On the hidden transient interphase in metal anodes: Dynamic precipitation controls electrochemical interfaces in batteries

Abstract

The solid-electrolyte interphase (SEI) formed on a battery electrode has been a central area of research for decades. This structurally complex layer profoundly impacts the electrochemical deposition morphology and stability of metal anodes. Departing from conventional approaches, we investigate metal dissolution-the reverse reaction of deposition-in battery environments using a state-of-the-art electroanalytical system combining a rotating-disk electrode and operando visualization. Our key finding is the presence of a transient SEI (T-SEI) that forms during fast discharging at high dissolution rates. We attribute T-SEI formation to local supersaturation and resultant electrolyte salt deposition. The T-SEI fundamentally alters the dissolution kinetics at the electrochemical interface, yielding a flat, clean surface. Unlike a classical SEI formed due to electrolyte decomposition, the T-SEI is "relaxable" upon removal of the enforced dissolution current; that is, the T-SEI dissolves back into the electrolyte when rested. The formation of T-SEI plays an unexpected critical role in the subsequent electrodeposition. When the metal is redeposited on a fully relaxed T-SEI surface, the morphology is remarkably different from that deposited on pristine or low-rate-discharged metal electrodes. Electron backscatter diffraction analysis suggests that the deposition occurs via growth of the original grains; this is in stark contrast to the isolated, new nuclei seen on standard metal electrodes without T-SEI formation. Using 3D profilometry, we observe a 42% reduction in surface roughness due to T-SEI formation. Our findings provide important insights into the kinetics at ion-producing electrochemical interfaces, and suggest a new dimension for engineering next generation batteries.

Keywords: batteries; interphase; metal anodes.

Fig. 1.

Classical SEI and T-SEI. (A) Schematic diagram showing the formation of a classical nontransient SEI caused by electrolyte decomposition. Due to its heterogeneous nature, metal growth upon the next charge is nonuniform. (B) Schematic diagram showing the formation of a Transient SEI. Under fast discharging conditions, the dissolution of the metal results in a steep local concentration gradient near the electrode, leading to local supersaturation and salt crystallization. A flat, uniform surface is produced because of the self-limiting nature of T-SEI dominated dissolution. The T-SEI completely dissolves over a characteristic relaxation time ${\tau }_{R,SEI}$ when the local supersaturation is no longer present after the discharge is stopped.

Fig. 2.

Understanding T-SEI formation dynamics using a RDE. Current density-voltage (J-V) curves of electrodissolution in (A) 20 mM, (B) 1 M, and (C) 3 M ZnSO₄ aqueous electrolytes. Time-dependent current measured in constant-potential electrodissolution at different potentials with no rotation: (D) 1 M and (E) 3 M ZnSO₄ aqueous electrolytes. The working electrode is Zn metal foil. The scan rate is 100 mV/s for all measurements.

Fig. 3.

Determination of the critical capacity Q_c for T-SEI formation. (A) Voltage profiles of electrodissolution in 3 M ZnSO₄ at different current densities. (B) Critical capacity Q_c dissolved before voltage spike plotted against reciprocal of current density for 3 M ZnSO₄ and 30 m ZnCl₂ electrolytes. (C) Optical images of the Zn metal working electrode at different times during 100 mA/cm² constant-current measurement [the green curve in panel (A)] in 3 M ZnSO₄ electrolyte.

Fig. 4.

Probing T-SEI properties using EIS. (A and B) EIS data represented on a Nyquist plot at different dissolution potentials. (C) Fitted values of charge transfer resistance as a function of potential and (D) fitted values of double-layer capacitance as a function of potential. The Zn metal anode is held at a series of potentials as detailed in the plots for the EIS measurements. Electrolyte: 3 M ZnSO₄.

Fig. 5.

Surface characterization of the metal electrode after full relaxation of the T-SEI. (A) Schematic diagram showing surface chemistry evolution associated with the T-SEI formation and relaxation. (B and C) FIB-SEM of the interface in (B) the Zn electrode discharged at high rate in 3.3 M ZnSO₄ and (C) pristine Zn. (D–F) EDX spectra of high-rate-discharged (yellow), low-rate-discharged (red), and pristine (blue) Zn electrodes showing oxygen (D), sulfur (E), and carbon (F) peaks. 1 mAh/cm² was discharged for both high-rate (5 V vs. Zn²⁺/Zn) and low-rate (0.1 V vs. Zn²⁺/Zn) samples. Electrolyte: 3 M ZnSO_4.

Fig. 6.

Influence of T-SEI formation on electrodeposition morphology. SEM images of (A and B) low-rate-discharged Zn electrodes at 0.1 V vs. Zn²⁺/Zn and (C and D) high-rate-discharged Zn electrodes at 5 V vs. Zn²⁺/Zn after T-SEI formation and relaxation. Discharge capacity: 0.5 mAh/cm². SEM images of redeposition morphology on (E and F) low-rate-discharged and (G and H) high-rate-discharged, T-SEI-formed/relaxed Zn electrodes. Redeposition condition: 0.5 mAh/cm² at a constant potential of −1 V vs. Zn²⁺/Zn. (I) EBSD image of a high-rate-discharged Zn electrode. (J) EBSD image of the redeposited Zn onto high-rate-discharged, T-SEI-formed/relaxed Zn electrode. The flat, and coarse-grained morphology observed is fundamentally different from redeposition on a low-rate-discharged Zn electrode. All experiments were carried out in 3 M ZnSO₄.