Revealing anisotropic lithiation control in silicon nanowires via a novel in situ TEM-based cross-sectional characterization method

Abstract

Silicon nanowires (Si NWs) hold great promise as high-capacity anode materials for next-generation batteries. However, their application is severely hindered by anisotropic lithiation, which leads to structural failure and rapid capacity fading. Here, we introduce a novel in situ transmission electron microscopy (TEM) cross-sectional analysis technique that enables real-time visualization and quantitative analysis of the radial structural evolution of one-dimensional (1D) nanomaterials under external stimuli. Applying this method to Si NWs, we uncover a two-tiered mechanism for regulating anisotropic lithiation in Si NWs. First, selecting axial orientations with high in-plane crystallographic symmetry can effectively facilitate uniform lithium (Li) diffusion and suppress directional expansion. Second, rational cross-sectional design, such as faceted-engineered geometries, further suppresses anisotropy by constraining the effective interfacial area and diffusion path length in fast-lithiation directions. These findings provide new insights into the control of anisotropic lithiation and offer a geometry-guided strategy for enhancing the structural stability and performance of Si-based anodes. Moreover, the methodology and anisotropy regulation principles established here are broadly applicable to other 1D nanomaterials.

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New concepts

This work introduces a novel in situ transmission electron microscopy (TEM)-based cross-sectional characterization technique for real-time, high-resolution observation of radial structural evolution in one-dimensional (1D) nanomaterials. It addresses the limitations of conventional characterization methods by enabling precise cross-sectional sample preparation and direct, real-time radial TEM imaging, offering previously inaccessible insights into the internal structural dynamics of 1D nanomaterials. Applied to silicon nanowires, this approach reveals how crystallographic orientations and cross-sectional geometry synergistically modulate their anisotropic lithiation behaviours. Specifically, axial orientations with higher in-plane crystallographic symmetry enable more uniform lithiation, while faceted cross-sectional geometries mitigate anisotropic lithiation by constraining the reaction interface area and diffusion path length along fast-lithiation directions. These findings uncover the previously unexplored yet critical role of cross-sectional geometry in governing anisotropic lithiation, offering new design strategies for high-performance anode materials. The developed technique provides a generalizable platform for real-time cross-sectional characterization, addressing key limitations of current methodologies and promoting the development of advanced 1D nanomaterials.

1. Introduction

Anisotropy in physical and chemical responses is a defining feature of many advanced materials, which critically impacts their structural stability and functional performance.^1–5 Silicon nanowires (Si NWs), as a highly promising anode material for lithium (Li) ion batteries due to their high theoretical capacity and natural abundance,^6–8 are a representative case where anisotropic behaviour significantly impairs functionality. Upon lithiation, Si NWs experience substantial anisotropic volumetric expansion, leading to fracture and even pulverization, ultimately reducing the overall performance and lifespan.^7,9,10 This remains a critical obstacle for their practical application as anode materials.

Studies on the anisotropic lithiation of Si NWs indicate that this anisotropy arises primarily from differences in interfacial mobility and reaction kinetics across various crystallographic planes. For example, lithiation kinetics and expansion rates are much faster along the 〈110〉 direction than the 〈111〉 and 〈100〉 directions.^9,11,12 This disparity arises from the lower energy barriers for Li-ion diffusion through the 〈110〉 plane and the relatively lower compressive stresses that develop at the phase boundary.^12,13 The directional lithiation results in significant anisotropic expansion, leading to non-uniform stress distributions and ultimately fracture. These findings highlight the critical role of interfacial evolution during lithiation in governing the anisotropic behaviour of Si NWs. Despite growing insights into the anisotropic lithiation of Si NWS, systematic investigations into their regulation or suppression through geometry design have been limited. This is mainly due to the lack of effective cross-sectional characterization techniques that enable direct observation of internal structural and interfacial evolution during lithiation. This technical gap not only constrains progress in Si NW research but also hinders the broader exploration of other one-dimensional (1D) nanomaterials, which often feature complex internal structures that evolve under external stimuli.^14–18

The current method for preparing cross-sectional samples involves embedding them in epoxy and subsequently sectioning them with diamond knives, microtome,¹⁹ or focused ion beam (FIB).²⁰ However, these methods fall short in implementing fixed-point analysis. Additionally, due to sample thickness, subsequent characterization predominantly depends on scanning electron microscopy (SEM),^9,11,21 which cannot provide high-resolution and accurate characterization of the cross-section. Transmission electron microscopy (TEM) serves as a vital tool for capturing structural evolution at the atomic scale. However, most in situ TEM studies on 1D nanomaterials adopt a side-view configuration that masks radial features and fails to resolve dynamic processes occurring at the reaction interfaces.

To overcome these limitations, we present a novel in situ TEM-based cross-sectional characterization technique that enables precise sectioning and real-time observation of the internal structural and interfacial evolution of 1D nanomaterials. In the present study, we apply this technique to study the anisotropic lithiation behaviour of Si NWs and how factors such as axial crystallographic orientation and cross-sectional geometry influence anisotropic lithiation-induced volumetric expansion and fracture. It is discovered that the axial crystallographic orientation determines the in-plane symmetry of Li diffusion pathways, with higher symmetry promoting more uniform lithiation across all directions and suppressing directional swelling. Meanwhile, the cross-sectional geometry further regulates anisotropic lithiation by modulating the effective lithiation interface area and diffusion path length along different crystallographic directions. Previous studies on Si NWs have primarily focused on their intrinsic properties, such as crystallography-dependent lithiation kinetics, with limited exploration of methods to regulate anisotropic lithiation. Our work highlights the synergistic effect of axial orientations and cross-sectional geometry in modulating anisotropic lithiation and offers new insights into the geometry design of anisotropic materials. While the present study focuses on Si NWs, the proposed cross-sectional characterization methodology and the insights gained here can be readily extended to a broad range of 1D nanomaterials.

2. Experimental

2.1. Cross-sectional TEM sample preparation

The method for preparing cross-sectional samples of the Si NWs for TEM characterization and in situ lithiation experiments is illustrated in Fig. 1. The Si NWs were epitaxially grown on Si wafers by chemical vapour deposition (CVD) via the vapour–liquid–solid (VLS) growth method. A small segment of the Si wafer bearing Si NWs was cleaved from the bulk Si wafer substrate, attached to an aluminium rod using conductive epoxy to ensure reliable electrical contact, and mounted on one side of the TEM–scanning tunnelling microscope (STM) holder. A tungsten (W) probe, which was prepared by electrochemical etching in NaOH under constant-voltage conditions, was fixed on the opposite, movable side. Using the piezo-manipulator of the STM holder, the position of the movable side is adjusted to bring the W probe into contact with the Si NW and eventually establish contact. Carbon deposition is then performed to attach the NW to the W probe, enabling it to pick up a short piece of the Si NW through lateral movement. To further shorten these small pieces of Si NWs and create thinner slices, both ends are further broken off under ultrahigh vacuum conditions. Subsequently, the W needle is tilted until the plane of the attached Si NW slice is perpendicular to the viewing direction, allowing for cross-sectional TEM analysis. This method enables precise acquisition of cross-sectional samples at any specific point along the 1D nanomaterials, providing detailed insights into their internal structures, growth mechanisms, and response to external stimuli. These insights are highly valuable for guiding the rational design, synthesis, and application of such materials. This technique is particularly suitable for the cross-sectional characterization of brittle functional 1D nanomaterials. When the extracted sample is sufficiently thin, atomic-resolution characterization of the cross-section can be achieved. An example of atomic-scale cross-sectional characterization performed on another 1D material, TiO₂ nanowires, is presented in Fig. S1.

Fig. 1 Schematic illustration and corresponding TEM images showing the preparation process of a cross-sectional Si NW sample for in situ TEM lithiation experiments. (a) Schematic illustration of the setup inside the TEM holder. (b) The W probe is positioned to contact a selected Si NW, followed by carbon deposition at the contact point to attach the W probe to the NW (scale bar: 200 nm). (c) A segment of the Si NW is picked up by manipulating the W probe (scale bar: 200 nm). (d) The other side of the segment is cut to shorten its length (scale bar: 200 nm). (e) The W probe is rotated by 90 degrees to align the TEM sample for cross-sectional observation. (f) The setup for the in situ TEM lithiation experiment on the cross-sectional sample. The right inset shows a TEM image of the prepared cross-sectional sample of a Si NW prior to lithiation (scale bar: 10 nm).

2.2. In situ TEM lithiation experiments

The in situ lithiation experiment was conducted using a Nanofactory TEM–STM holder inside a spherical aberration-corrected FEI TEM. All experiments were performed at room temperature. A piece of the Si NW sample was prepared as described in the Methods section and mounted on one side of the holder. A W probe was cut to expose a clean cross-section, which was then used to scratch the Li metal surface and collect fresh Li. The W probe with Li at the tip was mounted on the opposite side of the holder. Using the piezo-manipulator of the STM holder, the W probe was positioned to bring the Li metal into contact with the sample. Once the contact was established, a bias of 2 V was applied to drive electrochemical lithiation. Lithiation was observed using a bright-field TEM. During lithiation, the electron beam was blanked except for imaging and video recording. The schematic of the in situ TEM lithiation experiments is illustrated in Fig. 1f.

2.3. Modeling

The atomic model of the Si–Li system was constructed as follows. First, a unit cell of the Si crystal was generated with a space group of Fd3̄m, lattice constants a = b = c = 5.4307 Å, and angles α = β = γ = 90°. The generated Si crystal was then reoriented along the crystallographic directions x-[1̄1̄1], y-[1̄10], and z-[112] and subsequently replicated to dimensions of approximately 40 × 40 × 20 Å. Simultaneously, a Li structure with dimensions of approximately 40 × 40 × 20 Å was prepared by melting a body-centered cubic (BCC) Li crystal, followed by equilibration and quenching to obtain a stable amorphous configuration. Finally, the Si–Li structure was constructed by placing the amorphous Li structure atop the Si crystal along the z-direction, followed by structural optimization.

3. Results and discussion

3.1. Axial examination of the Si NWs

The TEM-based cross-sectional characterization technique developed in this study enables a precise, site-specific analysis of the Si NWs. To demonstrate the precision of this method in acquiring cross-sectional samples and to select appropriate positions for subsequent in situ lithiation studies, Si NWs were first produced via the chemical vapor deposition method, and cross-sections were obtained at targeted axial locations of the NWs for preliminary structural assessment. As shown in Fig. 2a, the Si NWs exhibit remarkable uniformity in both size and morphology. By extracting cross-sectional samples from three distinct regions along the NWs, it was found that the cross-sectional shape of the Si NWs varies along their growth direction.

Fig. 2 Axial cross-sectional shape evolution of a 〈111〉-oriented Si NW. (a) TEM image showing the side view of a 〈111〉-oriented Si NW (scale bar: 1 μm). (b) TEM image showing the three positions selected for cross-sectional analysis (scale bar: 1 μm). (c)–(e) Cross-sectional shapes of the Si NW in (b) at its rear end (c), middle (d), and front end (e), respectively (scale bar: 10 nm).

Fig. 2c–e illustrate the cross-sectional shapes of a Si NW with a 〈111〉 growth direction, captured at three representative locations along its axis (Fig. 2b). At the rear end (Fig. 2c), the NW exhibits a well-defined hexagonal cross-section, characterized by straight alternating short and long edges and sharply defined vertices. The corresponding diffraction pattern in the inset image reveals that the Si NW is encircled by six faceted {112} side facets. In the middle part of the NW (Fig. 2d), the hexagonal shape is preserved, while the vertices become more rounded. Near the front end (Fig. 2e), the cross-sectional shape changes from a hexagon encircled by six {112} planes to a polygon consisting of alternating {112} and {110} planes. Different regions of the NW correspond to various stages of its growth. The cross-sectional shape evolution over time illustrates the dynamic process of achieving structural stability and energy minimization during growth.^22–24 The front end of the NW represents its initial growth phase. During this early stage, the NW's cross-sectional shape is a polygon consisting of {110} and {112} planes, which is a configuration with a relatively high energy state. As growth progresses, the cross-section of the NW gradually transforms into a hexagonal shape enclosed by {112} planes, which is a more stable configuration. This change in the cross-sectional shape can be attributed to surface energy minimization.²⁵ Furthermore, when viewed from the 〈111〉 direction, there are six equivalent 〈112〉 directions, corresponding to six symmetrical {112} planes. Therefore, the hexagonal cross-section results not only from energy minimization but also from the inherent crystallographic symmetry of Si. The results indicate that the cross-section of the NW varies continuously along the axial direction. The middle and rear end sections correspond to the stable growth phase of NWs, where the structural and morphological characteristics reach equilibrium. Therefore, data from the middle section and the rear end should be prioritized for characterization, as they reflect the equilibrium state, ensuring a reliable evaluation of their properties.

3.2. Influence of NW axial orientations on anisotropic lithiation behavior

Beyond its pivotal role in characterizing the cross-sections of NWs at any location, this method also serves as a powerful tool for exploring the behaviors of the NWs under external fields. In this study, we conducted lithiation experiments on Si NWs to analyze their anisotropic lithiation behaviors and to investigate how these behaviors are influenced by growth directions and cross-sectional shapes. Si NWs with distinct axial orientations were subjected to varying degrees of lithiation and subsequently examined by their cross-sections. Fig. 3 compares the cross-sectional evolution of 〈112〉-oriented and 〈111〉-oriented Si NWs after similar lithiation durations. In both cases, the expansion of the Si NWs is dominated by a phase transformation process, in which the crystalline Si core is consumed to form the outer shell of the lithiated amorphous Li_xSi. However, the degree of anisotropic lithiation differs significantly between these two orientations.

Fig. 3 Influence of NW axial orientations on anisotropic lithiation behavior. (a)–(c) Cross-sectional evolution of the Si core in a 〈112〉-oriented Si NW at three representative lithiation stages (scale bar: 5 nm). (d) The atomic arrangement of Si viewed along the 〈112〉 direction. (e)–(g) Cross-sectional evolution of the Si core in a 〈111〉-oriented Si NW at three representative lithiation stages (scale bar: 10 nm). (h) The atomic arrangement of Si viewed along the 〈111〉 direction.

The 〈112〉-oriented Si NW, as shown in Fig. 3a–c, exhibits a roughly circular cross-sectional shape initially. Upon lithiation, the Si core shrinks rapidly along one direction, transforming its cross-sectional shape from circular to oval. The diffraction pattern indicates that this shrinkage of the Si core occurs primarily along the 〈110〉 crystal direction, with the 〈111〉 direction perpendicular to it. This anisotropic behavior suggests a faster lithiation rate along the 〈110〉 direction compared to the 〈111〉 direction, which is consistent with previous experiments and simulations.^9,12,26,27Fig. 3d illustrates the diamond cubic lattice viewing along the 〈112〉 direction. As shown in Fig. 3d, the {112} plane features two perpendicular 〈110〉 and 〈111〉 directions. Due to the presence of only two primary lithiation directions on the plane, with the lithiation rate in the 〈110〉 direction significantly higher than in the 〈111〉 direction, the NW exhibits pronounced anisotropic lithiation.

In contrast, 〈111〉-oriented Si NWs, despite having six symmetrical 〈110〉 directions on their cross-sectional plane, exhibit no obvious anisotropic lithiation behaviors. Fig. 3e–g displays the cross-sections of 〈111〉-oriented Si NWs at three lithiation stages. Initially, the Si NW presents a hexagonal cross-sectional geometry faceted by six {112} planes, with the six vertices aligned with the 〈110〉 direction. As lithiation proceeds, these facets become slightly concave, and in highly lithiated NWs, the inward curvature becomes more pronounced. However, the overall cross-sectional shape remains roughly unchanged, in stark contrast to the significant anisotropic lithiation observed in 〈112〉-oriented NWs. Fig. 3h shows the diamond cubic lattice viewed along the 〈111〉 axis, where the six 〈110〉 directions are bisected by six 〈112〉 directions, evenly distributed within the {111} plane. The high degree of symmetry is believed to balance Li-ion diffusion pathways and suppress directional anisotropy. These results show that axial orientation governs the in-plane symmetry of Li diffusion pathways. Therefore, selecting an axial orientation with high crystallographic symmetry in the cross-sectional plane is crucial for achieving uniform lithiation in Si NWs. It is important to note that high in-plane symmetry is the foundation for uniform lithiation. Lithiation uniformity depends not only on the degree of crystallographic symmetry but also on which specific low-index directions act as the main lithiation pathways and on the relative lithiation-rate differences among them. Therefore, the in-plane lithiation direction distribution needs to be coupled with the differences in lithiation rates along these directions to control the anisotropic lithiation. This is further discussed in Fig. S2 in the SI.

In addition, our cross-sectional characterization technique revealed an unusual interfacial evolution trend in 〈111〉-oriented NWs. Specifically, as shown in Fig. 3g, the six {112} facets that define the outer surface of the NW gradually curve inward, indicating a faster lithiation rate along the 〈112〉 direction compared to the 〈110〉 direction. This observation contradicts conventional understanding, which typically believes that lithiation proceeds more rapidly along the 〈110〉 direction than along the 〈112〉 direction.^12,27,28 Given that the 〈111〉-oriented Si NWs exhibit a hexagonal cross-sectional geometry, it is hypothesized that cross-sectional shape also plays a significant role in modulating local lithiation kinetics. To validate this hypothesis and further elucidate the underlying mechanism, we conducted comparative in situ lithiation experiments on 〈111〉-oriented Si segments with circular and hexagonal cross-sectional shapes, as detailed in the following section.

3.3. Influence of NW axial orientation on anisotropic lithiation behavior

While previous quasi-in situ experiments have provided valuable insights into the anisotropic behaviors of NWs with different axial orientations, they fall short in capturing the real-time interfacial dynamics during lithiation. In this section, we present in situ lithiation experiments on cross-sectional samples of Si NWs to investigate how cross-sectional geometry affects lithiation anisotropy, and, more importantly, how such anisotropy can be mitigated through structural and geometrical design.

Fig. 4 shows the comparison of the lithiation process of 〈111〉-oriented Si with circular and hexagonal cross-sections, respectively. Fig. 4a–c present a series of bright-field TEM images captured from SI Movie S1 illustrating the lithiation process and interfacial evolution in 〈111〉-oriented Si with an initial circular cross-section. The inner crystalline Si core is outlined in yellow, while the outer amorphous Li_xSi shell is marked by the dashed blue line. The white arrows in Fig. 4b and f indicate the 〈110〉 direction. Si with an initial circular cross-section exhibits pronounced anisotropic lithiation, with the 〈110〉 crystallographic direction demonstrating a significantly higher expansion rate than the 〈112〉 direction, ultimately forming a “flower-like” morphology. This anisotropy is quantitatively confirmed by the expansion measurements in Fig. 4d, which show that the expansion along the 〈110〉 direction (red line) significantly exceeds that along the 〈112〉 direction (black line). The expansion is measured by calculating the direction-resolved radial strain, which is derived from the change in the NW radius along each crystallographic direction over time. The pronounced anisotropic expansion generates strong strain gradients across the cross-section, leading to localized stress accumulation and the initiation of cracking (Fig. 4b). It is noteworthy that although lithiation along the 〈110〉 direction proceeds faster than along the 〈112〉 direction during the initial stages, the expansion rate in this direction progressively declines over time, eventually converging with that of the 〈112〉 direction. This transition is attributed to the coupled effects of stress accumulation in regions experiencing rapid expansion. In addition, the evolving stress field provides dynamic feedback that can accelerate, stall, or redirect the lithiation front.^29–33

Fig. 4 In situ lithiation experiments on 〈111〉-oriented Si with circular and hexagonal cross-sections. (a)–(c) Time-sequenced TEM images showing the lithiation process of the 〈111〉-oriented Si with an initial circular cross-section (scale bar: 50 nm). (d) Time-dependent lithiation-induced expansion along the 〈110〉 (red line) and 〈112〉 (black line) crystallographic directions in circular cross-sectioned Si. (e)–(g) Time-sequenced TEM images showing the lithiation process of the 〈111〉-oriented Si with an initial hexagonal cross-section (scale bar: 20 nm). (h) Comparison of lithiation-induced expansion over time along the 〈110〉 (red line) and 〈112〉 (black line) directions in hexagonal cross-sectioned Si.

In stark contrast, Si with an initial hexagonal cross-section exhibits uniform lithiation behaviors as shown in Fig. 4e–g (SI Movie S2). Initially, the cross-section displays a hexagonal shape enclosed by {112} planes, with vertices aligned along the 〈110〉 crystallographic direction. Upon lithiation, an amorphous outer shell forms, characterized by a grey contrast. As lithiation proceeds, the outer amorphous shell expands at the expense of the Si core, leading to a noticeable increase in the overall diameter. Notably, despite the expansion of the outer shell, the Si core maintains its structural integrity throughout lithiation. During the lithiation process, the Si core shrinks uniformly in all directions, indicating relatively uniform lithiation. Quantitative analysis of the expansion along the 〈110〉 and 〈112〉 directions (Fig. 4h) further confirms this isotropic lithiation behavior, as both directions exhibit nearly identical lithiation-induced expansion, with the expansion along the 〈112〉 directions slightly higher. The uniform lithiation across all directions leads to a more homogeneous stress distribution, thereby delaying the onset of crack initiation.

The experimental comparison presented above clearly demonstrates that the cross-sectional geometry of the Si NWs plays a significant role in modulating anisotropic lithiation behavior. Fig. 5a and b compare the morphological changes of the Si/Li_xSi interface at the apexes and side facets of the 〈111〉 Si NW with a hexagonal cross-section after an identical lithiation duration. The top part of Fig. 5a shows the initial state of the Si NWs’ apex, which is sharp and well-defined. As lithiation progresses, as shown in the bottom part of the image, the apex of the Si core becomes more rounded and less distinct, with a reduction of approximately 2.5% in diameter along the 〈110〉 direction. Compared to the reduction at the apex, the Si-core reduction at the side surface is more significant, reaching up to 5.6%. This is consistent with the observations discussed in the previous section, where the lithiation rate along the 〈112〉 direction is comparable to, or even faster than, that along the 〈110〉 direction in Si NWs with hexagonal cross-sectional geometry.

Fig. 5 Structural origin of balanced lithiation kinetics along the 〈110〉 and 〈112〉 directions in Si NWs with hexagonal cross-sections. (a) and (b) Morphological changes at the vertex (a) and side facet (b) of a 〈111〉-oriented Si NW before and after lithiation (scale bar: 2 nm). (c) Schematic illustration of primary lithiation directions in the cross-sectional plane of a 〈111〉-oriented Si NW with a hexagonal cross-section. (d) The schematic side view of a 〈111〉-oriented Si NW with a hexagonal cross-section, which is enclosed by six well-defined {112} facets. (e) Atomic structure of the Si {112} surface corresponding to the facets in (d). (f) Side view illustrating Li-ion diffusion pathways on the Si {112} facet.

To uncover the regulation of directional lithiation behavior by interfacial geometry, we conducted additional analyses, focusing on the interplay between cross-sectional geometry, Li-ion diffusion pathways, and interfacial reactions. As shown in Fig. 5c, when the fast-lithiation direction (as in the 〈110〉 direction) aligns with the geometric vertices, the contact area between the Li ions and the material is largely reduced, slowing the lithiation rate. Additionally, the longer diffusion path along the vertices further restricts the speed of lithiation, resulting in a more uniform lithiation rate and more homogeneous expansion across all directions. From a theoretical perspective, the total number of Li ions inserted per unit time is governed by both the diffusion flux J and the interfacial contact area A. According to Fick's first law of diffusion, the flux of Li ions during insertion can be described as:

where J is the diffusion flux (mol m⁻² s⁻¹), D is the Li diffusion coefficient (m² s⁻¹), and dC/dx is the concentration gradient of Li ions across the interface. Importantly, while this equation describes the local ion flux, the total number of Li ions inserted per unit time is directly proportional to both the flux and the contact area A:

This relationship indicates that, under the same diffusion coefficient and concentration gradient, a larger effective contact area significantly enhances the overall Li insertion rate. It should be noted that the simple flux expression based on Fick's law was introduced only to highlight the relationship between the Li insertion rate and interfacial area. At the applied potential (≈2 V), the Li transport is influenced not only by concentration gradients but also by a migration component induced by the electrochemical field, as described by the Nernst–Planck equation. While the electric field accelerates the Li insertion rate, the relationship between the insertion rate and the contact area still holds. Therefore, the contribution of electromigration is not discussed here.

In the case of Si NWs with a hexagonal cross-section, the six {112} facets collectively offer a significantly enlarged effective contact area for Li insertion compared to the geometrically confined vertex regions aligned with the 〈110〉 direction. Consequently, although the diffusion coefficient D along the 〈110〉 direction may be intrinsically higher, the substantially larger interfacial contact area along the 〈112〉 facet amplifies the overall Li-ion insertion rate. The resulting product J·A becomes comparable to, or may even surpass, that of the 〈110〉 direction. This geometric amplification effect provides a quantitative explanation for the experimental observation that expansion along the 〈112〉 direction in hexagonally faceted Si NWs proceeds at a rate comparable to, or slightly exceeding, that along the 〈110〉 direction. In contrast, in NWs with a circular cross-section, the contact areas for different crystallographic directions are approximately equal. As a result, the intrinsic differences in Li diffusion coefficients along various directions, such as the typically higher diffusivity along the 〈110〉 direction compared to the 〈112〉 direction, are no longer mitigated by geometric factors. This leads to a pronounced anisotropic lithiation behavior, where expansion occurs preferentially along the 〈110〉 direction, resulting in non-uniform volumetric changes, localized stress, and an increased likelihood of crack initiation.

In addition, it is revealed that the atomic structure of the {112} facets, which form the sidewalls of the hexagonal cross-sectional NWs, provides a highly favorable environment for Li insertion. As shown in Fig. 5d, the 〈111〉-oriented Si NW with a hexagonal cross-section is enclosed by six well-defined {112} facets. These {112} facets not only define the NW's shape but also serve as major Li-accessible interfaces. To further examine the lithiation behavior associated with these facets, an atomistic model of the Si {112} surface is constructed, as presented in Fig. 5e. The orange frames indicate the {111} planes. A magnified view of this model is provided in Fig. 5f, showing the Li insertion pathways. Previous studies have suggested that lithiation on both {110} and {112} planes of Si occurs predominantly through a ledge mechanism involving the layer-by-layer peeling of the {111} atomic facets.^34,35 From the atomic configuration shown in Fig. 5f, it is evident that the {112} surface exposes multiple {111} peeling planes, providing a high density of lithiation-active sites. This is consistent with the atomic-level characterization of the {112} facet shown in Fig. S3. These exposed planes facilitate fast and uniform lithiation across the {112} facet, which effectively balances the overall lithiation rate between the 〈110〉 and 〈112〉 directions, leading to the suppression of anisotropic expansion and the mitigation of stress localization during lithiation.

The results demonstrate that achieving uniform lithiation requires a cross-sectional design that strategically integrates geometric constraints with crystallographic orientation. The cross-sectional shape should be chosen by jointly considering the in-plane crystallographic symmetry and the direction-dependent lithiation kinetics, placing fast-lithiation directions at the vertices and slow-lithiation directions along the facets. In 〈111〉-oriented Si nanowires, six 〈110〉 directions are bisected by six 〈112〉 directions, symmetrically arranged within the {111} plane. When fast-lithiation directions such as the 〈110〉 direction are deliberately aligned with the vertices of a faceted cross-section, their effective interfacial contact area is minimized, and the diffusion path length is extended. This configuration suppresses lithiation along the 〈110〉 direction, despite its intrinsically high Li-ion diffusivity. In addition, aligning the slower-lithiating 〈112〉 direction along the flat facets increases the interfacial area for Li insertion and exposes multiple lithiation-active {111} atomic planes, thereby amplifying the overall Lithiation rate along the 〈112〉 direction. This configuration enables a balanced lithiation rate along different crystallographic directions. As a result, anisotropic lithiation and inhomogeneous stress distribution can be largely mitigated.

Our results highlight the importance of integrating geometric considerations into the design of anisotropic nanostructures. While circular cross-sections are typically regarded as ideal for isotropic materials, they fail to balance directional disparities in crystallographic properties in anisotropic systems such as Si. In these cases, rational cross-sectional design offers a promising strategy for regulating anisotropic behaviour and mitigating structural degradation. Beyond providing new insights into the control of anisotropy in anisotropic materials, this study also provides a powerful and generalizable methodology for investigating 1D nanomaterials, where internal structure and interfacial dynamics play a critical role in governing their performance. Relevant systems include alloy-type NWs undergoing phase transformations, core–shell semiconductor heterostructures with complex interfaces, and solid-solid interphase systems in solid–state batteries. These systems share a common need for resolving internal dynamic features that are inaccessible through conventional characterization or sample preparation methods. While the present study focuses on behaviours of 1D nanomaterials under an electric field, the proposed cross-sectional in situ TEM methodology could be readily extended to investigate structural evolution under other external stimuli. Nevertheless, the method has limitations. For instance, the technique is best suited to materials with dimensions compatible with TEM holders, and may not be directly applicable to larger or bulk samples.

4. Conclusions

In this study, we introduce a novel in situ TEM-based cross-sectional characterization technique that enables precise sectioning and real-time observation of the radial evolution of 1D nanomaterials under external stimuli. Using this approach, we investigate the anisotropic lithiation behaviours of Si NWs and how they are influenced by the axial orientation and cross-sectional geometry. Our results reveal that axial orientations with high in-plane crystallographic symmetry promote balanced Li-ion diffusion and mitigate directional swelling. Moreover, cross-sectional geometric design based on in-plane crystallographic symmetry can further modulate the local lithiation kinetics by constraining the lithiation rate along fast-lithiation directions. These findings offer valuable insights into the anisotropic lithiation control of Si NWs and open new possibilities for designing anode materials with higher performance and stability. In addition, by enabling direct, real-time tracking of internal structural and interfacial evolution, the newly proposed technique provides a powerful platform for the investigation and optimization of advanced 1D materials across a wide range of applications.

Author contributions

S. J. C.: conceptualization, data curation, formal analysis, investigation, visualization, writing, and funding acquisition. L. Z.: project administration, supervision, conceptualization, data curation, formal analysis, investigation, methodology, resources, writing, and funding acquisition. L. T. S.: project administration, supervision, resources, writing, and funding acquisition. H. L., K. L. L., and Q. Y. T.: formal analysis, validation, visualization, and writing. All authors discussed the results and contributed to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the results of this study are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nh00486a.

Acknowledgements

We acknowledge the support from the National Natural Science Foundation of China (Grant No. T2321002, 52271142 and 52401006), the Natural Science Foundation of Jiangsu Province (BK20230028 and BK20230863), China Postdoctoral Science Foundation (No. 2023M730550), and the Fundamental Research Funds for the Central Universities.

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