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www.advancedsciencenews.com www.advmat.de verified the fast kinetic of Li-ion transportation, and the LMS–AEA electrode with a high cathode mass loading of 13.91 mg cm−2 (compacting density is 4.5 g cm−3 and the thick- ness of electrode is 30.9 μm) eliminates the possibility that the capacity only originates from the interfacial electrochemical reaction between the LMS and the SSE (see the detailed infor- mation in Figures S3 and S4, Supporting Information). Fur- thermore, our LMS–AEA electrode exhibited superior plasticity, which enabled elastic recovery after releasing the pressure. As the apparent gap in porosity marked by the shadowed area in Figure 3b indicates, the porosity of Mo6S8 decreased to 11% under 360 MPa (the applied pressure of ASSLBs in real oper- ating conditions) and rebounded to 24% after the pressure was released (see the detailed information in Tables S6 and S7, Supporting Information). The excellent deformability of the LMS–AEA electrode is not only favorable in terms of achieving a dense electrode and good physical solid–solid interfacial con- tact but is also helpful for buffering the volume expansion of cathode materials during cycling. Thus, in the following sec- tion, LMS is selected as our priority material. To verify the phase transformation during charging, in situ X-ray diffraction (XRD) was performed for a tailor-made ASSLB (Figure 3c). The pristine electrode belongs to the Li0 phase (Mo6S8) (JCPDC: 89–5114) with three major peaks at 30.7° (121), 33.9° (212), and 34.8° (104). At the initial stage of the discharge, a new Li1 phase (LiMo6S8) [JCPDC: 81–0858, 30.4° (121), 33.7° (212), and 35.0° (104)] was generated along with a decrease in the intensity of the Li0, indicating the two-phase transforma- tion between Li0 and Li1. Subsequently, along with a second discharge plateau (2.05 V), the Li1 phase further transformed into a Li3 phase [JCPDS81-0859, 32.7° (212), 35.5°(104), and 36.8°(220)] and finally converted into Li4Mo6S8 (L4 phase) [JCPDS: 81–0860, 32.6° (212), 35.4° (104), and 36.7° (220)]. To further quantify the relative content of the different phases in the two-phase coexistence region, the normalized intensity based on the (212) peak was obtained, as shown in Figure 3d (middle). This revealed clear multiple two-phase coexist- ences that were consistent with the charge–discharge profiles (Figure 2d). The final discharge product of Li4 can only be reversibly converted into Li1 with a trace of Li0 phase remaining that corresponds with the very short plateau at 2.45 V at the end of the charge stage (circled in Figure 3c, right). The LMS–AEA electrode processes a step-wise electrochemical reaction with the multiple phase transformations in the first cycle, which can be divided into four stages in the order of stage I, stage II, stage III, and stage IV (Figure 3c, middle). During stages I and II, the increase in Li concentration promotes Li-ion transportation in the AEA electrode due to the formation of LixMo6S8 (x = 1, 3, 4) with a high diffusion coefficient DLi. The high Li concentra- tion phases gradually transformed into low phases during de- lithiation (stage III) (Li4–Li3–Li1). At the end of stage IV, an ioni- cally blocking interphase layer mainly consisting of Li0 formed since this is a Li-free and ionic-isolating phase, and its forma- tion on the interface of the electrode–electrolyte shut down the further phase transformation from Li1 to Li0 in the bulk elec- trode, much like a specific “ionic switch.” At this time, the de- lithiation process is blocked and Li1 is retained to a large extent. These results confirmed our hypothesis that the LMS–AEA electrode requires an initial partial pre-lithiation such that its Li ionic and electronic conductivities are qualified. Based on the above results and discussion, we have suc- cessfully demonstrated the proof-of-concept of the AEA elec- trode in ASSLBs. This AEA electrode can also work at room temperature (see the detailed information in Figures S5, Sup- porting Information). However, compared with conventional Li-ion batteries, the ESCelectrode of the LMS–AEA cathode was only 90 mA h g−1, which means that it had not yet realized its full potential in terms of energy density. To further display the advantages of the AEA electrode, a hybrid S8–Mo6S8 (S–LMS)- based AEA cathode was attempted, since an S cathode has a high theoretical capacity of 1,675 mA h g−1, with redox poten- tials ideally below the “ionic switch” of 2.45 V.[48–51] Given that a S8 cathode is electronically insulating, the conductive LMS served as an efficient electronically and ionically conducting network within the AEA electrode, with the weight percentages of S8 and Mo6S8 32.5 and 67.5 wt%, respectively. The transmis- sion electron microscopy (TEM) image of the hybrid S–LMS– AEA cathode (Figure 4a) revealed the coexistence of amorphous S8 (marked by the yellow-block) and crystalline Mo6S8 (marked by the green lines), the electron diffraction patterns are shown in Figure S6, Supporting Information, and the XRD patterns are shown in Figure S7, Supporting Information. Inside the electrode, the small Mo6S8 nanosheets were mutually intercon- nected and thus constructed a conductive network for transport of electrons/ions. Furthermore, the amorphous S8 was homog- enously dispersed, which was reflected by the 10–30 nm nano- domain distributed in the Mo6S8 framework that guarantees the desired electrons/ions transport in the insulating S8. Unlike conventional all-solid-state Li–S cathodes with three single- function materials (carbon/SSE/S) for forming three-phase reaction interfaces, our hybrid S–LMS–AEA cathode had an all- in-one ionically/electronically transporting two-phase reaction interface (LMS/S), which allows for avoiding the unbalanced transport between the electron (by carbon) and Li-ion (by the SSEs). Figure 4b shows the cross-sectional images and the ele- mental mapping of the hybrid S–LMS–AEA cathode and the Li10GeP2S12 SSE. The local element distribution of the phos- phorus (P) indicated a pure AEA cathode electrode without any SSE component (see the detailed information in Figures S8–S10, Supporting Information), which was confirmed by the obtained energy-dispersive spectrum, as shown in Figure 4c. Further- more, the volumetric density at the electrode level is highly dependent on the porosity, which can be reduced by applying pressure in ASSLBs. It is significant that our S–LMS–AEA electrode had a very high compacted filling rate of 91.8% at 360 MPa, which was due to the creep of the soft Mo6S8 and S8 (Figures 3b and 4d, see the detailed information in Tables S6 and S8, Supporting Information), the 8.2% residual porosity of which allowed for accommodating the volume expansion. Com- bining the advantages of AEA electrodes in terms of ESC, the theoretical volumetric density of the S–LMS–AEA electrode was estimated to be above 3565.3 W h L−1 (in terms of the volume of the Mo6S8 and S8), which is more than double that of the conventional S–C-LGPS (32.5 wt%, 17.5 wt%, 50 wt%) electrode with the same fraction of sulfur. Adv. Mater. 2021, 33, 2008723 2008723 (5 of 9) © 2021 Wiley-VCH GmbHPDF Image | Dense All-Electrochem-Active Electrodes for All-Solid-State Lithium Batteries
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