ϵ-VOPO.SUB.4 .cathode for lithium ion batteries

Abstract

The epsilon polymorph of vanadyl phosphate, ε-VOPO.sub.4, made from the solvothermally synthesized H.sub.2VOPO.sub.4, is a high density cathode material for lithium-ion batteries optimized to reversibly intercalate two Li-ions to reach the full theoretical capacity at least 50 cycles with a coulombic efficiency of 98%. This material adopts a stable 3D tunnel structure and can extract two Li-ions per vanadium ion, giving a theoretical capacity of 305 mAh/g, with an upper charge/discharge plateau at around 4.0 V, and one lower at around 2.5 V.

Claims

1. A method of making a lithium ion battery cathode, comprising: forming ε-VOPO.sub.4 particles having a diameter of 100-200 nm; coating the ε-VOPO.sub.4 particles with conductive carbon, to form coated ε-VOPO.sub.4 particles; adding a binder to the coated ε-VOPO.sub.4, particles to form a mixture; depositing the mixture on a current collector, wherein the deposited mixture has a capacity of at least 275 mAh per gram of Li.sub.2VOPO.sub.4 at a discharge rate of C/20.

2. The method according to claim 1, wherein the ε-VOPO.sub.4 particles are solvothermally generated.

3. The method according to claim 1, wherein the ε-VOPO.sub.4 particles are hydrothermally generated.

4. The method according to claim 1, wherein said coating comprises mixing the ε-VOPO.sub.4 particles with graphene nanoplatelets.

5. The method according to claim 4, wherein the graphene nanoplatelets have a surface area of at least 100 m.sup.2/g.

6. The method according to claim 1, wherein the binder comprises polyvinylidene fluoride.

7. The method according to claim 1, wherein the ε-VOPO.sub.4 particles are coated with carbon nanotubes.

8. The method according to claim 1, wherein the coated ε-VOPO.sub.4 particles have a coating thickness of 10 nm.

9. The method according to claim 1, wherein the ε-VOPO.sub.4 particles of the lithium ion battery cathode comprises a vanadium which is adapted to undergo a change in oxidation state of two between a charged state filled with intercalated lithium ions and a discharged state depleted of intercalated lithium ions.

10. The method according to claim 9, wherein the lithium ion battery cathode has a current-voltage profile which displays voltage plateaus on discharge at a discharge rate of C/20, at about 2.1 V, 2.25 V, 2.5 V, and 3.9 V.

11. The method according to claim 10, wherein the ε-VOPO.sub.4 particles have a capacity of at least 305 mAh per gram of Li.sub.2VOPO.sub.4 at a discharge rate of C/20.

12. The method according to claim 10, wherein the lithium ion battery cathode has a discharge capacity of at least 90% of a theoretical value for the discharge capacity of the ε-VOPO.sub.4 particles.

13. The method according to claim 9, wherein the ε-VOPO.sub.4 particles have a capacity of at least 290 mAh per gram of Li.sub.2VOPO.sub.4 at a discharge rate of C/20.

14. The method according to claim 1, wherein the ε-VOPO.sub.4 particles have an energy capacity of at least 850 mWh/g.

15. The method according to claim 1, wherein the conductive carbon comprises graphene particles, and the mixture comprises between 5% and 15% by weight of the graphene particles.

16. The method according to claim 1, wherein the mixture comprises at least 75% by weight ε-VOPO.sub.4 particles, at least 5% by weight graphene nanoplatelets, and at least 5% by weight of a binder.

17. The method according to claim 1, wherein the lithium ion battery cathode has at least two states, comprising: a first state in which at least 80 mol % of vanadium of the ε-VOPO.sub.4 particles is oxidized in a first oxidation state and associated with two lithium ions per vanadium, and a second state in which at least 80 mol % of the ε-VOPO.sub.4 particles is oxidized in a second oxidation state which differs by two from the first oxidation state.

18. The method according to claim 1, further comprising forming a lithium ion battery comprising: the lithium ion battery cathode; a lithium or lithium ion anode; an electrolyte adapted to operate at a battery potential of at least 4.5 V; and a supporting lithium salt.

19. A method of making a lithium ion battery cathode, comprising: combining ε-VOPO.sub.4, particles having a diameter of 100-200 nm coated with electrically conductive carbon particles, and a binder to form a mixture; and coating a current collector with a slurry of the mixture, wherein the ε-VOPO.sub.4 has a dual lithium ion exchange characteristic, having a capacity of about 125 mAh per gram of Li.sub.2VOPO.sub.4 at a discharge rate of C/20 while maintaining a voltage exceeding 3.7 V and a capacity of at least 275 mAh per gram of Li.sub.2VOPO.sub.4 at a discharge rate of C/20 while maintaining a voltage exceeding 1.6 V.

20. A lithium ion battery cathode, comprising a mixture of ε-VOPO.sub.4 particles having a size of 100-200 nm, coated with electrically conductive graphene, and a binder, on a current collector, having a capacity of at least 275 mAh per gram of Li.sub.2VOPO.sub.4 at a discharge rate of C/20.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:

(2) FIG. 1A shows morphological and structure characterization of ε-VOPO.sub.4 SEM image.

(3) FIG. 1B shows an XRD pattern with Rietveld refinement of the as-synthesized ε-VOPO4.

(4) FIG. 2 shows TEM images of ε-VOPO.sub.4 hand ground with graphene nanoplatelets for electrode preparation.

(5) FIG. 3A shows galvanostatic charge-discharge curves of ε-VOPO.sub.4 from 1.6 to 4.5 V at C/50.

(6) FIG. 3B shows cycle performance of ε-VOPO.sub.4 from 1.6 to 4.5 V at C/50, 1C=2 Li.

(7) FIG. 4 shows a CV curve profile of ε-VOPO.sub.4 at a scan rate of 0.02 mV/s.

(8) FIG. 5A shows galvanostatic charge-discharge curves of ε-VOPO.sub.4 from 1.6 to 4.5 V at C/20.

(9) FIG. 5B shows cycle performance of ε-VOPO.sub.4 from 1.6 to 4.5 V at C/20, 1C=2Li.

(10) FIG. 6A shows galvanostatic charge-discharge curves of ε-VOPO.sub.4 at the low voltage region, from 1.6 to 3.0 V.

(11) FIG. 6B shows cycle performance in the low voltage region, 1.6 to 3.0 V, of ε-VOPO.sub.4 at C/50, 1C=2Li.

(12) FIG. 7A shows cycling curves of ε-VOPO.sub.4 in the low voltage region, from 1.6-3.0 V, at different rates.

(13) FIG. 7B shows rate test capacities of ε-VOPO.sub.4 in the low voltage region, from 1.6 to 3.0 V.

(14) FIG. 8A shows galvanostatic charge-discharge curves of ε-VOPO.sub.4 at the high voltage region, from 3.0 to 4.5 V.

(15) FIG. 8B shows cycle performance in the high voltage region, 3.0-4.5 V, of ε-VOPO.sub.4 at C/50, 1C=2Li.

(16) FIG. 9A shows Cycle curves of ε-VOPO.sub.4 at high voltage region, from 3.0 to 4.5 V, at different current rates.

(17) FIG. 9B shows rate test capacities of ε-VOPO.sub.4 in the low voltage region, from 3.0 to 4.5 V.

(18) FIG. 10 is an exploded perspective view of a lithium secondary battery according to an embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(19) Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

(20) It will be understood that when an element is referred to as being “on” another element, it can be directly in contact with the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

(21) It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present embodiments.

(22) The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “or” means “and/or.” It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this general inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, are appropriate for use only if consistent with a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Thus, such resources as “Merriam Webster” (any version) are secondary to field of science-appropriate technical dictionaries and encyclopedias.

(23) One or more embodiments will now be described in detail with reference to the following examples. However, these examples are not intended to limit the scope of the one or more embodiments.

(24) The basis of the lithium-ion battery (LIB) uses lithium-ions to travel across the electrolyte and intercalate into the anode upon charge and into the cathode upon discharge [1]. Because they outperform competing primary batteries like lead-acid, alkaline, etc., as well as other rechargeable batteries such as nickel-metal hydride, nickel cadmium, etc., it is no wonder how the LIB has revolutionized and expanded the mobile electronics industry since 1991 [2]. While the performance and functionality of smartphones and laptops continue to improve, the development of LIBs need to catch up to match in terms of power and life cycle to expand into large energy storage applications.

(25) Currently, the cathode material in the market is dominated by LiCoO.sub.2. While it has an extremely high theoretical capacity of 274 mAh/g with an operating voltage around 3.6 V, the structure tends to undergo irreversible changes when more than 50% of the Li-ions are removed [3]. Environmental safety and the cost became major concerns as well, thus, drove the search for alternatives where the scarce and toxic cobalt is substituted by other metals such as in LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2, also known as NMC [4]. Extensive research for developing alternative cathode materials lead to study of metal phosphates, thus came the LiFePO.sub.4. This olivine material has garnered a great deal of attention that it has been commercialized for portable and stationary systems by A123, BAE Systems and in China. In fact, Hydro-Quebec, MIT and A123 further improved this material with high-power performance that can charge and discharge within minutes through nanosizing and carbon coating [5]. Through a collaborative effort from NECCES-I, a fundamental study on this phenomenon determined a metastable reaction mechanism of LiFePO.sub.4 which is why it can cycle at high rates [6].

(26) Another way to leverage in the stability of phosphates and to increase energy storage is to incorporate a second electron. Hautier et al. plotted the mean voltage for each per metal redox couple with respect to the capacity in a phosphate, giving way to consider two electron couples. Vanadium phosphate compounds were candidates within the acceptable voltage window [7]. The ε-VOPO.sub.4.Math.ε-LiVOPO.sub.4.Math.ε-Li.sub.2VOPO.sub.4 system has been regarded as one of the most promising and safe candidates to provide a two-electron reaction with a high theoretical capacity of 305 mAh/g and specific energy over 900 Wh/g [8, 15, 16]. This system has two redox potentials at useful potentials for storing energy, V.sup.3+.Math.V.sup.4+ at 4.0 V and V.sup.4+.Math.V.sup.5+ at 2.5 V. ε-VOPO.sub.4 was first synthesized by Lim et al. by heating monoclinic H.sub.2VOPO.sub.4 in oxygen, and Kerr et al. measured the electrochemical reversibility at the high voltage plateau at 4.0 V [9, 10]. Previously, the synthesis and characterization of ε-VOPO.sub.4 from two different phases of H.sub.2VOPO.sub.4 was reported, to discover that the electrochemical performance from the disordered tetragonal precursor was improved due to smaller particle size [11]. Fundamental studies were conducted on the structural evolution of ε-LiVOPO.sub.4 and two intermediate phases in the low-voltage regime identified using DFT calculations backed up with X-ray pair distribution function analysis and X-ray absorption near edge structure measurements [12]. The insertion of two Li-ions into ε-VOPO.sub.4 has been demonstrated, reaching the theoretical specific capacity of 305 mAh/g.

EXAMPLE 1

(27) Synthesis: ε-VOPO.sub.4 was synthesized by calcining the monoclinic H.sub.2VOPO.sub.4 precursor, as reported by Song et al. [11] Stoichiometric amounts of VCl.sub.3 (Sigma-Aldrich, 97%), and P.sub.2O.sub.5 (Sigma-Aldrich, ≥98%) were dissolved in 190 proof ethanol (Pharmco-AAPER). The solution was placed in a 4748 Type 125 mL PTFE-lined reactor (Parr Instrument Co.) and the reaction was set to 180° C. for 72 hours. The solvothermal product was collected by centrifugation and heated at 550° C. in flowing oxygen for 3 hours.

(28) Materials Characterization: For XRD measurements, a Bruker D8 Advanced X-ray diffractometer equipped with Cu Kα source, λ=1.54178 Å. The intensities were recorded within the 2θ range from 10° to 80° with 2θ steps of 0.02° from powder samples. The unit cell parameters were obtained by Rietveld refinement with the TOPAS program. Scanning electron microscopy (SEM) measurements were performed with a Zeiss Supra-55 VP field emission scanning electron, using both the secondary electron and InLens modes to determine the morphology and particle size, at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) imaging was performed using the FEI Titan 80-300 microscope with a field emission gun (FEG) and an image aberration corrector, operated at an acceleration voltage of 300 kV. The pristine material was dispersed on a copper grid coated with a lacey carbon film for high-resolution transmission electron microscopy (HRTEM) observation.

(29) Electrochemistry: The electrodes were prepared by mixing the active material, ε-VOPO.sub.4, with graphene nanoplatelets (surface area 750 m.sup.2/g, XG Sciences) as a carbon additive and polyvinylidene fluoride (PVDF, Aldrich) binder in a weight ratio of 75:15:10. The slurry was created by adding 1-methyl-2-pyrrolidinone (NMP, Aldrich) which was then laminated onto an aluminum foil 144 current collector and vacuum-dried overnight before use. The dried electrodes, of area 1.2 cm.sup.2, contained 8-10 mg of active material and were assembled in 2325-type coin cells in a He-filled glovebox with a pure lithium chip (thickness 0.38 mm, Aldrich) as the counter and reference electrode. The electrolyte used was lithium hexafluorophosphate (1 M LiPF.sub.6) dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) in 1:1 volume ratio with Celgard 2400 (Hoechst Celanese) as the separator. The electrochemical properties were investigated using the Bio-Logic VMP multichannel potentiostat. The cells were cycled galvanostatically in the high voltage region (3.0-4.5 V), low voltage region (1.6-3.0 V) and the whole voltage (1.6-4.5 V) window at C/50, where 1 C=2 Li or 305 mAh/g per gram of Li.sub.2VOPO.sub.4. Cells were also cycled at C/20 over the whole voltage range.

(30) Hydrothermal or solvothermal synthesis has many unique advantages because it offers good control over the sample's purity and crystallinity, easy to scale up and low cost. This method can keep the overall particle size small and size distribution narrow which are vital features for good cathodic electrochemical performance. FIG. 1A shows the as-synthesized ε-VOPO.sub.4 powder as nano-sized primary particles, ˜100-200 nm, that are cuboid in shape. This material matched well with earlier reported results from Chen et al., where ε-VOPO.sub.4 synthesized from monoclinic H.sub.2VOPO.sub.4 are made up of single crystals up to 200 nm [13]. Achieving small primary particles is important because it can improve the rate property for Li intercalation. Azmi et al. reported that smaller LiVOPO.sub.4 particle size results in easier lithium-ion diffusion with enhanced columbic efficiency by improving the capacity of lithium deintercalation upon discharge and the decreasing the lithium intercalation potential upon charge [14]. It is also observed in FIG. 1A, that the nano-sized ε-VOPO.sub.4 primary particles do not agglomerate nor form into secondary particles, providing a good surface area for the graphene or carbon nanotube additive to wrap around and assist in electron migration during the charge/discharge process.

(31) The lack of primary particle agglomeration is attributed to the choice of solvent used for synthesis. By using 190 proof ethanol, ε-VOPO.sub.4 results in loose particle morphology whereas 200 proof ethanol results in the formation of 2 μm balls as secondary particles. By using this solvothermal synthesis route, the precursor was successfully synthesized and calcined to produce pure crystalline ε-VOPO.sub.4, as seen in FIG. 1B, resulting in sharp and narrow peaks in the x-ray diffraction pattern. The observed pattern matched very well to the calculated pattern with no impurities or other vanadyl phosphate phases, resulting in a low R.sub.wp value of 4.55%.

(32) Normally, ball-mill treatment is necessary to break up any agglomeration and secondary particles and to reduce the particle size for good electrochemical performance. However, since the as-synthesized ε-VOPO.sub.4 is of nanometer size, there is no need to use this application, which helps preserve the crystal structure for better reversible intercalation chemistry. FIG. 2 shows HRTEM images of 75 wt. % ε-VOPO.sub.4 that was hand milled with 15 wt. % graphene nanoplatelets in a mortar and pestle before adding 10 wt. % PDVF and NMP for electrode preparation. FIG. 2 shows graphene nanoplatelets forming a conductive network between every single ε-VOPO.sub.4 primary particle. Upon closer inspection at FIG. 2B, HRTEM shows that the graphene nanoplatelets coated on the ε-VOPO.sub.4 particle is around 10 nm thick.

(33) Electrochemistry of ε-VOPO.sub.4:

(34) FIGS. 3A and 3B show ε-VOPO.sub.4 cycled in the whole voltage window from 1.6 V to 4.5 V at C/50, capable of achieving a high discharge capacity of 305 mAh/g for at least 50 cycles. FIG. 2A displays the desired characteristic plateaus at ˜4.0 V at the high voltage region and at ˜2.5, 2.25, 2.0 V at the low voltage region. The drop from the high voltage region to the low voltage region is a step-like curve and the hysteresis gap between the charge and discharge curve is very small. The high voltage region has a long plateau which extends the capacity to ˜150 mAh/g, equivalent to ˜1 Li. This corresponds to the redox potential of V.sup.3+ and V.sup.4+ where ε-VOPO.sub.4 becomes ε-LiVOPO.sub.4. The low voltage region has three plateaus at 2.5, 2.25 and 2.0 V which also extends the capacity to ˜150 mAh/g, corresponding to the second intercalation of lithium where ε-LiVOPO.sub.4 becomes ε-Li.sub.2VOPO.sub.4. The plateaus at the low voltage region has maintained step-like curves even after 35 cycles, suggesting good kinetics and the changes in the local structure may be reversible for easy Li intercalation.

(35) Cyclic voltammetry (CV) curves was measured in the voltage window of 1.6 V to 4.5 V to understand the redox process of ε-VOPO.sub.4 is shown in FIG. 4. There are four reduction peaks at certain voltages that correspond to four oxidation peaks at similar voltages. Each peak represents the reversible reaction between ε-VOPO.sub.4 and ε-Li.sub.2VOPO.sub.4 that correspond to the voltage plateaus found upon galvanostatic charge and discharge cycling. Starting from the OCV point at 3.9 V, there is a single oxidation peak at 3.7 V that indicates electrochemical lithiation from ε-VOPO.sub.4 to ε-LiVOPO.sub.4. As the scan rate test moves to the low voltage region, there are three additional oxidation peaks. Each of the peaks signify the transition from ε-LiVOPO.sub.4 to ε-Li.sub.2VOPO.sub.4 with intermediate stages in between. ε-LiVOPO.sub.4 becomes ε-Li.sub.1.5VOPO.sub.4 at ˜2.5 V, then it converts to ε-Li.sub.1.75VOPO.sub.4 at ˜2.25 V and finally becomes ε-Li.sub.2VOPO.sub.4 at ˜2.0 V. Reduction peaks appear as the voltage continues to sweep from the low to high voltage domain, indicating that the V.sup.5+ oxidation state of ε-VOPO.sub.4 was recovered from V.sup.3+ of ε-Li.sub.2VOPO.sub.4. From ε-Li.sub.2VOPO.sub.4, it becomes ε-Li.sub.1.75VOPO.sub.4 at ˜2.1 V, then ε-Li.sub.1.5VOPO.sub.4 at ˜2.25 V and ε-LiVOPO.sub.4 at ˜2.5 V. No further reaction takes place until ˜4.25 V where ε-LiVOPO.sub.4 further reduces to become ε-VOPO.sub.4.

(36) FIGS. 5A and 5B show that even at a faster rate, ε-VOPO.sub.4 can still deliver a discharge capacity of ˜305 mAh/g for up to 40 cycles at C/20. The long high voltage plateau extending past 100 mAh/g is preserved and each of the characteristic steps in the low voltage region are clearly sustained with no signs of diminishing for up to 30 cycles. In FIG. 4A, the drop from the high voltage region to the low voltage region evolved to a slope-like curve, which helps make up for the shorter high voltage plateau in the beginning but might indicate a little hysteresis. In subsequent cycles, the high voltage plateau slightly increases. The capacity of the 1.sup.st high voltage discharge plateau was ˜125 mAh/g and by the 35.sup.th cycle, it increased to ˜150 mAh/g which is equivalent to 1 Li. The low voltage region seems to show the opposite trend. As the high voltage plateau starts to increase in capacity, the low voltage steps start to decrease as well to maintain the overall discharge capacity at ˜305 mAh/g.

(37) Cycling of ε-VOPO.sub.4 has been separated at the high voltage region and the low voltage region to study the stability of the electrochemical curve and capacity without the influence of each other. FIGS. 6A and 6B show ε-VOPO.sub.4 cycled in the low voltage region to study how long-term cycling affects the shape and length of these three distinct plateaus at 2.5 V, 2.25 V and 2.0 V which is in agreement with DFT calculations confirming the two intermediate phases at x=1.5 and 1.75 in the low-voltage regime [12]. The initial discharge curve in FIG. 6A is different because the cell was discharged from OCV first, delivering more than 300 mAh/g. Even after 30 cycles, FIG. 6A shows that each voltage step is clearly distinguished, delivering a reversible capacity of ˜160 mAh/g, correlating to 1 Li. From then on, the cell was continuously charged and discharged in the low voltage window, from 1.6 V to 3.0 V. FIG. 6B shows that the low voltage steps maintained ˜160 mAh/g for up to 30 cycles with no sign of decay at all, suggesting good kinetics at the low voltage region. The charge-discharge profiles with steps agree with many reports, giving evidence to the existence of intermediate phases of ε-Li.sub.xVOPO.sub.4 where x=1.5 and 1.75.

(38) A rate test in the low voltage region was performed to study how faster cycling can affect the plateaus at 2.5 V, 2.25 V and 2.0 V, as shown in FIGS. 7A and 7B. To start the rate test at the low voltage region, the cell was first discharged from OCV to 1.6 V at C/50, delivering a discharge capacity of over 300 mAh/g. From C/50 to C/5, the low voltage plateaus still maintained a discharge capacity of ˜150 mAh/g with clearly defined step-like features, as shown in FIG. 7A. When the rate increased to 1C, the discharge capacity is still ˜150 mAh/g but the plateaus are more slope-in shape at slightly lower voltages. As the cycling rate increases, the difference between the charge and discharge capacities decreases, as shown in FIG. 7B. From C/50, the charge capacity is 175 mAh/g while the discharge is ˜150 mAh/g. When the rate increased to 1C, the charge and discharge capacities are ˜150 mAh/g, thereby increasing the coulombic efficiency to ˜100%. When cycled at the low voltage range, ε-VOPO.sub.4 can reversibly intercalate one full lithium ion at the low voltage region, even at faster cycling rates.

(39) FIGS. 8A and 8B show ε-VOPO.sub.4 cycled in the high voltage region, from 3.0 V to 4.5 V, to study how long-term cycling affects the shape and capacity. In this high voltage window, there is a plateau at ˜4.0 V that coincides with the V.sup.3+/4+ redox where ε-VOPO.sub.4.Math.LiVOPO.sub.4. This high voltage plateau delivers a reversible capacity of ˜140 mAh/g for up to 35 cycles which is close to 0.93 Li. This exceeds the previously reported results, where only 0.83 Li was inserted into ε-VOPO.sub.4 and 0.65 Li was inserted into ε-LiVOPO.sub.4. [11,12] After 30 cycles, the capacity slowly climbs to 150 mAh/g which corresponds to 1 Li which agrees with the trend in the galvanostatic charge-discharge curves in FIG. 4. Even after 30 cycles, FIG. 8B shows that ε-VPOPO.sub.4 delivers a reversible capacity of ˜150 mAh/g, correlating to 1 Li. This plateau is step-like with no signs of fading after many cycles, suggesting easy reversible intercalation.

(40) FIGS. 9A and 9B show how different rates can affect the high voltage plateau of ε-VOPO.sub.4 at ˜4.0 V. At C/50, the discharge capacity is around 130 mAh/g and the capacity decreases as the rate gets faster. By 1 C, the discharge capacity dropped to around 40 mAh/g. Despite the fast rate cycling of 1 C, the cell could deliver the high discharge capacity of 140 mAh/g after it was cycled back to C/50. This suggests that the structure was preserved, even at fast cycling, and can maintain a high discharge capacity when it was cycled back to C/50 from 1 C. It also seems that faster cycling leads to higher coulombic efficiency. It is evident that from C/50 to C, the coulombic efficiency increases. From C/25, some of the charge and discharge capacities are overlapping and by C/10, C/5 and C, the discharge capacities are practically the same as the charge. This means that at faster rates, it can de/intercalate lithium ion more efficiently.

Conclusions

(41) The optimized morphology and nano particle size of ε-VOPO.sub.4 is studied, as well as observations from transmission electron microscope to analyze the good carbon conductive network. By combining complementary characterization techniques of SEM, XRD and extensive electrochemical studies, the reversibility reaction of ε-VOPO.sub.4 as a cathode material for lithium-ion batteries is elucidated.

(42) Throughout this description all ranges described include all values and sub-ranges therein, unless otherwise specified. Additionally, the indefinite article “a” or “an” carries the meaning of “one or more” throughout the description, unless otherwise specified. Where a numerical limit or range is stated, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

(43) The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In this regard, certain embodiments within the invention may not show every benefit of the invention, considered broadly.

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