New membrane technology enables long-term gas analysis in batteries, revealing failure mechanisms
A graphene oxide-based separation membrane prevents solvent interference, enabling a realistic study of battery degradation over hundreds of hours.
image: Schematic of the MDEMS setup featuring a graphene oxide-based gas separation membrane (Left). The membrane selectively blocks volatile organic solvent molecules (e.g., DEC, EMC) from entering the mass spectrometer (MS), while allowing dissolved gases (e.g., H2, O2, CO, CO2, C2H4) generated in the NCM811-graphite full cell to be carried by an argon flow to the mass analyzer for detection. Corresponding gas evolution data (H2 and CO2) during cycling at 45℃ (Right). The results show that the combination of a carbon-coated NCM811 cathode and LiDFOB additive significantly suppresses gas generation compared to the unmodified cell, validating the strategy's effectiveness for enhancing battery stability.
Credit: ©Science China Press
Differential electrochemical mass spectrometry (DEMS) is a powerful tool for tracking gaseous products during battery operation, offering critical insights into reaction mechanisms and safety. However, when applied to lithium-ion batteries with organic electrolytes, conventional DEMS faces a major hurdle: the standard hydrophobic polytetrafluoroethylene (PTFE) membrane cannot block volatile solvent molecules like dimethyl carbonate (DMC) and diethyl carbonate (DEC). These solvents enter the mass spectrometer, causing signal interference, contaminating the ion source, and accelerating electrolyte dry-out, which leads to premature cell failure within just one to two days. This short testing window and unrealistic electrolyte conditions make it difficult to study long-term degradation mechanisms.
To address this, a team led by Professor Yuhui Chen at Nanjing Tech University designed a membrane-separated DEMS (MDEMS) system. The core of this innovation is a graphene oxide-based membrane that selectively allows small gas molecules (e.g., H2, CO2, C2H4) to pass through while effectively blocking larger organic solvent molecules.
This membrane design solves two problems simultaneously: it prevents solvents from interfering with detection and preserves the electrolyte composition by maintaining solvent vapor pressure inside the cell. Consequently, the MDEMS system supports stable operation for hundreds of hours, closely mimicking real-world battery conditions.
Using MDEMS, the team investigated how the additive lithium difluoro(oxalato)borate (LiDFOB) and a carbon coating on the cathode material affect gas generation in NCM811-graphite full cells. They found that while LiDFOB effectively suppresses gas evolution initially, it is gradually consumed over cycles as transition metal ions dissolved from the NCM811 cathode migrate to the graphite anode, damaging the solid-electrolyte interphase (SEI).
Applying a carbon coating to the cathode significantly suppressed metal ion dissolution, thereby protecting the SEI and reducing gas generation. The combination of LiDFOB and carbon coating synergistically delayed cell failure, with particularly notable performance at 45°C.
This study demonstrates that MDEMS is a robust platform for probing gas evolution mechanisms in batteries with volatile electrolytes. The technique is also applicable to other battery systems beyond lithium-ion. Future work will integrate MDEMS with other in-situ characterization methods to build a more comprehensive understanding of battery failure.