Researcher: Real-world performance evaluation of lithium-ion batteries

　　The key technologies for the development of many emerging applications in the future are inseparable from rechargeable batteries. In 2018 alone, there were 11,583 battery-related academic papers published. However, the conclusions and opinions in many papers have been exaggerated, leading to the phenomenon of "the old king sells his melons and boasts himself". There are two reasons for this phenomenon. One is that the researchers deliberately exaggerated, and the other is insufficient understanding of the battery testing system. Therefore, the test results in many laboratories do not represent the true situation of actual battery performance. In particular, many papers do not care about Coulombic efficiency (CE). You must know that if the cycle life of a commercial battery is to reach 500 times, the CE per cycle must be ≥99.96%.

　　1) When evaluating new battery anode materials in coin cells using metallic lithium as the negative electrode of the half-cell, usually the cutoff voltage for charging is set at 2.0 V or even 3.0 V vs. Li+/Li, which will result in high initial CE and High lithium removal capacity. However, according to the author's research, only the capacity in the voltage range of 0-0.8V is meaningful. Therefore, for most anode materials, the authors suggest that testing in the range of 0 to 0.8 V is sufficient; to understand the maximum delithiation capability or to study high-voltage anodes such as Li4Ti5O12 or TiNb2O7, the voltage range can be extended to 0–3.0 V.

　　2) High-rate performance has always been very important, which has been reported in many papers. It is highly dependent on the parameters of the material and electrode. The smaller the particle size of the material and the thinner the coating, the better the rate performance will be. However, this will result in low CE and low mass energy density. Moreover, the areal capacity of commercial batteries is about 3-4 mAh/cm2, so the battery should be measured at 1C rate at 3–4 mA/cm2 current density, while at 9 Measure 3C rate at –12 mA/cm2. In many literatures, despite obvious high-rate performance, the area capacity is far lower than this value, which cannot explain the problem.

　　3) In a lithium-ion full battery, all active lithium is provided by the positive electrode, and the total capacity loss determines the cycle life and actual energy density of the full battery. For graphite and Li4Ti5O12 anodes, the CE after the first cycle is close to 99.9%. However, for high-capacity anode materials reported in many literatures, the CE can only approach 99.5% after 10 or more cycles. Therefore, the total irreversible capacity loss should be calculated to predict the cycle performance of the material in the full battery. If the CE of the battery in each cycle is less than 99.5%, the author believes that the total capacity loss should be given when publishing the article.

　　[Article details]

　　Over the past 25 years, the average energy density growth rate of lithium-ion batteries has been less than 3%, and this growth rate will become slower and slower. From a historical perspective, due to the complex design of battery systems and high application performance requirements, energy density has never increased suddenly. Simply breaking the energy storage performance record of a material does not guarantee that new batteries can be commercialized in the short term. ation, so researchers should be aware of the collusive nature of developing batteries.

　　After 28 years of outstanding efforts by many scientists and engineers, the energy density of power batteries has now reached 300 Wh/kg, and the energy density of 3C consumer batteries has also increased from 90 Wh/kg to 730–750 Wh/L. We often read in articles that the energy density of a new energy storage device may be 2-10 times higher than current lithium-ion batteries, which means an energy density of 600-3000 Wh/kg or 1460-7500 Wh/L, although These values are very much needed, but to be honest, it's hard to achieve.

　　In addition to artificially exaggerating the data, a large part of it is due to the lack of standard test protocols for laboratory research, which leads to exaggerated performance. The development of effective laboratory standard testing protocols is of greater significance for the development of a new generation of lithium batteries using metallic lithium anodes. Therefore, researchers must understand the process parameters and standard testing protocols of actual batteries.

　　Recently, Lin et al. have pointed out (Nat. Commun. 9, 5262.) that the performance derived from a limited number of laboratory test reports does not represent the true situation of actual battery performance. In particular, many papers do not care about Coulombic efficiency (CE). It should be noted that commercial batteries require cycle stability of at least 500 cycles and maintain at least 99.96% CE. In the journal Joule, Chen et al. reported a set of button battery parameters and testing system based on the 300 Wh/kg soft pack battery level (Joule 3, this issue, 1094–1105.), which will help speed up the development of new materials. discovered, and mapped completely to a real battery. As mentioned in this article, a large number of papers on button cell testing conditions are not suitable for evaluating new materials and batteries, however, many authors do not have this knowledge.

　　Table 1. Process parameters of Li/NCA soft pack battery when the energy density is 385Wh/kg.

　　To this end, the author assembled a batch of lithium metal soft-pack batteries using NCA as the positive electrode and 50 mm lithium foil as the negative electrode. The parameters are shown in Table 1. Compared with the parameters previously reported by Chen, the design parameters of most materials are similar, except for the amount of electrolyte injected, and the selection of electrolyte and cathode. In this paper, the initial injected liquid electrolyte is 1.86 g/Ah, which is comparable to lithium-ion batteries. After in-situ solidification and the use of new electrolyte designs, the battery can be cycled more than 100 times with a capacity retention rate of more than 90%, as shown in Figure 1. The purpose of in-situ solidification is to reduce the continuous reaction between metallic lithium and liquid electrolyte, helping to reduce the weight ratio of the electrolyte.

　　Figure 1. Weight, volume ratio and electrochemical performance of each component in a real Li/NCA soft pack battery; (A) weight ratio, (B) volume ratio, (C) 0.3C at 2.75–4.3V voltage Battery charge and discharge curve, (D) battery capacity retention rate and Coulombic efficiency.

　　As Chen pointed out in Joule and Cao in the journal Nature Nanotechnology, electrolyte dosage, positive and negative electrode areas, N/P ratio and rate are important parameters that affect the performance of button batteries. In addition to process parameters, the author also summarized 10 factors that may cause deviation, error or low repeatability in button battery measurement, including material preparation, weighing, grinding, mixing, coating, accidental short circuit, button battery manufacturing, and measurement. Instrumentation, environmental controls, and experimental design. Therefore, great care should be taken when performing basic electrochemical performance testing using coin cells to obtain reliable, repeatable, and valuable data.

　　In addition to the above process parameters and experimental design, there are also the following problems, which may mislead the evaluation of new materials and new devices:

　　1) Voltage range: When using metallic lithium as the negative electrode of the half-cell and evaluating new battery negative electrode materials in button batteries, the cut-off voltage of charging is usually set at 2.0 V or even 3.0 V vs. Li+/Li, which will result in high Initial CE and high delithiation capability. However, according to the author's research, only the capacity in the voltage range of 0-0.8V is meaningful. Therefore, for most anode materials, the authors suggest that testing in the range of 0-0.8 V is sufficient. To understand the maximum delithiation capability or to study high-voltage anodes such as Li4Ti5O12 or TiNb2O7, the voltage range can be extended to 0–3.0V. Researchers should be aware that the high delithiation voltage of the negative electrode will lead to the low discharge voltage of the full battery. In some papers, high discharge capacity in a wide voltage range has some significance for basic research, but has no value for positive electrode applications.

　　2) Rate performance: High rate performance has always been very important. This has been reported in many papers. It is highly dependent on the parameters of the material and electrode. The smaller the particle size of the material and the thinner the coating, the rate performance will decrease. The better. However, this will result in low CE and low gravimetric energy density. Moreover, the areal capacity of commercial batteries is about 3-4 mAh/cm2, so the battery should be measured at 1C rate at 3–4 mA/cm2 current density, and at 9 Measure 3C rate at –12 mA/cm2. In many literatures, despite obvious high-rate performance, the area capacity is far lower than this value, which cannot explain the problem. Some authors use mA/g and A/g as the current density, while the authors of this article recommend using the area current density mA/cm2. This is because very low active material loading (mg/cm2) will lead to very high mA/g, which It has no reference value at all for practical applications. It is not surprising that some authors claim that their prepared electrodes can be cycled at a high rate of 100 C, which is equivalent to discharging or charging once in 36 s. However, assuming an area capacity of 3 mAh/cm2 and 100 C represents 300 mA/cm2, such a high current density may cause thermal runaway in a full cell. Additionally, when researchers claim high-rate performance, they often see that capacity retention at high rates is already low, while commercial applications require 80% capacity retention at the highest rate. For example, if the battery's capacity can maintain 80% at 3C, we can claim that the battery can charge and discharge stably at 3C.

　　3) Coulombic efficiency: In a lithium-ion full battery, all active lithium is provided by the positive electrode. The total capacity loss determines the cycle life and actual energy density of the full battery. For graphite and Li4Ti5O12 negative electrodes, the CE after the first cycle is close to 99.9%, while for many high-capacity anode materials reported in the literature, the CE can approach 99.5% only after 10 or more cycles. Therefore, the total irreversible capacity loss should be calculated to predict the cycle performance of the material in the full battery. If the CE of the battery in each cycle is less than 99.5%, the author believes that the total capacity loss should be given when publishing the article.

　　4) Energy density: The development of lithium-ion batteries can be seen as the history of increased battery energy density. It is always attractive and exciting when a new material or a new battery with high energy density is reported. of. But to avoid exaggeration, it should be known that when selecting negative and positive electrode materials, the theoretical energy density should be calculated using the Nernst equation based on the production data of reactants and products. It is known that the ratio of the actual energy density of the battery to the theoretical energy density of the electrochemical reaction is at most about 58%, so the actual energy density can be roughly estimated based on this ratio and the theoretical energy density. Of course, a truly meaningful calculation should include all materials based on reasonable data units, similar to the actual data provided in Table 1. It should be noted that some parameters in Table 1 can be modified further. When the reversible capacity and thickness of the positive electrode increase, the thickness of the separator, copper foil, and aluminum foil decreases, and the N/P ratio decreases from 2.0 to 1.2, the energy density of the battery will be significantly improved. These improvements are very possible, for example, the thinnest 3 mm lithium foil can be produced in the industry.

　　Recently, there have been several papers discussing the energy density in actual batteries, and these calculations clearly show that the actual energy density of the battery may be much lower than the values obtained from only rough estimates of the positive or negative active materials, so the reader should be very careful , don’t be too optimistic about the reported data. Especially when determining goals, actual calculations based on full parameters are of great help in judging whether the goals can be achieved. For example, for Li/NMC batteries (NMC: LiNixMnyCozO2), only when the reversible capacity of NMC exceeds 220 mAh/g , it is possible to achieve an energy density of 500 Wh/kg. This means that lithium-rich oxide cathodes may be more realistic, or the only feasible option to achieve the 500Wh/kg target.

　　Due to the diversity of electrode and coin cell designs, standardization does not have to be forced in basic research, however, researchers must understand that coin cell experimental design, cell fabrication and testing protocols have a significant impact on the results. Researchers can only make exciting claims after conducting solid experiments and proper data analysis.

　　It should be noted that in addition to basic electrochemical performance measurements, many other tests, characterizations and analyzes may also contain significant biases and problems. In order to obtain valuable and reliable data, each method must be understood and standardized. Doing so can Save time, improve research efficiency, and reduce the difficulty of technology transfer. In the long history of battery development, all progress is based on the ability to withstand the supervision of others and effective innovation.

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