This is a report from the 2022 Young Investigator Grant recipients.
Materials Staff Scientist
Materials Sciences Division
Lawrence Berkeley National Laboratory
Decarbonization is critical to solve the global climate crisis. To this end, improved forms of renewable energy generation and storage are needed. Electrochemical energy storage systems are particularly promising in this regard as they can provide high energy density over a long application lifetime. For example, Li-ion batteries (LIBs) have been successfully used in a wide range of technologies including portable electronic devices and electric vehicles (EVs). Their demand has grown rapidly with the global EV market, which is projected to expand from $105 billion in 2021 to $354.8 billion by 2028. [1] Adding to this demand is the need for more stationary energy storage installations, which are forecasted to rise from 9GW/17GWh in 2018 to 1,095GW/2,850GWh by 2040. [2]
It may therefore be difficult to satisfy these requirements using LIBs alone, especially given the supply chain issues and increasing prices associated with essential LIB components such as lithium (Li), cobalt (Co), and nickel (Ni). To resolve this problem, Na- and K-ion batteries (NIBs and KIBs) have emerged as low-cost alternatives. Sodium (Na) and potassium (K) resources are much more abundant that comparative Li resources. [3-4] Moreover, cathode materials for NIBs and KIBs generally do not require Co and Ni, thus significantly reducing their cost relative to the traditional layered oxide cathodes used for LIBs. [3-5] The use of aluminum (Al) as a current collector instead of copper (Cu) can also alleviate some of the costs associated with the anode materials used for NIBs and KIBs. [3-4, 6] When comparing these two categories of alternative energy storage systems, KIBs have several key advantages over NIBs. For one, a graphite anode can be used for K-ion intercalation by forming KC8, whereas Na-ion intercalation requires the use of hard carbon, which is less dense and more expensive than graphite. [7-8] Furthermore, K-ion cathodes typically have a higher working voltage than Na-cathodes owing to the low potential of K/K+ potential (2.93V vs. SHE). [8]
To enable increased adoption of KIBs, K-ion cathode materials with high energy density and long-cycling stability must be developed. Based on the success of the existing LIB technologies, there have been several studies of layered potassium transition metal oxides (KxMO2, where M is transition metal) as cathodes for KIBs. [9-14] Despite their ability to reversibly intercalate K ions, previous work has demonstrated two key shortcomings of layered oxide compounds when used as K-ion cathodes. [3-4, 15-16] Firstly, the compositions of most layered K-ion cathodes are deficient in their alkali ion content (x <1.0 In KxMO2), which reduces the available capacity in the rocking-chair battery systems. Secondly, the large size of the K ion leads to strong electrostatic K+-K+ interactions in the layered oxide structure, which gives rise to prominent sloping in the voltage profile that lowers the specific capacity and average voltage. [3, 6] To overcome these shortcomings, polyanionic compounds have been suggested as alternative K-ion cathodes. [6, 15, 17-20] These materials are promising as they typically adopt an open three-dimensional structural framework that can accommodate large K ions, leading to increased separation of K ions and therefore reduced electrostatic interactions between them. [6, 15]
Among the various polyanionic compounds, KVPO4F is particularly promising owing to its large specific energy (~450 Wh kg-1), high working voltage (>4.2 V), and fast K-ion diffusivity. [6, 21-24] Nevertheless, this material has some limitations associated with its low electronic conductivity and poor cycling stability. We have previously demonstrated that the high working voltage of KVPO4F can lead to decomposition of the K-ion electrolytes, causing growth of the insulating cathode-electrolyte interphase (CEI) layer which consequently increases the cell polarization. [25] One notable strategy to alleviate the capacity decay of KVPO4F is to use carbon coating or composites to protect the reactive surface and improve electronic conductivity. [26-29] For example, Liao et al. showed that carbon-coated KVPO4F nanoplates have enhanced ionic transport ability as well as improved electron conductivity while the uniform carbon coating layer reduces direct contact between KVPO4F and the electrolytes. Multi-component coatings have also been suggested to inhibit side reactions at the cathode-electrolyte interface. [30] He et al. demonstrated that a KVPO4F cathode with a multi-component coating displayed superior capacity retention of 86% after 200 cycles. [30]
In this work, we propose partial ionic substitution in KVPO4F to improve its cycling stability and rate capability. In particular, we focus on the synthesis and characterization of several KV(1-x)TixPO4+yF1-y compositions where and varied with the synthesis process. Through substitution of V/Ti and F/O, we found that the resulting materials display a smooth voltage profile with limited charging time at high voltage (> 4.8 V). As a result, less electrolyte decomposition was observed and therefore much of the corresponding interphase formation was avoided. We also found that oxygen substitution in KVPO4F unlocks additional K ions to be intercalated at a reasonably large voltage (~1.8 V vs. K/K+), in contrast to the voltage of these ions in the non-substituted KVPO4F (< 1.0 V vs. K/K+).
*Note that this work is submitted to an scientific journal for publication.
References
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[13] Z. Xiao, F. Xia, L. Xu, X. Wang, J. Meng, H. Wang, X. Zhang, L. Geng, J. Wu, L. Mai, Advanced Functional Materials 2022, 32, 2108244.
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[17] X. Lin, J. Huang, H. Tan, J. Huang, B. Zhang, Energy Storage Materials 2019, 16, 97.
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[19] H. Park, H. Kim, W. Ko, J. H. Jo, Y. Lee, J. Kang, I. Park, S.-T. Myung, J. Kim, Energy Storage Materials 2020, 28, 47.
[20] W. Ko, H. Park, J. H. Jo, Y. Lee, J. Kang, Y. H. Jung, T.-Y. Jeon, S.-T. Myung, J. Kim, Nano Energy 2019, 66, 104184.
[21] K. Chihara, A. Katogi, K. Kubota, S. Komaba, Chemical Communications 2017, 53, 5208.
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[26] Z. Liu, J. Wang, B. Lu, Science Bulletin 2020, 65, 1242.
[27] J. Liao, Q. Hu, X. He, J. Mu, J. Wang, C. Chen, Journal of Power Sources 2020, 451, 227739.
[28] J. Xu, J. Liao, Y. Xu, J. Li, C. Zhu, J. Lin, X. Zhou, Journal of Energy Chemistry 2022, 68, 284.
[29] J. Liao, X. Zhang, Q. Zhang, Q. Hu, Y. Li, Y. Du, J. Xu, L. Gu, X. Zhou, Nano Letters 2022, 22, 4933.
[30] X.-D. He, L.-M. Zhang, C.-H. Jiang, C.-H. Chen, Chemical Engineering Journal 2022, 433, 134634.
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