| Energy crisis and environmental pollution are major issues that human need to overcome.Thus,to develop new sustainable energy becomes the urgent affair.Secondary lithium ion battery(LIB)as an environment-friendly battery has been brought into focus nowadays.Cathode material is an important component of the LIB.Therefore,it is of significance to improve the performance of cathode materials,in order to realize the LIB which is suitable for commericial production with high performance.With the commercialization of material as the ultimate goal,this thesis studies the preparation methods and modifications on layered lithium cobalt nickel manganese(NCM)cathode material of LIB.And then,many characterization methods are used to analyze the structure,morphology and electrochemical performance of NCM.The effects on the morphology,structure and performance of NCM precursors through different preparation methods have been studied.It was shown that the uniform spherical-like precursors could be obtained by co-precipitation method under carefully controlled conditions,including the stirring speed,feed speed,reaction p H and concentration of complexing agents.The cathode material,synthesized undergoing a calcine process with lithium,shown the capacity of 140.6mAh/g with the cut-off voltage of 4.3 V at 1C.And the capacity retention ratio reached 97.7% after 200 cycles.This method and these results could be used to guide the practical production.The study on the controlling conditions during the material’s calcining process indicated that it was beneficial to the electrochemical performance through rationally increasing the temperature and time at the low temperature pre-sintering stage.The NCM with pre-sintered at 600°C for 6h showed a better comprehensive electrochemical performance.Then,by controlling of heating rate,temperature and calcination time,the specific capacity of NCM can be increased to 157mAh/g with voltage range of 3.0~4.3V at 1C rate.Furthermore,the capacity retention ratio reached 92.9% after 100 cycles at 1C when the cut-off voltage was raised up to 4.6V.The study also indicated that the cycle performance of NCM improves more significantly when the mole ratio of Li2CO3 and Li OH amount lithium sources was 95:5: capacity retention ratio reached up to 92.9% even after long time cycles of 500 times with the voltage range of 3.0~4.3V at 1C.These benchmarking results might also guide the practical production.Using appropriate amount of metal and nonmetal elements to modify NCM during preparation could improve the performance.Furthermore,the synthesis method was facile and effective enough for large scale production.In order to investigate the influences of modification on industrial-grade cathode materials,layered Ti-doped Li(Ni0.5Co0.2Mn0.3)1-x Tix O2 cathode materials have been synthesized via a facile solid state method.The doping results confirms that the material with moderate Ti doping had a better cycle and rate perfomance because of its lower cation ordering as revealed by Rietveld refinement of XRD.Mo-modified NCM was synthesized successfully via a solvent evaporation method using the water-soluble ammonium molybdate followed with a calcination method.This strategy enables both Mo-doping and Mo-coating.Mo not only intercalates into the crystal lattice of NCM in order to facilitate Li-ions diffusion,but also formed a film-like coating of Li-Mo-O compound on the surface to impede side reactions between electrode and electrolyte.Thus,its specific capacity,rate capability and cycle performance were improved simultaneously,especially in terms of longer cycling life(500 cycles).And optimal 1.0 wt.% Mo modification was found.Besides,through adding moderate amount of Li F into Li2CO3,doping F-to the NCM improves the cycle performance at room temperature.In addition,study had shown that,using special sintening synthesis method to mix different crystalline type materials with polyethylene glycol,one could obtain materials mixed with both layered and spinel type for electrodes directly.Besides,the tap density could be improved by mixing powders with differently sized NCM,particles followed with a secondary sintering process.Also,the specific volumetric capacity of the mixed sample(weight ratio of 9 μm: 6 μm: 3 μm=7:2:1),with the tap density of 2.57 g/cm3,increased to 394.3 mAh/cm3 at 1C,which was 8.5 %,22.2 %and 40.6 % higher than any single sized particles of 9 μm,6 μm and 3 μm,respectively.This finding could contribute to the industrial production of Li-Ni-Co-Mn-O cathode materials for lithium ion batteries. |
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