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 ZrO₂³ and Al₂O₃⁴ 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)