1 Introduction

Lithium-ion batteries (LIBs) have been established as one of the most attractive modern electronic devices and are widely used in portable electronics and battery-operated vehicles due to their superior energy and power densities [1,2,3,4,5]. Nevertheless, the current state-of-the-art LIBs based on lithium metal oxide cathodes and graphitic carbon anodes cannot meet the ever-increasing demands of the large-scale energy storage. Tremendous efforts have been devoted to developing other types of electrode materials [6,7,8,9]. Among the anode materials, silicon has been recognized as one of the most promising anode materials due to its superior theoretical capacity (4200 mA h g−1), natural abundance, and appropriate working potential (< 0.5 V vs. Li/Li+) [10,11,12,13,14,15,16,17,18,19].

Despite these merits, Si-based anode still suffers from the low intrinsic conductivity, drastic volume variation, and low initial coulombic efficiency (ICE) [20,21,22]. Among numerous strategies reported to date, Si/C composites with tunable structure can not only improve the overall electrical conductivity of the electrode but also accommodate the huge volume variation during rapid cycling [23,24,25,26,27,28,29,30,31,32,33]. In particular, the mesoporous and microporous properties of the carbon component generally release the induced strain to some extent [34,35,36,37]. However, new issues such as relatively low ICE values caused by increased surface area are still unsolved. The formation of unstable solid electrolyte interphase (SEI) as well as the irreversible reaction between carbonaceous component and electrolyte results in irreversible lithium ion consumption in the first lithiation process [38,39,40,41]. In full cells, high ICE is critical to meeting energy density requirement, since excessive side reactions in the first cycle will lead to excessive consumption of lithium ions from the cathode [42,43,44,45,46,47,48]. Typically, the ICE of Si/C composite anode is in the range of 65–85%, far below that of commercial graphite carbon anodes (90–94%) [13], which will seriously affect the large scales block industrial applications of silicon-based anodes. Consequently, major research on Si-based anodes should focus on how to suppress the irreversible reactions in the initial lithiation process thus effectively improve the ICE.

To date, great efforts have been made to boosting the ICE of Si-based anodes. However, a comprehensive investigate concentrate on the mechanism as well as the improvement approaches of ICE is lacking. In this review, we will first briefly present the main influencing factors of ICE, including electrical conductivity, structural stability, and specific surface area. Then, we present several typical designs, such as interface modification, surface conformal coating, and secondary structure design. Besides, instead of reasonable structural construction, some progress in artificially controlling initial lithiation process are also discussed. Figure 1 shows the main influencing factors of ICE and corresponding various strategies. At the end of the review, we summarize the key factors for boosting the ICE and propose personal perspectives on practical applications of Si-based anodes.

Fig. 1
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The main influencing factors of ICE and corresponding various strategies

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2 Influence factors of ICE

2.1 Interface properties

Silicon has a poor intrinsic electronical conductivity (10−3 S cm−1) and lithium diffusion coefficient (between 10−14 and 10−13 cm2 s−1) at room temperature, which will seriously decrease electron- and ion-transport kinetics [49, 50]. Surface coating is an effective way to improve the poor electrical conductivity. The coating material can promote the transport of lithium ions and electrons at the interface and prevent the active material from reacting with the electrolyte to consume excess lithium ions.

2.2 Structural stability

The SEI layer on the surface of the electrode is formed from the reductive decomposition of the organic electrolyte and works like a barrier between the electrolyte and electrode [51, 52]. If the electrode material experiences a large volume expansion during the first lithiation process, it will lead to the huge change of electrode surface area, and the exposed fresh electrode surface will react with the electrolyte and resulting in excess lithium ion consumption by the formation of a passivated SEI film. Therefore, the structural stability of the electrode material during initial lithiation process is critical for ICE.

2.3 Specific surface area

The specific surface area (SSA) of the electrode material is defined as the total contact area of the electrolyte with the active material per unit weight. In the structural design of Si-based anodes, the introduction of inner void is an effective method to accommodate the volume expansion [53,54,55]. Nevertheless, the electrode material (like core-shell or porous structure) with extra space will significantly increase the SSA. The increased SSA results in the increased contact area between electrolyte and active material, which will further lead to the irreversible electrode reaction as well as a decreased ICE values. Generally, the construction of secondary structure can effectively reduce the SSA.

3 Improvement strategies of ICE

In response to these factors, substantial efforts have been devoted to alleviating irreversible side reactions during the initial lithiation process. In this section, we first discuss the different structural designs for addressing the intrinsic low ICE of Si-based anode materials, including interface modification, surface conformal coating, and secondary structure design. Then, we briefly summarize recent progress to achieve high ICE from prelithiation technology. These distinctive strategies may provide new insight into the rational design of silicon-based electrodes.

3.1 Interface modification

Actually, the key to interface modification is to reduce the occurrence of side reactions and produce stable and electrochemically inert by-products through reasonable surface and interface design [56, 57]. In this section, numerous strategies including metal incorporation and graphitic carbon coating are discussed in detail.

Maier et al. [58] synthesized Ag-coated 3D macroporous Si through a facile magnesiothermic reduction and Ag-mirror reaction process (Fig. 2a). As shown in Figs. 2b and 3d, microporous pure silicon displayed the initial discharge and charge capacities of 3279 and 2416 mA h g−1. In contrast, the first discharge and charge capacities are 3585 and 2917 mA h g−1 for the Ag-coated 3D microporous silicon (Fig. 2c). Remarkably, compared with the 3D macroporous silicon, the ICE of the Ag-coated silicon electrode has improved from 73.7 to 81.4%. The significantly improved ICE is attributed to the fact that the surface-coated Ag can act as an electronic additive to improve the surface electrical conductivity of silicon, thereby reducing the side reactions originating from the formation of SEI on the electrode surface during the first charge and discharge processes. Very recently, Yang group [59] fabricated a satellite-like architecture via a thermal reduction process, where the silicon nanoparticles are encapsulated in the metal nanocrystals decorated porous carbon shell (Fig. 2d–f). In such design, the porous carbon shell not only prevents the direct contact between silicon and electrolytes, but also acts as a structural buffer to relieve the volume expansion. The metal nanocrystals can work as conductive agents to providing an electron pathway from the current collector to the surface area of active materials. The initial discharge capacities and ICE of the as-prepared Si@C-Fe, Si@pC, Si@pC-Ag, and Si@pC-Cu electrodes are shown in Fig. 2g. Interestingly, the Si@pC electrode shows the highest initial discharge capacity (2473 mA h g−1) and the lowest ICE (76.9%). Figure 2h shows the interface evolution of Si@pC and Si@pC-Ag electrodes during the initial lithiation process. The introduction of metal nanocrystals can effectively reduce side reactions with electrolytes and decrease the consumption of lithium ions in the initial lithiation process, thereby reducing the initial discharge capacity and further improve the ICE values.

Fig. 2
figure 2

Metal incorporation on Si-based anode surface. a Schematic illustration of Ag-coated 3D macroporous Si. Voltage profiles of 3D macroporous silicon (b) and Ag-coated 3D macroporous silicon (c). Reprinted with permission from ref. [58] Copyright 2010, Wiley-VCH. SEM (d), TEM (e), and HRTEM (f) images of Si@pC-Ag. g Initial discharge capacity and ICE of Si@C-Fe, Si@pC, Si@pC-Ag, and Si@pC-Cu (eight sets of data were counted for each sample). h Schematic illustration of the initial lithiation process of Si@pC (up) and Si@pC-Ag (down). Reprinted with permission from ref. [59] Copyright 2019, Royal Society of Chemistry

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Fig. 3
figure 3

Graphitic carbon coating on silicon interfaces. HRTEM images of AC-SNP (a) and GC-SNP (b). c Coulombic efficiency of AC-SNP and GC-SNP. dRSEI and RCT obtained from the fitting of the EIS curves. Reprinted with permission from ref. [60] Copyright 2019, American Chemical Society. e Illustration of a multiscale buffering engineering design. f TEM image of Si-C WBs. g Discharge and charge profiles of Si-C WBs. Reprinted with permission from ref. [61] Copyright 2019, American Chemical Society. h Preparation procedure of graphite/Si-porous carbon composite. i TEM image of the Si/C composite. j Discharge/charge curves of the Si/C composite. Reprinted with permission from ref. [62] Copyright 2014, Elsevier. k Schematic illustration of the preparation procedure of Si-G/C composite. l Elemental mapping of the Si-G/C. m Discharge and charge profiles of the graphite, Si-G/C, and nano-Si. Reprinted with permission from ref. [63] Copyright 2018, Royal Society of Chemistry

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It is known that the ICE of Si-based anode was greatly affected by the amorphous carbon coating. Amorphous carbon component undergoes irreversible side reactions with electrolytes, which in turn leads to irreversible capacity lose during the initial lithiation process. In contrast, the highly graphitic carbon coating not only is exhibiting high electrical conductivity but also has more stable interfacial properties with respect to the amorphous one. Therefore, extensive efforts have been dedicated to effectively compound silicon with graphitic carbon to improve the ICE value of the anode materials. Nava et al. [60] fabricated a routine CVD coating process to coating conformal carbonaceous shells onto silicon nanoparticles. The modified CVD approach enables precise control of the graphitization degree of the carbon coating. Figure 3 a and b show the TEM images of AC-SNP and GC-SNP composites; it can be seen that the silicon nanoparticles are encapsulated in a conformal carbon layer. The GC-SNP electrode displays an initial discharge capacity of 1960 mA h g−1 with the ICE value of 87% (Fig. 3c). The authors further performed electrochemical impedance spectroscopy (EIS) measurements to clarify the reasons for the difference in CE values. RSEI and RCT values obtained from the fitting of the EIS curves are shown in Fig. 3d. The average RSEI value of GC-SNP is 40% lower than that of AC-SNP, indicating that the higher conductivity of graphitic carbon leads to a decrease in the SEI layer impedance. The reason for the high ICE of GC-SNP is that the highly graphitic carbon coating not only facilitates the transport of lithium ions to the silicon active material, but also reduces irreversible consumption of lithium ions by defects in the carbon component. Recently, Hou et al. [61] reported a wool-ball-like design of S/C nanowire (NW) via RF-plasma and spray-drying treatment, and Fig. 3e shows the illustration of a multiscale buffering engineering design. The Si/C NW composite intertwined into wool-ball-like spherical frameworks with a diameter of 3–10 μm (Fig. 3f). Remarkably, the Si-C WB electrode delivers a high ICE of ∼ 90% (Fig. 3g), which is attributed to the stable SEI layer on the electrode surfaces and reversible Li+ insertion/extraction reaction in the first lithiation process. Li et al. [62] utilized a facile spray-drying/pyrolysis strategy to synthesize the graphite/silicon-porous core-shell structure (Fig. 3h). The TEM image indicated that the silicon with a size of 200 nm was embedded in the graphite layer (Fig. 3i). The discharge and charge capacities of the Si/C composite were 723.8 mA h g−1 and 538.1 mA h g−1, respectively, corresponding to the ICE of 75.2% (Fig. 3j). The excellent reversible Li+ storage capability of the Si/C electrode is attributed to the enhanced electrical conductivity and the formation of the stable SEI film. More recently, Xiao et al. [63] proposed a Si/graphite/carbon (Si-G/C) composite prepared by the evaporating and heat treatment process (Fig. 3k). Through elemental mapping of the Si-G/C, the carbon layer coated on the silicon nanoparticles was confirmed (Fig. 3l). Figure 3m shows the discharge and charge profiles of the graphite, Si-G/C, and nano-Si, with corresponding ICE of 93, 87.6, and 76%, respectively. Obviously, the ICE of the Si-G/C electrode was improved by the graphite core.

3.2 Surface conformal coating

As mentioned above, when the electrode material suffers large volume expansion during the initial lithium insertion process, electrolyte decomposes on the electrode surface and resulting in excess Li+ consumption by the formation of SEI film. [64,65,66,67] Surface conformal coating can effectively stabilize the volume change of active materials and reduce the irreversible side reactions during the initial lithiation process.

Yang et al. [44] constructed a novel Si@a-TiO2 core-shell nanostructure through a facile sol-gel approach (Fig. 4a). HRTEM images of the as-obtained Si@a-TiO2 are shown in Fig. 4b; it is clear that a large number of tiny TiO2 nanograins are uniformly dispersed on the surface of Si nanoparticles. Figure 4c shows the long-term cycling performance of the Si nanoparticles, Si@a-TiO2, and Si@c-TiO2 electrodes at a current density of 420 mA g−1. The Si@a-TiO2 electrode demonstrates an ICE value of 86.1%, much higher than that of Si@c-TiO2 (79%). It was found that conformal amorphous TiO2 shell prevented the consumption of lithium ions during the first cycle, resulting in a stable SEI film on the surface of the amorphous TiO2 shell.

Fig. 4
figure 4

Surface conformal coating of silicon anode. a Schematic illustration of fabrication process of Si@a-TiO2 composite. b HRTEM image of Si@a-TiO2 nanoparticle. c Cycling performance of the Si nanoparticle, Si@a-TiO2, and Si@c-TiO2 electrodes. Reprinted with permission from ref. [44] Copyright 2017, Wiley-VCH. TEM image (d) and elemental mapping (e) of nano-Si/cyclized-PAN. f Raman spectra of nano-Si/PAN and nano-Si/cyclized-PAN. g Cycling performance of nano-Si/cyclized-PAN electrode. Reprinted with permission from ref. [68] Copyright 2013, Wiley-VCH. h Schematic illustration of coating design on pSiMPs and structural evolution during cycling. i The voltage profiles of nC-pSiMPs. j Time-lapse images of the lithiation of nC-pSiMPs. Reprinted with permission from ref. [69] Copyright 2015, American Chemical Society

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Lee and co-workers [68] developed a conjugated polymer conformally coated silicon nanoparticles as high-performance silicon-based anode materials. Figure 4 d and e exhibited a conformal cyclized-PAN layer coating on the surface of silicon nanoparticles. The delocalized sp2 π D and G bands in the Raman spectra of the heat-treated sample demonstrate cyclization of the PAN coating (Fig. 4f). The nano-Si/cyclized-PAN electrode exhibits the ICE value of 70% (Fig. 4g), which is attributed to the conformal PAN coating that prevents the parasitic formation of SEI film and alleviates irreversible lithium ion consumption in the first cycle.

Instead of nanoscale structure of silicon source, there are also some progress to use micrometer-sized silicon as a source material. Cui group [69] reported a nonfilling carbon-coated porous silicon microparticle (nC-pSiMP) through thermal disproportionation and etching processes (Fig. 4h). The discharge and charge profiles in Fig. 4i indicate the high ICE and stable electrochemical behavior of nC-pSiMP electrode. Figure 4j demonstrates the TEM images of the in situ lithiation of the nC-pSiMP composite. As the lithiation process progressed, all the void spaces are occupied by lithiated silicon and no significant fracture of the carbon coating is observed. The nonfilling conformal carbon coating maintains the structural integrity of SiMPs as well as prevents continuous formation of SEI film, thus significantly increase the ICE value of the electrode material.

3.3 Secondary structure design

As described in the “Influence factors of ICE” section, the specific surface area (SSA) of the electrode material is one of the most important factors affecting ICE values. The SSA can be tuned by forming secondary structures with nanoscale structural units, thereby decrease the side reactions between active material and electrolyte [70,71,72]. Here we present four notable designs, including superparticle structure, pomegranate structural design, granadilla-like structure, and Si nanoparticle clusters.

The typical superparticle structure with lower SSA results in the inhibited interface side reactions, which is essential for increasing the ICE values. Zhang et al. [73] prepared a secondary superparticle composed of nanometer-sized Si, MgO, and SiO through a mechanical milling and spray drying process. After heat treatment and carbon coating, the C-SiO-MgSiO3-Si superparticle structure was finally synthesized (Fig. 5a). Low- and high-magnification SEM images confirmed the spherical morphology with a narrow size distribution of the obtained C-SiO-MgSiO3-Si-1100 superparticles (Fig. 5b, c). With the increase of reaction temperature, MgSiO4 phase gradually evolved into MgSiO3, and the in situ evolution of the MgSiO3 consumes the inactive SiO2 phase and acts as an electrochemically inert buffer layer to effectively alleviate volume change of active materials. Compared with silicon, the ICE of silicon oxide is relatively low, which is related to the irreversible formation of Li2O and lithium silicate during the first cycle. Figure 5d shows the coulombic efficiency of C-SiO-MgSiO4-Si and C-SiO-MgSiO3-Si superparticles; the ICE increased from 65.9 to 75.8% with the reaction temperature increased from 800 to 1200 °C. Notably, C-SiO-MgSiO3-Si-1100 exhibited the highest ICE value of 78.3%, which is higher than that of the previously reported Si-based anode materials. Furthermore, Guo group [74] synthesized the watermelon-like Si/C microspheres by the combination of spray drying and CVD processes, where the carbon-coated silicon nanoparticles are homogenously dispersed in flake graphite matrix (Fig. 5e). Taking advantage of the hierarchical buffer structure, the Si/C electrodes present the high ICE of 89.2% and average coulombic efficiency of 99.8% after several cycles (Fig. 5f, g). Following this achievement, the Guo group [28] further proposed a rational design of robust Si/C microspheres with silicon nanoparticles encapsulated in compact carbon component (Fig. 5h). As shown in Fig. 5i, the SEM image of Si/C microsphere confirms that the silicon nanoparticles are embedded in carbon framework and shows a microscale spherical morphology. The initial discharge and charge capacities were determined to be 711 and 640 mA h g−1 at a current density of 200 mA g−1, corresponding to an ultrahigh ICE of 90.5% (Fig. 5j). The high ICE of the robust Si/C anode should be attributed to the stable structure and lower SSA of the micron-sized spherical structure, which reduces the occurrence of interface side reactions during the initial lithiation process.

Fig. 5
figure 5

Fabrication of secondary superparticles. a Schematic illustration of the synthesis process of C-SiO-MgSiO3-Si superparticles. Low- (b) and high-magnification (c) SEM images of C-SiO-MgSiO3-Si-1100 superparticles. d Coulombic efficiency of C-SiO-MgSiO4-Si and C-SiO-MgSiO3-Si superparticles. Reprinted with permission from ref. [73] Copyright 2019, Elsevier. e Schematic illustration of the watermelon-inspired Si/C microspheres. f Discharge and charge profiles of Si/C microspheres. g The coulombic efficiency after 10 cycles at 0.5 C. Reprinted with permission from ref. [74] Copyright 2017, Wiley-VCH. h Schematic illustration of the synthesis process of Si/C microspheres. i SEM image of Si/C microsphere. j Initial discharge and charge profiles of Si/C mixture and Si/C microspheres. Reprinted with permission from ref. [28] Copyright 2019, American Chemical Society

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Inspired by the structure of a pomegranate, the Cui group [75] proposed a hierarchical structured silicon anode where nanoscale silicon particles are encapsulated in carbon shell and then ensemble of these nanoparticles in micrometer-sized carbon pouches. The outside carbon layer completely encapsulates the entire secondary particle, limiting the amount of SEI of the irreversible reactions in the first cycle (Fig. 6a). From the TEM image in Fig. 6b, it is clear that the silicon nanoparticles are encapsulated in the carbon framework. The Si pomegranate delivers a high ICE value of 82%, which is attributed to the lower SSA of pomegranate-like secondary structure (Fig. 6c). Similarly, Zhang et al. [76] prepared granadilla-like silicon microspheres via a facile CVD method. As shown in Fig. 6d, the authors firstly synthesized CaCO3@Si structure microspheres, in which the silicon nanoparticle are individually encapsulated in CaCO3 framework, followed by carbon deposition and removal of CaCO3 by HCl etching. Different from the previous reported two-step synthesis of pomegranate structure, CaCO3@Si microspheres were synthesized through a one-step method of carbon deposition. The single silicon nanoparticle was individually encapsulated in a hollow carbon sphere and assembled into a smooth spherical structure (Fig. 6e, f). As a result, the Si-granadilla electrodes expressed significant improved ICE of 80% (Fig. 6g). The outer carbon shell acts as an electrolyte blocking layer and the SEI film formed on the surface of carbon layer, which helps to reduce the irreversible electrode reaction, thereby increasing the reversible capacity of the active material.

Fig. 6
figure 6

Secondary structure design of silicon anode. a Three-dimensional view of one pomegranate microparticle before and after cycling. b TEM image of one silicon pomegranate particle. c Cycling performance of silicon pomegranate and other structures. Reprinted with permission from ref. [75] Copyright 2014, Nature Publishing Group. d Schematic illustration of fabrication process of silicon granadilla composite. SEM (e) and TEM (f) images of 50@Si-granadillas. g Charge/discharge curves of 50@Si-granadillas (inset shows the CV profiles of 50@Si-granadillas). Reprinted with permission from ref. [76] Copyright 2015, Wiley-VCH. h Schematic illustration of the preparation process of the porous Si. i TEM images of p-Si/C. j Voltage profiles of p-Si/C. Reprinted with permission from ref. [77] Copyright 2018, Elsevier. k Schematic illustration of the synthesis process of the SNC@C composite. l Initial coulombic efficiencies of different structures. m Cycling performances and coulombic efficiency of three electrodes. Reprinted with permission from ref. [78] Copyright 2019, Royal Society of Chemistry

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Jia et al. [77] synthesized a novel micrometer-sized silicon secondary structure with unique porous structure via microemulsion of silica nanoparticles and magnesiothermic reduction method (Fig. 6h). The HRTEM image in Fig. 6i shows that the carbon layer coating on the surface of silicon secondary superparticle is about 5 nm, and the inner void spaces can be observed clearly. The as-obtained p-Si/C electrode delivers a reversible capacity of 2500 mA h g−1 with the ICE value of 75% (Fig. 6j). The highly graphitic carbon shell serves as an electrolyte-blocking layer to induce the formation of SEI film on the surfaces of the secondary superparticles, thus alleviating the occurrence of irreversible side reactions at the interface in the first cycle.

More recently, Li et al. [78] constructed a novel SNC@Cs composite electrode, where Si nanoparticle clusters are encapsulated in hollow carbon capsules. As shown in the synthesis process in Fig. 6k, silicon clusters are encompassed by a polystyrene matrix, and then coating with a polydopamine layer, the SNC@Cs composites are finally synthesized after a carbonization process. This SNC@Cs composite structure can significantly reduce the specific surface area of the material, which will eventually influence the ICE value of yolk-shell structured anode materials. Among different yolk-shell structures, the h-SC electrode with the highest internal silicon clustering density shows the highest ICE value (81%), which is attributed to the reduction in the contact area between the electrode material and the electrolyte (Fig. 6l). Since the m-SC composite has a moderate silicon content, it exhibits high cycle stability than h-SC (Fig. 6m).

3.4 Prelithiation

Prelithiation can supplement the consumption of lithium in the cathode due to the formation of SEI during the initial lithiation process, thereby boosting the ICE of the anode material [79,80,81,82,83]. Prelithiation techniques can be divided into two main ways: electrochemical prelithiation and chemical prelithiation. We will discuss both methods in details in the following section.

In view of the electrochemical prelithiation, Kim et al. [84] proposed a delicate prelithiation scheme based on electrical shorting with lithium metal foil, which allows the ICE to reach as high as 94.9% (Fig. 7a). The highest ICE and cycle life can be obtained by controlling the final potential to be lower than that forms the SEI layer but above the one that the main alloying reaction performed. This prelithiation method can achieve delicate control of the degree of lithiation, thereby effectively solving the problem of low ICE of the silicon-based anode.

Fig. 7
figure 7

Prelithiation strategies to improve ICE. a Graphical illustration of prelithiation process of c-SiOx electrode and its scalable roll-to-roll process scheme. Reprinted with permission from ref. [84] Copyright 2015, American Chemical Society. b Schematic diagrams showing Si NPs react with melted Li to form LixSi NPs. Reprinted with permission from ref. [85] Copyright 2014, Nature Publishing Group. c Schematic diagram of the artificial SEI coating formed by reduction of 1-fluorodecane on the surface of LixSi NPs in cyclohexane. d TEM images of LixSi NPs after coating. e Voltage profiles of Si NPs/coated LixSi composite and Si NPs. Reprinted with permission from ref. [86] Copyright 2015, American Chemical Society

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In chemical prelithiation, prelithiation reagent can be mixed with various anode materials to provide additional lithium source to compensate the irreversible loss of lithium ions during the first lithiation process. The Cui group [85] produced a novel LixSi-Li2O core-shell nanoparticles via a one-step thermal alloying process (Fig. 7b). Li2O is helpful for forming a passivation shell without disturbing the whole structure of the electrode during the lithiation process, while LixSi can be used as a reactive prelithiation reagent to provide a localized lithium source. Following this achievement, the Cui group [86] further reported an artificial-SEI-protected LixSi NPs, in which LiF and lithium alkyl carbonate serve as an effective passivation layer that coat on the surface of LixSi NPs (Fig. 7c). TEM image shown in Fig. 7d indicated that each individual LixSi NP was wrapped in a uniform coating. As shown in Fig. 7e, the open-circuit voltage of Si NPs/coated LixSi composite is significantly lower than that of crystalline Si, indicating partial prelithiation of the Si NPs. Correspondingly, the ICE increased from 76.1 to 96.8%, confirming that the incorporation of the coated LixSi NPs can compensate for the irreversible capacity loss in Si NP anodes.

4 Conclusion and outlook

Silicon has been intensively regarded as the most promising anode materials for the next-generation lithium-ion batteries due to its high specific capacity and natural abundance. However, huge volume variation (> 300%) during lithiation and delithiation processes usually hinders the practical application of Si-based anodes. A common approach to address the issue is to compound silicon with different types of matrices. However, the high surface area of these composite structures significantly increases the irreversible reactions at the interface during the initial lithiation process. Accordingly, the ICE value of Si-based anodes is typically very low. In past decades, tremendous efforts have been made in eliminating the irreversible capacity loss during the first cycle. In this review, we focus on the influence factors of ICE, including electrical conductivity, structural stability, and specific surface area. We systematically summarize the recent advances in structural design and prelithiation method that could significantly improve the ICE of Si-based anodes.

Despite great progress, the exploration of high-ICE Si-based anode materials that could meet the needs of large-scale practical applications is still in its infancy. Firstly, although the metal/metal oxide interface modification can effectively alleviate the occurrence of side reactions at the interface, it usually reduces the energy density of the silicon anode material, and its cost is high as well as the synthesis is complicated. Next, the secondary structure can effectively reduce the SSA, but its fabrication process generally requires complex reaction conditions, and achieving an ICE value of over 80% is still a challenge. In addition, prelithiation is the most direct way to improve ICE, while the current prelithiation technology generally requires complex operations, making it difficult to meet the needs of scalable production.

In summary, future exploration of Si-based anodes should focus on structural and battery system designs to ensure high ICE toward full cells. Although various structural designs have been demonstrated to address the irreversible capacity loss in the first cycle, the widespread commercial application is still limited by production cost. Along these lines, combining silicon with graphite is the most feasible approach to achieve high ICE values. Meanwhile, scalable and feasible operations of the prelithiation procedure should be explored to address the challenge of ICE. It is expected that through extensive efforts, Si-based anode with satisfactory ICE can provide better service in next-generation energy storage devices.