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Recharge Cycle # Fig. 10. Plot of the number of duty cycles vs. the number of recharge cycles at various temperatures. Cells were charged at 120 mA to 2.4 V at 21~ during the recharge cycle for cells discharged at 21 and -20~ and to 2.1 V for the ceils discharged at 55~ Cells were discharged to a voltage limit of 1.5 V except as noted. (~) Cell discharged to 1.1 V at 21~ on the previous cycle. (.~ Cell discharged to 1.1 V at 21~ on this cycle.Capacity fade of cells operated at 21~ was much more rapid than at 55~ The shape of this fade curve is similar to that found previously for the Molicel TM discharged at high rates, greater than 300 mA (7). This is thought to be due to a conversion of high-capacity B-phase material of the cathode to a lower capacity m-phase at charge voltages greater than 2.1 V. Discharge results in reconversion of the s-phase to B-phase. The degree of reconversion is a function of the discharge current, the temperature of discharge, and the voltage the cell is discharged to. Discharge to 1.1 V at low current at higher temperatures maximizes this reconversion. This thesis is supported by the fact that discharging a cell to 1.1 V resulted in increased capacity. This is shown in Fig. 10 by the point marked (~ where on cycle 37 the cell was discharged to 1.1 V and the resulting number of duty cycles on recharge cycle 38 was increased.Capacity fade for cells at -20~ was more rapid than that for cells at higher temperatures, a-phase is formed on recharge to 2.4 V at 21~ However, at -20~ larger cell overpotentials than those at 21~ results in less reconversion of a-phase to B-phase. This is because the cathode does not 1569 get to as low a potential for a cell discharged to a cell voltage of 1.5 V as it does for cells discharged at higher temperatures. Cell capacity increase due to a-phase reconverted roB-phase can be seen in Fig. 10 at the point marked (.~. At cycle 35 the cell was discharged at 21~ to 1.1 V. On cycle 36 an increased number of duty cycles were recorded. ConclusionsCell rate performance, cell cycling, and most importantly CDLS tests have shown that X-75 cells can provide the necessary energy requirements for autofocus cameras. In particular, the electrolyte, 0.75M LiPF~ in 75/25 v/v 2MeTHF/EC, allowed over 75 h of camera operation for the life of the cell at -20~ This was wit]h an average of 1.5 h of camera operation per cell recharge. At ambient and higher temperatures even greater cell performance was achieved, e.g., at 21~ over 189 h and at 55~ 285 h of camera operation per cell life were obtained with over 3 h of use before cell recharging was required. Cell performance such as this should certainly meet the needs of even the most demanding photographers. However, it would be necessary that a full set of safety tests be completed before X-75 cells could be placed into this consumer application.

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