Ion‐Exchange‐Induced Phase Transition Enables an Intrinsically Air Stable Hydrogarnet Electrolyte for Solid‐State Lithium Batteries

Abstract Inferior air stability is a primary barrier for large‐scale applications of garnet electrolytes in energy storage systems. Herein, a deeply hydrated hydrogarnet electrolyte generated by a simple ion‐exchange‐induced phase transition from conventional garnet, realizing a record‐long air stability of more than two years when exposed to ambient air is proposed. Benefited from the elimination of air‐sensitive lithium ions at 96 h/48e sites and unobstructed lithium conduction path along tetragonal sites (12a) and vacancies (12b), the hydrogarnet electrolyte exhibits intrinsic air stability and comparable ion conductivity to that of traditional garnet. Moreover, the unique properties of hydrogarnet pave the way for a brand‐new aqueous route to prepare lithium metal stable composite electrolyte on a large‐scale, with high ionic conductivity (8.04 × 10−4 S cm−1), wide electrochemical windows (4.95 V), and a high lithium transference number (0.43). When applied in solid‐state lithium batteries (SSLBs), the batteries present impressive capacity and cycle life (164 mAh g−1 with capacity retention of 89.6% after 180 cycles at 1.0C under 50 °C). This work not only designs a new sort of hydrogarnet electrolyte, which is stable to both air and lithium metal but also provides an eco‐friendly and large‐scale fabrication route for SSLBs.


Synthesis of garnet and hydrogarnet electrolyte
The garnet electrolyte powder Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 was synthesized via a solid-state route reported by previous works [1] .To get the deep hydrated hydrogarnet electrolyte powder, hydrothermal treatment was utilized.
Firstly, garnet electrolyte powder was ultrasonically dispersed in deionized (DI) water with a content of 10 wt.%.
Then, the garnet electrolyte dispersion was transferred to stainless steel autoclaves with polytetrafluoroethylene (PTFE) lining, and the sealed autoclave was placed in an oven and heated to the target temperature for 6 hours.It is worth noting that the applied hydrothermal treatment temperature should not exceed 373K to avoid the generation of impurities such as La(OH) 3 .After cooling down naturally, the hydrogarnet electrolyte powder was finally collected by centrifugation.After washing with deionized (DI) water until the obtained filtrate was neutral, hydrogarnet electrolyte powder was obtained and dried at 80°C in an oven for 12 h.

Structural and compositional characterization methods
To detect the phase composition, XRD (X-ray diffraction) tests were conducted with a Bruke D8 Advanced Xray diffractometer.The morphology and the EDS (energy dispersive spectrometer) mapping of garnet powder, hydrogarnet powder, and PEO-xHG CPEs were conducted by a Zeiss Sigma 300 field emission scanning electron microscopy (SEM).The high-resolution TEM images and SEAD patterns of garnet and hydrogarnet electrolytes were obtained by FEI Tecnai G2 F20 TEM.Raman spectroscopy tests of garnet and hydrogarnet were conducted by a Renishaw inVia Raman spectrometer.Fourier-transform infrared (FT-IR) test results were obtained by Thermal Fisher Nicolet iS5 FT-IR spectrometer.The pH values of both garnet dispersion and hydrogarnet dispersion were obtained by a Horiba LAQUA F71G pH meter.The 6 Li magic angle spinning nuclear magnetic resonance (MAS NMR) tests were carried out with a Bruker Advanced NEO NMR spectrometer.
The 1 H- 7 Li heteronuclear correlation (HETCOR) MAS NMR experiment was carried out on a Bruker Avance III HD 500 M spectrometer (11.7 T).The resonance frequencies of 1 H and 7 Li are 500.1 MHz and 194.4 MHz respectively.Spectra were recorded at room temperature using a 4-mm Bruker magic angle spinning (MAS) probe at a spinning speed of 12 kHz.The TPPM15 pulse sequence used during the sampling period corresponds to a 160° pulse length of 5.3 us and a relaxation time of 1 s.The nutation frequency corresponding to the 1 H crosspolarization ramp energy is reduced from 36.2 kHz to 18.1 kHz.The optimized nutation frequency of 7 Li is 53.2 kHz, and the cross-polarization contact time is 1.25 ms.The chemical shifts of 1 H and 7 Li were referenced to H 2 O (= 4.7 ppm) and 1 M LiCl solution (= 0 ppm).
The aging process was conducted under full open ambient air for up to 2 years.The electrolyte powder was placed in Petri dishes and the Petri dishes were positioned inside a rack with only two sides and a top, to prevent dust from falling into the powder.After aging for 6 months, 1 year, and 2 years, part of the aged electrolyte powder was used for compositional and structural tests.

Theoretical calculation methods
All first-principles calculations are performed in the VASP software [2] using projection augment-wave (PAW) [3] Ab initio with periodic boundary conditions in the framework of density functional theory (DFT).The exchange-correlation energy is calculated using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerh of (PBE) [2] .These systems are assumed to be paramagnetic, and standard spin-polarized DFT calculations were performed throughout the study.Monkhorst-Pack grid is performed using 3×3×1 for structural optimization, and 9×9×1 is used for electronic calculations.The convergence criteria for the total energy and atomic forces are 10 -5 eV and 10 -4 eV•Å, respectively, within the above parameters.The minimum energy paths and the energy barriers for K-ion dissociation were obtained by the climbing image nudged elastic band (CINEB) method. [4]e free energy (ΔG) of each reduction step was obtained at zero bias potential using, [5] ΔG = ΔE + ΔE ZPE -TΔS where ΔE was the reaction energy, ΔE ZPE was the difference in zero-point energies, T was the temperature (298.15K) and ΔS is the reaction entropy.
The simulated SEAD was obtained by the ReciPro software package based on the CIF (crystallographic information file) optimized by DFT calculations.An electron beam with an accelerated voltage of 200 keV was used in the simulation.Band-valence energy landscape calculations were performed based on Bond_Str software using the same CIF files as SEAD simulations. [6]The mesh density was 80×80×80, and the d-max was selected to 5.0 angstrom.The activation energy of the iso-surface shown in BVEL maps is 1.5 eV.

Fabrication of PEO-hydrogarnet composite electrolyte
Firstly, 660 mg Polyethylene oxide (PEO, Aladdin, Mw=600 000) and 286 mg Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Aladdin, 99.99%) were dissolved in 5.65 mL deionized water to get a homogeneous PEO-LiTFSI solution.Then different amounts of nano-sized hydrogarnet powder (0, 25 mg, 50 mg, 100 mg, 200 mg, or 400 mg) were added to 2 mL PEO-LiTFSI solution, and the mixture was magnetically stirred for 12 hours to form a uniform slurry.After being cast into PTFE molds.the slurry was dried at 50°C under ambient air for 6 hours.Finally, the composite electrolyte was obtained after vacuum drying at 60°C for 12 hours.

Cell assembly and electrochemical measurements
In order to facilitate ion transportation inside the cathode, a composite LiFePO 4 (LFP) cathode containing PEO and LiTFSI was utilized.Firstly, 65 mg PEO and 35 mg LiTFSI were dissolved in 3 mL DI water.Then, 800 mg LFP powder (carbon coated, Shenzhen Dynanonic Co., Ltd.) and 100 mg Super P-Li (TIMCAL Graphite & Carbon Co., Ltd.) were added into the solution to form the cathode slurry.After magnetically stirring for 12 h, the slurry was tape-casted to alumina foil with a 150 μm blade, and the Al foil was heated under 60°C for 2 hours to get rid of water solvent.Finally, LFP composite cathodes were obtained after vacuum drying for 12 hours and the mass loading was 2-3 mg•cm -2 The Li|LFP batteries and lithium symmetry batteries were assembled within CR2032-type coin cells.The Lisymmetry batteries were fabricated by attaching two lithium sheets (Φ12mm) to both sides of the CPEs.The Li|LFP batteries were fabricated by attaching lithium sheets (Φ12mm) and LFP composite cathode (Φ12mm) to both sides of the CPEs.The assembled batteries were heated under 60°C for 12 hours to improve interfacial contact.
The charge/discharge tests of the cells were carried out using Land 2001A at 60°C.The electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), and potentiostatic polarization tests for lithium transference number measurement were conducted via a Biologic VSP electrochemical workstation.
For each CPE, the value of lithium transference number (t Li+ ) can be calculated by Bruce-Vincent method given by the following formula.

𝑡 𝐼 ∆𝑉 𝐼 𝑅 𝐼 ∆𝑉 𝐼 𝑅
Where ∆ is a small DC bias applied to polarize the sample,  is an initial value of current upon polarization with DC bias,  is a current reached in the steady-state for sample polarized with DC bias,  and  are resistances of solid electrolyte interface (SEI) before and after polarization, respectively.Figure S14.The 1 H- 7 Li HETCOR MAS NMR result of hydrogarnet.There is only one single peak in both 1 H and 7 Li projection observed, implying that only one lithium site remained as well as one proton position generated in this hydrogarnet structure.The broad peak in 1 H projection centered at +4 ppm approximately represents the formation of hydroxyls according to some previous reports. [7]Moreover, in the 1 H- 7 Li HETCOR MAS NMR result, the clear signal correlating the single 7 Li band with the 1 H band also supports the existence of lithium-hydroxyl interaction in the hydrogarnet structure., [8] the initial positions of protons are set in 48g sites and anchored by oxygens.Results of the structural optimization also supported this structure except for longer O-H band distances.Figure S16: This scheme showed the cell used for cold-pressed ionic conductivity measurement.the cell was correctly assembled, the whole cell was placed under a manual pellet hydraulic press machine and a pressure of 6 tons was applied to this configuration (Φ12mm rods).Subsequently, we tightened the pressure screws, ensuring that the pressure was maintained even after the press machine was removed.Both PEEK mold and insulator linings were applied in this cell to isolate the anode and cathode.Then EIS tests were conducted on this cell under different temperatures.7.9 The first line showed the content of metal elements calculated from the chemical formula of the garnet electrolyte.
Although some content deviation was observed, the Li/La, Li/Zr, and Zr/Ta ratios remained the same between the calculated results and the experimental results.Observed in the ICP results, Li content in hydrogarnet dropped to 22.7% of the pristine garnet, corresponding to the change from 6.5 lithium per garnet formula to 1.5 lithium per hydrogarnet formula.
It should be noted that, based on the TG results, a similar Li/H ratio can also be obtained by the following equation.It can be seen that for the lattice plane (310), the scattering factor of electron beams is an order larger than that of X-rays.

Figure S1 :
Figure S1: SEM images of both garnet and hydrogarnet electrolytes.After hydrothermal treatment under rationally selected conditions, no obvious alteration can be observed from SEM images.

Figure
Figure S2: EDS-Mapping images of both garnet and hydrogarnet electrolyte particles.After hydrothermal treatment under rationally selected conditions, no element segregation can be observed from EDS results.

Figure S3 :
FigureS3: SEM image of the resultant powders after hydrothermal treating garnet electrolyte powder under a higher temperature (453 K).A great number of needle-like impurities can be observed from SEM images.

Figure S4 :
FigureS4: XRD pattern of the resultant powder illustrated in FigureS3, impurity peaks belonging to La (OH) 3 were observed in this XRD pattern, suggesting a La loss may happen under a higher temperature.

Figure S5 :
Figure S5: XRD pattern of the calcinated (960 °C for 3 h) hydrothermal treated garnet electrolyte (hydrogarnet) powder.Prominent peaks of La 2 Zr 2 O 7 are observed after calcination, suggesting the hydrothermally treated garnet powder will decompose to La 2 Zr 2 O 7 accompanied by proton loss in the form of H 2 O.

Figure S6 :
Figure S6: High-resolution TEM images of both garnet and hydrogarnet electrolyte powder, the measured d 002 is 6.52 Å, in accordance with the lattice parameter (13.04 Å).

Figure S7 :
Figure S7: TEM images of the electrolyte particles selected for SEAD tests.Unsatisfying, in order to accumulate enough intensity of the characteristic spots of hydrogarnet, both garnet and hydrogarnet particles selected are sub-micron levers to accumulate sufficient intensity of the diffraction beam.As a consequence, double diffraction spots can be seen in both SEAD patterns obtained experimentally.Despite this, double diffraction spots of the I a3 ̅ d structure will only have even miller indices, spots of (310) and (510) are also prohibited, ensuring the persuasiveness of SEAD results.

Figure S8 :
Figure S8: (a) Raman spectra and (b) XRD patterns of hydrogarnet before and after being aged in ambient air for 2 years.From both Raman spectra and XRD patterns, no obvious signal of Li 2 CO 3 is observed in hydrogarnet powder even after a 2-year aging, in accordance with FT-IR results.Moreover, no obvious change happened to the hydrogarnet, indicating reliable phase stability of the hydrogarnet.

Figure S9 :
Figure S9: To confirm this prospect, both hydrogarnet and garnet powder were mixed with NMP solvent and heated for 6 hours under 80 °C.FT-IR spectra of the resultant powders were collected.The FT-IR spectra show that no obvious change of hydrogarnet is observed while prominent Li 2 CO 3 is generated in garnet powder, suggesting an improved stability of hydrogarnet toward NMP solvent.

Figure S10 :
Figure S10: Optical photos of the NMP solvent after contact with both hydrogarnet (the right bottle) and garnet (the left bottle) powder.The color change suggests the harmful effect of garnet electrolyte on NMP solvent, while this kind of harmful effect is absent for hydrogarnet.

Figure S11 :
Figure S11: Detailed coordination environment (a six-member ring parallel to [111] axis) of lithium ions.In the initial garnet structure, both 24d sites and 96h sites are occupied by lithium ions.However, for the hydrogarnet structure, only lithium ions in 12a sites are retained.

Figure S12 :Figure
Figure S12: FT-IR spectra of garnet electrolyte, acid-treated garnet, and hydrogarnet electrolytes after 14 days' aging.From the FT-IR spectra, signal of Li 2 CO 3 , although weaker than garnet, can be seen in the aged acid-treated garnet electrolyte.When compared to hydrogarnet electrolyte (where signals of Li 2 CO 3 are absent), the deficient stability of acid-treated garnet electrolyte is revealed.

Figure S14 :
FigureS14: Schemes of proton positions inside hydrogarnet structure.To realize the distribution of proton and get the final crystalline structure, structural optimization was carried out using first-principal calculations (DFT).According to neutron powder diffraction (NPD) results obtained by Orera, Alodia et al.,[8] the initial positions of protons are set in 48g

Figure S15 :
Figure S15: Lithium migration activation energy profile alone Y1-X2 route.The activation energy alone in this route passing through 4 protons is 550 meV.

Figure
Figure S17: (a) A scheme demonstrating the process preparing the PEO-xHG CPE based on the laboratory procedure.Consequently, free-standing PEO-xHG electrolyte membranes can be feasibly obtained; (b) SEM image of the PEO-20HG composite electrolyte, and the evenly distributed hydrogarnet particles in the PEO matrix can be observed.

Figure S20 :
Figure S20: The ionic conductivity of PEO-20HG, PEO-0HG, and PEO-20G CPEs under different temperatures.It can be seen that PEO-G20 CPE possesses the lowest ionic conductivity in the whole temperature range and even lower than that of PEO-0HG electrolyte, revealing the adverse effects of impurities in PEO-G20 on ionic conductivity and the incompatibility of garnet electrolyte toward water-based fabrication.

Figure S21 :
Figure S21: The stability of both garnet electrolyte powder and hydrogarnet electrolyte powder was tested by leaving the dispersion undisturbed for 24 hours and observing the sedimentation.As we can see from the photos, sedimentation was observed in the garnet dispersion after 30 minutes while hydrogarnet dispersion remained stable for nearly 24 hours.

Figure S22 :
Figure S22: XRD patterns of PEO-0HG and PEO-20HG.FWHM values of the listed peaks of PEO-20HG electrolytes are larger than those of PEO-0HG electrolytes, revealing a lower crystallinity of PEO-20HG electrolytes.

Figure
Figure S24: I-t curves of the Li-symmetry assembled PEO-20HG electrolyte before and after polarization (the insets showed the corresponding Nyquist plots).

Figure S25 :
Figure S25: Performance of lithium symmetry cells assembled with PEO-20HG CPE at a current density of 0.1 mA•cm -2 under 50 °C.Cell with PEO-20HG CPE can operate stable for more than 525 hours with an overpotential of 45 mV.

Figure S26 :
Figure S26: The charge and discharge curves of Li-LFP batteries assembled with PEO-20G CPE at different cycles.At the first 3 cycles at 0.2 C, the Li-LFP battery with PEO-0HG CPE demonstrated a relatively stable operation with a low specific capacity of 125 mAh•g -1 .However, when the charge rate increased to 1.0C, PEO-20G started to decompose severely.

Figure S27 :
Figure S27: The charge and discharge curves of Li|LFP batteries assembled with both PEO-0HG CPE and PEO-20HG CPE under different rates at 50 °C.Batteries with PEO-20HG demonstrated lower overpotentials and higher reversible capacities under different rates.

Table S1：ICP -
OES test results of both garnet and hydrogarnet electrolyte.

Table S2 :
Li .La Zr .Ta .O OH ≜ Li .La Zr .Ta .O The intensity of X-ray scattering and electron beam scattering for the hydrogarnet electrolyte of different lattice planes at the d range of 5.31 Å to 1.84 Å.