Effect of Al-Doping on the Electrochemical Performances of O3-Type Nanmc Cathode Material for Sodium-Ion Batteries

The emergence of Lithium-ion technology as a primary power source has revolutionized the global electric vehicle battery market, the high abundance, uniform geological distribution and similar electrochemistry of sodium, making sodium-ion batteries (NIBs) a promising LIB supplement in the large-scale energy storage applications1,2. Layered sodium transition metal oxide of O3-NaMO2-type such as NaCoO2, NaMnO2, NaNiO2, and NaFeO2, etc. have been investigated to show reversible Na-ion insertion within the applied potential limit 2,3. They suffer from their characteristic disadvantages, such as low redox potential of NaCoO2, complex phase transition of NaNiO2, electrolyte dissolution of Mn+2 of NaMnO2 and rapid capacity fading of NaFeO2 4,5. Therefore, the strategy of cation mixing to develop the multi-metallic oxides has been explored well to utilize the synergistic effects of all metal ions. O3-type layered NaNi0.5Mn0.3Co0.2O2 is considered as one of the most promising cathode materials for NIBs. O3-NaNi0.5Mn0.3Co0.2O2 as cathode material for SIBs delivers a 1st cycle capacity of 135 mAh g-1 with the 37% capacity fade at the end of 200th cycle at C/10 current rate. NaNi0.5Mn0.3Co0.2O2, synthesized by using simple solution combustion method followed by thermal treatment delivers an initial discharge capacity of 135 mAh g-1 at C/10 rate, which indicates a reversible insertion of ~50% sodium. However, it loses 37% of the initial capacity after 200 cycles due to structural deformation during sodiation/de-sodiation process. The irreversible phase transition due to structural deformation leads to sluggish kinetics, rapid capacity fade, and poor rate performance; thereby limit its wide practical applications. To mitigate structural instability and rapid capacity fading, doping of main-group metals within transition metal layers is an effective strategy.6-8 The partial substitution of Co3+ (0.545 Å) by Al3+ (0.535 Å) ions in the transition-metal layer to synthesize NaNi0.5Mn0.3Co0.2-xAlx (x=0.01, 0.02, 0.05) by solution combustion technique is an effective strategy to address the issue of structural deformation and thus to improve the performance of NaNi0.5Mn0.3Co0.2O2. The O3-type structure of the synthesized material with the R-3m space group was confirmed from XRD analysis. The synthesized materials show morphology of hexagonal plate-like primary structures aggregated to form secondary clusters. The galvanostatic charge-discharge studies carried out at C/10 rate in the voltage range of 2.0-4.0 V shows that the composition with an overall 2% Al doping (x=0.02) delivers much better capacity retention (~28% improvement than pristine NaNMC) even after 100 cycles than the other compositions studied (1% (x=0.01) and 5% (x=0.05) Al doping). Moreover, the NaNi0.5Mn0.3Co0.18Al0.02O2 shows the good capacity of around 80 mAhg-1 even at high C-rate of 5C rate, which is almost 72% of the initial capacity at C/10 rate. The improved electrochemical performance of the Al-substituted NaNMC is attributed to the enhanced structural stability of the sodium layered transition metal oxide achieved after the partial substitution of Co3+ by Al3+ ion. References 1. G. Zubi, R. Dufo-López, M. Carvalho and G. Pasaoglu, Renewable and Sustainable Energy Reviews, 89, 292 (2018). 2. N. Yabuuchi, K. Kubota, M. Dahbi, and S. Komaba, Chem. Rev., 114,11636 (2014). 3. J. Y. Hwang, C. S Yoon, I. Belharouak, and Y.K Sun, J. Mater. Chem. A, 4, 17952 (2016). 4. P. Vassilaras, A. J. Toumar, and G. Ceder, Electrochem. Commun., 38, 79 (2014). 5. M. H. Han, E. Gonzalo, G. Singh, and T. Rojo, Energy Environ. Sci., 8, 81 (2015). 6. M. Sathiya, K. Hemalatha, K. Ramesha, J-M. Tarascon, and A. S. Prakash, Chem. Mater, 24, 1846 (2012). 7. T. Hwang, J.-H. Lee, S. H. Choi, R.-G. Oh, D. Kim, M. Cho, W. Cho, and M.-S. Park, ACS Appl. Mater. Interfaces, 11, 30894 (2019). 8. H. Wang, R. Gao, Z. Li, L. Sun, Z. Hu, and X. Liu, Inorg Chem., 57, 5249 (2018).