From thick to thin: How boride film design affects battery performance

Voltage plateaus—those steady stretches in a charge or discharge curve—are key to determining a battery’s energy density. Yet adjusting electrode thickness often distorts them: thicker films hinder ion flow, causing polarization, while ultra-thin films may lose voltage stability. Material chemistry adds another layer of complexity, as different lithiation pathways—such as phase transitions versus solid solutions—respond differently to thickness changes. Borides, known for their high theoretical capacity and eco-friendly composition, are promising candidates, but their performance depends heavily on structural design. Due to these challenges, there is an urgent need to understand how thickness, composition, and reaction mechanisms interact in boride-based thin-film electrodes.

Researchers from Xiamen University and the University of Texas at Austin have mapped how boride film thickness shapes electrochemical performance in all-solid-state thin-film lithium batteries (TFLBs). Published (DOI: 10.26599/EMD.2025.9370062) June 11, 2025, in Energy Materials and Devices, the study blends advanced electrochemical testing with phase-field simulations to explain why thinner CoB and CoFeB films deliver higher capacity and stable cycling, while FeB films remain diffusion-limited regardless of thickness. This breakthrough clarifies long-standing questions about voltage plateau regulation and provides practical design rules for creating high-energy-density solid-state batteries with improved durability.

Using magnetron sputtering, the team fabricated CoB, FeB, and CoFeB films in multiple thicknesses, then tested them in both liquid-electrolyte and all-solid-state TFLBs. As thickness decreased, lithium-ion diffusion paths shortened, shifting lithiation from diffusion-limited to charge-transfer-dominated processes. For CoB and CoFeB, this shift yielded steeper voltage curves, more redox peaks, higher reversible capacity, and improved Coulombic efficiency. FeB films, however, showed simple voltage plateaus and low cycling stability regardless of thickness, due to inherently sluggish multiphase reaction kinetics.

Differential capacity mapping revealed that thick films suffered from rising overpotentials and fading redox activity over time, while thin films maintained stable peaks. Phase-field simulations confirmed that thick films developed steep lithium-salt concentration gradients that restricted ion penetration, whereas thin films achieved uniform ion distribution. This thickness-controlled mechanism held true in both solid-state and liquid-electrolyte battery configurations, proving its universality. The results unify thickness, material composition, and lithiation behavior into a single framework, offering a clear design pathway for stable, high-capacity thin-film electrodes.

“Thickness is more than a physical measurement—it’s a lever for controlling how a battery works,” said corresponding author Jie Lin. “By reducing thickness in CoB and CoFeB films, we shift lithiation toward surface-controlled reactions, which lowers polarization and extends cycling stability without sacrificing energy density. Our approach is simple, scalable, and adaptable to different chemistries, making it a practical tool for engineers seeking to optimize solid-state batteries. These findings answer fundamental questions about voltage plateau behavior and point the way toward more reliable high-energy storage devices.”

The discovery that electrode thickness can fine-tune voltage stability and performance offers a powerful, low-cost route to improve all-solid-state TFLBs used in microelectronics, wearable devices, and compact energy storage systems. Because the method relies on precise sputtering control rather than exotic materials or complex architectures, it can be readily integrated into existing manufacturing lines. Beyond borides, the same thickness–reaction principle could be applied to other alloy and conversion-type electrodes, enabling tailored designs for both solid-state and liquid-electrolyte batteries. By addressing the root causes of voltage instability, this strategy opens the door to longer-lasting, higher-capacity batteries across diverse applications.

Funding information

This work was supported by National Natural Science Foundation of China (Grant Nos. 52101273 and U22A20118), Natural Science Foundation of Fujian Province of China (Grant No. 2022J01042), and Fundamental Research Funds for Central Universities of China (Grant No. 20720242002).

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