UMD researchers report solution to high interfacial impedance hampering developing of high-performance solid-state Li-ion batteries
20 December 2016
Garnet-type solid-state electrolytes (SSEs) for Li-ion batteries offer a range of attractive benefits, including high ionic conductivity (approaching 1 mS cm−1 at room temperature); excellent environmental stability with processing flexibility; and a wide electrochemical stability window. However, development of high-performance solid-state Li-ion batteries (SSLiBs) using these materials has been hobbled by the the major challenge of the high solid–solid interfacial impedance between the garnet electrolyte and electrode materials.
Now, team of researchers at the University of Maryland Energy Research Center and A. James Clark School of Engineering report developing a solution to this problem. In a paper in Nature Materials, the researchers report effectively addressing the large interfacial impedance between a lithium metal anode and the garnet electrolyte by using ultrathin aluminium oxide (Al2O3) coating placed by atomic layer deposition.
With the garnet composition Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN) as the electrolyte material of choice (due to its reduced sintering temperature and increased Li-ion conductivity), the team observed a significant decrease of interfacial impedance from 1,710 Ω cm2 to 1 Ω cm2 at room temperature—effectively negating the lithium metal/garnet interfacial impedance.
Experimental and computational results showed that the oxide coating enables wetting of metallic lithium in contact with the garnet electrolyte surface and the lithiated-alumina interface allows effective lithium ion transport between the lithium metal anode and garnet electrolyte.
The researchers also demonstrated a working cell with a lithium metal anode, garnet electrolyte and a high-voltage cathode by applying the newly developed interface chemistry.
This is a revolutionary advancement in the field of solid-state batteries—particularly in light of recent battery fires, from Boeing 787s to hoverboards to Samsung smartphones. Our garnet-based solid-state battery is a triple threat, solving the typical problems that trouble existing lithium-ion batteries: safety, performance, and cost.—Liangbing Hu, co-corresponding author
Bruce Dunn, a UCLA materials science and engineering professor who was not involved in the research said the the work by the UMD team “effectively solves the lithium metal–solid electrolyte interface resistance problem, which has been a major barrier to the development of a robust solid-state battery technology”.
In addition, the high stability of these garnet electrolytes enable the team to use metallic lithium anodes, which contain the greatest possible theoretical energy density. Combined with high-capacity sulfur cathodes, this all solid-state battery technology offers a potentially unmatched energy density that far outperforms any lithium-ion battery currently on the market.Lithium-ion battery pioneer John B. Goodenough, Virginia H. Cockrell Centennial Chair in Engineering at the University of Texas (who was also unaffiliated with the study) commented:
Xiaogang Han et al. report that deposition of an ultrathin layer of Al2O3 on a dense garnet-based solid Li+ electrolyte by atomic-layer deposition allows dendrite-free plating/stripping of a lithium anode with a small impedance to Li+ transport across the lithium/garnet interface. This [finding] is of considerable interest to those working to replace the flammable liquid electrolyte of the lithium-ion rechargeable battery with a solid electrolyte from which a lithium anode can be plated dendrite-free when a cell is being charged.
The polycrystalline garnet-derived oxide Li7La3Zr2O12 has a bulk Li+ conductivity σLi ≈ 10-4 S cm-1 and is stable on contact with lithium metal at room temperature; the flammable liquid electrolyte has a σLi ≃ 10-1 S cm-1, but a lithium anode forms dendrites (whiskers) during charge of a rechargeable battery and the dendrites can grow across a thin liquid electrolyte to the cathode to give an internal short-circuit that ignites the electrolyte. Although a bulk σLi ≈ 10-3 S cm-1 can be obtained with a garnet by suitable doping, initial attempts to plate lithium from the garnet resulted in anode dendrites that penetrated the garnet grain boundaries to reach the cathode. This observation reinforced a generally held assumption that dendrites are inevitably formed on a lithium anode during plating and that blocking the dendrites by a solid electrolyte would result in a high impedance to Li+ transfer across the anode/electrolyte solid/solid interface. In addition, with more than three Li+ per formula unit, the electrolyte Li+ leaves the solid to react with oxide species in the grain boundaries of the garnet to create a large grain-boundary impedance to Li+ transport.
The [University of Maryland research team] has overcome these problems with two important innovations: first, the composition of the garnet framework was changed to allow fabrication at lower temperatures of a dense polycrystalline solid with tight grain boundaries while retaining a σLi ≈ 10-3 S cm-1; second, application of an ultrathin interphase Al2O3 film between the anode and the garnet electrolyte is shown to prevent the formation of dendrites on plating a lithium anode; therefore, a lithium/garnet interface can be fabricated so as to give a bonded interface with a low impedance to Li+ transport. Reaction of the Al2O3 interphase with Li+ from both the electrolyte and the anode transforms the thin Al2O3 interphase to a Li+ conductor.
The ability to plate a dendrite-free alkali-metal anode from a solid electrolyte can be accomplished where the alkali metal wets the electrolyte surface; it has recently been achieved by several groups with different solid electrolytes, but plating at high currents can be a problem. An electrolyte σLi ≈ 10-3 S cm-1 is not high enough for such a test or for high-power batteries; the problems of fabricating a low-impedance cathode/electrolyte interface and of a thin ceramic solid electrolyte of sufficient mechanical properties for a large-area membrane have yet to be solved.
This work was supported by the US Department of Energy ARPA-E RANGE (entitled “Safe, Low-Cost, High-Energy-Density, Solid-State Li-Ion Batteries”) and EERE (entitled “Overcoming Interfacial Impedance in Solid-State Batteries”).
Xiaogang Han, Yunhui Gong, Kun (Kelvin) Fu, Xingfeng He, Gregory T. Hitz, Jiaqi Dai, Alex Pearse, Boyang Liu, Howard Wang, Gary Rubloff, Yifei Mo, Venkataraman Thangadurai, Eric D. Wachsman & Liangbing Hu (2016) “Negating interfacial impedance in garnet-based solid-state Li metal batteries” Nature Materials (2016) doi: 10.1038/nmat4821