Abstract
Surface residual lithium compounds of Ni-rich cathodes are tremendous obstacles to electrochemical performance due to blocking ion/electron transfer and arousing surface instability. Herein, ultrathin and uniform Al2O3 coating via atomic layer deposition (ALD) coupled with the post-annealing process is reported to reduce residual lithium compounds on single-crystal LiNi0.6Mn0.2Co0.2O2 (NCM622). Surface composition characterizations indicate that LiOH is obviously reduced after Al2O3 growth on NCM622. Subsequent post-annealing treatment causes the consumption of Li2CO3 along with the diffusion of Al atoms into the surface layer of NCM622. The NCM622 modified by Al2O3 coating and post-annealing exhibits excellent cycling stability, the capacity retention of which reaches 92.2% after 300 cycles at 1 C, much higher than that of pristine NCM622 (34.8%). Reduced residual lithium compounds on NCM622 can greatly decrease the formation of LiF and the degree of Li+/Ni2+ cation mixing after discharge–charge cycling, which is the key to the improvement of cycling stability.
Highlights
Ni-rich cathodes (LiNix Coy Mn1−x−y O2, x ≥ 0.6, NCM) have attracted much attention for powering lithium-ion batteries for electric vehicles due to the high energy density and relatively low cost [1,2]
No extra peaks and no peak shifts are detected in the X-ray diffractometer (XRD) patterns of 2AL and 2AL-A samples, indicating that the atomic layer deposition (ALD) and post-annealing processes have no influence on the crystal structures of substrate materials
Ultrathin and uniform Al2 O3 coating via ALD coupled with the postannealing process has been performed to reduce the surface residual lithium compounds of
Summary
Ni-rich cathodes (LiNix Coy Mn1−x−y O2 , x ≥ 0.6, NCM) have attracted much attention for powering lithium-ion batteries for electric vehicles due to the high energy density and relatively low cost [1,2]. Single-crystal NCM usually exhibits good structural stability, it suffers from serious side reactions between surface residual lithium compounds (LiOH and Li2 CO3 ) and the electrolyte during discharge–. The surface residual lithium compounds can react with LiPF6 in the electrolyte to form LiF on the surface of NCM and produce CO2 , which increases the interfacial resistance and battery internal pressure, and leads to heat generation [6,7,8,9]. Surface residual lithium compounds can reduce the lifespan of batteries and induce serious safety issues. In order to reduce surface residual lithium compounds of NCM materials, washing with water is a widely used method in industrial manufacturing. After the washing process, surface residual lithium compounds are usually more rapidly formed when NCM materials are exposed to air again [7,13]. For liquid-solid reactions, some acids such as phosphoric acid, boracic acid, and oxalic acid have been applied to neutralize alkaline residual lithium compounds [16,17,18]
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