Surface Films on LIB (Lithium-ion Battery) Electrodes

Abstract

Graphite and silicon are the most commonly used negative electrodes for the present lithium-ion batteries (LIBs). At the working potential of these negative electrodes, the conventional carbonate-based electrolytes are not stable against reductive decomposition, therefore, decomposes and the decomposed products deposit as surface films. This electrolyte decomposition/film deposition is detrimental to the capacity and cost of cells because the costly positive electrodes (such as LiCoO2) should be oversized to afford Li+ ions and the equivalent amount of electrons that are irreversibly consumed in this process. Fortunately, however, the surface films are electronically insulating, such that any electrochemical reactions at the electrode/electrolyte interface are hindered due to negligible electron tunneling through the surface films; additional electrolyte decomposition/film deposition is prevented once they form at a certain thickness; the negative electrodes are passivated (protected from self-discharge and electrolyte reduction). The surface films are also known as Li+ ion conductor, such that they serve as a charge carrier enabling reversible Li+ reaction for negative electrodes without posing serious concentration polarization. The formation of passivating surface films on lithium metal also allows a commercialization of lithium primary batteries.1 In order for any surface films to effectively play the passivating role, they should uniformly cover the whole electrode surface. Ion transport across the surface films is critically important because Li+ ions should be crossed for electrode reactions. A thinner surface film is thus favored to minimize film resistance as far as it plays the passivating role. Thermal stability is another requirement because surface films are vulnerable to damage upon high-temperature exposure. Once being thermally degraded, the surface film loses its passivating ability to cause additional electrolyte decomposition and film deposition, which leads to cell polarization and eventual capacity fading. Moreover, sometimes, cell temperature increases due to heat evolution during this process, which can trigger a thermal runaway, in which the cell temperature increases uncontrollably because of continued exothermic reactions.2,3 Even if the roles of passivating surface films on negative electrodes are conceptually recognized, little is fully understood on the film forming mechanism, chemical composition, ion transport property, thermal stability, and changes evolved during charge/discharge. The surface films on positive electrodes are even more veiled. Intuitively, somewhat different film properties to those on the negative electrodes are assumed because these films are generated by electrochemical oxidation. This presentation addresses some of the above-mentioned issues. The short summary is listed in order. (i) Passivating ability: This is critically affected by the uniformity and coverage of surface films. The surface films generated from the commonly used electrolyte (LiPF6 dissolved in organic carbonate solvents) at a faster rate (higher charging current) are poorly passivating because of uneven coverage. Vinylene carbonate (VC) is very effective in that the surface film generated from this electrolyte additive is thin and uniform in thickness, thus highly passivating. (ii) Ion transport property of surface films on positive electrodes: it is far different to that on negative electrodes. The latter can be categorized as a Li+ conducting solid electrolyte film, whereas the former is not. Li+ ions are transported as a solvated form by solvent molecules in the former surface films. (iii) Thermal stability: The surface film on graphite generated from the carbonated-based electrolyte degraded at >60oC. When the graphite electrodes are not fully discharged, a self-repairing of the damaged film (additional electrolyte decomposition) is continued by taking Li+/electrons from the graphite electrode. Frequently, organic-rich surface films are thermally more stable than inorganic-rich films.4,5