锌镍电池电极材料氧化锌纳米化与表面包覆及其电化学性能

【作者】 李艳；

【导师】 郭绍义； 袁永锋；

【作者基本信息】 浙江理工大学 ， 机械设计及理论， 2010， 硕士

【摘要】 锌镍二次电池具有高能量密度、高功率密度、低成本和无污染等优点,是一种新型高性能绿色二次电池,在电动工具和便携式电子产品等方面具有广泛应用前景。锌镍电池未产业化的主要问题是循环寿命较低,原因是锌枝晶生长和锌电极变形。过去,研究工作主要集中在电解液添加剂、锌电极添加剂、隔膜筛选等领域。然而锌枝晶和锌电极变形等问题与ZnO的物理化学性质有密切关系,是ZnO/Zn在碱性电解液中的溶解和Zn在反复充放电过程中的电沉积所导致的。因此,对ZnO结构性质的研究也是提高锌镍电池电化学性能的一个重要途径。本文提出对ZnO纳米化和表面包覆来改善锌镍电池的循环寿命,制备了ZnO纳米线和纳米铋基化合物表面包覆的ZnO,并系统研究了两者的电化学性能和ZnO纳米线的微观形态衍变过程。采用ZnCl₂和Na₂CO₃水热反应合成了直径50-80 nm,长径比大于100的结晶良好的ZnO纳米线。对ZnO纳米线进行恒流充放电测试,结果表明,ZnO纳米线具有较好的循环稳定性,较高的放电中值电压和较低的充电中值电压。经过75个充放电循环,ZnO纳米线的放电容量仍保持在605 mAh g^-1,容量衰退率仅8.2%,而普通ZnO只有172 mAh g^-1,表现出明显改善的电化学性能。电化学性能的改善与ZnO纳米线的纳米效应和形态效应有关。循环伏安测试也说明了ZnO纳米线比普通ZnO具有更高的电化学活性。通过研究ZnO纳米线的微观形态衍变过程,可以发现在循环充放电过程中ZnO纳米线的形态未发生本质改变,只是变短变粗,近似成纳米棒的形状。这是由于较长的氧化锌纳米线在充放电循环过程中部分溶解断裂成纳米棒,纳米棒表面高活性导致外延生长形成的。同时纳米线一般是横卧在锌电极的内部和表面,其晶体自身最快生长方向<0001>和液相浓差极化诱导产生的最快生长方向（一般为垂直锌电极表面的方向）相垂直或倾斜,因而抑制了锌枝晶生长。此外,在充放电循环中,活性材料仍保持为纳米尺度使其仍具有纳米材料所具有的较高的电化学活性和充放电性能。通过Bi（NO₃）₃水解反应制备了含量从3.6 wt.%到14.8 wt.%的纳米铋基化合物表面包覆的ZnO,XRD和TEM发现氧化锌表面被直径约50 nm的颗粒（Bi₂O₃和BiO）包覆。表面包覆后,ZnO的循环稳定性,放电容量和平均利用率都有明显改善。其中含Bi 9.3 wt%的表面包覆ZnO放电容量增加了132 mAh g^-1,平均利用率提高了27 %,表现出较高的电化学活性。电化学性能的改善是因为铋在ZnO表面包覆降低了其与电解液的接触面积,减缓了ZnO在电解液中的溶解,保持了ZnO的电化学活性。表面包覆物纳米铋基化合物在初始循环后即被还原为金属铋,并始终以金属铋的形式包覆在氧化锌表面,能提高锌电极电导率,促进锌酸盐均匀沉积。因此,相对ZnO和Bi₂O₃的物理混合,纳米铋基化合物对ZnO表面包覆是有效利用锌电极添加剂,改善其电化学性能的有效方法。更多还原

【Abstract】 Zinc/Nickel （Zn/Ni） secondary battery has attractive advantages of high energy and power densities, low cost and no toxicity, is a new promising power source and has wide application for electric tools and new portable devices. However, widespread commercialization of the Zn/Ni battery has been prevented due to the serious problems such as Zn dendritic growth and shape change. By far, most researches on this field have focused on additives to the electrolytes or the zinc electrode, selection of stable separators and other techniques. Nevertheless, problems such as Zn dendritic growth and shape change have close relations with physical and chemical properties of ZnO, which results from dissolution of ZnO in alkalinity electrolyte and deposition of zincate during the charge/discharge cycles. Therefore, studying the structure and properties of ZnO is an effective method to enhance the electrochemistry performance of Zn/Ni secondary battery. This dissertation puts forward material design and surface modification of ZnO to improve cycle life of Zn/Ni battery. ZnO nanowires and surface-modified ZnO with nanosized Bi compounds are prepared, electrochemistry performance of them used as zinc electrode active material are investigated and the shape evolution of ZnO nanowires is discussed.ZnO nanowires were fabricated by a low temperature hydrothermal approach of ZnCl₂ and Na2CO3, and have diameters of 50-80 nm, lengths of 5-8 um and the length-diameter ratios of about 100. The electrochemical performance of ZnO nanowires were studied by the constant current charge-discharge test and the cyclic voltammetry test. The results of constant current charge-discharge test revealed that ZnO nanowires had better cycle stability, higher discharge midpoint voltage and lower charge midpoint voltage than the conventional ZnO. The discharge capacity of ZnO nanowires still remained 605 mAh g^-1 until the 75th cycle, while that of conventional ZnO was only 172 mAh g^-1. The fading rate of ZnO nanowires was only 8.2 % and displayed the ameliorative electrochemistry performance. The improved electrochemistry performance had relations with nanometer size effect and shape effect of zinc oxide. The cyclic voltammetry test also showed the ZnO nanowires had the higher electrochemistry activities compared to the conventional ZnO. By analysing the successional variation process of ZnO nanowires’microscopic shape, we found that the morphology of ZnO nanowires did not essentially change, only shortened and thickened, and turned into nanorods. This was the reason that part of long ZnO nanowires dissolved and ruptured to broke nanorods in the charging /discharging process, and high surface activeness of nanorods caused the epitaxial growth.What’s more, ZnO nanowires was parallel to Zinc electrode interior and surface, so the rapidest growth direction determined by the crystal growth habit was vertical or inclined to the accelerated growth direction induced by concentration polarization which was aroused by liquid-side mass transfer. The two growth modes competed and inhibited mutually, at last the zinc dendrite was impeded effectively. ZnO nanowires were nanomaterials after the certain charging and discharging cycles, so they still had higher electrochemistry activeness.Surface-modified ZnO with nanosized Bi2O3/BiO compounds was prepared by the hydrolyzation reaction of Bi（NO3）3. The characteristics of surface-coated ZnO were analyzed by transmission electron microscopy （TEM） and X-ray diffraction （XRD）, and the results revealed that some nanoparticles with about 50 nm in diameter were modified on ZnO. The constant current charge-discharge test and cyclic voltammetry of the ZnO electrode showed that bismuth modifying on the surface of ZnO enhanced the cycle stability of the electrode, maintained the electrochemical activity of ZnO, and increased the discharge capacity and average utilization respectively. In comparison with the untreated ZnO, discharge capacity and average utilization of ZnO coated by 9.3 wt.% Bi increased 132 mAh g^-1 and 27 %. The enhancement in the electrochemical performance was owing to the fact that Bi modified on ZnO decreased the contact area of active materials with the electrolyte, therefore, slowed the dissolution of ZnO in the electrolyte and maintained the electrochemical activity of ZnO. Nanosized bismith compounds were deoxidized to the metal bismith after the initial charge/discharge cycles, then bismith compounds kept the form of metallic Bi to modify on the surface of ZnO, which could raise the zinc electrode conductivity, promote zincate to deposit evenly and suppresse the zinc electrode corrosion. Compared with conventional mechanical mixture between ZnO and Bi2O3, surface modification with nanosized Bi compounds is one efficiency method to improve electrochemical performance of ZnO .更多还原