Oil bath chelation-assisted fabrication of nitrogen-doped carbon-coated Ni9S8/Ni3S2 composites for lithium-ion batteries

Introduction Lithium ion batteries (LIBs) are widely used in portable devices, such as cellular phones and laptop computers, because of their high energy density. Large LIBs for various industrial applications have recently attracted much attention and their development is ongoing [1]. LIBs include a positive electrode, such as LiCoO2, LiNi1-x-yMnxCoyO2, or LiMn2O4, a graphite negative electrode, and an electrolytic solution. The graphite is divided into two classes: natural and artificial graphite. Natural graphite is more cost effective than artificial graphite. But cell performances with natural graphite, for example cyclability and storage characteristics at high temperatures, is lower than with artificial graphite. Many researches suggest that deterioration of cell performance is related to decomposition of the Solid Electrolyte Interface (SEI). To improve cell performance with natural graphite, coating the graphite with ion exchange polymers such as polyacrylic acid (PAA) is effective [2]. The polymer may work as a pseudo SEI. On the other hand, the direct current resistance (DCR) of a cell with natural graphite coated with PAA tends to be high. LIBs, especially for Hybrid electrolytic vehicles require a low DCR, so it is necessary to develop new coating polymers to reduce the DCR. But it is very difficult to find new coating polymers because there are many ion exchange polymers, and we do not know the relationships between the structure of the ion exchange polymers and DCR. In recent years, computational chemistry has been growing; the field of biochemistry, especially medicines, is widely used to promote the efficiency of the research. But there are few examples of developing materials for LIBs, especially coating polymers, using computer simulation. In this research, we have investigated polymer functional groups and DCR values using computer simulation. We also have developed a new coating polymer which can reduce DCR. Experimental 1.Preparation of 18650 cells Natural graphite and a ion exchange polymer (0.5 wt%) were mixed in water as a solvent, and coating graphite was obtained by removing the solvent. Three types of polymers were used (PAA, and two types of polymers containing sulfonate). An 18650 type LIB includes a positive electrode, a polyolefin separator, an electrolytic solution, and a negative electrode which, in this study, was made of coated natural graphite. The DCR values of the 18650 cells were measured at 25°C. 2.Computer simulation In this study, the Gibbs free energy (ΔGr) values of the polymer’s ion exchange functional groups were calculated by ab initio molecular calculations, RB3LYP/6-31G+(d). The polymer molecule is very large, so it is difficult to calculate ab initio. On the other hand, the properties of a polymer reflect the properties of its segments. So in this research, ΔGr was calculated for the polymer segment shown in Fig. 1. 3.Synthesis of a new coating polymer A monomer containing functional groups of Fig. 3 and water as a solvent were mixed (weight ratio of monomer and water = 1:10) and 2,2'-Azobis (2-methylpropionamidine) dihydrochloride as the radical initiator was mixed. The mixture was heated in an oil bath at 60°C for 3 hours. Results and Discussion Fig. 2 shows the relationships between the DCR of the 18650 and ΔGr. The DCR and ΔGr had a well-defined correlation. ΔGr is defined by Eq. (1), where R is the gas constant and T is temperatures, and K is defined by Eq. (2), so ΔGr is related to the degree of disassociation. From the results, highly dissociated polymers existing in the negative electrode are advantageous in decreasing the DCR. The DCR of PAA is higher than that of sulfonate polymers. The reason for the high DCR of PAA is that PAA is low dissociate. To decrease DCR, a polymer which has a functional group that is low ΔGr seems to be effective. So in this study, many polymers which have ion exchange functional groups were assessed. In this research, polymer segment shown in Fig. 3 displayed the lowest ΔGr; the value was 139 kcal/mol. Consequently, a polymer with a segment of Fig. 3 was synthesized, and an 18650 cell was made and its DCR was evaluated. The polymer with segment of Fig. 3 was named Polymer (a). Fig. 4 shows the DCR of Polymer (a). The DCR was lower than that of the other polymers. This result suggests that the ΔGr of Polymer (a) is low; that is, it dissociates to a significant degree, so its DCR can be low.

Olivine LiMn0.4Fe0.6PO4 (LMFP) materials were synthesized by the modified sol–gel method with the addition of sucrose as an additional carbon source. Electrochemical properties of LMFP were advanced. The materials doped possess high reversible capacity of 107.4 mAh·g at 0.2 C-rate and excellent cycling stability (the capacity retention of 85.8% via 50 cycles), exhibiting the improved electrochemical properties. Introduction Currently, energy and environment are the most important topics. One of the greatest challenges is how to make use of Green Energy according to the strategy of sustainable development and the replacement of fossil fuels [1]. During the past decades, lithium ion batteries (LIBs) have dominated the portable electronic market and been applied to electric vehicles (EVs), hybrid EVs (HEVs) and smart grids, due to their high energy density and safety [2, 3]. All the time, lithium transition metal oxides are used as the main cathode active materials. However, these materials have some limitations for the intrinsic drawbacks, such as a poor thermal stability [4]. For this reason, the olivine-LiMPO4 (M =Mn, Fe, Co, Ni) compounds are drawing more and more interests owing to their low cost, environmentally benign nature, high capacity and thermal stability. As a promising candidate, lithium manganese phosphate LiMnPO4 is expected for its high energy density (700 Wh/kg) and aboundant resource [5]. Unfortunately, LiMnPO4 is being affected by the actual defect, such as slow lithium diffusion kinetics, low electronic and polaronic conductivity. In addition, the high structural strain at the phase boundary between charged and discharged phase and the instable structure due to Jahn-Teller distortion of Mn ion limit its application [6, 7, 8]. Hence, many efforts have been conducted to improve its properties. Recently, partial substitution by transition metal in LiMnPO4 is employed to achieve high energy density and stability. Indeed, some researches demonstrate an increase in kinetics when some of the Mn ions are replaced with Fe to form the solid solution LiMnxFe1-xPO4 [9]. Therefore, the solid solutions with an olivine structure is regarded as a promising material. What is more, the partial substitution of Mn ions by Fe ions can effectively improve the specific and rate capabilities, which may be attributed to the excellent contact area between active materials and electrolyte [10, 11]. In this work, we synthesize the LMFP utilizing Fe substitution by sol-gel method and investigate improved performance of LMFP, principally. LiMnPO4 sample is prepared as a comparison, as well. The results indicate that iron substituted lithium manganese phosphate exhibits distinctly improved electrochemical properties, compared to the simple lithium manganese phosphate. Experimental LiMn0.4Fe0.6PO4 was prepared by the modified sol-gel method, reported in a previous study [12]. All reagents were analytical grade. LiH2PO4, FeC2O4·2H2O, Mn(COOCH3)2·4H2O were dissolved in deionized water in the correct stoichiometric ratios to get a mixed solution, and a critic 5th International Conference on Environment, Materials, Chemistry and Power Electronics (EMCPE 2016) © 2016. The authors Published by Atlantis Press 342 acid solution was added to the preceding solution. Sucrose was added as an additional carbon source. After placed in oil bath at 80 °C for 10 h, the mixture was dried at 100 °C for 12 h in a vacuum oven. Finally, the gel was calcined at 700 °C for 10 h to obtain the composite. The structure was characterized by powder X-ray diffraction (XRD, Rint-2000, Rigaku, Japan) using Cu-Kα radiation over the 2θ range of 10°-80°. Electrochemical tests were performed by the coin cells (2032 type) that were assembled in an argon glove box. The cathode materials were formed from the active materials, a poly (tetrafluoroethylene) binder and acetylene black in 80:10:10 weight ratio. 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 vol. %) was used as the electrolyte between the lithium metal anode and the cathode. The capacities and cycle performances of the cells were carried out on a LAND CT2001A tester (Wuhan, China) in the voltage range of 2.5-4.5 V. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 10 kHz to 10 mHz, with a 5 mV a.c. input signal by CHI660C (Shanghai, China). Results and discussion 10 20 30 40 50 60 70 80 2 theta (degree) PDF#77-0178 In te ns ity (a .u .) x=1 x=0.4 LiMnxFe1-xPO4 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Nanoparticles are essential building blocks for many energyrelated applications, ranging from lithium ion batteries, catalysis, and photocatalysis to electrochromic windows. The ability to synthesize nanoparticles with well-defined size, shape, and structure is critically important. In recent years, there have been two main trends to improve the efficiency of nanoparticle synthesis by use of microwave or microfluidic reactors.1, 2 Microwave reactors provide a higher nanoparticle yield in shorter reaction times than conventional batch reactors,1 and microfluidic reactors have the separate advantage that reaction conditions, such as heat and mass transfer rates, can be independently controlled for small volumes of reaction solution.2 Integrating such miniaturized reactors with microwave heaters would combine the advantages of both and so improve nanoparticle synthesis. Several such devices have been developed for aqueous solutions, which are suitable for biochemical applications such as heating a polymerase chain reaction.3 However, water is not a suitable solvent for low-temperature, low-pressure synthesis of crystalline nanoparticles. Recently, colleagues and I have developed a microfluidic-microwave device, operating at 700–900MHz, which allows precise tuning of the temperature of non-aqueous solvents such as benzyl alcohol, n-butanol, and ethylene glycol.4 We used two independent non-contact methods to determine the temperature of benzyl alcohol droplets flowing in fluorocarbon-based oil. Infrared temperature imaging provided quantitative information about the microwave heating of the benzyl alcohol droplets and heat transfer from the droplets: see Figure 1. Additionally, we measured the microwave heating of the benzyl alcohol droplets by fluorescence imaging with high temporal resolution. We can heat the benzyl alcohol droplets to 50C in 15ms. We used our microfluidic-microwave device to synthesize tungsten oxide nanoparticles within benzyl alcohol droplets using the synthesis protocol for a conventional reaction in oil bath. Figure 1. Top: A 2D temperature map measured at the surface of the microwave-microfluidic device with an IR camera. Bottom: A schematic of the microfluidic-microwave device. (Copyright the Royal Society of Chemistry.4)

Hollow Co 3O 4 thin films as high performance anodes for lithium-ion batteries

Facile synthesis of flower-like and yarn-like α-Fe2O3 spherical clusters as anode materials for lithium-ion batteries

In this paper, fibrous-shaped of V6O13 were successfully synthesized via V2O5 and ethanol as reactants through the method of magnetic stirring in the oil bath, Compared to the prepared sample without mechanical force-driven, the sample prepared by this method has a fibrous morphology with good dispersion. It can be seen that as the speed increases, the width of the fibers decreases, the fiber length increases after SEM tests. After the electrochemical tests, it can be seen that the attrition rate of capacity will be successfully inhibited in the process of charging and discharging when the fibrous-shaped V6O13 as a lithium ion batteries cathode materials. When the stirring rate was 0 rpm and possessed a reversible capacity of 72 mAh/g after 50 deep cycles. By contrast, the stirring rate was 1200 rpm and possessed a reversible capacity of 102 mAh/g after 50 deep cycles. It increased 41.6%.

Electrostatic spray deposition of nanoporous CoO/Co composite thin films as anode materials for lithium-ion batteries

Traditional anodes for Li-ion batteries (LIBs) including graphite, TiO2 and Li4Ti5O12, generally have specific capacities less than 200 mAh g-1, which no longer meet rapidly increasing commercial demands. In recent years, various novel anode materials, such as metals, alloys, silicon, transition metal oxides (TMOs) and sulfides, have been widely studied because they can achieve much higher capacity than traditional anodes.[1] Transition metal phosphides (TMPs) have been investigated extensively owing to their high theoretical capacities and relatively low intercalation potentials vs Li/Li+.[2] In particular, cobalt phosphide (CoP), with a theoretical capacity of ∼894 mAh g-1 , has been proven to be one of the most promising candidates for LIB anodes.[3] To date, various CoP materials have been investigated,[4, 5] but their specific capacity and stability are still unsatisfactory due to low electric conductivity and fast structural degradation during high-rate or long-term charge/discharge processes. Herein, we report on a robust high-capacity anode based on CoP/reduced graphene oxide (rGO) nanocomposite.[6] We conduct a facile and versatile strategy involving an oil bath, freeze drying and phosphidation processes, allowing nanostructured CoP particles to be uniformly embedded in rGO nanosheet network. The resulting CoP/rGO nanocomposite can exhibit enough surface area and porosity, which can improve the electrolyte diffusion. Meanwhile, the rGO network can enhance the electrical conductivity and structural stability of active CoP anodes. Electrochemical measurements indicate that the CoP/rGO nanocomposite shows a high specific capacity over 1100 mAh g-1 at a current density of 100 mA g-1. A capacity retention of ~840 mAh g-1 is obtained when the current density increases to 2 A g-1, which reveals an excellent rate capability. The nanocomposite also shows a ultralong cycle life of 2000 stable cycles at a high current density of 2 A g-1. These results indicate that our strategy is very effective and versatile to improve TMP-based anodes for the development of state-of-the-art LIBs. [1] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today, 18 (2015) 252-264. [2] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci., 4 (2011) 2682-2699. [3] J. Yang, Y. Zhang, C. Sun, H. Liu, L. Li, W. Si, W. Huang, Q. Yan, X. Dong, Graphene and cobalt phosphide nanowire composite as an anode material for high performance lithium-ion batteries, Nano Res., 9 (2016) 612-621. [4] W. Wang, J. Li, M. Bi, Y. Zhao, M. Chen, Z. Fang, Dual function flower-like CoP/C nanosheets: High stability lithium-ion anode and excellent hydrogen evolution reaction catalyst, Electrochim. Acta, 259 (2018) 822-829. [5] X. Xu, J. Liu, R. Hu, J. Liu, L. Ouyang, M. Zhu, Self-Supported CoP Nanorod Arrays Grafted on Stainless Steel as an Advanced Integrated Anode for Stable and Long-Life Lithium-Ion Batteries, Chem. Eur. J., 23 (2017) 5198-5204. [6] Y. Yang, Y. Jiang, W. Fu, X. Liao, Y. He, W. Tang, F. Alamgir, Z.-F. Ma, Cobalt phosphide embedded in graphene nanosheet network as a high-performance anode for Li-ion batteries, Dalton Trans., (2019) DOI: 10.1039/C1039DT01240K

The effect of etching environment (opened or closed) on the synthesis and electrochemical properties of V2C MXene was studied. V2C MXene samples were synthesized by selectively etching of V2AlC at 90 °C in two different environments: opened environment (OE) in oil bath pans under atmosphere pressure and closed environment (CE) in hydrothermal reaction kettles under higher pressures. In OE, only NaF (sodium fluoride) + HCl (hydrochloric acid) etching solution can be used to synthesize highly pure V2C MXene. However, in CE, both LiF (lithium fluoride) + HCl and NaF+HCl etchant can be used to prepare V2C MXene. Moreover, the V2C MXene samples made in CE had higher purity and better-layered structure than those made in OE. Although the purity of V2C obtained by LiF+HCl is lower than that of V2C obtained using NaF+HCl, it shows better electrochemical performance as anodes of lithium-ion batteries (LIBs). Therefore, etching in CE is a better method for preparing highly pure V2C MXene, which provides a reference for expanding the synthesis methods of V2C with better electrochemical properties.

Preparation and electrochemical performance of LiFePO4/C microspheres by a facile and novel co-precipitation

CoS2 nanoparticles embedded in N-doped hollow carbon nanotubes as anode materials for high performance lithium-ion battery