Hierarchical SnO₂@PC@PANI composite via in-situ polymerization towards next-generation Li-ion capacitor by limiting alloying process with high energy, wide temperature performance, and cyclability

Abstract

Lithium-ion capacitors (LICs) are promising electrochemical energy storage devices with hybridization of the battery-type anode and capacitive-type cathode, bringing high energy density and high power density simultaneously in a single device. We report the extraction of activated carbon from the palmyra fruit, in-situ polymerization with polyaniline (PANI), and the making of different composite formulations with SnO₂ to be used as anode for LIC with palmyra fruit-derived activated carbon (PC) as the cathode. The SnO₂@PC@PANI electrode is restricted to 1 V vs. Li, which allows only the alloying reaction and eliminates the conversion process. An in-situ impedance study is performed to validate the formation and depletion of the solid electrolyte interface over the battery-type electrode upon cycling. Prior to the fabrication of the LIC, the SnO₂@PC@PANI is electrochemically pre-lithiated. The fabricated LIC with balanced mass loadings exhibits a mesmerizing electrochemical performance with a maximum energy density of ∼175 Wh kg^–1 at room temperature. The possibility of LIC in different climatic conditions is also analyzed by testing its electrochemical performance in different temperature conditions.

Introduction

Greenhouse gas emission is increasing tremendously due to increased fossil fuel consumption, escalating global warming, and other environmental impacts [1]. In this context, innovative steps should be taken to reduce its effect on the environment by exploring alternative renewable energy sources. We have different clean, renewable, and environmentally friendly energy sources such as solar, wind, tidal, etc. However, the storage of these forms of energy is a big challenge. Electrochemical energy storage is the most promising among the different energy storage devices. Since Sony commercialized the first Lithium-ion battery (LIB) in 1991, the research on batteries has increased exponentially to improve their performance. They have been significantly explored and are used in several electrical devices, including mobile phones, laptops, medical applications, etc. However, they still lack the power density required for high-end applications. Meanwhile, electrical double-layer capacitors (EDLCs) have a high power density but decreased energy density. In this context, there came the necessity for a single device with energy and power density. Hence, there arose the idea of Lithium-ion capacitors (LICs).

LIC is a hybrid supercapacitor that consists of a battery-type (faradaic reaction) anode paired with a capacitive-type (non-faradaic reaction) cathode. This hybrid device has a higher energy density than EDLCs and a high power density than LIBs. Hence, they can bridge the gap between EDLCs and LIBs [2], [3], [4], [5], [6]. The performance of LICs majorly depends on the battery type electrode, i.e., anode. Therefore, most of the research activities in LICs are carried out on battery-type electrodes [7], [8], [9], [10], [11], [12], [13], [14]. The exploration started with the insertion type [15] electrodes such as Li₄Ti₅O₁₂, graphite, etc. However, the lack of energy density for these insertion anodes makes people think of high-capacity alloying and conversion anodes [5,[16], [17], [18], [19], [20]]. The theoretical capacities of alloy/conversion-type anodes are more than (>600 mAh g^–1) that of the abovementioned insertion types [21]. Moreover, the operational potentials are moderate and have good safety characteristics. Among the different electrode materials, SnO₂ is one of the guaranteed candidates due to its high theoretical capacity (>750 mAh g^–1), abundance, and lower redox potential (∼0.3 V vs. Li) [22], [23], [24], [25]. Nevertheless, one of the main challenges to overcome is the volume variation (up to 300%) caused during cycling, which could lead to the instability of the cell. Many research efforts are being reported in which morphology-controlled nanoparticles are synthesized at various dimensions to alleviate the pulverization of electrodes during cycling [26], [27], [28], [29], [30]. In our work, we synthesized hierarchical flower-shaped SnO₂ nanostructures by a simple hydrothermal method and utilized them as anodes for LIC applications. The stability is improved by hybridizing with palmyra fruit-derived activated carbon (PC) and in-situ polymerized polyaniline (PANI). The polymer PANI enhanced the conductivity and flexibility of the SnO₂ electrode by stabilizing its volume variation and thereby provided better stability [31]. Mass loading between the battery and capacitive-type electrodes was optimized based on the half-cell performance (vs. Li) and subsequently pre-lithiated prior to the fabrication of the LIC. The better electrochemical performance was exhibited by both half-cell, and full-cell configurations with superior energy density, power density, and long-term cyclic stability even at different temperature conditions are discussed in detail. Moreover, while comparing with previously reported SnO₂-based LICs, our AC/SnO₂@PC@PANI LIC exhibits superior performance in terms of energy and power density [32], [33], [34], [35], [36].