This PDF file includes:

• fig. S1. An illustration showing the lithium symmetric coin cell used for the I-V electrokinetic measurements.
  
• fig. S2. Electrokinetic characteristics of viscoelastic electrolytes as a function of measurement time.
  
• fig. S3. Linear sweep voltammetry as a function of PMMA concentration at a scan rate of 1 mV/s.
  
• fig. S4. Schematic drawing of the experimental setup used to perform the visualization experiment for electrodeposition.
  
• fig. S5. Tracer particle velocities in the Newtonian liquid electrolyte (EC/PC–1 M LiTFSI) and a representative viscoelastic electrolyte (4 wt % PMMA in EC/PC–1 M LiTFSI) as a function of voltage.
  
• fig. S6. Complex shear viscosity at 25°C for viscoelastic liquid electrolytes containing varying concentrations of PMMA.
  
• fig. S7. EIS measurements and analysis for viscoelastic electrolytes with different polymer concentrations.
  
• fig. S8. Depth-profiling XPS spectra of a lithium metal surface after electrodeposition for 1 hour at a current density of 1 mA/cm².
  
• fig. S9. Photographic images showing evolution of various metal electrode/electrolyte interfaces during electrodeposition.
  
• table S1. Measured Li⁺ transference numbers and calculated limiting current densities in the optical cell (J₁) and O-ring coin cell (J₂) for electrolytes used in the study.

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