Properties of Carbon-coated SiO-C Pelletized Negative Electrodes for All-solid-state Batteries

Abstract

Carbon-coated SiO (SiO-C) pelletized electrode without binder materials and solid electrolytes worked as negative electrode materials for sulfide-based all-solid-state batteries. The SiO-C pelletized electrode exhibited the high capacity (ca. 1340 mAh g⁻¹) for all-solid-state batteries using lithium-ion conducting sulfide-based solid electrolyte at room temperature. The SiO-C pelletized electrode containing sulfide solid electrolyte exhibited superior cycle stability compared to the SiO-C pelletized electrode. Their SiO-C pelletized electrodes are promising as high-performance negative electrodes for all-solid-state batteries.

1. Introduction

Recently, all-solid-state batteries using lithium-ion conductive inorganic solid electrolytes are promised to improve safety and reliability of lithium-ion secondary batteries using organic liquid electrolytes. All-solid-state batteries are used in a wide temperature range and exhibit high-rate performance. Such all-solid-state batteries are expected to be applied to electric vehicles.¹ Sulfide solid electrolyte powder have excellent formability. Especially, Li₂S-P₂S₅-based sulfide pelletized solid electrolytes exhibit high lithium-ion conductivity on the order of 10⁻⁴–10⁻² S cm⁻¹ at room temperature.^2–4 Therefore, bulk-type all-solid-state batteries can be fabricated by stacking powdered electrode active materials and solid electrolytes.⁵

Silicon negative electrode materials with high theoretical capacity⁶ and low operating voltage⁷ are expected to achieve high energy density for LIBs using organic liquid electrolytes. However, when silicon is used as a negative electrode material, silicon particles undergo significant volume change (>300 %) during charge/discharge, respectively. The large volume change will cause the pulverization of silicon particles and loss of electrical contact between the silicon particles.

SiO negative electrode has been researched and developed to overcome their drawbacks, because SiO has smaller volume change (about 200 %)⁸ than that of silicon during charge/discharge. For example, SiO (Si : SiO₂ = 50 : 50 mol%) particles are consist of Si and SiO₂ particles. Si and SiO₂ are reduced to Li_3.75Si and Li₄SiO₄ during first charge, respectively.^9,10 The Li_3.75Si is oxidized to Si during first discharge. After the second cycle, the oxidized Si is rechargeable. Li₄SiO₄ compounds exhibit lithium-ion conductivity¹¹ and are inert for charge/discharge cycles.

Carbon-coated SiO (SiO-C) materials have been investigated as the negative electrodes in LIBs.^12–14 Coating carbon on the surface of SiO particles works as a conductive agent. The electron-conductive carbon may maintain the conductive path between the SiO particles and with the current collector.

Okuzawa et al. have reported that SiO-C sheet-type electrode without sulfide-based solid electrolytes worked as negative electrode materials for all-solid-state batteries.¹⁵ The SiO-C sheet-type electrode exhibited the high capacity (ca. 1500 mAh g⁻¹) at room temperature. On the other hand, it has not been reported that SiO-C pelletized electrodes without binder materials and sulfide-based solid electrolytes worked as rechargeable negative electrodes in all-solid-state batteries. As compared with SiO-C sheet-type electrodes prepared by wet-process using conventional polymer binders, SiO-C pelletized electrodes are prepared by a simple and low-cost process. Furthermore, SiO-C pelletized electrodes are promising for the utilization in the wide working temperature range.

In this study, SiO-C without binder materials and sulfide solid electrolytes and SiO-C+SE containing solid electrolytes (SE) were evaluated as the negative electrodes of sulfide all-solid-state batteries. The SiO-C pelletized electrode without binder materials and solid electrolytes exhibited the high capacity of about 1340 mAh g⁻¹ per unit weight of SiO during charge/discharge at room temperature. The SiO-C+SE pelletized electrode exhibited superior cycle performance compared to that of the SiO-C pelletized electrode.

2. Experimental

2.1 Preparation of sulfide-based amorphous inorganic solid electrolyte (a-Li₃PS₄) and all-solid-state half-cells

Li₂S (99.9 %, Mitsuwa) and P₂S₅ (99 %, Aldrich) were used as starting materials to prepare the amorphous solid electrolyte with the nominal composition of 75Li₂S-25P₂S₅ (a-Li₃PS₄). The a-Li₃PS₄ powder was synthesized by mechanical milling (MM) using a planetary ball mill device (Fritsch P-7) at a rotation speed of 530 rpm for 12 h at room temperature under a dry Ar atmosphere. A zirconia pot and zirconia balls were used for the ball milling. The zirconia pot was sealed in a stainless steel overpot. The volumes of the zirconia pot and number of balls were 45 cc and 500, respectively.

Figure 1 show the scheme of all-solid-state half-cell. All-solid-state half-cells were fabricated by uniaxial pressing (370 MPa) in an argon glove box using SiO-C (weight of C: 5 wt%, thickness of C: 40–50 nm, crystallite size of Si particles: ca. 4 nm) powder as the working electrode, Li-In alloy as the counter electrode, and a-Li₃PS₄ as the electrolyte.

Figure 1.

Image of all-solid-state half-cell.

SiO-C pelletized electrode before the charge/discharge test was observed by using a scanning electron microscope (SEM). The SEM observation was performed by using a scanning electron microscope (JEOL, JSM-6510) at an acceleration voltage of 12 kV.

SiO-C+SE powder was prepared by mixing SiO-C and a-Li₃PS₄ powder (SiO-C : a-Li₃PS₄ = 60 : 40 wt%) in an agate mortar for 20 minutes. The all-solid-state half-cell using SiO-C+SE powder was also assembled as shown in Fig. 1.

2.2 Electrochemical properties of SiO-C negative electrodes

Charge/discharge properties of SiO-C negative electrodes were measured using all-solid-state half-cells as shown in Fig. 1. Charge/discharge cycle measurements with a constant current (CC) density of 0.064 mA cm⁻² (20–25 mA g⁻¹-SiO) were carried out in the voltage range −0.6–0.9 V vs. Li-In by using a battery charge/discharge system (Hokuto Denko, HJ-1001 SD8) at room temperature. The charge process was performed in constant current–constant voltage (CCCV) mode. The cut-off current density for constant voltage (CV) processes was 0.0064 mA cm⁻² (2.0–2.5 mA g⁻¹-SiO). In this case, the lithium insertion to an electrode means a charge process.

The dQ/dV plot was obtained by the differentiation of a charge or discharge curve of cell with respect to capacity.

AC impedance measurements were performed using an electrochemical analyzer (Bio-Logic, VSP-300) with a frequency range of 10 mHz to 5 MHz and an applied amplitude voltage of 50 mV at room temperature.

3. Results and Discussion

Figure 2 shows SEM image of the pressed surface of the SiO-C pelletized electrode before the charge/discharge test in the all-solid-state half-cell. The SiO-C particles are about 5 µm in diameter. SiO-C particles are in direct contact with each other. This result is expected to form electrical conducting paths in the negative electrode layer.

Figure 2.

SEM image of the pressed surface for the SiO-C pelletized electrode before the charge/discharge test.

Figure 3 shows the charge/discharge curves of the SiO-C pelletized electrode (thickness of the pelletized electrode: ca. 10 µm) in the all-solid-state cell using the sulfide-based solid electrolyte at the 1st and 5th cycles. The SiO-C pelletized electrode exhibited the charge capacity of about 2101 mAh g⁻¹-SiO (6.61 mAh cm⁻²) and the discharge capacity of about 1340 mAh g⁻¹-SiO (4.21 mAh cm⁻²) at the 1st cycle. The electrode also exhibited the high discharge capacity of about 1173 mAh g⁻¹-SiO at the 5th cycle. These results indicate that the SiO-C pelletized electrode without binder materials and solid electrolytes can charge and discharge at room temperature. The first cycle coulombic efficiency (the first discharge capacity/the first charge capacity × 100 %) is approximately 64 %. The electrode reactions of the SiO materials have been shown in Eqs. 1–3.^9,10 Equations 1 and 2 correspond to the first charge and first discharge reactions, respectively. Equation 3 corresponds to the charge and discharge reactions for subsequent cycles.

  

\begin{align} &\text{Si} + \text{SiO$_{2}$} + \text{7.625Li$^{+}$} + \text{7.625e$^{-}$}\\ &\quad \to \text{1.5Li$_{3.75}$Si} + \text{0.5Li$_{4}$SiO$_{4}$} \end{align} (1)

  

\begin{equation} \text{Li$_{3.75}$Si}\to \text{3.75Li$^{+}$} + \text{3.75e$^{-}$} + \text{Si} \end{equation} (2)

  

\begin{equation} \text{3.75Li$^{+}$} + \text{3.75e$^{-}$} + \text{Si}\leftrightarrows \text{Li$_{3.75}$Si} \end{equation} (3)

Figure 3.

Charge/discharge curves of the SiO-C pelletized electrode in the all-solid-state cell using the sulfide-based solid electrolyte at the 1st and 5th cycles.

Assuming Eqs. 1–3, the theoretical first charge capacity is about 2300 mAh g⁻¹-SiO and the theoretical first discharge capacity is about 1710 mAh g⁻¹-SiO, resulting in the theoretical first cycle coulombic efficiency of about 73.8 %. The first cycle coulombic efficiency (64 %) of SiO-C pelletized electrode is lower than the theoretical first cycle coulombic efficiency (73.8 %). It is suggested that the low coulombic efficiency of the SiO-C pelletized electrode may be caused by side reactions (reductive reaction of a-Li₃PS₄ etc.) and the collapse of the conductive path. These points need further investigation. In contrast, the SiO-C pelletized electrode shows the high active material utilization of 78.4 % (1340 mAh g⁻¹-SiO/1710 mAh g⁻¹-SiO × 100 %). These results suggest that the lithium-ion transfer is smooth in the SiO-C pelletized electrode layer. It is considered that Li-Si alloy and Li₄SiO₄ compounds formed during first charge may work as lithium-ion conducting paths in SiO particles. The SiO-C pelletized electrode without binder materials and solid electrolytes are promising the high-capacity negative electrode for all-solid-state batteries.

Figure 4 shows the charge/discharge curves of the SiO-C+SE pelletized electrode (thickness of the pelletized electrode: ca. 20 µm) in the all-solid-state cell using the sulfide-based solid electrolyte at the 1st and 5th cycles. The SiO-C+SE pelletized electrode exhibited the first charge capacity of about 1995 mAh g⁻¹-SiO and the first discharge capacity of about 1430 mAh g⁻¹-SiO and the first cycle coulombic efficiency of about 72 %. The first cycle coulombic efficiency of approximately 72 % is very close to the theoretical first cycle coulombic efficiency of 73.8 %. The electrode also exhibited the high discharge capacity of about 1389 mAh g⁻¹-SiO at the 5th cycle. These results suggest that the SiO-C+SE negative electrode containing a-Li₃PS₄ can maintain ion and electron paths during the charge/discharge cycle.

Figure 4.

Charge/discharge curves of the SiO-C+SE pelletized electrode in the all-solid-state cell using the sulfide-based solid electrolyte at the 1st and 5th cycles.

Figure 5 shows the dQ/dV plots of SiO-C and SiO-C+SE pelletized electrodes in all-solid-state cells using sulfide-based solid electrolytes at the 5th cycle. The charge and discharge behavior are considered as follows.¹⁶ The two reduction peaks (A) in the voltage range of 0–0.3 V (vs. Li/Li⁺) during charge are due to Li-Si alloying reaction for both SiO-C and SiO-C+SE pelletized electrodes. The two oxidation peaks (B) in the voltage range of 0.2–0.6 V (vs. Li/Li⁺) during discharge are due to the delithiation reaction from the Li-Si alloy for both SiO-C and SiO-C+SE pelletized electrodes. The peak patterns for SiO-C and SiO-C+SE pelletized electrodes are similar to those of lithium insertion and desorption into the Si negative electrode in LIB with organic electrolytes. These results suggest that the electrodes reactions for both SiO-C and SiO-C+SE pelletized electrodes in all-solid-state cells are similar to the Si negative electrode reaction in lithium ion-conducting organic electrolytes.

Figure 5.

dQ/dV plots of (a) SiO-C and (b) SiO-C+SE pelletized electrodes in all-solid-state cells using sulfide-based solid electrolytes at the 5th cycle.

Figure 6 shows the cycle performance of discharge capacities of SiO-C and SiO-C+SE pelletized electrodes in all-solid-state cells using sulfide-based solid electrolytes. The discharge capacity of the SiO-C pelletized electrode decreased to about 766 mAh g⁻¹-SiO after 10th cycles. On the other hand, the SiO-C+SE pelletized electrode maintained the high discharge capacity of approximately 1266 mAh g⁻¹-SiO. The SiO-C+SE pelletized electrode exhibits good cycle stability. These results suggest that the SiO-C+SE negative electrode containing a-Li₃PS₄ can inhibit the capacity decrease because it can maintain some ion and electron paths in the negative electrode layer even if the volume of SiO-C changes during charge/discharge.

Figure 6.

Cycle performance of discharge capacities of SiO-C and SiO-C+SE pelletized electrodes in all-solid-state cells using sulfide-based solid electrolytes.

Therefore, AC impedance measurements were performed for all-solid-state half-cells using SiO-C and SiO-C+SE pelletized electrodes. The results of AC impedance measurements for all-solid-state half-cells were shown as Nyquist plots. Figure 7 shows Nyquist plots after the 1st, 2nd, and 5th charge cycles for all-solid-state half-cell using (a) the SiO-C pelletized electrode and (b) the SiO-C+SE pelletized electrode, respectively. The resistances of the high frequency range (>100 kHz) are assigned to the solid electrolyte layer, as shown in Figs. 7a and 7b.¹⁷ It is suggested that the arc after 100 kHz is the interfacial resistance between the SiO-C electrode and the solid electrolyte, including some of the interfacial resistance between the counter electrode Li-In and the solid electrolyte.¹⁸ The resistance of the solid electrolyte layer increases after the charge/discharge cycles, as shown in Figs. 7a and 7b. The interfacial resistance of the SiO-C pelletized electrode gradually increases with charge/discharge cycles. On the other hand, the interfacial resistance of the SiO-C+SE pelletized electrode hardly increases after the charge/discharge cycles. These results are consistent with the good cycle performance of the SiO-C+SE pelletized electrode in Fig. 6.

Figure 7.

Nyquist plots of (a) Li-In | a-Li₃PS₄ | SiO-C cell and (b) Li-In | a-Li₃PS₄ | SiO-C+SE cell after 1st, 2nd and 5th charge.

4. Conclusions

In this study, it was shown that carbon-coated SiO (SiO-C) pelletized electrode without binder materials can be charged and discharged as negative electrode in the all-solid-state cell. The SiO-C pelletized negative electrode without binder materials and solid electrolytes showed the high discharge capacity of about 1340 mAh g⁻¹-SiO during 1st charge/discharge in the sulfide-based all-solid-state cell at room temperature.

The interfacial contacts of SiO-C/SiO-C and SiO-C/solid electrolyte are described below. First, the carbon coating on the SiO-C surface is partially cracked during the fabrication process of all-solid-state half-cell. As a result, SiO particle/solid electrolyte interface and SiO particle/SiO particle interface are formed. So, ionic conduction paths are formed at the SiO particle/solid electrolyte interface. It is also considered that the formation of the SiO particle/SiO particle interface causes ion-electron transfer. These results need further investigation.

The SiO-C+SE pelletized electrode with solid electrolytes exhibited the high capacity of about 1430 mAh g⁻¹-SiO and good cycle performance. AC impedance measurements also showed that the interfacial resistance of the cell did not change significantly. The SiO-C material may be promising as the high-capacity negative electrode for all-solid-state batteries.

CRediT Authorship Contribution Statement

Naoya Ishii: Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Writing – original draft (Lead), Writing – review & editing (Supporting)

Naoki Kakinuma: Data curation (Supporting), Formal analysis (Supporting), Investigation (Supporting)

Hideyuki Morimoto: Conceptualization (Lead), Investigation (Equal), Supervision (Lead), Writing – original draft (Supporting), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: 23K04904

Footnotes

A part of this paper has been presented in ECSJ of Hokuriku Branch Spring Meeting in 2023 (Presentation T-03).

H. Morimoto: ECSJ Active Member

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