Low Temperature Research and Improvement on Lithium Ion Batteries

Abstract

Lithium ion batteries have been the dominant power sources in portable electronics and electric vehicles due to their high energy density, long lifetime and environmental benignity. However, it is a big challenge for lithium ion batteries to be used at low temperature. Compared with the performance at room temperature, lithium ion batteries deliver much less capacity and energy at low temperature. The poor low temperature performance restricts the usage of lithium ion batteries for power electronics and electric vehicles in the winter. The results of previous research, suggest that the poor low temperature performance is attributed to the freezing of the liquid electrolyte which leads to low ionic conductivity and high interfacial resistance, especially on the anode side. Other researchers tried using a solvent with low melting point to partially replace ethylene carbonate or adding some additives to the electrolyte to enhance the ionic conductivity or modify the interface to reduce the resistance and then deliver more capacity and energy.In this research, we investigated the discharge capacity, energy, pulse power and electrochemical impedance spectroscopy at different depths of discharge of NCM622/Graphite lithium ion batteries at varied temperatures. We found that the discharge energy and pulse power are sensitive to the temperature decreasing. For the NCN622/Graphite cell (2.5 mAh cm-2) with commonly used LiPF6-EC/EMC electrolyte, at -20 oC the cell is able to deliver 65% capacity of that at 30 oC with C/3 discharge current rate, while the delivered energy is only 60%. The 30 seconds pulse power became much worse at -20 oC compared with the power at room temperature. In our research the poor low temperature performance is not from the low ionic conductivity and interfacial resistance. The dominant factor is the charge transfer impedance from the cathode side of the cell.To improve the low temperature performance, we explored the addition inexpensive nano-particles as the additive into the electrolyte. The cell composed of this electrolyte is able to release more than 70% capacity and energy at -20 oC of that measured at 30 oC. The charge transfer resistance in the fully charge state decreased almost 80% compared to a cell with baseline electrolyte. Meanwhile, the additive has no effect on the long-term cycling (1000 cycles) and stability at high temperature (55 oC) Fig.1. Normalized capacity and energy. Fig.2. The EIS of the full cell at 30 oC and -20 oC. Figure 1