Unprecedented Cyclability and Moisture Durability of NaCrO₂ Sodium-Ion Battery Cathode via Simultaneous Al Doping and Cr₂O₃ Coating

Abstract

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO 2 remains one of the most attractive materials due to its reasonable level of capacity, nearly flat reversible voltages, and high thermal stability. However, the cyclic stability of NaCrO 2 needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that Cr 2 O 3 -coated and Al-doped NaCrO 2 , which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a Cr 2 O 3 shell and a Na(Cr 1−2x Al 2x )O 2 core, rather than xAl 2 O 3 /NaCrO 2 or Na 1/1+2x (Cr 1/1+2x Al 2x/1+2x )O 2 , through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either Cr 2 O 3 -coated NaCrO 2 without Al dopants or Al-doped NaCrO 2 without shells because of their synergistic contributions. As a result, Na(Cr 0.98 Al 0.02 )O 2 with a thin Cr 2 O 3 layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO 2 . In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of Cr 2 O 3 -coated Na(Cr 1−2x Al 2x )O 2 .

Unravelling air/moisture stability of cathode materials in sodium ion batteries: characterization, rational design, and perspectives

“…Ikhe et al studied the aqueous stability of Al-doped NaCrO 2 . 82 In Fig. 11a, NaCrO 2 particles displayed a strong affinity for water leading to dispersion and suspension upon soaking.…”

“…However, it is important to note that after reviewing various XPS analysis results in the literature, we have encountered instances of misinformation and conflicts arising from peak identification and fitting. 37,46,59,[77][78][79][80][81][82] To address this, below, we have undertaken a comparative analysis of XPS results, including our own research discussed in this review, in an effort to rectify any misleading analyses and offer perspectives and suggestions on XPS analysis. Regarding C 1s peak fitting, there is no doubt that the C-C peak at 284.8 eV is attributed to adventitious carbon from the ambient environment, commonly used as an internal calibration ref.…”

“…Although post-treatments after coating may increase costs and impact specific capacities, some coated materials exhibit remarkable air and electrolyte stability, alongside excellent long-term cyclability. 82 In summary, the pursuit of enhanced air stability of cathode materials requires a holistic approach, integrating both bulk structural medication and efficient coating strategies to produce air stable materials without compromising battery performance, cost or sustainability.…”

On the Electrochemical Properties of Carbon-Coated NaCrO2 for Na-Ion Batteries

“…Wang et al [22] investigated Cr 2 O 3 -coated NaCrO 2 and showed less deteriorated capacity fade at relatively high rates over bare NaCrO 2 . Similarly, Ikhe et al [23] have applied Cr 2 O 3 coating to Al-doped NaCrO 2, which also exhibited improved long-term cycle retention.…”

One‐step Surface‐to‐bulk Modification of NaCrO2 Cathode for Durable Sodium‐ion Batteries

“…Under a current density of 10 C, after 1000 cycles, it still has a specific capacity of 100.4 mAh g À 1 with a retention rate of about 84.8 %. [28] On this basis, Ikhe et al [29] combined Cr 2 O 3 coating with Al doping and found that the specific capacity had almost no attenuation after 1000 cycles. All the above studies show that surface coating is a very useful strategy to improve the stability of materials.…”