High elastic modulus polymer electrolytes suitable for preventing thermal runaway in lithium batteries

Abstract

A polymer that combines high ionic conductivity with the structural properties required for Li electrode stability is useful as a solid phase electrolyte for high energy density, high cycle life batteries that do not suffer from failures due to side reactions and dendrite growth on the Li electrodes, and other potential applications. The polymer electrolyte includes a linear block copolymer having a conductive linear polymer block with a molecular weight of at least 5000 Daltons, a structural linear polymer block with an elastic modulus in excess of 1×10 7 Pa and an ionic conductivity of at least 1×10 −5 Scm −1 . The electrolyte is made under dry conditions to achieve the noted characteristics. In another aspect, the electrolyte exhibits a conductivity drop when the temperature of electrolyte increases over a threshold temperature, thereby providing a shutoff mechanism for preventing thermal runaway in lithium battery cells.

1 . A polymer electrolyte, comprising:
 a linear block copolymer having,
 a Li-ion conductive non-crosslinked linear polymer block with a molecular weight of at least 5000 Daltons; 
 a structural linear polymer block with an elastic modulus in excess of 1×10 7  Pa; 
 and an ionic conductivity of at least 1×10 −5  Scm −1 . 
   
   
   
       2 - 20 . (canceled) 
   
   
       21 . The electrolyte of  claim 1 , wherein the copolymer is a triblock copolymer of polystyrene-block-polyisoprene-block poly(ethylene oxide) (S—I-EO) triblock copolymer. 
   
   
       22 . The electrolyte of  claim 1 , wherein the block copolymer is a polystyrene-block-poly(ethylene oxide) (SEO) diblock copolymer. 
   
   
       23 . The electrolyte of  claim 22 , wherein the poly(ethylene oxide) block has a molecular weight of at least 50,000 Daltons. 
   
   
       24 . The electrolyte of  claim 23 , wherein the polystyrene block has a molecular weight of at least 50,000 Daltons. 
   
   
       25 . (canceled) 
   
   
       26 . The electrolyte of  claim 1 , wherein the volume fraction poly(ethylene oxide) block is between 0.25-0.35. 
   
   
       27 . The electrolyte of  claim 1 , wherein the electrolyte is configured to dissolve a lithium salt at a first temperature and to precipitate the lithium salt at a higher temperature. 
   
   
       28 . (canceled) 
   
   
       29 . The electrolyte of  claim 27 , wherein the precipitation occurs at the threshold temperature between 100° C. and 120° C. 
   
   
       30 . (canceled) 
   
   
       31 . The electrolyte of  claim 27 , wherein the precipitation is accompanied by at least 5-fold drop of conductivity of the electrolyte comprising a lithium salt. 
   
   
       32 . (canceled) 
   
   
       33 . A method of making a polymer electrolyte, comprising:
 in an oxygen and moisture free environment,   forming a linear block copolymer having a Li-ion conductive linear polymer block with a molecular weight of at least 5000 Daltons and a structural linear polymer block with an elastic modulus of at least 1×10 7  Pa; and   incorporating a Li salt into the linear block copolymer;   wherein the resulting polymer electrolyte has a ionic conductivity of at least 1×10 −5  Scm −1 .   
   
   
       34 . (canceled) 
   
   
       35 . The method of  claim 33 , further comprising:
 after incorporating the Li salt into the block copolymer, exposing the block copolymer to a temperature above 25° C. in an annealing and/or casting process, wherein the annealing and/or casting process is performed below a threshold precipitation temperature without causing Li salt precipitation.   
   
   
       36 . The method of  claim 35  comprising performing annealing and/or casting at a temperature below 150° C. 
   
   
       37 . (canceled) 
   
   
       38 . The method of  claim 35 , wherein the block copolymer is a polystyrene-block-poly(ethylene oxide) (SEO) diblock copolymer, and wherein the annealing and/or casting is performed at a temperature range of between 90-110° C. 
   
   
       39 . The method of  claim 33 , wherein the method comprises:
 synthesizing the structural polymer block by living anionic polymerization;   adding a monomer of the conductive polymer block and a cryptand catalyst to the structural polymer block living anionic polymerization mixture;   allowing a diblock copolymerization reaction to proceed;   terminating the reaction;   precipitating and freeze-drying the resulting diblock copolymer product;   blending the freeze-dried diblock copolymer with the Li salt in a dry solution such that the Li salt is dissolved into the conductive polymer block; and   freeze-drying the polymer/salt solution.   
   
   
       40 . The method of  claim 39 , further comprising subjecting a portion of the freeze-dried polymer/salt solution to heat and pressure to form a freestanding polymer electrolyte film. 
   
   
       41 - 44 . (canceled) 
   
   
       45 . The method of  claim 33 , wherein incorporating a Li salt into the linear block copolymer comprises blending solutions of the lithium salt and the linear block copolymer in one or more liquid solvents, and subjecting the formed solution to heat to evaporate the one or more liquid solvents and to cast a copolymer film of desired dimensions, wherein said subjecting to heat dose not expose the formed solution to the threshold temperature above which lithium salt precipitation occurs. 
   
   
       46 . A battery cell, comprising:
 a Li anode;   a cathode; and   a solid phase polymer electrolyte as claimed in  claim 1  disposed between the anode and cathode.   
   
   
       47 . (canceled) 
   
   
       48 . The cell of  claim 46 , wherein the cell is configured for thermal run-away shutoff by providing a conductivity drop in the linear block copolymer responsive to a temperature increase of the block copolymer beyond a threshold temperature. 
   
   
       49 . The cell of  claim 48 , wherein the conductivity drop is at least 5-fold. 
   
   
       50 - 59 . (canceled) 
   
   
       60 . A method of operating a battery cell equipped with a thermal run-away shutoff, the method comprising:
 (a) providing a battery cell comprising a Li anode, a cathode, and a solid block copolymer electrolyte having a lithium salt dissolved therein, wherein the block copolymer electrolyte is configured for precipitating lithium salt above a threshold temperature;   (b) charging the battery cell; and   (c) discharging the battery cell, wherein   the battery cell is configured for cooling or for reduced heating once the electrolyte reaches the threshold temperature during battery cell operation, wherein said cooling or reduced heating is due to conductivity drop in the electrolyte associated with lithium salt precipitation.   
   
   
       61 . (canceled) 
   
   
       62 . The method of  claim 60 , wherein the threshold temperature is between 100° C. and 120° C. 
   
   
       63 . (canceled) 
   
   
       64 . The method of  claim 60 , wherein the block copolymer electrolyte comprises polystyrene-block-poly(ethylene oxide) (SEO) di-block copolymer wherein the volume fraction of poly(ethylene oxide) block is between 0.2 and 0.6. 
   
   
       65 . (canceled) 
   
   
       66 . A method of screening block copolymer electrolytes for a block-copolymer electrolyte suitable for preventing thermal runaway in a cell, the method comprising:
 (a) providing a plurality of block-copolymers having a lithium salt dissolved therein;   (b) measuring dependence of conductivity, SAXS, or light scattering profiles versus temperature for at least some of the provided block copolymers;   (c) based on the measurements obtained in (b), identifying the block copolymers which exhibit a drop in conductivity or exhibit evidence of salt precipitation in the SAXS or light scattering profile with increase of temperature above a threshold temperature.   
   
   
       67 . The method of  claim 66 , wherein the block copolymers provided in (a) have a conductivity of at least 1×10 −5  Scm −1  at least at some temperatures of the measured temperature range. 
   
   
       68 . The method of  claim 66 , further comprising identifying block copolymers exhibiting at least 5-fold drop in conductivity at a temperature range of about 90° C.-150° C.