Date Published: March 31, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Jianming Zheng, Joshua A. Lochala, Alexander Kwok, Zhiqun Daniel Deng, Jie Xiao.
The electrolyte is an indispensable component in all electrochemical energy storage and conversion devices with batteries being a prime example. While most research efforts have been pursued on the materials side, the progress for the electrolyte is slow due to the decomposition of salts and solvents at low potentials, not to mention their complicated interactions with the electrode materials. The general properties of bulk electrolytes such as ionic conductivity, viscosity, and stability all affect the cell performance. However, for a specific electrochemical cell in which the cathode, anode, and electrolyte are optimized, it is the interface between the solid electrode and the liquid electrolyte, generally referred to as the solid electrolyte interphase (SEI), that dictates the rate of ion flow in the system. The commonly used electrolyte is within the range of 1–1.2 m based on the prior optimization experience, leaving the high concentration region insufficiently recognized. Recently, electrolytes with increased concentration (>1.0 m) have received intensive attention due to quite a few interesting discoveries in cells containing concentrated electrolytes. The formation mechanism and the nature of the SEI layers derived from concentrated electrolytes could be fundamentally distinct from those of the traditional SEI and thus enable unusual functions that cannot be realized using regular electrolytes. In this article, we provide an overview on the recent progress of high concentration electrolytes in different battery chemistries. The experimentally observed phenomena and their underlying fundamental mechanisms are discussed. New insights and perspectives are proposed to inspire more revolutionary solutions to address the interfacial challenges.
The ever‐increasing energy demand and global environmental concerns have accelerated the efforts to develop low‐emission or zero‐emission electric vehicles (EVs) powered by high energy batteries.1 There is also increasing demand for high‐energy‐density battery systems for stationary wind and solar energy storage. Rechargeable lithium‐ion batteries (LIBs) and lithium (Li) metal batteries are considered the significant power sources to meet these demands. Depending on the specific applications, various batteries should find their way to fit into different systems. For example, the prioritory concerns of LIBs for hybrid electric vehicles (HEVs) or pure EVs are their energy density and safety properties. For storing renewable energy, reliabilty and cost are more important.2 While many research interests have been focused on materials chemistry,3 and electrolytes,4 the understanding of their derived interfaces has made much less progress due to the complexity of electrolyte decomposition in dynamic conditions and on various substrates with different surface properties. However, interfaces do play a critically important role in determining the mass flow and electrochemical kinetics, and thus the power, stability, and safety of LIBs.4
Concentrated electrolytes are attracting increasing amounts of interest due to the unique SEI properties identified recently in both non‐aqueous and aqueous types. Currently, the formation mechanisms and the constituents of the SEI derived from concentrated electrolytes can be categorized into three groups: 1) the considerably decreased irreversible solvent reduction due to the significantly reduced number and activity of free solvent molecules, 2) the thin and robust inorganic SEI film typically rich in LiF derived from the sacrificial anion reduction which significantly enhances the interfacial stability of various energy storage systems, and 3) the reversible SEI formation from the precipitation of the solute induced by the electrical field. As more research efforts are dedicated in this field, the understanding of the fundamental mechanisms in this intriguing system will be further deepened. MD and DFT simulations will be a complementary approach in assisting the massive screening of suitable concentrated electrolytes for various applications. Modeling the interface, although very challenging, may need more attention to promote further understanding of the SEI formation in concentrated electrolytes and on different electrode surfaces.