Designable ultra-smooth ultra-thin solid-electrolyte interphases of three alkali metal anodes

Dendrite growth of alkali metal anodes limited their lifetime for charge/discharge cycling. Here, we report near-perfect anodes of lithium, sodium, and potassium metals achieved by electrochemical polishing, which removes microscopic defects and creates ultra-smooth ultra-thin solid-electrolyte interphase layers at metal surfaces for providing a homogeneous environment. Precise characterizations by AFM force probing with corroborative in-depth XPS profile analysis reveal that the ultra-smooth ultra-thin solid-electrolyte interphase can be designed to have alternating inorganic-rich and organic-rich/mixed multi-layered structure, which offers mechanical property of coupled rigidity and elasticity. The polished metal anodes exhibit significantly enhanced cycling stability, specifically the lithium anodes can cycle for over 200 times at a real current density of 2 mA cm–2 with 100% depth of discharge. Our work illustrates that an ultra-smooth ultra-thin solid-electrolyte interphase may be robust enough to suppress dendrite growth and thus serve as an initial layer for further improved protection of alkali metal anodes.

*The DOD for Li in these reported literatures are no more than 1% unless otherwise specified, which are estimated according to the 1 cm 2 Li foil with 0.75 mm in thickness. While the DOD in this work for Li|Li and Li|S cells are 50% and 30%, respectively, as described in Methods.

of Na anodes treated by different strategies.
*The DOD for Na in these reported literatures are estimated according to the ~1 cm 2 Na foil with 1 mm in thickness unless otherwise specified.

Estimation of alkali metal ion surface concentration
The can be estimated and the results are listed in the Supplementary Table 3. As can be seen that after dissolution of 120 s, an average concentration of as high as 11 mol L -3 can be reached.

Calculation of Young's Moduli of SEI films
where F is the loading force, δ the indentation depth in the elastic deformation region, E the Young's modulus, ν the poison ratio (0.5), and R the tip radius (~8 nm). In addition, Wad is the adhesion work per unit area, which can be determined by the adhesion force, Fad, at jump-offcontact point on the force-displacement curve, divided by the contact area that is determined according to the maximum contact radius, a, at the point just before tip withdrawing on the force-displacement curve.
To obtain the adhesion force, Fad, associated with the tip-sample interaction in the elastic deformation region located in the very initial stage of tip indentation, force curves were recorded with withdrawing point carefully controlled in the elastic deformation region, avoiding tip running into the plastic deformation region. This is shown by Supplementary Fig.   7. The adhesion force, Fad, can be read out from the jump-off-contact, the work of adhesion is calculated by the equation: W'ad = Fad/2R 5 and the contact radius, a, calculated by the equation: a 2 =R. This allows calculate the work of adhesion per unit area, Wad, and thus Young's modulus by Supplementary Equation (3).

Calculation of thickness of SEI based on AFM indentation measurement
Basically, the total thickness of SEI by AFM indentation technique is determined

Calculation of the IR spectra and band assignments
IR spectra of a series of free molecules and their dimers were calculated with Gaussian 09 D.01 package 1 using B3LYP functional 2,3 . The basis sets 6-311++G (d, p) were employed for geometry optimization of the molecular structures. No imaginary frequencies were present after optimization. Theoretical vibrational frequencies are scaled by a factor of 0.967. Calculated IR spectra of CH3OCH2CH2OLi dimers are presented in Supplementary Fig. 12. Band from anion reduction products are assigned taking the information from literature works as reference.
Detailed band assignments are given in Supplementary Table 4. Although the free molecules, even their dimers, could be very different from those in the SEI layers, the similarities between the calculated and measured spectra allow us to propose that the CH3OCH2CH2OLi may be essential moiety in the SEI layer.

Proposed mechanism for electrochemical polishing of M A surface and formation of multi-layer structured SEIs
The electrochemical polishing of MA surface, involving anodic dissolution as the first step (stripping step), falls within the framework of general principle for electrochemical polishing of metallic surfaces in aqueous solutions, but distinct with the concurrent reduction of electrolyte and formation of insoluble SEI film which is indispensable for protecting MA surface after the polishing. A mechanism involving coupled viscous liquid layer 6 and SEI thin film is proposed to account for the polishing of the MA surface and formation of SEI. In the following, we provide a further discussion on the mechanism of two-step based electrochemical polishing, taking Li metal surface as an example.
In the stripping step, high-rate Li dissolution takes place, which generates an exceedingly high surface concentration of Li + so that a viscous layer is formed in the vicinity of the Li surface. The viscous layer is thinner at protrusions than at depressions due to difference in diffusion rate, favoring the preferential dissolution of Li from the protrusions. A primary SEI thin film is formed through electrolyte reduction, with the aid of high Li + concentration. DOL can be reduced to CH3OCH2CH2OLi and CH3CH2OCH2OLi: 3 Depending on the stripping potential ( Supplementary Fig. 1), the sequence of reactions and amount of organic and different inorganic products could vary considerably so that chemical composition of the SEI can be manipulated. At an optimum stripping potential, e.g.
1.0 V in the system of DOL/DME-LiTFSI, both the reduction of DOL and TFSI can proceed moderately to form oligmers of (ROLi)n which may be cross-linked by small inorganic molecules such as LiF and Li3N to create a network structure of organic-rich inner layer of SEI.
The proposed reactions are as follows: A possible structure with network-like organic-rich inner layer and inorganic-rich outer layer, i.e. the O-I type structured SEI, is schematically illustrated in the figure shown below.
Naturally, other types of SEI can be easily achieved by multi-step stripping-annealing process.
Supplementary Figure | Schematic of a possible SEI structure with network-like organic-rich inner layer and inorganic-rich outer layer, i.e. the O-I type of SEI structure.

The analysis of conductivity for M A + -rich SEI
The EIS spectra of different electrodes were fitted using the equivalent circuit proposed in Fig. 4h  used, which were obtained by electrochemical polishing of Li thin film coated Cu@Au electrodes after thoroughly removing residual Li by electrochemical dissolution. All Samples were washed thoroughly using the anhydrous DME solvent to remove residual electrolytes, and then transferred into the XPS or XANES chambers by using commercial air-isolating containers.
Fourier transform infrared (FTIR). FTIR measurements were performed on a Nicolet Avatar FTIR Spectrometer (Thermo Scientific) equipped with an MCT detector with a resolution of 4 cm -1 . All samples were washed thoroughly using anhydrous DME solvent to remove residual electrolytes and then placed in a sealed and Ar-filled FTIR cell without exposure to air. FTIR spectra were recorded in diffuse reflectance mode. Interferences from CO2 and H2O in air are suppressed by filling the testing chamber of the equipment with Ar. Spectra ranging from 400-4000 cm -1 with 32 accumulative scans were obtained. All FTIR dates were calibrated by Kubelka-Munk function before analysis.

Other related characterizations.
The open circuit potentials (OCP) of Li in DME/DOL (1/1, v/v) solvent and Li in 1 M LiTFSI/DME/DOL (1/1, v/v) electrolyte were measured using Li foil as working electrode. The OCP was read against a Pt wire whose potential was calibrated with respect to a Li foil electrode after prolonged soaking in 1 M LiTFSI/DME/DOL (1/1, v/v) electrolyte. The electrochemical impedance spectra (EIS) were measured before and after charge-discharge cycling using Autolab PGSTAT204 electrochemical analyzer (Metrohm) in the frequency range from1 MHz to 0.01 Hz with peak-to-peak amplitude of 5 mV. The morphologies of metal surfaces before and after cycling were characterized by scanning electron microscope (SEM, SU4800). For morphology measurement after cycling, cells were disassembled in the glove box followed by thoroughly washing the electrodes with anhydrous 41 DME to remove residual electrolytes. To avoid air exposure of samples, the dried samples were sealed in air-isolating containers in glove box and transferred quickly into the SEM equipment under the protection of Ar flow. Surface element analysis was performed by means of an energy dispersive X-ray fluorescence spectrometer (EDX). Optical images of the samples in this paper were obtained by 8 megapixel Apple iPhone 5s camera from a fixed position under constant lighting conditions.