During the first few cycles of a lithium-ion battery, the electrolyte reacts with both the anode and cathode to form a protective passivation layer. The formation of this passivation layer consumes part of the electrolyte, which can protect the electrode from corrosive damage; At the same time, the process of ion transport and diffusion passing through this membrane becomes a speed-controlling step in battery reactions. Aurbach and others conducted extensive research on the surface chemical processes of this phenomenon. At the beginning of the main chapters of their work, Aurbach and Cohen described that “electrochemistry and passivation phenomena in electrochemical systems are controlled by surface films and have been widely treated and studied over the years.” In lithium-ion batteries, the passivation film on the electrode surface is the key factor determining whether the battery can operate normally. Therefore, it is necessary to explain the origin of this work here and clearly introduce the properties and functions of passivation membranes. In lithium-ion batteries, the surface layer formed by the initial reaction between the electrode and the non-aqueous electrolyte is insulated, exhibiting very low electronic conductivity and very high ionic conductivity under the influence of an electric field. In non-aqueous solvents, metals react with electrolytes to form passivated films, a phenomenon first proposed by Vijh in 1968. Vijh research found that the bandgap of materials, including the semiconductor layer, can be estimated using the generation energy and bond energy, a finding that has attracted widespread attention in the field of electrochemistry. Later, Vijh defined these surface films generated during electrochemical reactions as “demetalized surfaces,” which are obtained by the reaction of electrodes and electrolytes, possessing semiconductor/insulator properties. They serve as the rate-controlled steps in electrochemical reactions, evolving from results obtained in lithium-ion batteries to broadly defined electrode reactions. Especially in 1974, the concept of active metal surfaces as the reaction center of batteries was proposed. As early as 1971, the role of these surface films during anodic dissolution and electrolysis of metals in non-aqueous solvents was reported, and further research was conducted in 1972. Therefore, the formation of surface films during electrode reactions, including what happens in battery systems, was reported and summarized in a 1974 work.
In 1979, Peled proposed that “in a practical non-aqueous battery system, the surfaces of alkali and alkaline earth metals would be covered by a surface film formed by the reaction of metal with the electrolyte.” He believes this passivation film is the speed-controlling step in the battery’s reaction. He basically reached the same conclusions as his predecessors, whose research applied to all electrode reactions. Therefore, it is believed that before Peled’s “rediscovery” of the role of surface films in electrode reactions, he had not read the relevant electrochemical literature and only narrowly focused on lithium batteries. The “demetalized surface” named by Vijh has been redefined by Peled as SEI film. Due to being limited to the battery field, most people do not understand the basic research process of surface film electrochemistry; except for Aurbach, who uses the term defined by Peled, he describes it. For the remainder of this section, we will introduce the nature, properties, and role of SEI films in electrode reactions. It should be remembered that the SEI membrane was not discovered by Peled, but was “rediscovered” and given a new name by him.
As mentioned earlier, metals, especially reactive alkali and alkaline earth metals, react with electrolyte components, so the electrode surface appears demetalized, meaning it is covered by a solid-state semiconductor/insulator layer. The SEI film formed on the surface of metallic lithium in batteries containing non-aqueous aprotic electrolytes proposed by Peled is just one example. The innovation of Peled’s work lies in comparing this film to a solid electrolyte filling the interfacial phase region. Peled also pointed out that the speed-controlling step of redox reactions on this surface is the diffusion of lithium ions through the SEI membrane. This view has been well explained in some previous reports by other researchers, such as Young’s classic work published in 1961, which proposed that most electrodes (especially anodes) surfaces are covered by an insulating layer. The chemical composition of SEI films is closely related to the electrolyte used, with a thickness of about 25~100 (1=0.1 nm), basically isolating electrons by 1. Ideally, the chosen electrolyte should help form an SEI film where the cation migration count is entirely contributed by lithium ions. The structure and composition of SEI films formed on the surface of metallic lithium in different electrolytes have been extensively studied, especially by Aurbach and others. Very useful knowledge and information about SEI membranes can also be obtained from books written by Young and Balbuena. Xu believes the performance of SEI films is mainly influenced by two factors:
(1) The static stability of the SEI film is related to the storage conditions of the battery;
(2) The dynamic stability of SEI films is related to their reversibility.
The formation of SEI films allows lithium electrodes to achieve static stability in non-aqueous solvents. However, SEI films also cause uneven surface morphology on the surface of lithium metal during lithium deposition. Therefore, during lithium deposition/intercalation, the distribution of surface current density through the surface is uneven, ultimately leading to the formation of lithium dendrites.
For lithium-ion batteries, the impact caused by uneven SEI film morphology on the electrode surface is not severe, and dendrite formation generally does not occur, unless under extreme conditions, such as lithium depositing on the carbon anode surface at extremely low temperatures. For lithium-ion batteries, the ideal SEI film should meet the following requirements:
(1) Electron migration number t=0;
(2) High ionic conductivity;
(3) Consistency of morphology and chemical components;
(4) Good adhesion on the surface of the anode (C, Si, Sn, etc.);
(5) High mechanical strength and good elasticity;
(6) Low solubility in electrolytes. In lithium-ion batteries, the SEI film is very important for the anode and also forms and exists on the cathode surface.