Removal of Interstitial H₂O in Hexacyanometallates for a Superior Cathode of a Sodium-Ion Battery

Abstract

Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for large-scale storage of electrical energy, provided a host cathode for Na can be found that provides the necessary capacity, voltage, and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease of synthesis and rigid open framework that enables fast Na(+) insertion and extraction. Here we report an intriguing effect of interstitial H2O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal composition Na2MnFe(CN)6·zH2O (Na2-δMnHFC). The newly discovered dehydrated Na2-δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of 150 mAh g(-1) in a sodium half cell and 140 mAh g(-1) in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C, the half-cell capacity is 120 mAh g(-1), and at 0.7 C, the cell exhibits 75% capacity retention after 500 cycles.

Effect of Concentrated Electrolyte on Aqueous Sodium-ion Battery with Sodium Manganese Hexacyanoferrate Cathode

“…3(d) and the lattice behavior in 17 m NaClO 4 aq. on cycle was in good agreement with the previous report 26 of the NMHCF cathode in non-aqueous electrolyte except the generation of new rhombohedral phase after 1 cycle. The aqueous circumstance may disturb the generation of rhombohedral anhydride.…”

“…3(b), two plateaus derived from Fe 2+ /Fe 3+ redox and Mn 2+ / Mn 3+ redox were observed on the first and second charge, and the first charge/discharge capacities were 124/116 mAh g ¹1 , respectively. These profiles and capacities were consistent with the previous reports 26,27 in non-aqueous electrolyte and the theoretical capacity (120 mAh g ¹1 ) of NMHCF hydrate, respectively. The reversible capacity on the NTP anode side was higher in 17 m NaClO 4 aq.…”

“…26,27 Briefly, Na 4 Fe(CN) 6 · 10H 2 O (99%, Sigma-Aldrich) and excess NaCl (99.5%, Nacalai Tesque) were dissolved in a solution of distilled water and ethanol. An aqueous solution of MnCl 2 ·4H 2 O (99.0%, Wako Pure Chemical Industries Ltd.) was dropped into the solution and stirred vigorously for 2 h. The suspension was filtered and washed by a water/ethanol solution three times, and a light green precipitate product was obtained.…”

“…Song. 26 On the basis of elemental analysis by ICP-AES for Mn and Fe, AAS for Na, and TGA for water content, the NMHCF molecular formula was determined as Na 1.24 Mn[Fe(CN) 6 ] 0.81 ·1.28H 2 O. Although the crystal water content in the sample depends on the temperature and humidity, 21 the theoretical capacity of this hydrate can be roughly estimated as 120 mAh g ¹1 on the basis of 1.24 Na + extraction/ insertion.…”

A Safer Sodium‐Ion Battery Based on Nonflammable Organic Phosphate Electrolyte

“…Similar to their Li counterparts,1 though sodium‐based system has similar electrochemical reaction characteristics compared to lithium‐based one, the larger ionic radius for sodium ion cause sluggish kinetics and volume change during Na storage, leading to lower capacity, poor cycling and rate properties of the Na storage materials. Recently, major efforts have been devoted to promote the electrochemical performance of Na storage materials, for example, Na x MO 2 ,2, 3, 4, 5, 6, 7, 8, 9, 10 polyanionic framework compounds,11, 12, 13, 14, 15, 16, 17, 18, 19 hexacyanoferrate,20, 21, 22, 23, 24, 25, 26, 27 for the cathode materials, and hard carbons,28, 29, 30, 31, 32, 33 alloys,34, 35, 36, 37, 38, 39, 40, 41 oxides,42, 43 sulfides37, 44, 45, 46 for the anode materials.…”