The lithium-air Li-O2 battery has a theoretical energy density much higher than that of the lithium-ion battery, and it has the highest energy density among the existing battery systems. If the weight of oxygen involved in the reaction is not considered, the energy density of lithium-air batteries can reach 11430W·h·kg-1, which is equivalent to fossil fuels. It is precisely because it is expected to become the driving power source for electric vehicles (EV), people’s attention and R&D investment in lithium-air batteries continue to increase. However, some factors make it difficult for the energy density of lithium-air batteries to reach the theoretical value. One of the reasons is that part of the discharge products of the battery reaction must be stored inside the battery, which requires that the weight of oxygen involved in the reaction must be included in the calculation of the energy density. In this chapter, the energy density actually obtainable by the secondary lithium-air battery will be used to compare with the energy density of the traditional battery system.
Since Plant invented the lead-acid battery in 1859, a variety of battery systems have come out. According to the calculation based on the mass of the active material and the open circuit voltage (OCV, 2.05V), the energy density of the lead-acid battery is about 171 w.h·kg -1, . In the past half century, the performance of this first practical rechargeable battery system has been greatly improved, but its energy density still cannot fully meet the application requirements of mobile devices and electric vehicles. Electric vehicles using lead-acid batteries as the driving power source cannot drive more than 100km when fully charged. The Institute of Industrial Technology (AIST) under the Ministry of International Trade and Industry of Japan began to apply for A battery research and development project for electric vehicles. After six years of research and development, the performance of lead-acid batteries, iron-air batteries, iron-nickel batteries, zinc-air batteries and sodium-sulfur batteries have been improved. According to reports, the improved lead-acid battery (140A.h module) has an energy density of 64.5wh·kg-1 (115W.hL -1, ) at a rate of 0.2C, and the capacity decay rate after 200 cycles is 55 %. The improved 2.46kw.h zinc-air battery module has the highest energy density of 131.5kw.kg -1, . Although the energy density is twice that of lead-acid batteries, the coulombic efficiency during charging and discharging is very low, only 37.0%. The energy density of the sodium-sulfur battery (108A.h module) at a rate of 0.2C is about 89.4W.h·kg-1 (265.5w.h.L -1, ), and the cycle life can reach 150 times (capacity attenuation to 80%). However, none of these systems can meet the application requirements of electric vehicles.
In 1989, more advanced rechargeable nickel-metal hydride batteries came into the world and commercialized. This battery consists of a metal alloy negative electrode that can absorb hydrogen, a nickel hydroxide positive electrode, and potassium hydroxide electrolyte. The energy density of 1kw·h Ni-MH battery module is about 60w.h.kg -1, (150W.h·L -1, ), and its volumetric energy density is higher than that of lead-acid batteries, and it has excellent cycle performance. Honda Motor Company This battery is applied to an electric car for the first time. After the battery is fully charged, the car can drive at a constant speed of 178.5km at a speed of 72km per hour. The total weight of the battery used in the car is 451kg, which accounts for about 30% of the vehicle’s weight. At present, nickel-hydrogen batteries have been widely used in hybrid vehicles.
In 1991, Sony realized the commercialization of lithium-ion batteries. This battery uniquely uses lithium-ion intercalable compounds as the positive and negative electrodes of the battery. The positive electrode material is usually a metal oxide with a layered structure, such as LiCoO2; while the negative electrode generally uses graphite. In the beginning, the energy density of small-sized lithium-ion batteries was still difficult to reach 100w·h·kg -1, , but it is now close to the level of 200W·h·kg -1, . In 1992, the United States and Japan both launched a research and development project for high-energy density batteries used in electric vehicles. In the early stage of the implementation of the US project (USABC project), the possibility of nickel-hydrogen batteries, lithium-sulfur batteries, lithium-ion batteries and lithium-polymer batteries as energy sources for electric vehicles was investigated, and lithium-polymer batteries were finally selected. The long-term goal of the USABC project is to develop a high-performance battery that has an energy density and power density of 200w.h·kg (300W.h·L -1, ) and 400w.kg at a rate of 1/3C, respectively. The energy density of the 2.2kW.h polymer lithium ion battery module developed in this project is about 155w.h·kg. The battery consists of a metal lithium negative electrode, a vanadium oxide positive electrode and a PEO (polyoxyethylene)-based polymer electrolyte. constitute. The New Sunshine Program (The New Sunshine Program) implemented in Japan aims to develop high-performance lithium-ion batteries for electric vehicles. The goal is that the energy density and power density of 3kw-h battery modules should reach 150 wh·kg, respectively. (300 WhL -1, ) and 400w.kg -1, . After 10 years of research and development, the mass energy density of the 3kw battery module has reached the standard, but the volume energy density is only 252W.h·L -1, , slightly lower than the original target . These battery materials for electric vehicle applications are basically the same as ordinary lithium-ion batteries. In the third stage of electric vehicle battery development, Japan’s New Energy and Industrial Technology Development Organiztion (NEDO) proposed a more challenging goal in 2006, namely, mass energy density and volume energy. The density should reach 700w.h.kg-1 and 2000W.h.L -1, respectively. This value is basically the same as the energy density of an internal combustion engine. In other words, when the battery is fully charged, the mileage of an electric vehicle can be comparable to that of a fuel vehicle. But batteries that can achieve such a high energy density (even if the primary battery is included) have not yet been developed.
The energy density of the battery depends on the electrode material used. For lithium-ion batteries, there are many types of electrode materials that can be combined. A battery with high energy density requires a negative electrode with low potential and high specific capacity and a positive electrode with high potential and high specific capacity. Although there are many kinds of negative electrodes with high specific capacity to choose from, no positive electrode with high potential and high specific capacity has been found. Therefore, the current capacity of lithium-ion batteries is mainly limited by the positive electrode material, and the highest energy density currently achieved is about 250w.h·kg -1, . In order to obtain a lithium ion battery with higher energy density, it is necessary to find a Li-M-O system with multivalent cations. If the M in LixMO2 can be converted from M4+ to M2+, when it is paired with a high specific capacity negative electrode, the energy density of the corresponding battery is expected to reach 600W.h·kg-1. Taking LixCr3O8 as an example, when Cr is reduced from Cro to Cri, its specific capacity can reach 400mA.hg1, which is 3 times that of LiCo02 material; but this compound pollutes the environment.
At present, the energy density of small lithium-ion batteries for mass production of mobile phones can reach up to 266 w.h·kg . If calculated based on the mass of the active material in these batteries, the energy density is about 400w.h·kg. Generally, the power density of a battery determines its energy density.