A uniform lithium ion flux and robust interphase enabled by an anion anchoring additive for high energy density Si-based anodes

Abstract

Silicon/graphite (Si/Gr) composite anodes offer a practical approach toward higher energy density lithium-ion batteries (LIBs), yet their performance commonly deteriorates due to interface and ionic transport driven reaction heterogeneity. Tortuous Li⁺ transport pathways induce non-uniform Li⁺ flux and localized overpotentials, which trigger spatially uneven interfacial reactions and the growth of organic-rich solid electrolyte interphase (SEI) layers with physicochemical instability. These coupled electrochemical and mechanical instabilities promote repeated SEI rupture/reformation, impedance growth, irreversible lithium loss, and electrode swelling. In this study, we report a facile electrode engineering strategy using sodium 4-vinylbenzenesulfonate as an interface stabilizing anion-anchoring additive (ISAA) for Si/Gr composite anodes. ISAA improves slurry dispersion via sulfonate-driven electrostatic stabilization, leading to a more homogeneous electrode microstructure. At the interface, anion anchoring establishes a Li⁺-enriched environment that enhances effective Li⁺ transport and homogenizes Li⁺ flux across the electrode. This regulated interfacial chemistry redirects SEI evolution toward a thin, LiF-rich inorganic interphase with superior physicochemical robustness, thereby suppressing interfacial degradation and mitigating irreversible swelling. Consequently, Si/Gr-ISAA anodes exhibit outstanding electrochemical performance in both half-cells and practical full-cell configurations. This study provides a practical approach for the development of high performance Si/Gr composite anodes for next-generation LIBs.

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Junghyun Choi

Junghyun Choi has been an Assistant Professor in the School of Chemical, Biological, and Battery Engineering at Gachon University since 2024. He received his PhD in the Department of Energy Engineering from Hanyang University in 2016. He previously worked as a Staff Researcher at the Samsung Advanced Institute of Technology (SAIT), a Staff Engineer at Samsung SDI, and a Senior Researcher at the Korea Institute of Ceramic Engineering and Technology (KICET). His current research focuses on the development of materials for alkali-ion batteries, advanced battery electrode technologies, and cell design for electric vehicle and energy storage system applications.

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1 Introduction

The rapid expansion of the electric vehicle (EV) market has driven an urgent demand for lithium-ion batteries (LIBs) that offer not only high energy density but also fast-charging capability and long cycle life under practical operating conditions.^1,2 To date, graphite has remained the predominant commercial anode material due to its excellent structural integrity, low cost, and high electrochemical reversibility.³ However, its limited theoretical capacity (372 mAh g⁻¹) fundamentally constrains further improvements in energy density.⁴ To overcome this limitation, silicon has emerged as a promising alternative due to its exceptionally high theoretical capacity (3579 mAh g⁻¹).^5,6 In practical applications, Si based materials such as pure Si, silicon oxides (SiO_x), silicon-carbon (Si/C) composites, and Si-alloys are typically blended with graphite to form silicon/graphite (Si/Gr) composite electrodes.^7,8 This composite strategy leverages the high capacity of silicon while mitigating its intrinsic challenges, such as severe volume expansion and poor cyclability, through the buffering effect and electrical conductivity of graphite.^9–11

Despite the promise of Si/Gr composite electrodes, gains in energy density are often offset at high silicon contents by rapid capacity fading arising from the intrinsic chemo-mechanical instability of silicon.¹² Such degradation in electrochemical performance is further aggravated by spatially heterogeneous electrochemical reactions originating from interfacial instability and non-uniform Li⁺ flux across the electrode.^2,13–15 The solid electrolyte interphase (SEI) forms on the anode surface via reductive decomposition of the electrolyte and comprises both organic and inorganic species, playing a critical role in interfacial stabilization.^16–18 Ideally, the SEI should suppress further electrolyte decomposition by offering physicochemical stability while simultaneously enabling rapid Li⁺ transport through high ionic conductivity.^6,19 However, in high energy density Si/Gr composite anodes, the SEI often lacks sufficient physicochemical integrity, resulting in sustained electrolyte consumption and progressive SEI thickening.²⁰ The resulting increase in interfacial resistance deteriorates charge transfer kinetics, while repeated SEI rupture/reformation coupled with silicon's volume change promotes electrode swelling and mechanical damage.²⁰ Under high mass loading and electrode densification, transport limitations become more severe, giving rise to non-uniform Li⁺ flux and localized current focusing.²¹ These coupled electro-chemo-mechanical effects accelerate interfacial degradation and intensify localized stress within the electrode, becoming even more pronounced at higher current densities.²²

To mitigate these degradation pathways, numerous strategies have been explored, including electrode architecture optimization, surface coatings, binder design, and the use of functional electrolyte additives.^23,24 Although these approaches can improve electrochemical performance and delay the onset of mechanical failure, they often overlook the underlying cause of degradation associated with the non-uniform interfacial electrochemical reactions arising from the intrinsic heterogeneity in Li⁺ transport and localized current density within Si/Gr electrodes.²⁵ As a result, interfacial instability accumulates during prolonged cycling, leading to impedance growth, loss of reversible Li⁺, and capacity fading.²⁶ These limitations highlight the need for a more fundamental strategy that directly regulates interfacial electrochemical reaction homogeneity for Si/Gr composite anodes. Therefore, it is necessary to develop a robust interfacial layer that offers both physicochemical stability and high ionic conductivity at the anode interface. In parallel, design strategies that assist in homogenizing Li⁺ flux throughout the electrode are also critically important for ensuring stable and uniform reaction kinetics.²⁷

In this study, we report a facile electrode engineering strategy employing sodium 4-vinylbenzenesulfonate (SVBS) as an interface stabilizing anion-anchoring additive (ISAA) to enhance the electrochemical performance of Si/Gr composite anodes. The aromatic groups of ISAA exhibit strong π–π interactions with carbonaceous components, enabling uniform distribution and robust anchoring of the additive throughout the Si/C-Gr electrode. Meanwhile, the anionic sulfonate (–SO₃⁻) moieties electrostatically coordinate Li⁺ ions and suppress anion mobility in the local interfacial environment. This increases the effective Li⁺ transference number and facilitates Li⁺ transport near the electrode surface.²⁸ In addition, a regulated interfacial environment promotes the formation of the LiF-dominated inorganic SEI, which provides effective passivation and lowers interfacial resistance.^29–32 By regulating local ion transport, interfacial chemistry, and electrode microstructure, ISAA suppresses interfacial degradation and irreversible electrode swelling, stabilizing the long-term cycling of high energy density Si/Gr composite anodes. It should be noted that the outstanding electrochemical performance is achieved via a simple electrode additive strategy tailored to Si/Gr composite anodes.

2 Experimental methods

2.1 Preparation of electrodes

Si/Gr composite electrodes were fabricated by blending artificial graphite (ShanShan) and Si/C (Hansol Chemical) in an 80 : 20 ratio. Super P as the conductive additive and a binder system consisting of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) in a 2 : 3 ratio were added to achieve a final electrode composition comprising 94 wt% active material, 1 wt% conductive additive, and 5 wt% binder. For the Si/Gr-ISAA anodes, 0.5 wt% of the active mass was substituted with the SVBS (Sigma-Aldrich) additive in powder form, which was directly added to the mixture during the homogenization process to ensure uniform distribution within the slurry. After homogenization via a Thinky mixer (ARE-310, THINKY), the mixture was cast onto a 9 µm thick copper current collector using a doctor blade. The resulting films were first treated at 80 °C in a convection oven, followed by calendering to 1.5 g cm⁻³ electrode density and overnight vacuum drying at 80 °C.

2.2 Materials characterization

The rheological properties of the pristine and Si/Gr-ISAA slurries were evaluated using a rotational rheometer (Discovery HR 20, TA Instruments). All measurements were conducted at a fixed solid content to ensure a direct comparison of slurry rheology without the influence of compositional dilution or concentration effects. Contact angle measurements were performed using deionized water as the probe liquid on the electrode surfaces to evaluate their surface wettability. The measurements were conducted using a contact angle goniometer (DSA100, KRUSS), with droplets placed directly onto the pristine Si/Gr and Si/Gr-ISAA electrode surfaces. The surface and interfacial cutting analysis system (SAICAS; SAICAS EN-EX, Daipla Wintes) was utilized to evaluate the cohesive mechanical properties of the electrodes. Universal testing machine (UTM) measurements were employed to assess the adhesive mechanical properties. Both analyses were conducted on calendered pristine Si/Gr and Si/Gr-ISAA electrodes. X-ray photoelectron spectroscopy (XPS) surface analysis was performed using an XPS spectrometer (PHI genesis, ULVAC PHI). XPS depth profiling was carried out separately using an XPS (K-alpha+, Thermo Scientific) equipped with an Ar⁺ sputter gun, with a sputter time of 20 s per cycle and an ion energy of 2 keV. Electrode resistance measurements were performed on the calendered electrodes using an electrode resistance measurement system (RM2611, Hioki). The morphologies of the pristine Si/Gr and Si/Gr-ISAA anodes were analyzed using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL) before cycling and after 30 cycles.

2.3 Cell assembly and electrochemical measurements

Electrochemical measurements were performed using CR2032-type coin cells. Both pristine Si/Gr and Si/Gr-ISAA electrodes exhibited areal capacities of 3.5 mAh cm⁻² and were employed consistently across all measurements. For half-cell assembly, the electrodes were fabricated into 12 mm diameter disks and paired with lithium metal counter electrodes prepared from 1.2 mm thick Li foil punched into 16 mm diameter disks. For full-cell assembly, composite NCM811 cathodes were prepared from a mixture of NCM811 active material, Super P conductive carbon, and poly(vinylidene fluoride) (PVdF, Solvay 5130) in a weight ratio of 96 : 2 : 2. The resulting slurry was uniformly cast onto 20 µm thick aluminum foil using a doctor blade, followed by drying in a convection oven at 110 °C for 30 min to remove residual solvent. The dried electrode sheets were subsequently punched into 10 mm diameter disks and calendered to achieve a target electrode density of 3 g cm⁻³. A commercial separator (Energy Tech Solution) was used in all cells. The electrolyte formulation consisted of 1.15 M LiPF₆ dissolved in EC : EMC : DMC (2 : 4 : 4 v/v), supplemented with 12.5 wt% fluoroethylene carbonate (FEC), 1 wt% vinylene carbonate (VC), and 1 wt% lithium difluorophosphate (LiPO₂F₂). Cell assembly was conducted inside an argon-filled glovebox under moisture and oxygen controlled conditions. Galvanostatic cycling of both half- and full-cells was carried out using a multi-channel battery tester (Neware, MIHW-200-160CH B). The operating voltage windows were set to 0.01–1.5 V (vs. Li/Li⁺) for half-cells and 2.5–4.2 V for full-cells. Prior to long-term cycling, all cells underwent a formation protocol consisting of three low-current charge/discharge cycles to stabilize the electrode–electrolyte interfaces. Electrochemical impedance spectroscopy (EIS) measurements were performed using a potentiostat/galvanostat system (VSP, Biologic) over a frequency range of 250 kHz to 10 mHz. To measure the Li⁺ transference number (t_Li⁺), the separator was modified using a deionized water-based solution containing 10 wt% of SVBS. The solution was uniformly drop-cast onto a separator, followed by drying at 80 °C under vacuum for 24 h to ensure complete removal of residual moisture. The measurements were then carried out using Li symmetric cells. The measurements were conducted via DC polarization (10 mV, 6 h) combined with impedance measurements recorded over 20 kHz–1 Hz and calculated using the following equation:
where I₀ and I_ss are the initial and steady-state currents, ΔV is the applied potential, and R_p,0 and R_p,ss are the interfacial resistances before and after polarization. Distribution of relaxation times (DRT) analysis was performed using impedance data collected over 1 MHz–0.02 Hz. The galvanostatic intermittent titration technique (GITT) was employed to evaluate the Li⁺ diffusion kinetics. The test consisted of a 0.5C current pulse for 10 min, followed by a 30 min relaxation period. The Li⁺ diffusion coefficient (D_Li⁺) was calculated using the simplified Fick's second law equation:
where τ is the pulse duration, m_B and M_B represent the mass and molecular weight of the active material, V_M and A signify its molar volume and active surface area and ΔE_S and ΔE_t denote the changes in steady-state voltage and transient voltage during the pulse, respectively.

2.4 Computational details

Density functional theory (DFT) calculations were conducted with the Vienna ab-initio simulation package (VASP).^33,34 The Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE) was chosen for the exchange-correlation functionals. Projector-augmented-wave potentials were used with valence configurations of 2p⁶3 s¹, 2 s¹, 2s²2p², 2s²2p⁴, 1 s¹ and 3s²3p⁶ for Na, Li, C, O, H and S, respectively. Plane waves with an energy cutoff of 350 eV were determined based on the convergence test within 0.01 eV per atom. The threshold for convergence of electronic self-consistent iterations was set to 10⁻⁶ eV per cell. The cell parameters and atomic positions were relaxed until the remaining force reached 1 × 10⁻² eV Å⁻¹.

The pseudo-two-dimensional (P2D) model, originally formulated by Doyle, Fuller, and Newman,^35,36 was employed to simulate the electrolyte-phase Li⁺ transport in a half-cell configuration. Steady-state simulations were performed at eight discrete C-rates (0.2C, 0.4C, 0.6C, 0.8C, 1.0C, 1.2C, 1.5C, and 2.0C) to map the rate-dependent concentration polarization behavior. All simulations were conducted using COMSOL Multiphysics.

3 Results and discussion

To achieve high energy density, Si/Gr composite electrodes are typically implemented with high areal loading and high electrode density. Under such practical electrode design and operating conditions, degradation of Si/Gr composite electrodes predominantly arises from inhomogeneous interfacial reactions. As ionic transport pathways become increasingly tortuous, effective Li⁺ transport into the inner regions of the electrode is significantly restricted. Li⁺ transport becomes kinetically favored near the electrode surface, resulting in non-uniform Li⁺ flux across the electrode. Furthermore, the spatially non-uniform Li⁺ flux leads to localized overpotentials at the electrode surface, accelerating parasitic interfacial reactions and the rupture of the SEI layer at earlier cycles, as schematically illustrated in Fig. 1a. In the absence of interfacial regulation, these localized electrochemical environments give rise to continuous electrolyte decomposition and the accumulation of organic-rich SEI species with inherently poor physicochemical integrity.^20,37 During repeated lithiation and delithiation, such vulnerable SEI layers undergo persistent fracture and reformation, resulting in irreversible Li⁺ trapping, and progressive impedance growth. Simultaneously, heterogeneous lithiation depths within silicon particles lead to pronounced internal stress accumulation, accelerating particle pulverization and interfacial delamination, which collectively manifest as irreversible electrode swelling, as depicted in Fig. 1c. In contrast, the negatively charged sulfonate groups in ISAA electrostatically coordinate Li⁺ ions near the electrode interface, enriching the local Li⁺ concentration and homogenizing the Li⁺ flux across the electrode (Fig. 1b). Such regulated interfacial ion transport suppresses localized current focusing and promotes uniform SEI formation. Moreover, the hydrophilic functional groups exhibit strong affinity toward polar carbonate solvents, facilitating a more homogeneous distribution of fluorinated electrolyte components such as FEC. This can favor the formation of a thin, LiF-rich SEI at the anode surface. The resulting mechanochemically robust SEI effectively protects the interface by suppressing continuous electrolyte decomposition while facilitating fast Li⁺ transport owing to its high ionic conductivity. Collectively, the synergistic effects of ISAA directly address both the electrochemical and mechanical origins of interfacial degradation in Si/Gr composite electrodes.

Fig. 1 Schematic of pristine Si/Gr and Si/Gr-ISAA electrodes. (a) Non-uniform Li⁺ flux and severe SEI growth in the pristine Si/Gr electrode. (b) Homogenized Li⁺ flux and stable SEI with ISAA. (c) Comparison of electrode-level expansion and current distribution.

The incorporation of ISAA leads to enhanced physicochemical properties of the electrode system, beginning at the slurry stage and extending to the electrode–electrolyte interface. As shown in Fig. 2a, the pristine Si/Gr and Si/Gr-ISAA slurries showed pseudoplastic behaviors with increasing shear rate.³⁸ The Si/Gr-ISAA slurry exhibits a lower apparent viscosity over the entire shear rate range, indicating improved processability and dispersion stability under practical solid loading conditions. This rheological improvement can be attributed to the dual functionality of ISAA. The aromatic moiety promotes adsorption on carbon surfaces through the van der Waals interactions (π–π interactions), while the sulfonate (–SO₃⁻) groups impart electrostatic repulsion and steric/electrostatic stabilization that suppresses agglomeration.³⁹ As a result, particle–particle contacts are effectively moderated, and the formation of local agglomerates of active materials and conductive additives is mitigated during slurry processing. Such homogeneous dispersion is essential not only for forming uniform electrode coatings but also for minimizing microstructural heterogeneity that can otherwise amplify localized current focusing and non-uniform interfacial reactions during cycling.^40,41 This improved dispersion at the slurry stage ensures uniform distribution of functional components, providing a critical basis for consistent electrochemical reactions throughout the electrode. In addition, a well-dispersed slurry contributes to improved mechanical properties of the electrode by ensuring a uniform binder distribution. To evaluate the mechanical properties of the Si/Gr composite electrodes, SAICAS and UTM measurements were conducted. The SAICAS measurements show an increase in horizontal force (F_H) corresponding to the cohesive strength of the Si/Gr anode prepared with ISAA (hereafter denoted as Si/Gr-ISAA), which indicates enhanced interparticle binding strength within the composite matrix, primarily driven by internal interactions such as hydrogen bonding and enhanced van der Waals forces (Fig. 2b and S1). The enhanced van der Waals interactions associated with the aromatic moiety of ISAA could be beneficial for enhanced interparticle cohesion within the Si/Gr-ISAA electrode.³⁹ In addition, UTM peel tests reveal improved adhesion strength between the electrode and the copper current collector for the Si/Gr-ISAA electrode, which is governed by the interfacial bonding at the electrode–current collector interface (Fig. 2c and S2). These results substantiate that the Si/Gr-ISAA electrodes exhibit outstanding mechanical integrity relative to the pristine Si/Gr anodes, as evidenced by higher adhesion and cohesion strengths. Such enhanced mechanical robustness helps maintain structural integrity during repeated volume changes, thereby stabilizing interfacial reactions and improving cycling performance. The sulfonate groups in ISAA provide functional surface modification to anode materials. Since electrolyte wettability critically affects the internal resistance of high-energy-density electrodes, the static contact angle of Si/Gr composite electrodes was measured to assess their wettability. Si/Gr-ISAA exhibited a lower contact angle of 47.9° compared to that of the pristine Si/Gr anode (69.7°), as shown in Fig. 2d and e. The polar surface functional groups of Si/Gr-ISAA improved electrolyte wetting and penetration into the electrode, facilitating rapid Li⁺ transport. This enhanced wettability enables more efficient electrolyte infiltration and uniform Li⁺ flux across the electrode, reducing ionic resistance. To clarify the homogeneous electrode microstructure and enhanced wettability, symmetric cells were fabricated. The ionic resistance (R_ion) in a tortuous electrode represents the impedance to Li⁺ migration through electrolyte permeated pore pathways within the electrode.⁴² The Si/Gr-ISAA anodes showed a markedly lower R_ion value of 9.98 Ω compared to the pristine Si/Gr anode (24.5 Ω), indicating facilitated ionic transport across the electrode with a uniform microstructure and improved electrolyte wettability (Fig. 2f). This result confirms that the combined effects of the improved microstructure and wettability directly contribute to enhanced ionic transport within the electrode. While both cations and anions contribute to ionic transport, efficient Li⁺ transport plays a dominant role in ensuring uniform electrochemical reactions at the anode. To further clarify this point, t⁺_Li was determined in Li symmetric cells using the Bruce–Vincent method (Fig. 2g and h).⁴³ Notably, the Si/Gr-ISAA cell exhibited a high t⁺_Li value of 0.59, markedly higher than that of the pristine Si/Gr composite anode (0.20) (Fig. 2i). This improvement is attributed to the anion anchoring nature of ISAA arising from its sulfonate moieties, which electrostatically coordinate solvated Li⁺ and partially suppress anion mobility in the local interfacial region.^44,45 This increased t⁺_Li quantitatively mitigates concentration polarization within the porous structure, thereby suppressing localized Li plating and current concentration. As a result, Li⁺ flux becomes more uniformly distributed within the Si/Gr-ISAA electrode, leading to faster reaction kinetics and more spatially homogeneous electrochemical reactions during cycling. Overall, the simultaneous improvements of Li⁺ transport kinetics and physicochemical integrity of Si/Gr-ISAA were confirmed.^46,47 The chemical composition and spatial distribution of the SEI layer are fundamentally determined by the local ionic environment at the interface. In general, the SEI consists of organic and inorganic species that facilitate Li⁺ transport while electronically passivating the electrode surface. However, the significant volume changes of silicon during cycling impose uniform Li⁺ flux and the formation of a vulnerable, organic-rich SEI. During cycling, continuous SEI rupture and reformation accelerate electrolyte decomposition and increase interfacial resistance, ultimately impairing Li⁺ transport kinetics and stability.^37,48 Therefore, establishing a chemically stable and mechanically robust SEI is essential for ensuring the long-term performance of Si/Gr composite anodes. Mechanical stress on the SEI leads to rupture and exposure of fresh Si surfaces.^49,50 These conditions induce non-uniform Li⁺ flux and the formation of a vulnerable, organic-rich SEI. During cycling, continuous SEI rupture and reformation accelerate electrolyte decomposition and increase interfacial resistance, ultimately impairing Li⁺ transport kinetics and stability.^37,48 Therefore, establishing a chemically stable and mechanically robust SEI is essential for ensuring the long-term performance of Si/Gr composite anodes. To investigate the SEI layer on the pristine Si/Gr and Si/Gr-ISAA electrodes, XPS analysis was conducted after the initial half-cell cycling. Fig. 3a and d show the C 1s XPS spectra of the pristine Si/Gr and Si/Gr-ISAA electrodes, respectively. The C 1s spectra were deconvoluted into four characteristic components corresponding to C–C (284.6 eV), C–O (286.4 eV), C=O (288.6 eV), and CO₃²⁻ species (289.9 eV).^51,52 The C–C component mainly originated from graphitic carbon in the active material and conductive carbon additives. For the pristine Si/Gr electrode, the C 1s spectrum showed a relatively high contribution from the C–C component, whereas the Si/Gr-ISAA electrode exhibited a reduced C–C intensity but increased contributions from oxygen-containing carbon species, including C–O and C=O components, as revealed by the relative peak area analysis (Fig. 3g). Meanwhile, the carbonate-related CO₃²⁻ component was slightly more pronounced for the pristine Si/Gr electrode, indicating a greater accumulation of carbonate-type electrolyte decomposition products.³⁰ These results suggested that the increased C–O and C=O contributions observed for the Si/Gr-ISAA electrode did not stem from uncontrolled carbonate-rich SEI formation, but rather from organic species coexisting within a LiF-rich inorganic SEI framework. This interpretation was further supported by the Li 1s and F 1s spectra. Compared with the pristine Si/Gr electrode (Fig. 3b and c), the Si/Gr-ISAA electrode (Fig. 3e and f) exhibited a markedly enhanced LiF-related signal, indicating the preferential formation of a LiF-rich SEI. Moreover, quantitative analysis of the Li 1s and F 1s peak areas (Fig. 3h and i) confirmed that LiF constitutes the dominant SEI component in the Si/Gr-ISAA electrode, whereas carbonate- and phosphate-derived species are relatively more pronounced in the pristine Si/Gr electrode.^29,53 The lower SEI resistance after cycling observed for the Si/Gr-ISAA electrode suggests the formation of a thinner and less resistive SEI layer, which facilitates efficient Li⁺ transport across the interface (Table 1). Furthermore, this reduced resistance implies that the SEI layer remains mechanically and chemically stable, effectively suppressing continuous SEI growth and electrolyte decomposition.

Fig. 2 (a) Viscosity as a function of shear rate for electrode slurries with and without ISAA. (b) Cohesion strengths from SAICAS and (c) electrode adhesion strengths from UTM. (d and e) Static contact angle images of pristine and Si/Gr-ISAA electrodes. (f) EIS spectra of electrode symmetric cells for R_ion. EIS spectra of Li symmetric cells with (g) pristine and (h) ISAA-coated separators before and after the polarization test and corresponding (i) t_Li⁺.

Fig. 3 (a–c) C 1s, Li 1s, and F 1s XPS spectra of the pristine Si/Gr electrode. (d–f) C 1s, Li 1s, and F 1s XPS spectra of the Si/Gr-ISAA electrode. (g–i) Quantitative comparison of SEI components derived from XPS peak fitting. (j and k) Atomic concentrations of Li, F, C, O and P for pristine Si/Gr and Si/Gr-ISAA electrodes, and (l) F/C atomic ratio as a function of sputtering time.

Table 1 Resistance parameters of pristine Si/Gr and Si/Gr-ISAA electrodes after 30 cycles

	R s (Ω)	R SEI (Ω)	R ct (Ω)
Pristine Si/Gr	1.8	4.6	10.6
Si/Gr-ISAA	1.7	3.9	8.3

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Under high-density and high-loading conditions typical of high-energy-density electrodes, reaction inhomogeneity and resistance gradients can develop along the electrode thickness, leading to a non-uniform distribution of stable SEI components. To analyze the depth-dependent chemical distribution of the SEI, XPS depth profiling was performed via Ar⁺ sputtering on the pristine Si/Gr and Si/Gr-ISAA electrodes (Fig. 3j and k). For both electrodes, the relative contents of Li and F gradually decreased during the transition from an inorganic-rich outer SEI layer to an organic-dominated inner SEI layer. Notably, the Si/Gr-ISAA electrode maintained higher Li and F contents with increasing sputtering time, while simultaneously exhibiting lower carbon, oxygen, and phosphorus contents across the entire sputtering range compared to the pristine Si/Gr electrode. This compositional trend provided strong evidence for the formation of a LiF-rich, inorganic-dominated SEI with suppressed organic species.⁵⁴ Furthermore, the relatively consistent LiF-rich composition throughout the sputtering depth indicates the formation of a more uniform SEI layer in the Si/Gr-ISAA electrode. Consistently, the evolution of the F/C atomic ratio as a function of sputtering time (Fig. 3l) confirmed the relative ratio between inorganic and organic species in SEI layers. The Si/Gr-ISAA electrode exhibited significantly higher F/C ratios with inorganic-rich SEI components, whereas the pristine Si/Gr electrode showed relatively lower F/C ratios due to the organic-rich SEI components. These results substantiate that the mechanochemically robust SEI layer for the Si/Gr-ISAA anodes could facilitate rapid Li⁺ transport kinetics with structural integrity at the interface, preventing continuous electrolyte decomposition.

To evaluate the electrochemical properties of the pristine Si/Gr and Si/Gr-ISAA anodes, galvanostatic charge/discharge measurements were performed using half-cells. The optimized ISAA content was identified as 0.5 wt% from cycling performance (Fig. S3). The Si/Gr-ISAA anode exhibited a slightly higher initial charge (delithiation) capacity of 541.0 mAh g⁻¹ with an initial coulombic efficiency of 90.7%, compared with the pristine Si/Gr anode (531.4 mAh g⁻¹, 90.4%), as shown in Fig. 4a. This result demonstrates that ISAA does not trigger additional parasitic reactions, while improving the initial coulombic efficiency and reversible capacity.⁵⁵ To evaluate the rate capability of the Si/Gr-ISAA electrode, the charge rate capability test was conducted up to 2.0C holding the discharge rate at 0.2C (Fig. 4b and c). The Si/Gr-ISAA anode exhibited higher specific capacities than the pristine Si/Gr anode up to 2.0C. The Si/Gr-ISAA anode exhibited a higher specific capacity of 430.7 mAh g⁻¹ at 2.0C compared to that of the pristine Si/Gr anode (408.4 mAh g⁻¹). The Si/Gr-ISAA anode showed reduced composite volume resistivity and lower interfacial resistance between the electrode and current collector (Fig. 4d and e). These improvements are primarily attributed to the more homogeneous electrode microstructure enabled by ISAA, which promotes uniform dispersion of conductive additives and more continuous percolation pathways throughout the composite. As a result, the electronically conductive network is better preserved during repeated lithiation/delithiation, even in the presence of substantial silicon volume changes, thereby contributing to enhanced long-term cycling stability. Overall, these results indicate improved electronic connectivity and a mechanically more coherent electrode architecture during prolonged cycling. EIS provides quantitative evidence for the stabilizing effect of ISAA on interfacial processes during cycling (Fig. 4f). Prior to cycling, the Si/Gr-ISAA electrode exhibited a significantly lower charge transfer resistance (R_ct) of 80.0 Ω compared to that of the pristine electrode (181.5 Ω). More importantly, the evolution of interfacial resistance during cycling differs markedly between the two electrodes. After repeated cycling, the growth of both the SEI resistance (R_SEI) and R_ct is effectively suppressed in the Si/Gr-ISAA anode compared to the pristine counterpart (Table 1). The impedance parameters were extracted by fitting the Nyquist plots with the corresponding equivalent circuits (Fig. S4), where a dual-interface model was employed to characterize the SEI layer and charge transfer resistance after cycling. Notably, the Si/Gr-ISAA anode retained a low R_ct of 8.3 Ω after cycling, whereas the pristine electrode exhibited a higher value of 10.6 Ω, indicative of more severe interfacial degradation. Beyond impedance stabilization, the beneficial effect of ISAA extends to the structural and electronic integrity of the electrode. These interfacial and structural stabilizations are directly reflected in the cycling performance. The Si/Gr anodes were evaluated at 0.5C (1C = 3.5 mAh cm⁻²) for 100 cycles (Fig. 4g). The Si/Gr-ISAA anode exhibited outstanding cycling performance with a capacity retention of 77.7% after 100 cycles. However, the pristine Si/Gr anode showed gradual capacity fading with a capacity retention of 68.1% after 100 cycles. Collectively, these results demonstrate that ISAA can facilitate Li⁺ transport and suppress interfacial degradation with enhanced physicochemical integrity, which is beneficial for the rate capability and cycling performance of high energy density Si/Gr composite anodes. Cation–organic compounds present on the electrode coating can contribute to ionic conductivity when the original cations are exchanged with Li⁺ ions. To elucidate whether the Na⁺ ions in ISAA can be replaced by Li⁺ and thereby enhance ionic conductivity at the electrode surface, DFT calculations were performed to investigate the role of ISAA at the atomic scale. Fig. 5a shows pristine SVBS without sodium. To determine the preferred positions of Na and Li, a single cation was initially placed adjacent to each of the three inequivalent oxygen sites, and their energies were compared. Fig. 5b and c show the relaxed structures with adsorbed Na and Li, respectively. For all Na and Li models, the cations preferentially form bonds with two oxygen atoms rather than coordinating with a single oxygen atom. Fig. 5d presents the cation adsorption energies of the models depending on the initially adjacent oxygen site. Interestingly, each cation exhibits similar adsorption energies regardless of the site, indicating site-independent behavior. Importantly, Li adsorption is more stable than Na adsorption in all cases. Therefore, ion exchange of Na with Li is predicted to be favorable, which is consistent with the XPS results (Fig. S5 and S6). Fig. 5e shows the integrated crystal orbital Hamiltonian populations (ICOHP) calculated to analyze the bonding strength between the cations and oxygen atoms. Fig. 5f and g display the COHP spectra for Na and Li, respectively. The more negative ICOHP values for Li indicate stronger Li–O interactions than Na–O interactions, supporting the preferential stabilization of Li at the sulfonate site. Collectively, these results suggest that ISAA can be converted in situ to Li-coordinated sulfonate motifs that act as immobilized anionic sites. Such anion anchoring can enrich Li⁺ near the interface while partially suppressing anion mobility, thereby increasing the effective Li⁺ transference and promoting more homogeneous Li⁺ flux during cycling. To elucidate the swelling behavior of Si/Gr composite electrodes, morphological analyses were conducted using the SEM backscattered electron (BSE) mode, comparing electrode microstructures before and after cycling. In the SEM images, the brighter regions correspond to Si active material, while the darker regions indicate graphite. The SEM images of the pristine Si/Gr electrode exhibited non-uniform particle distributions with agglomerated Si active materials (Fig. 6a). However, the Si/Gr-ISAA electrode displayed homogeneous and well-dispersed particle distributions (Fig. 6b). This initial morphological difference plays a decisive role in the electrode swelling behavior during electrochemical cycling. After 30 cycles, this initial heterogeneity in the pristine Si/Gr electrode exacerbated localized stress, leading to pronounced surface degradation characterized by severe microcrack formation and ruptured particle connectivity (Fig. 6c and S7). These mechanical failures are indicative of the localized current and non-uniform lithiation, triggered by the initially poor dispersion and unmodulated Li⁺ flux. In contrast, the Si/Gr-ISAA electrode retains a compact and integrated surface morphology with minimal crack initiation after cycling (Fig. 6d and S8). This demonstrates that the synergistic effect of improved initial dispersion and regulated interfacial Li⁺ transport effectively mitigates the accumulation of mechanical stress at both the particle and electrode levels. Cross-sectional SEM images further clarify the mitigated electrode swelling behavior of the Si/Gr-ISAA electrode. After 30 cycles, the pristine Si/Gr electrode showed non-uniform swelling throughout the entire electrode, attributed to localized stress accumulation caused by the inhomogeneous electrode microstructure. The swelling ratio of the pristine Si/Gr electrode was 62.8% (76.5 µm) compared to the initial electrode thickness (47 µm), as shown in Fig. 6e. In contrast, the Si/Gr-ISAA electrode exhibited alleviated electrode swelling behavior with a homogeneous electrode microstructure (Fig. 6d and f). The swelling ratio of the Si/Gr-ISAA electrode was 31.1% after 30 cycles. The suppression of electrode swelling originates from interfacial stabilization, homogenized Li⁺ flux, and uniform distribution of electrode components induced by ISAA. These effects promote efficient stress relaxation and enhance the mechanical integrity of the electrode, effectively mitigating structural rupture within the electrode. To substantiate the practical applicability of the Si/Gr-ISAA anodes, full cells were evaluated using NCM811 cathodes with an n/p ratio of 1.1. The full-cells assembled with pristine Si/Gr and Si/Gr-ISAA anodes exhibited similar initial voltage profiles (Fig. 7a). The initial discharge capacity of the full-cell with the Si/Gr-ISAA anode reaches 190.6 mAh g⁻¹ with an initial coulombic efficiency of 83.7%, comparable to the pristine counterpart (189.0 mAh g⁻¹ and 83.6%). Fig. 7b and c compare the charge and discharge rate capabilities of full-cells prepared with pristine Si/Gr and Si/Gr-ISAA anodes. The full-cells prepared with Si/Gr-ISAA anodes consistently delivered higher specific capacities and exhibited more stable voltage profiles during both charge and discharge than the full cells prepared with the pristine Si/Gr anodes, with the performance advantage becoming increasingly pronounced at elevated rates (Fig. S9 and S10). This trend indicates that, as full cell performance becomes increasingly limited by transport and interfacial polarization at high rates, ISAA-driven interfacial stabilization reduces kinetic losses by enabling more uniform Li⁺ flux and improved interfacial charge transfer kinetics. EIS analysis further substantiates the stabilizing role of SVBS in full-cell operation. At the initial state, the impedance characteristics of the full-cell prepared with the Si/Gr-ISAA anode are comparable to those of the pristine counterpart, indicating similar interfacial resistance components prior to cycling (Fig. 7d). However, after prolonged cycling, the growth of the Nyquist semicircle is markedly suppressed for the full cell with the Si/Gr-ISAA anode. Quantitative fitting reveals that the R_ct of the full cell with the Si/Gr-ISAA anode remains nearly unchanged, decreasing slightly from 9.6 Ω before cycling to 9.2 Ω after cycling, whereas the full-cell with the pristine Si/Gr composite anode exhibits a higher initial R_ct of 12.5 Ω that increases to 12.7 Ω after cycling, reflecting more pronounced interfacial degradation (Fig. 7e and f).

Fig. 4 (a) Initial charge–discharge voltage profiles in half-cells. Charge rate capability of (b) pristine Si/Gr and (c) Si/Gr-ISAA electrodes. (d and e) Electrode resistivity of pristine Si/Gr and Si/Gr-ISAA electrodes. (f) EIS spectra before and after cycling. (g) Cycling performance of pristine Si/Gr and Si/Gr-ISAA electrodes at 0.5C in a half-cell.

Fig. 5 DFT calculation results of SVBS with Na and Li cations. (a) Molecular structure of pristine SVBS without Na. Relaxed structures of (b) Na-absorbed and (c) Li-adsorbed SVBS. (d) Adsorption energies of Na and Li at different oxygen sites. (e) Calculated ICOHP values for Na–O and Li–O bonds. COHP spectra of (f) Na and (g) Li, indicating the stronger binding affinity of Li.

Fig. 6 (a and b) Top-view SEM images of pristine Si/Gr and Si/Gr-ISAA electrodes before cycling at low and high magnifications. (c and d) Top-view SEM images of pristine Si/Gr and Si/Gr-ISAA electrodes after 30 cycles at low and high magnifications. (e and f) Cross-sectional SEM images of pristine Si/Gr and Si/Gr-ISAA electrodes after 30 cycles, showing electrode thickness.

Fig. 7 (a) Initial charge–discharge voltage profiles in full-cells (b) charge rate capability of pristine Si/Gr and Si/Gr-ISAA electrodes. (c) Discharge rate capability of pristine Si/Gr and Si/Gr-ISAA electrodes. (d) EIS spectra before and after cycling. (e and f) Resistance values extracted from EIS spectra (g) Cycling performance of pristine Si/Gr and Si/Gr-ISAA electrodes measured at 1.0C.

As a direct consequence of this stabilized interfacial behavior, the full cell with the Si/Gr-ISAA anode exhibits markedly enhanced long-term cycling stability. When cycled at 1C (1C = 3.18 mAh cm⁻²), the full-cell with the Si/Gr-ISAA anode exhibited stable cycling performance over 300 cycles compared to the pristine counterpart (Fig. 7g). The full cell prepared with the pristine Si/Gr composite anode exhibited rapid capacity fading with a capacity retention of 35.8% after 300 cycles. This degradation is driven not only by the mechanical volume expansion of Si but also by non-uniform Li⁺ flux, which triggers localized current concentration. This electrochemical heterogeneity exacerbates mechanical strain and accelerates the irreversible depletion of the active Li-inventory, leading to poor cycle life. In addition, the cell with the pristine Si/Gr composite anode showed an average coulombic efficiency of 99.4% during cycling. In contrast, the full cell prepared with the Si/Gr-ISAA anode showed stable cycle performance, indicating a capacity retention of 51.0% and an average coulombic efficiency of 99.8% over 300 cycles. The higher CE of the Si/Gr-ISAA anode stems from the anion anchoring capability of ISAA, which promotes a homogenized Li⁺ flux and effectively mitigates the degradation problems. These results demonstrate that interfacial reinforcement via ISAA is beneficial for enhancing Li⁺ transport and maintaining structural integrity for high energy density Si/Gr composite electrodes, resulting in improved rate capability and cycle performance in full cells.

To track the in situ evolution of interfacial resistance during cycling, DRT analysis was employed to complement the EIS results. DRT provides improved separation of overlapping electrochemical processes by deconvolving impedance contributions according to their characteristic time constants, which is often difficult to resolve from conventional Nyquist plots alone (Fig. S11 and S12). Specifically, in situ DRT measurements were performed during the initial cycle (2.5–4.2 V) to monitor the dynamic evolution of resistance in pristine Si/Gr and Si/Gr-ISAA electrodes (Fig. 8). The resulting spectra were categorized into three distinct kinetic contributions, corresponding to contact resistance (P1), SEI-related transport resistance (P2), and charge transfer resistance (P3). This assignment is supported by the distinct time constant distribution, where P1, P2, and P3 appear in the high-, intermediate-, and low-frequency regions, corresponding to interparticle contact, SEI transport, and charge transfer processes, respectively. Moreover, their voltage-dependent evolution provides additional support: P2 varies with SEI modulation during lithiation, while P3 correlates with electrochemical reaction activity. This behavior is consistent with the proposed peak assignment.⁵⁶

Fig. 8 In situ DRT analysis of pristine Si/Gr and Si/Gr-ISAA electrodes in full-cells during charge and discharge. (a) In situ internal resistance of the pristine Si/Gr electrode. (b and c) DRT contour maps of the pristine Si/Gr full-cell during (b) charge and (c) discharge. (d) In situ internal resistance of the Si/Gr-ISAA electrode. (e and f) DRT contour maps of the Si/Gr-ISAA full-cell during (e) charge and (f) discharge. Contour maps of electrolyte Li⁺ concentration (C_e) as a function of distance from the current collector (CC) and C-rate, obtained from the P2D Newman model. (g) Pristine Si/Gr electrode and (h) Si/Gr-ISAA electrode. Dashed white lines denote the electrode–separator boundary (L_neg = 47 µm). The color scale represents C_e in mol m⁻³. The Si/Gr-ISAA electrode maintains a significantly more uniform C_e distribution across all C-rates, consistent with the reduced (1 – t_Li⁺) prefactor in the electrolyte mass conservation equation.

Fig. 8a and d present the in situ evolution of internal resistance for the pristine Si/Gr and Si/Gr-ISAA electrodes, respectively. During the initial charge (Fig. 8b and e), both electrodes show a decrease in net resistance, reflecting interfacial activation and improved charge-transfer/SEI transport. However, the pristine Si/Gr electrode maintains a substantially higher resistance throughout charge and exhibits limited relaxation, whereas the Si/Gr-ISAA electrode shows a more pronounced reduction, particularly in the interfacial (P2/P3) contributions. During discharge (Fig. 8c and f), the pristine electrode undergoes a stronger resistance rebound, suggesting accelerated interfacial polarization and destabilization of the nascent SEI, while the ISAA electrode exhibits a markedly attenuated rebound, implying improved reversibility of interfacial processes. The kinetic advantages of the Si/Gr-ISAA electrode were further corroborated by GITT analysis. As shown in Fig. S13, the Si/Gr-ISAA electrode consistently exhibited a higher Li⁺ diffusion coefficient across the examined state of charge range compared to the pristine electrode, which was well aligned with the reduced R_ct and stabilized R_SEI revealed by the in situ DRT analysis.

Fig. 8g and h present the contour maps of electrolyte Li⁺ concentration (C_e) as a function of distance from the current collector (CC) and C-rate for the pristine and ISAA electrode configurations. The dashed white lines indicate the electrode–separator boundary (at x = 47 µm). For the pristine electrode, a pronounced concentration gradient develops with increasing C-rate. The electrolyte near the CC becomes progressively depleted due to Li⁺ consumption at the electrode–electrolyte interface, while the high anion fraction (1 − t⁺_Li = 0.80) drives substantial anion accumulation near the CC, creating a self-reinforcing concentration gradient. At 2.0C, the minimum C_e drops to 704 mol m⁻³ (a 38.8% depletion from the initial concentration of 1150 mol m⁻³), yielding a concentration uniformity ratio C_e,min/C_e,max of 0.614. In marked contrast, the ISAA-modified electrode (t⁺_Li = 0.59) maintains a significantly more uniform C_e distribution across all C-rates. The elevated t⁺_Li reduces the anion transport fraction to (1 − t⁺_Li = 0.59) = 0.41, diminishing the source term in eqn (S1) by 49% relative to the pristine case. At 2.0C, the minimum C_e is 919 mol m⁻³ (20.1% depletion), with C_e,min/C_e,max = 0.800, representing a 30% relative improvement in concentration uniformity compared to the pristine electrode. The ISAA promotes the formation of a LiF-rich inorganic SEI layer with outstanding physicochemical stability. In addition, the anchored anions could boost Li⁺ transport kinetics, which regulates homogeneous electrochemical reactions across the electrode. In these regards, outstanding electrochemical performance can be achieved for the Si/Gr-ISAA anode in high-energy density LIBs.

4 Conclusions

In this study, we demonstrated a facile and effective electrode engineering strategy to stabilize Si/Gr composite anodes by introducing SVBS as an ISAA. The core of this strategy lies in anion anchoring, which fundamentally reconfigures the interfacial ionic environment. By increasing the Li⁺ transference number and promoting uniform Li⁺ flux across the electrode, the ISAA reduces localized polarization and helps equalize the reaction kinetics during cycling. Concurrently, this regulated ionic environment redirects the SEI evolution toward an inorganic-dominated interphase, yielding a thin, LiF-rich SEI with enhanced mechanochemical robustness. This interphase design strengthens interfacial passivation while maintaining efficient Li⁺ transport, which collectively mitigates continuous electrolyte decomposition and slows impedance growth. As a result, the Si/Gr-ISAA electrodes exhibited improved electrochemical stability with suppressed irreversible swelling and more stable interfacial resistance during prolonged cycles. Full cell evaluations further support the practical relevance of this approach, showing that interfacial reinforcement via ISAA can be implemented without complex processing while delivering enhanced electrochemical performance. The ISAA-modified configuration effectively mitigates electrolyte depletion near the current collector and maintains a more uniform ionic concentration gradient even under high-rate operation. Overall, this work provides a scalable design principle for next-generation LIBs by regulating interfacial ion distribution and SEI chemistry via anion anchoring to address the coupled electrochemical and mechanical degradation pathways of Si/Gr composite anodes.

Author contributions

J. Kim: writing – original draft, investigation, data curation, and formal analysis. S. Yu: writing – original draft, investigation, data curation, and formal analysis. H. Park, J. Yoon, C. Lee, R. Kang, and K. Park: investigation. S. Moon: science advice and data interpretation. P. Kim: science advice, data interpretation, and computational modelling. S. Sun: science advice and data interpretation. K. Roh: science advice and data interpretation. I. Jeong: conceptualization, supervision, project administration, funding acquisition, and data interpretation. D. Lee: conceptualization, supervision, project administration, and funding acquisition. J. Choi: conceptualization, supervision, project administration, and funding acquisition. J. Kim and S. Yu contributed equally to this work. All authors reviewed and approved the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ta01386d.

Acknowledgements

This work was supported by the Technology Innovation Program (RS-2024-00432015 and RS-2024-00446888) through the Korea Planning & Evaluation Institute of Industrial Technology(KEIT) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and the Gachon University Research Fund (202500870001).

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Footnote

† These authors contributed equally to this work.

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