CHEMICAL SCIENCE

The high-specific energy power battery charges at a new record for extremely fast charging


Recently, the team of Professor Wang Chaoyang of Pennsylvania State University in the United States has made another major breakthrough in lithium-ion battery fast charging technology, replacing the traditional electrolyte system with double salt, which not only improves the thermal stability of the electrolyte, but also greatly reduces the risk of lithium precipitation. On October 12, 2022, the article was published in Nature, a top international journal, under the title “Fast charging of energy-dense lithium-ion batteries.”

Background

At present, the energy density of commercial lithium-ion batteries (LIB) using high nickel ternary cathode and graphite anode has reached 250~300Wh/kg, which is widely used in the field of electrified transportation. However, mileage anxiety and charging anxiety are recognized as the core reasons that restrict the large-scale promotion and popularity of electric vehicles. In the context of the current rapid rise in lithium prices, reducing mileage anxiety by assembling large-capacity batteries (such as 100kWh, > 500km) is not an effective way. Ten minutes of rapid energy replenishment not only eliminates mileage anxiety but also can shrink the battery pack, which greatly reduces battery costs and raw material consumption. Lithium evolution has always been the biggest challenge to limit fast charging, and mechanically speaking, the root cause of lithium evolution is that one of the following three processes is hindered: 1) the transmission of lithium ions in the electrolyte; 2) Electrochemical reaction of lithium ions on the surface of graphite anode; 3) Solid phase diffusion of lithium ions inside graphite particles. Therefore, to speed up the transmission process and reaction rate inside the battery, that is, to suppress the occurrence of lithium analysis by coordinating the subversion of the battery structure, optimizing the battery material and thermal regulation strategy.

Outcome briefing

Recently, Professor Wang Chaoyang team of Pennsylvania State University in the United States has made another major breakthrough in lithium-ion battery fast charging technology, using double salt (0.6 M LiFSI +0.6 M LiPF6) to replace the traditional electrolyte system (1 M LiPF6), which not only improves the thermal stability of the electrolyte, but also greatly reduces the risk of lithium analysis, combined with the previous Wang Chaoyang team fast heat and asymmetric temperature modulation (Asymmetric Temperature Modulation, The ATM) method (i.e., heating to high temperature (~60°C) for fast charging before charging, discharging at room temperature) enables fast charging of high-energy-density lithium-ion batteries (265 Wh/kg) (~10 minutes of charging ~75% SOC) and is capable of stable cycles of up to 2,000 times.

Core content

Battery fast charging technology must be measured by 3 indicators at the same time: 1) charging time; 2) Charge specific energy; 3) The number of cycles under fast charging. Figure 1 compares the degree to which all fast charging techniques in the literature meet these three criteria. In response to the demand for electric vehicle power batteries, the U.S. Department of Energy put forward the requirement of at least 150Wh/kg specific energy for 15min (corresponding to the upper left rectangular area in Figure 1), and its ideal goal is to achieve 5min charge of 240Wh/kg at the same time can exceed 2000 stable cycles. As shown in Figure 1, although the battery using lithium titanate (LTO) negative electrode in the lower left corner can achieve 15C and 3000 cycles, its charge specific energy is less than 100Wh/kg, which is not suitable for the high battery specific energy requirements in the automotive field. In addition, the use of large batteries for flash charging of electric vehicles on the market belongs to the lower left corner area. For example, using a large battery with a range of 700 kilometers to flash 200 kilometers is only charged 30%, which is equivalent to 75 Wh/kg of the charging specific energy (30% of the 250Wh/kg battery has a calibrated energy density). In the upper right corner is the lithium metal battery (LMB) area, which has a high specific energy but a relatively long charging time. At present, only the development of graphite anode batteries is closest to the ideal goal of fast charging for automobiles. To reduce the risk of fast lithium recharge, different methods are used in different literature, such as replacing electrolytes, reducing electrode curvature, or using asymmetric temperature thermal control techniques.

Figure 1: Distribution of different batteries in the fast charge guidelines. Taking the charging time abscissa and charge specific energy as the ordinate coordinates, the circle size represents the number of cycles under fast charging conditions. a, the number of cycles is greater than 800 turns of the battery distribution. b, the number of cycles is less than 800 turns of the battery distribution.

The asymmetric temperature thermal regulation method (ATM) mentioned above contains two core contents: First, the battery is quickly preheated to 60 ° C or even higher before charging, and the increase in temperature can significantly accelerate the transmission process and reaction rate inside the battery, thereby avoiding lithium analysis; On the other hand, the battery is only at a high temperature of 60 ° C during fast preheating and extremely fast charging, and at room temperature under other application conditions, while the growth of the solid electrolyte interface film (SEI) is related to the time of exposure to high temperature (Figure 2a), which reduces the time of the battery at high temperature by means of ATM fast charging, effectively avoiding the aging caused by high temperature on the battery material. The researchers applied this method to fast charge batteries with 3.4 mAh/cm2 and 4.2 mAh/cm2 loads, even if the charging rate was reduced to 2C (24 minutes to 80%), the two batteries underwent capacity diving in ~70 and ~420 cycles, respectively (Figure 2b, c). This shows that even in the case of 60 ° C, the transmission performance of the electrolyte can not meet the needs of fast charging. First, because the conductivity and ion diffusivity of the electrolyte are less with the increase of temperature, it is only about 2 times that of 20 °C at 60 °C; Another reason is that the electrolyte transmission is limited, even when the initial cycle is not lithium precipitation, with the aging of the electrode and the consumption of the electrolyte, the battery will occur during the cycle of lithium precipitation, lithium metal quickly reacts with the electrolyte, resulting in further attenuation of reaction kinetics, and finally induce lithium analysis to lead to capacity diving.

Figure 2: Asymmetric temperature thermal regulation method for high-specific lithium-ion battery capacity decay curve. a, 1C charge and discharge cycle at 60°C due to aging caused by SEI growth. b, The ATM for battery applications with a surface load of 4.2 mAh/cm2 is charged to 100%, 75% and 75% SOC at 1C, 1.5C and 2C, respectively. c, ATM for battery applications with a surface load of 3.4 mAh/cm2 and a base electrolyte charged to 80%, 80%, and 70% SOC at 2C, 3C, and 4C, respectively. b, the solid line in c is the true attenuation curve of battery capacity, and the dashed line is the capacity decay curve calculated by a fitting only caused by SEI growth.

Based on the above problems, Wang Chaoyang’s team made targeted improvements to the design of electrolyte and electrode structure to improve the ion transport performance of the battery. First, the traditional electrolyte system (1M LiPF6) was replaced with double salt (0.6M LiFSI+0.6M LiPF6); Compared with LiPF6, LiFSI has a higher lithium ion migration number (0.56 vs. 0.38), that is, at the same magnification, LiFSI can reduce the electrolyte concentration polarization, improve the uniformity of lithium inlay reaction in the direction of electrode thickness, and reduce the risk of lithium precipitation. In addition, LiFSI has better thermal stability, and when combined with ATM methods, two-salt electrolyte systems are better than conventional electrolyte systems. Secondly, for the high specific energy graphite electrode porosity increased from the initial 0.26 to 0.35, although the electrolyte mass was increased, resulting in a 2% energy density loss, the tortuous degree of the ion transport path was greatly reduced, resulting in an increase in ion transport rate of ~40%.

The researchers applied the above improvements to lithium-ion batteries with a surface load of 3.4 mAh/cm2 and quickly charged the batteries again using the ATM method. The cycling results show that when the improved battery is charged to 75% SOC in 12 minutes in a 4C CCCV manner, the battery life can reach more than 900 cycles; When the fast charge capacity is reduced to 70% SOC, the charging time is only 11 minutes, and the battery life can reach more than 2000 cycles (Figure 3b). Experimental results show that ion transport resistance is the limiting factor for fast charging of high-specific energy lithium-ion batteries, and combined with the coordination of ATM method and electrolyte and electrode structure, this limitation can be successfully broken.

Figure 3: Fast charging performance of high-specific energy lithium-ion batteries after ATM application by using a two-salt electrolyte system, improving the electrode structure. a, Battery voltage and temperature rise curve under ATM method. b, fast charging capacity decline curve. c, fast charge charge than capacity

Finally, in order to verify the battery cooling requirements and safety under the above formula, structure and thermal control strategy, the researchers conducted a numerical simulation of the battery module of the electric vehicle, which was composed of 12 150Ah square cells using a double salt electrolyte system and a high porosity electrode monomer connected in series, with seamless contact between the cells, and the upper and lower ends of the module were forced by air convection to dissipate heat (Figure 4a). The simulation results show that at C/3 discharge, it takes 11.6min to drop the temperature to 40°C, which is close to 8min during the single battery test. Therefore, extremely reliable air-cooled heat dissipation is feasible in battery modules using ATM methods. For the high temperature safety of the battery at 65 ° C, because the battery itself is placed at high temperature for a short time, such as the total time of high temperature under 1000 fast charge cycles is 167h, accounting for only 0.167% of the 12-year service life of an electric vehicle, and the capacity loss caused by SEI growth at high temperature is also much less than the limit of 20%. Therefore, compared with the traditional liquid-cooled heat dissipation that requires the design of complex runners, compressed battery space, and the hidden danger of leakage, the thermal control module that only uses up-down and down-air cooling for heat dissipation has extremely high reliability and safety.

Figure 4: Electrochemical-thermally coupled simulation of a 150Ah square battery. a, battery structure, module form and forced air convection heat transfer conditions. b, 150Ah battery cell 4C charge C/3 discharge, the temperature distribution of position 3. c, The maximum and minimum temperature change curve of the battery during charge and discharge. d, the change curve during the battery voltage charge and discharge.

In summary, by using a double-salt electrolyte system, improving electrode porosity, and applying asymmetric temperature thermally regulated fast charging technology, the 265 Wh/kg high specific energy lithium-ion battery is charged to 75% SOC (or 70% SOC) for 12 minutes (or 11 minutes), and its stable cycle is as high as 900 times (or 2000 times). At the same time, the battery module that applies ATM method for 4C charging can ensure reliability and safety in the air-cooled heat dissipation system, providing a safe and effective way for the development of all-solid-state CTP battery packs. In addition, by combining the battery material and the thermal control structure to design together, the ATM method can be applied to a wider range of battery material systems such as the next-generation metal lithium anode or silicon anode. (Source: Science Network)

Related Paper Information:https://doi.org/10.1038/s41586-022-05281-0



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