Abstract
With the rise of electric vehicles and plug-in hybrid vehicles, the target service life of secondary batteries is at least 10 years, and such vehicles are expected to be widely popularized in the future. Non portable batteries also have a demand for long service life, which makes acceleration testing of secondary batteries highly anticipated. This article compares the acceleration testing of semiconductors, electronic components, and electronic devices, and elaborates on the current status and problems of acceleration testing for lithium-ion secondary batteries.
1. Introduction
The research and manufacturing of lithium-ion secondary batteries have made rapid progress in improving performance. Although ensuring safety is the most important issue for lithium-ion secondary batteries, the target lifespan for lithium-ion secondary batteries used in electric vehicles and plug-in hybrid vehicles (a growing and highly anticipated market) is 10 to 15 years. For non portable batteries, a minimum service life of 6 years is required. These demands will place increasing expectations on accelerated testing in the coming years. In contrast, in the field of accelerated testing of electronic components and devices, elucidating the degradation mechanisms and acceleration factors of parameters such as semiconductor/solder bonding and printed circuit board insulation can help advance life prediction technology. Unlike electronic components and devices, secondary batteries lack sufficient examples and system theories for degradation/life prediction and acceleration testing. This article describes the current situation and problems of accelerated testing of lithium-ion secondary batteries by comparing it with accelerated testing of semiconductors, electronic components, and electronic devices.
2. Definition of degradation
JIS C 8711 (covering lithium secondary batteries for portable devices) provides an example of secondary battery life assessment, where battery life termination is defined as the point in time when the capacity drops to 60% of the initial capacity during charge and discharge cycles. For the degradation of batteries stored at a constant temperature after being charged by a predetermined method, the values of 70% for single cells and 60% for battery packs are also defined. Manufacturers' degradation assessment standards are usually stricter than these values, such as using values like 80%. Due to the fact that the degradation rate of batteries during long-term storage is proportional to their state of charge (SOC), secondary batteries are designed to match the usage environment, enabling them to maintain low SOC and withstand long-term use. As long as the degradation value of the battery meets the set standards, it is considered to be working normally. The general view is that batteries are consumables and their capacity inevitably decreases. Figure 1 shows an example discharge curve of a lithium-ion secondary battery.
3. Degradation factors and mechanisms
Each component of lithium-ion secondary batteries has several degradation factors, and it is difficult to classify them simply. The possible reason for this difficulty is that there are few cases of fault analysis, as it is difficult to disassemble the battery without affecting its internal state, and the relationship between specific degradation mechanisms and battery life is not always clear. In contrast, the situation with electronic components is quite different. There are many fault analysis cases for electronic components, and with the improvement of analysis technology, reliability technology and life prediction technology have also been developed.
Figure 2 shows the structure of a lithium-ion secondary battery. According to reports, the increase in internal resistance caused by the growth of membranes on the electrode surface is a typical degradation factor of lithium-ion secondary batteries. Other possible phenomena include changes in the crystal structure of the active substance, as well as delamination at the electrode material or current collector interface. Due to some degradation phenomena being caused by electrolytes or membranes, it is necessary to estimate these reasons from characteristic evaluations or material structures. These types of degradation may coexist during charge discharge cycles. The methods for measuring the increase in internal resistance include direct current resistance method (DC-IR) and alternating current impedance method. Due to the limited number of fault analysis cases, the causal relationship between degradation phenomena and degradation sites is still unclear. However, the AC impedance method is a promising technique for measuring internal battery phenomena related to the aforementioned factors.
4. The impact of lithium-ion secondary battery usage environment on battery life
Storage temperature impact: Storage temperature is an important degradation factor for lithium-ion secondary batteries. The indoor usage environment temperature is expected to reach a maximum of about 40 ° C, while batteries in outdoor or mobile devices face more harsh environments. The long-term characteristics of current lithium-ion secondary batteries degrade rapidly at 40-60 ° C. Therefore, testing standards such as JIS 8711 and IEC 62660-1 (used for performance testing of single batteries in electric vehicles) stipulate that the long-term life test temperature should be between 40-45 ° C. To extend battery life, vehicle batteries are designed with a cooling mechanism to maintain battery temperature at no more than 45 ° C. Vehicle batteries still need to adapt to low temperatures of -20 ° C and below, as the internal resistance of secondary batteries usually increases and the capacity significantly decreases at this temperature. However, we are currently developing batteries with excellent low-temperature characteristics.
The impact of charging and discharging rates: Vehicle batteries have different charging and discharging rates during use, and the difference in charging and discharging rates can affect the degradation process. IEC 62660-1 specifies the charging and discharging rate mode for vehicle batteries during use. In contrast, household appliance batteries are often used in a continuously charged state, and maintaining high SOC is also a factor that leads to the degradation process. Due to the significant impact of market conditions on battery life, it is necessary to study accelerated testing conditions that can anticipate these market environments.
5. Secondary battery life testing and acceleration model
Overview of Testing Characteristics and Acceleration Models: Secondary battery life testing mainly focuses on two basic characteristics: storage life (calendar life) and charge discharge cycle life. The storage life is related to the temperature related degradation. The Arrhenius model is used as the acceleration model. It is believed that the storage life is determined by the increase of internal resistance caused by the growth of the facial mask on the electrode surface. The film growth is caused by chemical reaction. The accelerated test temperature is higher than the service temperature, but the maximum temperature of the lithium ion secondary battery is usually limited to 55-60 ° C, beyond which the acceleration effect will not be generated due to the progress of different chemical reactions. The extrapolation method based on the linear aging degradation is used for long-term life prediction (such as the case with a linear relationship with the square root of the aging time). However, in practical use, the charge discharge cycle life needs to be considered, and the Arrhenius model alone cannot fully express the actual situation, such as cases where the ratio of storage life to charge discharge cycle life is 9:1, and situations where storage temperature has a significant impact. In addition to the Arrhenius model, there are various acceleration models for electronic components, and their use and combination are determined by degradation factors. It is important to identify degradation factors such as humidity and repeated mechanical stress when creating acceleration models.
Limitations of acceleration models and challenges in battery life prediction: When using acceleration models for life prediction, factors are simplified as the main factors, resulting in significant errors in calculation results. Electronic components sometimes use triple calculation results as safety margin evaluations. However, the performance of battery products is relatively small compared to the required battery life margin, and accurate prediction is needed. Currently, it is extremely difficult to create a prediction method that determines a 10-year service life. However, as developers strive to ensure that the lifespan performance margin exceeds market demand, battery performance is expected to further improve.
6. Summary
Reasons for Difficulty in Accelerated Testing of Secondary Batteries
Compared with similar testing of electronic components or devices, long-term accelerated testing of secondary batteries is currently more difficult, mainly due to the following factors:
- Battery life prediction is usually not based on simulating harsher environments or multiplying safety factors like electronic components and devices, possibly due to a lack of sufficient margin for current battery life performance relative to market demand.
- There are still many unclear causal relationships between degradation phenomena and degradation factors, which may be due to the difficulty in fault analysis of degraded batteries and the limited number of cases.
- Due to the fast development speed of batteries, changes in material structure, and difficulties in fault analysis mentioned earlier, insufficient verification work has been done on the correlation between market degradation data.