Research and analysis on battery aging manifestations and factors
The rapid development of new energy has helped to alleviate the energy crisis and reduce environmental pollution, and also promoted the development of key technologies such as batteries, motors, and electronic controls.
Although the capacity and energy of lithium-ion power batteries have made great progress through scientific research, which has greatly increased the driving range of electric vehicles, there are still challenges. Battery aging affects the battery performance to a large extent, so research on battery aging is still necessary.
What is battery aging?
The battery aging is manifested in the continuous reduction of the available capacity and the continuous increase of the internal impedance with the increase of time, which affects the cruising range of the electric vehicle and the reduction of the output power. The battery aging is related to various factors such as SOC, temperature, charge and discharge rate, and DOD.
Battery aging is often associated with side reactions that occur in the battery, which tend to progress in an irreversible manner toward a more stable, lower-energy state. The battery aging causes the decomposition of the electrolyte, the loss of the cathode and anode active materials, and the loss of the recyclable lithium, which ultimately makes the battery unable to meet the actual needs.
The manifestations of battery aging
The manifestations of battery aging in the following aspects:
● Anode-electrolyte interface aging:
The interface between the electrolyte and the anode is a critical region where side reactions occur. Although a relatively stable SEI film is formed during the first charging process of the battery on the negative side, the negative stress of the battery changes greatly during the charging and discharging process, which will cause the rupture and reconstruction of the SEI.
Both of these phenomena lead to continuous thickening of the SEI layer, resulting in irreversible loss of cyclable lithium and electrolyte. In addition to the aging caused by SEI reconstruction, another important factor is the formation of dendrites, which can cause short-circuits in the electrodes and bring safety hazards.
● Cathode-electrolyte interface aging:
Aging on the cathode side is also related to surface film formation reactions and loss of active material activity, usually caused by side reactions between the cathode material and the electrolyte. In addition, the transition metal elements in the cathode material will dissolve out during the charging and discharging process of the battery, resulting in irreversible changes in the crystal structure, resulting in the loss of active materials, and the migration of transition metal ions to the surface of the anode SEI increases the thickness of the SEI film.
Generally battery aging in cathode is lower than battery aging in anode, but battery aging is accelerated when the cathode is operated at high voltage (about 4.5 V) in high voltage battery.
● Calendar and cyclic aging:
Battery aging is usually divided into calendar aging and cyclic aging. Calendar aging refers to the aging of the battery under specific storage conditions, that is, when the battery is not working. The main factors affecting the battery calendar aging include storage temperature and battery SOC.
Generally, the higher the temperature and the higher the SOC, the more serious the battery aging, and the calendar aging is related to the thermodynamic instability of the battery system. Cyclic aging is related to battery usage, including charge-discharge depth, charge-discharge rate, etc. Cyclic aging is not only related to thermodynamic stability but also to battery system dynamics.
Models commonly used to study battery aging
The cross-interaction of temperature, SOC, and charge-discharge rate makes it extremely challenging to create a suitable lifetime prediction model. The current models used to predict battery aging mainly include electrochemical-mechanical models, equivalent circuit models and semi-empirical models related to experimental data.
● Electrochemical-mechanical model:
The electrochemical-mechanical model mainly uses physical methods to describe battery aging phenomena. The model focuses on the formation of SEI and its evolution on the surface of the graphite anode. Electrochemical-mechanical models, detailing the underlying chemical processes, give good results in describing and predicting microscopic phenomena, exemplified by anode aging. However, it is still a big challenge to model the global battery aging at the macro level.
● Equivalent circuit model:
The equivalent circuit model is mainly based on electrochemical impedance spectroscopy (EIS) technology to study battery aging. EIS is considered a non-destructive characterization technique that can be used to understand electrochemical processes in batteries and how these chemical processes change over time.
The EIS can apply an AC sinusoidal current or voltage over a wide frequency range (from mHz to kHz or higher), measure the voltage or current response, respectively, and evaluate the equivalent battery impedance. The use of equivalent circuits helps to describe battery aging more generally and efficiently, but the disadvantage of this approach is that it requires a large amount of experimental data, making the model evaluation process time-consuming and labor-intensive.
● Semi-empirical model:
The semi-empirical model is the most widely used in practice. The semi-empirical model treats the battery as a black box, with calendar burn-in tests at different combinations of temperature and SOC. For the cycle battery aging test, different charge/discharge rates were used to evaluate the battery cycle life, and the internal resistance of the battery was tested according to the pulse resistance.
The semi-empirical model uses the trend change of the data obtained by the experiment to infer the change of battery capacity and internal resistance with time to predict the battery aging. All in all, semi-empirical models do not need to understand the physical and chemical changes in the battery in detail, but make life predictions for the battery as a whole. In this case, a large amount of basic data is also required.
Construction of battery calendar and cycle aging models
Lithium-ion battery aging can be divided into calendar aging and cycle aging. Many researchers have studied the battery’s calendar aging model with the NMC lithium-ion battery with a nominal capacity of 63 Ah as the research object.
Models show battery life under different operating conditions. The cycling performance of the battery was better at 10°C and 25°C, and as the temperature increased, the aging accelerated and the number of cycles decreased. SOC also has an impact on the battery cycle performance. The closer the battery is to 50% SOC, the less impact on the battery life is when the battery is shallowly charged and shallowly discharged. Similarly, the depth of discharge, charge-discharge rate and battery self discharge also have an impact on the battery.
The lithium-ion battery aging phenomenon depends on many factors, including temperature, initial state of charge (SOC), and charge-discharge rate C. High temperature, high SOC, and high charge-discharge rate will aggravate the lithium-ion power battery aging. Comparing the old and new batteries, the SOC and open circuit voltage (OCV) of the old battery decreased faster than that of the new battery, while the OCV hardly changed with the aging of the battery, and the resistance and temperature of the old battery were higher than those of the new battery.