How to simultaneously improve the energy density and cycle life of sulfur-based lithium-ion batteries

Construction of SPAN||Gr Battery of 1. N/P=0.6

It has been found that reducing the N/P ratio of SPAN||Gr cells may offer great potential for increasing energy density. Compared with the traditional LIBs, the SPAN||Gr pouch cell shows a significantly different mass ratio: Gr anode dominates (40%), surpassing the SPAN cathode and Electrolytes. This reversal stems from SPAN's higher specific capacity than Gr, and Gr quality needs to be improved to match the capacity. This phenomenon is likely to be prevalent in most high-capacity sulfur-based lithium-ion batteries. When the N/P ratio is 0.6, the SPAN mass exceeds Gr.

To realize the release of the energy density potential of the SPAN||Gr battery system, researchers designed and constructed a SPAN||Gr battery system with an N/P ratio of 0.6. By using a simple in-situ pre-lithionization method, ultra-thin lithium foils were attached to the surface of the SPAN node during battery assembly and spontaneously lithiated after the addition of Electrolytes. To ensure that the battery has sufficient lithium, the electrodes are preliminized using 20 μm ultrathin lithium foil.

 

Battery Electrolytes Adaptability of the 2. N/P<1.0

The choice of electrolyte is crucial for this sulfur-based battery in which lithium metal and Gr are used together. The LS electrolyte has a shuttle effect of polysulfides, and the battery has a charging abnormality during the charging process, whereas the LB is an ester electrolyte and is not compatible with the lithium metal. Therefore, the capacity retention rate is only 50% after 170 cycles.

The anion-mediated electrolyte LH designed by them is enriched with anions (mainly FSI anions) in its solvated structure, which facilitates the construction of stable SEI/CEI and inhibits the decomposition of polysulfides, thus ensuring the cycle stability of the cell. Consistent with N/P=1.1. The button battery retains 90% capacity after 250 cycles.

 

Interfacial and Electrochemical Kinetics of 3. N/P=0.6 SPAN||Gr Cells

When the concentration of anions in the electrolyte is higher, it is more favorable for the decomposition of anions to form SEI/CEI, and the SEI/CEI is mainly composed of inorganic components.The by-product LixPOyFz was detected in the Gr anode. Whereas no such by-product was found in the LH electrolyte, on the contrary, a higher content of LiF was detected. As an inorganic component, LiF has high stability and excellent electrochemical kinetics. For SPAN cathode, by-products were also detected in the LB electrolyte and the LiF content was low, whereas no by-products were detected in the Anode of the LH electrolyte and LiF was the main component.

The electrochemical dynamic characteristics of the cells can be compared by in situ EIS tests. The impedance changes of various electrochemical processes of the cell can be determined by detailed distribution analysis of relaxation time (DRT). It can be found that the RSEI of the cells circulating in LH is generally smaller than the RSEI of the cells in LB, which also validates the results of the interface detection. It is worth noting that in such a sulfur-based battery system with an ultra-low N/P ratio (N/P=0.6 ), the Rct exhibits significant fluctuations due to both Li+ intercalation and lithium deposition processes involved on the Gr anode. Both LB and LH electrolytes show a decreasing trend in Rct during battery charging from 0% SOC to about 60% SOC. This decrease is mainly due to two factors: On the one hand, the continuous embedding of Li+ in the Gr helps to reduce the Rct ; on the other hand, the Li2S with poor conductivity in the SPAN node gradually changes to the Li2S4 with more favorable dynamics. The synergistic effect of these two phenomena leads to a decrease in Rct. Conversely, when the battery is charged from 60% SOC to about 100% SOC, the anode materials transition to a less conductive S8, although the continued lithium deposition on the Gr still tends to lower the Rct. In this case, the change of the Rct node becomes the key factor that dominates the change of the Li+ node, leading to the increase of the value of the Li+ node. This phenomenon of Rct falling first and then rising with the increase of SOC is a typical characteristic of sulfur-based lithium-ion batteries. By comparing the Rct values of LH and LB systems, it is found that the Rct values of LH system are lower than those of LB system. This phenomenon may be attributed to the anion-enriched solvation structure in LH, which effectively lowers the desolvation barrier of Li+ and makes Li+ more easily desolvated, thus reducing Rct.

 

Deposition behavior of lithium on graphite

During the charging process, the deposition behavior of lithium at the negative position was significantly different among different electrolytes. In a conventional electrolyte (LB), lithium deposition presents a loose porous, non-uniform, and non-dense structure; In the local high concentration electrolyte (LH), the lithium deposition showed a compact and smooth morphology.

When charged to 100% SOC, the lithium deposition in the LB system showed a porous and non-uniform structure. The LH system forms a smooth and dense sedimentary layer. This flat lithium deposition can effectively guarantee the battery cycle life. Secondary electron images of G cross-sectional topography at 100% SOC in LH showed that lithium was uniformly deposited on Gr.

 

Battery performance under different cycle conditions

Different cycle conditions have a significant impact on battery performance. The capacity retention rate of the SPAN||Gr battery with N/P=0.6 is respectively 98% after one month of standing in the 100% SOC state. The results show that when lithium metal (100% SOC) is precipitated from the battery, self-discharge occurs to a certain extent, which may be caused by interface side reactions, but the self-discharge rate is low overall. (Capacity retention can be restored to more than 99% after cycling). This phenomenon indicates that the LH electrolyte has good compatibility with the battery system and does not cause a continuous and large number of side reactions during the standing process. Although the diffusion impedance of LH electrolyte is higher than that of LB system, its excellent interfacial dynamics ensures the stability of electrochemical performance.

Based on the N/P ratio of 0.6 of the battery system, it is an effective strategy to improve the cycle stability by controlling the DOD at 60% to avoid the deposition of lithium metal. This feature makes the battery system an ideal candidate for energy storage batteries with lithium pre-charged capabilities. The results show that the capacity decay of SPAN||Gr batteries is mainly due to the loss of active lithium. Therefore, supplementing with additional lithium in advance helps to extend the cycle life. The method of controlling the charge capacity to 60% of the initial capacity is adopted in this study. After 500 cycles switching to 100% DOD charge and discharge, the capacity retention rate can still reach 93% of the initial capacity. Low current charge/high current discharge strategy can effectively inhibit the growth of lithium dendrites. While this low-current charging strategy does not significantly improve cycle stability, the cells in the LH electrolyte system still maintain relatively stable cycle performance, which may be due to capacity attenuation due to SEI/CEI thickening.

 

SPAN||Gr Soft Pack Battery for 6. N/P=0.6

Pouch cell testing is a key means to evaluate the potential of batteries in practical applications. In this study, SPAN||Gr pouch cells with N/P ratio of 0.6 were prepared for systematic evaluation. First, a single-layer laminated soft-pack battery was assembled for testing. The results showed that, while the cell cycle capacity in the LB electrolyte decayed rapidly, the LH electrolyte system maintained 92% capacity retention after 300 cycles. When the electrolyte usage (E/C ratio) was 3.0 g Ah⁻¹, the capacity retention rate of the 1.0 Ah flexible pack battery was still 90% after 100 cycles in LH.

In this ultra-low N/P ratio battery system using both lithium metal and graphite as anode, the volume variation of the battery system is significantly different from that of the battery using lithium metal or graphite as anode alone. This study also tried to control the E/C ratio. When the E/C ratio was 2.0 g Ah⁻¹, the 2.0 Ah pouch cell was also able to perform stable cycle, and its energy density was increased by 24% compared with that of N/P=1.1.

---

---