鋰離子電池模型與電池穿刺熱行為分析

Abstract

由於鋰離子電池本身能量密度大、可攜式、壽命優良等特性，在各大產業皆相當依賴鋰離子電池的發展。例如在電動車的產業，電池更是扮演相當重要的角色，然而在台灣的能源產業上，希望結合電動車的發展，但是一直欠缺與歐洲汽車產業結合之整合能力。有著良好的電池產品，但卻往往不能順利的打進歐洲汽車市場，是相當可惜的，因此，本研究前半部份主要利用在歐洲各大車廠皆有使用之建模軟體Dymola將昇陽公司所研發之40138磷酸鋰鐵電池作分析，建立以Dymola為基礎之電池模組。而此模組內又分為熱電模型及壽命模型兩大部分作分析；電池最重要的特性不外乎為作用時之電池電壓曲線以及放電過程中之溫度變化，因此，本文之熱電模型利用了等效電路模型去模擬出電池在不同電流率之大小的情形下的表現狀況；另外透過壽命模型，分析此磷酸鋰鐵電池在不同充放電深度下之循環充放電之老化行為，並且加以預測，以及透過實際之電池放電實驗、分段放電實驗、循環老化充放電實驗去求取參數及驗證模型之可靠度。 另一方面，鋰離子電池之安全性一直以來都是大家所關心之議題，在一些極端情況下，如電動車遭受撞擊導致電受到擠壓而變形等，皆有可能對人身安全造成非常大之危害，因此本文針對與安全性習習相關之電池穿刺實驗作分析，利用有限元素軟體Ansys workbench建立一熱傳模型，可有效的模擬出電池在穿刺實驗過程中，電池內部短路所造成之熱失控結果，以及整體之溫度變化，並且利用與文獻結果之比較，驗證此熱傳模型之可靠性，並且提出一油封之方式，利用此熱傳模型證實油封電池提升電池整體之安全性之可行性，期待在未來可利用此熱傳模型針對更多鋰離子電池的安全性相關設計做分析。

Lithium ion batteries possess many advantageous characteristics, like of great power density、portable and having excellent cycle life, and are becoming widely used in electric vehicles (EV). In this study we use a numerical analysis software – Dymola, which is famous in Europe automobile industry, to build a lithium iron phosphate (LFP) battery model. The battery model can be combined with other system engineering model like power train system model in Dymola so that it can help us to enter the automobile market in Europe. Generally, the temperature of LFP batteries rises while discharging. If the battery operates beyond the appropriate temperature range, it will lead to a shorter cycle life of the battery. Moreover, as in many applications the cost of batteries pertains to a significant portion of the total cost, battery lifetime is critical for profitability. However some experiments, such as ageing, are expensive and time consuming and cannot be done comprehensively for every control parameters. Hence, this study aims to develop an accurate model to facilitate the analysis of battery performance. Besides, the safety issue for Li-ion battery is also what people always concerned. Nail penetration into a battery pack, resulting in a state of short-circuit and subsequent burning, is likely to occur in electric vehicle collisions. Therefore in this study, a reliable model to describe the thermal behavior for battery nail penetration test is also developed. In our battery model, we choose the equivalent circuit model (ECM) framework which resembles the physical processes of a battery with equivalent circuits such that it reduces the complexity of analysis. To prove the reliability of the model, we analyzes the 40138 LFP batteries produced by PSI by simulating the discharge curve and temperature evolution at different C-rate (0.5C, 1C, 2C,5C) of single cells and battery packs. The simulation results under various operation conditions are validated with experimental results. Moreover, we build an ageing model which can predict the long-term battery cycle behavior based on the short-term discharge cycle test of various depth of discharge. To describe the battery thermal behavior while nail penetrating, we build a heat transfer model by Ansys workbench. This model can capture the temperature evolution trend induced by internal short-circuit and thermal run away. Also, this nail penetration model is validated against a benchmark model for its reliability.