Ferroelectric SEI for Lithium Ion Batteries with Ultrahigh Rate Capability
Authors: Teranishi Takashi , Yoshikawa Yumi , Hayashi Hidetaka , Kishimoto Akira 
Published: February 2020

Lithium ion batteries with drastically enhanced power densities will enable excellent acceleration and better fuel economy, making them promising candidates as the next-generation battery for hybrid and electric vehicles. Higher power density; i.e., a capacity with faster charging/discharging times, is generally limited by the diffusion of Li+ in bulk compounds. A thin layer such as Al2O3 [1] decorated on the active materials acts as an artificial solid electrolyte interface (SEI), and improves the rate performance.

We claim that another breakthrough involves using the “ferroelectricity” of an artificial SEI to assist the Li ion diffusion. Dipole moments due to the polarization are generated in the ferroelectric SEI layer under a potential difference between two electrodes. Since the dipole moments are negatively charged at the active material–SEI interface, these negative charges might attract the positively charged Li+ as the battery charges and discharges. Such a “c
arrier
inducing effect of ferroelectricity”, would assist the smooth intercalation and deintercalation of Li ions, leading to an excellent power density.

LC particles were coated with ferroelectric BT layers via a simple liquid phase reaction, the sol–gel route. Various amounts of BT (0.1 to 15mol%) were loaded on the LC powder as a ferroelectric artificial SEI. The size of decorated BT nanoparticles was ca. 50 nm. A 2032-type coin cell was employed for the evaluation. The cells were cycled in a voltage window of 3.3–4.5 V. The cells were first charged and discharged at 0.1C (1C = 160 mA/g) for five cycles, and then the charge–discharge rate was increased stepwise to 10C (6 min. for 100% cha- and discharge)/100C (36 sec. for 100% cha- and discharge) for five cycles at each rate.

The initial capacity of the bare LC was ~187 mAh/g and the capacity was reduced significantly at above 1C, to 62 mAh/g at 35 cycles, 10C. This capacity corresponded to only 33% of the initial value. The composites with smaller BT loadings from 0.1–1 mol% exhibited relatively good retention of the discharge capacity at a high rate (10C); in particular, the 1 mol% BT had an excellent discharge capacity after 10C for five cycles (total 35 cycles) of 146 mAh/g, which is 78% of the initial capacity, versus as high as 238% of the capacity over the same cycles for the bare LC [2, 3].

To understand the origin of the enhancement in the high-rate capability, the impedances of the cells while charging and discharging were measured. The BT loading significantly reduced the resistance at lower frequencies; i.e., the resistance due to the cathode reaction, R
ct. For the BT 1 mol%, R
ct = 10.2 Ω, which was 1/12 that for the bare LC, R
ct = 127 Ω [4]. This significant reduction in R
ct could imply the promotion of the Li diffusivity at the electrode–electrolyte interface; i.e., at the BT SEI, attributed to the polarization due to the ferroelectric BT layer.

In-situ time-resolved dispersive X-ray absorption fine structure (DXAFS) analysis of the composites was also done to characterize the cobalt ion valence shift between oxidized and reduced states. Two types of artificial SEIs, ferroelectric BT and paraelectric Al2O3, were compared. The magnitude of the shift in the X-ray absorption energy at the peak of the white line, E
1, during charging and discharging at a 10 C rate, increased in the order of bare LC (0.264 eV) < Al2O3 1 mol% (0.497 eV) < BT 1 mol% (1.15 eV); the corresponding discharge capacities of the cells at 10 C [5]. All these results indicated the ferroelectric SEI effectively contributed to the enhancement in the high rate capabilities of the cell.

The ultrahigh rate cell performances of the LCs coated with various dielectric/ferroelectric SEIs will be also presented in the talk.

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