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Lithium iron phosphate (LiFePO4) is a very attractive positive active material because of features such as large discharge capacity of 160 mAh/g and has demonstrated long life even at high temperatures The current LiFePO4 chemistries, however, have shown to have poor high rate discharge capability. Therefore, in order to improve this chemistry, prototype lithium-ion cells with an improved LiFePO4/Gr electrode system have been developed. We made and evaluated 4Ah cells and 50Ah cells. The cells showed superior discharge performance with a flat voltage profile and maintain capacity retention of 98% even at high currents of 10CA. Furthermore, even after long term cycling at high ambient temperature of 45oC, the cells retain a much higher capacity than conventional LiMn2O4/Gr chemistry. We compared the new chemistry with identically sized conventional LiMn2O4/Gr system cells (same cell case and terminals). The new cells have almost the same charge/discharge energy (Wh) and power capability (W). However, the LiFePO4 cells showed much better life performance, especially at high temperature condition.