Anode Material Options Toward 500 Wh kg-1 Lithium-Sulfur Batteries

Abstract

Lithium-sulfur (Li-S) battery is identified as one of the most promising next-generation energy storage systems due to its ultra-high theoretical energy density up to 2600 Wh kg^-1 . However, Li metal anode suffers from dramatic volume change during cycling, continuous corrosion by polysulfide electrolyte, and dendrite formation, rendering limited cycling lifespan. Considering Li metal anode as a double-edged sword that contributes to ultrahigh energy density as well as limited cycling lifespan, it is necessary to evaluate Li-based alloy as anode materials to substitute Li metal for high-performance Li-S batteries. In this contribution, the authors systematically evaluate the potential and feasibility of using Li metal or Li-based alloys to construct Li-S batteries with an actual energy density of 500 Wh kg^-1 . A quantitative analysis method is proposed by evaluating the required amount of electrolyte for a targeted energy density. Based on a three-level (ideal material level, practical electrode level, and pouch cell level) analysis, highly lithiated lithium-magnesium (Li-Mg) alloy is capable to achieve 500 Wh kg^-1 Li-S batteries besides Li metal. Accordingly, research on Li-Mg and other Li-based alloys are reviewed to inspire a promising pathway to realize high-energy-density and long-cycling Li-S batteries.

Keywords: high energy density; lithium metal anodes; lithium-magnesium alloys; lithium-sulfur batteries; pouch cells.

Figure 1

Theoretical energy density of Li‒S battery with different anode materials at the first level of evaluation. The red line is the upper energy density limit at different anode theoretical specific capacities assuming the cell average voltage is 2.15 V.

Figure 2

Evaluation on the necessary electrolyte amount to reach the energy density target of 500 Wh kg⁻¹ and simultaneously infiltrate the practical electrodes at the second evaluation level. a) Schematic illustration of the structure of practical electrodes. The S cathode and the Li_4.4Si, Li₁₅Ge₄, and Li_4.4Sn anodes are porous and require electrolyte infiltration. The Li, Li₉Mg, and LiAl anodes can be directly used as integral foils. The relationship between the containable E/S ratio and the energy density using b) porous Li_4.4Si, Li₁₅Ge₄, and Li_4.4Sn anodes and c) non‐porous Li, Li₉Mg, and LiAl anodes. The vertical dash lines mark the containable E/S ratio for the target energy density of 500 Wh kg⁻¹. The horizontal dash lines mark the required E/S ratio for electrode filtration. If the containable E/S ratio is significantly higher than the required E/S ratio, the corresponding anode material is feasible to reach the energy density target at the second evaluation level.

Figure 3

Estimated energy density in practical Li–S pouch cells at the third evaluation level. The relationship between the containable E/S ratio, the areal S loading, and the energy density using a) Li, b) Li₉Mg, and c) Li_4.4Si. The dot–dash lines mark the relationship between the containable E/S ratio and the areal S loading at a given energy density. The dash lines represent the required E/S ratio for cell infiltration at different areal S loadings. The intersection point of the two lines represents the minimal electrolyte amount and areal S loading to reach the target energy density. d) Comparison of the containable E/S ratio and the required E/S ratio on different anode materials to reach the energy density target of 500 Wh kg⁻¹.

Figure 4

Review on Li–Mg alloy anodes in Li–S batteries. a) Calculated binary Li–Mg phase diagram. Reproduced with permission.^{[ 48 ]} Copyright 2019, Wiley‐VCH. b) Cyclic voltammograms of Li₇Mg₃ in nonaqueous electrolyte to demonstrate its stability. Reproduced with permission.^{[ 28 ]} Copyright 2001, Elsevier. c) Charge–discharge profiles of Li–S batteries with Li–Mg alloy anodes. d) Cycling performances of Li and Li–Mg anode in Li–S batteries. SEM images of e) Li metal anode and f) Li–Mg alloy anode after Li plating at 0.5 mA cm⁻² for 24 h. Reproduced with permission.^{[ 56 ]} Copyright 2019, Wiley‐VCH.

Figure 5

Review on protective alloy coating on Li metal anode. a) Schematic illustration of constructing an Al–Li alloy protective layer on Li metal anode by using ionic liquids. b) Cycling performance of Li–S batteries at 4 C with the Al–Li alloy protected anode. Reproduced with permission.^{[ 57 ]} Copyright 2020, Wiley‐VCH. c) EIS spectra of Li‐based anodes with Al protective layer at different lamination temperatures. Reproduced with permission.^{[ 58 ]} Copyright 2013, Elsevier. d) Cycling performance of Li‒S batteries at 0.5 C with Sn coated Li anode. Reproduced with permission.^{[ 59 ]} Copyright 2020, Elsevier.