Review—Solid Electrolytes for Safe and High Energy Density Lithium-Sulfur Batteries: Promises and Challenges

Since the first study on poly(ethylene oxide) (PEO) -based electrolyte for Li-S cells in the late of 1980s by DeGott,³⁷ several types of solid electrolytes have been investigated as Li-ion conducting materials for ASSLSBs, including those based on pure polymeric components,^38–49 ceramic electrolytes,^50–67 and composite electrolytes (mix of ceramic and other electrolytes).^68–70 Generally, PEs can be classified into two categories: solid polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs). In the former electrolyte system, lithium salts are dissolved in high-molecular-weight polymers containing high concentration of Lewis base groups such as ether (–O–), carbonyl (–C=O), and cyano (–C≡N), which serve as ligands for coordinating Li⁺ of the dissolved salt thus offering the necessary solvation energy for the polymer-lithium complex formation. In GPEs, low molecular weight components like small organic solvent (e.g., tetra(ethylene glycol) dimethyl ether,^39,71 DOL,⁷² carbonates^73–78), and ionic liquids^79,80 are added as plasticizers for improving the ionic conductivity of a polymer electrolyte. The chemical structures of the extensively studied lithium salt, polymer matrices, and low molecular weight plasticizers are summarized in Fig. 1. Another important class of solid electrolytes, the inorganic solid electrolytes, exhibit higher Li-ion conductivities at operating temperature (e.g., 10⁻² S cm⁻¹ at 25 °C for Li₁₀GeP₂S₁₂⁸¹) compared to SPEs; while SPEs have better processability and in principle are easier to be scaled up for large-format batteries, as demonstrated by the application of Bluecar from Blue Solutions in several cities (e.g., Paris, Bordeaux, Lyon, Indianapolis, etc.), which are equipped with a 30 kWh LiFePO₄/PEs/Li-metal battery pack, providing a 250 km driving range under normal urban use.⁸²

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Looking at the existing literature related to ASSLSBs, the cyclability and S utilization of the cells are important parameters for comparing the overall performances. The detailed information of ASSLSBs from literature is listed in Table I, in terms of the composition of the electrodes (cathode and anode), electrolyte formulation and conductivity of the electrolyte.

Interestingly, most of the research related to ASSLSBs report cell performances with relatively low S content (<60 wt%) and rarely mention the S loading (areal capacity), thus hampering a sensible comparison of the performances among all the available references. In general, the values of S content in liquid electrolyte based cells are higher compared to those in solid electrolytes (Fig. 2a), ascribed to the good wettability from liquid electrolytes, which allows the liquid solution of lithium salt and small molecular solvent to access easily the porous structures of the S electrodes. However, in the case of solid electrolytes, a large quantity of electrolyte is always required in the composition of the cathode to facilitate the ionic transport, thus the S content is hardly between 20−60 wt% in the total mass of the cathode. Though the S utilization at the first cycle for ASSLSBs could reach higher than 1000 mAh g⁻¹_sulfur (Fig. 2b), long-term cyclability still remains as a major concern. The reported cycle life for ceramic-and polymer-based cells are mostly lower than 200 cycles (Figs. 2c and 2d), while liquid-based cells could maintain more than 1000 cycles with good capacity retention (e.g., up to 1500 cycles with a specific capacity of 168 mAh g⁻¹_electrode⁸⁷).

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ASSLSBs with relatively long cycle life have been presented in recent studies; however, the cell performance at the end of the cycle life is not competitive with the liquid system. Tatsumisago et al.⁴⁸ reported that the Li-S cell using Li₂S-P₂S₅ glass-ceramic electrolytes could deliver a discharge capacity of 249 mAh g⁻¹_electrode after 200 cycles. Liang et al.⁶⁰ proved that the cycle life could be extended to 300 cycles with a good capacity of 336 mAh g⁻¹_electrode by introducing a new family of sulfur-rich compounds (Li₃PS_4+n) as active material. Hu et al.⁴⁵ reported that the polymer Li-S cell was found to maintain good capacity retention after 200 cycles by using PEO-based electrolyte containing a novel type of lithium salt ([(CF₃SO₂)(n-C₄F₉SO₂)N]Li). Recently, we evaluated the effect of lithium salt and ceramic fillers on the cycling performance of ASSLSBs using PEO-based electrolytes.^48,49 Compared with other SPEs, the electrolyte consisting of lithium bis(fluorosulfonyl)imide (LiFSI) in PEO showed good compatibility with Li metal electrode without sacrificing the ionic conductivity at the operating temperature.^48,94 By introducing Al₂O₃ or Li-ion conducting glass ceramic (LICGC) as inactive and active fillers, respectively, into the LiFSI/PEO polymer system, the electrochemical properties could be further improved. The composite polymer electrolyte (CPE) with nano-sized Al₂O₃ showed significant improvement of the Li/electrolyte interfacial properties, resulting in a good Coulombic efficiency (ca. 99%) of the Li-S cell, though accompanied by a relatively low S utilization (ca. 300 mAh g⁻¹_sulfur). More importantly, a high specific capacity of 1000 mAh g⁻¹_sulfur (areal capacity: 1.1 mAh cm⁻²) was achieved with CPE containing LICGC fillers. The Li-S cell with bilayer electrolytes (Li | Al₂O₃−CPE/LICGC−CPE | S) could deliver a discharge capacity of 900 mAh g⁻¹_sulfur at the first cycle with good capacity retention.⁴⁹

Application of solid electrolytes in Li-S batteries does not only facilitate the safe operation, but also benefits, in principle, the energy density at the cell level. Realistic calculations of energy density as a function of S loading, S utilization and electrolyte thickness will provide us useful guidance and key parameters for the design of ASSLSBs. Our previous calculation already demonstrated that solid electrolytes offer more room for improving the gravimetric energy density of Li-S batteries compared to liquid electrolytes based on the assumption that S utilization could reach 1000 mAh g⁻¹_sulfur in all cases.⁹⁵ However, in practice, the specific capacity of Li-S batteries usually decreases upon increasing the S content due to the electronically insulating properties of S and Li₂S. Therefore, we herein performed the calculation taking into account the variation in S utilization with different S contents and the results are presented in Fig. 3 (other parameters have been kept the same as our previous work⁷⁹). Not surprisingly, when comparing with LIB (Li⁰) cells, Li-S batteries could be competitive in terms of gravimetric energy density (Wh kg⁻¹) rather than in volumetric energy density (Wh L⁻¹). The latter is likely due the low-density cathode materials, as well as the high fraction of inert components (carbon) required due to the poor electronic conductivity of elemental sulfur. The effect of electrolyte thickness on energy density is more dominant in ceramic electrolyte than in polymer electrolyte, since the former one normally has 2 to 5 times higher density. For a cathode with S content of 40 wt% and achievable S utilization of 1100 mAh g⁻¹_sulfur (Fig. 3a), the minimum areal capacities of ca. 1.5 mAh cm⁻² for a polymer electrolyte with 50 μm thickness but a ceramic electrolyte with only 10 μm thickness are required for outperforming the LIB (Li⁰) technology. In addition, with increasing S content, both gravimetric and volumetric energy densities display little variation (Figs. 3a–3c) due to the reduced utilization of S. Under a possibly realistic but challenging S content (60 wt%, Fig. 3d), the gravimetric energy density for polymer electrolyte increases significantly with extending the degree of S utilization.

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ASSLSBs not only face the challenges coming from the solid electrolytes, like other types of all-solid-state batteries (e.g., low ionic conductivity, poor wetting properties), but also the intricate electrochemical redox reactions of S.^34,96 The main challenges and accompanying possible solutions are summarized below.

(1) The quest for highly Li-ion conductive solid electrolyte with good processability

(2) A good contact of solid/solid interphase

(3) An in-depth understanding of ASSLSBs

Li-S batteries have emerged as important candidates for future energy storage systems; especially for applications which remand high gravimetric energy densities. All-solid-state Li-S batteries (ASSLSBs) offer a chance not only to be safer than those using liquid electrolytes, but also to achieve higher gravimetric energy densities than the state-of-art Li-ion batteries. Current research works on ASSLSBs using either inorganic or organic polymer electrolytes or their composite mixture showed sufficiently high S utilization at initial cycles; however, the cyclability and areal capacity of ASSLSBs are still far from the practical requirements. The future development of ASSLSBs needs to overcome the challenges including in most cases low Li-ion conductivity, poor processability and solid-solid contact, lack of in-depth understanding of S redox chemistry in solid electrolytes, and the choice of ad-hoc techniques to probe in details the interfacial properties. With the possible solutions mentioned in this review and great efforts that are being dedicated to this field, we could anticipate that ASSLSBs will be an excellent choice for safe and high gravimetric energy storage system in the future. Moreover, we strongly believe that new findings, understandings, and insights into the Li-S chemistry, interfacial processes and cathode-anode interactions will synergistically bring ASSLSBs much closer to commercial reality.

This work was supported by GV-ELKARTEK-2016 from the Basque Government, and MINECO RETOS (Ref: ENE2015-64907-C2-1-R) from Spanish Government. X. J. thanks the Government of the Basque Country for funding through a Ph.D. Fellowship, and C.L. thanks the Juan de la Cierva scholarship (Ref: FJCI-2015-23898).

Heng Zhang 0000-0002-8811-6336

Chunmei Li 0000-0003-4438-0458