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Research background
Lithium-sulfur (Li−S) batteries, known for their high energy density and low material costs, are promising candidates for the next generation of batteries. However, their cycle life is limited by the polysulfide shuttle effect and dendrite growth on the lithium metal anode, along with side reactions. Most current research is conducted using coin cells, but as research advances, there is a growing consensus that pouch cells will be necessary for the commercialization of Li−S batteries. While there has been steady progress in improving the energy density of Li−S pouch cells in recent years, extending their cycle life and improving cycle stability remains challenging. Many strategies effective in coin cells do not work well in pouch cells. Currently, mAh-level pouch cells struggle to exceed 300 cycles while maintaining 80% or more of their capacity, and Ah-level pouch cells often last only a few dozen cycles. This indicates that existing methods have not fundamentally improved the cycle life of Li−S batteries, making long-cycle stability for Li−S pouch cells a significant challenge.
A recent article published in Energy & Environmental Science titled “Lithium−sulfur pouch cells with 99% capacity retention for 1000 cycles” presents a Li−S pouch cell with a sulfurized polyacrylonitrile (SPAN) cathode and a graphite (Gr) anode. The cell uses a simple in-situ prelithiation method to introduce lithium ions. In carbonate electrolyte, the SPAN cathode prevents the shuttle effect, while the Gr anode avoids dendrite growth and side reactions associated with lithium metal anodes. The study focuses on the pouch cell model, controlling cycling conditions to suppress active lithium loss and battery impedance increase. The resulting SPAN||Gr pouch cell demonstrates 1000 stable cycles with 99% capacity retention. Ah-level pouch cells can stably cycle 1031 times with 82% capacity retention and have passed multiple safety tests. This design shows promise for fundamentally improving the long-cycle stability of Li−S pouch cells.
Research questions
1. Cycle performance statistics of lithium-sulfur soft pack batteries and energy density calculation of SPAN positive electrode
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:
Statistical analysis of the cycling stability of different reported capacities of lithium-sulfur (Li-S) pouch cells reveals that for pouch cells with capacities less than 0.1 Ah and those between 0.1−1.0 Ah, their cycle life rarely exceeds 300 cycles while maintaining over 80% capacity retention. For pouch cells with capacities exceeding 1.0 Ah, the cycle life is often only a few dozen cycles. This indicates a lack of highly effective methods to fundamentally improve the cycle life and cycling stability of Li−S pouch cells. The proposed use of SPAN as the cathode and Gr as the anode in this work is expected to fundamentally mitigate the shuttle effect and the negative impacts associated with lithium metal anodes.
Additionally, a comparison of the energy density among three cathodes (LFP, NCM811, SPAN) shows that the SPAN cathode (including the mass of lithium required for lithiation) has the highest energy density. Therefore, this battery system can achieve high energy density while maintaining high cycling stability.
2. SPAN||Gr Features of the softpack battery
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:
The introduction of lithium in this battery system can be achieved through a simple in-situ prelithiation method. The battery system, using LB−015 electrolyte, shows poor rate performance at room temperature (1 C=1000 mA g−1). As the discharge rate increases, the battery’s capacity decay becomes more pronounced. However, the self-discharge rate of the system is less than 1% (after one month of storage).
In this single-layer pouch cell, the capacity gradually declines in a stable manner when the electrolyte is sufficiently supplied, with the capacity reducing to 80% of the initial value after 665 cycles. Additionally, at different stages of capacity decline, ultrasonic testing reveals good electrolyte wettability with no gas generation.
3. SPAN||Gr battery capacity attenuation factors
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:
Analysis of the capacity decay mechanism in this battery system reveals the presence of dead lithium and an increase in battery impedance. The microstructure of the electrode surfaces shows little difference across various stages of decay, with the anode exhibiting more degradation than the cathode in the later stages of cycling.
4. SPAN||Gr Soft pack battery SEI/CEI features
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 layer on the SPAN cathode and the SEI layer on the Gr anode both thicken, with the relative content of LiF increasing. This indicates a continuous consumption of active lithium ions, and as the SEI/CEI layers thicken, the battery impedance (specifically RSEI) also increases.
5. SPAN||Gr Volume change of the electrode of the soft-pack battery
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:
In-situ stress monitoring using FBG fiber Bragg grating, combined with observations of electrode thickness, reveals that the SPAN cathode undergoes significant volume changes, while the Gr anode experiences relatively smaller volume changes. However, for both the SPAN cathode and Gr anode, these volume changes cause the CEI/SEI layers to continuously crack and self-repair, which is the fundamental reason for their thickening.
Therefore, in the SPAN||Gr pouch cell, the primary causes of capacity decay are the loss of active lithium and the increase in battery impedance. The thickening of the SEI/CEI layers and the formation of dead lithium consume active lithium, while the thickening of the SEI/CEI layers leads to increased battery impedance.
6. Long cycle stable SPAN||Gr soft pack battery
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:
By controlling the depth of discharge (DOD) to 80%, the volume changes of the electrodes can be effectively mitigated. This pouch cell maintains 80% of its capacity after 1900 cycles.
Reducing the current density can prevent the significant formation of dead lithium. The pouch cell maintains 98% of its capacity after 500 cycles at 0.1 C, and 99% capacity retention after 500 cycles at 0.2 C.
Increasing the temperature to 45°C improves the kinetic properties of the battery system, with the pouch cell maintaining 99% capacity retention after 1000 cycles.
7. SPAN||Gr soft pack battery at Ah level
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:
A 1.1 Ah pouch cell, cycled at 0.5 C with 80% DOD, and with 3 low-rate cycles at 0.05 C every 100 cycles for recovery, maintained 82% capacity retention after 1031 cycles.
A 2.8 Ah pouch cell, cycled at 0.3 C with 80% DOD, and with 3 low-rate cycles at 0.05 C every 100 cycles for recovery, maintained 90% capacity retention after 211 cycles.
1.4 Ah SPAN||Gr pouch cell passed various safety tests, including over-discharge, overcharge, external short circuit, thermal box, and puncture tests, demonstrating that replacing the lithium metal anode with a Gr anode significantly improved the battery system’s safety.
Summary and Outlook
This work developed a Li−S pouch cell with SPAN as the cathode and graphite (Gr) as the anode, effectively avoiding the shuttle effect and addressing issues associated with lithium metal anodes. The results indicate that the capacity decay mechanism of this pouch cell is attributed to the loss of active lithium and an increase in battery impedance. The continuous thickening of the SEI/CEI layers and the formation of dead lithium contribute to the reduction of active lithium. Additionally, the thickening of SEI/CEI layers due to electrode volume changes also increases the battery’s impedance. By controlling cycling conditions, including depth of discharge (DOD), current density, and temperature, to mitigate the factors affecting capacity decay, a SPAN||Gr pouch cell (20 mAh) was successfully achieved with 1000 stable cycles and 99% capacity retention. The Ah-level pouch cells not only passed multiple safety tests but also demonstrated stable cycling for over 1000 cycles while maintaining over 80% capacity retention. This battery system holds promise for fundamentally improving the long-cycle stability of Li−S pouch cells and advancing the commercialization of lithium-sulfur batteries.
The article link is: https://pubs.rsc.org/en/content/articlelanding/2024/ee/d4ee02149e/unauth
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Company introduction
Welcome to Canrd Company (Canrd stands for “Creating Avenues for New Research Development”，website: www.canrd.com ). We specialized in:
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