Electrochemical Performance and Mechanisms of NaSn₂(PO₄)₃/C Composites as Anode Materials for Li-Ion Batteries

Abstract

NaSn2(PO4)3/C composites obtained by solid-state and pyrolysis reactions present high capacity retention and high-rate capability as anode materials for Li-ion batteries. The structure of NaSn2(PO4)3 was analyzed by combining X-ray diffraction and density functional theory to confirm the R3̅ space group. The composite is formed by submicrometer particles with a cube-like shape coated by pyrolytic carbon that improves the electronic percolation. The 119Sn Mössbauer spectroscopy shows the existence of Sn4+ with a more ionic character than SnO2, which can be related to the inductive effect of the phosphate groups. This technique was used in the operando mode to follow the reaction with lithium during the first discharge that is a crucial step for improving the performance. A two-step mechanism has been identified that consists of the irreversible transformation of the pristine material into Sn0 species for the first half of the discharge followed by reversible Li–Sn alloying reactions. The Mössbauer spectra of the Sn0 species differ from the spectrum of βSn because of their nanosize and the existence of chemical bonds with the sodium phosphate matrix formed during the conversion reaction. The first part of the discharge should be considered as a restructuration of the pristine material leading to the dispersion of Sn0 small particles in strong interaction with the phosphate matrix and providing good cyclability. Such a mechanism strongly differs from the insertion mechanism of NaTi2(PO4)3/C that contains a transition metal with the same oxidation state as Sn4+.

Metal Ti quantum chain-inlaid 2D NaSn2(PO4)3/H-doped hard carbon hybrid electrodes with ultrahigh energy storage density

“…1d. The Raman spectra of all samples show the same main features as the previously published spectrum of NaSn 2 (PO 4 ) 3 [34]. The strong peaks around 1000 cm −1 can be assigned to the intra molecular stretching modes of PO 4 .…”

“…Na super ionic conductor (NASICON)-type phosphates have also been widely investigated as anode materials for both Li-ion and Na-ion batteries because of their open framework structure and unique physical and chemical properties, such as, high ionic conductivity, mechanical stability, thermal stability [26]. Among them, Sn-based phosphates, such as SnP 2 O 7 [27,28], LiSn 2 (PO 4 ) 3 [29][30][31], NaSn 2 (PO 4 ) 3 [32][33][34] etc have been considered as a promising alternative to graphite as the anode materials owing to their natural abundance, recyclability, low working potential (less than 0.6 V vs. Li/Li + ), high theoretical capacity (993 mA h g -1 for Sn), more than twice of graphite (372 mAh g -1 ). However, the commercial application of those materials is limited by their low electrical conductivity and rapid capacity fading due to the large volume change during the charge-discharge process.…”

NaSn2(PO4)3 submicro-particles for high performance Na/Li mixed-ion battery anodes

“…The excellent long cycling stability and high Coulombic efficiency of A-NSP anode were verified at the higher current densities of 2 and 5 A g -1 for 200 cycle (Fig. 5d), which are much higher than previous reported results [12][13][14][15][16][17][18][19]. It is interesting that, when the A-NSP anode was cycled at the higher current density of 5 A g -1 , its capacity displays a good cyclability for the first 100 cycles, and very little change, but continuously increases after 100 cycles.…”

“…However, some Sn-based materials such as SnO 2 , SnS 2 , and Sn-based alloys, suffer from enormous volume expansion (≈300%) during the charge-discharge process and intermediate dissolution, resulting in poor cycling stability and capacity retention [20]. In a number of Sn-based materials, Na super ionic conductor (NASICON)-type Sn-based phosphates, such as SnP 2 O 7 [12,13], LiSn 2 (PO 4 ) 3 [14][15][16], NaSn 2 (PO 4 ) 3 [17][18][19] etc, have many advantages, such as natural abundance, high ionic conductivity (about 1.5 × 10 -5 S cm -1 ), low working potential (less than 0.6 V vs. Li/Li + ), high theoretical capacity (993 mA h g -1 for Sn), more than twice of graphite (372 mAh g -1 ). Compared to Sn-based alloys, NaSn 2 (PO 4 ) 3 (NSP) not only owns an open three-dimensional (3D) framework structure, but possesses excellent structural stability and good thermal stability, resulting in smaller volume change during the charge-discharge process.…”

“…6e. This structure consists of a three-dimensional framework of corner-sharing slightly distorted SnO 6 octahedra and PO 4 tetrahedra [19]. Fig.…”