LITHIUM SECONDARY BATTERY AND METHOD OF MANUFACTURE

Abstract

A lithium (Li) secondary battery having a Li buffer layer compressed between a Li metal anode and an electrolyte of the battery cell and a porous structure positioned between the Li metal anode and a current collector of the battery cell. The Li buffer layer is effective in preventing uncontrollable dendrite growth. The porous structure layer is effective in guiding the location of the Li deposition, thereby reducing the volume changes of the Li anode during the charge and discharge cycles of the lithium secondary battery.

Description

BACKGROUND OF THE INVENTION

In a typical solid-state lithium-ion battery, the anode and the cathode are separated by a solid electrolyte. During the charging and discharging of the battery, the lithium ions travel within the electrolyte between the anode and the cathode.

It is well known that the use of a lithium metal anode in a battery cell results in the growth of dendrites at the surface of the anode, as the battery cell experiences multiple charge/discharge cycles and/or a high-rate charge/discharge event. Dendrite growth leads to micro-shorts between the cathode and the anode, resulting in degradation of the battery cell.

Accordingly, what is needed in the art is an improved rechargeable lithium battery cell that is effective in controlling the growth of dendrites on the surface of the lithium metal anode, thereby prolonging the lifetime of the battery.

However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.

While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicant in no way disclaims these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.

The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.

SUMMARY OF THE INVENTION

In various embodiments, the present invention provides an improved secondary (rechargeable) lithium metal battery which exhibits stable charge/discharge performance and controlled volumetric change during charging and discharging of the battery.

In one embodiment, the rechargeable battery of the present invention comprises a solid-state electrolyte. In an additional embodiment, the rechargeable battery cell comprises a hybrid-electrolyte which includes a mixture of liquid and solid electrolyte.

In a particular embodiment, a lithium secondary battery is provided which includes, a lithium (Li) metal anode, a cathode layer and a Li-ion electrolyte layer positioned between the Li metal anode and the cathode layer. The battery further includes a Li buffer layer compressed between the Li metal anode and the Li-ion electrolyte layer, an anode current collector and a porous layer positioned between the Li metal anode and the anode current collector. The Li buffer layer is compressed between the Li metal anode and the electrolyte layer at a pressure of at least 0.1 MPa (megapascal) to maintain the porosity of the porous layer.

The cathode layer may further include, a cathode current collector and a cathode active material positioned between the cathode current collector and the electrolyte layer.

In a particular embodiment, the cathode active material may include LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) powder, carbon black conductive additive and lithium phosphate sulfur chloride (Li₆PS₅Cl) powder at a weight ratio of 70:5:25, respectively.

In general, the Li buffer layer provides high Li-ion conductivity and low electrical conductivity, and the Li buffer layer has a lower Li-ion conductivity than the electrolyte layer. In particular, the Li buffer layer may include lithium and one or more lithium compounds (LiX) selected from lithium chloride, (LiCl), lithium fluoride (LiF), lithium bromide (LiBr) and lithium iodide (LiI). In a particular embodiment, the concentration of the lithium compound in the Li buffer layer may be between about 10 mol % and about 70 mol %, and preferable between about 30 mol % and about 50 mol %. In an additional embodiment, the Li buffer layer may further include a composite comprising solid electrolyte (SE) and one or more lithium compounds (LiX) selected from lithium chloride, (LiCl), lithium fluoride (LiF), lithium bromide (LiBr) and lithium iodide (LiI).

The one embodiment, the electrolyte layer may be a solid Li-ion electrolyte. In particular, the electrolyte layer may be comprised of lithium phosphate sulfur chloride (Li₆PS₅Cl) powder. In an alternate embodiment, the electrolyte layer may be a hybrid-electrolyte comprising a mixture of liquid and solid Li-ion electrolyte.

The porous layer includes at least one mixed-ion-electron conductor and in particular, the porous layer may include Li_6.4Ga_0.2La₃Zr₂O₁₂ (LLZO) and carbon fiber.

While various methods and techniques may be employed in the manufacturing of the lithium secondary battery described above, in a particular embodiment a method for manufacturing the lithium secondary battery may include, providing a cathode layer, positioning an electrolyte adjacent to the cathode layer, positioning a Li buffer layer adjacent to the electrolyte layer, establishing a compressive stress between the Li buffer layer and the electrolyte layer, positioning a Li metal anode layer on the Li buffer layer, positioning a porous structure layer on the Li metal anode layer and positioning an anode current collector on the porous structure layer.

Various embodiments of the present invention provide for the use of a lithium metal anode in a lithium secondary (rechargeable) battery, which significantly improves the volumetric and gravimetric capacity of the battery. The embodiments also prevent inherent issues that are common in battery cells that utilize lithium metal anodes, such as dendrite formation and large volume changes during charge/discharge cycles.

The embodiments are applicable to numerous fields requiring rechargeable batteries, including but not limited to, electric vehicles, portable electronics and various other applications requiring high capacity, long cycle lifetime and minimum volume changes during charge/discharge cycles.

The invention accordingly provides improved rechargeable lithium battery that is effective in controlling the growth of dendrites on the surface of the lithium metal anode and mitigation of the overall volume changes of the battery using a Li buffer layer and a porous structure at the Li metal anode, thereby prolonging the lifetime of the battery

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:

FIG. 1A illustrates a lithium secondary battery during a charge cycle of the battery, in accordance with an embodiment of the present invention.

FIG. 1B illustrates the lithium secondary battery of FIG. 1A during a discharge cycle of the battery, in accordance with an embodiment of the present invention.

FIG. 2 is a graphical illustration of the change in retention rate of conductivity with respect to 0 mol % LiX relative to the concentration of LiX (mol) in the Li buffer layer, in accordance with an embodiment of the lithium secondary battery of the present invention.

FIG. 3 is a graphical illustration of the cell voltage (V) relative to the capacity of a first exemplary lithium secondary battery during a 1^st cycle, in accordance with an embodiment of the present invention.

FIG. 4 is a graphical illustration of the cell voltage (V) relative to the capacity of the first and a second exemplary lithium secondary battery during a 2^nd and 3^rd cycle, in accordance with an embodiment of the present invention.

FIG. 5 is an illustration of a cross-section from a scanning electron microscope (SEM) of the host structure of the porous layer of the lithium secondary battery, in accordance with an embodiment of the present invention.

FIG. 6 is a graphical illustration of the discharge capacity relative to the cycle number of the third exemplary lithium secondary battery, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part thereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the context clearly dictates otherwise.

In lithium metal batteries currently known in the art, the lithium metal anode is deposited on a current collector resulting in a volume change of the lithium metal anode and the overall battery cell, regardless of whether lithium buffer layers are employed on the lithium metal anode.

In various embodiments of the present invention, a porous structure comprising mixed-ion-electron conductivity in the anode of the lithium secondary battery and a lithium (Li) buffer layer are combined, with applied compressive stress, to form the lithium secondary battery of the present invention. The porous structure of the anode forms a scaffolding formation at the lithium metal anode layer that is effective in guiding the lithium deposition to a designated area, i.e., pre-existing pores in the porous structure. As such, the inventive embodiments allow for uniform deposition of the Li metal anode using the Li buffer layer and also provides for mitigation of the overall volume changes of the battery using the porous structure of the Li metal anode.

FIG. 1A illustrates a lithium secondary battery during a charging cycle of the battery 100, in accordance with an embodiment of the present invention. FIG. 1B illustrates the lithium secondary battery of FIG. 1A during a discharging cycle of the battery 100, in accordance with an embodiment of the present invention.

As shown in FIG. 1A and FIG. 1B, an all-solid-state rechargeable (secondary) lithium battery 100 in accordance with an embodiment of the present invention includes a cathode layer including a cathode current collector 105 and a cathode active material 110 positioned on the cathode current collector 105. The cathode active material 110 may be selected from oxides, including but not limited to, LiNi_xMn_yCo_zO₂, LiCoO₂, LiFePO₄. Alternatively, the cathode active material 110 may be selected from other Li composites, such as sulfides. In a specific embodiment, the cathode active material 110 may include LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) powder, carbon black conductive additive and lithium phosphate sulfur chloride (Li₆PS₅Cl) powder at a weight ratio of 70:5:25, respectively. However, this is not intended to be limiting and various other compounds and ratios are within the scope of the present invention to provide the cathode active material 110.

The lithium secondary battery further includes a solid lithium-ion electrolyte layer 115 positioned on the cathode active material 110, a Li buffer layer 125 positioned on the solid lithium-ion electrolyte layer 115. The lithium secondary battery further includes an anode layer including a lithium metal anode 130 positioned on the Li buffer layer 125, a porous layer 135 positioned on the Li metal anode 130 and an anode current collector 140 positioned on the porous structure layer 135. The Li buffer layer 125 is compressed between the Li metal anode 130 and the anode current collector 140.

In general, the Li buffer layer 125 provides a high Li-ion conductivity and low electrical conductivity. The Li buffer layer 125 may be comprised of a lithium alloy at a concentration of between 1%-99%. As such, the Li buffer layer 125 provides ion conductivity and electron insulating properties to inhibit the formation of dendrites on the lithium metal anode 130, thereby preventing micro-shorts from forming in the battery 100.

In a particular embodiment, the Li buffer layer 125 incudes lithium and one or more lithium compounds (LiX). The lithium compounds may include lithium chloride, (LiCl), lithium fluoride (LiF), lithium bromide (LiBr) and lithium iodide (LiI). Table I illustrates the relationship between the ionic conductivity and electrical conductivity based upon the mol ratio between lithium and fluoride in the Li buffer layer 125.

TABLE I
			Electronic
	mol ratio	Ionic conductivity	conductivity
	LiF:Solid Electrolyte	S/cm@25° C.	S/cm@25° C.
LiF-00	0:100	6.7 × 10−3	1.4 × 10−8
LiF-10	10:90	4.2 × 10−3	9.08 × 10−9
LiF-30	30:70	4.6 × 10−3	4.5 × 10−9
LiF-50	50:50	4.2 × 10−3	4.3 × 10−9
LiF-70	70:30	3.1 × 10−3	6.6 × 10−9
LiF-90	90:10	1.6 × 10−3	6.9 × 10−9
LiF-100	100:0	1.8 × 10−9	5.1 × 10−11

The graph 200 of FIG. 2 illustrates the change in retention rate of conductivity with respect to 0 mol % LiX relative to the change in concentration of LiX in the Li buffer layer 125. An ideal characteristic of the Li buffer layer 125 is to exhibit high ionic conductivity and low electrical conductivity. Since LiX has a lower ionic conductivity than the solid Li-ion electrolyte layer 115, a decrease in the ionic conductivity of the Li buffer layer 125, with increasing concentration of LiX, is inevitable. Accordingly, it is suitable to select a LiX concentration for the Li buffer layer 125 that can maintain the high ionic conductivity while also providing lower electrical conductivity. As such, in a particular embodiment, the concentration of LiX in the Li buffer layer 125 is between about 10 mol % and about 70 mol %, and more particularly between about 30 mol % and about 50 mol %.

Also shown in FIG. 1A and FIG. 1B, is a soft solid electrolyte interface (SEI) 120 which is naturally formed between the solid lithium-ion conductor 115 and the lithium buffer layer 125 during the charging and discharging cycles of the battery 100. The soft SEI interface 120 is permeable to lithium ions and is also electrically insulating.

In a particular embodiment, the anode current collector 140 may be comprised of one or more elements including, but not limited to, nickel (Ni), copper (Cu), or a composition of stainless steel (SUS). The porous structure layer 135 may be comprised of one or more mixed-ion-electron conductors. The lithium metal anode 130 may be comprised of high-capacity lithium.

In the battery 100 of the present invention, a compressive stress is established between the solid Li-ion conductor 115 and the lithium buffer layer 125. In a particular embodiment the compressive stress is between about 0.1 MPa and about 100 MPa.

Also shown in FIG. 1A, the porous structure layer 135 comprises a plurality of pores. As shown in FIG. 1B, the pores control the direction of the lithium plating (deposition) during the charging of the battery 100 by establishing compressive stress at the interface. In general, the porous structure layer 135 guides the location of the lithium deposition at the Li metal anode 130, thereby reducing the volume changes of the lithium metal anode 130 during the charge/discharge cycles by filling up the pores of the porous structure layer 135. The introduction of the porous structure layer 135 controls and minimizes the changes of the overall battery cell size, which contributes to the size stability when installing the battery 100 in electric vehicles (EVs) and hybrid electric vehicles (HEVs).

In confirmation of the effect of the Li buffer layer 125 in the battery 100 of the present invention, a pellet-type solid state battery is described and tested. In this exemplary embodiment, the active cathode material 110 was LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) powder, carbon black conductive additive and lithium phosphate sulfur chloride (Li₆PS₅Cl) powder at a weight ratio of 70:5:25, respectively, and 10 mg of the mixed powder was employed as the active cathode material 110. The solid Li-ion electrolyte layer 115 consisted of lithium phosphate sulfur chloride (Li₆PS₅Cl) powder, with 100 mg used for the solid Li-ion electrolyte layer 115. For the Li metal anode 130, 20 μm thick laminated foil and 10 μm thick copper (Cu) were used.

The cathode active material 115, solid Li-ion electrolyte layer 115 material and Li metal anode 130 material described above were placed in a ceramic cylinder with an internal diameter of approximately 10 mm and compressed with stainless cylinders to fabricate a pellet-type solid state battery.

Following the formation of the active cathode material layer 115, the Li buffer layer 125 was formed by compressing a 5 mg powder mixture uniformly dispersed on the solid Li-ion electrolyte layer 115, at a pressure of 100 MPa. The all-solid-state battery was then compacted at a pressure of 400 MPa. The resulting battery prepared by the procedure described above was then connected to a potentiostat, and constant current charge/discharge tests were performed at 0.1 mA with upper and lower cutoff voltages of 4.3V and 2.7V, respectively. Additionally, a compressive stress of 100 MPa was applied to the battery during the tests. The results of the tests are shown in FIG. 3 and FIG. 4

In FIG. 3 and FIG. 4, Example 1 310 is representative of a Li buffer layer 125 consisting of 30 mol % LiF, Example 2 405 is representative of a Li buffer layer 125 consisting of 50 mol % Lif and Reference 1 305 is representative of a Li buffer layer 125 consisting of 0 mol % LiF.

With reference to FIG. 3, in the graph 300 of the 1^st cycle, Example 1 310 started the first charge at approximately 3.5V. However, Reference 1 305 showed an unusual reaction 315 at about 2.0V. As shown, the 1^st cycle Coulombic efficiency was improved by 10% for Example 1 310 when compared to Reference 1 305. With reference to FIG. 4, in the graph 400 of the 2^nd and 3^rd cycles, Example 1 310 and Example 2 405 did not show noticeable capacity decay, at least to the 3^rd cycle. However, Reference 1 305, showed significant reduction in capacity in the 3^rd cycle. The results shown in FIG. 3 and FIG. 4 clearly indicate that the Li buffer layer 125 of Example 1 310 and Example 2 405 enables a uniform reaction on the Li metal anode 130 of the battery 100, which is a desirable feature for a rechargeable Li battery.

An exemplary embodiment for the fabrication of a battery which includes the porous mixed-ion-electron conductor layer 135 may involve the fabrication of a host structure. In the exemplary embodiment, materials of the structure may include an 8 μm thick copper foil as the cathode current collector 105, vapor grown carbon fiber as the host structure, Polyvinylidene DiFluoride (PVdF) as a binder and N-methylpyrrolidone (NMP) as solvent to prepare slurries with the compositions shown in Table II below.

TABLE II
Binder solvent: Mw: 880000 6 wt % in NMP
		Binder
Lot. No	VGCF (g)	Solvent (g)	LLZO (g)	NMP (g)	Total (g)
SuVG_l	1.5	3	0	40	44.5
SuVG_l	1.5	4.5	0	40	46
VGZ1_1	1.5	5	1	40	47.5
VGZ1_1	2.52	6.04	1	40	49.56

The slurries were uniformly mixed in a room temperature (25° C.) for 3 hours at 1000 rpm using a homogenizer. The prepared slurries were applied to the copper foil using a bar coater with 50 mil gap. Following the application of the slurry, the resulting electrode was dried for 12 hours at 120° C. using a vacuum dryer. The dried electrode was then punched to a circular electrode with a diameter of 10 mm and for 24 hours at 80° C. using a vacuum drier, similar to the drying step previously described. To provide the compression properties of the structure, the above host structure was clamped with stainless steel dies, compressed at 0.1 MPa using a hydraulic press and the porosity was then determined by structural analysis and the weight/thickness of the film. Confirmation of the pores present in the above-described host structure of the porous mixed-ion-electron conductor 135 were provided by a scanning electron microscope. The results of the scan are shown in FIG. 5.

An exemplary test battery was fabricated including the cathode layer 105, 110, solid Li-ion electrolyte layer 115 and Li metal layer 130, which were previously formed, with the above-described host structure on top. The layers were stacked and bonded with a compressive stress. In particular, the compressing pressure was controlled to between about 0.1 MPa and about 10 MPa. However, a lower pressure is desired for maintaining the porosity in the porous mixed-ion-electron conductor 135.

To verify the charge/discharge cycle results of the lithium secondary battery of the present invention, the exemplary test battery prepared by the above-described procedure was connected to a potentiostat and constant current charge/discharge tests were performed at 0.1 mA with upper and lower cutoff voltages of 4.3V and 2.8V, respectively. Additionally, the battery cell was compressed at 0.1 MPa during the tests to maintain the porosity of the porous mixed-ion-conductor layer 135. The results of the cycle test are shown in FIG. 6. As can be seen in the graph 600 of FIG. 6, after 30 cycles, Example 3 605, which includes the porous layer 135, shows 10 times more capacity compared with the Reference 1 305, clearly indicating that the cycle performance was improved by absorbing the volume change with charge/discharge reactions by the host layer of the porous layer 135.

The all-solid-state lithium secondary battery of the present invention provides for uniform lithium deposition to prevent the formation of dendrites, while also establishing a controlled growth of lithium in a specific direction and into a designated area of the battery cell. With these distinct features, the battery cell will have a prolonged lifetime with stable charge/discharge performance and controlled volumetric change of the battery cell.

While the illustrated embodiments are directed to a battery comprising a solid-state electrolyte, this is not intended to be limiting and it is within the scope of the invention for the battery cell to alternatively comprise a hybrid-electrolyte including a mixture of liquid and solid electrolyte.

The advantages set forth above, and those made apparent from the foregoing description, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.

Claims

1. A lithium secondary battery comprising: a lithium (Li) metal anode layer;a cathode layer;an electrolyte layer positioned between the Li metal anode and the cathode;a Li buffer layer compressed between the Li metal anode and the electrolyte layer;an anode current collector; anda porous layer positioned between the Li metal anode layer and the anode current collector.

2. The lithium secondary battery of claim 1, wherein the cathode layer comprises: a cathode current collector; anda cathode active material positioned between the cathode current collector and the electrolyte layer.

3. The lithium secondary battery of claim 2, wherein the cathode active material comprises LiNi0.6Mn0.2Co0.2O2 (NMC) powder, carbon black conductive additive and lithium phosphate sulfur chloride (Li6PS5Cl) powder at a weight ratio of 70:5:25, respectively.

4. The lithium secondary battery of claim 1, wherein the Li buffer layer provides high Li-ion conductivity and low electrical conductivity.

5. The lithium secondary battery of claim 1, wherein the Li buffer layer comprises lithium and one or more lithium compounds (LiX) selected from lithium chloride, (LiCl), lithium fluoride (LiF), lithium bromide (LiBr) and lithium iodide (LiI).

6. The lithium secondary battery of claim 1, wherein the Li buffer layer comprises lithium and one or more lithium compounds and wherein a concentration of the one more lithium compounds in the Li buffer layer is between about 10 mol % and about 70 mol %.

7. The lithium secondary battery of claim 1, wherein the Li buffer layer comprises lithium and one or more lithium compounds and wherein a concentration of the one more lithium compounds in the Li buffer layer is between about 30 mol % and about 50 mol %.

8. The lithium secondary battery of claim 1, wherein the Li buffer layer comprises lithium and solid electrolyte (SE) and one or more lithium compounds (LiX) selected from lithium chloride, (LiCl), lithium fluoride (LiF), lithium bromide (LiBr) and lithium iodide (LiI).

9. The lithium secondary battery of claim 1, wherein the Li buffer layer has a lower Li-ion conductivity than the electrolyte layer.

10. The lithium secondary battery of claim 1, wherein the electrolyte layer is a solid Li-ion electrolyte.

11. The lithium secondary battery of claim 1, wherein the electrolyte layer comprises lithium phosphate sulfur chloride (Li6PS5Cl) powder.

12. The lithium secondary battery of claim 1, wherein the electrolyte layer is a hybrid-electrolyte comprising a mixture of liquid and solid Li-ion electrolyte.

13. The lithium secondary battery of claim 1, wherein the porous layer comprises at least one mixed-ion-electron conductor.

14. The lithium secondary battery of claim 1, wherein the porous layer comprises Li6.4Ga0.2La3Zr2O12 (LLZO) and carbon fiber.

15. The lithium secondary battery of claim 1, wherein the Li metal anode layer comprises Li metal and copper (Cu).

16. The lithium secondary battery of claim 1, wherein the Li buffer layer is compressed between the Li metal anode and the electrolyte layer at a pressure of at least 0.1 MPa (megapascal).

17. A lithium secondary battery comprising: a lithium (Li) metal anode layer;a cathode layer;an electrolyte layer positioned between the Li metal anode and the cathode, wherein the electrolyte layer comprises solid lithium phosphate sulfur chloride (Li6PS5Cl);a Li buffer layer compressed between the Li metal anode and the electrolyte layer, wherein the Li buffer layer has a lower Li-ion conductivity than the electrolyte layer;an anode current collector; anda porous layer positioned between the Li metal anode layer and the anode current collector, wherein the porous layer comprises at least one mixed-ion-electron conductor and carbon fiber.

18. A method for manufacturing a lithium secondary battery, the method comprising: providing a cathode layer;positioning an electrolyte adjacent to the cathode layer;positioning a Li buffer layer adjacent to the electrolyte layer;establishing a compressive stress between the Li buffer layer and the electrolyte layer;positioning a Li metal anode layer on the Li buffer layer;positioning a porous structure layer on the Li metal anode layer; andpositioning an anode current collector on the porous structure layer.

19. The method of claim 18 wherein the electrolyte layer comprises solid lithium phosphate sulfur chloride (Li6PS5Cl) and wherein the Li buffer layer has a lower Li-ion conductivity than the electrolyte layer.

20. The method of claim 18, wherein the porous layer comprises at least one mixed-ion-electron conductor and carbon fiber.