Hydrothermal synthesis of LiFePO₄ as a cathode material for lithium batteries

Abstract

LiFePO₄ (space group: Pnma) was prepared by hydrothermal method at 170 °C. LiFePO₄ was prepared from precursor solutions consisting of FeSO₄ · 7H₂O, (NH₄)₂HPO₄, and three kinds of Li sources. LiCl, Li(CH₃COO), and LiOH · H₂O were used as Li sources. The pH of the precursor solution varied depending on Li source. The particle size, particle shape, and crystal texture of the obtained LiFePO₄ changed depending on pH. The electrochemical properties of the prepared LiFePO₄ were characterized as a cathode material for lithium batteries in an organic electrolyte at room temperature. The LiFePO₄ particle prepared from the precursor solution with Li(CH₃COO) was flake-like crystal (particle size: 1–2 μm) and had a preferred crystal orientation with a (020) texture. This LiFePO₄ exhibited a discharge capacity of 147 mA h g⁻¹, which was 85% of the theoretical capacity 170 mA h g⁻¹.

Introduction

LiFePO₄ has been prepared by various methods, in order to improve its electrochemical performance as a cathode material of rechargeable lithium batteries [1–6]. Usually, transition metal oxides such as LiCoO₂ and LiMn₂O₄ have been utilized as cathode materials for rechargeable lithium batteries. However, present cathodes have been suffered from a safety problem, which is due to oxygen release from the transition metal oxides under abuse conditions [7]. The oxygen release causes a rapid temperature increase in the battery, which sometimes leads to an explosion of battery. In order to realize real safety of rechargeable lithium batteries, new type cathode materials have to be developed. Recently, phosphate compounds have been paid much attention in a sense of safety and cost of cathode materials. Especially, LiFePO₄ has been intensively studied by many research groups [1–6]. The Li⁺ ion can be extracted/inserted from/into LiFePO₄ at the electrode potential of 3.5 V versus Li/Li⁺, and the theoretical discharge capacity of LiFePO₄ is 170 mA h g⁻¹. In addition, iron compounds are usually very cheap compared with other transition metal compounds. A main problem of LiFePO₄ is its low electronic conductivity. Several researchers reported that carbon-coating on LiFePO₄ particle was effective in improving its electrochemical performance [8–14]. On the other hand, high crystalline small particles of LiFePO₄ have been prepared by using various methods, such as hydrothermal process, sol–gel process, and so on [15–19]. In the case of hydrothermal process, the composition of precursor solution strongly influences on particle nature of LiFePO₄. Moreover, the oxidation state of iron ion during hydrothermal treatment is very important to obtain LiFePO₄ without any impurities [20]. In order to increase the utilization of LiFePO₄ for charge and discharge, smaller particle should be prepared under suitable conditions because the shorter electronic conduction path is helpful in reducing ohmic resistance within the particle.

The crystal texture of LiFePO₄ particle is also an important aspect, which is strongly related to electrode kinetics [21–24]. The Li⁺ ion diffusion takes place along b-axis in the crystal of orthorhombic LiFePO₄ (space group: Pnma) during charge and discharge, and the charge transfer (Li⁺ ion transfer) takes place mainly on the (020) plane [24]. Therefore, if a crystal with a (020) texture is obtained, faster electrochemical reaction can be achieved.

In this study, LiFePO₄ was prepared by hydrothermal method from FeSO₄ · 7H₂O, (NH₄)₂HPO₄, and three kinds of Li sources. LiCl, Li(CH₃COO), and LiOH · H₂O were used as Li sources. The pH of the precursor solution varied depending on Li source. The particle size, particle shape, and crystal texture of the obtained LiFePO₄ changed depending on the pH. The crystal characteristics and electrochemical properties of the prepared LiFePO₄ were investigated. Especially, the effects of crystal texture of LiFePO₄ on its electrochemical properties were discussed.

Experimental

LiFePO₄ was prepared by the hydrothermal method with three kinds of Li sources LiCl, LiCH₃COO, and LiOH · H₂O. The FeSO₄ · 7H₂O and (NH₄)₂HPO₄ were used as Fe and P sources, respectively. These starting materials were dissolved into ultra pure water. This mixed solution (40 mL) was put into Teflon-lines Parr reactor (inner volume: 120 mL). Table 1 shows the molar ratio of each compound and concentration of FeSO₄ for the hydrothermal process. Using these three different solutions, the hydrothermal process was performed at 170 °C for 12 h using a stainless steal autoclave. These processes were performed under N₂ atmosphere in order to prevent the oxidation of Fe²⁺ to Fe³⁺ [25]. After filtration, the prepared sample was dried under vacuum at 100 °C for 1 h. The prepared samples were characterized with X-ray diffraction (XRD). The charge and discharge measurements of prepared samples were conducted using 2016 coin-type cells. The cathode was prepared from a mixture of prepared sample, Ketjen black, and PTFE with weight ratio of 80:15:5. The anode was lithium metal and a mixed solvent of ethylene carbonate and diethylcarbonate with volume ratio of 1:1 containing 1.0 mol dm⁻³ LiClO₄ was used as the electrolyte. The charge and discharge tests were performed at 0.1 C rate (17 mA g⁻¹-LiFePO₄). The electrochemical test was conducted in an argon-filled glove box at room temperature.

Table 1 Molar ratio of each starting materials and concentration of FeSO₄, pH of the precursor solution, and yield of LiFePO₄

Results and discussion

The pH of precursor solution depended on lithium salt used as a starting material. In the case of LiOH, the precursor solution was most basic condition, and the pH was 8.99. The precursor solution prepared using LiCl as Li source was most acidic and the pH was 3.44. When Li(CH₃COO) was used as Li source, pH of the precursor solution was 5.11. The differences in Li source and pH of precursor solutions influenced on the particle shape of resulting LiFePO₄. Figure 1 shows the scanning electron micrographs of hydrothermally synthesized samples. Using LiOH as a Li source, the smallest particles were prepared. The particles were agglomerated, and the particle size was less than 1 μm. Among the three samples, the largest particle was obtained using LiCl as Li source. The particle was very thin and flake-like crystal, which was different from that prepared in the solution containing LiOH. On the other hand, the precursor solution with Li(CH₃COO) provided thick flake-like crystals. The particles were well dispersed, and the particle size was estimated to be 1–2 μm. In this way, the particle nature of LiFePO₄ strongly depended on Li source and/or pH of the precursor solution.

Fig. 1

Scanning electron micrographs of LiFePO₄ prepared from precursor solutions with LiCl (a), Li(CH₃COO) (b), and LiOH · H₂O (c)

Figure 2 shows XRD patterns of LiFePO₄ samples prepared from three precursor solutions. All of the peaks in three x-ray diffraction patterns can be assigned to those of orthorhombic LiFePO₄ (space group: Pnma) reported in the literature [16–18]. Therefore, it can be said that three samples are the single phase of LiFePO₄. However, the yield of LiFePO₄ (conversion ratio of Fe²⁺ in the precursor to LiFePO₄) depended on the acidic condition of the solution as shown in Table 1. When the pH of the precursor was higher than 5, the yield was more than 90%. However, the yield was as small as 55% in the case of precursor with LiCl because of the strong acid condition. Probably, the solubility of LiFePO₄ would be higher under acidic condition pH <4 at high temperatures (∼170 °C).

Fig. 2

X-ray diffraction patterns of LiFePO₄ prepared from precursor solutions with LiCl (a), Li(CH₃COO) (b), and LiOH · H₂O (c)

Although the all XRD patterns in Fig. 2 were assigned to the LiFePO₄ with olivine structure, the relative intensities of the peaks were completely different between three samples. The peak intensity ratio of the LiFePO₄ prepared from the precursor solution with LiOH was very similar to that reported in the literature [11–14], and the particle had a random crystal orientation. Indeed, the obtained particles were agglomerated polycrystalline particles as shown in Fig. 1c. However, the peak intensity of (200) of the LiFePO₄ prepared from the precursor solution with LiCl was strongest, which suggested that the LiFePO₄ flake particle had a preferred crystal orientation with a (200) texture. On the other hand, the (020) peak of the LiFePO₄ prepared from the precursor solution with Li(CH₃COO) was strongest, which suggested that the LiFePO₄ flake crystal had a preferred crystal orientation with a (020) texture. In this way, the crystal texture of LiFePO₄ was strongly influenced by Li source and/or pH of the precursor solution. These results suggested that the preferred crystal growth orientation changed depending on the pH of solution. The crystal texture of particles gives significant effects on the electrochemical properties of LiFePO₄. The following electrochemical measurements were conducted for these LiFePO₄ samples.

Figure 3 shows the charge and discharge curves of LiFePO₄ prepared from three different precursor solutions. The largest discharge capacity was achieved using Li(CH₃COO) as a Li source of LiFePO₄, which was 147 mA h g⁻¹. On the other hand, LiFePO₄ samples prepared from precursor solutions with LiCl and LiOH exhibited lower discharge capacities. As mentioned above, all LiFePO₄ samples prepared by hydrothermal method in this study did not contain any impurities, so that all samples were expected to show discharge capacities near to the theoretical one (170 mA h g⁻¹). However, the discharge capacity depended on Li source utilized as a starting material for hydrothermal synthesis of LiFePO₄. The differences among three samples were particle size, particle shape, and crystal texture. Particle size of active material usually influences on charge-discharge performance, because the Li⁺ ion diffusion and electronic conduction take place within the particle. In fact, as reducing the particle size, the diffusion length of Li⁺ ion and electronic conduction path are decreased, which reduce the diffusion resistance for Li⁺ ion and electrical resistance within the particle [11]. In the case of LiFePO₄, crystallographic phase transition takes place during charge and discharge, so that phase boundary diffusion may occur within the particle [16]. The rate of boundary diffusion depends on particle size and shape. By the way, LiFePO₄ is an olivine-type compound, and anisotropic crystal structure is involved [23]. Consequently, the crystal orientation of LiFePO₄ particle gives significant effects on its charge–discharge process. The Li⁺ ion diffusion takes place along b-axis in the crystal of LiFePO₄ during charge and discharge [21–24]. Furthermore, the charge transfer (Li⁺ ion transfer) takes place mainly on the (020) plane [24]. Therefore, the electrode reaction rate of LiFePO₄ can be improved by a preferential crystal orientation. If a tiny crystal with a (020) texture is obtained, faster electrochemical reaction can be achieved. In this way, the charge and discharge characteristics of LiFePO₄ should be discussed considering at least two factors, i.e., crystal texture and particle size. The LiFePO₄ prepared from the precursor solution with Li(CH₃COO) had a larger particle size than that prepared from the precursor solution with LiOH. However, the LiFePO₄ crystal prepared from the precursor solution with Li(CH₃COO) had a preferred orientation with a (020) texture while the LiFePO₄ prepared from the precursor solution with LiOH had random crystal orientations. The discharge capacity of LiFePO₄ prepared from the precursor solution with LiCl was the smallest among the three LiFePO₄ samples, due to the largest particle size and the (200) crystal texture. This crystal texture is not suitable for Li⁺ ion insertion and extraction, because the Li⁺ ion diffusion scarcely occurs along a-axis and the charge transfer does not take place on the (200) plane but (020) [24]. LiFePO₄ prepared from the precursor solution with LiOH had the smallest particle size, however, the crystal texture of the sample was random. The smaller particle of the LiFePO₄ is suitable for faster electrode reaction, but the poor (020) texture of LiFePO₄ is not good. From these results, it can be said that the crystal texture of LiFePO₄ particle is more dominant factor for the electrochemical performance. The crystal texture of LiFePO₄ particle was strongly influenced by the Li source and/or pH of the precursor solution. It was revealed that the weak acidic condition (pH ≈5) was preferable to obtain flake-like crystals of LiFePO₄ with a (020) texture, which showed high electrochemical reactivity.

Fig. 3

Charge and discharge curves of LiFePO₄ prepared from precursor solutions with LiCl (a), Li(CH₃COO) (b), and LiOH · H₂O (c). Charge and discharge measurements were carried out at 0.1 C rate

Figure 4 shows charge and discharge curves of LiFePO₄ prepared from the precursor solution with Li(CH₃COO) at various charge and discharge rates. The discharge capacity decreased with increasing charge and discharge rate. This is due to polarization in cathode. At 2.0 C rate, the discharge capacity was 32 mA h g⁻¹. This is smaller value compared with those of other cathode materials at 2.0 C (LiCoO₂, LiMn₂O₄, and so on) [26, 27]. The electronic conductivity of LiFePO₄ is smaller than those of other cathode materials. This property is not good for the high rate charge-discharge performance of lithium battery. In order to overcome this problem, smaller particles of LiFePO₄ with a (020) texture should be prepared. The particle size of LiFePO₄ does not only depend on Li source but also concentration of Fe²⁺ and PO ³⁻₄ ions in the precursor solution. Further optimization of hydrothermal condition is now in progress in our group and will be reported in due course.

Fig. 4

Charge and discharge curves of LiFePO₄ prepared from precursor solutions with Li(CH₃COO). Charge and discharge measurements were carried out at various rates

Conclusion

LiFePO₄ was prepared by hydrothermal method using three different Li sources, LiOH, Li(CH₃COO), and LiCl. The pH of the precursor solution varied depending on the Li source. The particle morphology and preferred crystal growth orientation of LiFePO₄ were strongly influenced by Li source and/or pH of the precursor solution. The electrochemical performance of LiFePO₄ depended on the crystal texture. The LiFePO₄ crystal with a (020) texture exhibited the highest charge and discharge capacities among the three samples. The discharge capacity of LiFePO₄ was 147 mA h g⁻¹ measured at 0.1 C rate (17 mA g⁻¹), however, the discharge capacity decreased with increasing discharge rate and became less than 40 mA h g⁻¹ at 2.0 C rate. This may be due to low electronic conductivity of LiFePO₄. In order to improve the electrochemical reactivity of LiFePO₄, smaller crystals with a (020) texture should be prepared by optimizing the hydrothermal condition.