Restriction lift date: 2022-06-05
Nanomaterial-based devices for advanced energy storage and delivery
Loading...
Files
Full Text E-thesis
Date
2021
Authors
McGrath, Louise M.
Journal Title
Journal ISSN
Volume Title
Publisher
University College Cork
Published Version
Abstract
The Internet of Things (IoT) scenario offers benefits such as reduced analysis costs, improved safety and will enable the prediction of future trends due to the multitude of wireless sensors acting as an environmental interface that provides data. Currently, non-rechargeable batteries act as the predominant energy source for today’s commercial wireless sensors, however their large sizes often hinders the miniaturisation of IoT sensors, thus reducing the potential application for these sensors. In addition, the high energy and power demands of the sensors dramatically reduce the lifetime of the primary batteries, which then requires their frequent replacement. Therefore, the ultimate challenge facing the mass distribution of wireless sensors is meeting the energy and power requirements to match the lifetime of the microdevices. Advancements towards energy harvester-powered sensors are hindered due to their low efficiency and limitations, therefore the need for hybrid systems comprising a significantly smaller rechargeable energy storage device coupled to energy harvesters are of interest to enable long life autonomous IoT sensors.
Li-ion rechargeable batteries (LiB) are the battery chemistry of choice and have been demonstrated with organic and solid-state electrolytes. LiBs provide high energy density (~270 W kg-1) in conventional organic electrolytes with modest cycle life (500 to 1,000) but are limited to low power densities (<1000 W kg-1) often caused by the presence of inactive binder and conductive additive materials in the anode and cathode, resulting in hampered device operation particularly during the energy intensive periods of sensor measurement and wireless communication. In addition, the utilisation of volatile organic carbonate based electrolytes within the battery raises a safety issue, therefore limiting their potential applications.
In order to circumvent this, solid-state Li microbatteries represent an alternative due to their increased intrinsic safety as no volatile organic components are utilised. In addition, they offer larger potential energy densities due to the removal of inactive binder and conductive additive materials, and in some cases utilisation of Li metal as an anode improves the energy density. They offer a significantly better cycle life when compared to organic electrolyte-based batteries (≥5,000 vs. <1,000). However, their drawbacks have limited their use in commercial systems, for example, the need to maintain thin electrodes (at the micron level) particularly for the low electronic conductivity oxide cathodes typically utilised. In addition, sluggish Li+ diffusion within the electrolyte and electrodes at room temperature often results in poor power capabilities and a significant potential drop during high current operation.
Based on the time constant (τ) for linear diffusion in a material of dimension L (τ= L^2/2D), where D is the diffusion coefficient, it can be estimated that the time taken for Li+ to diffuse in typical battery materials of micron dimension will be two to three orders of magnitude slower with a corresponding lower power capability than for a nanoscale (≤100 nm) material. Therefore, a small form factor capable of high current operation is critical in the development of the next-generation hybrid systems. Identification of an appropriate electrolyte which boasts a combination of organic and solid-state properties such as non-volatility, non-flammability and high ionic conductivity at room temperature which can exist in both liquid or solid form is desirable. Identification of such an electrolyte in addition to high power and energy density materials will allow for the realisation of IoT autonomous sensors.
An alternative class of electrolytes have been identified and discussed within this thesis: ionic liquids. They offer the aforementioned favourable properties in addition to large electrochemical windows which enables the use of various anode and cathode materials. Their non-volatility allows for easy integration into devices, and they offer the opportunity to electrodeposit certain electrode materials such as Ge, which further demonstrates their applicability and suitability as electrolyte materials. These hybrid properties are showcased during a thorough cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) analysis. The ability of the ionic liquid to form solid state analogues is demonstrated as a series of polymer gel electrolytes is synthesised using simple synthesis methods. The solid-state polymer gel electrolytes offer comparable ionic conductivities to those of conventional electrolytes (1 M LiPF6 in 1:1 v/v ethylene carbonate (EC):diethyl carbonate (DEC)) (1.9 mS cm-1 vs. 7.9 mS cm-1), with their ionic conductivities being up to 600 times greater than typical solid-state electrolytes such as lithium phosphorous oxy-nitride (LiPON).
The utilisation of direct current (D.C.) magnetron sputtering for the deposition of Ge is described, whereby uniform nanoscale thin films (~100 nm) are obtained. The compatibility of the Ge thin films with the ionic liquid electrolyte is demonstrated as typical amorphous CV data is obtained which correlates with the Ge analysed in organic-based electrolytes. Galvanostatic cycling of the Ge electrode in the ionic liquid achieved the highest reported Ge electrode capacities achieved in literature using the pyrrolidinium-based ionic liquid electrolyte. Capacities of ~900 mAh g-1 are obtained at 3.5 C (C = C-rate), whereby nanostructured electrodes in literature achieved a maximum capacity of 600 mAh g-1 at 1 C using similar electrolyte materials. The addition of carbonate additives such as vinylene carbonate (VC) prevented large scale delamination of the electrode material from the current collector as evidenced by SEM analysis.
The utilisation of electrodeposition to fabricate nanoscale V2O5 thin films allows for crystalline or amorphous to be obtained. Both amorphous and crystalline thin films exhibited compatibility with the liquid and polymer gel electrolytes, however, the crystalline V2O5 thin films exhibited higher power capabilities due to intercalation pseudocapacitance electrochemical kinetics. Long-term cycling (~400 cycles) are exhibited with minimal capacity fade (0.4% over 50 cycles at 5 C in the polymer gel electrolyte PG-60) with lithium metal anodes.
To assess the potential use of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C4mpyrTFSI)-based electrolytes with nanostructured electrodes, CV analysis was carried out with inverse opal (IO) TiO2 electrodes. Similar electrochemical performances to conventional organic electrolyte (1 M LiPF6 in 1:1 v/v EC:DEC) were observed. In addition, identical current densities (organic: -0.04 mA cm-2 vs. IL: ~ -0.04 mA cm-2 at 0.5 mV s-1) and peak positions were achieved in both solutions. Additive analysis was carried out the mitigate electrode swelling observed in additive-free electrolytes. Finally, polymer gel electrolyte analysis was carried out, whereby high coulombic efficiencies (>90%) were attained.
Prototype ionic liquid-based full cells consisting of V2O5 cathodes and Ge anodes are investigated. A reduction in the separator thickness results in more stable cycling across the C-rates as minimal capacity drop is observed in the vinylene carbonate (VC)-containing 260 versus a 900 m thick electrolyte cell ( ~0.3 vs. ~1 mAh g-1 per cycle at 0.5 C, respectively). Up to 700 cycles was achieved for both VC- and VC-free cells without significant capacity fade or cell failure. These cells are comparable to typical solid-state microbatteries, as capacities of 5.7 and 3.2 µAh at 0.2 C (cycles 11 to 20) and 1 C (cycles 41 to 50) respectively are demonstrated by the prototype, in addition to an average discharge capacity of 1.3 µAh at 5 C for cycles 600 to 650 is attained after significant C-rate analysis (rates from 0.2 C to 10 C utilised).
Description
Keywords
Germanium , Vanadium oxide , Titanium dioxide , Lithium metal , Ionic liquid , Pyrrolidinium , Polymer gel electrolyte , Lithium-ion , Microbattery , High-rate , Thin-film , Electrodeposition , Magnetron sputter
Citation
McGrath, L. M. 2021. Nanomaterial-based devices for advanced energy storage and delivery. PhD Thesis, University College Cork.