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|標題:||Characterization of Electrolytic Li3-3xFexPO4 Coatings for Thin Film Lithium Ion Batteries
|關鍵字:||電化學合成;Electrochemical synthesis;陰極極化曲線;沈積機構;Li3PO4;Li3-3xFexPO4;固態電解質;鋰離子導電度;活化能;非晶相FePO4;拉曼光譜;cathodic polarization curves;deposition mechanism;Li3PO4;Li3-3xFexPO4;solid electrolyte;lithium ionic conductivity;activation energy;amorphous FePO4;Raman spectroscopy||出版社:||材料工程學系所||摘要:||
1. 電化學沈積Li3PO4 (Li3-3xFexPO4, x = 0) 塗層當作電解質已經成功的被覆在白金電極上。經由儀器分析，結果顯示Li3PO4塗層在500℃時會由β-phase轉變至γ-phase，且在室溫下離子導電度可達到8.62×10-8 S cm-1，並提出Li3PO4的電化學沈積機構。
2. 經由LiNO3, NH4H2PO4及 Fe(NH4)2(SO4)2．6H2O的混合水溶液，成功的在白金電極上合成Li3−3xFexPO4 (x=0.20,0.45)塗層。經過交流阻抗分析，可以得到最佳的鐵離子添加量為x = 0.20，其在室溫下的導電性為1.77 × 10−7 S cm−1，同時發現Li+活化能會隨著鐵添加量增加而增加，從0.42 (x = 0)增加到0.62 eV (x = 0.45)。
3. FePO4 (Li3-3xFexPO4, x= 1)塗層可應用於防蝕、觸媒、污水純化、鐵電材料及鋰電池等用途上。在這裡一個新穎的方法，即經由電化學方法在Fe(NO3)3•9H2O及(NH4)2HPO4等比例的混合水溶液中成功的合成出非結晶性的磷酸鐵塗層。沈積塗層經由儀器分析，顯示沈積塗層為非結晶性Fe(OH)HPO4•H2O化合物，進一步在250℃內退火脫去一個結晶水形成Fe(OH)HPO4，最後在600℃內縮合成FePO4，並且在600℃以上形成hexagonal結構。經場發射掃瞄式電子顯微鏡觀察，塗層表面的奈米孔洞隨著溫度上升而逐步變大，晶粒也聚集成長。
4. FePO4經由50圈的充放電測試，結果顯示在退火300℃的FePO4塗層保有最佳的放電電容量260 mAh/g，相對於LiFePO4做為鋰電池正極材料擁有較高的電容量(理論電容量：170 mAh/g)，並且提出Li+在FePO4嵌入脫出的電化學反應機構。
In this study, the electrochemical syntheses of cathodic method was used to prepare the coating of Li3-3xFexPO4 films on Pt substrates. Also, the electrochemical mechanisms of deposition and the characterization of these films for lithium batteries were discussed. This dissertation contained four parts.
Ⅰ. Electrochemical deposition of Li3PO4 (Li3-3xFexPO4, x = 0) coating as the solid electrolyte has been carried out on Pt in LiNO3 and NH4H2PO4 aqueous solution. The coated specimens were characterized by X-ray diffraction (XRD), scanning electron micrographs (SEM), Field Emission Scanning Electron Microscope (FE-SEM), Fourier transform infrared spectrometer (FTIR) analysis and Electrochemical Impedance Spectroscopy (EIS). The orthorhombic β-phase Li3PO4 was existence until 500℃ transition to orthorhombic γ-phase. The lithium ionic conductivity of 100 nm crystalline Li3PO4 thin film was about 8.62×10-8 S cm-1 at 25℃. Also, the mechanism of electrolytic Li3PO4 coating on Pt was discussed in this article.
Ⅱ. Electrolytic Li3−3xFexPO4 ( x = 0.20,0.45) coating on Pt as the solid electrolyte has been carried out in the mixture of LiNO3, NH4H2PO4 and Fe (NH4)2(SO4)2．6H2O aqueous solution. The ionic conductivity of Li3−3xFexPO4 was investigated in terms of defect models with an iron level of 0 ≦ x ≦ 0.45. To determine the changes in ionic conductivity and activation energy of Li3−3xFexPO4 with iron content x, AC-IS measurements are carried out at temperatures from 25 to 70℃. The maximum ionic conductivity is 1.77 × 10−7 S cm−1 for x = 0.20 at room temperature, and the activation energy was increased from 0.42 to 0.62 eV with increasing iron contents.
Ⅲ. A novel method of FePO4 (Li3-3xFexPO4, x = 1) coatings on Pt by electrochemical synthesis in 0.01 M Fe(NO3)3•9H2O and 0.01 M (NH4)2HPO4 mixed aqueous solution was presented. After deposition, the coated specimens were further annealed and characterized by ICP-AES, XRD, FE-SEM, FTIR, and TG-DTA. It was found that the uniform as-deposited film was amorphous Fe(OH)HPO4•H2O, dehydrated into Fe(OH)HPO4 under 250℃, further condensed into FePO4 below 600℃, and fully crystallized at 600℃. Also, the sponge-like morphology of the annealed specimen was found full of nanopores and tuned with increasing temperature.
Ⅳ. The electrochemical properties of the iron phosphates were characterized with a voltage window of 0.2–2.5 V. Annealing at 300℃ had the excellent discharge capacity of 260 mAh/g after 50 cycles, while the cathode LiFePO4 has a theoretical capacity of 170 mAh/g. Based on ex situ Raman spectra, the electrochemical mechanism of FePO4 film with lithium upon cycling was proposed
|Appears in Collections:||材料科學與工程學系|
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