Synthesis And Modification Of ZnM₂O₄（M=Co, Fe, Mn） As Anode Materials For Lithium-ion Batteries

Compared with tradional graphite anode material for lithium-ion batteries(LIBs), transition metal oxides have attracted significant interests due to their high theoretical specific capacity, abundance and safety. It is known that cobalt oxides have better anodic performance than the other transition metal oxides. Unfortunately, inherent drawbacks such as expensive cost and toxicity limit the straightforward application of cobalt oxides. An effective way to resolve this problem is to replace one or all Co atom of cobalt oxide with cheaper and environmentally friendly metals, such as Zn, Ni, Fe and Mn. Moreover, owing to the synergic effects, these ternary transition metal oxides usually exhibit better reversible capacity. However, ZnM2O4 suffers from low conductivity, great volume change and capacity fading during operation. Aiming at overcoming these disadvantages of ZnM2O4, we attempted to improve the electrochemical performance of ZnM2O4 based on synthesis and modification. Main results in this thesis are shown as follows:(1) We propose a novel method to synthesize mesoporous ZnCo2O4/PPy by reflux and chemical polymerization method. The ZnCo2O4/PPy shows much better properties compared to ZnCo2O4. The discharge capacity of ZnCo2O4/PPy is 615 mAh·g–1 after 100 cycles at a current density of 0.1 A·g–1. When the current density increases to 0.2 A·g–1, the discharge capacity still retains 458 mAh·g–1 after 100 cycles, which is better than the discharge capacity of ZnCo2O4(289 mAh·g–1) and the commercial graphite(372 mAh·g–1). The enhanced electrochemical performance of ZnCo2O4 can be attributed to the PPy layer, which works as a conductive agent and buffer.(2) We develop a refluxing method to fabricate Ni doped ZnCo2O4. The doped Ni could improve the electrochemical performance of ZnxNi1-xCo2O4 due to the excellent rate performance of Ni-based oxides. Owing to the synergic effects of Zn, Ni and Co, the as-prepared Ni doped ZnCo2O4 anode exhibits significantly improved electrochemical properties. Among all the samples with different Ni content, Zn0.25Ni0.75Co2O4 exhibits best electrochemical properties, showing a reversible capacity of 1558.7 mAh·g–1 at a current density of 100 mA·g–1 after 30 charge-discharge cycles. It also maintains a stable rate capacity of 1056.2 mAh·g–1 even when the current density is as high as 2000 mA·g–1.(3) We firstly propose a novel strategy to fabricate three kinds of ZnFe2O4 with different exposed facets using NH4 F as structure-directing agent through a convenient hydrothermal method. It is found that the morphology of product is strongly affected by the hydrothermal time. According to the comparison of the three types of Zn Fe2O4, their electrochemical performance can be ranked as ―octahedra > truncated octahedra > cube‖. The results show that the crystal plane structure of electrode materials has great influence on the electrochemical performance.(4) We design a novel co-precipitation method to fabricate mesoporous ZnFe2O4 nanorods, in which H2C2O4 is used as precipitation agent. H2C2O4 could eliminate the solubility product difference of Zn2+ and Fe2+, which guarantees the co-precipitation of zinc oxalates and iron oxalates to form the precursor. When ZnCl2 and FeSO4·7H2O aqueous solution is injected into the H2C2O4 ethanol solution with a high concentration, FeSO4 reacts with H2C2O4 and thus forming a linear structure. Encouragingly, the as-synthesized ZnFe2O4 exhibits a good electrochemical performance as an anode material for LIBs, as it combines the advantages of mesoporous and nanorod-based nanostructure. A good Li ion storage performance for mesoporous ZnFe2O4 in LIBs is achieved, which remains 983 mAh·g–1 over 50 cycles at a current density of 100 mA·g–1.(5) We design a method to prepare mesoporous ZnMn2O4 microspheres with small size via controlling the viscosity of mixed solution. When the viscosity of mixed solution is suitable, the synthesized tiny particles could reunite to form mesoporous microspheres. When the addition of water is 15 ml during our expenriment, the viscosity of mixed solution is suitable to form mesoporous ZnMn2O4 microspheres. When evaluated as anode materials for LIBs, the mesoporous microspheres show superior cycling stability and rate capability. Even after 30 cycles, the reversible discharge capacity can still be remained at 1079 mAh·g–1 with a current density of 100 mA·g–1. When the current density is increased to 500 mA·g–1, the reversible discharge capacity still be remained at 748 mAh·g–1 after 80 cycles with a noticeable coulombic efficiency(97%).(6) We fabricate hierarchical structured ZnMn2O4 by using C4H4O6 KNa as structure-directing agent. Through controlling the content of C4H4O6 KNa and solvothermal time, we disscuss the possible formation mechanism. It is found that the hierarchical structured ZnMn2O4 could be prepared with 1 mmol of C4H4O6 KNa and 12 h of solvothermal treatment. We also disscuss the influence of calcined temperature on the electrical performance of hierarchical structure ZnMn2O4. When the calcined temperature is 350 oC, the example shows better cycle and rate performance.