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Reaction Mechanisms on Solvothermal Synthesis of Nano LiFePO4 Crystals and Defect Analysis Xiankun Huang, Xiangming He, Changyin Jiang, Guangyu Tian, and Yongzhong Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02009 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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Industrial & Engineering Chemistry Research

Reaction Mechanisms on Solvothermal Synthesis of Nano LiFePO4 Crystals and Defect Analysis

Xiankun HUANGa, Xiangming HEb,c*, Changyin JIANGb, Guangyu TIANc, Yongzhong LIUa,*

a Department of Chemical Engineering, Xi'an Jiaotong University, Xi'an 710049, P.R.China b Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, P.R. China c State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, P.R. China

* Corresponding authors: Email: [email protected] (Xiangming HE); [email protected] (Yongzhong LIU)

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ABSTRACT A solvothermal process is used to synthesize LiFePO4 nanomaterials for lithium ion batteries. Reaction parameters such as reaction temperature and residence time are explored to obtain the optimal LiFePO4 sample. A three-stage reaction mechanism is proposed to better understand the solvothermal synthesis process. XRD, SEM, and FTIR are used to investigate the prepared samples at different conditions. LiFePO4 formation reaction would occur at a temperature as low as 89˚C. Defect analysis results show that after four hours solvothermal treatment the concentration of lithium vacancy and Li-Fe antisite defects was too low to be detected. The charge-discharge data of the obtained LiFePO4 show that the carbon coated LiFePO4 samples prepared at 180˚C for four hours solvothermal treatment have a discharge capacity of 160.6 mAhg-1 at 0.1C discharge rate, and 129.6 mAhg-1 at 10C. Keywords: Solvothermal synthesis; Lithium iron phosphate crystals; Reaction mechanisms; Defect analysis

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1. Introduction LiFePO4 has been considered as one of the most promising cathode materials for lithium batteries for its stable structure, nontoxicity, safety and excellent electrochemical performances. However, low electronic conductivity (about 10-9 S cm-1)1 and low ionic conductivity (about 10-9 S cm-1)2 have limited its application in hybrid electric vehicles (HEVs) or electric vehicles (EVs)3. Various approaches have been investigated to improve its electronic conductivity, such as carbon coating4-6, higher valent cation doping7, 8, etc. Lithium ion is believed as diffusing along the [010] direction,9 reducing particle sizes10 and crystal morphology control11 have been confirmed as effective ways to improve LiFePO4 performances. Compared with conventional solid-phase synthesis methods12, hydrothermal/solvothermal synthesis methods have shown advantages in controlling particle sizes and crystal orientation in synthesizing many materials including LiFePO413-15. This synthesis process is a solution reaction, which is usually influenced by reactant concentration (including pH)16, 17, reactant sources18, 19, reaction time20,

21

, reaction temperature22, and reaction sequences23. Instead of hydrothermal

synthesis, in which reaction mechanisms have been intensively investigated so far, solvothermal synthesis route is able to produce products with smaller size particles like nanoparticles, nanoplates, nanorods11,

24, 25

, and dumbbell-like microstructures26, microflowers14, three-

dimensional (3D) porous microspheres27, 28 assisted by different solvents and surfactants. The solvothermal synthesis technique is, however, more complicated and sensitive to reaction parameters

22, 29-31

compared with the solid-phase methods. Reaction parameters,20, 21, 32such as

the type of raw materials, feeding sequences, reaction temperatures and reaction time have great 3

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impacts on the characteristics of the product LiFePO4 such as crystallinity, morphology and especially electrochemical performances. Hence, it is necessary to investigate the reaction mechanisms of this process to obtain better LiFePO4 materials and reduce the cost of the solvothermal synthesis process. Ethylene glycol (EG) is widely used in a LiFePO4 solvothermal process as a solvent or cosolvent for its significant positive influences19, 23, 25, 33. Consequently, in this work, LiFePO4 was synthesized by a solvothermal process by using ethylene glycol (EG) as solvent. The crystallinity, particle sizes, defects, and electrochemical performances of the prepared LiFePO4 evolution with reaction temperatures, time and acidity were investigated and the reaction mechanisms for LiFePO4 crystallization were also elucidated. 2. Experimental 2.1 Synthesis All the chemicals (AR grade) were purchased from Xilong Chemical Co., Ltd. They were used without any further treatment. The synthesis of LiFePO4 was carried out in a 50ml Teflon vessel, which was sealed in a stainless-steel autoclave. The molar ration of Li:Fe:P in the precursor solution was 2.7:1:1, and the concentration of LiFePO4 in the reaction solution was controlled to be 0.2M. LiOH·H2O, FeSO4·7H2O and H3PO4 were chosen as the sources of Li, Fe, P, and EG was chosen as solvent for this solvothermal synthesis. The typical feeding sequence was chosen: P → Fe → Li , i.e., LiOH·H2O and FeSO4·7H2O was dissolved into EG to form solutions individually; then H3PO4 was added into Fe solution slowly to make solution A; and the mixture A was added to Li solution dropwise slowly and stirred for 30 min; the green black slurry was transferred into the vessel in the end. The reactor 4


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was heated in an air oven setting at 120˚C, 140˚C, 160˚C, 180˚C for one minute and 180˚C for different residence time from one minute to ten hours. The heating and cooling rates were measured and collected. After the solvothermal reactions finished, the supernatants were collected and diluted to one fifth with deionized water and pH of the solutions were measured. The obtained precipitates were washed with deionized water and ethanol for several times and then dried at 60˚C for over 6h. In order to obtain carbon-coated LiFePO4/C powder with good electronic conductivity, the samples were mixed with 10wt% sucrose and sintered at 650˚C for 2h under argon atmosphere. 2.2 Characterization The samples were characterized by X-ray diffraction (XRD, Rigaku D/max 2550/PC) with a Cu-Kα radiation source. The morphologies of the samples were observed using scanning electronic spectroscopy (SEM, JSM-5600LV, JEOL, Japan). The pH values of the supernatants were measured by a microprocessor pH meter (PHS-25, Shanghai). Ferrous concentration of the supernatants was detected by an atomic absorption spectroscopy (AAS, SHIMADZU, AA-6880). FTIR spectra were collected in an IR spectrometer (PerkinElmer) at room temperature. 2.3 Electrochemical measurements The electrochemical performances were measured by using a 2032-type coin cell. The electrode was prepared by mixing a mixture of active materials, acetylene black, and polytetrafluoroethene (PTFE) binder in a mass ratio of 8:1:1. Pure metallic lithium was used as anode. The electrolyte was 1M LiPF6 dissolved in volume ratio of 1:1:1 with ethylene carbonate/dimethyl carbonate/ethylmethyl carbonate. Galvanostatical charging-discharging tests

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were carried out on a Land CT2001A cycler (Wuhan Kingnuo Electronic Co.) in a voltage range 2.5 - 4.2V. 3. Results and discussion 3.1 Influences of reaction parameters on solvothermal synthesis LiFePO4 3.1.1 Formation of LiFePO4 phase When H3PO4 was dripped into the EG solution of FeSO4, no precipitate was formed in this acidic environment. While the green black slurry was formed with Fe’s acid solution dripping into LiOH’s EG solution. An ex-situ method was developed to study the formation of olivine LiFePO4 during solvothermal synthesis process. Figure 1 shows the ex-situ XRD of the precursor’s phase composition with temperatures from 25˚C (ambient temperature) to 180˚C for residence time of one minute (All temperatures reported in this paper refer to the oven temperatures unless otherwise specified.). The precursor phase was clearly detected as amorphous until the oven temperature increased to 140˚C, as shown in Figure 1(a), (b), and (c). At 140˚C, the crystallized Fe3(PO4)2·H2O (PDF# 70-1794) and Li3PO4 (PDF# 45-0747) were indexed, which were slightly different with Chen30 and Ou20’s work as the dissoluble FeSO4 or Li2SO4 was washed by deionized water during the pretreatment step. Iron phosphate hydrates or iron phosphates and lithium phosphate were crystallized previously at 120~140˚C solvothermal condition. When the temperature increased to 160˚C, LiFePO4 phase appeared with little Fe3(PO4)2·H2O and Li3PO4. It indicates that the rapid formation of the LiFePO4 nuclei at 140~160˚C accompanied by Fe3(PO4)2·H2O and Li3PO4’s dissolution. Figure 1(e) shows stronger LiFePO4 diffraction peaks without any detectable crystallized impurity. More precursors were transformed into LiFePO4 crystals during the temperature intervals from 160˚C to 180˚C. 6


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Temperature is an important parameter in the solvothermal synthesis of LiFePO4. As we know, the specific heat capacities of air and EG are different (1.02 and 2.35 kJ /(kg·K), respectively) in spite of the thermal residence of the Teflon vessel and the chemical reaction heat. Therefore, the temperature of the slurry in the Teflon vessel would definitely not be consistent with that of air in the oven. Thus it is necessary to accurately obtain the slurry temperature to explore the real reaction mechanisms during the solvothermal synthesis of LiFePO4. We tracked the temperatures of the air in oven and the slurry in the Teflon vessel setting the oven at 180˚C for 10h, as shown in Figure 2(a). The temperature of air in oven increased rapidly, whereas the temperature of slurry increased in “S” shape. As we mentioned previously, LiFePO4 phase appeared when the oven was set at 160˚C and stayed for one minute. To get the real reaction temperature of the LiFePO4 crystal formation reaction, we collected the real situation again setting the oven at 160˚C for one minute, as shown in Figure 2(b). The highest temperature of the slurry is 89˚C instead, which means that the LiFePO4 nuclei could be formed as low as 89˚C, as shown in Figure 1(d) with EG as solvent during the solvothermal process compared with 105˚C during hydrothermal synthesis30. The morphology evolution of the samples synthesized at different oven setting temperatures and residence time are presented in Figure 3. Irregular large agglomerated particles were observed referred to amorphous precipitates in Figure 3(a), (b), and (c), whereas non-uniform smaller particles around 100 nm were formed on the surface of the big amorphous precursors at 160˚C, which referred to LiFePO4 phase in Figure 3(d). The LiFePO4 crystal particles (as shown in Figure 3(e)) became more uniform without any large particles when the oven temperature was set as 180˚C. 3.1.2 Growth and perfection of LiFePO4 crystals

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Residence time is the average time that a particle stays in a particular system, which is an important parameter in a chemical process. Here we use the term, residence time, to represent the time elapses in the oven at a setting temperature, i.e. the time for the slurry staying at a fixed temperature. At 180˚C we can obtain good crystallized and uniform LiFePO4 particles as shown in Figure 1(e) and Figure 3(e). Further investigations on the evolution of the LiFePO4 particles with time were carried out by collecting samples with different residence time at 180˚C. Figure 4 shows the XRD patterns of the LiFePO4 powder synthesized at 180˚C in various residence time intervals. The peaks near 45˚are better split with more than four hours, which indicates the crystallinity of the samples of (e), (f), (g), and (h) is better than that of (a), (b), (c), and (d). There is no crystallized impurity peaks detected for the particles with shorter residence time, even as short as one minute. It is different from the situation in the hydrothermal process20. Particles with more uniform sizes and cleaner surfaces were synthesized as shown in Figure 3(i), (j), (k), and (l) instead of (e), (f), (g), and (h). The average grain sizes of the samples synthesized at 180˚C for 4h, 6h, 8h, 10h are all around 80 nm. No apparent increase or decrease in grain sizes of samples synthesized by this solvothermal process are observed for the special physicochemical properties of EG, which are different from those synthesized by the hydrothermal process in Ou’s work20. Comparing with LiFePO4 nuclei forming process, it is difficult for LiFePO4 crystal growing in the EG solvent compared with water because of its special physicochemical properties such as higher viscosity, bigger molecular formula, reducibility at high temperature, etc. The XRD patterns of LiFePO4 samples which were after heat treatment at 650 ˚C for 2h has been shown in Figure S1 in the supporting information. 3.2 Reaction mechanisms of solvothermal synthesis LiFePO4

8


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Figure 5 shows the curves of pH value and Fe2+ concentration of supernatants change with the oven setting temperature and the residence time at 180˚C during the solvothermal reaction. The supernatant was collected when the temperature of the slurry was cooled to the room temperature. In this work, the data shows that the pH value was kept constant at 5.2 even the oven temperature was set as 120˚C, while Fe2+ concentration decreased slightly with the increase of tempereratures. This agrees with the XRD results presented in Figure 1 and the SEM images shown in Figure 3 that (a) was almost the same as (b). This might be explained as follows: when Fe’s acid solution was added to a high pH lithium solution dropwise, Fe2+-HxPO43- complexes and HxPO4x-3 turned to amorphous precipatates Fe3(PO4)2(s), Li3PO4(s) immediately and a green black slurry formed. In this context, the probable equilibrium reactions were as follows.

Fe3 (PO4 )2 (s) + 2H + ↔ 2FeHPO4 + Fe2+

(1)

FeHPO4 + H + ↔ FeH 2 PO4 +

(2)

FeHPO4 ↔ HPO4 2− + Fe2+

(3)

FeH 2 PO4 + ↔ H 2 PO4 − + Fe2+

(4)

Li3PO4 (s)+H + ↔ Li 2 HPO4 + Li +

(5)

Li 2 HPO 4 +H + ↔ LiH 2 PO4 +Li +

(6)

LiH 2 PO4 +H + ↔ H 3PO4 +Li +

(7)

H 3PO4 ↔ H 2 PO4 − + H +

(8)

H 2 PO4 − ↔ HPO4 2− + H +

(9)

HPO4 2− ↔ PO43- + H +

(10)

Fe2+ + H 2O ↔ Fe(OH)+ + H +

(11)

Fe(OH)+ + H 2O ↔ Fe(OH)2 (s) + H +

(12)

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When the oven temperature was lower than 120˚C, the pH value was kept constant at 5.2, which meant that the process was simply related to the dissolution of amorphous Fe3(PO4)2(s) and Li3PO4(s) as the oven temperature increased, and no more new materials, LiFePO4 for example,were produced. When the oven temperature was higher than 120˚C, all the upper reactions rates increased. More ferrous ions were released. The reaction (11) and (12) were enhanced too. More free ferrous ions hydrolyzed with water introduced by the raw materials; more Fe(OH)2 was formed; and more H+ was released. It led to decreasing pH values of supernatant with the increase of the oven temperature, as shown in Figure 5(a). The crystallizations of Fe3(PO4)2 and Li3PO4 were performed with the increase of the oven temperature from 120˚C to 140˚C, as shown in Figure 1(c). Some ferrous ions and lithium ions participated in the Fe3(PO4)2 and Li3PO4 crystallization reactions presented in the reaction (13) and (14). Furthermore, from Figure 5(a) we can see that the dissolution reaction of Fe3(PO4)2 is much more sensitive to the temperature compared with the hydrolization reaction of ferrous ions. Therefore, the concentration of ferrous ions increased when the temperature increased from 120˚C. Because of the crystallizations of Fe3(PO4)2 and Li3PO4, the pH value of the supernatant could not go back to 5.2 when the slurry was cooled to the room temperaure.

3Fe2+ +2PO3-4 ↔ Fe3 (PO4 )2 (c)

(13)

3Li+ +PO3-4 ↔ Li3PO4 (c)

(14)

When the oven temperature increased continuously from 140 ˚C to 160˚C, the slurry inside was energized to overcome the energy barrier of LiFePO4 formation reaction. The ferrous ions were at 10


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supersaturation state. Hence, the nuclei of LiFePO4 were rapidly formed along with the dissolutions of Fe3(PO4)2 and Li3PO4, as shown in the reaction (15).

Li+ +Fe2+ +PO4 3− ↔ LiFePO4 (s)

(15)

This process was named stage (I). Stage (I) continued until residence time lasted for one hour when the oven temperature was set to 180˚C, as shown in Figure 5(b). The pH value decreased from the original 5.2 to 2.42 on the 1h point, and the concentration of Fe2+ reached its maximum. After one hour, the residual Fe3(PO4)(s) was little so that the ferrous ions from the dissolution of Fe3(PO4)2 were not enough to retain the reaction (15). This would make the reactions (11) and (12) occur reversely to produce more Fe2+. Meanwhile, H+ and Fe2+ were consumed, and the pH value increased and the concentration of Fe2+ decreased in the next three hours. The main reactions in this period would be the reactions (11), (12) and (15), which is called the stage (II). The reaction rate was much lower than that in the first one hour for the lower concentration of Li+, Fe2+, and PO43-. Ostwald ripening reactions occurred during this stage besides the new crystallization reaction. It was also proved by Figure 6, which showed that the element ratio of the result samples changed slower in the stage (II). After four hours, the reactions step into the stage (III), where Fe(OH)2(s) was exhausted, and Li+, Fe2+ and PO43- were not enough to maintain the supersaturation state. Thus no more LiFePO4 was produced. The reaction (15) was in equilibrium. That is to say that the LiFePO4 crystal surface reach a dissolution-recrystallization equilibrium, and the structure was optimal. This can also be proved by our XRD Retiveld refinement cell parameters and ICP analysis results of the product LiFePO4 samples shown in Figure 6 and Figure 7. Some low energy facets might be 11



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formed, and the high energy facets disappeared instead. The crystal faces turned to be cleaner, as shown in Figure 3(i) - (l). The Rietveld refinements on XRD patterns of the LiFePO4 samples synthesized at 180˚C for various residence time intervals were performed. The lattice parameters of LiFePO4 samples were drawn in Figure 6 and Figure S2 in the supporting information. Our data show that the lattice parameters a, b and the cell volume V increased while c decreased against the residence time at one minute to four hours. This might be the result of the lack of lithium, which is also called lithium vacancy defects, or Li-Fe antisite defects. Therefore, LiFePO4 should be correctly written as LixFePO4 (x

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