Text/Da Zhuang
Editor/Da Zhuang
First, the challenge of low-temperature electrolyte for lithium-ion batteries
In a low temperature environment, the electrode reaction rate of lithium-ion batteries decreases, resulting in battery capacity degradation. This is caused by the decrease in the ionic conductivity of the electrolyte at low temperatures and the decrease in the reactivity of the active substance on the electrode surface.
Reduced lithium ion diffusion rate: At low temperatures, the viscosity of the electrolyte increases and the ion diffusion rate slows down. This results in limited transmission of lithium ions in the electrolyte, reducing the rate of storage and release of lithium ions, thereby reducing the effective capacity of the battery.
Reduced electrochemical reaction activity: At low temperatures, the rate of chemical reactions of the electrode material and electrolyte decreases. The electrochemical reaction during the discharge and charging process of the battery needs to occur on the electrode surface, and the activity of the electrochemical reaction is reduced under low temperature conditions, which limits the storage and release capacity of the active material in the battery.
Decreased lithium-ion insertion/de-intercalation efficiency: In low temperature environments, the lithium-ion insertion and de-intercalation efficiency of the electrode material decreases. The rate of charge transfer during insertion and de-intercalation slows down, resulting in reduced cycle stability and increased capacity decay. Lithium metal dendrite growth: At low temperatures, lithium metal dendrites may form on the surface of the anode of lithium-ion batteries. The growth of these dendrites can lead to short circuits and safety problems inside the battery, and further reduce the usable capacity of the battery.
At low temperatures, the conductivity of the electrolyte decreases, resulting in an increase in the internal resistance of the battery and limiting the migration rate of ions. This further affects the discharge performance and charging rate of lithium-ion batteries. Increased viscosity of the electrolyte: At low temperatures, the viscosity of the electrolyte increases significantly. An increase in viscosity causes the diffusion rate of ions in the electrolyte to slow down, reducing the conductivity of the electrolyte.
Formation of ionic-solvent pairing: In the electrolyte of lithium-ion batteries, lithium ions are usually paired with solvent molecules. At low temperatures, pairing formation becomes more stable, resulting in lithium ions binding more tightly to solvent molecules, slowing down the migration rate of ions.
Decrease in salt solubility: The solubility of some lithium salts at low temperatures decreases significantly, resulting in a decrease in ion concentration, which in turn reduces the conductivity of the electrolyte.
Ion-electrolyte solvent interaction: At low temperatures, the interaction between ions and electrolyte solvent becomes more intense. These interactions can reduce the fluidity of solvent molecules, which reduces the conductivity of the electrolyte.
At low temperatures, lithium metal dendrites in lithium-ion batteries will grow on the surface of the negative electrode, causing short circuits and safety problems in the battery. This is mainly caused by the decrease in conductivity of the electrolyte at low temperatures and the uneven deposition of lithium ions on the surface of the negative electrode.
Slowing ion migration rate at low temperatures: In a low temperature environment, the ion migration rate inside the battery slows down, resulting in the deposition process of ions on the surface or interface of the negative electrode becoming uneven. This uneven deposition encourages lithium metal to grow in the form of dendrites.
Increased interfacial instability: At low temperatures, the interface instability between the electrolyte inside the battery and the electrode material increases. This instability causes lithium ions in the electrolyte to accumulate on the surface of the negative electrode and form lithium metal dendrites.
Electrolyte composition and formulation: Additives and salts in the electrolyte affect the growth of lithium metal dendrites. Inappropriate additives and salt components may cause the electrolyte to be unstable at low temperatures, thereby promoting the formation of lithium metal dendrites.
Internal short circuit of the battery: The growth of lithium metal dendrites will cause a direct short circuit between the negative electrode and the positive electrode, which will impair the normal operation of the battery. This short circuit can lead to battery heating, capacity loss and even serious safety problems such as combustion and explosion.
Electrolyte depletion: The growth of lithium metal dendrites will lead to the irreversible loss of lithium ions in the battery, depleting lithium ions in the electrolyte. This will lead to battery capacity degradation and reduced cycle life, which in turn affects the reliability and service life of the battery.
Reduced safety: The growth of lithium metal dendrites increases the risk of heat generation and thermal runaway in the battery. The unstable interaction of lithium metal dendrites with electrodes or electrolytes may cause the temperature of the battery to rise, which in turn can lead to serious safety accidents such as overheating, combustion or explosion of the battery.
Reduced battery cycle life: The growth of lithium metal dendrites will lead to accelerated capacity decay of the battery during the charge-discharge cycle. The formation and expansion of dendrites can damage the internal structure of the battery, resulting in a significant reduction in the cycle life of the battery.
Second, the solution strategy
Adding additives that inhibit the growth of lithium metal dendrites is a common strategy to improve the performance and safety of lithium-ion batteries at low temperatures. These additives are able to adjust the chemical properties of the electrolyte and inhibit the dendrite growth of lithium metal, thereby reducing dendrite short circuits and safety risks inside the battery. Several common additives and their mechanisms of action will be discussed in detail here.
Lithium salt additive is a common additive that inhibits the growth of lithium metal dendrites. Commonly used lithium salt additives include lithium fluoride (LiF), lithium borate (LiBOB) and lithium sulfate (Li2SO4). These additives can inhibit dendrite growth by the following mechanisms:
Formation of stable solid electrolyte interface (SEI): lithium salt additives can form a stable solid electrolyte interface layer on the surface of the negative electrode, prevent direct contact between lithium metal and electrolyte, and reduce the formation of dendrites.
Adjust the chemical properties of the electrolyte: Lithium salt additives can adjust the solubility, ion migration rate and other chemical properties of the electrolyte, thereby reducing the tendency of dendrite formation.
Polymer additives are another commonly used class of additives that inhibit the growth of lithium metal dendrites. Common polymer additives include polyethylene oxide (PEO), polyacrylonitrile (PAN), and polyacrylic acid (PAA). These additives are able to pass through the following mechanisms
Formation of polymer lithium ion conduction layer: Polymer additives can form a lithium ion conduction layer on the surface of the negative electrode, limiting the dendrite growth of lithium metal. This polymer layer provides a more homogeneous interface of lithium deposition and reduces dendrite formation.
Adjust the viscosity and ion migration rate of the electrolyte: Polymer additives can increase the viscosity of the electrolyte and reduce the migration rate of ions, thereby reducing the formation of lithium metal dendrites.
In addition to a single additive, researchers often combine different additives to further improve the inhibition of lithium metal dendrite growth. Common additive combinations include the combination of lithium salt additives and polymer additives, as well as combinations of many different types of polymer additives.
Combination of lithium salt additives and polymer additives: lithium salt additives can provide a solid electrolyte interface layer to prevent direct contact with lithium metal; Polymer additives can form a lithium-ion conduction layer and adjust the viscosity and ion migration rate of the electrolyte. The combination of these two additives works synergistically to provide better inhibition of lithium metal dendrites.
Combination of multiple polymer additives: Different types of polymer additives have different properties and mechanisms of action. The combination of a variety of polymer additives can play a role in different aspects and improve the effect of inhibiting lithium metal dendrites. For example, combining a polymer with high lithium-ion conduction properties with a polymer with a viscosity adjustment can comprehensively improve the characteristics of the electrolyte and reduce the formation of dendrites.
In view of the problem of decreasing conductivity of electrolyte, the conductivity of the electrolyte can be improved by optimizing its formulation. One way is to select solvents and salts with high ionic conductivity.
For example, the use of solvents with low pour point, such as carbonate and ether solvents, can improve the fluidity and ion mobility rate of the electrolyte at low temperatures. In addition, choosing the right salt, such as lithium hexafluorophosphate (LiPF6) or lithium sulfuryl fluoride (LiFSI), can also improve the conductivity of the electrolyte.
The solvent of the electrolyte is one of the key factors affecting conductivity. In a low temperature environment, the choice of solvent needs to consider its low temperature fluidity and ion solubility. Common solvents include carbonates, ethers and esters. For example, carbonate solvents commonly used at low temperatures, such as ethylene glycol dimethyl ether (DME) and ethylene glycol diethyl ether (DEE), have lower viscosity and better ionic solubility, which can improve the conductivity of the electrolyte.
The salts in the electrolyte are key to ion conduction. Choosing the appropriate salt can improve the conductivity of the electrolyte. Common salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) and lithium hexafluorosulfonate (LiTFSI). These salts have high ionic conductivity and can increase the migration rate of ions in the electrolyte, thereby increasing the conductivity of the electrolyte.
Adding conductive additives is a common strategy when optimizing electrolyte formulations. These additives can improve the ionic conductivity in the electrolyte, thereby increasing the conductivity. Common conductive additives include carbonate additives, silicate additives and fluorate additives. These additives can increase the migration rate of ions in the electrolyte and improve the conductivity of the electrolyte.
The ratio of solvent to salt also affects the conductivity of the electrolyte. Proper solvent to salt ratio can improve the ionic conductivity of the electrolyte. A solvent ratio that is too high or too low can lead to an increase in the viscosity of the electrolyte or a decrease in ion concentration, reducing conductivity.
In order to cope with the problem of low temperature environment, reasonable temperature management and insulation design are also very important. By using a heating system or insulation layer, the operating temperature of the battery can be increased, and the fluidity and ion conduction performance of the electrolyte can be improved. In addition, considering the selection and layout of insulation materials in the battery design can reduce the negative impact of low temperatures on battery performance.
3. Experimental results and discussion
By comparing experiments with existing studies, we can verify the effectiveness of the above strategies. For example, the addition of additives that inhibit dendrite growth can significantly reduce the growth of lithium metal dendrites and improve the safety of batteries. Optimizing the electrolyte formulation can significantly improve the conductivity of the electrolyte and improve the discharge performance of lithium-ion batteries at low temperatures. Temperature management and insulation design can also effectively improve the low temperature performance of the battery.
4. Future prospects
Despite certain achievements, the performance and reliability of lithium-ion batteries in low-temperature environments still present challenges. Future research can be carried out in the following areas:
At present, inhibitors for the growth of lithium metal dendrites still need to be further improved and developed. Researchers can explore the use of novel additives to improve their effectiveness in inhibiting dendrite growth. Through a deep understanding of the dendrite growth mechanism, more effective additives can be designed and systematically verified experimentally.
The interface between the electrolyte and the individual components inside the battery has an important impact on low-temperature performance. Future studies can focus on the interfacial behavior of electrolytes at low temperatures, including electrode interfaces and solid electrolyte interfaces. By improving the stability and conductivity of the interface, the performance of lithium-ion batteries at low temperatures can be further improved.
In addition to optimizing electrolyte formulations, the development of new electrolytes can also be explored. For example, solid electrolytes are considered a potential solution to the problem of low temperatures. Researchers can develop solid electrolytes with high ionic conductivity and good low-temperature stability to replace traditional liquid electrolytes, thereby improving the performance of lithium-ion batteries at low temperatures.
Using computational simulation and modeling methods, it is possible to better understand how lithium-ion batteries work at low temperatures, and predict and optimize battery performance. Through simulation and modeling, suitable materials and cell structures can be selected and designed more quickly, accelerating the process of low-temperature battery development.
conclusion
Lithium-ion batteries face challenges such as capacity decay, conductivity degradation and lithium metal dendrite growth in low temperature environments. To address these problems, strategies such as adding additives to inhibit dendrite growth, optimizing electrolyte formulations to improve conductivity, temperature management, and insulation design are proposed. The effectiveness of these strategies is verified by comparing the experimental results with existing studies.
However, further research is needed to better understand how lithium-ion batteries work at low temperatures and to develop new materials and technologies to improve their performance and reliability. Future research can focus on the development of new electrolyte additives, optimization of electrolyte interfaces, development of new electrolytes, and simulation and modeling studies. These efforts will help improve the performance of lithium-ion batteries at low temperatures, expand their application areas, and promote the development of sustainable energy storage technologies.