Strategies for Approaching One Hundred Percent Dense Lithium-Ion Battery Cathodes

Abstract

Creating thick electrodes with low porosity can dramatically increase the available energy in a single cell and decrease the number of electrode stacks needed in a full battery, which results in higher energy, lower cost, and easier to manufacture batteries. However, existing electrode architectures cannot simultaneously achieve thick electrodes with high active material volume fractions and good power. These particle-based architectures rely on electrolyte transport within the pores of the cathode to fully lithiate active material particles during discharge. As cathode solid volume fractions approach 100%, batteries experience electrolyte depletion which leads to inaccessible cathode reaction sites (see Fig. 1A). The additional theoretical capacity that comes from increased cathode density, therefore, is impractical if that energy cannot be fully extracted.We combine experiments and simulations of high density and high thickness cathodes to understand the transport and performance trade-offs of LIBs as the cathode solid volume fraction approaches 100%, which we use to reveal the cathode properties needed to achieve high performance at high relative density and thickness. We use one- and two-dimensional simulations to compare the discharge performance of two cathode architectures, a traditional particle-based architecture and a continuous cathode architecture created via electrodeposition. In addition, a model with spaced diffusion-barriers explores the design space between these two architectures and elucidates the influence of increasing solid-diffusion length on discharge performance. We show that there is a large opportunity space for improved energy density at high relative densities by using new electrode manufacturing techniques to create continuous diffusion pathways and high diffusivities. Increasing the solid diffusion length from 4.78 µm to 55 µm in cathodes with high diffusivity leads to an increase in areal capacity (from 1.6 mAh/cm2 to 4.8 mAh/cm2) for a 110 µm thick, 95% dense LCO cathode discharged at a 1C rate.We also apply concepts and designs from these models to simulate the discharge performance of thick, high-density lithium-ion batteries with solid electrolytes to motivate even higher energy battery architectures. When discharged at a 1C rate, solid-state batteries with traditional particle-based composite cathodes (110 µm thick) cannot extract any energy at volume fractions above 94%, while batteries with high-diffusivity continuous cathodes and no solid electrolyte in the cathode region can achieve 3.8 mAh/cm2 at 95% solid volume fraction. These new cathode architectures which contain no electrolyte in the cathode region can significantly improve the gravimetric energy density of solid-state lithium-ion batteries.This work uses a comparative analysis of cathode architectures to explore the interdependent impact of solid volume fraction, solid-diffusivity, cathode thickness, and discharge rate on lithium-ion battery areal capacity. We should how a combination of high diffusivity and continuous solid-state diffusion pathways provides an exciting path for realizing ultra-dense and thick cathodes with high energy density. Figure 1