(Invited) An Inorganic-Rich but Low Lif Content Interphase for Fast Charging and Long Cycle Life Lithium Metal Batteries As Well As Evolution and Interplay of Lithium Metal Interphase Components
Authors: Yang Xiao-Qing 
Published: December 2024

The first part of this presentation will be focused on An Inorganic-rich but No LiF Interphase for Fast Charging and Long Cycle Life Lithium Metal Batteries. Li metal battery (LMB) with Li metal anode and LiNi1-x-yMnxCoyO2 (NMC) cathode represents the next generation high energy density electrochemical energy storage devices. A major challenge facing LMB is finding an electrolyte that is capable of forming good interphases and enabling stable cycling. A common practice is fluorinating the electrolyte to generate anion-derived interphase rich in LiF. Electrolytes based on this strategy can lead to long cycle life. However, they usually have low ionic conductivities and cannot enable the fast-charging capabilities. By using CsNO3 as a dual-functional additive, a good interphases on both NMC cathode and Li metal anode is formed for long cycle life of LMB. Such strategy allows the use of 1,2-dimethoxyethane as the single solvent which guarantees superior ion transport properties and hence fast charging capabilities. The cathode is protected by the nitrate-derived species. On lithium metal anode side, the gigantic Cs+ cations have weak interactions with the solvents, allowing the anions to enter the solvation sheath and contribute to the interphase formation. The resulting interphase is composed of anion-derived species as expected but surprisingly dominated by the inorganic species cesium bis(fluorosulfonyl)imide, or CsFSI, an interphase component that has never been reported. The active participation of Cs+ in the interphase formation suggests that Cs+ may be doing much more than just serving as electrostatic shield as commonly believed. In addition, the interphase has low content of LiF which is usually regarded as a key component for good interphase. Interestingly, the low content of LiF does not impact the quality of the interphase in any negative way. This is demonstrated by the good performance of LiNi0.8Mn0.1Co0.1O2||Li cell. Even with very high cathode loading (21mg/cm2 or 5mAh/cm2) and a low N/P ratio of 2, LMB cell can be cycled at 1C rate (~8mA/cm2) and retains more than 80% of initial capacity after 200 cycles. This work calls attention to re- examining the role of LiF in the interphase and the role of Cs-containing additive in the electrolyte.
The second part of this presentation will be focused on Evolution and Interplay of Lithium Metal Interphase Components of lithium metal batteries. LMBs have high energy densities and are crucial for clean energy solutions. The characterization of lithium metal interphase is fundamentally and practically important but technically challenging. Taking advantage of synchrotron x-ray which has the unique capability of analyzing crystalline/amorphous phases quantitatively with statistical reliability, we have studied the composition and dynamics of LMB interphase for a newly developed important LMB electrolyte that is based on fluorinated ether. Pair distribution function (PDF) analysis revealed the sequential role of anion and solvent in interphase formation during cycling. The relative ratio between Li2O and LiF first increases and then decreases during cycling, suggesting suppressed Li2O formation in both initial and long extended cycles. This work highlights the important role of Li2O in transitioning from anion-derived interphase to a solvent-derived one.

Acknowledgement

The authors are grateful to Dr. Zhiao Yu, Dr. Yuelang Chen, Prof. Zhenan Bao, and Prof. Yi Cui at Stanford University for providing the fluorinated ether based new electrolyte and the valuable scientific discussions, as well as to Dr. Jie Xiao and Jun Liu at pacific Northwest National Lab (PNNL) for scientific discussions. The theoretical calculation by Dr. Dacheng Kuai, Dr. Perez-Beltran, and Prof. Perla B. Balbuena is greatly appreciated. This work is supported by the Assistant Secretary for Energy Efficiency and Renewable Energy (EERE), Vehicle Technology Office (VTO) of the US Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) Program including the Battery500 Consortium under contract no. DE-SC0012704. This research used beamlines 5-ID, 7-BM, 23-ID- 2, 28-ID-2 of the National Synchrotron Light Source II, a US DOE Office of Science user facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract number DE-SC0012704. SEM measurements used the resources of the Center for Functional Nanomaterials, a US DOE Office of Science User Facility at BNL, under contract no. DE- SC0012704.