Role of Cathode-Electrolyte-Ferroelectric Interface for High Performance Lithium Ion Battery

Next generation lithium ion battery(LIB) should be endowed with a performance of high-speed chargeability and dischargeability. LiCoO2is commercially used as a cathode material of LIB but long period of time is generally needed to charge, which is originated from diffusion-rate-limitation of lithium ions. Usually, charge-discharge reaction is impeded by the side reaction at the electrode/electrolyte interface, where the cathode is coated by a solid electrolyte interface (SEI). The formation of SEI is well recognized in LIB and it mainly blocks intercalation/deintercaration of lithium ion into/from the cathode. In 2014, Teranishi et al. reported that LiCoO2 supported with ferroelectric BaTiO3 showed a good performance at high charge-discharge rate measurement.1,2 However, at the present time, the role of BaTiO3 in the improvement of charge-discharge speed is unknown. To make this point clear, we have fabricated epitaxial thin films and dots of BaTiO3 on single crystalline LiCoO2 films, evaluated the rate property of the charge and discharge of prepared samples, and examined the role of BaTiO3. Firstly, we prepared ‘Bare-LiCoO2’ which was LiCoO2 epitaxial thin films deposited on conductive SrRuO3/(100)SrTiO3 substrates by pulsed laser deposition method.Then we fabricated two types of BaTiO3/LiCoO2 epitaxial thin films. One is ‘Planer BaTiO3’, the other ‘Dot BaTiO3’. ‘Planer BaTiO3’ were coated by a sub-nm thickness of BaTiO3 on LiCoO2 surface. ‘Dot BaTiO3’ were partially coated by BaTiO3 nano-dots on LiCoO2 surface. We succeeded to obtain different shaped BaTiO3 by adjusting the P(O2) during deposition. Crystal structure of thin films were evaluated by high resolution X-ray diffraction (HRXRD) and cross sectional high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We also prepared coin cell(half-cell); Li│1mol/L LiPF6 EC:DEC (3:7 v/v) │LiCoO2 and measured cathode properties by successive charge-discharge measurements. Cut off potential was set 3.3 V - 4.2 V vs. Li+/Li and charge-discharge rate was investigated in the range of 1 C to 100 C. Out of plane XRD measurement showed that LiCoO2 104 was grown along (100)cSrRuO3//(100)SrTiO3 001 without any secondary phases and other orientations. HRXRD-RSM measurement clearly showed that all the prepared films were found to be epitaxially grown on (100)SrTiO3 substrates. From HAADF-STEM-EDS images, BaTiO3 layer was also found to be epitaxially grown on LiCoO2. All epitaxial relationships of each layers are expressed as follows; [001]BaTiO3//[104]LiCoO2//[001]SrRuO3//[001]SrTiO3, [100]BaTiO3//[0-14]LiCoO2//[100]SrRuO3//[100]SrTiO3 and [010]BaTiO3//[-114]LiCoO2//[010]SrRuO3//[010]SrTiO3. We performed to measure charge-discharge cycle for ‘Bare LiCoO2’ films. The charge-discharge curve was confirmed to be almost similar to the bulk one. 2 nm-‘Planer BaTiO3’ films showed lower discharge capacity at high C rate than ‘Bare LiCoO2’ one. Then, 1 nm-‘Planer BaTiO3’ films showed better performance at high C rate than that of ‘Bare LiCoO2’ and 2 nm-‘Planer BaTiO3’ films. On the other hand, ‘Dot BaTiO3’ films showed the best performance at high C rate, discharge capacity at 100 C only reduced by 40% of that at 1 C. Only ‘Dot BaTiO3’ films were still working at 100 C even though the other type films were not working under same measurement condition. Here, we will discuss about effect of film thickness of BaTiO3. 1 nm-‘Planer BaTiO3’ films (NOT fully covered on LiCoO2) worked as cathode however 2 nm-‘Planer BaTiO3’ one (fully covered on LiCoO2) did not work. It is considered that Li+ cannot penetrate into the inside of BaTiO3 grains however it could pass through grain boundaries. From the result of ‘Dot BaTiO3’ films, we expect that an enhancement of discharge capacity at high C rate was caused by BaTiO3/LiCoO2/electrolyte three-phase interfaces. It is informed that ‘electric field concentration’ may be occurred around the three-phase interfaces, then Li+are expected preferentially to pass around the three-phase interfaces. In summary, the origin of this enhancement by BaTiO3 was attributed to the three-phase interface due to an electric field concentration. The necessity of BaTiO3 for the enhancement of the charge-discharge performance is still unclear because similar reports using non ferroelectric ZrO2 3 and Al2O3 4 showed enhancement of Li+ intercalation. However, dischargecapacity ratio of 10 C/1 C in this study is better than these previous reports. 1. T. Teranishi et al., Appl. Phys. Lett., 105, 143904 (2014) 2. T. Teranishi et al., ECS Electrochem. Lett., 4 (12), A137 (2015) 3. D. Takamatsu et al., J. Electrochem. Soc., 160 (5), A3054 (2013) 4. I. D. Scott, et al., Nano Lett., 11, 414 (2011)