Bipolar stacked quasi-all-solid-state lithium secondary batteries with output cell potentials of over 6 V

OPEN SUBJECT AREAS: BATTERIES MATERIALS CHEMISTRY Bipolar stacked quasi-all-solid-state lithium secondary batteries with output cell potentials of over 6 V Takahiro Matsuo, Yoshiyuki Gambe, Yan Sun & Itaru Honma Received 9 May 2014 Accepted 25 July 2014 Published 15 August 2014 Correspondence and requests for materials should be addressed to I.H. (i.honma@tagen. tohoku.ac.jp) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. Designing a lithium ion battery (LIB) with a three-dimensional device structure is crucial for increasing the practical energy storage density by avoiding unnecessary supporting parts of the cell modules. Here, we describe the superior secondary battery performance of the bulk all-solid-state LIB cell and a multilayered stacked bipolar cell with doubled cell potential of 6.5 V, for the first time. The bipolar-type solid LIB cell runs its charge/discharge cycle over 200 times in a range of 0.1–1.0 C with negligible capacity decrease despite their doubled output cell potentials. This extremely high performance of the bipolar cell is a result of the superior battery performance of the single cell; the bulk all-solid-state cell has a charge/discharge cycle capability of over 1500 although metallic lithium and LiFePO4 are employed as anodes and cathodes, respectively. The use of a quasi-solid electrolyte consisting of ionic liquid and Al2O3 nanoparticles is considered to be responsible for the high ionic conductivity and electrochemical stability at the interface between the electrodes and the electrolyte. This paper presents the effective applications of SiO2, Al2O3, and CeO2 nanoparticles and various Li1 conducting ionic liquids for the quasi-solid electrolytes and reports the best ever known cycle performances. Moreover, the results of this study show that the bipolar stacked three-dimensional device structure would be a smart choice for future LIBs with higher cell energy density and output potential. In addition, our report presents the advantages of adopting a three-dimensional cell design based on the solid-state electrolytes, which is of particular interest in energy-device engineering for mobile applications. R enewable energy sources, which unlike exhaustible energy sources such as petroleum or natural gas, do not generate carbon dioxide that is considered the cause of global warming, are attracting considerable attention. Renewable energy sources, which exist in nature, are expected to provide clean energy, such as solar energy, wind energy, tidal energy, and geothermal energy. Energy storage devices that can store energy efficiently are essential for utilizing renewable energy. Lithium ion batteries (LIBs) with high energy density are an example of such devices and have attracted significant attention in recent times. LIBs are currently used not only for compact applications, e.g., as power sources for electronic devices but also for larger applications such as in electric vehicles and stationary power sources. Conventional LIBs use organic liquid electrolytes, and there is possibility of liquid leaks and dangers such as ignition. Such problems must be resolved for practical and safe use of LIBs. The use of all-solid-state secondary batteries that use solid electrolytes, which are flame resistant and do not pose the risk of liquid leaks, can be cited as a possible solution to these problems. In addition to the fact that there is no possibility of liquid leaks or dangers of ignition, all-solid-state lithium secondary batteries make it possible to design bipolar layer-built cells fabricated by layering batteries within a single package. The energy density in such batteries is expected to be higher than that in LIBs with organic liquid electrolytes. However, solid electrolytes used in all-solid-state secondary batteries pose certain issues. For instance, solid electrolytes that have sufficient ion conductivity and high stability when used with lithium metal electrode are not abundantly available, and it is difficult to achieve a good contact between the solid electrolytes and cathode materials1. In order to resolve such issues, our research group has been investigating the use of quasi-solid-state electrolytes. These materials are prepared by solidifying lithium-ion-conductive ionic liquids that are flame resistant and have high ionic conductivity, by utilizing the strong interaction on the surface of oxide nanoparticles2–5. Quasisolid-state electrolytes that contain SiO2 nanoparticles (particle diameter: 7 nm) as oxide nanoparticles and cation-bis(trifluoromethanesulfonyl)amide(TFSA)/Li-TFSA (cation: 1-ethyl-3-methyl imidazolium (EMI), N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME), N-methyl-N-propyl piperidinium (PP13)) SCIENTIFIC REPORTS | 4 : 6084 | DOI: 10.1038/srep06084 1 www.nature.com/scientificreports Table 1 | Quasi-solidification of [Li (G4)] [TFSA] with various oxide nanoparticles in x vol% (x 5 40, 50, 60, 75) (QSE 5 quasi-solid state electrolyte) SiO2 CeO2 c-Al2O3 a-Al2O3 ZrO2 Diameter/nm 40 vol% 50 vol% 60 vol% 75 vol% 7 10-30 5 20 50 5 10 QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE QSE Gel Gel Gel Gel as lithium-ion-conducting ionic solutions, have mechanical strength as well as transport properties similar to those of liquids; moreover, their electrical conductivity and self-diffusion coefficient are also not significantly different from those of liquids5. Furthermore, all-solidstate lithium batteries fabricated using such quasi-solid-state electrolytes have been confirmed to operate favorably at 0.1 C. We also considered quasi-solidifying glyme-lithium salt complexes ([Li (G4)] [TFSA]) having characteristics similar to those of ionic liquids, as reported by Watanabe and associates6–10. [Li (G4)] [TFSA] has been reported to be electrochemically stable up to 0 V (vs. Li/Li1) and has relatively high electrical conductivity. Further, the quasisolid-state electrolyte containing [Li (G4)] [TFSA] and SiO2 nanoparticles, which has transport characteristics similar to those of [Li (G4)] [TFSA] liquids, operates favorably in all-solid-state secondary batteries11. However, when constant polarization measurements for symmetrical cells fabricated using Li metal were conducted, quasisolid electrolytes containing [Li (G4)] [TFSA] indicated spike-wave amperometric responses under relatively small voltages11. This is believed to have occurred because of electrolyte penetration due to dendrite precipitation. Moreover, the electrical conductivity of these electrolytes needs to be improved further. It is necessary to inhibit dendrite precipitation and to design quasi-solid-state electrolytes with higher electrical conductivity for using them in all-solid-state secondary batteries. We attempted to design quasi-solid electrolytes with high stability to Li metal and excellent trans (...truncated)