Inverted vs. upright : how placement orientation governs thermal runaway behavior in overcharged prismatic lithium-ion batteries

Abstract

Lithium-ion batteries are typically installed either upright or inverted, and the selected placement orientation may significantly impact their failure behavior. The present study conducted overcharge experiments to systematically investigate the effects of battery orientation on thermal runaway phenomena. Multidimensional signals, including expansion force, voltage, temperature, and gas, were comprehensively analyzed during the thermal runaway process under two placement conditions. The results demonstrated that inverted batteries might exhibit poorer safety performance than upright batteries. Specifically, under overcharge conditions, inverted batteries exhibited more severe electrolyte leakage following the safety valve opening, leading to two significant adverse effects: (1) The reduction in lithium-ion migration pathways intensified plating at the anode, which further promoted the formation of lithium dendrites and potentially led to separator penetration; (2) Rapid side reactions of plated lithium dendrites generate substantial heat, raising the cell temperature and promoting separator shrinkage during the overcharge process. These factors collectively accelerated the onset of internal short circuits, thereby advancing the occurrence of thermal runaway. At a 1/3C charging rate, the internal resistance of inverted batteries was 799.76 % higher than that of their upright counterparts 30 min after the safety valve opened. Furthermore, internal short-circuiting and thermal runaway occurred at SOC thresholds that were 5.07 % and 4.53 % lower than those of the upright case. Overall, this study provides practical guidance for designing safe batteries with separate pole and valve arrangements and the placement direction of battery packs in engineering applications.