| 摘要: | 鎂有著低成本、低汙染、高地殼含量、高能量密度及高安全性的特點，使之成為吸引人的二次電池負極選項，但鎂離子電池發展面臨著兩大困難，其一為鎂的高化學活性使表面產生鈍化層，使電解液的發展受到相當程度的限制；其二為正極材料的開發，正二價的鎂離子帶有強大的靜電吸引力，導致離子擴散能力較差，使得現階段能夠以鎂離子進行嵌入嵌出反應的材料仍非常稀少。
本研究選用MoS2作為活性物質，在僅有鎂離子存在之APC電解液時，幾乎無法儲能，但添加鋰離子於電解液中便可使此材料進行儲能，且其電化學行為與MoS2作用於鋰離子電池相當類似，電化學性質會隨著鋰離子濃度的增加而提昇，電容量從45提升至166 mA h/g，充放電速度提升至1000 mA/g時，電容量從14提升至107 mA h/g，高速維持率從31% 提升至64%，在此材料中離子導電度扮演著左右其儲能性質的角色。
為提升鎂離子電池的實用性，本實驗以不含氯化物之Mg(BH4)2及乙二醇醚類溶劑 (Glyme-based solvent) 做系統性的探討，在不同基材、溫度、濃度、溶劑及鋰鈉鹽的添加皆有不同影響，其中添加LiBH4與NaBH4皆使其電化學性質戲劇性地增加。以MoS2及其石墨烯複合材，進行硼氫化物－乙二醇醚類雙鹽電解液性能比較，Diglyme溶劑之MBH雙鹽電解液性質最佳，而Triglyme及Tetraglyme由於黏度增高導致電容量下降，但考量安全性與實用性，高沸點之乙二醇醚類溶劑及不含氯化物之Mg(BH4)2仍有望取代APC電解液用於鎂離子電池。
本研究首次展示嵌入嵌出材料作為鈉鎂複合電池的可能性，但在低速充放電時電容量表現仍有待改善，但其高速維持率高達76%，相較鋰離子與鋰鎂複合電池系統顯示出更為優異的高速嵌入嵌出能力，從EIS分析可發現鈉鎂複合電池之電荷轉移阻抗極大，可提供改善鈉鎂複合電池系統之方向。;Magnesium is an attractive anode material for secondary batteries because of low cost, low pollution, abundance, high energy density and high safety. However, there are two major obstacle in developing ideal magnesium ion batteries (MIBs). The first one is the formation of passivation layer on the surface due to the high chemical activity of magnesium. This result in restrictions on selecting suitable electrolytes. The other one is the strong coulombic interaction between Mg2+ and the intercalation host, which makes the ion diffusion sluggish, creating a barrier for the development of the cathode materials for MIBs.
In this study, MoS2 is chosen as the active material which Mg2+ ion cannot intercalate/deintercalate into the host structure in the presence of APC electrolyte. After adding the lithium salt (LiCl) in the APC electrolyte, the Mg//MoS2 cell starts storing energy and its electrochemical behavior is very similar to MoS2 in lithium-ion battery system. The electrochemical performances significantly increase with the increase of lithium-ion concentration. At low concentration (0.1 M), the cell delivers reversible capacity of 45 mAh/g (at 25 mA/g), whereas at high concentration (0.7 M) the cell delivers superior capacity of 166 mAh/g. Similarly, the cell with high concentration of Li salt showed excellent high rate retention of 64% at 1000 mA/g, whereas the cell with low concentration of Li salt showed poor retention of 31%. The ion mobility is believed to play important role in electrochemical performance.
After showing the remarkable benefits of dual-salts electrolyte, it is worth expanding this concept to other electrolytes, which have the nature of high safety and environmental friendliness. Therefore, we introduce Mg(BH4)2 salt and Glyme-based solvents as electrolyte. The present study indicate that substrates, temperature, concentration, solvent and the additive effects the reversibility of Magnesium deposition and dissolution. The study showed addition of LiBH4 and NaBH4 can improve the electrochemical performance of MIBs to a greater extent. To investigate the properties of different dual-salts Mg(BH4)2 (MBH) electrolyte, MoS2/Graphene composite is chosen as the active material. Among them, MBH-diglyme electrolyte delivered highest capacity, whereas MBH-triglyme and MBH-tetraglyme electrolytes showed slightly decreased capacity owing to their high viscosity. Despite the electrochemical performance between MBH and APC electrolyte are similar, MBH electrolyte is expected to replace APC electrolyte for practical application.
This study reports the possibility of Na/Mg hybrid battery based on intercalated cathode material for the first time. Compared to lithium-ion and Li/Mg hybrid battery, the high rate retention of Na/Mg hybrid battery is much higher (up to 76%) but the capacity at low current density needs to be further improved. These results provide insight for further development of Na/Mg hybrid battery. |
