Comparing anode and cathode electrode/electrolyte interface composition and morphology using soft and hard X-ray photoelectron spectroscopy

• Depth profiling of the full anode SEI and cathode SPI was performed using hard and soft X-ray PES.
• In the innermost SEI, compounds like Li2O and lithium alkoxides are found.
• The SPI and the outermost SEI exhibit some similarities.
• The SEI thickness was estimated to two tens of nanometers, and the SPI thickness to a few nanometers.
• Evidence of binder and SEI/SPI mixing was observed.

Electrode/electrolyte interface depth profiling was performed on lithiated graphite and delithiated lithium iron phosphate electrodes after electrochemical cycling in a balanced full cell configuration containing a carbonate based LiPF6 electrolyte. The profiling was performed by synchrotron radiation based hard X-ray photoelectron spectroscopy, HAXPES, and soft X-ray photoelectron spectroscopy, SOXPES. In this way, the probing depth was varied over a wide range in the order of 2–50 nm. Both more surface and more bulk sensitive investigations than possible using traditional in-house X-ray photoelectron spectroscopy (XPS) could thus be performed. The composition and morphology of the lithiated graphite anode/electrolyte interface (solid electrolyte interphase, SEI) and the delithiated lithium iron phosphate cathode/electrolyte interface (solid permeable interface, SPI) were compared. In the vicinity of the highly reductive graphite active material in the SEI, low binding energy components like Li2O were found while no obvious composition gradients were observed in the SPI. Both in the cathode SPI and the anode SEI, significant amounts of C–O and P–F containing compounds were found to deposit during cycling. Evidence for mixing of the porous binder and other SEI/SPI components was observed in both the anode and cathode electrode/electrolyte interfaces. The lithiated graphite SEI was estimated to be of the order of two tens of nanometers, while the cathode SPI thickness was estimated to a few nanometers only.