Polymer-mediated Growth Of Nano-sized Lithium-rich Ternary Cathode Materials By Spray Drying-calcination

With the gradual improvement of people’s daily life, Mobile p Hones, tablet PCs, digital cameras and other intelligent consumer electronics products are becoming popular. However, the current batteries are not able to match the requirement of these devices due to their low energy density and limited operation time. The development of high energy density, long cycle life of lithium ion batteries is extremely urgent. Due to its ultra high capacity（>250m Ah/g）, excellent thermal stability and environmental benign, lithium-rich manganese-based cathode materials x Li2 Mn O3?（1-x）Li MO2（M=Ni, Co, Mn） have attracted extensive research attention. However, the some shortages, including high initial irreversible capacity, poor capability at high discharging rate, and poor cycling stability（capacity and operating voltage fade） etc, hindered its practically application of this type of material in the LIBs. It is becoming one of the most important topics in the field to overcome theses drawbacks.In this thesis, we attempted to prepare this material with a simple and efficient spray drying-calcination approach with assistance of polymer. The effects of the composition, the usage of polymer, co-precipitation time, spray drying conditions, calcination temperature and other synthesis conditions on the performance of cathode material have been investigated systematically. Furthermore, we tried to improve the rate capability and cycling stability of the material by doping with other elements and covering with some materials.First, we investigated the effects of Li content on the electrochemical performance of Lix Ni0.15Co0.15Mn0.7O2（x=1.1-1.8） through making the content of Ni, Co and Mn fixed. The results showed that material Li_1.4Ni_0.15Co_0.15Mn_0.7O₂ exhibited the highest capacity and better capacity retention. The first cycle discharge capacity is 134 m Ah/g at 1C rate. The capacity increased in the following cycles, and reached maximum（202m Ah/g） at 16 th cycles. After 100 charging-discharging cycles, its capacity retention could be up to 127%. After that, we investigated the effects of the proportions of nickel, cobalt and manganese in the materials on the electrochemical performance of Li1.4Nix Coy Mn1-x-y O2, it turned out that Li_1.4Ni_0.15Co_0.15Mn_0.7O₂ and Li1.4Ni0.2Co0.15Mn0.7O2 had the highest capacity and superior capacity retention. The first cycle discharge capacity of Li1.4Ni0.2Co0.15Mn0.7O2 is up to 206 m Ah/g, its capacity retention at 100 cycles at 1C rate could be 74%. Through analyzing the discharge curves of different cycles, we found the voltage fade of Li1.4Ni0.2Co0.15Mn0.7O2 is more serious. Meanwhile, the average discharge plateau retention of Li_1.4Ni_0.15Co_0.15Mn_0.7O₂ for 100 cycles at 1C rate is 86%, which is much higher than 74% of Li1.4Ni0.2Co0.15Mn0.7O2.Secondly, the effects of the synthesis conditions, including polymer variety and adding amount, p H value of the dispersion system, adding order of the chemicals, the synthesis reaction time, calcination temperature on the structure and performances of the materials have been investigated carefully. It was found that the addition of guar gum, carboxymethocel and hydroxyethyl cellulose（HEC） will result in the good dispersion and suspension of the system, thus resulting in enhanced capacity retention. SEM images demonstrated that the addition of 10 wt% HEC could reduce the agglomeration of precursor particles remarkably. It is revealed that the addition of HEC could increase the BET surface area, reduce Li+ diffusion resistance and charge transfer resistance, improving rate capability, cycling stability of the material. HEC-LNCM discharge capacity is high up to 250 m Ah/g and the capacity retention for 50 cycles increased from 53% to 92.7% at 0.1C rate. After 168 cycles at 0.5C, HEC-LNCM can deliver discharge capacity of 201.9m Ah/g, which is almost twice of that of the material without HEC addition（113.3m Ah/g）. As to higher rate 1C and 2C, withing the usage of HEC, the discharge capacity increased from 159 to 210 m Ah/g（100cycles） for 1C and 146 to 174 m Ah/g for 2C（50cycles）.Finally, we investigated the effects of element doping（Al, Na, K, Mg, Y） and Li4Mn5O12 coating on the performance of HEC-LNCM. The electrochemical tests showed that the doping with Na or Y could improve the cycling stability of the material. The improved performance of HEC-LNCM with Na and Y doping may be attributed to enhancement of structure stability. Li1.4Ni0.15Co0.15Mn0.70O2 first cycle discharge capacity is 220 m Ah/g at 0.5C rate and the capacity retention for 100 cycles is 79.5%. The first cycle discharge capacity of Li1.395Na0.005Ni0.15Co0.15Mn0.70O2 is 215 m Ah/g at 0.5C rate and the capacity retention for 100 cycles is 99.5%. Li4Mn5O12 coating can also improve cycling stability and maintain high capacity. Li4Mn5O12 can act as a protective layer protecting LNCM from electrolytes erosion and suppression the dissolution of transition metal ions, which are associated with the improved performance with Li4Mn5O12 coating. For coated material, its first cycle discharge capacity is up to 136 m Ah/g at 0.5C rate, and the capacity increased in the following cycles, reached to 200 m Ah/g at 10 th cycles, and the capacity retention is up to 140% at 100 th cycles. However, for uncoated material, its capacity retention at 100 cycles is only 80.3% although its initial capacity is up to 198 m Ah/g at 0.5C.