High Performance Na-Ion Batteries Based on Novel O3 Layered Oxide Cathode Materials

Abstract

Sodium-ion (Na-ion) batteries represent an attractive alternative to their lithium-ion counterparts, and are expected to offer some significant commercial advantages such as lower active material costs and improved safety characteristics [1,2]. In these systems Na-based layered oxides, NaxMO2(M = transition metal) have emerged recently as high performance and low cost cathode materials [3,4]. These oxide materials may be categorized into three main structural types using the classification proposed by Delmas and co-workers – i.e. O3, P2 and P3 types, where the alkali ions occupy either octahedral (O) or prismatic sites (P), respectively [5].In this study single-phase O3-NaaNi(1-x-y-z)MnxMgyTixO2 materials have been prepared using conventional solid-state processing methods. In these as-made compositions nickel and magnesium are included as M2+ while manganese and titanium are present as M4+. Special attention was given to the product composition, precursor mixing and synthesis conditions to ensure no unwanted NiO impurity content. Figure 1 shows the XRD pattern for a typical O3-Na layered oxide composition and confirms that the material is essentially phase pure.The electrochemical performance of the experimental O3-NaaNi(1-x-y-z)MnxMgyTixO2 phases was evaluated in full Na-ion test cells where the cathode materials were capacity balanced to a commercial hard carbon anode material. The electrolyte used was 0.5 M NaClO4 in propylene carbonate and testing was carried out at 30 oC.In Figure 2 we depict the low rate (C/10) voltage profile derived from a typical constant current charge-discharge cycle. The cathode active material cycles reversibly at a specific capacity of 163 mAh/g and the Na-ion cell generates an average discharge voltage of around 3.0 V. The specific energy performance for the cathode is comparable to commercial Li-ion electrode materials.Figure 3 shows a typical cycle life plot for a representative Na-ion cell. These data were collected at a C/10 charge/discharge rate using voltage limits of 4.2 and 1.0 V. Following 300 cycles the cell has retained over 70 % of the original discharge capacity. Most of this capacity loss is recoverable at lower discharge rates indicating importantly that the fade mechanism is associated with cell impedance changes rather than irreversible loss of active Na. Further performance data for the Na-ion system will be presented.