Investigations of Chalcogen-Cathodes and a Carbonitride-Anode for Alkali-Based Rechargeable Batteries
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In this thesis, we have attempted to understand the working principle of Li(Na)-S(Se) battery and following such understandings we have attempted towards the design of various S(Se)- cathode materials for the alkali based chalcogen batteries. In the final chapter, we have focussed on the development of anode materials for full Li-ion cell. The summary of the various chapters is as follows. Chapter 2 discusses about NaY-xS-PAni exhibiting remarkable electrochemical performance as a cost-effective sulfur cathode for rechargeable Li-S batteries. The superior electrochemical stability and performance of the NaY-xS-PAni is directly correlated to the novel NaY electrode structure in combination with the host polarity and ionic conductivity. The zeolite provides an optimum geometrical and chemical environment for precise confinement of the sulfur while the polyaniline coating provides electron conduction pathway along with extra polysulfide confinements. This cathode material exhibits very stable cycling for more than 200 cycles with relatively low specific capacity and modest rate capability. To develop a material for obtaining high specific capacity value we moved to carbon based host and the details are covered in chapter 3 and 4. To summarize Chapter 3, we have successfully extended the pressure induced capillary filling method for confinement of sulfur and selenium in the interior core of the MWCNTs. This method results in ultra-high loading yields of the chalcogens inside the MWCNTs. The ensuing composites S-CNT have been convincingly demonstrated as prospective cathodes in Li-S rechargeable batteries exhibiting very high specific capacities ~ 1000 mAh g-1 at C/10 current rates. The novelity of this host has been established by extending the work in encapsulating Se with the similar protocol and studying its electrochemical activity. The high efficiency of the Li-S/Se electrochemical reaction observed here is directly attributed to the efficacy of the encapsulation protocol of S/Se inside the CNTs. The polyselenides/polysulfides are completely confined within the precincts of the CNT cavity leading to an exceptionally stable battery performance at widely varying current densities. With the success of this encapsulation technique for the carbon based host, we developed another interconnected mesoporous microporous carbon host for sulfur encapsulation the details of which constitute the next chapter. In chapter 4, we have discussed here a novel S-cathode where the sulfur confining hierarchical carbon host synthesized using a sacrificial template can be very effectively employed for in Li-S rechargeable battery. The hierarchical mesoporous-microporous architecture comprising of both mesopores and micropores provide an optimal potential landscape which in turn traps high amounts of sulfur as well as polysulfides formed during successive charge-discharge cycles. The uniqueness of the carbon matrix translates to exceptionally stable reversible cycling and rate capability for Li. Such promising result with Li-S battery compelled us to check the performance with Na anode. This led to the development of intermediate temperature Na-S battery with JNC-S as the prospective cathode. It is envisaged that such materials design will be very promising in general for battery chemists especially for higher valent metal-sulfur systems (e.g. magnesium, aluminum). The host discussed here will be ideally suitable for introduction of dopants such as nitrogen, boron, thus enhancing it’s versatility as a heterodoped mesoporous-microporous host for varied applications. In all the preceding chapters, the focus was to encapsulate sulfur in some host structures. Chapter 5 deals with an alternative configuration for the Li-S battery that uses an oxide based interlayer to restrict the polysulfides. From the study discussed here, it can be concluded that NiOH-np/NiO-np can act as an efficient interlayer material for superior anode protection. The interlayer provides an anchor to hold back the polysulfides primarily on the cathode side by forming intermediates such as NiS3(OH) and NiS4(OH). Although, the specific capacity is less compared to the theoretically estimated value for S-cathode, the high cyclability coupled with extremely good rate capability performance makes this a very promising configuration of Li-S cell assembly for practical applications and deployment. The success of this strategy is expected to decrease the need for design of sophisticated S-scaffolds and lead to simpler Li-S rechargeable batteries. After an extensive discussion on development of cathodes for alkali based chalcogen batteries, we shifted gears and tried our hands in developing some eco-friendly anode materials. The details of graphitic carbonitride as an anode material for Li-ion cell has been discussed in chapter 6. To conclude, we have discussed here in detail the unique layered structure of the as-synthesized gCN and its impact on the intrinsic charge transport properties. Both factors eventually determine their electrochemical performance. The gCN discussed here is obtained using a very simple synthesis protocol in large yields from a very cheap organic precursor. The work highlights again the important role of chemical composition and structure on the functionality of the intercalation host. These have a strong bearing on the electronic charge distribution in the host and its eventual interaction with the intercalating ions. Compared to several non-trivial layered carbonaceous structures, the gCN interestingly displays 3-D ion transport. Additionally, it also sustains facile electron transport (2-D) despite the low concentration of carbon. In spite of the modest specific capacities as observed in case of the half cells, the gCN when assembled with (high) voltage cathodes in full Li-ion cells, the performance is quite encouraging. To the best of our knowledge this is for the first time that graphitic carbon nitrides have been demonstrated as an anode in full Li-ion cells. The potential of majority of the reported high surface area and high capacity complex carbonaceous structures in Li-ion cells are inconclusive. This is mainly due to the fact that the percentage of reports on full Li-ion cell performance is very rare. The full cell analysis of the gCN discussed here conclusively rules out the necessity of the requirement of high specific capacity materials in practical/commercial full cells. We envisage that the work discussed here will pave the way for synthesis of many such electrode materials from renewable resources resulting in the development of green and sustainable batteries. Overall we have been able to address some of the potential problems of Li-S and Li-ion battery systems. There is further scope of betterment with extensive study and this work opens the scope for it in future.
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