METHOD OF FABRICATING A COMPONENT MATERIAL FOR A BATTERY CELL

Abstract

A method is provided for fabricating a component material for a battery cell. The method comprises the steps of: providing a partially-fabricated battery cell, the partially-fabricated battery cell comprising a substrate having a planar deposition surface consisting of a first face of the substrate and a first battery component layer provided on the planar deposition surface, the substrate having a plurality of further surfaces, the planar deposition surface and the plurality of further surfaces defining the body of the substrate therebetween; wherein: the first battery component layer contains charge-carrying metal species and has an exposed surface; one or more electrically conductive or semi-conductive pathways extend through at least a portion of the substrate, each of the one or more pathways connecting the planar deposition surface to one of the plurality of further surfaces; and the partially-fabricated battery cell is held in position within a deposition chamber by a holding structure and each site of connection between one of the one or more pathways and the holding structure is electrically insulating; the method further comprising the step of depositing a second battery component layer on the first battery component layer, wherein the depositing comprises forming a plasma within the deposition chamber.

Claims

1. A method of fabricating a component material for a battery cell comprising the steps of: providing a partially-fabricated battery cell, the partially-fabricated battery cell comprising a substrate having a planar deposition surface consisting of a first face of the substrate and a first battery component layer provided on the planar deposition surface, the substrate having a plurality of further surfaces, the planar deposition surface and the plurality of further surfaces defining the body of the substrate therebetween; wherein: the first battery component layer contains charge-carrying metal species and has an exposed surface; one or more electrically conductive or semi-conductive pathways extend through at least a portion of the substrate, each of the one or more pathways connecting the planar deposition surface to one of the plurality of further surfaces; and the partially-fabricated battery cell is held in position within a deposition chamber by a holding structure and each site of connection between one of the one or more pathways and the holding structure is electrically insulating; the method further comprising the step of depositing a second battery component layer on the first battery component layer, wherein the depositing comprises forming a plasma within the deposition chamber.

2. The method according to claim 1, wherein the holding structure is electrically grounded.

3. The method according to claim 1, wherein the holding structure comprises a clamp ring that extends around at least a portion of the perimeter of the substrate.

4. The method according to claim 1, wherein the step of providing a mask within the deposition chamber, the mask comprising at least one aperture and being arranged such that the planar deposition surface faces towards the mask; wherein the mask is electrically isolated from the partially-fabricated battery cell.

5. The method according to claim 4, wherein the mask is electrically connected to the holding structure.

6. The method according to claim 4, wherein the at least one aperture defines an area that is at least 70% of the planar deposition surface.

7. The method according to claim 4, wherein the mask has an annular configuration.

8. The method according to claim 1, wherein the substrate comprises a second face opposed to the first face and one of the one or more pathways extends between the first face and the second face.

9. The method according to claim 8, wherein the substrate comprises a support layer for providing mechanical support to the partially-fabricated battery cell, the support layer having an electrical resistivity of 1000 Ω.Math.cm or less in a direction away from the planar deposition surface.

10. The method according to claim 8, wherein the support layer has a thickness of at least 100 μm.

11. The method according to claim 9, wherein the support layer comprises a material selected from the group consisting of: silicon, copper, iron-nickel-cobalt alloys, molybdenum, molybdenum-copper alloys, aluminium, and stainless steel.

12. The method according to claim 11, wherein the support layer comprises p-type or n-type silicon.

13. The method according to claim 1, wherein at least a portion of one of the one or more pathways is aligned with the planar deposition surface.

14. The method according to claim 13, wherein the planar deposition surface is provided by a current collector layer comprising a material selected from the group consisting of: Pt, Ni, Mo, Al, Au, W, Ti, stainless steel, nickel alloys, and indium doped tin oxide (ITO) and other electrically conducting metal oxides.

15. The method according to claim 14, wherein the current collector layer has a thickness in the range 0.05 to 1 μm.

16. The method according to claim 1, wherein the charge-carrying metal species of the first battery component layer comprises Li.sup.+ ions.

17. The method according to claim 1, wherein the first battery component layer comprises an electrode active material or an electrolyte.

18. The method according to claim 17, wherein the first battery component layer comprises a positive electrode active material.

19. The method according to claim 18, wherein the first battery component layer comprises a positive electrode active material selected from the group comprising: LiCoPO.sub.4, LiNi.sub.0.5Mn.sub.1.5O.sub.4, LiMn.sub.1−yCo.sub.yO.sub.2, LiMnPO.sub.4, LiMn.sub.2O.sub.4, LiCoO.sub.2, LiNiO.sub.2, LiNi.sub.1−yCo.sub.yO.sub.2, LiNi.sub.1−y−zMn.sub.yCo.sub.zO.sub.2 (e.g. LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2), LiFePO.sub.4, LiNiPO.sub.4, Li.sub.2NiPO.sub.4F, Li.sub.2CoPO.sub.4F, LiMnPO.sub.4F, Li.sub.2CoSiO.sub.4, Li.sub.2MnSiO.sub.4, FeF.sub.3, LiMn.sub.0.8Fe.sub.0.1Ni.sub.0.1PO.sub.4, Li.sub.1−xVOPO.sub.4, Li.sub.3V.sub.2(PO.sub.4).sub.3 and Li.sub.2FePO.sub.4F.

20. The method according to claim 1, wherein the first battery component layer comprises an electrode active material and the second battery component layer comprises an electrolyte.

21. The method according to claim 20, wherein the electrolyte comprises a material selected from the group consisting of: LiPON, Li.sub.3PO.sub.4, LiPBON, cation-doped Li.sub.7La.sub.3Zr.sub.2O.sub.12 (wherein the cation dopants may include tantalum, barium, niobium, yttrium, zinc, and combinations thereof), Li.sub.1.5Al.sub.0.5Ge.sub.1.5(PO.sub.4).sub.3, and Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3.

22. The method according to claim 1, the first battery component layer has a thickness in the range 1-40 μm.

23. The method according to claim 1, the second battery layer is deposited to a thickness in the range 0.3-20 μm.

24. The method according to claim 1, wherein the step of depositing the second battery component layer on the first battery component layer comprises forming a radio-frequency plasma.

25. The method according to claim 1, wherein the step of depositing the second battery component layer on the first battery component layer comprises a sputtering process.

Description

DETAILED DESCRIPTION

[0050] The invention will now be described by way of example only with reference to the following Figures in which:

[0051] FIG. 1 shows a schematic cross-sectional view of an example thin film battery structure, the fabrication of which includes the method according to an example of the present invention;

[0052] FIG. 2 shows a schematic cross-sectional view of an embodiment of a deposition system for use in an example of the method of the invention;

[0053] FIGS. 3a) and 3b) show Raman spectra obtained from a layer of lithium cobalt oxide respectively before and after deposition of a LiPON layer on an exposed surface of the lithium cobalt oxide layer, using a deposition process not according to the method of the invention;

[0054] FIG. 4 shows multiple Raman spectra obtained from different depths of a layer of lithium cobalt oxide after deposition of a LiPON layer on an exposed surface of the lithium cobalt oxide layer, using a deposition process not according to the method of the invention;

[0055] FIGS. 5a) and 5b) show Raman spectra obtained from a layer of lithium cobalt oxide respectively before and after deposition of a LiPON layer on an exposed surface of the lithium cobalt oxide layer, the deposition process being performed following an example of the method according to the invention.

[0056] FIGS. 6a) and 6b) show scanning electron micrographs obtained from a cross-section of a stack of battery component layers deposited on a semiconductor support layer. The electrolyte of the stack shown in FIG. 6a) was deposited according to an example of the method of the invention. The electrolyte of the stack shown in FIG. 6b) was not deposited according to the method of the invention.

[0057] FIGS. 7a) and 7b) are graphs of multiple charge-discharge cycles, shown as a plot of voltage against time, for thin film batteries built up on a semiconductor support layer. The electrolyte of the battery of FIG. 7a) was deposited according to an example of the method of the invention. The electrolyte of the battery of FIG. 7b) was not deposited according to the method of the invention.

[0058] FIGS. 8a) and 8b) are graphs of the electrical AC response used to determine the impedance of thin film batteries.

[0059] FIG. 1 shows a schematic cross-sectional view (not to scale) of a thin film structure 10, through the depth d of a multi-layer thin film stack 12 supported on the planar surface 14a of a support layer 14.

[0060] The layers of the stack 12 comprise layers suitable for implementing various components of a battery. The support layer 14 is a wafer of electrically semi-conductive or conductive material (such as p-type boron-doped silicon grown by the Czochralski method (CZ silicon)).

[0061] A first layer on the surface 14a is an adhesion layer 16, comprising a layer of titanium dioxide. Overlaying this is a layer providing a cathode current collector 18, formed from platinum. A layer of lithium cobalt oxide forms a cathode 20 covering the platinum layer 18. An electrolyte separator layer 22 comprising lithium phosphorous oxynitride (LiPON) is deposited on top of the cathode 20. An anode layer 24 overlies the electrolyte layer 22. Finally, an anode current collector 26 is provided over the anode 24. This structure is purely exemplary, however, and the structure may comprise more, fewer or other layers formed from other materials.

[0062] Once fabricated or otherwise obtained, the structure 10 can be cut to isolate individual battery elements or cells one from another, e.g. using a laser cutting technique.

[0063] With reference to FIG. 2, a deposition system 110 for use in a method according to the invention comprises a deposition chamber 112 in which are located a target 114 and a disc-shaped support layer 14. The support layer is held in position by a clamp ring 118 extending about the perimeter of the support layer. The clamp ring 118 comprises an electrically conductive material. An annular spacer 120 is provided between the support layer 14 and the clamp ring 118, the spacer consisting of a dielectric material (such as a Kapton® film or pyrolytic boron nitride), such that the clamp ring 118 and the support layer 14 are electrically insulated from each other.

[0064] A first battery component layer 20 is provided on the side of the support layer 14 that faces towards the target. Additional layers may be provided between the support layer 14 and the first battery component layer 20, as shown in FIG. 1. The support layer 14 provides mechanical support for the first battery component layer 20 and any intermediate layers. The support layer 14, the first battery component layer 20 and any intermediate layers together provide a partially fabricated battery cell.

[0065] In this example, the first battery component layer 20 has been patterned, that is, it is provided in the form of multiple discrete coplanar elements. However, in alternative embodiments, the first battery component layer may be unpatterned and co-terminous with the substrate.

[0066] An open mask 124 is provided between the support layer 14 and the target 114. The open mask 124 has an annular configuration having a central aperture that defines an area that is at least 70% of that of one face of the support layer 14. For example, the central aperture may have a diameter of 130 mm or 140 mm, while the support layer 14 may have a diameter of 150 mm.

[0067] The open mask 124 is electrically connected to the clamp ring 118 via connector pin 126, that is, the open mask is shorted to the clamp ring. The open mask 124 is separated from the partially fabricated battery cell by an annular spacer 127 consisting of a dielectric material. In alternative configurations, the open mask 124 may be spaced apart from the partially fabricated battery cell provided by support layer 14, the first battery component layer, and any intermediate layers, by a distance of about 0.1 mm.

[0068] The open mask 124 is provided by an electrically conductive material, for example a metal or metal alloy such as stainless steel or molybdenum, and typically has a thickness of 0.1 mm to 0.5 mm.

[0069] The clamp ring 118 is electrically grounded via one of the internal surfaces of the deposition chamber 112. Thus, both the clamp ring 118 and the open mask 124 are electrically grounded, while the partially fabricated battery cell is electrically floating within the deposition chamber.

[0070] A vacuum pump system 128 and a process gas delivery system 130 are in communication with the deposition chamber 112. A radio frequency plasma power source 132 is connected to the target 114.

[0071] In use, the partially fabricated battery cell is initially held outside the deposition chamber 112, while the chamber is evacuated to a pressure of 1×10.sup.−7 Torr by means of vacuum pump system 128. The partially fabricated battery cell, supported by clamp ring 118, and having open mask 124 located on the deposition surface (the mask being electrically isolated from the deposition surface), is then introduced into the deposition chamber 112, and positioned so that the mask 124 is between target 114 and the partially-fabricated battery cell, as shown in FIG. 2. The mask 124 is shorted to the clamp ring 118 by means of connector pin 126.

[0072] Controlled amounts of process and/or reactive gas are introduced into the deposition chamber by means of process gas delivery system 130. During the sputtering process the chamber pressure is typically 1×10.sup.−3 Torr. The plasma power source 132 is then activated to energize the process and/or reactive gas and form a radio frequency plasma in the deposition chamber 112.

[0073] The presence of the high-energy ions in the plasma causes material to be ejected from the target 114 to form a fine spray that becomes at least partly deposited on the exposed surface of the first battery component layer 20, so as to form a second battery component layer (not shown). That is, the second battery component layer is deposited through a sputtering process.

[0074] In certain embodiments, the target 114 is a lithium phosphate target (Li.sub.3PO.sub.4), the reactive gas is nitrogen, and the second battery component layer is LiPON.

[0075] Raman Measurements

[0076] FIG. 3a) shows a Raman spectrum obtained from a layer of lithium cobalt oxide provided on a semi-conductive support layer of p-type boron-doped silicon. Well-defined peaks are observed for this oxide layer at about 485 cm.sup.−1 and at approximately 595 cm.sup.−1.

[0077] FIG. 3b) shows a Raman spectrum obtained from the same section of the oxide layer as FIG. 3a), after deposition of a thin 200 nm LiPON layer on the exposed surface of the oxide layer. The LiPON deposition was not carried out according to the method of the present invention. Specifically, the support layer was not electrically floating, but was instead in electrical contact with the clamp ring that was used to hold it in place in the deposition chamber.

[0078] FIG. 3b) displays a well-defined peak at just below about 700 cm.sup.−1, while the prominence of the peaks originally observed in FIG. 3a) at about 485 cm.sup.−1 and at approximately 595 cm.sup.−1 is significantly reduced. This is thought to be due to the fact that, during the process of deposition of LiPON on the lithium cobalt oxide layer, Li.sup.+ ions have migrated out of the lithium cobalt oxide layer, resulting in attenuation of the characteristic peaks for lithium cobalt oxide at about 485 cm.sup.−1 and at about 595 cm.sup.−1, and the development of a new peak at just below about 700 cm.sup.−1 that is associated with the presence of a cobalt oxide-type (Co.sub.3O.sub.4) phase.

[0079] FIG. 4 shows multiple Raman spectra obtained from an oxide layer, originally provided in the form of lithium cobalt oxide, after deposition of a LiPON layer on an exposed surface of the lithium cobalt oxide layer. The LiPON deposition was not carried out according to the method of the present invention. Specifically, the electrically conductive support layer on which the lithium cobalt oxide layer was provided was not electrically floating, but was instead in electrical contact with the clamp ring that was used to hold it in place in the deposition chamber.

[0080] The spectra are obtained from different depths of the oxide layer (originally provided in the form of lithium cobalt oxide), such that the spectrum closest to the x axis of FIG. 4 corresponds to the portion of the oxide layer closest to the oxide/LiPON interface, while the spectrum furthest from the x axis corresponds to the portion of oxide layer closest to (and adjacent to) the support layer. That is, the further a spectrum is shown from the x axis of FIG. 4, the closer the corresponding portion of oxide layer is to the support layer.

[0081] Spectra taken from close to the support layer (e.g. spectra (a) and (b)) exhibit defined peaks at about 485 cm.sup.−1 and at about 595 cm.sup.−1, indicating the presence of lithium cobalt oxide (the peak at about 525 cm.sup.−1 in spectrum (a) is due to the support layer). There is no discernible presence of a cobalt oxide phase, which would be indicated by peaks at just below about 700 cm.sup.−1.

[0082] By contrast, spectra taken from close to the interface between the oxide layer and the deposited LiPON layer (e.g. spectra (c) to (i)) do not exhibit any discernible peak at about 485 cm.sup.−1 and at about 595 cm.sup.−1, but defined peaks are generally observed at just below about 700 cm.sup.−1. It is thought that the increased prominence of the peak at just below about 700 cm.sup.−1 in the portions of the oxide layer close to the interface with the deposited LiPON layer is an indication of delithiation of the lithium cobalt oxide during LiPON deposition, resulting in the presence of cobalt oxide-type (Co.sub.3O.sub.4) phase.

[0083] FIG. 5a) shows a Raman spectrum obtained from a layer of lithium cobalt oxide provided on an electrically conductive support layer. Well-defined peaks are observed at about 485 cm.sup.−1 and at approximately 595 cm.sup.−1.

[0084] FIG. 5b) shows a Raman spectrum obtained from the same section of the lithium cobalt oxide layer as FIG. 5a), after deposition of a LiPON layer on the exposed surface of the lithium cobalt oxide layer. The deposition of the LiPON layer was carried out according to an example of the method according to the present invention. The spectrum of FIG. 5b) is broadly similar to that of FIG. 5a), indicating that no significant compositional change has occurred in the lithium cobalt oxide layer during deposition of the LiPON layer.

[0085] Thickness Measurements

[0086] FIG. 6a) shows a scanning electron micrograph of a cross-section of a stack of battery component layers including a lithium cobalt oxide cathode, a LiPON electrolyte and an anode material, that have been deposited onto a semi-conductive support layer (a silicon wafer). The LiPON electrolyte layer was deposited according to an example of the method of the invention (that is, the partially-fabricated battery was insulated from the clamp ring during deposition of the electrolyte layer).

[0087] D1 denotes the thickness of the lithium cobalt oxide cathode layer (6.7 μm); D2 denotes the thickness of the LiPON electrolyte layer (2.4 μm); and D3 denotes the thickness of the anode layer (0.89 μm).

[0088] FIG. 6b) shows a scanning electron micrograph of a cross-section of a stack of battery component layers, including a lithium cobalt oxide cathode, a LiPON electrolyte and an anode material, that had been deposited onto a semi-conductive support layer (a silicon wafer). The LiPON electrolyte layer was not deposited according to the method of the invention (that is, the partially-fabricated battery was not insulated from the clamp ring during deposition of the electrolyte layer). Otherwise, the deposition conditions were chosen so as to match those used for the stack shown in FIG. 6a).

[0089] D1 denotes the thickness of the cathode layer (5.5 μm) and D3 denotes the thickness of the anode layer (1.1 μm).

[0090] It was found that the cathode and the anode layers were separated by an intermediate layer (D4) having a thickness of 4.01 μm. This layer appears to be a composite layer made up of a LiPON layer (D2) adjacent the anode and a reaction layer adjacent the cathode. This reaction layer does not have good ionic conductivity and its presence also increases the separation of the cathode and the anode, with the result that the internal resistance of the battery is increased.

[0091] Battery Cycling

[0092] FIG. 7a) shows a graph of multiple charge and discharge cycles measured from a thin film battery built up from layers deposited on a semi-conductive support layer (a silicon wafer), in which the LiPON electrolyte was deposited according to an example of the method of the invention. This confirms good performance of the battery.

[0093] FIG. 7b) shows the results of a similar attempt at battery cycling, carried out on a battery that was built up on a semi-conductive support layer (a silicon wafer), but for which the LiPON electrolyte was not deposited according to the method of the invention, that is, the support layer was not electrically insulated from the clamp ring. From this it may be seen that the battery failed to complete even a single cycle. This is thought to be due to the migration of lithium ions from the lithium cobalt oxide towards the deposited LiPON layer during the deposition process, resulting in a LiPON-reaction layer that increases the internal resistance of the battery. The impact of this increased internal resistance is observed clearly during charging, the battery reaches the cut-off voltage of 4V in only a few seconds and thus the battery was not able to be charged.

[0094] Impedance Measurements

[0095] Impedance measurements were carried out on batteries prepared by depositing the following layers on a support layer of boron-doped silicon: [0096] Adhesion layer: titanium dioxide; [0097] Cathode current collector layer: platinum; [0098] Cathode layer: lithium cobalt oxide cathode; [0099] Electrolyte layer: LiPON electrolyte; [0100] An anode material; [0101] An anode current collector layer.

[0102] The measurements were carried out using an Impedance Analyzer. The impedance was measured using an AC excitation signal with an amplitude of 10 mV across a frequency range of 1 MHz to 0.1 Hz. The response at each frequency was determined using a five second integration time. Seven frequencies were measured per decade with logarithmic spacing between the upper and lower frequency limits.

[0103] FIG. 8a) shows a Nyquist diagram for a battery in which the electrolyte was deposited according to an example of the method of invention (these results are indicated by the continuous line) and one for a battery not deposited in accordance with the method of the invention (these results are indicated by the dashed line). FIG. 8b) shows an enlarged portion of FIG. 8a).

[0104] In these diagrams, -Im(Z) (the imaginary part of the complex impedance) is plotted against Re(Z) (the real part of the complex impedance). As can be seen from FIG. 8b), both curves are made up of an arc-shaped portion adjacent the origin of the graph (middle to high frequencies) and a linear portion at higher values of the x and y axes (lower frequencies). The diameter of this arc-shaped portion of the curve in the middle-to-high frequency range is a function of the impedance of the charge transfer process occurring in the battery.

[0105] As may be seen from FIG. 8b), this impedance is about 1000Ω for the battery deposited according to a method of the invention. By contrast, the battery deposited without isolating the support layer from the sample holder has a significantly higher impedance of about 40000Ω, indicating the presence of a source of increased resistance within the battery.