Guorong Hu

Enhancing the Thermal and Upper Voltage Performance of Ni-Rich Cathode Material by a Homogeneous and Facile Coating Method: Spray-Drying Coating with Nano-Al2O3

The electrochemical performance of Ni-rich cathode material at high temperature (>50 °C) and upper voltage operation (>4.3 V) is a challenge for next-generation lithium-ion batteries (LIBs) because of the rapid capacity degradation over cycling. Here we report improved performance of LiNi0.8Co0.15Al0.05O2 materials via a LiAlO2 coating, which was prepared from a Ni0.80Co0.15Al0.05(OH)2 precursor by spray-drying coating with nano-Al2O3. Investigations by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy revealed that an Al2O3 layer is uniformly distributed on the precursor and a LiAlO2 layer on the as-prepared cathode material. Such a coating shell acts as a scavenger to protect the cathode material from attack by HF and serious side reactions, which remarkably enhances the cycle performance at 55 °C and upper operating voltage (4.4 and 4.5 V). In particular, the sample with a 2% Al2O3 coating shows capacity retentions of 90.40%, 85.14%, 87.85%, and 81.1% after 150 cycles at a rate of 1.0C at room temperature, 55 °C, 4.4 V, and 4.5 V, respectively, which are significantly higher than those of the pristine one. This is mainly due to the significant improvement of the structural stability led by the effective coating technique, which could be extended to other cathode materials to obtain LIBs with enhanced safety and excellent cycling stability.

Synthesis of LiNi0.8Co0.15Al0.05O2 with 5-sulfosalicylic acid as a chelating agent and its electrochemical properties

A spherical LiNi0.8Co0.15Al0.05O2 (LNCA) cathode material with excellent electrochemical performance for lithium-ion batteries is successfully synthesized with the precursor of Ni0.8Co0.15Al0.05(OH)2 (NCA) prepared by a continuous co-precipitation method. A more environmentally friendly chelating agent, 5-sulfosalicylic acid (SSA, H3L), stable as well as non-toxic, is adopted in our synthesis process for the first time instead of traditional NH3·H2O. The thermodynamics of the precipitation from the Ni(II)–Co(II)–Al(III)–SSA–H2O system at 298 K is systematically investigated through thermodynamics model analysis. The results demonstrate that the stoichiometric spherical Ni0.8Co0.15Al0.05(OH)2 precursor can be obtained at pH = 11–13, with the SSA concentration from 0.05 mol L−1 to 0.5 mol L−1. LiNi0.8Co0.15Al0.05O2 prepared from the precursor has an initial discharge specific capacity of 203.1 mA h g−1 at 0.1C and a capacity retention of 93.3% after 200 cycles when cycled at 1C between 3.0 and 4.3 V, as well as excellent rate capability. The electrochemical performances are superior to those prepared by using ammonia as the chelating agent. It is expected that the LiNi0.8Co0.15Al0.05O2 cathode material can be synthesized by a more environmentally friendly method.

Hierarchical LiMnPO4 assembled from nanosheets via a solvothermal method as a high performance cathode material

A series of LiMnPO4 nanoparticles with different morphologies have been successfully synthesized via a solvothermal method. The samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM). The results show that the morphology, particle size and crystal orientation are controllably synthesized by various precursor composite tailoring with various Li : Mn : P molar ratios. At 3 : 1 : 1, a Li+-containing precursor Li3PO4 is obtained while at 2 : 1 : 1, only a Mn2+-containing precursor involving Mn5(PO4)2[(PO3)OH]2·4H2O and MnHPO4·2.25H2O is detected. Especially, at 2.5 : 1 : 1, the precursor consists predominantly of a Mn2+-containing precursor with a minor amount of Li3PO4. From 2 : 1 : 1 to 3 : 1 : 1, the particle morphology evolves from sheet to spherical texture accompanied with the particle size reducing. In the presence of urea, highly uniform LiMnPO4 with a hierarchical micro-nanostructure is obtained, which is composed of nanosheets with a thickness of several tens of nanometers. Thus, these unique hierarchical nanoparticles with an open porous structure play an important role in the LiMnPO4 cathode material. At a concentration of 0.16 mol L−1 for urea, the hierarchical LiMnPO4/C sample assembled from nanosheets with the (010) facet exposed shows the best electrochemical performance, delivering higher reversible capacity of 150.4, 142.1, 138.5, 125.5, 118.6 mA h g−1 at 0.1, 0.2, 0.5, 1.0, 2.0C, respectively. Moreover, the composites show long cycle stability at high rate, displaying a capacity retention up to 92.4% with no apparent voltage fading after 600 cycles at 2.0C.

Novel synthesis of Mn3(PO4)2·3H2O nanoplate as a precursor to fabricate high performance LiMnPO4/C composite for lithium-ion batteries

Mn3(PO4)2·3H2O precursor was synthesized by a novel precipitation process using ethanol as initiator, and was lithiated to LiMnPO4/C composite via a combination of wet ball-milling and heat treatment. The as-synthesized precursor was plate-shaped with nanosize thickness. After heat treatment of the ball-milled mixture of Mn3(PO4)2·3H2O, NH4H2PO4, Li2CO3 and glucose, the well crystallized and highly pure LiMnPO4 with the particle size of about 100 nm and the carbon coating layer of 2 nm was obtained. The LiMnPO4/C composite fabricated at 650 °C delivers discharge capacities of 141.7 mA h g−1 at 0.05C, 119.9 mA h g−1 at 1C and 88.7 mA h g−1 at 5C. Meanwhile, it can retain 95.2% of the initial capacity after 100 cycles at 0.5C, revealing a quite good cycling stability. The method described in this work could be helpful in the development of LiMnPO4/C cathode materials for advanced lithium-ion batteries.

Red-blood-cell-like (NH4)[Fe2(OH)(PO4)2]·2H2O particles: fabrication and application in high-performance LiFePO4 cathode materials

A well-crystallized (NH4)[Fe2(OH)(PO4)2]·2H2O particle with a red-blood-cell-like (RBC-like) shape has been synthesized via a simple sonochemical method, and is employed as the precursor to fabricate LiFePO4/C as a cathode material for Li-ion batteries. The morphology evolution of (NH4)[Fe2(OH)(PO4)2]·2H2O from a nanoparticle to a nanoplate (∼50 nm in thickness) and to a RBC-like discoid is observed by increasing the reaction time. The formation mechanism of this kind of RBC-like shape can be understood as nucleation, crystallization, formation of thin plates, and self-assembly. The precursor and corresponding LiFePO4/C are characterized by XRD, SEM, TEM, charge–discharge tests and CV measurements. The results show that the morphology of the (NH4)[Fe2(OH)(PO4)2]·2H2O precursor turns from a RBC-like structure into an irregular structure and the size becomes much smaller for LiFePO4/C during the solid-state reaction process. The fabricated LiFePO4/C exhibits excellent rate performance with a discharge capacity of 113.4 mA h g−1 even at 10C and a capacity fading rate of 3.5% after 300 cycles. Thus, this kind of uniquely shaped (NH4)[Fe2(OH)(PO4)2]·2H2O shows potential application in high-power Li-ion batteries.

An investigation into LiFePO4/C electrode by medium scan rate cyclic voltammetry

AbstractA LiFePO4/C sample was prepared via solid state reaction and characterized with X-ray powder diffraction, scanning electron microscopy and charge–discharge test. Conductive carbon and highly crystallized LiFePO4 were embedded in each other to form the as-prepared LiFePO4/C, which exhibited an excellent rate capability and capacity retention. The LiFePO4/C electrode reaction was investigated by the method of medium scan rate cyclic voltammetry (CV) under temperature variation. The limit values for the CV redox peak potentials of the LiFePO4/C electrode scanned at different rates were obtained by curve fitting. The reversibility of the LiFePO4/C electrode was studied and found to be both scan rate and temperature dependent. A higher temperature led to a higher critical CV scan rate for a reversible LiFePO4/C electrode. In the electrode process, a higher temperature resulted in a smoother Fe3+/Fe2+ redox reaction, better reversibility, lower Rcv, smaller charge transfer resistance and higher Li+ ion diffusion coefficient at the cathode of the LiFePO4/C.

A novel design concept for fabricating 3D graphene with the assistant of anti-solvent precipitated sulphates and its Li-ion storage properties

As a potential precursor for the scalable production of graphene, graphene oxide (GO) is of great importance, which can be prepared by the classical Hummers method. However, it is not only troublesome for separating and purifying GO from the obtained mixed liquor but also technically difficult to avoid the agglomeration of individual graphene sheets during the subsequent reduction process. In this paper, we introduce a novel design concept for fabricating 3D graphene (3D-rGO) directly from the GO mixed liquor with the assistant of sulphates. By using an artful anti-solvent precipitation method, sulphates (Na2SO4 and the impurities) are firstly generated on the surface of GO nanosheets, which are then removed by simple water-washing at the end of the synthesis. The choice of this method in the fabrication of rGO is motivated by the water-soluble nature of Na2SO4, which circumvents the intrinsic problems associated with the prevailing methods for GO separation, purification and reduction, making our approach green, highly efficient and cost-effective. The overall microstructure and phase evolutions of the samples at each step are observed and the formation mechanism of the rGO layers is also proposed. When evaluated as an anode material for Li-ion batteries, the obtained 3D-rGO exhibits the excellent electrochemical properties. The present approach inspires a new way for the fabrication of rGO powder on a large scale.

Conductive Polymers Encapsulation To Enhance Electrochemical Performance of Ni-Rich Cathode Materials for Li-Ion Batteries

Ni-rich cathode materials have drawn lots of attention owing to its high discharge specific capacity and low cost. Nevertheless, there are still some inherent problems that desiderate to be settled, such as cycling stability and rate properties as well as thermal stability. In this article, the conductive polymers that integrate the excellent electronic conductivity of polyaniline (PANI) and the high ionic conductivity of poly(ethylene glycol) (PEG) are designed for the surface modification of LiNi0.8Co0.1Mn0.1O2 cathode materials. Besides, the PANI-PEG polymers with elasticity and flexibility play a significant role in alleviating the volume contraction or expansion of the host materials during cycling. A diversity of characterization methods including scanning electron microscopy, energy-dispersive X-ray spectrometer, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared have demonstrated that LiNi0.8Co0.1Mn0.1O2 cathode materials is covered with a homogeneous and thorough PANI-PEG polymers. As a result, the surface-modified LiNi0.8Co0.1Mn0.1O2 delivers high discharge specific capacity, excellent rate properties, and outstanding cycling performance.

Co/Mn-Coated LiNiO2 Cathode Materials by Solid-State Reaction at Room Temperature

Abstract Preparation of Co/Mn-coated LiNiO2 cathode materials by solid-state reaction at room temperature was described. The mechanism of Co/Mn hydrate coated Ni(OH)2 by solid-state reaction at room temperature was investigated. The structure, morphology and surface composition of the uncoated and coated particles were studied with XRD, SEM, TEM and EDS. The TEM result shows a thin film is coated on the surface of LiNiO2 particles. XPS results show Co/Mn elements are coated on the surface of LiNiO2 particles. SEM and EDS results indicate that the coating of Co/Mn layer uniformly covers the surface of spherical LiNiO2 particles. The electrochemical performances results show LiNiO2 cathode materials have a good cycleability by the Co/Mn coating.

Sodium Doping to Enhance Electrochemical Performance of Overlithiated Oxide Cathode Materials for Li-Ion Batteries via Li/Na Ion-Exchange Method

Overlithiated oxide cathode materials show high capacity but poor cycle stability and voltage attenuation. In this work, a concentration difference driven molten salt ion exchange strategy is used to replace a small quantity of lithium ions by sodium ions. With the entry of sodium ions, the interplanar spacing is increased and the structure is stabilized. The electrochemical properties of materials have been improved obviously. The powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, and transmission electron microscopy are used to detect the entry of sodium ions and structural changes. The modified materials display high discharge specific capacity, excellent cycling performance, and reduced voltage attenuation.