P-Type Stacking Dominated Electrochemical Process Enables Fast Na+ Transport for High-Energy P2/O3 Biphasic Cathodes

Abstract

Fabricating P2/O3 intergrowth structure in layered cathode materials is a viable strategy to improve the electrochemical property of sodium-ion batteries. Unfortunately, such biphasic materials have to bear obscure thermodynamic formation process and complicated structure-property associations between multiple phase transitions and Na⁺ diffusion kinetics at high state of charge. Here this issue is addressed by tailoring the crystalline domains of the P2 and O3 phase while reducing the residual alkali content in target P2/O3-Na_0.8Mg_0.06Ni_0.34Mn_0.54Ti_0.06O₂ cathode material, which consists of 24.26% P2 phase and 75.74% O3 phase. The thermodynamic phase distribution at atomic resolution and dynamic phase evolution identification are parsed out by experimental scanning transmission electron microscopy and FAULTS simulations. Moreover, the dislocations at phase boundary of the P2 and O3 crystalline domains serve to prevent O-type stacking and therefore allow most P-type stacking to dominate the electrochemical process in deep Na-depleted state, thereby facilitating Na⁺ diffusion kinetics to ensure high-rate capability. Consequently, the biphasic cathode material exhibits a high energy density of 534 Wh kg⁻¹ and a reversible capacity of 110 mAh g⁻¹ at 10 C. This work highlights the importance of thermodynamic phase modulation in improving the Na⁺ transport to obtain high-rate and high-energy P2/O3 biphasic cathode materials.