The synthesized LiZnPO4 nanoparticles were characterized at different temperatures (500°C, 600°C, and 700°C) using various techniques including XRD, FTIR, FESEM, XPS, and electrochemical analyses. X-ray diffraction (XRD) analysis of LiZnPO4 nanoparticles sintered at temperatures from 400 to 700°C revealed well-crystallized structures at 700°C, with preferred orientations along (202) and (020) planes. The Scherrer formula was employed to determine crystallite sizes, showing an increase from 55 nm at 400°C to 85 nm at 700°C. Fourier-transform infrared spectroscopy (FTIR) confirmed characteristic bonds within LiZnPO4, while scanning electron microscopy (SEM) exhibited morphological changes with sintering temperature, emphasizing the impact on size and aggregation. Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) confirmed elemental compositions and surface electronic states, respectively. Electrochemical experiments demonstrated promising performance, with cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance spectroscopy (EIS) revealing reversible redox processes and good rate capability.
Research Article
Structural, optical and electrochemical performance of LiZnPO 4 nanoparticles for supercapacitor applications
https://doi.org/10.21203/rs.3.rs-4295565/v1
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Nanoparticle Synthesis
Sintering Temperature
Crystallinity
Electrochemical Performance
Energy Storage
With the escalating costs and stringent emission regulations associated with gasoline-powered vehicles [1], the development of electric vehicles (EVs) has gained momentum. EVs offer numerous advantages, including environmental friendliness, reduced maintenance costs, and enhanced performance. Battery-operated EVs, in particular, boast extended driving ranges and higher efficiency. Nevertheless, the preference for batteries with greater energy density persists to meet the demands of extended journeys, despite their low specific power [2]. To circumvent the necessity for bulky battery packs in modern EVs, which are required to provide high power while mitigating degradation from acceleration and deceleration, alternative energy storage systems such as supercapacitors are being explored. Supercapacitors exhibit superior power density, longer cycle life, and higher cycling efficiency [3], making them an ideal complement to batteries in hybrid energy storage systems (HESS). By combining the high energy density of batteries with the high-power density of supercapacitors within a single unit, HESS optimally caters to both high energy and peak power demands [4]. Careful consideration is essential when integrating battery packs with supercapacitors to minimize transient currents outside of the battery. Furthermore, optimal utilization of supercapacitors can be achieved through the control of their energy content via power converters [5]. While batteries are typically sized to meet minimum mileage requirements for EVs, the sizing of supercapacitors should be calibrated to optimize performance, defined in this context as achieving the lowest operating costs.
LiZnPO4 nanoparticles have emerged as promising candidates for supercapacitor applications, offering exceptional electrochemical properties and stability. These nanoparticles, composed of lithium, zinc, phosphorus, and oxygen atoms, possess unique characteristics that make them highly desirable for energy storage systems. Their high surface area-to-volume ratio and tunable morphology enable efficient charge storage and rapid ion diffusion, crucial for enhancing the performance of supercapacitors.Recent studies have highlighted the potential of LiZnPO4 nanoparticles in improving the energy density, cycling stability, and rate capability of supercapacitors [6–8]. The synergistic effects arising from the combination of lithium and zinc ions within the phosphate framework contribute to enhanced electrochemical performance, making them a promising alternative to traditional electrode materials. Furthermore, the facile synthesis methods developed for LiZnPO4 nanoparticles, such as hydrothermal and sol-gel techniques, offer scalability and cost-effectiveness, paving the way for their practical implementation in energy storage devices. Moreover, the incorporation of these nanoparticles into composite structures with conductive materials enhances the overall conductivity and electrochemical performance of supercapacitor electrodes. One notable property of LiZnPO4 nanoparticles is their tunable morphology and size, which can be precisely controlled during synthesis, leading to enhanced electrochemical performance [9–12].
Additionally, their inherent stability against dissolution and structural degradation ensures prolonged cycling stability, critical for long-term device operation. LiZnPO4 nanoparticles demonstrate impressive charge storage capabilities, attributed to the reversible intercalation/deintercalation of lithium ions within the crystal lattice [13]. This mechanism enables high specific capacitance and energy density, essential metrics for evaluating supercapacitor performance. Moreover, the incorporation of zinc into the phosphate framework further enhances conductivity and facilitates electron transfer processes, contributing to improved charge/discharge kinetics and overall device efficiency [14–16].
In practical applications, LiZnPO4 nanoparticles offer a myriad of benefits, including rapid charging/discharging rates, prolonged cycle life, and enhanced safety compared to conventional supercapacitor materials. These attributes make them particularly well-suited for various energy storage applications, ranging from portable electronics to electric vehicles and renewable energy systems. Overall, the unique combination of properties exhibited by LiZnPO4 nanoparticles holds great promise for advancing the performance and reliability of supercapacitor technology, paving the way for next-generation energy storage solutions.
2.1 Materials used
The zinc nitrate (Zn(NO)3), lithium nitrate (LiNO3), ammonium dihydrogen phosphate (NH4H2PO4), nickel foam (Ni foam), sodium hydroxide (NaOH), hydrochloric acid (HCl), nitric acid (HNO3), polyvinylidene fluoride (PVDF), N-Methyl-2-pyrrolidone (NMP), carbon black, ethanol, and acetone were sourced from Sigma-Aldrich for the creation of LiZnPO4 nanoparticles. These materials were used as supplied, without undergoing additional purification steps, prior to their utilization in sample preparation and electrode assembly.
2.2 Synthesis of LiZnPO4 nanoparticles
The precise quantities of Zn(NO)3, LiNO3, and NH4H2PO4 were dissolved in deionized water with vigorous stirring. A 1 M solution of sodium hydroxide was also prepared in deionized water under stirring conditions, and then added drop-wise to adjust the pH of the solution to fall within the range of 8–9. Following this, the resulting precipitate was separated via centrifugation, subjected to several wash cycles, and subsequently dried at 150°C in a stream of H2 (5%) /N2 (95%). Finally, the powders underwent a heat treatment process under a flowing atmosphere of H2 (5%) /N2 (95%) within the temperature range of 400–700°C for a duration of 4 hours, resulting in the formation of a single-phase spherical structure.
2.3 Characterization Technique
The characterization and analysis of the LiZnPO4 nanoparticles at different temperatures (500°C, 600°C, and 700°C) involved the utilization of various analytical instruments and techniques. X-ray Diffraction (XRD) analysis was performed using the EMPYREAN PANalytical multipurpose X-ray diffractometer with copper Kα1 radiation (CuKα1) as the X-ray source, providing insights into the crystallographic structure and phase composition of the nanoparticles. Fourier-transform infrared (FTIR) transmission studies were conducted using the Nicolet 5DX FTIR spectrometer to analyze chemical functional groups and molecular vibrations. The structure, morphology, and elemental composition of the nanoparticles were assessed via Field Emission Scanning Electron Microscopy (FESEM) utilizing the GEMINI SUPRA 55VP Zeiss FESEM. X-ray Photoelectron Spectroscopy (XPS) with the PHI-VERSA PROBE III provided detailed surface analysis, revealing elemental composition and oxidation states. Electrochemical analyses were carried out using a three-electrode cell coupled to the potentiostat PGSTAT 204 from MetrohmAutolab, facilitating the study of electrochemical properties such as specific capacitance and cyclic stability. These advanced techniques collectively offer comprehensive insights into the structural, chemical, and electrochemical properties of the LiZnPO4 nanoparticles, thus enhancing their potential for diverse applications.
2.4 Preparation and Fabrication of Working Electrode
Ni-foam pieces sized at 1 cm x 2 cm underwent a thorough cleaning process involving deionized water, 1M nitric acid (HNO3), and 2M hydrochloric acid to remove oxides and contaminants. Following cleaning, the Ni-foam pieces were sonicated in ethanol for 15 minutes and subsequently dried in a hot-air oven at 80°C for eight hours. For electrode fabrication, a slurry consisting of 70% LiZnPO4 nanoparticles synthesized at 700°C, 20% activated carbon, and 10% polyvinylidene fluoride (PVDF) was meticulously mixed in a mortar and pestle. N-methyl-2-pyrrolidone (NMP) was incorporated to form a paste, which was evenly spread onto the cleaned Ni-foam substrate, covering an area of 1 cm². The coated electrode underwent air-drying for 60 minutes at 70°C, followed by heating in a hot-air oven at 90°C for 8 hours to ensure thorough drying and strong adhesion of the active material. This approach yields uniform and precisely controlled electrodes suitable for accurate electrochemical analysis.
2.5 Electrochemical Cell Setup
The electrochemical investigation of LiZnPO4 nanoparticles at temperatures ranging from 500°C to 700°C was carried out using a three-electrode cell configuration. The setup comprised a LiZnPO4-coated Ni foam serving as the working electrode, responsible for driving the electrochemical reactions within the test solution. A platinum coil was employed as the counter electrode, facilitating the flow of current and establishing a reference point for the reactions occurring at the working electrode. Additionally, an Ag/AgCl electrode functioned as the reference electrode, enabling precise potential measurements throughout the electrochemical analysis. All three electrodes were immersed in a 1.0 M potassium hydroxide (KOH) solution, serving as the electrolyte for the experimental investigations. This arrangement allowed for the thorough exploration of the electrochemical properties and behavior of the LiZnPO4 material under the specified experimental conditions.
3.1 Structural properties
The X-ray diffraction data for LiZnPO4 nanoparticles were sintered at temperatures ranging from 400 to 700°C in increments of 100°C, are depicted in Fig. 1a-d. Figure 1a-d illustrates that well-crystallized LiZnPO4 nanoparticles are obtained at 700°C, exhibiting a polycrystalline nature with peaks corresponding to (100), (111), (202), (020), (321), (123), (214), (602), (522), (710), (332), and (333) planes, with a preferred orientation along the (202) and (020) planes.As observed in Fig. 1a-c, the peak intensities are low at lower sintering temperatures, ranging from 400°C to 600°C. This is attributed to thermal activation at lower temperatures, which affects crystallinity and results in weaker diffraction signals. With increasing sintering temperature at 700 oC, the LiZnPO4 nanoparticles become more crystalline, as evidenced by the increased intensity of the (202) and (020) peaks, accompanied by a decrease in the corresponding Full Width at Half Maximum (FWHM). For comparison, the same figure depicts the position and relative intensity of peaks for crystalline LiFePO4 nanoparticles (JCPDS card No. 81-1173). The synthesized LiZnPO4 nanoparticles exhibit a well-crystallized olivine-type structure. The sintering temperature significantly influences the growth of (202) and (020) peaks, as depicted in Fig. 1a-d.
The crystallite size of LiZnPO4 was determined using the Scherer formula [17], as shown in Eq. (1)
where D represents the mean crystallite size, β denotes the full width at half maximum of the diffraction line, θ signifies the diffraction angle, and λ stands for the wavelength of the X-ray radiation. The crystallite sizes of the particles were assessed using the Scherrer equation, focusing on the (020) peaks observed in the XRD patterns [18]. To ensure accuracy in the calculation, instrumental broadening was accounted for. The (020) diffraction peak positions, normalized full width at half maximum (FWHM) values, and the calculated crystallite sizes of the LiZnPO4 particles are presented in Table 1. The crystallite size of the particles synthesized at 400°C was measured at 55 nm, showing an increase to 60 nm, 72 nm, and 85 nm as the reaction temperature was raised to 500°C, 600°C, and 700°C, respectively.
3.2 FTIR analysis
FT-IR spectra showcasing the LiZnPO4 nanoparticles synthesized via the simple root precipitation method at sintering temperatures ranging from 400 to 700°C are illustrated in Fig. 2a-c. Various bands within the structure of the LiZnPO4 nanoparticles were attributed to characteristic bonds, including Zn-O and P-O bonds observed at 515 cm− 1 and as a doublet at 490 cm− 1 (ν2). Furthermore, the presence of PO4− bonds in LiZnPO4 was indicated by characteristic bands at 594 cm− 1 (ν4) and Li–O bonds at 881 cm− 1 (ν1). Notably, it was anticipated that Li-O vibration bands would overlap with P-O bands in LiZnPO4 nanoparticles [19]. Particularly, the FTIR spectrum distinctly delineated the stretching and bending vibrations of the phosphate anion (PO4)3−. Stretching modes were observed at 881 cm− 1 (ν1), 515 cm− 1 (ν2), 1386 cm− 1 (ν3), while bending modes were detected at 594 cm− 1 (ν4) for the phosphate anion (PO4)3−. Additionally, bands at 2450 cm− 1 and 3449 cm− 1 were assigned to C-O stretching vibrations, as observed in Fig. 2a-d.
3.3 Surface morphological analysis
SEM images depicted in Fig. 3a-d illustrate LiZnPO4 nanoparticles were synthesized via the simple root precipitation method at sintering temperatures ranging from 400 to 700°C. Notably, Fig. 3a-b reveals the production of spherical LiZnPO4 nanoparticles measuring 58 nm in diameter at 400°C, with a minor portion of nanoparticles aggregating to form secondary submicron particles. The degree of aggregation and the presence of secondary particles increased at 500°C, yet the sintering process remained in a transitional phase from primary to secondary particles, evident by the presence of nanoparticle clusters surrounding the secondary particles.
Figure 3c displays the porous surfaces of LiZnPO4 nanoparticles, consisting of primary particles a few tens of nanometers in size when synthesized at 600°C, with individual nanoparticles or their clusters not readily observable. Subsequently, elevating the sintering temperature to 700°C led to increased densification of the secondary particles and grain growth. The measured grain sizes on LiZnPO4 porous particles were consistent with calculated values of crystallite aggregates, ranging from 60–75 nm to 80–95 nm with increasing sintering temperature.
The process of aggregation and sintering of nanoparticles became more pronounced with higher reaction temperatures. These findings underscore the significance of sintering temperature in controlling the size and morphological characteristics of the final LiZnPO4 product when utilizing the simple root precipitation method.
3.4 EDS analysis
The EDS analysis of LiZnPO4 nanoparticles were sintered at 400 to 700oC are shown in Fig. 4a-d. The EDS spectrum reveals the elemental compositions are the peaks corresponding to lithium (Li), zinc (Zn), phosphorus (P), and oxygen (O) confirm the presence of these elements in the nanoparticles. The high-intensity peaks observed may arise from the overlap of elemental peaks sharing similar binding energies at varying depth profiles. The analysis provides valuable insights into the chemical composition. Reinforce the material integrity, purity and quality of the LiZnPO4 nanoparticles, contributing to a better understanding of their properties and potential applications.
3.5 XPS study
Figure 5(a–e) depicts the surface electronic states of the O1s, Li1s, Zn 2p, and P2p regions of the LiZnPO4 nanoparticles at 700oC, captured through X-ray photoelectron spectroscopy (XPS) at high energy resolution. In Fig. 5b, the Li1s spectrum is observed to fit a single component. Moving to Fig. 5c, the Zn 2p spectrum exhibits two distinct components for the LiZnPO4 nanoparticle, identified as Zn 2p3/2 and Zn 2p1/2, with respective centers at 1022.73 eV and 1045.25 eV. Figure 5d illustrates the P2p spectrum, which is fitted to two components, with the second peak at 286.35 eV and the primary peak positioned at the lowest binding energy (BE) of 133.78 eV. The third peak, located at 140.24 eV, exhibits the highest BE. Lastly, the O1s region in Fig. 5e showcases a single component encompassing lattice oxygen P–O and Zn–O links, along with a narrow peak around 532.16 eV attributed to defects and oxygen [20, 21].
3.6 Electrochemical experiments
The performance of LiZnPO4 nanoparticles at 700oC electrode materials was examined using cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance spectrometry (EIS) in 1.0 M KOH solution as electrolytes at ambient temperature.
3.6.1 Cyclic voltammetry experiment
Figure 6a presents the cyclic voltammetry (CV) curves of the LiZnPO4 nanoparticles electrode, recorded in a 1.0 M KOH solution at scan rates of 5, 10, 25, 50, and 100 mVs− 1. The curves display pronounced redox peaks, suggesting the presence of pseudo-capacitance as the main mechanism for energy storage. The regularity of the CV curves suggests reversibility of the redox reaction. Moreover, increasing the scan rate leads to larger loop dimensions and higher redox peaks, indicating rapid redox processes occurring during scanning. Specific capacitance values were determined using Eq. (2).
$${ \text{C}}_{\text{s}\text{p}}=\frac{1}{\text{v}\text{m}({\text{V}}_{\text{a}}-{\text{V}}_{\text{c}})}{\int }_{{\text{V}}_{\text{a}}}^{{\text{V}}_{\text{c}}}\text{I}\left(\text{V}\right)\text{d}\text{V} \left({\text{F}\text{g}}^{-1}\right)$$
2
In Eq. (2), V represents the potential window, Csp stands for specific capacitance, υ denotes the scan rate, m represents the mass of the active material, I signifies the current, and Vc and Va indicate the voltage windows of the cathodic and anodic regions, respectively. At a scan rate of 5 mVs− 1, the specific capacitance reaches a peak value of 268.16 Fg− 1. As the scan rates incrementally increase to 10, 25, 50, and 100 mVs− 1, the specific capacitances decrease to 211.35, 168.45, 142.07, and 125.35 Fg− 1, respectively, as listed in Table 1.
3.4.2 Galvanostatic charge-discharge experiment
Figure 6(b) displays the galvanostatic charge-discharge (GCD) curves obtained at different current densities of 0.5, 1, 2, 3, 4, and 5 Ag− 1, respectively. These curves exhibit symmetry, suggesting excellent electrochemical performance of the electrodes and reversible redox processes. Furthermore, as the current densities increase, the corresponding charge–discharge times decrease. Specific capacitance values were determined using Eq. (3).
$${ \text{C}}_{\text{s}\text{p}}=\frac{\text{I}\varDelta \text{t}}{\text{m}\varDelta \text{V}} \left({\text{F}\text{g}}^{-1}\right)$$
3
where, 'm' represents the mass of the active material in kilograms, 'V' denotes the potential or voltage window in volts, 'I' signifies the current in amperes, and 'Δt' stands for the discharge time in seconds. The maximum specific capacitance value achieved for LiZnPO4 was 136.41 Fg− 1 at a current density of 0.5 Ag− 1. The calculated results are presented in Table (2). At current densities of 0.5, 1, 2, 3, 4, and 5 Ag− 1, the respective specific capacitances are 136.4, 132.7, 130.9, 120.0, and 109.1 Fg− 1. Based on the analysis, it can be inferred that when the current density increases to ten times its initial value, the specific capacitance retains approximately 80% of its initial value, indicating good rate capability of the material. This can be attributed to its unique stable structure and high conductivity, as implied by electrochemical impedance spectroscopy (EIS)[23].
3.4.3. Electrochemical Impedance Spectroscopy experiment
The equivalent specific capacitances determined via cyclic voltammetry (CV) are depicted in Fig. 7(a). At a scan rate of 5 mVs− 1, the specific capacitance measures approximately 268.16 Fg− 1, gradually decreasing to 125.35 Fg− 1 as the scan rate escalates to 100 mVs− 1. Figure 7(b) illustrates the retention of specific capacitance of the LiZnPO4 electrode at various current densities: 0.5, 1, 2, 3, 4, and 5 Ag− 1. Notably, the charge and discharge sectors exhibit symmetry, and specific capacitances were determined by discharge time using Eq. (2). Remarkably, the specific capacitance results calculated via galvanostatic charge-discharge (GCD), as shown in Fig. 7(b), closely mirror those obtained via CV in Fig. 7(a), indicating the reliability of these findings. At current densities of 0.5 Ag− 1 and 5 Ag− 1, the corresponding specific capacitances are 136.4 Fg− 1 and 109.1 Fg− 1, respectively, representing a modest 20% reduction in specific capacitance despite a tenfold increase in current density (Ag− 1)[24].
Additionally, an electrochemical impedance spectroscopy (EIS) test was conducted with LiZnPO4 nanoparticle electrodes, the results of which are presented in Fig. 7(d). To assess the varying resistance offered by the electrode materials, EIS measurements were performed across frequencies ranging from 1 Hz to 120 MHz. Figure 7(d) displays the Nyquist plot obtained for the first cycle and the 1500th cycle under open circuit potential. Two primary regions are discernible in the Nyquist plot: an inclined slope in the low-frequency region and a small semicircle in the high-frequency zone. The initial solution resistance of the LiZnPO4 nanoparticle electrode (Rs) is measured at 2.3 Ω, which is slightly higher than that observed after 1500 cycles of stability testing at 5 A/g current density, which registers at 2.081 Ω, as indicated in the inset of Fig. 7(d)[25].
X-ray diffraction analysis demonstrates the pivotal role of sintering temperatures in shaping the crystallinity and preferred orientation of LiZnPO4 nanoparticles, with optimal crystallization observed at 700°C. The Scherrer equation-based assessment reveals a progressive increase in crystallite size from 55 nm at 400°C to 85 nm at 700°C, indicating enhanced crystalline growth with elevated sintering temperatures. FTIR spectroscopy elucidates the characteristic bonding patterns within LiZnPO4 nanoparticles, affirming the presence of Zn-O, P-O, and Li-O bonds, contributing to structural stability. SEM imaging highlights the morphological evolution of nanoparticles with sintering temperature, transitioning from spherical particles to porous structures, indicating a temperature-dependent aggregation process. EDAX analysis confirms the elemental composition of LiZnPO4 nanoparticles, validating their chemical integrity and suitability for targeted applications. XPS investigation reveals distinct surface electronic states, providing insights into the chemical bonding environment and surface reactivity of the nanoparticles. Electrochemical experiments showcase the superior energy storage capabilities of LiZnPO4 nanoparticles, evidenced by pronounced redox peaks in cyclic voltammetry and symmetric charge-discharge curves in galvanostatic experiments. Specific capacitance values exhibit a gradual decrease with increasing scan rates and current densities, suggesting good rate capability and stability of the electrode material. Electrochemical impedance spectroscopy further corroborates the robust performance of LiZnPO4 nanoparticles, revealing low solution resistance and maintaining stability over prolonged cycling. These findings collectively underscore the temperature-sensitive synthesis route and the promising electrochemical properties of LiZnPO4 nanoparticles, laying a foundation for their potential utilization in energy storage applications.
Acknowledgments
The authors extend their gratitude to the University Grants Commission-South Eastern Regional Office (UGC-SERO), Hyderabad, India, for their financial support through project No. MRP-4892/14 (SERO/UGC). They also acknowledge the support of Adhiyamaan College of Engineering (Autonomous), Hosur, Krishnagiri; Government Engineering College, Bargur, Tamil Nadu, India; and the Department of Physics, Sri Ramakrishna College of Arts & Science, Coimbatore.
Declarations Ethics approval and consent to participate
Not applicable.
Consent for publication
The final version of the manuscript was reviewed and approved by all authors.
Availability of data and materials
The data that support the findings of this study are available from the corresponding author, Dr. R. Mariappan, upon reasonable request.
Competing of interests
The authors declare that they have no conflict of interest.
Authors Contributions
The First draft of the manuscript was written by E. Krishnamoorthy
Conceptualization, Methodology, Investigation, Data curation, Writing - review & editing by R. Mariappan
Material preparation, data collection and analysis were performed by G. Gowrisankar and R. Bakkiyaraj.
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Table (1) Specific capacitance values of LiZnPO4obtained usingCV curves at different scan rates
|
S.No. |
Scan Rate (mVs-1) |
Specific Capacitance (F/g) |
|
1 |
5 |
268.16 |
|
2 |
10 |
211.35 |
|
3 |
25 |
168.45 |
|
4 |
50 |
142.07 |
|
5 |
100 |
125.35 |
Table (2) Specific capacitance values of LiZnPO4obtained usingGCD curves at different current densities
|
S.No. |
Currentdensity (A/g) |
Specific Capacitance (F/g) |
|
|
1 |
0.5 |
136.4 |
|
|
2 |
1 |
132.7 |
|
|
3 |
2 |
130.9 |
|
|
4 |
3 |
120.0 |
|
|
5 |
4 |
116.4 |
|
|
6 |
5 |
109.1 |
|
No competing interests reported.