High temperature property of all-solid-state thin film lithium battery using LiPON electrolyte

doi:10.1016/j.matlet.2014.07.092

Materials Letters

Volume 134, 1 November 2014, Pages 237-239

https://doi.org/10.1016/j.matlet.2014.07.092 Get rights and content

Highlights

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The thin film lithium battery (Pt/LiCoO₂/LiPON/Sn_xN_y/Pt) was prepared.
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The discharge performances from the room temperature to 200 °C were studied.
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The battery maintained a good discharge performance at 150 °C.
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The heat treatment increased the ionic conductivity of LiPON.

Abstract

All-solid-state thin film lithium battery with the cell structure Pt/LiCoO₂/LiPON/Sn_xN_y/Pt was prepared. The total thickness of the battery was about 7.6 µm. The discharge capacity of the thin film battery was 163 µA h at 150 °C, about 87% of the initial discharge capacity at the room temperature, and remained a discharge capacity about 152 µA h at 200 °C. The influence of the temperature on the LiPON was also investigated exclusively. The results showed that the heat treatment of the LiPON films could reduce the adsorptive oxygen and defect polarization, and was responsible for the increase of the three-coordinated N atoms structural units which improved the ionic conductivity of LiPON.

Introduction

All-solid-state thin-film lithium batteries are often used as a power source for micro devices and suitable for the use in a high temperature environment (vs. lithium ion batteries using liquid electrolytes). As we know, Li metal is often used as a negative electrode in thin film lithium batteries. However, the fabrication and application of Li electrode are limited due to the safety problem arising from the low melting point of Li (181 °C) and its strong reactivity with moisture. Therefore some alternative negative electrode materials have been studied, such as Zn₃N₂ [1], alloyed Si [2], [3], SnO₂ [4], [5], silicon tin oxynitride (SiTON) [6]. Especially, and much attention has been paid to the tin-based materials such as tin nitrides (Sn_xN_y) [7].

Owning to a better durability Lithium phosphorus oxynitride (LiPON) could be used as the fine alternative to the traditional sulfides and oxides glassy electrolytes in all solid state thin film lithium battery. LiPON first reported by Bates et al. [8] exhibited a very low electronic conductivity, a relatively high ionic conductivity and a large stability electrochemical window voltage at room temperature and no phase change in wide temperature range.

In this work, the all-solid-state thin film lithium battery with the cell structure Pt/LiCoO₂/LiPON/Sn_xN_y/Pt was prepared using only magnetron sputtering and the discharge performances from the room temperature to 200 °C were studied. The influences of the heat treatment temperature on the property of LiPON were studied as well.

Section snippets

Experimental

The all-solid-state thin film lithium batteries were fabricated onto a quartz glass substrate by a sequence of magnetron sputtering processes. The Pt current collector layers were deposited by direct current (DC) magnetron sputtering (1.0 W/cm², 1 Pa). The LiCoO₂ layer was deposited by radio frequency (RF) magnetron sputtering of the LiCoO₂ target (2.6 W/cm², 2.5 Pa) and was then annealed at temperatures of 700 °C to obtain a fully crystalline phase. The LiPON electrolyte layer was deposited by RF

Results and discussion

Fig. 1 shows a cross-sectional SEM image of the thin film battery with the cell structure Pt/LiCoO₂/LiPON/Sn_xN_y/Pt. The thickness of Pt, LiCoO₂, LiPON and Sn_xN_y films are 0.3 µm, 3.4 µm, 2.7 µm and 0.9 µm, respectively, and thus the total thickness of this thin-film battery is about 7.6 µm.

The Charge and discharge curves of the thin film battery are presented in Fig. 2(a). The curves show that the thin film battery operates mainly in the potential range of 2.5 and 3.9 V without a typical voltage

Conclusion

In this work, the all-solid-state thin film lithium battery with the cell structure Pt/LiCoO₂/LiPON/Sn_xN_y/Pt was prepared and the discharge performances of the thin film battery were investigated at the temperature ranging from 20 °C to 200 °C. The results showed that the thin film battery maintains a good high-temperature discharge performance before 150 °C and the discharge capacity of the thin film battery was 163 µA h, about 87% of the initial discharge capacity at 20 °C. The influences of the

Acknowledgements

The work was supported by the National Natural Science Foundation of China (no. 201303262) and the Basic Scientific Research Program of National University of Defense Technology (no. 201205).

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Fig. 5(i) shows the N1s core-level spectra of the Li3PO4 samples treated for 60–180 min. All the spectra are deconvoluted into three peaks, which are attributed to the N bonds of doubly coordinated nitrogen (Nd: P–NP, peak position: 397.6–397.7 eV) [28,44–47], triply coordinated nitrogen (Nt: P–N<P2, 399.1 eV) [28,44–47], and O–NO group (403.4–403.5 eV) [44–46]. These comprise LiPON [28,44–47], confirming that LiPON was formed by the N2-plasma treatment of Li3PO4.
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Lithium phosphorus oxynitride (LiPON) is the most representative solid electrolyte in thin film battery applications. In addition, it has been used as an interfacial protective layer to improve the stability of cathode and anode materials. In this article, we review the effect of the process conditions on the structural and electrochemical properties of LiPON thin films. A common method to form LiPON thin films is radiofrequency (RF) sputtering; much research has been conducted to optimize the corresponding process parameters, such as RF power density, working pressure in nitrogen atmosphere, substrate temperature, substrate bias power, post-annealing, and sputtering target. Many studies have characterized LiPON films obtained with various process parameters, but significant differences have been observed in the reported trends. The most representative difference involves the N_t/N_d ratio, which has been reported to be either directly or inversely proportional to the ionic conductivity. Recently, controversial results have been obtained on the N-based local structure of LiPON thin films. The structural argument relies on the idea that nitridation promotes cross-linking via the formation of doubly and triply coordinated N bridges between P atoms. In addition to further research to clarify these issues, it is necessary to introduce new methods for the interpretation of data based on it.
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It has been stated in several studies that in reactive RF-sputtering, N2 is incorporated into the material structure by the substitution of bridging oxygen by doubly bonded nitrogen with phosphorus (P-N=P) up to 75% of its nitrogen atoms [19]. In addition, increasing the triple coordinated sites may increase the ionic conductivity further [20]. This is considered a decisive factor to enhance stability and ionic conductivity of LiPON compared to that of Li3PO4 [21].
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High temperature property of all-solid-state thin film lithium battery using LiPON electrolyte

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

Solid State Ion

Electrochim Acta

Mater Sci Eng, B

J Power Sources