金属锂电化学沉积溶解过程行为与机理研究

The safety and cycle performance of secondary metal lithium batteries relate closely to the formation of lithium dendrite during charge process. In detail, mossy-like dendrite on fresh deposition layer reacts with electrolyte rapidly due to high specific area and high chemical activity. Such behavior causes not only obvious consuming of metal lithium and electrolytes but also rapidly formation SEI and inner resistance. The thicker dendrite, or called as tree-like dendrite, causes inner short-circuit of metal lithium battery because such kind of dendrite can pierce SEI and membrane. Moreover, some dead lithium is also formed caused by breaking of the dendrite during discharge process of secondary metal lithium battery. The formation of dead lithium leads to massive consuming, rapid losing and pulverization of lithium negative electrode. Abovementioned processes happen repeatedly during charge/discharge cycle and lead to the poor columbic efficiency, poor safety performances, poor cycle performance of secondary metal lithium battery. .In the viewpoint of selectivity of electrochemical reaction, the improvement of cycle columbic efficiency is consistent with that of the depressing of formation and growth of lithium dendrite completely. In this project, the cycle columbic efficiency of lithium negative electrode is used as critical parameter for the evolution of effect of electrolytes ingredients, including solvents, support salts, polymer materials, additives, et.al, and used charge/discharge condition, such as current density, temperature. In fact, the formation and growth of lithium dendrite depend on the relative rate of all of elementary steps involved in whole electrode process of plating/stripping of metal lithium negative electrode (mass transfer, charge transfer, diffusion of lithium atom on solid surface, electro-crystallization, corrosion et.al) and practical operation current density. .In this project, the technique of (ultra)micro-electrode will be used for the illumination electrochemical mechanism and measurement of dynamic parameters of all of elementary steps involved in whole electrode process of plating/stripping of metal lithium negative electrode because of much of merits of technique of (ultra)micro-electrode, for example, low iR drop, short response time, high mass transfer rate and high S/N ratio. .Some other essential techniques, for example, in-situ optical microscopy, scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and Raman, will be used to observe and analyses the dendrite, SEI and information of each ingredients of electrolytes. Based on the relationships among, formation, growth and micro-morphology of lithium dendrite and basic information of elementary steps, the reason and mechanism of the change of coulomb efficiency will be illuminated at the level of the electrode reaction process in different electrolytes at different current density and temperature. Dynamic parameters of elementary steps of electrode process of electroplating/electro-stripping of metal lithium negative electrode will be used as critical standards for the optimization of ingredient of electrolytes and charge/discharge currents density in order to improvement the cycle columbic efficiency and improvement safety and stability of lithium secondary battery.

金属锂二次电池的安全性、循环性能与循环过程中金属锂负极表面上形成的枝晶和由枝晶引发的危害性后果(内短路、死锂等)和副反应(反复形成SEI)密切相关。从电化学反应选择性的角度看，提高金属锂循环库仑效率与抑制枝晶是一致的。本研究首先以金属锂的循环库仑效率为核心参数来评价典型电解液、操作电流密度等要素的效果及作用；进而采用现场显微光学、Raman、SEM、AES等技术分析枝晶、电解液以及SEI的信息，采用微电极技术解析金属锂沉积溶解的电极过程动力学机理，并测量其中各基元步骤动力学参数；在电极过程动力学层面上阐明电解液成分、操作电流密度、库仑效率、基元步骤动力学速度、金属锂枝晶的微观信息等要素之间的关联。以上工作的目的是基于电极过程动力学基元步骤的行为及参数来建立评价、筛选和优化电解液成分、操作电流密度等要素的方法和标准，从而抑制枝晶、提高金属锂循环库仑效率，改善金属锂二次电池安全性、循环性能。

工作背景：金属锂二次电源是高能量密度化学电源体系的代表之一。然而其安全性和循环稳定性是制约应用可能的关键障碍。提高安全性和循环稳定性归结为提高Li+/Li电对电化学氧化还原反应的库仑效率，以期不断接近100%。 其中的基本化学问题在于如何形成完美沉积层：即内部原子级致密、表面原子级光滑。.工作内容：基于基本常识：任何一个结局都有其形成历程，结局的成因也隐藏在这个历程中，不同条件通过历程中的环节对结局造成影响。由此，研究选择典型电解液条件(系列浓度醚类电解液、不同添加剂、碳酸酯类、局部高浓度 )和典型充放电电流密度序列，考察金属锂循环的表观特征&结局(库仑效率、循环寿命)、微观信息(形貌、SEI化学组成、元素深度分布)、动力学过程中基元步骤三者之间的相关程度，阐明相关性的物理化学原理和规律。.重要结果：1、在一定电解液条件下、电流密度下，临界晶核在生长过程中存在晶格重构("自圆滑")的可能， 它对最终晶核形状、沉积层微观形貌有显著影响；“自圆滑”时间越短，晶核在成长过程中越容易保持表面光滑；临界晶核形成需要一个“诱导期”，且“自圆滑”时间与“诱导期”的比值与晶核光滑程度、沉积层致密程度、表观库仑效率、循环寿命有显著关联，该比值越小意味着临界晶核在成长过程中越光滑、沉积层越致密。反之，则意味着临界晶核形成针状枝晶，沉积层也就越疏松。上述两种情况分别对应高库仑效率和第库仑效率。2、交换电流密度的稳定程度以及能量转移因子与沉积层微观形貌有明显关系，转移因子偏离B-V模型经典数值(0.5)意味着重组能较小，此时更容易形成光滑沉积层。3、增大迁移数可以显著增加出现“锂离子浓度梯度”区域的时间，有利于形成光滑沉积层，理论上迁移数等于1则可以在任意电流密度下形成光滑沉积层。4、SEI中各个元素的相对原子浓度随深度分布的规则程度越高则晶核、沉积层越光滑。.科学意义： 本研究从微观动力学层面上揭示了形成光滑晶核、光滑沉积层需要具备表观条件的物理化学原因；阐明了多个基元步骤的微观动力学参数与晶核、沉积层光滑程度之间的半定量相关性。