Surface Active Sites: An Important Factor Affecting the Sensitivity of Carbon Anode Material toward Humidity

Abstract

We report that various kinds of active sites on graphite surface including active hydrophilic sites markedly affect the electrochemical performance of graphite anodes for lithium-ion batteries under different humidity conditions. After depositing metals such as Ag and Cu by immersion and heat-treating, these active sites on the graphite surface were removed or covered and its electrochemical performance under the high humidity conditions was markedly improved. This suggests that lithium-ion batteries can be assembled under less strict conditions and provides a valuable direction toward lowering the manufacturing cost for lithium-ion batteries.To date, many kinds of anode materials for lithium-ion batteries have been studied. 1,2 However, graphitic carbons are still primarily available on the market, and this shows the importance of carbonaceous materials. A current research focus on the carbonaceous anode materials for lithium-ion batteries is the modification of graphite by, e.g., mild oxidation, deposition of metals or their oxides, coating with polymers, and other kinds of carbons on the graphite surface. 1-12 Nevertheless, few studies are devoted to investigate the sensitivity of anode materials towards humidity. 13,14 It has long been realized that the electrochemical performance of carbon anode materials is more sensitive to humidity than that of cathodic ones. When the humidity is high, anode materials easily absorb water resulting in rapid fading of the reversible capacity. Unfortunately, the water content in a manufacturing facility is very difficult to maintain at a desirably low level of essentially 0 ppm. If sensitivity of anode materials to humidity can be decreased, the requirements for controlling the atmosphere in the manufacturing unit will be lowered and lithium-ion batteries can be produced under less demanding conditions.In this paper, the sensitivity of graphitic carbon with hydrophilic active sites to humidity was studied, then the graphite surface was modified by covering or removing the surface active sites with deposited metals and the electrochemical performance of the composites prepared under high humidity condition was investigated. ExperimentalA natural graphite from mild oxidation ͑designated as LS17, d 002 3.351 Å, L c 120 Å and average particle size 17 m͒ as described in Ref. 8 was used for this study. Capacity measurements were performed as follows. A mixture of graphitic material and 5 wt % ͑based on the graphite͒ polyvinylidene fluoride ͑binder͒ was pressed into pellets with a diameter of ca. 1 cm. After drying under vacuum at 120°C overnight, the pellets were kept in an argon glove box at different humidities ͑Ͻ100 ppm and about 1000 ppm͒ for 1 h; later they were assembled into model cells under the same humidity conditions as the working electrode. Lithium foil was used as the counter and reference electrode, a solution of 1 M LiClO 4 in EC/ DEC ͑v/v = 3:7͒ as the electrolyte and a homemade porous polypropylene film as the separator. Electrochemical performance was measured g...

The Influence of Different Post‐Drying Procedures on Remaining Water Content and Physical and Electrochemical Properties of Lithium‐Ion Batteries

“…A further reason could be the water adsorption of graphite and especially its surface coating, which is applied to achieve capacities around 360 mAh g −1 . According to Fu et al, active hydrophilic sites at the surface of graphite exist, which adsorb water when exposed to a humidity higher than 1000 ppm . Compared with the separator, the anode shows an even greater water release after all post‐drying procedures, indicating a higher amount of weakly bound water.…”

“…The water, in turn, gets into the electrodes through various paths. Partially, it is already brought into the electrode structures by the initial materials, which store water from their surrounding atmosphere . In addition, water can be added during the liquid processing of the coatings .…”

The Impact of Absorbed Solvent on the Performance of Solid Polymer Electrolytes for Use in Solid-State Lithium Batteries

“…This has not been extensively studied in all-solid-state batteries, so this section will approach this question using data from systems that employ liquid electrolytes. In general, anodes tend to be more severely impacted by the presence of water than cathodes ( Fu et al., 2005 ). This is because when water is absorbed on the surface of the graphite anodes, C-OH sites that disrupt the formation of the SEI layer can be formed ( Fu et al., 2005 ).…”

Electrochemical performance of a novel surface modified spherical graphite as anode material for lithium ion batteries

“…However, co-intercalation of solvents, poor processing performance and poor rate capability due to the common flake shape limit the application of natural graphite. A variety of methods have been employed to tune the properties of natural graphitic materials, such as carbon coating, mild oxidation, deposition of metals and metal oxides and polymers [2][3][4][5][6][7][8]. However, while some progress has been made in each of these areas, only carbon coated natural graphite has been used in devices, and even in the case of carbon coating, it is not easy to control the uniformity of the product since the processing can easily destroy the carbon coating resulting in the separation of the coated film from the surface of natural graphite [8].…”