Selective Application of Electrolyte Additives on Anode or Cathode Investigated By on-Line Electrochemical Mass Spectrometry

Recently, many investigations have been devoted to the development of novel electrolyte additives for Li-ion batteries. In carbonate-based electrolytes, most additives are reduced preferentially during the first charging cycle to generate a solid-electrolyte interphase (SEI) on the anode.¹ Vinylene carbonate (VC) is the most widely investigated electrolyte additive for EC-based electrolytes, and its reduction has a well-characterized effect on the SEI on graphite anodes.^2,3 In contrast, lithium bis(oxalate) borate (LiBOB) has been shown to generate CO₂ and a cathode passivation film upon its oxidation that inhibits the oxidation of bulk electrolyte at the cathode.⁴

In this study, we employ our newly developed two-compartment Li-ion battery cell,^5,6 which provides a tight seal between the anode and the cathode compartment, to investigate the effect of VC and LiBOB selectively on the anode and cathode side. Our two-compartment cell features an edge-sealed Li⁺-ion conductive glass ceramic (Ohara glass), and we demonstrate that a lamination of the Ohara glass with a layered polypropylene-aluminum-foil can effectively suppress any gas and liquid diffusion between the two compartments. Figure 1 shows that the layered PP-Al-foil sealing can avoid continuous CO₂ (m/z = 44) evolution during OCV, in contrast to a PP-foil sealing. The potential independent CO₂ increase for the latter is caused by water permeation to the metallic lithium counter-electrode leading to OH^--catalyzed EC hydrolysis.⁵

We further use On-line Electrochemical Mass Spectrometry (OEMS)⁷ to investigate the effect of VC and LiBOB on the gas generation during the first three formation cycles of NMC/Li half cells, graphite/Li half cells, and NMC/graphite full cells. Our experimental approach is to separate the electrodes in the half cell arrangement, such that all gases detected by OEMS stem exclusively from either the NMC electrode or the graphite electrode. The influence of the lithium side is completely suppressed, and the effect of the additives can be monitored in a reductive (graphite) or oxidative (NMC) environment. Subsequently, we remove the edge-sealed gas diffusion barrier and investigate the gas evolution in NMC/graphite full cells in a one-compartment cell arrangement, to see the sum of gas generation from both electrodes. We quantify our OEMS results using a calibration gas, and give both, quantitative and mechanistic insights into the effect of VC and LiBOB on the gas evolution in Li-ion batteries.

Figure 2 highlights the combined gas evolution from the NMC cathode and the graphite anode measured by OEMS in a one-compartment cell arrangement using LP57 (1M LiPF₆ EC/EMC 3/7 wt/wt) without additives. The main gases evolved upon formation are ethylene C₂H₄ (m/z = 26), hydrogen H₂ (m/z = 2), and carbon dioxide CO₂ (m/z = 44). While H₂ results from the unavoidable presence of trace water (< 20 ppm), C₂H₄ is known to be the main gaseous product of EC reduction at the graphite anode. CO₂ evolution is only detected at the end of charge and is likely to originate from the oxidation of the carbonate-based electrolyte at the NMC cathode.

References

1. S. S. Zhang, J. Power Sources, 162, 1379 (2006).
2. D. Aurbach et al., Electrochimica Acta, 47, 1423 (2002).
3. B. Zhang et al., submitted (2015).
4. M. Xu, et al., J. Phys. Chem. C, 118, 7363 (2014).
5. M. Metzger et al., J. Electrochem. Soc., 167, A1123 (2015).
6. M. Metzger et al., J. Electrochem. Soc., 167, A1227 (2015).
7. N. Tsiouvaras et al., J. Electrochem. Soc., 160, A471 (2013).

Acknowledgement

The authors gratefully acknowledge BASF SE for financial support of this research through the framework of its Scientific Network on Electrochemistry and Batteries.