Synthesis, Characterization And Their Interfacial Properties Of Polymer Electrolytes For Secondary Lithium Batteries

The rechargeable lithium polymer batteries are considered as one of the best candidates for next generation power sources due to their high energy density, good cyclability, reliability and safety. PEO-based polymer electrolytes have received extensive attentions for their potential capability to be used as alternative candidate materials for the traditional liquid electrolytes in all solid-state rechargeable lithium polymer batteries. However, there are many problems that must be solved before these systems reach wide commercial utilization. Firstly, the battery performance is largely limited by the low ionic conductivity of PEO-based polymer electrolytes at room temperature. Therefore, it is important for PEO-based polymer electrolytes to enhance the ionic conductivity at room temperature. On the other hand, since lithium metal is used to be the anode material, the interface compatibility of polymer electrolyte with lithium metal severely affects the safety and cycle life of battery.In this dissertation, many efforts have been invested in the study of the problems above, and the results as follows:1. FTIR spectra of PEO-LiX (X=SCN, N(SO₂CF₃)₂, ClO₄) polymer electrolytes have been obtained for EO/Li ratios from 60:1 to 4:1 in order to investigate the interactions of ion-ion and ion-polymer. When a lithium salt is dissolved in the PEO with low dielectric constant, the ion association is commonly present and the degree of ion association varies with the type of anion from lithium salt. The results show that the ion association in the PEO-LiSCN system is more serious and at high concentration of LiSCN contact ion pairs, triple ions and dimers are main ion species. Furthermore, the crystalline phase of PEO was progressively transformed into amorphous phase since the large-size anions from lithium salt can play a role of plasticizer. LiN(SO₂CF₃)₂ with the largest size anion in this work possess the best plasticizing effect.From the ion transport of view, the results induced by the ion association and plasticizing role are opposites. The ion association necessarily diminishes the number of effective charge carrier and then leads to low ionic conductivity, whereas plasticizing role can change the phase composition of PEO and increase the content of the amorphous phase in which the highest ionic conductivity occurs. As a result, a maximum of conductivity is observed when the salt concentration is higher than a certain value. Based on the FTIR analysis, we commendably interpret the fact that ionic conductivities of PEO-LiX (X=SCN, N(SO₂CF₃)₂, ClO₄) polymer electrolytes follow the order LiN(SO₂CF₃)₂ > LiClO₄ > LiSCN at the same salt concentration.2. BMPyTFSI and BMImTFSI ionic liquids were synthesized by anion exchange reaction, then new gel polymer electrolytes containing ionic liquid were prepared by solution casting method. The electrochemical properties such as ionic conductivity, lithium transference number, electrochemical stability windows, and the compatibility with Li electrode were investigated with ac impedance, dc polarization, and linear sweep voltammetry techniques. The results show that the incorporation of BMPyTFSI or BMImTFSI to the P(EO)₂₀LiTFSI electrolyte improves the ionic conductivity over the entire temperature range investigated, but the greatest enhancement is at lower temperatures. The ionic conductivity of the P(EO)₂₀-LiTFSI electrolyte at 40℃with a BMIm⁺/Li⁺ mole fraction of 1.0 showed an increase of about two orders of magnitude reaching 4×10^-4 S/cm. In despite of decreasing for the lithium ion transference number with the increase of the amount of BMPyTFSI or BMImTFSI, the lithium ionic conductivity increased. The electrochemical stability and interfacial stability for these gel polymer electrolytes were significantly improved due to the incorporation of BMPyTFSI or BMImTFSI. At high concentration of ionic liquid, the electrochemical stability window reaches 5.2 V. The results suggest that the polymer electrolyte containing ionic liquid can be applied safely in 5 V lithium secondary batteries.The DSC and FTIR results show that the glass transition temperature (T_g) and the crystallinity obviously decrease with increase of the content of ionic liquid. These revealed ionic liquid can weaken the interaction among the polymer chains and accelerate the segmental motion of the PEO-based polymer electrolyte, thus the ionic conductivity of the gel polymer electrolytes increases.3. The passive layers formed on lithium at a nickel substrate in polymer electrolytes containing PC and BMPyTFSI were characterized by using XPS, FTIR as well as Ar ion sputtering technique. PEO seems to be rather inert to lithium and has no effect on the composition of the passive layer. The passive layers are mainly composed of the reduction products of organic plasticizer, anion from lithium salt, and impurities existing in electrolyte such as traces of O₂ and H₂O. The results show that PC was reduced on Li to ROCO₂Li and Li₂CO₃ species and the main reduction product of LiN(SO₂CF₃)₂ was LiF. Polymer electrolytes containing BMPyTFSI ionic liquid seemed to be stable with lithium and only formed a fewer passive layers mainly including LiF. LiSCN and LiBr salts have no effect on the composition of the passive layer. During the lithium deposition-dissolution process, there is no change for the composition and structure of the passive layer. It is very important to obtain good performance of battery.4. A spectroelectrochemical cell was designed and optimized, then the solid-solid interface between lithium electrode and polymer electrolytes was explored to characterize by using in situ micro-FTIR spectroscopy. The cyclic voltammetric results indicated that the reducing reactions of oxygen and water as well as the under-potential deposition (UPD) of lithium occur in the electrode/electrolyte interface in the different potential region. The infrared spectral changes observed during the CV process revealed that there is a direct correlation between the CV peaks and the magnitude of the infrared peaks. This change is most likely due to an increase or decrease in the infrared reflectivity induced by the formation of a thin layer at the Au/polymer electrolyte interface. It is shown that the infrared reflectivity from the solid-solid interface is very sensitive to the formation of the passive layer on the lithium electrodes. On the other hand, in situ FTIR results show that there is a sharply decrease in the amount of Li salt probed by the beam during the reducing process of water. The reason is that the reducing process leads to the loss of lithium ion from the surface of the working electrode and accompanies by the migration of anion into the bulk electrolyte. Optical micrographs obtained simultaneously also displayed directly the formation of the passive layer along with lithium deposition and dissolution process. It is correlated well with in-situ FTIR and electrochemical experiments.In situ FTIR results obtained from the polymer electrolyte containing PC after lithium deposition-dissolution process show that the surface chemistry of Li is dominated, as expected, by PC reduction to ROCO₂Li and Li₂CO₃ species. These are in agreement with the results obtained from ex situ experiments.