Main content:
Lithium batteries are widely used in portable electronic products and electric vehicles due to their advantages of high operating voltage, battery high energy density, long cycle life, and low cost.
However, under low temperature conditions, issues such as difficulty in charging, low discharge capacity, and short lifespan greatly limit its application in low-temperature environments. Therefore, it is imperative to explore the low-temperature failure mechanism of lithium batteries and improve their low-temperature performance.
This article summarizes the impact of low-temperature electrolyte in lithium batteries from the perspective of electrolyte design, and focuses on introducing strategies and mechanisms to improve the low-temperature performance of lithium batteries.
The importance of low-temperature performance in lithium battery
With the vigorous development of the new energy industry, people have increasingly stringent requirements for the scope and performance of lithium ion batteries. The working lithium ion battery temperature range is -20~55 ℃.
When the temperature is below -20 ℃, the cycling stability, service life, and safety of lithium ion batteries are seriously affected, making it difficult for them to work in low temperature environments in winter at high latitudes, and unable to meet the requirements of energy storage and release in ultra-low temperature fields such as polar regions and space.
With the increasing demand for batteries that can work stably under low temperature conditions, improving the low-temperature performance of lithium ion batteries has become a research focus in both academia and industry.
Although internal/external heating strategies can improve the stability of lithium ion batteries in low-temperature environments, such solutions often come with a trade-off between cost and portability, as well as a decrease in system energy density.
Therefore, developing batteries with inherent low-temperature tolerance has become an urgent need to achieve efficient operation of batteries in low-temperature environments in the future.
The working process of electrolyte in lithium battery
A typical electrolyte is mainly composed of salt, solvent, and electrolyte additive.
When lithium salt dissolves in a solvent, it dissociates into Li+and anions, forming a solvation sheath of Li+. The solvated Li+first migrates from the electrolyte to the electrode, and this mass transfer rate can be measured by ion conductivity. Secondly, when the solvated Li+migrates to the electrode/electrolyte interface, Li+undergoes a desolvation process. The interaction between Li+- solvent and Li+- anion in the Li+solvation structure is a key factor determining the kinetics of the desolvation process.
In addition, the reduction of solvation structure at the electrode/electrolyte interface determines the composition and structure of SEI, which in turn affects the transport characteristics of Li+through SEI. Therefore, the solvation structure largely determines the properties and interfacial behavior of the electrolyte, which in turn affects the low-temperature performance of LIBs. Macroscopically designing electrolyte formulations and microscopically adjusting solvation structures are key to improving ion conductivity, rapidly desolvation processes, and optimizing Li+transport on SEI. Based on the mechanism of electrolyte action, researchers have made great efforts in these three aspects.
Efforts made in low-temperature electrolyte in lithium battery
Before 2015, research mainly focused on adjusting and optimizing the traditional electrolyte formula for lithium-ion batteries, mainly from three perspectives: solvent formula, lithium salt solute, and additives, to improve the conductivity of lithium-ion battery electrolytes at low temperatures, reduce viscosity and melting point, and reduce resistance at various locations, in order to achieve the goal of improving low-temperature capacity performance.
After 2015, with the renewed emphasis on research on lithium metal batteries and a deeper understanding of the solvation mechanism of lithium batteries, researchers have discovered the decisive role of the desolvation process in low-temperature performance. In response to the desolvation process, they have designed electrolytes with special solvation structures to reduce the adverse effects of difficult desolvation at low temperatures on low-temperature performance.
People's pursuit of low-temperature performance has gradually shifted from simply pursuing better low-temperature capacity performance to hoping to achieve efficient and stable long-life low-temperature charge and discharge cycles. The understanding of the charge transfer mechanism of lithium ions in lithium batteries is constantly evolving, and there will inevitably be more and more electrolytes with special properties in the future. The exploration of low-temperature electrolyte in lithium batteries is far from over.
In fact, simply designing the low-temperature electrolyte to achieve truly usable low-temperature lithium batteries is far from enough. The properties of the electrode and electrode - electrolyte interface at low temperatures also have a significant impact on low-temperature performance.
The design of low-temperature lithium batteries needs to start from the perspectives of low-temperature electrolyte, interface structure, and overall electrode design, comprehensively considering the physical and chemical properties of the electrolyte, electrolyte solvation structure, interface passivation layer properties, and synergistic effects between battery structures. Through collaborative regulation between different designs, the low-temperature performance of lithium batteries can be optimized more comprehensively and efficiently.
The future of low-temperature electrolyte in lithium battery
Improving the electrochemical performance and service life of lithium ion batteries in low-temperature environments is an important issue for the future development of lithium ion batteries.
Electrolyte is an important factor limiting the low-temperature performance of lithium ion batteries. From the perspective of electrolyte design, researchers believe that future research should focus on the following aspects.
In depth study of the regulation principles of electrolyte micro interactions on macroscopic properties at the atomic and molecular scales, and further exploration of the essential effects of electrolyte solvation structure and ion transport mechanism on the low-temperature electrochemical performance of electrolytes, will contribute to the targeted design of new low-temperature electrolyte systems that meet the requirements.
An ideal low temperature electrolyte should have high ionic conductivity, low lithium ion desolvent energy barrier, and a stable SEI/CEI interface facial mask dominated by anion decomposition. However, in practical situations, these points often constrain each other, and it is very important to decouple the mutual constraint relationship between the above three in a reasonable way. Further explore methods to balance the aforementioned constraints and improve the performance of low-temperature electrolyte.
The types and quantities of electrolytes are vast. With the high-throughput screening capabilities of theoretical simulation and artificial intelligence technologies, molecular simulation methods can reveal micro mechanisms and explain macro phenomena from the electronic, atomic, and molecular levels.
Through data science methods, effective information can be further excavated from experimental and computational simulation results to establish electrolyte design strategies from micro to macro (i.e. bottom-up). Based on theoretical guidance and experimental attempts, targeted design and synthesis of potentially environmentally friendly low-temperature solvents, additives, and lithium salts used in low-temperature electrolyte are carried out to further enhance the low-temperature performance of lithium ion batteries.
Conclusion
When the temperature is below -20 ℃, the lithium ion battery discharge capacity will significantly decrease, and the cycle life will also be shortened, posing a huge challenge to the application of lithium ion batteries in low-temperature environments.
More seriously, the sharp decrease in conductivity and polarization enhancement at low temperatures can easily cause lithium deposition in the negative electrode, leading to continuous consumption of lithium ion battery electrolyte and potential safety hazards.
Therefore, it is urgent to develop low-temperature electrolyte with high ionic conductivity, high interface compatibility, and rapid desolvation ability to meet the use of lithium ion batteries in low-temperature environments.
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