The internal resistance of a lithium-ion battery mainly consists of three parts: ion resistance, electronic resistance, and contact resistance. To reduce the internal resistance of a lithium-ion battery, specific measures need to be taken for these three aspects.
The ion resistance of a lithium-ion battery refers to the resistance that occurs during the internal transmission of lithium ions. The migration rate of lithium ions and the electronic conductivity play equally important roles in the lithium battery, and the ion resistance is mainly influenced by the positive and negative electrode materials, the separator, and the electrolyte. To reduce ion resistance, the following points need to be considered:
Ensure good wetting properties of positive and negative electrode materials and electrolytes
When designing the electrode plates, it is necessary to select appropriate compaction density. Overly high compaction densities lead to poor electrolyte wetting, which in turn, increases ion resistance. For negative electrode plates, if the SEI film on the surface of the active substance is too thick during the initial charge and discharge, it will also increase ion resistance, and battery formation process needs to be adjusted to solve this problem.
The impact of the electrolyte
The electrolyte should have appropriate concentration, viscosity, and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between the positive and negative active materials of the electrolyte. At the same time, the electrolyte needs low concentration. Too much concentration is also detrimental to its flow and infiltration. The conductivity of the electrolyte is the most important factor that affects ion resistance and it determines the migration of ions.
The influence of the separator on ion resistance
The main factors influencing the ion resistance of the separator include the distribution of electrolyte in the separator, separator area, thickness, pore size, porosity, and tortuosity coefficient. For ceramic separators, it is also necessary to prevent ceramic particles from blocking the pores of the separator and to ensure that there is no excessive residual electrolyte to reduce the use efficiency of electrolyte.
The influencing factors of the electronic resistance of lithium-ion batteries are numerous and can be improved from material, process, and other aspects.
Positive and negative electrode plates
The factors influencing the electronic resistance of positive and negative electrode plates are mainly the contact between active materials and current collectors, the physical properties of active materials, and the parameters of electrode plates. Sufficient contact between active materials and current collectors should be established for better adhesion of positive and negative electrode pastes. The porosity of the active material itself, the by-products on the surface of the particles, and the uneven mixing with conductive agents can all result in a change in electronic resistance. The density of the active material is too small if the particle gap is too large, resulting in poor electronic conduction.
The factors influencing the electronic resistance of the separator are mainly the thickness, porosity, and by-products generated during the charging and discharging process. The first two factors are easy to understand. When disassembling the battery, a thick layer of brown substances can be often found on top of the separator. Inside it, the graphite negative electrode and its reaction by-products can cause blockage of the separator pores and reduce the battery's lifespan.
The current collector substrate
The material, thickness, width, and contact degree between the current collector and the electrode ear influence the electronic resistance. The current collector should use non-oxidized and passivated substrates, otherwise, the resistance will be affected. Poor welding between the copper and aluminum foils and the electrode ear will also affect electronic resistance.
Contact resistance is formed between the copper and aluminum foils and the active materials and requires special attention to the adhesion of the positive and negative electrode pastes.
When current passes through the electrodes, the electrode potential deviates from the equilibrium electrode potential, which is called electrode polarization. Polarization includes Ohmic polarization, electrochemical polarization, and concentration polarization. Polarization resistance refers to the internal resistance caused by the polarization of the electrodes during the electrochemical reaction. It can reflect the consistency inside the battery. However, it is not suitable for production because it is affected by operation and methods. Polarization resistance is not a constant but continuously changes over time during charging and discharging due to changes in the composition of the active material, electrolyte concentration, and temperature. Ohmic resistance follows Ohm's law and polarization resistance increases nonlinearly with increases in current density. The increase often occurs linearly with increasing log of current density.
The impact of structural design
In the structural design of lithium-ion batteries, in addition to the riveting and welding of battery structural components, the number, size, and position of battery electrode ears directly affect the internal resistance of the battery. To a certain extent, increasing the number of electrode ears can effectively reduce the internal resistance of the battery. The position of the electrode ear also affects the battery's internal resistance; the internal resistance of wound batteries is highest at the top of the positive and negative electrode plates, while stacked batteries are equivalent to parallel connection of tens of small batteries, which results in lower internal resistance.
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