Prussian Blue Analogues for Rechargeable Batteries

프러시안블루 유사체를 활용한 이차전지 연구

Abstract

Prussian blue analogues(PBAs) are comprised of cyano-bridged transition metal ions. The wide and unique open-framework structures of the PBAs enable reversible intercalation and deintercalation of various ions such as $Na^+$, $K^+$, $Mg^{2+}$, $Zn^{2+}$, etc. In addition, since PBAs are synthesized through coprecipitation reaction in aqueous solution at room temperature, they are produced economically and environmentally friendly. However, the formation of crystals proceeds rapidly, and defects such as vacancy and crystal water tend to be present in the crystals, thereby affecting key battery performance. Therefore, significant efforts to inhibit defects in PBAs have been made. In the case of vacancy, the reaction rate was controlled at the synthesis stage to reduce the formation of vacancy, and the crystal water was removed by heat treatment under vacuum. In addition, by adding transition metals that do not react within the structure of PBA, the structural instability during the electrochemical reaction was largely alleviated.

프러시안 블루 유사체(Prussian blue analogue; PBA)는 두 종류의 전이금속이 시안화물 리간드와의 배위결합을 통해 서로 연결되어 만들어진 구조체이다. PBA는 넓은 골격구조를 통해, 다양한 이온의 가역적 삽입/탈리를 가능하게 할 뿐 아니라, 두 종류의 전이금속이 반응하여 높은 비용량을 구현한다. 또한, PBA는 상온에서 수용액 상에서의 공침반응을 통해 합성 되기에, 경제적이며 친환경적으로 생산된다. 하지만, 결정의 형성이 빠르게 진행되며, 수용액 상에서 발생하기에 결정 내 공공격자결함(Vacancy)과 결정수(Crystal water)가 발생하기 쉬우며, 이는 전기화학적 성능에 영향을 미친다. 따라서 이러한 공공격자결함 및 결정수의 생성 억제를 통해 PBA의 전기화학성능 향상에 대한 연구가 활발하게 진행되고 있다. 공공격자결함의 경우 반응속도 제어를 통해 합성단계에서 제어 되며, 결정수는 합성 후 진공 열처리 및 산화제와 복합체 형성을 통해 제거할 수 있다. 뿐만 아니라 PBA의 구조 내에 비활성 전이금속 도핑을 통해 상기 결함들로 인해 PBA가 전기화학 반응 중에 겪는 구조적 불안정성을 해소할 수 있다.

Fig. 1. Schematic illustration of the general crystal structure and redox reactions for PBA.

Fig. 2. (a) The synthetic procedure of PBAs consisting of coprecipitation, dissociation, nucleation, and crystal growth. (b) TGA curves measured at a scan rate of 10 °C min^-1 from room temperature to 500 °C in N₂ atmosphere (conventional coprecipitation-based PBA: RC-Na₂CoFe(CN)₆, controlled crystallization-based PBA: CC-Na₂CoFe(CN)₆). (c) The XRD profiles of the RC-Na₂CoFe(CN)₆ and CC-Na₂CoFe(CN)₆. (d) Charge/discharge profiles and (e) long-term cycling stability at a rate of 1 C and 5 C, respectively (1 C=130 mA g^-1). Reproduced with permission.¹²⁾ Copyright 2015, John Wiley and Sons.

Fig. 3. (a) The structure of PBMN. (b) The galvanostatic charge–discharge profiles of PBM, PBN and PBMN when measured in the range of 2.0~4.0 V at 10 mA g^-1. (c) Cycling performance of PBM, PBN and PBMN when measured in the range of 2.0~4.0 V at 100 mA g^-1. Reproduced with permission.¹⁶⁾ Copyright 2014, The Royal Society of Chemistry.

Fig. 4. (a) The TGA curves and (b) IR spectra of air-dried and vacuum-dried Na₂MnFe(CN)₆. The TGA test was conducted at a heating rate of 5 °C min⁻¹ under N₂ atmosphere. Ex situ XRD patterns of (c) air-dried and (d) vacuumdried Na₂MnFe(CN)₆ at different states at the first cycle. (e) Rate capability of the vacuum-dried Na₂MnFe(CN)₆. (f) Cycling performance of vacuum-dried Na₂MnFe(CN)₆. Reproduced with permission.¹⁷⁾ Copyright 2015, American Chemical Society.

Fig. 5. (a) A schematic mechanism for the removal of coordinated water from RGOPC. (b) The galvanostatic charge–discharge profiles of PB, RGOPC1, RGOPC2 and RGOPC3 when measured in the rage of 2.0~4.0 V at 30 mA g^-1. (c) Cycling performance of PB, RGOPC1, RGOPC2 and RGOPC3 when measured in the rage of 2.0~4.0 V at 200 mA g^-1. Reproduced with permission.¹⁸⁾ Copyright 2015, The Royal Society of Chemistry.

Table 1. Elemental compositions of NaCoFe(CN)₆ from controlled crystallization (CC-NaCoHCF) and NaCoFe(CN)₆ from conventional coprecipitation reaction (RC-NaCoHCF). Reproduced with permission.¹²⁾ Copyright 2015, John Wiley and Sons

Table 2. The atom occupation and d-spacing of PB, RGOPC1, RGOPC2 and RGOPC3. Reproduced with permission.¹⁸⁾ Copyright 2015, The Royal Society of Chemistry.