Enhancement of electrochemical properties of niobium‐doped LiFePO4/C synthesized by sol–gel method

LiFePO₄/C and LiFe_1–xNb _xPO₄/C composites were synthesized using a sol–gel method. The influence of niobium doping on the constitution, morphology, and electrochemical properties of the samples was studied in detail. X‐Ray diffraction patterns indicate that appropriate Nb doping does not alter seriously the structure of LiFePO₄. Electrochemical characterization of the electrodes showed that the Li‐ion batteries based on LiFe_1–xNb _xPO₄/C electrode exhibited better charge/discharge performance than those based on LiFePO₄/C. The LiFe_0.95Nb_0.05PO₄/C‐based cell had the specific capacity of 157, 121, and 85 mAh/g at 0.2, 2, and 5 C, respectively, in comparison with 126, 94, and 52 mAh/g for the LiFePO₄/C cell. The results show that the addition of niobium promotes the electrochemical performance of the materials especially at high charge/discharge rates of the battery.     

Li3V2(PO4)3/C和Li2.5Na0.5V2(PO4)3/C的结构和电化学性能的比较研究

采用溶胶-凝胶法合成了锂离子正极材料Li3V2(PO4)3/C(LVP/C)及Li2.5Na0.5V2(PO4)3/C,并用XRD、循环伏安及交流阻抗等方法,研究了大量Na+掺杂对材料结构和电化学性能影响。结果表明,大量钠离子的掺杂会使LVP结构由单斜向菱方转变。掺杂化合物Li2.5Na0.5V2(PO4)3/C在0.5 C充电1 C放电时,首次放电容量为118 mAh.g-1,50次循环后容量保持率为92.4%,并发现与单斜LVP存在多个放电平台不同,Li2.5Na0.5V2(PO4)3/C仅在3.7 V处有一个放电平台。     

Studies of xLiFePO4·yLi3V2(PO4)3/C composite cathode materials with high tap density and high performance prepared by sol spray drying method

The xLiFePO₄·yLi₃V₂(PO₄)₃/C cathode materials are synthesized by a sol spray drying method. X-ray diffraction results reveal that the xLiFePO₄·yLi₃V₂(PO₄)₃/C (x,y?≠?0) composites are composed of LiFePO₄ and Li₃V₂(PO₄)₃ phases, and no impurities are detected. The samples show spherical particles with the size of 0.5–5 μm, and the tap densities of all the samples are higher than 1.5 g cm^?3. Electrochemical tests show that the xLiFePO₄·Li₃V₂(PO₄)₃/C (x,y?≠?0) composites exhibit much better performance than the single LiFePO₄/C or Li₃V₂(PO₄)₃/C. Among all the samples, 3LiFePO₄·Li₃V₂(PO₄)₃/C possesses the best comprehensive performance in terms of the discharge capacity, average working voltage, and rate capability. At 1, 5, and 10 C rates, the sample shows first discharge capacities of 152.0, 134.3, and 116.8 mAh g^?1 and capacity retentions of 99.2, 98.2, and 97.7 % after 100 cycles, respectively. The excellent electrochemical performance of micron-sized xLiFePO₄·Li₃V₂(PO₄)₃/C (x,y?≠?0) powders is owing to the homogeneous mixing of reactants at a molecular level by sol spray drying, the incorporation of fast ion conductor Li₃V₂(PO₄)₃, and the mutual doping in LiFePO₄ and Li₃V₂(PO₄)₃.     

Synthesis and characterization of LiFe0.99Mn0.01 (PO4)2.99/3F0.01/C as a cathode material for lithium-ion battery

The olivine-typed cathode materials of LiFePO₄were prepared via solid-state reaction under argon atmosphere and co-doped by manganese and fluorine to improve their electrochemical performances. The crystal structure, morphology, and electrochemical properties of the prepared samples were investigated using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, cyclic voltammetry, and charge–discharge cycle measurements. The result showed that the electrochemical performance of LiFePO₄ had been improved dramatically by Mn–F co-doping. The initial discharge capacity of LiFe_0.99Mn_0.01 (PO₄)_2.99/3F_0.01/C samples reached 140.2 mAh/g at 1C rate and only had a small amount of fading in 50 cycles.     

Electrochemical performance of LiFePO<Subscript>4</Subscript>/C doped with F synthesized by carbothermal reduction method using NH<Subscript>4</Subscript>F as dopant

The effect of fluorine doping on the electrochemical performance of LiFePO₄/C cathode material is investigated. The stoichiometric proportion of LiFe(PO₄)_1−x F_3x /C (x = 0.01, 0.05, 0.1, 0.2) materials was synthesized by a solid-state carbothermal reduction route at 650 °C using NH₄F as dopant. X-ray diffraction, scanning electron microscope, energy-dispersive X-ray, and X-ray photoelectron spectroscopy analyses demonstrate that fluorine can be incorporated into LiFePO₄/C without altering the olivine structure, but slightly changing the lattice parameters and having little effect on the particle sizes. However, heavy fluorine doping can bring in impurities. Fluorine doping in LiFePO₄/C results in good reversible capacity and rate capability. LiFe(PO₄)_0.95 F_0.15/C exhibits highest initial capacity and best rate performance. Its discharge capacities at 0.1 and 5 C rates are 156.1 and 119.1 mAh g⁻¹, respectively. LiFe(PO₄)_0.95 F_0.15/C also presents an obviously better cycle life than the other samples. We attribute the improvement of the electrochemical performance to the smaller charge transfer resistance (R _ct) and influence of fluorine on the PO₄³⁻ polyanion in LiFePO₄/C.     

Mechanism studies and electrochemical performance optimization on xLiFePO4·(1 ? x)Li3V2(PO4)3 hybrid materials

Hybrid materials xLiFePO₄·(1 − x)Li₃V₂(PO₄)₃ were synthesized by sol–gel method, with phenolic resin as carbon source and chelating agent, methylglycol as surfactant. The crystal structure, morphology and electrochemical performance of the prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), galvanostatic charge–discharge test and particle size analysis. The results show that LiFePO₄ and Li₃V₂(PO₄)₃ co-exist in hybrid materials, but react in single phase. Compared with individual LiFePO₄ and Li₃V₂(PO₄)₃ samples, hybrid materials have smaller particle size and more uniform grain distribution. This structure can facilitate Li ions extraction and insertion, which greatly improves the electrochemical properties. The sample 0.7LiFePO₄·0.3Li₃V₂(PO₄)₃ retains the advantages of LiFePO₄ and Li₃V₂(PO₄)₃, obtaining an initial discharge capacity of 166 mA h/g at 0.1 C rate and 109 mA h/g at 20 C rate, with a capacity retention rate of 73.3% and an excellent cycle stability.     

Synthesis of nanosized materials for lithium-ion batteries by mechanical activation. Studies of their structure and properties

This short review reports on the synthesis of nanosized electrode materials for lithium-ion batteries by mechanical activation (MA) and studies of their properties. Different structural types of compounds were considered, namely, compounds with a layered (LiNi_{1 − x − y} Co _x Mn _y O₂), spinel (LiMn₂O₄, Li₄Ti₅O₁₂), and framework (LiFePO₄, LiTi₂(PO₄)₃) structures. The compounds also differed in electronegativity, which varied from 10⁻⁴ S cm⁻¹ for LiCoO₂ to 10⁻⁹ S cm⁻¹ for LiFePO₄. The preliminary MA of mixtures of reagents in energy intensive mechanoactivators led to the formation of highly reactive precursors, and annealing of the latter formed nanosized products (the mean particle size is 50–200 nm). The local structure of the synthesized compounds and the composition of their surface were studied by spectral methods. An increase in the dispersity and defect concentration, especially in the region of the surface, improved some electrochemical characteristics. It increased the stability during cycling (LiMn₂O₄, at 3 V) and the regions of the formation of solid solutions during cycling (Li₄Ti₅O₁₂, LiFePO₄), led to growth of surface Li-ion conductivity (LiTi₂(PO₄)₃), etc. The mechanochemical approach was also used for the synthesis of core-shell type composite materials (LiFePO₄/C, LiCoO₂/MeO _x ) and materials based on two active electrode components (LiCoO₂/LiMn₂O₄).     

过渡金属铁配合物Fe(CO)_(5-x)(PR_3)_x的结构与性质的理论研究

采用密度泛函理论(DFT)对一系列低价铁化合物Fe(CO)_(5-x)(PR_3)_x(x=1~3,R=H,F,Me)的几何结构、电子结构、成键特点以及热力学性质进行了理论研究。结果表明引入膦配体后不会造成Fe(CO)x(PR_3)_(5-x)的几何结构畸变,为略扭曲的三角双锥形。自然键轨道(NBO)分析显示,膦配体与羰基铁基团间存在电荷转移,有效增强Fe-CO之间的共价作用。多数稳定结构Fe(CO)x(PR_3)_(5-x)的第一膦配体解离能要比第一羰基解离能低,预示Fe(CO)_(5-x)(PR_3)_x的反应活性比Fe(CO)5有明显提高。     

CeTiO4/g-C3N4复合材料的制备及其高效的光催化染料降解性能

采用简单固相法成功制备了CeTiO₄/g?C₃N₄?x(CTO/CN?x,x g为g?C₃N₄的添加量)复合材料,并通过X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线光电子能谱(XPS)、N₂吸附-脱附测试、紫外可见吸收光谱(UV?Vis)及电化学测试对材料进行表征。研究发现:CeTiO₄与g?C₃N₄层状纳米片紧密复合,并成功构建了界面异质结结构;形成CTO/CN?x复合相的光催化材料具有良好的可见光光响应性能,且光生空穴-电子对的分离和迁移率明显提高;通过太阳光模拟不同样品光催化降解有机污染物罗丹明B,降解140 min后复合材料CTO/CN?6表现出最高的光催化活性,反应速率常数为0.0202 min^-1。其活性增强的主要原因是异质结结构的构筑降低了CTO光生载流子的复合几率,提高了光生载流子的迁移速率。     

LiFePO4的软化学合成及锂快离子导体修饰

采用溶剂热法制备正极材料LiFePO_4,采用溶胶凝胶法制备Li_(0.5)La_(0.5)TiO_3(LLTO)粉体,并通过酒精悬浮法对LiFePO_4进行修饰,修饰量为LiFePO_4质量的1%~4%,获得了薄壁蜂窝状自组装结构的LiFePO_4上修饰有球状LLTO纳米颗粒的复合正极材料。通过进行充放电测试、交流阻抗测试及循环伏安测试,研究了不同修饰量对电池的充放电比容量、循环性能及可逆性的影响,发现当LLTO含量为3%(w/w)时,以2C和5C倍率放电相对于没有修饰LLTO的LiFePO_4的比容量分别提高29.7%和31.6%,30次循环之后,容量损失率较未改性前减小4.13%,循环伏安曲线上氧化还原峰之间的电位差仅为0.117 V,以3%的LLTO修饰改性的LiFePO_4显著提高了电池的倍率性能、循环性能和低温性能。     

尖晶石型LiMn2-xInxO4(x=0，0.01，0.02，0.05)的合成及电化学性能研究

以醋酸锰、氢氧化锂和三氧化二铟为原料,以柠檬酸为配位剂,采用溶胶-凝胶法制备了掺杂In的尖晶石LiMn2-xInxO4(x=0,0.01,0.02,0.05),采用XRD、SEM对目标材料进行了结构和形貌表征,采用恒流充放电、循环伏安(CV)以及交流阻抗(EIS)谱测试对材料进行了电化学性能表征,考察了不同In掺杂量对材料性能的影响。结果表明,当In掺杂量为1%时,LiMn1.99In0.01O4样品具有纯的尖晶石锰酸锂结构,在0.5C和3.4～4.35 V电压范围条件下,LiMn1.99In0.01O4的初始放电容量为119.9 mAh.g-1,经过1C 30次,2C 30次,再0.5C 5次循环后,其放电容量保持率为84.9%,显示了良好的电化学性能。掺杂1%的In的样品比未掺杂的样品具有更优的高温循环稳定性能。     

Al,B, and F doped LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> as cathode material of lithium-ion batteries

A series of the mixed transition metal compounds, Li[(Ni_1/3Co_1/3Mn_1/3)_1–x-y Al _x B _y ]O_2-z F _z (x = 0, 0.02, y = 0, 0.02, z = 0, 0.02), were synthesized via coprecipitation followed by a high-temperature heat-treatment. XRD patterns revealed that this material has a typical α-NaFeO₂ type layered structure with R3^- m space group. Rietveld refinement explained that cation mixing within the Li(Ni_1/3Co_1/3Mn_1/3)O₂ could be absolutely diminished by Al-doping. Al, B and F doped compounds showed both improved physical and electrochemical properties, high tap-density, and delivered a reversible capacity of 190 mAh/g with excellent capacity retention even when the electrodes were cycled between 3.0 and 4.7 V.     

过渡金属铁配合物Fe(CO)5-x(PR3)x的结构与性质的理论研究

采用密度泛函理论（DFT）对一系列低价铁化合物Fe（CO）_5-x（PR₃）_x（x=1~3,R=H,F,Me）的几何结构、电子结构、成键特点以及热力学性质进行了理论研究。结果表明引入膦配体后不会造成Fe（CO）_x（PR₃）_5-x的几何结构畸变,为略扭曲的三角双锥形。自然键轨道（NBO）分析显示,膦配体与羰基铁基团间存在电荷转移,有效增强Fe-CO之间的共价作用。多数稳定结构Fe（CO）_x（PR₃）_5-x的第一膦配体解离能要比第一羰基解离能低,预示Fe（CO）_5-x（PR₃）_x的反应活性比Fe（CO）₅有明显提高。     

Mo掺杂LiFePO4正极材料的第一性原理研究

基于第一性原理密度泛函理论计算了LiFePO₄和LiFe_1-xMo_xPO₄(x=0.005,0.01,0.015,0.02,和0.025)的电子结构和锂离子扩散能垒。结果显示掺杂后的LiFe_0.99Mo_0.01PO₄样品具有最大的(101)晶面间距,由此可知LiFe_0.99Mo_0.01PO₄沿[101]晶向具有最宽的锂离子扩散通道。未掺杂的LiFePO₄的锂离子扩散能垒为4.289eV,而掺杂后LiFe_0.99Mo_0.01PO₄降为4.274eV,经过计算得出掺杂样品LiFe_0.99Mo_0.01PO₄的锂离子扩散系数增为未掺杂LiFePO₄的1.79倍,表明Mo掺杂有利于改善LiFePO₄的锂离子扩散能力。态密度图显示,掺杂Mo后导带底附近的峰强度增强,对LiFePO₄电子导电性能的提高是有利的。因此,掺杂Mo有益于提高LiFePO₄的锂离子扩散能力和电子导电能力。结合我们的实验结果比较得知,在磷酸铁锂性能的改善上,相比电子导电能力,锂离子扩散能力的提高起到了更重要的作用。     

Synthesis, characterization, and electrochemical performance of LiFePO4/C cathode materials for lithium ion batteries using various carbon sources: best results by using polystyrene nano-spheres

Olivine LiFePO₄/C cathode materials for lithium ion batteries were synthesized using monodisperse polystyrene (PS) nano-spheres and other carbon sources. The structure, morphology, and electrochemical performance of LiFePO₄/C were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge–discharge tests, electrochemical impedance spectroscopy (EIS) measurements, and Raman spectroscopy measurements. The results demonstrated that LiFePO₄/C materials have an ordered olivine-type structure with small particle sizes. Electrochemical analyses showed that the LiFePO₄/C cathode material synthesized from 7 wt.% PS nano-spheres delivers an initial discharge capacity of 167 mAh g^-1 (very close to the theoretical capacity of 170 mAh g^-1) at 0.1 C rate cycled between 2.5 and 4.1 V with excellent capacity retention after 50 cycles. According to Raman spectroscopy and EIS analysis, this composite had a lower I _D/I _G, sp ³/sp ² peak ratio, charge transfer resistance, and a higher exchange current density, indicating an improved electrochemical performance, due to the increased proportion of graphite-like carbon formed during pyrolysis of PS nano-spheres, containing functionalized aromatic groups.     

不同碳源对多孔球形LiFePO4/C复合材料的影响

采用喷雾干燥-碳热还原法(SDCTM),分别研究了无机和有机碳源对锂离子正极材料LiFePO4/C形貌、结构及其充放电性能的影响。结果表明:以无机碳源炭黑制备的LiFePO4/C呈不规则球形,一次颗粒粒径在800nm左右,比表面积为2m2·g-1,0.1C放电比容量为107.3mAh·g-1。而以有机碳源制备的LiFePO4/C,其形貌较为规则,呈多孔球形结构,具有较高的比表面积和放电比容量。其中,以柠檬酸为碳源制备的多孔球形LiFePO4/C复合材料,其孔径均在50nm左右,比表面积可达32m2·g-1;在室温下,0.1C和10C首次放电比容量分别为158.8和87.2mAh·g-1,具有优异的循环性能和高倍率充放电性能。     

流变相法制备倍率性能优异的LiFePO4/C复合材料

以月桂酸为碳源和表面活性剂,氢氧化锂、碳酸锂和醋酸锂为锂源,采用流变相法制备LiFePO4/C复合材料。运用X射线衍射(XRD)、扫描电子显微镜(SEM)、粒度分析、恒流充放电测试、循环伏安以及交流阻抗测试等方法对复合材料进行表征。结果表明,不同的锂源对LiFePO4/C复合材料的结构和电化学性能均有很大影响,以氢氧化锂为锂源合成的LiFePO4/C材料展示出最佳的循环性能和倍率性能。该材料在0.1C下放电比容量为153.4 mAh.g-1,在大倍率10 C下,容量保持率仍可达76%,甚至10C下循环800次后,容量衰减率仅有4%,SEM结果显示该材料具有较小的粒径(～200 nm),且分布集中,有效提高了电子迁移速率,从而改进了LiFePO4/C的倍率性能。     

Matrix-Inducing Synthesis of SrxY10?x(SiO4)y(PO4)6?yO2: Eu3+ Micron Crystalline Coral Like Phosphors by Sol-Gel Composition of Hybrid Precursors

In the context, Sr_xY_10−x(SiO₄)_y(PO₄)_6−yO₂ doped with 1 mol%Eu³⁺ (x = 2, y = 6; x = 4, y = 4; x = 5, y = 3; x = 8, y = 0) were synthesized by using 3-aminopropyl-triethoxysilane (APES) as the sources of the silicate network. X-ray diagrams confirm that Sr_xY_10−x(SiO₄) _y(PO₄)_6−yO₂: Eu³⁺ solid solutions are formed as a pure apatitic phase. The SEM picture shows that there exist some novel unexpected coral like morphological structures. The luminescent intensity is the strongest for the host composition of Sr₄Y₆(SiO₄)₄(PO₄)₂O₂ although the effect of the composition on the luminescent intensity is little.     

溶胶-凝胶法合成Li1-xNaxMn2O4及其作为水系锂离子电池正极材料的电化学性能

用溶胶凝胶法合成了Na⁺离子掺杂的Li_1-xNa_xMn₂O₄(x=0,0.01,0.03,0.05)。X射线衍射图表明Na⁺取代Li⁺进入Li_1-xNa_x Mn₂O₄晶格中,扫描电镜图看出产物是粒径为100~300 nm的颗粒。恒流充放电测试结果表明,Li_0.97Na_0.03Mn₂O₄在2C倍率下循环100圈后放电容量保持率比未掺杂的LiMn₂O₄从51.2%提升到84.1%。循环伏安测试表明Na⁺离子掺杂降低了材料极化且增大了锂离子扩散系数。10C倍率下Li_0.97Na_0.03Mn₂O₄仍有79.0 mAh·g^-1的放电容量,高于未掺杂样品的52.1 mAh·g^-1。Na⁺离子掺杂可以稳定材料结构并提高锂离子扩散系数,从而提高LiMn₂O₄的电化学性能,是一种可行的改性方法。     

溶胶-凝胶法合成Li_(1-x)Na_xMn_2O_4及其作为水系锂离子电池正极材料的电化学性能

用溶胶凝胶法合成了Na+离子掺杂的Li_(1-x)Na_xMn_2O_4(x=0,0.01,0.03,0.05)。X射线衍射图表明Na+取代Li+进入Li_(1-x)Na_xMn_2O_4晶格中,扫描电镜图看出产物是粒径为100~300 nm的颗粒。恒流充放电测试结果表明,Li_(0.97)Na_(0.03)Mn_2O_4在2C倍率下循环100圈后放电容量保持率比未掺杂的LiMn_2O_4从51.2%提升到84.1%。循环伏安测试表明Na+离子掺杂降低了材料极化且增大了锂离子扩散系数。10C倍率下Li0.97Na0.03Mn2O4仍有79.0 m Ah·g-1的放电容量,高于未掺杂样品的52.1 m Ah·g~(-1)。Na+离子掺杂可以稳定材料结构并提高锂离子扩散系数,从而提高LiMn_2O_4的电化学性能,是一种可行的改性方法。