| As a high-energy and high-efficiency electrochemical power source, lithium ion batteries (LIBs) are widely used in small appliances and some electric vehicles (EVs). The electrochemical performance of sodium ion batteries (NIBs) is similar to that of LIBs, but they are ignored to some extent due to their lower energy density. In recent years, the extension of battery technology to large-scale energy storage becomes necessary for two reasons:1) the technology development of intermittent renewable energy such as wind and solar energies becomes more prevalent and integrated into the electrical grid.2) The electric grid requires higher-efficiency and higher-quality power/energy. SIBs are attracting extensive attentions because of their abundant resources of electrode materials and significantly low cost. The key research issue for NIBs is to develop cathode and anode materials with excellent electrochemical performances. Compared with other electrode materials, the transition-metal oxide materials are attractive because of their species diversity, structure stability and good electrochemical performances. In this thesis, we focus on the structure and electrochemical performance of the transition-metal oxide materials with three types of structures:tunnel, P2 and 03. In addition, a series of original researches are carried out for new type Prussian blue analogues as electrode materials.Chapter 1 gives a general introduction of the history, working mechanism and characteristic of NIBs. Then a detailed review of cathode and anode materials for NIBs is presented, in which the cathode materials are divided into three types: transition-metal oxide materials, polyanionic compounds and Prussian blue analogues. The anode materials are introduced in terms of their three different reaction mechanisms:intercalation and de-intercalation reaction, conversion reaction and alloying reaction.In Chapter 2, the experimental reagents and equipments used in this thesis are listed. A detailed procedure to fabricate half-cells for lithium and sodium ion batteries is also presented. The instruments for structural and electrochemical analyses are also described.In Chapter 3, a tunnel-type material Na4Mn9O18 is synthesized by a solid-state reaction method with precursors of different morphologies. The precursor of large particle size is harmful for mixture, because it causes the presence of the impurity so that its electrochemical property becomes poor. The morphology of precursor also influences the crystal growth direction, which would cause different structures and morphologies of the products. In addition, a Ti-substituted material Na4Mn9-xTixO18 (x=0,1,3,5) is also investigated. With the gradual substitution of Mn by Ti, the tunnel structure does not change but the lattice parameters increase gradually. The rise of the charge transfer resistance (Rct) at the electrode/electrolyte interface is suppressed with the Ti substitution.In Chapter 4, based on the composition of NaxM’yTi1-yO2, two P2-type oxides, Na0.67Ni0.33Ti0.67O2 and Na0.67Li0.22Ti0.68O2 are designed and synthesized by a sol-gel method. They are investigated as possible anode electrodes for sodium-ion batteries. Na0.67N1033Ti0.67O2 can deliver a reversible capacity of 120 mAh g"1 in the voltage range of 0.4-2.5 following an intercalation/de-intercalation mechanism. An excellent cycling performance and rate capability are achieved owing to its zero-strain characteristics and high ionic conductivity (10-4 S cm-1 at room temperature). About 63% of the initial capacity can be retained at 40C rate. P2-Na0.67Li0.22Ti0.68O2 also exhibits excellent cycling performance and rate capability. In addition, the addition of fluoroethylene carbonate in the electrolyte is also found necessary to improve the cell performance.In Chapter 5, we develop two Ti-doped O3-type cathode materials, i.e. binary NaNi0.5Mn0.5O2 and ternary Na[Ni0.4Fe0.2Mn0.4]O2. Both display far better electrochemical properties than literature data. With the Mn substitution with Ti, the crystal structure does not change but the lattice parameters increases gradually, which is beneficial for Na-ion de-intercalation and insertion. The optimal compositions NaNi0.5Mn0.3Ti0.2O2 can deliver a high reversible capacity in the voltage range of 1.5-4.2V, while Na[Ni0.4Fe02Mn0.2Ti0.2]O2 can deliver a high reversible capacity of 145 mA h g’1 with a retention of 84% after 200 cycles.In Chapter 6, we first discover that the influence factors of the crystal structure of Nax MO2 are not only the sodium content, but also the nature of transition metals. For Nao.44Mn02, a small quantity of Co doping changes the structure from tunnel-type to P2-type. Their electrochemical properties are also different. In addition to Co element, other transition metal elements such as Ni, Mg, Fe, Al also can influence the structure.In Chapter 7, by using a liquid nitrogen quenching process during the synthesis procedure, we obtain a Nao.67[Ni0.4Co0.2Mn0.4]O2 powder composed of mainly P2 phase and slightly O3 phase. This P2-O3 ternary cathode material shows high capacity, excellent cycling and rate performances without the abnormality of initial coulombic efficiency.In Chapter 8, a novel air-stable titanium hexacyanoferrate (Ti0.75Fe0.25[Fe(CN)6]0.96·19H:0) with a cubic structure is synthesized simply by a solution precipitation method. It is first demonstrated to be a scalable, low-cost anode material for lithium and sodium ion batteries exhibiting high capacity, long cycle life and good rate capability with a conversion reaction mechanism. A hydrothermal treatment leads to better crystalline and higher capacity. We also synthesize a similar hexacyanoferrate(Fe43Fe(CN)6·zH2O) and a series of its ramifications, including Fe2O3, amorphous carbon and Fe2O3/C composite. All of those samples can store lithium with different reaction mechanisms.At last, a brief summary of the innovations and shortcomings of the work in this thesis are presented in Chapter 9. We also point out the possible direction and aims of the related research in the future. |
PDF Full Text Request