Simultaneously improving sodium ionic conductivity and dendrite behavior of NaSICON ceramics by grain-boundary modification

Developing highly conductive and reliable solid-electrolytes (SEs) is still important for the advancement of solid-state sodium batteries. NaSICON-type polycrystalline SEs exhibit the dominance of grain-boundary resistance to the total resistance, which is mainly due to the thermal expansion anisotropy of NaSICON-type lattices. In this study, we modify the grain boundaries of NaSICON-type Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 (NZSP) by adding 2.5 mol% Na 3 LaP 2 O 8 (NLP) to counteract the effect of thermal expansion anisotropy. NLP does not serve as a sintering aid for NZSP because the sintering temperature and relative density of NZSP is not changed. The total conductivity of modified NZSP increases to 7.1 mS cm −1 at 25 °C, surpassing other reported polycrystalline oxide SEs. The critical current density of Na | modified NZSP | Na symmetric cells increases to 22 mA cm −2 . The cells can survive under long-term galvanostatic cycling up to 10 mA cm −2 , indicating the unprecedented dendrite tolerance. Remarkably, the main failure mode in these cells shifts from Na-dendrite short-circuiting to the loop of substantial polarizations and short-circuits. • NZSP is modified by adding 2.5 mol% NLP, resulting in σ total of 7.1 mS cm −1 at RT. • The effect of NLP is to reinforce the grain-boundary of NZSP. • NZSP:NLP shows high robustness against Na dendrite in Na.|NZSP:NLP|Na cells. • The CCD is up to 22 mA cm −2 , and long-term cycling is up to 10 mA cm −2 . • The cells show the loop of polarizations and short-circuits after dendrite.