What is the K value/open circuit voltage/polarization of lithium battery? What impact will it have on the battery?
K value refers to the voltage drop of the battery in unit time, usually expressed in mV/d, and is an indicator to measure the self discharge rate of lithium battery. OCV1 is measured at time t1. Measure OCV2 at time t2. K=（OCV1-OCV2)/（t2-t1）。 The K value of the battery with good performance is generally less than 2mV/d or 0.08mV/h.
Open circuit voltage
Open circuit voltage (OCV) refers to the terminal voltage of the battery in the open circuit state. The open circuit voltage of the battery is equal to the difference between the positive electrode potential and the negative electrode potential of the battery when the battery is in open circuit (i.e. when no current passes through the two poles), which is expressed in V open, i.e. V open= Ф+-Ф-， among Ф+、Ф- They are respectively the positive and negative electrode potentials of the battery. The open circuit voltage of the battery is generally less than its electromotive force, because the electrode potential established by the two poles of the battery in the electrolyte solution is usually not the equilibrium electrode potential, but the stable electrode potential. Generally, it can be approximately considered that the open circuit voltage of the battery is the electromotive force of the battery. The open circuit voltage of the battery will vary according to the materials of the positive and negative electrodes of the battery and the electrolyte. If the materials of the positive and negative electrodes of the battery are identical, the open circuit voltage will be the same no matter how large the battery volume is and how the geometric structure changes. In actual calculation, it can be seen that there is a voltmeter connected to the open circuit, and the voltmeter reading is the open circuit voltage.
Reaction equation：M2+ + 2e–=M Nernst equation：E=E0-RTln(am/am2+)
R is the gas constant, T is the reaction temperature, and a is the component activity or concentration. The open circuit voltage of the battery depends on the properties of the positive and negative electrode materials, electrolyte and temperature conditions, and is independent of the geometric structure and size of the battery.
The phenomenon that the potential deviates from the equilibrium potential when the battery has current passing through is called battery polarization. Overpotential is the difference between the actual potential and the balanced potential, which is used to measure the degree of polarization. The phenomenon of battery polarization exists in common batteries such as lead-acid batteries, lithium batteries and nickel hydrogen batteries.
According to the causes of polarization, polarization can be divided into three types: electrochemical polarization, concentration polarization and ohmic polarization.
1.Electrochemical polarization, also known as active polarization, is caused by the fact that the electrochemical reaction rate of positive and negative active substances is less than the rate of electron movement, and the response time is microseconds;
2. Concentration polarization is caused by the consumption of reactants, which causes that the electrode surface cannot be replenished in time (or some product accumulates on the electrode surface and cannot be evacuated in time). For example, the accumulation of hydrogen at the positive pole of the battery causes the electrode potential to deviate from the average value calculated according to the overall concentration before power on, and the response time is seconds;
3. Ohmic polarization is caused by electrolyte, electrode materials, diaphragm resistance and contact resistance between various components, which occurs instantaneously.
The above three polarizations are the resistance of electrochemical reaction. The internal resistance of the battery is the sum of ohmic internal resistance, electrochemical polarization internal resistance and concentration polarization internal resistance.
Factors affecting polarization
1.Internal influence of battery
(1) Influence of electrolyte: low conductivity of electrolyte is the main reason for lithium ion battery polarization. One of the ways to improve the large rate discharge capacity of electrolyte is to improve the conductivity of electrolyte.
(2) The influence of positive and negative electrode materials: the channel for large lithium ions of positive and negative electrode material particles to diffuse to the surface is lengthened, which increases the polarization effect, which can be solved through the nanometer way.
(3) Effect of conductive agent: When the content of conductive agent decreases, the polarization internal resistance increases rapidly, which makes the battery voltage quickly reduce to the discharge cut-off voltage.
(4) Influence of SEI film: The formation of SEI film increases the internal resistance of electrode/electrolyte interface, resulting in voltage hysteresis.
(5) Impact of battery design:
Electrode plate thickness: the increase of electrode plate thickness will increase the lithium ion diffusion path, resulting in greater concentration polarization.
Compaction density: when the compaction density is large, the contact between the material and the electrolyte decreases, resulting in concentration difference polarization and ohmic polarization.
2. External influence of battery
(1) Effect of SOC on polarization:
In the process of charging and discharging, the polarization voltage of 10% ＜ Bsoc ＜ 80% is relatively small and has little change;
When the SOC of battery charging is ＞ 80% and the SOC of battery discharging is ＜ 10%, the polarization voltage increases sharply in a short period of time, up to 300 V
(2) Influence of charging rate on polarization voltage:
The higher the charging rate is, the stronger the internal reaction of the battery is, the faster the migration speed of lithium ions is, the faster the polarization voltage value increases, and the less electricity can be charged;
If the charging rate is too high, the diffusion rate of lithium ions in the cell cannot meet the demand for current generation in a short time, and a large number of lithium ions will gather near the pole, resulting in a sharp rise in voltage. If the charging time is too long, lithium will crystallize, accelerating the decline of battery capacity.
(3) Effect of temperature on polarization:
When the SOC of battery is less than 30%, the open circuit voltage is obviously affected by temperature. The lower the temperature is, the higher the open circuit voltage is; When 30% ＜ SOC ＜ 80%, the change trend of open circuit voltage tends to be flat, and the value changes little;
With the increase of temperature, the activity of the battery increases, the polarization voltage changes slowly, and the available capacity increases slightly; At low temperature, the polarization voltage of the battery increases greatly, and there is an obvious accumulation phenomenon, which leads to the rapid reduction of the rechargeable capacity;
The polarization voltage of the battery has obvious accumulation. The greater the SOC, the more obvious the gap. The lower the temperature, the more serious the accumulation of polarization voltage.
Influencing factors of K value
K value is the physical quantity of self discharge speed of reaction lithium ion battery.
1. Depolarization effect
After the lithium ion battery is charged, the voltage gradually tends to be stable with the increase of time. When the depolarization is not complete, the test K value is too large, and misjudgment occurs.
2. Initial SOC state
OCV presents a curve change with the increase of SOC. When the slope K is large, it is conducive to the selection of K value.
3. Aging temperature and time
-According to Arrhenius equation, the increase of aging temperature can accelerate the chemical reaction, improve the self discharge rate and increase the K value. Increasing the temperature can shorten the screening cycle of K value and improve the production efficiency.
K=Ae (- Ea/RT) K is the rate constant
4. Test equipment
The accuracy of K value test shall reach 0.1mV, and the resolution shall be 0.01mV;
During the K value test, the same equipment or equipment with high consistency of pressure must be used for two OCV tests to ensure that the K value is true.
5. Physical micro short circuit
-Physical short circuit
Foreign matters pierce the isolation membrane, causing electronic conduction in the cell, forming an internal loop, and increasing the K value;
Type of foreign matter: conductor or semiconductor material such as metal or metal oxide
-Chemical dissolution short circuit
There is metal foreign matter M on the positive electrode surface or the diaphragm surface. Under high voltage, the metal foreign matter ionizes to form ion M -. The metal ion moves to the negative electrode under the action of electric field, and the reduction reaction takes place on the negative electrode surface to form crystal branches. The diameter grows along the direction of the electric field, piercing the isolation membrane and causing a short circuit.
Ranking of K value affected by foreign matters and metals: Cu ＞ Zn ＞ Fe ＞ Fe2O3
6. Chemical reaction
– Moisture impact
When there is H2O in the cell, it will first react with LiPF6 to produce corrosive gases such as HF; At the same time, it reacts with solvent to produce CO2 and other gases, causing battery expansion; HF will react with many substances in the battery, such as the main components of SEI, and destroy the SEI membrane; CO2 and H2O production; CO2 causes battery expansion, and the regenerated H2O participates in LiPF6, solvent and other reactions; Form a malignant chain reaction!
Consequences of SEI membrane destruction: 1) solvent enters the graphite layer and reacts with LixC6, causing impossible capacity loss; 2) The damaged SEI repair will consume Li+and solvent, further causing irreversible capacity loss.
Analysis method of K value defect
-K value defective analysis process
K value reconfirmation: it is easy to make a false judgment during K value test, and it is necessary to reconfirm the K value to determine whether it is a real defective product
Refrigeration impedance: confirm the ohmic impedance of lithium ion battery by using the rapid cooling method to determine whether it is a physical short circuit
Split: split to find short points and distribution rules to prepare for investigating the source of occurrence
Analysis: foreign matter analysis, analysis of foreign matter composition, size, etc
Occurrence source: according to the analysis results, combined with the characteristics of production process and equipment, investigate the occurrence source
-Foreign matter source and control
1) Raw material foreign matter
Foreign matters occur in positive and negative electrode materials, conductive agents, diaphragms, etc./iron removal equipment is added to strengthen the inspection of raw materials
2) Foreign matters during manufacturing
Environmental dust during manufacturing/strengthening 5S management in workshop
Key management of plate powder falling/plate adhesive storage state, mixing and extrusion
Cutting burr/cutter cycle management
Friction/removal of metal parts of equipment during winding
Improvement and periodic management of foreign matter splash/dust collection device for lug welding