Patent Publication Number: US-2022238932-A1

Title: Electrochemical cell monitoring assembly

Description:
INTRODUCTION 
     The present invention relates to a monitoring assembly and, more particularly, to a monitoring assembly for electrochemical cells of secondary lithium batteries. 
     A battery is a device that converts chemical energy into electrical energy by means of electrochemical reduction-oxidation (redox) reactions. In secondary or rechargeable batteries, these electrochemical reactions are reversible, which allows the batteries to undergo multiple charging and discharge cycles. 
     Secondary lithium batteries generally comprise one or more electrochemical cells that operate by reversibly passing lithium ions between a negative electrode and a positive electrode. The negative and positive electrodes are oftentimes disposed on opposite sides of a porous polymeric separator, and each of the negative and positive electrodes is typically carried on, or connected to, a metallic current collector. The negative and positive electrodes and the polymeric separator are infiltrated with an ionically conductive electrolyte that provides a medium for the conduction of lithium ions through the electrochemical cell between the negative and positive electrodes. In practice, an electrochemical potential is established between the negative and positive electrodes of the electrochemical cell by connecting their respective current collectors to one another via a controllable and interruptible external circuit. 
     During discharge of the electrochemical cell, the electrochemical potential established between the negative and positive electrodes drives spontaneous redox reactions within the electrochemical cell and the release of lithium ions and electrons at the negative electrode. The released lithium ions travel from the negative electrode to the positive electrode through the ionically conductive electrolyte, and the electrons travel from the negative electrode to the positive electrode via the external circuit, which generates an electric current. After the negative electrode has been partially or fully depleted of lithium, the electrochemical cell may be recharged by connecting the negative and positive electrodes to an external power source, which drives nonspontaneous redox reactions within the electrochemical cell and the release of the lithium ions and the electrons from the positive electrode. During recharge, the released lithium ions return to the negative electrode and may be plated on a surface of the negative electrode (e.g., on a surface of the negative electrode current collector), inserted or intercalated between lattice structures of the negative electrode material, or the lithium ions may react (e.g., alloy) with the negative electrode material to form lithium-containing intermetallic phases. 
     After repeated discharge and recharge cycles, the physical structure of various components of the electrochemical cell (e.g., the negative and positive electrodes and the porous polymeric separator) may be altered. In some instances, it may be desirable to diagnose and/or monitor changes in the physical structure of these components during operation of the electrochemical cell over time. 
     SUMMARY 
     A monitoring assembly for an electrochemical cell of a secondary lithium battery is disclosed. The monitoring assembly may comprise a porous sensory structure and a transducer. The porous sensory structure may be disposed between a major surface of a porous separator and a facing surface of a negative electrode layer. The porous sensory structure may include a sensory layer and a buffer layer. The sensory layer may be disposed on the major surface of the porous separator and may include an electrically conductive material. The sensory layer may be configured to produce a response to an input signal or to a physical stimulus. The buffer layer may be disposed between the sensory layer and the facing surface of the negative electrode layer. The buffer layer may be made of an electrically insulating material that electrically isolates the sensory layer from the facing surface of the negative electrode layer. The transducer may be configured to process the response produced by the sensory layer to generate an output signal indicative of a diagnostic condition of the porous separator or the negative electrode layer. 
     The electrically conductive material of the sensory layer may include particles of at least one of nickel, copper, titanium, silver, gold, or carbon. 
     The electrically insulating material of the buffer layer may include at least one of Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 , ZnO, Ta 2 O 5 , La 2 O 5 , HfO, lithium-lanthanum-zirconium oxide, a zeolite, polytetrafluoroethylene, aluminum alkoxide, zirconium alkoxide, titanium alkoxide, or diamond-like carbon. 
     The transducer may be configured to process the response produced by the sensory layer into an electrical signal, an electromagnetic signal, or a mechanical signal. 
     The transducer may be an antenna or a resonator. In such case, the transducer may be integrated into the sensory layer by patterning the electrically conductive material of the sensory layer into the form of the antenna or the resonator. 
     The transducer may be physically separate from the sensory layer. In such case, the transducer may be electrically coupled to the sensory layer via an electrical circuit. 
     The output signal generated by the transducer may be indicative of formation of a lithium dendrite that extends from the facing surface of the negative electrode layer and penetrates the sensory layer. 
     The porous sensory structure may include a first sensory layer disposed on the major surface of the porous separator, a first buffer layer overlying the first sensory layer, a second sensory layer overlying the first buffer layer, and a second buffer layer overlying the second sensory layer on the major surface of the porous separator. In such case, the output signal generated by the transducer may be indicative of formation of a lithium dendrite that extends from the facing surface of the negative electrode layer and penetrates the first sensory layer. The output signal generated by the transducer also may be indicative of a growth rate of the lithium dendrite. 
     The output signal generated by the transducer may be indicative of an electrical resistance of the sensory layer. In such case, the output signal generated by the transducer may be indicative of a compressive force or a tensile force applied to the sensory layer that results in physical deformation of the sensory layer. 
     The monitoring assembly may comprise a microcontroller configured to process the output signal generated by the transducer into data and to store the data in local memory as a function of time. In such case, the microcontroller may be electrically coupled to the sensory layer via an electrical circuit. 
     An electrochemical cell of a secondary lithium battery is disclosed. The electrochemical cell may comprise a positive electrode layer electrically coupled to a positive electrode current collector, a negative electrode layer spaced apart from the positive electrode layer and electrically coupled to a negative electrode current collector, a porous separator disposed between the positive electrode layer and the negative electrode layer, and a monitoring assembly. The porous separator may be disposed between the positive electrode layer and the negative electrode layer and may have a first major surface and an opposite second major surface. The first major surface of the porous separator may face toward a facing surface of the negative electrode layer. The monitoring assembly may include a porous sensory structure and a transducer. The porous sensory structure may be disposed between the first major surface of the porous separator and the facing surface of the negative electrode layer. The porous sensory structure may include a sensory layer and a buffer layer. The sensory layer may be disposed on the first major surface of the porous separator and may include an electrically conductive material. The sensory layer may be configured to produce a response to an input signal or to a physical stimulus. The buffer layer may be disposed between the sensory layer and the facing surface of the negative electrode layer. The buffer layer may be made of an electrically insulating material that electrically isolates the sensory layer from the facing surface of the negative electrode layer. The transducer may be configured to process the response produced by the sensory layer to generate an output signal indicative of a diagnostic condition within the electrochemical cell. 
     The electrochemical cell may comprise a microcontroller configured to process the output signal generated by the transducer into data and to store the data in local memory as a function of time. The microcontroller may be electrically coupled to the negative electrode current collector via a first electrical connector and may be electrically coupled to the positive electrode current collector via a second electrical connector. In such case, the microcontroller may be powered by an electrical potential difference between the negative electrode current collector and the positive electrode current collector. 
     The porous separator and the porous sensory structure may be infiltrated with a nonaqueous liquid electrolyte solution. 
     The sensory layer may include an electrochemically active reference electrode material. In such case, the electrochemically active reference electrode material may comprise at least one metal oxide of lithium iron phosphate, lithium titanium oxide, or lithium nickel manganese cobalt oxide. 
     The porous separator may be made of an ionically conductive polymeric material. In such case, the ionically conductive polymeric material may comprise at least one of polyethylene, polypropylene, polyamide, poly(tetrafluoroethylene), polyvinylidene fluoride, or poly(vinyl chloride). 
     The negative electrode layer may comprise at least one of graphite or silicon. Or the negative electrode layer may comprise a layer of lithium metal. In such case, the layer of lithium metal may comprise, by weight, greater than 97% lithium. 
     The above summary is not intended to represent every possible embodiment or every aspect of the present disclosure. Rather, the foregoing summary is intended to exemplify some of the novel aspects and features disclosed herein. The above features and advantages, and other features and advantages of the present disclosure, will be readily apparent from the following detailed description of representative embodiments and modes for carrying out the present disclosure when taken in connection with the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein: 
         FIG. 1  is a schematic side cross-sectional view of an electrochemical cell of a secondary lithium battery, the electrochemical cell includes a monitoring assembly that uses a porous sensory structure in combination with a transducer to diagnose and/or monitor certain conditions within the electrochemical cell, wherein the porous sensory structure includes a sensory layer disposed on a major surface of a porous separator and a buffer layer that electrically isolates the sensory layer from a facing surface of a negative electrode material layer; 
         FIG. 2  is a schematic side cross-sectional view of the porous sensory structure of  FIG. 1  during assembly of the electrochemical cell of  FIG. 1 , in accordance with one or more embodiments; 
         FIG. 3  is a schematic side cross-sectional view of a portion of an electrochemical cell including a monitoring assembly that uses a porous sensory structure in combination with a transducer to diagnose and/or monitor certain conditions within the electrochemical cell, wherein the porous sensory structure includes a first sensory layer disposed on a major surface of a porous separator, a first buffer layer overlying the first sensory layer, a second sensory layer overlying the first buffer layer, and a second buffer layer overlying the second sensory layer, and wherein the monitoring assembly is configured to diagnose and monitor lithium dendrite formation and growth within the electrochemical cell; 
         FIG. 4  is a schematic side cross-sectional view of a portion of an electrochemical cell including a monitoring assembly that uses a porous sensory structure in combination with a transducer to diagnose and/or monitor certain conditions within the electrochemical cell, wherein the porous sensory structure includes a sensory layer and a buffer layer, and wherein the transducer is integrated into the physical construction of the sensory layer; and 
         FIGS. 5, 6, and 7  are schematic side cross-sectional views of a portion of an electrochemical cell including a monitoring assembly that uses a porous sensory structure in combination with a transducer to diagnose and/or monitor certain conditions within the electrochemical cell, wherein the porous sensory structure includes a sensory layer and a buffer layer, the transducer is integrated into the physical construction of the sensory layer, and wherein the monitoring assembly is configured to diagnose volumetric changes in a porous separator and/or a negative electrode material layer of the electrochemical cell. 
     
    
    
     The present disclosure is susceptible to modifications and alternative forms, with representative embodiments shown by way of example in the drawings and described in detail below. Inventive aspects of this disclosure are not limited to the particular forms disclosed. Rather, the present disclosure is intended to cover modifications, equivalents, combinations, and alternatives falling within the scope of the disclosure as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The presently disclosed monitoring assembly may be incorporated into an electrochemical cell and used to monitor one or more diagnostic conditions of the electrochemical cell. The presently disclosed monitoring assembly includes a porous sensory structure, including a sensory layer and a buffer layer, disposed between a negative electrode layer and a porous separator layer of the electrochemical cell. The sensory layer includes an electrically conductive material and is configured to produce a response to certain input signals or other stimuli. The monitoring assembly also includes a transducer that is configured to transduce or otherwise process the response produced by the sensory layer into an output signal that can be used to diagnose certain conditions within the electrochemical cell. In assembly, the buffer layer is disposed between the sensory layer and the negative electrode layer and electrically isolates the sensory layer from the negative electrode layer during operation of the electrochemical cell. 
     In embodiments, the presently disclosed monitoring assembly may be configured to diagnose the formation of lithium dendrites on the surface of the negative electrode layer and may be configured to determine the penetration depth of the lithium dendrites and/or to track the growth rate of the lithium dendrites. Additionally or alternatively, the presently disclosed monitoring assembly may be configured to diagnose volumetric changes (e.g., expansion) of the negative electrode layer and/or the porous separator layer. In some embodiments, the sensory layer may be configured to act as a reference in measuring the individual electrochemical potentials of the negative electrode layer and a positive electrode layer of the electrochemical cell. 
     The transducer may be physically incorporated into the electrochemical cell, for example, by being integral with the sensory layer or by being incorporated into a microcontroller associated with the electrochemical cell. In other embodiments, the transducer may be part of an external control device that is electrically coupled to or otherwise in communication with the sensory layer. In embodiments where the electrochemical cell is combined with one or more electrochemical cells to form a secondary lithium battery, the external control device may be in the form of a battery controller, and together the secondary lithium battery and the battery controller may form a battery system that supplies power to an electric machine. In other embodiments, such as in laboratory environments, the external control device may be in the form of a battery testing or calibration machine, and the transducer may be part of the battery testing or calibration machine. 
     The term “transducer” as used herein means a device or structure that receives an input signal or stimulus (in one form of energy) and produces an output signal in response to the input signal or stimulus that may be in another form of energy. 
       FIG. 1  depicts a schematic side cross-sectional view of an electrochemical cell  10  that may be combined with one or more additional electrochemical cells to form a secondary battery, such as a lithium ion or lithium metal battery (sometimes referred to herein as a “lithium battery” for simplicity). The electrochemical cell  10  includes a positive electrode  12 , a negative electrode  14 , a porous separator  16  that electrically isolates the positive and negative electrodes  12 ,  14  from each other, and a monitoring assembly  18  including a porous sensory structure  19  disposed between and electrically isolated from the positive and negative electrodes  12 ,  14 . The positive electrode  12  includes a positive electrode material layer  20  deposited on a major surface of a positive electrode current collector  22 , and the negative electrode  14  includes a negative electrode material layer  24  deposited on a major surface of a negative electrode current collector  26 . In practice, the positive and negative electrode current collectors  22 ,  26  may be electrically coupled to a power source or load  28  via an external circuit  30 . 
     The positive electrode material layer  20  may comprise one or more electrochemically active materials that can undergo a reversible redox reaction with lithium, e.g., a material that can sufficiently undergo lithium intercalation and deintercalation, alloying and dealloying, or plating and stripping. 
     The negative electrode material layer  24  may comprise an electrochemically active material that can undergo a reversible redox reaction with lithium during operation of the electrochemical cell  10  and/or the negative electrode material layer  24  may comprise a layer of lithium (Li) metal or a lithium metal alloy. In some embodiments, the negative electrode material layer  24  may consist essentially of lithium metal and may comprise, by weight, greater than 97% lithium or, more preferably, greater than 99% lithium. In other embodiments, the negative electrode material layer  24  may comprise a material that can undergo the reversible insertion or intercalation of lithium ions or that can react with lithium to form a lithium-containing intermetallic compound. For example, in such case, the negative electrode material layer  24  may comprise graphite, silicon, or a silicon alloy. 
     The positive and negative electrode current collectors  22 ,  26  may comprise a material (e.g., a metal or a metal alloy) that can collect and reversibly passing free electrons to and from their respective electrode material layers  20 ,  24 . 
     The porous separator  16  is positioned between the positive electrode  12  and the porous sensory structure  19  of the monitoring assembly  18 . The porous separator  16  has a first major surface  32  that faces toward the positive electrode  12  and an opposite second major surface  34  that faces toward the porous sensory structure  19  and the negative electrode  14 . The porous separator  16  exhibits an open microporous structure and may comprise an organic and/or inorganic material that can physically separate and electrically insulate the positive and negative electrode material layers  20 ,  24  from each other while permitting the free flow of ions therebetween. The porous separator  16  may comprise a non-woven material, e.g., a manufactured sheet, web, or matt of directionally or randomly oriented fibers. The porous separator  16  may comprise a microporous polymeric material, e.g., a microporous polyolefin-based membrane or film. For example, the porous separator  16  may comprise a single polyolefin or a combination of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), poly(tetrafluoroethylene) (PTFE), polyvinylidene fluoride (PVdF), and/or poly(vinyl chloride) (PVC). In one form, the porous separator  16  may comprise a laminate of one or more polymeric materials, such as a laminate of PE and PP. The porous separator  16  may have a thickness in the range of 5 μm to 30 μm and a porosity in a range of 25% to 75%. 
     In assembly, the porous separator  16 , the porous sensory structure  19  of the monitoring assembly  18 , and the positive and negative electrode material layers  20 ,  24  of the electrochemical cell  10  may be infiltrated with an electrolyte (not shown). The electrolyte may comprise a material that can effectively conduct lithium ions through the porous separator  16  and the porous sensory structure  19 , and between the positive electrode material layer  20  and the negative electrode material layer  24  during operation of the electrochemical cell  10 . For example, the electrolyte may comprise a non-aqueous liquid electrolyte. In such case, the non-aqueous liquid electrolyte may comprise a solution including a lithium salt dissolved or ionized in a nonaqueous, aprotic organic solvent or a mixture of nonaqueous, aprotic organic solvents. Some suitable lithium salts that may be used to make the electrolyte include LiClO 4 , LiAlCl 4 , LiI, LiBr, LiSCN, LiBF 4 , LiB(C 6 H 5 ) 4 , LiAsF 6 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 , LiPF 6 , and combinations thereof. The nonaqueous, aprotic organic solvent in which the lithium salt is dissolved may be a cyclic carbonate (i.e., ethylene carbonate, propylene carbonate), an acyclic carbonate (i.e., dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate), an aliphatic carboxylic ester (i.e., methyl formate, methyl acetate, methyl propionate), a γ-lactone (i.e., γ-butyrolactone, γ-valerolactone), an acyclic ether (i.e., 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane), a cyclic ether (i.e., tetrahydrofuran, 2-methyltetrahydrofuran), or a combination thereof. As another example, the non-aqueous electrolyte may comprise a gel or plasticized polymer electrolyte. In such case, the non-aqueous electrolyte may comprise a polymer host material soaked with a non-aqueous liquid electrolyte solution. Some examples of suitable polymer host materials include poly(vinylidene) (PVdF), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), polyacrylates, and poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP). 
     The monitoring assembly  18  is configured to diagnose and/or monitor certain conditions within the electrochemical cell  10  that may affect the performance and/or cycle life of the electrochemical cell  10 , without interfering with the operation of the electrochemical cell  10 , in particular, without inhibiting the free flow of ions between the positive and negative electrode material layers  20 ,  24 . In the monitoring assembly  18 , the porous sensory structure  19  cooperates with at least one transducer  42  to sense and/or transmit information about certain aspects of the electrochemical cell  10  and/or its components that can be used to diagnose certain conditions within the electrochemical cell  10 . The porous sensory structure  19  includes at least one sensory layer  36  and at least one buffer layer  38  that electrically isolates the sensory layer  36  from a facing surface  40  of the negative electrode material layer  24 . 
     The sensory layer  36  is disposed on the second major surface  34  of the porous separator  16  and the buffer layer  38  is disposed between the sensory layer  36  and the facing surface  40  of the negative electrode material layer  24 . In  FIG. 1 , the transducer  42  is depicted as being physically separate from the sensory layer  36  and electrically coupled to the sensory layer  36  via an electrical connector  44 ; however, this is not necessarily required. In other embodiments, the transducer  42  may be integrated into the physical construction of the sensory layer  36 . In addition, in embodiments where the transducer  42  is physically separate from the sensory layer  36 , the transducer  42  may be in wireless communication with the sensory layer  36 , for example, via transmission and/or receipt of an electromagnetic signal (e.g., radio frequency (RF) radio waves, such as ultra-high frequency (UHF) radio waves). In some embodiments, the transducer  42  may be part of a microcontroller  46  associated with the electrochemical cell  10 . The microcontroller  46 , also referred to as a microcontroller unit, may be in the form of an integrated circuit including a central processing unit (CPU), random-access memory (RAM), flash memory, read-only memory (ROM), and/or an input/output (I/O) device. The microcontroller  46  may be associated with the electrochemical cell  10 , for example, by being physically incorporated into a housing or case (not shown) for the electrochemical cell  10 . In other embodiments, the transducer  42  may be part of an external control device (not shown) associated with the battery or another host. The external control device may be a computer including a central processing unit in the form of a microprocessor, memory, and one or more peripheral devices (e.g., a monitor and/or a battery testing device). 
     The transducer  42  and/or the microcontroller  46  optionally may be electrically coupled to the positive electrode current collector  22  and the negative electrode current collector  26  via an electric circuit  48 , and the inherent electrical potential difference between the current collectors  22 ,  26  may supply power for operation of the transducer  42  and/or the microcontroller  46 . In other embodiments, power may be supplied to the sensory layer  36 , the transducer  42 , and/or the microcontroller  46  by another power source, if desired. 
     The sensory layer  36  is porous and is configured to assist in diagnosing and/or monitoring certain conditions within the electrochemical cell  10  without interfering with the operation of the electrochemical cell  10 , e.g., without inhibiting the free flow of lithium ions therethrough. The sensory layer  36  is configured to produce a response to an input signal or to another stimulus received within the electrochemical cell  10 . For example, the sensory layer  36  may be configured to produce a response when exposed to a source of energy (e.g., an applied voltage, an electric current, and/or electromagnetic waves) or upon being subjected to a physical force, e.g., a compressive force or a tensile force, which may or may not result in physical deformation of the sensory layer  36 . The response produced by the sensory layer  36  may be an electromagnetic signal (e.g., radio waves), a mechanical signal (e.g., an acoustic signal), an electric current, an electric field, a magnetic field, a change in shape, or other type of response that can be used, for example, by the transducer  42 , the microcontroller  46 , and/or an external control device, to diagnose certain conditions within the electrochemical cell  10 . For example, the response produced by the sensory layer  36  may be used to diagnose lithium dendrite growth on the facing surface  40  of the negative electrode material layer  24  and optionally to indicate the extent of the lithium dendrite growth and the growth rate. Additionally or alternatively, the response produced by the sensory layer  36  may be used to diagnose volumetric changes in the porous separator  16  and/or the negative electrode material layer  24 . When the porous separator  16 , the negative electrode material layer  24 , and/or another component of the electrochemical cell  10  experiences a change in volume (i.e., expands or contracts in volume), the change in volume may impart a compressive or tensile force on the sensory layer  36 , which may eventually lead to deformation of the sensory layer  36 . 
     The sensory layer  36  includes at least one electrically conductive material, and the functionality of the sensory layer  36  may be predetermined based upon the way the electrically conductive material is physically arranged in the sensory layer  36 . In some embodiments, the sensory layer  36  (and the electrically conductive material thereof) may extend in a substantially continuous layer between the porous separator  16  and the negative electrode material layer  24  and may be commensurate in size with the area of the facing surface  40  of negative electrode material layer  24 . In other embodiments, the sensory layer  36  (and the electrically conductive material thereof) may be arranged in the form of one or more patterns or physical structures that can function as a transducer (e.g., an antenna, capacitor, inductor, resistor, or resonator) when exposed to an energy source. In the sensory layer  36 , the at least one electrically conductive material may be combined with one or more additional materials that may assist, for example, in deposition or adhesion of the sensory layer  36  on the second major surface  34  of the porous separator  16  or on the facing surface  40  of the negative electrode material layer  24  over the buffer layer  38 . 
     The electrically conductive material of the sensory layer  36  may comprise or consist essentially of at least one of nickel, copper, titanium, silver, gold, or carbon. In practice, the pores of the porous sensory layer  36  may be infiltrated with an electrolyte. The electrically conductive material may account for, by weight, 50% to 100% of the sensory layer  36  and, more preferably, greater than 90% of the sensory layer  36 , without accounting for the weight of electrolyte in the pores of the sensory layer  36 . The sensory layer  36  may be deposited on the second major surface  34  of the porous separator  16  or on the facing surface  40  of the negative electrode material layer  24  over the buffer layer  38  using a printing process (e.g., an ink jet printing, aerosol printing, screen printing, or low velocity plasma jet printing process), magnetron sputtering, ion beam sputtering, a thermal evaporation process, or an electroless deposition or plating technique. The sensory layer  36  may have a thickness in a range of 10 nanometers to 1 micrometer, or more preferably in a range of 25 nanometers to 200 nanometers. The thickness and density of the sensory layer  36  may be selected so that the sensory layer  36  is thick enough and dense enough to form at least one continuous electrically conductive pathway therethrough, without inhibiting the free flow of lithium ions. 
     In some embodiments, the sensory layer  36  may serve as a reference for monitoring the electrical potential of the positive and negative electrode material layers  20 ,  24 . In such case, the sensory layer  36  may include, in addition to the electrically conductive material, a reference electrode material, e.g., lithium iron phosphate, lithium titanium oxide, and/or lithium nickel manganese cobalt oxide. The reference electrode material may be dispersed throughout the sensory layer  36  or may be in the form of a separate layer overlying (or underlying) the sensory layer  36 . 
     The buffer layer  38  is porous and is configured to electrically isolate the sensory layer  36  from the facing surface  40  of the negative electrode material layer  24  without inhibiting the free flow of lithium ions therethrough. The buffer layer  38  may comprise or consist essentially of at least one electrically insulating material. The electrically insulating material of the buffer layer  38  may comprise at least one of an electrically insulating ceramic material (e.g., Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 , ZnO, Ta 2 O 5 , La 2 O 5 , HfO, lithium-lanthanum-zirconium oxide (Li—La—Zr—O), or a zeolite), an electrically insulating polymeric material (e.g., polytetrafluoroethylene), an electrically insulating composite material (e.g., aluminum alkoxide, zirconium alkoxide, titanium alkoxide), and/or diamond-like carbon. In the buffer layer  38 , the electrically insulating material may be combined with one or more additional materials that may assist, for example, in deposition or adhesion of the buffer layer  38  on the second major surface  34  of the porous separator  16  over the sensory layer  36  or on the facing surface  40  of the negative electrode material layer  24 . In practice, the pores of the porous buffer layer  38  may be infiltrated with an electrolyte. The electrically insulating material may account for, by weight, 50% to 100% of the buffer layer  38 , preferably greater than 90% of the buffer layer  38 , without accounting for the weight of the electrolyte in the pores of the buffer layer  38 . 
     The buffer layer  38  may be deposited on the second major surface  34  of the porous separator  16  over the sensory layer  36  or the buffer layer  38  may be deposited on the facing surface  40  of the negative electrode material layer  24  using an atomic layer deposition process, a molecular layer deposition process, sputtering, a thermal evaporation process, or an ion beam deposition process. The buffer layer  38  may have a thickness in a range of 5 nanometers to 100 micrometers. The thickness and density of the buffer layer  38  may be selected so that the buffer layer  38  is thick enough and dense enough to effectively electrically isolate the sensory layer  36  from the negative electrode material layer  24 , without inhibiting the free flow of lithium ions therethrough. 
     In assembly, the sensory layer  36  and the buffer layer  38  of the porous sensory structure  19  may be deposited on or otherwise formed on the second major surface  34  of the porous separator  16  or on the facing surface  40  of the negative electrode material layer  24  prior to sandwiching the porous sensory structure  19  between the facing surface  40  of the negative electrode material layer  24  and the second major surface  34  of the porous separator  16 . The porous sensory structure  19  may be deposited on the porous separator  16  or the negative electrode material layer  24 , for example, using a continuous roll-to-roll process. In embodiments where the porous sensory structure  19  is deposited on the porous separator  16 , the sensory layer  36  may be deposited on the second major surface  34  of the porous separator  16  and then the buffer layer  38  may be deposited on the second major surface  34  of the porous separator  16  over the sensory layer  36 . 
     Referring now to  FIG. 2 , in embodiments where the porous sensory structure  19  is deposited on the negative electrode material layer  24 , the buffer layer  38  may be deposited on the facing surface  40  of the negative electrode material layer  24  and then the sensory layer  36  may be deposited on the facing surface  40  of the negative electrode material layer  24  over the buffer layer  38 . In such case, a spacer  70  may be deposited or positioned over at least a portion of the negative electrode current collector  26  that extends beyond the negative electrode material layer  24  prior to deposition of the porous sensory structure  19  on the negative electrode material layer  24  and the spacer  70  over the negative electrode current collector  26 . The spacer  70  may be made of an electrically insulating material and/or a sacrificial material that may be removed from the negative electrode current collector  26  after formation of the porous sensory structure  19  on the negative electrode material layer  24 . 
     The transducer  42  is configured to transduce or otherwise process the response produced by the sensory layer  36  into an output signal (e.g., an electrical signal, an electromagnetic signal, or a mechanical signal) that is representative of or indicative of one or more diagnostic conditions within the electrochemical cell  10 . The output signal produced by the transducer  42  may be a signal that can be further processed by the microcontroller  46  or the external control device to diagnose certain conditions within the electrochemical cell  10 . The transducer  42  may be an antenna, a resonator, an electrical potential difference sensor (e.g., a voltage sensor, voltmeter, or electrometer), a current sensor (e.g., a galvanometer or an ammeter), a resistance sensor (e.g., an ohmmeter or Wheatstone bridge), an analog-to-digital converter, or a digital-to-analog converter. The transducer  42  may be an inductive-type sensor (an inductor), a capacitive-type sensor (a capacitor), and/or a resistive-type sensor (a resistor). An antenna can transduce or convert electromagnetic radiation (e.g., radio waves) into an electrical signal (that may be processed by another device, such as a receiver) and can transduce or translate an electrical signal emitted by another device (e.g., a transmitter) into electromagnetic radiation (e.g., radio waves). A voltage sensor can convert a measured electrical potential difference between two points in an electrical circuit into an output signal. A current sensor can convert a measured current flowing through an electrically conductive pathway into an output signal. An analog-to-digital converter can convert an analog signal into a digital signal, and a digital-to-analog converter can convert a digital signal into an analog signal. 
     The monitoring assembly  18  depicted in  FIG. 1  may be configured to diagnose formation of a lithium dendrite  50  on the facing surface  40  of the negative electrode material layer  24  that has grown though the buffer layer  38  and penetrated the sensory layer  36 . In such case, the transducer  42  may be in the form of an electrical potential difference sensor, a current sensor, or another type of sensor or actuator whose output signal can be used to diagnose formation of the lithium dendrite  50 . Prior to formation of the lithium dendrite  50 , the transducer  42  may continuously or periodically sense the electrical potential difference between the sensory layer  36  and the negative electrode material layer  24 , or the transducer  42  may sense the current flowing through the sensory layer  36 . In addition, the transducer  42  may periodically or continuously transmit an output signal indicative or representative of the sensed potential difference or current to the microcontroller  46  or to an external control device that can process the output signal transmitted by transducer  42  into data that can be stored and/or used to diagnose formation of the lithium dendrite  50 . For example, the microcontroller  46  may process the output signal generated by the transducer  42  into data and store the data in local memory as a function of time. When the lithium dendrite  50  penetrates the sensory layer  36 , the lithium dendrite  50  may complete an electric circuit between the sensory layer  36  and the negative electrode material layer  24 , which will necessarily change the electrical potential difference between the sensory layer  36  and the negative electrode material layer  24  and the amount of electric current flowing through the sensory layer  36 . In such case, the microcontroller  46  may affirmatively diagnose formation of the lithium dendrite  50  by comparing the most recent data received from the transducer  42  to the historical data stored in memory and calculating the difference between the recent data and the historical data. If the microcontroller  46  determines that the electrical potential difference between the sensory layer  36  and the negative electrode material layer  24  has significantly decreased, or if the current flowing through the sensory layer  36  has significantly increased, the microcontroller  46  may affirmatively diagnose formation of the lithium dendrite  50 . 
       FIG. 3  depicts another embodiment of a monitoring assembly  118  that may be used to diagnose and/or monitor certain conditions within an electrochemical cell of a secondary lithium ion or lithium metal battery, such as the electrochemical cell  10  of  FIG. 1 . The monitoring assembly  118  is similar in many respects to the monitoring assembly  18  depicted in  FIG. 1 , and a description of common subject matter generally may not be repeated here. As shown in  FIG. 3 , in this embodiment, the monitoring assembly  118  includes a first sensory layer  136  disposed on the second major surface  34  of the porous separator  16 , a first buffer layer  138  disposed on the second major surface  34  of the porous separator  16  over the first sensory layer  136 , a second sensory layer  152  disposed on the second major surface  34  of the porous separator  16  over the first buffer layer  138 , a second buffer layer  154  disposed on the second major surface  34  of the porous separator  16  over the second sensory layer  152 , and a transducer  142 . The first buffer layer  138  electrically isolates the first sensory layer  136  from the second sensory layer  152  and the second buffer layer  154  electrically isolates the second sensory layer  152  from the facing surface  40  of the negative electrode material layer  24 . The transducer  142  is physically separate from the first and second sensory layers  136 ,  152  and is respectively physically coupled thereto by first and second electrical connectors  144 ,  156 . 
     The monitoring assembly  118  depicted in  FIG. 3  may be used to detect formation of a lithium dendrite  150  on the facing surface  40  of the negative electrode material layer  24  that has grown though the second buffer layer  154  and penetrated the second sensory layer  152  or the first sensory layer  136 . In such case, the transducer  142  may be in the form of an electrical potential difference sensor, a current sensor, or another type of sensor or actuator whose output signal can be used to diagnose formation of the lithium dendrite  150 . The transducer  142  may periodically or continuously transmit an output signal  158  to an external control device  162 . The output signal  158  transmitted by the transducer  142  may be indicative or representative of the measured electrical potential difference between the negative electrode material layer  24  and the first and second sensory layers  136 ,  152  and/or the output signal  158  transmitted by the transducer  142  may be indicative or representative of the electric current flowing through the first and second sensory layers  136 ,  152 . The external control device  162  may be configured to process the output signal  158  transmitted by transducer  142  to diagnose formation of the lithium dendrite  150  and optionally to determine the depth of penetration of the lithium dendrite  150 . The thickness of the first and second sensory layers  136 ,  152  and/or the first and second buffer layers  138 ,  154  may be selected so that the monitoring assembly  118  can detect formation of lithium dendrites  150  that have reached certain predetermined depths of penetration in the overlying components of the electrochemical cell  10 . 
     The output signal  158  transmitted by the transducer  142  may be in the form of electromagnetic radiation (e.g., radio waves) or an electrical signal. When the lithium dendrite  150  first penetrates the second sensory layer  152 , the output signal  158  transmitted by the transducer  142  may be interpreted by the external control device  162  to affirmatively diagnose formation of the lithium dendrite  150 . In addition, the output signal  158  transmitted by the transducer  142  may indicate that the lithium dendrite  150  has grown to a sufficient size to penetrate the second sensory layer  152  but has not yet penetrated the first sensory layer  136 . If the lithium dendrite  150  continues to grow and subsequently penetrates the first sensory layer  136 , the output signal  158  transmitted by the transducer  142  may be interpreted by the external control device  162  to affirmatively determine that the lithium dendrite  150  has grown to a sufficient size to penetrate both the first and second sensory layers  136 ,  152 . In embodiments where the external control device  162  is configured to store historical data received from the transducer  142  in memory as a function of time, the external control device  162  may use the historical data to determine the growth rate of the lithium dendrite  150 . In this way, the monitoring assembly  118  may track the progressive growth of the lithium dendrite  150  as a function of time. 
     In embodiments, the external control device  162  may be configured to receive data from other components of the electrochemical cell  10 , e.g., from the positive and negative electrodes  12 ,  14  and/or the external circuit  30 . The external control device  162  may receive data related to various operating conditions of the electrochemical cell  10  from such components (e.g., current, cell voltage, and/or the individual electrical potentials of the positive and negative electrode material layers  20 ,  24 ), and may store the data in memory as a function of time. In such case, the external control device  162  may use the historical data saved in memory to determine the growth rate and to track the growth of the lithium dendrite  150  as a function of the current, charge rate, cell voltage, and/or the electrical potentials of the positive and negative electrode material layers  20 ,  24 . 
       FIG. 4  depicts another embodiment of a monitoring assembly  218  that may be used to diagnose and/or monitor certain conditions within an electrochemical cell of a secondary lithium ion or lithium metal battery, such as the electrochemical cell  10  of  FIG. 1 . The monitoring assembly  218  is similar in many respects to the monitoring assemblies  18 ,  118  depicted in  FIGS. 1 and 3 , and a description of common subject matter generally may not be repeated here. As shown in  FIG. 4 , in this embodiment, the monitoring assembly  218  includes a sensory layer  236  disposed on the second major surface  34  of the porous separator  16 , a buffer layer  238  disposed on the second major surface  34  of the porous separator  16  over the sensory layer  236 , and a transducer  242  that is integrated into the physical construction of the sensory layer  236 . The monitoring assembly  318  may be configured to obtain identifying information about the electrochemical cell  10  and/or may be configured to diagnose certain conditions occurring within the electrochemical cell  10 . The monitoring assembly  318  may be configured to diagnose volumetric changes in one or more components of the electrochemical cell  10  (e.g., the porous separator  16  and/or the negative electrode material layer  24 ), for example, that may alter the shape of the sensory layer  236 . 
     The transducer  242  may be a passive transducer or sensor that may be configured to generate an output signal  258  in response to an excitation signal  260  from an external control device  262 . The excitation signal  260  transmitted by the external control device  262  may be in the form of electromagnetic radiation (e.g., radio waves) or the excitation signal  260  may be in the form of an electric current (not shown) that is passed through the sensory layer  236 . The output signal  258  produced by the transducer  242  may be in the form of electromagnetic radiation (e.g., radio waves), an electric current, or an electric or magnetic field. Examples of passive transducers include antennas, capacitors, and inductors. The transducer  242  may be integrated into the sensory layer  236  by patterning the electrically conductive material of the sensory layer  236  into the shape of an antenna, capacitor, or inductor. The external control device  262  may process the output signal  258  generated by the transducer  242 , for example, in accordance with instructions stored in memory, to verify the identity of the electrochemical cell  10  of which the sensory layer  236  is a part or to diagnose certain conditions within the electrochemical cell  10 , for example, by comparing the information represented by the output signal  258  to information stored in the memory of the external control device  262 . 
       FIGS. 5, 6, and 7  depict another embodiment of a monitoring assembly  318  that may be used to diagnose and/or monitor certain conditions within an electrochemical cell of a secondary lithium ion or lithium metal battery, such as the electrochemical cell  10  of  FIG. 1 . The monitoring assembly  318  is similar in many respects to the monitoring assemblies  18 ,  118 ,  218  depicted in  FIGS. 1, 3, and 4 , and a description of common subject matter generally may not be repeated here. As shown in  FIG. 5 , in this embodiment, the monitoring assembly  318  includes a sensory layer  336  disposed on the second major surface  34  of the porous separator  16 , a buffer layer  338  disposed on the second major surface  34  of the porous separator  16  over the sensory layer  336 , a transducer  342  integrated into the physical construction of the sensory layer  336 , and a microcontroller  346 . The microcontroller  346  is electrically coupled to a first end of the sensory layer  336  via a first electrical connector  364  and is electrically coupled to a second end of the sensory layer  336  via a second electrical connector  366 . The sensory layer  336  includes an electrically conductive material in the form of a plurality of discrete electrically conductive particles  368 , and the transducer  342  may be defined by the arrangement of the electrically conductive particles  368  in the sensory layer  336 . The electrically conductive particles  368  in the sensory layer  336  may be configured to provide an electrically conductive pathway through the sensory layer  336  and between the first and second ends of the sensory layer  336 . 
     The monitoring assembly  318  depicted in  FIG. 5  may be configured to diagnose changes in the volume of the porous separator  16  and/or the volume of the negative electrode material layer  24 , for example, by detecting changes in the electrical resistance of the sensory layer  336 . When a known voltage is applied across the sensory layer  336 , for example, by the microcontroller  346 , an electric current will flow through the sensory layer  336  along the electrically conductive pathway defined by the electrically conductive particles  368  in the sensory layer  336 , and the amount of electric current flowing through the sensory layer  336  can be measured by the microcontroller  346 . The microcontroller  346  can then calculate the resistance of the sensory layer  336  based upon instructions stored in memory according to Ohm&#39;s Law. The microcontroller  346  may periodically or continuously measure the current flowing through the sensory layer  336  and, using such measurement, calculate the resistance of the sensory layer  336 . 
     Referring now to  FIG. 6 , when the negative electrode material layer  24  expands in volume, a compressive force will be applied to the adjacent sensory layer  336  by the negative electrode material layer  24 , which may increase the packing density of the electrically conductive particles  368  in the sensory layer  336 , resulting in a decrease in the electrical resistance of the sensory layer  336 . Referring now to  FIG. 7 , alternatively, when the negative electrode material layer  24  contracts in volume (or the porous separator  16  expands in volume), a tensile force will be applied to the adjacent sensory layer  336  by the negative electrode material layer  24 , which may decrease the packing density of the electrically conductive particles  368  in the sensory layer  336 , resulting in an increase in the resistance of the sensory layer  336 . The microcontroller  346  may compare the calculated resistance of the sensory layer  336  to historical resistance values stored in memory to diagnose volumetric changes of the porous separator  16  and/or the negative electrode material layer  24 . 
     These and other benefits will be readily appreciated by those of ordinary skill in the art in view of the forgoing disclosure. 
     While some of the best modes and other embodiments have been described in detail, various alternative designs and embodiments exist for practicing the present teachings defined in the appended claims. Those skilled in the art will recognize that modifications may be made to the disclosed embodiments without departing from the scope of the present disclosure. Moreover, the present concepts expressly include combinations and sub-combinations of the described elements and features. The detailed description and the drawings are supportive and descriptive of the present teachings, with the scope of the present teachings defined solely by the claims.