Heteroatom Doping: An Effective Way to Boost Sodium Ion Storage

Na‐Ion Batteries—Approaching Old and New Challenges

Abstract P2-type Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 cathode material has been synthesized via spray drying method and a two step solid state process. Electrochemical behavior of the prepared material as cathode material for sodium ion battery was investigated in different charge-discharge voltage ranges. The results indicated that the cycling performance of the P2-Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 cathode greatly depended on the voltage window. The material showed excellent reversibility between 2.0 V and 4.0 V with reversible capacity of 86 mAh g −1 (0.1 C) and 77 mAh g −1 (1 C). XRD analyses indicated that crystal structure of the P2-type Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 could be well maintained after long term cycling in 2.0–4.0 V. When the upper limiting voltage was increased to 4.5 V, the crystal structure of P2-Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 was irreversibly damaged due to over extraction of Na + in 4.0–4.5 V. On the other hand, when the cycling voltage range was between 1.6 V and 3.8 V, the discharge capacities increased to 135 mAh g −1 (0.1 C) and 108 mAh g −1 (1 C), respectively. However, the cycling stability in 1.6–3.8 V was not as excellent as that in 2.0–4.0 V. This maybe due to the lattice stress caused by the over insertion of Na + in the structure at lower voltage.

Electrochemical Properties of P2-Na2/3[Ni1/3Mn2/3]O2 Cathode Material for Sodium Ion Batteries

Polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) typically reveal a sudden failure in Li metal cells particularly with high energy density/voltage positive electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 (NMC622), which is visible in an arbitrary, time – and voltage independent, “voltage noise” during charge. A relation with SPE oxidation was evaluated, for validity reasons on different active materials in potentiodynamic and galvanostatic experiments. The results indicate an exponential current increase and a potential plateau at 4.6 V vs. Li|Li+, respectively, demonstrating that the main oxidation onset of the SPE is above the used working potential of NMC622 being < 4.3 V vs. Li|Li+. Obviously, the SPE│NMC622 interface is unlikely to be the primary source of the observed sudden failure indicated by the “voltage noise”. Instead, our experiments indicate that the Li | SPE interface, and in particular, Li dendrite formation and penetration through the SPE membrane is the main source. This could be simply proven by increasing the SPE membrane thickness or by exchanging the Li metal negative electrode by graphite, which both revealed “voltage noise”-free operation. The effect of membrane thickness is also valid with LiFePO4 electrodes. In summary, it is the cell set-up (PEO thickness, negative electrode), which is crucial for the voltage-noise associated failure, and counterintuitively not a high potential of the positive electrode.

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Poly(Ethylene Oxide)-based Electrolyte for Solid-State-Lithium-Batteries with High Voltage Positive Electrodes: Evaluating the Role of Electrolyte Oxidation in Rapid Cell Failure.

Reaching the Energy Density Limit of Layered O3-NaNi0.5Mn0.5O2 Electrodes via Dual Cu and Ti Substitution

© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. Suppressing the mobility of anionic species in polymer electrolytes (PEs) is essential for mitigating the concentration gradient and internal cell polarization, and thereby improving the stability and cycle life of rechargeable alkali metal batteries. Now, an ether-functionalized anion (EFA) is used as a counter-charge in a lithium salt. As the salt component in PEs, it achieves low anionic diffusivity but sufficient Li-ion conductivity. The ethylene oxide unit in EFA endows nanosized self-agglomeration of anions and trapping interactions between the anions and its structurally homologous matrix, poly(ethylene oxide), thus suppressing the mobility of negative charges. In contrast to previous strategies of using anion traps or tethering anions to a polymer/inorganic backbone, this work offers a facile and elegant methodology on accessing selective and efficient Li-ion transport in PEs and related electrolyte materials (for example, composites and hybrid electrolytes).

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ange.201905794

Suppressed Mobility of Negative Charges in Polymer Electrolytes with an Ether-Functionalized Anion.

The many advantages of Na-ion batteries (NIBs) in terms of availability and cost of raw materials compared with Li-ion batteries (LIBs) are hindered by the stability of Na-based electrodes. The most promising NIB positive electrodes are Co- and Ni-free sodium manganese rich layered oxides with the general formula (y < 0.33, TM = transition metal/s). Although their stability is greatly improved when doped with electrochemically inactive species such as Mg or Ti, the rationale behind this has not been understood to date. Here, we demonstrate how a given degree of TiIV doping (z = 0.1) helps to stabilize the crystal structure of sodium manganese rich layered oxides by absorbing electrochemically induced strain; a remarkable step forward on the quest of finding the best NIB positive electrode. In this case, any Mn–Ti substitution below z = 0.1 will not be enough to absorb the strain and substitutions above this value will increase the amount of Jahn–Teller active MnIII leading to destabilization of the crystal structure with poor electrochemical performance. The possibility of controlling structural and electrochemical properties by TM substitution is the starting point towards the design of electrode materials that will ultimately lead towards competitive Na-ion batteries.

Unraveling the role of Ti in the stability of positive layered oxide electrodes for rechargeable Na-ion batteries

The search for sodium ion battery cathodes has led to considerable interest in sodium layered oxides due to their attractive properties – e.g. flexibility, versatility, and intrinsically fast Na ion structural diffusion (leading to enhanced rate capability). Based upon our rational approach to material selection (i.e. doping a Mn-rich structure with small quantities of elements carefully selected to enhance performance properties), we present here two high performance materials synthesised via solid-state reaction: P2-Na2/3Mn0.9−xNixTi0.05Fe0.05O2 (x = 0.10 and 0.20, designated Ni 10% and Ni 20% respectively). Electrochemical characterisation demonstrated that, while both materials compare favourably with the literature, the energy density and cyclability of the Ni 10% material was superior to that of Ni 20%. Detailed examination of the two materials using a range of techniques (including in situ and ex situ X-ray and neutron diffraction, scanning and transmission electron microscopy, solid state NMR, and DFT simulations) provides a good understanding of their relative physiochemical nature and electrochemical behaviour, and demonstrates the power of our stoichiometric selection strategy. In this way this work provides details of both new, high-performance materials, and validates our rational design approach.

High performance P2 sodium layered oxides: an in-depth study into the effect of rationally selected stoichiometry

P2-Na2/3[Fe1/2Mn1/2]O2 layered oxide is a promising high energy density cathode material for sodium-ion batteries. However, one of its drawbacks is the poor long-term stability in the operating voltage window of 1.5-4.25 V vs Na+/Na that prevents its commercialization. In this work, additional light is shed on the origin of capacity fading, which has been analyzed using a combination of experimental techniques and theoretical methods. Electrochemical impedance spectroscopy has been performed on P2-Na2/3[Fe1/2Mn1/2]O2 half-cells operating in two different working voltage windows, one allowing and one preventing the high voltage phase transition occurring in P2-Na2/3[Fe1/2Mn1/2]O2 above 4.0 V vs Na+/Na; so as to unveil the transport properties at different states of charge and correlate them with the existing phases in P2-Na2/3[Fe1/2Mn1/2]O2. Supporting X-ray photoelectron spectroscopy experiments to elucidate the surface properties along with theoretical calculations have concluded that the formed electrode-electrolyte interphase is very thin and stable, mainly composed by inorganic species, and reveal that the structural phase transition at high voltage from P2- to "Z"/OP4-oxygen stacking is associated with a drastic increased in the bulk electronic resistance of P2-Na2/3[Fe1/2Mn1/2]O2 electrodes which is one of the causes of the observed capacity fading. 

https://www.nature.com/articles/s42004-022-00628-0.pdf

Role of the voltage window on the capacity retention of P2-Na2/3[Fe1/2Mn1/2]O2 cathode material for rechargeable sodium-ion batteries

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201908784

Oier Lakuntza

Papers

Suppressed Mobility of Negative Charges in Polymer Electrolytes with an Ether-Functionalized Anion.

Unraveling the role of Ti in the stability of positive layered oxide electrodes for rechargeable Na-ion batteries

High performance P2 sodium layered oxides: an in-depth study into the effect of rationally selected stoichiometry

Role of the voltage window on the capacity retention of P2-Na2/3[Fe1/2Mn1/2]O2 cathode material for rechargeable sodium-ion batteries

Inside Cover: Suppressed Mobility of Negative Charges in Polymer Electrolytes with an Ether‐Functionalized Anion (Angew. Chem. Int. Ed. 35/2019)