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A yearly allocation for the LCRC cluster has been requested with the
following updated information:
Submitter/PI: Paul Redfern
Project Name: Redoxshuttles
Division: MSD
Project title: Redox Shuttles and Additives for Lithium-Ion Batteries
Associated funding: EERE-DOE
Other Systems: Small Group Cluster
Science: There are several types of electrolytes that are used to improve the performance of the batteries.
Some electrolyte additives are used to improve lithium-ion battery performance by facilitating solid-electrolyte (SEI) formation, reducing irreversible capacity and gas formation, improving thermal stability, protecting cathode materials, improving electrolyte properties, decreasing flammability and providing overcharge protection. It is also desirable to develop additives that can stabilize electrolytes near 5 V in order to deliver energy at higher density and potential. Numerous additives have been developed that improve some aspect of battery performance, but many introduce negative side effects.
A second type is used to prevent dangerous overcharge of lithium-ion batteries occurs when charge from current forced through a cell exceeds its charge storing capability, leading to chemical and electrochemical reactions, rapid temperature rise, self accelerating reactions and explosion. Redox shuttle molecules are additives that are reversibly oxidized and reduced at potentials slightly higher than the end of charge cathode potential, locking the cathode potential at the oxidation potential of the redox shuttle and thereby preventing overcharging. The shuttling mechanism can be regarded as an internal short that can be repeated continuously due to reversible oxidation and reduction of the shuttle molecule. Redox shuttles with excellent performance and compatibility are critically important to the development of lithium-ion battery technology. Development of new redox shuttles is difficult and has been largely based on trial and error due to the multidisciplinary knowledge r
equired. Ideal redox shuttles should be electrochemically reversible, have a redox potential slightly higher than the cathode end of charge potential, be electrochemically stable, have good solubility and high diffusion coefficients, and be compatible with the cell, i.e., they should not adversely affect normal cell performance. The shuttle should not react with electrolytes, the solid-electrolyte interface, separators, current collectors, binders, etc. Interface resistance and thermal properties should also be considered.
Project description: Over the past year our DFT calculations predicted that each of the primary constituents of the inner inorganic SEI layer will sequester the manganese ions from the electrolyte in a lithium ion battery. Although these calculations are approximate, since they represent molecular reaction energies and not the solid phase, they bolster our hypothesis that the manganese ions can exchange with the lithium on the outer surface of the inner inorganic SEI layer. The influence of siloxane ring size on the coordination number of single Zn(II) and Ga(III) ions on a silica surface was investigated using density functional theory calculations. The theoretical results show how siloxane ring strain and metal ion charge density affect the formation of low-coordinated metal sites and can be used in future studies of catalyst design. Three fluorinated cyclic carbonates (FEC, TFPC, and TFP-PC-E) and three fluorinated linear carbonates (F-EMC, HF-DEC and TF-DEC) were screene
d in comparison with their non-fluorinated counterparts (EC and DMC) for voltage stability. For the linear carbonates, HF-DEC is the most stable, followed by F-EMC, TF-DEC and DMC. Mixtures of FEC and DMC in various ratios demonstrated that FEC is the more stable component in the binary formulations of FEC/DMC, although the beneficial effect is insignificant after the concentration of cyclic carbonate is increased over 50%, and wetting issue begins to arise. Testing of various salt concentrations did not result in great variation of the electrolyte stability, implying that the effect of the salt on the electrolyte voltage stability is minimal in dilute electrolytes. We also conducted a computational study of the synthesized MiPS and McPS and the commercially available TMS using the density functional method. Results suggest that McPS has a much lower oxidative stability than TMS and MiPS, consistent with experiment. New sulfone compounds with various substitution groups were
synthesized in our lab in order to investigate the impact of the substitution group on the oxidation stability of sulfone solvents when dissolved with LiPF6 salt. The poor experimental oxidation stability of McPS was verified by the DFT calculations. The oxidation potentials of TMS, MiPS, and McPS seem to correlate with the stabilities of the alkyl radical groups resulting from C-S bond cleavage of these three molecules. Substituent groups on the sulfone structure plays a significant role in its oxidative stability and should be considered for the development of new sulfone-based electrolytes for high voltage application. We performed a detailed examination the addition of water to tiron quinone using DFT. The slow step (with the highest energy barrier) seems to be the I1 to I2 reaction or the concerted B to I2 reaction. Thermodynamically, reaction energies for formation of C-C dimer and C-O-C dimer from substituted catechol and corresponding quinone are least negative for
4-chloro, intermediate for catechol and most negative for 4-cyano. This suggests that the 4-cyano will react to form polymers or other degradation products most easily. This is consistent with oxidation being reversible for 4-chloro, quasireversible for catechol and irreversible for 4-cyanocatechol.
Electric vehicles and energy storage require higher capacity and lower cost than can be achieved by lithium ion batteries. The lithium sulfur battery, with its high (theoretical) energy density, low cost and abundance of materials, is the best system to meet these goals. Several problems must be overcome before lithium sulfur batteries become practical. New cathodes, in which sulfur is impregnated in graphene or other carbon materials, improve cycling stability and electron conductivity. Ether based electrolytes such as TEGDME, DOL and DME have been found to be more suitable than carbonate based electrolytes. The electrolyte additive LiNO3 has been shown to form a stable SEI layer on the lithium anode resulting in a large performance increase. Dissolving Li2S8 in the electrolyte reduces cathode dissolution via Le Chatlier’s principle (mass buffering effect) and acts as a catholyte or dissolved cathode (electrochemical buffering effect). Addition of Li2S8 eliminates formatio
n of Li2S on the cathode which is insulating and increases volume by 50% leading to cathode collapse. LiNO3 decreases the Li+ transference number due to increased ionic strength of the solution while Li2S8 increases it. Lithium anodes have problems with dendrite formation, short circuit and thermal runaway, and have successfully been replaced by intercalation anodes. A lithium sulfur flow battery has been developed with a very concentrated carbon impregnated catholyte with improved volumetric capacity but requires a semi-solid silicon anolyte or other negative flow catholyte in order to be scalable.
Despite all of these advances, the polysulfide solubility problem has not been completely eliminated. Polysulfides dissolve at the cathode and migrate to the anode resulting in redox shuttle reactions, polarization and capacity fading. Reducing polarization leads to faster kinetics. Polysulfide anions are reduced by and chemically interact with the anode, acting as redox shuttles and leading to low charging efficiency, high self-discharge rates and permanent deposition of insoluble Li2S and Li2S2 on the anode surface. Protecting the anode with LiNO3 makes the following reaction reversible leading to higher charging efficiency and specific capacity.
S8 (insoluble) + 2Li → Li2S8 (soluble)
Problems remain using shuttle inhibitor LiNO3 because it is continuously consumed at the anode and is a strong oxidizing agent especially at high temperatures. LiNO3 is probably reduced at the lithium anode to insoluble LixOy species and oxidizes polysulfides to LixSOy species that passivate the anode. For example at the anode:
2Li + LiNO3 -> Li2O + LiNO2
Shuttle inhibitors that are safer and more effective than LiNO3 are needed to improve lithium sulfur battery performance. Possible shuttle inhibitors include but are not limited to lithium nitrite, potassium nitrate, potassium nitrite, cesium nitrate, cesium nitrite, barium nitrate, barium nitrite, ammonium nitrate, ammonium nitrite, dialkyl imidazolium nitrates, guanidine nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite octyl nitrite, nitromethane, nitropropane, nitrobutanes, nitrobenzene, dinitrobenzene, nitrotoluene, dinitrotoluene, nitropyridine, dinitropyridine, pyridine N-oxide, alkylpyridine N-oxides, and tetramethyl piperidine N-oxyl (TEMPO).
In a recent review a simplified discharge mechanism was proposed. Typical mechanisms involve steps in which multiple species get together in a reaction which is statistically impossible, or reduction reactions involving two or four electrons whereas all reductions actually involve one electron.
Charging involves either direct S8 electrochemical formation from Li2S or chemical reactions followed by electrochemical oxidation to S8. The resulting S8 may dissolve and react with Li2S or react with polysulfides.
Redox mediators play a crucial role in charging. Redox mediators are redox shuttles that activate insulating Li2S at lower voltage by getting oxidized at the electrode and then diffusing to and oxidizing Li2S. They are not detrimental to the cell and greatly increase its reversible capacity. Redox mediators can also be used during discharge where they reduce Li2S away from the electrode so that it forms a porous 3D network instead of a thin insulating layer on the electrode leading to a 6 fold increase in Li2S produced.
We propose to find transition states and calculate energy barriers for essential reactions of polysulfides during discharge. We will examine two step reduction, reductive dissociation, association and many other reactions during discharge. During charging, we will calculate transition states and barriers for chain growth and disproportionation reactions of polysulfides and their reaction with Li2s or S8. We will also examine novel redox mediators for both charge and discharge. In addition we will look for more effective shuttle inhibitors. Oxidation of redox mediators will be examined. Reduction of shuttle inhibitors by Li and oxidation of polysulfides by shuttle inhibitors will be studied. Structures will be optimized with b3lyp/631g(2df,p) so that accurate G4MP2 energies can be calculated. Frequency jobs will be performed to characterize minima and transition states. M05-2X density functional calculation will also be used as it has been shown to have the best overall agreem
ent with G4MP2. PCM will be used to estimate solvation free energies.
Software
We intend to use Gaussian 09 for quantum chemical calculations. In Gaussian 09 the DFT calculations scale well up to 64 processors, however the high-level ab initio methods scale only up to 16 processors. Therefore these calculations do require a longer execution time.
Publications
1) Role of Manganese Deposition on Graphite in the Capacity Fading of Lithium Ion Batteries, Daniel R. Vissers, Zonghai Chen, Yuyan Shao, Mark Engelhard, Ujjal Das, Paul Redfern, Larry A. Curtiss, Baofei Pan, Jun Liu, and Khalil Amine, ACS Appl. Mater. Interfaces 2016, 8, 14244-14251.
2) Effect of Siloxane Ring Strain and Cation Charge Density on the Formation of Coordinately Unsaturated Metal Sites on Silica: Insights from Density Functional Theory (DFT) Studies, Ujjal Das, Guanghui Zhang, Bo Hu, Adam S. Hock, Paul C. Redfern, Jeffrey T. Miller, and Larry A. Curtiss, ACS Catal. 2015, 5, 7177-7185.
3) Fluorinated Electrolytes for 5-V Li-Ion Chemistry: Probing Voltage Stability of Electrolytes with Electrochemical Floating Test, Meinan He, Libo Hu, Zheng Xue, Chi Cheung Su, Paul Redfern, Larry A. Curtiss, Bryant Polzin, Arthur von Cresce, Kang Xu, and Zhengcheng Zhang, Journal of The Electrochemical Society, 162 (9) A1725-A1729 (2015).
4) Alkyl Substitution Effect on Oxidation Stability of Sulfone-Based Electrolytes, Chi-Cheung Su, Meinan He, Paul Redfern, Larry A. Curtiss, Chen Liao,Lu Zhang, Anthony K. Burrell, and Zhengcheng Zhang, ChemElectroChem 2016, 3,790 –797.
5) Atomistically informed mesoscale model for growth and coarsening during discharge in Li-O2 batteries, M.J. Welland, K. C. Lau, P.C. Redfern, L. Liang, D. Zhai, D. Wolf, L.A. Curtiss, J. Chem. Phys. 143, 224113 (2015).
6) Structure and Stability of Lithium Superoxide Clusters and Relevance to Li-O2 Batteries, Ujjal Das, Kah Chun Lau, Paul C. Redfern, and Larry A. Curtiss, J. Phys. Chem. Lett. 2014, 5, 813-819.
7) Effect of the Size-selective Silver Clusters on Lithium Peroxide Morphology in Lithium-Oxygen Batteries" by Jun Lu, Lei Cheng, Kah Chun Lau, Eric Tyo, Xiangyi Luo, Jianguo Wen, Dean Miller, Rajeev Surendran Assary, Hau Wang, Paul Redfern, Huiming Wu, Jin-Bum Park, Yang-Kook Sun, Stefan Vajda, Khalil Amine, and Larry Curtiss, Nature Communications, 5, 4895, DOI: 10.1038/ncomms5895.
8) Molecular engineering towards stabilized interface: a novel electrolyte additive for high-performance Li-ion battery, Zhengcheng Zhang, Lu Zhang, Jinhua Huang, Kyrrilos Youssef, Paul Redfern, Larry Curtiss, and Khalil Amine, Journal of The Electrochemical Society, 2014, 161, A2262-A2267.
Industry partnership:
Project URL:
Current FY Hours Used: undetermined amount
New FY Requested allocation: 400000
Q1: 100000
Q2: 100000
Q3: 100000
Q4: 100000
Justification:
Storage requirements:
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