A 90 x – 180 y pulse echo sequence was used to minimize the probe background signal. 19F MAS NMR measurements were conducted with a 300 MHz Varian S Direct Digital Drive NMR spectrometer, (the 300 MHz spectrometer was used because of the availability of higher spinning speeds) at spinning speeds from 32 KHz to 37 KHz in 1.6 mm rotors. Tetramethylsilane (TMS) was used as the external reference.ġ9F MAS NMR (spin 1/2), is relatively commonly measured, yields strong signals and has a wide chemical shift range. Samples were packed in 4mm rotors and spun up to 12 - 14 KHz. A Varian Inova 500 MHz spectrometer and Doty low C/F background probe were used to perform 13C MAS NMR measurements. Because of low natural abundance and generally long spin-lattice relaxation time (T 1), spectrum acquisition on samples which have not been experimentally enriched in 13C takes a long time. Li/CF x experimental cells were constructed with the three types of CF x and they were discharged to various depth of discharge (DOD) under a 6 month discharge rate at 37☌.ġ3C MAS NMR (spin 1/2), is widely used, despite its relative paucity in naturally-occurring carbon (approximately 1%). CF x cathode mixes were prepared with raw CF x, conductive carbon, and binder materials. Unfortunately no further details were provided by the vendor. This study is focused on NMR investigations of these three different types of CF x materials to determine their structural/chemical differences, discharge mechanism and possible intermediate compound formation during discharge.ĬF x materials (F, C and G types) were prepared by fluorination of fibrous, amorphous coke and standard graphite at 300 - 600 ☌ respectively provided by commercial sources. Although the fundamentals of the electrochemical discharging process are known, the structure of the CF x cathode during the discharge and the mechanism of the defluorination process at the molecular level that might be responsible for the electrochemical differences are not known. At the same time LiF is formed and precipitates out of the structure during the course of discharge.Įlectrochemical studies have shown significant differences between each type of CF x: prepared from carbon with a fiber morphology, hereafter referred to CF x - F or F-type prepared from petroleum coke, hereafter referred to as CF x – C or C-type and prepared from graphite, hereafter referred to as CF x – G, or G-type. The CF x is converted into elemental carbon which is more conductive than CF x thereby lowering the internal impedance, improving the voltage regulation and the efficiency. NLi + (CF x) n → nLiF + nC(Overall reaction)
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