| 氧化还原媒介体对锂空气电池充放电过程作用机制研究 |
| 其他题名 | 材料工程
|
| 李阳
|
| 导师 | 锂空气电池
; 氧化还原媒介体
; LiI
; 痕量H2O
; 作用机制
|
| 2017-06
|
| 学位授予单位 | 中国科学院大学
|
| 学位授予地点 | 北京
|
| 学位专业 | 高性能动力电池的开发，是发展新能源汽车的重要技术支撑，锂空气电池因具有高的理论能量密度，有望用作未来电动汽车的动力电池。但是，锂空气电池仍然存在库仑效率低，循环性能差等问题。为了解决由电池的充电电压高导致的库仑效率低的问题，研究者在电池的电解液体系中引入可溶性催化剂氧化还原媒介体来促进放电产物Li2O2的分解。碘化锂（LiI）是目前广泛研究且存在争议的氧化还原媒介体。本文针对LiI作有机体系锂空气电池的氧化还原媒介体，探究了当有痕量水（ppm级）存在于电解液中，放电产物LiOH的生成和充电分解机制；对于理想无水电解液体系中，研究探索了充电过程中氧化态I-氧化分解传统锂空气电池放电产物Li2O2的作用机制，并设计了在锂空气电池中具有更高工作效率的I-氧化还原媒介体。研究工作主要分为以下两个方面：
（1）研究发现LiI作氧化还原媒介体的锂空气电池体系中，电解液中H2O含量为200 ppm和2000 ppm时电池的放电产物分别为LiOH和LiOH•H2O。进一步探索了在含H2O的条件下LiOH生成过程，并通过XRD和质谱分析技术研究了LiOH在充电过程中的分解机制，研究发现在充电过程中LiOH会被I2分解，但质谱数据证明LiOH与I2作用并没有O2产生。所以，在电池充电过程中LiOH并不能可逆的分解放出O2，而是LiOH与I2反应生成副产物LiIO3，而且生成的副产物积累在电池内部，严重影响锂空气电池的电化学性能。
（2）在LiI作氧化还原媒介体的理想无水锂空气电池体系中，采用质谱技术研究发现只有I2能够化学氧化分解Li2O2放出O2而I3-不能。同时，研究发现，在充电过程中，电解液中游离的I-或者Li2O2与I2反应生成的I-，很容易与I2结合生成I3-，钝化对Li2O2具有氧化作用的I2，降低Li2O2在充电过程中的分解效率从而影响电池的性能。受此启发，我们通过理论计算发现了化合物LiI(HPN)2，LiI(HPN)2在醚类电解液中具有弱的I-溶剂化作用，阳离子Li(HPN)2+可以作为电解液中I-的捕获剂，有效降低电解液中I-的浓度，从而提高I-在充电过程中向I2的转化效率和Li2O2的分解效率。实验结果表明，用化合物LiI(HPN)2代替LiI作锂空电池的氧化还原媒介体，可以显著提高Li2O2在充电过程中的氧化分解效率，同时电池的循环稳定性也有显著改善。
|
| 关键词 | 储能材料
|
| 摘要 | 中文 |
| 其他摘要 | It is highly required to develop high specific energy density battery systems for the development of electrical vehicles. The Li-O2 battery is a potential candidate to meet the demand due to its high theoretical energy density. However, Li-O2 batteries still suffer from poor round-trip efficiency and poor cycle life. To reduce the charge over-potential of Li-O2 batteries, redox mediators (RMs) were introduced to the electrolyte of Li-O2 batteries to promote the decomposition of discharge product Li2O2. Among the reported RMs, LiI is the most intensely studied one with lots of controversial reports. Here, we studied the Li-O2 battery employing LiI as RM. Firstly, we studied the discharge product LiOH and its decomposition mechanism during charge process of Li-O2 cell employing LiI as RM with trace of H2O impurity. Then we studied the oxidative mechanism of traditional discharge product Li2O2 by the oxidized I- (I3-, I2) in the anyhydrous Li-O2 cell employing LiI as RM. Moreover, we presented a design method of a more efficient I- redox mediator for Li-O2 cell. The main research contents are summarized as follows:
(1) We found that the corresponding discharge product was LiOH and LiOH•H2O when the H2O content was 200 ppm and 2000 ppm in Li-O2 cell employing LiI as RM. We studied the discharge mechanism of discharge product LiOH in Li-O2 cell contained trace H2O. We also studied the decomposition mechanism of LiOH during charge process by XRD and gas spectrometer (MS). The MS datas demonstrated that the discharge product LiOH was not reversibly oxidized to O2 during charge process. Instead, the reaction of LiOH and I2 generated side-product LiIO3, which accumulated and destroyed the electrochemical performance of the Li-O2 cell.
(2) In the ideal anyhydrous Li-O2 cell employed LiI as RM, the MS datas confirmed that the traditional discharge product Li2O2 can be oxidized by I2 rather than I3-. Meanwhile, we found that the free I- or the generated I- from Li2O2 oxidation can be complexed with effective I2 to give ineffective I3- species, reducing the oxidation efficiency of Li2O2 on charge. Inspired by this, we found a compound LiI(HPN)2, which has lower I- dissociation in ether electrolyte. So it can exhibit as an I- trap to reduce the concentration of I- in the electrolyte solution thus improve the generation of I2 species during charge and higher Li2O2 oxidation efficiency. Our work demonstrated that compound LiI(HPN)2 was a more efficient I- RM than LiI, which can largely enhance the Li2O2 oxidation efficiency during charge process and the cycling performance of Li-O2 cell. |
| 作者部门 | 仿生能源与储能系统团队 |
| 公开日期 | 2018-06-30
|
| 学位类型 | 硕士
; 学位论文
|
| 语种 | 中文
|
| 文献类型 | 学位论文
|
| 条目标识符 | http://ir.qibebt.ac.cn/handle/337004/9975
|
| 专题 | 仿生与固态能源系统研究组
|
| 作者单位 | 中国科学院大学
|
推荐引用方式
GB/T 7714 |
李阳. 氧化还原媒介体对锂空气电池充放电过程作用机制研究[D]. 北京. 中国科学院大学,2017.
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