Structural‐Defect‐Controlled Electrochemical Performance of Sodium Ion Batteries with NaCrO₂ Cathodes

Abstract

In this study, we report the effects of structural defects on the electrochemical cycle stability of Na‐ion batteries made of NaCrO2 cathodes. Three processing conditions are used to create NaCrO2 with different structural defects. One condition entails high‐energy ball milling of reactants before the solid‐state reaction at 900 °C, the second one has no high‐energy ball milling but simple mixing of reactants before the solid‐state reaction, and the last one has high‐energy ball milling after the solid‐state reaction. The NaCrO2 crystals derived from the process with high‐energy ball milling of reactants before the solid‐state reaction exhibit the best bulk conductivity and highest specific capacity with little capacity decay over 46 charge/discharge cycles. In contrast, the NaCrO2 crystals derived from the process without high‐energy ball milling have significant capacity decay, whereas the NaCrO2 crystals obtained from the process with high‐energy ball milling after the solid‐state reaction exhibits the lowest bulk conductivity and very low specific capacities. The different electrochemical properties exhibited by these NaCrO2 crystals have been ascribed to different extents of structural defects present in these crystals. This study unambiguously demonstrates that structural defects in NaCrO2 crystals are critical in achieving NaCrO2 electrodes with high specific capacity and excellent capacity retention.

Enhancing the Electrochemical Performance of NaCrO2through Structural Defect Control

“…22 In contrast, the typical synthesis condition for pristine NaCrO 2 is 900 °C with a holding time from 1 to 5 h only. 9,10,15,17,19,21 Thus, excluding the pristine NaCrO 2 synthesized at 900 °C with a 10 h holding time, 22 Table 2 shows that the BM-2h cell is the only pristine NaCrO 2 exhibiting capacity retention over the first 50 cycles larger than 90%, indicating the importance in design and control of the NaCrO 2 synthesis condition.…”

“…15 This irreversible capacity loss has been attributed to the irreversible migration of Cr ions from the octahedral sites in CrO 2 slabs to both tetrahedral and octahedral sites in the interslab layer. 15 Unfortunately, even for deintercalation of 0.5 mol of Na ions from NaCrO 2 to Na 0.5 CrO 2 , the specific capacity still exhibits gradual decay over charge/discharge cycles, 11,13,17 indicating the presence of capacity decay mechanisms even when intercalation and deintercalation are between NaCrO 2 and Na 0.5 CrO 2 .…”

“…17 Sawicki et al 17 have shown that reactants (in particular Na 2 CO 3 and Cr 2 O 3 ) with high-energy ball milling before solid-state reactions at 900 °C can form O3-NaCrO 2 with less misplacement of Cr 3+ at the Na + site with a longer Na−O bond length. Further, these O3-NaCrO 2 crystals exhibit little capacity degradation over 45 charge/discharge cycles at the rate of 0.25 C. 17 In contrast, they also reported that O3-NaCrO 2 with more misplacement of Cr 3+ at the Na + site and shorter Na−O bond length displays as much as 33% capacity degradation, 17 thus showing the importance of controlling the structural defects in the NaCrO 2 crystals for improving their capacity retention. However, in this previous study, 17 the effects of structural defects have only been studied up to 45 cycles.…”

“…Further, these O3-NaCrO 2 crystals exhibit little capacity degradation over 45 charge/discharge cycles at the rate of 0.25 C. 17 In contrast, they also reported that O3-NaCrO 2 with more misplacement of Cr 3+ at the Na + site and shorter Na−O bond length displays as much as 33% capacity degradation, 17 thus showing the importance of controlling the structural defects in the NaCrO 2 crystals for improving their capacity retention. However, in this previous study, 17 the effects of structural defects have only been studied up to 45 cycles. Thus, the effects on the long-term cycle stability are not clear yet.…”

“…Sodium-ion batteries (SIBs) have captured growing attention worldwide in the last several years. − Low cost and being an abundant resource make SIBs a competitive alternative to the current lithium-ion batteries (LIBs). Among various cathode materials investigated for SIBs, intercalation and deintercalation of Na in layered NaCrO 2 have been studied extensively. − One of the important advantages of NaCrO 2 is its high thermal stability because it is more stable than LiCoO 2 and LiFePO 4 . Accelerating calorimetry experiments have revealed that exothermic reactions do not take place for NaCrO 2 until 250 °C with little release of heat .…”