Properties Of Electrode/Electrolyte Interfaces In Lithium-ion Batteries

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.