Reducing Transformation Strains during Na Intercalation in Olivine FePO₄ Cathodes by Mn Substitution

Abstract

Sodium iron phospho-olivine, NaFePO 4 , is a promising cathode material for Na-ion batteries. However, Naion extraction and intercalation induce a large discontinuous volume change (>10 vol%), which leads to large transformation strains and may hamper the rate capability as well as the cyclic stability of NaFePO 4. In the Liion counterpart LiFePO 4 , the size of the discontinuous volume change can be effectively reduced by substitution of Mn onto the Fe-sites in the phospho-olivine structure, i.e. LiMn y Fe 1-y PO 4 solid solution. In this paper, we investigate the battery performance and phase evolution during Na-ion storage in a series of Mn y Fe 1y PO 4 (y = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8) cathode materials. Substitution of 10-20% of the Fe by Mn leads to enhance the capacity by >15%. Using operando powder X-ray diffraction, we discover in these materials, the charge-discharge transformation strains are significant reduced by stabilization of a Na z Mn y Fe 1-y PO 4 solid solution over a wide Na-compositional range. This effect even leads to a complete continuous solid solution transformation covering the entire charge process, which shows that the effect of Mn-substitution is a highly effective route for reducing intercalation strains even during intercalation of the relatively large Na-ion.

Expanded solid-solution behavior and charge-discharge asymmetry in NaxCrO2 Na-ion battery electrodes

“…For the battery cell employing liquid electrolyte (LE-cell), an electrode composite was prepared from conductive carbon, active material (NaCrO 2 ), and binder in a mass ratio of 2:6:2 as described in previous work [31][32][33][34]. Conductive carbon and binder (Graphite C-Nergy SFG6L, 99.5%, Imerys Graphite and Carbon, acetylene black VXC72, Cabot Corporation, and Kynar PVDF Arkena Inc.) were pre-dried at 60 • C in a vacuum oven for >4 h before being imported into the glove box [30].…”

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

“…220 In studies of battery materials, operando experiments allow following the changes in electrode structure during operation, and the use of PDF has revealed complex reactions, where disordered nanomaterials play a large role in the cycling processes. [221][222][223][224][225][226][227][228][229] Summary and future perspectives…”

Progress on Fe‐Based Polyanionic Oxide Cathodes Materials toward Grid‐Scale Energy Storage for Sodium‐Ion Batteries

“…investigated the performance of a series of olivine‐type Mn y Fe 1‐ y PO 4 during charge and discharge, concluding that Mn substitution reduces the lattice strain induced by Na + insertion and desorption effectively, in which 10–20% Mn substitution increased the capacity by more than 15% over FePO 4 (Figure 3b). [ 54 ] Initial reports about the electrochemical inactivity of maricite‐type NaFePO 4 and the absence of sodium‐ion diffusion channels in the closed structure is misleading. [ 55 ] The maricite phase powder with conductive carbon powder ball‐milled showed an initial discharge capacity of 142 mAh g –1 at 0.05 C (≈92% of the theoretical capacity) and the capacity retention rate of 95% after 200 cycles at 0.05 C (Figure 3c).…”

“…Reproduced with permission. [ 54 ] Copyright 2019, American Chemical Society. c) Galvanostatic discharge/charge profiles of maricite NaFePO 4 at 0.05 C and the inset showed the rate capacity from 0.05 C to 3 C. Reproduced with permission.…”

“…b) Comparison of discharge/charge curves between NaMn y Fe 1−y PO 4 and Mn y Fe 1−y PO 4. Reproduced with permission [54]. Copyright 2019, American Chemical Society.…”