Study of the initial stage of solid electrolyte interphase formation upon chemical reaction of lithium metal and N-Methyl-N-Propyl-Pyrrolidinium-Bis (Fluorosulfonyl) Imide

Chemical reaction studies of N-methyl-N-propyl-pyrrolidinium-bis(fluorosulfonyl)imide-based ionic liquid with the lithium metal surface were performed using ab initio molecular dynamics (aMD) simulations and X-ray Photoelectron Spectroscopy (XPS). The molecular dynamics simulations showed rapid and spontaneous decomposition of the ionic liquid anion, with subsequent formation of long-lived species such as lithium fluoride. The simulations also revealed the cation to retain its structure by generally moving away from the lithium surface. The XPS experiments showed evidence of decomposition of the anion, consistent with the aMD simulations and also of cation decomposition and it is envisaged that this is due to the longer time scale for the XPS experiment compared to the time scale of the aMD simulation. Overall experimental results confirm the majority of species suggested by the simulation. The rapid chemical decomposition of the ionic liquid was shown to form a solid electrolyte interphase composed of the breakdown products of the ionic liquid components in the absence of an applied voltage.

Extensive charge-discharge cycling of lithium metal electrodes achieved using ionic liquid electrolytes

The effect of extended cycling on lithium metal electrodes has been investigated in an ionic liquid electrolyte. Cycling studies were conducted on lithium metal electrodes in a symmetrical Li|electrolyte|Li coin cell configuration for 5000 charge–discharge cycles at a current density of 0.1 mA cm− 2. The voltage–time plots show evidence of some unstable behavior which is attributed to surface reorganization. No evidence for lithium dendrite induced short circuiting was observed. SEM imaging showed morphology changes had occurred but no evidence of needle-like dendrite based growth was found after 5000 charge–discharge cycles. This study suggests that ionic liquid electrolytes can enable next generation battery technologies such as rechargeable lithium-air, in which a safe, reversible lithium electrode is a crucial component.

Lithium metal borate (LiMBO3) family of insertion materials for Li-ion batteries: a sneak peak

Rechargeable lithium-ion battery remains the leading electrochemical energy-storage device, albeit demanding steady effort of design and development of superior cathode materials. Polyanionic framework compounds are widely explored in search for such cathode contenders. Here, lithium metal borate (LiMBO3) forms a unique class of insertion materials having the lowest weight polyanion (i. e., BO33-), thus offering the highest possible theoretical capacity (ca. 220 mAh/g). Since the first report in 2001, LiMBO3 has rather slow progress in comparison to other polyanionic cathode systems based on PO4, SO4, and SiO4. The current review gives a sneak peak to the progress on LiMBO3 cathode systems in the last 15 years highlighting their salient features and impediments in cathode implementation. The synthesis and structural aspects of borate family are described along with the critical analysis of the electrochemical performance of borate family of insertion materials.

Dendrites Inhibition in Rechargeable Lithium Metal Batteries

The specific high energy and power capacities of rechargeable lithium metal (Li⁰) batteries are ideally suited to portable devices and are valuable as storage units for intermittent renewable energy sources. Lithium, the lightest and most electropositive metal, would be the optimal anode material for rechargeable batteries if it were not for the fact that such devices fail unexpectedly by short-circuiting via the dendrites that grow across electrodes upon recharging. This phenomenon poses a major safety issue because it triggers a series of adverse events that start with overheating, potentially followed by the thermal decomposition and ultimately the ignition of the organic solvents used in such devices.

In this thesis, we developed experimental platform for monitoring and quantifying the dendrite populations grown in a Li battery prototype upon charging under various conditions. We explored the effects of pulse charging in the kHz range and temperature on dendrite growth, and also on loss capacity into detached “dead” lithium particles.

Simultaneously, we developed a computational framework for understanding the dynamics of dendrite propagation. The coarse-grained Monte Carlo model assisted us in the interpretation of pulsing experiments, whereas MD calculations provided insights into the mechanism of dendrites thermal relaxation. We also developed a computational framework for measuring the dead lithium crystals from the experimental images.

The zwitterion effect in ionic liquids : towards practical rechargeable lithium-metal batteries

Practical lithium-metal batteries are the ultimate goal of battery researchers. The addition of a zwitterionic compound (see Figure) to an ionic liquid electrolyte doped with a lithium salt results in a 100% enhancement of the current densities achieved in the cycling of a lithium-metal cell. This phenomenon arises due to increased lithium-ion mobility or a reduced solid electrolyte interphase layer resistance.

Effect of organic additives on the cycling performances of lithium metal polymer cells

Gel polymer electrolytes were prepared by immersing a porous poly(vinylidene fluoride-co-hexafluoropropylene) membrane in an electrolyte solution containing small amounts of organic additive. Three kinds of organic compounds, thiophene, 3,4-ethylenedioxythiophene and biphenyl, were used as a polymerizable monomeric additive. The organic additives were found to be electrochemically oxidized to form conductive polymer films on the electrode at high potential. By using the gel polymer electrolytes containing different organic additive, lithium metal polymer cells, composed of lithium anode and LiCoO2 cathode, were assembled and their cycling performance evaluated. Adding small amounts of a suitable polymerizable additive to the gel polymer electrolyte was found to reduce the interfacial resistance in the cell during cycling, and it thus exhibited less capacity fade and better high rate performance. Differential scanning calorimetric studies showed that the thermal stability of the fully charged LiCoO2 cathode was improved in the cell containing an organic additive.

Cycling performance of lithium metal polymer cells assembled with ionic liquid and poly(3-methyl thiophene)/carbon nanotube composite cathode

A poly(3-methylthiophene) (PMT)/multi-walled carbon nanotube (CNT) composite is synthesized by in situ chemical polymerization. The PMT/CNT composite is used as an active cathode material in lithium metal polymer cells assembled with ionic liquid (IL) electrolytes. The IL electrolyte consists of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and LiBF4. A small amount of vinylene carbonate is added to the IL electrolyte to prevent the reductive decomposition of the imidazolium cation in EMIBF4. A porous poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) film is used as a polymer membrane for assembling the cells. Electrochemical properties of the PMT/CNT composite electrode in the IL electrolyte are evaluated and the effect of vinylene carbonate on the cycling performance of the lithium metal polymer cells is investigated. The cells assembled with a non-flammable IL electrolyte and a PMT/CNT composite cathode are promising candidates for high-voltage–power sources with enhanced safety.

On the role of cyclic unsaturated additives on the behaviour of lithium metal electrodes in ionic liquid electrolytes

Three cyclic vinyl based additives, based respectively on oxygen, sulphur and fluorine, are tested for their ability to improve the cycling of lithium in a hostile ionic liquid medium. Oxygen based vinylene carbonate is found to offer the best protection of the lithium metal whilst allowing very consistent lithium cycling to occur. The vinylene carbonate based system under study is, however, imperfect. Lithium metal is deposited in a dendritic morphology, and vinylene carbonate is rapidly consumed during lithium cycling if it is present in a small quantity. Our results suggest that ionic liquid systems critically relying on a small amount of additive to protect a lithium electrode are not viable for long cycle life secondary batteries. It is suggested that an ionic liquid which itself is lithium metal compatible be used instead.

Ionic liquid electrolyte for lithium metal batteries : physical, electrochemical, and interfacial studies of N-Methyl-N-butylmorpholinium Bis(fluorosulfonyl)imide

The ionic liquid (IL) N-methyl-N-butylmorpholinium bis(fluorosulfonyl)imide (C₄mmor FSI) is examined from physical and electrochemical perspectives. Pulsed field gradient NMR spectroscopy shows that ion diffusivities are low compared with similar, non-ethereal ILs. Ionicity values indicate that above room temperature, less than 50% of ions contribute to conductivity.

Lithium cycling in symmetrical cells using a C₄mmor FSI-based electrolyte is best demonstrated at elevated temperatures. Specific capacities of 130 mAh g⁻¹ are achieved in a Li−LiFePO₄ battery at 85 °C. FT-IR spectroscopic investigations of lithium electrodes suggest the presence of alkoxide species in the solid electrolyte interphase (SEI), implying a ring-opening reaction of C₄mmor with lithium metal. In contrast, the SEI derived from N-methyl-N-propylpiperidinium FSI lacks the alkoxide signature but shows signs of alkyl unsaturation, and the activation energy for Li+ transport through this SEI is slightly lower than that for the C₄mmor-derived SEI. Our detailed findings give insight into the capabilities and limitations of rechargeable lithium metal batteries utilizing a C₄mmor FSI electrolyte.

In situ MRI of operating solid-state Lithium metal cells based on ionic plastic crystal electrolytes

Solid-state ion conductors based on organic ionic plastic crystals (OIPCs) are a promising alternative to conventional liquid electrolytes in lithium battery applications. The OIPC-based electrolytes are safe (nonflammable) and flexible in terms of design and operating conditions. Magnetic resonance imaging (MRI) is a powerful noninvasive method enabling visualization of various chemical phenomena. Here, we report a first quantitative in situ MRI study of operating solid-state lithium cells. Lithium ion transfer into the OIPC matrix during the ongoing discharge of the anode results in partial liquefaction of the electrolyte at the metal interface. The developed liquid component enhances the ion transport across the interface and overall battery performance. Displacement of the liquefaction front is accompanied by a faster Li transfer through the grain boundaries and depletion at the cathode. The demonstrated solid-liquid hybrid properties, inherent in many OIPCs, combine benefits of highly conductive ionic liquids with safety and flexibility of solids.

Lithium Containing Complex Metal Oxides and Metal phosphates:Investigations on their synthesis and various characterizations for rechargeable lithium battery applications

The work presented in the thesis is centered around two important types of cathode materials, the spinel structured LixMn204 (x =0.8to1.2) and the phospho -oIivine structured LiMP04 (M=Fe and Ni). The spinel system LixMn204, especially LiMn204 corresponding to x= 1 has been extensively investigated to understand its structural electrical and electrochemical properties and to analyse its suitability as a cathode material in rechargeable lithium batteries. However there is no reported work on the thermal and optical properties of this important cathode material. Thermal diffusivity is an important parameter as far as the operation of a rechargeable battery is concerned. In LixMn204, the electronic structure and phenomenon of Jahn-Teller distortion have already been established theoretically and experimentally. Part of the present work is an attempt to use the non-destructive technique (NDT) of photoacoustic spectroscopy to investigate the nature of the various electronic transitions and to unravel the mechanisms leading to the phenomenon of J.T distortion in LixMn204.The phospho-olivines LiMP04 (M=Fe, Ni, Mn, Co etc) are the newly identified, prospective cathode materials offering extremely high stability, quite high theoretical specific capacity, very good cycIability and long life. Inspite of all these advantages, most of the phospho - olivines especially LiFeP04 and LiNiP04 show poor electronic conductivity compared to LixMn204, leading to low rate capacity and energy density. In the present work attempts have been made to improve the electronic conductivity of LiFeP04 and LiNiP04 by adding different weight percentage MWNT .It is expected that the addition of MWNT will enhance the electronic conductivity of LiFeP04 and LiNiP04 with out causing any significant structural distortions, which is important in the working of the lithium ion battery.