Electrochemical studies of mixed valence NASICON

Sodium Extraction from NASICON-Structured Na3MnTi(PO4)3 through Mn(III)/Mn(II) and Mn(IV)/Mn(III) Redox Couples

“…The redox couple of V 4+ /V 3+ in Na 3 V 2 (PO 4 ) 3 is located at a voltage of ∼3.4 V vs Na + /Na. , The redox couple of Ti 4+ /Ti 3+ in NaTi 2 (PO 4 ) 3 has also been exploited, but its voltage is too low (∼2.1 V vs Na + /Na) to be used as a cathode for sodium-ion batteries . Besides the well-documented NASICON compositions of ATi 2 (PO 4 ) 3 (A = Li + , Na + ) and A 3 M 2 (PO 4 ) 3 (M = V 3+ , Fe 3+ , Cr 3+ ; A = Li + , Na + ), limited work has been conducted on mixed transition-metal phosphates with the NASICON structure, apart from the reports of Na 2 TiM­(PO 4 ) 3 (M = Cr 3+ , Fe 3+ , Ga 3+ , and Rh 3+ ). − The preliminary investigation of NASICON-structured Li 2 TiFe­(PO 4 ) 3 and Li 2 TiCr­(PO 4 ) 3 in lithium-ion batteries revealed electrochemical insertion of lithium ions proceeds through the reduction of Fe 3+ into Fe 2+ (∼2.8 V vs Li + /Li) and Ti 4+ into Ti 3+ (∼2.5 V vs Li + /Li). ,, The insertion of sodium ions into Na 1.5 Fe 0.5 Ti 1.5 (PO 4 ) 3 and Na 1+ x Ti 2– x Fe x (PO 4 ) 3 was reported recently; besides the plateau at about ∼2.1 V vs Na + /Na from the Ti 4+ /Ti 3+ redox couple, a continuous voltage decrease is observed between 2.6 and 2.2 V vs Na + /Na presumably because of the reduction from Fe 3+ to Fe 2+ . , However, the Fe 3+ and Ti 4+ mixed phosphate cannot provide high voltages due to the difficulty of oxidation of Fe 3+ to Fe 4+ in the NASICON framework. The design of M 2+ and Ti 4+ mixed phosphate would open a way to access high voltage with the help of the M 3+ /M 2+ and M 4+ /M 3+ redox couples, especially with M 2+ = Mn 2+ .…”

Single-Crystalline Bilayered V2O5 Nanobelts for High-Capacity Sodium-Ion Batteries

“…Fluorophosphate and fluorosulfate materials were also developed as cathode materials for Na-ion batteries. − We have successfully prepared α-MnO 2 , β-MnO 2 , and Na 0.7 MnO 2 as cathode materials for Na-ion batteries. Rechargeable Li-ion or Na-ion batteries rely on a metal oxide cathode host, from and into which alkali ions can be inserted and extracted reversibly. − The lithium ion is small enough to have an acceptable mobility and a solid-solution range in close-packed oxide–ion arrays. However, the larger sodium ion and its higher ionization potential require a more open framework for Na-ion intercalation and deintercalation. − On the basis of this principle, the NASICON structured Na 1+3 x Zr 2 (P 1– x Si x O 4 ) 3 with the hexagonal framework exhibited fast sodium-ion conductivity due to its unique open-frame M 2 (XO 4 ) 3 structure .…”

Na Superionic Conductor-Type TiNb(PO4)3 Anode with High Energy Density and Long Cycle Life Enables Aqueous Alkaline-Ion Batteries

“…Figure a,b depicts the CV curves of carbon-coated TNPO in aqueous LiClO 4 and NaClO 4 at 0.1, 0.2, 0.4, 0.8, and 1 mV s –1 . As shown, the potential polarization is small with an increased scan rate, implying a fast kinetics for both Li + and Na + intercalation reactions. − The corresponding log­( i )–log­( v ) for various peaks are plotted in Figure c,d. The slopes of log­( i )–log­( v ) plots for peak 1–6 in LiClO 4 are close to 1, indicating that a pseudocapacitive process dominates the entire Li + intercalation reaction. − The slopes of log­( i )–log­( v ) plots for peaks 1, 2, 4, and 5 in the NaClO 4 electrolyte are close to 1, while the slopes for peaks 3 and 6 are calculated to be around 0.5.…”

“…Although the reversible multielectron redox behavior in the TNPO electrode has been reported in the organic electrolyte, the detailed electrochemical properties such as rate performance and cycling behavior have not been revealed. − ,, The TNPO anode possesses a suitable theoretical capacity of 189 mA h g –1 and a satisfying operating potential (about 2.2 V vs Li + /Li) in the organic electrolyte, which indicates that it can be selected as a good candidate anode electrode material for the aqueous battery technology . Here, a three-electrode electrochemical cell was fabricated to evaluate the feasibility and durability of the carbon-coated TNPO anode and its ion kinetic behaviors in aqueous LiClO 4 and NaClO 4 electrolytes, where Ti mesh and Ag/AgCl (in saturated KCl aqueous solution) served as counter and reference electrodes, respectively.…”

“…For instance, vanadium-based phosphates (e.g., Na 3 V 2 (PO 4 ) 3 ) show poor cycle capability regardless of their high capacity, and titanium-based materials (e.g., LiTi 2 (PO 4 ) 3 and NaTi 2 (PO 4 ) 3 ) exhibit inferior rate ability. − Hence, novel NASICON anode materials with high capacity, excellent cycling, and ultrahigh-rate performance are needed to be explored to build practical batteries. TiNb­(PO 4 ) 3 (TNPO), as a member of NASICON-type materials, has been investigated in solid electrolytes by Goodenough and Quarton for many years. − It has not drawn much attention because of its awkward operation voltage and intrinsic poor electronic conductivity. Nonetheless, its satisfactory theory capacity (189 mA h g –1 ) and stable 3D framework make it a suitable electrode candidate for aqueous alkaline-ion batteries.…”