Abstract
Lithium-containing polymeric films and superionic inorganic lithium-conductors for protecting lithium metal electrodes, which lithium metal electrodes have little to no formation of SEI and dendrite growth.
Claims
1. A battery comprising: at least one Li electrode, said Li electrode coated with a lithium-containing polymeric film.
2. The battery of claim 1 wherein said Li-containing polymer film is LiEG (EG=ethylene glycol).
3. The battery of claim 1 wherein said Li-containing polymer film is LiHQ (HQ=hydroquinone).
4. The battery of claim 1 wherein said Li-containing polymer film is or LiGL (GL=glycerol).
5. The battery of claim 1 wherein said Li-containing polymer film is from the group consisting of LiEG (EG=ethylene glycol), LiHQ (HQ=hydroquinone), or LiGL (GL=glycerol) and combinations thereof.
6. A battery comprising: at least one Li electrode, said Li electrode coated with a superionic inorganic lithium-conductor.
7. The battery of claim 6 wherein said superionic inorganic lithium-conductor is a Li.sub.xAl.sub.yS film.
8. The battery of claim 6 wherein said Li electrode is coated with a plurality of superionic inorganic lithium-conductors.
9. The battery of claim 8 wherein said superionic inorganic lithium-conductors are Li.sub.xAl.sub.yS films having different ionic conductivities at room temperature (RT).
10. The battery of claim 9 wherein said superionic inorganic lithium-conductors have an ionic conductivity of 1.66×10.sup.−3 S/cm when the sub-cycle ratio m/n is 1:4.
11. A battery comprising: at least one Li electrode, said Li electrode coated with a lithium-containing polymeric film and a superionic inorganic lithium-conductor.
12. The battery of claim 11 wherein said Li-containing polymer film is LiEG (EG=ethylene glycol).
13. The battery of claim 11 wherein said Li-containing polymer film is LiHQ (HQ=hydroquinone).
14. The battery of claim 11 wherein said Li-containing polymer film is or LiGL (GL=glycerol).
15. The battery of claim 11 wherein said Li-containing polymer film is from the group consisting of LiEG (EG=ethylene glycol), LiHQ (HQ=hydroquinone), or LiGL (GL=glycerol) and combinations thereof.
16. The battery of claim 11 wherein said superionic inorganic lithium-conductor is a Li.sub.xAl.sub.yS film.
17. The battery of claim 11 wherein said Li electrode is coated with a plurality of superionic inorganic lithium-conductors.
18. The battery of claim 17 wherein said superionic inorganic lithium-conductors are Li.sub.xAl.sub.yS films having different ionic conductivities at room temperature (RT).
19. The battery of claim 18 wherein said superionic inorganic lithium-conductors have an ionic conductivity of 1.66×10.sup.−3 S/cm when the sub-cycle ratio m/n is 1:4.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0027] In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.
[0028] FIG. 1, which includes FIG. 1A illustrates an LMB cell during a discharge process, consisting of an Li metal anode, a cathode, a liquid organic electrolyte, a current collector for the Li anode, and a current collector for the cathode.
[0029] FIG. 1, which includes FIG. 1B illustrates an LMB cell during a charge process.
[0030] FIG. 2, which includes FIG. 2A illustrates an Li metal chip.
[0031] FIG. 2, which includes FIG. 2B illustrates the Li anode in an assembled cell, presenting bumpy surface covered by a layer of SEI.
[0032] FIG. 2, which includes FIG. 2C illustrates the occurrence of Li stripping (discharge process) starting first from bumpy sites.
[0033] FIG. 2, which includes FIG. 2D illustrates new occurrence of Li stripping from new bumpy sites once the old bumpy sites deplete and thereby are covered with some residuals of SEI.
[0034] FIG. 2, which includes FIG. 2E illustrates considerable formation of SEI over the Li anode after the first Li stripping, with the cost of consumption of the electrolyte and Li.
[0035] FIG. 2, which includes FIG. 2F illustrates Li dendrites formed during the subsequent Li plating (charge process), which are covered by an SEI layer.
[0036] FIG. 2, which includes FIG. 2G illustrates a much thicker SEI layer formed on Li surface after the second stripping, with the cost of much more consumption of the electrolyte and Li.
[0037] FIG. 2, which includes FIG. 2H illustrates remarkable accumulation of SEI layers over the significantly corroded Li anode after multiple Li stripping/platting cycles.
[0038] FIG. 3, which includes FIG. 3A illustrates how ALD enables inorganic materials to grow in a layer-by-layer manner at the atomic level.
[0039] FIG. 3, which includes FIG. 3B illustrates how MLD enables polymeric films to grow in a layer-by-layer manner at the molecular level.
[0040] FIG. 4, which includes FIG. 4A shows the proposed general MLD strategy for growing lithium-containing polymeric films and the proposed growth mechanisms.
[0041] FIG. 4, which includes FIG. 4B shows the MLD growth mechanism of LiEG.
[0042] FIG. 4, which includes FIG. 4C shows the MLD growth mechanism of LiHQ.
[0043] FIG. 4, which includes FIG. 4D shows the MLD growth mechanism of LiGL.
[0044] FIG. 5, which includes FIG. 5A shows the quartz crystal microbalance (QCM) measurement of the 200-cycle ALD Al.sub.2O.sub.3 grown on the crystal of QCM. The ALD Al.sub.2O.sub.3 growth presents a linear growth behavior, and the resultant film serves as the starting surface for QCM measurements of the growth of MLD polymeric films.
[0045] FIG. 5, which includes FIG. 5B shows the QCM measurement of the 100-cycle MLD LiGL growth on the pre-deposited ALD Al.sub.2O.sub.3 film, presenting a nearly linear growth.
[0046] FIG. 5, which includes FIG. 5C shows the enlarged view on the very beginning growth of the MLD LiGL in a few cycles on the pre-deposited ALD Al.sub.2O.sub.3 film.
[0047] FIG. 5, which includes FIG. 5D shows the enlarged view on the stable growth of MLD LiGL in three consecutive cycles.
[0048] FIG. 6, which includes FIG. 6A shows the image of pristine nitrogen-doped graphene nanosheets (N-GNS) by scanning electron microscopy (SEM)
[0049] FIG. 6, which includes FIG. 6B shows the SEM image of N-GNS covered by 20-MLD-cycle LiGL film.
[0050] FIG. 6, which includes FIG. 6C shows the elemental mapping on 20-MLD-cycle LiGL supported by N-GNS by energy dispersive X-ray spectroscopy (EDX).
[0051] FIG. 6, which includes FIG. 6D shows the high-resolution spectra of O1 s, C1 s, and Li1 s by X-ray photoelectron spectroscopy (XPS).
[0052] FIG. 7, which includes FIG. 7A shows the QCM measurement of the 30-cycle MLD LiEG growth on the pre-deposited ALD Al.sub.2O.sub.3 film, presenting a nearly linear growth.
[0053] FIG. 7, which includes FIG. 7B shows the enlarged view on the stable growth of the MLD LiEG in a few cycles on the pre-deposited ALD Al.sub.2O.sub.3 film.
[0054] FIG. 7, which includes FIG. 7C shows the QCM measurement of the 20-cycle MLD LiHQ growth on the pre-deposited ALD Al.sub.2O.sub.3 film, presenting a nearly linear growth.
[0055] FIG. 7, which includes FIG. 7D shows the enlarged view on the stable growth of the MLD LiHQ in a few cycles on the pre-deposited ALD Al.sub.2O.sub.3 film.
[0056] FIG. 8, which includes FIG. 8A schematically illustrates one stripping for the right Li electrode (or one plating for the left Li electrode) in a Li/Li symmetric cell.
[0057] FIG. 8, which includes FIG. 8B schematically illustrates one subsequent plating for the right Li electrode (or one subsequent stripping for the left Li electrode) in a Li/Li symmetric cell.
[0058] FIG. 9, which includes FIG. 9A illustrates an Li chip 910 uniformly covered with a coating 920 via an ALD or/and MLD process.
[0059] FIG. 9, which includes FIG. 9B illustrates that the assembled Li chip 911 becomes bumpy and that there are some fractures formed within the coating layer 920. The fractures are covered by some SEI 930.
[0060] FIG. 9, which includes FIG. 9C illustrates that Li stripping first starts from bumpy sites.
[0061] FIG. 9, which includes FIG. 9D illustrates that the bumpy coating layer 920 is flattened due to the Li stripping and the flattened coating helps produce uniform Li-stripping.
[0062] FIG. 9, which includes FIG. 9E illustrates that the coating layer 920 helps form uniform Li-plating and its exceptional ionic conductivity enables uniform Li deposition under the coating layer with little SEI and dendritic formation.
[0063] FIG. 10 illustrates the effects of LiEG coatings on the stability of the symmetric Li/Li cells due to different MLD cycles, compared to bare Li/Li cells in a carbonate electrolyte. The MLD-LiEG films were coated on both Li electrodes in Li/Li symmetric cells for different MLD cycles: 20, 40, and 60. The resultant LiEG-coated Li/Li symmetric cells were then named as LiEG20, LiEG40, and LiEG60, respectively. The results show that the LiEG40 symmetric cell has the best stability. The carbonate electrolyte is 1.2 M LiPF6 in 3:7 EC:EMC and controlled for 20 μL. The testing current density is 1 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0064] FIG. 11 illustrates the effects of the LiGL coatings on the stability of symmetric Li/Li cells due to different MLD cycles, compared to bare Li/Li cells in a carbonate electrolyte. The MLD-LiGL films were coated on both Li electrodes in Li/Li symmetric cells for different MLD cycles: 20, 40, and 60. The resultant LiEG-coated Li/Li symmetric cells were then named as LiGL20, LiGL40, and LiGL60, respectively. The results clearly show that the LiGL20 symmetric cell has the best stability. The carbonate electrolyte is 1.2 M LiPF6 in 3:7 EC:EMC and controlled for 20 μL. The testing current density is 1 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0065] FIG. 12 illustrates the effects of the 60-cycle MLD-LiGL coating on the stability of the symmetric Li/Li cells, compared to bare Li/Li cells in a carbonate electrolyte. The results show that the LiGL60 symmetric cell has a much better stability. The carbonate electrolyte is 1.2 M LiPF6 in 3:7 EC:EMC and controlled for 20 μL. The testing current density is 2 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0066] FIG. 13, which includes FIG. 13A illustrates the effects of the LiGL coatings on the stability of symmetric Li/Li cells due to different MLD cycles, compared to bare Li/Li cells in an ether electrolyte. The MLD-LiGL films were coated on both Li electrodes of the Li/Li symmetric cells for different MLD cycles: 10, 15, 20, 45, and 90. The resultant LiGL-coated Li/Li symmetric cells were then named as LiGL10, LiGL15, LiGL20, LiGL45, and LiGL90, respectively. The results clearly show that, under current density of 2 mA/cm.sup.2 and an areal capacity of 1 mAh/cm.sup.2, an MLD-LiGL coating thicker than 15-cycle MLD-LiGL is preferred and the thicker the lower of the overpotential of the MLD-coated Li/Li cells. The LiGL45 Li/Li cell has achieved the highest record of over 10,000 Li-stripping/plating cycles (over 10,000 testing hours). The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0067] FIG. 13, which includes FIG. 13B illustrates the effects of the LiGL coatings on the stability of symmetric Li/Li cells due to different MLD cycles, compared to bare Li/Li cells in an ether electrolyte. The MLD-LiGL films were coated on both Li electrodes of the Li/Li symmetric cells for different MLD cycles: 15, 20, and 60. The resultant LiGL-coated Li/Li symmetric cells were then named as LiGL15, LiGL20, and LiGL60, respectively. The results clearly show that, under current density of 5 mA/cm.sup.2 and an areal capacity of 1 mAh/cm.sup.2, an MLD-LiGL coating thicker than 20-cycle MLD-LiGL is preferred and the thicker the lower of the overpotential of the MLD-coated Li/Li cells. The LiGL60 Li/Li cell has achieved the highest record of over 20,000 Li-stripping/plating cycles (over 8,000 testing hours). The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0068] FIG. 13, which includes FIG. 13C illustrates the effects of the LiGL coatings on the stability of symmetric Li/Li cells due to different MLD cycles, compared to bare Li/Li cells in an ether electrolyte. The MLD-LiGL films were coated on both Li electrodes of the Li/Li symmetric cells for 100 MLD cycles. The resultant LiGL-coated Li/Li symmetric cells were then named as LiGL100. The results clearly show that, under current density of 7.5 mA/cm.sup.2 and an areal capacity of 1 mAh/cm.sup.2, the LiGL100 Li/Li cell has achieved the highest record of over 10,000 Li-stripping/plating cycles (over 2,600 testing hours). The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0069] FIG. 14, which includes FIG. 14A illustrates the comparison of bare Li/Li and LiGL60 symmetric cells in an ether electrolyte, under a current density of 2 mA/cm.sup.2 and an areal capacity of 2 mAh/cm.sup.2. The results show that the LiGL60 symmetric cell enables a lower stable overpotential. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0070] FIG. 14, which includes FIG. 14B illustrates the comparison of bare Li/Li and LiGL60 symmetric cells in an ether electrolyte, under a current density of 5 mA/cm.sup.2 and an areal capacity of 2 mAh/cm.sup.2. The results show that the LiGL60 symmetric cell enables a lower stable overpotential. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0071] FIG. 15, which includes FIG. 15A illustrates the comparison of bare Li/Li and LiGL60 symmetric cells in an ether electrolyte, under a current density of 2 mA/cm.sup.2 and an areal capacity of 4 mAh/cm.sup.2. The results show that the LiGL60 symmetric cell enables a lower stable overpotential. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0072] FIG. 15, which includes FIG. 15B illustrates the comparison of bare Li/Li and LiGL60 symmetric cells in an ether electrolyte, under a current density of 5 mA/cm.sup.2 and an areal capacity of 4 mAh/cm.sup.2. The results show that the LiGL60 symmetric cell enables a lower stable overpotential. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL.
[0073] FIG. 16, which includes FIG. 16A shows the surfaces of the cycled bare, LiGL10, LiGL20, and LiGL60 electrodes after 700 Li-stripping/plating cycles.
[0074] FIG. 16, which includes FIG. 16B shows the cross sections of the cycled bare, LiGL10, LiGL20, and LiGL60 electrodes after 700 Li-stripping/plating cycles.
[0075] FIG. 16, which includes FIG. 16C shows the XPS depth profiling on bare Li electrode after 10 Li-stripping/plating cycles.
[0076] FIG. 16, which includes FIG. 16D shows the XPS depth profiling on LiGL90 electrode after 10 Li-stripping/plating cycles.
[0077] FIG. 16, which includes FIG. 16E shows the XPS depth profiling on LiGL90 electrode after 50 Li-stripping/plating cycles.
[0078] FIG. 17, which includes FIG. 17A shows the SEM observations of the morphological changes of the bare Li electrode after 24-h stripping at 2 mA/cm.sup.2.
[0079] FIG. 17, which includes FIG. 17B shows the SEM observations of the morphological changes of the bare Li electrode after 24-h plating at 2 mA/cm.sup.2.
[0080] FIG. 17, which includes FIG. 17C shows the SEM observations of the morphological changes of the LiGL60 electrode after 24-h stripping at 2 mA/cm.sup.2.
[0081] FIG. 17, which includes FIG. 17D shows the SEM observations of the morphological changes of the LiGL60 electrode after 24-h plating at 2 mA/cm.sup.2.
[0082] FIG. 18, which includes FIG. 18A shows the SEM observations of the morphological changes of the bare Li electrode after 48-h stripping-plating (24-h stripping and 24-h plating) at 2 mA/cm.sup.2.
[0083] FIG. 18, which includes FIG. 18B shows the SEM observations of the morphological changes of the bare Li electrode after 48-h plating-stripping (24-h plating and 24-h stripping) at 2 mA/cm.sup.2.
[0084] FIG. 18C shows the SEM observations of the morphological changes of the LiGL60 electrode after 48-h stripping-plating (24-h stripping and 24-h plating) at 2 mA/cm.sup.2.
[0085] FIG. 18, which includes FIG. 18D shows the SEM observations of the morphological changes of the LiGL60 electrode after 48-h plating-stripping (24-h plating and 24-h stripping) at 2 mA/cm.sup.2.
[0086] FIG. 19, which includes FIG. 19A is a schematic illustration of Li.sub.xAl.sub.yS ALD processes for an embodiment of the present invention. Various Li.sub.xAl.sub.yS can be produced through tuning the sub-cycle ratio m/n. The different Li.sub.xAl.sub.yS compounds are named as (m:n)LiAlS, where m and n are the sub-cycles of the sub-ALD of Li—S and the sub-ALD of Al—S.
[0087] FIG. 19, which includes FIG. 19B shows how the ionic conductivity of Li.sub.xAl.sub.yS films varies with their compositions at room temperature (RT). (1:4)LiAlS enables the highest ionic conductivity, 1.66×10.sup.−3 S/cm at RT.
[0088] FIG. 20 illustrates the effects of (1:4)LiAlS on the stability of the symmetric Li/Li cells due to different ALD super-cycles, compared to bare Li/Li cells in a carbonate electrolyte. The ALD-(1:4)LiAlS films were coated on both Li electrodes of the Li/Li symmetric cells for different ALD cycles: 50, 100, 150, and 200. The resultant (1:4)LiAlS-coated Li/Li symmetric cells were then named as (1:4)LiAlS50, (1:4)LiAlS100, (1:4)LiAlS150, and (1:4)LiAlS200, respectively. The results clearly show that the ALD (1:4)LiAlS coatings perform stably when their ALD cycles are less than 150 super-cycles. The carbonate electrolyte is 1.2 M LiPF6 in 3:7 EC:EMC and controlled for 20 μL. The testing current density is 1 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0089] FIG. 21 illustrates the effects of the (1:4)LiAlS coatings on the stability of the symmetric Li/Li cells due to different ALD super-cycles, compared to bare Li/Li cells in an ether electrolyte. The ALD-(1:4)LiAlS films were coated on both Li electrodes of the Li/Li symmetric cells for different ALD super-cycles: 10, 20, 25, 75, and 100. The resultant (1:4)LiAlS-coated Li/Li symmetric cells were then named as (1:4)LiAlS10, (1:4)LiAlS20, (1:4)LiAlS25, (1:4)LiAlS75, and (1:4)LiAlS100, respectively. The results clearly show that the ALD (1:4)LiAlS coatings perform stably when their ALD cycles are more than 10 super-cycles of (1:4)LiAlS. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL. The testing current density is 2 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0090] FIG. 22 illustrates the effects of the (1:4)LiAlS50 coating on the stability of the symmetric Li/Li cells, compared to bare Li/Li cells in an ether electrolyte. The (1:4)LiAlS50 symmetric cell achieved a super stable cyclability close to 9000 Li-stripping/plating cycles. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL. The testing current density is 2 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0091] FIG. 23 illustrates a combined ALD-MLD approach for growing hybrid Li-conducting films for an embodiment of the present invention.
[0092] FIG. 24 illustrates effects of the hybrid Li-conducting film of bilayered LiGL15-(1:4)LiAlS50 coating on the stability of the symmetric Li/Li cells, compared to bare Li/Li cells in an ether electrolyte. The bilayered LiGL15-(1:4)LiAlS50 coating was deposited by the combined ALD-MLD approach. The bilayered LiGL15-(1:4)LiAlS50 symmetric cell achieved a super stable cyclability of 1,600 Li-stripping/plating cycles. The ether electrolyte is 1 M LITFSI in 1:1 DOL:DME and controlled for 20 μL. The testing current density is 2 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
DETAILED DESCRIPTION OF THE INVENTION
[0093] Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure, or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.
[0094] Principles Behind ALD and MLD
[0095] The embodiments of the invention may use the techniques of atomic and molecular layer deposition (ALD and MLD). ALD and MLD are two analogous vapor-phase thin-film techniques. Adopting different precursors, ALD exclusively enables the growth of inorganic materials at the atomic level while MLD enables the growth of pure or hybrid polymers at the molecular level. Thus, these two techniques are highly complementary. ALD and MLD both proceed with material growth through alternating gas-solid surface reactions as shown in FIG. 3.
[0096] To exemplify the unique mechanism of ALD, the model process of Al.sub.2O.sub.3 using trimethylaluminum (TMA) and water as precursors is illustrate in FIG. 3a. Two surface reactions are caused by TMA and water, respectively. After each of the two surface reactions, a purge is applied to avoid any direct reactions between TMA and water. The sequence of reaction/purge/reaction/purge constitutes an ALD cycle. With increased cycles, ALD builds up films on a substrate surface with a precisely controlled growth per cycle (GPC), typically 1 Å/cycle. Similarly, as illustrated in FIG. 3b using two homobifunctional organic precursors X and Y, MLD enables a GPC of typically a few angstroms per cycle. Due to their surface-controlled nature during deposition, ALD and MLD both can generate extremely uniform films with low roughness and coat high-aspect-ratio (HAR) structures conformally. For example, ALD has enabled uniform coatings on wafer-scale planar substrates and conformal coatings over nanoporous templates with a HAR of up to 10,000:1. Also, ALD and MLD both typically operate at low temperatures 300° C.).
[0097] Accordingly, in certain embodiments, the present invention provides protective films via ALD and MLD processes aimed at protecting Li anodes. The embodiments of the present invention concern novel Li-conducting films using ALD and MLD. The films and the ALD/MLD techniques may have the following aspects: (1). Forming a uniform and conformal coverage over any shaped Li anodes. (2). Enabling fast Li-ion transportation to Li anodes. (3). Having excellent mechanical strength to inhibit Li-dendritic growth. (4). Having exceptional elasticity and flexibility to accommodate the volume change of Li stripping and platting. (5). Being deposited on Li anodes directly at low temperatures less than 180° C. (6). Being stable in organic liquid electrolytes.
[0098] The Li-Containing Polymeric Films of the Present Invention
[0099] MLD Strategies
[0100] In certain aspects, the embodiments of the present invention concern MLD-deposited Li-containing polymeric films that improve Li-conduction as compared to existing over any MLD-deposited polymers. The Li-containing polymers may be LiEG (EG=ethylene glycol), LiHQ (HQ=hydroquinone), and LiGL (GL=glycerol). The MLD processes are illustrated in FIG. 4. To couple with these organic precursors (i.e., EG, HQ and GL), lithium tert-butoxide (LiO.sup.tBu or LTB) was used as the Li precursor.
[0101] FIGS. 4a-4d provide illustrations of preferred embodiments of the present invention using MLD to grow Li-containing polymers of LiEG (EG=ethylene glycol), LiHQ (HQ=hydroquinone), and LiGL (GL=glycerol) through coupling lithium tert-butoxide (LTB, LiO.sup.tBu) with EG, HQ or GL. (a).
[0102] The general MLD strategy for lithium-containing films and the proposed growth mechanisms of (b) MLD LiEG, (c) MLD LiHQ and (d) MLD LiGL.
[0103] Strategy Verifications Via QCM Measurements
[0104] To verify the feasibility of the proposed MLD processes, quartz crystal microbalance (QCM) was used to measure the growth of LiEG (FIG. 7(a,b)), LiHQ (FIG. 7(c,d)), and LiGL (FIG. 5(b-d)).
[0105] The MLD LiEG was operated at 150° C. with the time sequence of 3-120-0.1-120 s (LTB dose-purge-EG dose-purge). The LTB precursor was heated at 150° C. and EG was maintained at 50° C. during the MLD operation. It is found that the growth rate of LiEG is 17.5 ng/cm.sup.2, as illustrated in FIGS. 7a and 7b.
[0106] The MLD LiHQ was operated at 150° C. with a time sequence of 10-30-2-30 s (LTB dose-purge-HQ dose-purge). Both the LTB and HQ precursors were maintained at 150° C. during the MLD process. When changing the dose time of HQ from 0.5 s to 2 s, the growth rate increase from 6.06 to 12.04 ng/cm.sup.2, as illustrated in FIGS. 7c and 7d. However, when the dose time of HQ is increased to 3 s, the growth rate dropped dramatically. At the same time, it was found that the vapor pressure of HQ nearly vanished, even lower than the vapor using 0.5-s dose. This may be attributed to the evolution of HQ under a such high temperature of 150° C.
[0107] The MLD LiGL was operated at 150° C. with a time sequence of 3.0-60-2.0-60 s (LTB dose-purge-GL dose-purge). The LTB precursor was heated to 150° C. while GL was also maintained at 150° C. during the MLD process. The average mass gain (Δm=m.sub.1+m.sub.2) is ˜200 ng/cm.sup.2/cycle in the initiation region (˜30 cycles starting on an Al.sub.2O.sub.3 film, FIG. 5(c)) while ˜320 ng/cm.sup.2/cycle in the stable growth region (FIG. 5(d)). The stable region exhibits a highly repeatable GPC. Compared to EG and HQ, each GL molecule has three —OH functional groups so that they will not be terminated by the surface groups at the same time. The LiGL films were deposited on one type of nitrogen-doped graphene nanosheets (N-GNS) (FIG. 6(a)) to determine its growth per cycle (GPC) at 150° C. The N-GNS features its high surface area and thin wrinkles of <3 nm. Observing the thickness changes of the wrinkle of the N-GNS after 20 cycles (FIG. 6b) of the LiGL deposition using an SEM, it was concluded that the average GPC of the MLD LiGL is ˜2.7 nm/cycle, which is among the highest GPCs of all the MLD processes reported to date. In addition to the SEM images, elemental mapping was conducted on the 20-cycle LiGL-coated N-NGS using an energy dispersive X-ray spectroscopy (EDX) (FIG. 6c), which shows the distributions of N, C, and O. N is from N-GNS, C is from N-GNS and LiGL, and O is from LiGL. Consequently, EDX mapping revealed that the MLD LiGL coating over the N-GNS is very conformal and uniform.
[0108] The overall reaction of the MLD LiGL is as follows in Equation 1:
3LiO.sup.tBu+(CH.sub.2CHCH.sub.2)(OH).sub.3.fwdarw.(CH.sub.2CHCH.sub.2)(OLi).sub.3+3HO.sup.tBu (1)
[0109] Thus, the LiGL is supposed to have a unit structure of (CH.sub.2CHCH.sub.2)(OLi).sub.3 in its ideal MLD condition. XPS was employed to determine the composition of the deposited LiGL films on Si wafers. In FIG. 6d, the O1 s spectra show a strong peak at 529.6 eV and a weak peak at 531.4 eV, assigned to O.sup.2− in Li—O bonds and C—O—Li, respectively. The C1 s spectra show two evident peaks at 284.8 and 283.6 eV, which are related to C—C/C—H and Li—O—C groups, respectively. The two weak peaks at 288.7 and 287.1 eV are related to O═C—OH and C—O bonds, respectively. The Li 1 s XPS spectra shows only one peak at 53.3 eV and this should be attributed to Li—O. According to the XPS analyses, the deposited LiGL contains 31.69 at. % of Li, 28.73 at. % of C, and 39.59 at. % of O. The element contents of Li, C, and O are basically consistent to the postulation of the LiGL unit structure, (CH.sub.2CHCH.sub.2)(OLi).sub.3.
[0110] The MLD LiEG and LiHQ Growth were Investigated Using QCM at 150° C. as Shown in FIG. 7.
[0111] The overall reactions of the MLD LiEG and LiHQ are proposed in Equation 2 and Equation 3 as follows, respectively.
2LiO.sup.tBu+(CH.sub.2CH.sub.2)(OH).sub.2.fwdarw.(CH.sub.2CH.sub.2)(OLi).sub.2+2HO.sup.tBu (g) (2)
2LiO.sup.tBu+(C.sub.6H.sub.4)(OH).sub.2.fwdarw.(C.sub.6H.sub.4)(OLi).sub.2+2HO.sup.tBu (g) (3)
[0112] The growth temperatures of LiEG, LiHQ and LiGL are not limited to 150° C. and can be lower/higher than 150° C. In addition, the doses of the precursors (LTB, EG, HQ and GL) and purges also are adjustable and can be lower or higher times.
[0113] Effects of MLD Coatings on Electrochemical Performance of Li Anodes
[0114] To verify the beneficial effects of the MLD coatings of LiEG, LiHQ and LiGL, both LiEG and LiGL coatings on Li metal anodes in Li/Li symmetric cells (illustrated in FIG. 8) in two different electrolytes was investigated: one carbonate electrolyte is 1.2 M LiPF.sub.6 in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC=3:7 by weight) and one ether electrolyte is 1 M lithium bis(trifluoromethanesulfonyl) imide (LITFSI) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (DOL:DME=1:1 by volume). A Celgard 2325 membrane was used as the separator placed between the two Li electrodes.
[0115] The proposed protective mechanisms of the ALD/MLD coatings are illustrated in FIGS. 9(a)-9(e). FIG. 9(a) illustrates an Li chip 910 uniformly covered with a coating of the present invention 920 via ALD or/and MLD processes. FIG. 9(b) illustrates that the assembled Li chip 911 becomes bumpy and that there are some fractures formed within the coating layer. The fractures are covered by some SEI 930. FIG. 9(c) illustrates that Li stripping first starts from bumpy sites. FIG. 9(d) illustrates that the bumpy coating layer is flattened due to the Li stripping and the flattened coating helps produce uniform Li-stripping. FIG. 9(e) illustrates that the coating layer 920 helps form uniform Li-plating and its exceptional ionic conductivity enables uniform Li deposition under the coating layer with little SEI and dendritic formation.
[0116] As shown in FIG. 10 the LiEG coatings can be beneficial to sustain a longer cyclability of the coated Li/Li symmetric cells when a suitable film thickness is determined in 1.2 M LiPF.sub.6 in 3:7 EC:EMC. MLD cycles were controlled to control the deposited film thickness. It is apparent that the 40-cycle MLD-LiEG (i.e., LiEG40) performs the best among the investigated film thicknesses at a current density of 1 mA/cm.sup.2 and a capacity of 1 mAh/cm.sup.2 and outperforms the bare Li/Li cell. During the cell cycling, the Li stripping and plating were 1 hour, respectively.
[0117] Similarly, FIG. 11 illustrates the effects of the invented MLD LiGL coatings on Li metal electrodes in 1M LiPF.sub.6 in 3:7 EC:EMC. Using a current density of 1 mA/cm.sup.2 and a capacity of 1 mAh/cm.sup.2, among all the investigated film thicknesses of the 20-cycle MLD-LiGL (i.e., LiGL20) outperforms all the other cases in cyclability and sustains a low overpotential for the longest cycles.
[0118] As shown in FIG. 12, the effects of the 60-cycle MLD-LiGL coating on the stability of the symmetric Li/Li cells are compared to bare Li/Li cells in a carbonate electrolyte. The results show that the LiGL60 symmetric cell has a much better stability. The carbonate electrolyte is 1.2 M LiPF6 in 3:7 EC:EMC and controlled for 20 μL. The testing current density is 2 mA/cm.sup.2 and the testing capacity is 1 mAh/cm.sup.2.
[0119] Using a current density of 2, 5, and 7.5 mA/cm.sup.2 and a capacity of 1 mAh/cm.sup.2, results show that the MLD coatings performed much better in the ether electrolyte, 1 M LITFSI in 1:1 DOL:DME, as shown in FIG. 13. The results show that all the MLD-coated Li/Li cells have better stability than the bare Li/Li cell, accounting for lower overpotentials and remarkably much longer lifetimes. The results suggest that an MLD-LiGL coating thicker than 15 MLD cycles performs much better than a thinner MLD-LiGL coating in protecting Li anodes at a current density of 2 mA/cm.sup.2 (FIG. 13(a)). With increased current densities, thicker MLD-LiGL coatings are preferred (FIG. 13(b,c))
[0120] Using a current density of 2 and 5 mA/cm.sup.2 and a capacity of 2 mAh/cm.sup.2, results show that the MLD coatings could help maintain lower overpotentials than those of bare Li/Li symmetric cells in the ether electrolyte, 1 M LITFSI in 1:1 DOL:DME, as shown in FIG. 14.
[0121] Using a current density of 2 and 5 mA/cm.sup.2 and a capacity of 4 mAh/cm.sup.2, results show that the MLD coatings could help maintain lower overpotentials than that those of bare Li/Li symmetric cells in the ether electrolyte, 1 M LITFSI in 1:1 DOL:DME, as shown in FIG. 15.
[0122] Scanning electron microscopy (SEM) was used to observe the surfaces (FIG. 16(a)) and the cross sections (FIG. 16(b)) of the cycled bare, LiGL10, LiGL20, and LiGL60 electrodes after 700 Li-stripping/plating cycles at a current density of 2 mA/cm.sup.2 and an areal capacity of 1 mAh/cm.sup.2. Results show that LiGL coatings could protect Li electrodes from corrosion. The thicker the better of the protection effects. There was little SEI and Li dendrites observed from the surface and cross section of the LiGL60 electrode after 700 Li-stripping/plating cycles. In comparison, the bare Li electrode shows serious growth of SEI and dendritic structures.
[0123] XPS depth profiling results show that there has a very thick SEI layer formed on the bare Li after 10 Li-stripping/plating cycles (FIG. 16(c)). In contrast, results show that there has much less SEI formed on the LiGL-90 electrode after 10 (FIG. 16(d)) and 50 (FIG. 16(e)) Li-stripping/plating cycles.
[0124] SEM observations on both Li electrodes of bare Li/Li symmetric cells show that, after a 24-h Li-stripping on one Li electrode (FIG. 17(a)) while a 24-h Li-plating on another opposite Li electrode (FIG. 17(b)) at a current density of 2 mA/cm.sup.2 (i.e., an areal capacity of 48 mAh/cm.sup.2), the bare Li electrode after the 24-h Li-stripping (FIG. 17a(i)) was covered by craters (as circled by dashed lines) and bumps (the areas other than the circles) while there is a large amount of dendritic Li deposited on the bare Li surface after 24-h plating (FIG. 17b(i)). On the stripping side (FIG. 17(a)), the craters are smooth while the bumps are decorated with numerous micro-wells (or micro-holes) mainly in the range of 10-50 μm. These micro-wells are further enlarged to show more details (FIG. 17a(ii)-(iv)). As shown in FIG. 17a(iv), these micro-wells contain many dendritic structures (or micro-pillars). The craters might be the areas that have stripped Li first and then ceased while the bumps were the areas that did not strip Li at the very beginning but became the new areas for stripping after the craters ceased stripping. Thus, the stripping process on bare Li electrodes is not uniform and the stripping areas change with time. On the plating side, it is observed that the deposited Li was separated from the originally bare Li (FIG. 17b(ii)) while the originally bare Li surface was intact (FIG. 17b(iii)). The deposited Li (FIG. 17b(iv)) was in micron-sized dendritic structures that were squeezed together with clear boundaries. The bare Li surface and the dendritic structures should have been covered by one layer of SEI and the SEI layers have separated them from each other. The formation of this SEI layer consumed Li and the electrolyte.
[0125] SEM observations on both Li electrodes of the LiGL60 symmetric cells show that, after a 24-h Li-stripping on one LiGL60 electrode (FIG. 17(c)) while a 24-h Li-plating on another opposite LiGL60 electrode (FIG. 17(d)) at a current density of 2 mA/cm.sup.2 (i.e., an areal capacity of 48 mAh/cm.sup.2), the LiGL60 electrode after 24-h Li-stripping (FIG. 17(c)) was free of craters and bumps while the opposite LiGL60 electrode after 24-h Li-plating (FIG. 17(d)) was generally clean and smooth. These results show that the LiGL60 coatings have well protected both Li electrodes from corrosions; that is, there have no SEI formation and no Li dendritic growth. In addition, these results indicate that the LiGL coatings are ionically conductive but electronically insulating. There were fractures observed on both LiGL60 electrodes, which were created by the pressing force during assembling process.
[0126] SEM observations on both Li electrodes of bare Li/Li symmetric cells show that, after a 48-h stripping-plating (24-h Li-striping followed by 24-h Li-plating) on one Li electrode (FIG. 18(a)) while a 48-h plating-stripping (24-h Li-plating followed by 24-h Li-stripping) on another opposite Li electrode (FIG. 18(b)) at a current density of 2 mA/cm.sup.2 (i.e., an areal capacity of 48 mAh/cm.sup.2), the bare Li after 48-h stripping-plating (FIG. 18a) exhibited the similar morphology as shown in FIG. 17b and showed a thick dendritic Li layer on the top of the bare Li electrode, while the opposite bare Li after 48-h plating-stripping (FIG. 18b) has a similar appearance as shown in FIG. 17a and showed numerous craters and bumps covered by many micro-wells.
[0127] SEM observations on both Li electrodes of the LiGL60 symmetric cells show that, after a 48-h stripping-plating (24-h Li-striping followed by 24-h Li-plating) on one LiGL60 electrode (FIG. 18(c)) while a 48-h plating-stripping (24-h Li-plating followed by 24-h Li-stripping) on another opposite LiGL60 electrode (FIG. 18(d)) at a current density of 2 mA/cm.sup.2 (i.e., an areal capacity of 48 mAh/cm.sup.2), the LiGL60 electrode after 48-h stripping-plating (FIG. 18c) was clean on its surface and covered with numerous small pieces of LiGL coatings mainly in the range of 10-50 μm while the LiGL60 electrode after 48-h plating-stripping (FIG. 18d) was similarly clean and covered with small pieces of the LiGL film. The fractures between the LiGL pieces are 1-2 μm. These results well demonstrate that the LiGL coatings with suitable thicknesses are very effective to protect Li electrodes from dendritic growth and SEI formation.
[0128] The Li-Conducting Inorganic Films of Li.sub.xAl.sub.yS Via ALD
[0129] Another embodiment of the present invention concerns a super-ALD process for growing Li.sub.xAl.sub.yS in which the sub-cycle ratio of m/n, as shown in FIG. 19a, could be adjusted for different Li.sub.xAl.sub.yS films with accurately controlled compositions. The resultant Li.sub.xAl.sub.yS films have different ionic conductivities at room temperature (RT) and accomplished the highest ionic conductivity of 1.66×10.sup.−3 S/cm when the sub-cycle ratio m/n is 1:4, 750 times higher than that of the ALD Li.sub.3BO.sub.3—LiCO.sub.3 films reported for the highest ionic conductivity of 2.2×10.sup.−6 S/cm previously, as shown in FIG. 19b.
[0130] It has also been found that the effects of 1:4 Li.sub.xAl.sub.yS (i.e., (1:4)LiAlS) coating on Li metal electrodes in symmetric Li/Li cells in the carbonate and ether electrolytes are beneficial. Using the carbonate electrolyte of 1.2 M LiPF6 in 3:7 EC: EMC, as illustrated in FIG. 20, the (1:4)LiAlS coatings can evidently improve the cyclability of Li/Li cells with thin-film thicknesses (50-150 super-cycles corresponding to 5-15 nm), compared to the bare Li/Li cell. Using the ether electrolyte of 1 M LITFSI in 1:1 DOL: DME, as illustrated in FIG. 21 and FIG. 22, the (1:4)LiAlS coatings can dramatically improve the cyclability of Li/Li cells with thin-film thicknesses (10-100 super-cycles corresponding to 1-10 nm), compared to the bare Li/Li cell.
[0131] The Li-containing inorganic-organic hybrid films used in Li batteries of the present invention may be formed using both ALD and MLD. The embodiments of the present invention produce advanced hybrid inorganic-organic thin films on electrodes such as the cathode, anode or both through combining ALD and MLD, and the films enable multiple benefits in mechanical properties, ionic conductivity, stability in liquid electrolytes, and compatibility with liquid electrolytes.
[0132] As shown in FIG. 23, to achieve a desirable hybrid Li-conducting coating on an electrode of a battery, in a preferred embodiment, a combination of an MLD-deposited Li-conducting polymeric film with an ALD-deposited Li-conducting inorganic film may be used on the electrode. The Li-containing polymeric films may be LiEG, LiGL, and LiHQ and combinations thereof. Additionally, other aspects of the present invention provide superionic inorganic coating of (1:4)LiAlS using ALD. In other embodiments, the present invention combines the (1:4)LiAlS with one of the three Li-containing polymeric films (i.e., LiEG, LiGL, and LiHQ), as illustrated in FIG. 23. This allows for the tuning of the ALD/MLD sub-cycle ratios to achieve desired ionic conductivity, mechanical properties, and chemical stability.
[0133] It has also been found that the effects of the hybrid Li-conducting film of bilayered LiGL15-(1:4)LiAlS50 coating on Li metal electrodes in symmetric Li/Li cells in the ether electrolyte is beneficial. Using the ether electrolyte of 1 M LITFSI in 1:1 DOL: DME, as illustrated in FIG. 24, the bilayered LiGL15-(1:4)LiAlS50 coating can dramatically improve the cyclability of Li/Li cells with thin-film thicknesses (15-MLD-cycle LiGL and 50-super-cycle (1:4)LiAlS corresponding to ˜50 nm), compared to the bare Li/Li cell.
[0134] While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above-described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.
