Electrochemical and thermal characterization of lithium titanate spinel anode in C–LiFePO4//C–Li4Ti5O12 cells at sub-zero temperatures

C-LiFePO4 and C-Li4Ti5O12 electrodes were studied for their electrochemical performance at sub-zero temperatures with two different electrolytes, conventional and low temperature electrolytes. 18,650-size cylindrical cell using the conventional electrolyte 1 mol L-1 LiPF6 in EC + DEC displayed a discharge capacity of 1.01 Ah, when cycled between 1.0 and 2.8 V, during low 0.1 C charge-discharge rates at 25 degree C with 98%. Same cell with LiPF6 in EC/DEC electrolyte showed a discharge capacity of 0.97 Ah at 10 degree C during low 0.1 C charge-discharge rates and retained more than 75% of rated capacity at 5 C-rate. However, the cell failed at 8 C-rate due to poor ionic conductivity of the electrolyte. The 18,650 cell using the low temperature electrolyte 1 mol L-1 LiPF6 and 0.2 mol L-1 LiFSI in quaternary blend of aliphatic solvents such as PC + MP + EMC + 5% FEC displayed a similar electrochemical performance at 25 degree C as that of the cell with conventional electrolyte, but in contrast showed improved electrochemical performance down to -10 degree C due to improvement in ionic conductivity of the electrolyte. HPPC test performed on the C-LiFePO4//C-Li4Ti5O12 18,650 cell with low temperature electrolyte indicated the cells remain functional between 0 degree C and -10 degree C.