Concentrated Electrolytes for Enhanced Stability of Al-Alloy Negative Electrodes in Li-Ion Batteries

Abstract

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm −3 led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm −3 solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI − anions to form LiF from Li +-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth.

Exploring the Reversibility of Phase Transformations in Aluminum Anodes through Operando Light Microscopy and Stress Analysis

“…The potential-capacity profile exhibits a dip before the lithiation plateau, where the potential difference is defined as the nucleation potential by Wang et al [16] This potential is clearly below the equilibrium and represents the extra energy required for the β phase nucleation, [17] which may have a large contribution from the energy required to initiate deformation of the local α phase Al matrix. [18] Operando light microscopy monitors the electrode surface concurrently as (de-)lithiation processes are ongoing. The operando images at specific time points have been selected and are shown in Figure 2.…”

“…The potential‐capacity profile exhibits a dip before the lithiation plateau, where the potential difference is defined as the nucleation potential by Wang et al [16] . This potential is clearly below the equilibrium and represents the extra energy required for the β phase nucleation, [17] which may have a large contribution from the energy required to initiate deformation of the local α phase Al matrix [18] …”

Revealing electrolyte oxidation via carbonate dehydrogenation on Ni-based oxides in Li-ion batteries by in situ Fourier transform infrared spectroscopy

“…4c and d). This difference in the dehydrogenation of LP57 between coated and uncoated NMC622 demonstrates the effect of surface ''inert'' coating layers 76,77 such as Al 2 O 3 , 78,79 [84][85][86][87] NMC811 charged in the concentrated electrolyte (3.1 M LiPF 6 in EC/EMC), which contained fewer free EC or free EMC (molar fraction of free solvent in general concentrated electrolyte with dissociative salt is reported to be less than 10%), [88][89][90][91][92] did not show dehydrogenation or oligomerization products from EC or EMC in the FT-IR difference spectra (Fig. 5a) upon charging to 4.8 V Li , unlike in LP57 (with 1 M LiPF 6 ) where EC became dehydrogenated as discussed before (Fig.…”

Enhanced Cycling Performance of Ni-Rich Positive Electrodes (NMC) in Li-Ion Batteries by Reducing Electrolyte Free-Solvent Activity

“…Here we examine this oxide-mediated electrolyte oxidation mechanism 14,16,17,37,58 by employing highly concentrated carbonate-based electrolytes and electrolyte salts of different dissociation constants, and test if having fewer free carbonate molecules can potentially reduce the electrolyte reactivity with Ni-rich NMC. Highly concentrated electrolytes that typically have a salt molar concentration greater than ~3 M and a molar ratio of Li:solvent less than 1:4, have shown radically different properties such as reduced volatility/flammability 55,[59][60][61][62][63][64] , greater electrochemical stability window [65][66][67][68][69] , enhanced rate performance 56,68,[70][71][72] , and altered interfacial reactivity 61,[66][67][68]70,[73][74][75][76][77][78][79] from conventional electrolytes (~1 M). 13,80 These changes have been attributed to the reduction of free solvent molecules that do not coordinate with Li ions 62,[81][82][83] .…”