Overview of Inorganic Electrolytes for All-Solid-State Sodium Batteries

All-solid-state sodium batteries (AS3B) emerged as a strong contender in the global electrochemical energy storage market as a replacement for current lithium-ion batteries (LIB) owing to their high abundance, low cost, high safety, high energy density, and long calendar life. Inorganic electrolytes (IEs) are highly preferred over the conventional liquid and solid polymer electrolytes for sodium-ion batteries (SIBs) due to their high ionic conductivity (∼10-2-10-4 S cm-1), wide potential window (∼5 V), and overall better battery performances. This review discusses the bird's eye view of the recent progress in inorganic electrolytes such as Na-β"-alumina, NASICON, sulfides, antipervoskites, borohydride-type electrolytes, etc. for AS3Bs. Current state-of-the-art inorganic electrolytes in correlation with their ionic conduction mechanism present challenges and interfacial characteristics that have been critically reviewed in this review. The current challenges associated with the present battery configuration are overlooked, and also the chemical and electrochemical stabilities are emphasized. The substantial solution based on ongoing electrolyte development and promising modification strategies are also suggested.

Enhanced ionic conductivity of a Na3Zr2Si2PO12 solid electrolyte with Na2SiO3 obtained by liquid phase sintering for solid-state Na+ batteries

NASICON-type Na₃Zr₂Si₂PO₁₂ (NZSP) is supposed to be one of the best potential solid electrolytes with the characteristics of high ionic conductivity and safety for use in solid-state sodium batteries. Many methods have been used to enhance the ionic conductivity of NZSP, among which liquid phase sintering is a simple and rapid method. However, the transport mechanism of sodium ions in a NZSP electrolyte obtained by liquid phase sintering is not clear, and its application in solid-state batteries has not been confirmed. In this study, we synthesized NZSP with Na₂SiO₃ additives by liquid phase sintering to reduce the sintering temperature and improve the ionic conductivity. NZSP with 5 wt% Na₂SiO₃ (NZSP-NSO-5) achieves the highest ionic conductivity of 1.28 mS cm^-1 and the lowest activation energy of 0.21 eV. Furthermore, the DFT study proves the Na⁺ diffusion mechanism and the decline in activation energy after addition. Lastly, the Na/Na₃V₂(PO₄)₃ battery with a Na₂SiO₃-added NZSP solid electrolyte exhibits a remarkably extended cycling capacity of 96.6% capacity retention after being cycled at 0.1 C 100 times. The liquid phase sintering with addition of low melting point salt compounds to electrolyte powder represents a rapid and straightforward technique for improving other ceramic electrolytes.

The Origin of High Na+ Ion Conductivity in Na1+xZr2SixP3–xO12 NASICON Materials

Due to the high sodium ion conductivity, sodium super ionic conductors (NASICONs) are among the most promising candidates as solid electrolyte in solid state batteries and have therefore gained enormous...

NaFe2PO4(MoO4)2: A Promising NASICON-Type Electrode Material for Sodium-Ion Batteries

Searching for polyanionic electrode materials with high Na⁺ and electronic conductivity is pivotal to realize high-performance sodium-ion batteries. Here, we report a novel polyanionic-based NASICON-type compound, NaFe₂PO₄(MoO₄)₂ (NFPM), that does not crystallize in the common space group R-3c or C2/c but in the rare P2/c. The studies on bond valence sum maps show that NFPM has high Na⁺ conductivity because the large volumes of MoO₄ groups make the interstitial channels wider, thus making the energy barrier of Na⁺ diffusion decrease along these channels. Density functional theory calculations demonstrate that NFPM has high electronic conductivity because the contribution of Mo 4d orbitals on the formation of the bottom of the conduction band makes the connected MoO₄ groups take part in electron transport. Electrochemical tests exhibit that NFPM can deliver a capacity of ∼80 mAh g^-1 with good reversible cyclability utilizing the Fe³⁺/Fe²⁺ redox couple. In situ X-ray diffraction measurements indicate that NFPM undergoes one-phase reaction mechanism in the process of charge and discharge.

Insight into the diffusion mechanism of sodium ion-polaron complexes in orthorhombic P2 layered cathode oxide NaxMnO2

Using the density functional theory, we investigated the geometric, electronic structure, phase stability and electrochemical properties of a potential P2 layer orthorhombic cathode material NaxMnO2 (0 ≤ x ≤ 1) applied for sodium-ion batteries. Herein, we shed the light on the undeniable effect of the polaron formation and polaron migration on the diffusion of Na+ ions in the orthorhombic P2 layered oxides. Both GGA+U and HSE06 methods agree that, when a Na+ ion is removed from the fully charged state of NaMnO2, the accompanying polaron preferably forms at one of the third nearest Mn (3NN) octahedra to the Na vacancy, implying the oxidization of the Mn3+ ion at one of these 3NN sites to Mn4+. The positive polaron migrates simultaneously with the Na vacancy and would hinder the diffusion of Na ions. Two kinds of elementary diffusion processes, named parallel and crossing, have been explored which required almost same activation energy of about 423 meV (518 meV) by GGA+U (HSE06). In the fully discharged state, GGA+U and HSE06 methods indicate that the negative polaron forms at one of the second nearest Mn neighbours (2NN). The activation energy of 273 meV (327 meV) is needed for diffusion in a structure with a low Na concentration, which is much lower than that required for diffusion in the Na-rich regime. Consequently, Na+ ions can diffuse easier at lower Na concentrations. With the overall activation energy of 423 meV (518 meV), this material exhibits a faster ion diffusion in comparison with the prevailing lithium-based materials such as olivine phosphate.

Hybrid Functional Study on Small Polaron Formation and Ion Diffusion in the Cathode Material Na2Mn3(SO4)4

The crystal structure, electronic structure, and diffusion mechanism of Na ions in the cathode material Na₂Mn₃(SO₄)₄ are investigated based on the Heyd-Scuseria-Ernzerhof hybrid density functional method. The simultaneous motion model of polaron-sodium vacancy complexes was used to reveal the diffusion mechanism of Na ions in this material. Polaron formation at the Mn third-nearest neighbor to the Na vacancy was found. Two crossing and two parallel elementary diffusion processes of the polaron-Na vacancy complex were explored. The most preferable elementary diffusion process has an activation energy of 852 meV, which generates a zigzag-like pathway of Na-ion diffusion along the [001] direction in the whole material. Possessing a voltage of 4.4 V and an activation energy of 852 meV, Na₂Mn₃(SO₄)₄ is expected to be a good cathode material for rechargeable sodium ions.

A first-principles investigation of the influence of polyanionic boron doping on the stability and electrochemical behavior of Na3V2(PO4)3

Na₃V₂(PO₄)₃ (NVP) is one of the most promising candidates for use as cathodes in room-temperature sodium ion batteries owing to its high structural stability and rapid Na⁺ transportation kinetics. The cationic doping of foreign ions at Na or V sites in the NVP lattice has proven to be an effective approach for enhancing the electrochemical performance of NVP. In this work, we present a first-principles density functional theory investigation of the impact of polyanionic boron doping at P sites on the structural and electrochemical behavior of NVP. Our simulation results suggest that B doping considerably increases the structural stability of NVP while shrinking its lattice size to some extent. Since B donates far fewer electrons to connected O atoms, the surrounding V atoms become more positive, causing the operating voltage to increase with B content. However, the reduction in lattice size is not beneficial for the Na⁺ transportation kinetics. As demonstrated by a search for the transition state, a concerted ion-exchange mechanism is preferred for Na⁺ transportation, and increased B doping leads to a higher Na⁺ diffusion barrier. Improvements in electrochemical performance due to B doping see (Hu et al. Adv Sci 3(12):1600112, 2016) appear to originate mainly from the resulting increased electrical conductivity.

Diffusion mechanism of Na ion–polaron complex in potential cathode materials NaVOPO4 and VOPO4 for rechargeable sodium-ion batteries

Using the density functional method, we investigated the crystal and electronic structures and the electrochemical properties of Na_xVOPO₄ (x = 0, 1) and explored the diffusion mechanism of Na ions in these materials.