Enhancing structural stability of NaCrO2 by Nb-substituting for sodium-ion battery

As an attractive cathode material, the layered sodium chromium oxide NaCrO 2 (NCO) can provide a reversible capacity of around 120 mAh g -1 between 2.0-3.6 V, which corresponds to about 0.5 Na per formula is extracted/inserted. However, irreversible phase transition happens in Na x CrO 2 when x is less than 0.4 and the potential is exceeding 3.6 V vs. Na + /Na. To improve its high-potential structural stability, Nb 5+ is introduced to Cr 3+ site by a sol-gel method successfully in this study. The influences of Nb 5+ doping on the crystal structure and electrochemical properties are explored. As a result, a more stable structure as well as a higher ionic conductivity are achieved in Na 0.94 Cr 0.97 Nb 0.03 O 2 (NCO-Nb3), exhibiting better rate performance and cycling stability between 2.0-3.6 V and 2.0-3.7 V. When elevating the charging cut-off voltage to 3.8 V, 4.0 V, 4.4 V, the initial coulombic efficiencies of NCO-Nb3 are still significantly higher than those of pristine NCO. • Pentavalent Nb 5+ is substituted in Cr 3+ site successfully, and the latiice spacing of modified NaCrO 2 products is enlarged because of the decrease of the interlayer Na + content. • The structure stability and ionic conductivity are improved with the doping of Nb 5+ . • Enhanced rate performances and stable cycle performances are achieved in different voltage ranges. • When tested between 2.0-3.7 V, NCO-Nb3 can achieve 1000 cycles at 10 C while NCO even becomes electrochemically inactive after 300 cycles.