UC Riverside

To understand the baseline performance of lithium (Li) anode in liquid electrolytes, the electrochemical and physical properties of Li anode are studied with realistic parameters including: thin thickness (50 µm), practical areal capacity (1 to 4 mAh cm-2), practical areal current (0.5 to 2 mA cm-2), and low electrolyte/capacity ratio. Two different Li salts, lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), are used to probe the effects of the electrolyte chemistry and concentration. The cycling of Li/Li symmetric cells, combined with investigation using the scanning electron microscope, demonstrates that the soft-short of Li/Li cells is induced by the continuous volume expansion of Li electrodes during cycling instead of dendrites. The volume change of a Li electrode is dictated by the depth of deposition and stripping (i.e., areal capacity) and the electrolyte/capacity ratio, with no strong correlation with the type of Li salt and concentration. On the other hand, the average coulombic efficiency (CE) measurement demonstrates inherent correlation with the type of Li salt and its concentration in the electrolyte. Li electrode surface chemical analysis indicates that the fluoride-rich surface layer formed in the LiPF6 electrolyte can be detrimental to both CE and Li deposition-stripping overpotential.We report acrylonitrile (AN) as an effective additive in carbonate-based electrolytes to enable uniform and dense lithium (Li) deposition and to improve the coulombic efficiency of Li metal anode. Our electrochemical, spectroscopic, and theoretical study reveal that AN is cathodically electropolymerized on the Li surface prior to the electrochemical decomposition of the electrolyte during Li deposition. The resultant polyacrylonitrile artificial solid electrolyte interphase enables uniform nucleation and growth of Li deposition with significantly reduced side reactions. The effectiveness of the AN additive is demonstrated in 0.4 Ah Li||LiNi0.6Mn0.2Co0.2O2 pouch cells (using 50-μm Li anode, 3 mAh cm-2 cathode areal capacity, and 4g Ah-1 electrolyte) with excellent cycle stability under realistic charge-discharge condition. The hydrofluoric acid (HF) based side reaction in lithium (Li) hexafluorophosphate (LiPF6) electrolyte system hindered the direct application of LiPF6 electrolyte (in organic carbonate solvents) for Li metal batteries. In this study, we report phosphorus pentoxide (P2O5) as an effective additive in LiPF6 based electrolyte not only enable uniform and dense Li deposition but also mitigate the transition metal (TM) dissolution and cracking problems of LiNi0.6Mn0.2Co0.2O2 (NMC622) particle. The poor Li metal deposition behavior and the increasingly growth of cathode impendence caused the poor cycle life of the Li||NMC622 pouch cell in 1M LiPF6 electrolyte. The cycle life of the pouch cell in 1M LiPF6 electrolyte with P2O5 additive was greatly enhanced from 30 cycles to more than 230 cycles, with the capacity retention: 87.7% (230 cycles).