Study on Factors Affecting Water Resistance of Titanium Negative Electrode Materials for Na Batteries

Abstract

【Background】 In recent years, sodium-ion batteries, which use abundant Na ions as charge carries, have been attracting attention as an alternative to lithium-ion batteries. Sodium-ion batteries are expected to be used as large-scale energy storage devices due to moderate energy density, but air storage stability of the layered oxides is an important issue to be considered. In this study, air storage stability of titanium layered oxides with different chemical compositions and structures, P2 Na2/3Cr1/3Mg1/6Ti1/2O2, P2 Na0.67Li0.22Ti0.78O2, P2 Na2/3Cr2/3Ti1/3O2, and P3 Na0.58Cr0.58Ti0.42O2, is systematically examined, from which the factors affecting air storage stability is discussed.【Results and Discussion】 Figure 1 shows the X-ray diffraction patterns of P2 Na2/3Cr1/3Mg1/6Ti1/2O2 (P2 NaCrMgTi) before and after soaking in water for 30 minutes (and then dried after vacuum filtration). After the soaking test, although the profile of diffraction lines is broadened, its original crystal structure is retained. On the other hand, for P2 Na2/3Cr2/3Ti1/3O2 (P2-NaCrTi), and P3-Na0.58Cr0.58Ti0.42O2 (P3-NaCrTi), hydrated phases are formed. Figure 2 compares charge/discharge curves of these layered oxides after the soaking test. Electrodes with P2-NaCrMgTi, which have a minor change in crystal structure, shows the deterioration in initial Coulomb efficiency, but no reduction in discharge capacity is evidenced. In contrast, both P2-NaCrTi and P3-NaCrTi, where the hydrated phases were formed, shows the degradation in electrode reversibility. From these results, air storage stability of these different layered oxides is discussed in detail. Figure 1