Decoupling slab gliding and lattice contraction in Na layered oxides to enable high-voltage Na-ion batteries.

Abstract

Layered transition metal oxide cathodes (Na_xTMO₂) demonstrate a classic type of cathode for Sodium-ion batteries (SIBs), however their practical application faces a long-standing challenge of irreversible phase transitions at high voltages, which causes unsatisfied specific energy and cycling stability, particularly for P-type (Na⁺ located at prismatic sites) cathodes. This phenomenon is conventionally ascribed to the Na⁺ re-coordination from prismatic to octahedral (O-type) configuration upon Na⁺ extraction, whereby the TMO₂ slab gliding and abrupt c-lattice change are always coupled, and a straightforward solution to this situation remains elusive. Here, we reveal that, the TMO₂ slab gliding and the lattice contraction can be decoupled, and the rapid lattice contraction under high state-of-charge underlies the fundamental origin for the irreversible phase transitions. By pre-engineering 15.8% O-type stacking faults to a P-type Na_0.7Mn_0.8Ni_0.2O₂, the dramatic volume variation and irreversible phase transitions at high voltage (4.5 V vs. Na⁺/Na) can be primarily eliminated. This work advances the understanding on the phase transitions at deep desodiation states, and paves up a feasible way to realize high-energy layered oxides.