The performance and lifetime of lithium-ion batteries are strongly influenced by their composition. One category of critical components are electrolyte additives, which are included primarily to stabilize electrode/electrolyte interfaces in the battery cells by forming passivation layers. The presented study aimed to identify and study such an additive that could form a hydrogen-evolution-suppressing solid electrolyte interphase (SEI) in lithium-ion batteries based on aqueous electrolytes. A promising molecular additive, ethyl 2,2-difluoroacetate (EDFA), was found to hold the qualities required for an SEI former and was herein further analyzed electrochemically. Analysis of the battery cells were performed with linear sweep voltammetry and cyclic voltammetry with varying scan rate and EDFA concentrations. Results show that both 1 and 10 w-% EDFA in the electrolyte produced hydrogen-evolution-suppressing SEI:s, although the higher concentration provided no apparent benefit. Lithium-ion full-cells based on LiMn2O4 vs. Li4Ti5O12 active materials displayed poor, though partly reversible, dis-/charge cycling despite the operation of the electrode far outside the electrochemical stability window of the electrolyte. Inclusion of reference electrodes in the lithium-ion cells proved to be immensely challenging with unpredictable drifts in their electrode potentials during operation. To summarize, HER-suppressing electrolyte additives are demonstrated to be a promising approach to stabilize high-voltage operation of aqueous lithium-ion cells although further studies are necessary before any practical application thereof can be realized. Electrochemical evaluation of the reaction mechanism and efficiency of the electrolyte additives relies however heavily on the use of reference electrodes and further development thereof is necessary.

The research discussed is divided into two parts. The first part discusses the background work involved in preparation of the Li-ion cell testing stage. This includes the preparation of anodes using the doctor blade and a calendar mill, electrolyte preparation, test cell assembly, the Li-ion test cell design, and experiments performed to troubleshoot the cell. The second part deals with the cell testing experiments. Li-ion batteries are amongst the most promising rechargeable battery technology because of their high capacity and low weight. Current research aims at improving the anode quality to increase the capacity. The experiments discussed evaluate the traditional anode materials like SFG44 graphite and conducting grade graphite against the novel ones– and tin oxide (SnO2) based and carbon encapsulated tin based anodes. The solid electrolyte interphase formed on each anode was analyzed to understand the initial capacity fade leading to conditioning of the cell thus stabilizing its performance.

The rate of degradation of a lithium-ion battery depends on its use i.e. how it is charged and discharged. Physics-based models are used to represent the processes inside a cell as well as the degradation mechanisms. This thesis aimed to compare how the battery lifetime is affected when charging with different charging protocols using different battery models and degradation mechanisms. The investigated models are the Single Particle Model (SPM), the Single Particle Model with electrolyte (SPMe), and the Doyle-Fuller Newman model (DFN). The degradation mechanisms are solid electrolyte interphase (SEI), and lithium plating (LP). The used charging protocols are constant-current constant voltage(CCCV), positive pulsed current (PPC), and constant current (CC). Pulsed charging was included to investigate if the battery lifetime can be improved as in an experiment by Huang where pulsed charging increased the battery lifetime by 60%. To perform the simulations using the physics-based models, PyBaMM (PythonBattery Mathematical Modeling) was used. The simulations were performed for a lithium cobalt oxide (LCO) cell. Two types of SEI were implemented, solvent-diffusion limited and reaction limited. For the LP only irreversible LP was used.1200 cycles were simulated. Comparing the PPC and CC protocols, there were no significant changes between the degradation mechanisms for the different protocols. The results were the same for all the models, except for the results of the internal resistance. The conclusion is that for the PPC and CC protocols, the cell degrades the same although the PPC protocol used twice the C-rate. The PPC charging did not increase the battery lifetime. For the CCCV and CC protocols, there were some bigger differences between the protocols, but between the different models, there weren’t any significant differences. The CCCV degrades the cell faster for all degradation mechanisms and all models. Simulating one degradation submodel at a time resulted in a very small capacity fade for some submodels. Therefore, for future work, it is suggested to use several degradation submodels at the same time but also to try other degradation mechanisms or try PPC protocols with different frequencies and duty cycles. / Hur snabbt litiumjonbatterier degraderas beror på hur de används, laddas och laddas ur. Fysikbaserde modeller används för att representera processerna inuti cellen och även degraderingsmekanismerna. Denna studie har genomförts för att undersöka hur batteriets livslängd påverkas av olika laddningsprotokoll genom att använda olika batterimodeller och degraderingsmekanismer. Modellerna som användes är Singel-partikelmodellen (SPM), Singel-partikelmodellen med elektrolyt (SPMe) och Doyle-Fuller Newman-modellen (DFN). Degraderingsmekanismerna är fast elektrolytinterfas (SEI) och litiumplätering (LP). Laddningsprotokollen som användes är konstant ström konstant spänning (CCCV), positiv pulserande ström (PPC) och konstant ström konstant (CC). Protokollet för pulsad laddning inkluderades för att undersöka om batteriets livslängd kan förbättras som i ett experiment av Huang, där pulsad laddning ökade batteriets livslängdmed 60%. För att utföra simuleringar med fysikbaserade modeller användes PyBaMM(Pyhton Battery Mathematical Modeling). Simuleringarna utfördes för en lithiumkobaltoxid-cell (LCO). Två typer av SEI implementerades, lösningsmedelsdiffusion-begränsad och reaktions-begränsad SEI. För LP användes endast irreversibel LP.1200 cykler simulerades. Jämförande PPC- och CC-protokollen fanns det inga signifikanta förändringar mellan degraderingsmekanismerna för de olika protokollen. Resultaten vardesamma för alla modellerna, förutom resultaten av den interna resistansen. Slutsatsen är att för både PPC- och CC-protokollen så degraderades cellen på samma sätt, trots att PPC-protokollet använde dubbelt så hög C-faktor. PPC-protokollet ökade inte batteriets livslängd. För CCCV- och CC-protokollen fanns det några större skillnader mellan protokollen, men mellan de olika modellerna fanns det inga signifikanta skillnader. CCCV-protokollet försämrade cellen snabbare för alla degraderingsmekanismer och alla modeller. Att simulera en degraderingsmodell i taget resulterade i mycket små kapacitetsförluster. Därmed föreslås det att i framtida arbete använda flera degraderingsmodeller samtidigt men även testa andra degraderingsmekanismer eller PPC-protokoll med olika frekvenser och arbetscykler