Aging is often associated with failure, but strictly speaking, they are two different concepts. Aging refers to the deterioration of the performance parameters of a power lithium battery over time; it is a quantitative process, with key parameters including the battery's maximum usable capacity, internal resistance, and power. Failure, on the other hand, is the process by which the battery completely loses its ability to function; it is relatively short-lived and represents a qualitative change. The accumulation of aging is a significant cause of failure.
1. Factors affecting aging
The optimal operating temperature for power lithium batteries is 15℃-35℃, but in daily applications, it is impossible to fully meet the requirements of the battery. Therefore, the most common scenarios that affect battery aging are high temperature and low temperature.
Besides the environment, battery operating parameters can also accelerate or slow down aging, so the selection of cell charging and discharging parameters has a significant impact.
Under the external factors listed above, battery electrode materials and other components may undergo side reactions beyond normal charging and discharging during the electrochemical reaction process, leading to aging.
2. Typical aging process
The specific details of the aging process are closely related to the selection of positive and negative electrode materials, electrolyte, and separator. This article explains the general aging process and will not provide detailed explanations for specific materials.
2.1 High-temperature aging
50℃ to 60℃ is the upper limit of the allowable operating temperature range for general lithium-ion batteries. At higher temperatures, the electrolyte is more reactive and prone to decomposition. These decomposition products combine with the cathode material, consuming it; the cathode material corrodes, and the crystal lattice collapses due to insufficient support, reducing lithium-ion vacancies and decreasing the cathode's ability to hold lithium ions, thus resulting in a loss of battery capacity.
Meanwhile, the products of the positive electrode material reaction, floating in the electrolyte, may adhere to the surfaces of the positive and negative electrodes. The electrode surfaces are covered by substances that cannot participate in the charging and discharging process, hindering the smooth occurrence of the electrochemical process and increasing the internal resistance of the cell.
The effects of high-temperature processes on aging occur primarily at the positive electrode, with a relatively small impact on the negative electrode.
2.2 Low temperature aging
When the ambient temperature drops below 0°C, the performance of lithium-ion batteries begins to be significantly affected by the low temperature. The SiE film, or SiE layer, is a passivation film formed during the cell formation process between the negative electrode material and the electrolyte, serving to protect the negative electrode material.
During low-temperature operation, the SEI film grows, consuming some of the active lithium ions in the electrolyte. This reduces the concentration of conductive ions in the electrolyte, resulting in a permanent loss of usable battery capacity. The thickening of the SEI film increases the difficulty for lithium ions to penetrate the film and reach the negative electrode. Combined with the reduced concentration of conductive lithium ions, this leads to an increase in the cell's internal resistance.
Charging at low temperatures, especially with high charging current, can lead to another side reaction at the negative electrode: lithium deposition. At low temperatures, lithium-ion activity decreases, and forced charging causes excess lithium ions to accumulate around the negative electrode. These ions cannot penetrate the SEI film to intercalate and instead deposit on the negative electrode surface, forming a pure lithium layer. This process is prone to occur at excessively low charging temperatures and is irreversible. With repeated use, elemental lithium continues to accumulate, dendrites grow, and the risk of puncturing the separator increases steadily.
When lithium-ion batteries operate at low temperatures, aging issues primarily occur at the negative electrode. Side reactions also exist at the positive electrode, but their impact is not significant.
2.3 High Current Charging and Discharging
Discharging with a current exceeding the design discharge capacity has two main effects. First, the thermal effect of the current causes the battery temperature to rise, gradually exacerbating the side effects of high-temperature aging. Second, the large current causes an excessive amount of lithium ions to embed into the cathode material, impacting the material's stability.
High-current discharge also presents issues such as heat generation and the stability of the cathode material's insertion/extraction. Furthermore, excessive lithium ion transport to the anode, exceeding its capacity, leads to lithium deposition. This not only results in capacity loss but also increases the risk of thermal runaway during long-term use, causing even more serious damage.
2.4 Overvoltage and Undervoltage Charge/Discharge
Overvoltage charging and undervoltage discharging can both cause phase transitions in the cathode material, reducing the number of lithium-ion vacancies and affecting the maximum usable capacity of the cell.
2.5 Self-discharge
Self-discharge of battery cells occurs constantly, and it becomes more pronounced at higher temperatures and with higher charge levels. Self-discharge results in a loss of both reversible and irreversible capacity. The byproducts of self-discharge adhere to the electrode surface, blocking lithium-ion channels and reducing lithium-ion insertion sites, ultimately leading to permanent capacity loss in the battery cell.
Aging of 3 modules
Lithium-ion batteries are connected in series and parallel to form modules. The aging of the module is directly affected by the aging of the individual cells. In addition, the aging of the cells leads to a deterioration in the consistency between the cells, which means that the aging of the module is amplified on top of the aging of the cells.
Besides the effects of cell aging, the aging process of the module can be accelerated by factors such as vibration and oxidation corrosion of conductive components. Inside the module, the cells and busbars, as well as the busbars and module terminals, are connected by welding or screws to maintain tight contact and ensure resistance is within a reasonable range. However, the increased connection resistance caused by vibration and oxidation alters the distribution of resistance within the module. These changes can affect the measured cell voltage, thereby impacting the charging, discharging, and equalization processes of the cells.
Harsh environments and excessive operating parameters make the aging process more pronounced and easier for researchers to observe. In reality, the aging process occurs silently throughout the entire process. A battery cell has two lifespans: calendar life and cycle life. The name "calendar life" itself suggests the relentless aging process. Therefore, research on factors affecting battery cell aging focuses on mitigating the accelerated aging caused by improper operation. Currently, a common heating method for electric vehicles is a combination of heating and cooling systems, such as heat pump air conditioning, heat pipes, and phase change materials. In this type of system, heating occurs as the reverse of cooling, with heating and cooling essentially occurring within a single system. Switching between heating and cooling is achieved through a controller and the physical and chemical properties of the system during operation. While the technical name for this system is uncertain, we'll tentatively call it an integrated thermal management system. Another heating method, specifically designed for electric vehicles operating in cold environments, is a separate heating device unrelated to cooling; we'll call it an independent heating system for now.
Currently used independent heating systems mainly fall into two categories: resistance heaters and electrothermal film heaters. This article primarily discusses the relevant content of electrothermal film heating methods.
2. Types and working principles of electric heating films
Traditional heating films are primarily used in the construction industry as concealed heating systems. The electric heating film is embedded in walls or floors, and during cold seasons, it heats the space when powered on. Compared to traditional centralized heating, heating films can heat the space more evenly, providing a more comfortable experience.
The application of electric heating film in electric vehicles is a topic that has been gradually mentioned in recent years. No relevant standards have yet been found; the standards available for reference are mainly from the construction and household appliance industries, namely "JGJ319-2013 Technical Specification for Application of Low-Temperature Radiant Electric Heating Film Heating System" and "JGT286-2010 Low-Temperature Radiant Electric Heating Film".
Electric heating films can be classified according to the different encapsulation materials used: metal electric heating films, inorganic electric heating films (including carbon fiber electric heating films, ink electric heating films, etc.), and polymer electric heating films.
Metal heating films are the first generation of thin-film heating products. They utilize film-forming technologies such as vapor phase growth to attach a conductive metal material to an insulating material. Then, another layer of insulating material is applied to the metal layer, tightly encapsulating it and forming a thin, conductive film. When electricity is applied, the metal's internal resistance heats up, creating an electrothermal effect. Commonly used metal heating materials include copper and nickel. Different materials have different resistivities, operating voltages, and heating powers. Different metal materials, combined with different circuit designs, meet various user parameter requirements. The choice of metal material also directly affects the cost of the heating film.
Inorganic electrothermal films, where "inorganic" refers to the fact that the conductive material is inorganic, such as graphite, SiC, SiO2, conductive ink, carbon fiber, and other conductive silicates. Inorganic electrothermal films are made by mixing these inorganic conductive materials with flame retardants, film-forming agents, and other additives, and then coating them onto an insulating substrate to form a conductive film. When a voltage is applied across the heating film, the conductive layer, which is essentially a semiconductor, converts electrical energy into heat energy.
It should be noted that some inorganic conductive materials are brittle at room temperature. For example, SiO2 is a key component of glass, and these types of heating films must be coated onto rigid substrates for use as plate-shaped materials. Others, however, are flexible, such as conductive inks and carbon fibers. Inorganic materials are heat-resistant, have a long lifespan, and are readily available, making them a class of high-performance heating materials. However, their inability to bend and deform is a major factor limiting the use of inorganic heating films. Flexible inorganic heating films can achieve a wider range of applications.
Polymer electrothermal films are made by adding conductive particles to organic materials, processing them into thin film materials, and then encapsulating them. Alternatively, conductive materials are coated onto an insulating substrate to create an organic conductive film, which is then encapsulated with a polymer insulating material. Generally, their operating temperature is lower than that of inorganic electrothermal films. Among organic electrothermal films, silicone heating films, polyimide heating films, and PET heating films are widely used. Considering market demand and policy guidance, it is worrying whether the new energy vehicle industry can maintain its momentum and robust development if national and local subsidies are not implemented in 2020, given the precedent of a sharp drop in sales after the cancellation of subsidies in California, USA. Currently, the cost of power lithium batteries accounts for nearly half of the total vehicle cost. Therefore, power lithium battery companies must transform and upgrade, achieving product standardization, production automation, company scaling, and management informatization to strictly control battery costs.
● Power lithium battery companies achieve large-scale and standardized production in accordance with national standards (the three standards are still recommended standards and may be revised in a timely manner according to the development of the industry). The R&D costs and manufacturing costs are distributed, which can reduce PACK design costs and after-sales costs.
Wang Fang, chief expert of power lithium batteries at my country Automotive Technology Research Center, stated that inconsistent size specifications lead to poor interchangeability. Every time a car company produces a new product, battery companies have to redesign the battery pack, which is detrimental to cost reduction.
