Insights into the reactive sintering and separated specific grain/grain boundary conductivities of Li1.3Al0.3Ti1.7(PO4)3

Active Interphase Enables Stable Performance for an All‐Phosphate‐Based Composite Cathode in an All‐Solid‐State Battery

“…[ 46 ] It is worth noting that the standard Li 2 FeTi(PO 4 ) 3 XRD pattern (ICSD code: 151919) was used as the reference Pbna phase for the Rietveld refinement because the Pbna phase only containing LATP elements is not available in ICSD database. The refinement results show that the lattice constant c (20.8774 Å) of LATP is larger than that of the reported LATP sintered in air (20.8143 Å) (Table S1, Supporting Information), [ 34 ] suggesting the obtained rhombohedral LATP has a higher Ti/Al ratio due to the larger ionic radius of Ti 4+ (60.5 pm, coordination: 6) compared with that of Al 3+ (53.5 pm, coordination: 6). [ 47 ] This agrees with what Gellert et al have observed that heat treatment of Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 in Ar/H 2 atmosphere leads to an Al 3+ deficient LATP due to the formation of AlPO 4 .…”

“…It is worth noting that the grain boundary resistances in NASICON‐type compounds are usually ≈10 times higher than that of the bulk, implying the grain boundary resistance mainly determines the total ionic conductivity of the co‐fired LFP/LATP. [ 34,60,61 ] As the LFP/LATP interface is modified by a thick layer (40–100 nm) of LFTAP interphase, the ionically conductive nature of LFTAP is the determining factor for the sample to achieve the high σ i . The σ e of the sample was further verified to be 7.28 mS cm –1 at 60 °C by DC measurement, which is consistent with the EIS result (Figure S9a, Supporting Information).…”

“…The E a (Li) of the co‐fired LFP/LATP sample is in agreement with the reported total ionic activation energies of LATP. [ 34,64,65 ] The obtained E a (Li) is much lower than that of the lowest theoretical energy barrier of 0.60 eV in pure LFP, [ 66 ] suggesting that the ion transport inside the composite cathode is mainly through the NASICON‐type LATP and LFTAP phases. Besides, attributed to the shortened electronic diffusion path in the dense co‐fired LFP/LATP composite cathode, a low E a (e) of 0.07 eV is achieved.…”

“…[32] In particular, Li 1.15 Ti 1.85 Fe 0.15 (PO 4 ) 3 achieved ionic conductivity of 0.52 mS cm -1 at room temperature, which is comparable with that of LATP. [26,[32][33][34] Apart from their decent ionic conductivities, NASICON-type LiM 2 (PO 4 ) 3 materials consisting of transition metal ions are redox-active. [35][36][37] Patoux et al demonstrated the reversible Li + insertion/extraction of NASICONtype Li 2 FeTi(PO 4 ) 3 using the potentiostatic intermittent titration technique.…”

“…[30,33] In our previous work, highly ionically conductive LATP can be obtained at a low sintering temperature of 775 °C with the help of liquid phase sintering. [34] The reduced densification temperature of LATP is considered to have the ability to suppress the decomposition of LFP/LATP during the co-firing process. On the other hand, like many CAMs, the redox-reaction of electrochemically active interphase influences its ionic conductivity, [39,40] affecting the Li + transport kinetics and electrochemical behavior of the ASSB.…”