Studies On The Synthesis And Fluoride Surface Modification Of Lithium-rich Layered Cathode Materials

This synthesis and surface modification of a lithium-rich cathode material Li[Li0.2Ni0.17Co0.07Mn0.56]O2 were studied in this thesis. The mechanism for its electrochemical performance improvement surface coating was also investigated.Li[Li0.2Ni0.17Co0.07Mn0.56]O2 was synthesized by sol-gel method, and in the process of preparation of lithium source by changing the excess of this condition prepared a series of samples. The use of XRD, SEM, constant current charge discharge test methods characterization and electrochemical properties of physical properties of the prepared samples. The test results show that, the samples are the main diffraction peak of the space group R m six party structure, and the(006)/(012) and(018)/(110) is obviously independent to diffraction peaks of characterization of layered structure. The peak intensity with excess lithium source increases, indicating its crystalline is gradually getting better, integrity of crystal is gradually changed for the better. Lithium source mass fraction of excess of the crystalline 10 wt.% samples of the best particle, smooth surface, uniform size, good dispersion and particle size is about 200~300 nm, the cycle stability but also for the best.Lithium rich layered cathode materials due to the loss of oxygen charging phenomenon, in the first week of materials in the process, it will reduce the efficiency of the loss of large irreversible capacity, and reduce the cycle performance of the material is due to the ions rearrangement, insulation conductivity phase Li2MnO3 will cause the material existence the decline, also make the electrochemical properties of materials affected by the. In addition, when the voltage is charged to 4.8V, the electrolyte side reaction from the surface of the material and the oxidative decomposition, as well as the main factors that affect the electrochemical properties. In the method at present many to improve the electrochemical performance of material, surface coating for its easy operation, low cost and modification effect more obvious advantages as the most common method of modified materials. Many researchers have confirmed that can through the introduction of metal oxide coated layer enhances the stability of electrode/electrolyte interface. And recent studies have found that using metal oxide as the cladding layer of the active material in contact with the electrolyte and participate in the loop, metal oxide on the surface of the active material is easy to be with the HF electrolyte reaction to generate the corresponding metal fluorides. Additional studies prove, forming fluoride needs lower Gibbs free energy than formation of oxide. And studies have confirmed, fluoride is found in SEI film is stable, is not easy to decompose. So in this paper, using AlF3 and MgF2 as coating material surface modification of lithium rich layered materials.On the synthesis of pure phase of the original material basis, using easy to control and operate the chemical deposition method, AlF3 and MgF2 uniformly coated on the surface of Li[Li0.2Ni0.17Co0.07Mn0.56]O2 material, and respectively through the characterization and electrochemical performance of various physical properties test analysis, the samples were confirmed by using the results, fluoride on layered rich lithium cathode material surface coating modification, the main structure of the material after coating had no significant change, and only on the surface of the material form a layer of amorphous compounds, this layer can effectively avoid direct contact with the original sample and the electrolytic liquid, thereby enhances the electrochemical activity of samples. Among them, the optimal amount of AlF3 coated and MgF2 were 3 wt.% and 1 wt.% respectively. The electrochemical performance test results showed that, under the condition of room temperature, 3 wt.% AlF3 coated Li[Li0.2Ni0.17Co0.07Mn0.56]O2 after 50 cycles, the capacity to maintain in the 182mAhg- 1, the capacity loss is only 16.9%. 1 wt.% MgF2 coated Li[Li0.2Ni0.17Co0.07Mn0.56]O2 after 50 cycles, the capacity to maintain in the 188mAhg-1, the capacity retention rate reached 86%. In characterization of physical properties of the electrode sheet circulation after confirmed, rapid growth of AlF3 and Mg F2 could effectively inhibit SEI film, which is beneficial to the structure stability and promote removal of the lithium insertion process.In general, AlF3 and MgF2 fluoride coating plays a role as follows: 1, to reduce the circulation in the process of oxygen loss phenomenon and the electrolyte decomposition; 2, avoid direct contact materials and electrolyte, thereby inhibiting particle structure degradation, improve long-term cycle stability; 3, improve the conductivity of the active material, enhances the rate performance; 4, inhibited the rapid growth of impedance charge and discharge, prolong the cycle life of the battery; 5, improve the thermal stability of the materials, improve the battery safety.