System and method for pressure determination in a Li-ion battery

An electrochemical battery system in one embodiment includes a first electrode, a second electrode spaced apart from the first electrode, a separator positioned between the first electrode and the second electrode, an active material within the second electrode, a pressure sensor in fluid connection with the second electrode, a memory in which command instructions are stored, and a processor configured to execute the command instructions to obtain a pressure signal from the pressure sensor associated with the pressure within the second electrode, and to identify a state of charge of the electrochemical battery system based upon the pressure signal.

Cross-reference is made to U.S. Utility patent application Ser. No. 12/437,576 entitled “Li-ion Battery with Selective Moderating Material” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,592 entitled “Li-ion Battery with Blended Electrode” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,606 entitled “Li-ion Battery with Variable Volume Reservoir” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,622 entitled “Li-ion Battery with Over-charge/Over-discharge Failsafe” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,745 entitled “Li-ion Battery with Load Leveler” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,774 entitled “Li-ion Battery with Anode Coating” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,791 entitled “Li-ion Battery with Anode Expansion Area” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,822 entitled “Li-ion Battery with Porous Silicon Anode” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,873 entitled “Li-ion Battery with Rigid Anode Framework” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/463,024 entitled “System and Method for Charging and Discharging a Li-ion Battery” by Nalin Chaturvedi et al., which was filed on May 8, 2009; and U.S. Utility patent application Ser. No. 12/463,092 entitled “System and Method for Charging and Discharging a Li-ion Battery Pack” by Nalin Chaturvedi et al., which was filed on May 8, 2009, the entirety of each of which is incorporated herein by reference. The principles of the present invention may be combined with features disclosed in those patent applications.

FIELD OF THE INVENTION

This invention relates to batteries and more particularly to lithium-ion batteries.

BACKGROUND

Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.

When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials including BiF3(303 mAh/g, lithiated) and FeF3(712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira,Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).

Lithium/sulfur (Li/S) batteries are particularly attractive because of the balance between high specific energy (i.e., >350 Wh/kg has been demonstrated), rate capability, and cycle life (>50 cycles). Only lithium/air batteries have a higher theoretical specific energy. Lithium/air batteries, however, have very limited rechargeability and are still considered primary batteries.

A drawback that is common to many lithium ion batteries results from the fact that the chemistries incorporate phase-change materials that exhibit voltage plateaus dependent upon the particular cell chemistry, resulting in a very flat open-circuit potential (OCP) over the normal operating voltage of the cell. Battery state of charge (SOC), however, is typically estimated using a combination of two techniques: coulomb counting and OCP measurement.

Coulomb counting involves integrating the current that is passed to or from the cell to calculate the change in the cell's capacity. Errors in current measurement render this technique inaccurate over time, while side reactions in the cell lead to further deviations between the estimated and actual SOC. By measuring or estimating the OCP, or rest potential, of the cell, one may use OCP-SOC functional relationships to extract the SOC. The coulomb-counting technique tends to be more accurate at short times or when the current is high, while the OCP technique does better when the cell is at rest or the current is low. The two techniques of SOC estimation are typically combined in a number of different ways to obtain the most accurate estimate of SOC possible at all times.

Thus, flat or shallowly sloping OCPs, while providing some advantages, make accurate SOC estimation very difficult. Accordingly, for cells with a flat (or shallowly sloping) OCP, the OCP-SOC correlation technique does not provide the desired accuracy in determination of the cell SOC. Since coulomb counting alone is inherently inaccurate, a need exists for alternative SOC estimation techniques for lithium ion batteries.

What is needed therefore is a battery system and method that provides the advantages of chemistries which exhibit a flat or shallowly sloping OCP while providing a more accurate SOC determination. A system which could also be used to provide an indication of overpressure conditions in a cell would be beneficial.

SUMMARY

An electrochemical battery system in one embodiment includes a first electrode, a second electrode spaced apart from the first electrode, a separator positioned between the first electrode and the second electrode, an active material within the second electrode, a pressure sensor in fluid connection with the second electrode, a memory in which command instructions are stored, and a processor configured to execute the command instructions to obtain a pressure signal from the pressure sensor associated with the pressure within the second electrode, and to identify a state of charge of the electrochemical battery system based upon the pressure signal.

In accordance with another embodiment, a method of determining the state of charge of an electrochemical cell includes storing data indicative of the relationship between a range of pressures in an electrochemical cell and a range of states of charge for the electrochemical cell in a memory, generating a signal associated with the pressure within the electrochemical cell, receiving the signal associated with the pressure within the electrochemical cell; and identifying a state of charge of the electrochemical cell based upon the received signal and the stored data.

DESCRIPTION

FIG. 1depicts a lithium-ion battery system100including a lithium ion cell102, a memory104, and a processor106. Various command instructions, discussed in further detail below, are programmed into the memory104. The processor106is operable to execute the command instructions programmed into the memory104.

The lithium ion cell102includes a negative electrode108, a positive electrode110, and a separator region112between the negative electrode108and the positive electrode110. A pressure sensor114is in fluid communication with the positive electrode110. The negative electrode108includes active materials116into which lithium can be inserted, inert materials118, electrolyte120and a current collector122.

The negative electrode108may be provided in various alternative forms. The negative electrode108may incorporate dense Li metal or a conventional porous composite electrode (e.g., graphite particles mixed with binder). Incorporation of Li metal is desired since the Li metal affords a higher specific energy than graphite.

The separator region114includes an electrolyte with a lithium cation and serves as a physical and electrical barrier between the negative electrode108and the positive electrode110so that the electrodes are not electronically connected within the cell102while allowing transfer of lithium ions between the negative electrode108and the positive electrode110.

The positive electrode110includes active material126into which lithium can be inserted, a conducting material128, fluid130, and a current collector132. The active material126includes a form of sulfur and may be entirely sulfur. The conducting material128conducts both electrons and lithium ions and is well connected to the separator112, the active material126, and the collector132. In alternative embodiments, separate material may be provided to provide the electrical and lithium ion conduction. The fluid130, which may be a liquid or a gas, is relatively inert with respect to the other components of the positive electrode110. Gas which may be used includes argon or nitrogen. The fluid130fills the interstitial spaces between the active material126and the conducting material128. The fluid130is in fluid communication with the pressure sensor114.

The lithium-ion cell102operates in a manner similar to the lithium-ion battery cell disclosed in U.S. patent application Ser. No. 11/477,404, filed on Jun. 28, 2006, the contents of which are herein incorporated in their entirety by reference. In general, electrons are generated at the negative electrode108during discharging and an equal amount of electrons are consumed at the positive electrode110as lithium and electrons move in the direction of the arrow142ofFIG. 1.

In the ideal discharging of the cell102, the electrons are generated at the negative electrode108because there is extraction via oxidation of lithium ions from the active material116of the negative electrode108, and the electrons are consumed at the positive electrode110because there is reduction of lithium ions into the active material126of the positive electrode110. During discharging, the reactions are reversed, with lithium and electrons moving in the direction of the arrow144.

As lithium is reduced into the active material126, the volume of the active material126increases. This is depicted inFIG. 2by the increased size of the individual particles of active material126compared to the size of the individual particles of active material126in theFIG. 1. In the case of a Li/S battery, for example, the sulfur active material increases in volume by 80% as it becomes lithiated during battery discharge.

As the volume of the active material126increases, the pressure within the positive electrode110increases. The pressure within the electrode110is thus inversely related to the SOC of the electrochemical cell102. The pressure in the electrode110is sensed by the pressure sensor114and a signal indicative of the pressure is passed to the processor106. The pressure signal may be used to monitor the cell102for overpressure conditions which may occur if the cell is overcharged or over-discharged. The pressure signal may further be used to obtain an indication of the SOC of the cell102.

In one embodiment, the processor106executes command instructions stored within the memory106in accordance with a procedure150ofFIG. 3to identify the SOC of the cell102. At block152, the battery cell102is characterized. Characterization of the battery cell102identifies the relationship between the pressure in the electrode110and the state of charge of the cell102. At block154, the battery system100is used to supply power to a load, and recharged according to system procedures. If desired, a coulomb counter may be used to monitor the current flow into and out of the electrode110from the load/voltage supply. The coulomb counter may be an ammeter with an integration circuit and/or processor for integrating the current flow. The SOC of the battery system100may be estimated during operations based upon the coulomb counter.

At block156, the processor106obtains a signal from the pressure sensor114indicative of the pressure within the electrode110. The processor then correlates the obtained signal with the characterization data stored in the memory104at block158and determines the associated state of charge at block160.

The procedure150may be modified for various applications. By way of example, throughout the procedure150, the electrochemical cell102may continue to be used to provide a current or to be charged. If the cell102is being charged or discharged, the temperature102of the cell may be different from the temperature of the cell102at the time that the characterization data for the cell102was obtained at block152. Temperature variations may affect the pressure within the electrode110. Accordingly, a thermometer thermally connected to the electrode102or the fluid130may be provided. In such embodiments, the effect of temperature on the pressure within the electrode110may be characterized and the characterization data stored within the memory104. Accordingly, the identification of the cell state of charge may be corrected for the temperature of the electrode110at the time that the pressure signal was obtained from the pressure sensor114.

The procedure150may be performed for each electrochemical cell within a battery system to identify the SOC of the system without any interruption of battery system operation. Characterization of the relationship between the sensed pressure and actual SOC of the electrochemical cells may be updated periodically. By way of example, the pressure may be monitored during a procedure that identifies the capacity of the cell or during cell balancing, complete charge, or complete discharge of all individual cells.

In addition to battery systems such as the battery system100ofFIG. 1wherein the pressure within the electrode110is substantially isolated from the pressure within the electrode108, the procedure150may be used in a system wherein the electrodes are subjected to a common pressure. In such systems, accuracy of the SOC determination is optimized as the difference in volume change of the active materials is maximized.

In further embodiments, the pressure in the negative electrode may be used to identify the SOC of the electrochemical cell. By way of example, LiSi, typically used as an anode material, exhibits a large increase in volume during operation. In systems wherein the pressure within the electrodes is isolated from the pressure of other electrodes, multiple pressure sensors may be provided, each pressure sensor used in a different electrode.

In some embodiments, the pressure variation of an electrode may be reduced by provision of an expandable membrane such as in a variable volume reservoir. In such embodiments, strain sensors may be used in conjunction with the membrane to monitor volume changes of the variable volume reservoir, which is directly related to pressure changes in the electrode.