ABSTRACT
Sodium-ion batteries (SIBs) are the most promising alternative to lithium-ion batteries (LIBs) due to their low cost and environmental friendliness; therefore, enhancing the performance of SIBs' components is crucial. Although most of the studies have focused on single-phase cathode electrodes, these materials have difficulty in meeting the requirements in practice. At this point, composite materials show superior performance due to balancing different structures and are offered as an alternative to single-phase cathodes. In this study, we synthesized a Na0.44MnO2/Na0.7MnO2.05 composite material in a single step with cobalt substitution. Changes in the crystal structure and the physical and electrochemical properties of the composite and bare structures were studied. We report that even if the initial capacity is slightly lower, the rate and cyclic performance of the 1% Co-substituted composite sample (CO10) are superior to the undoped Na0.44MnO2 (NMO) and 5% Co-substituted (CO50) samples after 100 cycles. The results show that with the composite cathode phase transformations are suppressed, structural degradation is prevented, and better battery performance is achieved.
ABSTRACT
Na-ion batteries represent a promising complementary alternative to Li-ion batteries due to their high energy density and natural abundancy of Na. However, these batteries have short cycle life and extensive research activities on these batteries are required to understand the mechanism of such drawbacks. In this report, we investigate the capacity fading mechanism of Na0.44MnO2via ex situ X-ray diffraction, X-ray absorption spectroscopy, Fourier transform infrared spectroscopy and magnetization measurements. Our results show that the unit cell volume, the effective mass of Mn-O bonds, the number of Mn4+ ions and the effective magnetic moment decrease upon repeated cycling. We propose that some Mn4+ ions in the octahedral environment become Mn3+ ions in a square pyramidal environment, causing oxygen release upon cycling. Any free oxygen in the battery is expected to react with the electrolyte and cause capacity fade.
