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 13C
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, SiOx (~34%)
and neat silicon (66%) (Figure S3b),
compared to as-received Si NPs
that contain 21% of SiOx 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