The Vehicle Technologies Office (VTO) of the U.S. Department of Energy releases an annual report on vehicle battery research and development projects every year and puts it on the U.S. Department of Energy website for everyone to download for free. The reference material for this article is the 2017 Battery Research Progress Report. The research progress report for 2018 has not yet been released.

　　One of the goals of its vehicle battery research and development project (Battery Program) is to develop high-energy-density batteries for pure electric vehicles. The specific energy of a single battery core is greater than 350Wh/kg, and the cycle life is greater than 1,000 times. The project funded six companies, Envia systems, LGCPI, 24M, Farasis Energy, Amprius and SiNode Systems, to research battery cells. Among them, 24M mainly studies semi-solid lithium-ion batteries, SiNode Systems focuses on silicon/graphene composite materials with a gram capacity greater than 1000Wh/kg, and the remaining four companies study silicon anode lithium-ion batteries. The picture below is the target of the battery system and cells.

　　The following is a brief introduction to the research progress of six companies over the past 17 years:

　　Envia Systems

　　In 2017, we mainly completed the preparation of 11Ah soft-packed batteries (Cell-build 2#) with a specific energy of 280Wh/kg and a cycle of more than 500 cycles (C/3rate, CC-CV, 30℃, 4.35V to 2.3V). The 21Ah soft-packed battery cell (Baseline cell) with a specific energy of 245Wh/kg has a cycle life of more than 700 cycles (C/3 rate, CC-CV, 30℃, 4.35V to 2.3V). In 2018, 300Wh/kg 11Ah soft-packed batteries (Cell-build 3#) with a cycle life greater than 1,000 and 50Ah soft-packed batteries (Cell-build 4#) with a cycle life greater than 1,000 times will be prepared.

　　The cathode of the battery cell is a mixture of nickel-rich NCM and manganese-rich NCM, and the cathode is SiO/C (SiO content is greater than 50%). The first efficiency is improved through a pre-lithium process (in cooperation with Nanoscale Components). Among them, the electrolyte is produced by Daikin America, the coated separator is provided by Asahi Kasei, and the battery manufacturing is in cooperation with A123 Venture Technologies; the Si alloy is from 3M, and n-Si is from DuPont.

　　The project focuses on material optimization and selection, pre-lithium process, cell design, cell formation and testing methods. Circulation is one of the main challenges, and pulverization and fragmentation of the negative electrode are the main failure modes. Solutions include optimizing cell design (coating amount, density, N/P, etc.) and material selection (negative electrode, Positive electrode, conductive agent, electrolyte, separator).

　　Finding the cause of cycle failure is mainly carried out through failure analysis and cell disassembly (physical, chemical, structural, electrochemical analysis).

　　LGCPI

　　At the beginning of the project, a manganese-rich NCM/silicon anode system was selected, but the cycle performance was very poor, and it only cycled for 25 weeks. Subsequently, the switch was made to nickel-rich NCM. In 2017, the OCV, cycle and expansion of cells with SiO content of 10% and 30% were mainly compared. Compared with 10% SiO battery cells, 30% SiO battery cells have low OCV, poor cycle performance and large expansion force of 100% SOC. After reducing the SOC window, there is little difference in cycle performance between the two. It has not yet reached USABC’s goal, and will produce >50Ah cells in the future.

　　24M

　　24M Company mainly researches semi-solid lithium batteries. The positive electrodes mainly use NCM622 and 811. By improving the electrolyte, the low-temperature performance of the battery has been greatly improved. -20℃, 1/3C discharge capacity is 50% of room temperature capacity.

　　Farasis Energy

　　The project mainly achieves its goals by optimizing the cathode, silicon anode, pre-lithium process, electrolyte and conductive agent.

　　12 types of positive electrodes were screened, including high-voltage NCM, LMRNCM and nickel-rich NCM, with a specific capacity of 187-210 mAh/g; the negative electrode was mainly silicon alloy, SiO, silicon nanowires and silicon nanoparticles, with a specific capacity of >1000 mAh/g. .

　　Gen 1 battery cell capacity is 10~30Ah, specific energy is 300Wh/kg, no pre-lithium process is used, and it can reach 450 times by optimizing electrolyte circulation.

　　Through the pre-lithium process, the cycle performance of the silicon anode can be greatly improved (see the figure below). The final battery capacity of the project is 60Ah.

　　Amprius

　　The goal is to produce a soft-packed battery cell with a capacity of 40Ah, whose specific energy and energy density at EOL are 350Wh/kg and 750Wh/L respectively. The negative electrode uses silicon nanowires.

　　Cell performance target:

　　Available Energy Density @ C/3 Discharge Rate: 750 Wh/L

　　Available Specific Energy @ C/3 Discharge Rate: 350 Wh/kg

　　DST Cycle Life: 1,000 Cycles

　　Peak Discharge Power Density, 30 s Pulse: 1500 W/L

　　Peak Specific Discharge Power, 30 s Pulse: 700 W/kg

　　Peak Specific Regen Power, 10 s Pulse: 300 W/kg

　　Calendar Life: 15 Years

　　Selling Price @ 100K units: $100

　　Operating Environment: -30°C to +52°C

　　Normal Recharge Time: < 7 Hours

　　High Rate Charge: 80% ΔSOC in 15 minutes

　　Peak Current, 30 s: 400 A

　　Unassisted Operating at Low Temperature: > 70% UseableEnergy @ C/3 Discharge Rate at -20°C

　　Survival Temperature Range, 24 hours: -40°C to+ 66°C

　　Maximum Self-discharge: < 1%/month

　　Before this project, Amprius' battery cell energy density (BOL) was already greater than 700 Wh/L, 100% DOD, and the number of C/2 cycles was greater than 400 cycles. In order to achieve the goals of the project, the measures include: 1) adjusting the anode structure and using thinner foils and separators to increase energy density; 2) the cathode will change from LCO to NCM; 3) optimizing the anode structure to increase lifespan; 4) developing and improving Electrolyte formula for SEI film stability and cell performance; 5) Increase the size of the negative electrode and cell; improve the uniformity of silicon growth and deposition, and reduce defect density; a preparation method that can be expanded to larger sizes.

　　By adding additives that improve the stability of the SEI film in the electrolyte, the performance and life of the battery core are improved. In 2017, more than 100 electrolyte formulas were screened, including different additives, solvents and lithium salts. Focus on improving the gas production of the cathode under high SOC and high temperature. The electrolytes are screened by testing their cycling and storage performance at high temperatures (50°C). The cycle test system is: CC-CV at C/2 rate with 10% current taper and C/2discharge rate, over the full voltage range (2.85-4.25V).

　　The average capacity of Silicon-NCM cells sent for testing in 2017 was 10.6 Ah, the specific energy was 345 Wh/kg, and the energy density was 860 Wh/L.

　　Batteries with a capacity of 40Ah will be produced in 2018. Specifications are:

　　Rated Capacity: 45.9 Ah at C/3 rate

　　Vmax100 = 4.1V

　　Vmin0 = 2.5V

　　Cell weight = 450.7g

　　Cell size = 6.0 x 96 x 288 mm (body only)

　　348 Wh/kg and 921 Wh/L

　　SiNode Systems

　　SiNode's technology is a unique silicon alloy-graphene structure. In 2017, SiNode developed a new carbon coating method to stabilize the surface of SiOx active particles and the SEI film before graphene coating. Through carbon coating, the first efficiency of SiOx (i.e., >1500 mAh/g) exceeds 79%, which is very close to the target value of 80%. Now we can produce 2kg per month.

　　Partner A123 has been able to achieve laboratory-scale negative electrode (1000mAh/g) coating. The cycle performance of the two cells containing 10% and 20% FEC electrolyte is equivalent when the NCA positive electrode is prepared into a full battery.

　　summary

　　1) 350Wh/kg battery core, the positive electrode is made of high-nickel material, the negative electrode is SiO negative electrode, and the SiO content is 30~50%;

　　2) The battery core is a soft package with a capacity of 40~60Ah. The cycle performance is poor, a 300Wh/kg battery cell cycles about 500 times;

　　3) Adopt pre-lithium process to improve first efficiency. Optimize the electrolyte formula to improve SEI film formation stability and cell performance. Regulate the negative electrode structure and formula to reduce negative electrode breakage and powdering.

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