APPLICATIONS

Inventors: Gao Liu, Lei Wang and Sang-Jae Park

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This PCT Application claims priority to US Provisional Application Serial No.

61/543,629 filed October 5, 2011, which application is incorporated herein by reference as if fully set forth in its entirety.

STATEMENT OF GOVERNMENTAL SUPPORT

[0002] The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 between the U.S.

Department of Energy and the Regents of the University of California for the management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

Field of the Invention

[0003] Polyvinylidene difluoride (PVDF) has been the widely used binder materials for both cathode and anode electrodes. Water soluble styrene-butadiene rubber (SBR) binder has also been tested for LiFeP0 ₄ , LiCo0 ₂ and Graphite electrodes. Other binders have also been tested. The only other major commercial binder used in lithium ion battery electrodes, other than PVDF, are SBR type binders for graphite anode applications. However, the reported performance based on non-PVDF binders is lower than the performance for PVDF binder electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings. [0005] Figure 1 illustrates swelling test of Polyvinylidene difluoride (PVDF) and styrene- butadiene rubber (SBR) in the 1 M LiPF ₆ in EC/DEC (1 : 1 by weight) electrolyte according to an embodiment of the invention.

[0006] Figure 2 illustrates impedance of the swelled PVDF and SBR films after exposing to the electrolyte according to an embodiment of the invention.

[0007] Figure 3 illustrates the rate performance of electrodes made with PVDF binder and SBR binder according to an embodiment of the invention.

[0008] Figure 4 illustrates the cycling performance of electrode made with PVDF binder and SBR binder according to an embodiment of the invention.

[0009] Figure 5 illustrates SEM images of the SBR binder electrodes before and after cycling tests according to an embodiment of the invention.

[0010] Figure 6 illustrates a graphite particle coated with PVDF polymer layer by a solution based process according to an embodiment of the invention.

[0011] Figure 7 illustrates a schematic of an electrode cross-section made with PVDF coated graphite particles and SBR binder according to an embodiment of the invention.

[0012] Figure 8 illustrates rate performance of graphite electrode with different binders. PVDF coating is 2% of the weight of the graphite of the PVDF costing/SBR series (diamond) according to an embodiment of the invention.

[0013] Figure 9 illustrates cycling performance of PVDF coating graphite with SBR binder according to an embodiment of the invention.

[0014] Figure 10 illustrates polyethyleneoxide (PEO) derivatives used for active material particle coating according to an embodiment of the invention.

[0015] Figure 11 illustrates a graphite particle with coated cross-linked PEO polymer layer by the cross-linking process in Figure 10 according to an embodiment of the invention.

[0016] Figure 12 illustrates a schematic of an electrode cross-section made with PEO coated graphite particles and SBR binder according to an embodiment of the invention. DETAILED DESCRIPTION

[0017] In the discussions that follow, various process steps may or may not be described using certain types of manufacturing equipment, along with certain process parameters. It is to be appreciated that other types of equipment can be used, with different process parameters employed, and that some of the steps may be performed in other manufacturing equipment without departing from the scope of this invention. Furthermore, different process parameters or manufacturing equipment could be substituted for those described herein without departing from the scope of the invention.

[0018] These and other details and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.

[0019] Polyvinylidene difluoride (PVDF) has been the widely used binder material for both cathode and anode electrodes. However, the cost of PVDF binder is significantly higher than a styrene-butadiene rubber (SBR) polymer binder. PVDF and SBR binders bind to active material particles such as graphite in cathode and anode electrodes.

[0020] The PVDF binding force is much lower than that of the SBR binder to the lithium ion electrode materials and to current collectors. In addition, the swelling of PVDF in an electrolyte solution further compromises the PVDF binding force to the active materials. The SBR swells less than the PVDF in electrolyte solutions as illustrated in Figure 1 .

[0021] However, the swelling of the binder due to an electrolyte is an important factor for the lithium ion transport from the electrolyte to the active material particles. As such, the low swelling SBR has a high impedance while the swelled PVDF has a low impedance as evidenced in Figure 2.

[0022] Embodiments of the invention fully or partially replace PVDF with an SBR binder in the electrode laminates and at the same time retains and/or improves the performance characteristics of the SBR binder electrode in terms of rate, cycle, and storage life.

[0023] PVDF has been the widely used binder materials for both cathode and anode electrodes. A water soluble SBR binder has also been tested for LiFeP0 ₄ , LiCo0 ₂ and Graphite electrodes. Various other binders have also been tested. The only major commercial binder used in lithium ion battery electrodes other than PVDF is the SBR type of binder for graphite anode applications. However, the reported performance based on non-PVDF binders is lower than the performance for the PVDF binder electrodes. Thus, various embodiments of the invention describe the benefits of using a PVDF/SBR hybrid binder electrode system.

[0024] The commercially used SBR is a SBR aqueous suspension. SBR solid 50-100 nm round particles are suspended in a water solution. The SBR water suspension is made by an emulsion polymerization process. Surfactant molecules cover the surface of SBR particles. The surfactant produces the suspension properties of SBR nanometer (nm) sized balls in the water solution. The SBR nm particles provide the adhesion between the solid lithium-ion storage and conductive additive particles. However, it has been reported that increasing the amount of SBR binder degrades the performance of the graphite electrode based on the SBR binder.

[0025] An embodiment of the invention demonstrates that when SBR is dissolved in a toluene organic solvent, and the SBR/toluene solution is used to make graphite and acetylene black additive electrodes, the rate and cycling performance of the SBR based electrode is less than hoped for (see Figure 3, 4). Also, SBR delamination was detected after the cell was cycled (see Figure 5).

[0026] Based on literature reported data on the sensitivity of the amount of SBR used in a aqueous system, the less than hoped for results of SBR/toluene system, and the none swelling properties of the SBR binder as compared to PVDF binder, we concluded that the SBR binders block the lithium-ion movement from the electrolyte to the active material surfaces (in the case of graphite). We further propose the SBR blocking effect to lithium-ion movement will happen when SBR is used as a cathode binder. This Li-ion blocking effect is a major drawback for the use of SBR only as binder.

[0027] On the other hand, there are several major attainable benefits from using SBR low boiling point organic solvent (for example toluene, cyclohexane) solution to make slurry for lithium-ion electrode applications.

[0028] First, SBR is made in commodity scale at very low cost as compared to PVDF. Second, slurry made with low boiling point organic solvents can significantly decrease the energy used and shorten the time to dry the laminate. Third, lithium is stable in hydrocarbon solvents such as toluene, allowing for the inclusion of a stabilized lithium metal powder (SLMP) into the slurry mixing, coating and drying process. Thus, improvement is obtained because 1) this SLMP added electrode can reduce irreversible lithium loss during the formation; 2) allows for lithium ion battery formation without application of an external charge; 3) if a large amount of SLMP is added to the anode, it will prelithiate the anode to enable the use of none lithium containing low cost cathodes, such as CFx and Mn0 ₂ (electrolytic manganese dioxide) in rechargeable cells.

[0029] Various embodiments of this invention contemplate the use of SBR binder for both cathode and anode lithium ion electrodes. In one embodiment, graphite material is utilized as for the anode.

[0030] Lithium-ion battery electrode binders other than PVDF have not been widely accepted due to the reduced performance. Both rate and lifetime have been compromised compared to the electrodes made with PVDF based binder. However, the binding force of PVDF is not as strong as many of the other binders such as SBR. Various embodiments of the invention combine both the superior performance of the PVDF binder and the strong binding force of the SBR binder to form an electrode system that has increased or at least equivalent performance compared to the PVDF binder based electrode. In one embodiment, the improved SBR/PVDF based electrode will use far less PVDF binder relative to the PVDF only electrode to reduce the cost of the PVDF materials. Thus, various embodiments of the invention provide a new way to achieve comparable or improved results at a reduced cost.

[0031] Various embodiments use the combination of graphite and negative electrode laminate as an example to describe the approach. The approach can be applied to cathode materials and cathode laminates as well.

[0032] A thin layer of PVDF polymer is coated on the surface of graphite particles as shown in Figure 6. For example, PVDF is dissolved in N-methylpyrrolidone (NMP) solvent to form a PVDF/NMP solution. A certain amount of graphite particles is added to the solution and mixed thoroughly. The NMP will be evaporated to leave PVDF coated graphite particles. The PVDF coated graphite particle ranges from 10-0.0001% by weight to the graphite. The current result shows that from 0.75-3% of PVDF coating. A 1% PVDF coating by weight gives the best performance data.

[0033] Next, the PVDF coated graphite particles are mixed with SBR binder in either a water soluble format or SBR in organic solvent such as toluene to form a slurry. Acetylene black and/or carbon fiber can be part of a conductive additive to add into the slurry. If an organic solvent is used to form slurry, the organic solvent should not dissolve the PVDF coating. For example, a toluene solvent may be used. The slurry may then be utilized to coat an electrode laminate on a Cu current collector, and dried to form a negative electrode as shown in Figure 7. [0034] The SBR and PVDF do not mix, and PVDF does not dissolve in toluene. During the coating process, the PVDF remains on the surface of the graphite. The SBR is also phase separated from the PVDF in a micro-scale, when the electrode is made. Upon the addition of electrolyte to the electrode assembly, the PVDF will uptake electrolyte and swell. This swelled PVDF layer is in connection with the active material particle surface. The electrolyte swelled PVDF provides a channel for the lithium-ions to move between the electrolyte and active material surface. This invention will overcome the lithium-ion blocking effect exerted by a SBR coating on active material surface in SBR only systems.

[0035] The performance of PVDF coated graphite particles using SBR binder materials has shown much improved performance compare to the SBR binder alone system. Figure 8 shows the rate performance comparison among the three electrode compositions: 1) PVDF binder (orange square), 2) SBR binder (blue triangle) and 3) PVDF coating/SBR binder (diamond). The rate performance of 2% PVDF coated graphite performs as good as a PVDF binder based electrode, and far superior than the SBR binder without PVDF coating. The electrode made with PVDF coating graphite and SBR binder shows very good cycling performance as illustrated in Figure 9.

[0036] Possible surface modification agents for an anode include PVDF and related copolymer, polyether (such as polyethyleneoxide (PEO)) type of materials, polyvinylenecarbonate and its derivatives. Possible surface modification agents for cathode include PVDF,

polyvinylenecarbonate, and its derivatives.

[0037] Figure 10 shows the molecular structures of the PEO derivatives according to

embodiments of the invention. A thin layer of PEO type of polymer precursors (lc or 2c) is coated on the surface of graphite particles as shown in Figure 11. An example of the process is as follows. The lc polymer precursor (Figure 10) and a small amount of radical initiator (e.g. Azobisisobutyronitrile, (AIBN)) are dissolved in ether solvent to form a solution. A certain amount of graphite particles are added to the solution and mixed thoroughly. The ether will be evaporated to leave lc polymer precursor coated graphite particles. The lc polymer precursor is in a range from 10-0.0001% by weight of the graphite. The lc polymer precursor/graphite mixture is dispersed in a solvent, which does not dissolve thelc polymer precursor, (eg. Hexane). Heating or UV light will be used to initiate the crosslinking of the lc polymer precursor to form a permanent coating (Id) on the surface of the graphite, while the coated graphite particles are dispersed in the solvent.

[0038] This Id coated graphite particles will be mixed with SBR binder in aqueous based SBR dispersion or SBR in organic solvent such as toluene to form slurry. Acetylene black or/and carbon fiber can be part of the conductive additive to add into the slurry. The solvents used in this step should not dissolve the lc coating, as it is already crosslinked. This slurry is used to coat an electrode laminate on a Cu current collector, and dried to form the negative electrode as Figure 12.