【作者】 樊小勇；

【导师】 孙世刚；

【作者基本信息】 厦门大学 ， 物理化学， 2008， 博士

【摘要】 锡和锡基合金具有高的质量比容量和体积比容量,是下一代锂离子电池负极材料的研究热点之一。其主要缺陷在于嵌锂过程中体积膨胀导致活性材料粉化脱落,循环性能不好。目前解决的主要方法有:（1）制成纳米材料;（2）与活性或非活性元素合金化;（3）用活性或非活性材料包覆。除此之外,对集流体的表面形貌、多孔性和电极/电解质界面性质的深入认识也是提升合金电极电化学性能的重要内容。本论文发明了非氰化物电镀制备Cu₆Sn₅合金的方法,解决了一直以来难以用电镀法获得高锡含量的锡铜合金的难题。重点研究了不同集流体对锡基合金材料电化学循环性能的影响,和锡基合金电极与商业电解液的相容性,特别是运用电化学阻抗谱研究合金电极的相变过程和表面SEI膜的性质。主要研究内容和结果如下:（1）光滑铜片、泡沫铜和粗糙铜箔上锡铜合金的电化学性能。XRD结果指出光滑铜片、泡沫铜和粗糙铜箔上锡铜合金均由Cu₆Sn₅合金和少量纯Sn组成。充放电测试结果表明三种集流体上电镀获得的锡铜合金在放电过程中均在0.4V和0.1V附近给出Cu₆Sn₅的特征放电平台,但随着循环的进行逐渐消失。充放电循环结果显示粗糙铜箔上Cu₆Sn₅合金的容量和循环性能均优于其余两种。对经过不同循环次数的三种集流体上Cu₆Sn₅合金表面形貌进行分析,观察到光滑铜片上锡铜合金经过充放电循环39周后发生严重龟裂和脱落;泡沫铜上Cu₆Sn₅合金经过50周循环后也部分龟裂,但未发生明显脱落;粗糙铜箔上Cu₆Sn₅合金经过50周循环后,表面反而变得平滑,也无活性材料明显脱落。不同极化电位下的阻抗测试结果显示,Cu₆Sn₅合金在1.2V附近开始出现代表SEI膜的高频圆弧,在0.4V附近分别出现代表电荷传递和相变阻抗的中频和低频圆弧,但在0.3V附近相变阻抗圆弧基本消失,在0.1V附近又重新出现。Cu₆Sn₅合金放电不同容量后的阻抗谱研究指出,当放电5 mAh/g后Nyquist图中开始出现代表SEI膜的高频圆弧,当放电50 mAh/g后分别出现代表电荷传递和相变阻抗的中频和低频圆弧,当放电247 mAh/g后,低频圆弧转变为斜线,对应在0.4 V附近的放电结束。。对经过不同循环次数的Cu₆Sn₅合金电极的阻抗谱研究表明,电荷传递阻抗随着循环次数的增加先减小,后增大,说明了电极经历了一个从活化到逐渐失效的过程。（2）光滑铜片和粗糙铜箔上锡钴合金的制备及性质。XRD结果指出光滑铜片上电镀制得的锡钴合金为无定型态,而粗糙铜箔上电镀获得的锡钴合金为CoSn和Co₃Sn₂的复合物。充放电测试结果表明,光滑铜片和粗糙铜箔上电镀制得的锡钴合金在首次放电过程中均在0.22 V附近给出一个较长的放电电位平台,并在随后的循环中逐渐正移至0.4 V附近。充放电循环结果显示,粗糙铜箔上锡钴合金电极的循环性能明显优于光滑铜片上锡钴合金电极,经70周循环后容量无明显减小。表面形貌结果分析显示,光滑铜片上锡钴合金充放电循环20周后出现严重龟裂和脱落;但粗糙铜箔上锡钴合金经过70周循环后,表面变得平滑,活性材料未发生明显脱落。首次嵌锂过程的阻抗谱结果显示,锡钴合金电极在1.1V附近开始出现代表SEI膜阻抗的圆弧,在0.4 V附近开始出现代表电荷传递阻抗和相变阻抗的圆弧,当电极电位进一步降低至0.125V时,Nyquist图中代表电荷传递阻抗的中频圆弧消失,但低频圆弧仍然存在。对不同循环次数后的锡钴合金电极在0.05V处的阻抗谱分析表明,其电荷传递阻抗随着循环次数的增加不断增大,指示锂离子嵌入逐渐变得困难。（3）多孔铜集流体的制备及其表面电镀的锡铜和锡钴合金的电化学性能。运用氢气模版法电沉积制备得到有序多孔铜集流体,改变沉积条件调控孔的尺寸和壁厚。通过热处理增强多孔铜与基底间的结合力。分别以未热处理和热处理后的多孔铜为基底电镀制得锡铜合金。充放电结果显示热处理后的多孔铜上锡铜合金表现出较好的充放电性能,其首次放电容量735 mAh/g,首次充电容量571 mAh/g,经过50周循环后容量保持在342 mAh/g。不同温度条件下的EIS结果给出,开路电位时多孔铜上锡铜合金的Nyquist图均由一段曲率半径很大的圆弧组成。首次嵌锂过程中,当电极电位极化到1.2 V附近出现代表SEI膜阻抗的高频圆弧;到0.4 V附近,Nyquist图均转变为3段圆弧,即高频区域代表SEI膜阻抗的圆弧、中频区域代表电荷传递阻抗的圆弧和低频区域代表相变阻抗的圆弧。相变阻抗模拟结果显示,锡铜合金电极的相变电阻R_p在主要的相变电位区较小,其它电位下较大,但不同温度下R_p极小值出现的电位不同,随着温度的升高,其极小值电位正移。对于热处理后的多孔铜为基底电镀制备的锡钴合金,测得首次放电容量为726 mAh/g,首次充电容量为563 mAh/g,首次库伦效率为77.6%,从第2周循环开始到50周循环容量保持率为70%。多孔铜上锡钴合金的阻抗结果显示首次嵌锂过程中,当电极电位降低到0.4V附近,Nyquist图由高频区域代表SEI膜阻抗的圆弧、中频区域代表电荷传递阻抗的圆弧和低频区域代表相变阻抗的圆弧组成。与锡铜合金电极相似,锡钴合金的相变电阻同样在主要的相变电位区间内最小。当锡钴合金遭到突然的短路后,电极阻抗谱中出现了感抗弧,表征电极活性材料的不均匀性。首次嵌锂过程中感抗电阻R_L随着电极电位的降低而增大,而首次脱锂过程中随着电极电位升高感抗电阻R_L逐渐减小直至消失。本论文研究结果对于深入认识电极/电解质的界面性质和锡基合金的失效机理具有重要的基础理论意义。同时发明的非氰化物电镀制备锡基合金的方法、以及对集流体表面结构与电镀的锡基负极材料的功能之间的内在联系的深入认识,对于提升锡基合金负极材料的性能并应用于下一代锂离子电池具有直接的应用价值。更多还原

【Abstract】 Tin and tin-based alloys have been suggested as promising alternative anode materials for their high gravitational and volumetric capacity. The main disadvantage of tin and tin-based alloys is large volume expansion, which caused exfoliation, and poor cycleability. Up to now, the main methods to resolve this problem include using nano-materials, alloying with other active or inactive element, and coating with active or inactive materials. Besides these, the study of the surface morphology and porosity of alloy electrodes, and interfacial properties of electrode/electrolyte is also important to improve electrochemical performance. In this paper, we invented the method to prepare large tin content Sn-Cu alloy （Cu₆Sn₅ alloy） in cyanide-free solution. We have extensively studied the electrochemical performance, especially the relation of cycleability and current collectors with different structure, and the compatibility of electrode and commercial electrolyte. We also investigated the SEI film and phase transformation of these electrodes using electrochemical impedance spectroscopy. The main experiments and results are given as follow.（1） The electrochemical performance of Sn-Cu alloy on smooth copper sheet, copper foam and rough copper foil. The results of XRD indicated that all of them are comprised of Cu₆Sn₅ and pure Sn. Charge/discharge results revealed that they all appeared two potential plateaus at about 0.4 V and 0.1 V, which are the characteristic potential plateaus of Cu₆Sn₅ alloy. Charge/discharge results also revealed that the cycleability of Sn-Cu alloy on rough copper foil was better than that of other two electrodes. From SEM results, we observed that there appeared serious cracks and exfoliation on the surface of Sn-Cu alloy on smooth copper sheet after 39 cycles, and there are few cracks and no exfoliation for Sn-Cu alloy on copper foam and rough copper foil after 50 cycles. The Nyquist spectra of different electrode potentials indicated that the arc appearing in high frequency region represents the SEI film at 1.2 V. When the electrode potential was polarized to about 0.4 V, an arc in the middle frequency region and an arc in the low frequency region appear, corresponding respectively to charge transferring and phase transformation. The phase transformation impedance arc disappeared at about 0.3 V, and appeared again at about 0.1 V. The Nyquist spectra acquired at different stages of discharge under galvanostatic condition revealed that when the electrode is discharged for 5 mAh/g, the arc of high frequency arc associating to the impedance of SEI film is obeserved. When the electrode discharged 55 mAh/g, there appeared middle frequency arc and low frequency arc which represent charge transferring impedance and phase transformation impedance respectively. From the Nyquist spectra of the Sn-Cu electrode after different cycles, we can observe that the charge transferring impedance decreased first and then increased with the increasing of cycles, which revealed a process of activity to failure.（2） Preparation and properties of Sn-Co alloy on smooth copper sheet and rough copper foil. The results of XRD indicated that the Sn-Co alloy on smooth copper sheet is amorphous, and the Sn-Co alloy on rough copper foil is comprised of intermetallic composites of SnCo and Co₃Sn₂. Charge/discharge results revealed that one potential plateau at about 0.22 V is observed in all cases in the first cycle, and is shifted to 0.4 V in the second cycle. There appears no obvious decreasing for gravitational specific capacity after 70 cycles for Sn-Co alloy on rough copper foil, which is better than that of Sn-Co alloy on smooth copper sheet. From SEM results, we observed that there appeared serious cracks and exfoliation on the surface of Sn-Co alloy on smooth copper sheet after 20 cycles, and there are few cracks and no exfoliation for Sn-Co alloy on rough copper foil even after 70 cycles. The Nyquist spectra at different polarized potential indicated that there appeared a high frequency arc which represents the impedance of transferring through SEI film of lithium-ion at the potential of 1.1 V. When the electrode potential was polarized to about 0.4 V, there appeared a middle frequency arc and a low frequency arc which represent charge transferring impedance and phase transformation impedance respectively. When the electrode potential was polarized to 0.125 V, the middle frequency arc disappeared, which is different from that of Sn-Cu alloy electrode. From the Nyquist spectra of the Sn-Co electrode recorded in different cycles, we observed that the charge transfer impedance is gradually increased with the increase of cycles.（3） The synthesis of porous copper foam and the electrochemical performance of Sn-Cu and Sn- Co alloy on this porous copper foam current collector were studied. Variety of porous copper foams with highly open porous walls have been successfully sculptured using the gas evolved in an electrochemical deposition process. The pore sizes and wall structures of the foams are tunable by adjusting the deposition conditions. The contact between porous copper foams and substrate was strengthened by annealing porous copper foams at high temperature. Charge/discharge results revealed that the cycleability of Sn-Cu alloy on porous copper foams after annealing is better than that of the Sn-Cu alloy deposited on copper porous foam without annealing. The first discharge and charge capacity of Sn-Cu alloy on porous copper foams after annealing is 735 mAh/g and 571 mAh/g respectively, and the charge capacity remained 342 mAh/g after 50 cycles. The Nyquist spectra of different polarized potential at different temperature indicated that there appeared a high frequency arc which represents the impedance of transferring through SEI film of lithium-ion at the potential of 1.2 V. When the electrode potential was polarized to about 0.4 V, there appeared a middle frequency arc and a low frequency arc which represent charge transfer impedance and phase transformation impedance respectively. Simulation result revealed the phase transformation resistance is smallest at the potential region of phase transformation. Charge/discharge results revealed that the first discharge and charge capacity of Sn-Co alloy on porous copper foams after annealing is 726 mAh/g and 563 mAh/g respectively, and the first coulomb efficiency is 77.6%. After 50 cycles, the capacity remained 70% of that of the second cycle. When the electrode potential was polarized to about 0.4 V, the Nyquist spectra are comprised of a high frequency arc, a middle frequency arc and a low frequency arc which represent SEI film impedance, charge transfer impedance and phase transformation impedance respectively. Similar to Sn-Cu alloy, the phase transformation resistance is smallest at the potential region of phase transformation. When Sn-Co alloy electrode suffer Short circuit, there appear inductive loop in the Nyquist spectra, which caused by heterogeneity. The diameter of inductive resistance increased with the decreasing of electrode potential in the initial lithiation, and decreased with the increasing of electrode potential in the initial delithiation.The results of this thesis throw insight into electrode/electrolyte interface and failure mechanism of tin and tin-based alloy, and are of significance in developing relevant fundamental theory. The extensively study on the invention of cyanide-free electroplating tin-based alloy, and the intrinsic relation of surface structure of current collector and properties of them is also great importance in improving electrochemical performance of them and application in next generation lithium-ion battery.更多还原