A Novel Surface Modified Nano-Silicon As Promising Anode Material Lithium-Ion Batteries

Silicon has been considered as one of the most promising anode materials for LIBs in order to replace the conventional anode material (graphite). Investigations of the Li-Si binary system indicated at least theoretically, the highest specific capacity (4200 mAh/g), during the electrochemical lithium alloying (Li_4.4Si) of silicon with lithium ions. Unfortunately, Si shows a very low mechanical elasticity during the battery cycling (lithiation/delithiation), which causes a contact loss in the electrode and subsequently lead to a shorter cycle life and finally fast capacity fading. This is the major drawback of Si as an anode material and the reason why a general distribution within the lithium ion battery market couldn´t be established up to now.

The first idea proposed to overcome this shortcoming, is a new approach concerning the binder materials. In the research of energy storage materials, a new technology named biomimetics has been adapted to breakthrough the serious technical problem. The use of bioinspired binders for the silicon anode materials represents one basic approach of our present study. Namely, potato starch, gelatine and alginic acid from brown algae were studied as a binder. It was found that the potato starch was the best binder, exhibiting 1500 mAh/g at the 20^thcycle. This is due to the fact that amylopectin polysaccharides suppress the volume change of silicon and form a beneficial SEI film on the electrode during cycling.

The second idea proposed in this work is based on the formation of a nanoscale, 3D-connected mesoporous silicon by a magnesiothermic reduction process, providing short diffusion pathways for the lithium ions during lithiation / delithiation. The silicon-based cells were characterized by electrochemical investigations and the first results show that the mesoporous silicon-based cell exhibits a discharge capacity of 1200 mAh/g and a coulombic efficiency of 84% after the cycling precondition with 0.05C. The improvement of the electrical conductivity and the modification of the surface´s electrochemical properties is achieved by an adaption of a coating method, known as substrate-induced coagulation. The coating materials are distinguished between lithium reactive and nonreactive. Further, they can be subdivided, i.e. carbonaceous materials (nano-graphite, graphene), nano metals (Cu, Ni) and nano metal oxides [TiO₂, In(OH)₃, ITO(indium tin oxide)].