F. Cosandey

Electrochemical Performance of Acid-Treated Nanostructured LiMn1.5Ni0.5O4 − δ Spinel at Elevated Temperature

A surface treatment process based on a mild acidic solution was utilized to stabilize the surface of the LiMn 1.5 Ni 0.5 O 4-δ (LMNO) spinel cathode material and to improve its elevated temperature performance. To characterize the failure mechanism of the LMNO spinel at an elevated temperature (60°C), the effect of the Mn 3+ content and the charge/discharge state storage conditions were studied. It was shown that the existence of Mn 3+ is necessary for an improved elevated temperature performance. It was also identified that one of the main degradation mechanisms at an elevated temperature was the systematic impedance rise rather than the intrinsic capacity loss. The results of the charged state storage at 60°C demonstrated the worst condition for the spinel materials; however, the surface-treated materials presented an improved elevated temperature cycling and a much less impedance increase than the untreated spinel after 4 weeks of storage at 60°C. X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to characterize the effect of surface treatment on the crystal structure and morphology of the acid-treated material.

EELS Compositional Imaging of FeF2-C Nanocomposite Used as a New Positive Electrode in Li-Ion Batteries

Advances in Instrumentation and Techniques- Electron Energy Loss Spectroscopy for the 21st Century.

STEM/EELS Analysis of Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 Held at High Voltages

Ni rich Li(Ni0.8Co0.15Al0.05)O2 commonly known as NCA is being used commercially as Li-ion battery cathode material for its high discharge capacity. [1] The stability and related safety concerns at high voltage limit the use of NCA at 3.6V, where only 50% of Li can be extracted. Thus, only a part of theoretical capacity is achieved in this process. At high voltages (>4V), the layered structure of the bulk NCA does not change, however new surface phases are formed. To utilize the full potential of this material, high-voltage studies of surface phases, their chemical evolution and their mechanisms are needed. We present here the evolution of surface phases in NCA held at a constant voltage up to 4.75V.

STEM/EELS Analysis of Conversion Reactions in Cycled FeOF/C

Transition metal fluoride/carbon nanocomposites have been under extensive investigation in the past ten years due to their theoretical high capacity in the range of 500 to 800 mAh/g [1]. In this study, the structural changes of FeOF/C positive electrode during lithiation/delithiation have been studied as a function of number of cycles (up to 20) under constant cycling current of 50 mA/g and at 60°C. ADF-STEM imaging technique combined with electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED) were used to track the phase evolution of FeOF/C and chemical composition with bonding characteristics of phases present after cycling.

EELS Determination of Li Distribution and Fe Valence Mapping in Lithiated FeOF/C Nanocomposite Battery Materials

Amongst the techniques to investigate Li-ion battery materials, electron energy loss spectroscopy (EELS) play a unique role as Li distribution, chemical state and valence of transition metals (charge transfer) can all be determined with nanometer scale spatial resolution. In this work, we use EELS to investigate new positive electrodes for Li-ion batteries based on transition metal fluoride (FeF3, FeOF, FeF2, CuF2....)/C nanocomposites [1]. The high specific capacity in these new electrodes is obtained by using all the oxidation states of Fe from Fe to Fe during discharge cycles via a complete conversion process. In this study, we used Scanning Transmission Electron Microscopy (STEM) combined with EELS to determine the Li spatial distribution, its chemical state and the Fe valence state in FeOF/C nanocomposite electrodes during charge and discharge processes. This STEM-EELS analysis was done using a JEOL 2010F equipped with a Gatan GIF 200 spectrometer and with a Hitachi 2700 STEM equipped with an Enfina spectrometer. In order to minimize electron beam damage and F loss, the samples were cooled to LN2 temperatures and imaged with a total electron dose not exceeding 10 C/cm. Both lithiated (discharged) and delithiated (re-charged) FeOF/C nanocomposites electrodes were analyzed by EELS. The Fe valence state was obtained by measuring the Fe L3/L2 intensity ratio [2,3]. The L line intensities were obtained using either a 4.5 eV window or by taking the positive component of the EELS spectra second derivative. An ADF STEM image of a FeOF/C cathode material discharged to 1.5V is shown in Fig.1a with the corresponding low energy EELS signal (c.f. Fig. 1b) taken from area marked A revealing the superposition of the Li-K and Fe-M edges. The extracted Li-K edge has two prominent peaks whose energies are separated by 6.6 eV. In addition to the two prominent Li peaks, there is a third one located at a distance of 4.2 eV from the first peak. The existence of these peaks is indicative of the presence of two Li-base compounds (LiF) and a new Li-Fe-O-F cubic phase. The Li-K/Fe-M intensity map shown in Fig.1c from the area depicted in Fig.1a reveals the presence of Li and Fe rich phases with a spatial distribution in the 3-5 nm range. At this voltage the expected phases are LiF+Fe+LixFeOyFz [3]. At the surface, a 10-20 nm thick Li rich phase (c.f. Fig 1d) is observed corresponding to a mixed LiF-Li2CO3 solid electrolyte interface (SEI) surface layer. Upon lithiation, the Fe valence state decreases as represented by a decrease in the Fe-L3/L2 intensity ratio. At the lowest voltage of 0.8V, all Fe is in the metallic state. At the intermediate discharge voltage of 1.5 V, the Fe valence state is not uniform and the microstructure is composed of a mixture of high and low valence state phases as depicted in Fig. 2b. The O-K concentration map shown in Fig.2a has a similar distribution as the valence map of Fig. 2b which indicates that the oxygen rich phase is also the phase with highest valence state. A quantitative analysis of the Fe L3/L2 intensity ratios using standard model compounds (Fe, FeF2 and FeOF) as reference indicate a Fe valence state of 2.3 for this cubic LixFeOyFz phase. Upon recharge to 4.5 V, all the Fe in the electrode returns to its initial Fe valence state with the electrode material converting back to its initial rutile FeOF phase. [4].

EELS and HRTEM Analysis of Surface Phase in Nanostructured LiMn1.5Ni0.5O4 Battery Materials

The Ni doped LiMn1.5Ni0.5O4 spinel (Fd-3m) structure is an excellent candidate as Li-ion cathode material for high voltage battery applications due to its high capacity (> 130 mAh/g) with good room temperature cycling and high rate capabilities.. In order to improve the battery performance at high temperatures (60 C), we have introduced a surface modification process involving treatments in hydrofluoric (H-treated) or phosphoric (P-treated) acids followed by high temperature annealing. The surface modified LiMn1.5Ni0.5O4 spinel show drastically improved impedance and capacity retention with enhanced cycle life and rate capabilities over the untreated spinel. In this paper, we present EELS compositional and Mn valence map results as well as structural phase identification by combined HRTEM and image simulations showing the formation of a new surface phase induced by the surface acidic and annealing treatments.

EELS Analysis of Radiation Induced Structural Transformations in Fluoride Compounds

A new type of positive electrode for Li-ion batteries has been developed recently based on transition metal fluoride compounds (FeF3, FeOF, FeF2, CuF2, ...)[1]. In order to understand redox evolution and microstructural changes during discharge and recharge processes we are performing EELS analysis combined with high spatial resolution spectrum imaging. However, in view of the high beam sensitivity of fluoride compounds, it is necessary to understand the effect of radiation induced structural transformations caused by the high intense electron beam. In this study, we performed an EELS analysis of structural changes in fluoride compounds caused by irradiation with an intense nanometer-sized electron probe. One advantage of using EELS is that it can provide information on both the F loss and the associated changes in oxidation state of the transition metal. The results are presented in terms fluorine loss and electronic fine structure changes of the transition metals.

HRTEM and Diffraction Analysis of Surface Phases in Nanostructured LiMn1.5Ni0.5O4 Spinel

The Ni doped transition metal spinel LiMn1.5Ni0.5O4 is an excellent candidate as Liion cathode material for high voltage applications due to its high capacity (> 130 mAh/g) with good cycling and rate capabilities at room temperature. In order to improve its performance at high temperatures (60 °C), we have introduced a surface modification process involving treatments in hydrofluoric (H-treated) or phosphoric (P-treated) acids followed by high temperature annealing. The surface modified LiMn1.5Ni0.5O4 spinel show drastically improved impedance and capacity retention with enhanced cycle life and rate capabilities over untreated spinel. In this paper, we present HRTEM, diffraction and image simulation results on the formation of a surface phase induced by these surface acidic and annealing treatments.

A Nanoprobe Electron Diffraction Study of Surface Phases in LiCoO2

The LiCoO2 compound with trigonal structure (R-3m), is the most common positive electrode material used in Li-ion batteries. In this paper, we report nanoprobe electron diffraction results of two surface phases in LiCoO2 formed by two different heat treatments conditions. The nanoprobe diffraction patterns were taken at 200 kV with a Topcon TEM and using a 5nm probe size of convergence semi-angle α=2.4 mrad. Kinematical simulations of the nanoprobe diffraction patterns were done using JEMS program [1]. The diffraction patterns were analyzed with respect to published crystallographic data of stoichiometric LiCoO2 and Li deficient LixCoO2 phases [2, 3].

EELS Valence Mapping in Electron Beam Sensitive FeFx/C Nanocomposites

A new type of positive electrodes for Li-Ion batteries has been synthesized based on FeF{sub 2}/C and FeF3/C nanocomposites with particle size in the 8-12 nm range [1]. The measured high capacities rely on a complete reduction of Fe to its metallic state according to the following reaction: xLi{sup +}+xe{sup -} +Fe{sup x+}Fx = xLiF + Fe{sup 0}, where x=3 and x=2 for FeF3/C and FeF2/C respectively. This electrochemical reaction involves a change in valence state of Fe from 3+ or 2+ to 0 that can be determined uniquely by EELS from the peak energy of the L{sub 3} line and from the L{sub 3}/L{sub 2} line intensity ratio. In this paper, we report EELS mapping results on the electrochemical conversion processes and in particular the mapping of the Fe valence state before and after discharge. This work was performed with a Hitachi HF2000 equipped with a Gatan PEELS and with a FEI CM200 FEG TEM equipped with a Gatan GIF. Both instruments were operated in STEM mode at 200kV with an EELS collection half angle of {beta}=5 mrad and spectrum imaging software.