Methods for solid electrolyte interphase formation and anode pre-lithiation of lithium ion capacitors

A method of pre-doping an anode of an energy storage device can include immersing the anode and a dopant source in an electrolyte, and coupling a substantially constant current between the anode and the dopant source. A method of pre-doping an anode of an energy storage device can include immersing the anode and a dopant source in an electrolyte, and coupling a substantially constant voltage across the anode and the dopant source. An energy storage device can include an anode having a lithium ion pre-doping level of about 60% to about 90%.

BACKGROUND

Field

The present invention relates generally to electric energy storage devices, and more specifically, to a method of pre-doping an electrode of an energy storage device with ionic species.

Description of the Related Art

Lithium ion capacitors may be used to power a diverse range of electronic devices, including for example in wind power generation systems, uninterruptible power source systems (UPS), photo voltaic power generation, and/or energy recovery systems in industrial machinery and transportation systems. Lithium ion capacitors can have a variety of shapes (e.g., prismatic, cylindrical and/or button shaped). A lithium ion capacitor (LIC) can include an anode and a cathode immersed in an electrolyte which provides a transport of ionic species between the anode and the cathode. Lithium ion capacitors can be a type of hybrid ultracapacitor, exhibiting significant electrostatic and electrochemical energy storage. For example, electrical charge can be stored at an electrical double layer formed at an interface between an electrolyte and an electrode (e.g., between a lithium ion capacitor electrolyte and a lithium ion capacitor cathode). Electrical energy in a lithium ion capacitor may also be stored through adsorption of ionic species into an electrode (e.g., adsorption of lithium ions into a lithium ion capacitor anode). Lithium ions can be incorporated into the anode of the lithium ion capacitor through a pre-doping process.

A solid-electrolyte interphase (SEI) layer may form adjacent a surface of a lithium-ion capacitor anode. A solid-electrolyte interphase layer may form during an anode pre-doping process. For example, the solid-electrolyte interphase layer may form at least in part due to an electrochemical reaction at the anode surface involving an electrolyte solvent and/or an electrolyte salt. The solid-electrolyte interphase layer may electrically insulate the anode while allowing an ionic transport to the anode.

SUMMARY

Embodiments can include a method of pre-doping an anode of an energy storage device, the method including immersing the anode and a dopant source in an electrolyte, where the dopant source can include a source for lithium ions. The method can include coupling a substantially constant current between the anode and the dopant source.

In some embodiments, the energy storage device can include a lithium ion capacitor.

In some embodiments, the coupling can include coupling the substantially constant current between the anode and the dopant source for a duration of time to achieve a potential difference between the anode and the dopant source of about 0.01 Volts (V) to about 0.4 V. In some embodiments, the coupling can include coupling the substantially constant current between the anode and the dopant source for a duration of time to achieve an anode pre-doping level of about 60% to about 90%. In some embodiments, coupling the substantially constant current between the anode and the dopant source can include coupling a current source supplying a substantially constant current corresponding to a current C-rate of about C/72 to about C/144.

In some embodiments, the method can include forming a substantially homogeneous solid-electrolyte interphase layer adjacent the anode, where the solid-electrolyte interphase layer is substantially undisturbed subsequent to its formation.

In some embodiments, the method can include removing the dopant source from the electrolyte subsequent to coupling the substantially constant current across the anode and the dopant source. In some embodiments, the method can include immersing a cathode in the electrolyte, wherein immersing the dopant source can include immersing the dopant source to a side of the anode opposite that facing the cathode.

In some embodiments, the method can include performing a formation step subsequent to coupling the substantially constant current across the anode and the dopant source. The formation step can include applying a substantially constant voltage of about 2 Volts (V) to about 4.2 V between the anode and the dopant source. In some embodiments, performing the formation step can include applying the substantially constant voltage of about 2 Volts (V) to about 4.2 V between the anode and the dopant source for a duration of about 5 hours to about 75 hours.

Embodiments can include a method of pre-doping an anode of an energy storage device, the method including immersing the anode and a dopant source in an electrolyte, where the dopant source can include a source for lithium ions. The method can include coupling a substantially constant voltage across the anode and the dopant source.

In some embodiments, the energy storage device can include a lithium ion capacitor.

In some embodiments, coupling the substantially constant voltage across the anode and the dopant source can include coupling a voltage source supplying a substantially constant voltage of about 0.01 Volts (V) to about 0.4V. In some embodiments, coupling the substantially constant voltage across the anode and the dopant source can include coupling the substantially constant voltage for a duration of time to achieve an anode lithium ion pre-doping level of about 60% to about 90%.

In some embodiments, the method can include performing a formation step subsequent to coupling the substantially constant voltage across the anode and the dopant source. The formation step can include applying a substantially constant voltage of about 2 Volts (V) to about 4.2 V between the anode and the dopant source. In some embodiments, performing the formation step can include applying the substantially constant voltage of about 2 Volts (V) to about 4.2 V between the anode and the dopant source for a duration of about 5 hours to about 75 hours.

In some embodiments, the method can include forming a substantially homogeneous solid-electrolyte interphase layer adjacent the anode, where the solid-electrolyte interphase layer can be substantially undisturbed subsequent to its formation.

In some embodiments, the method can include removing the dopant source subsequent to coupling the substantially constant voltage across the anode and the dopant source. In some embodiments, the method can include immersing a cathode in the electrolyte, where immersing the dopant source can include immersing the dopant source to a side of the anode opposite that facing the cathode.

Embodiments can include an energy storage device having a cathode, an anode having a lithium ion pre-doping level of about 60% to about 90%; and a separator between the anode and the cathode configured to provide electrical insulation between the anode and the cathode.

In some embodiments, the device can include a dopant source to a side of the anode opposite that facing the cathode. In some embodiments, the dopant source can include lithium metal. In some embodiments, the device can include a second separator between the anode and the dopant source.

In some embodiments, the device can include a non-aqueous electrolyte conductive of lithium ions. In some embodiments, the anode can include graphite.

In some embodiments, the energy storage device can include a lithium ion capacitor.

For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages are described herein. Of course, it is to be understood that not necessarily all such objects or advantages need to be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that can achieve or optimize one advantage or a group of advantages without necessarily achieving other objects or advantages.

All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments will become readily apparent to those skilled in the art from the following detailed description having reference to the attached figures, the invention not being limited to any particular disclosed embodiment(s).

DETAILED DESCRIPTION

Although certain embodiments and examples are described below, those of skill in the art will appreciate that the invention extends beyond the specifically disclosed embodiments and/or uses and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention herein disclosed should not be limited by any particular embodiments described below.

Embodiments of the invention relate to lithium ion capacitors and methods of making these capacitors to facilitate improved capacitor performance. In one embodiment, the capacitors are made by pre-doping a capacitor anode using a constant voltage method. The constant voltage pre-doping method can include immersing a dopant source and the capacitor anode in an electrolyte. A constant voltage can be applied across the anode and the dopant source for a duration of time such that a solid-electrolyte interphase layer forms adjacent a surface of the anode. A voltage and/or a duration of a constant voltage pre-doping step can be selected to facilitate achieving a desired level of anode lithium ion pre-doping, for example facilitating a desired capacitor electrical and/or life cycle performance. The constant voltage pre-doping method can include providing a constant or substantially constant voltage of about 0.01 Volts (V) to about 0.4 V between the anode and the dopant source. In one embodiment, the capacitors are made by pre-doping a capacitor anode using a constant current method. The constant current pre-doping method can involve maintaining a constant current between a dopant source and the anode while the dopant source and the anode are immersed an electrolyte. The constant current can be maintained for a duration of time such that a solid-electrolyte interphase layer forms adjacent a surface of the anode. A current and/or a duration of a constant current step can be selected to facilitate achieving a desired voltage difference between the anode and the dopant source. The constant current pre-doping method can include maintaining a constant or substantially constant current between the anode and the dopant source such that a voltage difference of about 0.01 Volts (V) to about 0.4 V can be achieved between the anode and the dopant source. The constant current pre-doing method can include maintaining a constant or substantially constant current corresponding to a current C-rate of about C/24 to about C/144 between the anode and the dopant source. A voltage, a duration of a constant voltage pre-doping step, a current, and/or a duration of a constant current step can be selected to achieve a desired level of anode lithium ion pre-doping, including for example an anode lithium pre-doping level of about 60% to about 90%, such as to facilitate a desired capacitor electrical and/or life cycle performance. A suitable dopant source for the constant voltage and/or constant current pre-doping method provides a source of lithium ions. The constant voltage pre-doping method and/or the constant current pre-doping method can be performed in-situ such that the solid electrolyte interphase layer can be undisturbed subsequent to its formation. The constant voltage pre-doping method and/or the constant current pre-doping method may provide increased control in the formation of the solid-electrolyte interphase layer, facilitating formation of a more uniform and/or more stable solid-electrolyte interphase layer. Improved uniformity and/or stability of the solid-electrolyte interphase layer can improve a capacitance, resistance, and/or reliability performance of the lithium ion capacitor.

As described herein, and as shown inFIG. 1, a lithium ion capacitor (LIC)10can have an anode12in ionic communication with a cathode14. The anode12and the cathode14may be immersed in an electrolyte28, the electrolyte28providing a transport of ionic species between the anode12and the cathode14. The electrolyte28may include an electrolyte solvent and an electrolyte salt, the electrolyte salt including an anion and a cation. The electrolyte28may be a non-aqueous electrolyte conductive of lithium ions. For example, the electrolyte28may include a lithium salt and/or an ammonium salt. In some embodiments, the electrolyte28can include an aprotic organic solvent. The electrolyte solvent may provide a desired salt solubility, viscosity, and/or level of chemical and/or thermal stability for a temperature range. For example, the electrolyte solvent may include an ether and/or an ester. In some embodiments, the electrolyte solvent can include a propylene carbonate, a dimethyl carbonate, a vinylene carbonate, a diethylene carbonate, an ethylene carbonate, a sulfolane, an acetonitrile, a dimethoxyethane, a tetrahydrofuran, an ethylmethyl carbonate, combinations thereof, and/or the like. In some embodiments, an electrolyte salt can include hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(trifluoromethansulfonyl)imide (LiN(SO2CF3)2), lithium trifluoromethansulfonate (LiSO3CF3), combinations thereof, and/or the like.

The lithium ion capacitor10can include a separator24between the anode12and the cathode14. The separator24may be configured to permit a transport of ionic species between the anode12and the cathode14, while preventing an electrical short between the anode12and the cathode14. In some embodiments, a separator can be made of a porous electrically insulating material. In some embodiments, the separator can be a polymeric material. In some embodiments, the separator can be made of paper.

The anode12may include an anode current collector16and the cathode14may include a cathode current collector18. The anode current collector16and/or the cathode current collector18may be configured to facilitate an electrical connection between the anode and/or the cathode and an external circuit, respectively. A current collector (e.g., the anode current collector16and/or the cathode current collector18) can be made of a conductive material, including for example a metallic material. In some embodiments, the current collector can be made of an aluminum foil. In some embodiments, the current collector can be made of silver, copper, gold, platinum, palladium, and/or alloys of the metals. Other suitable conductive materials may also be possible. The current collector may have any suitable shape and/or dimension (e.g., a width, a length, and/or a thickness). For example, the current collector may have a rectangular or substantially rectangular shape (e.g., a rectangular aluminum foil). In some embodiments, the current collector can have a thickness of about 20 microns to about 100 microns. In some embodiments, the current collector can have a thickness of about 30 microns to about 50 microns, for example about 40 microns.

In some embodiments, the cathode14can include a first cathode electrode film22adjacent a surface30of the cathode current collector18. In some embodiments, the cathode14can include a second cathode electrode film adjacent a surface32of the cathode current collector18opposite that adjacent the first cathode electrode film22. The cathode14can include a cathode active material component. In some embodiments, the cathode electrode film22can be made of a cathode active material comprising a porous material. For example, the porous active material may provide a high surface area for the cathode14. In some embodiments, the porous material may comprise a porous carbon material, including but not limited to particles of activated carbon. The activated carbon can provide a porosity (e.g., a distribution of micropores, mesopores, and/or macropores) configured to facilitate a lithium ion capacitor performance.

In some embodiments, an cathode electrode film22can include a binder component and/or an additive component. In some embodiments, the binder component may provide structural support for the active electrode material. For example, the binder component may comprise one or more polymers. The polymers may provide a polymeric matrix support structure for one or more other components of the cathode electrode film22. In some embodiments, the binder component can comprise a fluoropolymer (e.g., polytetrafluoroethylene, PTFE), a polypropylene, a polyethylene, co-polymers thereof, and/or polymer blends thereof. In some embodiments, the cathode electrode film22can comprise at least one of a conductive additive component, for example to improve an electrical conductivity of the cathode electrode film22. The conductive additive component may comprise conductive carbon particles (e.g., graphite and/or graphene). Other active material components, binder components, and/or additive components may also be suitable.

Composition of a cathode electrode film22may be optimized to enable a desired lithium ion capacitor performance. For example, the composition of a cathode electrode film22may be configured to provide a desired capacitor capacitance and/or resistance, for example providing a desired device energy density and/or power density performance. In some embodiments, the composition of a cathode electrode film22may be configured to provide a desired cycling performance. In some embodiments, a cathode electrode film22can comprise from about 50% to about 99% by weight of a cathode active material component (e.g., activated carbon), including from about 60% to about 95% by weight. In some embodiments, a cathode electrode film22can comprise from about 1% to about 50% by weight of a binder component. In some embodiments, a cathode electrode film22can comprise up to about 30% by weight of an additive component, including for example a conductive additive component for promoting electrical conductivity of the cathode.

In some embodiments, the anode12can include a first anode electrode film20adjacent a surface34of an anode current collector16. In some embodiments, the anode12can include a second anode electrode film adjacent a surface36of the anode current collector16opposite that adjacent the first anode film20. An anode electrode film can be made of a material which can reversibly intercalate lithium ions. For example, an electrode film can comprise a carbon material which can reversibly intercalate lithium ions, including but not limited to a graphite material.

In some embodiments, an anode electrode film20can include an additive component and/or a binder component. For example, an anode electrode film20can include a conductive additive, such as an additive component for promoting electrical conductivity of the anode. In some embodiments, the conductive additive can be a conductive carbon additive, such as a conductive carbon black material. In some embodiments, a binder component of the anode electrode film20can include one or more polymers configured to provide a polymeric matrix support structure, including a fluoropolymer (e.g., polytetrafluoroethylene, PTFE), a polypropylene, a polyethylene, co-polymers thereof, and/or polymer blends thereof.

Composition of an anode electrode film20may be optimized to enable a desired lithium ion capacitor performance, for example, a desired energy density, power density, and/or cycling performance. In some embodiments, a anode electrode film20can comprise from about 50% to about 99% by weight of an active material component (e.g., graphite), including from about 60% to about 95% by weight. In some embodiments, an anode electrode film20can comprise from about 1% to about 50% by weight of a binder component. In some embodiments, a cathode electrode film2an anode electrode film20can comprise up to about 30% by weight of a conductive carbon additive component, including for example a conductive carbon black material.

A solid-electrolyte interphase (SEI) layer26can form adjacent a surface of a lithium ion capacitor anode12, for example during an anode pre-doping step. In some embodiments, the solid-electrolyte interphase layer26can form due to an electrochemical reaction involving an electrolyte solvent and/or an electrolyte salt at a surface of the lithium ion capacitor anode12adjacent to the electrolyte28. For example, the solid-electrolyte interphase layer26may form due at least in part to a decomposition of one or more components of the electrolyte28. The solid-electrolyte interphase layer26may provide a layer adjacent to the anode12which can provide electrical insulation for the anode12while being permeable to one or more ionic species.

In some embodiments, a lithium ion capacitor which includes a solid-electrolyte interphase layer26providing improved access of lithium ions to the anode12can provide a lithium ion capacitor10with an improved performance. Improved control in forming a solid-electrolyte interphase layer26has been found to facilitate formation of a solid-electrolyte interphase layer having an improved uniformity (e.g., increased homogeneity in a structure and/or composition of the SEI), a decreased thickness, an increased stability (e.g., a thermal and/or chemical stability), and/or a reduced resistance to a transport of lithium ions therethrough. In some embodiments, a solid-electrolyte interphase layer having an improved uniformity, a reduced thickness, an improved stability, and/or an increased permeability of lithium ions may facilitate a lithium ion capacitor having an improved capacitance, a decreased equivalent series resistance, and/or an improved reliability.

In some embodiments, a characteristic or parameter of an anode pre-doping process has been found to affect a characteristic of the solid-electrolyte interphase layer26. For example, a characteristic of the solid-electrolyte interphase26layer may depend at least in part on a level to which the anode12is incorporated with lithium ions in the pre-doping process, such as a percentage of available intercalation sites having an intercalated lithium ion (e.g., an anode lithium ion pre-doping level). In some embodiments, an improved control in one or more parameters of an anode pre-doping process was found to facilitate formation of a solid-electrolyte interphase layer having a reduced thickness, an improved uniformity, stability, and/or permeability of lithium ions, such as improved control in the rate at which and/or the level to which an anode12is pre-doped with lithium ions.

Referring toFIG. 2, in one embodiment, an anode42of a lithium ion capacitor40can be pre-doped using a constant voltage charging or constant voltage pre-doping process. Pre-doping a lithium ion capacitor anode42using a pre-doing process comprising a constant voltage pre-doping step may facilitate formation of a solid-electrolyte interphase layer having an increased uniformity, stability and/or permeability to lithium ions, thereby providing a lithium ion capacitor having an improved capacitance, a decreased equivalent series resistance and/or an improved cycling performance. For example, pre-doping an anode using a constant voltage pre-doping process may facilitate the formation of a pin-hole free or substantially pin-hole free solid-electrolyte interphase layer, thereby providing a lithium ion capacitor having a reduced degree of capacitance fade after a number of charge-discharge cycles. In some embodiments, a pre-doping process comprising a constant voltage pre-doping step may facilitate a reduced duration of time to achieve a desired level of anode pre-lithiation.

Pre-doping an anode42by applying a controlled voltage across the anode42and a dopant source46, for example instead of short circuiting the anode42and the dopant source46, was found to facilitate a formation of a solid-electrolyte interphase layer having an increased uniformity, thereby providing a lithium ion capacitor having decreased equivalent series resistance and/or improved cycling performance. A pre-doping process which includes applying a short circuit across the anode42and the dopant source46, instantaneously or substantially instantaneously creating a potential difference of about 0V between the anode42and the dopant source46, was found to provide a solid-electrolyte interphase layer having reduced uniformity, stability, and/or permeability to lithium ions, and/or providing a pre-doping process requiring a longer duration to achieve a desired level of anode pre-lithiation. For example, a solid-electrolyte interphase layer may form at a potential difference of about 1V. Reducing a potential difference between the anode42and the dopant source46instantaneously or substantially instantaneously to about 0V may provide a less controlled process of pre-doping of the anode42, for example providing a less controlled process of solid-electrolyte interphase layer formation during the pre-doping process.

In some embodiments, as shown inFIG. 2, an apparatus for pre-doping a lithium ion capacitor anode42can include a dopant source46and the anode42immersed in an electrolyte54(not shown). In some embodiments, the dopant source46can comprise a source of lithium ions. For example, the dopant source46can comprise a lithium metal. The dopant source46may be positioned to a side of the anode42. For example, the dopant source46may be placed to a side of the anode42opposite that facing the capacitor cathode44. In some embodiments, the pre-doping apparatus can include a separator48between the dopant source46and the anode42. The separator48may be configured to permit a transport of ionic species (e.g., lithium ions) between the anode42and the dopant source46, while preventing an electrical short between the anode42and the dopant source46. In some embodiments, the separator48can be made of a porous electrically insulating material (e.g., a material comprising a polymer, including a cellulosic material).

In some embodiments, pre-doping a lithium ion capacitor anode42can be performed in-situ. Referring toFIG. 2, in some embodiments, pre-doping a lithium ion capacitor42can be performed in a lithium ion capacitor cell40comprising the anode42, the dopant source46, a capacitor cathode44, and a separator48between the anode42and cathode44, and a separator48between the anode42and the dopant source46. The anode42, the dopant source46, the cathode44, and the separators48may be immersed in an electrolyte54(not shown). The dopant source46may be consumed during the constant voltage charging or constant voltage pre-doping step. In some embodiments, the dopant source46may be completely or substantially completely consumed during the constant voltage pre-doping step. In some embodiments, at least a portion of the dopant source46remains after the constant voltage pre-doping step, any remaining dopant source46being removed upon completion of the pre-doping process. In some embodiments, any remaining dopant source46can be removed from a lithium ion capacitor40and the lithium ion capacitor40can subsequently be sealed. For example, a solid-electrolyte interphase layer formed during an anode pre-doping process comprising a constant voltage pre-doping step can be undisturbed or substantially undisturbed subsequent to its formation. In some embodiments, the remaining dopant source46may not be removed until after completion of a formation step performed subsequent to the constant voltage pre-doping step, as described in more detail herein.

In some embodiments, a voltage source52can be positioned across the anode42and the dopant source46(e.g., a lithium metal electrode), the voltage source52providing a constant or substantially constant voltage across the anode and the dopant source46, the dopant source46. For example, the dopant source46may be coupled to a first electrode of a voltage source52, such as a positive electrode of the voltage source52, and the anode42may be coupled to a second electrode of the voltage source52, such as a negative electrode of the voltage source52, such that the voltage source52can maintain a desired potential difference between the dopant source46and the anode42. In some embodiments, a constant or substantially constant voltage can be applied across the anode42and the dopant source46for a duration of time to achieve a desired level of anode pre-lithiation. During a pre-doping process, dopants at the dopant source46may be released. For example, lithium metal at a dopant source46comprising a lithium metal electrode may be oxidized. Oxidation of the lithium metal may facilitate a release of lithium ions, thereby providing lithium ions for incorporation into the anode42.

A characteristic or a parameter of an anode pre-doping process may affect a characteristic of a solid-electrolyte interphase layer formed adjacent an anode surface during the anode pre-doping process. In some embodiments, a formation of a solid-electrolyte interphase layer adjacent the anode42can depend at least in part on a voltage applied across the anode42and the dopant source46, a duration in which the voltage is applied across the anode42and the dopant source46, and/or a desired level of dopant incorporation into the anode42. For example, a voltage value which is applied across the anode42and the dopant source46during an anode pre-doping process, a level of dopant incorporation into the anode42during the pre-doping process, and/or a duration for which the voltage value is applied across the anode42and the dopant source46may affect a thickness, a uniformity, a stability and/or a permeability of a solid-electrolyte interphase layer which can form adjacent an anode surface during the anode pre-doping process.

A voltage applied across the anode42and the dopant source46during a pre-doping process, a duration for which the voltage is applied across the anode42and the dopant source46, and/or a level of anode pre-lithiation may be determined based at least in part on a desired lithium ion capacitor performance. For example, an applied voltage, a duration in which the voltage is applied, and/or a level of anode pre-lithiation may be selected based at least in part on a desired lithium ion capacitor equivalent series resistance performance, a capacitor capacitance performance, and/or a capacitor cycling performance. In some embodiments, a cycling performance of the lithium ion capacitor may comprise a degree of capacitance fade exhibited by the capacitor after a number of charge-discharge cycles. For example, a voltage applied across the anode42and the dopant source46and a duration in which the voltage is applied may be selected based at least in part on a desired level of anode pre-lithiation, the level of pre-lithiation corresponding to a formation of a solid-electrolyte interphase layer adjacent a surface of the anode having desirable characteristics, thereby providing a lithium ion capacitor having an improved performance (e.g., a reduced equivalent series resistance, and/or improved cycling performance).

In some embodiments, a constant voltage pre-doping process includes applying a voltage of about 0.001 volts (V) to about 0.400 volts (V), including from about 0.01V to about 0.2V, across an anode42and a dopant source46. For example, a voltage of about 0.01V to about 0.4V may be applied across the anode42and the dopant source46. For example, a voltage of about 0.1V may be applied across the anode42and the dopant source46.

In some embodiments, a formation step can be performed subsequent to a constant voltage pre-doping step. A formation step may facilitate improvement and/or stabilization in a characteristic of a solid-electrolyte interphase layer formed during the pre-doping step. For example, the formation step can facilitate stabilization of a structural, thermal and/or chemical characteristic of the solid-electrolyte interphase layer, further improving a solid-electrolyte interphase layer uniformity, integrity and/or permeability to lithium ions. In some embodiments, the formation step can include application of a constant voltage across the anode42and the dopant source46for a period of time. In some embodiments, a formation step voltage of about 2 Volts (V) to about 5 V can be applied between the anode42and the dopant source46in the formation step. For example, a formation step voltage can be from about 2 V to about 4.5 V, including about 3V to about 4V, including from about 3.5V to about 4V. In some embodiments, the formation step voltage can be about 2 V to about 4.2 V. The formation step voltage can be applied for a duration of about 5 hours to about 75 hours, including from about 10 hours to about 50 hours. For example, a formation step voltage of about of about 3.5V to about 4V can be applied across the anode42and the dopant source46for about 10 hours to about 50 hours.

The dopant source46may be consumed during the formation step. In some embodiments, the dopant source46may be completely or substantially completely consumed during the formation step. Any remaining dopant source46can be removed upon completion of the formation step. In some embodiments, any remaining dopant source46can be removed from the lithium ion capacitor40and the lithium ion capacitor40can be subsequently sealed. The solid-electrolyte interphase layer present during the anode pre-doping process and/or during the formation step may advantageously be the same solid-electrolyte interphase layer through which lithium ions are transported in a subsequent charge and/or discharge of the lithium ion capacitor, the solid-electrolyte interphase layer being undisturbed or substantially undisturbed subsequent to its formation.

Referring toFIG. 3, in one embodiment, an anode82of a lithium ion capacitor80can be pre-doped using a process comprising a constant current pre-doping step. Pre-doping a lithium ion capacitor anode using a constant current pre-doping step was found to facilitate formation of a solid-electrolyte interphase layer having an increased uniformity, stability and/or permeability of lithium ions. For example, pre-doping an anode using a constant current pre-doping step facilitated the formation of a solid-electrolyte interphase layer having improved structural, thermal and/or chemical stability, and/or a solid-electrolyte interphase free or substantially free of pin-hole defects, thereby providing a lithium ion capacitor having a reduced degree of capacitance fade after a number of charge-discharge cycles. In some embodiments, a pre-doping process comprising a constant current pre-doping step may reduce a duration of time needed to achieve a desired level of anode pre-lithiation.

Pre-doping an anode82by applying a controlled current across the anode82and a dopant source86, for example instead of short circuiting the anode82and the dopant source86, facilitated formation of a solid-electrolyte interphase layer having an increased uniformity, thereby providing a lithium ion capacitor having decreased equivalent series resistance and/or improved cycling performance.

Referring toFIG. 3, in one embodiment, pre-doping a lithium ion capacitor anode82using a process comprising a constant current pre-doping step can be performed in a lithium ion capacitor cell80(e.g., performed in-situ), the lithium ion capacitor80comprising the anode82, a cathode84, and a separator88between the anode82and cathode84. In some embodiments, as shown inFIG. 3, the lithium ion capacitor80can include a dopant source86. The dopant source86may comprise a source of lithium ions. For example, the dopant source86can comprise a lithium metal (e.g., a lithium metal electrode). The dopant source86may be positioned to a side of the anode82. For example, the dopant source86may be positioned to a side of the anode82opposite the side facing the cathode84. In some embodiments, the lithium ion capacitor80can include a separator88between the dopant source86and the anode82. The separators88may be configured to prevent an electrical short between the anode82and the dopant source86or the cathode84, while being permeable to one or more ionic species (e.g., lithium ions). The separator88can be made of a porous electrically insulating material (e.g., a material comprising a polymer, including a cellulosic material). The anode82, the cathode84, the dopant source86, and the separators88may be immersed in an electrolyte94(not shown).

The dopant source86may be consumed during the constant current charging or pre-doping step. In some embodiments, the dopant source86may be completely or substantially completely consumed during the constant current pre-doping step. In some embodiments, at least a portion of the dopant source86remains after the constant current pre-doping step and the remaining portion of the dopant source86can be removed upon completion of the pre-doping process. In some embodiments, any remaining dopant source86can be removed from a lithium ion capacitor80and the lithium ion capacitor80can be subsequently sealed. For example, a solid-electrolyte interphase layer formed during an anode pre-doping process comprising a constant current pre-doping step can be undisturbed or substantially undisturbed subsequent to its formation. In some embodiments, the dopant source86may not be removed until after completion of a formation step performed subsequent to the constant current pre-doping step, as described in more detail herein.

In some embodiments, a current source92providing a constant or substantially constant current flow can be positioned across an anode82and a dopant source86(e.g., a lithium metal electrode). For example, the dopant source86may be coupled to a first electrode of a current source92and the anode82may be coupled to a second electrode of the current source92such that the current source92can maintain a desired current flow between the dopant source86and the anode82. In some embodiments, a current corresponding to a current C-rate of about C/24 to about C/144 (e.g., a current C-rate of about C/24 can correspond to a current such that the capacitor can be completely or substantially completely discharged in about 24 hours, and a current C-rate of about C/144 can correspond to a current such that the capacitor can be completely or substantially completely discharged in about 144 hours), including from about C/48 to about C/120, including from about C/72 to about C/96, may be maintained between the anode82and the dopant source86. In some embodiments, the current source92can provide a current corresponding to a current C-rate of about C/72 to about C/144. For example, a current corresponding to a current C-rate of about C/72 may be maintained between the anode82and the dopant source86.

In some embodiments, a constant or substantially constant current flow can be maintained between the anode82and the dopant source86for a duration of time. For example, a current flow may be maintained until a desired potential difference between the anode82and the dopant source86is achieved. In some embodiments, a potential difference between an anode86and a dopant source86may be reduced from a value at about an open circuit voltage value to a desired voltage value. For example, a current flow of a constant or substantially constant value may be maintained between the anode82and the dopant source86such that a potential difference between the anode82and the dopant source86may be reduced from about an open circuit voltage (OCV) (e.g., about 3V) to a desired potential difference. In some embodiments, an anode pre-doping process comprising maintaining a constant current flow between an anode82and a dopant source86can provide a method of controlled anode pre-lithiation. For example, a constant or substantially constant current can be maintained until a potential difference of about 0.01V to 0.4V, including about 0.01V to about 0.2V is achieved across the anode82and the dopant source86. In some embodiments, a constant or substantially constant current can be maintained until a potential difference of about 0.1V is achieved across the anode82and the dopant source86.

In some embodiments, an anode pre-doping process comprising a constant current charging step can provide increased control of an anode pre-lithiation level, and/or an improved control over formation of a solid-electrolyte interphase layer adjacent the anode. In some embodiments, a characteristic of a solid-electrolyte interphase layer formed adjacent the anode82during a pre-doping process can depend at least in part on a characteristic or parameter of the pre-doping process, including but not limited to a current flow maintained between the anode82and the dopant source86, a duration in which the current flow is maintained, and/or a desired level of dopant incorporation into the anode82. For example, a current value which is maintained between the anode82and the dopant source86during an anode pre-doping process, a level of dopant incorporation into the anode82during the pre-doping process, and/or a duration for which the current value which is maintained between the anode82and the dopant source86may affect a thickness, a uniformity, a stability and/or a permeability of a solid-electrolyte interphase layer which can form adjacent an anode surface during the anode pre-doping process.

A current maintained between the anode82and the dopant source86during a pre-doping process, a duration for which the current is maintained between the anode82and the dopant source86, and/or a level of anode pre-lithiation may be determined based at least in part on a desired lithium ion capacitor performance. For example, a current flow, a duration in which the current is maintained, and/or a level of anode pre-lithiation may be selected based at least in part on a desired lithium ion capacitor equivalent series resistance performance, a capacitance performance, and/or a cycling performance (e.g., a degree of capacitance fade exhibited by the capacitor after a number of charge-discharge cycles). For example, current maintained between the anode82and the dopant source86, and a duration in which the current is maintained may be selected based at least in part on a desired level of anode pre-lithiation, the level of pre-lithiation corresponding to a formation of a solid-electrolyte interphase layer adjacent a surface of the anode having desirable characteristics, thereby providing a lithium ion capacitor having an improved performance (e.g., a reduced equivalent series resistance, and/or improved cycling performance).

In some embodiments, a formation step can be performed subsequent to a constant current pre-doping step. A formation step may facilitate improvement and/or stabilization in a characteristic of a solid-electrolyte interphase layer formed during the constant current pre-doping step. For example, the formation step performed subsequent to the constant current pre-doping step can facilitate stabilization of a structural, thermal and/or chemical characteristic of the solid-electrolyte interphase layer, further improving a solid-electrolyte interphase layer uniformity, integrity and/or permeability to lithium ions. In some embodiments, this formation step can include application of a constant voltage across the anode82and the dopant source86for a period of time. In some embodiments, a formation step voltage of about 2 Volts (V) to about 5 V can be applied between the anode82and the dopant source86in the formation step. For example, a formation step voltage can be from about 2 V to about 4.5 V, including about 3V to about 4V, including from about 3.5V to about 4V. In some embodiments, the formation step voltage can be about 2 V to about 4.2 V. The formation step voltage can be applied for a duration of about 5 hours to about 75 hours, including from about 10 hours to about 50 hours. For example, a formation step voltage of about of about 3.5V to about 4V can be applied across the anode82and the dopant source86for about 10 hours to about 50 hours.

The dopant source86may be consumed during the formation step, including being completely or substantially completely consumed. In some embodiments, at least a portion of the dopant source86remains upon completion of the formation step, and the remaining dopant source86can be removed from the lithium ion capacitor80and the lithium ion capacitor80can be subsequently sealed. The solid-electrolyte interphase layer present during the constant current anode pre-doping process and/or during the formation step may advantageously be the same solid-electrolyte interphase layer through which lithium ions are transported in a subsequent capacitor charge and/or discharge, the solid-electrolyte interphase layer being undisturbed or substantially undisturbed subsequent to its formation.

In some embodiments, a voltage of a constant-voltage pre-doping step, a current of a constant current pre-doping step, and/or a duration of a pre-doping step can be selected to achieve a desired level of lithium ion pre-doping in a capacitor anode. For example, a level of lithium ion pre-doping can be selected to facilitate improved capacitor capacitance, resistance and/or cycling performance. In some embodiments, the voltage of a constant-voltage pre-doping process, the current of a constant-current pre-doping process, and/or the duration of the pre-doping process, can be selected to facilitate achieving a lithium ion pre-doping level of about 50% to about 90%, including about 60% to about 65%.

FIGS. 4 through 8show capacitance, resistance and/or cycling performances of one or more lithium ion capacitors pre-doped using one or more of the constant voltage pre-doping or constant current pre-doping processes described herein. Components of the one or more capacitors can have one or more compositions as described herein.

FIG. 4shows measured capacity performances of example anodes of lithium ion capacitors pre-doped using a pre-doping process comprising a constant voltage pre-doping process. Lithium ion capacitors were subject to a pre-doping process comprising a constant voltage pre-doping step (e.g., a pre-doping process including an example capacitor40as shown inFIG. 2), and the capacity of the lithium ion capacitors was subsequently measured.FIG. 4shows an increase in a measured capacity, in milliampere-hour (mAh), when a decreased voltage is applied during a constant voltage pre-doping process. As shown inFIG. 4, a lithium ion capacitor having an anode pre-doped using a constant voltage pre-doping step at a constant voltage of about 0.1V can have an anode capacity measurement (e.g., at about 18 mAh, larger than that of a lithium ion capacitor having an anode pre-doped using a constant voltage of about 0.2V (e.g., at about 14 mAh).

FIG. 4also shows a level of anode lithium ion pre-doping and corresponding voltage maintained during the constant voltage pre-doping step. As shown inFIG. 4, a level of lithium ion pre-doping (e.g., a level of lithium ion incorporation, a level of pre-lithiation) into an anode can be proportional to a constant voltage value applied during a pre-doping process, the level of lithium ion incorporation decreasing with an increasing constant voltage value. For example, a level of lithium ion pre-doping in an anode pre-doped using a constant voltage of about 0.4 Volts (V) can have a lower lithium ion pre-doping level than that of an anode pre-doped using a constant voltage of about 0.1V. A voltage applied in a constant voltage pre-doping step may be selected based on a desired level of anode lithium ion pre-pre-doping.

FIG. 5includes a table showing example measured lithium ion capacitor performance parameters of capacitors corresponding to some of the capacitor anodes shown inFIG. 4.FIG. 5shows measured lithium ion capacitor performance parameters corresponding to voltage values (e.g., “Pre-lithiation Voltage”) applied during a constant voltage pre-doping process. For each applied constant voltage listed inFIG. 5, a corresponding percentage of anode pre-lithiation (e.g., “% predoping”), a capacitance value measured in Farads (F) (e.g., “Capacitance, F”), an equivalent series resistance measured in Ohms (Ω) (e.g., “ESR, Ohms”), and an RC time constant calculated from the measured capacitance value and the measured resistance of the lithium ion capacitor, are listed. For values shown inFIG. 5, the lithium ion capacitor having an anode which was pre-doped using a constant voltage of about 0.1 V in a constant voltage pre-doping step exhibited the lowest RC time constant, and/or measured equivalent series resistance. As shown inFIG. 5, the anode of the capacitor having the lowest RC time constant was pre-doped to a lithium ion pre-doping level of about 60%.

FIG. 6shows cycling performance of some of the lithium ion capacitors ofFIG. 4. For example, a capacitance of the lithium ion capacitors was measured after cycling a number of cycles between a voltage of about 2.2 Volts (V) and 4.2 V, at a current C-rate of about 30 C (e.g., a current C-rate of about 30 C can correspond to a current such that the capacitor can be completely or substantially completely discharged in about 1/30 of an hour), in ambient conditions, to measure a percentage of reduction in measured capacitance as compared to an initial capacitance of the lithium ion capacitor prior to cycling (e.g., a capacitance fade performance).FIG. 6shows a general improvement in the cycling performance as voltage applied during a constant voltage pre-doping step decreases. For example, inFIG. 6, the lithium ion capacitor having an anode which was pre-doped using a constant voltage of about 0.1V in a constant voltage pre-doping step show a reduced degree of capacitance fade after a number of charge-discharge cycles, for example as compared to that of a capacitor having an anode pre-doped using a constant voltage of about 0.4V. A lithium ion capacitor exhibiting a reduced degree of capacitance fade, a reduced equivalent series resistance and/or a reduced RC time constant, may have a solid-electrolyte interphase layer having a decreased thickness, an increased uniformity, an improved stability and/or an increased permeability of lithium ions.

FIG. 7includes a table showing example measured lithium ion capacitor performance parameters corresponding to current flow rates (e.g., “C-rate”) maintained during a constant current pre-doping step of an anode pre-doping process. InFIG. 7, current maintained during a constant current pre-doping step is expressed as a rate at which the capacitor can be discharged and/or charged, a larger C-rate corresponding to a higher current. For example, a current corresponding to a C-rate of about C/48 is greater than a current corresponding to a C-rate of about C/96. For each maintained constant current listed inFIG. 8, a corresponding level to which lithium ions were incorporated into an anode of the lithium ion capacitor (e.g., “% pre-doping”), a capacitance value measured in Farads (F) (e.g., “Capacitance, F”), an equivalent series resistance measured in Ohms (Ω) (e.g., “ESR, Ohms”), and an RC time constant calculated from the capacitance value and the resistance of the lithium ion capacitor, are listed. For values shown inFIG. 7, decreasing a constant current flow rate maintained between an anode and a dopant source can reduce an equivalent series resistance, and/or RC time constant, of the lithium ion capacitor. For example, as shown inFIG. 7, capacitors having anodes pre-doped at reduced currents can demonstrate a reduced equivalent series resistance (ESR) and/or RC time constant. In some embodiments, a decreased current maintained during a constant current pre-doping step can facilitate increased lithium ion pre-doping level. As shown inFIG. 7, an anode pre-doped to a level of about 60% to about 65% can demonstrate a reduced equivalent series resistance (ESR) and/or RC time constant.

FIG. 8shows cycling performance of some of the lithium ion capacitors ofFIG. 7. For example, a capacitance of the lithium ion capacitors was measured after cycling a number of cycles between a voltage of about 2.2 Volts (V) and 4.2 V, at a current C-rate of about 30 C (e.g., a current C-rate of about 30 C can correspond to a current such that the capacitor can be completely or substantially completely discharged in about 1/30 of an hour), in ambient conditions to measure a percentage of reduction in measured capacitance as compared to an initial capacitance of the lithium ion capacitor prior to cycling (e.g., a capacitance fade performance).FIG. 8shows a general improvement in the cycling performance as current maintained during a constant current step decreases. For example the lithium ion capacitor having an anode which was pre-doped using a constant current charging step having a current C-rate of about C/96, as shown inFIG. 8, shows a reduced degree of capacitance fade after a number of charge-discharge cycles. In some embodiments, a lithium ion capacitor exhibiting a reduced degree of capacitance fade a reduced equivalent series resistance, and/or a reduced RC time constant, may have a solid-electrolyte interphase layer having a decreased thickness, an increased uniformity, an improved stability and/or an increased permeability of lithium ions.

An anode pre-doping process comprising a constant voltage pre-doping step and/or a constant current pre-doping step may provide an increased control over a degree of anode pre-lithiation. A voltage maintained in a constant voltage pre-doping step, or a current maintained in a constant current pre-doping step, can be selected based on a desired anode lithium ion pre-doping level. An improved control in a level to which lithium ions are incorporated into a lithium ion capacitor anode may provide an improved control in a formation of a solid-electrolyte interphase layer, facilitating for example an improved lithium ion capacitor performance. In some embodiments, an anode pre-doping process comprising a constant voltage pre-doping step and/or a constant current pre-doping step can facilitate formation of a solid-electrolyte interphase layer having an improved stability, uniformity, and/or permeability of lithium ions. An anode pre-doping process comprising a constant voltage pre-doping step and/or a constant current pre-doping step may provide a lithium ion capacitor exhibiting a reduced equivalent series resistance performance, a reduced RC time constant, and/or a reduced degree of capacitance fade after a number charge-discharge cycles.

Although this invention has been disclosed in the context of certain embodiments and examples, it will be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while several variations of the embodiments of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. It should be understood that various features and aspects of the disclosed embodiments can be combined with, or substituted for, one another in order to form varying modes of the embodiments of the disclosed invention. Thus, it is intended that the scope of the invention herein disclosed should not be limited by the particular embodiments described above.