The mechanical instability of the Solid Electrolyte Interphase (SEI) layer in lithium ion (Li-ion) batteries causes significant side reactions resulting in Li-ion consumption and cell impedance rise by forming further SEI layers, which eventually leads to battery capacity fade and power fade. In this paper, the composition-/structure-dependent elasticity of the SEI layer is investigated via Atomic Force Microscopy (AFM) measurements coupled with X-ray Photoelectron Spectroscopy (XPS) analysis, and atomistic calculations. It is observed that the inner layer is stiffer than the outer layer. The measured Young's moduli are mostly in the range of 0.2 to 4.5 GPa, while some values above 80 GPa are also observed. This wide variation of the observed elastic modulus is elucidated by atomistic calculations with a focus on chemical and structural analysis. The numerical analysis shows the Young's moduli range from 2.4 GPa to 58.1 GPa in the order of the polymeric, organic, and amorphous inorganic components. The crystalline inorganic component (LiF) shows the highest value (135.3 GPa) among the SEI species. This quantitative observation on the elasticity of individual components of the SEI layer must be essential to analyzing the mechanical behavior of the SEI layer and to optimizing and controlling it.

The use of electrolyte additives is one of the most effective and economical ways to improve battery performance by stabilizing the electrode/electrolyte interface. In this work, we identified that fluoroethylene carbonate (FEC), which is one of the important electrolyte additives, had different impacts on anode and cathode, by investigating a graphite anode and a LiMn 2 O 4 cathode through electrochemical analyses at room and elevated temperatures. In the anode side, the solid electrolyte interphase (SEI) layer derived from FEC exhibited a lower interfacial resistance and excellent thermal stability, showing excellent rate capability and improved cycle retention of cells. In contrast, poor cycling retention and a rapid increase in the interfacial resistance of the cathode were observed at elevated temperature. The poorer performance of the cathode in the FEC-containing cell at elevated temperature was attributed to the formation of a thicker surface layer and to increased Mn dissolution catalyzed by HF, which resulted from FEC dehydrofluorination initiated or accelerated by elevated temperature. Accordingly, it is suggested that the amount of FEC in a full cell must be optimized to minimize the adverse effects of FEC on cathode.It is well known that lithium-ion (Li-ion) batteries experience significant capacity fade during cycling or storage at elevated temperatures. However, the mechanisms responsible for the capacity fade at elevated temperatures are poorly understood because the capacity fade is caused by several interdependent factors. On anode side, most previous studies have identified the degradation of the solid electrolyte interphase (SEI) layer as the factor primarily responsible for the capacity loss seen at elevated temperatures. 1-4 Temperature-induced reactions, such as SEI decomposition, redox reaction, and electrolyte decomposition, cause changes in the morphology and composition of the SEI layer. 3 Deposition of transition metal ions, which are dissolved from the cathode active materials at elevated temperatures, onto the anode/electrolyte interface also contributes significantly to the degradation of the SEI layer. 5-7 Degradation of the SEI layer results in SEI reformation and growth, during which cyclable lithium ions are additionally consumed due to further electrolyte reduction reactions. Thus, the degraded SEI layer cannot sustain its original properties, which affects the degradation of anode performance. 1,3,7 For instance, an increase in the level of inorganic components present in the SEI layer can lower the ionic conductivity of the SEI layer, which hinders lithium ion transport into/from the anode. 4 While degradation of the SEI layer has been considered the main contributor to the fade in capacity at the anode side, several factors have been proposed as reasons for the capacity fade that comes from the cathode side. In particular, LiMn 2 O 4 shows the most severity in terms of (i) the dissolution of cathode materials due to the disproportionation reaction and hydrofluoric acid (HF) ...

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With the rapid growth of the volume of spent Li-ion batteries (LIBs), recycling of spent LIBs has attracted significant attention in recent years for future sustainability. In particular, there remains a great need for the development of a scalable and environmentfriendly separation process to recycle valuable cathode active materials from spent LIBs and electrode scraps. In this work, froth flotation technique was adopted to separate cathode active materials from a mixture of cathode and anode materials. To evaluate whether the recovered cathode materials maintain their functional integrity after the developed separation process, a variety of electrochemical analyses have been conducted systematically. The present work demonstrated that froth flotation process with kerosene enhanced separability of mixed electrode materials and the recycled cathode materials almost preserved their original electrochemical reactivity. Cycle performance (up to 200 cycles) and rate capability (up to 1 C) of the recycled cathodes were comparable to those of a pristine cathode. However, the higher polarization observed in the recycled cathodes was identified as a key challenge, and it needs to be addressed further. This work provides valuable insights into further development of a scalable froth flotation-based recycling process which can be implemented in a direct recycling process.