Effect of Al-Doping on the Electrochemical Performances of O3-Type Nanmc Cathode Material for Sodium-Ion Batteries
Authors: Kiran Kumar V , Ghosh Shuvajith , Biswas Sanjay , Martha Surendra Kumar 
Published: May 2020

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.

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