A sealed battery is actually a pressure vessel where electrochemical reaction takes place, and the inner pressure would cause the safety problems like leakage, and explosion and burning when the pressure is higher than the tensile strength of the battery shell. Compared with current adopted temperature monitoring method for battery safety management, the inner pressure is much more accuracy and prompt for monitoring the hazards because of the delay of heat transfer from the inner to the surface of the shell. Such inner pressure will certainly cause the deformation of the shell to produce surface tension stress, so the monitoring of such stress plays a signficant role in battery safety management. Herein, the suface stress changes of various lithium ion cells during cycles have been continuously online monitored using non-destructive strain gauges. The adherence of strain gauge to the shell surface has no influence on the electrochemical performance of the tested cells. The strain properties and temperature sensitivity of such strain gauges have been investigated in detail, and temperature correction has been conducted to eliminate the influence of temperature on the strain gauges. The relationship between strain and stress of a 304 stainless steel shell has been analyzed, and the effects of gauge substrate, temperature and state-of-charge (SOC) on the stress changes of Li(Ni_1/3Co_1/3Mn_1/3)O₂ cells have been investigated using different techniques. The surface stress of the open-circuit cells mainly originates from the thermal stress of the case when the ambient temperature ranges from 50 to 70 ºC, but the continuously increasing stress at 80 ºC is from the electrolyte decomposition in addition to the volume expansion of the electrode and thermal stress of the case. Noticeably, SOC has an obvious effect on the surface stress, and there is a residual stress which is closely realted to the capacity fade. The accumulated residue stress of the Li(Ni_1/3Co_1/3Mn_1/3)O₂ cell is 37.8 MPa, while the capacity degradation is 11.3% compared to the initial capacity after 100 cycles at the current rate of 1.0C. Proper fluorine substitution or PEDOT coating can effectively decrease the surface stress and residual stress to enhance the electrochemical performance. The facile method is not only effective for monitoring the stress changes of cathode materials, but for anode materials. In order to improve the electrochemical performance and decrease the volume changes of artificial graphite, the combination of Li₂SiO₃ and Li₂CO₃ have been in situ prepared to co-modify the graphite, and such co-modification retards and hinders the increase of the surface stress during cycles at various current densities. Via the co-modification, the residue stress of graphite decreases 34.4%, while the reversible capacity increases 16.4% after 50 cycles at the current rate of 1.0C. Proper coating by Si, SiO_y and Li₄Ti₅O₁₂ will also decrease the stress properties and increase the electrochemical performance of graphite. Therefore, the non-destructive strain-stress method is an effective way to monitor the strain and stress changes and take precautions against safety problems of current batteries, while some facile modificaiton methods might also be applicable for ameliorating the electrochemical performance and safety properties of other electrode materials for high performance batteries.

References:

1. Yao Xiao, Guixin Wang, Shuo Zhou, Yuhan Sun, Qiang Zhao, Yichao Gong, Tiecheng Lu, Chunhui Luo, Kangping Yan, Enhanced electrochemical performance and decreased strain of graphite anode by Li₂SiO₃ and Li₂CO₃ co-modifying, Electrochimica Acta, 2017, 223: 8-20.

2. Shuo Zhou, Guixin Wang, Yao Xiao, Qian Li, Dongmin Yang, Kangping Yan, Influence of charge status on the stress safety properties of Li(Ni_1/3Co_1/3Mn_1/3)O₂ cells, RSC Advances, 2016, 6, 63378–63389.