ARTICLE

J. Cent. South Univ. (2019) 26: 1443-1448

DOI: https://doi.org/10.1007/s11771-019-4100-0

Synthesis and electrochemical properties of Li₂FeSiO₄/C/Ag composite as a cathode material for Li-ion battery

TANG Yi-qun(唐轶群)¹, LIU Xi(刘喜)¹, HUANG Xiao-bing(黄小兵)^{1, 2},

DING Xiang(丁祥)¹, ZHOU Shi-biao(周诗彪)¹, CHEN Yuan-dao(陈远道)¹

1. Hunan Province Cooperative Innovation Center for the Construction &Development of Dongting

Lake Ecological Economic Zone, College of Chemistry and Materials Engineering,

Hunan University of Arts and Science, Changde 415000, China;

2. Key Laboratory of Preparation and Application of Environmental Friendly Materials of Ministry of Education, Jilin Normal University, Changchun 130103, China

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

Abstract: Li₂FeSiO₄ is deemed to be a potential candidate for large-scale applications because of its abundance, low cost and high safety, etc. Unfortunately, its low conductivity, resulting in poor rate performance, has become a main obstacle to its applications in power battery and energy storage system. In this work, C-Ag coated Li₂FeSiO₄ is introduced to improve the innate electronic conductivity and Li-ion diffusion ability. The results demonstrate that Li₂FeSiO₄/C/Ag composite exhibits better electrochemical performance. It possesses a specific discharge capacity of 152, 121, 108 mA·h/g at 0.2C, 5C and 10C, respectively. At the same time, the Li₂FeSiO₄ /C/Ag composite shows good cycle stability and a capacity retention ratio of 97.9% after 100 cycles at 1C.

Key words: lithium-ion batteries; cathode material; Li₂FeSiO₄; pitch; C-Ag coating

Cite this article as: TANG Yi-qun, LIU Xi, HUANG Xiao-bing, DING Xiang, ZHOU Shi-biao, CHEN Yuan-dao. Synthesis and electrochemical properties of Li₂FeSiO₄/C/Ag composite as a cathode material for Li-ion battery [J]. Journal of Central South University, 2019, 26(6): 1443-1448. DOI: https://doi.org/10.1007/s11771-019-4100-0.

1 Introduction

After Thomas research group first reported orthogonal structure Li₂FeSiO₄ as cathode material for lithium ion battery in 2015, Li₂FeSiO₄ has been investigated with wide interests. It has significant advantages over traditional layered LiCoO₂, spinel LiMn₂O₄ and olivine type LiFePO₄ cathode materials [1-3]: 1) High theoretical specific capacity (332 mA·h/g) can be obtained if two lithium ions could be discharged reversibly; 2) It is clear that there are abundant, cheap and environmentally-friendly Si and Fe in the Earth's crust; 3) Si—O bonds are more stable and the security of Li₂FeSiO₄ is much higher. However, Li₂FeSiO₄ material is characterized by low electronic conductivity (10^-14S/cm [4]) as well as low ionic diffusion coefficient (10^-14 cm²/S [5]), which leads to undesirable properties of the battery at high rate, especially when this material applys to powervehicles, large energy storage batteries and other areas, it will be greatly restricted.

At present, four main methods were used by research groups to solve this problem, including: 1) a composite electrode is formed to heighten the electronic conductivity of the material by surface coating or dispersing the conductive carbon materials [6-8]; 2) ion doping of Li₂FeSiO₄, such as Sn⁴⁺ [9], P⁴⁺ [10, 11], Al³⁺ [12], Y³⁺ [13], Ti⁴⁺ [14], Mg²⁺ [5, 15], V⁵⁺ [16], Cr³⁺ [17] and Zn²⁺ [18], enhances intrinsic conductivity of materials; 3) the nanosized Li₂FeSiO₄ was synthesized [19-22] to shorten the Li ion migration path and increase the contact area of the electrode active material with the electrolyte; 4) the preparation of porous Li₂FeSiO₄ [23, 24] is beneficial to the electrolyte directly filling the pore, shortening the transmission distance of lithium ions, and reducing the damage of the material structure caused by volume expansion during cycling.

In our previous research, Li₂FeSiO₄/C composite was prepared from high softening point coal pitch as carbon source. Furthermore, our research finds that carbon obtained by carbonization of aromatic asphalt during heat treatment has higher graphitization degree and better electrical conductivity, which improves the electronic conductivity of Li₂FeSiO₄ [25]. Based on this research background, we adopted solid phase reaction to synthesize the Li₂FeSiO₄/C/Ag composite and studied the effects of C-Ag coating on the structure, morphology and electrochemical properties of Li₂FeSiO₄.

2 Experimental

Li₂FeSiO₄/C/Ag composite was synthesized from lithium carbonate (Aladdin, 99.99%), ferrous oxalate dihydrate (Shenzhen Shuotian Science and Technology, 99.7%), silver nitrate (Sinopharm Chemical Reagent Co., Ltd, 99.8%) and pitch (BTR Battery Materials Co., Ltd.). Firstly, 0.04 mol of lithium carbonate, 0.04 mol of ferric oxalate dihydrate, 0.04 mol of silica, 1.5 g of pitch and 0.14 g of silver nitrate were dispersed into 60 mL of acetone, adding 211 g zirconia beads of 3 mm diameter into zirconia ball mill tank with milling for 12 h at 500 rad/min. Secondly, the precursor was prepared by drying in a vacuum circumstance at 80 °C for 2 h. Finally, the precursor was preheated under argon atmosphere for 5 h at 350 °C, followed by heating at 700 °C for 10 h to acquire the Li₂FeSiO₄/C/Ag composite cathode material. The fabrication procedure of Li₂FeSiO₄/C composite is similar to that of Li₂FeSiO₄/C/Ag composite without using silver nitrate as the starting materials.

The structure of the prepared samples was characterized by X-ray diffraction instrument (XRD, DX2700). The morphology was observed by using a scanning electron microscope (SEM, HITACHI S340). The material conductivity was investigated by four-point probe (RTS-9). Specific surface area meter (BET, SSA4200) was carried out to measure material specific surface area.

The electrode mixture consisted of active substance (Li₂FeSiO₄), Super-P and binder LA-132, and the mass ratio of all three in the electrode is 80:10:10. The preparation of the electrode sheet followed the following steps. First, Super-P was evenly dispersed in the binder LA-132. Then adding the active substance, the slurry was obtained after uniform mixing. The slurry was evenly coated on the aluminum foil with a medical scraper and made into a circle with a diameter of about 1.2 cm. In the end, the electrodes were baked in a vacuum oven for 16 h before use. The coin cells were prepared in dry glove boxes filled with argon by using lithium plate as anode, Celgard 2400 as separator and 1 mol/L LiPF₆/EC:DEC:DMC (V:V:V=1:1:1) as the electrolyte. Galvanostatic charge and discharge cycle tests were carried out on LAND equipment with a potential range from 1.5 to 4.8 V at 25 °C.

3 Results and discussion

Figure 1 shows the XRD profiles of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites. As observed, the main peaks of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites are consistent with those reported by NISHIMURA [26], which indicates that the addition of pitch and silver nitrate to the precursor does not affect the formation of Li₂FeSiO₄ phase. Compared with Li₂FeSiO₄/C composite, the diffraction peak strength of Li₂FeSiO₄/C/Ag composite at 2θ=38° and 2θ=44° was obviously enhanced, which is likely to be caused by the simple Ag in Li₂FeSiO₄/C/Ag composite. Test results of carbon sulfur analyzer show that the carbon contents of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composite materials were 12.1% and 12.9%, respectively. The Ag content of Li₂FeSiO₄/C/Ag composite is calcaculated to be about 1%.

Figure 2 shows the SEM images of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites, where Figure 2(a) shows Li₂FeSiO₄/C composite and Figure 2(b) shows Li₂FeSiO₄/C/Ag composite. It can be seen from the SEM images that Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites particles are all below 100 nm, suggesting that the nanoparticle size of silica raw material and the milling process are beneficial to the formation of small particles. In addition, Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites present agglomerating. The test results of BET indicate that the specific surface areas of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites were 94.5 and 86.3 m²/g, respectively. According to Ref. [27], Li₂FeSiO₄/C/Ag composite with larger specific surface area and smaller particle size has better performance in shortening the Li migration path and increasing the contact area of the electrode active material with the electrolyte. The electronic conductivities of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites were measured by four-probe, which were 5.98×10^-2 and 2.13×10^-2 S/cm, respectively. The results demonstrate that Ag coating could obviously improve the electrical conductivity of Li₂FeSiO₄/C composite.

Figure 1 X-ray diffraction profiles of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites

Figure 2 SEM images of Li₂FeSiO₄/C (a) and Li₂FeSiO₄/C/Ag composites (b)

Figure 3 depicts that the first profile of charge and discharge curves at various rates for Li₂FeSiO₄/C (a) and Li₂FeSiO₄/C/Ag (b) composites. As clearly seen, Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites have similar charge and discharge platforms corresponding to deintercalation lithium and intercalation lithium reactions. Meanwhile, the discharge voltage plateau decreases with the increase of current rate, demonstrating an increased polarization for both as-prepared Li₂FeSiO₄ samples [28]. In addition, the gap of Li₂FeSiO₄/C/Ag composite is narrower between charge voltage and discharge voltage at each rate than Li₂FeSiO₄/C composite, which indicates that Li₂FeSiO₄/C/Ag composite has less polarization [29].

Figure 3 First profile of charge-discharge curves at various rates for Li₂FeSiO₄/C (a) and Li₂FeSiO₄/C/Ag (b)

Figure 4 compares the rate capability of Li₂FeSiO₄/C and Li₂FeSiO₄/C/Ag composites. As the charge and discharge current increase, the difference of discharge specific capacity between Li₂FeSiO₄/C composite and Li₂FeSiO₄/C/Ag composite gradually increases. For instance, at a low rate of 0.2C, Li₂FeSiO₄/C/Ag composite material delivers a discharge specific capacity of 152 mA·h/g, and Li₂FeSiO₄/C composite material shows a discharge capacity of 140 mA·h/g. At high rate, especially, at 5C and 10C, Li₂FeSiO₄/C/Ag composite possesses the discharge specific capacity of 121 and 108 mA·h/g, respectively. While Li₂FeSiO₄/C composite material has the discharge specific capacity of 89 and 68 mA·h/g, respectively. The results reveal that Li₂FeSiO₄/C/Ag composites have good rate performance. The reason for this phenomenon could be due to that the presence of Ag inhibits the sintering and the agglomeration of the Li₂FeSiO₄ particles, leading to a decrease in crystallinity of Li₂FeSiO₄, fining the material particles and increasing the specific surface area. Thereby, the Li transport path is shortened and the contact area of the electrode active material with the electrolyte is increased. At the same time, the simple Ag coating on the surface of Li₂FeSiO₄ improves its electronic conductivity.

The long cycle property at 1C for Li₂FeSiO₄/C/Ag composite is shown in Figure 5. From the image, the initial discharge specific capacity of Li₂FeSiO₄/C/Ag composite was 140 mA·h/g at 1C. After the 100th cycle, the discharge capacity of Li₂FeSiO₄/C/Ag composite remained at 97.9%, which indicates that Li₂FeSiO₄/C/Ag composite possesses excellent cyclic properties.

Figure 4 Rate performance of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites

Figure 5 Long cycling performance at 1C for Li₂FeSiO₄/C/Ag composite

The Nyquist plots for Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites are displayed in Figure 6. The electrode testing was completed in 50% discharge state. There is a intermediate-frequency semicircle and a low-frequency straight line in the impedance spectrum. Among then, the intercepted resistance by the intersection of the high frequency region and the real axis represents the electrolyte resistance, corresponding to the ohm resistance (R_e), and the high-frequency semicircle shows the charge transfer resistance (R_ct). The oblique straight line at the low frequency region is in connection with the lithium ion diffusion coefficient [30-34]. By observing Figure 6, the charge transfer impedance of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites electrodes was 75 and 132 Ω, respectively. At low rate, the charge transfer impedance has little impact on the discharge capacity of the material, but at high rate, the charge transfer impedance makes a considerable difference to the discharge capacity of the material. The Li₂FeSiO₄/C/Ag composite electrode has relatively smaller charge transfer impedance. Therefore, it has better rate performance. The results of AC impedance further confirm that Li₂FeSiO₄/C/Ag composite has good electrochemical property.

4 Conclusions

Li₂FeSiO₄/C/Ag composite has been synthesized by solid phase method. The results reveal that Li₂FeSiO₄/C/Ag composites have distinguished multiplying performance and cycle performance compared with Li₂FeSiO₄/C composite. At high current densities of 5C and 10C, the discharge capacities of the composite are 121 and 108 mA·h/g, respectively. After 100 cycles at 1C, the retention rate of the discharge ratio is 97.9%. The main reasons for Li₂FeSiO₄/C/Ag composite with superior electrochemical properties could be summarized as the presence of Ag can inhibit the sintering and the agglomeration of the Li₂FeSiO₄ particles, thus fining the material particles and making the specific surface area larger, further shortening the Li⁺ migration path and increasing the contact area of the electrode active material with the electrolyte. In addition, the electronic conductivity of material has a considerable increase by coating of Li₂FeSiO₄ material with simple Ag.

Figure 6 AC impedance plots of Li₂FeSiO₄/C/Ag and Li₂FeSiO₄/C composites

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(Edited by FANG Jing-hua)

中文导读

Li₂FeSiO₄/C/Ag复合正极材料的合成与性能研究

摘要：Li₂FeSiO₄正极材料因其资源丰富、成本低、安全性能高等优点备受研究者关注，但其电导率低，倍率性能较差，极大地限制在动力和储能领域的应用。针对上述科学问题，本文采用固相法合成C和Ag共包覆的Li₂FeSiO₄，以提高其电子电导率和离子扩散系数。结果表明：与Li₂FeSiO₄/C复合物相比，Li₂FeSiO₄/C/Ag复合物具有较好的电化学性能，在0.2C、5C和10C下的放电比容量分别为152、121和108 mA·h/g；1C下循环100次后，其放电容量保留为97.9%。

关键词：锂离子电池；正极材料；硅酸亚铁锂；沥青；碳-银共包覆

Foundation item: Project(21771062) supported by the National Natural Science Foundation of China; Project(2016JJ2092) supported by the Hunan Provincial Natural Science Foundation, China; Project(2019013) supported by the Open Project Program of Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, China

Received date: 2018-10-27; Accepted date: 2019-03-04

Corresponding author: HUANG Xiao-bing, PhD, Associate Professor; Tel: +86-13762681279; E-mail:hxb220170@126.com; ORCID: 0000-0003-2217-0601