Investigation of the Lithium Solid Electrolyte Interphase in Vinylene Carbonate Electrolytes Using Cu||LiFePO₄Cells

Abstract

The influence of vinylene carbonate (VC) on the plating/stripping of lithium was investigated using Cu||LiFePO 4 cells. These cells allow for easy fabrication and in-situ generation of lithium, with no excess lithium to influence performance. Addition of VC to the electrolyte improves both capacity retention and efficiency. IR and XPS spectroscopy of the surface of the plated lithium suggests the presence of a significant amount of poly(VC) when the electrolyte (1.2 M LiPF 6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7, vol)) contains 5% of added VC. This suggests employing additives that generate polymeric species on the surface of lithium improves plating/stripping performance in carbonate electrolytes. The plating and stripping of the lithium metal negative electrode in non-aqueous electrolytes has been investigated for decades.1-3 In particular, carbonate solvents have relatively high voltage stability, making them desirable electrolytes for high-energy density lithium batteries.3-6 However, the efficiency of plating/stripping lithium in carbonate electrolytes does not meet requirements for commercial application (> 99.9%). 7,8 It is common to measure the plating/stripping efficiency of lithium by assembling Li||Cu cells. [9][10][11][12][13] In this cell design, a small amount of Li is cycled, with an excess reservoir of lithium present. One limitation of this cell design is the difficulty of controlling the design and construction of the solid electrolyte interphase 14 (SEI) on lithium, as the low reduction potential of the lithium metal electrode present during cell construction will cause immediate reaction with electrolyte upon exposure. Thus, a reaction between the electrolyte and the lithium metal electrode will occur before cycling begins. Further, the excess lithium within the cell can significantly increase the cycle life of the cell making it difficult to compare to commercial cells, with a limited supply of lithium.Contrary to Li||Cu cells, Cu||LiFePO 4 cells have air-stable components, facilitating their processing and assembly. 15,16 Further, the in-situ formation of lithium metal and low reactivity of LiFePO 4 ensures additives under investigation do not react with the electrode surface upon construction and are only reduced upon initial cycling. This affords the possibility for controlled design and construction of the SEI on lithium metal since the reduction of the electrolyte can be controlled by current density, cell potential, and the quantity of lithium plated. Finally, given that there is no excess lithium in Cu||LiFePO 4 cells, any observed improvements in capacity retention, Coulombic efficiency, or impedance should be applicable to other lithium metal based battery systems.Vinylene carbonate (VC) is a well known electrolyte additive for lithium-ion batteries, demonstrating exceptional performance for graphite and several cathode materials. [17][18][19][20][21][22][23][24] Further, the reaction products of VC with lithium have been investigated in detail, using Li||Ni ...

Synergistic Performance of Lithium Difluoro(oxalato)borate and Fluoroethylene Carbonate in Carbonate Electrolytes for Lithium Metal Anodes

“…9 The C1s spectra contain peaks associated with CO 3 at 289.9 eV, C-O at 286.7 eV and C-C/C-H at 285.0 eV consistent with the generation of a combination of ROCO 2 Li and Li 2 CO 3 , as observed by IR spectroscopy. 11,13,18 The O1s spectrum contains a broad beak centered at ∼531.8 eV, consistent with a mixture of C-O and C=O containing species. 11,13,18 A peak for Li 2 O is also observed at 528 eV in the O1s spectrum.…”

“…11,13,18 The O1s spectrum contains a broad beak centered at ∼531.8 eV, consistent with a mixture of C-O and C=O containing species. 11,13,18 A peak for Li 2 O is also observed at 528 eV in the O1s spectrum. 11,13,18 Further, The F1s spectra are very similar, containing peaks at 685 eV and 687 eV consistent with LiF and Li x PF y O z , respectively.…”

Anode‐Free Full Cells: A Pathway to High‐Energy Density Lithium‐Metal Batteries

“…The same group also reported that adding 5% vinylene carbonate (VC) to 1 m LiPF 6 in EC/EMC (3:7, vol) electrolyte led to a significant improvement in the capacity retention of Cu || LFP full cells (showing a LIRR of 95.1%). [ 52 ] The improvement was attributed to the formation of a polymeric SEI layer on the lithium surface.…”

Bisalt ether electrolytes: a pathway towards lithium metal batteries with Ni-rich cathodes

“…41 Repeated deposition and stripping in this manner presents the true reversibility of the electrolyte due to a fixed capacity of lithium in the cell. 71 In this work, AF cells with NMC-622 were used to test all electrolytes (Fig. 7c).…”