Electronic Confinement-Restrained <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>Mn</mi> <mi>Na</mi> <mo>·</mo></msubsup> <annotation>${\mathrm{Mn}}_{{\mathrm{Na}}}^{\mathrm{\cdot}}$</annotation></semantics> </math> Anti-Site Defects in Sodium-Rich Phosphates Toward Multi-Electron Transfer and High Energy Efficiency.

Abstract

Sodium (Na) super-ionic conductor structured Na₃MnTi(PO₄)₃ (NMTP) cathodes have garnered interest owing to their cost-effectiveness and high operating voltages. However, the voltage hysteresis phenomenon triggered by <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>Mn</mi> <mi>Na</mi> <mo>·</mo></msubsup> <annotation>${\mathrm{Mn}}_{{\mathrm{Na}}}^{\mathrm{\cdot}}$</annotation></semantics> </math> anti-site defects ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>Mn</mi> <mi>Na</mi> <mo>·</mo></msubsup> <annotation>${\mathrm{Mn}}_{{\mathrm{Na}}}^{\mathrm{\cdot}}$</annotation></semantics> </math> -ASD), namely, the occupation of Mn²⁺ in the Na2 vacancies in NMTP, leads to sluggish diffusion kinetics and low energy efficiency. This study employs an innovative electronic confinement-restrained strategy to achieve the regulation of <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>Mn</mi> <mi>Na</mi> <mo>·</mo></msubsup> <annotation>${\mathrm{Mn}}_{{\mathrm{Na}}}^{\mathrm{\cdot}}$</annotation></semantics> </math> -ASD. Partial replacement of titanium (Ti) with electron-rich vanadium (V) favors strong electronic interactions with Mn²⁺, restraining Mn²⁺ migration. The results suggest that this strategy can significantly increase the vacancy formation energy and migration energy barrier of manganese (Mn), thus inhibiting <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>Mn</mi> <mi>Na</mi> <mo>·</mo></msubsup> <annotation>${\mathrm{Mn}}_{{\mathrm{Na}}}^{\mathrm{\cdot}}$</annotation></semantics> </math> -ASD formation. As proof of this concept, an Na-rich Na_3.5MnTi_0.5V_0.5(PO₄)₃ (NMTVP) material is designed, wherein the electronic interaction enhanced the redox activity and achieved more Na⁺ storage under high-voltage. The NMTVP cathode delivered a reversible specific capacity of up to 182.7 mAh g^-1 and output an excellent specific energy of 513.8 Wh kg^-1, corresponding to ≈3.2 electron transfer processes, wherein the energy efficiency increased by 35.5% at 30 C. Through the confinement effect of electron interactions, this strategy provides novel perspectives for the exploitation and breakthrough of high-energy-density cathode materials in Na-ion batteries.