鈉離子電池Na3V2(PO4)3正極材料之製備與特性分析

Preparation and Characterization of Na3V2(PO4)3 Cathode Materials for Sodium-Ion Batteries

邱奕棠(Yi-Tang Chiu)

指導教授 ： 劉偉仁

中原大學/工學院/化學工程研究所/碩士(2017年)

https://doi.org/10.6840/cycu201700542

摘要

摘要

由於近年鋰離子電池的需求量提升，而其生產成本也隨著上升，原本就仰賴部分國家進口(南美、北美、中國)的鋰資源顯得更珍貴了。近年來，鈉離子電池開始被視為鋰離子電池的替代儲能裝置之一，由於其低成本和電化學相似性高。但是鈉離子電池的開發上，仍然存在著許多挑戰。由於鈉離子半徑(1.02 Å)大於鋰離子(0.76 Å)的離子半徑，這將影響離子的傳輸表現，也會導致結構變動與遷入遷出時的體積變化。本研究挑選了具有高度共價三維結構和高熱穩定性的Na3V2(PO4)3鈉電池正極材料作為研究主題，希望它能提供較佳的條件，以便於鈉離子進出結構框架。

本研究對NASICON (Sodium Superionic Conductor)結構之Na3V2(PO4)3 鈉電池正極材料進行階段性的改良: (1) 改良合成配方(2) 分散粉體團聚和堆疊 (3) F摻雜。經過實驗與測試，證實改良後有更優異的電化學特性。我們期許在缺乏鋰金屬的未來，NVP將成為具有潛力的鈉離子電池儲能材料。

首先，本篇論文使用溶膠凝膠法成功地製備了Na3V2(PO4)3，再對於合成配方做出調整與優化。在合成的過程中增加了鈉的比例並探討增加後Na3V2(PO4)3的電性表現。研究結果發現，提升比例後，電池的循環壽命隨之增加；另一方面，快速充放的表現也明顯提升了。在1.0A/g的快充下，過量鈉之樣品可傳遞61.1 mAh/g電量高於原樣品的46.9 mAh/g。
本研究使用溶膠凝膠法所製備出的NVP粉體有較不均勻的形貌，有團聚和堆疊的情況發生。後續經過一個分散團聚的流程，成功地減少粉體體團聚和堆疊情況，並提升了電性上的表現。在0.1A/g下，相較於未分散團聚的NVP粉體，第一圈可逆電容量從82.2 mAh/g提升到92.3 mAh/g。在0.1A/g下經過250圈循環壽命後，仍可保持76.9 mAh/g的電容量。

此外，本實驗合成了氟摻雜之NVP樣品並組裝成鈉離子半電池與全電池進行測試。X射線衍射和透射電子顯微鏡數據揭示了高純度NASICON磷酸鹽相和無定形碳的包覆，增強了複合電極材料的電子傳導性能。氟摻雜於Na3V2(PO4)3最佳摻雜濃度為0.15 mol％。藉中子粉末衍射後擬合數據，我們得到的化學成分為Na2.85V2(PO3.95F0.05)3。在掃描式顯微鏡和穿透式顯微鏡中，我們發現大量可見的孔隙率產生。 BET結果也與SEM和TEM一致，我們觀察到增加的表面積和降低的平均孔徑，證實空隙顯著改變了形貌。

最佳化氟摻雜NVP的半電池具有良好的穩定放電容量，在0.1A/g下250圈循環中，電容量和保持率分別為103 mAh/g和93％，高於未摻雜NVP的98 mAh/g和78％。氟摻雜後也增加了主體的導電性，對高速充放表現帶來明顯提升。此外，本實驗使用了最佳化氟摻雜NVP作為陰極和陽極材料製造對稱的鈉離子全電池。在0.1A/g下1000次循環中，展現良好可逆性和循環穩定性約為98％的容量保持率和100％的庫侖效率。

關鍵字

鈉離子電池 ； 溶膠凝膠法 ； NASICON磷酸鹽

並列摘要

Abstract

Recent years, the demand of green energy is rapidly increased. In the point of view, Li-ion batteries still stands as one of the most promising choices, particularly for the on-going massive demand for electric and plug-in hybrid vehicles. The main limiting factor of lithium-ion battery is limited resource of Lithium metal availability in the world. Hence, the researchers are focused on alternative device such as sodium-ion batteries and other batteries. Among that, Sodium-ion batteries began to be regarded as one of the alternative energy storage devices for lithium-ion batteries due to their low cost and similarity of electrochemical property. However, there are still many challenges for sodium-ion batteries. Since, the sodium ion radius (1.02 Å) is bigger than lithium ions (0.76 Å), that will affect the transport performance of ions and lead to structural changes, volume changes during the Na-ions extraction and insertion process. Na3V2(PO4)3 as a potential cathode material for sodium-ion batteries due to their peculiar advantages like that high theoretical energy density, more stable covalent network and high thermal stability and so on. In this regard, in the present investigation focused on synthesis and development of Na3V2(PO4)3 as dual electrode material for sodium-ion battery half and full-cell applications.

This research provides a step-by-step improvement for NASICON type Na3V2(PO4)3 sodium ion battery cathode material: (1). Modified synthetic formula (2). Dispersed powder agglomeration and stacked situation (3). F-doped NVP. In this research, The NVP and F-doped NVP nano-materials were successfully prepared by the sol-gel method. Subsequent through electrochemical performance test, confirmed that these improved samples have batter electrochemical properties. Therefore, the NVP and F-NVP are potential electrode materials for sodium- ion battery.

In the present work, Chapter-1 and Chapter-2 describes the basic idea of sodium-ion batteries, developments of cathode materials and recent research reviews. Chapter-3 explains the used chemicals purity, characterized instruments details with specification, preparation of Na3V2(PO4)3 and fabrication of sodium-ion half and full-cell details.

Chapter-4 a brief description of preparation and characterization of Na3V2(PO4)3 as a electrode materials for sodium-ion batteries. Na3V2(PO4)3 was prepared by sol-gel method with optimization of Na content. The results show that after increased the proportion of sodium, the battery cycle life becomes batter. On the other hand, the rapid charge and discharge performance is also significantly improved. The excess sodium sample can delivers a specific discharge capacity of 61.1 mAh/g which is higher than the original sample (46.9 mAh/g) at 1.0 A /g.

In this study, the bare NVP sample were observed for uneven aggregation and stacking. After a decentralized process, we successfully reducing the powder agglomerated and stacked situation, and enhance the electrochemical performance. After the modified process, the discharge capacity of first cycle increased from 82.2 mAh/g to 92.3 mAh/g. The modified sample delivers a specific discharge capacity of 76.9mAh/g over 250 cycles at 0.1 A/g.

In addition, the development of NVP was carried out by doping of fluorine. The fluorine content was optimized for sodium-ion batteries electrodes. X-ray diffraction and Transmission electron microscopy data unveiled high purity NASICON phosphate phases and amorphous carbon coating that enhances the electron conducting properties of the composite electrode material. The optimal doping concentration of F in Na3V2(PO4)3 is 0.15 mol %. By neutron powder diffraction data, we confirm that the chemical composition that we obtained from the fits is Na2.85V2(PO3.95F0.05)3. Subsequent to SEM, TEM, we found a lot of visible porosity generation. The BET results agree with the SEM & TEM, the increased surface area and the decreased average pore size were observed, confirming that the voids were significantly altered the morphology.

The half-cell of Na2.85V2(PO3.95F0.05)3 cathode exhibits a good stable discharge capacity of 98 and 93 % of capacity retention over 250 cycles at 0.1 A/g, which is higher than bare Na3V2(PO4)3 (92 mAh/g & 78%). The high rate capability of Na2.85V2(PO3.95F0.05)3 is also dramatically enhanced via increase the conductivity of host material by F-doping. Moreover, the symmetricalal Na-ion full-cell was fabricated using Na2.85V2(PO3.95F0.05)3 as cathode and anode materials. It was achieved that the good reversibility and superior cycling stability about 98 % of capacity retention with 100 % of coulombic efficiency at 1.0 A/g throughout 1000 cycles. Finally, Chapter-5 elucidates the conclusion of this investigation.

並列關鍵字

Sodium ion battery ； sol gel method ； NASICON phosphate

參考文獻

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