Internal resistance is a critical parameter for lithium batteries, directly influencing their power capability, efficiency, and overall lifespan. High internal resistance can lead to reduced usable energy, increased heat generation, and accelerated degradation. Understanding the multifaceted causes of high internal resistance – spanning material properties, manufacturing processes, usage conditions, and aging – is crucial for optimizing battery design and performance. We can broadly categorize these causes into ionic, electronic, and contact resistances.
1. Ionic Resistance: The Path for Lithium Ions
Ionic resistance refers to the opposition to the flow of lithium ions within the battery. Several factors can impede this movement:
Electrolyte Formulation and Quantity:
- An improper electrolyte formulation, such as one with a lithium salt concentration that’s too low or an imbalanced solvent ratio, can reduce the rate at which ions migrate.
- Similarly, if the electrolyte’s viscosity increases, especially in cold environments, it significantly slows down ion movement.
- Insufficient electrolyte volume means poor contact between the active materials and the electrolyte, directly increasing the internal resistance.
Electrode Compaction and Porosity:
- If electrodes are compacted too densely during manufacturing, their porosity (the volume of empty spaces within the material) is reduced. This limits the ability of the electrolyte to fully infiltrate the electrode, hindering ion transport.
- The porosity of the electrode is a vital indicator, as it directly impacts how much and how quickly the electrode can absorb electrolyte, profoundly influencing battery performance. Observing if the electrode becomes brittle or examining the material for breakage under an electron microscope, can help determine if it’s been over-compacted.
Separator Characteristics:
- A low porosity or excessive thickness of the separator (the permeable membrane between electrodes) directly increases the resistance to lithium-ion migration.
- Contamination or aging of the separator can block its pores with impurities. Furthermore, exposure to high temperatures can cause the separator to shrink or melt, severely obstructing ion transmission. The separator porosity is a key physical property tested to ensure optimal ion flow.
2. Electronic Resistance: The Flow of Electrons
Electronic resistance relates to the opposition to electron flow within the battery’s conductive components:
Conductivity of Electrode Materials:
- Some positive or negative electrode materials inherently possess poor electrical conductivity. For example, lithium iron phosphate (LFP) positive electrode material has low intrinsic conductivity. If it’s not adequately coated with carbon or modified through doping, it will lead to increased resistance for electron transfer.
Electrode Material Particle Size and Porosity:
- If the particle size of the electrode material is too large, it effectively lengthens the diffusion path for lithium ions, indirectly increasing overall resistance.
- Insufficient porosity within the electrode material itself can also hinder electrolyte infiltration, making it harder for ions to move.
Conductive Additives:
- An insufficient amount of conductive agent (such as carbon black), or its uneven dispersion throughout the electrode, results in an incomplete electronic conduction network. This disrupts the efficient flow of electrons within the electrode structure.
Ultimately, the impact of these factors—material properties, compaction density, the amount of conductive agent used, and current collector selection—is reflected in the pole piece (electrode sheet) resistance. Battery manufacturers routinely test the resistance of the pole piece to quantify its contribution to the overall internal resistance of the cell. The resistance of the pole piece generally changes with the pressure applied during manufacturing, as illustrated by various industry studies.
3. Contact Resistance: The Bridge Between Components
Contact resistance arises at the interfaces where different battery components meet, hindering the smooth flow of both ions and electrons:
- Active Material and Current Collector Interface: A large contact internal resistance between the active material and the current collector can be a significant issue. Strategies like using carbon-coated copper or aluminum foil are common to enhance conductivity at this interface.
- Current Collector and Tab Welding: If the welding connection between the current collector (e.g., aluminum or copper foil) and the battery tab is not firm or of high quality, it significantly increases contact resistance, creating a bottleneck for current flow out of the cell.
- Internal Cell Pressure: The internal pressure of the battery cell plays a subtle yet important role. If the pressure is too low, it can lead to poor contact between internal components. Conversely, if the pressure is excessively high, it might deform the separator, again affecting internal resistance.
High internal resistance in lithium batteries is a complex issue stemming from a confluence of factors related to materials, manufacturing processes, how the battery is used, and its natural aging over time.
Strategies to Reduce Internal Resistance
Mitigating high internal resistance requires a comprehensive approach, addressing each of the contributing factors:
Optimize Materials:
- Select electrode materials with inherently high electrical and ionic conductivity.
- Rationally design the pore structure of electrodes to maximize electrolyte infiltration and ion movement.
Improve Manufacturing Processes:
- Ensure uniform coating of electrode materials, preventing inconsistencies that can lead to resistive spots.
- Precisely control compaction density to achieve optimal balance between energy density and porosity.
- Optimize welding quality between various components to minimize contact resistance.
Adjust Electrolyte:
- Utilize high-conductivity electrolyte formulations that are specifically designed to maintain low viscosity and high ion mobility across the intended operating temperature range.
Avoid Abuse and Manage Usage:
- Prevent overcharging or over-discharging, as these can cause irreversible damage to electrode materials and increase internal resistance.
- Avoid high-temperature storage and operation, which accelerate degradation and can lead to internal resistance rise.
- Control the charge and discharge rate within recommended limits to prevent excessive heat generation and stress on battery components.
By systematically addressing these aspects, battery designers and manufacturers can significantly reduce the internal resistance of lithium batteries, leading to safer, more efficient, and longer-lasting energy storage solutions.