Improving Cycle Life through Fast Formation Using a Superconcentrated Phosphonium Based Ionic Liquid Electrolyte for Anode-Free and Lithium Metal Batteries

Abstract

Cell formation of lithium-ion cells impacts the evolution of the solid electrolyte interphase (SEI) and the cell cycle stability. Lithium metal anodes are an important step in the development of high energy density batteries owing to the high theoretical specific capacity of lithium metal. However, most lithium metal battery research has used a conventional lithium-ion formation protocol; this is time-consuming and costly and does not account for the different properties of the lithium metal electrode. Here, we have used a recently reported promising phosphonium bis­(fluorosulfonyl)­imide ionic liquid electrolyte coupled with an NMC622 high areal capacity cathode (>3.5 mAh/cm2) to investigate the effect of cell formation rates. A faster formation protocol comprised of a pulsed 1.25C current decreased the formation time by 56% and gave a 38% greater capacity retention after 50 cycles when compared to formation at C/20. Electrochemical impedance spectroscopy measurements showed that the fast formation gave rise to a lower resistance SEI. Column-like lithium deposits with reduced porous lithium domains between the particles were observed using scanning electron microscope imaging. To underline the excellent performance of these high energy density cells, a 56% greater stack specific energy was achieved compared to the analogous graphite-based lithium-ion cell chemistries.

Stable and Efficient Lithium Metal Anode Cycling through Understanding the Effects of Electrolyte Composition and Electrode Preconditioning

“…However, a high negative surface charge of −14.4 μC/cm 2 significantly stabilizes Li + and also decreases the FSI content, making the Li x FSI y ( x > y ) aggregates positively charged as a whole, and [C3mpyr] + and DME tend to be pushed away toward the bulk phase. This IL cation-to-Li x FSI y ( x > y ) aggregate replacement at high negative electrode polarization was previously seen for other superconcentrated IL electrolytes, which explains the increase of inorganic SEI components upon high current density preconditioning. , Moreover, we have also found that IL cation chemistry (e.g., phosphonium vs pyrrolidinium) may affect this process, making this replacement easier in an electrolyte with the weaker IL cation–surface and IL cation–other ions interactions, such as those ILs containing small phosphonium [P 1222 ] + (Figures S9 and S11–S13), as shown in the previous work . The detailed coordination analysis of lithium is discussed next.…”

“…8,45 Moreover, we have also found that IL cation chemistry (e.g., phosphonium vs pyrrolidinium) may affect this process, making this replacement easier in an electrolyte with the weaker IL cation−surface and IL cation− other ions interactions, such as those ILs containing small phosphonium [P 1222 ] + (Figures S9 and S11−S13), as shown in the previous work. 45 The detailed coordination analysis of lithium is discussed next.…”

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

“…The electrode potential varies in a working battery, so it is also worthwhile to explore the sensitivity of interfacial structures to the electrode potential. ,,− As mentioned previously, the CPMs model the electrode potential, electrode polarization, and charge distribution more reasonably than FCMs because the charge fluctuation of the electrode caused by the electrolyte is considered in CPMs. The CPM was first developed by Reed et al and further revised by Gingrich and Wilson. ,, In this method, electrode atoms carry a Gaussian charge distribution and the derivative of the electrostatic energy of the system with respect to electrode charges leads to the applied potential.…”

Interphase control for high performance lithium metal batteries using ether aided ionic liquid electrolyte

“…The mass of electrolyte was not included in the calculation to allow the comparison with literature reports. 27,40 The stack specific energy was determined to be 480 W h kg À1 and the energy density was 798 W h L À1 . The cell parameters are tabulated in Table S2 (ESI †).…”