| The coming decades will see an explosive demand for rechargeable lithium ion batteries (LIBs) due to the enormous usages of green renewable energy (energy storage) and the development of various electric vehicles (xEVs, energy consumption). However, the current commercial graphite anode cannot satisfy the growing needs for future LIBs because of its low theoretical capacity (372 mAh g-1) and poor rate capability. As a result, it is highly urgent to develop high performance anode materials. As one of the most promising alternative anodes, SnO2 has attracted much attention owing to its high capacity (1482 mAh g-1), safe Li-uptake potential and good processability. Unfortunately, SnO2 suffers from serious electrode degradation and quick capacity fading, which is caused by the large volume change. Although massive efforts have been undertaken to solve the above problem and some progress has been made, the results are still unsatisfying. The reason is that the mechanism of cycling performance degeneration is still unclear, as a result, the improvement of electrochemical performance is limited. Therefore, the thesis focuses on the mechanism of structure degeneration and a systematic research work has been carried out.Chapter 1 includes a general introduction about the research background and working principle of LIBs. After making a generalized summarization for the anode materials (insertion, conversion and alloying), a particular summary is provided on the research status of SnO2 based materials of LIBs.In chapter 2, the experimental reagents, equipment and methods used in this dissertation are introduced, followed by a detailed description of the material characterization methods (e.g. in-situ TEM) and electrochemical technology for LIBs.In chapter 3, we fabricated a rational design of 3D-staggered metal-oxide nanocomposite electrode (3D SnO2-MxOy) to avoid the aggregation and pulverization of SnO2. In this nanocomposite, various oxide nanocomponents are in a staggered distribution uniformly along three dimensions and across the whole electrode. The 3D-staggered SnO2-Fe2O3 nanocomposite delivers an initial specific discharge capacity of 1626.8 mAh g-1, and still maintains 1369.5 mAh g-1 after 50 stable cycles. More impressively, as high as 824.2 mAh g-1 can be retained even at a large current density of 8 A g-1. Additionally, the 3D-staggered SnO2-Co3O4 and SnO2-NiO composites with the similar structure also show good electrochemical performance.In chapter 4, from the point of atom migration and structure integrity view, we propose a novel approach of spatially-confined electrochemical reactions to enhance cycling stability. A highly dense SnO2-Fe2O3-Li2O (59.0:36.6:4.4 of weight ratio) nanocomposite anode, composed of multi-oxide nanoclusters uniformly and alternately, was developed to further improve the cycling performance of SnO2 based materials. The dense SnO2-Fe2O3-Li2O (59.0:36.6:4.4) nanocomposite anode delivers an initial volumetric discharge capacity of 6984.9 mAh cm-3 (1396.8 mAh g-1 for gravimetric capacity), and maintains 6034.5 mAh cm-3 (1206.9 mAh g-1,86.4% of the first discharge capacity) after 200 cycles, which is the highest volumetric capacity value reported so far. It’s worth noting that it is the first time to simultaneously achieve the high volumetric capacity and superior cycling performance. Equally impressively, as high as 4704.0 mAh cm-3 (940.8 mAh g-1) is retained even cycling at an ultra-large current density of 20 A g-1. With the support of the advanced spherical aberration correction TEM and in-situ TEM, it’s confirmed that spatially-confined lithiation/delithiation reactions are achieved, which improves the structure stability,In chapter 5, the Ni/SnO2 nanoclusters were first fabricated to obtain enhanced reaction dynamics and improve the reversibility of conversion reaction. The Ni/SnO2 nanocomposite clusters possess a high reversible capacity (820.5 mAh g-1) at a large current density of 1 A g-1 for more than 100 cycles. More impressively, large capacity of 841.9,806.6 and 770.7 mAh g-1 can still be maintained at high current densities of 2,5 and 10 A g-1 respectively, which corresponding to 96.8%,92.7% and 88.6% capacity retention relative to capacity at 1 A g-1. The high rate performance can be attributed to the enhanced kinetics of conversion reaction.In chapter 6, a SnO2-based nanocomposite (SnO2-Fe2O3-Li2O (66.7:28.9:4.4)) with ultra-high rate performance, super-long cycling performance and extremely high Coulombic efficiency was fabricated by special designing the microstructure of nanocomposite and tailoring the voltage window. In the voltage range of 0.005-1.2 V, the as resulted Sn (SnO) superfine nanoparticles are spatially confined by the surrounding Fe and Li2O, avoiding the migration of atom, In the meantime, the in-situ generated Fe establishes a well-organized conductive network, while the Li2O prevent the direct contact between active materials and electrolyte. The SnO2 based nanocomposite possesses a high reversible capacity (>420 mAh g-1) at a large current density of 1 A g-1 for more than 1200 cycles, corresponding only 0.016%/cycle of capacity decay. Large capacity of 350 mAh g-1 can still be maintained at a super-high current density of 80 A g-1, which corresponding to 67.3% capacity retention relative to the capacity at 1 A g-1. The Coulombic efficiencies of all the samples are nearly to 100%. Furthermore, the cycling performance of the full cell demonstrates that SnO2-based nanocomposite shows great application prospect.Finally, in chapter 7, an overview and the deficiency of the dissertation are summarized. Some prospects and suggestions on the possible future research are presented. |
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