非晶質碳材在動力鋰電池之研究

Abstract

現今的商用鋰離子動力電池負極材料主要以碳材為主，碳材擁有絕佳的穩定性、蘊藏量豐富、安全無毒性等特性。其中碳材又可分為石墨和非晶質碳(軟碳、硬碳)。這兩種碳材目前都被廣泛的應用在鋰離子動力電池負極上。因此，本研究為了瞭解石墨和軟碳在鋰離子動力鋰電池中的差異性，分別針對兩種材料的物理觀點(結構的特性)及電化學特性進行分析及探討。 首先，我們利用SEM和XRD的分析，探討石墨和軟碳在形貌及結構上的差異。結果顯示，石墨屬於球型，而軟碳的形貌並無均一性，呈現不規則的形貌分佈。由XRD結果顯示，在結構上，軟碳相較於石墨擁有較大的碳層間距(d-spacing)，此特性將有利於鋰離子在碳層中的嵌入/脫嵌。 再者，我們比較兩種材料在半電池(graphite/Li, soft carbon/Li cell)的特性。結果顯示，石墨因為其片狀及碳層排列整齊的結構，在第一圈充放電時有較低的不可逆電容。兩種材料所能容納的鋰離子量也有差異，石墨可和鋰離子生成LiC6，軟碳最多只能生成Li0.8C6。因此，石墨和軟碳在電容量及充放電曲線上有顯著的差異。 我們將石墨和軟碳分別搭配LiMn2O4 (LMn)和LiNi1/3Co1/3Mn1/3O2 (LNCM)混合的正極材料組成全電池進行分析。在第一圈的電性結果顯示LMn+LNCM/Graphite的全電池有較好的庫倫效率。在混合式脈衝功率特性(Hybrid Pulse Power Characterization；HPPC)和快充快放測試顯示，LMn+LNCM/Soft carbon的全電池在瞬間以及大電流充電時所顯現出來的電性優於LMn+LNCM/Graphite。在高溫(45℃)測試，軟碳在經過300圈充放電後，電容量仍有原本電容量的86.2%。 最後，利用交流阻抗法(AC Impedance)和直流內阻測試(Direct Current Internal Resistance；DCIR)分析軟碳和石墨極板在高溫下經過充放電後阻抗變化情形，結果顯示經過300圈充放電後軟碳的阻抗變大的程度遠小於石墨阻抗增加的程度。利用三極式全電池測試阻抗結果，更能明顯看出石墨負極界面阻抗明顯大幅上升。

Carbon materials are an important anode material for commercial high-power lithium ion batteries because they are chemically and thermally stable, abundant on earth and environmentally friendly. In general, two types of carbon are used as the anode material for high-power lithium ion batteries; they are graphitic carbon and non-graphitic carbon (including soft carbon and hard carbon). This research studies material characteristics of the graphitic and soft carbons and compared the electrochemical performance between the two carbon materials used as the anode for high-power lithium ion batteries. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the microstructure and the surface morphology of the carbon materials, respectively. The graphitic carbon is composed of spherical grains, but the soft carbon is non-uniform in shape. The soft carbon exhibits a graphite lattice (layer) distance (d-spacing) larger than the graphitic carbon. The larger d-spacing is beneficial to lithiation/ de-lithiation in the soft carbon anode. The study on the electrochemical performance shows that the graphite electrode exhibits a lower 1st cycle irreversible capacity than the soft carbon electrode. This can be ascribed to that the graphitic carbon has a more regular carbon layer structure. The lithium-graphite intercalation compounds (Li-GICs) formed in the graphite electrode has a composition of LiC6. However, the Li-GIC in the soft carbon electrode is composed of LixC6 with a maximum x value of 0.8. As a result, the two carbon electrodes have a significant difference in the charge/discharge performance. In addition, the full cell test using LiMn2O4 (LMn)+ LiNi1/3Co1/3Mn1/3O2 (LNCM) as the cathode shows that the graphite anode has a better 1st cycle coulombic efficiency. However, the soft carbon anode demonstrates better in the rate performance and the hybrid pulse power characterization (HPPC) test. The capacity retention of the LMn+LNCM/soft carbon full cell is about 86.2% after 300 test cycles at 45℃. The resistance change in the full cells was studied by AC impedance and direct current internal resistance (DCIR) study. The LMn+LNCM/soft carbon full cell has a smaller resistance increase rate after 300 test cycles. Base on the impedance test using a three-electrode full cell structure, the graphite anode exhibits a very large increase in the interfacial resistance.