A New Anion Receptor for Improved Interface Between the Lithium- and Manganese-Rich Layered Oxide Cathode and the Electrolyte

Abstract

Significant progress has been made in lithium-ion batteries for applications in electric vehicles and grid energy storage. The deep market penetration of the LIB technology would be further accelerated if methods to further increase energy density, stability and cycle life can be identified and developed. Although intensively studied in the last several decades, cathode materials are still considered a major limiting factor in LIB performance. In the past few years, remarkable efforts have been dedicated to studying a family of lithium- and manganese-rich layered oxides, with a general formula of Li1+x M1-x O2 (M=Mn, Ni, Co etc). [1] Depending on the composition, the oxides can deliver capacities exceeding 250 mAh/g at an average potential of about 4.6 V vs. Li+/Li. While they have the potential to drastically improve the energy density of lithium-ion batteries, there are several critical problems, including a large first-cycle irreversible capacity loss associated with the activation process, subsequent-cycle voltage decay characterized by a continuous decrease in average discharge voltage during cycling, transition-metal dissolution into the electrolyte, sluggish kinetics, and a high impedance at low state of charge (SOC), prevent their application in commercial batteries. In addition, the high charging potential of these materials along with the catalytic properties of the high-valent transition metals in the charged cathode oxidize carbonate solvents and decompose the electrolyte. [2] These reactive interactions between the electrode and the electrolyte, or side reactions, not only leave surface deposits onto the cathode, but also modify the intrinsic properties, including elemental, chemical and structural, of the cathode surface. While the stability of resulted cathode-electrolyte interface (CEI) layer, like solid electrolyte interface of anode, is largely influenced by the chemical nature (both organic and inorganic) and the distribution of the side reaction products, studies have shown that a build-up in CEI can increase the charge transfer resistance and deteriorate electrochemical performance. Previous research has demonstrated the important role of electrolyte additives in influencing the CEI and battery performance.[3] For example, boron-based anion receptors that can dissolve electrolyte decomposition byproducts, such as LiF, Li2O and Li2O2, are capable of enhancing long-term cycling of LIBs. To date, however, most studies have focused on the compositional engineering of additives. The general knowledge of how electrolyte additives influence the changes on the cathode particle surface and interfere with the electrolyte/electrode side reactions is nearly nonexistent. Such studies would have a profound impact on additive development that not only benefits the entire electrolyte community but also the cathode community. Herein, We report a new electrolyte additive, tris (2, 2, 2-trifluoroethyl) borate (TTFEB), to improve the discharge capacity, coulombic efficiency and cycle life of cells with a lithium- and manganese-rich layered cathode (in Fig1). TTFEB suppressed the growth of insulating inorganic byproducts at the cathode/electrolyte interface, inhibited the growth of surface film resistance, and alleviated the cell polarization during cycling. Furthermore, surface transition-metal (mostly Mn) reduction and structural transformation in cathode were mitigated when the electrode was cycled in a TTFEB containing electrolyte. The work highlights the importance of electrolyte additives in preserving the structure of cathode materials and helps us understand the underlying mechanism of the improved electrochemical performance enabled by electrolyte additives.