The aging of lithium ion batteries in off-grid photovoltaic (PV) energy systems is evaluated. Off-grid PV systems can improve their reliability and efficiency by storing the excess of energy produced during sunny days and using it when no other source of energy is available. Due to its high energy density, high efficiency, and constantly drop in prices, lithium ion batteries are the most suitable option to be integrated in the system as energy storage [1]. Although the impressive features, lithium ion batteries properties need to be studied further in order to achieve more efficient renewable energy systems [2]. One of the determinant property to be evaluated is the lifetime of the lithium ion battery, which is determined by aging factors [3]. In this work we have studied the capacity fade and impedance increase as aging factors.

State of charge (SOC) profiles corresponding to most common applications found in off-grid PV-systems were used to cycle NCA/graphite cylindrical for 8 months in the laboratory. Four SOC ranges were used to cycle the cells; Low ΔSOC (20% to 50%), middle ΔSOC (35% to 65%), high ΔSOC (65% to 95%), and and full ΔSOC (20% to 95%). Electrochemical techniques were used to characterize both capacity fade and impedance increase in full cells. Discharge at C/25 rate was performed to measure the capacity every 100 cycles. Impedance increase due to the solid electrolyte interface (SEI) formation and other unwanted process was determined by performing electrochemical impedance spectroscopy (EIS) measurements along with hybrid power pulse characterization techniques. Half cells and symmetrical cells were built to identify aging process on the NCA and graphite electrodes independently with the same electrochemical techniques described above.

From Figure 1-a) we can observe a relatively large capacity fade of the C/25 discharge capacity on cells cycled at high ΔSOC, and almost similar behavior was observed in cells cycled at full ΔSOC. Whereas, for cells cycled at low ΔSOC and middle ΔSOC the capacity fade is small in comparison. Comparison between dV/dQ curves show a shifting and growing of peaks as the number of cycles increase. These changes are more evident in cells cycled at high ΔSOC and full ΔSOC, Figure 1-b). The EIS measurements show a relatively large increase of impedance in the Nyquist plot for the cells cycled at high ΔSOC and full ΔSOC, along with a formation of a second semicircle at the mid frequency range, which is also observed for cells cycled at low ΔSOC and middle ΔSOC, Figure 1-c).

Bibliography

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[2] Y. Zhang, A. Lundblad, P. E. Campana, F. Benavente, and J. Yan, “Battery sizing and rule-based operation of grid-connected photovoltaic-battery system: A case study in Sweden,” Energy Convers. Manag., vol. 133, pp. 249–263, 2017.

[3] M. Klett, R. Eriksson, J. Groot, P. Svens, K. Ciosek Högström, R. W. Lindström, H. Berg, T. Gustafson, G. Lindbergh, and K. Edström, “Non-uniform aging of cycled commercial LiFePO4//graphite cylindrical cells revealed by post-mortem analysis,” J. Power Sources, vol. 257, pp. 126–137, 2014.