The aging of energy storage lithium batteries is a long-term gradual process, and the health of the battery is affected by various factors such as temperature, current rate, and cut-off voltage. At present, some achievements have been made in the research and modeling analysis of battery health status. Related research includes battery degradation mechanism and aging factor analysis, battery health management, battery status monitoring and estimation, battery life prediction, etc.

However, there is still a lack of a relatively complete summary and review of lithium battery state of health assessment. This paper systematically introduces the research status and progress of battery state of health from five aspects: definition, influencing factors, evaluation model, research difficulties and research significance of battery state of health.

1. Definition of energy storage battery health status

The battery SOH characterizes the ability of the current battery to store electrical energy relative to the new battery, and represents the state of the battery from the beginning of its life to the end of its life in the form of a percentage. There are many performance indicators of batteries. There are many definitions of SOH at home and abroad, but there is a lack of unity in concept. At present, the definition of SOH is mainly reflected in several aspects such as capacity, electricity, internal resistance, cycle times and peak power.

1 Capacity Definition SOH

There are most literatures on the definition of SOH by battery capacity decay, and the definition of SOH is given as follows:

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In the formula: Caged is the current capacity of the battery; Crated is the rated capacity of the battery.

2 Electricity definition SOH

The definition of SOH for electricity consumption is similar to the definition of capacity, because the rated capacity of the battery has actual effective capacity and maximum capacity, and the actual capacity of the battery is somewhat different from the nominal rated capacity, so some literatures define SOH from the perspective of battery discharge capacity.

In the formula: 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 it is also the reason for the further aging of the battery. Many literatures use the internal resistance to define the SOH.

In the formula: 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 using battery performance indicators such as capacity and internal resistance to define the SOH, there are also literatures that define the SOH of the battery by the number of remaining cycles of the battery.

 

In the formula: 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 types of batteries are relatively common in the literature. The definition of capacity and electricity is highly operable, but the capacity is the external performance of the battery, while the operability of the definition of internal resistance and remaining times is not strong. The internal resistance is related to SOC and temperature, and it is not easy to measure. The number of remaining cycles and the total number of cycles are not easy to measure. cannot be predicted accurately.

2. Factors affecting the health status of lithium batteries

In recent years, many domestic and foreign literatures have studied the aging mechanism and law of lithium batteries. It is generally believed that lithium ion deposition, SEI film thickening and loss of active materials are the main reasons for battery aging and capacity decay. Abuse of lithium batteries will accelerate battery aging, and normal charging and discharging of batteries will also affect battery health and accelerate battery aging.

1 Influence of temperature on battery SOH

Temperature is generally considered to be the main factor affecting the health of the battery. Temperature has a dual impact on the performance of the battery. On the one hand, high temperature will speed up the chemical reaction inside the battery and improve the efficiency and performance of the battery. At the same time, high temperature will also accelerate some irreversible chemical reactions. The reaction occurs, resulting in a decrease in the active material of the battery, causing aging and capacity decay of the battery. Experimental data show that high temperature will accelerate the growth of the SEI film of the battery electrode, and the difficulty of lithium ions penetrating the SEI film will increase, which is equivalent to an increase in the internal resistance of the battery.

2 Influence of charge and discharge current rate on battery SOH

The charge and discharge rate will affect the life of the battery. The Sony 18650 battery was tested for 300 cycles at three different discharge rates. At the same time, high-rate discharge will generate more heat inside the battery, which will accelerate the aging of the battery. It is observed under the electron microscope that the SEI film on the electrode surface of the high-rate discharge battery is thicker than that of the low-rate discharge.

3 Influence of depth of discharge 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 decay and aging analysis of the battery is carried out based on the total transfer energy. Gao Fei et al. analyzed the relationship between the accumulative transfer energy of the battery and the capacity decay of the battery through the cycle tests of different discharge depths of lithium batteries, and concluded that before the battery capacity decays to 85%, the accumulative transfer energy of the battery is in the deep charge and deep discharge and the battery capacity decay. The two modes of shallow charging and shallow discharging are basically the same. When the battery capacity decays to 85%~75%, the accumulative transfer energy and energy efficiency of the battery are better than the shallow charging and shallow discharging mode.

4 Influence of cycle interval on battery SOH

The battery charge-discharge cycle interval will also affect the battery aging process. The internal resistance of the charge-discharge battery is different for different cycle intervals. Therefore, the battery heat and reaction during the cycle are slightly different, which will affect the battery‘s health and aging in the long run. Therefore, some experts suggest that the battery SOC range is 20%~80%, which is beneficial 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, and inappropriate upper and lower voltage limits will affect the battery. The lower the discharge cut-off voltage, the greater the internal resistance of the battery, resulting in internal heating of the battery, increased side reactions, reduction of battery active materials and collapse of the negative graphite sheet, accelerated aging and capacity decay of the battery. Excessive charging cut-off voltage causes the internal resistance of the battery to increase, the internal heat of the battery increases, and the overcharge causes the “lithium precipitation” phenomenon of the negative electrode and the corresponding increase in side reactions, which affects the capacity and aging of the battery.

In summary, the operating temperature, charge-discharge rate, depth of discharge, cycle interval and charge-discharge cut-off voltage of the battery will all have an impact on the battery‘s health and life. At present, the research on the influencing factors of battery health status is in the qualitative research stage. Quantitative analysis of these influencing factors on battery aging and the coupling relationship between these factors are the difficulties of research and the research hotspot of battery health and life in the future.