Electrochemical characterization of Si composite anode with hard TiFeSi2 matrix for lithium secondary batteries

Abstract

As industries develop, various devices consuming larger energy appeared. The environmental pollution and the exhaustion of existing fossil fuels became more serious. Therefore alternative energy sources with higher efficiency and environmental friendliness are needed to be invented, and researches about lithium secondary batteries which are widely adopted to mobile devices have been proceeded actively for the last few decades. Among the component of lithium secondary batteries, the anode material is the least developed one and the carbon electrode which is used in commercial batteries has a gravimetric capacity of 372 mAh, far insufficient value to meet the need of fast-growing energy consumption and the miniaturization of devices. Si receives a lot of intensions as an alternative, and it has the highest gravimetric capacity (3600 mAh) among the candidates for anode material. However, when Si forms alloy with Li, an extremely large volume expansion of Si occurs and the pulverization of the electrode due to the stress by the expansion makes the ap-plication of Si to the electrode impossible. To solve this, the active-inactive system was invented and the inactive matrix means materials that can endure the stress generated from the volume expansion of Si without forming any Li-silicide phases. In this thesis, a hard material, the Si-TiFeSi2 composite was synthesized by rapid cooling method to apply TiFeSi2 as the inactive matrix. The microstructure of the composite was analyzed with XRD, SEM, TEM, and confirmed that nano-sized Si particles are uniformly dispersed within the TiFeSi2 matrix phase. It is also confirmed that Si is transformed into amorphous and the matrix phase remained with no change after the first cycle by charge-discharge test and ex-situ analyses. The results from FE-SEM and EDS line scanning analyses showed nano-cracks around Si particles revealing that the TiFeSi2 matrix successfully endured the stress by the volume expansion and made the Si-TiFeSi2 electrode to retain a high reversible capacity over 50 cycles.