Effects of Hybrid Solid Electrolyte on Internal Resistance of TiNb₂O₇/LiNi_0.5Co_0.3 Mn_0.2O₂ Cells

Abstract

Lithium-ion batteries have been required to enhance power density, high and low temperature performance, energy density, and safety due to electrification of various mobilities. We have developed hybrid solid electrolytes consisting of lithium-ion conducting solid particles and a gel polymer or liquid electrolyte as candidate to respond to these demands [1]. Our previous study revealed that hybrid solid electrolytes in the cell reduce the internal resistance, particularly at low temperatures. Moreover, we proposed the mechanism of the resistance reduction, which the lithium ion transference number was increased by attracting the cyclic carbonates such as PC and PF6 - anion on the surface of Lithium-ion conducting solid particles [2]. In this study, we have investigated the effect of hybrid electrolytes on the internal resistance of TiNb2O7(TNO)-based high-power lithium ion batteries. We prepared 1Ah TNO/LiNi0.5Co0.3Mn0.2O2 (NCM) pouch cells with hybrid electrolytes consisting of lithium-ion conducting solid electrolyte wetted by PF6-based liquid electrolyte. The solid electrolyte layer was formed on cathode as shown in Fig.1. NASICON-type Li1.2Zr1.9Ca0.1(PO4)3 (LZCP) was used as a lithium-ion conducting solid electrolyte because of its wide potential window, high chemical stability, and relatively low cost of raw materials. We compared electrochemical properties of the LZCP-based hybrid electrolytes cell to that of the liquid electrolyte cell and the Al2O3 (non-ion conductivity ceramic)-based hybrid electrolytes cell. The particle size and the film thickness of layer for Al2O3 were comparable to those for LZCP. Figure 2 shows DC resistance (DCR) of the three types of cells at various state of charge (SOC) conditions for 10 sec charging at 25℃ and -20℃. The DCR of LZCP-based hybrid electrolyte cell is smaller than that of liquid electrolyte in the whole SOC range at 25℃. At low temperature, the difference of DCR of two cells becomes remarkable, and DCR of LZCP-based hybrid electrolyte cell is about 20% lower than that of liquid electrolyte cell at -20℃. On the other hand, the DCR of the Al2O3-based hybrid electrolyte cell was a little higher than that of liquid electrolyte cell in the all temperature range. Therefore, the solid electrolyte contributes to the reduction of internal resistance especially at low temperatures. Typical AC-impedance spectra of the cells with the three kinds of electrolytes at 25℃ and -20℃ are shown in Figure 3. The impedance spectra exhibit a depressed semicircle, which can be interpreted as resulting from the interface resistance such as the charge-transfer process on the electrodes. The semicircle of the hybrid electrolytes cell is significantly smaller than that of liquid electrolyte cell and Al2O3-based hybrid electrolyte cell especially at -20℃. It is considered that the concentration overpotential near the cathode mainly decreased due to the increase of lithium ion transference numbers by using solid electrolyte to promote the supply of lithium ion to the electrode surface. On the other hand, the LZCP-based hybrid electrolytes cell and the Al2O3-based hybrid electrolyte cell have a larger bulk resistance than that of the liquid electrolytes cell at 25℃. This bulk resistance is mainly ascribable to lithium ion conductive resistance in liquid electrolyte. The lithium ion conductive resistance increased because the porosity was decreased by inserting of solid electrolyte layer or Al2O3 layer between electrodes. These results reveal that LZCP particles in the hybrid solid electrolyte reduces the charge transfer resistance between the electrolyte and active materials rather than the bulk ionic resistance in the electrolyte. The charge transfer resistance rapidly increases compared to the bulk ionic resistance with decreasing temperature. Therefore, it is considered that the DCR of LZCP-based hybrid electrolyte cell decreases especially at low temperatures. In this report, the detail of this hybrid electrolytes cell performance and the mechanism of resistance decreasing will be discussed. [1] K. Yoshima, Y. Harada and N. Takami, J. Power Sources, 302,283-290(2016). [2] T. Kusama, K. Yoshima, T. Sugizaki, K. Hoshina, T. Sasakawa, Y. Harada, and N. Takami, Abstract No A02-0172, The Electrochemical Society Meeting Abstract MA2019-01, Dallas, TX, May 27 (2019) Figure 1