Lithium-ion batteries using Li₄Ti₅O₁₂ (LTO) anode realizes high power, long life, and safety for automotive and stationary power applications because the LTO electrode has the advantages of nano-size particle, no lithium plating at quick-charging and low temperature conditions, outstanding safety on internal short-circuit, and thermal stability under high-temperature conditions[1,2]. Recently, LTO-based lithium-ion batteries have been investigated for low-voltage systems such as Start & Stop vehicle systems and the replacement of Pb-acid battery for stationary power supply because the LTO-based batteries have good voltage harmonization with the Pb-acid battery. We have been developing five series-connected LTO-based cells for 12 V low-voltage systems. LiMn_1-xFe_xPO₄ (LMFP) is a 4 V-class cathode with high thermal stability compared to such as LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂, and LiMn₂O₄. Lithium-ion cell with a combination of the LMFP cathode and the LTO anode shows a voltage plateau of 2.5 V for Mn³⁺/Mn²⁺ redox. We have been preliminarily studying a thin hybrid electrolyte based on a cubic garnet-type Li₇La₃Zr₂O₁₂(LLZ) as lithium-ion conducting ceramics for 12 V-class bipolar LTO/LMFP battery with liquid-free and separator-free [3]. The bipolar battery has advantages of size reduction and low-cost. In addition, LMFP and LTO can share the same aluminum current collector, which makes the bipolar structure simple. In this study, we report the electrochemical characteristics, the performance of LTO/LMFP cells, and the development of 12 V-class bipolar LTO/LMFP battery using the LLZ-based hybrid electrolyte.

    LiMn_0.85Fe_0.15PO₄ and LiMn_0.85Fe_0.1Mg_0.05PO₄ as the LMFP cathode were hydrothermally synthesized to obtain a small particle size of about 80 nm and no impurity phase. The discharge capacity of cathode increased from 140 to 160 mAh/g by using the LiMn_0.85Fe_0.1Mg_0.05PO₄ cathode. We fabricated 1 Ah and 25 Ah-class LTO/LMFP cells in order to investigate the cell performance. Discharge rate capability of the LTO/LMFP cell was improved to be comparable with that of the LTO/LMO cell. High-temperature cycle test exhibited that the LTO/LMFP cell was significantly superior to that of the LTO/LMO cell. The dissolution amount of Mn from the LMFP cathode at 55°C was two order of magnitude smaller than that from the LMO cathode. Fig.1 shows close-circuit voltage (CCV) at 0.2 C rate and open-circuit voltage (OCV) of the five series LTO/LMFP cells and 12 V Pb-acid battery during charge-discharge cycle. Five series LTO/LMFP cells exhibited good voltage harmonization with the 12 V Pb-acid battery in the voltage range between 10 V and 14 V and a smaller polarization compared with the Pb-acid battery. The LTO/LMPF cells are expected to be a replacement of Pb-acid battery in automotive and stationary power storage applications. Therefore, we fabricated the bipolar batteries constructed from five LTO/LMFP cells stacked together, which contained four bipolar electrodes and two monopolar electrodes as endplates for output voltage of 12 V. Cathodes and anodes in bipolar batteries are disposed in contact respectively with opposite sides of a common current collecting element forming a unitary bipolar structure. LTO and LMFP were coated with the LLZ hybrid electrolyte containing 4wt% polyacrylonitrile (PAN) gel polymer [3]. Fig.2 shows the cross section SEM image of LTO/LLZ/LMFP and typical charge-discharge curve of the bipolar battery. A few micrometers thickness of LLZ hybrid electrolyte layer was thin enough to reduce the internal resistance. This LLZ substrate did not only work for the lithium-ion conductor but also retain sufficient strength and electronic insulation even for a little amount of the PAN gel polymer and separator-free in the hybrid electrolyte layer. The bipolar battery exhibited the voltage plateau of 12.5 V, which is matched with the voltage of Pb-acid battery. Fig.3 shows discharge rate performance of the bipolar battery at various discharge rates. The discharge capacity retention at 5 and 20 C rates was 94 and 75%, respectively. The polarization of the bipolar battery was relatively small even at a high discharge rate. It was demonstrated that the bipolar battery had good performance in terms of discharge rate, low temperature, and cycle life. The performance of the 12 V-class bipolar battery was comparable with that of conventional lithium-ion batteries with a liquid electrolyte, indicating that 12 V-class bipolar LTO/LMFP battery using the LLZ-based hybrid electrolyte has enough performance for practical uses. We are now trying to develop larger bipolar batteries for low-voltage system applications. 

 

References

[1]N.Takami, H.Inagaki, T.Kishi, Y.Harada, Y.Fujita, K.Hoshina, J.Electrochem.Soc.,156, A128(2009)

[2]N.Takami, H.Inagaki, Y.Tatebayashi, H.Saruwatari, K.Honda, S.Egusa, J.Power Sources, 244,469 (2013)

[3]K.Yoshima, Y.Harada, N.Takami, J. Power Sources,302,283(2016)