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. LiCoO 2 is 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 LiCoO 2 supported with ferroelectric BaTiO 3 showed a good performance at high charge-discharge rate measurement. 1,2 However, at the present time, the role of BaTiO 3 in the improvement of charge-discharge speed is unknown. To make this point clear, we have fabricated epitaxial thin films and dots of BaTiO 3 on single crystalline LiCoO 2 films, evaluated the rate property of the charge and discharge of prepared samples, and examined the role of BaTiO 3 . Firstly, we prepared ‘Bare-LiCoO 2 ’ which was LiCoO 2 epitaxial thin films deposited on conductive SrRuO 3 /(100)SrTiO 3 substrates by pulsed laser deposition method.Then we fabricated two types of BaTiO 3 /LiCoO 2 epitaxial thin films. One is ‘Planer BaTiO 3 ’, the other ‘Dot BaTiO 3 ’. ‘Planer BaTiO 3 ’ were coated by a sub-nm thickness of BaTiO 3 on LiCoO 2 surface. ‘Dot BaTiO 3 ’ were partially coated by BaTiO 3 nano-dots on LiCoO 2 surface. We succeeded to obtain different shaped BaTiO 3 by adjusting the P (O 2 ) 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 LiPF 6 EC:DEC (3:7 v/v) │LiCoO 2 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 LiCoO 2 104 was grown along (100) c SrRuO 3 //(100)SrTiO 3 001 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)SrTiO 3 substrates. From HAADF-STEM-EDS images, BaTiO 3 layer was also found to be epitaxially grown on LiCoO 2 . All epitaxial relationships of each layers are expressed as follows; [001]BaTiO 3 //[104]LiCoO 2 //[001]SrRuO 3 //[001]SrTiO 3 , [100]BaTiO 3 //[0-14]LiCoO 2 //[100]SrRuO 3 //[100]SrTiO 3 and [010]BaTiO 3 //[-114]LiCoO 2 //[010]SrRuO 3 //[010]SrTiO 3 . We performed to measure charge-discharge cycle for ‘Bare LiCoO 2 ’ films. The charge-discharge curve was confirmed to be almost similar to the bulk one. 2 nm-‘Planer BaTiO 3 ’ films showed lower discharge capacity at high C rate than ‘Bare LiCoO 2 ’ one. Then, 1 nm-‘Planer BaTiO 3 ’ films showed better performance at high C rate than that of ‘Bare LiCoO 2 ’ and 2 nm-‘Planer BaTiO 3 ’ films. On the other hand, ‘Dot BaTiO 3 ’ 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 BaTiO 3 ’ 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 BaTiO 3 . 1 nm-‘Planer BaTiO 3 ’ films (NOT fully covered on LiCoO 2 ) worked as cathode however 2 nm-‘Planer BaTiO 3 ’ one (fully covered on LiCoO 2 ) did not work. It is considered that Li + cannot penetrate into the inside of BaTiO 3 grains however it could pass through grain boundaries. From the result of ‘Dot BaTiO 3 ’ films, we expect that an enhancement of discharge capacity at high C rate was caused by BaTiO 3 /LiCoO 2 /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 BaTiO 3 was attributed to the three-phase interface due to an electric field concentration. The necessity of BaTiO 3 for the enhancement of the charge-discharge performance is still unclear because similar reports using non ferroelectric ZrO 2 3 and Al 2 O 3 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)