Influence of surface modification on electrochemical performance of high voltage spinel ordered-LiNi_0.5Mn_1.5O₄ exposed to 5.3 v for 100 h before and after surface modification with ALD method

To quantify the effect of high voltage on the electrochemical properties of high-potential spinel ordered-LiNi_0.5Mn_1.5O₄, it was intentionally exposed to 5.3 V for 100 h that can ensure the decomposition of electrolyte. After this treatment, the bulk structure did not change, but electrochemical properties of the sample were severely degraded; polarization became large and capacity loss was substantial. Polarization was caused by formation of a thick insulating passivation layer on the surface of the sample that was measured by impedance spectroscopy. The capacity loss can be partially caused by incomplete phase transformation during discharging as a result of loss of electrical contact due to the presence of the thick passivation layer on the surface of particles. This indicates that the phase transformation depends on the applied current. The other cause for the capacity loss can be from the inactiveness of transition metals in the surface that was measured by XPS. Thick passivation layer on the surface can have inactive transition metals leading to permanent capacity fading. Hence, to control the electrode stability in high voltage spinel LiNi_0.5Mn_1.5O₄, a bare LNMO sample coated with Al₂O₃ by Atomic Layer Deposition (ALD) were prepared, then exposed to 5.3 V for 100 h. After this surface treatment, the Al₂O₃-coated sample showed much better electrochemical performance than the bare sample. During the exposure, the bare sample underwent intensive surface reactions with very large generated current density and large charge-transfer resistance. In contrast, the coated sample experienced much weaker surface reactions with low charge-transfer resistance even though the applied potential, 5.3 V was much higher than the stable upper voltage limit (∼4.5 V) of conventional electrolyte. The coating effectively protects the surface of the material from surface reactions such as oxidation of the electrolyte; therefore Al₂O₃-coated LNMO shows reasonable electrochemical properties after exposing at 5.3 V for 100 h. This finding demonstrates that detrimental effects of the exposure at high potential on the electrochemical properties strongly depends on surface characteristics. This understanding can be used to stabilize high-voltage positive electrode materials.