Effect of hydrostatic pressure on electrochemical performance of soft package lithium-ion battery for autonomous underwater vehicles

The soft package lithium-ion battery has been used as AUV (autonomous underwater vehicle) power supply because of its advantages such as high safety, high energy density and low self-discharge rate. However, the discharge mechanism of the cell at high hydrostatic pressure is still not clear. In this paper, the electrochemical performance of cells at different hydrostatic pressures (0.1 MPa, 30 MPa, 60 MPa, 90 MPa) was investigated comprehensively, and galvanostatic intermittent titration technique (GITT) was used to evaluate the lithium ions diffusion rate in the solid phase of the LiNi_0.8Co_0.1Mn_0.1O₂ electrode at different pressures. The internal resistance of the cell was quantitatively measured by electrochemical impedance spectroscopy and the parameter values of each impedance in the equivalent circuit were obtained. The results showed that the high hydrostatic pressure can improve the electrochemical performance of the cell by increasing the diffusion rate of lithium ions in the initial cycle stage. In addition, the hydrostatic pressure alleviates the overpotential loss caused by concentration polarization, and improves the cell closed circuit voltage and discharge capacity. Unfortunately, in the late cycle stage of the test, the cell capacity at high hydrostatic pressure declines seriously and the cell capacity degradation mechanism was further revealed by scanning electron microscopy and ex-situ X-ray diffraction. The prediction model of capacity loss (Q_loss) was established at high hydrostatic pressure based on Arrhenius law. The findings on the electrochemical performance and capacity degradation mechanism at high hydrostatic pressure can provide reference for reliability analysis of the battery pack of deep-sea AUV.