DVS_Bericht_393LP

Plasma-spray deposition of Na3Zr2Si2PO12 electrolyte for high
performance all solid-state sodium-ion battery
Xiao-Chen Bu, Nan Chen, Xiao-Tao Luo, Chang-Jiu Li *; State Key Laboratory for Mechanical Behavior of
Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an/China. Email address:
licj@mail.xjtu.edu.cn.
Abstract
All solid-state sodium-ion batteries (ASS-SIBs) have great potential for application to large-scale energy
storage devices due to their safety advantages by avoiding flammable organics and the abundance of sodium.
In this study, plasma spraying was used to deposit Na3Zr2Si2PO12 (NZSP) electrolyte for assembling
high performance ASS-SIBs. NZSP electrolyte layers were deposited at different spray conditions using
NZSP powders in different particle sizes. The factors influencing the microstructure and compositions of
NZSP layers were examined by characterizing the compositions of splat and cross-sectional microstructures
of the deposits. It was found that the preferential evaporation loss of Na and P elements occurs severely to
result in a large composition deviation from initial powders and spray particle size is key factor which dominates
their evaporation loss. The APS NZSP electrolytes present a dense microstructure with well bonded
splats which is attributed to low melting point of NZSP. The apparent porosity of the as-sprayed NZSPs was
lower than 3 %. The effect of annealing on the microstructure of APS NZSP was also investigated. The performance
of typical APS NZSP was also evaluated by assembling an ASS-SIB battery with APS NaxCoO2
(NCO), Na3Zr2Si2PO12 (NZSP) and Li4Ti5O12 (LTO) as cathode, electrolyte and anode, respectively. Results
showed that columnar-structured grains with a chemical inter-splat bonding were formed across the interfaces
between electrodes and electrolyte. There is no evidence of inter-diffusion of zirconium, cobalt and silicon
across the NCO/NZSP interface. With the preliminary battery, the solid electrolyte exhibited an ionic conductivity
of 1.21 × 10 -4 S cm -1 at 200 o C. The SIB can operate at 2.5 V with a capacity of 10.5 mA h g -1 at current
density of 37.4 μA cm -2 .
Keywords: Sodium-ion battery, NASICON electrolyte, plasma spray, capacity
1 Introduction
Recently sodium ion batteries (SIBs) have drawn great attention due to their great advantages: the abundance
of sodium (2.6 %, while lithium only accounts for 0.06 %), low mining costs and similar electrochemical
performance compared with lithium ion batteries [1-4]. The cost-effective manufacturing of SIBs with absolute
safety are beneficial for large-scale electrochemical energy storage system [2].
For traditional SIBs, liquid organic electrolyte are widely used, which still suffer from severe safety problems
such as liquid leakage and risk of fire [4, 5]. The solid-state Na-ion batteries are of great interest due to their
excellent safety advantage within a large operation temperature range and their potentially high energy densities
when compared to liquid counterparts [6, 7]. NASICON-type electrolytes with a general formula of
Na1+xZr2SixP3-xO12 (0≤x≤3) were first reported by Goodenough and attracted great attention for its high ionic
conductivity (10 -4 -10 -3 S/cm, RT) and chemical stability in air [4, 8]. Ran et al. found that the Sc and Ge codoping
of NZSP can stabilize the rhombohedral phase at RT and significantly boost the sodium ion transfer
[9]. For the synthesis of ceramic electrolyte, solid-state reaction is commonly used. However, the high
reacting temperature (>1200 o C) and long sintering duration (>10 h) may cause the element diffusion across
electrolyte/electrode interface [10-12]. Thus, it is difficult to make a compromise between the sintering temperature
of electrolyte and interface control of electrolyte/electrodes of ASSIBs.
Air plasma spraying (APS), as an efficient and economic coating technique, is theoretically capable of depositing
all kinds of ceramics which have a melting point [13]. For typical as-sprayed deposits of refractory ceramics,
more than 2/3 of the interface between splats are not bonded, which severely reduces the transportation
passage of ions and results in a much lower ionic conductivity along the thickness direction compared
with bulk material [13]. Yao et al. have reported that there exists a critical bonding temperature for certain
ceramic to achieve full bonding between adjacent splats during plasma spraying [14]. The critical bonding
temperature is proportional to the melting point of ceramic coating materials. With ceramic materials of a
melting point lower than 1450 o C, their critical bonding temperature is reduced to around or lower than the
normal temperatutre of 25 o C. This means that with such ceramic materials it becomes easy to deposit a
dense layer with well bonded splats. Fortunately, the NASICON-type electrolytes belong to such ceramic
materials. Besides the microstructure, the chemical composition is also crucial for the ionic conductivity of
NZSP. It has been proved that the extremely high temperature heating of plasma jet usually causes the preferential
evaporation of the constitute in complem ceramics with the high saturation vapor pressure and leads
to the deviation of chemical compositions from stoichiometric ratio of feedstock [15-17]. Zhang et al. reported
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