【电池】利用冷冻电镜技术从原子尺度解释电池界面膜的构效关系

对于高能量密度的电极材料，比如硅，锂金属，硫，只有准确找到其容量衰减机理，才能找到相应的解决方案，而由于技术条件的限制，我们对其衰减机理的理解目前只停留在在微米尺度，因此，迫切需要从更微观的角度去研究和分析其真正的衰减机理。

在锂电池中，固态电解质膜(SEI)对电池的循环性能有着巨大的影响，特别是对于高能量密度的电极材料，但关于SEI的构效关系一直存在着一些争论，为此，斯坦福大学的崔屹老师课题组利用冷冻电镜技术第一次从原子角度解释了电池界面膜的构效关系，研究发现，界面膜一般可分成马赛克模型和多层模型，而马赛克结构的SEI循环性能较差，而多层结构的循环性能较为优异，这主要是因为马赛克结构的SEI存在无机晶体成分空间分布不均匀的现象，而多层结构的SEI无机成分空间分布均匀，无机晶体成分的离子电导率较高，锂离子优先在此处沉积溶解，进而导致锂离子沉积不均匀的现象发生，随着循环次数的增加，枝晶不断产生，生长，最后断裂成没有电化学活性的死锂，导致容量的衰减和库伦效率的降低。因此，一种有效的提高高能量密度电极材料循环性能的解决策略是构建一个无机成分空间分布均匀的SEI，以确保锂离子在长期循环过程中均匀的沉积溶解，阻止枝晶的产生和生长。

Figure 1. Li Metal Deposition and StrippingMorphology with Mosaic and Multilayer SEI

Nanostructure

(A) Schematic of the mosaic SEInanostructure that forms on Li metal in EC/DEC electrolyte.

Crystalline grains are heterogeneouslydispersed within a polymeric and amorphous matrix.

(B) Top-view SEM image of Li metaldeposition in EC/DEC electrolyte.

(C) Top-view SEM image of stripped Li metalin EC/DEC electrolyte. Although the dendrite density appears decreased, thedendrite diameter is comparable with that of the deposited Li dendrite.

(D) Schematic of the multilayer SEInanostructure that forms on Li metal in 10 vol% FEC electrolyte. Organic andinorganic layers are formed in an ordered structure.

(E) Top-view SEM image of Li metaldeposition in 10 vol% FEC electrolyte.

(F) Top-view SEM image of stripped Li metaldeposition in 10 vol% FEC electrolyte. The dendrite diameter and density aresignificantly decreased compared with the deposited Li dendrites.

Figure 2. Non-uniform Li Stripping throughMosaic SEI

Figure 3. Uniform Li Stripping throughMultilayer SEI

Figure4. COMSOL Simulation of Li Dissolution through SEI Nanostructure

(A)Initial state of the modeled Li metal dendrite (500 nm diameter) with SEI layer(30 nm thick).

(B)Final structure of a stripped dendrite with uniform Li ionic conductivity throughthe SEI. 104 s

representsthe time needed to strip to 25% of the original diameter.

(C–F) Finalstructures of stripped dendrites with a defect (in red) introduced into the SEIwith 103(C), 1003 (D), 1,0003 (E), and 10,0003 (F) increase in ionic conductivity.Wavy structures withcurved surfaces on the stripped Li metal appear when theionic conductivity in the defect is

within1 order of magnitude (C). Notches appear when the ionic conductivity increasesbeyond 1

orderof magnitude (D–F).

相关论文发表在Joule:CorrelatingStructure and Function of Battery

Interphases at Atomic Resolution Using CryoelectronMicroscopy（DOI:10.1016/j.joule.2018.08.004）

投稿人：YYT

编辑：WJH

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