Improvement of Ionic Conductivity and Suppression of Gas Generation in LiNi0.5Mn1.5O4 Cathode Material By Using P-Ta Aqueous Coating Solution.
Authors: Kuruma Takuya , Yamamura Takayuki , Miura Takashi , Magori Daisuke , Ide Hitohiko 
Published: December 2024

With the increasing demand for electrification of mobility, there are high expectations in recent years for improving the quick-charging performance of batteries. However, conventional lithium-ion batteries have a limited recharging speed due to safety concerns, such as heat generation during rapid recharging.

Against this backdrop, batteries using lithium titanate (LTO), which has a spinel-type crystal structure with excellent charge acceptance, as the negative electrode have been attracting attention. The combination of lithium nickel manganese oxide (LNMO), which has the same spinel-type structure as the positive electrode, has the advantage of maximizing the high input/output characteristics of the battery. LNMO do not contain expensive cobalt and are not subject to resource constraints; moreover it can obtain a high energy density by utilizing the high discharge potential.

On the other hand, LNMO-based batteries have not yet been put into practical use due to technical issues concerning reliability, such as side reactions caused by metal leaching components from the cathode and gas generation due to electrolyte decomposition. Solutions have been proposed by reviewing the battery design, such as using highly concentrated electrolyte with excellent oxidation resistance and all-fluorinated solvents, but all of these solutions remain problems in terms of cost and input/output performance, and the original problem of the cathode has not been solved for a long time.

Although various coating technologies for active material particles have been investigated as a solution approach for cathode materials, issues related to controlling the coating composition and achieving coating homogeneity are well known. In this study, the aqueous coating solution "iconos™" developed by Mitsui Kinzoku was employed as a coating material. It is a liquid solution of elements known to be insoluble in water, including niobium, tantalum, titanium, and other elements that are potentially applicable to lithium ionic conductive materials. Compared to alkoxide liquid, it is inexpensive, can be stored in solution for a long period of time (six months or more), exhibits high stability over time, and is easy to handle. It can dissolve a wide range of elements according to the desired coating composition, and it is possible to obtain a uniform coat with the desired composition. Therefore, it can be used as a coating material source for cathode materials, including LNMO.

In this report, we select Li-P-Ta composition, which is a composite of lithium phosphate and lithium tantalate that can be used as a solid electrolyte, from the lineup of iconos™, and report the results of our attempt to form a solid electrolyte film layer on LNMO.

Tumbling fluid bed granulator Dryer / Coater System (Powrex) was used for the LNMO coating test, and an aqueous coating solution formulated to the desired Li-P-Ta composition was used as the spraying solution to obtain the desired coated LNMO particles. The resulting coated LNMO was dispersed in NMP solvent with conductive material (acetylene black) and binding agent (PVdF) in the ratio of 90:6:4, and coated onto Al foil and dried to obtain the electrode. The electrode was also prepared in the same way for LTO, the anode material. The electrodes were stacked via a separator and sealed in aluminum laminate to form a cell for evaluation. The electrolyte composition was selected to be 1M/LiPF6 EC:EMC=3:7 vol% solvent, which is a common electrolyte composition.

The Li-P-Ta layered LNMO coated with iconos™ showed higher gas suppression than uncoated LNMOs, with 17 cm3 g-1 of gas generation after 3.45 V144 hr float charge at a charge rate of 0.1 C at elevated temperature(55℃), half of the 40 cm3 g-1 of gas generation in the uncoated LNMO. This results from the high gas suppression effect of the Li-P-Ta coating. This is thought to be due to the Li-P-Ta layer functioning as a lithium ion conductive layer that prevents direct contact between the cathode active material and the electrolyte. This effectively suppresses the etching of the LNMO cathode material and the leakage of active metal species due to impurities in the electrolyte such as HF. The coated LNMO also exhibited superior output characteristics in terms of discharge characteristics. Figure 1, which plots discharge capacity against discharge rate, shows that coated LNMO has a higher linearity than uncoated LNMO. This is presumably due to the superior performance of the lithium ion conductor coating layer formed on the particle surface, which improves the lithium ion diffusion rate.

Figure 1