Preparation and Electrochemical Performance of Zinc-Doped Copper Fluoride

Abstract

To enhance the specific energy and rate performance of lithium primary batteries, the development of advanced cathode materials with superior electrochemical properties is essential. Fluorides, composed of light fluorine elements and multivalent cations, exhibit multi-electron reaction characteristics, possess a high theoretical voltage, and demonstrate high discharge-specific energy. However, owing to fluorine’s high electronegativity, which leads to the formation of strong ionic bonds with other elements, most fluorides exhibit poor electronic conductivity, thereby constraining their electrochemical performance when used as cathode materials. Copper fluoride (CuF₂) exhibits a high theoretical specific capacity and discharge voltage but is constrained by its large bandgap, poor electronic conductivity, and difficulties in synthesizing anhydrous CuF₂ materials, which significantly limit its electrochemical activity. In this study, zinc (Zn) was chosen as a dopant for copper fluoride. By combining theoretical calculations with experimental validation, the impacts of Zn doping on the structural stability and electrochemical performance of copper fluoride were comprehensively analyzed. The resultant highly active Zn-doped copper fluoride achieved a discharge specific capacity of 528.6 mAh/g at 0.1 C and 489.1 mAh/g at 1 C, showcasing superior discharge-specific energy and good rate performance. This material holds great potential as a promising cathode candidate for lithium batteries, providing both high specific energy and power density.

1. Introduction

Lithium primary batteries with lithium metal as the anode have garnered significant attention due to their high energy density, wide operating temperature range, long storage life, and maintenance-free characteristics. These batteries have been widely utilized in various fields, including defense and military, aerospace, smart healthcare, Internet of Things, etc. [1,2,3,4]. As devices that convert chemical energy into electrical energy, lithium primary batteries are mainly classified based on different cathode systems, such as lithium/thionyl chloride (Li/SOCl₂), lithium/manganese dioxide (Li/MnO₂), lithium/sulfur dioxide (Li/SO₂), and lithium/iron disulfide (Li/FeS₂) batteries. Since the anodes of lithium primary batteries are predominantly lithium metal, the specific energy and rate performance of these batteries are primarily determined by the cathode active materials. For example, Li/SOCl₂ batteries exhibit high specific energy but suffer from extremely low discharge rates (typically less than 0.01 C); Li/MnO₂ batteries demonstrate good discharge rates but possess relatively low energy density. To meet the demands of future electrical devices for longer battery life, lighter weight, and higher power, there is an urgent need to develop cathode material systems for lithium primary batteries with a higher specific energy and superior rate performance.

Fluorides, composed of the light element fluorine and multivalent cations, are capable of undergoing multi-electron redox reactions, thereby achieving higher energy densities compared to conventional single-electron systems. They represent one of the key pathways for developing next-generation high-specific-energy batteries [5,6]. For instance, carbon fluoride batteries have received considerable attention in recent years due to their high mass-specific energy. Copper fluoride (CuF₂) also possesses a high theoretical mass-specific energy (1874 Wh kg⁻¹) and volume-specific energy (7870 Wh L⁻¹). Moreover, unlike other metal fluorides, CuF₂ has a high theoretical discharge voltage (3.55 V), showing great potential for the development of high-specific-energy electrochemical energy storage devices [7,8,9,10]. However, similar to most metal fluorides, CuF₂ is limited by its large bandgap and poor electronic conductivity, which stem from its strong ionic bonding characteristics, typically leading to low electrochemical activity [11,12,13,14]. In the last century, research on CuF₂ cathode materials has been relatively scarce. This is because lithium ions diffuse slowly within CuF₂, the insulating lithium fluoride formed during the reaction hinders electron transport, active substances inside the material often fail to undergo electrochemical reactions and thus contribute to capacity, and synthesizing high-purity anhydrous CuF₂ remains challenging. Additionally, there is insufficient understanding regarding how to obtain anhydrous CuF₂ with a high electrochemical activity, and the products frequently contain impurities, such as CuF₂·H₂O and CuOHF, resulting in discharge specific capacities and voltages that are significantly lower than theoretical values.

Based on the above, this study focuses on the synthesis of highly electroactive CuF₂ materials. By doping to modulate the band structure and microstructure of CuF₂, both electron and ion transport rates are significantly enhanced, thereby effectively improving its electrochemical activity. The transition metal Zn was selected as the dopant, and the effects of Zn doping on the structural and electrochemical properties of CuF₂ were systematically investigated to develop high-specific-energy fluoride cathode materials with excellent electrochemical activity.

2. Experimental Methods

2.1. Material Preparation

Copper nitrate (Cu(NO₃)₂·3H₂O) and zinc nitrate (Zn(NO₃)₂) were accurately weighed and dissolved in anhydrous ethanol under stirring until complete dissolution, forming a homogeneous mixed solution. Ammonium fluoride (NH₄F) was separately dissolved in anhydrous methanol to prepare a clear solution. Subsequently, the NH₄F solution was slowly added dropwise into the mixed solution under continuous stirring. After standing for a sufficient period, the resulting precipitate was collected by centrifugation, washed thoroughly with anhydrous ethanol, and dried at an appropriate temperature to obtain the precursor. The precursor was then subjected to heat treatment under a controlled atmosphere. After cooling to room temperature, the Zn-doped CuF₂ material (Zn_xCu_1-xF₂) was successfully synthesized.

2.2. Battery Assembly

The active material (Zn_xCu_1-xF₂), conductive carbon black, and polyvinylidene fluoride (PVDF) binder were mixed in a weight ratio of 8:1:1 in N-methylpyrrolidone (NMP) solvent to form a homogeneous slurry. The slurry was uniformly coated onto aluminum foil using a doctor blade or similar method, and dried in a vacuum drying oven at a controlled temperature to completely remove the NMP solvent. The coated aluminum foil was subsequently rolled to enhance electrode density and punched into circular cathode sheets of appropriate size. The cathode sheets were further vacuum-dried at an elevated temperature to ensure the complete removal of residual moisture before use. Button-type simulated batteries were assembled in an argon-filled glove box with water and oxygen concentrations maintained below 10 ppm. Lithium metal sheets served as the anode, and 1 M LiCF₃SO₃ in an appropriate solvent was used as the electrolyte.

2.3. Material and Battery Testing

The microstructure of the prepared samples was characterized using a Zeiss Ultra Plus field emission scanning electron microscope (SEM, ZEISS, Obernkchen, Germany). Phase analysis of the samples was performed using a X’Pert PRO X-ray diffractometer (XRD, PANalytical B.V., Almelo, Netherlands) with a Cu Kα1 radiation source, over a diffraction angle range of 10°~80°. The microstructural analyses of the materials were conducted using a JEM-1400Plus transmission electron microscope (TEM, Japan Electronics Corporation, Tokyo, Japan). The ICP tests were conducted on the Prodigy 7 from LEEMAN LABS INC. Elemental distribution in the doped samples was analyzed using an ESCALAB-250Xi X-ray photoelectron spectrometer (XPS, Thermo Fisher, USA). The electrochemical performance of the button batteries was evaluated using a Land CT2001A battery testing system from Wuhan Land. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI760D electrochemical workstation (Chenhua, Shanghai, China). EIS measurements were performed in the frequency range of 100 kHz to 100 mHz, with an amplitude of 5 mV.

3. Results and Analysis

3.1. Theoretical Investigation of Zn-Doped CuF₂

The electrical conductivity of CuF₂ is closely related to its energy gap structure and valence bond characteristics. To gain insights into the effects of Zn doping, a theoretical investigation was conducted using the first-principles method. This study employed density functional theory (DFT) with the CASTEP program (2016.01) for calculations [15]. Electron exchange and correlation energies were evaluated within the generalized gradient approximation (GGA). Self-consistent field (SCF) calculations were performed with a convergence criterion of less than 10⁻⁶ eV in the root mean square change of electron density. Geometric optimization of the structure was carried out at the PBE (Perdew-Burke-Ernzerhof) level, with convergence criteria set at a force tolerance of 0.001 eV/Å and a maximum displacement of 0.001 Å. A 5 × 5 × 1 k-point grid was used for sampling the first Brillouin zone of all structures. Dispersion corrections were applied using the TS (Tkatchenko-Scheffler) method for DFT-D correction. After structural optimization, the space group of pure copper fluoride was identified as P21/C, with unit cell parameters of a = 3.3928 Å, b = 4.5973 Å, and c = 5.3925 Å. The supercell models of pure CuF₂ and Zn-doped CuF₂ are presented in Figure 1.

The electronic structure modulation induced by 15% Zn doping was investigated through density functional theory calculations. As evidenced by the PDOS (Partial Density of States) analysis (Figure 2), two prominent modifications were observed. The bandgap experienced a significant reduction from 3.845 eV to 2.72 eV (29.2% decrease), and distinctive spin-up polarized states emerged near the Fermi level. This electronic structure engineering can be attributed to the hybridization between Zn-3d orbitals and host matrix states, where the newly formed impurity levels effectively narrow the charge transition barrier.

Based on the above electronic structure analysis, we further calculated the formation energy of Zn doping using the following formula:

E_f = E_total − E_CuF2 − nE_Zn + nE_Cu

where E_f represents the formation energy, E_total is the total energy of the Zn-doped system, E_CuF2 is the energy of pristine CuF₂, E_Zn and E_Cu denote the chemical potentials of the Zn and Cu atoms, respectively, and n is the number of substituted atoms. The calculated formation energies are −0.462 eV for 5% doping and −0.643 eV for 15% doping, indicating that Zn doping is energetically favorable.

In first-principles calculations, population analysis is widely utilized to quantify and allocate the charge distribution of each atom in the system, referred to as the “local charge”. This analysis plays a critical role in understanding the chemical properties, electronic structure, and interatomic interactions of materials. Bond population can be used to evaluate the covalent and ionic characteristics of bonds. Typically, a high bond population indicates a strong covalent nature, whereas a low bond population suggests a dominant ionic interaction [16].

Table 1. Bond population distribution of Zn-doped CuF₂.

	Pure CuF2		5% Zn
Bond	P	L	P	L
Zn-F	-	-	0.19	2.042
Cu-F1	0.22	1.946	0.24	1.925
Cu-F2	0.21	1.954	0.23	1.933

Note: P stands for population, L for length.

summarizes the bond population distributions for pure CuF₂ and Zn-doped CuF₂, where Zn-F represents the bond between the Zn atom and its nearest neighboring F atom, Cu-F1 denotes the Cu-F bond closest to the Zn atom, and Cu-F2 refers to the Cu-F bond relatively farther from the Zn atom. As shown in Figure 3, the bond population of Zn-F is the smallest (0.19), while the introduction of Zn increases the bond populations of Cu-F1 and Cu-F2 to 0.24 and 0.22, respectively, which are higher than that of the Cu-F bond in undoped CuF₂. This indicates that trace Zn doping can modify the covalency of the Cu-F bond, potentially leading to a reduction in the working voltage of CuF₂.

3.2. The Influence of Zn Doping on the Structure and Morphology of CuF₂

Figure 4 displays the XRD patterns of the Zn_xCu_1-xF₂ (x = 0, 0.05, 0.1, 0.15, 0.2) series samples. As shown in Figure 4a, the main diffraction peaks of all samples are in good agreement with the characteristic peaks of CuF₂ (JCPDS no. 70-1936), and no diffraction peaks corresponding to ZnF₂ were observed. This indicates that no secondary phase of ZnF₂ was formed in the products. In the magnified view of 2θ = 26°~30° presented in Figure 4b, it can be seen that as the Zn doping concentration increases, the strongest peak of CuF₂ at 2θ = 27.658° gradually shifts to a lower angle. This phenomenon may be attributed to the fact that the ionic radius of Zn is 74 pm, which is slightly larger than that of Cu (73 pm). The incorporation of larger ions leads to an increase in the interplanar spacing of the crystal lattice, resulting in the observed shift of the diffraction peak to a lower angle. Figure 4c–d show the results of Rietveld refinement for selected samples of bare CuF₂ and Zn_0.1Cu_0.9F₂, the initial unit cell parameters of bare copper fluoride are a = 4.61 Å, b = 4.55 Å, and c = 3.30 Å, and the unit cell volume is 68.85 ų. The lattice parameters of Zn-doped CuF₂ are a = 4.63 Å, b = 4.58 Å, and c = 3.30 Å, and the lattice volume increases to 69.70 ų. These results confirm that Zn²⁺ has been successfully incorporated into the CuF₂ lattice, forming a solid solution. Simultaneously, ICP results show that Zn contents for the selected sample of Zn_0.1Cu_0.9F₂ and Zn_0.15Cu_0.85F₂ are about 9.71% and 14.94%, indicating that the synthesis method in this paper can successfully prepare samples with the designed stoichiometric ratios.

To further investigate the existence states of Zn²⁺ and Cu²⁺ in the sample, XPS was carried out on the Zn_0.15Cu_0.85F₂ sample, and the results are presented in Figure 5. In the Cu 2p spectrum, the peaks at 937.6 eV and 957.7 eV are assigned to the Cu 2p3/2 and Cu 2p1/2 orbitals of Cu²⁺, respectively, while the peaks at 944.6 eV and 964.3 eV correspond to the satellite peaks of Cu²⁺ [9]. In the Zn 2p spectrum, the peaks at 1023.8 eV and 1047.0 eV are attributed to the Zn 2p3/2 and Zn 2p1/2 orbitals of Zn²⁺, respectively. These findings confirm that, in the Zn_0.15Cu_0.85F₂ sample, the incorporation of Zn²⁺ does not alter the valence state of Cu²⁺, and Zn remains in the form of divalent ions.

SEM was employed to examine the microscopic morphology and particle size of the Zn_xCu_1-xF₂ (x = 0, 0.05, 0.1, 0.15, 0.2) series samples at a magnification of 30,000. The results are presented in Figure 6. All samples exhibited relatively uniform particle sizes and regular shapes, with no evidence of abnormal particle growth. However, varying degrees of agglomeration were observed in the particles of the Zn_0.05Cu_0.95F₂ and Zn_0.1Cu_0.9F₂ samples, whereas no significant agglomeration was detected in the Zn_0.15Cu_0.85F₂ and Zn_0.2Cu_0.8F₂ samples. These samples showed dispersion levels comparable to those of the original material, with individual particle sizes generally below 50 nm. To further analyze the elemental distribution on the sample surface, energy-dispersive X-ray spectroscopy (EDS) mapping was performed on the Zn_0.15Cu_0.85F₂ sample using a field emission scanning electron microscope. The results, shown in Figure 7, indicate that Cu, F, and Zn are homogeneously distributed across the sample surface, with no distinct regions of CuF₂ or ZnF₂ observed.

To further investigate the microstructural characteristics of the Zn_xCu_1-xF₂ series samples, TEM analysis was conducted on three representative samples: CuF₂, Zn_0.05Cu_0.95F₂, and Zn_0.15Cu_0.85F₂. The results are shown in Figure 8. For the undoped CuF₂ sample, the particles exhibit significant aggregation and stacking, which makes it challenging to identify their specific morphologies. In contrast, the Zn-doped samples (with 5% and 15% Zn doping) demonstrate better dispersion, smaller particle sizes, and irregular or strip-like shapes. These structural features are advantageous for enhancing lithium ion diffusion and transport.

3.3. The Impact of Zn Doping on the Electrochemical Properties of CuF₂

The Zn_xCu_1-xF₂ (x = 0, 0.1, 0.15, 0.2) series samples were fabricated into CR2025 coin cells. Electrochemical tests were conducted at an ambient temperature of 30 °C under charge–discharge rates of 0.1 C, 0.2 C, 0.5 C, and 1 C (1 C = 528 mA g⁻¹), with a voltage range of 1.5 V–4.2 V. The test results are presented in Figure 9. At 0.1 C, the initial discharge specific capacities of CuF₂, Zn_0.1Cu_0.9F₂, Zn_0.15Cu_0.85F₂, and Zn_0.2Cu_0.8F₂ were 501.4 mAh g⁻¹, 446.6 mAh g⁻¹, 528.6 mAh g⁻¹, and 479.2 mAh g⁻¹, respectively; the corresponding initial charge specific capacities were 232.2 mAh g⁻¹, 249.9 mAh g⁻¹, 301.48 mAh g⁻¹, and 261.6 mAh g⁻¹, respectively. The initial coulombic efficiencies were 46.3%, 56.0%, 57.0%, and 54.6%, respectively. Among these, Zn_0.15Cu_0.85F₂ exhibited the highest discharge capacity at this rate, surpassing the undoped material by 27.2 mAh g⁻¹. In contrast, the other doped samples showed lower capacities compared to the undoped sample. As the Zn doping level increased, the initial coulombic efficiency gradually improved, increasing by more than 10%, which indicates enhanced initial reversibility of the electrode materials.

At a higher charge-discharge rate of 0.2 C, Zn_0.15Cu_0.85F₂ still demonstrated the highest discharge specific capacity among the series samples, reaching 513.2 mAh g⁻¹. The discharge capacity trends of the other samples were consistent with those observed at 0.1 C. Notably, the initial coulombic efficiency of pure CuF₂ remained below 50% (45.2%), while the efficiencies of Zn_0.1Cu_0.9F₂, Zn_0.15Cu_0.85F₂, and Zn_0.2Cu_0.8F₂ were significantly higher, at 54.1%, 53.6%, and 53.2%, respectively.

At a charge–discharge rate of 0.5 C, the initial discharge specific capacities of CuF₂, Zn_0.1Cu_0.9F₂, Zn_0.15Cu_0.85F₂, and Zn_0.2Cu_0.8F₂ were 421.2 mAh g⁻¹, 367.0 mAh g⁻¹, 503.1 mAh g⁻¹, and 429.47 mAh g⁻¹, respectively; the corresponding initial charge specific capacities were 167.6 mAh g⁻¹, 184.6 mAh g⁻¹, 229.0 mAh g⁻¹, and 190.6 mAh g⁻¹, respectively. Compared to the undoped sample, the reversible specific capacities of the three doped samples increased by 17 mAh g⁻¹, 61.4 mAh g⁻¹, and 23 mAh g⁻¹, respectively. The trends in the initial discharge curves at 1 C were similar to those observed at all previous rates. For the three samples with doping levels less than 15%, the charge curve trends were comparable, while the charge voltage platform of Zn_0.15Cu_0.85F₂ was notably elevated, achieving an energy density of 543.2 Wh kg⁻¹, which is 96.9 Wh kg⁻¹ higher than that of the original material.

Although the charge specific capacity of Zn_0.2Cu_0.8F₂ was slightly lower than that of Zn_0.15Cu_0.85F₂, its charge energy density was comparable to that of Zn_0.15Cu_0.85F₂ and significantly exceeded that of CuF₂. It is worth noting that despite the lower discharge capacity of Zn_0.2Cu_0.8F₂ compared to the original material across all tested rates, its charging performance was markedly superior to that of the undoped sample, as evidenced by the elevated charging voltage and capacity. Additionally, during the initial charging process, the charging voltage of Zn_0.2Cu_0.8F₂ was higher than that of Zn_0.15Cu_0.85F₂.

All samples exhibited varying degrees of capacity attenuation and voltage drop with increasing current density, indicating that the electrochemical polarization within the battery increased as the rate rose. Compared to CuF₂, Zn_0.1Cu_0.9F₂ demonstrated improved rate performance, with the capacity difference decreasing from 127.3 mAh g⁻¹ to 100.7 mAh g⁻¹, and the voltage drop also reduced. Furthermore, the charging rate performance of this sample was significantly enhanced at this point, and the differences in charging voltage drops across various rates were minimized. When the Zn doping level was increased to 15%, the discharge capacities of Zn_0.15Cu_0.85F₂ at 0.1 C, 0.2 C, 0.5 C, and 1 C were 528.6 mAh g⁻¹, 513.2 mAh g⁻¹, 503.1 mAh g⁻¹, and 489.1 mAh g⁻¹, respectively. The capacity retention rates reached 97.1%, 95.2%, and 92.5%, and the capacity loss from 0.1 C to 1 C was only 39.5 mAh g⁻¹, less than 50% of the undoped sample. The charge and discharge voltage drops of Zn_0.15Cu_0.85F₂ at different rates were also improved, particularly with a noticeable reduction in voltage drop at high current densities. For Zn_0.2Cu_0.8F₂, the discharge capacities at different rates were 479.2 mAh g⁻¹, 441.8 mAh g⁻¹, 429.3 mAh g⁻¹, and 413.0 mAh g⁻¹, respectively. The discharge capacities at 0.2 C, 0.5 C, and 1 C were 92.2%, 89.6%, and 86.2% of the 0.1 C value, with a total capacity loss of 66.2 mAh g⁻¹. Although the discharge capacities of Zn_0.2Cu_0.8F₂ at all rates were lower than those of the undoped sample, its capacity retention rate and charge–discharge voltage drop performance were superior to those of the undoped sample. These results indicate that an appropriate amount of Zn doping can enhance the rate performance of the CuF₂ electrode, particularly improving its charge and discharge performance at high current densities.

The comparison of the initial discharge capacities of the sample Zn_0.15Cu_0.85F₂ in this work and that of some other fluoride and manganese oxide cathode materials studied in previous literature are listed in

Table 2. Comparison of capacity and voltage between Zn_0.15Cu_0.85F₂ and other fluoride and manganese oxide cathodes.

No.	Cathodes	Current	Voltage(V)	Capacity (mAh/g)	References
1	CuF2	0.1 C	2.7	413	[17]
2	CuF2/MoO3	0.1 C	2.5	483	[8]
3	Cu0.5Fe0.5F2	5 mAg −1	2.55	475	[18]
4	Cu0.5Fe0.5F2	0.02 C	2.65	485	[19]
5	Fe0.92Al0.08F3·0.33H2O/C	40 mA g−1	3.0	200	[20]
6	Cu0.1Ni0.9F2	C/10	1.75	548	[21]
7	MnO2	0.1 C	2.9	245.7	[22]
8	MnO2	20 mA g−1	2.8	235.5	[23]
9	Zn0.15Cu0.85F2	0.1 C	2.75	528.6	This work

. It can be found that, compared with some fluoride and manganese oxide cathodes reported in the literature, the Zn-doped CuF₂ cathode material synthesized in this paper shows obvious performance advantages. However, CuF₂ is unstable in high humidity and easily forms hydration products, resulting in a decrease in its energy density. Therefore, improving the air stability of CuF₂ is a research direction worthy of future attention, which is of great significance for its practical application.

Figure 10 shows the cyclic voltammetry (CV) curves of CuF₂ and Zn_0.15Cu_0.85F₂, tested over a voltage range of 1.0 V to 4.5 V at a scan rate of 0.1 mV/s in the reverse direction. The CV curves of both samples were similar, featuring a distinct reduction peak near 2.3 V, corresponding to the discharge process of Cu²⁺ reduction to Cu, and an oxidation peak near 3.9 V, corresponding to the charging process of Cu to Cu²⁺. The reduction/oxidation peak potentials for CuF₂ were 2.199 V and 3.912 V, respectively, with ΔE = 1.713 V, while those for Zn0.15Cu0.85F2 were 2.284 V and 3.899 V, respectively, with ΔE = 1.615 V. During the reduction process, the reduction peak area of the Zn_0.15Cu_0.85F₂ electrode was significantly larger than that of CuF₂, reflecting higher electrochemical activity and a higher discharge capacity. The reduction peak current for Zn_0.15Cu_0.85F₂ was 0.469 mA, compared to 0.371 mA for CuF₂, suggesting a more intense battery reaction. During the oxidation process, the peak area of Zn_0.15Cu_0.85F₂ was also larger than that of CuF₂, confirming its higher charging capacity, which is consistent with the previous test results.

To investigate the intrinsic effects of Zn²⁺ doping on the electrochemical kinetics of CuF₂, AC (Alternating Current) impedance measurements were conducted on the CuF₂ and Zn_0.15Cu_0.85F₂ electrodes. The Nyquist plots are shown in Figure 11a, with the corresponding equivalent circuit diagrams presented in the inset of Figure 11a. The Rs values for both electrodes are nearly identical, at 18.15 Ω and 16.44 Ω, respectively, indicating that the external conditions of the battery system are similar. The Nyquist curves exhibit two semicircles spanning the high-frequency and mid-frequency regions. The R1 value associated with the high-frequency semicircle is attributed to the electrode/electrolyte interface resistance, while the R2 value related to the mid-frequency semicircle reflects the charge transfer resistance [24]. For the electrode doped with 15% Zn ions, both R1 and R2 are smaller than those of the undoped CuF₂ electrode. The reduction in R1 may result from the refinement of active material particles caused by Zn²⁺ doping, which increases the contact area between the electrode and electrolyte. Furthermore, the charge transfer resistance (R2) of the Zn_0.15Cu_0.85F₂ electrode is significantly lower than that of the CuF₂ electrode, suggesting that the solid solution material exhibits enhanced kinetic properties during the charge transfer process. This improvement can be attributed to the formation of a specialized microstructure that facilitates efficient charge transport through the electrode to CuF₂.

$${\mathrm{D}}_{{\mathrm{Li}}^{+}}={\displaystyle \frac{{\mathrm{R}}^2{\mathrm{T}}^2}{2{\mathrm{n}}^4{\mathrm{A}}^2{\mathrm{C}}^2{\mathrm{F}}^4{\mathsf{\sigma }}^2}}$$

(1)

The lithium ion diffusion coefficients for the CuF₂ and Zn_0.15Cu_0.85F₂ electrodes were determined using Equation (1), where R, T, n, A, C, F, and σ represent the gas constant, absolute temperature (K), the number of electrons transferred per unit in the conversion reaction, the electrode surface area, the molar concentration of Li⁺, the Faraday constant, and the Warburg coefficient associated with Z’, respectively. Figure 8b illustrates the linear relationship between Z’ and ω^−1/2, with σ being calculated as the slope of the straight line. According to Equation (1), the lithium ion diffusion coefficients of the CuF₂ and Zn_0.15Cu_0.85F₂ electrodes were estimated to be 1.6 × 10⁻²¹ m²/s and 2.24 × 10⁻²¹ m²/s, respectively. These results indicate that the incorporation of Zn²⁺ effectively expanded the unit cell size of CuF₂, leading to an improvement in the lithium ion diffusion coefficient of the electrode.

To investigate the transformation mechanism during the discharge process of Zn-doped CuF₂, the discharge products of CuF₂ and Zn_0.15Cu_0.85F₂ were characterized using TEM. Figure 12d–f displays the TEM images and selected-area electron diffraction (SAED) patterns of the products after CuF₂ was discharged to 1.0 V. As observed in these figures, the discharge products of CuF₂ consist of nanoscale particles with relatively uniform sizes, approximately 4 nm in diameter. The lattice fringe spacing in the dark region of the magnified image (Figure 12e) is approximately 0.209 nm, corresponding to the (111) plane of Cu (JCPDS no. 70-3038), confirming that the particles in this region are Cu nanoparticles. The diffraction rings in the SAED pattern (Figure 12f) align perfectly with the crystal planes of Cu, further verifying the presence of Cu nanoparticles in the discharge products of CuF₂. Additionally, the diffraction ring corresponding to the (220) plane of LiF was detected, indicating the formation of LiF in the products. These findings confirm that the transformation reaction during the discharge process of CuF₂ follows the equation CuF₂ + 2Li → Cu + 2LiF, which is consistent with previous studies [25,26].

Figure 12a–c presents the TEM images and SAED patterns of the products after Zn_0.15Cu_0.85F₂ was discharged to 1.0 V. From Figure 12a,b, it can be seen that the lattice fringe spacings of 0.181 nm and 0.208 nm correspond to the (200) and (111) planes of Cu, respectively. In the SAED pattern (Figure 12c), the innermost diffraction ring originates from the (111) plane of Cu, while the remaining rings correspond to the crystal planes of Cu and LiF. Notably, no diffraction rings belonging to CuF₂ or ZnF₂ were observed, indicating that the discharge products consist of LiF and Cu. Since the discharge voltage exceeds the conversion voltage of ZnF₂, no Zn formation was detected, suggesting that the conversion processes of Zn and Cu do not occur simultaneously.

4. Conclusions

Owing to its strong ionic bond characteristics, CuF₂ exhibits a large bandgap and poor electronic conductivity, leading to low electrochemical activity and an actual capacity and voltage significantly lower than theoretical values. This study primarily investigates the effect of Zn doping on regulating the band structure and microstructure of CuF₂. Through theoretical calculations and experimental validation, it demonstrated that Zn doping effectively reduces the bandgap width and particle size of CuF₂, enhances the material’s electronic and lithium ion transport properties, and does not alter the conversion mechanism of CuF₂ during discharge. When the Zn doping level is 15%, the synthesized Zn_0.15Cu_0.85F₂ material exhibits superior electrochemical activity, achieving a discharge specific capacity of 528.6 mAh g⁻¹ at 0.1 C and maintaining 489.1 mAh g⁻¹ at 1 C, thereby showcasing excellent discharge specific energy and rate performance. This material holds promise as a novel lithium battery cathode material with both high specific energy and power.