Suppression of Self-Discharge in Aqueous Supercapacitor Devices Incorporating Highly Polar Nanofiber Separators

One of the major problems limiting the applications of electric double-layer(EDLC)supercapacitor devices is their inability to maintain their cell voltage over a significant period.Self-discharge is a spontaneous decay in charged energy,often resulting in fully depleted devices in a matter of hours.Here,a new method for suppressing this self-discharge phenomenon is proposed by using directionally polarized piezoelectric electrospun nanofiber films as separator materials.Tailored engineering of polyvinylidene fluoride(PVDF)nanofiber films containing a small concentration of sodium dodecyl sulfate(SDS)results in a high proportion of polarβphases,reaching 380.5%of the total material.Inducing polarity into the separator material provides a reverse-diode mechanism in the device,such that it drops from an initial voltage of 1.6 down to 1 V after 10 h,as opposed to 0.3 V with a nonpolarized,commercial separator material.Thus,the energy retained for the polarized separator is 37%and 4%for the nonpolarized separator,making supercapacitors a more attractive solution for long-term energy storage.

Regulating the synergy coefficient of composite materials for alleviating self-discharge of supercapacitors

Supercapacitor is an efficient energy storage device,yet its wider application is still limited by self-discharge.Currently,various composite materials have been reported to have improved inhibition on self-discharge,while the evaluation of the synergistic effect in composite materials is challenging.Herein,pairs of intercalation type pseudocapacitive niobium oxides are pre-lithiated and coupled to construct conjugatedly configured supercapacitors,within which the cathode and anode experience identical reaction environment with single type of charge carrier,thus providing ideal platform to quantify the synergistic effect of composite materials on the self-discharge process.By using titanium dioxide as the stabilizer,we have compared how the modes of forming composite would influence the selfdischarge performance of the active composite materials with similar ratio of the constituent materials.Specifically,core@shell Nb_(2)O_(5)@TiO_(2) composite using TiO_(2) as the shell shows significantly higher synergy coefficient(μ=0.61,defined as the value that evaluates the synergistic effect between composite materials,and can be quantified using the overall performance of the composite,performance of individual component as well as the ratio of the component.) than other control group samples,which corresponds to the highest retained energy of 63% at 100 h.This work is expected to provide a general method for quantifying the synergistic effect and guide the design of composite materials with specific mode of forming the composite.

Bipolar ionomer electrolytes with desirable self-discharge suppression for supercapacitors

Supercapacitors based on electric double layers are prone to serious self-discharge due to electrolyte ion desorption and the resulting energy loss severely limits the application range of supercapacitors.Rational design of polymer electrolyte systems to address this problem shows considerable generality and high feasibility.Herein,we reported a quasi-solid-state bipolar ionomer electrolyte prepared by an in-situ layer-by-layer ultraviolet-curing method,which has an integrated Janus structure with an intermediate binding layer.Based on the synergistic effect of confining impurity ions by ionizable groups and electrostatic repulsion to stabilize the electric double layers and superimposing synergies on both sides,the assembled device not only possesses ideal supercapacitor characteristics,but also exhibits an ultrahigh voltage retention of 71% after being left to stand for 100 h after being fully charged.Furthermore,through the quasi-in-situ energy dispersive X-ray spectroscopy linear scanning,the characteristics of ion diffusion in this ionomer electrolyte are revealed,suggesting its correlation with self-discharge behavior.

Recent research advances of self-discharge in supercapacitors:Mechanisms and suppressing strategies

Supercapacitors are one of the most promising energy storage devices in the fields of vehicle transportation,flexible electronic devices,aerospace,etc.However,the existed self-discharge that is the spontaneous voltage decay after supercapacitors are fully charged,brings about the wide gap between experimental studies and practical utilization of supercapacitors.Although eliminating the selfdischarge completely is not reachable,suppressing the self-discharge rate to the lowest point is possible and feasible.So far,the significant endeavors have been devoted to achieve this goal.Herein,we summary and discuss the possible mechanisms for the self-discharge and the underlying influence factors.Moreover,the strategies to suppress the self-discharge are systemically summed up by three independent but unified aspects:modifying the electrode,modulating the electrolyte and tuning the separator.Finally,the major challenges to suppress the self-discharge of supercapacitors are concluded and the promising strategies are also pointed out and discussed.This review is presented with the view of serving as a guideline to suppress the self-discharge of supercapacitors and to across-the-board facilitate their widespread application.

Mitigating self-discharge of carbon-based electrochemical capacitors by modifying their electric-double layer to maximize energy efficiency

Self-discharge is a significant issue in electric double layer energy storage, which leads to a rapid voltage drop and low energy efficiency. Here, we attempt to solve this problem by changing the structure of the electric double layer into a de-solvated state, by constructing a nano-scale and ion-conductive solid electrolyte layer on the surface of a carbon electrode. The ion concentration gradient and potential field that drive the self-discharge are greatly restricted inside this electric double layer. Based on this understanding, a high-efficiency graphene-based lithium ion capacitor was built up, in which the self-discharge rate is reduced by 50% and the energy efficiency is doubled. The capacitor also has a high energy density, high power output and long life, and shows promise for practical applications.

Correlation between self-discharge behavior and heteroatoms over doped carbon sheets for enhanced pseudocapacitance

For electric double layer supercapacitors,carbon materials originating from the purely physical energystorage mechanism limit the improvement in the capabilities of charge storage.To solve this problem,doping heteroatoms into carbon skeleton is a promising&charming strategy for enhancing electrochemical performance by providing the extra pseudocapacitance.However,the self-discharge behavior of such heteroatom-doped supercapacitors has been a challenging issue for a long time.Here,the porous carbon nanosheets with a tunable total content of heteroatoms are chosen as a demo to systemically decouple the correlation between the total content of heteroatoms and the specific capacitance as well as the self-discharge behavior.The capacitance changes in a range of 164–331 F g^(-1)@1 A g^(-1)with the increased total contents of doped heteroatom,strongly dependent on and sensitive to the total content of heteroatoms.The voltage retention rate and capacitance retention rate for the porous carbon nanosheets with a tunable total content of heteroatoms completely present a quick decline tendency as the increase in the content of heteroatoms,changing from 58%to 34%and 74%to 39%,respectively,indicative of a linear negative relationship.More importantly,the self-discharge mechanisms are elaborately explored and follow the combination of activation-and diffusion-controlled Faradic reactions.This work illustrates the diverse impacts of the doped heteroatoms on the electrochemical performance of supercapacitors,covering specific capacitance and self-discharge behavior,and highlights the importance of balancing the contents of doped heteroatoms in energy storage fields.

Influence of Four Factors on Discharge Capacity and Self-Discharge Rate of Iron Electrode

Ni-Fe rechargeable batteries possess the advantages of long cycle life, high theoretical specific energy, abundant raw material, low price and environmental friendship. It has a wide applied perspective. The advantages, disadvantages and preparation methods of iron electrodes were summarized. The influence of four factors on discharge capacity and self-discharge rate of iron electrode were discussed by means of orthogonal experiments, galvanostatic charges and discharges. The influences of graphite on the discharge capacity and self-discharge rate of iron electrode were the most remarkable, the most unapparent influences on the discharge capacity and self-discharge rate were HPMC (hydroxy propoxy methoxy cellulose) and sodium sulphide, respectively. The aim of the present research was to study the effects of graphite, HPMC and iron powder added in the electrodes, sodium sulphide added in the electrolytes on the discharge capacity and self-discharge rate of iron electrodes. The largest discharge capacity of the iron electrodes was 488.5 mAh/g-Fe at 66.4 mA/g-Fe in the first ten cycles, and the average self-discharge rate was 0.367% per hour.

Understanding the Coffee ring Effect on Self-discharge Behavior of Printed micro-Supercapacitors

Printed micro-supercapacitor exhibits its flexibility in geometry design and integration,showing unprecedented potential in powering the internet of things and portable devices.However,the printing process brings undesired processing defects(e.g.,coffee ring effect),resulting in severe self-discharge of the printed micro-supercapacitors.The impact of such problems on device performance is poorly understood,limiting further development of microsupercapacitors.Herein,by analyzing the self-discharge behavior of fully printed micro-supercapacitors,the severe self-discharge problem is accelerated by the ohmic leakage caused by the coffee ring effect on an ultrathin polymer electrolyte.Based on this understanding,the coffee ring effect was successfully eradicated by introducing graphene oxide in the polymer electrolyte,achieving a decline of 99%in the self-discharge rate.Moreover,the micro-supercapacitors with uniformly printed polymer electrolyte present 7.64 F cm^(-3)volumetric capacitance(14.37 mF cm^(-2)areal capacitance),exhibiting about 50%increase compared to the one without graphene oxide addition.This work provides a new insight to understand the relationship between processing defects and device performance,which will help improve the performance and promote the application of printed micro-supercapacitors.

Self-Discharge in Valve-Regulated Sealed Lead-Acid Batteries

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Self-discharge mitigated supercapacitors via hybrid CuO-nickel sulfide heterostructure for energy efficient, wireless data storage application

With the surge of demand for instant high power in miniaturized electronic and mechanical systems,supercapacitors(SCs)are considered as one of the viable candidates to fulfill the requirements.Thus,long-term resilience and superior energy density associated with self-discharge in SCs are obviously critical,but securing electrode materials,which can meet both benefits of SCs and persist charged potential for a comparatively prolonged duration,are still elusive.Herein,hierarchically refined nickel-sulfide heterostructure(CuO-NS)on CuO(CO)scaffold is achieved through optimized film formation,exhibiting a threefold improvement in the essential electrochemical characteristics and outstanding capacitance retention(∼5%loss).Self-discharge behavior and its mechanism are systematically investigated via morphological control and nanostructural evolution.Furthermore,significant mitigation of self-discharge owing to an increase in surface area and refined nanostructure is displayed.Remarkably,CuO-NS2(20 cycle overcoating)based SC can retain over 60%of the charged potential for a complete voltage holding and a self-discharge test for 16 h.An appealing demonstration of wireless power transmission in burst mode is demonstrated for secure digital(SD)card data writing,powered by SCs,which substantiates that it can be readily leveraged in power management systems.This enables us to realize one of the envisioned applications soon.

Mitigation on self-discharge behaviors via morphological control of hierarchical Ni-sulfides/Ni-oxides electrodes for long-life-supercapacitors

To cope up with the sustainable energy storage goals for supercapacitors(SCs),the self-discharge in SC electrodes is a significant hurdle,and thereby,nickel sulfide(NS)with high conductivity is adopted as a test vehicle for understanding the morphological evolution effects for long-life SCs.Herein,honeycomb-like NS is hierarchically formed over hydrothermally grown nickel oxide(NO)via successive ionic layer adsorption reaction(SILAR)method.Their heterostructure shows a fivefold improvement in specific capacitance from 348 F g^(−1) to 2077 F g^(−1)at 1 mV s^(−1) over bare NO.Furthermore,the remarkable upliftment of capacitance retention is achieved from 60.7%to 92.3%even after 3000 cycles via morphological control of NS/NO hetero-structure with the help of highly conductive NS.More importantly,the self-discharge behaviors and synergistic role of leakage current associated with morphological evolution via NS overcoating are studied in detail.In particular,the self-discharge mitigation from 45%(NO)to 35%(NS20/NO)due to the NS/NO heterostructure and the behind mechanism are ascribed to the activated-controlled Faradaic reaction coupled with a charge redistribution.This study emphasizes the potential importance of composite heterostructure by tuning the electrical conductivity and morphological adjustment NO via consecutive overcoating of NS through SILAR as a novel strategy.This enhances charge storage,redox kinetics,and the mitigation of self-discharge properties of the active electrode materials.For practical validation on sustainable energy storage,NS20/NO supercapacitors illuminate the LED for 35%longer than NO after one-time charging,potentially beneficial for the next generation SCs.

Separator coatings as efficient physical and chemical hosts of polysulfides for high-sulfur-loaded rechargeable lithium–sulfur batteries

Lithium-sulfur batteries(LSBs)are promising alternative energy storage devices to the commercial lithium-ion batteries.However,the LSBs have several limitations including the low electronic conductivity of sulfur(5×10^-30S cm^-1),associated lithium polysulfides(PSs),and their migration from the cathode to the anode.In this study,a separator coated with a Ketjen black(KB)/Nafion composite was used in an LSB with a sulfur loading up to 7.88 mg cm^-2to mitigate the PS migration.A minimum specific capacity(Cs)loss of 0.06%was obtained at 0.2 C-rate at a high sulfur loading of 4.39 mg cm^-2.Furthermore,an initial areal capacity up to 6.70 mAh cm^-2 was obtained at a sulfur loading of 7.88 mg cm^-2.The low Cs loss and high areal capacity associated with the high sulfur loading are attributed to the large surface area of the KB and sulfonate group(SO3^-)of Nafion,respectively,which could physically and chemically trap the PSs.

Capacity fading of spinel LiMn_2O_4 during cycling at elevated temperature

A normal spinel LiMn_2O_4 as cathode material for lithium-ion cells wascycled galvanostatically (0.2 C) at 55 deg C. To determine the contribution of each voltage plateauto the total capacity fading of the cathode upon repeated cycling, the capacities in each plateauwere separated by differentiation of voltage vs. capacity. The results how that the capacity fadingin the upper voltage plateau is more rapidly than that in the lower during discharging, while incharging process, it fades slower than that in the lower voltage range. The increased capacity shiftand aggravated self-discharge/electrolyte oxidation during discharging contribute to a high fadingrate in the upper step. Capacity shift also takes place during charging process, which againenhancing the fading rate of the lower voltage plateau. An increase in capacity shift, as a resultof an increase in polarization of the cell, plays a major role in determining the fading rate ineach voltage plateau, further reflecting the thickening of the passivation layer on the activeparticles, and the accumulation of electrolyte decomposition. The relative capacity loss formodified spinels is well correlated with the relative increase in the polarization of thehalf-cells, confirming the above causes for capacity fade of this kind of cathode material.

Realizing high-performance Zn-ion batteries by a reduced graphene oxide block layer at room and low temperatures

Rechargeable aqueous Zn-ion batteries (ZIBs) have attracted great attention due to their costeffectiveness,high safety,and environmental friendliness.However,some issues associated with poor structural instability of cathode materials and fast self-discharge hinder the further development of ZIBs.Herein,a new configuration is introduced by placing a reduced graphene oxide film as a block layer between the separator and the V2O5·nH2O cathode.This layer prevents the free diffusion of dissolved active materials to the anode and facilitates the transport of Zn ion and electrons,largely improving the cyclic stability and alleviating the self-discharge.Accordingly,the optimized battery delivers a remarkable capacity of 191 mAh g^-1 after 500 cycles at 2 A g^-1.Moreover,a high capacity of 106 mAh g^-1 is achieved after 100 cycles at-20℃.The strategy proposed is expected to be applicable to other electrode systems,thus offering a new approach to circumvent the critical challenges facing aqueous batteries.

Chloro-propylene Sulfite as Electrolyte Additive for Li/S Batteries

Chloro-propylene sulfite （CIPS） was employed as electrolyte additive of Li/S batteries for the first time. Linear potential sweep test showed that the CIPS keeps high electrochemical stability even under the voltage of 5.0V. Being used as electrolyte additive in Li/S batteries, CIPS displayed an excellent property for self-discharge prohibition. With CIPS additive the Li/S cells initial discharge capacity was 856.2 mAh·g^-1 and 830.8 mAh·g^-1 at the current density of 15 mA.g and 30 mA·g^-1 , after 30 cycles the discharge capacities were contained at as high as 753.8 mAh.g and 715.6 mAh·g^-1. By means of infrared spectra, TG/DTA experiment and element conten analysis the speculated reason of CIPS＇s novel function as additive was proposed.

A Generic Battery Model and Its Parameter Identification

A new dynamic model is developed in this paper based on the generic MATLAB battery model. The battery capacity is expressed as a function of the self-discharge rate, the discharge current, the cycling life and the temperature of the battery. The dependence of the model parameters on cycle life and temperature are estimated from the first order approximation. The detailed procedures and formula to extract the model parameters are presented and the extraction relies only on the discharge curves at two different discharge currents, at two different life cycles, and at two different temperatures. These discharge curves are typically provided in the battery manufacturer’s datasheet. The proposed model is verified for both nickel-metal hydride and lithium-ion batteries by comparing the calculated discharge curves with the results from the generic MATLAB model. The model is further validated for the Sinopoly lithium-ion battery (SP-LFP1000AHA) by comparing the model results with the discharge curves from the manufacturer’s datasheet at different discharge currents, different cycling numbers, and different temperatures. Simulation results show that the new model can correctly predict voltage separation beyond the nominal zone while maintaining the same level of accuracy as the generic MATLAB model in the exponential and nominal zones.

Is LiFePO₄Technology Ready for Internet of Things?

While self-sufficient sensors and actors are about to pave the way for a new computing class, associate Internet of Things applications will highly depend on efficient and reliable storage of electrical energy. Likewise the same is true for electrical based transport systems requiring light weights and high capacities. Recently, novel LiFePO4 based storage cell types with standardized form-factors have become available. These cells tend to be promising in terms of high energy densities, low self-discharge rates and long cycle lives. Anyhow, the aging behavior, maturity, statistical spread and reliability of these new cells have not been analyzed and modeled thoroughly. Therefore, we analyze and compare in this paper the self-discharge behavior, lifetime and reliability of different lithium-based battery cells using a dedicated test bench. We use temperature, voltage, current, and power cycling as acceleration and stress parameters.

Catalytic hosts with strong adsorption strength for long shelf-life lithium-sulfur batteries under lean electrolyte

Low electrolyte/sulfur ratio(E/S)is an important factor in increasing the energy density of lithium-sulfur batteries(LSBs).Recently,the E/S has been widely lowered using catalytic hosts that can suppress“shuttle effect”during cycling by relying on a limited adsorption area.However,the shelf-lives of these cathodes have not yet received attention.Herein,we show that the selfdischarge of sulfur cathodes based on frequently-used catalytic hosts is serious under low E/S because the“shuttle effect”during storage process caused by polysulfides(PSs)disproportionation cannot be suppressed using a limited adsorption area.We further prove that the adsorption strength toward PSs,which is unfortunately weak in commonly-used catalytic hosts,is critical for effectively hindering the disproportionation of the PSs.Subsequently,to verify this conclusion,we prepare a sulfur-doped titanium nitride(S-TiN)catalytic array host.As the adsorption strength and catalytic activity of TiN can be improved by S doping simultaneously,the constructed S/S-TiN cathodes under a low E/S(6.5μL·mg−1)exhibit better shelf-life and cycle-stability than those of S/TiN cathodes.Our work suggests that enhancing the adsorption strength of catalytic hosts,while maintaining their function to reduce E/S,is crucial for practical LSBs.

Mechanistic understanding of the role separators playing in advanced lithium-sulfur batteries

The lithium-sulfur battery is considered one of the most promising candidates for portable energy storage devices due to its low cost and high energy density.However,many critical issues,including polysulfide shuttling,self-discharge,lithium dendritic growth,and thermal hazards need to be addressed before the commercialization of lithium-sulfur batteries.To this end,tremendous efforts have been made to explore battery configurations and components,such as electrodes,electrolytes,and separators,among which the separator plays an especially critical role in addressing aforementioned issues.Thus,this review analyzes the mechanisms and interactions of these critical issues and summarizes both the function of separators and recent progress made towards remedying such issues.Additionally,promising directions for the development of separators in lithium-sulfur batteries are proposed.