Evaluation Of The Low Temperature Performance Of Lithium Manganese Oxide/Lithium Titanate Lithium-Ion Batteries For Start/Stop Applications

The Start/stop technology is one of the rapidly developing technologies in today’s automotive market world-wide since it can be virtually applied to every vehicle on the road and significantly reduce the fuel consumption and CO2 emission in the transportation sector. A system equipped with the Start/stop technology can automatically shut down and restart a vehicle’s internal combustion engine to reduce the engine idling time, thereby improving fuel economy. This technology is widely used in hybrid electric vehicles in which electricity from battery is used to propel the vehicle, but it can also be applied to nonelectric vehicles with a fuel economy gain ranging from 5 to 10 percent.Nowadays, lead-acid batteries are mainly used for high-volume Start/stop vehicles since lead-acid batteries have good low-temperature speci?c power(k W/kg) and low initial cost. However, compared with lead-acid batteries, lithium ion batteries(LIBs) are more compelling for Start/stop technology since they have(i) lower mass translating to lighter vehicle weight and higher fuel efficiency,(ii) lower internal resistance enabling more efficient energy recuperation, and(iii) longer cycle life ensuring longer battery warranty for customers. But, in spite of these advantages, LIBs face a challenge of delivering enough cold cranking power at-30 oC to start the engine, which is one of the most critical requirements for 12 V Start/stop batteries.Therefore, LIBs consisting of spinel lithium manganese oxide(LMO, formula: LiMn2O4) as the cathode electrode and lithium titanate(LTO, formula: Li4Ti5O12) as the anode electrode is of particular interest because the spinel structures of both LMO and LTO materials typically confer high power capability, and the sub-micrometer particle size of LTO would further boost the LTO’s power capability. In addition, the LMO/LTO battery possesses inherent advantages of low material cost, high safety, and lack of Li plating issues. Furthermore, LTO is also known to have excellent cycle life relative to conventional graphite anode materials. Thus, it is believed that the LMO/LTO battery is a promising candidate to meet the requirement of cold cranking power-30 oC for the Start/stop technology.The low temperature performance of LMO/LTO batteries with three different electrolytes were studied on a bilayer pouch cell format coupled with the reference electrode. In the different rates discharge/charge tests of LMO and LTO coin cells, LMO’s charge performance and LTO’s discharge performance were good at high rate, and it’s suitable for Start/stop application. In the 1C rate discharge tests of LMO/LTO cells using electrolytes A, B, and C at-30 oC, the LMO electrode potentials strikingly dropped much faster along the discharging process compared with their profiles at 30 oC and the end-of-discharge potentials of the LMO electrode were 3.1 V, 3.3 V, and 3.3 V vs. Li+/Li for electrolytes A, B, and C, respectively; But potentials of the LTO electrode remain at 1.6V vs. Li+/Li. In the EIS analysis, Rs and Rct of LMO electrode were larger than ones of LTO electrode. These results supported that it was the LMO electrode that limited the cell discharge performance at-30 oC. The higher Rct of LMO than that of LTO could be because the average particle size for LMO was almost one order of magnitude larger than that of LTO(10 mm vs. 1.2 mm). The larger particle size gives longer Li diffusion lengths and thus much larger Rct for LMO relative to LTO. In the 1C rate discharge tests of LMO/LTO cells using electrolytes A, B, and C at temperatures from 30 oC to- 30 oC. All three cells’ discharge capacities expectedly diminished with decreasing temperatures. It was found that the ratios of the discharge capacity at-30 oC to the value at 30 oC were correspondingly 64.64%, 86.97%, and 86.26% for cells using electrolytes A, B, and C. Furthermore, electrolyte conductivity measurement results demonstrated that compared with the electrolytes A and B, the electrolyte C was more suitable for Start/stop battery’s cold cranking applications since it not only had the lowest bulk resistance but also led to lower Rct for the both LMO and LTO electrodes. The outstanding low temperature performance of the electrolyte C could be attributed to its co-solvent, EA, which advantageously has low melting point and low viscosity. The developed LMO/LTO battery with electrolyte C can pass the USABC cold cranking test at-30 oC using an assumed 40 Ah battery pack.