| 摘要: | 本論文分兩部分，均以機械式熱處理法，分別將Li4Ti5O12 和Li4Mn5O12 塗佈 於商用LiCoO2 陰極材料表面，期能改善商用LiCoO2 陰極材料於高充放電截止電 壓及快速充放電速率下，循環穩定性不佳之缺點，以及電池於長循環測試後之熱 安全性。首先針對各製程材料之電池性能進行測試，進而求出最佳製程條件；而 後利用各項材料鑑定，對材料之各種物化性質進行探討；最後以循環伏安法分析 材料氧化還原性質，並利用交流阻抗分析電池內阻之變化。 於本論文中，吾人自行合成Li4M5O12 (M=Ti、Mn)作為塗佈物質，並用於處 理商用FMC-LiCoO2 陰極材料。吾人藉由機械式熱處理法將其塗佈於LiCoO2 材 料表面，藉由粉體緊密包覆形成一緊密的保護層，進而降低充放電過程中，電活 性物質與電解質液間之接觸機會，減緩電極材料因與電解質液反應所造成之電容 量衰退情形，由實驗結果發現，設定充放電截止電壓分別為4.40 V 和2.75 V， 充放電速率為0.2 C，以1.0 wt.% Li4Ti5O12 煆燒450 ℃，持溫10 小時改質 LiCoO2，其循環壽命為148 次，而以1.0 wt.% Li4Mn5O12 煆燒550 ℃，持溫10 小時改質之LiCoO2，則循環壽命為125 次(未改質LiCoO2 材料之循環壽命僅有 38 次)。以ESCA 分析LiCoO2 材料表層至90 nm 深處之元素縱深分佈，確實可 發現塗佈物質中的鈦及錳元素存在，由XPS 圖譜可知，吾人所使用之塗佈物質 與LiCoO2 反應而形成固態溶液。由DSC 測試結果可知，改質後材料之熱解溫度 提高且放熱量降低，顯示材料之熱穩定性已獲得改善。由鈷溶解度測試結果可 知，改質後材料之鈷溶解現象均較未改質材料低，顯示因鈷溶解現象所造成的電 容量衰退獲得改善。由循環伏安法測試可知，改質後材料之氧化還原峰變得較為 圓滑，顯示電極材料於充放電過程中之相變化程度可獲得減緩。由交流阻抗結果 可知，改質後材料可降低電解質液間之總電阻，顯示此表面塗佈技術可減緩電極 材料溶至電解質液中，進而增加電池循環穩定性。 Presently, LiCoO2 is the most widely used cathode material in commercially available Li-ion batteries, due to its high energy density and good cycle life performance. However, the phase transformation from a hexagonal to monoclinic phase, occurring between 4.1 and 4.2 V, induces a nonuniform volume change along the c direction (~2 % expansion). This change eventually induces strains and extended defects between and within the particles, leading to the disconnection of electrical contact between particles and increased cell capacity fading. To overcome this problem, the LiCoO2 cathode material was surface treated with the Li4M5O12 (M=Ti, Mn) particles by a simple mechano-thermal process. The Li4M5O12 (M=Ti, Mn) material possesses enhanced electrochemical activity, good reversibility, zero-strain insertion, a very flat discharge-charge plateau and high cycle stability during the charge–discharge process. The advantages of this compound led us to focus on preparing Li4M5O12 (M=Ti, Mn) material as a coating material on commercial LiCoO2 particles by a simple mechano-thermal process and studying its electrochemical cell performance when charged at higher voltages. A mixed metal oxide formed as a compact coating over the LiCoO2 cathode particle to suppress the capacity fading caused by reactions with the electrolyte. The Li4Ti5O12 and Li4Mn5O12 coated LiCoO2 cathode delivered excellent cyclability for 148 and 125 cycles, respectively, at a 0.2 C-rate between 4.40 and 2.75 V with charge retention to 80 % of FMC-LiCoO2. ESCA results revealed that the titanium and manganese ions of coating materials could be observed on the LiCoO2 surface. The XPS spectra showed the coating materials would react with LiCoO2 to form the LiMyCo1-yO2 (M=Ti, Mn) mixed metal oxide. The DSC results showed that the coated LiCoO2 significantly depressed exothermic activity and reduced heat generation at a highly delithiated state. In addition, Li4M5O12 (M=Ti, Mn) coated LiCoO2 has better thermal safety characteristics compared to the pristine LiCoO2 cathode material. The cobalt amounts dissolved in the electrolyte of the Li4M5O12 (M=Ti, Mn) coated LiCoO2 were less than the pristine one. Cyclic voltammetry revealed that the hexagonal-monoclinic-hexagonal phase transformations were retained for the coated cathode materials upon continuous cycling. Impedance spectra showed the electrolyte resistance of the coated cathode decreased ( Is this right, wouldn’t a film increase resistance) because the coating materials would form a thin-film on the cathode surface to protect the cathode from reacting with the electrolyte. |
