Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices.Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces,but also demands the overall device to be flexible in response to external fields.However,flexible energy storage devices inevitably occur mechanical damages(extrusion,impact,vibration)/electrical damages(overcharge,over-discharge,external short circuit)during longterm complex deformation conditions,causing serious performance degradation and safety risks.Inspired by the healing phenomenon of nature,endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices.Herein,this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices.Firstly,the main intrinsic self-healing mechanism is introduced.Then,the research situation of electrodes,electrolytes,artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed.Finally,the current challenges and perspective are provided.We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Layered double hydroxides as electrode materials for flexible energy storage devices

To prevent and mitigate environmental degradation,high-performance and cost-effective electrochemical flexible energy storage systems need to be urgently developed.This demand has led to an increase in research on electrode materials for high-capacity flexible supercapacitors and secondary batteries,which have greatly aided the development of contemporary digital communications and electric vehicles.The use of layered double hydroxides(LDHs)as electrode materials has shown productive results over the last decade,owing to their easy production,versatile composition,low cost,and excellent physicochemical features.This review highlights the distinctive 2D sheet-like structures and electrochemical characteristics of LDH materials,as well as current developments in their fabrication strategies for expanding the application scope of LDHs as electrode materials for flexible supercapacitors and alkali metal(Li,Na,K)ion batteries.

Graphene-based materials for flexible energy storage devices

The booming developments in portable and wearable electronics promote the design of flexible energy storage systems. Flexible supercapacitors and batteries as promising energy storage devices have attracted tremendous attention. As the key component of both supercapacitors and batteries, electrode materials with excellent flexibility should be considered to match with highly flexible energy storage devices. Owing to large surface area, good thermal and chemical stability, high conductivity and mechanical flexibility,graphene-based materials have been widely employed to serve as promising electrodes of flexible energy storage devices. Considerable efforts have been devoted to the fabrication of flexible graphene-based electrodes through a variety of strategies. Moreover, different configurations of energy storage devices based on these active materials are designed. This review highlights flexible graphene-based two-dimensional film and one-dimensional fiber supercapacitors and various batteries including lithium-ion, lithium–sulfur and other batteries. The challenges and promising perspectives of the graphene-based materials for flexible energy storage devices are also discussed.

Engineering Geometric Electrodes for Electric Field-Enhanced High-Performance Flexible In-Plane Micro-Supercapacitors

In plane micro-supercapacitors that are miniaturized energy storage components have attracted significant attention due to their high power densities for various ubiquitous and sustainable device systems as well as their facile integration on various flexible/wearable platform.To implement the micro-supercapacitors in various practical applications that can accompany solid state or gel electrolyte and flexible substrates,ions must be readily transported to electrodes for achieving high power densities.Herein,we show large enhancement in electrochemical properties of flexible,inplane micro-supercapacitor using sharp-edged interdigitated electrode design,which was simply fabricated through direct laser scribing method.The sharp-edged electrodes allowed strong electric field to be induced at the corners of the electrode fingers which led to the greater accumulation of ions near the surface of electrode,significantly enhancing the energy storage performance of micro-supercapacitors.The electric field-enhanced in-plane micro-supercapacitor showed the volumetric energy density of 1.52 Wh L^(−1)and the excellent cyclability with capacitive retention of 95.4%after 20000 cycles.We further showed various practicability of our sharp-edged design in micro-supercapacitors by showing circuit applicability,mechanical stability,and air stability.These results present an important pathway for designing electrodes in various energy storage devices.

Modeling and management performances of distributed energy resource for demand flexibility in Japanese zero energy house

Zero-energy buildings(ZEBs)can contribute to decarbonizing building energy systems,while the energy mismatch between energy demand and on-site stochastic generation in ZEBs increases the need for energy flexibility.This study proposed mixed-integer linear programming energy management schemes for optimizing the flexible scheduling of distributed energy resources,including battery energy storage,heat pump,and building thermal mass as a passive thermal energy storage system.With optimally designed objectives,this study used case studies to evaluate the flexibility potential provided by the demand-side management,considering dynamic characteristics of the process.The results showed that the proposed demand-side management for battery storage offers significant potential in increasing photovoltaic(PV)self-consumption and reducing operational costs.Cost reduction ratios of flexible dispatch of combined PV and battery storage systems exceed 15%.Flexible coupling of PV and heat pump systems for meeting hot water demand can reduce energy cost by more than 20%.The flexible coupling of the heat pump and PV system also had a significant impact on the power consumption pattern of domestic heat pumps,the load-shifting potential highly depends on the available PV generation and hot water demand.The optimal trade-off between thermal energy use and thermal comfort violation may not reduce the total energy used for space heating,the increased PV consumption helped reduce grid imports.The study provides insights into the energy flexibility behavior and efficiency of the proposed demand-side management for ZEBs,which is expected to provide guidelines for exploring demand-side flexibility.

Energy flexibility characteristics of centralized hot water system in university dormitories

The large-scale application of renewable energy is an important strategy to achieve the goal of carbon neutrality in the building sector.Energy flexibility is essential for ensuring balance between energy demand and supply when targeting the maximum penetration rate of renewable energy during the operation of regional integrated energy systems.Revealing the energy flexibility characteristics of centralized hot water systems,which are an important source of such flexibility,is of great significance to the optimal operation of regional integrated energy systems.Hence,in this study,based on the annual real-time monitoring data,the energy flexibility of the centralized hot water system in university dormitories is evaluated from the perspective of available storage capacity(C_(ADR)),recovery time(t_(recovery)),and storage efficiency(η_(ADR)),by the data-driven simulation method.The factors influencing the energy flexibility of the centralized hot water system are also analyzed.Available storage capacity has a strong positive correlation with daily water consumption and a strong negative correlation with daily mean outdoor temperature.These associations indicate that increased water use on the energy flexibility of the centralized hot water system is conducive to optimal dispatching.In contrast,higher outdoor temperature is unfavorable.The hourly mean value of the available storage capacity in spring and winter is found to be around 80 kWh in the daytime,and about twice that in summer and autumn.Recovery time is evenly distributed throughout the year,while t_(recovery)/C_(ADR)in spring and winter is about half that in summer.The storage efficiency was significantly higher in spring,summer,and winter than in autumn.The hourly mean storage efficiency was found to be about 40%in the daytime.The benefits of activating energy flexibility in spring and winter are the best,because these two seasons have higher available storage capacity and storage efficiency,while the benefit of activating energy flexibility is the highest at 6:00 a.m.,and very low from midnight to 3:00 a.m.

Highly elastic energy storage device based on intrinsically super-stretchable polymer lithium-ion conductor with high conductivity

Stretchable power sources,especially stretchable lithium-ion batteries(LIBs),have attracted increasing attention due to their enormous prospects for powering flexible/wearable electronics.Despite recent advances,it is still challenging to develop ultra-stretchable LIBs that can withstand large deformation.In particular,stretchable LIBs require an elastic electrolyte as a basic component,while the conductivity of most elastic electrolytes drops sharply during deformation,especially during large deformations.This is why highly stretchable LIBs have not yet been realized until now.As a proof of concept,a super-stretchable LIB with strain up to 1200%is created based on an intrinsically super-stretchable polymer electrolyte as the lithium-ion conductor.The super-stretchable conductive system is constructed by an effective diblock copolymerization strategy via photocuring of vinyl functionalized 2-ureido-4-pyrimidone(VFUpy),an acrylic monomer containing succinonitrile and a lithium salt,achieving high ionic conductivity(3.5×10^(-4)mS cm^(-1)at room temperature(RT))and large deformation(the strain can reach 4560%).The acrylic elastomer containing Li-ion conductive domains can strongly increase the compatibility between the neighboring elastic networks,resulting in high ionic conductivity under ultra-large deformation,while VFUpy increases elasticity modulus(over three times)and electrochemical stability(voltage window reaches 5.3 V)of the prepared polymer conductor.At a strain of up to 1200%,the resulting stretchable LIBs are still sufficient to power LEDs.This study sheds light on the design and development of high-performance intrinsically super-stretchable materials for the advancement of highly elastic energy storage devices for powering flexible/wearable electronics that can endure large deformation.

High Na-ion conductivity andmechanical integrity of anion-exchanged polymeric hydrogel electrolytes for flexible sodium ion hybrid energy storage

The polymeric gel electrolytes are attractive owing to their higher ionic conductivities than those of dry polymer electrolytes and lowered water activity for enlarged potential window.However,the ionic conductivity and mechanical strength of the Na-ion conducting polymeric gel electrolytes are limited by below 20 mS cm−1 and 2.2 MPa.Herein,we demonstrate Na-ion conducting and flexible polymeric hydrogel electrolytes of the chemically coupled poly(diallyldimethylammonium chloride)-dextrin-N,N′-methylene-bisacrylamide film immersed in NaClO_(4) solution(ex-DDA-Dex+NaClO_(4))for flexible sodium-ion hybrid capacitors(f-NIHC).In particular,the anion exchange reaction and synergistic interaction of ex-DDA-Dex with the optimum ClO_(4)−enable to greatly improve the ionic conductivity up to 27.63 mS cm−1 at 25◦C and electrochemical stability window up to 2.6 V,whereas the double networking structure leads to achieve both the mechanical strength(7.48 MPa)and softness of hydrogel electrolytes.Therefore,the f-NIHCs with the ex-DDA-Dex+NaClO_(4) achieved high specific and high-rate capacities of 192.04 F g^(−1)at 500 mA g^(−1)and 116.06 F g^(−1)at 10000 mA g^(−1),respectively,delivering a large energy density of 120.03Wh kg^(−1)at 906Wkg^(−1)and long cyclability of 70%over 500 cycles as well as demonstrating functional operation under mechanical stresses.

In-Situ Annealed Ti_(3)C_(2)T_(x) MXene Based All-Solid-State Flexible Zn-Ion Hybrid Micro Supercapacitor Array with Enhanced Stability

Zn-ion hybrid supercapacitors(SCs)are considered as promising energy storage owing to their high energy density compared to traditional SCs.How to realize the miniaturization,patterning,and flexibility of the Zn-ion SCs without affecting the electrochemical performances has special meanings for expanding their applications in wearable integrated electronics.Ti_(3)C_(2)T_(x) cathode with outstanding conductivity,unique lamellar structure and good mechanical flexibility has been demonstrated tremen-dous potential in the design of Zn-ion SCs,but achieving long cycling stability and high rate stability is still big challenges.Here,we proposed a facile laser writing approach to fabricate patterned Ti_(3)C_(2)T_(x)-based Zn-ion micro-supercapacitors(MSCs),followed by the in-situ anneal treatment of the assembled MSCs to improve the long-term stability,which exhibits 80%of the capacitance retention even after 50,000 charge/discharge cycles and superior rate stability.The influence of the cathode thickness on the electrochemical performance of the MSCs is also studied.When the thickness reaches 0.851μm the maximum areal capacitance of 72.02 mF cm^(−2)at scan rate of 10 mV s^(−1),which is 1.77 times higher than that with a thickness of 0.329μm(35.6 mF cm^(−2)).Moreover,the fab-ricated Ti_(3)C_(2)T_(x) based Zn-ion MSCs have excellent flexibility,a digital timer can be driven by the single device even under bending state,a flexible LED displayer of“TiC”logo also can be easily lighted by the MSC arrays under twisting,crimping,and winding conditions,demonstrating the scalable fabrication and application of the fabricated MSCs in portable electronics.

Regulating Thermogalvanic Effect and Mechanical Robustness via Redox Ions for Flexible Quasi‑Solid‑State Thermocells

The design of power supply systems for wearable applications requires both flexibility and durability.Thermoelectrochemical cells(TECs)with large Seebeck coefficient can efficiently convert lowgrade heat into electricity,thus having attracted considerable attention in recent years.Utilizing hydrogel electrolyte essentially addresses the electrolyte leakage and complicated packaging issues existing in conventional liquid-based TECs,which well satisfies the need for flexibility.Whereas,the concern of mechanical robustness to ensure stable energy output remains yet to be addressed.Herein,a flexible quasisolid-state TEC is proposed based on the rational design of a hydrogel electrolyte,of which the thermogalvanic effect and mechanical robustness are simultaneously regulated via the multivalent ions of a redox couple.The introduced redox ions not only endow the hydrogel with excellent heat-to-electricity conversion capability,but also act as ionic crosslinks to afford a dual-crosslinked structure,resulting in reversible bonds for effective energy dissipation.The optimized TEC exhibits a high Seebeck coefficient of 1.43 mV K−1 and a significantly improved fracture toughness of 3555 J m^(−2),thereby can maintain a stable thermoelectrochemical performance against various harsh mechanical stimuli.This study reveals the high potential of the quasi-solid-state TEC as a flexible and durable energy supply system for wearable applications.

Evaluating the impact of thermostat control strategies on the energy flexibility of residential buildings for space heating

Buildings can be operated in an energy-flexible manner while respecting occupant thermal comfort.This energy flexibility of building operations,both in time and quantity,can be harnessed by the electrical grid for load balancing.In the context of smart grid and intelligent buildings,the concept of energy flexibility in buildings broadens the existing demand management thinking from the top-down one-way control to two-way communications.This paper,extending studies on thermostat controls of heating and air conditioning systems for demand response,evaluates the impact of different control schemes on the energy flexibility of residential buildings.Two control strategies,Model Predictive Control(MPC)and Rule-Based Control(RBC),are investigated for a space heating system using co-simulation studies.Four indicators are introduced and adapted from the literature to assess the control performances of the strategies.Simulation results show that different flexibility indicators favour different control strategies in this case study.For demand response events of four hours,the MPC strategy presents two to three times of flexible energy than that of RBC.MPC also delivers 20%more of maximum power reduction during the events against RBC.The RBC strategy,on the other hand,is twice of MPC for flexible energy efficiency.This evaluation work can be beneficial to guide the control system design of new buildings or control retrofits of existing buildings that consider better grid-building interactions for the future.

Exploring Potential of Energy Flexibility in Buildings for Energy System Services

Buildings have both high as well as flexible energy demands and play an important role in the energy internet solution.The buildings’energy flexibility(BEF)is a widely recognized concept;however,how to unlock its potential is a relatively new research topic.In this paper,the authors provide an overview of the latest research related to BEF.An introduction to BEF is provided,methods developed for identifying and characterizing BEF are presented,and several key influencing factors are identified.The overview also covers various aggregation methods to scale up BEF impacts and service-oriented solutions for enabling BEF applications in different energy sectors.This work lays the groundwork for designing and developing seamless integration strategies for BEF use in both present and future energy systems.

A review of Danish integrated multi-energy system flexibility options for high wind power penetration

The current status of wind power and the energy infrastructure in Denmark is reviewed in this paper.The reasons for why Denmark is a world leader in wind power are outlined.The Danish government is aiming to achieve 100%renewable energy generation by 2050.A major challenge is balancing load and generation.In addition,the current and future solutions of enhancing wind power penetration through optimal use of cross-energy sector flexibility,so-called indirect electric energy storage options,are investigated.A conclusion is drawn with a summary of experiences and lessons learned in Denmark related to wind power development.

Development of a Soft Actor Critic deep reinforcement learning approach for harnessing energy flexibility in a Large Office building

This research is concerned with the novel application and investigation of‘Soft Actor Critic’based deep reinforcement learning to control the cooling setpoint(and hence cooling loads)of a large commercial building to harness energy flexibility.The research is motivated by the challenge associated with the development and application of conventional model-based control approaches at scale to the wider building stock.Soft Actor Critic is a model-free deep reinforcement learning technique that is able to handle continuous action spaces and which has seen limited application to real-life or high-fidelity simulation implementations in the context of automated and intelligent control of building energy systems.Such control techniques are seen as one possible solution to supporting the operation of a smart,sustainable and future electrical grid.This research tests the suitability of the technique through training and deployment of the agent on an EnergyPlus based environment of the office building.The agent was found to learn an optimal control policy that was able to minimise energy costs by 9.7%compared to the default rule-based control scheme and was able to improve or maintain thermal comfort limits over a test period of one week.The algorithm was shown to be robust to the different hyperparameters and this optimal control policy was learnt through the use of a minimal state space consisting of readily available variables.The robustness of the algorithm was tested through investigation of the speed of learning and ability to deploy to different seasons and climates.It was found that the agent requires minimal training sample points and outperforms the baseline after three months of operation and also without disruption to thermal comfort during this period.The agent is transferable to other climates and seasons although further retraining or hyperparameter tuning is recommended.

Photovoltaics and Energy Storage Integrated Flexible Direct Current Distribution Systems of Buildings:Definition,Technology Review,and Application

For a future carbon-neutral society,it is a great challenge to coordinate between the demand and supply sides of a power grid with high penetration of renewable energy sources.In this paper,a general power distribution system of buildings,namely,PEDF(photovoltaics,energy storage,direct current,flexibility),is proposed to provide an effective solution from the demand side.A PEDF system integrates distributed photovoltaics,energy storages(including traditional and virtual energy storage),and a direct current distribution system into a building to provide flexible services for the external power grid.System topology and control strategies at the grid,building,and device levels are introduced and analyzed.We select representative work about key technologies of the PEDF system in recent years,analyze research focuses,and summarize their major challenges&future opportunities.Then,we introduce three real application cases of the PEDF system.On-site measurement results demonstrate its feasibility and advantages.With the rapid growth of renewable power production and electric vehicles,the PEDF system is a potential and promising approach for largescale integration of renewable energy in a carbon-neutral future.

Worldwide carbon neutrality transition?Energy efficiency,renewable,carbon trading and advanced energy policies

Climate change and energy shortage crisis promptly necessitate achievement of sustainable development goals.However,there is no straightforward pathways for low-carbon transformation on building sectors,and energy/carbon trading and reverse promotion on decarbonization strategies are not clear.In this study,a literature enumeration method with dialectical analysis was adopted for state-of-the-art literature review and comparison.Low-carbon transformation pathways in buildings were holistically reviewed,with a series of integrated techniques,such as energy saving,clean energy supply,flexible demand response for high self-consumption,and even smart electric vehicle(EV)integration.Afterwards,energy/carbon flows and trading in building-related systems were provided,such as peer-to-peer energy trading,building and thermal/power grids,building and energyintegrated EVs,and carbon trading in buildings.Last but not the least,worldwide decarbonization roadmaps across regions and countries are analysed,to identify the most critical aspects and immediate actions on decarbonization.Results indicate that tradeoff strategies are required to compromise the confliction between insufficient feed-in tariff(FiT)incentives(low renewable penetration in the market)and great economic pressures(high investment in renewable systems).Low-carbon building pathway is further enhanced with first priority given to passive/active energy-saving strategies,onsite clean energy supply and then flexible demand response.Energy/carbon trading will significantly affect renewable energy utilization,and acceptance from end-users to actively install renewable systems or participate in EV interactions.Worldwide decarbonization pathways mainly focus on industries,transportation,buildings,renewable sources,carbon sink and carbon capture,utilization and storage(CCUS).This study can contribute to technical roadmaps and strategies on carbon neutrality transition in both academia and industry,together with advanced policies in grid feed-in tariff,energy/carbon trading and business models worldwide.

Optimized design and integration of energy storage in Solar-Assisted Ground-Source Heat Pump systems

The integrated use of multiple renewable energy sources to increase the efficiency of heat pump systems,such as in Solar Assisted Geothermal Heat Pumps(SAGHP),may lead to significant benefits in terms of increased efficiency and overall system performance especially in extreme climate contexts,but requires careful integrated optimization of the different system components.In particular,thermal storages take a fundamental role in optimizing the integration of renewable energy sources and the system operation.This work investigates the potential design optimization of a SAGHP system in a mountain site by exploring many different alternatives to optimize the mutual relationship between the solar field,the geothermal field and the water thermal storages.This is done through an original simulation-based multi-objective optimization framework considering energy efficiency and economic feasibility,which allows appraising the impact of the different design alternatives on the overall system performance and on the dynamics of the different system components.Results identify a set of optimized system configurations that optimize the integrated exploitation of the different thermal sources showing a potential increase of the overall system performance leading to 34%lower global cost compared to the initial design.High robustness of the optimal design solutions is reported with respect to the current context of high economic uncertainty.

Guest editorial:special section on energy storage systems and operational flexibility in power systems

The energy storage system(ESS) is becoming an important component in power systems to mitigate the adverse impact of intermittent renewable energy resources and improve power grid reliability and efficiency.However,storage devices driven by different technologies can have specific grid impacts.This special section is dedicated to reflecting the

Deep reinforcement learning for the optimized operation of large amounts of distributed renewable energy assets

This study utilizes machine learning and,more specifically,reinforcement learning(RL)to allow for an optimized,real-time operation of large numbers of decentral flexible assets on private household scale in the electricity domain.The potential and current obstacles of RL are demonstrated and a guide for interested practitioners is provided on how to tackle similar tasks without advanced skills in neural network programming.For the application in the energy domain it is demonstrated that state-of-the-art RL algorithms can be trained to control potentially millions of small-scale assets in private households.In detail,the applied RL algorithm outperforms common heuristic algorithms and only falls slightly short of the results provided by linear optimization,but at less than a thousandth of the simulation time.Thus,RL paves the way for aggregators of flexible energy assets to optimize profit over multiple use cases in a smart energy grid and thus also provide valuable grid services and a more sustainable operation of private energy assets.

Preparation of vanadium-based electrode materials and their research progress in solid-state flexible supercapacitors

Solid-state flexible supercapacitors(SCs)have many advantages of high specific capacitance,excellent flexibility,fast charging and discharging,high power density,environmental friendliness,high safety,light weight,ductility,and long cycle stability.They are the ideal choice for the development of flexible energy storage technology in the future,and provide a good prospect for energy storage applications.At present,solid-state flexible SCs are widely used for portable electronic equipment and wearable energy storage equipment,the research of them has become the focus of a growing number of researchers.Electrode material is the key part of SCs and always determines the electrochemical performance of SCs.It has been a hotspot and focus of research.Vanadium-based compounds are considered to be a promising electrode material for SCs because of variable valence,open structure,high theoretical capacity,and low price.Therefore,this study first gives an overview of solid-state flexible SCs,then reviews the current research status of vanadium-based electrode materials in solid-state flexible SCs,and proposes some strategies to solve some problems of vanadium-based electrode materials.