Alloy
electrode materials offer high capacity in lithium-ion batteries;
however, they exhibit rapid degradation resulting in particle disintegration
and electrochemical performance decay. In this study, the evolution
of lithium alloying-induced degradation due to electrochemomechanical
interactions is examined based on a multipronged electrochemical and
microstructural analysis. Copper–tin (Cu6Sn5) is chosen as an exemplary alloy electrode material. Electrodes
with compositional variations were fabricated, and electrochemical
performance was examined under varying conditions including voltage
window, C-rate, and short- and long-term cycling. Morphology and composition
analyses of pristine and cycled electrodes were conducted using micrography
and spectroscopy techniques. Alloying-induced electrode microstructural
evolution was probed using X-ray microtomography. The rapid capacity
fading was found to be caused by mechanical degradation of the electrode.
Driving the electrode to a lower potential (E ≈
0.2 V vs Li/Li+) induced Li–Sn alloy formation and
provided the characteristic large capacity; however, this led to a
large volume expansion and active particle cracking and disintegration.
Copper expulsion was found to be a consequence of the alloy formation;
however, it was not the primary contributor to the dramatic electrochemical
performance decay.
Prior work on Li-ion cells which were parallel-connected in a stack and subjected to long term cycling showed that non-uniform temperature distribution caused non-uniform and accelerated degradation. To elucidate the degradation mechanisms, electrochemical and post-mortem degradation analysis were performed. Electrochemical impedance spectroscopy analysis suggested that the main degradation mechanism for the middle cell was a solid electrolyte interface (SEI) layer growth. Nevertheless, post-mortem analysis using X-ray diffraction, optical microscope, and scanning electron microscopy paired with energy dispersive X-ray spectroscopy shows the presence of Li2CO3 in both baseline and middle cell anodes. This points towards a combined degradation mechanism of SEI layer growth and lithium plating. A combination of microstructural particle cracking and lithium plating is considered the main mechanism for blocking the anode’s porosity network, which hindered further lithium diffusion, resulting in the abrupt failure for the middle cell. The observation and quantitative analysis provides insight into the performance and reliability impacts of non-uniform conditions within lithium-ion batteries.
In operando 2D X-ray absorption near edge structure (XANES) imaging was performed near the Cu K-edge during cycling of Cu6Sn5 composite anodes for lithium ion batteries. Galvanostatic lithiation and delithiation with intermittent constant voltage holds near reaction plateaus show evolution of absorption spectra for active material particles. XANES spectra obtained from images taken during cycling were compared to standard spectra for Cu, Cu6Sn5, and Li2CuSn. Chemical composition was assessed for Cu-containing phases. Distinct Cu, Cu6Sn5, and Li2CuSn regions were identified for each voltage plateau. Mechanical degradation, electrode particle fracture and expansion were observed during delithiation. Movement of particles during cycling suggests that expansion also impacts the supporting secondary phases and the transport networks therein. These results demonstrate that spectroscopic X-ray imaging methods can clearly distinguish chemically distinct phases in alloy electrodes and have the versatility to observe the evolution of these phases during lithiation and delithiation.
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