The main factors affecting the internal resistance of lithium battery

With the use of lithium battery, the performance of the battery constantly decay, mainly manifested as capacity decay, internal resistance increase, power decline, etc., the change of battery internal resistance is affected by temperature, discharge depth and other use conditions.Therefore, this paper mainly expounds the factors affecting the battery internal resistance from the aspects of battery structure design, raw material performance, process process and use conditions.

Structural design impact

In the battery structure design, in addition to the riveting and welding of the battery structure itself, the number, size and position of the battery pole ear directly affect the internal resistance of the battery.To a certain extent, increasing the number of polar ears can effectively reduce the internal resistance of the battery.The pole ear position can also affect the internal resistance of the battery. The pole ear position in the head of the positive and negative electrode plate has the largest internal resistance, and compared with the coil battery, the laminated battery is equivalent to dozens of small batteries in parallel, and its internal resistance is smaller.

Performance impact of raw materials

1 Positive and negative electrode active material

The cathode material in the lithium battery is the Li storage side, which more determines the performance of the lithium battery. The cathode material mainly improves the electron conduction capacity between the particles through coating and doping.For example, after doping with Ni, the strength of the P-O bond is enhanced, the structure of LiFePO4 / C is stabilized, and the crystal cell volume is optimized, which can effectively reduce the charge transfer impedance of the cathode material.

And the simulation analysis of the electrochemical thermal coupling model shows that the large increase of the activation polarization, especially the negative electrode activation polarization, is the main reason of the serious polarization.Reducing the negative electrode particle size can effectively reduce the negative electrode activation polarization by 45% when the negative electrode solid phase particle size decreases by half.Therefore, in terms of battery design, the improvement of the positive and negative electrode material itself is also essential.

2. Conductor

Graphite and carbon black are widely used in the field of lithium battery because of their good performance.Compared with graphite conductive agent, the battery ratio performance of the positive electrode added carbon black conductive agent is better, because graphite conductive agent has a sheet particle morphology, causing the pore winding coefficient under the large ratio, which is easy to appear the phenomenon that Li liquid phase diffusion process limits the discharge capacity.The battery with CNTs has less internal resistance, because compared to the graphite / carbon black and the active material point contact, the fibrous carbon nanotube is in line contact with the active material, which can reduce the interface impedance of the battery.

Three episodes of fluid

Reducing the interface resistance between the fluid collector and the active material and improving the bond strength between them is an important means to improve the performance of lithium battery.The conductive carbon coating on the aluminum foil surface and the corona treatment of the aluminum foil can effectively reduce the interface impedance of the battery.Compared with ordinary aluminum foil, the use of carbon-coated aluminum foil can reduce the internal resistance of the battery by about 65%, and can reduce the increase of the battery in the process of use.

The AC internal resistance of aluminum foil treated with corona can be reduced by about 20%. In the commonly used 20%~90%SOC range, the DC internal resistance is small and the increase is gradually smaller with the increase of discharge depth.

4 Diaphragm

The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the diaphragm pores. The moisture absorption capacity of the diaphragm is the key to form a good ion flow channel. When the diaphragm has higher fluid absorption and porous structure, it can improve the conductivity, reduce the battery impedance and improve the multiplier performance of the battery.Compared with ordinary basic membrane, ceramic diaphragm and coated diaphragm can not only greatly improve the high temperature shrinkage resistance of the diaphragm, but also enhance the fluid absorption and wetting capacity of the diaphragm. Adding SiO2 ceramic coating on the PP diaphragm can increase the fluid absorption volume of the diaphragm by 17%.Painting 1 m of PVDF-HFP on the PP / PE composite diaphragm increased the diaphragm aspirate from 70% to 82%, and the cell internal resistance decreased by more than 20%.

Impact of process factors

1 The pulp

The uniformity of slurry dispersion during slurry closing affects whether the conductive agent can be evenly dispersed in the active substance and in close contact with it, which is related to the internal resistance of the battery.By increasing the high-speed dispersion, the uniformity of the slurry dispersion can be improved, and the smaller the internal resistance of the battery will be.Adding surfactant, the distribution uniformity of conductive agent in the electrode can be improved and the electrochemical polarization.

2 Tu cloth

Facial density is one of the key parameters of battery design. When the battery capacity is certain, increasing the extremely one-sided density is bound to reduce the total length of the fluid and diaphragm, and the ohmic internal resistance of the battery will be reduced. Therefore, within a certain range, the internal resistance of the battery decreases with the increase of surface density.During coating and drying, the migration of solvent molecules is closely related to the temperature of the oven, which directly affects the distribution of binder and conductive agent in the electrode, and then affects the formation of conductive grid inside the electrode. Therefore, the temperature of coating and drying is also an important process to optimize the performance of the battery.

3 Roll pressure

To a certain extent, the internal resistance of the battery decreases as the compaction density increases, because the compaction density increases, the distance between the raw material particles decreases, the more contact between the particles, the more conductive bridge and channels, the battery impedance decreases.The control of the compaction density is mainly achieved by the roller thickness.Different roller pressure thickness has a large impact on the internal resistance of the battery. When the roller pressure thickness is large, the contact resistance between the active substance and the collecting fluid increases due to the active material failing to roll down tightly, and the internal resistance of the battery increases.And the surface of the battery positive electrode with large roller pressure thickness after the battery cycle produces cracks, which will further increase the contact resistance between the surfactant of the electrode plate and the fluid collector.

4. Polar sheet turnover time

The different holding time affects the internal resistance of the positive plate. When the holding time is shorter, the internal resistance of the battery increases slowly due to the influence of carbon coating of lithium iron phosphate. When the holding time is longer (greater than 23h), the reaction of lithium iron phosphate and water and the bonding of the adhesive increase significantly.Therefore, the turnover time of the pole sheet should be strictly controlled in the actual production.

5 Injection

The ionic conductivity of the electrolyte determines the internal resistance and multiplier characteristics of the battery. The conductivity of the electrolyte is inversely proportional to the viscosity range of the solvent, and it is also affected by the lithium salt concentration and the anion size.In addition to the optimization of conductivity, the amount of fluid injection and the infiltration time after fluid injection also directly affect the internal resistance of the battery. Less amount of fluid injection or insufficient infiltration time will cause a large internal resistance of the battery, thus affecting the capacity of the battery.

Use condition effects

1 temperature

The effect of temperature on the size of the internal resistance is obvious, the lower the temperature, the slower the ion transmission inside the battery, the greater the internal resistance of the battery.Battery impedance can be divided into body phase impedance, SEI membrane impedance and charge transfer impedance. Body phase impedance and SEI membrane impedance are mainly affected by the electrolyte ionic conductivity, and the change trend at low temperature is consistent with the electrolyte conductivity change trend.Compared with the increase of body phase impedance and SEI membrane resistance at low temperature, the charge reaction impedance is more significant with the increase of temperature decrease. Below-20℃, the proportion of the charge reaction impedance in the total internal resistance of the battery reaches almost 100%.

2 SOC

When the battery is in different SOC, the internal resistance size is not the same, especially the dc resistance directly affects the power performance of the battery, reflecting the battery performance in the actual state: lithium battery dc resistance increases with the increase of battery discharge depth DOD, in 10%~80% of the discharge interval resistance size basically unchanged, generally in the deep discharge depth of internal resistance increased significantly.

3 Storage

With the increase of the storage time of lithium-ion battery, the battery keeps aging, and its internal resistance is constantly increasing.Different types of lithium batteries have different degrees of internal resistance change.After a long storage period of 9-10 months, the internal resistance increase rate of LFP batteries is higher than that of NCA and NCM batteries.The increase rate of internal resistance is related to storage time, storage temperature and storage SOC. Stroe et al. quantified the relationship between LFP / C batteries for 24 to 36 months (as follows):

Specifically, temperature in K, SOC in percentage, and time in months.

4 recurrence

Whether in storage or circulation, the impact of temperature on the battery internal resistance is consistent, the higher the cycle temperature, the greater the increase rate of internal resistance.Different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery increases with the improvement of charge and discharge depth, and the increase of internal resistance is proportional to the strengthening of charge and discharge depth.

In addition to the influence of charge and discharge depth, charging voltage is also affected: too low or too high charging voltage limit will make the electrode interface impedance, Zheng that LFP / C battery in the cycle of the optimal charging voltage is 3.9~4.3V, experiments found that too low limit voltage cannot form passivation film, and too high voltage limit will lead to electrolyte in the LiFePO4 electrode surface oxidation decomposition low conductivity product.

5 Other

Vehicle lithium battery will inevitably experience poor road conditions in practical application, but the study has found that the vibration environment of lithium battery in the application process has almost no impact on the internal resistance of lithium battery.

Internal resistance is an important parameter to measure the power performance of lithium ion and evaluate the battery life. The greater the internal resistance, the worse the multiplier performance of the battery, and the faster it increases in storage and recycling.The internal resistance is related to the battery structure, battery material characteristics and manufacturing process, and changes with the ambient temperature and charge state.Therefore, the development of low internal resistance battery is the key to improve the power performance of the battery, and it is of great practical significance to master the change law of internal battery life resistance