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C cathode material synthesized via lauric acid-assisted solid-state reaction
Electrochimica Acta 56 (2011) 2999–3005
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High power performance of nano-LiFePO4 /C cathode material synthesized via lauric acid-assisted solid-state reaction Fuquan Cheng a , Wang Wan a , Zhuo Tan a,b , Youyuan Huang a , Henghui Zhou a,∗ , Jitao Chen a , Xinxiang Zhang a a b
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, PR China
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Article history: Received 15 December 2010 Received in revised form 2 January 2011 Accepted 3 January 2011 Available online 15 January 2011 Keywords: Lithium-ion batteries LiFePO4 /C composite Lauric acid Solid-state reaction
a b s t r a c t A nano-LiFePO4 /C composite has been directly synthesized from micrometer-sized Li2 CO3 , NH4 H2 PO4 , and FeC2 O4 ·2H2 O by the lauric acid-assisted solid-state reaction method. The SEM and TEM observations demonstrate that the synthesized nano-LiFePO4 /C composite has well-dispersed particles with a size of about 100–200 nm and an in situ carbon layer with thickness of about 2 nm. The prepared nanoLiFePO4 /C composite has superior rate capability, delivering a discharge capacity of 141.2 mAh g−1 at 5 ◦ C, 130.9 mAh g−1 at 10 C, 121.7 mAh g−1 at 20 ◦ C, and 112.4 mAh g−1 at 30 ◦ C. At −20 ◦ C, this cathode material still exhibits good rate capability with a discharge capacity of 91.9 mAh g−1 at 1 ◦ C. The nanoLiFePO4 /C composite also shows excellent cycling ability with good capacity retention, up to 100 cycles at a high current density of 30 ◦ C. Furthermore, the effect of lauric acid in the preparation of nanoLiFePO4 /C composite was investigated by comparing it with that of citric acid. The SEM images reveal that the morphology of the LiFePO4 /C composite transformed from the porous structure to fine particles as the molar ratio of lauric acid/citric acid increased. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Increasing attention has been focused on lithium-ion batteries for application in electric vehicles (EV) and hybrid-electric vehicles (HEV) due to the energy crisis and environmental pollution. The specific requirements for these batteries in EV and HEV are high safety, low cost, long cycle life, and environmental compatibility. However, conventional cathode material LiCoO2 is limited to largescale applications due to cost and safety issues. Thus, the demand for the development of new cathode material that can meet these requirements has continuously increased. Since Padhi et al. [1] first reported lithium iron phosphate (LiFePO4 ) with olivine structure as a new cathode material for rechargeable lithium-ion batteries, LiFePO4 has become one of the most promising cathode materials for application in EV and HEV because of its high theoretical capacity (170 mAh g−1 ), safety, environmental friendliness, and low raw materials cost. However, the main obstacles for LiFePO4 are its low lithium ion diffusion coefficient and very low electronic conductivity. Over the past few years, a great deal of effort has been made to solve these problems by decreasing LiFePO4 particles to nanometer size [2–5], doping of supervalent ions [6–8], coat-
∗ Corresponding author. Tel.: +86 10 62757908; fax: +86 10 62757908. E-mail address: [email protected] (H. Zhou). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.007
ing with electronic conductive agents [9–11], or creating a fast ion-conducting surface phase [12]. For the application of LiFePO4 cathode in EV and HEV, high power performance is vital. A successful strategy in preparing high-power performance of LiFePO4 is to synthesize nano-LiFePO4 /C composite, wherein the nanometersized LiFePO4 particles are uniformly coated by a carbon layer with thickness of about 1–2 nm. Conventional synthetic methods to obtain the nano-LiFePO4 /C composite are low-temperature routes, such as hydrothermal [3], template-mediated approach [4], co-precipitation [13,14], sol–gel [15,16], and ionothermal method [17]. Although these routes are successful in preparing nanoLiFePO4 /C composite, the application of LiFePO4 in EV and HEV is still suffering from scaling up these approaches due to their high cost and time-consuming. In contrast, solid-state reaction in preparing LiFePO4 /C composite with in situ carbon layer is simple and repeatable. The in situ carbon layer has three functions: providing a highway for electron movement, inhibiting particle growth, and preventing formation of Fe2 O3 and Li3 Fe2 (PO4 )3 impurity phases. Many organic molecules, such as sugar [18] and citric acid [19], have been widely used to generate this in situ carbon layer. However, the in situ carbon layer produced from these organic molecules does not uniformly coat on LiFePO4 and cannot effectively suppress the growth of LiFePO4 crystallites, resulting in micrometer-sized LiFePO4 particles. To enhance the electrochemical performance of LiFePO4 /C composite obtained by solid-state method, additional treatments,
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such as ball milling or the addition/treatment of carbon are generally required. Further milling process is sensitive to the ball-milling time, ball-to-powder ratio, rotation speed, and size of balls [20,21]. Therefore, fabricating high-power performance nanometer-sized LiFePO4 /C composite by solid-state reaction without further treatment remains a significant challenge. In this study, we report the synthesis of high power performance nano-LiFePO4 /C composite by a one-step, lauric acid-assisted solidstate reaction without any additional treatment to improve the battery performance. Lauric acid, a saturated fatty acid, cheap, non-toxic, and safe to handle, has a dispersing effect and can form molten hydrocarbon, which implies it can favorably interact with raw materials during the sintering process. The structure, morphology, and electrochemical performance of the prepared nano-LiFePO4 /C were investigated. In addition, the effect of lauric acid on the preparation was studied by comparing it with that of citric acid.
and 4.0 V versus Li+ /Li at constant current–constant voltage mode on a battery cycler (LAND, CT2001A, China). Cyclic voltammetry measurements were performed with a three-electrode cell. The working electrode was prepared by the same procedure as that for the charge/discharge test, and lithium metal foil served as counter and reference electrode. Cyclic voltammetry was run on an electrochemical workstation (Shanghai Chenhua, CHI660D, China) in a glove box at a scan rate of 0.1 mV s−1 ranging from 2.0 to 4.0 V versus Li+ /Li. Electrochemical impedance spectroscopy (EIS) was performed with an Autolab PG302N electrochemical workstation (Netherlands). The sinusoidal excitation voltage applied to the coin cells was 5 mV, with frequency range from 100 kHz to 0.1 Hz.
2. Experimental
The thermogravimetry curves of all starting materials (except Li2 CO3 ) used to prepare LiFePO4 are shown in Fig. 1. As the temperature increased to 43.2 ◦ C (the melting point of lauric acid), the lauric acid started to melt. Above 43.2 ◦ C, the raw materials Li2 CO3 ,
The nano-LiFePO4 /C composite was directly synthesized from micrometer-sized precursor by a one-step lauric acidassisted solid-state reaction. Stoichiometric amounts of Li2 CO3 (0.0309 mol), FeC2 O4 ·2H2 O (0.06 mol), NH4 H2 PO4 (0.06 mol), and lauric acid (0.03 mol) were weighed (mol ratio of Li:Fe:P:lauric acid = 1.03:1:1:0.5) and placed in an agate bowl (250 mL). The mixture were milled in 20 mL acetone using a planetary ball mill with ball-to-powder ratio of 4:1 (ball-diameter 10 mm), rotating speed of about 200 rpm, and ball-milling time of 6 h. During the ball-milling process, the slurry concentration was about 50% (by weight). After the ball milling, the precursor mixture was heat treated at 350 ◦ C for 10 h and then 700 ◦ C for 15 h. All these heat treatments were conducted under a nitrogen atmosphere to avoid the oxidation of Fe2+ . The effect of lauric acid in the preparation of nano-LiFePO4 /C composite was investigated by comparing it with that of citric acid. Citric acid or its mixture with lauric acid was added to the precursors. The amount of citric acid remained constant and the content of lauric acid varied according to the molar ratio of lauric acid/citric acid, namely 0:1, 1:1, 2:1, and 5:1 (labeled as LFP-CA, LFP-11, LFP-21, and LFP-51, respectively). These precursors were treated with the same ball milling and heat treatment conditions above-mentioned to prepare LiFePO4 /C composite. Thermogravimetric analysis was carried out on a simultaneous thermal analysis apparatus (Q600, TA Instruments) from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 in nitrogen atmosphere. The crystallographic structure was identified with Xray diffraction (XRD, Rigaku, DMAX-2400) with Cu K␣ radiation. The XRD pattern was collected in the 2 range of 10–80◦ at a continuous scan mode with step size of 0.02◦ and scan rate of 2◦ min−1 . The morphology of these materials was observed with a fieldemission scanning electron microscope (SEM, FEI Nano430) and a high-resolution transmission electron microscope (HRTEM, FEI T20). The carbon content in LiFePO4 /C composite was determined by an elemental analyzer (Elementar Vario Micro Cube, Germany). Charge/discharge test was performed in standard 2032-type coin cells. The electrode was prepared by dispersing 80 wt.% LiFePO4 /C active material, 10 wt.% Super P carbon black, and 10 wt.% polyvinylidene fluoride binder in N-methylpyrrolidone to form a homogeneous slurry. The obtained slurry was then cast on the Al current collector and dried for 12 h in a vacuum oven at 120 ◦ C. The electrode was pressed under 6–10 MPa and punched into 12 mm diameter circular disks. The loading of the active materials was about 2.5 mg cm−2 . Standard 2032 coin cells were assembled in an Ar-filled glove box, using lithium metal foil as negative electrode and 1 M LiPF6 in EC/DMC/EMC (1:1:1 by weight) as electrolyte. Cells were charged and discharged between 2.0
3. Results and discussion 3.1. TG analysis
Fig. 1. The thermogravimetry curves of all starting materials (except Li2 CO3 ) recorded at a heating rate of 10 ◦ C min−1 .
Fig. 2. XRD pattern of the synthesized nano-LiFePO4 /C composite.
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FeC2 O4 ·2H2 O, and NH4 H2 PO4 were dispersed in the molten lauric acid. At temperatures between 150 and 250 ◦ C, excess lauric acid was removed, leaving only a lauric acid capping layer on the precursor surface [22]. The FeC2 O4 ·2H2 O lost lattice water under 220 ◦ C, and then FeC2 O4 then decomposed to FeO from 220 to 420 ◦ C, which reacted with Li2 CO3 and the decomposition product of NH4 H2 PO4 to form LiFePO4 [23]. Above 250 ◦ C, the lauric acid capping layer started to decompose and then formed the in situ car-
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bon coating layer, which could effectively inhibit particle growth in high-temperature heat treatment. 3.2. XRD and morphology analysis The XRD patterns in Fig. 2 demonstrate that the nano-LiFePO4 /C powder was well crystallized as an olivine structure without any detectable impurity phase. The composite can be indexed
Fig. 3. (a) SEM image of the solid reactants mixture of Li2 CO3 , FeC2 O4 ·2H2 O, NH4 H2 PO4 and citric acid created by planetary ball milling; (b) SEM image of synthesized nano-LiFePO4 /C composite; (c) TEM image of synthesized nano-LiFePO4 /C composite; (d) typical TEM image for individual LiFePO4 /C crystallites; and (e) typical HRTEM image for the carbon-coated LiFePO4 crystallites. Inset of (c) shows the selected-area electron diffraction.
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based on the orthorhombic Pnma space group. The lattice con˚ b = 6.004 A, ˚ and c = 4.690 A˚ were calculated stants of a = 10.322 A, using PowderX software. The results are very close to the stan˚ b = 6.010 A, ˚ c = 4.693 A) ˚ given by JCPDS dard data (a = 10.334 A, 83-2092. In addition, no diffraction pattern corresponds to impurity phases, such as Li3 PO4 , Li3 Fe2 (PO4 )3 , Fe2 O3 , and Fe2 P, which are frequently found in the LiFePO4 /C composite obtained through high-temperature treatment. The raw materials Li2 CO3 , FeC2 O4 ·2H2 O, and NH4 H2 PO4 used in the experiment are all micrometer-sized (SEM images are not shown here). Fig. 3(a) shows an SEM image of the solid reactant mixture of Li2 CO3 , FeC2 O4 ·2H2 O, NH4 H2 PO4 , and lauric acid created by planetary ball milling. After ball-milling treatment, these raw materials were mixed well with each other and the average particle size decreased to about 5 m. After high-temperature sintering, however, the micrometer precursor transformed to nanometersized LiFePO4 particles. Fig. 3(b) shows an SEM image of the nano-LiFePO4 /C composite prepared at 700 ◦ C with average particle size of about 100–200 nm. The TEM image shown in Fig. 3(c) illustrates that these nano-LiFePO4 were well-dispersed. These well-dispersed LiFePO4 /C particles were bound together by the in situ carbon layer to form a loose agglomerated structure, which was beneficial to the movement of electrons and permeation of the electrolyte. The selected-area electron diffraction pattern sug-
gests that the prepared LiFePO4 was highly crystalline (inset in Fig. 3(c)). The SEM and TEM images in Fig. 3(a)–(c) clearly indicate that the well-crystallized nano-LiFePO4 /C composite was directly prepared from micrometer-sized precursor by lauric acid assistedsolid state method. During the heat-treatment process, Li2 CO3 , FeC2 O4 ·2H2 O, and NH4 H2 PO4 reacted with each other and then formed nanometer-sized LiFePO4 crystallites, which have high surface energy and a tendency to agglomerate. Due to the formation of the lauric acid capping layer on the surface of precursors, the in situ carbon layer formed by decomposition of lauric acid uniformly deposited on the surface of the nano-LiFePO4 crystallites, and thus prevented the further growth of LiFePO4 particles during high-temperature sintering. High-resolution TEM analysis is a useful tool in analyzing the nature of the carbon layer in the interstitial particle/boundary region. Fig. 3(d) shows the magnified image of a single particle in Fig. 3(c). This particle had a regular shape and smooth surface. The HRTEM image in Fig. 3(e) clearly presents that the carbon layer was uniformly coated on the surface of LiFePO4 particles. The thickness of carbon layer was about 2 nm, which was favorable for lithium ion transport across the interface between LiFePO4 and electrolyte [5]. A distinct lattice fringe with an interplanar distance of 0.427 nm was observed, corresponding to the spacing of the (1 0 1) planes of the LiFePO4 particle. The carbon content of LiFePO4 /C composite
Fig. 4. (a) Rate capability at different current densities from 0.2 C to 30 C; (b) cycling life at different current densities from 1 C to 30 C (charging and discharging were performed with the same current density); (c) typical charge/discharge curves at 0.2 C under different temperatures from −20 ◦ C to 25 ◦ C; and (d) charge/discharge curves at various charge/discharge rates at −20 ◦ C; inset of Fig. 3(a) shows the corresponding charge/discharge curve.
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synthesized with and without lauric acid added to the precursor was 1.48 and 0.43 wt.%, respectively. Although the carbon content produced from lauric acid was very low (about 1.1 wt.%), it played a critical role in suppressing LiFePO4 particle growth. The prepared nano-LiFePO4 /C composite does not need any additional treatments to enhance its electrochemical performance, which greatly simplifies the synthetic process of preparing high-power nanoLiFePO4 /C composite. 3.3. Electrochemical characterization Fig. 4 shows the electrochemical performance of the nanoLiFePO4 /C cathode material. The electrochemical properties were investigated using CR2032 coin cells with lithium metal foil as the negative electrode. Fig. 4(a) shows the charge and discharge specific capacities observed in continuous cycling at rates varying from 0.2 ◦ C to 30 ◦ C between 2.0 and 4.0 V at 25 ◦ C, where 1 C corresponds to 170 mAh g−1 . At rate of 0.2 ◦ C, the nano-LiFePO4 /C composite delivered a high discharge capacity of 159.6 mAh g−1 , corresponding to 94.1% of the theoretical capacity. The nano-LiFePO4 /C composite had a high discharge capacity of 141.2 mAh g−1 and 130.9 mAh g−1 at rates of 5 ◦ C and 10 ◦ C, respectively. The inset in Fig. 4(a) shows the charge–discharge curves of the nano-LiFePO4 /C composite at different current densities. Although polarization increased and specific capacity decreased with increase in C rate, this LiFePO4 cathode material presented a discharge capacity of 121.7 mAh g−1 at 20 ◦ C with flat voltage plateaus. Even at high current density of 30 ◦ C, this cathode material could still deliver a specific capacity of 112.4 mAh g−1 , which was much higher than that of the LiFePO4 cathode material prepared by solid-state reaction in the literature [24,25]. These results indicate that the synthesized nano-LiFePO4 /C composite had superior power performance. The prepared composite also exhibited high cycling performance (see Fig. 4(b)). At 1 C rate, the discharge capacity of this material remained at 150.3 mAh g−1 at the 100th cycle, compared to the initial discharge capacity of 153.2 mAh g−1 at the 1st cycle. The discharge capacity loss ratio was 0.02% per cycle. At the high current density of 30 C, this composite still showed good cycling ability, with a discharge capacity loss of 6% at the 100th cycle. The low-temperature performance of nano-LiFePO4 /C composite was tested in low temperature cabinet, with temperatures at 25, 10, 0, −10, and −20 ◦ C. The charge and discharge processes were performed under the same temperature. Fig. 4(c) presents the charge–discharge curves of the prepared material at different low temperatures and at a rate of 0.2 C. The prepared material delivered a discharge capacity of 159.6, 150.5, 143.0, 131.6, and 113.7 mAh g−1 at 25, 10, 0, −10, and −20 ◦ C, respectively. The charge retention at −20 and 25 ◦ C was 75.5%, indicating that most of the capacity in the composite can be released in the low-temperature environment. At −20 ◦ C, this nano-LiFePO4 /C composite still exhibited excellent rate capability (Fig. 4(d)). The specific capacity at 0.5 ◦ C and 1 ◦ C rate were 100.8 mAh g−1 and 91.9 mAh g−1 , respectively. Thus, the nano-LiFePO4 /C composite synthesized by lauric acid assisted-solid state method has great prospect in application in HEV and EV in cold regions. To evaluate the electrode impedance upon cycling, Nyquist plots of coin cells at selected cycles with charge/discharge current density of 1 ◦ C were collected (Fig. 5(a)). EIS measurements were carried out under fully discharged state at 25 ◦ C. All the impedance spectra curves were composed of a depressed semicircle in high frequency region and a straight line in low frequency region. An intercept at the Zre axis at high frequency corresponds to the ohmic resistance of the electrolyte (Rs ). The depressed semicircle in the middle frequency range indicates the charge transfer resistance (Rct ). The inclined line in the low frequency range represents the Warburg impedance (Zw ), associated with lithium-ion diffusion in
Fig. 5. (a) Electrochemical impedance spectra of nano-LiFePO4 /C cathodes at selected cycle and (b) CV curves of synthesized nano-LiFePO4 /C cathodes at a scan rate of 0.1 mV s−1 .
the LiFePO4 bulk material. The charge transfer resistance at the 10th cycle was 20.5 cm2 . After 100 cycles at 1 ◦ C rate, Rct increased to 24.4 cm2 . This slight increase of Rct upon cycling indicates that contact between LiFePO4 and carbon layer is firm and the interface between the LiFePO4 /C particles and electrolyte is very stable. Fig. 5(b) shows the cyclic voltammogram of nano-LiFePO4 /C composite at a scan rate of 0.1 mV s−1 . The oxidation and reduction peaks are highly symmetric to each other. The ratios of Ipa /Ipc are close to 1, indicating good reversibility of lithium ion intercalation into and de-intercalation from the nano-LiFePO4 /carbon composite. The sharp oxidation and reduction peaks indicate excellent kinetics behavior in the LiFePO4 cathode. The improvement in the kinetics of lithium ion intercalation and de-intercalation could be attributed to the nanometer-sized particles that shorten the diffusion length of lithium ions in LiFePO4 [26]. The peak current and peak profile remain stable even after six cycles, which implies excellent cycling stability of the nano-LiFePO4 /C composite. Besides the peaks associated with phase transition between LiFePO4 and FePO4 , no peak appeared at 2.63 V (characteristic of Fe3+ in iron oxide), indicating that all the iron atoms in the nano-LiFePO4 /C composite were Fe2+ [27].
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3.4. Comparison between lauric acid and citric acid The above results reveal that lauric acid is a successful carbon additive in preparing nano-LiFePO4 /C composite and enhancing the electrochemical performance of the LiFePO4 electrode. To study the effect of lauric acid in hindering LiFePO4 particle growth, another organic molecule, citric acid, which usually acts as a chelating reagent in sol–gel [15] method and solid-state reaction [19], was introduced. According to the literature [19], 6 wt.% of citric acid was added to the precursor. The amount of citric acid remained constant, whereas the content of lauric acid varied according to the molar ratio of lauric acid to citric acid, which increased from 1:1 to 5:1. Fig. 6 shows the SEM image of LiFePO4 /C composite with lauric acid and/or citric acid as carbon additives. A porous structure with particles size about 10–20 m was observed in LFP-CA where only citric acid was added into precursors (Fig. 6(a)). The porous
structure was formed due to the vigorous gas evolution during decomposition of citric acid in sintering process [15]. There are still many surface apertures in the LFP-11 as the molar ratio increased to 1:1 (Fig. 6(b)). However, these surface apertures almost disappeared as the molar ratio increased to 2:1 (Fig. 6(c)). When the molar ratio increased to 5:1, the porous structure completely disappeared and the synthesized LiFePO4 /C composite has nearly homogeneous submicrometer particles, which is very similar to that of nano-LiFePO4 /C composite prepared with lauric acid as carbon additive (Fig. 3(b)). Fig. 7 compares the electrochemical behavior of LiFePO4 /C composite with lauric acid and/or citric acid as carbon additive. The electrochemical test was carried out between 2.0 and 4.0 V at rate of 1 C. The discharge capacity of LFP-CA, LFP-11, LFP-21, and LFP-51 were 142.7 mAh g−1 , 144.0 mAh g−1 , 146.8 mAh g−1 , and 149.8 mAh g−1 , respectively. An increasing capacity trend was observed as the ratio of lauric acid/citric acid increased. The elec-
Fig. 6. SEM image of LFP-CA (a), LFP-11(b), LFP-21(c), and LFP-51 (d).
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cycle even at a high current density of 30 C. At −20 ◦ C, the synthesized nano-LiFePO4 /C composite exhibited excellent rate ability and high capacity retention (75.5% retention compared with that at 0.2 ◦ C and 25 ◦ C). Due to its excellent rate capability and low-temperature performance, the nano-LiFePO4 /C cathode material synthesized by the one-step, lauric acid-assisted solid-state method has great promise for application in EV and HEV. Acknowledgements We gratefully acknowledge the financial support by the National Basic Research Program of China (No. 2009CB220100) and the National High Technology Research and Development Program of China (No. 2009AA035200). References Fig. 7. Charge and discharge curves of LFP-CA, LFP-11, LFP-21 and LFP-51 between 2.0 and 4.0 V at 1 C rate.
trochemical performance of the LiFePO4 /C composite is greatly affected by the carbon coating and particle size [28]. The carbon content in LFP-CA, LFP-11, LFP-21, and LFP-51 was 2.92 wt.%, 3.83 wt.%, 3.85 wt.%, and 3.94 wt%, respectively. LFP-CA and LFP-11 had porous structures, with particle size of about 10 m. Although the carbon content of LFP-11 was higher than that of LFP-CA by about 1 wt.%, the discharge capacity only increased from 142.7 to 144.0 mAh g−1 . LFP-51 and LFP-11 had almost the same carbon content, but the discharge capacity of LFP-51 was larger than that of LFP-11 by about 6 mAh g−1 . As shown in Fig. 6, the particle size of LFP-51 decreased to about 1 m, which was much smaller than that of LFP-11. The smaller particle size shortened lithium ion diffusion length, enhanced lithium ion intercalation/de-intercalation kinetics, and improved the discharge capacity at high current density. From the above analysis, we could conclude that the improvement of electrochemical performance was mainly attributed to the decrease of particle size caused by the addition of lauric acid. 4. Conclusions In this study, the nano-LiFePO4 /C composite has been directly synthesized from micrometer-sized precursor by lauric acidassisted solid-state reaction method. The pyrolysis of lauric acid generates a uniform in situ carbon layer, which could effectively suppress the growth of LiFePO4 particles. The SEM and TEM observations reveal that the LiFePO4 mediated by lauric acid consisted of nearly homogenous nanometer particles with size of about 100–200 nm. The prepared nano-LiFePO4 /C composite exhibited superior rate capability, with a discharge capacity of 149.3 mAh g−1 at 2 C, 130.9 mAh g−1 at 10 C, 121.7 mAh g−1 at 20 C, and 112.4 mAh g−1 at 30 C, respectively. In addition, this composite showed good stability with capacity loss of 6% at the 100th
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
High power performance of nano-LiFePO4 /C cathode material synthesized via lauric acid-assisted solid-state reaction Fuquan Cheng a , Wang Wan a , Zhuo Tan a,b , Youyuan Huang a , Henghui Zhou a,∗ , Jitao Chen a , Xinxiang Zhang a a b
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, PR China
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Article history: Received 15 December 2010 Received in revised form 2 January 2011 Accepted 3 January 2011 Available online 15 January 2011 Keywords: Lithium-ion batteries LiFePO4 /C composite Lauric acid Solid-state reaction
a b s t r a c t A nano-LiFePO4 /C composite has been directly synthesized from micrometer-sized Li2 CO3 , NH4 H2 PO4 , and FeC2 O4 ·2H2 O by the lauric acid-assisted solid-state reaction method. The SEM and TEM observations demonstrate that the synthesized nano-LiFePO4 /C composite has well-dispersed particles with a size of about 100–200 nm and an in situ carbon layer with thickness of about 2 nm. The prepared nanoLiFePO4 /C composite has superior rate capability, delivering a discharge capacity of 141.2 mAh g−1 at 5 ◦ C, 130.9 mAh g−1 at 10 C, 121.7 mAh g−1 at 20 ◦ C, and 112.4 mAh g−1 at 30 ◦ C. At −20 ◦ C, this cathode material still exhibits good rate capability with a discharge capacity of 91.9 mAh g−1 at 1 ◦ C. The nanoLiFePO4 /C composite also shows excellent cycling ability with good capacity retention, up to 100 cycles at a high current density of 30 ◦ C. Furthermore, the effect of lauric acid in the preparation of nanoLiFePO4 /C composite was investigated by comparing it with that of citric acid. The SEM images reveal that the morphology of the LiFePO4 /C composite transformed from the porous structure to fine particles as the molar ratio of lauric acid/citric acid increased. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Increasing attention has been focused on lithium-ion batteries for application in electric vehicles (EV) and hybrid-electric vehicles (HEV) due to the energy crisis and environmental pollution. The specific requirements for these batteries in EV and HEV are high safety, low cost, long cycle life, and environmental compatibility. However, conventional cathode material LiCoO2 is limited to largescale applications due to cost and safety issues. Thus, the demand for the development of new cathode material that can meet these requirements has continuously increased. Since Padhi et al. [1] first reported lithium iron phosphate (LiFePO4 ) with olivine structure as a new cathode material for rechargeable lithium-ion batteries, LiFePO4 has become one of the most promising cathode materials for application in EV and HEV because of its high theoretical capacity (170 mAh g−1 ), safety, environmental friendliness, and low raw materials cost. However, the main obstacles for LiFePO4 are its low lithium ion diffusion coefficient and very low electronic conductivity. Over the past few years, a great deal of effort has been made to solve these problems by decreasing LiFePO4 particles to nanometer size [2–5], doping of supervalent ions [6–8], coat-
∗ Corresponding author. Tel.: +86 10 62757908; fax: +86 10 62757908. E-mail address: [email protected] (H. Zhou). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.007
ing with electronic conductive agents [9–11], or creating a fast ion-conducting surface phase [12]. For the application of LiFePO4 cathode in EV and HEV, high power performance is vital. A successful strategy in preparing high-power performance of LiFePO4 is to synthesize nano-LiFePO4 /C composite, wherein the nanometersized LiFePO4 particles are uniformly coated by a carbon layer with thickness of about 1–2 nm. Conventional synthetic methods to obtain the nano-LiFePO4 /C composite are low-temperature routes, such as hydrothermal [3], template-mediated approach [4], co-precipitation [13,14], sol–gel [15,16], and ionothermal method [17]. Although these routes are successful in preparing nanoLiFePO4 /C composite, the application of LiFePO4 in EV and HEV is still suffering from scaling up these approaches due to their high cost and time-consuming. In contrast, solid-state reaction in preparing LiFePO4 /C composite with in situ carbon layer is simple and repeatable. The in situ carbon layer has three functions: providing a highway for electron movement, inhibiting particle growth, and preventing formation of Fe2 O3 and Li3 Fe2 (PO4 )3 impurity phases. Many organic molecules, such as sugar [18] and citric acid [19], have been widely used to generate this in situ carbon layer. However, the in situ carbon layer produced from these organic molecules does not uniformly coat on LiFePO4 and cannot effectively suppress the growth of LiFePO4 crystallites, resulting in micrometer-sized LiFePO4 particles. To enhance the electrochemical performance of LiFePO4 /C composite obtained by solid-state method, additional treatments,
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such as ball milling or the addition/treatment of carbon are generally required. Further milling process is sensitive to the ball-milling time, ball-to-powder ratio, rotation speed, and size of balls [20,21]. Therefore, fabricating high-power performance nanometer-sized LiFePO4 /C composite by solid-state reaction without further treatment remains a significant challenge. In this study, we report the synthesis of high power performance nano-LiFePO4 /C composite by a one-step, lauric acid-assisted solidstate reaction without any additional treatment to improve the battery performance. Lauric acid, a saturated fatty acid, cheap, non-toxic, and safe to handle, has a dispersing effect and can form molten hydrocarbon, which implies it can favorably interact with raw materials during the sintering process. The structure, morphology, and electrochemical performance of the prepared nano-LiFePO4 /C were investigated. In addition, the effect of lauric acid on the preparation was studied by comparing it with that of citric acid.
and 4.0 V versus Li+ /Li at constant current–constant voltage mode on a battery cycler (LAND, CT2001A, China). Cyclic voltammetry measurements were performed with a three-electrode cell. The working electrode was prepared by the same procedure as that for the charge/discharge test, and lithium metal foil served as counter and reference electrode. Cyclic voltammetry was run on an electrochemical workstation (Shanghai Chenhua, CHI660D, China) in a glove box at a scan rate of 0.1 mV s−1 ranging from 2.0 to 4.0 V versus Li+ /Li. Electrochemical impedance spectroscopy (EIS) was performed with an Autolab PG302N electrochemical workstation (Netherlands). The sinusoidal excitation voltage applied to the coin cells was 5 mV, with frequency range from 100 kHz to 0.1 Hz.
2. Experimental
The thermogravimetry curves of all starting materials (except Li2 CO3 ) used to prepare LiFePO4 are shown in Fig. 1. As the temperature increased to 43.2 ◦ C (the melting point of lauric acid), the lauric acid started to melt. Above 43.2 ◦ C, the raw materials Li2 CO3 ,
The nano-LiFePO4 /C composite was directly synthesized from micrometer-sized precursor by a one-step lauric acidassisted solid-state reaction. Stoichiometric amounts of Li2 CO3 (0.0309 mol), FeC2 O4 ·2H2 O (0.06 mol), NH4 H2 PO4 (0.06 mol), and lauric acid (0.03 mol) were weighed (mol ratio of Li:Fe:P:lauric acid = 1.03:1:1:0.5) and placed in an agate bowl (250 mL). The mixture were milled in 20 mL acetone using a planetary ball mill with ball-to-powder ratio of 4:1 (ball-diameter 10 mm), rotating speed of about 200 rpm, and ball-milling time of 6 h. During the ball-milling process, the slurry concentration was about 50% (by weight). After the ball milling, the precursor mixture was heat treated at 350 ◦ C for 10 h and then 700 ◦ C for 15 h. All these heat treatments were conducted under a nitrogen atmosphere to avoid the oxidation of Fe2+ . The effect of lauric acid in the preparation of nano-LiFePO4 /C composite was investigated by comparing it with that of citric acid. Citric acid or its mixture with lauric acid was added to the precursors. The amount of citric acid remained constant and the content of lauric acid varied according to the molar ratio of lauric acid/citric acid, namely 0:1, 1:1, 2:1, and 5:1 (labeled as LFP-CA, LFP-11, LFP-21, and LFP-51, respectively). These precursors were treated with the same ball milling and heat treatment conditions above-mentioned to prepare LiFePO4 /C composite. Thermogravimetric analysis was carried out on a simultaneous thermal analysis apparatus (Q600, TA Instruments) from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 in nitrogen atmosphere. The crystallographic structure was identified with Xray diffraction (XRD, Rigaku, DMAX-2400) with Cu K␣ radiation. The XRD pattern was collected in the 2 range of 10–80◦ at a continuous scan mode with step size of 0.02◦ and scan rate of 2◦ min−1 . The morphology of these materials was observed with a fieldemission scanning electron microscope (SEM, FEI Nano430) and a high-resolution transmission electron microscope (HRTEM, FEI T20). The carbon content in LiFePO4 /C composite was determined by an elemental analyzer (Elementar Vario Micro Cube, Germany). Charge/discharge test was performed in standard 2032-type coin cells. The electrode was prepared by dispersing 80 wt.% LiFePO4 /C active material, 10 wt.% Super P carbon black, and 10 wt.% polyvinylidene fluoride binder in N-methylpyrrolidone to form a homogeneous slurry. The obtained slurry was then cast on the Al current collector and dried for 12 h in a vacuum oven at 120 ◦ C. The electrode was pressed under 6–10 MPa and punched into 12 mm diameter circular disks. The loading of the active materials was about 2.5 mg cm−2 . Standard 2032 coin cells were assembled in an Ar-filled glove box, using lithium metal foil as negative electrode and 1 M LiPF6 in EC/DMC/EMC (1:1:1 by weight) as electrolyte. Cells were charged and discharged between 2.0
3. Results and discussion 3.1. TG analysis
Fig. 1. The thermogravimetry curves of all starting materials (except Li2 CO3 ) recorded at a heating rate of 10 ◦ C min−1 .
Fig. 2. XRD pattern of the synthesized nano-LiFePO4 /C composite.
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FeC2 O4 ·2H2 O, and NH4 H2 PO4 were dispersed in the molten lauric acid. At temperatures between 150 and 250 ◦ C, excess lauric acid was removed, leaving only a lauric acid capping layer on the precursor surface [22]. The FeC2 O4 ·2H2 O lost lattice water under 220 ◦ C, and then FeC2 O4 then decomposed to FeO from 220 to 420 ◦ C, which reacted with Li2 CO3 and the decomposition product of NH4 H2 PO4 to form LiFePO4 [23]. Above 250 ◦ C, the lauric acid capping layer started to decompose and then formed the in situ car-
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bon coating layer, which could effectively inhibit particle growth in high-temperature heat treatment. 3.2. XRD and morphology analysis The XRD patterns in Fig. 2 demonstrate that the nano-LiFePO4 /C powder was well crystallized as an olivine structure without any detectable impurity phase. The composite can be indexed
Fig. 3. (a) SEM image of the solid reactants mixture of Li2 CO3 , FeC2 O4 ·2H2 O, NH4 H2 PO4 and citric acid created by planetary ball milling; (b) SEM image of synthesized nano-LiFePO4 /C composite; (c) TEM image of synthesized nano-LiFePO4 /C composite; (d) typical TEM image for individual LiFePO4 /C crystallites; and (e) typical HRTEM image for the carbon-coated LiFePO4 crystallites. Inset of (c) shows the selected-area electron diffraction.
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based on the orthorhombic Pnma space group. The lattice con˚ b = 6.004 A, ˚ and c = 4.690 A˚ were calculated stants of a = 10.322 A, using PowderX software. The results are very close to the stan˚ b = 6.010 A, ˚ c = 4.693 A) ˚ given by JCPDS dard data (a = 10.334 A, 83-2092. In addition, no diffraction pattern corresponds to impurity phases, such as Li3 PO4 , Li3 Fe2 (PO4 )3 , Fe2 O3 , and Fe2 P, which are frequently found in the LiFePO4 /C composite obtained through high-temperature treatment. The raw materials Li2 CO3 , FeC2 O4 ·2H2 O, and NH4 H2 PO4 used in the experiment are all micrometer-sized (SEM images are not shown here). Fig. 3(a) shows an SEM image of the solid reactant mixture of Li2 CO3 , FeC2 O4 ·2H2 O, NH4 H2 PO4 , and lauric acid created by planetary ball milling. After ball-milling treatment, these raw materials were mixed well with each other and the average particle size decreased to about 5 m. After high-temperature sintering, however, the micrometer precursor transformed to nanometersized LiFePO4 particles. Fig. 3(b) shows an SEM image of the nano-LiFePO4 /C composite prepared at 700 ◦ C with average particle size of about 100–200 nm. The TEM image shown in Fig. 3(c) illustrates that these nano-LiFePO4 were well-dispersed. These well-dispersed LiFePO4 /C particles were bound together by the in situ carbon layer to form a loose agglomerated structure, which was beneficial to the movement of electrons and permeation of the electrolyte. The selected-area electron diffraction pattern sug-
gests that the prepared LiFePO4 was highly crystalline (inset in Fig. 3(c)). The SEM and TEM images in Fig. 3(a)–(c) clearly indicate that the well-crystallized nano-LiFePO4 /C composite was directly prepared from micrometer-sized precursor by lauric acid assistedsolid state method. During the heat-treatment process, Li2 CO3 , FeC2 O4 ·2H2 O, and NH4 H2 PO4 reacted with each other and then formed nanometer-sized LiFePO4 crystallites, which have high surface energy and a tendency to agglomerate. Due to the formation of the lauric acid capping layer on the surface of precursors, the in situ carbon layer formed by decomposition of lauric acid uniformly deposited on the surface of the nano-LiFePO4 crystallites, and thus prevented the further growth of LiFePO4 particles during high-temperature sintering. High-resolution TEM analysis is a useful tool in analyzing the nature of the carbon layer in the interstitial particle/boundary region. Fig. 3(d) shows the magnified image of a single particle in Fig. 3(c). This particle had a regular shape and smooth surface. The HRTEM image in Fig. 3(e) clearly presents that the carbon layer was uniformly coated on the surface of LiFePO4 particles. The thickness of carbon layer was about 2 nm, which was favorable for lithium ion transport across the interface between LiFePO4 and electrolyte [5]. A distinct lattice fringe with an interplanar distance of 0.427 nm was observed, corresponding to the spacing of the (1 0 1) planes of the LiFePO4 particle. The carbon content of LiFePO4 /C composite
Fig. 4. (a) Rate capability at different current densities from 0.2 C to 30 C; (b) cycling life at different current densities from 1 C to 30 C (charging and discharging were performed with the same current density); (c) typical charge/discharge curves at 0.2 C under different temperatures from −20 ◦ C to 25 ◦ C; and (d) charge/discharge curves at various charge/discharge rates at −20 ◦ C; inset of Fig. 3(a) shows the corresponding charge/discharge curve.
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synthesized with and without lauric acid added to the precursor was 1.48 and 0.43 wt.%, respectively. Although the carbon content produced from lauric acid was very low (about 1.1 wt.%), it played a critical role in suppressing LiFePO4 particle growth. The prepared nano-LiFePO4 /C composite does not need any additional treatments to enhance its electrochemical performance, which greatly simplifies the synthetic process of preparing high-power nanoLiFePO4 /C composite. 3.3. Electrochemical characterization Fig. 4 shows the electrochemical performance of the nanoLiFePO4 /C cathode material. The electrochemical properties were investigated using CR2032 coin cells with lithium metal foil as the negative electrode. Fig. 4(a) shows the charge and discharge specific capacities observed in continuous cycling at rates varying from 0.2 ◦ C to 30 ◦ C between 2.0 and 4.0 V at 25 ◦ C, where 1 C corresponds to 170 mAh g−1 . At rate of 0.2 ◦ C, the nano-LiFePO4 /C composite delivered a high discharge capacity of 159.6 mAh g−1 , corresponding to 94.1% of the theoretical capacity. The nano-LiFePO4 /C composite had a high discharge capacity of 141.2 mAh g−1 and 130.9 mAh g−1 at rates of 5 ◦ C and 10 ◦ C, respectively. The inset in Fig. 4(a) shows the charge–discharge curves of the nano-LiFePO4 /C composite at different current densities. Although polarization increased and specific capacity decreased with increase in C rate, this LiFePO4 cathode material presented a discharge capacity of 121.7 mAh g−1 at 20 ◦ C with flat voltage plateaus. Even at high current density of 30 ◦ C, this cathode material could still deliver a specific capacity of 112.4 mAh g−1 , which was much higher than that of the LiFePO4 cathode material prepared by solid-state reaction in the literature [24,25]. These results indicate that the synthesized nano-LiFePO4 /C composite had superior power performance. The prepared composite also exhibited high cycling performance (see Fig. 4(b)). At 1 C rate, the discharge capacity of this material remained at 150.3 mAh g−1 at the 100th cycle, compared to the initial discharge capacity of 153.2 mAh g−1 at the 1st cycle. The discharge capacity loss ratio was 0.02% per cycle. At the high current density of 30 C, this composite still showed good cycling ability, with a discharge capacity loss of 6% at the 100th cycle. The low-temperature performance of nano-LiFePO4 /C composite was tested in low temperature cabinet, with temperatures at 25, 10, 0, −10, and −20 ◦ C. The charge and discharge processes were performed under the same temperature. Fig. 4(c) presents the charge–discharge curves of the prepared material at different low temperatures and at a rate of 0.2 C. The prepared material delivered a discharge capacity of 159.6, 150.5, 143.0, 131.6, and 113.7 mAh g−1 at 25, 10, 0, −10, and −20 ◦ C, respectively. The charge retention at −20 and 25 ◦ C was 75.5%, indicating that most of the capacity in the composite can be released in the low-temperature environment. At −20 ◦ C, this nano-LiFePO4 /C composite still exhibited excellent rate capability (Fig. 4(d)). The specific capacity at 0.5 ◦ C and 1 ◦ C rate were 100.8 mAh g−1 and 91.9 mAh g−1 , respectively. Thus, the nano-LiFePO4 /C composite synthesized by lauric acid assisted-solid state method has great prospect in application in HEV and EV in cold regions. To evaluate the electrode impedance upon cycling, Nyquist plots of coin cells at selected cycles with charge/discharge current density of 1 ◦ C were collected (Fig. 5(a)). EIS measurements were carried out under fully discharged state at 25 ◦ C. All the impedance spectra curves were composed of a depressed semicircle in high frequency region and a straight line in low frequency region. An intercept at the Zre axis at high frequency corresponds to the ohmic resistance of the electrolyte (Rs ). The depressed semicircle in the middle frequency range indicates the charge transfer resistance (Rct ). The inclined line in the low frequency range represents the Warburg impedance (Zw ), associated with lithium-ion diffusion in
Fig. 5. (a) Electrochemical impedance spectra of nano-LiFePO4 /C cathodes at selected cycle and (b) CV curves of synthesized nano-LiFePO4 /C cathodes at a scan rate of 0.1 mV s−1 .
the LiFePO4 bulk material. The charge transfer resistance at the 10th cycle was 20.5 cm2 . After 100 cycles at 1 ◦ C rate, Rct increased to 24.4 cm2 . This slight increase of Rct upon cycling indicates that contact between LiFePO4 and carbon layer is firm and the interface between the LiFePO4 /C particles and electrolyte is very stable. Fig. 5(b) shows the cyclic voltammogram of nano-LiFePO4 /C composite at a scan rate of 0.1 mV s−1 . The oxidation and reduction peaks are highly symmetric to each other. The ratios of Ipa /Ipc are close to 1, indicating good reversibility of lithium ion intercalation into and de-intercalation from the nano-LiFePO4 /carbon composite. The sharp oxidation and reduction peaks indicate excellent kinetics behavior in the LiFePO4 cathode. The improvement in the kinetics of lithium ion intercalation and de-intercalation could be attributed to the nanometer-sized particles that shorten the diffusion length of lithium ions in LiFePO4 [26]. The peak current and peak profile remain stable even after six cycles, which implies excellent cycling stability of the nano-LiFePO4 /C composite. Besides the peaks associated with phase transition between LiFePO4 and FePO4 , no peak appeared at 2.63 V (characteristic of Fe3+ in iron oxide), indicating that all the iron atoms in the nano-LiFePO4 /C composite were Fe2+ [27].
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3.4. Comparison between lauric acid and citric acid The above results reveal that lauric acid is a successful carbon additive in preparing nano-LiFePO4 /C composite and enhancing the electrochemical performance of the LiFePO4 electrode. To study the effect of lauric acid in hindering LiFePO4 particle growth, another organic molecule, citric acid, which usually acts as a chelating reagent in sol–gel [15] method and solid-state reaction [19], was introduced. According to the literature [19], 6 wt.% of citric acid was added to the precursor. The amount of citric acid remained constant, whereas the content of lauric acid varied according to the molar ratio of lauric acid to citric acid, which increased from 1:1 to 5:1. Fig. 6 shows the SEM image of LiFePO4 /C composite with lauric acid and/or citric acid as carbon additives. A porous structure with particles size about 10–20 m was observed in LFP-CA where only citric acid was added into precursors (Fig. 6(a)). The porous
structure was formed due to the vigorous gas evolution during decomposition of citric acid in sintering process [15]. There are still many surface apertures in the LFP-11 as the molar ratio increased to 1:1 (Fig. 6(b)). However, these surface apertures almost disappeared as the molar ratio increased to 2:1 (Fig. 6(c)). When the molar ratio increased to 5:1, the porous structure completely disappeared and the synthesized LiFePO4 /C composite has nearly homogeneous submicrometer particles, which is very similar to that of nano-LiFePO4 /C composite prepared with lauric acid as carbon additive (Fig. 3(b)). Fig. 7 compares the electrochemical behavior of LiFePO4 /C composite with lauric acid and/or citric acid as carbon additive. The electrochemical test was carried out between 2.0 and 4.0 V at rate of 1 C. The discharge capacity of LFP-CA, LFP-11, LFP-21, and LFP-51 were 142.7 mAh g−1 , 144.0 mAh g−1 , 146.8 mAh g−1 , and 149.8 mAh g−1 , respectively. An increasing capacity trend was observed as the ratio of lauric acid/citric acid increased. The elec-
Fig. 6. SEM image of LFP-CA (a), LFP-11(b), LFP-21(c), and LFP-51 (d).
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cycle even at a high current density of 30 C. At −20 ◦ C, the synthesized nano-LiFePO4 /C composite exhibited excellent rate ability and high capacity retention (75.5% retention compared with that at 0.2 ◦ C and 25 ◦ C). Due to its excellent rate capability and low-temperature performance, the nano-LiFePO4 /C cathode material synthesized by the one-step, lauric acid-assisted solid-state method has great promise for application in EV and HEV. Acknowledgements We gratefully acknowledge the financial support by the National Basic Research Program of China (No. 2009CB220100) and the National High Technology Research and Development Program of China (No. 2009AA035200). References Fig. 7. Charge and discharge curves of LFP-CA, LFP-11, LFP-21 and LFP-51 between 2.0 and 4.0 V at 1 C rate.
trochemical performance of the LiFePO4 /C composite is greatly affected by the carbon coating and particle size [28]. The carbon content in LFP-CA, LFP-11, LFP-21, and LFP-51 was 2.92 wt.%, 3.83 wt.%, 3.85 wt.%, and 3.94 wt%, respectively. LFP-CA and LFP-11 had porous structures, with particle size of about 10 m. Although the carbon content of LFP-11 was higher than that of LFP-CA by about 1 wt.%, the discharge capacity only increased from 142.7 to 144.0 mAh g−1 . LFP-51 and LFP-11 had almost the same carbon content, but the discharge capacity of LFP-51 was larger than that of LFP-11 by about 6 mAh g−1 . As shown in Fig. 6, the particle size of LFP-51 decreased to about 1 m, which was much smaller than that of LFP-11. The smaller particle size shortened lithium ion diffusion length, enhanced lithium ion intercalation/de-intercalation kinetics, and improved the discharge capacity at high current density. From the above analysis, we could conclude that the improvement of electrochemical performance was mainly attributed to the decrease of particle size caused by the addition of lauric acid. 4. Conclusions In this study, the nano-LiFePO4 /C composite has been directly synthesized from micrometer-sized precursor by lauric acidassisted solid-state reaction method. The pyrolysis of lauric acid generates a uniform in situ carbon layer, which could effectively suppress the growth of LiFePO4 particles. The SEM and TEM observations reveal that the LiFePO4 mediated by lauric acid consisted of nearly homogenous nanometer particles with size of about 100–200 nm. The prepared nano-LiFePO4 /C composite exhibited superior rate capability, with a discharge capacity of 149.3 mAh g−1 at 2 C, 130.9 mAh g−1 at 10 C, 121.7 mAh g−1 at 20 C, and 112.4 mAh g−1 at 30 C, respectively. In addition, this composite showed good stability with capacity loss of 6% at the 100th
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