1. INTRODUCTION
The ongoing search for higher energy efficiency and performance has led to continuing advances in the development of lithium-ion batteries, which have become a cornerstone of modern technologies, especially in the fields of portable electronics, electric vehicles, and large-scale energy storage systems. Central to this pursuit is the continual innovation in electrode materials, which directly impacts the battery's capacity, stability, and overall lifespan [1,2].
Zinc oxide (ZnO) has attracted significant interest as an anode material for Li-ion batteries because of its unique properties and advantages [3,4]. ZnO offers a high theoretical capacity (approximately 978 mAh/g), which is considerably greater than that of traditional graphite anodes [5]. This high capacity is due to the ability of ZnO to undergo conversion reactions that allow for the storage of more lithium ions. Further, Zinc is an abundant element and ZnO is relatively easy and inexpensive to produce, making ZnO a cost-effective alternative to other anode materials, which is crucial for the large-scale production of batteries. ZnO can also be synthesized in various nanostructures (like nanoparticles, nanowires, and nanorods), each of which offers unique benefits in terms of surface area, electron transport, and lithium-ion diffusion pathways [6,7]. This flexibility allows ZnO-based anodes to be optimized for specific applications.
Despite these advantages, ZnO anodes also have shortcomings, particularly their cycle stability and volume expansion during the lithiation and delithiation processes [8,9]. ZnO has been the focus of extensive research as a promising anode material for Li-ion batteries, especially when doped with various elements. An appropriate doping strategy is a critical way of enhancing the performance of ZnO batteries. Doping introduces foreign atoms into the ZnO lattice, which can create free charge carriers (electrons or holes), thereby enhancing its electrical conductivity [10]. Zhang et al. reported that Aluminum (Al)-doped zinc oxide (ZnO) nanoparticles, particularly those with a 2% molar ratio of Al, exhibited superior reversible capacities compared to undoped ZnO under the same experimental conditions [11]. The Al-doped ZnO nanoparticles achieved reversible specific capacities of 418 mA·h·g−1 at a current density of 50 mA·g−1, likely due to enhanced electronic conductivity resulting from the optimal concentration of Al3+ ion doping.
Certain dopants can enhance the structural stability of ZnO during the lithiation and delithiation processes, reducing volume expansion and contraction that can lead to material degradation and capacity loss over time. Wang et al reported that manganese-doped zinc oxide (Zn0.8Mn0.2O) porous nanosheets, synthesized through a simple approach, showed enhanced cycle durability and reversible capacity [12]. The structure's cavities helped reduce volume expansion and improve electrical conductivity, leading to better electrochemical performance and stability. This study also offers an effective method of fabricating Zn0.8Mn0.2O nanostructures. Bresser et al. reported that Fe and Co doping of ZnO nanoparticles significantly improved their electrochemical performance, making them promising for use in advanced Li-ion anodes [13]. Although Co doping is more effective, Fe is preferable for environmental and economic reasons. Coating Fe-doped ZnO nanoparticles with carbon further enhances their electrochemical performance, particularly cycling stability, high-rate performance, and capacity retention [13].
This study focuses on the synthesis and application of ZnO nanocrystals (NCs) doped with magnesium (Mg), forming Zn1-xMgxO NCs, as an anode material for lithium-ion batteries. Doping ZnO with Mg alters its electronic structure, potentially leading to improved conductivity and enhanced electrochemical properties. In this study, we investigate the impact of varying Mg concentrations on the performance of Zn1-xMgxO NC anodes in Li-ion batteries, with a particular focus on the Mg composition range of x=0.05 to x=0.15.
The primary objective is to unravel the intricate relationship between Mg doping level-dependent electronic conductivity and key battery performance metrics, such as capacity, cycle stability, and the formation and characteristics of the solid-electrolyte interphase (SEI) layer. Employing a combination of experimental techniques, we meticulously analyze the electrochemical characteristics of the Zn1-xMgxO NC anodes. Through this comprehensive investigation, we aim to contribute to the development of more efficient, durable, and high-capacity lithium-ion batteries, paving the way for advanced energy storage solutions that are crucial for the future of sustainable energy technologies.
2. MATERIALS AND METHODS
2.1 Synthesis of Zn1-xMgxO NC
In this experimental procedure, we prepared precursor solutions using zinc acetate dihydrate and magnesium acetate tetrahydrate, each with a concentration of 5 mM. These solutions were combined in a volume of 300 mL of ethanol. The mixture was then subjected to continuous stirring at a temperature of 60 °C for a duration of 1 hour. The objective was to ensure homogenous mixing and proper dissolution of the precursors. We varied the atomic ratios of Mg to Zn in this mixture, experimenting with proportions of 0 (pure ZnO), 0.05, 0.1, and 0.15 to investigate the effects of different Mg doping levels on the properties of the final product, Zn1-xMgxO.
Separately, we prepared a solution of tetramethylammonium hydroxide pentahydrate (TMAH), weighing precisely 3.0998 g, and dissolved it in 100 mL of ethanol at room temperature. This solution was stirred for 1 hour to ensure complete dissolution of the TMAH, which acts as a precipitating agent in the synthesis process. After the hour of stirring, the TMAH solution was slowly added to the previously prepared zinc and magnesium acetate mixture. This addition was performed gradually, over a span of 15 minutes, to control the reaction kinetics and ensure uniform distribution of TMAH within the mixture.
Once the TMAH solution was fully incorporated, we allowed the reaction to continue for an additional 30 minutes. This step is crucial for the complete formation of Zn1-xMgxO NCs. Following this period, the reaction mixtures were left to cool down gradually at room temperature. This slow cooling process is important for the stabilization of the nanocrystals formed in the solution. The next step involved centrifugation of the cooled solutions.
2.2 Electrochemical characterization
To prepare the electrode material, Zn1-xMgxO NC powder, carbon black, and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 8:1:1. N-methylpyrrolidone (NMP) was used as the solvent to form a uniform slurry. This slurry was then evenly spread onto a copper foil using a specialized applicator. The coated foil was subsequently dried on a hot plate set at 80 °C for a duration of 12 hours. This step is critical to ensure the evaporation of the solvent and the formation of a consistent electrode film. To remove any remaining solvent, the electrode films were subjected to further drying in a vacuum oven at 60 °C overnight. This process ensures the complete removal of NMP, which is essential to the stability and performance of the electrode material.
For the battery assembly, CR2032 coin cells were constructed within an argon-filled glove box to prevent any moisture or air contamination. A lithium metal chip, with dimensions of 250 μm in thickness and 16 mm in diameter, was utilized as the counter electrode. Separation between the electrodes was maintained using a polyethylene separator, which measured 25 μm in thickness and 19 mm in diameter. The electrolyte employed was a solution of 1 M lithium hexafluorophosphate (LiPF6) in a 1:1 volume ratio mixture of ethylene carbonate and ethyl methyl carbonate. This particular electrolyte composition was chosen for its ability to facilitate efficient lithium-ion transport.
Electrochemical characterizations of the assembled coin cells were performed using a CS2350 CorrTest system. This involved a series of tests to evaluate the electrochemical properties and performance of the cells under various conditions, providing insights into the effectiveness of the Zn1-xMgxO anode material.
2.3 FET device fabrication and characterization
FETs were fabricated employing photolithography techniques on a gate dielectric layer of SiO2, with a thickness of 200 nm. A highly boron-doped silicon substrate with a resistivity ranging from 0.001 to 0.002 Ωcm was utilized as the gate electrode. The FET devices underwent a rigorous cleaning protocol beginning with UV ozone treatment, followed by sequential sonication in acetone, methanol, and deionized water to ensure the removal of organic contaminants. Subsequently, the Zn1-xMgxO precursor solution (10mg ml-1 in ethanol), was spin-coated at 2000 rpm for 30 seconds to form a film with a thickness of ~180 nm.
Electrical characterization was conducted in an inert argon atmosphere using an HP4145B semiconductor parameter analyzer to mitigate any atmospheric effects on measurement accuracy. For photoluminescence (PL) analysis, a 325 nm wavelength light source was employed to excite the Zn1-xMgxO NC films, which were encapsulated prior to measurement to prevent oxidation.
Structural characterization of the Zn1-xMgxO NCs was performed using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), providing insight into the crystalline nature and morphology of the synthesized materials.
3. RESULTS AND DISCUSSTIONS
When Mg is incorporated into ZnO, it alters the electronic structure of the material. Since Mg has a wider band gap than Zn, its addition to ZnO increases the overall band gap of the resulting Zn1-xMgxO NC compound. As a result, the material absorbs higher-energy (shorter wavelength) photons for electronic transitions to occur, which is observed as a shift in the optical absorption edge towards higher energy. The shift of the optical absorption edge to higher energy with increasing Mg composition (x) from 0.00 to 0.15 in Zn1-xMgxO NCs indicates a change in the band gap energy, as seen in Fig 1(a). Specifically, this shift suggests that the band gap is widening [14-16].
The X-ray diffraction (XRD) pattern of Zn1-xMgxO NCs is presented in Fig 1(b), indicating that Zn1-xMgxO NCs possess a hexagonal wurtzite crystal structure. Utilizing Scherrer’s formula, the estimated size of these Zn1-xMgxO NCs (x=0) is approximately 2.97 nm, and the size decreased to 2.52 nm with increasing Mg composition (x=0.15), consistent with previous studies.17 Peak position and intensity depending on Mg composition was not substantial. The lack of significant shifts in the peak positions suggests that the incorporation of Mg into the ZnO lattice does not drastically distort the crystal structure. Since the peak positions in XRD are sensitive to the lattice parameters, this indicates that the Mg ions are likely substituting for Zn ions in the lattice without causing major changes to the lattice dimensions. The absence of substantial changes in peak intensity can also suggest that Mg ions are being homogeneously distributed within the ZnO lattice. If Mg were clustering or forming separate phases, it could lead to more noticeable changes in the XRD pattern.
Substitution of Mg into Zn without substantial lattice distortion was also reflected in the surface morphology, which was measured using tapping-mode atomic force microscopy, as shown in Fig 2.
Small grains were densely populated on the substrate and shapes of the grains were similar, independent of Mg composition, with a roughness of 0.4~0.7 nm.
Mg composition-dependent defect energy levels were measured with photoelectron spectroscopy in air (PESA), as shown in Fig 3.
The energy of incident photons was varied within a range of 4.2 to 6.0 eV. To minimize surface charging, a weak UV intensity, approximately 50 nW/cm², was employed. The deeper energy levels (5.73 eV) with increased Mg composition (x=0.15) could indicate that Mg doping is introducing deeper defect states in the band structure of ZnO. These deeper defect states might be associated with changes in the electronic properties of the material, such as altered conductivity or optical absorption characteristics.
An increase in the PL intensity at 526 nm was observed with increasing Mg composition, as shown in Fig 4.
The 526 nm emission in ZnO is typically associated with green emission, often attributed to defect-related transitions within the material, such as oxygen vacancies or zinc interstitials. An increase in this emission's intensity with more Mg could indicate that Mg doping is influencing the concentration. If Mg doping leads to a higher density of defect states that facilitate radiative recombination, it might be seen as an increase in PL intensity at specific wavelengths.
We investigated Mg dependent FET mobility and threshold voltage using bottom-contact FET devices. In the saturation regime of transistor operation, the FET mobility (μ) and the threshold voltage (VT) are described by the Equation below.
Here Cox represents the capacitance of the gate dielectric, while Z and L denote the width and length of the channel, respectively. During the saturation transport regime, the saturated drain current (IDsat) at high drain voltage (VD=VG−VT) reaches a plateau as the width of the depletion region expands with increasing drain voltage, as detailed in the Equation.
The threshold voltage is the minimum gate voltage required to accumulate enough mobile carriers in the channel for conduction. Beyond this voltage, the drain current magnitude follows the above Equation. In the saturation regime, the threshold voltage is determined by the gate voltage at which the plot of IDsat1/2 versus VG intersects the yaxis at zero.
Variations in the threshold voltage magnitude are often linked to the density of electronic traps within the active layer of the FET. Applying gate voltage initially serves to fill these traps, especially the deeper ones. Once these deep traps are filled, mobile carriers near the gate dielectric become available, transitioning the FET device from the ‘‘off’’ to the ‘‘on’’ state, which occurs at voltages above the threshold voltage.
As the Mg composition increased from x = 0 to x = 0.15, the FET mobility decreased from 9.2 × 10-6 to 0.2 × 10-6 cm2V-1s-1, as shown in Fig 5.
Threshold voltage shifted to a more positive value from 4.0 to 21.0 V, indicating more traps are present at a higher Mg composition. The threshold voltage in FETs represents the gate voltage at which the device starts to conduct. A shift to more positive values indicates that a higher gate voltage is needed to accumulate enough carriers in the channel to start conduction. This shift can be due to an increase in trap states that need to be 'filled' before free carriers are available for conduction. Essentially, the initial applied voltage is used to 'fill' these traps, and only when they are sufficiently filled do additional carriers contribute to conduction.
Mg doping in Zn1-xMgxO can introduce new defect states or 'traps' in the material's band structure, as shown by the PESA and PL studies in Fig 3, and Fig 4, respectively. These traps can capture charge carriers (electrons in the case of n-type semiconductors), reducing the number of free carriers available for conduction. As Mg composition increases, the density of these traps may increase, leading to more pronounced carrier trapping.
The decrease in FET mobility with higher Mg content suggests that the carriers (electrons) face increased scattering or hindrance due to these defect states. Mobility is a measure of how quickly charge carriers can move through a material when an electric field is applied. The presence of more traps or defect states can impede this movement, thereby reducing mobility.
We assembled a Zn1-xMgxO NC half-cell battery configured with Li as anode and Zn1-xMgxO NC as cathode.
Fig 6(a) shows capacity as a function of the number of discharging cycles. In the initial cycles, as the Mg composition increased, the capacity decreased, however, after 5 cycles, the capacity became independent of the Mg composition, as shown in Fig 6(b) and 6(c). This trend was more significant with charging cycles, as shown in Fig 7.
During the initial charging cycles, Zn1-xMgxO with more Mg composition showed lower capacity, while after 5 cycles, the trend was altered. Nonetheless, ZnO had the largest capacity regardless of charging/discharging.
The decrease in capacity with increasing Mg composition in the initial cycles is likely due to the impact of Mg doping on the electrochemical properties of Zn1-xMgxO. Mg doping can influence electronic conductivity, the density of active sites for Li-ion intercalation, and the structural integrity of the material. With higher Mg content, the Zn1-xMgxO might have fewer active sites for Li-ion storage or poorer electronic conductivity, as demonstrated in Fig 5, leading to a lower initial capacity. The reduced conductivity can hinder the efficient movement of electrons during the electrochemical reactions of Li storage, leading to a lower initial capacity. The more pronounced effect of Mg composition on capacity during the initial charging cycles can be attributed to the different mechanisms at play during charging, such as the extraction of Li ions from the cathode material. The presence of Mg might initially impede this process more than during discharging, but similar to discharging, the system stabilizes after several cycles.
The capacity becomes independent of the Mg composition after a few cycles, which suggests a stabilization effect occurring within the electrode material. During the initial cycles, the electrode might undergo structural and chemical changes (such as the formation of an SEI layer, rearrangement of the crystal structure, or activation of inactive materials), which could lead to a convergence in capacity regardless of the initial Mg composition. Over successive cycles, the formation of the SEI layer becomes a critical factor. The SEI layer forms on the electrode's surface during the first few cycles and can significantly influence lithium-ion transport. While the SEI layer is ionically conductive, allowing Li-ions to pass through, it's electronically insulating, which can change the dynamics of how capacity is influenced by the electronic properties of the electrode. This can lead to structural and compositional changes in the material. These changes can affect the electrode's ability to store and release lithium ions, becoming more influential in determining capacity than the intrinsic electronic conductivity of the material.
Unlike Zn1-xMgxO, pure ZnO retained the highest capacity during cycles. Pure ZnO, without Mg doping, typically exhibits a more stable crystal structure and electronic conductivity. Its capacity is more directly related to these inherent properties, which do not undergo the same degree of alteration as in the doped material. As a result, ZnO may retain higher capacities consistently across all cycles. The formation and stability of the SEI layer in Zn1-xMgxO can be more variable due to the presence of Mg. In contrast, the SEI layer on pure ZnO might form more uniformly and be more stable, leading to more consistent capacity retention. In other words, while the doping of Mg in Zn1-xMgxO offers certain advantages, it also brings complexities that can impact capacity over multiple cycles. Pure ZnO, with its inherent stability and consistent properties, may not experience these complexities to the same extent, thus maintaining higher capacities across all cycles.
4. SUMMARY AND CONCLUSIONS
The electronic conductivity was found to be sensitive to changes in Mg composition. Consistently, the initial capacity decreased with an increase in Mg composition, aligning with the reduction in electronic conductivity due to Mg doping. However, with successive cycles, the capacity became independent of the electronic conductivity, an outcome attributed to the formation of the SEI and the conversion reactions. During the electrochemical reactions associated with Li storage, structural and electrical changes in Zn1-xMgxO are inevitable. As a result, a variety of factors, including electronic conductivity, begin to influence the capacity over time. Initially, electronic conductivity plays a major role in determining capacity. However, as the battery undergoes more cycles, other factors, such as the mechanical stability of the electrode, the integrity of the SEI layer, and the efficiency of lithium-ion transport, begin to exert a more significant influence on the capacity.
The clear dependence of size, mobility, and defect energy levels on Mg composition in Zn1-xMgxO underscores the importance of precise material engineering. By controlling the Mg doping level, it is possible to tailor these properties for specific applications, leading to optimized battery performance. This understanding can guide the development of materials with lower resistance, facilitating faster charging and discharging rates. More importantly, this study indicates that the initial capacity is closely tied to the conductivity of the Zn1-xMgxO material. By enhancing conductivity through optimal Mg doping, SEI formation and conversion reaction can be optimized for applications that require long-lasting power, such as electric vehicles and portable electronics. The formation of a stable and effective SEI layer is crucial for the long-term performance and safety of lithium-ion batteries. An optimized SEI layer can protect the electrode material, maintain ionic conductivity, and prevent further decomposition of the electrolyte. Improved conductivity in the electrode material can influence SEI formation dynamics, potentially leading to more stable and efficient cycling.
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