Elucidating Interface Reactions Coupled with Materials Degradation Mechanisms in Li-ion Batteries and Their Remedy by Electrolyte Additives

Lithium-ion batteries (LIBs) have become one of the most prevalent techniques for feasible and fascinating energy storage devices, however, they still face a significant challenge due to gas generation, which has been regarded as one of the fundamental barriers to the reversibility of battery chemistry. Electrolyte additive has been extensively used as an effective and economical practice. However, their selection has been conducted on an Edisonian trial-and-error, with little knowledge about the relationship between their molecular structure and reactivity as well as the electrochemical performance.

Combined with differential electrochemical mass spectrometry (DEMS), this thesis focuses on the gas generation and electrolyte decomposition mechanism as well as the remedy approaches by electrolyte additives.

Firstly, a DEMS is constructed to differentiate the speciation and source of each gas product in Li-metal batteries. It convincingly identifies that the active oxygen release from the layer structured LiNixCoyMnzO2 (NCM) as the source of instability.

Secondly, tripropyl phosphate (TPPC1), triallyl phosphate (TPPC2), and tripropargyl phosphate (TPPC3) are introduced with the purpose of revealing the structure-property relationship in LiNi0.8Co0.1Mn0.1O2 (NCM811)/artificial graphite (AG) pouch cells. It is found that the effectiveness of TPPC1, TPPC2, and TPPC3 in preventing gas generation and resistance growth as well as improving cycling performance can be described as TPPC3 > TPPC2 > TPPC1.

Thirdly, advanced carbonate-based electrolyte formulation containing vinylene carbonate (VC) and TPPC2/TPPC3 are reported to improve the high temperature performance of NCM811/AG pouch cells. Results reveal that TPPC2+VC and TPPC3+VC can well regulate the electrodes interface with more robust and homogenous films.

Finally, 1,3,2-dioxathiolane-2,2-dioxide (DTD), 4-methyl-1,3,2-dioxathiolane-2,2-dioxide (MDTD) and 4-propyl-[1,3,2] dioxathiolane-2,2-dioxide (PDTD) are introduced to build a robust interphase for NCM811AG pouch cells. As a result, DTD yields a more stable interphase due to more sulfates species can be produced in the interphase for the inchoate cycles, yielding a more cycling stability. However, PDTD outperforms the others after 950 cycles, only in which can sulfate species be detected on both of the cathode and anode.

The present research work will doubtlessly construct the principal foundation for the rational design of new electrolyte components for future battery chemistry.

锂离子电池（LIB）已成为可行且引人入胜的储能装置最流行技术之一，然而，由于气体的产生，它们仍然面临着重大挑战，被认为是电池化学可逆性的基本障碍之一。电解质添加剂已被广泛用作一种有效且经济的做法。然而，它们的选择是在爱迪生式的反复试验中进行的，对它们的分子结构和反应性以及电化学性能之间的关系知之甚少。

本论文结合微分电化学质谱（DEMS），重点研究了气体的产生和电解液分解机理以及采用电解液添加剂作为补救措施。

首先，构建了DEMS系统，以区分锂金属电池中每种气体产物的形态和来源。它令人信服地确定，从层状结构的LiNi_xCo_yMn_zO₂(NCM) 释放的活性氧是不稳定性的来源。

其次，引入了磷酸三丙酯（TPPC1）、磷酸三烯丙酯（TPPC2）和磷酸三炔丙酯（TPPC3），旨在揭示其用于NCM811/ AG软包电池中应用时的结构与性能的关系。 研究发现， TPPC1、TPPC2以及TPPC3 在防止气体产生，抑制电阻增长以及改善电池循环性能方面表现为 TPPC3 > TPPC2 > TPPC1。

第三，报道了基于碳酸亚乙烯酯 (VC) 和TPPC2/TPPC3的碳酸酯电解质配方，可改善 NCM811/AG 软包电池的高温性能。结果表明，TPPC2+VC和TPPC3+VC可以很好地调节电极界面，形成更坚固、更均匀的薄膜。

最后，引入DTD、MDTD，PDTD添加剂，用于为 NCM811/AG软包电池构建坚固的界面膜。结果表明，DTD 在循环早期产生的界面膜更稳定，因为其中含有更多的硫酸盐物质，从而使电池具有更优异的循环稳定性。然而，PDTD在950次循环后优于其他电解液体系，只有在PDTD系统中才能同时在正极和负极表面上检测到硫酸盐物种。

目前的研究工作无疑将为未来电池化学新型电解质成分的合理设计奠定主要基础。

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