Improve Cycle Efficiency Research Articles

Design optimization and off-design performance analysis of one-dimensional supercritical CO2 Brayton cycle

Supercritical carbon dioxide (SCO2) recompression Brayton cycle (RBC) is an encouraging technology for thermal power conversion and heat harvest for a wide range of heat sources due to its high-efficiency, low auxiliary power consumption, and compact size. However, most studies on its design and off-design performance analysis have been limited to the simple zero-dimensional component models without considering aerothermodynamics and ambient conditions. In this study, we developed a set of fully one-dimensional components models for SCO2 compressors, turbines, and heat exchangers, and, then, applied them to the multi-objective design optimization of a SCO2 cycle targeting a 50 MW net power output. The cycle off-design performance analysis methods and the control strategies were developed to countermeasure the performance degradation against the varying cooling water temperatures and part loads. The multi-objective cycle design optimization demonstrates that the cycle thermal efficiency and total product unit cost can be 0.476 and 11.652$/GJ, respectively, for the 50 MW SCO2 RBC cycle. The choke margin of the main compressor is 0.136 less than that of the re-compressor, and condensation can lead to a sharp decline in efficiency under high-flow conditions, causing the earlier onset of choking. The off-design performance analysis shows that the turbine inlet pressure control improves cycle efficiency below the design point of 317 K but does otherwise above the point. In contrast, inventory control strategies can maintain stable cycle efficiency and achieve adjustable net power output between 10 % and 110 %. The findings have the potential to advance further the commercial application of SCO2 cycles in high-temperature thermal energy systems.

Development of multilevel cascade layouts to improve performance of SCO2 Brayton power cycles: Design, simulation, and optimization

The multilevel cascade SCO2 Brayton cycles are developed based on the conventional recompression-reheat cycle to enhance thermodynamic performances using the multilevel cascade scheme. Mathematical models are proposed to characterize their thermal cycle efficiency. The results show that for three typical cases, the proposed multilevel cascade cycle with 3 compressors × 3 reheats × 2 turbines improve cycle efficiency by 2.56 %, 3.17 %, and 3.44 % in absolute change, while the other with 3 compressors × 3 reheats × 3 turbines further enhance by 3.17 %, 4.19 %, and 4.17 % in absolute change over the conventional cycle. The latter cycle as the optimal layout is chosen to perform the key parametric analysis. The cycle efficiency increases with increasing high-pressure turbine inlet temperature, while the main compressor inlet temperature has the opposite effect. It increases and then decreases with the high-pressure turbine inlet pressure, the main compressor inlet pressure, and the split ratio of the recompressor, respectively. Finally, the parametric global optimization is conducted to improve the cycle performance more deeply. It is demonstrated that the absolute efficiency can be further increased by 2.62 %, 1.81 %, and 1.09 % for the responding cases. The integrated configurations are also suggested for power generation systems with different applications.

Large Organic Polar Molecules Tailored Electrode Interfaces for Stable Lithium Metal Battery.

Lithium (Li) metal batteries (LMBs) are deemed as ones of the most promising energy storage devices for next electrification applications. However, the uneven Li electroplating process caused by the diffusion-limited Li+ transportation at the Li metal surface inherently promotes the formation of dendritic morphology and instable Li interphase, while the sluggish Li+ transfer kinetic can also cause lithiation-induced stress on the cathode materials suffering from serious structural stability. Herein, a novel electrolyte designing strategy is proposed to accelerate the Li+ transfer by introducing a trace of large organic polar molecules of lithium phytate (LP) without significantly altering the electrolyte structure. The LP molecules can afford a competitive solvent attraction mechanism against the solvated Li+, enhancing both the bulk and interfacial Li+ transfer kinetic, and creating better anode/cathode interfaces to suppress the side reactions, resulting in much improved cycling efficiency of LMBs. Using LP-based electrolyte, the performance of LMB pouch cell with a practical capacity of ~1.5 Ah can be improved greatly. This strategy opens up a novel electrolyte designing route for reliable LMBs.

Large Organic Polar Molecules Tailored Electrode Interfaces for Stable Lithium Metal Battery

AbstractLithium (Li) metal batteries (LMBs) are deemed as ones of the most promising energy storage devices for next electrification applications. However, the uneven Li electroplating process caused by the diffusion‐limited Li+ transportation at the Li metal surface inherently promotes the formation of dendritic morphology and instable Li interphase, while the sluggish Li+ transfer kinetic can also cause lithiation‐induced stress on the cathode materials suffering from serious structural stability. Herein, a novel electrolyte designing strategy is proposed to accelerate the Li+ transfer by introducing a trace of large organic polar molecules of lithium phytate (LP) without significantly altering the electrolyte structure. The LP molecules can afford a competitive solvent attraction mechanism against the solvated Li+, enhancing both the bulk and interfacial Li+ transfer kinetic, and creating better anode/cathode interfaces to suppress the side reactions, resulting in much improved cycling efficiency of LMBs. Using LP‐based electrolyte, the performance of LMB pouch cell with a practical capacity of ~1.5 Ah can be improved greatly. This strategy opens up a novel electrolyte designing route for reliable LMBs.

Investigation on the thermodynamic performance and machinery mass in the N2O-based mixtures recompression Brayton cycle for a nuclear-powered spacecraft

As a reliable energy system with a long service life, nuclear energy can produce massive power for spacecraft in deep space exploration. Due to the compact layout and excellent performance, the supercritical recompression Brayton cycle has received extensive attention and has become a hot research topic in thermoelectric conversion technology. The critical properties of the working medium are a constraint on the cycle performance. Adding additional gas into a pure S-N2O working medium can effectively change the properties of the critical point and improve the cycle performance. However, there is no standard for selecting appropriate binary mixture components to determine which additive gas can improve cycle efficiency. In this work, several gases are chosen as the potential N2O-based mixtures (N2O-O2, N2O-He, N2O-Kr, N2O-Ar, N2O-H2S, N2O-CO, N2O-SO2, N2O-CH4, N2O-C2H6, N2O-C3H8, N2O-C4H8, N2O-C4H10, N2O-C5H10, N2O-C5H12, N2O-C6H6) as the working medium in the system. The detailed sensitivity analysis reveals the influence of essential parameters (the split ratio, pressure ratio, selection minimum operating temperature, maximum operating temperature, and minimum operating pressure) on the thermodynamic efficiency and Brayton machinery unit mass. According to the analysis results, the thermal efficiency improves with an increase in the split ratio, pressure ratio, and maximum operating temperature and decreases in the selection minimum operating temperature and minimum operating pressure for all mixtures. The exergy efficiency also showed a similar variation trend. However, the influence of the maximum working temperature on it is different from that of the thermal efficiency, which decreases with the increase of the maximum working temperature. The cycles operated close to the critical area can provide better cycle performance, and N2O-He has the best thermal performance among all proposed N2O-based mixtures, which can be used as a potential choice as the working medium for future nuclear-powered spacecraft.

Enhancing performance of multi-pressure evaporation organic Rankine Cycle/Supercritical Carbon Dioxide Brayton cycle through genetic algorithm and Machine learning optimization

The Supercritical Carbon Dioxide Brayton combined cycle is one of the most important ways to enhance the performance of the cycle. Since the considerable exergy loss resulting from large temperature differences in the heat exchangers of combined cycles, the multi-pressure evaporation concept was adopted from Organic Rankine cycle research. Two novel multi-pressure evaporation configurations, namely parallel evaporation Brayton combined cycle and series evaporation Brayton combined cycle, are designed with the purpose of enhancing SCO2 Brayton cycle performance. The performance of the two new cycles was compared with that of the baseline cycle. Moreover, Printed Circuit Heat Exchangers were employed to improve the overall compactness of the cycles. The results from model calculations were utilized to train an Artificial intelligence neural network model using machine learning techniques which allowed to simplify the cycle and improves the computational speed. Subsequently, genetic algorithm was employed to conduct multi-objective optimization on the three cycles. Finally, a detailed exploration was conducted from the perspective of thermal and exergy to elucidate the reasons for the advantages and differences between the two new cycles compared to the baseline cycle. The results indicate that the series Brayton combined cycle exhibits the highest performance, and the combined cycle of multi-pressure evaporation helps to improve the performance of the combined cycle. Compared to the baseline cycle, the combined cycle efficiency of both series and parallel combined cycles has increased from 45.54% to 46.18% and 46.29%, respectively, while the bottom cycle efficiency has improved from 3.81% to 4.93% and 5.03%. The ratio of heat exchange area to net output power increased from 0.6051 to 0.6614 and 0.6740 for the parallel and series cycles, respectively. Thermal analysis shows that, the fundamental purpose of changing the layout is to improve the specific power per unit of working medium in the cycle. Exergy analysis shows that, the two new combined cycles improve cycle efficiency by reducing exergy losses in the intermediate heat exchanger. Compared to the parallel version, the series version's staged compression allows for a lower temperature difference in intermediate heat exchanger 1, resulting in smaller exergy losses and higher cycle efficiency. This work will contribute to an enhanced understanding of Supercritical Carbon Dioxide combined cycles within the academic community and the introduction of multi-pressure evaporation will further improve the performance of these important cycles.

Assessment of ternary CO2 mixtures as working fluids in supercritical Brayton cycles with floating critical points

The performance of the supercritical CO2 (sCO2) Brayton cycle deteriorates due to the mismatch between critical temperature and cold source temperature. An innovative solution based on ternary CO2 mixtures with adjustable compositions is proposed to expand the variation range of the critical temperature. An in-house MATLAB code is developed to investigate the design-point performance of the proposed system. The results indicate that the effect of improving cycle efficiency by using the modified floating critical point method becomes increasingly significant as the target critical temperature deviates from that of CO2. The CO2–H2S–Kr is screened out as the best ternary mixture, with the cycle efficiency being increased by over 8 % compared to the sCO2 cycle at very high (45 °C) and low (−15 °C) temperatures. The long-term performance is also evaluated based on the hourly historical weather data of two typical climate regions. The annual performance depends on both the average value and the distribution characteristics of the local ambient temperature. A relative improvement in annual average efficiency of 7%–9.56 % can be obtained depending on the regions. The present work is helpful to make the supercritical Brayton cycle give full play to its efficiency advantage in a wide cold source temperature range.

High dielectric filler for all-solid-state lithium metal battery

Lithium metal with its high theoretical capacity and low negative potential is considered one of the most important candidates to raise the energy density of all-solid-state batteries. However, lithium filament growth and its induced solid electrolyte decomposition pose severe challenges to realize a long cycle life. Here, dendrite growth in solid-state Li metal batteries is alleviated by introducing a high dielectric material, barium titanate, as a filler that removes the electric field gradients that catalyze dendrite formation. In symmetrical Li-metal cells, this results in a very small over-potential of only 48 mV at a relatively high current density of 1 mA cm−2, when cycling a capacity of 2 mA h cm−2 during 1700 h. The high dielectric filler improves the Coulombic efficiency and cycle life of full cells and suppresses electrolyte decomposition as indicated by solid-state nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) measurements. This indicates that the high dielectric filler can suppress dendrite formation, thereby reducing solid electrolyte decomposition reactions, resulting in the observed low overpotentials and improved cycling efficiency.

Applicability evaluation of internal heat exchanger in CO2 transcritical cycle considering compressor operation boundaries

CO2 transcritical cycle has been widely promoted as a green energy utilization technology. Internal heat exchanger (IHX) is under consideration for cycle efficiency improvement. However, the interaction between IHX and compressor in CO2 cycle has not yet been fully explored. In this regard, compressor operating boundaries are introduced as constraints to IHX applicability evaluation in CO2 cycle. It indicates that considering compressor limits, IHX can either benefit or attenuate the CO2 cycle performance, depending on operating conditions. By reducing optimal discharge pressure, IHX helps CO2 cycle avoid the upper pressure limit, further improving performance by 5%. Over a wide range, however, IHX increases compressor discharge temperature, indirectly resulting in up to 50% performance degradation. The major factors affecting the duality of IHX in CO2 cycle are compressor discharge temperature limit, IHX efficiency and motor heat loss. Finally, performance-oriented IHX design guidelines for mainstream single-stage CO2 applications are provided. For automotive air conditioner, optimum IHX efficiency improves cycle performance by up to 30% in cooling mode. By contrast, IHX contribution is negligible in heating mode. For heat pump water heater and refrigeration system, both can benefit from IHX, particularly when compressor discharge temperature tolerance is high.

Thermal energy storage as a way to improve transcritical CO2 heat pump performance by means of heat recovery cycles

Electrification of the heating sector is a major target of energy transition towards a more sustainable, efficient, and less polluted future. Heat pumps are considered more suitable than electrical heaters or fossil-fueled boilers; however, common refrigerants cause ozone layer depletion, which exacerbates the greenhouse effect. Natural refrigerants, such as CO2, perform comparably and even better than hydrofluorocarbons while minimizing the negative aspects. This study presents a model of a water-heater CO2 transcritical heat pump in a configuration that increases the overall coefficient of performance (COP) by introducing thermal energy storage (TES). The thermodynamic cycle was divided into two separate phases. After heating the TES (charging), warm water was used as the heating fluid in the evaporator to increase the evaporation temperature and pressure of CO2, which reduced the work of the compressor. As the water temperature decreased progressively, the discharge cycles improved the total COP. The case study focuses on dairy processes and suggests a more straightforward and cheaper method to improve cycle efficiency than the current processes, such as ejector-expansion systems or double compression.

Supercritical CO2 mixture Brayton cycle with floating critical points for concentrating solar power application: Concept and thermodynamic analysis

A novel concept of supercritical Brayton cycle with floating critical points is proposed to cope with the rapid deterioration of cycle performance at high heat sink temperatures. By coupling the power cycle with a distillation based composition tuning subsystem, the circulating mixture composition in the cycle can be altered dynamically to obtain the desired critical points at various ambient temperatures, thereby leading to a better match between the cold-end parameters of power cycle and the heat sink conditions. A thermodynamic model is established for the proposed novel system based on a recompression layout with single stage reheating. Four binary mixtures, including CO2–H2S, CO2–Butane, CO2–Isobutane, and CO2–Butene are employed. Simulation results indicate that an increasingly larger improvement in cycle efficiency compared to the pure CO2 cycle can be obtained as the ambient temperature increases from 20 to 50 °C. When the composition tuning process is not considered, the CO2–H2S cycle shows the best performance among the four mixture cycles, with an improvement between 1.3% and 7.4% being achieved over the entire temperature range. Several feasible schemes for composition tuning subsystem are proposed and compared in terms of energy consumption. Finally, the hourly efficiencies of the integrated system are evaluated based on typical daily weather conditions, with a piecewise control strategy for critical points. It is found that although the use of CO2–H2S causes relatively high energy consumption for distillation, a significant improvement in the daily average thermal efficiency of the integrated system can still be achieved, with efficiency being increased by 3.5%-5.1% in comparison with pure CO2 cycle on hotter days.

Thermo-economic analysis and multi-objective optimization of supercritical Brayton cycles with CO2-based mixtures

• A thermo-economic analysis of Brayton cycle with CO 2 -based mixtures is performed. • The Pareto fronts of 3 working fluids are obtained by multi-objective optimization. • SCO 2 -Xe cycle shows the best thermodynamic performance compared to others. • CO 2 -Kr shows the potential for development through thermo-economic analysis. The Supercritical carbon dioxide recompression Brayton cycle (SCO 2 RBC) is regarded as one of the most promising thermal power conversion systems and mixing other gases with CO 2 to change the critical point represents an effective strategy to improve the performance of the Brayton cycle. In this work, the feasibility of using xenon and krypton as additives to the SCO 2 RBC is investigated via thermodynamic analysis. The effects of the key parameters on the thermo-economic performance of recompression Brayton cycles using CO 2 , CO 2 /xenon (mass fraction 0.55/0.45) and CO 2 /krypton (mass fraction 0.755/0.245) as working fluids are discussed by means of parametric analysis. A non-dominated sorting genetic algorithm is employed to simultaneously optimize the cycle exergy efficiency and the levelized cost of energy. The total thermal conductance, the main compressor outlet pressure, pressure ratio and split ratio are selected as the decision variables. The optimum solutions with their corresponding decision variables are selected from the Pareto frontiers. The results show that the supercritical CO 2 /krypton cycle has the huge development potential. Specifically, the optimum exergy efficiencies of Brayton cycles using CO 2 , CO 2 /xenon and CO 2 /krypton as working fluids are 0.585, 0.646 and 0.664. The addition of xenon can improve cycle efficiency up to 12.08 % higher than SCO 2 RBC while the levelized cost of energy also increased by 10.04%. The cycle efficiency of the supercritical CO 2 /krypton cycle is 9.44% higher while the cost is 2.49% higher compared to the SCO 2 RBC. There are suitable decision variables to make the cost of the cycle using CO 2 / krypton lower than that using CO 2 while achieving the same exergy efficiency when the required exergy efficiency is in the range of 0.60-0.62. The exergy destruction distribution of Brayton cycle illustrates that high temperature recuperator is the highest exergy destruction component and using CO 2 -based binary mixtures can reduce the total exergy destruction effectively.

Study on Forced Flow Cooling of Superconducting Magnet for Compact Synchrotron

To cool the superconducting magnets of a compact synchrotron, a forced flow cooling system with cooling capacity of 10.16 watts at 4.5 K based on GM/J-T concept is designed. As a preliminary research of the cooling system, a research device is built. The research device shares a similar flow diagram with the cooling system, but the GM cryocooler is replaced by a cold helium pre-cooling section to searching the system parameters and improve the system response speed. For the research device, the pre-cooling section consumes 0.332 g/s of liquid helium and generates 0.565 g/s of liquid helium after throttling by the J-T valve. To elucidate the performance of the heat exchangers based on enthalpy balance, the cycle point parameters are picked. The results of the thermomechanical analysis show that the temperature, the temperature difference and the pressure of helium at inlet of the JTH section how to affect the cooling capacity of the system. The heat leakage analysis, the design parameters optimization and the cycle efficiency improvement in the system are also discussed in this paper.

Development of a Mathematical Model for Solid Fuel Gasification and Its Sensitivity Analysis

Within the framework of this study, a brief review of the gasification technology was carried out, the best types of blowing agents and gasification methods used in terms of efficiency and environmental safety were identified, and a mathematical model of a steam–oxygen gasifier was developed in the MS Excel software package. The authors paid special attention to the consideration of the effect of changing the input parameters of the syngas, such as the temperature and relative mass flow rate of steam and oxygen, on the heat of the combustion of the produced syngas. As a result of the research, methods for increasing the heat of the combustion of the syngas and the conditions for using the described methods were formulated. The work also revealed the optimal ratios of the blowing agents and solid fuel supplied for gasification and presented the output parameters of the produced generator gas, including the heat of combustion of the gas, the gas temperature, and the gasification efficiency. Computer simulation models of the gasifier and gasification process were the basis for the analysis of a combined cycle (CC) facility with an integrated solid fuel gasifier. The heat flow thermodynamic analysis shows that the gasification steam bleeding from the turbine is the best solution for the improvement of cycle efficiency.

Optical, thermal and thermo-mechanical model for a larger-aperture parabolic trough concentrator system consisting of a novel flat secondary reflector and an improved absorber tube

Using a larger-aperture parabolic trough concentrator can reduce cost and improve cycle efficiency. At present, the maximum output temperature of the typical parabolic trough concentrator systems is lower than the set 580 °C. Here a novel flat reflector was incorporated into the absorber tube to improve the intercept factor and optical efficiency and hence the overall performance. At the same time, the optimal diameter of the collector tube was also researched to meet the outlet temperature exceeding the set 580 °C and the absorber tube’s maximum temperature below 600 °C. A large-aperture parabolic trough concentrator (8 m aperture wide with 80° half rim-angle) is used as a case to demonstrate the effect of a flat reflector and advantages of the optimal absorber tube. Results obtained through simulations show that, in the large-aperture parabolic trough concentrator system with the flat reflector, the optimal diameter of the absorber tube was 70 mm for this design reaching 75% optical efficiency and 60.8% thermal efficiency at DNI = 1000 W/m2 and 53% daily average thermal efficiency for DNI = 400–1000 W/m2. Through adding a flat reflector, the optical efficiency and the uniformity were increased by 4–12% and 2–22% respectively, but the benefit gradually decreases with increasing the absorber tube diameter. To meet the requirement of outlet temperature more than 580 °C and absorber tube’s maximum temperature below 600 °C and reduce the thermal strain, a scheme of adjusting flow rate was incorporated.

Onset of heat transfer deterioration caused by pseudo-boiling in CO2 laminar boundary layers

In propulsion and power systems, operating pressures are increasing to improve cycle efficiency, often reaching conditions that approach or exceed the thermodynamic critical pressure of the working fluid. A known phenomenon that occurs at supercritical pressures is supercritical heat transfer deterioration (HTD). Although this phenomenon is well studied, there is still debate about the underlying physics. This paper studies pseudo boiling, the supercritical liquid–gas phase transition, as a possible mechanism that causes HTD, by means of computational fluid dynamics (CFD), boundary layer correlations, and thermodynamic analysis. To this end, we focus on forced laminar convection of supercritical CO2 over a flat plate, excluding alternative factors such as buoyancy or turbulence. In order to accurately represent the strongly non-linear near-critical fluid properties, we use tiny neural networks (TNN) in an interpretable machine learning approach. For use in CFD, TNN combine reference quality accuracy from look-up tables with the memory footprint of equations of state and curve fits. We demonstrate that pseudo boiling theory accurately indicates pressure and temperature regions susceptible to HTD. The simulations show the distinctive boiling curve, with a peak critical heatflux and a Leidenfrost minimum, demonstrating that pseudo boiling is, indeed, a sufficient physical mechanism to cause this effect, in absence of turbulence or boyancy. During HTD, we observe the formation of a supercritical gas film at the wall, with low density, low thermal conductivity, a high compressibility factor, and low viscosity. The boundary of the gaseous film is characterized by a peak in heat capacity, indicative of pseudo boiling, and a sudden transition to a dense supercritical liquid state. This suggests that the subcritical boiling crisis does have a direct representation at supercritical pressures through pseudo boiling, and that pseudo film boiling is a sufficient mechanism to cause HTD.

Parametric study of free-piston engine combustion under different compression ratios

Since free-piston thermal engines not bound by the motion of the rotating engine crankshaft as in existing engines, they are being examined as an option to conservative technologies by several scientific organisations around the world. In applications such as electrical energy production and hydrodynamic power production, free-piston engines are employed as an alternative to traditional techniques. Increased compression ratios allow improved cycle efficiency and raise in-cylinder temperatures, raising mechanical stress, pressures, and heat transfer inefficiencies, and these successfully calculate the optimum compression ratio. This research offers different in-cylinder flows that are computed using computational fluid dynamics for compression ratios ranging from 7 to 15, and estimates were performed to discover the ideal compression ratio for the finest engine firm profitability and characteristics.

Capacity-dependent configurations of S–CO2 coal-fired boiler by overall analysis with a unified model

Supercritical carbon dioxide (S–CO2) coal-fired power systems have been widely investigated due to its high efficiency and compactness. These power systems can range between 50 and 1000 MW and thus the corresponding boilers actually operate under dramatically different conditions. It is necessary to investigate the inherent relationship among different capacity S–CO2 boilers, which not only bring new theoretical understandings but also help improve practical designs. In this work, a unified model is proposed to investigate both thermal-hydraulic and thermodynamic performance. Furthermore, it is applied to evaluate the relationship between power capacity and boiler configuration. It is shown that small capacity furnaces suffer from overheating in the furnace wall and the optimized cooling wall is suggested to tackle this issue. Moreover, it is found that high pressure drop in the cooling wall of large capacity furnaces results in significant penalty on the cycle efficiency. For the popular 1000 MW boiler, an optimized configuration is suggested by moving the second reheating into the horizontal flue of boiler, achieving a lower pressure drop and an improved cycle efficiency from 35.4% to 50.09%. Finally, three recommendations for the capacity-dependent configurations are presented.

Investigation on the effect of mixtures physical properties on cycle efficiency in the CO2-based binary mixtures Brayton cycle

S–CO2 recompression Brayton cycle has attracted much attention with the development of compact heat exchangers. However, the critical point of CO2 is a constraint on the minimum cycle condition. Mixing additional gas into pure CO2 working fluids could reduce the critical point and bring improvement potential of the cycle efficiency. However it is not determined what kind of mixtures can improve cycle efficiency; and there is no criterion for choosing proper binary mixtures components. In this study, the feasibility of using mixtures physical properties as criterion to judge if CO2-based binary mixtures can improve cycle efficiency is discussed. The detail parametric study is performed to reveal the effects of mixtures physical properties on the performance of cycle efficiency. According to the analysis results achieved, three factors have been identified to have significant contributions to the improvement of the cycle efficiency: the entropy difference of a certain temperature range of CO2-based binary mixtures should be larger than that of S–CO2, the specific heat capacity within the temperature range of the compressor and cycle minimum pressure should be higher, and the maximum enthalpy of CO2-based binary mixtures should be lower than that of S–CO2.

Role of electrolyte in stabilizing hard carbon as an anode for rechargeable sodium-ion batteries with long cycle life

Hard carbon (HC) is an attractive anode material for grid-level sodium-ion batteries (NIBs) due to the widespread availability of carbon, its high specific capacity, and low electrochemical working potential. However, the issues of low first cycle Coulombic efficiency and poor rate performance of HC need to be addressed for it to become a practical long-life solution for NIBs. These drawbacks appear to be electrolyte dependent, since ether-based electrolytes can largely improve the performance compared with carbonate electrolytes. An explanation for the mechanism behind these performance differences is critical for the rational design of highly reversible sodium storage. Combining gas chromatography, Raman spectroscopy, cryogenic transmission electron microscopy, and X-ray photoelectron spectroscopy, this work demonstrates that the solid electrolyte interphase (SEI) is the key difference between ether- and carbonated-based electrolyte, which determines the charge transfer kinetics and the extent of parasitic reactions. Although both electrolytes show no residual sodium stored in the HC bulk structure, the uniform and conformal SEI formed by the ether-based electrolyte enables improved cycle efficiency and rate performance. These findings highlight a pathway to achieve long-life grid-level NIBs using HC anodes through interfacial engineering.