Unblocking “pinned” Li+ ions in novel KTiOPO4-structured LiVPO4F enables superior energy storage performance

Abstract

This study showcases the successful use of an elegant solid-state ion-exchange method to synthesize a KTiOPO₄-structured lithium–vanadium fluoride phosphate (KTP-LiVPO₄F), an electrode material yet unattainable through conventional synthesis routes. It demonstrates promising activity in the low-voltage domain, achieving a reversible capacity of 137 mA h g⁻¹, however, in the high-voltage region, it exhibits only 68 mA h g⁻¹ due to high activation barriers at certain Li sites. Introducing ∼15% of Na ions into the crystal structure of KTP-Li_1−xNa_xVPO₄F improves specific capacity up to 116 mA h g⁻¹ at an average potential of ∼4.0 V vs. Li⁺/Li. Density functional theory calculations suggest that Na ions partially occupy the “pinned” lithium sites, lowering activation energies which is further supported by molecular dynamics simulations as to increasing diffusion coefficients in Li_0.875Na_0.125VPO₄F. Finally, we demonstrate that co-cycling Li and Na ions within the VPO₄F framework provides a reversible capacity of about 128 mA h g⁻¹ at an elevated average potential of ∼4.2 V vs. Li⁺/Li. These findings pave the way for developing a novel concept of stable hybrid Li/Na-ion batteries with enhanced energy density, and also reveal an attractive strategy for using the “pillar” effect to increase the specific energy of electrode materials.