Sodium-ion batteries towards practical application through gradient Mn-based layer-tunnel cathode

Structure engineering on cathode materials is of great significance for sodium-ion batteries (SIBs) for large-scale practical applications. To achieve a long life-span, it is the major challenge to stabilize their internal bulk and surface structure. Herein, a layer-tunnel composite structure is employed by virtue of calcination chemistry. It is achieved via regulating competitive kinetics and thermodynamic differences between Na+ and Ti4+ when forming gradient layer-tunnel architecture with P2-type Mn-rich bulk and tunnel-type Ti-rich shell. This composite structure integrates superiorities of tunnel shell in stability and layered bulk in capacity. Meanwhile, the synergetic behavior in gradient layer-tunnel composite structure is also reflected by suppressing Mn2+ formation on tunnel-structure surface and P2-O2 layer-phase transformation of internal domain. The mechanical strength of holistic secondary particle is also improved during continuous Na+ (de)intercalation. The resulting Na0.6Mn0.95Ti0.05O2 enables sodium-ion batteries performing a cycle ability for capacity retention of 83.9% after 1000 cycles. More importantly, a high-loading electrode of 24.5 mg cm-2 with practical-target measurement shows a gravimetric energy density of 296.1 Wh kg-1, areal capacity of 2.6 mAh cm-2, and capacity retention 89.6% after 100 cycles. This study provides new perspective for bulk-structure design and construction of ultracyclicality cathode materials to reach the practical standard for SIBs.