Logo: to the web site of Uppsala University

Rechargeable batteries, particularly, lithium-ion batteries (LIBs) have proven to be stable and reliable energy storage devices over the past few decades. The rapid demands regarding battery applications and the pressure to move away from the fossil fuel era drive the search for new materials for better rechargeable batteries for electric vehicles, renewable energy storage, and portable electronics. In this context a deeper understanding of the electrochemical processes governing the electrochemical behaviour of batteries is required. This thesis work investigates the use of two titanium-based materials as negative electrode materials for lithium- and sodium-ion batteries. The focus is on identifying the redox reactions responsible for the electrochemical capacities observed for the materials. Having knowledge of the available redox reactions for new materials used in batteries is crucial in predicting whether they can compete with existing battery chemistries and be commercially viable.

One part of this thesis work examines the electrochemical behaviuor of a 2D titanium carbide, Ti₃C₂T_x, a member of the MXene family, in lithium- and sodium-ion batteries. The other part explores an A-site cation deficient Li_0.18Sr_0.66Ti_0.5Nb_0.5O₃ (L018STN) perovskite oxide, known for its high lithium-ion conductivity, in LIBs. The electrodes were electrochemically evaluated in pouch-cell batteries and analysed post hoc by means of X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. 

The results indicate that only the surface Ti(I), Ti(II), Ti(III), and Ti(IV) titanium species of the Ti₃C₂T_x flakes participate in the redox reactions and give rise to the electrochemical capacity. Furthermore, the restacking of individual flakes within the bulk of the Ti₃C₂T_x electrode limits the electroactive surface of a freestanding Ti₃C₂T_x electrode that is available for the redox reactions. The reversible capacities of Ti₃C₂T_x electrodes can be improved by long-term cycling (an effect known as capacity activation) and heat treatment, as the surface titanium species gradually oxidise to higher oxidation states, e.g., Ti(III) and Ti(IV), or transform to titanium oxides Ti_xO_y. 

The results for L018STN electrodes show that both titanium and niobium are redox active on over-lithiation, that is, when more than one Li⁺ was inserted per a vacant A-site. The structural reorganization during over-lithiation enabled access to diffusion paths for fast lithium-ion diffusion even when a high concentration of lithium was inserted into the structure. 

The findings of this thesis work thus indicate that a portion of the Ti₃C₂T_x electrode is electrochemically inactive when subjected to electrochemical cycling. This can be ascribed to its structure and two-dimensional nature. As a result, Ti₃C₂T_x cannot outperform existing negative electrodes for lithium- or sodium-ion batteries. The results obtained for L018STN provide valuable information on the lithium-ion diffusion behaviours in A-site cation deficient perovskite oxides. In a broader sense, this thesis work emphasises the significance of employing a multi-technique approach to obtain a good understanding of the underlying redox mechanisms when analysing battery materials.