Patent ID: 12237708

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.

Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the disclosure, so it should be understood that other equivalents and modifications could be made thereto without departing from the scope of the disclosure.

In addition, in the present disclosure, if it is deemed that a detailed description of a related known structure or function may obscure the subject matter of the present disclosure, the detailed description thereof will be omitted.

Throughout the specification, when a portion is referred to as “comprising” or “including” any element, it means that the portion may include other elements further, without excluding other elements, unless specifically stated otherwise. Furthermore, the term “control unit” described in the specification refers to a unit that processes at least one function or operation, and may be implemented by hardware, software, or a combination of hardware and software.

In addition, throughout the specification, when a portion is referred to as being “connected” to another portion, it is not limited to the case that they are “directly connected”, but it also includes the case where they are “indirectly connected” with another element being interposed therebetween.

In the specification, a secondary battery means one independent cell that has an anode electrode terminal and a cathode terminal and is physically separable. For example, one pouch-type lithium polymer cell may be considered as a secondary battery. However, the present disclosure is not limited to the kind of secondary battery.

FIG.1is a block diagram showing a charging apparatus100of a secondary battery according to an embodiment of the present disclosure.

Referring toFIG.1, the charging apparatus100of a secondary battery according to an embodiment of the present disclosure is a device for controlling charging of a secondary battery20and is electrically coupled to the secondary battery20.

The secondary battery20supplies an electrical energy required for a power system such as an electric vehicle and includes at least one battery cell. The battery cell may be, for example, a lithium-ion battery.

In the present disclosure, the battery cell is not limited to the lithium-ion battery, and a battery cell capable of charging and discharging may be used without limitation. The battery cells included in the secondary battery20are electrically connected in series and/or in parallel.

A switch30is installed on a current path for charging and discharging the secondary battery20. A control terminal of the switch30is provided to be electrically connected to a control unit120. The switch30is turned on or off in accordance with a duty ratio of a switching signal SS output from the control unit120. The switch30may be a field effect transistor or a mechanical relay.

The charging apparatus100of a secondary battery determines an internal state of the secondary battery20by using an electrochemical reduced order model (ROM) and adjusts the magnitude of a charging current applied to the secondary battery20in consideration of the internal state.

To this end, the charging apparatus100includes a sensing unit110, a control unit120, a memory unit130, and a communication unit140.

According to an embodiment, the internal state includes an average ion concentration and a surface ion concentration of an anode, a potential in the anode, a potential in an anode electrolyte, a side reaction rate of lithium ions, and a state of charge (SOC). Here, the average ion concentration and the surface ion concentration refer to an average ion concentration of active material particles and a surface ion concentration of active material particles.

The sensing unit110is configured to detect physical/electrical variables associated with the internal states of the secondary battery20at time intervals. The physical/electrical variables include voltage, current and temperature of the secondary battery20.

The sensing unit110includes a current measuring means111, a voltage measuring means112, and a temperature measuring means113.

The current measuring means111is provided to be electrically connected to the charge/discharge path of the secondary battery20. The current measuring means111is configured to detect a current flowing through the secondary battery20and output a first sensing signal SI representing the detected current to the control unit120. A Hall Effect sensor, a shunt resistor or the like may be used as the current measuring means111.

The voltage measuring means112is provided to be electrically connected to a cathode terminal and an anode terminal of the secondary battery20. The voltage measuring means112is configured to detect a voltage across the secondary battery20(that is, a potential difference between the cathode terminal and the anode terminal of the secondary battery20) and output a second sensing signal SV indicating the detected voltage to the control unit120. The voltage measuring means112includes a common voltage measuring circuit.

The temperature measuring means113is configured to detect a temperature of the secondary battery20and output a third sensing signal ST indicating the detected temperature to the control unit120. The temperature measuring means113may be a thermocouple.

The control unit120is operably coupled to the sensing unit110, the memory unit130, the communication unit140and the switch30. The control unit120, in hardware, may be implemented using at least one of application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), microprocessors and electrical units for performing other functions.

The control unit120is configured to periodically receive the first sensing signal SI, the second sensing signal SV and the third sensing signal ST output by the sensing unit110. The control unit120uses an analog-to-digital converter (ADC) included in the control unit120to convert each of the first sensing signal SI, the second sensing signal SV and the third sensing signal ST in analog form, which are received at each unit time, into a current value, a voltage value and a temperature value in digital form, and then store the converted value in the memory unit130. That is, in the memory unit130, a current history, a voltage history and a temperature history of the secondary battery20may be stored at each unit time.

The memory unit130is operably coupled to the control unit120. The memory unit130may store a program and various data necessary for executing control logics, explained later. The memory unit130may include, for example, at least one storage medium selected from a flash memory type, a hard disk type, a solid state disk (SSD) type, a silicon disk drive (SDD) type, a multimedia card micro type, a random access memory (RAM), a static random access memory (SRAM), a read-only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), and a programmable read-only memory (PROM).

The communication unit140may be communicatively coupled to a charger2. The charger2applies a charging current to the secondary battery20according to a request of the control unit120. The magnitude of the charging current is determined by the control unit120. The magnitude of the charging current is expressed in C-rate. The control unit120transmits a charging current adjustment message to the charger2in order to adjust (reduce) the magnitude of the charging current if the side reaction rate or the surface ion concentration in the anode reaches a preset upper limit or if the terminal voltage of the secondary battery20reaches a cutoff voltage, in consideration of the electrochemical model. Then, the charger2reduces the magnitude of the charging current according to the request of the control unit120. The charger2may be a charging station used to charge an electric vehicle, or a charger installed inside the electric vehicle.

The charger2includes, for example, an electronic control unit (ECU). The communication unit140may send and receive messages required to adjust the magnitude of the charging current to/from the ECU of the charger2. The communication unit140may communicate with the charger2through a wired network such as RS-232, a local area network (LAN), a controller area network (CAN) and a daisy chain and/or a short distance wireless network such as Bluetooth, Zigbee, Wi-Fi, etc. However, it is obvious that the present disclosure is not limited by the communication protocol.

The control unit120estimates the internal state of the secondary battery20, which includes an average ion concentration of the anode particles, an volume-averaged concentration fluxes, a surface ion concentration and a side reaction rate, by using an electrochemical reduced order model to reduce a charging time of the secondary battery20and reduce degrading of the secondary battery20as much as possible.

Hereinafter, the side reaction inside the secondary battery20, which is one of main causes degrading the secondary battery20, will be described in detail.

The charging time of secondary battery20may be simply reduced as the charging current increased. However, the increased charging current not only generates more heat but also accelerates the aging of the secondary battery20.

In many researches associated with the aging mechanism performed using lithium-ion batteries with nickel-manganese-cobalt oxide/graphite or lithium iron phosphate/graphite species, it has been revealed that side reactions occurring at the surface of anode particles under different operating conditions are a major cause of aging.

The side reaction is a reduction process between an electrolyte solvent (e.g., ethylene carbonate) and lithium ion at the surface of the anode particle. By-products of the side reaction form a very thin film that adheres to the surface of the anode particles. The corresponding film is called a solid electrolyte interphase (SEI) layer.

Initially, as the SEI layer is formed, further side reactions slows down. However, the side reactions take place continuously throughout the battery life because the anode always operates at the potential that is outside the stability window of the electrolyte.

The deposits produced by the side reactions are accumulated on the surface of the anode particles and result in the growth of a SEI layer. Particularly, the SEI layer at the anode particles located next to the separator grows faster among others and forms an extra deposit layer.

As a result, the ionic resistance of the SEI layers increases, and the surface area and pores of the active material accessible by lithium ions decrease. Since the SEI layers are electrical isolators that may completely isolate some anode particles from electrons, this leads to a loss of active anode material and finally capacity fade. In addition to the active material loss, the consumed ions and electrolyte solvents caused by the side reactions are another factors for capacity fade.

The side reactions are enhanced by favorite operating conditions like elevated temperatures and high SOC ranges. A high charging current also promotes the side reactions, which is analyzed later. When the temperature rises, the reaction kinetics of lithium ions and electrolyte solvents get increased, and as a result, more ions are passing through the SEI layer and reaching at the interface where the side reactions occur. Thus, the concentrations of both ions and solvents at the surface of the particles increase, which results in a high side reaction rate.

The effects of SOC ranges and magnitude of the charging current on the side reactions may be better explained with help of the relationship of potentials at the interface between the anode particle and the electrolyte.

A schematic diagram of the potential relationship at the anode side during charging is depicted inFIG.2.

When the secondary battery20is charged, two chemical reactions including a main reaction and a side reaction take place. The total reaction rate, jtotalLi, is expressed as a sum of both reaction rates as in Equation (1) below.
jtotalLi=j−Li+jsideLi(1)

Here, j−Liand jsideLirepresent reaction rates of the main reaction and the side reaction.

The main reaction rate, j−Li, produced by the main reaction at the interface between the anode particle and the electrolyte is a function of overpotential, η−and expressed by the Bulter-Volmer (B-V) equation as in Equation (2) below.

j-Li=as⁢i0⁡(exp⁡(αa⁢nFRT⁢η-)-exp⁡(-αc⁢nFRT⁢η-))(2)

Here, asis a specific surface area in which the main reaction occures. αaand αcare the anodic and cathodic transfer coefficients, which are assumed to be 0.5. n is the number of ions participating in the main reaction, which is equal to 1. R is the universal gas constant (8.3143 J mol−1K−1). i0is the exchange current density. T is the cell temperature.

The side reaction rate, jsideLi, may be calculated using the B-V equation as in Equation (3) below.

jsideLi=-i0,side⁢as⁢exp⁡(-αc,side⁢nside⁢FR⁢T⁢ηside)(3)

The overpotential (η−) in the B-V equation (2) above may be expressed as follows.

η-=φs--φe--Ueq,--RSEIas⁢jtotalLi(4)

Here, φs−and φe−are the electric potentials of the solid anode particle and the electrolyte, respectively.

The equilibrium potential of the anode, Ueq−, is a function of stoichiometric number that corresponds to the ratio between the ion concentration in solid phase and its maximum value. RSEIis the resistance of the SEI layer that causes a potential drop across the SEI. The potential drop may be expressed as in Equation (5) below.

VSEI=RSEIas⁢jtotalLi(5)

In Equation (3), nsideis the number of ions involved in the side reactions that is equal to 2. ηsideis the overpotential of side reactions and may be expressed as in Equation (6) below.

ηside=φs--φe--Ueq,side-RSEIas⁢jtotalLi(6)

Here, Ueq,sideis the equilibrium potential of the side reactions. The exchange current density of the side reactions, i0,side, is a concentration function of two reactants of the side reactions, namely lithium ions and electrolyte solvents such as ethylene carbonate (EC) molecules and may be expressed as in Equation (7) below.
i0,side=kside√{square root over (cs,surfcEC,Rs)}  (7)

Here, ksideis the kinetic rate constant for the side reactions. cs,surfand cEC,Rsare the concentrations of the lithium ions and the EC molecules at the surface of the anode particles, respectively. It is obvious that cEC,Rsis changed according to the kind of the electrolyte solvent.

φe−is regarded as the reference in order to analyze relationship to other potentials. When the battery is charged, the overpotential, η−, is negative because j−Liis negative due to the ion transport from the electrolyte to the anode.

Referring toFIG.2, when SOC is high, the ion concentration in the anode is high and the equilibrium potential, Ueq−, becomes small. Also, φs−also becomes small under the assumption that the overpotential, η−, is constant. As shown inFIG.2, when ηsidedecreases, the magnitude of the side reaction rate increases. Consequently, charging the secondary battery20in a high SOC range leads to a large rate of side reactions, which eventually accelerates degradation of the secondary battery20.

Also, when the secondary battery20is charged with a high current, the magnitude of the overpotential, η−, increases according to the B-V equation, which lowers the anode potential, φs−. Since the overpotential for side reactions, ηside, is the difference between the anode potential and the equilibrium potential, the magnitude of the overpotential for the side reactions increases, which leads to a high side reaction rate.

Meanwhile, CC/CV charging and pulse charging are used as classical charging methods. Among two charging methods, the CC/CV charging is the mostly used charging method. When the charging current increases, the charging time may be reduced. However, the charging time is not significantly reduced by a high charging current since it leads to the extension of CV mode. In the CC mode, a higher charging current leads to a quick increase of SOC. However, the terminal voltage reaches the cutoff voltage at a lower SOC. On the other hand, a high charging current leads to a high magnitude of overpotential of the side reactions. Accordingly, the side reaction rate is increased, and the cycle life is significantly reduced. In addition, an increase of the cutoff voltage may also significantly reduce the charging time because the CC charging period is extended and the average charging current in CV mode is also increased. However, the increased cutoff voltage increases the magnitude of the charging current in CC mode that leads to higher magnitude of overpotential of the side reactions, which increases the side reaction rate. In conclusion, increasing the charging current or the cutoff voltage of in the CC/CV charging method is not satisfied with the requirements for fast charging including short charging time and slow degradation speed.

On the other hand, the pulse charging method is another option being widely proposed. The pulse charging method may be categorized into unidirectional pulse charging and bidirectional pulse charging dependent upon the presence of resting or negative pulses. The charging time is determined by the mean value of the pulse charging current. Thus, the charging time cannot be reduced by only increasing the magnitude of the positive pulses. However, resting and negative pulses speed up relaxation of ion concentration gradients and reduction of the concentration overpotential in the anode, which suppresses formation of lithium plating. In addition, the bidirectional pulse charging with optimized frequency may significantly prevent lithium plating because deposited lithium dissolves during discharging and takes part in the main chemical reactions again. However, the bidirectional pulse charging has no positive or even detrimental effects on the performance and cycle life of the secondary battery20. For pulse current with the frequency larger than 10 Hz, the lithium-ion battery behaves like a low-pass filtering behavior because of the large capacitance of the battery, so the degradation of the battery is determined by the mean value of the pulse charging currents. Also, no differences in charging time and degradation speed between the pulse charging and the CC/CV charging are reported with the pulse frequency of 25 Hz. In addition, when the frequency is less than 10 Hz, pulse current cannot be completely buffered by the large capacitances of the battery, so the ion concentration gradient increases significantly and the anode potential becomes more negative, which increase the side reaction rate significantly.

On the other hand, the pulse charging method generates more heat compared with the CC/CV charging method, which causes a high side reaction rate. When the magnitude of the pulse charging current decreases, the ion concentration gradient and the ion concentration saturation may be effectively reduced. Thus, in the present disclosure, only the pulse charging current with decreased magnitude is employed for the range of high SOC to prevent the ion concentration from exceeding a saturation limit.

In the present disclosure, the charging method is designed in consideration of three parts as follows. First, a model that allows for estimation of physical variables such as the ion concentrations and the anode potentials in real time is provided, and estimation errors caused by model state error and measurement noises are reduced using advanced control. Second, the effects of the CC/CV charging method according to the developed model on the charging time and the degradation speed are analyzed. Third, the magnitude of charging current and the duration of pulses are determined considering different limiting factors.

First, the reduced order model (ROM) according to an embodiment of the present disclosure will be described.

When the lithium-ion battery is charged, lithium ions de-intercalated from the cathode particles are transported through the electrolyte to cause chemical reactions at the interface between the anode particles and electrolyte, and are finally intercalated into the anode particles. During this process, the electrons are transported through an external circuit. Similarly, reverse reactions are performed during discharging. This electrochemical process may be described mathematically using electrochemical, thermal and mechanical principles. The electrochemical principles include mass transport and electrochemical kinetics, and the thermal and mechanical principles are based on energy equations and mechanical stress-strain relationships.

Physical variables indicating the behavior of ions during the operation of the lithium-ion battery cannot be measured experimentally. One potential approach is to develop a model with governing equations that can estimate the physical variables through numerical analysis.

In addition, if degradation mechanisms are further combined, the model may predict the performance of the battery in a beginning of life (BOL) state, a middle of life (MOL) state and an end of life (EOL) state. Depending on the degree of the order, the model may be called a reduced low order model or a full order model.

A pouch-type lithium ion cell is made of stacked micro cells that are aggregated in a rolled or sandwiched form. If there is no difference in electrochemical characteristics among a plurality of micro cells, it may be assumed that one micro cell represents the behavior of the entire micro cells.

The micro cell has a sandwich structure in which a separator is interposed between the anode and the cathode in the thickness direction. The active materials of the lithium ion battery are made of metal oxides for the cathode and carbon for the anode. The shapes of the active materials are approximated with a spherical shape that are distributed uniformly in the space occupied by the active material layers. A schematic diagram of the single microcell cell is depicted inFIG.3.

Referring toFIG.3, when the micro cell is discharged or charged, lithium ions are de-intercalated from a lattice structure of the active material particle of any one electrode, then diffused to the surface of the active material particle, and then transported through the electrolyte and the separator to the surface of the active material particle of the other electrode. The transported lithium ions chemically react with electrons at the surface of the active material particle of the other electrode, then diffuse into the active material particles, and are intercalated into the lattice structure again. The electrons flow through an external circuit and complete the redox process.

The governing equations for the electrochemical principles including intercalation or de-intercalation of lithium ions, diffusion, ion transport, chemical reactions and resulting change of potentials are listed Table 1. The meanings of the variables and parameters of the governing equations may be understood from the Nomenclature defined in the beginning of the specification.

TABLE 1[Summary of full order model and reduced order model]Cell dynamicsFOMROMIon concentration in electrode∂cs∂t=Dsr2⁢∂∂r⁢(r2⁢∂cs∂r)dd⁢t⁢cs,ave+3⁢jLiRs⁢as⁢F=01r⁢∂cs∂r⁢|r=0=0⁢⁢and⁢⁢Ds⁢∂cs∂r⁢|r=Rs=-jLias⁢Fdd⁢t⁢qa⁢v⁢e+3⁢0⁢DsRs2⁢qa⁢v⁢e+452⁢jLi2⁢Rs2⁢as⁢F=03⁢5⁢DsRs⁢(cs,surf-cs,ave)-8⁢Ds⁢qa⁢v⁢e=-jLias⁢FIon concentration in electrolyte∂(ɛe⁢ce)∂t=∂∂x⁢(Deeff⁢∂∂x⁢ce)+1-t+0F⁢jLiċe= A*· ce+ B*· I y = C*· ce+ D*· I∂ce∂x⁢|x=0=∂ce∂x⁢|x=L=0Ohm’s law in electrode∂∂x⁢(σeff⁢∂∂x⁢φs)-jLi=0∂∂x⁢(∂∂x⁢φs)=jLiσeff-σeff⁢∂∂x⁢φs⁢|x=0=-σeff⁢∂∂x⁢φs⁢|x=L=IA-σeff⁢∂∂x⁢φs⁢|x=0=-σeff⁢∂∂x⁢φs⁢|x=L=IA∂∂x⁢φs⁢|x=δ_=∂∂x⁢φs⁢|x=δ_+δsep=0∂∂x⁢φs⁢|x=δ_=∂∂x⁢φs⁢|x=δ_+δsep=0Ohm’s law in electrode∂∂x⁢(κeff⁢∂∂x⁢φe)+∂∂x⁢(κDeff⁢∂∂x⁢ln⁢⁢ce)+jLi=0∂∂x⁢(∂∂x⁢φe)+jLiκeff=0∂∂x⁢φe⁢|x=0=∂∂x⁢φe⁢|x=L=0∂∂x⁢φe⁢|x=0=∂∂x⁢φe⁢|x=L=0Electrochemical kineticsjLi=as⁢i0⁢{exp[αc⁢n⁢FR⁢T⁢η]-exp[αc⁢n⁢FR⁢T⁢η]}jL⁢i=as⁢i0⁢n⁡(αa+αc)⁢FR⁢T⁢ηSOCSOC=1δ_⁢∫0δ_⁢(cs,ave-cs,max⁢S⁢t⁢o⁢i1⁢0⁢0)cs,max⁡(S⁢t⁢o⁢i1⁢0⁢0-S⁢t⁢o⁢i0)

Four partial differential equations (PDEs) listed in Table 1 describe physical variables depending on time and location. The physical variables include (1) an ion concentration (cs) in the active material particles derived from Fick's law that is a diffusion law of spherical particles, (2) an ion concentration (ce) in the electrolyte based on the preservation of lithium ions, (3) a potential (φs) in the active material layer derived from the ohm's law, (4) a potential (φe) in the electrolyte calculated using Kirchoffs law and ohm's law, and (5) a B-V equation describing electrochemical kinetics in the reaction interface.

Although accurate calculations of ion behavior during the operation of lithium-ion battery are possible, the full order model (FOM) shown at the left side of Table 1 is limited in its application because of its high computational load. The full order model is simplified to a reduced order model, called Pseudo-2D-ROM, where the electrodes are assumed to be composed of spherical particles with the same radius, which are in contact with each other. The two dimensions imply calculation of ion concentration in the particles and through the plane.

To solve the partial differential equations of the P2D-ROM, several other numerical analysis methods such as finite difference method (FDM), finite-element method (FEM) and finite-volume method (FVM) are available.

The P2D-ROM has two parts, and the calculation of ion concentration in the electrode and the electrolyte may be simplified by applying a polynomial approximation method and a state-space method, respectively. The computation time of the simplified P2D-ROM is reduced to one-sixth of the FOM computation time while the overall model accuracy is maintained. The equation for the simplified P2D-ROM is shown at the right side in Table 1. Hereinafter, unless otherwise stated, the reduced order model (ROM) refers to the simplified P2D-ROM.

A detailed description of the reduced order model for the FOM may be found in X. Li, M. Xiao, and S. Y. Choe. “Reduced order model (ROM) of a pouch type lithium polymer battery based on electrochemical thermal principles for real time applications.” Electrochimica Acta 97 (2013): 66-78. In an embodiment of the present disclosure, model parameters used in the P2D-ROM are listed in Table 2 below. The model parameters are stored in the memory unit130in advance.

TABLE 2model parameterNegativePositiveCategoryParameterelectrodeSeparatorelectrodeDesignThickness, δ (cm)50*10−425.4*10−436.4*10−4specificationsParticle radius, Rs(cm)1*10−41*10−4(geometry andActive material volume ratio, εs0.580.5volumePolymer phase volume ratio, εp0.0480.50.11fractions)Conductive filler volume ratio, εf0.040.06Porosity, εe0.3320.50.33Lithium ionMaximum solid phase temperature,16.1*10−323.9*10−3concentrationcs,max(mol cm−3)Stoichiometry at 0% SOC, Stoi00.1260.936Stoichiometry at 100% SOC, Stoi1000.6760.442Average electrolyte concentration, ce1.2*10−31.2*10−31.2*10−3(mol cm−3)Kinetic andExchange current density coefficient,12.96.28transportki0(A cm−2)propertiesCharge-transfer coefficient, αa, αc0.5, 0.50.5, 0.5Solid phase conductivity, σ (S cm−1)10.1Electrolyte phase Li+diffusion2.6*10−62.6*10−62.6*10−6coefficient, De(cm2s−1)Slid phase Li+diffusion coefficient, Ds,03*10−125.55*10−12(cm2s−1)Activation energy of Ds, Ea,D(J mol−1)4.5*1044.5*104Film resistance of SEI layer, RSEI,01000(Ω cm2)Activation energy of RSEI, Ea,R(J mol−1)3.8*104Bruggeman's posority exponent, p1.51.51.5Electrolyte phase ionic conductivity, κ15.8ce15.8ce(S cm−1)exp(−13472ce1.4)exp(−13472cel.4)Li+transference number, t+00.3630.3630.363EquilibriumNegative electrode (V)U_(x) = 8.00229 + 5.0647x − 12.578x½ − 8.6322 x 10−4x−1+potential2.1765 * 10−5x 3/2 − 0.46016 * exp(15 * (0.06 − x)) − 0.55364 *exp (−2.4326 * (x − 0.92)).where x = cs,surf−/cs,max−Positive electrode (V)The difference between OCV and the equilibrium potential of thenegative electrodeside reactionEquilibrium potential of side reaction, Ueq,side(V)0.4Kinetic rate constant for side reactions, kside(A cm mol−1)3.07*10−6Cathode symmetric factor of side reactions, αc,side0.7

Referring toFIG.1again, the control unit120determine the internal states of the secondary battery, including the average ion concentration (cs,ave) of the anode, the anode potential (φs−), the electrolyte potential (φe−) of the anode, the surface ion e concentration (cs,surf) of the anode and the volume-averaged concentration fluxes (qave) of the anode, by using the equations of the reduced order model defined in the Table 1. Here, cs,ave, φs−, cs,surfand qaveare for the active material particles in solid phase.

The control unit120also determines the state of charge (SOC) of the secondary battery from the average ion concentration (cs,ave) of the anode particles by using the SOC calculation equation in Table 1. For the parameter values included in the SOC calculation equation, the predefined data shown in Table 2 are referred to.

The control unit also determines the side reaction rate (jsideLi) using Equations (3) to (7) from the internal state of the secondary battery20, which is calculated from the reduced order model. For the parameters included in the equation for calculating the side reaction rate (jsideLi), the predefined data shown in Table 2 are referred to.

The control unit120also determines whether a charging current control condition, namely (i) whether the measured voltage value reaches the cutoff voltage, (ii) whether the surface ion concentration of the anode particles reaches its upper limit concentration or (iii) the side reaction rate reaches its upper limit rate, is satisfied.

Also, if any of the charging current control conditions is required, the control unit120reduces the magnitude of the charging current applied to the charging of the secondary battery20. The control unit120transmits a current adjustment message including the reduced charging current information to the charger2through the communication unit140to reduce the magnitude of the charging current applied to the secondary battery20by the charger2. If the charger2receives the current adjustment message from the control unit120, the charger2reduces the magnitude of the charging current with reference to the reduced charging current information included in the message.

For example, as shown inFIG.13, the control unit120refers to the profile defining the magnitude of the charging current according to the SOC, identifies a current, which corresponds to the sum (SOCk+ΔSOC) of the SOC (SOCk) at the present time when the charging current control condition is satisfied and the preset SOC variation amount (ΔSOC), from the profile, and determines the identified current as the reduced charging current.

Also, if the reduced charging current is determined, the control unit120transmits a current adjustment message including the reduced current information to the charger2through the communication unit140. Then, the charger2adjusts the magnitude of the charging current according to the reduced current information and applies the adjusted charging current to the secondary battery20.

Preferably, the current is adjusted repeatedly whenever the above condition is satisfied.

Meanwhile, the control unit120may repeatedly perform time update and measurement update for the internal state of the secondary battery20by using the extended Kalman filter so that the difference between the voltage of the secondary battery20estimated by the ROM and the measured voltage value measured through the voltage measuring means112is minimized The reduced order model combined with the extended Kalman filter is hereinafter referred to as ROM-EKF.

According to an embodiment of the present disclosure, the control unit120may be configured to repeatedly perform time update and measurement update for the internal state of the secondary battery using the extended Kalman filter, in which the state-space equation for the internal states of the secondary battery which includes the average ion concentration (cs,avek) of the anode particles, the volume-averaged concentration fluxes (qavek) of the anode particles and the surface ion concentration (cs,surfk) of the anode particles, and the output equation for the voltage of the secondary battery is defined as below, such that the difference between the estimated voltage (Vt) and the measured voltage value of the secondary battery is minimized

The state-space equation and the output equation are derived from the ROM and are defined using a discrete time model. Δt is the sampling time period in which time update and measurement update are repeated for the internal state of the secondary battery, and k and k−1 are time indexes.

[Discrete Equation]

cs,avek-cs,avek-1-3⁢jLi,k⁢Δ⁢⁢tRs⁢as⁢Fqavek-qavek-1-30⁢Ds⁢Δ⁢⁢tRs2⁢qavek-1-452⁢jLi,k⁢Δ⁢⁢tRs2⁢as⁢Fcz,zwfk=cs,avek+8⁢Rs⁢qavek35-Rs⁢jLi,k35⁢Ds⁢as⁢F
[Discrete State-Space Equation]

[cs,avekqa⁢v⁢ek]=[1001-30⁢Ds⁢Δ⁢tRs2]⁡[cs,avek-1qa⁢v⁢ek-1]+[-3⁢Δ⁢tRs⁢as⁢F-45⁢⁢Δ⁢⁢t2⁢Rs2⁢as⁢F]

(cs,ave:average ion concentration of anode particles, cs,surf: surface ion concentration of anode particles, qave: volume-averaged concentration fluxes of anode particles, jLi: lithium reaction rate, as: specific surface area of an electrode, F: Faraday constant, Ds: diffusion coefficient of anode particles, Rs: radius of spherical electrode particle, Δt: update interval)

[Output Equation]

Vt=φs+-φs--Rfilm⁢Iη=φs-φe+Ue⁢q⁡(cs,surf)+Rfilmas⁢jL⁢i

(Vt: voltage, φs+: solid phase potential of a cathode, φs−: solid phase potential of an anode, φe: electrolyte potential, Ueq:equilibrium potential, η: surface overpotential of an electrode, cs,surf: surface ion concentration of a solid phase particle, Rfilm: ohmic resistance in battery, I: current, A: cell area, RSEI: SEI resistance, as: specific surface area of an electrode, jLi: lithium reaction rate)

The output equation may be re-defined as follows to calculate the Jacobian applied to the extended Kalman filter. Here, it may be understood that the terminal voltage is a function of the surface concentration of solid phase particles. Upand Unare equilibrium potentials of the cathode and the anode.

Vt=η+-η-+φe+-φe-+Uρ⁡(cs,surf+)-Un⁡(cs,surf-)-RSEI⁢Aas⁢jLi-Rfilm⁢I

According to an embodiment of the present disclosure, when repeatedly performing time update and measurement update for the internal state of the secondary battery using the extended Kalman filter, the Jacobian defined by the following equation may be applied.

[Jacobian]

Hk=dVtkdxk=[H1,k⁢⁢H2,k⁢⁢H3,k⁢⁢H4,k]H1,k=-(dUnd⁢⁢cs,surf)k⁢(d⁢⁢cs,surf-d⁢⁢cs,ave-)k=-(dUnd⁢⁢cs,surf-)k,⁢H2,k=-(dUnd⁢⁢cs,surf-)k⁢(d⁢⁢cs,surf-dqme-)k=-(dUnd⁢⁢cs,surf-)k⁢8⁢Rs-35H3,k=(dUpd⁢⁢cs,surf)k⁢(d⁢⁢cs,surf-d⁢⁢cs,ave-)k=(dUpd⁢⁢cs,surf-)k,⁢H4,k=(dUpd⁢⁢cs,surf-)k⁢(d⁢⁢cs,surf-dqme-)k=(dUpd⁢⁢cs,surf-)k⁢8⁢Rs+35

FIG.4is a block diagram showing a reduced order model combined with an extended Kalman filter, namely ROM-EKF. Referring toFIG.4, the SOC is a ratio between the maximum number of ions that can be present in the active material particles and the number of ions that exist at the present. At any moment, the number of ions existing in the active material particles may be calculated based on the average ion concentration in the active material particles.

The dynamic error of the average ion concentration and the error of the SOC given by an initial value may further improved by the closed loop correction using the extended Kalman filter.

The control unit120predicts the internal states (cs,surf−, cs,ave−, qave) of the secondary battery20using the state-space equation of the extended Kalman filter. In addition, the control unit120determines the SOC, namely the state of charge of the secondary battery20, using the SOC calculation equation shown in Table 1 with the average ion concentration (cs,ave−) of the anode particles among the predicted internal states of the secondary battery20. Also, the control unit120determines the solid phase potential (φs−) of the anode particles and the electrolyte potential (φs−) of the anode among the internal states of the secondary battery20using the ROM, and determines the side reaction rate (jsideLi) of the secondary battery20. In addition, the control unit120determines the estimated voltage ({circumflex over (V)}t) of the secondary battery using the output equation of the extended Kalman filter, and corrects the internal states (cs,surf−, cs,ave−, qave) of the secondary battery20according to the difference between the estimated voltage ({circumflex over (V)}t) and the measured voltage (Vt).

If the state-space equation, the output equation and the Jacobian of the extended Kalman filter are defined, the time update and measurement update processes for the internal states of the secondary battery20is automatically executed using an extended Kalman filter algorithm known in the art, and this is not described in detail here.

Hereinafter, a charging method performed by the charging apparatus of a secondary battery above will be described with reference to a flowchart.

FIG.5is a flowchart for illustrating a charging method of a secondary battery according to an embodiment of the present disclosure.

First, in Step S10, the control unit120determines the magnitude of the charging current of the secondary battery20as a maximum value, and transmits a current adjustment message including the maximum charging information to the charger2through the communication unit140to start charging the secondary battery20. Here, the maximum value of current is determined in advance according to the specification of the secondary battery20and refers to the information previously stored in the memory unit130.

If the charger2receives the current adjustment message including the maximum charging information from the control unit120, the charger2applies a charging current corresponding to the maximum charging current to the secondary battery20to start constant current charging.

In Step S20, if charging of the secondary battery20is started, the control unit120predicts the internal states (cs,surf−, cs,ave−, qave) of the secondary battery20using the state-space equation of the extended Kalman filter.

In Step S30, the control unit120determines the state of charge (SOC) of the secondary battery20from the average ion concentration (cs,ave−) of the anode particles using the SOC calculation equation.

In Step S40, the control unit120determines the solid phase potential (φs−) and electrolyte potential (φs−) of the anode particles among the internal states of the secondary battery20using the ROM, and determines the side reaction rate of (jsideLi) of the secondary battery20using Equations (3) to (7).

In Step S50, the control unit120determines the estimated voltage ({circumflex over (V)}t) of the secondary battery using the output equation of the extended Kalman filter, and corrects the internal states (cs,surf−, cs,ave−, qave) of the secondary battery20according to the difference between the estimated voltage ({circumflex over (V)}t) and the measured voltage (Vt).

In Step S60, the control unit120determines whether a charging current control condition, namely (i) whether the measured voltage value (Vt) reaches the cutoff voltage, (ii) whether the surface ion concentration (cs,surf−) of the anode particles reaches its upper limit concentration or (iii) the side reaction rate (jsideLi) reaches its upper limit rate, is satisfied.

In Step S60, if it is determined that the charging current control condition is not satisfied, the control unit120maintains the magnitude of the charging current. On the other hand, if it is determined that the charging current control condition is satisfied, the control unit120proceeds to Step S70.

In Step S70, the control unit120reduces the magnitude of the charging current. Specifically, the control unit120identifies a charging current, which corresponds to the sum (SOCk+ΔSOC) of the SOC (i.e., SOCk) at the present time and the preset SOC variation amount (ΔSOC), with reference to the profile (FIG.13) defining the correlation between the magnitude of the current and the SOC, and determines the identified current as the reduced current. The data related to the profile is stored in the memory unit130in advance, and the magnitude of the charging current decreases as SOC increases as shown inFIG.13.

In Step S80, the control unit120transmits a current adjustment message including the reduced current to the charger2through the communication unit140. Then, the charger2applies a current corresponding to the reduced current to the secondary battery20.

In Step S90, if the charging current corresponding to the reduced current is applied to the secondary battery20, the control unit120predicts the internal states (cs,surf−, cs,ave−, qave) of the secondary battery20using the state-space equation of the extended Kalman filter as in Step S20. In addition, the control unit120determines the state of charge of the secondary battery20as in Step S30. Also, the control unit120determines the solid phase potential (qs−) and electrolyte potential (φs−) of the anode particles using the reduced order model as in Step S40, and determines the side reaction rate (jsideLi) of the secondary battery20using Equations (3) to (7). In addition, the control unit120determines the estimated voltage ({circumflex over (V)}t) of the secondary battery using the output equation of the extended Kalman filter as in Step S50, and corrects the internal states (cs,surf−, cs,ave−, qave) of the secondary battery20according to the difference between the estimated voltage ({circumflex over (V)}t) and the measured voltage (Vt).

In Step S100, the control unit120determines whether a charging current control condition, namely (i) whether the measured voltage value (Vt) reaches the cutoff voltage, (ii) whether the surface ion concentration (cs,surf−) of the anode particles reaches its upper limit concentration or (iii) the side reaction rate (jsideLi) reaches its upper limit rate, is satisfied.

In Step S100, if it is determined that the charging current control condition is not satisfied, the control unit120proceed to Step S110to maintain the magnitude of the charging current. On the other hand, if it is determined that the charging current control condition is satisfied, the control unit120proceeds to Step S70to reduce the magnitude of the charging current.

Meanwhile, after Step S110, the control unit120determines whether the charging stop condition is satisfied in Step S120. The charge stop condition is satisfied when the SOC of the secondary battery reaches a target value. The target value may be set before charging starts. For example, the target value is 100% SOC. In some cases, the target value may be set lower than 100% SOC.

In Step S120, if the charging stop condition is satisfied, the control unit120ends charging of the secondary battery20. However, if the charging stop condition is not satisfied, the control unit120proceeds to Step S90to continue charging. Thus, the magnitude of the charging current is repeatedly reduced whenever the charging current control condition is satisfied, until the charging stop condition is satisfied.

Example

The secondary battery used in the example of the present disclosure is a pouch-type lithium-ion battery having a capacity of 15.7 Ah. The active materials of the anode and the cathode are graphite and NMC (Li[MnNiCo]O2), respectively. The ROM-EKF is validated against the experimental data obtained during charging and discharging of the secondary battery. The magnitude of the charging current is adjusted to 1C, 2C, 3C, 4C, 5C and 6C at room temperature 25° C. Even at a high current, the temperature of the secondary battery is kept constant by a calorimeter designed in the laboratory. By doing so, the effects of the temperature on the charging and discharging characteristics are limited.

[Evaluation of the Terminal Voltage of the Secondary Battery]

FIG.6ashows a simulated terminal voltage and an experimental terminal voltage when the lithium secondary battery is charged, andFIG.6bshows a simulated terminal voltage and an experimental terminal voltage when the lithium secondary battery is discharged. InFIGS.6aand6b, a square/circle/triangle plottings and a solid/dashed line represent simulation data and experimental data, respectively. The results show that the terminal voltage estimated by the ROM-EKF is in a fairly good match with the experimental data.

In addition, tracking performance of the extended Kalman filter for estimation of SOC is tested for two cases, and their results are shown inFIGS.7aand7b.FIG.7ais a graph showing the change of SOC according to the magnitude of charging current and time, andFIG.7bis a graph showing the tracking performance of the extended Kalman filter when there is an initial SOC error. As shown in the figures, in one case, there is an initial SOC error, and in the other case, there is no initial SOC error. If no initial error is present, the ROM-EKF may estimate the SOC with an absolute error that is less than 5%. Also, even with 20% initial SOC error, the ROM-EKF is capable of tracking the SOC within 100 ms.

Meanwhile, even though there is a little bit of overshoot, this overshoot may be optimized by proper selection of the error covariance matrices of the EKF.

[Effects of CC/CV Charging on the Charging Time]

The CC/CV charging method is the simplest and widely used charging method. However, the increasing of charging current alone cannot significantly reduce the charging time because of extended duration of the CV charging mode. In addition, the increased charging current accelerates the degradation of the secondary battery. Thus, before proposing a new charging method, effects of CC/CV charging on the charging time and degradation, specifically side reactions, are firstly analyzed.

The charging time up to 100% SOC is determined by two factors. One factor is the magnitude of current applied during the constant current charging mode, and the other factor is the cutoff voltage in the constant voltage charging mode. Effects of the magnitude of the charging current on the SOC and the charging time in CC mode were studied experimentally using a pouch-type battery. The results are shown inFIGS.8aand8b, where the cutoff voltage was set to be 4.15 V. As shown inFIG.8a, a high charging current reduces the charging time, but the terminal voltage reaches the cutoff voltage of 4.15V even at a lower SOC. Thus, the maximum SOC to be charged at a given charging current during the CC mode is limited. Also, as shown inFIG.8b, the relationship between the charging current and the maximum chargeable SOC is inversely proportional to the charging current. That is, the higher the charging current, the lower the maximum chargeable SOC.

Likewise, effects of the CC charging combined with the CV mode on the charging time were also studied in another aspect. Here, a pouch-type battery is charged from 0% to 100% SOC. The charging current was varied from 0.5 C to 7 C and the cutoff voltage was set to be 4.15V. The charging time as a function of charging current is shown inFIG.9a, and the ratio between the charging time by CV charging and the charging time by CC/CV charging is depicted inFIG.9b. Referring toFIGS.9aand9b, if the charging current is less than 1.5C, the charging time is significantly reduced even with a slight increase of charging current. Also, the charging time is still reduced meaningfully even when the charging current is between 1.5C and 4.5 C. However, if the charging current rises over 4.5C, the charging time is not reduced meaningfully even though the charging current increases. In addition, as shown inFIG.9b, the ratio of charging time between the CV charging method and the CC/CV charging method increases with increasing of the charging current. This is because the higher the charging current is, the shorter does it take to reach the cutoff voltage even at a low SOC. Consequently, the charging time in the CV mode takes longer, which leads to a long overall charging time.

[Effects of CC/CV Charging on Side Reactions]

Effects of CC/CV charging on side reactions are investigated, where the SOC range and the charging current are varied. As discussed above, ion concentration heavily affects side reactions. The surface ion concentration of active material particles is estimated using the ROM-EKF, and its results are shown inFIG.10a. InFIG.10a, the x axis represents the coordinate in the direction of a through-plane of the anode. Each curve represents the surface ion concentration of different anode particles at a specific time. At the beginning of charging, the concentrations are uniformly distributed in the electrode (‘0s’ curve). As more ions are transported from the cathode, the ion concentration forms a high gradient gradually, reaches the maximum value after several minutes and then becomes less and finally zero after around 2200s. The ion concentration at the interface between the active material layer of the anode and the separator at 301s (at the end of CC charging) becomes higher than that at 2200s (at the end of CV charging). In addition, the value of surface ion concentration of a particle is dependent upon the location of the particle due to the limitation of diffusion rate and the ion concentration gradient in the electrolyte of the anode. The closer the particle to the separator is, the higher the surface ion concentration is.

The surface ion concentration of the particle just next to the separator in time domain is depicted inFIG.10b. An overshoot of the ion concentration is observed while the charging mode is changed until an equilibrium is reached, when the charging current is larger than 4 C. The overshoot is decreased in CV mode simply because of the decreased charging current. At the steady state when SOC reaches 100%, the ion concentration converges to a vicinity of a value. This implies that the anode particles cannot accept more lithium ions and the lithium ion concentration reaches saturation. The ion concentration value is 0.035 mol/cm3at SOC=100%, and thus the value is chosen as the saturated concentration, c*s, corresponding to an upper limit of the surface ion concentration of the anode particle in the charging method according to the present disclosure.

The high ion concentration caused by the overshoot leads to a low equilibrium potential that increases the magnitude of activation overpotential for side reactions and consequently promotes the side reactions. In addition, the excessive ions also increase the exchange current density of side reactions, i0,side.

Actually, the side reaction rate may be calculated using the B-V equation, as in Equation (3). The amount of ion loss, Cionloss, that represents the consumed ions by the side reactions is the same as the integration of side reaction rate, jsideLi, over the volume of the anode active material and the time as in Equation (8).

Cionloss⁡(τ)=∫x=0δn⁢(∫t=0τ⁢jsideLi⁡(l,t)⁢dt)⁢Adl(8)

Here, Cionlosshas a unit of Ah, δnis the thickness of the anode active material, τ is the total operating time, and A is the cross-section area of the lithium-ion battery.

In fact, it is obvious that the side reaction rate is dominantly affected by the overpotential in the B-V equation. The overpotential is the function of the charging current and the range of SOC, which is calculated at a cutoff voltage of 4.15V as shown inFIG.11a. In addition, the side reaction rate over time and the amount of ion loss versus SOC are shown inFIGS.11b,11cand11d. The magnitude of the overpotential increases with the increasing charging current until the terminal voltage reaches the cutoff voltage and then decreases in the CV mode. Accordingly, the side reaction rate tends to follow the shape of the overpotential, and the amount of consumed ions calculated by Equation (8) increases at high current. According to the calculation of the ion loss as a function of SOC, the amount of ion loss is relatively negligible in a low SOC range, but increases as the SOC increases.

When the SOC is less than 40%, a high charging current increases the side reactions but reduces the charging time. In addition, the relationship between the charging time and the side reaction rate is almost linear. Therefore, the contribution of the high charging current on degradation is not significant based on Equation (8) andFIG.11d. In fact, the increased charging current at the low SOC range does not significantly cause more ion losses in comparison to other ranges, but may contribute to reduce the charging time. This is valid only at a constant cell temperature. The side reaction rate becomes higher at elevated temperature.

In the middle SOC range, the relationship between the charging time and the magnitude of overpotential of side reaction becomes nonlinear, and the concentration overshoot appears in this SOC range, both of which accelerate the side reactions. Therefore, as the SOC increases, the relationship between the charging time and the side reaction rate becomes nonlinear, and the magnitude of the slope increases with the increase of charging current. As a result, a high charging current largely accelerates the battery degradation, as shown inFIG.11e.

In a high SOC range, the side reaction rate is very lower than that of the middle SOC range because of the continuously reduced charging current in the CV mode. However, the charging time takes longer than those in other SOC ranges. In addition, the equilibrium potential becomes lower because of the high ion concentration, and the overpotential gets higher, which causes more loss of ions. In this SOC range, the charging current still has a significant effect on lithium ion loss because of a longer charging time in the high SOC range and a higher side reaction rate caused by the higher ion concentration, as shown inFIG.11f.

[Design of the New Fast Charging Method]

The new charging method is designed based on the ROM-EKF that provides variables like a surface ion concentration of the anode particles and an anode potential. The variables are used to estimate the SOC and the side reaction rate. In order to activate the cutoff voltage, the terminal voltage is measured. A block diagram of the proposed fast charging method is depicted inFIG.12. The inputs for the ROM-EKF are the charging current, the terminal voltage and the temperature of the secondary battery. Once reference values for a requested SOC, cutoff voltage, an upper limit of the surface ion concentration in the anode and an upper limit of the side reaction rate in the anode are given, a charging protocol is generated by comparing the values with those of the estimated and measured. The charging protocol is used to control the charger to generate charging currents. The charging protocol includes a reduction schedule for the magnitude of the charging current. According to the charging protocol, a current reduction request is transmitted to the charger, and the charger reduces the magnitude of the charging current accordingly.

When a battery is being charged, the requested SOC (a target SOC) is one of conditions that stop charging. Meanwhile, other reference values are used to set the upper limitations that adjust the charging current to prevent from inducing degradation. A flowchart for the charging protocol according to the present disclosure is shown inFIG.5.

At the beginning, a maximum charging current is applied until one of the three reference variables reaches its upper limitation. Upon reaching the limitation, the charging current is reduced and kept as constant until the SOC is changed according to a predefined amount ΔSOC. This charging protocol is repeated until the conditions of stop charging are fulfilled.

As an example, experimental data obtained by measuring the change of the charging current according to the SOC in a state where the cutoff voltage of 4.15V is set is shown inFIG.13. InFIG.13, the circles present the experimental data. Firstly, the requested SOC is determined as one of conditions that stop charging. At the beginning of charging, the battery is charged with a maximum current of 7.6 C. The maximum current is the magnitude of a maximum charging current provided by the manufacturer. Once the terminal voltage reaches the cutoff voltage, the charging current is reduced to a lower level according to the given ΔSOC as shown inFIG.13.

The charging protocol may be optimized by considering other limitations that prevent degradation. The first limitation is the cutoff voltage. The manufacturer of the battery recommends 4.3V at the maximum charging current instead of 4.15V as the cutoff voltage, and the increasing effect of the cutoff voltage will be explained later. The second limitation is the calculated maximum surface ion concentration of the anode particle as explained above. The final limitation is the maximum side reaction rate selected at 40% SOC based on the result of analysis as shown inFIG.11d. The lithium ion loss does not significantly increase at 40% SOC. Under consideration of the four limitations, the SOC as a function of the charging current using the ROM-EKF with a side reaction rate according to the present disclosure is simulated and depicted inFIG.14. The results shown inFIG.14provide an important guideline how the charging current at different SOC should be determined for an optimal charging protocol that reduces the charging time and at the same time alleviates degradation.

Under the consideration of the limitations, several possible protocols are designed by combining the different limitations listed in Table 3, and each protocol is simulated using the ROM-EKF. As shown in the simulation results ofFIG.14, the charging current is limited as the SOC increases. At a low SOC range, an upper limit of the side reaction rate primarily limits the charging current at the first, and then the cutoff voltage of 4.15V is applied up to a middle range of SOC and continuously up to 100% SOC. In the CV mode with the cutoff voltage of 4.15V in a high SOC range, the surface ion concentration exceeds the upper limit and an overshoot occurs. Thus, the limitations are divided into two regions. In the region I, an upper limit of the surface ion concentration, c*s, is the first limitation that should prevent the overshoot of the surface ion concentration. In the region II, other three limitations are used to limit the charging current. Since the overshoot of the surface ion concentration of particles is caused by the mismatch of ions between transported and diffused, adding extra resting periods helps reduce the numbers of ions transported and give the ions in particles extra time to diffuse and to be intercalated into the lattice structure. Therefore, the duration of the resting is determined by considering the gradient of ion concentration in the anode active material. On the other hand, high charging currents larger than 5C may make the anode potential negative even at low SOC, which creates favorite conditions for lithium plating. Thus, 5C is preferably selected as the highest charging current even though the manufacturer recommends 7.6 C.

As an example, the simulated results of the charging protocol considering the upper limits of the side reaction rate, jside,maxLi, and the surface ion concentration, c*s, are shown inFIGS.15aand15d.FIGS.15aand15dinclude the simulation results of the current, terminal voltage, surface ion concentration and the side reaction rate, respectively. The surface ion concentration is not larger than the maximum allowed saturation concentration. Also, the side reaction rate is also limited up to a middle SOC range until the CV mode becomes active.

Five charging protocols are simulated and the resulting charging times are summarized in Table 3. Here, the two classical charging protocols, namely a charging protocol where CV charging is performed by the cutoff voltage of 4.15V after 1 C-CC charging and a charging protocol where CV charging is performed by the cutoff voltage of 4.15V after 5 C-CC charging, are compared. The charging time of the 1 C-CC/CV protocol takes about 71 min to fully charge the battery from 0% to 100% SOC. The FC-4.3V charging protocol and the FC-4.15V charging protocol using the upper limit of the surface ion concentration in the anode as the limitation and considering a cutoff voltage of 4.3V and 4.15V reduce the charging time to 44% and 52% of that by 1 C-CC/CV charging protocol, respectively. Here, the increased cutoff voltage has contributed to reduce the charging time.

TABLE 3Charging protocolCC/CV (1 C)CC/CV (5 C)FC-4.3 VFC-4.15 VFC-SRLimitation4.15 V4.15 V4.3 V, c*s4.15 V, c*sSide reactions, c*sCharging time71 min38 min31.5 min37.5 min40 min

The charging time by the FC-4.15V charging protocol is comparable to that of the 5 C-CC/CV charging protocol. The FC-4.15V charging protocol considers the cutoff voltage and the surface ion concentration as the limitations, and the charging time takes longer than that of the FC-4.3V charging protocol. This is because the FC-4.3V charging protocol reduces the time during the CC mode.

In addition, the simulation results of side reaction rate and consumed lithium ions of four charging protocols are shown inFIGS.16aand16b. InFIG.16a, the area enclosed by the side reaction rate represents the total consumed lithium ions. As shown inFIGS.16aand16b, when the cutoff voltage increases, the CC charging periods become extended, but the magnitude of side reaction rate becomes higher and the duration of the CC charging period takes longer. Consequently, the consumed lithium ion loss become increased. If the side reaction rate is further limited, the area becomes smaller and the ion loss becomes significantly reduced. However, the charging time takes longer.

In the present disclosure, the different charging protocols are implemented and experimentally evaluated using BIL (Battery-In-the-Loop) that facilitates to operate a test station with the designed controls in real time. The block diagram for the BIL is shown inFIG.17. The test station is designed to charge and discharge the secondary battery using a DC power supply and an electronic load that are connected in parallel to the secondary battery. The DC power supply and the electronic load are controlled by LabVIEW embedded in a PC. In addition, the secondary battery is placed in a designed calorimeter that dynamically rejects the heat generated by the battery. The calorimeter includes two thermal electric modules (TEMs), a bipolar power supply and a control algorithm. The control algorithm determines magnitude and direction of the current flowing into the TEMs. The TEMs have both cooling and heating functions and regulate the surface temperature of battery at set values. The maximum temperature variation becomes less than 1° C. even at a large charging current of 120 A. Thus, the calorimeter allows for minimization of the effects of the temperature on degradation of the battery.

The proposed charging method is implemented in the test station by integrating the ROM-EKF into the LabVIEW using a MATLAB script. The ROM-EKF facilitates estimation of the internal variables like SOC, surface ion concentrations and side reaction rate based on the current and the terminal voltage of the battery. The estimated internal variables are used to constrain the charging current and generate the charging protocol upon the requested SOC.

The secondary battery used for the experiments is a pouch-type large format lithium-ion battery, whose dimension is about 200 mm×150 mm×5 mm The capacity of the secondary battery is 15.7 Ah and the operating voltage is in the range of 2.5V to 4.15V.

After the implementation of the ROM-EKF in the test station, different charging methods are tested at the same test conditions. The test conditions are also used for the simulations. The experiment is repeated until 100 cycles. In each cycle, the lithium-ion battery is charged up to 100% SOC and then discharged with 1C current to a minimum cutoff voltage. The charging times of the five charging protocols in different SOC ranges are summarized inFIG.18a. The measured charging times are almost the same as those by the simulations. Compared with the normally recommended 1 C-CC/CV charging protocol, the other protocols may reduce the charging time by more than half in the low and middle SOC ranges. However, in the high SOC range, the designed charging methods may not reduce the charging time further. The charging time of the designed charging protocols are almost the same as that in the low SOC range but different in the middle SOC range because different limitations are applied. The FC-SR protocol that limits side reactions takes longer charging time than others by other limitations.

The capacity of the battery is measured after every 10 cycles using the 1 C-CC/CV charging and discharging method. A dimensionless capacity, Q+, is defined as the capacity of the aged cell over that of the fresh cell as in Equation (9) below.

Q*=Qa⁢g⁢e⁢dQf⁢r⁢e⁢s⁢h(9)

The dimensionless capacities of the charging protocols are shown inFIG.18b. The comparison between the FC-4.3V charging protocol and the FC-4.15V charging protocol shows that an increase of the cutoff voltage accelerates the aging speed. In addition, the limitation of the charging current by the surface ion concentration helps prevent the capacity fade. This is proved by the comparison between the FC-4.15V charging protocol and the 5 C-CC/CV charging protocol. The capacity fade by the FC-SR charging protocol is at least closest to the capacity fade by the 1 C-CC/CV charging protocol. However, the degradation speed of the FC-SR protocol is still larger than the degradation speed of the 1 C-CC/CV charging protocol. This can be explained by several possible reasons. Firstly, the lithium ion loss by the FC-SR protocol is slightly larger than that by the lithium ion loss by the 1 C-CC/CV charging protocol in the low SOC range. Secondly, the internal temperature of the battery by the FC-SR charging protocol is slightly higher than that by 1 C-CC/CV charging protocol because of the heat generated from the battery although the surface temperature of the battery is kept constant by the calorimeter.

In addition, the impedances at different charging protocols measured by the EIS are shown inFIGS.19aand19b. InFIG.19a, the left intercept between the impedance spectrum and the x-axis at high frequency represents an ohmic resistance. Also, the radius of the first semicircle represents an SEI resistance. Both of the ohmic resistance and the SEI resistance are extracted using an EIS equivalent circuit model. The growth of both resistances is directly related to power fade. The ohmic resistance of different charging protocols is almost the same as the ohmic resistance of a fresh cell. This implies that the side reactions do not contribute to a growth of the ohmic resistance. The growth of the SEI resistance after 100 cycles is dependent upon charging protocols. As shown inFIG.19b, the SEI resistance of the battery to which the FC-SR charging protocol is applied is comparable to the SEI resistance to which the 1 C-CC/CV charging protocol is applied.

In the above, an optimization of a charging method is proposed that takes charging time and degradation of the secondary battery into account. Effects of charging currents, cutoff voltages and internal variables on degradation are identified and analyzed. In order to find out an optimal charging protocol, the two internal variables, i.e., surface ion concentration and side reaction rate, are estimated by using a ROM-EKF. The upper limits of the surface ion concentration and the side reaction rate are used to limit the charging currents. The method of the present disclosure is implemented in a BIL and tested for 100 cycles, which verifies at least the battery capacity and the power fade.

Effects of different magnitudes of charging current on the charging time and the side reactions in CC/CV charging are different dependent upon SOC ranges. In a low SOC range, a high charging current increases the side reaction but reduces the charging time. Thus, the contribution of the magnitude of the charging current on the aging speed is not significant. In middle and high SOC ranges, the charging current has a significant influence on the aging speed.

The proposed charging method is designed using the ROM-EKF with a side reaction rate model, where cutoff voltage, saturation of ion concentration and maximum side reaction rate are used to limit the charging currents. The method of the present disclosure reduces about half of the charging time compared with the normal 1 C-CC/CV charging method. Increased cutoff voltage decreases the charging time, however increases the capacity and power fade. The limitation by the surface ion concentration helps prevent the capacity and power fade. The charging method limited by the surface ion concentration and the side reaction rate is the best one among others with respect to the charging time and the degradation.

The charging apparatus of a secondary battery according to an embodiment of the present disclosure may be included in an electric-driven apparatus. The electric-driven apparatus includes various devices receiving power from a secondary battery pack, such as smart phones, tablet PCs, laptop computers, electric vehicles, hybrid vehicles, plug hybrid vehicles, electric bicycles, drones, power storage devices, uninterruptible power supplies, and the like.

In addition, the charging apparatus of a secondary battery according to the present disclosure may be included in a secondary battery management system for controlling charging and discharging of the secondary battery.

In the description of the various exemplary embodiments of the present disclosure, it should be understood that the element referred to as ‘unit’ is distinguished functionally rather than physically. Therefore, each element may be selectively integrated with other elements or each element may be divided into sub-elements for effective implementation control logic(s). However, it is obvious to those skilled in the art that, if functional identity can be acknowledged for the integrated or divided elements, the integrated or divided elements fall within the scope of the present disclosure.

The present disclosure has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description.

INDUSTRIAL APPLICABILITY

The charging apparatus and method of a secondary battery according to the present disclosure is designed using a ROM with a side reaction rate model, where the cutoff voltage, the saturation of surface ion concentration in the anode, and the maximum side reaction rate are used to limit the charging currents. The method of the present disclosure reduces about half of the charging time compared with the normal 1 C-CC/CV charging method. The limitation by the surface ion concentration in the anode helps prevent the capacity and power fade. The charging method limited by the surface ion concentration and the side reaction rate is the best one among the tested charging methods with respect to the charging time and the degradation.