Mg²⁺ substitution effect on the electrochemical performance of LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0.0, 0.01, 0.02, 0.03, 0.04, 0.05) cathode materials for LIBs

The research on energy storing devices is advancing every day. Their capacity depends on different factors, such as precursors, synthesis methods, etc. One of the research studies has shown that the type of precursor used in wood-based carbon electrodes affects their performance: denser precursors increase mass loading and introduce more defects in microstructure while incorporating hierarchical porosity enhances storage capabilities for Li-ion batteries.¹ Copper sulfide (CuS) has emerged as a promising material for aqueous ammonium-ion batteries (AAIBs), demonstrating superior NH⁴⁺ storage performance and stability improvements through electrolyte adjustments.² In addition, hybrid batteries³ utilizing coated Co₉S₈/CNT microspheres⁴ and monoclinic WO₃ nanospheres⁵ show enhanced conductivity and ion diffusion for AAIBs in microbial fuel cells.⁶ Meanwhile, studies continue to explore the efficiency improvements of Li–S⁷ and Zn-ion batteries.⁸ A one-step synthesis of hierarchical porous carbon has enabled stable performance over 50 000 cycles in symmetric supercapacitors while achieving an energy density of 79 Wh kg⁻¹ in zinc-ion hybrid capacitors.^3,9

Lithium-ion batteries (LIBs) are highly appropriate for large-scale applications due to their high energy, low power density, high cycle life, low cost, and eco-friendly nature.^10,11 Hence, the nickel-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathode material turned out to be the best alternative to LiCoO₂ that is widely used as a cathode in LIBs.^12–14 Ni-, Co-, and Mn-based layered solid oxides [LiMO₂ (M = Ni, Co, Mn)] have spinel structures with higher specific capacitance suitable for developing advanced cathode materials.¹⁵ Nickel-rich LiNi_0.8Co_0.1Mn_0.1O₂ has a high specific capacity of ∼200 mA h g⁻¹, a specific energy of >275 mA h g⁻¹, and reversible capacities of >180 mA h g^−1.16–18 The layered structure of the Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathode material exhibited poorer results than theoretical values due to Li⁺/Ni²⁺ cation disorder.^19–21 Furthermore, cycling is irreversible and poor, which creates thermal instability.²² This problem can be minimized with the doping of transition metals by changing the preparation conditions, thus improving the electrochemical performance.²³ Hence, this research aims to ensure improved material stability and specific capacitance under safe conditions.²⁴ The operating voltage used is 3.0–4.6 V, applicable for mobile applications for storing charge.^25–29 

This study introduces the innovative use of Mg-doped LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials, synthesized via the sol–gel auto-combustion method,³⁰ to enhance lithium-ion battery performance. The strategy involves systematically substituting Mg to improve structural, morphological, and electrochemical proper-ties. Key findings include the confirmation of a rhombohe-dral–hexagonal structure, increased initial discharge capacities, and improved retention capacity and impedance characteristics, particularly for the x = 0.03 composition. These results highlight the potential of Mg-doped LiNi_0.8Co_0.1Mn_0.1O₂ for high-performance battery applications. The compositions x = 0.01, 0.02, 0.03, 0.04, and 0.05 in LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ are denoted NCM, NCM-1, NCM-2, NCM-3, NCM-4, and NCM-5, respectively. The chart of synthesis and characterization is shown in Chart 1.

The cathode materials LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (where x = 0.01, 0.02, 0.03, 0.04, and 0.05) were prepared with a sol–gel approach using Li₂CO₃, NiO, CoO, and MnO.²⁰ Citric acid and ammonia solutions were used as chelating and precipitating components that made changes in the chemical reaction and pH value, respectively. The initial stirring was performed at 80 °C for a few minutes, and stirring continued at 130 °C for 10 h. The proper mixing and drying of the solution result in gel whose organic residues can be separated by heating at 500 °C in the air @ 5 °C/min for 6 h. The resultant is powdered and again sintered in the air @ 850 °C for 18 h to get the final composition. A Rigaku XRD, Carl Zeiss FESEM, and Thermo Nicolet 6700 FTIR and the KBr pellet method were used to determine the structural and DC resistive properties of the sample.

90 wt. % of the active material, 7 wt. % of carbon black, and 3 wt. % of polyvinylidene fluoride were mixed. The resultant was then mixed with n-methyl pyrrolidone to get a slurry product. The product was pasted on aluminum foil and dried at 30 °C for 12 h using a hot air oven, and then CR 2032 coin cells were prepared. The Grammy interface was used for the first cycle study using the coin cells.

Thermogravimetry analysis (TG) and thermal galvanometry analysis (DTA) are the two main techniques used for thermal analysis. From TG/DTA, the crystallization temperature and phase change are determined. The TG curve of the synthesized cathode material LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0.0, 0.01, 0.02, 0.03, 0.04, and 0.05) is shown in Figs. 1(a)–1(f), indicating significant weight loss in the base material in the temperature range between 100 and 420 °C.

The weight is continuously lost by removing the residual water and decomposition of the nitrates. The transformation is endothermic near 420 °C, where considerable weight loss occurs with the evaporation of CO₂. The weight loss was minimum between 420 and 500 °C. After 500 °C, diffusion of lithium takes place with the decomposition of LiO into the MO (M = Ni_, Co_, Mn, and Mg) precursor,³¹ resulting in a layered structure. Above 500 °C, the curve became flat, indicating the negligible weight loss in the oxide formation. This thermal study concludes that the required material LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ can be prepared by calcination at 550 °C.

The crystalline nature of LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0.0, 0.01, 0.02, 0.03, 0.04, and 0.05) can be identified with the help of powder XRD analysis, scanning from 10° to 80° with a step size of 0.008° as shown in Figs. 2(a)–2(f), which agrees well with JCPDS card no 35–1782. The samples had a rhombohedral–hexagonal system of space group $R3̄m$ of LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ cathode materials at angles of about 2θ ≈ 21.6° and 2θ ≈ 23.2°. The XRD data are listed in Table I, which shows the decreased value of lattice constant “a” from 2.8721 to 2.8680 Å and hence the unit cell volume of 101.41–100.83 ų with Mg substitution. This is due to the smaller ionic radii of the substituent Mg²⁺ (0.72 Å) than the host Li⁺ (0.76 Å) and Ni²⁺ (0.69 Å).²⁸ The c/a ratio in the range 4.93–4.94 Å is greater than 4.9, indicating a higher degree of hexagonal structure.^29,32 The average crystallite size also varies from 20.31 to 43.28 nm. Here, 003 is the highest peak used for crystallite size, as listed in Table I.

There is splitting of XRD peaks near (006)/(012) and (108)/(110). Next, the increasing intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ from 2.195 to 2.386 with doping indicates a lower degree of cation mixing, as is indicated in Table I along with cell volumes, lattice parameters, occupancy of Li ions, and refinement factors.

LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (where x = 0.0, 0.01, 0.02, 0.03, 0.04, and 0.05) are calcined at 850 °C after synthesizing by a sol–gel auto-combustion method, as shown in Figs. 3(a)–3(f). Their corresponding EDS spectra are shown in Figs. 4(a)–4(f).

The crystallite size was 220–350 nm, and they had a round shape with slight agglomeration, which increased with the doping concentration of Mg. The concentration decreases the round shape. The size difference between the particles increases the surface area, charge transfer, capacity, and ion diffusion pathway, thereby improving the electrochemical performance of the electrode material.

The EDS results of the elements Ni, Co, Mn, Mg, and O in the compound LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ are presented in the histogram Fig. 5. EDS is unable to detect the lithium due to its low atomic number.

Fourier transform infrared spectroscopy (FT-IR) is the most potent scientific instrument for detecting the internal structure of the molecules and the nature of chemical bonds in the molecules. Simply, it can fingerprint all types of molecules. FT-IR analyses solids, liquids, and gases and works on the Michelson interferometer principle. The FT-IR of the synthesized materials LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (where x = 0.0, 0.01, 0.02, 0.03, 0.04, and 0.05) is carried out using a Shimadzu IR-Prestige 21 FT-IR 5300 for the local environments of cations within 400–1200 cm⁻¹, the images of which are shown in Figs. 6(a)–6(f). LiO₆ and MO₆ are trigonally distorted layers in the crystal structure of LiNi_0.8Mg_0.1Co_0.1Mn_0.1O₂ layered oxides. The formation of LiO₆ octahedra of the synthesized material is identified with the Li–O stretching vibration in the band around 480 cm⁻¹. The broad absorption band observed in all the samples may be due to mixed vibration bands of Ni–O, Mg–O, Co–O, and Mn–O bonds, as evident from the spectral data in Table II. Compared with Mg substituted samples, a better band shift toward a higher wavenumber was observed in the x = 0.03 sample.

Layered LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0 and 0.03) cathode materials displayed a wide electrochemical stability window, as shown by the Cyclic Voltammetry (CV) data, which are used to study the electrochemical properties and estimate cycle efficiencies. The current (mA) vs voltage (V) plots from the cyclic voltammetry data are shown in Figs. 7(a) and 7(b), indicating the lithium-ion intercalation. The characteristic of the CV curves demonstrated that lithium could reversely intercalate and de-intercalate into electrodes. The inappropriate compositional stoichiometry of layered oxide cathode material samples plays a vital role in the performance of the LIBs. The composite exhibits good cycling performance in the potential range from 2.5 to 4.4 V. The anodic current of more than 3.0 V potential and better cycling performance indicate that the cathode material is more valuable. These results revealed that layered oxide LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0 and 0.03) cathode materials have their potential value in enhancing battery performance and could be utilized in lithium-ion batteries.

The performance of the layered LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0.0 and 0.03) cathode materials is systematically investigated. These materials’ specific capacity and cyclability are measured by galvanostatic charge/discharge testing. Electrochemical testing is performed with a coin cell CR2032, and the electrode under test is made with 90% active material, 7% carbon black, and 3% PVDF. The results are shown in Fig. 8(a) and 8(b). The substituted samples’ first and fifth cycle charge/discharge capacities are 219.11/214.84 and 237.18/233.57 mA h g⁻¹, respectively, with a retention capacity of 97.4% and 96.48%, respectively. These values are summarized in Table III. The charge/discharge at a slow current rate of 0.1 C for all samples with a charge cutoff voltage of more than 4.4 V and discharge cutoff voltage of 3.0 V is observed. These observations are similar to those reported in the literature. It is concluded that these compounds are promising materials for rechargeable Li-ion batteries. Finally, we conclude that the Mg substituted x = 0.03 material is the optimum level for good capacity and cyclability in LiNi_0.8Co_0.1Mn_0.1O₂.

The investigated compounds, particularly the material with Mg substitution at x = 0.03, show promise as rechargeable Li-ion battery materials, offering optimal levels of capacity and cyclability.

Nyquist plots are commonly used in electrochemical impedance spectroscopy (EIS) to study the electrical behavior of a system, often in the context of batteries or electrochemical cells. The linear portion, often extending from the end of the semicircle toward the low-frequency region, is associated with diffusive processes, such as lithium-ion diffusion in battery materials. The slope of this line provides information about the diffusion kinetics. If you observe a Nyquist plot with both a semicircle and a linear tail, it suggests that the electrical response involves both charge transfer and diffusion-controlled processes. In the context of the statement provided, Mg doping is reported to reduce the transfer resistance (Rct). This reduction in Rct is often desirable as it indicates improved conductivity and faster charge transfer within the material. It may manifest as a smaller or altered semicircle on the Nyquist plot. The conclusion drawn is that Mg-doped material exhibits good impedance characteristics. The reduction in transfer resistance contributes to enhanced conductivity, making the material more suitable for electrochemical applications, such as in batteries.

The impedance Nyquist plot for the synthesized material (NCM) is depicted in Fig. 9. From the figure, we can see a semicircle region and a linear curve. This linear curve is attributed to the lithium diffusion and semicircle transfer resistance. With Mg doping, the transfer resistance is reduced, which will help increase conductivity. We conclude that Mg-doped synthesized material shows good impedance characteristics.

Substituting magnesium (Mg) into lithium nickel cobalt manganese oxide (LiNi_0.8Co_0.1Mn_0.1O₂) cathode materials in lithium-ion batteries can have several potential benefits and impacts on the performance of the battery. Mg substitution can enhance the structural stability of the cathode material. This helps in reducing the degradation of the material over multiple charge–discharge cycles, leading to improved cycling stability of the battery. Mg substitution may contribute to increasing the energy density of the battery. This can result in a higher capacity and longer-lasting battery, which is desirable for various applications, including electric vehicles and portable electronics. Mg substitution can also have positive effects on the thermal stability and safety of lithium-ion batteries. Improved thermal stability is crucial in preventing thermal runaway reactions and mitigating safety risks associated with overheating. Depending on the availability and cost of materials, Mg substitution may offer a more cost-effective alternative than other cathode materials. This can contribute to reducing the overall cost of lithium-ion batteries. Mg substitution may lead to better voltage stability during charge and discharge cycles. This can result in a more stable voltage profile, improving the overall performance and reliability of the battery. Mg is more abundant and less toxic than some other elements used in cathode materials. Substituting Mg can have positive environmental implications, making the battery materials more sustainable. Mg substitution may enable the cathode material to operate at higher voltages, which can contribute to achieving higher energy densities and improved overall battery performance.

In conclusion, the performance of cathode materials in lithium-ion batteries can be significantly enhanced through novel compositions and synthesis methods. This study has demonstrated that substituting Mg into LiNi_0.8Co_0.1Mn_0.1O₂ to form LiNi_0.8−xMg_xCo_0.1Mn_0.1O₂ (x = 0.0, 0.01, 0.02, 0.03, 0.04, and 0.05) using the sol–gel auto-combustion approach is effective. The materials synthesized at 850 °C for 18 h exhibited a rhombohedral–hexagonal structure, with particle sizes ranging from 200 to 320 nm and a slight agglomeration morphology. Among the compositions, the x = 0.03 material showed the most promising results, with initial discharge capacities of 214.84 and 233.57 mA h g⁻¹ at a rate of 0.1 C in a voltage range of 3.0–4.6 V and improved retention capacity of 96.48% over five cycles. The cyclic voltammetry results confirmed better cycling stability and higher anodic current for the Mg-doped samples. Moreover, electrochemical impedance spectroscopy revealed that Mg substitution reduced transfer resistance, enhancing the material’s conductivity and overall electrochemical performance. These findings underscore the potential of Mg-doping in optimizing cathode materials for lithium-ion batteries.

The authors would like to express their gratitude to Andhra University, India, and Tribhuvan University, Nepal, for giving access to the laboratories for the synthesis and characterization of the sample and other resources for our work.

The authors have no conflicts to disclose.

We have no human or other animal participants in our study. We strictly follow all the ethical standards of the institutional and/or national research committee and the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

D. Parajuli: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). N. Murali: Conceptualization (equal); Data curation (equal); Methodology (equal); Validation (equal); Visualization (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.