A Post-Mortem Study of Stacked 16 Ah Graphite//LiFePO₄ Pouch Cells Cycled at 5 °C

Herein, the post-mortem study on 16 Ah graphite//LiFePO4 pouch cells is reported. Aiming to understand their failure mechanism, taking place when cycling at low temperature, the analysis of the cell components taken from different portions of the stacks and from different positions in the electrodes, is performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS). Also, the recovered electrodes are used to reassemble half-cells for further cycle tests. The combination of the several techniques detects an inhomogeneous ageing of the electrodes along the stack and from the center to the edge of the electrode, most probably due to differences in the pressure experienced by the electrodes. Interestingly, XPS reveals that more electrolyte decomposition took place at the edge of the electrodes and at the outer part of the cell stack independently of the ageing conditions. Finally, the use of high cycling currents buffers the low temperature detrimental effects, resulting in longer cycle life and less inhomogeneities

Ethylene carbonate-free electrolytes for Li-ion battery: Study of the solid electrolyte interphases formed on graphite anodes

We investigated the Solid Electrolyte Interphase (SEI) formed onto graphite using adiponitrile/dimethyl carbonate-based electrolytes with either lithium difluoro(oxalato)borate (LiDFOB) or lithium bis(fluorosulfonyl)imide (LiFSI), with or without fluoroethylene carbonate (FEC) as SEI-forming additive, by a combination of X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and scanning electron microscopy. Without FEC, LiDFOB leads to a more protective SEI layer than LiFSI. FEC leads to improvements in both cases and, in combination with LiDFOB, allows forming an SEI richer in LiF with an overall impedance lower than without FEC. It also prevents the dissolution of the SEI upon cycling. For the graphite electrodes cycled with the LiFSI electrolytes, the interface is greatly influenced by the presence of FEC. Nevertheless, with or without FEC, the SEI layer thicknesses decreases upon cycling. In presence of FEC though, this effect is mitigated, but localized exfoliation of graphite was observed after 50 cycles

Multisalt chemistry in ion transport and interface of lithium metal polymer batteries

Solvent-free solid-state polymer electrolytes (SPE) that go beyond the barriers like intrinsic low ionic conductivity, slow ion dynamics, and unstable electrode-electrolyte interphase will be fundamental for realizing the next generation of safe and high-performance lithium metal batteries. Hereby, cross-linked solid polymer electrolyte (XSPE) networks based on multisalt chemistry are synthesized using photopolymerization reaction, which outshine the conventional single salt-based XSPEs. By introducing the multisalt chemistry, an enhanced Li+ ion transport (ionic conductivity and short residence time) via anion mediated transfer (AMT) and improved interfacial characteristics (e.g., stable Li|electrolyte interphase, smooth Li-deposition) are demonstrated. Furthermore, a three-times increase in Li+ ion transference number and nearly one order of magnitude increment in diffusion coefficient are achieved. Using theoretical calculations, we propose an AMT-based ion conduction pathway in multisalt-based XSPEs. Besides, the superior electrochemical performance of multisalt-based XSPEs compared to single salt-based polymer electrolytes in Li-metal polymer batteries (LMPB) using C-LiFePO4 and LiNi0.8Co0.15Al0.05O2 cathodes are successfully demonstrated

Adiponitrile-based electrolytes for high voltage, graphite-based Li-ion battery

Operating high voltage lithium-ion batteries (LIB) is still an obstacle due to the limited anodic stability of state-of-the-art alkyl carbonates-based electrolytes which incorporate ethylene carbonate (EC). Thus, we replace here the widely used ethylene carbonate (EC)/dimethyl carbonate (DMC) solvent formulation by adiponitrile (ADN)/DMC (1/1, wt./wt.), to enable room temperature electrolyte formulations with high anodic stabilities. The possibility of operating graphite with 1 M LiDFOB & 1 M LiFSI ADN/DMC (1/1, wt./wt.) without additive is evidenced, with a clear advantage for the LiDFOB electrolyte. The addition of fluoroethylene carbonate (FEC) as a SEI additive results in improved graphite electrode performance in both cases and, less expectedly, in improved anodic stabilities. Cathodes operating above 4.3 V vs Li+/Li have been paired with graphite as well and allowed improved rate capability as compared to graphite half-cells. The safety of the electrolytes versus a charged graphite anode is improved as compared with state-of-the-art, EC-based electrolytes

Compatibility of Various Electrolytes with Cation Disordered Rocksalt Cathodes in Lithium Ion Batteries

Cation disordered rock salt cathode materials have gathered increased research interest over the last couple of years due to their high specific capacity and wide array of element combinations. It is still unclear whether the capacity fading observed for this type of material is solely due to the occurrence of anionic redox reactions and consequent material degradation or also due to the side reactions between the cathode material and the carbonate-based electrolyte. In order to address it, this study compares the differences in electrochemical performance of a rock salt Li1.25Fe0.5Nb0.25O2 cathode and cathode electrolyte interphase (CEI) formation in both lithium metal and lithium ion cells by using a conventional carbonate-based electrolyte and an ionic liquid-based electrolyte. Thereby, the ionic liquid electrolyte promotes capacity retention, whereas the organic carbonate-based electrolyte leads to increased capacity fading and ineffective CEI formation. Severe side reactions between the carbonate-based electrolyte and the cathode material are characterized by poor Coulombic efficiency and result in continuous inner resistance growth, ongoing gas evolution, and the coverage of the cathode surface with electrolyte degradation products like LiF and Li2CO3. This study shows the mismatch of carbonate-based electrolytes with the Li1.25Fe0.5Nb0.25O2 cathode and offers a strategy that can be also applied for the improvement of performance of other disordered rock salt cathode materials

Solid Polymer Electrolytes for Lithium Metal Battery via Thermally Induced Cationic Ring-Opening Polymerization (CROP) with an Insight into the Reaction Mechanism

We report the synthesis of solid polymer electrolytes (SPEs) using a thermally induced and a lithium salt catalyzed cationic ring-opening polymerization (CROP) technique. A synergistic approach using two salts such as lithium tetrafluoroborate-LiBF4 and lithium bis(trifluoromethane sulfonyl)imide-LiTFSI has assured a complete monomer to polymer conversion and fast reaction kinetics during the CROP process. The initiation mechanism of lithium salt-induced CROP is elucidated using molecular dynamic simulation, quantum chemical calculation, real-time FT-Raman spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetry–mass spectrometry analysis techniques. The cross-linked 3D network of ethylene oxide based SPE is prepared without the use of any solvents or external catalysts. In particular, a mixture of poly(ethylene glycol) diglycidyl ether, LiBF4, and LiTFSI in appropriate proportions after a baking process produced a freestanding, flexible, and nontacky film. The synthesized SPEs exhibit low glass transition temperature (0.1 mS cm–1), and excellent oxidation stability (>5.5 V vs Li/Li+). The SPE is polymerized directly onto a carbon-coated LiFePO4 cathode film and successfully cycled in a lithium metal battery configuration at 40 and 60 °C. As evidence, the SPE is galvanostatically cycled against a high-voltage LiNi1/3Mn1/3Co1/3O2 cathode, and the preliminary results indicated exciting characteristics in terms of specific capacity and Coulombic efficiency

Improving the graphite/electrolyte interface in lithium-ion battery for fast charging and low temperature operation: Fluorosulfonyl isocyanate as electrolyte additive

Nowadays, the demand for high energy density, fast-charging and wide-temperature range lithium-ion batteries has increased significantly. The Solid Electrolyte Interphase (SEI) protecting layer, formed at the interface between the graphite anode and the electrolyte is a key parameter for fast kinetics and wide temperature operation, especially to enable fast charge of cells including graphite anodes. In this work, fluorosulfonyl isocyanate (FI) is used as a novel SEI film forming electrolyte additive for graphite anode. Due to its high reduction potential (above 2.8 V vs. Li+/Li), FI is reduced prior to the carbonate-based electrolyte, yielding a conductive SEI on the surface of the graphite. The SEI is made of a thick and protective inorganic inner layer that prevents the growth of the outer organic layer. As a result, the resistance of the graphite/electrolyte interface is dramatically decreased. Therefore, Li/graphite cells with 2 wt% FI exhibit excellent rate performance at room temperature (20 °C) and low temperature (0 °C and −20 °C), compared to those with the reference liquid electrolyte (LP30)

Interactions of cation disordered rocksalt cathodes with various electrolytes

Despite them missing a layered structure, cation disordered rocksalt (DRX) cathode materials have risen in research interest in the last couple of years, not only because of their high specific capacity but also for the possibility to lift the limitation on specific transition metals like Ni, Co and Mn.(1-3) Many of them involve the combination of a redox inactive d0 stabilizing element like Nb.(4, 5) The utilization of anionic redox reaction in this material class is believed to be the cause of the materials poor electrochemical performance, while possible other factors like unfavourable side reactions with commonly used carbonate-based electrolytes or other cell components are less investigated.(6, 7)In order to address this fact, the electrochemical performance as well as the formation of the cathode electrolyte interphase (CEI) of a Li1.25Fe0.5Nb0.25O2 DRX cathode was investigated in Lithium metal and Lithium ion cells. Thereby, the use of a carbonate-based electrolyte and an ionic liquid-based electrolyte were compared. Severe side reactions occur between the cathode material and the carbonate-based electrolyte which leads to a strong capacity fading and poor Coulombic efficiency. An ongoing decomposition of electrolyte components on the cathode materials surface during charge and discharge cycling covers the material with degradation products like Li2CO3 and LiF which causes resistance growth and is accompanied by dangerous gassing in the cells. On the other hand, the ionic liquid electrolyte shows only negligible degradation and can promote capacity retention.(8)The mismatch of carbonate based electrolytes with the Li1.25Fe0.5Nb0.25O2 cathode shown here can be applied to other DRX materials which opens new approaches for improvements in their performance.(8)Reference

A Post-Mortem Study of Stacked 16 Ah Graphite//LiFePO₄ Pouch Cells Cycled at 5 °C

Herein, the post-mortem study on 16 Ah graphite//LiFePO4 pouch cells is reported. Aiming to understand their failure mechanism, taking place when cycling at low temperature, the analysis of the cell components taken from different portions of the stacks and from different positions in the electrodes, is performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS). Also, the recovered electrodes are used to reassemble half-cells for further cycle tests. The combination of the several techniques detects an inhomogeneous ageing of the electrodes along the stack and from the center to the edge of the electrode, most probably due to differences in the pressure experienced by the electrodes. Interestingly, XPS reveals that more electrolyte decomposition took place at the edge of the electrodes and at the outer part of the cell stack independently of the ageing conditions. Finally, the use of high cycling currents buffers the low temperature detrimental effects, resulting in longer cycle life and less inhomogeneities