近年来锂离子电池产业发展迅速，但其在电滥用、热滥用及机械滥用等情况下极易发生热失控，进而导致火灾事故的发生，安全性问题亟需解决。由于电池在使用、运输等场景下易发生倾斜，造成安全阀放置方向改变，致使电池热失控燃烧行为演变过程更为复杂。因此，本文选取18650型三元锂电池为研究对象，采用自主搭建的锂电池热失控实验台，研究单体及电池模组在不同倾斜角度、荷电状态下的热失控燃烧行为演变规律，揭示电池模组内热失控传播路径及热量传递机制。研究结果为评估电池火灾危险性、制定安全预警策略提供了重要的理论支撑。

    本文研究了不同倾斜条件下单体电池热失控过程中的表面温度、温升速率、质量损失等特征参数变化，掌握了不同倾斜角度、荷电状态对锂电池热失控特征参数的影响规律，确定了单体电池热失控燃烧行为演变规律主要分为四个阶段：安全阀开启、气体释放、火焰喷射及火焰熄灭阶段。研究结果表明，热失控表面温度峰值与荷电状态成正比，其中当表面温度达到140~220 ℃时，电池发生热失控现象；基于对热失控临界条件的分析，得到了倾斜角度与电池安全阀开启时间成反比。通过对比分析电池热失控前后质量变化及残留物形貌特征，确定了随电池荷电状态的逐渐增大，质量损失越多，壳体破损程度越严重；发现了倾斜角度的变化可导致电池热失控时表面易形成熔孔和裂纹，为能量释放提供额外途径。

    分析了单体电池热失控火焰形态及火焰温度等参数的变化，得到了倾斜条件下锂电池燃烧行为的影响规律。研究结果表明，荷电状态为25%的锂电池热失控过程仅向外喷射烟气，其余高于25%的电池热失控过程均产生不同剧烈程度的火花及燃烧现象。随着荷电状态的增加，热失控火焰高度及面积显著增加；而火焰平均高度随倾斜角度的增加呈先减小后上升的趋势。通过对比不同倾斜角度和荷电状态下火焰峰值温度的变化规律，发现火焰温度随垂直距离的增大呈现降低趋势；其中在不同荷电状态下，火羽流温度距喷射口2 cm处温度最高，且火焰峰值温度与荷电状态呈线性增长关系；另外，除荷电状态为25%外，其余荷电状态的火焰峰值温度均远高于铝制外壳660 ℃的熔点；随倾斜角度的增大，火焰峰值温度呈现先降低后上升的趋势。

    分析了电池模组热失控传播现象及燃烧行为，揭示了倾斜条件下电池组热失控传播路径及传递机制。研究结果表明，在相同的加热功率条件下，随着荷电状态的逐渐减小，热失控传播能力减弱，传播时间增加。其中当荷电状态为0%的电池模组不会发生热失控传播，而荷电状态为50%及100%的情况下则会触发相邻电池发生热失控，并形成模组内单体间的热失控传播现象，其传播时间分别为131 s、23 s；不同倾斜条件下，与热源临近的单体电池热失控时间随倾斜角度的增大而减小，而与该电池相邻的电池则呈现出先上升后下降的规律，其中当荷电状态为100%时，竖直向上与竖直向下的电池模组单体间完成热失控传播时间为20 s左右，水平放置的电池模组所需传播时间较长。通过对比分析热失控过程中质量损失变化规律，可知荷电状态为0%时，电池模组未发生热失控传播，而临近热源的单体电池发生了安全阀开启现象，造成5%的质量损失，其他荷电状态（50%和100%）的电池模组发生热失控传播，质量损失分别为24%、41%。不同倾斜角度下，水平放置（90°）的电池模组质量损失最少。通过分析热失控过程中的火焰形态，发现了当满荷电状态的电池模组竖直向上放置时，其热失控火焰高度及面积最大；倾斜角度改变了火焰形态及蔓延方向，从而改变了热失控在模组内的传播。

    In recent years, the lithium-ion battery industry has been developing rapidly, but it is very prone to thermal runaway in the case of electrical, thermal and mechanical abuse, which in turn leads to fire accidents, and the safety problem needs to be solved urgently. As the battery is easy to tilt in use, transport and other scenarios, resulting in a change in the direction of the safety valve, resulting in a more complex evolution of thermal runaway combustion behaviour. Therefore, this paper selects 18650 lithium ternary batteries as the research object, and adopts the independently constructed lithium battery thermal runaway experimental bench to study the evolution of thermal runaway combustion behaviour of single and battery modules at different tilt angles and charge states, and to reveal the thermal runaway propagation paths and heat transfer mechanisms within the battery module. The results of the study provide important theoretical support for assessing the fire risk of batteries and formulating safety warning strategies.

    In this paper, we investigated the changes of surface temperature, critical conditions, mass loss and other characteristic parameters in the thermal runaway process of single battery under different tilt conditions, mastered the influence of different tilt angle and charge state on the characteristic parameters of thermal runaway of Li-ion batteries, and determined the evolution of thermal runaway combustion behaviour of single battery is mainly divided into four phases: battery expansion, gas release, flame jet and flame quenching phase. The results show that the thermal runaway surface temperature peak is proportional to the state of charge, in which when the surface temperature reaches 140~220 ℃, the battery thermal runaway phenomenon; based on the analysis of the critical conditions of thermal runaway, it is obtained that the tilt angle is inversely proportional to the time of opening the safety valve of the battery. By comparing and analysing the mass change and the morphological characteristics of the residue before and after the thermal runaway, it was determined that with the gradual increase of the charge state of the battery, the more mass is lost and the more serious the shell is broken; and it was found that the change of tilt angle can lead to the formation of fusion holes and cracks on the surface of battery during the thermal runaway, which can provide an additional pathway for the release of energy.

    The changes of parameters such as flame pattern and flame temperature of thermal runaway of single battery were analysed, and the influence law of combustion behaviour of lithium battery under tilted conditions was obtained. The results show that the thermal runaway process of lithium batteries with a charge state of 25% only emits smoke to the outside, and the rest of the thermal runaway process of batteries with a charge state higher than 25% produces sparks and combustion phenomena with different degrees of intensity. With the increase of charge state, the height and area of thermal runaway flame increase significantly; and the average flame height decreases and then increases with the increase of tilt angle. By comparing the change rule of peak flame temperature under different tilt angle and charge state, it was found that the flame temperature showed a decreasing trend with the increase of vertical distance; among them, the fire plume temperature was the highest at 2 cm from the injection port under different charge states, and the peak flame temperature showed a linear growth relationship with the charge state; in addition, except for 25% of the charge state, the peak flame temperature of the rest of the charge state was much higher than the flame temperature of the aluminium shell at 660 ℃. The melting point of aluminium shell is 660 ℃; with the increase of the tilt angle, the peak flame temperature shows the trend of decreasing and then increasing, in which the flame temperature is significantly lower than 0° when the tilt angle is 180°.

    The thermal runaway propagation phenomenon and combustion behaviour of the battery module are analysed, and the thermal runaway propagation path and transfer mechanism of the battery module under tilted conditions are revealed. The results show that under the same heating power condition, the thermal runaway propagation ability decreases and the propagation time increases with the gradual decrease of the charge state. The thermal runaway propagation does not occur when the charge state is 0% of battery module, while the charge state of 50% and 100% triggers the thermal runaway of adjacent batteries and forms the thermal runaway propagation phenomenon between monomers in the module, with the propagation time of 131 s and 23 s, respectively; the thermal runaway time of monomer batteries adjacent to the heat source decreases with the increase of the tilt angle in different tilting conditions, while batteries adjacent to the battery show the thermal runaway propagation path and the transmission mechanism. When the charge state is 100%, it takes 23 s to complete the thermal runaway propagation between the vertical upward battery modules, and only 19 s for the vertical downward (180°) module. By comparing and analysing the change rule of the mass loss during the thermal runaway process, it can be seen that when the charge state is 0%, the battery module does not have the thermal runaway propagation, and the safety valve opening phenomenon occurs for the single battery close to the heat source. The safety valve opening phenomenon occurred in the battery near the heat source, resulting in 5% mass loss, and the thermal runaway propagation occurred in the battery module of other charge states (50% and 100%), with mass losses of 24% and 41%, respectively. The battery module placed horizontally (90°) had the least mass loss at different tilt angles. By analysing the flame pattern during thermal runaway, it was found that the height and area of the thermal runaway flame were the largest when the fully charged battery module was placed vertically upwards; the tilt angle changed the flame pattern and the direction of flame propagation, which accelerated the thermal runaway propagation in the module.

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