Atomic Layer Deposition of Lithium Borate and Borophosphate Thin Films for Lithium-Ion Battery Applications
Authors: Verhelle Tippi , Dhara Arpan , Henderick Lowie , Dendooven Jolien , Detavernier Christophe 
Published: February 2024

Lithium-ion batteries (LiBs) are widely used in portable electronic devices and electric vehicles because of their high energy density and long cycle life. However, their performance is limited by unwanted side reactions that occur at the electrolyte-electrode interface during charge/discharge cycles.
Previous studies have indicated that the application of a conformal and ultra-thin protective coating on the electrode surface through atomic layer deposition (ALD) is a promising strategy to prevent these undesired side reactions [1]. Due to their high stability, metal phosphates are deemed a promising class of materials for protective coatings, and several studies have focused on ALD of metal phosphates [2], with lithium phosphate as a particularly relevant material. However, the low ionic conductivity, as evidenced by the reported value of 1.4 x 10-10 S cm-1 with an activation energy of 0.92 eV for crystalline Li3PO4 films deposited by ALD [3] may hinder the flow of lithium ions through the coating. It has been shown, albeit using other deposition techniques, that the inclusion of borate species can enhance the ionic conductivity of phosphate materials, including lithium phosphate [4]. However, lithium borates exhibit a more narrow electrochemical stability window. Here, we investigate the combination of lithium phosphate and lithium borate into a new, intermixed ALD coating, i.e. lithium borophosphate, aiming to increase the ionic conductivity of the protective coating while preserving as much as possible its electrochemical stability.
To date, only one ALD process for lithium borate has been reported by Kazyak et al. [5], which resulted in films with significant carbon incorporation. In this work, we have developed an ALD process for depositing lithium borate with minimal contaminants (< 5 at.% as determined by XPS) using LiHMDS, H2O and TMB as precursors. Through-plane impedance spectroscopy measurements were used to determine the ionic conductivity. For a lithium borate ALD film with a thickness of 81 nm deposited at 250°C, an ionic conductivity of 1.60 ± 0.03 x 10-7 S cm-1 was obtained at 30 °C, with an activation energy of 0.58 ± 0.01 eV.
By combining this lithium borate ALD process with the well-known lithium phosphate ALD process [6], the composition of the film could be precisely controlled, resulting in tailored electrochemical properties. Cyclic voltammetry was employed to evaluate the electrochemical stability of the lithium borophosphate mixtures. The coatings were deposited on top of an ion-blocking material and cycled to high voltages. Initial results indicate that the stability of the coating material decreases as the amount of lithium borate to lithium phosphate cycles increases.
These promising preliminary results demonstrate that ALD is a valuable tool in developing high-performance battery technologies, enabling the deposition of new protective coatings with controllable electrochemical properties. Finding the optimal lithium borophosphate coating will require a trade-off between ionic conductivity and electrochemical stability and will ultimately depend on the intended application.

References

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Kazyak, E., J. Mater. Chem. A 6, 19425 (2018).

Hämäläinen, J., J. Electrochem. Soc. 159, A259 (2012).

Figure 1