All-solid-state lithium battery is a promising candidate of post lithium ion batteries because it can realize many  attractive properties, such as higher safety, higher energy density and longer lifetime.

However, it is very difficult to form cathode/solid-state electrolyte interface in all-solid-state lithium batteries by currently-used electrode fabrication methods such as slurry coating and chemical vapor deposition, resulting in poor electrochemical performance.  

Therefore a new technique appropriate for the solid-solid interface in all-solid-state batteries is required.

 Aerosol deposition method (AD method) has been known as a technique of ceramics deposition process using a room temperature impact consolidation. When an aerosol, for example a colloid of ceramic fine particles dispersed in Ar-gas, is sprayed in a vacuum chamber, a ceramic layer is formed on a target substrate. This AD method is able to form a ceramic film uniformly and rapidly at room temperature without high temperature heat treatment. Actually, it has been reported by Iwasaki et al. that a cathode/solid-state electrolyte interface for all-solid-state lithium batteries can be formed at room temperature using AD method¹⁾.

On the other hand, Li⁺-ion conducting additives such as Li₃BO₃ are being focused as effective supporting materials to form good electrochemical interface between solid cathode and solid electrolyte²⁾. However, Li₃BO₃ requires a high heat treatment at around 800 °C to form Li⁺-conducting pathways between solid cathode and solid electrolyte. This strongly limits the cathode materials applicable to LLZ solid electrolyte.

Thus, we focused on the combination of AD method and application of Li₃BO₃, which is expected to form good solid-solid electrochemical interface for all-solid-state batteries at room temperature.

 In this study, LCO/LBO composite cathodes were formed on Al doped Li₇La₃Zr₂O₁₂(LLZ) by AD method, and the electrochemical performance of them was investigated as all-solid-state-lithium batteries.

Experimental

LLZ-pellets were prepared by a solid-state technique using lithium hydroxide, lanthanum hydroxide, zirconium oxide, and g-alumina. By AC impedance measurement, the LLZ-pellet was confirmed that the Li ionic conductivity was at least 10^-4 S cm^-1 at room temperature. A cathode material LiCoO₂ (LCO) as aerosol powder was prepared by a sol-gel method using lithium acetate, cobalt acetate, and citric acid. Li₃BO₃(LBO) was synthesized by a solid-state technique using lithium carbonate and boron oxide before the LBO particles were pulverized by a planetary-ball milling. The LCO/LBO composite as aerosol powder was prepared by calcining a mixture of LCO and pulverized LBO at 800 °C for 2 hr.

 Fig. 1 shows schematic illustration of the AD method apparatus. The apparatus was consisted of an Ar gas cylinder, a vacuum chamber, and a rotary-pump. The LLZ pellets was fixed on a sample holder in the vacuum chamber. The aerosol powder, LCO, LCO/LBO composite, or a mixture of LCO and LBO, was introduced into a stainless gas-tube connected with a nozzle in the vacuum chamber. When the ultimate vacuum of the chamber reached 70 Pa using the rotary-pump, the aerosol powder was sprayed into the pellet in the vacuum chamber by flowing Ar gas regulated to 0.6 MPa into the stainless gas-tube. The thick cathode layer was formed on LLZ-pellet by repeating the above AD operations.

 The thickness of cathode layer was measured with a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDS).

 The electrochemical performance was investigated as all-solid-state-lithium batteries with lithium metal anode on the opposite side of LLZ-pellet.

Results

Fig.2 shows the cross-sectional SEM and EDS images of LCO cathode layer and LCO/LBO composite cathode layer on LLZ-pellet by repeating the AD operations 20 times. The thickness of LCO cathode layer and LCO/LBO composite cathode layer were estimated at about 6 mm and 10 mm respectively from a distribution of Co mapping date. Fig.3 shows the charge-discharge caves of the all-solid-state lithium batteries produced by AD method. The open-circuit voltage immediately after the batteries assembly was 0.3 V. The discharge capacity till 2.5 V after the initial charge was 100 mA h g^-1. The discharge capacity was increased in 120 mA h g^-1in second cycle, but the discharge capacity was gradually decreased in third cycle and after cycles.

 The electrochemical utilization of LCO cathode layer without LBO was 20 %, but the utilization of LCO/LBO cathode layer was 70 %. The result shows that Li⁺-conducting pathways were increased by LBO being interposed between LCO particles. But the decrease in the capacity shows that LCO came off LBO or LLZ by the volume change during charge/discharge process of LCO particles. In this study, all-solid-state batteries was experimental produced by AD method and performed as a rechargeable lithium batteries.