Quantum view of Li-ion high mobility at carbon-coated cathode interfaces

Abstract

Lithium-ion batteries (LIBs) are among the most promising power sources for electric vehicles, portable electronics and smart grids. In LIBs, the cathode is a major bottleneck, with a particular reference to its low electrical conductivity and Li-ion diffusivity. The coating with carbon layers is generally employed to enhance the electrical conductivity and to protect the active material from degradation during operation. Here, we demonstrate that this layer has a primary role in the lithium diffusivity into the cathode nanoparticles. Positron is a useful quantum probe at the electroactive materials/carbon interface to sense the mobility of Li-ion. Broadband electrical spectroscopy demonstrates that only a small number of Li-ions are moving, and that their diffusion strongly depends on the type of carbon additive. Positron annihilation and broadband electrical spectroscopies are crucial complementary tools to investigate the electronic effect of the carbon phase on the cathode performance and Li-ion dynamics in electroactive materials.

Keywords: Electrochemical energy storage; Electrochemical materials science; Electrochemistry; Materials application; Materials science.

Graphical abstract

Figure 1

Broadband electrical spectroscopy results Real permittivity (ε′, A) and conductivity (σ′, B) spectra on frequency of the four different samples containing LiCoO₂, PVDF and: without carbon (No C, blue circles), carbon Super P (SP, green triangles), carbon nanospheres (XC, red diamonds), and carbon nanotubes (NT, black squares). Markers represent experimental data and dashed lines are the fitting results. Concentration of effective mobile lithium ions ($n_{{Li}^{+}eff}$, C) and lithium diffusion coefficient (D_Li⁺, D) as a function of the carbon employed into the cathode electrode. Error bars (2 sigma) are calculated on the basis of the fitting error and experimental data accuracy. Dotted lines in C represent the total lithium ion concentration in the electrodes with (red line) and without (blue line) carbon.

Figure 2

Positron lifetime spectra of LiCoO₂ cathodes The spectra of SP, XC, NT and No C cathodes are normalized at the same high peak after subtracting the source and spurious components (NT spectrum is multiplied by a factor of 5 to avoid overlapping with XC spectrum). The inset shows the intensity of the second lifetime component I₂ as a function of the carbon employed to coat the cathode grains (see Table 1). Error bars are calculated on the basis of the experimental errors.

Figure 3

(A) Coincidence Doppler broadening of the annihilation radiation of the studied cathode materials Distribution of the annihilation peak intensity $I\left(p_L\right)$ (normalized in area) as a function of the momentum $p_L$ of the annihilation pair in the longitudinal direction of detection. (B and D) Momentum distribution $N\left(p_L\right)$ associated with the high momentum electron contribution, i.e., the chemical fingerprint of the annihilation site (NT distribution is translated adding 0.25 $N\left(p_L\right)$ units to prevent overlapping). (C) S parameter evolution as a function of the positron implantation energy (or mean implantation depth, upper frame) in LCO cathodes and in carbon (black dashed line). The dashed lines through the experimental data in panel c correspond to a VEPFIT fit.