担载纳米硅的锂-碳复合微球作为锂二次电池负极

金属锂由于其极高的理论比容量(3860mAh·g~(-1),2061mAh·cm~(-3))和低的还原电势(相对于标准氢电极(SHE)为-3.04 V)等特点,成为了高能量密度锂电池负极材料的极佳选择之一。从上个世纪七十年代开始,科研工作者便开始了金属锂负极的研究,然而,由于金属锂与电解液反应严重,镀锂过程体积膨胀大,且在循环中易生成枝晶,以金属锂为负极的电池循环稳定性差,而且容易短路从而带来安全隐患。因此金属锂做为锂电池负极的商业化推广最终没有成功。在本工作中,我们在前期设计的锂-碳纳米管复合微球(Li-CNT)中引入了纳米硅颗粒制备了硅颗粒担载的锂-碳复合球(LiCNT-Si)。实验发现,纳米硅颗粒的加入不仅提高了锂-碳复合微球的载锂量(10%(质量百分含量)的硅添加量使得比容量从2000 mAh·g~(-1)提高到2600 mAh·g~(-1)),降低了锂的沉积/溶解过电势,有利于引导锂离子回到复合微球内部沉积,大大提高了材料的循环稳定性。同时,担载了纳米硅颗粒的锂-碳复合球也继承了锂-碳复合微球循环过程中体积膨胀小,不长枝晶的优点。而且添加的纳米硅颗粒还填充了Li-CNT微球中的孔隙,减少了电解液渗入复合微球内部腐蚀里面的金属锂,进一步提高了材料的库仑效率。以添加10%硅的锂碳复合材料作为负极,与商用磷酸铁锂正极组成全电池,在常规酯类电解液中1C (0.7 mA·cm~(-2))条件下能稳定循环900圈以上,库仑效率为96.7%,大大高于同样条件下测得的Li-CNT复合材料(90.1%)和金属锂片(79.3%)的库仑效率。因此,这种通过简单的熔融浸渍法即可制备的,具有高的比容量和长的循环稳定性的锂硅-碳复合材料具有较大的潜能成为高能量密度电池的负极材料,尤其适用于锂硫、锂氧这种正极不含锂源的电池体系。

Silicon-loaded Lithium-Carbon Composite Microspheres as Lithium Secondary Battery Anodes

Lithium metal is the most promising anode material for Li (ion) batteries from the viewpoint of energy density because of its high theoretical specific capacity (3860 mAh∙g^-1, 2061 mAh∙cm⁻³) and low reduction potential (−3.04 V vs standard hydrogen electrode (SHE)). Lithium has been used as an anode material for lithium metal batteries since the 1970s. However because of the serious reaction between Li and non-aqueous electrolytes, the large volume expansion during Li plating, and the formation of Li dendrites during cycling, Li batteries with Li metal anodes show very low Coulombic efficiency (CE) and are easily short-circuited. This limits the widespread commercialization of Li metal anodes for Li batteries. Motivated by our previous study on the development of a Li carbon nanotube (Li-CNT) composite anode material, in this study, we prepared a Si-loaded Li carbon nanotube composite (Li-CNT-Si) via a facile molten impregnation method. The introduction of Si nanoparticles increased the Li content of the composite, thus increasing its specific capacity (the specific capacity of the Li-CNT composite increased from 2000 mAh∙g^-1 to 2600 mAh∙g^-1 with the addition of 10% Si (mass fraction)). Moreover, Si nanoparticles decreased the polarization for Li plating/stripping, resulting in an improved electrochemical performance. The Li-CNT-Si composite showed the merits of the Li-CNT composite with the advantages of limited electrode volume expansion and negligible Li dendrite formation during cycling. Furthermore, the Si nanoparticles filled the pores inside the Li-CNT microspheres, thus preventing the electrolyte from flowing into the microspheres to corrode the Li metal present inside them. Hence, the incorporation of Si nanoparticles improved the CE of the composite anode. When the 10% Si-loaded Li-CNT-Si composite was used as an anode and coupled with a commercial LiFePO₄ cathode, the resulting battery showed more than 900 stable cycles in an ether-based electrolyte at a charge/discharge rate of 1C (0.7 mA∙cm^-2) corresponding to a CE of 96.7%, which is considerably higher than those of the Li-CNT (90.1%) and Li metal foil (79.3%) anodes obtained under the same conditions. We believe that the Li-CNT-Si composite prepared in this study is a promising anode material for Li secondary batteries having high energy density, particularly for those employing Li-free cathodes, e.g., Li-sulfur and Li-oxygen batteries.