LITHIUM ANODE INTERFACE DESIGN FOR ALL-SOLID-STATE LITHIUM-METAL BATTERIES

Abstract

All-solid-state lithium-metal batteries (ASSLBs) have attracted intense interest as the next generation of energy storage devices due to their high energy density and safety. However, the Li dendrite growth and high interface resistance remain challenges due to the lack of understanding of the mechanism. Inserting a solid-electrolyte interlayer/interphase (SEI) with high lithiophobicity, high ionic conductivity, and low electronic conductivity at the Li/SSE interface can solve these problems. However, how lithiophobicity, ionic conductivity, and electronic conductivity of the interlayers affect the lithium dendrite suppression capability of the SEI has not been systematically investigated yet but is critical for ASSLBs. The main goal of this dissertation is to propose a comprehensive interface design principle/frame by considering the impacts of interlayer lithiophobicity, electronic/ionic conductivity, and porosity to Li striping/plating behavior. A combination of modeling and experiments was used to validate the design principle. The developed principle could help to resolve the electrolyte reduction and void formation issues in all-solid-state batteries. The design principle can be applied to different solid electrolytes that have different reactivity against Li, which was presented in the 3rd-6th Chapters for detail. The interlayer design principle opens opportunities to develop safe and high-energy ASSLBs.In the 3rd chapter, we investigated the correlation among ionic and electronic conductivities, lithiophobicity, and Li plating stability in the Li7N2I-Carbon Nanotube (LNI-CNT) interlayer. LNI solid electrolyte has a high ionic conductivity of 3.1 × 10–4 S cm–1 and a low electronic conductivity, high lithiophobicity, and high electrochemical stability against Li, while CNT has a high lithiophobicity, high electronic conductivity, and low tap density. Therefore, mixing LNI with CNT at different ratios can form porous lithiophobic interlayers with variable ionic and electronic conductivity. The 90 μm LNI-5% CNT interlayer enabled Li to plate on the Li/LNI-CNT interface (rather than the SSE/LNI-CNT interface) and then reversibly penetrate into/extract from the porous LNI-CNT interlayer during Li plating/stripping. The 3-dimensional Li/LNI-5% CNT interlayer contact achieved by well-controlled Li nucleation and growth enabled Li/LNI/Li cell to charge/discharge at a high current density of 4.0 mA cm-2 and a high capacity of 4.0 mAh cm-2 for > 600 hours. We also reported that a stable Li plating/stripping cycle can be achieved if the Li nucleation region in the interlayer is smaller or equal to the Li growth region in the interlayer (from the Li anode). This study represents a comprehensive interlayer design for ASSLBs with a significantly improved dendrite suppression capability and reversibility. In the 4th chapter, we develop an LNI-Mg interlayer to increase the Li dendrite suppression capability of Li//Li cells with Li6PS5Cl solid electrolyte. LNI-25%Mg interlayer can form gradient electronic conductivity inside the interlayer due to Mg migrating from the interlayer to the Li anode during activation, which can reduce the interlayer thickness and enhance the Li dendrite suppression capability. The migration of Mg was attributed to the formation of LiMg solid solution. It was found that the gradient electronic conductive LNI-Mg interlayer has better Li dendrite suppression capability than the homogeneous electronic conductive LNI-CNT interlayer due to more constrained Li plating region and mitigated electrolyte reduction. As a result, 18.5 µm LNI-25%Mg interlayer enables Li4SiO4@NMC811/LPSC/Li full cells with an areal capacity of 2.2 mAh cm-2 to be charged/discharged for 350 cycles at 60 oC with capacity retention of 82.4%. This study promotes the development of ASSLBs with higher energy density. In the 5th chapter, we combined experimental techniques and simulation methods to investigate the relationship between the interlayer’s ionic/electronic conductivity ratio, lithiophobicity, and Li plating/striping behavior in carbon-based interlayers. Firstly, we screen the carbon materials based on their ionic/electronic conductivity ratio and lithiophobicity. Li stripping/plating mechanisms were identified in different carbon materials from simulations. Secondly, we predict the critical current density of the interlayer based on the boundary condition of avoiding Li nucleation during Li plating and void formation during striping. Finally, guided by the theoretical prediction, we optimized the ionic/electronic conductivity and lithiophobicity of the carbon-based interlayer by dopping with CuO. The CuO-CNF-M (M= Mg or Ag) interlayer in situ converts to Cu-Li2O-CNF SEI/LiM structure during Li plating. The optimized SEI with ionic conductivity of 0.41 S/m and electronic conductivity of 3.3×10-3 S/m coupling with LiM anode (in-situ formed during Li plating) enables lithium-free NMC811||Cu cell to achieve long cycle life. This work represents a valuable attempt to promote the development of high-performance Li anode interlayer with a joint effort of simulations and experiments. In the 6th chapter, we design a P and I rich SEI for halide electrolytes. Halide electrolytes have the advantage of matching with high-voltage cathodes due to the high thermodynamic oxidation potential. However, they are unstable against Li anode due to their strong reactivity with Li and the formation of electronic conductive metal. In this chapter, we propose and verify critical overpotential as a criterion for Li dendrite growth. By tuning the composition of the SEI, we reduce the overpotential to lower than critical overpotential using P and I containing SEI. The P and I containing SEI with a high ionic/electronic conductivity ratio of the SEI enable Li/LYbC/Li cells to cycle at the current density of 0.1 mA cm-2 with a capacity of 0.05 mA cm-2 for more than 220 hours without a short circuit. This work represents a valuable attempt to achieve Li-stable halide electrolyte.