Patent Application
20190036122
The present invention is directed towards novel electrodes for an energy storage device, particularly a lithium battery, comprising as ingredients a specific active material and a specific electricity conducting material, so that it is possible in particular to view the charge status of said electrode via a simple change in colour thereof.
The invention pertains to an energy storage device comprising at least one such electrode.
The field of the invention can be defined as the field relating to energy storage devices and notably to lithium batteries.
Energy storage devices are conventionally electrochemical batteries operating along the principle of electrochemical cells capable of delivering an electrical current by means of the presence in each thereof of a pair of electrodes (respectively a positive electrode and negative electrode) separated by an electrolyte, the electrodes comprising specific materials able to react via a redox reaction after which electrons are produced resulting in the electrical current and the production of ions circulating from one electrode to the other via an electrolyte.
The most frequently employed batteries governed by this principle are lithium batteries such as lithium-ion batteries.
From an operating viewpoint, lithium-ion batteries are based on the principle of intercalation-desintercalation or complexation-decomplexation of lithium within the constituent materials of the electrodes of the electrochemical cells of the battery (these materials also possibly being called active materials).
More specifically, the reaction causing the production of current (i.e. when the battery is in discharge mode) involves the transfer, via a lithium ion conducting electrolyte, of lithium cations arriving from a negative electrode which insert themselves in the acceptor network of the positive electrode, whilst electrons derived from the reaction at the negative electrode power the external circuit with which the positive and negative electrodes are connected.
Among the active materials able to be included in the composition of at least one of the electrodes, these can be inorganic compounds able to receive lithium ions in their network that are in the process of being charged or discharged depending on the polarity of the electrodes; or organic compounds which, via a redox reaction, are able to complex said lithium ions.
These active materials are generally used in association with an electricity conducting additive such as carbon particles (carbon black in particular), and also with an additive ensuring cohesion of the electrode in which the active material and the conducting additive are included, said cohesion additive possibly being a polymeric binder, the association of these ingredients leading to a dense, opaque mixture.
With such electrodes comprising said mixture, it is therefore not possible to conduct colorimetric monitoring to determine the charge status of the electrode, when a change in status appears as a change in colour of the active material, said colour change particularly occurring with electrochromic materials, since the electricity conducting additive masks the colour change of the active material.
The need is therefore felt for a simple manner to estimate the charge status of a battery via simple colorimetric monitoring thereof, notably in the field of portable electronic equipment.
This need is henceforth met by the authors of the present invention, who have discovered that by associating a specific electricity conducting additive with a specific active material in an electrode, it has become possible to conduct estimation of charge status via mere colorimetric monitoring thereof.
The invention therefore pertains to an electrode for energy storage device comprising an electrochromic material as active material and metal nanowires as electricity conducting additive.
By active material, in the foregoing and in the remainder hereof, it is meant as is conventional the material that is directly involved in the insertion-desinsertion and/or complexation-decomplexation reactions of the ions acting in the energy storage device, these ions being lithium ions when the device is a lithium battery.
By associating an active material of the electrochromic material type with metal nanowires as electricity conducting additive, it is possible to monitor the changes in colour of said electrochromic material as a function of the charge status of the electrode, without such changes being masked by the electricity conducting material as is particularly the case when it is in the form of carbon black.
For example, the electrode is deposited on a transparent substrate allowing viewing of changes in colour as a function of charge status. This substrate can ensure the function of a current collector.
The transparent substrate can be in glass or a flexible plastic material, optionally coated with an electricity conducting layer e.g. a layer in electricity conducting ceramic such as a layer of indium tin oxide.
The active material, in this invention, is an electrochromic material i.e. a material able to change colour when an electric charge is applied thereto which, in the context of the operating of the energy storage device, means when this active material is discharging.
This electrochromic material can be an inorganic material such as graphite, TiO2 in bronze form (sometimes designated as TiO2—B), a vanadium oxide such as V2O5, V3O7, a mixed oxide of lithium and titanium such as Li4Ti5O12 (sometimes designated by the abbreviation LTO), lithium phosphates such LiFePO4 (sometimes designated by the abbreviation LFP).
This material may also be an organic compound and more specifically an organic compound comprising at least one electron acceptor group such as a carbonyl group for example.
This type of compound, since it comprises an electron acceptor group and is therefore able to be reduced, can therefore be included in the composition of a positive electrode when the energy storage device in the process of discharging or in the composition of a negative electrode when the energy storage device is in the process of charging.
In particular, said compound can be an aromatic compound such as a perylene compound comprising at last one electron acceptor group such as a carbonyl group or imide group, one specific compound meeting this definition being perylene-3,4,9,10-tetracarboxylic dianhydride (symbolized by the abbreviation PTCDA) meeting following formula (I):
The active material may also be an organic compound comprising at least one electron donor group such as a carboxylate group for example.
This type of compound, since it comprises an electron donor group and is therefore able to be oxidized, can therefore be included in the composition of a negative electrode when the energy storage device is in the process of discharging, or in the composition of a positive electrode when the energy storage device is in the process of charging.
More specifically, it may be an aromatic compound such as perylene or phenylene compound, comprising at least one electron donor group such as a carboxylate group, and more particularly a lithiated carboxylate group, specific compounds which come under this definition meeting one of following formulas (II) or (III):
Advantageously, the electrochromic material is included in the electrode in a proportion of 45 to 99%, preferably 80% to 98% by weight relative to the total weight of the electrode.
As mentioned above, the electrodes of the invention also comprise metal nanowires as electricity conducting additive.
In the foregoing and in the remainder hereof by «nanowire» it is meant generally a wire having a thickness of between 1 and 100 nanometres but the length of which may reach up to 10 micrometres.
Aside from the fact that nanowires do not mask changes in colour of the electrochromic material, they can ensure good electronic conduction and exhibit very low percolation thresholds (approximately one percent) in the electrodes.
These metal nanowires can be nanowires in a metal selected from among copper, nickel, silver, gold, platinum, titanium, palladium, zinc, aluminium and alloys thereof, the metal advantageously being selected as a function of the range of working potentials of the active material.
For example:
More specifically, the nanowires can be nanowires in copper or nanowires in gold.
From a geometric viewpoint, they may advantageously have a form factor corresponding to the ratio of nanowire length to nanowire diameter ranging from 10 to 1000000, for example higher than 30.
Advantageously, the nanowires are contained in the electrode in a proportion of 0.1 to 20%, preferably from 0.1 to 6% by weight relative to the total weight of the electrode.
In addition, the electrodes of the invention may comprise a binder, such as a polymeric binder e.g. polyvinylidene fluoride (known under the abbreviation PVDF), a mixture comprising carboxymethylcellulose (known under the abbreviation CMC) with a latex of styrene-butadiene type (known under the abbreviation SBR) or with polyacrylic acid (known under the abbreviation PAA), this binder contributing towards improving the resistance of the electrode.
Therefore, from a structural viewpoint, the electrode may be in the form of a composite material comprising a matrix of polymeric binder(s) in which fillers are dispersed composed of the active material and of the metal nanowires.
The electrodes of the invention are intended to be included in the composition of energy storage devices such as:
Therefore, the invention also pertains to an energy storage device such as a lithium battery comprising at least one electrochemical cell comprising two electrodes of opposite polarity, a positive electrode and negative electrode respectively, separated by an electrolyte, at least one of the electrodes being an electrode such as defined above namely an electrode comprising an electrochromic material as active material and metal nanowires as electricity conducting additive.
The characteristics defined above for the electrodes of the invention can be applied to energy storage devices comprising said electrodes.
For example, when the electrochromic material is an organic compound comprising at least one electron attractor group, the electrode of the invention can be a positive electrode i.e. an electrode acting as cathode (therefore the site of reduction), when the generator delivers current (i.e. when it is in the process of discharging) or can be a negative electrode acting as cathode (therefore the site of reduction) when the generator is in the process of charging.
For example, when the positive electrode as electrochromic material comprises an organic compound comprising at least one electron attractor group, the negative electrode may notably be a lithium metal electrode.
Each of the electrodes is generally in contact with a current collector.
In particular, the current collector—for the electrode comprising an electrochromic material as active material and metal nanowires as electricity conducting additive—can be a transparent substrate on which the electrode can be deposited, this transparent substrate possibly being a substrate in glass for example or in a flexible plastic material optionally coated with an electricity conducting layer e.g. an electricity conducting layer in ceramic such as a layer of indium tin oxide.
The current collector may also be in the form of a metal foil or mesh e.g. in copper or aluminium.
The two electrodes of opposite polarity (namely the positive electrode and negative electrode) are separated by an electrolyte and more specifically an ion conducting electrolyte e.g. lithium ions (when the device is a lithium battery), sodium ions (wen the device is a sodium battery), magnesium ions (when the device is a magnesium battery) or organic ions (when the device is a battery operating with organic ions).
This ion conducting electrolyte can be a liquid electrolyte comprising at least one salt in one or more solvents e.g. a lithium salt (when the device is a lithium battery), a sodium salt (when the device is a sodium battery), a magnesium salt (when the device is a magnesium battery), or a salt comprising an organic ion (when the device is a battery operating with organic ions).
As examples of lithium salt, mention can be made of LiClO4, LiAsF6, LiPF6, LiBF4, LiRfSO3, LiCH3SO3, LiN(RfSO2)2, Rf being chosen to be F or a perfluoroalkyl group having 1 to 8 carbon atoms, lithium bis(trifluoromethanesulfony)imide (known under the abbreviation LiTFSI), lithium bis(oxalato)borate (known under the abbreviation LiBOB), lithium bis(perfluorethylsulfonyl)imide (also known under the abbreviation LiBETI), lithium fluoroalkylphosphate (known under the abbreviation LiFAP), lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (known under the abbreviation LiTDI).
As examples of solvents, mention can be made of:
In addition, the electrolyte may be caused to impregnate at least one separator element arranged between the two electrodes of the battery.
As a variant, the ion conducting electrolyte can be a polymer electrolyte or gelled electrolyte.
For example, a device conforming to the invention is a lithium battery comprising an electrochemical cell comprising:
More specifically, the electrolyte may comprise a lithium salt LiPF6 and a mixture of carbonate solvents e.g. a ternary mixture comprising ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate.
Another example conforming to the invention is a lithium battery comprising an electrochemical cell, said electrochemical cell comprising:
Finally, another example conforming to the invention is a lithium battery comprising an electrochemical cell comprising:
Advantageously, the devices of the invention are packaged in a transparent casing to allow viewing of the electrode comprising an electrochromic material and hence the changes in the colour thereof as a function of the charge status of the electrode concerned. It is specified that the casing surrounds the constituent parts of the battery namely the electrodes and the electrolyte.
This transparent casing can be in polyethylene for example or polyethylene terephthalate or in polypropylene.
The devices of the invention are particularly suitable for the fields of application in which direct viewing of the charge status of the batteries is an advantage, such as is the case with portable electronic equipment e.g. a mobile telephone, technical textiles, timepieces.
Finally, the invention pertains to the utilisation of metal nanowires in an electrode for energy storage device such as a lithium battery comprising an electrochromic material as active material to view to the charge status of said electrode via a change in colour thereof.
The characteristics of the metal nanowires, of the electrode and of the electrochromic material defined above can be applied to this utilisation.
Other characteristics will become better apparent on reading the following additional description referring to an example of embodiment of electrodes and batteries conforming to the invention.
Evidently, the following example is merely given to illustrate the subject of the invention and is in no manner limiting thereof.
This example illustrates the preparation of two batteries conforming to the invention, each of these batteries, as illustrated in the exploded view in appended
a) Preparation of Copper Nanowires
First, a solution of 2000 mL of NaOH at 15 mol·L−1 is prepared in a 3-litre round bottom flask by dissolving 1200 g of NaOH in 2000 mL of deionized water (hereafter called first solution).
In parallel, a solution of copper nitrate at 0.2 mol·L−1 is prepared by adding 4.65 g of Cu(NO3)2 to 100 mL of deionized water.
This solution is added to the first solution, after which the addition is made of 30 mL ethylenediamine (EDA) and 2.5 mL hydrated hydrazine (35 weight %).
The reaction medium is heated to 80° C. for one hour under vigorous agitation.
The solution changes from a royal blue colour to a reddish-brown colour indicating the formation of copper metal nanowires.
The nanowires are collected by centrifugation and washed in an aqueous solution with 3 weight % hydrazine and finally stored in a bottle containing a solution of same type (3 weight % hydrazine) under an argon atmosphere to prevent oxidation thereof.
The copper nanowires obtained have a form factor (corresponding to the ratio between the length and diameter of the nanowires) higher than 30, with a length estimated by scanning electron microscopy of about 5 μm and diameter estimated by scanning electron microscopy of about 150 nm.
b) Preparation of Electrodes Conforming to the Invention
Initially, a suspension is prepared comprising copper nanowires prepared at above-mentioned step a), polyvinylidene fluoride (PVDF) and N-methyl pyrrolidone (NMP) at respective weight contents of 1%, 0.5% and 98.5%, the whole being dispersed for 1 hour in a sonotrode. Next, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and PVDF/NMP are added to the suspension, the mixture being dispersed using a Dispermat mixer. The resulting ink comprises 86 weight % PTCDA, 4 weight % copper nanowires and 10 weight % polyvinylidene fluoride (weight percentages being expressed relative to the total weight of these three ingredients).
In a first variant, the above-mentioned ink is deposited on copper foil then dried in an oven at 55° C. for 24 hours. A circular piece 14 mm in diameter is cut out using a punch, said piece then being dried in a Buchi at 80° C. for 48 hours, the resulting piece forming a positive electrode (called first electrode) deposited on a copper collector.
In a second variant, the above-mentioned ink is deposited by spraying, using an airbrush gun, onto a transparent wafer coated with a template formed of a glass substrate coated with an indium tin oxide layer. The resulting piece is then dried in an oven at 55° C. for 24 hours, after which a second positive electrode is obtained.
c) Preparation of the Batteries Conforming to the Invention
A first battery conforming to the invention is prepared from the first electrode defined under paragraph b) above.
More specifically, this first battery is a battery of button cell type respectively comprising:
A second battery conforming to the invention is prepared from the second electrode defined under paragraph b) above.
More specifically, this second battery is a battery of PouchCell type respectively comprising:
This second battery is placed in a flexible transparent casing in polyethylene so that it is possible to view changes in colour of the positive electrode as a function of the charge status thereof.
This second battery is subjected to a discharging process which, from a chemical viewpoint, corresponds to reducing the PTCDA, and the change in colour of the positive electrode is examined through the transparent flexible casing.
As and when it discharges, a red colour is seen to change to a much darker colour tending towards purple.
With the electrode and batteries of the invention, it is thus possible to view the change in charge status of the battery simply via a mere change in colour of the positive electrode.
In parallel, the electrochemical performances of the batteries conforming to the invention were also tested.
A charge/discharge profile was therefore determined for the first battery as compared with a battery not conforming to the invention in which the copper nanowires were replaced by carbon black of Super P type, the proportions of the ingredients of the positive electrode respectively being 75 weight % for PTCDA, 20 weight % for carbon black and 5 weight % for PVDF.
The curves are given in
They exhibit a relatively similar profile evidencing the fact that the copper nanowires do not perturb the electrochemical performance of the batteries.
A cyclic voltammetry test was also conducted with the first battery, whereby cycling was performed at between 1.8 V and 3.2 V vs. Li+/Li0 at a scanning rate of 0.1 mV·s−1, the cycling curve being illustrated in
This example illustrates the preparation of a battery of button cell type conforming to the invention, said battery comprising:
This battery was prepared along the same modalities as those set forth in Example 1, with the exception that the positive electrode in Example 1 becomes the negative electrode in this Example 2, and the positive electrode of this Example comprises LiFePO4.
A cyclic voltammetry test was carried out with this battery, whereby cycling was performed at between 0.2 V and 1.5 V vs. Li+/Li0 at a scanning rate of 0.01 mV·s−1, the cycling curve being illustrated in
A charge/discharge profile was also determined at C10 rate with this battery, 5 charge/discharge cycles being carried out, the curves of 5 cycles being given in
This example illustrates the preparation of a battery of button cell type conforming to the invention, said battery comprising:
The active material Li4Ti5O12 is an electrochromic material able to change from a white colour to a dark blue colour as a function of the charge status thereof.
This battery was prepared along the same modalities as those set forth in Example 1 with the exception that the positive electrode was prepared in accordance with the following protocol.
A solution containing copper nanowires, polyvinylidene fluoride (PVdF) and N-methylpyrrolidone (NMP) was prepared by dispersion using a sonotrode for 1 hour, this solution comprising 1 weight % copper nanowires, 0.5 weight % PVDF and 98.5 weight % NMP. Li4Ti5O12 was added thereto and the mixture dispersed in a Dispermat. The composition of the coloured ink obtained was approximately 90% Li4Ti5O12, 4% copper nanowires and et 6% PVdF. This ink was coated onto copper collectors then dried in an oven at 55° C. for 24 hours. An electrode 14 mm in diameter was cut out with a punch and dried in a Buchi at 80° C. for 48 hours.
A charge/discharge profile at C10 rate was also determined for this battery, 5 charge/discharge cycles being performed; the curves of 5 cycles are given in
The trend in specific capacitance C (in mAh/g) was also determined as a function of the number of cycles N, the results being given in
| Number | Date | Country | Kind |
|---|---|---|---|
| 16 51173 | Feb 2016 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/053055 | 2/10/2017 | WO | 00 |
