Solid solution Na4TiyMn9-yO18 for sodium-ion batteries and a new strategy to enhance its electrochemical performance

Sodium-ion batteries are advantageous in terms of lower cost and sustainability compared to lithium-ion batteries. Sodium, manganese, and titanium are abundant, low-toxicity elements and very promising for electrode materials. The tunnel-type structure in certain manganese oxides facilitates the easy diffusion of sodium ions into the framework. Still, the Mn(IV)/Mn(III) redox pair could initiate degradation mechanisms and lead to battery failure. To improve the electrochemical performance, this study investigates the progressive substitution of manganese(IV) ions by titanium(IV). A series of Na 4 Ti y Mn 9-y O 18 (0.0 ≤ y ≤ 5.0) was synthesized. X-ray diffraction (XRD) patterns and density functional theory (DFT) calculations reveal that replacing Mn⁴⁺ by Ti⁴⁺ expands the unit cell volume. Titanium(IV) ions substituting for manganese(IV) ions can lead to some improvement in electrochemical cycling stability, although this enhancement is limited. This study also explores the blending of a tunnel-type oxide (Na 0.5 Ti 0.1 Mn 0.9 O 2 ) with a NASICON-type phosphate (Na₃V₂(PO₄)₃) as a new strategy to achieve better cycling stability and larger capacity. • Titanium ions in tunnel-type Na 4 Ti x Mn 9-x O 18 expand the unit cell and stabilize the electrode material. • DFT calculations reveal that the oxidation of the Mn 3 + ion in octahedral coordination is more favorable. • The new electrode comprising tunnel-type Na 0.5 Ti 0.1 Mn 0.9 O 2 and NASICON-type Na 3 V 2 (PO 4 ) 3 exhibits superior performance.