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The development of rechargeable batteries with higher energy densities and longer lifetimes represents a major challenge in enabling the shift from fossil fuel-powered to electric vehicles. Preventing the parasitic reactions between the electrodes (positive and negative) and the electrolyte solution, is essential for enabling longer lasting lithium-ion batteries. In this work, solution- and solid-state nuclear magnetic resonance (NMR) methodologies are developed for studying the electrode-electrolyte reactions. The decomposition reactions of the electrolyte solution at positive electrodes, layered transition metal oxides (LiMO2, M = Ni, Mn, Co or Al, e.g. LiCoO2, LCO; LiNixMnyCo1-x-yO2, NMC), are investigated by a combination of solution NMR spectroscopy and operando gas measurements. The soluble products formed at LCO electrodes are identified, and reaction mechanisms are proposed to rationalise the formation of the observed species. The proposed mechanisms are confirmed by isotopic labelling and by comparing the decomposition products formed at the positive electrode to those formed in a controlled setup simulating specific battery conditions. This methodology is then extended to NMC electrodes with various compositions (i.e., different ratios of Ni, Mn and Co), which revealed that the mechanisms for electrolyte decomposition were the same for all compositions, but the onset voltage for the Ni-rich materials was lower. At the negative electrode, the reduction and deposition of the electrolyte solution at the metal surface results in the formation of a passivating layer, the solid electrolyte interphase (SEI). The interface between a lithium metal electrode and the SEI is studied by dynamic nuclear polarisation (DNP) enhanced solid-state NMR. The signals from SEI components are selectively enhanced in 1H, 7Li and 19F NMR spectra via an Overhauser DNP mechanism, and the proximity of the SEI species to the metal can be inferred from the relative DNP enhancements of the signals. The effect of temperature, magnetic field strength, microwave power and sample dilution on the enhancement are also explored, to understand what the limitations are when using this mechanism to study the lithium metal-SEI interface.

Non-aqueous electrolyte liquids such as carbonate solvents have been widely employed in the commercial lithium-ion batteries and in the development of next-generation rechargeable batteries. The decomposition products of the organic electrolyte and additive molecules contribute to the formation of solid electrolyte interphases (SEIs) on the electrode surface, which have key impacts on battery's electrochemical performance. The rational engineering of electrolyte systems demands precise understanding of the electrochemical reaction pathways as well as the decomposition products of the electrolytes. Mass spectrometry (MS) is a well-established molecular analytical approach that can provide critical information for unambiguous structure assignment based on its mass-resolving power. In recent years, the application of MS for battery research has been expanding rapidly, providing valuable insights about the chemical species generated during battery operation and electrolyte evolution. This review aims to summarize the recent advances of MS technique-based analysis of electrolyte decomposition products as well as SEIs, and thus to demonstrate the high utility of MS methods for characterization of battery systems.

Lithium-ion technology is considered as outstanding candidate for implementation in high energy density applications. Adjusting the cycling conditions of electrodes and monitoring the undergoing reactions are necessary to maximize their potentiality and ensure high performance and safe operation for end-users. Herein, in situ electrochemical impedance spectroscopy (EIS), direct current (DC) resistance and differential voltage analysis (DVA) are complementarily used to understand and predict the lifetime of LiNi0.6Mn0.2Co0.2O2 (NMC622) vs graphite coin cells cycled at different upper cut-off voltage (UCV). Lithium de/intercalation reactions in graphite, phase transitions in NMC and the formation of electrode-electrolyte interphases have been identified by DVA. Combined with EIS and DC resistance, the occurrence of these reactions has been monitored upon cycling. The main findings indicate that despite observing other detrimental phenomena (charge transfer resistance increase or irreversibility of NMC622 phase transitions), the different solid electrolyte interphase (SEI) formation and resistance with UCV are most relevant factors affecting cycle life. The loss of lithium inventory is the main cause of the capacity fade. The need of a stable SEI to delay the continuous electrolyte consumption is highlighted. The combined information provided by these techniques can be leveraged by battery management systems to optimize cell performance while cycling.

Most commercially available lithium ion battery systems and some of their possible successors, such as lithium (metal)-sulfur batteries, rely on liquid organic electrolytes. Since the electrolyte is in contact with both the negative and the positive electrode, its electrochemical stability window is of high interest. Monitoring the electrolyte decomposition occurring at these electrodes is key to understand the influence of chemical and electrochemical reactions on cell performance and to evaluate aging mechanisms. In the context of lithium-sulfur batteries, information about the analysis of soluble species in the electrolytes—besides the well-known lithium polysulfides—is scarcely available. Here, the irreversible decomposition reactions of typically ether-based electrolytes will be addressed. Gas chromatography in combination with mass spectrometric detection is able to deliver information about volatile organic compounds. Furthermore, it is already used to investigate similar samples, such as electrolytes from other battery types, including lithium ion batteries. The method transfer from these reports and from model experiments with non-target analyses are promising tools to generate knowledge about the system and to build up suitable strategies for lithium-sulfur cell analyses. In the presented work, the aim is to identify aging products emerging in electrolytes regained from cells with sulfur-based cathodes. Higher-molecular polymerization products of ether-based electrolytes used in lithium-sulfur batteries are identified. Furthermore, the reactivity of the lithium polysulfides with carbonate-based solvents is investigated in a worst-case scenario and carbonate sulfur cross-compounds identified for target analyses. None of the target molecules are found in carbonate-based electrolytes regained from operative lithium-titanium sulfide cells, thus hinting at a new aging mechanism in these systems.

Li-S cells can have high gravimetric energy densities above 300 Wh kg−1 when the electrodes and cell components are optimized. Low electrolyte/sulfur mass ratios or more generallly, the relative amount of electrolyte in a Li-S cell have an especially high impact on the achievable gravimetric energy density. A negative side effect of low electrolyte/sulfur ratios are low cycle numbers due to electrolyte decomposition and the possibility that electrolyte becomes inaccessible at the lithium metal anode when the lithium becomes more and more porous during cycling. Electrode thickness measurements were performed during cycling for various cell chemistries such as lithium-sulfur (Li-S) with different cathode sulfur loadings and porosities, lithium-hard carbon (Li-HC), lithium-silicon (Li-Si), prelithiated HC-sulfur (LiHC-S), prelithiated Si-sulfur (LiSi-S) and Li-ion. The thickness measurements provided information about mechanical stress and irreversible thickness changes. The thickness measurements also helped to explain different electrolyte decomposition behavior and they can be used to discuss the impact of thickness changes on gas analysis. The electrolyte decomposition of the Li-S standard electrolyte based on lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME): dioxolane (DIOX) with LiNO3 was examined by online mass spectrometry (MS) within Li-S, Li-HC, Li-Si, LiSi-S and LiHC-S cells. Several electrolyte decomposition products were verified by post-mortem gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) of Li-S cells with cycled electrolyte.

Aiming to develop a cost effective means to store large amounts of electric energy, NiFe batteries were produced and tested under galvanostatic conditions at room temperature. Multiple regression analysis was conducted to develop predictive equations that establish a link between hydrogen evolution and electrode manufacturing conditions, over a wide range of electrode/electrolyte systems. Basically, the intent was to investigate the incidence of lithium hydroxide and potassium sulphide as electrolyte additives on cell performance. With this in mind, in-house built Fe/FeS based electrodes were cycled against commercially available nickel electrodes on a three electrode cell configuration. A 3 × 4 full factorial experimental design was proposed to investigate the combined effect of the aforementioned electrolyte additives on cell performance. As a consequence, data from 144 cells were finally used in conducting the analysis and finding the form of the predictive equations. Our findings suggest that at the level of confidence alpha = 0.05, the presence of relatively large amounts of the soluble bisulphide would enhance the performance of the battery by reducing electrolyte decomposition.

Multiple regression analysis was used to investigate the effect of bismuth sulphide and iron sulphide as anode additives for NiFe cells. With this in mind, in-house made Fe/FeS/Bi2S3 based electrodes were cycled against commercially available nickel electrodes. A simplex centroid design was used to investigate the combined effects of any of the aforementioned additives on cell performance. The manuscript ends with an initial look at electrolyte systems as a means to further improve the performance of our cells. Finally, our findings support the idea that HS- ions improve the overall performance of NiFe cells.

The effect of temperature on surface film formation on nickel electrode was studied in 1 mol dm−3 bis(perfluoroethylsulfonyl)imide dissolved in propylene carbonate by atomic force microscopy (AFM) and ac impedance spectroscopy. Cyclic voltammetry measurements revealed that electrolyte decomposition reactions are accelerated at elevated temperatures, especially at 60 and 80 °C. In situ AFM measurements showed that the film formation is fast and the resulting surface film is thicker at 80 °C than at room temperature. Furthermore, it was confirmed by ac impedance measurements that the resistance of surface film was very low at elevated temperatures. These results were discussed in relation to superior cycling characteristics of lithium deposition and dissolution at the elevated temperatures.