A New Strategy to Build a High‐Performance P′2‐Type Cathode Material through Titanium Doping for Sodium‐Ion Batteries

Abstract

1901912 (1 of 10) Herein, Ti 4+ in P′2-Na 0.67 [(Mn 0.78 Fe 0.22 ) 0.9 Ti 0.1 ]O 2 is proposed as a new strategy for optimization of Mn-based cathode materials for sodium-ion batteries, which enables a single phase reaction during de-/sodiation. The approach is to utilize the stronger Ti-O bond in the transition metal layers that can suppress the movements of Mn-O and Fe-O by sharing the oxygen with Ti by the sequence of Mn-O-Ti-O-Fe.It delivers a discharge capacity of ≈180 mAh g −1 over 200 cycles (86% retention), with S-shaped smooth charge-discharge curves associated with a small volume change during cycling. The single phase reaction with a small volume change is further confirmed by operando synchrotron X-ray diffraction. The low activation barrier energy of ≈541 meV for Na + diffusion is predicted using first-principles calculations. As a result, Na 0.67 [(Mn 0.78 Fe 0.22 ) 0.9 Ti 0.1 ]O 2 can deliver a high reversible capacity of ≈153 mAh g −1 even at 5C (1.3 A g −1 ), which corresponds to ≈85% of the capacity at 0.1C (26 mA g −1 ). The nature of the sodium storage mechanism governing the ultrahigh electrode performance in a full cell with a hard carbon anode is elucidated, revealing the excellent cyclability and good retention (≈80%) for 500 cycles (111 mAh g −1 ) at 5C (1.3 A g −1 ).reactions. [3][4][5] Although SIBs have intrinsic drawbacks, such as their low operation voltages relative to those of LIBs and the difficulty of ready insertion of sodium ions because of the larger size of Na + ions (1.02 Å) compared with Li + ions (0.76 Å), these difficulties can be mitigated with a high capacity to compensate for the low operation voltage. [5] Layered cathode material for SIBs (Na x MeO 2 ) has received particular attention owing to their relatively high capacity and structural stability. Layered Na x MeO 2 (x = 0.5-1 and Me; transition metal) consist of MeO 2 layers sharing edges with MeO 6 octahedra were classified into two groups based on structure: trigonal prismatic (P type: P2, P3, and P′2) and octahedral (O type: O3). [6] The differences in these structures are attributed to sodium ions being respectively located at the district trigonal prismatic or octahedral crystallographic sites sandwiched between the MeO 2 sheets. Among them, many works have introduced Mn-based cathode materials, mainly P2-type materials, in which sodium ions are located at prismatic sites with an AABB oxygen stacking sequence, because of their low cost, good performance, and nontoxicity. [7,8] Recently, many works about P2-type materials have been investigated such as Na x MnO 2 , [7,9] Na x CoO 2 , [10] Na x VO 2 , [11] Na x [Ni,Mn] O 2 , [12] Na x [Fe,Mn]O 2 , [13] Na x [Ni,Fe,Mn]O 2 , [14] Na x [Mg,Mn] O 2 , [15] Na x [Ni,Mg,Mn]O 2 . [16] P2-type materials crystallize in a hexagonal structure; however, the transition metal layers can be distorted at higher temperature with stabilization in an orthorhombic structure (represented as P′2) albeit with the same chemical composition. [17,18] Many works have introduced Mn-based c...

Mitigation of Jahn–Teller distortion and Na+/vacancy ordering in a distorted manganese oxide cathode material by Li substitution

“…The c parameter increases along with the (002) and (004) peaks shiing to lower angles due to the repulsive force between the adjacent oxygen layers during Na + extraction, and the ab plane contracts as the (112) peak shis to higher angles because of the smaller ion radius of Mn at the valence state of 4+. 19,30 The OP4 phase starts to appear upon extraction of $0.39 mol Na + and remains till full charge, as conrmed by the typical OP4 (004) peak at $17.5 in Fig. 2 and illustrated in Fig.…”

“…Particularly, the distorted P 0 2-Na 0.67 MnO 2 with an orthorhombic structure caused by the Jahn-Teller (J-T) effect exhibits higher capacity than the undistorted P2-Na 0.67 MnO 2 . [15][16][17][18][19][20] However, the J-T distortion, derived from high spin Mn 3+ (t 2g 3 -e g 1 ), causes an abnormal bond length change of O-Mn-O in [MnO 6 ] octahedra, large lattice strain and anisotropic volume change during the charge and discharge processes, resulting in Na + /vacancy ordering and rapid degradation of structural and electrochemical properties of P2/P 0 2type Na 0.67 MnO 2 . 15,[21][22][23] Due to the lattice distortion in P 0 2-Na 0.67 MnO 2 , a high specic capacity of $200 mA h g À1 can be obtained at the initial cycles, but followed by rapid capacity decay because of the drastic J-T effect during Na + extraction/ insertion.…”

“…[24][25][26][27] When doped with Ti, P 0 2-Na 0.67 [(-Mn 0.78 Fe 0.22 ) 0.9 Ti 0.1 ]O 2 delivers a capacity of $180 mA h g À1 and exhibits reversible Na + extraction and insertion for 200 cycles with a retention of 86%. 19 With the introduction of Al into P2-Na 0.67 Al x Mn 1Àx O 2 , the complex phases of P 0 2 + P2 are electrochemically converted to the undistorted P2, leading to better capacity retentions. The sample doped with 10 mol% Al yields a specic capacity of 175 mA h g À1 and long cycling stability exceeding 200 cycles with 85% capacity retention.…”

The Cathode Choice for Commercialization of Sodium‐Ion Batteries: Layered Transition Metal Oxides versus Prussian Blue Analogs

“…With rational composition design and structure optimization, introducing minor quantities of cationic substitution is the most utilized and effective way, and it is capable of affording a significant improvement of the overall electrochemical performance, even the air stability 146. Sacrificial salt compensation and surface modification could effectively endow layered Na x MO 2 materials with improved Coulombic efficiency and longer cycle life 25,147. The exploration of P‐ and/or O‐type mixed phases, of which several materials with both satisfactory performance and air stability are reported,14,148 is inspiring a new perspective on the development of layered Na x MO 2 .…”

“…NaCrO 2 can deliver a highly reversible capacity of ≈120 mAh g −1 , while LiCrO 2 is electrochemically inactive 11. Different cathode materials, including transition metal (M) oxides (Na x MO 2 , x ≤ 1),12–26 hexacyanoferrates (HCF) or Prussian blue and its analogs (PBAs),27–32 polyanionic compounds,33–47 and organic compounds48–57 have been widely studied for SIBs. The substantial growth of exploration on full cell systems, which serve as a bridge between laboratory studies and practical application, clearly reveals the unprecedented interest in and expectation for the commercialization of SIBs (Figure 1c).…”