利用新穎製程和金屬摻雜提升二氧化鈦負極材料及磷酸鋰錳正極材料於高功率鋰離子電池之應用潛力

Exploration of TiO2 Anode Materials and LiMnPO4 Cathode Materials through Novel Fabrication Method and Cation Substitution for High-rate Li-ion Batteries

楊浩(Yang, Hao)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2018年)

摘要

鋰離子二次電池由於具有優異的循環穩定性及較高的能量密度而成為最有潛力的能源儲存系統。到目前為止，提升正極/負極材料的結構穩定性、能量密度、功率密度及安全性仍是開發電池中甚為重要的一塊，增加電池的安全性可以減少電池爆炸的可能性，而在開發的材料之中，負極的二氧化鈦及正極的磷酸鋰錳具有優異的安全性，本研究將聚焦於利用奈米化粉體、形貌控制、表面改質及異質元素摻雜改善這兩種能源材料的電學性質。
在負極材料，由於銳鈦礦的二氧化鈦之工作電壓高於1V，避免惰性膜的大量生成，相較其他高安全性的負極材料(例如: 鈦酸鋰)具有更高的理論電容量(334 mAhg-1)。然而二氧化鈦的缺點係其較差的導電度和導離子率，所以研究的第一部分將聚焦於藉由結合粒徑縮小、形貌控制及元素摻雜提升二氧化鈦的電學性質。在水熱法中，將五價鈮元素摻雜入空心狀的二氧化鈦可以製備中片狀的二氧化鈦奈米粉末，片狀結構具有適合鋰離子脫嵌的(001)面，並展現出優異的電容量(127 mAhg-1,10 C)及循環穩定性(10,000圈,20C)。藉由分析不同速率下的循環伏安圖，發現其優異的電化學性能是來自表面偽電容的存在，大幅提升了鋰離子脫嵌材料表面的速度。在第二部分，研究致力於了解二氧化鈦在鈉離子電池的發展潛力，藉由在溶膠凝膠法中加入對苯二酸可以製備出具有均勻孔洞的片狀二氧化鈦，並具有10奈米的粒徑。在電性上，較高結晶度的片狀二氧化鈦展現出優異的電容量及穩定度(53 mAhg-1,30C)。藉由非臨場的XRD及XPS分析，發現到二氧化鈦會因為鈉離子的嵌入而逐漸結構扭曲並且非晶化，雖然展現出Ti4+/Ti3+的可逆反應，但也伴隨著Ti金屬的出現。雖然起初二氧化鈦的電容量來自於偽電容，但隨著活化反應出現了明顯的脫嵌現象，證明片狀二氧化鈦具有可以穩定儲存鈉離子的結構。
在正極材料，橄欖石結構的磷酸鋰錳具有優異的熱穩定、便宜及良好的循環穩定性等優點，磷酸鋰錳具有小於電解液裂解的電壓(4.1V)及較高的能量密度(701 Whkg-1)，未來具有取代磷酸鋰鐵的潛力。然而其具有極差的導電度及導離子率，必須藉由粒徑縮小、表面鍍碳及元素摻雜來改善電化學性質。在本研究中，首先利用溶劑熱法在常壓高溫下製備磷酸鋰錳的奈米粉末，並添加對苯二胺抑制粒徑成長，對苯二胺是一個具有平面狀結構的有機鹼，可以吸附氫離子並減少氫氧化鋰的添加量。當磷酸鋰錳以蔗糖作為碳源時，可以展現穩定的電容量(134 mAhg-1, 0.1C)。另外，在合成時，將對苯二胺與對苯二醯氯聚合，經過高溫燒結後，可以在磷酸鋰錳表面直接生成均勻的氮摻雜碳膜。研究證明，氮摻雜碳膜具有較高的導離子率及較佳的附著性，更提升磷酸鋰錳的電化學性質。進而磷酸鋰錳的電學性質可以藉由摻雜四價釩離子而大幅提升(157 mAhg-1,0.1C及106 mAhg-1, 20C)，釩離子抑制晶粒大小並使其展現出適合鋰離子脫嵌的(020)面，摻雜小於二價錳離子半徑的四價釩離子，可以收縮晶格及產生陽離子空缺。在臨場的同步輻射分析中，發現在電化學循環過程中，釩離子具有V5+/V4+及V4+/V3+兩種價數轉換，使得磷酸鋰錳有更高的理論電容量。另外，在脫鋰的過程中，磷酸鋰錳因摻雜釩而展現出更迅速的非晶化反應，這表示磷酸鋰錳可以快速的扭曲結構，使得鋰離子能迅速的進出材料內部。
本研究利用發展出的新穎製程，並將粉末奈米化、形貌控制、表面改質及元素摻雜等技術引入製程，使二氧化鈦及磷酸鋰錳展現出極度優異的電化學性能，這代表高安全性的正負極材料在未來的高功率電池的發展上有極大的優勢。

關鍵字

鋰離子電池 ； 鈉離子電池 ； 二氧化鈦 ； 磷酸鋰錳 ； 摻雜 ； 結構設計

並列摘要

Li-ion batteries (LIBs) are the most promising energy storage system because of their long cycle life and high energy density. However, it is vital for anode and cathode materials which store Li-ions in their host structure to pursue higher energy density, power density, and safety. Increasing the stability of batteries prevents the damage as well as the explosion of batteries. Therefore, it is aimed to develop high-capacity and high-rate LIBs on high-safety anode/ cathode materials, anatase TiO2 and olivine LiMnPO4, by using the strategies of nano technology, morphology control, surface coating, and cation doping/substitution.
For anode materials, anatase TiO2 is a high-safety material as the operating voltage is above 1 V (v.s. Li/Li+) higher than the voltage range to form severe SEI layer. Anatase TiO2 has the high theoretical capacity (334 mAhg-1) among other high-safety materials, yet using the strategies as mentioned above is essential since it has a poor electrical conductivity and Li-ion diffusivity. The first part in this study aims to enhance the cycling life and rate capability of anatase TiO2 by combining the size reduction, morphology control, and cation substitution into one step. Nb-substituted TiO2 nanoplates (Nb-TiO2) were synthesized from hollow TiO2 in a hydrothermal process. Substituting large amounts of Nb5+ into anatase TiO2 promotes the morphology transformation from hollow to plate. Nb-TiO2 nanoplates with (001) preferred orientation show superior rate capability of 127 mAhg-1 at 10 C and cycling stability of 10,000 cycles at 20 C. By applying the cyclic voltammetry in wide scan rates, the mechanism of high rate is due to the enhanced pseudocapacitance which promotes fast Li-ion insertion/ extraction behaviors near the surface of Nb-TiO2.
In the second part, the study aims to improve the utility potential of anatase TiO2 in LIBs and SIBs (Na-ion batteries). Therefore, a novel preparation method is invented. The anatase TiO2 nanoplates with 10 nm particle sizes and uniform pores are produced by pyrolysing titanium-terephthalate hybrid materials. In SIBs, high-crystallinity anatase exhibits good rate capability, delivering 53 mAhg-1 at 30 C. Ex-situ XRD and XPS analysis show that anatase TiO2 forms metallic Ti and amorphous sodium titanate which is reversible with Ti4+/Ti 3+ redox reaction. Pseudocapacitance is found to comprise most capacity in the first cycle, and then the insertion capacity will enhance after activation, which proves that anatase TiO2 is a suitable host for accommodating Na-ion.
For cathode materials, LiMnPO4 is one of the olivine materials which show the features of high thermal stability, cost efficiency, and cycle life. LiMnPO4 with higher operating voltage (4.1 V v.s. Li/Li+) and energy density (701 Whkg-1) is expected to replace the commercialized LiFePO4. However, LiMnPO4 suffers from poor electric conductivity (< 10-9 Scm-1) and Li-ion diffusivity (< 10-14 cm2s-1) which limit its potential. Therefore, coating carbon, reducing particle size, and doping cation into LiMnPO4 is essential to improve the electrochemical performance. So far, LiMnPO4 is generally synthesized from the hydrothermal process since the produced particles are highly crystalline and small, but the yields are meager.
The study of LiMnPO4 in the first part reveals a novel preparation method, called diamine-assisted polymerization method to synthesize nano LiMnPO4 coated with homogenous N-doped carbon derived from polyamides. The p-phenylenediamine (PPD) is added into the synthesis process to suppress the particle growth. Moreover, PPD maintains the reaction pH, preventing the impurity formation. When coated carbon is prepared with sucrose, the LiMnPO4/C prepared with large amounts of PPD exhibits 134 mAhg-1 at 0.1 C. To cover a more homogenous and conductive carbon on LiMnPO4, PPD and acyl chlorides are in-situ polymerized into aromatic and semi-aliphatic polyamide. N-doped carbon pyrolyzed from the polyamide allows a fast Li-ion migration into the LiMnPO4. It is demonstrated that N is bonding with P and Mn on the LiMnPO4 surface, decreasing the contact resistance of carbon. Thus, LiMnPO4/N-doped C exhibits superior cycling performance.
In the final part, V4+ is substituted into LiMnPO4 to improve rate capability and relieve the strain during phase transition. V4+ acts to suppress the grain growth and promote (020) preferred orientation. The accommodation of V4+ on Mn site is accompanied with Mn vacancy. The carbon-coated LiMn1-2xVxPO4 exhibits a superior rate capability of 157 mAhg-1 at 0.1 C and 106 mAhg-1 at 20 C. In-situ XANES reveals that a continuous V3+/4+ and V4+/5+ redox reactions occur in the range of 2.0-3.5 V and 3.5-4.3 V. The V3+/4+ redox reaction promotes the solid-solution reaction and additional capacity below 3.5 V. Furthermore, in-situ XRD shows that the LiMn1-2xVxPO4 undergoes a fast crystalline-to-amorphous reaction. The formation of a metastable amorphous phase with wide Li contents will relieve the interfacial strain, which explains why the olivine with sluggish phase transition can exhibit fast lithiation/ delithiation after substituting with V4+.
Therefore, both anatase TiO2 and olivine LiMnPO4 exhibit outstanding performance, especially on rate capability by using novel preparation methods and several improving strategies. The electrochemical mechanisms are completely revealed which becomes the foundation to develop high-safety materials with superior performance.

並列關鍵字

Li-ion battery ； Na-ion battery ； Titanium oxide ； Lithium manganese phosphate ； Doping ； Architecture design

參考文獻

[1] H. Yang, C.-K. Lan, J.-G. Duh, Journal of Power Sources, 288 (2015) 401-408.

[2] M. Gaab, N. Trukhan, S. Maurer, R. Gummaraju, U. Müller, Microporous and Mesoporous Materials, 157 (2012) 131-136.

[3] M. Sabo, W. Böhlmann, S. Kaskel, J. Mater. Chem., 16 (2006) 2354-2357.

[4] Z. Wang, X. Li, H. Xu, Y. Yang, Y. Cui, H. Pan, Z. Wang, B. Chen, G. Qian, Journal of Materials Chemistry A, 2 (2014) 12571.

[5] A. Tekiel, J.S. Prauzner-Bechcicki, S. Godlewski, J. Budzioch, M. Szymonski, The Journal of Physical Chemistry C, 112 (2008) 12606-12609.