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, rate performance, and coulombic efficiency.

BACKGROUND OF THE INVENTION

For batteries two main types of batteries may be considered, solid state batteries, and liquid electrolyte batteries. A solid-state battery relates to battery technology that uses solid electrodes and a solid electrolyte. Materials proposed for use as solid electrolytes in solid- state batteries include ceramics (e.g. oxides, sulfides, phosphates), and solid polymers. Solid- state batteries have found various uses and have certain advantages. Examples of solid state batteries can be found in US 2018/301741 A1 and CN 108 963 222 A. US 2018/301741 A1 recites a lithium-sulfur battery, especially ones that are flexible for wearing about an appendage of a wearer. Such batteries have a lithium metal anode, a sulfur cathode comprising sulfur, a conductive carbon, a lithium supertonic solid-state conductor, and a dendritic or hyperbranched polymer binder, an electrolyte flexible film layer between the lithium metal anode and the sulfur cathode, and a current collector positioned on the sulfur cathode opposite the electrolyte layer (see e.g. fig. 1). CN 108 963 222 A recites a solid composite electrolyte electrode active material. The solid-state composite electrolyte electrode active material layer comprises an oxide solid-state electrolyte material and an electrode material having lithium ion storage characteristics and typically a conductive additive and/or binder. A manufacture method thereof comprises apply forming a solid composite electrolyte electrode active material layer sheet. Albeit a semi-solid state is mentioned, therein the solid-state composite electrolyte electrode active material layer is still applied.

Liquid electrolyte batteries make use of a liquid. For instance lithium-ion batteries contain lithium salts. The salts are in a solvent, such as an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate. A liquid electrolyte acts as a conductive pathway for the movement of cations passing from the negative to the positive electrodes during discharge, and vice versa during charging. Typical electronic conductivities of liquid electrolyte are relatively low, whereas ionic conductivities are relatively high.

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 always 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.

There is an on-going need to improve a capacity, an energy density, prevent ion depletion, charging speed, and cycling performance of power supply units. In addition prior art devices tend to have too many inactive parts and/or too large inactive part. Some of these devices suffer from internal mechanical stress, capacity loss, and shorting of cycle life. In this respect Si could be considered as anode material, but it is often not suited in view of its large volumetric expansion when forming Li _x Si _y (such as LF _. - Si). Also SiC composites are considered, but these have limited capacity. In general, loss of contact and rupture of the passivating solid electrolyte interphase (SEI) is detrimental to the operation of the battery.

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

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 power supply unit comprising at least two electrodes, selected from at least one anode and at least one cathode, or element functioning as such, such as anode active material and cathode active material, respectively, at least two current collectors each individually in electrical contact with at least one electrode, at least one separator, a liquid electrolyte soaking the anode and the cathode and the separator, the liquid electrolyte comprising Li-salt and/or Na-salt, typically dissolved in the solvent, such as ethylene carbonate/diethylene carbonate, characterized in that the at least one electrode comprises solid Li-ion or Na-ion conducting additives, preferably with a conductivity of <10 ^_1 S/cm, preferably <3*10 ^-2 S/cm (@ 20 °C), more preferably <10 ^-2 S/cm, such as KG ⁴ to 5*10 ^-3 S/cm. The liquid electrolyte at least partly and typically fully, in as far as (mechanically) possible, surrounds and infiltrates the electrodes as well as the separator (see e.g. fig. 1). As the electrodes and separator are porous the liquid electrolyte typically fully soaks these. Further the liquid electrolyte allows ions to move back and forth to the respective electrodes, through the separator. The present solid Li-ion or Na- ion conducting additives may be incorporated in the least one electrode, such as by mixing them into an electrode slurry, and thereafter applying them to substrate material or current collector, they may be introduced therein, such as by depositions techniques, or the electrode may comprise these additive in another way, and combinations thereof. The solid Li-ion or Na-ion conducting additives may be provided in more than one electrode, preferably in all electrodes. The present electrodes, each individually, may be partly porous, such as with an open pore volume (relative to a total volume of the electrode) of 5-70 vol/vol%, preferably 10-50 vol.%, more preferably 15-45 vol.%, such as 30-40 vol.%, e.g. 30 vol.%. The present electrodes are typically made or provided as a matrix material, comprising various components, typically in particulate form and adhered together, therewith providing open, porous volume. The present electrode may comprise an active material, the present ion conductive additive, an electron conductive material, such a carbon black, and binder, typically in a mass:mass ratio of (1-5 active material):(0.1-0.5 ion conductive additive):(0.2- 1.0 electron conductive material):(0.2-1.0 binder), such as 0.77/0.05/0.08/0.1 active material/solid additive from this invention/ carbon black/binder, preferably optimized in view of the active material, and preferably in a small amount. The present current collector may be integrated in the present power supply unit, may be incorporated therein, may form part thereof, such as a part of a housing thereof, and combinations thereof. The present invention provides a more homogenous ionic current distribution between the active material particles, a (relative) reduction of (volume of) inactive parts, such as current collectors and separators, the option of thicker electrodes, homogeneous distribution of electrolyte decomposition products, a longer cycle life, improved rate performance, reduced mechanical stress, improved capacity, and increased energy density, provided by improved charge transport, both ionic and electronic through the present stable power unit electrode morphology and active material. Also use of scarcely available materials may be minimal. Typical further characteristics of Li-ion or Na-ion batteries do not change significantly. On top of the above mentioned beneficial effects, the solid lithium ion or Na-ion conductive particles make it more straightforward to achieve satisfactory performance and thereby reduce the need for electrode porosity. This facilitates battery preparation, but also provides a better performance when the active material expands (during operation), or when a passivating film decreases either the electrode porosity, or the active material contact with the electrode, or the particles provide an ion transporting bridge across the film.

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

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

In an exemplary embodiment the present power supply unit is a liquid electrolyte battery. In an exemplary embodiment of the present power supply unit solid Li-ion or Na-ion conducting additive may be selected from crystalline material, polycrystalline material, and anisotropic material, the material comprising octahedra and/or tetrahedra sharing common corners, edges, or facets, wherein octahedra are preferably selected from MYr,, wherein M may be a metal, such as Ti, Ge, Si, Al, La, Zr, Sn, and Sr, or a non-metal, such as P, and wherein Y may be O, S, or a halogen, and wherein tetrahedra are preferably selected from XY4, wherein X is preferably selected from P, Zr, and Si, and wherein Y may be O, S, or a halogen, or combinations thereof.

In an exemplary embodiment of the present power supply unit M in MY ₆ may preferably be selected from Ti, Ge, Si, Al, La, Zr, Sn, Sr, such as in Lii _+x M2 _+y X3- _z Oi2 (0<x,y,z<3), such as in Lii _. sAlo _. sGei PCL^ and in LLTisOn, in Nai+ _x M2+ _y X3-zOi2 (0<x,y,z<3), such as in Nai _. sAlo _. sGei PCL^ and in Na ₄ Ti ₅ 0i ₂ ,from garnet-type oxide compounds (Li-La^Zr Oi ), from titanates, such Lao _.57 Lio _.29 Ti03 _, and from argyrodite type compounds (LLPSsX (X = Cl, Br, I, F)), and combinations thereof.

In an exemplary embodiment of the present power supply unit the additives may be provided as particles.

In an exemplary embodiment of the present power supply unit the additive particles have an equal or similar average size as the active material particles, such as 0.9-1.1 an average size of the material particles, such as about 1 pm.

In an exemplary embodiment of the present power supply unit the particles of active material may have an average size of 10-50000 nm, preferably 20-250 nm, such as 50-100 nm.

In an exemplary embodiment of the present power supply unit the at least one electrode may comprise silicon, such as Si particles.

In an exemplary embodiment of the present power supply unit the at least one electrode may comprise 2-20 mass.% solid Li-ion or Na-ion conducting additives, preferably 4-17 mass.% additives, more preferably 5-15 mass.% additives, such as 7-12 mass.% additives.

In an exemplary embodiment of the present power supply unit the at least one electrode may comprise intercalation materials, such as LFP (LiFePCL) particles or NMC (Li _a Ni _b Mn _c Co _d 0 ₂ or Na _a Ni _b Mn _c Co _d O) particles, wherein 0<(b,c,d)£l and l£a<2, or more general X _a MCh wherein X=Li or Na and l£a<2 and M a single metal or combination of metal ions.

In an exemplary embodiment of the present power supply unit the at least one electrode may comprise 2-20 vol.% solid Li-ion conducting additives.

In an exemplary embodiment of the present power supply unit at least one electrode thickness may exceed 120 micron.

In an exemplary embodiment of the present power supply unit the power supply unit may be a Li-ion or Na-ion battery, or battery pack, or car battery, or storage, or combination thereof.

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 Figs. 1 and 2 show schematics of the invention and prior art.

Fig. 3a shows half-cell voltage versus time.

Fig. 3b shows results of impedance spectroscopy.

Figure 4. Rate capability of silicon electrodes with fillers added.

DETAILED DESCRIPTION OF FIGURES The figures are detailed throughout the description, and specifically in the experimental section below.

In the figures:

1 lithium ion

2 positive electrode

3 negative electrode

4 separator

5 liquid electrolyte

6 current collector

7 active material

8 electron conductive additive, e.g. carbon black

9 ionic conductive additive according to the invention 11 surface layer of the active material

Figure 1 shows schematics of a lithium ion battery in operation. Lithium ions 1 move between positive electrode (cathode) 2 and negative electrode (anode) 3, which are separated by a porous medium, i.e. separator 4, soaked with liquid electrolyte 5. Current collectors 6 supply the electrodes with electrons. For Na-ion batteries a similar layout may be used.

Figure 2 shows a schematic drawing of a electrode for use in a battery. Top row shows pristine, as prepared electrodes, second row shows cycled/aged electrodes. During operation, after cycling, passivating layers will form on the active material surface. Also the active material undergoes volume changes upon lithiation. These processes hamper proper ionic conductive pathways between active material particles throughout the electrode and the bulk liquid electrolyte. The addition of ionic conductive particles is found to preserve ionic pathways and the internal resistance reduced.

Figure 3a shows that using PTFE binder two free standing, self-supporting electrodes were made with a thickness of approximately 300 micron and 36% vol porosity using LiFePCf active material. This thickness is three times the industrial practice. When these electrodes are charged versus lithium metal counter electrode at a current equal to one tenth of the theoretical capacity (175mAh/g) available. Inventors observe that electrodes made according to current standards reach 25% of the available capacity (average of 5 samples), whereas the electrodes made with 5% solid ionic conductive additives (in this case LAGP), reach 70% of the available capacity. This is almost a 3 fold improvement. Furthermore the overpotential is greatly reduced, hence the energy dissipation is less, so less heat development, faster charging characteristics, less side reactions, and thus a longer cycle life. To further study the overpotential improvement inventors performed EIS, shown below. It is observed that for all frequencies (10 ⁵ to 10 Hz) the impedance of the electrode layer is greatly reduced (over 10 fold).

Fig. 3b shows Impedance spectroscopy results for symmetric LiFePCE electrode cells, i.e. LiFePCE vs LiFePCE. Hence only the resistance of the cathode is probed. The method for making the freestanding electrodes is the same as the other, however PTFE binder is used in combination with isopropanol solvent. Freestanding electrodes could be made much thicker (generally) as they do not suffer from delamination upon drying. Other methods for making freestanding are available and can also be used in combination with the present invention.

Figure 4. Rate capability of silicon electrodes with fillers added. The biggest improvement is observed for the ionic conductive additive

Si/LAGP>Si/LT0>Si/Ti0 ₂ >Si/Al ₂ 0 ₃ >Si>Si/CU (LAGP). The positive effect is clearly related to limited ionic conductivity. The invention delivers 2x more capacity with respect to the standard electrode at C/5 and 3x better after 20 cycles.

The results are equally applicable to Na-ion batteries.

Methods of preparation

All electrodes are prepared using a conventional slurry casting method. The dry components ((a)active material, (b)the present ion conductive additive, (c) an electron conductive (carbon black (Super P, TIMCAL)) and (d) binder) are mixed by ball-milling. Solvent (binder dependent) is added to form a viscous slurry. The slurry was casted on clean copper or aluminium foil with an applicator having a slit thickness of 100 pm. The coating was dried at 60°C in vacuum oven for 12h and cold-pressed under 2 tons for 1 minute.

For the silicon anodes the active material is crystalline silicon powder with mean particle size distribution of 100 nm and 99% purity as specified by Alfa Aesar. The ratio for the above different components is 0.55/0.05/0.2/0.2, for active material/ionic conductive additive/electron conductive additive/binder. As a binding agent carboxymethyl cellulose was used with an average molar weight of 90,000 g/mol, acquired from Sigma Aldrich. To dissolve the binder 2 ml of buffer solution of citric acid/sodium hydroxide/sodium chloride solution with fungicide (pH=3, Fluka) was used. For this electrode a copper foil current collector was used (cleaned with 0.1M oxalic acid and ethanol). For standard silicon electrodes the preparation method was the same, solely lacking the ionic conductive additive. The cathode material was carbon-coated LiFePCE from Phostech with an average particle size of 140 nm. Conventional type cathode electrodes were prepared through mixing a slurry of LiFePCE, Carbon Black (Super P), PVDF (polyvinylidene fluoride, Solvay), with a mass ratio of 0.80/0.10/0.10 respectively, in N -methylpyrrolidone (NMP). Again the ionic conductive additive weight fraction was 5%, leading to a ratio of 0.75/0.05/0.1/0.1. These slurries were casted on aluminium current collectors.

The pouch cells/coin cells were assembled in an argon filled glove box, in order to avoid reactions with oxygen and moisture (<0.1 ppm O2 and <2 ppm H2O) using a polymer separator (Celgard 2250). A disk cut from lithium metal foil, equal to size of the working electrode, served as a counter electrode, unless otherwise indicated. As an electrolyte 1.0 M L1PF6 in 1:1 v/v ethylene carbonate (EC) and diethyl carbonate (DEC), with the addition of 3%wt vinyl carbonate (VC) and 20%wt fluoroethylene carbonate (FEC). These liquid additives are used to stabilize the passivation layer on the silicon active material.

Results

To elucidate the improvements of so-called porous silicon carbon anodes a range of model electrodes were studied. Prior art conformal coatings have shown appreciable cyclability. Now a range of silicon porous electrodes were prepared with additives mixed into the slurry. These additives are chosen as copper, inert alumina, titanium oxide, and Lii _.5 Alo _.5 Gei _.5 (P04)3 referred to as LAGP. These electrodes were tested at varying c-rates to test whether these additives beneficially influence the electrodes charge transport network (see fig. 4). All the above additives showed improvement with respect to the bare material. The copper composite exhibits an especially high capacity in the first few cycles, but performs no better from cycle 12 and onwards. The electronic conductivity of this electrode is considered superior. The biggest improvement is observed for the ionic conductive additive LAGP.

The above is equally applicable to Na-ion batteries.