【作者】 李斯蓉；

【导师】 陈春华； Jeff Dahn；

【作者基本信息】 中国科学技术大学 ， 材料学， 2014， 博士

【摘要】 环境污染和能源危机使得绿色能源技术得到了迅猛的发展,锂离子电池由于能量密度高、循环寿命长、环境污染小等优势受到广泛关注。除了在手机、数码相机、笔记本电脑等传统便携设备上的使用,近年来人们也开始广泛研发锂离子电池在电网储能和电动汽车方面的应用。不同于数码产品上的应用,电网储能和动力电池对锂离子电池的能量密度提出了更高的要求。本论文主要围绕高比能二次锂电池电极材料展开研究,涉及到V2O5、Fe-V复合氧化物、尖晶石Li1.05Mn1.95O4、5V高电压LiNi0.5Mn1.5O4等材料的制备,LiNi0.5Mn1.5O4材料的掺杂、表面包覆改性研究,以及LiNi0.5Mn1.5O4||Li4Ti5O123V全电池、LiNi0.42Mn0.42Co0.16O2||Li4Ti5O122.5V全电池性能优化探索等。第一章,我们介绍了锂离子电池的工作原理,对常见锂离子电池正负极材料做了简单介绍,然后介绍了目前锂离子电池常见电化学测试表征,重点对Dalhousie大学的高精度充放电测试仪进行了介绍,并总述了该测试手段的研究进展。第二章介绍了本论文涉及到的实验药品、实验方法以及实验设备,重点介绍了JeffDahn课题组进行扣式电池组装的流程。第三章以静电喷雾沉积的方法制备了三维多孔结构Fe掺杂的V205薄膜和Fe-V复合氧化物薄膜,这种三维多孔薄膜可以提高电解液与电极材料的浸润性,缩短材料中锂离子的传输距离。Fe掺杂的V205能够显著提高层状结构的稳定性,在2.0-4.0V电压范围内有着更好的循环容量保持率和倍率性能。Fe-V复合氧化物中,晶化的Fe2V4O13薄膜在1.0-4.0V之间表现出从结晶性向无定形变化的特征,但在2.5-4.0V之间结构非常稳定。无定形Fe2V4O12.29薄膜由于Fe和v混合价态带来的更高的电子导电性,在1.0-4.0V区间具有优异的循环及倍率性能。第四章以热醇解法制备得到纳米Mn3O4,将其与LiCH3COO·2H2O混合进行固相烧结制备了微米级别的Li1.05Mn1.95O4粉末。Li1.05Mn1.95O4||Li半电池具有非常优异的倍率性能和高温循环性能,室温下5C的放电容量为98.4mAh/g,55℃1C循环100次后容量能保持首次容量的90.5%。此外,该材料还具备非常优异的低温循环性能,-20℃时的放电容量可稳定在84.5mAh/g。循环伏安法测试锂离子的表观扩散速率得到扩散系数DLi+从25℃时的10-10cm2/s降低至-20℃时的10-12cm2/s。第五章先通过共沉淀法制备了Ni0.25Mn0.75(OH)2,再与Li2CO3固相烧结得到纯相的尖晶石LiNi0.5Mn1.5O4材料,利用高精度充放电电仪对LiNi0.5Mn1.5O4||Li半电池在不同倍率及温度下的库伦效率和充放电曲线滑移进行了研究。实验结果表明,LiNi0.5Mn1.5O4||Li半电池的库伦效率与测试温度、电极材料比表面积以及倍率大小相关。LiNi0.5Mn1.5O4||Li半电池一直存在较大的充放电曲线滑移,说明LiNi0.5Mn1.5O4表面电解液氧化等副反应在不断发生。此外,对比碳酸乙酯(EC)和碳酸二乙酯(DEC)溶剂,碳酸乙酯(EC)和碳酸二甲酯(DMC)作为电解液溶剂可以在一定程度上提高LiNi0.5Mn1.5O4||Li半电池的库伦效率,减少副反应的发生。库伦欠效率CIE/一次循环时间的研究结果显示副反应速率与倍率大小无关,只与反应温度、电极材料的比表面积相关。第六章对LiNi0.5Mn1.5O4材料分别进行了掺杂和包覆改性研究,以共沉淀法对LiNi0.5Mn1.5O4材料进行了Al、Co、Fe、Cr掺杂,利用羧甲基纤维素钠制备的浆料对LiNi0.5Mn1.5O4进行了ZnO、Al2O3包覆,并对以上改性措施在LiNi0.5Mn1.5O4||Li半电池中进行了研究。实验结果表明,Al、Co、Fe、Cr掺杂对半电池库伦效率没有明显改善,充放电曲线滑移现象与未掺杂样品相比没有缓解,但是半电池的循环稳定性得到很大提高。ZnO、Al2O3包覆LiNi0.5Mn1.5O4的样品SEM图片显示包覆效果非常均匀,但相比未包覆样品在LiNi0.5Mn1.5O4||Li半电池中未表现出优势,充放电曲线的滑移及库伦效率都与未包覆样品相近。此外,我们发现P4332相LiNi0.5Mn1.5O4中Mn3+的消除能够显著提高半电池在高温条件下的循环稳定性,但对库伦效率影响不大。第七章对LiNi0.5Mn1.5O4||Li4Ti5O123V全电池的衰减机理进行了研究。构建了LiNi0.5Mn1.5O4||Li4Ti5O12“背对背电池”,结合LiNi0.5Mn1.5O4||Li4Ti5O12全电池的电化学行为,成功验证了LiNi0.5Mn1.5O4和Li4Ti5O12电极之间的相互作用。LiNi0.5Mn1.5O4正极表面电解液氧化产生的产物会向Li4Ti5O12电极表面发生迁移,引起额外的电子得失从而造成Li4Ti5O12电极的滑移。LiNi0.5Mn1.5O4容量控制的LiNi0.5Mn1.5O4||Li4Ti5O12全电池随循环存在着容量衰减。Li4Ti5O12容量控制的LiNi0.5Mn1.5O4||Li4Ti5O12全电池在短时间内容量保持稳定,但是这种情况下依然存在电解液的氧化以及正负极间的相互作用,一旦电解液消耗完毕即会造成电池容量快速衰减。LiNi0.5Mn1.5O4容量控制的LiNi0.5Mn1.5O4||Li4Ti5O12全电池在C/20、C/2、1C和2C不同倍率下的库伦效率研究表明,库伦效率与充放电电流密度相关。LiNi0.5Mn1.5O4||ILi4Ti512O全电池中副反应主要是由于电解液在正极材料表面的氧化引起的,这种副反应产生的电流大约在C/2000左右。不同倍率下的测试结果表明该全电池体系更适合在大倍率条件下循环,2C时发挥80%以上容量可循环500次。第八章对LiNi0.5Mn1.5O4||Li4Ti5O123V全电池的性能优化进行了探索,包括对LiNi0.5Mn1.5O4的Al、Co、Fe、Cr掺杂,ZnO.Al2O3表面包覆,不同电解质盐,以及电解液添加剂的使用等等。Al,Co,Fe,Cr掺杂的LiNi0.5Mn1.5O4在LiNi0.5Mn1.5O4||Li4Ti5O123V全电池中没有对库伦效率、放电容量保持率、充放电曲线滑移起到改善作用,未能有效控制全电池中的副反应。ZnO、Al2O3包覆的LiNi0.5Mn1.5O4可以有效抑制正负极间的相互作用,减小充放电曲线的滑移,提高全电池的库伦效率。对比四种常用电解质盐在LiNi0.5Mn1.5O4||Li4Ti4Ti5O12全电池中的应用发现,LiPF6相比LiC104,LiBOB和LiBF4具有最高的库伦效率、最小的充放电曲线滑移,因此在这四种盐中最适合用于LiNi0.5Mn1.5O4||Li4Ti5O12体系。使用1wt%氟化叔丁醇锂(LiO-t-C4F9)添加剂能够显著提高LiNi0.5Mn1.5O4||Li4Ti5O123V全电池的循环稳定性,但是相对于空白电池而言,其库伦效率更低、充放电曲线滑移更快、自放电现象更严重,这些现象表明这种添加剂并没有减少LiNi0.5Mn1.5O4表面的副反应,因此并不是一种好的添加剂。相反,1wt%铝酸六氟三异丙酯(Al(HFiP)3)添加剂虽然没有能够提高全电池的循环容量保持率,但是在电池满充贮存实验中的自放电程度更小,说明它在一定程度上减少了LiNi0.5Mn1.5O4电极表面副反应的发生。第九章对LiNi0.42Mn0.42Co0.16O2||Li4Ti5O122.5V全电池的电化学性能优化措施进行了探索,发现该2.5V全电池中也存在正负电极之间的相互作用。在1.0-2.5V电压区间循环时Li4Ti5O12存在着滑移,将充电截止电压从2.5V降低至2.27V可以显著减小这种滑移,有效抑制全电池中副反应的发生。电池在高温下循环时会加速电池中副反应的发生,使Li4Ti5O12出现更大的滑移。在1.0-2.5V区间,Li4Ti5O12的滑移使得Li4Ti5O12电池容量随循环有-定增加,而1.0-2.27V区间L14T15O12不存在滑移使电池有着稳定的放电容量。此外,本章还发现在该全电池中使用2wt%碳酸亚乙烯酯(VC)作为添加剂时,Li4Ti5O12和Li4Ti5O12电极的滑移都有所加剧,意味着2wt%VC不适用于提高该体系的电化学性能。第十章对本论文的创新及不足之处做了简单总结,并对未来工作进行了展望。最后,本论文还涉及了负极MnO/C材料的制备与电化学性能表征,在附录-中给予介绍。更多还原

【Abstract】 Recently, many green energy technologies have been regarded as a promising approach to address both energy crisis and environmental pollution. Due to their high energy density, long cycle life and environmental friendly, Li-ion batteries have been widely used in portable devices such as telephone, digital camera and laptop. Besides, researchers are also exploring the application of Li-ion batteries in energy storage systems and electrical vehicles, which have a higher requirement for energy density. This thesis focuses on high energy density cathode materials for Li-ion batteries, including the preparation of V2O5, Fe-V oxide thin films, spinel Li0.05Mn1.95O4and LiNi0.5Mn1.5O4powders. It also involves in the doping and surface coating of LiNi0.5Mn1.5O4, the study of LiNi0.5Mn1.5O4||Li4Ti5O123V full cells and LiNi0.42Mn0.42Co0.16O2||Li4Ti5O122.5V full cells.Chapter1gives a general introduction of the working mechanism of Li-ion batteries, some typical cathode and anode materials as well as some conventional electrochemical examinations. Then a detailed introduction of high precision charger (HPC) in Dalhousie University is presented. A summary of recent researches using the HPC for Li-ion batteries is conducted as well.In Chapter2, we introduce the experimental reagents, methods and equipments used in the project of this thesis. A detailed procedure of coin-cell fabrication in Dahn lab is presented as well as some electrochemical, structure and morphology analysis.In Chapter3, Fe doped V2O5and Fe-V oxides thin films are prepared by the electrostatic spray deposition technique. Such a three-dimensional structure allows the electrolyte to soak well into the active material and facilitate the kinetics of lithium-ion transport. After introducing Fe3+into the V2O5structure, the stability of the layered structure can be improved, leading to an improved cycling and rate performance in the voltage range of2.0-4.0V. The crystalline Fe2V4O13thin film performs a structural degradation in the voltage range of1.0-4.0V while it is very stable during2.5-4.0V. The amorphous Fe2V4O12.29thin film shows good cycling performance and rate capability during1.0-4.0V due to the enhanced electronic conductivity caused by the existence of mixed valence states of Fe and V.In Chapter4, nanometer-sized Mn3O4powder is prepared in an oil-bath synthesis process with diethylene glycol as the solvent. Then micrometer sized Li1.05Mn1.g5O4powders are synthesized by the mixture of Mn3O4and LiCH3COO·2H2O. The L1.05M1.95O4||Li cells exhibit a high rate performance with a specific capacity of98.4mAh/g at5C at room temperature. A stable cycling performance is observed at55℃that90.5%of its initial capacity can be obtained after100cycles at1C. Furthermore, the Li1.05Mn1.95O4sample also shows much stable cycling performance at low temperatures with a specific capacity of84.5mAh/g at-20℃. The diffusion coefficients of lithium ion measured by CV method show a drop from10-10cm2/s at25℃to10-12cm2/s at-20℃.In Chapter5, LiNi0.5Mn1.5O4is prepared by a solid state reaction from a mixture of Li2CO3and Nio.25Mno.75(OH)2, which is obtained by a co-precipitation method. An accurate coulombic efficiency (CE) study of LiNi0.5Mn1.5O4||Li cells by HPC using different surface area electrodes cycled at different temperatures and C-rates is performed. CE is found to be dependent on temperature, specific surface area and C-rate for LiNi0.5Mn1.5O4||Li cells. The charge and discharge profile of the LiNi0.5Mn1.5O4||Li cells slips to large relative capacities with continued cycling, indicating serious parasitic reactions inside the cells. EC/DMC as the electrolyte solvent increases the CE of LiNi0.5Mn1.5O4||HLi cells compared to EC/DEC. Measurements of the CIE/(time of a cycle) show that parasitic reactions continue at the same rate independent of the cycling current.In Chapter6, a study of doping and surface coating for LiNi0.5Mn1.5O4is performed. Al, Co, Fe and Cr substituted LiNi0.5Mn1.5O4samples are successfully synthesized from a hydroxide precursor.2wt%ZnO and2wt%Al2O3coated LiNi0.5Mn1.5O4is made from a carboxymethyl cellulose containing slurry. A study of the CE of LiNi0.5Mn1.5O4||Li using the substituted LiNi0.5Mn1.5O4shows that there is no "magic" improvement in the CE of cells using the transition metal substituted samples. The charge and discharge slippage increases with cycles, indicating serious parasitic reactions occurring inside all the cells. SEM results show that ZnO or Al2O3is uniformly coated on the surface of LiNi0.5Mn1.5O4. But both electrodes show the same capacity retention and close CE versus cycle number compared to uncoated LiNi0.5Mn1.5O4electrode. The capacity retention is improved while no improvement in CE is observed for the LiNi0.5Mn1.5O4||Li cells by comparing the ordered P4332phase LiNi0.5Mn1.5O4with or without Mn3+In Chapter7, the capacity degradation mechanism of LiNi0.5Mn1.5O4||Li4Ti5O12cell is investigated by a designed "back-to-back" cell. The electrode-electrode interactions that occur in LiNi0.5Mn1.5O4||Li4Ti5O12Li-ion cells are examined. Species created at the LiNi0.5Mn1.5O4electrode by electrolyte oxidation migrate to the Li4Ti5O12electrode, cause excess charge consumption and lead to the slippage of the Li4Ti5O12electrode. Therefore, LiNi0.5Mn1.5O4-limited LiNi0.5Mn1.5O4||Li4Ti5O12cells show rapid capacity fading while Li4Ti5O12-limited cells cycle without loss for a short while before the electrolyte is exhausted. Measurements of CE of LiNi0.5Mn1.5O4-limited LiNi0.5Mn1.5O4||Li4Ti5O12cells at C/20, C/2,1C and2C show that the CE is dependent on cycling current due to a significant parasitic current determined to be close to C/2000for these cells. Besides, it revealed that LiNi0.5Mn1.5O4||Li4Ti5O12cells prefer to cycle well at2C that500cycles could be obtained with80%of its initial capacity.In Chapter8, we apply some strategies to improve the CE of LiNi0.5Mn1.5O4||Li4T15O12cells, such as Al、Co、Fe and Cr substituted LiNi0.5Mn1.5O4, ZnO or Al2O3coated LiNi0.5Mn1.5O4, different electrolyte salts and the use of electrolyte additives. There is no improvement for Al, Co, Fe and Cr substituted LiNi0.5Mn1.5O4from the aspect of CE and charge/discharge slippage. ZnO or Al2O3coated LiNi0.5Mn1.5O4can effectively decrease the interactions between LiNi0.5Mn1.5O4and Li4Ti5O12electrodes and increase the CE of the LiNi0.5Mn1.5O4||Li4Ti5O12cells. Compared to LiClO4LiBOB and LiBF4, LiPF6gives the best CE and the least slippage for LiNi0.5Mn1.5O4||Li4Ti5O12cells. LiNi0.5Mn1.5O4||Li4Ti5O12cells with1wt%LiO-t-C4F9electrolyte additive show improved capacity retention versus cycle number compared to control cells. However their CE, capacity end point slippages and self-discharge are worse than control cells, indicating that this additive increased, not reduced, the rate of parasitic reactions occurring at the LiNi0.5Mn1.5O4electrode. Therefore, although this additive initially looks "good" it turns out to be poor when all factors are considered. By contrast,1wt%Al(HFiP)3does decrease the rate of parasitic reactions at the positive electrode based on the results of storage experiments.In Chapter9, the electrochemical performance of LiNi0.42Mn0.42Co0.16O2||Li4Ti5O12cells are studied, the electrode-electrode interactions between positive and negative electrodes are detected in such2.5V full cell. Li4Ti5O12electrode shows slippage in the voltage range of1.0-2.5V while no slippage is observed during1.0-2.27V, indicating the decreased parasitic reactions. Elevated temperature is found to accelerate the Li4Ti5O12slippage. The slippage of Li4Ti5O12electrode leads to a gradual capacity increase in the voltage range of1.0-2.5V while a constant capacity is obtained without Li4Ti5O12slippage at1.0-2.27V. Besides,2wt%Vinylene carbonate (VC) is shown to be negative for the LiNi0.42Mn0.42Co0.16O2||Li4Ti5O12cells due to the increased charge and discharge slippage from both LiNio.42Mn0.42Co0.16O2and Li4Ti5O12electrodes.In Chapter10, we give an overview of the innovation and deficiencies of this thesis. Some prospects and suggestions for the future work are presented as well.Finally, there is still some work of MnO/C as anode materials for Li-ion batteries involved in this thesis, which is given in the appendix1.更多还原