High-Modulus Hexagonal Boron Nitride Nanoplatelet Gel Electrolytes for Solid-State Rechargeable Lithium-Ion Batteries

Abstract

Solid-state electrolytes based on ionic liquids and a gelling matrix are promising for rechargeable lithiumion batteries due to their safety under diverse operating conditions, favorable electrochemical and thermal properties, and wide processing compatibility. However, gel electrolytes also suffer from low mechanical moduli, which imply poor structural integrity and thus an enhanced probability of electrical shorting, particularly under conditions that are favorable for lithium dendrite growth. Here, we realize high-modulus, ion-conductive gel electrolytes based on imidazolium ionic liquids and exfoliated hexagonal boron nitride (hBN) nanoplatelets. Compared to conventional bulk hBN microparticles, exfoliated hBN nanoplatelets improve the mechanical properties of gel electrolytes by 2 orders of magnitude (shear storage modulus ∼5 MPa), while retaining high ionic conductivity at room temperature (>1 mS cm −1 ). Moreover, exfoliated hBN nanoplatelets are compatible with high-voltage cathodes (>5 V vs Li/Li + ) and impart exceptional thermal stability that allows high-rate operation of solid-state rechargeable lithium-ion batteries at temperatures up to 175 °C.

Nanocomposite Ionogel Electrolytes for Solid‐State Rechargeable Batteries

“…NIEs have shown relatively high ionic conductivity as a solid-state electrolyte, with many examples exhibiting ionic conductivity over 1 mS cm −1 . [12,[16][17][18][19] In general, the ionic conductivity of ionogels is primarily influenced by the inherent IL viscosity, salt concentration, matrix loading, and temperature. For NIEs, however, interactions between the gelling matrices and ILs also play an important role because the large surface area of nanoscale materials provides greater interactions with ILs, which can contribute to the dissociation of charge carrier salts and ILs and thus an increase in the number of free ions.…”

“…For instance, hexagonal BN nanoplatelets enhance the mechanical modulus of ionogels by two orders of magnitude without compromising ionic conductivity compared to hexagonal BN microparticles at the same matrix loading. [12] As a result, NIEs based on hexagonal BN nanoplatelets and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI)/LiTFSI have concurrently achieved high room temperature ionic conductivity (>1 mS cm −1 ) and mechanical strength (storage modulus as high as 5 MPa). Similarly, nanoscale matrices have played a key role in reinforcing polymer-inorganic hybrid NIEs.…”

“…Multiple NIEs have exhibited decomposition temperatures over 300 °C, particularly when employing inorganic matrix materials such as SiO 2 , [26] TiO 2 , [21] BaTiO 3 , [27] and hexagonal BN. [12] Since these inorganic matrix materials are relatively refractory, the onset decomposition temperatures of NIEs are commonly limited by the ILs, but residual solvents and trapped Reproduced with permission. [9] Copyright 2017, Royal Society of Chemistry.…”

“…For example, NIEs based on hexagonal BN nanoplatelets and EMIM-TFSI/LiTFSI exhibited anodic stability of 5.3 V (vs Li/Li + ), which is higher than that of the IL electrolyte alone (4.2 V vs Li/Li + ). [12] Moreover, NIEs based on BMIM-TFSI/LiTFSI showed enhanced anodic stability (5.2 V vs Li/Li + ) when using SiO 2 nanoparticle matrices. [18] Polymer-inorganic hybrid NIEs based on imidazolium ILs have also demonstrated improved anodic stability upon the incorporation of nanoscale additives.…”

“…Herein, we discuss recent developments surrounding nanocomposite ionogel electrolytes (NIEs) in the context of solidstate rechargeable batteries. Diverse nanoscale matrix materials are considered including oxides, [6][7][8][9][10][11] boron nitride (BN), [12,13] metal-organic frameworks (MOFs), [14] and garnets. [15] Section 2 describes the relationships between nanoscale material structure and ionogel properties, including ionic conductivity, mechanical moduli, thermal properties, and electrochemical stability.…”