What are the other "lightning points" in the industrial application of silicon-based negative electrodes?

【Achievement Introduction】

Silicon has long been considered a promising alternative to conventional graphite anodes in high-energy lithium-ion batteries due to its high gravimetric capacity. However, intrinsic problems such as severe volume expansion during cycling have plagued the development of batteries using silicon anodes. While great progress has been made in the laboratory to address these issues, most silicon-based batteries in the industry, where the silicon anode is made of silicon oxide or Si-C composites, contain only a small amount of silicon.

Here, Professor Jaephil Cho of Ulsan National Institute of Science and Technology and Professor Jaekyung Sung of Gyeongsang University consider the many aspects of anodes that hinder the development of practical silicon-based batteries. First, the authors discuss the importance of electrode expansion in practical battery design, especially its impact on battery energy density. Then, the dependence of the cutoff voltage on the Si content in the silicon-based anode is emphasized. At the same time, the calendar life, safety, and cost-effectiveness of silicon-based batteries are also considered, as well as the comparison with graphite-based batteries, and a common testing protocol is recommended to estimate the commercial viability of silicon-based anodes.

The relevant research results were published in Nature Energy under the title “Issues impeding the commercialization of laboratory innovations for energy-dense Si-containing lithium-ion batteries”.

【Core Content】

Conventional lithium-ion batteries have reached the limit of their energy density and cannot meet the rapidly growing demand for long-range electric vehicles. Due to the high gravimetric capacity of silicon (3592 mAh g-1), there have been many attempts to introduce silicon-based materials into conventional graphite anodes to increase the energy density. However, mixing silicon-based materials (SiOx and Si-C composites) with graphite has only achieved limited success in industry. Due to the inherent problems of Si anodes that lead to performance degradation, the use of Si in commercial anodes is still very limited (<5 wt%) (Figure 1).

In fact, most studies on silicon-based anodes focus on improving cycling performance under relatively mild test conditions, but the test conditions must include active materials with high mass loading, low mass and volume fractions of inactive ingredients such as binders and carbon additives, densely packed electrodes with low porosity, and electrolytes with low content. In addition, calendar aging, storage conditions, stacking pressure, safety, and cost-effectiveness are also important factors for practical applications, but have rarely been considered in previous studies.

Despite the increasing number of active collaborations between academia and industry in developing Si-based anodes, there is still no comprehensive set of practical performance metrics to evaluate strategies for Si-containing batteries. In addition, previous reviews have mainly focused on the progress in material development, and few reports have discussed in detail the challenges of Si-containing batteries from a practical perspective.

In academia, energy density is usually calculated based on active materials only, while in industry, energy density must be considered at the cell level. The values ​​obtained without taking into account actual conditions (such as electrode expansion and inactive components) are artificially overestimated. In addition to this overestimation, several important factors affecting the practical application of silicon anodes, such as the reduction of discharge cut-off voltage and calendar life, are easily overlooked by many researchers.

Figure 1. Types of commercial silicon-based 18650 batteries. The silicon content and specific capacity of the negative electrodes used in commercial 18650 silicon-based batteries produced by various battery companies.

Effect of electrode expansion on battery energy density

Battery energy density can be maximized when the active components occupy as limited a battery space as possible. However, in commercial battery designs, any free space within the hard or soft shell is typically used to accommodate battery expansion (Figure 2a, b). Since conventional graphite anodes have low electrode expansion of less than 20%, graphite-based batteries require relatively small free space to accommodate expansion during battery operation. Graphite-based batteries have low swelling and small free space, with low theoretical specific capacity, but can still achieve reasonable energy density. However, silicon-containing batteries inevitably require larger free space than graphite batteries because their electrodes expand more, which will reduce energy density and weaken the high specific capacity of silicon-based anode heat generation, but most studies ignore the dependence of energy density on free space within the battery when discerning electrode expansion.

In addition to free space, the electrode density or electrode porosity should also be considered. Prado et al. showed that the swelling of silicon-graphite (30 wt% silicon content) electrodes is highly dependent on their density (Figure 2c). In general, silicon-containing electrodes with lower bulk density (higher electrode porosity) show lower swelling, as they have more space to accommodate volume expansion. However, higher electrode porosity not only requires thicker electrodes (to maintain the same loading), but also requires more electrolyte to fill the pores, thereby reducing the battery energy.

To clarify the effect of electrode expansion on energy density, the authors estimated the energy density of commercial batteries composed of SiOx or silicon nanoparticles and graphite with different silicon contents. The results showed that Si-mixed electrodes had higher specific capacity than SiOx-mixed electrodes, but they also had higher porosity. Therefore, the two types of electrodes have similar expandability. At the same time, batteries with SiOx exceeding 10%SiOx have higher energy density than commercial graphite batteries. However, it is worth noting that the energy density of silicon-containing batteries with more than 20% SiOx in the negative electrode is reduced. This is because higher electrode expansion requires more free space, which reduces the number of stacks in the limited space of the pouch cell, resulting in a reduction in battery capacity.

In addition to the irreversible expansion, the anode also suffers from in-plane (x-direction) expansion as well as out-of-plane (z-direction) expansion. The graphite anode is negligible, but due to the large anisotropic expansion of Si, the in-plane expansion may be severe, but due to the lack of understanding of this issue, this in-plane expansion has been ignored in academia.

Figure 2. Volume expansion of silicon anode and related issues.

Cut-off voltage and its dependence on silicon content

Figure 3a shows the delithiation voltage curves of anodes prepared with graphite, silicon-graphite (5wt%Si), and Si-carbon (35wt%Si) in the literature. Due to the relatively high working potential and amorphous nature of Si, the voltage curve of the silicon-containing anode is more inclined compared to the graphite anode. As shown in Figure 3a, most of the total capacity of graphite is provided in the low-voltage region (below 0.3 V), while the silicon-containing anode has a large capacity contribution up to 1.5 V. Compared with graphite, the inclined voltage curve of the silicon-based anode means that a higher delithiation voltage cutoff is required to use its maximum capacity, and the silicon-containing battery requires a lower cutoff voltage than the graphite battery during discharge to achieve its full capacity.

Figure 3b shows the anode voltage cutoff and the corresponding full-cell voltage cutoff in previous Si-containing battery studies, which depends on the battery design, including the N/P ratio, battery size, and target energy density. The content of active lithium ions also varies depending on the battery design, resulting in different cutoff voltages even when electrodes composed of the same material are used. In addition, it is worth noting that a low discharge cutoff voltage increases the anode potential, which causes side reactions such as Cu dissolution in the anode current collector, which may lead to serious problems including additional SEI formation, increased battery resistance, non-uniform current density, and safety issues. Therefore, the cutoff voltage of Si-containing batteries is crucial, but this issue has not been considered in detail in most studies related to Si material development.

Figure 3. Effect of cut-off voltage on the performance of silicon-based anode batteries.

Importance and Assessment of Calendar Life

Calendar aging is a time-dependent performance degradation and is an intrinsic behavior. The cycle life aging of silicon-containing batteries mainly originates from the undesirable chemical reaction between the newly exposed negative electrode surface (caused by SEI rupture) and the electrolyte. Conventional graphite batteries generally show moderate tolerance to HF without significant battery degradation. However, Si-based materials can be etched by HF, resulting in the loss of active Si material and the generation of H2O, gas and soluble silicon species.

To evaluate calendar aging, long-term experiments involving testing periods of more than one year are required. Instead of traditional long-term tests, Kalaga et al. evaluated the calendar life of anode cells containing 15 wt% Si and compared them with graphite cells by using a constant potential holding method. This study clearly showed that the addition of Si significantly increased the side reaction current compared with the silicon-free graphite cell. In addition, the addition of fluoroethylene carbonate (FEC) seemed to suppress the side reactions during the voltage holding period for both cells. However, the capacity loss of the silicon-containing cell with FEC after calendar aging was almost twice that of the silicon-containing cell without FEC (Figure 4d). It is well known that FEC can produce HF by dehydrofluorination at high temperatures, which indicates that the FEC additive induces higher capacity loss in the silicon-containing cell after calendar aging at high temperatures. Considering that FEC is often used as an additive to improve the cycle life of Si cells, the stability issue affecting the calendar life of the cell including FEC is another key challenge in the design of silicon-containing cells.