Electrochemical Performance of Large-Grained NaCrO₂ Cathode Materials for Na-Ion Batteries Synthesized by Decomposition of Na₂Cr₂O₇·2H₂O

Abstract

The solid-state reaction has been widely employed as the standard procedure to prepare oxide cathode materials for sodium-ion batteries. However, it involves multiple steps and consumes much energy. In this work, we report a facile method to synthesize a large-grained O3−NaCrO 2 cathode by directly reducing sodium dichromate dihydrate (Na 2 Cr 2 O 7 • 2H 2 O) under a hydrogen atmosphere. Owing to its unique large particle morphology, the as-prepared NaCrO 2 exhibits a high tap density of 2.55 g cm −3 . The compact NaCrO 2 shows excellent electrochemical performance with a high reversible capacity of 123 mAh g −1 at 0.1C, a high capacity retention of 88.2% after 500 cycles at 2C, and an outstanding rate capability of 68 mAh g −1 at 20C. The performance is attributed to a stable structure from the distinctive morphology with small specific surface area to suppress interfacial side reactions and rapid Na-ion diffusion channels with a highly (110)-oriented crystal structure. Ex situ X-ray diffraction and cyclic voltammetry tests demonstrate the consecutive and reversible phase transition mechanism with facile Na + migration. Importantly, the obtained cathode material exhibits an excellent performance in sodium-ion full cells with hard carbon as the anode.

Grain Boundary Design of Solid Electrolyte Actualizing Stable All‐Solid‐State Sodium Batteries

“…The charge/discharge profile and cycling performance at the rate of 1 C (1 C = 125 mA g −1 ) are provided in Figure 5d,e with initial ten cycles at 0.3 C in the voltage range of 2.3–3.6 V. The first charge/discharge curves show a typical wide voltage plateau of around 2.9 V at 0.3 C and several narrow higher voltage plateaus, which are in accordance with the previous literature. [ 17 ] High Coulombic efficiency of 99.9% is enabled during charge/discharge cycles for 1755 times, indicating high reversibility of the battery with a longer cycle life than those of NaCrO 2 ‐based sodium ion batteries using liquid electrolytes. [ 17 ] A reversible capacity of 110.9 mA h g −1 is delivered at 1 C with a high retention of 87% at the 1755th cycle (Figure 5e).…”

“…[ 17 ] High Coulombic efficiency of 99.9% is enabled during charge/discharge cycles for 1755 times, indicating high reversibility of the battery with a longer cycle life than those of NaCrO 2 ‐based sodium ion batteries using liquid electrolytes. [ 17 ] A reversible capacity of 110.9 mA h g −1 is delivered at 1 C with a high retention of 87% at the 1755th cycle (Figure 5e). The rate performance of the NaCrO 2 /NZSP‐0.2Mg/Na battery was evaluated at the rates of 0.3–5 C. The charge/discharge profile in Figure 5f reveals that the discharge voltage decreases from the stabilized 2.9 V at 0.5 C on the 11th cycle to 2.7 V at 5 C on the 61th cycle, representing small voltage output decay and excellent rate capability.…”

Enhancing the Electrochemical Performance of NaCrO2through Structural Defect Control

“…This is better than pristine NaCrO 2 , which typically exhibits capacity retention after 50 cycles between 63 and 87% 11,[13][14][15]18,20,21 with only one exception showing 92% capacity retention. 22 The reason for this exception is not obvious but may be related to its substantially long holding time (10 h) at 900 °C when synthesizing pristine NaCrO 2 . 22 In contrast, the typical synthesis condition for pristine NaCrO 2 is 900 °C with a holding time from 1 to 5 h only.…”

“…22 By reducing sodium dichromate dehydrate under a hydrogen atmosphere, Wang et al 22 have synthesized large-grained NaCrO 2 particles (1−80 μm) which exhibit a high reversible capacity of 123 mA h/g at 0.1 C, a high capacity retention of 88% after 500 cycles at 2 C, and an outstanding rate capability of 68 mA h/g at 20 C. The improved cycle stability has been ascribed to the reduced side reactions because of the decreased specific surface area of large-grained particles, while the outstanding rate capacity has been attributed to the greatly enhanced growth of {010} active facets that allow fast intercalation and deintercalation of Na ions. 22 The fourth strategy in improving the cycle stability of NaCrO 2 is to fabricate nanocrystal-constructed ultralong NaCrO 2 nanowires via electrospinning. 23 It has been shown that such nanowires offer a superior capacity retention of 88% after 300 cycles at 2 C while also delivering an outstanding rate capability (∼87 mA h/g at 50 C).…”

Aliovalent-doped sodium chromium oxide (Na0.9Cr0.9Sn0.1O2 and Na0.8Cr0.9Sb0.1O2) for sodium-ion battery cathodes with high-voltage characteristics

“…In contrast to O3-LiCrO 2 , which suffers from the hindrance of Li + diffusion due to Cr 6+ ions generated during charging, 22 O3-NaCrO 2 shows reversible Na + intercalation/deintercalation within a reasonable voltage window. [22][23][24][25][26][27][28][29][30][31] Unless the high-voltage cutoff (E high ) exceeds ca. 3.6 V, the hexagonal O3 phase of NaCrO 2 is reversibly transformed to a monoclinic P 0 3 type via the monoclinic O 0 3 phase and delivers a capacity of ca.…”