2, Uppsala University, Uppsala, , Sweden
Characterizing interfaces and interphases in primarily lithium-ion batteries is in one way simple but the result can be difficult to interpret. It is simple in the sense that it is easy to take a battery apart and then use different techniques to study the composition and morphology of interfaces of electrodes and separators post mortem. It is, however, difficult to study in situ how an interface forms and evolves during battery operation.
The SEI (Solid Electrolyte Interphase) on negative electrodes can be described as a mixture of inorganic and organic compounds where the inorganic compounds are formed closer to the electrode surface. The layer is a consequence of the low potential – close of that of lithium – where for lithium-ion batteries the reduction of the thermodynamically instable organic solvent (below 0.8V vs. Li+/Li) is taking place. There are even descriptions of the SEI consisting of an inner, more dense inorganic layer, where electrons can tunnel through until a certain thickness of the layer has been obtained where the SEI becomes electronically insulating but where ions can penetrate. How the different SEI-compounds interplay to form a well-functioning layer is not yet clear. This presentation aims at bringing some light on these complex issues based on the combination of different techniques.
On the surface of a positive electrode is also influenced by the electrolyte composition (and then primarily of the electrolyte salt) but in a different way compared to the negative electrode. In general the surface film is thinner and the reactions can more be described as corrosion reactions.
The presentation willinclude a description of how the crosstalk in the redox chemistry between the electrodes during battery operation will influence the interfacial chemistry, respectively. What is the difference to a the interface compositions in so-called half-cells compared to those for full-cell Li-ion and Na-ion batteries. Typical chemistries involve graphite and silicon as negative electrode materials and different nickel, cobalt and manganese oxide materials as positive electrode materials.