High-Rate, Ultralong Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene

Abstract

Nitrogen-doped graphene (NG) is a promising conductive matrix material for fabricating high-performance Li/S batteries. Here we report a simple, low-cost, and scalable method to prepare an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside the NG sheets (S@NG). We show that the Li/S@NG can deliver high specific discharge capacities at high rates, that is, ∼ 1167 mAh g(-1) at 0.2 C, ∼ 1058 mAh g(-1) at 0.5 C, ∼ 971 mAh g(-1) at 1 C, ∼ 802 mAh g(-1) at 2 C, and ∼ 606 mAh g(-1) at 5 C. The cells also demonstrate an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle), which is among the best performance demonstrated so far for Li/S cells. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles. With a high active S content (60%) in the total electrode weight, the S@NG cathode could provide a specific energy that is competitive to the state-of-the-art Li-ion cells even after 2000 cycles. The X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to the well-restored C-C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The results indicate that the S@NG nanocomposite based Li/S cells have a great potential to replace the current Li-ion batteries.

Leaf‐Like Graphene‐Oxide‐Wrapped Sulfur for High‐Performance Lithium–Sulfur Battery

“…However, the specific areal capacity in most previous reports13, 16, 20, 21, 23, 26, 28, 30, 39, 40, 43, 45, 51 is lower than 2 mAh g −1 due to the low sulfur loading in the electrodes (see Table S1 in the Supporting Information and corresponding discussion). Moreover, some previous superior cycling performances over 500 cycles20, 43, 45, 51 are achieved by low sulfur loading, which is usually lower than 1 mg cm −2 (Table S1, Supporting Information), while in our present work, the electrode with 2.7 mg cm −2 sulfur loading can still run over 600 cycles. In addition, the leaf‐like GO/S composite with a sulfur content of 75 wt%, which is corresponding to sulfur loading of 1.2 mg cm −2 (sulfur loading = 2 × 75% × 80% = 1.2 mg cm −2 ) in the electrode, can still run over 1000 cycles with an extremely low capacity decay of 0.038% per cycle (see Figure 6).…”

“…The battery delivers a reversible discharge capacity of 1250 mAh g −1 at current rate of 0.1 C that slowly reduced to 1130 mAh g −1 at 0.2 C , 930 mAh g −1 at 0.5 C , 753 mAh g −1 at 1 C , and 685 mAh g −1 at 2 C . Even at high rate of 4 C (6680 mA g −1 ), the leaf‐like GO/S cathode still can deliver a capacity of 468 mAh g −1 , which is almost the best rate performance of C/S composite cathode 8, 10, 11, 12, 14, 15, 16, 17, 18, 19, 20, 21, 22, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 45, 46, 48, 51, 52, 53…”

“…However, the insulating nature of sulfur (S) and its reaction products (i.e., Li 2 S), the large volume expansion from S to Li 2 S, along with the dissolution of lithium polysulfide intermediates (i.e., Li 2 S x , 4 ≤ x ≤ 8) into liquid electrolyte and the consequent shuttling effect between the anode and cathode, makes it generally display poor rate ability, limited cycle life and severe self‐discharge 1, 2, 3, 4, 5, 6, 7. Therefore, a variety of strategies have been pursued to circumvent the sulfur cathode problems, including optimization of organic electrolytes8, 9 and fabrication of sulfur‐conductive polymer composites10, 11 and sulfur–carbon‐based composites 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45. Among these approaches, porous‐carbon/sulfur composites12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 are more attractive because porous carbon can i...…”

“…Therefore, the achieved rate performances in most previous reports about GO/S composites are generally below the rate of 2 C (1 C = 1675 mA g −1 ) 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35. On the contrary, reduced GO (RGO) or graphene (G) with higher electronic conductivity was also employed to prepare RGO/S or G/S composites for improving rate performance 36, 37, 38, 39, 40, 41, 42, 43, 44. However, RGO or G does not have the abundant functional groups that can bind sulfur and its discharge products.…”

“…However, RGO or G does not have the abundant functional groups that can bind sulfur and its discharge products. In addition, similar with previous reports about porous‐carbon/S composites,12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 the sulfur content in the GO/S and RGO/S (or G/S) composites and sulfur loading on the electrode is still low 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44. In addition, some efforts34 have been made to mix high conductivity carbon nanotube (CNT) with GO to improve the conductivity of GO.…”