Patent Publication Number: US-2023155174-A1

Title: Lithium metal battery electrolytes including flame retardant additives

Description:
INTRODUCTION 
     The present disclosure relates to secondary lithium metal batteries and, more particularly, to liquid electrolytes for lithium metal batteries that enhance the flame retardant performance thereof. 
     Electrochemical cells of secondary lithium batteries generally include a negative electrode and a positive electrode spaced apart from one another by a porous separator. The negative and positive electrodes and the porous separator are infiltrated with an ionically conductive electrolyte that provides a medium for the conduction of lithium ions between the negative and positive electrodes during discharge and recharge of the electrochemical cell. Electrolytes of lithium batteries generally comprise a lithium salt dissolved or dispersed in one or more aprotic organic solvents and may be formulated to exhibit certain desirable properties over a wide operating temperature range. Such desirable properties may include high ionic conductivity, low viscosity, non-flammability, a wide electrochemical stability window, and chemical compatibility with the other components of the electrochemical cell. 
     Manufacturing defects, aging, and/or certain abuse conditions may impair the thermal stability of secondary lithium batteries. Certain conditions that increase the internal temperature of lithium batteries may set-off undesirable events and/or chemical reactions within the batteries that may lead to further undesirable heat generation. Because components of lithium batteries and/or of their surrounding environment may be flammable, it may be desirable to incorporate materials into the internal components of such batteries that can effectively inhibit propagation of combustion chain reactions. 
     SUMMARY 
     An electrolyte for a lithium metal battery is disclosed. The electrolyte includes a nonaqueous aprotic organic solvent, a lithium salt dissolved in the nonaqueous aprotic organic solvent, and from 1% to 10%, by volume, of a flame retardant additive. The flame retardant additive is at least one of an organophosphate compound, an organophosphite compound, organophosphonate compound, or a phosphazene compound. 
     The flame retardant additive may include at least one of tris(2,2,2-trifluoroethyl) phosphate; tris(2,2,2-trifluoroethyl) phosphite; triphenyl phosphite; diethyl ethylphosphonate; diethyl phenylphosphonate; 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2,4,6-triazatriphosphorine; or pentafluoro(phenoxy)cyclotriphosphazene. 
     The nonaqueous aprotic organic solvent may include at least one of dimethoxy ethane, dimethoxy propane, ethylene carbonate, propylene carbonate, or fluoroethylene carbonate. The nonaqueous aprotic organic solvent may constitute, by volume, from 10% to 90% of the electrolyte. 
     The electrolyte may include a diluent. The diluent may be at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE); 1,2,2,2-tetrafluoroethyl methyl ether; n-Butyl-1,1,2,2-tetrafluoroethyl ether; 1H,1H,2′H,3H-Decafluorodipropyl ether; bis(2,2,2-trifluoroethyl)ether; ethyl 1,1,2,2-tetrafluoroethyl ether; difluoromethyl-2,2,3,3,3-pentafluoropropyl ether; 2,2,2-trifluoroethyl 1,1,2,2-tetrafluoroethyl ether; difluoromethyl 2,2,3,3-tetrafluoropropyl ether; propyl 1,1,2,2-tetrafluoroethyl ether; 1,1-difluoroethyl 2,2,2-trifluoroethyl ether; isopropyl 1,1,2,2-tetrafluoroethyl ether; bis(2,2-difluoroethyl)ether; or 1,1,2,2-tetrafluoroethyl isobutyl ether. The diluent may constitute, by volume, from 10% to 89% of the electrolyte. 
     The lithium salt may be at least one of lithium bis(fluorosulfonyl)imide; lithium hexafluorophosphate; lithium perchlorate; lithium tetrachloroaluminate; lithium iodide; lithium bromide; lithium thiocyanate; lithium tetrafluoroborate; lithium tetraphenylborate; lithium hexafluoroarsenate; lithium trifluoromethanesulfonate; lithium bis(fluorosulfonyl)imide; lithium bis(trifluoromethylsulfonyl)imide; lithium bis(oxalate) borate; lithium difluoro(oxalato)borate; lithium tris(pentafluoroethyl)-trifluorophosphate; lithium bis(trifluoromethyl)-tetrafluorophosphate; lithium tetrafluorooxalatophosphate; lithium tris(trifluoromethyl)trifluorophosphate; lithium trifluoromethanesulfonate; or lithium nitrate. A molar concentration of the lithium salt in the electrolyte may be in a range of from 0.5 M to 5.0 M. 
     An electrochemical cell for a lithium metal battery is disclosed. The electrochemical cell includes a lithium metal negative electrode, a positive electrode spaced apart from the negative electrode, and an electrolyte in ionic contact with the lithium metal negative electrode and the positive electrode. The positive electrode includes a transition metal oxide that can undergo a reversible intercalation of lithium ions. The electrolyte includes: a nonaqueous aprotic organic solvent, a lithium salt dissolved in the nonaqueous aprotic organic solvent, and 1% to 10%, by volume, of a flame retardant additive. The flame retardant additive is at least one of an organophosphate compound, an organophosphite compound, organophosphonate compound, or a phosphazene compound. 
     The flame retardant additive may be an organophosphate compound represented by the chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and R 3  are each individually a C1-C4 alkyl group or an aryl group having 5 to 6 carbon atoms. In such case, the flame retardant additive may be tris(2,2,2-trifluoroethyl) phosphate. 
     The flame retardant additive may be an organophosphite compound represented by the chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and R 3  are each individually a C1-C4 alkyl group or an aryl group having 5 to 6 carbon atoms. In such case, the flame retardant additive may be at least one of tris(2,2,2-trifluoroethyl) phosphite or triphenyl phosphite. 
     The flame retardant additive may be an organophosphonate compound represented by the chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and R 3  are each individually a C1-C4 alkyl group or an aryl group having 5 to 6 carbon atoms. In such case, the flame retardant additive is at least one of diethyl ethylphosphonate or diethyl phenylphosphonate. 
     The flame retardant additive may be an aryl phosphazene compound represented by the chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  are each individually a fluorine atom, a C1-C4 alkyl group, a C1-C4 alkoxy group, an aryl group having 5 to 6 carbon atoms, or an aryloxy group having 5 to 6 carbon atoms. In such case, the flame retardant additive may be at least one of 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2,4,6-triazatriphosphorine or pentafluoro(phenoxy)cyclotriphosphazene. 
     The transition metal oxide of the positive electrode may be a high-nickel content lithium nickel cobalt manganese oxide having the chemical formula: LiNi 1-a-b Co a Mn b O 2 , wherein a=0.1-0.2 and b=0.1-0.2. In such case, the nonaqueous aprotic organic solvent in the electrolyte may be at least one of dimethoxy ethane or dimethoxy propane, and the electrolyte may comprise, by volume, less than 5% of a carbonate-based organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluoroethylene carbonate. 
     The lithium salt in the electrolyte may be at least one of lithium bis(fluorosulfonyl)imide; lithium hexafluorophosphate; lithium perchlorate; lithium tetrachloroaluminate; lithium iodide; lithium bromide; lithium thiocyanate; lithium tetrafluoroborate; lithium tetraphenylborate; lithium hexafluoroarsenate; lithium trifluoromethanesulfonate; lithium bis(fluorosulfonyl)imide; lithium bis(trifluoromethylsulfonyl)imide; lithium bis(oxalate) borate; lithium difluoro(oxalato)borate; lithium tris(pentafluoroethyl)-trifluorophosphate; lithium bis(trifluoromethyl)-tetrafluorophosphate; lithium tetrafluorooxalatophosphate; lithium tris(trifluoromethyl)trifluorophosphate; lithium trifluoromethanesulfonate; or lithium nitrate. 
     The lithium metal negative electrode may comprise, by weight, greater than 97% lithium. 
     The electrolyte may include a diluent. In such case, the diluent may be at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE); 1,2,2,2-tetrafluoroethyl methyl ether; n-Butyl-1,1,2,2-tetrafluoroethyl ether; 1H,1H,2′H,3H-Decafluorodipropyl ether; bis(2,2,2-trifluoroethyl)ether; ethyl 1,1,2,2-tetrafluoroethyl ether; difluoromethyl-2,2,3,3,3-pentafluoropropyl ether; 2,2,2-trifluoroethyl 1,1,2,2-tetrafluoroethyl ether; difluoromethyl 2,2,3,3-tetrafluoropropyl ether; propyl 1,1,2,2-tetrafluoroethyl ether; 1,1-difluoroethyl 2,2,2-trifluoroethyl ether; isopropyl 1,1,2,2-tetrafluoroethyl ether; bis(2,2-difluoroethyl)ether; or 1,1,2,2-tetrafluoroethyl isobutyl ether. 
     In aspects, the electrolyte may include, by volume, from 15% to 60% of the nonaqueous aprotic organic solvent, from 30% to 80% of the diluent, and from 5% to 10% of the flame retardant additive. 
     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 metal battery. 
     
    
    
     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 liquid electrolytes are formulated for use in electrochemical cells of lithium metal batteries to help prevent propagation of thermal runaway temperatures, for example, by quenching, suppressing, and/or terminating combustion chain reactions within the electrochemical cells. 
       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 lithium battery, such as a lithium metal battery. The electrochemical cell  10  includes a positive electrode  12 , a negative electrode  14  spaced apart from the positive electrode  12 , an ionically conductive liquid electrolyte  16  that provides a medium for the conduction of lithium ions between the positive electrode  12  and the negative electrode  14 , and a porous separator  18  that electrically isolates the positive and negative electrodes  12 ,  14  from each other while allowing lithium ions to pass therethrough. The positive electrode  12  is disposed on a major surface of a positive electrode current collector  20 , and the negative electrode  14  is disposed on a major surface of a negative electrode current collector  22 . In practice, the positive and negative electrode current collectors  20 ,  22  may be electrically coupled to a power source or load  24  via an external circuit  26 . 
     The electrolyte  16  infiltrates the pores of the porous separator  18  and is in physical and ionic contact with the positive and negative electrodes  12 ,  14 . The electrolyte  16  is formulated to facilitate the transport of lithium ions between the positive and negative electrodes  12 ,  14  and, when the electrochemical cell  10  is exposed to thermal runaway temperatures (e.g., temperatures greater than about 80° C.), the electrolyte  16  is formulated to prevent thermal runaway propagation, for example, by quenching, suppressing, and/or terminating combustion chain reactions within the electrochemical cell  10 . 
     The electrolyte  16  comprises an organic solvent, a lithium salt dissolved in the organic solvent, a flame retardant additive, and optionally a diluent. The organic solvent in the electrolyte  16  may include one or more nonaqueous aprotic organic solvating solvents. The one or more solvating solvents in the organic solvent of the electrolyte  16  may be selected to facilitate dissolution of the lithium salt in the electrolyte  16  and to inhibit formation of lithium dendrites on the negative electrode  14  during repeated charging cycles of the electrochemical cell  10 . In aspects, the one or more solvating solvents may comprise dimethoxy ethane (DME), dimethoxy propane (DMP), ethylene carbonate (EC), propylene carbonate (PC), and/or fluoroethylene carbonate (FEC). In aspects, the one or more solvating solvents may consist essentially of dimethoxy ethane, dimethoxy propane, or fluoroethylene carbonate. The organic solvent may constitute, by volume, from about 10% to about 90% of the electrolyte  16 . When the electrolyte  16  does not include a diluent, the organic solvent may constitute, by volume, from about 90% to about 99% of the electrolyte  16 . When the electrolyte  16  includes a diluent, the organic solvent may constitute, by volume, from about 10% to about 89% of the electrolyte  16 . 
     In aspects where the electrochemically active material of the positive electrode  12  is formulated for charging to high voltages (e.g., greater than 4.3V vs. Li/Li), the organic solvent of the electrolyte  16  may be substantially free of carbonate-based organic solvents. For example, in aspects where the electrochemically active material of the positive electrode  12  comprises a high-nickel content layered oxide, the organic solvent of the electrolyte  16  may include, by volume, less than 5%, less than 2%, or less than 1% carbonate-based organic solvents. Examples of carbonate-based organic solvents include: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluoroethylene carbonate. 
     The optional diluent in the electrolyte  16  may be selected to provide the electrolyte  16  with a desirable combination of low viscosity and high ionic conductivity. In aspects, the diluent may be a fluorinated ether. For example, the diluent may comprise 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE); 1,2,2,2-tetrafluoroethyl methyl ether; n-Butyl-1,1,2,2-tetrafluoroethyl ether; 1H,1H,2′H,3H-Decafluorodipropyl ether; bis(2,2,2-trifluoroethyl)ether; ethyl 1,1,2,2-tetrafluoroethyl ether; difluoromethyl-2,2,3,3,3-pentafluoropropyl ether; 2,2,2-trifluoroethyl 1,1,2,2-tetrafluoroethyl ether; difluoromethyl 2,2,3,3-tetrafluoropropyl ether; propyl 1,1,2,2-tetrafluoroethyl ether; 1,1-difluoroethyl 2,2,2-trifluoroethyl ether; isopropyl 1,1,2,2-tetrafluoroethyl ether; bis(2,2-difluoroethyl)ether; and/or 1,1,2,2-tetrafluoroethyl isobutyl ether. In aspects, the diluent may consist essentially of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. When present in the electrolyte  16 , the diluent may constitute, by volume, from about 10% to about 89% of the electrolyte  16 . 
     For example, in aspects where the electrolyte  16  includes the organic solvent and the diluent, the electrolyte  16  may include, by volume, 15% to 60% organic solvent, 30% to 80% diluent, and 5% to 10% flame retardant additive. 
     The lithium salt in the electrolyte  16  may be selected to exhibit good solubility in the organic solvent. In aspects, the lithium salt may comprise lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 , or LiFSI); lithium hexafluorophosphate (LiPF 6 ); lithium perchlorate (LiClO 4 ); lithium tetrachloroaluminate (LiAlCl 4 ); lithium iodide (LiI); lithium bromide (LiBr); lithium thiocyanate (LiSCN); lithium tetrafluoroborate (LiBF 4 ); lithium tetraphenylborate (LiB(C 6 H 5 ) 4 ); lithium hexafluoroarsenate (LiAsF 6 ); lithium trifluoromethanesulfonate (LiCF 3 SO 3 ); lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 ); lithium bis(trifluoromethylsulfonyl)imide (LiN(CF 3 SO 2 ) 2 , or LiTFSI); lithium bis(oxalate) borate (LiB(C 2 O 4 ) 2 , or LiBOB); lithium difluoro(oxalato)borate (LiBF 2 (C 2 O 4 ), or LiODFB); lithium tris(pentafluoroethyl)-trifluorophosphate (LiPF 3 (C 2 F 5 ) 3 , or LiFAP); lithium bis(trifluoromethyl)-tetrafluorophosphate (LiPF 4 (CF 3 ) 2 ); lithium tetrafluorooxalatophosphate (LiPF 4 (C 2 O 4 ), or LiFOP); lithium tris(trifluoromethyl)trifluorophosphate (LiPF 3 (CF 3 ) 3 ); lithium trifluoromethanesulfonate (LiSO 3 CF 3 ); and/or lithium nitrate (LiNO 3 ). In aspects, the lithium salt may consist essentially of lithium bis(fluorosulfonyl)imide. 
     The molarity or molar concentration of the lithium salt in the electrolyte  16  (moles of lithium salt per liter of electrolyte  16 , M) may be in a range of from 0.5 M to 5.0 M. In aspects where the organic solvent of the electrolyte  16  consists essentially of the one or more solvating solvents and does not include a diluent, the electrolyte  16  may be a high-salt-concentration electrolyte (HCE) and the molar concentration of the lithium salt in the electrolyte  16  may be greater than 2.0 M or greater than 2.5 M, less than 5.0 M or less than 4.0 M, or from 2.0 M to 5.0 M or from 2.5 M to 4.0 M. In aspects where the organic solvent of the electrolyte  16  includes one or more solvating solvents and a fluorinated ether diluent, the electrolyte  16  may be a localized high-salt-concentration electrolyte (LHCE) and the molar concentration of the lithium salt in the electrolyte  16  may be greater than 0.5 M or greater than 1.0 M, less than 2.5 M or less than 2.0 M, or from 0.5 M to 2.5 M or from 1.0 M to 2.0 M. 
     The flame retardant additive in the electrolyte  16  comprises one or more phosphorus (P)—containing compounds that thermally decompose when exposed to thermal runaway temperatures (e.g., temperatures greater than about 80° C., greater than about 120° C., or greater than about 250° C.). Without intending to be bound by theory, it is believed that, at thermal runaway temperatures, the flame retardant additive in the electrolyte  16  can thermally decompose to produce certain gaseous species that quench, remove, or interfere with free radical-initiated combustion chain reactions occurring in the electrochemical cell  10 . 
     The flame retardant additive may comprise an organophosphate compound, an organophosphite compound, organophosphonate compound, and/or a phosphazene compound. In aspects, the flame retardant additive in the electrolyte  16  may be an alkyl or aryl organophosphate compound represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and/or R 3  may be C1-C4 alkyl groups and/or aryl groups having 5 to 6 carbon atoms, e.g., phenyl groups (—C 6 H 5 ). In aspects, the organophosphate compound of formula (1) may be a fluorinated organophosphate compound. In such case, R 1 , R 2 , and/or R 3  in the organophosphate compound of formula (1) may be C1-C4 polyfluoroalkyl groups and/or polyfluoroaryl groups. Examples of C1-C4 polyfluoroalkyl groups include polyfluoromethyl groups (e.g., trifluoromethyl groups, —CF 3 ) and/or polyfluoroethyl groups. 
     In aspects, R 1 , R 2 , and R 3  in the organophosphate compound of formula (1) each may be a trifluoroethyl group (e.g., —CH 2 CF 3 ). In such case, the flame retardant additive may comprise tris(2,2,2-trifluoroethyl) phosphate (TFEPO), CAS No. 358-63-4, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     In aspects, the flame retardant additive in the electrolyte  16  may be an alkyl or aryl organophosphite compound represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and/or R 3  may be C1-C4 alkyl groups and/or aryl groups having 5 to 6 carbon atoms, e.g., phenyl groups (—C 6 H 5 ). In aspects, the organophosphite compound of formula (3) may be a fluorinated organophosphite compound. In such case, R 1 , R 2 , and/or R 3  in the organophosphite compound of formula (3) may be C1-C4 polyfluoroalkyl groups and/or polyfluoroaryl groups. Examples of C1-C4 polyfluoroalkyl groups include polyfluoromethyl groups and/or polyfluoroethyl groups. 
     In aspects, R 1 , R 2 , and R 3  in the organophosphite compound of formula (3) each may be a trifluoroethyl group (e.g., —CH 2 CF 3 ). In such case, the flame retardant additive may comprise tris(2,2,2-trifluoroethyl) phosphite (TFEP), CAS No. 370-69-4, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     As another example, R 1 , R 2 , and R 3  in the organophosphite compound of formula (3) each may be a phenyl group (—C 6 H 5 ). In such case, the flame retardant additive may comprise triphenyl phosphite (TPP), CAS No. 101-02-0, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     In aspects, the flame retardant additive in the electrolyte  16  may be an alkyl or aryl organophosphonate compound represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and/or R 3  may be C1-C4 alkyl groups and/or aryl groups having 5 to 6 carbon atoms. For example, R 1 , R 2 , and/or R 3  in the organophosphonate compound of formula (6) may be methyl groups (—CH 3 ), ethyl groups (—CH 2 CH 3 ), and/or phenyl groups (—C 6 H 5 ). In aspects, the organophosphonate compound of formula (6) may be a fluorinated organophosphonate compound. In such case, R 1 , R 2 , and/or R 3  in the organophosphonate compound of formula (6) may be C1-C4 polyfluoroalkyl groups and/or polyfluoroaryl groups having 5 to 6 carbon atoms. 
     In aspects, R 1 , R 2 , and R 3  in the organophosphonate compound of formula (6) each may be an ethyl group (—CH 2 CH 3 ). In such case, the flame retardant additive may comprise diethyl ethylphosphonate (DEEP), CAS No. 78-38-6, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     As another example, in the organophosphonate compound of formula (6), R 2  may be a phenyl group (—C 6 H 5 ) and R 1  and R 3  may be ethyl groups (—CH 2 CH 3 ). In such case, the flame retardant additive may comprise diethyl phenylphosphonate (DEPP), CAS No. 1754-49-0, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     In aspects, the flame retardant additive in the electrolyte  16  may be an alkyl or aryl phosphazene compound. For example, the flame retardant additive in the electrolyte  16  may be an aryl phosphazene compound represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , R 3 , R 4 , R 5 , and/or R 6  may be fluorine atoms (F), C1-C4 alkyl groups, C1-C4 alkoxy groups, aryl groups having 5 to 6 carbon atoms, and/or aryloxy groups having 5 to 6 carbon atoms. For example, R 1 , R 2 , R 3 , R 4 , R 5 , and/or R 6  in the aryl phosphazene compound of formula (9) may be methyl groups (—CH 3 ), methoxy groups (—OCH 3 ), ethyl groups (—CH 2 CH 3 ), ethoxy groups (—OCH 2 CH 3 ), phenyl groups (—C6H 5 ), and/or phenoxy groups (—OC 6 H 5 ). In aspects, the aryl phosphazene compound of formula (9) may be a fluorinated aryl phosphazene compound. In such case, R 1 , R 2 , R 3 , R 4 , R 5 , and/or R 6  in the aryl phosphazene compound of formula (9) may be fluorine atoms (F), C1-C4 polyfluoroalkyl groups, C1-C4 polyfluoroalkoxy groups, polyfluoroaryl groups having 5 to 6 carbon atoms, and/or polyfluoroaryloxy groups having 5 to 6 carbon atoms. 
     In aspects, in the aryl phosphazene compound of formula (9), R 2  may be an ethoxy group (—OCH 2 CH 3 ) and R 1 , R 3 , R 4 , R 5 , and R 6  may be fluorine atoms (F). In such case, the flame retardant additive may comprise 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2,4,6-triazatriphosphorine, CAS No. 33027-66-6, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     As another example, in the aryl phosphazene compound of formula (9), R 2  may be a phenoxy group (—OC 6 H 5 ) and R 1 , R 3 , R 4 , R 5 , and R 6  may be fluorine atoms (F). In such case, the flame retardant additive may comprise pentafluoro(phenoxy)cyclotriphosphazene, CAS No. 33027-68-8, and may be represented by the following chemical formula: 
     
       
         
         
             
             
         
       
     
     The flame retardant additive may constitute, by volume, greater than 1% or greater than 2%, less than 10% or less than 6%, or from 1% to 10% or from 2% to 6% of the electrolyte  16 . In aspects, the flame retardant additive may constitute, by volume, about 5% of the electrolyte  16 . 
     The porous separator  18  is configured to physically separate the positive electrode  12  and the negative electrode  14  from one another while permitting lithium ions to pass therethrough. The porous separator  18  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 electrodes  12 ,  14  from each other while permitting the free flow of ions therebetween. The porous separator  18  may comprise a non-woven material, e.g., a manufactured sheet, web, or mat of directionally or randomly oriented fibers. The porous separator  18  may comprise a microporous polymeric material, e.g., a microporous polyolefin-based membrane or film. For example, the porous separator  18  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  18  may comprise a laminate of one or more polymeric materials, such as a laminate of PE and PP. In aspects, the porous separator  18  may include a ceramic coating and/or a heat-resistant material coating, which may be disposed on one or both major facing surfaces of the porous separator  18  and may alumina (Al 2 O 3 ) and/or silica (SiO2). The porous separator  18  may have a thickness in a range of from 5 μm to 30 μm and a porosity in a range of from 25% to 75%. 
     The positive electrode  12  is porous and 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. In one form, the positive electrode  12  may comprise an intercalation host material that can undergo the reversible insertion or intercalation of lithium ions. In such case, the intercalation host material of the positive electrode  12  may comprise a layered oxide represented by the formula LiMeO 2 , an olivine-type oxide represented by the formula LiMePO 4 , a spinel-type oxide represented by the formula LiMe 2 O 4 , a tavorite represented by one or both of the following formulas LiMeSO 4 F or LiMePO 4 F, or a combination thereof, where Me is a transition metal (e.g., Co, Ni, Mn, Fe, Al, V, or a combination thereof). In another form, the positive electrode material  12  may comprise a conversion material including a component that can undergo a reversible electrochemical reaction with lithium, in which the component undergoes a phase change or a change in crystalline structure accompanied by a change in oxidation state. In such case, the conversion material of the positive electrode  12  may comprise sulfur, selenium, tellurium, iodine, a halide (e.g., a fluoride or chloride), sulfide, selenide, telluride, iodide, phosphide, nitride, oxide, oxysulfide, oxyfluoride, sulfur-fluoride, sulfur-oxyfluoride, or a lithium and/or metal compound thereof. Examples of suitable metals for inclusion in the conversion material of the positive electrode  12  include iron, manganese, nickel, copper, and cobalt. In aspects, the positive electrode  12  may include an electrochemically active material in the form of a high-nickel content layered oxide: LiNi 1-x M x O 2 , where M=Co, Mn, and/or Al, and wherein x≤0.4. For example, the positive electrode  12  may include a high-nickel content lithium nickel cobalt manganese oxide (NCM): LiNi 1-a-b Co a Mn b O 2 , wherein a=0.1-0.2 and b=0.1-0.2. 
     The electrochemically active material of the positive electrode  12  may be intermingled with a polymeric binder to provide the positive electrode  12  with structural integrity. Examples of polymeric binders include polyvinylidene fluoride (PVdF), ethylene propylene diene monomer (EPDM) rubber, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid, and mixtures thereof. The positive electrode  12  optionally may include particles of an electrically conductive material, which may comprise very fine particles of, for example, high-surface area carbon black. The electrochemically active material(s) may account for, by weight, from 50% to 90% of the positive electrode  12 , the polymeric binder may account for, by weight, from 5% to 30% of the positive electrode  12 , and the electrically conductive material may account for, by weight, 5% to 40% of the positive electrode  12 . 
     The negative electrode  14  may be in the form of a nonporous layer of lithium metal. In such case, the negative electrode  14  may comprise a lithium metal alloy or may consist essentially of lithium (Li) metal. For example, the negative electrode  14  may comprise, by weight, greater than 97% lithium or greater than 99% lithium. As such, in aspects, the negative electrode  14  does not comprise other elements or compounds that undergo a reversible redox reaction with lithium during operation of the electrochemical cell  10 . For example, in aspects, the negative electrode  14  does not comprise an intercalation host material that is formulated to undergo the reversible insertion or intercalation of lithium ions or an alloying material that can electrochemically alloy and form compound phases with lithium. In addition, in aspects, the negative electrode  14  does not comprise a conversion material or an alloy material that can electrochemically alloy and form compound phases with lithium. Examples of materials that may be excluded from the negative electrode  14  of the present disclosure include carbon-based materials (e.g., graphite, activated carbon, carbon black, and graphene), silicon and silicon-based materials, tin oxide, aluminum, indium, zinc, cadmium, lead, germanium, tin, antimony, titanium oxide, lithium titanium oxide, lithium titanate, lithium oxide, metal oxides (e.g., iron oxide, cobalt oxide, manganese oxide, copper oxide, nickel oxide, chromium oxide, ruthenium oxide, and/or molybdenum oxide), metal phosphides, metal sulfides, and metal nitrides (e.g., phosphides, sulfides, and/or nitrides or iron, manganese, nickel, copper, and/or cobalt). In aspects, the negative electrode  14  does not comprise a polymeric binder. Examples of polymeric binders that may be excluded from the negative electrode  14  of the present disclosure include polyvinylidene fluoride (PVdF), ethylene propylene diene monomer (EPDM) rubber, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyacrylic acid. The negative electrode  14  may have a thickness in a range of from 5 micrometers to 600 micrometers. 
     The positive and negative electrode current collectors  20 ,  22  may be in the form of thin and flexible porous or non-porous electrically conductive metallic substrates and may comprise a metallic material that is capable of collecting and reversibly passing free electrons to and from their respective electrodes  12 ,  14 . The term “metallic,” as used herein refers to a material that predominantly comprises one or more metals. As such, a metallic material may comprise a single metal, more than one metal (in alloy form or otherwise), or both one or more metals and one or more other non-metal components in elemental or compound form. For example, the positive and negative electrode current collectors  20 ,  22  may comprise an electrically conductive metal or metal alloy, e.g., a transition metal or an alloy thereof. In aspects, the positive electrode current collector  20  may comprise aluminum (Al), nickel (Ni), or an iron (Fe) alloy (e.g., stainless steel) and the negative electrode current collector  22  may comprise copper (Cu), nickel (Ni), an iron (Fe) alloy (e.g., stainless steel), or titanium (Ti). Other electrically conductive metals may of course be used, if desired. 
     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.