Comprehensive comparison of fuel cell and lithium battery

For new energy vehicles, the battery is the key, which also determines the category of the new energy vehicle. In electric vehicles, lithium batteries are currently the most mature, stable performance, and most widely used power batteries.

   With the continuous development of fuel cells, a preliminary consensus has been formed in the industry. In the future, new energy vehicles will coexist with lithium batteries as the main power battery and new energy vehicles with fuel cells as the main power battery.

   On July 10 this year, the China Association of Automobile Manufacturers released the June automobile production and sales data. China's auto production and sales were 1.895 million and 2.056 million, down 17.3% and 9.6% year-on-year, representing a 12-month consecutive decline in sales.

   But in terms of new energy vehicles, in June, the production and sales of pure electric vehicles were 113,000 and 129,000 respectively, an increase of 78.0% and 106.7% respectively over the same period of the previous year.

   The production and sales of fuel cell vehicles were completed 508 and 484, respectively, a year-on-year increase of 9.8 times and 14.6 times.

   From January to June, the production and sales of pure electric vehicles were 493,000 and 490,000 units, an increase of 57.3% and 56.6% respectively over the same period of the previous year.

   The production and sales of fuel cell vehicles were 1,170 and 1,102 respectively, an increase of 7.2 times and 7.8 times respectively over the same period of the previous year.

From the above data and the data of previous years, it is a general trend that new energy vehicles will become the mainstream of my country's automobile industry in the future. Then, as the core of the two new energy mainstream vehicles in the future, what are the characteristics of the two power batteries and their performance comparisons? How?

lithium battery

   Lithium battery mainly refers to a type of battery that uses lithium element as the main active material in the electrode material, which mainly includes two types of lithium metal batteries and lithium ion batteries. The lithium batteries mentioned in this article are mainly lithium-ion batteries.

   Lithium ion battery is a kind of secondary battery, which mainly relies on the movement of lithium ions between the positive electrode and the negative electrode to work, and is a battery that can be charged and discharged. The structure of a lithium-ion battery mainly includes a positive electrode, a separator, a negative electrode, an electrolyte, and a battery casing.

   Positive electrode: generally lithium manganese oxide or lithium cobalt oxide, lithium nickel cobalt manganese oxide material (commonly known as ternary), pure lithium manganese oxide and lithium iron phosphate are gradually faded out due to large volume, poor performance or high cost.

   Diaphragm: It is a specially formed polymer film with a microporous structure that allows lithium ions to pass freely, but electrons cannot pass.

   Anode: generally graphite, or carbon with a similar graphite structure.

   Electrolyte: It is the carrier of ion transmission in the battery, which is generally composed of lithium salt and organic solvent. Its main function is to conduct lithium ions between the positive and negative electrodes of the lithium battery.

  Battery shell: It is divided into steel shell (square type is rarely used), aluminum shell, nickel-plated iron shell (for cylindrical batteries), aluminum plastic film (soft packaging), etc., mainly used to protect the battery.

Lithium-ion batteries mainly include lithium cobalt oxide, lithium manganate, lithium nickelate, ternary lithium, lithium iron phosphate, etc. according to the positive electrode material. At present, the more mature lithium iron phosphate batteries and ternary lithium batteries for vehicles are the former. The representative is BYD, which is Tesla.

The fuel cell

  The fuel cell is a chemical device that directly converts the chemical energy of the fuel into electrical energy, also known as an electrochemical generator.

   It is based on the electrochemical principle, that is, the working principle of the galvanic cell, isothermally converting the chemical energy stored in the fuel and oxidant into electrical energy, so the actual process is an oxidation-reduction reaction.

  The fuel cell is mainly composed of three parts, the electrode, the electrolyte and the external circuit.

The fuel cell electrode is the electrochemical reaction site where the fuel undergoes oxidation reaction and the oxidant undergoes reduction reaction. It mainly includes an anode and a cathode. The thickness is generally 200-500mm. Its structure is different from ordinary battery flat electrodes. It is a porous structure. The purpose is to improve fuel The actual operating current density of the battery.

  The electrolyte plays the role of transferring ions and separating fuel gas and oxidizing gas. In order to prevent the mixing of two gases from causing a short circuit in the battery, the electrolyte is usually a dense structure.

  The external circuit is generally composed of bipolar plates. The bipolar plates have the functions of collecting current, separating oxidants and reducing agents, and channeling reaction gases. Its performance mainly depends on its material characteristics, flow field design and processing technology.

Commonly used fuel cells can be divided into proton exchange membrane fuel cells (PEMFC), solid oxide fuel cells (SOFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC) and alkaline fuels according to their different electrolytes. Battery (AFC).

Proton exchange membrane fuel cell (PEMFC) is currently a relatively mature and widely used fuel cell due to its multiple performance advantages, including low battery operating temperature and fast start-up speed. It accounts for global shipments and megawatts of shipments. leading position.

  The fuel of the fuel cell is mainly hydrogen, methanol and other hydrocarbons. The fuel cell in this article mainly takes hydrogen fuel cell as an example for analysis.

  Comparison of the two batteries in all directions

   is also a new energy battery. The lithium battery inputs/outputs electric energy. In fact, it stores the input electric energy first, and then outputs the electric energy through the output device when it is used.

  The fuel cell is actually equivalent to the internal combustion engine of a traditional car. The internal combustion engine burning oil is only an energy conversion device, not an energy storage device; the fuel cell burning hydrogen is also an energy conversion device, not an energy storage device.

   Lithium batteries are energy storage devices, so strictly speaking, fuel cells are not batteries, but engines.

   Therefore, the fuel cell is a power generation device, and the lithium battery is an energy storage device. The following table is a comprehensive comparison of the two power batteries. The comparison factors include comprehensive performance, cost, policy support, resource constraints, environmental protection, and degree of commercialization.

 

Energy Density

   Energy density (Energy density) refers to the amount of energy stored in a certain space or mass matter. The energy density of a battery is the electric energy released by the average unit volume or mass of the battery.

   The energy density of the battery is divided into the energy density of the single cell and the energy density of the battery system. The energy density of the battery system is lower than that of the single cell.

The lithium battery system is a closed system. Due to the characteristics of the lithium element, the ternary lithium battery with the highest energy density among the lithium batteries has been taken as an example, and its single energy density is only 1.08MJ/kg (the battery pack system attenuates 20% ).

   If the energy density of lithium batteries is to be increased in the future, it will need to rely on breakthroughs in all-solid-state battery technology, but the upper limit of its energy density is not high.

The fuel cell system is an open system, and its energy density essentially depends on the amount of hydrogen storage. The energy density of hydrogen itself is 143MJ/Kg, and the energy density of the current fuel cell system exceeds 350wh/kg. With the advancement of hydrogen storage technology in the future, There is still a lot of room for energy density improvement.

 

   power density

   Power density is the ratio of the maximum output power of a power battery to the mass or volume of the battery system.

If the lithium battery system increases its output power to enable high-power discharge, the general solution is to increase the number of batteries, which will increase the weight of the entire battery system at the same time, even if Tesla uses the ternary battery with the best energy density at present, its battery The components weigh close to half a ton.

   Therefore, the high power discharge of the lithium battery system is not compatible with the high cruising range, and the power density increase is limited.

   Fuel cell can essentially be understood as a chemical power generation system using hydrogen as the raw material, so the output power is relatively stable. Generally, in order to maximize the discharge power, it is only necessary to add a power battery system. For example, Toyota Mirai is equipped with a nickel-hydrogen battery.

  The fuel cell system is an open power system. The output power is easy to increase, and the additional battery will not increase too much weight. The power density of Toyota Mirai has reached 2036W/kg.

safety

   Regardless of whether it is a pure electric vehicle equipped with a lithium battery or a vehicle equipped with a fuel cell, as long as it is a car, safety is the most important indicator.

   Lithium battery is a closed energy system. In principle, it is difficult to be compatible with high energy density and safety. If you simply pursue high energy density, then the entire lithium battery system is equivalent to ***.

   Therefore, in the current mainstream process route, the low energy density lithium iron phosphate is safer, and the battery does not start to decompose until the temperature reaches 500 to 600 degrees. It basically does not need too much protective auxiliary equipment.

   The ternary battery used by Telsa has a high energy density, but it is not resistant to high temperatures. It will decompose at 250 to 350 degrees and is poor in safety.

  The solution is to connect more than 7000 batteries in parallel, which greatly reduces the risk of leakage and explosion caused by a single battery.

   However, if you analyze the accident of a Tesla car, it is either a minor collision or a static situation, but the battery is on fire, so there are still many problems in its safety.

  The fuel cell itself is very safe. After it is used in vehicles, its safety mainly comes from the hydrogen storage system.

   But through a large number of experiments, it has been proved that compared with gasoline and natural gas, the two common combustible gases for vehicles, the safety of hydrogen is not bad.

  Moreover, the hydrogen storage devices for vehicles now use carbon fiber materials, which can be undamaged in the 80KM/h multi-angle crash test.

   Even if a car accident causes a leak, due to the high concentration required for hydrogen explosion, it generally starts to burn before the explosion, but it is difficult to explode.

  Moreover, the hydrogen is light in weight. The hydrogen that overflows the system will rise up quickly after it catches fire, which protects the body and passengers to a certain extent. Therefore, as commercialization advances, its overall security is controllable.

   reliability

  The reliability of the battery refers to the probability of the battery losing its power storage capacity due to an accident.

   The reliability of lithium batteries has a lot to do with their safety issues, but it is not a concept. Safety accidents of lithium batteries will inevitably lead to the loss of electrical energy storage capabilities.

   But the loss of energy storage capacity of lithium batteries is not always caused by safety accidents, such as battery failure due to capacity "diving".

  The lithium battery system is composed of hundreds of individual cells assembled in series and parallel, so the unreliability of the entire battery system will be greatly magnified.

   Judging from the data accumulated in domestic pure electric vehicles, the reliability of the lithium battery system is currently not satisfactory.

Fuel cells have been used in the space shuttle since the 1970s. The third-generation AFC (nominal/limit power 7.0/12.0 KW) produced by the International Fuel Cell Corporation (IFC) later became the standard power source for the American space shuttle. .

   Most of the conventional submarines currently in or about to enter service around the world use PEMFC (Proton Exchange Membrane Fuel Cell) as the main power battery system.

   New conventional submarines in Russia, South Korea, Australia, Israel, and Italy all use PEMFC fuel cell technology, and large-scale PEMFC stacks have developed to a high degree of perfection and reliability in terms of technology alone.

   Therefore, fuel cells have extremely high reliability.

  Ambient temperature adaptability

   Due to the wide range of areas where vehicles are used, temperature adaptability is very important for new energy vehicles, and what temperature range it can adapt to depends on the power battery itself.

  Currently, the performance of lithium batteries will not be affected in the living environment above zero, but the problems that occur below zero are problems that need to be solved urgently.

  The low-temperature performance of lithium batteries mainly depends on the effect of temperature on the conductivity of the electrode material, the ion diffusion coefficient and the conductivity of the electrolyte.

  The viscosity of the electrolyte increases at low temperatures and the conductivity decreases, resulting in a sharp increase in battery polarization. Especially when the lithium battery is close to zero, its performance drops sharply, and it can hardly work normally at -20°C.

  Furthermore, frequent charging and discharging at low temperatures will seriously deteriorate the life of the lithium battery, and easily lead to the precipitation of lithium on the negative electrode, which may cause safety hazards.

   After the fuel cell is started, due to the working principle of the battery itself, the temperature of the fuel cell stack will quickly stabilize in the normal operating temperature range of 80~90℃ even at a very low ambient temperature.

   Toyota and Honda’s fuel cell vehicles have already started at -30°C, but for fuel cells, it is still necessary to continue to improve their performance at low temperatures. -40°C is the main goal in the future.

   related costs

   Power battery involves all aspects of costs, including consumption costs, the cost of the battery itself, and the cost of infrastructure.

  Lithium battery

Lithium batteries consume electricity, and their cost is mainly electricity. Lithium battery cars generally consume about 17 kilowatt-hours per hundred kilometers. According to the electricity price, it is calculated from 0.5 yuan (home charging) to 2.2 yuan / kilowatt-hour (commercial), and its consumption The cost is 8.5~37.4 yuan/100 kilometers.

 

   The cost price of the lithium battery itself, calculated according to relevant data, is around 8-9 yuan/kwh.

 

  The infrastructure for lithium battery vehicles is mainly charging stations. At present, the investment in infrastructure and power distribution facilities for a single charging station is about 4.3 million yuan.

   Fuel cell aspect

  The fuel cell consumes hydrogen, and the cost of consumption is the price of hydrogen consumption.

At present, the selling price of hydrogen in commercial hydrogen refueling stations in my country is between 30-120 yuan/kg (Shanghai Yilan hydrogen refueling station hydrogen price is 40-45 yuan/kg), calculated on the basis of a passenger car’s battery life of 100 kilometers per kilogram , And its cost is 30-120 yuan/100 kilometers.

   The cost of fuel cell itself is related to the output. Because foreign fuel cell vehicles are more mature than domestic ones, the cost is relatively low.

  According to calculations based on relevant data, when the output of fuel cells in foreign countries reaches 500,000 units, the cost can be reduced to 40 US dollars/kW. At present, due to the small output in China, the battery cost is between 10,000 and 15,000/kW.

   The infrastructure for fuel cell vehicles is a hydrogen refueling station. The construction cost of a hydrogen refueling station is related to its hydrogen refueling capacity. Generally speaking, the greater the hydrogen refueling capacity, the higher the overall investment price of the hydrogen refueling station.

  According to public information, the investment scale of domestic hydrogen refueling stations with a hydrogen supply capacity of 500kg/d is between 12 and 18 million.

  The cost of Nanhai Ruihui hydrogen refueling station is 15.5 million yuan, the cost of Foluo road hydrogen refueling station is 12.5 million yuan, and the cost of Hanlan Songgang Chantan Road hydrogen refueling station is 29.85 million yuan.

   At present, the highest construction price of hydrogen refueling stations in my country is the Shanghai Yilanjinshan hydrogen refueling station. This hydrogen refueling station is currently the world's largest hydrogen refueling station with a total investment of 55 million yuan.

Policy Support

   Regardless of whether lithium battery vehicles or fuel cell vehicles, due to their high costs in various aspects, the early development requires strong support from the state.

   On March 26, 2019, the Ministry of Finance, the Ministry of Industry and Information Technology, the Ministry of Science and Technology, and the Development and Reform Commission jointly issued the "Notice on Further Improving the Financial Subsidy Policy for the Promotion and Application of New Energy Vehicles."