“10 MINUTE LTO ULTRAFAST CHARGE PUBLIC TRANSIT EV BUS FLEET OPERATIONAL DATA-ANALYSIS OF 240,000 KM, 6 BUS FLEET SHOWS VIABLE SOLUTION"

Long battery charge times and low battery charge-discharge cycle life are the two major limitations holding back the commercialization of electric vehicles. In order to resolve these problems, a robust battery system was developed by Microvast Inc. The batteries can be charged in less than 10 minutes and rapidly charged and discharged up to 20,000 times, while still maintaining more than 80% of the original capacity. 6 City Buses utilizing the Microvast battery systems have been tested in commercial operation in Chong Qing, China for more than one year. The batteries are still in good condition. The improved Li4Ti5O12 negative electrode material gives the Microvast battery system its excellent properties.


1．LpTO Technology
As a negative electrode material, Li 4 Ti 5 O 12 (LTO) has been well documented with good stability and high charge-discharge rates. However, the gas generation in the charge-discharge cycles of LTO batteries has been a fatal drawback of the batteries, leading to the degradation of the batteries and limited use of such batteries in the market.
The investigation found that, in most cases, the gas generation in the batteries occurs because of the chemical reactions started on the surface of Li 4 Ti 5 O 12 material. In the charge-discharge process, the electrolyte solvent reacts with the Li 4 Ti 5 O 12 generating reductive lithium alcoholate, which can be oxidized on the positive electrode in the charged state to form H 2 O, CO, CO 2 , C 2 H 4 , and C 3 H 6 , which lead to much faster battery degradation. In order to resolve the problem, scientists at Microvast developed a new technology to enrich a layer of inert material LpTO material A battery with LpTO negative electrode material and NiCoMnOx positive electrode material has been tested for up to 18 months. The results, shown in Figure 3, reveal that, after 25,000 cycles, the capacity retention is still about 75%.

Application of LpTO
Li-ion batteries are beginning to appear in both the electric vehicle and smart grid energy storage markets. Although they are now entering markets, they still face high lifetime total-cost-of-ownership challenges, blocking true mass market development. The LpTO battery, with its long cycle life, brings an alternative solution, which is much more cost effective.
Combining the ultrafast charging solutions now available with the long cycle life LpTO technology can transform the electric vehicle industry. Two particularly attractive applications are the Shuttle Bus and the City Bus, which are typically operated on high use and fixed loop routes every day. With LpTO battery technology, the bus or shuttle can be ultrafast charged every loop, taking less than 10 minutes per charge. Because the battery is charged in each loop, a very small battery pack can be used, meeting a single loop energy requirement as opposed to a whole day energy requirement that would require a much larger battery pack. The advantage can be found in the following example: Compare a 30 unit small bus fleet equipped with LTO battery technology with that of one equipped with LFP battery technology. . We find that a small bus equipped with an LTO battery that can be ultrafast charged, has the following advantages: 1) a smaller pack is less expensive, 2) a smaller pack weighs less reducing strain on the bus chassis and freeing up space, 3) reduced weight increases vehicle efficiency, and 4) reduced charging infrastructure investment is required due to higher charging port efficiency. The same circular route is run by each of the buses each day. The route is approximately 20 km (14 miles) long with an average 5 or 6 trips taken each day. Fast charging between routes takes place at a central location; fast charging times average about 10 minutes at charge power levels of approximately 400 kW.
Over the approximately 380 days run, the 62051 bus was charged approximately 1930 times. Over this time, battery capacity has decreased only slightly and we estimate a pack life of at least 15,000 cycles allowing for a 70% end of life capacity retention.
The following information reflects data captured from bus 62051 during its operation between March 18, 2011 and March 31, 2012.
1) Current during Operation (Fig. 6) While driving, the current from the battery pack was recorded. Negative data indicates energy regeneration during braking or downhill driving. A maximum 380A or about 3.5 C discharge was observed.
2) Number of Charges (Fig. 7) The number of charges was about 2,000 with the average energy added per charge of 20 -40kWh. The average duration of each charge was less than 10 minutes, with the minimum charge duration being approximately 3 minutes, and the maximum charge duration being approximately 14 minutes. Charge power was, on average, approximately 400kW.  Battery temperatures were recorded (Fig. 8)

Analysis of Battery Modules
In April, 2012, modules were removed from the buses and analyzed for capacity retention, increased impedance, and constant current capacity. These test results are shown below in Figure 9, 10, and 11.

1) Capacity Retention
In order to evaluate capacity retention, 5 cells were removed from the module and evaluated.
The nominal 1 C capacity of the 5 cells was tested at 1 C and compared with original delivery capacity data: In order to test for impedance gain, initial impedance data was compared with that of the 5 removed cells. The impedance rose from 0.662 mOhm initially to 0.676 mOhm. An increase of about 2% was observed.

3) Constant Current Capacity
When testing for constant current capacity loss, the ratio of initial capacity of 5C constant current charging was compared with that of the removed cells. Nominal constant current capacity decreased from 90.2% to 88.6%. Figure 11. Impedance increase of used cells Based on the results of the tested cells and modules, the data shows very good results after 2,000 ultrafast charging cycles.

Conclusion
Beginning with research and development on LpTO anode material, Microvast has introduced a battery that appears to offer long cycle life even during ultrafast charge events. Tests of buses utilizing the LpTO chemistry and ultrafast charge appear to validate the long cycle life of the Microvast cells. Further analyses on removed battery modules supports the bus test data.