The Investigation Of LiNi_xCo_yMn_1-x-yO₂ As Cathode Materials And Their Corresponding Electrolyte Additive For Lithium-ion Batteries

Recently, LiNi_xCo_yMn_1-x-yO₂ material has become one of the promising cathode material systems for their high capacity, low cost, environmental-friendly, good thermal stability and stable cyclic performance in the lithium ion batteries. In this thesis, two kinds of material in this family, such as LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_0.4Co_0.2Mn_0.4O₂ with excellent electrolyte performance have been synthesized. Their structure, surface properties, thermal stability and storage performance were also studied in detail by various electrochemical methods and spectral techniques, including structural analysis, surface analysis, thermal analysis and electrochemical techniques. In particularly, the origination of the first irreversible capacity of LiMn_1/3Ni_1/3Co_1/3O₂,and the storage performance of LiNi_0.4Co_0.2Mn_0.4O₂ were also emphasized.In addition, we have designed and set up Differential Electrochemical Mass Spectrometer (DEMS) for the first time in the lab for Li-ion batteries study, e.g. investigation of the improvement mechanism of VEC as the electrolyte additive.LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_0.4Co_0.2Mn_0.4O₂ cathode were prepared by sol-gel method and a mixed-hydroxide method with the sintering temperature of 850-900℃, respectively. The oxidation state of Ni, Co and Mn are +2, +3 and +4, both of them have good thermal stability. The electrochemical performances of the materials are excellent, especially at high charging voltage. They all delivered a capacity over 200 mAh/g in the 1^st discharge process when circling at 18 mA/g between 2.5 V and 4.6 V, and still maintain more than 70% after charge-discharge 60 cycles.However LiNi_1/3Co_1/3Mn_1/3O₂ loses 23% capacity in the first cycle, it is a big problem which affect its practical application. In order to make clearly of this origination, we took the NO₂BF₄ as oxidize, quantitatively extracted some lithium ions from the material which could avoid the interference of carbon, PVDF and current collector in the electrochemical analysis. The reaction is:LiNi_xCo_yMn_1-x-yO₂+ m NO₂BF₄→Li_1-mNi_xCo_yMn_1-x-yO₂+mNO₂+mLiBF₄.The investigated results of this series of chemical delithium sample shown that LiNi_1/3Co_1/3Mn_1/3O₂ is able to keep its single phase during delithiation process. Oxidation states of Ni and Co increase to +4 when charging the material to high voltage (V≥4.3V) i.e. Ni³⁺→Ni⁴⁺ and Co³⁺→Co⁴⁺.These changes happened at the first cycle are partly irreversible, which may cause negative effects on the electrochemical performance of the material. Simultaneously, the active oxygen species such as O₂^-,O^-,O₂^2- will lose some electrons to form O₂,or immigrate to the surface of material at higher charging voltage. Parts of them even release from the delithiated materials. These irreversible changes also break the balance of charge and species in the original materials, affecting insertion of lithium ions into delithiated materials when it is discharged. These results will be very helpful to understand the structural changes of the materials and chemical activity of the lattice oxygen species during the electrochemical delithiation process in LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials.The storage stability of LiNi_0.4Co_0.2Mn_0.4O₂ material was studied by comparing four materials which stored at different conditions (like exposing to air, or placing in glove box, or storing in a box maintained at 35℃and 45% HR, or storing in a airtight bottle without CO₂ which was also put into an incubator) for two and eight months. It is proved that the surface reaction between samples and CO₂/H₂O is so negligible that it could not affect the electrochemical performance of the materials. However the sample stored in the glove box has shown the worst cycle stability because of the oxygen release from the material surface. After that the Mn and Co ions will decrease their oxidation states, take part in the electrode reaction, and dissolve into electrolyte. The regular ion arrangement on the surface may be destroyed, and the cycle performance of the material becomes poorer. So keeping the storage atmosphere same as the synthesis condition would be a scientific choice for batteries materials. There is no especial request like vacuum-pack for LiNi_0.4Co_0.2Mn_0.4O₂ material which will be ??very valuable for its practical use.In the end of this thesis, the reason of improvement of VEC as an electrolyte additive was also investigated through the DEMS method. It is proved that adding 2 % VEC to LiPF₆/EC+DMC electrolyte can significantly improve the cyclic performance of LiNi_0.8Co_0.2O₂ cathode material at elevated temperature such as 50℃. It works through former reacted with OPF3 before EC and DMC, which would suppress the decomposition of electrolyte, the possible reactions are:In summary, the products of OPF₂OR_a,the net high-polymer molecule from self-polymerized of VEC, and OPFR_bOCOOCH₂CHFCHCH₂ from the unavoidable reaction between DMC and OPF₂OR_a are helpful in forming a stable SEI layer.It could prevent subsequent decomposition of the electrolyte during charge/discharge process, and improve the electrochemical performance of the batteries.