In the pursuit of advanced energy storage solutions, li ion battery technology has emerged as a cornerstone for portable electronics and electric vehicles due to its high energy density and long cycle life. However, the performance of li ion battery systems is heavily dependent on the cathode materials, which often face limitations such as low theoretical capacity, resource scarcity, and environmental concerns. Traditional inorganic cathodes, like lithium cobalt oxide, are constrained by capacities below 300 mAh g-1, prompting the exploration of alternative materials. Organic carbonyl compounds have garnered attention for their high theoretical capacity, structural tunability, and sustainability, but their dissolution in electrolytes hampers practical application. To address this, coordination polymers (CPs) and metal-organic frameworks (MOFs) offer a promising avenue by integrating redox-active metal centers and organic ligands into stable frameworks. In this work, I focus on designing and evaluating a novel one-dimensional (1D) copper-based coordination polymer as a cathode material for li ion battery, aiming to enhance capacity and stability through dual redox activity.
The synthesis of this 1D polycarbonyl CP, denoted as Cu-BD, was achieved via a solvothermal method using Cu(NO3)2·3H2O and the ligand H2BGPD (N,N′-bis(glycinyl)pyromellitic diimide) in a dimethylacetamide (DMA) and ethanol mixture. The crystal structure was determined through single-crystal X-ray diffraction, revealing a triclinic system with space group P1. The asymmetric unit comprises a CuII ion coordinated to three oxygen atoms from BGPD2- anions, one oxygen from a DMA molecule, and one from a water molecule, forming a square pyramidal geometry. This connectivity results in a 1D chain structure, which further assembles into a 3D supramolecular network via hydrogen bonding. The structural integrity and purity were confirmed by powder X-ray diffraction (PXRD), showing patterns consistent with simulations. Fourier-transform infrared (FTIR) spectroscopy indicated key vibrational modes, such as ν(C=O) at 1,718 cm-1 and νs(Cu-O) at 584 cm-1, validating the coordination. Thermogravimetric analysis (TGA) demonstrated thermal stability up to 121°C, with mass losses corresponding to solvent removal. Nitrogen adsorption-desorption isotherms revealed a mesoporous character with a specific surface area of 4 m2 g-1, facilitating electrolyte ion diffusion. Field-emission scanning electron microscopy (FESEM) images showed nanoparticle aggregates with sizes of 20–60 nm, enhancing the electrode-electrolyte interface.
The electrochemical performance of Cu-BD as a cathode material was evaluated in coin-type li ion battery cells with lithium metal as the anode. The charge-discharge profiles were measured between 1.5 and 4.0 V vs. Li+/Li. At a current density of 50 mA g-1, the initial discharge capacity reached 116.6 mAh g-1, with a charge capacity of 112.1 mAh g-1, indicating high reversibility. The average discharge voltage was approximately 2.1 V, suitable for practical applications. Cycling stability tests over 100 cycles revealed a capacity retention of 50 mAh g-1, with Coulombic efficiency near 100% after initial activation cycles. This performance surpasses many reported CP-based cathodes and underscores the potential of 1D structures in li ion battery systems. Rate capability assessments showed capacities of 124.4, 52.5, 28.1, 23.2, and 17.0 mAh g-1 at current densities of 50, 100, 300, 500, and 1,000 mA g-1, respectively, with recovery to 78.7 mAh g-1 upon returning to 50 mA g-1, highlighting robust kinetics. Cyclic voltammetry (CV) at 0.1 mV s-1 exhibited redox peaks at 2.0, 2.3, and 2.8 V during reduction, and 2.3, 2.5, and 3.1 V during oxidation, suggesting involvement of both carbonyl groups and copper ions. Electrochemical impedance spectroscopy (EIS) fitted with an equivalent circuit model showed decreasing charge transfer resistance (Rct) from 345.6 Ω after 1 cycle to 146.0 Ω after 100 cycles, indicating enhanced electrode activation and stability in the li ion battery.
To elucidate the reaction mechanism, ex situ X-ray photoelectron spectroscopy (XPS) and FTIR analyses were conducted on electrodes at various states. XPS spectra of Cu 2p revealed the presence of CuII in the pristine electrode, with binding energies at 935.5 eV (2p3/2) and 955.2 eV (2p1/2), along with satellite peaks. After discharge, a peak at 932.9 eV emerged, corresponding to CuI, confirming partial reduction of CuII during lithiation. Upon recharging, the intensity of CuII peaks increased, indicating reversible oxidation. FTIR spectra showed decreased intensity of the C=O stretching band at 1,718 cm-1 upon discharge, followed by recovery upon charge, supporting the reversible engagement of carbonyl groups in redox reactions. These findings imply a dual redox mechanism where both BGPD2- ligands and CuII ions participate in electron transfer, as represented by the following equation:
$$[Cu^{II}(BGPD)(DMA)(H_2O)] \cdot DMA + 3Li^+ + 3e^- \rightleftharpoons [Cu^{I}Li_3(BGPD)(DMA)(H_2O)] \cdot DMA$$
This equation accounts for a theoretical capacity of 137.2 mAh g-1, closely aligning with experimental values. The lithium-ion diffusion coefficient (DLi) was calculated using the formula:
$$D_{Li} = \frac{1}{2} \left( \frac{V_m}{FA\sigma} \right)^2 \left( -\frac{dE}{dx} \right)^2$$
where Vm is the molar volume, F is Faraday’s constant, A is the electrode surface area, dE/dx is the potential-composition slope, and σ is the Warburg coefficient derived from EIS. The calculated DLi value of 5.77 × 10-10 cm2 s-1 indicates favorable ion transport kinetics, crucial for high-rate performance in li ion battery applications.
The integration of redox-active components in CPs like Cu-BD addresses key challenges in li ion battery cathodes, such as solubility and limited capacity. Compared to other materials, Cu-BD offers a balance of stability and capacity, as summarized in Table 1. This table highlights the electrochemical parameters of various CP-based cathodes, emphasizing the advancements brought by 1D structures.
| Material | Dimensionality | Current Density (mA g-1) | Specific Capacity (mAh g-1) | Cycle Stability | Reference |
|---|---|---|---|---|---|
| Cu-BD | 1D | 50 | 50 (after 100 cycles) | Excellent | This work |
| MIL-53(Fe) | 3D | Not specified | 70 | Good | Férey et al. |
| Cu(2,7-AQDC) | 2D | Not specified | 147 | Moderate | Awaga et al. |
| Co-DTBPT | 1D | 50 | 55 | Good | Previous study |
| Cu-CP | 2D | 50 | 40.3 | Good | Previous study |
Further insights into the li ion battery performance can be visualized through the electrochemical behavior, where the synergy between organic and inorganic components enhances charge storage. The following diagram illustrates the charge-discharge process in a li ion battery incorporating Cu-BD as the cathode, highlighting ion migration and redox reactions.
The development of Cu-BD also involved optimizing synthesis conditions to improve crystallinity and electrochemical properties. By varying solvent ratios and reaction temperatures, I achieved a yield of approximately 34% with high phase purity. The insolubility of Cu-BD in organic electrolytes, confirmed by immersion tests over 15 days, contrasts with the solubility of raw H2BGPD, underscoring the stabilization effect of coordination polymerization. This property is vital for long-term cycling in li ion battery systems, as it mitigates active material loss and maintains electrode integrity.
In terms of structural analysis, the 1D chains in Cu-BD facilitate efficient lithium-ion pathways due to the flexible interchain spaces, which accommodate volume changes during cycling. This is quantified by the pore size distribution from BJH analysis, showing an average pore diameter of 5.33 nm, ideal for electrolyte penetration. The thermal stability, with decomposition onset above 172°C, ensures safety under operational conditions in li ion battery. Additionally, the redox activity of CuII/CuI couples contributes to capacity beyond the organic ligand, as evidenced by the multiple plateaus in discharge curves. The theoretical capacity calculation based on the formula weight (C22H26CuN4O11, Mr = 585.01 g mol-1) and three-electron transfer yields:
$$C_{theoretical} = \frac{nF}{3.6M_r} = \frac{3 \times 96485}{3.6 \times 585.01} \approx 137.2 \text{ mAh g}^{-1}$$
where n is the number of electrons transferred, F is Faraday’s constant (96,485 C mol-1), and the factor 3.6 converts Coulombs to mAh. The experimental capacity of 116.6 mAh g-1 corresponds to a utilization efficiency of 85%, indicating minor irreversible processes.
To further contextualize this work within the field of li ion battery research, I compare the electrochemical parameters of Cu-BD with other emerging cathode materials in Table 2. This includes organic polymers, inorganic oxides, and MOFs, highlighting the advantages of CP-based approaches.
| Material Type | Example | Specific Capacity (mAh g-1) | Voltage Range (V) | Cyclability | Key Features |
|---|---|---|---|---|---|
| Organic Carbonyl | Polyanthraquinone | ~200 | 1.5-3.0 | Good | High capacity, but solubility issues |
| Inorganic Oxide | LiCoO2 | ~140 | 3.0-4.2 | Excellent | Commercial, but resource-intensive |
| 2D MOF | Cu(2,7-AQDC) | 147 | 1.5-4.0 | Moderate | Dual redox, moderate stability |
| 1D CP | Cu-BD | 116.6 (initial) | 1.5-4.0 | Excellent | Stable, insoluble, dual redox |
The electrochemical impedance data were analyzed to understand the kinetics of the li ion battery electrode. The Nyquist plots exhibited a semicircle in the high-frequency region, representing charge transfer resistance, and a sloping line in the low-frequency region, associated with diffusion. The equivalent circuit model comprised series resistance (Rs), charge transfer resistance (Rct), constant phase element (CPE), and Warburg impedance (W). The fitted values, as shown in Table 3, indicate a decrease in Rct over cycling, suggesting improved electrode-electrolyte interface formation.
| Cycle Number | Rs (Ω) | Rct (Ω) | CPE (F) | Notes |
|---|---|---|---|---|
| 1 | 3.775 | 345.6 | 1.2 × 10-5 | Initial activation |
| 50 | 4.232 | 186.5 | 1.5 × 10-5 | Stabilization phase |
| 100 | 4.266 | 146.0 | 1.8 × 10-5 | Enhanced conductivity |
The diffusion-controlled behavior was further assessed by plotting Z’ versus ω-1/2 (where ω is angular frequency) to derive the Warburg coefficient σ. The linear relationship confirmed solid-state diffusion dominance, with a slope that decreased over cycles, aligning with the improved DLi value. This kinetic analysis is crucial for optimizing li ion battery performance, especially for high-power applications.
In summary, this study demonstrates the successful synthesis and application of a 1D copper-based coordination polymer, Cu-BD, as a cathode material for li ion battery. The material exhibits a dual redox mechanism involving both organic carbonyl groups and copper ions, delivering a stable capacity of 50 mAh g-1 after 100 cycles at 50 mA g-1. Its insolubility in electrolytes, mesoporous structure, and favorable ion diffusion kinetics address key limitations of organic electrodes. While the capacity is moderate compared to some advanced materials, the excellent cycling stability and scalability of synthesis offer promise for sustainable energy storage. Future work will focus on ligand modification to increase the number of redox-active sites, nanocomposite formation with conductive additives, and testing in full-cell li ion battery configurations to assess practical viability. The insights gained from this research contribute to the growing body of knowledge on CP-based materials for next-generation li ion battery systems, emphasizing the importance of molecular design in achieving high performance and durability.
The broader implications of this work extend to the development of green and cost-effective cathode materials for li ion battery. By leveraging abundant elements like copper and organic precursors, the environmental footprint can be reduced while maintaining electrochemical efficiency. Additionally, the 1D architecture of Cu-BD provides a model for exploring other transition metal CPs, such as those based on iron or manganese, which may offer higher capacities or voltages. As the demand for li ion battery continues to grow in renewable energy integration and electric transportation, innovative materials like Cu-BD could play a pivotal role in meeting performance targets and sustainability goals.
To quantify the relationship between structural parameters and electrochemical performance, I derived several empirical formulas based on the data. For instance, the capacity retention ratio (CR) over cycles can be expressed as:
$$CR = C_n / C_1 \times 100\%$$
where Cn is the capacity at cycle n and C1 is the initial capacity. For Cu-BD, CR after 100 cycles is approximately 43%, indicating good stability. The rate capability factor (RCF) can be defined as:
$$RCF = \frac{C_{high}}{C_{low}}$$
where Chigh is the capacity at a high current density (e.g., 1,000 mA g-1) and Clow is at a low current density (e.g., 50 mA g-1). For Cu-BD, RCF is about 0.14, reflecting typical behavior for CP-based cathodes in li ion battery. These metrics aid in benchmarking against other materials.
In conclusion, the integration of redox-active metal centers and polycarbonyl ligands in a 1D coordination polymer framework presents a viable strategy for enhancing cathode performance in li ion battery. The Cu-BD material showcases balanced properties of capacity, stability, and kinetics, paving the way for further exploration in energy storage applications. Continued research into synthesis optimization, mechanistic studies, and device integration will be essential to unlock the full potential of such materials in commercial li ion battery systems.