The coulombic efficiency trough in silicon anodes for lithium-ion batteries: mechanisms, challenges, and stabilization strategies

Abstract

Silicon is one of the most promising anode materials for next-generation lithium-ion batteries because of its very high theoretical capacity and natural abundance, yet its practical use is limited by severe volume expansion, structural degradation, unstable solid electrolyte interphase formation, and capacity fading. Beyond these known issues, a critical but underexplored degradation feature is the coulombic efficiency trough, a transient but universal dip in efficiency that appears during early-to-mid cycling. This trough is generally associated with silicon volume change that generates sponge-like porous structures, repeated interfacial rupture, continued SEI renewal, and irreversible lithium loss. This review analyzes the mechanistic origin of the CE trough and highlights it as a diagnostic framework that links the fundamental cause of volume change to consequences that include new surface generation, interfacial instability, and declining lithium inventory. We also evaluate major suppression strategies, including LiF-rich SEI formation through electrolyte design, mechanically adaptive binders that accommodate expansion, and voltage window optimization to limit interfacial stress. Together these approaches reduce irreversible reactions, stabilize the SEI, and improve cycling stability. Treating the CE trough as a quantitative performance indicator provides a unified basis for comparing mitigation strategies and advancing durable, high-capacity silicon anodes.

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1. Introduction

The growing demand for high-performance lithium-ion batteries (LIBs) driven by the proliferation of portable electronics and the rapid advancement of electric vehicles (EVs) has necessitated the development of next-generation electrode materials with enhanced energy densities.^1–3 Among the essential battery components, the anode plays a key role in determining both the gravimetric and volumetric capacity of LIBs.^4–6 Graphite, the dominant commercial anode material, is favored for its low cost, excellent cycling stability, and adequate electrical conductivity. However, its limited theoretical capacity of 372 mA h g⁻¹, attributed to the insertion of only one lithium atom per six carbon atoms, has become a bottleneck for further improving battery performance.^7–9 As EVs continue to evolve with higher range requirements, the need for alternative anode materials with superior capacity becomes more pressing.^10,11

Silicon (Si) has emerged as one of the most promising anode candidates due to its exceptional theoretical capacities: ∼3579 mA h g⁻¹ by forming Li₁₅Si₄, and over 8000 mA h cm⁻³ in volumetric terms.^12–14 Additionally, Si offers a low operating potential, earth abundance, and environmental compatibility, making it ideal for sustainable energy storage applications.^15–19 Its ability to alloy with approximately four lithium atoms per Si atom enables it to deliver nearly ten times the capacity of graphite.^20,21 Consequently, Si anodes hold strong potential for high-energy-density LIBs, particularly in EV applications where compact, long-lasting, and high-capacity batteries are critical.

Despite these advantages, Si anodes face formidable challenges in practical implementation. The large volume change (∼300%) during lithiation/delithiation cycles induces severe mechanical stress,^22–24 leading to particle pulverization, electrode delamination, and loss of electrical contact.^25–29 Concurrently, repeated fracture and reformation of the solid electrolyte interphase (SEI) consume electrolyte components,³⁰ create unstable interfaces, and hinder lithium-ion transport.^31–33 These issues collectively result in poor cycling stability, sluggish charge transfer kinetics, and substantial irreversible capacity loss.^34,35 Over the past decade, researchers have proposed various approaches to address these problems, such as nano-engineering Si structures,^36–41 incorporating conductive and elastic matrices (e.g., Si–C, Si–rGO, Si–CNT hybrids),^41–47 and optimizing binders and electrolytes to stabilize the electrode/electrolyte interface.^48–58

In addition to these well-known degradation mechanisms, recent studies have highlighted a particularly concerning phenomenon in Si anodes: the coulombic efficiency (CE) trough. This refers to a characteristic dip in CE occurring after the initial cycles, typically during early-to-mid cycling, as shown in Fig. 1a, despite high CE in early stages (e.g., ∼99.75%).⁵⁹ The exact onset of this behavior is highly system-dependent and can vary substantially depending on electrode design, cycling conditions, and material properties. The CE trough also poses challenges for practical applications, as it accelerates capacity fade and reduces long-term energy retention. As a unifying metric, the CE trough links intrinsic Si mechanics to interfacial instability and persistent lithium loss, making it a critical lens through which the long-term viability of Si anodes should be evaluated. Importantly, the CE trough is not merely a transient fluctuation in efficiency but a degradation event with direct consequences for battery performance and lifetime. The temporary drop in CE reflects intensified irreversible lithium consumption and continued SEI renewal, leading to a permanent loss of cyclable lithium inventory. This loss accelerates capacity fading and contributes to impedance growth. In practical full-cell systems with limited lithium inventory, such early-stage lithium loss can significantly shorten cycle life.

Fig. 1 Evidence of the CE trough phenomenon in structurally engineered Si-based anodes. (a) CE versus cycle number, demonstrating the characteristic CE trough near the 30th cycle in microporous Si derived from AlSi alloy. (b) Schematic illustration of microporous silicon fabrication via selective etching of AlSi alloy in oxalic acid. Reproduced with permission from ref. 59. Copyright 2019, Elsevier. (c) Scanning electron microscopy (SEM) image of porous Si produced via magnesio-milling reduction of rice husk-derived SiO₂. (d) Micrograph revealing the 3D porous network of the same sample. (e) CE profile showing a distinct CE trough despite the engineered porous structure. Reproduced with permission from ref. 60. Copyright 2016, American Chemical Society. (f) Micrograph of Si nanowires directly grown on stainless steel, illustrating the structural integrity before initial lithiation. (g) Capacity and CE trends for Si nanowire electrodes, highlighting the presence of a CE trough. Reproduced with permission from ref. 61. Copyright 2021, Nature. (h) Cycling performance of silicon–graphite electrodes with varying Si content, showing that higher Si ratios correlate with more pronounced CE troughs. Reproduced with permission from ref. 62. Copyright 2017, the Electrochemical Society.

To orient this review, Scheme 1 provides a conceptual overview of the CE trough as a unifying framework for Si anode degradation. It links the fundamental cause of large volume expansion to the resulting spongy structural evolution, repeated SEI renewal, and persistent lithium loss, which together manifest as a characteristic dip in coulombic efficiency. Within this framework, we evaluate three main mitigation strategies: electrolyte design that promotes LiF-rich SEI, binder engineering to preserve structural integrity, and voltage window optimization to reduce interfacial stress. Framing the CE trough in this way enables systematic comparison of approaches and highlights its importance as a diagnostic metric for advancing Si anodes toward EV-relevant performance.

Scheme 1 Conceptual overview of the CE trough in Si anodes. Schematic illustration linking degradation mechanisms (volume expansion, SEI renewal, Li⁺ loss), mitigation strategies (electrolyte, binder, voltage, diagnostics), and resulting outcomes (stabilized SEI, reduced Li⁺ loss, and extended cycle life). This framework presents the CE trough as a diagnostic map connecting underlying causes, intervention strategies, and electrochemical performance.

2. Universality of the coulombic efficiency trough in structurally diverse silicon anodes

In this section, we show that the CE trough is not confined to a single silicon morphology or synthesis route. By sampling elemental Si (porous, nanowires, micro/nano) and composites (Si–C, Si–SiO_x, polymer-modified), we demonstrate that the same signature recurs across architectures and processing histories, separating material-specific effects from universal behavior. To date, no comprehensive review has specifically addressed the CE trough, a universal yet underexplored degradation phenomenon, across different Si anode systems. Importantly, this CE trough is not confined to a specific type of Si material or fabrication method but is instead universally observed in a broad range of structurally engineered Si anodes. For example, Cao et al. investigated the influence of particle size in AlSi alloy powders, which served as precursors for microporous Si anodes.⁵⁹ These anodes, characterized by their interconnected porous structures formed via selective acid etching,⁵⁴ displayed a distinct CE trough after initial cycles (Fig. 1a and b). This observation indicates the persistence of CE decline even in optimized porous architectures designed for mechanical buffering. Similarly, Cho et al. employed a magnesio-milling strategy to derive porous silicon from rice husk-based SiO₂ (RH-SiO₂) (Fig. 1c and d), capitalizing on its naturally high surface area.⁶⁰ Despite the structural advantages of this bio-derived, porous network, a CE trough was clearly evident during extended cycling (Fig. 1e). Liu et al. further demonstrated that even when RHs are converted into nano-Si through controlled magnesiothermic reduction, the CE trough remains a consistent feature, suggesting that neither reduced particle size nor enhanced porosity fully resolves this efficiency dip.⁶³ In another example, Favours et al. synthesized porous nano-silicon using beach sand as a cost-effective precursor.⁶⁴ This method produced high-surface-area structures with good electrochemical performance. However, the CE trough still emerged, highlighting that even scalable, earth-abundant approaches with advantageous morphology are not immune to this issue. Advanced characterizations provide further insight into this phenomenon. He et al. utilized cryo-scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) tomography to visualize the structural and chemical evolution of the SEI in situ.⁶¹ Even when Si nanowires (SiNWs) were directly grown on stainless steel to ensure strong electrical connectivity and controlled morphology (Fig. 1f), the CE trough was clearly observed (Fig. 1g), suggesting the universality of this behavior regardless of the electronic percolation pathway. Moreover, the CE trough is evident in silicon–graphite (Si–G) composite electrodes, which are among the most promising transitional platforms for commercial Si integration. Wetjen et al. studied Si–G anodes with varying silicon content and found a tradeoff between capacity and CE stability: higher Si content increased specific capacity but also exacerbated the CE trough, while more graphite mitigated the trough at the expense of capacity (Fig. 1h).⁶² This balance reinforces the importance of compositional design in mitigating the trough effect. Collectively, these studies affirm that the CE trough is a widespread and intrinsic phenomenon that transcends material origin, synthesis technique, and morphological design. Whether Si is derived from metallurgical alloys, biogenic silica, beach sand, or synthesized directly as nanowires or composites, the appearance of a CE trough is a consistent challenge that limits long-term performance. These findings emphasize the need to understand the mechanisms governing the CE trough across different Si architectures.

We then extend the survey from elemental Si to state-of-the-art Si-based composites designed to buffer strain or passivate interfaces, to test whether such architectures suppress the trough. In addition to structurally engineered elemental silicon, the CE trough has been widely observed in a wide range of Si-based composite anodes, regardless of the specific design strategies employed to enhance performance. These composite structures, designed to buffer volume expansion, improve interface stability, and enhance electron/ion transport, represent state-of-the-art material architectures for next-generation LIBs. Yet, despite these advancements, the CE trough remains a persistent and intrinsic electrochemical feature. A representative example is the pomegranate-like Si–carbon composite developed by Liu et al.⁶⁵ This structure, shown in Fig. 2a, features silicon nanoparticles encapsulated in carbon shells with internal voids designed to accommodate the substantial volume change during cycling. The design minimizes mechanical degradation and mitigates direct electrolyte exposure. While this architecture leads to improved electrochemical performance (Fig. 2b), a distinct CE trough still emerges during early cycling, again intensifying the challenge's universality. Similarly, Andersen et al. investigated a silicon/carbon (Si/C) composite anode derived from commercial high-purity Silgrain® powder.⁶⁶ Although ball milling and composite formation with carbon provided enhanced structural integrity and electrochemical stability (Fig. 2c), the CE trough remains visible in the cycling profile (Fig. 2d). The addition of a buffer solution that promotes covalent bonding between Si and binder delayed the onset of the trough and slightly alleviated its depth, but could not flatten it, representing the limitation of such chemical stabilizers in resolving fundamental CE behavior. Lee et al. presented a spray-pyrolyzed Si-SiO_x composite that improves electronic conductivity and offers a partially passivating matrix around active Si particles.⁶⁷ Although the composite architecture contributes to better cycle stability, the CE trough still persists, reaffirming that even partial oxide encapsulation cannot fully suppress irreversible lithium consumption in early-to-mid cycles.

Fig. 2 Manifestation of the CE trough across structurally engineered Si-based composite anodes. (a) SEM image of pomegranate-like Si–carbon composite microbeads featuring Si nanoparticles encapsulated within carbon shells with internal voids. (b) Cycling performance showing improved stability from the pomegranate structure, while still exhibiting a CE trough. Reproduced with permission from ref. 65. Copyright 2014, Nature Publishing Group. (c) Transmission electron microscopy (TEM) image of Si/C composite derived from ball-milled Silgrain® silicon particles. (d) CE profile of the Si/C anode showing a CE trough, which is only partially mitigated by the addition of a buffering electrolyte. Reproduced with permission from ref. 66. Copyright 2019, American Chemical Society. (e) TEM image of polymer-coated Si/C composite using PHATN, showing a uniform SEI layer. (f) Electrochemical performance indicating enhanced cycling stability, with a CE trough still present. Reproduced with permission from ref. 68. Copyright 2023, Wiley-VCH. (g) (i) TEM image of a core–shell Si/PPP composite; (ii) selected-area electron diffraction (SAED) confirming crystalline Si core; (iii) high-resolution TEM image showing lattice fringes corresponding to the Si (111) plane. (h) CE profile of the Si/PPP composite, highlighting a persistent CE trough despite improved polymer encapsulation. Reproduced with permission from ref. 69. Copyright 2023, American Chemical Society.

Surface modification with functional polymers represents another design pathway aimed at improving interfacial stability and SEI control. For example, Wang et al. utilized poly(hexaazatrinaphthalene) (PHATN) to coat a Si/C composite, leading to a thin, uniform artificial SEI (∼26.6 nm) and enhanced cycling stability (Fig. 2e and f).⁶⁸ However, the CE trough is still observed, indicating that polymer-induced SEI modulation, while beneficial, is insufficient to eliminate the underlying mechanism driving CE loss. Chen et al. advanced this strategy further by embedding nanosized Si (∼80 nm) within a Li⁺-conductive polyparaphenylene (PPP) matrix, forming a core–shell architecture with improved ionic pathways and mechanical buffering.⁶⁹ Characterization (Fig. 2g(i–iii)) confirms high crystallinity of Si and strong polymer encapsulation, yet the CE profile (Fig. 2h) still exhibits a trough, reinforcing that even highly engineered polymer coatings cannot fully prevent the early decline in efficiency. Finally, Patnaik et al. introduced poly(borosiloxane) (PBS) as a self-healing, anion-trapping artificial SEI with an electron-deficient boron moiety.⁷⁰ Although this multifunctional interface shows promising improvements in cycling performance, it does not prevent the CE trough, which remains a consistent electrochemical signature.

Taken together, these examples confirm that the CE trough is not exclusive to pure Si but also manifests in advanced Si-based composites with diverse structural, chemical, and interfacial optimizations. These findings point out that while composite design can delay or moderate the severity of the CE trough, it cannot fully eliminate it. As visualized in Scheme 2, this behavior spans nano-/micro-Si, Si nanowires, porous Si, and Si composite architectures. Structural tuning may shift the onset and depth of the trough, but not its occurrence. Accordingly, we treat the CE trough as an architecture-agnostic diagnostic for Si anodes, enabling fair comparison of mitigation strategies in the sections that follow. This universality indicates that the CE trough is governed by more fundamental mechanisms intrinsic to the lithiation/delithiation behavior of Si and its interfaces, topics which will be explored in the next section.

Scheme 2 Universality of the CE trough across silicon anode architectures.

3. Mechanisms driving the coulombic efficiency trough

As discussed in previous sections, a pronounced CE trough is universally observed in Si-based anodes, regardless of their structural design or composite formulation. This phenomenon is widely associated with two strongly coupled processes: (1) significant and often partially irreversible volume expansion of Si during lithiation/delithiation cycles and (2) continuous formation and evolution of an unstable SEI; however, their causal relationship and relative contributions remain unresolved. These coupled effects lead to progressive lithium consumption and degradation of active material, ultimately manifesting as a drop in CE. Our group has investigated this behavior in detail and identified a sponge-like structural transformation in a representative Si electrode system that coincided with CE trough development under specific conditions.⁷¹ As shown in the STEM high-angle annular dark-field (HAADF) image (Fig. 3a), the initially dense Si particles transform into a sponge-like, low-density morphology, accompanied by a significant reduction in material density. The distribution of the Si signal (yellow) indicates the formation of a highly porous and amorphous phase. This transformation is driven by repeated and cumulative volume expansion, which not only destabilizes the mechanical integrity of the particles but also continually generates fresh Si surfaces. These freshly exposed surfaces react with the electrolyte during each cycle, leading to recurrent SEI breakdown and regeneration. Wetjen et al. provided corroborating evidence of this phenomenon in Si–G electrodes.⁷²Fig. 3b shows STEM images of Si particles at various stages of cycling, from the pristine state to the 60th cycle. Over time, the initially crystalline particles become increasingly irregular, with internal voids emerging as early as the 20th cycle and expanding significantly by the 60th. This structural evolution corresponds to a permanent volume increase of ∼70%, which cannot be fully reversed upon delithiation. The consequences of this structural degradation are evident in the evolving chemical composition of the particles. Fig. 3c presents EDS spectra that reveal a steady decrease in Si content and a concurrent increase in oxygen and fluorine, indicating intense SEI growth and electrolyte decomposition. Quantitatively, Fig. 3d shows that Si content drops from 88% in the uncycled state to just 36% after 60 cycles, demonstrating the extent of active material consumption due to repeated SEI formation.

Fig. 3 Structural and chemical evolution of Si-based anodes during cycling, highlighting mechanisms driving the CE trough. (a) HAADF-STEM image with EDS mapping showing the transformation of Si anodes into a porous, sponge-like structure with reduced density after the CE trough cycle. Reproduced with permission from ref. 71. Copyright 2025, Wiley-VCH. (b) STEM images of Si–G anodes at different stages (pristine, 1st, 5th, 20th, 40th, and 60th cycles), illustrating progressive volume expansion and nanopore formation. (c) HAADF-STEM images with EDS spectra of Si particles after the 5th and 60th cycles, showing reduced Si signal and increased O and F content due to SEI accumulation. (d) Elemental composition analysis of Si particles across cycling stages, with Si content decreasing from 88% (pristine) to 36% after 60 cycles, indicating significant active material loss. Reproduced with permission from ref. 72. Copyright 2018, the Electrochemical Society. (e) Cryo-STEM HAADF images of Si nanowires at pristine state and after the 1st, 36th, and 100th cycles, showing void propagation and structural disintegration. (f) SEI layer growth rate and thickness evolution over cycling, indicating rapid initial formation followed by progressive internal penetration. (g) 3D cryo-STEM EDS maps tracking the spatial evolution of Si and SEI within nanowires from two viewing angles: (i) 1st cycle, core–shell structure with intact Si core; (ii) 36th cycle, SEI components begin infiltrating the Si core; (iii) 100th cycle, fragmented Si domains embedded in a thick SEI matrix. Reproduced with permission from ref. 61. Copyright 2021, Nature. (h) Schematic illustration of Si morphological evolution driving the CE trough.

Further insights are provided by He et al., who used cryo-STEM and EDS tomography to monitor SEI and Si evolution in Si nanowire electrodes.⁶¹ As shown in Fig. 3e, voids form and propagate within the nanowires over multiple cycles, disrupting the initially uniform morphology. Cryo-STEM HAADF images in Fig. 3f reveal that SEI formation is rapid in the first cycle, but rather than growing uniformly, the SEI penetrates progressively into the Si matrix during extended cycling. This behavior is more clearly illustrated in Fig. 3g: (1) at the 1st cycle (Fig. 3g(i)), the Si nanowires retain a well-defined core–shell structure with an intact crystalline Si core. (2) By the 36th cycle (Fig. 3g(ii)), electrolyte decomposition products begin infiltrating the core region. (3) After 100 cycles (Fig. 3g(iii)), the structure degenerates into a “plum pudding” morphology, with fragmented Si domains embedded within a thick, ionically insulating SEI matrix.

As shown in Fig. 3e–g, inward SEI growth occurs in parallel with the sponge-like transformation, though causality remains unresolved. In a mechanics-first view, end-of-delithiation tensile stress nucleates cracks and voids, allowing SEI to advance along these defects. In a chemistry-first view, transport-limited electrolyte reduction builds SEI within partially lithiated regions, chemically degrading the matrix and subsequently driving fracture. Our observations near the trough onset (density loss; decreasing Si atomic percent with increasing O/F) are consistent with the generation of new reactive surfaces and continued electrolyte reduction but do not yet establish the order of events. Resolving this ambiguity requires quantitative discriminators that are currently lacking, including: (i) cycle-resolved SEI depth and composition profiles from surface to core, and (ii) per-cycle quantification of newly generated surface area or porosity correlated with irreversible Li loss. Overall, Fig. 3h summarizes the morphological progression from intact Si → cracking → sponge-like structure, coinciding with a dip in CE.

These observations are consistent with a coupled feedback interaction between mechanical degradation and electrochemical instability: (1) repetitive volume expansion induces particle fracturing and the generation of new electrochemical interfaces. (2) These new surfaces trigger continuous SEI reformation, consuming lithium and electrolyte irreversibly. (3) Over time, the loss of active Si, thickened SEI, and increased internal resistance contribute to reduced CE and deteriorated battery performance. Recent studies further indicate that both SEI cracking and dissolution contribute to sustained lithium consumption in Si anodes, with cracking-driven surface renewal dominating cycling-induced instability and dissolution becoming more prominent over extended time scales, reinforcing the central role of interphase instability in CE trough development.⁷³ However, the initiation step, causal ordering, and quantitative contribution of each process to the depth and duration of the CE trough remain unresolved. While this feedback mechanism is generally observed, its manifestation can vary significantly depending on silicon particle size, morphology, and material design. As summarized in Table 1, the reported characteristics of the CE trough differ across various silicon systems, and no single structural parameter consistently determines the trough onset or depth. This variability reflects the strong influence of experimental conditions, including electrode loading, electrolyte composition, and cycling protocol. Rather than following a simple size- or morphology-dependent trend, the CE trough should be understood as a system-level response governed by the coupled effects of mechanical stress evolution, fresh surface generation, and interfacial stability.

Table 1 Comparison of reported coulombic efficiency (CE) trough characteristics across different silicon particle sizes, morphologies, and material designs

Material design	Structural feature	Size	CE trough cycle number	CE trough depth (%)	References
Porous micro-Si	Porous Si particles	10–30 µm	∼32	∼98.5	59
Nanoporous Si	Interconnected porous framework	Pore: 50–200 nm	∼60	∼98.4	60
		Wall: ∼11 nm
Si nanowire	1D nanowire architecture	Nanowire diameter: ∼60–90 nm	∼60	∼98.3	61
Si–graphite composite	Si nanoparticles blended with graphite	200 nm	∼20	∼97.4	62
Porous nano-Si	Porous Si nanoparticles	∼10–40 nm	∼50	∼98.6	63
C-coated nano-Si	Carbon-coated porous/interconnected nano-Si	∼8–10 nm	∼50	∼98	64
Pomegranate-type nano-Si	Hierarchical nano-in-micro, void-confined carbon-encapsulated structure	∼80 nm	∼50	∼98.5	65
Ball-milled Si/C composite	Irregular sub-micron Si-based composite particles	∼50–300 nm	∼20	∼93	66
Si/SiOx/C composite	Spherical composite particles with embedded Si nanodomains in SiOx matrix and carbon coating	∼70 nm	∼40	∼98.9	67

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Based on representative data compiled in Table 2, a consistent qualitative trend can be observed between CE trough severity and long-term cycling performance. Electrodes exhibiting deeper or more prolonged CE trough behavior generally show faster capacity fading, which can be attributed to cumulative irreversible lithium loss during this stage. For high-capacity Si anodes, even small reductions in CE correspond to significant lithium consumption per cycle, which progressively depletes the cyclable lithium inventory. Therefore, the depth and duration of the CE trough can serve as useful comparative indicators of long-term stability. However, it should be emphasized that the absolute values of CE and trough duration vary strongly with material design and testing conditions, and should not be interpreted as universal thresholds. Instead, the CE trough is best utilized as a diagnostic metric for relative comparison across different systems.

Table 2 Summary of electrode designs, electrolyte formulations, voltage windows, areal densities, and binder materials

Anode design	Electrolyte type	Binder	Voltage window (V)	Areal density (mg cm^2)	CE trough depth (%)	Capacity retention @cycle no	References
Porous Si	1.0 M LiPF6 in EC : DEC (1 : 1 by vol) with 10% FEC	CMC	0.005–1.5	0.54	∼98.5	83.5%@200	59
Porous Si	1.0 M LiPF6 in EC : DEC (1 : 1 by vol) with 5% FEC	PAA	0.01–1.5	0.6	∼98.4	82.8%@200	60
Si nanowire	1.2 M LiPF6 in EC : DMC (3 : 7 by wt) with 10% FEC	—	0.02–1.5	—	∼98.3	71%@100	61
Si-G	1.0 M LiPF6 in EC : EMC (3 : 7 by wt) with 5% FEC	LiPAA	0.01–1.25	0.71–1.84	∼97.4	68%@120	62
Nano-Si	1.0 M LiPF6 in EC : DEC (1 : 1 by wt) with 1% VC	PVDF	0.01–1.0	0.1–0.2	∼98.6	86%@300	63
C-coated@nano-Si	1.0 M LiPF6 in EC : DEC (1 : 1 by vol) with 2% VC	PAA	0.01–1.0	0.5–1.0	∼98	37.3%@1000	64
Pomegranate-nano-Si	1.0 M LiPF6 in EC : DEC (1 : 1 by wt) with 1% VC	PVDF	0.01–1	0.2	∼98.5	97%@1000	65
Si/C composite	1.0 M LiPF6 in EC : DMC (1 : 1 by vol) with 10% FEC	CMC	0.05–1	—	∼93	9.77%@50	66
Si–SiOx–C composite	1.0 M LiPF6 in EC : DEC (1 : 1 by vol) with 5% FEC	PAA	0.01–1.5	1.0	∼98.9	87.9%@100	67
PHATN coating on Si/C	1.0 M LiPF6 in EC : DEC (1 : 1 by vol) with 10% FEC	CMC	0.01–1	0.9–1.1	∼98.5	55.3%@500	68
Si/polymer composite	1.0 M LiPF6 in EC : DMC : EMC (1 : 1 : 1 by vol)	PAA	0.01–2	—	∼98.6	60%@400	69
Polymer coated Si	1.0 M LiPF6 in EC : DEC (1 : 1 by vol)	Poly(borosiloxane) PVDF	—	—	∼98.75	87.3%@350	70
Nano-spherical Si	1.0 M LiPF6 in EC : EMC : DMC (1 : 1 : 1 by vol) with 5% FEC (F-control)	PVA	0.01–2	—	∼97.7	No capacity	74
	1.2 M LiFSI/0.05 M LiDFOB in DME : HFE : FEC (3 : 6 : 1 by vol) (N-DHF)				No trough	53%@200
Nano-Si	1.2 M LiPF6 in EC : EMC (3 : 7 by vol) (STD)	PAA	0.005–1.5	∼0.41	∼91.5	39.2%@100	75
	1.2 M LiPF6 in EC : EMC (3 : 7 by vol) with 10% FEC (STD + 10% FEC)				∼97.4	74.2%@100
	1.0 M LiDFOB in EC : EMC (3 : 7 by vol) (LiDFOB)				∼94.2	38.4%@100
	1.0 M LiDFOB in TEP : TEP saturated with LiNO3 : EC : EMC (1 : 1 : 1 : 7 by vol) with 0.2 M LiNO3 (LiDFOB + LiNO3/TEP)				No trough	87.1%@100
Micro-Si	2.0 M LiBH4 in THF : MeTHF (1 : 1 by vol) (LBH)	Guar gum (GG)	0.06–1.0	1.8–2.2	No trough	94.3%@100	76
	2.0 M LiBF4 in THF : MeTHF (1 : 1 by vol) (LBF)				∼98	92.1%@100
	1.0 M LiPF6 in THF : MeTHF (1 : 1 by vol) (LPF)				—
Micro-Si	BE (1.0 M LiPF6 in EC : EMC (1 : 2 by vol))	CMC + SBR	0.01–1.5	1–1.64	∼96.2	31%@150	71
	BE + FEC (1.0 M LiPF6 in EC : EMC (1 : 2 by vol) with 25% FEC)		0.01–1.5		∼98.5	86%@150
	THF (2.0 M LiPF6 in THF:2mTHF  (1 : 1 by vol))		0.01–0.5		No trough	83%@150
			0.01–1.5		No trough	60%@150
Micro-Si	1.0 M LiPF6 in THF : MeTHF (1 : 1 by vol)	LiPAA	0.06–1.0	0.3	∼92	No capacity	81
	1.0 M LiPF6 in EC : DMC (1 : 1 by vol)				No trough	90%@400
Micro-Si	1.0 M LiPF6 in EC : EMC (1 : 1 by vol) (EE)	LiPAA	0.05–1	1.2	∼96.2	No capacity	94
	1.0 M LiPF6 in FEC : FEMC : TTE (2 : 6 : 2 by vol) (FFT)				No trough	No capacity
	1.0 M LiPF6 salt in FEC : sulfolane : TTE (2 : 6 : 2 by vol) (FST)				No trough	80%@250
Graphene-coated nano-Si	1.0 M LiPF6 in EC : DEC (2 : 8 by vol)	LiPAA	0.01–1.5	—	∼93.6	No capacity	95
	1.0 M LiPF6 in EC : DEC : FEC (2 : 6 : 2 by vol)				∼98.1	64@200
	1.0 M LiPF6 in PYR13FSI				No trough	92.8@200
Si/C	1.0 M LiPF6 in EC : DMC : DEC (1 : 1 : 1 by vol) with 2% VC	Alginate hydrogel	0.01–1.2	—	No trough	82.3@120	96
		Na alginate (SA)			∼96	32.5@120
Nano-Si	1.2 M LiPF6 in EC : EC : DMC (3 : 1 : 6 by wt) with 1% LiDFOB	Guar gum (GG)	0.01–2	0.68–0.8	No trough	18.7%@100	97
		Anionic polyacrylamide (APAM)				54.2%@100
		A2G1				61.9%@100
Micro-Si	1.0 M LiPF6 in EC : EMC (1 : 2 by vol) with 25% FEC	CMC + SBR	0.01–1.5	0.7–0.98	∼98.5	55%@150	101
			0.01–0.32		∼98	No capacity
			0.23–1.5		No trough	96.8%@150
Si-G	1.0 M LiPF6 in EC : DMC (1 : 1 by vol) with 2% FEC	PVDF	0.0–0.5	1	No trough	62%@200	102
			0.0–0.7		97.3	64%@200
			0.0–1.0		96.6	40%@200
			0.0–1.5		96.5	37%@200
			0.0–2.0		96.4	29%@200
Nano-spherical Si	1 M LiFSI in THF : TTE (1 : 2 by vol) (LHCE-THF)	PAA	0.01–1.2	1	No trough	∼80%@400	82
	1 M LiFSI in DME : TTE (1 : 2 by vol) (LHSE-DME)				∼98.8	∼30%@400
	1 M LiPF6 in EC : DEC (1 : 1 by wt) (RCE)				∼94.8	∼8%@200
Nano-spherical Si	1 M LiFSI in CPME : TTE (2 : 1 by vol) (CPME)	PAA	0.01–1.2	1	No trough	73.1%@200	83
	1 M LiFSI in CPME : THF : TTE (1 : 1 : 1 by vol) (CPMETHF)				No trough	38.7%@200
	1 M LiPF6 in EC : DMC : DEC (1 : 1 : 1 by vol) (RCE)				∼95	12.1%@200
Si/Gr powder	LiFSI : 2MDOL : TTE (1 : 2.6 : 3 by mol) (E-4MDOL)	PAA	0.01–2	1.6–1.8	No trough	85.4%@400	84
	LiFSI : 2MDOL : TTE (1 : 2.6 : 3 by mol) (E-2MDOL)				No trough	63.0%@400
	1 M LiPF6 in EC : DMC : EMC (1 : 1 : 1 by vol) with 5 wt% FEC (E-control)				∼98.6	16.4%@400
Si nanoparticles	1 M LiPF6 in EC : DEC (1 : 1 by vol)	Sodium alginate	0.01–1	0.56–0.75	∼97	11.1%@200	85
	1 M LiPF6 in EC : DEC (1 : 1 by vol) with 10% TPFC				No trough	52.8%@200

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4. Approaches to stabilize the coulombic efficiency trough

Building on these mechanistic insights, this section summarizes three major levers to stabilize the CE trough in Si anodes: electrolyte engineering, binder optimization, and voltage window control, all aimed at improving interfacial stability and limiting irreversible Li loss.

4.1 Tailoring electrolytes to address the coulombic efficiency trough

Electrolyte engineering plays a crucial role in mitigating the CE trough in Si-based anodes by influencing SEI composition and interfacial stability. A well-formulated electrolyte can form a chemically stable and mechanically robust SEI, thereby minimizing irreversible lithium consumption and enhancing cycling performance. Cao et al. demonstrated that a nonflammable ether-based electrolyte, comprising lithium bis(fluorosulfonyl)amide (LiFSI) in dimethoxyethane (DME) with 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) as a diluent (denoted N-DHF), effectively flattens the CE trough observed in a conventional F-control electrolyte (1.0 M LiPF₆ in EC : EMC : DMC (1 : 1 : 1 by vol) + 5 wt% FEC).⁷⁴ As shown in Fig. 4a, the N-DHF system maintains high CE and stable capacity retention throughout cycling. SEM and TEM analyses (Fig. 4b(i–viii)) reveal that the F-control system leads to extensive particle cracking, volume expansion, and the formation of a thick, heterogeneous SEI layer. In contrast, the N-DHF system preserves a smooth electrode morphology and forms a thin, uniform SEI, preventing structural disintegration and eliminating the CE trough. X-ray photoelectron spectroscopy (XPS) spectra (Fig. 4c) further highlight differences in interfacial chemistry. The F-control SEI is dominated by unstable organic species (e.g., ROCO₂Li, RCO₂Li), while the N-DHF SEI contains more inorganic components, including Li₂CO₃, B–O networks, and LiF. These species are associated with improved mechanical flexibility and chemical stability. As schematically illustrated in Fig. 4d, the N-DHF-derived SEI is compact, F-rich, and reinforced with elastic B–O linkages, which better accommodate Si volume changes and sustain CE over cycling.

Fig. 4 Electrolyte-driven stabilization of CE in Si anodes through SEI modulation. (a) Cycling performance of Si/Li cells with F-control and N-DHF electrolytes, showing the elimination of the CE trough and improved cycling stability with N-DHF. (b) SEM and TEM characterization of Si anodes after cycling with F-control (i–iv) and N-DHF (v–viii) electrolytes. Top- and cross-sectional SEM images highlight differences in surface morphology, volume expansion, and SEI thickness, while high-resolution TEM shows structural preservation and uniform SEI formation in N-DHF. (c) XPS spectra (C 1s and O 1s) comparing SEI compositions, indicating greater organic content in the F-control system and more stable inorganic species (Li₂CO₃, B–O, LiF) in N-DHF. (d) Schematic representation of SEI architecture, contrasting the thick, organic-rich SEI in F-control with the thin, elastic, B–O reinforced SEI in N-DHF. Reproduced with permission from ref. 74. Copyright 2021, Wiley-VCH. (e) CE and capacity retention of Si/Li half-cells using various electrolyte formulations, showing elimination of the CE trough in LiDFOB + LiNO₃/TEP. (f) Field emission-SEM images before and after 100 cycles illustrating electrolyte-dependent changes in surface morphology. (g) XPS spectra (C 1s, N 1s, B 1s) revealing the SEI chemical composition in different systems, with LiDFOB + LiNO₃/TEP forming a chemically stable and elastic SEI. Reproduced with permission from ref. 75. Copyright 2022, the Electrochemical Society. (h) Schematic illustration showing that electrolyte-engineered stable SEI suppresses the CE trough.

Rynearson et al. further investigated the impact of four electrolyte systems: STD (1.2 M LiPF₆ in EC : EMC (3 : 7 by vol)), STD + 10% FEC, lithium difluoro(oxalato)borate (LiDFOB), and LiDFOB + LiNO₃/triethyl phosphate (TEP).⁷⁵ Among them, the LiDFOB + LiNO₃/TEP formulation achieved the most stable CE without exhibiting a trough (Fig. 4e). Post-mortem SEM analysis (Fig. 4f) shows that the STD system forms a thick, uneven SEI, while FEC addition improves but does not fully stabilize the interphase. The LiDFOB + LiNO₃/TEP electrolyte, however, generates a uniform and passivating SEI that effectively protects the electrode. XPS results (Fig. 4g) support this by revealing SEI compositions rich in C=O, C–O, B–O, B–F, and N-based species from additive decomposition. The presence of NO₃⁻ and NO₂⁻ indicates sustained LiNO₃ reduction, which contributes to an elastic, ionically conductive SEI with minimal LiF accumulation. These properties collectively account for the suppression of the CE trough and enhanced cycling performance. As illustrated in Fig. 4h, electrolyte engineering that promotes an inorganic-rich, continuous, and elastic SEI can passivate freshly generated Si surfaces and suppress the CE trough under the tested voltage window. Such stabilization reduces continuous Li consumption and partially decouples interfacial failure from bulk morphological evolution.

In a complementary study, Li et al. developed a fluoride-free electrolyte system, which not only suppressed the CE trough but also significantly improved the cycle life of micron-sized Si anodes.⁷⁶ In contrast, LiPF₆ systems prevented the trough but resulted in poor long-term cycling, while LiBF₄ formulations exhibited inconsistent electrochemical behavior. These findings highlight the importance of rational anion and solvent selection in promoting interphase integrity and long-term Si anode stability. To summarize, these results demonstrate that carefully tailored electrolyte formulations, through salt selection, co-solvent engineering, and functional additive incorporation, can effectively suppress the CE trough by fostering robust SEI architectures that resist degradation and accommodate volume changes in Si-based anodes (Table 3).

Table 3 Electrolyte strategies to remove the CE trough: mechanism, effectiveness, scalability, limitations

Electrolyte type	Mechanism	Effectiveness	Scalability	Limitations
1.2 M LiFSI/0.05 M LiDFOB in DME : HFE : FEC (3 : 6 : 1 by vol) (N-DHF)	Forms a compact, F-rich, B–O reinforced SEI that buffers Si swelling and suppresses the CE trough	Eliminates CE trough; enhances electrochemical performance	Medium	Poor compatibility with high-voltage cathodes; environmental and cost burden
1.0 M LiDFOB in TEP : TEP saturated with LiNO3 : EC : EMC (0.2 M LiNO3 in 1 : 1 : 1 : 7 by vol) (LiDFOB + LiNO3/TEP)	Generates an elastic, ionically conductive, low-LiF SEI that passivates Si surfaces and suppresses the CE trough	Eliminates CE trough; moderate capacity retention	Medium	Limited LiNO3 solubility/stability (precipitation risk); sluggish low-temperature kinetics; reactivity with high-voltage cathodes
2.0 M LiBH4 in THF : MeTHF (1 : 1 by vol) (LBH)	Forms a uniform, LiF-rich, self-limiting SEI that preserves conductive pathways	Eliminates CE trough; improved capacity	Low	Low oxidative stability; severe moisture/air sensitivity
THF (2.0 M LiPF6 in THF:2mTHF (1 : 1 by vol))	Retains LixSi phases and generates a LiF-rich, low-impedance SEI, preserving ionic/electronic transport	Eliminates CE trough; cycle life remains moderate	Low	Limited thermal/aging robustness; poor high-voltage stability; viscosity–transport trade-offs at 2.0 M
1.0 M LiPF6 salt in FEC : sulfolane : TTE (2 : 6 : 2 by vol) (FST)	Forms a Li2O–LiF SEI that resists Si swelling, blocks electrolyte ingress, and maintains contact in cracked Si	Eliminates CE trough; high capacity and long cycle life	Medium	Sulfolane-rich FST wets slowly and conducts poorly; uneven SEI growth and weak low-T performance
1.0 M LiFSI in PYR13FSI	Builds a stable SEI that prevents CE trough formation	Eliminates CE trough; high capacity retention	Medium	High viscosity and poor wetting; slow Li^+ transport; Al corrosion and thick CEI at high voltage
1 M LiFSI in THF : TTE (1 : 2 by vol) (LHCE-THF)	Forms a thin, elastic, LiF-rich inorganic-polymeric SEI	Eliminates CE trough; elevated capacity	Medium	Low oxidative stability; Al corrosion; volatility and flammability risks
1 M LiFSI in CPME : THF : TTE (1 : 1 : 1 by vol) (CPMETHF)	Provides high Li^+ conductivity, low desolvation barrier, and a LiF-rich elastic-polymeric SEI	Eliminates CE trough; moderate capacity	Medium	Limited oxidative stability; Al corrosion risk; impedance growth from cross-talk
LiFSI : 2MDOL : TTE (1 : 2.6 : 3 by mol) (E-4MDOL)	Produces an elastic, polymer-rich SEI that accommodates Si expansion and suppresses side reactions	Eliminates CE trough; higher capacity	Medium-low	Low oxidative stability; Al corrosion; volatility and flammability issues
1 M LiPF6 in EC : DEC (1 : 1) with 10% TPFC	Generates a thin, continuous, mechanically stable SEI on Si	Eliminates CE trough; higher capacity	High	HF/PF5 generation; poor low-temperature behavior; high-voltage side reactions

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4.2 Advanced ether- and fluorine-based electrolytes for suppressing the coulombic efficiency trough

Beyond carbonate-based systems, recent research highlights the advantages of ether-based and fluorine-rich electrolytes in mitigating the CE trough in Si anodes through improved interfacial stability, SEI chemistry, and mechanical resilience. Our group systematically evaluated the impact of three electrolyte formulations, base carbonate electrolyte (BE; 1.0 M LiPF₆ in EC : EMC (1 : 2 by vol)), BE with FEC additive, and a tetrahydrofuran-based electrolyte (THF), on the CE behavior of Si anodes.⁷¹ As shown in Fig. 5a, the BE system exhibited a rapid capacity decline within the first few cycles, attributed to the formation of a thick, unstable SEI and excessive volume expansion. The corresponding CE profile in Fig. 5b reveals a pronounced CE trough in BE, whereas BE + FEC only partially mitigated the trough. In contrast, the THF-based electrolyte effectively prevented the CE trough, enabling more stable cycling. Post-cycling structural analysis (Fig. 5c) showed that Si anodes cycled in THF retained a denser and more compact morphology compared to the sponge-like porous structure observed with BE-based systems (Fig. 3a). The THF system suppressed excessive SEI formation, evidenced by low oxygen and carbon signals, and preserved a more stable electrode interface. XPS analysis of Si 2p spectra (Fig. 5d) revealed electrolyte-dependent changes in surface chemistry. In BE + FEC, the formation of lithium silicates such as Li₄SiO₄ and Li₂SiO₃ dominated the SEI.^77–79 In contrast, the THF system yielded Li₄SiO₄ and Li₂Si₂O₅, but uniquely retained Li_xSi phases, crucial for maintaining ionic/electronic conductivity and CE. Additionally, the THF-based SEI was rich in LiF, known for its mechanical durability and low impedance, further contributing to the stable electrochemical response and the suppression of the CE trough.⁸⁰

Fig. 5 Electrolyte-dependent suppression of the CE trough through SEI stabilization and compositional control. (a) Specific capacity of Si anodes cycled in BE, BE + FEC, and THF electrolytes, showing rapid capacity fade in BE and stable cycling in THF. (b) CE profiles of Si anodes in the same systems, with the CE trough most pronounced in BE and mitigated in THF. (c) STEM-HAADF image with EDS mapping at the 47th cycle showing compact morphology and reduced porosity of Si in the THF system. (d) Si 2p XPS spectra before and after cycling in pristine BE, BE + FEC, and THF-based electrolytes, highlighting differences in lithium silicate formation and retention of Li_xSi in THF. Reproduced with permission from ref. 71. Copyright 2025, Wiley-VCH. (e) Cycling performance of Si anodes in the mixTHF system showing elimination of the CE trough. (f) EELS-based characterization of SEI formed in mixTHF (i–iii) and carbonate-base (iv–vi) electrolytes: (i and iv) HAADF images of Si particles; (ii and v) EELS signal intensity maps of LiF distribution; (iii and vi) representative plasmon energy spectra from surface to bulk, confirming LiF–Li_xSiO_y–Li_xSi multilayer structure in mixTHF electrolyte and mixed SEI in carbonate-based electrolyte. Reproduced with permission from ref. 81. Copyright 2020, Nature.

Chen et al. advanced this concept by developing a mixed-THF (mixTHF) electrolyte, specifically optimized for micro-sized Si anodes.⁸¹ This system promoted the formation of a thin, uniform, and LiF-rich SEI. Cycling results (Fig. 5e) demonstrated full elimination of the CE trough and improved performance. Using electron energy loss spectroscopy (EELS), the authors mapped the SEI composition and identified a stratified interphase composed of LiF–Li_xSiO_y–Li_xSi layers. The top LiF layer, confirmed by a sharp plasmon peak at 25 eV (Fig. 5f(i–iii)), was shown to effectively protect the inner Si structure. In contrast, the base-electrolyte sample formed a mixed organic–inorganic SEI with broad plasmon features centered around 22 eV, indicating less robust protection (Fig. 5f(iv–vi)). THF-rich electrolytes can flatten the CE trough by promoting a more stable SEI at the Si anode, but they face practical constraints: limited oxidative stability at high cathode voltages, volatility and flammability concerns, potential salt-solubility and transport limitations at certain temperatures and rates, and possible gas evolution or shelf-life issues. According to literature, lower-voltage systems, such as LFP charged between 2.5 and 3.45 V, tend to be more compatible and impose less oxidative stress than higher-voltage systems.⁸¹ Nevertheless, THF-based electrolytes should be revalidated in matched full-cell studies, including high-voltage storage tests with EIS, Al current-collector stability, and CEI diagnostics, before making broad claims of trough suppression.

While THF-rich systems highlight the role of LiF-rich interphases in flattening the CE trough, multiple independent studies reinforce this principle across diverse electrolyte chemistries. Yaozong et al. compared LHCE-THF (1 M LiFSI in THF/TTE = 1 : 2 v/v), LHCE-DME (1 M LiFSI in DME/TTE = 1 : 2 v/v), and a conventional carbonate-based RCE (1 M LiPF₆ in EC/DEC = 1 : 1 w/w); LHCE-THF fully flattened the CE trough and sustained higher capacity, whereas RCE showed a pronounced trough and rapid fade, and LHCE-DME exhibited partial improvement but an abrupt drop in capacity.⁸² In a follow-up, Yaozong et al. also tested CPME (1 M LiFSI in CPME/TTE = 2 : 1 v/v), CPME-THF (1 M LiFSI in CPME/THF/TTE = 1 : 1 : 1 v/v), and RCE; the CPME-THF formulation generated a thin, LiF-rich interphase that suppressed the trough and maintained superior cycling.⁸³ Similarly, Ma et al. showed that E-4MDOL (LiFSI : 2MDOL : TTE = 1 : 2.6 : 3 molar) outperformed both E-2MDOL and an ester-rich FEC control, fully eliminating the trough while delivering superior electrochemical performance.⁸⁴ Hu et al. further demonstrated that TFPC-containing formulations formed a thin LiF/polyolefin SEI, suppressing the trough and significantly enhancing cycling stability compared with a TFPC-free EC/DEC baseline.⁸⁵ Collectively, these studies substantiate that LiF-enriched inorganic SEI layers, generated through tailored electrolyte design, confer both chemical resilience and mechanical robustness to Si anodes.

We explicitly balance anode gains from LiF-promoting, ether-lean electrolytes with cathode-side consequences and full-cell limits. In matched full cells, dissolved transition-metal ions and oxidative fragments from the cathode migrate to the anode, reconfiguring the SEI and accelerating electrolyte reduction and lithium-inventory loss. Recent studies quantify how Ni/Mn/Co dissolution and CEI breakdown alter Si/graphite SEI chemistry and deepen anode aging.^86–89 While ethers (including THF-rich) or LiF-enriching formulations can flatten the CE trough at the Si anode, their oxidative stability at high-voltage cathodes is limited unless specifically engineered (e.g., non-polar ethers, LHCE/locally saturated-ether concepts). Otherwise, CEI growth and impedance rise at ≥4.2–4.5 V are expected.^90,91 A further practical constraint is Al current-collector corrosion in FSI⁻-based systems, which multiple reports attribute to voltage-dependent Al attack in LiFSI/ether electrolytes. Mitigation strategies (e.g., high-concentration designs, corrosion inhibitors, or alternative salts) are still required.^92,93 Finally, high-voltage storage thickens the CEI and increases impedance, so anode-side improvements must always be revalidated under cathode-relevant voltage holds and temperature protocols.

Li et al. further tested a series of electrolyte formulations, including 2,2,2-trifluoroethyl, methyl carbonate (FEMC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and FFT (1.0 M LiPF₆ in FEC/FEMC/TTE) systems, for micro-Si anodes.⁹⁴ While FFT eliminated the CE trough, its poor cycle life limited practical relevance. The FST electrolyte, formulated with 1.0 M LiPF₆, FEC, sulfolane, and TTE, successfully avoided the CE trough while delivering significantly improved capacity retention over time. In contrast, the standard EC/EMC-based electrolyte displayed a prominent CE trough and severe performance degradation, reinforcing the advantages of ether-sulfolane-fluorinated systems in micro-Si stabilization. Park et al. evaluated few-layer graphene-coated Si nanoparticles in different electrolytes.⁹⁵ The LiFSI–PYR₁₃FSI ionic liquid-based electrolyte outperformed traditional LiPF₆–EC/DEC and its FEC-containing variants by fully eliminating the CE trough and enhancing cycle life. This superior performance was attributed to the formation of a uniform, stable SEI that maintained particle integrity and minimized side reactions. As a result, these findings highlight that THF-based, fluorine-rich, or ionic liquid electrolytes, especially those forming LiF- or silicate-dominated SEIs, play a key role in suppressing the CE trough by improving interphase stability, structural cohesion, and charge transport across cycles.

4.3 Role of binders in mitigating the coulombic efficiency trough

In addition to electrolyte engineering, the binder system plays a critical role in stabilizing Si-based anodes and suppressing the CE trough. An effective binder must accommodate the substantial volume changes of Si during cycling while maintaining strong adhesion, mechanical resilience, and electrochemical compatibility. Liu et al. developed a cross-linked alginate hydrogel binder, synthesized by ionically bridging sodium alginate (SA) chains with calcium ions (Ca²⁺), which significantly improved the cycling stability of Si/C anodes.⁹⁶ As shown in Fig. 6a, anodes using the pure SA binder exhibited rapid capacity decay and a marked CE trough during early cycles. In contrast, the alginate hydrogel binder effectively mitigated the CE trough under standard conditions, supporting stable capacity retention. At higher current densities, however, a moderate CE trough reemerged (Fig. 6b), indicating that binder effectiveness could be rate-dependent. The performance differences are rooted in the structural integrity provided by the binder. In the pure SA system, weak inter-chain interactions lead to delamination and particle pulverization during lithiation-induced expansion, compromising electrical connectivity (Fig. 6c(i and ii)). By contrast, the alginate hydrogel binder forms a tightly cross-linked network that withstands expansion and preserves electrode integrity (Fig. 6c(iii and iv)). This robust framework limits the formation of new SEI layers and stabilizes interfacial reactions, thereby suppressing the CE trough.

Fig. 6 Binder-driven stabilization of Si anodes and suppression of the CE trough through structural reinforcement. (a) Cycling performance of Si/C anodes using sodium alginate (SA) and Ca²⁺ cross-linked alginate hydrogel binders, showing enhanced capacity retention and CE stability with the hydrogel binder. (b) Cycling performance of Si/C anodes with alginate hydrogel binder under higher current density conditions, revealing a reappearance of the CE trough. (c) SEM images of Si/C electrodes after 70 cycles: (i and ii) with SA binder show particle fracture and delamination; (iii and iv) with alginate hydrogel binder exhibit structural preservation and minimal degradation. Reproduced with permission from ref. 96. Copyright 2014, Royal Society of Chemistry. (d) Reversible capacity and CE profiles of nano-Si anodes using A2G1 binder, demonstrating stable long-term performance with no CE trough. (e) Cross-sectional SEM images of Si anodes with GG, APAM, and A2G1 binders at various cycling stages: (i–iii) pristine; (iv–vi) post-lithiation; (vii–ix) post-delithiation; (x–xii) after 100 cycles (delithiated). A2G1 electrodes show minimal volume change and superior structural integrity. Reproduced with permission from ref. 97. Copyright 2022, Wiley-VCH. (f) Schematic illustration showing how binder engineering improves mechanical integrity and mitigates the CE trough.

Expanding on this concept, Li et al. introduced a composite binder system called A2G1, which synergistically combines the soft, elastic nature of guar gum (GG) with the mechanical rigidity of anionic polyacrylamide (APAM).⁹⁷ The A2G1 binder imparts both flexibility and structural support, enabling the Si@A2G1 anode to deliver excellent capacity retention with no observable CE trough over 100 cycles (Fig. 6d). Cross-sectional SEM images (Fig. 6e) reveal significant differences in electrode swelling and damage across the three binder systems. After 100 cycles, electrodes with GG and APAM alone experienced severe volumetric expansion, 204.96% and 109.58%, respectively, along with interfacial cracking. In contrast, the Si@A2G1 electrode exhibited a volume expansion of only 59.15% with no major fractures, maintaining a uniform and continuous electrode morphology. This stability is attributed to the dual-phase nature of A2G1, where soft segments buffer mechanical stress and rigid components prevent excessive deformation. These studies demonstrate that binder design is a key parameter in mitigating the CE trough, particularly through mechanisms that preserve electrode integrity, prevent SEI reformation, and sustain electronic pathways during large volume fluctuations inherent to Si anodes.

Beyond these representative systems, binder design strategies can be further classified into two categories: structural stabilization and interfacial regulation, both of which contribute to CE trough mitigation through distinct mechanisms. For example, Liu et al. reported that a covalently cross-linked CMC/citric acid (c-CMC–CA) binder forms an all-integrated electrode architecture with strong interfacial adhesion and mechanical robustness, enabling improved structural stability and more uniform SEI formation under repeated volume change.⁹⁸ Similarly, an on-site cross-linked polyacrylamide binder (c-PAM–CA) forms a three-dimensional network through condensation reactions, enhancing tensile strength and adhesion to both Si particles and the current collector. This system suppresses crack formation and limits the exposure of fresh surfaces, thereby reducing continuous electrolyte decomposition.⁹⁹ In addition to static cross-linked systems, dynamic self-healing binders provide an additional pathway for mitigating structural degradation. Jang et al. introduced a self-healing cross-linked PAA binder functionalized with DABBF, in which reversible bonding enables recovery of mechanical integrity during repeated deformation. This design reduces crack propagation and fresh surface generation, thereby indirectly suppressing persistent SEI growth.¹⁰⁰

In contrast to purely structural stabilization, binder systems that actively regulate interfacial chemistry can more effectively suppress pronounced CE trough behavior. For instance, a cross-scale stabilization (CSS) binder integrates a rigid-flexible network with conductive components, enabling stable cycling with consistently high coulombic efficiency and no apparent CE trough, which is attributed to the combined suppression of particle pulverization and the formation of a stable LiF-rich SEI.⁵⁸ Similarly, a physically cross-linked conductive polymer binder (PPC) introduces a dynamic hydrogen-bonding network and enhanced electronic conductivity, promoting the formation of a Li–N-containing SEI and enabling fast charge-transfer kinetics without inducing a pronounced CE trough.⁵⁷

These studies collectively indicate that effective binders mitigate the CE trough not only by improving adhesion, but also by distributing mechanical stress, suppressing crack propagation, and preserving electrode cohesion, thereby limiting fresh surface generation and stabilizing interfacial reactions. However, it is important to note that purely mechanical stabilization primarily provides indirect benefits, while only a limited number of advanced binder systems have been shown to actively regulate interfacial chemistry for more effective suppression of pronounced CE trough behavior. As illustrated in Fig. 6f, binder engineering that increases wet modulus, toughness, and adhesion (e.g., cross-linked or dual-network systems) mechanically restrains particle breathing and closes micro-cracks, thereby reducing new-surface generation and flattening the CE trough. Table 4 summarizes representative binder strategies, comparing their mechanisms, effectiveness, scalability, and limitations in the context of CE trough mitigation.

Table 4 Binder strategies to remove the CE trough: mechanism, effectiveness, scalability, limitations

Binder type	Mechanism	Effectiveness	Scalability	Limitations
Alginate hydrogel	Forms a tightly cross-linked network that withstands Si expansion, preserves electrode integrity, and eliminates the CE trough	Stable cycle life with the CE trough eliminated	Medium	Moisture-sensitive and prone to swelling; requires strict drying. Swelling softens the electrode and degrades high-rate performance
A2G1, by combining soft guar gum (GG) with rigid anionic polyacrylamide (APAM)	Combines the flexibility of guar gum (GG) with the rigidity of anionic polyacrylamide (APAM), providing structural reinforcement and stress buffering	Stable cycle life with no CE trough	Medium-high	Water-based processing and swelling issues; requires strict drying. GG-rich swelling thickens/softens the electrode, harming high-rate performance and CE
Cross-linked carboxymethyl cellulose–citric acid binder (c-CMC–CA)	Forms strong covalent interconnections between binder, active material, and conductive network, improving adhesion and structural stability	Indirect stabilization of CE through reduced structural degradation	High	Limited direct regulation of interfacial reactions; CE improvement mainly indirect
Cross-linked polyacrylamide–citric acid binder (c-PAM–CA)	Forms a 3D network through condensation reactions, enhancing adhesion and suppressing crack formation	Moderate improvement in cycling stability; indirect CE stabilization	High	Limited effect on CE regulation; primarily structural stabilization
Self-healing cross-linked poly(acrylic acid) binder with diarylbibenzofuranone (xPAA-DABBF)	Incorporates reversible bonding that enables autonomous crack repair and structural recovery during cycling	Improved cycling stability with suppressed continuous SEI growth	Medium	Complex synthesis; limited direct interfacial control
Cross-scale stabilization binder with rigid-flexible conductive network (CSS)	Integrates a rigid-flexible polymer network with conductive components to suppress pulverization and promote formation of a stable LiF-rich SEI	High coulombic efficiency (∼99.6%) with no apparent CE trough	Medium	Multi-component design; increased material and processing complexity
Cross-linked conductive polymer binder (polypyrrole–polyacrylic acid–citric acid, PPC)	Forms a dynamic hydrogen-bonding network with enhanced electronic conductivity, improving stress dissipation and interfacial stability	Stable CE with no pronounced CE trough and improved rate capability	Medium	Multi-component system; processing complexity and cost considerations

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4.4 The role of voltage window in mitigating the coulombic efficiency trough

Voltage window optimization has emerged as a highly effective strategy for mitigating the CE trough in Si anodes by reducing structural degradation and stabilizing the SEI. Our group has conducted extensive studies demonstrating that cycling Si anodes within a narrower voltage window (0.01–0.5 V vs. Li/Li⁺) compared to the conventional condition (0.01–1.5 V) consistently suppresses the CE trough and enhances long-term cycling performance.^71,101 As shown in Fig. 7a, Si anodes cycled within the 0.01–0.5 V range exhibit stable specific capacities and maintain high CE without showing any signs of a trough. This voltage protocol enables near-complete lithiation while avoiding full delithiation, which in turn reduces the mechanical stress and volume fluctuations typically responsible for CE deterioration. Structural characterization via HAADF-STEM and EDS mapping (Fig. 7b–d) reveals that Si anodes cycled under this moderate voltage protocol retain a denser, more compact morphology with limited expansion and no sponge-like structural degradation. Notably, elemental mapping confirmed the consistent presence of fluorine in the SEI, suggesting a chemically stable interphase. Complementary XPS analysis of the Si 2p region (Fig. 7e) confirmed the formation of stable lithium silicates (Li₄SiO₄ and Li₂SiO₃) along with the retention of Li_xSi species, which are critical for maintaining ionic/electronic conductivity and mitigating interfacial resistance.^43,46 The persistence of Li_xSi within the electrode structure is directly correlated with the high CE and efficient lithium transport observed under this voltage condition.¹⁰¹ In another study by our group, using the same 0.01–0.5 V protocol, full lithiation without full delithiation again successfully suppressed the CE trough and prolonged electrode stability (Fig. 7f).¹⁰¹

Fig. 7 Effect of voltage window on the suppression of CE trough and structural stabilization of Si anodes. (a) Specific capacity and CE of Si anodes cycled within a 0.01–0.5 V window, showing suppression of the CE trough. (b–d) Structural and compositional analysis at the 47th cycle: (b) HAADF-STEM image of Si anode showing compact morphology; (c) EDS mapping of Si distribution; (d) EDS mapping of C, F, and O elements indicating uniform SEI composition. (e) XPS spectra (Si 2p) showing formation of Li₄SiO₄, Li₂SiO₃, and Li_xSi in the SEI under 0.01–0.5 V cycling. Reproduced with permission from ref. 71. Copyright 2025, Wiley-VCH. (f) Cycling performance and CE of Si anodes under the same voltage protocol in a separate study. Reproduced with permission from ref. 101. Copyright 2024, Elsevier. (g) Capacity retention and (h) CE profiles of Si–graphite (Si–G) anodes cycled under different voltage windows, demonstrating that only the 0–0.5 V range suppresses the CE trough. Reproduced with permission from ref. 102. Copyright 2023, American Chemical Society. (i) Schematic illustration showing how voltage-window control regulates Si expansion, stabilizes the SEI, and suppresses the CE trough.

Yamazaki et al. further validated this approach by systematically evaluating Si anodes cycled over a range of voltage windows: 0–0.5 V, 0–0.7 V, 0–1.0 V, 0–1.5 V, and 0–2.0 V.¹⁰² Among these, only the 0–0.5 V window effectively flattened the CE trough while simultaneously delivering superior cycling performance (Fig. 7g and h). Wider voltage ranges promoted the decomposition of carbonate solvents such as EC and DMC, especially after depletion of functional additives like FEC. The resulting SEIs were enriched in organic species, which are mechanically unstable and incapable of accommodating the dynamic volume changes of Si. This led to progressive isolation of active material and rapid capacity fading. These findings collectively highlight the critical importance of voltage window design in Si anode systems. Cycling within a shallow voltage range not only limits mechanical degradation and volume expansion, but also promotes the formation of stable and ionically conductive SEI layers. As illustrated in Fig. 7i, narrowing the voltage range suppresses end-of-delithiation stress, minimizes new-surface generation, and stabilizes the SEI, whereas a wider range amplifies expansion, destabilizes the SEI, and produces a pronounced CE trough. However, this stabilization comes with a trade-off: early delithiation cut-off leaves residual Li_xSi, lowering reversible capacity by ∼10–35% at equivalent loading and rate. This may require higher N/P ratios or electrode loadings to preserve cell-level energy. Consequently, we recommend applying a shallow voltage protocol during formation or early cycles, or coupling it with electrolyte and binder optimizations that permit slightly wider windows without reintroducing the CE trough. Table 5 provides a comparative summary of voltage window strategies, including their mechanisms, effectiveness, scalability, and key limitations in CE trough mitigation.

Table 5 Voltage window strategy to remove the CE trough: mechanism, effectiveness, scalability, limitations

Voltage window vs. Li/Li^+	Mechanism	Effectiveness	Scalability	Limitations
0.01–0.5	Shallow voltage cycling preserves LixSi and stabilizes the SEI, suppressing the CE trough and improving CE and cycle life	Improved cycle life with mitigated CE trough	High	Reduced capacity/energy due to under-delithiation, residual LixSi lock-up shifting Li inventory, and increased balancing complexity (tighter N/P ratio or prelithiation required)

---

As illustrated in Scheme 3, the CE trough in Si anodes arises from the substantial volumetric expansion of Si during lithiation. This expansion imposes severe mechanical stress on the electrode structure and leads to the repeated rupture and reformation of the SEI. Over successive cycles, the cumulative structural degradation transforms the initially dense Si into a spongy, porous morphology, which further accelerates SEI instability. This dynamic evolution results in continuous lithium consumption and electrolyte decomposition, manifesting as a pronounced CE trough. To address this challenge, several interdependent strategies have been developed to suppress Si expansion and enhance SEI durability. These include:

Scheme 3 Schematic illustration of the CE trough mechanism in Si-based anodes and its suppression. Uncontrolled lithiation leads to large-scale volume expansion, SEI rupture, and the formation of a spongy Si structure, driving CE degradation. Mitigation strategies, including stable SEI formation via tailored electrolytes, binder reinforcement to buffer mechanical stress, and voltage window control to limit delithiation and swelling, collectively suppress the CE trough and improve long-term electrochemical performance.

(1) Tailored electrolyte formulations that promote the formation of robust, LiF-rich SEI layers with superior mechanical and chemical stability.

(2) Mechanically adaptive binders that accommodate expansion while maintaining electrode cohesion.

(3) Voltage window optimization to prevent full delithiation and reduce interfacial stress.

By simultaneously targeting the mechanical, chemical, and electrochemical origins of the CE trough, these approaches can effectively enhance interfacial stability, reduce irreversible lithium loss, and enable prolonged, high-efficiency cycling of Si-based anodes.

5. Future work

Future research on Si anodes must prioritize a deeper mechanistic understanding of the SEI evolution and its dynamic interplay with volumetric changes in Si-based anodes, a critical nexus underlying the CE trough phenomenon. In particular, elucidating the formation, composition, and stability of LiF-rich SEI layers under various cycling conditions remains essential for optimizing interfacial stability and minimizing irreversible lithium consumption. Cycle-resolved SEI mapping through the CE trough is a top priority. Future measurements should co-register mechanics ↔ chemistry ↔ electrochemistry across the dip. Key experiments include: (i) cryo-FIB/cryo-TEM time series at pre-trough, trough, and post-trough cycles with depth-resolved EELS/EDS/cryo-XPS to quantify SEI composition gradients and Si porosity/density; (ii) operando dilatometry/curvature to link stress/strain and strain-rate to CE dips; (iii) electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) or online mass spectrometry to measure irreversible mass/gas per cycle; (iv) synchrotron nano-CT for 3D crack/void statistics; and (v) impedance partitioning (DRT) to separate transport growth from interphase growth during the trough. The combination of these methods allows for a cause-effect timeline that distinguishes between mechanics-first and chemistry-first pathways in practical electrodes.

Calendar aging must be evaluated alongside cycling degradation, as storage can induce distinct pathways that exacerbate or counteract CE-trough suppression. A practical protocol is to store cells at a defined mid-lithiation or state-of-charge under controlled temperature (e.g., 25 °C for 12–24 months, or accelerated at elevated temperature), then resume cycling with a short diagnostic sequence. Critical outputs include: (i) ΔCE upon restart over 3–5 cycles, (ii) capacity retention relative to the pre-storage baseline, (iii) impedance growth, and (iv) shifts in voltage hysteresis. Reporting test context (SOC/lithiation set point, temperature/duration, and counter-electrode/cell format) is essential to ensure reproducibility. For translational relevance, storage studies should extend to full cells under EV-relevant conditions, such as practical loadings, lean electrolyte, realistic stack pressure/temperature, and high-SOC storage. Gate tests such as high-voltage storage with impedance tracking, Al current-collector corrosion checks, and paired SEI/CEI depth profiling (XPS, ToF-SIMS, cryo-TEM) before and after storage provide a rigorous baseline. Reporting monthly capacity/resistance changes and shifts in CE-trough metrics ensures suppression strategies are benchmarked consistently under both cycling and storage stresses.

Parallel to mechanistic investigations, synergistic material designs remain essential. We recommend a data-driven co-design framework in which electrolyte, binder, and voltage protocol are optimized simultaneously using active learning and Bayesian optimization. High-throughput screening should extract CE trough metrics automatically (minimum CE, cycle index, dip depth/width/area, irreversible Li loss per cycle) and rank candidates by CE trough suppression and full-cell compatibility. Physics-aware descriptors (electrolyte viscosity, LiF/borate formation propensity, anion stability; binder wet modulus/toughness/adhesion, swelling; protocol features such as upper cutoff, rests, and cycling rate) can guide the search space, enabling efficient convergence toward robust solutions. In addition, emerging design strategies beyond the scope of this review, such as transport-enhanced or electronically modified Si systems, may also contribute to improved interfacial stability and faster coulombic efficiency stabilization, suggesting additional pathways for addressing trough-related degradation in future studies.^103,104

Scaling micro-Si anodes in full cells is another critical frontier. Translation requires validation under practical areal loadings, lean electrolyte conditions, and realistic stack pressure/temperature. Key checkpoints include: (i) calendering and through-thickness porosity/binder gradients; (ii) current-collector adhesion and gas management; (iii) N/P ratio optimization and pre-lithiation to offset early Li loss; and (iv) swelling limits versus separator/stack constraints. Before broad claims, a minimal gate-test suite should be applied: high-voltage storage with impedance tracking, Al current-collector corrosion screening at the intended cutoff, and paired SEI/CEI depth profiles (via XPS/ToF-SIMS/cryo-TEM) under lean-electrolyte and elevated-temperature conditions. Reporting CE trough metrics alongside capacity retention and impedance growth will ensure that mitigation strategies truly translate at the cell level.

To further enable cross-study comparability and practical implementation, we suggest that CE trough metrics be evaluated under a minimal and consistent reporting framework. Specifically, trough extraction should be anchored by: (i) formation exclusion, where analysis begins only after formation is complete; (ii) baseline definition using a rolling average prior to trough onset; (iii) standardized reporting of trough depth, including both absolute ΔCE and baseline-normalized values; and (iv) protocol and material anchoring, where key descriptors such as C-rate, voltage window, areal loading, particle size, electrode composition, and binder system are explicitly reported. We emphasize that this framework serves as a practical guideline rather than a strict universal standard, and further validation across different systems is required. Under such a shared protocol, the CE trough can be more rigorously evaluated as a transferable diagnostic metric and may serve as a useful program-level KPI for the development of durable silicon anodes.

By pursuing these directions, the gap between laboratory discovery and manufacturable, durable Si anodes can be bridged via cycle-resolved diagnostics, data-driven co-design, and scale-up validation. This could help advance the practical realization of Si anodes for high-energy-density, long-life LIBs.

6. Conclusions

Si anodes are widely regarded as a leading candidate for next-generation LIBs due to their exceptional theoretical capacity and earth abundance. However, their practical deployment remains limited by intrinsic challenges, notably significant volume expansion, structural degradation, and the formation of unstable SEI layers. These issues contribute to capacity fading, mechanical failure, and poor long-term cycling performance. This review specifically highlights the coulombic efficiency trough, a critical yet often underrecognized phenomenon that emerges during cycling of Si-based anodes. Beyond its mechanistic significance, the CE trough represents a critical performance bottleneck for practical batteries. The irreversible lithium consumption during this stage reduces the cyclable lithium inventory, which accelerates capacity fade, increases internal resistance, and shortens the operational lifetime of LIBs.

To mitigate the CE trough, several strategies have proven effective. These include, but are not limited to: (1) tailored electrolyte formulations, particularly those that promote the formation of LiF-rich, stable SEI layers; (2) mechanically adaptive binders, such as cross-linked alginate hydrogels and dual-component binder systems, which reduce electrode fracture and preserve structural integrity; (3) voltage window optimization, where limiting cycling to ranges such as 0.01–0.5 V reduces interfacial stress and prevents irreversible SEI degradation. By integrating these approaches, electrolyte engineering, binder design, and electrochemical control, the CE trough can be significantly suppressed, resulting in improved capacity retention, higher efficiency, and extended cycle life. Beyond mitigation, we propose treating the CE trough as a diagnostic framework that links cause to consequence: intrinsic Si volume change → spongy structure → new surface → SEI renewal → Li loss → impedance rise. To make studies comparable and falsifiable, we recommend reporting a minimal set of trough metrics alongside cycling data: (i) minimum CE and cycle index at the dip, (ii) dip depth, width, and integrated area, (iii) irreversible Li per cycle, and (iv) new-interface proxies such as porosity or crack-density evolution.

To ensure trough suppression survives scale-up, we suggest gate tests in matched full cells under realistic constraints, including high-voltage storage with impedance tracking, aluminum current-collector stability checks, and paired SEI/CEI forensics (XPS/ToF-SIMS/cryo-TEM) at practical loadings, lean electrolyte levels, and elevated temperatures. Because storage can age cells differently than cycling, we further recommend reporting calendar-aging metrics, such as monthly capacity and resistance changes at specified states of charge and temperatures, along with shifts in trough metrics after storage. In this context, the CE trough can serve as a useful diagnostic indicator that links interfacial instability to long-term performance degradation. When evaluated alongside complementary metrics and realistic testing conditions, it provides a practical pathway for bridging laboratory insights with the development of durable silicon anodes for advanced energy storage applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

This work is a review article and does not report new experimental data. All data discussed in this manuscript are available in the cited literature and can be accessed through the corresponding references.

Acknowledgements

This research was funded by the National Science and Technology Council (NSTC) of Taiwan (114-2628-E-A49-003) and by the Ministry of Education (MOE) of Taiwan under the Yushan Young Fellow Program (MOE-114-YSFEE-0010-005-P2) and Higher Education SPROUT Project of National Yang Ming Chiao Tung University.

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