posted on 2020-12-01, 23:29authored byPeer Bärmann, Bastian Krueger, Simone Casino, Martin Winter, Tobias Placke, Gunther Wittstock
Because
of their high specific capacity and rather low operating
potential, silicon-based negative electrode materials for lithium-ion
batteries have been the subject of extensive research over the past
2 decades. Although the understanding of the (de)lithiation behavior
of silicon has significantly increased, several major challenges have
not been solved yet, hindering its broad commercial application. One
major issue is the low initial Coulombic efficiency and the ever-present
self-discharge of silicon electrodes. Self-discharge itself affects
the long-term stability of electrochemical storage systems and, additionally,
must be taken into consideration for inevitable prelithiation approaches.
The impact of the crystalline Li15Si4 phase
is of great interest as the phase transformation between crystalline
(c) and amorphous (a) phases not
only increases the specific surface area but also causes huge polarization.
Moreover, there is the possibility for electrochemical over-lithiation
toward the Li15+aSi4 phase
because of the electron-deficient Li15Si4 phase,
which can be highly reactive toward the electrolyte. This poses the
question about the impact of the c-Li15Si4 phase on the self-discharge behavior in comparison
to its amorphous counterpart. Here, silicon thin films used as model
electrodes are lithiated to cut-off potentials of 10 mV and 50 mV versus Li|Li+ (U10mV and U50mV) in order to systematically
investigate their self-discharge mechanism via open-circuit
potential (UOCP) measurements and to visualize
the solid electrolyte interphase (SEI) growth by means of scanning
electrochemical microscopy. We show that the c-Li15Si4 phase is formed for the U10mV electrode, while it is not found for the U50mV electrode. In turn, the U50mV electrode displays an almost linear self-discharge
behavior, whereas the U10mV electrode
reaches a UOCP plateau at ca. 380 mV versus Li|Li+, which is due to
the phase transition from c-Li15Si4 to the a-LixSi phase. At this plateau potential, the phase transformation at
the Si|electrolyte interface results in an electronically more insulating
and more uniform SEI (U10mV electrode),
while the U50mV electrode displays a less
uniform SEI layer. In summary, the self-discharge mechanism of silicon
electrodes and, hence, the irreversible decomposition of the electrolyte
and the corresponding SEI formation process heavily depend on the
structural nature of the underlying lithium–silicon phase.