DESC:
TECHNICAL FIELD
The present disclosure relates to the field of electrochemical energy and storage. Particularly, the present disclosure relates to electrode material comprising at least one lead-based oxide and an electrochemical device having said electrode material. Also disclosed are methods of synthesis of lead-based oxides, method of forming a battery having said electrode material, a battery pack, etc.

BACKGROUND AND PRIOR ART
Batteries store chemical energy and converts the same to an electrical energy. The process involves flow of electrons from one electrode to another, providing an electric current through an external circuit. In order to balance the flow of electrons, the charged ions also flows through an electrolyte in contact with both the electrodes. Different reactions take place depending on the type of electrodes and electrolytes used in a battery and these determine the effect of a battery.

Some batteries are primary or disposable while others are secondary or rechargeable. Following is a list of points on some of the prior art documents:

In 2003, Tarascon group reported the Cumulative (Copper) Displacement (Lithium) Insertion (CDI) reaction in Cu2.33V4O11 [1].
Long back in 1987, before SONY’s commercial use of LIB in 1991, or use of carbon (unavailability for sodium insertion) or dendrite-risky Na metal, the low voltage Na-Pb alloying reaction was used commercially in a patent by Showa Denko K.K., Hitachi, Ltd. And Allied-Signal, Inc. to show long life of a sodium-ion cell with NaxCoO2 [9].
Tin was widely used as Li-alloying center of choice in amorphous glass, perovskite, and numerous other structures as lithium-ion battery anodes following Fujifilm Celltec Co. Ltd.’s 1994 seminal patent on use of tin-based ATCO glass in camera batteries as LIB anode with energy density higher than graphite [8].
Conversion reaction in nano-sized transition-metal oxides (e.g. Co3O4, Fe2O3) for negative-electrode materials for lithium-ion batteries (reported in 2000) [10].
Na2Ti3O7 was reported as the lowest voltage (ca. 0.3 V) oxide insertion anode for sodium-ion batteries (circa 2011) [11].

Although lithium-ion and sodium-ion batteries are widely used, there is still a need to develop batteries having high performance. One can look at various options to develop batteries such as the use of alternate materials used in anode or cathode, the mechanism by which they are used as electrode materials and yet retaining potential/even a greater potential compared to conventionally known batteries. The present disclosure achieves this and overcomes the limitations associated with the prior art by providing an electrode material, batteries having said electrode material and its applications thereof.

SUMMARY OF THE DISCLOSURE
The present disclosure relates to electrode material comprising non-alkali metal ion, non-transition metal ion, particularly at least one lead-based oxide. The disclosure can be used in electrodes to make safe electrochemical devices such as lithium-ion and sodium-ion batteries, particularly in applications where a high-energy density is preferred to a high-power delivery. Thus, the electrode material of the present disclosure provides for high electrochemical performance. The electrochemical performance of the lead-based oxides is explored at extremely low current and in the widest voltage range.

In an embodiment of the present disclosure, the electrode material comprises at least one lead-based oxide selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the electrode is anode or cathode or both.

In an embodiment of the present disclosure, the lead-based oxide show capacity values of over 300 mAh/g at low current value of at least 10mA/g.

In an embodiment of the present disclosure, the lead-based oxide show capacity values of over 300 mAh/g at low current value of at least 10mA/g in the first cycle or after first charge.

The present disclosure also relates to a method of synthesis of lead-based oxide used as electrode materials in an electrochemical device, wherein the lead-based oxide is synthesized comprising steps of:
preparing a precursor solution of metal nitrate(s); and
mixing and heating the precursor solution of metal nitrate(s) to initiate drying and combustion, resulting in the formation of lead-based oxide; and
optionally calcining the lead-based oxide in air.

In an embodiment of the present disclosure, the lead-based oxide is PbTi3O7.

In an embodiment of the present disclosure, the lead-based oxide is PbLi2Ti6O14.
In an embodiment, PbLi2Ti6O14 of the present disclosure shows high-capacity values at low current and low voltages.
In an embodiment, PbLi2Ti6O14 of the present disclosure is tested at low current and its capacity is significantly higher than any previous report on such compounds.
The present disclosure also relates to a method of forming an electrochemical device comprising the electrode material, wherein the method comprises:
forming an anode comprising at least one lead-based oxide; and forming a cathode electrode in contact with the anode electrode through an electrolyte, resulting in the electrochemical device; or
forming a cathode electrode comprising at least one lead-based oxide and forming an anode electrode in contact with the cathode electrode through an electrolyte, resulting in the electrochemical device; or
forming anode or cathode comprising at least one lead-based oxide and bringing them in contact with each other through an electrolyte, resulting in the electrochemical device.

The present disclosure also relates to a method of improving electrochemical performance of electrochemical device, said method comprises step of employing electrode material comprising at least one lead-based oxide selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3 in the device.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the disclosure may be readily understood and put into practical effect,
reference will now be made to exemplary embodiments as illustrated with reference to
the accompanying figures. The figures together with detailed description below, are
incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the
present disclosure where:

Figure 1: provides (a) Rietveld refinement of XRD (Cu Ka) pattern of PbTi3O7 synthesized by solution combustion in a single step at 500 oC (top) and annealed at 900 oC for 2 hours (bottom). (b) SEM, and TEM SAED micrographs of PbTi3O7 made by solution combustion method and annealed for 2 hours at 900 oC. Experimental data points (red circles, annealed; blue circles, single step combustion synthesis), calculated pattern (black line), their difference (green line), and Bragg reflections of possible phases (red sticks, PbTi3O7; blue sticks, PbTiO3) are shown. Inset shows the representative crystal structure of PbTi3O7 along b axis (Pb = black balls, TiO6 = distorted cyan octahedra).

Figure 2: illustrates synthesis and structure of MLi2Ti6O14 family (M = divalent ion): (a) Rietveld refinement of XRD (Cu Ka) pattern of members of the MLi2Ti6O14 family (M= Sr, Ba, Pb) formed by solution combustion method (900 oC, 2h). Experimental data points (grey dots), calculated pattern (black line), their difference (blue line), and Bragg reflections (red bars) of the respective isostructural phases are shown. (b) Microscopy analysis: (left panel) SEM-EDX images showing homogenous element distribution; (middle panel) TEM bright-field images of a single crystallite; (right panel) HRTEM images primarily showing lower index planes, and SADP showing clear diffraction spots indexed to their respective zone axes. (c) projects a labeled diagram of the MLTO structure showing the occupied polyhedra and vacant sites. (d) illustrates the lithium migration paths overlaid on the MLTO (M = divalent ion) structure with 2D tunnels. The fluorescent yellow polyhedra depict presence of alkali ions.

Figure 3: shows electrochemical performance of PbTi3O7 in CMC binder: Galvanostatic voltage-capacity profiles (a, d), differential capacity (b, e), and cycling stability/ capacity retention (c, f) plots of PbTi3O7 versus lithium (solution combustion PbTi3O7) and sodium (annealed PbTi3O7) respectively.

Figure 4: depicts charge storage mechanism of Li in PbTi3O7. Bright field TEM micrographs of (a) pristine PbTi3O7, and (b, c) ex-situ samples obtained after (de)lithiation post charge (2.5 V). XRD (Cu Ka) pattern of ex situ PbTi3O7 samples obtained at different voltage points in the 10th cycle. The scale bars in (a), (b) and (c) indicate 100 nm, 20 nm, and 2 nm respectively. The vertical dashed lines in (e) indicates the Bragg positions of metallic Pb.

Figure 5: (a) BVSE analysis of the migration barrier of divalent M ions in MLTO family. (b-d) Electrochemical performance of MLTO at low currents (C/50) in a wide voltage window (0.01 – 2.5 V). (i = interstitial site, s = highest energy saddle point). (PbLTO = PbLi2Ti6O14, BaLTO = BaLi2Ti6O14, SrLTO = SrLi2Ti6O14).

Figure 6: illustrates impact of exiting lead ion on PbLTO’s electrochemical performance in the subsequent cycles, and its indication on structure evolution: Voltage profiles at C/100 of PbLTO in (a) full window (0.01 – 3V), and (b) restricted window (0.5 – 3V). Electrochemical performance in the (c) upper window (1 – 3V), and (d) lower window (0.01 – 1V) after one full discharge. (PbLTO = PbLi2Ti6O14).

DETAILED DESCRIPTION OF THE DISCLOSURE
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent product/methods do not depart from the scope of the disclosure.

Definitions:
Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

As used herein, the singular forms ‘a’, ‘an’ and ‘the’ include both singular and plural referents unless the context clearly dictates otherwise.

The term ‘comprising’, ‘comprises’ or ‘comprised of’ as used herein are synonymous with ‘including’, ‘includes’, ‘containing’ or ‘contains’ and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. Likewise, certain terms may be interchangeably used throughout the specification and thus have the same meaning even when they are referred interchangeably.

The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed.

The present disclosure relates to electrode material comprising non-alkali metal ion, non-transition metal ion, particularly at least one lead-based oxide. The disclosure can be used in electrodes to make safe electrochemical devices such as lithium-ion and sodium-ion batteries, particularly in applications where a high-energy density is preferred to a high-power delivery. Thus, the electrode material of the present disclosure provides for high electrochemical performance. The electrochemical performance of the lead-based oxides is explored at extremely low current and in the widest voltage range.

In an embodiment of the present disclosure, the lead-based oxide is selected from a group comprising at least one of PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the electrode is anode or cathode or both.

In an embodiment of the present disclosure, the electrode is anode.

In an embodiment of the present disclosure, the electrode is cathode.

In an embodiment of the present disclosure, the lead-based oxides show capacity values of over 300 mAh/g at low current value of at least 10mA/g.

In an embodiment of the present disclosure, the lead-based oxides show capacity values of over 300 mAh/g at low current value of at least 10mA/g in the first cycle or after first charge.

In an embodiment of the present disclosure, the electrode comprising lead-based oxides show capacity values of over 300 mAh/g at low current value of at least 10mA/g.

In an embodiment of the present disclosure, the electrode comprising lead-based oxides show capacity values of over 300 mAh/g at low current value of at least 10mA/g in the first cycle or after first charge.

In an embodiment of the present disclosure, the lead-based oxide is PbTi3O7.

In an embodiment of the present disclosure, the lead-based oxide is PbLi2Ti6O14.
In an embodiment, PbLi2Ti6O14 of the present disclosure shows high-capacity values at low current and low voltages.
In an embodiment, PbLi2Ti6O14 of the present disclosure is tested at low current and its capacity is significantly higher than any previous report on such compounds.
The present disclosure also relates to an electrochemical device comprising the electrode material comprising at least one lead-based oxide selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the electrochemical device is a battery.

In another embodiment of the present disclosure, the battery is a primary or secondary battery.

In an embodiment of the present disclosure, the electrode is anode or cathode or both.

In an embodiment of the present disclosure, the battery comprises:
an anode;
a cathode; and
electrolyte.
wherein the anode or cathode or both comprise at least one lead-based oxide selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the electrolyte is selected from a group comprising lithium-based electrolytes, sodium-based electrolytes and combinations thereof.

The present disclosure also relates to a method of synthesis of lead-based oxides which are used as electrode materials in an electrochemical device.

In an embodiment of the present disclosure, the lead-based oxides are synthesized by wet solution combustion synthesis route.

In an embodiment of the present disclosure, the method of synthesis of lead-based oxide comprise:
preparing a precursor solution of metal nitrate(s); and
mixing and heating the precursor solution of metal nitrate(s) to initiate drying and combustion, resulting in the formation of lead-based oxide; and
optionally calcining the lead-based oxide in air.

In an embodiment of the present disclosure, the lead-based oxide is selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the metal nitrate is selected from a group comprising lead nitrate, lithium nitrate, titanyl nitrate, zirconium nitrate and combinations thereof.

In an embodiment of the present disclosure, the precursor solution is initially heated at about 150 oC followed by heating at about 500 oC.

In an embodiment of the present disclosure, the lead-based oxide is calcined in air at about 800-900 oC for 2 h.

In an embodiment of the present disclosure, the method of synthesis of lead-based oxides requires fuel selected from a group comprising glycine, urea, sucrose, ascorbic acid, citric acid and combination thereof along with metal nitrates(s) which act as oxidizers.

In an embodiment, the present disclosure relates to synthesis of PbTi3O7.

In an embodiment of the present disclosure, the method of synthesis of PbTi3O7 comprise:
preparing a precursor solution of lead nitrate and titanyl nitrate; and
mixing the precursor solution of lead nitrate and titanyl nitrate followed by heating to initiate drying and combustion, resulting in the formation of PbTi3O7; and
optionally calcining the PbTi3O7 in air.

In an embodiment of the present disclosure, the lead nitrate and the titanyl nitrate is taken at a ratio of about 1:3.

In another embodiment, the present disclosure relates to synthesis of PbLi2Ti6O14.

In an embodiment of the present disclosure, the method of synthesis of PbLi2Ti6O14 comprise:
preparing a precursor solution of lead nitrate, lithium nitrate and titanyl nitrate; and
mixing the precursor solution of lead nitrate, lithium nitrate and titanyl nitrate followed by heating to initiate drying and combustion, resulting in the formation of PbLi2Ti6O14; and
optionally calcining the PbLi2Ti6O14 in air.

In an embodiment of the present disclosure, the lead nitrate, lithium nitrate and the titanyl nitrate is taken at a ratio of about 1:2:6..

The present disclosure also relates to a method of forming an electrochemical device having electrode material comprising at least one lead-based oxide.

In an embodiment of the present disclosure, the lead-based oxide is selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the electrode is anode or cathode or both.

A method of forming an electrochemical device comprising electrode material comprising at least one lead-based oxide, wherein the method comprise:
forming an anode comprising at least one lead-based oxide; and forming a cathode electrode in contact with the anode electrode through an electrolyte, resulting in the electrochemical device; or
forming a cathode electrode comprising at least one lead-based oxide and forming an anode electrode in contact with the cathode electrode through an electrolyte, resulting in the electrochemical device; or
forming anode or cathode comprising at least one lead-based oxide and bringing them in contact with each other through an electrolyte, resulting in the electrochemical device.

In an embodiment of the present disclosure, the lead-based oxide is selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the method of forming an electrochemical device comprising electrode material comprising at least one lead-based oxide comprises:
forming an anode electrode comprising at least one lead-based oxide; and
forming a cathode electrode in contact with the anode electrode through an electrolyte, resulting in the electrochemical device.

In an embodiment of the present disclosure, the method of forming an electrochemical device comprising electrode material comprising at least one lead-based oxide comprises:
forming an anode electrode; and
forming a cathode electrode comprising at least one lead-based oxide in contact with the anode electrode through an electrolyte, resulting in the electrochemical device.

In an embodiment of the present disclosure, the method of forming an electrochemical device comprising electrode material comprising at least one lead-based oxide comprises:
forming an anode electrode comprising at least one lead-based oxide; and
forming a cathode electrode comprising at least one lead-based oxide in contact with the anode electrode through an electrolyte, resulting in the electrochemical device.

In an embodiment of the present disclosure, the lead-based oxide is selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.

In an embodiment of the present disclosure, the electrochemical device is a battery.

In another embodiment of the present disclosure, the battery is a primary or secondary battery.

In an embodiment of the present disclosure, the electrode is anode or cathode or both.

In one embodiment of the present disclosure, the cathode is a conventional electrode when anode comprises at least one lead-based oxide.

In an embodiment of the present disclosure, the cathode electrode is selected from a group comprising but not limiting to LiCoO2, LiFePO4 and NMC compounds.

In another embodiment of the present disclosure, the anode is a convention electrode when cathode comprises at least one lead-based oxide.

In one embodiment of the present disclosure, both anode and cathode comprise at least one lead-based oxide.

In an embodiment of the present disclosure, the electrolyte is selected from a group comprising, lithium-based electrolytes, sodium-based electrolytes and combinations thereof.

The present disclosure also relates to a method of improving electrochemical performance of electrochemical device, said method comprises step of employing electrode material comprising at least one lead-based oxide selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3 in the device.

The present disclosure also relates to a battery pack comprising one or more cells wherein each cell comprises electrode material comprising at least one lead-based oxide.

In an embodiment, the present disclosure finds application in, but not limiting to, electronic and automotive devices.

In an embodiment, the electrode materials show high electrochemical performance at extremely low current and in the widest voltage range. The electrode materials act by mechanisms but not limiting to Cumulative Displacement Insertion Alloying (CDI-A) and Reversible Conversion Intercalation Alloying (RCIA), that includes displacement and conversion reaction. In the CDI-A insertion reaction, Pb(II) ion loss is compensated by positive charge of lithium ion.

In an embodiment of the present disclosure, the lead-based oxide show capacity values of over 300 mAh/g at low current value of at least 10mA/g.

In an embodiment of the present disclosure, the lead-based oxide show capacity values of over 300 mAh/g at low current value of at least 10mA/g in the first cycle or after first charge.
In an embodiment, PbLi2Ti6O14 of the present disclosure shows high-capacity values at low current and low voltages.
In an embodiment, PbLi2Ti6O14 of the present disclosure is tested at low current and its capacity is significantly higher than any previous report on such compounds.
The present disclosure also relates to use of lead-based oxides as electrode materials.

Lead-containing oxide materials such as PbTi3O7 and PbLi2Ti6O14 (under low voltage/current) structures have not been used so far in rechargeable batteries for energy storage. Thus, in the present disclosure, lead-based oxide compounds are used to reversibly store charge by different mechanisms. The primary idea behind charge storage is either a structure collapse, then lead-alkali metal alloying; or (ir)reversible removal of Pb and its displacement by lithium in the parent structures.

In an embodiment, lead based compounds is used as safe and energy dense anodes for alkali-ion rechargeable batteries. As a proof-of-concept study, both PbTi3O7 and PbLi2Ti6O14 are used in rechargeable lithium-ion and sodium-ion batteries. The advantages include easy synthesis and ambient atmosphere stability. In doing so, the aim to repurpose variety of Pb containing oxides with any structure for charge storage in rechargeable batteries. Besides, these phases are weatherproof, and non-toxic as compared to pristine lead metal or its binary oxides that have been used long back in charge storage.

In another embodiment of the present disclosure, phase pure PbTi3O7 is synthesized by a low-temperature single-step solution combustion method. PbTi3O7 is introduced as an anode for secondary alkali-ion (Li-ion, Na-ion) batteries and exhibits capacity values of about 350 mAh/g (current rate=50 mA/g) and about 300 mAh/g (current rate=10 mA/g) for lithium and sodium insertion respectively. Lithium insertion follows a new mechanism that is different, and more efficient than sodium insertion in PbTi3O7. These mechanisms are completely different than lithium or sodium insertion in compounds like Na2Ti3O7.

In another embodiment of the present disclosure, PbLi2Ti6O14 is synthesized by a two-step solution combustion method. Its electrochemical performance is tested at very low current (C/100) in wide voltage window (low voltage cutoff, 0.01 V). Capacity values of over 300 mAh/g (after 1st charge) and about 200 mAh/g (after 15th cycle) are observed when tested for Li-ion batteries. It is the highest reported values so far for PbLi2Ti6O14 and among other members of the MLi2Ti6O14 family (where M = Sr, Ba, Pb etc.). Further, the electrochemical performance is tested under different voltage windows to explore the above unusual findings.

In another embodiment of the present disclosure, the superior performance of both PbTi3O7 and PbLi2Ti6O14 revolves around the lead ion and its (ir)reversible escape from the oxide frameworks. Structural changes occurring during charge storage are different for lead-based oxides, depends on the inserting ion and is key to the quantity (energy density), and quality (power density) of the final electrochemical performance. Thus, lead-based oxide stores charge and are useful in rechargeable non-aqueous alkali-ion batteries.

In an embodiment, the present disclosure finds application in making safe lithium-ion and sodium-ion batteries, particularly in applications where a high-energy density is preferred to a high-power delivery. Most viable use can be for but not limiting to portable electronics especially camera batteries, batteries in automotive applications, etc.

It is to be understood that the foregoing description is illustrative not a limitation. While considerable emphasis has been placed herein on particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure, certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments may be practiced and to further enable those of skill in the art to practice the embodiments. Accordingly, following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES
EXAMPLE 1: SYNTHESIS OF PbTi3O7 and PbLi2Ti6O14
PbTi3O7 and PbLi2Ti6O14 are prepared by wet solution combustion synthesis route. Respective metal nitrate precursors are used as oxidizer (O), glycine as fuel (F) (O/F =1) and the precursor solution is heated at 500 oC to initate drying and combustion. The key nitrate solution, TiO(NO3)2, is freshly prepared in an ice bath [2]. PbTi3O7 forms during the initial combustion step as soon as the combustion flame sweeps through the thick precursor slurry. Both the lead-based oxides are calcined in air at 900 oC for 2 h. Phase analysis is performed using X-Ray Diffraction performed (at 25 oC) using a PANalytical instrument (employing Cu Ka source).

Figure 1 and Figure 2 show the Rietveld refinement of these XRD patterns done using GSAS software [3]. PbTi3O7 and MLi2Ti6O14 (M=Sr, Ba, and Pb) form in monoclinic (P21/m) and orthorhombic (Cmca) crystal structures respectively. SEM micrograph (Fig. 1b, left) shows the arrangement of grains in a porous formation due to the gases released during the combustion step. PbTi3O7 is made of chains of edge shared units of 3 TiO6 octahedra directed along a axis (Fig. 1a, inset). These chains are corner shared leaving space for Pb ions. Fig. 2a represents the MLi2Ti6O14 structure. There is space for lithium insertion in empty interlayer 8f positions and in tunnels along c axis (Fig. 2c). Fig. 2d shows possible lithium ion flow pathway connecting the above sites. The 1D tunnel interconnects through the empty 8f site, creating a 2D migration path.

EXAMPLE 2: ELECTROCHEMICAL PERFORMANCE OF PbTi3O7 and PbLi2Ti6O14
The active material, combustion synthesized PbTi3O7 and PbLi2Ti6O14, is hand mixed with carbon black (Super P) and aqueous binder (Sodium Carboxymethyl Cellulose, NaCMC) in a 80:10:10 (w/w) ratio. Distilled water (500 µl/100mg of dry weight) is added and the resulting slurry was drop cast onto precut 12 mm or 16 mm coupons made from SS304 or battery grade Cu foils. After drying at 60oC to remove water, the coatings were weighed and vacuum-dried in Büchi glass-oven at 120oC overnight and are transferred into an Ar-filled glovebox (MBraun LabStar GmbH, O2 and H2O levels below 0.5 ppm). The coatings are used against respective M alkali metal in 2032 type Coin cells or SS304 Swagelok cells. Electrolytes are acquired commercially or made in-house. 1M LiPF6 dissolved in 1:1:3 v/v % of ethylene carbonate/propylene carbonate/dimethyl carbonate (EC/PC/DMC) is used as the lithium electrolyte (Chameleon Reagent). 1M NaPF6 dissolved in 0.45:0.45:0.1 v/v % EC/PC/DMC is used as electrolyte for sodium-ion batteries. The battery testing is performed with a Biologic BCS-805/BCS-810 or Neware BTS4000 series battery cyclers in different voltage windows without any rest time in between charge and discharge.

Post cycling, the Swageloks are disassembled inside glovebox and the recuperated electrodes are washed using anhydrous DMC. The material is scratched from the washed electrode, dispersed in DMC, and drop cast onto TEM grids inside the glovebox and are dried in the antechamber overnight before further examination. Ex situ coatings are directly used for X-ray analysis for confirmation of phases evolved during cycling.

Electrochemical performance of PbTi3O7
Figure 3 shows the electrochemical performance of PbTi3O7 versus lithium and sodium in CMC binder. Lithium de-insertion (at 50 mA/g) gave capacity of around 350 mAh/g in the first charge and around 200 mAh/g in the 110th charge (Fig. 1c). Sodium de-insertion (at 10mA/g) gave capacity of around 300 mAh/g in the first charge that faded to less than 100 mAh/g after 130 cycles (Fig. 1f). The dQ by dV plots (Fig. 1b, and 1e) show typical signs of (de)alloying reactions of lead with lithium, and sodium. The difference in capacity and sluggishness of the sodium (de)insertion indicates a different reaction pathway.

Figure 4 shows XRD profiles and TEM images of ex situ samples at different voltages and (de)lithiation levels in the 10th cycle. These results depict the charge storage mechanism of lithium in PbTi3O7. Ex situ XRD shows the amount of the original structure (Fig. 4a) reduces as lead metal increases during lithiation (discharge) (Fig. 4e). The most striking observation is the reappearance of the original PbTi3O7 phase during delithiation (charge). Some unreacted lead particles (diameter ~ 10 nm) that have not reinserted in the reformed PbTi3O7 structure, which can also be seen at the end of charge (Fig. 4b and c). Based on complete structure reversal of PbTi3O7, one of the two new charge storage mechanism of lithium are proposed to be active:

PbTi3O7 structure completely breaks after lithiation

New Mechanism: Reversible Conversion Intercalation Alloying (RCIA)

Reversible Reactions:

Pb(II) ?Ti?_3 O_7 + 2?Li?^++ 2e^-? Pb(0)+TiO_2+ ?Li?_2 O

TiO_2+ x?Li?^+ + xe^-? ?Li?_x TiO_2

Pb + y?Li?^++ ye^-? ?Li?_y Pb (y<4.4)

PbTi3O7 breaks and converts to lead and titanium/lithium oxides. Lead and TiO2 (de)alloy and (de)insert lithium respectively. All processes must reverse for the pristine PbTi3O7 structure to reform.

But RCIA reaction is unlikely as evidence of reversible conversion reaction in form of a characteristic single voltage plateau is absent at any point during lithium (de)insertion voltage profile (Fig. 3a).

PbTi3O7 structure does not break after lithiation

New Mechanism: Cumulative (Lead) Displacement (Lithium) Insertion – Alloying (CDI-A)

Reversible Reactions:

Pb(II) ?Ti?_3 O_7 + 2?Li?^++ 2e^-??Li?_2 ?Ti?_3 O_7+Pb(0)

?Li?_2 ?Ti(IV)?_3 O_7 + x?Li?^++ xe^-? ?Li?_(2+x) ?[?Ti?_(1-x) (IV) ?Ti?_x (III)]?_3 O_7

Pb + y?Li?^++ye^-? ?Li?_y Pb (y<4.4)

PbTi3O7 does not break. During discharge, two lithium displace Pb(II) ion in PbTi3O7 to form Li2Ti3O7, as two electrons reduce outgoing Pb(II) ion into lead metal (pure CDI). Li2Ti3O7 may be isostructural to PbTi3O7 or end up in a slightly reorganized form with lots of empty space after Pb(II) ion leaves. This structure may further (re)insert lithium conventionally by Ti(IV)/Ti(III) reduction (CDI + conventional intercalation). The lead metal (de)alloys with lithium (completing the CDI-A). All above reactions must reverse for the pristine PbTi3O7 structure to reform at end of charge. None of the processes need a plateau which is also not present in the voltage profile, further strengthening the above described CDI-A reaction hypothesis.

The lead (de)alloying signature, which is seen in both the RCIA and CDI-A reaction, is clearly seen in the dQ/dV plots (Fig. 3b and 3e).

Sodium insertion in PbTi3O7 is simpler. The mechanism involves conventional irreversible conversion of PbTi3O7 (to lead, oxides of titanium/lithium), reversible (de)insertion and (de)alloying of lithium-ion in TiO2 and lead.

Pb(II) ?Ti?_3 O_7 + 2?Na?^++ 2e^-? Pb(0)+TiO_2+ ?Na?_2 O (irreversible)

TiO_2+ x?Na?^++ xe^-? ?Na?_x TiO_2

Pb + y?Na?^++ye^-??Na?_y Pb (y<3.75)

Appearance of a conversion plateau during the first discharge clearly supports above mechanism. Further, absence of the plateau confirms irreversibility of this conversion step and no chance of original structure comeback.

Electrochemical performance of PbLi2Ti6O14

Lithium reversibly (de)inserts in the MLi2Ti6O14 (M = Sr, Ba, Pb) (MLTO) structure [4, 5, 6]. From Bond Valence Site Energy (BVSE) calculations, the incoming lithium is predicted to fill the empty 8f sites (in case of divalent M) and the sites (8c/8e) in the tunnels that already have lithium (16g site). Ti(IV)/Ti(III) redox is involved. The capacity delivered by MLTO is space limited as the structure, despite having six titanium ions, cannot accommodate six lithium. A maximum of 4 lithium insertion has been reported in the MLTO family for M = Pb [2].

The electrochemical performance of the MLTO structure was explored at extremely low current and in the widest voltage range for the following reasons:

The soft and polarizable nature (6s2 lone pair) of divalent lead ion is unique to MLTO structure and may mobilize the lead ion. The M ion mobility in MLTO structure is also not been investigated so far.

BVSE calculations performed on the MLTO structure show a very low migration barrier (0.484 eV) for inter M ion hop for M = Pb, which is comparable to inter lithium hop (0.23 V) in the structure (Fig. 5a). This hints at mobility of lead ion. Inter M ion hop takes place through an intermediate interstitial site, and two saddle point regions having the maximum energy. This energy varies with the M ion (Ba > Sr > Pb). This is unsurprising as BaLTO has an average Ba-O bond length that is less than the sum of Ba’s ionic radii [7]. This imparts residual compressive strain in BaLTO which also gives a high bond valence of Ba.

The alloying tendency of Pb ion and its occupancy in the interlayer region may further aid its outward mobility.

The above hypothesis is supported from the electrochemical performance of MLTO at low current (C/50 or C/100; C = 220 mA/g) in different voltage ranges. Thus, key conclusions include:

When all members of the MLTO with divalent M are discharged at very low C/50 current, only in the case of PbLTO, lead ions could be removed (Figure 5b-d). While similar removal of the Sr and Ba ions are not seen, observed charge capacities are very high due to the wide voltage window. The reason behind the removal of Pb ion and not the Sr or Ba ions could be from the fact that Sr and Ba ions are strong acids and hold the basic framework structure more tightly than Pb. While Pb ion can readily leave the structure, Sr/Ba ions need a much higher over potential with respect to Li than Pb ion to exit.

Interestingly, the first charge curve has a voltage profile that is like the one obtained during the normal cycling of PbLTO above 0.5V occuring without forcing out Pb. Subsequent cycles retain the nature of charge/discharge profile. This indicates that the structure may remain intact post lead loss. Pb ion exit reduces the particle size and smoothens the profiles.

A first charge capacity of more than 300 mAh/g is observed that is higher than theoretical capacity (of 220 mAh/g) and is the highest reported for PbLTO or in MLTO system at C/100 (Fig. 6a). It is double the highest charge capacity reported so far (when Pb ion stays intact). After few cycles, the charge capacity stabilizes at nearly 200 mAh/g which is close to the PbLTO’s theoretical capacity. This may show that Pb ion exit lifts the space limit in the MLTO structure by freeing up space for additional lithium ions to fill in. A redox change of nearly all 6 Titanium is likely.

Capacity observed below 0.3 V (Fig. 6a) is around 220 mAh/g and is much higher than that required for 2 electron Pb(II) ion to Pb(0) metal reduction (73 mAh/g). Beyond this, the rest 150 mAh/g of charge is consumed in LixPb alloy formation. But as the subsequent cycles do not feature the characteristic 0.5 V alloying signature, the alloying process is irreversible. This irreversible drainage of lead is further confirmed by fading of the voltage plateau below 0.3 V after a few cycles. The exiting lead cannot get back into the structure and decorates in the vicinity either as lead metal or in the form of LixPb alloy.

Slow cycling at C/100 to lower cutoff of 0.5 V does not drain out lead ions, and shows normal behavior reported so far for PbLTO (Fig. 6b). Slow C/100 cycling PbLTO to deep discharge levels (lower cutoff of 0.01V) is an extremely necessary step to drain out Pb ions.

Low capacity (around 150 mAh/g) is observed in a higher voltage window post lead removal using one slow C/100 deep discharge step (Fig. 6c). This emphasizes that repeated slow discharge is required to eliminate all Pb ions and make space for more lithium ions to (de)intercalate.

The alloying reaction of lithium with lead coming out of the structure is irreversible. This is confirmed from the very low capacity obtained at smaller voltage window (0.01 – 1V) post one slow C/100 discharge step (Fig. 6d).

Based on the above, the following mechanism of lithium insertion at slow current in PbLi2Ti6O14 is proposed:

Pb(II) ?Li?_2 ?Ti?_6 O_14 + 2?Li?^++ 2e^- ? ?Li?_2 ?Li?_2 ?Ti?_6 O_14+Pb(0) (irreversible CDI, few cycles)

Pb + y?Li?^++ ye^-? Pb (y<4.4) (irreversible, few cycles)

?Li?_2 ?Li?_2 ?Ti(IV)?_6 O_14 + x?Li?^++ xe^-? ?Li?_(2+x) ?Li?_2 ?[?Ti?_(1-x) (IV) ?Ti?_x (III)]?_6 O_14 (reversible)

Lithium ions displace Pb ions in PbLi2Ti6O14, which take electrons and reduce to Pb metal. Subsequently, Pb metal alloys with lithium. Above two reactions are irreversible and complete over the course of few cycles. Lithium ions further reversibly (de)intercalate in the PbLTO structure, in additional voids arising from the exit of Pb ions. This extra space permits all six-titanium redox to light up resulting in near theoretical capacity of 220 mAh/g.

REFERENCES

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,CLAIMS:
1. An electrode material comprising at least one lead-based oxide selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.
2. The electrode material as claimed in claim 1, wherein the electrode material is anode or cathode or both.
3. The electrode material as claimed in claim 1, wherein the electrode material is anode.
4. The electrode material as claimed in claim 1, wherein the electrode material is cathode.
5. The electrode material as claimed in claim 1, wherein the lead-based oxide show capacity values of over 300 mAh/g at low current value of at least 10mA/g.
6. The electrode material as claimed in claim 1, wherein the lead-based oxide show capacity values of over 300 mAh/g at low current value of at least 10mA/g in the first cycle or after first charge.
7. The electrode material as claimed in claim 1, wherein the lead-based oxide is PbTi3O7.
8. The electrode material as claimed in claim 1, wherein the lead-based oxide is PbLi2Ti6O14.
9. An electrochemical device comprising electrode material as claimed in claim 1.
10. The electrochemical device as claimed in claim 9, wherein the electrochemical device is a battery.
11. The electrochemical device as claimed in claim 9, wherein the battery is a primary or secondary battery.
12. A method of synthesis of lead-based oxide used as electrode materials in an electrochemical device, wherein the method comprises:
? preparing a precursor solution of metal nitrate(s); and
? mixing and heating the precursor solution of metal nitrate(s) to initiate drying and combustion, resulting in the formation of lead-based oxide; and
? optionally calcining the lead-based oxide in air.
13. The method as claimed in claim 12, wherein the lead-based oxide is selected from a group comprising PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.
14. The method as claimed in claim 12, wherein the metal nitrate is selected from a group comprising lead nitrate, lithium nitrate, titanyl nitrate, zirconium nitrate and combinations thereof.
15. The method as claimed in claim 12, wherein the precursor solution is initially heated at about 150 oC followed by heating at about 500oC.
16. The method as claimed in claim 12, wherein the lead-based oxide is calcined in air at about 800-900oC for 2 h.
17. The method as claimed in claim 12, wherein the method of synthesis of lead-based oxide requires fuel selected from a group comprising glycine, urea, sucrose, ascorbic acid, citric acid and combination thereof, along with metal nitrates(s) which act as oxidizers.
18. The method as claimed in claim 12, wherein the lead-based oxide is PbTi3O7.
19. The method as claimed in claim 18, wherein PbTi3O7 synthesis comprise:
? preparing a precursor solution of lead nitrate and titanyl nitrate;
? mixing the precursor solution of lead nitrate and titanyl nitrate followed by heating to initiate drying and combustion, resulting in the formation of PbTi3O7; and
? optionally calcining the PbTi3O7 in air.
20. The method as claimed in claim 19, wherein the lead nitrate and the titanyl nitrate is taken at a ratio of about 1:3.
21. The method as claimed in claim 12, wherein the lead-based oxide is PbLi2Ti6O14.
22. The method as claimed in claim 21, wherein PbLi2Ti6O14 synthesis comprise:
? preparing a precursor solution of lead nitrate, lithium nitrate and titanyl nitrate;
? mixing the precursor solution of lead nitrate, lithium nitrate and titanyl nitrate followed by heating to initiate drying and combustion, resulting in the formation of PbLi2Ti6O14; and
? optionally calcining the PbLi2Ti6O14 in air.
23. The method as claimed in claim 22, wherein the lead nitrate, lithium nitrate and the titanyl nitrate is taken at a ratio of about 1:2:6.
24. A method of forming an electrochemical device comprising electrode material claimed in claim 1, wherein the method comprises:
? forming an anode comprising at least one lead-based oxide; and forming a cathode electrode in contact with the anode electrode through an electrolyte, resulting in the electrochemical device; or
? forming a cathode electrode comprising at least one lead-based oxide and forming an anode electrode in contact with the cathode electrode through an electrolyte, resulting in the electrochemical device; or
? forming anode or cathode comprising at least one lead-based oxide and bringing them in contact with each other through an electrolyte, resulting in the electrochemical device.
25. The method as claimed in claim 24, wherein the lead-based oxide is selected from a group comprising at least one of PbTi3O7, PbZr3O7, PbLi2Ti6O14, PbLi2Zr6O14, Pb0.5Ti2(PO4)3, Pb0.5Zr2(PO4) and PbFe3O(PO4)3.
26. The method as claimed in claim 24, wherein the electrolyte is selected from a group comprising lithium-based electrolytes, sodium-based electrolytes and combinations thereof.
27. A method of improving electrochemical performance of electrochemical device, said method comprises step of employing electrode material as claimed in claim 1.
28. The method as claimed in claim 27, wherein the electrochemical device is a battery.
29. The method as claimed in claim 28, wherein the battery is a primary or secondary battery.
30. A battery pack comprising one or more cells wherein each cell comprises electrode material as claimed in claim 1.