Competitive lithium and sodium intercalation into sodium manganese phospho-olivine NaMnPO4 covered with carbon black

Abstract

In this contribution we provide novel data on the reversible lithium and sodium ion intercalation into a sodium-manganese phospho-olivine NaMnPO₄, when it is used as a cathode in model lithium-ion cells. The ion-exchange reaction involving the participation of KMnPO₄·H₂O dittmarite as precursor was chosen for the preparation of NaMnPO₄. The NaMnPO₄ particles were covered with carbonaceous materials to improve the electrical conductivity and electrolyte wetting. The procedure includes ball-milling of NaMnPO₄ with conductive carbon black additives Super C/65, followed by thermal treatment. The mechanically treated samples consist of well crystallized phospho-olivine phase NaMnPO₄ free of any anti-site defects and disordered carbon species with graphite like medium-range order. The composite NaMnPO₄/C material manifests a reversible capacity between 80–85 mA h g⁻¹ in model lithium cells versus lithium anode. Prior to the electrochemical test, the chemical inertness of NaMnPO₄ in the lithium electrolyte is studied by soaking phospho-olivines in the solution of LiPF₆ in EC:DMC. The mechanism of the reversible intercalation/deintercalation cycling is investigated using ex situ X-ray powder diffraction, TEM and high-angle annular dark field STEM analysis, infrared spectroscopy and electron paramagnetic resonance spectroscopy (EPR). The study demonstrates, for the first time, that NaMnPO₄ is able to intercalate reversibly both Na⁺ and Li⁺ ions following the chemical reaction Li_xNa_1−xMnPO₄ ↔ Li_0.0Na_0.5MnPO₄ (0.25 ≤ x ≤ 0.45).