FIELD OF THE INVENTION

The present invention is in the field of high electric energy density storage devices, typically Li-comprising power units, which may be used as a battery pack, a car battery, an electrical energy storage device in general, and may be incorporated in portable electronics. It is aimed at improving power unit performance, e.g. in terms of cycle life and coulombic efficiency.

BACKGROUND OF THE INVENTION

A lithium-ion battery may be used for energy storage, which may be a type of rechargeable battery. Lithium-ion batteries are widely used, such as for portable electronics, electric vehicles, and electrical energy storage devices. In the batteries, lithium ions may move back and forth, from the negative electrode to the positive electrode during discharge, and vice versa when charging. For rechargeable cells, the term cathode designates the electrode where reduction is taking place during the discharge cycle; for lithium-ion cells, the positive electrode is referred to as cathode, which typically is the lithium-based one. Li-ion batteries may use an intercalated lithium compound as one electrode material. The batteries have certain advantages over other electric energy storage device, such as a relatively high energy density, low self-discharge, and no memory effect. Typical density characteristics are a specific energy density of up to 900 kJ/kg, a volumetric energy density of up to 2230 J/cm ³ , and a specific power density of up to 1500 W/kg. Performance of the batteries can be improved, such as in terms of life extension, energy density, safety, costs, and charging speed.

The anode, such as a Li-metal anode, may suffer from various drawbacks, which are typically irreversible. On the anode, a layer may be formed, which limits anode efficiency and increases cell impedance and reduces capacity. Also, plating of metallic lithium on the anode may lead to a loss of cyclable lithium. Also, lithium dendrite formation may take place on the anode, which dendrites may accumulate and pierce the separator, causing a short circuit leading to heat, fire or explosion.

Efforts for solving the above dendrite problem as well as the volume expansion of the lithium metal anode in lithium metal batteries have been directed towards electrolyte additives, solid-state electrolytes, and lithophilic anodic hosts to retard or even eliminate the above-mentioned problems. These strategies aim at stopping the dendrite nucleation in a first stage and preventing dendrite growth in a second stage.

The present invention therefore relates to an improved power supply unit with an improved anode and a method for forming said anode, which solves one or more of the above problems and drawbacks of the prior art, providing reliable results, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more limitations of power supply units of the prior art and methods of making these and at the very least to provide an alternative thereto. In a first aspect the present invention relates to a method of producing a power supply unit with a novel and stable 3D nonconductive lithium metal anode achieved by a metal electrode (usually referred to as current collector) covered by a porous layer comprising a dielectric material, such as a barium titanate (BTO) covered copper. A densely and uniformly plating of Li metal in the microporous anodic structure is achieved. As the dielectric is electrically nonconductive, lithium ions can only deposit on the metal of the electrode or on the plated Li substrate. Effective regulation of lithium ion distributions is achieved when these ions approach the anode surface and thus a uniform Li deposition and dendrite formation are prevented. The presence of the dielectric material is found to lead to an increased lithium Coulombic efficiency compared to a planer metal host. In an example, BTO was employed as a nonconductive 3D microporous high dielectric host. Li metal deposition is observed to occur homogeneously filling the 3D host lowering electrolyte decomposition and lowering the interfacial resistance. This is found to result in a stable cycling behavior. As a result the present anode provides enhanced electrochemical performance of lithium metal batteries (LMBs).

The present electrodes with a microporous layer comprising a high dielectric material have been repeatedly tested showing a strongly improved battery cycling (of over 700 cycles) and higher Coulombic efficiencies of above 99.9%, even without electrolyte additives that could improve these even more. It was found that electrical field gradients near the electrodes were mitigated by the dielectric material. Also dendrite growth of Li could no longer be observed. Also electrolyte decomposition, increase in internal resistance over time, and safety were found to have been improved.

In the present method the power supply unit comprising an anode, a cathode, and an electrolyte in electrical contact with the anode and cathode, the method comprising providing the anode, wherein the anode is a metal electrode, providing a dielectric material, typically in crystalline or semi crystalline form, such as BTO, providing fine particles, such as by milling the dielectric material into fine particles, such as with a 10 nm-20 pm size, dissolving the particles in a solvent, and depositing a layer of the particles onto the anode.

The present power supply unit, obtainable by the present method, comprises an anode, a cathode, an electrolyte in electrical contact with the anode and cathode, the electrolyte preferably comprising a Li-salt in a solvent, or a solid electrolyte, and optionally a separator, characterized in that the anode comprises a metal provided with a microporous layer of dielectric material, wherein the dielectric material comprises >50 wt% of material with a relative permittivity Sr of > 80 (@ 300K). The present power supply unit may also be used as power storage unit, and may typically be rechargeable. The separator may be provided as a separate element, or may be combined with the electrolyte, e.g. a layer comprising both electrolyte and separator. Suitable separators may be selected from fibrous material, such as glass fibre, particulate material, such as glass beads, and combinations thereof.

Figure 1 shows a schematic diagram of lithium metal plating and stripping, (a) lithium metal plating/stripping process on planer copper (prior art);(b) lithium metal plating/stripping process on a BTO layer on copper foil. In the prior art, Li ⁺ ions are deposited on the solid substrate electrode, such as a copper electrode (al). A metallic Li deposit is formed, typically in irregular patterns (a2). Soon Li-metal dendrites start to appear (a3). When cycling these dendrites may be covered by a solid electrolyte interphase (SEI) layer, severely hampering functioning. The present invention provides a microporous layer of particulate dielectric, such as BTO, on the solid substrate (bl). The Li metal is now formed in the micropores of the dielectric layer (b2) and when cycling between loading and unloading can easily be dissolved as electrolyte (b3).

The present invention is also subject of a scientific article of Wang, Wagemaker, et al., which article and its contents are incorporated by reference. Details of experiments, background, and the invention may be found there.

Accordingly, the present invention can be seen as a method of producing a power supply unit, the power supply unit comprising an anode, a cathode, and an electrolyte in electrical contact with the anode and cathode. Said method comprising providing the anode, wherein the anode is a metal electrode, providing a dielectric material, such as barium titanate, providing particles of the dielectric material, such as by milling the dielectric material into fine particles, dissolving the particles in a solvent, and depositing a layer of the particles onto the anode.

It should be understood that the solution may in this example be a suspension and dissolved particles may be seen as a particulate in said suspension.

Also separately from this example and compatible with all embodiments, the method may comprise depositing a layer of dielectric material particles on a copper current collector, wherein in the method lithium is deposited interstitially between the particles within the deposited layer at the current collector and/or interstitially between the particles within the deposited layer. This beneficially allows for a homogeneous electric field which increases the effective prevention of dendrite formation.

According to a further aspect of a power supply unit, also separately from this example and compatible with all embodiments, a deposited layer of dielectric material particles is provided on a copper current collector, that is to say a copper anode, wherein the battery unit is arranged such that, during battery cycling, lithium is deposited interstitially between the particles within the deposited layer at the current collector, and/or interstitially between the particles within the deposited layer.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

In an exemplary embodiment of the present method the dielectric material may be milled into particles with an average particles of 10 nm-50 pm, preferably 20 nm-10 pm, more preferably 100 nm-8 pm, even more preferably 250 nm-7 pm, such as 500 nm-5 pm.

In an exemplary embodiment of the present method the dielectric material may be milled into particles with a BET specific surface area of >1 m ² /g, preferably >5 m ² /g, more preferably >10 m ² /g, such as >20 m ² /g.

In an exemplary embodiment of the present method the dielectric material layer may have a porosity of 10-90 % (v/v), preferably 20-80%, more preferably 30-70%, even more preferably 35-66%, such as 40-60%, e.g. 45-55%. It has been found that such high porosities provide a good storage capacity for Li, which in turn improves the specific capacity and energy density of the power supply unit.

In an exemplary embodiment of the present method the solvent may be a water-miscible solvent, preferably a dipolar aprotic solvent, such as dimethylformamide, dimethyl sulfoxide, and N-Methyl-2-pyrrolidone (CAS Nr. 872-50-4).

In an exemplary embodiment of the present method the solvent may comprise a binder, such as a polymeric material, preferably a thermoplastic polymer, such as a fluoropolymer, such as PVDF (CAS 24937-79-9).

In an exemplary embodiment of the present method the solvent may comprise a gas forming species, such as bicarbonate. After deposition of the slurry comprising the present dielectric particle and the gas forming species on the anode, the gas forming species is removed, such as by increasing he temperature during a period of time, thereby contributing to the porosity of the dielectric layer formed.

In an exemplary embodiment of the present method the solvent, dielectric particles, and optionally binder material and gas forming species, may be mixed into a slurry before depositing, such as in a particle:binder:gas forming species ratio (mass:mass:mass) of 0.8:0.2:9 to 7.5:1.5:1, such as (5:1:4). The mass:mass ratio of particle:binder is typically from 1:1 to 20:1, such as 2:1 to 10:1.

In an exemplary embodiment of the present method the deposited layer may have a thickness of 1-500 pm, preferably 2- 80 pm, more preferably 5-70 pm, such as 10-60 pm.

In an exemplary embodiment the present method may further comprise increasing temperature in a vacuum oven above > 60°C.

In an exemplary embodiment of the present method the electrode material may be selected from copper.

In an exemplary embodiment of the present method the dielectric particles may comprise >50 wt% of material with a relative permittivity Sr of > 80 (@ 300K), preferably >250, more preferably > 500, even more preferably > 1000.

In an exemplary embodiment of the present method the dielectric material may be selected from titanates, such as barium titanate, aluminium titanate, copper titanate, calciumcopper titanate, calcium titanate, strontium titanate, lead titanate, zircone titanate, lanthanium titanate, and from niobates,.

In an exemplary embodiment of the present power supply unit the dielectric material may comprise >50 wt% of material with a relative permittivity Sr of > 80 (@ 300K), preferably >250, more preferably > 500, even more preferably > 1000.

In an exemplary embodiment of the present power supply unit the microporous layer may have a thickness of >10 nm, preferably 20nm-500 pm, more preferably 100 nm-100 pm, such as 200 nm-10 pm.

In an exemplary embodiment of the present power supply unit the dielectric material may be selected from titanates, such as barium titanate, aluminium titanate, copper titanate, calcium copper titanate, calcium titanate, strontium titanate, lead titanate, zircone titanate, lanthanium titanate, and from niobates,.

In an exemplary embodiment of the present power supply unit the microporous layer may be a microporous layer, such as with pore sizes of 10 nm-200 pm, preferably 20 nm-20 pm, more preferably 100 nm-8 pm, even more preferably 250 nm-7 pm, such as 500 nm-5 pm.

In an exemplary embodiment of the present power supply unit the metal may be selected from Cu, Al, Ni, stainless steel, and combinations thereof.

In an exemplary embodiment of the present power supply unit the BET specific surface area of the microporous layer may be >1 m ² /g, preferably >5 m ² /g, more preferably >10 m ² /g, such as >20 m ² /g.

In an exemplary embodiment of the present power supply unit the dielectric material layer may have a porosity 10-90 %

(v/v), preferably 20-80%, more preferably 30-70%, even more preferably 35-66%, such as 40-60%, e.g. 45-55%.

In an exemplary embodiment of the present power supply unit the power supply unit may be a Li-ion battery, or a battery pack, or car battery, or a storage, or incorporated in portable electronics.

The invention will hereafter be further elucidated through the following examples which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF THE FIGURES

Figure 1 shows a schematic diagram of lithium metal plating and stripping, (a) lithium metal plating/stripping process on planer copper (prior art);(b) lithium metal plating/stripping process on a BTO layer on a copper foil.

Figure 2 shows Lithium Coulombic efficiency measurements for Li/Cu cells with different covering layers. Lithium Coulombic efficiencies at various current densities 1(a) 1 mA crrr ² ; 1(b) 2 mA crrr ² , 1(c) 4 mA crrr ² , with a fixed lithiation capacity of 1 mAh cm ^-2 . 1(d) Lithium Coulombic efficiency at a current density of 4 mA cm-2 with a high areal capacity of 4 mAh crrr ² .

Figure 3a-f shows SEM morphologies of electrodes after depositing 1 mA h crrr ² lithium at a current density of 1 mA crrr ² with different covering layers. (3a,3d) with planar copper;

(3b,3e) with high dielectric material BTO); (3c,3f) with low dielectric material AO. Insets are the digital images of relative electrodes.

DETAILED DESCRIPTION OF FIGURES

The figures are detailed throughout the description, and specifically in the experimental section below.

Examples

Preparation of electrodes and electrochemical tests

Commercial BTO and AI2O3 (AO) powder were bought. BTO and AO were ball milled at 450 rpm, during 20 h. BTO and AO were then both mixed with polyvinylidene fluoride (PVDF) and NH4HCO3 in a 5:1:4 ratio using N-methyl-2-pyrrolidone (NMP) as a solvent.

The obtained slurry was used for depositing on a copper anode. After deposition both electrodes were put in a vacuum oven at 80 °C to remove NH4HCO3 and to obtain a microporous structure.

Coin-shaped battery cells were assembled using as-prepared electrodes with lithium metal, PE (Celgard 2300) and conventional carbonate electrolyte (1M LiPF ₆ in 1:1 v/v EC: DMC) were used as separator and electrolyte. Galvanostatic cycling was performed by deposition of Li onto the Cu working electrode with different current densities up to a fixed capacity (1 mAh/cm ² to 4 mAh/cm ² ), followed by Li stripping at different current densities up to 1 V. To prepare LTO cathodes, active material was mixed with carbon Super P and PVDF in a mass ratio of 8:1:1, and NMP was used as the solvent. The Li/LTO half cells were cycled. Impedance measurements of coin cells were carried out on Autolab between 10 kHz and 0.01 Hz. Cyclic voltammograms (CVs) were recorded.

Characterization of the Materials and the Electrodes

Lithium metal-plating electrodes were prepared by discharging the coin cell at a 1 mAh/cm ² capacity. Cross-section SEM samples were made at 2-10 kV (secondary electron). Nitrogen adsorption-desorption isotherms were recorded using an automatic surface area and porosity analyzer (Micromeritics) at 77 K. Particle size distribution was measured using Microtrac S3500.

Results and Discussion

Average particle sizes of milled BTO and AO were found to be about 8 pm as shown by SEM and Dynamic Light Scattering. The BET specific surface areas were found to be 23.59 and 23.08 m ² g ¹ for BTO and AO, respectively.

After depositing 1 mAh crrr ² lithium at a current density of 1 mA crrr ² , cells with BTO layer were more stable and showed uniform lithium electrodeposition without lithium dendrites being formed. It was found that freshly deposited lithium metal was well-confined in the 3D microporous structure of the BTO layer and no loose or mossy structures were found. In contrast, direct Li deposition on the Cu foil showed islands and dendritic structures). In cells with an AO layer lithium dendrites are found in the top of the layer, with visible Li dendrites in a diameter of 2 pm, which is even worse than planar copper.

Coin cells cycling performance of BTO and AO electrodes

Lithium coulombic efficiency (CE) was measured. 1 mA-hour crrr ² of Li metal was plated onto the Cu foil working electrode, followed by complete stripping of the Li metal from the Cu foil. The BTO microporous layer electrode exhibits a higher lithium CE and longer cycling life span at all current densities compared to the AO layer. After a few cycles the CE easily reached about 99.99% for the next 700 or more cycles.

It was found that a uniform electric field was achieved in the whole structure without any covering layer on the planar copper electrode. However, around the lithium metal, especially at the tip point the electric field was high (6.17xl0 ⁴ V/m) causing lithium dendrite formation on the copper foil. When using AO it was found that the electric field was even higher (9.17xl0 ⁴ V/m). This result is confirmed with SEM images (figs. 3a-f). When using BTO the electric field was even smaller the background (1.5xl0 ³ V/m). Therewith a lithium dendrite free morphology was achieved. It was found that metal-Li deposition showed a flat and dense morphology, without the observation of dendritic Li (Figure la-d). Li was found to deposit from the bottom of the electrode and fill up the BTO microporous layer. The dense deposition of lithium within the BTO layer is found to reduce side reactions.

Possible recipes are presented below.

1. Normal copper current collector without a coating layer, BTO coated glass fiber as the separator, and lithium metal using a liquid electrolyte to form a power storage unit. A high dielectric material is put in or on the separator to prevent dendrite growth on both electrodes. Commercial BTO powder was bought and BTO was ball milled at 450 rpm, during 20 h. BTO was then mixed with polyvinylidene fluoride (PVDF) and NH4HCO3 in a 5:1:4 ratio using N-methyl-2-pyrrolidone (NMP) as a solvent. Next, glass fiber was immersed in the obtained slurry. After that, the BTO coated glass fibers were put in a vacuum oven at 80 °C to remove NH4HCO3 and to obtain a microporous structure. Coin-shaped battery cells were assembled using a copper current collector with lithium metal, the BTO coated glass fiber and conventional carbonate electrolyte (1M LiPF ₆ in 1:1 v/v EC: DMC) were used as separator and electrolyte. Galvanostatic cycling was performed by deposition of Li onto the Cu working electrode with different current densities up to a fixed capacity (1 mAh/cm ² to 4 mAh/cm ² ), followed by Li stripping at different current densities up to 1 V.

2. Melting lithium metal and high dielectric material together as the electrodes and assembling a symmetrical cell using solid-state electrolyte with some high dielectric material additive. Use is made of a of high dielectric material in solid-state lithium metal plating.

Lithium metal was melted in a nickel crucible at 300 °C and commercial BTO nano-powder was put in with a ratio of 1:1 to make a homogeneous mixture of BTO and lithium. The obtained composite was used as an electrode for lithium metal plating. Symmetrical Solid-state batteries were assembled using as- prepared electrodes with solid-state electrolyte LiePSsCl. Galvanostatic cycling was performed by deposition of Li onto the BTO/Li working electrode with different current densities up to a fixed capacity (0.1 mAh/cm ² to 2 mAh/cm ² ), followed by Li stripping at different current densities up to 1 V.