Manickam Minakshi Sundaram

In order to take advantage of the increasing sophistication of technology for harnessing renewable energy resources, serious attention must be paid to how to store and re-access this energy. Electrochemical storage, in the guise of batteries, supercapacitors and pseudocapacitors, has attracted much attention as a viable option for enhanced energy storage applications. But in order for these technologies to be implemented successfully we need to find materials that perform better and are relatively easy to synthesise. Bimetallic transition metal oxides are materials that are readily synthesised and may be multifunctional, i.e. have a role at the electrochemical atomic level as well as the device level. In order for these materials to work efficiently in new generation systems based on sodium and lithium they also need to be mesoporous. This can be achieved by trying to find synthetic techniques that produce specific, highly regulated nanostructures or by adding a ‘templating’ agent during the bulk synthesis step. We have investigated the simple hydrothermal preparation of a number of nickel cobaltate (NiCo2O4) materials using polymer templates, eggshell membrane (ESM) and poly methyl methacrylate (PMMA), as potential electrode materials for supercapacitors. The ESM was expected to act as a fibrous, random polymeric template while the PMMA should produce a much more ordered material. Electrochemical testing showed that the different templates have led to changes in material morphology and these have resulted in a difference in electrochemical properties. Templated materials had an increased specific capacitance than non-templated and the choice of template could influence the capacitance by as much as 30 %.

An environmentally friendly sodium transition metal phosphate (NaMn1/3Co1/3Ni1/3PO4) has been synthesized via combustion route with carbon coating on the surface. Energy storage devices based on sodium have been regarded as an alternative to the traditional lithium-based system because they are abundant in nature, inexpensive and safe. Sodium transition metal phosphate served as an active electrode material for both aqueous and nonaqueous hybrid supercapacitors. The electrochemical behavior of phosphate vs. activated carbon in the fabricated hybrid device exhibits both faradaic and nonfaradaic processes. Reversible Na+ de-intercalation/intercalation and desorption/adsorption occur within a safe voltage window for both aqueous and nonaqueous electrolytes. The asymmetric device shows redox peaks in the cyclic voltammetry and sloping profiles in the charge-discharge curves while providing excellent capacity retention. A detailed study on the electrochemistry and materials perspective (using microscopy and surface analyses) with an emphasis on the reaction mechanism has been reported.

The availability of an efficient and low cost battery is the key for developing practical electric vehicles (EV). The currently available nickel-metal hydride battery could be a good candidate for EV but it is too expensive and not environmentally acceptable for EV applications. Rechargeable lithium ion batteries that use non-aqueous (organic solvents) electrolytes have been available in the market for over a decade are the most attractive power sources that are vital to meet the challenge of global warming, greenhouse gas emissions and fossil fuel consumption. These can be readily used for powering consumer electronic devices. However, it is quite difficult to make a large lithium battery which is both safe and inexpensive. This is due to the reactivity of the electrode materials with the non-aqueous electrolytes i.e. thermally unstable. In order to realize a perfect safety even at high temperature, non-aqueous (organic) electrolyte may be replaced by aqueous electrolyte system. In the case of non-flammable (aqueous) electrolyte, lithium hydroxide may have an advantage in terms of high conductivity that lowers the charge transfer resistance and cell impedance. The Zn|LiOH|MnO2 battery chemistry reported in this work delivered 142 mAh/g and the cell was rechargeable for multiple cycles. Alternatively, Polyvinylpyrrolidone (PVP) coated MnO2 showed improved discharge capacity of 200 mAh/g but a larger amount of PVP coating causes a decrease in capacity to 83 mAh/g. The incorporation of Bi2O3 + TiS2 (3 wt% each) additives into the MnO2 cathode was found to improve the overall cell performance, this is partly due to the suppression of proton insertion.

Considering the safety, cost and low ohmic resistance issues with respect to the non-aqueous electrolytes that are generally used in rechargeable lithium ion batteries, aqueous electrolytes attract wide interest. A traditional Zn-MnO2 battery in which potassium hydroxide (KOH) has been replaced with a lithium hydroxide (LiOH) electrolyte is discussed. As lithium intercalation materials are of special interest as cathode in rechargeable batteries, the new concept has been extended to use lithium nickel phosphate as cathode for aqueous rechargeable batteries. Here, we show reversible extraction and insertion of lithium from and into olivine LiNiPO4 in aqueous solutions. These cells are found to be cheap, rechargeable and safe. The unique sol-gel synthesis has been used to synthesize LiNiPO4 which has been characterized in order to evaluate a new potential cathode for aqueous rechargeable batteries.

A new class of rechargeable manganese dioxide electrode (MnO2) in aqueous electrolyte is described. Intercalation of lithium from the LiOH electrolyte into the vacant sites of a host MnO2 has been achieved electrochemically is good news. The formation of a lithium carbonate layer from a LiOH electrolyte acts as a barrier for protons while permitting lithium ion insertion in aqueous solutions forming lithium intercalated manganese dioxide (LixMnO2) upon discharge. This novel mechanism may be a key in transferring primary to secondary batteries using LiOH as electrolyte.

The redox behavior and surface characterization of manganese dioxide (MnO2) containing titanium disulphide (TiS2) as a cathode in aqueous lithium hydroxide (LiOH) electrolyte battery have been investigated. The electrode reaction of MnO2 in this electrolyte is shown to be lithium insertion rather than the usual protonation. MnO2 shows acceptable rechargeability as the battery cathode. The influence of TiS 2 (1,3 and 5 wt. %) additive on the performance of MnO2 as a cathode has been determined. The products formed on reduction of the cathode material have been characterized by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM). It is found that the presence of TiS2 to ≤ 3 wt. % improves the discharge capacity of MnO 2. However, increasing the additive content above this amount causes a decrease in its discharge capacity.

The redox behavior and surface characterization of manganese dioxide (MnO2) containing titanium boride (TiB2) as a cathode and Zn as an anode have been investigated in aqueous saturated lithium hydroxide (LiOH) electrolyte battery. The electrode reaction of MnO2 with TiB2 in this aqueous electrolyte is shown to be lithium insertion rather than the usual protonation. The influence of TiB2 additive (1, 3 and 5 wt.%) on the performance of MnO2 as a cathode and its cycling ability have been determined.