FIRST-PRINCIPLES STUDY ANALYSIS OF BATTERY PERFORMANCE ON BORON AND FLUORINE CO-DOPED LINI0.815CO0.15AL0.035O2 (NCA) CATHODE MATERIAL

• Main Author: Hendra Widyadharma, Putu
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/55076
• Institution: Institut Teknologi Bandung

LiNi1-x-yCoxAlyO2 (NCA) is the superior cathode battery candidate to be applied in any sector because of its major advantages likes high average operation voltage (3.7 V), high theoretic specific capacity (279 mAh g-1) that has been proven to have 200 mAh g-1 capacity in actual condition, ability to work in the temperature up to 60ºC, and high cyclability up to 2500 cycles. But, the high nickel content in NCA induced several problems, such as high cation mixing probability, low capacity retention, and low thermal stability that leads to safety problems. It inhibits the use of NCA in the wider applications. Doping is then used to modified the NCA structure to overcoming the problem. It is proven that doping modification could prevent cation mixing, reduce the crystal lattice distortion, prevent microcracks and decrease the oxygen evolution. Even more, doping could improve the battery performance likes increasing battery working voltage and capacity retention. Co-doping modification is actually become the promising doping method to have synergetic effect of cation and anion doping to more improve both the battery performance and capacity retention. Even though the partial doping of boron and fluorine has been reported to improve the performance of NCA battery, co-doping boron and fluorine to NCA is ever done yet. Hence, this study is focused on analyzing the battery performance of NCA cathode material that is co-doped by boron and fluorine. Battery performance analysis is performed using first principles study based on density functional theory (DFT) with Vienna Ab Initio Simulation Package (VASP) program. Hubbard correction is brought to compensate the strong correlated electron in nickel and cobalt atom. Moreover, nonlocal van der Waals correction with code optB86b and rev-vdW-DF2 is used to accommodate the van der Waals interaction of the material, especially under delithiation states. The maximum error of 1,22% in lattice parameter a and 0,56% in lattice parameter c has come as the result that is in agreement with experimental value. Furthermore, the inclusion of van der Waals correction could produce the good prediction of the crystal lattice revolution during delitiation. The average operating voltage calculation using Hubbard and van der Waals optB86b of NCA has the result of 3,69 V that is in agreement with experiment average voltage of 3,7 V on NCA cathode. This leads to conclusion that the van der Waals functional with code optB86b provide the best result that fit the experiment value. Electronic structure of NCA is calculated using PBE functional to provide better data that consistent with experimental data. In average, NCA hold the band gap energy of 0,41 eV that is in accordance with band gap energy of LiNiO2 in the same family structure. It is happened probably due to the same of high nickel content which are owned by both. Hereafter, the changes in projected density of states (PDOS) of transition metal ions indicates the different electronic structure between the same metal ions. It is also indicating the change in electronic structure during the delithiation process in NCA. Using both the revolution of PDOS and magnetic moment of each transition metal atoms, it concludes the oxidation mechanism of the transition metal ions during delithiation process. The oxidation is firstly observed in Ni3+ and Ni2+ion to become Ni4+. Later, in lower that 34% of lithium in the structure, the delithiation process followed by the Co3+ oxidation to Co4+. This phenomenon is confirmly observed in experiment. Dopant sites optimization is done to discover the correct boron doping condition in NCA structure. Using substitution energy data of boron in nickel and cobalt site, it shows that the boron doping in the structure could only become spontaneous in the nickel site substitution. Boron tends to occupy the nickel sites with substitution energy of -1,62 eV compared with the cobalt sites that has substitution energy of 0,51 eV. Doping 3,5% of boron and 4% of fluorine are performed on standard NCA materials. As a result, emerge of Ni2+ions in standard NCA structure can be avoided as shown by the decrease in the number of Ni2+ ion from 9% to 0%. An increase in the crystal lattice parameter c of the NCA structure also observed consistently up to 0,13 Å in all delithiation states that indicating a possible increase in the rate of lithium-ion diffusion. In addition, the decrease of band gap value from 0,41 eV in standard NCA to 0,20 eV in boron and fluorine co-doped NCA also indicated an increase in electron diffusivity of the NCA material. These results shows the role of co-doping boron and fluorine in NCA materials to improve structural stability and potentially provide better battery performance compared with standard NCA.