Study of Irreversible Capacities for Li Insertion in Hard and Graphitic Carbons

Abstract

The irreversible capacity of carbon electrodes for lithium is a critical parameter which must be minimized in practical Li ion cells. This paper reports studies of the origin of the irreversible capacity in high-capacity carbons derived from sugar, and for comparison, in graphitic carbons made from mesocarbon microbeads. Tablet electrodes (-0.4 mm thick) of sugar carbon or of mesocarbon microbeads (mixed with 10% pitch) were prepared by pyrolysis of the precursors at 1050°C under a vacuum of about 10 mTorr. Tablets exposed to different gases were used in carbon/Li coin cells, in order to study the effect of different gas exposures on irreversible capacities, Crr, for Li insertion. Carbon electrodes exposed only to argon or nitrogen demonstrated a dramatic reduction in irreversible capacity: C,,. 50 and 10 mAh/g for sugar carbon and mesocarbon microbead tablet cells, respectively, compared to C 180 and 30 mAh/g for the corresponding conventional doctor-blade spread electrode cells where the electrodes had been exposed to laboratory air for several days. Tablet electrodes were also exposed to C02, 02, water, steam, and air, respectively, for different periods of time. Tablets exposed to each of these reactive gases showed a dramatic increase in irreversible capacity. The irreversible capacity for hard carbons results from two sources: (i) electrolyte decomposition on nominally clean carbon surfaces and (ii) reactions with surface groups which form on carbons exposed to reactive gases. The amount of irreversible capacity from the latter source depends on the gas exposure time and involves reactions with species such as hydroxyl, carboxyl functional groups, or adsorbed water. lnfroduction Lithium-ion cells currently represent the state of the art in small rechargeable batteries' due to their higher voltage, higher energy density, and longer shelf life compared with conventional rechargeable batteries. The technology is based on the use of suitably chosen lithium intercalation compounds for the electrodes. Usually a lithium transition metal oxide is used for the cathode and a carbonaceous material for the anode. One of the problems arising from the use of carbon anodes is the irreversible reaction during the first discharge or first lithiation. The capacity related to the irreversible reaction is unusable, because Li consumed in the first discharge cannot be reversibly recovered. In commercial Li-ion cells, the loss of lithium is normally compensated by the use of excess cathode material; therefore, the irreversible capacity leads to a decrease in energy density and an increase in cost. Irreversible reactions can also cause evolution of gases.2 This is particularly undesirable when cells are held in thin-walled flat cases, because the generation of gases causes swelling.It is widely recognized that part of the irreversible capacity arises from electrolyte decomposition followed by the formation of a passivation layer2 or solid electrolyte interface (SEI)3 on the carbon surface. Matsumura et al.4 showed that a large ...

Investigation of Pre-lithiation in Graphite and Hard-Carbon Anodes Using Different Lithium Source Structures

“…5), different colors, corresponding to different states of lithiation can be observed, which appear due to the changes in the optical properties of these lithiated compounds as more and more Li is inserted into the graphite crystal lattice. This color transformation feature in graphite is the 'stage formation', which is a stepwise formation of a periodic pattern of unoccupied graphitic layer gaps, which can be described by stage index n (I, II, III, IV), with the Stage-I being the most lithiated state (Stage-n is a sequence of n graphite sheets with one intercalate host layer of Li + ), 29,30 Fig. 6.…”

From an Enhanced Understanding to Commercially Viable Electrodes: The Case of PTCLi4 as Sustainable Organic Lithium‐Ion Anode Material

“…This higher discharge capacity results partially from a slightly higher irreversible capacity at potentials above the potential plateau, which is in good agreement with the recorded CVs (Figures 3c and 6a), indicating that the MWCNTs may trigger the electrolyte decomposition at higher potentials and/or an extended presence of functional surface groups and defects in case of the MWCNTs-1 (see also Figures S4a and S5b, Supporting Information, in comparison). [36,37,42] Nevertheless, also the reversible capacity is substantially higher for the optimized electrode composite containing MWCNTs-1 (≈150 vs 120 mAh g −1 or about 135 vs 110 mAh g −1 , if excluding the carbon additive contribution), accompanied by a greatly full paper increased capacity retention after 200 cycles (90% vs 80%). In addition to the enhanced electrode architecture and realization of a percolating electronically conducting network by using 1D MWCNTs instead of nanospherical SP (see Figures 5 and 4d-f, respectively), we may also assume that the noncovalent interaction between the PTCLi 4 molecules and the MWCNTs, leading to a facilitated electron and charge transfer [45][46][47] along the π-π stacking direction, [31,48] advantageously affects the electrochemical performance of such electrodes.…”

Superhigh capacity and rate capability of high-level nitrogen-doped graphene sheets as anode materials for lithium-ion batteries

“…Both the nitrogen-doped graphene sheets and the graphene sheets have the similar charge/discharge curves [22][23][24]. The presence of the plateau at about 0.9 V in the first cycle could be attributed to the formation of solid electrolyte interface (SEI) film on the surface of the graphene sheets [10,23,38,39]. The first reversible specific capacity of nitrogen-doped graphene sheets is as high as 1123 mAh g −1 , which is much higher than that of the graphene sheets (around 848 mAh g −1 ).…”