Studies of manganese based oxides as alternative electrode materials for lithium based energy storage devices

Lithium ion based energy storage devices have captured a significant share of the market for energy storage devices owing to their high energy and power density. However the cost of raw materials and their availability warrants a search for alternative materials with improved energy densities and rate capabilities. Manganese based oxide materials are attractive as alternative choices for electrode materials. However, they encounter a host of issues which result in poor performance in systems based on the Li+ ion shuttle. One of the issues is structural degradation. It is well known that spinel LiMn2O4 undergoes structural degradation when lithium is inserted into the octahedral voids of the structure owing to the change of average valence of manganese from +3.5 to +3. This leads to a Jahn-Teller distortion induced structural relaxation that leads to the formation of a new tetragonal phase which is responsible for poor capacity retention. However if one can manage to keep the average manganese valence to above +3.5 during lithium insertion, it would be possible to prevent JT distortion and associated structural changes thus making it possible to access the voltage offered by the Mn3+/Mn4+ redox couple. This thesis sheds new light on the existing understanding about the effect of substitutions on performance of two well-known spinel compounds LiMn2O4 (substitution with nickel) and Li4Mn5O12 (substitution with titanium). Both substitutions were found to prevent phase transformation during cycling resulting in better capacity retention. Different synthesis methods are employed to prepare samples with different morphologies and the effect of morphology on performance is also investigated. It was found that the performance of the spinel LiNi0.5Mn1.5O4 was affected by the space group of the crystal structure as well as morphology in the voltage range of 2.3 – 3.3V. Nano-structuring was found to play a significant role in improving the performance of spinel LiNi0.5Mn1.5O4. The findings reveal that nickel substitution in LiNi0.5Mn1.5O4 was a more attractive option to pursue. Finally full cell devices were assembled using the best performing material as working electrode to demonstrate the feasibility of manganese based oxide materials as attractive alternative electrode materials for rechargeable energy storage devices based on the lithium ion.DOCTOR OF PHILOSOPHY (MSE

Enhanced cycling stability of o-LiMnO2 cathode modified by lithium boron oxide coating for lithium-ion batteries

The effect of lithium boron oxide (LBO) coating on the electrochemical performance of orthorhombic LiMnO2 (o-LiMnO2) cathode for lithium-ion batteries is investigated. o-LiMnO2 synthesized via solid state synthesis technique is modified with LBO addition. The presence of LBO is identified via Fourier transform infrared spectroscopy analysis. o-LiMnO2 is observed to transform to a spinel-like phase during cycling which undergoes capacity fading. Studies indicate that the presence of 1-2 wt% LBO results in an improved capacity and better capacity retention with cycling. The pristine sample reveals a maximum specific capacity of 172 mAhg(-1), whereas the LBO-modified samples display about 189.1 mAhg(-1) in the cycling tests conducted at a rate of 50 mAg(-1) in the voltage range of 2-4.5 V. After 70 cycles, the LBO-modified LiMnO2 displayed higher capacity retention of 175 mAhg(-1) as compared to the pristine sample that exhibited 130 mAhg(-1). By analyzing the charge-discharge behavior, it is observed that the capacity obtained from lithium insertion into the tetrahedral sites of the spinel structure is more or less constant throughout the cycling and that the bulk of the capacity loss is resulting when lithium is inserted into the octahedral sites of the spinel structure. Impedance measurement reveals a reduced charge-transfer resistance for the LBO-modified samples suggesting that the presence of LBO is countering capacity loss arising from insertion of lithium into the octahedral sites thus contributing to the overall cycling stability

All carbon based high energy lithium-ion capacitors from biomass : the role of crystallinity

We report all carbon-based high energy Li-ion capacitor from environmentally threatening bio-source, prosopis juliflora. The pyrolyzed carbon exhibits a few layers of graphene-like structure and tubular morphology with multiple inherent heteroatoms like N, S, and Ca. Presence of such heteroatoms are certainly beneficial to the betterment of electrical conductivity, and pore generation which eventually results in an enhancement in capacity/capacitance of carbonaceous materials. The electrochemical pre-lithiation strategy is used to mitigate the irreversibility observed, and eventually employed as a negative electrode in a hybrid configuration. This LIC delivered a high energy density of ∼216 and 185 Wh kg−1 at ambient (25 °C) and elevated temperature (55 °C) conditions, respectively. Further, ∼94% initial capacity is retained after 5000 cycles with minimum fading of 0.0013% per cycle at ambient temperature. This results clearly demonstrate that the surface functionality and heteroatom doping with tubular structure synergistically facilitates the Li+ and electron transport properties to realize higher energy density for this fascinating all carbon-based Li-ion capacitor.National Research Foundation (NRF)This study was supported by the National Research Foundation of Korea grant funded by the Korea government (Ministry of Science, ICT) (No. NRF-2011-C1AAA0010030538). Also, the work was financially supported by NTU-HUJ Create Phase II which is a joint research programme between the Hebrew University of Jerusalem (HUJ, Israel) and Nanyang Technological University (NTU, Singapore) with CREATE (Campus for Research Excellence and Technological Enterprise) funding from National Research Foundation of Singapore (NRF,Singapore). VA thank the financial support from the Science & Engineering Research Board (SERB), a statutory body of the Department of Science & Technology, Govt. of India through Ramanujan Fellowship (SB/S2/RJN-088/2016)

Unveiling the Fabrication of “Rocking-Chair” Type 3.2 and 1.2 V Class Cells Using Spinel LiNi_0.5Mn_1.5O₄ as Cathode with Li₄Ti₅O₁₂

We first report the possibility of using spinel LiNi_0.5Mn_1.5O₄ (LNM) nanofibers as cathode for constructing high (∼3.2 V) and low (∼1.2 V) voltage “rocking-chair” type Li-ion cells with spinel Li₄Ti₅O₁₂ (LTO) by utilizing Li-insertion into tetrahedral (Ni^2+/4+) and octahedral (Mn^4+/3+) sites of LNM. We also explored the possibility of constructing symmetric cells (LNM/LNM) with a working potential of ∼1.8 V. Among the three configurations investigated, LNM/LTO (electrochemically prelithiated) is found superior in terms of high rate capability, cyclability, and good capacity retention characteristics. High performance spinel LNM nanofibers are prepared via a scalable electrospinning technique and their Li-insertion properties are evaluated in half-cell assembly. The half-cell studies are used for adjusting the mass balance while fabricating “rocking-chair” Li-ion cells

Layered NaxMnO2+z in sodium ion batteries-influence of morphology on cycle performance

Due to its potential cost advantage, sodium ion batteries could become a commercial alternative to lithium ion batteries. One promising cathode material for this type of battery is layered sodium manganese oxide. In this investigation we report on the influence of morphology on cycle performance for the layered NaxMnO2+z. Hollow spheres of NaxMnO2+z with a diameter of similar to 5 mu m were compared to flake-like NaxMnO2+z. It was found that the electrochemical behavior of both materials as measured by cyclic voltammetry is comparable. However, the cycle stability of the spheres is significantly higher, with 94 mA h g(-1) discharge capacity after 100 cycles, as opposed to 73 mA h g(-1) for the flakes (50 mA g(-1)). The better stability can potentially be attributed to better accommodation of volume changes of the material due to its spherical morphology, better contact with the added conductive carbon, and higher electrode/electrolyte interface owing to better wetting of the active material with the electrolyte

Investigation of the electrochemical and thermal stability of an ionic liquid based Na 0.6 Co 0.1 Mn 0.9 O 2 /Na 2.55 V 6 O 16 sodium-ion full-cell

Electrolytes based on non-flammable and electrochemically and thermally stable ionic liquids (ILs) are rendered promising alternatives to the conventionally applied organic electrolytes for lithium as well as sodium ion batteries (SIBs). In this study the electrochemical performance and thermal stability of a SIB full-cell containing an IL based electrolyte is evaluated and compared to a reference system employing a conventional organic electrolyte. Compatibility of the IL electrolyte with the electrode materials Na0.6Co0.1Mn0.9O2 (NMO) and Na2.55V6O16 (NVO) is assured by SIB half-cell studies. In NMO/NVO full-cells the IL electrolyte outperforms the organic electrolyte in terms of cycling stability and columbic efficiency, reaching a retention of 76% after 100 cycles. Studies at 75°C show that, in contrast to the system based on the organic electrolyte, the IL-based SIB is capable of operating at elevated temperatures. Further, for the first time the superior safety of an IL-based SIB full-cell over the organic analogue is proven using Accelerating Rate Calorimetry (ARC) underlining the benefits of the IL based electrolyte.NRF (Natl Research Foundation, S’pore)Published versio

Synthesis and physicochemical characterization of room temperature ionic liquids and their application in sodium ion batteries

Sodium ion batteries (SIBs) based on IL electrolytes have attracted great attention, particularly in large-scale energy storage systems for renewable energy due to the abundance of sodium and the excellent safety resulting from the use of non-flammable ionic liquid (IL) electrolytes. In this article, a series of 15 functionalized room temperature ionic liquids (RTILs) suitable as electrolytes is presented. Special emphasis was laid on the purity of the synthesized RTILs and a consistent and uniform characterization of their physicochemical properties. Evaluation of the viscosity, conductivity, and thermal and electrochemical stabilities resulted in clear structure–property relationships, rendering the ether functionalized RTILs most promising for application in SIBs. Electrochemical investigations of the ether functionalized IL electrolytes in SIB half cells (Na0.6Mn0.9Co0.1O2 as cathode material) proved their compatibility with a SIB system. Stable cycling performance was achieved with the piperidinium based RTIL IL 6 outperforming the organic electrolyte by far with a retention of 81% after 350 cycles. These results show the suitability of RTILs to enhance the performance of SIB systems and serve as a basis for the design of high performance IL electrolytes.NRF (Natl Research Foundation, S’pore)Accepted versio

Synthesis of high volumetric capacity graphene oxide-supported tellurantimony Na- and Li-ion battery anodes by hydrogen peroxide sol gel processing

High-charge-capacity sodium-ion battery anodes made of Sb2Te3@reduced graphene oxide are reported for the first time. Uniform nano-coating of graphene oxide is carried out from common sol of peroxotellurate and peroxoantimonate under room temperature processing. Reduction by hydrazine under glycerol reflux yields Sb2Te3@reduced graphene oxide. The electrodes exhibit exceptionally high volumetric charge capacity, above 2300mAhcm-3 at 100mAg-1 current density, showing very good rate capabilities and retaining 60% of this capacity even at 2000mAg-1. A comparison of sodiation and lithiation shows that lithiation exhibits better volumetric charge capacity, but surprisingly only marginally better relative rate capability retention at 2000mAg-1. Tellurium-based electrodes are attractive due to the high volumetric charge capacity of Te, its very high electric conductivity, and the low relative expansion upon lithiation/sodiation

Effect of conducting salts in ionic liquid electrolytes for enhanced cyclability of sodium-ion batteries

The electrochemical performance of ionic liquid electrolytes containing different sodium salts dissolved in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) evaluated in a half-cell configuration using spherical P2-Na0.6Co0.1Mn0.9O2+z (NCO) cathodes are reported. Among the various electrolytes investigated, sodium bis(fluorosulfonyl)imide (NaFSI) (0.5 M) in BMPTFSI shows the best electrochemical performance with a significant improvement in cycling stability (90% capacity retention after 500 cycles at 50 mA g–1 in a half cell versus Na metal anode) compared with conventional NaClO4 (1 M) in ethylene carbonate/propylene carbonate electrolytes (39% retention after 500 cycles). Cyclic voltammetry (CV) studies reveal that ionic liquid electrolytes are stable up to 4.8 V versus Na/Na+. When NaFSI and NaTFSI are used as conducting salts, X-ray photoelectron spectroscopy results prove that the cathode electrolyte interface (CEI) is composed of components resulting from the decomposition of the TFSI anion and the deposition of the BMP cation. On the other hand, the CEI layer of the electrode cycled in an electrolyte containing NaClO4 in BMPTFSI follows a different pathway of TFSI decomposition and consists mainly of sodium fluoride. Similarly, plating studies were used to understand the stability of different ionic liquids in contact with metallic sodium. It was found that the excellent capacity retention for the electrolyte consisting of NaFSI salt is related to the formation of a stable CEI and solid electrolyte interphase layers.NRF (Natl Research Foundation, S’pore)Accepted versio