What is a battery cycle?
In the process of use, the actual available capacity of lithium ion battery, relative to its rated capacity at the factory, will continue to decrease, that is, capacity attenuation occurs. Any adverse side-reaction that consumes lithium ions may cause a change in the lithium ion balance in the battery, which is irreversible and can accumulate through multiple battery cycle, adversely affecting battery performance.
Factors affecting the lithium-ion battery cycle times
Battery charging and discharging once is called a battery cycle, battery cycle life is an important index to measure the performance of battery life. The underlying cause of the factors affecting thelithium-ion battery cycle life is the decreasing amount of lithium ions involved in energy transfer.
In some cases, the amount of lithium in the battery does not decrease, but rather the “activated” lithium ions become trapped in some places or the transmission channel is blocked, and cannot freely participate in the charge and discharge process. Therefore, several factors affecting the lithium-ion battery cycle times including:
Plating of lithium metal
For lithium cobalate and graphite system, anode graphite becomes the “short board” side in the battery cycle process is more common. If the negative electrode is insufficient, the cell may not plate the lithium before battery cycle, but after hundreds of cycles, the positive electrode structure changes little, but the negative electrode structure is seriously damaged and can not fully receive the lithium ions provided by the positive electrode, resulting in a premature decline in capacity.
The plating of metal lithium generally occurs on the surface of the negative electrode. When lithium ions migrate to the surface of the negative electrode, part of the lithium ions do not enter the active substance of the negative electrode to form stable compounds, but obtain electrons and deposit on the surface of the negative electrode to become metal lithium, and no longer participate in the subsequent battery cycle process, resulting in a reduction in capacity.
For example, when overcharging or negative material is insufficient, the negative pole cannot accommodate lithium ions migrated from the positive pole, resulting in the plating of lithium metal; When charging at high rate, too many lithium ions reach the negative electrode in a short time, resulting in channel blockage and plating.
Decomposition of cathode materials
Lithium metal oxides of cathode materials will continue to decompose during long-term use, producing some electrochemical inert substances and some combustible gases, destroying the capacity balance between electrodes, resulting in irreversible loss of capacity.
SEI film on electrode surface
In the first cycle, the electrolyte will form a layer of solid electrolyte (SEI) film on the electrode surface. The SEI film will consume lithium ions during the formation process, and the SEI film is not stable. It will constantly break during the battery cycle, expose the new negative surface and react with the electrolyte to form a new SEI film.
This will cause continuous loss of lithium ions and electrolytes, resulting in a decrease in battery capacity. In addition, the SEI membrane lithium ion diffusion channel may be blocked, which will also lead to the reduction of battery capacity.
In the continuous process of battery cycle, the battery electrolyte will continue to decompose and volatilize, leading to the reduction of the total amount of electrolyte, can not fully infiltrate anode and cathode materials. And the charge and discharge reaction is not complete, resulting in the decline of the actual use capacity.
In addition, if there is a certain amount of water in the electrolyte, the water will react with LiFP6 to produce LiF and HF, which in turn destroys the SEI film and generates more LiF, resulting in the deposition of LiF and continuous consumption of active lithium ions, resulting in the reduction of battery cycle life.
Diaphragm blocking or damage
In the process of lithium ion battery cycle, diaphragm gradually drying up failure is also a reason for capacity decline. Due to the drying of the isolation film, the ohmic resistance of the battery increases, leading to the blockage of the charge and discharge channel, and incomplete charge and discharge. The capacity of the battery can not return to the initial state, greatly reducing the capacity and service life of the battery.
Shedding of cathode and anode materials
The active substances of the anode and cathode are fixed to the substrate by means of a binder. In the long-term use process, due to the failure of the binder and the battery by mechanical vibration and other reasons, the positive and negative active substances constantly fall off into the electrolyte solution, which leads to the continuous reduction of active substances that can participate in the electrochemical reaction, the battery cycle life continues to decline.
The type of material
when you choose the material with poor circulation performance, even if the process is reasonable and perfect, the battery cycle is bound to be unable to guarantee; But when you choose a good material, even if there are a few problems later on, the cycling performance may not be too bad.
From the material point of view, the cycle performance of a full battery is determined by the worse of the two, the cycle performance of the positive electrode and the electrolyte matching, or the battery cycle performance of the negative electrode and the electrolyte matching.
Material cycle performance is poorer, on the one hand, is likely to be in the process of crystal structure change too fast to continue to complete the intercalated-li lithium; on the other hand, may be due to the active substance and its corresponding electrolyte can’t generate dense and uniform SEI film, active material and electrolyte premature side effects, and make the electrolyte excessive consumption and thus affect the battery cycle.
In cell design, if one pole is confirmed to choose the material with poor cycling performance, the other pole does not need to choose the material with better cycling performance.
Positive and negative electrode compaction
If the positive and negative electrode compaction is too high, although the energy density of the cell can be improved, the cyclic performance of the material will also be reduced to a certain extent. Theoretically, the greater the compaction, the greater the damage to the material structure, and the material structure is the basis to ensure the battery cycle.
In addition, the cell with higher positive and negative electrode compaction is difficult to guarantee higher liquid retention, which is the basis for the cell to complete the normal cycle or more cycles.
Coating film density
Considering the influence of film density on battery cyclewith a single variable is almost an impossible task. Inconsistent film densities result in either capacity differences or in the number of winding or lamination layers of the cell.
For cells of the same type and capacity and material, reducing the film density is equivalent to increasing the number of winding or laminating layers by one or more layers, and the corresponding increased diaphragm can absorb more electrolyte to ensure battery cycle.
Considering more thin film density can increase rate performance of the batteries, the pole piece and naked batteries baked in addition to the water will be easier, too. Of course, when the density of the film is too thin, coating of error may be more difficult to control, larger particles active in the substances may also cause negative effect to coating, rolling.
More layer means more foil and diaphragm, that in turn means higher costs and lower energy density. Therefore, the evaluation also needs to be balanced.
Objective conditions of the test
External factors such as charging and discharging ratio, cut-off voltage, charging cut-off current, overcharge and discharge in the test, temperature of the test room, sudden interruption in the test process, contact resistance between the test point and the cell will more or less affect the battery cycle performance of the test results.
In addition, different materials have different degrees of sensitivity to the objective factors mentioned above. Unified test standards and understanding of the characteristics of common and important materials should be enough for daily work.
As the barrel principle, many factors affecting the performance of the battery cycle, the final decisive factor, is the shortest board in many factors. At the same time, there are also interactive effects among these influencing factors. In the same material and manufacturing capacity, the higher the battery cycle, often means the lower the energy density, find just to meet the needs of customers, as far as possible to ensure the consistency of the cell, is the most important task for the professional lithium-ion battery company.
Test method for lithium ion battery cycle times
At present, the test methods used by lithium-ion battery manufacturers to evaluate the lithium ion battery cycle are generally through continuous charging and discharging cycle tests, which require a long test cycle. Lithium ion battery standards generally stipulate the cycle life requirements and testing methods. The test requirements for lithium ion battery cycle life in China are shown in the table.
The existing test standard for lithium ion battery cycle in China
Battery cycle test data analysis
In general, battery cycle test generates a lot of data, which can obtain a lot of information. What can we do to analyze and process these data?
The charge-discharge curve shows the voltage, current, and capacity of a battery changing with time during charging and discharging. The main data recorded in the charge-discharge test is the time evolution of current and voltage, and the subsequent analysis is basically a process of extracting data from the charge-discharge curve for re-analysis.
Typical charge and discharge curves are shown in the figure. As the battery cycle progresses, the battery capacity attenuates and the charge and discharge curves change.
Charge and discharge coulomb efficiency
Coulomb efficiency, also known as charging efficiency CE, refers to the ratio of the Discharge capacity of the battery to the charging capacity in the same cycle, i.e. CE= Discharge capacity/ Charge capacity*100%. The power input from charging is often not used to convert the active substance into the charging state, but partly consumed, so the Coulomb efficiency is usually less than 100%. Coulomb efficiency is an important battery parameter, which is closely related to the loss of reactive lithium.
dQ /dV curve
Charge and discharge the lithium ion battery, and record the charge and discharge parameters, especially the electric quantity and voltage data. The physical meaning of the dQ/dV curve is also very simple, that is, the capacity contained in the material per unit voltage range.
dQ/dV curve mainly show the cathode active material in the phase transition in the process of charging and discharging, according to the data of the battery, we can find out the different characteristics of the dV/dQ curve peak corresponds to phase change, and then according to the variation of loop dQ/dV curve trend, we are able to qualitative infer the cause of lithium ion battery irreversible capacity loss, It provides reference for the design of lithium ion battery.
Constant voltage charging current and time
Lithium ion batteries usually discharge at different current during use and often cannot experience a complete and stable discharge process. This incomplete discharge process will affect the subsequent charging process.
The battery charging process is generally in cross-current-constant voltage mode(CC-CV), consisting of two continuous processes: CC charging and CV charging, constant current until the battery voltage reaches the nominal maximum voltage. The battery then enters a constant-voltage charging mode, and the charging voltage remains constant until the charging current gradually decreases to the cutoff current.
The dynamic characteristics of CV phase can well reflect the health information of the battery whether the battery is fully discharged or not. In addition, charging data in the CV phase can be fully monitored by BMS. Therefore, the kinetics of CV charging can be used to study the aging law of batteries.
Since the rate of current change during constant voltage charging is closely related to the relevant time constant, the current time constant of CV charging cycle can be used to study the battery aging state.