Abstract
Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li+/Li0 and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode–electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anion-intercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF6, LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g–1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF6-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g–1. The poor performance in LiPF6-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in LixPFy, POxFy, LixPOyFz, and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI- and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF6-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.
Highlights
The increasing need to harness renewable energy sources has created a necessity for safe and cost-effective energy storage systems that are suited for large-scale stationary use
graphite dual-ion batteries (GDIBs) offer a maximum capacity of ∼140−150 mAh g−1; combining this with a high operational voltage (∼4.5 V vs Li+/Li0) and an optimized cell design results in a competitive energy density ranging from 210 to 260 Wh L−1.4,6 The energy density is lower compared to most Li-ion systems (∼400 Wh L−1), but is compensated by the environmental and cost benefits associated with GDIBs, as the use of expensive transition metal oxides can be eliminated and cheaper resources can be used.[4,6]
The cyclic voltammograms (CV) in Figure 1 revealed the inherent dependence of the reversibility and kinetics of electrochemical anion intercalation on the chemical nature of the anion and its concentration in the electrolyte
Summary
The increasing need to harness renewable energy sources has created a necessity for safe and cost-effective energy storage systems that are suited for large-scale stationary use. Typical examples of battery chemistries employed in such applications include lead-acid, nickel−cadmium, nickel−metal hydride, sodium−sulfur, and redox flow batteries.[1,2] Among the emerging technologies that are considered promising for stationary energy storage are dual-ion batteries (DIBs)[3,4] and in particular graphite dual-ion batteries (GDIBs).[5] GDIBs offer a maximum capacity of ∼140−150 mAh g−1; combining this with a high operational voltage (∼4.5 V vs Li+/Li0) and an optimized cell design results in a competitive energy density ranging from 210 to 260 Wh L−1.4,6 The energy density is lower compared to most Li-ion systems (∼400 Wh L−1), but is compensated by the environmental and cost benefits associated with GDIBs, as the use of expensive transition metal oxides can be eliminated and cheaper resources can be used.[4,6]. The mechanism of charge storage in the positive electrode in GDIBs relies on the reversible intercalation of anions into the layered structure of graphite. Reduction of graphite takes place with concurrent Li+ intercalation according to eq 2
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