熱電池電解質-熔融鹽混合物之第一原理分子動力學模擬分析

First Principles Molecular Dynamics Analysis on the Molten Electrolytes of Thermal Batteries

紀尚甫(Shang-Fu Chi)

指導教授 ： 洪哲文

國立清華大學/工學院/動力機械工程學系/碩士(2014年)

摘要

本論文利用古典以及第一原理分子動力學模擬方法，進行研究混合多元鹽類於高溫熔融狀態下的運動行為，計算其離子傳導率以及熱傳導率結果，進行交互驗證以評估其熔融電解質對於熱電池效能之影響；最後再與現有文獻實驗結果進行比較，以檢驗不同方法的優缺點以及適合操作狀況。
熱電池與一般電化學電池最大不同地方，係為利用外部熱源，將電解質迅速加熱至高溫熔化狀態並激發此電池。影響熱電池效能關鍵因素除了陰陽極材料之外，最重要的為熔鹽電解質的離子傳導率與熱傳導率，此二項因素為最直接的影響發電性能。本研究利用建立系統等容定溫模型 (NVT constrain)，在離子傳導率的部分，同時使用古典分子動力學以及第一原理分子動力學搭配Nernst-Einstein relation作計算;在熱傳導率計算上使用反向非平衡態分子動力學方法 (RNEMD method)。不同於實驗，以施加溫差的方式提供熱通量，該方法則是利用提供已知的熱通量來觀察其溫度變化，相較於其他一般的非平衡態分子動力學方法 (NEMD method)或是平衡態分子動力學的Green-Kubo方法有更好的收斂性。
最後嘗試利用分子動力學方法模擬共晶材料模擬材料降溫熔融態至固態的過程，用以觀察其不連續的區域或點來評估熔點位置，以期能建立一個完整的熔融鹽模擬工具，作為未來選用電解質材料的重要參考，並可達到優化性能設計之目的。

關鍵字

熱電池 ； 分子動力學 ； 第一原理分子動力學 ； 離子傳導率 ； 熱傳導率 ； 熔點

並列摘要

The main propose of this thesis is to simulate blend molten salts by molecular dynamics (MD) and first principles molecular dynamics (FPMD) techniques. It is followed by calculating the ionic conductivity and thermal conductivity in order to analyze how the molten electrolyte affect the operation of a thermal battery. Furthermore, we compare our simulation results with literatures to verify the pros and cons of different methods.
According to the literature survey, whether a thermal battery is good or not is based on the discharging life and efficiency. The important factors to the battery efficiency are ionic conductivity and thermal conductivity. They both will affect the ionic transportation. The battery life time and the melting point are also influential factors. In the simulation of ionic conductivity, we use both classical molecular dynamics and first principles molecular dynamics to simulate the behavior at high temperature. Then we calculate the ionic conductivity with Nernst-Einstein relation and thermal conductivity with reversed non-equilibrium molecular dynamics (RNEMD) algorithm. Comparing to some other non-equilibrium molecular dynamics (NEMD) methods, RNEMD has been found to have better convergence.
Finally, we try to simulate the process of cooling down temperature of eutectic-salts electrolyte and observe the region of discontinuation to evaluate the melting point. It is expected to build a complete software tool for molten salt simulation to replace the costive in experiments and to optimize the battery design in a more efficient way.

並列關鍵字

thermal battery ； molecular dynamics ； first principles molecular dynamics ； ionic conductivity ； thermal conductivity ； melting point

參考文獻

1. Sandia National Laboratories.

2. Guidotti, R.A. and P. Masset, Thermally activated (“thermal”) battery technology: Part I: An overview. Journal of Power Sources, 2006. 161(2): p. 1443-1449.

3. Masset, P. and R.A. Guidotti, Thermal activated (thermal) battery technology: Part II. Molten salt electrolytes. Journal of Power Sources, 2007. 164(1): p. 397-414.

4. Masset, P., Iodide-based electrolytes: A promising alternative for thermal batteries. Journal of Power Sources, 2006. 160(1): p. 688-697.

5. Masset, P., A. Henry, J.-Y. Poinso, and J.-C. Poignet, Ionic conductivity measurements of molten iodide-based electrolytes. Journal of Power Sources, 2006. 160(1): p. 752-757.