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Large-scale nickel-manganese-cobalt/lithium manganese oxide (NMC/LMO):graphite pouch cells from the General Motors Chevrolet Volt program were tested on Dalhousie's Ultra-High Precision Cycler after being stored for 2 years at 30% state of charge. The difference in the amount of charge that the cells required to reach the end of charge from cycle to cycle, termed charge end point capacity slippage, was initially very high, but became very low after a few charge-discharge cycles. After the stabilization period, the charge end point capacity slippage rates were found to be relatively insensitive to temperature changes. Yet the capacities of the cells continued to fade at an increasing rate with increasing cell temperature. This capacity fade modeled well with respect to the square root of time (t 1/2 ), consistent with the solid electrolyte interface (SEI) thickness growth model on the negative electrode. An Arrhenius relationship was fitted for the SEI growth and used to predict the life dependency on temperature. The potential cell failure modes of these cells were compared with those found for large-format NMC442/graphite pouch cells from another automotive supplier.

Twenty LiCoO 2 /Graphite cells with ages between 0 and 12 years having different cycling histories but identical chemistry and construction were obtained from Medtronic Inc. These cells presented a unique opportunity to learn about aging mechanisms in Liion cells. The cells were studied using the ultra-high precision chargers (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS) and differential voltage versus capacity (dV/dQ vs Q) measurements. Even after 12 years of operation at 37 • C, the cells still retained 80% of their initial capacity and their coulombic efficiency was 0.99985 when measured at C/20 and 40 • C. The capacity loss of the aged cells could be explained by loss of lithium inventory through growth of the solid electrolyte interphase (SEI) at the negative electrode which led to impedance increase. There is no evidence of any active material loss due to electrical disconnect in these cells suggesting the cells have excellent electrodes. A low upper cutoff voltage (4.075 V) is crucial to the long lifetime of these cells due to electrolyte oxidation reactions at the positive electrode, revealed by the UHPC experiments. Comparing similar cells that were cycled to either 4.075 or 4.175 V showed that those cycled to higher voltages under the same conditions failed significantly sooner. These results suggest that electrolyte additives that reduce electrolyte oxidation at the positive electrode, will prolong the cycle and calendar life of these Li-ion cells.

A Li-ion battery anode based on few-layer graphene flakes and ultra-small Si nanoparticles shows a remarkable stability during cycling (0.04% capacity fading per cycle). Our approach offers a viable approach to develop new generation Li-ion battery anodes.