Constructing Pure Si Anodes for Advanced Lithium Batteries

Abstract

ConspectusWith the escalating demands of portable electronics, electric vehicles, and grid-scale energy storage systems, the development of next-generation rechargeable batteries, which boasts high energy density, cost effectiveness, and environmental sustainability, becomes imperative. Accelerating these advancements could substantially mitigate detrimental carbon emissions. The pursuit of main objectives has kindled interest in pure silicon as a high-capacity electroactive material, capable of further enhancing the gravimetric and volumetric energy densities compared with traditional graphite counterparts. Despite such promising attributes, pure silicon materials face significant hurdles, primarily due to their drastic volumetric changes during the lithiation/delithiation processes. Volume changes give rise to severe side effects, such as fracturing, pulverization, and delamination, triggering rapid capacity decay. Therefore, mitigating silicon particle fracture remains a primary challenge. Importantly, nanoscale silicon (below 150 nm in size) has shown resilience to stresses induced by repeated volume changes, thereby highlighting its potential as an anode-active material. However, the volume expansion stress not only affects the internal structure of the particle but also disrupts the solid-electrolyte interphase (SEI) layer, formed spontaneously on the outer surface of silicon, causing adverse side reactions. Therefore, despite silicon nanoparticles offering new opportunities, overcoming the associated issues is of paramount importance.Thus, this Account aims to spotlight the significant strides made in the development of pure silicon anodes with particular attention to feature size. From the emergence of nanoscale silicon, the following nanotechnology played a crucial role in growing the particle through nano/microstructuring. Similarly, bulk silicon microparticles gradually surfaced with the post-engineering methods owing to their practical advantages. We briefly discuss the special characteristics of representative examples from bulk silicon engineering and nano/microstructuring, all aimed at overcoming intrinsic challenges, such as limiting large volume changes and stabilizing SEI formation during electrochemical cycling. Subsequently, we outline guidelines for advancing pure silicon anodes to incorporate high mass loading and high energy density. Importantly, these advancements require superior material design and the incorporation of exceptional battery components to ensure compatibility and yield synergistic effects. By broadening the cooperative strategies at the cell and system levels, we anticipate that this Account will provide an insightful analysis of pure silicon anodes and catalyze their practical applications in real battery systems.

Figure 1

Basic characteristics of the pure Si anode. (a) Practical gravimetric and volumetric energy densities of graphite, Si/C, SiO_x, and pure Si anodes paired with the NCM811 cathode at the cell level (unit: Wh kg^–1 and Wh L^–1 for gravimetric energy density and volumetric energy density, respectively.) (b) Comparison of the cell thickness between graphite|NCM811 and pure Si|NCM811. (c) Current market share of Si-based materials and (d) cost comparison of graphite, Si/C or SiO_x, pure SiNP, and pure SiMP. (e) Schematic diagram for volumetric expansion, the anisotropic property, and crack formation of crystalline Si during the lithiation process. (f) Main degradation mechanism of Si anodes originates from the large volume expansion of Si during lithiation. (g) Electrochemical lithiation curves of Si (black solid line) and graphite (green dashed line) anodes (inset: DFT voltage profiles for the lithiated reaction into each (111), (110), and (111) plane of crystalline Si). Adapted with permission from ref (15). Copyright 2011 American Chemical Society. (h) Critical size (D_c) for occurring particle fracture upon lithiation and volume expansion.

Figure 2

Bulk Si engineering for the advanced SiNP anode. (a) Synthetic route of porous Si from metallurgical Si through ball-milling, stain-etching, the corresponding TEM image of porous Si particles, and HAADF-STEM tomography. Labeled colors indicate the separated pore connections. Reproduced with permission from ref (25). Copyright 2014 American Chemical Society. (b) Schematic illustration showing the synthetic process of the Si-based materials@LTO. Reproduced with permission from ref (1). Copyright 2015 the authors. Published by Royal Society of Chemistry under a Creative Commons Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0) License. (c) Schematic of the lithiation/delithiation process (left) and Li diffusion (right) in the Si anode and Ge-doped Si anode. Reproduced with permission from ref (30). Copyright 2019 the authors. Published by American Association for the Advancement of Science under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License.

Figure 3

Nanostructuring for the advanced nanoscale Si anodes. (a) Schematic of morphological changes that occur in SiNWs during electrochemical cycling. (b) TEM data for SiNWs at different stages of the first charge. Reproduced with permission from ref (33). Copyright 2008 Springer Nature. (c) Schematic illustration of the Si@void@C electrode and magnified schematic of an individual Si@void@C particle showing that SiNP expands without breaking the carbon coating or disrupting the SEI layer on the outer surface. (d) In situ TEM images of Si@void@C in a pristine state (0 s) and after full lithiation. Adapted with permission from ref (34). Copyright 2012 American Chemical Society. (e) Time-lapse in situ TEM images of 2DSi@C and (f) snapshots of deformation morphologies predicted by the chemomechanical model for two lithiation cycles. Reproduced with permission from ref (2). Copyright 2018 the authors. Published by Springer Nature under a Creative Commons Attribution 4.0 International (CC BY 4.0) License.

Figure 4

Microstructuring for the advanced microscale Si anode. (a) Schematic of silicene flowers features high tap density, three-dimensional electron/lithium ion transport channels, reduced lithium ion diffusion length, and limited variable SEI. Reproduced with permission from ref (37). Copyright 2017 American Chemical Society. (b) Schematic illustrating the lithiation/delithiation process of the ant-nest-like microscale porous Si particles showing inward volume expansion and stable Si framework retention during cycling. Reproduced with permission from ref (43). Copyright 2019 the authors. Published by Springer Nature under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. (c) Schematic illustration of a hollow porous Si sphere (HPSS) and magnified TEM images of pristine, fully lithiated, and delithiated HPSS@C particles, which illustrate a thickened shell (22% expansion after lithiation) and pore filling/restoration showing no structural collapse. Reproduced with permission from ref (45). Copyright 2018 the authors. Published by Springer Nature under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. (d) Electron energy loss spectroscopy (EELS) elemental mapping of mixed amorphous–crystalline Si (MACS) for Si (orange) and boron (green). Reproduced with permission from ref (4). Copyright 2023 Royal Society of Chemistry.

Figure 5

Bulk Si engineering for the advanced SiMP anode. (a) Schematic for the reliable structure of graphene cage-encapsulated SiMP after cycling. (b) In situ TEM time-lapse images of graphene cage-encapsulated SiMP during lithiation. Reproduced with permission from ref (48). Copyright 2016 Springer Nature. (c) Graphical representation of the operation of the PR-PAA binder to dissipate the stress during repeated volume changes of SiMPs. Reproduced with permission from ref (53). Copyright 2017 American Association for the Advancement of Science. (d) Schematic illustration of an integrated binder-conductive material system through functionalization. (e) Lithiation limited cycling of the bare SiMP anode and constructed SiMP anode at 0.5 C with state-of-charge (SOC) control to 50%.

Figure 6

Outlook and perspective to achieve a practical pure Si anode for advanced batteries. Adapted with permission from ref (57). Copyright 2020 the authors. Published by Elsevier under a Creative Commons Attribution 4.0 International (CC BY 4.0) License; ref (58), copyright 2021 the authors. Published by Oxford University Press under a Creative Commons CC BY License.