| Rechargeable sodium-ion batteries (SIBs) have long been considered as an ideal candidate for large-scale electric energy storage (EES) applications, because of their low cost and wide availability of sodium resources. However, a severe difficulty in this technology development is to find suitable host materials for larger sodium ion. Although layered transition-metal oxides have certain Na storage capability, their electrochemistry performance is far from battery applications. Therefore, creating stable host frameworks with large Na storage capacity is a hot topic in the energy storage technologies. This thesis was aimed at exploring high capacity and cycle-stable cathodes based on layered transition-metal oxides and optimizing their sodium storage performances by compositional adjustment and bulk-doping in the layered lattices. Also, the sodium storage mechanism and phase transformations of these layered compounds were evaluated. The main results and conclusions are as follows:1. To solve the problems of stepped discharge, low capacity and poor cycleability, elemental substitution was used to construct stable layered structures with abundant sodium inserting sites and accordingly the reaction mechanism was also revealed. The experimental results demonstrate that Co, Fe, Sn-substitution is favorable for structural stabilization of the 03-type oxides and for enhancing the sodium storage performance. Though parallel analysis and compositional optimization, we obtained three compounds with best electrochemical performances:O3-NaCo0.1Nio.4Mn0.5O2 can deliver an initial capacity of 136 mAh g-1 with 93% capacity retention over 100 cycles; O3-NaNi0.5Mn0.4Sn0.1O2 gave an initial capacity of 117 mAh g-1 with a capacity retention of 92.3% over 50 cycles; O3-NaFe0.2Mno.4Ni0.4O2 delivered an initial capacity of 131 mAh g-1 with a 95% capacity retention after 30 cycles. The mechanistic study of elemental substitution revealed that Co-substitution exacerbated the gliding of transition-metal slabs (MO2), while Sn, Fe-substitution suppressed the change of Na/vacancy ordering. In addtion, Fe-substitution can change the sodium intercaltion mechanisum (P3-P3" change to P3-OP2 conversion) and enhance sodium storage kinetics.2. To smooth the discharge profiles and enhance the high voltage cycleability of P2-type oxides, Co/Al substitution was used to stabilize the layered structure of the P2-phase and to improve the cycling performance. The experimental results show that P2-Na0.67tMn0.65Ni0.15Co0.2JO2 delivered a reversible capacity of>140 mAh g-1 with smoothed discharge plateau and a considerably improved cycleability of an 88% capacity retention over 50 cycles. Al-doping further improves structural stability, P2-Nao.67[Mno.65Ni0.15Co0.15Al0.05]02 gave a reversible capacity of 129 mAh g-1 and a 95.4% capacity retention after 50 cycles. Structural evolution characterization revealed that the enhanced cycleability was resulted from the improved reversibility of P2-O2 transformation in high voltage region after Co/Al-doping. For Fe-Mn-based oxides, Ni-substitution is an effective method for stabilizing the sodium storage performance. The experimental results show that Nao.67Nio.15Feo.2Mn0.65O2 cathode with Ni replacement for Fe can delivered not only a reversible capacity of 208 mAh g-1, but also gave a 71% capacity retention over 50 cycles and a capacity of 119 mAh g-1 even at 8C rate. Furthermore, the structural evolution was found to go through a reversible P2-OP4 conversion. Similar to the O3-phased layered oxides, the P2-Na0.74Co02 cathode also exhibits a stepped discharge profile due to multi-phase conversions. It was found that LiMn-cosubstitution can greatly suppress the phase conversion and enhance the rate capability of P2-Nao.74Co02 cathode. The experimental results demonstrated that Nao.74[(Li1/3Mn2/3)1/3Co2/3]02 can realize a capacity of up to 80 mAh g-1 even at 10 C (2400 mA g-1) rate, corresponding to 69% of initial capacity. In addition, Li1/3Mn2/3-cosubstitution also obviously stabilize the layered structure. The working mechanism of Li doping was evidenced to stabilize the valence of Mn4+, thus keeping the layered framework stable in repeated cycles. In addition, Li1/3Mn2/3-cosubstitution might induce the electrochemical reaction of O2-ion, leading to an additional sodium storage capacity of the layered oxides.3. A novel families of A2M2XO6 and A3M2XO6 (A=Na, Li; M=+2 cations; X= Sb, Bi, Nb, Ru, etc.) were proposed in this thesis, which possesses honeycomb ordering structure and favorable sodium insertion properties. In this honeycomb ordering structure, M as an active center favors sodium intercalation while X as an inert component stabilizes the crystal structure. Furthermore, the honeycomb ordering of M/X render X stabilizes crystal structure uniformly. As an example, Na3Ni2SbO6 gave a reversible capacity of 117 mAh g-1 and exhibited excellent cycleability and rate capability with 70% of its reversible capacity remained over 500 cycles at a high rate of 2 C and the reversible capacity of 90 mAh g-1 delivered even at high rate of 30 C (6000 mA g-1). The mechanistic study demonstrated that Ni2+/Ni3+ are electrochemically active while Sb remains inert during the Na insertion/extraction processes, and the honeycomb ordering uniformly buffer the structural distortion. It was also found that the structural evolution of Na3Ni2SbO6 layered compound occurred through the reversible O3-P3-O1 three-phase transformations in cycling and Mg-substitution can suppress the P3-O1 conversion and changes the reaction route with 03-P3 two-phase conversion. Mg-substituted Na3Ni1.5Mg0.5Sb06 cathode gave an initial capacity of 116 mAh g-1 with>92.2% capacity retention over 100 cycles, which is obviously superior to unsubstituted cathode. We believe that our results obtained in this thesis may offer a new avenue to develop advanced cathode materials. |
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