Search

The Li/CFx cell has been traditionally used for low rate applications at discharge rates of C/1000 or lower. The development of cells capable of operating at discharge rates closer to C/10 is underway for communication applications. The Li/CFx cell is known to produce a significant amount of thermal power during high rate discharge and cathode swelling that can result in mechanical deformation of the cell. Previously, we characterized the thermal power during discharge in both coin cells and large format cells and proposed that the discharge is a two phase reaction with the direct formation of LiF. In this paper we measure the mechanical cathode swelling as a function of discharge depth, rate, and temperature. Discharged cathodes are then characterized using X-ray diffraction, scanning electron microscopy, and N2 adsorption surface area measurements. These results support a mechanism where the discharge product is LiF deposited on the internal surfaces of the carbon layers left behind after electrochemical reduction with this deposition leading directly to the measured cathode swelling. The recrystallization of LiF observed by X-ray diffraction over a period of days results in a long time constant thermal power that can be measured after discharge in large format cells. [ABSTRACT FROM AUTHOR]

Abstract: The effect of quaternary ammonium on discharge characteristic of Li/O2 cells was studied by using Super-P carbon as air cathode, a 0.2molkg−1 LiSO3CF3 1:3 (wt.) PC/DME solution as baseline electrolyte, and tetrabutylammonium triflate (NBu4SO3CF3) as an electrolyte additive or a co-salt. Results show that Li/O2 cells can run normally in an electrolyte with NBu4SO3CF3 as the sole conductive salt. However, such cells suffer lower voltage and capacity as compared with those using the lithium ionic baseline electrolyte. This is due to the larger molar volume of quaternary ammonium cation, which results in less deposition of oxygen reduction products on the surface of carbon. When used as an electrolyte additive or a co-salt, the ammonium is shown to increase capacity of Li/O2 cells. The plot of differential capacity versus cell voltage shows that the Li/O2 cell with ammonium added has broad and scatted differential capacity peaks between the voltages of two reactions of “2Li+O2 →Li2O2” and “2Li+Li2O2 →2Li2O”. This phenomenon can be attributed to the phase transfer catalysis (PTC) property of quaternary ammonium on the second reaction. Due to inverse effects of the cation geometric volume and the PTC property of ammonium ions on the discharge capacity, there is an optimum range for the concentration of ammonium. It is shown that the addition of NBu4SO3CF3 increases discharge capacity of Li/O2 cell only when its concentration is in a range from 5mol% to 50mol% vs. the total of Li/ammonium mixed salt, and that the optimum concentration is about 5mol%. In this work we show that the addition of 5mol% NBu4SO3CF3 into the baseline electrolyte can increase discharge capacity of a Li/O2 cell from 732mAhg−1 to 1068mAhg−1 (in reference to the weight of Super-P carbon) when the cell is discharged at 0.2mAcm−2. [Copyright &y& Elsevier]

Abstract: Discharge characteristic of Li/O2 cells was studied using galvanostatic discharge, polarization, and ac-impedance techniques. Results show that the discharge performance of Li/O2 cells is determined mainly by the carbon air electrode, instead by the Li anode. A consecutive polarization experiment shows that impedance of the air electrode is progressively increased with polarization cycle number since the surfaces of the air electrode are gradually covered by discharge products, which prevents oxygen from diffusing to the reaction sites of carbon. Based on this observation, we proposed an electrolyte-catalyst “two-phase reaction zone” model for the catalytic reduction of oxygen in carbon air electrode. According to this model, the best case for electrolyte-filling is that the air electrode is completely wetted while still remaining sufficient pores for fast diffusion of gaseous oxygen. It is shown that an electrolyte-flooded cell suffers low specific capacity and poor power performance due to slow diffusion of the dissolved oxygen in liquid electrolyte. Therefore, the status of electrolyte-filling plays an essential role in determining the specific capacity and power capability of a Li/O2 cell. In addition, we found that at low discharge currents the Li/O2 cell showed two discharge voltage plateaus. The second voltage plateau is attributed to a continuous discharge of Li2O2 into Li2O, and this discharge shows high polarization due to the electrically isolating property of Li2O2. [Copyright &y& Elsevier]

Abstract: Carbothermal treatment was used to improve the discharge rate performance of primary lithium/carbon monofluoride (Li/CF x with x =1) batteries. The treatment was carried out by heating a mixture of CF x and carbon black (CB) just below the decomposition temperature of CF x under nitrogen for 2h. In the treatment, poly(vinylidene fluoride-co-hexafluoropropylene) (Kynar) was used as a fluorinated polymer binder to press the CF x /CB mixture into pellets. It was shown that the content of Kynar significantly affected the discharge performance of the resulting treated-CF x (T-CF x ). This can be attributed to the catalytic effect of HF formed by the pyrolysis of Kynar on the decomposition of CF x and on the reaction of CB with the volatile fluorocarbons formed by the decomposition of CF x . The discharge performance of T-CF x cathode was also affected by the temperature of carbothermal treatment and by the ratio of CF x to CB. In this work the best result was obtained from a treatment conducted at 470°C on a 87CF x /10CB/3Kynar (by weight) mixture. In the discharge condition of C/5 and 20°C, the Li/CF x cell with such-obtained T-CF x cathode showed about 95mV higher voltage than the control cell while retaining nearly the same specific capacity. Impedance analyses indicate that the improved discharge performance is mainly attributed to a reduction in the cell reaction resistance (R cr) that includes an ohmic resistance related to the ionic conductivity of the discharge product shell and a Faradic resistance related to the processes of charge-transfer and Li+ ion diffusion in the CF x reaction zone. [Copyright &y& Elsevier]

Abstract: In this work we demonstrate that the thermal treatment of CF x cathode material just below the decomposition temperature can enhance discharge performance of Li/CF x cells. The performance enhancement becomes more effective when heating a mixture of CF x and citric acid (CA) since CA serves as an extra carbon source. Discharge experiments show that the thermal treatment not only reduces initial voltage delay, but also raises discharge voltage. Whereas the measurement of powder impedance indicates the thermal treatment does not increase electronic conductivity of CF x material. Based on these facts, we propose that the thermal treatment results in a limited decomposition of CF x , which yields a subfluorinated carbon (CF x−δ ), instead of a highly conductive carbon. In the case of CF x /AC mixture, the AC provides extra carbon that reacts with F2 and fluorocarbon radicals generated by the thermal decomposition of CF x to form subfluorinated carbon. The process of thermal treatment is studied by thermogravimetric analysis and X-ray diffraction, and the effect of treatment conditions such as heating temperature, heating time and CF x /CA ratio on the discharge performance of CF x cathode is discussed. As an example, a Li/CF x cell using CF x treated with CA at 500°C under nitrogen for 2h achieved theretical specific capacity when being discharged at C/5. Impedance analysis indicates that the enhanced performance is attributed to a significant reduction in the cell reaction resistance. [Copyright &y& Elsevier]

Abstract: dc-polarization and ac-impedance techniques were used to analyze the discharge characteristic of a primary Li/CF x cell. In most cases, impedance spectrum of a Li/CF x cell shows a suppressed semicircle followed by a sloping straight line. The semicircle is shown to present a cell reaction resistance (R cr), which reflects an ohmic resistance (mainly, ionic conductivity of the discharge product shell) and a charge-transfer process. It is shown that the overall resistance of a Li/CF x cell is dominated by the CF x cathode, whose resistance is further dominated by the R cr that is found to be extremely sensitive to the temperature. Therefore, the low temperature performance and rate capability of a Li/CF x cell are mainly determined by the CF x cathode. In addition, based on the discharge curve and open circuit voltage (OCV) recovery of a Li/CF x cell, we proposed a “core-shell” model consisting of a shrinking “CF x core” and a growing “product shell” for the discharge process of CF x cathode. The “product shell” plays an important role in the discharge performance of Li/CF x cells. [Copyright &y& Elsevier]