Layered Electrode Materials With High-Performance For Sodium-ion Batteries

With the development of global economy,how to obtain a high-performance energy storage device becomes an urgent problem in clean energy fields.Because of the similar working principle with lithium ion batteries,sodium ion batteries(SIBs)with low cost and nontoxicity have been regarded as an attractive alternative for large-scale energy storage.But the low specific capacity and instability of electrode materials remain formidable hurdles in developing SIBs.In this context,the layerd materials with different components are synthesized as electrodes to reveal the underlying bottlenecks and build better SIBs with high energy density.The main contents in this thesis are summarized as follows.(1)We systhesized a series of Na0.67Ni0.3Mn0.7xTixO2(x=0,0.1 and 0.2)cathode materials and the Ti4+ions were intentionally employed to substitute for Mn4+ in Na0.67Ni0.3Mn0.7O2.The original Na+/vacancy ordering would be disturbed in Na0.67Ni0.3Mn0.7O2 upon substitution to further manipulate their electrochemical performance.The electrochemical tests,CV and TEM-SAED characterizations were carried out to understand the structure and performance relationship.We found that the Na+/vacancy ordering is not only affected by the types of transition metal,but also determined by the contents of the substituted transition metal ions.As a consequence,the optimized Na0.67Ni0.3Mn0.6Ti0.1O2 electrode with Na+/vacancy disordering displays best rate capability(80 mAh/g at 1 C after 200 cycles)of all.(2)The glide of the MO6 slabs and large volume change can result in the complicated and irreversible phase transition upon Na extraction.Hence,in order to investigate the detailed structural evolution,STEM and EIS were employed to investigate the atomic-scale structural changes and surface stability at different charge and discharge states in the long cycle.The results showed that the Ti-substituted Na0.67Ni0.3Mn0.7 electrodes can transform to a series of reversible intermediate phases at high desodiation levels and the reasonable Ti substitution for transition metal ions could enhance the surfacial and structural stability of Na0.67Ni0.3Mn0.7O2 electrodes in during cycling.(3)The P2-type layered oxides usually had a lower capacity in full cells because of lower Na content and the consumption of Na in carbon anode and at the carbon-electrolyte interface(SEI formation)during the formation cycle.To increase the initial Coulombic efficiency,we designed a symmetric P2-Nao.67Co0.35Fe0.15Mn0.45O2 for full SIBs using its intrinsic two different redox voltages.As cathode,it delivered a working voltage of 2.8 V,about 120 mAh/g initial capacity and still above 100 mAh/g at 0.5 C after 100 cycles.As anode,its’ working voltage was about 1.3 V and the stable capacity was about 90 mAh/g at 0.1 C after 200 cycles.The galvanostatic intermittent titration technique(GITT)results showed the apparent Na+ion diffusion coefficient is about 10-10 cm2/s.The symmetric SIBs with Na0.67Co0.35Fe0.15Mn0.45O2 had ahout 70 mAh/g reversible capacity and 1.5 V working voltage.(4)Different with the half cells,the performance of the as-prepared full SIBs was affected by many factors,such as mass ratio of anode vs.cathode,types of electrolytes and additives.So we tested the symmetric SIBs with Na0.67Co0.35Fe0.15Mn0.45O2 as cathode and anode materials in diferent N/P ratios(N/P ratios=1.0,1.2,1.4,1.6,1.8)and different types of electrolytes(PC:EC(1:1)/NaClO4,PC(2%FEC)/NaClO4,PC(5%FEC)/NaPF6).We found that the symmetric full SIBs had the best electrochemical performance(initial capacity was 80 mAh/g and reversible capacity was 70 mAh/g at 0.1 C after 50 cycles)with the N/P ratios=1.6 in PC(5%FEC)/NaPF6 electrolyte.