Effective Suppression of Irreversible Li Metal Deposits at High Charging Rates or Low Temperatures Influenced by SEI Properties in Lithium Ion Batteries

摘要

Lithium ion batteries (LIB) are a key technology for portable electronic and the most promising energy source to power electrical vehicles (EV). However, the current LIBs show major drawbacks for the automotive application with regards to the energy density and fast charging ability. Taken together, these two limitations creating the so-called 'range anxiety' for EV users, which is a reason for the slow adaption in mass market. With future perspective to charge EVs with up to 350 kW [1], the limitations for fast charging must be addressed. Breaking down fast charging on material level, low temperatures and high charging currents reduce Li+ conductivity in the electrolyte and the electrodes. Accordingly, charging at low temperatures and high currents leads to increased overpotentials, which might lower the interfacial potential of the graphite anode below 0 V vs Li/Li+ [2]. Consequently the reduction of Li ions to Li metal on the anode surface is more likely, known as Li plating. Even if Li metal plating is considered as a big safety risk due to the possible short circuit, it was shown that advanced aging can lead to Li metal deposition at higher temperatures [3,4]. Thus, Li plating must be considered as unwanted but also as an unavoidable aging mechanism in many cases. Assuming Li metal is unavoidable during the lifetime of any Li ion battery, it is of interest to understand the plating behavior in detail, in particular, Li plating is divided in reversible and irreversible plating. Reversible Li plating remains electrochemical available, either by stripping in the discharge step or by re-intercalation into graphite in the rest step [5,6]. Furthermore, irreversible Li plating is responsible for permanent Li losses which directly translate to capacity losses. The concept of reversible and irreversible Li metal plating is known. However, the main factors determining the ratio between reversible and irreversible Li plating are not investigated yet. In this study the impact of the SEI is in the focus of the investigation. Film forming additives are used to change the properties of the SEI. The cells are cycled at Li plating conditions. Electrochemical methods are used to quantify both, the reversible and irreversible plating. The first results suggest a strong dependency of the SEI properties on the ratio between reversible and irreversible Li metal plating. The understanding of the complex system might be used to shift the present limits of fast charging. In case of a fully reversible system Li plating might be tolerated under fast charging conditions due to the temporarily character.