No abstract

Spatially dependent low-temperature to roomtemperature degradation mechanisms for Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 / Li x C 6 (NMC532/graphite) large format 50Ah Li-ion batteries were investigated. First, highly stressed regions of the cathode/ anode are found to be exacerbated by extreme conditions (i.e., lowtemperature cycling). The severe electrochemical polarization of large 50Ah electrodes at low temperature leads to substantial Li 0 deposition and severe gassing at the regions of high stress (i.e., high curvature, edges, and electrode ripples). A series of analytical techniques (e.g., SEM, XPS, GC-MS, and Raman spectroscopy) found that Li 0 plating (charge) or corrosion (storage) leads to severe gassing and decomposition products (including carbides). The expansion/contraction and extreme polarization during lowtemperature cycling, was found to cause a ripple-type Li 0 deposition on the electrode. Multilocation liquid nitrogen (N 2 ) Raman spectroscopy of electrodes indicates significant quantities of Li 0 deposition reside at ripple peaks (high-stress region) and are found negligible at ripple troughs. Postmortem analysis discovered two failure scenarios that originate from low-temperature cycling, either nonthermal runaway venting or an internally shorted thermal runaway. It was found in the first case (storage) that LiC 6 −Li 0 undergoes severe corrosion and gassing during storage conditions (i.e., no movement, current, and temperature) and proceeds to trigger thermal runaway and ejection of materials (∼2 weeks). The second case (RT cycling after low temperature) resulted in nonthermal runaway overpressurized venting of the cell and release of detectable quantities of flammable/toxic gases (e.g., CO 2 , CO, CH 4 , and C 2 H 2 ). The second event was found to be caused by competing reactions (i.e., Li 0 stripping, Li 0 corrosion, and severe gassing). This study finds that low-temperature Li 0 plating and LiC 6 −Li 0 corrosion results in severe gassing, which exacerbates highly stressed regions (i.e., electrode buckling) and greatly compromises safety of the application via nonthermal runaway venting when cycled (e.g., stripping of Li 0 and gassing) and catastrophic thermal runaway when resting under storage (e.g., larger quantities of Li x C 6 −Li 0 corrosion).

The leading cause for safety vent rupture in alkaline batteries is the intrinsic instability of Zn in the highly alkaline reacting environment. Zn and aqueous KOH react in a parasitic process to generate hydrogen gas, which can rupture the seal and vent the hydrogen along with small amounts of electrolyte, and thus, damage consumer devices. Abusive conditions, particularly deep discharge, are known to accelerate this "gassing" phenomena. In order to understand the fundamental drivers and mechanisms for such gassing behavior, the results from multiphysics modeling, ex-situ microscopy and operando measurements of cell potential, pressure and visualization have been combined. Operando measurements were enabled by the development a new research platform that enables a cross-sectional view of a cylindrical Zn-MnO 2 primary alkaline battery throughout its discharge and recovery. A second version of this cell can actively measure the in-cell pressure during the discharge. It is shown that steep concentration gradients emerge during the cell discharge through a redox electrolyte mechanism, leading to the formation of high surface area Zn deposits that experience rapid corrosion when the cell rests to its open circuit voltage. Such corrosion is paired with the release of hydrogen and high cell pressure -eventually leading to cell rupture.

Nanostructured hybrids that physically encapsulate highly morphable, high capacity Li-ion battery anodes can potentially enable much longer cycle life than straightforward deployment of the same chemistry.

Electroanalytical techniques are specialized tools with high-sensitivity that when combined with electron transfer theory can shed light on the mechanisms of highly complex, heterogeneous, multi-step reactions – including SEI formation on LiB anodes.