Effect of the positive electrode and of mechanical stresses on the cyclability of a Li-ion cell containing silicon-based negative electrode

Résumé

The rise of silicon and nickel manganese cobalt layered oxides (NMC) as new negative and positive electrode materials for Li-ion batteries appear the question of their integration in full cells. Indeed, silicon lithiation induces a high material expansion, which leads to significant swelling and mechanical stresses at the anode and cell level. Additionally, interphases formed at the surface of the electrodes during the first cycle (SEI and CEI) play an essential role in the operation and ageing of the cell. Recent studies showed the occurrence of a SEI-CEI crosstalk influenced by the nature of the positive electrode. Thus, this study aims at understanding the role of mechanical constraints and nature of the positive electrode on the capacity retention of a high performance silicon carbon graphite composite electrode (Si-C/G) in a Li-ion cell. In case of cylindrical 18650 cells, strain gauges confirmed the rigidity of the casing and measured a maximum pressure of 4.3 MPa; in addition, internal cell components thickness change was captured at 8 different states of charge (SOC) during a cycle by in situ 3D imaging with X-Ray micro computed-tomography (voxel size 1.6μm) combined with a specific image treatment. For bi-layer pouch cells, operando swelling was measured using an in-house high precision (< 0.1μm) compression set-up with simultaneous pressure and thickness recording as well as dynamic pressure regulation system. Combining these unique experimental techniques and the modelling of Si-C/G active material swelling in function of the SOC we were able to provide insights in porosity changes of anodes for the two cell formats. Then three cathode materials were compared (NMC622, NMC811 and LCO) leading to a better capacity retention with NMC811 and then NMC622. NMC811 contributes firstly to minimize the maximum swelling in contrary to LCO. Post-mortem analyses of positive and negative electrodes harvested from pouch cells were carried out by combining Electrochemical Impedance Spectroscopy (EIS), X-ray Photoelectron Spectroscopy (XPS), Time of Flight Secondary-Ion Mass Spectrometry (Tof-SIMS) and Inductively Coupled Plasma (ICP) mass spectroscopy. For all type of cells, the same fading mechanism occurs. A continuous growth of the SEI has been highlighted at the anode side, trapping lithium ion but without charge transfer increase. At the cathode side, an increase of the charge transfer was observed for all cells correlated to CEI thickening upon cycling. The better capacity retention in NMC811 cells was found to be related to a lower amount of SEI formed upon cycling. In another hand, faster degradation of LCO cells originates from a more significant thickening of both the CEI and SEI. Various approaches used in this work could easily be applied to develop other Si-C composite or anode formulation with limited swelling, to optimize cell design and the integration of this new generation of cells which will finally promote higher energy density of Li-ion battery. The study of the mechanical stresses at particle level might also provide significant insight to understand and limit the swelling at electrode level. A direct correlation between the mechanical stress and the SEI evolution is still under debate.

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