A lithium ion battery cell

Technical field

The present disclosure relates to a lithium ion battery cell.

Background art

The use of rechargeable (secondary) batteries is becoming increasingly prevalent, due in part to trends in society such as vehicle electrification and pervasive use of mobile consumer electronics. Since their commercial introduction in 1991, lithium ion batteries (Li-ion) batteries have been widely adopted in a variety of applications due to favorable properties such as high energy density, low self-discharge or little or no memory effect.

A Li-ion battery cell typically comprises a cathode and an anode, with an electrolyte and a separator arranged between the electrodes. Typically, the cathode comprises an intercalation material, which is a solid host network capable of reversibly storing lithium guest ions. Most commercialised cathode materials are based on transition metal oxides, such as the lithium cobalt oxide used in the first commercial lithium ion batteries, although cathodes based on polyanion compounds such as lithium iron phosphate are now also commercially available. The anode typically comprises a carbon material such as graphitic or hard carbon, which is capable of intercalating lithium between its graphene planes. The separator may typically be a single- or multi-layer porous polyolefin membrane, sometimes coated with one or more ceramic layers. Electrolytes for lithium ion batteries may typically be based on organic carbonates such as ethylene carbonate, with additives such as lithium salts used to optimise the electrolyte properties.

However, there remains a need to develop lithium ion batteries with improved properties. Summary

The inventors have identified a number of shortcomings with prior art lithium ion batteries. In order to expand the range of applications to which Li-ion batteries are suited, there is a need to improve the cost and performance of the batteries. Some appropriate performance metrics that are desirable to be improved include, but are not limited to, charge rate, discharge rate, energy density, specific energy, power density, and specific power of the batteries. Some of the raw materials used in the manufacture of Li-ion batteries are relatively scare and thus expensive, particularly some of the transition metals used in cathode manufacture, and there is a desire to transition to the use of cheaper, more abundant materials.

During the first charge and discharge (first cycle) of a lithium ion battery, the electrode materials react with the electrolyte to form a passivating solid-electrolyte interface (SEI) at the interface between the electrolyte and the electrodes. The SEI is mainly formed from the decomposition products of the electrolyte which involves the consumption of lithium ions originating from the cathode and the anode. This may give rise to an irreversibly reduced capacity of the battery since the lithium ions withdrawn from the cathode and anode for SEI formation are rendered unusable during the subsequent operation of the battery. However, the SEI is important to the battery since the presence of the SEI prevents further undesirable decomposition of electrolyte. In addition, a cathode electrolyte interphase layer (CEI) may be formed on the cathode, however the CEI layer does not consume a significant amount of lithium ions.

The present disclosure aims at balancing the irreversible losses of lithium ions from the anode and cathode, respectively, and to improve the reversible capacity of lithium ion batteries by the provision of a lithium ion battery cell comprising an anode layer as described below.

According to a first aspect there is provided a lithium ion battery cell comprising a cathode layer comprising a positive electrode active material with the formula Li _x Ni _a IVInbCo _c l\/ldO2-yAy, where 0.95<x<1.05, a+b+c+d=l, 0.5<a<0.98, preferably 0.83<a<0.98, or more preferably 0.80<a<0.98, M is either absent or comprises one or more metal dopants, y<0.1, and where A is one or more of S, N, F, Cl, Br, I, and P. The lithium ion battery cell comprises an anode layer comprising a negative electrode active material comprising graphite and 1-3 wt% Si and/or SiO _x , wherein 0<x<2, and an electrolyte comprising a solvent, wherein the solvent comprises ethylene carbonate, ethyl methyl carbonate or dimethyl carbonate or a combination thereof. For the anode, Si could be present as Si, SiOx, SiC, or Si-carbon composite, and could be mixed with graphite (Gr). Graphite could be the synthetic and natural graphite mixing, and the SG:NG ratio could be 100:0 to 50:50, preferably 90:10 to 70:30. The Si contents could range from 1% to 5%, preferably 2% to 3%.

The advantage of the proposed lithium ion battery cell that the irreversible losses of lithium ions are balanced between the anode layer and the cathode layer. Both the high nickel content NMC (nickel manganese cobalt) cathode layer and the silicon material (Si or SiO _x ) of the anode layer, and in particular the high content NMC cathode layer both have a low first cycle efficiency due to relatively high irreversible losses of lithium. By a balancing the irreversible losses between the anode and cathode side, respectively, the reversible capacity of the lithium ion battery is improved.

The negative electrode active material may comprise Si particles, the particles having a Dso particle size of 200 to 50 nm, preferably 175 to 75, most preferably 150 to 100 nm, or SiO _x particles, the particles having a Dso particle size of 1-20 pm, preferably 5-15 pm. The above Dso particle sizes of Si and SiO _x provide for a desired performance of the lithium ion battery cell according to the present disclosure.

The electrolyte may further comprise a first lithium salt, wherein the first lithium salt comprises LiPFe, I BF4, I CF3SO3, Li(CF3SO2)2N, UCIO4 or a combination thereof and preferably a second lithium salt, wherein the second lithium salt comprises lithium difluoro(oxalate)borate, LiBOB, lithium difluoro(oxalato)borate, LiDFOB, lithium difluorobis(oxalato)phosphate, LiDFOP, lithium tetrafluoro(oxalate)phosphate, LTFOP, or a combination thereof. The advantage of the first salt is that it provides a desired ionic conductivity of the electrolyte. The advantage of the second salt is that it is aiding the build-up of the SEI at the Si/SiO _x of the anode layer.

The electrolyte may further comprise an additive, wherein the additive comprises fluoroethylene carbonate or vinyl carbonate, or a combination thereof. The advantage of the proposed additives is to stabilize the surface of the cathode and the anode by aiding the buildup of the SEI at the Si/SiO _x of the anode layer.

The negative/positive capacity ratio of the cathode to the anode may be 1.0-1.1, preferably 1.02-1.05. Thus, a high performance of the lithium ion battery cell (the negative and positive electrode capacity being expressed as Ah/cm ³ ) because of balanced electrochemical reactions is provided.

The negative electrode active material may comprise a first graphite material with a particle size, Dso, in the range 10-20 pm, preferably 14-18 pm, a second graphite material with a particle size, D50, of 2-8 pm, preferably 3-6 pm, and Si and/or SiO _x particles with a particle size, D50, of 1- 5 pm, preferably 2-3 pm. The advantage of using different particle sizes of the first graphite material, the second graphite material and/or the Si and/or SiO _x particles is that it provides for a higher package ratio of the anode layer as the smaller particles can fit into voids between the larger particles.

The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure.

Hence, it is to be understood that the herein disclosed disclosure is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps. Brief description of the drawings

Figure 1 shows Al doping content for charge and discharge vs first cycle efficiency in %.

Figure 2 illustrates Al doping content for cycle life at 45 °C.

Figure 3 is a table which illustrates a capacity test for CAM and AAM combinations, showing real capacity.

Detailed description

Aspects of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings. The battery cell disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

A lithium ion battery cell comprises at least one cathode assembly, at least one anode assembly, an electrolyte, and optionally a separator. The electrolyte is arranged in contact with the cathode and anode in order to provide ion transport within the cell. The separator, if present, is primarily intended to provide a physical barrier between the cathode and anode, whilst still permitting ion transport. The cell typically comprises a single cathode assembly and single anode assembly, but may comprise multiple cathodes, i.e. two or more cathodes, such as three, three or four cathodes, and/or multiple anodes, i.e. two or more anodes, such as three, four or five anodes. The contents of the cell may be housed in a casing. The cell may be of any design known in the art, such as a cylindrical, prismatic or pouch cell. The cell is preferably a cylindrical cell.

Depending on the application, single battery cells may be used, or the cells may be arranged into battery packs. A battery pack comprises a plurality of battery cells, i.e. two or more battery cells, such as from about two to about 20 000 cells, such as from about 10 to about 10 000 cells, such as from about 100 to about 1000 cells. A battery pack may comprise a plurality of cells arranged in series and/or parallel. A battery pack may comprise further components such as a battery management system and a pack housing to enclose the battery pack components.

A cathode assembly comprises a current collector and at least one cathode layer. The current collector may comprise, consist essentially of, or consist of a metal foil, such as an aluminium foil, a copper foil, a stainless steel foil, or combinations thereof. The current collector may preferably be aluminium foil. A cathode layer is arranged on at least one surface of the current collector, alternatively on both surfaces of the current collector. The cathode layer comprises cathode active material, binder, and optionally further additives such as conductive additive. The binder may be any binder known in the art, such as for example PVDF (polyvinylidene fluoride), SBR (styrene-butadiene rubber), CMC (carboxymethylcellulose), or combinations thereof. The binder preferably consists essentially of PVDF. The conductive additive may be for example a carbon material such as carbon black, carbon nanotubes (multi wall or single wall), graphitic particles or graphite particles.

NMC is among the most popular cathode materials for lithium ion batteries, due in part to it having a specific energy comparable to lithium cobalt oxide (LCO) despite lower cost. Increasing the proportion of nickel in the NMC provides further improved energy/power density and may therefore be desirable, but may result in capacity degradation with repeated use. Cobalt is often added to the cathode material in an amount of about 10 mol% to obtain stability in the cathode material and obtain improved performance by balancing magnetic frustration. For example in case of NMC 811, the cobalt content is normally 10 mol%. However, cobalt is both scarce and toxic, and it would therefore be desirable to be able to reduce the cobalt content in the cathode active material, while maintaining stability and performance and avoiding cathode corrosion.

Accordingly, the cathode active material according to the present disclosure comprises of nickel manganese cobalt (NMC), Li _x Ni _a MnbCo _c MdO2-yAy, where 0.95<x<1.05, a+b+c+d=l, and 0.5<a<0.98, preferably, 0.80<a<0.98, or 0.83<a<0.98, and where M is either absent or comprises one or more metal dopants, n is the absolute value of the oxidation number of A, z<0.1, and where A is one or more of S, N, F, Cl, Br, I, and P.

The preferred metal dopant is typically aluminum (Al), but other dopants may be used, such as Zirconium (Zr), Yttrium (Y), Tungsten (W), Barium (Ba), Calcium (Ca), Strontium (Sr), Magnesium (Mg), Titanium (Ti), Boron (B), Bismuth (Bi), Molybdenum (Mo), etc. The amount of the dopants, such as Al dopants, should preferably be in the range of 0.005 to 0.05, such as 0.005 to 0.02. Adding the dopants in said amount increases the life cycle properties at higher temperatures, and the doping could make the lower 1 ^st cycle efficient. When increasing Ni contents in the CAM, higher specific capacity could be attained, but high-temperature properties would be lost, such as the Retention capacity of life cycle or Storage at the high temperature. Accordingly, adding some dopants will replace the transition metal to improve the crystal structure, thus mitigating this loss.

The cathode assembly may be prepared by coating the current collector with a slurry comprising the cathode active material, binder and optional additives in a suitable solvent. A suitable solvent may be a polar aprotic solvent such as NMP (N-methyl-2-pyrrolidone). Any suitable coating methods known in the art may be used. Following the coating, the cathode assembly may be further processed as is conventional in the art, such as by drying and calendaring.

An anode assembly comprises a current collector and at least one anode layer. The current collector may comprise, consist essentially of, or consist of a metal foil, such as a copper foil, an aluminium foil, a stainless steel foil, or combinations thereof. The current collector may preferably be copper. An anode layer is arranged on at least one surface of the current collector, alternatively on both surfaces of the current collector. The anode layer comprises anode active material, binder, and optionally further additives such as conductive additive. The binder may be any binder known in the art, such as for example SBR (styrene-butadiene rubber), CMC (carboxymethylcellulose), PVDF (polyvinylidene fluoride), or combinations thereof. The binder preferably consists essentially of a combination of SBR and CMC. The conductive additive may be for example a carbon material such as graphitic particles or graphite particles.

According to the present disclosure, the lithium ion battery comprises an anode layer comprising a negative electrode active material comprising graphite and 1-3 wt% Si and/or SiO _x , wherein 0<x<2.

The Si and/or SiO _x may be in particulate form. The Si particles may have a D50 particle size of 200 to 50 nm, preferably 175 to 75, most preferably 150 to 100 nm. SiO _x particles may have a D50 particle size of 1-20 pm, preferably 5-15 pm. By "D50 particle size" is meant the average particle size distribution (PSD). In one example, the anode layer comprises a negative active material comprising graphite and Si particles. In another example, the anode layer comprises a negative active material comprising graphite and SiO _x particles. In yet an example, the anode layer comprises a negative active material comprising graphite and a mixture of Si and SiO _x particles.

The negative electrode active material may comprise a first graphite material with a particle size, D50, of 10-20 pm, preferably 14-18 pm, a second graphite material with a particle size, D50, of 2-8 pm, preferably 3-6 pm, and Si and/or SiO _x particles with a particle size, D50, of 1-5 pm, preferably 2-3 pm. In one example, the negative electrode material comprises about 70-80 wt% of the first graphite material, 20-30 wt% of the second graphite material and 1-3 wt% of Si and/or SiO _x particles.

It is known that the higher amount of silicon in the anode layer, the higher energy/power density of the lithium ion battery. However, a large amount of silicon in the anode also increases the cost of the anode and thereby the total cost of the lithium ion battery cell. Another drawback by providing silicon in the anode layer is that an anode layer comprising silicon suffers from a relatively high amount of irreversible losses, thereby providing a low first cycle efficiency, during the first charge and discharge cycle of the lithium ion battery cell. In addition, silicon increases the resistance of the electrolyte by self-pulverization due to expansion during charging of the battery cell, thereby creating hollow spaces within the electrode. A SEI layer is formed on the new surface exposed during the expansion. In addition, side reactions may occur. This all together consumes electrolyte and increases the resistance within the lithium-ion battery cell.

As described above, during the first charge and discharge cycle of a lithium ion battery, the electrode materials reacts with the electrolyte to form a passivating solid-electrolyte interface (SEI) at the interface between the electrolyte and the electrodes. Most of these irreversible reactions occur during the first charge and discharge cycle of the lithium ion battery cell. Irreversible reactions may also occur during subsequent charge and discharge cycles, but to a much lesser extent as compared to the losses occurring during the first cycle.

The irreversible reactions occurring at the anode layer are as follows:

2Li + 3SiO2 -> 5Si + Li2SiO3 4Li +2SiO2 -> Si + Li4SiO4

4Li + 5SiO2 -> Si + 2Li2Si20s

As noted above, by increasing the proportion of nickel in the NMC cathode an improved energy/power density is provided. However, it is known that a cathode layer comprising NMC having a high nickel content, such as > 50%, preferably > 83% nickel in combination with a graphite anode, suffers from a high amount of irreversible losses during the first charge/ discharge cycle of the lithium ion battery cell.

It has been found that by combining a cathode layer comprising a high Ni content such as > 50%, preferably > 83% nickel, with an anode layer comprising a negative electrode active material comprising graphite and 1-3 wt% Si or SiO _x , wherein 0<x<2, the irreversible losses are balanced between the cathode and the anode. The reason for this is that the irreversible losses are substantially evenly distributed between the anode layer and the cathode layer such that substantially the same amount of lithium is consumed at the anode layer and cathode layer, respectively. By balancing the irreversible losses between the anode layer and cathode layer, the reversible capacity of the lithium ion battery is improved.

The anode assembly may be prepared by coating the current collector with a slurry comprising the anode active material, binder and optional additives in a suitable solvent. The anode layer of the present disclosure also comprises and 1-3 wt% Si and/or SiO _x , wherein 0<x<2. A suitable solvent may be a polar protic solvent such as water. Any suitable coating methods known in the art may be used. Following the coating, the anode assembly may be further processed as is conventional in the art, such as by drying and calendaring.

The electrolyte may comprise a solvent, a salt and optionally further additives. The lithium ion battery cell of the present disclosure comprises a solvent comprising ethylene carbonate, ethyl methyl carbonate or dimethyl carbonate or a combination thereof. The salt may comprise, consist essentially of, or consist of an inorganic lithium salt soluble in the solvent at relevant concentrations. In one example, the lithium ion battery cell of the present disclosure comprises an electrolyte comprising a first lithium salt, wherein the first lithium salt comprises LiRFe, Li BF4, UCF3SO3, LifCFsSChhN, UCIO4 or a combination thereof and preferably a second lithium salt, wherein the second lithium salt comprises lithium difluoro(oxalate)borate, LiBOB, lithium difluoro(oxalato)borate, LiDFOB, lithium difluorobis(oxalato)phosphate, LiDFOP, lithium tetrafluoro oxalato phosphate, LTFOP, or a combination thereof.

Further additives may be used, such as additives to stabilize the solvent/electrode interfaces, suppress gas formation, stabilize the cell against high voltage and overcharge, and decrease flammability. Such additives may be used in suitable concentrations. In one example, the additive comprises fluoroethylene carbonate or vinyl carbonate, or a combination thereof.

The battery cell may preferably comprise a separator arranged between the cathode assembly and the anode assembly. Any suitable separator known in the art may be used. The separator may comprise, consist essentially of, or consist of a porous polymer film. The polymer may be a polyolefin such as polyethylene (PE), polypropylene (PE), Polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), a polyester such as polyethylene terephthalate (PET), or a combination thereof. The separator may comprise a single layer or may be multi-layer, such as bilayer or trilayer. Further layers may comprise, consist essentially of, or consist of porous polymer films as described above, and/or may comprise, consist essentially of, or consist of ceramic material such as AI2O3, SiCh, TiCh, MgO, CaCCh, and combinations thereof.

The person skilled in the art realizes that the present disclosure is not limited to the embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed disclosure, from a study of the disclosure, drawings and the appended claims.

Examples

Example 1, Comparative experiment of Cathode Active Material (CAM) preparation

Different cathode active materials (CAMs) were prepared and tested for efficiency. Six types of cathode active material were prepared, which have Ni80/Col0/Mnl0 and Ni90/Co5/Mn5 with 3 different amounts of Al dopant such as 0.5mol%, lmol%, 2mol% in the conventional manner of manufacturing process, as shown in Table 1 below. The slurry was prepared with the following recipe: CAM:conductive: binder = 96:2:2 (solvent : NMP); Conductive : Carbon black + CNT (50:50); Binder : PVdF.

Table 1 below shows the results for the different CAMs in view of the different compositions. As seen in Table 1, if the Al content in the CAM was increased, the 1st cycle efficiency (specific discharge capacity) was reduced, but the life cycle property at high temperatures is improved. This is illustrated in the graphs of Figures 1 and 2, where Figure 1 illustrates the 1st cycle efficiency for different Al doping contents, and Figure 2 illustrates the cycle life at 45 °C for different Al doping contents.

Table 1, and Figures 1 and 2 illustrate the coin half-cell results. This test was conducted under the following conditions:

The coin cell type was 2032 type, using Li metal for the anode side.

1st cycle : charge - 0.1C up to 4.3V (Constant current), cut-off condition C/200 (Constant voltage)

1st cycle : discharge - 0.1C down to 3.0V

Life cycle : charge - 0.5C up to 4.3V (Constant current), cut-off condition C/200 (Constant voltage)

Life cycle : discharge - 1.0C down to 3.0V

Life cycle : checking capacity retention after lOOcycles

Table 1 Example 2, Comparative experiment of Anode Active Material (AAM) preparation

Different anode active materials (AAMs) were prepared and tested for efficiency. Six types of anode active material were prepared, which have different amounts of SiOx such as 0%, 1%, 3%, 5%, 10%, 20%. The slurry eas prepared by the following recipe: AAM:CMC:SBR = 97:1.5:2.5 (solvent : water); Binder : CMC, SBR.

The coin half-cell results are shown in Table 2 below. It may be seen that if we increase the SiOx content in AAM, we will lose the 1st cycle efficiency (specific discharge capacity).

This test above was conducted under the following conditions:

The coin cell type is 2032 type, using Li metal for the cathode side.

1st cycle : charge - 0.1C down to 0.005V (Constant current), cut-off condition C/200 (Constant voltage)

1st cycle : discharge - 0.1C up to 1.5V

Table 2

The 1st cycle result of Gr and Si by the coin half-cell method is show in Table 3 below.

Table 3 Example 3, Real capacity test for CAM and AAM combinations

In this example, a single side pouch cell with CAM 4, 5, 6 vs AAM 1~6 was prepared. N/P ratio was 1.05, and L/L of Cathode is 15mg/cm2. The outline of the experiment is shown in the table of Figure 3, in which also the real capacity for each DOE can be found. It is shown that even though the Al doping amounts were increased, the same cell capacity with Gr. System may be attained, which means that it will not lose the capacity. Thus, this method may prevent the cell capacity from being reduced due to the initial efficiency reduction of cathodes, which have high Al doping amounts and implement the same cell capacity.