The aging of lithium ion batteries is a long-term gradual process, and the health status of batteries is affected by many factors, such as temperature, current ratio, cut-off voltage and so on. At present, some achievements have been made in the research and modeling decomposition of battery health status, including battery degradation mechanism and decomposition of aging factors, battery health management, battery condition monitoring and estimation, battery life prediction, etc.
However, there is still a lack of comprehensive induction and review on the health assessment of lithium ion batteries. In this paper, the research status and progress of battery health status are analyzed systematically from five aspects: the meaning, influencing factors, evaluation model, research difficulties and research significance of battery health status.
1. Battery Health Status
Battery SOH represents the current battery phase and the capacity of the new battery to store electric energy. It represents the state of the battery from the beginning to the end of its life in the form of percentage, and is used to quantitatively describe the performance state of the current battery. Battery performance indicators are many, SOH has a variety of meanings at home and abroad, the concept of the lack of unity, the current meaning of SOH is the current capacity, electricity, internal resistance, cycle times and peak power and other aspects.
1. Capacity Meaning SOH
Most literatures have used the meaning of battery capacity attenuation SOH, and the meaning of SOH is given as follows:
Where: Caged is the current battery capacity; Crated indicates the rated battery capacity.
2. Electric quantity meaning SOH
The meaning of SOH is similar to that of capacity. The rated capacity of a battery has the actual effective capacity and the maximum capacity. The actual capacity of a battery is different from the nominal rated capacity.
Where, Qaged- Max is the maximum discharge of the current battery; Qnew-max Indicates the maximum discharge capacity of a new battery.
3, internal resistance meaning SOH
The increase of internal resistance of batteries is an important manifestation of battery aging and the reason for further aging of batteries. Many literatures use the meaning of internal resistance SOH.
Where: 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. Remaining cycles Meaning SOH
In addition to battery performance indicators such as capacity and internal resistance (SOH), battery remaining cycles (SOH) are also used in the literature.
Where: Cntremain is the number of remaining battery cycles; Cnttota is the total number of battery cycles.
The SOH meanings of the above four kinds of batteries are common in the literature. The meaning of capacity and quantity of electricity is highly operable, but the meaning of capacity is the external performance of the battery, while the meaning of internal resistance and remaining times is not strong operable. The internal resistance is related to SOC and temperature, and is not easy to measure. The remaining cycles and total cycles cannot be accurately predicted.
2. Factors affecting the health status of lithium ion batteries
In recent years, there has been a lot of literature research on the aging mechanism and law of lithium ion batteries at home and abroad, and it is generally believed that lithium ion deposition, SEI film thickening and active material loss are the critical causes of battery aging and capacity attenuation. The abuse of lithium-ion batteries will accelerate battery aging, and the normal charging and discharging of batteries will also affect the health status of batteries and accelerate battery aging.
1. Influence of temperature on battery SOH
Temperature are generally considered to be the critical factors that affect the battery health status and temperature have a dual effect on the performance of the battery, on the one hand, the high temperature will speed up the battery's internal chemical reaction, improve the efficiency and performance of the battery, and high temperature can accelerate some irreversible chemical reactions take place, resulting in a decrease of battery of the active material, cause aging and damping capacity of the battery. According to experimental data, high temperature will accelerate the rise of SEI film of battery electrode, and the difficulty of lithium ion penetrating SEI film is increased, which is equivalent to the increase of internal resistance of battery.
2. Influence of charge-discharge current ratio on battery SOH
The charge and discharge rate will affect the life of the battery. The SONY 18650 battery was tested for 300 cycles with three different discharge rates. The battery capacity decreased by 9.5%, 13.2% and 16.9% respectively, and the battery internal resistance increased by 12.4%, 18.3% and 27.7% respectively. The SEI film on the electrode surface of high power battery discharge is thicker than that of low power battery discharge under electron microscope.
3. The influence of discharge depth on battery SOH
Battery charging and discharging depth has an impact on battery health and aging. It is believed that the battery has a cumulative total transfer energy, and the capacity attenuation and aging decomposition of the battery are carried out based on the total transfer energy. Goofy, through to the lithium ion battery cycle detection of different depth of discharge, the accumulation of decomposition battery energy transfer and the relationship between the battery capacity attenuation, it is concluded that battery capacity decay to 85% before the battery has the energy transfer in deep deep filling put with a pale imitation put this two modes are basically the same, when the battery capacity attenuation to 85% ~ 75%, In terms of energy transfer and energy efficiency, the deep charging and discharging mode is better than the shallow charging and discharging mode.
4. Influence of cycle interval on battery SOH
The battery charging and discharging cycle range also affects the battery aging process. Different charging and discharging cycle ranges have different internal resistances. Therefore, the battery heating and reaction are slightly different during the cycle, which will affect the health and aging of the battery in the long term. Therefore, some experts suggest that the battery SOC range should be 20%~80%, which is conducive to battery health and cycle life.
5. Influence of charge-discharge cut-off voltage on battery SOH
Overcharge and overdischarge of the battery will affect the health of the battery. Improper upper and lower voltage limits will affect the battery. The lower the discharge cut-off voltage is, the greater the internal resistance of the battery will be, which will cause the internal heating of the battery, and at the same time cause the addition of side reactions, the decrease of the active substance of the battery and the collapse of the negative graphite layer, which will accelerate the aging and capacity attenuation of the battery. High charging cut-off voltage causes the increase of battery internal resistance, and the increase of heat in the battery. Excessive charging causes the negative electrode to generate "lithium evolution" phenomenon and the corresponding side reaction, which affects the capacity and aging of the battery.
To sum up, battery operating temperature, charge and discharge rate, discharge depth, cycle interval and charge and discharge cut-off voltage will affect the health state and life initiation of the battery. At present, the research on the influencing factors of battery health state is in the qualitative research stage. The quantitative decomposition of these influencing factors to battery aging and the mutual coupling relationship of various factors are the difficulties of the research, and also the hot spot of battery health and life in the future.
Iii. Lithium ion battery health status assessment model
The health status of lithium-ion batteries can not be obtained by straight measurement, but can be obtained by model evaluation. Battery aging and health are affected by many factors. At present, the health status assessment models of lithium-ion batteries are mainly electrochemical model, equivalent circuit model and relevant empirical model.
1. Electrochemical model
Based on the electrochemical reaction mechanism of the battery, the electrochemical model decomposed the changes of the health state during the operation of the battery, and considered the influence of the aging factors of the battery on the internal and external state variables (temperature, current ratio, cut-off voltage, etc.). Research on electrochemical models of lithium-ion batteries includes complex electrochemical models based on SEI mechanism model, electrochemical first principle model, single factor and multi-factor synthesis electrochemical models, etc.
2. Equivalent circuit model
From the point of view of battery electrotechnics, the equivalent circuit model combined with a large number of state data decomposition, the lithium ion battery is equivalent to a basic circuit model, and the circuit model is used to evaluate the health status of the battery. The basic equivalent circuit models of lithium-ion batteries include Rint model, RC model and Thevenin model. PNGV model and GNL model are improved models based on Thevenin equivalent circuit model.
3. Relevant empirical models
Through decomposition, fitting, trial and error of a large number of experimental data, relevant empirical formulas and statistical solutions, the related empirical models can obtain the changes of battery performance state, and summarize the changing rules of battery health state. There are also relevant empirical models of battery impedance and battery capacity estimation.
Iv. Difficulties in the research on the health status of lithium ion batteries
The health status and life of lithium ion battery research is more and more attention, but the battery SOH research is still in the primary state, there are three important reasons.
1. Long research period and strict control of test conditions
Lithium ion battery cycle life cycle is long, battery aging test cycle is very long, in the detection process, temperature, charge and discharge current and charge and discharge cut-off voltage should be strictly controlled, and every other time according to the time of the battery aging condition to be evaluated.
2. Difficulty in monitoring and disassembling the internal status of the battery
SOH research of lithium-ion batteries involves internal state variables of batteries, such as internal temperature, electrolyte concentration and internal resistance of batteries in the electrochemical model. However, it is very difficult to accurately monitor the internal state of batteries, and these state variables need to be quantitatively decomposed, which makes the SOH research of batteries difficult to deal with.
3. Coupling of various influencing factors
Battery operating temperature, charge and discharge rate and discharge depth are all factors that affect battery aging and life, and these factors are common purposes, battery SOH research requires decoupling of various influencing factors. However, these factors are interrelated, and the decoupling condition is difficult to control, so it is difficult to carry out decoupling decomposition at present.
V. Research significance of health status of lithium ion battery
Battery SOH research is difficult and progress is slow, but SOH research is of great 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. Significance for battery management
The battery management system estimates the SOC and remaining power of a battery, which are related to the battery capacity. If the battery management system can learn the aging rule and health status of a battery, it can manage the battery in its full life cycle.
2. Significance for battery use and maintenance
SOH research is beneficial to grasp the factors affecting battery aging and supply theoretical command for battery use and maintenance. For the use and maintenance of batteries, understanding the factors affecting battery aging can reduce the damage of high and low temperature, overcharge and overdischarge of batteries. Knowing the current health status of the battery helps you determine the potential risks and service life of the battery and provides a reference for battery maintenance and replacement.
3. Significance for battery economic evaluation
The accurate evaluation of SOH is of great significance to the economic evaluation of batteries. The different use scenarios, use modes and maintenance means of lithium-ion batteries cause the difference of battery life, which makes the economic evaluation of battery use cost and economic benefit different. Through SOH research, the battery aging model is established to provide support for the decomposition of battery economic supply data, and provide effective assistance for the company's investment decision, government policy making and industrial development planning.
