Role of Lattice Flexibility on Electrochemical Ion Insertion: Structural Dynamics in WO₃ vs WO_2.9

The demand for high energy and high power energy storage devices drives the search for electrode materials that can exhibit high capacities and fast kinetics. One strategy to achieve these properties is through the use of ion insertion-type electrodes that can undergo at least 1 e - transfer (for high specific capacity) with little structural change (for fast kinetics). To investigate this strategy, we examined the ion insertion behavior of WO 3 with the related oxygen deficient Magnéli phase, WO 2.9 . WO 2.9 contains crystallographic shear planes, which are planes of edge-sharing WO 6 octahedra within a corner-sharing framework. The presence of crystallographic shear planes is proposed to increase ion insertion kinetics because they decrease the lattice flexibility of the material. To test this hypothesis, we performed cyclic voltammetry in aqueous and non-aqueous electrolytes as well as galvanostatic intermittent titration technique (GITT) and galvanostatic charge discharge (GCD) to study the kinetics H + and Li + insertion into WO 3 and WO 2.9 . In acidic electrolytes, WO 3 inserts more H + , achieving a capacity of 41 mAh/g, compared to WO 2.9 which has a capacity of 17.5 mAh/g. In non-aqueous electrolytes, WO 2.9 can insert more Li + than WO 3 , achieving a capacity of 127 mAh/g at a rate of C/10 compared to WO 3 , which has a capacity of 100 mAh/g. During Li + insertion, WO 2.9 exhibits a diffusion coefficient of 10 -8 cm 2 /s compared to WO 3 , which has a diffusion coefficient of 10 -10 cm 2 /s. To measure the lattice flexibility during H + insertion, we performed operando AFM experiments. We measured the local change in thickness of tungsten oxide electrodes in aqueous acidic electrolyte during electrochemical cycling. We observe that WO 2.9 undergoes less deformation during proton insertion and de-insertion than WO 3 . However, it undergoes higher deformation per e - /formula unit. To investigate the lattice flexibility and structural changes during Li + insertion and de-insertion, we performed in situ XRD and observed that WO 2.9 undergoes a small volumetric contraction of < 0.6% during the insertion of 1.14 Li + per formula unit. This study reports the benefits and tradeoffs of utilizing non-stoichiometric oxides like WO 2.9 as electrodes for insertion-type high power energy storage.