Enhancement of the Rate Capability of LiFePO4 by a New Highly

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Enhance the rate capability of LiFePO4 by a new highly-graphitic carbon coating method Jianjun Song, Bing Sun, Hao Liu, Zhipeng Ma, Zhouhao Chen, Guangjie Shao, and Guoxiu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02567 • Publication Date (Web): 30 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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ACS Applied Materials & Interfaces

Enhance the rate capability of LiFePO4 by a new highly-graphitic carbon coating method Jianjun Song, †,‡,§ Bing Sun,‡ Hao Liu, ‡ Zhipeng Ma, † ,§ Zhouhao Chen, † ,§ Guangjie Shao,*,† ,§ and Guoxiu Wang*,‡ †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, China. ‡

Centre for Clean Energy Technology, School of Mathematics and Physical Sciences, Faculty of

Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia. §

Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical

Engineering, Yanshan University, Qinhuangdao 066004, China

ABSTRACT: Low lithium ion diffusivity and poor electronic conductivity are two major drawbacks for the wide application of LiFePO4 in high-power lithium ion batteries. In this work, we report a facile and efficient carbon coating method to prepare LiFePO4/graphitic carbon composites by in-situ carbonization of perylene-3, 4, 9, 10-tetracarboxylic dianhydride during calcination. Perylene-3, 4, 9, 10-tetracarboxylic dianhydride containing naphthalene rings can be easily converted to highly-graphitic carbon during thermal treatment. The ultrathin layer of highly-graphitic carbon coating drastically increased the electronic conductivity of LiFePO4. The short pathway along [010] direction of LiFePO4 nanoplates could decrease the Li+ ion diffusion

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path. In favor of the high electronic conductivity and short lithium ion diffusion distance, the LiFePO4/graphitic carbon composites exhibit an excellent cycling stability at high current rates at room temperature and superior performance at low temperature (-20 oC).

KEYWORDS: Graphitic carbon, low temperature, high rate, electronic conductivity, lithium iron phosphate.

1. Introduction In recent years, lithium ion batteries (LIBs) have been widely used to power portable electronic devices such as mobile phones, laptops, and personal digital gadgets. However, the current LIBs technology must be further advanced for enhancing the energy and power densities to expand the application in all-electric vehicles (EVs) and hybrid electric vehicles (HEVs). As a LIBs cathode material, olivine structured lithium iron phosphate (LFP) has become one of the most promising candidates in the field of battery research since the first report by Goodenough et al. in 1997.1 Compared with commercial layered LiCoO2 material, LFP possesses many advantages such as superior capacity retention, environmental friendliness, low cost, high thermal stability and nontoxicity.1-3 The major problems of LFP however originate from the fact of its low lithium ion diffusivity and poor electric conductivity,1, 4 which lead to its undesirable high-rate and low-temperature performances, retarding its wide applications in energy storage systems. Various methods have been proposed to addressing these issues, including surface coating with electronic conductive layer,2, 5-7 metal doping,8-9 and optimizing morphology and particle size.10-13


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Carbon coating has been extensively regarded as an efficient way to enhance the conductivity of battery material. A thin uniform carbon layer can remarkably improve the performance of LFP. Numerous research has proved that homogeneous carbon coating layers can be achieved by organic carbon sources, like glucose and citric acid.14-16 We consider that they are advantageous in having a small wetting angle and a good extension with LFP surface during the carbonization process at high temperatures. The in-situ coating methods can also form a uniform carbon layer on LFP through the self-polymerization process.16-17 Chi et al. synthesized a uniform carbon nanoshell on LFP via the 3-aminophenol-formaldehyde polymerization process, which could deliver high discharge capacities of 117 and 110 mAh g-1 at 5 and 10 C rates, respectively.18 Our previous study also showed that homogeneous carbon coating can be achieved by an esterification reaction between ethylene glycol and citric acid, which evidently ameliorated the electrochemical performances of LFP.11, 19 However, most of the above-mentioned carbon layers are amorphous carbon. The structure of the carbon can notably affect the electrochemical performance of LFP/C composites. Compared with amorphous carbon, graphitic carbon is more beneficial for contributing to the electronic conductivity of LFP. Therefore, many researchers have attempted to combine LFP with highly graphitic carbon materials, such as carbon nanotubes (CNTs) and graphene, to boost its electronic conductivity.4,

20-29

Yang et al. succeeded in

synthesizing one-dimensional core-shell LFP@CNT nanowire composites by a facile sol-gel route. The composites could deliver a capacity of 160 mAh g-1 at 17 mA g-1, and 65 mAh g-1 at a high current density of 8500 mA g-1.30 Zhu and his co-workers prepared a LFP/rGO hybrid using a homogeneous coprecipitation method combined with thermal treatment. Such a composite exhibited a high specific capacity of 166 mA h g-1 at 0.6 C, and the capacity remained 139 mA h g-1 at 11.8 C.31 Nevertheless, LFP was not conformably encapsulated by the graphitic carbon


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in most of above composites, and the uncoated areas remained vulnerable to low conductivity.32 Considering that controlling crystal orientation along the (010) facet can effectively enhance lithium ion diffusion of LFP by decreasing the Li+ diffusion length,33 the in-situ uniform coating of graphitic carbon onto LFP nanoplates with highly oriented (010) facets is greatly desired for achieving excellent electrochemical performance. Herein, we report a facile and efficient carbon coating method to prepare highly-graphitic carbon-wrapped, (010) facet-orientated LFP nanoplates (LFP/GC) through in-situ carbonization of Perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA), and their application as a high power LIBs cathode material. PTCDA with naphthalene rings can be easily converted to highlygraphitic carbon during heat treatment, as shown in Figure 1.34 To the best of our knowledge, there is no report on its application on LIBs by now. In this case, LFP nanoplates are uniformly coated by ultrathin graphitic carbon layers (2-3 nm). The thin homogeneous graphitic carbon layer can evidently facilitate a higher electrochemical reactivity and reversibility by ensuring good electronic conductivity and fast lithium ion diffusion for LFP electrodes. Due to the high electronic conductivity and improved lithium ion diffusion, the LFP/GC composites delivers outstanding cycling performance and rate capability with a discharge capacity of 143.8 mAh g-1 at the 5 C after 300 cycles, corresponding to 97.9% capacity retention. 2. Experimental 2.1 Preparation of electrode materials All chemicals used were analytical grade without further purification. LFP nanoplates were synthesized through a solvothemal method, and ethylene glycol (EG) was used as solvent. Typically, LiOH (0.045 mol) was firstly dissolved in 25 ml EG. Afterwards, H3PO4 (0.015 mol)


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was slowly introduced into the solution. Then 25 ml FeSO4 (0.015mol) solution was added and stirred for another 30 min. After that, the obtained mixture was transferred into a 100 mL Teflonlined stainless steel autoclave and heated at 180 oC for 18 h. After the autoclave was cooled down naturally to room temperature, the obtained products were washed with ethanol and deionized water for several times and dried at 80 oC overnight. The LFP/GC was synthesized by a simple ball milling method. Typically, LFP nanoplates (0.5 g) and PTCDA (0.11 g) were intensively mixed through a ball milling process at 240 rpm for 5 h, and then heated at 700 oC for 4h under a nitrogen atmosphere. In this calcination process, the LFP/PTCDA could be converted to LFP/GC by in-situ carbonization of PTCDA. The synthesis process is schematically illustrated in Figure 1. To investigate the effect of the graphitic carbon formed by PTCDA, the carbon-coated LFP composite through the esterification reaction between EG and citric acid were also synthesized for comparison. These have shown fairly good electrochemical performance in previously reported work.11 Typically, 0.13 g citric acid was firstly dissolved in 10 ml de-ionized water, and then 0.5g LFP nanoplates and 0.15 g EG were added. The mixture was kept at 90 oC to obtain a dark blue gel. Finally, the gel was heat treated at 700 oC for 4 h in nitrogen atmosphere. 2.2 Material characterizations X-ray diffraction (XRD) was performed with a Rigakud/MAX-2500/pc X-ray diffractometer with a scanning speed of 5° min-1 in the 2θ range from 15 to 55°. The carbon contents of asprepared samples were calculated based on themogravimetric Analysis (TGA) (Pyris Diamond, PerkinElmer Thermal Analysis), which were carried out from room temperature to 700 °C under air atmosphere at the rate of 10 °C min-1. The graphitization degree of the carbon coating was


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investigated by high-resolution transmission electron microscope (model JEM2010) and Raman spectroscopy (Renishaw, InVia Raman Microscope). Scanning electron microscope (SEM) was carried out in Zeiss Supra 55VP at 10 KV. 2.3 Electrochemical measurements The working electrodes for electrochemical testing were prepared by mixing 80 wt. % active materials, 10 wt. % poly(vinylidene fluoride) (PVDF) and 10 wt. % acetylene black in N-methyl2-pyrrolidone (NMP) solvent. The obtained slurry was spread onto aluminium foil, dried in a vacuum oven at 120 oC overnight. The loading of active material is about 1.5 mg cm-2. The electrolyte for cells consisted of 1 M LiPF6 in EC/DEC (1:1, v/v). The cells were charged and discharged over a potential range of 2.4 V-4.2 V (versus Li/Li+) on a LAND CT2001A testing system. The specific capacities were calculated based on the total mass of the composites. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested on CHI 660E electrochemical workstation. CVs were carried out at a scanning rate of 0.5 mV s-1 between 2.4 and 4.2 V. Electrochemical impedance spectroscopy (EIS) were carried out by applying an amplitude of 5 mV over a frequency range from 100 kHz to 0.01 Hz. 3. Results and discussion The X-ray diffraction (XRD) patterns of the LFP/C and LFP/GC composites are shown in Figure 2a. The diffraction peaks of samples are well indexed to an orthorhombic olivine structure with the space group Pnma (JCPDS 83-2092), and no other impurity phases are detected. Raman spectra of the obtained composites show intense D (disordered) and G (graphitic) bands (Figure 2b). The D band originates from the disorder in sp2-type carbon, and the G band can be associated with the tangential stretching (E2g) mode of graphite. The peak area


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ratio of D band to G band (AD/AG) can be used as estimation of the graphitization degree. Obviously, LFP/GC composites exhibit a lower value of AD/AG (2.17) than that of LFP/C (3.09). The results well illuminate that the carbon layer resulting from in-situ carbonization of PTCDA shows a higher graphitization degree, and thus it can improve the electronic conductivity of the composites. The carbon contents of LFP/C and LFP/GC composites were analysed by TG analysis (Figure S1) and calculated to be 4.2 wt. % and 3.7 wt. %, respectively. The morphology and particle size of the as-prepared composites are investigated by SEM and TEM. Figure 3a shows that the LFP are composed of uniform nanoplates with a small thickness (about 25 nm), a length in the range of 100-120 nm and a width of about 50-70 nm. The investigation of its crystal orientation by selected area electron diffraction (inset of Figure 3b) proves that the exposing crystal face is (010) plane, so the short length along the b-axis is beneficial to the rapid Li+ transport during Li+ intercalation/deintercalation process. Both LFP/C (Figure 3c) and LFP/GC (Figure 3e) retain plate-like morphology and similar particle size after carbon coating. From the HRTEM image in Figure 3d, we can observe that the LFP/C nanoplates were coated by an amorphous carbon layer with the thickness about 2-3 nm. On the contrast, the LFP/GC were covered by a homogeneous graphitic carbon layer with similar thickness (Figure 3f and Figure S2). The HRTEM results clearly demonstrate that the PTCDA can provide a uniform and highly-graphitic carbon coating layer on the surface of LFP nanoplates, which significantly improves the electrochemical performance of LFP. The interplanar spacings of 0.47 nm in the HRTEM images is well matched with the d-spacing of (001) planes of LFP, and perfect lattice fringes prove the good crystalline state of the sample. Figure 4 shows the charge/discharge curves of LFP/C and LFP/GC and the corresponding cycling performance at various rates. The LFP/C electrodes present discharge capacities of


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129.4, 123.8, 119.1, 103.6, 89.1 and 75.9 mAh g-1 at 0.5, 1, 2, 5, 10 and 20 C, respectively. While the LFP/GC electrodes deliver 159.8, 156.9, 153.1, 143.5, 127.1 and 106.9 mAh g-1 at the corresponding C-rates. Noticeably, LFP/GC electrode possesses much higher reversible capacity and more flat potential plateaux than that of LFP/C, especially at high rate. The in-situ generated highly-graphitic carbon can ensure the electrons transfer and alleviate the polarization in the charge/discharge process, thus greatly enhancing the electrochemical performance. The longterm cycling performance of LFP/GC electrode at higher current rates is shown in Figure 4d. The LFP/GC nanoplates exhibit superior cycling stability with a discharge capacity of 143.8 mAh g-1 at the 5 C after 300 cycles, corresponding to 97.9% capacity retention. Even at high rates of 10 C and 20 C, it still shows high discharge capacities of 131.4 mAh g-1 and 103.3 mAh g-1 after 300 cycles, corresponding to 77.3 % and 60.8% of theoretical capacity. Figure S3 compares the electrochemical performance of LFP/GC composites with some previously reported graphitic carbon-modified LFP materials. Those materials delivered fairly good rate performance by modifying LFP with CNTs and graphene.4,

21-27, 30-31

Clearly, the LFP/GC

nanoplates exhibits a better performance than most of those composites because of the conformal coating of graphitic carbon. As with high-rate performance of an electrode material, the low-temperature capability is also significant among increasing demands of LIB applications. Figure 5 shows the rate capability and typical charge/discharge profiles of the LFP/C and LFP/GC at -20 oC. The LFP/GC electrode can deliver a high discharge capacity of 145.3 mAh g-1 at 0.1 C, while LFP/C only presents 127.9 mAh g-1 at the same current density. With the increase of current density, the LFP/GC displays more outstanding performance with initial discharge capacities of 133.4 and 119.5 mAh g-1 at 0.2 and 0.5 C. Even at 2 C, it still can deliver a high discharge capacity of 82.7


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mAh g-1. The low temperature performance of as-prepared LFP/GC composites are also better than previously reported values.35-38 Obviously, the low temperature performance can also be enhanced by the uniform graphitic carbon coating. Compared with other LFP/C composites, the admirable performance of LFP/GC composite can be attributed to the uniform coating of graphitic carbon resulted from the in-situ carbonization of PTCDA. Different from CNT and graphene, PTCDA can homogeneously disperse on the surface of each LFP nanoplate. During heat treatment, it can be converted in-situ to a uniform highly-graphitic carbon layer due to its naphthalene ring structure. In this case, every LFP nanoplate can be conformably encapsulated by a thin graphitic carbon layer. Obviously, this is an ideal carbon coating because the uncoated areas will remain vulnerable to the low conductivity for LFP in those heterogeneous coatings. The uniform coating of highly-graphitic carbon can markedly enhance the electronic conductivity of electrode material and ensure sufficient electron supply, which can play a great effect on the rate performance of LFP.39 Meanwhile, lithium ions can be easily inserted or extracted in the framework of LFP through the thin graphitic carbon during the intercalation and deintercalation process.30-31 Conceptually, the crucial step during the charging/discharging processes is that a transferred electron must be reciprocally compensated by the extraction/insertion of Li+ to keep the total charge balanced. If a transferred electron cannot meet a diffusing Li ion, it will cause a surplus of Li+. And thereafter, it will limit the diffusion of the following lithium ion owing to the electrostatic repulsion. Hence, the improved electronic conductivity by the uniform coating of graphitic carbon can also promote the lithium ion diffusion at the same time. From the high electronic conductivity and enhanced Li+ diffusivity, superior rate capability and cycling performance was obtained.


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The Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were used to further analyze electrode performance of LFP/GC nanoplates, as shown in Figure 6. The higher redox peak values of the LFP/GC nanoplates suggest its higher redox kinetics activity due to enhanced electronic conductivity by highly-graphitic carbon coating. The LFP/GC electrode also exhibits a smaller potential separation between reduction and oxidation peaks than that of LFP/C, which is associated with its higher reversibility. Both Nyquist plots of LFP/C and LFP/GC nanoplates present a depressed semicircle in the moderate frequency region and a straight line in the low frequency region, which are relevant to a charge transfer process and a Warburg diffusion process, respectively. The fitted Nyquist plots and the plot between Z′ and reciprocal root square of the lower angular frequencies (ω-1/2) are shown in Figure S4. The LFP/GC nanoplates exhibit a lower charge transfer resistance (Rct, 136 Ω) than that of LFP/C nanoplates (371 Ω) owing to improved electronic conductivity by graphitic carbon coating. The lithium ion diffusion coefficients of LFP/GC and LFP/C were calculated to be 1.67×10-13 and 4.99×10-14 cm2 s-1 (see Supporting Information Figure S4b for more details). The EIS results clearly proved that the uniform graphitic carbon coating can remarkably improve the electronic conductivity and lithium ion diffusivity, and thus remarkably enhanced electrochemical performance of LFP/GC. 4. Conclusions Uniform graphitic carbon coated LiFePO4 nanoplates were successfully synthesized through the in-situ carbonization of Perylene-3, 4, 9, 10-tetracarboxylic dianhydride by calcination. The as-prepared composites exhibited superior electrochemical performances with outstanding cycling stability at high rates at room temperature and remarkable low temperature performance. The excellent electrochemical performances can be ascribed to the uniform and ultrathin


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highly-graphitic carbon coating layer, which noticeably enhances the electron transport and Li+ diffusion during the charge/discharge process. Therefore, the graphitic carbon coating through in-situ carbonization of Perylene-3, 4, 9, 10-tetracarboxylic dianhydride is an effective way to improve the electrochemical performance of LiFePO4 for high power application. Furthermore, other compounds containing naphthalene rings can also be used as applicable graphitic carbon resources for the synthesis of high-performance lithium ion battery materials. ASSOCIATED CONTENT Supporting Information. Additional TEM image, TGA and EIS analysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Prof. Guangjie Shao. Tel.: +86-335-8061569; Fax: +86-335-8059878. E-mail: [email protected] Prof. Guoxiu Wang. Tel.: +61 2 95141741; Fax: +61 2 95141460. E-mail: [email protected] Acknowledgements This project was financially supported by the Natural Science Foundation of State key Laboratory of China and partially supported by China Scholarship Council (No. 201508130079) and the Postgraduate Innovation Project of Hebei Province (00302-6370013). References


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Figure 1 Schematic illustration of the carbonization and preparation process for the LFP/graphitic carbon composites.


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Figure 2 XRD profiles (a) and Raman spectra (b) of LFP/C and LFP/GC composites.


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Figure 3 Typical SEM images of (a) LFP, (c) LFP/C and (e) LFP/GC nanoplates. (b) TEM image of LFP nanoplates and the corresponding selected area electron diffraction (inset). HRTEM image of LFP/C nanoplates (d) and LFP/GC nanoplates (f).


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Figure 4 Initial charge/discharge curves of LFP/C (a) and LFP/GC (b) electrodes at various rates. (c) Rate performance of LFP/C and LFP/GC electrodes. (d) Cycling performance of LFP/GC nanoplate electrode at 5, 10 and 20C, respectively.


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Figure 5 Typical charge-discharge profiles of the LFP/C (a) and LFP/GC (b) electrodes at -20 o C. (c) The rate capability of the LFP/C and LFP/GC electrodes at -20 oC.


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Figure 6 (a) Cyclic voltammetry cures of LFP/C and LFP/GC electrodes with a scanning rate of 0.5 mV s-1. (b) Nyquist plots of LFP/C and LFP/GC electrodes


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TOC Figure


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Enhancement of the Rate Capability of LiFePO4 by a New Highly

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