Lithium-ion batteries, as one of the most successful chemical energy storage batteries in the world today, are not only located in consumer electronic products, but also expanded into the electric vehicle field. However, lithium-ion batteries with such excellent performance are very sensitive to temperature and low temperature. Lithium-ion battery will lead to reduced electrical performance, even lead to lithium-ion battery can not be used, low-temperature charging will lead to lithium dendrite, in order to improve the low-temperature performance of lithium-ion batteries, the majority of scientific researchers put forward a variety of measures, such as The amorphous electrolyte technology proposed by Marta Kasprzyk et al. of Warsaw University of Technology extends the use temperature of the electrolyte to -60°C. The ethyl acetate-based electrolyte proposed by Prof. Xia Yongyao of Shanghai University will be used for special materials for batteries. The use temperature is further reduced to -75°C. Of course, not all scholars have focused their attention on the electrolyte. Guangsheng Zhang et al. of the University of Pennsylvania designed a battery with a built-in Ni heater. The battery is from -40°C. It only takes 112 seconds to return to normal temperature, which greatly improves the convenience of using lithium-ion batteries at low temperatures.
The key to improving the low-temperature performance of lithium-ion batteries is the improvement of the low-temperature performance of electrolytes. The viscosity of conventional commercial lithium-ion battery electrolytes will rapidly increase at low temperatures, and the electrical conductivity will drastically drop. We use a common commercial lithium-ion battery electrolyte LB303. For example, the ionic conductivity at room temperature is about 10mS/cm, but at -40°C, the electrical conductivity drops sharply to 0.02mS/cm, which seriously affects the low-temperature discharge performance of lithium-ion batteries, thus increasing the temperature of lithium-ion batteries at low temperatures. The key to performance is to improve the low temperature performance of the electrolyte.
How to improve the low-temperature performance of lithium-ion battery electrolytes, Janak Kafle of the University of Wisconsin Milwaukee, US, believes that we do not need to add special additives in the electrolyte. By adjusting the electrolyte solvent ratio, we can significantly improve the electrolyte Low-temperature performance. Janak Kafle's research shows that cyclic carbonate solvents reduce the electrolyte's low temperature performance, while linear solvents can improve the electrolyte's low temperature performance.
The following figure shows the molecular structure and some basic physical and chemical parameters of some common lithium-ion battery solvents. From the figure we can see that the EC is a ring structure in the common solvent, and EC can help to form a better stability in the negative electrode. SEI membrane, so we hope to add more EC in the electrolyte, but the higher melting point (38°C) and high viscosity characteristics of EC will lead to excessively low EC conductivity when the EC is added too much, affecting the electrolyte. Low-temperature performance. Linear solvents, such as DMC, EMC, etc., have relatively low viscosity and good electrochemical stability. Therefore, in order to improve the low-temperature performance of lithium-ion battery electrolytes, we usually use a variety of solvent mixtures. To improve the low-temperature performance of electrolytes, for example, MC Smart of the US Jet Propulsion Laboratory has expanded the operating temperature range of SAFT's DD-size batteries (9Ah) to -50-40 by optimizing the electrolyte solvent ratio. °C (-40 °C, the specific energy of C/10 is still up to 95Wh/kg), making it able to meet the requirements of performing Mars exploration tasks.
In order to study the effect of different solvent ratios on the low temperature performance of the electrolyte, Janak Kafle of the University of Wisconsin-Milwaukee designed a variety of electrolytes (as shown in the following table, the test cell was NCM111 (0.93 mAh/cm2) cathode/graphite cathode. Button battery, test system is 25 °C, 1C full charge, set aside for 2h at low temperature, so that the battery reaches the thermal equilibrium after 5C discharge), from the test results, the battery's low temperature discharge capacity is very dependent on the electrolyte solvent ratio, When the proportion of the cyclic solvent exceeds 40%, the discharge capacity of the electrolyte at a low temperature is significantly reduced.
The figure below shows the discharge capacity at low temperature of batteries using different EC ratio electrolytes. From the figure we can clearly observe that the battery discharge capacity at low temperatures is significantly increased with the increase of the ratio of the ring solvent EC addition. reduce.
The following figure shows the effect of different ratios of short-chain solvents on the cell's low-temperature discharge capacity (because EC is only added in a small proportion of 20-30% throughout the experiment, EC has little effect on the low-temperature performance of the cell, so In the study together, we can note that with the increase of short-chain solvent, the battery's low-temperature discharge capacity has significantly increased. This does not actually meet our conventional understanding, because DMC and EC The melting point of 3 °C and 38 °C, respectively, does not significantly reduce the melting point of the electrolyte, indicating that there must be other factors that affect the low temperature performance of the electrolyte.
In order to analyze the key factors affecting the low-temperature performance of the electrolyte, we need to return to the first table of this paper. We note that electrolyte 11# can only discharge about 80% of electrolyte 12# at -20°C. The only difference between the two electrolytes is that 2% VC additive is added to electrolyte 12#, while 2% VC additive does not significantly change the electrolyte conductivity, and more importantly, this Some of the VCs will undergo reductive decomposition during cell formation. Therefore, we can infer that the key factor that leads to better low-temperature performance of electrolyte 12# is the formation of a better SEI film.
The table below compares the proportions of the C, O, F, and P elements in the SEI films formed in electrolytes 9, 10, and 12. From the table, we can note that the biggest difference between these different SEI films is in F. Element, the content of F element in the SEI film formed in the electrolyte 9# is about 70%, while the F element content in the SEI film formed in the electrolyte 10# and 12# is only 10% and 16%, and we It is known that more LiF means smaller Li+ diffusion resistance and therefore better discharge performance.
From the above analysis, it is not difficult to find that the focus of the conflict has shifted from the low-temperature conductivity of the electrolyte to the composition of the negative SEI film. When the SEI film is formed in the lithium ion battery, components in the electrolyte decompose on the surface of the negative electrode. The resulting porous structure. The porosity and density of the SEI film have a significant effect on the performance of the battery. Too high a porosity does not prevent further reaction of the electrolyte on the negative electrode surface, while a too high density results in significant diffusion of Li+ therein. The following table shows the impedance fitting results of several different electrolyte-formed SEI films at 25°C and -20°C. From the table, we notice that the ohmic resistance Rs changes relatively slowly when the temperature decreases, and The diffusion resistance R and the charge exchange resistance Rcte of the Li+ in the SEI film are greatly changed. This shows that the decrease of the ionic conductivity of the electrolyte is not the main reason for the low temperature performance of the battery, and the key factor that really causes the degradation of the battery low temperature performance. In the interface diffusion and charge exchange impedance increases.
It is not difficult to see from the above analysis that the low-temperature conductivity of the electrolyte has no large impact on the low-temperature performance of the lithium-ion battery, and the composition and structure of the negative-electrode SEI film have important effects on the low-temperature performance of the battery. The SEI film should contain more LiF, thus reducing the diffusion resistance of Li+ in the SEI film. In general, more chain solvents, such as EMC and DMC, less cyclic solvents, such as EC, can effectively improve The low-temperature performance of lithium-ion batteries, but in order to form a more stable SEI film, we still need to add a small amount of EC and PC.