Electronic States Tailoring and Pinning Effect Boost High-Power Sodium-Ion Storage of Oriented Hollow P2-Type Cathode Materials

Abstract

Fierce phase transformation and limited sodium ion diffusion dynamics are critical obstacles that hinder the practical energy storage applications of P2-type layered sodium transition metal oxides (Na x TMO2). Herein, a synergistic strategy of electronic state tailoring and pillar effect was carefully implemented by substituting divalent Mg2+ into Na0.67Ni0.33Mn0.67O2 material with unique oriented hollow rodlike structures. Mg2+substitution can not only facilitate the anionic oxygen redox reactions and electronic conductivity through increasing the electronic states at Femi energy but also act as pillars within TMO2 layers to alleviate the severe phase transformation to improve structure stability. Moreover, the oriented hollow structure incorporating sufficient buffer spaces and rationally exposed electrochemically active facets effectively alleviates the stresses induced by low volume changes of 8% and provides more open channels for Na+ ion diffusion without crossing multiple grain boundaries. Hence, the Na0.67Mg0.08Ni0.25Mn0.67O2 cathode showed a superior rate capability with high energy density and cycling stability for sodium-ion storage. The underlying mechanisms of these achievements were deciphered through diversified dynamic analysis and the first principle calculations, providing new insights into P2-type Na x TMO2 cathodes for the infinite prospect as an alternative to lithium-ion batteries.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

“…Additionally, our group implemented a synergistic strategy involving electronic-state tailoring and the pining effect by substituting Mg 2+ in Na 0.67 Ni 0.33 Mn 0.67 O 2 , which has a unique oriented hollow rod-like structure. 373 As shown in Fig. 45f and g, the oriented hollow structure provides sufficient buffer space and abundant exposed electrochemical active surface, which reduced the stress caused by an 8% low volume change and provided more open channels for Na + ion diffusion without crossing multiple grain boundaries.…”

Bismuth-doping boosting Na+ diffusion kinetics of layered oxide cathode with radially oriented {010} active lattice facet for sodium-ion batteries

“…To alleviate the problems, exposing sufficient {010} facets of primary particles is one effective way to provide additional ion transport channels for enhancing Na + diffusion kinetics. , A surfactant-assisted synthesis method was used to effectively promote inducing the growth of the exposed {010} active planes, by lowering the high surface energy of the {010} facets with the addition of surfactants of sodium dodecyl sulfate and polyvinylpyrrolidone. , A surfactant-free coprecipitation method was able to construct a porous and hollow architecture with exposed {010} facets. − Furthermore, rationally designing secondary microparticles assembled with primary nanoparticles is effective at improving diffusion kinetics by shorting the ion diffusion pathway. , However, the diffusion of Na + into the electrolyte is difficult as it entails crossing through multiple grain boundaries due to the random agglomeration of primary particles . In addition, the nanoparticles with large specific surface areas in secondary microparticles easily initiate surface parasitic reactions with electrolyte, accelerate the degradation of the cathode material, and thus result in capacity decay after long cycling. − Therefore, simultaneous combination of the oriented crystal facets and the nanoplatelet-containing microspheres could greatly enhance both the structural stability and Na + diffusion kinetics.…”

Predominant P3-Type Solid–Solution Phase Transition Enables High-Stability O3-Type Na-Ion Cathodes

“…Generally, high Na content can screen the electrostatic repulsion effect between the two oxygen sheets, leading to a reduction in the d (O–Na–O) interlayer space, which contributes to the emergence of an O3-type structure. On the contrary, low Na content leads to a larger d (O–Na–O) interlayer space, therefore promoting the formation of a P2-type structure. , …”

“…On the contrary, low Na content leads to a larger d (O−Na−O) interlayer space, therefore promoting the formation of a P2-type structure. 18,19 O3-type NaNi 0.5 Mn 0.5 O 2 can exhibit high capacity that benefited from high Na content and adequate Ni 2+/4+ redox couple. Nevertheless, it suffers irreversible phase transformation with great volume variation at elevated cutoff voltages, which is detrimental for cycling stability.…”