Abstract
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).
Highlights
The development of advanced lithium-ion batteries (LiBs), such as generation 3b in the European strategic energy technology (SET) plan,[1] is still of the highest priority for the fastest-growing energy-storage applications, that is, electric vehicles and for large-scale storage and many others
While no salt residues were observed in the electrolyte in the studied concentration range, some negligible number of solid residues could be spotted after several days at the bottom of the glass vial containing the lithium bis(fluorosulfonyl)imide (LiFSI):ethylene carbonate (EC) 1:2 electrolyte
It is well known that oxidation reaction products and transition metals dissolved from the cathode active material may migrate to the anode and be involved in side reactions and solid electrolyte interphase (SEI) formation.[3,7,52−54] In this context, Figures 8d and S10 show that Ni can be detected on the surface of the Si/graphite electrode cycled with the LP40 electrolyte, while its concentration is visibly lower for the LiFSI:EC electrolyte
Summary
The development of advanced lithium-ion batteries (LiBs), such as generation 3b in the European strategic energy technology (SET) plan,[1] is still of the highest priority for the fastest-growing energy-storage applications, that is, electric vehicles and for large-scale storage and many others. The reasons for this performance difference were not investigated further, but may be due to cell resistance increase caused by the deposition of electrolyte decomposition products on different cell components[7] or the loss of contact between the Al current collector and the LNMO particles.[41] The results in Figure S7 indicate that the compatibility of the pure LiFSI:EC 1:2 electrolyte with the high-voltage cathodes has a limit.
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