包覆於正極材料之高分歧高分子對於鋰離子電池熱失控行為探討

Abstract

在18650型石墨/鋰鎳鈷錳氧系統的電池穿刺實驗中，當MBMI此種高分歧高分子被應用在Li(Ni0.4Co0.2Mn0.4)O2正極材料的表面包覆上時，不但抑制了電池熱失控行為的發生，而且趨緩了穿刺短路所伴隨迅速的電位下降。本研究為了瞭解MBMI高分子所提供的電池保護機制，分別針對MBMI高分子自身電化學行為、添加MBMI高分子的充飽電正極材料熱升溫過程中相變化行為、化學熱生成以及焦耳熱生成這四個部分進行討論。 首先，MBMI高分子的表面包覆會產生鈍化層的功效，使得正極氧化材料表面不易生成SEI膜。就熱升溫過程中的充飽電正極材料相變化方面而言，MBMI高分子的表面包覆並不能改變正極材料整體相轉換的起始溫度，所以可以推斷MBMI高分子的保護機制發生在相變化起始溫度之前。最後，藉由DSC和充飽電的極板阻抗分析來討論化學熱和焦耳熱的生成。當MBMI高分子包覆正極材料表面之後，因為SEI的生成銳減，導致電池熱失控中初始放熱量也隨之大幅降低。此外，當鋰離子電池充飽電後，具有MBMI高分子包覆的正極極板自身阻抗也會因為溫度的升高而大幅提升。在熱升溫過程中，MBMI高分子的包覆使得正極材料表面產生一類絕緣層，大幅減少焦耳熱生成。最後，藉由化學熱以及焦耳熱生成相關的實驗數據，可以模擬出熱失控過程中電池內部溫度的改變。其中，焦耳熱僅用以啟動並維持電池內部的升溫，而化學熱生成卻是造成電池內部溫度急遽增加的主要關鍵。 綜合上述討論，在電池熱失控行為中，必須要提供初始的能量，才可以誘導後續的反應繼續發生，所以在這個連鎖反應中，SEI層的裂解和焦耳熱的生成成為一必需的能量。對於包覆MBMI高分子的正極而言，熱失控行為開始啟動時，由於焦耳熱、以及源自於電解液和氧化物表面反應的化學熱生成量降低，熱失控行為得以被抑制住，進而提升了鋰離子電池的安全性。

A hyper-branched polymeric material, known as MBMI polymer, has been used as surface coating of Li(Ni0.4Co0.2Mn0.4)O2 cathode particles, and it is shown to effectively suppress thermal runaway and slow down voltage drop of graphite/ Li(Ni0.4Co0.2Mn0.4)O2 18650-battery during the nail-penetration test. In order to investigate the protection mechanism enabled by the MBMI polymer coating, there are four parts to be discussed in this study, including the electrochemical stability of MBMI polymer coating, the structure of the charged cathode materials by in-situ synchrotron XRD analysis during heating, the analysis of the chemical heat generation, and the analysis of the joule-heating generation. Firstly, MBMI polymer coating passivates the Al surface and suppresses the formation of SEI deposit. Similar “passivating” effect can be anticipated to take place on the cathode oxide surface when it is coated with MBMI polymer. For the structure changes of the charged cathode material, in-situ synchrotron XRD study indicates that the polymer coating does not affect the layered-to-spinel phase transition. It is apparent that the suppressing mechanism is operative below the temperature of oxygen releasing from the oxide particles. Finally, the chemical heat and joule-heating generations are investigated by the analysis of DSC and resistances. When coating MBMI polymer on the surface of the cathode material, the less SEI formation leads to the less initial chemical heat released. Additionally, the higher temperature accompanies the much higher resistance of the charged electrode. It reduces the joule-heat generation via the growth of the insulation-like layer during heating process. Finally, from the experimental results associated with chemical heat generation and joule-heating, the changes of internal battery temperature during the thermal runaway process are simulated. The joule-heating only offers energy to initiate and keep the internal temperature increasing, while the chemical heat generation is the key to cause the internal temperatures to increase dramatically. During thermal runaway process, it needs a primary energy to initiate the subsequent reactions. Hence, it is a chain reaction which the necessary energy is provided by the decomposition of SEI and joule-heating. For MBMI polymer coated on the cathode, it is inferred that as a result of the MBMI polymer cathode coating, the reductions of both the joule-heating releasing and the chemical heat generation by electrolyte-oxide interfacial reactions during the initial stages of heating-up contributes to suppression of thermal runaway.