Robin Lundström

Cycling NMC cathodes to high voltage causes a severe c -lattice contraction, which leads to poor capacity retention and cycle life. Limiting the upper cut-off voltage (UCV) mitigates this degradation. In this study, we examine whether overlithiation can serve as a strategy to stabilize the c -lattice while allowing operation at higher voltage. We choose NMC 532 as a model system and synthesize both standard NMC 532 and overlithiated NMC 532 (Li-rich 532) from the same precursor to compare their performance. We find that Li-rich 532 achieves superior capacity retention and, after extended cycling, delivers a specific capacity comparable to NMC 532 cycled to 4.4 V. Furthermore, the Li-rich 532 exhibits fewer anti-site defects, resulting in enhanced rate capability. These findings suggest that strategic overlithiation of NMC cathode materials coupled with a moderate UCV cycling is a viable method for improving NMC cathode performance, particularly for applications requiring both high stability and fast-charging capabilities.

Triethyl phosphate (TEP) is a cheap, environmentally benign, and non‐flammable electrolyte solvent, whose implementation in lithium‐ion batteries is held back by its co‐intercalation into graphite anodes, resulting in exfoliation of the graphite structure. In this work, the electrode‐electrolyte interface behaviour of electrolytes containing up to 100 % TEP is investigated and correlated to electrochemical performance. High capacity and stable cycling are maintained with up to 30 % TEP in carbonate ester‐based electrolytes, but above this threshold the reversibility of Li ⁺ intercalation into graphite drops sharply to almost zero. This represents a potential route to improved battery safety, while TEP can also improve safety indirectly by enabling the use of lithium bis(oxalato)borate, a fluorine‐free salt with limited solubility in traditional electrolytes. To understand the poor performance at TEP concentrations of >30 %, its solvation behaviour and interfacial reaction chemistry were studied. Nuclear magnetic resonance spectroscopy data confirms changes in the Li ⁺ solvation shell above 30 % TEP, while operando gas analysis indicates extensive gas evolution from TEP decomposition at the electrode above the threshold concentration, which is almost entirely absent below it. X‐ray photoelectron spectroscopy depth profiling of electrodes demonstrates poor passivation by the solid electrolyte interphase above 30 % TEP and significant graphite exfoliation.

Vinylene carbonate (VC) is the most commonly applied performance-enhancing electrolyte additives in Li-ion batteries to date. Despite numerous studies, there is a lack of consensus regarding the various reaction pathways of VC and their implications. VC has primarily been observed to either polymerize forming poly(vinylene carbonate) (poly(VC)) or decompose releasing major amounts of CO₂, two seemingly contradictory processes. Herein, we present evidence of additional reaction pathways of VC highlighting its role as a H₂O scavenging agent. In contrast to the typical electrolyte solvent ethylene carbonate, VC reacts much more rapidly with water impurities, especially when in contact with hydroxides, forming products less likely to influence cell performance. Efficient removal of water and hydroxides is essential to preserve the stability of Li-ion electrolyte solvent and salt, hence guaranteeing a long lifetime of the battery. Model studies pinpointing reaction pathways of electrolytes and additives, as presented herein, are critical not only to improve modern Li-ion cells but also to establish design principles for future battery chemistries.

Electrolyte additives are indispensable to enhance the performance of Li-ion batteries. Lithium bis(oxalato)borate (LiBOB) has been explored for many years, as it improves both cathode and anode performance. No consensus regarding its reaction mechanisms has, however, been established. A model operando study combining attenuated total reflection infrared spectroscopy (ATR-FTIR), electrochemical quartz crystal microbalance (EQCM), and online electrochemical mass spectrometry (OEMS) is herein presented to elucidate LiBOB reduction and electrode/electrolyte interphases thus formed. Reduction of the BOB^- ion sets in at ∼1.8 V with solid lithium oxalate and soluble oxalatoborates as the main products. The reduced BOB^- ion also reacts with itself and its environment to evolve CO₂, which in turn impacts the interphase formed on the negative electrode. This study provides further insights into the reduction pathways of LiBOB and how they contribute to the interphase formation.

Ethylene carbonate (EC) is the archetype solvent in Li-ion batteries. Still, questions remain regarding the numerous possible reaction pathways of EC. Although the reaction pathway involving direct EC reduction and SEI formation is most commonly discussed, EC ring-opening is often observed, but seldomly addressed, especially with respect to SEI formation. By applying Online Electrochemical Mass Spectrometry, the EC ring-opening reaction on carbon is found to start already at ∼2.5 V vs Li⁺/Li as initiated by oxygenic carbon surface groups. Later, OH⁻ generated from H₂O reduction reaction at ∼1.6 V further propagates EC to ring-open. The EC reduction reaction occurs <0.9 V but is suppressed depending on the extent of EC ring-opening at higher potentials. Electrode/electrolyte impurities and handling conditions are found to have a significant influence on both processes. In conclusion, SEI formation is shown to be governed by several kinetically competing reaction pathways whereby EC ring-opening can play a significant role.

Silyl groups are included in a wide range of electrolyte additives to enhance the performance of state-of-the-art Li-ion batteries. A recognized representative thereof is tris-(trimethylsilyl)phosphate (TMSPa) which, along with the similarly structured phosphite, has been at the center of numerous electrolyte studies. Even though the silyl group has already been widely reported to be specifically reactive towards fluorides, herein, a reactivity towards several Lewis bases typically found in Li-ion cells is postulated and investigated with the aim to establish a more simplified and generally applicable reaction mechanism thereof. Both gaseous and electrolyte soluble reactants and products are monitored by combining nuclear magnetic resonance and injection cell-coupled mass spectrometry. Experimental observations are supported by computational models. The results clearly demonstrate that the silyl groups react with water, hydroxide, and methoxide and thereby detach in a stepwise fashion from the central phosphate in TMSPa. Intermolecular interaction between TMSPa and the reactants likely facilitates dissolution and lowers the free energy of reaction. Lewis bases are well known to trigger side reactions involving both the Li-ion electrode and electrolyte. By effectively scavenging these, the silyl group can be explained to lower cell impedance and prolong the lifetime of modern Li-ion batteries.

The “ethylene carbonate (EC)–propylene carbonate (PC) mystery” has puzzled electrochemists for decades. Surprisingly, the minor structural difference between PC and EC, a methyl vis‐à‐vis a proton, prevents PC unlike EC to form a stable solid electrolyte interphase (SEI) on carbon (C), which along with the popularity of PC has impeded the development of Li‐ion batteries with many years. Despite several hypotheses, the fundamental reason remains debated largely due to the lack of sufficient experimental evidence. Herein, SEI formed as a result of EC and PC reductions are analyzed by two state‐of‐the‐art operando techniques, online electrochemical mass spectrometry and electrochemical quartz crystal microbalance with dissipation monitoring. Although both EC‐ and PC‐based electrolytes appear to have virtually identical reaction pathways, PC is reduced much more extensively than EC and forms a much thicker SEI. However, while the SEI derived from EC remains on the electrode, PC reduction products redissolve in the electrolyte leaving the bare C electrode behind. The presented study illustrates the complex scheme of competing electro‐/chemical reactions behind SEI formation and provides further scientific details needed to eventually form a consensus of the processes governing electrode/electrolyte interphases in Li‐ion batteries.

Online Electrochemical Mass Spectrometry (OEMS) is capable of monitoring both partial and total gas pressure of Li-ion cells. Herein, the development and validation of an OEMS system along with detailed calibration protocols and limits of detection sensitivity are showcased. A full cell based on LiCoO2/Graphite cell during overcharge to 4.9 V vs. Li/Li+ at 50 °C is investigated and the results are compared to LiCoO2/LiFePO4 and Graphite/LiFePO4 cells in order to differentiate between gases forming at the anode and cathode. The release of O2 from LixCoO2 (x < 0.4) during both charge and discharge demonstrates that its degradation is dependent on composition rather than potential. Combining partial and total pressure measurements provides a clear advantage when detailing major and minor gas reactions as well as when determining unaccounted gases. The methodology presented herein provides a technical basis for deeper understanding of degradation reactions in batteries and electrochemical systems alike.