Elsevier

Energy Storage Materials

Volume 15, November 2018, Pages 458-464
Energy Storage Materials

All-in-one lithium-sulfur battery enabled by a porous-dense-porous garnet architecture

https://doi.org/10.1016/j.ensm.2018.08.009Get rights and content

Abstract

Li-S batteries, while promising, face tremendous challenges due to the infinite volume change of the lithium anode, the constantly evolving solid-electrolyte interface, and the polysulfide shuttling effect. Herein we report a novel all-in-one cell design introduced by a porous-dense-porous trilayer garnet electrolyte. Both lithium anode and the sulfur cathode are infiltrated in the porous garnet framework, resulting in the first reported solid-state all-in-one battery. The interconnected 3D garnet electrolyte provides ion pathways throughout the whole cell while the conformal coating of carbon nanotubes and infiltrated lithium metal form continuous pathways for electrons. The all-in-one cell design has the following advantages: (1) continuous pathways for Li+ and electrons that lead to a lower resistance, (2) all-solid-state lithium metal anode eliminating the formation of an SEI, (3) seamless contact between the garnet and lithium metal introduced by a ZnO surface treatment that results in small interface resistance, (4) low local current density at an applied areal current density due to high contact area of the 3D porous structure, (5) locally confined volume changes for both anode and cathode, avoiding dead Li and dead S formation, (6) the use of a thin dense ceramic electrolyte as a separator enabled by the trilayer architecture, which is otherwise impossible due to inherent brittleness of thin film ceramics, (7) unique approaches for cell manufacturing and packaging made available by the all-in-one design, which bring about new opportunities. As a proof of concept, we demonstrated the all-in-one Li-S battery which completely eliminates lithium polysulfide shuttling and lithium dendrite penetration, leading to a high efficiency and safe operating battery system. With both lithium and sulfur infused in the porous layer of the solid-state electrolyte, the proposed Li-S battery achieves a high capacity of over 1200 mAh/gS and nearly 100% coulombic efficiency, demonstrating the advantages of the all-in-one design.

Introduction

Lithium sulfur (Li-S) batteries have been considered as one of the most promising next generation energy storage devices, especially for emerging electric vehicles [1], [2], [3], [4], [5]. Unlike the state-of-art lithium ion (Li-ion) batteries, the Li-S battery is based on the conversion reaction between lithium and sulfur, as opposed to the lithium intercalation/deintercalation mechanism, which leads to an exceptionally high theoretical energy density of over 500 Wh/kg [6], [7], [8], [9], [10] Despite the high energy density, there are still many obstacles to tackle for the application of Li-S batteries. On the anode side, lithium metal suffers from the problems of infinite volume change, constant solid-electrolyte interface (SEI) formation and reformation, and dendrite growth. [11], [12], [13] On the cathode side, the lithium polysulfide shuttling effect plagues the cycling performance of Li-S batteries [14], [15], [16], [17], [18] Much research effort has been devoted to solving the problems caused by the lithium metal anode, including the employment of electrolyte additives, [13] engineering of separators, [19], [20], [21], [22], [23] and theoretical studies of lithium deposition [24], [25] Tremendous progress has also been achieved in avoiding the polysulfide shuttling effect, including the engineering of electrolytes to modify anode surfaces, [26], [27], [28] modification of cathode structure and chemistry to host sulfur, [29], [30], [31], [32], [33], [34], [35], [36] utilization of new separator material to block lithium polysulfides [37], [38], [39], [40], [41], [42], and the employment of smaller sulfur molecules to avoid the formation of lithium polysulfide [43], [44], [45], [46], [47]. With the successful integration of improvements to lithium metal anode and sulfur cathodes, prolonged cell cycling with a high coulombic efficiency has been demonstrated. However, there are still critical challenges in the application of Li-S batteries to achieve typical industry performance goals such as specific capacity of 3 mAh/cm2, current density of 3 mA/cm2, and 500–1000 cycles due to limitations of the above problems.

Solid-state electrolytes (SSEs) in principle can provide a definitive solution to the problems of lithium dendrite growth and lithium polysulfide shuttling, since the nature of ceramic electrolytes can physically prevent the chemical species from crossing the separator. Various types of SSEs have been studied for use in a solid-state lithium metal battery [48], [49], [50], [51]. Among those, garnet type electrolyte has shown excellent performance due to its high ionic conductivity and chemical and electrochemical stability [51], [52], [53], [54], [55], [56]. In this study, we introduce a solid-state Li-S battery design based on a triple layer garnet type ceramic electrolyte [57], where a thin dense layer of garnet is sandwiched by two porous layers. Both the lithium and sulfur are infiltrated into opposing porous sides of the garnet electrolyte and are separated by the dense center layer of garnet, forming an all-in-one battery for the first time. The all-in-one battery has the potential to address the aforementioned challenges in Li-S batteries due to the following reasons: (1) solid state Li metal can be used due to its eletrochemical compatibility with garnet, meaning no SEI formation is involved in the battery system; (2) volume changes are re-distributed in the small pores, thus avoiding formation of dead Li and S products; (3) the dense ceramic layer makes it impossible for Li2Sx to migrate to from the cathode to the anode, thus completely eliminating the shuttling effect. As a proof of concept, the solid-state all-in-one Li-S battery based on trilayer garnet was developed, which shows high capacity of 1200 mAh/gsulfur and stable coulombic efficiency of nearly 100%. The utilization of the solid-state electrolyte also prevents the growth of lithium dendrites, enabling much safer battery operation. The high energy density, all-in-one configuration, solid-state Li-S battery introduces a new route for future energy storage systems.

The trilayer garnet based solid electrolyte was synthesized via a tape casting technique [57]. The porous layers are 50–70 µm thick and have a porosity of 66%, while the dense layer separating the anode and cathode has a thickness of 10–30 µm (Fig. 1a). On the anode side of the porous layer, the pores are filled with lithium metal using our previously reported zinc oxide (ZnO) coating as an interface layer, [58] forming a 3D solid-state lithium metal anode. On the cathode side, sulfur and carbon nanotubes (CNTs) are infused with a solution-based method, resulting in a 3D sulfur cathode. Note that the cathode side of pores are not fully occupied by sulfur in order to leave room for the volume change between sulfur and lithium sulfides upon cycling. With the porous-dense-porous trilayer structure, the lithium ion transports along the interconnected garnet structure in the porous layer and through the dense layer of the solid-state electrolyte to react with sulfur on the cathode side and form lithium sulfide during discharge, filling the pores on the cathode side (Fig. 1b). During the charge cycle of the battery, the discharge product lithium sulfide decomposes to lithium metal and elemental sulfur, freeing the pores on the cathode and refilling the pores on the anode (Fig. 1c). Therefore, with the trilayer structure of the Li-S battery, only lithium ions transfer between anode and cathode, eliminating the problem of lithium polysulfide shuttling. Moreover, both the anode and the cathode are infused into the garnet framework, forming the all-in-one structure of the full cell. With the 3D lithium host and sulfur host, the stripping/plating of lithium metal and the formation/decomposition of lithium sulfide are confined in rigid 3D solid-state framework, thus preventing battery short circuit caused by lithium dendrite growth and cathode volume expansion. Furthermore, it removes the issue of infinite volume change with lithium metal anodes. This unique design of the all-in-one battery structure shows significant potential for high energy density, high coulombic efficiency, and safe lithium metal batteries.

The fabrication and characterization of the trilayer garnet based solid-state electrolyte is shown in Fig. 2. The trilayer garnet was synthesized via a highly scalable tape casting method (Fig. 2a) and the resulting tapes are cut to coin cell size and sintered for cell fabrication (Fig. 2b). The full-scale SEM image of the trilayer electrolyte shows a 15 µm highly dense center layer with 50 µm thick porous layers on either side (Fig. 2c). Focusing on the dense layer, there are no pinholes that bridge the anode side and the cathode side, making it impossible for dendrites to physically grow through and polysulfide to diffuse across (Fig. 2d). On the borderline between the dense layer and the porous layer, the garnet grains of the porous layer are strongly bound to the dense layer with no gaps in between (Fig. 2e). The good connection between the dense layer and the porous layer ensures that lithium ions can be successfully transported between the cathode and anode without introducing extra impedance. From the top view of the porous layer, many 5–8 µm sized pores can be observed (Fig. 2f), leading to a high porosity of 66%. Such a high porosity provides enough space for the infiltration of lithium and sulfur, thus leading to a high energy density battery system. While the structure of the solid-state electrolyte is complicated, the chemistry is still maintained. The X-ray diffraction (XRD, Fig. 2g) of the synthesized trilayer electrolyte matches perfectly with the cubic phase of Li6.5La3Zr1.5Nb0.5O12 (LLZN), ensuring remarkable ion conductivity of the electrolyte. The porous-dense-porous architecture enables a unique all-in-one design for solid-state lithium metal batteries (Fig. S1).

With the trilayer garnet framework, lithium metal is infused into the anode side of the porous layer to form a 3D solid-state lithium metal anode (Fig. 3). The surface of the porous is first coated with 20 nm zinc oxide by atomic layer deposition (ALD) to modify the wettability of molten lithium on the surface of garnet. Lithium metal is carefully polished to remove the oxide layer on the metal surface before being attached flat on top of the ALD coated trilayer garnet (Fig. 3a). Lithium is then gradually heated to melting and infused into the porous framework. Under 180 °C the surface of the lithium starts to melt and forms a rough surface, and the non-uniformity of the melting hinders the infiltration of lithium, leaving the porous garnet structure empty with lithium still on top (Fig. 3b). With further heating to 240 °C all lithium metal is melted and is drawn into the porous garnet media due to capillary force, eventually filling all the pore space to form the 3D lithium metal anode (Fig. 3c). After removing most of the excessive lithium on the surface, the amount of lithium utilized in cell assembly (i.e., the anode loading) is weighed and determined to be 4.3 mg/cm2. SEM images reveal the structure of the resulting 3D lithium metal anode (Fig. 3d-f). With a thin excess lithium layer on top, most lithium metal has been infiltrated into the anode side of the trilayer garnet framework without penetrating the dense layer (Fig. 3d). Focusing on the anode side, lithium metal fills all the pore space in the 3D anode host, including the deepest pores close to the dense layer (Fig. 3e). With the ZnO ALD coating on the porous garnet, the surface can be completely wetted by lithium metal, leaving no gap between the garnet and the lithium (Fig. 3f), ensuring seamless contact between SSE and the anode and low interfacial impedance. The 3D solid-state lithium metal anode lays the foundation for high energy density solid-state all-in-one lithium metal batteries.

On the cathode side sulfur is introduced into the porous garnet via a solution-based method (Fig. 4). CNT is first coated onto the surface of the porous garnet by repeated infusion of CNT ink until the surface of the trilayer garnet is visually black, thus ensuring electronic conductivity in the 3D cathode host (Fig. 4a-b). Molten sulfur is then infiltrated into the 3D cathode host to form the 3D sulfur cathode (Fig. 4c-d) with a sulfur loading of 5.4 mg/cm2. The morphology of the trilayer after cathode infiltration was studied by SEM and it confirms the presence of sulfur in the pores (Fig. 4e-j). After the infiltration of CNTs, the CNT can be observed densely coating the surface of the porous garnet (Fig. 4e), which forms an interconnected network on the surface of the garnet leading to a mixed electron/ion conducting framework on the cathode. With sulfur infiltrated in the cathode, the CNT network is covered by the sulfur cathode and is no longer visible (Fig. 4f). Note that the pores on the cathode side are only partially filled intentionally, which leaves room for volume expansion during discharge to prevent structural damage. Fig. 4e-h shows the elemental mapping of the trilayer after infiltration of cathode and anode. Both zirconium (Fig. 4h) and lanthanum (Fig. 4i), as components of the garnet solid electrolyte, are evenly distributed throughout the whole structure, indicating the full coverage of electrolyte in the battery. On the other hand, the sulfur is only found on the cathode side, which means no crack in the dense layer for polysulfide shuttling or dendrite penetration (Fig. 4j). With both lithium and sulfur in the porous structure, the trilayer architecture ensures the high efficiency and high energy density of the solid-state Li-S battery (Fig. S2).

With both lithium and sulfur in the trilayer garnet framework, the all-in-one Li-S battery has been fabricated. The electrochemical performance of the solid-state Li-S battery is shown in Fig. 5a-c. The anode side was tested with an all solid-state structure, while a very small amount of liquid electrolyte (<1 µL/mgS) was added to the cathode side to ensure contact between the cathode and solid electrolyte. The electrochemical impedance spectrum shows a total impedance of less than 800 Ω cm2 (Fig. 5a), which is impressively low for a solid-state full battery at room temperature. The constant current cycling (50 mA/gS) shows a typical Li-S two plateau behavior with an ultra-high capacity of ~1200 mAh/gS (Fig. 5b), notably without any polysulfide shuttle effect which plagues the lithium-sulfur chemistry in liquid systems. The first plateau corresponds to the formation of high order Li2Sx, while the second plateau corresponds to the conversion from high order Li2Sx to lower order Li2Sx and solid discharge product Li2S/Li2S2. The high interfacial contact between the garnet electrolyte and sulfur cathode in the porous structure leads to the full utilization of the sulfur cathode, resulting in an ultra-high discharge capacity. Moreover, the solid-state Li-S battery shows stable behavior with small capacity decay and nearly 100% coulombic efficiency after 50 cycles (Fig. 5c). The rate performance has also been studied (Fig. S3). The cell can be stably cycled under elevated current densities (300 mA/gS) with a capacity of 400 mAh/gS. The high capacity is recovered after the current density is turned back down to 50 mA/gS.

The discharge product has been characterized via ex-situ XPS (Fig. S4). After discharge, the distinct peaks corresponding to Li2S and Li2S2 can be observed on the XPS spectrum, which are typical discharge products in Li-S batteries. After recharge, the peaks of discharge products disappear and only elemental sulfur can be seen on the spectrum, indicating rechargeability of the solid-state Li-S battery. The morphology of the trilayer cell after discharge has also been investigated via post-mortem SEM (Fig. 5d). After discharge, the lithium side of the trilayer is more vacant, leaving only a small amount of lithium metal to be seen. After further cycles, with lithium transferred to the cathode side, more empty space is formed on the anode side (Fig. S5). The remaining lithium metal forms a coating on garnet surface, forming a continuous pathway for the transport of Li ions and electrons. Therefore, it ensures the transport of both electrons and lithium ions for the subsequent cycles. On the sulfur side, the pores are much more filled compared to as-assembled case (Fig. 4d), indicating the volume expansion of the sulfur after discharge.

Since all components are included in the trilayer structure, the energy density of the whole cell can be easily calculated. The demonstrated cell shows a remarkable 272 Wh/kgcell energy density as calculated in Table S1, which is superior compared to state-of-art Li-ion batteries (LFP, LCO, NMC, etc. Fig. 5e). Moreover, there is significant potential to further increase the energy density without any improvement in cathode chemistry performance. To simplify fabrication, the demonstrated cell had 70 µm thick electrode layers on both sides and we used a thicker 30 µm dense layer. However, minor improvements to the garnet structure can greatly increase energy density. 421 Wh/kgcell can be achieved by capacity matching the electrode layers (with 5% excess lithium metal) and decreasing the dense electrolyte layer thickness to 10 µm (Fig. 5e). It should be noted as well that the porosity of the porous layer can be tuned by changing the amount of the pore former, which can further improve the energy density to 630 Wh/kgcell. (Fig. 5f). Other than the ultra-high energy density that has been achieved using a coin cell, the scale-up of the trilayer Li-S battery has also been demonstrated. Solid-state Li-S battery with a dimension of 5 cm × 5 cm in a pouch cell (Fig. S6) has been fabricated and it shows stable performance as well (Fig. 5g). The high energy density and excellent stability of the trilayer Li-S battery makes it one of the most promising candidates for future energy storage systems.

Additionally, since both the anode and cathode are inside the trilayer garnet framework, it resolves the safety issues introduced by employment of lithium metal anode. In conventional lithium metal battery, when lithium is accidentally exposed in the atmosphere, the whole anode is rapidly oxidized, leading to the death of the battery. In the all-in-one solid-state battery, even though both the anode and cathode are exposed (Fig. 5h, pouch cell cut open), only the electrodes on the surface of the porous media can be oxidized, thus the battery can still fully function. More impressively, the all-in-one battery can fully function for as long as 48 h with continuous exposure to ambient air. The high performance and excellent safety of the solid-state all-in-one Li-S battery is a promising new design for the application of solid-state lithium metal battery.

Section snippets

Conclusion

In summary, a high energy density solid-state all-in-one lithium metal battery has been achieved for the first time using a trilayer garnet based solid-state electrolyte. Both the cathode and the anode are infiltrated into the porous layer and are separated by a dense layer of ceramic electrolyte. The all-in-one cell design ensures continuous pathways for Li+ and electrons that lead to a lower resistance, all solid-state lithium metal anode with low interfacial impedance, and low local current

Fabrication of garnet trilayer

The garnet trilayers were fabricated at previously reported [55]. Isopropanol (21 wt%), toluene (21 wt%), fish oil (0.5 wt%), and the prepared LLZ (30 wt%) were weighed into a bottle with YSZ grinding media and milled for 24 h. Subsequently, benzyl butyl phthalate (BBP, 6.5 wt%), polyvinyl butyral (PVB, 5.0 wt%) were added and milled for another 24 h. After the second day of milling was complete 10 μm cross-linked PMMA spheres (16 wt%) were added as porogens. This slurry was milled for 1 hour

Acknowledgements

The authors would like to thank NASA and ARPA-E for financially supporting this work under the NASA Advanced Energy Storage System Project within the Game Changing Development Program of the Space Technology Mission Directorate Project (contract #NNC16CA03C) and the ARPA-E Robust Affordable Next Generation Energy Storage Systems program (Contract No. AR-DE0000384 and AR-DE0000787). We would also like to acknowledge the characterization facilities at the University of Maryland including the

Conflict of interest

Eric D. Wachsman, Liangbing Hu, and Gregory T. Hitz founded a company to commercialize solid-state batteries. However, all results reported herein were performed at the University of Maryland under federal sponsorship.

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