Figure 3. Half-cell performance data of the TzG/Si@Cu
electrodes. Galvanostatic charge-discharge cycling with potential
limitation (GCPL) uses constant current mode at C/8 within a voltage
range of 0.01-1.2 V vs. Li+/Li. a , Specific
capacities normalized to Si mass and Coulombic efficiencies (CE) vs.
cycle number. b , Potential vs. specific capacity curves for
consecutive de-/lithiation cycles.
low conductivity in the pristine, unlithiated state. While an in-depth
study of the electrical conductivity of lithiated TzG/Si@Cu is out of
the scope of this work, XPS results for TzG@Cu suggest that the first
lithiation cycle leads to n-doping of the TzG polymer by N 1s signal
shifting to higher binding energy (Figure S8),26-28analogous to lithiated graphite,29,30 and, hence, to
an increase of its conductivity above and beyond the effect achieved by
the conducting additive in TzG/Si/CB@Cu.
We subjected the TzG/Si@Cu electrode to de-/lithiation cycling at C/8 in
a half-cell set-up recording specific capacity (Figure 3). In the first
lithiation cycle, we observe the formation of the SEI at a comparatively
low CE value of ~70%, meaning that the surface of Si
NPs is readily accessible for lithium and for the electrolyte viathe pore channels of the TzG polymer. Similar Coulombic efficiencies
have been reported for nanosized silicon particles with a large external
surface area. Microsized silicon particles with a smaller ratio of
exposed surfaces show higher initial CE values but suffer from limited
lithiation capacities and poor cycling performance.31In the third cycle, the CE rises above 97% and steadily improves up to
99.5% which means that SEI formation is completed early on. The
specific capacity observed during the first lithiation exceeds the
theoretical value for Li15Si4(3579 mA h g-1) due to the formation of the
SEI.32-34 Starting with the second cycle, no excess
capacity is recorded and, hence, the newly formed SEI does not
contribute to the measured discharge capacity. The recorded specific
lithiation capacity in the second cycle is
~3500 mA h g-1, and it remains at a
record high in subsequent cycles (e.g.~3450 mA h g-1 in the fifth cycle). In
comparison, the TzG/Si/CB@Cu electrode prepared together with carbon
black does not perform better than the additive-free electrode (Figure
S14). Taking the specific lithiation capacity in the second cycle as a
baseline, TzG/Si@Cu retains 81.0% of its specific capacity after 50
cycles (2862 mA h g-1) and 69.1% after 100 cycles
(2443 mA h g-1). Conversions of these values to areal
and volumetric capacities can be found in Figure S15.
For comparison, we prepared two electrodes using conventional binders
and CB additive, PVdF/Si/CB@Cu and PAA/Si/CB@Cu (Figure
S16).21,35 These conventional formulations allow a
total Si mass loading of up to 60% without compromising the performance
of the electrode, compared to 75% of Si in the TzG/Si@Cu system.
Overall, the specific capacity of TzG/Si@Cu
Table 1. Relative molar concentrations of elements as
determined by peak integration using X-ray photoelectron spectroscopy
(XPS) data recorded at 1486.6 eV for TzG/Si@Cu electrodes at various
stages of de-/lithiation cycled at a constant current of
0.32 mA cm-2 (i.e. C/8 for the capacity of Si, within
0.01-1.2 V vs. Li+/Li). (Calculated based on XPS C 1s,
Si 2p N 1s, O 1s, Li 1s, F 1s and P 2p spectra).