The hottest lithium battery development highlights

There are many bright spots in the development of lithium batteries: silicon carbon cathode, all solid state, lithium titanate, soft bag, super capacitor...

Abstract: the development of new energy vehicle industry still has a long way to go. The development of this industry needs not only policy promotion, but also scientific and technological innovation. We believe that each of the lithium battery technology schools mentioned above has its applicable application fields, and these technologies will continue to be improved in the industrialization practice

the development of new energy vehicle industry needs the power of scientific and technological innovation. Each lithium battery technology school has its applicable application fields, which will be gradually improved in the industrialization practice in the future

according to the data released by the Ministry of industry and information technology, the output of new energy vehicles in China in 2016 was 517000, accounting for only about 1.84% of the total output of the automotive industry (28.119 million). Compared with traditional fuel vehicles, new energy vehicles are still in the early stage of development, and there are still some problems, such as short mileage, long charging time, insufficient safety, etc. In the structure of electric vehicles, batteries play an important role. We judge that in the future, batteries will pay more attention to high specific energy, lightweight, fast charging, safety and other properties. The industry is also trying various materials to develop suitable batteries for the different applications in table 15

we have done some research on the development trend of power lithium batteries in the next few years, and share it as follows. Readers are welcome to interact and discuss with us

trend 1: negative electrode will be the key to improve energy density

on April 6, 2017, the Ministry of industry and information technology, the national development and Reform Commission and the Ministry of science and technology jointly issued the medium and long term development plan for the automotive industry. In recent years, the technical level of new energy vehicles in China has been greatly improved, the industrial scale has expanded rapidly, and the industrial chain has become increasingly perfect, which is expected to become a breakthrough to seize the opportunity and catch up with the development. We believe that the introduction of the plan is of great significance for standardizing and guiding the development of the new energy vehicle industry. For power lithium batteries, the plan proposes that by 2020, the annual production and sales of new energy vehicles will reach 2million, the specific energy of power battery cells will reach more than 300 watt hours/kg, strive to achieve 350 Watt hours/kg, the specific energy of the system will strive to reach 260 watt hours/kg, and the cost will be reduced to less than 1 yuan/watt hour. By 2025, new energy vehicles will account for more than 20% of vehicle production and sales, and the specific energy of power battery system will reach 350 Watt hours/kg. We believe that the above goals will promote the industry to improve the performance of power batteries through the application of new materials and process improvement

at present, the cathode materials of lithium-ion batteries mainly include graphite carbon, silicon composite, tin carbon composite, titanium oxide, metal lithium, etc. the comparison of main performance indicators is shown in table 16. Among them, the specific capacity of silicon/TiN/carbon and metal lithium has been improved compared with graphite, which will be a new choice for high specific energy lithium batteries in the future, while titanium oxide (lithium titanate material) mainly improves the cycle and fast charging performance

many foreign battery companies have long begun to try new negative electrode materials. For example, although the probability of electromechanical appearance is not very high, such as Panasonic, enevate, ampere, etc. have begun to produce lithium-ion batteries with silicon carbon composites as negative electrodes, ionic, Oxis, sionpower, etc. have begun to produce lithium sulfur batteries, solidenergy, etc. have begun to explore high nickel ternary batteries with lithium metal as negative electrodes, and the energy density has reached 400-500wh/kg, as shown in Figure 15

trend 2: soft pack batteries will be the trend of battery lightweight

at present, lithium-ion batteries mainly fall into three categories: square, cylindrical and soft pack. The square and cylindrical shells mainly use hard shells such as aluminum alloy and stainless steel, while the soft pack shell generally adopts strain control or beam displacement control, using aluminum plastic film, which is their essential difference, as shown in Table 17, In terms of performance, the main advantages of the soft packed lithium battery include:

(1) good safety performance: the electrolyte of the soft packed battery leaks less, and the soft packed battery will blow up and crack in the case of potential safety hazards, unlike the hard shell battery, which will explode if the internal pressure is too high

(2) light weight: the weight of soft packed battery is 40% lighter than that of steel shell square battery with the same capacity, and 20% lighter than that of aluminum shell square battery

(3) large battery capacity: soft bag saving means that the system volume controlling the operation of the experimental machine is 20%, which is 50% higher than that of the steel shell battery with the same specification and size, and 20-30% higher than that of the aluminum shell battery

(4) good cycle performance: the cycle life of soft packed battery is longer, and the attenuation of 100 cycles is 4% - 7% less than that of aluminum shell

(5) small internal resistance: the internal resistance of the soft packed battery is smaller than that of the lithium battery, and the domestic minimum can be less than 35m Ω, which greatly reduces the self consumption of the battery

(6) flexible design: the shape can be customized according to customer needs, and it can be made thinner. The ordinary aluminum shell can only be 4mm, and the soft bag can be 0.5mm

we believe that in terms of performance, soft pack batteries have more comprehensive advantages and broad application prospects in the future:

1) soft pack batteries are lighter and thinner, suitable for portable applications with high requirements for space or thickness, such as 3C consumer electronics

2) although the unit capacity of the square battery is high, it is heavy and large, and the soft pack battery has obvious advantages in energy density. At present, the single cell is also developing in the direction of large capacity and high magnification, which will more meet the requirements of mobile power supply in new energy vehicles and other fields

3) although the production process of cylindrical batteries is mature and the advantage of energy density is obvious, because the capacity of single cells is very small, battery packs with the same capacity need more cells. For example, the battery pack capacity of Tesla models is 85kwh, and there are about 7000 18650 cylindrical cells, which requires very high requirements for the battery management System BMS, and the consistency of cells is difficult to control, Therefore, the energy density of the system decreases significantly compared with that of single cells, while the capacity of soft packed batteries is large, and the requirements for BMS are low. Between the limited supporting capacity of most BMS in China and even in the world, large capacity battery packs will not use cylindrical batteries too much. In addition, the safety of cylindrical batteries is poor due to high energy density, and the hard shell leads to high internal pressure, which often leads to explosion events, The soft pack battery has obvious advantages

more than 60% of the global power battery manufacturers adopt a square battery structure, and representative models include Mitsubishi iMiEV, BMW I3, Toyota pruis, etc; The Panasonic 18650 battery used by Tesla models is the representative of cylindrical power battery; The proportion of soft pack batteries is higher than that of cylindrical batteries. The representative models are Nissan LEAF, general Chevrolet Volt, BMW, etc., as shown in table 18. The main suppliers are Japan AESC, LG Chemical, A123, etc

in recent years, domestic mainstream electric vehicle enterprises such as Dongfeng, Zhongtai, BAIC new energy, BYD and so on have also begun to try soft pack power batteries. The main suppliers are ATL, micro macro power, Dover, AVIC lithium, carnet new energy and so on. Recently, Yiwei lithium energy, Shandong Hengyu, Suzhou Yuliang and so on have also begun to vigorously invest in the construction of soft pack power battery production lines. We believe that with the development of battery routes, the penetration rate of soft packs will continue to increase, and the market share is expected to reach 40% - 50% in the future. The relevant targets for the layout of soft pack batteries include (but are not limited to): polyfluorodox, Yiwei lithium energy, etc

trend 3: lithium titanate batteries lead the era of rapid charging of lithium batteries

at present, the commercialized cathode materials of lithium batteries mainly use graphite carbon, and its index performance advantages are relatively balanced, as shown in Figure 16, but the potential of carbon electrode is close to that of metal lithium. When the battery is overcharged, it is easy to precipitate metal lithium on the surface of carbon electrode to form lithium dendrites, which will pierce the diaphragm and cause short circuit, resulting in potential safety problems. At the same time, when the carbon negative battery is formed for the first time, the electrode surface is reduced to form a surface facial mask (SEI film). The formation and repair process of SEI film will consume electrolyte, resulting in the increase of irreversible capacity and the intensification of battery attenuation, which will affect the service life of the battery. Lithium titanate just makes up for some defects of graphite and has the following advantages:

1) high safety, high lithium insertion potential (1.55 catch the national policy and market opportunity of vigorously developing new energy vehicles V), no lithium dendrites, and extremely high thermal stability in both charged and discharged states

2) long life, zero strain material, extremely long life (up to more than 30000 times), not the SEI film in the traditional sense of Chen

3) fast charging, spinel structure, ten times the diffusion coefficient of lithium ion, and the charging time is only 6-15 minutes

4) wide temperature range, -40-60 ℃ can be charged and discharged, DOD ≥ 60%

5) low cost, aluminum foil can be used as collector, and the production yield is high

at present, domestic enterprises that have the production capacity of lithium titanate batteries include Huzhou Weihong Power Co., Ltd., Zhuhai Yinlong new energy Co., Ltd., CITIC guoanmeng Guli Power Technology Co., Ltd., Anhui Tiankang (Group) Co., Ltd., and their products should be applied to fast charging pure electric buses, plug-in hybrid electric buses, dual source trolleybuses, etc

trend 4: all solid-state batteries will break through the bottleneck of energy density

all solid-state lithium batteries have the advantages of high safety performance, long cycle life, wide operating temperature range, no diaphragm, etc. in addition, compared with traditional lithium-ion batteries, they also have the following advantages:

① solid electrolyte has a higher electrochemical window, which is suitable for high-voltage electrode materials, and can be connected in series through multi-layer stacking technology, Obtain higher output voltage

② all solid-state lithium batteries use solid materials, with higher packaging efficiency, and are prepared by physical methods, similar to semiconductor technology, with mature production methods

③ thin film batteries and flexible batteries can be prepared, which can be applied to intelligent wearable and implantable medical devices in the future. At present, the main disadvantages of all solid state batteries are small monomer capacity, low conductivity and temporarily high cost

in the future, if we want to increase the energy density of lithium batteries to more than 500wh/kg, we need new revolutionary power storage technologies, such as lithium sulfur batteries and lithium air batteries. The negative poles of these two types of batteries are metal lithium, and the positive active substances are sulfur or air respectively. The research of rechargeable lithium sulfur battery and lithium air battery has gone through decades. Because liquid electrolyte has been used before, the stability of metal lithium anode is difficult to solve. With the application of solid-state battery, this problem will be overcome

domestic expansion in the field of all solid-state lithium batteries is still in its infancy, and the following enterprises are mainly promoting industrialization:

1) Jiawei Co., Ltd. has successfully developed high-performance all solid-state lithium batteries (organic and inorganic doping) by cooperating with scientists from first-class universities such as Harvard and using material genome technology. At present, a pilot line has been built for small batch production, with an energy density of 300wh/kg, The company will invest billion to build phase I production capacity of 200-250 million ampere hours. It is expected to be put into operation at the end of 2017 or the beginning of 2018, which can achieve an annual income of 2 billion yuan. The subsequent plan is to invest another billion to build phase II production capacity of 1 billion ampere hours, which can achieve an annual income of billion yuan

2) Zhongtian Technology Group signed an agreement on "development of high-performance solid-state lithium metal batteries" with Qingdao Institute of bioenergy and process, Chinese Academy of Sciences, in May 2015. It plans to cooperate for three years to develop all solid-state lithium batteries with a high safety monomer capacity of 10Ah, with an energy density of more than 250wh/kg

3) Ganfeng lithium signed the "pilot technology research and development of high lithium ion conductivity solid electrolyte materials" with Ningbo Institute of materials technology and engineering, Chinese Academy of Sciences in November 2016