Studies on charging lithium-ion cells at low temperatures

The charge and discharge voltage profiles obtained with the conventional constant current - constant voltage charging and pulse charging protocols at low temperatures were studied. There are two peaks in the charge and subsequent discharge voltage profiles, which suggests that lithium plating can occur in two stages during the low-temperature charging. The first stage may occur shortly after the charging at - 20 degrees C while the second stage occurs in the later part of charging. The lithium deposited on the negative surface during the low-temperature charging can rapidly diffuse into the graphite negative at room temperature while it cannot diffuse into the graphite negative at - 20 degrees C. It appears that the lithium solid diffusion in the graphite negative is the ultimate rate-limiting factor for charging lithium-ion cells at low temperatures. The effects on cell properties such as cell capacity, rate capability, impedance, and cycle life from low-temperature charging with different charging protocols were also studied. Directions for improving the low-temperature charge capability of the lithium-ion cells and designing low-temperature charging protocols are discussed. (C) 2006 The Electrochemical Society.