Designing Solid Electrolytes for Rechargeable Solid-State Batteries

Abstract

Lithium-ion battery (LIB) is an indispensable energy storage device in portable electronics, and its applications in electric vehicles and grid-level energy storage are increasing dramatically in recent years due to high demands. To meet energy demands and address fire hazards, next generation batteries with better safety, higher energy density, and longer cycle life have been actively investigated. In this thesis, works on polymer and ceramic solid electrolytes to improve safety and energy density of rechargeable solid-state batteries are discussed. In the first section, a flexible composite solid electrolyte is presented. Since ceramic electrolytes have high conductivities but are fragile, and polymer electrolytes are easy to process but have low conductivities, we propose a composite structure that combines these advantages. A vertically aligned and connected ceramic electrolyte is realized through the ice-templating method to improve the ionic conduction. Then a polyether-based polymer electrolyte is added to make the composite electrolyte flexible. Specifically, vertically aligned and connected LATP and LAGP nanoparticles (NPs) in the polyethylene oxide (PEO) matrix are made. The conductivity reaches 0.52 × 10-4 S/cm for LATP/PEO, and 1.67 × 10-4 S/cm for aligned LAGP/PEO composite electrolytes, which are several times higher than that with randomly dispersed LATP/LAGP NPs in PEO. Compared to the pure PEO electrolyte, the mechanical and thermal stabilities of the composite solid electrolyte are higher. The LFP-LAGP/PEO-Li cell with 148.7 mAh/g during the first discharge at 0.3C has over 95% capacity retention after 200 cycles. This method opens a new approach to optimize ion conduction in composite solid electrolytes for solid-state batteries. In the next section, polyether-based polymer electrolytes such as PEO and PEG are studied. Polyether-based electrolytes are electrochemically unstable above 4 V, restricting their use with high voltage cathodes such as NMC for high energy density. A technique involving atomic layer deposition (ALD) of Al2O3 to stabilize the polyether-based electrolyte with 4 V class cathodes is described. With a 2 nm Al2O3 coating, the capacity retention stays at 84.7% after 80 cycles and 70.3% after 180 cycles for the polyether-based electrolyte. Without the coating, the capacity drops more than 50% after only 20 cycles. This study opens new opportunity to develop safe electrolytes for lithium batteries with high energy density. In the final section, we propose a new polymer electrolyte, a poly(vinylidene fluoride) (PVDF) polymer electrolyte with organic plasticizer dimethylformamide (DMF), which possesses compatibility with 4V cathode for high energy density and high ionic conductivity (1.2×10-4 S/cm) at room temperature. This polymer electrolyte can be used as a supplement for the polyether-based electrolytes we discussed in the first two sections. In this polymer electrolyte, palygorskite ((Mg,Al)2Si4O10(OH)) nanowires are introduced to form composite solid electrolytes (CPE) to enhance both stiffness and toughness of PVDF/DMF-based polymer electrolyte. With 5 wt % of palygorskite nanowires, the elastic modulus of the PVDF-DMF CPE increases from 9.0 MPa to 96 MPa, and its yield stress increases by 200%. We further demonstrate that full cells composed of Li(Ni1/3Mn1/3Co1/3)O2 (NMC 111) cathode, PVDF-DMF/palygorskite CPE, and lithium metal anode, can be cycled over 200 times at 0.3 C, with 97% capacity retention. Moreover, the PVDF-DMF electrolyte is nonflammable, making it a safer alternative to the conventional liquid electrolyte. Our work illustrates that the PVDF-DMF/palygorskite CPE is a promising electrolyte for solid state batteries with better safety and cycling performance. Collectively, we study the polyether-based polymer electrolyte and ceramic electrolyte to combine their advantages through the ice-templating method in a battery, use ALD technique to stabilize polyether-based electrolyte for high energy density, and propose an alternative PVDF/DMF-based polymer electrolyte with nanowire additives for high energy density and stable cycling, contributing to the rechargeable solid-state batteries, with better safety, higher energy density and better cycling stability.