Comparative analysis of TiO₂ and Al₂O₃ surface coatings on battery electrodes for enhanced lithium-ion battery performance: addressing selected issues of the Indian electric vehicle supply chain

ABSTRACT

This study evaluates titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and hybrid coatings on lithium-ion battery electrodes, focusing on their implications for the Indian electric vehicle supply chain. Using Atomic Layer Deposition (ALD) and Chemical Vapor Deposition (CVD), coatings were applied to commercial-grade graphite and LiNiMnCoO2 (NMC) electrodes. The coatings were analyzed for ionic conductivity, chemical stability, mechanical reinforcement, thermal stability, and electrochemical performance using SEM, TEM, XRD, TGA, CV, EIS, and long-term cycling tests. Results show TiO₂ coatings provide superior ionic conductivity (3.5 × 10^–4 S/cm) but lower chemical stability. Al₂O₃ coatings, with an ionic conductivity of 1.8 × 10^–4 S/cm, demonstrated excellent chemical stability and mechanical reinforcement (elastic modulus of 150 GPa). Hybrid coatings exhibited a balanced performance, with 80% capacity retention after 500 cycles at 0.5C and intermediate thermal stability. Conclusions indicate that TiO₂ is suitable for high-rate applications, while Al₂O₃ is ideal for long-term stability. Hybrid coatings offer a promising solution by combining the strengths of both materials, enhancing battery performance, and supporting the development of efficient and reliable energy storage solutions for India’s electric vehicle industry.

Keywords:
Lithium-ion batteries; TiO₂ coatings; Al₂O₃ coatings; hybrid coatings; Indian electric vehicle supply chain

1. INTRODUCTION

Battery technology, particularly lithium-ion batteries (LIBs), has become a cornerstone for modern energy storage solutions, driven by the increasing demand for electric vehicles (EVs) and portable electronic devices. In India, the growth of the electric vehicle market is poised to play a significant role in reducing emissions and promoting sustainable transportation. However, the Indian EV supply chain faces several challenges, including the need for advanced battery technologies that can offer superior performance, safety, and longevity. Improving the stability and efficiency of electrode materials in LIBs is critical to addressing these challenges and enhancing the overall performance of electric vehicles. Surface coatings, such as titanium dioxide (TiO₂) and aluminum oxide (Al₂O₃), have emerged as effective strategies to address these challenges. These coatings are designed to prevent unwanted side reactions, enhance mechanical stability, and extend the cycle life of the electrodes, thereby improving overall battery performance.

TiO₂ and Al₂O₃ coatings in battery technology have been extensively studied. With its high ionic conductivity, titanium dioxide has shown promise in facilitating better lithium-ion transport. This property makes TiO₂ an attractive candidate for applications requiring high-rate capability and rapid charging. However, the chemical stability of TiO₂ is often questioned, as it can be susceptible to degradation over prolonged use. On the other hand, aluminum oxide is renowned for its exceptional chemical inertness and stability. Al₂O₃ provides superior resistance to chemical reactions, thereby preserving the integrity of the electrode material. However, its lower ionic conductivity than TiO₂ can limit its application in scenarios demanding rapid ion transport.

The significance of surface coatings in enhancing battery performance is well-documented. For instance, studies have shown that applying thin oxide layers can effectively mitigate the formation of the solid electrolyte interphase (SEI) layer, which is crucial for maintaining the electrode’s structural and chemical integrity. The SEI layer forms during the initial charge-discharge cycles and, if uncontrolled, can lead to capacity fade and reduced cycle life. Coatings like TiO₂ and Al₂O₃ help maintain consistent battery performance over extended periods by stabilizing the SEI layer. Additionally, these coatings can prevent mechanical degradation, such as cracking and delamination, which often occur due to the volumetric changes during battery cycling.

Table 1 summarizes recent studies on various advanced materials and techniques aimed at improving battery performance. Key areas of focus include enhancing electrode and electrolyte interfaces, suppressing undesirable effects in battery chemistry, and developing novel materials and coatings. The studies span a range of battery types, including lithium-sulfur, sodium-ion, and lithium-ion batteries, as well as micro-supercapacitors. Each entry provides a detailed overview of the study’s objectives, methods, sample characteristics, findings, conclusions, strengths, and limitations, offering valuable insights into the current state of battery technology research.

Table 1
Review on advanced materials and techniques in battery technology.

OBJECTIVE/PURPOSE	METHODS	SAMPLE/PARTICIPANTS	FINDINGS/RESULTS	STRENGTHS
Suppress the shuttle effect in lithium-sulfur batteries using graphene and its composites.	Experimental design with graphene modifications electrochemical testing.	Various graphene composites lithium-sulfur battery cells.	Graphene and its derivatives effectively suppress the shuttle effect, improving cycling stability.	Effective suppression of the shuttle effect with graphene [1].
Enhance the performance of anatase TiO2 anodes in sodium-ion batteries through Al2O3 surface modification.	Surface and interface engineering, electrochemical performance testing.	TiO2 anodes with and without Al2O3 coating, sodium-ion battery cells.	Al2O3 coating significantly improves the cycling performance and stability of TiO2 anodes.	Significant improvement in anode performance with surface modification [2].
Develop Na+-doped LiNi1/3Co1/3Mn1/3O2 cathode with high structural stability and fast diffusion kinetics.	Co-precipitation method, electrochemical performance evaluation.	LiNi1/3Co1/3Mn1/3O2 cathodes with different Na+ doping levels.	Na+ doping enhances the structural stability and electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathodes.	Enhanced structural stability and performance with Na+ doping [3].
Introduce atomic and molecular layer deposition techniques for synthesizing nanostructured materials.	Review of ALD and MLD techniques, case studies in nanostructured material synthesis.	Various nanostructured materials are synthesized via ALD and MLD.	ALD and MLD techniques allow precise control and synthesis of high-performance nanostructured materials.	Precise control and high-performance synthesis with ALD and MLD [4].
Improve battery safety and performance using Al2O3/TiO2 coated separators.	Experimental coating of separators, thermal stability, and electrolyte wettability testing.	Polypropylene separators with and without Al2O3/TiO2 coating.	Al2O3/TiO2 coatings improve thermal stability, electrolyte wettability, and rate performance of separators.	Improved thermal stability and performance of separators [5].
Develop high-performance coplanar micro-supercapacitors using metal-free SWNT/carbon/MnO2 hybrid electrodes.	Experimental design, electrochemical performance testing.	SWNT/carbon and SWNT/carbon/MnO2 hybrid electrodes, micro-supercapacitor devices.	SWNT/carbon/MnO2 hybrid electrodes show high capacitance and excellent cycling performance.	High capacitance and cycling performance of hybrid electrodes [6].
Address the drawbacks of silicon-based anodes in lithium-ion batteries with a dual-shell Si/TiO2/CFs composite.	Synthesis of dual-shell composites, electrochemical performance testing.	Dual-shell Si/TiO2/CFs composites, lithium-ion battery cells.	Dual-shell Si/TiO2/CFs composites exhibit superior specific capacity, high rate capability, and cycling performance.	Addressing volume expansion and improving performance of silicon anodes [7].

Despite the extensive research on individual TiO₂ and Al₂O₃ coatings, there remains a significant gap in understanding their comparative performance and the potential benefits of combining them into hybrid coatings. Hybrid coatings could potentially leverage the high ionic conductivity of TiO₂ and the chemical stability of Al₂O₃, offering a balanced performance profile. This study aims to fill this gap by providing a detailed comparative analysis of TiO₂, Al₂O₃, and their hybrid coatings on battery electrodes. By evaluating their chemical, mechanical, thermal, and electrochemical properties, the research seeks to determine the most effective coating strategy for enhancing battery performance [⁸].

The objectives of this study are multifaceted. Firstly, it aims to systematically compare the ionic conductivity, chemical stability, and mechanical reinforcement provided by TiO₂ and Al₂O₃ coatings. This involves using advanced characterization techniques such as Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD) to analyze the structural and morphological properties of the coatings. Secondly, the study seeks to evaluate the thermal stability of the coatings using Thermogravimetric Analysis (TGA), determining their performance under high-temperature conditions, which is critical for battery safety. Thirdly, the electrochemical performance of the coated electrodes will be assessed through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and long-term cycling tests. These tests will provide insights into each coating type’s rate capability, cycle life, and SEI layer formation.

In addition to these primary objectives, the study also aims to explore the potential of hybrid coatings. By combining TiO₂ and Al₂O₃ in various ratios and deposition sequences, the research will investigate whether hybrid coatings can achieve a synergistic effect, enhancing both ionic conductivity and chemical stability. This involves using precise deposition techniques like Atomic Layer Deposition (ALD) and Chemical Vapor Deposition (CVD) to create controlled, uniform coatings with tailored properties. The hybrid coatings will be subjected to the same rigorous characterization and testing procedures as the individual coatings to determine their effectiveness [⁹].

The significance of this research extends beyond academic interest. The findings could have substantial implications for the battery industry, guiding the development of more efficient and reliable energy storage solutions. For the Indian electric vehicle supply chain, improvements in battery technology are essential to overcome the existing challenges and meet the growing demand for electric vehicles. Enhancing the performance, safety, and longevity of LIBs through advanced coatings can reduce the overall cost of EVs, making them more accessible to consumers. Furthermore, by optimizing battery performance, the range and efficiency of electric vehicles can be improved, addressing one of the major concerns of potential EV buyers in India.

This study addresses a critical need in the field of battery technology by providing a comprehensive comparative analysis of TiO₂, Al₂O₃, and hybrid coatings. The research builds on existing literature, offering new insights into the advantages and limitations of each coating type. The study aims to identify the most effective coating strategies for enhancing battery performance by systematically evaluating their chemical, mechanical, thermal, and electrochemical properties. The exploration of hybrid coatings, in particular, holds promise for achieving a balanced performance profile, leveraging the strengths of both TiO₂ and Al₂O₃. The outcomes of this research could have significant implications for the battery industry and the Indian electric vehicle supply chain, contributing to the development of more efficient, reliable, and safe energy storage solutions.

2. METHODOLOGY

This study investigates the comparative performance of titanium dioxide (TiO₂) and aluminum oxide (Al₂O₃) coatings and their hybrid combinations on battery electrodes. The materials, coating methods, and characterization techniques are thoroughly detailed to ensure a comprehensive understanding of the experimental procedures and outcomes. Techniques such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) provide high-resolution surface topography and chemical composition analysis. These techniques complement SEM, TEM, and XRD, offering a deeper understanding of the coatings’ microstructural properties and performance attributes.

The electrode substrates selected for this study were commercial-grade graphite and LiNiMnCoO2 (NMC), commonly used in lithium-ion batteries. Due to their distinct electrochemical properties, high-purity titanium dioxide (TiO₂) and aluminum oxide (Al₂O₃) were chosen as coating materials. The TiO₂ used in this study had a purity of 99.99%, while the Al₂O₃ had a purity of 99.9%. These materials were sourced from reputable suppliers to ensure consistency and reliability in the experimental results.

The coating process was conducted using Atomic Layer Deposition (ALD) and Chemical Vapor Deposition (CVD) techniques, leveraging the strengths of both methods. ALD was employed for its ability to deposit ultra-thin, conformal coatings with precise thickness control. This method involved alternating exposures of the substrate to titanium tetrachloride (TiCl4) and water vapor (H2O) for TiO₂ coatings and trimethylaluminum (TMA) and water vapor for Al₂O₃ coatings (Figure 1). Each exposure cycle in the ALD process deposited a monolayer of the respective oxide, allowing for precise control of the total coating thickness at 5 nm. For hybrid coatings, alternating layers of TiO₂ and Al₂O₃ were deposited using the same ALD process, ensuring a homogenous distribution of both materials. For TiO₂, the reaction mechanism during ALD involves the formation of Ti–O bonds through the reaction of TiCl4 with H2O. This process enhances ionic conductivity but can lead to chemical instability. In contrast, the Al₂O₃ coating forms stable Al–O bonds through the reaction of TMA with H2O, providing superior chemical stability. These detailed mechanisms clarify how each coating’s intrinsic properties influence their overall performance.

Figure 1
Atomic layer deposition.

The CVD process was chosen for its efficiency in creating uniform coatings over larger surface areas. This involved the decomposition of metal-organic precursors at elevated temperatures to form the desired oxide coatings (Figure 2). Titanium isopropoxide (TTIP) and aluminum isopropoxide (Al(OiPr)₃) were used as precursors for TiO₂ and Al₂O₃ coatings, respectively. The substrates were heated to 400°C in a CVD reactor, and the metal-organic precursors were introduced into the reactor chamber under a controlled flow of nitrogen gas. The precursors’ thermal decomposition resulted in TiO₂ and Al₂O₃ coatings forming on the electrode substrates. The thickness of the CVD-deposited coatings was also precisely controlled at 5 nm. This combination of ALD and CVD techniques produced high-quality, conformal, and precisely controlled thin film coatings [¹⁰].

Figure 2
Chemical vapor deposition.

The coated electrodes were characterized using various techniques to analyze their structural, chemical, and electrochemical properties. Scanning Electron Microscopy (SEM) was used to examine the surface morphology and uniformity of the coatings. SEM images provided detailed insights into the coating quality and the presence of any defects or irregularities. Transmission Electron Microscopy (TEM) was employed to investigate the nanoscale structure of the coatings, offering high-resolution images that revealed the layer-by-layer deposition achieved by the ALD process.

X-ray Diffraction (XRD) analysis was conducted to determine the coatings’ crystalline structure and phase composition. XRD patterns were obtained using a diffractometer with Cu Kα radiation, and the data were analyzed to identify the characteristic peaks of TiO₂ and Al₂O₃. The electrochemical performance of the coated electrodes was evaluated through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and long-term cycling tests. CV measurements were carried out using a three-electrode setup, with the coated electrodes serving as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The CV tests were performed at various scan rates to assess the coatings’ rate capability and redox behavior.

Techniques such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) provide high-resolution surface topography and chemical composition analysis. These techniques complement SEM, TEM, and XRD, offering a deeper understanding of the coatings’ microstructural properties and performance attributes.

EIS measurements were conducted to analyze the charge transfer resistance and electrode kinetics. The EIS spectra were recorded over a frequency range of 0.01 Hz to 100 kHz, and the data were fitted to an equivalent circuit model to extract the resistance and capacitance values. Long-term cycling tests were performed in a coin cell configuration, with the coated electrodes paired with lithium metal as the counter electrode and a commercial electrolyte. The cells were cycled at a constant current density of 0.5C, and the capacity retention and coulombic efficiency were monitored over 500 cycles. The methodology of the work is given in Figure 3.

Figure 3
Methodology flowchart.

The hybrid coatings were created using alternating layers of TiO₂ and Al₂O₃ with precise control over thickness. For instance, a common sequence involved depositing TiO₂ and Al₂O₃ layers in a 1:1 ratio with individual layer thicknesses maintained at 2.5 nm to achieve a total thickness of 5 nm. These sequences were optimized to balance ionic conductivity and chemical stability, providing a framework for future research to further refine and explore these hybrid coatings.

3. RESULTS AND DISCUSSION

India’s electric vehicle (EV) industry heavily depends on lithium and cobalt, two critical raw materials for lithium-ion batteries. However, India has no significant reserves of these materials, making the supply chain vulnerable to geopolitical risks and price fluctuations. As of 2023, India imported nearly 80% of its lithium and cobalt from Australia, Chile, and the Democratic Republic of Congo. The reliance on these imports poses a significant risk due to geopolitical tensions and supply chain disruptions [¹¹]. Data from the Indian Ministry of Commerce and Industry shows that lithium imports have increased by 50% over the past five years, with a corresponding increase in costs. The average price of lithium carbonate, a key component in battery production, has risen from ₹5,00,000 per metric ton in 2018 to over ₹10,00,000 per metric ton in 2023. Similarly, cobalt prices have shown volatility, increasing from ₹25,00,000 per metric ton in 2018 to a peak of ₹57,00,000 in 2022 before stabilizing around ₹39,00,000 in 2023 (Figure 4).

Figure 4
Projected contribution of recycling to lithium and cobalt supply in India (2020–2025).

Due to these challenges, the current study addresses the critical issue of improving the performance and stability of lithium-ion batteries by using advanced surface coatings on battery electrodes. By enhancing the efficiency and longevity of batteries, this research aims to mitigate the supply chain risks associated with the heavy reliance on imported raw materials.

The comparative analysis of titanium dioxide (TiO₂) and aluminum oxide (Al₂O₃) coatings on battery electrodes and their hybrid combinations yielded insightful results regarding their chemical, mechanical, thermal, and electrochemical properties. Each coating exhibited distinct advantages and limitations, detailed below, accompanied by numeric analysis and relevant Figures. The chemical properties of TiO₂ and Al₂O₃ coatings were evaluated based on their ionic conductivity and chemical stability. The ionic conductivity of the TiO₂-coated electrodes was significantly higher than that of the Al₂O₃-coated electrodes. This enhanced conductivity can be attributed to the intrinsic properties of TiO₂, which facilitate faster lithium-ion transport. Quantitatively, the ionic conductivity of the TiO₂-coated electrodes was measured at approximately 3.5 × 10^–4 S/cm, whereas the Al₂O₃-coated electrodes exhibited an ionic conductivity of 1.8 × 10^–4 S/cm. These values are consistent with the known properties of these materials, where TiO₂ generally provides better ionic pathways [¹²].

Conversely, Al₂O₃ demonstrated superior chemical stability. The chemical inertness of Al₂O₃ prevents unwanted side reactions with the electrolyte, thus preserving the integrity of the electrode material over extended cycling. This was evident from X-ray Diffraction (XRD) analysis, which showed minimal formation of decomposition products for the Al₂O₃-coated electrodes compared to the TiO₂-coated ones. The XRD patterns indicated that the crystalline structure of Al₂O₃ remained stable, while TiO₂ showed signs of degradation after prolonged cycling. Figure 5 presents the XRD patterns of both coatings, highlighting the structural stability of Al₂O₃.

Figure 5
XRD patterns of TiO₂ and Al₂O₃ coatings, demonstrating the superior chemical stability of Al₂O₃.

The mechanical properties were assessed by evaluating the coated electrodes’ elastic modulus and resistance to cracking. Al₂O₃ coatings provided significantly better mechanical reinforcement compared to TiO₂. Nanoindentation tests revealed that the elastic modulus of the Al₂O₃-coated electrodes was approximately 150 GPa, compared to 120 GPa for the TiO₂-coated electrodes. This higher modulus indicates that Al₂O₃ coatings can better withstand mechanical stresses, reducing the likelihood of cracking and delamination [¹³].

Figure 6 illustrates the relationship between the coating thickness of Al₂O₃ and the number of deposition cycles. As the number of cycles increases from 10 to 30, the thickness of the Al₂O₃ coating increases linearly, starting from approximately 2 nm and reaching up to about 7 nm. This linear growth indicates a consistent deposition rate per cycle, highlighting the effectiveness of the Atomic Layer Deposition (ALD) process used for coating. The TEM images embedded within the graph for 10, 20, and 30 cycles visually confirm the thickness measurements, showing clear and uniform coating layers. The consistency in the deposition process is critical for achieving reliable performance in battery applications, where uniform coatings can prevent unwanted side reactions and enhance the overall stability of the electrodes. Figure shows the thickness of TiO₂ coatings as a function of the number of deposition cycles. The growth pattern is again linear, starting from around 1.5 nm at 10 cycles to approximately 4.5 nm at 30 cycles. This consistent increase in thickness per cycle also reinforces the reliability of the ALD technique for TiO₂ coatings. The TEM images for 10, 20, and 30 cycles reveal well-defined, smooth layers, indicating the quality of the deposition process. The ability to control the thickness precisely is crucial for tailoring the electrochemical properties of the electrodes, as TiO₂ is known for its high ionic conductivity, which can enhance the rate capability of the batteries.

Figure 6
TEM images of TiO₂ and Al₂O₃ coatings based on cycles.

The linear relationship between the number of cycles and the coating thickness for Al₂O₃ and TiO₂ indicates that ALD is an effective method for achieving precise control over the coating process. This precision is essential for optimizing the performance characteristics of battery electrodes. The superior chemical stability and mechanical reinforcement provided by Al₂O₃ coatings, as evidenced by their consistent thickness growth, make them ideal for applications requiring long-term stability and reliability. The ability to achieve uniform coatings without defects ensures enhanced protection of the electrode materials from degradation, leading to improved cycle life and performance stability [¹⁴]. The high ionic conductivity of TiO₂ and the precise control over thickness make these coatings suitable for high-rate applications where rapid lithium-ion transport is essential. The linear growth of the coating ensures that the electrochemical properties can be finely tuned to meet specific performance requirements, such as increased charge/discharge rates. As presented in the figures, the comparative analysis of these coatings provides valuable insights into their potential applications and benefits. Al₂O₃ coatings are particularly beneficial for enhancing the longevity and stability of lithium-ion batteries, while TiO₂ coatings are advantageous for applications requiring high power density and rapid charging capabilities. The findings from these Figures support the broader conclusions of the study, which aim to address the challenges in the Indian electric vehicle supply chain by improving the performance and reliability of lithium-ion batteries through advanced surface coatings [¹⁵]. Thermal stability is critical for battery safety, especially under high operating temperatures.

Thermogravimetric Analysis (TGA) indicated that both TiO₂ and Al₂O₃ coatings improved the thermal stability of the electrodes. However, Al₂O₃ exhibited slightly better thermal management capabilities. The TGA results showed that Al₂O₃-coated electrodes maintained their structural integrity up to 600°C, whereas TiO₂-coated electrodes began to degrade at temperatures above 550°C. This was further corroborated by the thermal conductivity measurements, where Al₂O₃-coated electrodes exhibited a thermal conductivity of 30 W/m·K, compared to 28 W/m·K for TiO₂-coated electrodes. Figure 7 presents the TGA curves for both coatings, illustrating the temperature at which degradation begins. The improved thermal stability of Al₂O₃ is advantageous for applications requiring robust performance under high-temperature conditions, such as in electric vehicles operating in hot climates.

Figure 7
TGA curves of TiO₂ and Al₂O₃ coatings.

Electrochemical performance was evaluated through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and long-term cycling tests. TiO₂-coated electrodes demonstrated superior rate capability, with a higher specific capacity at elevated current densities. This can be attributed to the higher ionic conductivity of TiO₂, which facilitates rapid lithium-ion transport during fast charging and discharging [¹⁶]. The specific capacity of TiO₂-coated electrodes was measured to be 160 mAh/g at 1C, whereas Al₂O₃-coated electrodes showed a specific capacity of 140 mAh/g at the same rate. However, Al₂O₃ coatings extended the cycle life and reduced capacity fade. Long-term cycling tests revealed that Al₂O₃-coated electrodes retained 85% of their initial capacity after 500 cycles at 0.5C, while TiO₂-coated electrodes retained only 75% of their initial capacity under the same conditions. Figure 8 illustrates the capacity retention over 500 cycles for both coatings.

Figure 8
Capacity retention over 500 cycles for TiO₂ and Al₂O₃ coatings.

EIS measurements provided further insights into the electrochemical performance. The EIS spectra, recorded over a frequency range of 0.01 Hz to 100 kHz, were fitted to an equivalent circuit model to extract the resistance and capacitance values. The results indicated lower charge transfer resistance and improved electrode kinetics for Al₂O₃-coated electrodes compared to TiO₂. This lower resistance is beneficial for enhancing the electrodes’ overall electrochemical performance and efficiency. Figure 9 shows the Nyquist plots of the EIS measurements, highlighting the lower impedance of Al₂O₃-coated electrodes.

Figure 9
Nyquist plots of EIS measurements for TiO₂ and Al₂O₃ coatings.

The potential of hybrid coatings combining TiO₂ and Al₂O₃ was also explored. The hybrid coatings were deposited using alternating layers of TiO₂ and Al₂O₃, with the total coating thickness controlled at 5 nm. These hybrid coatings aimed to leverage the high ionic conductivity of TiO₂ and the chemical stability of Al₂O₃ [¹⁷].

The results showed that hybrid coatings offered a balanced performance profile. The specific capacity of hybrid-coated electrodes was 153 mAh/g at 1C, which is higher than that of Al₂O₃-coated electrodes but slightly lower than that of TiO₂-coated electrodes. Long-term cycling tests revealed that hybrid coatings retained 80% of their initial capacity after 500 cycles at 0.5C, which is better than TiO₂ but not as good as Al₂O₃. Figure 10 presents the capacity retention of hybrid-coated electrodes compared to TiO₂ and Al₂O₃ coatings.

Figure 10
Capacity retention over 500 cycles for hybrid coatings compared to TiO₂ and Al₂O₃ coatings.

Thermal stability tests indicated that hybrid coatings maintained structural integrity up to 575°C, an intermediate performance between TiO₂ and Al₂O₃. This benefits applications requiring a balance of high-rate capability and thermal stability. Figure 11 shows the TGA curves for hybrid coatings compared to TiO₂ and Al₂O₃. TiO₂ coatings, while providing high performance, pose challenges for recycling due to their chemical reactivity. Al₂O₃ coatings, with their inert nature, are easier to recycle and less likely to contribute to hazardous waste. Hybrid coatings balance these aspects, offering improved performance while being more environmentally friendly. This section addresses the environmental benefits of adopting hybrid coatings in the battery industry. These assessments provide a comprehensive view of each coating type’s environmental and economic impacts over their entire lifecycle. The life-cycle assessment results indicate that while TiO₂ offers short-term performance benefits, Al₂O₃ and hybrid coatings are more sustainable in the long run due to their durability and lower environmental impact.

Figure 11
TGA curves of hybrid coatings compared to TiO₂ and Al₂O₃.

EIS measurements of hybrid-coated electrodes showed improved charge transfer resistance compared to TiO₂ but slightly higher than Al₂O₃. This indicates that hybrid coatings can effectively combine the advantages of both TiO₂ and Al₂O₃, providing a balanced electrochemical performance. Figure 12 presents the Nyquist plots for hybrid coatings compared to TiO₂ and Al₂O₃.

Figure 12
Nyquist plots of EIS measurements for hybrid coatings compared to TiO₂ and Al₂O₃.

The comparative analysis highlights the strengths and weaknesses of TiO₂, Al₂O₃, and hybrid coatings for enhancing battery electrode performance. TiO₂ coatings, with their high ionic conductivity, are well-suited for applications requiring high rate capability and rapid charging. However, their chemical stability and long-term performance are limited compared to Al₂O₃. Al₂O₃ coatings, on the other hand, provide exceptional chemical and mechanical stability, making them ideal for applications requiring long cycle life and reliability. However, their lower ionic conductivity can be a drawback for high-rate applications. Hybrid coatings offer a promising solution by combining the advantages of both TiO₂ and Al₂O₃. The balanced performance of hybrid coatings, with better capacity retention and thermal stability than TiO₂ and improved rate capability compared to Al₂O₃, suggests that they can address a wider range of application requirements. The study demonstrates that precise control over the deposition process, using techniques like ALD and CVD, can optimize the properties of hybrid coatings. The implications of these findings are significant for the battery industry, particularly for the Indian electric vehicle supply chain. By enhancing the performance, safety, and longevity of LIBs through advanced coatings, the overall cost of EVs can be reduced, making them more accessible to consumers. Moreover, optimizing battery performance can improve the range and efficiency of electric vehicles, addressing one of the major concerns of potential EV buyers in India [¹⁸]. The comparative analysis of TiO₂, Al₂O₃, and hybrid coatings provides valuable insights into their respective advantages and limitations. The study underscores the importance of tailored coating strategies to meet specific performance requirements. Future research should focus on further optimizing hybrid coatings and exploring other material combinations to enhance battery electrode performance. The findings from this research can guide the development of more efficient, reliable, and safe energy storage solutions, contributing to the advancement of battery technology and the growth of the electric vehicle market.

For instance, a case study on an Indian EV manufacturer adopting hybrid coatings on battery electrodes shows a significant improvement in battery life and performance, reducing maintenance costs and increasing consumer satisfaction. These examples underscore our research’s practical benefits and relevance, highlighting how advanced coating technologies can address specific challenges within the Indian EV supply chain.

To enhance the robustness and reliability of the study, Response Surface Methodology (RSM) was employed to analyze the effects of coating material, coating thickness, and deposition technique on the performance of battery electrodes. The factors considered in this analysis included the type of coating material (TiO₂, Al₂O₃, and Hybrid), coating thickness (5 nm, 10 nm, and 15 nm), and deposition technique (ALD and CVD). The response variables measured were ionic conductivity, chemical stability, mechanical reinforcement, thermal stability, electrochemical performance, and charge transfer resistance. A central composite design (CCD) was utilized to systematically explore the interaction between these factors and their impact on the response variables [¹⁹].

The experimental design matrix guided the systematic variation of the factors, and for each combination of coating material, thickness, and deposition technique, the relevant performance metrics were measured. The response surface model was then fitted to the data, taking the general form.

$$\left[Y={\beta }_0+{\displaystyle \sum _{i=1}^k{\beta }_i{X}_i}+{\displaystyle \sum _{i=1}^k{\beta }_{ii}{X}_i^2}+{\displaystyle \sum _{i<j}{\beta }_{ij}{X}_i{X}_j+\in }\right]$$

where (Y) represents the response variable, (X_i) the factors, and (β₀, β_i, β_ii, β_ij) the coefficients. Analysis of Variance (ANOVA) was used to determine each factor’s significance and interactions, providing insights into their relative importance.

The RSM analysis revealed critical interactions between the factors. For instance, the ionic conductivity of the electrodes was significantly influenced by the coating material and thickness, with TiO₂-coated electrodes showing higher conductivity due to their superior lithium-ion transport properties. However, the chemical stability, indicated by XRD patterns, showed that Al₂O₃ coatings were more stable, resisting degradation better than TiO₂. Mechanical reinforcement, assessed through nanoindentation tests, demonstrated that Al₂O₃ provided greater mechanical strength than TiO₂, as evidenced by a higher elastic modulus. Thermal stability tests using TGA showed that Al₂O₃-coated electrodes maintained integrity at higher temperatures than TiO₂-coated electrodes. Electrochemical performance, including specific capacity and capacity retention over 500 cycles, indicated that while TiO₂ coatings offered higher initial capacity, Al₂O₃ coatings ensured better long-term stability and lower capacity fade. Hybrid coatings, combining TiO₂ and Al₂O₃, presented a balanced performance profile, leveraging the high ionic conductivity of TiO₂ and the chemical stability of Al₂O₃ [²⁰].

Contour plots and 3D surface plots generated from the RSM analysis visually demonstrated these effects, aiding in identifying optimal coating strategies. The optimization process, guided by the response surface models, suggested that hybrid coatings deposited using ALD with a thickness of around 10 nm could provide an optimal balance of performance characteristics. This comprehensive RSM analysis highlighted the individual strengths of TiO₂ and Al₂O₃ coatings and showcased the potential of hybrid coatings in enhancing battery electrode performance, contributing to the development of more efficient and reliable energy storage solutions for the electric vehicle industry. The comparative performance analysis of titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and their hybrid coatings on battery electrodes revealed significant insights into their thermal stability and charge transfer resistance. These properties are critical for enhancing the performance, safety, and longevity of lithium-ion batteries (LIBs) used in electric vehicles (EVs).

The analysis of ionic conductivity and specific capacity provided further insights into the performance of these coatings. The ionic conductivity of TiO₂, Al₂O₃, and hybrid coatings was assessed at different thicknesses, revealing that TiO₂-coated electrodes exhibited the highest ionic conductivity due to its superior lithium-ion transport properties (Figure 13). Specific capacity measurements indicated that electrodes coated using Atomic Layer Deposition (ALD) demonstrated higher specific capacities compared to those coated using Chemical Vapor Deposition (CVD) techniques (Figure 14).

Figure 13
Ionic conductivity vs. coating thickness and material.

Figure 14
Specific capacity vs. coating thickness and deposition technique.

Thermal stability is crucial for battery safety, especially under high operating temperatures. The study measured the thermal stability of coatings at various thicknesses (5 nm, 10 nm, and 15 nm) using thermogravimetric analysis (TGA). The results indicated that Al₂O₃-coated electrodes demonstrated the highest thermal stability, withstanding temperatures up to 610°C at a thickness of 15 nm. In contrast, TiO₂-coated electrodes exhibited lower thermal stability, degrading at around 560°C at the same thickness [²¹]. Combining TiO₂ and Al₂O₃, hybrid coatings showed intermediate thermal stability, maintaining integrity up to 585°C. The higher thermal stability of Al₂O₃ is attributed to its exceptional chemical inertness and structural stability, which prevent decomposition at elevated temperatures. The hybrid coatings leveraged the advantages of both materials, providing a balanced thermal performance (Figure 15).

Figure 15
Thermal stability vs. coating thickness and material.

Charge transfer resistance is a key parameter influencing the electrochemical performance of battery electrodes. Electrochemical impedance spectroscopy (EIS) was used to evaluate the resistance at different coating thicknesses. Al₂O₃-coated electrodes displayed the lowest charge transfer resistance, indicating superior electrode kinetics and lower energy losses during charge and discharge cycles. Specifically, the resistance values for Al₂O₃-coated electrodes were measured at 40 Ohms (5 nm), 42 Ohms (10 nm), and 44 Ohms (15 nm). TiO₂-coated electrodes showed higher resistance values of 50 Ohms (5 nm), 52 Ohms (10 nm), and 54 Ohms (15 nm). Hybrid coatings again presented intermediate characteristics, with resistance values of 45 Ohms (5 nm), 47 Ohms (10 nm), and 49 Ohms (15 nm). The lower charge transfer resistance in Al₂O₃ coatings can be attributed to the material’s ability to maintain stable and efficient pathways for lithium-ion transport. Hybrid coatings benefited from the combination of TiO₂’s ionic conductivity and Al₂O₃’s chemical stability, resulting in a balanced electrochemical performance (Figure 16).

Figure 16
Charge transfer resistance vs. coating thickness and material.

The observed trends in thermal stability and charge transfer resistance can be attributed to the inherent properties of the coating materials. Al₂O₃’s superior chemical stability and inertness contribute to its higher thermal stability, preventing decomposition and maintaining structural integrity at higher temperatures [²²]. On the other hand, TiO₂’s higher ionic conductivity facilitates efficient lithium-ion transport, but its lower chemical stability compared to Al₂O₃ results in higher charge transfer resistance and lower thermal stability. Hybrid coatings effectively combine the strengths of both materials, providing balanced performance by leveraging TiO₂’s conductivity and Al₂O₃’s stability. The comprehensive analysis of these coatings underscores the importance of material selection and optimization in battery technology. By enhancing the thermal and electrochemical properties of battery electrodes, these advanced coatings can significantly improve the performance, safety, and longevity of LIBs, thereby supporting the growth and adoption of electric vehicles.

Lifecycle assessments indicate that while TiO₂ offers high ionic conductivity, its lower chemical stability may lead to higher replacement rates. Conversely, Al₂O₃’s chemical stability enhances the longevity of batteries, reducing the frequency of replacements and thereby lowering long-term costs. Hybrid coatings, by combining the strengths of both materials, present an economically viable and environmentally sustainable solution for large-scale production. These findings support the adoption of advanced coating technologies to improve the overall sustainability of the Indian EV industry [²³].

Future research could explore using other metal oxides, such as ZrO2 or HfO2, and their combinations with TiO₂ and Al₂O₃. Additionally, advanced deposition techniques like molecular layer deposition (MLD) could provide even greater control over coating properties, enhancing battery performance. Investigating the integration of these new materials and techniques will be crucial for the continued advancement of battery technology.

4. CONCLUSION

This study has provided a comprehensive comparative analysis of TiO₂, Al₂O₃, and hybrid coatings on lithium-ion battery electrodes, with significant implications for the Indian electric vehicle supply chain. The findings indicate that TiO₂ coatings offer superior ionic conductivity (3.5 × 10^–4 S/cm), making them suitable for high-rate applications, but their chemical stability is lower than that of Al₂O₃. Al₂O₃ coatings, with an ionic conductivity of 1.8 × 10^–4 S/cm, demonstrate excellent chemical stability and mechanical reinforcement, boasting an elastic modulus of 150 GPa. Hybrid coatings combine the benefits of both materials, achieving a balanced performance with 80% capacity retention after 500 cycles at 0.5C and intermediate thermal stability. Thermal stability tests indicated that Al₂O₃-coated electrodes maintained integrity up to 600°C, while TiO₂-coated electrodes began to degrade above 550°C.

The study underscores the potential of hybrid coatings in enhancing battery performance by leveraging the high ionic conductivity of TiO₂ and the chemical stability of Al₂O₃. These advancements directly address several critical issues in the Indian electric vehicle supply chain. Enhanced battery performance and longevity can reduce the overall cost of electric vehicles (EVs), making them more accessible to consumers. Improved battery efficiency and safety can increase the range and reliability of EVs, alleviating consumer concerns and boosting adoption rates. Additionally, the use of advanced coatings can lead to a reduction in the frequency of battery replacements, thereby lowering maintenance costs and environmental impact.

By optimizing battery technology, this research supports the development of more efficient and reliable energy storage solutions, essential for the growth of India’s EV market. The findings can guide battery manufacturers in India to adopt advanced coating technologies, enhancing the competitiveness of Indian-made EVs in the global market. For instance, lithium-ion batteries can constitute up to 40% of an EV’s total cost. Improving battery performance can lower this percentage, making EVs more economically viable. Moreover, these improvements can reduce India’s dependence on imported raw materials by increasing the efficiency and lifespan of existing batteries, aligning with the country’s sustainability and economic goals.

Future research should further optimize hybrid coatings’ ratios and deposition sequences and explore other novel materials that could offer even greater performance enhancements. Additionally, investigating these coatings’ long-term environmental and economic impacts will be crucial for their sustainable implementation in the battery industry. For example, hybrid coatings with 10 nm thickness deposited via Atomic Layer Deposition (ALD) showed optimal balance, suggesting this configuration could be a standard for future applications. This study lays the groundwork for significant advancements in battery technology, essential for the proliferation of electric vehicles in India.

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Publication Dates

• Publication in this collection
  20 Sept 2024
• Date of issue
  2024

History

• Received
  13 July 2024
• Accepted
  02 Aug 2024