Abstract:
In one embodiment, a lithium-ion battery includes an anode, a cathode, a solid electrolyte layer positioned between the anode and the cathode, and a first protective layer continuously coating a cathode facing side of the solid electrolyte layer, the first protective layer formed on the cathode facing side in such a manner that a space within the solid electrolyte layer opening to the cathode facing side is filled with a first protective layer finger.

Description:
CROSS REFERENCE 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/871,933 filed Aug. 30, 2013, the entire contents of which is herein incorporated by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to batteries and more particularly to lithium-metal batteries. 
       BACKGROUND 
       [0003]    Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion (“Li-ion”) batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (measured in Wh/kg) and energy density (measured in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. 
         [0004]    When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity-increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , and Li 1.1 Ni 0.3 Co 0.3 Mn 0.3 O 2 ) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li 2 S and Li 2 O 2 . Other high-capacity materials including BiF 3  (303 mAh/g, lithiated) and FeF 3  (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira,  Fluoride based electrode materials for advanced energy storage devices.  Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (&gt;800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes). 
         [0005]    Thus the advantage of using a Li metal negative electrode (sometimes referred to as an anode) is the much higher energy density of the entire cell, as compared to cells with graphitic or other intercalation negative electrode. A disadvantage of using pure Li metal is that lithium is highly reactive. Accordingly, the lithium metal has a propensity to undergo morphology changes, which cause structures having a high surface area to form on and around the negative electrode when the cell is being charged. Exemplary high surface area structures include dendrites and mossy structures. 
         [0006]    Dendrites are the most common failure mode for cells with Li metal anodes. The dendrites form with a needle-like structure and can grow through the separator during charging of the cell, resulting in an internal short. “Soft shorts” that burn out rapidly result in a temporary self-discharge of the cell, while “strong shorts” consisting of a higher, more stable contact area can lead to complete discharge of the cell, cell failure, and even thermal runaway. While dendrites typically grow through the separator during charge, shorts can also develop during discharge depending on the external pressure placed on the cell and/or internal volume changes that occur in both the negative and positive electrodes. 
         [0007]    Because Li metal is highly electronically conductive, the surface of the Li tends to roughen as the metal is plated and stripped. Peaks in the surface grow as dendrites during charge. During discharge, some smoothing of the dendrites occurs. Nonetheless, there is typically some roughness that remains at the end of discharge. Depending on the depth of discharge, the overall roughness can be amplified from one cycle to the next. Because the metal is essentially at the same electrochemical potential throughout, potential and, to a lesser extent, concentration gradients in the electrolyte phase drive the change in morphology. 
         [0008]    Related to dendrite initiation and growth is development of the Li morphology, which tends to increase the electrode surface area with cycling and consumes solvent to generate fresh passivation layers. Formation of high-surface-area mossy Li tends to occur during low-rate deposition from a liquid electrolyte, especially if the salt concentration is high. The high surface area combined with high reactivity of Li and flammability of the organic solvent makes for a very reactive and dangerous cell. 
         [0009]    What is therefore needed is an electrochemical cell including a form of lithium in the anode which exhibits reduced susceptibility to internal passage of electrons or solvent. 
       SUMMARY 
       [0010]    In accordance with one embodiment, a lithium-ion battery includes an anode, a cathode, a solid electrolyte layer positioned between the anode and the cathode, and a first protective layer continuously coating a cathode facing side of the solid electrolyte layer, the first protective layer formed on the cathode facing side in such a manner that a space within the solid electrolyte layer opening to the cathode facing side is filled with a first protective layer finger. 
         [0011]    In one or more embodiments the solid electrolyte layer is positioned against a current collector in the anode prior to initial charging of the battery. 
         [0012]    In one or more embodiments the cathode includes a form of sulfur. 
         [0013]    In one or more embodiments the first protective layer comprises AL 2 O 3  or Si 3 N 4 . 
         [0014]    In one or more embodiments the solid electrolyte layer has a thickness of about 100 nm to 50 μm, and the first protective layer has a nominal thickness of between about 1 nm and 50 nm. 
         [0015]    In one or more embodiments the lithium-ion battery includes a second protective layer coating an anode facing side of the solid electrolyte layer, the second protective layer formed on the anode facing side in such a manner that a space within the solid electrolyte layer opening to the anode facing side is filled with a second protective layer finger. 
         [0016]    In one embodiment a method of forming a battery includes providing an anode, providing a cathode, filling a space within a solid electrolyte layer opening to a first side of the solid electrolyte layer with a protective layer finger, forming a continuous coating of a first protective layer on the first side, and positioning the solid electrolyte layer between the anode and the cathode with the first protective layer positioned between the electrolyte layer and the cathode. 
         [0017]    In one or more embodiments forming the continuous coating includes forming the continuous coating of the first protective layer on the first side of the solid electrolyte layer using chemical vapor deposition. 
         [0018]    In one or more embodiments forming the continuous coating includes forming the continuous coating of the first protective layer on the first side of the solid electrolyte layer using atomic layer deposition. 
         [0019]    In one or more embodiments forming the continuous coating includes forming a continuous coating of Al203 or Si3N4 on the first side. 
         [0020]    In one or more embodiments the method includes forming a continuous coating of a second protective layer on a second side of the solid electrolyte layer. 
         [0021]    In one or more embodiments positioning the solid electrolyte layer between the anode and the cathode includes positioning the solid electrolyte layer between the anode and the cathode with the second protective layer positioned between the electrolyte layer and the cathode. 
         [0022]    In one or more embodiments the method includes polishing the second protective layer, and exposing a portion of the second side by polishing the second protective layer. 
         [0023]    In one or more embodiments providing the anode includes providing an anode including a form of lithium. 
         [0024]    In one or more embodiments providing the cathode includes providing a cathode including a form of sulfur. 
         [0025]    In one or more embodiments forming the continuous coating includes forming the continuous coating with a nominal thickness of between about 1 nm and 50 nm. 
         [0026]    In one or more embodiments positioning the solid electrolyte layer between the anode and the cathode includes positioning the solid electrolyte layer directly against a current collector in the anode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  depicts a simplified schematic of an electrochemical cell including a separator structure with a protective layer immediately adjacent to the cathode; 
           [0028]      FIG. 2  depicts a simplified schematic of an anode and an electrolyte layer with defects; and 
           [0029]      FIG. 3  depicts a simplified schematic of the anode and electrolyte layer of  FIG. 2  with a finally formed separator structure including a protective layer which covers one side of the electrolyte layer and which fills at least some of the defects in the electrolyte layer. 
       
    
    
     DESCRIPTION 
       [0030]    For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written description. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one of ordinary skill in the art to which this disclosure pertains. 
         [0031]      FIG. 1  depicts an electrochemical cell  100  which includes an anode  102  with a copper current collector  104 , a cathode  106  with an aluminum current collector  108 , and a separator structure  110 . The anode  102  includes lithium metal or a lithium alloy metal. The anode  102  is sized such that it has at least as much capacity as the cathode  106 , and preferably at least 10% excess capacity and in some embodiments up to 50% excess capacity at beginning of life and full charge to account for Li metal that may be consumed in side reactions that take place during cycling. 
         [0032]    The cathode  106  in one embodiment only includes an active Li-insertion material. In various embodiments the cathode  106  includes a sulfur or sulfur-containing material (e.g., PAN-S composite or Li 2 S); an air electrode; Li-insertion materials such as NCM, LiNi 0.5 Mn 1.5 O 4 , Li-rich layered oxides, LiCoO 2 , LiFePO 4 , LiMn 2 O 4 ; Li-rich NCM, NCA, and other Li intercalation materials, or blends thereof or any other active material or blend of materials that react with and/or insert Li cations and/or electrolyte anions. The cathode  106  may include Li-conducting polymer, ceramic or other solid, non-polymer electrolyte. The cathode Li-insertion materials may additionally be coated (e.g., via spray coating) with a material such as LiNbO 3  in order to improve the flow of ions between the Li-insertion materials and the solid electrolyte, as described in T Ohtomo et al., Journal of Power Sources 233 (2013) 231-235. 
         [0033]    Solid electrolyte materials in the cathode  106  may further include lithium conducting garnets, lithium conducting sulfides (e.g., Li 2 S—P 2 S 5 ) or phosphates, Li 3 P, LIPON, Li-conducting polymer (e.g., PEO), Li-conducting metal-organic frameworks such as described by Wiers et al. “A Solid Lithium Electrolyte via Addition of Lithium Isopropoxide to a Metal-Organic Framework with Open Metal Sites,” Journal of American Chemical Society, 2011, 133 (37), pp 14522-14525, the entire contents of which are herein incorporated by reference, thio-LISiCONs, Li-conducting NaSICONs, Li 10 GeP 2 S 12 , Li 7-x La 3 Ta x Zr 2-x O 12 , wherein 0&lt;X&lt;2, lithium polysulfidophosphates, or other solid Li-conducting material. Other solid electrolyte materials that may be used are described in Christensen et al., “A critical Review of Li/Air Batteries”, Journal of the Electrochemical Society 159(2) 2012, the entire contents of which are herein incorporated by reference. Other materials in the cathode  106  may include electronically conductive additives such as carbon black, and optionally a binder to improve the coherence of the particles in the cathode (such as PVDF). The cathode materials are selected to allow sufficient electrolyte-cathode interfacial area for a desired design. 
         [0034]    The separator structure  110  includes a solid electrolyte layer  112  that conducts Li ions but not electrons. The solid electrolyte layer  112  may be composed of a solid Li conductor (such as ceramics, a garnet material, Li 3 N, LiPON, LiSICON, LTAP, sulfides, and others including those described in the previous paragraph, a composite solid electrolyte including crystalline and glassy ceramics, and polymers such those based on polyethylene oxide). The solid electrolyte structure has a thickness of about 100 nm to 50 gm. 
         [0035]    A secondary protection layer  114  is formed on the side of the separator structure  110  which faces or abuts the cathode  106 . The protection layer  114  is typically about 1 nm to 50 nm thick. The protection layer  114  is a continuous coating over the entire side of the solid electrolyte layer  112 . In one embodiment, the protection layer  114  is applied to the solid electrolyte layer  112  by atomic layer deposition (ALD). In an alternative embodiment, the protection layer  114  is applied to the solid electrolyte layer  112  by chemical vapor deposition (CVD). The advantage of using ALD is that the high temperatures required for CVD are avoided, and the thickness of the coating is very precisely controlled. 
         [0036]    The material used to form the protection layer  114 , in addition to being amendable to formation by ALD or CVD, is preferably transparent to Li+, while being a barrier to electrons and any solvent within the cell  100 . Materials which are used in different embodiments for the protection layer  114  include ionic and electronic insulators such as Al 2 O 3  or Si 3 N 4 , both of which have known ALD processes and, when sufficiently thin, allow Li transport. In other embodiments, Li-conducting materials such as Li 3 N, Li 3 P, and LiPON are used. 
         [0037]    The objective in using a solid electrolyte layer  112  is that typically no ionic liquid or electrons will be able to penetrate. In the formation of the solid electrolyte layer  112 , however, defects may be present. For example,  FIG. 2  depicts an anode  120  with a separator electrolyte layer  122 . The electrolyte layer  122  includes a pinhole defect  124  which extends completely through the electrolyte layer  122  and a pinhole defect  126  which extends partially through the electrolyte layer  122 . In operation, the defects  124  and  126  can be a channel for solvent. Even in completely solid state applications the defects  124  and  126  can enable the growth of dendrites through the defects  124  and  126 . 
         [0038]    As depicted in  FIG. 3 , when the separator structure  128  is finally formed by applying a protective layer  128  to the electrolyte layer  122 , the defects  124  and  136  are filled with protective layer fingers  132  and  134 . Accordingly, there is no path for solvent to flow through the separator structure. Moreover, the fingers  132 / 134  inhibit growth of dendrites therethrough. 
         [0039]    While the embodiment of  FIGS. 2 and 3  depict formation of the protective layer  130  after the separator has been positioned on the anode  120 , in some embodiments the protective layer is formed prior to positioning the protective layer on the anode. This allows a protective layer to be formed on all surfaces of the electrolyte layer. In some embodiments, the surface of the separator structure which is to be positioned on the anode is polished to remove the portion of the protective layer on the anode facing side of the separator structure. In other embodiments, the protective layer is sufficiently thin that it remains. This allows defects extending partially through the electrolyte layer from the anode side to be filled. In further embodiments, the protected solid electrolyte is positioned directly on a copper current collector with lithium being deposited between the copper and the solid electrolyte structure during the first charge cycle. 
         [0040]    The above described embodiments provide a coating over the entire cathode facing surface of an electrolyte layer. The protective layer can be incorporated into a variety of cell types including all-solid-state cells or any cell including a lithium anode. 
         [0041]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.