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

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.