Method for increasing the cycle life of a lithium-ion battery system

Abstract

A method for increasing the cycle life of a cell-encapsulated type lithium-ion battery system used in a product (8), the method comprising: Providing the cell-jacketed type lithium-ion battery system having at least one negative electrode (30) and at least one positive electrode (20), wherein the at least one negative electrode (30) comprises an alloy, the alloy consisting of about 9 atomic percent Bi, about 64 atomic percent Sn, and about 27 atomic percent In and having a melting point below 150 degrees Celsius and above a normal operating temperature of the cell-jacketed type lithium-ion battery system; periodically heating the at least one negative electrode (30) to a temperature near the melting point for a period of time sufficient to substantially remove any cracks in the at least one negative electrode (30); and Cooling the at least one negative electrode (30) to a temperature below the melting point, wherein the periodic heating of the at least one negative electrode (30) comprises: coupling an electrical bus (80, 80') to the at least one negative electrode (30); and Introducing an electrical current from the electrical bus (80, 80') through the at least one negative electrode (30); Maintaining the electric current for a period of time sufficient to substantially regenerate any cracks in the negative electrode (30); and Decoupling the electrical bus (80, 80') from the at least one negative electrode (30).

Description

[0001] The field to which the disclosure belongs relates to a method for increasing the cycle life of a lithium-ion battery system.

[0002] Lithium-ion batteries are a type of rechargeable battery in which a lithium ion moves between a negative electrode and a positive electrode. Lithium-ion batteries are commonly used in consumer electronics. In addition to their use in consumer electronics, lithium-ion batteries are experiencing growing popularity in defense, automotive, and aerospace applications due to their high energy density.

[0003] The process of lithium ion introduction and release can result in large-scale expansion and contraction in some high-capacity negative electrodes. This expansion and contraction can approach 300 percent, which can make the positive and negative electrodes vulnerable to cracking as the battery cycles between charge and discharge.

[0004] KR 10 2006 0 098 137 A discloses a lithium secondary battery using gallium alloy electrodes. The gallium alloy is applied directly to a current collector without a bonding layer. Cracking occurring during the application of the gallium alloy, which is caused by the reaction of gallium with lithium ions, can be eliminated by performing a temperature control step following the application, which melts the gallium alloy and eliminates the cracks.

[0005] The object of the invention is to provide a method by which it is possible in a simple and reliable manner to reduce cracking of negative electrodes in lithium battery cells.

[0006] The object is solved by the subject matter of claim 1. Advantageous developments of the invention are described in the subclaims.

[0007] An exemplary embodiment includes a product having a positive electrode and a negative electrode made of a low-melting-point alloy with a melting point below about 150 degrees Celsius that reacts with lithium. By periodically heating the negative electrodes near their melting points, any cracks associated with lithium ion introduction and release during normal use of the product can be repaired.

[0008] Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, in which: Fig. 1 is a schematic diagram of a cell-encapsulated type lithium-ion battery with a negative electrode according to an exemplary embodiment; Fig. Figure 2A is a binary phase diagram of a Li-Sn system; Fig. Figure 2B is a binary phase diagram of a Li-In system; Fig. 2C is a binary phase diagram of a Li-Bi system; Fig. Figure 3A is a binary phase diagram of a Bi-Sn system; Fig. 3B is a binary phase diagram of an In-Sn system; and Fig. 4 is a pseudo-binary phase diagram of a Sn-In-Bi system.

[0009] An exemplary embodiment includes a negative electrode that may be formed from a material that reacts with a relatively large amount of lithium. Furthermore, the composition of the material that reacts with a relatively large amount of lithium is such that it can be subsequently repaired by heating the negative electrodes to a temperature near the respective melting points of the materials, whereby the cracks can be substantially "healed" or otherwise repaired. This may result in a longer-lasting negative electrode.

[0010] Regarding the need for heating to repair the negative electrodes, it is also desirable that the materials that react with lithium forming the negative electrode have a relatively low melting point to prevent deterioration of the battery or electrode assembly in which the electrode may be used due to the aforementioned heating. In an exemplary embodiment, the melting point should be above the operating temperature for the battery system in which it is used, but should be below about 80 degrees Celsius unless the cell uses a non-conventional electrolyte (i.e., one that does not consist of organic acid ester solvent components that thermally decompose at the negative electrode at temperatures above about 80 degrees Celsius).

[0011] An exemplary embodiment for the use of a negative electrode in a product 8 is shown in Fig. 1, which shows a plan view of a lithium-ion battery 10 that may be used in automotive applications and includes an electrode assembly 12 and a cell casing 14 that may be molded with an interior region 16 for receiving the electrode assembly 12. In other words, Fig. 1 shows a prismatic lithium-ion battery 10 with cell casing, which includes the aforementioned electrode assembly 12. The components of the electrode assembly 12 and the cell casing 14 are illustrative of the basic components and are not intended to be shown in the correct orientation or to the correct scale.

[0012] The electrode assembly 12 may include a first electrode layer 20, a second electrode layer 30, and a separator 40 (or a solid electrolyte layer (not shown)) disposed between the first and second electrode layers 20 and 30 to prevent short circuits between the first and second electrode layers 20 and 30 and to allow only lithium ions to pass therethrough. The electrode assembly 12 may be formed by winding the first electrode layer 20, the separator layer 40 (or solid electrolyte layer), and the second electrode layer 30 into a jelly-roll-type structure.Alternatively, the first electrode layer 20, the separator 40 (or the solid electrolyte layer), and the second electrode layer 30 may be sequentially positioned in a stacked structure (not shown), or the first electrode layer 20, the separator 40 (or the solid electrolyte layer), and the second electrode layer 30 may be wound into a roll (not shown). Moreover, as shown in FIG. Fig. 1, the first electrode layer 20 is a positive electrode 20, while the second electrode layer 30 is a negative electrode 30, although the reverse arrangement is conceivable. A liquid electrolyte 45 may be introduced into the interior region 16 of the cell enclosure 14 before the cell enclosure 14 is sealed.

[0013] A positive tab 50 and a negative tab 52, which may be electrically connected to the respective electrodes 20, 30 of the electrode assembly 10, may be attached such that a predetermined length thereof is exposed outside the housing cell casing 14. Portions of the electrode tabs 50, 52 that come into contact with the housing cell casing 14 may be wrapped with an insulating tape (not shown).

[0014] The positive electrode 20 may be formed by coating a strip-shaped metal layer, such as a positive collector, with a coating containing a positive active material present at the positive collector, such as, but not limited to, LiFePO 4 or LiMnO 2, or another positive electrode active material as a main component. The coating may also include a binder and a conductive material. The positive electrode 20 may be electrically connected to the positive tab 50 and wrapped with insulating tape (not shown).

[0015] In one embodiment, the separator 40 may be made of a polyethylene film, a polypropylene film, or a combination thereof. The separator 40 may be formed to be wider than the positive and negative layers 20 and 30 to prevent short circuiting between the positive and negative layers 20 and 30.

[0016] In one embodiment, the liquid electrolyte 45 may comprise solid lithium salt electrolytes such as, but not limited to, LiPF 6 , LiBF 4 or LiClO 4and organic solvents, such as carbonates. The liquid electrolyte 45 conducts lithium ions, which act as a carrier between the negative electrode 30 and the positive electrode 20 when the battery 10 conducts an electrical current through an external circuit.

[0017] The cell casing 14 can be formed from a wide variety of materials that are both flexible and heat-sealable, preventing oxygen or water vapor from entering. The cell casing 14 can be a laminate material composed of aluminum and plastic.

[0018] Both the active material in the positive electrode 20 and the active material in the negative electrode 30 are materials into and out of which lithium can migrate. When a cell discharges, the lithium leaves the negative electrode 30 and reacts with the positive electrode 20. When the cell charges, the reverse process occurs: lithium is released from the positive electrode 20 and forced to react with the negative electrode 30.

[0019] The negative electrode 30 may, according to an exemplary embodiment, be formed by coating a strip-shaped metal layer, such as a negative collector, with a negative active material in the form of particles or thin films of alloys of tin, bismuth, and indium, as further described below. The negative electrode 30 may be electrically connected to a negative tab 52 and wrapped with insulating tape (not shown). In an exemplary embodiment, the strip-shaped metal layer is formed from copper and could alternatively be formed from nickel.

[0020] The lithium introduction and removal process results in large volume expansion and contraction of the negative electrode, which is made of metals or metal alloys that react with lithium 30. This expansion and contraction of metals and metal alloys during Li introduction and removal can be on the order of several hundred percent, which can make the negative electrode materials 30 susceptible to cracking as the battery cycles between charge and discharge.

[0021] To substantially repair these cracks, according to the exemplary embodiments herein, heat may be applied to the negative electrode 30 at a temperature near the melting point of the alloy. At this temperature, the cracks previously formed by volume expansion and contraction may be substantially "regenerated" or "repaired" by softening the negative electrodes.

[0022] To achieve this, according to an exemplary embodiment, as also in Fig. 1, heat from the closely coupled engine block 70 can be used in products 8, such as the lithium-ion battery 10, in which the negative electrode 30 is formed from an alloy with a sufficiently low melting point. Although an engine block 70 in Fig. 1, the battery 10 having the self-regenerating negative electrodes 30 can be used in hybrid vehicles or fuel cell type electric vehicles.

[0023] According to another exemplary embodiment (also in Fig. 1), a pair of spaced-apart electrical buses 80 are provided with resistive heating wires 100 therebetween. In one exemplary embodiment, the buses 80, 80' may extend through a hole 82 in the cell casing 14. In these exemplary embodiments, the wires 100 may or may not be in direct contact with the negative electrode 30. Current flowing through the wires 100 may heat the negative electrode 30 to a temperature near the melting point of the alloy to remelt the alloy and repair the cracks formed therein. The current is then interrupted, allowing the negative electrode 30 to cool below its melting point, with the negative electrode 30 once again available for use.

[0024] Elements that have been found to react with a relatively large amount of lithium and are potentially available for use in the negative electrode 30 include tin (Sn), bismuth (Bi), and indium (In). However, each of these elements has relatively high melting points above 150 degrees Celsius. By properly alloying elemental tin, bismuth, and indium, the melting point of the alloy can be lowered to below 150 degrees Celsius and possibly below 100 degrees Celsius. This, in turn, allows the cracks formed in the negative electrode 30 during normal use cycles to be regenerated by raising the temperature of the negative electrode 30 to a temperature near its melting point of approximately 100 degrees Celsius while remaining below 80 degrees Celsius.

[0025] In a specific exemplary embodiment, the negative electrode 30 may be formed from low-melting-point lithium-reactive alloys of M-Sn, where M is a metal selected from Bi and In, or both Bi and In, alloyed with Sn. A low-melting-point lithium-reactive alloy, for purposes herein, may be a material that is solid at battery operating temperatures but has a melting point below a predetermined maximum temperature, here a maximum of about 150 degrees Celsius, or more preferably below 100 degrees Celsius or less.

[0026] The Fig. 2A, Fig. 2B and Fig. 2C show binary phase diagrams for a Li-Sn alloy system, a Li-In alloy system, and a Li-Bi alloy system. Fig. 3A and Fig. 3B shows binary phase diagrams for a Bi-Sn alloy system and an In-Sn alloy system. Finally, Fig. 4 a pseudo-binary phase diagram for a Bi-Sn-In alloy system.

[0027] As the Fig. 2A, Fig. 2B and Fig. 2C, none of the proposed alloys of Li-Sn, Li-In, or Li-Bi achieve melting or eutectic points below 150 degrees Celsius. A eutectic or eutectic mixture, for purposes herein, is defined as a mixture of two or more metals in such percentages that the melting point is as low as possible and, further, that all the constituents crystallize simultaneously from molten liquid solution at that temperature. Such simultaneous crystallization of a eutectic mixture is known as a eutectic reaction, the temperature at which this occurs being the eutectic temperature, and the composition and temperature at which this occurs being the eutectic point.

[0028] How Fig. However, as shown in Figure 3A, a eutectic point at 139 degrees Celsius can be achieved for an alloy comprising roughly 57% Bi and 43% Sn (in atomic percent). Moreover, as shown in Fig. 3B, a eutectic point can be reached at 120 degrees Celsius for an alloy comprising roughly 49% In and 51% Sn (in atomic percent). In addition, as Fig. 4 shows that a eutectic point of 59 degrees Celsius can be achieved for an alloy containing roughly 9.5% Bi in an alloy consisting of Sn, In, and Bi. Additionally, each of these diagrams confirms melting points of various alloys below approximately 150 degrees Celsius. Fig. 3A, Fig. 3B and Fig. 4 thus show that various alloys of a Sn-In-Bi system may be available for use as a negative electrode 30 in products 8, such as lithium ion battery systems 10 described herein, where it is desired that a negative electrode 30 has the ability to react with a relatively large amount of lithium and has a relatively low melting point well below 150 degrees Celsius so that the negative electrode can be "regenerated" or repaired as described above.

[0029] A specific exemplary composition of a negative electrode 30 used by Fig. 3A is an alloy composition of about 42 atomic percent Bi and 58 atomic percent Sn. In addition, a specific exemplary composition of a negative electrode 30 derived from Fig. 3B, an alloy composition of about 50 atomic percent In and 50 atomic percent Sn. Also shown is a specific exemplary composition of a negative electrode, which is Fig. 4, an alloy composition of about 9 atomic percent Bi, about 64 atomic percent Sn and about 27 atomic percent In.

[0030] While a specific exemplary embodiment discloses the use of the negative electrode 30 in a cell-encapsulated type lithium-ion battery system 10, such a product 8 should be considered non-limiting. The negative electrodes formed from low-melting point alloys that react with lithium may find application in any other type of conventional lithium-ion battery system.

Claims

[1] A method for increasing the cycle life of a cell-encapsulated type lithium-ion battery system used in a product (8), the method comprising: Providing the cell-jacketed type lithium-ion battery system having at least one negative electrode (30) and at least one positive electrode (20), wherein the at least one negative electrode (30) comprises an alloy, the alloy consisting of about 9 atomic percent Bi, about 64 atomic percent Sn, and about 27 atomic percent In and having a melting point below 150 degrees Celsius and above a normal operating temperature of the cell-jacketed type lithium-ion battery system; periodically heating the at least one negative electrode (30) to a temperature near the melting point for a period of time sufficient to substantially remove any cracks in the at least one negative electrode (30); and Cooling the at least one negative electrode (30) to a temperature below the melting point, wherein the periodic heating of the at least one negative electrode (30) comprises: coupling an electrical bus (80, 80') to the at least one negative electrode (30); and Introducing an electrical current from the electrical bus (80, 80') through the at least one negative electrode (30); Maintaining the electric current for a period of time sufficient to substantially regenerate any cracks in the negative electrode (30); and Decoupling the electrical bus (80, 80') from the at least one negative electrode (30). [2] The method of claim 1, wherein periodically heating the at least one negative electrode (30) comprises: Heating the at least one negative electrode (30) outside the battery system from the normal operating temperature to a temperature near the melting point for a period of time sufficient to melt the at least one negative electrode (30) and thereby substantially repair the cracks in the negative electrode (30). [3] The method of claim 1, further comprising: Determining one or more alloys of at least two metals, at least one of which has the ability to react with lithium and has a melting point below 150 degrees Celsius; Determining which of the one or more alloys has a desired mixture of lithium ion absorbing capabilities and low melting point at a particular size and shape for use as a negative electrode (30) in the product; forming a negative electrode (30) having the determined alloy content; and Inserting at least one of the negative electrodes (30) into an inner portion of the battery system. [4] The method of claim 3, wherein the at least two lithium absorbing metals comprise Sn alloyed with at least one of Bi or In.

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