Importance of Composite Electrolyte Processing to Improve the Kinetics and Energy Density of Li Metal Solid-State Batteries

Abstract

Solid-state batteries with a Li metal anode and polymer-ceramic electrolytes hold the promise to boost safety and energy density, provided that stable conductive interfaces are achieved. Here, by focusing on the composite electrolyte system LLZO-PEO(LiTFSI), i.e., Li-ion conductive doped-Li7La3Zr2O12 (LLZO) garnet fillers embedded within the Li-ion conductive poly(ethylene oxide) matrix, we report on the impact that electrolyte processing has on its mesostructure and ultimately on full cell kinetics. Two distinct solvent-based routes are used to prepare composite membranes with a fixed composition of 10 vol % LLZO but with different filler distributions and particle sizes (18 vs 1 μm). Both membranes have a similar Li-ion conductivity of ∼0.4 mS·cm-1 at 70 °C as the bulk polymer matrix drives the macroscopic Li-ion transport. However, when assembled with Li metal/LiFePO4 (LFP) electrodes, the resultant cells show distinct performances. The capacity is enhanced, from 139 to 150 mAh·g-1 at C/10 and from 60 to 97 mAh·g-1 at C/2, when the 1-μm-size LLZO fillers are homogeneously distributed within the membrane. In addition, by minimizing the electrolyte thickness, the capacity can be further enhanced to 168 mAh·g-1 at C/10, which is nearly the theoretical capacity of LFP (170 mAh·g-1). A set of electrochemical and structural characterization techniques, such as galvanostatic cycling, cyclic voltammetry, electrochemical impedance spectroscopy, potentiostatic intermittent titration, and scanning electron microscopy, allow one to identify the electrolyte-Li metal interface stability as the dominating source of the rate capability behavior of the full cells. LLZO fillers with a high surface area and even distribution within the PEO matrix are the key to allow fast and reversible Li-ion flux by minimizing Li concentration gradients. These characteristics are crucial to maximize battery kinetics and capacity utilization. Finally, the design rules of solid-state batteries containing composite electrolytes are proposed to fulfill the industry requirements for practical applications.