PERFORMANCE LITHIUM ION BATTERIES

Technical Field of Invention The present invention relates to a gel polymer binder used for high performance lithium ion batteries.

More specifically, the present invention relates to a gel polymer binder containing polyvinyl alcohol or polyhydroxyethyl methacrylate units and a polyfluorene- phenylene copolymer, which is conjugated with carboxylic acid for cross linking and electrolyte reception.

Prior Art

Due to the use of silicon (Si) as the negative electrode material instead of the conventional graphite anode, significant volume changes (>400%) occur during lithiation, which is quite harmful for the cycle stability of Lithium Ion Batteries (LIBs). The mechanical stress caused by this repeated volume change breaks the anode composite and separates the components of LIBs from each other; this, in turn, causes a weak electrical contact between the Si particles and leads to a significant degradation of the electrode during the cycle. Even worse, this negative effect becomes more apparent when the electrode is generated for practical, high energy cells which require the active agents to be highly loaded on the current collector.

Generally, a Si-based composite anode is generated by using a compound comprising Si particles (active agent), carbon (conductive agent) and a polymeric binder; and most researches are directed towards developing and modifying various pure forms; silicon nanotubes, nanoporous silicon and Si thin thrombocytes are used for preventing the recycling capacity of Si-based electrode materials. Even though silicone materials have improved electrochemical performance, the wide surface areas of Si structures arising from their small sizes usually increase the surface failure percentages, therefore several problems need to be solved in order to commercialize these for LIB’s. Such failures cause weak cycles and consequently, supportive inactive compounds such as binders and conductive agents, which consume the capacity, in excessive amounts are required for establishing contacts among the particles. Furthermore, the synthesis of nano sized Si materials requires complex preparation procedures, thereby inevitably increasing production costs.

The chemical cross linking of alginate, acidic and hyper branched polymeric bonds is proposed as increasing the stability of LIBs through Si-based electrodes. On the other hand, even though the chemically cross linked polymers which are connected to each other in three dimensions exhibit high mechanical resistance against tension and unrecoverable deformation due to strong interaction among polymer chains, the nature of the chemical cross linking also increases the rigidity of the polymer. This causes a gradual and irreversible breaking of the grid, thereby causing the cell performance to be faulty during long term battery operation.

The polymers used as binders in electrodes can suppress the volumetric expansions and the resulting internal stresses created by the reaction of the commercially used graphite with lithium, and can maintain the structure. However, the binding polymers (CMC, PVDF, EPDM) used for graphite in metals, which are tried instead of graphite and which exhibit over 200% volumetric change as a result of their reaction with lithium, do not possess the flexibility and binding performance that can tolerate the volumetric changes exhibited due to the metals (Al, Si, Sn, Sb) being able to hold more lithium. Therefore, the use of polymers in metals exhibiting such high volumetric changes, which are to be used as active agents in the electrodes of lithium ion batteries, poses great significance. Another problem experienced with the electrode active materials used in lithium ion batteries is that they are not sufficiently conductive and, accordingly, that the electrode structure deforms at high charge -discharge speeds. For this reason, additives which increase conductivity are added to the electrode active agents. These additives can be carbon-based materials (such as carbon black, graphene, super-P conductive) or conductive polymers (PT, PANI). However, the addition of carbon-based materials in addition to the polymers used as binders for the purpose of increasing conductivity increases costs and the weight of the battery, and nowadays it became important to provide conductive properties to the polymers used as binders.

So, it is of great importance that the polymer or polymers to be used to are flexible, conductive and quite reasonable in terms of production costs.

In the known art, polymeric binders for lithium ion batteries have been developed.

In the US patent document numbered US2013288126 of the known art, a family of carboxylic acid groups containing fluorene copolymers is used as binders of silicon particles in the fabrication of negative electrodes for use with lithium ion batteries. In the document, the polyvinylalcohol ether backbone structure of the conductive polymer binder is also present.

In the US patent document numbered US2012119155 of the known state of the art, an improved polymeric binder for forming silicon electrodes is disclosed. This polymer includes a poly 9, 9-dioctylfluorene and 9-fluorenone copolymer.

In the International patent document numbered WO2016145341 of the known art, electrode materials are disclosed that include an electrode active material combined with a binder that is formed of cross linked polymeric material, optionally polyvinyl alcohol cross linked with lithium tetraborate. The developed material optionally includes silicon, silicon carbon composites, lithium alloys, lithium metal oxides, lithium metal phosphates, transition metal oxides, nitride materials and fluorine materials. In the US patent document numbered US2016164099 of the known art, an anode for use in a lithium-ion battery consists of a polymeric gel binder made of at least two polymers having carboxylic acid groups and silicon particles. The anode of said invention contains polyvinyl alcohol. The exemplary patent documents focus on the formation of cross linked gel by using polyvinyl alcohol groups against polyacrylic acid. In order to optimize its electronic and mechanical properties, first, fluorene (F) has been included in order to form the electronic structure of the polymer, which is a polyfluorene type of conductive polymer, and different groups have been used to be copolymerized in order to increase the adhesion strength between the polymer binders.

Yet, the gel polymer binders included in the disclosed documents are not flexible or conductive and they have high production costs. Therefore, it is necessary to develop the gel cross polymeric binder of the present invention.

Objectives and Brief Description of the Invention The objective of the present invention is to provide a gel polymeric binder with cross links, which is flexible, conductive and which incurs low production costs.

The objective of the present invention is to provide a new gel polymeric binder with cross links, which is induced by an interaction between the polyfluorene- phenylene carboxylic acid (PF-co-PPC) copolymer and polyvinyl alcohol (PVA) units.

Another objective of the present invention is to provide a gel polymeric binder which has a high charging capacity.

Yet another objective of the present invention is to provide a gel polymeric binder which has both ionic and electrical conductivity in a single polymer chain. The present invention provides a new polymeric binder with cross links, which is induced by an interaction between the polyfluorene-phenylene carboxylic acid (PF-co-PPC) copolymer and polyvinyl alcohol (PVA) units. PF-co-PPC has a conjugated aromatic structure which has carboxylic acid groups for the hydrogen bonds which can partake in the specific interactions with the hydroxyl group of PVA. PF-co-PPC/PVA gives Si composite electrodes with high mechanical resistance on Si and provides a battery performance which is distinctly improved when compared to conventional binders. The developed Si-based composite anode provides a perfect cyclic performance with a high charging capacity. Detailed Description of the Invention

In the present invention, conjugated (conductive) polymer which includes carboxylic acid groups is obtained through polymerization with suitable duration, temperature and catalysts, by using various derivatives of dioctylfluorene boronic ester and various derivatives of dibromobenzoic acid (Diagram 1). The polymers obtained herein are Ri and R ₂ , methylene groups and their C number can vary between 1 - 17. The m number (polymer repeating unit) of the obtained polymer can vary between 1 -1.000.000 (Formula 1).

Diagram 1:

As an alternative to the reaction illustrated in Diagram 1, the synthesis and application of polymethacrylic acid (PMAA) polyhydroxyethylmethacrylate (PHEMA) exhibit similarities to the polymer mentioned in Formula I (Formula II).

The conductive polymer with carboxylic acid content as shown in Formula I and Formula II is handled with a strong base (NaOH or KOH) and is converted into sodium/potassium carboxylate salt form (Diagram 2).

Diagram 2:

The polymer which is converted into sodium/potassium carboxylate salt form is cross linked with a polymer containing hydroxyl group, such as polyvinyl alcohol or polyhydroxyethylmethacrylate, through a thermal process at 25-250 ⁰ C, thereby obtaining a porous structure. The molecular weight of the polyvinyl alcohol or polyhydroxyethylmethacrylate (commercial polymers), which are used herein, should be between 1-150.000. The surface area of the porous structure has been measured via the BET porosimetry method and has been found to be 100- 800 m ² /g. Thanks to the created pores, the silicon used in the lithium ion battery has been prevented from degrading due to volumetric expansion. The obtained cross linked material is shown in formula IV.

In the present invention, electrodes are produced by using the conductive and flexible polymers from which the electrode active agents are synthesized. The synthesized polymer has conductive and flexible groups. The polymers which are commercially used in lithium batteries are not flexible or conductive. Thanks to the conductivity provided to the electrode via the conductive polymer of the present invention, the carbon based materials which are commercially used in electrodes to increase conductivity are not used herein. Moreover, the electrode structure maintains its integrity thanks to the flexibility provided by the developed polymer. Because, due to the operating mechanism of the battery, Li ⁺ ions move between anode and cathode electrodes, they settle inside these electrode structures and this causes volumetric swelling in the electrode structure. The structure, which is not flexible enough, breaks and disbands due to swelling. The flexible polymers obtained with the present invention are able to compensate such volumetric swellings. Thus the active material is able to maintain its structure during the swelling and shrinking observed when the battery is charged or discharged. This enables the electrode to endure long cycles. Furthermore, the capacity of the electrode is mostly utilized since the structure is prevented from being pulverized. This, in turn, extends the life and capacity of the battery.