- Email: [email protected]
C composite from mechanical activation using sucrose as carbon source
Electrochimica Acta 54 (2009) 2861–2868
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Process investigation, electrochemical characterization and optimization of LiFePO4 /C composite from mechanical activation using sucrose as carbon source Ke Wang, Rui Cai, Tao Yuan, Xing Yu, Ran Ran, Zongping Shao ∗ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 1 August 2008 Received in revised form 16 October 2008 Accepted 10 November 2008 Available online 18 November 2008 Keywords: Lithium-ion battery Cathode Sucrose LiFePO4 Mechanical activation
a b s t r a c t LiFePO4 /C composite was synthesized by mechanical activation using sucrose as carbon source. Highenergy ball milling facilitated phase formation during thermal treatment. TG–DSC and TPR experiments demonstrated sucrose was converted to CHx intermediate before completely decomposed to carbon. Ball milling time, calcination temperature and dwelling time all had significant impact on the discharge capacity and rate performance of the resulted power. The optimal process parameters are high-energy ball milling for 2–4 h followed by thermal treatment at 700 ◦ C for 20 h. The product showed a capacity of 174 mAh/g at 0.1C rate and around 117 mAh/g at 20C rate with the capacity fade less than 10% after 50 cycles. Too low calcination temperature or insufficient calcination time, however, could result in the residual of CHx in the electrode and led to a decrease of electrode performance. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, there is a great interest in rechargeable lithiumion battery for applications in electric and electric-hybrid vehicles and dispersed energy storage systems, which require much larger battery size than the portable electrical devices, such as cameras and laptop computers. Such applications also have stricter requirements on safety, high-rate performance, and volumetric/gravimetric energy density. It means LiCoO2 , the currently widely applied positive electrode (cathode) material in commercial lithium-ion batteries, should be replaced. Orthorhombic lithium iron phosphate, LiFePO4 , as first demonstrated by Padhi et al. [1], is attracting considerable yet still increasing attention as a safe, low-cost and high-capacity cathode material for lithium-ion battery [2–8], especially for application in electric and hybrid-electric vehicles. Similar to Li4 Ti5 O12 anode, the lithium insertion/extraction into LiFePO4 cathode involves twophase mechanism [9–12]. On lithium extraction, LiFePO4 changes to FePO4 , a material with a similar structure and a cell volume difference of only ∼7% to LiFePO4 [1]. Such characteristics ensure an excellent cycling stability. LiFePO4 also features a favorable theoretical capacity around 170 mAh/g, a flat voltage curve with a plateau around 3.5 V vs. Li/Li+ , high thermal and chemical stability, a wide safety margin of usage for many organic
∗ Corresponding author. Tel.: +86 25 83172256; fax: +86 25 83365813. E-mail address: [email protected] (Z. Shao). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.11.012
electrolytes, low material cost, low toxicity and improved safety [1]. The main drawbacks of the olivine-type LiFePO4 include low electronic conductivity and low lithium-ion diffusivity. Serious polarization is typically observed under high charge/discharge rate, which results in a poor rate capacity. To overcome these drawbacks, three strategies are frequently applied: (i) reducing the grain size to lessen the diffusion distance for lithium ion inside the electrode bulk [13–15], (ii) doping the lattice with foreign cations to increase the intrinsic electronic conductivity and lithium-ion diffusivity [16–18] and (iii) surface modification such as carbon coating to increase the electronic conductivity consequently the interfacial reaction kinetics [19–22]. On the other hand, the electrochemical performance of LiFePO4 cathode also depends on a number of factors, e.g., phase purity, particle size and morphology, which are closely related with the synthesis technique and process parameters. Recently, a variety of techniques have been exploited for the synthesis of high-performance LiFePO4 [23–27]. Mechanical activation, which involves the blending of ingredients by high-energy ball milling followed by thermal treatment at high temperature, has turned out to be a versatile technique with high capability for scale up [28–30]. The ball milling can reduce the particle size and create close contact of the reactants. Thereby, a decrease in the thermal treatment time and temperature is usually experienced during the following calcination to obtain a pure phase product. It then effectively avoids undesirable grain growth. Mechanical activation process also allows the in situ formation of carbon layer over LiFePO4 when carbon black or a suitable organic/polymeric
2862
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
compound is incorporated as the carbon source during the synthesis. Besides performing as the coating material for LiFePO4 , the carbon also acts as a reducing agent to suppress the oxidation of Fe2+ to form Fe3+ impurities during the synthesis. The carbon may further act as a diffusion barrier by blocking the particle contact, thereby suppressing the grain growth. Improved performance at high charge/discharge rate has been observed for the LiFePO4 /C composite as compared to the LiFePO4 prepared by the same technique but without adopting carbon additive [31]. Among various carbon raw materials for the coating layer, sucrose was found to be an excellent candidate [5]. Numerous studies have been conducted in literature on the synthesis of high-performance LiFePO4 /C composite by applying sucrose as the carbon source [5,32–34]. However, most of the investigations were focused on the electrochemical performance of the as-synthesized samples. Very limited attention was paid to the synthesis process. Such information is, however, important guidance for further optimizing the material properties. In this study, the synthesis of LiFePO4 /C composite cathode by a mechanical activation process applying sucrose as the carbon source was systematically investigated. Importance was paid to the synthesis process and the influence of process parameters on the electrochemical performance of the product. High electrochemical performance was achieved for the LiFePO4 /C composite synthesized under optimal conditions. 2. Experimental 2.1. Material synthesis LiFePO4 /C was synthesized by a mechanical activation process followed by high-temperature calcination. Li2 CO3 , FeC2 O4 ·2H2 O and NH4 H2 PO4 (all chemicals of >99% purity) were applied as the raw materials and sucrose was selected as the carbon source. They were weighed in stoichiometric amounts according to the nominal LiFePO4 to C weight ratio 94 to 6, assuming the sucrose following the thermal pyrolysis reaction C12 H22 O11 → 12C + 11H2 O. The ingredients together with 60% acetone were magnetically stirred at room temperature for 5 min, followed by high-energy ball milling in a zirconia container with zirconia balls (ball-topowder weight ratio 10 to 3) at room temperature for different periods under an argon atmosphere using a Pulverisette-6 planner mill at 400 rpm. The as-obtained slurry was dried by evaporating the acetone liquid media under vacuum. Finally the solid precursors were fired at temperatures ranging from 500 to 900 ◦ C for various hours in a tube furnace purging with pure nitrogen atmosphere. 2.2. Electrode fabrication The electrochemical cycling performance of LiFePO4 or LiFePO4 /C powders was tested with coin-shaped cells using metallic lithium film as the counter and reference electrode at room temperature (∼30 ◦ C). The cells are based on the configuration of Li metal (−)|electrolyte|LiFePO4 (+) with a liquid electrolyte (1 M solution of LiPF6 in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1, v/v). Microporous polypropylene film (Celgard 2400) was used as the separator [35]. Slurries composed of 85 wt.% LiFePO4 with 10 wt.% conductive Super P (NCM HERSBIT Chemical Co., Ltd., China) and 5 wt.% LA-132 binder (Chengdu Organic Chemicals Co., Ltd., China) in de-ionized water were prepared, which were deposited on current collectors of 6 m thick aluminum foils by blade. The electrode was dried under vacuum at 100 ◦ C for 12 h before electrochemical evaluation. Cell assembly was conducted in a glove box filled with pure argon.
2.3. Characterization The crystal structures of the synthesized powders were examined by X-ray diffraction (XRD) using a Bruker D8 advance diffractometer with filtered Cu K␣ radiation. The experimental diffraction patterns were collected at room temperature by step scanning at the range of 10◦ < 2 < 80◦ . The crystallite size was calculated based on the Scherrer equation from the study of the Bragg angle and half bandwidth of the index peaks. The mean value of crystallite sizes calculated based on [1 0 1], [1 1 1], [2 1 1] and [3 1 1] diffraction planes was adopted as the crystallite size of assynthesized LiFePO4 . The particulate morphology was examined by Environmental Scanning Electronic Microscope (ESEM, QUANTA-2000). The specific surface area of the samples was characterized by N2 adsorption at the temperature of liquid nitrogen using a BELSORP II instrument. Prior to analysis, the samples were put in vacuum at 200 ◦ C for 3–5 h to remove the surface adsorbed species. Thermogravimetry–differential scanning calorimetric analyses (TG–DSC) were performed using a NETZSCH STA 409 PC in the range of 25–1000 ◦ C at a heating rate of 10 ◦ C/min under flowing argon atmosphere. Temperature programmed reaction (TPR) experiments were conducted using a U-type quartz reactor. About 0.02 g sample in 40–60 mesh was dropped into a quartz tube reactor and a constant helium flow at 20 ml/min [STP] was introduced. The temperature of the reactor was programmatically increased. The effluent gas from the reactor was introduced to a mass spectroscope (QIC 20) for in situ compositional analysis. Before the electrochemical performance test, the cells were activated through one charge/discharge cycles with a small current (0.1C, 1C = 170 mAh/g) using a NEWARE BTS-5V50mA computercontrolled battery test system. The charge/discharge characteristics of the cells were recorded over the potential range between 2.0 and 4.2 V at different range of 0.1–50C at 30 ◦ C. Cyclic voltammetry tests were performed over the potential range of 2.0–4.2 V using a Princeton Applied Research PARSTAT 2273 advanced electrochemical system at the scanning rate 0.1–1 mV/s. 3. Results and discussion 3.1. Synthesis The reactants, a physical mixture of stoichiometric amounts of NH4 H2 PO4 , Li2 CO3 , FeC2 O4 ·2H2 O and C12 O11 H22 , demonstrated the color of light yellow, a typical color of FeC2 O4 ·2H2 O. It changed steadily to light green with increasing ball milling time during the mechanical activation. This implies that certain type of interaction between the reactants appeared during the high-energy ball milling. It is well known that Fe(H2 O)6 2+ has a color of light blue. Hereby, the ball milling likely led to the interaction of the hydroxide groups in sucrose with FeC2 O4 ·2H2 O. Fig. 1 shows the XRD patterns of the precursor after the ball milling at room temperature for various hours. Before ball milling, the characteristic diffraction peaks of FeC2 O4 ·2H2 O were detected. However, an amorphous structure at room temperature was observed when the precursor was ball milled for 2 h or longer. This implies that the crystallite phase of the FeC2 O4 ·2H2 O was destroyed by reaction or certain type of interaction between FeC2 O4 ·2H2 O and other reactants existed after the ball milling, in accordance with the color change after the ball milling. During the thermal treatment, the cracking of sucrose and the phase reaction among NH4 H2 PO4 , Li2 CO3 and FeC2 O4 ·2H2 O to form LiFePO4 could happen simultaneously or separately. Different formation mechanism may result in significantly different anode morphology or phase structure, consequently, differed electrochemical behavior. The thermal decomposition behavior of sucrose
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
2863
Fig. 1. XRD patterns of the LiFePO4 /C precursor after the ball milling at room temperature for various hours. Fig. 3. TPR curves of sucrose under a nitrogen atmosphere with a heating rate of 10 ◦ C/min.
Fig. 2. TG–DSC curves of sucrose under a nitrogen atmosphere with a heating rate of 10 ◦ C/min.
was first investigated. As shown in Fig. 2 is the TG–DSC curves of sucrose under a nitrogen atmosphere with a heating rate of 10 ◦ C/min. The weight loss started at ∼215 ◦ C, mainly happened between 220 and 400 ◦ C, and finished around 700 ◦ C. The final weight loss reached 84%. The DSC curve showed a sharp endothermic peak around 194 ◦ C corresponding to a zero weight loss, which was assigned to the cleavage of glucose ring in sucrose. Another
endothermic peak was observed around 230 ◦ C accompanied with a sharp weight loss in the TG profile. There were two small successive endothermic peaks at 296 and 450 ◦ C, respectively. The appearance of several successive endothermic peaks in the DSC profile suggests that the thermal cracking was conducted in several successive steps. For the thermal decomposition of sucrose, a theoretical weight loss 58% should be expected if carbon and water were the sole products. The actual weight loss 84% implies that only 38% carbon in sucrose was successfully converted to solid carbon. On other words, ∼62% carbon was lost in the form of gaseous products, i.e., CO and CO2 . The overall thermal decomposition reaction of sucrose could be expressed as C12 O11 H22 → 4.56C + 7.44(CO/CO2 ) + 11(H2 O/H2 ). If assuming decomposition behavior of sucrose in the solid precursor was similar to pure sucrose, the actual carbon content in the derived LiFePO4 /C composite is only around 2.3 wt.%. To further exploit the thermal decomposition reaction of sucrose, a TPR was conducted. As shown in Fig. 3, there was almost no any gaseous species produced at T < 220 ◦ C, agreed well with the TG–DSC results. A broad water peak and a CO2 peak were observed within the temperature range of 220–550 ◦ C, and 250–500 ◦ C, respectively. A CO peak was also appeared alongside with the CO2 peak. At T > 550 ◦ C, a large and broad H2 peak was appeared, however, no other peak was detected in the same temperature zone. Since carbon is the only final solid residual, the hydrogen was then produced from the thermal decomposition of CHx . Above results
Fig. 4. TG–DSC curves of the LiFePO4 /C precursor under a nitrogen atmosphere with a heating rate of 10 ◦ C/min (a) without the ball milling precursor and (b) the ball-milled precursor.
2864
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Fig. 5. TPR profiles of the solid precursor under a nitrogen atmosphere with a heating rate of 10 ◦ C/min (a) without the ball milling precursor and (b) the ball-milled precursor.
demonstrated that the sucrose first cracked to CHx intermediate during the thermal decomposition, which was further converted to carbon at relatively high temperature of >550 ◦ C. To exploit the role of the high-energy ball milling in the phase formation and its effect on the thermal decomposition behavior of the solid precursor, TG–DSC and TPR of the solid precursors with and without the ball milling process were conducted. As shown in Fig. 4, without the ball milling process, the weight loss started at 162 ◦ C. A sharp weight loss appeared between 162 and 230 ◦ C, which accounted for a weight loss of about 25%. Another obvious weight loss amounting to ∼20% of initial weight appeared between 230 and 420 ◦ C. Between 420 and 800 ◦ C and mainly between 550 and 650 ◦ C there was additional modest weight loss amounting to ∼6%. The ball-milled precursor showed somewhat different decomposition behavior. One significant difference is that the weight loss initiated at room temperature. In combination with the color change after the ball milling, such weight loss was attributed to the evaporation of dehydrated water from FeC2 O4 ·2H2 O, created during the high-energy ball milling process. Another significant difference is that the sharp weight loss between 550 and 650 ◦ C was disappeared. As shown in Fig. 5, for the precursor without the ball milling process, there were almost no gaseous decomposition products at T < 180 ◦ C, however, a big water peak was observed between 60 and 130 ◦ C for the ball-milled sample. Such different behaviors are in well agreement with the TG–DSC results. It further supported that the hydrated water in FeC2 O4 ·2H2 O was released during the high-energy ball milling process. Starting at around 180 ◦ C and happening mainly between 180 and 380 ◦ C, a strong and broad water peak was appeared for the solid precursor without the ball milling process, which was ascribed to the thermal decomposition of sucrose and the dehydration of FeC2 O4 ·2H2 O. Between 220 and 260 ◦ C, CO and CO2 decomposition peaks with similar intensity were appeared simultaneously. In connection with the thermal decomposition behavior of the sucrose, they were ascribed to the thermal decomposition of sucrose via the reaction C12 O11 H22 → CHx + CO + CO2 + H2 O. As compared to pure sucrose, the CO and CO2 decomposition peaks become sharper and moved to a lower temperature zone. It suggests that the formation of solid mixture facilitated the thermal decomposition of sucrose. Ammonia peak, attributed to the thermal decomposition of NH4 H2 PO4 to NH3 and H3 PO4 , was detected between 160 and 500 ◦ C. Between 380 and 430 ◦ C, sharp peaks of CO and CO2 with similar intensity appeared simultaneously, which were attributed to the thermal decomposition of FeC2 O4 via the reaction FeC2 O4 → FeO + CO + CO2 . A big and broad hydrogen peak was also observed in the same tem-
perature zone, which was ascribed to the thermal dehydrogenation of CHx to C and H2 . Between 600 and 700 ◦ C, hydrogen, CO and CO2 peaks with low intensity were appeared simultaneously, which were assigned to the thermal decomposition of Li2 CO3 to Li2 O and CO2 , and the reaction of CO2 with CHx to form CO and H2 , and so on, via the reactions as follows. Li2 CO3 → Li2 O + CO2 Li2 O + FeHPO4 → LiFePO4 + H2 O H2 O + CHx → CO + H2 CO2 + CHx → CO + H2 When the solid precursor was subjected for high-energy ball milling, the reactants were mixed thoroughly and had close contact. The ball milling process also led to the dehydration of FeC2 O4 ·2H2 O with the formation of free water as demonstrated previously. Such free water can be easily evaporated. It then well explained the observation of the water peak between 60 and 150 ◦ C for the ballmilled sample. Starting at 180 ◦ C, additional water desorption peak appeared, which was produced from the thermal decomposition of sucrose. Two small ammonia peaks were observed between 50 and 180 ◦ C, while they were not detected for the solid precursor without the ball milling process during the synthesis. It suggests that the high-energy ball milling also facilitated the thermal decomposition of NH4 H2 PO4 . Between 180 and 400 ◦ C, relatively broad ammonia peak alongside with the CO2 and CO peaks appeared. The CO2 peak was found to have much stronger intensity than CO, different from the decomposition behavior of the precursor without the ball milling process. It was attributed to the thermal decomposition of sucrose alongside with the reaction of NH4 H2 PO4 and Li2 CO3 (Li2 CO3 + NH4 H2 PO4 → LiH2 PO4 + CO2 ), facilitated by the high-energy ball milling. Between 400 and 450 ◦ C, sharp CO and CO2 peaks with similar densities appeared, which were ascribed to the thermal decomposition of FeC2 O4 to FeO, CO and CO2 . At T > 500 ◦ C, a broad hydrogen peak was also appeared without the formation of other decomposition product, in connection with the thermal decomposition behavior of sucrose, it was assigned to the decomposition of CHx . Based on the TG–DSC and TPR results, a calcination temperature of around 700 ◦ C was necessary in order to achieve the complete reaction of the reactants in the solid precursor without the ball milling process, while a similar temperature was required for the ball-milled precursor allowing the CHx completely decomposed to
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Fig. 6. XRD of the as-prepared LiFePO4 /C composites by solid-state reaction at 700 ◦ C for 20 h (a) without the high-energy ball milling process and without sucrose adopted, (b) with sucrose as carbon source during the synthesis but without the high-energy ball milling process and (c) with the high-energy ball milling process and also applying sucrose as carbon source during the synthesis.
carbon. Thereby, 700 ◦ C was selected for the thermal treatment of solid precursors. As shown in Fig. 6 are the XRD of the as-prepared LiFePO4 /C composites by solid-state reaction at 700 ◦ C for 20 h with or without the mechanical activation process, and LiFePO4 synthesized without the high-energy ball milling process and without sucrose adopted during the synthesis. All three samples showed a well-crystalline single-phase LiFePO4 . For each sample, all its diffraction peaks can be indexed well based on orthorhombic structure with the space group pnmb. No carbon diffraction peaks were detected for both LiFePO4 /C composites with or without the ball milling process during the synthesis. This implies the formed carbon coating layer over LiFePO4 surface was in an amorphous phase. 3.2. Electrochemical performance Fig. 7 shows the first charge/discharge profiles at 0.1C rate (current density of 17 mAh/g) for the samples of LiFePO4 electrode prepared by solid-state reaction of NH4 H2 PO4 , Li2 CO3 and FeC2 O4 ·2H2 O at 700 ◦ C for 20 h (sample I), LiFePO4 /C composite prepared from the similar procedure of sample I but applying sucrose
Fig. 7. The corresponding first charge/discharge profiles at 0.1C rate for the samples of LiFePO4 electrode prepared by solid-state reaction.
2865
Fig. 8. The discharge curves of the LiFePO4 /C composite at various discharge/charge rates varied from 0.1 to 50C.
as carbon source during the synthesis (sample II), and LiFePO4 /C composite prepared by the similar procedure of sample II with high-energy ball milling of the solid precursor for 2 h during the synthesis (sample III). The sample I showed the first discharge capacity around 110 mAh/g, similar to the literature result for a pure LiFePO4 prepared by a similar solid-phase reaction process [36]. When sucrose was introduced as a carbon source during the preparation, the as-obtained LiFePO4 /C composite showed a capacity around 140 mAh/g. It is generally believed that the carbon additive had several functions during the synthesis of LiFePO4 . Besides its main function to improve the electronic conductivity by performing as surface coating layer, it also effectively prevents the oxidation of Fe2+ to Fe3+ and acted as a diffusion block to suppress the LiFePO4 grain growth. The measured BET surface area of samples I and II was 13.6 and 23.6 m2 /g, respectively. It suggests that the sucrose indeed increased the surface area of the resulted LiFePO4 . Both the increase in surface area and the improvement in electronic conductivity may account for the improved discharge capacity of LiFePO4 /C composite as compared to conventional LiFePO4 . When both high-energy ball milling (mechanical activation) and carbon additive were introduced during the synthesis of LiFePO4 , the surface area further increased to 28.3 m2 /g. The first discharge capacity increased obviously to 174 mAh/g, which reached the theoretical value for a LiFePO4 cathode (170 mAh/g) Within experimental error. This implies all LiFePO4 in the electrode became electrochemically active. The sample also demonstrated excellent reversible capacity around 97%. Thereby, the high-energy ball milling together with in situ carbon coating is highly effective to increase the electrochemical activity of LiFePO4 cathode. As shown in Fig. 8 are the discharge curves of the LiFePO4 /C composite at various discharge/charge rates varied from 0.1 to 50C. For accurate comparison of the rate performance during discharging, the cell was recharged at 0.1C rate to assure identical initial conditions for all the discharges [37,38]. The discharge capacities decreased from 171 mAh/g at 0.1C to 117 mAh/g at 20C. Even at 50C, a capacity of 81 mAh/g was still maintained, which is even higher than 76 mAh/g of the LiFePO4 /C composite without the ball milling process during the synthesis at 5C rate. These results are among the best for LiFePO4 /C composite electrode reported ever in literature. By a similar synthesis process Kim et al. reported a first discharge capacity of around 164 mAh/g at 0.1C rate and ∼120 mAh/g at 1C rate [33]. The cycling characteristics of the as-prepared LiFePO4 /C cathode material at different discharge rate are shown in Fig. 9. Favor-
2866
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Under the condition of semi-infinite and infinite diffusion process, the peak current Ip and the square root of scanning rate () has the relationship of Eq. (1) [39]. 1/2
Ip = 2.69 × 105 S · Co · Do
· n2/3 · 1/2
(1) (cm2 ),
Co is where S is the total surface area of the anode material the concentration of lithium ions in the cathode (mol/ml), Do is the average diffusion coefficient of lithium ion (cm2 /s), n is the number of electrons transferred per molecule during the intercalation, it is 1 for LiFePO4 , and is the scanning rate (s−1 ).The surface area of the anode material can be calculated by S = Sm × no
(2) (cm2 /mol)
and no is the where Sm is the mole surface area of anode amount of the active material in the electrode (mol). If assuming the electrode material is in sphere shape, the mole surface area can be calculated by Fig. 9. The cycling characteristics of the as-prepared LiFePO4 /C cathode material at different discharge rates.
able cycle performance was demonstrated. At all discharge rates, the capacity fade after 50 cycles was less than 10%. The good long-term cycling behavior of the LiFePO4 /C composite at various discharge rates confirms the excellent reversibility of lithium insertion/extraction reactions. Cyclic voltammograms of the LiFePO4 /C electrode at the scan rates of 0.1, 0.2, 0.5 and 1 mV/s between 4.2 and 2.0 V are shown in Fig. 10a. The oxidizing peak and the reduction peak are highly symmetric to each other. The ratios of Ipa /Ipc are close to 1, implying that the good reversibility of lithium intercalation into and de-intercalation from the carbon-coated LiFePO4. There is a linear relationship between Ipa and square root of the scan rate, as shown in Fig. 10b. The slope k was found to be 1.446 × 10−2 A s0.5 /V0.5 , where A is the current in Ampere, s is the time in second and V is the voltage. CV is a quasi-static electrode characterization technique. In an electrochemical process of LiFePO4 /C electrode, the anodic process corresponds to the lithium-ion de-intercalation from LiFePO4 bulk with the simultaneous diffusion of electron from the electrode to the current collector, while the cathodic process corresponds to the Li+ intercalation into FePO4 and acquiring electron from the current collector. Diffusion coefficient of lithium ions (Do ) can be roughly estimated by cyclic voltammetry since the intercalation and de-intercalation is also a redox process.
Sm =
3Vm r
(3)
where Vm is the mole volume of the electrode (ml/mol) and r is the radius of the particle (cm). The electrical charge Q transferred during charge/discharge process can be related with Co by Q = nFVm × Co
(4)
is in linear relationship with n1/2
with the slope of k, the Since Ip lithium diffusion coefficient Do can be expressed by the following equation [40]: Do =
r 2 k2 69.9n3 Q 2
(5)
The electrical charge that transferred through the active material during the charge/discharge process could be took as its capacity, which can also be calculated from the weight amount of the active material and the theoretical specific capacity. It can also be calculated from the discharge/charge curves or be obtained by integrating the CV curves with respect to time. Here, Do was calculated based on theoretical capacity, i.e., Q = 0.95C. As shown in Fig. 11a is the SEM of the as-synthesized LiFePO4 powder. It shows that LiFePO4 had quasi-sphere shape with an average particle size of 1.5 m, i.e. r = 7.5 × 10−5 cm. Together with the k value of 1.446 × 10−2 A s0.5 /V0.5 , the lithium diffusion coefficient Do of ∼8.93 × 10−13 cm2 /s was calculated according to Eq. (5). The reported value for a pure LiFePO4 is ∼1.8 × 10−14 cm2 /s [41]. It
Fig. 10. (a) Cyclic voltammograms of the LiFePO4 /C electrode at the scan rates of 0.1, 0.2, 0.5 and 1 mV/s between 4.2 and 2.0 V and (b) a linear relationship between Ipa and square root of the scan rate.
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
2867
Fig. 11. SEM photos of the as-synthesized LiFePO4 powder (a) 700 ◦ C, (b) 600 ◦ C and (c) 800 ◦ C.
then implies that the lithium-ion diffusion coefficient was greatly improved by applying sucrose as a carbon source in combination with high-energy ball milling, which may be the cause for the excellent high-rate performance of the LiFePO4 /C composite prepared in this study. 3.3. Process optimization To exploit the effect of process parameter on the electrochemical performance of the derived LiFePO4 , various ball milling times, thermal treatment temperatures and times were investigated in the synthesis of LiFePO4 /C. As shown in Table 1, the ball milling of 2–4 h resulted in the maximum surface area. The further increase in milling time, however, led to a steady decrease in surface area and increase in grain size. It is well known that the high-energy ball milling could result in a close contact of the reactants, which facilitated the solid-phase reaction, therefore reduced the sintering temperature and time for obtaining a pure phase LiFePO4 . However, it can also break down the reactant particles and increase the surface activity. It means the particle sintering could be accelerated during the successive thermal treatment when the reactants were over ball milled. The thermal treatment temperature was found to have most significant impact on the surface area, the higher the calcination temperature the lower the surface area. As shown in Fig. 11a–c, the increase of calcination temperature resulted in obvious size growth of the particles. The calcination time also had an
impact on the surface area, the longer the cacination the lower the surface area, as shown in Table 1. The discharge capacities at various rates for the LiFePO4 /C composites prepared at various conditions were then measured with the results also listed in Table 1. The discharge capacity and rate performance of the as-prepared LiFePO4 /C composite was found highly dependent on the process parameters. The best performance was achieved at a ball milling time of 2–4 h followed by heat treatment at 700 ◦ C for 20 h. X-ray diffraction pattern demonstrated a pure phase LiFePO4 was already formed for the 600 ◦ C calcinated sample. SEM examination demonstrated it had a mean particle size of 0.5 m, smaller than the 700 ◦ C calcinated one of 1.5 m (Fig. 11b). A smaller particle size means a shorter lithium diffusion distance, which is beneficial for high-rate discharge and charge processes. However, the capacities at both low and high discharge rates were reduced as compared to the 700 ◦ C calcinated one. The poor electrochemical performance of the 600 ◦ C calcinated sample was then contributed from the coating layer, which was likely in the state of CHx based on the TGA and TPR experiments. Since CHx has poor electronic conductivity it would have a serious block effect on the electronic transfer. The poor electrochemical performance of the 800 ◦ C calcinated sample was attributed to its large grain size (around 2.2 m), as shown in Fig. 11c, due to the sintering. Relatively poor electrochemical performance was also observed for the LiFePO4 /C composite calcinated at 700 ◦ C for 15 h. It was likely that some of carbon in the coating layer was still in the form of CHx due
2868
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Table 1 Effect of various process parameters on the surface area and discharge capacity and rate performance of the as-synthesized LiFePO4 /C composite. Milling time (h)
Calcination temp. (◦ C)
Calcination time (h)
BET area (m2 /g)
2 4 6 10 6 2 2
700 700 700 700 800 600 700
20 20 20 20 20 20 15
28.3 30.1 24.3 22.5 20.6 31.8 31.1
to the insufficient calcination time. As shown in Table 1, the ball milling time also had a significant impact on the discharge capacity and rate performance of the derived LiFePO4 /C composite. The optimal ball milling time was 2–4 h. The further increase of ball milling time resulted in obvious decrease of capacity at both low and high discharge rates. It was believed that the over ball milling accelerated the LiFePO4 sintering during the following high-temperature treatment. To summary, ball milling time, heat treatment temperature and time should be specifically optimized in order to achieve a high-performance LiFePO4 /C composite synthesized by mechanical activation adopting sucrose as carbon source. 4. Conclusions Olivine-type LiFePO4 /C composite was successfully synthesized by mechanical activation followed by thermal treatment. Ball milling time, thermal treatment temperature and time all have significant impact on the electrochemical performance of the derived LiFePO4 /C composite. The high-energy ball milling facilitated the phase reaction between reactants and the thermal decomposition of sucrose. The sucrose was first cracked to CHx intermediate, which was then further converted to solid carbon at 700 ◦ C or higher for a sufficient time. Too low calcination temperature for example 600 ◦ C, and too short calcination time for example 15 h, resulted in the residual of undecomposed CHx , which seriously blocked the electron transfer and consequently decreased the electrochemical performance of LiFePO4 /C composite. Over ball milling or over heating, however, resulted in grain growth of LiFePO4 electrode during the following thermal treatment, a decrease in electrochemical performance was then experienced. Thereby, the process parameter should be optimized in order to obtain a high electrochemical performance. At a ball milling time of 2–4 h, thermal treatment at 700 ◦ C for 20 h, the derived LiFePO4 /C composite showed a discharge capacity of 174 mAh/g at 0.1C rate, it reached the theoretical value for LiFePO4 within experimental error. It also showed a highrate performance with a discharge capacity of 117 mAh/g at 20C rate. The performance of the derived LiFePO4 /C composite, prepared by mechanical activation was very sensitive to the process parameters. Suppressing the grain growth and avoiding CHx are the keys to achieve high-rate performance LiFePO4 /C composite cathode. Acknowledgements This work was supported by National Basic Research Program of China under contract no. 2007CB209704. Dr. Zongping Shao also acknowledges the financial support from Chinese Ministry of Education via the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT0732).
Discharge rate 0.1C
0.2C
1C
2C
5C
174.6 173.8 156.7 143.3 120.6 134.6 142.1
164.5 165.3 147.3 128.1 112.4 123.5 131.6
153.8 154.2 132.1 114.3 84.3 111.7 122.8
142.7 143.1 120.4 103.8 – 99.8 109.4
134.6 132.5 111.2 92.3 – – –
References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] A. Yamada, S.C. Chung, K. Hinokuna, J. Electrochem. Soc. 148 (2001) A224. [3] H. Huang, S.C. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) A170. [4] G. Arnald, J. Garche, R. Hemmer, S. Strobele, C. Vogler, M. Wohlfahrt-Mehrens, J. Power Sources 119–121 (2003) 247. [5] S. Franger, F. Le Cras, C. Bourbon, H. Rouault, Electrochem. Solid-State Lett. 5 (2002) A231. [6] K. Zaghib, P. Charest, A. Guerfi, J. Shim, M. Perrier, K. Striebel, J. Power Sources 134 (2004) 124. [7] K. Striebel, A. Guerfi, J. Shim, M. Armand, M. Gauthier, K. Zaghib, J. Power Sources 119–121 (2003) 951. [8] J. Barker, M.Y. Saidi, J.L. Swoyer, Electrochem. Solid-State Lett. 6 (2003) A53. [9] J.L. Allen, T.R. Jow, J. Wolfenstine, J. Solid-State Electrochem. 12 (2008) 1031. [10] L. Laffont, C. Delacourt, P. Gibot, M.Y. Wu, P. Kooyman, C. Masquelier, J.M. Tarascon, Chem. Mater. 18 (2006) 5520. [11] C.S. Wang, U.S. Kasavajjula, P.E. Arce, J. Phys. Chem. C 111 (2007) 16656. [12] J.L. Allen, T.R. Jow, J. Wolfnstine, Chem. Mater. 19 (2007) 2108. [13] L. Hui, Z.L. Tang, S.H. Luo, Z.T. Zhang, Prog. Chem. 19 (2007) 1460. [14] Y. Wang, G.Z. Cao, Adv. Mater. 12 (2008) 2251. [15] K.F. Hsu, S.Y. Tsay, B.J. Hwang, J. Mater. Chem. 14 (2004) 2690. [16] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Nat. Mater. 1 (2002) 123. [17] T. Nakamura, Y. Miwa, M. Tabuchi, Y. Yamada, J. Electrochem. Soc. 154 (2007) A1108. [18] N. Jayaprakash, N. Kalaiselvi, Electrochem. Commun. 9 (2007) 620. [19] C.H. Mi, Y. Cao, X.G. Zhang, X.B. Zhao, H.L. Li, Powder Technol. 181 (2008) 301. [20] J.K. Kim, G. Cheruvally, J.H. Ahn, H.J. Ahn, J. Phys. Chem. Solid 69 (2008) 1257. [21] R. Dominko, M. Gaberscek, A. Bele, D. Mihailovic, J. Jamnik, J. Eur. Ceram. Soc. 27 (2007) 909. [22] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee, K.T. Kang, H.G. Kim, Solid State Commun. 129 (2004) 311. [23] W. Ojczyk, J. Marzec, K. Swierczek, W. Zajac, M. Mdenda, R. Dziembaj, J. Molenda, J. Power Sources 173 (2007) 700. [24] G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo, N. Penazzi, J. Power Sources 160 (2006) 516. [25] C.W. Kim, M.H. Lee, W.T. Jeong, K.S. Lee, J. Power Sources 146 (2005) 534. [26] S.T. Myung, S. Komaba, N. Hirosaki, H. Yashiro, N. Kumagai, Electrochim. Acta 49 (2004) 4213. [27] D. Jugovic, M. Mitric, N. Cvjeticanin, B. Jancar, S. Mentus, D. Vskokovic, Solid State Ionics 179 (2008) 415. [28] X.Z. Liao, Z.F. Ma, L. Wang, X.M. Zhang, Y. Jiang, Y.S. He, Electrochem. Solid-State Lett. 7 (2004) A522. [29] J.K. Kim, J.W. Choi, G. Cheruvally, J.U. Kim, J.H. Ahn, G.B. Cho, K.W. Kim, H.J. Ahn, Mater. Lett. 61 (2007) 3822. [30] H.C. Kang, D.K. Jun, B. Jin, E.M. Jin, K.H. Park, H.B. Gu, K.W. Kim, J. Power Sources 179 (2008) 340. [31] H.C. Shin, W.I. Cho, H. Jang, J. Power Sources 132 (2004) 235. [32] S.L. Bewlay, K. Konstantinov, G.X. Wang, S.X. Dou, H.K. Liu, Mater. Lett. 58 (2004) 1788. [33] J.K. Kim, G. Cheruvally, J.H. Ahn, J. Solid-State Electrochem. 12 (2008) 799. [34] A.D. Spong, G. Vitins, J.R. Owen, J. Electrochem. Soc. 152 (2005) A2376. [35] G.X. Wang, J.J. Xu, M. Wen, R. Cai, R. Ran, Z.P. Shao, Solid State Ionics 179 (2008) 946. [36] H. Huang, S. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) 170. [37] H.C. Shin, S.B. Park, H. Jang, K.Y. Chung, W.I. Cho, C.S. Kim, B.W. Cho, Electrochim. Acta 53 (2008) 7946. [38] S.W. Oh, H.J. Bang, S.T. Myung, Y.C. Bae, S.M. Lee, Y.K. Sun, J. Electrochem. Soc. 155 (2008) A414. [39] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applications, 2nd edition, 2003. [40] S.Q. Liu, B.L. Gong, K.L. Huang, G. Zhang, S.C. Li, Chin. J. Power Sources 5 (2007) 382. [41] P.P. Prosini, M. Lisi, D. Zane, M. Pasquali, Solid State Ionics 1 (2002) 45.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Process investigation, electrochemical characterization and optimization of LiFePO4 /C composite from mechanical activation using sucrose as carbon source Ke Wang, Rui Cai, Tao Yuan, Xing Yu, Ran Ran, Zongping Shao ∗ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 1 August 2008 Received in revised form 16 October 2008 Accepted 10 November 2008 Available online 18 November 2008 Keywords: Lithium-ion battery Cathode Sucrose LiFePO4 Mechanical activation
a b s t r a c t LiFePO4 /C composite was synthesized by mechanical activation using sucrose as carbon source. Highenergy ball milling facilitated phase formation during thermal treatment. TG–DSC and TPR experiments demonstrated sucrose was converted to CHx intermediate before completely decomposed to carbon. Ball milling time, calcination temperature and dwelling time all had significant impact on the discharge capacity and rate performance of the resulted power. The optimal process parameters are high-energy ball milling for 2–4 h followed by thermal treatment at 700 ◦ C for 20 h. The product showed a capacity of 174 mAh/g at 0.1C rate and around 117 mAh/g at 20C rate with the capacity fade less than 10% after 50 cycles. Too low calcination temperature or insufficient calcination time, however, could result in the residual of CHx in the electrode and led to a decrease of electrode performance. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, there is a great interest in rechargeable lithiumion battery for applications in electric and electric-hybrid vehicles and dispersed energy storage systems, which require much larger battery size than the portable electrical devices, such as cameras and laptop computers. Such applications also have stricter requirements on safety, high-rate performance, and volumetric/gravimetric energy density. It means LiCoO2 , the currently widely applied positive electrode (cathode) material in commercial lithium-ion batteries, should be replaced. Orthorhombic lithium iron phosphate, LiFePO4 , as first demonstrated by Padhi et al. [1], is attracting considerable yet still increasing attention as a safe, low-cost and high-capacity cathode material for lithium-ion battery [2–8], especially for application in electric and hybrid-electric vehicles. Similar to Li4 Ti5 O12 anode, the lithium insertion/extraction into LiFePO4 cathode involves twophase mechanism [9–12]. On lithium extraction, LiFePO4 changes to FePO4 , a material with a similar structure and a cell volume difference of only ∼7% to LiFePO4 [1]. Such characteristics ensure an excellent cycling stability. LiFePO4 also features a favorable theoretical capacity around 170 mAh/g, a flat voltage curve with a plateau around 3.5 V vs. Li/Li+ , high thermal and chemical stability, a wide safety margin of usage for many organic
∗ Corresponding author. Tel.: +86 25 83172256; fax: +86 25 83365813. E-mail address: [email protected] (Z. Shao). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.11.012
electrolytes, low material cost, low toxicity and improved safety [1]. The main drawbacks of the olivine-type LiFePO4 include low electronic conductivity and low lithium-ion diffusivity. Serious polarization is typically observed under high charge/discharge rate, which results in a poor rate capacity. To overcome these drawbacks, three strategies are frequently applied: (i) reducing the grain size to lessen the diffusion distance for lithium ion inside the electrode bulk [13–15], (ii) doping the lattice with foreign cations to increase the intrinsic electronic conductivity and lithium-ion diffusivity [16–18] and (iii) surface modification such as carbon coating to increase the electronic conductivity consequently the interfacial reaction kinetics [19–22]. On the other hand, the electrochemical performance of LiFePO4 cathode also depends on a number of factors, e.g., phase purity, particle size and morphology, which are closely related with the synthesis technique and process parameters. Recently, a variety of techniques have been exploited for the synthesis of high-performance LiFePO4 [23–27]. Mechanical activation, which involves the blending of ingredients by high-energy ball milling followed by thermal treatment at high temperature, has turned out to be a versatile technique with high capability for scale up [28–30]. The ball milling can reduce the particle size and create close contact of the reactants. Thereby, a decrease in the thermal treatment time and temperature is usually experienced during the following calcination to obtain a pure phase product. It then effectively avoids undesirable grain growth. Mechanical activation process also allows the in situ formation of carbon layer over LiFePO4 when carbon black or a suitable organic/polymeric
2862
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
compound is incorporated as the carbon source during the synthesis. Besides performing as the coating material for LiFePO4 , the carbon also acts as a reducing agent to suppress the oxidation of Fe2+ to form Fe3+ impurities during the synthesis. The carbon may further act as a diffusion barrier by blocking the particle contact, thereby suppressing the grain growth. Improved performance at high charge/discharge rate has been observed for the LiFePO4 /C composite as compared to the LiFePO4 prepared by the same technique but without adopting carbon additive [31]. Among various carbon raw materials for the coating layer, sucrose was found to be an excellent candidate [5]. Numerous studies have been conducted in literature on the synthesis of high-performance LiFePO4 /C composite by applying sucrose as the carbon source [5,32–34]. However, most of the investigations were focused on the electrochemical performance of the as-synthesized samples. Very limited attention was paid to the synthesis process. Such information is, however, important guidance for further optimizing the material properties. In this study, the synthesis of LiFePO4 /C composite cathode by a mechanical activation process applying sucrose as the carbon source was systematically investigated. Importance was paid to the synthesis process and the influence of process parameters on the electrochemical performance of the product. High electrochemical performance was achieved for the LiFePO4 /C composite synthesized under optimal conditions. 2. Experimental 2.1. Material synthesis LiFePO4 /C was synthesized by a mechanical activation process followed by high-temperature calcination. Li2 CO3 , FeC2 O4 ·2H2 O and NH4 H2 PO4 (all chemicals of >99% purity) were applied as the raw materials and sucrose was selected as the carbon source. They were weighed in stoichiometric amounts according to the nominal LiFePO4 to C weight ratio 94 to 6, assuming the sucrose following the thermal pyrolysis reaction C12 H22 O11 → 12C + 11H2 O. The ingredients together with 60% acetone were magnetically stirred at room temperature for 5 min, followed by high-energy ball milling in a zirconia container with zirconia balls (ball-topowder weight ratio 10 to 3) at room temperature for different periods under an argon atmosphere using a Pulverisette-6 planner mill at 400 rpm. The as-obtained slurry was dried by evaporating the acetone liquid media under vacuum. Finally the solid precursors were fired at temperatures ranging from 500 to 900 ◦ C for various hours in a tube furnace purging with pure nitrogen atmosphere. 2.2. Electrode fabrication The electrochemical cycling performance of LiFePO4 or LiFePO4 /C powders was tested with coin-shaped cells using metallic lithium film as the counter and reference electrode at room temperature (∼30 ◦ C). The cells are based on the configuration of Li metal (−)|electrolyte|LiFePO4 (+) with a liquid electrolyte (1 M solution of LiPF6 in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1, v/v). Microporous polypropylene film (Celgard 2400) was used as the separator [35]. Slurries composed of 85 wt.% LiFePO4 with 10 wt.% conductive Super P (NCM HERSBIT Chemical Co., Ltd., China) and 5 wt.% LA-132 binder (Chengdu Organic Chemicals Co., Ltd., China) in de-ionized water were prepared, which were deposited on current collectors of 6 m thick aluminum foils by blade. The electrode was dried under vacuum at 100 ◦ C for 12 h before electrochemical evaluation. Cell assembly was conducted in a glove box filled with pure argon.
2.3. Characterization The crystal structures of the synthesized powders were examined by X-ray diffraction (XRD) using a Bruker D8 advance diffractometer with filtered Cu K␣ radiation. The experimental diffraction patterns were collected at room temperature by step scanning at the range of 10◦ < 2 < 80◦ . The crystallite size was calculated based on the Scherrer equation from the study of the Bragg angle and half bandwidth of the index peaks. The mean value of crystallite sizes calculated based on [1 0 1], [1 1 1], [2 1 1] and [3 1 1] diffraction planes was adopted as the crystallite size of assynthesized LiFePO4 . The particulate morphology was examined by Environmental Scanning Electronic Microscope (ESEM, QUANTA-2000). The specific surface area of the samples was characterized by N2 adsorption at the temperature of liquid nitrogen using a BELSORP II instrument. Prior to analysis, the samples were put in vacuum at 200 ◦ C for 3–5 h to remove the surface adsorbed species. Thermogravimetry–differential scanning calorimetric analyses (TG–DSC) were performed using a NETZSCH STA 409 PC in the range of 25–1000 ◦ C at a heating rate of 10 ◦ C/min under flowing argon atmosphere. Temperature programmed reaction (TPR) experiments were conducted using a U-type quartz reactor. About 0.02 g sample in 40–60 mesh was dropped into a quartz tube reactor and a constant helium flow at 20 ml/min [STP] was introduced. The temperature of the reactor was programmatically increased. The effluent gas from the reactor was introduced to a mass spectroscope (QIC 20) for in situ compositional analysis. Before the electrochemical performance test, the cells were activated through one charge/discharge cycles with a small current (0.1C, 1C = 170 mAh/g) using a NEWARE BTS-5V50mA computercontrolled battery test system. The charge/discharge characteristics of the cells were recorded over the potential range between 2.0 and 4.2 V at different range of 0.1–50C at 30 ◦ C. Cyclic voltammetry tests were performed over the potential range of 2.0–4.2 V using a Princeton Applied Research PARSTAT 2273 advanced electrochemical system at the scanning rate 0.1–1 mV/s. 3. Results and discussion 3.1. Synthesis The reactants, a physical mixture of stoichiometric amounts of NH4 H2 PO4 , Li2 CO3 , FeC2 O4 ·2H2 O and C12 O11 H22 , demonstrated the color of light yellow, a typical color of FeC2 O4 ·2H2 O. It changed steadily to light green with increasing ball milling time during the mechanical activation. This implies that certain type of interaction between the reactants appeared during the high-energy ball milling. It is well known that Fe(H2 O)6 2+ has a color of light blue. Hereby, the ball milling likely led to the interaction of the hydroxide groups in sucrose with FeC2 O4 ·2H2 O. Fig. 1 shows the XRD patterns of the precursor after the ball milling at room temperature for various hours. Before ball milling, the characteristic diffraction peaks of FeC2 O4 ·2H2 O were detected. However, an amorphous structure at room temperature was observed when the precursor was ball milled for 2 h or longer. This implies that the crystallite phase of the FeC2 O4 ·2H2 O was destroyed by reaction or certain type of interaction between FeC2 O4 ·2H2 O and other reactants existed after the ball milling, in accordance with the color change after the ball milling. During the thermal treatment, the cracking of sucrose and the phase reaction among NH4 H2 PO4 , Li2 CO3 and FeC2 O4 ·2H2 O to form LiFePO4 could happen simultaneously or separately. Different formation mechanism may result in significantly different anode morphology or phase structure, consequently, differed electrochemical behavior. The thermal decomposition behavior of sucrose
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
2863
Fig. 1. XRD patterns of the LiFePO4 /C precursor after the ball milling at room temperature for various hours. Fig. 3. TPR curves of sucrose under a nitrogen atmosphere with a heating rate of 10 ◦ C/min.
Fig. 2. TG–DSC curves of sucrose under a nitrogen atmosphere with a heating rate of 10 ◦ C/min.
was first investigated. As shown in Fig. 2 is the TG–DSC curves of sucrose under a nitrogen atmosphere with a heating rate of 10 ◦ C/min. The weight loss started at ∼215 ◦ C, mainly happened between 220 and 400 ◦ C, and finished around 700 ◦ C. The final weight loss reached 84%. The DSC curve showed a sharp endothermic peak around 194 ◦ C corresponding to a zero weight loss, which was assigned to the cleavage of glucose ring in sucrose. Another
endothermic peak was observed around 230 ◦ C accompanied with a sharp weight loss in the TG profile. There were two small successive endothermic peaks at 296 and 450 ◦ C, respectively. The appearance of several successive endothermic peaks in the DSC profile suggests that the thermal cracking was conducted in several successive steps. For the thermal decomposition of sucrose, a theoretical weight loss 58% should be expected if carbon and water were the sole products. The actual weight loss 84% implies that only 38% carbon in sucrose was successfully converted to solid carbon. On other words, ∼62% carbon was lost in the form of gaseous products, i.e., CO and CO2 . The overall thermal decomposition reaction of sucrose could be expressed as C12 O11 H22 → 4.56C + 7.44(CO/CO2 ) + 11(H2 O/H2 ). If assuming decomposition behavior of sucrose in the solid precursor was similar to pure sucrose, the actual carbon content in the derived LiFePO4 /C composite is only around 2.3 wt.%. To further exploit the thermal decomposition reaction of sucrose, a TPR was conducted. As shown in Fig. 3, there was almost no any gaseous species produced at T < 220 ◦ C, agreed well with the TG–DSC results. A broad water peak and a CO2 peak were observed within the temperature range of 220–550 ◦ C, and 250–500 ◦ C, respectively. A CO peak was also appeared alongside with the CO2 peak. At T > 550 ◦ C, a large and broad H2 peak was appeared, however, no other peak was detected in the same temperature zone. Since carbon is the only final solid residual, the hydrogen was then produced from the thermal decomposition of CHx . Above results
Fig. 4. TG–DSC curves of the LiFePO4 /C precursor under a nitrogen atmosphere with a heating rate of 10 ◦ C/min (a) without the ball milling precursor and (b) the ball-milled precursor.
2864
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Fig. 5. TPR profiles of the solid precursor under a nitrogen atmosphere with a heating rate of 10 ◦ C/min (a) without the ball milling precursor and (b) the ball-milled precursor.
demonstrated that the sucrose first cracked to CHx intermediate during the thermal decomposition, which was further converted to carbon at relatively high temperature of >550 ◦ C. To exploit the role of the high-energy ball milling in the phase formation and its effect on the thermal decomposition behavior of the solid precursor, TG–DSC and TPR of the solid precursors with and without the ball milling process were conducted. As shown in Fig. 4, without the ball milling process, the weight loss started at 162 ◦ C. A sharp weight loss appeared between 162 and 230 ◦ C, which accounted for a weight loss of about 25%. Another obvious weight loss amounting to ∼20% of initial weight appeared between 230 and 420 ◦ C. Between 420 and 800 ◦ C and mainly between 550 and 650 ◦ C there was additional modest weight loss amounting to ∼6%. The ball-milled precursor showed somewhat different decomposition behavior. One significant difference is that the weight loss initiated at room temperature. In combination with the color change after the ball milling, such weight loss was attributed to the evaporation of dehydrated water from FeC2 O4 ·2H2 O, created during the high-energy ball milling process. Another significant difference is that the sharp weight loss between 550 and 650 ◦ C was disappeared. As shown in Fig. 5, for the precursor without the ball milling process, there were almost no gaseous decomposition products at T < 180 ◦ C, however, a big water peak was observed between 60 and 130 ◦ C for the ball-milled sample. Such different behaviors are in well agreement with the TG–DSC results. It further supported that the hydrated water in FeC2 O4 ·2H2 O was released during the high-energy ball milling process. Starting at around 180 ◦ C and happening mainly between 180 and 380 ◦ C, a strong and broad water peak was appeared for the solid precursor without the ball milling process, which was ascribed to the thermal decomposition of sucrose and the dehydration of FeC2 O4 ·2H2 O. Between 220 and 260 ◦ C, CO and CO2 decomposition peaks with similar intensity were appeared simultaneously. In connection with the thermal decomposition behavior of the sucrose, they were ascribed to the thermal decomposition of sucrose via the reaction C12 O11 H22 → CHx + CO + CO2 + H2 O. As compared to pure sucrose, the CO and CO2 decomposition peaks become sharper and moved to a lower temperature zone. It suggests that the formation of solid mixture facilitated the thermal decomposition of sucrose. Ammonia peak, attributed to the thermal decomposition of NH4 H2 PO4 to NH3 and H3 PO4 , was detected between 160 and 500 ◦ C. Between 380 and 430 ◦ C, sharp peaks of CO and CO2 with similar intensity appeared simultaneously, which were attributed to the thermal decomposition of FeC2 O4 via the reaction FeC2 O4 → FeO + CO + CO2 . A big and broad hydrogen peak was also observed in the same tem-
perature zone, which was ascribed to the thermal dehydrogenation of CHx to C and H2 . Between 600 and 700 ◦ C, hydrogen, CO and CO2 peaks with low intensity were appeared simultaneously, which were assigned to the thermal decomposition of Li2 CO3 to Li2 O and CO2 , and the reaction of CO2 with CHx to form CO and H2 , and so on, via the reactions as follows. Li2 CO3 → Li2 O + CO2 Li2 O + FeHPO4 → LiFePO4 + H2 O H2 O + CHx → CO + H2 CO2 + CHx → CO + H2 When the solid precursor was subjected for high-energy ball milling, the reactants were mixed thoroughly and had close contact. The ball milling process also led to the dehydration of FeC2 O4 ·2H2 O with the formation of free water as demonstrated previously. Such free water can be easily evaporated. It then well explained the observation of the water peak between 60 and 150 ◦ C for the ballmilled sample. Starting at 180 ◦ C, additional water desorption peak appeared, which was produced from the thermal decomposition of sucrose. Two small ammonia peaks were observed between 50 and 180 ◦ C, while they were not detected for the solid precursor without the ball milling process during the synthesis. It suggests that the high-energy ball milling also facilitated the thermal decomposition of NH4 H2 PO4 . Between 180 and 400 ◦ C, relatively broad ammonia peak alongside with the CO2 and CO peaks appeared. The CO2 peak was found to have much stronger intensity than CO, different from the decomposition behavior of the precursor without the ball milling process. It was attributed to the thermal decomposition of sucrose alongside with the reaction of NH4 H2 PO4 and Li2 CO3 (Li2 CO3 + NH4 H2 PO4 → LiH2 PO4 + CO2 ), facilitated by the high-energy ball milling. Between 400 and 450 ◦ C, sharp CO and CO2 peaks with similar densities appeared, which were ascribed to the thermal decomposition of FeC2 O4 to FeO, CO and CO2 . At T > 500 ◦ C, a broad hydrogen peak was also appeared without the formation of other decomposition product, in connection with the thermal decomposition behavior of sucrose, it was assigned to the decomposition of CHx . Based on the TG–DSC and TPR results, a calcination temperature of around 700 ◦ C was necessary in order to achieve the complete reaction of the reactants in the solid precursor without the ball milling process, while a similar temperature was required for the ball-milled precursor allowing the CHx completely decomposed to
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Fig. 6. XRD of the as-prepared LiFePO4 /C composites by solid-state reaction at 700 ◦ C for 20 h (a) without the high-energy ball milling process and without sucrose adopted, (b) with sucrose as carbon source during the synthesis but without the high-energy ball milling process and (c) with the high-energy ball milling process and also applying sucrose as carbon source during the synthesis.
carbon. Thereby, 700 ◦ C was selected for the thermal treatment of solid precursors. As shown in Fig. 6 are the XRD of the as-prepared LiFePO4 /C composites by solid-state reaction at 700 ◦ C for 20 h with or without the mechanical activation process, and LiFePO4 synthesized without the high-energy ball milling process and without sucrose adopted during the synthesis. All three samples showed a well-crystalline single-phase LiFePO4 . For each sample, all its diffraction peaks can be indexed well based on orthorhombic structure with the space group pnmb. No carbon diffraction peaks were detected for both LiFePO4 /C composites with or without the ball milling process during the synthesis. This implies the formed carbon coating layer over LiFePO4 surface was in an amorphous phase. 3.2. Electrochemical performance Fig. 7 shows the first charge/discharge profiles at 0.1C rate (current density of 17 mAh/g) for the samples of LiFePO4 electrode prepared by solid-state reaction of NH4 H2 PO4 , Li2 CO3 and FeC2 O4 ·2H2 O at 700 ◦ C for 20 h (sample I), LiFePO4 /C composite prepared from the similar procedure of sample I but applying sucrose
Fig. 7. The corresponding first charge/discharge profiles at 0.1C rate for the samples of LiFePO4 electrode prepared by solid-state reaction.
2865
Fig. 8. The discharge curves of the LiFePO4 /C composite at various discharge/charge rates varied from 0.1 to 50C.
as carbon source during the synthesis (sample II), and LiFePO4 /C composite prepared by the similar procedure of sample II with high-energy ball milling of the solid precursor for 2 h during the synthesis (sample III). The sample I showed the first discharge capacity around 110 mAh/g, similar to the literature result for a pure LiFePO4 prepared by a similar solid-phase reaction process [36]. When sucrose was introduced as a carbon source during the preparation, the as-obtained LiFePO4 /C composite showed a capacity around 140 mAh/g. It is generally believed that the carbon additive had several functions during the synthesis of LiFePO4 . Besides its main function to improve the electronic conductivity by performing as surface coating layer, it also effectively prevents the oxidation of Fe2+ to Fe3+ and acted as a diffusion block to suppress the LiFePO4 grain growth. The measured BET surface area of samples I and II was 13.6 and 23.6 m2 /g, respectively. It suggests that the sucrose indeed increased the surface area of the resulted LiFePO4 . Both the increase in surface area and the improvement in electronic conductivity may account for the improved discharge capacity of LiFePO4 /C composite as compared to conventional LiFePO4 . When both high-energy ball milling (mechanical activation) and carbon additive were introduced during the synthesis of LiFePO4 , the surface area further increased to 28.3 m2 /g. The first discharge capacity increased obviously to 174 mAh/g, which reached the theoretical value for a LiFePO4 cathode (170 mAh/g) Within experimental error. This implies all LiFePO4 in the electrode became electrochemically active. The sample also demonstrated excellent reversible capacity around 97%. Thereby, the high-energy ball milling together with in situ carbon coating is highly effective to increase the electrochemical activity of LiFePO4 cathode. As shown in Fig. 8 are the discharge curves of the LiFePO4 /C composite at various discharge/charge rates varied from 0.1 to 50C. For accurate comparison of the rate performance during discharging, the cell was recharged at 0.1C rate to assure identical initial conditions for all the discharges [37,38]. The discharge capacities decreased from 171 mAh/g at 0.1C to 117 mAh/g at 20C. Even at 50C, a capacity of 81 mAh/g was still maintained, which is even higher than 76 mAh/g of the LiFePO4 /C composite without the ball milling process during the synthesis at 5C rate. These results are among the best for LiFePO4 /C composite electrode reported ever in literature. By a similar synthesis process Kim et al. reported a first discharge capacity of around 164 mAh/g at 0.1C rate and ∼120 mAh/g at 1C rate [33]. The cycling characteristics of the as-prepared LiFePO4 /C cathode material at different discharge rate are shown in Fig. 9. Favor-
2866
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Under the condition of semi-infinite and infinite diffusion process, the peak current Ip and the square root of scanning rate () has the relationship of Eq. (1) [39]. 1/2
Ip = 2.69 × 105 S · Co · Do
· n2/3 · 1/2
(1) (cm2 ),
Co is where S is the total surface area of the anode material the concentration of lithium ions in the cathode (mol/ml), Do is the average diffusion coefficient of lithium ion (cm2 /s), n is the number of electrons transferred per molecule during the intercalation, it is 1 for LiFePO4 , and is the scanning rate (s−1 ).The surface area of the anode material can be calculated by S = Sm × no
(2) (cm2 /mol)
and no is the where Sm is the mole surface area of anode amount of the active material in the electrode (mol). If assuming the electrode material is in sphere shape, the mole surface area can be calculated by Fig. 9. The cycling characteristics of the as-prepared LiFePO4 /C cathode material at different discharge rates.
able cycle performance was demonstrated. At all discharge rates, the capacity fade after 50 cycles was less than 10%. The good long-term cycling behavior of the LiFePO4 /C composite at various discharge rates confirms the excellent reversibility of lithium insertion/extraction reactions. Cyclic voltammograms of the LiFePO4 /C electrode at the scan rates of 0.1, 0.2, 0.5 and 1 mV/s between 4.2 and 2.0 V are shown in Fig. 10a. The oxidizing peak and the reduction peak are highly symmetric to each other. The ratios of Ipa /Ipc are close to 1, implying that the good reversibility of lithium intercalation into and de-intercalation from the carbon-coated LiFePO4. There is a linear relationship between Ipa and square root of the scan rate, as shown in Fig. 10b. The slope k was found to be 1.446 × 10−2 A s0.5 /V0.5 , where A is the current in Ampere, s is the time in second and V is the voltage. CV is a quasi-static electrode characterization technique. In an electrochemical process of LiFePO4 /C electrode, the anodic process corresponds to the lithium-ion de-intercalation from LiFePO4 bulk with the simultaneous diffusion of electron from the electrode to the current collector, while the cathodic process corresponds to the Li+ intercalation into FePO4 and acquiring electron from the current collector. Diffusion coefficient of lithium ions (Do ) can be roughly estimated by cyclic voltammetry since the intercalation and de-intercalation is also a redox process.
Sm =
3Vm r
(3)
where Vm is the mole volume of the electrode (ml/mol) and r is the radius of the particle (cm). The electrical charge Q transferred during charge/discharge process can be related with Co by Q = nFVm × Co
(4)
is in linear relationship with n1/2
with the slope of k, the Since Ip lithium diffusion coefficient Do can be expressed by the following equation [40]: Do =
r 2 k2 69.9n3 Q 2
(5)
The electrical charge that transferred through the active material during the charge/discharge process could be took as its capacity, which can also be calculated from the weight amount of the active material and the theoretical specific capacity. It can also be calculated from the discharge/charge curves or be obtained by integrating the CV curves with respect to time. Here, Do was calculated based on theoretical capacity, i.e., Q = 0.95C. As shown in Fig. 11a is the SEM of the as-synthesized LiFePO4 powder. It shows that LiFePO4 had quasi-sphere shape with an average particle size of 1.5 m, i.e. r = 7.5 × 10−5 cm. Together with the k value of 1.446 × 10−2 A s0.5 /V0.5 , the lithium diffusion coefficient Do of ∼8.93 × 10−13 cm2 /s was calculated according to Eq. (5). The reported value for a pure LiFePO4 is ∼1.8 × 10−14 cm2 /s [41]. It
Fig. 10. (a) Cyclic voltammograms of the LiFePO4 /C electrode at the scan rates of 0.1, 0.2, 0.5 and 1 mV/s between 4.2 and 2.0 V and (b) a linear relationship between Ipa and square root of the scan rate.
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
2867
Fig. 11. SEM photos of the as-synthesized LiFePO4 powder (a) 700 ◦ C, (b) 600 ◦ C and (c) 800 ◦ C.
then implies that the lithium-ion diffusion coefficient was greatly improved by applying sucrose as a carbon source in combination with high-energy ball milling, which may be the cause for the excellent high-rate performance of the LiFePO4 /C composite prepared in this study. 3.3. Process optimization To exploit the effect of process parameter on the electrochemical performance of the derived LiFePO4 , various ball milling times, thermal treatment temperatures and times were investigated in the synthesis of LiFePO4 /C. As shown in Table 1, the ball milling of 2–4 h resulted in the maximum surface area. The further increase in milling time, however, led to a steady decrease in surface area and increase in grain size. It is well known that the high-energy ball milling could result in a close contact of the reactants, which facilitated the solid-phase reaction, therefore reduced the sintering temperature and time for obtaining a pure phase LiFePO4 . However, it can also break down the reactant particles and increase the surface activity. It means the particle sintering could be accelerated during the successive thermal treatment when the reactants were over ball milled. The thermal treatment temperature was found to have most significant impact on the surface area, the higher the calcination temperature the lower the surface area. As shown in Fig. 11a–c, the increase of calcination temperature resulted in obvious size growth of the particles. The calcination time also had an
impact on the surface area, the longer the cacination the lower the surface area, as shown in Table 1. The discharge capacities at various rates for the LiFePO4 /C composites prepared at various conditions were then measured with the results also listed in Table 1. The discharge capacity and rate performance of the as-prepared LiFePO4 /C composite was found highly dependent on the process parameters. The best performance was achieved at a ball milling time of 2–4 h followed by heat treatment at 700 ◦ C for 20 h. X-ray diffraction pattern demonstrated a pure phase LiFePO4 was already formed for the 600 ◦ C calcinated sample. SEM examination demonstrated it had a mean particle size of 0.5 m, smaller than the 700 ◦ C calcinated one of 1.5 m (Fig. 11b). A smaller particle size means a shorter lithium diffusion distance, which is beneficial for high-rate discharge and charge processes. However, the capacities at both low and high discharge rates were reduced as compared to the 700 ◦ C calcinated one. The poor electrochemical performance of the 600 ◦ C calcinated sample was then contributed from the coating layer, which was likely in the state of CHx based on the TGA and TPR experiments. Since CHx has poor electronic conductivity it would have a serious block effect on the electronic transfer. The poor electrochemical performance of the 800 ◦ C calcinated sample was attributed to its large grain size (around 2.2 m), as shown in Fig. 11c, due to the sintering. Relatively poor electrochemical performance was also observed for the LiFePO4 /C composite calcinated at 700 ◦ C for 15 h. It was likely that some of carbon in the coating layer was still in the form of CHx due
2868
K. Wang et al. / Electrochimica Acta 54 (2009) 2861–2868
Table 1 Effect of various process parameters on the surface area and discharge capacity and rate performance of the as-synthesized LiFePO4 /C composite. Milling time (h)
Calcination temp. (◦ C)
Calcination time (h)
BET area (m2 /g)
2 4 6 10 6 2 2
700 700 700 700 800 600 700
20 20 20 20 20 20 15
28.3 30.1 24.3 22.5 20.6 31.8 31.1
to the insufficient calcination time. As shown in Table 1, the ball milling time also had a significant impact on the discharge capacity and rate performance of the derived LiFePO4 /C composite. The optimal ball milling time was 2–4 h. The further increase of ball milling time resulted in obvious decrease of capacity at both low and high discharge rates. It was believed that the over ball milling accelerated the LiFePO4 sintering during the following high-temperature treatment. To summary, ball milling time, heat treatment temperature and time should be specifically optimized in order to achieve a high-performance LiFePO4 /C composite synthesized by mechanical activation adopting sucrose as carbon source. 4. Conclusions Olivine-type LiFePO4 /C composite was successfully synthesized by mechanical activation followed by thermal treatment. Ball milling time, thermal treatment temperature and time all have significant impact on the electrochemical performance of the derived LiFePO4 /C composite. The high-energy ball milling facilitated the phase reaction between reactants and the thermal decomposition of sucrose. The sucrose was first cracked to CHx intermediate, which was then further converted to solid carbon at 700 ◦ C or higher for a sufficient time. Too low calcination temperature for example 600 ◦ C, and too short calcination time for example 15 h, resulted in the residual of undecomposed CHx , which seriously blocked the electron transfer and consequently decreased the electrochemical performance of LiFePO4 /C composite. Over ball milling or over heating, however, resulted in grain growth of LiFePO4 electrode during the following thermal treatment, a decrease in electrochemical performance was then experienced. Thereby, the process parameter should be optimized in order to obtain a high electrochemical performance. At a ball milling time of 2–4 h, thermal treatment at 700 ◦ C for 20 h, the derived LiFePO4 /C composite showed a discharge capacity of 174 mAh/g at 0.1C rate, it reached the theoretical value for LiFePO4 within experimental error. It also showed a highrate performance with a discharge capacity of 117 mAh/g at 20C rate. The performance of the derived LiFePO4 /C composite, prepared by mechanical activation was very sensitive to the process parameters. Suppressing the grain growth and avoiding CHx are the keys to achieve high-rate performance LiFePO4 /C composite cathode. Acknowledgements This work was supported by National Basic Research Program of China under contract no. 2007CB209704. Dr. Zongping Shao also acknowledges the financial support from Chinese Ministry of Education via the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT0732).
Discharge rate 0.1C
0.2C
1C
2C
5C
174.6 173.8 156.7 143.3 120.6 134.6 142.1
164.5 165.3 147.3 128.1 112.4 123.5 131.6
153.8 154.2 132.1 114.3 84.3 111.7 122.8
142.7 143.1 120.4 103.8 – 99.8 109.4
134.6 132.5 111.2 92.3 – – –
References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] A. Yamada, S.C. Chung, K. Hinokuna, J. Electrochem. Soc. 148 (2001) A224. [3] H. Huang, S.C. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) A170. [4] G. Arnald, J. Garche, R. Hemmer, S. Strobele, C. Vogler, M. Wohlfahrt-Mehrens, J. Power Sources 119–121 (2003) 247. [5] S. Franger, F. Le Cras, C. Bourbon, H. Rouault, Electrochem. Solid-State Lett. 5 (2002) A231. [6] K. Zaghib, P. Charest, A. Guerfi, J. Shim, M. Perrier, K. Striebel, J. Power Sources 134 (2004) 124. [7] K. Striebel, A. Guerfi, J. Shim, M. Armand, M. Gauthier, K. Zaghib, J. Power Sources 119–121 (2003) 951. [8] J. Barker, M.Y. Saidi, J.L. Swoyer, Electrochem. Solid-State Lett. 6 (2003) A53. [9] J.L. Allen, T.R. Jow, J. Wolfenstine, J. Solid-State Electrochem. 12 (2008) 1031. [10] L. Laffont, C. Delacourt, P. Gibot, M.Y. Wu, P. Kooyman, C. Masquelier, J.M. Tarascon, Chem. Mater. 18 (2006) 5520. [11] C.S. Wang, U.S. Kasavajjula, P.E. Arce, J. Phys. Chem. C 111 (2007) 16656. [12] J.L. Allen, T.R. Jow, J. Wolfnstine, Chem. Mater. 19 (2007) 2108. [13] L. Hui, Z.L. Tang, S.H. Luo, Z.T. Zhang, Prog. Chem. 19 (2007) 1460. [14] Y. Wang, G.Z. Cao, Adv. Mater. 12 (2008) 2251. [15] K.F. Hsu, S.Y. Tsay, B.J. Hwang, J. Mater. Chem. 14 (2004) 2690. [16] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Nat. Mater. 1 (2002) 123. [17] T. Nakamura, Y. Miwa, M. Tabuchi, Y. Yamada, J. Electrochem. Soc. 154 (2007) A1108. [18] N. Jayaprakash, N. Kalaiselvi, Electrochem. Commun. 9 (2007) 620. [19] C.H. Mi, Y. Cao, X.G. Zhang, X.B. Zhao, H.L. Li, Powder Technol. 181 (2008) 301. [20] J.K. Kim, G. Cheruvally, J.H. Ahn, H.J. Ahn, J. Phys. Chem. Solid 69 (2008) 1257. [21] R. Dominko, M. Gaberscek, A. Bele, D. Mihailovic, J. Jamnik, J. Eur. Ceram. Soc. 27 (2007) 909. [22] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee, K.T. Kang, H.G. Kim, Solid State Commun. 129 (2004) 311. [23] W. Ojczyk, J. Marzec, K. Swierczek, W. Zajac, M. Mdenda, R. Dziembaj, J. Molenda, J. Power Sources 173 (2007) 700. [24] G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo, N. Penazzi, J. Power Sources 160 (2006) 516. [25] C.W. Kim, M.H. Lee, W.T. Jeong, K.S. Lee, J. Power Sources 146 (2005) 534. [26] S.T. Myung, S. Komaba, N. Hirosaki, H. Yashiro, N. Kumagai, Electrochim. Acta 49 (2004) 4213. [27] D. Jugovic, M. Mitric, N. Cvjeticanin, B. Jancar, S. Mentus, D. Vskokovic, Solid State Ionics 179 (2008) 415. [28] X.Z. Liao, Z.F. Ma, L. Wang, X.M. Zhang, Y. Jiang, Y.S. He, Electrochem. Solid-State Lett. 7 (2004) A522. [29] J.K. Kim, J.W. Choi, G. Cheruvally, J.U. Kim, J.H. Ahn, G.B. Cho, K.W. Kim, H.J. Ahn, Mater. Lett. 61 (2007) 3822. [30] H.C. Kang, D.K. Jun, B. Jin, E.M. Jin, K.H. Park, H.B. Gu, K.W. Kim, J. Power Sources 179 (2008) 340. [31] H.C. Shin, W.I. Cho, H. Jang, J. Power Sources 132 (2004) 235. [32] S.L. Bewlay, K. Konstantinov, G.X. Wang, S.X. Dou, H.K. Liu, Mater. Lett. 58 (2004) 1788. [33] J.K. Kim, G. Cheruvally, J.H. Ahn, J. Solid-State Electrochem. 12 (2008) 799. [34] A.D. Spong, G. Vitins, J.R. Owen, J. Electrochem. Soc. 152 (2005) A2376. [35] G.X. Wang, J.J. Xu, M. Wen, R. Cai, R. Ran, Z.P. Shao, Solid State Ionics 179 (2008) 946. [36] H. Huang, S. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) 170. [37] H.C. Shin, S.B. Park, H. Jang, K.Y. Chung, W.I. Cho, C.S. Kim, B.W. Cho, Electrochim. Acta 53 (2008) 7946. [38] S.W. Oh, H.J. Bang, S.T. Myung, Y.C. Bae, S.M. Lee, Y.K. Sun, J. Electrochem. Soc. 155 (2008) A414. [39] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applications, 2nd edition, 2003. [40] S.Q. Liu, B.L. Gong, K.L. Huang, G. Zhang, S.C. Li, Chin. J. Power Sources 5 (2007) 382. [41] P.P. Prosini, M. Lisi, D. Zane, M. Pasquali, Solid State Ionics 1 (2002) 45.