Online Electrochemical Mass Spectrometry on Large-Format Li-Ion Cells
Authors: Misiewicz Casimir , Lundström Robin , Ahmed Istaq , Lacey Matthew , Brant William Robert , Berg Erik J. 
Published: February 2024

Significant progress has been made in real-time analysis methodologies for batteries, particularly with regards to the development of Operando Electrochemical Mass Spectrometry (OEMS) [1]. OEMS provides unprecedented selectivity and sensitivity in analysing the release of volatile species, enabling the detection of side reactions during battery cycling which can provide crucial information about the Solid Electrolyte Interface (SEI) and processes that limit battery life. However, most research thus far has been conducted on model battery systems (~1 mAh) that do not accurately reflect conditions within large-format commercial cells (~10,000 mAh) particularly in terms of electrode areas, electrolyte volumes, and stack pressures. Additionally, the atmosphere within the cell is altered through either continuous flow [2] [3] (in the open approach) or purging [4] (in the semi-closed approach [5]). In both cases, gas is being withdrawn from the cell (10% of cell volume/measurement point) during operation and replaced with a carrier gas (usually Ar). This dilutes the existing environment and prevents further reactions, making it difficult to compare the results to real-life situations.
In this work, the development and validation of an entirely closed OEMS system that can be interfaced with large-format commercially available batteries is showcased. Instead of replacing the removed gas, a small amount (0.2 % of cell volume/measurement point) is periodically removed, causing a gradual pressure drop in the cell. Experiments are limited by a minimum internal cell pressure of 700 mbar due to the sensitivity of the pressure sensor and vapour pressures of the electrolyte. Apart from this pressure drop, the internal cell environment is therefore relatively constant allowing us to observe side reaction products that have occurred within a realistic cell environment.
We additionally provide a detailed description of an adapter design for interfacing large format prismatic cells with our intermittently closed OEMS (ICEMS) setup. The adapter is placed over a hole in the cell (produced with a drill) and can be easily scaled for different cell geometries. Using this ICEMS setup, we provide operando pressure and gassing data from a calendared BAK PHEV2 cell. The pressure within the cell changes depending on the level of lithiation in the cathode (NMC) and anode (graphite), revealing the multiple phase transitions that occur in both materials during cycling. These transitions can be divided into four regions of interest, where graphite dominates regions I and III (with large pressure increases during charge) and NMC dominates regions II and IV (with pressure decreases during charge). By analysing the gas produced during these transitions, it is clear that structural changes within both electrodes result in side reactions. When graphite undergoes large volume changes (from 1L to 2L and 2 to 1, regions I and III), H2 and C2H4 are released as SEI reformation products, while the c lattice collapse in NMC (during the H2 to H3 transition, region IV) results in CO2 evolution (through O2 release that subsequently reacts with the electrolyte). In addition, we suggest that Lewis bases formed on the graphite through the reduction of electrolyte species ring open EC, forming CO2 [6]. The results of this study suggest that the internal chemistry of commercially available prismatic rechargeable batteries can be studied effectively using ICEMS without significantly altering the battery's chemistry.
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