Article pubs.acs.org/IECR
Facile Synthesis and Electrochemical Performance of LiFePO4/C Composites Using Fe−P Waste Slag Hanchang Kang, Guixin Wang,* Heyi Guo, Miao Chen, Chunhui Luo, and Kangping Yan College of Chemical Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: A facile synthesis route has been developed to prepare LiFePO4/C composites by using Fe−P waste slag from the industrial production of yellow phosphorus. The processes included reclaiming Fe and P in the form of a ferroalloy, preheating the Fe−P with lithium salts, and complementary phosphorus source in air to produce a precursor, and calcining the precursor with glucose in Ar to obtain the products. The reaction process and electrochemical performance were investigated with various techniques. LiFePO4/C with 5.9 wt % carbon exhibits enhanced power capability, low polarization, high reaction activity and reversibility. The discharge capacities are 150, 147, 131, 124, 112, and 93 mAh/g at different current rates of 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively. The recovery rate at 0.1 C is 98.9% after 130 cycles at the given rates. The results are comparable to that of the LiFePO4/C prepared using FePO4 or other Fe salts, which indicates the applicability of the novel simple way put forward in this work to convert industrial waste into energy materials for scaling up based on low cost.
1. INTRODUCTION With the continuous decrease of fossil resources and increase of environmental pollution, electric vehicles become more important in the energy economy. Because of the unique advantages such as safe characteristics, thermal stability, and environmental compatibility, olivine-type LiFePO4 is a promising cathode material of lithium ion batteries for electric vehicles, power tools, or standby power sources.1−9 The main limitations of LiFePO4 are the low electronic conductivity5,10 and the slow lithium ion diffusivity through the LiFePO4/ FePO4 interfaces.11,12 Therefore, various carbonaceous materials, particularly organic carbon sources, are extensively applied to improve the electrochemical performance of LiFePO4.9,13−17 LiFePO4 are traditionally synthesized by using iron oxides such as FeC2O4·2H2O18,19 or Fe2O3.20,21 To improve the mixing uniformity of iron and phosphorus salts, FePO422−24 is popularly used in current production of LiFePO4. However, the high cost of FePO4 is the main obstacle to the wide applications of LiFePO4. Therefore, to develop novel raw materials for the synthesis of LiFePO4 with low cost is of great concern. As an intermetallic compound with the elements of Fe and P mixing well, Fe−P alloy has the similar composition with FePO4 except for oxygen, which makes it favorable to the production of LiFePO4. Furthermore, Fe−P alloy can be obtained plentifully from the commercial manufacture of yellow phosphorus.25−27 Its quantity of output in China had amounted to over 140 thousand tons in 2010 (0.1−0.2 ton of Fe−P can be generally obtained from the production of 1 ton of yellow phosphorus depending on the composition of the used raw materials), which leads the cost of such Fe−P being no more than 500 $/ton, far lower than that of current commercial FePO4 (∼4000 $/ton). In addition, such Fe−P normally has 50−75 wt % Fe, 18−30 wt % P, and a minor amount of impurities such as Ca, Mn, V, Ti, and Si depending on the used raw materials.25−27 Consequently, Fe−P alloy is a competitive candidate for the synthesis of LiFePO4 from the points of © 2012 American Chemical Society
resource recycling, economic benefit, and ecological environment. The oxidization of P plays an important role in the synthesis of LiFePO4 because Fe−P alloy has no elemental oxygen and the valence of the P is low. Our early research results confirmed that it is feasible to synthesize LiFePO4 using Fe−P via oxygen permeation,28 but the precise control of oxygen content makes it difficult for the scale production of LiFePO4. To solve these problems, a facile method is put forward to synthesize LiFePO4/C by preheating the Fe−P mixture in air and calcining with glucose in Ar. The electrochemical performance of the as-prepared LiFePO4/C and the effects of carbon content were investigated.
2. EXPERIMENTAL SECTION 2.1. Materials Used in the Experiment. Fe−P powder applied in this work was provided by a chemical factory in Southwest China, which comes from the phosphorus waste slag in the electrothermal reduction process for producing yellow phosphorus. The phosphorus waste slag was first separated into Fe−P slag (mainly Fe−P alloy) and furnace slag (mainly CaSiO3 and Ca3Si2O7) by the method of melting crystallization. Fe−P slag was further ball milled and gas flow crushed to obtain Fe−P powder with a particle size below 2000 mesh (about 8.0 μm), which is discussed in the section “results and discussion” in detail. Other raw materials, including Li2CO3 (battery grade, ∼99.65%), LiOH·H2O (battery grade, ∼99.2%), NH4H2PO4 (AR, ∼99.0%), and D-glucose (AR, ∼99.0%), were used to synthesize LiFePO4/C composites. 2.2. Materials Characterization. The chemical composition of the Fe−P powder was qualitatively analyzed by X-ray fluorescence (XRF, Shimadzu XRF-1800, Japan). Considering Received: Revised: Accepted: Published: 7923
January 10, 2012 May 15, 2012 May 21, 2012 May 21, 2012 dx.doi.org/10.1021/ie300088p | Ind. Eng. Chem. Res. 2012, 51, 7923−7931
Industrial & Engineering Chemistry Research
Article
Figure 1. Synthesis route of LiFePO4/C samples using Fe−P waste slag.
the second precursor was calcined at ∼700 °C for 5−7 h in a quartz tube furnace full of Ar. A clean route can be achieved by recycling the by-produced NH3, CO2, and CO to synthesize the valuable NH4HCO3 fertilizer and Li2CO3, and the synthesis route is described in Figure 1.
the half-quantitative characteristics of the XRF results, the molar ratio of Fe to P, significant for the matching ratio of raw materials for the synthesis of LiFePO4/C, was further quantitatively determined by inductively coupled plasmaatomic emission spectrometry (ICP−AES, Thermo Electron Corporation, USA). The solution for ICP−AES analysis was obtained by dissolving the Fe−P in hot aqua regia. The phase structures of the Fe−P powder and the synthesized samples were examined by X-ray diffraction (XRD, Philips X’Pert Pro, Holland) with a step of 0.04°/s within the range of 10−65° using Cu Kα radiation at the power of 40 kV × 100 mA. The morphology, particle size, crystalline properties, surface structure, and element composition of the samples were observed by using field-emission scanning electron microscopy (HITACHI S-4800, Japan), high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-TWIN, USA), and selected area electron diffraction (SAED). The HRTEM system is equipped with electron diffraction and energy dispersive X-ray spectroscopy. The carbon content in the synthesized samples was determined on a carbon−sulfur analyzer (CS-902, China). 2.3. Facile Synthesis of LiFePO4/C Composites Using the Fe−P. According to the result of ICP−AES, the atomic ratio of Fe to P in the Fe−P powder is 1.5, and the compositional formula of such Fe−P was named as Fe1.5P for simplification. The Fe1.5P powder was mixed well with Li2CO3, LiOH·H2O and NH4H2PO4 in a molar ratio of 2:1:1:1 according to reaction 1. The mixture was ground thoroughly with a dispersive solution of ethanol to form the first rheological phase precursor in an agate mortar, which was thereafter transferred to a stainless steel plate and preheated at 500−550 °C for 4−6 h in a muffle oven in air. After being cooled to ambient temperature, the preheated precursor was further finely ball-milled with different amounts of glucose in ethanol to form the second rheological phase precursor. Finally,
2Fe1.5P + Li 2CO3 + LiOH·H 2O + NH4H 2PO4 + 4O2 → 3LiFePO4 + NH3 ↑ + CO2 ↑ + 3H 2O↑
(1)
2.4. Electrochemical Measurements. The electrochemical performance was investigated at room temperature by using 2032 coin cells via recording galvanostatic charge/discharge curves, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The as-synthesized LiFePO4/C composite powders were finely ground and then mixed well with 15 wt % conductive acetylene black and 5 wt % commercial LA132 binder (Chengdu Indigo Power Sources Co. Ltd., China). The mixture was further mechanically homogenized in an agate mortar to form viscous slurry, and coated on a cleaned aluminum foil current collector. After being dried at 100 °C under vacuum for 10 h, the foil was laminated and cut into 1.2 cm2 wafers as working electrodes. Metal lithium was used as both the counter electrode and the reference electrode. In an argon-filled glovebox, 2032 button cells were assembled by sandwiching a Celgard 2300 microporous separator between the working electrode and the metal lithium. The electrolyte was 1.0 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1:1:1 in vol., Shenzhen Capchem Chemicals Co. Ltd., China). Galvanostatic charge/discharge measurements were performed in a voltage range of 2.4−4.2 V versus Li+/Li on a Neware battery-testing instrument (Shenzhen Neware Technology Ltd., China). The CV and EIS measurements were carried out on an electrochemical workstation including a PAR 7924
dx.doi.org/10.1021/ie300088p | Ind. Eng. Chem. Res. 2012, 51, 7923−7931
Industrial & Engineering Chemistry Research
Article
and 0.15 ton of Fe−P were by-produced when 1 ton of phosphorus was manufactured. According to the ratio of Fe to P obtained from the XRF result, the maximum theoretical contents of Fe and P in all the byproducts were calculated to be 1.63 wt % and 0.75 wt %, respectively. In fact, the real contents are lower than the calculated values because of the existence of impurities. The contents of Fe and P in Fe−P powder increase, respectively, to 55.58 wt % and 25.63 wt % by melting crystallization, which indicates the improvement of Fe−P purity. As a result, the recovery ratios of Fe and P from all the byproducts in the production of yellow phosphorus are at least 81.4% and 81.6%, respectively. The major component of the Fe−P powder determined by XRF is Fe and P (∼81 wt %). Apparently, because of the special treatment process, the purity of the as-obtained Fe−P slag is relatively high. C and O (∼13 wt %) are consumed during the process for synthesizing LiFePO4, which have no effect on the performance of the as-synthesized LiFePO4 samples. Other impurities such as Ca, Mn, Ti, Cu, Al, V, and Si amounting to ∼6 wt % may affect the electrochemical performance. It is noteworthy that most of the metallic elements, such as Mn, Ti, Cu, Al, and V, can contribute to the electrochemical performance of LiFePO4.6,31−36 The Fe−P powder was dissolved in hot aqua regia, which was applied to analyze the composition by ICP−AES. It was found that 98 wt % powder was dissolved in the solution, which was estimated to be Fe−P and the compounds of Ca, Mn, Ti, Cu, Al, and V. The remainder 2 wt % of undissolvable substance is ultrafine white powders and may be SiO2, CaSiO3, or Ca3Si2O7, which are electrochemically inactive materials and disadvantageous to the electrochemical performance of LiFePO4 because of their extremely low conductivity and high chemical stability. However, the concentration of such impurities can be reduced by remelting crystallization or zone melting crystallization. 3.2. Carbon Content Analysis. Carbon has a positive effect on the electrochemical performance of LiFePO4, which is affected by carbon source and carbon content.14 Glucose is an effective and cheap organic carbon source for improving the electronic conductivity of LiFePO4.14,37,38 To demonstrate the feasibility of the novel facile route and obtain the optimal carbon content, various LiFePO4/C products with different carbon contents were synthesized under the same preparation conditions, except the glucose amount added to the preheated precursor. According to the carbon content determined by carbon−sulfur analysis, as summarized in Table , the final products are labeled as “blank”, “1”, “2”, “3” and “4”, with the carbon content increasing. 3.3. Phase Structure Analysis. The XRD patterns of the Fe−P powder, the preheated precursor, the sample "blank", and sample 3 are compared in Figure 2. According to the XRD analysis results, the Fe−P powder is composed of orthorhombic FeP (JPCDS card number 78-1443) and hexagonal Fe2P (JPCDS card number 01-1200).28 In the preheated precursor obtained in air, there are obvious indexed peaks of Fe2P, Fe2O3 (JPCDS card number 16-0653), Li3PO4 (JPCDS card number 12-0230), Li3Fe2(PO4)3 (JPCDS card number 47-0107) and LiFeP2O7 (JPCDS card number 37-0236), indicating the composition of the preheated precursor becomes complicated when the preheating temperature increases from ∼400 °C28 to above 500 °C. The existence of Fe2O3, Li3PO4, Li3Fe2(PO4)3, and LiFeP2O7 indicates the reaction of the Fe1.5P with the lithium salts and the O2 in air, while some phosphates are further deoxidized to produce pyrophosphates. The indexed
273A potentiostat/galvanostat and a signal recovery model 5210 lock-in-amplifier controlled by a Powersuit software (Princeton Applied Research, USA). Lithium ion diffusion coefficient (DLi+) was calculated according to the function of the impedance and the square root of frequency in low frequency region of EIS data.
3. RESULTS AND DISCUSSION 3.1. Recovery of Fe and P Elements. In the typical electrothermal reduction process for producing yellow phosphorus, three raw materials, i.e. phosphate rock, silica, and coke, are mainly used, and the critical reactions are as follows.29,30 1400 ∼ 1500 ° C
4Ca5(PO4 )3 F + 18SiO2 + 30C ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 3P4 ↑ + 18CaSiO3 + 2CaF2 + 30CO↑
(2)
1400 ∼ 1500 ° C
6CaF2 + 7SiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 3SiF4 ↑ + 2Ca3Si 2O7
(3)
Iron oxides always exist in phosphate rock, which are reduced to Fe in the reductive atmosphere in the furnace, and the produced Fe reacts further with P to form Fe−P compounds. 1400 ∼ 1500 ° C
4FexO + 4C + P4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 4FexP + 4CO↑
(4)
where, x is the atomic ratio of Fe to P, and usually ranges from 0.5 to 3 according to the composition of the raw materials. In fact, such Fe−P is generally a mixture composed of Fe3P, Fe2P, FeP, or FeP2. In the high temperature furnace, the byproducts are a melting mixed solution of Fe−P slag and furnace slag (mainly CaSiO3 and Ca3Si2O7). Depending on the density difference of the Fe−P slag (5.6 to 6.0 g/cm3) and the furnace slag (≤3.0 g/cm3), Fe−P is enriched at the bottom of the furnace. On the other hand, due to the melting point difference of the Fe−P (1200−1400 °C) and the furnace slag (
