Huazhong University of Science and Technology's Yunhui Huang and Zhen Li, EES: Lithium-Sulfur Pouch Cells: 99% Capacity Retention After 1,000 Cycles

In July 2024, doctoral student Huangwei Zhang from the research group of Yunhui Huang and Zhen Li published a paper titled “Lithium-sulfur pouch cells with 99% capacity retention for 1000 cycles” in the journal Energy & Environmental Science (impact factor > 32.4). The study developed lithium-sulfur pouch cells with 99% capacity retention over 1,000 cycles.

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Research Background

Lithium-sulfur (Li?S) batteries have high energy density and low material costs, making them a promising next-generation battery system. However, the shuttle effect of polysulfides, as well as dendrite growth and side reactions in the lithium metal anode, limit the cycle life of this battery system. Moreover, most current research is conducted on coin cells, but as research progresses, it is generally believed that the commercialization of lithium-sulfur batteries requires using pouch cells as the research model. In recent years, lithium-sulfur pouch cells have made steady progress in improving energy density, but research on extending cycle life and enhancing cycle stability remains challenging. Many strategies that work well in coin cells are less effective in pouch cells. Currently, mAh-level pouch cells struggle to achieve over 300 cycles while maintaining 80% or more of their capacity, and Ah-level pouch cells often have a cycle life of only a few dozen cycles, indicating that existing methods have difficulty fundamentally improving the cycle life of lithium-sulfur batteries. Achieving long-cycle stability in Li?S pouch cells is therefore highly challenging.

Recently, an article titled "Lithium?sulfur pouch cells with 99% capacity retention for 1000 cycles" was published in Energy & Environmental Science. The article reports the construction of a Li?S pouch cell using sulfurized polyacrylonitrile (SPAN) as the cathode and graphite (Gr) as the anode. Lithium ions were introduced through a simple in-situ prelithiation method. In carbonate-based electrolytes, using a SPAN cathode can avoid the shuttle effect, while using a Gr anode can prevent dendrites and side reactions associated with lithium metal anodes. The study, based on a pouch cell model, demonstrated that by reasonably controlling the cycling conditions to suppress the loss of active lithium and the increase in battery impedance, a SPAN||Gr pouch cell with stable cycling for 1000 cycles and 99% capacity retention could be obtained. Ah-level pouch cells could stably cycle for 1031 cycles with 82% capacity retention and passed multiple safety tests. This design holds promise for fundamentally improving the long-cycle stability of Li?S pouch cells.

Research Problem

1. Cycle Performance Statistics of Lithium-Sulfur Pouch Cells and Energy Density Calculation of SPAN Cathode

Fig. 1 (a) Cycle number and capacity retention of the Li?S pouch cells with different capacity. (b) Comparison of energy density between LFP, NCM811, and SPAN.

Key Points:

1. By analyzing the cycle stability of different capacity lithium-sulfur pouch cells reported in the literature, it is observed that pouch cells with a capacity of less than 0.1 Ah and those in the range of 0.1?1.0 Ah typically struggle to exceed 300 cycles while maintaining over 80% capacity retention. Pouch cells with capacities greater than 1.0 Ah often only have a cycle life of a few dozen cycles. This indicates a lack of highly effective methods to fundamentally improve the cycle life and stability of lithium-sulfur pouch cells. The proposed use of SPAN as the cathode and graphite (Gr) as the anode in this work is expected to fundamentally address the shuttle effect and the negative impacts associated with lithium metal anodes.
2. Additionally, a comparison of the energy densities of three different cathodes (LFP, NCM811, SPAN) shows that the SPAN cathode (which includes the mass of lithium needed for lithiation) has the highest energy density. Therefore, this battery system can maintain a high energy density while also achieving good cycle stability

2. Characteristics of SPAN||Gr Pouch Cells

Fig. 2 Characteristics of the SPAN||Gr pouch cell. (a) Structural schematic of the SPAN||Gr pouch cell. (b) Rate performance. (c) Voltage profiles of the SPAN||Gr pouch cell rests for one month at 50% SOC. (d) Cycling performance and corresponding capacity decay stages. (e) The ultrasonic transmission image of the long cycle SPAN||Gr pouch cell at different capacity decay stages.?

Key Points:

1. The lithium source for this battery system can be introduced through a simple in-situ prelithiation method. The system uses LB?015 electrolyte, which shows poor rate performance at room temperature (1 C = 1000 mA g?1). As the rate increases, the capacity degradation of the battery becomes more pronounced. The self-discharge rate of the battery is less than 1% after one month of storage.
2. In a single-layer pouch cell with sufficient electrolyte, the capacity degrades steadily, reaching 80% of the initial value after 665 cycles. At different stages of degradation, ultrasonic detection reveals good electrolyte wetting with no gas generation.

3. Factors Contributing to Capacity Degradation in SPAN||Gr Pouch Cells

Fig. 3 Capacity decay mechanism of the SPAN||Gr pouch cell. (a) Differential capacity?potential plot of the long cycle SPAN||Gr pouch cell. (b) ElS data for the SPAN||Gr pouch cell. (c) 7Li NMR spectrum of delithiated Gr anodes at different decay stages. (d, e) XRD patterns of delithiated (d) Gr anodes and (e) SPAN cathodes at different decay stages. (f) FTIR spectra of SPAN cathodes. (g) The capacity loss ratio of Gr anode and SPAN cathode. (h) ICP data for delithiated electrodes.

Key Points:

An analysis of the capacity degradation mechanism in this battery system reveals the presence of dead lithium and an increase in battery impedance. The microstructural morphology of the electrodes shows little difference across different stages of degradation. However, in the later stages of cycling, the degradation of the anode exceeds that of the cathode.

4. Characteristics of SEI/CEI in SPAN||Gr Pouch Cells

Fig. 4?Characteristics of SEI/CEI. (a?d) HRTEM images of delithiated (a) Gr anode at 5% capacity decay stage, (b) Gr anode at 20% capacity decay stage, (c) SPAN cathode at 5% capacity decay stage, and (d) SPAN cathode at 20% capacity decay stage. (e?h) F 1s XPS spectra of delithiated (e) Gr anode at 5% capacity decay stage, (f) Gr anode at 20% capacity decay stage, (g) SPAN cathode at 5% capacity decay stage, and (h) SPAN cathode at 20% capacity decay stage.

Key Points:

As the number of cycles increases, the CEI (Cathode Electrolyte Interface) on the surface of the SPAN cathode and the SEI (Solid Electrolyte Interface) on the surface of the Gr anode both thicken. The relative content of LiF increases, indicating continuous consumption of active lithium ions. Additionally, as the SEI/CEI layers thicken, the battery impedance (specifically R_SEI) also increases.

5. Volume Changes of Electrodes in SPAN||Gr Pouch Cells

Fig. 5 Volume changes and stress evolution of electrodes in the SPAN||Gr pouch cell. (a) Schematic of in?situ?optical fiber sensor derived monitoring. (b) Stress curves of electrodes in the SPAN||Gr pouch cell and corresponding voltage profile. (c, d) SEM cross?sectional images of (c) Gr anodes and (d) SPAN cathodes at 100% DOD and 80% DOD. Scale bars are 50 μm. (e) Schematic of capacity decay mechanism for the SPAN||Gr pouch cell.?

Key Points:

1. Using FBG (Fiber Bragg Grating) optical fiber sensors for in-situ stress monitoring, combined with observations of electrode thickness, it is found that the SPAN cathode experiences significant volume changes, while the volume change in the Gr anode is relatively small. However, both SPAN and Gr electrodes undergo continuous cracking and self-repair of the CEI/SEI layers due to volume changes, which is the fundamental reason for the thickening of these layers.
2. For SPAN||Gr pouch cells, the loss of active lithium and the increase in battery impedance are the main causes of capacity degradation. The continuous thickening of the SEI/CEI layers and the generation of dead lithium consume active lithium, while the thickening of the SEI/CEI layers leads to an increase in battery impedance.

6. Long-Cycle Stability of SPAN||Gr Pouch Cells

Fig. 6?High cycle stability SPAN||Gr pouch cells. (a) Cycling performance of the SPAN||Gr pouch cell at 80% DOD. (b, c) Cycling performance of the SPAN||Gr pouch cells at (b) 0.1 C and (c) 0.2 C. (d) Cycling performance of the SPAN||Gr pouch cells at different temperatures.?

Key Points:

1. By controlling the depth of discharge (DOD) to 80%, the volume changes of the electrodes can be effectively mitigated. The pouch cell cycled to 1900 cycles retains 80% of its capacity.
2. Reducing the current density can prevent the extensive formation of dead lithium. A pouch cell cycled at 0.1 C for 500 cycles retains 98% of its capacity, while a pouch cell cycled at 0.2 C for 500 cycles retains 99% of its capacity.
3. Raising the temperature to 45°C improves the kinetic properties of the battery system. The pouch cell cycled to 1000 cycles retains 99% of its capacity.

7. Ah-Level SPAN||Gr Pouch Cells

Fig. 7?Ah?level SPAN||Gr pouch cells. (a, b) Cycling performance of the (a) 1.4 Ah and (b) 2.8 Ah SPAN||Gr pouch cells. (c) Digital photo of smartphone charging by the pouch cells. (d) The 6.1 Ah SPAN||Gr pouch cell. (e) Safety testing of the 1.4 Ah SPAN||Gr pouch cells.

Key Points:

1. A 1.4 Ah pouch cell cycled at 0.5 C with 80% DOD, with a recovery of 3 cycles at 0.05 C every 100 cycles, achieved a capacity retention of 82% after 1031 cycles.
2. A 2.8 Ah pouch cell cycled at 0.3 C with 80% DOD, with a recovery of 3 cycles at 0.05 C every 100 cycles, achieved a capacity retention of 90% after 211 cycles.
3. The 1.4 Ah SPAN||Gr pouch cell has passed various safety tests, including over-discharge, over-charge, external short-circuit, thermal chamber, and puncture tests, indicating a significant improvement in safety when replacing lithium metal anodes with graphite anodes.

Research Summary

This work developed a Li?S pouch cell with SPAN as the cathode and graphite (Gr) as the anode. This battery system can avoid the shuttle effect and eliminate issues associated with lithium metal anodes. The results indicate that the capacity degradation mechanism of this pouch cell involves the loss of active lithium and an increase in battery impedance. The continuous thickening of the SEI/CEI and the formation of dead lithium contribute to the reduction of active lithium. Additionally, the thickening of SEI/CEI due to electrode volume changes also increases the battery impedance. By controlling cycling conditions, including depth of discharge (DOD), current density, and temperature, to mitigate the impact of capacity degradation factors, a SPAN||Gr pouch cell (20 mAh) with 1000 stable cycles and 99% capacity retention was successfully achieved. Ah-level pouch cells not only passed multiple safety tests but also cycled stably for over 1000 cycles while maintaining more than 80% capacity retention. This battery system is expected to fundamentally improve the long-cycle stability of Li?S pouch cells and contribute to the commercialization of lithium-sulfur batteries.

Article link: https://pubs.rsc.org/en/content/articlelanding/2024/ee/d4ee02149e/unauth

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