Scalable Production of Nano‑Perovskite K(Mn0.95Ni0.05)F3 Cathode via EDTA‑Assisted Homogeneous Precipitation for High‑Performance Potassium‑Ion Batteries

Abstract

Potassium‑ion batteries (KIBs) offer a promising alternative to lithium‑ion systems due to abundant potassium resources and higher operating potentials. We report the first scalable synthesis of a nano‑perovskite cathode, K(Mn_0.95Ni_0.05)F₃, by an EDTA‑assisted homogeneous precipitation route. The method yields monodisperse particles (~100 nm) with a concentration‑gradient Ni distribution that suppresses Mn dissolution and enhances ion transport. Depositing these particles on multi‑walled carbon nanotubes (MWCNTs) creates a nanocomposite that markedly improves electronic conductivity. Electrochemical tests show that the K(Mn_0.95Ni_0.05)F₃/MWCNT cathode delivers 106.8 mAh g⁻¹ at 35 mA g⁻¹ after 60 cycles, with a 92.6 % capacity retention and a Coulombic efficiency exceeding 99 %. Electrochemical impedance spectroscopy confirms low charge‑transfer resistance and stable SEI formation, underpinning the observed cycle life. These results demonstrate that the concentration‑gradient perovskite framework is a viable route to high‑performance, scalable KIB cathodes.

Introduction

The rapid expansion of portable electronics and electric vehicles has spurred extensive research into alternatives to lithium‑ion technology. While LIBs face material scarcity and cost challenges, potassium offers a more earth‑abundant resource and can operate at higher potentials, translating into higher energy density. However, KIB development lags behind, largely due to limited cathode options and the sluggish kinetics of K⁺ intercalation.

Perovskite fluorides are attractive because of their robust crystal frameworks and high‐voltage platforms. Their main drawback—poor electronic conductivity—has been mitigated by composite strategies, yet most synthesis routes involve harsh conditions or toxic reagents. Here we leverage a gentle, EDTA‑mediated homogeneous precipitation that controls Mn release, produces uniform nanoparticles, and allows Ni substitution to create a concentration‑gradient structure that stabilizes the lattice and facilitates K⁺ diffusion.

Materials and Methods

Raw Materials

All reagents were analytical grade: EDTA‑2Na (98 %), Mn(CH₃COO)₂·4H₂O (99 %), Ni(CH₃COO)₂·4H₂O (99.9 %), KF (99 %), MWCNTs (>95 %), PVDF (Arkema), and NMP (99 %).

Synthesis of K(Mn_0.95Ni_0.05)F₃

Six millimoles of EDTA‑2Na and 5.25 mmol of Mn acetate were dissolved in a 1:1 water/ethanol mixture, followed by the addition of 20 mmol KF to form solution A. Ni acetate (6 mmol) was introduced dropwise under stirring, allowing 30 min of reaction and 12 h of standing. The precipitate was collected by centrifugation, washed with ethanol and water, and dried at 60 °C. Varying Ni content (6.0–6.5 mmol) produced KMnF₃, K(Mn_0.975Ni_0.025)F₃, and the target K(Mn_0.95Ni_0.05)F₃ phases.

Fabrication of K(Mn_0.95Ni_0.05)F₃/MWCNT Composite

MWCNTs (0.1 g) were dispersed in a 1:1 water/ethanol solution via sonication (0.5 h) and then combined with solution A. Subsequent steps mirrored the particle synthesis, yielding a uniform nanocomposite.

Characterization

Phase purity and crystallinity were assessed by XRD (Cu Kα, 10–70°). Surface composition was examined by XPS (Al Kα). Morphology was observed by TEM (Tecnai G2 F20). Elemental ratios were quantified via ICP‑AES. Electrochemical performance was evaluated in coin cells with a 0.85 M KPF₆ in EC/DEC (1:1) electrolyte. Galvanostatic cycling (4.2–1.2 V vs K/K⁺) and rate tests (35–280 mA g⁻¹) were conducted. EIS (10⁵–10⁻² Hz) probed interfacial kinetics.

Results and Discussion

Structural Confirmation

XRD patterns confirm the formation of the perovskite structure; Ni incorporation shifts peaks to higher angles, indicating lattice contraction. ICP‑AES shows Mn:Ni ratios matching theoretical values (Table 1). TEM reveals monodisperse particles with average diameters of 100 nm for the Ni‑doped sample, a significant reduction from 150 nm in the undoped KMnF₃. HRTEM and line‑scan analysis demonstrate a uniform Ni distribution on the surface and a Mn concentration gradient toward the core, consistent with the proposed precipitation mechanism.

Composite Morphology and Composition

SEM/TEM images of the K(Mn_0.95Ni_0.05)F₃/MWCNT composite show well‑dispersed nanoparticles tightly wrapped by MWCNTs, creating a continuous conductive network. XRD displays the characteristic (002) peak of MWCNTs at ~26°, confirming their presence. XPS spectra reveal C–F bonds at 293.3 and 295.9 eV, indicating covalent attachment of the fluoride lattice to the carbon surface, which facilitates electron transfer.

Electrochemical Performance

In galvanostatic cycling at 35 mA g⁻¹, the composite delivers 106.8 mAh g⁻¹ after 60 cycles, retaining 92.6 % of its initial capacity. The unmodified K(Mn_0.95Ni_0.05)F₃ exhibits rapid capacity fade, underscoring the role of MWCNTs in enhancing conductivity and stabilizing the solid‑electrolyte interface. Rate capability tests show reversible capacities of 90 mAh g⁻¹ at 140 mA g⁻¹ and 60 mAh g⁻¹ at 280 mA g⁻¹, with full recovery upon returning to 35 mA g⁻¹. CV curves display overlapping oxidation and reduction peaks, indicative of good reversibility.

Electrochemical Impedance

Nyquist plots reveal a high‑frequency semicircle (SEI resistance), a mid‑frequency semicircle (electron transport through the composite), and a low‑frequency tail (charge‑transfer resistance). The composite shows a markedly reduced charge‑transfer resistance compared to the pristine material, confirming that the MWCNT network facilitates electron flow and mitigates the intrinsic band‑gap limitations of the fluoride cathode.

Conclusions

We have introduced a scalable, environmentally benign synthesis route for a concentration‑gradient nano‑perovskite cathode, K(Mn_0.95Ni_0.05)F₃, via EDTA‑assisted homogeneous precipitation. Depositing this material on MWCNTs yields a composite cathode that delivers >100 mAh g⁻¹ at moderate rates and maintains 92.6 % capacity after 60 cycles, with low charge‑transfer resistance. This strategy offers a blueprint for designing high‑performance, low‑cost KIB cathodes and can be extended to other fluoride‑based systems.