【作者】 庄全超；

【导师】 孙世刚；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 博士

【摘要】 锂离子电池中电极表面SEI膜对电池的电化学性能有重要影响，是锂离子电池研究的热点之一。本文系统、深入地研究了石墨负极，LiCoO₂正极，尖晶石Li2Mn₂O_及其Ni、Fe、Ti掺杂产物正极表面SEI膜在首次充放电中的形成过程和性质、嵌锂电极动力学和热力学以及嵌锂过程物理机制。重点探讨了温度、充放电过程、电极电位和电解液种类等对SEI膜成膜机制、材料电子电导率、电荷传递过程和感抗产生机制的影响规律。主要研究结果如下：（1）石墨负极表面SEI膜的成膜机制。研究发现，对于扣式电池体系，石墨负极首次阴极极化过程中电化学阻抗谱（Nyquist图）的高频区域半圆除了与SEI膜的形成有关外，还与集流体与集流体之间的接触阻抗有关。在三电极模拟电池中，这一接触阻抗可以被完全消除，因此从石墨负极在首次阴极极化过程中EIS谱特征及其变化可有效地研究SEI膜的成膜机制。研究结果指出，在1MLiPF₆-EC：DEC：DMC电解液中，水溶性粘合剂石墨负极和油性粘合剂石墨负极SEI膜的成膜电位区间不同，前者主要在0．8～0．55 V之间，而后者则在1．0～0．6V之间形成。电解液中甲醇杂质的含量极大地影响石墨负极的性能，当甲醇杂质含量小于0．1％时对石墨负极的充放电循环可逆性基本不发生影响，但大于0．5％则十分显著。提出甲醇杂质对石墨负极性能影响的机制为：甲醇在2．0 V左右还原生成甲氧基锂在石墨负极表面上形成一层初始SEI膜，不能有效地钝化电极表面，导致EC的过度还原分解进而影响SEI的成膜过程。发现高温（60℃）下在1MLiPF₆-EC：DEC：DMC电解液中添加5％VC，可抑制石墨负极首次阴极极化过程中电解液的过度分解，从而改善石墨负极／电解液界面的稳定性。在电化学循环伏安扫描4～10周范围内，SEI膜电阻随循环扫描周数的增加近似线性增长，但石墨负极／电解液界面总阻抗反而减小，归因于电荷传递电阻的降低。石墨负极在经历电化学循环扫描后，其活性材料表层发生粉化和无定形化，但石墨材料仍然保持完整的层状本体结构。（2）LiCoO₂正极／电解液界面性质。研究指出，LiCoO₂正极的组成及其制备工艺对其EIS谱的特征有重要影响。当LiCoO₂正极的组成为80％的活性材料、10％的PVDF-HFP粘合剂、7％的石墨和3％碳黑（质量百分比）时，从EIS谱中可观察到与Li_xCoO₂电子电导率相关的半圆。提出LiCoO₂正极在充放电过程中的LiCoO₂／Li_1-xCoO₂局域浓差电池模型，较好地解释了Li／LiCoO₂电池体系的感抗来源。发现锂离子在LiCoO₂电极中的嵌脱过程可较好地用兰格谬尔嵌入等温式和弗鲁姆金嵌入等温式描述，定量测定了LiCoO₂正极中锂离子嵌脱过程中的物理化学参数。得到电荷传递反应的对称因子α=0．5；在1MLiPF₆-EC：DEC：DMC及1M LiPF₆-PC：DMC+5％VC电解液中，锂离子迁移通过SEI膜的离子跳跃能垒平均值分别为37．74和26．55 kJ／mol，电子电导率的热激活化能平均值分别为39．08和53．81 kJ／mol，以及嵌入反应活化能平均值分别为68．97和73．73 kJ／mol。（3）尖晶石LiMn₂O₄及其掺杂产物的合成与表征。采用溶胶-凝胶法合成尖晶石LiMn₂O₄及其Ni、Fe、Ti掺杂产物，运用EIS研究了所合成材料正极界面特性的温度效应。发现EIS谱的高频区域半圆是由两个半圆重叠而成，分别与SEI膜和活性材料的电子电导率有关。在首次充放电过程中，改变温度不引起SEI阻抗明显的变化；然而高温（55℃）则使充放电过程中活性材料电子电阻和电荷传递电阻增大，归因于放电过程中尖晶石结构内Mn-Mn原子间距快速增大，远远超过充电过程中Mn-Mn原子间距的收缩。同样，提出尖晶石LiMn₂O₄正极中存在LiMn₂O₄/Li_1xMn₂O₄和Li_0.5Mn₂O₄/Li_0.5-xMn₂O₄两种局域浓差电池的模型，解释了Li／LiMn₂O₄电池体系充放电过程中的感抗来源。研究发现，镍和铁的掺杂虽然不引起SEI膜阻抗明显变化，但都使正极活性材料电子电阻和电荷传递电阻在高温下的放电过程中随电极电位降低而增长的速度变慢。钛的掺杂不仅导致SEI膜阻抗增大，降低活性材料的电子电导率，同时保持正极活性材料电子电阻和电荷传递电阻在高温下的放电过程中随电极电位降低而快速增长。钛掺杂还能够有效地抑制感抗的产生。在充分考虑导电剂对嵌锂过程影响（即电子传输过程的影响）的基础上，提出了嵌锂过程的物理模型。本论文的研究结果对深入认识电极表面SEI膜的成膜机制和充放电中锂的嵌入脱出过程、以及发展相关的基础理论具有重要价值，同时对于指导新型正极材料的开发，发展高比能绿色二次电池具有重要意义。更多还原

【Abstract】 The passive layer, which is generally called the solid electrolyte interface layer （SEI layer）, covered on both anode and cathode of a Li-ion battery plays a key role in the electrochemical property of the lithium ion battery, and has attracted extensive attentions in past years. The formation mechanisms of SEI film in the first charge-discharge process, the thermodynamics and kinetics as well as the physical mechanism of lithium intercalation were systematically investigated in this thesis. The emphasis was put upon the effects of temperature, charge-discharge process, electrode potential and electrolyte on the mechanism of SEI formation. The electronic properties of active materials, the charge transfer process and the mechanism of inductance formation were also thoroughly discussed. The main results are summarized below.（1） The mechanism of SEI formation on graphite anode. It has found that, in a coin cell, the arc appearing in high-frequency range （HFA） observed in the Nyquist diagram recorded in the first lithiation of graphite anode depends not only on the SEI film, but also on the interface contact problem between the electrode and current collector. It has demonstrated that such contact problem can be eliminated completely in a three-electrode cell system. Thus the mechanism of SEI formation can be investigated through the EIS features and their evolution recorded in the first lithiation of graphite materials. The results showed that, the SEI can be formed between 0.8-0.55 V in the 1M LiPF₆-EC:DEC:DMC electrolyte on a graphite anode using water-soluble binder （F-103）, while in the potential region of 1.0-0.6 V using PVDF-HFP as binder. It has demonstrated that trace （< 0.1% ） methanol contaminant may not affect the electrochemical performance of graphite electrode, whereas a significant deterioration is observed when methanol contaminant exceeds 0.5%. Based on experimental data and analysis, a mechanism of the deterioration of electrochemical performance of graphite electrode caused by methanol contaminant was proposed as below: lithium methoxide was generated through methanol reduction near 2.0 V and deposited on graphite electrode surface to form an initial SEI layer, which can not passivte efficiently electrode surface and caused excess decomposition of ethylene. At high temperature （60℃）, the excess decomposition of electrolyte in the first lithiation of graphite anode can be avoided by adding 5%VC （volume ratio） to the 1M LiPF₆-EC:DEC:DMC electrolyte. As a consequence the interface stability between the graphite anode and electrolyte is improved. It has determined that the resistance of the SEI film is increased almost linearly during prolonged electrochemical cycling within 4-10 cycles. However the total interface resistance between the graphite anode and the electrolyte solution is decreased due to the decrease in charge transfer resistance. After having subjected to electrochemical cycling, the surface of the active material was exfoliated, pulverized and become amorphous, but the bulk of the active material keeps unchanged.（2） Properties of LiCoO₂ electrode/electrolyte interface in Li-ion batteries. It has illustrated that, the common EIS features of a LiCoO₂ cathode depend strongly on its composition and preparation procedure. When the LiCoO₂ cathode composition was 80 weight percent （wt %） LiCoO₂ powder, 10 wt % ployvinylidene fluoride binder, 3 wt % carbon black and 7 wt% graphite, an arc in Nyquist plots relating to electronic properties of the material can be observed. The results showed that the inductive loop observed in the impedance spectra of the LiCoO₂ cathode in Li/LiCoO₂ cells is originated from the formation of a Li_1-xCoO₂/LiCoO₂ concentration cell. Moreover, it has demonstrated that the lithium-ion insertion-deinsertion in LiCoO₂ hosts can be well described by both Langmuir and Frumkin insertion isotherms; the symmetry factor of charge transfer was measured at 0.5. In 1M LiPF₆-EC:DEC:DMC or 1M LiPF₆-PC:DMC+5%VC electrolyte, the energy barrier for the ion jump was evaluated at 37.74 and 26.55 kJ/mol, the thermo active energy of electronic conductivity jump was determined at 39.08 and 53.81 kJ/mol, and the active energy of lithium intercalation was obtained at 68.97 and 73.73 kJ/mol, respectively.（3） Synthesis and characterization of spinel LiMn₂O₄ and its doped compounds. The spinel LiMn₂O₄ and its doped compounds with Ni、Fe and Ti were synthesized by sol-gel methods, and the effects of temperature and doping elements on the spinel LiMn₂O₄/electrolyte interface were investigated by EIS. It has found that the high frequency arc in Nyquist plots consists of two overlapping semicircles that relate respectively to SEI film and electronic conductivity of active material. The influence of varying temperature on resistance of SEI film is small, while the electronic resistance and charge transfer resistance are increased rapidly at high temperature （55℃） in the first charge-discharge process, attributing to the contraction of hopping length （Mn-Mn interatomic distance） is smaller than the expansion of hopping length in the charge-discharge process. As for the LiCoO₂ cathode, the inductive loop observed in the impedance spectra of the LiCoO₂ cathode in Li/LiCoO₂ cells has been interpreted by a model of formation of a LiMn₂O₄/Li_1-xMn₂O₄ and a Li_0.5Mn₂O₄/Li_0.5-xMn₂O₄ concentration cells. It has also demonstrated that the doping with Ni and Fe affects slightly the resistance of SEI film, but it can slow down the increasing rate of the electronic and charge transfer resistances with decreasing electrode potential in charge-discharge processes. The doping of Ti results not only in increase of the resistance of SEI film, but also in decrease of electronic conductivity of active materials. The doping of Ti can maintain the fast increase of the electronic and charge transfer resistances with the decrease of electrode potential in the charge-discharge process. Furthermore, the doping of Ti has eliminated the inductance formation of spinel LiMn₂O₄ electrode. Based on important effects of conduct additives on lithium intercalation-deintercalation process, a model of physical mechanism involved in lithium intercalation is proposed.The results of this thesis throw insight into the SEI film formation mechanisms and lithium intercalation-deintercalation process, and are of significance in developing relevant fundamental theory. The study is also of great importance in exploitation of new cathode materials and in development of green rechargeable batteries with high-specific energy density.更多还原