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Iron-assisted carbon coating strategy for improved electrochemical LiMn0.8Fe0.2PO4 cathodes
Electrochimica Acta 212 (2016) 800–807
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Iron-assisted carbon coating strategy for improved electrochemical LiMn0.8Fe0.2PO4 cathodes Hongyu Liua , Li Rena,* , Jiashen Lib,* , Hongna Tuoa a b
College of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China School of Materials, The University of Manchester, Manchester M13 9PL, UK
A R T I C L E I N F O
Article history: Received 20 May 2016 Received in revised form 28 June 2016 Accepted 10 July 2016 Available online 11 July 2016 Keywords: Lithium manganese phosphate Iron Carbon layers Solvothermal method
A B S T R A C T
An iron-assisted carbon coating strategy is developed to guide the formation of uniform and highly graphitized carbon layers on surfaces of the LiMn0.8Fe0.2PO4 (LMFP) to yield cathode materials with improved electrochemical performance. A small amount of iron oxalate is added as a catalyst precursor, which decomposes into ferrous oxide (FeO) at high temperature. During the calcination process, FeO is reduced to iron (Fe) that helps to transform amorphous carbon into graphitized carbon which is deposited uniformly and tightly on surfaces of LMFP materials. The impact of Fe atoms on the formation of highly graphitized carbon layers as well as the electrochemical performances of the resulting LMFP/Fe/ Carbon (LMFP/Fe/C), is evaluated. Compared to LMFP/C without iron oxalate, LMFP/Fe/C exhibited substantial discharge capacity and better rate and cycling performances. Discharge capacities of 152.3, 141.9, 132.1, 105.6 and 76.0 mAh g1 are recorded at 0.2, 0.5, 1, 5 and 10 C, respectively. The retention capacity remained 78.6% at 10 C after 60 cycles. Furthermore, the conductivity and the lithium ion diffusion processes of LMFP/Fe/C are improved. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have gained great attention as renewable green energy and occupied a prime position in broad applications ranging from portable electronic devices to electric vehicles [1,2]. The cathode material is an important component in the successful operation of LIBs. The olivine-type LiMPO4 (M = Fe, Mn etc.) cathode materials have been reported to exhibit higher capacity and superior safety coupled with low cost [3–5]. Compared with LiFePO4 (LFP) material, the LiMnPO4 (LMP) is considered as a better candidate for rechargeable LIBs because of its larger potential window of 4.1 V versus that of Li/Li+ (3.4 V for LFP) [6]. However, the low electrical conductivity (<1010 S cm1) and the slow lithium-ion diffusion (<1016 cm2 S1) have limited its practical applications [7,8]. To overcome the drawbacks of LMP, many modification methods have been attempted, including particle-size minimization [9–11], Mn-site doping [12–14], and carbon coating [15–18]. The particle-size minimization method shortens the lithium-ion diffusion path and increases the electrode surface area for interfacial charge transfer that leads to improved
* Corresponding author. Tel.: +86 22 26582054. E-mail addresses: [email protected] (L. Ren), [email protected] (J. Li). http://dx.doi.org/10.1016/j.electacta.2016.07.049 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
ionic diffusion efficiencies. Ye et al. [19] have synthesized nano LiMn0.9Fe0.1PO4 by the solvothermal method and found that the introduction of cationic ions into the crystalline LMP can lead to stable crystal structure and excellent electrochemical performance. Hong et al. [20] have demonstrated the higher rate capability and cycle performance of LIBs cathode when Mn2+ is partially substituted by Fe2+. Carbon coating is an effective way to improve the electronic conductivity of LMP and its kinetic properties [21–24]. The carbon content, distribution and uniformity of the coverage, carbon surface area, and the degree of graphitization (the sp2/sp3 or graphitized/disordered ratio IG/ID) are the key factors to affect the electrochemical performance of LMP cathode [25–27]. However, the most conventional carbon sources and coating technologies are far from the requirements to yield high-performance cathodes. Graphene is alternatively exploited due to its high specific surface area and excellent conductivity [28], but graphene is difficult to be dispersed and its interaction with LMP particles is weak. Thus, direct mixing of graphene with LMP is not the best approach to improve the electrochemical performance of LMP cathodes. Metal compounds such as iron oxides and iron sulfates can guide and catalyze the formation of graphitized carbon from various carbon sources [29–33]. Amorphous carbon could nucleate and grow into graphene layers depositing on metal surfaces [34].
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Furthermore, LiFePO4/graphene was synthesized using glucose and FeSO4 as the carbon source and catalyst precursor, respectively [35]. High-quality carbon layers were also coated on LiFe0.6Mn0.4PO4/C cathode material, where both the carbon and metal catalyst were mixed uniformly and were randomly distributed among the cathode particles. After the calcination process, the graphene layers were grown not only on the surface of the cathode particles but also in places among particles. Besides, some graphene sheets stretched out from the cathode particles yielding no positive contribution to the electrochemical performance [36]. Therefore, the adding and spatial sequence of the metal catalyst and carbon sources is a very important factor in acquiring highly graphitized carbon coatings only on the surface of cathode materials. In this paper, LiMn0.8Fe0.2PO4 (LMFP) was synthesized by the solvothermal method. Iron oxalate was selected as metal catalyst precursor and mixed with LMFP for the synthesis of LMFP/Fe/ Carbon (LMFP/Fe/C). By adding iron oxalate, and glucose in the desired order, the mechanism of forming the uniform and highly graphitized carbon layers was particularly investigated. The synthesized materials were characterized by various analytical methods and their electrochemical performances as LIBs cathodes were measured. 2. Experimental 2.1. Synthesis of LMFP and LMFP/Fe/C composite The LMFP particles were synthesized by the solvothermal method. The Mn/Fe mixture (Mn/Fe = 4:1, n/n, MnSO4H2O (AR grade), FeSO47H2O (AR grade)) and H3PO4 (85 wt%)were dissolved
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in a mixture of ethylene glycol-water (5:3, v/v) as the solvent in presence of ascorbic acid as the reducing agent. LiOHH2O was then added dropwise into the mixture with a molar ratio of Li:Mn/Fe:P fixed at 3:1:1. After vigorous stirring for 30 min under a nitrogen atmosphere, the resulting suspension was transferred into a 200 ml Teflon-lined stainless steel autoclave and was heated at 240 C for 4 h. After the autoclave was cooled down to room temperature, the gray precipitate was washed off several times with water and ethanol. To prepare LMFP/Fe/C, iron oxalate as the catalyst precursor was mixed with LMFP and the molar ratio between PO43+ and iron oxalate was kept at 200:1. Subsequently, the mixture was dried at 200 C in vacuum for 4 h. The obtained powder (LMFP/FeO) was then mixed with glucose and the mass ratio between the powder and carbon contained in glucose was kept to 93:7. Finally, the mixture was calcined at 650 C for 8 h in a tube furnace under a nitrogen atmosphere. The composite material LMFP/Fe particles with carbon coating was abbreviated as LMFP/Fe/C. For comparative purposes, the LMFP particles with carbon coating (LMFP/C) without iron oxalate were also produced using the same method. 2.2. Characterization The phase purity and crystal structure of the samples were characterized by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer with Cu Ka radiation set between 5 and 80 at a scan rate of 5 /min. The morphology and structure were determined by a NanoSEM450 scanning electron microscope (SEM) and a Tecnai G2 F20 transmission electron microscopy (TEM). The carbon analysis was conducted by DXR Microscope Raman spectroscopy with a 532 nm wavelength laser.
Fig. 1. The schematic preparation of the LMFP/Fe/C. (a) Iron oxalate was converted to ferrous oxide and attached on surface the of LMFP particles during the drying process, (b) ferrous oxides were reduced to iron atoms by carbon during the sintering process, (c) under the catalytic guidance of Fe atoms, carbon atoms nucleated and grew into graphitized carbon layers on surfaces of the LMFP particles, (d) LMFP was coated with uniform graphitized carbon layers.
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2.3. Electrochemical measurements The working electrodes were prepared using the following mixture: 80% (wt%) of the active materials, 10% (wt%) Surper-P and 10% (wt%) polyvinylidene fluoride (PVDF) dissolved in Nmethyl pyrrolidinone (NMP). The slurries were coated on aluminum (Al) foil and dried at 120 C in a vacuum oven for 12 h to remove the solvent before pressing. The pellet was rolled to enhance adhesion between the electrode laminate and the Al foil then was punched into round disks of 14 mm in diameter. The electrochemical measurements were performed with CR2430 coin-type cells with lithium as anode in an electrolyte composed of (1 mol L1 LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethyl methyl carbonate (EMC) (EC:DMC:EMC = 1:1:1, v/v/ v)). The half-cell was assembled in a glove box filled with pure argon. After resting for 4 h, the charge-discharge tests were performed on a program-controlled Battery Test System (Land, Wuhan, China) using voltages ranging from 2.0 to 4.6 V at various constant current rates and the rate of charge and discharge kept the same. The cyclic voltammetry (CV) and electrochemical impedance measurements (EIS) measurements were performed on a CHI650 B Electrochemical Workstation (Chenghua, Shanghai, China). The CVs were measured in the potential window of 2.0 to 4.6 V and EIS over frequency ranges of 102–105 Hz at 5 mV excitation potential. 3. Results and discussion 3.1. Phase structure The LMFP/Fe/C composite was synthesized schematically displayed in Fig. 1. As shown in Fig. 1a, after LMFP particles were synthesized by solvothermal method, they were firstly mixed with iron oxalate. Then the mixture were heated to transfer iron oxalate into ferrous oxide (FeO) which attached to the surface of LMFP particles (1). The obtained LFMP/FeO powder was then mixed with glucose and dried. So far, FeO and glucose were sequentially coated on the surface of LMFP particles. Then the mixture was calcined in a tube furnace under a nitrogen atmosphere. Firstly, glucose was thermally carbonized to be amorphous carbon. FeO was then reduced to Fe by carbon during the calcination process (Fig. 1b). Secondly, Fe atom acted as the catalyst to nucleate the carbon atoms and which grow into graphitized carbon layers on the LMFP particles surfaces (Fig. 1c). This process was induced by the interaction between carbon and Fe atoms which rearranged the carbon atoms into graphitized carbon layers, and directly coated them on the surfaces of LMFP particles (Fig. 1d). Owning to the catalytic role of Fe, the degree of carbon graphitization was improved. Moreover, the carbon layers are quite thin and uniform. Meanwhile randomly graphitized carbon among the LMFP particles is hardly observed because there is no Fe outside of LMFP particles. Consequently, the overall quality of the carbon coating was optimized. All of these were very important for optimized cathode materials. (1)
The phase composition and crystallinity of the LMFP/Fe/C and LMFP/C materials were analyzed by XRD, and the results were shown in Fig. 2. All the diffraction peaks are in agreement with the standard LMFP (JCPDS NO.13-0336) with no additional visible peaks due to impurities. This indicates that LMFP/Fe/C and LMFP/C may be well-indexed to the orthorhombic LMFP phase with a Pmnb space group, and both the samples are readily prepared via the solvothermal method [37,38].
Fig. 2. XRD patterns of the LMFP/Fe/C and LMFP/C samples.
Fig. 3 presents SEM images of the LMFP/Fe/C and LMFP/C materials prepared by the solvothermal synthetic method and carbon coating procedure. Both samples synthesized in ethylene glycol-water mixed solvents show relatively uniform rod-like shape particles with sizes around 200 nm in diameter. The addition of iron oxalate and glucose do not influence the particle morphology. LMFP/Fe/C (Fig. 3a) showed clear particles surrounded with homogeneous carbon layers. Almost all carbon were coated on the surface of LMFP particles. Few graphitized carbon films could be observed which were outside of LMFP/Fe/C. Compared with LMFP/Fe/C, LMFP/C particles depicted in Fig. 3b are not as clear as LMFP/Fe/C and they are mixed together with pellet carbon layers. There are more carbon layers among particles. Therefore, the addition of iron oxalate promoted the directional growth of the carbon layers, resulting in highly graphitized carbon that tightly packs the particles. TEM and HRTEM measurements were utilized to further investigate the microstructure of LMFP/Fe/C and LMFP/C. Fig. 4a and b illustrate TEM and HRTEM images for the LMFP/Fe/C. The graphitized carbon layers of 2.2 nm thickness were uniformly and tightly coated on the surfaces of LMFP/Fe/C particles. The lithium ions can easily cross the thin carbon layers into the framework of LMFP [39]. Compared to LMFP/Fe/C, the carbon layers deposited on the LMFP/C are thick and non-uniform (Fig. 4c and d). Moreover, some carbon films were stretched out from LMFP particles which have no positively contribute to the electrochemical performance (Fig. 4c). Depending on location, the thicknesses of the carbon layers vary between 4.1-5.7 nm on different parts of the particles. Hence, high-quality carbon coatings are obtained by adding iron oxalate as the catalyst precursor. The synthesized LMFP/Fe/C and LMFP/C materials were subjected to further characterization by Raman spectroscopy, a sensitive technique for probing carbon structures. As shown in Fig. 5, the peak located at 951 cm1 is assigned to the stretching vibration in PO43 group while the two strong peaks at 1350 cm1 and 1610 cm1 are attributed to D-band (disorder band) and Gband (graphitized band) of carbon, respectively [40,41]. Both samples showed strong G band in comparison with D band, indicating the highly ordered graphitized structures of the samples. The IG/ID intensities of LMFP/Fe/C and LMFP/C are respectively 1.20 and 1.11, suggesting that iron can improve the quality of the carbon layers and the degree of graphitization [42]. 3.2. Electrochemical performance To demonstrate the benefit of adding iron oxalate on the electrochemical performance, the LMFP/Fe/C and LMFP/C
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Fig. 3. SEM images of (a) LMFP/Fe/C and (b) LMFP/C samples.
Fig. 4. (a) TEM and (b) HRTEM images of LMFP/Fe/C, (c) TEM and (d) HRTEM images of LMFP/C.
electrodes containing 7 wt% carbon are subjected to chargedischarge testing in the voltage window of 2.0-4.6 V at different rates. As shown in Fig. 6a and b compare the initial discharge profiles of both materials at 0.2, 0.5, 1, 5 and 10C (1C = 170 mAh g1) at 25 C. It can be seen that all the electrodes exhibit two discharge voltage platforms related to the Mn3+/Mn2+ and Fe3+/Fe2+ redox couples. However, those corresponding to LMFP/Fe/C (Fig. 6a) display higher and longer potential plateau at respective discharge rate. This indicates that the LMFP/Fe/C possesses an easier twophase transformation process between the LMFP and Mn0.8Fe0.2PO4 (MFP) compared to LMFP/C. As the discharge rates increase, the LMFP/Fe/C delivers respective discharge capacities of 152.3, 141.9, 132.1, 105.6 and 76.0 mAh g1 at the first cycle, whereas the discharge capacities obtained with LMFP/C are 150.4, 138.9, 128.4, 85.9 and 59.0 mAh g1 under the same conditions. The specific discharge capacities of LMFP/Fe/C recorded at different discharge rates look improved if compared with those obtained with LMFP/C, especially at high rates. Furthermore, the cycle performance of LMFP/Fe/C is better than that of LMFP/C at corresponding charge-discharge rates (Fig. 6c and d). The rule of
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Fig. 5. Raman spectra of the LMFP/Fe/C and LMFP/C samples.
the capacity retention is summarized in Fig. 6e, showing that the capacity retention of LMFP/Fe/C is about 78.6% at 10C after 60 cycles while that of LMFP/C is only 39.1% under same conditions. The coulombic efficiency fades of the LMFP/C is much faster as the rate increases (Fig. 6f). The improved electrochemical activity and rate performance of LMFP/Fe/C are attributed to the compact and uniformly formed carbon layers on the LMFP surfaces, which enhances the effective utilization of the active materials [43]. The improved cycling stability is attributed to the highly graphitized carbon layers, which reduces the stranded lithium ion in carbon layers during the cycle and ensure the enough lithium ion transference number in the charge and discharge process. The optimized carbon layers are attributed to the catalysis of iron. The addition of iron to improve the quality of the carbon coatings is an effective way to increase the cycle stability and coulombic efficiency of LMFP. The CVs of the LMFP/Fe/C and LMFP/C samples were measured at voltages ranging from 2.0-4.6 V at different scan rates (0.1-0.5 mV s1) and under 25 C (Fig. 7). Both the materials exhibit two pairs of redox peaks corresponding to Fe2+/Fe3+ and Mn2+/Mn3+ [44,45]. The LMFP/C shows broader redox peaks, suggesting the lower electrode kinetics. The LMFP/Fe/C, on the other hand, depicts sharper and stronger redox peaks, indicating improved electrode kinetics. The estimated voltage separation between the redox peaks corresponding to Fe2+/Fe3+ in LMFP/Fe/C and LMFP/C are 0.18/0.42, 0.29/0.49, 0.37/0.60, 0.44/0.66 and 0.49/
0.72 V at scan rates of 0.1, 0.2, 0.3, 0.4 and 0.5 mV s1, respectively. The redox peaks corresponding to the Mn2+/Mn3+ in LMFP/Fe/C and LMFP/C are respectively 0.42/0.68, 0.56/0.81, 0.63/0.96, 0.73/0.97 and 0.78/0.99 V at the same scan rates. The smaller voltage separation suggests a better reversibility of the lithium insertion/ extraction processes in LMFP/Fe/C. The excellent electrochemical performance obtained with LMFP/Fe/C is attributed to the carbon layers completely and firmly coating on the particles, resulting in elevated graphitization degree, reduced polarization and improved electrical conductivity. It is due to iron oxalate, which guides the formation of the graphitized carbon during the calcination process. EIS is an effective way to examine the kinetic factors underlying the electrochemical performance. The Nyquist plots of the LMFP/ Fe/C and LMFP/C materials in the frequency range of 102–105 Hz and 5 mV excitation potential are shown in Fig. 8a. The Nyquist plots are measured at full charge state after 30 cycles at 0.2C. The spectra depict depressed semi-circle at high-to-medium frequency and an inclined line in the low-frequency region. The intersection of Z0 axis at higher frequency refers to the ohmic resistance (Rs) of electrodes and electrolyte. The semicircle at high-to-medium frequency corresponds to the charge transfer resistance (Rct) at the interface between the cathode and the electrolyte. The inclined line observed at low-frequency is attributed to the Warburg impedance (Ws), which restricts the lithium ion diffusivity within the samples [46,47]. The plots are fitted by Z-view based on the equivalent circuit inserted in Fig. 8a. The LMFP/Fe/C electrode
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Fig. 6. The first discharge profiles of (a) LMFP/Fe/C and (b) LMFP/C, the cycling performance of (c) LMFP/Fe/C and (d) LMFP/C, (e) the capacity retention of LMFP/Fe/C and LMFP/C at different rates, (f) the coulombic efficiency of LMFP/Fe/C and LMFP/C at the first charge-discharge at different rates.
shows lower Rs and Rct (Table 1), indicating a reduced charge transfer polarization resistance for LMFP/Fe/C. The conductivity and dynamic of delithiation/lithiation processes are substantially improved due to the addition of iron oxalate. The apparent lithium-ion diffusion coefficient (DLi) of the electrodes is calculated from the inclined lines at low-frequency using Eqs. (2) and (3) [48]. D = (R2T2)/(2A2n4F4C2s2)
(2)
Z0 = Rs + Rct + sv1/2
(3)
Where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons transferred, F is the Faraday constant, C is the concentration of the lithium ion, and s is the Warburg factor associated to Z0 obtained by linear fitting the plot between Z0 and reciprocal square root of the angular frequency v (Fig. 8b). The resulting lithium-ion diffusion coefficients of the two electrodes are compared in Table 1. The DLi of LMFP/Fe/C (2.15 1014 cm2 s1) is larger than that of LMFP/C (1.65 1014 cm2 s1). The good lithium ion diffusion obtained with LMFP/Fe/C is attributed to its excellent formed carbon layers. Due to the uniform and a highly graphitized carbon layers, the lithium ions
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Fig. 7. The CV curves of (a) LMFP/Fe/C and (b) LMFP/C at different scan rates.
can easily cross them into the framework of LMFP, hence improving the ionic diffusion efficiency.
5. Conclusion An iron-assisted carbon coating strategy is developed to guide the formation of uniform and the highly graphitized carbon layer on LMFP particles. Iron oxalate and glucose were sequentially coated on LMFP from inside to outside. During the calcination process and thanks to the presence of Fe atoms, uniform and a highly compacted graphitized carbon layers are formed on surfaces of the LMFP nanoparticles. Compared to LMFP/C, the LMFP/Fe/C material exhibited a better discharge capacity and better rate and cycling performances. The LMFP/Fe/C showed excellent discharge capacities of 152.3, 141.9, 132.1, 105.6 and 76.0 mAh g1 at 0.2, 0.5, 1, 5 and 10C, respectively. Even at increased rate of 10C, the retention capacity remained 78.6% after 60 cycles. The LMFP/Fe/C also displayed improved conductivity and lithium-ion diffusion, promising for high-performance electrochemical LMFP materials at lower costs.
Acknowledgment This work was partially supported by National Nature Science Foundation of China (51203041).
Fig. 8. (a) EIS of the LMFP/Fe/C and LMFP/C samples after 30 cycles at 0.2 C and the corresponding equivalent circuit used for fitting the experimental EIS data, (b) the relationship between Z0 and the square root of the frequency (v1/2) in the lowfrequency region.
Table 1 The simulation results of the electrochemical impedance and lithium ion diffusion coefficients. Sample
Rs (V)
Rct (V)
s
DLi (cm2 s1)
LMFP/Fe/C LMFP/C
8.52 8.80
34.26 78.67
37.76 43.07
2.15 1014 1.65 1014
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Iron-assisted carbon coating strategy for improved electrochemical LiMn0.8Fe0.2PO4 cathodes Hongyu Liua , Li Rena,* , Jiashen Lib,* , Hongna Tuoa a b
College of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China School of Materials, The University of Manchester, Manchester M13 9PL, UK
A R T I C L E I N F O
Article history: Received 20 May 2016 Received in revised form 28 June 2016 Accepted 10 July 2016 Available online 11 July 2016 Keywords: Lithium manganese phosphate Iron Carbon layers Solvothermal method
A B S T R A C T
An iron-assisted carbon coating strategy is developed to guide the formation of uniform and highly graphitized carbon layers on surfaces of the LiMn0.8Fe0.2PO4 (LMFP) to yield cathode materials with improved electrochemical performance. A small amount of iron oxalate is added as a catalyst precursor, which decomposes into ferrous oxide (FeO) at high temperature. During the calcination process, FeO is reduced to iron (Fe) that helps to transform amorphous carbon into graphitized carbon which is deposited uniformly and tightly on surfaces of LMFP materials. The impact of Fe atoms on the formation of highly graphitized carbon layers as well as the electrochemical performances of the resulting LMFP/Fe/ Carbon (LMFP/Fe/C), is evaluated. Compared to LMFP/C without iron oxalate, LMFP/Fe/C exhibited substantial discharge capacity and better rate and cycling performances. Discharge capacities of 152.3, 141.9, 132.1, 105.6 and 76.0 mAh g1 are recorded at 0.2, 0.5, 1, 5 and 10 C, respectively. The retention capacity remained 78.6% at 10 C after 60 cycles. Furthermore, the conductivity and the lithium ion diffusion processes of LMFP/Fe/C are improved. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have gained great attention as renewable green energy and occupied a prime position in broad applications ranging from portable electronic devices to electric vehicles [1,2]. The cathode material is an important component in the successful operation of LIBs. The olivine-type LiMPO4 (M = Fe, Mn etc.) cathode materials have been reported to exhibit higher capacity and superior safety coupled with low cost [3–5]. Compared with LiFePO4 (LFP) material, the LiMnPO4 (LMP) is considered as a better candidate for rechargeable LIBs because of its larger potential window of 4.1 V versus that of Li/Li+ (3.4 V for LFP) [6]. However, the low electrical conductivity (<1010 S cm1) and the slow lithium-ion diffusion (<1016 cm2 S1) have limited its practical applications [7,8]. To overcome the drawbacks of LMP, many modification methods have been attempted, including particle-size minimization [9–11], Mn-site doping [12–14], and carbon coating [15–18]. The particle-size minimization method shortens the lithium-ion diffusion path and increases the electrode surface area for interfacial charge transfer that leads to improved
* Corresponding author. Tel.: +86 22 26582054. E-mail addresses: [email protected] (L. Ren), [email protected] (J. Li). http://dx.doi.org/10.1016/j.electacta.2016.07.049 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
ionic diffusion efficiencies. Ye et al. [19] have synthesized nano LiMn0.9Fe0.1PO4 by the solvothermal method and found that the introduction of cationic ions into the crystalline LMP can lead to stable crystal structure and excellent electrochemical performance. Hong et al. [20] have demonstrated the higher rate capability and cycle performance of LIBs cathode when Mn2+ is partially substituted by Fe2+. Carbon coating is an effective way to improve the electronic conductivity of LMP and its kinetic properties [21–24]. The carbon content, distribution and uniformity of the coverage, carbon surface area, and the degree of graphitization (the sp2/sp3 or graphitized/disordered ratio IG/ID) are the key factors to affect the electrochemical performance of LMP cathode [25–27]. However, the most conventional carbon sources and coating technologies are far from the requirements to yield high-performance cathodes. Graphene is alternatively exploited due to its high specific surface area and excellent conductivity [28], but graphene is difficult to be dispersed and its interaction with LMP particles is weak. Thus, direct mixing of graphene with LMP is not the best approach to improve the electrochemical performance of LMP cathodes. Metal compounds such as iron oxides and iron sulfates can guide and catalyze the formation of graphitized carbon from various carbon sources [29–33]. Amorphous carbon could nucleate and grow into graphene layers depositing on metal surfaces [34].
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Furthermore, LiFePO4/graphene was synthesized using glucose and FeSO4 as the carbon source and catalyst precursor, respectively [35]. High-quality carbon layers were also coated on LiFe0.6Mn0.4PO4/C cathode material, where both the carbon and metal catalyst were mixed uniformly and were randomly distributed among the cathode particles. After the calcination process, the graphene layers were grown not only on the surface of the cathode particles but also in places among particles. Besides, some graphene sheets stretched out from the cathode particles yielding no positive contribution to the electrochemical performance [36]. Therefore, the adding and spatial sequence of the metal catalyst and carbon sources is a very important factor in acquiring highly graphitized carbon coatings only on the surface of cathode materials. In this paper, LiMn0.8Fe0.2PO4 (LMFP) was synthesized by the solvothermal method. Iron oxalate was selected as metal catalyst precursor and mixed with LMFP for the synthesis of LMFP/Fe/ Carbon (LMFP/Fe/C). By adding iron oxalate, and glucose in the desired order, the mechanism of forming the uniform and highly graphitized carbon layers was particularly investigated. The synthesized materials were characterized by various analytical methods and their electrochemical performances as LIBs cathodes were measured. 2. Experimental 2.1. Synthesis of LMFP and LMFP/Fe/C composite The LMFP particles were synthesized by the solvothermal method. The Mn/Fe mixture (Mn/Fe = 4:1, n/n, MnSO4H2O (AR grade), FeSO47H2O (AR grade)) and H3PO4 (85 wt%)were dissolved
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in a mixture of ethylene glycol-water (5:3, v/v) as the solvent in presence of ascorbic acid as the reducing agent. LiOHH2O was then added dropwise into the mixture with a molar ratio of Li:Mn/Fe:P fixed at 3:1:1. After vigorous stirring for 30 min under a nitrogen atmosphere, the resulting suspension was transferred into a 200 ml Teflon-lined stainless steel autoclave and was heated at 240 C for 4 h. After the autoclave was cooled down to room temperature, the gray precipitate was washed off several times with water and ethanol. To prepare LMFP/Fe/C, iron oxalate as the catalyst precursor was mixed with LMFP and the molar ratio between PO43+ and iron oxalate was kept at 200:1. Subsequently, the mixture was dried at 200 C in vacuum for 4 h. The obtained powder (LMFP/FeO) was then mixed with glucose and the mass ratio between the powder and carbon contained in glucose was kept to 93:7. Finally, the mixture was calcined at 650 C for 8 h in a tube furnace under a nitrogen atmosphere. The composite material LMFP/Fe particles with carbon coating was abbreviated as LMFP/Fe/C. For comparative purposes, the LMFP particles with carbon coating (LMFP/C) without iron oxalate were also produced using the same method. 2.2. Characterization The phase purity and crystal structure of the samples were characterized by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer with Cu Ka radiation set between 5 and 80 at a scan rate of 5 /min. The morphology and structure were determined by a NanoSEM450 scanning electron microscope (SEM) and a Tecnai G2 F20 transmission electron microscopy (TEM). The carbon analysis was conducted by DXR Microscope Raman spectroscopy with a 532 nm wavelength laser.
Fig. 1. The schematic preparation of the LMFP/Fe/C. (a) Iron oxalate was converted to ferrous oxide and attached on surface the of LMFP particles during the drying process, (b) ferrous oxides were reduced to iron atoms by carbon during the sintering process, (c) under the catalytic guidance of Fe atoms, carbon atoms nucleated and grew into graphitized carbon layers on surfaces of the LMFP particles, (d) LMFP was coated with uniform graphitized carbon layers.
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2.3. Electrochemical measurements The working electrodes were prepared using the following mixture: 80% (wt%) of the active materials, 10% (wt%) Surper-P and 10% (wt%) polyvinylidene fluoride (PVDF) dissolved in Nmethyl pyrrolidinone (NMP). The slurries were coated on aluminum (Al) foil and dried at 120 C in a vacuum oven for 12 h to remove the solvent before pressing. The pellet was rolled to enhance adhesion between the electrode laminate and the Al foil then was punched into round disks of 14 mm in diameter. The electrochemical measurements were performed with CR2430 coin-type cells with lithium as anode in an electrolyte composed of (1 mol L1 LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethyl methyl carbonate (EMC) (EC:DMC:EMC = 1:1:1, v/v/ v)). The half-cell was assembled in a glove box filled with pure argon. After resting for 4 h, the charge-discharge tests were performed on a program-controlled Battery Test System (Land, Wuhan, China) using voltages ranging from 2.0 to 4.6 V at various constant current rates and the rate of charge and discharge kept the same. The cyclic voltammetry (CV) and electrochemical impedance measurements (EIS) measurements were performed on a CHI650 B Electrochemical Workstation (Chenghua, Shanghai, China). The CVs were measured in the potential window of 2.0 to 4.6 V and EIS over frequency ranges of 102–105 Hz at 5 mV excitation potential. 3. Results and discussion 3.1. Phase structure The LMFP/Fe/C composite was synthesized schematically displayed in Fig. 1. As shown in Fig. 1a, after LMFP particles were synthesized by solvothermal method, they were firstly mixed with iron oxalate. Then the mixture were heated to transfer iron oxalate into ferrous oxide (FeO) which attached to the surface of LMFP particles (1). The obtained LFMP/FeO powder was then mixed with glucose and dried. So far, FeO and glucose were sequentially coated on the surface of LMFP particles. Then the mixture was calcined in a tube furnace under a nitrogen atmosphere. Firstly, glucose was thermally carbonized to be amorphous carbon. FeO was then reduced to Fe by carbon during the calcination process (Fig. 1b). Secondly, Fe atom acted as the catalyst to nucleate the carbon atoms and which grow into graphitized carbon layers on the LMFP particles surfaces (Fig. 1c). This process was induced by the interaction between carbon and Fe atoms which rearranged the carbon atoms into graphitized carbon layers, and directly coated them on the surfaces of LMFP particles (Fig. 1d). Owning to the catalytic role of Fe, the degree of carbon graphitization was improved. Moreover, the carbon layers are quite thin and uniform. Meanwhile randomly graphitized carbon among the LMFP particles is hardly observed because there is no Fe outside of LMFP particles. Consequently, the overall quality of the carbon coating was optimized. All of these were very important for optimized cathode materials. (1)
The phase composition and crystallinity of the LMFP/Fe/C and LMFP/C materials were analyzed by XRD, and the results were shown in Fig. 2. All the diffraction peaks are in agreement with the standard LMFP (JCPDS NO.13-0336) with no additional visible peaks due to impurities. This indicates that LMFP/Fe/C and LMFP/C may be well-indexed to the orthorhombic LMFP phase with a Pmnb space group, and both the samples are readily prepared via the solvothermal method [37,38].
Fig. 2. XRD patterns of the LMFP/Fe/C and LMFP/C samples.
Fig. 3 presents SEM images of the LMFP/Fe/C and LMFP/C materials prepared by the solvothermal synthetic method and carbon coating procedure. Both samples synthesized in ethylene glycol-water mixed solvents show relatively uniform rod-like shape particles with sizes around 200 nm in diameter. The addition of iron oxalate and glucose do not influence the particle morphology. LMFP/Fe/C (Fig. 3a) showed clear particles surrounded with homogeneous carbon layers. Almost all carbon were coated on the surface of LMFP particles. Few graphitized carbon films could be observed which were outside of LMFP/Fe/C. Compared with LMFP/Fe/C, LMFP/C particles depicted in Fig. 3b are not as clear as LMFP/Fe/C and they are mixed together with pellet carbon layers. There are more carbon layers among particles. Therefore, the addition of iron oxalate promoted the directional growth of the carbon layers, resulting in highly graphitized carbon that tightly packs the particles. TEM and HRTEM measurements were utilized to further investigate the microstructure of LMFP/Fe/C and LMFP/C. Fig. 4a and b illustrate TEM and HRTEM images for the LMFP/Fe/C. The graphitized carbon layers of 2.2 nm thickness were uniformly and tightly coated on the surfaces of LMFP/Fe/C particles. The lithium ions can easily cross the thin carbon layers into the framework of LMFP [39]. Compared to LMFP/Fe/C, the carbon layers deposited on the LMFP/C are thick and non-uniform (Fig. 4c and d). Moreover, some carbon films were stretched out from LMFP particles which have no positively contribute to the electrochemical performance (Fig. 4c). Depending on location, the thicknesses of the carbon layers vary between 4.1-5.7 nm on different parts of the particles. Hence, high-quality carbon coatings are obtained by adding iron oxalate as the catalyst precursor. The synthesized LMFP/Fe/C and LMFP/C materials were subjected to further characterization by Raman spectroscopy, a sensitive technique for probing carbon structures. As shown in Fig. 5, the peak located at 951 cm1 is assigned to the stretching vibration in PO43 group while the two strong peaks at 1350 cm1 and 1610 cm1 are attributed to D-band (disorder band) and Gband (graphitized band) of carbon, respectively [40,41]. Both samples showed strong G band in comparison with D band, indicating the highly ordered graphitized structures of the samples. The IG/ID intensities of LMFP/Fe/C and LMFP/C are respectively 1.20 and 1.11, suggesting that iron can improve the quality of the carbon layers and the degree of graphitization [42]. 3.2. Electrochemical performance To demonstrate the benefit of adding iron oxalate on the electrochemical performance, the LMFP/Fe/C and LMFP/C
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Fig. 3. SEM images of (a) LMFP/Fe/C and (b) LMFP/C samples.
Fig. 4. (a) TEM and (b) HRTEM images of LMFP/Fe/C, (c) TEM and (d) HRTEM images of LMFP/C.
electrodes containing 7 wt% carbon are subjected to chargedischarge testing in the voltage window of 2.0-4.6 V at different rates. As shown in Fig. 6a and b compare the initial discharge profiles of both materials at 0.2, 0.5, 1, 5 and 10C (1C = 170 mAh g1) at 25 C. It can be seen that all the electrodes exhibit two discharge voltage platforms related to the Mn3+/Mn2+ and Fe3+/Fe2+ redox couples. However, those corresponding to LMFP/Fe/C (Fig. 6a) display higher and longer potential plateau at respective discharge rate. This indicates that the LMFP/Fe/C possesses an easier twophase transformation process between the LMFP and Mn0.8Fe0.2PO4 (MFP) compared to LMFP/C. As the discharge rates increase, the LMFP/Fe/C delivers respective discharge capacities of 152.3, 141.9, 132.1, 105.6 and 76.0 mAh g1 at the first cycle, whereas the discharge capacities obtained with LMFP/C are 150.4, 138.9, 128.4, 85.9 and 59.0 mAh g1 under the same conditions. The specific discharge capacities of LMFP/Fe/C recorded at different discharge rates look improved if compared with those obtained with LMFP/C, especially at high rates. Furthermore, the cycle performance of LMFP/Fe/C is better than that of LMFP/C at corresponding charge-discharge rates (Fig. 6c and d). The rule of
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Fig. 5. Raman spectra of the LMFP/Fe/C and LMFP/C samples.
the capacity retention is summarized in Fig. 6e, showing that the capacity retention of LMFP/Fe/C is about 78.6% at 10C after 60 cycles while that of LMFP/C is only 39.1% under same conditions. The coulombic efficiency fades of the LMFP/C is much faster as the rate increases (Fig. 6f). The improved electrochemical activity and rate performance of LMFP/Fe/C are attributed to the compact and uniformly formed carbon layers on the LMFP surfaces, which enhances the effective utilization of the active materials [43]. The improved cycling stability is attributed to the highly graphitized carbon layers, which reduces the stranded lithium ion in carbon layers during the cycle and ensure the enough lithium ion transference number in the charge and discharge process. The optimized carbon layers are attributed to the catalysis of iron. The addition of iron to improve the quality of the carbon coatings is an effective way to increase the cycle stability and coulombic efficiency of LMFP. The CVs of the LMFP/Fe/C and LMFP/C samples were measured at voltages ranging from 2.0-4.6 V at different scan rates (0.1-0.5 mV s1) and under 25 C (Fig. 7). Both the materials exhibit two pairs of redox peaks corresponding to Fe2+/Fe3+ and Mn2+/Mn3+ [44,45]. The LMFP/C shows broader redox peaks, suggesting the lower electrode kinetics. The LMFP/Fe/C, on the other hand, depicts sharper and stronger redox peaks, indicating improved electrode kinetics. The estimated voltage separation between the redox peaks corresponding to Fe2+/Fe3+ in LMFP/Fe/C and LMFP/C are 0.18/0.42, 0.29/0.49, 0.37/0.60, 0.44/0.66 and 0.49/
0.72 V at scan rates of 0.1, 0.2, 0.3, 0.4 and 0.5 mV s1, respectively. The redox peaks corresponding to the Mn2+/Mn3+ in LMFP/Fe/C and LMFP/C are respectively 0.42/0.68, 0.56/0.81, 0.63/0.96, 0.73/0.97 and 0.78/0.99 V at the same scan rates. The smaller voltage separation suggests a better reversibility of the lithium insertion/ extraction processes in LMFP/Fe/C. The excellent electrochemical performance obtained with LMFP/Fe/C is attributed to the carbon layers completely and firmly coating on the particles, resulting in elevated graphitization degree, reduced polarization and improved electrical conductivity. It is due to iron oxalate, which guides the formation of the graphitized carbon during the calcination process. EIS is an effective way to examine the kinetic factors underlying the electrochemical performance. The Nyquist plots of the LMFP/ Fe/C and LMFP/C materials in the frequency range of 102–105 Hz and 5 mV excitation potential are shown in Fig. 8a. The Nyquist plots are measured at full charge state after 30 cycles at 0.2C. The spectra depict depressed semi-circle at high-to-medium frequency and an inclined line in the low-frequency region. The intersection of Z0 axis at higher frequency refers to the ohmic resistance (Rs) of electrodes and electrolyte. The semicircle at high-to-medium frequency corresponds to the charge transfer resistance (Rct) at the interface between the cathode and the electrolyte. The inclined line observed at low-frequency is attributed to the Warburg impedance (Ws), which restricts the lithium ion diffusivity within the samples [46,47]. The plots are fitted by Z-view based on the equivalent circuit inserted in Fig. 8a. The LMFP/Fe/C electrode
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Fig. 6. The first discharge profiles of (a) LMFP/Fe/C and (b) LMFP/C, the cycling performance of (c) LMFP/Fe/C and (d) LMFP/C, (e) the capacity retention of LMFP/Fe/C and LMFP/C at different rates, (f) the coulombic efficiency of LMFP/Fe/C and LMFP/C at the first charge-discharge at different rates.
shows lower Rs and Rct (Table 1), indicating a reduced charge transfer polarization resistance for LMFP/Fe/C. The conductivity and dynamic of delithiation/lithiation processes are substantially improved due to the addition of iron oxalate. The apparent lithium-ion diffusion coefficient (DLi) of the electrodes is calculated from the inclined lines at low-frequency using Eqs. (2) and (3) [48]. D = (R2T2)/(2A2n4F4C2s2)
(2)
Z0 = Rs + Rct + sv1/2
(3)
Where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons transferred, F is the Faraday constant, C is the concentration of the lithium ion, and s is the Warburg factor associated to Z0 obtained by linear fitting the plot between Z0 and reciprocal square root of the angular frequency v (Fig. 8b). The resulting lithium-ion diffusion coefficients of the two electrodes are compared in Table 1. The DLi of LMFP/Fe/C (2.15 1014 cm2 s1) is larger than that of LMFP/C (1.65 1014 cm2 s1). The good lithium ion diffusion obtained with LMFP/Fe/C is attributed to its excellent formed carbon layers. Due to the uniform and a highly graphitized carbon layers, the lithium ions
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Fig. 7. The CV curves of (a) LMFP/Fe/C and (b) LMFP/C at different scan rates.
can easily cross them into the framework of LMFP, hence improving the ionic diffusion efficiency.
5. Conclusion An iron-assisted carbon coating strategy is developed to guide the formation of uniform and the highly graphitized carbon layer on LMFP particles. Iron oxalate and glucose were sequentially coated on LMFP from inside to outside. During the calcination process and thanks to the presence of Fe atoms, uniform and a highly compacted graphitized carbon layers are formed on surfaces of the LMFP nanoparticles. Compared to LMFP/C, the LMFP/Fe/C material exhibited a better discharge capacity and better rate and cycling performances. The LMFP/Fe/C showed excellent discharge capacities of 152.3, 141.9, 132.1, 105.6 and 76.0 mAh g1 at 0.2, 0.5, 1, 5 and 10C, respectively. Even at increased rate of 10C, the retention capacity remained 78.6% after 60 cycles. The LMFP/Fe/C also displayed improved conductivity and lithium-ion diffusion, promising for high-performance electrochemical LMFP materials at lower costs.
Acknowledgment This work was partially supported by National Nature Science Foundation of China (51203041).
Fig. 8. (a) EIS of the LMFP/Fe/C and LMFP/C samples after 30 cycles at 0.2 C and the corresponding equivalent circuit used for fitting the experimental EIS data, (b) the relationship between Z0 and the square root of the frequency (v1/2) in the lowfrequency region.
Table 1 The simulation results of the electrochemical impedance and lithium ion diffusion coefficients. Sample
Rs (V)
Rct (V)
s
DLi (cm2 s1)
LMFP/Fe/C LMFP/C
8.52 8.80
34.26 78.67
37.76 43.07
2.15 1014 1.65 1014
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