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  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
  • H01M10/44—Methods for charging or discharging
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  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
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  • H01M4/483—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides for non-aqueous cells
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  • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
  • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
  • H01M4/621—Binders
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Definitions

• the present invention provides an electrode and an electrochemical cell, such as a lithium ion battery, comprising the electrode, together with methods for using the electrode within the electrochemical cell.
• an electrochemical cell such as a lithium ion battery
• the most intuitive and commonly used approach to increase the rate performance of an electrode material is to create a nanosized or porous (and often hierarchical) structure. This minimizes the lithium ion solid-state diffusion distance, enabling more rapid lithium ion transport through the electrode and increasing the surface area of electrode materials in contact with electrolyte. Carbonaceous hierarchical structures and carbon-coating are also frequently employed to improve electronic conductivity, which is another prerequisite for high-rate applications.
• Li 4 Ti 5 0i 2 lithium titanate; LTO
• LTO lithium titanate
• SEI solid-electrolyte interphase
• a high-rate lithium ion cell using electrode materials comprising niobium tungsten oxide can operate above 1.3 V.
• the average voltage of Nbi 6 W 5 0s 5 is 1.57 V vs Li7Li and the average voltage of NbisWi 6 0 93 is 1.67 V vs Li7Li). Operating in this voltage range negates the need to perform an initial formation cycle, simplifying the cell
• a typical lithium-ion cell comprising a graphite electrode operates below 1.3 V and must undergo an initial formation cycle before the cell is sealed. Typically, this formation cycle takes place at elevated temperature, for example 60 °C, in order to allow degassing to occur. This adds signification time and cost to the cell manufacturing process.
• LiN(CF3SC>2)2 LiTFSI
• LiN(CF3SC>2)2 LiTFSI
• aluminum can be used as the current collector instead of the more expensive copper while avoiding LiAI alloying potentials ( ⁇ 0.3 V vs. Li7Li).
• niobium tungsten oxide as an electrode material, for example in a high-rate lithium cell, allows the open-circuit voltage to be used as a measure of state-of-charge. This has the potential to provide a simple and reliable BMS, which may prove to be a significant advantage for high power/fast charging applications.
• the electrochemical cell may contain a counter electrode and an electrolyte, and optionally the electrodes are connectable to or are in connection with a power supply.
• the method may be a method of charging and/or discharging an electrochemical cell at a current density of at least 750 mA-g 1 , such as at least 800 mA-g 1 .
• the working electrode may comprise the niobium tungsten oxide in particulate form.
• the niobium tungsten oxide may have a primary particle size of at least 1 pm.
• particulate niobium tungsten oxides decreases side reactions and mitigates the problems of gas evolution and the associated swelling or pressure build-up observed with nano-LTO electrodes. Moreover, particulate niobium tungsten oxides can be quickly and easily prepared using solid state synthesis.
• the favorable lithium diffusion properties allow micrometer-sized particles of niobium tungsten oxide to be used at extremely high rates.
• the molar ratio of Nb 2 C>5 to WO 3 in the working electrode may be from 6:1 to 1 :15. For example, from 8:5 to 1 1 :20.
• the working electrode may comprise a mixture of niobium tungsten oxide and an additional active material.
• the working electrode may comprise a mixture of niobium tungsten oxide and LTO.
• the ratio of niobium tungsten oxide to LTO may be from 95:5 to 5:95 by weight.
• the ratio may be from 90:10 to 10:90 by weight, from 80:20 to 20:80 by weight, from 70:30 to 30:70 by weight, from 60:40 to 40:60 by weight or the ratio of niobium tungsten oxide to LTO may be 1 :1 by weight.
• a lithium ion battery comprising one or more electrochemical cells of the invention. Where there are a plurality of cells, these may be provided in series or parallel.
• Figure 1 is a wide-field SEM image of (A) Nbi6W 5 0s5 and (B) NbisWi6093 at a low
• Figure 2 is an atomic structure and SEM image showing (A-C) Nbi 6 W 5 0s 5 and (D-F) NbisWieOgs.
• the scale bars shown in (B) and (E) are 5 pm.
• the scale bars shown in (C) and (F) are 10 pm.
• Figure 6 is a comparison of Cu foil to carbon-coated Al foil current collector.
• Fig. 6A shows Nbi6W 5 C>55 cycled for 1000 cycles under constant current discharge and charge at 10C (first 200 cycles) and 20C (last 800 cycles); Cu foil at 1.0 V (topmost line); Cu foil at 1 .2 V (second from top); C-AI at 1.0 V (third from top) and C-AI at 1 .2 V (bottommost line).
• Fig. 6A shows Nbi6W 5 C>55 cycled for 1000 cycles under constant current discharge and charge at 10C (first 200 cycles) and 20C (last 800 cycles); Cu foil at 1.0 V (top
• 6B shows discharge and charge profiles between 3.0 and 1 .0 V for the 100th cycle at 10C; order of curves at 100 mA-h-g 1 : C-AI charging curve (topmost); Cu foil charging curve (second from top); Cu foil discharging curve (third from top) and C-AI discharging curve (bottommost).
• Figure 8 shows overpotential in a Li
• Figure 12 shows (A) relative changes in lithium diffusion as a function of open-circuit voltage (Voc) and (B) open-circuit voltage vs. closed-circuit voltage (Vcc) from galvanostatic intermittent titration technique (GITT) measurements showing the“thermodynamic” electrochemical profiles at C/20 rate with a 12 h rest period at each point, reaching a full discharge in approximately one month.
• Voc open-circuit voltage
• Vcc open-circuit voltage vs. closed-circuit voltage
• GITT galvanostatic intermittent titration technique
• Figure 15 shows the capacity versus cycle number for Nb 2 Mo 3 0i 4 at 0.05C and from 1.0 to 3.0 V vs. Li7Li.
• Figure 19 shows discharge and charge voltage profiles of full cells comprising a Nbi 6 W 5 0s 5 anode and (A) LiFeP0 4 , (B) LiMn 2 0 4 , and (C) NMC622 cathode.
• Figure 19(D-E) shows the rate performance of a cell comprising a Nbi 6 W 5 0s 5 anode and NMC622 cathode; order of cures in (E): capacity (topmost, sloping); coulombic efficiency (lower, horizontal).
• the invention generally provides an electrode comprising a niobium tungsten oxide, an electrochemical cell comprising the electrode, and the use of the cell, for example, in a lithium ion battery, at high C-rates of 5C during charging and/or discharging.
• Electrodes comprising NbsWgO ⁇ (Montemayor ef a/.); Nb 26 W 4 0 77 (Fuentes ef a/.); Nbi 4 W 3 0 44 (Fuentes ef a/.); and Nb x Wi -x 0 3-x/2 , wherein 0 ⁇ x ⁇ 0.25 (Yamada et el.), have been previously described. However, the capacity of the materials against the C-rate was not measured.
• the high-rate application may also be described by reference to (gravimetric) current density, for example where the current density is at least 800 mA-g 1 or 1000 mA-g 1 .
• the working electrode comprises Nbi6W 5 C>55, Nbi8Ws069, Nb 2 WOs, NbisWi6093, or Nb 22 W 2 oOn5, or combinations thereof .
• the working electrode comprises
• the working electrode comprises a mixture of niobium tungsten oxide and an additional active material.
• the additional active material may be an additional metal oxide.
• the working electrode may comprise a mixture of niobium tungsten oxide and an additional active material selected from lithium titanate (LTO; U 4 Ti 5 0i 2 ), titanium niobium oxides (for example TiNb 2 0 7 ), titanium tantalum oxides (for example TiTa 2 0 7 ), tantalum molybdenum oxides (for example TasWgO ⁇ ) and niobium molybdenum oxides (for example Nb 2 Mo 3 0i 4 ).
• LTO lithium titanate
• U 4 Ti 5 0i 2 titanium niobium oxides
• titanium tantalum oxides for example TiTa 2 0 7
• tantalum molybdenum oxides for example TasWgO ⁇
• niobium molybdenum oxides for example Nb 2 Mo 3 0i 4
• the working electrode does not have a porous nor hierarchical structure.
• the electrode material may have a specific surface area of less than 20 m 2 -g 1 , less than 10 m 2 -g 1 , less than 5 m 2 -g 1 , less than 3 m 2 -g 1 , less than 2 m 2 -g 1 or less than 1 m 2 -g 1 .
• the specific surface area of the electrode material may be known, or it may be determined using standard techniques such as N 2 adsorption isotherm analysis and Brunauer-Emmett- Teller (BET) theory.
• the working electrode may have a porous structure.
• the working electrode may have a specific surface area of at least 50 m 2 -g 1 , at least 60 m 2 -g 1 , 70 m 2 -g 1 , 80 m 2 -g- 1 , 90 m 2 -g- 1 , 100 m 2 -g- 1 , 150 m 2 -g- 1 , 200 m 2 -g- 1 , 300 m 2 -g- 1 , or 400 m 2 -g- 1 .
• the overpotential is below 100 mV; however, at 5 mA (10C) it rises to 200 mV and increases to ca. 700 mV at 100C.
• the overpotentials in the symmetric cell closely match those observed in the electrochemical cycling curves of Fig. 4A and 4D. This suggests that the extremely high rates for a bulk electrode are approaching the limits of Li metal plating/stripping and/or lithium ion desolvation and transport in carbonate ester electrolytes at room temperature, i.e., a significantfraction of the ohmic drop seen during fast charging results from the Li metal and not the complex oxide electrode materials.
• radiofrequency (rf) pulse the net magnetization loses coherence due to T 2 relaxation; thus, the time period following this pulse, which includes the first PFG pulse (to encode spin position), must be shorter than T 2 .
• a second 90° rf pulse is applied which stores the net magnetization along the z axis prior to the diffusion time, D, allowing the observed species to diffuse for a time commensurate with the comparatively longer spin-lattice (Ti) value, as no T 2 relaxation occurs.
• a short spoiler gradient SINE.100 is applied to remove residual transverse magnetization.
• a third 90° rf pulse is applied, followed by a PFG pulse to decode spin position.
• a PFG pulse to decode spin position.
• Sufficiently long delays between PFG and radiofrequency pulses were used (> 0.5 ms) to minimize eddy currents in the diffusion measurements.
• the gradient strength, g was varied from 0.87 to 1800 or 2300 G-crrr 1 , and 16 gradient steps were acquired using‘opt’ shaped pulses with 1024-4096 transients.
• the opt shape is a composite pulse that starts with a quarter of a sine wave, followed by a constant gradient, and ends with a ramp down (Fig. 9).
• The‘opt’ gradient pulses provide the largest gradient integral for a given time period, maximizing the range of diffusion coefficients we could assess in this experiment.
• 4Nbi8Wi6C>93 is approximately 700 ps at room temperature vs. 1.9 ms at 453 K.
• the increase in T 2 allowed the use of longer gradient pulses, d, that were necessary to measure diffusion coefficients in the solid oxides.
• Li6 . 3Nbi6W 5 C>55 shows two-component behavior with lithium transport as rapid as 4.3x10 -12 m 2 -s 1 at 333 K (Fig. 1 1 ). Assuming Arrhenius behavior and the measured activation energy of 0.23 eV, the room temperature lithium diffusion coefficient is estimated to be 2.1 x10 -12 m 2 -s 1 (Table 1 ).
• Li7TM Li7TM where the diffusion then drops by about two orders-of-magnitude toward 2.0 Li7TM. This suggests that the inherent range of the niobium tungsten oxide electrode materials for high-rate multiredox extends to approximately 1 .5 Li7TM.
• the diffusion coefficients on the order of 10 -12 to 10 13 m 2 -s 1 measured for these materials are consistent with the values required to achieve full lithiation of 6 to 19 pm particles on a 60C timescale.
• Information on electrode thermodynamics, including phase transitions, and lithium kinetics can in principle be extracted from GITT measurements by tracking the voltage evolution after a brief current pulse as lithium diffuses and the chemical potential equilibrates within the electrode/particles.
• Reliable quantitative diffusion coefficients, Du are, however, difficult to extract from GITT alone. In order to determine a diffusion coefficient from GITT
• titania, niobia, and graphite When compared strictly on the basis of theoretical 1.0 U+/TM reaction and crystallographic density of the active material, titania, niobia, and graphite all display theoretical charge densities of greater than 800 A-h-L 1 (Fig. 14).
• the bulk, unoptimized niobium tungsten oxides presented here maintain volumetric charge densities of greater than 500 A-h-L 1 at 1 C and up to 400 A-h-L 1 at 20 C, volumetric performance that even the most optimised versions of T1O2, Nb 2 0s, LTO cannot achieve. This is not to say that the compounds presented cannot be improved by methods such as nanostructuring, calendaring, or carbon-coating as demonstrated by e.g.
• NMC622 showed an average voltage of 3.8 V and a practical capacity of 175 mA-h-g 1 under these conditions (Fig. 17A).
• LiFeP0 4 showed an average voltage of 3.4 V and a practical capacity of 165 mA-h-g 1 (Fig. 17B), and LiMn 2 0 4 showed an average voltage of 4.0 V and a practical capacity of 120 mA-h-g 1 (Fig. 17C).
• Electrochemical characterisation was conducting using a stainless steel 2032 coin cell and glass microfiber separator as described above. A Li metal disk was used as anode in half-cell geometry. The electrolyte was 150 pL LP30. No additives were used.
• niobium tungsten oxides as high-rate anode materials, full cells were produced using the commercially-available high-rate cathode materials NMC622, LiFeP0 4 and LiMn 2 0 4 . Electrochemical measurements were conducted using a stainless steel 2032-coin cell and glass microfiber separator in the same manner as for Nb- WsOss and NbisWieOgs described above. The commercial cathode material, Li-250 carbon and PVDF were ground together as described above to prepare an 80/10/10 electrode
• the rate performance of the NMC622 full cell was evaluated in a range of current densities from C/5 to 20C.
• the NMC622 full cell maintained a capacity of 125 mA-h-g 1 at 20C, greater than 75% capacity retention relative to the capacity at C/5 (Fig 19D).
• the NMC622 full cell was cycled at 10C charge and 10C discharge rates for 300 cycles. Under these conditions, a capacity of 120 mA-h-g 1 was observed after
• a full cell comprising a 3:7 (Nbi 6 W 5 0 55 :LT0) anode and an NMC622 cathode was prepared in a stainless steel 2032-coin cell with glass microfiber separator in the same manner as for Nbi 6 W 5 0 55 and NbisWi6093 described above.
• the Nbi6W 5 C>55, LTO, Super P carbon and PVDF were ground together to prepare an 80/15/5 active material/carbon/polymer anode.
• the NMC622, Li250 carbon and PVD were ground together as described above to prepare an 80/10/10 active material/carbon/polymer electrode which served as cathode.
• the electrolyte was 150 pL LP30. No additives were used
• Nb 2 Mo 3 0i 4 displays an average voltage around 2.0 V.
• Nb 2 Mo 3 0i 4 may be used in a voltage window of, for example 3.0 to 1.4 V, or 3.0 to 1.8 V, and still provide a high capacity at a high rate whilst avoiding or minimizing reaction with the electrolyte. This is advantageous as many prospective solid electrolyte materials react with low voltage anodes and this could be mitigated using a Nb2Mo30i 4 anode.
• Wagemaker et al., J. Am. Chem. Soc. 2001 , 123, 1 1454 -1 1461. Wagemaker, et al., Chem. - Eur. J. 2007, 13, 2023 -2028.

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