The interface chemistry of Li x Mn 2 O 4 electrodes in carbonate-based electrolytes has been investigated using X-ray photoelectron spectroscopy, infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy following cycling or storage in ͗LiMn 2 O 4 ͉ethylene carbonate/dimethyl carbonate LiPF 6 /LiBF 4 ͉Li͘ cells. No significant changes were found in the elemental composition of surface films formed on cycled and stored samples, suggesting that surface-film formation is not governed by processes associated with cell cycling. The amount of surface species increases with storage time and cycle number at ambient temperature, where LiF, Li x PF y O z products and some polyether-type polymeric compound could be identified as reaction products on the cathode surface. A lithium-rich manganese oxide layer develops on the surface of the cathode particles under continued storage and cycling. The thickness of the surface layer decreases rather than increases with storage at a higher state-of-charge. More carbon compounds are preserved on the electrode surface using LiBF 4 rather than LiPF 6 as electrolyte salt.LiMn 2 O 4 is one of the most interesting cathode materials for use in Li-ion batteries. Many aspects have been investigated thoroughly during the last decade, but mainly concern bulk properties, i.e., cycling performance, capacity fade, structure, and synthesis. 1-3 The interface between cathode and electrolyte has so far not been researched extensively, even though it could well be responsible for capacity loss, and ultimately, cell failure.The electrochemical reactions in the cell occur at the electrode interfaces, followed by mass transport into the bulk of the electrode, with structural changes as a result. Unwanted side reactions can therefore take place as the electron meets the Li ϩ ion at or near the surface of the cathode particle. Spontaneous reactions such as selfdischarge and decomposition of the cathode material can also create a reactive surface, where solvent and salt can participate in reactions during storage and cycling.The formation of a solid electrolyte interphase ͑SEI͒ accompanies and influences the electrochemical processes occurring during cell charge and discharge. It can protect the surface from further reactions, but can also increase the cell impedance and thus decrease its cycling efficiency.It has been shown that an intermediate migration step occurs through a surface film between electron transfer at the particle surface and diffusion into the bulk, for the most commonly used Li-ion battery cathode materials ͑LiCoO 2 , LiNiO 2 , and LiMn 2 O 4 ͒. 4,5 This surface film has been suggested to be formed electrochemically during the first cycles. 6,7 In some cases, the cathode material has been shown to have a pristine surface film of Li 2 CO 3 , which could be a residue from the synthesis precursors, or a product of reactions between CO 2 in the atmosphere and the active cathode powder. Recent theories relating to the formation of cathode surface layers involve the electroch...
