Factors affecting the cycle life of lithium-ion batteries

With the development of science and technology and the maturity of technology, the application of lithium-ion batteries has become more and more extensive. Lithium-ion batteries have the advantages of high cell voltage, relatively light weight, and environmental friendliness, but after multiple cycles of charging and discharging cycles, performance such as battery capacity decreases. Under the same conditions, the faster the battery capacity decays, the lower the battery quality will be. The cycle performance of lithium-ion batteries is an important indicator of its quality. Many standards for lithium-ion batteries have cycle life items.

The charging and discharging cycle of lithium-ion batteries is a complex physical and chemical reaction process, and its cycle life influence factors are various. On the one hand, it is related to the characteristics of the battery itself, such as the design, manufacturing process, and material performance degradation; on the other hand, it is related to the external influence of the battery during use, such as the use environment and the charging and discharging system. The following will analyze the factors that affect the cycle life of lithium-ion batteries.

1 Introduction to the structure and principle of lithium-ion batteries
Lithium-ion batteries are mainly composed of anode and cathode materials, electrolyte, separator, current collector and battery casing. The anode and cathode materials are composed of two different lithium ion intercalation compounds. When charging, lithium ions are extracted from the positive electrode and inserted into the negative electrode through the separator through the electrolyte. The opposite is true when discharging. During the first charge and discharge of a lithium-ion battery, a passivation film can be formed on the interface between the negative electrode and the electrolyte. It acts as a diaphragm between the electrode and the electrolyte. It is an electronic insulator but an excellent conductor of lithium ions. Lithium ions can be freely inserted and extracted through the passivation layer. It has the characteristics of a solid electrolyte, so this passivation film Known as "solid electrolyte interface" (solid electrolyte interface), referred to as SEI membrane [1]. Lithium ion battery charge and discharge electrode reaction [2] is:

2 The influence of design and manufacturing process
In the battery design process, the choice of materials is the most important factor. Different materials have different performance characteristics, and there are also gaps in the performance of the batteries developed. The cycle performance of the positive and negative materials is good, and the cycle life of the battery will be long. In terms of ingredients, pay attention to the amount of positive and negative materials added. Generally speaking, the capacity of the negative electrode is generally required to be excessive relative to that of the positive electrode during the design and assembly process. If it is not excessive, lithium will precipitate out of the negative electrode during the charging process, forming lithium dendrites, which affects safety. There is too much excess of the negative electrode relative to the positive electrode, and the positive electrode may be excessively delithified, causing the structure to collapse.

The electrolyte is also a very important factor in the influence of the reversible capacity of the battery. The process of removing and inserting lithium ions from the electrode material is always the process of interaction with the electrolyte, and this interaction has an important influence on the change of the interface condition and internal structure of the electrode material [3]. The electrolyte will be lost during the process of interacting with the positive and negative materials. In addition, part of the electrolyte will be consumed when the battery is formed into SEI film and pre-charged. Therefore, the type of electrolyte and the amount of electrolyte also affect the battery life.

The manufacturing process of lithium-ion batteries mainly includes: positive and negative electrode ingredients, coating, sheeting, winding, shelling, liquid injection, sealing, forming, etc. In the battery production process, very strict requirements are imposed on each step of the process. Any process that is not well controlled may affect the battery cycle performance.

In the process of mixing positive and negative electrodes, attention should be paid to the amount of binder added, stirring speed, slurry concentration, temperature and humidity, and to ensure that the materials can be evenly dispersed.

In the coating process, under the premise of ensuring the high specific energy of the battery, reasonable control of the positive (negative) electrode coating amount, and appropriate reduction of the electrode thickness is beneficial to reduce the battery decay rate [4]. The coated pole piece needs to be further compacted with a roller press. A suitable positive electrode compaction density can increase the discharge capacity of the battery, reduce the internal resistance, reduce the polarization loss, and prolong the cycle life of the battery.

When winding, the wound cell should be tight and not loose [5]. The tighter the separator and the positive and negative electrodes are rolled, the smaller the internal resistance, but when the winding is too tight, it will be difficult to wet the pole piece and the separator, resulting in a smaller discharge capacity; too loosely rolled will cause excessive expansion of the pole piece during the charging and discharging process , Increase the internal resistance, reduce the capacity, and shorten the cycle life.

3 The influence of aging and decay of battery materials
The charge-discharge cycle of lithium-ion batteries is the process of lithium ions being extracted and moved back and forth between the positive and negative materials through the electrolyte. During the cycle of lithium-ion batteries, in addition to oxidation-reduction reactions at the positive and negative electrodes, there are also a large number of side reactions. If the side reactions of the lithium-ion battery can be reduced to a low level, so that the lithium ions can always smoothly go back and forth between the positive and negative materials through the electrolyte, the cycle life of the lithium-ion battery can be increased.

The movement of lithium ions from the positive electrode to the negative electrode must pass through the SEI film covering the carbon negative electrode. The quality of the SEI film directly affects the cycle life of the battery. Foreign scholars have studied the aging and decay of battery materials relatively early, especially the research on SEI membranes. The main research method is to analyze the stability and decay mechanism of battery materials through battery life experiment data combined with electrochemical characterization methods [6].

The stability of the SEI film has an important influence on the stability of the battery. The SEI film is unstable and easily precipitates lithium metal, which will cause the negative electrode active material to quickly decline. The lithium battery that forms a stable SEI film can be stored for more than 4 years under high temperature conditions [7]. D. Aurbach et al. [8] disassembled the lithium cobalt oxide battery after the cycle, and analyzed the positive and negative plates through SEM, XRD and other experiments, and attributed the capacity decline to the continuous consumption of Li+ in the negative electrode SEI film and the formation of positive LiCoO2 and HF. LiF interface film and other irreversible side reactions. P. Ramadas et al. [9] established a capacity decline model by describing the process of lithium ion loss caused by the continuous growth of the negative electrode SEI film during the charge-discharge cycle. S. Sankarasubramanian et al. [10] established a capacity decay model including solvent diffusion and SEI film growth mechanism, and concluded that the capacity decay has a linear relationship with SEI film thickness and battery aging time.

Huang Haijiang [11] conducted research on aluminum-plastic film lithium-ion batteries that have undergone 200 charge-discharge cycles, and the results show that the battery discharge capacity gradually decreases, and the internal resistance and thickness gradually increase. After disassembling batteries with different cycles, experimental observations showed that after 200 cycles, many cracks appeared on the surface of the positive electrode, and the average particle size decreased; the negative electrode showed that the SEI film became thicker, and lithium and lithium compounds were precipitated at the end of the cycle. The extraction and insertion of lithium ions will cause internal stress in the crystal lattice. Under this internal stress fatigue, the LiCoO2 will form cracks and eventually reduce the particle size.

J. Vetter et al. [12] conducted an in-depth analysis of the aging mechanism of battery internal materials with charge and discharge cycles, and reviewed the stability of the crystal structure of the electrode material, the side reaction of the active material and the electrolyte, and the degradation of the binder performance. Will affect the battery capacity and power performance, and summarize the causes and effects of the aging of the positive and negative electrodes. For the negative electrode material, in addition to the increase in impedance due to the deterioration of the contact between the anode components due to the formation and growth of the SEI film, the main factors are: the solvent is embedded in the C electrode and the gas is generated, which causes the C particles to rupture and the volume change during the cycle. The resulting contact between active material particles deteriorates, and the precipitated lithium metal reacts with the electrolyte to accelerate aging. The reasons and effects of aging and decay of cathode materials are shown in Figure 1.

Figure 1 Causes and effects of aging of cathode materials

Common electrolyte components are solvents (commonly used are alkyl carbonates, such as EC, DEC, DMC, etc.), lithium salts (commonly used are LiPF6, LiBF4, etc.) and various additives [13]. The process of removing and inserting lithium ions from the positive and negative materials always interacts with the electrolyte, and complex redox reactions will occur on the interface due to this effect, and even produce gas or solid products, which will deplete the electrolyte. The gas will increase the internal pressure of the battery and cause the battery to deform, and the solid product will form a passivation film on the surface of the electrode, which will increase the polarization of the battery and reduce the output voltage of the battery. These factors will have a negative impact on battery capacity and safety, and ultimately affect the cycle life of the battery [14]. Adding additives can effectively improve the cycle performance of lithium-ion batteries, such as adding trace additives anisole to the EC/DEC solvent system [15].

The nature of the positive and negative current collectors will also affect the capacity and cycle life of the battery. The commonly used current collector materials for the positive and negative electrodes of lithium-ion batteries are aluminum and copper, both of which are corrosive metal materials. The formation of a passive film after the current collector is corroded, poor adhesion, local corrosion (pitting) and general corrosion will increase the internal resistance of the battery, resulting in a loss of capacity and a decrease in discharge efficiency. The adhesion and corrosion resistance can be enhanced by pretreatment methods such as acid-alkali etching and conductive coating [16].

4 The impact of battery usage environment
The use environment of lithium-ion batteries is also very important for its cycle life. Among them, the ambient temperature is a very important factor. Too low or too high ambient temperature will affect the cycle life of lithium batteries.

Chen Jitao et al. [17] studied the charge and discharge performance of C/LiCoO2 lithium-ion batteries at -20 ℃. The results show that the discharge performance of the battery deteriorates at low temperatures, the 0.2 C discharge capacity is only 77% of the normal temperature capacity, and the 1 C discharge capacity is only 4% of the 0.2 C discharge capacity. At low temperatures, the constant voltage charging time increases, and the charging performance also deteriorates significantly.

The main reasons for the decrease in discharge capacity of lithium-ion batteries at low temperatures include: poor electrolyte conductivity, poor wettability and/or permeability of the separator, slower lithium ion migration, and charge transfer at the electrode/electrolyte interface The rate slows down and so on [18]. In addition, the impedance of the SEI film increases at low temperatures, which slows the speed of lithium ions through the electrode/electrolyte interface. The reason for the increase in the impedance of the SEI film is that it is easier for lithium ions to escape from the negative electrode at low temperature, and it is more difficult to insert. When charging, metal lithium will appear and react with the electrolyte, forming a new SEI film covering the original SEI film, which increases the impedance of the battery and causes the capacity of the battery to decrease [19].

Li Lianxing et al. [20] conducted 300 charge-discharge cycle experiments on the same batch of lithium batteries at 60 ℃ and normal temperature. In the initial stage, the battery showed a higher discharge capacity at 60 ℃. However, as the cycle progresses, the battery capacity decays faster, the cycle stability decreases, and even the battery swells in the later stage. The charge-discharge cycle of lithium-ion batteries is unstable at high temperatures. High temperature leads to increased electrochemical polarization of the battery's electrodes and gas generation, causing swelling. At the same time, the charge transport resistance increases and the ion transport dynamics performance decreases.

At present, most lithium-ion batteries use LiPF6 as the electrolyte. Due to the impure electrolyte or the catalytic decomposition of conductive salt with trace water, the electrolyte contains a certain acidic substance HF [21]. HF will react with the main components ROLi and ROCO2Li in the SEI film to form LiF and deposit on the surface of the negative electrode. The SEI film containing LiF will hinder the migration of lithium ions. At the same time, the generated high-resistance material will insulate and isolate the graphite particles. With the progress of high-temperature charging and discharging, the performance of the negative electrode will gradually deteriorate and eventually lead to battery failure [18].

Equipment using lithium-ion batteries may be subjected to conditions such as vibration, shock, and collision during transportation or normal operation. Some lithium batteries charge and discharge when communicating with the system and receive data information according to a certain frequency. The frequency when the device vibrates may interfere with the battery frequency, causing chip data errors or triggering the protection circuit action [22]. Under strong vibration or shock, the tabs, external connections, terminals, solder joints, etc. of the lithium-ion battery may break or fall off, and the active material on the battery pole pieces may also peel off [23], which will affect the life of the battery or even Create a dangerous situation.

5 The influence of the charging and discharging system during the cycle
The use process of lithium ion batteries is the process of charge and discharge cycles. The size of charge and discharge current, the choice of charge and discharge cut-off voltage, and which charge and discharge methods are used also have a very important impact on the cycle life of lithium ion batteries. Blindly increasing the working current of the battery, increasing the charge cut-off voltage, lowering the discharge cut-off voltage, etc. will degrade the battery performance.

Different electrochemical systems have different charge-discharge cut-off voltages. During the charging process of the lithium-ion battery, when the charge cut-off voltage is exceeded, it is considered that an overcharge has occurred. K. Maher et al. [24] set the charge cut-off voltage of LiCoO2 batteries from 4.2V to 4.9V in turn, and performed X-ray diffraction and Raman spectroscopy experiments on the electrode materials after the experiment, which showed that both the graphite anode and the lithium cobalt oxide anode After changing the structure, by testing the entropy curve of different SOC of the electrode after charging with different cut-off voltage, it is also found that the electrode material has changed in structure. When a lithium-ion battery is overcharged, the excess lithium ions extracted from the positive electrode will be deposited or embedded on the negative electrode, and the deposited active lithium easily reacts with the solvent, releasing heat to increase the temperature of the battery. The positive electrode is heated to liberate oxygen, which makes the electrolyte easy to decompose and generate a lot of heat [25]. When the discharge voltage of the lithium battery is lower than the discharge cut-off voltage, overdischarge is formed. In the process of over-discharge, lithium ions will be excessively extracted from the negative electrode, and it will be more difficult to re-intercalate during the next charge. Yu Zhongbao et al. [26] overdischarged the battery with MCMB as the negative electrode and LiCoO2 as the positive electrode to 0 V, the copper foil current collector was severely corroded, the negative electrode SEI film was destroyed, and the SEI film formed again had poor performance, which made the negative electrode The impedance increases and the polarization increases. The discharge capacity and charge-discharge efficiency of the battery are greatly reduced during the cycle after over-discharge.

Li Yan et al. [27] studied the discharge of 18650 lithium-ion batteries at different rates. The results show that the battery capacity decay increases almost proportionally with the increase of the charge and discharge rate. The high-rate cycling LiCoO2/graphite-based lithium-ion battery has a serious capacity degradation. Through analysis, it is concluded that the basic reason for the serious capacity decline is the change of the positive electrode material structure and the thickening of the negative electrode surface film, which leads to the decrease of Li+ quantity and the blockage of the diffusion channel. In the case of high current discharge, ions need to be inserted and removed quickly from the positive and negative electrodes, and the reaction speed is very fast. Through experimental analysis, Tang Zhiyuan et al. [28] concluded that because the battery needs to release a large capacity in a relatively short period of time during high current discharge, the electrode reacts rapidly and violently, and some lithium ions are too late to deintercalate or pass through the discharge process of the negative electrode material. it's over. In addition, the battery tabs may fuse under high current conditions, and equipment components may also be damaged.

6 Conclusion
Through analysis, it can be known that there are many factors that affect the cycle life of lithium-ion batteries, no matter in the design, manufacture or use process. The application of lithium-ion batteries has become more and more extensive, and the demand for lithium batteries has put forward higher requirements in terms of quantity and quality. The cycle life directly affects the service time and quality of lithium-ion batteries, so it is very necessary for manufacturers to study its influencing factors. Only in the process of R&D and production, all factors that affect cycle life are well grasped, can enterprises take the initiative in the fierce market competition. Consumers should pay attention to the characteristics of lithium-ion batteries during use, and use the batteries correctly according to the instructions in the manual.