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Innovations in Lithium Ion Battery Technologies: A Conversation with Will West, Nancy Dudney, and Andrew Westover Downloaded via 46.161.60.18 on March 1, 2019 at 02:28:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ithium ion batteries are widely used in rechargeable energy-based consumer and industrial products; however, there is still a need for increased energy density and therefore improved storage capacity for such applications as increasing electric vehicle range and improved backup power storage paired with renewable energy sources. While graphite is among the most commonly used anode materials in lithium batteries, a 30% increase in energy density can be achieved simply by substituting the graphite-based anodes for those comprised of lithium metal. Despite the simplicity of this idea, the practicality of implementing lithium metal anodes is limited by the propensity for extended tree- or needle-like lithium structures (“dendrites”) to form over repeated charging cycles. These dendrites damage the internal structure of the battery and thus present significant technological and safety challenges for their everyday use. Alternative battery materials such as sodium and magnesium are being investigated; however, there still lies great promise in further innovation of lithium-based battery technologies if the propensity for dendrite formation can be overcome. One of the proposed tactics for mitigating damage caused by lithium dendrites is through development of electrolytes that can prevent or confine their growth. The formation of lithium dendrites upon battery cycling has been reported for a variety of liquid and inorganic solid-state electrolytes; however, there have been promising reports of the use of lithium phosphorus oxynitride, or Lipon, as a solid-state electrolyte that limits dendrite growth, thus improving battery durability. The mechanism for dendrite suppression in Lipon electrolyte is not well understood, and shedding light on this question would lend important insights for the optimization of lithium battery materials. In their new ACS Energy Letters article (DOI: 10.1021/acsenergylett.8b02542), Dudney, Westover, and coworkers at Oak Ridge National Laboratory probe the mechanism of Lipon’s effectiveness of dendrite growth suppression. Through construction of a modified “Li-free” battery using a lithium cobalt oxide cathode, copper current collectors, and Lipon electrolyte with an artificial Lipon−Lipon interface, they concluded that the absence of irregularities or interfaces in the solid-state electrolyte plays an important role in explaining how dendrite growth can be suppressed. To expand upon the findings of this study and provide insight into the challenges currently facing lithium ion battery design, we spoke with cocorresponding authors Dr. Andrew Westover and Dr. Nancy Dudney of Oak Ridge National Laboratory, as well as an independent expert in the field, Dr. Will West from NASA Jet Propulsion Laboratory. Conversation with Will West (Figure 1): © XXXX American Chemical Society
Figure 1. Dr. Will West is currently a Technologist at NASA Jet Propulsion Laboratory (https://electrochem.jpl.nasa.gov/?page= West).
Can you provide some background on the challenge of Li metal dendrite formation to battery design? What are some promising strategies that have been adopted to address this challenge? Lithium metal anodes offer almost an order of magnitude greater specific capacity relative to conventional graphitic anodes in state-of-the-art lithium ion cells. Given this important attribute, the use of lithium metal anodes for liquid electrolytebased rechargeable lithium battery cells has been explored for decades but has been hindered by the formation of lithium dendrites that deposit at the anode upon repeated charge/ discharge cycling. These dendrites can become detached from the anode, resulting in loss of capacity, or can penetrate the separator, thereby shorting the cell with catastrophic failure of the cell. In a move toward greater safety, several decades ago, solid electrolytes were examined as candidates to replace flammable organic solvents, with the notion that solid-state materials would also have the added benefit of being more resistance to dendrite penetration. This hypothesis was proved in the early 1990s by Oak Ridge National Laboratory in solid-state thin film batteries using lithium phosphorus oxynitride (Lipon) as the solid electrolyte. Lipon has been recognized for decades as a remarkable solid electrolyte with outstanding reductive and oxidative electrochemical stability as well as for its ability to resist dendrite penetration even after tens of thousands of deep charge/discharge cycles. Received: February 15, 2019 Accepted: February 27, 2019
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DOI: 10.1021/acsenergylett.9b00358 ACS Energy Lett. 2019, 4, 786−788
Energy Focus
Cite This: ACS Energy Lett. 2019, 4, 786−788
Energy Focus
ACS Energy Letters Scaling up Lipon-based thin film cells to sizes needed for applications such as portable electronics or transportation is very difficult given the need to prepare Lipon by way of sputter deposition. Naturally, much work has been expended attempting to find alternative solid electrolytes with sufficient ionic conductivity and electrochemical stability that could be prepared by methods more amenable to mass production. While several solid electrolyte candidates have been identified with sufficient ionic conductivity and electrochemical stability that can be prepared in bulk form, the performance of solid-state cells prepared from these materials suffers from the same pernicious issue observed for liquid electrolyte cells: formation and propagation of lithium dendrites through the electrolyte during cycling. Some progress has been made to reduce dendrite-induced shorting (e.g., limiting the current density during charging). However, no magic bullet has yet been identified to stop dendrite formation under practical operational conditions in large-format cells, and rationalizing the lone success of Lipon has eluded researchers. How does this work reported in ACS Energy Letters f it into the f ield, and to what extent could it shape future directions? Given the well-known success of Lipon to suppress lithium metal dendrite formation in thin film solid-state batteries, the question arises: What attribute (or combinations of attributes) of Lipon halts dendrite propagation? Is it due to the nature of preparation by way of sputter deposition, is it due to the unique metastable composition, or is it perhaps due to the amorphous, grain-free surface, or something else? In this article, the authors report a key finding: insertion of an internal Lipon−Lipon interface in the solid electrolyte allows for lithium dendrite formation lateral to the electrodes within the Lipon−Lipon interface without penetration of the dendrite between the electrodes. The implications are clear: the composition or deposition methodology of Lipon is not responsible for dendrite tolerance because dendrites can be easily propagated laterally within the Lipon electrolyte. Rather, it is the property that the glassy electrolyte is virtually featureless and defect-free that prevents dendrite penetration. When Lipon has an artificial defect inserted (in the case reported by the authors, a few nanometers of lithium carbonate between the two Lipon layers), lithium dendrites can grow quite easily along the defect. Does the significance of these f indings potentially extend beyond thin film batteries into other battery systems? While the reported work uses the thin film battery test format, the findings can in principle be extended to large-format solid electrolyte cells, informing a strategy for developing dendriteinhibition structures. On the basis of these results, the anode− solid electrolyte interface will likely have much greater dendrite tolerance if the solid electrolyte is kept as free as possible from grain boundaries, chemical inhomogeneities, and other such defects. Could you please describe other technical challenges to solid-state lithium battery development such as for optimizing energy density, longevity, ef ficiency, and materials availability? Advancements reported by research groups around the world suggest that practical solid electrolyte batteries useful for largescale applications are on the near horizon. This technology will almost certainly displace conventional lithium ion batteries given the numerous advantages offered by solid-state electrolyte and lithium metal anode. However, it is unlikely that these lithium metal anode solid-state batteries will use a solid electrolyte drop-in replacement for the liquid electrolyte. In fact, the successfully commercialized solid-state battery will very
likely bear few structural similarities to the present-day lithium ion cells and will be challenging to manufacture. The solid-state cells may employ multiple layers of various solid electrolyte powders, compositionally graded layers, supporting membranes, and thin, low-defect glassy passivation electrolyte layers to block dendrites, as reported in the article. The solid electrolyte−cathode interface will also require attention to allow for high current densities for practical solidstate cells. Unlike conventional lithium ion cathodes that are readily infiltrated with liquid electrolyte, the solid electrolyte will not simply permeate the cathode and will have to be judiciously tailored to allow for ionic conductivity without sacrificing electrode-specific energy or electronic conductivity. Conversation with Nancy Dudney and Andrew Westover (Figures 2 and 3):
Figure 2. Dr. Nancy Dudney is currently Corporate Fellow and Group Leader in the Chemical Science Division at Oak Ridge National Laboratory (https://www.ornl.gov/staff-profile/nancy-jdudney). Photo courtesy of Oak Ridge National Laboratory.
Figure 3. Dr. Andrew Westover is currently a Staff Research Scientist at Oak Ridge National Laboratory. Photo courtesy of Oak Ridge National Laboratory.
What is the motivation for this work and the significance of your f indings reported in ACS Energy Letters? Ultimately our goal is to help make batteries that both have higher energy density and are safer than current batteries. Safety can be improved by replacing the flammable liquid electrolyte in most Li ion batteries with an inorganic solid-state electrolyte. However, they must be paired with significant improvements in 787
DOI: 10.1021/acsenergylett.9b00358 ACS Energy Lett. 2019, 4, 786−788
Energy Focus
ACS Energy Letters
these include key experiments on the nature of the Li−Lipon interface and alternative synthesis routes for Lipon.
energy density to motivate them economically. Using a Li metal anode with a solid-state electrolyte can give this needed boost. The first successful Li metal battery was the solid-state thin film Li metal battery using Lipon as an electrolyte. Since then, there have been several efforts to develop large-scale all-solidstate Li metal batteries, but virtually every other inorganic electrolyte has been plagued with Li “dendrites” or Li penetration through the electrolyte. Overcoming this problem in a high-capacity battery cell is critical to realizing solid-state Li metal batteries. One of the key differences between Lipon and other solidstate electrolytes is its homogeneous glassy morphology. Although there are several hypotheses, there has been no clear scientific study demonstrating why Lipon works. Our work directly proves that the homogeneous nature of Lipon without grain boundaries or voids of any kind is one of the reasons that it can suppress Li “dendrites”. This understanding helps define what properties a solid-state electrolyte needs to enable a commercially viable all-solid-state Li metal battery. Lipon clearly brings promise for its ability to suppress Li metal dendrite growth, but are there any technical challenges presented by its use as a solid-state electrolyte? Why is it not more ubiquitously used now? Lipon itself is a perfectly viable solid-state electrolyte with Li metal batteries and has been commercialized on a small scale. One of the most successful examples of this is Front Edge Technology Inc., which currently sells thin film batteries using Lipon. The challenge with using Lipon on a larger scale is twofold. First, currently Lipon can only be produced using radio frequency (RF) magnetron sputtering. Whereas this can be implemented on a large scale, the processing can be slow and therefore costly to make. The cost of a full cell at present is acceptable for niche applications but not for mass market electronics or electric vehicles. Improvements in sputtering technology may overcome these issues. The second greater challenge is the development of a high-capacity solid-state cathode that can be paired with Lipon. The cathodes used in thin film batteries are also made via RF magnetron sputtering and have been limited to about 10 μm in thickness. Cathodes that can meet modern energy demands for electric vehicles and electronics must be thicker at minimum and processed more economically. What do you see as the greater implications for the findings reported in your ACS Energy Letters article? Because the large-scale processing of Lipon is so difficult, researchers have been working on the development of other solid-state electrolytes including LLZO, Li3PS4, and LPS glass, among others. Many of these materials can be made with largescale manufacturing techniques that have been developed for decades in the glass and ceramics industries. None of these other electrolytes however have been able to reproduce the Li dendrite suppression capability of Lipon. Without a clear understanding as to what makes Lipon different, it will be difficult to reproduce its performance. Our work gives insight into what makes Lipon special and gives insight into how to engineer other solid-state electrolytes so that they can suppress Li penetration. Could you comment on any follow-up studies and/or f uture directions? This work is one of a series of pieces we are working on trying to understand what makes Lipon work and will allow us to either produce Lipon on a commercially viable scale or reproduce the qualities of Lipon in another solid-state electrolyte. Some of
Christina M. MacLaughlin, Development Editor
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ACS Publications
AUTHOR INFORMATION
Notes
Views expressed in this Energy Focus are those of the authors and not necessarily the views of the ACS. The author declares no competing financial interest.
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DOI: 10.1021/acsenergylett.9b00358 ACS Energy Lett. 2019, 4, 786−788
