چكيده به لاتين
In this work, the effect of fluorine element on two important materials as electrodes for lithium and sodium ion batteries was studied and investigated. In the continuation of the effect, the fluorine element alone was studied as an active material. The properties and theoretical behavior of materials were investigated using density functional theory (DFT). The first category are layered metal oxide materials with the formula Li/Na M O2 (M is one or more transition metals), which are among the first commercialized electrodes and have the property of low capacity but high rate-capability. The second category of silicate materials with the chemical formula Li2MSiO4 (M one or two transition metals) which have high capacity but low rate-capability were studied. Three ranges of combinations of active matyerials were considered for the study, which include the primary raw material, alloying and doping. At first, to compare the effect of alloying and doping, raw oxide and silicate active materials were calculated. Then the calculations of alloying and doping effect of fluorine were studied in them. After calculations, physical properties such as structural stability, ion stablishments, interatomic forces, electrical and magnetic properties were extracted, analyzed and evaluated. In the following, the electrochemical properties of materials such as capacity, cyclability, voltage and rate capability were investigated and analyzed.
One of the existing challenges for the use of electrodes in rechargeable batteries is the relatively low structural stability, cyclability and applicable current rate. In this research, using electron transport, charge distribution and density of states (DOS) obtained from the calculations performed using DFT, a quantitative method was presented to understand the rate acceptability of these materials. This comparison was presented between all known cathodes. This theory can justify the behavior observed in this study and other extensive studies. Based on this theory, four approaches were proposed to investigate the rate-capability of these materials, which are (a) calculating the electronic transfer barriers, (b) considering the state of the inserted and the extracted ion of the particles as a semiconductor junction and calculating the resulting difference from the valence and conduction bands, (c) considering the bands resulting from the 3d orbitals of transition metals and their positive effect on electron transfer and (d) the explicit calculation of electron conduction and its investigation in the semiconductor junction.
