Microbial pyrazine diamine is a novel electrolyte additive that shields high-voltage LiNi1/3Co1/3Mn1/3O2 cathodes

Abstract

The uncontrolled oxidative decomposition of electrolyte while operating at high potential (> 4.2 V vs Li/Li⁺) severely affects the performance of high-energy density transition metal oxide-based materials as cathodes in Li-ion batteries. To restrict this degradative response of electrolyte species, the need for functional molecules as electrolyte additives that can restrict the electrolytic decomposition is imminent. In this regard, bio-derived molecules are cost-effective, environment friendly, and non-toxic alternatives to their synthetic counter parts. Here, we report the application of microbially synthesized 2,5-dimethyl-3,6-bis(4-aminobenzyl)pyrazine (DMBAP) as an electrolyte additive that stabilizes high-voltage (4.5 V vs Li/Li⁺) LiNi_1/3Mn_1/3Co_1/3O₂ cathodes. The high-lying highest occupied molecular orbital of bio-additive (DMBAP) inspires its sacrificial in situ oxidative decomposition to form an organic passivation layer on the cathode surface. This restricts the excessive electrolyte decomposition to form a tailored cathode electrolyte interface to administer cyclic stability and enhance the capacity retention of the cathode.

Scheme 1

Structural significance and mode of action of DMBAP additive to stabilize LIB cathodes.

Figure 1

Theoretical evaluation of electrolyte components and DMBAP bio-additive. HOMO–LUMO band energy comparison between the electrolyte components ethylene carbonate (EC), diethyl carbonate (DEC), and DMBAP additive, respectively with their corresponding DFT optimized structures.

Figure 2

LSV studies and XPS characterization after LSV studies. (a) Oxidative linear sweep voltammogram from 0.0 to 6.0 V versus Li/Li⁺ at a scan rate of 1.0 mVs⁻¹ and (b) XPS spectra recorded after LSV measurements to determine the fate of DMBAP additive after oxidative decomposition in comparison to the control system.

Figure 3

Charge–discharge studies. (a) Comparison between charge–discharge performance of control system and DMBAP-based cathodic half-cell at varying current-rates (rate studies), (b) long cycling performance of DMBAP-based cathodic half-cell at 1C rate against the control system with no additive, (c) comparison of capacity retention between the DMBAP additive-based half-cell and control system, (d) comparison of overpotential between the control system and DMBAP-based cathodic half-cells during long cycling at 1C-rate, and (e) reversible capacity versus cycle number comparison between DMBAP-based and control electrolyte-based full cells, respectively.

Figure 4

Impedance Spectroscopy Studies. Nyquist impedance profile comparison for DMBAP-based cathodic half-cell and control system (a) after fabrication and (b) after CV studies, respectively. DEIS 3-D Nyquist profiles after delithiation half-cycle of (c) DMBAP-based cathodic half-cell and (d) control system-based cathodic half-cell. (e) The best fit Equivalent Electrical Circuit Model (EECM) for computational simulation of 3D-Nyquist Impedance profiles for both systems, and (f) CEI impedance (R_CEI) versus potential (V) comparison profiles during lithiation half-cycle in case of DMBAP-based cathodic half-cell and control system, respectively.

Figure 5

Study of surface morphology of LiNi_1/3Mn_1/3Co_1/3O₂ cathodes upon storage in electrolyte. SEM micrographs of (a) pristine cathode, (b) cathode stored in electrolyte with no additive, and (c) cathode stored in electrolyte with DMBAP additive, respectively.