However, many papers exaggerate their conclusions and viewpoints, exhibiting a tendency towards self-praise. This phenomenon arises for two reasons: firstly, researchers deliberately exaggerate; secondly, there is a lack of understanding of battery testing systems. Therefore, many laboratory test results do not accurately reflect the true performance of actual batteries. In particular, many papers neglect coulombic efficiency (CE). It's important to understand that for a commercially viable battery to achieve a cycle life of 500 cycles, the CE per cycle must be ≥99.96%.
To this end, Researcher Li Hong from the Institute of Physics, Chinese Academy of Sciences, published a paper entitled "Practical Evaluation of Li-Ion Batteries" in the international journal *Joule*. He also assembled a batch of pouch lithium batteries and analyzed their various parameters and performance characteristics, aiming to help more researchers better identify problems and further bridge the gap between the laboratory and industry. The author argues that while there may not be a single standard for laboratory testing systems, researchers must understand that the experimental design, battery fabrication, and testing methods for button batteries have a significant impact on the results. Only by clarifying certain concepts and parameters can researchers collaborate and jointly promote development in the next generation of lithium batteries using lithium anodes. This article is concise and well-written, making it a highly recommended read.
[Author's suggestion]
1) When using lithium metal as the negative electrode in a half-cell and evaluating new battery negative electrode materials in coin cells, the charging cutoff voltage is typically set at 2.0V or even 3.0V vs. Li+/Li, which results in high initial CE and high delithiation capability. However, according to the authors' research, only the capacity within the voltage range of 0–0.8V is meaningful. Therefore, for most negative electrode materials, the authors suggest that testing within the 0–0.8V range 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.
2) High rate performance has always been crucial, as reported in numerous papers. It is highly dependent on material and electrode parameters; smaller particle sizes and thinner coatings generally result in better rate performance. However, this leads to lower CE (Cellular Energy Conversion) and lower mass energy density. Furthermore, the areal capacity of commercial batteries is approximately 3–4 mAh/cm². Therefore, batteries should be measured at 3–4 mA/cm² for 1C and 9–12 mA/cm² for 3C. In many studies, despite exhibiting significantly high rate performance, the areal capacity is far below this value, which is not indicative of a high rate performance.
3) In lithium-ion full cells, all active lithium is supplied by the positive electrode, and the total capacity loss determines the cycle life and actual energy density of the full cell. Regarding graphite and Li4Ti5O12 anodes, the CE (cycle efficiency) after the first cycle is close to 99.9%, while many high-capacity anode materials reported in the literature only achieve a CE close to 99.5% after 10 or more cycles. Therefore, the total irreversible capacity loss should be calculated to predict the material's cycle performance in a full cell. If the battery's CE is below 99.5% in each cycle, the authors believe that the total capacity loss should be provided in the published article.
[Article Details]
Over the past 25 years, the average energy density of lithium-ion batteries has increased by less than 3%, and this rate of increase is slowing down. Historically, due to the complexity of battery system design and the high performance requirements of applications, energy density has never seen a sudden surge. Simply breaking a record for the energy storage performance of a material does not guarantee that a new battery can be commercialized in a short period of time. Therefore, researchers should be aware of the complicity in battery development.
Thanks to the remarkable efforts of many scientists and engineers over the past 28 years, the energy density of power lithium batteries has now reached 300Wh/kg, and the energy density of 3C consumer batteries has increased from 90Wh/kg to 730–750Wh/L. We often read in articles that a new energy storage device may have an energy density 2–10 times higher than current lithium batteries, which means an energy density of 600–3000Wh/kg or 1460–7500Wh/L. While these values are very impressive, frankly speaking, they are very difficult to achieve.
Besides the artificial exaggeration of data, a significant portion of the discrepancy stems from a lack of standardized testing methods in laboratory research, leading to exaggerated performance figures. Developing effective standardized laboratory testing methods is of paramount importance for the development of next-generation lithium-ion batteries using lithium metal anodes. Therefore, researchers must understand the process parameters and standard testing methods of actual batteries.
Recently, Lin et al. pointed out (Nat. Commun. 9. 5262.) that the performance obtained from a limited number of laboratory test reports cannot represent the true performance of actual batteries. In particular, many papers do not focus on coulombic efficiency (CE), while commercial batteries require cycle stability of at least 500 cycles and at least 99.96% CE. In the journal Joule, Chen et al. reported a set of coin cell parameters and testing system based on 300Wh/kg pouch lithium batteries (Joule 3. this issue, 1094–1105.), which will help accelerate the discovery of new materials and fully map them to real batteries. As mentioned in this article, the testing conditions for coin cells in a large number of papers are not suitable for evaluating new materials and new batteries; however, many authors are unaware of this knowledge.
Table 1. Process parameters of Li/NCA soft-pack lithium batteries with an energy density of 385Wh/kg.
To this end, the authors assembled a batch of lithium metal pouch batteries using NCA as the positive electrode and 50 mm lithium foil as the negative electrode, with parameters shown in Table 1. Compared to the parameters previously reported by Chen, the design parameters for most materials are similar, except for the amount of electrolyte injected and the selection of the electrolyte and positive electrode. In this paper, the initial injected liquid electrolyte was 1.86 g/Ah, comparable to that of lithium batteries. After insitu solidification and the use of the new electrolyte design, the battery can cycle more than 100 times with a capacity retention exceeding 90%, as shown in Figure 1. The purpose of insitu solidification is to reduce the continuous reaction between metallic lithium and the liquid electrolyte, which helps to reduce the weight ratio of the electrolyte.
Figure 1. Weight and volume ratios of components and their electrochemical performance in a real Li/NCA pouch lithium battery; (A) weight ratio, (B) volume ratio, (C) charge-discharge curves of the battery at 0.3C under voltages of 2.75–4.3V, (D) capacity retention and coulombic efficiency of the battery.
As Chen in Joule and Cao in Nature Nanotechnology pointed out, electrolyte content, positive and negative electrode areas, N/P ratio, and rate capability are important parameters affecting coin cell performance. In addition to process parameters, the authors summarized 10 factors that can lead to measurement bias, errors, or low repeatability in coin cells: material preparation, weighing, grinding, mixing, coating, accidental short circuits, coin cell manufacturing, measuring instruments, environmental control, and experimental design. Therefore, extreme caution should be exercised when performing basic electrochemical performance tests using coin cells to obtain reliable, repeatable, and valuable data.
In addition to the aforementioned process parameters and experimental design, the following issues may mislead the evaluation of new materials and devices:
1) Voltage Range: When using lithium metal as the negative electrode in a half-cell and evaluating new battery negative electrode materials in coin cells, the charging cutoff voltage is typically set at 2.0V or even 3.0V vs. Li+/Li, which results in high initial CE and high delithiation capability. However, according to the authors' research, only the capacity within the 0-0.8V voltage range is meaningful. Therefore, for most negative electrode materials, the authors suggest that testing within the 0-0.8V range is sufficient. To understand the maximum delithiation capability or to study high-voltage negative electrodes, such as Li4Ti5O12 or TiNb2O7, the voltage range can be extended to 0–3.0V. Researchers should be aware that a high delithiation voltage of the negative electrode leads to a low discharge voltage of the full cell. While high discharge capacity over a wide voltage range may have some significance for basic research in some papers, it is of no value for positive electrode applications.
2) Rate Performance: High rate performance has always been crucial, as reported in numerous papers. It is highly dependent on material and electrode parameters; smaller particle sizes and thinner coatings generally result in better rate performance. However, this leads to lower CE (Cellular Energy Density) and lower gravimetric energy density. Furthermore, the areal capacity of commercial batteries is approximately 3-4 mAh/cm². Therefore, batteries should be measured at 3–4 mA/cm² for 1C and 9–12 mA/cm² for 3C. In many studies, despite significant high rate performance, the areal capacity is far below this value, which is inconclusive. Some authors use mA/g and A/g as current densities, while this paper recommends using the areal current density mA/cm². This is because extremely low active material loading (mg/cm²) results in very high mA/g, which is of no practical value. Some authors claim their electrodes can cycle at 100C, equivalent to a 36-second discharge or charge cycle, which is not surprising. However, assuming an areal capacity of 3 mAh/cm², 100C represents 300 mA/cm². Such a high current density could lead to thermal runaway in a full cell. Furthermore, when researchers claim high-rate performance, they often observe low capacity retention at high rates, while commercial applications require 80% capacity retention at the highest rate. For example, a battery can only be claimed to be stable at 3C if it retains 80% of its capacity.
3) Coulombic efficiency: In lithium-ion full cells, all active lithium is supplied by the positive electrode. The total capacity loss determines the cycle life and actual energy density of the full cell. For graphite and Li4Ti5O12 anodes, the CE after the first cycle is close to 99.9%, while many high-capacity anode materials reported in the literature only achieve a CE close to 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 a full cell. If the CE of the battery is lower than 99.5% in each cycle, the authors believe that the total capacity loss should be given when publishing the article.
4) Energy Density: The development of lithium batteries can be seen as a history of increasing battery energy density. Reports of new materials or new batteries with high energy density are always attractive and exciting. However, to avoid exaggeration, it's important to understand that when selecting negative and positive electrode materials, the theoretical energy density should be calculated using the Nernst equation based on the data on reactant and product formation. It is well known that the ratio of the actual energy density of a 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, truly meaningful calculations 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 further modified. When the reversible capacity and thickness of the positive electrode increase, and the thickness of the separator, copper foil, and aluminum foil decreases, reducing the N/P ratio from 2.0 to 1.2, the battery energy density will significantly improve. These improvements are very possible; for example, the thinnest 3mm lithium foil can be industrially produced.
Recently, several papers have discussed the energy density in real-world batteries. These calculations clearly show that the actual energy density of a battery may be far lower than the values obtained from rough estimates based solely on the active materials of the cathode or anode. Therefore, readers should be very careful not to be overly optimistic about reported data. Especially when setting targets, actual calculations based on all parameters are very helpful in determining whether a target is achievable. For example, regarding Li/NMC batteries (NMC: LiNixMnyCozO2), an energy density of 500Wh/kg can only be achieved if the reversible capacity of NMC exceeds 220mAh/g. This means that lithium-rich oxide cathodes may be more realistic, or even the only feasible option to achieve the 500Wh/kg target.
Due to the diversity of electrode and coin cell designs, standardization is not mandatory in basic research. However, researchers must understand that the experimental design, fabrication, and testing methods of coin cells have a significant impact on the results. Researchers should only make exciting announcements after conducting reliable experiments and appropriate data analysis.
It should be noted that, in addition to basic electrochemical performance measurements, many other tests, characterizations, and analyses may also contain significant biases and problems. To obtain valuable and reliable data, it is essential to understand and standardize each method. Doing so can save time, improve research efficiency, and reduce the difficulty of technology transfer. Throughout the long history of battery development, all progress has been built on a foundation of effective innovation and the ability to withstand scrutiny from others.
Let me conclude with this saying: In the great waves of life, only the gold remains; in the whirlwind of battle, only the victor reigns supreme!
