Crystal chemistry of Mg substitution in NaMnPO₄olivine: concentration limit and cation distribution

Abstract

Metal ion substitution in phospho-olivines is an effective way to improve their performance as electrode materials in lithium ion and alternative sodium ion batteries. In this contribution, we examine in detail the crystal structure of Mg-substituted NaMnPO. The preferential occupancy of the alkaline M1 position by Mg ions has been found for the first time - a phenomenon which appears to be opposite to the case of Mg-substituted LiMnPO, where Mg and Mn reside in the M2 position. Mg solubility in NaMnPO is limited in the range of 0.10 < Mg/(Mg + Mn) < 0.15 mole part. Mg-substituted NaMnPO is prepared at 200 °C by ionic exchange reactions involving the participation of mixed dittmarite salts, KMnMgPO·HO. The structural aspects of Mg substitution in NaMnPO are studied by combination of powder X-ray diffraction using the Rietveld analysis with IR and electron paramagnetic resonance spectroscopy. The morphologies of precursors and target olivines are examined by means of SEM and EDS. In order to understand the crystal chemistry of Mg-substituted NaMnPO, we use solid solutions between LiMnPO and LiMgPO as references. The reference compositions of LiMnMgPO are prepared using the same KMnMgPO·HO precursors as in the case of Mg-substituted NaMnPO.

High‐Abundance and Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems, Progress, and Key Technologies

“…Mg solubility in NaMnPO 4 is limited in the mole ratio range of 0. www.advancedsciencenews.com < 0.15. [155] NaMnPO 4 /C composite was prepared through an ion-exchange reaction and ball milling processes. [156] The as-prepared compound consists of well-crystallized olivine-phase NaMnPO 4 with disordered carbon species and no antisite defects, delivering a reversible capacity between 80 and 85 mAh g −1 in model lithium cells (Figure 13b).…”

Storage performance of Mg2+ substituted NaMnPO4 with an olivine structure

“…Recently we have demonstrated that the Mg solubility in NaMnPO 4 is restricted up to 15 mol%. 8 This is in contrast to the lithium analogues, forming solid solutions having compositions such as LiMn 1Àx Mg x PO 4 without any concentration constrains. 9,10 The difference between Mg-substituted NaMnPO 4 and LiMnPO 4 analogues arises from their structure peculiarities.…”

“…9,10 The difference between Mg-substituted NaMnPO 4 and LiMnPO 4 analogues arises from their structure peculiarities. 8 For sodium manganese phospho-olivine, it is found out that Mg 2+ occupies preferentially M1 olivine site instead of the usual M2 sites and the magnesium charges are compensated by creation of corresponding metal vacancies in the M1 position: [Na 1À2x/(1Àx) Mg x/(1Àx) ] M1 [Mn] M2 PO 4 . 8 In lithium manganese phospho-olivine, Mg 2+ ions substitute the Mn 2+ ions, which leads to a signicant improvement in the electrochemical performance of LiMnPO 4 due to a reduction of the lattice mismatch between the lithiated and delithiated phases, as well as to a modication of the Mn 3+ /Mn 2+ redox couple.…”

“…8 For sodium manganese phospho-olivine, it is found out that Mg 2+ occupies preferentially M1 olivine site instead of the usual M2 sites and the magnesium charges are compensated by creation of corresponding metal vacancies in the M1 position: [Na 1À2x/(1Àx) Mg x/(1Àx) ] M1 [Mn] M2 PO 4 . 8 In lithium manganese phospho-olivine, Mg 2+ ions substitute the Mn 2+ ions, which leads to a signicant improvement in the electrochemical performance of LiMnPO 4 due to a reduction of the lattice mismatch between the lithiated and delithiated phases, as well as to a modication of the Mn 3+ /Mn 2+ redox couple. 9,11 Although the electrochemical performance of LiMnPO 4 in lithium ion batteries is a subject of intensive studies during the last years, the properties of NaMnPO 4 with respect to Li + and Na + intercalation are poorly understood.…”

“…The Mg substituted phospho-olivines are prepared on the basis of ionic exchange reactions involving participation of Mg-substituted KMnPO 4 -$H 2 O dittmarites as structural template. 8 In order to improve the conductivity, the phosphate particles were furthermore covered with activated carbon through ball-milling followed by thermal treatment at 400 C. This is a standard procedure for LiMnPO 4 in order to achieve a Mn 2+ /Mn 3+ plateau at 4.1 V at high reversibility. 6 The storage performance of phosphoolivines is analyzed in sodium and lithium half-ion cells, as well as in full-ion cells versus bio-mass derived activated carbon and spinel Li 4 Ti 5 O 12 as anodes.…”