Ni-Composite Microencapsulated Graphite as the Negative Electrode in Lithium-Ion Batteries I. Initial Irreversible Capacity Study

Abstract

Since Sony introduced the first commercial cell in 1990 that used a carbon material in lieu of lithium metal as the negative electrode, 1 rechargeable Li-ion batteries have become readily available worldwide. Of various carbon materials, graphite is favored because it exhibits a high specific capacity, a most desirable charge potential profile, and superior cycling behavior. However, the irreversible reactions that take place with lithium during the first cycle have been one of the very persistent problems associated with the use of graphite. In commercial Li-ion cells, the loss of lithium due to these irreversible reactions is normally compensated for by using excess cathode material, however, this leads to a decrease in the specific energy density and thus an increase in the cell cost. 2,3 These irreversible reactions can also cause gas evolution, which may result in some very serious safety issues, such as cell can buckling, cell venting, electrolyte spillage, and even fire. 2,4 It is widely known 2-5 that the irreversible capacity can originate from electrolyte decomposition followed by the formation of a solid electrolyte interface (SEI) film on the external surface of the graphite. However, it can also arise from solvated lithium intercalation between the graphene layers and subsequent reduction of the solvent, 6-7 because the formation of a solvated lithium/graphite compound like Li x (sol) y C 6 is more favorable than the formation of Li x C 6 during insertion. 6 The irreversible capacities in both cases are largely dependent on the external surface area of the electrode, 6 because the exposed surface is where the irreversible reactions take place. In fact, it has been shown that the irreversible capacity associated with the edge surfaces of graphite is substantially larger than that associated with the basal plane surfaces due to solvent cointercalation into the layer spacing. 7 The irreversible capacity associated with the use of graphite also varies considerably with different electrolytes, and especially with different solvents. It has been shown 7-9 that the irreversible reactions are worse in propylene carbonate (PC)-based electrolyte than in ethylene carbonate (EC)-based electrolyte, and PC alone can cause severe degradation of the graphite structure by a process called "exfoliation." 7-11 The results by Ogumi et al. 12 indicated that PC begins to decompose at ca. 1 V vs. Li/Li ϩ and continues to decompose during the remainder of the intercalation process. It has further been suggested that this decomposition occurs continuously without forming a stable passive SEI layer on the edge surfaces of the graphite particles. [7][8][9][10][11] The net effect is that the PC solvent continuously cointercalates with Li-ions into the graphene layers and subsequently reduces, which gives rise to a large irreversible capacity during cycling. 9,11 Nevertheless, PC is still attractive for use as an electrolyte in Li-ion batteries, especially for low-temperature operation, because of its high salt solubility and lo...

Discharge characteristics of silver vanadium oxide cathodes

“…In the Equation (1), the faradaic impedance is a linear combination of charge transfer resistance (R CT ) and diffusion impedance (modified Warburg) defined by the first and second terms on the right side of the equation respectively. Separating the modified Warburg impedance into a real part (Z re ) and an imaginary part (Z im ) and then differentiating Z im and Z re gives the slope of the Nyquist plot in the diffusion controlled region as [19] where…”

“…The solution resistance was found by measuring the DC resistance of a T-cell (three electrode cell) with only the electrolyte and current collectors [18][19]. The resistance of lithium metal was estimated by measuring the DC resistance of a T-cell using lithium metal as both working and counter/reference electrodes.…”

“…This was analyzed using a T-cell, a three electrode cell assembly where a foil of pure metallic lithium acts as both working and counter/reference electrodes [18,19]. EIS studies were done on this T-cell and the results are shown in Figure 6b, which shows a small semi-circle in the high-mid frequency regions accounting for the migration of lithium ions through the SEI film formed on the surface of lithium metal [20].…”

“…The first was based on the modified EIS approach developed by Haran et al [26] and later validated by Yu and others [19]. The second was based on the equivalent circuit approach as outlined by Nobili et al [21] and Levi et al [22].…”

The dependence of natural graphite anode performance on electrode density

“…This technique was first proposed by Ho et al [24] to investigate lithium diffusion in tungsten trioxide thin films. From this method, the Warburg prefactor σ is represented following equation [25];…”