LiMn1.5Ni0.5O4, referred to herein as “LMNO”, has been explored as an electrode material for lithium ion batteries (LIBs) due to its improved cycling behavior relative to the pristine spinet. The nominal cost, enhanced thermal stability, enhanced rate capability owing to its three-dimensional structure, and the operating voltage window of LMNO make it a potential candidate for use in hybrid electric vehicles (HEV). However, it has not gained commercial usability in HEV due to high capacity fade during cycling at elevated temperatures and Mn3+ dissolution by HF. High capacity fade during cycling is also a problem with other currently available electrode materials such as LiCoO2, LiMn2O4, Li4Ti5O12, Li2MnO3, and LiNiMnCoO2.
Doping LMNO with ions has been considered to better the core properties of LMNO for enhanced electrochemical performance. However, doping alone cannot significantly improve the cycleability and capacity retention of LMNO or other LIB electrodes because it cannot avoid dissolution of Mn3+ ions by HF. Several researchers have used wet chemical methods including sol-gel methods to coat protective film over pristine LMNO. Although the protective coating improved cycling life and capacity retention of LMNO, there was always an unfortunate trade-off between decreasing the capacity and increasing cycle life of the battery. In these studies, the films were not conformally coated on the particle surfaces, and it was difficult to precisely control the thickness of the coating. The increased thickness causes increased mass transfer resistance that delays the movement of species, electrons, and ions.
A sol-gel method has been used to form an iron oxide coating on carbon nanorods and on SnO2 particles, but the resulting coatings were not sufficient to solve the afore-mentioned problems.
Disclosed herein is an electrode comprising at least one electrode particle, the at least one electrode particle comprising: a source of lithium ions, a coating of an oxide of a transition metal on the surface of the electrode particle, and the transition metal ions and/or the elemental transition metal doped under the surface of the electrode particle.
We have now discovered: a method of transition metal doping of lithium ion battery (LIB) electrode particles while simultaneously, using atomic layer deposition (ALD) on the LIB electrode particles, forming an ultra-thin film coating of the transition metal oxide on the LIB electrode particles so as to effect a synergistic or synergetic result. We have also discovered a product formed by the disclosed method, the product exhibiting the synergetic effect of the transition metal doping. In one embodiment of the invention, the transition metal is iron, and the ultra-thin film coating is iron oxide. In other embodiments, the transition metal doped may be, for example, cobalt or nickel, and the simultaneously deposited ultra-thin coating is cobalt oxide or nickel oxide, respectively.
In a fluidized bed reactor by ALD, we coated large quantities of LIB electrode particles with ultra-thin iron oxide films. We also disclose our discovery of a unique phenomenon of iron ions, and possibly elemental iron, entering the lattice structure of the LIB electrode particles during the ALD coating process. Herein we disclose evidence that the combined effect of the surficial partial doping of iron ions and/or elemental iron into the LIB electrode particles, along with the conductive optimal ultrathin coating of iron oxide films has significantly enhanced cycleability and reduced capacity fade of the LIB electrodes.
In one embodiment of the invention, the doping of iron ions and/or elemental iron into the LIB electrode particles during ALD comprises a penetration of the ionic Fe and/or elemental iron into the lattice structure of the LIB electrode particles. In another embodiment of the invention, during initial ALD cycles, the ionic Fe and/or elemental iron saturates a plurality of structural defects in the LIB electrode particle lattice structure, and then participates in formation of an ultrathin film of iron oxide on the LIB electrode particle surface.
In another embodiment of the invention, the doping of ionic Fe and/or elemental iron during the ALDcoating process is near the surface of the LIB electrode particles. The expression “near the surface,” as used herein means that some of the doped ionic Fe and/or elemental iron are on the surface of LIB electrode particles and some of the doped ionic Fe and/or elemental iron have penetrated beneath the surface of the LIB electrode particles, and some of the doped ionic Fe and/or elemental iron have penetrated inside the lattice structure of the LIB electrode particles.
The invention inter al/a includes the following, alone or in combination. One embodiment of the invention is an electrode particle comprising: a source of lithium ions, a coating of iron oxide on the surface of the electrode particle, and iron ions and/or elemental iron doped under the surface of the electrode particle.
In another embodiment of the invention the source of lithium ions comprises LiMn1.5Ni0.5O4 (LMNO).
In another embodiment of the invention the source of lithium ions comprises at least one of LiCoO2, LiMn2O4, Li4Ti5O12, Li2MnO3, Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC), and a layered LiMO2 component or a spinel LiM2O4 component, wherein “M” is predominantly Mn and/or Ni.
In yet another embodiment of the invention, the source of lithium ions comprises LiNixCoyAlzOa, for example, LiNiCoAlO2, or LiNi0.8Co0.15Al0.05O2.
Another embodiment of the invention is an electrode comprising particles comprising LiMn1.5Ni0.5O4, a coating of iron oxide on the surface of the particles, and elemental iron or iron ions doped under the surface of the particles.
An electrode according to an embodiment of the invention may comprise a metal or a carbon substrate at least partially coated with a mixture comprising a plurality of electrode particles, each electrode particle comprising a source of lithium ions, a coating of iron oxide on the surface of the electrode particle, and iron ions and/or elemental iron doped under the surface of the electrode particle; and a polymer binder. Almost any other transition metal may be suitable for use in an embodiment of the disclosed invention.
In various embodiments of the invention, ALD may be used to deposit an ultra-thin coating of almost any transition metal oxide onto the LIB electrode particles, while simultaneously doping the atoms or ions of that same transition metal beneath the LIB particle surface. In one embodiment of the invention, a LIB electrode particle comprises an iron oxide film coating of from about 0.1 nanometer to about 500 nanometers.
In another embodiment of the invention, a LIB electrode particle comprises an iron oxide film coating of from about 0.2 nanometer to about 1 nanometer.
In another embodiment of the invention, a LIB electrode particle comprises an iron oxide film coating of from about 0.2 nanometer to about 200 nanometer.
In another embodiment of the invention, a LIB electrode particle comprises an iron oxide film coating of from about 0.4 nanometer to about 100 nanometers.
In another embodiment of the invention, a LIB electrode particle comprises an iron oxide film coating of from about 0.6 nanometer to about 50 nanometers.
In another embodiment of the invention, a LIB electrode particle comprises an iron oxide film coating of about 0.6 nanometer.
Another embodiment of the invention is an electrode comprising: a metal or a carbon substrate at least partially coated with a mixture comprising a plurality of electrode particles, each electrode particle comprising a source of lithium ions, a coating of iron oxide on the surface of the electrode particle, and iron ions and/or elemental iron doped under the surface of the electrode particle; and a polymer binder.
Another embodiment of the invention is an electrode according to paragraph [0014], wherein the metal or carbon substrate is at least partially coated with a mixture comprising, respectively, an 80:10:10 weight percent (wt. %) mixture of LiMn1.5Ni0.5O4, carbon black, and a polymer binder.
Another embodiment of the invention is an electrode according to paragraph [0014], wherein the metal or carbon substrate is least partially coated with a mixture comprising from about 10 wt. % LiMn1.5Ni0.5O4 to about 90 wt. % LiMn1.5Ni0.5O4, carbon black, and a polymer binder.
Yet another embodiment of the invention is a method of preparing, in a fluidized bed reactor by ALD, electrode particles comprising: a source of lithium ions, a coating of iron oxide on the surface of the electrode particle, and iron ions doped under the surface of the electrode particle.
Yet another embodiment of the invention is a method of preparing, in a fluidized bed reactor by ALD, electrode particles comprising: a source of lithium ions, a coating of a transition metal oxide, such as, for example, nickel oxide (NiO) or cobalt oxide (CoO) on the surface of the electrode particle, and, the transition metal ion doped under the surface of the electrode particle. For example, respectively, nickel ions or cobalt ions are doped under the surface of the electrode particle. That is, Ni ion doping with NiO film coating would occur for a NiO ALD process; and Co ion doping with CoO film coating would occur for a CoO ALD process.
The disclosed process of simultaneously coating and doping LIB electrode particles in a single step using an ALD process can be used to prepare many different lithium ion battery electrodes, with little or only routine experimentation needed to adjust parameters.
The present invention has many advantages. The ALD coated samples prepared by the disclosed method demonstrated both higher initial capacity and longer cycle life with improved stable performance for more than 1,000 cycles of electrochemical cycling at room temperature and at 55° C.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of illustrative embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
A description of preferred embodiments of the invention follows. It will be understood that the particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. At the outset, the invention is described in its broadest overall aspects, with a more detailed description following.
The present invention is directed to conformal films of iron oxide coated on LMNO particles in a fluidized bed reactor by means of atomic layer deposition.
The inventors of the disclosed subject matter herein report their discovery of the synergetic or synergistic effect of the combination of forming by ALD an electrochemically active transition metal oxide film, for example, an iron oxide film coating on LIB particles, for example, LiMn1.5Ni0.5O4 (LMNO) particles, while simultaneously doping transition metal ions, for example, iron ions or elemental iron into the LMNO particles. In one embodiment of the invention, the doping of iron ions or elemental iron into the LMNO particles is surficial and partial. The elemental iron or the ionic Fe penetrates into the lattice structure of LMNO during initial ALD cycles. The elemental Fe or the ionic Fe further penetrates into the lattice structure of LMNO with the increasing number of ALD coating cycles. After the structural defects of the LMNO lattice are saturated, the transition metal, for example, iron (elemental Fe or ionic Fe) participates in formation of ultrathin films on LMNO particle surface. Owing to the conductive nature of iron oxide film, having the optimal film thickness of approximately 0.6 nm, the initial capacity improved by about 17% at room temperature (RT) and by about 24% at elevated temperature of 55° C. at 1C cycling rate. We herein disclose that the synergy of doping of LMNO particles with Fe ions surficially, that is, near the particle surface, combined with the conductive and protective nature of the optimal iron oxide film leads to unexpectedly high capacity retention (˜98% at RT and ˜95% at elevated temperature) even after 1,000 cycles at 1C cycling rate.
Iron oxide films coated on LiMn1.5Ni0.5O4 particles: Different numbers of iron oxide ALD coating cycles were applied on the surfaces of LMNO particles (4-5 μm, NANOMYTE® SP-10, NEI Corporation). ALD reaction was carried out in a fluidized bed reactor by atomic layer deposition. Ferrocene and oxygen were alternately dosed into the reactor at a temperature of about 450° C. for 10 (10Fe), 20 (20Fe), 25 (25Fe), 30 (30Fe), 40 (40Fe), 80 (80Fe), and 160 (160Fe) cycles. The transmission electron microscopy (TEM) image of an uncoated (UC) LMNO particle, shown in
In order to confirm the diffusion and distribution of iron inside the particle structure, about 80 nm thick thin section across the center of the 160Fe sample particle was cut using focused-ion beam (FIB) and elemental mapping was performed using energy dispersive x-ray spectroscopy (EDS).
The Fe content on the LMNO particles was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). As shown in
Based on the surface area of particles, percentage of Fe in the 160Fe sample obtained from ICP-AES, and assuming the oxide films being Fe3O4, the expected thickness of the ultrathin film was found to be about 6 nm. However, the TEM analysis showed the film thickness to be only 3 nm. This discrepancy also indirectly supported that Fe had entered the lattice structure of LMNO. To our knowledge, this unique phenomenon, doping of iron in LMNO has not been previously reported as having occurred during an ALD coating process.
The ALD reaction was carried out for 10 (10Fe), 30 (30Fe), 80 (80Fe), and 160 (160Fe) cycles,
The PXRD patterns of pristine and modified samples confirm the existence of cubic spinel structure. All the main diffraction peaks are sharp, which indicates that the tested samples are well-crystallized. The pattern for the UC differs significantly from the 160Fe sample. For the 160Fe sample, the main peaks are not so sharp and some of the peaks have a significant shift in their position, indicating a significant amount of Fe was doped or diffused into the LMNO structure. The weak reflections observed at around 18.2°, 30°, and 57.5° in the 160Fe sample are absent in the 10-Fe sample and only 30° peak in 30Fe and 80Fe. The presence of Fe3O4 was confirmed for the case of 160Fe by the additional peaks at 30° and 57.5°, which are consistent with reported results. The PXRD patterns, consistent with the SAED pattern, indicate that the iron oxide ALD coated LMNO does not have the same phase as its uncoated counterpart. X-ray photoelectron spectroscopy (XPS) results shown in
To get a better insight into the nature of Fe3O4 phase, Mössbauer spectroscopy was carried out for the 160Fe sample, since this sample had substantial amount of Fe for reliable Mössbauer signal. The room temperature (25° C.) Mössbauer spectrum of 160Fe of broad sextet indicates hyperfine magnetic component together with a central quadrupolar doublet, as shown in
In summary, the results from PXRD, TEM-SAED, STEM-EDS, and XPS strongly suggest that for the initial cycles of ALD, such as 5Fe and 10Fe, instead of deposition of thin film of iron oxide on the surface of the LMNO, some amount of Fe doping occurred. “Fe doping,” as the expression is used herein, means that in some valance state, iron ions penetrated into the lattice structure of LMNO. Although not being bound by theory, we believe that both the ALD formation of the Fe3O4 ultra-thin film stops and doping stops, which cessations could be due to saturation of surface defect sites. With increment in iron oxide ALL) cycles. Fe3O4 can be further oxidized to provide γ-Fe2O3, as here in the case of 160Fe.
Electrochemical testing: The charge-discharge analysis was carried out in a 3.5 V-5 V voltage range.
For these conditions, almost all the iron oxide ALD coated cells showed higher initial discharge capacity than the UC. We believe that the increased discharge capacity of iron oxide coated samples can be attributed to a synergetic effect between the doped Fe ion in the LMNO and conductive iron oxide overlayer on the LMNO particles.
In
The diffusional and kinetic overpotential, solid electrolyte interphase (SEI) layer induced resistance, and contact/olunic resistance are the main cause of the voltage drop in a typical LIB. The term “overpotential” relates to a cell's voltage efficiency. The existence of overpotential relates to a loss of energy as heat. The ultrathin iron oxide ALD film can significantly alter most of these causes of the voltage drops. However, if the Li concentration ratio between the particle surface and the bulk is not affected by the coating, then the overpotential cause by the diffusional forces remains unchanged. The layer formed on the electrode surface (known as solid permeable interface) is usually much thinner than the SEI layer formed on the anode surface, and its thickness increases with charge-discharge cycling and the temperature.
The 10Fe samples showed higher initial capacity than the 20Fe and 25Fe, which is in agreement with the different C rate results; however in the long run, the capacity declined very significantly. This could be explained by the same reason that the Fe doped into the near surface structure of the LMNO helped improve the initial capacity of the material and the iron oxide coating, which occurred after more ALD cycles (as in 20Fe and 25Fe) gave stability to the material. The 80Fe sample showed poor stability over the testing time of 1,000 charge-discharge cycles. The reason could be that it has relatively thicker coating than other coated samples. The thick film induces more stresses during lithium ion insertion and deinsertion. These increased stresses combined with more mass transfer resistance of Lr due to the relatively thick films as compared to 30Fe/40Fe lead to poorer performance of the 80Fe sample, With increase in charge-discharge cycling, less Li+ inserted into electrode due to the increasing thickness of the SEI layer on lithium. This would explain the worst performance of the 80Fe sample.
The drawback of coating on electrode particles is slower species transport. Consequently, a demonstration of performance improvement via ALD coatings at high C rate is significant because the diffusivity of ions in the solid phase becomes significant as the input current increases. Also, the inside temperature of a cell increases with faster charge-discharge cycle rate, and that also increases the stress level due to developed concentration gradient inside particles. There is also a possibility of phase transition at the particle surface from over-lithiation during this cycling process. Therefore, in order to examine the performance of these coated cells, they were cycled at 2C rate, shown in
The interface change due to ALD thin film coating was further investigated using electrochemical impedance spectroscopy (EIS). See
Table 1A at room temperature and Table 1B at 55° C. below provide impedance parameters using equivalent circuit models for electrodes made of pristine, 10Fe, 20Fe, 25Fe, 30Fe, 40Fe, 80Fe coated LiMn1.5Ni0.5O4 particles:
Table 1 provides the list of all the fitted parameters value obtained after fitting the impedance curves to an equivalent circuit . The semicircle from the impedance analysis of all the cells was fitted using a combination of two R|C units (resistodcapacitor) to represent surface-film and charge-transfer resistance, R(f+ct). For clarification, the lines in resultant impedance curves were not obtained after fitting the equivalent circuit to the impedance curves. One semicircle was observed for the UC and the iron oxide ALD coated cells, as shown in
EIS study was also performed at high temperature (55° C.), as shown in
The 30Fe cell showed the best results among all the other cells tested. With increase in charge-discharge cycling, the charge-transfer and the film resistance increased, and the difference between the UC cell and the coated cells grew significantly. For example, after 1,000 charge-discharge cycles, the combined film and charge transfer resistance of the 30Fe was 173.9 Ω, while it was 300.3 Ω for the UC cell, which was greatly increased from the value of the fresh cell. The resistance values explain that the kinetics of the surface film developed on the electrodes. Rohm values for the UC sample and the other samples are not the same. The difference could be due to the structure modification of LMNO by iron doping and iron oxide coating. The 30Fe sample performs the best as compared to the other samples. This is because of the lowest charge transfer and film resistance of the 30Fe sample. For the 20Fe sample, the film was just not thick enough to provide good protection as compared to the optimal coating of 30Fe. Lower charge transfer and film resistance could also mean that more Li+ ions are available at the 30Fe electrode surface, thereby compensating for increased diffusion resistance. The lower film resistance is due to the conductive iron oxide film coating. The trend of the charge transfer and the film impedance values confirm that the 30Fe sample has the optimal ultrathin coating of iron oxide.
Pellets of only the UC (uncoated), 20Fe, 30Fe, 40Fe, 80Fe, and 160Fe particles were prepared for the conductivity measurements. The ac complex plane impedance analysis was used for the experiment and the same impedance analyzer was used to obtain the impedance curves shown in
where, σ0 is the pre-exponential factor, kB is the Boltzmann contant, T is the absolute temperature, and Ea is the activation energy for Li ion movement.
Conclusions: We have successfully demonstrated that the cycle life and the capacity retention of LMNO can be significantly improved by the synergistic or synergetic effect of ultrathin film coating of iron oxide by ALD combined with the simultaneous Fe ionic doping near the surface of the LMNO particles. The ionic Fe penetration into the lattice structure of LMNO was verified by cross-sectional STEM-EDS of iron oxide coated samples and the ultra-thin iron oxide films were directly observed by TEM. Mössbauer and XPS results confirmed the valance state of the iron for the ALD coated samples. It can be seen that the 30Fe sample has a high initial capacity of 143 mAh/g, which is about 25% higher than that of the UC sample. It shows 93% capacity retention after 1,000 cycles at room temperature. More importantly, at elevated temperatures, the 30Fe sample performs the best as compared to the UC sample and other iron oxide coated samples. We herein report for the first time the synergistic effect of doping and thin film coating on LMNO particles.
The foregoing data shows that an ALD coating of iron oxide provided much better improvement in performance of LMNO than what could potentially be due to only doping effect. ALD has the potential to prepare these ultrathin electrochemically active films with optimal thickness and synergetic effect of simultaneous conductive coating and element doping, providing novel electrodes that are durable as well as functional at high temperature and fast cycling rates. Further in depth analysis of this novel product and method could provide a major breakthrough in solving the current problems in the field of energy storage.
The ALD coating was carried out in a fluidized bed reactor, by methods known in the art and described below. There are filters employed to contain the particles in the reactor, while allowing only gas to pass. Ferrocene (99% pure, from Alfa Aesar®) and oxygen (99.9%) were used as precursors, and were delivered into the reactor in alternate doses at 450° C.. Ferrocene was delivered into the reactor using a heated bubbler and nitrogen was used as a carrier gas. Then N2 was used to purge the reactor to remove any unreacted ferrocene and by-products. After that, O2 was fed into the reactor, followed by another N2 purge. All lines were heated to 120° C. to avert any vapor deposition. In one embodiment of the disclosed method, the steps are substantially as follows.
Step 1. The particle ALD coating was carried out in a fluidized bed reactor (FBR). The FBR is connected to a steel plate and it is balanced on four large springs which helps isolate the vibration generated from two three-phase AC Fibro-motors (Martin Engineering, IL, USA). The vibration frequency is controlled by speed controller (VS1MX, Baldor Drives, St. Louis, Mo.). The LiMn1.5Ni0.5O4 particles (NANOMYTE® SP-10, 4-5 μm, NEI Corporation. USA) to be coated were introduced into reactor. Filters were employed to contain the particles in the reactor, while allowing only gas to pass. Then the temperature was increased to a point using a split-furnace (CM Furnaces Inc., NJ, USA) wherein the surface reaction could occur. After that, the minimum fluidization velocity was measured using LabView® program and then the particles were outgassed for 5 hrs at 250° C.
Step 2. Ferrocene (99% pure, from Alfa Aesart, USA) and oxygen gas (99.9%, Airgas Inc., St. Louis, Mo.) were used as precursors, and were delivered into the reactor in alternate doses at 450° C. Ferrocene was delivered into the reactor using a heated bubbler (115° C.) and nitrogen gas (99,99%, Airgas Inc., St. Louis, USA) was used as a carrier gas. Then N2 was used to purge the reactor to remove any unreacted ferrocene and by-products. All the gases were controlled using mass flow controllers (MKS Instruments Inc., USA). After that, O2 was fed into the reactor, followed by another N2 purge, All lines were heated to 150° C. to avert any vapor deposition inside the lines. The ALD process has two half-reactions for the two precursors to complete one ALD cycle. ALD process was controlled precisely using a custom-made LabView® program. First, the ferrocene was introduced into the reactor to react with the native hydroxyl groups present on the surface, and then the unreacted precursor and any by-products were flushed out of the reactor using nitrogen (inert) gas. The reactor was purged with vacuum, and then only the second precursor was introduced in the reactor. Again, the unreacted precursor and any by-products were flushed out of the reactor using inert gas. The reactor was purged with vacuum.
Step 3. We repeated the process described in step 2.
Step 4. We stopped the reaction cycles. First, we cooled down the reactor and the precursor bubbler to room temperature, meanwhile passing the nitrogen gas through the reactor. The product of the disclosed process comprises doped and coated LiMn1.5Ni0.5O4 particles, an embodiment of which is shown schematically in
An 80:10:10 wt. % mixture of LiMn1.5Ni0.5O4, carbon black (Super P conductive, 99+%, Alfa Aesar, USA) and polymer binder poly(vinylidene fluoride) (Alfa Aesar, USA) was used to prepare electrodes. The slurry of the mixture was spread on the aluminum-foil, and then it was dry-heated at 120° C. The electrode discs were made after punching the coated foil. The reference/counter electrode was Li metal (99,9% trace metal basis, Sigma-Aldrich, USA) and LiPF6 (1 mol/L in a mixed solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate with a volume ratio of 1:1:1, MTI Corporation) in all the cells prepared. The CR2032 cells fabrication was carried out in an Ar-filled glove box. An embodiment of the disclosed electrode is schematically shown in
The charge-discharge analysis was carried out using an 8-channel battery analyzer (Neware Corporation) for 3.5 to 5 V potential range at various C rates, and at different temperatures (room temperature and 55° C.). The electrochemical impedance spectroscopy of the prepared cells were carried out using an IviumStat impedance analyzer. The EIS analysis was performed at 5 mV and 0.01-1M Hz frequency range. Conductivity measurements were carried out using the same analyzer for cold pressed pellets of the samples. The pellets were coated with Ag (paste from Sigma Aldrich) on both sides to act as the blocking electrodes. These pellets were vacuum-dried at ˜85° C. for 6 hr. The analyses were performed for a range of 1 Hz to 1 MHz and at 1 mV. The test temperature range was 20° C. to 55° C. The spectra were analyzed using Zview software (Scribner Associates, Inc.). The conductivity tests were performed to compare the coated and uncoated samples and to examine the conductive nature of the coating with respect to the substrate only. Necessary steps were taken to make sure that all the cells and pellets were exposed to the same conditions for their respective batches of experiments.
Inductively coupled plasma-atomic emission spectroscopy was used to quantify the mass percent of iron on the particles. The iron oxide films were verified using a FEI Tecnai F20 field emission gun high resolution TEM equipped with energy dispersive X-ray spectroscopy (EDS) system. To check the Fe element distribution within the particles, about 80 nm thick thin section across the center of the particle was prepared by focused ion beam, using an FEI Q3D dual-beam system. The thin section was subsequently checked by a JEOL 2010F TEM in both TEM mode and scanning TEM mode at 200 kV acceleration voltage.
The crystal structure of the uncoated and coated particles was determined via powder X-ray diffraction (Phillips Powder Diffractometer, CuKα radiation, λ=1.5406 Å). The PXRD analysis was performed using a scan rate of 2°/min and a step size of 0.2°.
The selective area electron diffraction (SAED) patterns obtained, aligned along their main axis, from the UC and the 160Fe samples shown in
X-ray photoelectron spectroscopy (XPS, Kratos Axis 165) was used to study the oxidation state of Fe by employing Al K (α) excitation, operated at 150 W and 15 kV. The peaks were corrected with C 1s at 284.6 eV. The values for the Fe2p3/2 peak reported in the literature differ by 0.9 eV between two extreme values 710.6 eV and 711.5 eV. As shown in
For 40Fe, a faint peak at 710.1 eV represents Fe3O4. That could explain an overall slightly better result of 30Fe compared to 40Fe. Also, a small peak broadening at 707 eV was observed in the XPS spectra of 30Fe and 40Fe indicates that there could be Fe with different valance state in the 30Fe and 40Fe sample. There is also a small satellite peak at ˜717 eV in the 30Fe and 40Fe samples, which indicates the existence of Fe2O3, as reported previously. Iron at an intermediate oxidation state in Fe3O4 with a mixture of Fe(II) and Fe (III) presents a BE value of 710.2 eV. The Fe3+ component of Fe 2p3/2 in γ-Fe2O3 is at 710.1 eV. The peak-shifts for 30Fe, 40Fe, and 80Fe samples are because the main difference between the two sets of samples is coordination of the Fe3+cations. In the α-compounds, the crystal structure is oriented in such a way that all of the cations are octahedrally coordinated. In the γ-compounds, on the other hand, three-quarters of the Fe3+cations are octahedrally coordinated whereas the other quarter of the cations are tetrahedrally coordinated. This also explains the satellite peaks, as proven in literature, the XPS Fe 2p spectrum of 40Fe possesses smaller satellite intensity as compared with that of 30Fe due to the larger Fe 3d to O 2p hybridization in 40Fe, which has higher amount of γ-Fe2O3. The formation of a conformal iron oxide, e.g., Fe2O3 ALD layer on the surface can act as an artificial solid permeable interface (SPI) layer and helps prevent electrolyte decomposition at higher voltage. The PXRD and SEED results indicate that Fe in some form of oxidation state has penetrated into the lattice structure of LMNO.
Fe Mössbauer spectroscopy was performed on the as-prepared, chemically oxidized, and different state-of-charge electrode materials in transmission aeometry using a constant acceleration spectrometer equipped with a 57Co (25 mCi) gamma source embedded in Rh matrix. The instrument was calibrated for velocity and isomer shifts with respect to α-Fe foil at room temperature. The resulting Mössbauer data were analyzed using Lorentzian profile fitting by RECOIL software.
Recent studies have shown that there were oxygen vacancies in Fd3m disordered structure of LMNO. It was also said that these defects were in the diffusion path of lithium ions (Li+). During iron oxide ALD process, a typical cycle involves introducing gaseous precursors into the reactor system in a sequence ensuring no mixing of both the precursors. It is proposed that during the ferrocene dose, half surface reaction that leads to formation of Fe ions on the surface of I:NINO could penetrate inside the lattice structure in some valance state due to the defects present in Li+ diffusion path. This occurred during initial cycles of iron oxide ALD. After the defect sites were saturated with Fe, conformal iron oxide film would form on the surface of LMNO particles. The doping of Fe near the surface of the coating coupled with the ultrathin optimal iron oxide are responsible for significant enhancement of cycleability, improvement in initial specific capacity and high capacity retention. This also helps explain the discrepancy between the observed thickness of iron oxide films on LMNO particle surfaces by TEM and the calculated film thickness based on iron content on LMNO particles from ICP-AES analysis.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 62/297,817 filed on Feb. 20, 2016, the teachings of which are incorporated herein by reference in their entirety.
Filing Document | Filing Date | Country | Kind |
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PCT/US2017/018642 | 2/21/2017 | WO | 00 |
Number | Date | Country | |
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62297817 | Feb 2016 | US |