Surface chemistry of LiNi_0.5Mn_1.5O₄ particles coated by Al₂O₃ using atomic layer deposition for lithium-ion batteries

The effects of depositing ultrathin (<1 nm) Al₂O₃ coatings on LiNi_0.5Mn_1.5O₄ (LNMO) particles using atomic layer deposition (ALD) are presented. Promising electrochemical performance of the Al₂O₃ ALD coated LNMO at 30 °C is demonstrated in not only significantly improved coulombic efficiency, cycle retention, and rate capability, but also in dramatically suppressed self-discharge and dissolution of transition metals. Combined analyses by electrochemical impedance spectroscopy, ex-situ X-ray photoelectron spectroscopy, and ex-situ time-of-flight secondary ion mass spectrometry reveal that the solid electrolyte interphase layer on the Al₂O₃ ALD coated LNMO is much thinner and contains fewer organic species than the one on the bare LNMO. This difference originates from the suppression of the side reaction at high voltage by the Al₂O₃ ALD protective coating. Also, fluorination of Al₂O₃ ALD layer upon repeated charge-discharge cycling is confirmed, and this can account for the capacity increases during the initial charge-discharge cycles. Finally, it is also demonstrated that a full LNMO/Li₄Ti₅O₁₂ battery incorporating the Al₂O₃ ALD coated LNMO outperforms the one incorporating only bare LNMO.