鋰離子電池矽銅複合負極材料製備與分析

Abstract

本篇論文之主要目的為開發以矽為基材的新型鋰離子負極材料，雖然矽擁有高於傳統石墨(372 mAh/g)的比電容量(~3000 mAh/g)，但是由於充放電時伴隨劇烈的體積膨脹與固有的低導電性，導致循環壽命不長，使矽負極商業化之路受到阻礙。

為了克服矽電極本質上的問題，我們使用兩種不同的方法合成矽銅複合材料。一種是以氯化亞銅為先驅物的流體化還原法，另外一種是以甲醛為還原劑的無電鍍法。雖然兩種方法都能成功的將銅還原，但使用流體化還原法的複合材料，其銅的披覆性不好，並且只能形成一矽加銅的混合物。儘管極版的導電性變好，但所呈現的電化學表現依然很差，這可能是因為此系統並不能忍受矽電極於充放電時所帶來的體積膨脹。 相反地，從掃瞄式電子顯微鏡可發現由無電鍍法合成的複合材料，銅具有很好的披覆效果，因此循環壽命相較於矽而言，提升了許多。為了增加銅層的機械強度，流體化化學氣相沈積將原來的複合材料再鍍上一碳層。結果顯示經由無電鍍反應及熱處理，電極的循環壽命於1000mAh/g的放電深度可以大幅的增加至六十圈循環，遠超過矽電極的八圈。

經由流體化化學氣相沈積熱處理後的複合材料，會產生一個新的物質——銅化矽。臨場X光繞射實驗顯示銅化矽對於鋰而言並非完全為一鈍性物質。最後，我們將於本論文中檢驗純相銅化矽電極之電化學性質。

The main purpose of this study is to explore new anode materials based on silicon for lithium-ion battery. Although silicon possesses a higher theoretical capacity (~3000 mAh/g) than graphite (372 mAh/g), the dramatic volumetric variation during cycling and intrinsic low conductivity, which resulted in structural instability and poor cyclability, obstruct its commercial application. Si-Cu composite materials are developed by two different methods to overcome the inherent problems of silicon. One is fluidized-bed type reduction (FB-reduction) with the precursor of CuCl powder, and the other is electroless plating in which formaldehyde was served as a reducing agent. Copper has been successfully reduced by both synthesized routes; however, the quality of coating was not satisfactory for FB-reduction and only a “Si + Cu” mixture was formed. Poor electrochemical performance hence has been observed for Si-Cu composites by FB-reduction due to inability to tolerate the volume expansion of silicon, in spite of the enhancement of electrode conductivity. Contrarily, scanning electron microscope (SEM) images show that more conformity and uniformity of coating can be achieved by using electroless plating and the cyclability, as compared with pure Si electrode, has been thereby improved. To enhance mechanical strength of the copper layer, fluidized-bed chemical vapor deposition (FB-CVD) technique has been carried out to coat a further carbon film on Si-Cu composites. Results show that the electrode made by electroless plating and heat treatment, comparing with Si electrode (<8 cycles), can be greatly improved to 60 cycles without fading at the discharge capacity of 1000 mAh/g. A new material copper silicide (Cu3Si) is found for Si-Cu composites after heat treatment in FB-CVD. In-situ X-ray diffraction shows that Cu3Si is a partially inactive material in the reaction of lithium. Moreover, electrochemical performance of single phase Cu3Si electrode has been studied.