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Page 2 of 10 Zhu et al. Energy Mater. 2025, 5, 500034 https://dx.doi.org/10.20517/energymater.2024.201

INTRODUCTION
With the advancement and growing prominence of energy materials, numerous energy devices have been
developed to satisfy the increasing demands of modern society. Solid-state lithium batteries (SLBs) that
employ solid-state electrolytes (SSEs) are attractive due to their high energy density and enhanced safety
[1-5]
compared to widely used lithium-ion batteries (LIBs) . Among them, garnet-type Li La Zr O (LLZO)
2
7
3
12
SSEs and their derivatives are promising because of their high chemical stability and wide electrochemical
[6]
window , but the low compactness and inferior ionic conductivity of cubic-phase LLZO (c-LLZO) cannot
[7-9]
meet the current practical requirements of SLBs.
Elemental doping is widely used to stabilize the phase structure by reducing Li content to induce Li
vacancies, thus increasing the ionic conductivity of LLZO SSEs. The main doping methods include
Li-site [10-13] , La-site [14,15] , Zr-site [16-18] , and multi-site [19-23] doping, and most reports focus on Zr-site doping.
-4
5+
Ohta et al. found that the ionic conductivity of LLZO is increased to 8 × 10 S cm when the Nb content is
-1
0.25 per formula unit (pfu) . Thompson et al. prepared the Ta-doped LLZO through a hot pressing
[16]
method, and obtained the highest compactness of ~97% and the highest ionic conductivity when the lithium
vacancy number is 0.5 (Li La Zr Ta O , LLZTO) . Mukhopadhyay et al. calculated the Li-O bond
[17]
12
1.5
0.5
3
6.5
lengths for different Ta-doping samples, showing that the Li-O bonds of LLZTO are longer than the other
+
[18]
samples to facilitate faster Li migration and higher ionic conductivity . As a result, LLZTO stands out
among various electrolytes due to its superior overall properties.
High-temperature sintering is the most critical step for densifying the LLZO pellets, including atmospheric
sintering , hot pressing , electric-field assisted sintering , and plasma sintering . Atmospheric sintering
[24]
[25]
[26]
[27]
enables the mass production of LLZO pellets but is hindered by issues such as non-uniform sintering and
material waste. In contrast, methods such as hot pressing, electric-field assisted sintering, and plasma
sintering are effective for producing high-quality ceramics, but are not widely adopted due to their complex
equipment requirements and high costs. For the widely used atmospheric sintering method, it has been
reported that compaction pressure (before sintering) greatly influences the ceramic quality of sulfide
electrolytes [28,29] . However, specific studies on oxide SSEs, particularly garnet-based SLBs, remain lacking.
In this work, the LLZTO pellets are compacted under different pressures and then sintered at 1,250 °C for
4 h (LLZTO-x, where x is the compaction pressure, MPa). The compacted LLZTO pellets show different
morphologies and compositions, and the impact of compaction pressure on the performance of garnet-
based SLBs is investigated. Among the samples, the Li|LLZTO-600|Li symmetric cell and LiFePO (LFP)|
4
LLZTO-600|Li full cell both exhibit the best performance, which is ascribed to the dense structure and
restrained lithium loss upon sintering. This work elucidates the influence of compaction pressure on the
formation and properties of LLZTO pellets, which could inspire the development of analogous SSEs and
SLBs.

EXPERIMENTAL
Materials preparation
The LLZTO SSEs were prepared using a solid-state synthesis. The Li CO , La O , ZrO , and Ta O
2
3
2
2
2
3
5
precursors were weighed according to the stoichiometry of LLZTO, and 15% excess Li CO was weighed to
3
2
supplement the Li loss upon high-temperature synthesis. Subsequently, 20 g precursor, 40 g Zr beads, and
30 mL ethanol were added into the ball mill jars, which were placed in a high-energy ball miller (8000D,
-1
Spex SamplePrep, USA) and kept at 8,000 r min for 1 h. The mixed powders were transferred to a vacuum
oven set at 80 °C and heated for 12 h. The powders were ground after drying, and sintered in a muffle