Trans. Nonferrous Met. Soc. China 22(2012) 1766-1770

Thermodynamics analysis of LiFePO₄ pecipitation from Li-Fe(II)-P-H₂O system at 298 K

HE Li-hua, ZHAO Zhong-wei, LIU Xu-heng, CHEN Ai-liang, SI Xiu-fen

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

Received 13 July 2011; accepted 27 September 2011

Abstract: Thermodynamics of the precipitation from Li-Fe(II)-P-H₂O system at 298 K was investigated. The results demonstrate that LiFePO₄ can be formed at room temperature under pH value of 0-11.3, and the impurities Li₃PO₄ and Fe(OH)₂ will be yielded at pH value above 11.3 and 12.9, respectively. The optimum pH value for LiFePO₄ precipitation is 8-10.5. Considering the low rate of phase transformation kinetics, metastable Li-Fe(II)-P-H₂O system was also studied. The results indicate that equimolar ratio of co-precipitation precursor Fe₃(PO₄)₂·8H₂O and Li₃PO₄ cannot be obtained at the initial molar ratio 1:1:1 and 1:1:3 of Li:Fe:P. In contrast, equimolar ratio of the co-precipitation precursor can be yielded by adjusting the pH value to 7-9.2, matching the molar ratio 3:1:1 of Li:Fe:P, meaning that Li⁺-excess is one of the essential conditions for LiFePO₄ preparation by co-precipitation method.

Key words: lithium iron phosphate; lithium ion batteries; Li-Fe(II)-P-H₂O system; thermodynamics; co-precipitation

1 Introduction

Since lithium iron phosphate, LiFePO₄, was first reported as a positive electrode for rechargeable lithium ion batteries in 1997 by PADHI et al [1], it has been actively investigated and considered one of the most promising cathode materials for lithium ion batteries due to its low cost, nontoxic, high lithium intercalation voltage, high theoretical specific capacity and great thermal stability [2,3]. Conventionally, LiFePO₄ powders are prepared by solid-state synthesis with their repeated grinding and a long period of high temperature operations. However, this approach has some conspicuous disadvantages such as large particle size, broad particle size distribution, and impurities. In contrast, wet chemical preparation routes, such as hydrothermal, solvothermal, sol-gel, emulsion-drying and co-precipitation methods, have an indisputable advantage over solid-state reactions in achieving better homogeneity and mixing the starting compounds on molecular level. Especially, precipitation and/or hydrothermal approach, as a commercially feasible process, has attracted much attention recently. For instance, PARK et al [4,5] and YANG et al [6] synthesized the single phase of LiFePO₄ successfully via co-precipitation method. YANG et al [7] and CHEN and WHITTINGHAM [8] synthesized LiFePO₄ directly in a hydrothermal reaction, and no impurities were detected. Additionally, similar studies repeatedly demonstrated that the precipitation and hydrothermal methods are promising synthesis methods for LiFePO₄ [9-12].

Nevertheless, precipitation and hydrothermal methods suffer from the impurities formation, such as Li₃PO₄, Fe₈(PO₄)₂, Fe₂O₃, Li₃Fe₂(PO₄)₃ [4,10], which have predominant influence on the electrochemical performances. And the experimental conditions for LiFePO₄ preparation via wet chemical routes in the present researches were almost obtained on experience, and the conditions varied with different processes and/or experimental devices which are of a certain extent randomicity and blindness. So, if the conditions, under which pure LiFePO₄ can be obtained in solution medium, could be predicted, it will guide us to later researches and practical production.

In this work, the thermodynamics of the precipitation from Li-Fe-P-H₂O system is studied, aiming to understand the precipitation characteristics. Meanwhile, the predominance-area and optimum conditions for the formation of LiFePO₄ will be predicted theoretically.

2 Data treatment and calculation model

In this work, the precipitation processes are based on the LiOH-FeSO₄-H₂PO₄-H₂O system, in which 19 ionic species are assumed to exist in the solution to a significant extent: H⁺, OH^-, PO₄^3-, HPO₄^2-, H₂PO₄^-, H₃PO₄(aq), SO₄^2-, HSO₄^-, H₂SO₄(aq), Li⁺, LiOH, LiHPO₄^-, Fe²⁺, Fe(OH)⁺, Fe(OH)₂(aq), Fe(OH)₃^-, Fe(OH)₄^2-, FeHPO₄(aq), FeH₂PO₄⁺, and 5 species as insoluble compounds were assumed to exist: Li₃PO₄(s), Fe(OH)₂(s), Fe₃(PO₄)₂·8H₂O(s), FeHPO₄(s), LiFePO₄(s). These inventory leads to 19 equilibriums listed in Table 1, each of them gives rise to an independent equilibrium equation from the mass action law.

Table 1 Thermodynamic equilibrium equations of precipitation system at 298 K

The equilibrium reactions (20) and (21) are related to FeHPO₄(s) and LiFePO₄(s) which have no solubility product available in literatures. However, the K_sp can be evaluated by the following equation:

Δ_rG^Q_m=-RTln K                              (20)

where Δ_rG^Q_m is the standard Gibbs free energy of the reaction, R is the gas constant (8.314J K/mol), and T is temperature in Kelvin.

The Gibbs free energy value of FeHPO₄(s) can be estimated by the method in Ref. [16], with the result of -1201.77 kJ/mol. And the Gibbs free energy value of LiFePO₄(s), which was calculated in our previous work [17], is -1517.7 kJ/mol. Using Eq. (22), the K_sp values, which are 10^-5.89 and 10^-24.6 for FeHPO₄(s) and LiFePO₄(s), respectively, are calculated.

At the same time, the charge balance and 4 mass balance equations are given as follows:

[H⁺]+[Li⁺]+2[Fe²⁺]+[Fe(OH)⁺]+[FeH₂PO₄⁺]

=[OH^-]+3[PO₄^3-]+2[HPO₄^2-]+[H₂PO₄^-]+

2[SO₄^2-]+[HSO₄^-]+[LiHPO₄^-]+[Fe(OH)₃^-]+

2[Fe(OH)₄^2-]                              (21)

[Li]_T=[Li⁺]+[LiOH]+[LiHPO₄^-]                 (22)

[Fe(II)]_T=[Fe²⁺]+[Fe(OH)⁺]+[Fe(OH)₂(aq)]+

[Fe(OH)₃^-]+[Fe(OH)₄^2-]+[ FeHPO₄(aq)]+

[FeH₂PO₄⁺]                               (23)

[P]_T=[PO₄^3-]+[HPO₄^2-]+[H₂PO₄^-]+[H₃PO₄(aq)]+

[LiHPO₄^-]                                (24)

[S]_T=[SO₄^2-]+[HSO₄^-]+[H₂SO₄(aq)]             (25)

where [M] is the equilibrium concentration for each species; [Li]_T, [Fe(II)]_T , [P]_T and [S]_T are designated as the total concentration of Li, Fe, phosphate and sulphate in equilibrium conditions; while [Li]ⁱ_T, [Fe(II)]ⁱ_T , [P] ⁱ_T and [S]ⁱ_T are defined as the initial total concentrations of Li, Fe, phosphate and sulphate before the precipitation of solid phases.

The calculation process based on the Newton– Raphson iteration method was carried out by using Microsoft Excel.

3 Results and discussion

3.1 Li-Fe(II)-P-H₂O system

Figure 1 shows the molar ratio of solid species diagram and solubility diagram of the Li-Fe(II)-P-H₂O system. Three predominance-areas exist at the given pH value. At 0≤pH≤11.3, LiFePO₄(s) appears to be the stable single phase, and its solubility decreases basically with increasing pH value. At 11.3≤pH≤12.9, a two-phase domain of LiFePO₄(s)+Li₃PO₄(s) is observed, showing that no pure LiFePO₄ can be formed. Furthermore, another two-phase domain, where Fe(OH)₂(s) and Li₃PO₄(S) coexist (1:1 in molar ratio), is found in the pH value range of 12.9 to 14. Note that no coexistent region of Fe₃(PO₄)₂·8H₂O(s)+Li₃PO₄(s) appears, indicating that Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s) exist in a metastable state and can react with each other, accompanied by the formation of LiFePO₄. The results are consistent with the previous studies that Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s) must be an intermediate in the formation of LiFePO₄ [18-20].

Therefore, it is clear from the above discussion that the pH value is one of the key factors affecting LiFePO₄ precipitation in solution medium. For example, LIU et al [21] reported that LiFePO₄ just was prepared by hydrothermal method at pH=6.3-9.04, but not at pH=5.4 and 11.04. Coincidentally, LEE and TEJA [19] showed that pure LiFePO₄ was synthesized at pH=6.5 and 9.22; LiFePO₄, iron oxide and Li₃PO₄ coexisted at pH=10.62; and a mixture of iron oxide (Fe₃O₄) and lithium phosphate (Li₃PO₄) was obtained at pH=14.32. Note that LiFePO₄ cannot be prepared at strong acid pH condition in the above practical experiments, which seems to be in conflict with the thermodynamic computation in this work. This can be reconciled by recognizing the similar principle of the hydroxyapatite precipitation that phosphate appears mainly in the form of H₃PO₄ and/or H₂PO₄^- in strong acid aqueous medium, causing too low reaction probability of Ca₅(PO₄)₃(OH) crystallization to nucleate [22], whereas it was the same for LiFePO₄. Using the method in Ref. [22], the reaction probability of LiFePO₄ crystallization was calculated and is shown in Fig. 2. The results show that the reaction probability increases dramatically with increasing pH value.

Fig. 1 Molar ratio of solid species diagram (a) and solubility diagram (b) of Li-Fe(II)-P-H₂O system ([Fe(II)]ⁱ_T=[P]ⁱ_T=0.1 mol/L, [Li]ⁱ_T=1 mol/L)

PARK et al [5] and YANG et al [6] reported that only green color co-precipitated powders (Fe₃(PO₄)₂·8H₂O and Li₃PO₄) were obtained via co-precipitation method at 60 ℃. Moreover, LEE and TEJA [19] found that large irregular Fe₃(PO₄)₂·8H₂O particles were synthesized at room temperature, and both LiFePO₄ and Fe₃(PO₄)₂·8H₂O particles were obtained at 120 ℃, whereas at 300 ℃ and higher temperatures, only LiFePO₄ was yielded, indicating that temperature also played an important role in the formation of LiFePO₄. This was because the processes of LiFePO₄ precipitation were controlled by phase-transformation kinetics, so it was favorable for the intermediate products converting to LiFePO₄ phase at high temperature, and some research inferred that there was a critical temperature [23].

Fig. 2 Growth unit concentration and probability (P) of LiFePO₄ crystallization

The nucleation rate of LiFePO₄ was affected by many factors, such as kinetic coefficient, temperature, supersaturation, interfacial free energy. It was possible to obtain LiFePO₄ at low temperature and broad pH range via special treatment, e.g. adding nucleator and surfactant, precipitation at high supersaturated solution.

3.2 Metastable Li-Fe(II)-P-H₂O system

According to the above investigation on precipitation from Li-Fe(II)-P-H₂O system and practical experiments, we know that in the actual precipitation process of LiFePO₄, it is easy to form the transition phases, e.g. Fe₃(PO₄)₂·8H₂O and Li₃PO₄, at the initial stage. In order to obtain pure LiFePO₄, precipitation parameters should be controlled precisely and strictly, such as pH value, temperature. Fe₃(PO₄)₂·8H₂O and Li₃PO₄ can be yielded at low temperature, meanwhile, it provides us with another idea for preparing LiFePO₄. In fact, many works had been done to synthesize LiFePO₄ by co-precipitation method [4-6]. In the process, obtaining equimolar ratio of Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s) was the basic prerequisite for preparing pure LiFePO₄. Based on this situation, it is necessary for us to have an investigation on the metastable Li-Fe(II)-P-H₂O system, in which we assume that the LiFePO₄ does not exist.

Figure 3(a) shows the solubility diagram of the metastable Li-Fe(II)-P-H₂O system in the initial concentration of 0.1 mol/L [Li]ⁱ_T, [Fe(II)]ⁱ_T, and [P] ⁱ_T. Four predominance areas can be found. At 3≤pH≤6.6, Fe₃ (PO₄)₂·8H₂O(s) appears to be the stable single phase. A two-phase domain of Fe₃ (PO₄)₂·8H₂O(s)+Li₃PO₄(s) is defined in pH value range of 6.6 to 10.45, in which its solubility decreases as the pH value increases. At pH≥10.45, another two-phase domain, where Li₃PO₄(s) and Fe(OH)₂(s) coexist, is found. However, the molar ratio of solid species from Fig. 3(b) suggests that it is impossible to obtain equimolar ratio of Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s) in the whole range of pH value theoretically, and the calculation result is consistent with the experimental results [24].

Fig. 3 Solubility (a) and molar ratio (b) of solid species of metastable Li-Fe(II)-P-H₂O system ([Fe(II)]ⁱ_T=[P]ⁱ_T=[Li]ⁱ_T= 0.1 mol/L)

In order to obtain equimolar ratio of Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s), we reckoned that the results will be improved under phosphate or Li⁺-excess conditions, and the molar ratio of solid species diagrams with the molar ratio 1:1:3 and 3:1:1 of Li:Fe:P are shown in Figs. 4(a) and (b), respectively. Figure 4(a) suggests that it is also hard to obtain equimolar ratio of Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s), indicating the effect of phosphate-excess is not obvious. In contrast, equimolar ratio of Fe₃(PO₄)₂·8H₂O(s) and Li₃PO₄(s) can be obtained by adjusting the pH value to 7-9.2 with the molar ratio 3:1:1 of Li:Fe:P. This result is consistent with the experimental pH value in Refs. [25-27]. And the optimal conditions for Fe₃(PO₄)₂·8H₂O(s)+Li₃PO₄(s) co-existing precursor are in the neutral or slightly basic/acid solution medium.

Fig. 4 Molar ratio of solid species of metastable Li-Fe(II)-P-H₂O system with P^-- or Li⁺-excess: (a) [Fe(II)]ⁱ_T=[Li]ⁱ_T=0.1 mol/L, [P]ⁱ_T=0.3 mol/L; (b) [Fe(II)]ⁱ_T= [P]ⁱ_T=0.1 mol/L, [Li]ⁱ_T=0.3 mol/L

4 Conclusions

Thermodynamic analysis shows that LiFePO₄ can be obtained at 298 K and pH=0-11.3, with its optimum precipitation pH=8-10.5. So it is possible to obtain pure LiFePO₄ through wet chemical preparation routes. However, because of the low phase transformation kinetic rate, it is hard to obtain LiFePO₄ at low temperature. In this case, the metastable Li-Fe(II)-P-H₂O precipitation system was also studied and the results indicate that Li⁺-excess is essential to obtain equimolar ratio of Fe₃(PO₄)₂·8H₂O and Li₃PO₄ for LiFePO₄, and the optimal conditions are in the neutral or slightly basic/acid solution medium.

References

[1] PADHI A K, NANJUINDASWAMY K S, GOODENOUGH J B. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries [J]. J Electrochem Soc, 1997, 144(4): 1188-1194.

[2] WHITTINGHAM M S, SONG Y, LUTTA S, ZAVALIJ P Y, CHERNOVA N A. Some transition metal (oxy) phosphates and vanadium oxides for lithium batteries [J].J Mater Chem, 2005, 15(33): 3362-3379.

[3] PADHI A K, NANJUINDASWAMY K S, MASQUILIER C, OKADA S, GOODENOUGH J B. Effect of structure on the Fe³⁺/Fe²⁺ redox couple in iron phosphates [J]. J Electrochem Soc, 1997, 144(5): 1609-1613.

[4] PARK K S, SON J T, CHUNG H T, KIM S J, LEE C H, KIM H G. Synthesis of LiFePO₄ by co-precipitation and microwave heating [J]. Electrochem Commun, 2003, 5(10): 839-842.

[5] PARK K S, KANG K T, LEE S B, KIM G Y, PARK Y J, KIM H G. Synthesis of LiFePO₄ with fine particle by co-precipitation method [J]. Mater Res Bull, 2004, 39(12): 1803-1810.

[6] YANG M R, KE W H, WU S H. Preparation of LiFePO₄ powders by co-precipitation [J]. J Power Sources, 2005, 146(1-2): 539-543.

[7] YANG S F, ZAVALIJ P Y, WHITTINGHAM M S. Hydrothermal synthesis of lithium iron phosphate cathodes [J]. Electrochem Commun, 2001, 3(9): 505-508.

[8] CHEN J J, WHITTINGHAM M S. Hydrothermal synthesis of lithium iron phosphate [J]. Electrochem Commun, 2006, 8(5): 855-858.

[9] LIU H W, YANG H M, LI J L. A novel method for preparing LiFePO₄ nanorods as a cathode material for lithium-ion power batteries [J]. Electrochem Acta, 2010, 55(6): 1626-1629.

[10] ARNOLD G, GARCHE J, HEMMER R, STROBELE S, VOGLER C, MEHRENS M W. Fine-particle lithium iron phosphate LiFePO₄ synthesized by a new low-cost aqueous precipitation technique [J]. J Power Sources, 2003, 119-121(2): 247-251.

[11] CHEN J J, VACCHIO M J, WANG S J, CHERNOVA N, ZAVALIJ P Y, WHITTINGHAM M S. The hydrothermal synthesis and characterization of olivines and related compounds for electrochemical applications [J]. Solid State Ionics, 2008, 178(31-32): 1676-1693.

[12] WU S H, HSISO K M, LIU W R. The preparation and characterization of olivine LiFePO₄ by a solution method [J]. J Power Sources, 2005, 146(1-2): 550-554.

[13] SPEIGHT J G. Lange’s handbook of chemistry [M]. 6th ed. New York: McGraw-Hill, 2005.

[14] KOTRLY S, SUCHA L. Handbook of chemical equilibria in analytic chemistry [M]. New York: Ellis Horwood, 1985.

[15] NRIAGU J O. Stability of vivianite and ion-pair formation in the system Fe₃(PO₄)₂-H₃PO₄-H₂O [J]. Geochim Cosmochim Acta, 1972, 36(4): 459-470.

[16] LA LGLESIA A. Estimating the thermodynamic properties of phosphate minerals at high and low temperature from the sum of constituent units [J]. Estudios Geológicos, 2009, 65(2): 109-119.

[17] ZHAO Z W, LIU X H. Thermodynamic analysis of Li-Fe-P-H₂O system [J]. The Chinese Journal of Nonferrous Metals, 2006, 16(7): 1257-1263. (in Chinese)

[18] AIMABLE A, AYMES D, BERNARD F, CRAS F L. Characteristics of LiFePO₄ obtained through a one step continuous hydrothermal synthesis process working in supercritical water [J]. Solid State Ionics, 2009, 180(11-13): 861-866.

[19] LEE J, TEJA A S. Characteristics of lithium iron phosphate (LiFePO₄) particles synthesized in subcritical and supercritical water [J]. J Supercrit Fluids, 2005, 35(1): 83-90.

[20] UCHIYAMA H, IMAI H. Preparation of LiFePO₄ mesocrystals consisting of nanorods through organic-mediated parallel growth from a precursor phase [J]. Cryst Growth Des, 2010, 10(4): 1777-1781.

[21] LIU J L, JIANG R R, WANG X Y, HUANG T, YU A S. The defect chemistry of LiFePO₄ prepared by hydrothermal method at different pH values [J]. J Power Sources, 2009, 194(1): 536-540.

[22] BOISTELLE R, VALERO I L. Growth units and nucleation: The case of calcium phosphates [J]. J Cryst Growth, 1990, 102(3): 609-617.

[23] TENG F, SANTHANAGOPALAN S, ASTHANA A, GENG X B, MHO S I, YASSAR R S, MENG D D. Self-assembly of LiFePO₄ nano dendrites in a novel system of ethyleneglycol–water [J]. J Cryst Growth, 2010, 312(23): 3493-3502.

[24] DELACOURT C, POIZOT P, MORCRETTE M, TARASCON J M, MASQUELIER C. One-step low-temperature route for the preparation of electrochemically active LiMnPO₄ powders [J]. Chem Mater, 2004, 16(1): 693-699.

[25] LI P, HE W, ZHAO H S, WANG S P. Biomimetic synthesis and characterization of the positive electrode material LiFePO₄ [J]. J Alloys Compd, 2009, 471(1-2): 536-538.

[26] YANG R, SONG X P, ZHAO M S, WANG F. Characteristics of Li_0.98Cu_0.01FePO₄ prepared from improved co-precipitation [J]. J Alloys Compd, 2009, 468(1-2): 365-369.

[27] RUAN Y L, TANG Z Y. Synthesis of LiFePO₄/C cathode material through a solution-phase approach and its electrochemical performance [J]. Acta Chim Sinica, 2008, 66(6): 6680-684.

室温下Li-Fe(II)-P-H₂O体系共沉淀制备LiFePO₄的热力学分析

何利华，赵中伟，刘旭恒，陈爱良，司秀芬

中南大学 冶金科学与工程学院，长沙 410083

摘  要：对Li-Fe(II)-P-H₂O体系共沉淀制备LiFePO₄进行热力学研究。在常温条件下，当pH值为0~11.3时可生成LiFePO₄；而当pH值大于11.3和12.9时，会依次生成Li₃PO₄ 和 Fe(OH)₂。LiFePO₄共沉淀的最佳pH值为8~10.5。考虑到低温条件下共沉淀前躯体向LiFePO₄相转变的动力学速度较慢，对亚稳态Li- Fe(II)- P-H₂O体系进行研究。结果表明，当共沉淀体系的初始Li与Fe与P摩尔比为1:1:1或1:1:3时，不能得到等摩尔量的Fe₃(PO₄)₂·8H₂O和Li₃PO₄沉淀；而当摩尔比为3:1:1、溶液pH值为7~9.2时，能够制备出符合化学计量比的前躯体。这表明采用共沉淀法制备LiFePO₄的一个重要条件是溶液中要存在过量的Li⁺。

关键词：磷酸铁锂；锂离子电池；Li-Fe(II)-P-H₂O系；热力学；共沉淀

(Edited by YUAN Sai-qian)

Foundation item: Project (2007CB613603) supported by the National Basic Research Program of China

Corresponding author: ZHAO Zhong-wei; Tel: +86-731-88830476; E-mail: zhaozw@csu.edu.cn

DOI: 10.1016/S1003-6326(11)61385-X