Ta₂O₅ Coating as an HF Barrier for Improving the Electrochemical Cycling Performance of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ at Elevated Temperatures

Abstract

The high-voltage spinel LiNi 0.5 Mn 1.5 O 4 cathode material suffers from the rapid degradation of electrochemical cycling performance at elevated temperatures, which prevents its successful commercialization. Herein, we show that coating the surface of this material with Ta 2 O 5 , which has high resistance against hydrofluoric acid (HF) attack, is an effective way to improve its electrochemical cycling performance. A Ta 2 O 5 -coated LiNi 0.5 Mn 1.5 O 4 half-cell shows a capacity retention of ∼93% and a Coulombic efficiency of ∼98% after 100 cycles at 55 °C, compared to the corresponding values of ∼76% and ∼95% measured for the bare LiNi 0.5 Mn 1.5 O 4 half-cell. The detailed structural analysis of the Ta 2 O 5 -coated LiNi 0.5 Mn 1.5 O 4 shows that a small amount of Ta 5+ ions diffuse into the 16c site on the cathode surface during the coating process, as directly observed by Cs corrected scanning transmission electron microscopy. The modification of the LiNi 0.5 Mn 1.5 O 4 surface with Ta 5+ , together with the residual Ta 2 O 5 coating, stabilizes the surface structure during cycling, leading to reduced Ni and Mn dissolution as well as formation of the solid electrolyte interface (SEI). In contrast, LiNi 0.5 Mn 1.5 O 4 coated with HF scavengers, such as Al 2 O 3 , shows only limited improvement in cycling performance after prolonged cycling at 55 °C, due to the consumption of the surface coating by reaction with HF, which leaves LiNi 0.5 Mn 1.5 MnO 4 unprotected against HF attack. KEYWORDS: lithium ion batteries, LiNi 0.5 Mn 1.5 O 4 , coating, Ta 2 O 5 , scanning transmission electron microscopy (STEM)

On the Durability of Protective Titania Coatings on High‐Voltage Spinel Cathodes

“…In Figure 8b there is a clear Ti peak that is not visible in the LNMO spectrum. The TiO 2 ‐coating has been reported to be offering only temporary protection from HF attack, as it will be fluorinated [41] . Furthermore, Cheng et al [33] .…”

“…The TiO 2coating has been reported to be offering only temporary protection from HF attack, as it will be fluorinated. [41] Furthermore, Cheng et al [33] reported that the TiO 2 -coating they deposited by ALD on an LCO electrode was no longer detectable by XPS after 5 charging cycles. They attributed the loss of Ti 2p signal to instability due to participation in the redox reactions.…”

“…From previous studies, it is clear that TiO 2 shows great promise as a protective surface coating on LNMO. TiO 2 is, however, found to be a HF scavenger rather than a HF barrier and will therefore react with HF and only offer temporary protection from HF attack [41] . It is furthermore not well understood how the TiO 2 ‐coating is changing over time, and how robust it is under long‐term cycling, as the majority of published studies do not emphasize post‐mortem characterization but rather focus on electrochemical characterization, and mainly in half‐cell configuration only [42] .…”

“…TiO 2 is, however, found to be a HF scavenger rather than a HF barrier and will therefore react with HF and only offer temporary protection from HF attack. [41] It is furthermore not well understood how the TiO 2 -coating is changing over time, and how robust it is under long-term cycling, as the majority of published studies do not emphasize post-mortem characterization but rather focus on electrochemical characterization, and mainly in half-cell configuration only. [42] The large excess in available Li from the often employed thick Li metal electrodes will mask certain degradation effects, such as capacity decay caused by Li inventory loss.…”

Large-scale synthesis of lithium- and manganese-rich materials with uniform thin-film Al2O3 coating for stable cathode cycling

“…Strategies such as surface coating, elemental doping and morphology control, are generally used to address the above issues. Directly coating on the interface is an effective way to suppress the interfacial reactions [23][24][25][26][27][28][29][30][31]. A number of approaches, such as spray pyrolysis [32], sol-gel method [33], and vapor deposition method [34], have been employed to inhibit the electrolyte decomposition.…”