Energy storage technology plays a crucial role in the implementation of renewable energy and electrical vehicles. Na-ion battery, which relies on earth-abundant and low-cost Na resource, shows great potential for such applications. 

As an indispensable part of Na-ion battery, cathode materials with various structures have been extensively explored and investigated. Among them, layered Na_2/3MnO_2+σ with P2 structure is considered a promising candidate due to its high capacity, high volumetric energy density as well as spacious Na⁺ diffusion pathway. However, the original P2 structure in Na_2/3MnO_2+σ will transform through oxygen layer gliding at high charging voltage/low Na⁺ content, leading to serious capacity decay upon cycling. The multiple voltage plateaus during charge and discharge, which can be assigned to the ordering of Na⁺/vacancy in the structure, also brings adverse effect on cycle life. It has been reported that Li substitution in the transition metal sites (Na_y[Li_xM_1-x]O₂) is effective in suppressing the structural transformation and relieving phase transitions for both P2 and O3 type layered transition metal oxides, thereby improving the cycle performance ^1-6. Among these studies, it is worth to mention Xu’s surprising discovery from nuclear magnetic resonance (NMR) spectroscopy ². They reported that the Li are not fixed in the transition metal sites. A majority of them will migrate to the Na layer when the cell is charged above 4.1 V to a higher potential of 4.4 V, while most Li diffuse back to the TM layer upon discharging to 2.0 V. The stability of the P2 structure with Li-substitution is attributed to the electrostatic attraction between adjacent TM layers induced by Li in the Na layer at high voltage. In other words, it is the Li reserve in the Na layers that hold the TM layers together. In this study, we investigate whether it is possible to stabilize the P2 structure by directly adding Li into Li_xNa_2/3MnO_2+σ (0 ≤ x ≤ 0.2).

Li_xNa_2/3MnO_2+σ (0 ≤ x ≤ 0.2) were synthesized with the same Na/Mn ratio but with different Li content via conventional solid state method. The first benefit of Li addition is that the material is more stable in ambient conditions: Na_2/3MnO_2+σ undergoes structural change when left in air for a period of time, mainly due to reaction with moisture, while the Li-added samples remain unchanged. In addition, electrochemical behaviors are also changed. Li addition suppresses the phase transitions of Na_2/3MnO_2+σ. Even though the overall capacity is decreased with Li addition, cycle stability is improved. For example, capacity of Na_2/3MnO_2+σ decreases from 208.5 mAh/g to about 52.7 mAh/g (25.3% retention) after 100 cycles in the voltage range of 1.5~4.3 V at 15 mA/g. Li_0.1Na_2/3MnO_2+σ on the other hand retains about 61.6% of its capacity after continuous Na⁺insertion/extraction for 100 cycles. Coulombic efficiency and rate performance are also improved with Li addition. Further structural characterizations to understand the reason for the improvements are underway and a detailed discussion will be presented at the meeting.

Reference:

(1) Van Nghia, N.; Ou, P.-W.; Hung, I. M. Electrochimica Acta 2015, 161, 63.

(2) Xu, J.; Lee, D. H.; Clément, R. J.; Yu, X.; Leskes, M.; Pell, A. J.; Pintacuda, G.; Yang, X.-Q.; Grey, C. P.; Meng, Y. S. Chemistry of Materials 2014, 26, 1260.

(3) Xu, J.; Liu, H.; Meng, Y. S. Electrochemistry Communications 2015, 60, 13.

(4) Karan, N. K.; Slater, M. D.; Dogan, F.; Kim, D.; Johnson, C. S.; Balasubramanian, M. Journal of the Electrochemical Society 2014, 161, A1107.

(5) Yabuuchi, N.; Hara, R.; Kajiyama, M.; Kubota, K.; Ishigaki, T.; Hoshikawa, A.; Komaba, S. Advanced Energy Materials 2014, 4.

(6) Kim, D.; Kang, S.-H.; Slater, M.; Rood, S.; Vaughey, J. T.; Karan, N.; Balasubramanian, M.; Johnson, C. S. Advanced Energy Materials 2011, 1, 333.