具高電容量之鋰離子電池磷酸鹽正極材料合成與特性

Synthesis and Characterization of Phosphate-based Cathode Materials with High Capacity for Li ion Batteries

郭慧通(Hui-Tung Kuo)

指導教授 ： 劉如熹

國立臺灣大學/理學院/化學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.02797

摘要

論文內容分為三個部分: (1) 利用濕式化學法以LiI控制不同之化學劑量以進行LiFePO4之合成，配合電化學之研究結果，提出合理之反應機制並以最佳之合成條件其產物進行導電碳材之外添加技術，發現具多孔性結構之C/LiFePO4並可有效提升其電化學之表現；(2) 利用固態反應合成法配合掺雜(doping)之技術以改變不同之燒結氣氛合成LiMn0.35Co0.2Fe0.45PO4，並探討Co2P之析出對於產物之導電性與其電化學特性之關係性以探討最佳之合成條件可得到最大之電容量；(3) 利用凝膠反應(sol-gel method)合成法於還原性之氣氛下合成LiVOPO4並改變不同之燒結溫度，結果發現LiVOPO4可經結構相變之過程轉變至具最高電容量之Li3V2(PO4)3，並利用多種分析技術以探討其中之結構相變過程及其電化學表徵之差異性。

關鍵字

鋰離子電池 ； 正極材料 ； 鋰鐵磷酸鹽 ； 結構相變

並列摘要

This thesis mainly focused on the synthesis and characterization of phosphate-based polyanion group (for example, LiFePO4, LiMn0.35Co0.2Fe0.45PO4, LiVOPO4, and Li3V2(PO4)3) cathode materials with high capacity. It is divided into three parts:
In the first part, we used the wet-chemistry method to control the amounts of LiI mixed with as-prepared FePO4 for synthesizing LiFePO4. Then, the electrochemical method is used to process the suitable products and conditions for preparing the LiFePO4. For the results, we proposed a reasonable mechanism for forming the pure phase of LiFePO4. We found that the amount of LiI greatly affected the purities of products. Then, we used the additive method for preparing the carbon-coated LiFePO4 with cavity structure to enhance the electrochemical performance successfully. LiFePO4/C sample showed constant values of current density during potential cycling up to 35 cycles as compared to LiFePO4 suggesting that the LiFePO4/C have better and stable electrochemical performance. The obtained maximum capacity for LiFePO4/C can be approached to the theoretical capacity of 170 mAh/g. Moreover, Brunauer-Emmett-Teller (BET) measurements of LiFePO4 and LiFePO4/C showed surface area of 6.7 and 50 m2/g, respectively. The transmission electron microscopy (TEM) showed the formation of cavity architecture, which may be due to the release of H2O and CO2 gases during the heating process of white sugar. It is reasonable to believe that the the unique microstructure are improved charge transfer kinetics and the smaller particle size achieved by the controlling LiI in wet-chemistry method.
In the second part, we used the solid state reaction method combined with the ball-milling technique to synthesize carbon-coated LiMn0.35Co0.2Fe0.45PO4 and sintered in different atmosphere (Ar and 5% H2/Ar). The conductivity of Li(Mn0.35Co0.2Fe0.45)PO4/C sintered at 600-800 °C in 5% H2/Ar is increased as the temperature is increased. The O K-edge X-ray absorption near edge spectrum (XANES) demonstrates that content of hole carriers is increased in Li(Mn0.35Co0.2Fe0.45)PO4/C as the amount of Co2P increased. We also observed that the capacity of Li(Mn0.35Co0.2Fe0.45)PO4/C is increased with sintering temperature, and it exhibited a maximum capacity of 166 mAh/g at 700 °C. It was found that the enhancement in the discharge capacity of sintered Li(Mn0.35Co0.2Fe0.45)PO4/C was as a result of its higher electrical conductivity under 5% H2/Ar atmosphere as compared with Ar atmosphere.
In the third part, we report the transformation of α-LiVOPO4 to α-Li3V2(PO4)3, leading to an enhancement of capacity. The α-LiVOPO4 sample was synthesized by a sol-gel method, followed by sintering at 550–650 oC in a flow of 5% H2/Ar. The structural transformation of a triclinic α-LiVOPO4 structure to a monoclinic α-Li3V2(PO4)3 structure was observed at higher sintering temperatures (700–800 oC in a flow of 5% H2/Ar). The α-Li3V2(PO4)3 phase was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and X-ray absorption near edge spectrum (XANES) techniques. The valence shift of vanadium ions from +4 to +3 states was observed using in situ XANES experiments at V K-edge. The structural transformation is ascertained by the shape changes in pre-edge and near edge area of X-ray absorption spectrum (XAS). It was observed that the capacity was enhanced from 140 mAh/g to 164 mAh/g via structural transformation process of LiVOPO4 to Li3V2(PO4)3.

並列關鍵字

Li ion battery ； cathode material ； LiFePO4 ； structure transformation

參考文獻

[4] Dell, R. M.; Rand, D. A. J. Understanding Batteries, 1st ed RSC Paperbacks,

Cambridge: Royal Society of Chemistry, 2001.

[8] Brock, W. H. The Norton History of Chemistry, in Norton History of Science, R. Porter (Ed), New York: W. W. Norton & Company, 1993.

[11] Josephson, M. Edsion: A Biography, Reprint ed, Chicago:John Wiley & Sons, 1992.

[14] Robinson, A. Science 1974, 184, 554.