All silicon lithium-ion batteries

Sammanfattning: Lithium-ion batteries have been widely used as power supplies for portable electronic devices due to their higher gravimetric and volumetric energy densities compared to other electrochemical energy storage technologies, such as lead-acid, Ni-Cd and Ni-MH batteries. Developing a novel battery chemistry, ‘‘all silicon lithium-ion batteries’’, using lithium iron silicate as the cathode and silicon as the anode, is the primary aim of this Ph.D project. This licentiate thesis is focused on improving the performance of the silicon anode via optimization of electrolyte composition and electrode formulation. Fluoroethylene carbonate (FEC) was investigated as an electrolyte additive for silicon composite electrodes, and both the capacity retention as well as coulombic efficiency were significantly improved by introducing 10 wt% FEC into the LP40 electrolyte. This is due to the formation of a stable SEI, which mainly consisted of FEC decomposition products of LiF, -CHFOCO2-, etc. The chemical composition of the SEI was identified by synchrotron radiation based photoelectron spectroscopy. This conformal SEI prevented formation of large amounts of cracks and continues electrolyte decomposition on the silicon electrode. An alternative lithium salt, lithium 4,5-dicyano-2-trifluoromethanoimidazole (LiTDI), was studied with the silicon electrode in this thesis. The SEI formation led to a rather low 1st cycle coulombic efficiency of 44.4%, and the SEI layer was found to contain hydrocarbon, ether-type and carbonate-type species. Different to conventional composite silicon electrodes, which require heavy and expensive copper current collector, a flexible silicon electrode, consisted of only silicon nanopowder, Cladophora nanocellulose and carbon nanotube, was facilely prepared via vacuum filtration. The electrode showed good mechanical, long-term cycling as well as rate capability performance.