(Invited) Controlling the Ordering of Na⁺ and Vacancy By Ti-Doping in P2-Na_0.67Co_1-XTi_xO₂ for High Energy and Power Density Cathode Materials in Sodium-Ion Batteries

Abstract

Sodium-ion batteries (NIBs) for large power sources having kWh or MWh scale of energy are highly interesting alternatives for lithium-ion batteries because of abundant sodium resources in the earth and high chemical similarity between Li and Na ions. However, due to the high atomic weight of Na+ and the low working potential of NIBs, the energy and power densities of NIBs are also required to be improved for wide applications. Despite the similar chemical behavior of Na ions in the rechargeable batteries, the high performance of the active material in NIBs is an essential research- topic for the realization of NIBs. Especially, Na ion having a larger diameter than Li-ion in the P2 type layered structure has a distinct Na+/vacancy ordering behavior presenting multiple voltage plateaus during sodium ion insertion and extraction in the electrochemical cycling. This different behavior from lithium-ion batteries makes the sodium ion diffusion complex in the solid matrix due to the pass-through of various stable Na+/vacancy arrangements. This presentation covers an improvement in the electrochemical performance of the cathode material, Na0.67Co1-xTixO2 having P2 type crystalline structure by using various amounts of Ti doping to control Na/vacancy ordering for mitigating the Na ion diffusion in the solid matrix as well as for extending the de-sodiation range to 4.4 & 4.5 V vs. Na/Na+. The proposed material having Ti doping presents a highly relieved an electrochemical Na+/vacancy ordering behavior from the that of Na0.67Co1-xO2 having a severe capacity loss at high potential because of irreversible phase transitions. In particular, 10 % Ti-doping in the Co content highly advances the cycleability by showing 115 mAh g-1 at the 100th cycle, as well as the rate capability by reaching 108 mAh g-1 at a current density of 1000 mA g-1. The detail diffusion kinetics and phase behaviors from the prepared materials are investigated by a systematic analysis combination comprising TEM, in-situ XRD, and electrochemical methods.