Elec­tro­chem­ic­al per­form­ance of KTA – More than a photon­ic ma­ter­i­al?

The world needs more energy, preferably in a form that is clean and renewable. This requires sophisticated energy-storage strategies that go beyond the presently used lithium-ion batteries. In this context, there is an intense search for new electrode materials that provide high working voltages, thermal stability, and energy densities, as well as fast ion (de)intercalation.

Potassium titanyl phosphate (KTiOPO₄, KTP) has been suggested as a promising electrode material for alkali-ion batteries. However, the characteristic photochromic damage of KTP, the so-called gray tracking, could also be a limiting factor for battery applications: Since it is related to thermally stable defect-trapped electrons, it could, in fact, affect the electron flux.

This motivated Adriana Bocchini from the Theoretical Materials Physics Group headed by Wolf Gero Schmidt in collaboration with other Paderborn physicists and chemists to explore computationally the applicability of potassium titanyl arsenate (KTiOAsO₄, KTA) as electrode material in potassium-ion batteries. KTA belongs to the KTP-type family materials, but is less prone to gray tracking.

Using density-functional theory (DFT), KTA cathodes/anodes were modeling by K-deficient K_1-xTiOAsO₄ (x = 0.0 - 1.0) and K-doped KTiOAsO₄K_x (x = 0.0 - 0.5), respectively. Average voltages, the gravimetric energy densities as well as the volume distortions were calculated. In addition, the K (de)intercalation dynamics was studied. The results suggest KTA to be indeed an excellent material for both cathodes and anodes in potassium-ion batteries.

In fact, the material is demonstrated to combine higher average working voltages than KTP (up to 3.8 V) with a modest volume expansion (shrinkage) upon K (de)intercalation (both below 8%), and a fast K-ion (vacancy) diffusion, especially along the [001] direction, where the activation energies were found to be less than 0.5 eV.

The results have recently been published in Physical Review Materials, see https://journals.aps.org/prmaterials/pdf/10.1103/PhysRevMaterials.6.105401