All-Solid-State Lithium-Sulfur Battery Based on a Nanoconfined LiBH₄Electrolyte

Abstract

In this work we characterize all-solid-state lithium-sulfur batteries based on nano-confined LiBH 4 in mesoporous silica as solid electrolytes. The nano-confined LiBH 4 has fast ionic lithium conductivity at room temperature, 0.1 mScm −1 , negligible electronic conductivity and its cationic transport number (t + = 0.96), close to unity, demonstrates a purely cationic conductor. The electrolyte has an excellent stability against lithium metal. The behavior of the batteries is studied by cyclic voltammetry and repeated charge/discharge cycles in galvanostatic conditions. The batteries show very good performance, delivering high capacities versus sulfur mass, typically 1220 mAhg −1 after 40 cycles at moderate temperature (55 • During the past decade, the quest for promising next generation energy storage systems has led significant attention to secondary batteries with high specific energy such as lithium/sulfur (Li/S) batteries (1675 mAhg −1 sulfur at 2.15 V), 1-6 which have 3 to 5 times higher energy densities than commercial state-of-art Li-ion batteries, e.g. 7 The inexpensive, abundant and environmentally benign nature of sulfur makes this battery more appealing for large scale application purposes (e.g. transportation, portable and residential applications) than other metal-ion battery systems. However, there are still numerous scientific and technical challenges for practical application. During discharge in Li/S batteries, lithium ions move spontaneously through the electrolyte from the negative electrode, typically lithium metal, silicon or tin-based compounds, to the positive sulfur electrode and S is ultimately reduced to form Li 2 S, while electrons flow through the external circuit. During charge, Li 2 S is oxidized back to S and Li + by applying an external voltage. The overall electrochemical reaction is:However, the electrochemical reduction of sulfur in Li/S battery occurs through the formation of a series of intermediate lithium polysulfides, Li 2 S x (2 ≤ x ≤ 8). 8,9 These intermediates are soluble in most liquid organic solvents/electrolytes and shuttling between the sulfur cathode and Li anode results in fast self-discharge during storage and low coulombic efficiencies during charging. Therefore, a grand challenge for Li/S batteries is to suppress this mechanism, for example by encapsulation or coating of the sulfur electrode, 10-14 use of impermeable membranes, 15 and/or the use of suitable electrolytes that minimize the solubility and diffusivity of the polysulfides. [16][17][18] Another possibility is to use fast ion-conducting solids, i.e. solid electrolytes with an ionic conductivity (σ ∼ 10 −4 to 10 −1 Scm −1 ) comparable to that of standard liquid electrolytes (e.g. σ ∼10−2 Scm −1 for 1.0 M LiFP 6 in organic solvent). Nanoconfinement of LiBH 4 in nanoporous carbon scaffolds has been reported by various groups to improve the BH 4 − rotational diffusivity and lithium mobility at room temperature.46-49 However, due to their high electronic conductivity, nano-scaffolds of carbon are not ver...

Progress of the Interface Design in All‐Solid‐State Li–S Batteries

“…After confining LiBH 4 into the ordered mesoporous silica, the phase transition temperature was reduced to 90 °C and the ionic conductivity at room temperature can reach up to 0.1 mS cm −1 . Further an ASS Li–S battery based on this confined LiBH 4 electrolytes were reported and exhibited very high sulfur utilization and good cycle stability (more 1200 mAh g −1 after 40 cycles at 0.03C) at a moderate temperature—55 °C …”

“…The sulfide‐based electrolytes are the most promising SSEs for ASS Li–S batteries and were widely used to construct ASS batteries for the high ionic conductivity, good compatibility with sulfur and easy fabrication of cells and cathode, which has been elaborated in the above part . In the past years, most of the composite cathodes applied in ASS Li–S batteries were prepared by mechanically mixing SSEs, conductive additives and sulfur . According to the report from Tatsumisago and co‐workers, the different fabrication methods for cathode can impose a significant influence in battery performance .…”

“…According to the calculation from Ceder and co‐workers, most SSEs are stable against Li 2 S, except LiBH 4 , Li 3 N, and LiH . Due to the instability of LiBH 4 at a high potential, interphases can be formed at the first cycle inducing an ultrahigh specific capacity, fortunately the interphases are relatively stable and enable the proper work of LiBH 4 ‐based ASS Li–S batteries at following cycles . The interface between sulfide‐based SSEs and sulfur/Li 2 S cathode has been ignored for the natural compatibility, but the recent studies have shown that intriguing interphases between sulfur and SSEs can form …”

A Comprehensive Understanding of Lithium–Sulfur Battery Technology

“…With respect to the use of LiBH 4 , its P63/mmc phase is stable at temperatures above 110 °C so that it can deliver an ionic conductivity of ≈1 mS cm −1 at 120 °C . In this perspective, Das et al investigated the performance of all‐solid‐state Li–S batteries using LiBH 4 solid electrolyte. The cell composed by sulfur/carbon cathode and lithium foil anode could deliver a reversible capacity of about 1200 mA h g −1 at 0.03 C even at a temperature as high as 55 °C.…”

Investigation of the Reversible Lithiation of an Oxide Free Aluminum Anode by a LiBH4 Solid State Electrolyte

“…Recent interest in utilizing LiBH 4 as a solid-state electrolyte was established through the work of Orimo [4], who demonstrated that the ionic conductivity of lithium can be greater than 1 mS/cm at temperatures above the orthorhombic to hexagonal phase transition that occurs at 380 K [5]. This work has been expanded to achieve high conductivity in LiBH 4 based solid electrolytes through the addition of Li halide salts [6][7][8], nanoconfinement [9][10][11][12], nanoionic destabilization [13,14], ion substitution [15][16][17][18][19][20][21], and eutectic formation [22].…”