In Situ Atomic‐Scale Investigation of Structural Evolution During Sodiation/Desodiation Processes in Na3V2(PO4)3‐Based All‐Solid‐State Sodium Batteries

Abstract Recently, all‐solid‐state sodium batteries (Na‐ASSBs) have received increased interest owing to their high safety and potential of high energy density. The potential of Na‐ASSBs based on sodium superionic conductor (NASICON)‐structured Na3V2(PO4)3(Na3VP) cathodes have been proven by their high capacity and a long cycling stability closely related to the microstructural evolution. However, the detailed kinetics of the electrochemical processes in the cathodes is still unclear. In this work, the sodiation/desodiation process of Na3VP is first investigated using in situ high‐resolution transmission electron microscopy (HRTEM). The intermediate Na2V2(PO4)3 (Na2VP) phase with the P21/c space group, which would be inhibited by constant electron beam irradiation, is observed at the atomic scale. With the calculated volume change and the electrode–electrolyte interface after cycling, it can be concluded that the Na2VP phase reduces the lattice mismatch between Na3VP and NaV2(PO4)3 (NaVP), preventing structural collapse. Based on the density functional theory calculation (DFT), the Na+ ion migrates more rapidly in the Na2VP structure, which facilitates the desodiation and sodiation processes. The formation of Na2VP phase lowers the formation energy of NaVP. This study demonstrates the dynamic evolution of the Na3VP structure, paving the way for an in‐depth understanding of electrode materials for energy‐storage applications.


Figure
Figure S2|XRD pattern of the Na 3 VP/NZSP sample.

Figure
Figure S3|Schematic diagram showing the structures of Na 3 VP and NZSP.

Figure
Figure S4|TEM images of the pristine interface between Na 3 VP and NZSP..

Figure
Figure S6|In situ XRD patterns of Na 3 VP during first cycle with a voltage window of 2.7 -4.0 V vs. Na + /Na at the C-rate of 0.4 C. Figure S7|The intensity line profiles of Na 3 VP during desodiation.

Figure
Figure S8|SAED images of the Pt electrode.

Figure
Figure S9|The intensity line profiles of Na 2 VP during sodiation.

Figure
Figure S10|TEM images of the interface between Na 3 VP and NZSP after a cycle.

Figure
Figure S11|Resistance measurements of the sample at 4 V for 15 minutes.

Figure
FigureS12|TEMimages of the interface between Na 3 VP and NZSP after three cycles.

Figure
Figure S13|SAED images of Na 3 VP and the distances between two diffraction spots during

Figure
Figure S14|SAED images of Na 2 VP and the distances between two diffraction spots during

Figure
Figure S15|SEM images of the TEM sample with Pt wires deposited with the FIB system.

Figure
Figure S16|STEM images of the sample and EDS mapping of the V signals.

Figure
Figure S17|HRTEM images of cathode.

Figure
Figure S18|Schematic diagram illustrating the evolution of the Na 3 VP crystal structure during the

Figure
Figure S19|Migration energy profiles of the Na + ion in the (a) Na 2 VP and (b) Na 3 VP crystals, as

Figure
Figure S20|SEM images showing the preparation of the TEM sample and the deposition of

Figure S3 .
Figure S3.Schematic diagram showing the structures of Na 3 VP and NZSP.

Figure S4 .
Figure S4.TEM images of the pristine interface between Na 3 VP and NZSP.

Figure S5 .
Figure S5.Cycling performances of Na 3 VP-NZSP-Na cells operating at 25 °C.(a-b) performance of a cell operating with 35.2 μA cm −2 : (a) Charge and discharge curves for stable cycles (no.3-100); (b) Discharge capacity and Coulombic efficiency for each cycle.(c-d) Performance of a cell with a current density of 127.7 μA cm −2 : (c) Charge and discharge curves for stable cycles (no.3-100); (d) Discharge capacity and Coulombic efficiency for each cycle.(e) Performance of a cell operating with different current densities: corresponding discharge capacity and Coulombic efficiency for each cycle.(Nano Energy 2019, 65, 104040)

Figure S8 .
Figure S8.SAED images of the Pt electrode.(a) Pristine Pt electrode.(b) Pt electrode after the sample was desodiated.

Figure S10 .
Figure S10.TEM images of the interface between Na 3 VP and NZSP after a cycle.(a) Interface of the sample outside the in situ area (with Na 2 VP phase).(b) Interface of the sample within the in situ area (without Na 2 VP phase).The white dashed lines indicate the interfaces between Na 3 VP and NZSP.

Figure S11 .
Figure S11.Resistance measurements of the sample at 4 V for 15 minutes.Resistance of the sample during (a) desodiation and (b) sodiation.Resistance of the sample from 0 to 4 minutes during (c) desodiation and from 10 to 15 minutes during (d) sodiation.

Figure S12 .
Figure S12.TEM images of the interface between Na 3 VP and NZSP after three cycles.(a) Interface of the sample outside the in situ area (with Na 2 VP phase).(b) Interface of the sample within the in situ area (without Na 2 VP phase).The white dashed lines indicate the interfaces between Na 3 VP and NZSP.

Figure S13 .
Figure S13.SAED images of Na 3 VP and the distances between two diffraction spots during desodiation.(a-d) SAED images of Na 3 VP during the desodiation process at 4 V for 15 minutes.The distances between two diffraction spots circled in red lines are shown on the right of the corresponding SAED images.

Figure S14 .
Figure S14.SAED images of Na 3 VP and the distances between two diffraction spots during sodiation.(a-d) SAED images of Na 3 VP during the sodiation process at 4 V for 15 minutes.The distances between two diffraction spots circled in red lines are shown on the right of the corresponding SAED images.

Figure S15 .
Figure S15.SEM images of the TEM sample with Pt wires deposited with the FIB system.(a) The full image of the TEM sample and Pt wires.(b-d) Enlarged images showing the contacts of the sample and Pt wires.

Figure S16 .
Figure S16.STEM images of the sample and EDS mapping of the V signals.The STEM image of the sample during (a) the first cycle, (b) the second cycle and (c) the third cycle, and their corresponding EDS mapping of V.The white dotted lines represent the interface between Na 3 VP and NZSP.

Figure S17 .
Figure S17.HRTEM images of cathode.Crystal structures of cathode after (a) the first cycle, (b) the second cycle and (c) the third cycle.

Figure S18 .
Figure S18.Schematic diagram illustrating the evolution of the Na 3 VP crystal structure during the desodiation process, along with the corresponding formation energy (black digits) and the desodiation energy (blue digits).

Figure S19 .
Figure S19.Migration energy profiles of the Na + ion in the (a) Na 2 VP and (b) Na 3 VP crystals, as well as the corresponding initial-state (IS), transition-state (TS), and final-state (FS) structures.

Figure S20 .
Figure S20.SEM images showing the preparation of the TEM sample and the deposition of electrodes with the FIB system.(a) The surface of the Na 3 VP/NZSP sample.(b) Protective layer of Pt deposited on the sample surface.(c) Trenches milled on both sides of the TEM sample.(d) In situ TEM sample after the low-kV cleaning process.(e) Deposition of the TEM sample with Pt wires on the top and bottom sides of the sample.(f) An enlarged SEM image of the in situ TEM sample.