This invention relates to surface-modified alkali metal materials. In particular, though not exclusively, this invention relates to a method of making a surface-modified alkali metal material for electrochemical use, to surface-modified lithium materials for electrochemical use, and to an electrode, electrode assembly or electrochemical cell comprising the materials.
Metallic lithium is known to be a potentially useful electrode material, since it has low electrode potential (3.05V against Normal Hydrogen Electrode) and high electrochemical equivalent (3,884 Ah/g). It is widely used in primary cells.
However, metallic lithium has limited application in secondary cells because of the formation of finely dispersed residues which do not have electric contact with the bulk of electrode metallic lithium. Such residues take the form of dendritic and mossy metallic lithium and do not participate in electrochemical reactions.
The formation of dispersed residues takes place during charge and discharge of the cell. In practice, formation of finely dispersed lithium leads to a number of negative effects such as quick battery capacity fade and possible internal shorts that could result in fire (X. B. Cheng, et al., Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review, Chem. Rev. 117 (2017), 10403-10473 DOI: 10.1021/acs.chemrev.7b00115).
It may be possible to avoid or mitigate formation of finely dispersed lithium residues by providing a special coating on the surface of a metallic lithium electrode. Such coatings may be referred to as barrier or protection layers. Barrier layers prevent or mitigate formation of finely dispersed lithium, whilst still permitting electrochemical reactions.
A range of materials can be used as barrier layers such as lithium alloys and solid-state lithium ion conductive coatings of different types: polymer, ceramic, polymer-ceramic, etc.
Barrier layers on metallic lithium can be formed by a variety of methods such as thermal deposition, magnetron sputtering, chemical solution deposition, polymerisation, etc. The choice of method is determined by the desired material properties of the barrier layer. The method itself must provide formation of a layer with good coverage and adhesion to metallic lithium.
U.S. Pat. No. 6,911,280 discloses a lithium electrode protected by a solid electrolyte layer of LiPON, produced by spraying lithium phosphate or by surface treatment of Li with phosphoric acid. Two other documents, RU 2 579 357 and RU 2 596 023 C1 describe lithium electrodes with sprayed layers on lithium made of Si, Ge, C, Al and Au. Barrier layers on the lithium electrode in these cases were produced by vacuum sputtering.
There is also a disclosure in the prior art of an anode material made of metallic foil with contact tabs so that metallic lithium can be either sprayed or rolled on its surface, optionally followed by coating the metallic lithium with a barrier layer (RU 2 596 023 C1).
Finally, there is also known a method of forming a barrier layer on the surface of a lithium electrode by magnetron vacuum sputtering of a material chosen from the following group: Si, Ge, C, Al, Au. (RU 2 579 357 C1).
Nevertheless, there remains a need in the art for lithium (and indeed other alkali metal) electrode materials comprising effective and cost-efficient protective/barrier layers.
From one aspect, the invention provides a method of making a surface-modified alkali metal material for electrochemical use, the method comprising bringing a barrier agent into frictional contact with an alkali metal substrate to form a tribochemical barrier layer on the substrate.
It has been found that barrier layers formed in this manner are effective and cost- and energy-efficient to produce.
The field of tribochemistry is concerned with chemical and physiochemical changes of matter due to the influence of mechanical energy. Tribochemical mechanisms are manifold, highly complex, interrelated and not well understood (Kalin, Mitjan. “On the Evaluation of Thermal and Mechanical Factors in Low-Speed Sliding.” Tribology of Mechanical Systems: A Guide to Present and Future Technologies. Ed. Jože Vižintin, Mitjan Kalin, Kuniaki Dohda, and Said Jahanmir. ASME Press, 2004.).
The term “tribochemical barrier layer” is used herein to refer to an adherent coating that results from frictional contact of the barrier agent with the substrate. The barrier layer may be formed as a result of mechanical or chemical phenomena, or a combination thereof.
The barrier layer permits electrochemical reactions with the alkali metal substrate during electrochemical use, whilst mitigating or preventing the formation of dispersed residues of the alkali metal substrate (in particular in the context of a secondary cell).
From another aspect, the invention comprises a surface-modified alkali metal material obtainable by any method in accordance with the invention.
From yet another aspect, the invention provides a surface-modified alkali metal material for electrochemical use, the material comprising an alkali metal substrate bearing a tribochemical barrier layer.
From still another aspect, the invention comprises an electrode, electrode assembly or electrochemical cell comprising a surface-modified alkali metal material in accordance with the invention.
In an aspect and various embodiments of the invention, the surface-modified alkali metal material for electrochemical use is made by a method comprising bringing a barrier agent into frictional contact with an alkali metal substrate to form the tribochemical barrier layer on the substrate.
The barrier agent may comprise any material capable of forming the tribochemical barrier layer upon being brought into frictional contact with the alkali metal substrate.
In various embodiments, the barrier agent and/or tribochemical barrier layer may include one or more materials that capable of conducting ions derived from the alkali metal substrate.
Suitably, the barrier agent may be metallic, i.e. comprise or optionally consist of one or more metals. Additionally, or alternatively, the barrier agent may be non-metallic, i.e. comprise or optionally consist of one or more non-metals.
Suitably, the barrier agent may be capable of forming an alloy or compound with at least a part of the alkali metal substrate.
In various embodiments, the barrier agent may comprise a metal compound, optionally an alkali metal compound.
In various embodiments, the barrier agent may comprise Li3N, Si, Zn, Al, C, S, P2S5, SiS2, Li2S, Li3PS4, Li3PO4 or combinations thereof. Advantageously, the barrier agent may comprise Si and/or Li3N.
Advantageously, the barrier agent may be particulate. This enhances the surface area of the barrier agent and facilitates frictional engagement and tribochemical mechanisms.
Advantageously, the particle size of the barrier agent may be selected so as not to change the mechanical continuity of the substrate.
Suitably, the volume-based average particle size of the barrier agent may be in the range of from a fifth or a tenth of the thickness of the substrate.
In various embodiments, the average or median particle size may be in the range of from 0.5 to 50 μm, such as in the range of from 1 to 30 μm, or even in the range of from 2 to 20 μm, e.g. 5 to 15 μm.
An average or median particle size may be determined on a volume basis.
Frictional contact can be achieved in a variety of ways. The barrier agent may be moved towards the substrate, or vice versa, or both.
Conditions for achieving an adherent coating on the alkali metal substrate may vary to a degree depending on the chosen alkali metal substrate and the barrier agent.
In general, bringing the barrier agent into frictional contact with the substrate may comprise forcing together the barrier agent and the substrate. Conveniently, the barrier agent and one or more planar faces of the substrate may be forced together.
In various embodiments, the barrier agent and the substrate may be forced together with a force in the range of from 0.1 to 1.0 kg/cm2 substrate, optionally in the range of from 0.2 to 0.8 kg/cm2, or even in the range of from 0.3 to 0.7 kg/cm2.
Additionally, or alternatively, bringing the barrier agent into frictional contact with the substrate may comprise sliding or rubbing between the barrier agent and the substrate, optionally whilst the barrier agent and the substrate are forced together. Conveniently, the method may comprise sliding or rubbing between the barrier agent and one or more planar faces of the substrate.
In various embodiments, sliding or rubbing between the barrier agent and the substrate may be performed with a reciprocating motion.
Optionally, the reciprocating motion may have an amplitude in the range of from 1 to 5 mm and/or a frequency in the range of from 0.1 to 10 Hz.
In various embodiments, the barrier agent and the substrate may be slid or rubbed together for a period in the range of from 2 to 10 minutes. Suitably, the period may be 3 to 8 minutes, for example 4 to 7 minutes.
Advantageously, the substrate may be affixed and an applicator may be employed to force the barrier agent against the substrate and optionally to slide or rub the barrier agent along the substrate (optionally whilst continuing to force the barrier agent against the substrate). The applicator may comprise a smooth or roughened application surface. One example of a suitable applicator is a plate.
Conveniently, the substrate may be sheet-like and affixed onto a flat surface with one planar face exposed. The barrier agent may then be forced and optionally slid or rubbed against the exposed face, for example using an applicator.
Conveniently, bringing the barrier agent into frictional contact with the substrate may additionally or alternatively comprise impinging a stream of fluid bearing the barrier agent onto the substrate. Suitably, the fluid may pressurised. Conveniently, the fluid may be a gas.
Alkali metal is prone to passivation. Accordingly, the method may comprise removing a passivation layer from the alkali metal substrate. This may suitably be done before bringing the barrier agent into frictional contact with the substrate.
To prevent or mitigate passivation the method may be performed in a suitably inert atmosphere. Optionally, the atmosphere may comprise or consist essentially of argon.
However, it has surprisingly been found that the method can also be performed in an atmosphere comprising nitrogen or dry air.
Advantageously, the frictional contact may take place in the absence of solvents or additives.
Alternatively, the frictional contact may take place in the presence of one or more solvents or additives. Suitable additives may include monomeric species capable of polymerisation on contact with the alkali metal substrate. Examples of suitable solvents or additives include dioxolane, ketones, ethers, and unsaturated compounds.
The alkali metal substrate employed in aspects and embodiments of the invention comprises alkali metal and, optionally, a support. The alkali metal substrate may consist of metal/alloy or may be a composite comprising alkali metal.
The alkali metal may advantageously comprise lithium, sodium, lithium alloy, solidum alloy, potassium, potassium alloy, or combinations thereof. Preferably, the alkali metal may comprise or consist of lithium metal or a lithium alloy. In one embodiment, the alkali metal consists essentially of lithium.
Conveniently, the alkali metal substrate may comprise or consist of a foil of the alkali metal.
Where present, the support of the alkali metal substrate may provide additional mechanical stability thereto. Suitably, the support may be polymeric. Advantageously, the support may be fibrous, for example a non-woven material. The alkali metal may be deposited onto the support, for example as described in WO/2016/122353. Additionally, or alternatively, the alkali metal may be calendared onto or into the support.
In various embodiments, the alkali metal support may be permeable with through-pores. Alternatively, the alkali metal support may be impermeable without through-pores.
Suitably, the alkali metal substrate may be sheet-like with opposed planar faces defining a thickness therebetween. Advantageously, a thickness defined between opposed faces of the alkali metal substrate may be in the range of from 1 to 500 μm, such as in the range of from 10 to 150 μm, or even in the range of from 15 to 80 μm.
Optionally, the alkali metal substrate may comprise one or more connectors or collectors for electrochemical connection, e.g. in an electrochemical cell. The alkali metal substrate may thus constitute an electrode.
The tribochemical barrier layer is an adherent coating that results from frictional contact of the substrate with a barrier agent. The barrier layer may be formed as a result of mechanical or chemical phenomena, or a combination thereof.
The barrier layer may of course comprise a barrier agent as defined anywhere herein. Additionally, or alternatively, the barrier layer may comprise a tribochemical product derived from the barrier agent.
In various embodiments, the barrier layer has a thickness in the range of from 0.5 to 10 microns. Suitably, the thickness may be in the range of from 1 to 8 microns, for example in the range of from 2 to 5 microns.
Advantageously, the barrier layer may cover substantially the entirety of the substrate.
In various embodiments, the substrate may be sheet-like with opposed faces and the barrier layer may be applied to one or both faces.
Suitably, the barrier layer may be continuous, although an intermittent barrier layer may also be of use in some embodiments.
Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to”, and do not exclude other components, integers or steps. Moreover, the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible.
To further illustrate the invention, one or more non-limiting embodiments of the invention will now be described in the following experimental section with reference to the accompanying drawings in which:
Lithium foil is treated in a glove-box under argon, or nitrogen, or in a dry room. Metallic lithium foil is positioned on a flat surface of a material neutral to lithium, such as stainless steel. The surface of lithium is prepared by removing any possible contamination from its surface. A simple brush can be used for that purpose. After that a layer of powder material such as Si or Li3N is applied on the surface in a thin even layer. To initiate a tribochemical reaction a stainless-steel plate is applied and moved across the surface in a reciprocating way. The surface of the stainless-steel plate can have different level of roughness to provide more efficient conditions for tribochemical reaction on the surface of metallic lithium. The friction energy of movement is thus transferred into tribochemical treatment of lithium. The processing time could take from 0.5 to 10 min with the pressure between the stainless-steel plate and lithium foil being in the range from 0.01 to 1 kg/cm2.
Barrier Layer by Tribochemical Treatment with Si Powder—Dry Air Atmosphere
All work on tribochemical treatment of the surface of lithium foil with silicon was carried out in a glove box in the atmosphere of dry air. The H2O content was in the range of 20 to 40 ppm. A lithium foil with a thickness of 100 microns was placed on the surface of a stainless steel plate and secured. Then, the surface of metallic lithium was mechanically cleaned by using a stainless steel brush and/or scraper. After removing contaminants from the lithium surface, a uniform layer of silicon powder was applied. The median volume size of the Si particles was estimated to be in the range of from 5 to 15 microns. On the surface of the lithium foil with a layer of silicon powder was laid a stainless steel plate with a rough surface. To carry out the tribochemical reaction between metallic lithium and silicon powder, the rough plate was pressed to lithium foil with a pressure of 0.1-0.2 kg/cm2 and brought in a reciprocal and progressive movement with amplitude of 1-5 mm and a frequency of 1-10 Hz. The resulting tribochemical reaction (tribochemical treatment of lithium foil) was carried out for 2-3 minutes.
After tribochemical treatment, the powder of unresponsive silicon was removed from the surface of the lithium foil. After tribochemical treatment, the surface of the lithium foil was dark grey. The thickness of the surface layer was assessed by weight by the difference in the mass of lithium foil before and after the tribochemical treatment. The thickness of the formed barrier layer was 1.5 microns.
Electrodes of the right size were cut from lithium foil with a barrier layer and they were further pressed through plastic film at a pressure of 100 kg/cm2.
Symmetrical lithium cells (Li/electrolyte/Li) were then assembled from the resulting lithium electrodes. Also for comparison we assembled similar symmetrical cells but with lithium electrodes without a barrier layer.
Two layers of Celgard separator 3501 were used. The electrolyte was a solution 1.0M LiClO4 in sulfolane (SI). The galvanostatic polarization of the cells was carried out at a temperature of 30 C. The voltage range at cathodic and anodic polarization was limited by +/−500 uV with the current density being 0.2 mA/cm2.
The amount of electricity in cathodic deposition and/or anodic dissolution of lithium was equal to 1.0 mAh/cm2.
Studies have shown (
Barrier Layer by Tribochemical Treatment with Li3N Powder—Nitrogen Atmosphere
The formation of the Li3N barrier layer was carried out in a similar way described in Example 1, except that the treatment of the surface of lithium foil was carried out in an airtight reactor under nitrogen, which was purged by nitrogen gas at a speed of 6 l/min.
The median volume size of the particles in the Li3N powder (used instead of the powder of Example 1) was estimated to be in the range of from 5 to 15 microns.
Studies have shown (
Barrier Layer by Tribochemical Treatment with a Mixture of Si and Li3N Powder—Nitrogen Atmosphere
A barrier layer of Si—Li3N was formed in a similar way as described in Example 2.
The median volume size of the particles in the Si and Li3N powder (used instead of the particles of Example 2) was estimated to be in the range of from 5 to 15 microns.
Studies have shown (
Barrier Layer by Tribochemical Treatment with Si Powder—Argon Atmosphere
The formation of the Si barrier layer was carried out in a similar way as described in Example 2, except that the treatment of the surface of lithium foil was carried out in an airtight reactor under dry argon atmosphere.
The median volume size of the particles in the Si powder (used instead of the particles of Example 2) was estimated to be in the range of from 5 to 15 microns.
Studies have shown (
Barrier Layer by Tribochemical Treatment with P2S5 Powder—Nitrogen Atmosphere
The formation of the P2S5 barrier layer was carried out in a similar way as described in Example 2.
The median volume size of the P2S5 powder (used instead of the particles of Example 2) was estimated to be in the range of from 5 to 15 microns.
Studies have shown (
Number | Date | Country | Kind |
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2000467.7 | Jan 2020 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2021/050593 | 1/13/2021 | WO |