Effect of 12 Crown 4 on the Electrochemical Intercalation of Lithium into Graphite

Abstract

The cathodic decomposition of 1M LiCIO4 PC/EC electrolyte in presence of various amounts of 12 crown 4 during the first lithiation of graphite is investigated galvanostatically. The decomposition reaction is greatly reduced after addition of 12 crown 4 and reaches a limit at an optimum 12 crown 4 concentration of -0.35M. The reduction of electrolyte deeomposition after addition of 12 crown 4 is attributed to a significant decrease in gas evolution and possibly to the formation of a more stable SEI film.In the last decade, much research effort has been focused on the search for suitable carbon materials as an alternative anode for lithium rechargeable batteries. Since graphite has the highest theoretical capacity of all carbons known today it is a desirable candidate for the anode. The theoretical capacity of a graphite anode is 372 mAh g-i based on the stoichiometry of LiC6. 1 A major problem in using graphite as the anode is the massive electrolyte decomposition during the first lithiation process, at least in a propylene carbonate (PC) based electrolyte. 24 This necessitates the presence of both extra lithium and electrolyte sources in the cell and reduces the apparent capacity. In our previous study, 4 we proposed a mechanism for electrolyte decomposition in 1M LiCIO4 propylene carbonate/ethylene carbonate (PC/EC) (50/50) electrolyte. The mechanism involved two types of process: oneelectron reduction of PC or EC to form SEI film (Eq. 3) and two electron reduction of PC or EC to form propylene or ethylene gas (Eq. 2 and 5).The reduction scheme for propylene carbonate is[1]Electrochemical, I e, Li + [PCmLi+] -~ (m -1) PC + propylene (g) + Li2CO3[2] Radical termination [PCmLi+] -~ Li alkyl carbonates (SEI) [3] Chemical, 1 e mPC + Li + --> [PCmLi +] + nC > [PCmLi+Cn] [4] Chemical, le, Li + [PCmLi § > propylene (g) + Li2CO3 + nC + (m -1)PC [5]Recently, we 4 and others 5 showed that crown ethers are good reagents for suppressing the electrolyte decomposition. It was suggested, ~ although without any experimental evidence, that the excessive electrolyte decomposition was the result of the creation of new carbon surfaces due to the exfoliation caused by co-intercalation of the solvent molecules such as PC. Crown ethers prevent co-intercalation of PC and hence the massive electrolyte decomposition.We report here our preliminary studies of the effect of 12 crown 4 on the electrolyte decomposition in terms of 12 crown 4 concentrations and reaction pathway. ExperimentalThe electrolyte was 1M LiCLO4 (Baker) dissolved in a 50/50 mixture of PC (Aldrich) and EC (Aldrich). It was prepared as described earlier. 4 12 crown 4 (Aldrich) was purified by first drying over activated molecular sieves for 24 h followed by distillation. The molecular sieves (type 4A, Anachemia) were activated by heating them in a vacuum oven at 200~ for two days. The water content of the electrolytes was measured using Karl-Fisher titration and was 15 ppm for 1M LiCIO4 in PC/EC and 50 ppm for 12 crown 4.The carbon electrode was made of KS15 (Lonza) arti...

Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries

“…378 The authors suggested that, unlike most of the additives that obey the empirical rule of the LUMO energy level, a crown ether might not be chemically involved in the formation of the SEI but rather affect this process indirectly by means of preferential solvation of lithium ions. Thus, the exclusion of PC molecules from the solvation sheath rather than the reductive decomposition of crown ethers is responsible for the reduced irreversible process at 0.80 V. 398 Their hypothesis was confirmed by the work of Aurbach and co-workers, who performed detailed FT-IR investigations on the graphitic anode surface that was cycled in electrolytes containing 12-crown-4. 250 Since the peaks corresponding to the possible reduction products of 12-crown-4 were absent in the spectra, they concluded that the effect of crown ether was not due to its participation in the buildup of the SEI.…”

Electrochemical Scanning Tunneling Microscopy Observation of Highly Oriented Pyrolytic Graphite Surface Reactions in an Ethylene Carbonate-Based Electrolyte Solution

“…Graphite materials have been extensively studied for use as negative electrodes in secondary lithium batteries. − At the graphite electrode, lithium ions are intercalated into graphite upon charging and deintercalated upon discharging. The phase transition mechanism upon electrochemical Li intercalation has been investigated using X-ray diffraction 6-9 and Raman spectroscopy …”

Enhancement of photocurrent generation and open circuit voltage in dye-sensitized solar cells using Li+ trapping species in the gel electrolyte

“…Polymer electrolytes containing crown ethers are usually employed in secondary lithium batteries. In general, the addition of crown ethers has been demonstrated to improve the ionic conductivity at low concentrations affecting the rate of the Li electrode reaction [24][25][26] Shi et al employed an electrolyte containing the ionic liquid 1,2-dimethyl-3-propylimidazolium iodide and 18-crown-6 ether in DSSC. The devices containing crown-ethers exhibited a small enhancement in the short-circuit photocurrent.…”