LI Wan-Long LI Yue-Jiao,* CAO Mei-Ling QU Wei QU Wen-Jie CHEN Shi,2 CHEN Ren-Jie,2,* WU Feng,2

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Synthesis and Electrochemical Performance of Alginic Acid-Based Carbon-Coated Li3V2(PO4)3Composite by Rheological Phase Method

LI Wan-Long1LI Yue-Jiao1,*CAO Mei-Ling1QU Wei1QU Wen-Jie1CHEN Shi1,2CHEN Ren-Jie1,2,*WU Feng1,2

(1;2)

Li3V2(PO4)3/C(LVP/C) cathode materials were successfully prepared by a rheological phase method using alginic acid as the carbon source. The X-ray diffraction (XRD) patterns demonstrate that all the samples contain pure LVP with the same monoclinic structure. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that LVP/C materials have a uniform particle size. The LVP/C sample with 10% () alginic acid shows the best cycling stability. It delivers a discharge capacity of 117.5 mAh?g?1(3.0–4.3 V), which can be maintained at 116.5 mAh?g?1after 50 cycles at a rate of 0.1. Its capacity retentions of 99.1% (3.0–4.3 V) and 76.8% (3.0–4.8 V) after 50 cycles are prominently higher than those of pristine Li3V2(PO4)3, which are 89.7% (3.0–4.3 V) and 62.39% (3.0–4.8 V). These outstanding electrochemical performances are mainly attributed to the alginic acid-based carbon coating, which can increase the electronic conductivity of materials and buffer the mechanical damage of the active materials during the Li ion insertion/extraction process, thus improving the electrochemical performance of the LVP/C samples.

Lithium ion battery; Rheological phase method; Alginic acid; Li3V2(PO4)3/C composite

1 Introduction

Lithium ion batteries (LIBs) have attracted more and more attention as one of the most potential energy storage devices for energy storage systems (ESSs) and electric vehicles (EVs) due to their high security, high power density, and long cycle life1–4. For the LIB system, the cathode material is an important factor of the safety and electrochemical performance5–11. Thus, developing novel cathode materials with high security, high power density, and low cost is essential for the development of the next generation of LIBs. Among the different kinds of cathode materials, phosphate-based materials such as, Li3V2(PO4)3(LVP) have attracted tremendous attention owing to their large theoretical specific capacity, good thermal stability, and high operating voltage12–19. However, the practical applications of LVP have been restricted by its poor electronic and ion conductivity because ofthe separation of VO6octahedra by PO4tetrahedra20–22.

Various approaches have been attempted to solve these obstacles, including doping with metal atoms, minimizing the particle size, and coating with carbon materials18,23–30. Doping of metal ions into LVP composites may have an adverse effect and the method is not easy to control. Minimizing the particle size would decrease the volume density of the materials and would be difficult for large-scale production. Therefore, carbon coating is considered to be a more convenient and effective strategy. A carbon coating can improve the surface electrical conductivity, control the particle size, and minimize the contact area between the electrode material and the electrolyte.

Although the carbon coating method have been used to improve the structure and electronic conductivity of electrode materials, suitable carbon source is very difficult to be achieved31–33. Alginic acid, a polysaccharide refined from seaweed, possesses many ecofriendly properties such as biocompatibility, non-toxicity, and availability. Some results show that alginic acid can be carbonized into porous carbon, which shows excellent electrical conductivity34,35.

The structures and properties of materials are significant influenced by the synthesis methods. At present, researchers have proposed various synthesis methods to prepare LVP/C composites, such as a solid state method, hydrothermal synthesis method, and sol-gel method36–38. In contrast to the complicated and high-cost solid state and sol-gel methods, as a novel soft chemistry process, the rheological phase method is more simple and efficient. In our previous work39–41, we successfully used the rheological phase method to synthesize a homogeneous LVP material. The material had higher crystallinity, minimal agglomeration, and a smaller particle size with uniform distribution. The electrochemical performance of the material, including the rate capacity and cyclability, was excellent. In a rheological phase system, the solid-phase reactants and solvent are adequately mixed together and the solid matter is uniformly distributed in the liquid substance, which can significantly increase the contact between the two phases and effectively improve the utilization of their interfaces. As a result, the energy exchange between the two phases becomes easier39,42. At present, more and more attention has been focused on the rheological phase method to prepare electrode materials owing to the advantages discussed above43–46.

Here, we use alginic acid as a carbon source to synthesis LVP/C composites through a rheological phase method. The effects of the carbon coating on the structure and electrochemical performance of LVP were carefully investigated.

2 Experimental

2.1 Materials synthesis

TheLi3V2(PO4)3/C composites were synthesized by a rheological phase method using Li2CO3(Sigma-Aldrich,≥ 99%), V2O5(Sigma-Aldrich, ≥ 98%), NH4H2PO4(Sigma- Aldrich,≥ 99%), and alginic acid (Acros,≥ 95%). Stoichiometric amounts of Li2CO3, V2O5, NH4H2PO4, and alginic acid were ground for 0.5 h in a mortar until they were well mixed. The mass fraction of alginic acid was adjusted to be 0%, 10%, 15%, or 20%. Deionized water was then added slowly and the mixture was ground to obtain a rheological state. The precursor was obtained through transferring the rheological phase material to a hermetic vessel and remained at 80 °C for 12 h. After drying at 60 °C for 4 h, the precursor was initiallyheat treatment under argon flow at 350 °C for 3 h. A bead machine was used to press the mixture for 10 min under the pressure of 8 MPa. Finally, the mixture was sintered at 750 °C for another 6 h under an argon atmosphere to yield the LVP/C composite (Fig.1).

Fig.1 Schematic diagram of the synthesis process.

2.2 Material characterization

X-ray diffraction (XRD) measurements are performed using a diffractometer (Rigaku) with a CuKradiation source (= 0.154 nm). The morphology of the particles was determined by scanning electron microscopy (SEM, FEI Quanta 250) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010).

2.3 Electrochemical measurements

The electrochemical behavior of the LVP/C composites was examined using a coin-type (CR2025) cell battery which were assembled under high purity argon gas and the anode electrode was pure lithium foil. The working electrode was prepared by mixing the active material, carbon black, and polyvinylidene fluoride (PVDF) binder with a mass ratio of 8 : 1 : 1. The generated slurry was spread onto Al foil and dried at 80 °C for 24 h. The mass loading of the active material on each disk is about 3.68 mg?cm?2. The separator of the cell batteries used Celgard 2300 membrane and the electrolyte was 1 mol?L?1Lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of 1 : 1) (Fosai New Materials Co., Ltd., JiangSu, China). The galvanostatic charge-discharge measurements were performed at 3.0–4.3 V (1= 130 mAh?g?1) and 3.0–4.8 V (Li/Li+) at different currents under a Land CT2001A charge-discharge analysis instrument (Wuhan, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out in an electrochemical workstation CHI660A (Chenhua Co., Ltd., Shanghai, China). CV tests were carried out between 3.0–4.8 V at a scanning speed of 0.1 mV?s?1. EIS tests were performed over a frequency range of 0.01 Hz to 100 kHz. All the tests were conducted at room temperature.

3 Results and discussion

The crystal structure of the LVP/C samples was confirmed using the XRD pattern. The XRD patterns of the pristine LVP and LVP/C composites with different C content are shown in Fig.2. The three characteristic peaks at 2= 20.60°, 24.26°, 29.30° were assigned to the monoclinic LVP with a21/space group. This indicated that all the prepared samples had the same structure and there were no impurity generated during the carbon coating process. At the same time, compared with other samples, the diffraction peak intensity of the sample with 10% alginic acid is sharper,which shows a higher crystallinity and a more complete crystal structure. As the amount of the carbon coating gradually increases, the peak intensity gradually decreases.

Fig.2 XRD patterns of all the samples: pristine LVP and the LVP/C with alginic acid coating of 10, 15, and 20%.

Table 1 lists the crystal cell parameters and volume of the four samples obtained using JADE software fitting. Compared with the pristine sample, coating with carbon resulted in an increase of the unit cell parametersand, and a decrease of; the volume of the unit cell increased. Therefore, the addition of carbon source has influence on the growth of the material without changing its crystal structure.

The morphologies of the LVP and LVP/C composites were depicted in Fig.3. The LVP particles with poor crystallinity are agglomerated together and show an obvious layered accumulation. The large secondary particles with a size of 4 μm are distributed extremely randomly. By comparison, the morphologies of the carbon-coated LVP particles show an apparent improvement. The outline of the samples is clearer and the size is more uniform. The average particle size of the LVP/C-10% composite is approximately 0.8 μm, which is much smaller than the LVP sample. However, with the increase of carbon content, the size of the particles increases gradually and their morphologies become irregular. The electrochemical properties of the materials may be influenced by these factors.

The microstructure of the sample was investigated by TEM and HRTEM images. As shown in Fig.4(a), the LVP particle is uniformly coated with a carbon layer. From the image in Fig.4(b) we can estimate that the carbon layer is approximately 8–9 nm thick. The HRTEM images in Fig.4(c, d) clearly show the crystal texture of LVP and the amorphous texture of the coated carbon. It is believed that the uniform carbon coating can control the growth of the LVP particles、increase the electronic conductivity of materials, buffer the mechanical damage of the active materials during the Li ion insertion/ extraction process and shorten the Li+diffusion pathway, thus improving the electrochemical performance and prolonging the cycle life of the LVP/C samples.

Fig.5(a) illustrates the initial charge/discharge curves of the LVP and LVP/C composites between 3.0 and 4.3 V. The samples show three flat charge plateaus at 3.61, 3.67, and 4.1 V and corresponding three flat discharge plateaus at 4.07, 3.65, and 3.56 V. The three charge plateaus correspond to the phase transition processes of Li3V2(PO4)3to LiV2(PO4)3(= 2.5, 2.0, and 1.0). Due to the existence of Li2.5V2(PO4)3, the extract of the first Li+requires two steps. Conversely, three discharge plateaus correspond to the process of the reinsertion of two Li+ions, which accompanying the phase transition processes of LiV2(PO4)3to LiV2(PO4)3(= 2.0, 2.5, 3.0). In the potential range of 3.0–4.3 V at 0.1, all of the three coated samples showhigher capacity than pristine LVP. The LVP/C-10% composite shows excellent performance with an initial capacity of 117.5 mAh?g?1in discharge and 127.6 mAh?g?1in charge, which is approaching the theoretical capacity (132 mAh?g?1), and the coulombic efficiency is 92.1%. By comparison, the initial capacity of LVP is only 99.7 mAh?g?1in discharge and 105.8 mAh?g?1in charge. When the content of the alginic acid coating is 20%, the discharge capacity of the LVP/C-20% sample is only 102.8 mAh?g?1and the coulombic efficiency is 88.8%. These results illustrate that the discharge capacity of the LVP/C samples increases after alginic acid coating, but too much alginic acid content may decrease the energy density and impair its electrochemical performance.

Table 1 Lattice parameters of the four different samples.

Fig.3 SEM images of all the samples

(a) LVP/C coated with 10% alginic acid, (b) LVP/C coated with 15% alginic acid, (c) LVP/C coated with 20% alginic acid, (d) pristine LVP.

Fig.4 TEM (a, b) and HRTEM (c, d) images of the LVP/C-10% sample.

Fig.5 Initial charge-discharge profiles of all the samples at 0.1C in a cut-off potential range of 3.0–4.3 V (a) and 3.0–4.8 V (b); Cycle curves of all the samples at 0.1C in a cut-off potential range of 3.0–4.3 V (c) and 3.0–4.8 V (d).

Fig.7 CV curves of all the samples between 3.0 and 4.8 V.

Fig.8 Electrochemical impedance spectra of all the samples after 3 cycles (3.0–4.8 V).

After 3 cycles (3.0–4.8 V), EIS measurements were performed in the frequency range of 10?2–106Hz. As shown in Fig.8, the impedance plots of the samples are composed of a depressed semicircle at high-frequency and a straight line at the low-frequency. The depressed semicircle at the high-frequency is consistent with the charge transfer resistance at the electrode/electrolyte interface. The formation of the inclined line at the low-frequency represents the Warburg impedance related to the diffusion of the Li+into the active mass. By comparison, after alginic acid coating, all the resistances decreased, indicating that the alginic acid coating improves the conductivity and diffusion coefficient of the cathode materials and contributes to the extraction/insertion of Li+.

The charge transfer resistance in the high-frequency region of the pristine sample (Fig.8) is much higher than that of the alginic acid-coated samples and is calculated to be 146.9 Ω. The LVP/C-10% sample shows the lowest charge transfer resistance of 47.99 Ω, indicating the lowest polarization performance and the fastest reaction rate. The impedance value of the sample increases gradually with increasing amount of alginic acid, hindering the electrochemical performance.

4 Conclusions

In this paper, we successfully synthesized a series of LVP/C composites with alginic acid as the carbon source through a rheological phase method. The coating amount of the carbon source was optimized, and the influence of different coating amounts on the microstructure and electrochemical properties of the materials was investigated detailedly. The XRD patterns reveal that all the prepared samples have a pure LVP phase with the same monoclinic structure. The electrochemical performance tests show that coating with alginic acid can significantly improve the charge/discharge capacity, the cycle stability, the rate capability, and the structural stability of Li3V2(PO4)3. The optimum alginic acid coating amount is 10%. In a potential window of 3.0–4.3 V, the initial discharge capacity of LVP/C-10% can reach 117.5 mAh?g?1, and the capacity can be maintained at 116.5 mAh?g?1after 50 cycles at 0.1, with a better capacity retention rate of up to 99.1% compared with the LVP sample (89.7%). According to the rate performance results, the LVP/C-10% sample shows an improved capacity retention rate of 97.8% (3.0–4.3 V) whereas the LVP sample only achieves a capacity retention rate of 93% after 50 cycles. The reasons for this improved electrochemical performance are closely related to the lower charge transfer resistance values, which results from the suitable carbon coating. In summary, this work presents a facile synthetic route for the preparation of alginic acid-coated Li3V2(PO4)3composites that show great promise in the field of cathode materials for LIBs.

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流變相法制備海藻酸基碳包覆Li3V2(PO4)3材料的電化學(xué)性能

李萬隆1李月姣1,*曹美玲1曲 薇1屈雯潔1陳 實1,2陳人杰1,2*吳 鋒1,2

(1北京理工大學(xué)材料學(xué)院，環(huán)境科學(xué)與工程北京市重點實驗室，北京 100081；2北京電動車輛協(xié)同創(chuàng)新中心，北京 100081)

以海藻酸為碳源，采用流變相法制備出碳包覆改性的Li3V2(PO4)3/C (LVP/C)正極材料。X射線衍射(XRD)結(jié)果顯示所合成樣品均為標(biāo)準(zhǔn)的單斜結(jié)構(gòu)Li3V2(PO4)3。掃描電子顯微鏡(SEM)和透射電子顯微鏡(TEM)圖像顯示所合成的LVP/C活性材料顆粒尺寸較均勻。海藻酸質(zhì)量分?jǐn)?shù)為10%的LVP/C樣品展現(xiàn)出最優(yōu)的循環(huán)穩(wěn)定性。0.1放電電流下，首次放電容量為117.5 mAh?g?1，50周循環(huán)后容量保持在116.5 mAh?g?1。LVP/C-10%材料在3.0–4.3 V和3.0–4.8 V電壓范圍內(nèi)循環(huán)50周后的容量保持率分別為99.1%和76.8%，明顯優(yōu)于未包覆的LVP材料。海藻酸基碳包覆層可以有效增加材料的電子導(dǎo)電性、緩沖活性材料在脫嵌鋰過程產(chǎn)生的機械損傷，進而提高材料的電化學(xué)性能。

鋰離子電池；流變相法；海藻酸；磷酸釩鋰/碳復(fù)合材料

O646

10.3866/PKU.WHXB201705293

March, 27, 2017;

May 23, 2017;

May 29, 2017.

Corresponding authors. CHEN Ren-Jie, Email: chenrj@bit.edu.cnTel: +86-10-68912508. LI Yue-Jiao, Email: lyj@bit.edu.cn; Tel.: +86-10-68912528.

The project was supported by the National Key Research and Development Program of China (2016YFB0100204), National Natural Science Foundation of China (21373028), Joint Funds of the National Natural Science Foundation of China (U1564206), and Major achievements Transformation Project for Central University in Beijing, Beijing Science and Technology Project (D151100003015001).

國家重點研發(fā)計劃項目(2016YFB0100204)，國家自然科學(xué)基金項目(21373028)，國家自然科學(xué)聯(lián)合基金項目(U1564206)和中央在京高校重大成果轉(zhuǎn)化項目(D151100003015001)資助