Magnetism and ion diffusion in honeycomb layered oxide K_2Ni_2TeO_6

In the quest for developing novel and efficient batteries, a great interest has been raised for sustainable K-based honeycomb layer oxide materials, both for their application in energy devices as well as for their fundamental material properties. A key issue in the realization of efficient batteries based on such compounds, is to understand the K-ion diffusion mechanism. However, investigation of potassium-ion (K ^+ ) dynamics in materials using e.g. NMR and related techniques has so far been very challenging, due to its inherently weak nuclear magnetic moment, in contrast to other alkali ions such as lithium and sodium. Spin-polarised muons, having a high gyromagnetic ratio, make the muon spin rotation and relaxation ( μ ^+ SR) technique ideal for probing ions dynamics in these types of energy materials. Here we present a study of the low-temperature magnetic properties as well as K ^+ dynamics in honeycomb layered oxide material K_2Ni_2TeO_6 using mainly the μ ^+ SR technique. Our low-temperature μ ^+ SR results together with complementary magnetic susceptibility measurements find an antiferromagnetic transition at T_N≈ 27 K. Further μ^+ SR studies performed at higher temperatures reveal that potassium ions (K ^+ ) become mobile above 200 K and the activation energy for the diffusion process is obtained as E_a=121 (13) meV. This is the first time that K ^+ dynamics in potassium-based battery materials has been measured using μ ^+ SR. Assisted by high-resolution neutron diffraction, the temperature dependence of the K-ion self diffusion constant is also extracted. Finally our results also reveal that K-ion diffusion occurs predominantly at the surface of the powder particles. This opens future possibilities for potentially improving ion diffusion as well as K-ion battery device performance using nano-structuring and surface coatings of the particles.