Parasitic gas evolution in lithium ion battery (LIB) cells especially occurs within the first charge cycle, but can also take place during long-term cycling and storage, thereby, negatively affecting the cell performance. Gas formation is influenced by various factors, such as the cell chemistry and operating conditions, thus, demanding fundamental studies in terms of interphase and gas formation (gas volume and composition) and electrolyte consumption. Gas analyses in terms of mass spectrometry of gaseous products are regularly performed, however, usually using custom-made cell designs with a high excess of electrolyte. Here, a gas sampling port (GSP) is incorporated in a commercial small-scale multilayer pouch cell in a simple post-production process and systematically evaluated as proof-of-principle approach towards effective electrolyte additive research under practically relevant conditions, i.e., when applying a limited amount of electrolyte per cell capacity. The GSP-based LIB pouch cell design allows the voltage-dependent identification and separation of formed gases, while a clear correlation between electrolyte reduction peaks, observed in differential capacity profiles, and the onset of gas evolution is demonstrated. In summary, the novel GSP-based pouch cell setup benefits from the possibility of multiple time-, cell voltage- or state-of-charge-dependent gas measurements, without significantly influencing the original cell performance.
Silicon (Si) has attracted much attention to be applied
as a negative
electrode (N) material for lithium ion batteries (LIBs) with increased
energy density. However, the huge volume changes during (de-)lithiation
of the Si, accompanied with the breakdown of the initially formed
solid electrolyte interphase (SEI), result in the gradual consumption
of active lithium and electrolyte and, hence, a poor cycling performance
of LIBs with Si-based N. The addition of various electrolyte additives
was proven to be able to reduce the active lithium consumption by
the formation of a more effective/flexible and, therefore, better
protecting SEI on the Si. Within this study, we synthesize the new
electrolyte additive lactic acid O-carboxyanhydride
(lacOCA), which is designed to incorporate two different moieties
within its structure, that both show to function as effective SEI
additives. The addition of small amounts of 2 wt % of lacOCA to the
baseline electrolyte significantly improves the electrochemical performance
of NMC-111||Si full cells in terms of discharge capacity retention
and Coulombic efficiency. The lacOCA also outperforms the comparable
additives lactide and diethyl dicarbonate, which are chosen to individually
represent the moieties incorporated within the lacOCA structure, proving
the synergistic effect of the two different moieties, when in one
molecule. Ex situ investigations of the SEI by means of X-ray photoelectron
spectroscopy and attenuated total reflectance Fourier transform infrared
spectroscopy reveal that the SEI formed by lacOCA is mainly composed
of poly(lactic acid) and lithium carbonate, which enables a significant
reduced consumption of active lithium during charge/discharge cycling
of the NMC-111||Si full cells.
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