Lithium ion batteries have been continuously developed for over forty years. In 1976, Whittingham proposed and produced the first lithium-ion battery. The solar cell used titanium sulfide as the positive electrode material and metallic lithium as the negative electrode material. Due to the unstable chemical properties of metallic lithium, its safety was extremely poor. In 1980, Mizushima et al. discovered that using lithium cobalt oxide as the positive electrode material for solar cells can improve their stability. In 1983, Yoshino et al. proposed the first modern lithium-ion secondary solar cell using lithium cobalt oxide as the positive electrode and soft carbon as the negative electrode, and conducted safety tests on it in 1986, laying an important foundation for the commercialization of Sony’s lithium-ion batteries.
In the solar cell industry, cycle life and calendar life are two important indicators for evaluating the lifespan of solar cells. The cycle life refers to the number of charging and discharging cycles that a solar cell can withstand before its capacity decays to a certain specified value under a certain charging and discharging system. If the solar cell undergoes a complete charging and discharging cycle, then this cycle number is also known as the “standard cycle number”. Calendar lifespan refers to the period of time from the production date of a solar cell to the end of its lifespan (when the capacity of the solar cell decays to a certain specified value), usually measured in years. During this period, the solar cell may undergo different stages such as shelving, aging, high and low temperature, cycling, and working condition simulation. Corresponding to the cycle life and calendar life of solar cells, the aging of solar cells is defined as cycle aging and calendar aging, respectively. Generally speaking, the faster the aging rate of solar cells, the shorter their lifespan. The aging of solar cells can be quantitatively analyzed through changes in solar cell capacity, internal resistance, and other parameters. It can also be directly observed by dissecting and analyzing the changes in internal components of solar cells to explore the causes of solar cell aging. In most studies, in order to avoid permanent damage to solar cells caused by dismantling them, The researchers used incremental capacity curves, differential voltage curves, and constant current charging curves Identify the internal changes of lithium-ion batteries and further describe the aging mechanism of solar cells.
Overall, domestic and foreign researchers have conducted extensive research on the aging and influencing factors of various types of lithium-ion batteries. In terms of cyclic aging, the aging rate of lithium-ion batteries is influenced by both external environment and usage conditions. Specific influencing factors include temperature, operating range of state of charge (S0C), number of cycles, charging and discharging current, voltage, etc; In terms of calendar aging, temperature, state of charge, and time are key factors affecting the aging rate of lithium-ion batteries. The relevant research on various influencing factors is as follows:
(1) Temperature
Among all the influencing factors, temperature has the greatest impact on lithium-ion batteries. Currently, most research on the effect of temperature on the aging of lithium-ion batteries is conducted at high temperatures (above room temperature). For example, Markevich et al. studied the degradation of carbon negative electrodes at 60 ° C and 80 ° C; Gabrisch et al. investigated the degradation of UC0O2 and LiMn cathode materials at 65 ° C and 75 ° C; Handel et al. conducted a detailed study on the thermal degradation of electrolytes at high temperatures and analyzed the influence of solar cell shell materials on the thermal degradation of electrolytes. The above research focuses on the effect of high temperature on individual components in lithium-ion batteries. For a complete solar cell, the effect of high temperature on the aging of the solar cell is the result of the joint action of various components, among which the formation of solid electrolyte interphase (SEI) is the main reason for the capacity attenuation and impedance increase of the solar cell. In addition, Bodenes et al. cycled and stored specially designed Li (Ni, Mn, Co) 02/C high-temperature lithium-ion batteries at 85 ° C and 120 ° C, and found that at extremely high temperatures, in addition to the formation of solid electrolyte interphase, The poor re insertion of lithium ions caused by the formation of the adhesive layer on the positive electrode surface is also an important reason for the aging of solar cells.
Unlike high temperatures, which often accelerate the aging of lithium-ion batteries, the impact of low temperatures on the aging of lithium-ion batteries depends on whether solar cells mainly experience cyclic aging or calendar aging. In terms of cyclic aging, Waldmann et al. studied the aging mechanism of 18650 nickel manganese cobalt batteries during charging and discharging at 1C cycle rate at -20 ° C-25 ° C using electrochemical methods and anatomical analysis. They found that the aging of solar cells at low temperatures is mainly caused by the electroplating of anode lithium, and as the temperature decreases, The aging of solar cells will accelerate. Petz et al. also obtained the same conclusion in the low-temperature (-22 ° C -25 ° C) cycling aging experiment of 26650 lithium iron phosphate.
A recent study suggests that when 18650 lithium cobalt oxide batteries are placed in an environment of -10 ° C and cycled at different rates of 0.2C-1C, electroplating (lithium deposition) of anode lithium remains the main aging mechanism, and over long cycles, electroplating lithium may react with electrolytes, Causing the formation of intermediate phases in solid electrolytes. Unlike in cycles where low temperatures accelerate the aging of lithium-ion batteries, when lithium-ion batteries mainly experience calendar aging, the aging rate of solar cells slows down due to the unfavorable effects of low temperatures on material diffusion and chemical reactions. Li et al. conducted aging experiments on 26650 lithium iron phosphate batteries at temperatures of 20 ° C, 40 ° C, and 60 ° C, and Liu et al. conducted aging experiments on 18650 lithium cobalt oxide batteries This viewpoint has been validated by temperatures of 10 ° C, 25 ° C, and 45 ° C. Currently, no experimental studies on calendar aging of lithium-ion batteries have been found at extreme low temperatures. In addition, the formation of intermediate phases in solid electrolytes is considered the main mechanism of low-temperature calendar aging. Due to the lower temperature, dense solid electrolytes have been formed in the intermediate phase The phase is neither easy to dissolve nor easy to break, but instead provides protection for the electrodes, causing the capacity of solar cells to decay more slowly over time.
(2) State of Charge
In terms of cyclic aging, the effect of state of charge on the aging of solar cells can be divided into the effects of depth of charge (DOD) and average state of charge (meanS0C) on the aging of solar cells. Omar et al. investigated the effect of charge and discharge depth (20%, 40%, 60%, 80%, and 100%) on the cyclic aging of lithium-ion batteries at temperatures ranging from 20 ° C to 25 ° C. The results showed that as the charge and discharge depth increased, the capacity decay and impedance increase of solar cells increased. The same conclusion has been expressed regarding the research on nickel manganese cobalt lithium manganese oxide mixed electrode lithium-ion batteries and the effect of charging and discharging depth on the capacity degradation of electric vehicle batteries. Belt et al. conducted cyclic aging experiments on 18650 nickel manganese cobalt and lithium manganese oxide batteries under SOY, studying the effect of average state of charge (30%, 60%, and 90%) on the aging of solar cells. The results showed that when the average state of charge was 90%, the capacity degradation rate of solar cells was the fastest, followed by 30%, When the average state of charge is 60%, the capacity decay rate of solar cells is the slowest, which seems to indicate that there is no clear correlation between the average state of charge and the capacity decay rate of solar cells. However, the impedance of solar cells increases with the average state of charge. However, Saxena et al. found in their study on the cyclic aging of LiC002/graphite batteries that, compared to the 20% -80% and 40% -100% state of charge, the solar cells aged the slowest at 0% -60% cycles. This conclusion seems to suggest that the lower the average state of charge, the more slowly the solar cells aged, The slower the cycling aging rate of solar cells. Therefore, when modeling the cycle life of solar cells, the average state of charge can still be considered as one of the influencing factors.
In terms of calendar aging, the state of charge affects the aging of lithium-ion batteries by affecting the potential of the anode and cathode. This is mainly related to the formation of the intermediate phase in solid electrolytes. A high cathode potential can easily cause electrolyte oxidation and transition metal dissolution at the cathode interface, while a low anode potential can aggravate electrolyte reduction at the anode, thereby promoting the formation of the intermediate phase in solid electrolytes. Due to the higher state of charge of solar cells, the higher the cathode potential and the lower the anode potential, it is generally believed that as the state of charge increases, the aging rate of lithium-ion batteries will become faster. Eddahech et al. found that four types of lithium-ion batteries (nickel manganese cobalt, nickel cobalt aluminum, lithium manganese oxide, and lithium iron phosphate) exhibit different states of charge (30%, 65%, and 100%) at different temperatures (30 ° C, 45 ° C, and 60 ° C) The aging situation during storage and the research results of Schmit et al. on the aging of 18650 nickel manganese cobalt batteries at 20 ° C both confirm this viewpoint. However, Keil et al. conducted calendar aging experiments on three different lithium-ion batteries (LiNiCoA102/graphite, LiNiCoMn02/graphite, LiFePCU/graphite) at three temperatures: 25 ° C, 40 ° C, and 50 ° C, However, no strong correlation was observed between calendar aging and state of charge.
(3) Time
For lithium-ion batteries that mainly experience cycle aging, the effect of time on battery aging can correspond to the effect of cycle number on solar cell aging, as well as the effect of actual operating time on solar cell aging. As the number of cycles increases, the aging of solar cells often shows a trend of early acceleration and basically unchanged in the middle and late stages. In order to compare the effects of various cycling conditions on the aging rate of solar cells, researchers often use the equivalent full cycle (EFC) as the horizontal axis to draw a schematic diagram of battery capacity decay and impedance increase. From these literature, it can be seen that when the cycle range of solar cells is small, the capacity degradation rate of solar cells will decrease as the number of equivalent full cycles increases; When solar cells are cycled at a greater discharge depth, the capacity degradation rate often reaches a turning point around 85% of the initial capacity, and the capacity degradation of the battery accelerates with the increase of the equivalent full cycle number. When the cycle life of a solar cell is calculated in years or months, the aging rate of the solar cell will become faster as the actual operating time increases. This is because in the initial stage, the cycle period of the solar cell is less, and then due to the decay of the solar cell capacity, the cycle period becomes more.
For lithium-ion batteries that mainly experience calendar aging, based on the theory that the growth of solid electrolyte interphase can serve as a protective layer to slow down the further consumption of active lithium in the later stage of aging, many researchers believe that there is a square root relationship between storage time and capacity decay of lithium-ion batteries during calendar aging.
(4) Current/voltage of charge and discharge
In cyclic aging, in addition to temperature and state of charge, the charging and discharging current and cut-off voltage are also important factors affecting the aging rate of solar cells. It is generally believed that high charging and discharging currents have a greater impact on the aging of solar cells than low charging and discharging currents, and the impact of charging currents on the aging of solar cells is greater than that of discharging currents. For example, Kei et al. tested three 18650 lithium-ion batteries with different charging and discharging schemes, and the results showed that although the impact of charging and discharging currents on the cycle life of different solar cells was significantly different, the lifespan of solar cells was more affected by high charging currents than by high discharging currents, and the larger the current, the greater the impact, The faster the aging of solar cells. Su et al. compared the effects of seven different factors, including charge and discharge currents, on the aging of solar cells using principal effects analysis and analysis of variance, and obtained the same conclusion. However, due to the impact of charging and discharging currents on the heat generation of solar cells, even if most solar cell experiments are conducted at room temperature or in a constant temperature box, the temperature of the solar cells is not controlled, and higher currents will still lead to higher solar cell temperatures. Therefore, the experimental results will be affected by temperature interference. In a relatively new study, Barcelona et al. excluded the influence of temperature and state of charge on battery aging by controlling the temperature and charge/discharge range of solar cells. Finally, they concluded that the cyclic aging of solar cells does not depend on the current rate.
The study on the effect of voltage on the aging of lithium-ion batteries shows that high charging cutoff voltage will accelerate the aging of solar cells, especially capacity degradation; And low discharge cutoff voltage can also affect the aging of solar cells, especially power attenuation. The research results of Gao et al. also indicate that there is a critical charging cutoff voltage, and when the charging stress exceeds the critical value, the aging rate of solar cells will be greatly accelerated.