Abstract
The microstructure of lithium-ion battery electrodes significantly influences the electrochemical performance of electrode active materials. This study explores the effect of different electrode structures, specifically double-layer electrodes, on the performance of lithium iron phosphate battery (LFP battery). A double-layer coating technology was employed to construct LFP electrodes with varying compositions and pore structures. By incorporating ammonium bicarbonate as a pore-forming agent, the electrode coating exhibits a gradually increasing porosity from the current collector to the electrode surface. The results indicate that the double-layer electrode with an increasing porosity gradient shows superior electrochemical performance, particularly at high discharge rates.
Introduction
With the rapid development of renewable energy and the electrification of transportation, lithium-ion batteries (LIBs) have emerged as the primary energy storage solution for portable electronics, electric vehicles (EVs), and grid-scale energy storage systems. Among various cathode materials, lithium iron phosphate battery (LFP battery) stands out due to its high safety, long cycle life, and environmental friendliness. However, the low energy density and poor rate capability of LFP battery remain challenges that need to be addressed.
One effective strategy to enhance the energy density and rate capability of LIBs is to optimize the electrode structure. The electrode microstructure, including porosity, thickness, and composition, directly affects the ion transport, electron conductivity, and overall performance of LIBs. This study focuses on the impact of electrode structure, particularly the introduction of a double-layer electrode design, on the electrochemical performance of LFP battery.
Electrode Structure and Design
Single-Layer vs. Double-Layer Electrodes
Traditional LIB electrodes are typically single-layer structures, where the active material, conductive additive, and binder are mixed and coated onto a current collector in a single step. While this method is straightforward and widely adopted, it has limitations in terms of rate capability and energy density, particularly for thick electrodes.
In contrast, double-layer electrodes offer several advantages. By varying the composition and porosity of each layer, tailored ion and electron transport pathways can be created, leading to improved electrochemical performance. Specifically, a double-layer electrode with a higher porosity gradient from the current collector to the electrode surface can facilitate faster ion diffusion and electrolyte infiltration, thereby enhancing rate capability.
Pore-Forming Agent
The porosity of an electrode plays a critical role in determining its electrochemical performance. A higher porosity facilitates electrolyte infiltration and ion diffusion, while a lower porosity can improve electron conductivity and mechanical stability. To create a porosity gradient within the electrode, pore-forming agents can be used. In this study, ammonium bicarbonate (NH4HCO3) is selected as the pore-forming agent due to its ease of decomposition during electrode processing, leaving behind interconnected pores without residual impurities.
Experimental Methods
Electrode Preparation
The LFP electrodes were prepared using a slurry-coating method. LFP powder, super P carbon black (as a conductive additive), and polyvinylidene fluoride (PVDF) binder were mixed in N-methyl-2-pyrrolidone (NMP) solvent at various mass ratios. For double-layer electrodes, a pore-forming agent (NH4HCO3) was added to the upper layer slurry.
Table 1: Single-Layer Electrode Composition
| Sample | LFP (g) | Super P (g) | PVDF (g) | NMP (mL) |
|---|---|---|---|---|
| P1 | 8.5 | 0.5 | 0.5 | 20 |
| P2 | 8.5 | 1.0 | 0.5 | 20 |
| P3 | 8.5 | 0.5 | 0.5 | 20 |
Table 2: Double-Layer Electrode Composition
| Layer | LFP (g) | Super P (g) | PVDF (g) | NH4HCO3 (g) | NMP (mL) |
|---|---|---|---|---|---|
| Bottom | 8.5 | 1.0 | 0.5 | 0 | 10 |
| Top | 8.5 | 1.0 | 0.5 | 1.0 | 10 |
The slurries were coated onto aluminum foil current collectors using a doctor blade and dried at 80°C. The electrodes were then calendared to a desired thickness and punched into discs for battery assembly.
Battery Assembly and Testing
CR2016-type coin cells were assembled in an argon-filled glovebox. The LFP electrodes served as the cathode, lithium metal as the anode, and 1 M LiPF6 in ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) (1:1:1 by volume) as the electrolyte. Electrochemical performance was evaluated using a battery tester and an electrochemical workstation. Cyclic voltammetry (CV), galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy (EIS) were performed to assess the performance of single-layer and double-layer electrodes.
Results and Discussion
Electrode Composition Optimization
To determine the optimal composition for the LFP electrodes, single-layer electrodes with varying LFP:Super P:PVDF ratios were evaluated. the initial charge-discharge profiles of selected electrodes at 0.1 C.
The electrode with an 85:10:5 LFP:Super P:PVDF ratio (P2) exhibited the highest specific capacity and coulombic efficiency, indicating that a balance of active material, conductive additive, and binder is crucial for optimal performance.
Porosity Gradient in Double-Layer Electrodes
Double-layer electrodes were prepared with a higher porosity gradient in the top layer achieved by incorporating NH4HCO3 as a pore-forming agent. the SEM images of single-layer and double-layer electrodes.
The double-layer electrode exhibits a distinct porosity gradient, with larger pores on the top surface. Mercury intrusion porosimetry confirmed this porosity gradient.
Electrochemical Performance
Rate Capability
Rate capability tests were performed at various C-rates ranging from 0.1 C to 5 C. the rate capability of single-layer and double-layer electrodes.
The double-layer electrode with a porosity gradient demonstrated significantly higher discharge capacities at high C-rates, particularly at 5 C, where it retained a capacity of 40 mAh/g compared to nearly zero capacity for the single-layer electrode. This improvement can be attributed to the faster ion diffusion facilitated by the porosity gradient.
Cycling Stability
Cycling stability was evaluated at 1 C over 100 cycles. the cycling performance of single-layer and double-layer electrodes.
The double-layer electrode exhibited better cycling stability, retaining over 97% of its initial capacity after 100 cycles, compared to 87% for the single-layer electrode. This suggests that the porosity gradient not only enhances rate capability but also improves cycling stability.
Electrochemical Impedance Spectroscopy (EIS)
EIS measurements were performed to gain insights into the charge transfer kinetics. the Nyquist plots of single-layer and double-layer electrodes before and after cycling.
The double-layer electrode exhibited a smaller charge transfer resistance (Rct) both before and after cycling, indicating faster charge transfer kinetics. This is consistent with the improved rate capability and cycling stability observed in electrochemical tests.
Conclusion
This study investigated the impact of electrode structure, particularly the introduction of a double-layer design with a porosity gradient, on the electrochemical performance of LFP battery. The results demonstrate that the double-layer electrode with a porosity gradient significantly enhances rate capability and cycling stability compared to a conventional single-layer electrode. The porosity gradient facilitates faster ion diffusion and electrolyte infiltration, leading to improved performance at high C-rates. Furthermore, the double-layer design maintains good cycling stability, retaining over 97% of the initial capacity after 100 cycles at 1 C. These findings highlight the potential of tailored electrode structures in improving the performance of LFP battery for high-power applications.