be drawn: (1) Network pores of BPC were almost completely filled by LiFePO4@C nanoparticles, which were formed by a crystalline LiFePO4 core with size ranging from 58.6 nm to 80.1 nm and a carbon shell with a thickness of approximately 2 nm; (2) Two kinds of LiFePO4@C nanoparticles existed on the wall surface of BPC. One was grown from the active point of BPC and had a particle size of approximately 60 nm, whereas the other was probably deposited from the solution and had a particle size approximately 200 nm to 500 nm; (3) Double electrical conductive network (local network formed by carbon shell in LiFePO4@C and network formed by BPC in LiFePO4@C/C) formed in the composites improved the electrical properties of samples from 2.59 × 10-6 S·cm-1(A-0) to 5.76 × 10-2 S·cm-1(A-20). In addition, a series of schematic models were used to describe the formation process of the electrical conductive network. The improved mixture rule was also used to discuss the electrical conductive mechanism of LiFePO4@C/C composites; (4) Synergy effect of electric double layer energy storage produced by BPC and lithium-ion extraction/insertion energy storage by LiFePO4 reduced the charge/discharge capacity reduction rate of the composites, and a charge/discharge capacity of 114.2/110.5 mA·h·g-1 for sample A-5 at 10C was obtained; (5) BPC can have a dramatic advantage on the reduction of the interfacial resistance, and can 17
increase the exchange current density. The former was converted from 86.72 Ω (sample A-0) to 37.58 Ω (sample A-20) and the latter converted 0.30 mA (sample A-0) to 0.70 mA (sample A-20), both changes were beneficial to kinetic behavior and facilitated the charge/discharge process.
Acknowledgments This work is supported by the Planning of the Growth of Young Teachers (Hunan University China), the Science and Technology Planning Project of Hunan Province, China (2012WK3023), and the Science and Technology Planning Project of Changsha City, Hunan Province, China (K1109117-11).
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(2010) 1259-1269. [8] N. Omar, M.H. Daowd, P.V.B. Bossche, et al. Rechargeable Energy Storage Systems for Plug-in Hybrid Electric Vehicles-Assessment of Electrical Characteristics[J]. Energies, 5 (2012) 2952-2988. [9] X.B. Hu, Y.J. Huai, Z.J. Lin, et al. A (LiFePO4-AC)/Li4Ti5O12 hybrid battery capacitor[J]. J. Electrochem. Soc., 154 (2007) A1026-A1030. [10] Z.H. Deng, X.B. Hu, J.S. Suo, et al. A high rate, high capacity and long life (LMO-AC)/Li4Ti5O12 hybrid battery-supercapacitor[J]. J. Power Sources, 187 (2009) 635-639. [11] J.H. Jiang, L. Zhang, X.Y. Wang, Nancy Holm, Kishore Rajagopalan, F. L. Chen, S.G. Ma. Highly ordered macroporous woody biochar with ultra-high carbon content as supercapacitor electrodes[J]. Electrochim. Acta, 113(2013)481-489. [12] W.W. Gong, L. Wang, D. Y. Li, P. Z. Gao, H. N. Xiao. Preparation and Characterization of Biomorphic Porous Carbon of for Supercapacitor [J]. China Ceram., 48(8)(2012)18-22, 42. [13] W.W. Gong. Study on Electrochemical Properties of Nano LiFePO4 and Porous Carbon carried Nano LiFePO4 Materials [D]. Hunan: Hunan University. 2012, 16. [14] Y.M. Zhu, S.Z. Tang, H.H. Shi, et al. Synthesis of FePO4·xH2O for fabricating submicrometer structured LiFePO4/C by a co-precipitation method[J]. Ceram. Int., 40(2014) 2685-2690. [15] P.Z. Gao, W.W. Gong, D.Y. Li, L. Wang, H.N. Xiao. Preparation and Electrochemical Properties of Nano LiFePO4/C with a Core-shell Structure through Modified In-Situ Restriction Polymerization Method[J]. Rare. Metal. Mat. Eng., 41(3) (2012) S201-204. [16] Man-Soon Yoon, Mobinul Islam, Soon-Chul Ur. The role of impurities on electrochemical properties of LiFePO4 cathode material[J]. Ceram. Int., 39 (2013)S647–S651. [17] W. W. Gong, P.Z. Gao, W. X. Wang. Characterization and oxidation properties of biomorphic porous carbon with SiC gradient coating prepared by PIP method[J]. Ceram. Int., 37(2011) 1739-1746. [18] Lihua He, Xuheng Liu, Zhongwei Zhao. Non-isothermal kinetics study on synthesis of LiFePO4 via carbothermal reduction method[J]. Thermochim. Acta 566 (2013) 298-304. [19] C. Delacourt, C. Wurm, L. Laffont, J.-B. Leriche, C. Masquelier. Electrochemical and electrical properties of Nb- and/or C-containing LiFePO4 composites [J]. Solid State Ionics, 19
177(3–4)( 2006) 333-341 [20] R.K. Goyala, K.R. Kambalea, S.S. Nenea, et al.. Fabrication, thermal and electrical properties of polyphenylene sulphide/copper composites [J]. Mater. Chem. Phys. , 2011, 128:114-120. [21] S Beckman, B.A Cook, M Akinc. An analysis of electrical resistivity of compositions within the Mo–Si–B ternary system part II: Multi-phase composites[J]. Mater. Sci. Eng., A .2001; A299: 94–104 [22] Y. Han, X.T. Dong, C. Zhang, et al. Hierarchical porous carbon hollow-spheres as a high performance electrical double-layer capacitor material [J]. J. Power Sources, 211 (2011) 92-96. [23] R. Xu, Z.L. Tang, J.R. Li, et al. Optimization of LiFePO4-AC/Li4Ti5O12 Hybrid Cells [J]. Rare Metal Mat Eng, 38 (2009) 1095-1098. [24] C. Chen, G.B. Liu, Y. Wang, J.L. Li, H. Liu. Preparation and electrochemical properties of LiFePO4/C
nanocomposite
using
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nanoparticles
by
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of
Fe3(PO4)2·8H2O at low cost[J]. Electrochim. Acta,113 (2013) 464-469. [25] Ma T.T., Wang Z. X. , LI X. H., Guo H. J., Fang
J., Peng W. J., Hu Q. Y., Zhang Y. H..
Electrochemical performance of LiMn2O4/activated carbon composite material [J]. J. S.-Cent. Univ. Natl. (Nat. Sci. Ed.)(China), 43(1)(2012)17-22 [26] F. Gao, Z.Y. Tang. Kinetic behavior of LiFePO4/C cathode material for lithium-ion batteries[J]. Electrochim. Acta, 53 (2008) 5071-5075. [27] H.C. Shin, W.I. Cho, H. Jang. Electrochemical properties of the carbon-coated LiFePO4 as a cathode material for lithium-ion secondary batteries[J]. J. Power Sources, 159 (2006) 1383-1388. [28] H.H. Chang, C.C. Chang, C.Y. Su, et al. Effects of TiO2 coating on high-temperature cycle performance of LiFePO4-based lithium-ion batteries[J]. J. Power Sources, 185 (2008) 466-472. [29] Y. Cui, X.L. Zhao, R.S. Guo. High rate electrochemical performances of nanosized ZnO and carbon co-coated LiFePO4 cathode[J]. Mater. Res. Bull., 45 (2010) 844-849. [30] H.B. Shu, X.Y. Wang, Q. Wu, et al. Ammonia assisted hydrothermal synthesis of monodisperse LiFePO4/C microspheres as cathode material for lithium Ion batteries[J]. J. Electrochem. Soc., 2011, 158(12): A1448-A1454. [31] H.B. Shu, X.Y. Wang, Q. Wu, et al. The effect of ammonia concentration on the morphology and electrochemical properties of LiFePO4 synthesized by ammonia assisted hydrothermal 20
route[J]. Electrochim. Acta, 76 (2012) 120-129.
Figure and table captions
Figure 1 TEM (a) and HRTEM (b) images of LiFePO4@C nanoparticles Figure 2 SEM images of LiFePO4@C particles growing on the wall surface of BPC and EDS spectra of particles A and B Figure 3 XRD patterns of standard LiFePO4, LiFePO4@C, and LiFePO4@C/C composites Figure 4 Schematic models describing the formation of double electrical conductive network in composites Figure 5 Initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate Figure 6 Platform of initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate Figure 7 Charge/ Discharge performance of LiFePO4@C/C composites at various charge rates Figure 8 Electrochemical cycling behavior of LiFePO4@C/C composites at 1C Figure 9 Cyclic voltammetry spectroscopy of LiFePO4@C/C composites and BPC matrix with a scan rate of 0.1 mV s−1 Figure 10 EIS of LiFePO4@C and LiFePO4@C/C composites Figure 11 Equivalent circuit used for fitting the experimental EIS data
Table 1 Structure parameters of LiFePO4 in LiFePO4@C and LiFePO4@C/C composites Table 2 Data of volume fraction, electrical conductivity and in-factor I of LiFePO4@C/C composites Table 3 Electrode kinetic parameters obtained from equivalent circuit fitting of experimental data of LiFePO4@C/C composites 21
Figures
Carbon shell 58.6 nm
Carbon shell LiFePO4 core 80.1 nm
LiFePO4 core
Figure 1 22
2nm
A
BPC
B LiFePO4@C
(a) Cross-section of BPC in composites
(b) Wall surface of BPC in composites
O P
Fe
Intensity (a.u.)
C Fe
Cu
Elem P Fe O
Particle 'A'
O P
Fe
C Fe
0
Cu
Elem P Fe O
mol% 13.6 14.1 53.04
Particle 'B'
2
4
6
8
10
Energy (keV)
(c) EDS spectra of particles A and B Figure 2 23
mol% 11.28 11.64 44.22
12
(121)
A - Li3PO4 C - Amorphous C
(260)
(340)
(311) (222)
A
(022)
C
(211)
(031)
(131)
BB
(011) (120) (021)
(020)
Intensity(a.u.)
B
B - Fe2P2O7
LiFePO4@C/C LiFePO4@C Standard LiFePO4
10
20
30
40
50
60
2θ(o)
70
80
Figure 3
Local network
Local network
5% BPC (sample A-5)
10% BPC (sample A-10) LiFePO4@C
LiFePO4@C/C
Figure 4
24
Thoroughly network
15% BPC (sample A-15)
4.2
Potential/V vs Li+/Li
4.0 3.8 3.6 3.4 3.2
A-20
3.0
A-15
2.8
A-10 A-5
2.6 2.4
A-0
0
20
40
60
80
100
120
Capacity/mAhg-1
140
160
Figure 5
Potential/V vs Li+/Li
3.55 3.50 3.45
A-20
A-15
A-0
A-5
3.40 3.35 3.30 15
20
25
30
35
40
Capacity/mAhg -1
Figure 6 25
A-10
45
50
150
A-5
A-5
130
130
Capacity/mAhg-1
Capacity/mAhg-1
140
A-10
120 110
A-15 A-20
100 90 80
0.1C
A-0
140
A-0
0.5C
1C
2C
4C
6C
C-rate
8C
A-10
120 A-15
110 A-20
100 90 80
10C
0.1C
(a) Charge curve
0.5C
1C
Capacity/ mA h g
-1
140 120 100 A-0 A-5 A-10 A-15 A-20
80
0
10
20
30
Cycle times
Figure 8
26
6C
(b) discharge curve
Figure 7
60
2C
C-rate
40
50
8C
10C
5 3
4
A-10 A-15
2 1
3
Current/μA
Current/mA
5
Epc A-0 A-5
4
A-20
0 -1 -2
2 1 0 -1 -2
-3
-3
Epa
-4 2.0
2.4
2.8
3.2
3.6
-4
4.0
Voltage/V
2.0
2.5
3.0
Voltage/V
(a) LiFePO4@C/C
(b) BPC matrix
Figure 9
40 35
A-20
30
A-15
Z''(ohm)
25 A-10
20 A-5
15 10 5 0
A-0
0
20
40
60
80
Z'(0hm)
100
Figure 10
Rs
CPE Rct
Zw
Figure 11
27
3.5
4.0
Tables
Table 1 Cell parameters sample c(Å)
volume(Å3) Space group
A(Å)
b(Å)
LiFePO4 @C
6.002
10.332 4.693 291.025
Pnmb
LiFePO4@C/C
6.005
10.333 4.693 291.199
Pnmb
Standard LiFePO4
6.008
10.334 4.693 291.392
Pnmb
Table 2 Samples
A-0
A-5
A-10
A-15
A-20
BPC
σ (S·cm-1)
2.59 × 10-6
5.56 × 10-4
1.15 × 10-3
4.50 × 10-2
5.76 × 10-2
0.38
VC
0
0.39
0.58
0.69
0.76
1.00
VLFP@C
1.00
0.60
0.42
0.31
0.24
0
I
-
0.93
1.10
0.67
0.74
-
* ρBPC = 0.288g·cm-3 [17] ρ LFP = 3.577 g·cm-3 (theoretical value)
Table 3
Samples
Rs (Ω)
Rct (Ω)
i 0(mA)
A-0
1.95
86.72
0.30
A-5
1.78
72.64
0.35
A-10
1.54
53.63
0.54
A-15
1.32
40.25
0.67
A-20
1.46
37.58
0.70
28
Electrochemical performance of LiFePO4@C composites with biomorphic porous carbon loading and Nano-core-shell structure Peng-Zhao Gao, Ling Wang, Dong-Yun Li, Bing Yan, Wei-Wei Gong
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)00719-6 http://dx.doi.org/10.1016/j.ceramint.2014.04.164 CERI8532
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
9 March 2014 29 April 2014 29 April 2014
Cite this article as: Peng-Zhao Gao, Ling Wang, Dong-Yun Li, Bing Yan, Wei-Wei Gong, Electrochemical performance of LiFePO4@C composites with biomorphic porous carbon loading and Nano-core-shell structure, Ceramics International, http://dx.doi.org/ 10.1016/j.ceramint.2014.04.164 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical Performance of LiFePO4@C Composites with Biomorphic Porous Carbon Loading and Nano-Core-Shell Structure Peng-Zhao Gaoa*, Ling Wanga, Dong-Yun Lib, Bing Yana, Wei-Wei Gonga a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China
b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
*
Corresponding author. Tel.: +86 731 88822269; fax: +86 731 88823554.
Email: [email protected]
Abstract Effect of biomorphic porous carbon (BPC) addition on the composition, microstructure, and electrochemical performance of LiFePO4@C/C composites was investigated. Results indicated that network pores of BPC were almost completely filled by LiFePO4@C nanoparticles, which were formed by an olivine structure LiFePO4 core with size that ranged from 58.6 nm to 80.1 nm and an amorphous carbon shell with a thickness of approximately 2 nm. Double electrical conductive networks formed in the composites improved the electrical properties of samples from 2.59 × 10-6 S·cm-1 (sample A-0) to 5.76 × 10-2 S·cm-1 (samples A-20). Synergy effect of electric double layer energy storage produced by BPC and lithium-ion extraction/insertion energy storage by LiFePO4 clearly reduced the capacity reduction rate of composites, and obtained a charge/discharge capacity of 114.2/110.5 mA·h·g-1 (samples A-5) at 10C. Moreover, addition of BPC showed a significant advantage in reducing the interfacial resistance of the electrode reaction in composites from 86.72 Ω (samples A-0) to 37.58 Ω (samples A-20). The electrical conductive mechanism of LiFePO4@C/C composites is discussed.
1
Keywords LiFePO4@C/C composites; nanostructures; electrochemical properties; double electrical conductive network; synergy effect
1. Introduction Lithium-ion battery has gained popularity in the portable electronics market and is under development for electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs)[1] because of its high energy density and low self-discharge rate. However, the low power density and defective charge/discharge rate performance of lithium-ion battery limits its use in high rate charge/discharge applications [2]. An electric double layer capacitor can form an electric double layer capacitance interface for energy storage depending on polarized electrode and electrolyte. The formation of an electric double layer capacitance interface leads to high power density and excellent rate performance, but the energy density is relatively low. Numerous recent studies have attempted to overcome the drawbacks of single storage component and develop a new kind of high-energy storage device by combining two energy storage mechanisms
[3]
. At present, two ways of combining two energy storage devices are available,
namely, the “outside” and “inside” combinations. In the outside combination, two monomers are combined by a power management system into an energy storage component to achieve larger capacity and power[4]. However, this method has many shortcomings such as having too-large bulk, low specific power, and complex system management and control[5]. In the inside combination, lithium-ion extraction/insertion energy storage and electric double layer capacitance energy storage 2
mechanism are realized in the same electrode, thus ensuring monomer consistency and reduced system management complexity. The presence of lithium-ion extraction/insertion energy storage and electric double layer capacitance energy storage mechanism in the same electrode is an example of inside combination. Current studies on lithium-ion extraction/insertion energy storage and electric double layer capacitance energy storage mechanism in the same electrode is primarily oriented towards the combination of lithium-inserted compounds and activated carbon (AC). In charge and discharge processes, lithium-inserted compounds can function as lithium-ion extraction/insertion energy storage, whereas AC can be the electric double layer capacitance energy storage because of its high specific surface area and electrochemical stability. When the battery is charging, lithium ion can be extracted from the lattice of the cathode made up of lithium-inserted compounds that are spontaneously inserted into the lattice of the anode material through electrolyte. AC on the cathode surface forms an electric double layer as a result of charge adsorption and solvation ion reconstruction. When the battery is discharging, the lithium ion can extract from the anode material spontaneously and insert into the lattice of the cathode of lithium-inserted compounds through the electrolyte, and the charge gathered around the bipolar surface will produce displacement current because of the reconstruction[6]. Hu
[7]
prepared LiFePO4 with AC (LAC) and direct-mixed LAC (DMLAC) composite cathode
materials with different LiFePO4 contents. Hybrid battery-capacitors LAC/Li4Ti5O12 and DMLAC/Li4Ti5O12 with a Li4Ti5O12 anode were subsequently assembled. The overall electrochemical performance of the hybrid battery capacitors was best when the content of LiFePO4 in the composite cathode materials ranged from 11.8 wt.% to 28.5 wt.%. In addition, the hybrid
3
battery-capacitor devices showed good life cycle performance at high rates. The capacity loss of the DMLAC/Li4Ti5O12 hybrid battery-capacitor device at 4C was no more than 4.8% after 1000 cycles, and did not exceed 9.6% after 2000 cycles at 8C. Omar[8] investigated the performance of various lithium-ion chemistries used in PHEVs and compared them with several other rechargeable energy storage system technology, such as lead-acid, nickel-metal hydride, and electrical double layer capacitors. The nickel manganese cobalt oxide cathode stands out with high energy density up to 160 Wh/kg, compared with 70 Wh/kg to 10 Wh/kg for lithium iron phosphate cathode, as well as 90 and 71 Wh/kg for lithium nickel cobalt aluminum cathode and lithium titanate oxide anode battery cells, respectively. The values are considerably higher than those of lead-acid (23 Wh/kg to 28 Wh/kg) and nickel-metal hydride (44 Wh/kg to 53 Wh/kg) battery technologies. Hu[9,10] used LAC as cathode material, and Li4Ti5O12 as the anode material to form the LAC/Li4Ti5O12 system. The hybrid battery-capacitor LAC/Li4Ti5O12 has the advantage of having the high-rate capability of a hybrid capacitor AC/Li4Ti5O12 and the high battery capacity of LiFePO4/Li4Ti5O12. The hybrid battery-capacitor LAC/Li4Ti5O12 is an energy storage device in which the capacitor and secondary battery coexist in one cell system. Few studies have reported on the LiFePO4@C compound with biomorphic porous carbon (BPC), which has highly ordered macroporous structures, controlled specific area, and excellent electric double layer energy storage properties [11,12]. In the present paper, the inside combination compound of LiFePO4@C/C composites was prepared through a modified in-situ restriction polymerization method, using BPC as matrix[13]. The effect of BPC content on the composition, microstructure, and electrochemical performance of LiFePO4@C/C composites was studied by controlling the BPC content. Interesting results obtained are discussed in the succeeding sections. 4
2. Experimental process 2.1. Raw materials The BPC was self-made and smashed into small particles with a size of 50 ± 10 μm and a specific area of 771.6 m2·g-1[13]. Subsequently, BPC particles were dried at 80 °C until a constant weight was reached. Lithium acetate (CH3COOLi⋅2H2O), ferric chloride (FeCl3⋅6H2O), ammonium dihydrogen phosphate (NH4H2PO4), aniline (ANI, C6H5NH2), ascorbic acid (C6H8O6), and ethanol (CH3CH2OH) were used in the experiment. All reagents are of analytical grade. Deionized water was used as main solvent. Ferric chloride, ANI, and ascorbic acid were obtained from the Chinese Medicine Group Chemical Reagent Co., Ltd. Lithium acetate was from Tianjin Kermel Chemical Reagent Co., Ltd. Ammonium dihydrogen phosphate was from Guangdong Taishan Chemical Plant Co., Ltd., and ethanol was from Changsha Antaeus Fine Chemical Industrial Co., Ltd.
2.2. Synthesis of FePO4@polyaniline (PANI)/C Ferric chloride aqueous solution was dropped into ammonium dihydrogen phosphate and ANI mixture aqueous solution at a rate of 10 ml·h-1 under rapid stirring to produce FePO4@PANI. BPC particles with a mass fraction of 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% (the fraction was relative to the weight of the obtained LiFePO4) were subsequently added into the mixture under rapid stirring. The FePO4@PANI/C material was obtained when the mixture was stirred for 10 h at room temperature[13]. The obtained powder was centrifuged and washed thrice with deionized water and dried at 80 °C until a constant weight was obtained.
5
2.3. Synthesis of LiFePO4@C/C composites An appropriate content of FePO4@PANI/C powder was added into the lithium ethanol solution. The solution was stirred for 2 h, and then ascorbic acid was added into the suspension. The mixture was stirred for 8 h at 60 °C. The final precipitates were centrifuged and washed thrice with deionized water and ethanol until a neutral pH was obtained. The precipitate was then dried at 80 °C until a constant weight was obtained. Finally, the precursor was sintered at 700 °C for 3 h to obtain LiFePO4@C/C composites[13]. The samples were labeled as A-0, A-5, A-10, A-15, and A-20, according to the BPC addition.
2.4. Characterization of materials The interfacial nanostructures of LiFePO4@C material was identified by high resolution transmission electron microscopy (HRTEM, JEM-3010, Jeol). The surface morphology of LiFePO4@C/C composites was characterized by scanning electron microscopy (SEM, JSM-6490LV, Jeol). The phase identification of LiFePO4@C/C composites was performed by X-ray diffraction (XRD, Rigaku D/max2200 VPC) using monochromic Cu Kα radiation of 0.15405 nm operated at 40 kV and 40 mA. The conductivity of LiFePO4@C/C composites was measured by 61/2 digital multimeter (34401A, Agilent) and electrical resistivity instrument (GM-II, Shanxi Coal and Chemical Institute). The electrochemical measurement was performed with the CR2025 button cell. The cathode electrode was created using the following procedures. LiFePO4@C/C active material was mixed 6
with acetylene black as conducting agent, and polyvinylidene fluoride as binder in a weight ratio of 80:10:10. After blending in N-methyl pyrrolidone, the mixed slurry was spread uniformly on an aluminum foil and dried in vacuum for 12 h at 120 °C. With the lithium metal anode as anode, a Celgard2400
microporous
membrane
as
separator,
and
1 mol/L
LiPF6
in
ethylene
carbonate–dimethyl carbonate (1:1 in volume) as electrolyte, the cell was assembled in a nitrogen-filled (RH<3%) glove box. The charge/discharge characteristics and cycle voltammetry test were examined using LAND battery program control cell tester (LAND CT2001) between 2.5 and 4.0 V, with a theoretical current of 170 mAh/g and various rates (vs. Li/Li+) at room temperature. The electrochemical impedance spectroscopy (EIS) was performed on CHI660C electrochemical station (CH Instruments, Inc., Shanghai, China) with a voltage between 2.5 and 4.1 V at a scanning rate of 0.1mV/s. The EISs were potentiostatically measured at the open circuit voltage (OCV) of the cell with an alternating current oscillation of 5 mV amplitude over the frequencies in the range 105 Hz to 10-2 Hz. Stable OCV was obtained by cycling the cell at a constant charge/discharge rate to the desired value and subsequently leaving the cell at open circuit for 10 min. The collected EISs were fitted using Zviw 2.0 software (Scribner Associates Company).
3. Results and discussion 3.1. Structure and morphology of LiFePO4@C/C composites TEM images of LiFePO4@C nanoparticles (sample A-0) are shown in Figure 1. Each primary crystallite is completely coated by a carbon layer to form a core-shell structure, where the carbon shell is derived from the carbonization of Polyaniline(PANI)[13]. The particle sizes range from 58.6 nm to 80.1 nm (Fig. 1a). The particles are connected to each other by the carbon shell to form a 7
local network that helps improve the electrical conductive properties of LiFePO4 [14]. The HRTEM image shown in Fig. 1b reveals that the inner LiFePO4 material (black) is uniformly coated with a carbon layer (gray) with a thickness of approximately 2 nm.
Insert Figure 1 here
SEM images of LiFePO4@C particles existing in the BPC are shown in Figure 2. The network pores of BPC are almost completely filled by LiFePO4@C particles, which helps improve specific capacity and electrical properties of the material (Fig. 2a)[14]. Two types of LiFePO4@C particles exist on the wall surface of BPC (Fig. 2b). One type is grown from the active point of BPC surface and labeled as A with a particle size of approximately 60 nm [15], whereas the other type is deposited from the solution and labeled as B with a particle size of approximately 200 nm to 500 nm[14]. According to the previous researches [14, 15], in a simple precipitation process, particle formation begins with the nucleation, growth, and then coalescence of micro-particles in solution. In this experiment, two different nucleation sites exist, one appears in solution, and the other exists on the active point of BPC surface. For the nucleus formed in solution, ions can deposit and then grow on the surface of it; subsequently, micro-particles in solution can also coalesce with each other. It means that two processes work and produce the big particles of approximately 200 nm to 500 nm (type B). As for the nucleus formed on the active point of BPC surface, only half surface of the nucleus expose to the solution, and therefore the opportunity of ions deposited on the surface of it is noticeable reduced, also the micro-particles formed on the active point of BPC surface are stabled and cannot coalesce with each other. As a result, tiny particles of approximately 60 nm are obtained on the surface of BPC (type A). 8
The EDS spectra of particles A and B are shown in Fig. 2C. The molar ratio of Fe:P:O for both types of particles are nearly 1:1:4 and is very close to the composition of LiFePO4 (Fig. 2b). Particles A and B were obviously LiFePO4@C[13].
Insert Figure 2 here
The XRD patterns of LiFePO4@C/C composites are shown in Figure 3. An olivine orthorhombic structure for both samples(LiFePO4@C and LiFePO4@C/C composites) is presented, as opposed to the standard diffraction peak pattern (space group Pmnb (62), JCPDS #40–1499, Figure 3). In addition, unexpected Li3PO4, Fe2P2O7 phases (labeled as “A” and “B”) are observed in LiFePO4@C/C composites, these are derived from the decomposition of LiFePO4, similar to the results of Soon–Chur Ur
[16]
. The peaks of Li3PO4 and Fe2P2O7 phases are too weak to change the
main crystal structure of LiFePO4. No carbon peak is detected in LiFePO4@C, suggesting that the carbon shell from PANI was weak. Combined with the TEM images, the possible reason may be that the carbon shell is amorphous [15]. In addition, a weak diffraction peak of BPC is tested in LiFePO4@C/C (labeled as “C”). The weak diffraction peak indicates that the carbon from BPC was amorphous [17]. In addition, lattice constants of LiFePO4 in LiFePO4 @C and LiFePO4@C/C are calculated (listed in Table 1) and the negligible change is observed for the two samples. In addition, all data obtained are extremely closed to Zhao’s result [18], which also indicate that the olivine orthorhombic structure for both samples is obtained.
Insert Figure 3 here 9
Insert Table 1 here
3.2. Electrical conductivity of LiFePO4@C/C composites The electrical conductivity values of LiFePO4@C/C composites are listed in Table 2. The values indicates that the addition of BPC has a significant effect on the improvement of electrical conductivity of LiFePO4@C/C composites, and the value of sample A-15 and sample A-20 are close to each other.
Insert Table 2 here
Carbon material mainly acts as a conductor in LiFePO4/C composites and improves electrical conductivity[14]. A local conductive network produced by carbon shell in core-shell structure LiFePO4@C in composites can better improve the electrical properties of LiFePO4 (Fig. 1a), compared with that in Masquelie’s result
[19]
. With the increase of BPC content, a thoroughly
electrical conductive network produced by BPC in composites is formed slowly. When the BPC content is sufficient to form the thoroughly conductive network (content of BPC equals to 15% in this study), further increase in the BPC content will have little effect on the improvement of electrical conductivity of the composites, which can explain why the electrical conductivity of samples A-15 and A-20 are close to each other. In this study, a series of schematic models (Figure 4) is used to describe the form process of the two kinds of electrical conductive networks in the composites (local network formed by carbon shell in LiFePO4@C and thoroughly network formed by BPC in LiFePO4@C/C).
10
Insert Figure 4 here Current study on electrical conductivity and the mechanism of composites are focused on
multi-phase composites, in which one phase works as the matrix, and the remaining phase distribute evenly among it. If only one phase involves an electrical conductive material, then the conductivity is mainly affected by the content of the conductive phase, the key is the formation of percolation network [20]. If the electrical conductive phase exceeds one, improved mixing rules can be applied to investigate the effect of electrical properties and volume fraction of every conductive phase on the electrical properties of the composites
[21]
. However, reports on the double electrical network
structure cathode composites are few. In this study, the improved mixing rules are used to explore the influence of BPC addition on double electrical networks and the combination of LiFePO4@C nanoparticles and BPC. All calculations assume that LiFePO4@C nanoparticles work as a matrix, and that BPC are uniformly distributed within the matrix. Given that the low content of impurity Li3PO4 and Fe2P2O7, the author ignores the effect of impurity content on the electrical properties of the composites [16]. Equation (1) is used in the following form[21]: ln σC = I (VClnσC +VLFP@ClnσLFP@C)
(1)
In the equation, σC represents the electrical conductivity of composites, VC represents the volume fraction of BPC, σC represents the electrical conductivity of BPC, VLFP@C represents the volume fraction of LiFPO4@C, σLFP@C represents the electrical conductivity of LiFPO4@C, and I represents the in-factor of stack density of composites during electrical property test and the combination of LiFPO4@C and BPC. The results are shown in Table 2. Table 2 shows that the value of I, which initially increases and then decreases with the increase in 11
BPC content in composites. BPC has low density and large size (compared with that of LiFePO4). BPC addition can reduce the stack density and increase the porosity of composites. This characteristic has a negative influence on the electrical properties of composites, but the excellent electrical property of BPC has a positive influence on the electrical properties of composites. Furthermore, an increase in BPC content in the composites results in more LiFePO4@C particles held in the pores of BPC, which improves the combination of LiFePO4@C particles with BPC and results in an increase in electrical properties. The effects that increase electrical properties and the quantitative relationship between the compositions and electrical properties of composites will be studied in future studies.
3.3. Galvanostatic charge/discharge behavior of LiFePO4@C/C composites The initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate are shown in Figure 5. The capacity of the composites decreased gradually with increasing BPC content (Fig 5). According to the literature[22], porous carbon can play the role of electric double layer energy storage during charge/discharge cycles, thus increasing the charge/discharge capacity of the composites to some extent. However, this phenomenon is not observed in the current experiment probably because the specific surface area of the BPC we used is relatively small[12], and the storage capacitance of electric double layer may not be sufficient to offset the capacity loss caused by the reduction of LiFePO4@C content. In addition, at the end of the charging/discharging platform, the increasing and decreasing trends gradually change with increasing BPC content, which indicates that the proportion of electric double layer energy storage increases with increasing BPC content[23].
Insert Figure 5 here 12
The initial charge/discharge platform curves of LiFePO4@C/C composites at 0.1C rate are shown in Figure 6. The platform voltage difference decreases with increasing BPC content, which shows that the increase in BPC content in composites can decrease polarization rate [13].
Insert Figure 6 here
The charge/discharge curves of LiFePO4@C/C composites at various charge rates are shown in Figure 7. The charge/discharge capacity decreases with increasing charge current rate for all samples, and this trend is more readily observed in sample A-0 compared with other samples. The
charge/discharge
capacity
of LiFePO4
closely
corresponds to the lithium-ion
extraction/insertion energy storage itself and depends on the migrated number of lithium-ion per mole during the charge/discharge process. For sample A-0 (LiFePO4@C), larger charge/discharge capacity results in a faster rate of capacity reduction with increasing current rate, which indicates that the migrated lithium-ion number of LiFePO4@C nanoparticles decreases quickly during high current charging/discharge processes and results in a rapid decrease in lithium-ion extraction/insertion energy storage, as indicated by the low electrical conductivity of LiFePO4@C nanoparticles and the low lithium-ion migration rate of LiFePO4 [19]. Given that the BPC content increases, the charge/discharge capacity reduction rate of composites slowly decrease. When charging/discharge current rate equals 10C, the charge/discharge capacity are 114.2/110.5 mA·h·g-1 for sample A-5 and 103.7/95.4 mA·h·g-1 for sample A-0, these data appears a slight higher compared to that of Chen’s results [24]. The results may have been determined by the electric double layer energy storage created by 13
BPC addition during the charge/discharge process[22]. The electric double layer energy storage helps to release the current loaded rate and result in a slow decrease of the migrated lithium-ion number of LiFePO4@C nanoparticles during charging/discharge process, which in turn slows down the capacity decrease rate. The synergy effect of electric double layer energy storage produced by BPC and lithium-ion extraction/insertion energy storage by LiFePO4 reduces the charge/discharge capacity reduction rate of composites.
Insert Figure 7 here
Figure 8 shows the electrochemical cycling behavior of LiFePO4@C/C composites at 1C. All samples have good cycling capabilities and sample A-5 owns the best performance with a discharge capacity of 124 mAh g−1 after 50 cycles and the capacity retention rate of it is higher than 95%.
Insert Figure 8 here
3.4. Cyclic performance of LiFePO4@C/C composites The cyclic voltammetry curves of LiFePO4@C/C composites and BPC matrix with a scan rate of 0.1 mV s−1 are illustrated in Figure 9. Compared with the curves in Figs. 9a and b, LiFePO4@C (sample A-0) exhibits a sharp redox peak during cycling, which indicates that a redox reaction happened, and that the lithium extraction/insertion is a single reversible energy storage mechanism. The ratio between cathodic (ipc) and anodic (ipa) peak currents is approximately equal to one, which indicates that the sample has a good reversibility during the redox reaction process. The cathodic (Epc) and anodic (Epa) peak voltages are approximately 3.35 and 3.49 V, respectively. The potential 14
difference between Epc and Epa is approximately 0.14 V, which is consistent with the initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate, as shown in Figure 5. As shown in Fig. 9a, the obvious redox peak is observed with the increase in BPC content, but the peak broadens and the height of the peak is gradually reduced. The curve of sample A-20 is similar to that of BPC in Fig. 9b. The larger proportion of electric double layer capacitance effect results in an approximately rectangular shape for the cyclic voltammetry curve of BPC[25]. The broadening peak of the samples A-5, A-10, A-15, and A-20 in Fig. 9a indicates that the samples produce an electric double layer capacitance effect in charging and discharging process, and that the effect of the electric double layer capacitance increases with increased amount of BPC[11].
Insert Figure 9 here
3.5. Electrochemical impedance spectra (EIS) of LiFePO4@C/C composites The EIS of LiFePO4@C/C composites is shown in Figure 10. A simplified equivalent circuit model (Figure 11) is constructed to analyze the impedance spectra in Fig. 10. The EIS of the composites consists of a semicircle at high frequency and an inclined line of nearly 45 °C at low frequency (Fig. 10). The semicircle shows the charge transfer resistance (Rct in Fig. 11), double-layer capacitance, and the intercept in the high frequency region of the Z’ real axis corresponds to ohmic resistance (Rs in Fig.11), which represents the resistance of the electrolyte and electrode material. The inclined line of 45 °C to the real axis in low frequency region represents the Warburg impedance (Zw in Fig.10) of long-range lithium-ion diffusion in the LiFePO4 particles[26–28]. The constant phase element (CPE in Fig. 11) represents the double layer capacitance used in Fig. 9. Zviw2.0 software is used to fit the curves in Fig. 10. 15
The parameters in the equivalent circuit are calculated and listed in Table 3. From the table, we can see that value of Rs does not change, but that of Rct decreases dramatically as the BPC content increases. The charge transfer resistance of LiFePO4@C is 86.72 Ω, whereas that of sample A-20 is only 37.58 Ω. A small Rct is beneficial to the kinetic behavior in the course of charge/discharge process and result in better electrochemical performance of the active materials.
Insert Figure 10 here Insert Figure 11 here
The exchange current density is a parameter that indicates the reversibility of the electrode. A high exchange current density value indicates a small resistance of the electrode reaction, thus making the charge/discharge process easier. The exchange current density (i0) of the composites is calculated from the following equation [29–31]
: i0 = RT / n FRct
(2)
where n represents the number of electron transferred per molecule during the intercalation, and is 1 for LiFePO4@C, R represents the gas constant (8.314J K−1·mol−1), T represents the absolute degree (K), and F represents the Faraday Constant (9.64 × 104 C/mol). All calculated results are listed in Table 3, which shows that the increase in BPC content, the value of i0 increases from 0.30 mA (sample A-0) to 0.70 mA (sample A-20). Therefore, BPC addition can reduce the resistance of the electrode reaction and facilitate the charge/discharge process.
16
Insert Table 3 here
4. Conclusions In the present study, the effect of BPC content on the composition, microstructure, and electrochemical performance of LiFePO4@C/C composites was examined by controlling the BPC content. The following conclusions could be drawn: (1) Network pores of BPC were almost completely filled by LiFePO4@C nanoparticles, which were formed by a crystalline LiFePO4 core with size ranging from 58.6 nm to 80.1 nm and a carbon shell with a thickness of approximately 2 nm; (2) Two kinds of LiFePO4@C nanoparticles existed on the wall surface of BPC. One was grown from the active point of BPC and had a particle size of approximately 60 nm, whereas the other was probably deposited from the solution and had a particle size approximately 200 nm to 500 nm; (3) Double electrical conductive network (local network formed by carbon shell in LiFePO4@C and network formed by BPC in LiFePO4@C/C) formed in the composites improved the electrical properties of samples from 2.59 × 10-6 S·cm-1(A-0) to 5.76 × 10-2 S·cm-1(A-20). In addition, a series of schematic models were used to describe the formation process of the electrical conductive network. The improved mixture rule was also used to discuss the electrical conductive mechanism of LiFePO4@C/C composites; (4) Synergy effect of electric double layer energy storage produced by BPC and lithium-ion extraction/insertion energy storage by LiFePO4 reduced the charge/discharge capacity reduction rate of the composites, and a charge/discharge capacity of 114.2/110.5 mA·h·g-1 for sample A-5 at 10C was obtained; (5) BPC can have a dramatic advantage on the reduction of the interfacial resistance, and can 17
increase the exchange current density. The former was converted from 86.72 Ω (sample A-0) to 37.58 Ω (sample A-20) and the latter converted 0.30 mA (sample A-0) to 0.70 mA (sample A-20), both changes were beneficial to kinetic behavior and facilitated the charge/discharge process.
Acknowledgments This work is supported by the Planning of the Growth of Young Teachers (Hunan University China), the Science and Technology Planning Project of Hunan Province, China (2012WK3023), and the Science and Technology Planning Project of Changsha City, Hunan Province, China (K1109117-11).
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Figure and table captions
Figure 1 TEM (a) and HRTEM (b) images of LiFePO4@C nanoparticles Figure 2 SEM images of LiFePO4@C particles growing on the wall surface of BPC and EDS spectra of particles A and B Figure 3 XRD patterns of standard LiFePO4, LiFePO4@C, and LiFePO4@C/C composites Figure 4 Schematic models describing the formation of double electrical conductive network in composites Figure 5 Initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate Figure 6 Platform of initial charge/discharge curves of LiFePO4@C/C composites at 0.1C rate Figure 7 Charge/ Discharge performance of LiFePO4@C/C composites at various charge rates Figure 8 Electrochemical cycling behavior of LiFePO4@C/C composites at 1C Figure 9 Cyclic voltammetry spectroscopy of LiFePO4@C/C composites and BPC matrix with a scan rate of 0.1 mV s−1 Figure 10 EIS of LiFePO4@C and LiFePO4@C/C composites Figure 11 Equivalent circuit used for fitting the experimental EIS data
Table 1 Structure parameters of LiFePO4 in LiFePO4@C and LiFePO4@C/C composites Table 2 Data of volume fraction, electrical conductivity and in-factor I of LiFePO4@C/C composites Table 3 Electrode kinetic parameters obtained from equivalent circuit fitting of experimental data of LiFePO4@C/C composites 21
Figures
Carbon shell 58.6 nm
Carbon shell LiFePO4 core 80.1 nm
LiFePO4 core
Figure 1 22
2nm
A
BPC
B LiFePO4@C
(a) Cross-section of BPC in composites
(b) Wall surface of BPC in composites
O P
Fe
Intensity (a.u.)
C Fe
Cu
Elem P Fe O
Particle 'A'
O P
Fe
C Fe
0
Cu
Elem P Fe O
mol% 13.6 14.1 53.04
Particle 'B'
2
4
6
8
10
Energy (keV)
(c) EDS spectra of particles A and B Figure 2 23
mol% 11.28 11.64 44.22
12
(121)
A - Li3PO4 C - Amorphous C
(260)
(340)
(311) (222)
A
(022)
C
(211)
(031)
(131)
BB
(011) (120) (021)
(020)
Intensity(a.u.)
B
B - Fe2P2O7
LiFePO4@C/C LiFePO4@C Standard LiFePO4
10
20
30
40
50
60
2θ(o)
70
80
Figure 3
Local network
Local network
5% BPC (sample A-5)
10% BPC (sample A-10) LiFePO4@C
LiFePO4@C/C
Figure 4
24
Thoroughly network
15% BPC (sample A-15)
4.2
Potential/V vs Li+/Li
4.0 3.8 3.6 3.4 3.2
A-20
3.0
A-15
2.8
A-10 A-5
2.6 2.4
A-0
0
20
40
60
80
100
120
Capacity/mAhg-1
140
160
Figure 5
Potential/V vs Li+/Li
3.55 3.50 3.45
A-20
A-15
A-0
A-5
3.40 3.35 3.30 15
20
25
30
35
40
Capacity/mAhg -1
Figure 6 25
A-10
45
50
150
A-5
A-5
130
130
Capacity/mAhg-1
Capacity/mAhg-1
140
A-10
120 110
A-15 A-20
100 90 80
0.1C
A-0
140
A-0
0.5C
1C
2C
4C
6C
C-rate
8C
A-10
120 A-15
110 A-20
100 90 80
10C
0.1C
(a) Charge curve
0.5C
1C
Capacity/ mA h g
-1
140 120 100 A-0 A-5 A-10 A-15 A-20
80
0
10
20
30
Cycle times
Figure 8
26
6C
(b) discharge curve
Figure 7
60
2C
C-rate
40
50
8C
10C
5 3
4
A-10 A-15
2 1
3
Current/μA
Current/mA
5
Epc A-0 A-5
4
A-20
0 -1 -2
2 1 0 -1 -2
-3
-3
Epa
-4 2.0
2.4
2.8
3.2
3.6
-4
4.0
Voltage/V
2.0
2.5
3.0
Voltage/V
(a) LiFePO4@C/C
(b) BPC matrix
Figure 9
40 35
A-20
30
A-15
Z''(ohm)
25 A-10
20 A-5
15 10 5 0
A-0
0
20
40
60
80
Z'(0hm)
100
Figure 10
Rs
CPE Rct
Zw
Figure 11
27
3.5
4.0
Tables
Table 1 Cell parameters sample c(Å)
volume(Å3) Space group
A(Å)
b(Å)
LiFePO4 @C
6.002
10.332 4.693 291.025
Pnmb
LiFePO4@C/C
6.005
10.333 4.693 291.199
Pnmb
Standard LiFePO4
6.008
10.334 4.693 291.392
Pnmb
Table 2 Samples
A-0
A-5
A-10
A-15
A-20
BPC
σ (S·cm-1)
2.59 × 10-6
5.56 × 10-4
1.15 × 10-3
4.50 × 10-2
5.76 × 10-2
0.38
VC
0
0.39
0.58
0.69
0.76
1.00
VLFP@C
1.00
0.60
0.42
0.31
0.24
0
I
-
0.93
1.10
0.67
0.74
-
* ρBPC = 0.288g·cm-3 [17] ρ LFP = 3.577 g·cm-3 (theoretical value)
Table 3
Samples
Rs (Ω)
Rct (Ω)
i 0(mA)
A-0
1.95
86.72
0.30
A-5
1.78
72.64
0.35
A-10
1.54
53.63
0.54
A-15
1.32
40.25
0.67
A-20
1.46
37.58
0.70
28