a b s t r a c tThe effect of the Si electrode morphology (amorphous hydrogenated silicon thin films -a-Si:H as a model electrode and Si nanowires -SiNWs electrode) on the interphase chemistry was thoroughly investigated by the surface science techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS analysis shows a strong attenuation and positive shift of the Si 2p peaks after a complete charge/discharge performed in PC-and EC:DMC-based electrolytes for both electrodes (a-Si:H and SiNW), confirming a formation of a passive film (called solid electrolyte interphase -SEI layer). As evidenced from the XPS analysis performed on the model electrode, the thicker SEI layer was formed after cycling in PC-based electrolyte as compared to EC:DMC electrolyte. XPS and ToF-SIMS investigations reveal the presence of organic carbonate species on the outer surface and inorganic salt decomposition species in the inner part of the SEI layer. Significant modification of the surface morphology for the both electrodes and a full surface coverage by the SEI layer was confirmed by the scanning electron microscopy (SEM) analysis.

XPS, RBS, and NRA have been combined to study the mechanisms of Li-ion electrochemical intercalation in MoO 3 thin films prepared by thermal oxidation of molybdenum metal. A direct anaerobic and anhydrous transfer was used from a glovebox (O 2 and H 2 O < 1 ppm), where the samples were electrochemically treated at selected potentials between 1.7 and 3.2 V versus Li + /Li, to the XPS analysis chamber. The thermal oxide film grown at T ) 450 ( 10 °C and P(O 2 ) ) 100 ( 10 mbar for t ) 5 min consisted of a 20 nm thick MoO 3 outer layer overlying a 13 nm thick inner layer of lower oxides (Mo 2 O 5 and MoO 2 ). Combined RBS/NRA analysis allowed the dosing of intercalated lithium and the determination of the composition of the lithiated phases. Li 0.50 MoO 3 , Li 1.20 MoO 3 , and Li 0.21 MoO 3 were obtained after intercalation at 2.58 and 1.73 V and deintercalation at 3.2 V, respectively, showing that ∼1.2 mol of Li can be initially intercalated in the potential range 1.7-3.2 V (capacity of 223 mA h/g), and ∼0.2 mol of Li per mol of MoO 3 is trapped in the oxide matrix after the initial stages of intercalation. The XP Mo3d core level spectra evidenced the reduction of Mo 6+ ions to Mo 5+ ions after intercalation at 2.58 V and further to Mo 5+ and Mo 4+ ions after intercalation at 1.73 V with resulting Mo 6+ /Mo 5+ /Mo 4+ ratios of 53:47:00 at % and 37:39:24 at %, respectively. Reoxidation of molybdenum is observed after deintercalation but 40 at % Mo 5+ subsist at 3.2 V due to the trapping of lithium strongly bonded to the oxide matrix. The Li1s core level (at E B ) 55.80 eV) is most intense at 1.73 V and does not vanish at 3.2 V. Broadening of the Mo3d core level peaks are assigned to the distortion of the oxide matrix. Changes of the electronic structure after intercalation result from the occupation of the Mo4d states (at E B ) 1.0 eV) originally empty in the pristine oxide. The XP C1s and O1s core level spectra show the irreversible formation of a solid electrolyte interphase (SEI) layer including lithium carbonate and Li-alkoxides.

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Iron oxide (mostly α-Fe 2 O 3 ) model thin-film electrodes were prepared by thermal oxidation of pure metal iron substrates at 300 ± 5 °C in air and used for comprehensive investigation of the lithiation/delithiation mechanisms of anode material undergoing an electrochemical conversion reaction with lithium ions. Surface (X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS)) and electrochemical (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) analytical techniques were combined. The results show that intercalation of Li in the Fe 2 O 3 matrix and solid electrolyte interphase (SEI) layer formation both precede conversion to metallic iron and Li 2 O upon lithiation. Depth profile analysis evidences stratification of the converted thin-film electrode into fully and partially lithiated outer and inner parts, respectively, due to mass transport limitation. The SEI layer has a stable composition (Li 2 CO 3 with minor ROCO 2 Li) but dynamically increases/decreases in thickness upon lithiation/delithiation. Conversion, proceeding mostly in the outer part of the electrode, causes material swelling accompanied by SEI layer thickening. Upon delithiation, lithium is trapped in the deconverted electrode subjected to shrinking, and the SEI layer mostly decomposes and reduces in thickness after deconversion. The nonreversibility of both conversion and surface passivation mechanisms is demonstrated.

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Potts, S.E.; Schmalz, L.; Fenker, M.; Díaz, B.; Swiatowska, J.; Maurice, V.; Seyeux, A.; Marcus, P.; Radnóczi, G.; Tóth, L.; Kessels, W.M.M.