| 摘要: | 摘要
隨著石化能源的廣泛使用與依賴，石化燃料漸漸枯竭造成能源危機，此外也對環境造成不少汙染，因此政府積極推動再生能源 (如：太陽能與風能等) 作為替代能源，再加上電子行動裝置與電動車的蓬勃發展，儲能系統無疑是很重要的關鍵。儘管鋰離子電池已是目前最普遍應用的儲能裝置，仍有不少缺點如資源有限與地表分布不均造成的高成本，需審視鋰離子是否合適應用於大規模的electrical energy storage system (EES)。近年來，地表資源蘊存豐富與成本低的鈉離子電池逐漸受到重視，其原理也與鋰離子電池相似，使得鈉離子電池極具發展潛能。
具有三維NASICON (Na-superionic conductor) 型架構的Na3V2(PO4)2F3 提供足夠的空間來容納鈉 (Na+) 離子，還具有高能量密度和優異的循環性能，適合作為鈉離子電池的陰極材料。然而，低導電性造成較差電容維持率 (Rate Capability) 的缺點阻礙實際的應用。為解決此問題，我們藉由結合溶膠－凝膠法 (Sol-gel) 與碳-熱還原 (Carbon-thermal reduction)，合成出不同比例的鎂 (Mg2 +, x = 0, 0.01, 0.05, and 0.1) 摻雜在碳包覆的Na3V2-xMgx(PO4)2F3。我們詳細研究了鎂離子在Na3V2(PO4)2F3 / C 結構和電化學性能中的作用。在V 位取代的 Mg2+ 離子通過產生電子缺陷和增加 Na+ 離子在晶格晶體中的離子擴散途徑來增加電子電導率和離子電導率。Na3V1.95Mg0.05(PO4)2F3 / C電極在10 C下實現了80 mAh / g的優異倍率性能；此外，500次循環後保留容量仍達到88%，平均庫侖效率為99.9%。這一成果與Mg摻雜的Na3V2(PO4)2F3/C的多重效應有關：(1)提高體電子電導率； (2)促進Na+ 離子在晶體結構中的擴散； (3)減小晶體尺寸和粒徑： (3)提高結構穩定性。
除了透過Mg2+ 原子摻雜外，將Ca2+ 離子取代到 NVPF 晶體結構裡也能達到優化Na3V2(PO4)2F3 的電化學性值。儘管 Ca2+ 離子尺寸較大，XRD 分析結果顯示晶體結構沒有變化。此外，Na3V1.95Ca0.05(PO4)2F3 / C 電極在 0.1 C 和 10 C 下的最高電容量分別為124 mAh / g 和 86 mAh / g，主要因為晶格的擴張與更小的顆粒尺寸
;ABSTRACT
The energy crisis and high levels of pollution resulting from fossil fuels have boosted the government to develop renewable energies (i.e., solar tide and wind). The lithium-ion battery is an energy storage system that has been widely applied for mobile phone, laptop computer, and electric vehicles. However, lithium resources are expensive and geographically constrained, making the application of lithium-ion in massive scale electrical energy storage systems (EES) needs to be reconsidered. On the other hand, sodium batteries are currently getting much attention due to global abundance and low cost. Moreover, the working principle of sodium-ion batteries and lithium-ion batteries are alike, making them promising to be developed as an energy storage system.
Na3V2(PO4)2F3/C with a 3D NASICON (Na-superionic conductor) type framework provides sufficient space to accommodate Na+ ions, making the material potentially a cathode material. Na3V2(PO4)2F3/C also has high energy density and excellent cycling performance. However, capacity reduction and inferior rate capability due to low electronic conductivity make this material difficult to implement practically. Here, sodium storage delivered excellent performance was realized by substituting the Mg2+ ion into the Na3V2(PO4)2F3/C structure, synthesized by a combination of sol-gel and carbon-thermal reduction. We studied the role of magnesium ions in the Na3V2(PO4)2F3/C structure and electrochemical properties in detail. The Mg2+ ion, which is substituted at the V site, increases the electronic conductivity and ionic conductivity with resulting hole and broadening the ionic diffusion pathway for Na+ ions in the lattice crystal. An excellent rate capability of 80 mAh g-1 at 10 C was achieved by Na3V1.95Mg0.05(PO4)2F3/C electrode; In addition, the retention capacity still reaches 88% after 500 cycles and the average coulombic efficiency is 99.9%. This achievement is related to multiple effect of Mg-doped Na3V2(PO4)2F3/C : (1) elevating the bulk electronic conductivity; (2) boosting the Na+ ion diffusion in the crystal structure; (3) reducing the crystal size and particle size: (3) improving the structural stability.
Besides the Mg2+ as a dopant atom, the electrochemical enhancement of Na3V2(PO4)2F3 was conducted by Ca2+ doped Na3V2(PO4)2F3. Despite the large size of the Ca2+ ion, the XRD pattern showed no change in the crystal structure. Moreover, the Ca2+ doped NVPF (x = 0.05) electrode delivered the highest capacities 124 mAh g-1 and 86 mAh g-1 at 0.1 C and 10 C, respectively, due to the enlargement of the crystal lattice and smaller particle size. |
