Bulk-type all-solid-state rechargeable lithium batteries using oxide solid electrolytes (Ox-SSBs) require low-resistive electrode-solid electrolyte composite.  Both electrode and solid electrolyte are fragile and hard ceramics and then they need a densification process to enlarge charge transfer reaction sites at the interface.  Those composites have been developed by heating processes such as co-sintering [1], spark plasma sintering [2, 3], fusion bonding using low-melting solid electrolyte [4], etc.  However, one of the crucial problems to apply such heating process is a formation of mutual diffusion layer at the interface.  Formation of mutual diffusion layer can work as a driving force of solid-solid adhesion, but at the same time, that can retard charge transfer reaction at the interface, resulting in the reducing of the battery power density.  Therefore, it is important to examine interfacial structure and interfacial resistance by a formation of mutual diffusion layer in detail. 

This work focuses on LiNi_1/3Co_1/3Mn_1/3O₂ (NMC: cathode) / LATP (solid electrolyte) interface.  The NMC shows smaller volume change (less than 1%) during charge-discharge reactions among the various positive electrode materials [5].  Also, LATP is one of the highest Li⁺ conductive solid electrolytes (~10^-4 S cm^-1) [6].  Here, the NMC films were deposited by pulsed laser deposition (PLD) on mirror-polished LATP sheets (150 μm in thickness, Ohara Inc.) at different temperature as a model system of sintered LATP/NMC interface.

Experimental

NMC films were deposited on one side of LATP sheets by PLD at 700 °C or 900 °C, which will be denoted as NMC-700 and NMC-900, respectively.  Bare side of the LATP sheet was immersed in a liquid electrolyte, where both reference and counter electrodes of lithium were immersed.  Electrochemical properties of those NMC/LATP half-cells were measured by cyclic voltammetry (CV) and AC impedance spectroscopy.  Structure analyses of NMC/LATP interface were carried out by XRD, TEM, EDX, and EELS.

Results and discussion

Thin films of NMC prepared at 700 °C or 900 °C on Pt/Ti-coated SiO₂ substrates showed almost same redox reactions at around 3.6-3.8 V in liquid electrolyte, and the charge transfer resistance (R_ct) at 4.0 V was 200-350 Ω cm² in both cases.  However, they showed quite different reactivity on the LATP sheets.  Fig. 1(a) shows CVs of the NMC-700 and NMC-900 measured at 1 mV s^-1.  The NMC-700 appeared redox peaks at around 3.6-3.8 V as with the film in liquid electrolyte, while the NMC-900 did not show any redox peaks.  The R_ct of NMC-700 at 4.0 V was 315 Ω cm², while that of NMC-900 was 114000 Ω cm².   These results suggest that the R_ct of NMC/LATP strongly depend on the sintering temperature.

Fig. 1(b) shows the EDX line profile of O, Ti, P, Ni, Co, and Mn around the interface of NMC-900.  Also, integrated intensity ratio of L edge peaks, L3-edge/L2-edge, in Co (L3/L2-Co) and Mn (L3/L2-Mn) are shown below, which corresponds to the valence state of Co and Mn [7].  A mutual diffusion layer was observed around the interface, but its thickness of 30 nm was same with the case of NMC-700.  XRD analysis detected impurity peaks in case of the NMC-900 while not NMC-700/LATP, indicating that mutual diffusion layer is crystallized.  Also, both EDX and EELS analyses suggested that Co in the NMC films looks to diffuse toward the LATP as a reduced valence at higher sintering temperature.  Based on these results, we will conclude that both structural disorder in NMC around the interface and crystallization of mutual diffusion layer will be key issues to increase the R_ct of NMC/LATP interface at higher sintering temperature.

Acknowledgement

This work was supported by JST-ALCA.

References

[1] E. Kobayashi, et al., Electrochem. Commun., 12, 894 (2010).  [2] A. Aboulaich, et al., Adv. Energy Mater., 1, 179 (2011).  [3] G. Delaizir et al., Adv. Funct. Mater., 22, 2140 (2012).  [4] S. Ohta et al., J. Power Sources, 238, 53 (2013).  [5] T. Ohzuku and Y. Makimura, Chem. Lett., 30, 642 (2001).  [6] J. Fu, Solid State Ionics, 96, 195 (1997). [7] Z. L. Wang, J. S. Yin, and Y. D. Jiang, Micron, 31, 571 (2000).