Improved Electrochemical Performance Using Transition Metal Doped ZnNi/Carbon Nanotubes as Conductive Additive in Li/CF_x Battery

Abstract

Lithium/carbon fluoride (Li/CF_x) batteries are promising for specialized applications due to their high theoretical capacity (>865 mAh·g⁻¹) and energy density. However, their practical deployment is hindered by the intrinsically low conductivity of CF_x and sluggish reaction kinetics. While conventional conductive additives improve electron transport, their physical mixing with active materials yields weak interfacial contacts and fails to catalytically facilitate C–F bond cleavage. To address these dual limitations, this study proposes a dual-functional conductive-catalytic additive strategy. We engineered zinc-nickel/carbon nanotube (ZnNi/CNT) composites modified with transition metal dopants (Fe, W, Cu) to integrate conductive networks with nanoscale-dispersed catalytic sites. Fe-doped ZnNi/CNT (ZnFeNiC) emerged as the optimal system, delivering a discharge plateau of 2.45 V and a specific capacity of 810.3 mAh·g⁻¹ at 0.1 C. This performance is attributed to Fe-doping accelerates Li⁺ diffusion, and promotes reversible Ni redox transitions (Ni²⁺↔Ni⁰) that catalyze C–F bond dissociation. This work establishes a design paradigm for high-performance Li/CF_x batteries, bridging the gap between conductive enhancement and catalytic activation.

1. Introduction

Lithium/carbon fluoride (Li/CFx) batteries have garnered significant attention in the field of specialized power sources due to their high theoretical specific capacity (>865 mAh·g⁻¹) and energy density [1,2]. Nevertheless, the intrinsically low electrical conductivity and sluggish reaction kinetics of CFx materials significantly hinder their high-rate performance and capacity utilization [3,4]. Conventional approaches primarily involve incorporating conductive carbon materials (e.g., Super P, carbon nanotubes) to construct an electronic conduction network [5,6]. However, mechanically mixed carbon additives often exhibit weak interfacial contact with active materials, which limits their ability to catalyze C-F bond cleavage at the nanoscale level. As a result, this leads to a relatively low discharge voltage plateau for the battery and a substantial loss of theoretical energy density [7,8].

The aforementioned issues could be addressed through the functional design of conductive additives [9,10]. Studies indicate that carbon-based materials doped with transition metals could effectively catalyze the oxygen reduction reaction process [11,12,13]. Integrating a conductive network with transition metal-based catalytic active sites to construct bifunctional conductive additives holds promise for synergistically enhancing both the conductivity and kinetics in Li/CF_x batteries [12,14,15]. It is important to note that current research predominantly focuses on the catalysis of oxygen electrode reactions (e.g., ORR/OER), whereas investigations into the catalytic mechanism underlying C-F bond cleavage in Li/CF_x batteries remain insufficient [16,17]. The dissociation of C-F bonds entails a complex process involving fluorine atom extraction and carbon skeleton reduction, which exhibits a substantially higher energy barrier compared to the oxygen reduction reaction. Consequently, it is imperative to explore design principles for specific catalytic sites tailored for C-F bond activation.

In response to the aforementioned challenges, this study proposes a dual-functional conductive-catalytic additive design strategy: employing zinc-nickel bimetal/carbon nanotube (ZnNi/CNT) as a functionalized conductive substrate and constructing highly dispersed nanoscale catalytic sites via transition metal doping (Fe/W/Cu), thereby achieving an integrated design of conductive networks and catalytic active sites. Specifically, the ZnNi bimetal in conjunction with the one-dimensional conductive network of CNT ensures high-speed electron transfer, while the dopants optimize the electronic structure of Ni active centers, enabling reversible Ni²⁺/Ni⁰ cycling that catalyzes C-F dissociation. Notably, the Fe-doped system (ZnFeNiC) effectively facilitates the reversible valence state conversion of Ni active centers (Ni²⁺ ↔ Ni⁰), enhances the lithium ion diffusion coefficient, and achieves a high discharge platform of 2.45 V and a capacity of 810.3 mAh/g at 0.1 C. This work establishes a design paradigm for high-performance conductive additives in Li/CF_x batteries.

2. Results and Discussion

To verify the crystal structure and microstructural characteristics of the prepared samples, XRD and Raman analyses were performed. As shown in Figure 1a, the XRD peak positions of the three samples match those of nickel metal (PDF#04-0850) and zinc nickel carbide (PDF#28-0713), with no significant impurity phases detected. Furthermore, a diffraction peak at 26.3° was observed in all three samples, corresponding to graphitized carbon (PDF#41-1487). This indicates that all three samples contain graphite-like carbon materials. Peaks corresponding to iron, tungsten, or copper were not detected, likely due to their low concentrations in the samples, where these elements may exist in the form of dopants [18,19,20].

Figure 1b presents the Raman spectra of the prepared product. The characteristic peak observed below 1200 cm⁻¹ corresponds to the peak position attributed to nickel [21,22,23]. Upon closer inspection of Figure 1b, the peak at 1350 cm⁻¹ corresponds to the D peak of the carbon material, which is associated with the structural disorder of the carbon material. The peak at 1580 cm⁻¹ represents the G peak of the carbon material, indicating the degree of graphitization [24,25]. The relatively weak intensities of the D and G peaks for the three aforementioned products may arise from the interaction at the metal-carbon interface between the transition metal and the carbon nanotube [26]. This interaction alters the electronic structure of the carbon nanotube, thereby influencing the spectroscopic properties of the carbon material.

The microscopic morphology of the prepared samples was characterized by SEM and TEM, as illustrated in Figure 2. Upon observation, it is evident that all three samples exhibit a tubular structure with a relatively small diameter, confirming the presence of the carbon nanotube (CNT) structure. Figure 2a,b reveal that the Fe-doped zinc-nickel/CNT (ZnFeNiC) composite displays a relatively uniform distribution, with metal particles visibly attached to the surface of the CNTs. Figure 2c,d depict the morphology of the W-doped zinc-nickel/CNT (ZnWNiC) composite material, where the CNTs also demonstrate a uniform distribution. Figure 2e,f present the SEM images of the Cu-doped zinc-nickel/CNT (ZnCuNiC) composite material. In Figure 2e, the composite appears as a relatively dense block-like aggregation. By examining the magnified image (Figure 2f), the distribution of the CNTs becomes apparent. Based on these observations, it could be concluded that different types of transition metal doping in zinc-nickel/CNT composites do not significantly alter the microscopic structure of the composites. The microstructure of the ZnFeNiC samples was further characterized by TEM, as illustrated in Figure 2g,h. In Figure 2g, a bamboo-like hollow carbon nanotube structure is clearly visible, with the diameters of the carbon nanotubes consistently measuring approximately 100 nm. Upon examining the HRTEM image (Figure 2h), distinct diffraction lattice fringes are observable, which correspond to the (200) crystal plane of Ni. The EDS elemental mappings shown in Figure 2i–n demonstrated that the elements C, N, Ni, Zn, and Fe were uniformly distributed throughout the ZnFeNiC sample. Figure S1 presents the elemental composition of various elements as determined by EDS energy spectra analysis. Given the established roles of Zn and Ni as essential components for the tip-growth mechanism of carbon nanotubes [27], it is reasonable to infer that these elements served as catalysts, promoting the formation of the CNT structure in this study. The presence of Fe indicates the successful doping of iron in the sample.

To further investigate the surface chemical composition and elemental valence states of zinc-nickel/carbon nanotube composites doped with different transition metals, XPS analyses were performed, and the results are presented in Figure 3. To elucidate the test results, the XPS spectra of C 1s, Ni 2p, Zn 2p, W 4f, Fe 2p, and Cu 2p for the prepared samples were deconvoluted into their respective chemical states. The deconvolution results of C 1s reveal four distinct states: C-C, C-N, C=O, and O-C=O [28,29,30]. The C-C and C-N bonds are primarily associated with the structural characteristics of the carbon-based materials in the composites. The fitted peaks of Ni 2p correspond to Ni⁰, Ni²⁺, and its satellite peak, while the fitted peaks of Zn 2p are attributed to Zn 2p1/2 and Zn 2p3/2 [31,32,33,34]. Furthermore, the XPS fitting results of W 4f, Fe 2p, and Cu 2p confirm the presence of W, Fe, and Cu doping in the three prepared samples [35,36,37]. From the N 1s spectra of the composites (Figure S2), it is evident that the three types of nitrogen peaks do not exhibit significant variations with the change in doping element. This may be attributed to the relatively low concentration of the doping element, leading to only minor alterations [17,38].

To evaluate the electrochemical performance of the three products, they were employed as conductive additives in Li/CF_x batteries, as illustrated in Figure 4 and Figure S1. Figure 4a,b depict the constant current discharge curves of the three products at 0.1 C and 1 C, respectively. Under the 0.1 C discharge condition, it is evident that the Li/CF_x battery with ZnFeNiC as the conductive additive exhibits the highest discharge voltage platform at 2.45 V, delivering a specific capacity of up to 810.3 mAh·g⁻¹. The battery with ZnCuNiC as the conductive additive shows the second-highest voltage platform at 2.42 V, with a specific capacity of 745.2 mAh·g⁻¹. In contrast, the corresponding discharge voltage platform of ZnWNiC is the lowest, reaching only 2.40 V, with a specific capacity of merely 663.9 mAh·g⁻¹. At the 1 C discharge condition, the Li/CF_x battery with ZnFeNiC as the conductive additive continues to demonstrate the highest specific capacity, whereas the electrodes with ZnWNiC and ZnCuNiC exhibit relatively lower specific capacities. Regardless of whether the discharge condition is 0.1 C or 1 C, the Li/CF_x battery with ZnFeNiC as the conductive additive consistently achieves the highest discharge specific capacity. In contrast, Figure S4 illustrates the electrochemical performance of commercial carbon nanotubes, ZnNiC, and commercial Super P when used as conductive additives.

Figure 4c displays the CV curves of the three electrodes at a sweep rate of 0.05 mV s⁻¹. It is evident that the peak positions of the Li/CF_x battery during the discharge process are consistently within the range of 2.0 V to 2.3 V, corresponding to the transformation of fluorinated carbon into graphitized carbon [39,40]. Figure 4d illustrates the dQ/dV curve associated with the constant current discharge curve of the three electrodes at 0.1 C. The three samples exhibit a comparable peak voltage range. Notably, the Li/CF_x battery with ZnFeNiC as the conductive additive demonstrates the highest peak voltage position, closely aligned with the discharge plateau voltage. This suggests that the battery capacity of this configuration is predominantly supplied by the plateau capacity, and ZnFeNiC effectively enhances the discharge reaction activity. Figure S3 presents the dQ/dV curve corresponding to the constant current discharge curve of the three electrodes at 1 C. The peak voltage range for all three electrodes lies between 1.8 V and 2.0 V, consistent with the voltage platform observed in the discharge curve.

The diffusion coefficient was determined using the GITT under various discharge conditions to further investigate the kinetic characteristics of the three electrodes. As shown in Figure 4e,f, the GITT test results and the corresponding lithium-ion diffusion coefficient distributions for the three electrodes under 0.1 C and 1 C discharge conditions are presented. The diffusion coefficient was calculated based on the following formula [41,42]:

$${\mathrm{D}}_{{\mathrm{L}\mathrm{i}}^{+}}={\displaystyle \frac{4}{\mathsf{\pi }\mathsf{\tau }}}{\left({\displaystyle \frac{{\mathrm{m}}_{\mathrm{B}}{\mathrm{V}}_{\mathrm{M}}}{{\mathrm{M}}_{\mathrm{B}}\mathrm{S}}}\right)}^2{\left({\displaystyle \frac{\mathsf{\Delta }{\mathrm{E}}_{\mathrm{s}}}{\mathsf{\Delta }{\mathrm{E}}_{\mathsf{\tau }}}}\right)}^2,\left(\mathsf{\tau }\ll {\displaystyle \frac{{\mathrm{L}}^2}{\mathrm{D}}}\right)$$

Here, D denotes the diffusion coefficient of Li⁺ (cm² s⁻¹), τ represents the relaxation time (s), m_B corresponds to the electrode mass (g), S indicates the electrode area (cm²), MB and VM are the molar mass (g mol⁻¹) and molar volume (cm³ mol⁻¹) of the electrode material, respectively. ΔEs refers to the potential change induced by the current pulse perturbation (V), and ΔEτ signifies the voltage change during constant current discharge (V).

Under the 0.1 C discharge condition, the lithium-ion diffusion coefficients of the three composite electrodes during the discharge process were determined to range from 10⁻⁸ to 10⁻¹⁵ cm² s⁻¹. Especially for the ZnFeNiC electrode, the average diffusion coefficient is 10⁻¹⁰ cm² s⁻¹. It remained consistently higher than that of the other two samples throughout the discharge process, with a stable distribution observed. This suggests that the ZnFeNiC electrode exhibits a higher lithium ion diffusion rate. Additionally, it indicates that doping with Fe elements enhances the reaction kinetics of the electrode. At a discharge rate of 1 C, the ZnFeNiC electrode still demonstrates a higher diffusion coefficient (exceeding 10⁻¹⁰ cm² s⁻¹) compared to the other two electrodes. Furthermore, this diffusion coefficient is considerably higher than that of the original CF_x electrode [3,6] and surpasses that of the carbon-coated CF_x electrode [7].

This further confirms that even at elevated discharge rates, the ZnFeNiC electrode maintains its rapid lithium ion diffusion capability and superior reaction kinetics.

In order to further investigate the influence of different transition metal-doped zinc-nickel/carbon nanotubes as conductive additives on the reaction kinetics of Li/CF_x batteries, EIS tests were performed on each electrode after various discharge rates [43]. Figure 5a,b presents the EIS test results for the three electrodes (ZnFeNiC, ZnWNiC, and ZnCuNiC) after discharge at 0.1 C and 1 C, respectively. The test outcomes align with the electrode’s corresponding electronic circuit diagram, where each component is assigned specific calculated values. The data were fitted using the equivalent circuit illustrated in Figure 5c. In this circuit, Rs represents the Ohmic resistance (related to the electrolyte and contacts), Rf denotes the resistance of the SEI film, CPE1 corresponds to the constant phase element associated with the SEI capacitance, Rct is the charge transfer resistance, CPE2 represents the double-layer capacitance as another constant phase element, and W stands for the Warburg diffusion impedance [42,44]. In these electronic components, Rct represents the charge transfer resistance of the fabricated electrodes.

Figure 5d presents the fitted charge transfer resistance (Rct) values for the prepared electrodes. It is evident that the ZnFeNiC electrodes exhibit the minimum Rct values across all discharge rates, suggesting that the charge migration speed of this electrode is significantly faster compared to the other two electrodes.

To investigate the electrochemical reaction mechanism of the electrode, ex situ XPS tests were performed on Li/CFx batteries using ZnFeNiC as the conductive additive. To better elucidate the test results, the acquired XPS spectral data were further analyzed and processed, as illustrated in Figure 6. As shown in Figure 6, the peak position of Zn 2p remained largely unchanged during the battery discharge process and consistently existed in its elemental state. This suggests that the zinc element primarily functions as a catalyst in the battery discharge reaction without directly participating in redox processes. During the battery discharge reaction process, the content of Ni⁰ increases, suggesting that a portion of the nickel has actively participated in the discharge reaction. This contributes additional lithium-ion storage capacity to the Li/CF_x battery. As evidenced by the XPS spectra of fluorine and carbon elements, the content of C-F bonds decreases gradually, whereas the contents of Li-F and C-C bonds increase progressively [38,45]. These observations indicate that the final products of the battery discharge reaction are lithium fluoride and graphitized carbon.

To verify the phase composition and microstructural characteristics of the electrodes after the reaction, XRD and SEM analyses were performed on the three electrodes after discharge, as shown in Figure 7 and Figure S2. Figure 7 illustrates the phase composition and crystal structure of the electrode materials for the three electrodes discharged at 0.1 C and 1 C rates. The discharge products of all three electrodes consist of LiF (PDF#04-0857) and a broad peak of carbon materials [38]. The predominant growth planes of LiF are the (200) plane and the (311) plane. The XRD test results are consistent with the XPS test results (Figure 6), thereby supporting the proposed electrode discharge reaction:

CF_x + xLi⁺ + xe⁻ → C + xLiF

Figure S5 displays the SEM images of the three electrodes after discharge under various conditions. The carbon nanotube structure remains observable in the composite electrode post-discharge. Notably, for the electrode utilizing ZnFeNiC as the conductive additive, the lithium fluoride particles generated after discharge are smaller and more uniformly distributed. Furthermore, under the 1 C discharge condition, it is observed that the lithium fluoride particles grow homogeneously around the carbon nanotubes, thereby highlighting the stability and uniformity of the electrode structure.

3. Experimental Section

3.1. Preparation of ZnFeNiC/ZnWNiC/ZnCuNiC

The products obtained in this study were all synthesized via a one-step pyrolysis method in a tube furnace with an Ar atmosphere. After thoroughly grinding and mixing the raw materials, they were placed into the tube furnace and heated at a rate of 10 °C min⁻¹ until reaching 700 °C. The furnace was then cooled to room temperature. The resulting black samples were collected as the final products. The specific raw materials and corresponding product nomenclature used in this study will be detailed in the following section.

The raw materials used were 0.05 g of ammonium iron (III) oxalate trihydrate, 0.05 g of zinc nitrate hexahydrate, 0.9 g of nickel nitrate hexahydrate, and 1.67 g of melamine. After the reaction, the resulting transition metal-based composite, consisting of Zn, Fe, Ni, and carbon nanotubes (CNTs), was collected and designated as ZnFeNiC. Similarly, for the synthesis of ZnWNiC, tungsten hexachloride (0.05 g) was used as the tungsten source, while the remaining components and preparation process were identical to those of ZnFeNiC. Likewise, for ZnCuNiC, copper sulfate pentahydrate (0.05 g) served as the copper source, with all other raw materials and synthesis steps remaining the same as those for ZnFeNiC.

3.2. Materials Characterization

The crystal structure and microstructural characteristics of the products were characterized by X-ray diffraction (XRD, D/max-2200PC, Cu Kα radiation) and Raman spectroscopy (Renishaw Invia). The morphological features were examined using scanning electron microscopy (SEM, S-4800). The surface chemical composition and elemental valence states were analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD). XPS analysis was performed after calibrating all spectra to the C 1s peak of adventitious carbon at 284.6 eV.

3.3. Electrochemical Characterization

Electrochemical characterization was conducted after assembling coin-type batteries (CR2032) with an Ar-filled glove box, with both H₂O and O₂ concentrations maintained below 0.01 ppm. In this study, fluorinated graphite was used as the cathode material, while the synthesized products (ZnFeNiC, ZnWNiC, ZnCuNiC) served as conductive additives. Sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were employed as binders. The fluorinated graphite, synthesized products (ZnFeNiC, ZnWNiC, ZnCuNiC), CMC, and SBR were mixed in a mass ratio of 7:2:0.5:0.5 to prepare the electrode. After thoroughly grinding the mixture in deionized water to form a homogeneous slurry, it was coated onto copper foil and dried at 80 °C for 24 h. Following drying, the electrode materials were cut into circular disks with a diameter of 10 mm. Lithium metal foil was used as the counter electrode, and a polypropylene film served as the separator. The LiPF₆ (1 M) dissolved in EC/DEC (1:1 in volume) was used as the electrolyte.

The assembled batteries were allowed to rest for more than 24 h before any electrochemical testing. To evaluate the electrochemical performance of the final products, constant current discharge tests and galvanostatic intermittent titration technique (GITT) were performed using a battery testing system (NEWARE BTS-5V, Neware Technology Co., Ltd.). The electrochemical window for testing was 1.5 to 3.0 V. Additionally, electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range of 0.05 Hz to 100 kHz, and cyclic voltammetry (CV) was carried out at a scan rate of 0.05 mV s⁻¹, both using an electrochemical workstation (CHI660E, Shanghai Chenhua).

4. Conclusions

This study successfully resolves the dual challenges of electrical insulation and sluggish kinetics in lithium/carbon fluoride (Li/CF_x) batteries through rationally designed Fe-doped zinc-nickel/carbon nanotube composites (ZnFeNiC). As a dual-functional conductive-catalytic additive, ZnFeNiC delivers exceptional electrochemical performance with a high discharge plateau of 2.45 V and a specific capacity of 810.3 mAh·g⁻¹ at 0.1 C, which represents a 22–35% improvement over W/Cu-doped counterparts. The performance enhancement is mechanistically attributed to three experimentally verified effects: accelerated lithium-ion diffusion evidenced by GITT measurements, catalytic facilitation of C-F bond dissociation through reversible Ni²⁺/Ni⁰ transitions confirmed by ex situ XPS, and homogeneous LiF nucleation observed in SEM analysis after discharge that prevents electrode structural degradation. The ZnFeNiC conductive framework ensures efficient electron transport, reducing interfacial charge transfer resistance as quantified by EIS. This work provides a practical material design solution for high-energy Li/CF_x batteries.