Abstract:
A method for producing a hybrid electrolyte including preparing a housing, positioning a solid lithium ion conductor in the housing, and at least partially filling the housing with an organic liquid lithium ion conductor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to co-pending U.S. Provisional Patent Application Ser. No. 61/531,342 and 61/531,330, both filed on Sep. 6, 2011, and co-pending U.S. Provisional Patent Application Ser. No. 61/531,822, filed on Sep. 7, 2011, and incorporates the same herein in their respective entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The novel technology relates generally to electrochemistry, and, more particularly, to an electrolyte system for an electrochemical cell. 
       BACKGROUND 
       [0003]    The use of organic liquid electrolytes poses a challenge for further development of current lithium-ion battery technology due to the flammable liquid nature of the electrolyte that gives rise to safety problems, solvent leakage, and a tight electrochemical window. For these reasons, many lithium-ion conducting materials, such as polymer, polymer-gel, ionic liquid, and inorganic solids, have been investigated as alternative electrolytes for a lithium ion (Li-ion) battery. Among them, fast Li-ion conducting inorganic solid materials have been given attention as alternative candidates because of their advantages over liquid and polymer electrolytes such as their high Li-ion conductivity over 10 −4  S/cm, their wide electrochemical window (0-7 V vs. Li + /Li 0 ), and their good chemical stability with highly reducing and oxidizing electrodes. 
         [0004]    For these reasons, many fast Li-ion conducting solids, such as sulfide glass, glass-ceramics, and oxy-sulfide glasses, have been developed. Due to their sulfurous character, they generally yield higher Li-ion conductivity than oxide compounds. However, they are generally very unstable in an air atmosphere, which gives rise to difficulty in handling. There are a few oxide compounds that yield high Li-ion conductivity up to 10 −3  S/cm. These include the NASICON type: Li1.3Ti1.7Al 0.3 (PO 4 ) 3 , Garnet type: Li 7 La 3 Zr 2 O 12 , and LLTO type: Li 3x La (2/3)-x( )(1/3)-2x TiO 3  in this case. 
         [0005]    Use of these fast Li-ion conducting solid materials as electrolytes has been intensively and extensively studied in the design of solid-state batteries that use a solid anode, cathode, and electrolyte. However, even with the high ionic conductive solid electrolytes, it has been a struggle to coax solid electrolyte battery to obtain similar specific capacity, rate capability, and cycle life to those of liquid electrolyte battery cells. One of common problems is that there is a large capacity decay after the first charge (or discharge) of the cell. Even at a very small current rate, the capacity and cycle life are limited. 
         [0006]    Recent studies show that the major problems arise from the interfacing of a solid electrolyte with a solid electrode rather than simple the use of the solid electrolyte. To solve this problem, coating of ceramic on the surface of electrode particles has been performed to minimize the electrode/electrolyte interface resistance. However, the electrochemical performance has not been competitive to that of the cell in liquid electrolyte. Thus, there is a need for an improved electrolyte system for electrochemical cells. The present novel technology addresses this need. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of an electrochemical intersection. 
           [0008]      FIG. 2  is a schematic diagram of a hybrid electrochemical cell of the present novel technology. 
           [0009]      FIG. 3  is a graphic illustration of impedance versus pressure for the system of  FIG. 2 . 
           [0010]      FIG. 4A  graphically illustrates specific capacity as a function of voltage for the system of claim  2 . 
           [0011]      FIG. 4B  graphically illustrates impedance change for the system of  FIG. 2  after cycle charging. 
           [0012]      FIG. 5  is a graph of initial impedance for various electrochemical cell configurations for the system of  FIG. 2 . 
           [0013]      FIG. 5B  schematically illustrates the conduction path through the hybrid electrolyte of  FIG. 2 . 
           [0014]      FIG. 5C  schematically illustrates the conduction path through a prior art solid state electrolyte. 
           [0015]      FIG. 6A  is a first graph of charge/discharge curves for various electrochemical cell configurations of  FIG. 2 . 
           [0016]      FIG. 6B  is a second graph of charge/discharge curves for various electrochemical cell configurations of  FIG. 2 . 
           [0017]      FIG. 7A  is a first graph of impedance curves for various electrochemical cell configurations. 
           [0018]      FIG. 7B  is a second graph of impedance curves for various electrochemical cell configurations. 
           [0019]      FIG. 8A  is a first graph of the change in voltage over time for the electrochemical cell of  FIG. 2  at various temperatures. 
           [0020]      FIG. 8B  is a second graph of the change in voltage over time for the electrochemical cell of  FIG. 2  at various temperatures. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    For the purposes of promoting an understanding of the principles of the novel technology, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
         [0022]    Two problems with the above-described electrochemical cell designs remain to be addressed: 1) the coating materials are not soft enough to match the volume change of the electrode materials during Li insertion/extraction on discharge/charge of the cell; and 2) there may be an intrinsic problem of using an inorganic solid as the electrolyte for a Li-ion battery. 
         [0023]    Following the above mentioned topic, the question is raised as to whether fast Li-ion conducting inorganic solids can work as an electrolyte if the interface problems are addressed and/or eliminated. Therefore, to minimize the problem of the solid electrolyte/solid electrode interface, the present novel technology relates to the addition of Li-ion conducting liquid between a solid electrode and a solid electrolyte. The use of liquid at the point of contact between a solid electrolyte and a solid electrode is also convenient to accommodate the volume change of electrode during Li insertion or extraction. 
         [0024]    For relatively easy handling and synthesis, Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 3  was selected in one embodiment to be the a solid electrolyte. As Li-ion conducting liquid, LiPF 6  in EC/DEC was selected as an organic electrolyte. With the use of Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 3  as a solid electrolyte and LiPF6 in EC/DEC as a liquid electrolyte, a stable electrochemical window is only 2.5-4.5 V vs. Li + /Li 0 . To remove any other side effects such as the decomposition of liquid and solid electrolyte, LiMn 2 O 4  was chosen as the material for both positive and negative electrodes.  FIG. 1  shows that the Fermi energy of Li 2 Mn 2 O 4  and Mn 2 O 4  are located in a stable window of both liquid and solid electrolytes. Hence, the charged and discharged shape of this material does not overlap the electrochemical intersection of solid and liquid electrolyte. As will be seen below, the interface impacts the electrochemical performance of a lithium ion cell. 
         [0025]    Preparation of Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 3  was modified as follows. A stoichiometric mixture of Li 2 CO 3 , Al 2 O 3 , TiO 2  and (NH 4 ) 2 PO 4  was ground and heated in a platinum crucible at 300° C. for 2 hours and 900° C. for 2 h. The material was reground into fine powder using a ball mill for 2 hours by a wet milling process. The dried powder was reheated at 900° C. for 2 hours and then ball milled again for 5 h. The resultant milled powder was pressed into pellets. The pellets were fired at 1050° C. for 2 hours and cooled to room temp. The ionic conductivity of the prepared pellets was measured to be 1.03×10 −3  S/cm. 
         [0026]    Preparation of LiMn 2 O 4  was accomplished as follows. A stoichiometric mixture of Li 2 CO 3  and MnO 4  was ground and heated at 350° C. for 2 hours and then heated at 850° C. for 24 hours, followed by natural cooling. 
         [0027]    In the preparation of electrodes for an all-solid-state cell and a hybrid electrolyte cell, the LiMn 2 O 4  was mixed with the solid electrolyte and carbon in a weight ratio of 25:25:3 by using agate mortar and pestle. For each electrode a symmetric cell ten mg of mixture was used. 
         [0028]      FIG. 2  schematically illustrates a solid state cell  10 . Electrolyte powder (20 mg) was pelletized under the 1.75 Tones inside the aluminum tube with inside diameter of 6.4 mm. An electrode powder of 10 mg for each side was added to the pelletized electrolyte layer and pressed under the same pressure one by one. Three layers were hand pressed and were sandwiched by two stainless steel cylinders with a 6.4-mm-diameter. The cell  10  was charged and discharged at constant current of 0.05 mA at the temperature of 20° C. 
         [0029]    After the electrolyte powder (20 mg) was pelletized under the 1.75 Tone inside the aluminum tube, 2 mg of liquid electrolyte was added between each electrode and electrolyte layers of all solid state symmetric cell of LiMn 2 O 4 /Li 1.5 Ti 1.7 Al 0.5 (PO 4 ) 3 /LiMn 2 O 4  in argon filled dry box. The electrode powders (10 mg) for each side were added to the pelletized electrolyte and liquid electrolyte layer then those were pressed together at 2 Tone into a three layered pellet of 6.4-mm-diameter. The experiment was performed under a hand pressure vise with stainless steel current collectors on both sides. 
         [0030]    The electrodes  15  for the coin cell  10  were fabricated from a 70:20:10 (wt %) mixture of active material, carbon as the current conductor and polytetrafluoroethylene as binder. The mixture was rolled into thin sheets and punched into 7-mm-diameter circular disks as electrodes. The typical electrode mass and thickness were 5-10 mg and 0.03-0.08 mm. The electrochemical cells  10  were prepared in standard 2016 coin-cell hardware with lithium metal foil used as both the counter and reference electrodes. The electrode disks  15  and cells  10  were prepared in an argon glove box. The electrolyte  25  used was 1M LiPF 6  in a 1:1 ethylene carbonate/diethyl carbonate. 
         [0031]      FIG. 3  shows the Electrochemical Impedance Spectroscopy (EIS) of the LiMn 2 O 4  electrode  15 /Solid Electrolyte  25 /LiMn 2 O 4  electrode  15  cell  10  at different pressures, 350, 700, and 1300 psi, respectively. Any pressure higher than 1300 psi risks rupture of the Al 2 O 3  tube. Only one semicircle is observed for all samples. The left intercept of the semicircle with real axis corresponds to the solid electrolyte resistance (R SE ). The semicircle corresponding to the solid electrolyte is not generally observed in the high frequency region over 1 MHz due to its low resistance. 
         [0032]    The size of the semicircle reflects the interface resistance (R IR ) between solid electrolyte particles or electrolyte/electrode particles. The total resistance (R SE +R IR ) of the samples is therefore obtained from the right intercept of the semicircle with the real axis in the plots. The total conductivity (σ t ) of the cell  10  can be calculated from the measured total resistance (R SE +R IR ) of the cell  10 . 
         [0033]    The impedance spectroscopy clearly shows that the total resistance of the cell  10  decreases as pressure increases. Both of the bulk and grain boundary resistances decrease at higher pressure. This is because higher pressure provides better contact between the solid electrolytes (reducing R SE ) and between the electrolyte/electrode (reducing R IR ). 
         [0034]      FIG. 4A  shows the charge and discharge voltage test for the all-solid-state battery cell  10  composed of LiMn 2 O 4 /S.E./LiMn 2 O 4 . The pressure of 1300 psi is kept during the measurement to minimize the resistance of the cell  10 . The cell  10  displays a smooth charge voltage curve during the first charge of the cell  10 . The capacity reached 120 mAh/g. A current rate of 0.02 mA was selected. When a current higher than 0.02 mA is applied, a proper charge/voltage curve could not be maintained, which is common among other all-solid-state battery cells  10 . Although elevated pressure was applied to the cell  10  a second time, the resulting capacity and cycle-life were not acceptable even at the low current rate of 0.02 mA. 
         [0035]      FIG. 4B  shows impedance profiles of the as-prepared all-solid-state cell  10  after charging to 1.6 V. There is a dramatic change of the impedance spectra after the first charging to 1.6 V. Two semicircles are observed at high frequency and low frequency region. The left intercept of the high frequency semicircle with real axis is the same as that of the as-prepared sample, which indicates that the solid electrolyte resistance (R SE ) doesn&#39;t change even after charging the cell  10 . 
         [0036]    During the charging process, Li-ion extracts from Li 1-x Mn 2 O 4  in the cathode  30  and in the anode  35  Li-ion inserts into the Li 1+x Mn 2 O 4 . It is commonly known that there is a large interface resistance between the intercalation electrode  15  and solid electrolyte  25 . As a result, two semicircle regions can be regarded as the resistances at Li 2 Mn 2 O 4 /SE and Mn 2 O 4 /SE interfaces. The increase in interface resistance during the first charge will be the likely cause of decrease of the capacity following discharge and charge of the cell  10 . 
         [0037]    The interface resistance between solid electrode  15  and solid electrolyte  25  is quite a challenge for the all-solid-state battery  10 . Although they initially have good contact under high pressure, the volume change of the electrode during Li insertion/extraction on charging/discharging is a critical problem. To address this problem, many studies have been done on coating ceramic onto the surface of the electrode materials to solve this problem. However, their performance is not comparative with that of liquid electrolyte. Therefore, adding a very small amount of liquid  80 , just enough to make good contact between the solid electrolyte/solid electrode  25 , 15  allows volume adjustment during cycling of the cell  70 . 
         [0038]    When 20 mg of solid electrolyte  75  and 10 mg of each electrode  15  are used, 2 mg of liquid electrolyte  80  is used between each electrode  15  and electrolyte  75 , and the cell  70  is pressed under 1300 psi.  FIG. 5  shows the impedance spectra of the hybrid electrolyte cell  70  compared with the solid electrolyte cell  10 . The liquid  80  may be added to a solid electrolyte body  25 , or may incorporate a plurality of inorganic particles  75  suspended in an organic liquid matrix  80   
         [0039]    The size of the semicircle corresponding to the interface resistance decreases in the hybrid cell  70 . In addition, the electrolyte resistance indicated by the left intercept of the semicircle with real axis also decreases to 80 ohm compared to 420 ohm of the solid electrolyte cell  10 . So, total resistance decrease from 850 ohm to 110 ohm. 
         [0040]    Even under high pressure, there will always be space between solid electrolyte particles  75 , electrode  15  particles, and between solid electrolyte  25 /solid electrodes  15  in general. The addition of liquid electrolyte  80  fills the gap between any of these solid particles. This can provide better Li-ion mobility in the hybrid cell  70 .  FIGS. 5B and 5C  show the pathways of Li-ion in solid electrolyte cells  10  and hybrid cells  70 . The total conductivity is calculated to be 2.84×10 −4  S/cm for the solid electrolyte cell  10  and 2.03×10 −3  S/cm for the hybrid cell  70 . The ionic conductivity of 2.03×10 −3  S/cm for the hybrid cell  70  is similar to the ionic conductivity, 1.0×10 −3  S/cm, of Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 3  pellet, but smaller than that (2.0×10 −2  S/cm) of liquid electrolyte  80 . 
         [0041]    Even though less than 10 wt % of liquid electrolyte  80  was used in the novel material, the cell  70  was tested to ensure that electrochemical performance arises from the hybrid electrolyte  45  (combination of solid  25  and liquid electrolyte  80 ) and not just from the liquid electrolyte  80 . Thus, the hybrid electrolyte cell  70  was prepared with non-Li-ion conductive Al 2 O 3  particles instead of using Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 3 . Proper impedance data was indistinguishable over the noise. Further, the hybrid cell  70  could not be charged or discharged with Al 2 O 3  even at very low current rate of 0.005 mA/cm 2 . This supports that the liquid electrolyte  80  typically doesn&#39;t penetrate the solid electrolyte pressed pellet  25  (the solid electrolyte material  25  may be present in the form of a solid body, plurality of particles, a plurality of particles formed into a green body, a plurality of particles sintered into a unitary body, or the like). 
         [0042]      FIG. 9  illustrates one embodiment of a hybrid electrolyte  45 , a housing  100  defined by a wherein which a solid lithium ion conductor  25  (in particulate form) is distributed and an organic lithium ion conducting liquid  80  is likewise distributed therein. The nonconducting matrix  100  may be a porous polymer, a foam, a gel, a fibrous matrix, or the like. In other embodiments, the housing  100  may be a coin shell, a cylinder, or the like, and may be formed of a polymer, a ceramic, a metal, a composite, or the like. 
         [0043]      FIG. 6A  shows the five cycles of charge and discharge voltage curves of the hybrid electrolyte cell  70 . Compared with the pure solid electrolyte cell  10  of  FIG. 4A , the hybrid cell  70  provides much better second discharge and following cycle capacity. This capacity is observed to be better than that of the pure liquid electrolyte  80  coin cell  90  in  FIG. 6B . 
         [0044]    Both sides of electrodes  15  are LiMn 2 O 4  as anode  35  and cathode  30 . In liquid electrolyte  80 , the electrode spinel LiMn 2 O 4  gives rise to an electrode-electrolyte reaction. The electrode surface disproportionation reaction 2Mn 3+ =Mn 2+ +Mn 4+  results in dissolution of the Mn 2+  from the electrode  15  into the electrolyte  80 . This reaction, unless suppressed, gives an irreversible capacity loss of the electrode  15  and migration of the Mn 2+  across the electrolyte  15  to the anode  35  during charge and blocks Li-ion insertion into the anode  35 . This eventually leads to poor cycle life of the cell  90  using LiMn 2 O 4  electrode  15 . 
         [0045]    In addition to poor cycle life, a large capacity loss between first charge and discharge is commonly observed in liquid electrolyte  80 . This is also observed in  FIG. 6C . On the other hand, the hybrid electrolyte cell  70  gives a larger first charge capacity, but it provides less capacity loss in the following discharge. This means that the less use of liquid electrolyte  80  improves the electrochemical properties of a LiMn 2 O 4  electrode  15 . 
         [0046]    This shows the advantage of the use of the hybrid electrolyte  45  over pure solid electrolyte  25  and liquid electrolyte  80 . Solid electrolyte  25  was used for the major electrolyte part to improve safety of batteries, and use liquid electrolyte  80  for minor part to provide better interface between solid electrode  15  and solid electrolyte  25 . The smaller Li-ion conductivity of a solid electrolyte  25  compared to that of liquid  80  can be a problem for high current rate battery applications, but the hybrid system  45  combines the advantages of both to minimize current rate limitations. 
         [0047]    Another advantage of the use of a hybrid electrolyte system  45  over the use of pure liquid electrolyte  80  is that this hybrid system  45  can behave as a self-safety device when sudden higher temperature is applied.  FIG. 8  shows that during the charge of the coin cell  90  that used liquid electrolyte  80 , the temperature increases and then the charge voltage drops. This would indicate the reaction between electrode  15  and electrolyte  80  occurs, which if not continued would cause catastrophic failure of the battery  90  producing gas then fire. However, for the cell  70  with a hybrid electrolyte  45 , when the temperature increases, the voltage drops and thus the cell  70  stops. 
         [0048]    While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.