Capacity Fading Mechanism in All Solid-State Lithium Polymer Secondary Batteries Using PEG-Borate/Aluminate Ester as Plasticizer for Polymer Electrolytes

Authors

  • Fuminari Kaneko,

    1. Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan)
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  • Shinta Wada,

    1. Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan)
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  • Masanobu Nakayama,

    Corresponding author
    1. Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan)
    • Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan).
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  • Masataka Wakihara,

    1. Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan)
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  • Jun Koki,

    1. Central for Advanced Materials Analysis Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan)
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  • Shigeki Kuroki

    1. Department of Chemistry and Materials Science Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku Tokyo 152-8552 (Japan)
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Abstract

Solid-state lithium polymer secondary batteries (LPB) are fabricated with a two-electrode-type cell construction of Li|solid-state polymer electrolyte (SPE)|LiFePO4. Plasticizers of poly(ethylene glycol) (PEG)-borate ester (B-PEG) or PEG-aluminate ester (Al-PEG) are added into lithium-conducting SPEs in order to enhance their ionic conductivity, and lithium bis-trifluoromethansulfonimide (LiTFSI) is used as the lithium salt. An improvement of the electrochemical properties is observed upon addition of the plasticizers at an operation temperature of 60 °C. However, a decrease of discharge capacities abruptly follows after tens of stable cycles. To understand the origin of the capacity fading, electrochemical impedance techniques, ex-situ NMR and scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) techniques are adopted. Alternating current (AC) impedance measurements indicate that the decrease of capacity retention in the LPB is related to a severe increase of the interfacial resistance between the SPE and cathode. In addition, the bulk resistance of the SPE film is observed to accompany the capacity decay. Ex situ NMR studies combined with AC impedance measurements reveal a decrease of Li salt concentration in the SPE film after cycling. Ex situ SEM/EDS observations show an increase of concentration of anions on the electrode surface after cycling. Accordingly, the anions may decompose on the cathode surface, which leads to a reduction of the cycle life of the LPB. The present study suggests that a choice of Li salt and an increase of transference number is crucial for the realization of lithium polymer batteries.

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