Comparison of dynamic lithium-ion batteries with fuel cells

With the continuous development of the economy, the global car ownership is increasing. The large consumption of fuel makes the energy crisis become more and more serious, and the automobile exhaust emission makes the environmental pollution become more and more serious. Therefore, in recent years, many automobile enterprises focus on the research of new energy vehicles, trying to replace the use of internal combustion engine. The development of new energy vehicles is getting faster and faster. Electric vehicles represented by Tesla enjoy great popularity, while fuel cell vehicles represented by Toyota Mirai have also attracted much attention recently. Lithium-ion batteries and fuel cells, as the core components of electric vehicles and fuel cell vehicles, play a decisive role in their development.

Dynamic lithium-ion batteries have been commercialized to some extent. In 2016, domestic power battery shipments were 28GWh, lithium iron phosphate battery was still the main market share of 20GWh, ternary material battery was only 6.3gwh, the others were lithium manganate, lithium titanate, nimh battery, supercapacitor and other materials battery. The number of domestic lithium battery manufacturers is expected to be around 150, and the top three in terms of shipments are byd, CATL and watma. Fuel cells are not yet commercially available. Only a few related enterprises, such as Beijing ehuatong, xinyuan power, saic motor and wuhan institute of technology, are involved.

Below, we will introduce the characteristics of lithium ion battery and fuel cell from the aspects of working principle, performance and safety, so as to deepen our understanding and understanding of them.

First, the working principle of lithium ion battery and fuel cell

Lithium ion battery is a kind of energy storage device. The commonly used lithium ion battery can be divided into lithium iron phosphate (LFP) battery, terncm battery and lithium manganate (LMO) battery according to the positive electrode materials. Take lithium iron phosphate battery as an example: when discharging, iron phosphate in the positive electrode and lithium ions transferred from the negative electrode through the electrolyte and electrons transferred from the external circuit combine to form lithium iron phosphate. Lithium embedded in the graphite layer of the negative electrode discharges and becomes lithium ions and electrons transferred to the positive electrode through the electrolyte and external circuit respectively.

The fuel cell is essentially a generator whose fuel and oxidant are converted into electricity by electrochemical reaction without combustion. Therefore, fuel cells are not limited by carnot cycle, and energy conversion efficiency is high. Fuel cells can be as efficient as 60 percent when used as a power conversion unit, and as efficient as 80 percent when used as a cogeneration unit. Fuel cells are divided into basic fuel cell (AFC), phosphoric acid fuel cell (PAFC), solid oxide fuel cell (SOFC), molten carbonate fuel cell (MCFC) and proton exchange membrane fuel cell (PEMFC) according to their electrolytes. Different types of fuel cells application field in different environments, and the use of the use of the proton exchange membrane fuel cell temperature range for the room temperature to 80 ℃ or so, at present in the fuel cell car using basic are of this type. In a proton exchange membrane fuel cell (pemfc), for example, when generating electricity, the positive oxygen and the hydrogen ion from the negative electrode and the electrons from the external circuit are combined to form water, and the negative hydrogen atom loses electrons and becomes hydrogen ion and electrons are transferred to the positive electrode through the electrolyte and the external circuit respectively.

Ii. Main technical characteristics of lithium-ion batteries and fuel cells

performance

The reversible electromotive force of a lithium-ion battery is determined by the chemical reactions that occur within the fuel cell. For an electrochemical reaction, its reversible emf can be calculated by the formula as follows:

The change in gibbs free energy of the electrochemical reaction in the standard state reflects the thermodynamic possibility of the electrochemical reaction, which is determined by the nature of the reaction itself, the concentration of reactants and products, and the reaction temperature. N is the number of electrons transferred per mole of reactant, F is Faraday's constant. Under the standard condition, the reversible emf of the fuel cell is about 1.25V. For lithium ion batteries, the reversible electromotive force is constantly changing due to the constant change in the structure of positive and negative electrode materials during the reaction process. The reversible electromotive force of the battery has a corresponding relationship with the degree of reaction. Therefore, the charged state of the battery can be judged by measuring OCV according to the ocv-soc curve. The performance curves of fuel cells and lithium-ion batteries in actual use are shown in table 2.

The energy density

Electric cars are powered entirely by batteries, with more emphasis on the ability to continue driving after charging, thus paying more attention to the energy density of the battery. The improvement of energy density of lithium-ion batteries is limited by the theoretical bottleneck of battery materials. At present, lithium iron phosphate (LFP) is the main positive electrode material of domestic electric steam-powered power batteries, while graphite is still the main negative electrode material, whose specific energy is about 90~140Wh/kg. Fuel cells, on the other hand, are power-generating devices with far higher energy densities than lithium-ion batteries. In terms of the driving range of the whole vehicle directly corresponding to the energy density, the driving range of Tesla, the top luxury electric car, has just reached 500km. The typical fuel cell vehicles represented by Toyota Mirai and hyundai ix35 all have a continuous driving range of over 500km. So fuel cells are better at energy density than lithium-ion batteries.

life

The performance of both fuel cells and lithium-ion batteries deteriorates with increasing battery life. In addition, the starting and stopping, acceleration and deceleration conditions of the car account for a large part of the total working conditions, which makes the range of working current of the battery wide and the rate of current change very large, which will undoubtedly shorten the battery life. Therefore, it is one of the key problems to study the life of power fuel cell and lithium ion battery.

The cost of

At present, the cost of domestic lithium ion battery system is about 1800 yuan /kWh, and the cost of fuel cell stack (excluding fuel system and other accessories in the system) is about 5000 yuan /kW. For an ordinary car, let's say it is an electric car, with a power configuration of 60kWh (BYDE6 configuration of 60kWh), its cost is 96,000 yuan. If it is a fuel cell car, the power configuration is 100kW(Toyota Mirai configuration is 114kWh), and the cost of the electric reactor is around 500,000 yuan.

The cost of fuel cell is much higher than lithium ion battery, which is the bottleneck of fuel cell development. It is generally believed that the high cost of fuel cells is mainly due to the use of precious metal Pt, but the actual cost of Pt is calculated as follows: the current level of higher Pt load is 0.4mg/cm2, and its electrical performance level is 1600Ma@0.6V/cm2, that is, 0.96w /cm2. For a 100kW fuel cell system, the Pt content used is 41.67g. The price of Pt is calculated according to 500 yuan /g, and the cost of using Pt is 41.67*500=20833 yuan. The cost of Pt is only about 4% of the total cost of a 100kW fuel cell reactor that costs more than 500,000 yuan. The cost of fuel cells is mainly due to the immaturity of materials and system technologies. However, with the development of commercialization, the cost of fuel cells is bound to drop dramatically.

Safety and regulations

The safety of power battery is the first problem to be considered and solved in the development of electric vehicle. To improve the safety of dynamic lithium ion batteries, a series of technical measures should be established, including material, battery and key components, system security and so on. With the large scale and grouping use of single battery, the safety of dynamic lithium ion battery system is facing new challenges. The fuel for fuel cells is hydrogen, which is flammable and explosive, so there is widespread concern about its safety. In fact, hydrogen is no less safe than gasoline and natural gas.

Single-cell fuel cells have fewer safety features than lithium-ion cells. The system integration level fuel cell system is more complex than the lithium ion battery system. With the use of combustible gas hydrogen, there is an additional leakage protection design for hydrogen. Because of the need to prevent the impact of inadequate proton exchange membrane wetting, it is necessary to monitor the internal resistance to monitor the changes in the internal humidity. The safety design of fuel cells and lithium-ion batteries is shown in table 4.

The reductant and oxidant of the dynamic lithium-ion battery are stored in the same device, separated by a membrane only a micron thick, while the reductant and oxidant of the fuel cell are placed outside the battery. In principle, fuel cells are safer than lithium-ion ones. Through a series of safety precautions, the safety of both batteries is acceptable.

In order to ensure the safety of power batteries, the state has formulated a series of standards for power lithium-ion batteries and fuel cells, so as to ensure the safety and reliability of power batteries. As shown in table 5, fuel cells have fewer standards than lithium ion batteries, with earlier release time, and their compliance with the status quo is not as good as that of lithium ion batteries. There is a corresponding specification for shaping test of electric vehicles, GB/ t18388-2005 specification for shaping test of electric vehicles, and the specification for fuel cell vehicle, as a necessary standard for shaping new energy vehicle products in the automotive industry, urgently needs to be launched.

Iii. Prospects and prospects

Overall, fuel cells are superior to lithium ion batteries in terms of energy density, life and safety. In terms of cost, fuel cells can't compete with lithium-ion batteries. At present, the key technologies of lithium ion battery are energy density enhancement, safety, thermal management, system integration and optimization control, etc. The key technologies of fuel cell are durability, cold start, system integration and optimal control, etc. Both fuel cells and lithium-ion batteries have plenty of room for improvement. For lithium ion battery, if its energy density can be further improved, cycle life can be longer, it is also an excellent driving energy. If the cost of fuel cells can be reduced, they can truly serve as an alternative to gasoline/diesel fuel. The improvement of energy density is faced with the bottleneck of basic subject field, and it is very rare to have a qualitative improvement; The cost reduction can be solved through commercialization. So in the short term, lithium-ion batteries are better than fuel cells; In the long term, fuel cells are more promising than lithium-ion batteries.

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