lithium battery

Gradient Design For High Energy With High Power Lithium Batteries

Charge transport is a key process that dominates battery performance, and the microstructure of the cathode, anode, and electrolyte plays a central role in directing ion and/or electron transport within the battery. Rational design of key battery components with different microstructures along the charge transport direction to achieve optimal local charge transport kinetics can compensate for the reaction polarization and thus accelerate the development of electrochemical reaction kinetics.

Yu Guihua of the University of Texas at Austin, Esther S. Takeuchi of Stony Brook University, and Wang Huanlei of the Ocean University of China introduced the principle of the charge transport mechanism and its decisive role in battery performance and then discussed the relationship between charge transport regulation and battery microstructure design. correlation between. Then, the design strategies of gradient cathode, Li metal anode, and solid electrolyte are summarized. Finally, future directions and prospects for gradient design are provided to achieve practically usable batteries with high energy and high power density.

The authors summarize gradient battery designs that focus on tuning charge transport behavior to improve energy/power density. In porous electrodes, Rion and Re need to be minimized between separator/electrode and electrode/current, respectively, and gradient pore structure and gradient electron conductivity designs are used to tune Rion and Re, respectively.

Lithium battery anode and cathode design

Gradient active species also facilitate the development of reaction kinetics by shortening the integrated ion diffusion distance during electrochemical reactions. The introduction of the lithium host provides a lithium diffusion pathway for non-porous lithium metal anodes, and the gradient design of lipophilicity and electric field in the composite lithium anode can suppress the “top growth” mode and lithium dendrite growth by guiding the diffusion and nucleation of lithium ions. The gradient or asymmetric design also endows the solid-state electrolyte (SSE) with a wider electrochemical window, better interfacial stability, higher ionic conductivity, and mechanical strength.

Although great progress has been made in the ongoing research into gradient design of key battery components, certain scientific challenges remain unsolved, requiring more fundamental research.

(1) Scientific research. Physics-based models are a valuable tool to gain insight into physical processes such as reaction kinetics, charge transport kinetics, and structural evolution beyond experimental data. Advanced modeling using the latest theoretical developments and experimental datasets is essential to elucidate the reaction kinetics of gradient battery components. Characterizing the microstructure and its evolution during operation is essential to understanding the performance and guiding the advanced design of critical components.

(2) Engineering optimization. Mass-loaded electrodes are urgently needed to achieve the energy density target of 500 Wh kg-1. For Li-NMC cells, the minimum cathode loading to achieve this is 30-40 mg cm-2. Thick electrodes require new fabrication techniques due to slow charge transport kinetics and weakened mechanical strength. More extensive methods are needed to precisely control the structure of low tortuosity electrodes, and the interplay between microstructure and performance should be carefully evaluated to provide further guidance for advanced battery design. For the Li anode, to pair with a 30–40 mg cm-2 cathode, the amount of Li metal needs to be reduced to ≈4 mg cm-2, corresponding to a thickness of 75 µm (assuming N/P ratio = 2).

To this end, the weight fraction of the lithium host needs to be minimized, and in this regard, light carbonaceous or polymeric hosts with dual gradients of lipophilicity and electronic conductivity are advantageous in terms of energy density. Meanwhile, the mechanical robustness and even flexibility of the reduced-thickness lithium host remains a key requirement. On the other hand, the fabrication process of complex lithium needs to be more scalable and cost-effective.

Furthermore, besides ionic conductivity and interfacial stability, another key parameter that determines the reaction kinetics and energy density of solid-state batteries is the thickness of the SSE. Theoretical analysis shows that SSE needs to be 20 µm or thinner to compete with current polymer separators. Designing thin SSEs with ideal gradients and sufficient mechanical strength is crucial for realizing high-energy/power solid-state batteries.

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