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Electrode degradation processes at various Li-ion batteries' state-of-health (SoH 100 %, 80 %, 50 %, and 30 %) and cycling temperatures (5°C, 23°C, and 45°C) were investigated. For this purpose, the standard format of Li-ion cylindrical 18,650 batteries with Si-Graphite negative and LiNi0⋅8Co0⋅1Mn0⋅1O2 (NMC811) positive electrodes were cycled with registering battery parameters and the electrochemical impedance spectrum were recorded after every 200 cycles. Once reaching their end-of-life, electrodes from cycled batteries were subjected to post-mortem analysis. NMC811 positive electrode was observed to crack during the charge and discharge processes, suffered by irreversible phase transition, transition metal dissolution, cathode electrolyte interphase growth, and cation mixing. The Si-Graphite negative electrode material was also affected by crack formation, layer exfoliation, solid electrolyte interphase (SEI) recompositing, Li dendrite growth, transition metal contamination, and Si dissolution. Degradation of components leads to an increase of the contact resistance, Li+ diffusion limitations, reduction of active materials participating in Li-ion storage and, as a result, capacity fade that finally rendered the battery utilization unfeasible. Degradation processes can be detected by capacity fade and impedance growth of the full battery. High temperature accelerates electrode degradation processes when low temperature leads to SEI and Li dendrite growth.

Rechargeable batteries with solid polymer electrolytes (SPEs), Li-metal anodes, and high-voltage cathodes like LiNixMnyCozO2 (NMC) are promising next-generation high-energy-density storage solutions. However, these types of cells typically experience rapid failure during galvanostatic cycling, visible as an incoherent voltage noise during charging. Herein, two imidazolium-based zwitterions, with varied sulfonate-bearing chain length, are added to a poly(epsilon-caprolactone-co-trimethylene carbonate):LiTFSI electrolyte as cycling-enhancing additives to study their effect on the electrochemical stability of the electrolyte and the cycling performance of half-cells with NMC cathodes. The oxidative stability is studied with two different voltammetric methods using cells with inert working electrodes: the commonly used cyclic voltammetry and staircase voltammetry. The specific effects of the NMC cathode on the electrolyte stability is moreover investigated with cutoff increase cell cycling (CICC) to study the chemical and electrochemical compatibility between the active material and the SPE. Zwitterionic additives proved to enhance the electrochemical stability of the SPE and to facilitate improved galvanostatic cycling stability in half-cells with NMC by preventing the decomposition of LiTFSI at the polymer-cathode interface, as indicated by X-ray photoelectron spectroscopy (XPS).

The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi0.33Mn0.33Co0.33O2 (NMC111), and LiNi0.5Mn1.5O4 (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metal–Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111-Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (4.7 V vs Li+/Li).

The objective of this report was to implement battery cycling and an intermittent current interruption (ICI) method for determining battery resistance into a simple lithium-ion battery model in the finite element methods (FEM) program COMSOL Multiphysics, andevaluate how accurately the model reflects the behaviour of voltage and internal resistance with respect to experimental results. The ICI technique consists of repeating the steps of first having a longer charging period and then having a short current interruption, where the internal resistance is calculated from the voltage drop that occurs when the current is turned off. The model was evaluated against measurements, made with the same technique (ICI), on assembled NMC-graphite batteries. Codes written in the statistical programming language “R” were used to process the data from both COMSOL and the experiments. Both the batteries and the model were constructed with a reference electrode, to enable measurement of each electrode by itself. The results as documented in this report show that it is possible to simulate the measurement technique in COMSOL, but that both the resistance and voltage profiles differed quite a lot from the behaviour of the tested batteries. The resistance of the positive electrode did however give good results and it was possible to improve the model by changing some parameters. The magnitude of the resistance, which was already quite close, could be improved by changing the porosity and particle size, and the voltage profiles were improved when using voltage-data achieved from the real measurements.