钒氧化物电极材料晶格呼吸现象原位探测及其反应机制研究

In Situ Observation and Mechanism Investigation of Lattice Breathing in Vanadium Oxide Cathode

作为锂离子电池正极材料,层状钒氧化物具有优异的物理特性和良好的储锂性能,因此被广泛研究与应用于工业生产与日常生活的各个方面.然而,在脱嵌锂的过程中,V2O5凝胶等层状钒氧化物的层状结构存在"晶格呼吸"现象.这种现象导致了电极材料的钝化,并进一步导致电池容量快速衰减.采用了原位X射线衍射(XRD)来研究一种层状钒氧化物(VOx)在充放电过程中的"晶格呼吸"现象,揭示了其独特的相变过程.在充放电过程中,原位XRD对应的二维衍射图显示出三个不同的阶段,分别对应三个固溶反应.放电过程中,三个阶段的衍射峰都向高角度偏移,表明在锂离子嵌入过程中,VOx的层间距存在持续收缩过程.(001)层间距大小随充放电过程的变化图进一步证明了这三个过程的不连续性.这些发现揭示了这类材料在电极反应过程中的晶体结构变化规律以及造成其容量衰减的原因.

As cathode materials in lithium-ion batteries, layered vanadium oxides have been extensively studied and used in many aspects varying from industrial production to our daily life, due to their excellent physical property and gorgeous lithium storage performance. During lithiation/delithiation, layered vanadium oxides such as V205 xerogel （with a bilayer structure）, undergoes ＂lattice breathing＂ which leads to the deactivation of electrode materials and fast capacity fading, which limits its large-scale application. In this work, VOx is used as the cathode material of lithium-ion batteries to study the ＂lattice breathing＂ phenomenon. The phase evolution has been observed and studied via in situ method. The X-ray diffraction （XRD） patterns show typical （001） diffraction peaks characteristic of vanadium oxide xerogel structure and also confirm the good crystallinity. This compound with crystal parameters of a=4.56A, b=14.87 A, c=12.38A, α=117.26°, β=96.02°, γ= 81.86°, forms a triclinic structure. Results of scanning electron microscope （SEM） and transmission electron microscope （TEM） further verify the layered structure of VOx. The thermo gravimetric analysis （TGA） at air and nitrogen atmosphere shows that the carbon content of the sample is about 2.4 wt% and the water content is about 2.1%. As lithium-ion battery cathode the initial discharge capacity of the compound is about 136 mA·h/g at a current density of 100 mA/g, with a capacity retention of 92.6% after 50 cycles. To study the lithium storage mechanism of VOx, electrochemical discharge/charge processes are further investigated by in situ XRD. It is found that the lattice plane diffraction displays three different stages linked during the insertion and deinsertion of lithium ions, indicating three solid solution reactions. During discharge process, the three diffraction changes show continuous shifts to higher diffraction angles, demonstrating three different continuous contraction processes with the insertion of lithium ions. Nevertheless, the evolution of the （001） peak is swift during the beginning and the end of discharge, in contrast to the slow deviation of the intermediate process. In the whole process, the diffraction pattern displays periodic changes, confirming the reversibility of the reaction process. The corresponding calculations of d001 during the discharge/charge process prove the notable discontinuity between these three stages. In addition, cycling experiments conducted at the higher and the lower temperature indicate that the electrochemical performance of this compound is highly sensitive to temperature.