Silver-coated LiVPO4F composite with improved electrochemical performance as cathode material for lithium-ion batteries

Nano-structured LiVPO₄F/Ag composite cathode material has been successfully synthesized via a sol–gel route. The structural and physical properties, as well as the electrochemical performance of the material are compared with those of the pristine LiVPO₄F. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal that Ag particles are uniformly dispersed on the surface of LiVPO₄F without destroying the crystal structure of the bulk material. An analysis of the electrochemical measurements show that the Ag-modified LiVPO₄F material exhibits high discharge capacity, good cycle performance (108.5 mAh g⁻¹ after 50th cycles at 0.1 C, 93% of initial discharge capacity) and excellent rate behavior (81.8 mAh g⁻¹ for initial discharge capacity at 5 C). The electrochemical impedance spectroscopy (EIS) results reveal that the adding of Ag decreases the charge-transfer resistance (R_ct) of LiVPO₄F cathode. This study demonstrates that Ag-coating is a promising way to improve the electrochemical performance of the pristine LiVPO₄F for lithium-ion batteries cathode material.     

LiVPO4F: A new active material for safe lithium-ion batteries

In this paper, we report the latest findings for the new lithium vanadium fluorophosphate cathode material, LiVPO₄F. High quality samples have been prepared using a carbothermal reduction approach and extensive electrochemical and DSC measurements have been performed. In graphite based lithium-ion cells, the LiVPO₄F demonstrates reversible specific capacity behavior approaching theoretical. The lithium-ion system operates with an average discharge voltage around 4 V, low polarization and with good rate capability. These results indicate that the active material possesses exemplary electrochemical performance and may well be suitable as a replacement for LiCoO₂ in commercial lithium-ion cells. DSC measurements on charged cathodes indicate the thermal stability behavior expected for a phosphate based active material.     

Cation mixing （Li0.5Fe0.5）2SO4F cathode material for lithium-ion batteries

We study the crystal structure of a triplite-structured (Li_0.5Fe_0.5)SO₄F with full Li⁺/Fe2⁺ mixing. This promising polyanion cathode material for lithium-ion batteries operates at 3.9 V versus Li⁺/Li with a theoretical capacity of 151 mAh/g. Its unique cation mixing structure does not block the Li⁺ diffusion and results in a small lattice volume change during the charge/discharge process. The calculations show that it has a three-dimensional network for Li-ion migration with an activation energy ranging from 0.53 eV to 0.68 eV, which is comparable with that in LiFePO₄ with only one-dimensional channels. This work suggests that further exploring cathode materials with full cation mixing for Li-ion batteries will be valuable.     

Synthesis of PANI/rGO composite as a cathode material for rechargeable lithium-polymer cells

Graphene oxide (GO) was synthesized by an improved Hummers method and then reduced with NaBH₄; GO became rGO with regular layered structure. Polyaniline (PANI)/rGO composite was prepared by a adsorption double oxidant method with rGO as a template. Some physical characterization methods (Fourier transform infrared spectroscopy analysis, X-ray diffraction, scanning electron microscope, and transmission electron microscope) were used to analyze the morphology and crystallinity of the composite. The electrochemical properties were characterized by cyclic voltammetry, impedance spectroscopy, galvanostatic charge/discharge, and rate capability. The first discharge specific capacity of the rPANI/rGO and PANI/rGO was 181.2 and 147.8 mAh/g. After 100 cycles, the capacity retention rate was still 90.2 and 88.9% separately, and the coulombic efficiency of batteries is close to 100%. These results demonstrate the composite has exciting potentials for the cathode material of lithium-ion battery.     

The synthesis, structure, and electrochemical properties of a novel rods-shaped Li6V10O28 for lithium-ion batteries

Rods-shaped Li₆V₁₀O₂₈ powders were synthesized by rheological phase reaction. The ratio of the Li/V of the product sintered at 600 °C for 8 h was characterized by inductively coupled plasma. The structure, composite, and morphology of the product have been investigated by X-ray diffraction, scan electron microscope, transmission electron microscopy, and X-ray photoelectron spectrometry, respectively. After charge–discharge test using the product as the cathode material of lithium-ion batteries, the product calcined at 600 °C for 8 h exhibited an initial high discharge specific capacity of 212.4 mAh/g at a rate of 1.0 mA/cm² in a potential range of 2.0 and 4.4 V, and its stabilized capacity still remained 167.7 mAh/g after 30 cycles, which indicates that the rods-shaped Li₆V₁₀O₂₈ are promising cathode materials in lithium-ion batteries.     

Effect of carbon coating on the electrochemical performance of LiFePO<Subscript>4</Subscript>/C as cathode materials for aqueous electrolyte lithium-ion battery

Organic electrolyte is widely used for lithium-ion rechargeable batteries but might cause flammable fumes or fire due to improper use such as overcharge or short circuit. That weakness encourages the development of tools and materials which are cheap and environmental friendly for rechargeable lithium-ion batteries with aqueous electrolyte. Lithium iron phosphate (LiFePO₄) with olivine structure is a potential candidate to be used as the cathode in aqueous electrolyte lithium-ion battery. However, LiFePO₄ has a low electronic conductivity compared to other cathodes. Conductive coating of LiFePO₄ was applied to improve the conductivity using sucrose as carbon source by heating to 600 °C for 3 h on an Argon atmosphere. The carbon-coated LiFePO₄ (LiFePO₄/C) was successfully prepared with three variations of the weight percentage of carbon. From the cyclic voltammetry, the addition of carbon coatings could improve the stability of cell battery in aqueous electrolyte. The result of galvanostatic charge/discharge shows that 9 % carbon exhibits the best result with the first specific discharge capacity of 13.3 mAh g^?1 and capacity fading by 2.2 % after 100 cycles. Although carbon coating enhances the conductivity of LiFePO₄, excessive addition of carbon could degrade the capacity of LiFePO₄.     

花状磷酸铁锂的聚乙二醇辅助水热合成及其电化学性能

通过聚乙二醇辅助水热法制备了厚度为200 nm的片状磷酸铁锂晶体,并由此自组装为花状磷酸铁锂颗粒．聚乙二醇在水热体系中作为共溶剂使用,它能有效地降低磷酸铁锂片的厚度,并且作为软模板,使磷酸铁锂片自组装成花状结构．这样的花状磷酸铁锂虽然没经过碳包覆改性,在锂离子电池中仍具有高达140 mAh/g的放电容量,并且表现出优异的循环性能,在循环50次后,容量未出现衰减．这种未经碳包覆的磷酸铁锂材料表现出良好的电化学性能．     

Synthesis and lithium-ion storage performances of LiFe<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>PO<Subscript>4</Subscript>/C nanoplatelets and nanorods

Carbon-coated olivine-structured LiFe_0.5Co_0.5PO₄ solid solution was synthesized by a facile rheological phase method and applied as cathode materials of lithium-ion batteries. The nanostructure’s properties, such as morphology, component, and crystal structure for the samples, characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer, Emmett, and Teller (BET) determination, X-ray photoelectron spectroscopy (XPS), and the electrochemical performances were evaluated using constant current charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results indicate that nanoplatelet- and nanorod-structured LiFe_0.5Co_0.5PO₄/C composites were separately obtained using stearic acid or polyethylene glycol 400 (PEG400) as carbon source, and the surfaces of particles for the two samples are ideally covered by full and uniform carbon layer, which is beneficial to improving the electrochemical behaviors. Electrochemical tests verify that the nanoplatelet LiFe_0.5Co_0.5PO₄/C shows a better capacity capability, delivering a discharge specific capacity of 133.8, 112.1, 98.3, and 74.4 mAh g^?1 at 0.1, 0.5, 1, and 5 C rate (1 C?=?150 mA g^?1); the corresponding cycle number is 5th, 11th, 15th, 20th, and 30th, respectively, whereas the nanorod one possesses more excellent cycling ability, with a discharge capacity of 83.3 mAh g^?1 and capacity retention of 86.9% still maintained after cycling for 100 cycles at 0.5 C. Results from the present study demonstrate that the LiFe_0.5Co_0.5PO₄ solid solution nanomaterials with favorable carbon coating effect combine the characteristics and advantage of LiFePO₄ and LiCoPO₄, thus displaying a tremendous potential as cathode of lithium-ion battery.     

Structure change analysis in γ-Fe2O3/carbon composite in the process of electrochemical lithium insertion

It was previously shown that γ-Fe₂O₃/carbon composite synthesized by the aqueous solution method exhibit good high-speed charge/discharge and cycle characteristics as the cathode material in lithium-ion rechargeable batteries. We examined the crystal structure of γ-Fe₂O₃ in γ-Fe₂O₃/carbon composite by X-ray diffraction and the Rietveld analysis before and during electrochemical insertion of lithium ions. Before insertion, the sample has a spinel structure belonging to the Fd3¯m space group with the following iron occupancies: 8a site, 0.92, 16c site, 0 and 16d site, 0.87. During insertion, iron occupancy at 16d site remains virtually constant, at 8a site decreases from 0.92 to 0, and at 16c site increases from 0 to 0.53. These results suggest that, during insertion, iron migrates from 8a to 16c site. In the most highly lithiated sample, iron occupancy at 8a site decreases to 0 and occupancies at 16c and 16d sites were not equalized. Thus, the crystal structure for this sample belongs not to the Fm3¯m space group that represents the rock salt structure, but rather to the Fd3¯m space group.     

Synthesis of LiCoPO4 as a cathode material for lithium-ion battery by a polymer method

A polymer method has been used to synthesize high operation voltage LiCoPO₄ cathode material. Thermogravimetric analysis and differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM),galvanostatic charge–discharge test and cyclic voltammetry (CV) are used to study the LiCoPO ₄ . The results show LiCoPO₄ has a well-crystallized olivine structure with submicron size. In the range of 3.0–5.1 V, the initial discharge capacities of polymer material are 97.3, 91.5, and 86.5 mAh g^?1 at 0.1, 0.2. and 1 C, respectively. Thus, the polymer method has a great potential in preparing electrode materials for lithium-ion batteries.