Since 1990 lithium-ion (Li-ion) batteries have been commercially available and currently remain as the technology of choice for applications where high energy densities are required. Sodium-ion (Na-ion) technology is similar in many ways to Li-ion technology, but is still in its initial stage. Current research trends are mostly based on Na-ion based technology due to several commercial advantages, including lower cost, greater sustainability and improved safety characteristics.^{1, 2} Among different types of cathode materials available, layered oxides such as Na_xMO₂ (M = transition metal) have shown great promise in terms of both cost and performance.^3,4 According to Delmas et al, these layered oxides are classified into different structures, the most common of which are O3, P2 and P3.⁵ In these descriptions, the O and P refer to an octahedral (O), or prismatic (P), coordination of the Na-ions, and the number refers to the number of layers in the unit cell. P2-type Na-Ni-Mn-O has been considered as a suitable cathode material for the modern day requirement of high power and energy applications due to their low cost, easy synthesis and high theoretical capacity of greater than 250 mAhg^-1. However this material has the serious problem of capacity fading due to structural instability. Cationic substitution and surface coating was an efficient strategy to enhance the electrochemical performance by improving the structural stability and preventing Mn³⁺ dissolution into electrolyte at higher voltages. In this work a novel P2-type Na_0.5Ni_0.33Cu_0.07Mn_0.67O₂ was synthesized using the single step conventional solid state method and studied as cathode material for sodium ion batteries. The presence of Cu in the lattice structure enhanced the capacity retention to 83% after 100 cycles along with smooth voltage plateau in the high voltage region as shown in Figure 1. Surface coating with MgO increased the specific capacity of the material in the extended voltage window of    2.0 – 4.5V. The MgO coated material shows a smooth voltage plateau without any phase gliding in the higher voltage as in Figure 2. The capacity retention after 70 cycles is found to be 77.6% with enhanced performance. Hence the MgO coated Na_0.5Ni_0.26Cu_0.07Mn_0.67O₂is studied as a novel cathode for room temperature Na-ion batteries.

References:

[1] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonz, T. Rojo, Energy Environ. Sci. 5, 2012, 5884.

[2] J. Barker, M.Y. Saidi, J. Swoyer, Electrochem. Solid St. 6, 2003, A1.

[3] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2, 2012, 710.

[4] M. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Func. Mater. 23, 2013, 947.

[5] C. Delmas, C. Fouassier, P. Hagenmuller, Physica B+C, 99, 1980, 81.

Figure 1(a). Cycle stability of Na_0.5Ni_0.26Cu_0.07Mn_0.66O₂ at 0.25C from 2.0 - 4.25V. ( Inset figure shows the XRD pattern and unit cell diagram of metal substituted sample). (b) Cyclic stability of MgO coated Na_0.5Ni_0.26Cu_0.07Mn_0.66O₂ in high voltage of 2-4.5V