Investigation Of Electrode/Electrolyte Interfacial Reactions Involving In Li Ion Batteries

The interfacial reactions are the key issues that relate to cycling ability, lifetime,chemical and physical stability, and irreversible capacity of a lithium ion battery (LIB). Inaddition, studies of interfaces of LIB are of significance in revealing the structure ofnonaqueous interfaces and developing relevant models and theories. The main scope of thisdissertation is developing an approach to use FTIRS (in situ, ex situ), EQCM, XPS, andToF-SIMS to investigate the interfacial reactions of LIB. We have tried to investigate theinterfaces of Sn, Sn-Co alloy, graphite, Cr₂O₃ and Cr₂S₃ anodes as well as V₂O₅ cathode. Themain experiments and results are given follow:1.Sn and Sn-Co alloy anodes were prepared by electroplating. The betterelectrochemical performance on Sn-Co alloy than Sri anode confirms that inactive Cocomponent can buffer against volume change of Sn component during the alloying/dealloyingprocess.2. The interfacial reactions of Sn thin film anode in 1 M LiPF₆/EC+DMC were in situinvestigated by MFTIRS and EQCM. When electrolyte is reduced, the measured massaccumulated per mole of electrons (mpe) values are smaller to the theoretical ones. However,in alloying/dealloying process, the measured mpe values are higher than the theoretical values.The lithiation/delithiation process was characterized by MFTIRS through the desolvation/solvation effect. The solvation/desolvation effect varies the concentration free solvent (sol:EC, DMC) and solvated solvent (Li(sol)_n⁺) as well as causes the shifts of IR bands (C=O, C-O,C-H). In situ MFTIRS studies revealed that Li(sol)_n⁺ species rather than free solvent wasreduced on Sn anode, and the reductive products of electrolyte are ROCO₂Li.3. The interfacial properties of Sn-Co alloy and graphite film anodes in 1 MLiPF₆/EC+DMC were investigated by using in situ and ex situ MFTIRS. In situ MFTIRSresults on Sn-Co alloy anode confirm that FTIRS is efficient to characterize thelithiation/delithiation process through desolvation/solvation effects. Taking the advantage ofgraphite thin film electrode with a high IR reflectivity which is prepared by spin coating, insitu IR spectra with an excellent signal-to-noise ratio are obtained. As the potential regions of electrolyte reduction and lithiation processes overlap partly on graphite anode, the change ofin situ IR spectra is frequently described to reduction of electrolyte and formation of SEI layer,rather than lithiation process. However, our in situ IR results suggest the variations of spectraare caused by the intercalation process. EIS studies confirm that the SEI layer is formed on acycled Sn-Co and graphite anodes, and ex situ MFTIRS determines that the layer consists ofROCO₂Li.3. Cr₂O₃ thin films grown by thermal oxidation of Cr metal were investigated as anodematerial for LIB in 1 M LiClO₄/PC. The initial capacity is larger than the theoretical capacitybecause of the decomposition of electrolyte. The stable charge/discharge capacity of460 mAh·g^-1 was obtained in the 3^rd-10^th cycles. XPS and PM-IRRAS reveal the maincomposition of SEI layer is Li₂CO₃. This chemical composition is stable but there arevariations of the surface contents during the conversion/deconversion process. The volumeexpansion on the lithiated sample, evidenced by ToF-SIMS, presumably generates cracks inthe SEI layer that are filled by the immediate decomposition of electrolyte, thus increasing thesurface content in Li₂CO₃. The volume shrink of the delithiated oxide, also evidenced byToF-SIMS, is thought to generate the loss of fragments of the SEI layer due to compressivestress. ToF-SINS results demonstrate that the conversion/ deconverson processes of Li withCr₂O₃ are limited, most likely by mass transport, even for ultra-thin films.4. Cr₂S₃ film grown by thermal treatment of Cr metal under H₂S atmosphere was testedas an anode material for LIB for the first time. The intensities of both cathodic and anodiccurrent peaks in CV curves are declined with increasing cycling number, which is partly dueto the exfoliation of active species. The strong irreversible cathodic peak, assigned to thereductive electrolyte decomposition, is not observed on Cr₂S₃ film. XPS studies indicate thatthe main composition of the SEI layer is Li₂CO₃ species, and its thickness is thinner than thatof cycled Cr₂O₃ anode. The XPS signal of S2p on the cycled sample is dramatically attenuatedfor the following two reasons: 1 .part of Cr₂S₃ particles are exfoliated from the sulfide film; 2.the surface is covered by SEI layer. The XPS studies suggest that Li is trapped and thevalence of Cr decreases for 3+ to 2+ after the electrochemical cycles.5. Interfacial reactions on nanostructured V₂O₅ thin film cathode were ex situinvestigated by utilization of the XPS technique. The CV results evidence that Li intercalation is quasi reversible in this range of potential 2.8-3.8 V. With the increasing of cycle number,the change of the V2p_3/2 core level peak results from the decrease of the higher bindingenergy peak at 517.9 eV (V⁵⁺)and the increase of the lower binding energy peak at 516.5 eV(V⁴⁺), which is due to the partial reduction of V from 5+ into 4+. This proportion decrease ofV⁵⁺ and V⁴⁺ reveals that Li is trapped in the oxide film. XPS and PM-IRRAS of V₂O₅ thinafter 15 charge-discharge cycles in 1 M LiClO₄/PC suggest the SEI layer consist of mainlyLi₂CO₃ species. However the formation of SEI is not as easy as on Cr₂O₃ anode, for both XPSand EIS do not detect the formation of SEI layer after 1 CV cycle.The results of this dissertation throw insight into electrode/electrolyte interfacialreactions, and are of significance in developing relevant fundamental theory. The present IRresults provide firstly an approach to probe the lithiation/delithiation process of LIB by in situFTIR reflection spectroscopy, and are also of importance for the analysis of other LIB systems,especially the powder electrode materials. The detail research on Cr₂O₃ anode by XPS andToF-SIMS, is helpful to understand the electrochemical processes and improve theirelectrochemical performance of transition oxide materials, which are intensively studiedrecently.