The aging of lithium batteries is a long-term gradual process. The health of batteries is affected by many factors such as temperature, current rate, and cut-off voltage. At present, research and modeling analysis of battery health status have achieved certain results. Relevant research includes battery degradation mechanism and aging factor analysis, battery health management, battery condition monitoring and estimation, and battery life prediction.
However, there is still a lack of comprehensive induction and review of lithium battery health assessment. This paper systematically introduces the research status and progress of battery health status from five aspects: definition of battery health, influencing factors, evaluation model, research difficulties and research significance.
Ⅰ. the battery health status definition
The battery SOH characterizes the ability of the current battery to store electrical energy relative to the new battery, and represents, in percentage, the state of the battery from the beginning of its life to the end of its life, used to quantitatively describe the performance state of the current battery. There are many performance indicators of batteries. There are many definitions of SOH at home and abroad, and there is a lack of uniformity in concept. At present, the definition of SOH is mainly reflected in several aspects such as capacity, power, internal resistance, cycle number and peak power.
1. Capacity definition SOH
The literature on the definition of SOH using battery capacity attenuation is the most, and the SOH is given as follows:
Caged is the current capacity of the battery; Crated is the rated capacity of the battery.
2, the power definition SOH
The power consumption definition SOH is similar to the capacity definition because the rated capacity of the battery has the actual effective capacity and the maximum capacity. The actual capacity of the battery is somewhat different from the nominal rated capacity. Therefore, there is a literature to define the SOH from the perspective of the battery discharge capacity.
Qaged-max is the maximum discharge capacity of the current battery; Qnew-max is the maximum discharge capacity of the new battery.
3. Internal resistance defines SOH
The increase of the internal resistance of the battery is an important manifestation of the aging of the battery, and is also the cause of further aging of the battery. Many documents use internal resistance to define SOH.
REOL is the internal resistance at the end of the battery life; Rc is the internal resistance of the current battery; Rnew is the internal resistance of the new battery.
4, the number of remaining cycles defines SOH
In addition to defining SOH using battery performance indicators such as capacity and internal resistance, there are also literatures that define the SOH of the battery with the number of cycles remaining in the battery.
Cntremain is the number of remaining cycles of the battery; Cnttota is the total number of cycles of the battery.
The SOH definitions of the above four batteries are more common in the literature. The capacity and power definitions are operability, but the capacity is the external performance of the battery, while the internal resistance and the remaining number define the operability is not strong, the internal resistance is related to SOC, temperature, difficult to measure, the number of remaining cycles and the total number of cycles Unable to predict accurately.
Ⅱ. the factors affecting the health status of lithium batteries
In recent years, there have been many literatures at home and abroad to study the aging mechanism and laws of lithium batteries. It is generally believed that lithium ion deposition, SEI film thickening and active material loss are the main reasons for battery aging and capacity decay. The abuse of lithium batteries will accelerate the aging of the battery, and the normal charging and discharging of the battery will also affect the health of the battery and accelerate the aging of the battery.
1. Effect of temperature on battery SOH
Temperature is generally considered to be the main factor affecting the health of the battery. Temperature has a double impact on the performance of the battery. On the one hand, high temperature will accelerate the chemical reaction speed inside the battery, improve the efficiency and performance of the battery, and the high temperature will accelerate some irreversible chemistry. The reaction occurs, causing a decrease in the active material of the battery, causing aging and capacity decay of the battery. There are experimental data showing that high temperature will accelerate the growth of the SEI film of the battery electrode, and the difficulty of lithium ion penetrating the SEI film is increased, which is equivalent to the increase of the internal resistance of the battery.
2. Effect of charge and discharge current rate on battery SOH
The charge-discharge rate will affect the battery life. The battery is tested for 300 cycles at three different discharge rates. The battery capacity is reduced by 9.5%, 13.2%, and 16.9%, respectively. The internal resistance of the battery is increased by 12.4% and 18.3%, respectively. 27.7%, at the same time, high-rate discharge will generate more heat inside the battery, accelerate battery aging, and the SEI film on the surface of the electrode with high-rate battery discharge is observed to be thicker than the low-rate discharge under the electron microscope.
3. Influence of discharge depth on battery SOH
The depth of charge and discharge of the battery has an impact on the health and aging of the battery. It is believed that the battery has accumulated total transfer energy, and the capacity attenuation and aging analysis of the battery are performed based on the total transfer energy. Gao Fei et al. analyzed the relationship between the cumulative transfer energy of the battery and the battery capacity attenuation by cycling the different discharge depths of the lithium battery. It is concluded that before the battery capacity decays to 85%, the accumulated energy of the battery is deep deep and deep. The shallow charge and shallow release are basically the same in the two modes. When the battery capacity is attenuated to 85%~75%, the energy transfer and energy efficiency of the battery are better than the shallow charge and deep release mode.
4. Influence of cycle interval on battery SOH
The battery charge and discharge cycle interval will also affect the battery aging process. The internal resistance of the corresponding charge and discharge battery is different in the cycle interval. Therefore, the battery heat and reaction are slightly different during the cycle, which will affect the health and aging of the battery for a long time. Therefore, some experts recommend that the battery SOC range from 20% to 80%, which is beneficial to battery health and cycle life.
5. Effect of charge and discharge cutoff voltage on battery SOH
Overcharging and overdischarging of the battery can have an impact on battery health. Inappropriate voltage limits and lower voltage limits have an impact on the battery. The lower the discharge cut-off voltage, the larger the internal resistance of the battery, causing internal heat generation of the battery, causing an increase in side reactions, a decrease in battery active material and collapse of the negative graphite sheet, and accelerated aging and capacity decay of the battery. Excessive charging cut-off voltage causes the internal resistance of the battery to increase, and the internal heat of the battery increases. Overcharging causes the anode to produce "lithiation" and the corresponding side reactions increase, which affects the capacity and aging of the battery.
In summary, the battery operating temperature, charge and discharge rate, depth of discharge, cycle interval and charge and discharge cutoff voltage will have an impact on the health and life of the battery. At present, the research on the influencing factors of battery health status is in the qualitative research stage. The quantitative analysis of battery aging and the coupling relationship of various factors are the difficult points of research, and also the hotspot of battery health and life research in the future.
Ⅲ. lithium battery health assessment model
The health status of the lithium battery cannot be obtained by direct measurement. The battery health status can be obtained through model evaluation. The aging and health of the battery are affected by many factors. At present, the health evaluation model of the lithium battery mainly has an electrochemical model and an equivalent circuit model. And experience models three.
1. Electrochemical model
The electrochemical model analyzes the health state changes during battery operation from the electrochemical reaction mechanism of the battery, and considers the influence of the aging factor of the battery on the internal and external state variables (temperature, current multiplier, cutoff voltage, etc.) of the battery. Lithium battery electrochemical model research includes complex electrochemical models based on SEI mechanism model, electrochemical first principle model, single factor and multi-factor integrated electrochemical model.
2. Equivalent circuit model
The equivalent circuit model is based on the electrical field of the battery, combined with a large number of state data analysis, the lithium battery is equivalent to a basic circuit model, and the battery model is used to evaluate the health of the battery. The basic equivalent circuit model of lithium battery includes Rint model, RC model and Thevenin model. PNGV model and GNL model are improved models based on Thevenin equivalent circuit model.
3. Empirical model
The empirical model obtains the change of battery performance state through a large number of experimental data analysis, fitting, trial and error, empirical formula and statistical processing, and summarizes the changes of the health state of the battery, mainly including the battery impedance empirical model and the battery capacity estimation empirical model.
Ⅳ. the difficulty of research on the health status of lithium batteries
The research on the health status and life of lithium batteries is getting more and more attention. However, the SOH research of batteries is still in the primary state, mainly for the following three reasons.
1. Long research period and strict control of experimental conditions
The cycle life of the lithium battery is long, and the battery aging test period is very long. During the test, the temperature, charge and discharge current, and charge and discharge cutoff voltage need to be strictly controlled, and the aging condition of the battery needs to be evaluated at regular intervals.
2, battery internal state monitoring and analysis difficulties
The SOH study of lithium batteries involves internal state variables of the battery, such as the internal temperature of the battery, electrolyte concentration and internal resistance in the electrochemical model. It is very difficult to accurately monitor the internal state of the battery. It is also necessary to quantitatively analyze these state variables. It is difficult to solve the problem of SOH research of the battery.
3. Coupling of various influencing factors
The temperature, charge and discharge rate and depth of discharge of the battery are all factors affecting the aging and life of the battery, and these factors are synergistic. The research of battery SOH requires decoupling various influencing factors. However, these factors are interrelated and the decoupling conditions are difficult to control. It is currently difficult to perform decoupling analysis.
V. Research significance of lithium battery health status
Battery SOH research is difficult and progress is slow, but SOH research has high value for battery use, maintenance and evaluation, and can provide basis and reference for planning, policy and industrial development, which is of great significance.
1, the significance of battery management
The battery management system estimates the state of charge of the battery and the remaining capacity is related to the capacity of the battery. If the battery management system can grasp the aging rule and health status of the battery, it will help it to manage the battery life cycle.
2, the significance of battery use and maintenance
SOH research is helpful to grasp the factors affecting battery aging and provide theoretical guidance for battery use and maintenance. For the use and maintenance of the battery, understanding the factors affecting the aging of the battery can reduce the use of the battery, such as high and low temperature and overcharge and over discharge; knowing the current health status of the battery can help determine the inherent danger and life of the battery. Provide a reference for battery maintenance and replacement.
3. Significance of battery economic evaluation
The accurate evaluation of SOH is of great significance to the economic evaluation of the battery. The application scenarios, usage methods and maintenance methods of lithium batteries cause differences in battery life, which makes the economic evaluation of battery use cost and economic benefit different. The establishment of battery aging model through SOH research provides data support for analyzing the economics of batteries, and will provide effective support for enterprise investment decision-making, government policy formulation and industrial development planning.
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