Rational design of Ti-based oxygen redox layered oxides for advanced sodium-ion batteries

Abstract

Considering Mn⁴⁺ (3d₃)-based cations, various layered oxides (A[A_yM_1−y]O₂, where A and M refer to alkali metals and transition metals, respectively) exhibiting oxygen-redox reactions have been investigated extensively to achieve high energy density in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Because M redox activity is determined by the energetics of crystal field theory for oxides, we systematically design a Ti⁴⁺ (3d₀)-based Na layered oxide, i.e., Na[Li_1/3Ti_1/3Cr_1/3]O₂, which features a cation–anion-coupled redox reaction based on the following four rational steps. First, we demonstrate a Ti-based Li layered oxide (Li[Li_1/3Ti_2/3]O₂) undergoing deintercalation at an extremely high voltage to deliver immense oxygen redox capacities for LIBs. Second, we rationally design a Na[Li_1/3Ti_2/3]O₂ layered oxide that shows a high-voltage trend at ≈4.4 V vs. Na⁺/Na with a high capacity of ≈300 mA h g⁻¹. Third, it is unambiguous that unhybridized O 2p-electrons are vital for compensating the charge imbalance induced by Na removal because of the rigid Ti⁴⁺ electronic structure. Fourth, two types of cations are incorporated into the M layer, i.e., redox-inactive (Ti) and redox-active (Cr) cations. Na[Li_1/3Ti_1/3Cr_1/3]O₂ is considered superior as it exhibits phase stability when utilizing the oxygen-redox activity and maintains the initial charge character upon cycling.