Patent ID: 12237477

DETAILED DESCRIPTION

One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers'specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

The battery systems described herein may be used to provide power to various types of electric vehicles (xEVs) and other high voltage energy storage/expending applications (e.g., electrical grid power storage systems). Such battery systems may include one or more battery modules, each battery module having a number of battery cells (e.g., lithium-ion (Li-ion) electrochemical cells) arranged and electrically interconnected to provide particular voltages and/or currents useful to power, for example, one or more components of an xEV. As another example, battery modules in accordance with present embodiments may be incorporated with or provide power to stationary power systems (e.g., non-automotive systems).

Based on the advantages over traditional gas-powered vehicles, manufactures, which generally produce traditional gas-powered vehicles, may desire to utilize improved vehicle technologies (e.g., regenerative braking technology) within their vehicle lines. Often, these manufacturers may utilize one of their traditional vehicle platforms as a starting point. Accordingly, since traditional gas-powered vehicles are designed to utilize 12 volt battery systems, a 12 volt lithium ion battery may be used to supplement a 12 volt lead-acid battery. More specifically, the 12 volt lithium ion battery may be used to more efficiently capture electrical energy generated during regenerative braking and subsequently supply electrical energy to power the vehicle's electrical system.

As advancements occur with vehicle technologies, relatively high voltage electrical devices may also be included in the vehicle's electrical system. For example, the lithium ion battery may supply electrical energy to an electric motor in a mild-hybrid vehicle. Often, these high voltage electrical devices utilize voltage greater than 12 volts, for example, up to 48 volts. Accordingly, in some embodiments, the output voltage of a 12 volt lithium ion battery may be boosted using a DC-DC converter to supply power to the high voltage devices. Additionally or alternatively, a 48 volt lithium ion battery may be used to supplement a 12 volt lead-acid battery. More specifically, the 48 volt lithium ion battery may be used to more efficiently capture electrical energy generated during regenerative braking and subsequently supply electrical energy to power the high voltage devices.

Thus, the design choice regarding whether to utilize a 12 volt lithium ion battery or a 48 volt lithium ion battery may depend directly on the electrical devices included in a particular vehicle. Nevertheless, although the voltage characteristics may differ, the operational principles of a 12 volt lithium ion battery and a 48 volt lithium ion battery are generally similar More specifically, as described above, both may be used to capture electrical energy during regenerative braking and subsequently supply electrical energy to power electrical devices in the vehicle.

Accordingly, to simplify the following discussion, the present techniques will be described in relation to a battery system with a 12 volt lithium ion battery and a 12 volt lead-acid battery. However, one of ordinary skill in art is able to adapt the present techniques to other battery systems, such as a battery system with a 48 volt lithium ion battery and a 12 volt lead-acid battery.

As generally set forth above, the present disclosure relates to limiting lithium plating at anodes of lithium ion batteries. For example, the present disclosure provides, among other things, certain systems and approaches for monitoring and mitigating such lithium plating, based on experimental observations. When a lithium ion battery charges, it may be advantageous to limit certain charge parameters to lessen the likelihood of lithium plating at the anodes of the lithium ion batteries. To reduce the likelihood of lithium plating while still maintaining an efficient charge rate, limits to various charge parameters may be dynamically altered to correspond to measured parameters (e.g., charge current, temperature, or state of charge of the lithium ion battery) presently experienced by the lithium ion battery, which may affect the lithium ion battery's propensity toward experiencing lithium plating.

With the preceding in mind, the present disclosure describes systems and methods for controlling charging operations of a battery system to prevent the lithium ion batteries from experiencing lithium plating on anodes of the lithium ion batteries. Traditionally, to combat lithium plating, lithium ion battery manufacturers have provided a current limit for charging operations of lithium ion batteries. However, these current limits are often overly conservative for the specific circumstances surrounding a lithium ion battery, which may result in inefficient charging operations by unnecessarily limiting charge current levels. In addition, such charge current limits are often established based on constant current charging. While constant current charging is useful in certain applications such as in EVs and PHEVs, other applications such as hybrid vehicles (e.g., mild hybrids, micro hybrids) may, in some instances, use pulse charging. Indeed, it is now recognized that pulsed charging currents may have different lithium plating dynamics than continuous charging currents. It is also now recognized that the current limits for such pulsed charging currents can more appropriately be determined using the systems and methods described herein, as opposed to basing the current limits on continuous charging current limits.

In accordance with the present disclosure, a battery management system may utilize an advanced method that prevents plating and estimates plating in a more accurate manner as compared with traditional methods. The battery management system of the present disclosure thereby maximizes the battery performance in service while preventing plating and related aging. As an example, it is now recognized that the “plating current,” or the current at which lithium plating at the anode occurs, is a strong function of both temperature and state of charge for pulsed charging regimes.

Various methods to construct one or more models used by the battery management system to quantify and prevent plating are also described herein. For example, the present disclosure provides an electrochemical model that explicitly accounts for the lithium plating reaction. The model enables both determination of plating onset and quantification of plated lithium. The battery management systems described herein may, for instance, measure operating parameters of the lithium ion batteries and, using such a model, control the charging operations to avoid operating parameter values of the lithium ion batteries that may result in an increased likelihood of lithium plating on the anodes based not only on instant values of the monitored parameters, but also based on quantified lithium plating, the state of the lithium ion battery based on historical usage, and so forth, generated by the model.

To help illustrate,FIG.1is a perspective view of an embodiment of a vehicle10, which may utilize a regenerative braking system. Although the following discussion is presented in relation to vehicles with regenerative braking systems, the techniques described herein are adaptable to other vehicles that capture/store electrical energy with a battery, which may include electric-powered and gas-powered vehicles.

As discussed above, it would be desirable for a battery system12to be largely compatible with traditional vehicle designs. Accordingly, the battery system12may be placed in a location in the vehicle10that would have housed a traditional battery system. For example, as illustrated, the vehicle10may include the battery system12positioned similarly to a lead-acid battery of a typical combustion-engine vehicle (e.g., under the hood of the vehicle10). Furthermore, as will be described in more detail below, the battery system12may be positioned to facilitate managing temperature of the battery system12. For example, in some embodiments, positioning a battery system12under the hood of the vehicle10may enable an air duct to channel airflow over the battery system12and cool the battery system12. The battery system12may include or be associated with a battery management system that is programmed to perform monitoring, estimation, and control of the battery system12to limit lithium plating, as described in further detail below.

A more detailed view of the battery system12is described inFIG.2. As depicted, the battery system12includes an energy storage component14coupled to an ignition system16, an alternator18, a vehicle console20, and optionally to an electric motor22. Generally, the energy storage component14may capture/store electrical energy generated in the vehicle10and output electrical energy to power electrical devices in the vehicle10.

In other words, the battery system12may supply power to components of the vehicle's electrical system, which may include radiator cooling fans, climate control systems, electric power steering systems, active suspension systems, auto park systems, electric oil pumps, electric super/turbochargers, electric water pumps, heated windscreen/defrosters, window lift motors, vanity lights, tire pressure monitoring systems, sunroof motor controls, power seats, alarm systems, infotainment systems, navigation features, lane departure warning systems, electric parking brakes, external lights, or any combination thereof. Illustratively, in the depicted embodiment, the energy storage component14supplies power to the vehicle console20and the ignition system16, which may be used to start (e.g., crank) an internal combustion engine24.

Additionally, the energy storage component14may capture electrical energy generated by the alternator18and/or the electric motor22. In some embodiments, the alternator18may generate electrical energy while the internal combustion engine24is running More specifically, the alternator18may convert the mechanical energy produced by the rotation of the internal combustion engine24into electrical energy. Additionally or alternatively, when the vehicle10includes an electric motor22, the electric motor22may generate electrical energy by converting mechanical energy produced by the movement of the vehicle10(e.g., rotation of the wheels) into electrical energy. Thus, in some embodiments, the energy storage component14may capture electrical energy generated by the alternator18and/or the electric motor22during regenerative braking. As such, the alternator18and/or the electric motor22are generally referred to herein as a regenerative braking system.

To facilitate capturing and supplying electric energy, the energy storage component14may be electrically coupled to the vehicle's electric system via a bus26. For example, the bus26may enable the energy storage component14to receive electrical energy generated by the alternator18and/or the electric motor22. Additionally, the bus26may enable the energy storage component14to output electrical energy to the ignition system16and/or the vehicle console20. Accordingly, when a 12 volt battery system12is used, the bus26may carry electrical power typically between 8-18 volts.

Additionally, as depicted, the energy storage component14may include multiple battery modules. For example, in the depicted embodiment, the energy storage component14includes a lead acid (e.g., a first) battery module28in accordance with present embodiments, and a lithium ion (e.g., a second) battery module30, where each battery module28,30includes one or more battery cells. In other embodiments, the energy storage component14may include any number of battery modules. Additionally, although the first battery module28and the second battery module30are depicted adjacent to one another, they may be positioned in different areas around the vehicle. For example, the second battery module30may be positioned in or about the interior of the vehicle10while the first battery module28may be positioned under the hood of the vehicle10.

In some embodiments, the energy storage component14may include multiple battery modules to utilize multiple different battery chemistries. For example, the first battery module28may utilize a lead-acid battery chemistry and the second battery module30may utilize a lithium ion battery chemistry. In such an embodiment, the performance of the battery system12may be improved since the lithium ion battery chemistry generally has a higher coulombic efficiency and/or a higher power charge acceptance rate (e.g., higher maximum charge current or charge voltage) than the lead-acid battery chemistry. As such, the capture, storage, and/or distribution efficiency of the battery system12may be improved.

To facilitate supply of power from the battery system12to the various components in the vehicle's electrical system (e.g., HVAC system and vehicle console20), the energy storage component14(i.e., battery module) includes a first terminal32and a second terminal34. In some embodiments, the second terminal34may provide a ground connection and the first terminal32may provide a positive voltage ranging between 7-18 volts.

In still further embodiments, the energy storage component14may only include a single battery module, such as the lithium ion battery module30. In such embodiments, the lithium ion battery module30may have electrical characteristics that enable it to replace a traditional lead-acid battery. By way of example, the lithium ion battery module30may be a 12V starter battery.

As illustrated, the energy storage component may be associated with a battery management system (BMS)36. As used herein, the BMS36generally refers to control components that control operation of the battery system12, such as relays within the battery module or switches in the alternator18. Additionally, the BMS36may be disposed within the lithium ion battery module30(e.g., within a housing of the module), or the BMS36may be remote to the lithium ion battery module30, as depicted inFIG.2. As also shown inFIG.2, the operation of the lithium ion battery module30, and indeed the energy storage component14, may be controlled by the BMS36. For example, the BMS36may regulate an amount of electrical energy captured/supplied by each battery module28or30(e.g., to de-rate and re-rate the battery system12), perform load balancing between the battery modules28,30, control charging and discharging of the battery modules28,30(e.g., via relays or DC/DC converters), determine a state of charge of each battery module28,30and/or the entire energy storage component14, activate an active cooling mechanism, activate a short circuit protection system, and the like.

Accordingly, the BMS36may include a memory38and a processor40programmed to execute control algorithms for performing such tasks. More specifically, the processor40may include one or more application specific integrated circuits (ASICs), one or more field programmable gate arrays (FPGAs), one or more general purpose processors, or any combination thereof. Additionally, the memory38may include volatile memory, such as random access memory (RAM), and/or non-volatile memory, such as read-only memory (ROM), optical drives, hard disc drives, or solid-state drives. In some embodiments, the BMS36may include portions of a vehicle control unit (VCU) and/or a separate battery control module. Additionally, as depicted, the BMS36may be included separate from the energy storage component14, such as a standalone module. In other embodiments, the BMS36may be included within the energy storage component14.

Additionally, as depicted inFIG.2, the lead-acid battery28and the lithium ion battery module30are connected in parallel across the first terminal32and the second terminal34to enable charging and discharging of the batteries. As described above, the battery terminals32and34may output the power stored in the energy storage component14to provide power to the vehicle's electrical system. Further, the battery terminals32and34may also input power to the energy storage component14to enable the lead-acid battery28and the lithium ion battery module30to charge, for example, when the alternator18generates electrical power through regenerative braking.

To provide more detail as to the charging process of the lithium ion battery module30,FIG.3illustrates a schematic view of components of the lithium ion battery module30. For simplicity, the lithium ion battery module30is depicted schematically to show the various internal components of a constituent battery cell contained within the lithium ion battery module30. In other words, the internals of the lithium ion battery module30shown inFIG.3are simplified to facilitate discussion. However, it should be appreciated that there are a number of additional components internal to the lithium ion battery module that are not specifically shown, such as a plurality of battery cells, bus bars used to connect the battery cells, and so forth.

Turning briefly toFIG.4, for instance, a cutaway view of a lithium ion battery cell42with a spiral wound cell structure is illustrated. Specifically, the battery cell42has a cell housing or cell casing44, which may take many forms but is illustrated as cylindrical. Other embodiments of the lithium ion battery cell42may have, for example, a prismatic shape or a pouch configuration for the cell casing44.

The lithium ion battery cell42also includes a first cell terminal46and a second cell terminal48. While shown as being positioned at opposing sides of the cell casing44, other embodiments may have the terminals46,48positioned on the same side of the cell casing44. The first and second cell terminals46,48of the lithium ion battery cell42enable electrical connection with other lithium ion battery cells, as well as to the first and second terminals32,34of the lithium ion battery module30. As shown, the lithium ion battery cell42includes an anode50and a cathode52separated by a separator54, which are wrapped in a spiral wound configuration within the lithium ion battery cell42. An electrolyte enables shuttling of lithium ions56between the anode50and the cathode52across the separator54, as schematically shown inFIG.3.

For example, the second terminal34may electrically couple to the cathode52(e.g., via the first cell terminal46) and the first terminal32may electrically couple to the anode50(e.g., via the second cell terminal48). Accordingly, the lithium ion battery cell42depicted inFIG.4may be electrically coupled to the alternator18and the BMS36in a similar manner to the schematic representation of the lithium ion battery module30depicted inFIG.3. Indeed, the term “lithium ion battery” may be used herein to refer to the battery module30and the battery cell42unless otherwise specified.

The anode50, cathode52, and separator54(in addition to an electrolyte) function to enable lithium ion shuttling, intercalation, and deintercalation to deliver and receive electrical energy. The anode50includes an anode active material including carbon (e.g., graphite), silicon, silicon dioxide, lithium titanate, or any other suitable material. The cathode52may include any appropriate cathode active material, for example lithium metal oxides and lithium mixed metal oxides of cobalt, manganese, nickel, aluminum, iron or any other suitable material, or phosphates of iron. Non-limiting examples include lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), and the like. The separator54may be a polymeric material (e.g., polypropylene and/or polyethylene material) that provides electrical separation between the anode50and the cathode52while allowing the lithium ions56to pass through in an unobstructed manner.

Referring again toFIG.3, during a charging operation, the lithium ions56are driven toward and intercalated into the anode50, and are driven away and deintercalated from the cathode52due to the electrochemical characteristics of the anode active material and the cathode active material. For example, during a charging operation of the lithium ion battery module30, an electric current60is applied to the lithium ion battery module30. The charging operation may use power generated by the internal combustion engine24or power generated by regenerative braking to apply the current60. Application of the electric current60causes lithium ion deintercalation at the cathode52and lithium ion insertion at the anode50. As the lithium ions56travel to the anode50along a path62via an electrolyte, a state of charge of the lithium ion battery module30increases.

To control charging of the lithium ion battery module30, the BMS36may receive inputs from sensors and may control application of power from a power source (e.g., the alternator18) to the cathode52. To facilitate this control, the lithium ion battery module30may include a temperature sensor64and/or additional sensors66communicatively coupled to the BMS36. The additional sensors66may measure, for example, voltage and/or current applied to the first terminal32and/or the second terminal34.

In accordance with present embodiments, the charging control performed by the BMS36may be based on feedback generated by the temperature sensor64and/or the additional sensors66, as well as a number of other factors. For example, the BMS36may perform charging control to maximize charging efficiency while minimizing the likelihood of irreversible lithium plating at the anode50. In general, several factors may influence lithium plating on the anode50, and the model-based control utilized by the BMS36may account for such factors. For example, in a general sense, as a state of charge of the lithium ion battery module30increases, a likelihood of lithium plating also increases. Similarly, as the state of charge of the lithium ion battery module30decreases, the likelihood of lithium plating decreases. Additionally, greater current levels applied to the lithium ion battery module30may increase the likelihood of lithium plating, a decrease in voltage at an interface between the separator54and the anode50may increase the likelihood of lithium plating, and a decrease in temperature of the lithium ion battery module30may also increase the likelihood of lithium plating. Accordingly, the BMS36may control the application of power by the alternator18to the lithium ion battery module30in a number of ways to limit the likelihood of lithium plating on the anode50in view of the factors described above.

For example, under certain conditions, lithium plating may occur on the anode50at an interface between the anode50and the separator54as the lithium ions56are deposited on the material that makes up the anode50. For instance, charging the lithium ion battery module30at −40 degrees Celsius may result in a heightened likelihood of lithium plating on the anode50. Lithium plating may result in battery degradation as the lithium ions56deposited on the anode50effectively remove active lithium ions56from future electrochemical reactions within the lithium ion battery module30. Accordingly, a lifespan of the battery30may be significantly reduced due to lithium plating on the anode50during charging operations. Therefore, it may be desirable to avoid the certain conditions that result in a greater likelihood of lithium plating on the anode50by controlling the application of power from the alternator18to the cathode52during regenerative braking or when the alternator18converts mechanical movement of the internal combustion engine24into electrical power to charge the energy storage component14.

For example, the current60applied during a charging operation may be limited to different levels depending on the state of charge of the lithium ion battery module30, the temperature of the lithium ion battery module30, a duration and/or frequency of charging pulses applied to the battery30, or any other factor that may influence the likelihood of lithium plating on the anode50. Because these conditions may change over time, the BMS36may dynamically control the power output of the alternator18into the lithium ion battery module30during the charging operation. Therefore, the likelihood of the anode50experiencing lithium plating may not correspond to a specific level of the current60. Rather, the likelihood of lithium plating at a specific level of the current60may vary based on the various factors discussed above.

To illustrate the functionality of the BMS36,FIG.5is a process flow diagram describing a method70for controlling charging operations using an electrochemical model72of the lithium ion battery module30. Initially, at block74, the electrochemical model72of the lithium ion battery module30may be generated. The electrochemical model72of the lithium ion battery module30may estimate any number of parameters relating to the lithium ion battery module30. As an example, the electrochemical model72may be used to identify the onset of lithium plating at the anode50, to obtain reaction kinetics for lithium plating at the anode50, to estimate the amount of lithium plating at the anode50, or a combination thereof. Additionally or alternatively, the electrochemical model72may be used to determine current limits for pulsed charging currents applied to the lithium ion battery module30, to develop a quantitative current map to prevent lithium plating at the anode50during charging operations, and similar features.

By way of example, the electrochemical model72may be an extensive mathematical model that solves for dependent variables, such as electrolyte concentration, electrolyte potential, solid concentration, solid potential, reaction rate, local current density, etc., using governing equations based on porous electrode theory and concentrated electrolyte theory, among others. The electrochemical model72may be based on, or include fundamental equations relating to lithium transport between the anode50and the cathode52.

To help illustrate,FIG.6is a schematic representation of the electrochemical model72, or at least a portion thereof, of the various battery cell components described above. As shown, the electrochemical model72includes an anode framework76, a cathode framework78, and a separator framework80, where each framework represents one or more equations (e.g., a system of equations) that model lithium behavior at different electrochemically active regions of the battery cell42. The anode framework76and the cathode framework78of the electrochemical model72include solid phase and interface portions, and the separator framework80includes a liquid phase portion. The solid phase portions of the anode and the cathode frameworks76,78include equations relating to lithium diffusion in electrochemically active particles (e.g., the anode and cathode active materials). The equations in the solid phase portion are also governed by the solid phase Ohm's law. The liquid phase portion of the separator framework80relates to lithium ion diffusion and migration in the liquid phase (the electrolyte). This portion is governed by the liquid phase Ohm's law.

The interface portions of the anode and the cathode frameworks76,78include one or more Butler-Volmer equations to model the behavior of lithium at the interface between the anode50and the separator54, and the interface between the cathode52and the separator54. It is now recognized that, from the standpoint of generating the electrochemical model72to account for lithium plating behavior, the interface between the anode50and the separator54is the most likely area where lithium plating will occur.

FIG.7illustrates a schematic of the major reactions involved in lithium plating at the anode50, which in this embodiment is assumed to be graphite-based. The reactions include the main reaction of lithium insertion into the layered structure of carbon. This may be considered an electrochemical reaction to produce inserted lithium82(e.g., lithium carbide). This electrochemical reaction has no impact on cell degradation. The reactions also include the plating reaction, which is electrochemical in nature and produces solid lithium particles84. The solid lithium particles84may be reinserted into the anode50(e.g., by re-oxidation and insertion), or may undergo further reaction. When the solid lithium particles84are reinserted into the anode50, this has no impact on cell degradation. The other reactions of the solid lithium particles84include further reaction with the electrolyte, the reaction being chemical in nature to produce a passivation layer86of oxidized lithium particles88. The production of the passivation layer86accelerates aging of the battery cell42by causing capacity loss and an increase in resistance. As also shown, the solid lithium particles84may undergo a physical reaction to accumulate and form plated lithium90. This reaction is substantially irreversible and will cause capacity loss and possible thermal runaway issues for the battery cell42.

Returning toFIG.6, in accordance with present embodiments, the anode framework76includes Butler-Volmer equations that consider the lithium plating reaction. Equation (1) describes the Butler-Volmer kinetics of the plating reaction, which may enable the BMS36to determine the onset of lithium plating and monitor the progress of the plating reaction. In an embodiment, equation (1) is as follows:

ip=av⁢i0,p[cl,refc1⁢exp⁢(αa⁢F⁢ηpR⁢T)-clcl,ref⁢exp⁢(-ac⁢F⁢ηpR⁢T)](1)
where ipis the plating current (A/m3), αvis the specific surface area of active material (m2/m3), i0,pis the plating exchange current (A/m2), Cl,refis the reference lithium salt concentration (1M), clis the lithium salt concentration (mol/m3), ααa is the anodic charge transfer current, F is the Faraday constant, np is the plating over potential (V), R is the universal gas constant, Tis temperature, and ac is the cathodic charge transfer current.

Equation (2) calculates the plated lithium as a result of the plating reaction. In an embodiment, equation (2) is as follows:

∂εL⁢i∂t=-ML⁢iρLi⁢F⁢ip+f⁡(k,εL⁢i)(2)
where εLiis the plated lithium volume fraction, t is time, MLiis the atomic mass of lithium (g/mol), and pLiis the density of lithium metal (g/m3). In equation (2), the first term is contributed by the plating reaction, which may be positive or negative depending on the plating over potential ηp. The second term is contributed by side reactions. If the side reaction is physical accumulation, the term can be positive or negative. If the side reaction is chemical in nature (e.g., reaction with electrolyte), the term will be always negative.

To demonstrate the manner in which equations (1) and (2) relate to plating dynamics,FIGS.8,9, and10present a situation where a constant current 100 A 10 s pulse was applied to a lithium ion battery cell at T=0° C. and SOC=50%. Unless otherwise noted, the battery simulations described herein are based on a battery having NMC as the cathode active material, and graphite as the anode active material.FIG.8shows the various potentials of the anode50(as a function of position relative to the current collector (CC) and the separator (Sep)) for the initial state when current is zero. The ULi+/Li=0V, is the reference potential. The potential of the negative electrode, V−=OCVgraphite, is about 120 mV, which corresponds to the open circuit voltage of a graphite anode at 50% SOC. Since there is no current, the over potential for the main anode reaction, η−=V−OCVgraphite=0. As a result the over potential for the plating reaction, ηp=V−−0[V]>0 indicating that lithium plating has not happened.

Note that in equation (1) when ηp>0, the anodic term in the equation dominates and there is no lithium plating.

FIG.9illustrates the various potentials at the end of the charge pulse. Here, the negative electrode potential, V1, is significantly lowered to maintain a negative over potential η−and thus supports reacting current in the anode50. Meanwhile, the over potential for the lithium plating reaction, ηp, goes negative as well and then plating happens (the cathodic term in equation (1) wins over the anodic term).

FIG.10illustrates the evolution of plated lithium which grows along charging time. In other words, longer charging current pulses result in a larger volume fraction of plated lithium. Here, lithium is plated more at the interface of the anode50and the separator54than at the interface between the anode50and the current collector which is reasonable because there is a resistance for lithium ions to migrate and diffuse from separator to collector and reactions are favored at the anode50and the separator54.

The plating may be reversed (e.g., via re-insertion of lithium ions into the graphite anode through electrochemical reactions) when the charging stops, as shown inFIGS.11and12.FIG.11illustrates the various electrical potentials across the anode50after the charging current has stopped for 1 s. As shown, the plating over potential instantly goes back to positive and thus the anodic reaction of equation (1) will dominate and the plated lithium converts to lithium ions and re-insert in graphite through two steps of electrochemical reactions.FIG.12demonstrates that the plated lithium has gone away along with relaxation time. It should be noted thatFIGS.11and12illustrate an ideal case where no side reactions occur (the second term of equation (2) is zero). In the ideal case, the plated lithium will not cause any negative effects if they are re-inserted into graphite. However, in reality the side reactions, either physical or chemical, can never be zero. Once the lithium is plated there will always some loss of lithium and will cause some degree of battery degradation. This is one of the reasons plating may cause battery aging.

Returning now toFIG.5, the BMS36may use battery measurements and the electrochemical model72to monitor and/or quantify lithium plating, at block100. It should be noted that the electrochemical model72may be used directly in performing the acts according to block100, or may be used to construct, for example, look-up tables, current limit maps, and the like, which are used directly to monitor and/or quantify lithium plating. The look-up tables, current limit maps, or similar features (e.g., an equivalent circuit model) may reduce the computational cost associated with performing the acts of block100, which may be desirable to allow the BMS36to control the operation of the lithium ion battery module30in a more efficient manner. For example, a time cost for dynamically changing charging parameters may be reduced by simplifying the algorithmic structure associated with monitoring and subsequently controlling the charging of the lithium ion battery module30to prevent lithium plating.

As an example, a look-up table generated using the electrochemical model72may relate various measurable factors of the lithium ion battery module30to evaluate aspects of lithium plating at the anode50. For example, one or more look-up tables may provide a mechanism for relating the temperature, state of charge, current pulse duration and level, and the like, of the battery cells42of the lithium ion battery module30to the likelihood of lithium plating. Accordingly, one or more look-up tables may enable the BMS36to identify the onset of lithium plating on the anode50, and to quantify lithium plating on the anode50based on the electrochemical model72.

Thus, at block100, measurable factors of the lithium ion battery module30may be applied to the electrochemical model72, to one or more look-up tables based on the electrochemical model72, to an equivalent circuit model based on the electrochemical model72, or the like, to identify lithium plating onset, to monitor lithium plating kinetics, to quantify lithium plating, or any combination thereof. Again, the measurable factors may include, but are not necessarily limited to, state of charge, temperature, and similar factors.

Subsequently, at block102, charging operations may be controlled by the BMS36based on the monitored and/or quantified lithium plating. For example, the BMS36may control the alternator18, or other feature responsible for delivering current to the lithium ion battery module30, to adjust the charging current limit and/or pulse duration provided to the lithium ion battery module30. Such control may be performed in response to determining that quantified lithium plating or observed lithium plating kinetics meet a predetermined criterion.

As set forth above with respect to the acts associated with block100, the electrochemical model72may be used to generate a number of useful tools, such as look-up tables, equivalent circuit models, current limit maps, and so forth. Indeed, as noted, it is now recognized that the current limit associated with pulsed charging operations is a strong function of a state of charge (SOC) of the lithium ion battery module30(or, more simply, battery cell42). Indeed, the present disclosure enables the determination of current limits for preventing lithium plating, as well as lithium plating quantification, using the electrochemical model72.

FIG.13is a process flow diagram of a method110of using the electrochemical model72to generate current limit maps for specific lithium ion batteries for eventual control of the batteries to balance lithium plating and battery performance. At block112, a lithium ion battery (e.g., battery cell or battery module) having a particular chemistry (a particular combination of positive and negative active materials and electrolyte) is subjected to three-electrode tests at different SOC levels of the battery. Overpotentials for the negative and positive electrodes (anode and cathode) of the battery are obtained using the three-electrode tests, which enables a determination of the true plating current limit for the battery.

The electrochemical model72may be calibrated using such test results, and at block114, the trained electrochemical model72is used to determine the negative overpotential for a simulated battery of similar construction. This enables the plating current limit as a function of SOC to be determined for the battery. It should be noted that the battery simulation may be performed using a battery simulation system constructed using the test results obtained at block112, using various battery design parameters, and so forth. The battery simulation system may be constructed in a modeling environment (e.g., a physics-based modeling environment) such as COMSOL Multiphysics®, run on a programmable device by programming the device and the environment with particular battery characteristics, appropriate electrochemical models, and so forth.

Once the battery simulation system is appropriately programmed, various parameters can be varied, including SOC and temperature, among others, to model the behavior of a simulated battery. Indeed, as shown at block116, a current limit map to prevent lithium plating may be generated.FIGS.14and15are example plots of modeled plating current limits as a function of SOC at 25° C. and −25° C., respectively. As shown, the plating current limits for pulsed charging currents are a strong function of SOC, while the plating current limit for continuous charging currents does not vary with SOC but has an overall much lower threshold. As can be seen from these plots, current limits for preventing plating are a function of temperature, SOC, and current duration.

Top charge voltage is a standard specification for a lithium ion battery, because overcharge may cause over oxidation of the cathode and plating at the anode. The top voltages normally are different for continuous and pulse charging conditions. In the plots, the “top voltage” curve represents the current limit for a 10 s charge pulse to reach a top voltage of the battery (e.g., 4.2V).

FIGS.14and15also demonstrate that a single simple maximum voltage limit specification may not necessarily prevent lithium plating. Only under very specific conditions, such as high SOC and room temperature is the current limit based on top voltage lower than the current limit for preventing plating as defined for a continuous charge specification.

At block118, the electrochemical model72may also be used to quantify plating as a function of SOC under a particular set of conditions. For example, the given set of conditions may represent conditions often experienced when the battery is used in a particular application (e.g., in a particular type of vehicle or under certain driving conditions). Lithium plating quantification may be an important factor in control strategy for the lithium ion battery module30in embodiments where it may be desirable to balance the amount of lithium that is allowed to plate on the anode50with desired charging parameters for the lithium ion battery module30.

In accordance with certain embodiments, determining the amount of plating may allow for the generation of a degradation map, as shown at block120.FIG.16is an example of such a degradation map, which compares the quantity of plated lithium for a fixed 10 s charge pulse at various SOC levels. As shown, the plated lithium varies with SOC and charged As. At high SOC, less lithium plating is observed because the charge current has been cut by an upper voltage limit. At low SOC without a voltage limit, the plated lithium varies with SOC. This indicates there is room in controlling charging operations to minimize the plating while keeping the simple control of a single voltage limit, for example when the battery is estimated to be within a certain SOC range. A temperature and SOC-dependent current de-rating (or charging) strategy could be applied to minimize the plating. For example, less lithium plates on the anode50when the battery cell is charged at 50% SOC than at 30% SOC.

The illustrated method110ofFIG.13also includes controlling charging operations at block122. According to block122, the charging operations may be controlled based on the current limit map, the monitored lithium plating kinetics, quantified lithium plating, the degradation map, or any combination of these, in response to changes in various monitored parameters. As set forth above, the monitored parameters may include charging current levels, a SOC of the battery, a temperature of the battery, pulse durations, and so forth. Specifically, the BMS36may monitor the parameters, compare the monitored values of these parameters against outputs of the electrochemical model72(which may include any one or a combination of the current limit maps, degradation maps, look-up tables, and so forth), and adjust charging operations for the lithium ion battery module30accordingly.

While described in terms of regular pulses applied to the lithium ion battery module30(or to the lithium ion battery cell42), the performance evaluation based on standard pulses can be quite different from the performance evaluation based on driving cycle because the real-world driving cycles often include irregular charge and discharge pulses. Rest time and the sequence of charge and discharge may play a role in ultimate performance. Accordingly, the BMS36may also utilize the electrochemical model72to monitor the performance of the lithium ion battery module30over time.FIGS.17and18illustrate driving profile simulation results for battery voltage and battery current, respectively, at a constant temperature T=25° C. and T=0° C. in which the temperature rise due to internal heating has been ignored. In this simulation, the lithium ion battery module30has 13 lithium ion battery cells in series representing a 48V system application.

Because battery cell resistance is much higher at low temperature, the battery voltage spans in a wider range at 0° C. than at 25° C. Due to the system voltage cutoff (54V is assumed), the battery at 0° C. is not capable of accepting all regen currents.FIG.19illustrates the evolution of the average volume fraction of plated lithium corresponding to the simulated driving profiles. It is seen that at T=25° C. there is no plating accumulation. However, for T=0° C., the regen has induced some lithium plating. As illustrated, the last pulse has contributed significantly to the total plating. The driving cycle simulation is believed to be consistent with the current limit maps ofFIGS.14and15. Indeed, lithium plating may not necessarily lead to capacity loss as long as the lithium can be re-inserted into the anode50. However, if there are certain side reactions and the side reactions can be quantified, the capacity loss contributed by lithium plating can be estimated by the BMS36using the electrochemical model72. Estimating the capacity loss in this manner may provide an accurate measure of aging of the lithium ion battery module30.

The electrochemical model72described herein may also be useful for simulating characteristics, including lithium plating, for a variety of battery cell designs. Indeed, a variety variables exist for cell level design such as coating thickness, porosity, materials formula, particle size. The optimal design depends on balance of requirement and cost. For example, thicker coatings allow more active materials packed in limited space favoring higher energy density and lower cost. Accordingly, one major difference between a “power cell” and an “energy cell” is the thickness of active material coated onto the current collector of the cell.

Accordingly, the effect of coating thickness may be modeled using the electrochemical model72to acquire a measure of how energy cell vs power cell design may affect the current limit to prevent lithium plating.FIGS.20and21depict current limit maps for T=25° C. and 0° C., respectively, for relatively thin (power cell) and relatively thick electrode (energy cell) designs, where the energy cell design has a coating thickness that is approximately four-fold greater than the power cell design. As shown, the current limit is significantly affected by the active material thickness. This demonstrates that thicker coatings cause higher diffusion resistance and the negative over potential has a sharper slope from the electrode/separator interface to the electrode/collector interface. There is a higher chance plating occurs at the electrode/separator interface. Table 1 uses 50% SOC as an example to demonstrate the extent of current limit reduction when using an energy cell design versus a power cell design.

TABLE 1Comparison of modeled current limits for a power cell(thin coating) and an energy cell (thicker) designT = 25° C.T = 0° C.PowerEnergyPowerEnergyCurrent limit ofdesigndesigndesigndesignContinuous charge140A92A33A26Ato prevent plating1 s pulse to183.4A157.0A47.7A43.3Aprevent plating10 s pulse to180.7A138.3A46.7A42.6Aprevent plating30 s pulse to152.2A109.7A46.6A39.9Aprevent plating10 s pulse to300A228A130A108Areach 4.2 V

One or more of the disclosed embodiments, alone or on combination, may provide one or more technical effects including decreasing the likelihood of lithium plating on the anode50of the lithium ion battery module30. For example, the disclosed electrochemical model may be used to study how plating occurs and analyze factors that influence plating in a battery cell. For example, current limits for preventing plating generated using the disclosed electrochemical model can be quite different from current limits based on maximum voltage limits, and may more accurately predict battery cell behavior when accounting for lithium plating in battery charging operations. The technical effects and technical problems in the specification are exemplary and are not limiting. It should be noted that the embodiments described in the specification may have other technical effects and can solve other technical problems.

While only certain features and embodiments have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the disclosed subject matter. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.