【Background】
The energy density of lithium-ion batteries (LIBs) is increasingly difficult to meet the needs of electric vehicles, low-altitude vehicles, and mobile electronics, among others. The urgent need for higher energy density (>350 Wh kg−1) and lower cost (<100 $/Wh) has driven the research and development of new LIB materials. The industry has witnessed updates and iterations on the positive side (increasing Ni content and battery energy density), but progress on the negative side has been slow: since the 90s of the 20th century, carbon-based anodes (graphite, soft/hard carbon) have been dominant.
Alloy anodes (such as Si, Sn, Ge, Al, etc.) have moderate lithium storage potential and high gram capacity, which can greatly improve the energy density of the battery, but the volume expansion in the lithium intercalation seriously restricts its cycle stability. Numerous studies of such anodes, such as silicon, often rely on "preset porosity" to alleviate this problem. As increasing the Si content in the anode to increase the energy density of the battery becomes inevitable, this will inevitably lead to more severe volume expansion, making it inevitable to preset more pores. Although porosity is crucial, it is still unclear how the amount, distribution and type of porosity affect the mechanical, electrochemical and geometric response of alloy anodes during cyclic work. Therefore, it is an urgent task to deeply understand, skillfully handle and rationally design the porosity of alloy anodes.
【Job Profile】
Recently, Liu Wei of Sichuan University and Nikhil Koratkar of Rensselaer Polytechnic Institute and others published a paper entitled "Densification of Alloying Anodes for High Energy Lithium-Ion Batteries: Critical Perspective on Inter- Versus" in the internationally renowned journal Advanced Science Intra-Particle Porosity". Taking the silicon carbon anode as an example, this paper quantitatively analyzes the influence of silicon content and preset pores in the silicon carbon composite particles on the energy density of the battery by constructing a pouch battery model in practical scenarios, and then promotes and establishes a reasonable design scheme for the inter-particle and intra-particle pores of alloy anode technology. In this paper, we further discuss the realization of intelligent pore design and its synthesis methods, introduce the characterization methods that can be used to quantify/monitor pores, and point out the problems that need to be further solved in the pore design process of alloy anodes, and point out the development direction of high-energy-density LIBs alloy anode materials in the future.
【Content Description】
1. Effect of negative electrode porosity on battery energy density
Fig. 1 Monolithic cell model for calculating the energy density of the battery, the effect of the negative electrode porosity (Panode) on the energy density of the battery is due to the fact that the pore volume (Vpore) of the negative electrode itself will significantly affect the amount of electrolyte (melectrolyte), while the Vanode can be subdivided into intragranular pores (Vintra) and interparticle pores (Vinter).
First of all, the author takes the silicon carbon anode as an example and introduces C-Si||NCM 811 Laminated Pouch cell cell model (Figure 1). When the positive electrode (NCM811, 4 mAh/cm2 double-sided, 16 μm aluminum foil, compaction density 3.4 g/cc), separator (15 μm, 50% porosity) and negative current collector (8 μm copper foil) are fixed, the active coating of the negative electrode is the only variable affecting the energy density of the battery. melectrolyte), the volume of the negative electrode (Vanode), and the pore volume in the negative electrode (Vpore).
The pore volume in the negative electrode is composed of two parts (Vpore= Vintra+Vinter):(1) The pore volume (Vintra) located inside the particle, which can only be controlled in the material synthesis stage. (2) The volume of pores (Vinter) located between the particles, which can be controlled by electrode roll compaction. Therefore, the negative electrode porosity can also be expressed as: Panode = Pintra + Pinter = Vintra/Vanode + Vinter/Vanode.
Fig.2 C-Si||NCM 811 Cell weight/volume energy density, (a) electrode rolling is often used to control the overall porosity (Panode) and compaction density (ρanode) of the negative electrode; (b) Fully dense C-Si particles and the energy density of LIB with them as the negative electrode; (c) The porosity in the porous electrode/porous particle obtained by delithiating the dense Li15Si4 film or Li15Si4 particle after complete lithiation is the ideal electrode pore (Panode) and intra-particle pore, respectively; (d) Pintra is designed based on the expansion of carbon (10%) during lithium intercalation; (e) Pintra is designed taking into account both the expansion of carbon (10%) and the expansion of silicon (280%) during lithium intercalation.
Figure 2a illustrates the process of electrode rolling modulation of anode and porosity of anode. After the graphite anode is rolled to anode = ≈ 1.6 g cm−3 or Panode ≈26% is a common parameter for actual commercial batteries, and the introduction of silicon to construct C-Si particles in the anode can reduce the weight of the anode and improve the energy density of the battery. However, the content of silicon in C-Si, the intragranular porosity (Pintra) and the intergranular porosity (Pinter) of C-Si particles have important effects on the energy density of LIBs.
First, Figure 2b shows the mass/volume energy density (EV, EG) of LIBs at different Si content (0~100 wt%) and anode compaction density (anode) of a fully solid silicon-carbon particle (Pintra=0). Under the same silicon content, the increase in anode directly leads to an increase in EG, which is due to the decrease in the pore volume (Vpore) in the negative electrode, resulting in a decrease in the amount of electrolyte and the overall weight loss of the battery. The study found that when the Si content was more than 30 wt%, the Si dosage was further increased, and the income of Eg and Ev became very flat, which confirmed the rationality of the extensive development of C-Si anode technology with a Si content of < 30 wt%.
However, theoretically, the lithiation process will inevitably cause volume expansion, such as graphite to LiC6 will produce ~10% expansion, and silicon lithiation (Si to Li15Si4) volume change is as high as 280%. In order to avoid particle pulverization/electrode swelling, it is advisable to preset a reasonable pore volume (Vintra) in the particle (commercial synthetic graphite anode materials follow this idea to extend their cycle life). Figure 2c illustrates the pore design concept considering the maximum volume variation of silicon: the porosity of the porous electrode/porous particle obtained by delithiating the dense Li15Si4 film or Li15Si4 particle after complete lithiation is the optimal electrode pore (Panode) and intra-particle pore (Pintra), respectively. Of course, the Pintra value in silicon-carbon particles is related to the Si and C content (φC, φSi).
Figure 2d shows the silicon-carbon particles of porous carbon + solid silicon designed to accommodate the volumetric expansion (Vintra=10% VCSiφC) of carbon-intercalated lithium. Compared with solid C-Si particles, the introduction of the pore volume of the particles significantly decreases the EV of the battery, which is due to the increase in the volume of the negative electrode (Vanode). However, this model does not take into account the volume expansion of silicon-lithium alloying, and the Vintra will even decrease (C decreases) as the silicon content increases, which is obviously not suitable for high-silicon systems.
Figure 2e further illustrates the design of porous carbon + porous silicon silicon for the resulting porous carbon + porous silicon considering both carbon-intercalated lithium and silicon-lithium alloyed volume expansion (Vintra = 10% VCSiC + 280% VCSiSi). At this time, the internal pores of the particles are just enough to accommodate the expansion of C and Si lithiation at the same time. A significant change is that the volume and mass energy density of the cell are significantly reduced, and this is more prominent in the high-silicon system. This is because the higher the Si content, the larger the reserved pore space, and the volume of the negative electrode and the amount of electrolyte also increase. With this pore design, the highest achievable energy densities are 411 Wh/kg and 1308 Wh/L.
2. Design from opening to sealing
In the above analysis, the introduction of electrode porosity will inevitably increase the amount of electrolyte, resulting in a decrease in the energy density of the battery. The intergranular pores are inevitably completely infiltrated by the electrolyte, but the intragranular pores (Vintra) can be designed and prepared into a closed-cell structure, so that the pores can be introduced without increasing the amount of electrolyte, and a high energy density can be achieved (Fig. 3a). The authors introduced a closed cell ratio (fully closed cell at =1) to quantitatively analyze the energy density of the cell when the pores in the particle were fully introduced (Figure 3bc). At this point, three ideal electrode conditions for alloy anodes can be seen:
(1) Pinter=20%, and the active particles are approximately densely packed hexagonal;
(2) Vintra=10%VCSiC+280%VCSiSi,此时嵌锂膨胀由颗粒内孔隙完全消化;
(3) =1, the pores in the particles at this time do not increase the amount of electrolyte at all;
At the same time, the most ideal condition (MIC) of the negative electrode, the most stable and highest energy density C-Si anode can be achieved under the MIC condition, and the three conditions correspond to high electrode compaction, precisely controlled pore content and distribution in particles, and excellent particle surface coating, respectively. Fig. 3d shows the theoretical energy density of different components of C-Si anode under MIC conditions, and the highest energy density can be as high as 437 Wh/kg and 1244 Wh/L (pure silicon anode).
The electrolyte-to-capacity ratio (E/C, measured in g/Ah, Fig. 3e) is commonly used in industry to quantify the amount of electrolyte, and the MIC on the left represents the minimum electrolyte volume of 1.2~1.4 g/Ah for each C-Si system, which is basically consistent with the actual value of ≲2 g/Ah in commercial cells. However, the E/C ratio could not capture the difference in the amount of electrolyte required in different Si content systems, and the authors proposed to introduce a pointer of electrolyte-to-silicon ratio (E/Si, in ml/g, Fig. 3f) to design the electrolyte dosage of the C-Si anode. For example, if the desired energy density > 300 Wh/kg, you need to control the E/Si ratio < 23 ml/g, and so on 350 Wh/kg corresponds to an E/Si ratio of <10 ml/g, and 400 Wh/kg corresponds to an E/Si ratio of < 5 ml/g. The authors emphasize that in low-Si systems, it is even more effective to increase the energy density of the cell by decreasing the E/Si ratio than directly increasing the amount of silicon, and that reducing the E/Si ratio can be achieved by increasing the intra-particle closed porosity (η approaching 1) and reducing the inter-particle void (Pinter).
Fig.3 The degree of opening/closing of pores in particles can greatly affect the mass and energy density of the battery. (a) The intra-particle pores (Vintra) can be reduced from open to closed, which can reduce the amount of electrolyte and further improve the energy density; (b, c) the change of cell mass and energy density caused by the change of the inner pores of the particles from completely open (0) to completely closed (1); (d) the effects of different silicon content and compaction density on the energy density of the battery under the condition of complete closed-cell; (e) Mass-energy densities with different electrolyte-to-capacity ratios (E/C) and (f) electrolyte-to-silicon ratios (E/Si) under optimal conditions (MIC).
Furthermore, the author proposes that the above pore design concept is applicable to all alloyed composite anodes, including the Sn/Ge/Al/Sb/Si-based anode systems of Li/Na/K ion batteries, and the authors have made the predicted values of different alloy systems (corresponding to different gram capacity and expansion rate), different inter/internal porosity and the actual battery energy density into an editable file with built-in algorithms, which readers can use to evaluate and predict the energy density of the actual battery of each alloy anode system.
3. Engineering and characterization of particle porosity
The key influence of porosity on the energy density of a cell has been described above, and the intra-particle porosity (Pintra) directly determines the energy density and performance of the material, so it needs to be tightly controlled during the material synthesis stage, which includes three main goals: low specific surface area, high compaction density, and high closed porosity (Figure 4a). Although nano-silicon can avoid particle breakage, its high specific surface area and low tap density pose significant challenges. At present, many methods have been developed to assemble nanosilicon nanoparticles (SiNPs) into micron-scale secondary particles from the bottom up (Fig. 4b), such as spray drying to assemble C-Si secondary particles, high-temperature and high-pressure hydrothermal reaction, powder compaction granulation, mechanical fusion, and other strategies.
Top-down reconstitution of particles is another way to prepare anode particles with nanofeatures (Figure 4c), where the obtained particle size will remain in the micron range, but the encapsulated Si has a nanostructure and behaves similarly to SiNPs. There are two main strategies for the implementation of this method, either to prepare a porous Si backbone first, or to deposit Si into a porous carbon framework, both of which ultimately need to be coated to construct a closed-cell structure.
Fig.4 Synthesis goals and implementation strategies of alloyed anode particles: (a) The synthesis of materials needs to be strictly controlled in terms of specific surface area, powder density and particle pore opening/closure; The synthesis strategies mainly include (b) bottom-up assembly of primary nanoparticles into secondary micron particles, (c) top-down construction of frameworks for encapsulation into micron particles, and (d) inactive excipients to enhance the electrochemical performance of particles.
The performance of the alloyed anode is also affected by the inactive excipients (conductive agents, binders, and electrolytes) (Figure 4D). The adhesive modification of the silicon interface can effectively improve the bonding strength between the particles / between the particles and the conductive agent, and improve the electrode cycling performance. The chemical composition of the electrolyte can promote a benign SEI film and inhibit side reactions to stabilize the silicon particles. Modifications to current collectors, separators, or solid electrolytes will also contribute to the long-life service of stable, high-capacity silicon-carbon particles.
The authors point out that assessing the distribution, size, and geometry of pores requires a corresponding characterization approach. The tap density (TD) and electrode-based compaction density (CD) of the powder can only be considered as indirect or semi-quantitative methods for evaluating the porosity of the electrode. Advanced characterization tools accurately capture the pore structure and evolution of materials in both powder and electrode states. Microscopic methods such as SEM/TEM/AFM can observe the morphology and characteristics of surface pores. Liquid/gas ingress methods, such as mercury injection (MIP) and liquid nitrogen adsorption (LNA), provide direct quantitative characterization of open pores. Characterization methods such as small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and X-tomography reconstruction can monitor and quantitatively evaluate the dynamic evolution of electrode/particle pores. Theoretical simulations, such as finite element method (FEM), molecular dynamics (DFT), density functional (DFT), etc., can help to understand the distribution and dynamic evolution of pores of particles. The combined use of these characterization methods can provide further guidance for particle design.
【Conclusion】
High-capacity alloy anodes are recognized as the cornerstone of next-generation LIBs, but their huge volume expansion is a major obstacle. In this paper, we take carbon-silicon anode particles as a typical case to re-examine the role of intra/inter/inter-pore structures in particles, and emphasize that the pore structures introduced to accommodate expansion (such as pore size, volume, geometry and spatial distribution) will have a significant impact on the energy density and performance of the battery, so it is necessary to strictly quantify and control the pores of particles and electrodes. With the continuous development of advanced characterization and simulation methods, the understanding of the evolutionary behavior of closed pores in particles, such as growth, proliferation, aggregation, or collapse, will continue to advance. In general, the core technology of closed and dense alloy anode technology is the core technology of high specific energy batteries, and pore engineering will play a key role in improving the energy density and cycle life of the battery.
Yiteng Luo, Yungui Chen, Nikhil Koratkar, Wei Liu, Densification of Alloying Anodes for High Energy Lithium-Ion Batteries: Critical Perspective on Inter- Versus Intra-Particle Porosity, Advanced Science, 2024.
https://doi.org/10.1002/advs.202403530
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