Overcharge Protection Research Articles

On the Stability and Reactivity of Phenothiazine-Based Redox Shuttles for Overcharge Protection

The performance of aromatic compounds as redox shuttles for overcharge protection in lithium-ion batteries is variable and difficult to predict. Redox shuttles can decompose in battery electrolyte in their neutral and oxidized forms, both of which are present during overcharge protection. The reasons for differing stability in compounds with only slight structural differences is often unclear, and the exploration of decomposition of redox shuttles has been severely limited, restricting our ability to design improved versions of redox shuttles. To better understand the stability and reactivity of redox shuttles (also relevant to the improvement of positive electrode materials in non-aqueous redox flow batteries) our research has focused on measuring the stability of neutral and oxidized forms of redox shuttle candidates as well as using a variety of spectroscopic methods to analyze the byproducts of decomposition, both from radical cations generated in model solvents and electrolytes from postmortem analysis of failed batteries. Figure 1

3,7-Bis(trifluoromethyl)-N-Ethylphenothiazine: A Highly Soluble, Highly Stable Redox Shuttle and Electro-Active Material for Non-Aqueous Redox Flow Batteries

3,7-Bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) is one of several derivatives of phenothiazine prepared in our laboratory for evaluation as redox shuttles for overcharge protection in lithium-ion batteries. We varied alkyl substituents at the N position and incorporated electron-withdrawing substituents at the 3 and 7 positions. Derivatives with ethyl and iso-propyl groups at the N position were the most stable, and those with chloro and trifluoromethyl substituents were the most robust in overcharging lithium-ion batteries. BCF3EPT showed the highest rate performance when dissolved at 0.5 to 1.5 M in 1.2 M LiPF6 in EC/EMC in graphite/LFP coin cells. At 0.08 M BCF3EPT, batteries lasted over one year in overcharge at a charging rate of C/10. When compared to dimoethoxybenzene-based redox shuttles, the radical cation of BCF3EPT – generated by chemical oxidation with tris(2,4-dibromophenyl)aminium hexachloroantimonate – was more stable in battery electrolyte. The stability of the radical cation form of redox shuttles is critical to overcharge protection and limited effects of the redox shuttle on SEI formation, so the stability of the radical cation of BCF3EPT may explain its extended performance in side-by-side overcharging experiments. In addition to overcharge protection studies, we also studied BCF3EPT as an electron-donating electro-active material in non-aqueous redox flow batteries including one example in which BCF3EPT and trimethylquinoxilane (TMeQ) were incorporated into 1 M LiBF4 in PC and tested in a stationary coin cell following a setup used by Brushett’s group, testing concentrations up to 0.4 M of both species, the anolyte being the limiting factor in high concentration batteries. Lastly, we have focused on the scale-up of BCF3EPT, which we prepare in three steps from commercially available phenothiazine. So far the first and second transformations have yields of more than 95%, and the last step - the trifluoromethylation – can be conducted in over 90% yield. Figure 1

A Roadmap to Design Robust Redox Shuttles for Lithium-Ion Batteries

In designing robust, effective redox shuttles for overcharge protection in lithium-ion batteries, a variety of factors are necessary to consider and evaluate. The stability and reactivity of the radical cation form of a redox shuttle candidate are critical to extensive overcharge performance. Radical cations are subject to a variety of reactions because of their unpaired electron and positive charge, making them subject to nucleophilic attack and dimerization, among other reactions including disproportionation or premature reduction. Equally as important is the stability of the neutral form of the redox shuttle, which may decompose through reaction with electrolyte, SEI, and/or be reduced at the anode/electrolyte interface. Neither stability nor reactivity is easy to predict, although an understanding of organic chemistry can lead to some general guidelines for the design of less reactive species. Density functional theory (DFT) calculations can be used to predict energies of reaction and have been employed by Curtiss, Dahn, and our group. DFT calculations can also be used to compute oxidation and reduction potentials, helping to identify promising candidates before synthesizing materials. This presentation focuses on our work using DFT calculations to help us identify promising redox shuttles and to gain a better understanding of their reactivity and includes experimental data in support of some of the proposed mechanisms of decomposition.

A Highly Soluble Organic Catholyte for Non‐Aqueous Redox Flow Batteries

AbstractA phenothiazine derivative with high solubility in carbonate solvents containing lithium salts showed extensive overcharge protection and, as a result, has been evaluated as a catholyte for non‐aqueous redox flow batteries. We report the testing of 3,7‐bis(trifluoromethyl)‐N‐ethylphenothiazine as a catholyte and 2,3,6‐trimethylquinoxaline as the anolyte in redox flow batteries containing 0.05, 0.15, and 0.35 M active material and found the longest capacity retention over about 60 cycles at 0.15 M. To our knowledge, this is the most soluble catholyte candidate with a robust radical cation.

Redox Shuttles for Lithium-Ion Batteries at Concentrations up to 1 M Using an Electroactive Ionic Liquid Based on 2,5-di-tert-butyl-1,4-dimethoxybenzene

In this work, we designed two redox shuttles with high solubility (up to 1 M) in conventional carbonate-based lithium-ion battery (LIB) electrolytes. At this high concentration, redox shuttles ensure improved overcharge protection than lower concentrations. We developed electroactive imidazolium salts by modifying imidazolium with 2,5-di-tert-butyl-1,4-dimethoxybenzene. Two salts with the cation 1-(3-(2,5-di-tert-butyl-1,4-methoxyphenoxy)propyl)-3-methyl-1H-imidazol-3-ium (EMIm) were synthesized using either hexafluorophosphate (DDB-EMIm-PF6) or bis(trifluromethanesulfonyl)amide (DDB-EMIm-TFSI)) anions. The electrochemical properties of DDB-EMIm-PF6 and DDB-EMIm-TFSI dissolved in ethylene carbonate : diethyl carbonate (EC:DEC), in the presence of either LiPF6 or LiTFSI, were evaluated. Cyclic voltammetry showed a compatible potential (∼3.85 V vs. Li/Li+) for use in LIBs using LiFePO4 as cathodes. Electrolytes using 0.1 M of DDB-EMIm-PF6 or 0.3, 0.7 and 1 M of DDB-EMIm-TFSI were prepared and evaluated in Li/LiFePO4 (LFP) test cells to demonstrate overcharge protection. Electrochemical cycling at C/10 showed an overcharge protection for all concentrations of the redox ionic salts under 100% overcharge conditions. Among these salts, DDB-EMIm-TFSI, at a concentration of 0.7 M, was effective in shuttling excess current for over 200 cycles, representing over 6000 operating hours, while maintaining nominal values for the discharge capacity of LiFePO4.

A Highly Soluble Redox Shuttle with Superior Rate Performance in Overcharge Protection

ABSTRACTThe demand for a stable and compatible redox shuttles for use in lithium-ion batteries has prompted us to explore strategies to tune and improve the properties of redox shuttles. We have studied over 50 new diarylamine derivatives synthesized in our laboratory including one compound in which we introduced trifluoromethyl groups (–CF3) at the positions para to the nitrogen atom in N-ethylphenothiazine (EPT). The high electronegativity of the CF3 group raises the oxidation potential, and its incorporation also significantly increases solubility in battery electrolyte. Here we report 3,7-bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) as a new redox shuttle, which we have observed to have the highest reported solubility in battery electrolyte of all redox shuttles that maintain extended overcharge performance. We have compared its performance with 1,3-di-tert-butyl-2,5-dimethoxybenzene (DBB), EPT, and other robust redox shuttles. In our hands, overcharge cycling of BCF3EPT far surpasses any reported redox shuttle, and – because it can be dissolved at higher concentrations – it tolerates faster charging rates than both DBB and EPT.

1,4-Bis(trimethylsilyl)-2,5-dimethoxybenzene: a novel redox shuttle additive for overcharge protection in lithium-ion batteries that doubles as a mechanistic chemical probe

A novel redox shuttle additive, 1,4-bis(trimethylsilyl)-2,5-dimethoxybenzene, is shown to deliver superb overcharge protection of LiFePO4 electrode in Li-ion batteries.

On the Stability and Reactivity of Redox Shuttles in Their Neutral and Radical Cation Forms

ABSTRACTThe performance of aromatic compounds as redox shuttles for overcharge protection in lithium-ion batteries is quite variable and is often difficult to predict. Redox shuttles may decompose in battery electrolyte in their neutral and radical cation forms, both of which are present during overcharge protection. While hundreds of compounds have been evaluated as redox shuttle candidates and a few have stood out as top performers, the reasons for increased stability over similar candidates with slightly different structures is often unclear, and the exploration of decomposition of redox shuttles has been severely limited, restricting our ability to design improved versions of redox shuttles that do not suffer from the same reactions in lithium-ion batteries. To better understand the stability and reactivity of redox shuttles (also relevant to the improvement of positive electrode materials in non-aqueous redox flow batteries) our research has focused on measuring the stability of neutral and oxidized forms of redox shuttle candidates as well as using a variety of spectroscopic methods to analyze the byproducts of decomposition, both from radical cations generated in model solvents and electrolytes from postmortem analysis of failed batteries.

An organophosphine oxide redox shuttle additive that delivers long-term overcharge protection for 4 V lithium-ion batteries

Redox shuttle additives are used to protect Li-ion batteries from overcharge.

Biphenyl as overcharge protection additive for nonaqueous sodium batteries

Sodium batteries are considered as a promising low-cost alternative for currently used lithium batteries.

N-substituted phenothiazine derivatives: how the stability of the neutral and radical cation forms affects overcharge performance in lithium-ion batteries.

Phenothiazine and five N-substituted derivatives were evaluated as electrolyte additives for overcharge protection in LiFePO4 /synthetic graphite lithium-ion batteries. We report on the stability and reactivity of both the neutral and radical-cation forms of these six compounds. While three of the compounds show extensive overcharge protection, the remaining three last for only one to a few cycles. UV/Vis studies of redox shuttle stability in the radical cation form are consistent with the overcharge performance: redox shuttles with spectra that show little change over time exhibit extensive overcharge performance, whereas those with changing spectra have limited overcharge protection. In one case, we determined that a C-N bond cleaves upon oxidation, forming the phenothiazine radical cation and leading to premature overcharge protection failure; in another case, poor solubility appears to limit protection.

Influence of the solid electrolyte interphase on the performance of redox shuttle additives in Li-ion batteries – A rotating ring-disc electrode study

Redox shuttle electrolyte additives (RSAs) can be applied for reversible overcharge protection of batteries. Their successful operation involves their oxidation at the cathode and reduction at the anode. The most common anodes in lithium-ion batteries are graphite or amorphous carbon, which are normally covered with a solid electrolyte interphase (SEI). The reduction of RSAs at these anodes is in apparent contradiction with the common understanding of the SEI, which is thought to be electronically insulating. In this communication the reduction behaviour of ferrocene and 2,5-di-tert-butyl-1,4-dimethoxybenzene is studied at un-filmed and SEI-filmed electrodes. It is found that it depends strongly on the type of RSA and/or composition of the SEI. The rotating ring-disc electrode (RRDE) is introduced as a powerful diagnostic tool to study the reaction mechanism of RSAs in general and the influence of the SEI in particular.

Design of novel additives and nonaqueous solvents for lithium-ion batteries through screening of cyclic organic molecules: an ab initio study of redox potentials.

We have screened 142 cyclic organic compounds in search of novel functional additives and nonaqueous solvents for use in lithium-ion batteries through the use of ab initio calculations, and have determined redox potentials for all molecules. We have estimated the range of variation in oxidation potentials through heteroatom replacement and structure modification. By analyzing the oxidative properties of these compounds, we have shown that substituted heteroatoms and isomers in cyclic organic molecules can induce large variations of oxidation potentials more than 2.3 V. Calculations of reduction potentials show that the substituted heteroatoms, isomers, and the number of double bonds in cyclic organic molecules can change reduction potentials more than 1.7 V. Additionally, we have identified seventeen and twelve compounds that could serve as additives in eliciting film formation on cathodes and anodes, respectively. We have also identified five compounds that could function in the overcharge protection of over-lithiated layered oxide cathodes. Finally, we have identified five compounds that could serve as electrochemically stable solvents for high-voltage (>6 V vs. Li/Li(+)) applications. These inductive screenings and their analyses and findings extend our empirical knowledge into quantitative estimation in the design of materials.

Design of Semiconductor Thermoelectric Generation Control System Based on MCU

Based on the characteristics of semiconductor thermoelectric generation, this paper takes STC12C5410AD MCU as the core, and designs an intelligent lead-acid battery charging and discharging controller. It implements the intelligent switch, overload overcharge protection of charging way, and can automatically switch between power generating system and battery according to battery load circuit. It realizes the effective transformation of heat into electricity, intelligent storage and utilization.

Novel Energy Storage System, bindbattery™, with an Intrinsic Overcharge Protection Capability

CONNEXX SYSTEMS developed a novel hybrid energy storage system, bindbattery™, with a unique overcharge protection capability, high power and high energy capability and long cycle life at low cost without complex battery management system. bindbattery™ consists of lithium-ion battery units and aqueous electrolyte battery units. The two units are connected in parallel and form a virtual cell. The multiple virtual cells are connected in series and in parallel to meet specifications of each application. An aqueous electrolyte battery could be chosen from a nickel-hydride battery, lead-acid battery or other form of aqueous batteries depending on requirements of applications. Hybridization with an aqueous electrolyte battery makes bindbattery™ tolerable to abusive conditions such as reverse directional high current pulse or overcharge current. Benefits of bindbattery™ include: 1) Safety: The lithium-ion battery is protected from the devastating overcharge by aqueous batteries connected in parallel. The aqueous battery unit consumes overcharge energy through the aqueous electrolyte decomposition (electrolysis). The oxygen evolved at the positive electrode recombines with hydrogen on the negative electrode and reforms water. By connecting the lead-acid battery and lithium-ion battery in parallel, the lead-acid battery effectively protects lithium-ion battery from overcharge by absorbing the overcharge current through the chemical reaction. Thus, bindbattery™ does not require complex electronic protection circuit to protect from overcharge. In other words, the bindbattery™ configuration makes the battery system intrinsically safe. This simplifies construction of the bindbattery™, and reduces both the cost and the weight and makes the battery suitable for large scale stationary applications. 2) Power performance: As a result of high power characteristics of both high power lithium-ion batteries and lead-acid batteries, bindbattery™ has a high power capability. 3) Cycle life improvement: The lead-acid battery is an inexpensive battery but its cycle life is poor compared to the lithium-ion battery. The lithium-ion battery has higher discharge voltage than the lead-acid battery, therefore lithium-ion battery discharges prior to the lead-acid battery in bindbattery™. Thus bindbattery™ provides an excellent cycle life in the same manner as the lithium-ion battery. 4) High energy density: bindbattery™ can provide a larger energy compared to the lead-acid battery alone as a result of the hybridization with lithium-ion battery. 5) Low temperature performance improvement: The lead-acid battery has excellent low temperature capability. At subzero temperature, lithium-ion battery is not able to discharge immediately. In bindbattery™, the lead-acid battery discharges first and provides warm-up time for the lithium-ion battery. 6) Float charge capability: Like other aqueous batteries, bindbattery™ can be used in the float-charge mode. 7) Cost improvement: Use of lead-acid battery as an aqueous battery units lowers the cost significantly. Elimination of electrical protection switches and battery management units contribute to the cost reduction. These unique features make bindbattery™ the most suitable energy storage system for stationary applications including storage system for renewable energy, demand shifting and frequency regulation. We developed 20 kWh bindbattery™ module for stationary applications such as solar energy or wind energy storage. The module will comprise a larger scale storage system by connecting multiple of the modules in parallel or in series. The simplified control method and low cost also make bindbattery™ a remarkable choice for car applications.

Controlling Oxidation Potentials in Redox Shuttle Candidates for Lithium-Ion Batteries

Overcharge, a condition in which cell voltage rises to undesirably high potentials, can be prevented in lithium-ion batteries by incorporating redox shuttles into the battery electrolyte. Although extensive overcharge protection has been demonstrated in batteries with LiFePO4 cathodes, the redox shuttles that work in these batteries are incompatible with higher voltage cathodes. Designing stable additives with higher oxidation potentials is necessary to protect high voltage batteries from overcharge. Toward that goal, we synthesized diarylamines with varied structures, including fused heteroaromatic ring systems and electron-withdrawing substituents. We found that trends in oxidation potentials correlated with those in calculated adiabatic ionization potentials. Some diarylamine derivatives protected batteries from overcharge with varying degrees of success.

Design and Implementation of New Smart Battery Management System

The purpose of the battery management system is to improve the efficiency and prolong cycling life of the battery pack which is connected in series, SH79F329 is used as the main control IC of the hardware. The system can detect the voltage, current, temperature of the battery by AD sampling, and can calculate the SOC by the value of current and the way of SOC algorithm,as well as communicating with the host computer. and it has been implemented in the process of charging equalization, overcharge protection, overdischarge protection and a series of other processes. It proves that this design has high reliability and real-time performance in the test.

Cathodes with intrinsic redox overcharge protection: A new strategy towards safer Li-ion batteries

Overcharge safety is the most crucial problem facing especially large-sized lithium-ion batteries (LIBs) packs owing to the inevitable inhomogeneity of charge-state for each cell. We propose a fresh safety strategy to avoid overcharging, i.e. the use of a solid-state anti-overcharge additive to perform an intrinsic overcharge protection. The mechanism is triggered from a solid-state composite cathode obtained by typically mixing a pre-selected transition-metal oxide with a certain cathodes thus constituting a composite cathode. The effectiveness of this strategy is demonstrated with an example of LiCoO2/CuO (95:5 by weight) composite, which exhibits high-potential (up to 5 V vs. Li+/Li) and long-period (month-level) anti-overcharge performance. It is found that the additive does not hurt the electrochemical cycling performance under normal operational conditions. This novel safety strategy is simple and flexible, which may open a new window to develop safer LIBs systems.

Syntheses of novel delocalized cations and fluorinated anions, new fluorinated solvents and additives for lithium ion batteries

To improve the properties of rechargeable lithium ion batteries, like conductivity, SEI-formation, thermal and electrochemical stability, low and high temperature performance and safety new electrolyte salts, novel solvents (co-solvents) and additives have been synthesized. All new anions, solvents and additives contain fluorine proving the importance of this element for the electrolyte system. Tetrafluoroborates having bulky delocalized nitrogen-, phosphorus and sulfur-centered counter-cations containing tetramethylguanidyl substituents, like [(Me2N)2CNC(NMe2)2]+, have been prepared to improve the conductivity in polymer electrolytes. The hitherto unknown lithium sulfonate, MeOCF2CF2SO3Li, has been successfully synthesized along with further analogs, and also MeOCF2CF(CF3)SO3Li was obtained, both from precursors, FO2SCF2C(O)F or FO2SCF(CF3)C(O)F accessible by ring opening reactions from the respective sultones. For the lithium salt CF3OCF(CF3)SO3Li, a new simple synthetic pathway was found where CF3OCFCF2 and SO2F2 were used as precursors. Novel possible redox shuttles, namely (CF3)5C6OLi and fluorinated pyridine-N-oxides have been prepared. A neutral cyclic carben-PF5 adduct turned out to be a very effective overcharge protection additive. The family of cyclic and acyclic carbonates playing a key-role as electrolyte solvents in lithium ion batteries could be extended by derivatives of 1,1,1,4,4,4-hexafluorobutandiol. Reaction products from perfluoropropene oxide and alcohols, ROC(F)CF3C(O)OR (R = CH2CF3, CH2CH2, CH(CF3)2) were obtained according to new optimized methods. New cyclic sulfonamides synthesized from FO2SCF2C(O)F and FO2SCF(CF3)C(O)F could be successfully identified as versatile electrolyte additives.

Organic Redox Active Molecular Engineer for NRF

:Non-aquoes redox flow battery is a unique energy storage technology. Unlike traditional battery systems, the active energy storage material is in mobile phase and could basically be stored anywhere separately. This gives a lot of flexibility in terms of capacity, power and safety control, making it an idea candidate for large scale stationary energy storage application.Material development for this technology is crucial for possible implementation. Molecular engineer of organic redox active molecules could lead to tunable physical and electrochemical properties, including solubility in organic solvents, molecular mobility, redox potential, and electrochemical reversibility, which are key factors to energy storage applications. Specifically, one family of dimethoxybezene based organic molecules will be discussed in this talk. Dimethoxybezene based molecules have drawn increasing attentions due to their excellent electrochemical activities, especially as redox shuttle additives and possible non-aqueous flow battery catholytes, which both require redox active molecules to be the key functional components. Due to the organic synthesis feasibility, structural improvements could be conducted to intentionally tune the physical or electrochemical properties of dimethoxybezene. For instance, ANL-2 was developed with improved solubility in carbonate based electrolytes and excellent overcharge performance as redox shuttle additive. The success of ANL-2 was based on the previous attempts using various design strategies to improve the solubility of DDB, including asymmetric ANL-1, and DBMOEB (1,4-di-tert-butyl-2,5-bis(2-methoxyethoxy)methoxy-benzene). Other examples are ANL-3 and ANL-4 molecules, with focus on increasing the redox potentials. ANL-3 is able to provide 4.8 V vs Li/Li+ overcharge protection, highest ever reported in the literature. ANL-4, on the other hand, exhibited 4.5 V vs Li/Li+ redox potential and good electrochemical reversibility.