生物质基硅纳米颗粒的可控制备和储锂性能研究

Silicon (Si) is a promising anode material for next generation lithium batteries (LIBs) because of its low Li-uptake potential and the large theoretical capacity of 4,200 mAh/g, which is ten times higher than that of commercial graphite anodes (372 mAh/g). However, the practical application of Si as an anode material generally suffers from serious drawbacks such as the intrinsic low electrical conductivity and extreme volume changes (> 300%) during Li alloying and dealloying processes, causing severe pulverization of active Si material and rapid capacity fade upon cycling. Reducing the size of Si to the nanoscale has been demonstrated to be a promising strategy to mitigate the physical strain and prevent pulverization of Si active materials during lithiation/delithiation process. Forming conductive coatings or hybrid structures are effective approaches to enhance the performance of silicon anode battery by improving the conductivity of Si anodes. However, the Si nanoparticles (NPs) produced by the existing methods such as chemical vapor deposition, laser ablation are usually complex, costly, and energy demanding and are also difficult to scale up to cater to mass production. Moreover, Si NPs prepared via current methods have a broad particle size distribution. Biomass, such as bamboo, maize and rice are Si accumulating plants which can act as the “natural factory” to absorb a vast quantity of Si from the soil in the form of water-soluble silicic acid and then convert into hydrate SiO2 NPs in the roots, branches and leaves by biomineralization synthesis. In this project, we will investigate the possibility to recycle waste biomass such as bamboo leaves, rick husks, maize leaves as sustainable resources to produce ultrafine Si NPs by thermally decomposing the organic matter, followed by magnesiothermic reduction. The effect of element Si distribution and structures in the biomass on the size, surface, pore volume and size of the as-produced SiO2 NPs will be revealed. A molten salt assisted magnesiothermic reduction process will be developed to produce Si NPs from biomass-derived SiO2 NPs. The effect of different molted salt on the final morphology, surface area, pore size and size will be disclosed. To achieve a better rate capability, the biomass-derived ultrafine Si NPs are further converted into carbon coated Si and reduced graphene oxide (RGO) nanocomposites by the layer-by-layer assembly method. The double protection rendered by the carbon shell and RGO minimizes volume changes, avoids direct contact between Si and electrolyte, and yields high electrical conductivity. This project describes a simple, low cost, and scalable approach for ultrafine Si NPs production enabling the use of waste biomass as sustainable sources towards high-performance Si-based anodes for LIBs.

硅具有高的理论储锂容量，是下一代锂电池理想的负极材料之一，但是硅材料脱嵌锂体积膨胀大、电导率低和循环性能差等问题，限制了其广泛应用。硅纳米化和复合化是提高硅负极电池性能的有效途径。针对纳米硅合成成本高、形貌和结构可控性差的问题，本项目以“废弃生物质”为原料，利用生物质中生物矿化形成的纳米氧化硅水合物，低成本、可控制备氧化硅纳米颗粒。研究稻壳、玉米叶和竹叶等生物质中硅的分布情况和聚集形态对氧化硅纳米颗粒形貌、表面积和孔容的影响。在熔盐介质中，利用镁热还原反应，将生物质产生的氧化硅纳米颗粒转化为硅纳米颗粒。研究熔盐介质对镁热还原反应过程以及纳米硅形貌和结构的影响。制备碳包硅/石墨烯复合材料，利用石墨烯和碳层的协同效应提高硅的导电性和循环稳定性，促进 “生物废弃物”在锂离子电池领域中的高附加值应用。

硅（Si）的理论储锂容量高达4200 mAh/g，有望替代石墨类碳材料成为下一代锂离子电池负极材料，然而 Si负极脱嵌锂过程中体积变化大（420 %），致使Si活性材料粉化，循环性能差。纳米Si材料可缓解脱嵌锂过程中产生的应力，改善Si材料的循环稳定性。然而现有的纳米硅制备方法存在成本高和工艺复杂等问题。低成本、大规模制备纳米硅，对开发高性能硅/碳负极材料和促进高容量的锂离子电池发展具有重要的意义。. 本项目围绕生物质基纳米二氧化硅和纳米硅的可控制备和储锂性能开展研究。研究不同种类的含硅生物质（稻壳、竹叶和玉米叶）和不同产地稻壳制备的二氧化硅在产率和形貌的差异。探究了生物质空气热处理条件（温度和升温速度等）对二氧化硅形貌的影响规律。通过砂磨处理能够进一步减小纳米二氧化硅的粒度和改善其均匀性，研发粒径小于50 nm的二氧化硅纳米颗粒可批量化制备工艺。生物质基纳米二氧化硅经过镁热反应能转化为硅纳米，纳米硅的尺寸主要依赖于二氧化硅的尺寸，二氧化硅的尺寸越小，获得的纳米硅尺寸也越小。进一步认识了硅纳米颗粒尺寸对纳米硅储锂性能的影响，并制备了纳米硅/碳复材料改善纳米硅的电化学性能。不同粒径的纳米硅的电化学性能分析表明，颗粒尺寸适中时（50-100 nm）纳米硅的储锂性能较优。采用葡萄糖水热包裹和后续炭化方式将纳米硅“焊接”在碳纳米管上构建三维分级硅/碳复合材料有效改善了纳米硅的电化学性能。上述研究为纳米硅的低成本和大规模制备提供了理论依据，有利于加速高容量硅/碳负极在高比能锂电池领域的应用，推动高比能电池和电动汽车等产业的发展。另外，发展了镁热制备纳米碳化硅和多孔微米硅的路线，为后续的研究工作提供了新的思路。除此之外，我们也设计了一些碳、金属氧化物和合金的负极材料，对发展高性能负极材料有一定参考价值。