High transference number enabled by sulfated zirconia superacid for lithium metal batteries with carbonate electrolytes

The prospect of increasing the energy density has promoted research on lithium metal batteries. Yet, avoiding the uncontrolled growth of lithium dendrites and the resulting interfacial instability to ensure the practical viability of the given battery technology remains a considerable challenge. Here, we report coating the separator with sulfated zirconia superacid to achieve a high lithium ion transference number of 0.92 and compelling cycle life when a full-cell paired with a LiNi0.82Co0.07Mn0.11O2 cathode was tested in a carbonate electrolyte under practical operating conditions. The exceptionally high transference number is attributed to strengthened binding of the PF6− anion of the lithium salt with the superacid. Furthermore, a trace amount of water bound to the superacid reacts with PF6− to induce a mechanically stable solid-electrolyte-interphase (SEI) layer rich in LixPOyFz. This study demonstrates the benecial effect of the superacid on emerging post-lithium-ion batteries by immobilizing the anion of the salt as well as modifying the SEI composition.

levels of the lowest unoccupied molecular orbitals (LUMOs) of carbonates are su ciently low 30,31 to induce facile reductive decomposition and consequently a weak irregular organic-dominant SEI layer.
Li ion transport in a liquid electrolyte is affected by multiple parameters including salt dissociation, the degree of solvation, and the viscosity of the solvent, and these parameters are often interrelated to each other. Moreover, Li ion transport and the spatial distribution thereof at the interface are largely in uenced by the physicochemical properties of the SEI layer, and the composition of this layer can affect Li nucleation and subsequent growth behavior 32,33 . This rationale led us to pursue an approach by which the dissociation of the Li salt is tuned to obtain a high Li ion transference number, and the dissociated anion can yield an SEI layer with high stability upon reaction with the electrolyte solvents. In this vein, we adopted sulfated zirconia (denoted as S-ZrO 2 ) because S-ZrO 2 can function as a solid acid to promote the dissociation of the Li salt most commonly used in LIBs, namely lithium hexa uorophosphate (LiPF 6 ), by strongly binding with the PF 6 − anion. A solid acid is a class of superacid and exhibits extremely high acidity, even several orders of magnitude higher than that of pure sulfuric acid. The extraordinary acidity of S-ZrO 2 arises 34,35 from the inductive effect of the sulfate complex with its high electron-withdrawing ability, which enhances the a nity of the superacid for a Lewis base. We applied a coating of S-ZrO 2 superacid nanoparticles to the separator with the aim of distributing the superacid effect over the entire electrode area while simultaneously benchmarking the concept of a ceramic nanoparticle coating to minimize the thermal shrinkage that commercial LIBs are prone to experience at high temperatures. This approach enabled the transference number to reach a value as high as 0.92 as a result of the superacid effect, and this, along with the modi ed SEI layer involving liberated PF 6 − anions, resulted in highly reversible and sustainable cyclability of the Li plating-stripping process. In previous studies, solid superacids were implemented [36][37][38] in polyethylene oxide (PEO) polymer electrolytes to improve the ionic conductivity or transference number; however, they have not been used in liquid-based cells at all. The present investigation points to the unique advantage offered by superacids: simultaneously high values for both ionic conductivity and transference number, unlike solid-state electrolytes that have a high transference number but are affected by moderate ionic conductivity particularly at the interface, and, needless to mention, poor processability. These bene cial properties are rooted in the physical state of superacid solids; i.e., being solid in nature but functioning in the liquid state.

Synthesis and Coating of Sulfated Zirconia Superacid
The S-ZrO 2 nanoparticles were synthesized via a sol-gel process. Brie y, 1.0 M zirconium npropoxide in anhydrous n-propanol was hydrolyzed in a mixture of sulfuric acid and distilled water, followed by condensation at 50 °C for 1 hour. The compound in the gel state was washed with anhydrous ethanol and centrifuged to accomplish dehydration. Finally, the centrifuged sample was calcined at 600 °C for 3 hours. Details of the synthetic procedure are provided in the Methods Section. X-ray diffraction (XRD) analysis ( Figure 1a) showed that the synthesized compound had a single tetragonal phase belonging to the P4 2 /nmc space group (ICSD collection code = 66787) without impurities. The FTIR pro le of the S-ZrO 2 exhibited peaks at 1213, 1130, 1041, and 995 cm −1 (Figure 1b), which were assigned 39 to the sulfate ion coordinated to Zr 4+ in a bidendate con guration. Thermo-gravimetric analysis (TGA) indicated that the sulfate anion content is 10.0 wt% (Supplementary Figure 1). In addition, the N 2 adsorption-desorption isotherm of S-ZrO 2 revealed its surface area to be 104 m 2 g −1 , and its primary particle size to be in the range 5−10 nm, as visualized by scanning electron microscopy (SEM) ( Figure 1c). After measuring the transference number and assessing the cycling performance with a Li-Li symmetric cell setting by varying the sulfate contents in the range of 2−10 wt% (Supplementary Figure 2), the sample with sulfate content of 10 wt% was chosen as the main sample for subsequent experiments.
The polyethylene (PE) separator was coated with the prepared S-ZrO 2 nanoparticles via a simple doctor blading process. The coating was uniformly spread across the entire separator (Figure 1d where , I ss , I 0 , R ss , R 0 , and ∆V represent the Li-ion transference number, steady-state current, initial current, steady-state resistance, initial resistance, and applied voltage (5 mV Figure 10), the peak of the F 1s branch appeared at 687.2 eV and the peaks of the P 2p branch were observed at 136.2 and 134.1 eV. This revealed an SEI composition containing Li x PO y F z , which is consistent with that of the immersed S-ZrO 2 superacid described above, along with the fact that the interior of the SEI layer is rich in inorganic components. It should be noted that the complete removal of trace water from most carbonate electrolytes is impossible. In our experiment, the formation of PO y F z − was preserved even after the electrolyte was stored over molecular sieves to reduce the amount of trace water, implying that the observed effect of the S-ZrO 2 superacid on the SEI composition is consistent regardless of the amount of trace water present in typical carbonate electrolytes. In addition, the XRD pro les of S-ZrO 2 remained unchanged (Supplementary Figure 11), thus supporting the catalytic role of the S-ZrO 2 superacid in increasing the transference number of the Li ion as well as inducing the PO y F z -rich SEI layer.

Conclusion
Interfacial stability is of key importance for the reliable operation of Li metal anodes and is greatly affected by the Li ion transference number and SEI characteristics. In spite of the widely accepted consensus in this regard, achieving the desired properties related to these parameters without sacri cing the volumetric energy density is no trivial matter; for instance, the integration of additional components that increase the Li ion transference number is mostly foreign to the current LIB technology and therefore demerits the use of a Li metal anode after comprehensive evaluation.
In this sense, the approach introduced in the present study, namely the incorporation of the S-ZrO 2 superacid, is effective yet realistic because the application of a coating of ceramic material to the separator is being adopted in most of the cells that are currently commercially available. Moreover, the effect of the given approach is substantial as the S-ZrO 2 superacid enhances binding with the anion of the Li salt, which increases the mobility of the Li ion apart from inducing a more stable SEI layer. While this view of the superacid-anion interaction was supported by Raman and 31 P NMR analyses, its outcome was electrochemically validated in both symmetric-and full-cell con gurations. Taking a broad perspective, the solid-state nature of the superacid warrants its easy integration into a cell and we thus foresee the universal applicability of the positive effects of the superacid to a wide range of rechargeable batteries beyond LMBs.

Methods
Synthesis of sulfated ZrO 2 superacid and ZrO 2 particles. 70 wt% zirconium n-propoxide (Sigma-Aldrich) in 20 mL of propanol (Sigma-Aldrich) was rst mixed with 62.2 mL of anhydrous n-propanol (99.7% purity, Sigma-Aldrich). To this solution, 1.10 mL of sulfuric acid (99.999% purity, Sigma-Aldrich) and 14.81 mL of distilled water were added sequentially for 10 min at 50 °C until the vortex disappeared. The solution was then aged at the same temperature for another hour. Next, 100 mL of ethanol (≥99.5%, Sigma-Aldrich) was added and the solution was centrifuged at 6000 rpm for 10 min to switch the solvent from water to alcohol to minimize particle growth during drying and calcination. This washing via centrifugation was repeated three times. The produced powder was dried at 80 °C for 12 hours to remove residual alcohol, followed by calcination at 600 °C for 3 hours to produce the designated crystal structure. For S-ZrO 2 with 2 and 6 wt% sulfate, the same procedure was adopted except that 0.28 and 0.55 mL of sulfuric acid was added, respectively. The bare ZrO 2 particles were synthesized based on the same procedure as S-ZrO 2 particles except that sulfuric acid was not added.
Coating where I SS is steady state current, I 0 is initial current, R SS is steady state resistance, R 0 is initial resistance, and ΔV is voltage applied (5 mV in the current study). Modi ed PITT analysis was conducted by applying constant voltage for one hour at every 2 mV while the current was monitored. The measured potential ranged from 0 to −50 mV vs. Li/Li + . To test the stability of the SEI layer, EIS analysis was performed after leaving the symmetric cells for a different number of days after undergoing one cycle of Li plating and stripping (C-rate=0.1C, capacity of each plating and stripping=3 mAh). These measurements were recorded in the frequency range of 10 −2 −10 6 Hz with an amplitude of 10 mV.
Characterization of S-ZrO 2 and ZrO 2 particles. XRD (Empyrean, PANanalytic) and FTIR (VERTEX 70, Bruker) analyses were carried out to characterize the crystal structures and chemical bonds of the S-ZrO 2 and ZrO 2 particles, respectively. TGA (TG/DTA 6300, PerkinElmer) was used to evaluate the sulfate content in S-ZrO 2 . SEM analysis (JSM-7000F, JEOL) was performed to visualize the morphology of the Li metal deposits on the surface. Raman analysis with a laser excitation wavelength of 532 nm (Senterra Grating 400, Bruker) was carried out for the electrolytes in which the different separators were soaked for 24 hours to monitor the ion-to-solvent interaction. 31 P solid NMR analysis (ASCEND, Bruker) was conducted for the S-ZrO 2 and ZrO 2 particles soaked in the electrolyte for 24 hours. The conditions for 31 P solid NMR analysis were such that the spinning frequency, dwell time, pre-scan delay, recycle delay, excitation pulse length, and excitation pulse power were 11 kHz, 6.3 msec, 6.3 msec, 0.5 sec, 5.0 msec, and 50 W, respectively. XPS (Scienti c K-ALPHA, Thermo Fisher) was used to elucidate the SEI components.
DFT calculation. Geometrical optimizations and energy calculations were performed without symmetry restriction using the B3LYP hybrid density functional implemented in the GAUSSIAN 09 software package 51 . The 6-311+G(d, p) basis sets were adopted for all the atoms. Frequency calculations at the same basis sets were performed to determine the nature of a stationary point as a true local minimum.

Declarations
Data availability The data that support the plots within this paper and other ndings of this study are available from the corresponding authors upon reasonable request.