鋰離子電池矽與氧化鋯複合負極材料製備與分析

Synthesis and Characterization of Zirconia-Silicon Composite Anode Materials for Lithium-ion Batteries

郭家邦(Chia-Pang Kuo)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02571

摘要

本論文的主要目的為開發以矽為主體的鋰離子二次電池負極材料，並且與氧化鋯形成複合材料以提高其振實密度及矽負極或矽碳複合負極的體積電容量密度。矽，擁有極高的理論質量電容量密度(大於3600 mAh/g)、豐富的蘊藏量豐富及安全無毒性的特性，此外，隨著半導體產業成熟所伴隨的提煉技術進步，使其相對價格便宜，為目前極有可能取代石墨(372 mAh/g)，成為新世代高電容量鋰離子電池的負極材料之一。但由於充放電時體積劇烈膨脹與低導電度，以及固相電解液介質(SEI)的負面影響，而造成極板結構不穩定以及充放電的電容不可逆性。這些原因使得鋰電池在應用上受到限制，為了克服這些問題，我們使用溶膠凝膠法製作矽與氧化鋯的複合材料，並且利用不同的鍍碳方式，改善氧化鋯不導電的特性，從而製作出一具高振實密度以及高充放電穩定性的矽基複合負極材料。
合成的概念在於將奈米級的矽顆粒與正丙醇鋯(氧化鋯的前驅物)混合均勻於異丙醇溶劑中，使用溶膠凝膠法於其中生成氧化鋯膠體，如此便可以得到矽與氧化鋯複合膠體。將此膠體於高溫鍛燒作緻密化處理，即可得到矽與氧化鋯複合負極材料。鍛燒後得到的複合粉體仍可維持其高孔隙度，並利用這些孔隙體積和氧化鋯的強機械強度，緩衝矽材在充放電過程中的不可逆膨脹。
首先，先從作出高孔隙度的氧化鋯氣凝膠(aerogel)著手，在不使用超臨界流體乾燥法的情況下(常見於製作高孔隙度的凝膠)，簡化實驗流程，調整溶膠凝膠法製作凝膠中的各種重要因素，如前驅物濃度、粉體鍛燒溫度、含水量、凝膠狀態等，達到製程最佳化的目標，並且得到高孔隙度的氧化鋯粉體。此外，在真空環境下鍛燒，達到高溫鍛燒(大於500度)仍可維持粉體孔隙度的結果。之後，以這些實驗參數為基礎，將奈米級矽顆粒加入凝膠混合均勻，乾燥後將膠體作高溫鍛燒處理得到高孔隙度的矽-氧化鋯粉體。
此外，兩種鍍碳方式分別為果糖鍍碳以及瀝青鍍碳。前者為溶膠過程中，直接浸泡穩定的矽與氧化鋯膠體(鍛燒前)於果糖溶液中，使得果糖溶液可以均勻地分散於高孔隙度的膠體中，並且在矽表面形成果糖包覆，在高溫鍛燒後裂解成碳層包覆並得到矽-氧化鋯-碳複合材料，實驗結果顯示，經過鍍碳處理可以提升導電度並大幅降低矽基電極阻抗。後者為將矽與氧化鋯膠體先進行第一步低溫鍛燒(400度)，得到矽與氧化鋯粉體後再與瀝青丙酮溶液混合，乾燥後做第二次鍛燒，將瀝青裂解成碳而得到矽-氧化鋯-碳複合材料，實驗結果顯示，瀝青分解的碳能填滿孔洞體積，以達到降低表面積並降低SEI膜生成而導致的不可逆。
此外，兩種不一樣的奈米級矽原料的使用，也於本論文中探討。實驗結果顯示，矽-氧化鋯-碳複合負極樣品在50圈的充放電後，仍然可以維持70%的電容量。

關鍵字

鋰離子二次電池 ； 矽 ； 溶膠凝膠法 ； 氧化鋯氣凝膠 ； 複合負極材料

並列摘要

The main purpose of this research is to develop a high tap-density anode material based on silicon for lithium ion batteries. Silicon, in addition to its abundance on earth and its environmentally-friendly property, it also possesses a high theoretical capacity ( > 3600 mAh/g) compared to graphite (372 mAh/g). However, the dramatic volumetric variations during cycling and intrinsic low conductivity result in structural instability and poor cyclability. Moreover, the irreversibility caused by solid electrolyte interphase (SEI) formation accelerates the capacity fading as well. In order to solve those problems, we use sol-gel process to make a porous zirconia-silicon composite material and use different carbon coating process to improve the electronic insulation property of zirconia.
To synthesize Si-ZrO2 composite, nano-sized Si is dispersed in iso-propanol and at the same time, the zirconia gel forms by sol-gel method with zirconium propoxide as precursor. After deriving Si-ZrO2 gel, high temperature treatment is conducted to have porous Si-ZrO2 powder. The pore volume and strong mechanical property of zirconia are utilized to buffer the irreversible expansion of Si during cycling.
The preliminary work is to make a porous zirconia which can provide sufficient pore volume for buffering expansion of Si. This research has developed a rather-simple process (without supercritical drying, which is typically regarded as indispensable drying method for deriving aerogel) by controlling several important factors in sol-gel process, such as concentration of precursor, calcination temperature, water content, and gel state to derive porous zirconia. Besides, high-temperature treatment under vacuum environment can preserve more pore volume than that under 3% H2/N2 environment ( T > 500℃ ). After all the factors are well studied, the silicon is mixed with zirconia aerogel and then calcined to have porous Si-ZrO2 composite powder.
Besides, carbon coating methods consist of fructose carbon coating and pitch carbon coating. The former one is directly soaking stable Si-ZrO2 gel in fructose solution and the fructose solution can permeate into the porous gel and form a fructose layer on Si surface. The fructose layer decomposes into carbon layer after high temperature treatment. The experimental result indicates that the impedance of Si-ZrO2 electrode decreases a lot with increasing conductivity of Si-ZrO2-C. Different from fructose carbon coating, pitch carbon coating adopted two-step calcinations. Si-ZrO2 proceeds the 1st calcination under low temperature (400℃) to have Si-ZrO2 powder and then the powder is mixed with pitch in acetone solution. After drying process, the collected powder proceeds the 2nd calcination to have Si-ZrO2-C. The experimental result shows that the dissociated carbon indeed fills the pore volume and reduces the surface area of porous Si-ZrO2 structure to improve the irreversible capacity from SEI formation.
Last, two kinds of nano silicon with different sizes and qualities are used to form Si-ZrO2-C composite. The experimental result shows that Si-ZrO2-C electrode can retain 70% of 1st cycle charge capacity after running 50 cycles.

並列關鍵字

Li-ion batteries ； Silicon ； Sol-gel process ； zirconia aerogel ； composite anode material

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