【作者】 张露露；

【导师】 黄云辉；

【作者基本信息】 华中科技大学 ， 材料学， 2012， 博士

【摘要】 聚阴离子型正极材料磷酸钒锂(Li3V2(PO4)3,LVP)因具有结构稳定、电化学性能优良、安全性能好等优点而备受关注。然而,由于LVP结构中的VO6八面体被PO4四而体分离,导致该材料的电导率偏低,限制了其大规模应用。本文以提高LVP材料的电化学性能为主要目标,通过包覆、掺杂等方法对其进行改性研究。结果表明：采用固相法、溶胶凝胶法和喷雾干燥法制备Li3V2(PO4)3/C的工艺优化结果如下：当葡萄糖用量为15 wt%时,采用固相法在700℃烧结合成的Li3V2(PO4)3/C材料电化学性能最优,且经循环后其单斜结构仍保持不变。该材料在2.5-4.5 V和3.0-4.5 V区间循环稳定性好,但容量偏低；在2.5-4.8 V和3.0-4.8 V区间循环容量高,但稳定性差。以草酸和葡萄糖为双碳源,利用溶胶凝胶法制备了Li3V2(PO4)3/C复合正极材料。当草酸和V2O5为化学计量比时,额外加入葡葡糖能显著影响Li3V2(PO4)3的电化学性能。其中,添加15 wt%的葡萄糖所得产物的电化学性能最优,在0.1 C和10C下的放电比容量分别为171.2 mAhg-1和118.9 mAh g-1,远大于不添加葡萄糖样品的比容量(145.3 mAh g-1/0.1 C和13.7 mAh g-1/10 C)。采用喷雾干燥法制备LVP/C时,添加15 wt%羧甲基纤维素钠(CMC)制备的球形Li3V2(PO4)3/C电化学性能最优。CMC的加料顺序对Li3V2(PO4)3的形貌、振实密度及电化学性能影响显著。将原料预烧后进行喷雾造粒形成的是实心球结构,即LVP/C(A);而将原料直接进行喷雾造粒形成的则是空心球结构,即LVP/C(B)。而且,当以CMC为碳源时,虽然产生了Na3V2(PO4)3、Li2NaV2(PO4)3和Li4(P2O7)等杂相,但在3.0-4.8 V间循环,其单斜结构仍能得以保持。与LVP/C(B)相比,LVP/C(A)具有更优的电化学性能和更高的振实密度。对Li3V2(PO4)3进行了Nb和Si的包覆改性,发现：Nb改性Li3V2(PO4)3时,Nb没有进入Li3V2(PO4)3晶格,而是以β-NbOPO4形态存在于LVP颗粒表面。Nb引入后,由于颗粒尺寸减小和电导率提高,使得Li3V2(PO4)3的电化学性能显著改善。与未改性的Li3V2(PO4)3/C相比,Nb改性的样品Li3V1.97Nb0.03(PO4)3/C在容量、倍率性能和循环稳定性等方面均有显著的改善,在低倍率(0.1 C)下可释放出高达182 mAh g-1的比容量,循环50圈后其容量保持率为84.7%；其高倍率(1 C和5 C)放电容量分别为160mAh g-1和125mAh g-1。SiO2改性Li3V2(PO4)3/C的研究表明：V在纯LVP/C和LVP/C-2Si样品中的价态均为+3价；SiO2仅包覆在Li3V2(PO4)3/C颗粒表面,并未改变Li3V2(PO4)3的单斜结构。低含量(2 wt%)SiO2的引入能显著提高Li3V2(PO4)3的电化学性能(尤其是高倍率性能),原因是SiO2包覆能有效抑制V在电解液中的溶解,提高Li3V2(PO4)3|的结构稳定性,降低电荷转移电阻。对Li3V2(PO4)3进行了Fe和Na的掺杂改性,发现：Fe掺杂LVP/C时,Li3-xV2-yFe2+y(PO4)3、LiFePO4和FePO4共存于体系中。与纯LVP/C相比,Fe掺杂Li3V2(PO4)3/C复合材料的容量、循环性能和倍率性能得到了明显的提高,这是因为Fe引入后,材料的颗粒尺寸减小、电荷转移电阻降低以及结构稳定性得到了提高。Na掺杂LVP对其化学行为影响显著,在3.85 V附近出现的电化学反应平台和氧化/还原峰证实了Li3-xNaxV2(PO4)3或其它一些含Na+的电化学活性物质的存在。采用两步固相烧结法制备了一系列不同V掺杂量的LiFe1-xVxPO4/C(x=0,0.03,0.05,0.07,0.10,0.20,0.50,1.00)样品。结果表明,V掺杂能显著提高LiFePO4的电化学性能。当V掺杂量为5 at%时,材料的电化学性能最优,在5.0 C下循环50圈后仍能释放出高达129 mAh g-1的比容量,且在所有倍率下的容量保持率均高于97.5%。XAS分析表明,LiFe0.95V0.05PO4/C样品中V的价态介于+3和+4价之间。当V含量较低时(x≤0.03),V能进入LiFePO4晶格；而当V含量较高时(0.05<x<1.00),过量的V则以Li3V2(PO4)3和LiVOPO4杂相形态存在更多还原

【Abstract】 As polyanion cathode material for lithium ion batteries, lithium vanadium phosphate (Li3V2(PO4)3, LVP) has attracted much attention due to the stable structure, excellent electrochemical performance and high safety. However, the poor intrinsic electronic conductivity of LVP, resulting from the separation of VO6 octahedra by PO4 tetrahedra in their structures, limits its practical application in lithium-ion batteries. This paper presents a thorough study of modification on LVP by doping and coating, with the purpose to improve their electrochemical performances. The results are as follows:Li3V2(PO4)3/C (LVP/C) composites were successfully synthesized via solid-state reaction, sol-gel process and spray-drying process, and the process parameters had been optimized. For solid-state reaction, the LVP/C composite with 15 wt% glucose as carbon source and sintered at 700℃, showed the best electrochemical performance, and also exhibited a stable monoclinic structure after cycling. This cathode material exhibited good cycle stability but low capacity when cycling between 2.5-4.5 V and 3.0-4.5 V; whereas showed high capacity but poor cycle stability when cycling between 2.5-4.8 V and 3.0-4.8 V. For sol-gel process, glucose, as a second carbon sources, has significant influence on the electrochemical performance of LVP when oxalic acid and V2O5 are in a stoichiometric ratio. The LVP/C sample prepared with 15 wt% glucose exhibits the best electrochemical performance with discharge capacity as high as 171.2 mAh g-1 at 0.1 C and 118.9 mAh g-1 at 10 C, which is much higher than that for the LVP/C sample without glucose (145.3 mAh g-1/0.1 C and 13.7 mAh g-1/10C). For spray-drying process, the effect of the amount and addition sequence of CMC on the performance of LVP was investigated. The LVP/C sample prepared with 15 wt% CMC exhibits the best electrochemical performance. Controllable spherical LVP/C composites were obtained, i.e., solid spherical particles for LVP/C(A) prepared by adding CMC after pre-sintering, and hollow spherical particles for LVP/C(B) prepared by adding CMC before pre-sintering. XRD results reveal that some impurities, such as Na3V2(PO4)3, Li2NaV2(PO4)3 and Li4(P2O7), appear when sodium salt of caboxy methyl cellulose (CMC) acts as carbon source, but the monoclinic structure of LVP remains after cycling between 3.0 and 4.8 V. Compared to LVP/C(B), LVP(A) presents better electrochemical performance at any C-rate and higher tap density.The effect of niobium and silicon coating on the electrochemical performance and mechanism of LVP was studied. Our results show that, for Nb-incorporated LVP, (3-NbOPO4 existed on the surface of LVP particles, rather than doped into the crystal lattice. The particle size is reduced and the electrical conductivity is enhanced for LVP after Nb-incorporation, therefore, electrochemical performance of LVP is remarkably improved. Compared with pristine LVP/C, the Li3V1-xNbxPO4)3/C (x=0.03) composite shows significant improvement in capacity, rate capability and cyclability, which delivers an initial discharge capacity as high as 182 mAh g-1 at 0.1 C with a capacity retention ratio of 84.7% after 50 cycles, and an excellent rate-capability of 160 mAh g-1 at 1 C and 125 mAh g-1 at 5 C. For SiO2modified LVP, the valence of V in both the pristine and SiO2-coated LVP are close to+3.SiO2 coating on the surface of LVP particles does not change the monoclinic structure of LVP. Furthermore, the electrochemical performance of LVP/C, especially high C-rate performance, can be significantly improved by low-level (2 wt%) SiO2 coating, which can suppress vanadium dissolution in the electrolyte, improve structural stability and reduce charge-transfer resistance.The effect of Fe and Na doping on the electrochemical performance and mechanism of LVP was also investigated. The experimental results show that, for Fe-doped LVP, Li3-xV2-yFe2+y(PO4)3, LiFePO4 and FePO4 co-exist in the Fe-doped LVP/C composite. Compared with pristine LVP/C, significant improvement in capacity, cycling stability and rate capability in LVP/C-Fe are achieved, which is attributed to reduced particle size, decreased charge-transfer resistance and enhanced structural stability of LVP. For Na-doped LVP, Na shows a notable effect on electrochemical behavior of LVP cathode material. The charge/discharge plateaus and the anodic/cathodic peaks around 3.85 V for LVP/C(A) and LVPC(B) electrodes confirmed the existence of Li3-xNaxV2(PO4)3 or some other electrochemically-active composites containing Na-A series of LiFe1-xVxPO4/C(x= 0.0.03,0.05,0.07,0.10.0.20.0.50,1.00) samples have been successfully prepared using a two-step solid-state reaction route. It is found that V-incorporation significantly enhances the electrochemical performance of LFP. Particularly, the LFP/C sample with 5 at% vanadium exhibited the best performance, i.e. a specific discharge capacity of 129 mAh g-1 at 5.0 C after 50 cycles and all capacity retention ratio was above 97.5% at C-rate from 0.1 to 5.0 C. X-ray absorption spectroscopy (XAS) results showed that the valence of V in LiFe1-xVxPO4/C (x= 0.05) is between+3 and+4. It was confirmed that the samples with x≤0.03 are in single phase while the samples with 0.05≤x＜1.00 contain two impurity phases:Li3V2(PO4)3 and LiVOPO4.更多还原