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It can be said that the lifespan of lithium-ion batteries is a trilogy of external stress applications, changes in the microstructure of the battery cell, and the performance of the external characteristics of the battery cell. And an external characteristic may correspond to several microscopic changes, such as the addition of internal resistance, which is affected by both the growth of SEI film and the decrease in the total amount of lithium ions in the system; And a microscopic change may also bring about several changes in external characteristics, such as electrolyte decomposition, which may lead to an increase in resistance and a decrease in open circuit voltage. There is a similar relationship between the use of external stress and changes in microstructure. If the charging cutoff voltage is too high, it may cause lithium plating on the anode and may also cause changes in the lattice structure of the cathode active material; Anodic lithium plating may be the result of high charging voltage or the impact of low charging temperature. In an article, the combination of micro and macro, the correspondence between external stress and microstructure, the correspondence between micro changes and battery external characteristics, and the comprehensive presentation of factors affecting battery cell life on a single page are believed to form a comprehensive understanding of the issue of lithium-ion battery life. Of course, some of the narratives in this article only represent the current viewpoints in some studies. The complex electrochemical processes inside lithium-ion batteries may not be conclusive in many explanations, but rather inferences based on experimental phenomena. For the time being, we will integrate non contradictory viewpoints together.

The lifespan of lithium-ion batteries can be considered using two concepts: calendar lifespan and cycle lifespan. The cycle life refers to the time it takes for the battery to reach its end of life during the operating cycle or conventional cycle process; Calendar lifespan refers to the time required for a battery to reach its end of life in an open circuit state at a certain reference temperature, that is, the lifespan of the battery in standby state. Both belong to conventional applications. Generally, the lifespan of power type batteries is important to consider changes in internal resistance, while energy type batteries are important to consider capacity degradation.

The third scenario of the lifespan of lithium-ion batteries is the short-term rapid decline in battery life caused by improper operation, accidents, and abuse. The following content is mixed together for discussion.

1. The causes of aging described from a microphysical perspective

1.1 Anode

The reactions related to battery life that occur on the graphite anode side mainly include the formation, development, damage, and repair process of SEI film, as well as lithium elemental electroplating reactions.

1) The two sides of the SEI membrane block side reactions and consume lithium ions

At present, commercial lithium-ion batteries, regardless of ternary, lithium iron phosphate, lithium manganese oxide and other positive electrode materials, are equipped with graphite materials as the negative electrode. The graphite negative electrode is not stable and compatible with the electrolyte. At the beginning of contact, a solid passivation film called the solid electrolyte interface (SEI film) is formed, which isolates the electrolyte from the graphite. At the same time, the gaps on the film allow for the entry and exit of lithium ions. Meanwhile, in terms of electronic conductivity, it is also an insulator that does not allow electrons to pass through. It can be said that such properties are very ideal. Therefore, the SEI membrane is an important structure for the stable electrochemical performance of lithium-ion batteries.

The formation of SEI film is crucial during the initial charge and discharge process of the battery, and it still exhibits a faster growth rate in the following cycles compared to other cycles in its lifespan. SEI is formed on the surface of graphite by lithium ions and solvents (EC/DMC), trace amounts of water, HF, etc., forming a porous layer containing polymers and inorganic salts. The growth of SEI film continues to occur within several cycles after the first charge and discharge. The growth of SEI is influenced by several factors such as the amount/composition of electrolyte, charging voltage/current, and temperature. Therefore, every battery manufacturer carefully designs the charging and discharging parameters to form a uniform and dense SEI layer. The position of the SEI membrane is shown in the following figure.

SEI is not static during the calendar and cycle life of batteries. Without any improper use, SEI will gradually grow, gradually increase thickness, and there will be a certain proportion of damage. At the damaged location, the electrolyte and graphite come into direct contact again, and a new SEI layer is reconstructed.

SEI film plays an important role in the aging process of batteries. On the one hand, high-quality SEI film is a necessary condition for batteries to have a long cycle life; On the other hand, during the formation and repair of SEI, lithium ions are used as raw materials, which inevitably consumes the amount of lithium ions in the system; During the use of SEI, some of the pores collapse and deform due to stress, making the ion pathway no longer smooth. These microscopic changes cause the battery to exhibit phenomena such as increased internal resistance, decreased capacity, and decreased charging capacity, leading to a decline in its lifespan.

2) Anodic lithium plating

Lithium plating is not an inevitable phenomenon in the working process of lithium-ion batteries, and current research is not particularly thorough. However, the mainstream view is that the basic reason for the formation of anode lithium plating is the accumulation of a large number of lithium ions on the anode, which cannot smoothly embed into the graphite layered structure, causing the ions to deposit after receiving electrons on the electrode surface, forming lithium elemental accumulation, also known as dendrite growth. Dendritic growth is considered an important contributing factor to thermal runaway. On the one hand, if the amount of dendrite growth accumulates is large enough, it may puncture the diaphragm, causing a short circuit between the positive and negative poles, directly leading to uncontrolled heating. On the other hand, lithium is a very reactive metal that can undergo violent reactions at lower temperatures. When the battery experiences self heating and accumulates too much heat, causing a significant temperature rise, lithium may undergo violent reactions, which is considered a major cause of uncontrolled heating.

The operation that may form a large amount of lithium ion anode surface aggregation is considered an important issue that is prone to occur during the charging process, specifically in the following three situations: low-temperature charging, overvoltage charging, and overcurrent charging.

1.2 Cathode

The most important source of ions in lithium-ion batteries, apart from a small portion present in the initial electrolyte, is the cathode material. Lithium ions are stored in the lattice structure of materials and can be detached or embedded during charging and discharging processes. Under normal application conditions, there are two important forms of aging of cathode materials over time. One is the collapse of the lattice structure, which leads to a decrease in the total amount of active substances caused by the detachment of local materials from the overall structure; The second is the consumption of side reactions between electrolytes and cathode materials. As a result, the number of lithium ions that can be released and the number of vacancies for storing lithium ions decrease accordingly. If improper operation and abuse occur, the cathode material will experience large-scale crystal rupture due to various stress applications, resulting in a large amount of loss of active substances in a short period of time.

The cathodic damage at the microscopic level mentioned above is directly manifested as a decrease in capacity in the external characteristics of the battery; Due to local changes in the lattice structure, the pathway for ions to enter and exit is blocked, at least prolonging the diffusion path of ions in the solid structure, resulting in an increase in the internal resistance of the battery.

1.3 Electrolytes

Electrolytes and electrode materials are not perfectly compatible, and electrolyte and anode graphite need to be protected by SEI passivation film to reduce reaction probability; There are always trace side reactions between the cathode material and it, and as the temperature increases, the reaction tends to intensify. These side reactions will consume electrolytes, reduce conductive ions, and produce side reaction gases.

If the applied voltage is too high, exceeding the voltage window that the electrolyte can withstand, it will intensify the decomposition process of the electrolyte. The decomposition products also contain combustible gases, which can damage the conductivity of the electrolyte.

Electrolyte, as a pathway for the movement of lithium ions between the positive and negative electrodes inside a battery, the viscosity of the electrolyte and the density of lithium ions in the electrolyte directly affect the rate of charge transfer and the degree of obstruction to the ion movement rate. This obstacle manifests externally as the resistance of lithium-ion batteries.

2. Aging reasons described from external characteristics

The external factors that affect battery performance and lifespan mainly include temperature, voltage, current, and depth of charge and discharge.

1) Temperature

Temperature can almost be said to be the most important environmental factor affecting lithium-ion batteries. Lithium ion batteries are electrochemical power sources, and their application process relies entirely on the ability of electrochemical reactions, and temperature determines the activity level of most chemical reaction processes. Scientists have already provided quantitative descriptions of this. The Arrhenius equation provides an equation for temperature and chemical reaction rate, suggesting an exponential relationship between chemical reaction rate and temperature.

K is the rate constant, R is the molar gas constant, T is the thermodynamic temperature, Ea is the apparent activation energy, and A is the pre exponential factor (also known as the frequency factor).

At high temperatures, the activity of electrochemical reactions increases, and the battery exhibits better performance characteristics, such as enhanced discharge capacity and reduced internal resistance. Looking inward, the high temperature intensifies the internal side reactions of the battery, and the reaction between the electrolyte and the positive and negative electrode materials consumes electrode materials and lithium ions. The loss of this electrode material and electrolyte is permanent, causing a decrease in capacity and an increase in internal resistance.

Low temperature, in contrast to high temperature, reduces the electrochemical activity inside the battery system, reduces the level of side reactions, and also reduces the conductivity of active substances. If high current discharge occurs at low temperatures, the electrode material may fail to meet the load requirements and cause structural damage to the cathode material; Low temperature and high current charging may lead to issues with anode lithium plating and dendrite growth.

2) Voltage

Charging overvoltage is an important reason for abnormal attenuation of lithium-ion batteries. The following figure shows the relationship curve between the charging cutoff voltage and cycle life of a ternary lithium battery cell. It can be seen that the voltage difference of only 0.15V has an impact on the battery life.

As mentioned in the previous section on electrolytes, electrolytes have their own determined voltage window. Beyond the limit of this window, the proportion of electrolyte decomposition caused by voltage will greatly increase. And overvoltage charging, for the battery anode, is about trying to stuff too many lithium ions into its limited small room, which may be crowded or the road may get stuck; For the cathode, excessive lithium ions are driven away from the anode lattice structure, affecting structural stability and causing local collapse. Excessive lithium ions on the anode that cannot be embedded will deposit on the electrode surface, forming a dangerous lithium plating problem.

When the discharge current of the battery is too high, a large amount of lithium ions need to pass through the SEI film for a short period of time, which may cause large-scale damage to the film structure, leading to the detachment of the old film layer, which is a large-scale repair process and also a process that consumes a large amount of lithium ions. A large number of lithium ions come to the cathode, and if they want to squeeze into the lattice in a short period of time and the diffusion speed cannot keep up, it will cause channel blockage and the lattice structure will be impacted.

The thermal effect caused by high discharge current will further expand the impact range of high current discharge. The heat dissipation capacity cannot keep up, the temperature rise of the battery is too high, electrolyte decomposition, and membrane melting may all occur consecutively. Therefore, the impact of current on the lifespan is not only due to the impact of lithium ions on the internal microstructure, but also through the use of heat. Therefore, the cooling capacity of battery systems varies, and the same high current has a very different impact on their lifespan.

If the charging current is too high, charging is a process in which lithium ions detach from the positive electrode lattice and embed into the graphite layered structure. Rapid detachment from the lattice will have an impact on the stability of the structure. If one wants to quickly embed into the anode, the diffusion rate cannot keep up, and lithium ions will deposit on the surface of the anode.

The loss of lithium ions during high current charging and discharging is reflected in the lifespan parameters, which are the attenuation of available capacity and the increase of charging and discharging resistance. The damage to the electrode structure leads to capacity loss.

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