K Ions Migrate via a Cog-Wheel Mechanism in Solid Electrolyte Vanadium Pyrophosphates: Fact or Fiction?

Solid-state batteries have gained attention for their potential to achieve higher energy density and improved safety. This study investigates K2(VO)3(P2O7)2, focusing on the influence of polyanion dynamics on ion conduction and the cog-wheel mechanism driving K+ migration at low- and high-temperature phases. At the low-temperature phase, the rotational motion of the cluster anion (P2O7) is minimal, exhibiting a soft cradle effect. Subtle angular tilts create an extended PxOy network, forming open channels that facilitate smooth K+ diffusion with a lower energy barrier (0.35 eV). However, at the high-temperature phase, cooperative reorientation of the polyanion is more pronounced, leading to frequent PxOy bond rearrangements and enhancing diffusion. However, this increased mobility also introduces structural instability, resulting in a higher energy barrier (0.45 eV). The cog-wheel mechanism emerges at the high-temperature phase, where coupled cation–anion dynamics influence ion mobility. The vibrational properties of cations and anions further support this mechanism. Additionally, we obtained a linear correlation between relaxation time and ionic conductivity across different phases. The relaxation behavior of the cluster anion with temperature indicates that the rotational relaxation time decreases with increasing temperature from 1.6 ns (500 K) to 0.02 ns (1200 K), exhibiting Arrhenius behavior. Thermal energy introduces lattice vibrations and transient pathways, influencing ion mobility while reshaping structured diffusion channels. The estimated diffusion coefficient and conductivity of this material at room temperature are 5.31 × 10–12 cm2 s–1 and 1.03 × 10–4 mS cm–1, respectively. It also exhibits an average discharge potential of 3.24 V (vs K+/K) and a theoretical specific capacity of 85 mA h g–1, with minimal volume change (<1%) during complete potassium deintercalation. These findings provide insights into how polyanion dynamics influence ion transport and structural stability in potassium-based solid-state batteries.