Evolution of Interfacial Phenomena Induced by Electrolyte Formulation and Hot Cycling of Anode-Free Li-Metal Batteries

Even though the anode-free lithium metal battery (AFLMB) is the next promising energy-storage device because of its high energy density, the irreversible interfacial phenomena at the Cu/electrolyte interface are the key failure mechanisms of the AFLMBs. To the best of our knowledge, the interfacial phenomena associated with the capacity fading of AFLMBs are not reported yet. Herein, hot cycling combined with electrolyte formulation using 1% tris(trimethylsilyl) phosphite (TMSP) additive has been used as a protocol to systematically investigate the Cu/electrolyte interfacial phenomena of Li nucleation, compactness of deposited Li, inactive Li ("dead Li" and Li for solid–electrolyte interphase (SEI) formation "SEI-Li"), electrolyte decomposition, and SEI layer formation using a scanning electron microscope, scanning electron microscope-focused ion beam, titration gas chromatography, in situ gas chromatography–mass spectroscopy, and X-ray photoelectron spectroscopy, respectively. The synergy of hot cycling and the TMSP additive lowers the nucleation barrier in Li/Cu from 76.2 to 21.7 mV and increases the compactness of deposited Li from 32.3 to 43.8% and decreases "dead Li" from 45.18 to 10.86% and SEI-Li from 31.04 to 12.49% of anode-free Cu/NMC111. Because of the synergy effects, the Cu/NMC111 cell provides over 100 cycles with an average coulombic efficiency (avg. CE) of 98.6% for the first 60 cycles and a capacity retention (CR) of 60.4% at the end of the 60th cycle at 0.2/0.5 mA cm–2 charger/discharge current density, while the cell with 1 M LiPF6 EC/DEC at 25 °C offers only an avg. CE of 92.8% for the first 15 cycles with a CR of 40.4% at the 15th cycle. The results suggest that the synergy of hot cycling and electrolyte formulation decreases the surface area of deposited Li and stabilizes the interface that suppresses inventory Li loss. This work probes the Cu/electrolyte interfacial phenomena, attributed to the failure mechanisms of AFLMBs. It also sheds light on suitable strategies to stabilize the Cu interface for stable cycling of AFLMBs.