吾人採用固態高溫法合成，以碳酸鋰、草酸亞鐵和磷酸二氫銨作為起始物並加入硝酸鑭作為摻雜用之金屬來源，採用丙二酸與癸二酸兩種有機酸作為碳源製備LiFe0.99La0.01PO4/C鋰離子複合陰極材料。吾人選用X光繞射分析儀(XRD)、總有機碳分析儀(TOC)、微分掃描熱量分析儀(DSC)、掃描式電子顯微鏡(FE-SEM)、穿透式電子顯微鏡(TEM)、四點探針導電度計、拉曼光譜儀、慢速循環伏安法等儀器對於此項材料之物性與電化學分析。 實驗結果可知，以高溫固態法摻雜1 mole%金屬鑭改質LiFePO4於873 K煆燒12小時為最佳製程條件；由XRD結果顯示摻雜後之材料其晶體結構不變，且有效提升整體導電度(~10-5 S cm-1)；電池性能測試結果，當丙二酸60 wt.%為碳源和癸二酸36 wt.%為碳源時，初始電容量分別為151 和145 mAh g-1，相對於未改質之LiFePO4，其初始電容量僅有100 mAh g-1，可見材料經改質後初始電容量有效提升。由SEM和TEM觀察中得知，材料經碳處理後可得3~6 nm的碳膜，且顆粒表面經EDS分析出只含碳，間接證實金屬鑭以摻雜方式而非塗佈方式改質LiFePO4。拉曼光譜儀分析結果顯示，材料經丙二酸60 wt.%碳處理後ID/IG值為96.8，以癸二酸36 wt.%處理後ID/IG值為93.2，且由SAED分析佐證當ID/IG值越小時材料含似石墨化碳程度越高。熱穩定性方面，以丙二酸60 wt.%處理LiFe0.99La0.01PO4時，其放熱量為103.9 J g-1；若以癸二酸36 wt.%為碳源時，總放熱量為93.7 J g-1。CV測試結果顯示，氧化與還原峰僅差0.26 V，顯示極化現象很小，可佐證當丙二酸和癸二酸為碳源時，長循環測試結果分別為513次(C.R.=80 %)與485次(C.R.=96 %)，如此優異的循環穩定度。由以上結果顯示，適當的摻雜金屬鑭與碳添加量確實能有效地提高導電度和放電電容量，進而增進循環穩定度。 In order to enhance the capability of LiFePO4 materials, we attempted to coat carbon and dope lanthanum-ion by a high temperature solid-state method. The starting materials of the LiFe1-xLaxPO4/C composite were synthesized using lithium carbonate, iron (II) oxalate dihydrate, ammonium dihydrogen phosphate and lanthanum nitrite. The purity of LiFe0.99La0.01PO4/C was confirmed by XRD analysis. Galvanostatic cycling and conductivity measurements were used to evaluate the material’s electrochemical performance. A galvanostatic charge-discharge study was carried out between 4.0 and 2.8 V. The best cell performance was delivered by the sample doped 1 mole% lanthanum-ion and coated with 60 wt.% malonic acid and/or 36 wt.% sebasic acid. Its first-cycle discharge capacity was 151 mAh g-1 at a 0.2 C-rate with malonic acid as a carbon source and 145 mAh g-1 with sebasic acid. And the cyclicability of cathodes coated by malonic acid and sebasic acid are 513 cycles(C.R.=80 %) and 485 cycles(C.R.=96 %). The residual carbon in the composite was measured by total organic carbon (TOC) and Raman spectral analysis. The actual carbon content of LiFePO4 was 1.65 wt.％ with the addition of 60 wt.% malonic acid and 4.69 wt.％ with the addition of 36 wt.% sebasic acid. The doped samples were measured by Raman spectral analysis. The intense broad bands at 1330 and 1600 cm-1 are assigned to the D and G bands of residual carbon in LiFe0.99La0.01PO4/C composites, respectively. The lowest peak intensity ratio (ID/IG) of the doped powders was 0.968 for malonic acid and 0.932 for sebasic acid. The products’ morphologies were analyzed by SEM and TEM/SAED/EDS. Compared to bare LiFePO4, the particle size of the products decreased as the amount of malonic acid and/or sebasic acid added was increased. However, adding too much malonic acid or sebasic acid contributed to an increase in particle growth. The EDS carbon map shows a uniform distribution of carbon in the sample on the surface of the composite particles. The DSC patterns were fully charged for 4.5 V, the test range between 373 K to 673 K in N2 atmosphere. The total exothermic heat when malonic acid was used as the carbon source was only 103.9 J g-1 and 93.7 J g-1 when sebasic acid was used. The TEM/EDS results show that the particles in the dark region are LiFe0.99La0.01PO4 with a trace of carbon and the grayish region are carbon only. To produce LiFe0.99La0.01PO4 with carboxylic acid and sebasic acid as a carbon source not only increase the overall conductivity (~ 10-5 S cm-1) of the materials, but also enhances the cell performance and cyclability.
