Analyzing the Effect of Electrolyte Quantity on the Aging of Lithium-Ion Batteries

Abstract

Despite a substantial impact on various economic and cell technology factors, the influence of electrolyte quantities is rarely addressed in research. This study examines the impact of varying electrolyte quantities on cell performance and aging processes using three different electrolytes: LP57 (1 M LiPF6 in ethylene carbonate:ethyl methyl carbonate (EC:EMC 3:7 w/w), LP572 (LP57+2 wt.% vinylene carbonate (VC)) and LP57 + absVC (18.351 mg VC). Comprehensive analytical post mortem investigations revealed that continuous excessive electrolyte decomposition determines the performance of cells using LP57, leading to enhanced irreversible lithium-ion loss and interphase thickening with increasing electrolyte volume. Impedance rise due to the growth of the interphase was also identified as the cause of degrading cell performance with rising amounts of LP572, attributed to an increasingly pronounced consumption of VC rather than electrolyte aging effects. By varying the electrolyte quantity while maintaining a constant amount VC within the cell system, the differences in cell performance were minimized, and observed deteriorating effects were suppressed. This study demonstrates the sensitive interdependence of electrolyte volume and additive concentration, practically affecting aging behavior. Comprehensively understanding the characteristics of each individual electrolyte component and tailoring the electrolytes to cell-specific cell properties proves to be crucial to optimize cell performance.

Keywords: aging mechanisms; electrolyte characterization; electrolyte quantity; lithium‐ion batteries; post‐mortem analysis.

Figure 1

Normalized discharge capacities of NMC622||G large‐format pouch cells (5 Ah) with LP572 electrolyte at different vfs varying from 1.01 to 1.58. Life cycle test of 1000 cycles with one cell per vf was performed at 1C (a) and check‐up cycles for every 50th cycle at C/10 (b) in the voltage range of 2.9–4.2 V.

Figure 2

Discharge capacities of NMC622||AG small‐format pouch cells (200 mAh) with LP57 (a), LP572 (b) and LP57+absVC (c) electrolyte at different vfs varying from 1.0 to 1.8 (0.2 steps). Electrochemical cycling was conducted at 1C for 300 cycles in the voltage range of 2.9–4.2 V.

Figure 3

Charge and discharge curves at cycle 1, 30, and 300 after formation of the NMC622||AG small‐format pouch cells (200 mAh) with LP57 + absVC (top line) and LP572 (bottom line).

Figure 4

Mass fractions of volatile electrolyte components of the NMC622||AG small‐format pouch cells (200 mAh) with LP57 (a), LP572 (b), and LP57+absVC (c) at various vfs (1.0–1.8) after electrochemical aging. Low‐concentrated components are displayed enlarged with original VC mass fractions additionally presented transparently. Quantification was performed by means of GC‐FID.

Figure 5

Reaction scheme for the electrochemical reduction pathways of EMC^{[ 48} , ^{50 ]} and EC.^{[ 48} , ⁵¹ , ^{52 ]} Proposed chemical reactions of alkoxide triggered EMC transesterification^{[ 50} , ^{51 ]} (c) and alkyl dicarbonate formation via EC^{[ 48} , ^{51 ]} (d) and EMC^{[ 57 ]} (e).

Figure 6

Determined absolute amounts of volatile electrolyte components of the NMC622||AG small‐format pouch cells (200 mAh) with LP57 (a), LP572 (b), and LP57 + absVC (c) at various vfs (1.0–1.8) after electrochemical aging. Low‐concentrated components are displayed enlarged with original VC amounts additionally presented transparently. Quantification was performed by means of GC‐FID.

Figure 7

Reaction scheme for alkoxide scavenging by VC (a) according to Sasaki et al.,^{[ 62 ]} VC reduction with subsequent polymerization (b) according to Zhang et al.^{[ 63 ]} and alkoxide scavenging by CO₂ (c) according to Zhang et al.^{[ 20 ]}

Figure 8

Quantified lithium contents in the active material of anode (bottom) and cathode (top) samples after electrochemical aging for 300 cycles plotted against the vf for the electrolytes LP57 (brown), LP572 (yellow), and LP57+absVC (green).

Figure 9

SEM investigations of the surface morphology of the pristine (a) electrochemically aged negative electrodes with LP57 at vf 1.0 (b), vf 1.4 (c), and vf 1.8 (d) as well as LP572 at vf 1.0 (e), vf 1.4 (f), and vf 1.8 (g). Smaller images show parts of a magnified area.