Enhanced Cyclic Performance of High-Ni LiNixCoyMn(1-x-y)O2 By Lithium Boron Oxide Glass Coating at High Voltage
Authors: Hashigami Satoshi , Kato Yukihiro , Yoshimi Kei , Yoshida Hiroyuki , Inagaki Toru , Doi Takayuki , Inaba Minoru 
Published: February 2020

Lithium ion batteries with high energy density have a vital role as power sources, such as portable devices, plug-in hybrid vehicles and electric vehicles (EVs). The requirement for high capacity has grown recently for extending the drive range of EVs. High capacity cells are also expected for use in electric power grids with the aim of load frequency control and supply-demand control.

Among many other materials, Li[NixCoyMn(1-x-y)]O2 is regarded as a promising cathode material for industrial application due to its enhanced capacity, especially under high voltage. However, severe capacity fading at high voltage was observed. Secondary particles often generate cracks during charge-discharge cycling, due to the repetition of the expansion and contraction of each primary particle. Such strain-induced cracking has been considered to be one of the major degradation mechanisms for the cathode.

Spray pyrolysis is a simple one-step synthesis technique to obtain various kinds of functional oxide powders. We have been working on the synthesis method for many years. Recently, it has been confirmed that any micro-cracks were not detected in the Li[Ni0.5Co0.2Mn0.3]O2 particles prepared by spray pyrolysis after the cycling because these secondary particles have a small size between 100 nm and 2 μm. However, Li[Ni0.5Co0.2Mn0.3]O2 cathode materials prepared by spray pyrolysis still have the drawback of a rapid capacity fading due to the cycling. Our goal is to suppress the harmful side reaction between the cathode material and the electrolyte. In this study, we tried coating high-Ni Li[Ni0.5Co0.2Mn0.3]O2 prepared by spray pyrolysis with lithium boron oxide glass. This prevented direct contact between electrode and electrolyte.

Stoichiometric amounts of various nitrates were dissolved in deionized water. The solution was used for spray pyrolysis. The reaction furnace consisted of four independent heating zones. The collected powder was heat-treated at 850ºC for 8 h in air. The precursor of the lithium boron oxide glass was synthesized by dissolving LiBO2 in deionized water. Lithium boron oxide glass was coated using ethanol on the surface of Li[Ni0.5Co0.2Mn0.3]O2 particles. The samples with 0, 2 and 5 wt.% LiBO2 were denoted as NCM, CB2-NCM and CB5-NCM. The tests were performed galvanostatically at an 1C rate between 2.5 and 4.5 V at 30 ºC.

The XRD patterns of NCM and CB-NCMs were not changed, indicating that the coating treatment did not change the material structure and the lithium boron oxide may exist in an amorphous state. We investigated the capacity retention for CB2-NCM and CB5-NCM at 1C in 1 M LiPF6/EC+DMC (1:2). An improved electrochemical properties under high voltage associated with the lithium boron oxide glass were shown. First, boron coating enhanced the capacity of Li[Ni0.5Co0.2Mn0.3]O2. The initial discharge capacity of CB5-NCM (172 mAh g-1) was higher than that of NCM (158 mAh g-1). Second, CB-NCMs exhibited better cyclic performance than NCM. The discharge capacity retention on the 50th cycle of the CB5-NCM electrode gave 85.6% of the initial discharge capacity, which was much higher than that of NCM (53.8%). Third, the coulombic efficiencies of CB-NCMs approached a steady-state above 99.5%.

The particle shape and size were not changed significantly by boron coating, and NCM and CB-NCMs exhibited similar surface morphologies. In order to confirm the formation of the coating structure in the materials, X-ray Photoelectron Spectroscopy was used to study the surface compositions. A B1s peak was detected in the CB5-NCM. In addition, a coating layer was observed on the surface of CB5-NCM before and after cycling from the Transmission Electron Microscope images. These results indicated that the lithium boron oxide glass layer was formed on the Li[Ni0.5Co0.2Mn0.3]O2 surface.

In order to investigate the bulk structure, we examined the valence states of Ni of NCM and CB-NCMs; (i) before cycling, (ii) after the 35th cycle, (iii) 3.8 V at the 2nd cycle, and (iv) 3.8 V at the 50th cycle by X-ray Absorption Near Edge Structure. The valences of Ni of NCM and CB-NCMs were identical when the samples were in the same charge state. These results mean that the surface of the cathode plays an important role in electrochemical properties.

AC impedance measurements were examined to analyze the charge transfer resistance. Nyquist plots of CB-NCMs showed smaller arc assigned to the charge transfer reaction than that of NCM. It was considered that the reason for the relatively smaller impedance value for the coated cathodes was that the boron coating prevented the direct contact between the cathode and electrolyte, and therefore suppressed side reactions.