Xiukang Yang

Excellent cycle performance of Co-doped FeF3/C nanocomposite cathode material for lithium-ion batteries

Fe1−xCoxF3 (x = 0, 0.03, 0.05, 0.07) compounds are synthesized via a liquid-phase method. To further improve their electrochemical properties, a ball milling process with acetylene black (AB) has been used to form Fe1−xCoxF3/C (x = 0, 0.03, 0.05, 0.07) nanocomposites. The structure and performance of the samples have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), charge–discharge tests, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and the galvanostatic intermittent titration technique (GITT). It is found that Co-doping significantly improves the electrochemical performance. Fe0.95Co0.05F3/C exhibits excellent electrochemical performance with discharge capacities of 151.7, 136.4 and 127.6 mA h g−1 at rates of 1C, 2C and 5C in the voltage range of 2.0–4.5 V vs. Li+/Li, and its capacity retentions remain as high as 92.0%, 92.2% and 91.7%, respectively, after 100 cycles. Co-doping could decrease the charge transfer resistance, increase the lithium diffusion coefficient during the lithiation process and improve the electrochemical reversibility. The preparation of Co-doped FeF3/C offers a new method to improve the performance of FeF3: cationic doping, which is a significant step forward for developing high-power lithium batteries.

Suppressed capacity/voltage fading of high-capacity lithium-rich layered materials via the design of heterogeneous distribution in the composition

Lithium-rich layered materials, Li1+xM1−xO2 (M = Mn, Ni, Co), have been under intense investigation as high-performance cathode materials for lithium ion batteries due to their high discharge capacity, low cost and environmental benignity. Unfortunately, the practical uses of these oxides have so far been hindered by their severe capacity and voltage fading during high voltage cycling (>4.5 V vs. Li/Li+). In an attempt to overcome these problems, herein, a novel lithium-rich Li1.14[Mn0.60Ni0.25Co0.15]0.86O2 microsphere with heterogeneous distribution in the composition has been reasonably designed and successfully synthesized via a co-precipitation method. The chemical composition in the spherical particle is gradually altered by increasing the Mn concentration while reducing the Co content from the particle center to the outer layer. At the same time, the Ni content remains almost constant throughout the particle. The coin cell with the heterogeneous cathode material delivers a high discharge capacity of over 230 mA h g−1 between 2.0 V and 4.6 V, and shows excellent cyclic stability due to the continuous increase of the stable tetravalent Mn towards the outer surface of the spherical particles, corresponding to 93.8% capacity retention after 200 cycles at 0.5 C. More importantly, the as-prepared material exhibits a significantly lower discharge voltage decay compared with conventional materials, which may mainly be ascribed to the suppression of the layered-to-spinel transformation in the Co-rich/Mn-depleted regions of the spherical particle. The capacity and voltage fading of the lithium-rich layered material are simultaneously suppressed by the special architecturual design, and the results here will shed light on developing cathode materials with special structures and superior electrochemical properties for high-performance lithium ion batteries.

Porous hollow α-Fe2O3@TiO2 core–shell nanospheres for superior lithium/sodium storage capability

Porous hollow α-Fe2O3@TiO2 core–shell nanospheres for use as anode materials in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) have been successfully fabricated by a simple template-assisted method, which has been rarely reported before. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption–desorption isotherms reveal that the as-prepared α-Fe2O3@TiO2 is composed of a hollow inner cavity and an outer shell with massive mesopores. This porous hollow structure is capable of buffering the large volume variation of α-Fe2O3 during cycling and preventing the electrode from pulverization and aggregation, as well as providing sufficiently large interstitial space within the crystallographic structure to host alkalis (Li and Na). As a consequence, this hybrid composite exhibits outstanding electrochemical properties, e.g., high specific capacity, excellent cyclability, satisfactory rate performance, and splendid initial coulombic efficiency for both LIBs and SIBs.

Synthesis and characterization of a Li-rich layered cathode material Li1.15[(Mn1/3Ni1/3Co1/3)0.5(Ni1/4Mn3/4)0.5]0.85O2 with spherical core–shell structure

A Li-rich layered cathode material Li1.15[(Ni1/3Co1/3Mn1/3)0.5(Ni1/4Mn3/4)0.5]0.85O2 with a spherical core–shell structure was firstly synthesized by a co-precipitation route. In this material, the Li1.15[Ni1/3Co1/3Mn1/3]0.85O2 core was completely encapsulated by a Li1.15[Ni1/4Mn3/4]0.85O2 shell. The structure and morphology of the as-prepared core–shell structured material were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results indicate that the core–shell structured material has a typical layered structure with the existence of a Li2MnO3-type integrated component. Spherical morphologies with an inner core and outer shell layer are clearly observed by SEM. A half cell using the core–shell structured cathode material showed a high capacity of 242 mA h g−1 at a rate of 0.1 C in a voltage range of 2.0–4.8 V. Especially, the core–shell structured cathode material represents excellent lithium intercalation stability compared to the Li1.15[Mn1/3Co1/3Mn1/3]0.85O2 core, and an improved rate capability compared to the Li1.15[Ni1/4Mn3/4]0.85O2 shell. A synergetic effect of the positive attributes of the two materials is achieved by the formation of the core–shell architecture. Therefore, the as-prepared core–shell structured Li1.15[(Mn1/3Ni1/3Co1/3)0.5(Ni1/4Mn3/4)0.5]0.85O2 is very effective for improving the electrochemical behavior of Li-rich layered cathode materials in the high-performance lithium ion batteries.

A facile synthesis of Fe3O4 nanoparticles/graphene for high-performance lithium/sodium-ion batteries

In this study, a facile, simple, and inexpensive co-precipitation method is used to fabricate diamond-like Fe3O4 nanoparticle/graphene composites for use as lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) electrode materials. In our synthesis, high-temperature treatment and complicated procedures and apparatus are avoided. Physical characterizations reveal that the as-prepared product is composed of a large fraction of diamond-like Fe3O4 nanoparticles uniformly distributed on thin graphene nanosheets. Compared to bare Fe3O4 and most of the previously reported studies, the as-obtained Fe3O4/graphene composite exhibits greatly enhanced electrochemical properties for both LIBs and SIBs, including excellent reversible capacity, superior cyclability and good rate performance. Specifically, when tested as an anode for LIBs, the Fe3O4/graphene composite shows specific capacity of 1430 mA h g−1 after 100 cycles at 200 mA g−1. The initial discharge capacity tested in SIBs is 855 mA h g−1, and after 40 cycles, the discharge capacity stabilizes at ∼210 mA h g−1 for 250 cycles. The excellent performance can be attributed to the greatly improved electrical conductivity, large surface area and excellent stability of the electrode material.

Facile synthesis and performances of nanosized Li2TiO3-based shell encapsulated LiMn1/3Ni1/3Co1/3O2 microspheres

LiMn1/3Ni1/3Co1/3O2 microspheres covered by a nanosized Li2TiO3-based shell are prepared by a facile synthesis method. First, a controllable TiO2 nano-layer is grown on the surface of a spherical Mn1/3Ni1/3Co1/3CO3 precursor, and then the resultant TiO2@LiMn1/3Ni1/3Co1/3O2 hybrid is synchronously transformed in situ into a hierarchical Li2TiO3@LiMn1/3Ni1/3Co1/3O2 microsphere through a solid-phase reaction. It has been found that the hierarchical Li2TiO3@LiMn1/3Ni1/3Co1/3O2 microspheres exhibit a good rate capability with a discharge capacity of 92.3 mA h g−1 even at higher rates of 20 C between 3.0 and 4.3 V. Besides, they possess excellent cyclic stability especially at high rates, with a capacity retention of 90.3% after 500 cycles at a 20 C rate. The enhanced electrochemical performance of the hierarchical Li2TiO3@LiMn1/3Ni1/3Co1/3O2 at high rates is attributed to the stable and fast Li+-conductor characteristic of the nanosized Li2TiO3-based shell. Thus, the Li2TiO3@LiMn1/3Ni1/3Co1/3O2 microspheres will be a promising cathode material for lithium-ion batteries with high power density and excellent cycling performance.

Effect of magnesium doping on properties of lithium-rich layered oxide cathodes based on a one-step co-precipitation strategy

A Mg-doped lithium-rich layered oxide material is successfully synthesized via a co-precipitation process and a subsequent high-temperature solid state method. The chemical composition, structural characteristics and elemental distribution of the Mg-doped lithium-rich layered oxide material are investigated by inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDXS). The results show that Mg is effectively and equably doped into the samples, which can replace partially Li+ in the cathode material. The electrochemical properties of the Mg-doped lithium-rich layered oxide material are evaluated and compared with those of the conventional Li1.5[Mn0.75Ni0.25]O2+δ cathode material. It can be found that the Mg-doped lithium-rich layered oxide material exhibits excellent electrochemical performance. It can deliver a high initial discharge capacity of 248.6 mA h g−1 and an improved initial coulombic efficiency of 87.9% at 0.1C with a cut-off voltage of 2.0–4.6 V. Moreover, the capacity retention reaches a relatively high value of 94.2% after 200 cycles at 0.5C. In contrast, the capacity retention of conventional Li1.5[Mn0.75Ni0.25]O2+δ is only 59.6%. In addition, the Mg-doped lithium-rich layered oxide material also shows excellent rate capability, which can display a discharge capacity of 130.1 mA h g−1 even at 10C and a capacity retention of 92.6% after 100 cycles at 5C. The enhanced electrochemical properties of the Mg-doped lithium-rich layered oxide material could be attributed to the introduction of Mg, which can effectively mitigate the structural deterioration of the material and facilitate the diffusion coefficient of Li+ during cycling.

Spherical concentration-gradient LiMn1.87Ni0.13O4 spinel as a high performance cathode for lithium ion batteries

A novel spherical concentration-gradient material with an average composition of LiMn1.87Ni0.13O4 is successfully synthesized via a co-precipitation route, in which the homogeneous LiMn2O4 core is encapsulated by a continuously Ni increasing concentration-gradient layer, and the composition of the outmost layer of the spherical LiMn1.87Ni0.13O4 is LiMn1.5Ni0.5O4. The physicochemical and electrochemical performances of the spherical LiMn1.87Ni0.13O4 sample are investigated by X-ray diffraction (XRD) and electrochemical tests, and using a scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscope (EDXS). The results show that the LiMn1.87Ni0.13O4 sample has a typical Fd3m spinel structure. It can be found from the cross-sectional SEM images and EDXS analysis that the LiMn1.87Ni0.13O4 particles are quite homogeneous without any apparent gap between the inner core and the outer concentration-gradient layer. Especially, the LiMn1.87Ni0.13O4 sample has excellent performance at an elevated temperature. It delivers a discharge capacity of 108.2 mA h g−1 between 3.0 and 4.4 V vs. Li/Li+ with a retention of 90.2% over 200 cycles at a rate of 0.5 C (74 mA g−1) at 55 °C. Besides, it has an exceptional capacity of 129.1 mA h g−1 between 3.0 and 4.9 V with a retention of 91.9% over 100 cycles at a rate of 0.5 C at 55 °C. Apparently, the LiMn1.87Ni0.13O4 sample shows excellent capacity stability even at an elevated temperature, i.e. 55 °C, where a traditional LiMn2O4 sample inevitably fails. Thus, the LiMn1.87Ni0.13O4 sample with a homogeneous LiMn2O4 core material and an isotropy concentration-gradient outer layer shell will be a promising cathode material for advanced lithium ion batteries.

Dependence of structure and temperature for lithium-rich layered-spinel microspheres cathode material of lithium ion batteries

Homogeneous lithium-rich layered-spinel 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 microspheres (~1 μm) are successfully prepared by a solvothermal method and subsequent high-temperature calcinations process. The effects of temperature on the structure and performance of the as-prepared cathode material are systemically studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), galvanostatical charge/discharge and electrochemical impedance spectra. The results show that a spinel Li4Mn5O12 component can be controllably introduced into the lithium-rich layered material at 750°C. Besides, it has been found that the obtained layered-spinel cathode material represents excellent electrochemical characteristics. For example, it can deliver a high initial discharge capacity of 289.6 mAh g−1 between 2.0 V and 4.6 V at a rate of 0.1 C at room temperature, and a discharge capacity of 144.9 mAh g−1 at 5 C and 122.8 mAh g−1 even at 10 C. In addition, the retention of the capacity is still as high as 88% after 200 cycles, while only 79.9% for the single-phase layered material. The excellent electrochemical performance of the as-prepared cathode material can probably be attributed to the hybrid structures combining a fast Li-ion diffusion rate of 3D spinel Li4Mn5O12 phase and a high capacity of the layered Li-Mn-Ni-Co-O component.

Sheet-like structure FeF3/graphene composite as novel cathode material for Na ion batteries

A sheet-like structure FeF3/graphene composite is successfully synthesized by a novel and facile sol–gel method. The structure and electrochemical performance of the as-synthesized FeF3/graphene composite are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and electrochemical measurement. The results indicate that the FeF3 nanosheets are loaded on the surface of the graphene sheets to form the sheet-like structure hybrid. Fourier transform infrared (FTIR) spectrum confirms that C–F bonds exist in FeF3/graphene composite, and it further indicates that a chemical bond between FeF3 and graphene has been formed and FeF3 can preferably stick to the surface of the graphene. The FeF3/graphene composite as cathode material of rechargeable Na ion batteries (NIB) exhibits a fairly high initial discharge capacity of 550 mA h g−1 at 0.1 C, and it still keeps a capacity of 115 mA h g−1 after 50 cycles at 0.3 C at a range of 1.0–4.0 V for NIB.