Electrode materials for sodium ion batteries: A cheaper solution for the energy storage

The high abundance of sodium and its low cost compared to lithium renewed the high interest on sodium ion batteries as cheaper solution for electrochemical energy storage. Indeed, sodium ion batteries have recently proposed as possible candidates for stationary and mobile energy storage devices. Herein, we present two new intercalation materials, which shown excellent electrochemical features in terms of charge-discharge, reversibility, coulombic efficiency and high specific discharge capacity. The first one is the lamellar oxide material Na_0.67Co_2/3Ni_1/9Mn_2/9O₂, prepared by a soft chemistry route (SolGel) crystallizes in the rhombohedral system with layered type structure. This compound offers a specific capacity of 110 mAhg^-1 when cycled between 2.0 and 4.2 V vs. Na⁺/Na. The electrodes exhibited a good capacity retention and a coulombic efficiency exceeding 99.4%, as well as a reversible discharge capacity of 140 mAhg^-1 when cycled between 2.0 and 4.5 V. The second investigated electrode material is the phosphate Na_1.5Ti_1.5Fe_0.5(PO₄)₃. In addition to its benign effect on the environment, this material delivers a specific discharge capacity of 120 mAh^-1 with two extraction/insertion plateaus at 2.3 V and 2.2 V. The open framework of this sample leads a high structural stability during cycling making this material a good candidate for long-term energy storage material.