Electroplating transition metal oxides

Abstract

The present disclosure generally relates to a method for electroplating (or electrodeposition) a transition metal oxide composition that may be used in gas sensors, biological cell sensors, supercapacitors, catalysts for fuel cells and metal air batteries, nano and optoelectronic devices, filtration devices, structural components, and energy storage devices. The method includes electrodepositing the electrochemically active transition metal oxide composition onto a working electrode in an electrodeposition bath containing a molten salt electrolyte and a transition metal ion source. The electrode structure can be used for various applications such as electrochemical energy storage devices including high power and high-energy primary or secondary batteries.

Description

FIELD OF THE INVENTION

The present disclosure generally relates to a method for electroplating (or electrodeposition) a transition metal oxide composition that may be used in gas sensors, biological cell sensors, supercapacitors, catalysts for fuel cells and metal air batteries, nano and optoelectronic devices, filtration devices, structural components, energy storage devices such as primary and secondary batteries. Transition metal oxides can also be used to coat battery electrodes to make Li batteries safer at high voltages. In one such exemplary embodiment, the transition metal oxide composition is an anodically active or a cathodically active material and is incorporated into a monolithic porous open-cell structure adapted for use as, or as a component of, a primary or secondary battery. The present disclosure also describes how to manufacture highly pure metal oxide as a final product using low purity starting precursors. Using less refined, lower purity precursors, the present invention can reduce the final cost of the metal oxide. The method proposed herein demonstrates that high purity traditional transition metal oxides (TMOs) that perform as good as or in some cases better than TMOs can be synthesized and formed into an electrode from starting materials with purities as low as 50%. This technology should enable the direct use, or preliminary purifications, of some common ores of transition metals (TM).

BACKGROUND OF THE INVENTION

Lithium ion batteries (LIBs) have spurred great interest in the realm of energy crisis owing to their high energy densities. They are comprised of a cathode, which host Li ions, a Li⁺ containing electrolyte submerged into a separator and an anode in which Li ions can be inserted into and de-inserted from. LIBs are ubiquitous devices where they find useful applications ranging from watches, cell phones, laptops and electric vehicles. In a commercial LIB, graphite has been used as an anode where Li ions can be inserted at around 0.1-0.2V (versus Li+/Li). In a typical commercial LiCoO₂ electrode if coupled with graphite anode delivers an average output potential of 3.7V. This electrode configuration is one of the most widely used battery architecture in the LIB market. Graphite anodes, however are limited with a specific capacity (350 mAh/g) where next generation LIBs will require high density anode active materials. For this particular reason, metal oxides are promising alternatives for anode applications in LIBs. They exhibit three times higher specific capacities (1200 mAh/g) than that of the commercially available anodes (see Shijiao Sun, Xiangyu Xhao, Meng Yang, Zhaoyin Wen, Xiaodong Shen, Hierarchically ordered mesoporous Co₃O₄ materials for high performance li-ion batteries, Scientific Reports 6, Article number 19564, 2016). Although a LIB consisting of Co based metal oxide anode delivers a low output potential (2V), this feature has attracted chip and optic industries where lower operating voltages are required in order to protect electronics. It is also plausible to increase the output potential with Mn based metal oxide as an anode active material.

Transition metal oxides are long sought alternative anode active materials for lithium ion batteries owing to their high capacities. Myriad of methods have been tried to manufacture transition metal oxides. Conventional techniques such as wet chemical processes, solid state synthesis, hydrothermal, vapor-based depositions, electrodeposition in aqueous media have all been used to obtain metal oxides. For example, Z.-S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li and H.-M. Cheng, ACS Nano, 4, 3187 (2010) reported Co₃O₄ anode material prepared by a wet chemical process. Their material showed drastic capacity fade even after 10 cycles due to volume expansion and low conductivity. In addition, the reported method is time consuming and economically not viable for large scale production. Their reported poor electrochemical performance is also another bottleneck. Furthermore, this synthesis technique has a multi-step protocol in which the final material has to be mixed with other additives such as carbon, binder which is usually followed by coating the active particles via slurry casting machines. These steps require rigorous care and minor mistakes can cost money and time.

SUMMARY OF THE INVENTION

In the present invention, a new technique is described, which is both time and cost effective, and delivers good electrochemical performance. The present invention eliminates the use of conductive carbon and binder additives and solely relies on a highly pure active material plated on a 3D substrate or planar electrode. Compared to traditionally manufactured TMO's requiring high temperature processes, this method can be used to synthesize active material with a mild heat treatment (e.g., a 2 h treatment at 450° C. under atmospheric conditions), or even without any heat treatment at all. Both of these advantages greatly reduce the cost of the final product. Even without heat treatment, due to low temperature environment one can get less crystalline, but highly nano-sized final particles with desired structure. Furthermore, unlike the conventional way of making metal oxides, high purity of starting precursors can be substituted with low purity precursors (down to 50% purity), which decreases the cost of the final metal oxide product.

In general, the present invention discloses a method of electrodepositing a transition metal oxide, doped transition metal oxide or sodiated transition metal oxide onto the surface of a working electrode comprising the steps of:

(a) immersing a working electrode into a molten salt electrolyte comprising a transition metal ion source in the presence of an inert atmosphere,

(b) electrodepositing an electrochemically active transition metal oxide onto a surface of the working electrode from the molten salt electrolyte at a temperature in excess of the melting temperature of the molten salt electrolyte,

(c) removing the working electrode from the bath,

(d) rinsing the electrodeposited transition metal oxide,

(e) followed by heat treatment of the electrodeposited transition metal oxide.

In a preferred embodiment, the transition metal oxide is Co₃O₄, CoO, MnO₂, Mn₂O₃, Mn₃O₄ or mixture of Mn and Co metal oxide Mn_xCo_yO_z where x, y and z range from 0.1 to 4.

The molten salt electrolyte can comprise a hydroxide salt, a halide salt, a nitrate salt, a sulfate salt or a combination thereof. Preferably, the molten salt electrolyte comprises a hydroxide salt selected from the group consisting of NaOH, KOH, and mixtures thereof.

In a preferred embodiment the molten salt comprises NaOH and Co(OH)₂ the electrodeposited transition metal oxide is Na_xCo_yO₂, wherein x is between 0.1 to 1 and y is between 1 to 0.1. In another preferred embodiment, molten salt comprises NaOH and MnCl₂, and the electrodeposited transition metal oxide is Na_xMn_yO₂, wherein x is between 0.1 to 1 and y is between 1 to 0.1.

The electrodeposited transition meal oxide is preferably conformally coated onto the working electrode. In a preferred embodiment, the working electrode is a porous nanostructured component and the transition metal oxide is conformally coating onto the porous nanostructured component. The working electrode used for electrodeposition of the transition metal oxides can also be porous with 3D interconnected pore structures.

The thickness of the electrodeposited transition metal oxide preferably ranges from about 10 nm to about 100 μm. The electrodeposited transition metal oxide material can also be in the form of a powder, and wherein the powder can be scraped off.

The electrodeposition is carried out at relatively low temperatures ranging from 150° C. to 600° C. Preferably, the electrodeposition temperature is from about 300° C. to about 500° C.

In another embodiment of the present invention, the method of electrochemical deposition of transition metal oxides, doped transition metal oxides and sodiated transition metal oxide, comprises the steps of: making a plating bath, comprising NaOH, KOH, or fused melts of an NaOH/KOH eutectic mixture, dissolving a transition metal, providing a conductive substrate, and depositing an electrochemically active transition metal oxide material on the substrate at relatively low temperatures ranging from 150° C. to 600° C. The conductive substrate used for electrodeposition preferably comprises at least one of nickel, tungsten, copper, gold, platinum, titanium, and carbon. In a preferred embodiment, the conductive substrate used for electrodeposition is porous with 3D interconnected pore structures.

One of the embodiments of this invention was demonstrated by synthesizing Co₃O₄ particles. In this example, three-dimensional stainless-steel fibers (SSF) were used as the working electrode. In order to obtain conformal coating of each fiber with Co₃O₄, Applicants' electroplating method is applied, which will be detailed later. These fibers cannot be coated conformally with conventional methods such as slurry casting. One way to synthesize Co₃O₄ conformaly on a 3D substrate has been demonstrated via aqueous media electrodeposition methods. (see Spataru, N.; Terashima, C.; Tokuhiro, K.; Sutanto, I.; Tryk, D. A.; Park, S.-M.; Fujishima, A. J. Electrochem. Soc. 2003, 150, E337) However, this method has the drawback of nonuniform coating and isolated particles at the edges of substrate. These problems prevent the use of such methods to scale up at industrial levels. Electroplating in non-aqueous molten salts are on the other hand offers better ionic conductivity and better ion diffusion. High surface area of the 3D substrates provides enhanced Li-ion transportation due to better electrode-electrolyte interface. The interior pores of SSF provide free void spaces to suppress structural strain as a result of Li insertion/extraction processes. Thus, unlike other studies such as the ones reported by G. Huang, S. Xu, S. Lu, L. Li, H. Sun, ACS Applied Materials & Interfaces, 6, 7236 (2014), Y. Fan, H. Shao, J. Wang, L. Liu, J. Zhang, C. Cao, Chemical Communications, 47, 3469 (2011), Y. Li, B. Tan, Y. Wu, Nano Letters, 8, 265 (2008), X. W. Lou, D. Deng, J. Y. Lee, J. Feng, L. A. Archer, Advanced Materials, 20, 258 (2008) and N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X. Hu, X. Kong, Q. Chen, The Journal of Physical Chemistry C, 116, 7227 (2012), significantly better cycle life and relatively stable voltage responses are easily achieved. The synthesis temperature of the metal oxide is as low as 150° C. This enables the formation of uniform, crack free, interconnected Co₃O₄ nano-flakes which directly adhere to the substrate. Of particular significance is that this technique renders the ability to control crystallinity, morphology and surface area of the active materials by just adjusting the electrodeposition parameters such as applied potential and interval time between each pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Cyclic voltammetry of NaOH/KOH bath containing Co(OH)₂ at 5 mV/s scan rate at 170° C. Peak broadening when SSF is used as working electrode is due to high surface area of SSF. Two current scales were drawn for comparison.

FIG. 2A. Voltage pulses and open circuit voltage (OCV) recorded in each cycle.

FIG. 2B. Current responses obtained from voltage pulses.

FIG. 3. XRD patterns of Co₃O₄ nano-flakes plated on SSF and after heat treatment at 450° C. Asterisks show the peaks from pure SSF.

FIG. 4. A-) HRSEM figure of Co₃O₄ nano-flakes obtained from KOH/NaOH eutectic bath and heat treated at 450° C. B-) An image showing the fiber and interconnected Co₃O₄ nano-flakes. C-) A HRSEM figure showing conformal plating of Co₃O₄ across the electrode. See the far bottom fibers. Inset figure shows the bare SSF before electroplating. D-) Photographs of the electroplated SSF and Ni foam.

FIG. 5. Charge-discharge profiles of Co₃O₄ containing half-cell constructed in a coin cell versus Li foil. The cell was cycled between 0.01V and 3V at different rates showing excellent power capabilities. Inset figure shows stable cycle life of the battery for the 70 cycles.

FIG. 6. Cycling and power capabilities of a full Li-ion cell employing electroplated LiCoO₂ and Co₃O₄ electrodes exhibiting outstanding power capabilities and cycling.

FIG. 7. Charge-discharge profiles of electroplated Mn based anode cycled versus Li metal. The electrode cycled between 0.01V and 3V potentials at C/2 rate. First and second cycles were shown in solid and long dashed lines, respectively.

FIG. 8. Cyclic voltammetry of NaOH/KOH bath containing 50% pure Co(OH)₂ at 5 mV/s scan rate at 170° C.

FIG. 9. Charge-discharge profiles of Co₃O₄ obtained from low purity Co(OH)₂ precursor. The half-cell constructed in a coin cell versus Li foil. The cell was cycled between 0.01V and 3V.

DETAILED DESCRIPTION OF THE INVENTION

Electroplating of a transition metal oxide (TMO) was carried out using a 3-electrode system where a working electrode, a counter (Ni foil), and a pseudo reference (Co metal) electrode were immersed into a eutectic solution, which is also called a molten salt, containing a transition metal ion source.

The transition metal ion source can be aluminum, copper, chromium, cobalt, manganese, nickel, silver, gold, tin, platinum, zinc, tungsten, tantalum, rhodium, molybdenum, titanium, iron, zirconium, vanadium, hafnium, and the alloys thereof. The transition metal ion source in the plating bath can further comprise at least one of an oxide doping agent selected from the group consisting of Al₂O₃, AlOH₃, and combinations thereof.

The working electrode can be an electrically conductive material selected from the group consisting of electrically conductive carbon, metal, metal alloys, metallic ceramics, oxides, polymers, and combinations thereof. Preferably, the working electrode is an electrically conductive metal selected from the group consisting of aluminum, copper, chromium, cobalt, manganese, nickel, silver, gold, tin, platinum, zinc, tungsten, tantalum, rhodium, molybdenum, titanium, iron, zirconium, vanadium, hafnium, and the alloys thereof.

The eutectic system can provide a relatively low synthesis temperature. The eutectic temperature is known as the melting point which is lower than any composition made up of the mixture. Above the eutectic temperature, the liquid phase is generally called molten salt. In the present invention, the term of molten salt system is used to define all liquid phases including eutectic composition. For electrodepositing on a working electrode, a molten salt system should have low temperature to protect the working electrode and possess high solubility of transition metal sources. Low-temperature molten salt is usually selected by checking the eutectic points in the phase diagrams. The molten salt bath is prepared with at least one of the following chemicals: including hydroxides (KOH, NaOH, RbOH, CsOH etc), halides (KF, KCl, NaCl, NaF, NaBr, KBr, NaI, KI, AlCl₃ etc), nitrates (NaNO₃, KNO₃), nitrites (NaNO₂, KNO₂), and sulfates (Na₂SO₄, K₂SO₄). A Ni crucible was used as the reaction vessel and a glass lid was used to hang abovementioned electrodes into the eutectic solution. All three electrodes were connected to a power supply which provides sufficient voltage or current densities where electrochemically active transition metal oxide materials were produced. These materials are of great interest to the battery, optic, catalyst, sensor and supercapacitor industries.

FIG. 1 shows the Cyclic voltammetry (CV) of a molten salt system (KOH/NaOH eutectic mixture) which contains CoOH₂ (98% purity) species. The working electrode is employed as both SSF and Pt foil to evaluate if there are any impurities possibly dissolving from SSF that can be deposited. As can be seen from the CV profiles, in both cases Co′ ions are being oxidized at relatively similar potentials i.e. above 0.6V. For SSF case, the broad oxidation peak is due to the high surface area of the fibers compared to Pt foil. Both profiles resemble each other smoothly suggesting that no impurities are present in the fibers. The sharp peak appearing at around 1.2V is due to the oxidation of OH— groups originating from KOH and LiOH salts, which are the major components of the molten salt. From these results, Applicants demonstrated that starting with 0.6V, any voltage pulses up to 1.4V will produce Co₃O₄ nano-flakes. Low potential (e.g. 0.6-0.7V) pulses will require longer on-time while high potential (e.g. 1.4V) needs less on-times. Applicants find the optimum voltage pulse to be 1.2V with is on-time. All voltage values are reported versus Co/Co²⁺ pseudo reference electrode. During electroplating, the rest time is crucial as this affects the conformal coating of SSF with Co₃O₄ nano-flakes. As can be seen from FIGS. 2A and 2B, approximately 2 minutes rest time between each voltage pulse was sufficient to allow Co²⁺ ions to diffuse to the pores of the SSF. Stable current responses in FIG. 2B are an essential indicator of conformal Co₃O₄ deposition. If the rest time is less than 2 minutes, the OCV readings would rise and current responses would diminish which would lead to non-conformal coating and surface clogging of Co₃O₄ electrode which are parameters that would lead to catastrophic electrochemical failure of the cell.

The molten salt bath temperature typically is around 150-170° C. which is achieved by mixing NaOH and KOH salts. The molar ratio of NaOH/(NaOH+KOH) was 0.515 in order to get to the eutectic point. One can still melt the salt mixture with different molar ratio at the expense of increasing the temperature. The eutectic mixture can be also reinforced by using CsOH and/or RbOH to extend low temperature solubility of molten salt. It should be also noted if the molten salt is achieved by only melting NaOH, the final product will form sodiated metal oxides which are receiving great attention as they can be used as cathode materials for Na-ion batteries. See, for example, YasuhikoTakahashi, YoshitoGotoh, JunjiAkimoto, Single-crystal growth, crystal and electronic structure of NaCoO₂, Journal of Solid State Chemistry Volume 172, Issue 1, April 2003, Pages 22-26, incorporated by reference herein. The Co source could be CoO, CoOH₂, CoCl₂, CoSO₄, or Co(NO₃)₂ salts. In one example, 110 g of KOH, 50 g NaOH, and 8 g CoOH₂ are added to the bath and monitored until they are dissolved thoroughly. The color of the melt changed from transparent color to blue as the divalent Co′ ions are coordinated by hydroxide ions. This is followed by immersing the 3-electrode lid into the molten salt. Afterwards, 1.2V (versus cobalt wire reference electrode) potential pulses were applied. The pulse range is from 100 μs to 20 s where a SSF, Ni foil and Co wire are used as working, counter, and pseudo reference electrodes, respectively. Between pulses, there was an open circuit voltage period (ranging from 3 s to 2 minutes). This resting period allows ions to move into the voids of SSF, so that conformal deposition can be achieved. The number of deposition cycles (duty cycle refers to on/off time) determines the loading of the sample. Constant voltage or current densities will also lead to the formation of the metal oxide; however, the electroplated material will not cover the 3D substrate conformally. Nevertheless, one can use this method to obtain powder form of the metal oxide. In Li-ion battery language, the Li ion insertion voltage is defined as the lithiation potential. The lower the lithiation potential for an anode material, the better output potential for a battery would be. This is because overall voltage of a battery is given by the equation of V_cell=V_cathode−V_anode. By simply changing the transition metal to a Mn source, one also can produce Mn₃O₄ material which has lower lithiation potential leading to higher voltage outputs in a full cell configuration.

To obtain Al doped Co₃O₄, in one example, 110 g of KOH, 50 g NaOH, 8 g CoOH₂, and 0.4 g Al₂O₃ or AlOH₃ were added to the bath and monitored that they were dissolved thoroughly. Afterwards, 1.2V (versus cobalt wire reference electrode) potential pulses were applied. The pulse range is from 100 μs to 20 s where a SSF, Ni foil and Co wire are used as working, counter and pseudo reference electrodes, respectively. Between pulses, there was an open circuit voltage period (ranging from 3 s to 2 minutes). This resting period allows ions to move into the voids of SSF thereby conformal deposition is achieved. To obtain Mg doped Co₃O₄, 110 g of KOH, 50 g NaOH, 8 g CoOH₂, and 0.4 g MgOH₂ were added to the bath and monitored that they were dissolved thoroughly. Afterwards, 1.2V (versus cobalt wire reference electrode) potential pulses were applied. The pulse range is from 100 μs to 20 s where a SSF, Ni foil and Co wire are used as working, counter and pseudo reference electrodes, respectively. Between pulses, there was an open circuit voltage period (ranging from 3 seconds to 2 minutes). This resting period allows ions to move into the voids of SSF thereby conformal deposition is achieved. To obtain Mn and Co co-plated MnCo₂O₄, 110 g of KOH, 50 g NaOH, 4 g CoOH₂, and 2 g MnCl₂ were added to the bath and monitored that they were dissolved thoroughly. Afterwards, 1.2V (versus cobalt wire reference electrode) potential pulses were applied. The pulse range is from 100 μs to 20 s where a SSF, Ni foil and Co wire are used as working, counter and pseudo reference electrodes, respectively. Between pulses, there was an open circuit voltage period (ranging from 3 s to 2 minutes). This resting period allows ions to move into the voids of SSF thereby conformal deposition is achieved. Different ratio of Mn, Co will lead different stochiometric of Mn_xCo_yO_z product. See for example, Guoyong Huang, Shengming Xu, Zhenghe Xu, Hongyu Sun and Linyan Li, Core-Shell Ellipsoidal MnCo2O4 Anode with Micro-/Nano-Structure and Concentration Gradient for Lithium-Ion Batteries, ACS Appl. Mater. Interfaces, 2014, 6 (23), pp 21325-21334, incorporated herein by reference.

After electroplating metal oxide on the working electrode, the electrode is quickly rinsed with deionized water thoroughly to ensure no residual KOH or NaOH salts remain. Since CoOH₂ is not soluble in water, a chelating agent, such as citric acid, would help to dissolve Co²⁺. If CoOH₂ traces are not removed, during heat treatment this would form another type of Co₃O₄ particles that are not formed during electroplating. Removal of Co′ ions can be simply done by immersing the electrode into an approximately 0.4M citric acid aqueous solution for 1 minute. The rinsing can also be performed by using pure eutectic solution, where no Co species are present, to dissolve the CoOH₂ particles dried on the working electrode. At the end of either rinsing procedure, a warm water rinse is performed where the electrode is left in warm water for 20 minutes in order for water to diffuse into the pores of the fibers. This ensures full dissolution of any residual LiOH and KOH salts. Subsequently, the electrode was dried in an oven and heated up to 450° C. for 2 h under ambient atmosphere. The heat treatment can go up to higher temperatures as long as the substrate is stable. Applicants have treated the samples from 300° C. to 600° C. and found the optimum temperature to be around 450° C. FIG. 3 portrays the XRD patterns of the final Co₃O₄ nano-flakes electroplated onto SSF. It has a cubic crystal structure with a phase group of Fd3m and all diffraction peaks can be indexed with the standard Joint Committee on Powder Diffraction Standards (JCPDS) card no. 42-1467. The peak broadening seen in the pattern is due to nano-sized Co₃O₄ flakes. The XRD patterns of bare SSF and Co₃O₄ plated SSF showed that the peaks corresponding to plain SSF are shifted towards to higher degrees suggesting lattice shrinkage of SSF. This is due to the heat treatment of SSF at 450° C. for 2 h and exposure to the oxidative nature of molten salt.

FIG. 4 contains several High-Resolution Scanning Electron Microscopy (HRSEM) images along with optical photographs of the final products with different substrates. FIG. 4A displays desert rose like flakes that are connected to each other. The figure revealed that flakes have 100 nm wall thicknesses. Higher deposition potentials lead to larger crystalline sizes and thereby higher surface areas. Higher temperature and higher ion concentration can also lead to higher nucleation of metal oxide. FIG. 4B shows the coating thickness which was calculated to be around 10 μm, which lead to high power densities. FIG. 4C shows the conformal Co₃O₄ plating on SSF. Images of the fibers collected at different depth of fields (located far behind the front fibers) showed Co₃O₄ active materials. Inset figure of FIG. 4C shows the HRSEM image of plain fibers.

One aspect of this invention is also elimination of conductive carbon and polymeric binder additives where both additives decrease volumetric energy densities of the battery. In addition, they increase the production cost of active materials for Li-ion batteries. Although Applicants prefer to utilize 3D stainless steel fiber scaffolds as a working electrode, it can be any geometrical shape as long as it is conductive. A conductive electrode is required in order for electron transfer. For example, the following materials can be used as working electrodes: Ni foil, commercial Ni foam, Ni—Cr mesh, stainless steel, platinum, and copper foils. Having such flexibility to choose working electrode during electroplating allows this technique to be implemented in currently used commercial production lines immediately. FIG. 4D presents two 3D scaffolds, namely SSF and Ni foam, which are electroplated with Co₃O₄ nano-flakes. One can also synthesize powder form of metal oxide on a metal substrate, highly conductive graphite, or polymeric components. After deposition the materials can be scraped off and grinded for further processing.

One example to obtain Co₃O₄ is to mix 110 g KOH, 50 g NaOH and 8 g CoOH₂ together and heat up to 170° C. under inert atmosphere to prevent hydrolysis of molten salts and oxidation of Co²⁺. For this purpose, Applicants used a glove box specially designated for electroplating experiments. Using the same setup, it is also possible to produce the same metal oxide outside of the glove box with an inert atmosphere (supplied by either nitrogen or Argon gases) pressure more than 1 atm. The pressure is an important factor as it will disable any hydrolysis of KOH or LiOH salts or oxidation of CoOH₂ species present in the molten salt. Similar to Co₃O₄, different Mn based oxides (Mn_xO_y) were produced also with the same eutectic mixture except that CoOH₂ was replaced with MnCl₂. Depending on the applied potential, different forms of manganese oxides can be obtained. Among the Mn oxides having higher than Mn²⁺ oxidation state, MnO₂ is the most stable in the air. However, one can obtain Mn₃O₄ or Mn₂O₃ as well by changing the deposition parameters. The MnCl₂ source can be replaced with MnSO₄, MnOOH, Mn₂O₃ or MnO. Instead of applying anodic electrodeposition (1.2V versus Co metal pseudo reference electrode), cathodic electrodeposition of MnO₂ can also be attained. In the presence of KMnO₄, Mn⁷⁺ ions can be reduced to Mn⁴⁺ and form MnO₂ by applying cathodic potential of 1.2V versus Co metal. Many transition metals can be doped into Mn or Co based metal oxides with the appropriate stoichiometry. These transition metals can be Al, Fe, Cr, Ti, Ni, V, Cu, Zn or other non-transition metals like Mg and Ca. A low temperature environment is crucial as this will lead to working electrode options to expand and protect the electrode from adverse effect of high temperature.

In one experiment, Applicants employed stainless steel fibers (SSF) as the 3D scaffold working electrode. The plating procedure is as follows: 1.2V (versus Co/Co²⁺) voltage pulses for is on-time followed by 2 minutes rest between each voltage pulses. This ensured transition metal ions to diffuse inner pores of 3D scaffold leading to conformal plating. Approximately, 10 cycles of these pulse plating cycles resulted in a 2 mAh/cm² loading electrodes with around 1200 mAh/g specific capacity, i.e. more than 90% capacity of reported practical values in the literature referenced in this provisional patent application. If any of the plating parameters are changed, the electrochemical performance of final product would change dramatically. For example, if the rest time between each pulse is 1 minute then the conformal plating is not achieved leading to poor electrochemical performance. The electrochemical charge and discharge profiles of Co₃O₄ on SSF were plotted in FIG. 5. The coin cell was constructed with Co₃O₄ anode deposited onto SSF versus Li foil and separated with a commercially available 25 um polymer separator. The conventional electrolyte was used which has the formulation of 1.15M LiPF₆ salt dissolved in EC/EMC/DEC at the ratio of 20/60/20 v/v. The lithiation and delithiation potentials are in agreement with practical potentials reported by G. Huang, S. Xu, S. Lu, L. Li, H. Sun, ACS Applied Materials & Interfaces, 6, 7236 (2014). Color coded charge-discharge profiles in FIG. 5 displays the different Co₃O₄ delithiation C-rates. A C-rate is a measure of the rate at which a Li-ion battery is discharged or charged relative to its practical capacity. For example 1C charge rate means that the charge will take 1 hour or 2 C charge rate means that the charge will take 30 minutes. Even at high rates (10 C-discharging in 6 minutes) the material holds 80% of the initial capacity, an outstanding feature compare to available literature reported by Z.-S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.-M. Cheng, ACS Nano, 4, 3187 (2010), X. Hu, H. Huang, J. Zhang, J. Shi, S. Zhu, N. Su, RSC Advances, 5, 99899 (2015), Y. Fan, H. Shao, J. Wang, L. Liu, J. Zhang, C. Cao, Chemical Communications, 47, 3469 (2011) and N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X. Hu, X. Kong, Q. Chen, The Journal of Physical Chemistry C, 116, 7227 (2012). Inset figure of FIG. 5 exhibits excellent cycle life in a half cell where almost no capacity fade was observed at C/2 rates (slow rate is usually used for cycle life assessments as it correctly establishes the structural robustness). This electrode delivers approximately 2 mAh/cm² practical capacity.

In another example, a Co based full cell was assembled employing an electroplated LiCoO₂ cathode and a pre-cycled Co₃O₄ anode. Pre-cycling was a necessary step to avoid Li plating which exacerbates the cycle life of the battery even in the first 10 cycles. As can be seen from FIG. 6, the output potential of the battery is around 2-2.2V during discharge. This potential can be used for chip and optic industries where less than 2.5V output potentials are required. Currently, these industries are employing either NiMH or NiCd low voltage (1.2V) secondary batteries. Ni based batteries are toxic in addition to their memory effect which necessitates frequent full discharge and possess high self-discharge which requires recharging after storage. Power tests revealed that 70% specific capacity was retained at 10 C (6 minutes discharge). Cycle life tests were carried out for around 50 cycles and no capacity degradation was observed. This can be seen in the inset of FIG. 6.

In another example, the molten salt is achieved only by melting NaOH, the final product will form sodiated metal oxides which are receiving great attention as they can be used as cathode materials for Na-ion batteries. The Co source could be CoO, CoOH₂, CoCl₂, CoSO₄, or Co(NO₃)₂ salts. In one example, 50 g NaOH, and 0.5 g CoOH₂ are added to the bath and monitored until they are dissolved thoroughly. The color of the melt changed from transparent color to blue as the divalent Co′ ions are coordinated by hydroxide ions. This is followed by immersing the 3-electrode lid into the molten salt. Afterwards, 1.2V (versus cobalt wire reference electrode) potential pulses were applied. The pulse range is from 100 μs to 20 s where a SSF, Ni foil and Co wire are used as working, counter, and pseudo reference electrodes, respectively. Between pulses, there was an open circuit voltage period (ranging from 3 s to 2 minutes). This resting period allows ions to move into the voids of SSF, so that conformal deposition can be achieved. The number of deposition cycles (duty cycle refers to on/off time) determines the loading of the sample. Changing the applied potential (starting from 0.6V to 1.4V) will lead to different stochiometric sodiated metal oxide. Similarly, in another example, 50 g NaOH, and 0.5 g MnCl₂ are added to the bath and monitored until they are dissolved thoroughly. 1.2V (versus cobalt wire reference electrode) potential pulses were applied. The pulse range is from 100 μs to 20 s where a SSF, Ni foil and Co wire are used as working, counter, and pseudo reference electrodes, respectively. Between pulses, there was an open circuit voltage period (ranging from 3 s to 2 minutes). This resting period allows ions to move into the voids of SSF, so that conformal deposition can be achieved. One can tune the plating parameters e.g. constant or pulse voltages, constant or pulse currents or combination of them in order to achieve different morphology and structure of the final material.

Mn-based transition metal oxide was also obtained in another embodiment of the present invention. In order to obtain MnO₂, 110 g KOH, 50 g NaOH and 8 g MnCl₂ was mixed together and heated up to 170° C. under inert atmosphere. The plating procedure is as follows: 1.2V (versus Co/Co²⁺) voltage pulses for is on-time followed by 2 minutes rest between each voltage pulses. This ensured transition metal ions to diffuse into inner pores of the 3D scaffold leading to conformal plating. One striking feature of Mn-based anode material is that it has lower lithiation potential which ultimately increases the output potential of a full cell favoring its application even to consumer electronics. FIG. 7 shows the voltage profiles of the first (black) and second (red) cycles of Mn₃O₄ electrode tested versus Li foil in a coin cell. The potentials and capacities resemble the data reported by Jie Yue, Xin Gu, Liang Chen, Nana Wang, Xiaolei Jiang, Huayun Xu, Jian Yang and Yitai Qian, J. Mater. Chem. A, 2014, 2, 17421-17426. Based on the aforementioned Mn₃O₄ studies, a 3V output potential is anticipated if employed in a full cell versus LiCoO₂. Although the conductivity, and hence the power capabilities of Co based anode materials are better than Mn-based ones, the lower lithiation potential provides much higher output potentials. Applicants are synthesizing Mn—Co composites that are expected to have low lithiation potentials without compromising the power capabilities. Other transition metal doping, such as Al³⁺, and metal ion doping, e.g. Mg²⁺, Ti′ can further stabilize the structure.

In another example, FIG. 8 shows the CVs of two NaOH/KOH molten salt systems containing low purity (50%) of Co(OH)₂ in which the impurities were 25% CoSO₄, 25% MgOH₂ in which the bath temperature was set to 170° C. The plating procedure is as follows: 1.2V (versus Co/Co²⁺) voltage pulses for is on-time followed by 2 minutes rest between each voltage pulses. This ensured transition metal ions to diffuse inner pores of 3D scaffold leading to conformal plating. Approximately, 10 cycles of these pulse plating procedures resulted in a 2 mAh/cm² loaded electrodes with around 1200 mAh/g specific capacities, i.e. more than 90% capacity of reported practical values in the literature referenced in this provisional patent application. The 50% pure Co(OH)₂ powders were prepared by mixing 98% Co(OH)₂ powders with the appropriate percentages of CoSO₄ and Mg(OH)₂, commonly found impurities in Co(OH)₂ (see Processing Considerations for Cobalt Recovery from Congolese Copperbelt Ores, B Swartz, S. Donegan, S. Amos, Hydrometallurgy Conference 2009, The Southern African Institute of Mining and Metallurgy, 2009). The weight percentages of the CoSO₄ and Mg(OH)₂ impurities were 1/1. In the CV study, the Pt foil working electrode is employed to evaluate if there is any side reactions stemming from impurities of Co(OH)₂ precursors. As can be seen from CV profile, Co′ ions are started to be oxidized at relatively similar potentials of the bath containing highly pure CoOH₂ i.e. above 0.6V. Both CV profiles in FIGS. 8 and 1 resemble each other suggesting that no impurities are present during electroplating. As can be seen from FIG. 9, discharge capacity of Co₃O₄ electrode obtained from low impurity of Co(OH)₂ delivered identical values which is around 1000 mAh/g in the second cycle. Voltage profiles, also resembled each other suggesting that CoSO₄ and Mg(OH)₂ impurities do not have adverse effect on the final product of Co₃O₄. The electrochemical discharge profiles of Co₃O₄ plated from low purity CoOH₂ precursor on SSF were plotted in FIG. 9. The coin cell was constructed with Co₃O₄ anode deposited onto SSF versus Li foil and separated with a commercially available 25 um polymer separator. The conventional electrolyte was used which has the formulation of 1.15M LiPF₆ salt dissolved in EC/EMC/DEC at the ratio of 20/60/20 v/v. The lithiation and delithiation potentials bode well with the Co₃O₄ anode that was obtained from high purity CoOH₂ precursors.

In conclusion, the technique described above has several advantages and these are listed below:

It eliminates the use of binder and conductive carbon additives which increases the volumetric energy density of the battery

Provides faster way to obtain final electrode to be utilized in battery applications.

One can get the final product in less than 20 minutes.

Can convert metal hydroxide precursors to metal oxide counterparts (e.g. CoOH₂—Co₃O₄) at temperatures as low as 150° C.

By just changing the voltage and deposition parameters, this technique allows for tailoring the morphology and texture of desired metal oxide.

Single step synthesis which phases out any intermediate steps reported in above articles. These steps increase the manpower need and hence cost.

Preserving stable voltage and capacities, and high power capabilities of a battery without conductive additives.

Side products of the synthesis is 99.9% Co metal which can be channeled to other applications for industrial purposes.

Since the electroplating is element specific, the precursors do not need to be high purity, a requirement for conventional methods which spikes up the cost of the product.

Claims

1. A method of electrodepositing a transition metal oxide onto the surface of a working electrode comprising the steps of: (a) immersing a working electrode into a molten salt electrolyte comprising a transition metal ion source, NaOH and KOH, in the presence of an inert atmosphere,(b) electrodepositing an electrochemically active transition metal oxide onto a surface of the working electrode from the molten salt electrolyte at a temperature in excess of the melting temperature of the molten salt electrolyte,(c) removing the working electrode from the bath,(d) rinsing the electrodeposited transition metal oxide,(e) followed by heat treatment of the electrodeposited transition metal oxide, wherein the transition metal oxide is selected from the group consisting of aluminum oxide, copper oxide, chromium oxide, cobalt oxide, manganese oxide, nickel oxide, silver oxide, gold oxide, tin oxide, platinum oxide, zinc oxide, tungsten oxide, tantalum oxide, rhodium oxide, molybdenum oxide, titanium oxide, iron oxide, zirconium oxide, vanadium oxide, hafnium oxide, and mixtures thereof.

2. The method of claim 1, wherein the transition metal oxide is Co3O4, CoO, MnO2, Mn2O3, Mn3O4 or a mixture of Mn and Co metal oxide MnxCoyOz where x, y and z range from 0.1 to 4.

3. The method of claim 1, wherein the molten salt electrolyte further comprises a a halide salt, a nitrate salt, a sulfate salt or a combination thereof.

4. The method of claim 3, wherein the molten salt electrolyte comprises at least one of (i) a further hydroxide salt selected from the group consisting of RbOH, and CsOH, (ii) a halide salt selected from the group consisting of KF, KCl, NaCl, NaF, NaBr, KBr, NaI, KI, and AlCl3, (iii) a nitrate salt selected from the group consisting of NaNO3, and KNO3, (iv) a nitrite salt selected from the group consisting of NaNO2, and KNO2, and v a sulfate salt selected from the group consisting of Na2SO4, and K2SO4.

5. The method of claim 4, wherein the molten salt comprises NaOH and Co(OH)2, and the electrodeposited transition metal oxide is NaxCoyO2, wherein x is between 0.1 to 1 and y is between 1 to 0.1.

6. The method of claim 4, wherein the molten salt comprises NaOH and MnCl2, and the electrodeposited transition metal oxide is NaxMnyO2, wherein x is between 0.1 to 1 and y is between 1 to 0.1.

7. The method of claim 1, wherein the working electrode comprises an electrically conductive material selected from the group consisting of electrically conductive carbon, metal, metal alloys, metallic ceramics, oxides, polymers, and combinations thereof.

8. The method of claim 7, wherein the working electrode is an electrically conductive metal selected from the group consisting of aluminum, copper, chromium, cobalt, manganese, nickel, silver, gold, tin, platinum, zinc, tungsten, tantalum, rhodium, molybdenum, titanium, iron, zirconium, vanadium, hafnium, and the alloys thereof.

9. The method of claim 1, wherein the transition metal ion source comprises at least one of cobalt, manganese, nickel, copper, iron, chromium, vanadium, titanium, molybdenum, and tungsten, and combinations thereof.

10. The method of claim 9, wherein the transition metal ion source in the plating bath further comprises at least one of an oxide doping agent selected from the group consisting of Al2O3, AlOH3, and combinations thereof.

11. The method of claim 1, wherein the transition metal oxide is conformally coated onto the working electrode.

12. The method of claim 11, wherein the working electrode is a porous nanostructured component and wherein the transition metal oxide is conformally coated onto the porous nanostructured component.

13. The method of claim 12, wherein the working electrode used for electrodeposition of the transition metal oxides is porous with 3D interconnected pore structures.

14. The method of claim 1, wherein the electrodeposition temperature is in a range of from 150° to 600° C.

15. The method of claim 14, wherein the electrodeposition temperature is in a range of from 300° to 500° C.

16. The method of claim 1, wherein the thickness of the electrodeposited transition metal oxide ranges from 10 nm to 100 μm.

17. The method of claim 1 wherein the electrodeposited transition metal oxide material is in the form of a powder and wherein the powder can be scraped off.

18. A method of electrochemical deposition of a transition metal oxide, the method comprising: making a plating bath comprising NaOH and KOH, dissolving a transition metal, providing a conductive substrate, and depositing an electrochemically active transition metal oxide material on the substrate at temperatures ranging from 150° C. to 600° C., wherein the transition metal oxide is selected from the group consisting of aluminum oxide, copper oxide, chromium oxide, cobalt oxide, manganese oxide, nickel oxide, silver oxide, gold oxide, tin oxide, platinum oxide, zinc oxide, tungsten oxide, tantalum oxide, rhodium oxide, molybdenum oxide, titanium oxide, iron oxide, zirconium oxide, vanadium oxide, hafnium oxide, and mixtures thereof.

19. The method of claim 18, wherein the conductive substrate used for electrodeposition comprises at least one of nickel, tungsten, copper, gold, platinum, titanium, and carbon.

20. The method of claim 19, wherein the conductive substrate used for electrodeposition is porous with 3D interconnected pore structures.