A comparison study between the lithium sulphur battery and fuel cell in new energy vehicle applications

In recent years, the development of new energy vehicles has been rapid to solve the emerging energy depletion and carbon emission problems. Lithium-sulfur batteries and fuel batteries have become the most competitive two development directions in the new generation of power batteries for electric vehicle applications. Lithium-sulfur battery has a high specific capacity, low production cost and is environmentally friendly. However, some problems include low coulomb efficiency and short life span in the process of lithium-sulfur battery production. Fuel cells are also environmentally friendly, energy-efficient and drive efficiently. But fuel cells are extremely expensive, and their longevity problems regarding the membranes and electrode materials and supporting infrastructure, such as the hydrogen and oxygen storage obstacles, must also be overcome. This paper focuses on analyzing the two new energy vehicle power resource principles, advantages and disadvantages of two kinds of typical batteries, the current development bottleneck, and respectively put forward suggestions for future development.


Introduction
With the development of sustainable energy, the development of renewable energy gradually increases, but due to the consideration of electricity demand, electrochemical energy storage is still an efficient, convenient and stable way.In recent years, with the development of aerospace, unmanned aerial vehicles and new energy vehicles, people's requirements for batteries have gradually increased, and the existing commercial lithium-ion batteries have been unable to meet the needs of future development.Lithiumsulfur batteries (LSBs) have attracted a lot of attention for their high theoretical energy.LSB is a battery made of metal lithium cathode and composite sulfur cathode.Lithium battery has a high theoretical specific capacity and high energy density, abundant sulfur reserves and low price.The non-memory effect of LSB can also ensure its energy has a continuous and stable output.
In recent years, many teams have designed and optimized lithium-sulfur batteries to improve their energy density and cycle stability, such as increasing a load of sulfur on the cathode and nitrogen doping.The electrolyte was optimized to improve its electrochemical performance.The anode material was optimized, and the SEI film was synthesized to protect the anode and reduce the shuttle effect and the generation of lithium dendrites [1].
Hydrogen energy and fuel cells have a broad application prospect and have great potential in the fields of automobiles, spacecraft and ships, but fuel cells are also faced with cost and durability problems.Research shows that fuel cells to reduce carbon dioxide emissions and reduce the application of fossil energy has made a great contribution.With the development of technology, the cost has been gradually controlled, and recent work has also led to significant improvements in fuel cell components.
In this paper, LSB and fuel cells are reviewed, respectively.Firstly, the mechanism of lithium-sulfur battery is briefly described, and then the advantages and disadvantages of lithium-sulfur battery are discussed.Then, the methods to reduce dendritic generation and the improvement of electrolytes to slow down the shuttle effect are reviewed.Finally, the two are compared, and the future development direction has prospected.

Working principle of LSB
In the discharge process of LSB, the elemental sulfur in the ring S8 is removed through the ring opening reaction and reacts with Li.The long-chain Li2S8 is converted to the short-chain Li2S.Two discharge platforms accompany the process.The high potential discharge platform is 2.45V-2.1V.In this process, a large number of S8 is converted into S4 2-, while the low potential discharge is 2.1V-1.7V, in which a large number of S4 2-is converted into S2 2-and S 2-.And different conversion degree also corresponds to different capacitance [2].The discharge reaction equation is shown in figure 1: The whole discharge process is as follows: S8+16Li→8Li2S (1) Positive reduction reaction:

Advantages and disadvantages of LSB
2.2.1.Advantages of LSB.Firstly, LSB has a high theoretical specific capacity (1675 mAh.g -1 ) and energy density (2600 Wh. kg -1 ) because of the high charge-discharge capability of sulfur elements.
During the discharge process, each sulfur atom can transfer two electrons and more electrons than metal ions.Further, the thermal power performance of LSB is very stable because the chemical properties of the sulfur element are stable, and it is a coronal structure composed of eight sulfur atoms.In addition, the cost of LSB is low.Sulfur elements are abundant in nature, and their cost is relatively low [3].

Disadvantages of LSB.
Firstly, the sulfur resistance of LSB is high up to 5 × 10 -30 S. cm -1 , and changes in the structure and shape of the intermediate lead to instability of the contact between the sulfur electrodes.The shuttle effect of polysulfide compounds in LSB causes the loss of active substances and the capacity and the Coulomb efficiency to decrease rapidly, shortening battery life [4].During the charging and discharging process in LSB, the volume of the electrodes expands up to 80%, which results in less contact between the collector and the electrodes, and the specific capacity of the batteries decreases rapidly [5].Additionally, serious self-discharge of sulfur may also be the cause of battery capacity decline in LSB.And some LSB deposited on the negative Li-ion pole are liable to form lithium dendrites, which cause safety problems for the batteries.Finally, insulated Li2S is come into being on the surface of the lithium negative in LSB, which increases the impedance of the electrodes, resulting in poor cycle and capacity decay of the batteries [6].

Challenges for LSB
Owing to the uneven distribution of the electric field, the dense charge deposits at the bulges of lithiumsulfur batteries are faster during the deposition process, which results in the growth of lithium dendrites [7].The boost of lithium dendrites breaks through the diaphragm, causing the short circuit of positive and negative electrodes to produce local high temperatures, which causes the battery system of ether electrolytes to explode and fire [8].

Future research prospects of LSB
Firstly, Inhibiting the boost of lithium dendrites and solving root cause problems Inhibiting the boost of lithium dendrites is helpful to promote the recycling function of batteries, the safety of batteries and decelerate the decay of battery capacity.Currently, the main methods are adding physical, or chemical resist film on the surface of the lithium negative, improving the lithium negative's matrix structure, and protecting the lithium negative in situ by using additives during the cycle.This method can improve the surface flexibility of the electrodes and relieve the local pressure caused by dendrites and inhibit dendrite growth by affecting the concentration distribution of lithium ions.The physical protective film is a commonly used method, which is simple and easy to use but requires high ionic conductivity and mechanical strength of the film [9].Secondly, regulating electrolyte composition to make the lithium surface more stable.The high reactivity between lithium metal and electrolyte always covers a solid electrolyte interface (SEI) membrane.One way to protect the negative lithium electrode is to construct a highly stable artificial SEI membrane [2,10].Electrolyte additives are a good method, which eliminates the steps of electrode pretreatment compared to surface protection and is simpler and easier to implement.LiF can be introduced into SEI membranes because of its good lithophile properties and uniform deposition of lithium ions on the surface of the guide electrodes.The lithium surface can also be stabilized by increasing the organic components in SEI through changes in the composition of the electrolyte.In addition, the additive LiNO3 used in combination with lithium-sulfur electrolytes (ether electrolytes) can also prevent lithium deposition and enhance the stability of the interface due to its Li-N bond [11].
Finally, the development of flame-retardant electrolytes.Optimizing the composition of the electrolyte, such as replacing the flammable hazardous components in the electrolyte, or increasing the flame retardant additives such as phosphorus or halogen molecules, can reduce the risk of battery ignition.The addition of flame retardant in the electrolyte can prevent the battery from igniting at high temperatures during a short circuit.The selection of flame retardant needs to be considered in combination with its electrochemical window [12].
Though the evolution of lithium-sulfur batteries has increased some hope for evidence-based deterioration and high energy storage, most of them are focused on experimental research, but there are few theoretical studies.The shortcomings and challenges of LSB indicate that the material and structure of LSB still have numerous of difficulties to surmount, and it is important to establish a complete theoretical system for the study of LSB [13].

Basic principles of fuel cells
The fuel cell is primarily made up of three sections: a positive electrode, a negative electrode and an electrolyte.The principle is that hydrogen oxidation and oxygen reduction reactions occur respectively on both sides of the electrolyte diaphragm.In operation, the oxidized gas is sent to the cathode, and the fuel goes to the anode, which provides electricity through the hydrogen and oxidation reactions.The process is as follows: Hydrogen enters the battery's anode, where it reacts as a catalyst, releasing electrons into positively charged hydrogen ions, which travel through the electrolyte to the cathode.Electrons enter the circuit and form a current that releases electricity.A catalyst in the cathode causes hydrogen ions to combine with oxygen in the air to form water, the only by-product of the cell's chemical reaction [14].

Fuel cell systems
The composition of the hydrogen fuel cell is relatively complex.In addition to the reactor, it includes fuel supply, electricity management, hydrothermal management and control subsystems, etc.The main system components are composed of an air compressor, hydrogen bottle and other components, which, together with the reactor, constitute the power generation system of the fuel cell.This article chooses to introduce the component section of the fuel cell.

Membrane electrode assembly.
The membrane electrode (MEA) is the kernel component of the hydrogen fuel cell system, is the main area of the occurrence of electrochemical reaction, is to undertake proton and electron transfer material, generally composed of the cathode diffusion layer and cathode catalyst layer, electrolyte membrane, anode gas diffusion layer and the anode catalyst layer.At present, there have been mature research results on the first generation (GDE) and the second generation (CCM) in China [14].The first generation of technology is hot pressing, and the second generation of CCM is widely used, which has the advantages of high platinum utilization and durability.At present, the third generation is under development.The ordered membrane electrode technology can make MEA has the maximum reactive area and pore connectivity, so it can improve the utilization of catalyst, which is now a hot research field.

Proton exchange membrane (PEM).
The Proton exchange membrane is a kind of solid polymer membrane, the core component of the proton exchange membrane fuel cell.It acts the role of isolating the reactants at different electrodes, conducting protons and isolating electrons.Its main types are divided into perfluoro sulfonic acid proton exchange membrane, partially fluorinated proton exchange membrane, and fluorine-free proton exchange membrane.
The structure of the perfluoro sulfonic acid proton exchange membrane is a carbon-fluorine structure composed of polytetrafluoroethylene as the main chain.Fluorine atoms with large radius can provide a protective layer for the vicinity of the carbon-carbon bond.Because of the relatively high bond energy of the carbon-fluorine bond, the chemical stability and mechanical strength of the proton exchange membrane are good.At the same time, the sulfonic acid base group is not difficult to gather together to shape ion-rich regions, so it can form a proton-conducting channel to promote the proton conductivity of PEM [15].
Partially fluorinated proton exchange membranes are made using partially substituted fluoride instead of perfluorosulfonic acid resin or mixing fluoride with inorganic matter.The oxidation environment greatly influences the membrane's properties, so the perfluorinated main chain with good stability should be used, with the sulfonic acid group as the proton exchange group.
Fluorine-free proton exchange membrane is a polymer membrane composed of carbon and hydrogen elements characterized by stable performance, low cost and high conductivity.Fluorine-free proton exchange membrane is mainly made of aromatic polymer containing benzene ring structure, in which sulfonic acid group is obtained by sulfonation, such as sulfonated polyaromatic ether sulfone (SPES), sulfonated polyimide (SPI), etc [16].

Electrocatalyst.
One of the materials at the core of the fuel cell, the catalyst, reduces the reaction's activation energy and promotes the redox process at the electrode.Currently, the three common catalysts are platinum, low platinum, and non-platinum catalyst.The most important commercial catalyst is Pt/C, a supported catalyst composed of Pt nanoparticles dispersed onto a carbon powder carrier.At present, the preparation methods of platinum-based and palladium-based noble metal hollow catalysts for direct alcohol fuel cells include the template method, displacement reaction method and template-free method.Studies have shown that Pt catalyst with special morphology has high catalytic activity, stability, and heat.It is a potential electrocatalyst in many fields, such as material preparation, activity characterization and mechanism exploration.Besides Pt/C, cathode materials can be catalyzed by other binary alloys [17].Core-shell catalysts take the active component of the catalyst as the shell and the transition metal element as the core, which has a high utilization rate of precious metals and catalytic activity of oxygen reduction [17].

Gaseous diffusion layer.
The main role of the gas diffusion layer in a fuel cell reactor is to support the catalyst and provide gas, proton, electron and water channels for electrode reaction.It has good electrical conductivity, high chemical stability, and thermal stability.Common also should have appropriate pore structure, flexibility, surface smoothness, and high mechanical strength.These properties are critical to the electrocatalytic activity of the catalyst layer and the reactor energy conversion and are the embodiment of GDL structure and material properties.
Current studies have shown that the addition of adhesives and compression deformation can affect the permeability of the gas diffusion layer under certain conditions, thus slowing down the flow velocity.Electrochemical corrosion also damages the water management of the gas diffusion layer, and the destruction of its microporous layer structure weakens its water management ability.With the requirement of carbon peak, how to reduce carbon has become an important topic [18].At present, the mature types of MPL carbon materials are still limited, but the application of low-dimensional carbon materials and the modification of common carbon materials and other new ideas show that there is still numerous of room for development in the domain of carbon materials.The runner's improvement can effectively remove the water transport in the runner to enhance gas transmission and weaken flooding.

Bipolar plate.
The kernel structural component of the reactor is the bipolar plate.Its role is to make the distribution of gas more uniform and can achieve the effect of drainage, heat conduction, and conduction.The bipolar plate is a very important part, and its flow structure greatly impacts the utilization rate of reactive substances and battery performance.The most commonly used materials for bipolar plates are hard carbon plates, composite bipolar plates, and metal bipolar plates.Graphite BPs are widely used because of their excellent performance, but their high cost and harsh application conditions; therefore, low-cost and easy-to-use metal BPs have become a development hotspot.The problem of metal corrosion also urgent to be solved.Studies have shown that Cr2AlC coating can significantly promote the corrosion resistance of stainless steel bipolar plates, so the preparation of many coatings is also the focus of current research [18].In addition, there are also problems of insufficient air tightness and electrical conductivity of the bipolar plate.Impregnation or optimization of the preparation process can enhance the air tightness of the bipolar plate, but it may not meet the expectations, thus increasing the production cycle.To optimize the conductivity of the composite bipolar plate, the interface contact resistance and volume resistance should be reduced.

Comparison between two batteries
Fuel cells, as longer-lived forms, naturally have significant advantages.The hydrogen fuel cell is a device that converts the chemical energy of hydrogen into electric energy directly, efficiently and pollution-free.It has the advantages of high energy density, high energy conversion efficiency and pollution-free.Therefore, new energy vehicles using fuel cells as power batteries have the advantages of long driving range, short fuel filling time, fuel extraction from water, etc.At present, fuel cell vehicles sold in the market after hydrogenation (3~5 min), the mileage is more than 500 km, and the performance is far better than lithium-ion battery electric vehicles LSB, by contrast, take more charging time, reducing the overall efficiency of the vehicle.However, its commercial application is seriously hindered by the low conductivity of sulfur ions, polysulfide shuttle effect and the growth of lithium dendrite.Lithium metal storage also restricts the development of LSB.Whether optimizing SEI film or using lithium alloy, cost should also be considered in the mass production of lithium-sulfur batteries.
But the cost of fuel cell new energy is higher, which is one of the main factors hindering the development of new energy vehicles.The fuel in the batteries uses platinum, a metal that costs more, as a catalyst for chemical reactions.Second, fuel cells don't last long.Platinum catalysts are gradually consumed in the use process, so the life of the whole fuel cell is gradually shortened, indirectly increasing the cost of fuel cell new energy vehicles.As a reliable and convenient energy storage method, electrochemical energy storage can ensure the balance of power supply.Although all kinds of lithium batteries have shortcomings, such as poor safety and harsh production conditions, they can provide stable energy storage conditions in the case of overproduction.This benefits from the high theoretical specific capacity (1675 mAh/g) and high energy density (2600 Wh/kg) of lithium batteries to make up for the shortcomings of lithium batteries.Moreover, the rich source of sulfur can provide a stable supply of raw materials.In addition, lithium-sulfur batteries have no memory effect.
In conclusion, among the many new generations of high-energy density power batteries, the new generation of lithium-ion batteries is the most expected to be commercialized in the short term.Although fuel cell vehicle has a certain application field and a good application prospect, the key technical bottleneck that is difficult to solve in the short term will always restrict its commercialization.

Conclusion
Although the further promotion of new energy vehicles still faces many problems, but from the perspective of energy, environment, national strategy and other analysis, we can know that the broad prospect of new energy vehicles is beyond doubt.Lithium-sulfur batteries and fuel cells, the two most promising batteries, each have advantages and disadvantages.The low conductivity of sulfur ion, polysulfide shuttle effect, lithium dendrite growth and lithium metal storage also restrict the development of lithium-sulfur batteries, but their high theoretical specific capacity and energy density are very attractive.Fuel cells have also run into bottlenecks due to the high cost of catalysts and other factors, but they can be recharged quickly, making the car superior in terms of endurance.
Overall, the commercial availability of lithium-sulfur batteries and fuel cells requires further scientific and technological development and more research, with emphasis on their overall design.In future development, the two can complement each other.Satisfactory and reliable lithium-sulfur batteries require continuous optimization of sulfur cathodes, lithium anodes and electrolytes to meet market demands.In addition, simple, sustainable and scalable manufacturing methods for lithium-sulfur battery components are needed, as cost-effectiveness is important for industrial-scale production.It is believed that lithium-sulfur battery technology can be applied to transportation and large-scale grid energy storage shortly.Fuel cells also need to control the production cost of fuel cells, establish a unified