IDENTIFICATION OF SURFACE-FILMS FORMED ON LITHIUM IN PROPYLENE CARBONATE SOLUTIONS
IDENTIFICATION OF SURFACE-FILMS FORMED ON LITHIUM IN PROPYLENE CARBONATE SOLUTIONS
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DOI：
10.1149/1.2100722
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发表时间：
1987-07-01
影响因子：
3.9
通讯作者：
YEAGER, E
中科院分区：
文献类型：
--
作者：
AURBACH, D;DAROUX, ML;YEAGER, E
用FTIR、IR和XPS研究了锂在LiC104、LiAsF、liiso ~ CF等碳酸丙烯酯溶液中形成的薄膜。在一定条件下，在初始表面膜中检测到的主要成分是烷基碳酸锂（RCO~ Li， R=烷基）。另一种碳酸烷基溶剂碳酸二乙酯与锂反应生成碳酸乙基锂CH3CH~ CO3Li。除了溶剂还原，XPS测量给出了盐还原反应的指示。LiC104、LiAsFG和LiSO。~ CF.~被锂还原成卤化物离子，并在锂表面检测到卤化物离子。讨论了烷基碳酸盐形成的两种可能机制。一种是碳酸丙烯与OH-等碱性物质的亲核反应，另一种是金属锂对碳酸丙烯进行单电子还原，然后进行自由基终止反应。当存在高浓度的水时，碳酸酯与水进一步反应形成碳酸锂。在没有机械稳定表面膜的锂表面，如锂/汞合金表面，还原反应被认为是通过一个整体的双电子过程进行的，主要产物是碳酸锂。锂电极在各种极性非质子溶剂体系中被观察到高度稳定，在热力学基础上，锂应该与之反应(1)。人们普遍认为，这种稳定性的起源是动力学的，是由活性金属与电解质、溶剂和/或杂质（如水）反应在锂表面形成的膜引起的。这些固体电解质（SE）薄膜(2)抑制了锂电极与溶液的进一步自发反应，并且是一次锂电池具有优异保质期的原因，锂电池利用了广泛的非水溶剂，包括线性和环醚，烷基碳酸酯和氧卤化物(3)。在二次锂电池中，锂溶解/沉积过程的不可逆性限制了循环寿命，这里也可能起薄膜形成反应的作用。目前公认的是，锂的渐进电子隔离发生在电池的充电和放电过程中，并提出了两种理论来解释这一点。Peled等人(3)提出的一种观点认为，锂沉积物形成于表面薄膜的顶部，并在随后的溶解中由于锂连接的丢失而被电子隔离，而锂连接可以穿透薄膜。Brummer等人(4)提出的另一种理论认为，在电溶解反应完成之前，快速的晶间腐蚀导致金属颗粒在电子上被隔离（再次通过表面膜）。目前的研究涉及碳酸丙烯（PC）溶液中锂的薄膜形成过程的调查，并利用光谱技术，主要是傅里叶变换红外光谱（FTIR），来确定表面薄膜的主要成分。多种技术已被用于研究有机电解质中锂的成膜现象。元素组成、厚度和电学性能已被研究（3-7），但表面膜的形成机制和化学结构仍存在不确定性。锂盐在碳酸丙烯中的高溶解度，以及它的高介电常数、低挥发性和缺乏毒性，引起了人们对含有这种溶剂的体系的特别关注。使用碳酸丙烯酯电解质的电池在放电时具有较高的电流密度，但锂电池的循环效率较差。
FTIR, IR, and XPS have been used to study the films formed on lithium in propylene carbonate solutions of LiC104, LiAsF,, and LiSO~ CF~. Over a range of conditions, the main components detected in the initial surface films were lithium alkyl carbonates (RCO~ Li, R= alkyl). Another alkyl carbonate solvent, diethyl carbonate, was found to react with lithium to form lithium ethyl carbonate, CH3CH~ CO3Li. In addition to solvent reduction, XPS measurements gave indication of salt reduction reactions. LiC104, LiAsFG, and LiSO.~ CF.~ were reduced by lithium to form halide ions, which were detected on the lithium surface. Two possible mechanisms for the formation of alkyl carbonates are discussed. One is the nucleophilic reaction of propylene carbonate with basic species such as OH-, while the other involves one-electron reduction of propylene carbonate by lithium metal, followed by free radical termination reactions. When high concentrations of water were present, lithium carbonate was formed by further reaction of the alkyl carbonates with water. On lithium surfaces without a mechanically stable surface film, such as those of lithium/mercury amalgams, the reduction reaction is believed to proceed by an overall two-electron process, and the primary product is lithium carbonate.Lithium electrodes are observed to be highly stable in a variety of polar aprotic solvent systems with which lithium, on a thermodynamic basis, should be reactive (1). It is generally accepted that this stability is kinetic in origin and is caused by a film formed on the lithium surface by reaction of the active metal with the electrolyte, solvent, and/or impurities (eg, water). These solid electrolyte (SE) films (2) inhibit further spontaneous reaction of the lithium electrode with the solution and are the reason for the excellent shelf-life of primary lithium storage batteries which utilize a wide range of nonaqueous solvents, including linear and cyclic ethers, alkyl carbonates, and oxyhalides (3). In secondary lithium cells, the cycle life is limited by the irreversibility of the lithium dissolution/deposition processes, and here also film formation reactions may play a role. It is currently accepted that progressive electronic isolation of the lithium occurs upon charging and discharging of the cells, and two theories have been proposed to account for this. One, proposed by Peled et al.(3), suggests that lithium deposits are formed on top of the surface films and are electronically isolated on subsequent dissolution by the loss of the lithium connections, which penetrate the films. An alternative theory, proposed by Brummer et al.(4), suggests that rapid intergranular corrosion causes the metal grains to become isolated electronically (again by surface films) before the electrodissolution reaction is completed. The present study has involved the investigation of the film formation processes occurring on lithium in propylene carbonate (PC) solutions and has made use of spectroscopic techniques, primarily Fourier transform infrared spectroscopy (FTIR), to identify the major components of the surface films. A variety of techniques have been used to study film formation on lithium in organic electrolytes. Elemental composition, thickness and electrical properties have been investigated (3-7), but there are still uncertainties about the formation mechanisms and chemical structure of the surface films. The high solubility of lithium salts in propytene carbonate, together with its high dielectric constant, low volatility, and lack of toxicity have prompted special attention to systems containing this solvent. Batteries with propylene carbonate electrolytes exhibit high current densities for discharge, but poor cycling efficiency for the lithium …