Electrode/Electrolyte Interfaces Reaction Mechanisms Studies Of LiFePO₄

The charge/discharge capacity, cycle ability and rate capability are associated with the electrochemical reactions at the electrode/electrolyte interfaces, so it is necessary to understand the electrode/electrolyte interfaces reaction mechanisms for clarifying the capacity fading mechanisms, improving the charge/discharge capacity and the rate capability of the electrode material. Olivine LiFePO₄ is at present a very good candidate as a cathode material for lithium-ion batteries. However, there is still some debate about the interpretation of the electrochemical impedance spectroscopy（EIS） Nyquist plots of Li FePO₄ electrodes in the literature.In the present paper, impedance spectra for lithium ion deinsertion and insertion in the LiFePO₄ electrode with different percent of graphite materials as conductive additive were obtained at different potentials during first charge-discharge cycles at room temperature. The results revealed that the characteristic Nyquist plot of LiFePO₄ electrodes is strongly influenced by the content of conductive additive. With the increase of the content of conductive additive, the characteristic Nyquist plot of the LiFePO₄ electrode change from one semicircle and an inclined line to two semicircles and an inclined line at room temperature. When the content of conductive additive in the Li FePO₄ electrode is below 30 wt%,the Nyquist plot gives a semicircle in the high frequency region and an inclined line in the low frequency region, and the semicircle in the high frequency region cannot ascribed to charge transfer process but to contact resistance between the electrode material and current collector. However, at intermediate degrees of intercalation, the Nyquist plot of the Li FePO₄ electrode with 50 wt% conductive additive consists of three well separated parts, namely, two semicircles and an inclined line. It was also found that the depressed semicircle in the high frequency region can be ascribed to contact resistance between the electrode material and current collector, while the semicircle in the middle frequency region can be attributed to inter-particle resistance and charge transfer process. In addition, the variations of the semicircle in the middle frequency region as a function of the electrode potential mainly due to charge transfer process.The variations of EIS features of the Li FePO₄ electrode were also investigated with the temperature range of-20²5 ℃. The results shown that, during the first delithiation, three semicircles were observed in the Nyquist plots of LiFePO₄ electrode at 5 ℃. It was found that, the common EIS features of LiFePO₄ electrode are related to the temperature, the semicircle observed in the Nyquist diagram which related to the contact resistance between the electrode material and current collector and the semicircle related to the SEI film overlap each other to form one semicircle at higher temperatures. According to the above results, it can be concluded that the high frequency semicircle in the Nyquist plot of the LiFePO₄ electrode at room temperature not only have been assigned to the contact resistance between the electrode material and current collector, but also related to the SEI film. The experimental EIS data was simulated by a suitable equivalent circuit, and the active energies of lithium ion intercalation-deintercalation reaction of LiFePO₄ electrode were also calculated.On the other hand, LiMn_0.8Fe_0.2PO₄/C composite material was synthesized by high-energy-milling method. The delithiation/lithiation process and cycle ability of LiMn_0.8Fe_0.2PO₄/C composite eletcrode were investigated by EIS and galvanostatic cycling test. The results showed that the LiMn_0.8Fe_0.2PO₄/C eletcrode material had a typical Olivine structure and good electrochemical properties. The first discharge capacity was 142.2 mAh g-1, and it became 144.8 mAh g-1 after 100 cycles. The EIS results indicated that the Nyquist plot of LiMn_0.8Fe_0.2PO₄/C electrode contains three distinct features, which consist of one semicircle, one arc and one line. And the semicircle in the high frequency region can be ascribed to contact resistance between the electrode material and current collector, and the migration of lithium ions through the SEI films. The arc in the middle frequency region can be attributed to charge transfer process.