A Fast-Charging Study on the Lifetime of Power-Optimized Lithium Titanite Batteries By Electrochemical Impedance Spectroscopy
Authors: Hosen Md Sazzad , Kalogiannis Theodoros , Jaguemont Joris , Berecibar Maitane , Van Mierlo Joeri 
Published: December 2020

In order to meet the growing demand of extended lifetime and fast charging capability, Li-ion batteries (LiBs) have been optimized and further developed since the beginning. From a wide choice of LiB technologies, Li4Ti5O12 (LTO) anode are known for its fast charging characteristics which is a key requirement of present electrical vehicles (EVs). The longer lifetime is a crucial criterion too to make it a popular choice for EVs and hybrid electric vehicles (HEVs). Since, the aging of Li-ion batteries depend on several degradation factors related mostly to used active materials, electrolyte etc., two types of LTO battery cells are selected for cycle life aging investigation in this research. Fast charging is selected as the key variation in the cycling conditions to understand the battery aging under accelerated lithiation process. Electrochemical impedance spectroscopy (EIS) is used along with typical capacity check during tests to characterize the batteries during lifetime.

In this study, two types of LTO batteries were cycled under a long cycling campaign. Both the cells are power optimized and commercially available. The pouch shaped EIG 5Ah cell has a LiNixCoyAl1-x-yO2 (NCA) cathode material and a specific power of more than 2000 W/kg. On the other hand, TOSHIBA manufactured LTO is a prismatic cell, which operates at a nominal voltage of 2.3V, has a specific power of more than 1200 W/kg and it has a cathode material possibly of LiNixCoyMn1-x-yO2 (NMC). These batteries were cycled with four fast charging conditions at room temperature and with 90% and 80% depth-of-discharge (DoD), respectively. LTO 5Ah cell was cycled with 0.5C to 5C charging currents against 1C discharge current, where C-rate refers to the cell’s nominal capacity. Whereas, LTO 23Ah was tested in the range of 1C to 8C charging current versus 2C discharge rate. The ongoing lifetime test campaign is already more than 2-yr long, however, aging characteristics is quite different. The lifetime tests and the capacity measurement are done with PEC ACT0550 battery cycler and the EIS is performed with the Biologic’s MPG-205 equipment. The performance tests were done in the frequency based on full equivalent cycles (FEC) of which, FEC is taken as the nominal charge Ah throughput from a single charge-discharge cycle. All the tests were performed inside controlled environment temperature chambers (CTS).

The capacity fade characteristics during aging, is found to be insignificant for the EIG cell as it has a negative degradation which basically means an improved state of health (SoH). A SoH can be defined as the percentile of actual or latest capacity divided by the beginning of life (BoL) value. All the aging conditions have experienced capacity gain for the EIG cell and even after 4000 FEC, no degradation is found. This proves both the fast charging suitability of this cell and longer lifetime as well. On the contrary, TOSHIBA LTO 23Ah cell has shown charging C-rate dependency on the capacity fade. Cells cycled with 2C rate aged faster (6%) than the other conditions. This is little unusual thus further investigation is necessary. While other conditions with different charging speed have moderate degradation with the maximum of 2.5% fade for 4C charge-cycling after 4800 FEC. The ongoing cycling investigation of LTO 23 Ah cell will have better illustration of the degradation characteristics in future.

The impedance growth is analyzed after obtaining the parameters through an equivalent circuit model (ECM). The quantitative black box type modeling was done by EC-lab software by choosing a one-RQ circuit where the Nyquist plot is fitted by ohmic resistance (R0), charge transfer resistance (Rct) and constant phase elements (CPEs, Q). The results show that LTO 5Ah has a maximum ohmic resistance increase of 26% for 1C charge condition while the charge transfer resistance growth is gradually higher for the higher C-rate cycling eventually reaching 84% growth for this cell. To the opposite, the TOSHIBA cells have insignificant R0 increase and have decreasing Rct growth by C-rate.

In general, the lower impedance growth in both the cells indicate their long lifetime and good performance under fast charging conditions. The characterization parameters in terms of capacity and power fade investigated in this study, can be used as model input parameters to do prognosis and/or diagnosis of LTO-anode based cells as state of charge/health/power/energy prediction.

Figure 1