Investigation of Na Ion Dynamics in P2-Type Na2/3[Ni1/3Ti2/3]O2: A Combination of Quasi-Elastic Neutron Scattering and First-Principles Molecular Dynamics Study
Authors: Chen Qian , Lai Wei 
Published: February 2020

While the lithium-ion batteries are dominating the portable electrical energy storage market owing to the high energy density, their implementation in large-scale energy storage is likely to be constrained by the price and availability of lithium resources. With overwhelming advantages of huge redundancy and low cost, the sodium ion batteries provide a potential solution to the increasing needs of the large-scale static energy storage for the intermittent renewable energy resources such as wind and solar energy. Thus, the materials for sodium-ion batteries have been attracting increasing research interests in recent years.
In the search for the suitable electrode materials for sodium-ion batteries, the layered sodium transition metal (TM) oxide materials (NaxTMO2) are commonly studied following the successfully-commercialized example of their Li analogs (LixTMO2). Among these layered Na insertion materials, the P2-Type Na2/3[Ni1/3Ti2/3]O2 have been found to exhibit good electrochemical properties as either positive or negative electrode in batteries1. Such bi-functionality is due to the coexistence of the high redox potential couple Ni2+/ Ni3+ and the low redox potential couple Ti4+/ Ti3+, bringing further advantages of lower manufacture cost and simpler design.
The dynamics of ionic charge carriers in electrodes, i.e., the sodium ions, are critical to the performance of the battery. In this study, we investigated the dynamics of Na ions in P2-Type Na2/3[Ni1/3Ti2/3]O2 with a combination of the experimental and computational tools in the similar time and length scale to complement each other. The quasi-elastic neutron scattering (QENS) experiments were conducted in the temperature range of 400 to 700K. The Na self-diffusion properties were derived from the broadening of the elastic peak (quasi-elastic features) in the small Q range of 0.2 to 2 Å-1. The experimental results provided the verification for the first-principles molecular dynamics (MD) simulations based on density functional theory (DFT). The intermediate scattering function in the time domain was calculated based on the MD trajectory to compare with the experimental results. The diffusion behavior of the Na ions can be described by the Singwi-Sjolander jump model2, in which the Na ion jumps, with an average jump distance d, take much less time than the residence time τ when the ions vibrate in certain sites. Such integrated experimental and computational study provides a better understanding of Na diffusion mechanism in the material as well as insights in designing electrode materials with better performance.
References:
(1) R. Shanmugam and W. Lai "Na2/3Ni1/3Ti2/3O2: "Bi-Functional" Electrode Materials for Na-Ion Batteries," ECS Electrochemistry Letters
2014, 3, A23-A25. http://dx.doi.org/10.1149/2.007404eel
(2) K. S. Singwi and A. Sjolander "Diffusive Motions in Water and Cold Neutron Scattering," Physical Review
1960, 119, 863-871. http://dx.doi.org/10.1103/PhysRev.119.863