All-Solid-State Thin-Film Secondary Battery with Lithium-Free Anode for High-Temperature Tolerance
Authors: Suzuki Akiyoshi , Sasaki Shunsuke , Kimura Isao , Jimbo Takehito 
Published: February 2020

Introduction
Recently, smart device which enables IoT (internet of things) has induced a strong demand for high density packaging and miniaturization to fulfill high performance and functionalities simultaneously, and all-solid-state thin-film secondary batteries (TFBs) are most promising to be used as small-format and stand-alone power supplies for smart device due to intrinsically safe, high reliability and long cycle life. TFB is fabricated by forming each layers (i.e. cathode, electrolyte, anode and current collector) using physical vapor deposition process1,2), and metallic lithium is currently the most used material as an anode material in TFB.
However, TFB using metallic lithium anode have thermal constraint3,4). For example, the solder-reflow process needs a thermal treatment at up to 250 deg.C, and thermal specification for automotive device is up to 150 deg.C, whereas the melting point of metallic lithium is around 180 deg.C. Therefore, other anode material in place of metallic lithium is necessary to improve high-temperature tolerance of TFB. In this work, we investigated TFB with amorphous silicon (a-Si) thin-film as an anode, and its battery performance.

Experimental
Lithium cobalt oxide (LCO) film was prepared by RF and DC hybrid power magnetron sputtering method on platinum film as current collector. Ar was used as sputtering gas, and process pressure was kept at 1.6 Pa. Furthermore, LCO was deposited with applying each bias power to the substrate for investigating the planarization. After deposition, LCO was annealed at 600 deg.C under atmospheric pressure by lamp heating system. Solid electrolyte (LiPON) film was prepared by RF magnetron reactive sputtering method with only N2 gas5). a-Si film as an anode was prepared by sputtering method using boron-doped silicon target. Finally, fabricated cell was encapsulated, and then “lithium-free anode” TFB was completed.

Results and discussion
TFB cell was fabricated with 3-μm-thick LCO, 2-μm-thick LiPON and 200-nm-thick a-Si. Figure 1 shows result of cycle performance under each SOC (state of charge), which is controlled by modulating charging cut-off voltage (3.8, 4.1, 4.2 V). Actual capacity of a-Si can be estimated from result of discharge capacity, and it was found that good cycle performance was obtained when actual capacity of a-Si anode is controlled to 2300 mAh/g. It might be considered that irreversible capacity is generated by breaking a-Si anode in a few cycles due to large volume change in results of 2750 and 3000 mAh/g.
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Figure 1