Principal Factors that Contribute to the Gas Formation in High Voltage Cathode Systems of Lithium Ion Batteries - a Discussion about the Interaction Between Carbon Black and the Electrolyte -

Interface reactions, especially those that lead to gas evolution, between carbon black (CB) and electrolyte in the positive electrode at potentials over 4.5 V have been studied. Key measures to suppress these undesirable reactions and sequential side reactions between the components of the cathode and the electrolyte will be discussed.

 In this study, we focused on the investigation of the behavior of CB at high voltages in combination with active material and binder, a topic that has not been subject to a lot of research so far. We believe that conductive agents of the positive electrode like CB are subject to anion intercalation, similar to what was observed for a graphite positive electrode ^[1]. The reactions between different CB and the electrolyte in the presence of active material was studied by a research team comprising of a carbon and an active material producer.

 The first part of the study will focus on the quantification of the gas that is formed due to the presence of the carbon material. Various conductive agents such as acetylene black (AB), furnace black (FB), specially customized AB as well as graphite (GR) were examined.

 The experiments revealed that in the high voltage system the gas evolution was induced by both, the cathode active material and the conductive agents. Mass spectroscopic analysis indicated that the source of the gas was oxidized electrolyte. The specific amount of gas that was generated due to the presence of the CB was found to be 8 to 15 times higher than the portion that originated from the cathode active material LiNi_0.5Mn_1.5O₄(LNMO) itself, Fig. 1.

 In order to interpret the above findings, the electrochemical performance of the different carbon materials at high voltages was analyzed in more detail utilizing carbon positive electrode/Li-metal half cells (carbon cell).  At a potential of 5.0 V vs. Li⁺/Li at 60 ^oC the amount of gas detected was directly proportional to the degree of reversibility of the cathodic/anodic current as summarized in Fig. 2. Furthermore, this reversibility tends to be higher when the anion intercalation/de-intercalation reaction is more dominant than the decomposition of some impurities, and we conclude that the anion intercalation is an important factor for the gas evolution at high voltages.

 In a second experiment, the carbon cells were discharged stepwise in the voltage region of 5.2 to 4.0 V in decrements of 0.2 V (CC followed by CV). Although the carbon cell was being discharged, most of the cells required the application of a charge current in order to keep the voltage at the desired level during the CV phase.

 In order to explain the above results an interaction between the carbon additive, the electrolyte and the cathode material is considered. We assume that the charged (= anion intercalated) CB in a high voltage cathode system is very unstable and tends to release the anions back to the electrolyte which can be considered as a “self-discharge”.

 As the discharged CB is in contact with the fully charged cathode material it can be recharged via this solid-solid interface and discharge itself at the same time again via the solid-liquid interface (local battery system within the cathode).  This complex reaction that includes the active material reduction and electrolyte decomposition continues until the electrode potential reaches the 4.0V level where the anion intercalation is stopped.

 The study of a larger range of commonly used conductive agents allowed us also to examine the influence of specific material properties on the gas formation phenomena. Further details shall be presented at the meeting.

Reference

[1] J. A. Seel and J. R. Dahn, J. Electrochem. Soc., 147(2000) 892.