Synthesis, Modification And Electrochemical Characterization Of Li-Co-O As Cathode Material For Lithium-ion Batteries

As a new generation green non-pollution battery, Lithium-ion batteries are important power sources for portable electronics, such as laptop computers, cellular phones and camcorders. In addition, they are also considered as the promising candidates for using in the electric vehicles in the future. The cathode material is known to exert a major influence on the performance of Li-ion batteries. LiCoO₂ is the primary cathode material in the current generation of commercially available Li-ion batteries for its simple technological process, high energy density, high operating voltage and long cycle-life. But LiCoO2 as cathode material has some drawbacks, such as the lower practical capacity (only 60⁷0% of theoretical capacity), shortage of cobalt sources, high cost of LiCoO₂, poor resisted overcharge capacity and just passable thermal stability. To overcome the above shortcomings, basing on studying of LiCoO₂, we researched several metal substituents(both electrochemically-active and -inactive) in LiCoO₂. Another approach to improve the cycling stability of cathode materials is by surface coating treatment. The compounds of LiCoO₂ were synthesized by semi-solid-state method, wet moist chemical method, Sol-gel method. The products were tested by XRD, TG-DTA, XPS and electrochemical methods. It was shown that all the products was single-phase and layered structure. The sol-gel method has the advantages of good stoichiometric control and the production of active submicronized particles in a relatively shorter processing time at lower temperatures. The method of semi-solid-state to prepared LiCoO2 using LiOH?H2O and CoOOH as raw material was more saving energy than traditional solid state method. Aim to investigate the structural stability of LiCoO2, the powers have been soaked in a series of acid solution with different pH. The results indicated Li ion is easier to lose than Co ion in the LiCoO2 structure. And when pH>5 the LiCoO2 structure is more stabile relatively. The surfaced-alkali of LiCoO2 would influence the electrochemical properties, to avoid this harmful effect the distilled water was used to wash and soak the basic-LiCoO2, the electrochemical results was shown that LiCoO2 treated by distilled water had more excellent electrochemical performances and its cycle efficiency enhanced 30%. In addition, the element analysis to determinate the content of Li, Co in the raw materials and the products by acidmetry, complesometric titration and spectrophotometric method is described in this paper. The methods were proved to be simple and accurate. In this work, we studied the structural and electrochemical properties of Al-substituted LiCo1-yAlyO2(X=0.1,0.2,……0.6) cathode materials synthesized by a sol-gel method from citrate precursors. It was shown by XRD that the layered pure phase of LiCo1-yAlyO2 were made for y<0.5. the highest solubility for the Al doping is about 50% in 750oC keeping 12h. With the increasing content of Al in LiCo1-yAlyO2, the crystal structure was changed. Lattice constant a decreased, and lattice constant c increased, respectively. Electrochemical performances research indicated that the first charge/discharge capacity of LiCo0.7Al0.3O2 was 145.2/128.5 mAh·g-1, and has the higher working voltage than LiCoO2.Although cyclic efficiency enhanced with increasing the cycle number, the capacity was faded at the same time. The layered LiCo1/3Ni1/3Mn1/3O2 cathode material for lithium ion batteries was synthesized by sol-gel route. XRD analysis, IR, SEM,XPS and electrochemical tests were used to characterize the composition, structure, appearance and electrochemical performances of the aimed material. The XRD indicated that the layered pure phase LiCo1/3Ni1/3Mn1/3O2 was obtained under 800oC for 20h in air. The SEM showed that the synthesized material has a good micro-morphology, uniform spherical particle size(d≈0.3μm) and a very narrow grain size distribution. Electrochemical performances research indicated that the LiCo1/3Ni1/3Mn1/3O2cycled between 2.7⁴.3V had charge/discharge capacity of 168/160 mAh·g-1,169/162 mAh·g-1,165/160 mAh·g-1,163/158 mAh·g-1 in the previous four cycles respectively and had good cycle properties. The XPS showed that the predominant oxidation states of Ni, Co, and Mn in the compound were +2, +3 and +4, respectively. The different raw materials and amount would influence the valence of transition metal in the compound. To avoid the harmful interactions between LiCo1/3Ni1/3Mn1/3O2 and electrolyte, the LiCo1/3Ni1/3Mn1/3O2 cathode materials were modified by applying surface coating of TiO2, AlPO4 and LiCoO2 at the different conditions. The coated-LiCo1/3Ni1/3Mn1/3O2 was tested by XRD, SEM and electrochemical test. The results shown that coated-LiCo1/3Ni1/3Mn1/3O2 had the same structure as the before at the experiment conditions, and TiO2-coated materials when the atomic ratio of Ti/M=0.08 showed the more excellent electrochemical performances than the others in this work. TiO2-coating improve the cycling stability of LiCo1/3Ni1/3Mn1/3O2 cathode material. But AlPO4-coating and LiCoO2-coating materials were not reached the expected results.