Electrochemically Active Interlayers for Lithium Ion Batteries

Abstract

This disclosure is geared toward a lithium ion battery cell system. The system includes an anode current collector in contact with an anode, a separator in contact with the anode, an electrolyte, a cathode, an additive layer situated between the separator and the cathode, and a cathode current collector. The additive layer may include at least one of a transition metal sulfide, a transition metal oxide, and a transition metal phosphate.

Claims

1. A battery cell system comprising: an anode current collector in contact with an anode; a separator in contact with the anode; an electrolyte; a cathode; an additive layer situated between the separator and the cathode, wherein the additive layer comprises at least one of a transition metal sulfide, a transition metal oxide, and a transition metal phosphate; and a cathode current collector in contact with the cathode.

2. The battery cell system of claim 1, wherein the anode comprises a lithium metal anode, a graphite anode, a silicon anode, a lithiated graphite (LixC6, x<1), a lithiated Silicon (LixSi, x<4.4), or a combination thereof

3. The battery cell system of claim 1, wherein the anode comprises a lithium metal anode.

4. The battery cell system of claim 3, wherein the cathode comprises sulfur.

5. The battery cell system of claim 1, wherein additive layer comprises at least one of TiS.sub.2, MnO.sub.2, LiMn.sub.3O.sub.6, LiMn.sub.8O.sub.16, V.sub.2O.sub.5, LiV.sub.3O.sub.8, or LiFePO.sub.4.

5. The battery cell system of claim 1, wherein additive layer comprises ?-MnO.sub.2.

6. The battery cell system of claim 1, wherein additive layer comprises TiS.sub.2.

7. The battery cell system of claim 6, wherein the separator comprises a polypropylene membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 schematically illustrates prior art battery cell system.

[0009] FIG. 2 schematically illustrates an embodiment of a battery cell.

[0010] FIG. 3a schematically illustrates an embodiment of a first sulfur discharge step of a battery cell.

[0011] FIG. 3b schematically illustrates an embodiment of a second sulfur discharge step of a battery cell.

[0012] FIG. 4a schematically illustrates an embodiment of a first MnO.sub.2 discharge step of a battery cell.

[0013] FIG. 4b schematically illustrates an embodiment of a second MnO.sub.2 discharge step of a battery cell.

[0014] FIG. 4c schematically illustrates an embodiment of a third MnO.sub.2 discharge step of a battery cell.

[0015] FIG. 5 is a graph showing the cell cycling discharge capacities vs. the cycle number for the coin cells using a Control Cathode or a Hybrid Sulfur/TiS.sub.2 Cathode.

[0016] FIG. 6 include cell discharge voltage profiles against the percent delivered capacity in embodiments of the invention.

[0017] FIG. 7 is a graph showing sulfur discharge efficiency according to embodiments of the invention.

[0018] FIG. 8 is a graph showing the the cycling capacity retention according to embodiments of the invention.

[0019] FIG. 9 is a graph showing the cyclic voltammetry (CV) performed on a embodiment of a coin cell with ?-MnO.sub.2 cathode.

[0020] FIG. 10 is a graph showing the 1.sup.st cycle discharge and charge voltage profiles of embodiments of the invention.

[0021] FIG. 11 is a graph showing the cell cycling behavior over 35 cycles of an embodiment.

[0022] FIG. 12 is a graph showing the first discharge voltage profiles vs. the theoretical capacity of embodiments of the invention.

[0023] FIG. 13 is a graph showing the second discharge voltage profiles vs. the theoretical capacity of embodiments of the invention.

[0024] FIG. 14 is a graph showing the sulfur utilization vs. the cycle number of embodiments of the invention.

DETAILED DESCRIPTION

[0025] This disclosure provides embodiments of battery cells in which elechtrocehmically an active transition metal sulfide, oxide, or phosphate is an additive in the battery cell. Instead of mixing the additive in with the cathode material as shown in FIG. 1, the additive interacts with the cathode of the battery cell as a discrete layer.

[0026] FIG. 2 schematically illustrates an embodiment of a battery cell, and includes a cathode current collector, a cathode, an additive layer, a separator, anode, and an anode current collector.

[0027] The cathode current collector is typically a conductive layer that may comprise a non-reactive metal such as silver, gold, platinum or aluminum.

[0028] The cathode may be made from a material comprising sulfur. In one embodiment the cathode comprises sulfur. In addition to sulfur the cathode may include at least one of carbon and a fluropolymer. In certain embodiments the carbon may be carbon black. In certain embodiments the fluorpolymer may be polyvinylidene difluoride. In one embodiment the cathode comprises sulfur, carbon black, and polyvinylidene difluoride. In one embodiment the ratio of sulfur:carbon:polyvinylidene difluoride is 60:30:10.

[0029] The additive layer is an active interlayer situated between the separator and the cathode and may comprise at least one of a transition metal sulfide, a transition metal oxide, and a transition metal phosphate. In certain embodiments, the additive layer comprises at least one of TiS.sub.2, MnO.sub.2, LiMn.sub.3O.sub.6, LiMn.sub.8O.sub.16, V.sub.2O.sub.5, LiV.sub.3O.sub.8, LiFePO.sub.4, or combinations thereof.

[0030] A separator such as for example a polyolefin separator may be incorporated with the cell system. The separator may be between the cathode and the anode current collector. In one embodiment, the separator is in direct contact with the additive layer. In one embodiment, the separator is be between the additive layer and the anode.

[0031] The anode may be for example silicon, graphite, carbon, graphene, combinations thereof, or any material known to be used for the anode. The carbon may be in the form of for example a nanotube. The anode when supplied with lithium thus may comprise lithium metal, lithiated graphite, or lithium-Si alloy. In certain embodiments, the anode comprises a lithium metal anode, a graphite anode, a silicon anode, a lithiated graphite (LixC6, x<1), a lithiated Silicon (LixSi, x<4.4), or a combination thereof. In one embodiment the anode is lithium metal.

[0032] The anode current collector may comprise one or more of a variety of materials that can collect current from the anode, contact the anode without being reduced, and allow alkali ions from the anode to pass therethrough. Some non-limiting examples of suitable anode current collector materials include reduced, or pure, copper, nickel, brass (70% copper, 30% zinc), a suitable cermet material (e.g., a Cu/NaSICON, a Cu/LiSICON, etc.), and one or more other suitable materials.

[0033] Additionally, the battery cell may include an electrolyte. The electrolyte suitable in the present cell system may include any electrolyte know in the art. The electrolyte may comprise a liquid, a solid, or a polymer gel-type electrolyte. Specific examples may include but are not limited to a non-aqueous liquid or a solid polymer electrolyte that contains a dissolved lithium salt. In certain embodiments the electrolyte includes a lithium hexafluorophosphate solution in ethylene carbonate and dimethyl carbonate. In one embodiment the electrolyte comprises Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dioxolane and dimethoxyethane (DOL/DME).

[0034] Other components of the lithium battery may include an external encapsulating shell, a cathode terminal, and an anode terminal.

[0035] This approach allows the use of the conventional and commercially available metal oxide or phosphate cathode materials and additives, contributing to an improved sulfur cell performance without increasing the cost associated with the synthesis and process procedures. In addition, the selected additives are also electrochemically active with higher redox potentials than that of the Sulfur system and contributing to the cell capacity, which partially compensates the gravimetric capacity density lost due to the extra weight introduced by the additive. Furthermore, since these additives have high discharge voltage (within 3V to 4V range), their contribution to the energy density (in Wh/g) is even higher. For a sulfur battery, the typical cycling voltage limit is between 2.6-3.0V (charge) to 1.8V (discharge). Within this voltage range, the selected additives are inert electrochemicallyprovide no capacity to the overall cell energy density. In this approach, a wider voltage window is used to activate the additive layer, especially during cell charge. Depending on the material, the charge voltage limit can be increased to as high as 4.0V. Due to the oxidative stability of the ether based electrolyte, the upper voltage limit may be controlled no higher than 4.0V or as low as possible to just get the additive cathode activated. Possibilities of using for example metal oxide material with cycling voltage higher than 4.0V also exist when high-voltage stable electrolytes are used, such as Fluoro-substituted ether based solvents or carbonate based solvents.

[0036] FIG. 2 shows the present invention that maintains the advantages and minimizes the disadvantages from the prior art designs. With this approach, the positive impact of TiS.sub.2 and metal oxide (for example ?-MnO.sub.2) additive on the Sulfur cell performance is maximized: i) by using the wider cell cycling voltage window the reversible capacity of TiS.sub.2 (1.6V to 2.6V) and ?-MnO.sub.2 (1.6V to 4.0V) is fully utilizable contributing to the cell energy density ii) TiS.sub.2, ?-MnO.sub.2, and carbon included in the TiS.sub.2 or MnO.sub.2 layer can act as the polysulfide absorber either physically or chemically to minimize the shuttling effect of polysulfide intermediate, resulting in high Coulombic efficiency; iii) the discrete layer may act as a physical barrier to slowdown the migration of polysulfide from the cathode to the anode while still allowing the lithium ion diffusion; iv) due to the electronic conductivity nature of the transition metal sulfides or the Li-ion conductive ability of the transition metal oxide composite, the TiS.sub.2 or MnO.sub.2 discrete layer can act as a conductive pathway for Li.sub.2S (the discharge product of Sulfur electrode), which were deposited on the surface of the cathode matrix during polysulfide reduction, to be re-oxidized more efficiently for higher sulfur utilization in the subsequent charge cycles, leading to higher sulfur utilization for high energy density and long cycle life; v) the presence of discrete conductive TiS.sub.2 or MnO.sub.2 composite layer on the top of the Sulfur cathode may allow the usage of high sulfur electrode loading with higher active Sulfur % cathode formulation, resulting in higher cell energy density and low cost; vi) under the charged state, the high electrochemical potential of the MnO.sub.2 additive may facilitate the conversion of soluble polysulfide into the insoluble sulfur which helps to maintain Sulfur electrode stability with minimized self-discharge. Furthermore, the presence of a discrete TiS.sub.2 layer on the top of the Sulfur electrode, the Sulfur utilization can be improved from ?40-50% to up to ?70-80%a net 30% improvement from ?670-837 mAh/g to 1172-1339 mAh/g as shown in the following examples.

[0037] The mechanism of cell cycling for an embodiment using a discrete TiS.sub.2 additive layer is schematically described in FIG. 3. FIG. 3a demonstrates the 1.sup.st step sulfur discharge between 2.5V to ?2.0V where electrolyte soluble polysulfide intermediates Li.sub.2S.sub.n (n=8 to 3) are formed. At this stage, the polysulfide products dissolve in the electrolyte and migrate out of the sulfur cathode matrix and diffuse into the TiS.sub.2 composite layer that contains TiS.sub.2 (or partially reduced TiS.sub.2), carbon additive and binder. The polysulfide will interact with TiS.sub.2 and carbon to be absorbed either physically or chemically. Depending on the efficiency of the TiS.sub.2 layer and polysulfide interaction, small amount of polysulfide can still pass through the TiS.sub.2 and the separator layers to reach the anode surface. In this situation, partial shuttling effect could still exist, but with the reduced scope. During this stage, TiS.sub.2 will also be partially reduced to produce Li.sub.1-zTiS.sub.2 (0<z<1) and contribute capacity to the cell discharge. FIG. 3b shows the 2.sup.nd stage discharge from 2.0V to 1.6V, where the high order soluble polysulfide intermediates (n=8 to 3) are further reduced to the low order insoluble polysulfide products (x=2 to 1) which deposit on the surface of the cathode and the TiS.sub.2 layer, especially on the electrolyte/Sulfur cathode interface. The final product is Li.sub.2S (x=1), which is an insulator and insoluble in electrolyte. At this stage, the partially discharged Li.sub.1-zTiS.sub.2 (0<z<1) will be fully discharged to LiTiS.sub.2. Due to the insulating nature of Li.sub.2S, the additional Sulfur or polysulfide reduction on the cathode surface could be blocked in a regular Sulfur cell, resulting in low Sulfur utilization. However, the discrete conductive TiS.sub.2 layer in current design, which is in direct contact with the sulfur cathode, provides an alternative electron conduction pathway to the Li.sub.2S covered cathode interface for additional sulfur and polysulfide reduction, leading to the improved sulfur utilization.

[0038] During the cell charging, the above processes are just reversed. Again, due to the electronic and ionic conductive nature of the TiS.sub.2 discrete layer, the efficiency of Li.sub.2S oxidation to polysulfide intermediate and then to sulfur can be improved. At the same time, the LiTiS.sub.2 will be charged to TiS.sub.2 providing extra cell capacity.

[0039] The mechanism of cell cycling for an embodiment using a discrete ?-MnO.sub.2 additive layer is schematically described in FIG. 4. FIG. 4a demonstrates the 1.sup.st step MnO.sub.2 discharge between 4.0V to 2.4V where ?-MnO.sub.2 is lithiated to form Li.sub.yMnO.sub.2 (0<y<1) contributes to the cell discharge energy density. Due to the high potentials, Sulfur electrode is not discharged at this stage. FIG. 4b demonstrates the 2.sup.nd step discharge between 2.4V to ?2.0V where electrolyte soluble polysulfide intermediates Li.sub.2S.sub.n(n=8 to 3) are formed. At this stage, the polysulfide products dissolve in the electrolyte and partially diffuse into the MnO.sub.2 composite layer that contains ?-MnO.sub.2, carbon additive and binder. The polysulfide will interact with Li.sub.yMnO.sub.2 and carbon to be absorbed either physically or chemically. The chemical interaction between polysulfide and lithiated ?-MnO.sub.2 may form SSO.sub.3 bond that immobilizes the polysulfide and thus prevents the polysulfide diffusion to the anode surface. If the insufficient MnO.sub.2 material was present, small amount of polysulfide could still pass through the Li.sub.yMnO.sub.2 and the separator layers to reach the anode surface. In this situation, partial shuttling effect could still exist but with the reduced scope. FIG. 4c shows the 3.sup.rd stage discharge from 2.0V to 1.6V, where the high order soluble polysulfide intermediates (n=3 to 8) are further reduced to the low order insoluble polysulfide products (x=1 to 2) that deposit on the surface of the cathode and the LiMnO.sub.2 layer, especially on the electrolyte/Sulfur cathode interface. The final product is Li.sub.2S (x=1), which is an insulator and insoluble in electrolyte. At this stage, the partially discharged LiyMnO.sub.2 (0<y<1) will be fully discharged to LiMnO.sub.2. Due to the insulating nature of Li.sub.2S, the additional Sulfur or polysulfide reduction on the cathode surface could be blocked in a regular Sulfur cell, resulting in low Sulfur utilization. However, the discrete conductive MnO.sub.2 layer in current design, which is in direct contact with the sulfur cathode, provides an alternative electron and Li-ion conduction pathway to the Li.sub.2S covered cathode interface for additional sulfur and polysulfide reduction, leading to the improved sulfur utilization. In addition, the chemical interaction of MnO.sub.2 layer with dissolved polysulfide provides another mechanism to reduce the shuttling effect besides a physical barrier for polysulfide diffusion.

[0040] During the cell charging, the above processes are just reversed. Again, due to the electronic and ionic conductive nature of the MnO.sub.2 discrete layer, the efficiency of Li.sub.2S oxidation to polysulfide intermediate and then to sulfur can be improved. At the same time, the LiMnO.sub.2 will be charged to LiMn.sub.3O.sub.6.sup.24 to the charge voltage of up to 4.0V providing extra cell capacity for the subsequent cycles.

EXAMPLE 1

Control Cathode and Hybrid Cathode

[0041] All chemicals used in this example were used as they were received without further purification. To make a hybrid cathode and a control cathode, Sulfur, TiS.sub.2, Super C65 conductive carbon black, and polyvinylidene difluoride (PVDF) (5 wt % in N-methyl-2-pyrrolidone (NMP)) were combined according to the weight ratios given in Table 1 and mixed thoroughly in a mortar and pestle to form a uniform mixture.

TABLE-US-00001 TABLE 1 Carbon PVDF Sulfur TiS.sub.2 (weight (weight Example (weight ratio) (weight ratio) ratio) ratio) Control Cathode 60 0 30 10 (Sulfur cathode) Hybrid Cathode 60 20 30 10 (Sulfur Cathode mixed with TiS.sub.2 additive)

[0042] NMP was added to the mixture which was further homogenized until a uniform slurry was obtained. After a uniform slurry was obtained, the slurry was coated onto an aluminum foil with a doctor blade, and the coated sample was dried in air for 24 hours, followed by another 24 hours in a vacuum oven at 50? C. Each dried sample punched into two 1.27 cm.sup.2 disks to be used as electrodes. CR2032 sized coin cells were assembled with the punched cathode, a lithium foil as counter electrode, polypropylene membrane as separator, and (LiTFSI) in DOL/DME (1:1 volume %) electrolyte. The cells were tested with an Arbin electrochemical station in Galvanostatic mode. The current density was chosen to be C/5 and the voltage range was set between 2.6 and 1.6V.

[0043] FIG. 5 is a graph showing the cell cycling discharge capacities vs. the cycle number for the coin cells using the Control Cathode or the Hybrid Cathode. The discharge capacity is normalized by the Sulfur weight within the cathode. The data show a benefit effect of S:TiS.sub.2 hybrid cathode cells over the control cell in terms of lower cycling capacity fade. However, the initial discharge capacity for both cases are very similar (between 700-800 mAh/g Sulfur or 42-48% Sulfur utilization)indicating no improvement in Sulfur utilization by just physically mixing TiS.sub.2 in the Sulfur electrode.

EXAMPLE 2

TiS.SUB.2 .Discrete Layer over Sulfur Cathode

[0044] Sulfur cathodes (Control Cathode) were produced as in Example 1. TiS.sub.2 in NW slurry was coated on a polypropylene membrane to form a discrete layer. This discrete TiS.sub.2 layer on membrane was then assembled within the cell stack with the TiS.sub.2 side facing the Sulfur cathode and in direct contact with the Sulfur cathode as shown in FIG. 2 and FIG. 3. The formulation of Sulfur electrode and the discrete TiS.sub.2 layer is fixed as: Active (either S or TiS2):Carbon:PVDF=60:30:10. The theoretical capacity ratio between Sulfur and TiS.sub.2 can be adjusted by modifying the loading of either Sulfur cathode or coated TiS.sub.2 layer on the separator. Examples of this ratio change are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Capacity % Coin Cell # TiS.sub.2 Discrete Layer (TiS.sub.2/[S + TiS.sub.2]) 1 Control Cathode No 0 2 Control Cathode No 0 3 Yes 3.7 4 Yes 4.5 5 Yes 16 6 Yes 17.5 7 Yes 23.1 8 Yes 25.5

[0045] The cells were discharge/charge cycled under C/5 rate (based on the total S+TiS.sub.2 capacity). The cell discharge voltage profiles against the percent delivered capacity (theoretical cell capacity=100%) are shown in FIG. 6. All cells with the discrete TiS.sub.2 layer resulted in higher active cathode material utilization than the Sulfur only cells. The majority of this cathode efficiency improvement may be due to the higher sulfur discharge efficiency as show in FIG. 7.

[0046] In FIG. 7, it is assumed that the TiS.sub.2 material delivers 100% of its theoretical capacity. By subtracting this value from the total cell capacity, the sulfur electrode utilization in all cases can be estimated as shown in FIG. 7 with a linear relationship against the % TiS.sub.2 capacity. The more relative amount of TiS.sub.2, the higher the Sulfur utilization will be realized. The Sulfur utilization is improved from ?50% for the control cell to up to ?80% within this experiment. If TiS.sub.2 discharge efficiency is less than 100%, then the Sulfur utilization could be even higher than the projected value. The cycling capacity retention of the control cells (Cells 1 and 2) vs. the cells with high TiS.sub.2 capacity ratio (Cells 7 and 8) is shown in FIG. 8. Similar to Example 1 (Hybrid Cathode, Sulfur Cathode mixed with TiS.sub.2 additive), cells with TiS.sub.2 coated separator design exhibited lower capacity fade.

[0047] As seen in these examples, sulfur utilization improvement may be achieved by using a discrete layer of TiS.sub.2 (Cells 3-8). Using hybrid cathodes alone (Sulfur Cathode mixed with TiS.sub.2 additive), as demonstrated in Example 1, does not realize this improvement, indicating the Sulfur cathode/electrolyte interface may play a part of the positive STiS.sub.2 interaction. The presence of electronic and ionic conductive TiS.sub.2 layer at the Sulfur cathode and electrolyte interface may facilitate the conversion of soluble polysulfide reduction to form the insoluble lithium sulfide final product.

Example 3

?-MnO.SUB.2 .cathode

[0048] ?-MnO.sub.2 material electrochemical performance was tested by preparing a ?-MnO.sub.2 cathode. The ?-MnO.sub.2 cathode was prepared by mixing ?-MnO.sub.2 with Super C65 conductive carbon black and PVDF binder in the weight ratio of 85:10:5 in a slurry using NMP as solvent. The electrode was tape casted on Al foil and air dried for 24 hours inside a dry room (maximum dew point ?40? C.), then dried at 50? C. in an oven for 24 hours. Coin cells with lithium as anode and 1.0M LiTFSI in DOL:DME=1:1 v/v ratio containing 1 wt % +LiNO.sub.3 as electrolyte were prepared in order to simulate the chemical environment of Sulfur battery. FIG. 9 shows the cyclic voltammetry (CV) performed on a coin cell with ?-MnO.sub.2 cathode. The numbers 1, 2, and 3 in the figure indicates cycle 1, 2, and 3 respectively.

[0049] The CV data indicate the reversible cycling of the ?-MnO.sub.2 electrode within the voltage window of 1.8V to 4.0V vs. Li/Li.sup.+. It also indicates the increasing of the rate capability of this material upon cycles with current density increased from cycle 1 to cycle 2 for charging and from cycle 2 to cycle 3 for discharging. In both cases, the peak potential position also shifted to a higher voltage value. After the above CV test, the cell was placed on cycling test by discharge and charge under ?0.6C rate. The 1.sup.st cycle discharge and charge voltage profiles are shown in FIG. 10. The sloped voltage profiles are observed for both discharge and charge. FIG. 11 presents the cell cycling behavior over 35 cycles. Clearly, the cycling capacity increased in the initial 5 cycles that is consistent with the CV data. The cell cycling is very reversible with delivered discharge capacities stabilized at ?190 mAh/g. This test indicates that ?-MnO.sub.2 can be cycled reversibly within the ether-based electrolyte (same as used for Sulfur batteries) between 1.8V to 4.0V. Under this discharge condition, the MnO.sub.2 electrode delivered 92% of its theoretical capacity (205 mAh/g assuming 0.67 Li intercalation) if the reversible reaction follows the following equation.

LiMn.sub.3O.sub.6+2Li3LiMnO.sub.2

Example 4

?-MnO.SUB.2 .Discrete Layer Over Sulfur Cathode

[0050] ?-MnO.sub.2 was coated on a polyolefin separator to form a discrete layer with a formulation as in Example 3. This discrete ?-MnO.sub.2 layer was then assembled within a cell stack with the ?-MnO.sub.2 side facing the Sulfur cathode and in direct contact with the Sulfur cathode as shown in FIG. 2 and FIG. 4. For comparison, the Sulfur cathode formulation is fixed as: S:Carbon:PVDF=60:30:10 for both control cells and the layered testing cells. To check the electrode design effect, a coin cell using the hybrid mixture of Sulfur and ?-MnO.sub.2 was prepared and tested using formulation of S:MnO.sub.2:Carbon:PVDF=30:40:20:10 to match the active material ratio used for the discrete layer cells. The coin cell construction information and the testing voltage limit for each coin cell are summarized in Table 3.

TABLE-US-00003 TABLE 3 Cell Theoretic Capacity % Sulfur ?-MnO.sub.2 (mAh) Theoretical Cell Testing ?-MnO.sub.2 Loading Loading ?- Total Capacity Voltae Cell # layer (mg/cm.sup.2) (mg/cm.sup.2) Sulfur MnO.sub.2 Cell by ?-MnO.sub.2 Range 1 Yes 1.13 0.80 2.39 0.42 2.81 14.9% 1.8 V-4.0 V 2 Yes 1.21 0.71 2.56 0.37 2.94 12.6% 1.8 V-4.0 V 3 No 1.34 0.00 2.84 0.00 2.84 0.0% 1.8 V-2.6 V 4 No 1.12 0.00 2.37 0.00 2.37 0.0% 1.8 V-2.6 V 5 Mixture 0.90 1.20 1.91 0.31 2.22 14.0% 1.8 V-4.0 V

[0051] The first discharge voltage profiles vs. the theoretical capacity are shown in FIG. 12. For control cells (cells 3 and 4), a typical sulfur cell voltage profile is shownone voltage plateau at ?2.3V represents sulfur reduction to polysulfide and a 2.sup.nd voltage plateau at ?2.1V represents the reduction of polysulfide to form Li.sub.2S.sub.2 and LiS.sub.2. Under C/5 discharge rate, the control cells delivered ?52% of the theoretical capacity. However, the cells with the discrete MnO.sub.2 layer (cells 1 and 2) showed 3 distinct voltage plateau regions. The 1.sup.st discharge voltage region at ?3.0V representing the formation of Li.sub.yMnO.sub.2 where 0<y<1. The 2.sup.nd and the 3.sup.rd voltage plateaus represent the sulfur cell discharge as described above. The hybrid mixture cell (cell 5) also showed similar voltage profiles as cells 1 and 2. To clearly show the MnO.sub.2 discharge voltage plateau, the 2.sup.nd cycle discharge voltage profiles of all the cells are shown in FIG. 13. Overall, the cells with discrete MnO.sub.2 layer delivered an average of ?64% of its theoretical capacity, while the hybrid cell delivered 57% of its theoretical capacity in the 1.sup.st discharge. Assuming the MnO.sub.2 portion of the cathode delivered 100% of its theoretical capacity (205 mAh/g), the worst case Sulfur utilization for the layered cells and hybrid mixture cell can be estimated as shown in Table 4. For both the 1.sup.st and 2.sup.nd discharges, the mixture cell (cell 5) delivered the same Sulfur utilization as control Sulfur cells (cells 3 and 4). While for cell 1 and cell 2 with discrete MnO.sub.2 layer, higher Sulfur utilization (?5% or ?84 mAh/g Sulfur) is observed. Considering this as the worst-case scenario, the % improvement could be much higher if MnO.sub.2 did not deliver 100% of its theoretical capacity.

TABLE-US-00004 TABLE 4 Capacity Sulfur Utilization (% Cell ?-MnO.sub.2 by Sulfur (mAh/g) Theoretical) # layer 1.sup.st discharge 2.sup.nd discharge 1.sup.st discharge 2.sup.nd discharge 1 Yes 1013 859 61 51 2 Yes 943 872 56 52 3 No 843 700 50 42 4 No 904 795 54 47 5 Mixture 848 785 51 47

[0052] For comparison, the sulfur utilization vs. the cycle number for all the cells is shown in FIG. 14. The data indicate that the cells with the ?-MnO.sub.2 discrete layer maintain higher Sulfur utilization than the Sulfur control cells throughout the cycling test; while the hybrid mixture cell exhibited similar Sulfur utilization.

[0053] The above examples demonstrate the use of discrete ?-MnO.sub.2 layer on top of Sulfur electrode to promote Sulfur utilization and improve the cell cycle life. This higher Sulfur utilization cannot be achieved by just mixing Sulfur with the MnO.sub.2 in a hybrid electrode design as demonstrated here. In addition, the widening of the cell cycling voltage window allows the ?-MnO.sub.2 to be electrochemically active as shown here, contributing to the deliverable cell energy density that compensate the gravimetric energy density lost due to the extra weight introduced by ?-MnO.sub.2 discrete layer. The improved cell cycling results also demonstrate the compatibility of ?-MnO.sub.2 and the ether-based electrolyte within the charge-discharge voltage window.