Investigation of Na2/3Ni1/3Ti2/3O2 as Novel Bi-Functional Electrode Material for Non-Aqueous, Room Temperature Na-Ion Batteries

Developing long life, low-cost energy storage solutions based on earth abundant minerals is crucial for renewable energy sources, smart electric grid networks, portable electronics and automotive sector. Na-ion batteries, engineered to operate under ambient conditions with non-aqueous electrolytes, offer much promise in that they do not have resource constraints¹and by selecting appropriate materials they can be fabricated into high voltage electrochemical cells, similar to Li-ion chemistry. 

The objective of the current work is to study the electrochemical properties of a new oxide material with the composition Na_2/3Ni_1/3Ti_2/3O₂ (SNTL)² as a “bi-functional” intercalation-type electrode for Na-ion battery applications. The presence of two transition metals in this layered oxide enables us to activate two redox couples (Ni^2+/3+ and Ti^4+/3+) of different species in the same material and thereby provides the opportunity to use it as anode or cathode depending on the potential window. This approach of making both electrodes (anode and cathode) using the same material can simplify electrode design and potentially reduce manufacturing costs for commercial applications.  

The solid state synthesis conditions of SNTL have been optimized to make phase-pure oxide with minimal particle size as shown in Figure 1a and Figure 1b. 

Galvanostatic cycling shows that up to 50% of sodium ions can be removed reversibly by oxidizing nickel from 2+ to higher oxidation state without any significant capacity loss. This contributes to a sloping voltage profile in the range of 3-4.2 V (Figure 1c) with solid solution-type mechanism involving no phase transitions. 

Sodium insertion into pristine SNTL, due to its sodium deficient nature, activates the Ti^4+/3+ redox couple. This leads to again a sloping voltage profile with an average voltage of 0.7 V vs. Na/Na⁺ as shown in Figure 1d. The reaction has been confirmed to be of solid solution-type using ex-situ XRD measurements. Thus, SNTL can be used to fabricate 3V intercalation-based Na-ion batteries.  

Wet chemistry-based synthetic routes (sol gel) are being currently explored to reduce the particle size, decrease the diffusion length scale of electrons and thereby improve the electrochemical performance under higher currents. Fabrication and testing of solid state batteries using SNTL material as active electrodes (anode and cathode) and Na₃Zr₂P₁Si₂O₁₂ material (NASICON)³as fast sodium conducting solid electrolyte is also currently under progress. 

References

1. M.D.Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-Ion Batteries, Advanced Functional Materials, 2013, 8, 947-958

2. Y.J. Shin, M.Y. Yi, Preparation and structural properties of layer-type oxides Na_xNi_x/2Ti_1-x/2O₂, Solid State Ionics, 2000, 132, 131-141

3. Goodenough JB, Hong HYP, Kafalas JA., Fast  Na⁺-Ion transport in skeleton structures. Mater Res Bull 1976, 11(2), 203-220