All-solid-state lithium batteries consisting of the Li
5.5PS
4.5Cl
1.5 electrolyte combined with the
S/
L-FeS
2 cathode and Li-In anode were fabricated and cycled at various current densities between 0.4 V and 2.4 V (
vs. Li-In) to investigate the electrochemical performances.
Fig. 2a shows the charge/discharge profiles for the 1
st and 50
th cycles of the corresponding solid-state batteries using
L-FeS
2 and
S-FeS
2 materials as cathode cycled at 0.1 mA/cm
2 at room temperature. During the initial discharge process, both batteries show clear discharge plateaus at ~0.8 V (
vs. Li-In). Moreover, the
L-FeS
2 and
S-FeS
2 cathodes deliver discharge capacities of 686.3 and 703.9 mAh/g for the 1
st cycle, respectively. During the following initial charge progress, two charge plateaus located at ~1.2 and ~1.8 V (
vs. Li-In) are observed in the charging profiles. These two voltage plateaus are associated with the electrochemical reaction between Fe and Li
2S and the oxidation reaction of the remaining Li
2S to S, respectively. For both
L- and
S-FeS
2 electrodes
, the discharge capacity is much higher than the charge capacity during the 1
st cycle, which may be related to the decomposition of sulfide electrolyte during charge/discharge processes. As shown in
Fig. 2b, both
L- and
S-FeS
2 cathodes show a fast drop in discharge capacities during 50 cycles. As a typical conversion electrode material, FeS
2 suffers fast capacity degradation during cycling due to the large volume expansion in lithium batteries using liquid organic electrolytes [
38]. For all-solid-state batteries using FeS
2 electrode materials and solid electrolytes, this situation becomes even worse due to the poor solid-solid contact and more intense volume variations during cycling in the battery [
35,
39]. However, the
S-FeS
2 cathode still delivers much higher discharge capacities compared to the
L-FeS
2 cathode, suggesting that lowering the particle size of the FeS
2 electrode material is helpful to achieve higher capacities. Fig. S3 (Supporting information) shows the SEM images of the surface section of the cathode mixture part of the assembled
S-FeS
2/Li
5.5PS
4.5Cl
1.5/Li-In and
L-FeS
2/Li
5.5PS
4.5Cl
1.5/Li-In batteries. The cathode mixture with
S-FeS
2 shows superior solid-solid contact than that of the
L-FeS
2. For the
S-FeS
2/Li
5.5PS
4.5Cl
1.5 cathode mixture, intimate contact between different particles is observed, while clear cracks and voids are detected in the SEM image of the
L-FeS
2/Li
5.5PS
4.5Cl
1.5 cathode mixture. The good solid-solid interface contact produces fast Li-ion transport with small interfacial resistance, while poor solid-solid interface contact lowers Li-ion mobility and yields huge interfacial resistances. Therefore, the
S-FeS
2 electrode delivers higher discharge capacities and slightly better cyclability than that the
L-FeS
2 electrode at room temperature. Furthermore, the rate capability of both batteries was also verified at different charge/discharge current densities. As shown in
Fig. 2c, the discharge capacity of the
L-FeS
2/Li
5.5PS
4.5Cl
1.5/Li-In battery was 950.7 mAh/g at 0.1 mA/cm
2, 729.9 mAh/g at 0.2 mA/cm
2, 538.1 mAh/g at 0.5 mA/cm
2, 346.8 mAh/g at 1.0 mA/cm
2 and 244.7 mAh/g at 2.0 mA/cm
2, respectively. In contrast, the
S-FeS
2/Li
5.5PS
4.5Cl
1.5/Li-In battery delivers discharge capacities of 950.9 mAh/g at 0.1 mA/cm
2, 787.0 mAh/g at 0.2 mA/cm
2, 622.7 mAh/g at 0.5 mA/cm
2, 434.4 mAh/g at 1.0 mA/cm
2, and 288.0 mAh/g at 2.0 mA/cm
2, respectively.
S-FeS
2 electrode delivers much higher discharge capacities than that of the
L-FeS
2 at higher charge/discharge current densities. FeS
2 materials with smaller particle sizes mean short diffusion lengths, resulting in better electrochemical performances, especially at large current densities. Moreover, superior battery performance is also associated with solid/solid interfacial diffusions. To investigate the Li-ion dynamics in the
L- and
S-FeS
2 cathode mixture, GITT was performed on both
L-FeS
2/Li
5.5PS
4.5Cl
1.5/Li-In and
S-FeS
2/Li
5.5PS
4.5Cl
1.5/Li-In batteries at room temperature. As shown in
Figs. 2d and
e,
S-FeS
2 shows smaller overpotentials and faster Li-ion diffusion rates than that
L-FeS
2 at different charge/discharge states, which agrees well with the previous analysis.