I. Lithium battery internal resistance basic concepts and definitions
1、Basic Concept
The internal resistance of lithium battery refers to the internal obstruction of the battery to the current. The main source of internal resistance is the resistance and polarization effect of the electrolyte, positive and negative materials and components such as diaphragm inside the battery. The size of the internal resistance has a direct impact on the working condition of the battery. A smaller internal resistance can make the battery output a more stable voltage and current during operation, thus improving the overall performance of the battery. On the contrary, a larger internal resistance will lead to more heat in the discharge process, reducing the efficiency and service life of the battery.
2, the classification of lithium battery internal resistance
The internal resistance of lithium battery can be divided into ohmic internal resistance and polarization internal resistance.
(1) Ohmic internal resistance
Ohmic internal resistance is mainly composed of electrode material, electrolyte, diaphragm resistance and contact resistance of each part, which is related to the size, structure and connection of the battery. The ohmic internal resistance reflects the inherent resistance inside the battery, which always exists in the battery charging and discharging process.    
(2) Polarization internal resistance
Polarization resistance is the resistance that appears at the moment of loading current, which is the sum of various factors inside the battery to hinder the transmission of charged ions. Polarization internal resistance can be further divided into electrochemical polarization and concentration polarization. Electrochemical polarization is caused by the limited rate of chemical reaction on the electrode surface, while concentration polarization is caused by the difference in ion concentration in the electrolyte.
(3) Calculation of internal resistance
The precise calculation of internal resistance is quite complicated and will keep changing during the use of the battery, and the size of internal resistance is usually estimated by experiment or experience. Usually, the larger the volume of the lithium battery, the smaller its internal resistance, this is because a larger battery volume can provide more spacious ion transmission channels, reducing the resistance of ion transmission. Lithium battery internal resistance is one of the important indicators of battery performance, which is affected by a variety of factors.

2 the lithium battery internal resistance on the performance of the battery
1, lithium battery discharge
In terms of discharge performance, the larger the internal resistance of lithium batteries in the discharge process, the more voltage is lost, resulting in lower output voltage. In addition, a larger internal resistance will also limit the size of the output current, the size of the lithium internal resistance directly affects the battery's discharge capacity. In scenarios requiring high power output, such as electric vehicle acceleration or high load operation of electronic equipment, lithium batteries with large internal resistance may not be able to meet the demand, resulting in performance degradation.
2. Lithium battery charging
In terms of charging performance, internal resistance also plays a key role. When the internal resistance is large, the charging current will be limited, resulting in slower charging. This not only extends the charging time, but also may affect the charging efficiency of the battery. Especially in the wide application of fast charging technology, reduce internal resistance becomes the key to improve charging performance.
3, internal resistance also affects the self-discharge rate of lithium batteries    
Self-discharge refers to the loss of power due to internal chemical reaction when the battery is not connected to the external circuit. The higher the internal resistance, the higher the self-discharge rate, which will also cause rapid loss of power when the battery is not in use.
4, the internal resistance of lithium battery temperature characteristics of the impact
Lithium batteries generate heat when working, and the larger the internal resistance, the more heat generated. This may not only cause the battery temperature to rise, affecting the stability and safety of the battery.
5, the internal resistance of lithium battery cycle life also has a non-negligible effect
Cycle life refers to the durability of the battery during the charge/discharge cycle. The greater the internal resistance, the greater the loss of the battery in the cycling process, leading to a gradual decline in battery performance, and ultimately affecting the service life of the battery.

4, reduce the internal resistance of lithium battery, improve battery performance optimization measures
1, optimize the battery materials: the selection of positive and negative materials and electrolyte with high conductivity and low polarization effect can effectively reduce the internal resistance of the battery. At the same time, by improving the production process and formula, improve the purity and uniformity of materials, can also further reduce the internal resistance.
2、Improve the structure of the battery: by optimizing the design of the battery's pole piece, the thickness of the diaphragm and the distribution of the electrolyte, etc., the internal resistance and polarization effect of the battery can be reduced, thus lowering the internal resistance.
3, the application of battery management system (BMS) management to reduce internal resistance: BMS can accurately control and monitor the battery to avoid problems such as over-charging, discharging and overheating, in order to keep the internal resistance of the battery at a lower level.
4, through the use of advanced thermal management technology to reduce the internal resistance of lithium batteries: through the use of advanced thermal management technology to reduce the internal resistance of lithium batteries, such as liquid cooling, air-cooling, etc., you can effectively control the operating temperature of the battery to reduce the increase in resistance due to the rise in temperature.