Reduction of bis (Oxalato)Borate on a High Surface Area Carbon Electrode
- PDF / 493,394 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 184 Views
1127-T03-11
Reduction of bis(Oxalato)Borate on a High Surface Area Carbon Electrode. John Flynn and Carl Schlaikjer Tracer Technologies Inc., 20 Assembly Square Drive, Somerville, Massachusetts 02145
ABSTRACT Lithium bis(oxalato)borate (LiBOB) has gained widespread interest as an electrolyte salt for lithium ion batteries because of its high conductivity, low cost, thermal stability, and adequate solubility in many organic solvents [1]. Cyclic voltammetric data taken on platinum [2] and carbon [3] indicate electrochemical stability over a wide potential range. We show that bis(oxalato)borate (BOB) can be reduced at about 1.75 volts anodic to lithium, by discharging electrolytes at low current density (0.1 mA/cm2) on high surface area carbon electrodes containing a mixture of acetylene and Ketjen carbon blacks. The evidence includes discharge profiles and 11B NMR data. The behavior of discharge plateaus indicates that BOB is reduced to a soluble species with electrolytic properties, and the appearance of a broad 11 B NMR peak in the electrolyte indicates that the reduced species undergoes extensive exchange. INTRODUCTION The graphite negative electrodes of lithium ion rechargeable cells depend for their satisfactory operation on the formation of an insoluble salt film on the surface of the graphite through which lithium ions may pass, but which prevents the extensive reduction of the solvent. This now familiar “solid electrolyte interphase” (SEI) [4] forms during the first charge cycle in lithium-intercalated graphite negative electrodes. The charge which must be used to form this protective layer represents the irreversible capacity noted during the first charge cycle. In electrolytes based on carbonate esters, the SEI is lithium carbonate, which begins to form as soon as the potential of the graphite electrode falls below about one volt versus lithium [5]. The purpose of this work was to study the electrochemical reduction of BOB on a high surface area carbon electrode, rather than the carbon or metallic electrodes previously studied. The high surface area electrode resembles more closely the carbon negative electrodes used in lithium ion cells. EXPERIMENT LiBOB was prepared in a manner similar to that described by Yu et al. [6]. 21.65 grams (516 mmol) of lithium hydroxide monohydrate (98% reagent), 31.91 grams (516 mmol) of boric acid (99.5+%), and 130.12 grams (1,032 mmol) of oxalic acid dihydrate (99%, reagent), all obtained from Sigma-Aldrich, were used without further purification. The reagents were ground together in a 500 mL crown glass mortar in the air until the mixture turned to a paste. The paste was then heated in a 250 mL Pyrex beaker to 250°C. in an air-filled oven for six hours. During this time, the unreacted oxalic acid, which decomposes at 189°C [7], was removed. The residue
was taken up in Aldrich reagent grade acetone, the remaining unreacted reagents allowed to settle, the supernatant siphoned into a 250 mL roundbottom flask, and the acetone removed using a Büchi Rotovap. The flask was move
Data Loading...
