Rechargeable batteries are considered as the ideal choice to store energy harvested from renewable sources, power the vehicles, and decarbonize the energy generation. The practical application of rechargeable batteries for electric vehicles requires higher energy density batteries than commercial Li-ion batteries (LIBs) to solve the “range anxiety” problem. This requirement brought recent attention to redeveloping Li metal batteries (LMBs) that were commercialized prior to LIBs in the late 1980s. Paired with conversion-based cathodes or commercialized intercalation-based cathodes, Li metal batteries can boost energy densities by more than three folds. However, due to the high reactivity and electro-plating/stripping working mechanism of Li metal anode, achieving high and efficient electrochemical reversibility of Li metal anode during the battery operation is challenging. To tackle the challenges, researchers have found that electrolyte engineering and designs are the keys to achieving high-performance and practical LMBs.
Lately, scientific advancements in electrolyte engineering have enabled LMBs to cycle longer and more efficiently. As Li metal is both chemically and electrochemically reactive, all the possible battery electrolytes become thermodynamically unstable against Li metal anode. This thermodynamic instability results in the almost inevitable formation of intricate solid-electrolyte interphases (SEIs) at the Li/electrolyte interface. Typically, SEIs are composed of both organic and inorganic insoluble decomposition products of the electrolyte Li+ solvation shells. The SEIs are found to be perhaps the most critical components in determining the electrochemical performances of Li metal. Extensive research has been conducted to engineer electrolytes to understand and control the properties of the SEIs to fabricate high-performance LMBs. There has been a growing pursuit for creating inorganic-rich SEIs within the community to improve Li metal anode performance. Despite the growing interest in this type of engineering principle, the fundamental functions of the inorganics in SEIs are yet understood.
Since inorganic compounds in the SEIs have been showing improved electrochemical performances of Li metal anode, our scientific curiosity started with the question of “If the inorganics are truly beneficial to Li metal anode, then simply adding inorganics to Li metal cells should do the same.” After contemplating the practical ways to incorporate inorganics into Li metal cells, we came up with a suspension electrolyte design, a mixture of insoluble inorganic with liquid electrolytes, to scrutinize the effects of the universal inorganic, specifically Li2O, to Li metal anode. In this way, we could closely study how the prevalent inorganic of Li2O affects the SEI evolution on Li metal anode, mainly due to the vital interplay between the electrolytes and SEIs on Li metal anode. Notably, we have found that Li2O impacts the Li+ solvation environment of the electrolyte, in which the modified Li+ solvation environment in the suspension electrolyte signifies a different SEI evolution on Li metal anode. As a corollary, we were able to identify crucial aspects of inorganics for Li metal anode: “the inorganics can impact the Li+ solvation environment of the liquid electrolytes that affect the SEI evolutions on Li metal anode.”
Through theoretical and empirical analyses on the Li2O suspension electrolytes, we were able to differentiate the unique properties of the suspension electrolyte from the conventional liquid electrolytes used for Li metal anode (Fig. 1). Additionally, we were also able to determine the features of Li2O for Li metal anode in the following.
(1) Li2O modifies the Li+ solvation environment through the interfacial interactions between the Li2O surface and its surrounding Li+ solvation shells of the liquid electrolyte.
(2) Li2O creates a weakly solvating environment by decreasing Li+-solvent and increasing Li+-anion coordinations.
(3) Li2O facilitates desolvation of solvated Li+.
(4) Li2O attracts fluorinated species and dissociated Li+.
(5) Li2O induces inorganic-rich and anion-derived SEIs on Li metal anode.
(6) Li2O promotes a formation of temporally and electrochemically stable interphases on Li metal anode.
(7) Li2O suppresses the dendritic growth of Li.
(8) Li2O is a beneficial inorganic to SEIs of the Li metal anode.
Consequently, the outcomes of this study provide new insights into the inorganic species in the SEIs on Li metal anodes. Li+ solvation environment, at least near the SEI layer, must be different from the bulk electrolyte to further affect electrochemical performances of Li metal anode. Hence, figuring out exclusive and universal features of advantageous inorganics to SEIs of Li metal anode will considerably contribute to the scientific advancement for developing and realizing practical LMBs near future.
As the development of the suspension electrolytes for Li metal anode is currently at a nascent stage, the suspension electrolyte design opens a new direction to engineering the electrolytes for LMBs. We believe that the concept of synthesizing heterogeneous electrolytes (Fig. 2), which deviates from the conventional way of using homogeneous electrolytes, is worth investing endeavours in creating better and controlling SEIs to realize high-performance and reliable LMBs near the future.
More details can be found in our article published in Nature Materials
Kim, M.S., Zhang, Z., Rudnicki, P.E. et al. Suspension electrolyte with modified Li+ solvation environment for lithium metal batteries. Nat. Mater. (2022).
Contributors: Mun Sek Kim, Zewen Zhang, Paul E. Rudnicki, Professor Yi Cui
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