Dependence of Initial Capacity Irreversibility on Oxygen Framework Chemistry in Li-Rich Layered Cathode Oxides

The undesirable capacity loss after first cycle is universal among layered cathode materials,which results in the capacity and energy decay.The key to resolving this obstacle lies in understanding the effect and origin of specific active Li sites during discharge process.In this study,focusing on Ah-level pouch cells for reliability,an ultrahigh initial Coulombic efficiency(96.1%)is achieved in an archetypical Li-rich layered oxide material.Combining the structure and electrochemistry analysis,we demonstrate that the achievement of high-capacity reversibility is a kinetic effect,primarily related to the sluggish Li mobility during oxygen reduction.Activating oxygen reduction through small density would induce the oxygen framework contraction,which,according to Pauli repulsion,imposes a great repulsive force to hinder the transport of tetrahedral Li.The tetrahedral Li storage upon deep oxygen reduction is experimentally visualized and,more importantly,contributes to 6%Coulombic efficiency enhancement as well as 10%energy density improvement for pouch cells,which shows great potentials breaking through the capacity and energy limitation imposed by intercalation chemistry.

基于Dmol3模块的不同表面活性剂对煤尘润湿性的影响

煤尘防治一直是煤矿井下亟需解决的现实难题,以喷雾技术为基础,通过添加抑尘剂实现对煤尘的有效沉降具有较好的应用前景。但制备抑尘剂的过程中,原材料表面活性剂的遴选往往以实验方式进行,存在效率较低、成本较高以及实验条件苛刻等技术难题。随着分子模拟技术已相当成熟并广泛应用于物理、化学、生物等多个领域,成为物质结构分析、过渡态搜索等重要手段,因此利用分子模拟技术,借助量子化学分析中前线轨道能量差作为表面活性剂润湿性的主要判据,以表面活性剂分子的静电势极值大小和氢键数量为辅助手段,对快速渗透剂T、十二烷基苯磺酸钠(SDBS)、椰油酰胺丙基甜菜碱(CAB)的煤尘润湿性进行讨论。通过表面张力、表面活性剂毛细反渗透实验和接触角实验对上述表面活性剂的润湿性进行宏观论证;并在分子层面通过建立煤/水/表面活性剂动力学模型进行分子动力学分析,观察不同表面活性剂体系下各分子间的相互运动趋势,并对模拟结果进行相对浓度、均方位移和径向函数分布分析,在分子层面对不同的表面活性剂的润湿性能进行验证。结果表明:快速渗透剂T的前线轨道能量差与水分子形成的氢键数目为5.077 eV、1098个,SDBS为4.387 eV、1046个,CAB为2.077 eV、1029个;而3者的水溶液对煤尘的最小表面张力、接触角和煤尘渗透高度分别是23.80 mN/m、27.12°、4.1 cm;26.83 mN/m、32.69°,3.2 cm;29.94 mN/m、39.61°、1.4 cm。随着轨道能量差的减小,表面活性剂溶液对煤尘的润湿效果也逐步下降,表面活性剂前线轨道能量差与其润湿性有直接联系,表面活性剂轨道能量差越大,与水分子形成的氢键数量越多,则表面活性剂溶液的润湿性越好,这与相对浓度、径向函数等分析也相一致,实验结果与与分子模拟结果展现出相同的变化规律,验证了模拟的合理性。

Oxygen-defects evolution to stimulate continuous capacity increase in Co-free Li-rich layered oxides

Though oxygen defects are associated with deteriorated structures and aggravated cycling performance in traditional layered cathodes,the role of oxygen defects is still ambiguous in Li-rich layered oxides due to the involvement of oxygen redox.Herein,a Co-free Li-rich layered oxide Li_(1.286)Ni_(0.071)Mn_(0.643)O_(2)has been prepared by a co-precipitation method to systematically investigate the undefined effects of the oxygen defects.A significant O_(2)release and the propagation of oxygen vacancies were detected by operando differential electrochemical mass spectroscopy(DEMS)and electron energy loss spectroscopy(EELS),respectively.Scanning transmission electron microscopy-high angle annular dark field(STEMHAADF)reveals the oxygen vacancies fusing to nanovoids and monitors a stepwise electrochemical activation process of the large Li_(2)MnO_(3)domain upon cycling.Combined with the quantitative analysis conducted by the energy dispersive spectrometer(EDS),existed nano-scale oxygen defects actually expose more surface to the electrolyte for facilitating the electrochemical activation and subsequently increasing available capacity.Overall,this work persuasively elucidates the function of oxygen defects on oxygen redox in Co-free Li-rich layered oxides.

Characterization of Li-rich layered oxides by using transmission electron microscope

Lithium-rich layered oxides(LrLOs) deliver extremely high specific capacities and are considered to be promising candidates for electric vehicle and smart grid applications. However, the application of LrLOs needs further understanding of the structural complexity and dynamic evolution of monoclinic and rhombohedral phases, in order to overcome the issues including voltage decay, poor rate capability, initial irreversible capacity loss and etc. The development of aberration correction for the transmission electron microscope and concurrent progress in electron spectroscopy, have fueled rapid progress in the understanding of the mechanism of such issues. New techniques based on the transmission electron microscope are first surveyed, and the applications of these techniques for the study of the structure, migration of transition metal, and the activation of oxygen of LrLOs are then explored in detail, with a particular focus on the mechanism of voltage decay.

Silicon lithium-ion battery anode with enhanced performance： Multiple effects of silver nanoparticles

Silicon has been regarded as one of the most promising next generation lithium-ion battery anode. How- ever, the poor cyclic stability of the Si based anode has severely limited its practical applications, which is even worse with high mass loading density （〉1 mg cm^-2 ）. A new concept has been developed to enhance the electrochemical performance of the Si nanoparticle anode. Silver nanoparticles are composited with the silicon nanoparticles in a facile way for the first time. It is found that the mechanical properties of the Si electrode have been significantly improved by the incorporation of the silver nanoparticles, leading to enhanced cyclic performance. With the Si/Ag mass ratio of 4：1, the reversible specific discharge capacity is retained as l 156 mA h g^-1 after 100 cycles at 200 mAg^-1, which is more than three times higher than that of the bare silicon （318 mA h g^-1 ）. The rate performance has been effectively improved as well due to excellent electron conductivity of the silver nanoparticles.

Surface yttrium-doping induced by element segregation to suppress oxygen release in Li-rich layered oxide cathodes

Doping electrochemically inert elements in Li-rich layered oxide cathodes usually stabilizes the structure to improve electrochemical performance at the expense of available capacity.Here,we use an element segregation principle to realize a uniform surface doping without capacity sacrifice.On the basis of Hume-Rothery rule,element yttrium is chosen as a candidate dopant to spontaneously segregate at particle surface due to mismatched ionic size.Combined with X-ray photoelectron spectroscopy and electron energy loss spectroscopy mapping,yttrium is demonstrated uniformly distributed on particle surface.More importantly,a significant alleviation of oxygen release after surface doping is detected by operando differential electrochemical mass spectrometry.As a result,the modified sample exhibits improved reversibility of oxygen redox with 82.1%coulombic efficiency and excellent cycle performances with 84.15%capacity retention after 140 cycles.Postmortem analysis by transmission electron microscopy,Raman spectroscopy and X-ray diffraction reveal that the modified sample maintains the layered structure without a significant structure transformation after long cycles.This work provides an effective strategy with a series of elements to meet the industrial application.

Scalable Synthesis of Hierarchical Antimony/Carbon Micro-/Nanohybrid Lithium/Sodium-Ion Battery Anodes Based on Dimethacrylate Monomer

A facile scalable synthesis of hierarchical Sb/C micro-/nanohybrid has been addressed in this work, which possesses the advantages of both micrometer and nanometer scale structures as lithium-ion battery anode. Difunctional methacrylate monomers are used as solvent and carbon source as well. Liquid precursor of antimony（III） n-butoxide is dissolved in the resin monomer solution, and further incorporated into the cross-linking polymer network via photo polymerization. Through calcination in argon/hydrogen atmosphere, antimony nanoparticles are in situ formed by carbothermal reduction, and homogeneously embedded in the in situ formed micrometer sized carbon matrix. The morphology, structure, crys- tallinity, spatial dispersion, composition, and electrochemical performance of the Sb/C micro-/nanohybrid are systemati- cally investigated. The cyclic and rate performance of the Sb/C micro-/nanohybrid anode have been effectively improved compared to the pure carbon anode. A reversible capacity of 362 mAh g-1 is achieved with a reasonable mass loading density after 300 cycles at 66 mA g-1, corresponding to capacity retention of 79%. With reducing mass loading density, the reversible capacity reaches 793 mAh g-1 after 100 cycles. Moreover, the electrochemical performance of Sb/C micro-/nanohybrid as sodium-ion battery anode is also investigated in this study.