Annealing of LiCoO 2 films on flexible stainless steel for thin film lithium batteries
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ADVANCES IN BATTERY TECHNOLOGY: MATERIAL INNOVATIONS IN DESIGN AND FABRICATION
Annealing of LiCoO2 films on flexible stainless steel for thin film lithium batteries Yibo Ma1, Mu Chen1, Yue Yan1,a), Youxiu Wei1, Weiming Liu1, Xiaofeng Zhang1, Jiaming Li1, Ziyi Fu1, Jiuyong Li1, Xuan Zhang1 1
Beijing Engineering Research Center of Advanced Structural Transparencies for the Modern Traffic System, Beijing Institute of Aeronautical Materials, Beijing 100095, China a) Address all correspondence to this author. e-mail: [email protected] Received: 27 June 2019; accepted: 18 September 2019
The LiCoO2 films were directly deposited on stainless steel (SS) using medium-frequency magnetron sputtering, and the effects of annealing parameters, such as ambiences, temperatures, holding times, and heating rates, were systematically compared based on surface morphologies, crystal structures, and electrochemical properties. The results demonstrate that an aerobic atmosphere with 3.5 Pa is the most important parameter to maintain the performance of LiCoO2 films. The influence of the annealing temperature (>550 °C) ranks second because the formed (101) or (104) planes of LiCoO2 facilitate Li+ migration. A short holding time of 20 min and a moderate heating rate of 3 °C/min are selected to reduce the oxidation or inter-diffusion between the LiCoO2 films and the SS substrate. Finally, the optimal annealing process is confirmed and corresponds to the initial discharge capacity of 37.56 lA h/(cm2 lm) and the capacity retention of 83.81% at the 50th cycle.
Introduction Researchers now pay more attention to all-solid-state thin film lithium batteries (TFLBs) due to their potential applications such as main power in identification cards, metal–oxide– semiconductors, and flexible electronic paper displays [1]. The fabrication of LiCoO2 films as cathodes in TFLBs is a successful choice due to their excellent electrochemical properties and maturity of manufacturing [2]. Compared to the traditional hard substrates, such as Si or mica, TFLBs on stainless steel (SS) are flexible; that is, they can be charged/ discharged in a bending state [3] and, thus, can be applied in wearable devices [4, 5]. Since the as-grown LiCoO2 films prepared by magnetron sputtering (MS) at room temperature are in an amorphous state, a post-annealing process is indispensable to obtain crystalline LiCoO2. Traditional post-annealing methods for LiCoO2 films include tube furnace heating [6], rapid thermal annealing [7, 8, 9, 10, 11], in situ heating [12], two-step heating [13], and plasma-assisted treatment [14, 15]. In this article, annealing is performed in the tube furnace due to the uniform temperature distribution and low cost. The deposition of LiCoO2 films by MS is always in an anoxic state; in order to keep the ideal stoichiometric ratio of Co:O and an ideal layered structure, an aerobic environment is
ª Materials Research Society 2019
required in the post-annealing process. But the SS substrate is inevitably oxidized and the oxidation layer formed on both sides of
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