Electrochemical cell

Abstract

An electrochemical cell (100) comprising an alkali metal anode (110) and a solid electrolyte (140), the cell (100) comprising: a glass, ceramic or glass-ceramic solid electrolyte (140) having a roughened surface, which is held in electrical and mechanical direct contact with the alkali metal anode (110) by an applied pressure (P); and a rigid housing (160), wherein the rigid housing (160) exerts the pressure (P).

Description

Methods for manufacturing high-performance, long-life batteries using an alkali metal, particularly lithium or sodium, as an anode and a solid electrolyte are provided.Batteries based on lithium chemistry are increasingly prevalent applications requiring high energy density, such as automobiles and many other consumer goods such as telephones and cameras. Most lithium chemical batteries use lithium in a suitable receptor / donor material that is capable of receiving and accepting lithium ions during the charging and discharging cycles. The electrodes are then separated by a porous polymeric film separator and immersed in an organic lithium ion conducting liquid electrolyte. Apart from the high energy densities, such lithium ion batteries lose a minimal amount of charge when not in use and do not exhibit any memory effects.However, the energy density of existing rechargeable batteries can be further improved by using metallic lithium electrodes. However, lithium metal can react with and consume the liquid electrolytes common in lithium ion batteries, leading to a progressive reduction in cell capacity over several charge-discharge cycles. This lithium reactivity has led to the development of solid electrolytes which both conduct lithium ions and serve for physical and electrical insulation of anode and cathode.Although these lithium batteries may provide a higher energy density than lithium ion batteries, they may often exhibit a progressive reduction in energy density after repeated charge / discharge cycles. In many cases, the reduction in energy density may be sufficient to cancel the initial advantage over lithium ion batteries.US 2017 / 0 092 981 A1 describes a method for producing a solid state battery, comprising heating a flux disposed between a solid ceramic electrolyte and a Group 1 metal. The flux may be heated to roughen a surface of the solid ceramic electrolyte and melt and adhere the group 1 metal to the surface of the solid ceramic electrolyte.US 2011 / 0 287 292 A1 describes a solid battery including a wound solid electrolyte / electrode unit and a case housing the solid electrolyte / electrode unit, wherein a pressurized liquid is filled between the inner circumferential surface of the case and the solid electrolyte / electrode unit.It can be considered an object to generally improve the long-term power density, in particular of lithium-metal batteries and alkali metal batteries.The object is achieved by an electrochemical cell according to claim 1. methods are also described with which the electrochemical cell according to the invention is produced.This specification relates to improving the long-term energy density of secondary rechargeable batteries using an alkali metal, for example, lithium or sodium, as an electrode and a solid electrolyte as both an ion conductor and a separator. The processes can be used separately, and better results are achieved by the combination of the processes.According to an exemplary method, a surface of the solid electrolyte is roughened by any suitable means including, but not limited to, mechanical abrasion, laser ablation, and chemical etching to obtain a greater effective surface area on the electrolyte than is possible with a smooth planar surface. The degree or severity of roughening is determined both by the improvement in the energy density and by the tendency of the generally glassy or ceramic solid electrolyte to fracture under mechanical loading.According to an exemplary method, the alkali metal electrode material is pressed against the surface of the solid electrolyte under pressure. As a result of the applied pressure, the alkali metal flows over a large area and fills the depressions in the roughened solid electrolyte surface substantially completely, with the result that the electrode takes up substantially the entire larger effective surface. The initial pressure is maintained during operation of the battery and serves both to ensure continuity of electrical contact between the alkali metal electrode material and the fixed electrode during discharge.A high capacity alkali metal anode, an electrochemical cell having the above characteristics, can be produced by assembling under a pressure greater than the flow stress of the alkali metal anode, the alkali metal anode and a roughened solid electrolyte.The foregoing practices are applicable to a number of solid electrolyte families including, without limitation; sulfide, oxide, and oxysulfide glasses, glass-ceramics and ceramics; garnets; anti-perovskites; perovskites; and materials having a structure analogous to the NASICON (sodium (NA) SuperIonic CONDUCTOR) structure, of which LATP (Li 1.3 Al 0.3 Ti 1.7( PO 4)3 is a non-limiting example.FIGS. 1A, B, C schematically illustrate a part of the anode and the solid electrolyte in a metal anode battery. In particular, the figures illustrate how the arrangement of atoms in the metallic anode as a finished battery (FIG. 1A ) initially discharges, which leads to a partial loss of the electrode contact (FIG. 1B ) and then to a substantially complete loss of the electrode contact (FIG. 1C ).FIGS. 2A, B schematically illustrate the reduction of the electrode contact area of a metallic anode battery after repeated discharge and charge cycles.FIG. 3 schematically illustrates the surface of a solid electrolyte mechanically roughened by linear movement of an abrasive in contact with the surface.Figures 4A, B compare the discharge power of two Li-Li cells. FIG. 4A illustrates the behavior of a conventionally fabricated cell, while FIG. 4B illustrates the behavior of a cell fabricated according to the methods of this specification.FIG. 5 illustrates, in partial section, a representative prismatic cell that may integrate a roughened solid electrolyte and operate under elevated pressure.Batteries having cells using alkali metal anodes, generally lithium and sodium, have a great potential for applications where high energy density batteries provide performance enhancements. These applications may involve, for example, transportation applications such as in electric or hybrid cars and trucks, and consumer electronics applications such as computers and telephones.Lithium ion batteries usually use liquid electrolytes, the electrolytes customary in lithium ion batteries reacting slowly with lithium metal. As a result, lithium batteries using liquid electrolytes lose their capacity over relatively few charging and discharging cycles. For this reason, lithium anode and alkali metal anode batteries generally use solid electrolytes. These solid electrolytes can also serve as a separator that maintains electrical isolation between the cell anode and the cathode and prevents dendritic shorting of the cell and overheating or thermal leakage of the cell. However, a metal anode battery with solid electrolyte is liable to fail prematurely due to contact failure.In a Li battery with solid phase electrolyte, an interface is established between the solid Li metal and the solid phase electrolyte. The solid phase electrolyte may be, inter alia, a sulfide or oxysulfide glass or glass ceramic. Since a solid phase electrolyte promotes coulombic efficiency near the unit, the battery does not lose capacity over time. However, the solid-solid interface contact is prone to contact failures, wherein during discharge sufficient Li is removed from the interface so that the Li metal electrode loses electrical contact with the solid electrolyte over a large portion of the electrolyte surface, resulting in a rapid increase in internal resistance, followed by cell failure due to the resulting high local current densities.Due to such contact loss, conventionally processed metal anode batteries are limited to a capacity of about 200 μAhcm -2( microampere hours per square centimeter). Such capacity is sufficient for the thin-film "coin cell" batteries, which are often used as embedded power sources in consumer electronics. However, this capacity, 200 μAhcm -2, is significantly less than the capacity of 1000 μAhcm -2, or greater, which is required for lithium metal anode batteries as a replacement for lithium ion batteries in high current and high power batteries, for example in hybrid or battery vehicles, or for battery powered hand tools or garden tools such as drilling machines, saws, lawn trimmers and mowers, and many more.The detachment of metal anode atoms from the anode-solid electrolyte interface is schematically illustrated in FIGS. 1A-C. FIG. 1A is a fragmentary view of a portion of a metal anode battery 10; the metal anode 12 is comprised of a plurality of metal atoms 20, shown in simplified form as circles in the figure, shown in crystalline atomic arrangement with the innermost layer of atoms contacting a solid electrolyte 14 (atomic details not shown). Most alkali metals assume a body centered cubic structure at room temperature or about 25° C., so that the illustrated atomic arrangement may be representative of a {100} plane.In the following discussion, for example, the battery 10 is considered a lithium battery only and without limitation, such that metal atoms 20 comprising a metal anode 12 are lithium atoms in a lithium anode. In the discharge, some of the original lithium atoms 20 are oxidized, releasing electrons to an external circuit (not shown), and generating lithium ions, which are transported through the solid electrolyte 14 to an appropriate cathode (not shown). The passage of lithium ions through the solid electrolyte 14 is represented as ion flux 16. As shown in FIG. 1B, since the oxidation of lithium atoms 20 is non-uniform on the smooth surface 114 of the solid electrolyte, continued operation or discharge of the battery 10 will produce a battery 10' having local voids 120 on the smooth solid electrolyte surface 114 on which no lithium atoms are present. As the discharge continues, the electrolyte area occupied by voids continues to increase until a large number of individual voids accumulate in macroscopic voids, resulting in nearly complete loss of electrical contact in some areas of electrolyte 14, as shown in FIG. 1C. Although other areas of the solid electrolyte surface may be present in which contact with lithium ions is maintained, the local current density drastically increases at these locations and promotes battery failure with continued discharge.Thus, the sustained loss of contact area between the electrode and electrolyte determines the capacity of the battery and not the total volume or mass of lithium metal in the electrode. If the battery discharge is terminated before a battery failure, the battery can be substantially recharged to its original capacity, since the cavities are filled during the charging process. However, the deposition of lithium atoms can only take place in regions in which the electrical contact between electrode and electrolyte is maintained. The lithium deposition thus does not take place uniformly in the entire electrolyte region, but at least first in local regions in which the contact between electrode and electrolyte is maintained, which leads to locally high voltages which have a negative effect on the service life of the battery.In the foregoing discussion, solid electrolyte 14 has been characterized as having a "smooth" surface 114. The term 'smooth' generally denotes the difference between an artificially produced surface and, as discussed below, an artificially and deliberately roughened surface. Two categories of solid electrolytes may be used, solid electrolytes made from powder or powder consolidated layers, typically by sintering the powder, and melt processed layers made from a liquid melt. Typically, the melt-processed layers have at least one glassy or amorphous character, while the powder-processed layers typically have a ceramic character. Generally, including melt-processed glassy solid electrolytes, the typical root mean square (RMS) surface roughness range is 0.001-0.1 micrometers, while a sintered ceramic layer may have a roughness of about 0.5 micrometers to 1 micrometer. Generally, the term 'roughened' refers to a solid electrolyte surface and refers to an artificially roughened solid electrolyte surface. In a melt processed vitreous sheet, the roughened surface may have an RMS roughness of from 0.5 microns to 10 microns, while in a powdered layer the RMS roughness may be between 2 and 10 microns.A later phase of this process is schematically depicted in FIG. 2A, which illustrates a near-production cell having a metallic anode 12, a cathode 30, and a solid electrolyte 14 therebetween. A suitable cathode may be made without limitation from NMC, a mixed oxide formulation having the general composition Li[Ni y Co (1-2y) Mn y] O 2. During operation, lithium is incorporated into the NMC during the discharge and is removed from the NMC during a subsequent charging process. This macroscopic representation corresponds to the structure on the atomic level shown in FIG. 1A. The same cell is shown in Figure 2B in a substantially discharged state after a discharge cycle indicated generally by the arrow labeled +i, representing the transport of lithium ions (Li+) through the solid electrolyte 14. The cell of Figure 2B now comprises a metallic anode 112 having a roughened or 'undulated' surface in contact with a smooth solid electrolyte surface 114 and a significantly thicker cathode 30'. As shown in FIG. 2B, the arrangement of the remaining contact pads between the partially oxidized anode 112 and the electrolyte surface 114 is shown as a series of regularly spaced contact pads 45, but for clarity only. The arrangement of the contact pads may take any configuration, regular or irregular, depending on the cell chemistry, the original anode geometry, the discharge rate, the cell temperature, or other parameters that may affect the oxidation of the anode.In the configuration shown in FIG. 2B, contact between the anode 112 and the solid electrolyte 14 is established across a plurality of substantially discrete contacts 45, rather than across the entire smooth solid electrolyte surface 114, as shown in FIG. 2A for the cell in its manufactured state. The extent of such reduction of contact may be significant. As an example, a lithium anode cell with a smooth solid electrolyte at a current draw of 1000 μAcm -2( microamperes per square centimeter) showed a reduction in apparent electrode area by a factor of 4 after oxidation of about 200 μAhcm -2 lithium, i.e., the apparent anode area was 25% of the nominal anode area, resulting from localization of lithium oxidation. The significant reduction in the current-carrying section of the anode leads to local current densities which exceed the nominal current density by a factor of 4 or more. This in turn promotes further localization of lithium oxidation and generates local voltages during the subsequent charge cycle. These local voltages may be sufficient for the solid electrolyte to break and may result in an internal electrical short circuit within the cell.In one embodiment, it has been determined that increasing the effective surface area of the solid electrolyte upon direct contact with the alkali metal anode increases the life of metal anode cells. In general, when an alkali metal intercalation cathode is used, the opposite surface of the solid electrolyte in direct contact with the cathode may be smooth. As a non-limiting illustration of such roughening, solid electrolyte 14 is illustrated in a practical perspective illustration in FIG. 3 after mechanical roughening on its alkali metal anode contact surface 114. This mechanical roughening may be suitably performed by contacting an abrasive article (not shown) with the previously smooth surface 114, applying pressure to the abrasive article, and then pulling or pushing the abrasive article in the direction shown by arrow 60. The resulting surface, shown in Figure 3, comprises portions of the remaining smooth surface 114 having linear grooves or grooves 50 aligned parallel to arrow 60. For graphic clarity, the grooves 50 are shown V-shaped and extend over the entire output surface 114. However, those skilled in the grinding art will appreciate that the abrasive particles in a grinding wheel may wear and break to thereby produce alternating groove cross-sections and grooves that extend only partially across the initially smooth surface 114. Suitable abrasive articles may be bulk material, consolidated particles such as in an abrasive disc, or abrasive particles supported by a paper or cloth backing such as in 'abrasive paper', or loose abrasives directed at a shallow angle to surface 114 by, for example, an air stream. Less directional mechanical roughening can also be achieved by "sandblasting" if the abrasive particles are directed at an almost normal angle to an electrolyte surface. Suitable abrasives may include diamond, carbides, carborundum, alumina and silicaMany solid electrolyte compositions have been proposed. These may include, without limitation, sulfide, oxide and oxysulfide glasses and partially crystallized glass ceramics with glass formers such as P 2 S 5, SiS 2, GeS 2, P 2 O 5 glass modifiers such as Na 2 S, Li 2 S; ceramic phases such as Li 3 PS 4, Li 7 P 3 S 11 and Li 10 P 2 XS 12( X=Ge, Sn, Si); garnets such as Li 7 La 3 Zr2O12(LLZO); anti-perovskites such as X3YO, wherein X may be Li or Na and Y may be Cl or Br; Perovskites such as Li 0.67-x La 3x TiO 3( LLTO); and phosphates such as Li 1.3 Al 0.3 Ti 1.7( PO 4)3( LATP), and materials having an analogous structure (sodium (NA) SuperIonic CONDUCTOR) to that of NASICON, of which LATP (Li 1.3 Al 0.3 Ti 1.7( PO4)3is a non-limiting example.Although the foregoing discussion has focused on mechanical abrasion to roughen the solid electrolyte surface, other approaches to surface roughening including acid etching with suitable protic solvents such as alcohols or organic acids, aggressive acid such as HF, and selective laser ablation with suitable gas, chemical, solid state or fiber lasers may also be effectively employed. It is to be expected that the roughening method chosen is determined by the character (glass, ceramic or glass ceramic) of the solid electrolyte and its composition. The nature of the surface will, of course, affect the nature of the surface with etched and laser-deposited surfaces with overlapping more equiaxed pits than the elongated grooves shown in Figure 3. All of these methods increase the effective surface area of the solid electrolyte and may be used singly or in combination.The degree of roughening is determined by the competing requirements for increasing the electrolyte surface area, it being taken into account that all the above-mentioned possible separator materials have a limited ductility and are therefore at risk of breakage in the event of overloading. The fracture is promoted by the presence of defects including surface defects or cracks as shown in Fig. 3. The fracture stress of materials with limited ductility can be estimated by means of a fracture-mechanical approach. Generally, the fracture mechanics indicate that a material breaks at a stress of σ:where C is of the material and the crack geometry and a is the length of a surface crackThus, modeling, experimentation, or sampling requires balancing the competing requirements of increasing the effective surface area of the electrolyte by roughening the electrolyte surface without roughening the surface so much that the ability of the electrolyte to withstand operating loads is unduly limited. As mentioned above, a suitable range of RMS surface roughness for a solid electrolyte layer or film may be from 2-10 microns for powdered layers or from 0.5-10 microns for melt processed layers when this roughness is measured according to the protocol(s) for measuring the RMS roughness parameter. Because the breaking properties of the possible solid electrolytes differ from each other, some compositions may be suitably roughened more aggressively, resulting in a greater increase in the solid electrolyte surface area while ensuring that the solid electrolyte remains sufficiently robust to withstand the stresses during operation without breaking.In a further embodiment, the metal anode can be connected to the solid electrolyte under elevated pressure. Conventional smooth surfaced lithium metal anode cells are mounted and operated under a pressure of 0.3 MPa to 0.7 MPa. As shown in FIG. 4A, such a cell may fail when discharged at a current density of 1000 μAcm -2( curve I, ordinate 200, scaled in μAcm -2) after about 800 seconds (abscissa 400, scaled in seconds), indicated by the rapid increase in cell voltage (curve V, ordinate 300, scaled in millivolt, mV). This corresponds to a cell capacity of about 200 μAhcm -2. The cell voltage increased due to the development of a large internal resistance by almost complete loss of the electrical contact between glass and Li metal electrode (as shown in FIG. 1C ).The performance of a cell combining a lithium metal anode with a roughened surface electrode mounted and operated under a pressure of about 10 MPa is shown in FIG. 4B. The cell whose performance is illustrated in FIG. 4B is thus operated at a pressure that is more than 10 times that of the cell whose performance is illustrated in FIG. 4B. Again, the cell is discharged at a current density of 1000 μAcm -2( curve I, ordinate 200, scaled in μAcm -2) but the voltage for that cell is stable (curve V, ordinate 300, scaled in millivolt, mV) over a period of 8 hours (abscissa 400', scaled in hours). This corresponds to a cell capacity of about 8000 μAhcm -2 or 40 times the capacity of the cell, the performance of which is shown in FIG. 4A. It will be appreciated that a cell with a roughened electrode operating at higher pressure provides a significant improvement in durability.A useful parameter for characterizing the properties of an electrochemical cell is the critical current density, called CCD for short, which is generally defined as "the amount of current per unit area of the electrode at which an abrupt change in a variable of an electrolytic process occurs.". Generally, moderate roughening of the electrolyte surface, as may be achieved with 1200 grade abrasive paper at a manual pressure between about 5 N and 20 N, combined with an increase in cell operating pressure to about 10 MPa, results in a cell having at least 25% more CCD over an equally sized cell having a smooth electrolyte surface at a pressure in the range of 0.3 MPa to 0.7 MPa.Without being bound by any particular theory, it appears that the operating pressure of about 10 MPa of the aforementioned test cell is greater than room temperature, or about 25° C., wherein the yield stress of the lithium metal is between about 0.85 MPa and 1 MPa. Thus, the higher pressure may promote mechanical deformation of the lithium metal anode, thus forcing the fresh electrode material into the voids formed at the electrode-electrolyte interface, thereby at least partially restoring electrical and mechanical contact between electrode and electrolyte during operation. This means that a maximum benefit of the increasing pressure is achieved when the pressure exceeds the yield stress of the electrode metal and that a minimum further benefit results from a further increase in the operating pressure. Thus, it appears that the pressure used in the above example, about 10 MPa, can be reduced to about 3 MPa without losing the advantages of such extended working pressure. However, it should be noted that a higher overpressure may be required for the narrow, steep, slit-shaped abrasions shown in FIG. 3 than for the less steep depressions which are produced by laser ablation or chemical etching. Similar considerations may also apply to sodium metal electrodes, however, it may be assumed that sodium has a lower yield stress than lithium, so that cells with sodium metal electrodes at even lower pressures may provide better performance than are suitable for lithium metal electrode cells.In a first embodiment, such high pressure cells can be manufactured and assembled into a battery contained within a deformable housing such as a pouch or a thin walled container and maintained under pressure by an external pressure source. Alternatively, in a second embodiment, the cells may be assembled into a battery that is contained within a rigid, robust container and is subjected to the desired pressure during manufacture. A sufficiently robust container furthermore permanently applies the required pressure, since lithium atoms are reversibly transferred from the anode to the counterelectrode (cathode).A representative arrangement applicable to both embodiments is illustrated in Figure 5, which illustrates in partial section a prismatic cell 100 having a labelled alkali metal anode 110, a solid electrolyte 140 and a striped cathode 120 disposed in intermediate layers. The battery can be connected to an external circuit via the tabs 130 (cathode) and 130' (anode). The housing 160 may be a soft bag or a robust, stable housing. If the housing 160 is a soft bag, the pressure P may be applied by an external source, for example hydraulically. If the housing 160 is a robust housing, the various intermediate layers may be subjected to a pressure P during battery assembly and the pressure maintained by inserting the battery assembly under pressure into the close-fitting housing 160. For example, the housing 160 may be assembled without limitation from two parts that serve to transmit the pressure P to the battery assembly. After full pressure has been reached, the parts can then be fastened, for example, to the flanges 162 and 164 (only fragmentarily shown) to the housing 160 while the pressure P is maintained between 3 MPa and 10 MPa. Selection of the metal anode, lithium or sodium metal, can affect selection of the appropriate cathode material. For example only and without limitation, the cathode 120 may comprise the mixed oxide composition LiNi y Co (1-2y) Mn y] O 2( NMC). Anode 110 may be a metallic anode comprised of lithium metal or sodium metal and any commonly occurring impurities. The alkali metal of the anode may be electrochemically deposited mechanically, by thermal evaporation, or after battery assembly. Such electrodeposition may be accomplished by charging the cathode with LiMO 2( where M may be Mn, Co or Ni) and electrochemically plating Li on a copper current collector by oxidizing LiMO 2 to Li 1-x MO 2.It should be noted that the solid electrolytes described have limited ductility and may be brittle in some compositions. These solid electrolytes are not well suited for spirally wound or track wound battery geometries, especially since during battery manufacture an appreciable tensile load must be applied to the wound layers in order to exert the intended pressure on the alkali metal anode. However, if a solid electrolyte composition can bear these stresses during manufacture without damaging or compromising intended use, wound battery geometries can also be used.

Claims

An electrochemical cell (100) comprising an alkali metal anode (110) and a solid electrolyte (140), the cell (100) comprising: a glass, ceramic or glass ceramic solid electrolyte (140) having a roughened surface maintained in electrical and mechanical direct contact with the alkali metal anode (110) by an applied pressure (P); and a rigid housing (160), the rigid housing (160) applying the pressure (P).The electrochemical cell (100) of claim 1, wherein the alkali metal anode (110) is one of lithium or sodium.The electrochemical cell (100) of claim 1, wherein the RMS roughness of the solid electrolyte (140) is in the range of from 2 micrometers to 10 micrometers for a powder processed film electrolyte or from 0.5 micrometers to 10 micrometers for a melt processed film electrolyte and is selected not to affect the ability of the cell (100) to absorb stresses in operation without breaking the solid electrolyte (140).The electrochemical cell (100) of claim 1, wherein the alkali metal anode is a lithium metal anode (110); and wherein the roughened surface is maintained in electrical and mechanical direct contact with the metal anode (110) by an applied pressure (P) between 3 MPa and 10 MPa.The electrochemical cell (100) according to claim 4, wherein the solid electrolyte (140) is a sulfide glass or an oxysulfide glass or a sulfide glass ceramic or oxysulfide glass ceramic.

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