Patent Application
20170194672
This disclosure relates to lithium ion batteries and methods for making the same, and in particular to methods for improving low temperature lithium ion battery performance and lithium ion batteries with improved rate capacity profiles.
Hybrid vehicles (HEV) and electric vehicles (EV) use chargeable-dischargeable power sources. Secondary batteries such as lithium-ion batteries are typical power sources for HEV and EV vehicles. Certain types of lithium-ion secondary batteries use conductive metal and metal-based alloy material as the anode electrode. Lithium ion batteries having metal or metal alloy anodes suffer rapid capacity fade, poor cycle life and poor durability and exhibit lower discharge retention rates at elevated C rates as well as irregular dependence of discharge capacity retention percentage as a function of C rate. One cause of the decrease in discharge capacity retention percentage is due to damage in the electrode microstructure caused by multiple battery cycling evidenced the development of delamination sites and large cracking networks propagating in the structure. This deteriorative phenomenon leads to electrode delamination, loss of porosity, electrical isolation of the active material, rapid capacity fade and ultimate cell failure
A method for preparing a lithium ion battery having at least one unit cell in which the unit cell has a cathode, a separator, an electrolyte and a metal based anode that includes a metal based alloy overlaying a metal current collector. The metal based alloy has alloying particles present therein. The method includes the step of applying a high C-rate discharging current to the unit cell, the high C-rate discharging current (CHD) sufficient to secure conductive pathways in at least one structure present in the unit cell, wherein the high C-rate discharging current (CHD) applied is greater than a high C-rate operating current (CO) passing through the metal-based anode during use and the step of charging the unit cell with the application of a high C-rate charging current after the application of the discharging current has been discontinued
These and other aspects of the present disclosure are disclosed in the following detailed description of the embodiments, the appended claims and the accompanying figures.
The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:
Lithium ion batteries using metal-based alloy anodes suffer from poor rate capability. This poor rate capability limits the applications for such batteries. Based on rate capability analysis, it has been found that batteries having metal-based alloy anodes exhibit a monotonous decrease in performance as C-rate increases. It is believed that this phenomenon is due, at least in part to the compromise in electronic conductive pathways defined through the anode structure to the current collector which become compromised due to one or more of cracking and delamination of the active material, solid electrolyte interface (SEI) layer overgrowth due to side reactions, electrolyte decomposition. Each of these, in turn, can lead to increases in resistance. Impairment of the electronic conductive pathway can cause lithium ion diffusion to become blocked or impaired.
To address the poor energy density of carbon based electrodes, alternative active materials with higher energy densities are desired. Alloying particles such as silicon, tin, germanium and their oxides and alloys are non-limiting examples of materials that may be added to an electrode active material layer to improve its energy density, among other benefits. In certain applications, electrodes can be constructed that have regions of carbon-based material such as graphite as well as regions containing alloying particles.
Electrode materials such as silicon, germanium, or tin react with lithium via a mechanism different from that of graphite. Lithium forms alloys with electrode material such as silicon in a process that involves breaking the bonds between host atoms, causing dramatic structural changes in the process. Since alloying materials such as silicon, germanium, or tin do not constrain the reaction, anode materials that form alloys can have much higher specific capacity than intercalation electrode materials such as graphite. Anode-active materials such as silicon, germanium, tin and the like suffer from rapid capacity fade, poor cycle life and poor durability. One primary cause of this rapid capacity fade is the massive volume expansion of these materials (typically up to 300%) and structural changes due to lithium insertion. Repeated volume expansion of materials such as silicon can cause particle cracking and pulverization when the silicon has no room to expand, which leads to delamination of the active material layer from the current collector, electrical isolation of the fractured or pulverized active material, capacity fade due to collapsed conductive pathways, and increased internal resistance over time.
The present disclosure is predicated on the unexpected discovery that assembly and treatment processes such as those disclosed can produce metal alloy anodes and associated lithium ion batteries that exhibit a stable, flat dependence of discharge capacity retention as a function of C-rate.
As depicted in
The metal-based anode employed can be one composed of a metal alloy, more particularly a copper based metal alloy. Non-limiting examples of materials that alloy with copper to produce the metal based anode in the unit cell include at least one of tin, molybdenum, niobium, tungsten, tantalum, iron. Such alloy materials can be present in the alloy in suitable ratios based on cell requirements. The metal alloy can also include alloying particles. The alloying particles can be silicon-based, germanium-based or tin-based, for example. The silicon-based particles can be silicon, a silicon alloy, a silicon/germanium composite, silicon oxide and combinations thereof. The tin-based particles can be tin, tin oxide, a tin alloy and combinations thereof. Other high energy density materials known to those skilled in the art are also contemplated. As discussed above, this high capacity for lithium ions results in large volume expansions of the alloying particles.
Where desired, the metal-based anode can have an active material coating such as graphite and the like on the metal alloy structure. In the anode as disclosed, a metal alloy based material can be in overlying relationship to a current collector and can function as an active material in the unit cell. It is also contemplated that an electroactive material such as graphite can be deposited on the alloy material. The current collector composition and thickness can vary based on cell requirements. In certain embodiments, the current collector can be a metal foil material such as copper. The active material can be any suitable lithium based composition.
The various structures present in the unit cell can have such thicknesses and configurations as dictated by battery conditions and performance requirements. The electrolyte composition and additive as well as the porosity of the active material can vary based on unit cell requirements.
The lithium ion battery preparation method as disclosed will include the step of applying a C-rate discharging current to the prepared unit cell as at reference numeral 30. The value of the C-rate discharging current that is employed is one that activates the alloying particles at an activation speed (AS) such that the activation speed during discharge is greater than the rate at which the alloying particles contract upon delithiation during discharge, referred to as the contraction rate (Rc). The application of the specified C-rate discharging current results in an anode exhibiting decreases in fracture or pulverized active material, decreases in capacity fade due to collapsed conductive pathways, and reduction in increased internal resistance over time. Without being bound to any theory, it is believed that application of C-rate discharging current at defined value (CD) will result in delithiation while maintaining the alloying particles in an expanded volume.
In certain embodiments such as the embodiment of the method as depicted in
The C-rate discharging current (CD) such as the high C-rate discharging current (CHD) can be applied for a time interval suitable to reduce the state of charge (SOC) in the associated unit cell to a target depleted level and/or to secure electronic conductive pathways in a least one structure present in the anode such as the alloy material overlying the current collector. In certain embodiments, it is believed that conductive pathways can be secured by application of C-rate discharging current (CD) and/or high C-rate discharging current (CHD) for an interval sufficient to reduce SOC to a level of 10% maximum charge. In certain instances, SOC will be reduced to a level less than 5% of maximum charge. In many instances, the SOC will be reduced to 0% of maximum charge. It is understood that the total discharge interval can vary based on factors such as the particular capacity of the associated cell and/or the C-rate value employed.
In the method 10′ depicted in the flow chart in
In the method 10′ as depicted in
It has been found that one cycle of C-rate discharge current application at the defined rates followed by charging to 100% SOC administered prior to operation of the associated battery provides a lithium ion battery that demonstrates improved discharge capacity retention during cycling over that which occurs in routine battery operation of similarly structured battery units.
The present disclosure also contemplates methods for preparing a lithium ion battery that includes at least two discharge/charge iterations. One non-limiting example of such a method is outlined in
Once the unit cell reaches the lowered state of charge value and high C-rate discharging current (CD1) is discontinued, the application of charging current (CC) can be applied to the unit cell as at reference numeral 50″. This can occur at rate values between C/20 and 1 C and proceeds for an interval sufficient to provide the unit cell with an elevated state of charge. In certain embodiments, it is contemplated that rate values between 3 C and 6 C can be employed during the charging step. The elevated state of charge may be any value that is above the previously lowered stated of charge value previously achieved. In certain embodiments, the elevated state of charge achieved in this process step will be at or above 90% of maximum SOC for the unit cell; while in other embodiments, the elevated state of charge will be a value at or near 100% maximum SOC for the unit cell. The interval for application of the charging current (CC) is dependent on factors such as the C-rate value of the charging current (CC) applied, the capacity and/or configuration of the specific unit cell, or both.
Once the unit cell reaches the elevated state of charge, the application of charging (CC) can be discontinued and a high C-rate discharging current (CD2) applied as at reference numeral 60″ of
The decremented high C-rate discharging current (CD2) will be applied until the SOC in the unit cell is reduced to a state of charge at or less than the lowered state of charge previously achieved. Once the second lowered state of charge has been reached, application of the decremented high C-rate discharging current (CD2) is discontinued as depicted in
Once the second lowered state of charge is reached, the unit cell can be charged to an elevated state of charge as at reference numeral 80″. This can occur by application of a charging current (CC) having a C-rate value. In certain applications, the C-rate value can be between C/20 and 1 C; in other applications it is contemplated that the C-rate of the charging current can be between 3 C and 7 C. The elevated state of charge achieved can be at or above 90% of the maximum SOC; while in other embodiments, the elevated state of charge will be a value at or near 100% of the maximum SOC. As with prior charging cycles, the interval for application of the charging current (CC) can vary based on factors such as the C-rate value of the charging current (CC), the capacity and/or configuration of the specific unit cell, or both. The charging current (CC) applied in this subsequent charging step can have the same value as that applied previously or can differ from that charging current initially applied.
The charging and discharging steps can be repeated though multiple iterations sequentially reducing the high C-rate discharging current with each iteration until the high C-rate discharging current applied has a defined lower value. The defined lower value is greater than 0.1 C. This is depicted at reference numeral 90″ in
The sequential reductions in discharging current can be in any suitable decreasing sequence; non-limiting examples include equal value intervals, logarithmic intervals, inverse logarithmic intervals and the like. One example of a decreasing discharge sequence would be discharge proceeding for one cycle each at 7 C, 5 C, 3 C, 1 C, 0.1 C.
The method disclosed can also include multiple charging step iterations in the values previously noted. The multiple charging step iterations can be incremented as desired or required. In certain embodiments, the charging step iterations can progress from lowest to highest in increments similar to the increments employed in the high C-rate discharge steps.
The lithium ion battery that may be produced by the method as disclosed may be one that includes a unit cell having a metal-based anode having a current collector, and an active material structure formed of a suitable metal alloy and a electroactive material coating the surface of the metal alloy structure. The resulting anode is characterized by at least one area defined as a spongy region having a conductive network of a suitable metal with alloying particles present within the network defined charge conduit extending through the active material layer to the current collector.
A schematic cross-sectional depiction of a representative anode 102 and associated structure 100 present in a lithium ion battery prepared according to methods known in the prior art after five plus operative cycles is depicted in
As depicted in
In contrast, a cross-sectional representation of anode 202 and associated structure 200 produced according the preparation method as disclosed herein after five-plus operative cycles is depicted in
The lithium ion battery that incorporates anode 202 exhibits stable discharge capacity retention over five plus cycles.
To further illustrate the invention as disclosed herein, attention is directed to
A representation of anode 102 after initial charging is depicted in
A representative illustration of the anode 102 after initial slow discharge is depicted in
In the method disclosed herein, discharge occurs at an activation speed (AS) that is greater than the contraction rate (RC) of the alloying particles such that the volume of the alloying particles subsequent to discharge is at least greater than the volume of the alloying particles in the anode as manufactured. In certain embodiments it is believed that the volume of the alloying particles subsequent to discharge is essentially equal to the volume of the alloying particles upon full lithiation upon charging.
Without being bound to any theory, it is believed that rapid delithiation from of the lithiated alloying particles maintains the apparent particle size of alloying particles resulting in good electronic conductivity between alloying particles 152 to (copper) 118 and less volume loss of entire active materials layers 104 and 110 including 152.
A lithium ion battery is prepared applying a standard formation cycle to unit cells. A rate capability check is performed in sequential order from 0.1 C to 5 C rate on the resulting unit cell. The rate capabilities are illustrated in
A lithium ion battery is prepared applying the standard formation cycle to unit cells as outlined in Comparative Example I. A high C-rate discharging current is having a value of 7 C is applied to the unit cell for an interval sufficient to produce a state of charge in the unit cell of 0%. The unit cell is charged back to 100% state of charge by application of a charging C-rate of 1 C for and interval sufficient to achieve 100% SOC. A rate capability check is performed on the resulting cell in sequential order from 5 C to 0.1 C a intervals of 5 C, 3 C, 2 C, 1 C, 0.5 C and 0.1C. The rate capabilities are illustrated in
A lithium ion battery is prepared applying the standard formation cycle to unit cells as outlined in Comparative Example I. A high C-rate discharging current is having a value of 3 C is applied to the unit cell for an interval sufficient to produce a state of charge in the unit cell of 0%. The unit cell is charged back to 100% state of charge by application of a charging C-rate of 20/C for and interval sufficient to achieve 100% SOC. A rate capability check performed on the resulting unit cell in sequential order from 5 C to 0.1 C rate indicates that unit cell performance is similar to that demonstrated in Example I.
A lithium ion battery is prepared applying the standard formation cycle to unit cells as outlined in Comparative Example I. An initial high C-rate discharging current having a value of 7 C is applied to the unit cell for an interval sufficient to produce a state of charge in the unit cell of 0%. The unit cell is charged back to 100% state of charge by application of a charging C-rate of 1 C for and interval sufficient to achieve 100% SOC. The unit cell is then subjected to application of a discharging current having a value of 5 C followed by application of a charging C-rate of 1 C. Discharging and charging steps are repeated over several cycles using discharge rates of 3 C; 1 C and 0.1 C respectively. A rate capability check performed on the unit cell in sequential order from 5 C to 0.1 C respectively indicates that unit cell performance is similar to that demonstrated in Example I.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.
