Safety First: A Multidimensional Strategy Analysis for Enhancing Lithium-ion Battery Safety

 Application and Safety Issues of Lithium-ion Batteries: Lithium-ion batteries are widely used due to their high energy density, high output power, and high average output voltage. However, accidents caused by battery failures occur every year, and yet few people actively understand the safety risks. Therefore, identifying and mitigating the safety hazards of lithium batteries is crucial. The main content of the article: First, it analyzes the phenomenon of thermal runaway and discusses various monitoring systems. Then, it emphasizes the application of Fiber Bragg Grating sensors (FBG) in real-time detection of battery data. Finally, it summarizes methods to reduce safety issues with lithium batteries, including the use of electrode surface coatings, electrolytes, separators, and suppression of lithium dendrite growth. These contents are of reference value for future lithium battery safety research.

1.Introduction

The application and safety issues of lithium-ion batteries are becoming increasingly prominent: The development of renewable energy is a trend of the times, batteries are ubiquitous in life, and lithium-ion batteries are widely used and key to the development of new energy fields. However, in recent years, their overheating problem has affected the development of electric vehicles, and battery safety has received attention. Research direction and purpose of the article: Scientists use various technologies to improve the safety of lithium-ion batteries. Currently, safety monitoring research on battery thermal runaway prediction and early warning methods is a popular direction. The article aims to summarize related advanced methods and introduce the latest research progress.

2.Current methods to improve safety factors

Causes of safety accidents: When lithium-ion batteries are improperly used (such as overcharging, overheating, impact, short circuit), the temperature abnormally increases, triggering internal chemical reactions, producing gases and smoke. The safety valve opens, and the temperature further increases, which may lead to combustion or explosion.

Ways to improve safety: There are mainly two approaches: monitoring and avoiding safety accidents, and upgrading battery structure or replacing problematic components.

Specific methods to improve the safety of lithium-ion batteries

Preventing thermal runaway

Thermal runaway principle: The exothermic reactions within the battery materials cause the battery to heat up rapidly, releasing chemical energy. Overheating can be caused by various factors such as structural deformation, short circuits, overcharging, component aging, and cooling system failures. The high energy density of the battery and the use of flammable electrolytes increase the risk of thermal runaway.

Cooling system: Scientists have developed battery thermal management systems (BTMS), including air cooling and liquid cooling systems, but both have disadvantages. A hybrid cooling system combines the advantages of both, allowing for better regulation and management of battery heat dissipation. The specific choice should be determined based on the situation.

Table 1. Comparison of Air, Liquid, and Hybrid Cooling BTMS

Fiber Bragg Grating Sensor (FBG)

Monitoring Principle: By real-time monitoring of various symptoms of batteries, it prevents safety hazards. Modern methods often involve monitoring heat flow or detecting electrode cracks as indirect reflections of the battery's condition, whereas FBG sensors can directly or indirectly measure the internal and external temperature and strain responses of the battery. They study electrolyte degradation through the interaction of light carried by optical fibers with the surrounding chemical environment.

Advantages: FBG sensors have characteristics such as minimally invasive, resistant to electromagnetic interference, and insulation. They can still accurately provide data under high temperatures and pressures. When indicators reach critical values, operations can be adjusted or terminated in a timely manner to improve the safety of battery usage.

Table 2. FBG Indicator Table

Improving Battery Separators for Stable Battery Performance

Separator Role and Design Challenges: The separator in a battery acts as a physical barrier to prevent direct contact between the positive and negative electrodes and to hold the electrolyte to facilitate ion movement. The design must balance mechanical durability with porosity or transport properties, which poses a challenge when used in large-scale battery systems.

Improvement Methods: Current research is primarily focused on modifying commercial polyolefin (PP) separators, such as coating or grafting organic/inorganic compounds, and treating the surface with heat-resistant compounds. Electrospinning technology can also be used to manufacture nanofiber separators, which can enhance thermal stability. Adding hydrophilic materials can improve performance and inhibit the growth of lithium dendrites.

Non-flammable polymer electrolyte

Issues with traditional electrolytes and directions for improvement: Traditional electrolytes may experience thermal runaway under extreme conditions, leading to oxidation, mixing of electrode materials, and even explosions. Improvements need to take into account the physical and chemical properties and stability of both the electrolyte and the electrodes. Solid polymer electrolytes (SPEs) are the future trend, as they are non-leaking, have high mechanical strength, and are stable, which can reduce the volume changes of electrode materials.

Table 3. Types and Characteristics of SPEs

Characteristics and flame retardants of SPEs: Different SPEs have different advantages, such as high conductivity and adjustable size for polyethylene oxide SPEs; good thermal stability and non-flammability for polysiloxane SPEs. Most SPEs require the addition of flame retardants, with inorganic materials being safer and lower in cost, which can improve the performance of SPEs and inhibit the growth of lithium dendrites. However, SPE research is relatively new, applications are limited, and commercial electrolytes still cannot be replaced.

Table 4. Different Flame Retardants and Their Properties

Inhibition of Lithium Dendrite Growth

Formation and Hazards of Lithium Dendrites: Lithium dendrites are caused by the uneven deposition of lithium ions during their migration between the positive and negative electrodes. This can lead to electrode expansion, reduced coulombic efficiency, decreased battery capacity, and deterioration of safety performance, ultimately resulting in battery failure.

Inhibition Methods: Inhibition can be approached from both the electrolyte and the lithium metal anode. Adding additives to the electrolyte can enhance the function of the solid electrolyte interface (SEI) layer, such as lithium polysulfides and lithium nitrate, which can effectively inhibit the formation of lithium dendrites. From the electrode perspective, three-dimensional structured lithium anodes can reduce the volume change of the anode, such as graphene composite electrodes. Some new types of SEI layers can also effectively inhibit the growth of lithium dendrites.

Surface Coated Electrode Method

Role and Application of Surface Coating: Surface coating is the primary technology for protecting the cathode and enhancing the thermal stability of cathode materials. It can suppress phase transitions and improve the conductivity of the material. Nickel-cobalt-manganese ternary (NMC) cathode materials, when using surface coating technology, can improve microstructure, electrochemical performance, thermal conductivity, ion diffusion coefficient, and thermal stability, reducing internal structural damage and increasing cycle stability, preventing metal ion leaching.

Specific Methods and Effects: For instance, employing a "coating + impregnation" synthesis method at room temperature for coating specific materials, or using sol-gel technology to create a uniform coating on the cathode surface at low temperatures, can significantly enhance cycle stability.

Table 5. The Impact of Surface Coating Technologies on Lithium-Ion Batteries

3.Summary

Method Classification: Methods to improve the safety of lithium-ion batteries can be broadly classified into two categories. One category involves real-time monitoring of battery parameters as an early warning system to prevent safety incidents, while the other category focuses on improving the internal materials or structure of the battery.

Specific Measures and Effects

In the first category, the Battery Thermal Management System (BTMS) can prevent thermal runaway. A hybrid BTMS offers the best cooling effect, although it is complex in structure and expensive. Fiber Bragg Grating (FBG) sensors can monitor battery temperature, strain, and pressure in real-time, enabling quick identification of overheating or abnormal conditions.

In the second category, researchers have improved the safety of lithium-ion batteries by modifying the separator, electrolyte, suppressing the growth of lithium dendrites, and treating the cathode surface.

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