In Situ Polymerization on a 3D Ceramic Framework of Composite Solid Electrolytes for Room‐Temperature Solid‐State Batteries

Abstract Solid‐state batteries (SSBs) are ideal candidates for next‐generation high‐energy‐density batteries in the Battery of Things era. Unfortunately, SSB application is limited by their poor ionic conductivity and electrode‐electrolyte interfacial compatibility. Herein, in situ composite solid electrolytes (CSEs) are fabricated by infusing vinyl ethylene carbonate monomer into a 3D ceramic framework to address these challenges. The unique and integrated structure of CSEs generates inorganic, polymer, and continuous inorganic–polymer interphase pathways that accelerate ion transportation, as revealed by solid‐state nuclear magnetic resonance (SSNMR) analysis. In addition, the mechanism and activation energy of Li+ transportation are studied and visualized by performing density functional theory calculations. Furthermore, the monomer solution can penetrate and polymerize in situ to form an excellent ionic conductor network inside the cathode structure. This concept is successfully applied to both solid‐state lithium and sodium batteries. The Li|CSE|LiNi0.8Co0.1Mn0.1O2 cell fabricated herein delivers a specific discharge capacity of 118.8 mAh g−1 after 230 cycles at 0.5 C and 30 °C. Meanwhile, the Na|CSE|Na3Mg0.05V1.95(PO4)3@C cell fabricated herein maintains its cycling stability over 3000 cycles at 2 C and 30 °C with zero‐fading. The proposed integrated strategy provides a new perspective for designing fast ionic conductor electrolytes to boost high‐energy solid‐state batteries.

was obtained from Umicore. All chemicals were used as received, except AIBN, which was recrystallized, and the degree of hydration of ZrO(NO3)2. H2O was determined by performing TGA analysis before it was used, as shown in Figure S1.

Synthesis of materials
Li6.4La3Zr1.4Ta0.6O12 (LLZT) powder was synthesized by following the sol-gel method. [1] Stoichiometric amounts of LiNO3 (10 mol% excess), La(NO3)3.6H2O, ZrO(NO3)2. H2O, and TaCl5 were dissolved in 100 mL deionised water (DI water) containing citric acid and ethylene glycol (1:1 molar ratio). The number of moles of citric acid was double the total number of S5 moles of cations. This solution was stirred for several hours and evaporated at 80 °C; subsequently, it was transferred to a vacuum oven and fully dried at 80 °C. The obtained powder was treated at 400 °C for 10 h in air atmosphere and then ground to obtain a fine powder. The fine powder was calcined at 900 °C for 12 h in air atmosphere.
Na3.3Zr1.7La0.3(SiO4)2(PO4) (NZLSP) particles were prepared by following a modified solgel method. [2] First, TEOS was dissolved in a mixture of DI water and ethanol in the molar ratio were used, and the number of moles of citric acid was double the total number of moles of cations.
Na3V1.95Mg0.05(PO4)3@C (NVMP@C) was synthesized as described in a previous report. [3] In brief, stoichiometric amounts of NaOH (10 mol% excess), NH4VO3, NH4H2PO4, Mg(CH3COO)2 were added to DI water containing a certain amount of citric acid. The number of moles of acid was equal to the number of moles of NH4VO3. The above solution was stirred for 4 h at 80 °C and then dried at 80 °C for 10 h in a vacuum oven to obtain a powder. Then, the powder was pretreated at 350 °C for 4 h in air atmosphere, followed by carbonization at 800 °C for 8 h in Ar:H2 mixed atmosphere (95:5 vol%) inside a tube furnace.
All powders were stored in a vacuum desiccator for further use.

Preparation of composite solid electrolytes (CSEs)
LLZT, PVP as a surfactant, PVB as a binder, BBP as a plasticizer, and starch as a porous agent were mixed in a solution containing ethanol and acetone (5: NZLSP was prepared by following the same process, except NZLSP powder and NaNO3 as precursors were used instead of LLZT powder and LiNO3, respectively. Electrolyte precursor solutions were prepared by dissolving the designed amount of LiTFSI or NaTFSI salt in 1 g of VEC monomer with 0.5 wt.% of AIBN as a thermal initiator, 5.0 wt.% of PETA as a cross-linking agent, and 50 µL of FEC additive. Then, 20 µL of this solution was injected into the 3D ceramic framework thrice, followed by heating at 70 °C for 10 h in an oven to facilitate the occurrence of the in-situ polymerization reaction. For comparative purposes, the solid polymer electrolyte (SPE) was synthesized using a similar procedure to that of the CSE; however, Whatman® glass fiber was employed in place of the 3D ceramic framework.

Material characterization
The crystal structure of the synthesized materials was investigated using an X-ray

Electrochemical characterization
Electrochemical characterization was performed using CR2032-type coin cells, which were assembled in an Ar-filled glove box. The ionic conductivity (σ) of the CSE in a symmetric cell was measured using Equation 1 and the electrochemical impedance spectroscopy (EIS) technique in an electrochemical workstation (Zive SP2). The symmetric cells were fabricated by placing an inorganic ceramic film on a stainless-steel (SS) disc, after which the monomer solution was injected following the above procedure. Another SS disc was inserted, and then, the CR2032 cells were assembled. Finally, the assembled cells were treated in an oven at 70 °C for 10 h. (1) where t is the thickness, S is the interaction surface area of the CSE, and R is its resistance.
According to Equation 2, the activation energy (Ea) of the CSEs is calculated from the slope of the log σ vs. 1/T plot, as follows: where σ is the ionic conductivity, A is a pre-exponential constant, R is the universal gas constant, and T is the temperature.
The Li|3D-LLZT-CSE|SS asymmetric cells were prepared by following the same above process, except one of the two SS discs was replaced with a 16 mm lithium chip. The Additionally, the Li-ion transference numbers ( + ) of the CSEs were calculated with the Bruce and Vincent method, [5] which is expressed as Equation 3, by using a single-step chronoamperometry technique with an overpotential of 10 mV at 30 °C applied to the Li|3D-LLZT-CSE|Li lithium symmetric cells.

S9
(3) where Io and Ro denote the current and resistance before polarization, respectively; Iss and Rss are the steady-state current and resistance after polarization, respectively; and ∆V is the applied overpotential.
The Li + plating/stripping behavior of CSE in the Li|3D-LLZT-CSE|Li symmetric cell was measured to investigate the stability of the CSE in the presence of Li metal by using a battery cycler (WonATech-WBCS 3000).
The cathode materials (NCM811 or NVMP@C) were blended with Super P carbon (Timcal) and 2 wt.% PVDF or 2 wt.% PAA in NMP solvent as a binder in a weight ratio of 7:2:1 to obtain a homogeneous slurry. Thereafter, the slurry was cast onto an Al foil as a current collector by using a doctor blade. Then, the slurry was dried in a vacuum oven at 80 °C for 12 h. Subsequently, circular electrodes measuring 14 mm in diameter were punched from the foil. To prepare the sodium-ion batteries, we started by wiping the kerosene off Na cubes stored in kerosene (99.8% purity, Sigma-Aldrich). Then, we cut these cubes into smaller cubes and pressed them to obtain approximately 150-μm-thick Na foils. Then, discs measuring 16 mm were punched from these foils and used as anodes. To confirm the performance of as- mAh g -1 and 117.6 mAh g -1 , respectively, which corresponds to 1 C. The obtained specific capacity was calculated based on the masses of the cathode materials. All electrochemical characterizations were performed in a temperature chamber at 30 °C.

Computational method
The density-functional-theory (DFT) calculations were implemented using an ultra-soft pseudo-potential and the generalized gradient approximation in the Heyd-Scuseria-Ernzerhof exchange-correlation functional (HSE) in Quantum Espresso package. [6] The structures were relaxed using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm with force and energy convergences of less than 10 −5 Ry/Bohr and 10 −5 Ry, respectively. The nudged elastic band (NEB) method was used to calculate the Li migration barrier. The electrostatic potential S11 maps and natural bond orbital (NBO) charge were calculated using Gaussian 09. The VESTA and XcrySDen programs were used for visualization. [7] S4 2. Tables   Table S1. Rietveld    Therefore, the degree of hydration was calculated as follows: 2 , and 2 are the molecular masses of ZrO2, ZrO(NO3)2. H2O, and H2O, respectively. . % 1000ᴼ is the weight percentage of the residual samples at 1000°C. The calculated degree of hydration is 5.52, meaning that the nominal chemical formula of the precursor is ZrO(NO3)2(5.52H2O).         S32 Figure S20. High-resolution XPS spectra of (a) C 1s and (b) F 1s of Na electrode after cycling test.