Weak-Interaction Environment in a Composite Electrolyte Enabling Ultralong-Cycling High-Voltage Solid-State Lithium Batteries.

Poly(vinylidene fluoride) (PVDF)-based solid electrolytes with a Li salt-polymer-little residual solvent configuration are promising candidates for solid-state batteries. Herein, we clarify the microstructure of PVDF-based composite electrolyte at the atomic level and demonstrate that the Li+-interaction environment determines both interfacial stability and ion-transport capability. The polymer works as a "solid diluent" and the filler realizes a uniform solvent distribution. We propose a universal strategy of constructing a weak-interaction environment by replacing the conventional N,N-dimethylformamide (DMF) solvent with the designed 2,2,2-trifluoroacetamide (TFA). The lower Li+ binding energy of TFA forms abundant aggregates to generate inorganic-rich interphases for interfacial compatibility. The weaker interactions of TFA with PVDF and filler achieve high ionic conductivity (7.0 × 10-4 S cm-1) of the electrolyte. The solid-state Li||LiNi0.8Co0.1Mn0.1O2 cells stably cycle 4900 and 3000 times with cutoff voltages of 4.3 and 4.5 V, respectively, as well as deliver superior stability at -20 to 45 °C and a high energy density of 300 Wh kg-1 in pouch cells.

Defect Strategy in Solid-State Lithium Batteries.

Solid-state lithium batteries (SSLBs) have great development prospects in high-security new energy fields, but face major challenges such as poor charge transfer kinetics, high interface impedance, and unsatisfactory cycle stability. Defect engineering is an effective method to regulate the composition and structure of electrodes and electrolytes, which plays a crucial role in dominating physical and electrochemical performance. It is necessary to summarize the recent advances regarding defect engineering in SSLBs and analyze the mechanism, thus inspiring future work. This review systematically summarizes the role of defects in providing storage sites/active sites, promoting ion diffusion and charge transport of electrodes, and improving structural stability and ionic conductivity of solid-state electrolytes. The defects greatly affect the electronic structure, chemical bond strength and charge transport process of the electrodes and solid-state electrolytes to determine their electrochemical performance and stability. Then, this review presents common defect fabrication methods and the specific role mechanism of defects in electrodes and solid-state electrolytes. At last, challenges and perspectives of defect strategies in high-performance SSLBs are proposed to guide future research.

Dielectric Filler-Induced Hybrid Interphase Enabling Robust Solid-State Li Metal Batteries at High Areal Capacity.

The fillers in composite solid-state electrolyte are mainly responsible for the enhancement of the conduction of Li ions but barely regulate the formation of solid electrolyte interphase (SEI). Herein, a unique filler of dielectric NaNbO3 for the poly(vinylidene fluoride) (PVDF)-based polymer electrolyte, which is subjected to the exchange of Li+ and Na+ during cycling, is reported and the substituted Na+ is engaged in the construction of a fluorinated Li/Na hybrid SEI with high Young's modulus, facilitating the fast transport of Li+ at the interface at a high areal capacity and suppressing the Li dendrite growth. The dielectric NaNbO3 also induces the generation of high-dielectric ß phase of PVDF to promote the dissociation of Li salt. The Li/Li symmetrical cell exhibits a long-term dendrite-free cycling over 600 h at a high areal capacity of 3 mA h cm-2. The LiNi0.8Mn0.1Co0.1O2/Li solid-state cells with NaNbO3 stably cycle 2200 times at 2 C and that paired with high-loading cathode (10 mg cm-2) can stably cycle for 150 times and exhibit excellent performances at -20 °C. This work provides a novel design principle of fillers undertaking interfacial engineering in composite solid-state electrolytes for developing the safe and stable solid-state lithium metal battery.

Determining the Role of Ion Transport Throughput in Solid-State Lithium Batteries.

Solid-state lithium metal batteries (SSLMBs) are promising candidates for high-energy-density energy storage devices. However, there still lacks an evaluation criterion to estimate real research status and compare overall performance of the developed SSLMBs. Herein, we propose a comprehensive descriptor, Li+ transport throughput ( φ L i + ${{\phi{} }_{{{\rm L}{\rm i}}^{+}}}$ ), to estimate actual conditions and output performance of the SSLMBs. The φ L i + ${{\phi{} }_{{{\rm L}{\rm i}}^{+}}}$ is defined as molar number of Li+ passing through unit area of electrode/electrolyte interface in an hour (mol m-2 h-1 ) during cycling of battery, which is a quantizable value after taking complex aspects including cycle rate, electrode areal capacity and polarization into account. On this basis, we evaluate the φ L i + ${{\phi{} }_{{{\rm L}{\rm i}}^{+}}}$ of liquid, quasi-solid-state and solid-state batteries, and highlight three key aspects to achieve high value of φ L i + ${{\phi{} }_{{{\rm L}{\rm i}}^{+}}}$ via building highly efficient cross-phase, cross-gap and cross-interface ion transport in the solid-state battery systems. We believe that the new concept of φ L i + ${{\phi{} }_{{{\rm L}{\rm i}}^{+}}}$ provides milestone guidelines towards large-scale commercialization of SSLMBs.

Inhibiting Formation and Reduction of Li2 CO3 to LiCx at Grain Boundaries in Garnet Electrolytes to Prevent Li Penetration.

Poor ion and high electron transport at the grain boundaries (GBs) of ceramic electrolytes are the primary reasons for lithium filament infiltration and short-circuiting of all-solid-state lithium metal batteries (ASLMBs). Herein, it is discovered that Li2 CO3 at the GBs of Li7 La3 Zr2 O12 (LLZO) sheets is reduced to highly electron-conductive LiCx during cycling, resulting in lithium penetration of LLZO. The ionic and electronic conductivity of the GBs within LLZO can be simultaneously tuned using sintered Li3 AlF6 . The generated LiAlO2 (LAO) infusion and F-doping at the GBs of LLZO (LAO-LLZOF) significantly reduce the Li2 CO3 content and broaden the energy bandgap of LLZO, which decreases the electronic conductivity of LAO-LLZOF. LAO forms a 3D continuous ion transport network at the GB that significantly improves the total ionic conductivity. Lithium penetration within LLZO is suppressed and an all-solid-state LiFePO4 /LAO-LLZOF/Li battery stably cycled for 5500 cycles at 3 C. This work reveals the chemistry of Li2 CO3 at the LLZO GBs during cycling, presents a novel lithium penetration mechanism within garnet electrolytes, and provides an innovative method to simultaneously regulate the ion and electron transport at the GBs in garnet electrodes for advanced ASLMBs.

Stable Interface Chemistry and Multiple Ion Transport of Composite Electrolyte Contribute to Ultra-long Cycling Solid-State LiNi0.8 Co0.1 Mn0.1 O2 /Lithium Metal Batteries.

Severe interfacial side reactions of polymer electrolyte with LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathode and Li metal anode restrict the cycling performance of solid-state NCM811/Li batteries. Herein, we propose a chemically stable ceramic-polymer-anchored solvent composite electrolyte with high ionic conductivity of 6.0×10-4  S cm-1 , which enables the solid-state NCM811/Li batteries to cycle 1500 times. The Li1.4 Al0.4 Ti1.6 (PO4 )3 nanowires (LNs) can tightly anchor the essential N, N-dimethylformamide (DMF) in poly(vinylidene fluoride) (PVDF), greatly enhancing its electrochemical stability and suppressing the side reactions. We identify the ceramic-polymer-liquid multiple ion transport mechanism of the LNs-PVDF-DMF composite electrolyte by tracking the 6 Li and 7 Li substitution behavior via solid-state NMR. The stable interface chemistry and efficient ion transport of LNs-PVDF-DMF contribute to superior performances of the solid-state batteries at wide temperature range of -20-60 °C.