Mechanochemical Synthesis of K_xMn[Fe(CN)₆] and CNT Composite for High-power Potassium-ion Batteries

Abstract

This study introduces a facile mechanochemical synthesis of K_xMn[Fe(CN)₆] (KMnHCF) and carbon nanotube (CNT) composite (KMnHCF@CNT) as a positive electrode material for potassium-ion batteries. The KMnHCF@CNT, synthesized by a simultaneous process of mechanochemical synthesis and carbon compositing, shows a homogeneous composite and achieves a much higher electron conductivity of 7.16 × 10⁻¹ S cm⁻¹ than the KMnHCF and CNT mixture (2.35 × 10⁻² S cm⁻¹) synthesized by the two-step process. The improved electron conductivity demonstrates reduced carbon content in the electrode and excellent rate performance of maintaining 80 mAh g⁻¹ at 20 C in potassium cells.

Potassium-ion batteries (KIBs) have recently attracted considerable attention as an alternative to Li-ion batteries for various energy storage applications.^1–3 Their significant advantages include abundant raw material resources and potentially high power density. The potential high-power performance is due to the weak Lewis acidity of K⁺ ions compared to Li⁺ ions, which enables faster ionic diffusion in electrolytes.¹ Therefore, the development of high-power electrode materials is essential to achieve high-power KIBs.

Prussian blue analogues (PBAs) have emerged as one of the most promising positive electrode materials for KIBs due to their high reversible capacity, high working voltage, and long cycle life.^4–6 These open-framework structures, composed of transition metal hexacyanoferrates, provide large channels that allow fast ion diffusion. However, the low electronic conductivity of PBA and the consequent high carbon content in the electrode result in low power density and low energy density of the electrode.

Carbon coating and carbon composites are widely accepted methods to overcome the low electronic conductivity of electrode materials, as demonstrated in LiFePO₄.^7,8 Thus, researchers have developed these methods for PBAs. Moritomo et al. synthesized glucose-treated Na_xMn[Fe(CN)₆] (NaMnHCF) at 300 °C in a vacuum to improve rate capability.⁹ The glucose-treated NaMnHCF demonstrated improved rate performance. Nevertheless, the low heat treatment temperatures due to the limited thermal stability of NaMnHCF would be insufficient to form highly conductive carbon coating, and the authors hypothesized that this improvement in rate performance is mainly the result of improved contact with conductive carbon. You et al. used a precipitation synthesis method to crystallize Na_xFe[Fe(CN)₆] (NaFeHCF) on carbon nanotubes (CNT).¹⁰ Although the NaFeHCF@CNT composite achieved impressive low-temperature and rate performance, the synthesis method is applicable only to specific PBAs like Na_xFe[Fe(CN)₆] or K_xFe[Fe(CN)₆] because it utilized the decomposition of Na₄[Fe(CN)₆] as a single precursor.

In general, PBAs are synthesized by precipitation method in aqueous solutions.^11,12 However, direct synthesis of homogeneous carbon composite PBAs in aqueous solutions has several challenges, such as the dispersibility of carbons and controlling PBA nucleation on the carbon surface. Indeed, our preliminary results showed inhomogeneous composite formation by precipitation method in a solution with CNT dispersion (Fig. S1). Thus, solvent-free synthesis method would have advantages to synthesizing carbon composite PBAs. According to the literature, PBAs can also be synthesized by mechanochemical method.^13–15 In this study, we introduce a one-step mechanochemical synthesis of KMnHCF-CNT composite (KMnHCF@CNT), demonstrating a feasible and flexible approach to carbon composite PBA synthesis.

KMnHCF samples were synthesized using the mechanochemical method (see Supporting Information for detailed methods). Briefly, K₄[Fe(CN)₆]·3H₂O and multiwall CNT dispersion were mixed and dried. The obtained powder was ball-milled with MnCl₂·4H₂O at 300 rpm for 12 h. The sample was then washed with water and dried at 100 °C to obtain MC-KMnHCF@CNT. MC-KMnHCF was synthesized using the same procedure without multiwall CNT.

First, the synthesized samples were investigated through powder XRD and Rietveld refinement using GSAS-II software.¹⁶ The XRD patterns before washing revealed that both MC-KMnHCF and MC-KMnHCF@CNT exhibited strong peaks of KCl and minor peaks of unreacted K₄[Fe(CN)₆]·3H₂O, as well as monoclinic KMnHCF (Fig. S2). Thus, the following reaction (Eq. 1) should mechanochemically proceed thorough ball-milling.

  

\begin{align} &\text{K$_{4}$Fe(CN)$_{6}{\cdot}$3H$_{2}$O} + \text{MnCl$_{2}{\cdot}$4H$_{2}$O} \\&\quad \to \text{K$_{6-4y}$Mn[Fe(CN)$_{6}$]$_{y}{\cdot}{}n$H$_{2}$O} + \text{2KCl}\\ &\qquad + \text{$(4-n+3y)$H$_{2}$O} + \text{$(1-y)$K$_{4}$[Fe(CN)$_{6}$]${\cdot}$3H$_{2}$O} \end{align} (1)

After washing, the impurity peaks disappeared, indicating that PBA could be synthesized without crystalline impurities (Fig. 1). All peaks were indexed with space group P2₁/n, which is the typical structure of K-rich PBA.^6,17,18 Rietveld refinement indicated that MC-KMnHCF and MC-KMnHCF@CNT have some K⁺ and [Fe(CN)₆]⁴⁻ vacancies, and their respective estimated compositions were K_1.76Mn[Fe(CN)₆]_0.94 and K_1.82Mn[Fe(CN)₆]_0.94 (Fig. S3, Tables S1 and S2), which deviate from the stoichiometric one of K₂Mn[Fe(CN)₆]. Furthermore, the estimated composition of KMnHCF@CNT (K_1.82Mn[Fe(CN)₆]_0.94) was in good agreement with ICP-AES data (K_1.78Mn[Fe(CN)₆]_0.92), confirming the reliability of the Rietveld refinement. Based on the composition, the Fe and Mn oxidation states were +2 in both samples, indicating no oxidation during synthesis. The [Fe(CN)₆]⁴⁻ vacancy formation would be due to rapid nucleation/crystal growth, as reported in the literature.^19,20 In order to further decrease the number of vacancies, using an excess amount of K₄[Fe(CN)₆]·3H₂O and optimizing the ball-milling speed could be effective and are under investigation. Finally, elemental analysis was used to estimate water and CNT content, and the KMnHCF@CNT composition was estimated as K_1.76Mn[Fe(CN)₆]_0.94·0.79H₂O with 5.85 % of CNT.

Figure 1.

XRD patterns of K_xMn[Fe(CN)₆] synthesized by mechanochemical method (MC-KMnHCF) and K_xMn[Fe(CN)₆] and CNT composite (MC-KMnHCF@CNT) with calculated curves obtained by Rietveld refinement.

The morphology of MC-KMnHCF and MC-KMnHCF@CNT was examined with SEM. The MC-KMnHCF sample showed highly agglomerated primary particles of <100 nm to form micrometer-size secondary particles (Fig. 2a). The KMnHCF@CNT sample also presented agglomerated primary particles of <100 nm and CNTs to form secondary particles (Fig. 2b). Moreover, some CNTs connected the secondary particles to each other. Incorporating carbon nanotubes within KMnHCF@CNT will likely enhance total electron conductivity and enable more uniform current distribution in electrodes.

Figure 2.

SEM images of (a) MC-KMnHCF and (b) MC-KMnHCF@CNT.

Initially, MC-KMnHCF was tested with a composite electrode whose composition was AM : AB : PVdF = 80 : 10 : 10 wt%. In this setup, the KMnHCF electrode showed almost no reversible capacity (Fig. S4), possibly due to low electronic conductivity. Thus, we mixed MC-KMnHCF and 5 wt% CNT by ball milling to prepare a control sample (MC-KMnHCF/CNT mixture).

Both MC-KMnHCF/CNT mixture and MC-KMnHCF@CNT composite electrode demonstrated reversible charge/discharge curves with two plateaus and a reversible capacity of over 110 mAh g⁻¹ at 0.1 C (1 C = 155 mA g⁻¹ based on the theoretical capacity of K₂Mn[Fe(CN)₆]) (Figs. 3a and 3b). The lower obtained capacity than the theoretical capacity would be mainly due to self-discharge caused by electrolyte decomposition, which has been recently studied.^21,22 The MC-KMnHCF/CNT mixture showed a rapid potential drop with increased current density to 1 C and no reversible capacity at 5 C or higher rates (Fig. 3a). In contrast, the KMnHCF@CNT composite exhibited smaller potential drops at high rates and maintained reversible capacity of 80 mAh g⁻¹ at 20 C (Fig. 3b). The superior rate capability of KMnHCF@CNT composite would be due to enhanced electron conductivity. The detailed mechanisms of the enhanced rate capability are under investigation and will be reported elsewhere.

Figure 3.

Charge/discharge curves of (a) MC-KMnHCF/CNT mixture and (b) MC-KMnHCF@CNT composite electrodes in K cells discharged at various current densities from 0.1 C to 20 C (1 C = 155 mA g⁻¹) and charged at 0.1 C. (c) Variation of discharge capacity of MC-KMnHCF@CNT and MC-KMnHCF/CNT electrodes at different current densities.

Next, we measured the electric conductivity of MC-KMnHCF@CNT composite and MC-KMnHCF/CNT mixture without the additional carbons or binders. The MC-KMnHCF@CNT showed a higher conductivity of 7.16 × 10⁻¹ S cm⁻¹ than MC-KMnHCF/CNT mixture (2.35 × 10⁻² S cm⁻¹). Thus, the well-composited morphology of KMnHCF@CNT is highly effective in improving electron conductivity.

In summary, this study demonstrates a facile mechanochemical synthesis of KMnHCF@CNT composite. The synthesized composite demonstrated excellent rate capability, showing a reversible capacity of 80 mAh g⁻¹ at 20 C. This approach provides a flexible and feasible path towards the synthesis of carbon composite of PBAs.

Acknowledgment

This study was partially funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Program: Data Creation and Utilization Type Materials Research and Development Project (JPMXP1122712807), the JST through the A-STEP program (Grant No. AS282S001d), CREST (Grant No. JPMJCR21O6), and JSPS KAKENHI (Grant No. JP18K14327, JP20J13077, JP20H02849, JP21K14724, JP21K20561, JP22K14772, and JP23K13829). TH thanks Tokuyama Science Foundation research grant.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.24978534.

• 1) This study introduces a facile mechanochemical synthesis of K_xMn[Fe(CN)₆] (KMnHCF) and carbon nanotube (CNT) composite (KMnHCF@CNT) as a positive electrode material for potassium-ion batteries. The KMnHCF@CNT, synthesized by a simultaneous process of mechanochemical synthesis and carbon compositing, shows a homogeneous composite and achieves a much higher electron conductivity of 7.16 × 10⁻¹ S cm⁻¹ than the KMnHCF and CNT mixture (2.35 × 10⁻² S cm⁻¹) synthesized by the two-step process. The improved electron conductivity demonstrates reduced carbon content in the electrode and excellent rate performance of maintaining 80 mAh g⁻¹ at 20 C in potassium cells.

CRediT Authorship Contribution Statement

Tomooki Hosaka: Conceptualization (Lead), Formal analysis (Equal), Investigation (Equal), Supervision (Lead), Visualization (Equal), Writing – original draft (Lead)

Nobuhito Noro: Formal analysis (Lead), Investigation (Equal), Visualization (Equal)

Shinichi Komaba: Conceptualization (Supporting), Funding acquisition (Lead), Supervision (Equal), Writing – review & editing (Supporting)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

MEXT: JPMXP1122712807

Japan Science and Technology Agency: AS282S001d

Japan Science and Technology Agency: JPMJCR21O6

JSPS: JP18K14327

JSPS: JP20J13077

JSPS: JP20H02849

JSPS: JP21K14724

JSPS: JP21K20561

JSPS: JP22K14772

JSPS: JP23K13829

Footnotes

T. Hosaka: ECSJ Active Member

S. Komaba: ECSJ Fellow

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