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Zhu et al. Energy Mater. 2025, 5, 500034 https://dx.doi.org/10.20517/energymater.2024.201 Page 3 of 10

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furnace at 950 °C for 10 h with a heating rate of 2 °C min . After natural cooling, the ground pre-sintered
powders were placed into a mold and pressed under the pressure of 50, 150, 300 and 600 MPa. The
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compacted pellets were then heated at 1,250 °C for 4 h with a heating rate of 5 °C min , and the LLZTO
pellets were obtained after natural cooling.

Materials characterization
The crystal structure was analyzed using an X-ray diffraction (XRD) instrument (D8-A25, Bruker AXS,
Germany) with Cu Kα radiation. The morphology and element distribution were examined by a scanning
electron microscope (SEM) (SU-70, Hitachi, Japan) equipped with an energy dispersive spectroscopy (EDS)
detector. The elemental composition and valence states were measured using an X-ray photoelectron
spectroscopy (XPS) equipment (Escalab Xi+, Thermo Fisher, USA).

Electrochemical measurements
To measure the ionic and electronic conductivity, the Ag slurry was evenly sprayed on both sides of the
polished LLZTO pellet. After drying, the Ag|LLZTO|Ag symmetric cells were tested on an electrochemical
workstation (PARSTAT 3000A, Princeton, USA). The electrochemical impedance spectrum (EIS) plots
were recorded with an amplitude potential of 10 mV and a frequency range of 10 to 10 Hz. The applied
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voltage of the steady-state current measurement was 200 mV. The LLZTO pellets were polished in an
Ar-filled glove box (H O < 0.01 ppm and O < 0.01 ppm) to assemble the Li|LLZTO|Li symmetric cells. To
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improve the interface contact, a drop (~10 μL) of liquid electrolyte containing 1M lithium
hexafluorophosphate (LiPF ) dissolved in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate
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(EC/EMC/DMC) (1:1:1 by volume) was applied to both sides of the LLZTO pellets. No solid electrolyte was
added to the LFP cathode, but the above trace amount of liquid electrolyte was used to ensure good contact
between LLZTO and LFP. The LFP|LLZTO|Li full cells were assembled in similar steps as the symmetric
cells. To prepare the cathode sheets, LFP, polyvinylidene fluoride (PVDF), and acetylene black (8:1:1 by
mass) were dissolved in N-methyl pyrrolidone (NMP) to form the slurry, which was then cast on aluminum
foils and vacuum-dried at 85 °C for 12 h. The electrodes were cut into circle sheets with a diameter of 12
mm, and the mass loading of each sheet was ~1.5 mg. All electrochemical tests were performed at room
temperature except for the activation energy tests.

RESULTS AND DISCUSSION
Figure 1A shows the XRD patterns of the different LLZTO pellets after sintering. It can be seen that all the
characteristic peaks of each sample match well with the c-LLZO, and no Li-deficient-phase impurity La Zr O
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(at ~28°-29°) is observed. Compared with the standard c-LLZO, the peaks of the samples shift to higher
angles because the radius of Ta is smaller than that of Zr , and the lattice constant becomes smaller after
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the replacement of Zr by Ta . Since the peaks of Li La Ta O are close to those of c-LLZO, each sample
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shows different degrees of peak splitting in the range of 30°-60° representing the Li La Ta O . As
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compaction pressure increases, the characteristic peaks and the peak splitting are more obvious, indicating
the more Li La Ta O and the higher crystallinity of SSEs. As shown in Figure 1B, the calculated grain size
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and micro-strain along the (422) facet are compared quantitatively. As compaction pressure rises, the grain
size first increases and then decreases, while the micro-strain decreases and then increases. This indicates
that higher compaction pressure leads to greater deformation, smaller particles, and denser structure.
Despite the similar sintering conditions, obvious morphology differences can be observed between the
LLZTO pellets. For the LLZTO-50 pellet [Figure 1C], numerous pores are present in the worm-like
structure due to insufficient contact between grains. For the LLZTO-150 pellet [Figure 1D], the grains are in
contact with each other but are not completely densified, resulting in irregular hexagonal particles due to