锂硫电池外特性及其机理研究

发布时间：2018-04-02 22:01

本文选题：锂硫电池　切入点：自放电特性　出处：《青岛科技大学》2017年硕士论文

【摘要】：锂硫电池是目前已知采用固态材料为活性物质的比能量最高的二次电池体系。然而,多硫化物穿梭效应导致的容量衰减快、库仑效率低、自放电率高及产热等问题限制了锂硫电池的实用化进程。本文通过对锂硫软包电池的放电深度(DOD),环境温度,搁置时间及放电倍率等因素进行控制,测试并分析电池自放电特性、产热特性及内阻特性等外特性及其规律,并进一步通过原位四电极法分析了电池循环过程中内部电解液电导率变化情况,探究影响锂硫电池特性的影响因素和相关机制。实验结果表明,锂硫电池的自放电特性和其电化学反应机制与过程产物密切相关。未循环过的锂硫电池因其没有生成多硫化物而几乎没有自放电行为,已循环过的锂硫电池在不同的放电深度下具有不同的自放电特性。在放电深度(DOD)为0时,自放电最大,随着放电深度增加,自放电率不断减小直到低电压平台处自放电较为平稳,并在放电末尾处自放电最小。此外,自放电与环境温度、搁置时间皆呈正相关性。在低温及放电末尾处贮存已循环过的锂硫电池受自放电行为的影响更小。对锂硫电池充放电产热特性研究结果表明:在大电流充放电时,表面不同位置会产生不同程度的温度变化,电池靠近极耳的中上部区域较开始时温升最大;随着循环次数增多、环境温度降低、充放电电流增大,电池的最大温升变大;在一次循环中,前期循环时放电时温升较大,最易出现最大温升处为DOD30%阶段,随着循环次数增多,充电温升超过放电温升,充电结束时成最大温升处;在一定温度范围内,较高温度对电池的热稳定性有积极作用,随着温度的升高,此时电池内部极化小,充电时最大温升更稳定。通过阶跃法和阻抗法测试了锂硫电池内阻特性,对比发现锂硫电池内阻变化规律与电池表面温度的变化规律有很好的吻合度,电池内阻的增加或降低直接影响电池温度的变化。原位四电极法测试研究发现,电池内部电解液阻抗放电时随着放电深度的增加而增加,在30%DOD时达到最高值,此阶段对应于高价态多硫化物的生成以及向低价态多硫化物的转化,电解液中多硫化物浓度不断增加并达到最高值。当继续放电时之后阻抗降低;对应于低价态多硫化物向Li2S2和Li2S的转化,可溶性多硫化物的浓度不断降低。随着循环次数的增多,在相同DOD条件下电解液的阻抗增加,作者认为在循环过程中电解液溶剂被消耗,以及多硫化物的不可逆溶解导致电池电解液阻抗的增加。
[Abstract]:Lithium-sulfur batteries are known to use solid materials as active substances with the highest specific energy of the secondary battery system.However, the problems such as fast capacity attenuation, low Coulomb efficiency, high self-discharge rate and heat production due to polysulfide shuttle effect limit the practical process of lithium-sulfur batteries.In this paper, the discharge depth, ambient temperature, shelving time and discharge rate of lithium-sulfur soft-clad battery are controlled, and the characteristics of self-discharge, heat generation and internal resistance of the battery are tested and analyzed.Furthermore, the change of electrolyte conductivity during battery cycle was analyzed by in situ four-electrode method, and the influencing factors and related mechanisms were explored.The experimental results show that the self-discharge characteristics and electrochemical reaction mechanism of lithium-sulfur batteries are closely related to the process products.The uncirculated lithium-sulfur battery has almost no self-discharge behavior because it does not produce polysulfide. The circulating lithium-sulfur battery has different self-discharge characteristics at different discharge depths.When the discharge depth (DOD) is 0, the self-discharge is maximum. With the increase of discharge depth, the self-discharge rate decreases continuously until the self-discharge at the low voltage platform is more stable, and the self-discharge at the end of the discharge is the smallest.In addition, self-discharge is positively correlated with ambient temperature and shelving time.At low temperature and at the end of discharge, the effect of self discharge behavior on the storage of circulating lithium sulfur batteries is less than that at the end of discharge.The results of the study on the thermal characteristics of lithium-sulfur battery show that when the charge and discharge are high, the temperature changes in different positions of the surface will be different, and the temperature rise in the middle and upper part of the battery near the polar ear will be the highest at the beginning, and with the increase of cycle times,The maximum temperature rise of the battery increases with the decrease of ambient temperature and the increase of charge / discharge current. In the first cycle, the maximum temperature rise during the early cycle is larger, and the maximum temperature rise is at the DOD 30% stage, with the increase of the cycle times.The temperature rise of charge exceeds the temperature of discharge and becomes the maximum temperature rise at the end of charging. In a certain temperature range, higher temperature has a positive effect on the thermal stability of the battery. With the increase of temperature, the internal polarization of the battery is small.Maximum temperature rise during charging is more stable.The internal resistance characteristics of lithium-sulfur battery were measured by step method and impedance method. It was found that the variation of internal resistance of lithium-sulfur battery was in good agreement with the change of cell surface temperature, and the increase or decrease of internal resistance of battery directly affected the change of battery temperature.In situ four-electrode method, it was found that the impedance discharge of electrolyte increased with the increase of discharge depth and reached the highest value at 30%DOD, which corresponds to the formation of high-valence polysulfide and the transition to low-valence polysulfide.The concentration of polysulfide in electrolyte is increasing and reaching the highest value.When the discharge continues, the impedance decreases, and the concentration of soluble polysulfide decreases continuously, corresponding to the conversion of low valence polysulfide to Li2S2 and Li2S.With the increase of the number of cycles, the impedance of the electrolyte increases under the same DOD conditions. The authors believe that the electrolyte solvent is consumed during the cycle and the irreversible dissolution of polysulfide leads to the increase of the impedance of the electrolyte.
【学位授予单位】：青岛科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM912

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