Dual redox-active copper hexacyanoferrate nanosheets as cathode materials for advanced sodium-ion batteries

Highlights

• Crystallization controlled synthesis were performed for optimized CuHCF cathode.

• Both Cu and Fe are redox-active in CuHCF cathodes.

• The initial coordinate water inhibit the redox activity of both transition metals.

• Pair distribution function (PDF) analysis was used to detect framework structures.

Abstract

Prussian Blue Analogs (PBAs) such as copper hexacyanoferrate (CuHCF) are traditional intercalation cathodes for rechargeable Na-ion batteries. However, the electrochemical performance of these PBAs suffers from insufficient activation and sharp performance deterioration. Here, the insufficient electrochemical activity and frame deformation issues in the CuHCF cathodes were investigated to enhance their specific capacity and improve their cycling stability. The high-crystallinity CuHCF nanosheets (CuHCF-P) with low-water-content were synthesized by a pyrophosphate-assistant co-precipitation method. It has a highly reversible 1.5-Na insertion/desertion capability with a specific capacity of ~120 mAh g⁻¹ at 0.1 C, which is the highest among all the CuHCF cathodes reported. First-principle study and XPS detection demonstrate that both Cu and Fe are redox-active centers in CuHCF-P cathode, which facilitates a high Na⁺ storage capability. And the decrease of water content in CuHCF framework increases Fe/Cu 3d-orbital occupy-sites, which activates both of the transition-metals. Furthermore, the full cells fabricated with as-prepared CuHCF-P cathode and commercial hard carbon anode exhibit excellent performances with a reversible capacity of 109 mAh g⁻¹ at 0.1 C over 200 cycles.

Introduction

Prussian Blue Analogs (PBAs), intrinsic open framework structure materials with wide migration channels, are typical insertion electrode materials for Na-ion batteries [1], [2], [3], [4]. A traditional PBAs are generally described as A_xM[Fe(CN)₆]_y•□_1-y•mH₂O [5]. Fe and M coordinate with six cyanide groups by Fe-C and M-N bonds, respectively, forming an octahedral geometry. Insertion cation A and partial H₂O molecules occupy the inner space of the framework, which alters the open framework lattice into cubic, monoclinic, rhombohedral, or triclinic structures [6], [7], [8]. The open frameworks provide a large insertion space of approximately 4.6 Å and a wide migration channel of 3.2 Å along the 〈100〉 direction [9]. Thus, this open framework facilitates the fast transport of large-radius Na-ions for electrochemical insertion/extraction. When both M and Fe in PBA framework participate in redox reactions, they would go through a two-electron transferring and contribute to a higher specific capacity. However, these PBA materials are usually suffered from insufficient activation with a much lower capacity than the theoretical value [10]. Also, the excessive intercalation ions would lead to severe frame distortion of PBA frameworks and cause sharp performance deterioration of PBA-based batteries [11,12].

Copper-based PBA (CuHCF, copper hexacyanoferrate) is assembled with Cu-NC-Fe skeletons, generally contain high amounts of [Fe(CN)₆] vacancies and H₂O molecules [13]. They have high redox potential around 3.5 V for Na⁺ storage, but experience 0.8-Na reversible reaction corresponds to a specific capacity below 60 mAh g⁻¹ [14], [15], [16], [17], [18]. CuHCF is partially deactivated, there should expect more for CuHCF. Cu and Fe are potential electrochemical active centers in some metallic-organic composite materials as the Cu^+/2+ and Fe^2+/3+ redox couples [10,19]. According to the previous report, both Cu and Fe can participate in a redox reaction for 1.5-Li storage [20,21], but the CuHCF frames collapse rapidly in few cycles [22]. Notably, the frame volume strain is more severe for the large-radius alkali ions' insertion. Thus the CuHCF frame stability would be worse after massive Na-ion insertion [23]. Although some preliminary efforts such as in-situ doping or coating were performed to activate the redox reaction sites and stabilize its structure [24]. An inherent understanding of the deactivated mechanisms is urgently demanded, as well as improving Na storage performances.

Here, a novel monoclinic CuHCF nanosheet (CuHCF-P) with 50% excesses specific capacity and stable cyclic performance are reported. Pair distribution function (PDF) analysis was performed to investigate the fine structural characteristics of these frameworks. A First-principle study and XPS detection illustrate the electrochemical inactivity mechanisms. And it demonstrates that the initial coordinate water in the frames is responsible for this inactivation behavior. The optimized CuHCF-P is prepared by a pyrophosphate assistant co-precipitation method. It has a low-content of H₂O and exhibits superior high specific capacity (117.9 mAh g⁻¹) with 1.5 Na⁺ insertion/desertion reaction per formula and excellent cyclic stability over 300 cycles. Further, the as-prepared CuHCF nanosheets are compatible with a commercial hard carbon anode. The full-cell exhibits a high reversible capacity of 109 mAh g⁻¹ and excellent capacity retention of 96.6% over 200 cycles.