橄欖石結構之磷酸鹽磷酸鋰鈷 (LiCoPO4) 具有高氧化還原電位 (4.8 V)、高能量密度 (800 Wh/Kg) 和穩定的結構特性，是一種很有前景的高壓鋰離子電池陰極材料。儘管LiCoPO4具有如此多優異的特點，但卻仍然面臨著電子電導率低、鋰離子擴散速率差等挑戰。在鋰離子擴散過程中，鋰離子必須克服LiCoPO4表面附近與塊材中的能障。一般來說，表面能障為被認為是速度決定步驟。為了提高LiCoPO4的電化學性能，金屬離子摻雜已被證明能夠改善材料性能，實驗研究已報導鐵摻雜可以提高LiCoPO4的電化學性能。因此，我們以DFT計算鐵摻雜LiCoPO4為研究重點，從微觀角度闡明了材料電化學性質改善的緣由。 本研究通過第一原理計算探討LiCoPO4摻雜鐵離子前後的晶體結構、電子結構和鋰離子擴散路徑。根據以往實驗研究報導中最佳的電化學表現，將鐵的摻雜濃度設定為x = 0.2 (LiFe0.2Co0.8PO4)。在摻雜鐵離子之後，透過觀察計算求得之晶格結構，確認穩定的橄欖石結構能將摻雜鐵離子所導致之晶體變形限制在局部空間內。並進一步計算摻雜鐵籬子LiCoPO4的電子結構，透過與純LiCoPO4之計算結果進行比較，確認摻雜鐵離子能有效降低LiCoPO4材料之能隙與提升導電性。為了研究LiCoPO4的表面性質，計算了不同 Miller indices表面的表面能。結果表明在 (0 1 0) 表面為能量最穩定的表面，因此選擇此面來做接下來的計算。採用Nudged Elastic Band (NEB) 方法計算了材料在塊材與接近表面的最小能量路徑和能量屏障。計算結果表明通過鐵摻雜改性可以有效降低鋰離子擴散的能障。此外，亦進一步探討極化子和鋰離子在材料中同時遷移的擴散過程。 ;The olivine phosphate LiCoPO4 is a prospective cathode material in high voltage lithium ion batteries with high energy density (800 Wh/kg), high redox potential (4.8 V), and stable crystal structure. Despite these excellent properties, it still faces challenges, such as poor lithium ion diffusivity and low electronic conductivity. In lithium ion diffusion process, lithium ion must overcome the diffusion energy barrier near to the surface and in the bulk of LiCoPO4. In general, the surface barrier is regarded as the rate-determining step. To enhance the electrochemical performance of LiCoPO4, metal ion doping has been demonstrated to be able to improve material properties. Experimental studies have reported that Fe doping can improve the electrochemical performance of LiCoPO4. Therefore, this study focused on using density functional theory (DFT) calculation to elucidate the origin of electrochemical performance improvements for Fe doped LiCoPO4 from a microscopic viewpoint. In this work, the structural stability, electronic structure, and lithium ion diffusion barrier of LiCoPO4 before and after doping with Fe ions were calculated with DFT calculations. The Fe doping concentration is set to x = 0.2 (LiFe0.2Co0.8PO4), according to the best electrochemical performance in a previous experimental study. The electronic structure of Fe doped LiCoPO4 was calculated and compared to that of pristine LiCoPO4. To investigate the surface properties of LiCoPO4, the surface energies for surfaces with different indices were calculated. The results showed that (0 1 0) surface was the most stable surface, and was therefore chosen for further investigation. Lithium diffusion energy barriers were calculated via the nudged elastic band (NEB) method for lithium diffusion in the bulk and near the surface of the material. The calculation results showed that the energy barrier of lithium ion diffusion is reduced due to Fe doping modification. Furthermore, the phenomena of polaron and lithium ion simultaneous migration were discussed as well.
