Structure solution of metal-oxide Li battery cathodes from simulated annealing and lithium NMR spectroscopy

Discerning the arrangement of transition metal atoms in Li­[Ni_<i>x</i>Mn_<i>y</i>Co_<i>z</i>]­O₂ cathode materials has remained an open problem for many years despite the commercial importance of some stoichiometries and the even more promising characteristics of others. We present a method for structural determination in this class of cathode materials. A simple definition of the total energy, based on the chemical principle of electroneutrality, is used in combination with a simulated annealing algorithm to generate model structures. The method reproduces the well-known structure of Li­[Li_1/3Mn_2/3]­O₂ and produces structures of the disordered Li­[Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>}]­O₂ phases (where <i>x</i> = 0.02, 0.1, 0.33) that are verified by detailed ⁷Li NMR spectra. For each Li­[Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>}]­O₂ phase, the solution is found to be heavily disordered, yet retaining significant ion pairing. Since the underlying notion of favoring charge-neutral regions is generic, we anticipate its utility in a much broader family of materials

The effect of ionic aggregates on the transport of charged species in lithium electrolyte solutions

In this investigation we focus on the problem of modelling the transport of the charged species (lithium ions) in electrolyte solutions with moderate and high salt concentrations (0.1M to &gt;2M), and consider the Nernst-Planck equation as a model of such processes. First, using a combination of magnetic resonance imaging (MRI) and inverse modelling (IM) we demonstrate that at higher concentrations the Nernst- Planck equation requires negative transference numbers in order to accurately describe the concentration profiles obtained from experiments. The need for such a physically inconsistent constitutive relation indicates the loss of validity of the Nernst-Planck equation as a model for this process. Next we consider the formation of ion pairs and clusters as a possible effect responsible for the appearance of negative transference numbers and derive an extended version of the Nernst-Planck system which accounts for these additional species. However, a careful analysis of this model reveals that incorporation of ion-pairing effects into the modelling will not change the transference numbers inferred from the experimental data via inverse modelling. This demonstrates that physical effects other than formation of ion pairs and clusters must be incorporated into the Nernst-Planck model in order for it to correctly describe ion transport at higher salt concentrations. One prime candidate for such effects is the motion of the reaction surface resulting from dendrite growth

A Polymer-Rich Quaternary Composite Solid Electrolyte for Lithium Batteries

All-solid-state batteries continue to grow as an alternative to replace the traditional liquid-based ones not only because they provide increased safety but also higher power and energy densities. However, current solid-state electrolytes are either ceramics that are brittle but highly conducting (e.g. Li0.33La0.55TiO3, LLTO) or polymer electrolytes that are poorly conducting but form flexible films with desired mechanical properties (e.g. Poly(ethylene oxide):Lithium bis(trifluoromethanesulfonyl)imide, PEO:LiTFSI). In this work, we have developed quaternary composite solid-state electrolytes (CSEs) to combine the benefits of the two types along with Succinonitrile (SN) as a solid plasticizer. CSEs with different compositions have been fully characterized over the whole compositional range. Guided by neural network simulation results it has been found that a polymer-rich CSE film gives the optimal ionic conductivity (>10−3 S cm−1 at 55 °C) and mechanical properties (Tensile strength of 16.1 MPa; Elongation-at-break of 2360%). Our solid-state coin-type cell which employs our in-house made cathode shows good cycling performance at C/20 and 55 °C maintaining specific discharge capacity at 143.2 mAh g−1 after 30 cycles. This new approach of formulating quaternary CSEs is proven to give the best combination of properties and should be universal and be applied to other CSEs with different chemistry

Adaptive Smooth Variable Structure Filter Strategy for State Estimation of Electric Vehicle Batteries

Battery Management Systems (BMSs) are used to manage the utilization of batteries and their operation in Electric and Hybrid Vehicles. It is imperative for efficient and safe operation of batteries to be able to accurately estimate the State of Charge (SoC), State of Health (SoH) and State of Power (SoP). The SoC and SoH estimation must remain robust and accurate despite aging and in presence of noise, uncertainties and sensor biases. This paper introduces a robust adaptive filter referred to as the Adaptive Smooth Variable Structure Filter with a time-varying Boundary Layer (ASVSF-VBL) for the estimation of the SoC and SoH in electrified vehicles. The internal model of the filter is a third-order equivalent circuit model (ECM) and its state vector is augmented to enable estimation of the internal resistance and current bias. It is shown that system and measurement noise covariance adaptation for the SVSF-VBL approach improves the performance in state estimation of a battery. The estimated internal resistance is then utilized to improve determination of the battery’s SoH. The effectiveness of the proposed method is validated using experimental data from tests on Lithium Polymer automotive batteries. The results indicate that the SoC estimation error can remain within less than 2% over the full operating range of SoC along with an accurate estimation of SoH

<i>Ex Situ</i>²³Na Solid-State NMR Reveals the Local Na-Ion Distribution in Carbon-Coated Na₂FePO₄F during Electrochemical Cycling

The potential Na-ion cathode material Na₂FePO₄F is investigated here by <i>ex situ</i> ²³Na solid-state nuclear magnetic resonance (ssNMR) in order to characterize the structure and ion mobility as a function of electrochemical cycling. The use of fast magic angle spinning (MAS) speeds of 65 kHz allows for the collection of high-resolution ²³Na NMR spectra that reveal two unique peaks at +450 and −175 ppm, corresponding to the two crystallographically unique Na sites in the material of interest. Two-dimensional NMR exchange spectroscopy results reveal that chemical exchange between the Na ions residing in distinct environments has a maximum hopping rate of ∼200 Hz. The collection of one-dimensional NMR spectra as a function of electrochemical cycling reveals the reproducible formation of a new peak at +320 ppm in the ²³Na NMR spectrum at all intermediate states of charge. The appearance of this resonance at +320 ppm is attributed to the fully oxidized (NaFePO₄F) phase that is present even upon initial electrochemical oxidation. The simultaneous existence of both the pristine and oxidized phases suggest formation of two distinct phases upon charging, consistent with a two-phase desodiation mechanism. This two-phase arrangement of Na ions persists for multiple charge/discharge cycles and is congruent with high reversibility of Na (de)­intercalation in Na₂FePO₄F cathodes. These findings imply that the Na₂FePO₄F framework is incredibly structurally stable with a robust intercalation process, despite a lack of ideal sodium-ion kinetics

Differentiating Lithium Ion Hopping Rates in Vanadium Phosphate versus Vanadium Fluorophosphate Structures Using 1D ⁶Li Selective Inversion NMR

The electrochemical performance of lithium ion batteries is strongly correlated with the ion dynamics within the electrode structures. This study characterizes Li ion hopping rates and energy barriers in the layered phase, Li₅V­(PO₄)₂F₂, using ⁶Li selective inversion (SI) NMR measurements. Li₅V­(PO₄)₂F₂ has six crystallographically distinct lithium sites giving the possibility of fifteen exchange partners between nonequivalent lithium environments. Here, ⁶Li 1D SI measurements over a variable temperature range were used to quantify the time scales and energy barriers of ion mobility for several ion pairs observed to participate in ion hopping. The rates determined in this material are similar in range to the previously determined rates found in tavorite Li₂VPO₄F yet considerably slower than results from both α-Li₃V₂(PO₄)₃ and α-Li₃Fe₂(PO₄)₃. A detailed analysis of the structural features that enhance or inhibit fast ion mobility is discussed. This includes a consideration of the bond valence density maps of the diffusion pathway. Comparison of the ion mobilities in the phosphates and fluorophosphates shows how the gains in redox potential come at the expense of fast ion mobility, meaning that any improvements to the energy output of the lithium ion battery through higher voltage may be compromised due to slow charge/discharge rates

Structure and electronic structure evolution of P2-NaxCoO2 phases from X-ray diffraction and 23Na magic angle spinning nuclear magnetic resonance

P2-Na 0.70 CoO 2 is considered as a model material for positive electrode application in Na-ion batteries. In this paper, we report an in-depth study and characterization of P2-Na x CoO 2 system, in order to understand the material evolution from the point of view of structure at different scales and electronic properties upon charge up to high voltage (4.6 V). Using a combination of ex-situ and operando XRD and ex situ 23 Na MAS NMR we discuss the structural changes occurring due to the deintercalation of Na + ions from the interlayer slabs and the change in the electronic structure and magnetic properties. The XRD study allows discussing the general evolution in relation with previous works. The novelty lies here in the observation for the first time of an ordered phase for x=1/3 appearing between above 4.3 V followed by a disordering in the slabs stacking for higher voltages. The combination of the data obtained by the different techniques allowed the interpretation of the NMR shift and shape evolution versus the Na content. This study reveals a complex behavior due to the presence of localized and delocalized electrons whose relative proportions is changing versus Na content