Next-generation anode materials for metal-ion batteries

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Jain, Rishabh
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Electronic thesis
Mechanical engineering
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Lithium-ion battery technology is one of the greener alternatives of non-renewable energy sources for overcoming burgeoning energy demand. However, current-state-of-art lithium-ion batteries (LIBs) face critical challenges among which low performance, scarcity, and uneven distribution of lithium metal in the earth’s crust are major ones. The low-performance challenge can be overcome by utilizing alloy-based or conversion-based chemistries instead of intercalation/de-intercalation chemistry, used in a current-state-of-art LIBs, to store a large number of lithium atoms in one cycle. Scarcity challenges can be overcome by exploring beyond LIBs and two important candidates are sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs). Therefore, various alloy-based and conversion-based materials have been explored not only for the fabrication of efficient lithium-ion but also for sodium-ion and potassium-ion batteries. Constant development in the fabrication of innovative materials led to the introduction of various new innovative materials which has not been explored before for battery application. Tellurene and phosphorene are two such materials. In this Ph.D. work, I have explored tellurene and phosphorene as electrode materials for the fabrication of all three alkali-ion batteries. In the first aspect of my work, we fabricated tellurene (Te) based LIBs, SIBs, and KIBs. Through density functional theory calculations (DFT) and in-situ transmission electron microscopy (TEM) technique, we confirmed that Te when reacts with lithium (Li), sodium (Na), and potassium (K) led to the formation of A2Te type product where A is either Li or Na or K. Significant difference in crystal property of product was observed during in-situ TEM measurement wherein Li2Te showed single-crystal property whereas Na2Te and K2Te showed polycrystalline nature. Nudged elastic band calculations (NEB) and ab-initio molecular dynamics (MD) confirmed isotropic diffusion of Li atoms on the surface and inside few-layer Te sheets. However, the diffusion of Na atoms and K atoms was highly anisotropic. This allowed us to present a hypothesis stating that if the diffusion of alkali atoms is isotropic, the reaction between Te and alkali atoms will happen simultaneously in all directions due to which no grain boundaries will form during the reaction. Te-based LIBs were able to deliver a moderate initial capacity of ~ 350 mAh g-1, higher than Te-based SIBs ( ~ 160 mAh g-1) and Te-based KIBs (~ 150 mAh g-1). However, huge volume expansion in Te during chemical reaction led to pulverization/delamination of Te electrode due to which capacity degradation was observed in all three alkali-ion batteries. Te-based LIBs were able to deliver a stable capacity of ~ 120 mAh g-1 after 50 cycles proving that gravimetrically Te does not provide any advantage over conventional graphite used in current-state-of-art LIBs. However, the volumetric picture was different wherein Te-based LIBs were able to deliver a very high initial capacity of ~ 1800 mAh cm-3 with a stable capacity of ~ 700 mAh cm-3 (2x graphite) after 50 cycles. Pulverization/delamination was still observed and to overcome this issue, graphene coating over Te was used. Te was able to retain over ~ 85% of its initial capacity after 100 cycles in presence of graphene. In the second aspect of my work, I have explored phosphorene for battery application. Before us, phosphorene has already been explored for LIBs and SIBs and delivers a very high capacity of ~ 2600 mAh g-1 corresponding to Li3P/Na3P alloy formation. Hence, we explore phosphorene for only KIBs with an expectation that it will lead to a similar type of alloy product. Phosphorene and phosphorene carbon composite was synthesis through a liquid-phase exfoliation method. Two carbon-based materials were utilized, reduced graphene oxide (rGO) and single-walled carbon nanotubes (sCNTs), to overcome the pulverization/delamination challenge. Phosphorene was able to deliver a very high initial capacity of ~ 1200 mAh g-1 but rapid capacity decay to ~ 50 mAh g-1 was observed in subsequent cycles. rGO and sCNTs were used to provide mechanical buttress to the electrode wherein rGO proved to be a better candidate in preventing delamination of phosphorene electrode because of its similar dimensionality with phosphorene. Few-layer phosphorene/reduced graphene oxide (FLP-rGO) showed a stable capacity of ~ 550 mAh g-1 after 50 cycles with ~ 97% columbic efficiency. Using DFT calculations, ex-situ X-ray photoelectron spectroscopy, and X-ray diffraction techniques, we confirmed that phosphorene when reacts with K led to the formation of K4P3 alloy. FLP-rGO performed well even in a full-cell configuration while utilizing spherical potassium cobalt oxide (s-KCO) as a cathode. Throughout my Ph.D. work, we have utilized nano-sized tellurene and phosphorene for the fabrication of alkali-ion batteries. The long-standing debate over utilizing nano-sized over micro-sized electrodes for battery fabrication concludes that battery engineers are reluctant in utilizing nano-sized material due to its low first cyclic columbic efficiency, low volumetric capacity, high fabrication cost, and complex experimental synthesis. Alloy or conversion-based materials were expected to work better in their nano-sized form, however, a growing body of works indicates that micro-sized material can be utilized with proper electrode design, binder modification, and electrolyte engineering. Therefore, micro-sized tellurene and phosphorene can potentially be explored in the battery field to enhance their industrial significance.
August 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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