ARTICLE

J. Cent. South Univ. (2019) 26: 1521-1529

DOI: https://doi.org/10.1007/s11771-019-4108-5

Na₂FePO₄F/C composite synthesized via a simple solid state route for lithium-ion batteries

HU Hai(胡海)^{1, 2}, WANG Yu(王誉)^{1, 2}, HUANG Yan(黄妍)^{1, 2},SHU Hong-bo(舒洪波)^{1, 2}, WANG Xian-you(王先友)^{1, 2}

1. School of Mechanical Engineering and School of Chemistry, Xiangtan University,Xiangtan 411105, China;

2. National Base for International Science & Technology Cooperation, National Local Joint Engineering

Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of

Electrochemical Energy Storage and Conversion, Xiangtan 411105, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Using low-cost FePO₄·2H₂O as iron source, Na₂FePO₄F/C composite is prepared by alcohol-assisted ball milling and solid-state reaction method. The XRD pattern of Na₂FePO₄F/C composite demonstrates sharp peaks, indicating high crystalline and phase purity. The SEM and TEM images reveal that diameter of the spherical-like Na₂FePO₄F/C particles ranges from 50 to 300 nm, and HRTEM image shows that the surface of Na₂FePO₄F/C composite is uniformly coated by carbon layer with a average thickness of about 3.6 nm. The carbon coating constrains the growth of the particles and effectively reduces the agglomeration of nanoparticles. Using lithium metal as anode, the composite delivers a discharge capacities of 102.8, 96.4 and 90.3 mA·h/g at rates of 0.5C, 1C and 2C, respectively. After 100 cycles at 0.5C, a discharge capacity of 98.9 mA·h/g is maintained with capacity retention of 96.2%. The Li⁺ diffusion coefficient (D) of Na₂FePO₄F/C composite is calculated as 1.71×10^{–9 cm2}/s. This study reveals that the simple solid state reaction could be a practical and effective synthetic route for the industrial production of Na₂FePO₄F/C material.

Key words: lithium-ion batteries; Na₂FePO₄F/C composite; alcohol-assisted ball milling; solid state reaction; spherical-like particles

Cite this article as: HU Hai, WANG Yu, HUANG Yan, SHU Hong-bo, WANG Xian-you. Na₂FePO₄F/C composite synthesized via a simple solid state route for lithium-ion batteries [J]. Journal of Central South University, 2019, 26(6): 1521-1529. DOI: https://doi.org/10.1007/s11771-019-4108-5.

1 Introduction

For several decades, LIBs have been investigated and utilized in energy storage system and portable electronic devices due to the outstanding electrochemical performance [1–4]. So far, the advanced cathode materials become the key issues for development of high performance LIBs. At present, commercial cathode materials including LiCoO₂, LiMn₂O₄ and LiFePO₄ [5–8] have been applied to electronic device. LiFePO₄ has been considered a promising material owing to its non-toxic, abundant resource and safety. However, the demand for lithium is increasing in the recent years due to rapid growth of electric vehicle and mobile electronic devices, thus lithium resource diminishes quickly. In contrast to lithium, sodium resource is abundant and sodium’s radius is closer to lithium within alkali metal [9–13]. Therefore, sodium and iron-based materials have attracted more attention as promising candidates for cell cathode. For instance, NaFeO₂ [14], NaFeF₃ [15] and NaFePO₄ [16] have been used as cathode material. Among these materials, Na₂FePO₄F stands out due to its special two-dimensional channels structure and small volume change (3.7%) on the electrochemical reaction [17].

Na₂FePO₄F was firstly reported by ELLIS et al [18], and then further investigated by researchers for LIBs [17, 19-22]. Such phosphate presents high working voltage (3.6 V vs. Li/Li⁺) and considerable safety advantage because of the induction of PO₄^3– and the strong electro-negativity of F^– [23]. In LIBs system, the reaction mechanism of Na₂FePO₄F cathode can be explained by a Li/Na exchange [24, 25]. Na⁺ extraction from and Li⁺ insertion in the lattice occurs during the first few cycling process, namely the Na⁺ site is replaced by Li⁺ in Na₂FePO₄F crystallographic. The NaFePO₄F intermediate phase exhibits a volume expansion of unit cell compared to LiFePO₄F, thus facilitating the insertion/extraction of Li⁺. The chemical reaction equations is described as follows: Na₂FePO₄F→ NaFePO₄F+Na⁺+e, and then: NaFePO₄F+Li⁺+e→ (Li, Na)FePO₄F. Meanwhile, Li⁺ insertion/ extraction occurs at the graphite counter electrode. Even in the presence of Na⁺, the graphite anode is electrochemically inactive in the electrolyte. Therefore, it may be promising that Na₂FePO₄F is utilized as cathode of LIBs for electric vehicle and energy storage devices.

As one of phosphates, electrical conductivity of Na₂FePO₄F is relative low, which leads to low specific capacity, unsatisfying cycling life and rate capability. As known to all, carbon coating is an effective way to enhance the electrical conductivity. In most cases, carbon sources such as citric acid [20], acetylene black [22], and ascorbic acid [26], are applied to improve the electrical conductivity of Na₂FePO₄F. Reported synthesis methods are solid-state reaction [18, 22], sol-gel [20], ion thermal [17], hydrothermal [19], and spray-drying methods [27]. Among these methods, solid state method is usually adopted in practical because of easy operation and simple synthetic procedure. But there are some disadvantages including non-uniform carbon coating and serious agglomeration. To solve the problem, a modified method, by which reactants are mixed by ball milling in alcohol and calcinated at high temperature, is put forward to synthesize Na₂FePO₄F/C composite. The as-prepared composite is composed of nanoparticles (50–300 nm), the surface of which is uniformly coated by thin carbon layer, and a small amount of particles come together. The structure, morphology and electrochemical performance of Na₂FePO₄F/C composite are studied in detail.

2 Experimental

2.1 Material synthesis

Na₂FePO₄F/C composite was synthesized by a modified solid state reaction from stoichiometric amounts of FePO₄·2H₂O, NaHCO₃, NaF and glucose. The precursors were mixed by ball milling for 3 h in alcohol. The mixture was dried at 70 °C for 12 h. The obtained powder was pre-sintered at 350 °C for 3 h, and further sintered at 600 °C for 8 h under Ar.

2.2 Material characterizations

The crystal structure of Na₂FePO₄F/C composite was tested over 2θ interval of 10°–80° by Rigaku D/MAX-2500 X-ray diffraction using Cu K_a radiation (λ=1.54178 ) and a graphite monochromator with a scan rate of 5°/min. The morphologies and microstructure were analyzed by using a JEOL JSM-6610 scanning electron morphologe (SEM) and JEM-2100F transmission electron microscope (TEM). Energy-dispersive X-ray spectroscopy (EDX) was performed on the TEM system with an EDX attachment. Raman spectra were collected using Macro-Raman spectrometer (Renishaw invia Raman microscope). Elemental analysis on carbon was performed by Vario EL III elemental analyzer. The specific surface area was determined using the Brunnauer- Emmett-Teller (BET) surface adsorption method (BET micromeritics tristar 3000).

2.3 Electrochemical measurements

Coin-type cells (2025 type) were made in the argon-filled glove box, where oxygen and moisture concentrations were both less than 5×10^–6. The electrode was composed of 70% cathode material, 20% acetylene black, and 10% polyvinylidene fluoride (PVDF) binder. The mass of the positive active material is about 3.57 mg/cm² per electrode. The testing cells were composed of as-prepared cathode, lithium foil negative electrode, Celgard 2300 separator, and 1 mol/L LiPF₆/(V(EC): V(DMC)=1:1) as the electrolyte. Electrochemical tests were performed at 25 °C under 2.0–4.5 V utilizing the galvanostatic charge/discharge unit of NetWare battery system BTS-XWJ-6.44S-00052. Specific capacity was calculated based on the mass of active materials. Cyclic voltammetry (CV) experiments were carried out at different scan rates using a VersaSTAT3 electrochemical workstation (Princeton, America). EIS was also performed on a VersaSTAT3 electrochemical workstation.

3 Results and discussion

3.1 Materials characterizations

Figure 1(a) displays the XRD pattern of Na₂FePO₄F/C composite. The XRD pattern of Na₂FePO₄F/C matches well with the standard pattern (ICSD No: 167044) of the orthorhombic structure with the Pbcn space group [18]. The single phase Na₂FePO₄F is similar to the structure of Na₂FePO₄(OH) [28], analogous to Na₂MgPO₄F [29] and Na₂CoPO₄F [30]. No impurity peaks are detected, indicating high phase purity. Moreover, carbon peaks are not observed in the diffraction pattern of sample, implying amorphous state of carbon. Figure 1(b) displays the crystal structure model of Na₂FePO₄F. Apparently, dioctahedral Fe₂O₇F₂ units, composed of sharing-face FeO₄F₂ octahedra, are connected with F atoms to construct chains. These chains are linked by PO₄ to form [FePO₄F] layers which provide two-dimensional channels for ion movement [18, 19, 31, 32].

The SEM image (Figure 2(a)) reveals that the size of Na₂FePO₄F particles is in the range of 50–300 nm. The particles are homogeneous- dispersed, and few particles conglomeration is found between nearby particles. The TEM image (Figure 2(b)) confirms that spherical-like Na₂FePO₄F/C composite is composed of small primary nanoparticles. Further investigation into Na₂FePO₄F/C composite is performed using HRTEM as shown in Figure 2(c), and the fast Fourier transform (FFT) graph is displayed in the inset of Figure 2(c). Clear lattice fringes are observed from Figure 2(c), reflecting that Na₂FePO₄F/C sample is well crystalline. The crystal fringes with a spacing of about 0.393 nm associated with (121) plane of Na₂FePO₄F (d₁₂₁= 3.933 ). Moreover, the outer of nanoparticles is uniformly encapsulated by carbon layer of about 3.6 nm thickness, by which small particles connect to form excellent conductive network. The signals of Na, Fe, P, O and F can be determined in Figure 2(d), and the Cu peaks come from a copper collector. The atomic mole ratios are obtained by EDX result,x(Na) : x(Fe) : x(P) : x(F)=10.97 : 5.26 : 5.43 : 5.38, which is near to the theoretical value of 2 : 1 : 1 : 1. The carbon fraction in Na₂FePO₄F/C is determined to be 2.35% by carbon & sulfur analyzer.

Raman spectrum is applied to analyzing carbon species on Na₂FePO₄F/C composite (Figure 3(a)). The strong D and G peaks are observed at 1348 and 1587 cm^–1, respectively. Generally, the G-band is attributed to graphitic carbon, and the D-band is related to disorder carbon. I_D/I_G is usually used to identify the degree of carbon disorder. The calculated I_D/I_G is 0.89, which is bigger than that of using mixed carbon source [33]. The result shows disorder carbon is the major component of carbon layer. Additionally, it is noted that Raman scatting by PO₄^3– in the range of 300–1100 cm^–1 is not observed and the Raman signals of carbon are mainly found with G and D bands. The analyses further confirm the purity phase of Na₂FePO₄F, which is consistent with XRD result. The N₂ adsorption–desorption of Na₂FePO₄F/C is displayed in Figure 3(b). The isotherm with a hysteresis loop appeared at high pressure (p/p_o>0.8) belongs to a typical of IV isotherm. A surface area value of 24.7 m²/g is calculated by means of BET method.

Figure 1 XRD pattern of Na₂FePO₄F/C composite (a) and crystal structure of Na₂FePO₄F with space group Pbcn, viewed along [100] (b)

Figure 2 SEM image (a), TEM (b) and HRTEM (c) images and energy-dispersive X-ray spectrum (d) of Na₂FePO₄F/C composite

3.2 Electrochemical properties

The as-prepared samples have been tested in lithium-ion batteries. Figure 4 shows charge/ discharge curves of Na₂FePO₄F/C in the first five cycles at 0.1C (12.4 mA/g). The initial charge/ discharge capacity are 109.2 and 120.9 mA·h/g, respectively, and the columbic efficiency is 110.7%. The high columbic efficiency is probably because of the excessive amount of ions (Li⁺/Na⁺) insert into vacancies (Na site)of the Na₂FePO₄F/C crystal compared with the charge process, similar results have been reported by ELLIS et al [18]. The two obvious charge platforms and the two discharge platforms appear at 3.0 and 3.7, 2.9 and 3.6 V, respectively. After three cycles, the ion-exchange has completed and NaLiFePO₄F phase is formed [19, 22]. The curve shows a repetitive loop of charging and discharging, and the reversible discharge capacity of 117.8 mA·h/g can be retained.

Figure 3 Raman spectrum (a) and N₂ adsorption- desorption isotherm (b) of Na₂FePO₄F/C composite

Figure 4 Charge/discharge curves of Na₂FePO₄F/C composite at 0.1C

The charge/discharge curves of Na₂FePO₄F/C composite at the various rates are displayed in Figure 5(a). The discharge capacities gradually decrease with increasing discharge current density, meanwhile, the electrode polarization increases. Figure 5(b) shows the rate capability with a sustain discharge capacity of 118.5, 102.8, 96.4, 90.3 and 55.7 mA·h/g at 0.1C, 0.5C, 1C, 2C and 5C, respectively. When the current density is reduced back to 0.1C, the capacity is almost equal to the original value (118.5 mA·h/g). The rate performance in our work is superior to the recently reported works. For example, CUI et al [33] synthesized Na₂FePO₄F/C by a carbothermal reduction, and the electrode exhibited discharge capacity of 74 mA·h/g at 1C and 52 mA·h/g at 2C. BRISBOIS et al [34] prepared Na₂FePO₄F/C microsphere using spray-drying method, and a discharge capacity of 78 mA·h/g at 1C rate was achieved. Figure 5(c) displays the cycling performance of the Na₂FePO₄F/C composite at 0.5C. An initial discharge capacity of 102.8 mA·h/g is delivered and 98.9 mA·h/g is maintained after 100 cycles with capacity retention of 96.2%. The Na₂FePO₄F/C cathode reported by CUI et al [33] delivered a capacity of 110 mA·h/g at 0.1C, and only 91% of capacity could be retained after 30 cycles. Apparently, the Na₂FePO₄F/C composite in our work exhibits better cycling performance.

Figure 6(a) shows CV curves of Na₂FePO₄F/C composite at different scan rates of 0.1 to 1 mV/s. As can be seen, a pair of symmetrical redox peaks can be found, indicating good reversibility of the electrode. Na₂FePO₄F/C cathodes show two anodic peaks located at about 3.0 V and about 3.7 V corresponding to catholic peaks at about 2.9 V and about 3.6 V at a scan rate of 0.1 mV/s, which are consistent with the two voltage platforms in the charge/discharge curves, respectively. The two pairs of redox peaks probably represent the two reversible phase transformations of NaLiFePO₄FNaLi_0.5FePO₄F and NaLi_0.5FePO₄FNaFePO₄F based on the mechanism of NaLiFePO₄F electrode reaction [24, 25]. As the scan rate increasing, the anodic peaks move to higher value and the corresponding cathodic peaks move to lower value. But the small potential interval (ΔE_p) between redox peaks (0.29 V for 0.1 mV/s and 0.67 V for 1 mV/s ) implies fine reversibility and rate capability [35]. Furthermore, the linear relationship between ν^1/2 and I_p was examined in Figure 6(b), indicating a diffusion controlled process for the electrode reaction. Li⁺ diffusion coefficient (D) could be calculated using the Randles-Sevcik equation as:

            (1)

Figure 5 Charge/discharge curves at different rates (a), rate performance (b) and cycling stability cycled at 0.5C rate (c) of Na₂FePO₄F/C composite

where m represents the mass of cathode; F is the Faraday constant, R is the general gas constant; T represents the absolute temperature; n is the number of electrons per mole (n=1); A represents the area (A=0.785 m²); D represents Li⁺ diffusion coefficient and C is the Li⁺ concentration calculated from the lattice parameters of Na₂FePO₄F. In Figure 6(a), anodic peak (B) and catholic peak (D) fade slowly at a high scan rate (≥0.8 mV/s). The phenomenon is similar to that of the work reported by RUI et al [36]. Thereby, anodic peak (A) and catholic peak (C) might behave as a solid solution behavior, which could be utilized to determine the real Li⁺ diffusion coefficient. The calculated values according to anodic peak (B) and catholic peak (D) could be regarded as the apparent result. The Li⁺ diffusion coefficients (D) are calculated as follows: 1.40×10^{–9 cm2}/s (peak A), 4.96×10^{–10 cm2}/s(peak B) and 1.71×10^{–9 cm2}/s (peak C), and 3.70×10^{–10 cm2}/s (peak D).

Figure 6 CV curves at different scan rates of Na₂FePO₄F/C in range of 2.0–4.5 V vs Li⁺/Li (a) and linear relationship between square root of scan rate ν^1/2 and peak current I_p (b)

EIS is further used to explore the electrochemical performance of Na₂FePO₄F/C composite. The Nyquist plot is given in Figure 7(a). The high-frequency semicircle is ascribed to SEI film resistance, and the semicircle in the middle- frequency is related to the charge transfer resistance. The inclined line at the low frequency is the Warburg impedance (Z_w) which is associated with the Li⁺ diffusion into the electrode. Figure 7(b) is the equivalent circuit used to fit EIS. Here, R_e is the solution resistance, R_sf is SEI resistance and R_ct is charge transfer resistance. CPE₁ and CPE₂ represent double layer capacitance of solid electrolyte interface (SEI) and the capacitance at the electrode- electrolyte interface, respectively. W_s is the Warburg element. Apparently, the fitting pattern is in good agreement with the experimental EIS data. The fitted impedance parameters are listed in Table 1. As seen in Table 1, R_ct and R_sf values of Na₂FePO₄F/C electrode are 199.4 and 250.7 Ω, respectively, suggesting that redox reaction of electrolyte/electrode takes place easily.

Figure 7 Nyquist plot of Na₂FePO₄F/C electrode (a) and equivalent circuit model (b)

Table 1 Impedance parameters of fitting equivalent circuit

4 Conclusions

The Na₂FePO₄F/C composite with uniform particle distribution is prepared by solid state reaction. The EIS result indicates that Na₂FePO₄F/C electrode has small charge-transfer resistance. The Na₂FePO₄F/C composite as cathode materials for LIBs exhibits improved cycling stability and rate performance. It exhibits a discharge capacity of 102.8 mA·h/g at 0.5C, and superior capacity retention of 96.2% after 100 cycles. Discharge capacity of 96.4 mA·h/g at 1C and 90.3 mA·h/g at 2C are achieved. The improved electrochemical performance is ascribed to small size of nanoparticles and uniform carbon-coating. Spherical-like nanostructures lead to the increased interface of electrode/electrolyte and shortened transport distances of both electron and Li⁺. Thus, the preparation of Na₂FePO₄F/C composite is an effective method to enhance the performance of Na₂FePO₄F for the application in LIBs.

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(Edited by FANG Jing-hua)

中文导读

简单固相法合成锂离子电池用Na₂FePO₄F/C复合材料

摘要：选用二水合磷酸铁为铁源，经乙醇辅助球磨和固相反应制备了氟磷酸亚铁钠/碳复合材料。X-射线衍射证实产品有高的结晶度和相纯度。扫描电镜和透射电镜照片显示，球形氟磷酸亚铁钠粒子的粒径分布在50~300 nm之间；从高分辨透射电镜图可以看出，在氟磷酸亚铁钠/碳复合材料的表面包覆了一层厚度为3.6 nm的碳层。碳层的包覆能有效地遏制氟磷酸亚铁钠粒子的长大及粒子的团聚。 以锂片作为负极组装半电池，在0.5C，1C，2C倍率下，复合材料的放电比容量分别为102.8，96.4，90.3 mA·h/g。0.5C循环100次后电池的放电比容量为98.9 mA·h/g，容量保持率为96.2%。从循环伏安曲线计算得到氟磷酸亚铁钠/碳复合材料的锂离子扩散系数为1.71×10^{–9 cm2}/s。显然，固相法是制备锂离子电池正极材料用氟磷酸亚铁钠/C复合材料的有效方法。

关键词：锂离子电池；氟磷酸亚铁钠/碳复合材料；乙醇辅助球磨；固相反应；球形粒子

Foundation item: Projects(51472211, 51502256) supported by the National Natural Science Foundation of China; Projects(2016GK4005, 2016GK4030) supported by the Strategic New Industry of Hunan Province, China; Project(13C925) supported by the Research Foundation of Education Bureau of Hunan Province, China

Received date: 2018-09-30; Accepted date: 2019-03-07

Corresponding author: WANG Xian-you, PhD, Professor; Tel: +86-731-58293377; E-mail: wxianyou@yahoo.com; ORCID: 0000-0001- 8888-6405