Designing new electrode materials for energy devices by integrating ab initio computations with experiments

Abstract

Utilization of Ni2+/Ni4+ double redox couple in electrochemical reactions has been tested as a way to gauge useful properties such as high capacity in electrode materials. The feasibility of a Ni2+/Ni4+ active redox couple is confirmed in a new layered electrode material, Lio.gNi045Ti5502. First principles calculations combined with experiments show that the degree of cation disordering in the material arising from both synthesis conditions and the electrochemical reaction is critical in performance of this material as the electrode. In an attempt to fully utilize Ni2+/Ni4+ double redox couple, Li2NiO2 in the Immm structure was successfully synthesized and its electrochemical behavior upon delithiation was evaluated. The material shows a high specific charge capacity of about 320 mAh/g and discharge capacity of about 240 mAh/g at the first cycle. The stability of Li2NiO2 in the Immm structure is attributed to the more favorable Li arrangement possible as compared to a Li2NiO2 structure with octahedral Ni. The electrochemical data, first principles calculation and EXAFS analysis all indicate that the orthorhombic Immm structure is rather prone to phase transformation to a close-packed layered structure during the electrochemical cycling.

(cont.) The possibility of stabilizing the orthorhombic Immm structure during the electrochemical cycling by partial substitution of Ni is also evaluated. First principles computations of some chemically substituted materials identified Pt substitution as a way of stabilizing the Li2(Ni,M)O2 composition in the Immm structure but found no elements that would likely stabilize the material upon Li removal. The second part of the thesis is focused on designing high rate capable electrode materials. We systematically investigated several of the factors that influence the migration barrier for Li motion in layered oxides with the 03 structure using first principles methods, and found that the two dominant effects are the Li slab spacing which determines the compressive stress on Li when it is in the tetrahedral site, and the electrostatic repulsion Li experiences there from the transition metal ion. The other factors investigated (non-transition metal doping, Li-metal site exchange) can be reduced to the effect they have on the electrostatic and Li-slab factor. We have used these first principles results as key strategies for increasing the rate capability of layered cathode materials and applied them to Li(NiO.5MnO.5)02, a safe, inexpensive material that has been thought to have poor intrinsic rate capability.

(cont.) Structural modification of Li(Nio.sMno.5)2 according to first principles guideline leads this novel material to retain its capacity at high rates in agreement with the theoretical predictions. The rate capability tests show that even at a 6C discharge rate (C= 280mA/g) it can deliver over 180mAh/g. The best electrochemical data published for this material shows that it can deliver about 130mAh/g at a 2C rate, and there is no data available for a rate as high as 6C. The electron microscopy shows that the particle size of this material is about two times bigger than- the conventional Li(Nio.5Mno.5)O2. This implies that with proper engineering optimization in processing (i.e. synthesis temperature, time etc.) this material can show even better rate capability.