One-pot synthesis of high-capacity silicon anodes via on-copper growth of a semi-conducting, porous polymer

Jieyang Huang^[a], Andréa Martin^[a], Anna Urbanski^[b], Ranjit Kulkarni^[a], Patrick Amsalem^[c], Moritz Exner^[a], Johannes Müller^[c], David Burmeister^[a], Norbert Koch^[c],[d], Torsten Brezesinski^[e], Nicola Pinna^[a], Petra Uhlmann^[b], Michael J. Bojdys*^[a],[f]

[a] Dr. J. Huang, Dr. A. Martin, Dr. R. Kulkarni, M. Exner, D. Burmeister, Prof. Dr. N. Pinna, Prof. Dr. M. J. Bojdys Institut für Chemie and IRIS Adlershof Humboldt-Universität zu Berlin Brook-Taylor-Str. 2, 12489 Berlin, Germany E-mail: m.j.bojdys.02@cantab.net

[b] Dr. A. Urbanski, Prof. Dr. P. Uhlmann Leibniz-Institut für Polymerforschung Dresden (IPF) e. V Institut für Physikalische Chemie and Physik der Polymere 01069 Dresden, Germany

[c] Dr. P. Amsalem, J. Müller, Prof. Dr. N. Koch Institut für Physik and IRIS Adlershof Humboldt-Universität zu Berlin Brook-Taylor-Str. 2, 12489 Berlin, Germany

[d] Prof. Dr. N. Koch Helmholtz-Zentrum Berlin GmbH Albert-Einstein-Str. 15, 12489 Berlin

[e] Dr. T. Brezesinski Institute of Nanotechnology Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

[f] Prof. Dr. M. J. Bojdys Department of Chemistry King’s College London Britannia House Guy’s Campus, 7 Trinity Street, London, SE1 1DB, UK

Supporting information for this article is given via a link at the end of the document.

Abstract: Silicon-based anodes with lithium ions as charge carriers have the highest predicted theoretical specific capacity of 3579 mA h g^-1 (for Li₁₅Si₄). Contemporary electrodes do not achieve this theoretical value largely because conventional production paradigms rely on the mixing of weakly coordinated components. In this paper, a semi-conductive triazine-based graphdiyne polymer network is grown around silicon nanoparticles directly on the current collector, a copper sheet. The porous, semi-conducting organic framework (i) adheres to the current collector on which it grows via cooperative van der Waals interactions, (ii) acts effectively as conductor for electrical charges and binder of silicon nanoparticles via conjugated, covalent bonds, and (iii) enables selective transport of electrolyte and Li-ions through pores of defined size. The resulting anode shows extraordinarily high capacity at the theoretical limit of fully lithiated silicon. Finally, we combine our anodes in proof-of-concept battery assemblies using a conventional layered Ni-rich oxide cathode.

Introduction

Cheap, high-performance, and safe energy storage solutions are needed to address the increasing demand for portable electronics and the transition to electric mobility. Lithium-ion batteries have replaced conventional secondary battery technology (like nickel-cadmium and nickel-metal hydride batteries) due to their high energy densities and stable capacity retention in the charged state.¹Lithium metal anodes were quickly replaced by lithium-intercalated graphitic materials in order to avoid the formation of dendrites that resulted in short circuiting of the two electrodes. However, while lithium-graphite intercalates are safer and more stable they have only a tenth of the energy density of lithium metal.² Silicon is a good active material for Li-ion anodes because it has a superior theoretical specific capacity of 3579 mA h g^-1 (or 8340 mA h cm^-3) for Li₁₅Si₄, and – unlike some transition metals – it is not toxic and abundant. Moreover, its alloying reaction with lithium triggered at 0.3 V vs. Li⁺/Li, prevents the formation of lithium around the anode during charging – a detrimental process observed for graphite-based Li-ion batteries known as “lithium plating” – and allows the use of Si electrodes under harsher conditions.³ The large number of lithium atoms that silicon can store, however, induces large volume changes during charge/discharge cycling (>300%).³ The mechanical stress induced by these drastic volume changes leads to the pulverization of the silicon active material, loss of contact of the electrode film with the current collector, and loss of overall mechanical integrity of the whole electrode. Repeated cycles of expansion and contraction of silicon expose pristine silicon surfaces and induce the reformation of solid electrolyte interfaces (SEI). This process contributes to the gradual consumption of lithium and electrolyte, and it limits the diffusion of charge carriers through the expanding SEI.^3-7 It is difficult to counter these detrimental mechanical and chemical changes to silicon-based electrodes because conventional methods of electrode assembly rely on the mixing of multiple components that are held together by weak, dispersive forces.

In laboratory settings, some strategies were developed to address the inherent flaws of these multi-component assemblies. For example, shaping silicon into hierarchical, nano-sized, or porous structures buffers some of its dramatic volume expansion during lithiation.⁷On the downside, nanostructured silicon is susceptible to restructuring during battery operation, and its larger specific surface promotes reactions with the electrolyte to form more of SEI. In other approaches silicon particles are encapsulated in a carboneous matrix,⁸ or they are coated with metal oxides.⁹ However, encapsulation of silicon requires supplementary components that do not meaningfully contribute to the capacity of the electrode and might necessitate the addition of agents that enhance electrical conductivity. Such modifications of the active material prior to electrode assembly have proven too time consuming, low yielding and expensive and, hence, none of these methods have found their way into commercial processes to date.

In this work, we present a departure from the current “blend-and-bake” paradigm of electrode manufacture. In a one-pot process, we embed silicon nanoparticles (Si NPs) in a covalently-linked, porous, semi-conducting polymer matrix whose growth is initiated and templated by the current collector (Cu) itself. The covalent bonds of the organic matrix contribute to a superior mechanical and chemical resistance of our electrode films. The overall π-conjugated backbone of the triazine-based graphdiyne (TzG) polymer enables the transport of electrons from the active material to the current collector. Since the polymerization is promoted by the reactive metal surface of the current collector, the resulting polymer/silicon composite (TzG/Si) adheres strongly to it.¹⁰ In summary, the covalent polymer matrix acts at the same time as (i) a strong binder, as (ii) an electrical conductor, and as (iii) a semi-permeable membrane that enables transport of ions and electrolyte but prevents the migration of homogeneously dispersed silicon nanoparticles even under harsh conditions. This facile method yields silicon-based anodes (TzG/Si@Cu) of superior performance that suffer little mechanical and electrochemical deterioration from the inherent volume expansion of silicon during lithiation-delithiation and that drastically limit the detrimental loss of lithium and electrolyte at the SEI.

Results and Discussion

Electrodes of TzG/Si@Cu are prepared by dissolving the organic monomer 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine and dispersing Si NPs in pyridine in a 25%:75% weight ratio, respectively. The reaction mixture is then transferred onto a copper foil (Figure 1a; Supplementary Information Section S1, Scheme S1 and S2). Residual Cu(II) and Cu(I) species on the untreated copper surface initiate the polymerizationvia a Glaser‐type oxidative coupling reaction.^11-13 The polymerization is driven to completion and the pyridine is removed by evaporation.^{10 13}C cross‐polarization / magic‐angle‐spinning (CP/MAS) solid-state NMR (Figure 1b) shows the characteristic signals of a triazine-based graphdiyne polymer;¹⁰ the triazine carbon at ~170 ppm and the diyne-bridges at 75-85 ppm. An additional signal seen at ~30 ppm is attributed to O₂SiMe₂ surface groups originating from the preparation of these commercially available Si NPs. This is corroborated by ²⁹Si single-pulse-magic-angle-spinning (SP-MAS) solid-state NMR (Figure S1) and Fourier transform infrared (FT-IR) spectra (Figure S2).^14-16 The Raman spectrum of TzG/Si@Cu (Figure 1c) shows stretching bands of diyne C≡C at 2209 cm^-1, of triazine C=N at 1411 cm^-1, of phenyl C=C at 1604 cm^-1, and of crystalline Si-Si bonds at 518 cm^-1.^10,17,18

X‐ray photoelectron spectroscopy (XPS) performed on c-axis oriented layers of TzG/Si@Cu show all expected carbon environments in the C 1s region that are observed for the neat TzG polymer (Figure S3a).¹⁰ In addition, Si 2p spectra show the presence of surface silicon oxide, SiO_x (~34%) and neat silicon (66%) (Figure S3b), compared to as-received Si NPs that contain 21% of SiO_x and 79% of Si(0) environments (Figure S3d ).^19-22 In summary, spectroscopic analysis confirms the formation of a covalent, conjugated, triazine-based polymer network around chemically unchanged Si NPs.

For comparison, we have prepared three different types of electrode systems via the same one-pot method described above but with varying compositions to carefully test the effects of individual components: (i) growing a film of TzG on Cu, we obtain TzG@Cu, (ii) growing TzG in the presence of Si NPs we get TzG/Si@Cu (in ratio 25/75 wt%), and (iii) TzG/Si/CB@Cu (in ratio 20/60/20 wt%) is produced by growing TzG around Si NPs and a conventional electronically conductive additive, carbon black (CB) (Supplementary Information Section S1.5).

Scanning electron microscopy (SEM) images reveal the morphology of pristine TzG/Si@Cu electrodes. The material grown on the copper support adopts a porous, sponge-like, and homogeneous morphology as seen top-down (Figure 1e) and from cross-sections of the electrode film (Figure 1f). Cross-sectional SEM imaging at lower magnifications shows films of TzG/Si with a thickness of ~25 µm that adhere well to the Cu substrate with no apparent gaps (Figure S4). More detailed transmission electron microscopy (TEM) energy-filtered mapping on TzG/Si films shows a homogenous distribution of carbon and silicon on the nanometer level (Figure 1g). On the nanoscale, the electrode film consists of Si NPs homogenously embedded in an organic polymer matrix of TzG. Residual Cu nanoparticles (less than 1 wt%) can be seen within the polymer matrix that stem from the TzG polymerization process (Figure 1h; a comparison of TEM images of TzG@Cu and of pristine Si NPs can be found in Figure S5).¹⁰ Overall, individual Si NPs are enclosed by the conjugated, polymer and held cooperatively as a film on the current collector.

We have shown previously that neat, unmodified triazine-based graphdiyne polymers are narrow band-gap semiconductors (E _g,elec = 1.84 eV and conductivity of 1.2 µS cm^-1 at RT) with moderate porosities (N₂ BET surface area of 124 m² g^-1 at 77 K).^10,23 Hence, the composite TzG/Si on Cu foil (TzG/Si@Cu) has a promising combination of chemical, electrical, and structural features for electrochemical energy storage applications.

In the following (Supplementary Information Section S2.1), we discuss the electric and electrochemical performance of TzG-based electrodes and the effects of the TzG polymer on the formation of the SEI. For the three electrode systems TzG/Si@Cu, TzG/Si/CB@Cu, and TzG@Cu (i) we compared the bulk conductivities of the unlithiated, “as-synthesized” electrodes (Figure S6), (ii) we recorded cyclic voltammetry (CV) curves (Figure S7), and (iii) we performed ex-situ XPS measurements probing the electrode surfaces to a depth of approx. 10 nm after a number of de-/lithiation cycles (Figure 2a; Figure S8, S9, S10 and S13; Table S1; details of the XPS spectra fitting method are described in Supplementary Information section S1.7). During the first lithiation of the TzG/Si@Cu electrode, the resulting